Dynamic simulations of ground-coupled heat pumps using high-resolution analytical models

This paper reports on an algorithm for hourly simulation of ground-coupled heat pump (GCHP) systems. This algorithm integrates the newest analytical models for coefficient of performance (COP) of heat pumps and for heat transfer of ground heat exchangers (GHEs). These analytical models make the algorithm computationally fast and efficient. The algorithm overcomes two limitations restricting the accuracy and precision of the GCHP simulations. First, the simulation studies used analytical models being insufficient for modelling the underground heat transfer. Second, most of the simulations have used empirical approaches to modelling heat pumps. The algorithm comprises seven independent equations for solving seven system variables, thus including C₁₄⁷ = 3432 simulation scenarios. The algorithm is applied to hourly 50-year simulations of a GCHP for illustrating the computation efficiency of the algorithm. The simulation cases show that the COP and seasonal COP of GCHPs depend heavily on the mutual-cause coupling between the ground-side load and the temperature field in the ground. This coupling effect renders the high-resolution simulation highly necessary and this algorithm is compared with an algorithm using a conventional finite line-source model for borehole GHEs. Because conventional finite line-source model neglects the heat capacity of grouting material results in large variations in Tf,out. Comparison results show that the difference in maximum Tf,out during the 50-year is larger than 1.2°C, a temperature difference that may lead to a discrepancy in the design length of GHEs because the length of GHEs is very sensitive to the used extremes of Tf,out. Thus, in the applications with large-variation load, the effect of heat capacity of grouting material will be more significant