Experimental investigation of thermal energy storage with latent and sensible heat in solar air collector

In this study, it is aimed to increase the thermal efficiency and usability of solar air collector (SAC) by sensible and latent heat storage methods. In order to be able to make simultaneous comparisons, two separate test setups consisting of three collectors, two heat storage collectors with different materials, were prepared with conventional flat absorber plate collector. The first experimental set-up consists of a collector with a flat plate absorber, only phase change material (PCM) and PCM+honeycomb combination. The second experimental set-up consists of a collector with flat absorber plate, cherry stone and milled cherry stone. Both heat storage methods were tested separately. PCM and honeycomb were combined in order to increase the low heat conduction coefficient, which is one of the chronic problems of PCMs. The cherry stone as an agricultural waste was examined in the form of core and powder. The experiments were carried out under natural and forced convection conditions at flow rates between 0.008 and 0.048 kg/s. In addition, latent heat storage (with PCM) collector is modeled numerically with ANSYS Fluent. The samples with various mixing ratios for the combination of PCM and cherry stone powder were subjected to differential scanning calorimetry (DSC) test. In the experiments using PCM, the day time thermal efficiency was increased by 8.4% to 9.0% and the daily thermal efficiency was increased by 2.6% to 22.3%. With the use of aluminum honeycomb, the charge-discharge time of the PCM is significantly shortened, while the PCM temperature has increased by up to 10°C. In the latent heat storage experiments, the equalization of the inlet-outlet temperature after sunset was approximately 1 hour for the collector with flat absorber plate and for 6-10 hours for the collectors with PCM. However, the efficiencies of the collectors using only PCM and PCM+honeycomb were very close to each other. In the collectors using the cherry stone and powder, the daytime thermal efficiency decreased up to -4.7% depending on the low flow rate during the charging period and up to 10.6% at high flow rates. With the use of sensible heat storage, the daily thermal efficiency was increased from 2.8% to 18.7%. The equalization of the inlet-outlet temperature of the sensible heat storage collectors lasted longer than 5-6 hours after the sunset compared to the collector with flat absorber plate. The thermal efficiencies of the cherry stone and powder collectors were very close to each other. In the results of differetial scanning calorimeter (DSC), it was determined that cherry stone powder and PCM gave results parallel to the mixture ratios and could be used easily. Moreover, the results of numerical modeling were parallel to the experimental data. The experimental results showed that the use of PCM and PCM+honeycomb as concealed heat storage material in designed SACs and the use of cherry stone and powder as sensible heat storage material could significantly increase the collector thermal efficiency and could be used for a long time after sunset.