Heat extraction from single ended glass absorber tubes

Abstract

The efficiencies of solar thermal collectors are degraded if there are appreciable temperature differences between the solar absorbing surface and the heat extraction fluid. This problem has been well researched for flat plate collectors, but little has been done on the many possible heat extraction methods for all-glass tubular evacuated collectors. Measurements are reported here for assorted heat removal schemes in single-ended glass tubes with the non-uniform solar input around the tube simulated electrically. The results for two schemes with the flowing water in direct contact with the inside of the glass tube are consistent with a simple laminar flow model, with the heat input assumed to be uniform around the tube. Results for more complex schemes where the water is contained in a metal pipe are also given, and some are shown to possess unacceptably high temperature penalties. It is also shown that a simple fluid-in-glass scheme where the natural buoyancy forces alone keep the cold ingoing and hot outgoing streams apart should give acceptable performance.