A numerical study into effects of intermittent pump operation on thermal storage in unsaturated porous media

Highlights: • Numerical study of intermittent pump operation on thermal storage system was presented. • The media's cumulative heat increases during the first 12-h on mode and decreases in the second 12 h. • Sand loses about half of the total moisture in the media after ten days with 70 W/m³ flow rate. • The intensity of heat increase is reduced by increasing the rest time of the heat pump. • A 0.5 kJ reduction in heat is achieved at the end of the tenth day by two-hour reduction of pump operation. In this paper, the effects of varying soil properties and intermittent pump operation on the performance of the thermal storage system are examined using finite element numerical simulation. Furthermore, 10 days of intermittent operation, which involved different modes of pump operation like permanent on-and-off mode condition, 12-h active mode and 12-h off mode and unequal hours of pump operation are simulated in a single U-shaped pipe for both emitting and extracting modes and the cases have been selected for the various types of soil such as sand, silt, clay, and sandstone. According to the results, if the pump operates continuously, the heat increase is lower than which the pump switches off every 12 h, due to the initial condition of re-launching the system. Also, the process of increasing the saved heat in unsaturated clay for several modes of pump operation in emitting mode indicated that for every two-hour reduction of pump operation the media approximately faces 0.5 kJ/m³ reduction of saved heat at the end of the tenth day. As the results indicate, the comparison of saved heat in four kinds of soil indicates that the highest saved heat is associated with sandstone and the lowest contributed to sand. So, the saved heat at the end of the tenth day for sandstone in the mode of 8–16 is approximately 15.5 kJ/m³ and this value for sand is 10 kJ/m³.