Utility Week

16 | JUNE 2021 | UTILITY WEEK Countdown to COP Analysis temperature sensor runs down the full length of the tank and when more needs to be heated, a pump draws cold water from the bottom and injects it at the top. White says this cold water "rapidly obtains the sur- rounding water temperature, which causes that hot water layer to grow and expand down through the tank, pushing the thermocline down, so it means we can boost the tank from 20 per cent full to 30 per cent full and 40 per cent full, all the way down to 100 per cent." Among other things, White said this capability unlocks headroom to store surplus energy, either locally generated or from the power grid: "There's a lot of Mix- ergy tanks connected to solar PV systems, and the ben- efit of that is the customer can heat exactly what they need in the morning. So, say they only have one shower in the morning, they might heat the tank to 20 per cent full, have a shower which might use 10 or 12 per cent of the cylinder, and then they've created 90 per cent of a tank's, worth of cold water available for sinking solar energy into it." He says most hot water tanks can store between 120 and 300 litres of water, which equates to 8 to 14kWh of energy storage. As well as providing benefits on an individual level, White says the smart controls also allow Mixergy to coor- dinate the actions of numerous cylinders and operate them as a "virtual distributed battery". "At a macro scale, we do that today," he remarks. "We offer frequency services through Centrica Business Solu- tions, which is amazing. I think we're the only hot water product on the market at the moment that has National Grid approval to do frequency response." Latent heat storage In contrast to sensible heat, latent heat refers to energy that is absorbed or released by a substance during a change in its state of matter – or phase – without chang- ing its temperature. Andrew Bissell, chief executive of Sunamp, says that while heating up a kilogram of water by one degree Cel- sius requires approximately one watt-hour of energy, melting ice into water, going from minus one degree Cel- sius to plus one, requires 80 watt-hours – or 40 times as much per degree. The heat batteries developed by Sunamp leverage this phenomenon to store large amounts of energy in a small volume using specially designed phase-change materi- als. "If you think about it that way, that phase change is storing a great deal of energy. And what's really inter- esting is when you go the other way – you resolidify the material – you get all the energy back," he says. The heat batteries developed by Sunamp are primar- ily composed of the sodium trihydrate acetate – a salt that was already used in chemical handwarmers. These handwarmers use another phenomenon known as sub-cooling, where a substance cools to below its usual freezing point while remaining a liquid. This hap- pens because the salt is contained within a smooth plas- tic bag that offers no seed for the formation of crystals. The bag also contains a small, domed metal disk, which when "clicked" introduces such a seed. The salt rapidly solidifies, releasing heat from the phase change and rais- ing its temperature to its freezing point. But Bissell says this salt and the other phase change materials available when Sunamp set out to develop a heat battery still suffered from degradation due to the way they crystallised: "They would degrade over a num- ber of cycles from being good on cycle one to not so good on cycle ten to being completely spent on 20 or so and that's no good." To solve the problem, Sunamp enlisted the help of the University of Edinburgh's School of Chemistry, including professor Colin Pulham, an expert on crystallisation. Pulham and his colleagues developed a "chemical nucleator" that is "the same shape in crystallographic terms as the sodium acetate trihydrate but which has a higher melting point" and performs an equivalent func- tion to the "clicky disk" in handwarmers. They also developed a polymer solution that acts as a "chemical habit modifier" that ensures the substance crystalises as the desired salt hydrate and not the anhydrous salt that is responsible for degradation. He says testing has shown the material can be cycled over 40,000 times, while retaining 95 per cent of its origi- nal capacity. By using phase change materials, Bissell says these heat batteries can provide the same amount of storage as a 210-litre hot water cylinder in something around two- thirds smaller: "We can pack 40kWh of storage into an airing cupboard and deliver time shi™ed space heating." Sunamp is also exploring the possibility of using sub- cooling to provide long-duration heat storage. Although this doesn't make sense over short periods because heat is lost as the material initially cools; once it reaches ambient temperature the losses cease. Bissell acknowledges that this may never be economi- cally viable for interseasonal storage: "You've got a store that gets one cycle a year, so every kilowatt-hour in that store has got to be really, really cheap – way cheaper than even the lowest cost I can reasonably project our phase change materials getting to. And then you've still got to have a heat exchanger in there and a tank to contain. He continues: "That said, I think there's probably an intermediate point where we will see it happen and that's in the domain of storing between a week and a month and we may need a lot of that because of this thing I think the German call the 'Dunkelflaute' – the high pres- sure in winter when the skies are leaden and the wind doesn't blow, and we had that this year for example. "And there you don't need interseasonal. You need to be able to have a store that you can discharge for a week or two weeks and then recharge when the conditions get better again." With this in mind, Bissell says Sunamp is also exploring thermo-chemical storage, which uses revers- ible chemical reactions to store and release heat energy. "You always want to be able to obsolete yourself and if thermo-chemistry is going to come through and become big, we want to be on the right side of that," he says. Tom Grimwood, energy editor The full version of this article is available to Utility Week subscribers at: https://utilityweek.co.uk/the-state-of-storage-2/ continued from previous page "If you have a cycle life that is an order of magnitude larger than lithium-ion, there's a lot of space there to be more cost- effective over the lifetime." MILES FRANKLIN, LEAD ENGINEER, GRAVITRICITY

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