Integrated model for comparison of one- and two-pipe ground-coupled heat pump network configurations

Several network configurations are possible when designing the interior portion of centralized ground-coupled heat pump systems. In this study, three different configurations are examined: Two-pipe networks with either a direct-return or a reverse-return and one-pipe systems. One-pipe networks typically require less piping than two-pipe systems. However, heat pump energy consumption might be higher because the inlet temperature to the heat pumps tends to increase (in cooling) or decrease (in heating) along the network. In the current study, a versatile integrated modeling tool is developed in the TRNSYS environment to study the energy consumption (pumps and heat pumps) of each type of network. A control method for one-pipe systems, based on the bore field return temperature, is also proposed. The tool is first compared to detailed individual models in annual simulations where it is shown to give good results. The results obtained with four different case studies indicate that the total annual energy consumption of one-pipe networks is up to 5% higher than two-pipe networks even though the pumping energy is 4% to 53% lower than two-pipe networks. No significant differences are observed in the required borehole length.