Solar power density

The power density of photovoltaics is 10-100x higher than that of biomass. The surface area need for the same power output is orders of magnitude less. A comparison is also made with the power densities of fossil fuel. This is important, because as one notes from Fig. 1.9 even the solar power densities do not satisfy the power need requirements of modern cities. 

Insolation - The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. 

Solar cells have been used extensively and successfully to power sateUites in space since the late 1950s, where their high power-to-weight ratio and demonstrated rehabiUty are especially desirable characteristics. On earth, where electrical systems typically provide large amounts of power at reasonable costs, three principal technical limitations have thus far impeded the widespread use of photovoltaic products solar cells are expensive, sunlight has a relatively low power density, and commercially available solar cells convert sunlight to electricity with limited efficiency. Clearly, terrestrial solar cells must be reasonably efficient, affordable, and durable. International efforts are dedicated to obtaining such devices, and a number of these activities have been reviewed (1). 

The intrinsic material fabricated at the frequencies reported above was incorporated in p -i-n solar cells [493]. The p- and n-layer were prepared by the conventional 13.56-MHz discharge. The device quality films indeed yield good solar cells, of 10% efficiency, as is shown in Figure 60. This cell is manufactured with a 500-nm-thick t -layer made at 65 MHz with a power density of 42 mW/cm , resulting in excellent properties. The deposition rate still is 2-3 times higher than... 

FIG. 61. Solar cell performance parameters as functions of power density for cells made at 65 MHz (a) yjc, efficiency, Vqc- and fill factor (FF) (b) spectral response. 

FIG. 62. Normalized solar cell efficiency as a function of illumination time for different power densities as obtained by continuous illumination of 1000-W/m" AM 1.5 light. The initial efficiencies of the four cells were 9%, 109f. 9c. and 69c for 28-. 42-. 57-. and 113-mW/cm power density, respectively. 

The logistics requirements are quite different. Whereas biomass is optimally used when processed decentrally, the power density of solar energy requires processing at large centralized units. 

Rh2 is the rate of production (moles/s) of hydrogen in its standard state per unit area of the photoelectrode. The standard Gibbs energy AG° = 237.2 kj/mol at 25°C and 1 bar, and Pt is the power density (W/m ) of illumination. The numerator and denominator have units of power and hence, as in the case of photoelectrochemical solar cells, the photoconversion efficiency is the ratio of power output to the power input. 

In May 2006, the University of Neuchatel in Switzerland announced a technique using a film created for use in space that yields power densities of 3,200 watts per kilogram. There is also interest in SSP in Japan, where the JAXA space agency has hopes to launch a satellite that will spread into a sizable solar array capable of beaming 100-kilowatts of microwave or laser power to Earth. 

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. 

Fig. 15.12. Daily variation in electrolytic hydrogen production rate (1), the solar array temperature (2), and radiation power density (3). Single crystalline silicon solar cells, SPE electrolyzer location, Cape Canaveral, Florida. The time scale denotes minutes elapsed from 5 a.m. (Reprinted from Yu. I. Khar-kats, Electrochemical Storage of Solar Energy, in Environmental Oriented Electrochemistry, C. A. C. Sequeira, ed., Fig. 5, p. 477, copyright 1994. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)...

The more rectangular the. //E-curve appears, the higher the FF is and the better charge carriers are collected within the device. Finally, the conversion efficiency is defined as the ratio between electrical power density delivered by the solar cell under standard illumination conditions and the power density of the incident light Pnght... 

These types of batteries fulfill the requirements for reversible storage clusters, which are needed for buffering fluctuations of solar or wind power plants. Here, cost, safety and energy density, rather than power density, are the primary considerations. 

Incident solar flux 101 mW/cm2 Relative edge power density 130 mW/cm2... 

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