Photo by Zbynek Burival on Unsplash
We are currently adopting ultra-low-carbon energy sources at a lightning pace, and over the next few decades, they will become our primary source of electricity. But as we transition away from fossil fuels, grid-level energy storage becomes of utmost importance. You see, in order for an energy grid that uses mostly renewable power to remain stable and avoid experiencing regular blackouts, we need huge batteries capable of storing enormous amounts of energy harvested on sunny or windy days. However, these megabatteries are astronomically expensive and have a significant environmental impact due to their mining and chemical leaching. We need to find a solution if we want to meet our emissions targets without further damaging the planet or draining our wallets. Luckily, some brilliant researchers from the University of Illinois have a solution: recycling abandoned oil wells and turning them into geothermal batteries!
A thermal battery is similar to a thermos flask. It heats up a material wrapped in a highly insulting outer case to store energy and then converts this thermal energy back into electricity to discharge. Abandoned oil wells have a similar setup. These vast underground cavities don’t have any geothermal heating, but they do contain sponge-like rocks and minerals with excellent thermal conductivity, meaning they can store a large amount of thermal energy very easily. Moreover, the rock layers surrounding this porous rock are highly insulating, keeping any heat trapped. So, in some ways, geology has already built the thermal battery for us, hence why this concept has been dubbed a “geothermal battery.”
But it also turns out that the rest of the infrastructure needed to turn the abandoned oil well into a fully functional geothermal battery is already in place. A vast network of underground pipes and pumps has already been built and abandoned by the oil industry. As such, very little modification is needed to turn it into a geothermal battery.
So how would such a battery work? Well, either waste heat from nearby industries or surplus renewable energy will heat water to around 50 degrees Celsius (122 degrees Fahrenheit). This will then be pumped into the cavity of the well. Down there, it will heat the surrounding rock, effectively charging the battery. When power is needed, it will pump out the water, which will still be close to its original temperature. Then, using thermocouples, the thermal energy in the water will be extracted as electricity, and the now-cold water will be ready to be reheated and returned to the battery.
This idea has floated around for a while, but no one has really looked into how much it would cost or even how efficient it would be. That was until these genius researchers from Illinois showed up. They built a geothermal battery using an abandoned oil well and then analysed the results to see if this could become a viable technology. What they found was surprising!
In their test, they injected water preheated to 50 degrees Celsius (122 degrees Fahrenheit) 3,000 feet down into an old oil well, pumped the water back up, and extracted the energy from it. Ultimately, this geothermal battery had an overall efficiency of 82%. That means that if you put 1 kWh of energy into it, you can get 0.82 kWh back out. This was far higher than many expected, putting it on par with other lithium-ion alternative battery technologies.
The study also found that this system was economically viable, as it can produce an “average overall net cost of electricity generation of $0.138 per kilowatt-hour.” This is a weird way to assess the cost of a battery, as usually you would calculate the cost per kWh of battery capacity, not the cost per kWh it produces. This is likely because the study was looking at charging the battery with free waste heat energy from local industry. Using this technology in such a way makes it more akin to an energy source than a renewable battery, so it makes sense to measure it as such.