Is plain old gravel a solution to low-cost, grid-scale energy storage challenges? This is one of the questions being explored by such companies such as Isentropic in England and Energy Cache in the U.S. Isentropic uses gravel and compressed argon gas to create temperature differentials/ thermodynamic processes, in two connected but separate tanks where the gravel is held. The two tanks are separated by a reversible heat pump that expands the gas in one tank and compresses it in the other, creating temperature differentials of 500 degrees Celsius and -160 degrees Celsius, of which energy is stored in the gravel. During times of high energy demand, the process is reversed and the energy that was stored in the gravel is released.
Another technology underway is the Pennsylvania company Energy Cache’s “ski lift” system that has received heavy investments from Bill Gates. This system uses gravel that is used in the energy production and storage via a motor generator, buckets, storage slots, and a lift system as shown:
Watch at: http://www.youtube.com/watch?v=G3nz_kU604s&feature=youtu.be
Both systems, provided they work as planned, are cheap, simple, efficient, and easily set up (not as many permits and regulations) alternatives to large-scale energy storage.