| Solar energy seasonal thermal storage system with buried pipes can be used to solve the problem of the mismatch between seasonal distribution of solar energy and heating demand.The system is widely used in heating projects because it can raise the solar heating rate of heating system and enhance its stability.The heat exchange characteristics of ground heat exchanger(GHE)play key roles on the performance of seasonal thermal storage system,and the operating strategy and thermal property of the soil both have distinct effects on the heat exchange characteristics of GHE.But existing literatures reported little about the detailed regularit ies of their effects,so a further study on it was carried out in this paper.Remolded loess in Lanzhou was chosen to be the thermal storage media,and the effects of compaction process parameters on thermal properties of remolded loess were studied with experimental method.A numerical model of a small thermal storage apparatus with horizontal buried pipes was established by Computational Fluid Dynamics(CFD)software FLUENT,the thermal and physical properties of remolded loess in Lanzhou was optimized and assigned to the model.The simulation results of the model were proved to be precise by comparing with experimental results of the apparatus.Based on the simulation results of the model,the relationship between parameters of intermittent operating modes and the heat exchange efficiency of GHE was studied and a mathematical correlation between them was fitted.Furthermore,a scaling model of a large solar energy seasonal thermal storage system with horizo ntal buried pipes was established by CFD software FLUENT.The pattern of the effects of operating strategy on the heat storage characteristics of the system w ere studied based on numerical simulation results.The experimental results indicate that there is a significant correlation between the compaction parameters and thermal properties of remolded loess i n Lanzhou.Under a certain pressure,when the moisture content of remolded loess changes from 6% to 19%,there is a positive correlation between thermal conductivity and moisture content.With the increase of moisture content,the thermal conductivity increases rapidly at first and then the increment becomes smaller gradually.With a certain moisture content,there is also a positive correlatio n between thermal conductivity and pressure,when the pressure increases,the increment of thermal conductivity becomes greater.The maximum measurement value of thermal conductivity of remolded soil is 1.78 W/(m·K),an empirical formula of thermal conductivity of remolded soil is fitted based on the measurement results of thermal conductivity.The simulation results of the numerical model of the small thermal storage apparatus with horizo ntal buried pipes shows that parameters of intermittent operating modes have significant effects on the heat exchange characteristics of the heat exchanger,Under a certain intermittence frequency,when proportion of intermittence increases gradually,the heat exchange power of the buried pipe increases continuously,and the growth rate gradually increases,too.Under a certain proportion of intermittence,when the number of intermittence increases in the range of 2 to 12,the heat exchange power of the buried pipe increases but its increment decreases gradually.The simulation results of the Numerical scaling model of thermal storage system with horizo ntal buried pipes shows that the start time of the GHE has sig nificant effects on the heat storage performance.Under continuous heat storage mode,when the start time delays from 8:00 to 18:00,heat stored per day decreases,a nd the decrement becomes greater,meanwhile,the heat exchange power increases linearly.In this research,a quantitative relationship between thermal conductivity and compaction parameters of remolded loess in Lanzhou was fitted for the first time.The relationship between intermittent operating parameters of heat storage process and heat exchange power of heat exchanger was obtained.The heat storage characteristics of solar energy thermal storage system with horizo ntal buried pipes under intermittent operating modes were also studied in this research.For the solar energy thermal storage system using remolded loess in Lanzhou as thermal storage media,this research can provide important references for the design of the system and the optimization of its operating strategy. |
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