The influence of environmental and geometrical factors on air-ground tube heat exchanger energy efficiency

More and more efficient solutions of thermal insulation of buildings result in an increasing role of ventilation in the energy balance of buildings. This leads to a necessity for seeking unconventional heat sources, as well as development of the exhaust air heat recovery methods. The use of heat accumulating potential of the ground perfectly fits into this trend, allowing natural pre-heating and pre-cooling of the inlet air. There appears to be very limited research and published data on their thermal performance in Poland. This paper introduces a developed method of air-ground heat exchanger (AGHX) performance evaluation together with its validation and research results of conducted simulation. The AGHX model (based on a quasi 3D finite elements method) allows analysis of energy performance dependence on a wide range of parameters including AGHX geometrical configuration, mode of operation and environmental factors. The simulation results indicate that the analyzed parameters in various degrees affect the thermal efficiency of AGHX; various is also the nature of their impact. For some of them it is possible to set a value to maximize heat or cold yield (pipe diameter and placement depth, number of parallel pipes, bypass system, soil thermo-physical parameters, ground area shading and ground surface cover). In other cases, the influence of parameters has an asymptotic nature for which the maximum heating or cooling efficiency is achieved for parameter values tending to infinity (pipe length and distance between parallel pipes).