Photovoltaic

Photovoltaic (PV) systems use semiconductor materials for the direct conver­sion of light into electricity by the photoelectric effect, which was first observed by Heinrich Hertz in 1887 and explained by Albert Einstein in 1905. The amount of electricity produced by the photoelectric effect is a function of semiconductor composition and the intensity and wavelength of solar radiation available to the PV device (Hertz, 1887; Einstein, 1905). By 1954, three researchers at Bell Laboratories had developed the first practical “solar battery”—a PV cell that converted 6% of the incident solar radiation to elec­tricity (Perlin, 2004). Advances in the research and development of PV devices have steadily produced increases in conversion efficiency, with the present world record at 43.5% (Figure 1.1).

Initially a high-value source of electricity used for space applications with total production capacities measured in watts, the global PV industry now provides an installed capacity of more than 40 GW and is growing about 25% annually (REN21, 2011). PV technologies are used in a variety of collector designs, including flat panels positioned at a fixed tilt or on Sun-following trackers, integrated into building designs (building-integrated PV, or BIPV) and deployed in concentrating PV (CPV) systems, as shown in Figure 1.2. The amount of solar radiation available to each of these collector modes and orientations requires special consideration when assessing historical solar resources or when forecasting operational system performance.

The modular nature of PV systems is well suited to rooftop distributed generation, where electrical power is produced near the point of use, but is also scalable for larger, utility-scale central power generation, which requires electricity transmission. Understanding the spatial variability of solar radiation is important for the success of both distributed – and central-generation systems. PV systems have a very fast response to changes in solar radiation (settling time for individual cells is ~ 10 ms). Therefore, the temporal variations in solar radiation must be characterized to design and operate a PV system that can provide the most stable power output.

Photovoltaic devices are based on single – and multicrystalline silicon (most prevalent), amorphous silicon, microcrystalline silicon, or polycrystalline thin – film materials such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Multijunction PV devices have achieved the highest energy – conversion efficiencies. In late 2012, the world record for PV cell efficiency was 43.5% for a GaInP/GaAs/GaLnNAs(Sb) (Kurt, 2012). To predict electrical – power output, each PV technology requires specific information about the broadband amount and spectral distribution of solar irradiance available to the device (Figure 1.3). Because the performance of PV devices depends on several environmental factors, standards have been developed for rating PV modules based on reference test conditions, including standards for the spectral distri­bution of solar irradiance (ASTM International,; Myers, 2011).

Electrical power is the product of voltage (V) and current (I). The power produced by a PV device is characterized by an I-V curve. As shown in Figure 1.4, the maximum power point on an I-V curve is determined by the PV device voltage and current characteristics corresponding to amount of incident solar irradiance, electrical load, and device temperature. The short-circuit current varies proportionally with incident solar irradiance (Figure 1.5), and the power output decreases with increasing device temperature (Figure 1.6). The semiconductor materials used in a PV device fundamentally determine these response characteristics.

Подпись: 4 j Solar Energy Forecasting and Resource Assessment

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FIGURE 1.1 Chronology of improvements in PV-cell efficiencies according to device technology since 1976. (Courtesy of NREL Image Gallery, http://www. nrel. gov/ncpv/images.) This figure is reproduced in color in the color section.

 

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(b) Подпись: (a) Fixed-tilt PV arrays

Подпись: (c) 1-Axis tracking PV arrays Подпись: (d) Thin-film PV roof shingles

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(e) Concentrating PV on 2-axis tracker (f) Building integrated PV

FIGURE 1.2 Examples of commercially available PV systems for producing electricity in a variety of applications: (a) fixed-tilt PV arrays; (b) polycrystalline PV modules; (c) fixed-tilt PV arrays; (d) thin-film PV roof shingles; (e) concentrating PV on 2-axis tracker; (f) building – integrated PV. (Courtesy of NREL Image Gallery, http://images. nrel. gov.) This figure is repro­duced in color in the color section.

1.2.1. Concentrating Solar Power

Concentrating solar power (CSP; defined here to exclude CPV) converts solar radiation to thermal energy to produce steam that powers an electrical generator or to operate an external combustion engine/generator combination. This utility-scale application relies on direct (beam) solar radiation, as described below, to generate tens to hundreds of megawatts of electrical power from

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FIGURE 1.3 Spectral response functions of selected PV materials illustrating their selective abilities to convert solar irradiance to electricity. (Courtesy of Chris Gueymard.) This figure is reproduced in color in the color section.

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Voltage

FIGURE 1.4 PV system performance characteristics determined by short-circuit current (Isc) and open-circuit voltage (Voc), and maximum power point (Pmax). This figure is reproduced in color in the color section.

a CSP system. There are several methods for concentrating solar radiation on a thermal receiver to produce working temperatures from 500°C to more than 1000°C (Figure 1.7). Solar-power towers use hundreds to thousands of helio­stats (2-axis Sun-tracking mirrors) to reflect solar radiation onto a central tower-mounted receiver. The receiver is an efficient heat exchanger used to transfer solar-thermal energy to a working fluid, typically a molten salt, stored

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Voltage

FIGURE 1.5 PV-array short-circuit current (Isc) is proportional to solar irradiance incident to the module. Open-circuit voltage is much less dependent on irradiance level. This figure is reproduced in color in the color section.

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Solar Irradiance (Watts / Square meter)

FIGURE 1.6 Combined effects of solar irradiance and array temperature on PV-array power output. This figure is reproduced in color in the color section.

in large tanks. The heat is used to drive a turbine generator in a manner similar to that in conventional fossil-fueled power stations.

Linear trough collector technologies rely on parabolic mirrors or a series of Fresnel reflectors to concentrate direct solar radiation onto a tubular receiver aligned at the collector’s line of focus. These modular designs are mounted on 1-axis solar trackers usually oriented north/south and rotated east to west during the day to continuously focus direct solar radiation onto a linear receiver tube. A heat-transfer fluid circulates through the receiver tube into a series of heat exchangers where the fluid is used to generate high-pressure superheated steam before returning to the solar collector. The steam is used by a turbine generator to make electricity.

Dish Stirling engines are mounted at the focal point of a parabolic-dish reflector that is continuously aligned with the Sun by a 2-axis tracker. The heat-transfer fluid in the receiver is heated to 250°C-700°C for use by an

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(c) Dish Stirling engine (d) Linear Fresnel collector

FIGURE 1.7 Examples of CSP systems for converting high levels of DNI to heat and electricity (a) parabolic trough collector; (b) power tower and heliostats; (c) dish sterling engine; (d) linear Fresnel collector. (Courtesy of NREL Image Gallery, http://images. nrel. gov.) This figure is reproduced in color in the color section.

external combustion Stirling engine to generate electrical power. Providing high efficiencies, modular parabolic-dish systems are scalable to meet the needs of communities for distributed power and those of electrical utilities for central generation. As with all CSP technologies, dish Stirling systems require resource information for direct (beam) solar irradiance.

Updated: August 2, 2015 — 8:53 pm