An earlier Future Friday post was titled Batteries: The Key to Renewable Energy Expansion. A better title would have been Energy Storage: The Key to Renewable Energy Expansion. Batteries are just one of many energy storage technologies currently in use. One of the oldest and most well established technologies is pumped hydro-power. Pumped hydro systems move water to a higher elevation during times of excess power, and release it to drive a turbine generator during times of increased demand. Compressed air energy storage (CAES) systems operate on the same principle as pumped-hydro, using low-cost, excess power to store energy in the form of compressed air, which is released to drive a generator when power is needed. Other technologies include thermal storage (e.g., ice making for cooling) and flywheels that store energy as rotational inertia. To learn more, the Energy Storage Association has detailed information on each of these technologies.
The trade-off with renewable energy sources such as solar and wind is that even though the fuel (i.e., sun and wind) is free the production is intermittent. Intermittency is the primary roadblock to the expansion of renewable generation resources. One way to address the intermittency problem is to store solar and wind energy in times of excess generation for times when production does not meet demand.
Advanced Rail Energy Storage (ARES) is currently testing a new, large-scale energy storage technology in Tehachapi, CA. When power prices and demand are low the system drives electric rail cars carrying a load up an incline. When demand and prices are high gravity pulls the trains back down hill turning the onboard electric motor into a generator. When fully built out each train will haul a 230 ton load uphill, and be the equivalent of a 2 MW generator on the way down. ARES’ website states that systems can be scaled as large as 3 GW, with 16-24 GWh of storage capacity. Scalability is also a function of geography. The system runs best on grades of 6 – 8%, and the longer the track, the more energy the system can store. Currently, the system returns 86% of the energy used to drive the train uphill, though ARES believes this number can be improved.
ARES has already met with some commercial success. They are working with Valley Electric, a Nevada-based electric co-op to build a 50 MW system with 12.5 MWh of storage capacity. The goal is to have the system up and running by 2016.