Theoretical and Experimental study of a Multi-pass two-glass cover solar air heater under transient conditions

Abstract

The major problems experienced with solar air heaters are their non-reliability as their operation largely depends on weather conditions which keep on changing. This study was aimed at providing and experimentally validating a transient based model for predicting the performance of multi-pass solar air heaters operating on changing weather conditions. A transient prediction model was developed by considering the thermal capacities of the collector components and the flowing air. Differential equations were developed by considering the energy interactions and balances for the various components of the collector, and then solved nUmerically. The developed model was tested with input data of insolation, ambient temperature, wind velocity and solar time and then validated experimentally by use of a single collector which was designed to accommodate all the four flow arrangements investigated ( SPM, DMPM, DTPM and TPM).

The time constant of the collector constructed was determined experimentally to be 27 minutes. The theoretical collector performance results indicated a transient behavior for changing weather conditions. This was' also confirmed by the experiment carried out whose data were recorded and plotted at intervals of two minutes Based on air temperature rise and collector efficiency both the developed theoretical model and experimental set up indicated that the triple pass mode is superior to the other modes, with the single pass mode ranked the lowest in performance. The developed ~UlputaticmaI prediction model had a standard deviational error of 1.8 to 2.1 % as compared to the experimental values which had an error of2.7 to 9.6%. The TPM had the lowest error (1.8%) while the SPM had the highest (2.1%). When compared to other models and experiments, the results were in good agreement. The developed model confirmed that air temperature rise in the collector is a function of available solar insolation and prevailing weather conditions such as; cloud cover, ambient temperature and wind.

It was found that, in actual sense thermal solar collectors do expenence transient conditions and the developed transient model was necessary, and is expected to reduce the day long experiments that need to' be carried out to. acquire the performance characteristics of solar air heaters under changing weather conditions.