Investigation of the Reduced Performance of a Collector Array with Direct Flow Vacuum Tubes
Abstract
TU Berlin and ISFH investigated the reduced performance of a collector array with direct flow vacuum tubes and an aperture area of 258 m². A transient simulation of the collector loop has been performed using the TRNSYS software package. The simulation showed that the measured daily energy gain is only 60% of the gain indicated by the simulation. Measurements showed partial stagnation in several collectors of the array where the outlet temperature of single vacuum tubes fluctuates between the collector loop temperature and the boiling temperature of the fluid. The process of partial stagnation was investigated in detail at the ISFH. The results indicate no significant reduction in collector efficiency under partial stagnation. The main cause of partial stagnation is the reduced volume flow, thus indicating hydraulic problems in the investigated collector array. A reduction in the volume flow can reduce the collector efficiency due to an internal thermal coupling in the coaxial absorber pipes. Infrared photography indicated problems with the vacuum of several tubes as well as damage in the pipe insulation. By integrating the detected reduction factors in the simulation model, the measured collector gain shows good accordance to the simulated collector gain. The results point out the importance of a purge system in vacuum tubes as well as the need to improve the hydraulic layout of the whole collector system.