Epic Systems has drilled about 6,100 wells, 90 to 240 meters (about 300 to 800 feet) deep, that heat and cool about 40 buildings.
Summer heat from a 3.5-megawatt data center is stored in the bedrock and heats offices, hot water and walkways in winter.
In Bonn, a heat storage system in the groundwater has supplied 60 to 80 percent of the winter heat to offices, a hotel and a data center since 2009.
Heat all year round, demand in winter
Servers in data centers produce heat around the clock, every day of the year. Cooling accounts for about 30 to 40 percent of data centers' electricity use. The heat that is removed is usually released straight into the air.
Data centers have become more energy efficient. We have previously written about how a 550 percent increase in data centers led to only 6 percent higher electricity consumption. But the heat from the servers remains.
Buildings in most of the US need heat only about four months a year. Sending server heat directly to a neighbor therefore works in February, but does no good in July.
One way is to store the heat in the ground. That is what Moones Alamooti, assistant professor of energy and petroleum engineering at the University of North Dakota, writes in The Conversation. A little way below the surface, the temperature stays close to the local annual average all year. It can be around 10°C (50°F).
6,100 wells in Wisconsin
Software company Epic Systems, which makes electronic medical record systems for health care, has built such a system over two decades. On its campus in Verona, Wisconsin, Epic has drilled about 6,100 wells, 90 to 240 meters (about 300 to 800 feet) deep. They are in four borefields and heat and cool about 40 buildings with a total of about 840,000 square meters (about 9 million square feet) of floor space.
Among other things, the system cools a 3.5-megawatt data center, according to the US Department of Energy. A stormwater lake and a lake in an old quarry are also connected to the network.
The rock holds the heat for months
Standard geothermal heat pumps draw heat from the ground in winter. Epic's system does more than that. In summer, warm water is pumped down through the wells and heats the rock around them.
Rock conducts heat slowly. A little heat leaks away right away. But when a large volume of rock is heated, the heat can only escape through the edges of the heated area. That is why most of it stays for several months.
In winter, water is pumped back through the same wells and picks up the stored heat. Heat pumps raise the temperature. The heat is then used to warm offices, provide hot water for the cafeterias and melt snow on the walkways.
The heat from the servers is about 60°C (140°F). That is too low to drive a turbine and make electricity. But it is enough to heat buildings with the help of heat pumps.
A quarter less energy
Epic's buildings use about 25 percent less energy than comparable buildings in the same climate, according to the Department of Energy. The University of Wisconsin-Madison has monitored the system since it was built. Among other things, the researchers measure air and ground temperatures and how the groundwater moves.
James Tinjum, a professor of engineering at the university, has calculated how much energy the system requires from manufacturing to demolition, including the diesel used for drilling. According to his calculation, the energy savings paid back that energy in just over a year.
Old oil wells and German data centers
The same principle can be used in more places. The US has more than 141,000 documented abandoned oil and gas wells. Alamooti and colleagues have calculated how unused wells in California could become heat storage without replacing the steel casing.
A German study examined 19 data centers in Germany. Of these, 13 are located where the ground is suitable for storing heat in the groundwater. In Frankfurt, six data centers could cover about 13 percent of the city's residential and commercial heating demand.
In Bonn, the method is already in use. Since 2009, a heat storage system in the groundwater has supplied 60 to 80 percent of the winter heat to an area with offices, a hotel, a health center and a data center.
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