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Renewables: Heat pumps

Renewables

Unlocking the potential of heat pumps in cold weather

By Metro Commercial

Amid the global shift towards sustainable and zero-emission energy at the point of use, heat pumps have emerged as a key technology in the transition.

Heat pumps regularly draw attention and scrutiny, particularly regarding their performance in cold weather conditions and their suitability for the UK climate.

A recent study carried out by the Regulatory Assistance Project and Oxford University and published in Joule magazine provides clearer findings on the efficiency of air source heat pumps in cold climates. The study addresses concerns surrounding the efficiency of air-source heat pumps in sub-zero temperatures, drawing insights from recent field studies that provide real-world performance data. In this article we review what the Regulatory Assistance Project discovered.

 |  Renewables in buildings

At the heart of the discussion lies the efficiency of heat pumps, measured by the coefficient of performance (COP). The COP represents the ratio of useful heat output to energy consumed. Generally ranging between 3 and 6, the COP indicates that 3 to 6 units of heat are generated for every unit of electricity used.

While ground-source heat pumps maintain high COPs even in cold weather, air-source heat pumps, the primary focus of this commentary, have shown remarkable performance, even in extreme conditions.

Performance in mild cold climates

Focusing on mild European winters with average January temperatures above -10°C, RAP research analysed seven field studies spanning different climatic zones and heat pump configurations. These studies were conducted in Switzerland, Germany, the United Kingdom, the United States, Canada, and China.

Importantly for the UK, temperatures ranging from 5°C to -10°C, and RAP found the actual the mean COP across all systems was 2.74, demonstrating efficiency well above that of fossil heating and electric resistance alternatives.

Extreme cold climate performance

In extremely cold climates, where temperatures approach -30 °C, specially engineered "cold-climate heat pumps" come into play. Field studies in Finland and the United States showcased COPs above 2 even at temperatures as low as -20°C. In Minnesota, central-ducted cold-climate air-source heat pumps maintained COPs between 1 and 2 during heat-pump-only operation below -12°C. In Alaska, the Oak Ridge National Laboratory tests showed a COP of 2.0 at -25°C and 1.8 at -35°C.

Strategies for improved performance

Regulatory Assistance Project found that key strategies to optimise heat pump performance included reducing the heat delivery temperature. Lowering the water supply temperature, especially in hydronic systems, can significantly enhance heat pump efficiency. While concerns about the need for back-up heating in extreme conditions persist, field studies indicate that back-up heating was generally engaged only when temperatures dropped below -10°C. Above this threshold, heat pumps effectively provide the required heat at high efficiency.

Hybrid systems and future considerations

The potential need for hybrid systems in the coldest climates raises questions about efficiency and the high output capacity required at very low temperatures. Further research is needed to explore the specific value of hybrid-type heating systems. Efforts to improve the thermal performance of the building to minimise load, and encourage demand response can significantly reduce the the impact of peak heating loads on energy systems.

A bright future for heat pumps

The resaearch by Regulatory Assistance Project demonstrates that in insulated buildings air-source heat pumps can be remarkably efficiency in mild and extreme cold climates. Their use in countries with colder climates, such as Norway, Sweden, and Finland, shows their effectiveness in providing heating solutions. The data presented in this commentary supports the widespread adoption of air-source heat pumps as part of global decarbonisation efforts.

While ground-source heat pumps and hybrid systems may find significant value in the coldest climates, existing technology proves successful in most areas with space heating demand, subject to thorough heating system design and high-quality installation. The future of heat pumps looks promising, contributing substantially to the transition to sustainable and efficient heating technologies worldwide.

Heat pumps are ideally suited to well insulated buildings.

This research was carried out by Duncan Gibb a senior advisor at Regulatory Assistance Project, Dr Jan Rosenow, Dr Richard Lowes at Regulatory Assistance Project, and Professor Neil Hewitt at Belfast School of Architecture and Built Environment at Ulster University. The research article was published in Joule Magazine here.

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