A great fraction (20-40%) of primary energy is required for building air conditioning, so the use of renewable energy sources is increasing. The geothermal energy for Heating, Ventilating and Air Conditioning (HVAC) systems can be used considering an Earth-to-Air Heat eXchanger (EAHX). This work analyses the performance of an EAHX through a mathematical model (2D), as a function of diameter and length of the air ducts. The problem is solved with finite element method. A case study office building is analyzed. The air conditioning plant is characterized by fan-coil units and primary air; the EAHX is positioned upstream the Air Handling Unit (AHU) to pre-cool/pre-heat the air. The building is virtually placed initially in six Italian cities (different climatic zones according to Italian regulation DPR 412/93) and subsequently in eight worldwide cities according to Köppen climate classification. The following parameters are calculated: air temperature variation and thermal efficiency of the EAHX; the decreasing of cooling and heating capacity of the coils into the AHU. The best results refer to a duct length of 100 m for Ottawa (warm-summer humid continental climate, 65% capacity reduction), the worst ones for Rio de Janeiro (tropical wet and dry climate, maximum 24% reduction).
The employment of an earth-to-air heat exchanger as pre-treating unit of an air conditioning system for energy saving: a comparison among different worldwide climatic zones