Solar wind ocean energy

sean faris

Solar wind ocean energy

sean faris

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Abstract
AI

The paper explores the potential of utilizing ocean thermal energy in conjunction with solar and wind systems to create hybrid energy solutions. It discusses the underlying principles of photovoltaic cells, including the role of p/n junctions in generating electric fields, and examines the necessary components for a functional energy system. Additionally, it presents models and case studies focused on developing countries with suitable ocean thermal resources, highlighting geographic and temperature conditions that facilitate energy generation from ocean sources.

Figures (410)

FIGURE 1.1. p—njunction of the PV cell. The two semiconductors behave like a battery, creating an electric field at the surface where they meet, called the p/n junction. This is a result of the flow of electrons and holes. The electrical field forces the electrons to move from the semiconductor toward the negative surface to carry current. At the same time, the holes move in the opposite direction, toward the positive surface, where they wait for incoming electrons [8]. gia 7 Rae eer

power output of PV cells, they have to be connected together to form larger units called modules. The modules, in turn, can be connected to form larger units called arrays, which can be interconnected to produce more power. By connecting the cells or modules in series, the output voltage can be increased. On the other hand, the output current can reach higher values by connecting the cells or modules in parallel.

A solar energy system consists of several components, as shown in Figure 1.3. Several PV modules are connected in series or in parallel, depending on the voltage and energy requirements of the PV array. The direction of the PV arrays toward the sun might be controlled by the position motors allowing the motion of the panels horizontally and/or vertically. The sun tracking system takes the data from sun sensors or photodiodes and processes the data to determine the motor positions for the best illumination position of the PV array directed to the sun. This position detection and adjustment is similar to a mechanical power point tracking technique [18]. On the other hand, due to the voltage- current and current-power characteristics of the PV array, the electrical maximum power point (MPP) may vary based on operating current and voltage. Therefore, an electrical maximum power point tracking (MPPT) system is needed to operate the PV array at the MPP. This can be done through a DC/DC converter. Tyo Le LL ee oe et Li le 8 ee LL OL Ld ee ew ee ee nee bn L] Lee che ne TOUT ow mee ee se ee eeae

FIGURE 1.5 Current-power curves of the PV Cell for different short-circuit current values.

FIGURE 1.4 Current—voltage curves of the PV cell for different short-circuit levels.

FIGURE 1.6 (a) Series and (b) parallel connection of identical cells.

FIGURE 1.7 Single-diode model of solar cell equivalent circuit. also KNOWN as a singie-diode model. In this model, open-circuit voltage and short-circuit current are the key parameters. The short-circuit current depends on illumination, while the open-circuit voltage is affected by the material and temperature. In this model, Vy is the temperature voltage expressed as Vy = kT /q, which is 25.7 mV at 25°C. The ideality factor « generally varies between 1 and 5 for this model. The equations defining this model are

FIGURE 1.9 Model for a single solar cell.

The I-V characteristic of the solar cell can be alternatively defined by [21]

FIGURE 1.10 A typical I-V curve for a solar cell. where G, is ambient irradiation and A is cell area.

Operational Nominal and Standard Conditions for PVs

FIGURE 1.12 (a) Influence of the ambient irradiation and (b) influence of the cell temperature on the cell I-V characteristics.

FIGURE 1.13 (See color insert following page 80.) The structure of the sun tracking system based on angle of the light using two photodiodes. a a Dynamic range is the parameter that the sun tracking system responds to in threshold limits. The distance between both modules does not have to limit the dynamic range. If the dynamic range is increased, the sensitivity of the current differences decreases. Therefore, between the dynamic range and the sensitivity, there is a trade-off. As a solution, two

FIGURE 1.14 (See color insert following page 80.) (a) x-Axis sensor module structure; (b) y-axis sensor module structure.

FIGURE 1.15 Block diagram of the sensor output evaluation and motor control for sun tracker.

FIGURE 1.17 Incremental conductance algorithm flow-chart diagram.

FIGURE 1.18 Operating point trajectory of incremental conductance—based MPPT.

FIGURE 1.19 Normalized IV, PV, and |dP/dV| characteristics of a PV array. 1.5.2 Perturb and Observe—-Based MPPT

FIGURE 1.20 A typical I-P curve of a PV array.

it they rollow the opposite trend, the current reference should be reversed. The initial value of the current can be zero or any closer value to the Iypp, that is, the current at the MPP. On the other hand, choosing an appropriate incremental step (Alter) can quickly approach the operating point to MPP. This process should be repeated until the maximum available power from the PV array is found and extracted. The oscillation can be minimized with smaller perturbation steps. However, small step sizes cause the slow response of the MPP tracker [33]. The solution to this problem can be found using variable perturbation steps as addressed in [32,34-36]. In this method, the perturbation size should become smaller while approaching the MPP.

FIGURE 1.22 A typical I-V curve with linearized functions.

FIGURE 1.23 Linearized I-V characteristics-based MPPT controller. 1.5.4 Fractional Open-Circuit Voltage-Based MPPT

FIGURE 1.24 Implementation of fractional open-circuit-based MPPT. 1.5.5 Fractional Short-Circuit Current-Based MPPT

FIGURE 1.25 Implementation of fractional short-circuit-based MPPT. MPP is never perfectly matched, as suggested by Equation 1.36, since it is an approxima- tion of the current of the MPP. The PV array can be periodically swept from open circuit

FIGURE 1.26 Membership function for inputs and outputs of fuzzy logic controller.

Fuzzy Rule Base Table TABLE 1.3 and goes to zero while becoming closer to the MPP. Generally, the output of the fuzzy logic controller is the change in duty ratio AD of the power converter. This change in the duty ratio can be looked up in a look-up table such as Table 1.3 [50], right after E and AE are calculated and converted to the linguistic variables.

FIGURE 1.27 Implementation of fuzzy logic controller—-based MPPT.

1.5.8 Ripple Correlation Control-Based MPPT There are voltage and current ripples on the PV array due to the switching events of the connected power electronic converter. Therefore, the power output of the PV array may also contain ripples. These ripples are used in ripple correlation control (RCC) [66] to perform MPPT. RCC correlates the time derivative of the time-varying PV array power p with the time derivative of the time-varying PV array current i or voltage @ to drive the power gradient to zero, in order to track the MPP. The operating point is below the MPP (V < Vypp or! < Iypp) if v or iis increasing (0 > 0

FIGURE 1.30 Implementation of RCC-based MPPT technique.

FIGURE 1.32 DC link capacitor droop control. The computation of PV array power is not required in this method. However, when compared to the methods that detect the power directly, this method has lower accuracy [72], since the response of the DC-voltage control loop of the inverter directly affects its response. where V is the voltage across the input voltage of the power converter and Vjink is the voltage across the DC link. If Viink is fixed, the extracted power of the PV array can be controlled by varying the input current of the inverter. The voltage Vjink can be kept constant as long as the power required by the inverter does not exceed the maximum available power from the PV array while the current is increasing; otherwise Vjink droops. The current control command of the inverter (Ipeak) is at its maximum value and the PV array operates at the MPP right before that point. To achieve the MPP, d is optimized to bring Ipeak to its maximum value in order to prevent Viink from drooping. This droop can be prevented by feeding the AC system line current back. Se ny ee a ly a rr i er : Os ie ee:

FIGURE 1.31 Implementation of current sweep-based MPPT.

FIGURE 1.33 (a) Bypass diodes connected across each cell and (b) bypass diodes connected across a string 0 cells.

FIGURE 1.35 I-V characteristics under different shading conditions.

FIGURE 1.34 I-V characteristics for PV strings in shaded condition.

FIGURE 1.36 MPP relocation under shaded condition. Therefore, Voc can be written as

FIGURE 1.37 (See color insert following page 80.) Pyypp, FE, and y variations versus the shading factor S. where Eo is the illumination of unshaded cell and As is the shaded area. Figure 1.37 gives the MPP, fill factor, and efficiency under various shading factor values. The efficiency and fill factor suddenly drop for shading factors larger than a certain point. The power performance of the shaded PV cell almost linearly decreases as the shading factor increases [75].

FIGURE 1.38 Power electronic interfaces classification based on inverter utilization and stage/module configurations.

FIGURE 1.39 Conventional PV system technology using centralized inverter system topology.

FIGURE 1.41 Multistring inverters topology.

FIGURE 1.42 Isolated DC/DC converters: (a) H-bridge DC/DC converter, (b) series resonant H-bridge DC/DC converter, and (c) push-pull DC/DC converter [77].

FIGURE 1.43 PV array current for push-pull converter. FIGURE 1.44 Transformer output voltage in the push-pull converter.

FIGURE 1.45 Output voltage of the push-pull converter.

FIGURE 1.46 Fly-back current-fed inverter. The fly-back current-fed inverter shown in Figure 1.46 can be controlled to provide a rectified sine-wave output current into the inverter and to track the MPP [82]. The current into the fly-back converter is discontinuous and hence a buffer capacitor should be used to eliminate both low- and high-frequency ripples.

FIGURE 1.48 Inverter input voltage in the fly-back converter.

In topologies shown in Figure 1.52a and b, instead of using a full bridge inverter for the DC/AC conversion stage, a half bridge inverter can also be used. In this way, the number of switching elements can be reduced and the controller can be simplified; however, for the DC bus two-series connected capacitor is required to obtain the mid-point. This mid-point of two series connected capacitors will be used as the negative terminal of the AC network of the half bridge configuration. These half bridge inverter topologies are illustrated in Figure 1.53a and b.

FIGURE 1.50 Inverter output voltage with LC filter.

FIGURE 1.52 (a) Boost converter with full-bridge inverter and (b) buck converter with full bridge inverter. The current and voltage waveforms of a nonisolated two-stage converter in which a boost converter is followed by a full bridge inverter (Figure 1.52a) are demonstrated in Figures 1.54 through 1.57. The inductor of the boost converter is 0.1 mH, the capacitor of the boost converter is 5.6 mF, and the nominal output voltage of the PV array is 48 V. The switching frequency of the boost converter is 10 kHz. The 48 V output of the PV array

FIGURE 1.51. Aseries resonant DC/DC converter and its grid-connected inverter.

FIGURE 1.53 (a) Boost converter connected to a half bridge inverter and (b) buck converter connected to a half bridge inverter.

IGURE 1.54 PV array output current for boost converter/inverter topology.

FIGURE 1.55 Boost converter output voltage or inverter input voltage for boost converter/inverter topology.

FIGURE 1.56 Inverter output voltage for boost converter/inverter topology.

FIGURE 1.57 The filtered inverter output voltage.

FIGURE 1.58 Single-stage inverter for multiple modules. A typical single-stage inverter for multiple modules is depicted in Figure 1.58, which is the simplest grid connection topology [85]. The inverter is a standard voltage source PWM inverter, connected to the utility through an LCL filter. The input voltage, gen- erated by the PV modules, should be higher than the peak voltage of the utility. The

FIGURE 1.59 A multilevel converter topology. ORR na Reinie en gaye eoeme wee Mtn pe Sein irene En, Me ae Rea ee Reem ee, eet SNe eee ee pecie RS ot eh three-level inverter can be expanded into five, seven, or even more levels by adding more modules and switches [87,88]. This allows for further reduction of the harmonic distor- tion. Drawbacks of this topology are the high number of required semiconductors and imbalanced loading of the different PV strings. Therefore, maximum power transfer from each individual string can be difficult to reach, especially when moderate shading occurs. To obtain a positive voltage at the inverter’s output terminals, upper switches S; and S» should turn on, while to obtain a negative voltage the lower switches 53 and S4 should conduct. In order to obtain zero voltage, the two intermediate switches should be turned on, that is, Sz and $3. The DC bus voltage should be greater than grid voltage amplitude in order to transfer power to the grid. Thus, a higher number of PV modules or individual boost DC/DC converters can be employed to achieve the desired voltage levels. In this

system, each of the PV strings is connected to the neutral point of the grid through the capacitors. This results in reducing the capacitive ground currents. In addition, the negative effect of these currents on electromagnetic compatibility is decreased [89,90]. However, a disadvantage of this topology is to load the DC source only during a half cycle, which increases the size of the decoupling capacitors and increases the cost of the inverter. The output voltage is 0, +Vac, or —Vac for a or 2a degrees, as shown in Figure 1.61 [91]. By controlling the a interval, the output voltage of the inverter can be controlled. TM laes: eedene eeopicen cil Ble mnnbencdkermn easier: wean hoa eaweeemoen «eo

FIGURE 1.61 HBDC inverter output voltage.

