A numerical study of a wastewater-air source hybrid heat pump for domestic use

Abstract

To face the ongoing climate crisis, the Dutch government adopted a number of policies and measures to limit its greenhouse gas emissions. It is estimated that 10% of Dutch annual emissions are caused by the built environment, a large portion of which come from two sources: space heating and domestic hot water.
The use of heat pumps is said to meaningfully contribute to the electrification of domestic heat supply. However, heat pumps face intermittency issues during unfavourable climate conditions when heating is most needed, thus limiting their potential of being standalone heating systems.
A hybrid heat pump - meaning a single heat pump operating with multiple renewable heat sources - could efficiently provide heat all year round with maximised performance. However, literature on the subject is sparse and non-systematic. Therefore, conclusions on the system performance and its variability cannot be drawn.
Thus, the aim of this project was to create a modular numerical model of a hybrid heat pump system to analyse its performance. This was achieved by individually modelling and experimentally validating each component.
The chosen heat sources were wastewater and air. The numerical model showed satisfactory correlation with experimental results for each component, particularly the condenser of the heat pump, air source and wastewater storage bag. Improvements could be made in the evaporator of the heat pump, boiler and spiral heater.
After analysis of its yearly performance, it may be concluded that the modelled hybrid heat pump is not currently a worthwhile investment for a typical Dutch home. However, with a higher capacity heat pump, alongside other component and control optimisations, the system can be an all-electric alternative to many gas-heated homes.