Heinrich Hertz first discovered the photoelectric effect in 1887, and Albert Einstein explained this phenomenon in 1905. Photovoltaic (PV) systems use the photoelectric effect of semiconductor materials to directly convert light into electrical energy. The composition of the semiconductor and the intensity and wavelength of the effective solar radiation received by the photovoltaic device will affect the power generation of the photovoltaic device (Hertz, 1887; Einstein, 1905). Until 1954, three researchers from Bell Labs developed the first practical “solar cell.” The battery can convert 6% of incident solar energy into electrical energy (Perlin, 2004). As R&D activities continue to make progress, the conversion efficiency of photovoltaic installations has also increased, and the current world record is 43.5% (Figure 1).
Photovoltaic technology was first applied in the space field as a kind of expensive power source, and the power generation capacity at that time was only a few watts.
At present, the installed capacity of the photovoltaic industry has exceeded 40GW and is growing at a rate of 25% per year (REN21, 2011). As shown in Figure 2, photovoltaic power generation technology systems have many forms, including flat-panel photovoltaic systems fixedly installed on slopes or solar trackers, systems integrated with architectural design (Building Integrated Photovoltaics, also known as BIPV), and poly Photovoltaic (CPV) system. The amount of solar radiation received is related to the receiving form and direction of the above-mentioned system. This correlation needs to be taken into account when evaluating historical solar resources or making predictions.
(a) Fixed inclined plane photovoltaic array; (b) Polycrystalline photovoltaic modules; (c) 1-axis tracking photovoltaic array; (d) Roof tile membrane type; (e) 2-axis tracking concentrating photovoltaic;(f) Photovoltaic building integration
The rooftop distributed photovoltaic power generation system makes good use of the modular characteristics of the photovoltaic system. The electricity generated by the system can be used nearby, and can be expanded to a public centralized power generation system through a transformer. To fully utilize distributed and centralized power generation systems, it is essential to understand the spatial variation of solar radiation. Photovoltaic systems are very sensitive to changes in solar radiation (the correction time of a single cell is about 10us). Therefore, in order to make the photovoltaic system produce stable power output, it is necessary to understand the characteristics of solar radiation over time during system design and operation.
The semiconductor materials used in photovoltaic devices are: monocrystalline silicon and polycrystalline silicon (the most common), amorphous silicon, microcrystalline silicon and polycrystalline thin film materials, such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Multi-junction photovoltaic devices have the highest energy conversion efficiency. At the end of 2012, the GalnP/GaAs/GaLnNAs (Sb) multi-junction battery with a conversion efficiency of 43.5% set a new world record (Kurt, 2012). Predicting the power generation of photovoltaic systems requires specific information such as the spectral distribution of solar radiation and the number of broad-spectrum bands that can be obtained by photovoltaic equipment (Figure 3). Since the performance of photovoltaic devices depends on several environmental factors, some standards for evaluating photovoltaic panel performance have been developed on the basis of benchmark test conditions, including standards for the spectral distribution of solar irradiance (American Society for Testing and Materials; Myers, 2011).
Power is the product of voltage (V) and current (l). The volt-ampere curve (I-V curve) can describe the power generation of photovoltaic equipment. As shown in Figure 4, incident solar radiation, electrical load, and equipment temperature determine the voltage and current characteristics of photovoltaic equipment, and then determine the maximum power point of the volt-ampere curve. The short-circuit current changes in proportion to the incident solar radiation (Figure 5), and the output power will decrease as the temperature of the device rises (Figure 6). The semiconductor materials used in photovoltaic devices fundamentally determine the above response characteristics.
