Solar Concentration Ratio Research Articles

On the study of hydrogen production from water using solar thermal energy

Theoretical thermal efficiency of hydrogen production by one-step water splitting utilizing solar heat at high temperatures is calculated. Carnot efficiency is assumed for the conversion of effective work input, and the solar collection efficiency is considered for the total energy input. The overall efficiency shows its maximum in the range of temperature between 1500 and 2700 K depending upon the solar concentration ratio and the method of product separation. The technical feasibility of direct splitting method is discussed on the basis of those calculated results.

Color-corrected Fresnel lens for solar concentration

A new linear convex Fresnel lens with its groove side down is described. The design philosophy is similar to that of the highly concentrating two-focal Fresnel lens but includes a correction for chromatic aberration. A solar-concentration ratio as high as 80 is achieved. For wide-acceptance angles, the concentration nears the theoretical maximum.

Efficiency of tandem solar cell systems as a function of temperature and solar energy concentration ratio

This paper presents the results of a comprehensive theoretical analysis of tandem photovoltaic solar cells as a function of temperature and solar concentration ratio. The I– V characteristics of the solar cells were assumed to be governed by the relation I = I 0( e qV/AkT − 1) with I 0 = K e −E G /BkT and A = B The overall efficiencies of tandem cell stacks consisting of as many as 24 cells having energy gaps in the 0.7 to 3.6 eV range were calculated for temperatures of 200, 300, 400 and 500 K and for illumination by an AMO solar spectrum having concentration ratios C = 1, 100, 500 and 1000 suns. For ideal diodes ( A = B = 1), the calculations show that the optimized overall efficiency has a limiting value η opt ≈ 70% for T = 200 K and C = 1000. For T = 300 K and C = 1000, this limiting efficiency approaches 60%. Most of the gain in efficiency occurs with the number of semiconductors in the tandem system between six and ten, e.g. for T = 300 K and C = 1000, an optimized six cell system has a theoretical limit efficiency of about 53%. Calculations were also conducted for the A = B = 2 case (non-ideal diode behavior); in this case the limiting value of η for a 24 cell system is about 65% at 200 K and 55% at 300 K.

Composite semiconductors: Selective absorbers of solar energy

We have produced composite semiconductors by cosputtering CaF 2 with either Ge or Si and measured their optical constants. Their optical behavior can be used described as being similar to that of the parent semiconductor, with the same energy gap but with a reduced, concentration dependent index of refraction. The normal specular reflectance of films sputtered on mirrored surfaces was measured. These data were used to compute the solar absorptance α s and thermal emittance ε th. It was found that α s ≈ 0.7 and ε th ≈ 0.06 with a weak dependence on composition, thickness and operating temperatures. Thus at T = 500° C conversion efficiencies of 50% are currently possible at solar concentration ratios C of 7–8 and about 70% for C ≈ 40 and improved performance can be expected with continuing research.

The effect of non-direct insolation on the radiative performance of trapezoidal grooves used as solar energy collectors

Some possible applications of solar energy require heat at temperatures above those associated with flat-plate collection but below those usually associated with an economical focusing collector. Collection temperatures in this range may be economically obtained using moderately concentrating, non-focusing, trough-like collectors. In this paper the radiative performance (the effective solar absorptivity and concentration ratio) of moderately concentrating collectors, composed of east-west aligned parallel, trapezoidal grooves, is examined. Both direct and non-direct insolation performance is discussed. Results are presented indicating performance degradation due to non-direct insolation. Finally, nomographs are presented which allow one to determine the radiative performance of the grooved collectors when exposed to various combinations of direct and non-direct insolation.

Absorptance, emittance, and thermal efficiencies of surfaces for solar spacecraft power

An expression for the thermal efficiency of a surface used as a receiver for spacecraft solar-power generation is derived in terms of the solar absorptance, hemispherical emittance, and concentration ratio. Experimental equipment is described in which direct measurements may be made up to 1000°C by using a carbon-arc-image furnace to simulate the sun. Emittance and absorptance data are presented on six polished metals and four plasma flame sprayed coatings from 200 to 800°C. It is shown that thermal conversion efficiencies as high as 40 percent at 800°C can be realized with refractory coatings and with a solar radiation concentration ratio of 50 or 6.9 watts per square centimeter.