Optimum number and arrangement of evacuated-tube solar collectors under various operating conditions

Evacuated-tube solar collectors are featured by high outlet temperatures with a relatively high thermal efficiency and no direct environmental impacts. However, improper selection of the number of collectors and their arrangement, can lead to undesirable increase in the footprint and capital cost resulted from manufacturing and installation processes. The present work aims at optimizing the total number of tubes and their arrangement for large arrays of evacuated-tube collectors that are used to serve a heat driven cycle working at a relatively high inlet temperature, such as adsorption and absorption chillers and desalination systems. A deduced expression for the thermal efficiency of the evacuated-tube collector has been validated experimentally for different series and parallel arrangements. The experimental validation indicates the capability of the expression to predict the exit water temperature with a maximum relative error of 2%. Optimization charts for the optimum number and arrangement of tubes are produced at different climatic and operating conditions. Effects of the solar irradiance, mass flow rate and temperature rise on the minimum number of tubes and the corresponding arrangement are investigated and discussed. The optimization tactic used in the present study reveals that a reduction in the total number of tubes up to 41% can be achieved leading to a considerable saving in the initial cost.