Technology Overview: Thermal Energy Storage (TES)

October 17, 2013
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In the context of California’s permanent load shifting (PLS) program, the term “Thermal Energy Storage” (TES) refers to the storage of cooling capacity in order to shift electrical demand to off-peak hours. Typical thermal energy storage systems work in conjunction with air distribution systems and rely on water chillers or ice-making machines to produce cooling capacity. The minimum set of equipment required in a TES system includes a cold storage tank along with the pipes, valves, and pumps to control the exchanges between the HVAC system and the storage tank

Image: Large chilled water TES tank. Burns and McDonnell

Types of Thermal Energy Storage (TES) Systems

Several types of TES technologies are included in the PLS program. Three notable types are particularly well-established and are based on the medium used to store the cooling capacity:

  • Chilled water systems use water as the storage medium, and have the largest footprint due to the space required for large water storage tanks.
  • Ice storage systems use ice as the storage medium, and require a smaller footprint than chilled water systems as they offer a greater density for thermal storage.
  • Phase-change material systems use eutectic salts as storage medium, and also require a smaller footprint and offer greater storage density than chilled water systems.

TES systems can be used for a variety of facility types, but are typically most economical in commercial and industrial facilities that use electricity under time of use rates. TES systems take advantage of the differences between on-peak and off-peak electricity rates and demand charges.  The shift in electricity consumption contributes to the cost effectiveness of installing and operating TES systems.

Thermal Energy System Cost-Related Benefits

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Thermal Energy Storage (TES) is the most popular way to implement the demand-side energy management strategy known as 'Permanent Load Shifting' (PLS).

TES systems help building owners to lower their annual operating costs. TES systems can:

  • Reduce Power Consumption - With thermal energy storage, chiller operation is typically shifted to a period of time when ambient temperatures are cooler, such as the night. As a result, chillers operate more efficiently and consume less energy for a given cooling load.
  • Lower Usage Costs – Time of use (TOU) rates have been established by many utilities as a means of reflecting higher-cost electricity supply, and can encourage customers to use less energy during peak hours. Operating chillers when off-peak rates are in effect lowers the cost of electricity used for cooling.
  • Lower Demand Charges - Most industrial and commercial utility rates include monthly “demand charges” based on the maximum power (kW) used by the customer each month during certain parts of the day. By operating the TES system, the chiller plant runs during the utility's off-peak period, when demand charge rates are often significantly lower than the on-peak period. Thus, shifting the chiller load to the lower rate period lowers the total monthly demand charges.   

Lower Capital Equipment Purchases - TES systems are often a cost-effective alternative to purchasing expensive chiller plant equipment in new construction and facility expansion projects. If configured properly, smaller-capacity chillers can be used to store an amount of thermal energy equivalent to what larger-capacity chillers could produce operating without the TES system. Utilizing a TES tank can lower capital outlays.


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