Gravity Power is testing a novel kind of energy storage with potential gigawatt-scale capacity – that uses simple mechanics and gravity underground – at a first test site in Santa Barbara. Their Gravity Power Modules would marry traditional heavy rig drilling technology with renewable energy storage.
At utility-scale, the pumped storage would begin with drilling thousands of feet underground, large enough to accommodate an 18 foot diameter storage shaft and a 6 foot diameter return pipe.
Here’s how it works, in the elegant words of Powermag
“At the bottom of the shaft is a large concrete piston fitted to the shaft, called the “weight stack.” Also bored into the ground is a parallel but smaller-diameter “return pipe” that is connected to the main shaft at the top and bottom.
Finally, the entire volume is filled with water and tightly sealed—air is compressible and its presence reduces the system effectiveness. In essence, the position of the weight stack in the shaft determines the amount of energy stored.
During the energy storage process, off-peak electricity is used to power a pump that pushes water down the return pipe that will raise the weight stack from the bottom of the deep storage shaft.
During a peak electricity demand period, the weight stack is released, which pushes the water up the return pipe, reversing the direction of rotation of the pump-turbine and producing electricity, much as in a typical pumped storage hydroelectric plant.”
CEO Jim Fiske envisions that his Gravity Power Modules would be installed in clusters to produce the amount of energy desired. The storage capacity of a 7 acre site could amount to more than 2 GW (2,000 MW) depending on the depth and diameter of the shafts.
The Gravity Power Module has a conversion efficiency that looks likely to be in the 75% to 80% range once it is tested at full scale, at installation costs a little higher pumped hydro, around $150/kWh for a system capable of storing about 200 MWh.
Pumped hydro installation has installation costs of around $100/kwh. But it can be controversial because, like hydro-electricity itself, pumped hydro can impact a natural habitat for fish. More than half the states that have renewable energy standards do not allow hydro to qualify as renewable because of the ecological damage.
New pumped hydro projects face formidable permitting obstacles, despite the need to add more energy storage as we move to a clean power economy. The Gravity Power Module could be one of the solutions.
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