Plugging the gap

Britain is already a world leader when it comes to renewables. But with demand for electricity set to increase as more people buy into EVs and heat pumps, energy storage technologies will be crucial, Jasmin Fox-Skelly reports

By its very nature, energy generated from renewables is variable and unreliable. Solar panels don’t work when it’s cloudy, or during night-time hours. In 2022, Europe experienced a ‘wind drought’ that lasted for weeks. On the flip side, sometimes a lot more energy is generated from renewables than is needed.

‘The point at which someone wants to run a bath or heat their home isn’t necessarily the point at which the wind is blowing or the sun is shining, so we need some means of plugging that gap,’ says mechanical engineer Rob Barthorpe at the University of Sheffield, UK.

However, storing surplus energy is easier said than done. On a cold winter’s day in the UK the country can consume up to 50GW of electricity. And this demand is only set to increase. So, what are the options?

Currently, rechargeable lithium-ion batteries – invented 30 years ago – are the most established technology. Found in every laptop and mobile phone, lithium batteries usually consist of a cobalt oxide cathode and a graphite carbon anode.

However, there is a worldwide shortage of lithium; by 2035 the lithium supply gap is predicted to reach 1.1m t – or 24% less than demand.^[1] Meanwhile, metals such as cobalt, nickel and manganese are toxic and can potentially contaminate water supplies and ecosystems. Li-batteries also suffer from short lifespans and can set on fire.

‘Some of the rare metals you need to make an electrical battery are very scarce and involve mining in very delicate areas of the world,’ says Barthorpe. ‘Lithium-ion batteries also only have a 10- to 15-year life expectancy and over course of that lifetime the amount of energy you can get from them degrades quite markedly.’

However, new types of batteries are in development. StorTera, based in Edinburgh, Scotland, UK, is currently building a prototype megawatt scale flow battery, which can operate for up to eight hours. Flow batteries consist of two liquid electrolytes separated by a membrane. When charging or discharging, ions move from one component to another component through the membrane. The single liquid flow battery (SLIQ) developed by StorTera uses lithium polysulfide as an electrolyte and is designed to be fully recyclable or reusable at the end of its life, minimising its overall environmental impact. According to StorTera, SLIQ is the most energy dense flow battery under development, at over 200Wh/L, and as the internal liquid electrolyte is refreshed continuously, the battery lifetime is up to 30 years.

‘Lithium-ion batteries degrade due to a number of factors,’ explains Brenda Park, Director of StorTera. For example, electrolytes can decompose or react with lithium ions, reducing the number of Li-ions that can shuttle between the electrodes. Cathodes can also deteriorate, and a process called lithium dendrite formation can occur where extra lithium ions accumulate on the anode surface, stopping them from being absorbed into the anode.

‘In a lithium-ion battery there is no maintenance method to prevent these effects or to overcome these problems,’ says Park. ‘However, in SLIQ, the patented flushing and dosing mechanism can refresh the battery stack, refresh the catholyte and refresh the electrolyte. This technology reduces the standard battery degradation mechanisms.’

Meanwhile Form Energy, based in Somerville, Massachusetts, US, is developing ‘iron-air’ batteries. The battery essentially stores energy by rusting and un-rusting iron in a cycle. While discharging, the battery ‘breathes in’ oxygen from the air and converts iron metal to rust. Then when charging, an electrical current is used to convert the rust back to iron, while the battery ‘breathes out’ oxygen. The active components of the battery system are some of the safest, cheapest and most abundant materials on the planet: iron, water, and air. According to the company, the batteries operate at less than 1/10th the cost of lithium-ion battery technology and can deliver more than 100 hours of power in one cycle.

The point at which someone wants to run a bath or heat their home isn’t necessarily the point at which the wind is blowing or the sun is shining, so we need some means of plugging that gap,
Rob Barthorpe Senior Lecturer in mechanical engineering, University of Sheffield, UK

Another option is hydrogen, which can be generated cleanly using renewable electricity during peak production hours, and then stored until needed. Researchers at the University of Bristol, UK, are currently working alongside energy company EDF UK to develop the world’s first hydrogen storage demonstrator at UKAEA’s Culham Science Centre in Abingdon, Oxfordshire. Electricity will first be used to split water into hydrogen by electrolysis. The hydrogen will then be chemically bonded to depleted uranium (238U) to form heavy-metal hydrides. The process is completely reversible, and the uranium quickly gives up the stored hydrogen when heated.

Barthorpe, meanwhile, is leading a multi-institution project looking at thermal energy storage (TES) as a solution for heating homes. TES works by heating or cooling a storage medium so that the stored energy can be used later. In its simplest form, this could mean heating a water tank for later use. Barthorpe’s team is looking at two different types of advanced thermal energy storage technologies – Phase Change Materials (PCM) and Thermochemical Storage (TCS).

PCMs are substances that absorb heat when they change from an ordered state to a disordered state, for example, from a solid to a liquid, or a liquid to a gas. They then release this heat when changing back again. For example, when heat energy is applied to water, the temperature increases; however, when the water gets close to boiling point, something strange happens. Despite putting more and more energy in, the temperature begins to flatline. That’s because the energy is diverted into pushing the tightly compressed lattice of molecules apart instead. This large amount of latent heat or energy can then be released by reversing the process, and ‘changing the state’ of the material from liquid to solid.

