Numerical Research On A Domestic Solar Heating System With Underground Cylinder Thermal Storage

The solar heating system with underground seasonal storage is a new work in solar energy utilization research.By storing solar energy in non heating season and extracting it for indoor heating in heating season,the system can balance the differences between solar energy supply and heat load demand in time.However,the solar heating system with underground seasonal storage has been little studied and applied in China,this research has tried to present a preliminary study on the application of the system in the heating climate zone of northern China.The solar heating system with underground seasonal storage consists of collector,underground water tank,heating room,auxiliary heating system and water pump and other components.Neglecting the auxiliary heating,the optimal capacity of underground water tank depends on the amount of heat collected by the solar collector,the required load and economic factors.Therefore,the matching of the solar collector area size with the water tank volume is the key factor which affects the solar fraction and system economy.On the basis of reasonably determining the area of collector and the volume of water tank,this paper will be mainly focus on the analysis of the heat storage characteristics of the water tank without insulation layer.Taking a rural residential building in Taiyuan area as an example in this paper,the three-dimensional numerical model of underground heat storage tank and surrounding soil was established by FLUENT software.Besides,the heating load demand was analyzed by the Dest software.Using the user-defined function in FLUENT,the heating load data and the solar radiation intensity are transformed into the dynamic boundary conditions of the water tank and the collector inlet.In order to know the heat storage characteristics of the underground water tank and the surrounding soil for many years,at the same time for the sake of reducing the calculation time,small scaled models based on similarity theory are established.In this paper,the temperature distribution of the tank and the surrounding soil under the conditions of different height-diameter ratio,different soil types,different burial depth and different collector area are compared.Besides,the solar fraction and contribution rate of soil thermal storage are defined to characterize the heat storage and heat extraction.The simulation results show that the underground heat storage tank with the height to diameter ratio of 2: 1 is the best,and the clay is more suitable as the heat storage medium around the underground heat storage tank,and the area size of the collector is an important factor which affects heat storage and heat extraction.Furthermore,the solar fraction for storage tank with 8 meter’s burial depth is relatively higher comparing with the 4 meter’s one.