CAES plants compress air and store it underground when energy demand is low and then extract the air to create electricity when demand is high. But startup costs currently limit commercial development of these projects, the scientists said.
The researchers proposed a new geothermal-assisted compressed-air energy storage system that makes use of depleted oil and gas wells - the Environmental Protection Agency estimates there are around 3.9 million in the United States - and found it can improve efficiency by 9.5% over the existing technology. This means a larger percentage of the energy stored in the system can be recovered and turned into electricity, potentially boosting profits for operators.
"This improvement in efficiency can be a game changer to justify the economics of compressed-air energy storage projects," said Arash Dahi Taleghani, professor of petroleum and natural gas engineering at Penn State and corresponding author on the study. "And on top of that, we could significantly avoid the upfront cost by using existing oil and gas wells that are no longer in production. This could be a win, win situation."
Reusing depleted oil and gas wells would allow operators to access geothermal heat in hot rock formations underground, eliminating upfront costs of drilling new wells and potentially making the technology more appealing to industry, the scientists said.
Gases like compressed air increase in pressure as temperatures increase, meaning the heated wells could potentially store more energy, according to Taleghani. When electricity is needed, the heated, compressed air is released, driving a turbine to produce power.
"Without taking advantage of the geothermal setup, you could not get enough encouraging numbers," Taleghani said, explaining that the team used numerical modeling simulations to find that placing CAES systems in abandoned oil and gas wells significantly increased the air temperature in the systems. "And on top of that, drilling new wells may not justify the economics of this type of storage. But by combining these two factors, and by going back and forth through modeling and simulation, we found this could be a very good solution."
Energy storage options like CAES are particularly important in the transition to clean energy, according to the researchers, because they help address the intermittent nature of renewable sources. By storing excess renewable energy and releasing it when needed, energy storage contributes to grid stability and reliability.
"The problem is that sometimes when we need energy, there is no sunshine or there is no wind," Taleghani said. "That's a big barrier against further expansion of most of the renewable energy that is available to us. That's why it's very important to have some storage capacity to support the grid."
Repurposing depleted oil and gas wells may also help mitigate potential environmental impacts of abandoned wells and potentially provide new job opportunities in areas with rich energy industry traditions, the researchers said.
In Pennsylvania alone, regulators estimate there are hundreds of thousands of orphaned and abandoned wells. If these wells are improperly plugged, or damaged, they can leak methane into the atmosphere and groundwater.
"If we use existing wells, we are basically hitting two birds with one stone," Taleghani said. "First, we are sealing these wells. That stops any potential leaks. And then if we are repurposing these wells for energy storage, we are still using the infrastructure that is in place in these communities. It can potentially maintain employment in the area and allow communities to be part of the energy future."
This research was conducted as part of the Repurposing Center for Energy Transition (ReCET) at Penn State. The center seeks to repurpose fossil energy infrastructure for energy transition applications, especially in legacy energy communities.
Also contributing from Penn State were Derek Elsworth, G. Albert Shoemaker Chair in Mineral Engineering and professor of energy and geo-environmental engineering, and Qitao Zhang, a postdoctoral scholar, both in the John and Willie Leone Family Department of Energy and Mineral Engineering.
Research Report:Underground energy storage using abandoned oil and gas wells assisted by geothermal
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