HEAT PUMPS | COMPACT ENERGY STORES STORAGE BEATER One barrier to heat pumps in smaller homes is the requirement for a large water cylinder to store hot water. Midsummers Jez Climas looks at how compact thermal-energy storage systems could solve the problem I n November 2020, the UK Prime Minister announced an eye-catching plan to ramp up heat-pump installations across the nation, as part of an initiative to decarbonise domestic heating. The headline target was for 600,000 heat pumps a year to be installed by 2028. New-build homes might account for around 100,000 of these installations, but the majority will have to be retrofitted in existing homes. In many ways, this looks like a challenging target, as current levels of installations although climbing sharply are still in the low tens of thousands a year. There are, however, reasons to be optimistic, as the problem is mostly not a technical one. Domestic air source heat pumps (ASHPs) are a mature technology and are being produced in quantity by large multinational companies such as Mitsubishi, Samsung, Daikin and Vaillant. Installing a monobloc heat pump may require more work, but its not much more technically challenging than a boiler once installers have built up some familiarity with the technology. Nonetheless, there are a number of small technical and practical issues to overcome for many heat-pump retrofit projects. These were identified by the Department for Business, Energy and Industrial Strategy as part of its Electrification of heat demonstrator project a national project in which heat pumps have been installed in a significant number of homes to better understand the issues and investigate solutions (see panel, Heat pump challenges to overcome). Heat storage One of the participants in the project is Sunamp, an innovative Scottish heat-storage company that has developed a phase-change heat battery system that enables much more compact storage of energy for hot water. It produces a range of heat batteries, designed to act as replacements for direct and indirect hot-water cylinders. The heat battery is essentially a block of sodium acetate with a melting point of 58C (PCM58). Energy is stored by melting the PCM58 using heat from an electric heating element or an indirect coil run through the store. To extract the heat as hot water, a second coil is in the block The size of the Sunamp heat battery compared with a hot-water cylinder cold water goes in, hot water comes out just below the melting point of the PCM58, at 55C. The heat batteries have a smaller surface area and are encased in vacuum insulation panels, so their standing losses are small compared with a cylinder ~30W depending on size. The indirectly heated heat batteries work with a range of heat pumps currently on the market, and could be a facilitating technology for heat pumps to be deployed in more dwellings. While a 210-litre cylinder is around 550mm in diameter and 1,500mm high, a heat-battery unit of equivalent storage capacity is 365mm wide x 575mm deep x 856mm high little more than half the size of a standard domestic washing machine. The fact that it is a thermal store, rather than an indirect cylinder, means there are two heat-transfer processes: 65C low temperature hot water (LTHW) to 58C PCM to 55C water. For an indirect cylinder, it is one: 60C LTHW to 55C water. So, to get the same temperature of hot water, the heat pump needs to run hotter in hot-water mode, with a resultant drop in coefficient of performance. This is somewhat offset by the fact that heat-battery units do not need a legionella pasteurisation cycle and have lower standing heat losses. Installation Installation of a heat battery as hot-water storage with a heat pump is only slightly more complex than installing an unvented cylinder; not 50 August 2021 www.cibsejournal.com CIBSE Aug 21 pp50-51 Heat battery.indd 50 23/07/2021 12:38