Heat pumps will play a key role in transitioning heating to fossil-free sources. However, improvements in energy efficiency and cost reduction are still needed.
Current vapour-compression heat pumps are built upon the Evans-Perkins cycle which was originally designed for refrigeration applications. Once hot liquid refrigerant has transferred energy to the central heating system, it leaves the condenser with sensible heat which can be utilised. Here we report a modified and flexible Evans-Perkins heat pump cycle integrating heat recovery and storage which is then used as an ancillary heat source for the heat pump’s operation.
It operates in a quasi-two-stage mode to theoretically save up to 20% in compressor power consumption compared with single-stage cycles. We built a prototype with off-the-shelf parts and demonstrate a practical 3.7% power saving at a heat production temperature of 35°C. Power saving will further increase with heat supply temperature. We also qualitatively show that hot refrigerant exiting the condenser can be directly used for defrosting the evaporator, providing additional energy saving.
Total heat energy consumed by domestic and industrial applications accounts for almost half of global final energy consumption in 2021, contributing more than 40% (13.1 Gt) of global energy-related CO2 emissions in 2020. To meet the net-zero emissions target by 2050, 600 million heat pumps should be installed annually by 2030. As of 2020, only 180 million are used globally, accounting for 7% of building demand.
Among heat pump products, air source heat pumps (ASHP) are the cheapest, but still remain 3–4 times more expensive than a gas boiler both in the UK and Germany. Improvements in energy performance and cost reduction are critical to facilitate the uptake of heat pumps to replace fossil fuel-based products.
The heating capacity and coefficient of performance (COP) of ASHPs decrease as the outdoor temperature drops. Therefore, most ASHPs require an auxiliary heating system. The research community has made continuous efforts to develop new methods to improve heat pumps’ performance, particularly for applications with high temperature lift.
One way to address this challenge is to use multi-stage heat pumps. They compress the refrigerant vapour from evaporating pressure to condensing pressure through multi-stages to improve the COP by reducing the work of compressor, including cascade cycle systems and two-stage cycle systems.
In cascade cycles, the high and low-pressure stages use different refrigerants and are coupled together through an intermediate exchanger. In a two-stage cycle, the same refrigerant is used in the entire heat pump system, so the heat exchanger between the low-pressure and high-pressure stages can be replaced with a phase separator tank (i.e., flash tank).
In different designs, the flash tank can be used for intercooling, sub-cooling, flash gas removal, or the combination of them. Depending on the layout, the compression process in such systems is a two-stage compression with a certain degree of intercooling.