| Abstract |
5th generation district energy systems, also known as thermal energy networks or geothermal networks, are an important technology for efficiency utilization of thermal energy. These networks use water-source heat pumps and an ambient temperature loop to exchange energy with a borehole field to meet building heating and cooling loads. The borehole field acts as a heat exchanger to cool or heat fluid used to maintain a desired building temperature. A semi-transient, reduced-order model, called HEATNETS, has been developed at NREL that captures the flow of energy within this described thermal energy network. HEATNETS is designed to be an open-source code aiding in modeling and predicting performance for geothermal networks. The research presented here is part of the Learning from the Ground Up (LeGUp) project, studying the first utility-led networked geothermal installations occurring in Massachusetts. HEATNETS uses either measured or modeled building loads to simulate and predict parameters such as the borehole field temperatures and heat pump power which are all connected via the ambient thermal loop. In this work, a comparison of HEATNETS and a well-known code, TRNSYS, has been completed for one of the proposed geothermal networks, as verification process. Hourly data provided from the TRNSYS simulation were building loads, pumping power, heat pump power, temperature entering and leaving the borehole field, and mass flow rates. The hourly temperatures were used to create a linear regression model utilized in HEATNETS to estimate the borehole field heat exchange. The building loads and mass flow rates were directly read into HEATNETS. The pumping power, heat pump power, borehole temperatures, and coefficients of performance were all simulated and calculated by HEATNETS, allowing for direct comparison of the thermal energy transfer, rather than also comparing control systems responses. Results prove that HEATNETS can be used as a predictive tool for the performance of a full geothermal network system. |