Phase-Change Materials for Heat Storage

Storing heat in a home has usually meant storing hot water, or storing warmth in the clay bricks of an electric storage heater. Phase-change materials (PCM) do something different: they store heat inside a material that melts as it charges and freezes as it gives the heat back. Energy Saving Trust describes the two families of heat battery plainly: one type "stores heat in a material that changes from a solid to a liquid and back, called phase change materials (PCM)", while the other stores heat in a material such as a ceramic with no phase change at all¹.

The practical payoff is size. Sunamp, the Scottish maker whose Thermino heat batteries dominate the UK's PCM offering, states that because the energy is stored as latent heat, its heat batteries take up to four times less space than an equivalent hot water cylinder². Scottish Government research guidance notes that phase-change materials are particularly well suited to small apartments because of their high volumetric latent heat storage capacity and small footprint³. An independent 2024 product analysis, though, found only a limited range of PCM products available in the heating market, so choice is narrow⁴.

Thermal storage using phase change rather than hot water

Almost every British home that stores heat today stores it in one of two ways. The first is water: a thermal store holds heated water in a large, well-insulated cylinder, often called a buffer or accumulator tank, and can keep heat in the form of hot water for several hours⁸. The second is solid mass: an electric storage heater charges a core, often made from heavy clay blocks or ceramic bricks, and releases that warmth through the day¹⁰.

A phase-change heat battery belongs to a third category. Electricity North West, describing the storage landscape for homes, splits storage systems into two categories: thermal stores and batteries¹². Within the battery category, the phase-change type holds its energy not by making something hotter, but by changing its state. The material inside is solid when discharged; charging it melts it; drawing heat from the unit lets it freeze again¹.

This matters for where such a unit can go. Heat pumps, now the standard low-carbon heating route, require a water storage cylinder for hot water, in the same way older gas systems did¹³. In properties such as flats with space constraints, a heat battery that stores energy in a phase-change material can stand in for that cylinder, as Energy Systems Catapult noted when describing Sunamp's technology being used in exactly those settings¹⁴.

How melting and freezing store and release heat

The physics is simple to state. Phase-change materials absorb, store and release large amounts of latent heat when changing state between solid and liquid: heat is absorbed as the material melts and released as it freezes⁵. Sunamp's own support information describes the same cycle in its Thermino batteries: the Plentigrade phase-change material absorbs and releases thermal energy during a melting and freezing process¹⁵.

The useful comparison is with sensible heat, the kind a storage heater uses. A storage heater works by charging up when electricity is cheaper, then releasing heat gradually through the next day¹⁶. Its bricks get hot and then cool, and the stored heat is released into the room through vents¹⁷. Nothing changes state: the bricks are solid before, during and after. The same is true of the non-PCM type of heat battery, which stores heat in a ceramic with no phase change¹.

Latent heat is far more concentrated than sensible heat. That is why an independent analysis of PCM batteries describes them as absorbing and releasing energy when they change state, for example from liquid to solid, and notes that this property is what attracts commercial interest⁴. A material that melts at a chosen temperature will hold a large amount of energy at that temperature, and hold it there, rather than sliding from very hot to cold as water or bricks do.

The cycle also behaves differently on discharge. A hot water cylinder delivers progressively cooler water as it empties. A PCM unit holds its output near the material's melting point until most of the stored heat is gone, because the material freezes at a fixed temperature and releases its latent heat at that temperature throughout the process⁵.

Four times the energy in a quarter of the room

The headline claim, from the maker, is consistent across Sunamp's materials. Thermino heat batteries use phase-change materials to store heat in a much smaller volume than water can¹⁸, and because the energy is stored as latent heat, the batteries take up to four times less space than an equivalent hot water cylinder². On the material side, Sunamp states that melting and freezing its Plentigrade P58 formulation stores up to four times more energy than heating and cooling hot water⁵.

These are maker figures, and they should be read as such: they describe Sunamp's own products and material, not the whole PCM field. But the direction of travel is corroborated independently. Scottish Government research guidance on electricity network constraints highlights phase-change materials' high volumetric latent heat storage capacity as the reason they suit small apartments³, and Energy Systems Catapult's account of Sunamp's technology notes its use in flats with space constraints instead of hot water cylinders¹⁴.