FIGURE 1.62 Output inductor current for an HBDC inverter ee ee In configuration Figure 1.64b, the strings can operate at their individual MPP; therefore a better overall efficiency is expected. APV system composed of strings with an individual DC/DC converter, a common grid-connected inverter, and the generation control circuit (GCC) is shown in Figure 1.65, which consists of two buck—boost converters with a common inductor [92]. The left leg of the inverter can control the voltage across each string individu- ally. The upper switch along with the freewheeling diode in the lower switch and the input

FIGURE 1.64 Configurations of dual-stage inverters for multiple PV modules: (a) modules with a common dual-stage inverter, and (b) strings with individual DC/DC converter and a common grid-connected inverter. FIGURE 1.63 Output voltage for an HBDC inverter.

FIGURE 1.65 Utility interactive PV inverter. inductor plus the capacitors across the strings forms a buck—boost converter. Similarly the opposite arrangement forms another buck—boost converter topology. The right leg of the inverter controls the current through the output inductor, and hence the current injected into the utility grid. The amplitude of the grid current is determined by an MPPT system. Therefore, MPPT controls the voltages across each string in order to achieve maximum power. This topology is a good solution for a multimodule system since it increases the overall efficiency of the system without adding extra components. It can also be expanded to multiple modules, by adding more chopper stages. A modified form of the previously mentioned topology, which consists of a buck-boost

FIGURE 1.67 Voltage of the inductor at the buck—boost stage for modified GCC topology. FIGURE 1.66 Modified structure of the topology given in Figure 1.65.

FIGURE 1.69 Input voltage of the inverter (output voltage of the DC/DC converter). FIGURE 1.68 Input switch current.

Another topology for multimodule multistring interfaces is shown in Figures 1.71 and 1.72 [80,93]. The inverter in Figure 1.71 consists of three boost converters, one for each PV string, and a common half bridge PWM inverter. The circuit can also be constructed with an isolated current- or voltage-fed push-pull or full-bridge converter [93], similar to the circuit in Figure 1.72, and a full-bridge inverter in order to connect to the utility. The voltage across each string can be controlled individually [80,93].

FIGURE 1.71 Topology of the power electronics of the multistring inverter.

FIGURE 1.72 Topology of the power electronic interface for a three-string inverter.

FIGURE 1.73 PV/battery connection—type 1. In the connection topology shown in Figure 1.74, the battery bank is connected to the output of the PV panel in parallel, instead of the cascaded connection. In this parallel connection, the DC/DC converter at the battery bus is never disabled, since it should be operated in either insulated or noninsulated conditions. The DC/ AC inverter and the DC/DC converter should manage the load demands and capture the maximum available power from the PV panel. This topology requires a more complicated control strategy in comparison with the

FIGURE 1.75 PV/battery connection—type 3.

connection shown in Figure 1.74, since it requires synchronized operation of the converters. In the case of sufficient solar insulation, the DC/DC converter should charge the battery. The PV panel should supply power to the load through the DC/AC inverter. In the case of no insulation, the stored energy in the battery should be transferred to the load through the DC/DC converter and the DC/AC inverter. This topology requires a bidirectional DC/DC converter to charge and discharge the battery.

FIGURE 1.78 Positive output multiple input buck—boost converter.

FIGURE 1.79 Flowchart of PV sizing procedure.

Percentages of the Estimated System Losses

FIGURE 1.80 Power circuit of a residential PV system. A PV system consisting of PV panels, power conditioners, and a storage device is shown in Figure 1.80 with household loads and grid connection. ADC/DC converter is required to track the MPP, and a bidirectional power electronic interface, which is able to operate as a DC/AC inverter or a AC/DC rectifier, is required in order to charge the battery pack from grid or utility and discharge the battery to the load or grid.

FIGURE 1.81 Operation mode 1 for the proposed residential system.

FIGURE 1.84 Operation mode 4 for the proposed residential system. FIGURE 1.83 Operation mode 3 for the proposed residential system.

FIGURE 1.86 System configuration of the grid connected PV system. FIGURE 1.85 Pattern of daily operation of the proposed residential system.

FIGURE 1.87 The boost converter circuit. The PV panel output current (Ipy) can be controlled by controlling the converter current (Ic). This can be done by current mode control of the boost converter.

FIGURE 1.88 The implementation block diagram of Equation 1.84. The block diagram given in Figure 1.88 is the implementation of Equation 1.84

FIGURE 1.89 Current control scheme of the boost converter.

1.9.1.3. Bidirectional Inverter/Converter The power of the PV panels and batteries is transferred to the load and utility grid if the power flow direction is from the DC bus to the AC bus. If the power flow direction is from AC side to DC side, this means that the grid power is used to recharge the batteries. Meanwhile, the load is also powered by grid power. SE a I a A I I Mabe ater a In Figure 1.90, Ve is the capacitor voltage, which i is equal to the grid voltage Vu, Io is the current of the inductance, which is equal to the converter current, and Vo is the output voltage of the converter. PWM signals are applied to the switch gates, which are produced by comparing a triangle carrier signal (—1 to 1) with the control voltage v1. Therefore, the output voltage may alternate between +Vy and —Vjp.

9.1.3.1 Current Control for the Bidirectional Inverter/Converter The converter current should be controlled in order to control the power transfer if the grid voltage is assumed to be constant. The voltage of the capacitance is a disturbance input for the system. To eliminate the effect of the disturbance, this value can be subtracted from the control signal. However, since the disturbance signal is alternating periodically, adding or subtracting this value may cause stability problems. Therefore, a feed-forward controller can be used to damp the effect of the disturbances. Figure 1.92 shows the block diagram of the closed-loop controller and the bidirectional controller. i eT a, NO, ee et , ce ft a a i kn Ment. i ear

The output active power over a period can be calculated by

FIGURE 1.95 PV array voltage. In order to increase the efficiency, instead of connecting solar panels using only one power tracker, separate maximum power trackers can be used, since each solar panel may have

FIGURE 1.100 Distributed solar panel and power tracker configuration. The electrical power system of a solar EV is shown in Figure 1.101, which consists of two different DC voltage buses for the traction and auxiliary power requirements [106]. This topology also contains an external outlet to charge the batteries in the case of a plug-in hybrid EV. Ultracapacitors can also be used to capture the braking energy at higher efficiencies and may help overcome the transient conditions of the vehicle, such as acceleration or hill climbing, due to their faster response times. Utilizing ultracapacitors will also reduce the

FIGURE 1.101 Block diagram of the electrical system of a solar hybrid EV.

The outputs from the power trackers are connected in parallel to a main bus at 96 V. The power trackers keep the solar array operating at its highest efficiency by conditioning the input and maintaining the MPP of the solar cells. The output controller monitors the batteries’ state of charge to ensure maximum power transfer to the battery pack. A 150 ke pack of Delphi lead-acid battery pack is connected to the 96 V bus to run the motor and various electrical peripherals. The motor and the motor controller are developed by New Generation Motors, Inc. Ly py ey ee es eS, By pee eens Pets oe ee cle Cer Lis ey A eh er eT me a, Me be ty |

FIGURE 1.103 Layout of the Solar Miner II electrical system.

FIGURE 1.104 (See color insert following page 80.) SAUV II. (Courtesy of Solar Powered Autonomous Underwater Vehicle (SAUV II), Falmouth Scientific, Inc., available online: http://www.falmouth.com/ DataSheets /SAUV.pdf) The measurement data are directly acquired by an on-board microprocessor, and stored and transmitted to a computer system in the laboratory by a direct line initially and later by wireless telemetry. This enables testing of the telemetry system and comparing data received via telemetry, sent by the microprocessor. PC software on the laboratory computer

monitors, analyzes, and displays the data in real time. It also provides the capability to analyze the data statistically over longer periods of time in order to develop histograms to assist later planning strategies for energy utilization and battery charging. A variable and controllable load is integrated into the system, which allows for reasonable simulation of battery discharging, simulating expected AUV energy usage profiles. A block diagram of the system is shown in Figure 1.105. tone sac pa tes

FIGURE 1.106 Solar boat Korona. (Courtesy of R. Leiner, Proceedings of Africon, pp. 1-5, October 2007.)

FIGURE 1.107 Block diagram of energy chain and additional inputs to the IMS.

1.9.4.1 Solar Power Satellite Energy System

FIGURE 1.109 Schematic of the propulsion system of a solar-rechargeable aircraft.

FIGURE 1.110 (See color insert following page 80 for [a].) (a) Solar-powered aircraft and (b) aircraft power train. (Modified from Y. Perriard, P. Ragot, and M. Markovic, IEEE International Conference on Electric Machines and Drives, pp. 1459-1465, May 2005.)

FIGURE 1.112 (See color insert following page 80.) The solar-powered air vehicle. (Modified from K.C. Reinhardt, et al., Proceedings of the IECEC 96 Energy Conversion Engineering Conference, Vol. 1, pp. 41-46, August 1996.)

FIGURE 1.111 Electrical system of the proposed airplane.

The power and propulsion system of the UAV is shown in Figure 1.113.

Wind speed is one of the most important parameters in determining the available wind power. Therefore, it is important to accurately monitor and measure it. Wind speed distri- bution can be measured by a cup anemometer. The cup anemometer works on the principle of simple measurement of the number of revolutions per minute. Data acquisition systems are used to store data for a long period, for instance one year [3].

FIGURE 2.2 Specific wind power due to wind speed variation. = . = Information collected from anemometers is useful for measuring the wind rose, which shows the information of wind speed distributions in various directions. Depending on the number of sectors, various wind rose resolutions exist. Usually the compass is divided into 12 sectors, one for each 30° of the horizon, but more precise anemometers have 24 or more sectors. Figure 2.3 shows an example of a wind rose.

Wind Power Classes Si a I aaa Another important factor for wind turbine siting is roughness of terrain. There are rough- ness classes, which explain the relation between wind speeds and landscape conditions. Roughness class varies from 0 to 4, where class 0 represents water surfaces and open terrains with smooth surfaces, and class 4 represents very large cities with tall build- ings and skyscrapers. Two key parameters regarding the different geographical locations affect the wind turbine siting: the friction coefficient and the roughness classification [5]. The friction coefficients are based on different terrain characteristics and are shown in Tahle 72.2.

FIGURE 2.4 (See color insert following page 80.) Wind speed before and after the turbine.

FIGURE 2.5 Maximum theoretical extracted power from wind.

FIGURE 2.6 The power curve for a wind turbine.

design) and to generate output power. Usually it is 3m/s for smaller turbines and 5-6 m/s for bigger ones. The power curve can be divided into a few regions. These regions are presented in Figure 2.7 and are separated by cut-in, nominal, and cut-out speeds. Once the rotor of the turbine starts spinning, it can be assumed that the amount of produced output power is linearly increasing with the speed.

FIGURE 2.8 Power coefficient curve of a wind turbine. The power coefficient cy depends on the specific design of the wind turbine (especially the particular aerodynamic structure of the blades). Each wind turbine has its own powe! coefficient c, that depends mainly on the tip speed ratio }. As shown in Equation 2.15, the tip speed ratio depends on the geometry of the wind turbine, its rotational speed, and the length of the blades:

FIGURE 2.9 Speed versus power with respect to different pitch angle values. where w is the rotor speed, R is the length of the blades, and v denotes the wind speed. In addition, the power coefficient depends on the pitch angle, whichis defined as the angle between the blade surface and the plane of the wind rotor, as shown in Figure 2.9. There is a great difference between an onshore and offshore wind turbine’s power coefficient. For onshore turbines, for example, the blades are designed such that the optimal tip speed is limited to roughly 50-70 m/s, because the blade tips cause excessive acoustical noise and can cause damage at higher speeds. On the other hand, the noise does not play an important role in offshore turbines and higher speeds lead to slightly higher optimal values of Cp [10].

FIGURE 2.10 (See color insert following page 80.) Turbine output power for different wind turbine diameters.

FIGURE 2.11 Connection scheme of a typical small wind turbine. c NS ( ( ( ( ( c ( ' [ ( ( ' c 1 ( { g a Large wind turbines are of MW power range. These turbines incorporate complex systems, and wind farms typically consist of several to hundreds of those large turbines. One of the world’s biggest wind turbines is located in Emden, Germany [13], built by the German company Enercon as an offshore turbine. The price of 1 kW installed power from a large wind turbine is significantly less than the price of 1kW from a small turbine. Currently, the installed price of a large turbine is about $500/kW, and the price of energy is around 30-40 cents/kWh, depending on the site and turbine size. Enercon turbines have a power output of 5MW with a rotor blade diameter of 126m, sweeping an area of over 12,000 m2 [13].

