THERMAL STORAGE | HOT WATER TANKS SMART WATER Hot water tanks have the potential to store renewable energy, balance the electricity grid, and reduce carbon emissions and utility bills. Alex Smith speaks to Mixergy to find out how the hot water cylinder has the potential to become the smartest technology in the home 500 Energy cost (/MWh) 400 Figure 1: The half-hour periods when the price of electricity fell below gas in the last year 300 200 100 0 System sell price (/MWh) T 14/03/2021 01/02/2021 22/12/2020 11/11/2020 01/10/2020 21/08/2020 11/07/2020 31/05/2020 20/04/2020 10/03/2020 -100 Gas (/MWh) he past decade has seen the demise of the hot water tank in many homes, as improvements in combi boilers which combine delivery of hot water for space heating and domestic hot water mean occupiers can have efficient instantaneous hot water without the need for a storage tank. Hot water cylinders are still needed in larger homes, however, where more hot water is required. More than 450,000 hot water tanks are still sold annually, and the governments target of 600,000 heat pumps installed by 2028 will boost the market, as heat pumps require cylinders with heat exchangers to deliver hot water. The potential to store energy is another benefit of a hot water cylinder. As well as enabling energy to be stored from local renewables, they could store excess energy generated by wind and solar on the National Grid. Innovate UK and the Department for Business, Energy and Industrial Strategy (BEIS) has been carrying out trials to see how tanks can integrate renewables in the home and connect with other tanks to offer a demand side response (DSR) to the National Grid. One of the sectors pioneers is Mixergy, a spin-out firm from Oxford Universitys Energy and Power Group. Its hot water tanks featured in a BEIS DSR pilot where connected tanks were heated by the Grid when national electricity demand was low and excess energy was being generated by renewables. Storing this excess energy in a large-scale battery of hot water tanks allowed more electricity on the Grid to be generated using wind and solar. Building on this pilot, Mixergy is now remotely operating 700 of its customers tanks to offer a DSR service for the Grid. The tanks provide a stabilising service for the Grid, says Mixergy CEO Pete Armstrong. The frequency response service is quite rare and hardly changes heating patterns. The typical cost of a frequency event is only 1-5 pence, and it only lasts a few minutes and usually adds to heating that is required anyway. Hot water tanks are the lowest-hanging fruit in terms of energy storage, he adds. Theres a huge dormant capacity in hot water tanks. Of the 27 million homes in the UK, about 40% have a tank as part of a system boiler or direct electric. Collectively, this represents about 85GWh of energy storage, which is just over nine times the size of Dinorwig Power Station in Wales [which provides rapid-response capability in the Grid], says Armstrong. Armstrong says using hot water tanks as energy storage batteries is a lot cheaper than using lithium batteries, and use far less resources in their manufacture. Lithium-ion batteries are expensive and take a decade to pay for themselves, by which point theyre dead, he says. Batteries are incredibly material intensive, containing around 80kg of cobalt, lithium and copper compared with 30kg of, predominantly, stainless steel and polyurethane in a hot water tank with equivalent energy storage density. How it works Mixergys hot water tank technology differs from traditional tanks by heating from the top of the tank. It uses stratification to keep hot water at the top of the tank separated from the cold water below. An electric immersion heater and heating coil from the system boiler is installed at the top of the tank. A cold water mains feed enters the bottom of the tank and a pump takes this water, internally, to the top of CASE STUDY Mixergy tanks were installed in 78 East Devon District Council properties, alongside a Vaillant air source heat pump, PV panels, and a PV diverter, to allow the solar panels to charge the hot water tank. Insulation was topped up in the roof to improve the ability to retain heat, and radiators were upgraded. lifted homes from EPC band E to B at a cost of around 15,000 per property. 48 April 2021 www.cibsejournal.com CIBSE April 21 pp48-50 Mixergy smart water tank.indd 48 26/03/2021 17:30