Standing losses are the other half of the density story. Sunamp states that its PCM heat batteries have a minimal heat loss rate, less than a quarter of a conventional hot water cylinder's relative heat loss, thanks to vacuum insulation². A cylinder that loses heat faster needs more input to stay ready; a store that holds its heat with less loss stretches each unit of cheap or self-generated electricity further.

From the storage heater to a domestic heat battery

The idea of a British household charging a store overnight and living off it through the day is not new. Electric storage heaters have done it for decades: they contain bricks capable of storing large amounts of heat, charged during off-peak periods such as Economy 7 and Economy 10 tariffs¹⁹, and some models use a clay core system instead of ceramic bricks²⁰. Storage heaters fell out of favour in the 80s and 90s as electricity prices increased and people wanted their heat on a more flexible schedule²¹, and manual storage heaters have since been phased out and replaced by more efficient automatic models²².

The storage heater's weakness was never the idea of storing heat; it was the crudeness of the store. The bricks release their heat whether or not anyone wants it: static storage heaters release heat in two ways using natural convection, a top flap releasing hot air once charged plus a constant background release²⁰. A National Energy Action case study of a Dimplex Quantum installation found properties with 20-year-old storage heaters designated "hard to heat" despite meeting the Decent Homes Standard, precisely because the old heaters could not hold and release heat on demand²³. Households like the one in Home Energy Scotland's case study, which relied on electric storage heaters before switching, experienced the same inflexibility²⁴.

Phase-change heat storage is the modern answer to that old problem: a store that holds a large amount of heat in a small volume, keeps it with low losses, and releases it when hot water is drawn. But it is a young market. The independent 2024 product analysis found a limited range of PCM products available for the heating market⁴, so the technology a householder can actually buy today is dominated by one maker's range. For more on how these products have performed in real homes, see heat battery trials and field results.

Inside Sunamp batteries: the Plentigrade compound

Plentigrade is Sunamp's name for its phase-change material. The maker describes it as a high-performance phase change technology platform that delivers heating or cooling, and explains the mechanism in household terms: it absorbs and releases thermal energy during a melting and freezing process, similar to the gel in a pocket handwarmer⁵. Sunamp's Thermino support pages confirm that the Plentigrade phase-change material in its heat batteries absorbs and releases thermal energy during that melting and freezing cycle¹⁵.

Energy Systems Catapult, which has worked with Sunamp on heat battery projects, describes the underlying technology as a patented phase change material²⁵. The patent position matters for the market's shape: because the material is proprietary to one company, the UK's PCM heat storage offering is effectively a single-manufacturer field, which is part of why the independent analysis found only a limited range of products⁴.

The handwarmer analogy is worth pausing on, because it captures both the promise and the limit. A pocket handwarmer holds a small amount of heat at a fixed, comfortable temperature, and releases it steadily as its gel freezes. A heat battery does the same at domestic scale. What the analogy also shows is that the material's melting point fixes the temperature the store works at: the battery is designed around a chosen phase-change temperature, and its behaviour follows from that choice.

Thermino models explained: ePlus, xPlus and TS

Sunamp's domestic product line is the Thermino range: compact, energy-efficient thermal storage systems that use high-performance phase-change material to deliver reliable, mains-pressure hot water, in the maker's own description¹⁵. Within the range, Sunamp names three variants: the ePlus, the xPlus and the TS.

The ePlus is described by Sunamp as a compact heat battery that can be powered by the grid²⁶. The range's stated role is as a hot water replacement: Sunamp says a heat pump, or even a modern electric storage heater, plus a Thermino battery (ePlus, xPlus or TS) can replace a combi boiler¹⁸. That is a significant claim for households with no cylinder and no room for one, because the alternative heat-pump route requires a water storage cylinder for hot water¹³.

All three share the same storage principle: Plentigrade phase-change material in a modular, compact and vacuum-insulated unit, as Sunamp describes it². For a direct comparison with a traditional cylinder, see heat battery vs hot water cylinder, and for how Sunamp's products line up against the other entrant in this field, Sunamp vs Caldera heat batteries.

Suitable settings for PCM systems: apartments, heat pumps, solar thermal

The clearest independent statement on suitability comes from the Centre for Sustainable Energy: heat batteries are most suitable for smaller homes that don't have space for an outdoor heat pump unit⁶. Scottish Government guidance goes further for the material itself, citing phase-change materials' high volumetric latent heat storage capacity and small space footprint as reasons they are particularly well suited to small apartments³.