FIGURE 2.13 Induced EMF of a three-phase BLDC generator.

FIGURE 2.14 Diode rectifier connected to a BLDC generator. ee eee ee eee eee Tee Figure 2.14 shows the equivalent circuit of the BLDC generator connected to a diode rectifier. This is the simplest way of using the BLDC machine for wind applications because no switch is necessary to control the phase current. The full bridge rectifies the induced voltages of variable frequencies caused by the variable wind speeds. Basically, the waveform of the induced EMF is converted by the diode rectifier to DC voltage, regardless of the input shape. Usually these types of wind turbines are connected to batteries; therefore, the rectified electrical power is used to charge the battery. Ls as es ee ae er ad T T \N oY. gle. Le LL ew ne ww Cw 1. ODT TWN ...... 2... kb.

FIGURE 2.15 (See color insert following page 80.) Back EMF, real current, and reference current waveform of a BLDC machine.

Since the real-time waveform of the induced EMF in the BLDC generator is not a sinu- soidal waveform, it contains harmonics. The trapezoidal waveform is generated by the machine design. All the harmonics have to be included, and in the single-phase BLDC generator, the average power can be expressed as [17-20]

FIGURE 2.17 (a) BLDC generator connected to an active rectifier. (b) Phase voltage-current waveform. FIGURE 2.16 Back EMF and voltage phasor diagram.

FIGURE 2.19 Basic schematic of a small wind turbine for household application. (Modified from W.D. Jones, IEEE Spectrum, 43 (10), 2006.)

FIGURE 2.18 Performance graph of a Skystream small wind turbine with a BLDC generator.

FIGURE 2.21 (See color insert following page 80.) Back EMF induced by the BLDC phases.

FIGURE 2.20 Block diagram of a wind turbine with a BLDC generator. In order to investigate the behavior of a wind turbine with a BLDC generator, a small- ale model, shown in Figure 2.20, is analyzed. The wind turbine produces mechanical que that drives the BLDC generator. In this example, the pitch angle is 0; therefore, there no pitch angle controller and the wind speed is constant at 9m/s. The BLDC generator ectrical parameters are: stator resistance R, = 0.2 Q, stator inductance Ls = 8.5 mH, flux duced by magnets ¢~ = 0.175 Wb, and back EMF flat area is 120°, where the flat area repre- nts the duration when the wave has a constant amplitude in Figure 2.21. The mechanical arameters of the generator are: inertia J = 0.05 kg m2, friction factor F = 0.005 Nms, and umber of poles p = 4. The trapezoidal back EMF induced by the BLDC generator is shown for phases A, B, and

generator shaft speed in radians per second is given in Figure 2.23. The main disadvan- tage of the BLDC generator is the nonsinusoidal-induced back EMF and nonsinusoidal current including harmonic components. Therefore, there are some ripples in the induced electromagnetic torque due to the distorted current waveform. The generator speed is proportional to the induced torque of the machine, since the

FIGURE 2.23 BLDC generator speed. This type of machine can be used in both fixed and variable speed applications. The PMSG is very efficient and suitable for wind turbine applications.

The current and torque equations in abc frame can be expressed as where L, and Lg are the q- and d-axis inductances, R is the stator resistance, ig and ig are the q- and d-axis currents, 0; and vg are the q- and d-axis voltages, w, is the rotor angular speed, i is the flux amplitude induced in the stator by PMs of the rotor, and T, is the electromagnetic torque.

FIGURE 2.24 PMSG with a rectifier /inverter. NETS RETA SS SSAC SS Bi SL Figure 2.25 shows a PMSG where the PWM rectifier is placed between the generator and the DC link, and PWM inverter is connected to the utility. In this case, the back-to- back converter can be used as the interface between the grid and the stator windings of the PMSG [35]. The turbine can be operated at its maximum efficiency and the vari- able speed operation of the PMSG can be controlled by using a power converter that is able to handle the maximum power flow. The stator terminal voltage can be controlled in

FIGURE 2.26 Block diagram of the simulated wind-turbine-driven PMSG.

several ways [36]. In this system, utilizing the field orientation control (FOC) allows the generator to operate near its optimal working point in order to minimize the losses in the generator and power electronic circuit. However, the performance depends on the knowl- edge of the generator parameter that varies with temperature and frequency. The main drawbacks are the cost of the PMs that increases the price of the machine, and the demag- netization of the PMs. In addition, it is not possible to control the power factor of the machine [14] @ ee ee ee a 4 =e Bete: ig = ee c4< em Specie

Zephyros Wind Turbine Technical Specifications

FIGURE 2.27 Wind speed versus output power curve of the Zephyros wind turbine.

EEE OO The mechanical torque produced by the wind turbine, which is the input torque for the PMSG, is represented in Figure 2.29. It should be noted that the transient response of the torque for the first fractions of the simulation is due to the turbine dynamics and the stall condition of the generator at time zero. The speed of the generator increases as the torque increases and reaches the steady-state condition as the torque : settles. m1 2 1 Cet fal Tha wAmmom. ore oF 1 ARN Tm) mm Wie

for synchronous machines in similar applications. If the induction machine is connected to the grid, the required reactive power can be provided by the power system [40]. The induction machine may be used in cogeneration with other synchronous generators or the excitation can be supplied from capacitor banks (only for stand-alone self-excited generators application) [41-46]. The self-excitation phenomenon in an induction machine can be easilv explained using

FIGURE 2.31 Voltage of the first switch of the inverter.

IGURE 2.32 Phase-to-phase voltage at the transformer output.

FIGURE 2.33 Induction machine electrical equivalent circuits.

FIGURE 2.35 Induction generator voltage and frequency control with load regulation. 2.7.3.3 Improved Voltage and Frequency Control with a VSI

FIGURE 2.34 Conventional control scheme for the induction generator. 1s used to meet the power demand at the desired power factor. The main purpose of an impedance controller is to keep the total real and reactive powers at a constant level. In the case of any load changes, the impedance controller will absorb the active or reactive power, which has not been used by the load. The excess power will be redirected to Rg, and will be consumed on this resistance. Thus, the total reactance of the system is adjusted as a function of the duty cycle of the chopper switch as shown in Figure 2.35. On the other hand, the excess power, which is not absorbed by the load, will be released through the resistor. Therefore, the prime mover requires no regulation and can always be operated at the required power, voltage, and frequency [56]. The imnedance controller can reoulate the voltage and frequency of the induction cen-

FIGURE 2.38 Induction machine controlled by a back-to-back inverter. Wound rotor induction machines can be supplied from both the rotor and stator sides. The speed and the torque of the wound rotor induction machine can be controlled by Figure 2.39 presents a topology that consists of a DFIG with AC/DC and DC/AC convert- ers, as a four-quadrant AC/AC converter using isolated gate bipolar transistors (IGBTs) connected to the rotor windings. In the DFIG topology, the induction generator is not a squirrel cage machine and the rotor windings are not short circuited. Instead, the rotor windings are used as the secondary terminals of the generator to provide the capability of controlling the machine power, torque, speed, and reactive power. To control the active and reactive power flow of the DFIG topology, the rotor side converter (RSC) and GSC should be controlled separately [59-62]. TTATW..... J] ... 4... *.. J... 214°... 2... 2 210 *.. 2. 2 2 2 LL 2 1°] J CO. 2 dL et de ltl | kw el let nw tl Yt Od)

FIGURE 2.40 (a) DFIG steady state and (b) dynamic equivalent electrical circuits. In the rotor circuit, two voltage-fed PWM converters are connected back-to-back, while the stator windings are directly connected to the AC grid side, as shown in Figure 2.40. The direction and magnitude of the power between the rotor windings and the stator windings can be controlled by adjusting the switching of the PWM signals of the inverters [68-70].

regulating voltages from both the rotor and the stator sides of the machine. The DFIG can be considered as a synchronous/asynchronous hybrid machine. In the DFIG, like the synchronous generator, the real power depends on the rotor voltage magnitude and angle. In addition, like the induction machine, the slip is also a function of the real power [63]. DFIG topology offers several advantages in comparison to systems using direct-in-line converters [64,65]. These benefits are as follows:

FIGURE 2.41 Power transfer of DFIG by controlling the slip: (a) power transfer to DFIG and (b) power transfer from DFIG. where Hyp is the wind turbine shaft inertia, AQ, is the wind turbine angular speed devi- ation, and T,, is the wind turbine prime torque from the wind. The mechanical torque Tm that drives the generator shaft can be described as the twist positions between the turbine shaft and generator rotor shaft and can be obtained from the following equation:

FIGURE 2.42 Torque-speed characteristic of the DFIG.

FIGURE 2.43 Operation modes of the DFIG: (a) subsynchronous operation w < ws and (b) oversynchronous operation © > Ws.

FIGURE 2.44 Total power and power flow of DFIG. 2.7.3.7 Voltage and Frequency Control Using Energy Storage Devices

FIGURE 2.45 Voltage and frequency control using energy storage. Having additional energy storage decreases the system inertia, improves the behavior of the system in the case of disturbances, compensates transients, and therefore improves the overall system efficiency [78]. However, it adds to the initial cost of the system and requires periodical maintenance depending on the storage devices.

FIGURE 2.46 Energy storage with SMES and batteries for wind applications.

FIGURE 2.47 Energy storage with bidirectional converter in DFIG systems. The produced excess energy of a wind generator can also be converted to kinetic energy and stored in this form by using a flywheel. In this approach, the flywheel can be driven by another induction generator and this induction generator can be controlled by a cascaded bidirectional rectifier/inverter topology [89]. This energy can be converted to electrical energy for future use if the power production of the wind generator is not sufficient. The flywheel energy storage systems can be used as power regulators over short periods of time for electrical power quality improvement and to alleviate the volt- age and frequency fluctuations. Therefore, the storage system’s lifetime and efficiency can be increased [90-93]. These systems improve the power quality of the energy produced by wind generators and, therefore, they can be connected to the grid with less fluctuations and harmonics [94].

FIGURE 2.49 Multiple-level energy storage in DFIG system. Lite VIVUCK Uldetaril Ul a LTING blo oLlOWiIL LL PlStc 42-J9U. LIM Wilh LULUITIOC priryUeuUce mechanical torque that drives the induction generator. In Figure 2.50, RSC stands for the rotor side converter and GSC stands for the grid sid converter. The wind turbine produces the mechanical torque Ti that drives the inductiot generator with the rotor speed of wy. B represents the pitch angle, Wreference represent the reference rotor speed (1p.u.), Vgc represents the DC bus voltage, Iabc_(Stator bus) Stan for the phase currents measured from the stator windings, Ipc (RSC bus) is the phase cur rents measured from the rotor side, Iabe (Gsc bus) 1s the phase current of the GSC, Qreg i the reference reactive power, Q(Grid bus) is the measured reactive power that is injectec to the grid, Vabc (Grid bus) is the phase voltages measured from the grid bus, Iabe_(Gsc bus is the GSC’s output phase current, and I) ref is the reference q-axis current generatec by GSC. The pitch angle controller, the RSC controller, and the GSC controller for this applicatiot

FIGURE 2.50 Block diagram of a DFIG topology. For the pitch angle controller, the actual rotor speed is subtracted from the reference rotor speed and the error is processed by a PI controller. The output of the PI controller might be higher than the maximum pitch angle value, depending on the speed difference. Therefore, the output of the PI controller is limited using a saturation block. The pitch angle controller block diagram is shown in Figure 2.51. This controller helps reduce the rotor speed to a reasonable value in high wind speed conditions.

to track the reference power. The reference rotor current [qr ref is produced by the PI regu- lator, which must be injected to the rotor side by the RSC. The electromagnetic torque Tem is produced by this current component (according to Equation 2.94). A current regulator reduces the error between the actual and the reference q-axis current Ij, to zero. On the other hand, the d-axis component of the current is related to the output voltage of the RSC or the reactive power produced /absorbed by this converter (Equation 2.97). Either the volt- age regulation or the reactive power regulation can be used to determine the reference d-axis current Ij; ye¢- The current regulator also uses the error between this reference current and the measured d-axis current. The output of the current regulator is the voltage that must be produced by the RSC.

