Keepers of the wind in the Pacific Northwest
DOI: 10.1063/PT.4.0578
By Rachel Berkowitz
During his 10-year odyssey among the Ionian Islands, Trojan War hero Odysseus was lucky enough to meet Aeolus , the keeper of the winds. Aeolus showed him how to trap wind power to use later, whenever it was wanted:
Aeolus . . . flayed me a prime ox-hide to hold the ways of the roaring winds, which he shut up in the hide as in a sack—for Jove had made him captain over the winds, and he could stir or still each one of them according to his own pleasure. He put the sack in the ship and bound the mouth so tightly with a silver thread that not even a breath of a side-wind could blow from any quarter.
Scientists at Pacific Northwest National Laboratory (PNNL) and the Bonneville Power Administration (BPA) are also planning to trap and store energy generated by wind turbines. Instead of an ox hide, they will make use of the abundant porous rock in the Columbia River Basalt Group, a series of ancient solidified lava flows that extend across Washington, Oregon, and Idaho.
How to store wind energy underground
Wind generation capacity connected to the BPA electrical grid is expected to nearly double, to 6000 MW, by the end of 2013. But the peak generating times—that is, when the wind is strongest and produces the most power—aren’t necessarily the same times that the electricity is needed. So the energy generated needs to be stored.
The PNNL team, led by geologist B. Pete McGrail, is beginning a feasibility study on storing wind energy in the basalt’s ‘brecciated’ areas—high-permeability regions where two lava flows joined together. When it’s available, wind power will be used to compress air or to heat water. Those forms of stored energy will then be injected into the basalt. The impermeable bulk of the lava flow will thus correspond to the silver thread that Aeolus used to bind his ox-hide bag.
When power is needed, the pressurized air or hot water is returned to the surface. Pressurized air can provide about 40% of the power needed to drive a combustion turbine; the remaining 60% is supplied by natural gas. With geothermal storage, the hot water is flashed to steam and used to drive an engine that turns a generator.
To determine how to trap compressed air or heated water, scientists will use information from the lab’s extensive studies of supercritical carbon dioxide sequestration in basalt formations. Those experiments have already generated ideas about how to inject material and how to determine whether it stays in place.
For compressed air, ‘the thing we have to worry about is whether we have similar kinds of interactions [with the basalt rock] that we see in CO2. Do carbonate minerals form from injecting high-pressure air?’ asks McGrail. For example, reactions can happen in the basalt that consume the oxygen in compressed air, which can create problems in the turbines.
Economics are the key
The hope is that capturing and storing wind energy will improve overall energy efficiency. Storing wind energy geothermally would ‘be a game-changer,’ according to McGrail. Excess power from wind turbines could inject additional hot water into the reservoir and raise the temperature, thereby reducing the energy losses that arise in geothermal plants due to the large temperature differential from fluid to rock.
Compressing air for storage also results in a loss of heat, which needs to be replaced when the compressed air is expanded to generate electricity. ‘If instead of burning fossil fuels, you can store the heat that you would otherwise have lost on the compression cycle and restore that heat when you expand again . . . you increase overall efficiency,’ says Stephen Knudsen of the BPA.
McGrail expects that geothermal storage will operate on a cycle of weeks to months. However, the maximum amount of time the plant can run while using stored compressed air is 10–12 hours.
In the upcoming year McGrail and Knudsen will be conducting a detailed technical and economic analysis of the compressed air and geothermal options. They hope to identify an optimal storage location and an estimate of how much it would cost to implement.
“It’s not just generation or just transmission that matters: it’s the commercial model that will deliver cost-effective storage to market, for companies that will invest and need to get a return of their investment,” adds Knudsen. Issues to address include the expectations for plant flexibility, withdrawal, ramp rates, and storage volumes, and the economics of a storage facility when it is integrated into the Pacific Northwest power system.
And with the support of eight companies that have a broad mix of technical, economic, and regulatory experience in the power industry, McGrail and Knudsen have every reason to expect a positive report from the feasibility study.
Aeolus, keeper of the winds, may not be on call for the Pacific Northwest as he was for Odysseus, but it looks like PNNL and the BPA are doing a fine job without him.