The second natural fit is alongside a heat pump. An independent analysis notes that when combined with heating solutions such as heat pumps, PCM batteries can have several benefits such as increased efficiencies and reduced peak loads⁴. Because a heat pump needs a cylinder for hot water¹³, a compact PCM store can serve the hot water side of the system where space is short, and can buffer the heat pump's output against the grid's peak periods.

On the generation side, thermal stores can be used with solar, wind, biomass and hydro renewable systems¹², and a thermal store can hold heat from multiple sources, such as solar thermal panels or a wood-fired boiler⁹. A PCM store charged by solar thermal through the day and drawn on in the evening is, in principle, the same shift-and-hold logic that storage heaters apply to cheap night-time electricity¹⁶. Under ECO4 delivery guidance, electric storage heaters and electric heating systems are classed as renewable heating systems when installed in a home which has solar PV, or receives solar PV as part of the same project²⁷, a recognition that shows how schemes treat storage paired with generation.

Certification issues and the SAP shortfall

For all its promise, PCM hot water storage has a specific barrier in the UK's grant landscape. The Warm Homes Social Housing Fund's scheme guidance states that PCM heat batteries for hot water "cannot currently be installed through the scheme until there is a formally approved convention or they are a formally recognized technology category in SAP"⁷. The Warm Homes Local Grant policy guidance says the same: they cannot be installed until there is a formally approved convention or formal recognition in SAP²⁸.

SAP, the Standard Assessment Procedure, is the calculation that determines a home's energy rating and, with it, the measured benefit a grant scheme can claim for a measure. A technology outside SAP's recognised categories delivers no improvement in the assessment, however well it performs in practice. This is the same recognition problem that affects other emerging heat storage products, and it is why heat batteries appear in some schemes' general language but not in their eligible measures tables.

The wider policy conversation does acknowledge the technology. ECO guidance lists storage in several forms, including heat stored in the fabric of the building, hot water storage, electric battery storage and heat batteries (phase change materials)²⁹, and Energy Saving Trust's response to the Treasury's VAT consultation on energy saving materials refers to both conventional thermal storage, such as additional tanks for hot water, and more innovative heat battery technologies³⁰. So the technology is named in policy documents, but for the two Warm Homes schemes the SAP gap is decisive. For households, the practical consequence is that a PCM heat battery is currently a self-funded purchase rather than a grant-supported one.

PCM storage and self sufficiency: the real benefits and limits

What a PCM heat battery gives a household is time-shifting. It lets a home charge a store when electricity is cheap, whether that is an off-peak tariff or a period of surplus generation, and draw on it later, in the same way a storage heater charges up when electricity is cheaper and releases heat through the day¹⁶. With low standing losses, less than a quarter of a cylinder's relative heat loss by the maker's figure², more of the bought or generated energy survives to be used. Paired with solar thermal or a heat pump, it can hold renewable heat until it is needed, which is the definition of thermal energy storage itself⁸.

What it does not do is make a home independent of the grid or of a supplier. A heat battery stores energy; it does not generate it. The ePlus is described by its maker as powered by the grid²⁶, and the whole model depends on electricity arriving at a price and a time that makes charging worthwhile. A household with a PCM store is still a customer of an electricity supplier, still exposed to tariff design, and still dependent on the grid's off-peak pricing holding its shape.

There is also a dependence on the manufacturer. The UK's PCM heat storage market is effectively one company's patented material²⁵ and a limited range of products⁴, so a buyer is tying a long-lived part of their heating system to one firm's continued existence and parts supply. That risk is real in this sector, and it is covered in detail on energy technology company failures. Set against it, the technology's physical durability is a genuine strength: a material that melts and freezes does not corrode like a cylinder or degrade like a chemical battery cell, and the cycle is the same every time⁵.

For the right home, a flat or small house with no cylinder space and no room for an outdoor unit, PCM heat storage is one of the few ways to get cylinder-grade hot water from a unit the size of a slim cupboard. For a household weighing it against the alternatives, the comparisons at heat battery or heat pump and the wider picture at emerging home energy technology set it in context.