Energy-saving experimental geothermal battery that heats homes came out of Oak Ridge
A modern remix of old technologies that cuts home energy bills has the potential to utterly transform homes in the future, and the system was created in East Tennessee's own Oak Ridge National Lab.
Scientists have developed prototype geothermal “batteries” that, unlike conventional batteries, actually tap and store the heat energy of the Earth to provide heating, cooling and hot water.
The really attractive side? Unlike natural gas or oil furnaces, there are no emissions and no household pollutants like carbon monoxide.
“This is not a small thing,” said Bob Wyman co-founder of Dandelion, a home geothermal company. “This is something you might see installed in tens of millions of homes around the country.”
The geothermal battery is a device that uses water tanks, the ambient heat of the Earth and heat pumps (like those you might find in a refrigerator) to maintain a reservoir of hot or cold water that can be used to heat or cool the house. By tapping into and storing the Earth’s heat, the geothermal battery can run at high efficiency regardless of the weather.
“I don’t know if you’ve ever visited Ruby Falls Cave or Mammoth Cave,” said study author Xiaobing Liu, “If you go in the summertime you feel immediately that it’s very cold. In the wintertime, it’s warm.”
That's because the temperature underground is fairly constant. Most of the sun’s energy is absorbed by the ground during the day. Soil holds heat better than air and releases this energy slowly at night. This keeps the temperature underground stable year-round.
In Tennessee it's roughly 50 to 60 degrees Fahrenheit at a depth of 10 feet. Liu’s prototype enables buildings to tap into this heat to keep internal temperatures consistent.
“In the summer you can remove heat from your house to the ground. In the winter you can attract heat from the soil and raise the temperature [inside],” explained Liu.
If this sounds familiar that’s because it is. A version of ground-heat exchangers is available on the market for installation, albeit at considerable cost.
“It’s not the cost of the material, it’s the cost of driving a drill out to somebody’s house,” said Bob Wyman. He explained that most ground heat exchangers require specialized equipment to drill the holes for underground pipes. These pipes are deep, up to 300 feet, to get enough surface area to allow the water or air to become the temperature of the ground.
But Liu’s geothermal device does several things differently. First, it uses a shallow hole, cutting the expensive boring or digging process substantially. Second, the device uses a “heat pump,” a device like one in your refrigerator or air conditioner to push heating or cooling energy into the water for storage. The pump is active when renewable energy is plentiful, and electricity is cheap.
Third, and most important, this design allows home energy systems to store extra heat within an inner tank that incorporates “phase changing material” to boost the heat capacity of the tank. Shifting from a solid to a liquid takes far more energy than simply raising or lowering the temperature. Phase change materials can be customized to absorb more heat at specific temperatures.
“As long as you pick the temperature (of the phase change) right, it massively increases the BTUs you can store in the device,” said Wyman.
In a 1,000-gallon tank with a phase change material, you potentially could store the same heat energy as 124,000 gallons of water, Wyman explained. The heat pump can store more energy in the battery than water would otherwise be able to store. And because the Earth is a more consistent temperature than the outside air, the heat pump works far less hard to draw energy into the battery.
Phase change materials have been in use for at least a decade in diverse applications, from medical care to home climate control.
“None of these technologies themselves are new,” said Sunil Mehendale, associate professor of mechanical engineering at Michigan Tech. “But combining all these technologies together in one structure, that’s what brings the novelty.”
“I would really like to see how a full-scale device stacks up against current technology,” Mehendale added.
Liu’s initial results are promising. But it’s a long way away from being put in a home. Full-scale tests are only in the planning stages. If it works as predicted, then home heating and cooling will become much better for your wallet, the climate and your health.
“Work like this is vital to the future well-being of our society,” said Wyman, “It’s really at the forefront of creating a world that we want our grandchildren to live in."
Source: Knoxville News Sentinel, by Vincent Gabrielle
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Published February 7, 2020