Example three-cylinder arrangement to meet NHS HTM.The heat pump and solar use a dual coil cylinder with the heat pump on the bottom and the solar on top. With this arrangement, there is no thermal conflict between the heat pump and solar adds what it can.
The entire tank is heated to 45C by the heat pump and then receives solar top-up. A purge pump would run once per day to use the after heater to heat up the preheater for sterilisation to prevent legionella.
With a calculated carbon footprint equivalent to 31 million tonnes of CO2, 62% of which can be attributed to its wider supply chain, the NHS must reduce emissions by 8% each year through until 2036. This means stakeholders across government, NHS and industry need to be working together in order to provide new standards for procurement and innovation to ensure sustainable practices.
One area that accounts for a consistent and definitive need within the NHS is the provision of hot water. A core facet of hygienic working practices and the provision of healthy spaces for treatment and care, heating water can be an energy-intensive process, accounting for as much as 30% of daily energy demands. It is also a provision that can immediately deliver a reduction in carbon emissions and operational costs once implemented, whether a building is new construction, or more likely a refurbishment of existing property within the NHS estates,
For more sustainable domestic hot water (DHW) systems, we expect healthcare facilities will look for hot water to be generated at a commercial scale via multiple localised systems that each provide hot water to wash hand basins and showers for a dedicated area. This has been the most common use case seen across recent pandemic-driven refurbishments for example. This typically will see systems arranged on a ward-by-ward basis, but appliances may also supply hot water to local shared spaces where appropriate. Such systems will demand hot water be stored and distributed at a minimum temperature of 60°C. Working flows could be required up to 75°C with minimum return temperatures not below 50°C.
The overriding issue is that NHS health technical memoranda (HTM) 04-01* states that, preheat vessels cannot be used unless they can be guaranteed to preheat to a minimum of 45°C. This could immediately preclude for instance the use of solar thermal as a lone source for preheat, although it matches well to the minimum working flow temperature for preheat that we would design into a system utilising the current generation of air source heat pump. Solar thermal would be more effective for the provision of top-up heating. Awareness of this facet of the HTM has historically been either ignored or missed, so solar thermal has been deployed in a number of health facilities, often successfully, and Adveco has been servicing hospital solar systems for more than a decade in such locations as Merseyside and Greater Manchester. That said, we are aware of systems that have not met the requirements of the HTM leading to costly decommissioning.
To achieve net zero and meet NHS mandates for DHW a typical sustainable application would see hot water generated via a pair of parallel-arranged hot water cylinders to ensure redundancy. These would be sized to provide 60% of the hot water demand for the area served based on a peak 15-minute duration loading. The heat into the cylinder would come from a combination of solar thermal collectors and an air source heat pump working in conjunction to guarantee the preheat temperature.
The design of the system allows for the heat pump to contribute to the incoming cold water. That ensures 45°C while taking full advantage of the COP of the heat pump. The solar thermal is employed after the heat pump to heat the water from 45°to 50 or 60°C depending on the time of year. We calculate the solar contribution over the year, with a system able to provide about 10% of the peak output when conditions are at their worst - short days with low sun and heavy cloud cover – rising to 30% under optimal conditions.
Should temperatures of up to 75°C be required, a high-temperature electric heat source would be used. Whilst typically an immersion, we would suggest specifying an electric boiler to avoid issues of scale in hard water areas, also well as delivering further system redundancy since the boiler will incorporate multiple immersions within its chassis. The electric boiler tops up what the solar could not manage. Still, by delivering preheat through both the ASHP and solar thermal system, demands on more costly electric heating are considerably reduced, if not avoided entirely. While higher temperatures will reduce solar efficiency, it is still useful to offset the high-grade heat than the low-grade/high COP heat pump heat.