The new hotness in energy storage: gravel

Can a humble system of gravel and a heat pump provide a breakthrough for utility-scale energy storage? British startup Isentropic thinks so, and this week announced that they’ve raised $22 million in project funding and an equity investment from the Energy Technologies Institute (ETI), a collaboration between the U.K. government and companies in the energy industry.
I covered Isentropic back in 2009, and back then the company was looking for a $5 million Series B round. The five-year-old company, previously raised a Series A round from Credit Suisse Securities Europe and won a £250,000 ($380,112) research grant from The Carbon Trust.
The innovation behind Isentropic’s idea is an advanced heat pump connected to a super simple, low cost energy storage design. Heat pumps are basically engines that can work in reverse and Isentropic’s device can store and release energy when needed. Founders and engineers Jon Howes and James Macnaghten developed the design of the heat pump a decade ago, and then brought on Mark Wagner as chairman to help with business direction.
An isentropic process (hence the name) is a thermodynamic process that can be reversed. Chairman Mark Wagner told me back in 2009 that the key to the company’s heat pump is that it can be reversed extremely efficiently, and has an isentropic efficiency (reversible efficiency) of 99 percent.
Energy storage comes into play because the Isentropic team decided to connect their heat pump to an energy storage system using two silos full of plain old gravel (or any cheap mineral or particle that can hold heat and cold well). Using the heat pump, the system compresses argon gas to produce a temperature differential and deposits heat and cold into the two separate large silos of gravel. Energy is stored in the gravel and when the process is reversed, it can be released.
The benefit of such a system are that it could be very low cost, quick and easy to set up, and could be built in a variety of locations. Utilities are looking to add more forms of energy storage to the power grid, to store energy when it’s not being used immediately, but many energy storage options are expensive and can only be built in certain locations (in elevations or underground reservoirs). Batteries are among the most expensive options, and the most location-specific choices are technologies like pumped hydro (push water up a hill and let it fall back down) and compressed air (stuff air into a container or underground hole and then release it).
Isentropic isn’t the only startup looking to develop a super low cost utility energy storage system using gravel. Energy Cache is a startup backed by Bill Gates, Claremont Creek Ventures, and Idealab that is building a system that carries buckets of gravel up and down a hill and stores energy in much the same way that pumped hydro does. Energy Cache is currently building a pilot system in Irwindale, Calif.
Isentropic will use part of the funds to build a demonstration project — sized at 1.5 MW — of its energy storage system at a primary substation owned by U.K. power company Western Power Distribution. This idea and technology is still in the very early stages, and it will take significant funds to scale it up to see if it works on a large scale.