Utility Week

14 | JUNE 2021 | UTILITY WEEK Speical report on storage Project director Ian Innes says this is enough to power three million homes for an entire day: "You need a lot of batteries to match that capability." "This is a big step up," he adds. "As a single project, it's very signi• cant." Innes says SSE Renewables has planning permission for up to 1500MW of generation, but the exact • gure is yet to be • nalised. Among other things, it will depend on what kind of support is o‡ ered by government. As a mature technology, Innes says pumped hydro storage is "well proven" and "very reliable". Neverthe- less, SSE is still aiming to make improvements over the previous generation of pumped hydro by using variable Š ow speeds and new construction methods enabled by modern tunnel-boring machinery. The company expects the roundtrip e‹ ciency of the plant to be around 85 perŽcent. Construction is currently slated to begin in 2024, with the plant becoming operational in 2030. While other projects are being developed, the poten- tial for pumped hydro storage in the UK is limited by the availability of suitable locations. However, one idea being considered is to access stor- age in neighbouring countries with geographies that lend themselves better to pumped hydro, namely Nor- way, which research has suggested could build another 20GW of generation capacity in addition to the large amount it already possesses. The UK's • rst interconnector to the country – the 1.4GW North Sea Link – is currently being built by National Grid and Statnett and is expected to be com- pleted later this year. Gravitational storage Gravitational energy storage operates on a similar princi- ple to pumped hydro but using solid weights rather than water. Miles Franklin, lead engineer at Gravitricity, says the system it is developing replaces pumps with winches: "By liž ing the weights, we transfer energy from an elec- tricity grid to the gravitational potential energy of the weights. And then when the energy is needed back, the system runs in reverse, the motors act as generators and then we Š ow power back to the grid." He says its system o‡ ers the fast sub- second response times of lithium-ion batteries but without capacity deg- radation between cycles, meaning that, like pumped hydro, it can be operated for decades. "The long lifetime and the long cycle life feeds into a low levelised cost of storage," he says. "We're not claim- ing that we can be cheaper than lithium-ion on day one. That's not the expectation. But 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-e‡ ective over the lifetime." The company recently commissioned a 250kW dem- onstrator at the Port of Leith in Edinburgh and is in the early stages of testing. The demonstrator takes the form of an above ground tower, but for commercial plants the company is initially looking to reuse abandoned mine shaž s, "which obviously o‡ er a big cost saving because it's a big existing hole in the ground". Franklin says di‡ erent con• gurations will be suitable for di‡ erent applications. A system with a heavier single weight would have a higher generation capacity mak- ing it better for "short-duration, high-power balancing", while one with multiple smaller weights would have a higher storage capacity making it suitable for "peak shaving". He says there are lots of old mine shaž s in the UK but "the massive caveat is that a fairly small number of those are known to be in a good condition for us to consider them as an early system." It is for this reason that Gravitricity is looking for places like eastern Europe and South Africa as possible locations for their • rst storage systems in mine shaž s. Franklin says that although "people's instinct is that sinking new shaž s will be astronomically expen- sive", the company's research suggests this is also feasi- ble, although the shaž s would need to be shallower at around 150 to 200 metres deep. He says similar volumes of energy storage could be achieved by using multiple weights, stacking them at the bottom and then putting them to one side once they have been liž ed to the top: "Then there's the challenge of maintaining continuous output – and that's where we have some speci• c patents to achieve that." He continues: "If you want to then double the mass of the system, you double a signi• cant proportion of the cost of the system. But if you just have two weights and don't double the liž ing of the system, then there's a route there to scaling energy much cost-e‡ ectively." Compressed air energy storage (CAES) This technology works by compressing air from the atmosphere and storing it an underground cavern. When the energy is needed, the air is allowed to expand, driv- ing a turbine. The main challenge that arises is the changing tem- perature of the air during this process. The heat gener- ated as it is compressed needs to be removed before it Analysis continued from previous page "When you go off shore there's a lot more space than on land and if you have a lot more space you can install a lot more of your systems, and that's your way of scalingup." Alexander Habeder, head of business development, Siemens Energy "Hydrogen is a very clean gas, but it is diffi cult to produce, it's diffi cult to manage. It's the opposite to air. Air is easy to fi nd, air is very easy to store, air is totallyinert." Javier Cavada, chief executive, Highview Power "When you go off shore there's a lot more your systems, and that's your way of scalingup."

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