Implementation and validation of ground-source heat pump system models in an integrated building and system simulation environment

Despite the low energy consumption and lower maintenance benefits of ground-source heat pump (GSHP) systems, little work has been undertaken in detailed analysis and simulation of such systems. Long-term transient ground heat transfer significantly affects the performance of these systems. Annual and multi-year simulation consequently becomes an invaluable tool in the design of such systems-both in terms of calculating annual building loads and long-term ground thermal response. The EnergyPlus program, which makes use of variable time-step sizes in its simulation of building systems, was extended to allow multi-year simulations. Models of a water-source heat pump and a vertical borehole ground-loop heat exchanger have been implemented in EnergyPlus. The ground heat exchanger model uses Eskilson's "g-junctions" to model response to time-varying heat fluxes and has been extended to include a computationally efficient variable time-step load aggregation scheme. The performance of this model has been compared with an analytical line source approximation. For a steady periodic input that included pulsated heat extraction, the model agreed with the analytical solution to within 2 degrees C The heat pump model was able to predict power and heat transfer rates over a wide range of operating conditions to within +/- 10% of published data. Experimental data from the Oklahoma State Hybrid Ground Source Heat Pump Laboratory have been used to validate both the heat pump and ground heat exchanger models. System simulation results were compared with five days of experimental data. The results showed an average error in the predicted ground heat transfer rate of less than 6% and average errors in the predicted heat pump power and the predicted source-side heat transfer rate of less than 3% and 4%, respectively. Using these models, it is possible to represent GSHP systems in a flexible way and examine their performance over the extended periods required for proper analysis.