The irresistible allure of old mine shafts

UK energy start-up Gravitricity announced February 7 it had received a GBP650,000 ($903,400) grant from Innovate UK, the UK government's innovation agency, for a plan to use old mines shafts to store energy.

The idea is relatively simple: A massive weight would be raised up the mine shaft using a winch powered by electricity at times of low power prices. The weight could then be released quickly or slowly depending on the power requirement, with the potential energy of the falling weight used to drive a generator.

The idea is a variation on two themes and another example of the seemingly irresistible attraction old mine shafts hold for energy development companies. It seems impossible not to gaze down into the darkness and think 'how can this large hole in the ground be put to use?'

Gravitricity's project is similar to one proposed by US company Gravity Power in 2011. That envisaged a huge piston in a mine shaft filled with water and was dubbed the Gravity Power Module. The idea was that cheap electricity was used to pump water into the shaft raising the piston, which would then be released when required, forcing the water back through a hydro turbine to generate power.

The concept fell afoul of the problem of scale. While it is possible, say in a car engine, to engineer a near-perfect piston, it is much harder to make a massive one with the same level of precision, and particularly where the piston casing is formed by the walls of an old mine shaft.

While this concept was a bit off the wall, mine shafts and hydro power hold significant attraction. A mine effectively represents the lower reservoir of a pumped hydro station with the surface the higher reservoir. The mine shaft is the race tunnel. There are plans to turn the Prosper-Haniel coal mine in North Rhine  Westphalia, Germany, into a 200-MW hydroelectric reservoir. In the UK, proposals for pumped storage have been made that would make use of old mines and slate quarries in Snowdonia.

However, Gravitricity's concept depends on the potential energy of a weight rather than elevated water. It avoids the piston problem of the Gravity Power Module, but maybe not all problems with scale, as the weight is envisaged at up to 2,000 tons.

In abandoning the hydro element, it is more closely related to another US project, ARES -- Advanced Rail Energy Storage. That envisages using trains to store potential energy from cheap electricity. All that is required is a hill and an electrified railway; storage involves driving a train up the hill and electricity is generated from the train's motion back down. Gravity is again the driving force.

As with all these projects, they will capture "green power," which they may well do, but the reality is that unless they are tied to generation from a particular variable source -- for example a wind farm -- their storage of electricity will be somewhat more indiscriminate.

Projects using old mine shafts also inevitably claim to have regenerative powers for mining communities, but again a storage facility might generate power, but it won't generate anything like the number of jobs that a mine would.

Gravitricity says the innovation funds will enable them to start building a scale demonstrator later this year, and find a site to install a full-scale prototype by 2020. The company plans models from 1 to 20 MW. Once they have proven the technology in old mines, the company plans to sink new shafts to store energy wherever it is required.

This is a selling point in that unlike pumped hydro, it would not be dependent on the existence of a particular topography, but sinking a new mine shaft will be significantly more expensive than rehabilitating an old one. As the technology advances, the cost of drilling will reduce significantly, the company says.

Unlike batteries, the Gravitricity system can operate for decades without any degradation or reduction in performance, according to Managing Director Charlie Blair. However, it's a bit much to claim there would be no wear and tear on a mechanical system. The company also says its system would be more responsive than pumped hydro. Maybe so, but it would not be more responsive than a battery.

All forms of storage have their pros and cons, and Gravitricity's might find its niche, but for the moment it looks like a much bigger challenge than the MW-scale batteries being turned out in easily-transportable transport container-sized boxes, even if they don't last forever.