Avoiding equipment damage from lack of cooling and high peak demand charges are among the many challenges faced by businesses. One solution is thermal storage, which maintains cooled liquid in large tanks for use during emergencies or to reduce energy use and save money in peak demand situations. During a power outage, Intel used its chilled water storage system to keep its servers operating for more than 15 minutes until the main chillers could be restarted. Pumps and fans continued operating because they were on backup power.
Thermal storage is also used to save money on peak demand charges by allowing cooling systems to operate at night, when energy prices are cheaper. During the day, thermal storage can provide some of the cooling, shifting chiller operation to lower cost, off-peak hours. Because chillers used for thermal storage run for long periods during off-peak hours, they can be downsized, producing the same amount of cooling while saving money on the capital costs.
The technology
Thermal storage systems use water, ice or some other type of liquid coolant, which is sent through a heat exchanger and stored in a tank. Reduced thermal storage efficiency (due to long storage times) and space constraints are limitations with large storage tanks. The high capital costs are offset by requiring smaller chillers and operating during lower cost, off-peak hours. Chiller power is also reduced during simultaneous tank discharge by raising the temperature set point of the supplied water and/or decreasing the flow rate of the water passing through the chiller which saves money.
Stored ice allows smaller quantities of water at a lower temperature. This saves space and reduces insulation surface area requirements. Once frozen, only a very small flow of cooling water is needed to maintain the ice, reducing the amount of energy required. The stored cooling capacity is available immediately and can be used as a backup when the chillers are offline.
At a large Phoenix data center facility, four-inch polyethylene spheres containing water float in a solution of water and glycol inside storage tanks. During the night, this solution is run through chillers and pumped back into the tanks, freezing the water inside the spheres. During the day, the chilled solution is pumped through a heat exchanger, chilling water in a separate loop used in the data center. The system provides a total of 24,000 ton-hours of thermal storage.
Going Forward
Thermal storage was popular in the 1980s and is being revived as a valuable resource for customers and APS. By assisting customers like you in your efforts to shift energy away from peak hours, you can participate in time-of-use plans and save money by using more energy during lower-cost, off-peak hours. It’s beneficial to serve our customers with the abundant solar energy that is generated in the middle of the day. We are currently helping several customers evaluate thermal storage for their operations and offer rebates (in addition to the financial savings that come from participating in time-of-use plans).