Hot water To ensure system longevity and return
on investment, fluid within the solar collectors must be correctly managed. If left in the panel it can overheat, stagnate, and leave collectors irreparable. This can be avoided by incorporating Drain Back into solar system designs. This gravity flow approach reduces pump capacity requirements and energy use of the pump station to a minimum and will automatically drain fluid if power is cut, without the need for working components. This makes solar thermal systems with Drain Back low maintenance, with long operational lifespans. Fluid refresh is, on average, required every eight years but may last much longer.
Though not applicable to private care homes, NHS health technical memoranda (HTM) 04-01 states any preheat vessels must be guaranteed to preheat to a minimum of 45°C. This effectively precludes the use of solar thermal as a lone source for preheat due to annual variation in production, but this does match the minimum working flow temperature for preheat that would be designed into a system utilising the current generation of air source heat pump. Under such conditions, a typical sustainable application would see a cylinder sized to meet the storage requirements of the building’s hot water demands with the heat coming from a combination of an air source heat pump and solar thermal collectors working in conjunction to guarantee the preheat temperature. The heat pump, operating at optimal efficiency at lower temperatures will preheat the 5°C cold feed to 45°C, at which point the solar thermal is employed to further raise temperature to 50 or 60°C depending on the time of year. Working together, the renewables can offset the majority of the electrical costs otherwise required to heat the water, even during periods of peak demand.
All electric, all good?
Despite the pressure to address carbon emissions in building stock in the UK, we are still waiting for clear advice at a government policy level. The final decision on energy solutions remains unresolved. Do you electrify all equipment now on the basis that the grid will become zero carbon, or hold out for the option of carbon-free gas such as hydrogen, which in terms of infrastructure change and refurbishment would be potentially quicker, cheaper and less disruptive?
42 As indicated, if your building has a
gas connection and has high hot water demands, it remains the most cost-effective option. Additionally, new gas-fired appliances operate with ever-reduced emissions, and most are ready to accept the initial proposed 20 per cent hydrogen blends in the gas grid as early as 2024 without requiring any alteration. ‘Hydrogen ready’ units are, with a replacement of the burner and pre-mixer, even capable of burning 100 per cent hydrogen, but that scenario is some time away. Should hydrogen be accepted by the government as a function of net zero, we would not expect 100 per cent feeds to be in place nationally until 2040, with the grid changeover beginning in the early to mid-2030s. Retaining an existing gas connection, therefore, provides a degree of futureproofing should green gas technology be embraced.
What is clear, though, is that the latest building regulations (Part L, 2021) have radically revised the carbon intensity of electricity from 519g CO/kWh ten years ago to just 136 today. Gas in the same period has fallen from 240 to 233. While the regulations do not yet exclude gas, they do advantage the adoption of electricity. We have demonstrated that renewables have a critical role in reducing the carbon emissions of a system, as well as offsetting the costs of heating water with direct electricity. Gas-based hot water applications are, by a factor of 3.8, currently cheaper to operate than direct grid-electric systems. Using heat pumps can offset 25-35 per cent of those energy costs, but this still leaves a considerable excess operating charge because of the need to provide top-up energy for safe operating temperatures. Historically, additional system top-up was provided by electric immersions, which for backup purposes and occasional peaks in demand, while more expensive, was acceptable. The shift to fully electric systems has put a greater onus on technology that was never designed to provide primary heat. The costs are excessive and, as we indicated, should such technology be deployed in a hard water area, it can rapidly develop scale, leading to permanent damage in a remarkably short time. For this reason, we recommend the replacement of immersion technology with smaller electric boilers that are both more efficient, and, because they operate in a closed loop, will avoid the issues of systems scaling up.
Perhaps the most detrimental issue we
see today as a result of replacing gas with electricity is the propensity to oversize the new electric system, replacing gas appliances with electric alternatives with like-for-like capabilities. Hot water systems have been inherently oversized in the past due to a lack of understanding of application design or concerns over providing suitable backup to ensure system continuity. The result of oversizing is, however always the same – unnecessary capital costs for system supply and installation, and ongoing excess operational costs associated with higher energy demands and, therefore, greater carbon emissions. This can best be avoided by understanding your building’s actual hot water demands and designing the replacement to meet those specific needs. There is an art to designing hot water systems, but real, actionable data is priceless. When considering options for introducing sustainability, the best advice we can give is to understand your needs first. Live metering is an easy, non-intrusive way of securing the valuable operational data you need to make informed decisions that deliver on expectations to lower carbon emissions without incurring unforeseen costs.
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Bill Sinclair
Bill Sinclair brings more than twenty years’ industry experience in mechanical engineering. Having joined Adveco Ltd in 2011 as technical manager, he today holds responsibility for product and system development, as well as managing the company’s bespoke design and engineering teams. Bill is currently Chair of the Commercial Heating Technical Committee for the Industrial and Commercial Energy association (ICOM).
www.thecarehomeenvironment.com April 2023
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