Performance Evaluation of Horizontal Air-Ground Heat Exchangers for Conditioning Systems

The need to improve the energy performance of buildings led to a remarkable development of innovative solutions for the air conditioning and heating of buildings. In particular, horizontal air-ground heat exchangers (HAGHE) offer a significant contribution in reducing energy consumption for ventilation, using the thermal energy stored underground. By the use of energy stored in the ground, it is possible to pre-heat or pre-refresh the ventilation air, in winter and summer, respectively. This is really interesting in applications for industrial, commercial and education buildings where keeping under control the Indoor Air Quality is dramatically important. A good ventilation needs to guarantee an high refresh air mass flow rate per person but it means more thermal power needed for the conditioning treatment. Experimental measurements show that the ground temperature below a certain depth, remains relatively constant throughout the year. This is due to the fact that the temperature fluctuations at the surface of the ground are diminished as the depth of the ground increases because of the high thermal inertia of the soil. Therefore, at a sufficient depth, the ground temperature is always higher than that of the outside air in winter and is lower in summer. The difference in temperature between the outside air and the ground can be used as a preheating means in winter and pre-cooling in summer by operating a ground heat exchanger. The efficiency of total system depends on several elements: the total pipe length, depth of installation, climatic data, ground properties. The 3D geometric model was created with tetrahedral and hexahedral elements grid more dense at the pipe and more sparse in the trench and in the ground, carried out through a parallelepiped (4 x 4 x 27 m3). The air flow rate was set at four different values: 150, 250, 350, 450 m3/h. The simulations were performed in unsteady annual conditions. The experimental climatic data were air temperature and velocity, ground temperature and total solar radiation of the year 2002. Climatic data were recorded by a climatic station, located in Lecce. The simulations have shown a significant advantage in using these exchangers as they can limit the variations in temperature of inlet air to the heat pumps, with significant benefits in terms of performance. In particular, in not-extreme climatic conditions, the pre-treatment of ventilation air could be enough bypassing the following heat pump.