What is the science behind our Plentigrade phase change material

By Dr David Oliver, Materials Development Manager, Sunamp | May 2022

Given the UK Finance Minister’s Spring Statement 2022 announcing a cut of value-added tax (VAT) from 5% to zero on the installation of efficient systems such as solar panels, heat pumps and insulation, it is likely that we will see an uptick in interest for our heat batteries coming fast up the track.

And with that swell of interest, we expect and welcome a raft of intelligent questions from both professional installers and the growing community of green technology early adopters, driven by environmental concerns, a knowledgeable enthusiasm for the technology and, increasingly, economic factors.

If you fall into one of these camps, it is likely you already know why heat batteries are grabbing headlines, not just because of their key role in reducing carbon footprints and energy costs, but because their compact size frees up valuable cupboard space.

You are also probably pretty conversant with the theory of how heat batteries work and why they are a crucial component of a green and sustainable energy solution in both a domestic or commercial setting. But here’s a useful link if you need a quick explainer for a friend or colleague.

But what you may not be up to speed with within such a fast-changing environment is why and how the Phase Change Material (PCM) in a heat battery is so fundamental and why the PCM formulation may well be pivotal in your choice of heat battery.

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, engineers use a range of conventional PCMs such as waxes, paraffins and plant-derived oils to power thermal batteries and other heating and cooling applications. But these PCMs have drawbacks – they are carbon-based, have low energy density, are costly because many of the bio-based oils are used in competitive industries like animal feed and they also have questionable sustainability. Add in the rather alarming fact that they are also combustible, and it was clear to Sunamp that there was a need in the market for a re-think.

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 highly patented PCM formulations form the backbone of our Plentigrade platform. Our PCMs have an enviable high energy density, avoid costly or scarce components and there is no question over their sustainability as the salts are either mined, are by-products of the existing chemical industry or are simple commodity chemicals with diverse supply chains.

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 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, Materials Development Manager, Sunamp