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PNNL studies storing wind power underground

A Pacific Northwest National Laboratory scientist is looking deep underground to give wind power the flexibility that could make it more practical.

In a joint project of the Department of Energy national lab in Richland and Bonneville Power Administration, Pete McGrail is looking at the possibility of using the volcanic rock beneath Eastern Washington to store energy.

“This particular storage technology is a long shot,” said Steve Knudsen, BPA account executive. But the potential benefits to the Northwest power system are significant enough to make it worth pursuing, he said.

Mid-Columbia winds blow the hardest in the spring, but that also is when high waters make power from hydroelectric generation abundant.

This spring, wind generators had to shut down because more power was being produced than the electric grid could handle. Water couldn’t be diverted from hydroelectric generators instead to reduce power production without creating potentially harmful water conditions for salmon.

“That’s what got me interested,” said McGrail, a Battelle fellow.

Other researchers are working on better batteries to store power. But that is not a big enough solution for the BPA power-production region, which could use 100 times the capacity.

Wind generation capacity interconnected to the BPA system is expected to almost double to 6,000 megawatts by the end of 2013, leading to more periods with more generation than the grid can handle.

There may be months in the spring and summer with 1,100 megawatts of excess power from all sources, both because of transmission constraints and also because demand is low in that season, McGrail said. That’s almost equal to the generation of the Columbia Generating Station, the nuclear power plant near Richland.

Turning to earth science seems the most viable option to McGrail to store power equal to a month’s generation from a nuclear plant, he said. In addition, it might be useful for helping balance power supply during the frequent short term peaks and dips in wind power generation.

“It may not be economical. That’s what we have to find out,” he said. “But in terms of scale that’s where we have to go.”

Most of the wind farms in southeastern Washington and northeastern Oregon sit on land above the Columbia River Basalt Extent, where lava flows left layers of solid basalt that sandwich other layers with as many holes as a sponge.

McGrail already is part of a research team doing field studies near Wallula to determine the feasibility of storing large volumes of carbon dioxide, a predominant culprit implicated in climate change, in the pockmarked layers between basalt.

Those same layers may be useful for storing compressed air. It’s some of the most suitable geology for the project in the world, Knudsen said.

Excess power would be used to compress air from 14.7 pounds per square inch in the atmosphere to 1,000 to 2,000 pounds per square inch and push it underground. When more power is needed, the compressed air could be brought to the surface, heated slightly, and used to spin a turbine to produce electricity.

Theoretically it could return 70 percent of the energy used for storage. The process could be done every few hours, with a short cycle preferred to keep chemical reactions underground from dropping oxygen levels too low to run turbines efficiently. But it might also be practical for storage for months at a time, Knudsen said.

The technologies involved are relatively well known, he said. Similar storage of compressed air is done in Germany and Alabama, but those use caverns formed from leaching salt out of underground salt formations.

Using the region’s basalt flows would cost less to develop because caverns would not have to be carved out.

It also could work more efficiently, McGrail said. As compressed air is pumped out of the salt formations, the remaining air depressurizes and the system becomes less efficient. But in basalt layers, water would be pushed out of the way to make a bubble of pressurized air. When the pressurized air is pumped out, the water would collapse back on the remaining air and help maintain pressure.

“In theory, we can keep the efficiency up,” McGrail said.

However, with no large, empty space to store the pressurized air, research will have to show that the air can be added and removed quickly and that the basalt will not fracture.

McGrail also is looking at basalt to use periodic excess power generation to improve underground thermal energy storage.

Power can be generated by pumping up water heated in basalt three or four miles underground. It’s warm enough to put through a heat exchanger to boil an organic fluid such as butane or propane, which have lower boiling points than water, and spin a turbine to produce electricity.

However, the process requires injecting cooled water back into the underground reservoir, which gradually cools off the rock that heats the water. The cycle gradually becomes less efficient.

The excess power produced during times of high wind power production – power that otherwise would be wasted – could be used to heat the water before it’s injected into the ground to keep the geothermal process efficient. During months when power would go to waste otherwise, the geothermal system essentially would be run in reverse to maintain its efficiency for power production the remainder of the year.

McGrail is conducting a yearlong feasibility study to determine if the compressed air and thermal storage in basalt are technically and economically viable. In addition to BPA, nine companies with a mix of technical, economic and regulatory experience also are supporting the study.

In a potential second phase, a site could be picked and exploratory test wells drilled.