COVER STORY: ALTERNATIVE BATTERY TECHNOLOGIES plus power will be drawn from the grid to raise the underground weights closer to ground level. When the time comes to inject energy back into the grid, the weights can be released for a burst of power generation. The firm says the output duration can be between 15 minutes and eight hours. Although this is similar to pumped hydro it has one extra benefit — an almost instant (one-second) response to fluctuations, as well as a potential degradation-free operational lifespan of 50 years. Innovate UK, the British government funded agency, awarded the start-up a £650,000 ($1 million) grant earlier this year. A full scale demonstrator will be developed this year and the firm hopes to install a full scale prototype by 2020. Managing director Charlie Blair says the difference between pumped hydro is that “we don’t need a mountain with a loch or lake at the top, and we can react much faster”. He says the biggest single cost is the hole, and that is why the start-up is developing its technology using existing mine shafts, in the UK and also in South Africa. He reckons that as the technology advances, the cost of drilling will reduce significantly and will allow them to sink purpose-built shafts wherever they are required. The firm plans to build models from 1MW to 20MW. It says its total cost of ownership is far lower than the equivalent installation of a lithium ion storage facility.
Train power
A similar gravity propelled energy storage system is being developed in the hills of Tehachapi in California close to the Mojave desert. Tehachapi Pass Wind Farm is one of the first large-scale wind farms installed in the US with a capacity of around 700MW. The firm, known as ARES — Advanced Rail Energy Storage — uses rail cars carrying heavy blocks of concrete that are pushed to the top of a grade using excess power from renewable energy plants during off-peak hours when electricity demand is low. Similar to the Gravitricity model, when the grid requires energy to meet periods of high demand, the rail cars are released down the hill, generating electricity through regenerative braking. The company says the system can respond to increases or decreases in
www.batteriesinternational.com
Although the system requires sunshine to generate power, it doesn’t necessarily require the sun to be shining on the panels the whole time. The problem of intermittency has been avoided. demand in seconds. The time taken for powering the system will be longer than the Gravitricity model, given there is the question of momentum being built up with increasing speed. The firm says it has a charge/discharge efficiency of 80% — roughly the same as Gravitricity — which is roughly that of pumped hydro and can deliver constant power for periods of up to eight hours. The company has tested a pilot system on a 268 metre track and the company has been granted permission by the Nevada Public Utilities Commission to build the energy storage system in Nevada’s mountains. The US state of Nevada is ranked around third in the country in terms of existing solar installations. The firm envisages that by the 2020s a fleet of automated 300 tonne electric-traction-drive shuttle trains could be moving up and down a 7.2% grade slope, providing 50MW of power to balance the California electricity grid. The 34 shuttle units on the ARES system will operate on a 9.2 km track with an elevation differential between top and bottom of 640 meters). The firm’s ambitions are much greater, however. The rail track model used is scalable — shuttle trains can run in parallel and installations could range from 100MW with 200MWh of storage capacity, up to larger 2-3GW regional energy storage systems with 16-24GWh energy storage capacity. Jim Kelly, chief executive of the firm and a former senior vice president for Southern California Edison, says the system can be deployed at around half the cost of other available storage technologies.
Under the Dutch thinking, the maglev train would be travelling in a sealed, vacuum tight tunnel running at 2,000 km/h around a circular track with a 2.5 km radius. This energy
ARES pilot track. Rail cars carrying heavy blocks of concrete are pushed to the top of a grade using excess power from renewable energy plants. On release they return the energy stored.
Maglevs too
Another version — still only at the theoretical level — is being explored in the Netherlands. This takes the concept of the kinetic energy flywheel further on a maglev train. A maglev train — magnetic levitation — use two sets of magnets, one set to repel and push the train up off the track as in levitation, then another set to move the floating train ahead at great speed taking advantage of the lack of friction.
Hawaiian Electric and Amber Kinetics are testing the 8 kW/32kWh storage system for local grid reliability and support and aid in the integration of renewable energy. One 8 kW unit can power approximately 25 homes for one hour
Batteries International • Spring 2018 • 65