FIGURE 2.54 Active power output to the grid side. SLI GALUVE Pees JIU VV UG 1d LIC AOU LUE ULI oo MU») OLIVVV IL IIL i Ey 427i bo UVUVlalleu as shown in Figure 2.54. The reactive power flow that is also measured from the grid bus of Figure 2.50 is shown in Figure 2.55. The reference reactive power is set to zero and the actual reactive power flow tracks its reference with a small amount of oscillations as shown in Figure 2.55. The reactive power is controlled by the RSC, which was described earlier. Since the reactive power regulation is selected as in Figure 2.52, the grid bus voltage is also regulated. If the grid bus voltage regulation was preferred in the control loop, the necessary reactive power would be injected /absorbed to/from the grid.

FIGURE 2.55 Reactive power output to the grid side.

FIGURE 2.56 DC bus voltage between the converters.

FIGURE 2.57 (See color insert following page 80.) Three-phase voltage measured from the grid side bus. 2.8 Synchronous Generators

FIGURE 2.58 Three-phase current measured from the grid side bus.

FIGURE 2.59 Synchronous machine q- and d-axis equivalent electrical circuits.

FIGURE 2.61 Synchronous generator with a DC/DC converter for field voltage control.

FIGURE 2.62 Synchronous generator with DC link regulation. FIGURE 2.63 Current loop for the DC/DC converter duty cycle. the current—torque relationship of the generator in order to capture maximum power from the wind turbine.

Comparison of Three Wind Turbine Concepts Source: Modified from M.R. Dubois, H. Polinder, and J.A. Ferreira, Proceedings of the Conference on Wind Energy in Cold Climates, Matane, Canada, 2001. Notes: +, strength; —, weakness. TABLE 2.6

FIGURE 3.2 Tidal fences can be mounted (a) at the entrance of bays, (b) between the main land and an island and (c) between two islands. Sometimes, even the top of the fence could be simultaneously used as roads con- necting two islands. The structure of a sample tidal fence with a road on it is shown in Figure 3.3. This fence structure can be used in channel blocking locations illustrated in Figure 3.2.

FIGURE 3.4 Water flows inside to the tidal pond.

FIGURE 3.3 Tidal fence and bridge structure. (Redrawn from “Ocean Energy,” Technical Report of the US Department of Interior, Minerals Management Service.)

FIGURE 3.6 (See color insert following page 80.) La Rance tidal power station. (From Dam of the tidal power plant on the estuary of the Rance River, Bretagne, France, image licensed under the Creative Commons Attribution 2.5 License. With permission.)

FIGURE 3.7 Gravitational effect of the sun and the moon on tidal range. molecule to the moon/ sun. Due to the less distance between the moon and the earth, the moon exerts 2.17 times greater force on the tides as compared to the sun [8]. As a result, the tide closely follows the moon during its rotation around the earth, bulging along the axis pointing directly at the moon. When the sun and the moon are in line, whether pulling on the same side or on the opposite side, their gravitational forces are combined together and this results in a high “spring tide” [6]. When the moon and the sun are located at 90° angle to each other, their gravitational force pulls the water in different directions, causing the bulges to eliminate each other’s effect. This results in a smaller difference between high and low tides, which is known asa “neap tide” [6]. The concepts of spring tide and neap tide are shown in Figure 3.7. The period between the two spring tides or neap tides is around 14 days, half of the lunar cvcle. The range of a spring tide is commonly about twice as that of a neap tide, whereas

FIGURE 3.9 Funneling effect in an estuary entrance.

reaches coasts or continental shelves, bringing huge masses of water into narrow bays and river estuaries along a coastline. At some sites, the tidal flow can be heightened to more than 10m by the complex resonance effects. Bay of Fundy, in Canada, where the greatest tides in the world can be found, and the Severn Estuary, in England, are famous examples of this effect [6]. In these areas, two high tides occur in one day, called semidiurnal tide, with a tidal cycle of about 12h and 25 min, as shown in Figure 3.8 [9]. This daily variation is quasisinusoidal.

FIGURE 3.10 Difference in height between high and low tides. where E is the energy (J), g is the acceleration of gravity (m/s*), p is the seawater density (approximately 1022 kg/m* for seawater), A is the surface area of the basin (m7), z is the vertical coordinate of the ocean surface (m), and h is the water head (m). The average value for the acceleration of gravity is g = 9.8m/s*. Therefore, for seawater

FIGURE 3.13 Process of the flood generation approach. IGURE 3.12 Process of the ebb generation approach

FIGURE 3.14 Process of the two-way generation approach.

FIGURE 3.16 Operation process of a tidal lagoon. FIGURE 3.15 Schematic diagram of a tidal lagoon.

FIGURE 3.18 Bulb turbine inside. Bulb turbine is one of the most popular turbines utilized in tidal barrages. The La Rance tidal plant in France uses bulb turbines. It is similar to a bulb, as shown in Figures 3.17 and 3.18. All the essential turbine components, such as turbine bearing and shaft seal box, as well as the generator are all placed inside a bulb. During its operation, water flows around the turbine, passing the generator, guide vanes, blades, and draft tube into tail channel. It has normally three to five blades made of stainless steel. Cincea wratar miict hho nravoentoad fram flavwzinga thraioh thea tirhinea dimnoe maintenance ite

FIGURE 3.22 Hydro turbine speed control model.

FIGURE 3.23 Block diagram of hydraulic turbine and its control system.

where Gol'w is the approximate effective water starting time for small perturbations around the operating point. the operating point. Detailed hydro turbine models can be implemented by considering other parameters such as the traveling wave models, surge tank effects, elastic water column conditions, multiple penstocks, and other mechanical parameters [14-17]. However, the model presented here is adequate for most of the analysis and control design procedures.

FIGURE 3.27 A typical hydro turbine governor model. Tr is the reset time or dashpot time constant.

FIGURE 3.26 Hydro turbine and speed control linear model.

FIGURE 3.29 Hydraulic turbine model. The hydraulic turbine and governor system consists of a hydraulic turbine, a servomotor, and a PID-based governor. The hydraulic turbine is modeled as Figure 3.29. This model is the Simulink implementation of Figures 3.25 and 3.26, and Equation 3.8. The parameters beta and tw in this model are speed deviation damping coefficient and water starting time,

FIGURE 3.31 Complete turbine, governor, and servomotor model. LE SETIELALOL il OLGET tO Ila talil ad LXE SerleratOl OULPUL VOLIdEe. The reference power generation of the turbine is selected as 0.8 and the reference speed is aken as unity (1). From Figure 3.28, itis seen that the hydraulic turbine and governor system roduces a mechanical power and this mechanical power is applied to the generator’s rotor haft. The output of the synchronous generator is connected to the grid through an RL ircuit, representing the transmission line. For 22s of simulation the rotor speed settles at 1, vhereas the rotor speed deviation is damped to 0 after some small oscillations, as seen from ‘igures 3.32 and 3.33. The generator voltage is shown in Figure 3.34, which is controlled by he excitation voltage controller. The mechanical power supplied to the generator’s shaft,

FIGURE 3.35 Turbine mechanical power. Instead of using costly dams, which potentially occupy too much space, the other approach is to take advantage of the horizontal flow of tide current. In this way, the kinetic energy of the moving unconstrained tidal streams is utilized to drive the turbines, without dams, in a similar way to harvest wind energy from wind. In this concept, instead of collecting the ebb tidal water in a dam and using its potential energy, the energy of moving tidal current is harvested.

3.3.3.1 Energy Calculation for Tidal Current Energy Harvesting Technique

where V is the speed of the water, m is the mass of water, which is given by FIGURE 3.37 Generator active power.

FIGURE 3.39 Conceptual tidal current turbine used for tidal energy harvesting. EEE The power coefficient, Cp, is the percentage of power that the turbine can extract from the water flowing through the turbine. According to the studies carried out by Betz [18], the theoretical maximum amount of power that can be extracted from a fluid flow is about 59%, which is referred to the Betz limit. Tha thrict farce anniiaed tra the ratar can ho avnroccoed hry The thrust force applied to the rotor can be expressed by

FIGURE 3.40 Three tidal turbine fundamental types: (a) horizontal axis system; (b) vertical axis system; (c) linear lift based system. ” Horizontal axis turbines have their blades rotating around a main shaft pointed into th direction of tidal current flow. Between blades and the shaft, a hub works to transfer anc convert captured stream energy to the rotational energy. The hub is mostly designed t¢ accommodate the pitch control that allows turning of the blades to their optimum orienta tion in the tidal stream. Gearbox is another critical component commonly used in horizonta axis turbine system. The gear increases the low speed of the turbine shaft to the desirec operating speed of the generator shaft, while the generator converts the mechanical rotatior energy into electric power. The projects based on horizontal axis turbines are described it the following paragraphs. 1. Hammerfest Strom’s E-Tide Project As shown in Figure 3.41, the so-called Blue or E

FIGURE 3.42 (See color insert following page 80.) A SeaGen turbine installed at Strangford Lough. (Courtesy of Seaflow and SeaGen Turbines, Marine Current Turbines Ltd., available at: http://www.marineturbines.com, March 2008.)

IGURE 3.43 (See color insert following page 80.) SeaGen turbine farm. (Courtesy of Seaflow and SeaGen urbines, Marine Current Turbines Ltd., available at: http:/ /www.marineturbines.com, March 2008.)

FIGURE 3.44 (See color insert following page 80.) A generating system with a horizontal axis turbine, Rotech Tidal Turbine. (Courtesy of Rotech Tidal Turbine (RTT), Lunar Energy'", Harnessing Tidal Power, available at: http://www.lunarenergy.co.uk/)

FIGURE 3.45 Ducted turbine cross section.

FIGURE 3.47 (See color insert following page 80.) Position transformation of SST to its maintenance position. (2 Water is started to be pumped out of the main body, (b) swing arm started to roll, (c) rotors are rising to the wate surface, (d) rotors reach a stable position on the sea surface. (Courtesy of Semi-Submersible Turbine, TidalStrear Limited “The Platform for Tidal Energy,” available at: http: / / www.tidalstream.co.uk/)

FIGURE 3.46 (See color insert following page 80.) Semisubmersible turbine. (Courtesy of Semi-Submersible Turbine, TidalStream Limited “The Platform for Tidal Energy,” available at: http:/ /www.tidalstream.co.uk/)

FIGURE 3.48 (See color insert following page 80.) Comparison of SST with a wind turbine for the same power rating. (Courtesy of Semi-Submersible Turbine, TidalStream Limited “The Platform for Tidal Energy,” available at: http://www.tidalstream.co.uk/)

FIGURE 3.49 Operation of a bidirectional horizontal axis turbine.

FIGURE 3.50 (See color insert following page 80.) Stingrays mounted on the seabed. (Courtesy of Engineering Business Ltd., UK, “Stingray tidal stream generator,” available at: http://www.engb.com, March 2008.)

FIGURE 3.52 Equivalent circuit of synchronous generator. where @m is the machine angular velocity and is the electric angular velocity. Based on Equations 3.26 and 3.27, speed regulation is achievable through controlling the frequency. The equivalent circuit and the phasor diagram of the synchronous generator are shown in Figure 3.52. In this equivalent circuit, Eo is the equivalent induced voltage (back emf), Z is the load impedance, E is the output voltage, X is the synchronous reactance, and I is the stator current for one phase. Equation 3.28 is the general voltage equation for the synchronous generator:

FIGURE 3.53 PMSM vector control scheme. In ideal conditions, mathematical equations of vector control of PMSM can be expressed as PMSM can be actively controlled by vector control of rotor magnetic field orientation to get better torque characteristics. Figure 3.53 shows the vector control of a PMSM. PMSM is a synchronous machine with a rotor made of PM, so its vector control is much simpler than that of asynchronous machines [30]. Tn ideal conditions. mathematical equations of vector control of PMSM can be expressed as

FIGURE 3.54 Variable speed operation of a synchronous generator. Figure 3.55.

The rotor winding is connected to the grid through “back-to-back” converters. A DC-link capacitor is placed, as energy storage, between these two converters to decrease the voltage ripple in the DC link. The machine side converter is used to control the torque or the speed of the DFIG and also the power factor at the stator terminals. The main objective for the grid side converter is to keep the DC-link voltage constant [26], as shown in Figure 3.56. Figure 3.57 presents one-phase equivalent circuit of the DFIG. FIGURE 3.57 Equivalent circuit of DFIG. (Modified from S. Tnani et al., “A generalized model for double fed induction machines,” IASTED Conference, 1995.)

