Enhancing seasonal performance of heat pumps integrated into building ventilation systems using a multi-stage volume enlarger: A comparative study across European regions

The buildings sector is one of the largest energy consumers in the EU, accounting for approximately 40 % of final energy consumption and around 36 % of greenhouse gas emissions. European governments are implementing various measures to reduce these figures. One effective approach to lowering energy consumption is to enhance the energy efficiency of HVAC systems in buildings. Extending the control range of heat pumps can significantly improve their seasonal energy efficiency, thereby reducing carbon dioxide emissions and enhancing building sustainability. Modern heat pump capacity control commonly involves a variable-speed compressor and an electronic expansion valve. In this study, the authors introduce an additional control component—variable volume in the active loop of the heat pump. The paper evaluates the performance of a variable-volume heat pump integrated into a ventilation system across different European regions (Vilnius, Helsinki, and Milan). A comparison is made between the conventional control approach, which uses a compressor and expansion valve, and an enhanced control method that incorporates an additional device to regulate the heat pump circuit volume. Based on data from prior experiments—specifically, the dependencies of the heat pump heat output and efficiency on compressor speed, expansion valve position, and circuit volume—a control algorithm was developed with Matlab to simulate heat pump performance in a ventilation system over a typical meteorological year for each city. Key findings highlight that the use of additional volume control in the heat pump loop significantly extends the unit’s operational time compared to conventional control methods. Specifically, operational time increased from several percent to as much as 69.97 % across the regions analysed, positively impacting seasonal performance. These results underscore the critical importance of optimizing heat pump control strategies, as they not only enhance energy efficiency but also contribute to reducing reliance on auxiliary heating and lowering overall carbon emissions. Furthermore, despite these advantages, the study notes a significant limitation: the current lack of commercially available tools for implementing dynamic volume adjustment in heat pumps, which presents challenges for practical application in the field. •Climatic conditions significantly affect heat pump performance.•Multi-stage volume increase for longer continuous operation to 69.97 %.•The seasonal coe