A phase-change material (PCM) absorbs and releases energy when it changes phase, for example, from solid to liquid. Applying energy in the form of heat to a solid will eventually melt it. If you then cool the liquid, it will freeze, releasing the energy used to melt it. This cycle repeats (Figure 1).
When heat is first applied to a solid, its temperature increases: this is called sensible heat. Once the solid starts melting, its temperature stops increasing and stays the same, even though heat is still being absorbed; this is called latent heat. Once it has completely melted, its temperature starts to increase again. The converse happens when a liquid is cooled: its temperature decreases until it starts to freeze and remains constant until it’s completely frozen, when its temperature starts to decrease again.
Figure 1: The cycle of melting and freezing for a PCM
A century ago, the observation of a constant temperature during melting confused scientists as they thought that heat was mysteriously disappearing. Joseph Black, a Scottish physicist and chemist at the University of Edinburgh, realised that it was a different type of heat, and he called it latent (meaning “hidden”) heat. Through his experiments, he demonstrated that ice requires a lot more heat to melt than it does to go from -10 °C to the point just before melting. To increase the temperature of ice at -5 °C to water at +5 °C requires 10 times as much energy as raising the temperature of water from +5 °C to +15 °C.
Ice has been used for millennia for thermal energy storage, with Romans and Egyptians using it. Emperor Nero even used it to chill his wine, and ice banks are used worldwide, including in Chicago’s district cooling networks. Although ice is great for keeping our drinks cold, the temperature for using its latent heat is too hot or even too cold for many uses. It is not cold enough for freezing applications and it is also too cold to provide heat – although ice and water are used for cooling and heating, respectively, it is only the sensible heat that is used, not the latent heat of the phase change.
Can anything be a phase change material?
Technically, yes. There is always an energy associated with a phase change but it is not necessarily always a lot of energy. There are many different types of PCMs, with different melting temperatures and latent heat values. Organic PCMs include paraffins, waxes and oils, and have a relatively low latent heat (in terms of energy per unit volume).
Other types of organic PCMs are fatty acids and esters, and sugar alcohols. Inorganic PCMs include salt hydrates, which are what Sunamp typically use. Furthermore, PCMs are usually used over the solid-liquid transition, but Sunamp is also developing solid-solid PCMs. In a solid-solid PCM the 3D arrangement of the atoms changes to a different arrangement well below the substance’s melting temperature, and there is an energy associated with that solid-solid transition.
So what makes a good phase change material?
When it comes to long-lasting, high storage capacity and the ability to store a large amount of energy in a material, having a reversible freeze-melt behaviour is crucial. It’s important that when a PCM is melted and then frozen, it will keep returning to its original solid form over thousands of cycles. When developing PCMs in the lab, we look for a flat melting plateau on a time-temperature graph (Figure 2), indicating absorption of energy at one specific temperature. We also look for a flat freezing plateau, which indicates the heat being released at a specific temperature. This means we get more usable heat out of the PCM than we would with one that has a sloping freezing plateau. An effective PCM should also provide similar energy output with each cycle, while an ineffective one will show a decrease in energy over time. At Sunamp, our rigorous approach to developing and testing PCMs means we offer a minimum guarantee of 10 years on our P58 Thermino products.
Figure 2: An example of good PCM behaviour: a flat plateau indicates the PCM melting and absorbing heat at one specific temperature.
It is also ideal for the PCM to be compatible with metals commonly used in heat exchangers, such as copper, aluminium and stainless steel, as well as with common plastics like polypropylene.
Other characteristics of an ideal PCM are a high thermal conductivity so that heat is transferred efficiently to and from the PCM; and to be made from inexpensive chemicals that are simple and safe to handle in large quantities. For example, our flagship P58 PCM is made from sodium hydroxide and acetic acid, two of the biggest commodity chemicals in the world.
Why are our PCMs better than our competitors?
Well, number one, it is not just about the chemistry. If we want to make good thermal energy storage products, we need to make products that work well with the PCM. Our unique selling point lies in the integration of chemistry and engineering, utilising an iterative process to create durable, long-lasting products, with a notably better understanding of PCMs than competitors.
Our PhD chemists have been working with PCMs for many years, and we have an extensive understanding of the underlying chemistry of PCMs – knowing how the crystals form and how to change the crystal-growing process during the transition from liquid to solid. We also have access to a lot of sophisticated analytical equipment through our collaboration with the University of Edinburgh, amongst others, which other companies do not have. Additionally, we have a strict testing regime where we test our PCMs for 10,000 cycles or more to ensure that, at the end of the development process, we have produced a reliable, well-performing PCM.
Take our flagship P58 PCM for example (which melts at 58 °C). This is based on sodium acetate trihydrate, which is the same material that’s in handwarmers. This has low cyclability and longevity, with less than 100 cycles (you’ll see a white solid build up in a handwarmer with repeated use). Dr David Oliver, now Head of R&D at Sunamp, solved this problem during his PhD with Professor Colin Pulham at the University of Edinburgh. Our P58 PCM retains 100% energy capacity after 10,000 cycles and >95% energy capacity after over 40,000 cycles, eliminating the stability problems.
PCM sometimes gets a bad name as there are several manufacturing companies selling substandard, poor-quality PCM. We want people to know that if you buy a Sunamp heat battery, our P58 PCM inside outperforms handwarmer mixes or other PCMs at this temperature. Academic groups in universities worldwide are still working on this material, trying to reach the point that we achieved with P58 10 years ago.
We strive to use non-toxic materials in our heat batteries, and we also start from an environmentally friendly base. There are manufacturers around the world that only use organic materials such as bio-waxes, and bio-oils and claim they are being environmentally friendly. However, these products are often associated with some sort of food. Some companies have taken soya oil, for example, and turned it into PCM. However, this takes away a food source from the world. Other companies use products that have a direct impact in terms of deforestation in some countries. This is why we stay away from organic PCMs where we can.
The future of phase change materials at Sunamp
Over the last 7 years the Materials team at Sunamp has grown from one Materials chemist to a team of 8, which allows us to focus on more aspects of PCM development than just formulations. For example, we focus on materials compatibility of a PCM with a wide range of metals and plastics; accelerated life cycle testing of PCMs; scaling up PCM from lab scale to factory scale; rigorous Quality Control testing; and supporting Business Development Managers with our technical knowledge. Every year, we also host two undergraduate Masters students from the University of Edinburgh’s School of Chemistry in their Industrial Placement year. We are also funding our sixth PhD student in 15 years, also in the School of Chemistry at the University of Edinburgh.
The Materials team are developing other PCMs and have been doing so for over 10 years in a wide range of temperatures from -74 °C up to >500 °C. For example, we have P74 (a PCM that melts at 74 °C) deployed at Turku, Finland on a Horizon 2020 project which acts as an interface between the district heating network and several University of Turku buildings.
Our P89 PCM is central to what we’re doing in the Thames Mobile Heat Project, and will therefore be used in large format Central Bank products. We also have a slew of customers waiting to sample our P285 PCM in the various sizes of Central Bank Minis and Central Banks that are in development.
Learn more about our Plentigrade phase change material.
Get in touch with us to know more.
Our PhD chemists have been working with PCMs for many years, and we have an extensive understanding of the underlying chemistry of PCMs – knowing how the crystals form and how to change the crystal-growing process during the transition from liquid to solid. We also have access to a lot of sophisticated analytical equipment through our collaboration with the University of Edinburgh, amongst others, which other companies do not have. Additionally, we have a strict testing regime where we test our PCMs for 10,000 cycles or more to ensure that, at the end of the development process, we have produced a reliable, well-performing PCM.
Dr. Kate Fisher
