Evaluation of a Ground Source Heat Pump System for Varying Loads and Return Water Temperatures using Different System Performance Factors

A quasi-steady state mathematical model of a ground source heat pump has been developed. Heating and cooling is delivered to the building using fan coil units, which are either in constant operation (Strategy 1) or switched on/off as water flow to the coil is diverted (Strategy 2). The performance of the system is evaluated using a system performance factor (SPF). It was observed that values for SPF4 (which includes all components) decreases for decreasing building load. For strategy 2, the energy saving associated with SPF4 are greater than Strategy 1, but savings are diminished at higher loads. Improved system performance was observed at lower return water temperatures in heating and higher return temperatures in cooling. However, in both cases the capacity of the fan coil units is reduced. The temperature compensation strategy developed resulted in an improved SPF4 for Strategy 1 and a less significant improvement for Strategy 2.