Stashing heat underground to warm a winter day

Here in Minnesota, where winter warmth is sometimes hard to come by, a major experiment to store waste heat for use during cold winter months is under way.

If it works, it could have far-reaching implications for the nation's utilities, universities, and large commercial developments.

In the largest project of its kind, the University of Minnesota is beginning to test the feasibility, cost effectiveness, and environmental impact of storing heated water over time in an underground aquifer some 800 feet below the Twin Cities.

The plan of university engineers and geologists is to pump some of the existing water out, raise its temperature in a heat exchanger, and pump it back into the aquifer for a storage period of anywhere from eight days at the beginning to 60 days later on, when the water temperature will be hiked to 302 degrees F.

The university will use steam from its heating plant to warm the water. But project director James O'Gara says the potential exists for using not only waste heat but solar energy as well.

''Now the cost effectiveness of solar energy in the North is very dependent on having good storage, and man-made storage systems tend to be very costly,'' he says.

Those working on the project, which was built in December and subjected to warm-water testing for the first time in May, expect that as much as 80 percent of the heat injected into the aquifer can be preserved.

The university hopes by the experiment to eventually reduce its heating bills by 30 percent and cut its need for buying more boilers.

But project director O'Gara says the technology should be transferable by the mid-'80s to other large users of heat located over similarly slow-moving aquifers.

''Our job is to bring this from an art to the point where it could be commercialized,'' he says.

But 4he $2 million project, financed by the US Department of Energy, is not yet as good as done.

''There is tremendous risk in this type of project - that's why the federal government has been interested in putting the money up front,'' observes O'Gara. ''Most of the theories about the potential for aquifer storage are basically on paper.''

Indeed, in the early days of testing last month, one of the pumps used to inject the heated water back undeRground jammed. And university scientists testing for chemical changes in the water are finding that the heat tends to produce magnesium and calcium capbonates, which cause scaling in the heat exchanger and which could clog pore space in the aquifer and well sites.

O'Gara, who is an engineer, says that project leaders are looking into the possibility of softening the water or filtering it as a solution to that problem. He does not count it a major obstacle.

''The problems so far are soluble with present engineering and equipment,'' he insists.

Meanwhile, the university is not alone in looking at the efficiency of underground energy storage. China, a number of Western European countries, and a few other American universities have embarked on similar projects.

But many have been looking at the possibility of preserving cool rather than warm energy, and Minnesota's is the only test facility designed to go as high as 300 degrees F.

To launch the experiment, project leaders had to seek a waiver in Minnesota law that forbids injection of pollutants (including heat) into a natural resource. No fewer than nine monitoring wells have been built to keep a constant check on water quality in the course of the research.

''We're investigating high temperature thermal storage because very little research has been done on it,'' explains O'Gara. ''And nationally the high temperatures would probably be much more beneficial once the research is proven.''

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