The influence of different ground covers on the heating potential of earth-to-air heat exchangers

Ten years' hourly measurements of air and ground temperature values at various depths below bare and short grass soil at Dublin Airport have been used in order to investigate the impact of different ground surface boundary conditions on the efficiency of a single and a multiple parallel earth-to-air heat exchanger system. The heating potential of both these systems buried under bare soil has been assessed and compared with the heating potential of the same systems buried under short-grass-covered soil. The results of this comparison revealed that soil surface cover might be a significant controllable factor for the improvement of the performance of earth-to-air heat exchangers. The heating system consists of a single pipe or multiple parallel pipes laid horizontally, through which ambient or indoor air is propelled and heated by the bulk temperature of the natural ground. The dynamic thermal performance of these systems during the winter period and their operational limits have been calculated using an accurate numerical model. Finally, a sensitivity analysis was performed in order to investigate the effect of the main design parameters, such as pipe length, pipe radius, air velocity inside the tube and the depth of the buried pipe below the earth's surface, on the system heating capacity. Cumulative frequency distributions of the air temperature at the pipe's exit have been developed as a function of the main input parameters.