FIGURE 3.58 Rotating dq-axis reference frame system of a DFIG. Equations of the machine are

FIGURE 3.60 Output power versus rotating speed for a tidal energy generator. Ina variable speed generating system, the MPPT is one of the most important parts of the system. MPPT methods are implemented by controlling the speed of the tidal generator to ensure that the maximum available energy from the tidal current is captured. It can be seen in Figure 3.61 that with variable current speed (from V; to V;,) we can get the maximum value (from P; to P,) with variable turbine speed (from w1 to w,). By connecting all these

FIGURE 3.59 Variable-speed direct-driven (gearless) turbine.

FIGURE 3.61 Generator power curves at various tidal speeds. top points, we can get the optimal line. The goal is to keep the operating point on that line dynamically.

FIGURE 3.63 Block diagram of a MPPT controller.

FIGURE 3.62 Control method based on current speed measurements.

Applying the chain rule [39], Equation 3.53 can be expressed by

FIGURE 3.65 Block diagram of a MPPT control system.

the drawbacks of MPPT methods that are based on look-up table or water current speed calculation. The duty ratio (D) of a buck converter, in steady-state operation, can be expressed as The duty ratio (D) of a buck converter, in steady-state operation, can be expressed as

FIGURE 3.67 Grid connection and controls of tidal current power conditioning system.

FIGURE 3.68 A simplified PLL principle. o ensure efficient and reliable operation, grid-connected power converters are required. For the voltage and frequency synchronization, the phase-locked-loop (PLL) method can oe utilized. The operation schematic of the PLL method is depicted in Figure 3.68. PLL is 1 control system that has an output signal with a fixed relation to the phase of the input reference signal. The PLL mechanism can be realized using analog or digital circuits. In ooth mechanisms, a phase detector, a variable electronic oscillator, and a feedback sensor re required.

Current track control FIGURE 3.70 Grid-connected current control.

In the case of any fault suchas under- or overvoltage of the grid, overcurrent or overheat of the power electronic devices, and undervoltage of the generator output, the grid-connected generation system may not detect the power outage immediately and self-cut from the grid [41]. As a result, the tidal current power generation system and near-by loads form a stand- alone island that is not fed by the grid [42]. In general, the islanding may harm the entire power system and the consumer side. Islanding affects voltage and frequency stability. It can also contribute to phase asynchrony. Therefore, detection of islanding by appropri- ate detection circuit is very essential for the tidal energy conversion systems. Figure 3.69 demonstrates an islanding detection system. ST ccs oll Be ee wee at ny Veeco eddicaceee Dee es Meow eee Ebest Vliet ain ond ccs TOT PF ec

FIGURE 3.72 Block diagram of the outer loop control system.

decrease the amplitude and phase of the error. The summation of the corrected values of the given current and the current signal produced by the SPLL forms the reference current of the inner loop. Thus, the inner and outer loops cooperate to track the given current [41]. The reference point to produce the sinusoidal signal is provided by the zero-crossing of the grid voltage. Consequently, the SPLL is used to generate the given current in phase with the grid voltage. The control system block diagram is shown in Figure 3.71 [42], where Iyer is the reference current injected to the grid, Ipredicted is the predicted grid current, and Upredicted is the prediction of the grid voltage. a iy a. ae

FIGURE 3.73 Phasor diagram for synchronized grid and generator. ——erveeee— ECG EE —e———— ——— Trier rer rrr OO CD eae Ore For the frequency and voltage regulation, back-to-back converters can be used for grid connection. In this method, the output of the generator is rectified and then inverted again vith desired amplitude, phase, and frequency specifications. In this method, Clarke or -ark transformations can be used. In Clarke transformations, the ABC phase quantities are onverted to af stationary frame. The reference and measured of quantities are compared, ind using controllers, the appropriate PWM signals can be generated for the converters. The Park transformations involve the similar procedure. The difference is the conversion yf ABC phase quantities into dq axis quantities instead of af quantities. By filtering the nput signals such as three-phase voltages, the phase angle of the utility grid voltage can ye estimated.

FIGURE 4.2 System-level diagram; the land DC/DC converter is moved offshore. Another option is installing an offshore transformer as seen in Figure 4.3. This would increase the power transmission capability, since the higher voltage transmission will result in less transmission losses, since, for the same power rating, the current will be lower with a higher transmission voltage level. However, this case does not allow for DC transmission. As an alternative, boost DC/DC converters can be installed after the AC/DC converter of the generator. This allows a high-voltage DC transmission link as illustrated in Figure 4.4. In this case, both transmission losses will be kept at a minimum and only the single-line DC transmission through the ocean water will be needed. The disadvantage of this con- figuration is that more power components will be used with some additional losses and additional cost will be required to install the boost converter.

power is transmitted from ocean to land, a DC/DC converter or a tap-changing transformer is used for voltage regulation. Depending on the utilized voltage regulation system, a DC/AC inverter is used before or after the voltage regulator. The voltage synchronization is provided by the inverter and the output terminals of the inverter can be connected to the grid.

FIGURE 4.5 Characteristics of a wave. (Modified from WJ. Jones and M. Ruane, “Alternative electrical energy sources for Maine, Appendix I, Wave Energy Conversion by J. Mays,” Report No. MIT-E1 77-010, MIT Energy Laboratory, July 1977.)

FIGURE 4.3 System-level diagram with the offshore transformer.

FIGURE 4.4 System-level diagram with the high voltage DC transmission link.

FIGURE 4.7 Directions of motion of a floating body. (Modified from WJ. Jones and M. Ruane, “Alternative electrical energy sources for Maine, Appendix I, Wave Energy Conversion by J. Mays,” Report No. MIT-E1 77-010, MIT Energy Laboratory, July 1977.)

FIGURE 4.6 Side view of a fixed body in waves showing incident, reflected and transmitted waves. (Modi- fied from WJ. Jones and M. Ruane, “Alternative electrical energy sources for Maine, Appendix I, Wave Energy Conversion by J. Mays,” Report No. MIT-E1 77-010, MIT Energy Laboratory, July 1977.)

FIGURE 4.8 Spinning the air-driven turbines using wave power. (Modified from “Ocean Energy,” Report of the U.S. Department of Interior Minerals Management Service.) Figure 4.8 shows the operating principle of an offshore application, which consists of a floating buoy with an air chamber and an air-driven generator. In this system, when the waves hit the body, the water level inside the channel of the buoy increases. This increase in water level applies a pressure to the air in the air chamber. When the air is pressurized, it applies a force to the ventilator turbine and rotates it. This turbine drives the electric generator, creating electricity at its output terminals. When the waves are pulled back to the ocean, the air in the air chamber is also pulled back as the water level in the buoy channel decreases. Due to the syringe effect, the turbine shaft rotates into the contrary direction, still producing electricity. |

FIGURE 4.9 Linear generator—based buoy-type wave energy harvesting method. ~ The generator AC voltage starts at zero when the buoy is in its lowest position, increases until the buoy reaches its highest position at the top of the wave and descends back to zero as the buoy stops. The preferable geometry of a buoy is a cylindrical shape since it can act as a point absorber and intercept waves coming from different directions. When the wave rises, the buoy pulls the generator piston by the rope. When the wave subsides, the generator will be drawn back by the spring that stores the mechanical energy in the first case. Thus, the electric generation is provided during both up and down motion.

FIGURE 4.11 A schematic illustrating the fixing of the Salter Cam WEC device. Salter Cam is shown in Figures 4.10 and 4.11. Salter Cam, also known as “the nodding duck,” features an outer shell that rolls around a fixed inner cylinder that is activated by the incoming waves. The power can be captured through the differential rotation between the cylinder and the cam. In this application, the motion of the cam is converted from wave into a hydraulic fluid, and then the hydraulic motor is used to convert the pressurized hydraulic fluid into rotational mechanical energy. Consequently, the rotational mechanical energy is converted to electricity by utilizing electric generators. As an intermediate step,

FIGURE 4.13 Forces affecting a Salter Cam.

FIGURE 4.14 Channeled ocean wave to a reservoir to spin the turbines.

FIGURE 4.15 Air-driven turbines using the wave power [14]. (a) Upcoming wave starts filling the chamber, (b) air is compressed by rising water, and (c) air is pulled back by retreating waves.

FIGURE 4.16 Falnes buoy-shaped wave-activated turbine. A wave contouring raft whose joints are formed by hydraulic pumps operating on the differential motions of the linked rafts as shown in Figure 4.18 was proposed by Wooley and Platts. The details of this method were discussed in Ref. [4]. This method has been experimented by Wavepower Ltd of Southampton, England. The individual rafts are built at a length of one-quarter of the wavelength of the waves that, on the average, contribute the greatest amount of energy. The rafts would be somewhat wider than their length in the direction of the advancing wave. scl Soe Aas | baad AS A. eorernlawalkla Avs. Be th ate alas lites tA. Ait Be ae Lames LA cise

FIGURE 4.17 Air pressure ring buoy. (a) Water level increases and air is taken out from the upper outlets. (b Water level decreases and air is pulled back from the upper inlets.

FIGURE 4.18 Wave contouring raft. In this subsection, some of the most common wave power turbines that are coupled to the rotational generators for wave energy applications are discussed. These devices are generally installed within fixed structures to the shoreline. Since they are fixed, they provide an appropriate frame for wave forces that come against them so that they can have high conversion efficiencies.

FIGURE 4.21 Turbine efficiency as a function of flow coefficient. where k is the proportion constant. The air power is proportional to the air velocity as

FIGURE 4.22 Output torque waveform of the Wells turbine with sinusoidal input airflow.

where 9 is the air density, v is the mean axial flow velocity, Up is the circumferential velocity at mean radius (rg), b is the rotor blade height, | is the chord length, Ap is the total pressure drop between before and after the turbine, To is the output torque, and z is the number of rotor blades [25]. The Cy-® characteristics vary with respect to the different blade angles. The Cy value decreases if y increases in the stall-free region. This shows that the larger y results in better starting conditions. Besides, the stall point increases with y as well as the flow coefficient at no-load condition. The input coefficient Ca is higher at y = 0 than nonzero y values. This is caused by the rotor geometry. If y is different from zero, the Ca value is negative at small flow coefficients. This means that the rotor acts as a fan at smaller inlet angles. Thiic tha avaragcad narfarmancoa af tho rnatar aonac dawn wundaran ncrillating flaws can.

FIGURE 4.23 Turbine using self-pitch-controlled blades for WEC.

FIGURE 4.24 Overall view of wave energy turbine with self-pitch-controlled blades. Here, I is the inertia moment of the rotor, t is the time, and Ty, is the loading torque. For given values of I, T,,, and To, Equation 4.16 can be numerically solved. At the beginning, this gives the starting characteristics of the turbine and provides the running characteristics at the asymptotic condition. The turbine performance can be calculated as a mean efficiency when the solution is in the asymptotic condition:

FIGURE 4.25 Double-regulated turbine model with control system and rotor dynamics. Hence, the turbine mechanical torque is also affected by these four variables as where m is the turbine torque (p.u.), nt is the turbine efficiency, and w is the speed (p.u.). Head, speed, guide vanes, and runner blades are the parameters affecting the discharge and efficiency of the turbine as

FIGURE 4.26 Nonlinear model of hydraulic turbine. 3.4.4 Other Types of Turbines Used for WEC

The turbine rotor with v = 0.7 hub-to-tip ratio is placed at the center of the test sec- tion. Then the turbine rotor is tested at a constant rotational speed under steady-state conditions. The performance of the turbine is evaluated in terms of the turbine angular speed, w, turbine output torque, To, flow rate, Q, and total pressure drop between the inside and outside of the air chamber, Ap. It should be noted that all of the turbines are self-starting [35].

FIGURE 4.28 Impulse turbine with self-pitch-controlled guide vanes by link motion (ISGV).

FIGURE 4.29 Impulse turbine with fixed guide vanes.

FIGURE 4.30 Schematic of the air turbine-type wave power generator system. owever, it should be noted that efficiency is not the only important factor for optimal tur- dine selection in WEC. In addition, turbine characteristics vary with the efficiency of the air chamber, which is the ratio of the power of the OWC and the incident wave power [35]. Air- ‘low generated by the OWC is irregular due to the irregular natural behavior of ocean waves. hus, it is important to clarify the turbine characteristics under irregular flow conditions. The irregular test wave is based on The International Ship Structure Congress spectrum, which is specific for irregular wave behavior and is used typically in ocean applications [36]. AWEC system with an air turbine is presented in Figure 4.30, which shows the upcoming emi a ce cee ie ee eee Boe Ble oe oe Bae! eee es eee Ve Rage Coe le ae ee eee oe tle ce seeds Bile caste

Using the turbine and air chamber efficiencies, the overall conversion efficiency of the wave energy device is solidity at rp = 1.z/(20rp). In Equation 4.38, the first and second terms on the left-hand side are the inertia and loading terms and the right-hand side is the torque produced by the turbine. Using Equation 4.38, the turbine behavior at the starting conditions can be calculated as a function of Ka* and vz if the loading characteristics, torque coefficient, and geometrical specifications of the rotor are given. The operation characteristics after the start-up can be obtained assuming a constant rotor speed. The mean output and input coefficients Co and Cj can be calculated, respectively, as

FIGURE 4.31 Comparison efficiency of wave energy converters with different turbines.

