POWER | APPLICATION REPORT
HOIST STORES POWER, SAVES PLANET?
The equation is simple: U=mgh. Potential energy is mass times the acceleration of gravity times the height you raise it to. Every science student learns it in school. If you apply it to a hoist that has lifted a weight it tells you how much energy is stored in that system – and also how much energy you can get back when you lower the weight back down. Any hoist that has regenerative braking can do it. When it is holding a load suspended, it is an energy store. “Energy storage is the holy grail of
renewable energy,” says Robin Lane, commercial director, Gravitricity, a company which intends to solve exactly that problem in a theoretically very simple way: by lifting weights. As is well known, renewable energy sources are intermittent. Wind towers make electricity only when wind is blowing. Solar does it only when there is daylight. Demand, though, does not follow those timings. So electricity from renewables needs somehow to be stored. Batteries are the standard way. They
are limited, they are expensive, they need elements that are environmentally destructive to obtain; and they leak their stored charge. Feed a kilowatt-hour into a lithium-ion battery, and two weeks later it may have only two-thirds of a kilowatt-hour to give back. They are not the answer. Successful schemes exist using hydro
power. The UK has four pumped storage hydropower stations, two in Wales, two in Scotland, and they provide 2,800MW of storage between them. The principle is simple. Water is pumped to a mountaintop reservoir using surplus electricity in times of low demand; when demand is high it generator at the bottom of the mountain where it re-makes the electricity. It works
R Gravity energy storage is simple in principle
Q Gravitricity’s proof-of-concept tower in Edinburgh
in other words it gives back most of the electricity that is put into it. The only disadvantage is that you need a nearby mountain. Two companies intend to use the same
water, and hoists instead of pumps. Both companies have built proof-of-concept designs. Energy Vault, based in Switzerland, plans
to raise weights of several tons to the top of towers or specially-designed warehouse-like buildings. Gravitricity, based in Edinburgh, plans to lower weights of up to a 1,000 tons down disused mineshafts or, eventually, purpose-built holes in the ground. “A big advantage of mineshafts is that
they are deep,” says Lane. “The U=mgh equation shows that you need as great a drop as possible to generate power. You can or 800m, or even a kilometre. In contrast, the highest building in the world, the Burj Khalifa in Dubai, is 828m high and you could So the ‘h’ in that energy-storage equation is In 2020 Gravitricity built a proof-of-concept model in Edinburgh – ironically a tower rather lifted and lowered by a hoist. Huisman,” says Steve Kirk, lead mechanical induction motor with variable frequency drive, and it performed well. We obtained a than pumped hydro; and unlike with Lithium batteries there were no losses of power during standstill. Speed of response is also
Q A possible site for Gravitricity’s East European prototype
important; your stored power must go on-line as soon as the grid signals that it is needed. We easily obtained the fraction-of- a-second responses required.” The next stage is a full-scale prototype.
Gravitricity is considering two possible sites in the Czech Republic. “East Europe is decommissioning many coal mines at present. Those in the UK from the 1980s Again, they will use Huisman hoists. “We have a great working relationship with them, from back when we started,” says Kirk. hoists will be constructed to order, but from standard components: such hoists are used by mining concerns already. The drive chains that is well within standard technology.” “The main requirements for the hoists will be large capacity – up to 1000t – and that the speed of descent be easily controllable,” says Kirk. “Variable frequency drive of regeneration will have to be compatible with the G99 requirement, which is the legislation for such things as voltage control and stability of power that is being fed back into the national grid. Braking will be controlled mainly by the motor and gearbox torques.” The weights will be lowered at around half a metre a second, which for the 800m depth of the proposed Czech prototype will give an output for 1,600 seconds, which is about half an hour, (re)generating around 4Mw equity and a similar amount of grant funding; we hope to begin Series A fundraising this year,” says Lane. “We are working on detailed design work for the Czech project now, with construction beginning next year to start operations within 18 months.”
26 | May 2022 |
www.hoistmagazine.com
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