Using heat pumps for efficient hot water generation on lower temperatures
When it comes to the provision of domestic hot water (DHW) within commercial buildings, there is no doubt that using heat pumps plays a fundamental role in the drive to improve sustainability. However, with most heat pumps operating at low flow temperatures, how best can we utilise heat pumps to their full potential and ensure they are operating effectively and efficiently?
Heating and hot water are significant contributors to a building’s carbon emissions. They create nearly a third (32%) of the total carbon emissions in the UK. Luckily, the technology to generate hot water in commercial buildings in a more energy efficient, renewable way is already out there – in the form of heat pumps.
What makes heat pump technology energy efficient?
Heat pumps are an energy-efficient way of producing hot water. The majority of heat pumps currently used in the UK are air source heat pumps (ASHPs). Air source heat pumps work by changing low-grade heat into useable heat by using a small amount of electrical energy to transfer the ambient heat in the air outside of a building and move it inside, where, at a higher temperature it can then be used to maintain a supply of hot water.
Delivering up to 4 kWh of energy for every 1 kWh of electricity used to power it, air source heat pumps can be 300 to 400% more efficient than boilers alone. But the benefits don’t end there. Once correctly installed by a qualified installer, with maintenance in line with the manufacturer’s recommendations, a heat pump could last between 10-25 years or more.
Types of ASHPs
Air source heat pumps are divided into two types: split and monobloc units. A split heat pump has both an outside unit incorporating the heat exchanger and compressor with pipework containing refrigerant connecting to the internal hydro box. A monobloc heat pump contains all the refrigerant components in the outdoor unit, leaving more space inside the property, making them easier to install and maintain.
With an air source heat pump there are various factors that need to be taken into account to ensure the heat pump is performing efficiently when it comes to generating hot water.
DHW: a hybrid approach
Heat pumps offer an effective and efficient means of heating domestic water. The heated water is transferred to a hot water cylinder, much the same as with a conventional indirect hot water system. However, therein lies the potential problem. As most heat pumps offer the highest efficiency at lower flow temperatures of 55°C, many heat pumps used for the storage of domestic hot water will need to be supplemented by another means of heating to raise the stored water temperature to 60°C, to prevent Legionella bacterial growth and associated health risks. Here, a possible option would be to employ a high-temperature heat pump that can produce heat between 60 and 80ºC by using a different refrigerant from a conventional model.
Another solution would be to deploy a bivalent (hybrid) approach with a secondary heat source via a gas boiler, electric boiler, immersion heater, etc to lift the DHW temperature. In this setup, the heat pump will run as much as possible where it can meet the demand. However, achieving an efficient hybrid hot water approach depends on harmoniously balancing the different system elements together to ensure they work with each other and not against.
Care must be given as this can reverse heat the air source heat pump, via the indirect coil and cause the ASHP to switch off. In such applications, the boiler will take over. This can subsequently reduce carbon savings.
Solution: Preheat
To overcome this challenge and find the most efficient solution, it’s advisable to take the time to design a system in such a way that it maximises heat pump contribution performance while taking temperature limitations into account. Here, a simple yet effective solution would be to pre-heat the cold feed via a Plate Heat Exchanger (PHE) heated by an air source heat pump; for example, a 70kw ASHP connected to a suitably sized PHE would enable the system to pre-heat the cold feed to a 500 litre calorifier. This would, in turn, lift the temperature to 45°C and mean that the secondary heat source would only need to manage a 15ºC temperature lift. This would also protect the heat pump from potential issues of being “back-fed” with higher DHW temperatures from other sources.
An example of this would be a recent Hamworthy Heating project that is currently underway at a school in the South of England. Utilising the Hamworthy 18kW Tyneham monobloc air source heat pump as the primary source, which will be connected to a plate heat exchanger, the system will effectively preheat the cold feed to a Powerstock PS500 highperformance glass-lined calorifier. In addition, three vertically stacked Wessex Modumax MK3 condensing boilers have been specified to efficiently manage the temperature uplift.
One of the lightest commercial heat pumps on the market, the Tyneham monobloc air source heat pump range features a coefficient (COP) rating of up to 4.85. Featuring seven models in the range with six nominal outputs of between 14 to 70kW, Tyneham heat pumps use R32 refrigerant to deliver increased efficiencies.
Conclusion
With increasing awareness of our environmental impact, maximising energy efficiency plays a key role in achieving carbon savings. Whilst there is no doubt that heat pumps play a vital role in helping to reduce our carbon impact, when it comes to utilising them for the generation of efficient DHW, the system design and set up might need some fine-tuning. To meet this challenge and achieve real carbon savings, careful design factors need to be considered on how best to design a system in such a way that it maximises heat pump performance, particularly whilst taking these temperature limitations into account.
Want to know more?
Contact Hamworthy for a free site survey to review your current heating and hot water system and see how you could benefit from a low carbon hybrid heating system.