Reducing Ventilation Energy Demand by Using Air-to-Earth Heat Exchangers

Abstract

Air-to-Earth heat exchangers (earth tubes) utilize the fact that the temperature in the ground is relatively constant during the year. By letting the air travel through an air-to-earth heat exchanger before reaching the house’s ventilation air intake the air gets preconditioned by acquiring heat from the soil in the winter, and by rejecting heat to the soil in the summer. There are few studies showing how large the energy saving would be by using earth tubes. The existing studies and models are adapted to a warm climate like India and Southern Europe. Few studies are made for a Nordic climate. To be able to use earth tubes efficiently, different parameters need to be optimized. The parameters that have the largest effect are length, depth, and diameter of the earth tube , as well as the air velocity inside the tube. To analyze this influence, a numerical model has been created in the simulation program Comsol Multiphysics 4.0a. Weather data for Stockholm, Sweden was used for all simulations. The soil type was chosen to be clay and the material of the duct was polyethylene. The parameters were varied one at a time and compared to a base case consisting of a 10 m long duct placed at a depth of 2 m and with a diameter of 20 cm. The air velocity in the duct for the base case is 2 m/s and the corresponding volumetric flow rate is 60 l/s. Results show that longer heat exchangers with a smaller diameter, lower air velocity and buried at a deeper depth gives a larger energy saving. The increase in efficiency that comes from a deeper placed earth tube levels out at depth over 3.5 m. The decrease in efficiency that comes from an increase of the diameter of the duct levels out at diameters of 60 cm. The total energy saving for one year increased by 70 % for a 20 m long earth tube compared to a 10 m long earth tube. The energy saving for the base case is 525 kWh/year for the heating season and 300 kWh/year for the cooling season. This corresponds to an energy saving of 5 % for heating and 50 % for cooling compared to a case where no earth tube is used.