FIGURE 4.34 (See color insert following page 80.) Average power and efficiency variation versus load resistance.

These solutions can substituted within the system efficiency equation given in Equation 4.53; then it is observed that system efficiency is a complex function of resistance R. By controlling the value of the equivalent load resistance of R, the system efficiency can be maximized.

FIGURE 4.36 Generator power. —— EE e—ee—e——e—eeeeeee ee The generator power and generator voltage are both in sinusoidal form as shown in Figures 4.36 and 4.37, respectively. The generator power has a biased sinusoidal waveform. In order to regulate the generator’s output power and voltage, the output of the linear generator terminals is first converted to the DC voltage and then filtered. Considering the large oscillations in the output, a large capacitor is required to achieve satisfactory voltage regulation performance. A boost DC-DC converter operating in the current control mode is employed, since the input voltage to boost the converter is the output voltage of the rectifier and that is fixed. Current control mode [38] helps optimum loading of the linear generator

FIGURE 4.35 System-level configuration of the linear generator and power converters.

where the Lwave is the wavelength and is defined as

rope to a buoy, which is on the ocean level and moves up and down and can be dragged towards different directions. Once a wave results in the motion of the buoy, the buoy pulls the PM piston. The PM moves up and down within the fixed stator windings. This also generates an electromagnetic field generating the electricity. The schematic of this energy conversion device is illustrated i in Figure 4.41. SS a i fe ee © FF fF . FF... me hm i Ne

FIGURE 4.40 Approximate magnetic flux intensity (Wb/m*) created by PMs.

FIGURE 4.42 Three-phase diode rectifier schematic where a, b, and c are the linear generator phases.

FIGURE 4.44 Buoy power and output power versus current density. where f is the output frequency, p is the number of salient poles, and n is the rotation speed of the rotor shaft in rpm. Therefore, for a two salient pole machine, the synchronous speed would be 3600 rpm [48], since the output frequency of the system should be 60 Hz for grid connection. The armature excitation generates AC and this current is converted to DC so that armature exciter acts as a DC supply for field winding. The armature exciter has three phase windings in order to get the improved efficiency of three-phase rectification in comparison to single-phase rectification.

FIGURE 4.46 Coordinates and rotor position for stator winding. FIGURE 4.45 The traveling flux wave of stator winding.

where the tangential speed of the rotor, s, is given by

FIGURE 4.47 Total stator surface and q slots around the periphery.

FIGURE 4.49 Stator core design and stator winding arrangement per phase.

FIGURE 4.50 The schematic of brushless generator wound rotor with armature and field exciter systems. where Day is the average diameter of the winding.

FIGURE 4.51 Flux density B versus field strength H. SLIGTIEVELIGLIRE UO LE] In the directly driven turbine topology, the active parts of the machine are surrounded by the periphery of the blades instead of placing them on the axis of the turbine as in the classical methods. This idea is studied in [50-52], which have presented successful results for marine propulsion. Some of those rim-driven propellers are used for autonomous underwater vehicles or vessel propulsion. A similar machine has been tested with a 50-W rating turbine in [49]. Ortltt ate, Pr eoe elect ait caer > nieaneents TEULG GALIM EYL PEs Some older technologies are based on the coupling of an axial flow turbine with an electric generator similar to wind turbines. A gearbox is required to shift up the typical rotor speed (10-20 rpm) to the speed of a conventional generator (above 500-1000 rpm). However, direct drive is an attractive solution to eliminate the complexity, failure rate, efficiency, and maintenance cost [49].

FIGURE 4.52 Radial magnetic flux electric generator that is placed on the periphery of the horizontal axis turbine blades, (a) frontal view and (b) lateral view.

FIGURE 4.53 (See color insert following page 80.) Cross-sectional view of two poles of the generator.

where B;, is the flux density remnant of the magnets, B is the ratio of magnet width to pole width (Lm/ Ipole), Rsm is the ratio of air gap diameter to magnet outer diameter (D/Dm), p is the number of pair poles, |r is the magnets relative recoil permeability, Mm and hg are the magnet and air gap heights, respectively. The slotless machine equivalent can be obtained by Carter factor [55]. Carter factor is a parameter to include the slotting effect on the air gap flux. If By, p, and Rsm are known hp can be solved to calculate the required magnet height. The slot height, hs, can be calculated as follows for a given loading current Ar, [54]:

FIGURE 4.54 Electrical phasor diagram for y = 0. where f is the frequency of the field iron, Pre, is the iron losses per mass unit at the given frequency of fo, Bmax, is the flux density, and b and c are 1.5 and 2.2, respectively, for a typical high-quality Fe-Si-laminated steel. The mechanical losses are neglected in this model. Mechanical losses are affected by the choice of the bearing technology to compensate for the axial drag of the turbine and to keep the gap between the stator and rotor fixed.

FIGURE 4.55 Unity volume and thermal equivalent circuit. where S is the heat exchange surface and h is the convective transfer coefficient. The surface resistance is used within the model of the convective heat exchanges as

The output of the induction generator is in the form of three-phase AC. The voltage can be controlled to maintain constant output voltage by adjusting the excitation current [59,60]. Thus, the output voltage can be fixed regardless of the load current and speed variations. A controller can be employed to determine the magnitude and frequency of the excitation current. The excitation current should be supplied to the stationary windings from which it is induced into the short circuited rotor windings. i a en, a i a: ee: i : en: en: ee ci: an, Laer In an induction machine, the synchronous speed and the angular velocity of the rotor can be expressed as

FIGURE 4.57 (See color insert following page 80.) 8/6 pole SRG cross-sectional view, (a) stator of the SRG and (b) rotor of the SRG.

FIGURE 4.58 Linear switched reluctance machine cross-sectional view. Instead of using linear generators or conventional rotating generators, piezoelectric/ electrostrictive materials can also be used for WEC applications. A new device named Energy Harvesting Eel (Eel) is developed by Taylor et al. [64] to convert mechanical energy of ocean waves into electricity for powering remotely located sensors or devices used for .3.5.7 Ocean Energy Conversion Using Piezoelectric/Electrostictive Materials

FIGURE 4.59 Eel system structure fixed to the sea bottom.

FIGURE 4.60 Eel movement behind the bluff body.

FIGURE 4.61 Switched resonant power converter. impractical for use in Hel. The switched resonant-power conversion technique is used for electrical power extraction from the polymers. Using electrical and mechanical resonant systems, we can overcome the low coupling factor k, = d3!(Y/e) when piezoelectric devices are used as power generators. Eel motion has a very low frequency such as 1-2 Hz, so direct electrical resonance is not practical, since it requires very large inductor values. Therefore, switched resonant power conversion may overcome this limitation. Using this conversion, this technique is capable of high-efficiency operation at the 1-2 Hz range using reasonable inductor values [64]. Figure 4.61 shows the circuit schematic of the switched resonant power converter used for Eel energy harvesting [64]. Tn thie circiiit Rv renrecente the dielectric loce recictance af the niezoelectric (PVIDB) Cr

FIGURE 4.62 The interface structure for induction generator-based wave energy converters. JULUIrtuiirv-e aaa co ) Ve Vea Pe Xi Qe Een MLtito aa ala SEMEL LLY ru VV L £44 CULE LO VA LA to different periods and heights of waves [67]. A typical grid connection interface for use with generator applications is presented ir Figure 4.63. Although using several units has an effect on obtaining better overall wave forms, further conditioning is necessary for grid connection. Several WEC units have outpu voltages with different amplitudes, frequencies, and phases. DC bus voltage variations car be lowered if many units are connected together. After the rectification stage, capacito. tanks should be placed for better suppression of the DC bus voltage variations. The capac itor bank acts as a short time energy buffer for the sustainability of energy transfer to the grid, if any of the units does not meet any incident wave for a short time. These capacitor: are also used for the determination of the neutral point and the output filter of the inverter The third stage consists of a six-pulse insulated gate bipolar transistor (IGBT) inverter. Thi: inverter should be controlled to synchronize the inverter output voltage with grid quanti ties such as voltage amplitude and frequency. The synchronization is generally providec

FIGURE 4.63 Grid connection interface for applications using different generators.

FIGURE 4.65 Energy buffer system using a hydraulic PTO system with induction generator. enerator [4]. 1Nis also acts aS a Mechanical energy Durter [Oo]. The schematic configuration of the hydraulic PTO system is shown in Figure 4.65. The iydraulic PTO is composed of a hydraulic piston, a high-pressure accumulator, and a iydraulic motor. The high-pressure accumulator smoothes the mechanical power fluctu- tions caused by the wave power absorbers; thus power input to the induction generator vill be smoother but may have more or less fluctuations. The electrical storage may help in moothing the residual power fluctuations [68]. However, it would be better to handle the :ower smoothing at the same conversion stage, if the dimensions and cost of the conversion nd storage would stay in a reasonable range. STATCOM could also be used as an electrical

FIGURE 4.66 Induction generator in series with a full converter as grid interface technology eee ee ee eee ny ee ON Ne Ne EEE EE — ee eee eee ene eee ee ee EE DC/AC converter for grid and generator stator windings connection points [73]. This converter connection is called back-to-back converter, which is in series with rotor windings and shunt on the grid wires. With the aid of the hydraulic smoothing stage, the generator speed does not change too much, so the converters have lower ratings in comparison to the full converter in series [73]. However, doubly fed topology is not suitable for direct drive. The rotor side converter is used to control the power output of the system and the voltage (or reactive power) measured at grid connection terminals. The grid side converter generates or absorbs the reactive power by regulating the voltage of the DC bus

capacitor [74]. In other words, the rotor side converter controls magnetizing current and electromagnetic torque, while the grid side converter is controlled similar to STATCOM control [72]. In this topology, the rating of the converter limits the range of the variable speed operation. So it does not have as much capability as the full converter in the active control. The larger control margins can be reached by increasing the converter ratings, which in turn increase the cost. Thus, the full converter provides more flexibility of control. The system performance can be the same as a fixed speed generator in a STATCOM [73]. This structure is very common in wind farms, and the same technology can be implemented

FIGURE 4.68 Overall grid connection diagram of an SRG. An overall diagram of the grid connection interface for a SRG is given in Figure 4.68 [75]. The torque is a function of the angular position of the rotor due to the variable reluctance of SRGs. The phase currents of the SR generator should be controlled by the power electronic controller according to the certain positions of the rotor. In order to control the torque and transfer the available power to the grid, the magnitude and the waveform of the phase currents should be regulated by the power electronic interface. The safe operation of the

FIGURE 4.70 Block diagram of the operating principle of grid-connected SRG. power is first supplied from the capacitor, while transistors are conducting. The capacitor voltage turns at a point slightly before and after the aligned position. So, the bulk of the winding conduction period comes after the alignment. Current begins to rise while the rotor poles approach the stator poles of the next phase, which is going to be excited [77]. The phase current increases with extracting energy from the DC-link capacitor till the switches are turned off. The turn off event happens at the commutation angle, which is determined by the control circuit.

FIGURE 4.69 Power electronic converter and controller for SRG grid connection.

FIGURE 4.71 Power electronic interface schematic for piezoelectric/electrostrictive generators. The current in the switches is commuted to the diodes, when the switches are turned off till the current reaches zero. The terminal voltage then reverses by the freewheeling diodes. During this defluxing period, the DC-link capacitor receives the returned generated power. The direction of energy flow switches from the machine to the capacitor resulting in the increase of capacitor voltage.

FIGURE 4.72 (See color insert following page 80.) Pelamis WEC device at sea (Ocean Power Delivery Ltd). (Courtesy of R. Henderson, Renewable Energy, 31 (2), 271-283, 2006.)

