Utility Week

UTILITY WEEK | JUNE 2021 | 11 Speical report on storage "What that allows us to do is two things," Harper explains. "First of all, by separating those two elements from one another we can independently vary the capa- bilities of the system from both a power and an energy perspective. This is how we can get to comparatively long discharge durations at comparatively low cost, because if you just need more hours of storage, you don't need to double the entire system, you just double the amount of electrolyte. "The other thing it allows us to do is, because you're • owing the electrolyte through the cell stack, you can very discretely control that electrochemical charge and discharge process in ways that are not possible with con- ventional battery cells. That's part of how we get the very, very durable high cycle life out of our batteries, because we're controlling that charge and discharge reaction a lot more discretely than you would see in a conventional battery cell." Harper says these characteristics mean the com- pany's batteries are well suited to "ultra-high through- put" applications. "There's no impact of the number of discharge cycles on the performance of this battery. And what that means is instead of looking at applications that might cycle 100 or 200 times a year to meet the peak requirements of the electric grid, we can be cycling these batteries two to ƒ ve a day and still have that long-run performance remain in place," he„says. While lithium-ion batteries are liable to catch ƒ re fol- lowing damage or failure, Harper says • ow batteries are not subject to the same risk, with the liquid electrolyte more likely to act as an extinguisher. Round-trip e† - ciency is, however, at around 70 per cent on average. Harper is also conƒ dent that vanadium, which is a relatively abundant element, can be supplied in su† - cient quantities to meet demand: "There are resources all around the world that are currently not being extracted." The market for vanadium is closely tied to the steel industry, with most being produced as a by-product of iron mining and added to steel alloys to strengthen them. It is also a by-product of the burning and reƒ n- ing of heavy fuel oil. "There are tremendous reserves of vanadium sitting in waste piles waiting to be used," he adds. He says the battery systems Invinity Energy currently manufactures are "slightly more expensive on a ƒ rst cost basis" than equivalent lithium-ion systems but their greater durability means they are ultimately cheaper on a per-megawatt-hour basis over their full lifespan of 20 to 30 years, and he also expects to achieve signiƒ cant cost reductions over the coming years. Ultracapacitors Like the regular capacitors commonly found on printed circuit boards, ultracapacitors store energy as electro- static charge at the surface of two separated electrodes. However, ultracapacitors have a slightly diŽ erent structure, enabling what's called "double-layer" capaci- tance. The electrodes are also made of "activated car- bon", with many small pores that massively increase their surface area to "over 1,500 square metres per gram", according to Sebastian Pohlmann, vice president of innovation at Skeleton Technologies. This allows them to achieve higher capacitances, measured in thousands of farads rather than microfar- ads or millifarads. Pohlman says lithium-ion batteries still have "up to 30 times the energy density or even more when compared to today's ultracapacitors". The ultracapacitors Skeleton Technologies produces can currently store 7„ watt-hours per kilogram, although they are hoping to raise this to 15 and then 20. But ultracapacitors also have a much higher power density, meaning "you can charge them up in seconds and discharge them in a second". They suŽ er from far less degradation, giving them a long lifetime of "over a million charge/discharge cycles", and have an even higher roundtrip e† ciency of more than 99 per cent "even with very fast charge/discharge cycles". These low losses as heat mean "they can mostly get away without cooling". Pohlman says they therefore excel at delivering fre- quent, short bursts of power as is o— en needed for fre- quency response. He says they are ideally suited to providing synthetic inertia – a form of fast-acting fre- quency response that seeks to mimic the eŽ ects of syn- chronous generators such as coal and gas plants that contain large spinning turbines rotating in harmony with the frequency of the power grid. Voltage control is another potential use case. He says ultracapacitors can be easily re cycled, can operate at a wider range of temperatures than lithium- ion batteries and are also safer. However, Pohlman says he does not see ultracapaci- tors as competitors to lithium-ion batteries, "rather a complementary technology that can actually help lith- ium-ion batteries to be more e† cient" when combined into hybrid systems. To the extent that ultracapacitors can replace lithium- ion batteries, he says it is in applications where "batter- ies are just used because nothing else is there and then continued overleaf "There are tremendous reserves of vanadium sitting in waste piles waiting to be used." Matt Harper, chief commercial o cer, Invinity Energy, says there are plentiful resources to make flow batteries "You can charge ultracapacitors up in seconds and discharge them in a second." Sebastian Pohlmann, vice president, innovation at Skeleton Technologies

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