What is the science behind our Plentigrade phase change material

Originally written by Dr David Oliver, former head of R&D at Sunamp | May 2022. Updated October 2026.

Phase change material sits at the heart of every Sunamp thermal battery. But what makes one PCM suitable for reliable thermal storage while another may struggle in a real product?

The answer lies not simply in whether a material can melt and crystallise, but in how much useful energy it can store, how reliably it can cycle, how effectively heat can move in and out of it, and how well it works within the wider thermal storage system.
Sunamp’s Plentigrade® platform brings together materials science, testing and product engineering to address those challenges.

So, what’s the science, and in particular the chemistry, behind Plentigrade, Sunamp’s unique and heavily patented PCM platform?

Generically, all PCMs are substances that absorb and release large amounts of latent heat when they go through a change in their physical state. And latent heat is simply the energy associated with a “change of state” process such as melting and freezing.

For ice or salt to melt, for example, heat is applied until the solid reaches its specific melting point, for example 0°C for water or 800°C for salt. And then, even while heat continues to be applied, the temperature remains the same and the accumulating energy is diverted into molecular change, pushing the tightly compressed lattice of molecules apart, and absorbing “hidden” (latentem in Latin) or latent energy.

Similarly, when the process is reversed and the “change of state” is from liquid to solid, the disordered, free-floating molecules generate another round of latent energy as they self-assemble back into a tight, ordered lattice, releasing energy as they do so.

With this tremendous natural phenomenon of latent heat at our disposal, how have we scientists harnessed it?

It’s all a question of controlling and manipulating it and the answer is – PCMs.

PCMs are not new. Ice is a simple PCM and way back in 400BC the Persians built yakhchals in the desert, a kind of evaporative cooler which cooled the air through the evaporation of water and was the precursor of the modern fridge.

Today, thermal storage systems use a wide range of PCM chemistries, including organic materials such as waxes and paraffins, as well as inorganic materials such as salt hydrates. Each chemistry brings its own characteristics and trade-offs, including energy density, operating temperature, flammability, sourcing, cycling behaviour and compatibility with the wider system.

That means the useful question is not simply which PCM family a material belongs to, but whether its properties make it suitable for the application and product it needs to work within.

The solution we came up with in the University of Edinburgh’s labs and, since then, at the Sunamp R&D facilities was a range of water and salt hydrate syntheses, with game-changing additives to help the PCM melt cleanly and evenly and solidify with the right crystal structure.

These patented formulations form the backbone of the Plentigrade® platform. Their value lies not in a single property, but in the combination of energy density, operating temperature, cycling behaviour, material compatibility and system performance required for real thermal storage applications.

Its green and safety credentials are impressive too, for example, Plentigrade P58 is a food-grade product. It’s certainly not advised that anyone eats it – but sodium acetate trihydrate – its core ingredient – is actually a flavouring used in salt and vinegar crisps. There’s definitely no combustion risk and the PCM can also be safely recycled back into the ground if necessary.

However, it will be many decades into the future before we need to think about recycling as we have put Plentigrade through the most rigorous testing we could envisage to demonstrate and evidence the product’s stability and endurability. This involved the use of cutting edge science facilities, like the UK’s synchrotron facility, the Diamond Light Source, to probe our PCMs with high-intensity X-rays during the phase transition itself.

This gave Sunamp the confidence and data (and a peer-review publication) to know how to make a PCM last indefinitely. To demonstrate this in action, we challenged ourselves to put Plentigrade P58 through 40,000 cycles of phase changes. Given an average heat battery with Plentigrade PCM would cycle through two phase change processes a day, that’s a clean bill of health, with no loss of stability or noticeable deterioration for around 50 years. In fact, Plentigrade P58 didn’t even fail at 40,000 cycles, we just decided that was more than enough testing.

But we’re not resting on our laurels. The PCM used in the heat batteries currently being installed, mainly our Thermino range for hot water, by residential customers and social landlords in the UK and in early adopter residential and commercial buildings in New York State in the USA, is predominantly Plentigrade P58. We are also developing and testing a range of other Plentigrade products with varying melting points for different applications such as cooling, low-grade waste heat recovery and industrial heat.

If you are one of our technically minded readers and would like further details on the specific properties of Plentigrade PCMs across the range, such as PCM temperatures, energy over an operating temperature range and densities, you may find this overview diagram interesting.

Dr David Oliver has a doctorate in Chemistry, specialising in Phase Change Materials (PCMs) from The University of Edinburgh. He is behind the development and implementation of our game-changing PCM as part of our Plentigrade technology platform.