Solar irradiance efficiency

Earth s deserts receive 250-300 W/m2 of solar irradiance. If solar eneigy could be used with 10% efficiency, 4% of the sunlight falling on the deserts would provide all the energy used in the world in 2000. The solar cell17 described here has a conversion efficiency close to 7%. 

In practical terms this means that ket is large enough to yield a good value of e at solar irradiation intensities and at generally accessible concentrations of B. However, the extent to which the oxidation of B can be driven uphill, Ey, is generally modest (0.A - 0.5 V at open-circuit) compared to Eg = 1.1 eV for Si. Small values of Ey give low overall optical energy conversion efficiency. 

These units are claimed to provide conversion efficiencies of up to 25%, or even 30% if the irradiation level is high enough. High irradiation is defined as 200-300 suns (1 sun = 0.1 W/cm2), which correspond to 20-30 W/cm2 of solar irradiation. To obtain such levels of solar irradiation, the solar light has to be focused 200-300 times. For a light concentration ratio of 100x, the high-performance PV cells are claimed to have a conversion efficiency of 28%. 

The capacity of a collector is expressed in terms of its peak power production (Wp). This is the amount of electric power that a PV module is able to generate when it receives 1,000 watts per square meter of vertical solar irradiation at 25°C cell temperature. This is also called one sun. If this level of insolation existed for 24 h every day of the year, each m2 of collector area would receive 8,760 kWh/yr. The actual rate of power generation is naturally less. The value of Wp/ m2 of a module is also called its power density or efficiency. Therefore a 10% efficient module when receiving one sun will generate 0.1 kWp/m2 and a 15% will generate 0.15 kWp/m2. 

The power output of solar modules is expressed in terms of their "peak" power output (Wp). This is the amount of electric power that a module generates when it receives 1.0 kW/m2 of vertical solar irradiation at 25°C module temperature (also called one sun). Therefore, a 10% efficient module receiving one sun, will produce 100 Wp/m2. If this solar irradiation exists for 8 hours per day, this module will generate 0.8 kWh/m2/day. Naturally both the number of sunny hours of the day and the intensity of insolation vary (see Figures 1.29 and 1.30 and www.bpsolar.com). In addition the angle of irradiation and/or cell temperature can also be less than optimum. 

Solar conversion efficiency The ratio of the Gibbs energy gain per unit time per nf of surface exposed to the sun to the solar irradiance, E, integrated between X = 0 and X = 00. 

From Equation (5), it is clear that the basic parameter that decides the light harvesting ability of the photocatalyst is its band gap. The ideal limiting efficiencies for conversion of solar radiation calculated by Equation (5) as a function of the band-gap wavelength for standard AM 1.5 solar irradiation in a single band-gap device are represented in Figure 9. 

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