ISFH investigated the impact of the internal coupling in vacuum tube collectors with coaxial absorber pipes. Measurements show an efficiency reduction of 10% if reducing the flow rate from 78 kg/m~2h to 31 kg/m~2h for a collector group with 60 parallel coaxial vacuum tubes at one-sided connection. For a more detailed investigation a simulation model of a vacuum tube collector was developed. The simulations show non-linear temperature profiles along the inner and outer tubes with the maximum temperature not at the fluid outlet but inside the outer pipe. This non-linearity increases with decreasing flow rates. Significantly higher temperatures if compared to the standard flow direction are observed if the fluid is directed from the outer to the inner pipe. The model was extended to simulate a complete collector group with 60 vacuum tubes connected in parallel, whereat the tubes are operated with an experimentally determined flow distribution. The simulation results confirm the measured efficiency decrease at low flow rates. It was found, that the main reason for this is the coupling effect in the vacuum tubes, which is more pronounced at unequal flow distribution. Internal coupling in the header increases the efficiency reduction just slightly if not connected one-sided. As a consequence, low flow rates should be avoided for coaxial tube collectors, thus restricting the possible operation conditions.
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