Heat Pumps Today

20 The government has identified heat pumps as offering a solution to achieving the UK’s binding net zero target by 2050. Andy Green, Technical Director at Baxi Heating, looks at how applying hybrid systems that integrate heat pumps and high efficiency traditional technologies can help building operators meet their financial and environmental commitments Getting the balance right with hybrid solutions The building services industry has an important part to play in helping the UK achieve its commitment to reduce carbon emissions by 78% by 2035, leading to net zero emissions by 2050. With heat in buildings accounting for a third of total UK emissions, the way in which we heat our public and commercial building stock must change. At the time of writing, we are still awaiting the UK government’s Heat and Buildings Strategy. But while the policies are yet to be clarified, it’s clear that action is required now to identify all achievable opportunities to achieve energy and emissions savings from existing heating systems. As the national electricity grid becomes greener and cleaner, electrification of heat and the use of heat pumps is one approach to drive down the carbon footprint of commercial buildings. Indeed, we are pleased to be adding commercial air source heat pumps (ASHPs) to our comprehensive portfolio of heating and hot water solutions. Other methods to propel the energy transition are likely to include decarbonisation of the gas grid (using hydrogen), and heat networks (in dense urban environments). While heat pumps can bring huge potential benefits in commercial premises, the solution is likely to be more complex and must be tailored every time to meet the individual requirements. For this reason, combining ASHPs with traditional high e¥ciency boilers or water heaters in a hybrid system is increasingly proving an e¦ective approach in commercial buildings. Meeting peak demand Take hotels, which traditionally have a high demand for domestic hot water (DHW) generation. The hot water usage in these October | November 2021 H Y B R I D H E A T P U M P S buildings is usually characterised by an extremely high peak in the morning for guests’ showers followed by a long period of low demand. Typically, a second peak will occur towards the end of the day with end of service in the hotel restaurant and the cleaning down period for the kitchens. If the system were designed around wholly electrical supplies, for example, a larger thermal store would be required to meet peak demand, due to longer recovery times. This would then have a financial impact, as operational costs would rise compared with, say, a conventional system based on direct-fired gas condensing water heaters. Legionella control would also be a consideration as only a small portion of ASHPs can at present e¥ciently generate temperatures high enough to store domestic hot water above legionella temperatures (60°C or higher). What’s more, finding su¥cient space to store the large storage vessels could prove problematic, from a logistical standpoint. After a particularly hard eighteen months, hotels may well prefer to use this space to provide an additional room for paying guests. Hybrid systems How, then, can a hybrid system overcome these challenges? Many hotels traditionally use direct-fired condensing water heaters as one of the most e¥cient means of meeting the sudden peaks in hot water demand as they can deliver large volumes of water in a short time. A major benefit of this approach is that it reduces energy loss between peak periods as the system will only use energy when hot water is required. However, if we integrate a heat pump and water heater in a hybrid design, 52% of the peak load can be met by renewable pre-heat, achieving a significant emissions reduction compared with a 100% natural gas approach. This method also greatly reduces the requirement for storage described in the wholly electric approach. A case in point is a hotel that requires 6,000 litres of DHW in a 90-minute peak period with eight hours between each peak. The electrical supply on site suits a maximum heat output of 45kW which can be used to pre-heat a small storage vessel – 2,000 litre storage in this example – sized to deliver half the peak demand. The storage vessel, which can be charged in between the peak periods, then feeds the direct-fired water heater which will meet the peak hot water demand. The hot water demand is met e¥ciently and more sustainably, with no issues surrounding system losses, legionella control or space. Stepwise approach In addition to the environmental benefits, hybrid solutions enable building owners and operators to choose how and when to replace equipment components. Taking a phased approach to heat decarbonisation Andy Green