Optimum discharge pressure correlation based on experimental data for R744 direct expansion ground source heat pump in space heating applications
Optimum discharge pressure control in a transcritical carbon dioxide direct expansion ground source heat pump (CO2-DX-GSHP) can further improve the system’s thermal performance. To achieve the highest coefficient of performance (COP) possible, accurate and reliable correlations based on empirical data are essential. These correlations ensure the optimum operation of CO2-DX-GSHP systems. The literature offers several correlations for transcritical CO2 systems using numerical models for air-source and water heating applications; however, only a few have developed correlations for space heating based on experimental data. In this study, the optimum discharge pressure of a CO2-DX-GSHP was evaluated experimentally for space heating. For this purpose, a fully instrumented CO2-DX-GSHP test bench at CanmetENERGY Research Centre was used. Several polynomial-based correlations are then formulated for the system’s COP over a wide range of discharge pressure and for specific supply and return water temperatures suitable for space heating applications. Results indicate that the COP could be improved by 50% compared to cases without optimal control for the space heating application.