Thermal analysis of vertical ground exchangers of heat pumps

The heat rate absorbed from the ground by a vertical ground exchanger of a heat pump unit is considered. The aim is to investigate the time variation of this energy rate for a set of parameters. The analyzed set of alternatives encompasses arrangements of the exchanger tubes, values of the temperature of the heat carrier, thermal parameters of the ground, periodic operation of the compressor, and, when appropriate, different values of seepage velocity. To achieve this goal, the transient temperature distributions in the soil surrounding the ground exchanger are evaluated. The calculations are carried out using both PATRAN-THERMAL, a commercial finite volume code, and FEMCONV, an in-home FEM package. Characteristic features of the latter are discussed briefly along with some results of simulations for a ground exchanger with tube-in-tube (Field-type) elements. It is shown that in every case, the heat rate absorbed from the ground depends on the season and reaches the minimum value in the second part of winter. As expected, a strong influence of the arrangement of the exchanger tubes and the motion of moisture is observed. It is shown that if the prices of the electric energy are variable during a day, it may be profitable to operate an HP unit compressor in a periodic regime. The approximate values of the heat pump unit coefficient of performance, defined as the ratio of heat output of HP and compressor driving power, are evaluated. It is pointed out that this coefficient depends on the heat carrier temperature, and therefore this temperature may also be a subject of optimization calculations.