Gas Heat Conduction in Evacuated Flat-Plate Solar Collectors: Analysis and Reduction

In stationary heat-loss experiments, the thermal losses by gas conduction of an evacuated flat-plate solar collector (EFPC) were experimentally determined for different values of interior gas pressure. The experiments were carried out with air and argon in the pressure range from 10−3 to 104 Pa. For air, loss reduction sets in at 100 Pa, whereas at 0.1 Pa heat conduction is almost completely suppressed. Using argon as filling gas, gas conduction is reduced by 30 percent (compared to air) at moderate interior pressures of 1000 Pa. With decreasing pressure this reduction is even greater (50 percent reduction at 10 Pa). A theory was developed to calculate thermal losses by gas conduction in an EFPC: Fourier’s stationary heat conduction equation was solved numerically (method of finite differences) for the special geometry of the collector. From kinetic gas theory a formula for the pressure dependency of the thermal conductivity was derived covering the entire pressure range. The theory has been validated experimentally for the gases air and argon. Calculations for krypton and xenon show a possible gas conduction loss reduction of 60–70 percent and 75–85 percent (with respect to air, depending on gas pressure), corresponding to a reduction of the overall collector losses of up to 40 percent.