Internal thermal coupling in direct-flow coaxial vacuum tube collectors

This investigation covers the impact of low flow rates on the efficiency of coaxial vacuum tube collectors. Measurements show an efficiency reduction of 10% if reducing the flow rate from 78 kg/m2 h to 31 kg/m2 h for a collector group with 60 parallel vacuum tubes with a coaxial flow conduit at one-sided connection. For a more profound understanding a model of the coaxial tube was developed which defines the main energy fluxes including the internal thermal coupling. The tube simulations show a non-linear temperature profile along the tube with the maximum temperature in the outer pipe. Due to heat transfer to the entering flow this maximum is not located at the fluid outlet. The non-linearity increases with decreasing flow rates. The experimentally determined flow distribution allows simulating the measured collector array. The simulation results confirm the efficiency decrease at low flow rates. The flow distribution has a further impact on efficiency reduction, but even at an ideal uniform flow, a considerable efficiency reduction at low flow rates is to be expected. As a consequence, low flow rates should be prevented for coaxial tube collectors, thus restricting the possible operation conditions. The effect of constructional modifications like diameter or material variations is presented. Finally the additional impact of a coaxial manifold design is discussed.