Volume 55

Performance of Solar Flat Plate Collectors with s-CO2 as Heat Transfer Fluid Wasim Ashraf, Maddali Ramgopal

Abstract

In the present work, a simple numerical model is developed to estimate the performance of a solar flat plate collector (FPC) with water and CO2 as heat transfer fluids. An experimental test rig validates the model with water as heat transfer fluid (HTF). Though water is an excellent working fluid, it cannot be used in places where the ambient temperature falls below 0oC. Though antifreeze solutions can be added to water, this decreases the overall efficiency of the collector. In this context, using supercritical CO2 (s-CO2) in place of water can offer enhanced performance in addition to an extended operating temperature range. Hence in this study, the analysis is done with s-CO2 and water by considering four different cases under different operating conditions. The performance of both the HTFs is compared by using dimensionless quantities such as Reynolds number, Nusselt number, and Prandtl number. It has been observed that the outlet temperature and thermal efficiency of solar water heating system (SWHS) achieved are superior when the rate of mass flow is decreased, and the diameter of the riser tubes is made smaller. It has also been observed that substituting s-CO2 with water as the HTF in a solar FPC improves the overall performance. The solar collectors containing s-CO2 have high operating pressures, phase-changing potential with the change of ambient temperature, and considerable property variations around the critical point of the fluid, which are the major challenges for designing this type of SWHS. Thus, these subjects demand an extensive investigation.

Keywords Flat Plate Collector, Solar Water Heating System, Thermal efficiency, Supercritical-CO2