FIGURE 4.74 Electrical interconnection of Demo-Plant—in Oregon. (Redrawn from O. Siddiqui and R. Bedard. “Feasibility assessment of offshore wave and tidal current power production: A collaborative public/private partnership (Paper: 05GM0538),” EPRIsolutions, CA, Proceedings of the IEEE Power Engineering Society 2005 Meeting Panel Session, June 2005.) nO As shown i in Figure 4. 74, the commercial system uses a total of four clusters, each one ontaining 45 Pelamis units (i.e., 180 total Pelamis WEC devices), connected to subsea cables. iach cluster consists of three rows with 15 devices per row. The other state designs are ganized in a similar manner with four clusters. The number of devices per cluster varies uch that each plant produces an annual energy output of 300,000 MWh/yr. The four subsez ables connect the clusters to shore as shown in Figure 4.75. The electrical interconnection ot he devices is accomplished with flexible jumper cables, connecting the units in mid-water ‘he introduction of four independent subsea cables and the interconnection on the surface srovide some redundancy in the wave farm arrangement [2]. Pelamis was used to establish the cost model for a commercial scale (300,000 MWh/yr) ATANTYTA Laan T axgaliwad crAct rAMmMNnnANnaNnte aro ehnurmn in Biowroa A WE. T7Q] Tha caAct hveoa-daurnr

FIGURE 4.73 (See color insert following page 80.) General layout of the Pelamis WEC device

FIGURE 4.76 (See color insert following page 80.) Cost pie graph of the Pelamis WEC device.

FIGURE 4.75 Electrical interconnection of Demo-Plant—Oregon Example.

FIGURE 4.77 Operating principle of the Wave Dragon; waves overtop the ramp, water stored in a reservoir above the sea level and hydro turbines rotate as the water discharges. Teamwork Technology located in the Netherlands is the developer company of the AWS, which is an offshore submerged device. The device is activated by the oscillations of static pressure caused by the surface waves. Figure 4.80 shows the operating principle of the AWS [80].

The AWS is an air-filled cylinder of which the floater is a body heaving up and down to or from the fixed bottom part. The waves create a pressure difference on the top of the device resulting in the force that moves the active part of the device. High water pressure causes the chamber volume to reduce if the wave crest is above the AWS. The floater heaves due to the action of the chamber pressure if the trough is above. Here “trough” represents the concave between the two wave crests. The behavior of the air in the chamber is similar to a spring with a variable stiffness by pumping water in or out of the chamber [80,91]. PM linear synchronous machines are used within the AWS applications as direct drive energy conversion devices. Using an auxiliary energy storage or conditioning, the efficiency of this type of energy conversion can be improved.

FIGURE 4.79 (See color insert following page 80.) Front view of the Wave Dragon with wave reflectors in the sides and the reservoir in the middle. (Courtesy of Wave Dragon ApS, available online at http://www. wavedragon.net/)

The water dampers used within the AWS are actuated when the floating part approaches the mechanical end-stops. These dampers reduce the velocity of the floater and avoid the strong collision. Water dampers are also actuated with the electric generator when the floater does not have enough force required for adequate control of the movement of the AWS [91].

FIGURE 4.81 View of the WSE, a multipoint absorber. (Courtesy of M. Kramer, “The wave energy con- verter: Wave Star, a multi point absorber system,” Technical Report, Aalborg University and Wave Star Energy, Bremerhaven, Denmark.)

FIGURE 4.82 (See color insert following page 80.) WSE position in storm protection mode. (Courtesy of M. Kramer, “The wave energy converter: Wave Star, a multi point absorber system,” Technical Report, Aalborg University and Wave Star Energy, Bremerhaven, Denmark.) Se ee ae Se ee eee ee Se ES Se Se Se a Se fee ee eee mee sone ne he hydraulic fluid to common transmission system, and drive a hydraulic motor. When a wave rolls in, the floating absorbers in one side of the Wave Star are lifted up. “onsequently, the floating absorbers of the other side will be lifted, until the wave subsides. There are individual hydraulic cylinders for each floating absorber. A piston in the cylinder ipplies pressure to the hydraulic fluid, inside the common transmission system of the levice, when an absorber is lifted up. This pressure drives a hydraulic motor. This hydraulic notor drives a generator that produces electric power. Since the device has a length of several wavelengths, the system will operate continuously to harvest energy. Thic device ic ctorm-nrotected Diuiringo the ctarm neriodc the ahcoarhere can he lifted 1n Mii

FIGURE 5.1 (See color insert following page 80.) A general overview of an OTEC system.

FIGURE 5.2 (See color insert following page 80.) An offshore OTEC application.

FIGURE 5.3 (See color insert following page 80.) A nearshore OTEC application.

FIGURE 5.4 Operational block diagram of closed-cycle OTEC. (Redrawn from Natural Energy Laboratory o: Hawaii Authority, USA, available at: http:/ /www.nelha.org, March 2008.) working fluid remains in a closed system and is continuously circulated following a Carnot cycle. This technology is essentially similar to the standard refrigeration or air conditioning systems; however, it operates in the reverse direction of the Carnot cycle. Refrigerators and air conditioners consume electricity to create temperature difference, whereas OTEC takes advantage of the - temperature difference to generate electricity. aes =a + 1 - ee? . . a es

FIGURE 5.5 Schematic diagram of energy transfer. Here, S is the entropy. Based on Rudolf Clausius, the entropy is defined as The amount of thermal energy transferred between the cold reservoir and the system is

FIGURE 5.6 Temperature-entropy diagram for an ideal Carnot cycle.

Consequently, the efficiency of a nonideal OTEC system would be FIGURE 5.7 Temperature-entropy diagram for a generalized thermodynamic cycle.

FIGURE 5.10 Comparison of mixture and pure fluid with biofouling.

FIGURE 5.9 Comparison of mixture and pure fluid.

FIGURE 5.11 Block diagram of open cycle. In an open-cycle OTEC process, which is shown in Figure 5.11, seawater functions as the working fluid. The boiling temperature of water is a function of pressure [13]. It drops as the pressure decreases. The first step is boiling the warm shallow water by placing it in a low-pressure container of about 2% atmospheric pressure at sea level. Then the expanding steam drives a low-pressure turbine coupled to an electrical generator. The 5.3.3.1 Structure and Principles of Open-Cycle Systems

FIGURE 5.12 General structure of the hybrid OTEC process. (Redrawn from Natural Energy Laboratory of Hawaii Authority, USA, available at: http: / /www.nelha.org, March 2008.) goes to another vaporizer to release its heat to vaporize the working fluids, like ammonia, in a method similar to the closed-cycle evaporation process. An effervescent two-phase, two-substance mixture is obtained by physically mixing the second fluid with the warm seawater. The evaporated second working fluid is separated from the steam/seawater, which is recondensed similar to the closed cycle. The low-pressure turbine is easily driven by the phase change of the seawater/ammonia mixture. The vaporized fluid then drives a turbine to produce electricity, and the steam condenses within the heat exchanger and turns into desalinated water.

An evaporator is a heat exchanger, which vaporizes a substance from its liquid state to its gaseous state, as shown in Figure 5.15. The liquid working fluid (low temperature) is fed FIGURE 5.14 Parallel heat exchanger operation.

into the U-type tubes, absorbing heat from the outside heat source (passing warm seawater), which will turn it to gas. Witch Wilt TUrTL It tO eds. In an evaporator, the main heat transfer passage is conduction, not the radiation or convec- tion. The conduction heat transfer can be defined as the transfer of thermal energy through direct molecular interaction within a medium or between mediums in direct physical contact without the flow of the medium material. The rate af heat trancfer can he eyvnrecced hu Newton’e canglinge law [320 21]: The rate of heat transfer can be expressed by Newton’s cooling law [30,31]:

i a I A I I In Figure 5.16, Tyi is the temperature of inlet hot water to the evaporator, Tho is the temperature of outlet hot water from the evaporator, po is the pressure of outlet hot water, Pni is the pressure of inlet hot water, where Ppi = Pho = Ph, and my is the mass flow rate of hot water. Qh; is the heat quantity of inlet hot water, Qho is the heat quantity of outlet hot water, Twi is the temperature of inlet working fluid, Two is the temperature of outlet working fluid, pwi is the pressure of inlet working fluid, pwo is the pressure of outlet working fluid, and my is the mass flow rate of working fluid.

FIGURE 5.18 Nonlinear statics for heat exchange values of the water versus working fluid.

FIGURE 5.19 Schematic diagram of a condenser. 5.4.3 Condenser

FIGURE 5.22 Block diagram of the OTEC system controller. Heat quantity is used as the state variable for the upper control system of the OTEC sys- tem since it is more robust to the pressure and flow rate variations [37]. If the temperature is chosen as the state variable, the overall system becomes more complicated, since the temperature is very sensitive to the pressure and mass flow rate. Moreover, using the heat quantity as the state variable, highly nonlinear relationship between enthalpy and temper- ature /pressure can be eliminated. The power generation of the OTEC system is a function of the heat quantity difference between turbine inlet and turbine outlet working fluids. Therefore, the power generation can be controlled by controlling the heat quantity instead of temperature, because electric power can be calculated directly using the heat quantity. The nonlinear separation controller can easily be implemented in the OTEC systems and it is simplerin compare to other controller strategies based on the physical models [37—40]. The

FIGURE 5.24 Construction procedure of the nonlinear separation model: (a) identification of nonlinear statics function, F and (b) identification of linear dynamics function, G(s). As mentioned earlier, the nonlinear separation controller is designed using the mirror image of the nonlinear separation model. The nonlinearity can be eliminated by the inverse of the nonlinear statics as well as the residual characteristics of the linear dynamics compensated by the inverse dynamics. PI controllers are used to compensate the modeling errors and disturbances and they are used in parallel for the dynamic compensation.

FIGURE 5.25 Steam turbine speed governing system. Similar to the hydraulic turbine control, the main duty of the governor is to control the gate opening. A controller consists of a speed governor, a speed relay, a servomotor, and governor-controlled valves can be used to control the steam turbine [41]. The speed control system of a steam turbine is shown in Figure 5.25. i en nS a a ee ee, ee ee Se ey ee, ey MO mee 2 Ree Ce eh aoe 2 es: (yf 5.6 Control of a Steam Turbine

FIGURE 5.27 Steam turbine and governor coupled to a synchronous generator

feedback is used to represent the speed relay. The integrator with time constant T'sy and direct feedback represent the servomotor. The servomotor moves the valves and is physi- cally large, particularly on large units. Rate limiting and position limits of the servomotor are shown at the integrator, which represents the servomotor.

FIGURE 5.28 Complete governor, steam turbine, and shaft model.

FIGURE 5.29 Speed governing system model.

The excitation voltage control system in Figure 5.27 determines the appropriate exci- tation voltage level by using the qd-axis components of the generator stator voltage and the reference output voltage by a PI controller. The dq-axis components of the voltage are obtained using Park Transformations from ABC phase voltage quantities. The simulation time is set to 70s. Figures 5.31 and 5.32 show the rotor speed and rotor speed deviation. It can be seen that the rotor speed settles at 1p.u. while the rotor speed fe own ge an

The available OTEC thermal resources throughout the world can be summarized as follows:

Developing Nations with Adequate Ocean-Thermal Resources 25 km or Less from the Shore

FIGURE 5.37 Multipurpose applications of OTEC systems. Deep seawater discharged from an OTEC plant contains high concentrations of nutrients that are depleted in surface waters due to biological consumption, and it is relatively free of pathogens. This is an excellent medium for growing phytoplanktons (microalgae), such as Spirulina, a health food supplement, and a variety of commercially valuable fish and shellfishes, such as salmon and lobster, which thrive in the nutrient-rich, cold seawater [45]. Suitable mixing of the warm and cold water discharges in various ratios can provide

FIGURE 1.2 (a) PV cell, (b) module, and (c) array.

FIGURE 1.13 The structure of the sun tracking system based on angle of the light using two photodiodes

FIGURE 1.14 (a) x-Axis sensor module structure; (b) y-axis sensor module structure.

FIGURE 1.37 Pyypp, FEF, and y variations versus the shading factor S.

FIGURE 1.104 SAUV II. (Courtesy of Solar Powered Autonomous Underwater Vehicle (SAUV II), Falmouth Scientific, Inc., available online: http://www.falmouth.com/DataSheets/SAUV.pdf)

FIGURE 1.110 (a) Solar-powered aircraft. (Modified from Y. Perriard, P. Ragot, and M. Markovic, IEEE International Conference on Electric Machines and Drives, pp. 1459-1465, May 2005.)

FIGURE 1.112 The solar-powered air vehicle. (Modified from K.C. Reinhardt, et al., Proceedings of the IECEC 96 Energy Conversion Engineering Conference, Vol. 1, pp. 41-46, August 1996.)

