Friday, July 19, 2024

Do air source heat pumps work in cold weather?

Vital Energi experts tell all

Air source heat pumps are one of the most effective technologies for reducing carbon. They harness the natural heat energy present in the air to warm up a space. They work by extracting heat from the outside air and transferring it into a building. The pump uses a working fluid (refrigerant) to absorb heat from the outdoor air, which is then compressed to increase its temperature. This refrigerant is then circulated through a heat exchanger to distribute this heat into a building.

So, if they absorb heat from the air, how are they affected when it’s cold outside?

You might think that if the temperature outside falls below zero, the heat pump will stop working, however even in cold air, there is still sufficient heat for the heat pump to absorb and convert into useful energy.

We had a chat with Vital Energi’s Elliott Sharpe (Strategy & Partnerships Director), Dave Wilkinson (Design Director), Chris Green (Engineering Director), and Liam Grice (Senior Engineer), who advised how you can maximise a heat pump’s efficiency during cold weather, the best location for it, how you can prepare it for cold weather, and more.

How efficient is an air source heat pump in cold weather?
When we talk about efficiency of an air source heat pump (ASHP), we often consider how many units of heat we get from an ASHP for each unit of electricity used. Because it gets its energy from the surrounding air, we might get 3 units of heat for every 1 unit of electricity, so an efficiency of 300% in the summer months. As the outdoor temperature drops as we head towards winter, the efficiency of an ASHP does reduce. As this reduces, we could find a situation where the heat pump is producing 1 unit of heat for every 1 unit of electricity, which is 100% efficient, this might not sound all that bad, but heat pumps can deliver much higher efficiencies than this when deployed correctly. 

How do you maximise efficiency during winter? 
Generally, during cold weather, the heat pump is operating at its worst efficiency when most heat is required. Careful design of the heat pump and the system it is connected to is important to maximise the efficiency. Understanding that heat pumps may have a reduced capacity at low temperatures is important in correctly sizing the heat pump to cope with this. If you buy a 300kW boiler, that boiler is 300kW all year round. That’s not quite the case for an ASHP, which might provide 300kW in a +10°C ambient temperature, and only 150kW during the coldest of days. The efficiency of heat pumps increases as their supply temperature reduces, so its beneficial to design heat emitters to operate effectively at lower temperature e.g. –underfloor heating can work on a 45C flow. Including buffer vessels or thermal storage can also provide flexibility which can lower the cost of heating from the heat pump. Optimisations of the heat pumps defrost cycle are essential for achieving the best winter performance and should be carefully considered.

Is there anything you can do to prepare your heat pump for cold spells?
Absolutely! The main objective should be to ensure that the heat pump is operating as it should. Proactive maintenance will assist with this, where the entire system should be inspected, refrigerant and oil levels checked, and any issues addressed. Consider scheduling maintenance before winter begins to ensure optimal performance.
In cold ambient temperatures, the heat pump will enter a defrost cycle more often. For optimum efficiency and performance, the air source collector should be unrestricted and free from debris. If you notice any excessive ice build-up on the air source collector it might suggest that the defrost cycle may not be functioning correctly.

ASHP being crane lifted to the roof of a Westminster City Council building

Is there an optimum position or location where an ASHP will perform better during colder months?
ASHPs use fans to move ambient air over the collector. Any restrictions to the air flow will reduce performance. The air is cooled as it passes over the collector, so it is important to minimise air recirculation. Manufactures guidance should be followed to make sure there is sufficient free space around the heat pump and consider any additions such as acoustic panels and their impact on air circulation. Computational Fluid Dynamics (CFD) modelling is a useful investment to make during the design process to make sure the air source collector position is optimised.

Are there specific refrigerants or technologies that can improve winter performance?
When choosing a heat pump, it’s crucial to assess its suitability for various operating conditions over its lifespan. Understand your load profile to determine peak performance needs. If the ambient temperature drops below the design threshold, the system’s heat output and efficiency will decrease. Consider these factors carefully to select the optimal heat pump technology and refrigerant that precisely meets your requirements.

How can defrost cycles be optimised to reduce energy consumption and output limitations?
Typically, when ambient temperatures are below 7ᵒC, ASHPs will need to regularly complete a defrost cycle to remove frost which forms on the coil surface as moisture from the air freezes. The process requires energy to melt the frost, and generally the heating output is reduced during this time, so it is critical the process is completed quickly and efficiently. This can be optimised as part of the commissioning and O&M activities, making sure all temperature probes are fitted correctly and the settings are correct so the defrost process is not more frequent and longer than needed. It is a balance though, because not defrosting correctly causes severe performance issues. If all the frost has not melted and drained away, this will refreeze and eventually create ice which blocks the coil. Manual intervention is often then needed to get the system running optimally again.

Are there any energy conservation measures that can be implemented during the winter months which will help a heat pump run more efficiently?
There is a term often used which is ‘fabric first’. What this means is, the first port of call for any project should be to try and reduce your energy demands first, before looking into any new technology. This fabric first approach could be improving the performance of your windows, to reduce how much heat you lose. It could mean adding more insultation to walls and ceilings. All of these measures will result in your site/building requiring less energy.

Click here to discover more about heat pumps.

This article appeared in the May 2024 issue of Energy Manager magazine. Subscribe here.

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