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Storing Energy in the Earth with 90% Efficiency.
George Harvey
Energy storage is getting to be a fascinating subject, partly because there are so many ways to do it. When we drive a conventional car, we are storing electricity in the car’s battery to start it next time we go out. When we open a door, we might be storing energy in a spring that closes when we let go. If we stack cord wood, we are storing energy in the form of biofuel. There are so many ways to store energy that we don’t often think about.
We have many ways to store energy created by renewable generators. We can store heat in bricks or water. Flywheels can be used to store energy. And in some parts of this country, special trains store energy by hauling carloads of stones to a high point on a rail system and release it by generating electricity as they coast downhill. Pumped storage works by pumping water to a high reservoir so it can be used to generate electricity as it flows back down to a lower reservoir. And, of course, we have big batteries. The ways to store energy seem endless.
Many people have been looking into using abandoned underground sites. Disused mines can be used as reservoirs for pumped storage. And what about the caverns that have been created by removing salt from salt domes? They can be huge, and putting air into them under pressure can be a way to store energy – when the air is released, it can drive a turbine.
Now I can tell readers about a way to store energy I have never seen before. It is to use an abandoned gas well to store water under immense pressure. The water goes into the cracks in the earth that were created by fracking, and it actually pushes up on the geological formations above it. The pressure comes from the weight of the Earth, so the force can be maintained for a very long time. When valves are opened allowing the water to come back to the surface, it comes out in a jet powerful enough to drive Pelton turbines.
Pelton turbines have been around for a very long time. They are very small, relative to their power output, and they are typically over 90% efficient. They do not use much water, but they do need very high pressure. Because of these attributes, a very small turbine can use very little water to produce a lot of electricity, provided it is sited properly. The typical setup is a small turbine at the foot of a mountain, on which there is a lake hundreds or thousands of feet higher.
Sage Geosystems (Sage), a company based in Houston, Texas, has engineered a way to put all this together in a nice little package called EarthStore™ (ES) to store energy. Sage has tested the system and has some data on it.
The original turbine for the test was a Pelton turbine designed to be turned by water at 3,000 pounds per square inch (psi). Sage redesigned it to take 3,500 psi. We are talking about a lot of pressure.
As a small test, the test site produced one megawatt (MW) of electricity for half an hour. It also showed that it could produce 200 kilowatts for 18 hours. Sage pointed out that the test showed the promise for larger systems of 2 MW or 3 MW, and possibly much larger.
The round-trip efficiency of the system was 70% to 75%. And the levelized cost of storage shows that it is competitive with lithium-ion batteries or pumped storage.
All things considered, Sage’s ES system has advantages that the other storage systems do not have. Lithium-ion batteries and pumped storage take a fair amount of land. The former is used in what looks like self-storage sites. The later uses reservoirs that are rather large. Lithium uses scarce materials. By contrast, the ES site would only use a single reservoir, or possibly none, and the housing for the system might actually be completely out of sight, because the turbine is so small.
Some people might be worried that the Sage ES system could cause earthquakes because they can be used at old fracking sites. That is not of great concern, because the sites used for the ES would be carefully chosen, avoiding the fracking sites associated with earthquakes.
One other thing we might mention is that the Sage ES system is not geothermal storage, but mechanical. That is something the other articles I have seen on the system get wrong.
We are excited to see where this technology is going.
Sage Geosystems’ website is https://www.sagegeosystems.com.
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