Waste Heat Recovery in Data Centers


A Hot Topic

Design Topic

The information technology arena is at a crossroads. While technologies can make the world a more efficient place, the facilities that support such transformative advancements have historically been energy intensive.

As server densities have increased, cooling demands have also grown to handle the increased heat rejection of the servers. Research by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and others has demonstrated that servers will operate reliably at room temperatures when coupled with an enclosed hot aisle. These findings have opened the door to new techniques in data center heat rejection, including the potential for data center waste heat recovery. To take advantage of this, however, designers and engineers need to adjust how they approach data center conditioning – thinking in terms of heat rejection (and recovery), as opposed to simply cooling.

The concept of heat recovery is not new, and has been proven in other high energy use facilities such as laboratories. Typical laboratory systems take energy from the exhaust air to precondition outside air. A similar concept exists for energy recovery ventilators in office buildings. However, the application of waste heat recovery in data centers is impacted by two major obstacles: the scale of the installation and the physical separation (or isolation) of the data center to adjacent facilities which could utilize the rejected heat.

Data centers can consume large amounts of power coupled with a tremendous amount of waste heat.  Comparatively, a typical office building uses much less heat. Therefore, a data center will often have a significant surplus of waste heat, which may not justify the increased cost of distribution systems when paired with a typical office building. The best design approach is to couple the data center with other facilities or systems that have a high heating demand in the form of a laboratory or central campus heating system.


At the Energy Systems Integration Facility (ESIF) at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) in Golden, Colorado, SmithGroupJJR’s architects and engineers have taken steps to do just that. As one of the nation’s most efficient data centers (PUE of 1.06), the ESIF building uses waste heat from the data center to provide heating throughout the building and its associated research laboratory. Waste heat from servers is collected in the form of 95F degree water, which is then transferred to the building heating water system.

The building’s demand for heating exceeds the data center waste heat available, but only during peak design conditions and with the initial data center load. As the data center grows, its volume of waste heat will eventually surpass that of the building needs. Even with initial data center loads, the building does not require the same level of heating year round. In summer months, the laboratory is the primary user of heat, but at a much reduced level. To maximize the potential waste heat recovery, the project made provision for connection to a future low-temperature campus heating system.

The NREL ESIF project represents what is possible when data center waste heat is integrated into building and campus heating water systems. The approach not only increases the efficiency of the data center, it increases the efficiency of the building while conserving our limited resources.  While it is not possible for all data centers to take advantage of this approach, this technique for waste heat recovery is not limited to data centers alone. Manufacturing facilities with large volumes of process waste heat may also benefit from a similar approach to provide supplemental heating to adjacent buildings. In the end, the goal is the same – to increase energy efficiency and recover as much waste heat as possible.


Eric Kirkland
Energy Systems

Eric Kirkland