The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.