The performance of vertical ground source heat pumps (GSHPs) in hot dry climates Saudi Arabia as a case study

Nowadays the use of renewal energy has become a fundamental choice in most developed and developing countries, in order to reduce the energy demand and CO2 equivalent emissions. In order to reduce this demand, conventional air-conditioning technology should be replaced by more efficient renewable energy systems, such as those employing ground source heat pumps (GSHPs). The GSHP is cooling/heating system that transfer heat to or from the ground benefiting from the fact that the underground temperature remains almost constant all year-round at ten metres in depth or more. This means that the effect of the ambient temperature is limited and the difference in temperature between what is considered desirable (inside the building) and the surrounding medium (underground soil) is small compared to the outside temperature. In a hot and dry country such as Saudi Arabia, air-conditioning systems consume seventy per cent of the electrical energy. In order to reduce this demand, the performance of ground source heat pumps (GSHPs) compared to the conventional air source heat pump (ASHP) systems which have a poor performance when the air temperature is high. In Saudi Arabia, this can be as much as 50 °C. Unfortunately, the majority of the previous research into GSHPs has been focused on cold regions. Therefore, for the first time in such a hot/dry climate, the performance of ground source heat pumps (GSHPs) has been evaluated compared to the conventional air source heat pump (ASHP) systems in this type of climate. Both systems were comprehensively modelled and simulated using the Transient System Simulation (TRNSYS) and Ground Loop Design (GLD). In order to assess this configuration, an evaluation of a model of a single-storey office building, based on the climatic conditions and geological characteristics that occur in the city of Riyadh in Saudi Arabia, was investigated. The period of evaluation was twenty years to determine the Coefficient of Performance (COP), Energy Efficiency Ratio (EER) and power consumption. Generally energy efficiency ratio is calculated by taking ratio of cooling or heating output in BTU to electrical energy input in W. The simulation results show that the GSHP system has high performance when compared to the ASHP. The average annual COP and EER were 4.1 and 15.5 for the GSHP, compared to 3.8 and 11 for the ASHP respectively, and the GSHP is a feasible alternative to the ASHP, with an 11-year payback period, with an 18% total cost saving over the simulation period and 36% lower annual energy consumption. The TRNSYS model shows that despite the positive results of the modelling, the high rate of the underground thermal imbalance (88%) could lead to a system failure in the long term. In addition, Ground Loop Design software has been employed to conduct a sensitivity analysis of 12 parameters that most affect the behaviour of the system in order to determine the near-optimum design of a GSHP system. Thus, the entering water temperature (EWT) was selected to be the performance measure of the system. The results showed that the most important four design parameters are: the thermal conductivity, soil temperature, building load, and fluid flowrate. And the length of the ground heat exchanger and power consumption in the proposed design could be reduced by 15% and 1.12% respectively, compared to the baseline system. Despite the high underground thermal imbalance and increase in the initial capital costs of GSHP, because of the extra expensive drilling costs for the ground loop heat exchanger and piping, the feasibility of GSHP system is worthy of investigation. These results can be used to simplify the design of buildings in similar arid regions worldwide.