Some materials are even better at absorbing latent energy than water. ‘In our case we make use of fatty acid esters that absorb a lot of heat when they melt and release a lot of heat when they return from a liquid to solid,’ says Barthorpe. The unit works in the following way. When renewable energy sources – such as solar and wind – are in abundance, electricity is used to heat water in the home. This hot water then passes through a box to a heat exchanger, which transfers the heat onto the phase change material, melting it.

‘When it comes to you wanting to run a bath in your home or use the radiator to heat your property, you simply pass cool water through the heat exchanger, which causes the material to solidify. This releases energy which heats your home,’ explains Barthorpe.

Thermochemical Storage (TCS), on the other hand, is potentially even more exciting, because energy can be stored for an extremely long time with no standing losses.

TCS involves taking an active material, usually a salt hydrate, and passing warm dry air over it. When it dries out the material takes in energy, and when rehydrated the material releases heat.

‘The big advantage is once you dry out the material, as long as you keep it dry, which is fairly easy to do, that thermal chemical potential is there for as long as you want – days, weeks, or even months,’ says Barthorpe.

This means in principle you could have a large unit in your basement or garage that could store energy in good weather and keep it there until the wind drops and you need that heat again.

Barthorpe’s team is currently leading Project ADSorB, which is developing a prototype thermal energy storage system that makes use of PCM and TCS technologies developed by the Centre for Renewable Energy Systems (CREST) at Loughborough University, UK. They will trial the technology in two new homes within the University of Nottingham’s Creative Energy Homes (CEH) campus – a seven-house development that provides a living test-site for energy research.

And they are also looking at ways to adapt the technology so modular thermal energy stores can be slotted into homes as additional elements alongside existing energy systems, as part of a retrofit or a new build scheme. 

Elsewhere, Sunamp, based in East Lothian, Scotland, UK, is also developing thermal batteries based on phase change materials. Their active material, Plentigrade, is formed from various water and salt hydrate syntheses, with additives to ensure it solidifies with the right crystal structure.

‘You need your material to crystallise reliably and repeatedly within a defined temperature range, so we use what are called nucleation agents to ensure we get good and predictable nucleation within the materials,’ says David Oliver, Head of Materials at Sunamp. Other additives ensure the materials melt cleanly and evenly.

‘We tend to think of melting as a simple process, but salt hydrates can be challenging as they can form anhydrous salts when melting as opposed to a clean melt,’ says Oliver. ‘So instead of going from a solid to a liquid, you go from a solid to a new solid and a liquid. Over repeated cycling this would cause your PCM to degrade and separate, leading to a distributed temperature range of melting and reduced latent heat.’

According to the company, the batteries store up to four times more energy than heating and cooling hot water alone, allowing smaller and more powerful units that can meet the needs of people in their homes. A recent study also showed that Plentigrade can last for 40,000 cycles of phase changes unchanged, suggesting a lifespan of 50+ years.^[2] Lithium-ion batteries, in comparison, tend to degrade after 5-10,000 cycles.

The environmental credentials are also good. The salts are either mined, are byproducts of the existing chemical industry or are simple commodity chemicals with diverse supply chains. Plentigrade is also a food-grade product, and sodium acetate trihydrate – its core ingredient – is a flavouring used in salt and vinegar crisps. There’s no combustion risk, and the PCM can be safely recycled back into the ground if necessary.

Sunamp has received funding to trial a new variant of its advanced thermal storage system in 100 homes across the UK, where it will be paired with household energy systems to tackle periods of low renewables generation on the grid.

In the future, the likelihood is there will be a mix of storage solutions. Large scale batteries and gravitational storage schemes could store electricity for the national grid, while thermal heat storage devices could help reduce the energy demand of our homes, easing the burden on the grid at peak times. Such technologies can’t come too soon. In 2020, 43% of UK electricity came from a mix of wind, solar, bioenergy and hydroelectric sources.^[3] In the last quarter of 2021, wind power alone contributed 26.1%, with the government aiming to generate enough wind energy to be able to power every home in the UK by 2030.

Power of gravity

Currently more than 90% of the world’s energy-storage capacity is in reservoirs. When electricity demand is low, water is pumped up a mountain, and when it is high the water is released downhill, where it drives a turbine, generating power. However, some researchers are adapting the concept slightly to develop a new generation of energy storage based on the same principles as pumped hydro, but with solids, or high-density liquids, instead of water.

For example, rather than pumping water uphill and releasing it downhill, Gravitricity, based in Scotland, UK, aims to use clean energy to lift weights on a winch. When energy is needed, the weights are released, and the resulting kinetic energy is used to power a generator and create electricity. Gravitricity recently concluded a demonstration of the technology by heaving a 50t block up a tower. It now plans to raise and lower single 1000t blocks inside disused mine shafts.

Meanwhile RheEnergise, based in Montreal, Canada, has come up with a concept based on pumped hydropower, using a fluid the company has invented called R-19. According to RheEnergise, R-19 is an environmentally safe, mineral-rich fluid more than two and half times denser than water. At times of low energy demand, the fluid is pumped to storage tanks at the top of a hill. Then later, when electricity is needed by the grid, the fluid is released, whereupon it passes through turbines. As the fluid is much denser than water, RheEnergise says the system can create electricity from gentle slopes, without requiring the steep dam walls or high mountains necessary for traditional hydropower. This could open up around 700 sites across the country – a total of 7GW of energy storage.

References
1 https://www.bcg.com/publications/2022/the-lithium-supply-crunch-doesnt-have-to-stall-electric-cars
2 D.E Oliver et al, CrystEngComm, 2021, 23, 700.
3 https://www.nationalgrid.com/stories/energy-explained/how-much-uks-energy-renewable