Electric air-to-water heat pumps will play an essential role in the global shift towards decarbonised heat. A plethora of design options for this technology exist, each with performance and cost characteristics that are correlated. This gives rise to a critical question: should manufacturers prioritise higher-performance, higher-cost heat pumps to potentially reduce wider energy infrastructure costs but increase upfront expenses for end-users? Or should they focus on more affordable, yet lower-performance alternatives? Researchers from the Clean Energy Processes (CEP) Laboratory at Imperial addressed this question as part of the IDLES Programme in a recent article published in the newly launched open-access journal Cell Reports Sustainability, offering a deeper understanding of how different heat-pump designs influence national decarbonisation pathways. Here, they share some of their findings.
What inspired your team to pursue this research?
The team’s research on heat pump designs considered factors such as heat exchanger sizes, compressor types, and working fluids. As detailed in the article, these components have a pivotal influence on the performance and economic viability of heat pumps. Recognising the inherent correlation between heat pump cost and performance, the research sought to identify designs with a balance between performance and affordability that maximised benefits to the UK energy system’s decarbonisation pathways. It is noteworthy that, until now, there has been no study quantifying how heat-pump design performance-cost trade-offs affect the broader energy system in terms of national electricity generation capacity and heat decarbonisation cost.
What sets your methodology apart?
The whole-energy system models used to inform national decarbonisation pathways often simplify the cost and performance of heating technologies to single parameters. The approach taken in this recent work integrates detailed models capturing heat pump performance and cost metrics into a whole UK energy system model, enhancing its capabilities to not only optimise network infrastructures, but also advise on optimal technology designs aligned with system-wide objectives.
What is the effect of designing domestic heat pumps appropriately?
Our findings demonstrate that heat-pump design decisions significantly influence both their cost and performance. For instance, the investment cost (excluding installation) of a domestic electric air-to-water heat pump was shown to range from around £500/kWth to approximately £990/kWth, while its coefficient of performance (COP) can more than double from 1.6 to 3.8. The question that arises is: among all the available heat pump designs, which one is associated with the minimal heat decarbonisation cost to the whole energy system?
Figure 1. Electric air-to-water heat pump COP at nominal operating conditions versus specific retail price for different designs based on alternative compressor types, working fluids, and heat exchanger (condenser and evaporator) areas.
What are the key energy system implications of heat pump design?
When different heat pump designs were compared in terms of their implications for the wider UK energy system, it was shown that enabling the installation of progressively higher-performance but also higher-cost heat pumps always leads to a reduction in the required national electricity generation capacity. Higher-performance/higher-cost heat pumps (defined as having COP > 3.2), which are often designed with rotary vane or scroll compressors and large heat exchangers, can lead to a substantial reduction in the required electricity generation capacity of up to 50 GW (20%) compared with lower-performance/lower-cost heat pumps (COP < 2.8), thus reducing the needs for expansion of the power grid and technology-uptake rate requirements, both of which are known challenges.
However, as the heat pump COP increases, the end-use heat pump cost (and, therefore, investment) required also rises, leading to an optimal design that minimizes the total system transition cost towards decarbonisation. Beyond this optimal design, further COP improvements become financially ineffective, and those resources could be more effectively allocated elsewhere in the energy system. Specifically, we show that from an energy system perspective, it is overall cost-optimal to design heat pumps with nominal COP in the range of 2.8–3.2, which typically have a specific cost below 650 £/kWth, and simultaneously to invest in increased capacities of renewable energy generation technologies and batteries, in the first instance, followed by open-cycle gas turbines (OCGT) and combined-cycle gas turbines (CCGT) with carbon capture and storage (CCS).
What broader significance do these findings hold?
The study demonstrates the need for nuanced predictive and decision-making tools capable of accounting for the diversity of heat pump designs and how these can affect real technology performance and cost characteristics. By considering both heat pump performance and cost implications within the broader energy system context, manufacturers and policymakers can make informed choices that advance decarbonisation goals while ensuring affordability for end-users.
Given that optimal heat pump designs can enable a reduction of up to 20% in required electricity generation capacity and a decrease of up to 10% in total system transition costs compared to lower- or higher-performance alternatives, it can be concluded that heat-pump design selection holds equal or even greater significance for the decarbonisation of the UK energy system when compared to the inclusion of other technologies that are considered crucial to the future evolution of the system, such as CCGT with CCS, batteries, and bioenergy with carbon capture and storage (BECCS), which are here projected to represent about 10%, 7% and 1% of the UK’s total installed electricity generation capacity in 2050.
Figure 2. Structure of paper novel methodology and the impact of different domestic electric air-to-water heat pump designs of varying COP on the installed electricity generation capacity by technology in the UK energy system in 2050 and system cost of heat decarbonisation.
How do your findings influence UK heat pump policies?
By understanding the trade-offs between performance and cost that can arise from design choices made by heat pump manufacturers, and their impact on the UK energy landscape, policymakers, energy planners, manufacturers, installers and end-users can make informed decisions that accelerate the adoption of low-carbon heating technologies. Our study suggests that investing in specific heat-pump designs, along with complementary energy system enhancements in renewable energy generation and storage capacities, can lead to significant reductions in total system transition costs and can facilitate the transition to a more sustainable energy future. These results can be translated into certain energy policy measures (e.g., grants, subsidies, regulation) that encourage consumers to buy heat pumps that meet specified design and cost criteria, and manufacturers to make design choices that are optimal from a whole-energy system perspective.
Join authors Professor Christos N. Markides and Dr. Andreas V. Olympios for a special webinar on this topic on 26th April. More info and registration here.