Energy storage in unsaturated soil using pore air flow

Mineral grains in soil are surrounded by pores, the porosity of sandy soil being typically 25-35 % of its volume. Above the water table, such pores are filled with air. The heat stored in the mineral grains is transferred to the surrounding air in the pores. Field experiments on energy storage in unsaturated soil using pore air flow were carried out during 1990 - 1991 in Southern Finland. High volumes of pore air, for example 3 900 m3/h in this study, can be produced from a well installed to a depth of 5-7 m, where natural temperatures remain at 5-6°C throughout the year in Finland. The influence of the pumping can be detected far from the surroundings of the well; a 20 Pa pressure difference was measured in the soil 50 meters from the well in this study. For numerical simulation the multiphase, multicomponent model TOUGH was used, simulating the coupled transport of water, air and heat in gas and liquid phases. Heat transfer due to convection within air was found to be considerable in comparison to conduction. The injection flow rate mainly affects the radius of influence and the temperature of the injection air the temperature distribution near the well. Comparison of the results with a hypothetical situation where air is extracted from the soil under natural conditions shows that the benefit due to air injection reduces in 3 months to about 1 °C. Further studies are needed to take into account the subzero temperatures during the winter period. The possibility of using the model THETA to simulate the flow of air in a porous medium was also studied. The model is microcomputer based and simulates the coupled flow of fluid, heat and solute in saturated porous media. The simulations show that the compatibility of those two models seems to be fairly good. The actual suitability of microcomputer simulations by THETA depends on degree accuracy desired. A markedly better fit with experimental data was observed when more accurate ambient temperature data were used in THETA simulations. Most of the energy flux into the soil was caused by solar energy conduction. From the technical point of view heat can be stored in summer and cold in winter by pumping ambient air into the soil through a well. From the economical point of view this is however not viable. Heat derived from industry could be used with better efficiency. Natural heat storage in the unsaturated zone maintains the produced pore air at temperatures of at least 3-5 °C throughout the winter. From the technical point of view the method can be applied to keep lawns and sports grounds free of snow and unfrozen when occasional snow cover is acceptable. Sufficient data concerning the availability and temperature of pore air for calculations of the use of pore air flow directly into houses as indoor air, to heat greenhouses or to cool air-raid shelters were obtained in the study.