Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

Gravel batteries are one of the newest concepts for storing power. The concept is new enough that, at this point, there are not yet any facilities to demonstrate the process and prove its effectiveness. The term “gravel battery” itself is somewhat misleading. This form of power storage is much more akin to compressed air storage systems rather than it is to chemical batteries.

Fundamentally, the gravel battery system can be understood as a giant heat pump. The process uses a heat engine with pistons and valves rather than turbo systems to compress the gas in its system. This prevents a number of thermal losses that are otherwise present in other kinds of compression systems. The overall efficiency of the system is expected to be in the range of 70 – 80 percent, which is similar to that of pumped hydro.

Argon gas is compressed and then pumped into an insulated silo filled with “lightly processed (crushed, graded and cleaned) mineral particulate,” i.e. gravel. As the gas moves through the silo, it heats the gravel to over 900 degrees F (500 degrees C) while cooling the gas to ambient temperature. The gas is then passed through a second gravel-filled silo which returns it to normal pressure while cooling the second silo down to -256 degrees F (-160 degrees C). The process can be reversed in order to run a generator to supply electricity when additional power is needed.

Although the process is still theoretical, Isentropic, the company that has developed this technology, is developing a demonstration facility to prove that the process will work as expected. This pilot plant will have two storage tanks, each 7 meters (23 feet) tall and 7 meters in diameter, which will provide a storage capacity of 16 megawatt-hours. Although it is still very much an experimental system, if the technology proves to perform as expected, then it could provide a new and inexpensive method for storing power.

Gravel is a very inexpensive material, and it is readily available anywhere in the world. It also serves as a very effective thermal mass which can go through a wide range of temperatures without changing phase (unlike a fluid boiling at high temperature, for example) or breaking down. Because there is no reliance on refrigerants or on extremely complicated systems, gravel batteries offer the potential for inexpensive power storage that can be installed wherever it is needed.

The thermal capacity of the gravel battery means that it can store energy for long periods of time, as well. The designers say that, if left alone, a gravel battery would still retain half of its energy even after 3 years. This could be useful for dealing with seasonal supply variability from renewable sources or with demand fluctuations in power needed by the grid.

Gravel batteries can be installed in modular fashion, allowing for large-scale installations. One advantage that the company is promoting for gravel batteries is that, like several other of the industrialized power storage systems (including batteries, flywheels, and even some compressed air systems) they can be located anywhere. Compared to a pumped hydro storage system, Isentropic’s founder, Jonathan Howes, notes that a gravel battery facility with similar power storage capacity would require only 1/300th of the space.

As with pumped hydro storage, gravel batteries are probably not especially good for fast-response grid load balancing, and would likely be used to supplement base-load energy demand.

Main photo credit: Phlat Phield Photos; diagram credit: Isentropic