Performance Investigation And Numerical Modeling Of A Phase Change Material Integrated Flat Plate Solar Air Heater Coupled With An Earth Air Heat Exchanger

Due to the limitation of shallow soil temperature,the outlet air temperature of earth air heat exchanger(EAHE)is generally relatively low in winter,which is difficult to meet the space heating demand of building.In previous research,the outlet air temperature was mainly increased by using conventional flat-plate solar air heater(SAH)to reheat the outlet air of EAHE.However,owing to intermittent and fluctuating solar radiation,the developed system(EAHE and SAH coupled system)suffers some drawbacks,such as unstable outlet air temperature,overheating and running period being limited to daytime.To cope with this situation,phase change material(PCM)was proposed for use in the SAH coupled with the EAHE.To assess the thermal performance of the proposed system,an experimental setup was built and a series of experimental tests were conducted.Based on this,the numerical model of the coupled system was established and validated.And the parameter influence on the thermal performance of the coupled system in winter was studied based on the model.The main results show:(1)The coupled system is able to effectively suppress the impact of fluctuating solar radiation on outlet air temperature,avoid overheating.Compared with the conventional flat-plate solar air heater,the PCM-flat-plate solar air heater maintained the outlet air temperature in the range of 33~oC to 35.8~oC for nearly 4.5h,and decreased the maximum outlet air temperature by 8~oC by utilizing the characteristic that PCM's phase change temperature variation range is relatively small.(2)The PCM-flat-plate solar air heater can continue to operate at night,which improves the thermal performance stability of the coupled system.Compared with the conventional flat-plate solar air heater,the PCM-flat-plate solar air heater can store excessive solar energy in the form of latent heat during the day and release it at night,thus prolong the working period of the coupled system by nearly 5.5h.(3)The thermal performance of the coupled system changes with the change of airflow,PCM's phase change temperature,PCM's thermal conductivity,PCM's phase change latent heat and emissivity of absorber surface.In this study,when the airflow rate increased from 100kg/h to 300kg/h,the maximum outlet air temperature and the working period of the coupled system decreased by 5.3~oC and 4h respectively.The total heating capacity and renewable energy utilization rate of the coupled system increased by 18.6MJ(varied from 23.3MJ to 41.9MJ)and 13.6%(varied from 60.2%to 73.8%)respectively.Moreover,reducing the PCM's phase change temperature,PCM's thermal conductivity and emissivity of absorber surface as well as increasing the PCM's phase change latent heat can prolong the working period of coupled system;besides that,reducing the PCM's phase change temperature and emissivity of absorber surface as well as increasing PCM's phase change latent heat can increase the total heating capacity of coupled system.And the renewable energy utilization rate of the coupled system can be increased by reducing the PCM's phase change temperature and emissivity of absorber surface.Based on the results of this paper,the future research direction should focus on further optimizing the geometry of the coupled system,and evaluating the indoor environment when the system is actually applied to building heating.