Solar input

The three main industrial methods used to produce salt are the solar evaporation method, mining of rock salt, and solution mining. The solar evaporation method is the oldest process used to obtain salt. This method is applied in geographic areas with high solar input and low... 

The Earth s elliptical orbit causes the distance between the Earth and the Sun (the Earth s radius vector) to vary by 3.39% from perihelion (closest) to aphelion (farthest). These variations in distance cause the intensity of solar radiation at the top of the atmosphere to vary as 1/R2, where R is the radius vector. Thus the solar input at the top of the atmosphere varies from 1414 Wm 2 (in December) to 1321 Wm 2 (in July). Additional variations in solar intensity, or brightness, result from the solar sunspot cycle, and even solar oscillations. These slight variations in the solar output are usually accounted for in the calculation of solar energy available at the top of the atmosphere, or the total extraterrestrial solar radiation, referred to as ETR. The ETR has only been monitored from space since the early 1970 s, or almost three solar sunspot cycles. Excellent histories of ETR measurements and analysis are provided in Frohlich3 and Gueymard.4... 

Although the total absorbed and emitted energy of the surface-atmosphere system balance at 240 W m , the fluxes of processes transferring energy within the system, between the surface and atmosphere and within the atmosphere itself, can exceed the net 240 W m solar input. Actually, about 70 of the 240 W m in absorbed insolation is absorbed by trace constituents, aerosols, and clouds in the atmosphere (Ramanathan et al., 1987). In addition, since the Earth s observed average surface temperature is 288 K, about 390 W m is radiated upward from the surface, so that trace atmospheric constituents must be responsible... 

This is the total solar input which may be recovered in part as biomass, wind energy, OTEC, direct water photolysis, photovoltaic energy, etc. 

A 1964 assessment is 2 x 10 EJ for the Gulf Stream alone. If 0.1%ofthe solar input power is used for tropieal waters with aeeessible temperature differences > 22° C (area 60 x 10 km ), the continuous power production is 12 x 10 GWe year=378 x 10 EJ/year. 

In larger, pumped systems the variable volume/variable flow rate method ensures optimum operating conditions the collector flow rate is increased as the solar radiation input increases thus the output temperatme remains near constant. If there is a hot water draw-off from the tank during a no-sun period (e.g., at night), the tank water level is allowed to drop, that is, no make-up water is admitted (which would reduce the tank temperatme), until the next day s solar input period. 

Another alternative is to use mirrors or to concentrate the sun on the cells. Recently a team led by Allen Barnett (University of Delaware) announced an efficiency of 42.8% obtained by concentrating 20 suns onto a beam splitter that divided the solar input into three spectral bands and imaged each of these bands onto a cell optimized for that band. 

The stagnation temperature (Ts) is the maximum temperature reached by a surface exposed to sunlight. It is reached when the losses from thermal radiation, convection, and conduction equal the solar input. Under vacuum conditions, convection losses are eliminated, conduction losses can be negligible, and the stagnation temperature is given by... 

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