ASHRAE ST-16-018

Abstract

Scope: Most commercial ground-source heat pump (GSHP) systems in the United States are in a distributed configuration. These systems circulate water or an anti-freeze solution through multiple heat pump units via a central pumping system, which usually uses variable-speed pumps. Variablespeed pumps have potential to significantly reduce pumping energy use; however, the energy savings in reality could be far lower than its potential due to improper pumping system designandcontrols. In this paper,asimplified hydronicpumping system was simulated with the dynamic Modelica models to evaluate three different pumping control strategies. The pumping control strategies include two conventional control strategies: one strategy is to maintain a constant differential pressure across either the supply and return mains and the other is to maintain a constant differential pressure at the most hydraulically remote heat pump. There is also an innovative control strategy that adjusts system flow rate based on the demand of each heat pump. The simulation results indicate that a significant overflow occurs at part-load conditions when the variable-speed pump is controlled to maintain a constant differential pressure across the supply and return mains of the piping system. On the other hand, an underflow occurs at part-load conditions when the variable-speed pump is controlled to maintain a constant differential pressure across the furthest heat pump. The flow-demand-based control can provide needed flow rate to each heat pump at any given time and with less pumping energy use than the two conventional controls. Finally, a typical distributed GSHP system was studied to evaluate the energy saving potential of applying the flow-demand-based pumping control strategy. This case study shows that the annual pumping energy consumption can be reduced by 64% using the flow-demandbasedcontrolcomparedwith using the conventional pressurebased control to maintain a constant differential pressure across the supply and return mains.