In a couple of years, utilities will do just about anything to be able to store excess power coming from their future renewable sources, such as wind or solar. The good news is that lately several solutions have emerged from privately-funded startups. One of the solutions found that storing energy inside gravel-filled containers is a good idea, and Isentropic Energy, its creator, is really going to build a prototype.
The Pumped Heat Electricity Storage (PHES) is all about pumping heated and pressurized gas in an insulated gravel-filled container. Basically, their system is made of two such containers, one of them heated up to 500 degrees Celsius and the other one frozen down to -160 degrees Celsius in the process, just like in the picture above.
Johnatan Howes, Isentropic Energy’s CEO, describes how the PHES works: “The Isentropic PHES system utilizes a highly reversible heat engine/heat pump to pump heat between two insulated storage vessels containing gravel. A gas is employed in this process from which work is extracted. At first, the compressed gas is made to pass through one of the stores.
This heat[s] up the gravel up to 500 degree[s] Celsius and during heat exchange, the temperature of the gas almost drops to ambient condition. In the second step, this gas is made to expand to its original pressure and as a result the temperature drops to -160 degree[s] Celsius.
It is then passed through the other store and it exchanges heat with the gravel by direct contact. In this process, the gas is warmed back to its original temperature. The energy supply is mainly by employing a motor as it not only supplies electrical power to it but also acts as an energy storage element. The cycle is reversed to release the energy. As the energy passes from hot to cold, it powers a generator from which electricity is produced.”
The claimed efficiency is said to be around 72 to 80 percent, and the cost per kilowatt somewhere around $55 (not the cost of the stored electricity, but the cost of the storage itself). It could nevertheless get cheaper if scaled up to utility sizes, down to about $10 per kWh. The sizes is also not a problem, since its 8 x 16 x 7 meters would comply with spaces available to utilities.
A 16 MWh pilot storage plant is being planned to be built during the next two and a half years, with an estimated cost of about $16 million. Although several other companies had been developing more complex energy storage systems, I guess just because this one is simpler would make it a success in some parts of the world, where building costs would be considerably smaller.