Reducing ventilation energy demand by using Air-to-Earth heat exchangers: Part 2 - System design considerations

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. A numerical model has been developed using Comsol Multiphysics 4.0.a in order to study earth tubes with multiple ducts. Both the spacing between ducts as well as the number of ducts is simulated. Finally, results have been extrapolated to mimic an installation in a building with a large ventilation demand. 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. For the cases where the duct spacing was investigated, results showed that the outlet temperature of the earth ducts changed only marginally for the three cases simulated. The energy saving per duct showed a slight increase as the spacing was increased. For the cases with different number of ducts, the energy saving increases with increasing number of ducts. However, the increase in energy saving is less than the increase in heat transfer area. The case study considering a building with a large ventilation energy demand, several configurations of earth tube installations have been investigated. Results showed that the best configuration is a case with a small velocity, small duct diameters, long ducts installed at as deep in the earth as possible. However, Once the depth goes below 3.5 m, the increase in energy saving is marginal. For the building having a ventilation air flow demand of 1000 liters/s, a configuration of 33 parallel ducts with a duct diameter of 20 cm and a spacing of 1 meter gave the greatest energy saving. For this configuration, a total energy saving of 34.2 % is possible. © Springer-Verlag Berlin Heidelberg 2013.