Opportunity Knocks with Thermal Storage

By Jon Schwartz, Associate Principal, Energy and Power Solutions Group, Carter & Burgess, Inc., Fort Worth, Texas -- Consulting-Specifying Engineer, 6/1/2005

Control of a facility's cooling-load profile can provide financial and operational advantages: reduced energy costs, utility rebates/revenue, less and easier maintenance and increased capacity, among others. What's the secret? Chilled-water thermal-energy storage (TES).

The process involves producing and storing chilled water during low-cost or low-cooling-load periods, then using this stored cold water during peak-load periods. Such a system is recommended for consideration when a facility's peak cooling load occurs for a short or limited duration. In addition, if there are any utility-demand penalties or opportunities, or a major expansion is planned, TES should be evaluated. That being said, engineers must be aware that TES requires significant space for installation.

The TES theory involves a stratified chilled-water tank. Taking advantage of density differences, water of varying temperatures is separated into two layers: warm on top and cold on the bottom, with no hard barrier or baffle required. The separation area is called the thermocline. The key to maintaining the thermocline and warm/cold separation is the use of low-speed laminar diffusers at the top and bottom of the tank. These diffusers are ASHRAE-standard design elements available from various TES manufacturers.

Stratified chilled water, where space is available, offers advantages as compared to ice-storage systems. These advantages are related to simple design and operation—using standard, self-balancing chillers at standard temperatures without de-rating capacity or efficiency, which typically results in lower first costs.

In the deregulated energy market, an improved or controlled load profile is more attractive to providers, as a flattened, higher load-factor profile can be achieved. The chiller load can be moved around as needed to fit new tariffs or real-time pricing, allowing the building to become a market driver rather than a market victim. In addition, demand is reduced, which can significantly lower utility bills. Also, by running chillers at night, opportunities for better efficiencies (e.g., lower wet-bulb temperature and reduced part loading on chillers) are achievable.

With TES, a central plant can be reduced in size, resulting in fewer chillers, pumps and towers and less switchgear. This helps to offset the costs of TES and can also reduce maintenance costs, as there are fewer pieces of equipment. Service procedures can also be improved if chillers are run at night, as maintenance staff can attend to them during the day when they are not operating, further reducing maintenance costs. In addition, very low load conditions can be handled with the TES system rather than at inefficient operating points on a chiller.

TES can also increase the available capacity of the central plant, as maximum load would occur by having some of the chillers off. If additional capacity is required, chillers could simply be started during the on-peak period. Of course, doing so limits pumping capacity and the ability to recharge the TES system during the off-peak period.

Overall, a stratified chilled-water system is an option that needs to be considered if your goals are to reduce operating costs and improve system operations.