The idea is to use gravity instead of rechargeable batteries. Trains powered by wind turbines and photovoltaic solar plants would pull heavy loads up a hill. When an electric utility gives the signal that it needs power, the train would be released to roll back down the hill, generating a steady supply of juice much the way a hybrid electric car recharges its batteries.
ARES hopes the tests support its claim that steel wheels on steel tracks are more efficient than other storage projects, including pressured air storage, rechargeable batteries and the most common method: pumping water uphill and letting it flow through a hydroelectric generator.
Key measures of ARES' success will be how quickly its design responds to demand and how much energy is lost in the process. If successful, the system could make renewable energy more reliable and less prone to spikes in output.