Comparative study of fluid-in-metal manifolds for heat extraction from single ended evacuated glass tubular collectors

An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies η₀ of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for high-temperature (>100°C) heat extraction.