Temperature distribution and performance of ground-coupled multi-heat pump systems for a greenhouse

The primary objective of a greenhouse is to produce good plant-growth conditions such as temperature and humidity. One of the hot issues for the greenhouse is to provide an appropriate heating system which can achieve favorable temperature condition and save energy. In this study, the performance of a ground-coupled multi-heat pump system for the greenhouse heating was investigated. The ground-coupled multi-heat pump system was composed of GLHX (ground loop heat exchanger) and multi-heat pump unit which had one outdoor unit and two or more indoor units. The temperature distribution within the greenhouse using the ground-coupled multi-heat pump system was represented relatively uniform comparing to when the conventional heating system and GCHP system were adopted, because the capacity of each indoor unit could be changed linearly according to the variation of load. The temperature difference between the maximum and minimum temperatures and the standard deviation of inside temperature for the greenhouse were 2.1 °C and 1.2 °C, respectively. It is necessary to develop the multi-heat pump unit which can be operated with high performance at relatively low temperature setting conditions. The system COP of the ground-coupled multi-heat pump unit decreased greatly at part load condition due to relatively high power consumption of the ground circulation pump. Therefore, it is suggested that a control algorithm of the ground circulation flow rate has to be developed to maximize energy saving by applying the ground-coupled multi-heat pump system to the greenhouse.