Data centers make heat all year long – here’s how the ground can hold onto it until winter

The ground absorbs and releases heat much more slowly than air. At depths of a few hundred feet, the temperature remains stable year-round, close to the local yearly average. For example, in Verona, Wisconsin, the ground stays around 50 degrees Fahrenheit (10 degrees Celsius) all year. This property is why basements often feel cool even on hot summer days. The ground’s ability to retain heat at a steady temperature makes it useful for storing excess heat generated by activities like running data centers, which produce heat continuously throughout the year. Unlike air, which heats up and cools down quickly, the ground acts as a natural thermal battery, keeping warmth available for later use.

Epic Systems, a health software company in Verona, Wisconsin, has developed a system that stores heat underground for later use. The company drilled approximately 6,100 wells, each 300 to 800 feet deep, to create a geothermal network. This system heats and cools about 40 buildings on its campus using waste heat from a 3.5-megawatt data center. The system works by pumping heated water from the data center through pipes deep underground in summer, warming the surrounding rock. In winter, the process reverses: cooler water is pumped back through the pipes, absorbs the stored heat, and returns to the surface to warm buildings. This approach is unusual because it stores heat in summer and retrieves it months later, addressing the timing mismatch between when heat is produced and when it is needed.

Underground heat storage relies on the principle that heat escapes slowly from large volumes of material. When heat is injected into a large volume of rock, most of it remains trapped because heat can only escape through the edges of the warmed zone. In summer, water heated by servers is pumped through underground pipes, warming the rock around them. Rock is a poor conductor of heat, which helps retain the warmth. In winter, water is pumped back through the same pipes, absorbing the stored heat and carrying it to the surface. The water returns at a lukewarm temperature, around 59 degrees Fahrenheit (15 degrees Celsius), which is still useful for heating buildings. This method does not require a tank or battery; the rock itself acts as the storage medium.

The heat recovered from underground storage is used for various purposes. At Epic Systems, the heat warms offices, supplies hot water to dining halls, and even melts snow off sidewalks. While the temperature of the recovered heat (about 140 degrees Fahrenheit or 60 degrees Celsius) is not high enough to generate electricity, it is efficient for heating applications. A heat pump can move large amounts of heat with relatively little electricity, making this method energy-efficient. The system reduces the campus’s energy use by about 25% compared to similar buildings in the same climate. However, if more heat is stored underground each summer than is retrieved in winter, the rock gradually warms over time, reducing the system’s effectiveness.

One major challenge of underground heat storage is the risk of overheating the rock. If more heat is injected into the ground than is extracted, the rock around the boreholes slowly warms up year after year, eventually reducing the system’s ability to store heat effectively. To address this, Epic Systems drilled additional boreholes as the campus expanded, spreading the heat over a larger volume of rock. Another challenge is the need for proximity: the stored heat is only useful for buildings located near the boreholes. Additionally, the scale of modern data centers, which can range from 100 to 1,000 megawatts, poses a challenge for underground heat storage systems designed for smaller facilities.

The U.S. has over 141,000 documented orphaned oil and gas wells that could potentially be repurposed for underground heat storage. Researchers have proposed converting these idle wells into thermal storage systems without replacing their steel casings. This approach could reduce costs and environmental impact by reusing existing infrastructure. However, most states currently regulate wells either as oil and gas wells or as plugged wells, with no clear category for heat storage. Researchers have suggested creating a streamlined permit process to allow wells to be used for this additional purpose. Repurposing these wells could help manage the growing heat generated by data centers while reducing energy costs and water use in local communities.