Sage Geosystems Inc, a Houston-based startup, has found a novel method to store renewable energy in the ground and retrieve it on demand as geothermal energy. The firm achieves this using old oil and gas wells, putting them to use yet again, a press release said.

The oil industry has used this approach for many decades, where steam is injected into the shale deposit and left there for many hours to heat the leftover oil. The added heat makes the oil less viscous and easier to be pumped out.

Renewable energy may be replacing oil wells but they can still be repurposed. Image: Kiyoshi Tanno/iStock 

Sage Geosystems also uses the same approach but uses renewable energy as an input. Instead of oil, the startup is interested in the heat from the wells, which helps multiply the efficiency of the energy generation system. 

How does the setup work?

Sage deployed its technology at an oil field in Texas for six months. As a first step, the company sent down deep drilling mud at high pressure into the rock. This was aimed at pushing slim fractures in the rocks apart.

During the day, excess renewable energy was used to pump water into the wells at high pressure, and the valve was closed to keep the water in. When the electricity demand was higher at night, the valve opened up, and the excess pressure from underground sent the water up through the pipes, which were used to spin a turbine and generate electricity. 

Sage found that the system works at 70-75% efficiency and can generate 200 kilowatts of energy over 18 hours or even one megawatt for 30 minutes. The team also measured seismic activity but found no traces during its operations. 

Increasing the efficiency of the system

The water is pumped into oil wells that run several hundred and a few thousand feet deep. At these depths, the temperature of the surrounding rock is significantly higher than the ground, reaching 180-220 degrees Celsius. The water storage in such wells transfers the heat to the water, which is also let out when the valve is opened.

By putting a heat exchanger in the harvest setup, Sage managed to get even more energy out of the system than it had put in, as the battery efficiency now rose to 200%.

Further, the company proposes that in areas where multiple wells are located close, the heat exchanger can remove the heat from the water, which can then be pumped into an adjacent well to heat up. The measured fluid losses in Sage's setup were just two percent, and recycling the water makes the system even more sustainable in the long run.

The estimated cost of renewable energy storage in such a format is at par with pumped hydro storage and lithium-ion batteries. However, it does not require creating new infrastructure or mining new lithium in the process.

“We have cracked the code to provide the perfect complement to renewable energy, said Cindy Taff, CEO of Sage Geosystems, in the press release.

“The opportunities for our energy storage to provide power are significant. We can interconnect with power grids or develop island/microgrids with a cleaner energy solution that is proven and ready to scale.”