FIGURE 2.10 Turbine output power for different wind turbine diameters.

FIGURE 2.4 Wind speed before and after the turbine.

FIGURE 2.15 Back EME real current, and reference current waveform of a BLDC machine.

FIGURE 2.57 Three-phase voltage measured from the grid side bus.

FIGURE 2.21 Back EMF induced by the BLDC phases.

FIGURE 3.42 A SeaGen turbine installed at Strangford Lough. (Courtesy of Seaflow and SeaGen Turbines, Marine Current Turbines Ltd., available at: http://www.marineturbines.com, March 2008.)

FIGURE 3.6 La Rance tidal power station. (From Dam of the tidal power plant on the estuary of the Rance River, Bretagne, France, image licensed under the Creative Commons Attribution 2.5 License. With permission.)

FIGURE 3.43 SeaGen turbine farm. (Courtesy of Seaflow and SeaGen Turbines, Marine Current Turbines Ltd., available at: http://www.marineturbines.com, March 2008.)

FIGURE 3.44 A generating system with a horizontal axis turbine, Rotech Tidal Turbine. (Courtesy of Rotech Tidal Turbine (RTT), Lunar Energy ’”, Harnessing Tidal Power, available at: http:/ /www.lunarenergy.co.uk/)

FIGURE 3.46 Semisubmersible turbine. (Courtesy of Semi-Submersible Turbine, TidalStream Limited “The Platform for Tidal Energy,” available at: http:/ /www.tidalstream.co.uk/)

FIGURE 3.47 Position transformation of SST to its maintenance position. (a) Water is started to be pumped out of the main body, (b) swing arm started to roll, (c) rotors are rising to the water surface, (d) rotors reach a stable position on the sea surface. (Courtesy of Semi-Submersible Turbine, TidalStream Limited “The Platform for Tidal Energy,” available at: http://www.tidalstream.co.uk/)

FIGURE 3.48 Comparison of SST with a wind turbine for the same power rating. (Courtesy of Semi-Submersible Turbine, TidalStream Limited “The Platform for Tidal Energy,” available at: http:/ /www.tidalstream.co.uk/)

FIGURE 3.50 Stingrays mounted on the seabed. (Courtesy of Engineering Business Ltd., UK, “Stingray tidal stream generator,” available at: http://www.engb.com, March 2008.)

FIGURE 4.34 Average power and efficiency variation versus load resistance.

FIGURE 4.38 Reference current tracking and power output for the linear PM generator

FIGURE 4.53 Cross-sectional view of two poles of the generator.

FIGURE 4.57 8/6 pole SRG cross-sectional view, (a) stator of the SRG and (b) rotor of the SRG.

FIGURE 4.72 Pelamis WEC device at sea (Ocean Power Delivery Ltd). (Courtesy of R. Henderson, Renewable Energy, 31 (2), 271-283, 2006.)

FIGURE 4.79 Front view of the Wave Dragon with wave reflectors in the sides and the reservoir in the middle. (Courtesy of Wave Dragon ApS, available online at http:/ /www.wavedragon.net/)

FIGURE 4.76 Cost pie graph of the Pelamis WEC device.

FIGURE 4.82 WSE position in storm protection mode. (Courtesy of M. Kramer, “The wave energy con- verter: Wave Star, a multi point absorber system,” Technical Report, Aalborg University and Wave Star Energy, Bremerhaven, Denmark.)

FIGURE 5.1 A general overview of an OTEC system.

FIGURE 5.2 An offshore OTEC application.

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An Overview of the PV System

IJSRD - International Journal for Scientific Research and Development

solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV) or indirectly using concentrated solar power (CSP). The concentrated solar power system uses lens or mirrors and tracking systems to focus a large area of sunlight into a small beam. The process of conversion of light into electric current by the photovoltaics is known as the photovoltaic effect. Solar photovoltaic arrays are subjected to partial shading and rapid fluctuations of shading, in most of the portable applications. By partially shaded cells, the residual energy generated, either cannot be collected if the diode is bypassed, or impedes collection of power from the remaining fully illuminated cells, if the diode is not bypassed. The PV system is capable of maximizing the power generated by every PV cells in the PV panel. This system comprises of an array of parallel connected PV cells, a low-input voltage step up power converter and a simple bandwidth MPPT (Maximum Power Point Tracking) tracker. The MPPT operation is to adjust photovoltaic interfaces, so that the operating characteristics of the load and the photovoltaic array match at the maximum power point, no matter what the stand-alone or grid connected photovoltaic applications are.

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Solar cell

Gideon Mutabazi

A solar cell made from a monocrystalline silicon wafer with its contact grid made from busbars (the larger strips) and fingers (the smaller ones) A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels.

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How Solar Cells Work

Pancratius Benkbenk

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TO STUDY THE SOLAR CELL AND ITS PROGRESS IN THREE GENERATION

IJETRM Journal

ijetrm journal , 2021

A solar cell is an electrical system that uses the photovoltaic effect to transform light energy into electrical energy. A solar cell is a p-n junction diode in its most basic form. Solar cells are a type of photoelectric cell, which is characterized as a device whose electrical characteristics change when exposed to light, such as current, voltage, or resistance. Individual solar cells can be combined to form modules known as solar panels. The common single-junction silicon solar cell can produce a maximum open circuit voltage of approximately 0.5V to 0.6V. These solar cells are very small. When combined into a large solar panel, large amounts of renewable energy can be generated.

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Solar photovoltaic cells

Amit Yogiraj

Renewable energy sources i.e., energy generated from solar, wind, biomass, hydro power, geothermal and ocean resources are considered as a technological option for generating clean energy We know that the solar cell industry has grown quickly in recent years due to strong interest in renewable energy and the problem of global climate change .Cost is one of the important factor in the success of any solar technology. Today's solar cells are simply not enough efficient and are too expensive to manufacture for large-scale electricity generation..

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Solar Photovoltaics

ANIL RAI

Solar Photovoltaics, 2018

The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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PV cell

Gypsy Moon

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Solar Cells

Katja Vozel

2006

Solar cells are one of the biggest sustainable methods of energy and have the ability to convert radiated light into electricity. This article provides an overview of what a solar cell (or also known as photovoltaic is (PV), inorganic solar cells (ISC), or photodiode), the different layers included within a module, how light is converted into electricity, the general production of inorganic solar cells, and what ideal materials (typically semiconductors) are used for it.

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Solar Energy and PV Systems

Adel M Sharaf

International Journal of Photoenergy, 2014

The utilization of solar photovoltaic (PV) systems has gained a tremendous momentum due to decreasing costs of PV arrays and interface systems by as much as 50% during the last five years. The advancements on electric utility grid interface systems and utilization of PV arrays in standalone local power generation and smart buildings with storage battery and back-up hybrid systems are increasing the PV system utilization as the emerging form of renewable/alternative energy source. In many countries, the government has instituted special incentives and tax credits as well as feed-in tariff and energy purchase back legislation programs in order to promote and encourage manufacturers and consumers and boost new investments in solar PV energy use in different sectors.

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Solar Photovoltaic System

Bipin Mokal

International Journal of Advance Research and Innovative Ideas in Education, 2020

A solar PV System is a system which converts solar energy into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the photovoltaic effect. In solar PV system, solar inverter is used to convert the output from direct to alternating current. Solar PV system is very reliable and clean source of electricity which suit a varied range of applications. As the demand for solar electric systems grows, progressive builders are adding solar Photovoltaic (PV) as an option for their customers.

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Book Chapter on "Photovoltaic Solar energy - From Fundamentals to Applications: Silicon Solar Cell Device Structures"

Ngwe Zin

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Solar PV - Equipping the Future

Joshua Kripas, Christopher Benner

2020

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a report on types of solar cells and their uses

AVINASH DASH

solar cells and their uses

The Earth endures a marvellous supply of solar power. The sun, an average star, is a mixture activator that has existed blazing for over 4 billion years. It supports enough strength in individual minutes to supply the globe's energy needs for individual old age. Sooner or later, it supports more strength than our current population would expend at 27 ages. In fact, "The amount of energy from the sun beautiful the earth over a three-epoch ending is equivalent to the strength stocked in all relic strength beginnings." Solar energy is a free, tireless resource, still controlling it is somewhat creative. Considering that "the first proficient solar containers were fashioned inferior 30 times gone by," we have come to a great distance. The extension of solar professional guests designing singular and distinguishing energy from undepletable source methods for individual families means skilled is not any more an excuse not to contemplate energy from the sun for your home. The biggest jumps in effectiveness reached "accompanying the onset of the transistor and following semiconductor science. There are various benefits of photovoltaic solar power that manage "individual of ultimate hopeful energy from undepletable source sources in the globe." It is non-polluting, has no exciting parts that manage break below, requires little support and no project, and has a history of 20-30 age with depressed running costs. It is exceptionally singular cause no big installation is necessary. Remote extents can surely produce their supply of power by constructing as limited or as abundant of order as wanted. Solar power generators are utterly delivered to houses, schools, or trades, where their congregation demands no extra growth or land district, and their function is dependable and quiet. As communities evolve, more solar radiation competency may be added. Solar energy is the most wanted contemporary in underdeveloped countries, the fastest increasing segment of the photovoltaic's display. People deny or be denied power as the sunlight subdues the land, making solar power the apparent strength choice. "Governments are judgment allure modular, distributed type ideal for contents the energetic needs of the thousands of detached villages in their nations." It is much more efficient than the enlargement of expensive capacity lines into detached extents, place communities do not have the money to finance common power. There are only two basic disadvantages to utilizing energy from an undepletable source: the amount of light part of every 24 hours and the cost of supplies.

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Abd Elhady 2017 Energy Proceedia An Understanding of the operation of Silicon PV

salama M abdelhady

As the photons of the solar irradiance fall on the free electrons of the PV cells, it mobilizes the electrons, according to Einstein’s description, and causes a flow of electric current. It has been found that the electric current from a silicon PV cell is proportional to the incident solar radiation such that there should not be a limit of the mobilized free electrons. Accordingly, the performance of such PV cell represents a very confusing phenomena, because the number of free electrons from a PV cell is limited while the generated electric current should have unlimited number. It has been shown that considering the electric current as a flow of electrons is contradicted by many experimental measurements. On the other hand; there is a postulated nature of electric current as a flow of electromagnetic waves of electric potential that may solve such contradictions. So, the PV cell should be considered as a device through which the incident solar radiation of a convenient energy which depends on its wavelength, may gain its junction’s electric potential during the flow through it. So, increasing the incident solar radiation will lead to increase the output current without any limitation.

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Fundamentals of Photovoltaic Modules and Their Applications

Adélio de Lima Bana

Preface related with field exposures. The thermal modelling and results of various configurations of PV/T systems, including air collectors, water heaters, distillation systems and dryers, have been discussed in Chapter 7. The energy and exergy analysis on the basis of embodied energy of materials used for fabrication of different components of PV/T systems has been highlighted in Chapter 8. Chapter 9 deals with the net CO 2 mitigation, carbon credit and climate change. The techno-economics of the solar systems has been discussed in Chapter 10. SI units have been used throughout. Appendices have been given at end of the book. This book aims to provide a great insight into the subject, particularly to learning students/professionals doing self-study. In spite of our best efforts, some errors might have crept into the text. We fully welcome valuable suggestions and comments from all readers for further improvement of the book in the next edition.

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PV Cell – Working Principle and Applications How Powerful is Solar Power?

Tien Nguyen

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Messeger 2003 - photovoltaic systems engineering

xhei kazaferi

It is our fervent hope that the engineers who read this book will dedicate themselves to the creation of a world where children and grandchildren will be left with air they can breathe and water they can drink, where humans and the rest of nature will nurture one another.

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CLEAN ELECTRICITY FROM PHOTOVOLTAICS Imperial College Press

isai garcia

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Photovoltaic applications

Cepisca Costin, Horia Andrei

Journal of Materials Processing Technology, 2007

The strong social implications of the energy systems, the complex links between the technical systems, and the connections to environmental aspects, have imposed strong efforts in the development of researches on the electricity production from renewable sources. Photovoltaic (PV) systems, converting sunlight energy into electrical energy, are characterised by modularity, functional autonomy and long duration of operation. This paper presents a detailed analysis of real-case applications of PV systems, already in operation for some years, with the focus on the technologies adopted, energy and data management and characterisation of the PV system performance.

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Principles of Solar Cells, LEDs and Diodes e role of the PN junction

Miodrag Dedeic

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