junction producing the lowest current. Therefore, current density, which has an inverse relationship with bandgap energy, should ideally be equal between the junctions. However, multijunction designs are intrinsically based on a range of bandgaps, so material selection involves a compromise between matched current densities and ideal coverage of bandgaps. Layer thickness can be optimized to minimize losses [7]. Power conversion efficiency is defined as the ratio of electrical power output from the cell to the power of the sunlight striking the surface of the cell. This can be visualized as the ratio of areas under the absorbance spectra and AM1.5G sunlight spectrum (Figure 2). The ultimate achievable efficiency between a theoretical system
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Considering seasonal weather patterns, the mean annual insolation of an area can be used to predict the input power density for a solar installation. Within the United States, California, Arizona and the neighboring southwestern states offer the highest levels of insolation in the country. This is reflected in the United States solar production capacity: California, Arizona, Nevada, New Mexico, Colorado and Texas comprise 86% of the country’s installed photovoltaic power production capacity respectively [14]. Worldwide, there is less of a correlation between insolation and installed capacity. Germany is the world leader in photovoltaic installations, and represents 32% of the world’s peak photovoltaic power capacity of 102GW; however, its typical insolation levels are relatively low. The United States, containing large areas with above-average insolation levels, had a peak photovoltaic power capacity of 8043MW as of 2012, 8% of the worldwide total [15]. This suggests that on a global scale, photovoltaic installations are driven by economic incentives (which are abundant in Germany), above geographical practicality. Typically, utility-scale photovoltaic power plants are smaller in power capacity than the average fossil fuel plant; the current largest photovoltaic installation is the Agua Caliente plant in Arizona, with a peak capacity of 290MW, compared with the average United States coal plant capacity of 570MW [16,17].
Outlook
Utility-scale photovoltaics must compete not only with other renewable technologies, but also conventional fossil fuel sources. Considering the fluctuating power output of solar systems, fuel burning generators are generally better-suited to meet the needs of the grid.
Considering seasonal weather patterns, the mean annual insolation of an area can be used to predict the input power density for a solar installation. Within the United States, California, Arizona and the neighboring southwestern states offer the highest levels of insolation in the country. This is reflected in the United States solar production capacity: California, Arizona, Nevada, New Mexico, Colorado and Texas comprise 86% of the country’s installed photovoltaic power production capacity respectively [14]. Worldwide, there is less of a correlation between insolation and installed capacity. Germany is the world leader in photovoltaic installations, and represents 32% of the world’s peak photovoltaic power capacity of 102GW; however, its typical insolation levels are relatively low. The United States, containing large areas with above-average insolation levels, had a peak photovoltaic power capacity of 8043MW as of 2012, 8% of the worldwide total [15]. This suggests that on a global scale, photovoltaic installations are driven by economic incentives (which are abundant in Germany), above geographical practicality. Typically, utility-scale photovoltaic power plants are smaller in power capacity than the average fossil fuel plant; the current largest photovoltaic installation is the Agua Caliente plant in Arizona, with a peak capacity of 290MW, compared with the average United States coal plant capacity of 570MW [16,17].
Outlook
Utility-scale photovoltaics must compete not only with other renewable technologies, but also conventional fossil fuel sources. Considering the fluctuating power output of solar systems, fuel burning generators are generally better-suited to meet the needs of the grid.