CARBON AND ENERGY REDUCTION
Good water heaters can be major help in infection prevention.
decarbonisation pathway for their individual estates.
High and low temperature heat pumps When considering DHW generation, there are a number of ways in which ASHPs can be used. Let’s consider possible ASHPs’ design strategies using different types of heat pumps – low temperature and high temperature. Low temperature ASHPs can be used with direct electric or direct gas-fired solutions to raise the DHW to safe temperatures. The direct electric approach is more likely to be the option of choice, but would involve higher volumes of stored DHW – certainly compared with low-storage direct gas-fired water heaters, which the building may previously have relied on (see Figure 1). If considering this solution, it’s important
to consider potential issues relating to available space and weight for the larger cylinders – particularly when dealing with rooftop or non-basement plant rooms. Controlling Legionella within the larger volumes of stored water will also need to be carefully monitored and managed. High temperature ASHPs are capable
of delivering the high flow temperature required to meet the design temperature for sanitary hot water. The advantage of using HT heat pumps is that it avoids the need for an alternative form of technology to store the DHW above Legionella temperatures (60 °C or higher). This makes it a truly low-to-zero carbon solution. This option also offers greater design
flexibility, and requires less space, for a much simpler design, and easier installation. However, it should be noted that the coefficiency of performance of heat pumps falls off at higher temperatures, affecting the real-world efficiency, and subsequently operating costs.
56 Health Estate Journal June 2023
An offsite prefabricated interface unit.
Bivalent approach The low-carbon credentials of heat pumps are well established in new-build and some existing buildings, but capital expenditure and operating costs may influence the design strategy decision when dealing with older NHS buildings. While the NHS’s aim is to transition to zero carbon technologies, full decarbonisation could take some time to achieve. So, for practical reasons, a balance may need to be struck to meet the year-round requirements for reliable, efficient, sanitary hot water. On projects where an all-electric solution is not considered suitable, a bivalent approach to hot water generation using two energy sources should not be overlooked as an important step on the Net Zero path. With refurbishment projects, for example, where the natural gas supply might be maintained, there is the opportunity to use ASHPs to preheat direct gas-fired water heaters (DGFWH). (see Figure 2). Integrating ASHPs and DGFWHs in a
bivalent system can provide a practical solution to the project limitations previously described, while meeting hot water demand more sustainably, and making significant progress towards decarbonisation. Further, many DGFWHs are compatible with the projected 20% hydrogen blend into the natural gas network, enhancing the sustainability of the installation. In time, the remaining gas use can be cut by using green hydrogen models. In this way this approach offers a practical opportunity for important immediate efficiency gains and emission reduction in older NHS buildings.
Advantages from a design perspective From a design perspective, there are
advantages to be gained from using high-efficiency DGFWHs over indirect DHW systems (boiler calorifiers) and direct electric systems – storage, for one. DGFWHs have greatly reduced storage compared with other systems, which means less weight – and fewer issues if being sited within roof top plant rooms. Importantly, energy usage is also reduced, along with associated emissions, as there is less water to maintain at temperature. Legionella is another case in point. Some DGFWHs come with in-built anti- Legionella functions as standard, which make control of Legionella far more straightforward, reducing maintenance time for Estates and Facilities managers. Those DGFWHs with smart return temperature sensor technology have the ability to provide further energy and emission savings by reducing the time required to complete the pasteurisation process. As a final point, it is generally accepted that where a design includes renewable technology, such as heat pumps, project costs are higher. Whether planning to install a complete
plant room to serve a new-build hospital, or refurbish the hot water heating system in an existing building, it’s worth exploring the use of offsite solutions. Prefabrication makes everything easier – installation is faster and simpler, onsite time and labour are reduced, health and safety are improved, and quality assurance is enhanced. How can the offsite technique be applied for DHW solutions? The options range from the largest containerised plant rooms to prefabricated modules that are purpose-designed to provide a high-quality plug and play solution. For example, if planning on integrating ASHPs in the next phase of the refurbishment
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