Experimental study on the thermal performance of a grey water heat harnessing exchanger using Phase Change Materials. (February 2020) - Mazhar, Abdur Rehman
Abstract: To integrate heat extraction and storage into a single unit along with decoupling of demand and supply, Phase Change Materials (PCMs) can be used to harness heat from grey water (GW). A simple heat exchanger linking both the GW and incoming mains cold water (CW) in a counter flow arrangement with a PCM is experimentally tested. To enhance the thermal conductivity of the PCM, metallic copper fins are placed throughout the cross-section of the pipes. The charging with a GW temperature of 325 K and the discharging with a CW temperature of 285 K, of the PCM is investigated. The influence of the mass flow rates of both the fluids is investigated by varying it between discrete values of 0.1 kg/s and 0.05 kg/s. Similarly the operation strategy of the heat exchanger is varied between the solo operation of the GW and CW compared with the simultaneous flows of both. Finally two different PCMs; with a melting temperature of 298 K (RT-25) and 315 K (RT-42) are also tested. The mass flow rate is proportional to performance with 0.1 kg/s showing the best results but being less influential for RT-25 as compared to RT-42. In RT-25 most heat is transferred as latent heat with a higher phase change rate whilst RT-42 transfers sensible heat. The solo operation strategy of non-simultaneous GW and CW flow is more effective as this exchanger is meant to decouple demand and supply. The ability of RT-25 to retain heat over the long term is also greater compared to RT-42. Ullage issues and corrosion concerns of the metallic container and pipes are also dominant over the long-term usage of these PCMs. Highlights: Experimental testing of grey and cold water flow in a PCM exchanger is performed. A mass flow rate of 0.1 kg/s has better charging and discharging of the PCM. A PCM with a melt temperature of 298 K outperforms one with that of 315 K. About 20 kg of the PCM RT-25 is melted in 2700s from bottom to top. Compared to melting, freezing is a slower and less heat intensive process.