In 1996, the U.S. Department of Veterans Affairs North Texas Health Care System contracted DN Tanks to construct a 3.3 MG Thermal Energy Storage (TES) tank for the purpose of saving energy costs by taking advantage of the time-of-use electric rates. The TES system was designed to shift the electric load of the chillers and associated cooling equipment from the on-peak periods (daytime), to the off-peak periods (night time). Carter-Burgess provided the design of the TES System that included TES system that included a stratified chilled water storage tank with a thermal energy storage capacity of 24,600 ton-hrs of cooling. The TES tank was integrated into the district chilled water system that provides cooling for this 84 acre health care campus with over 2.5 million square feet of conditioned space.
In 2000, the electric utility market in Texas was deregulated and the time-of-use electric rate was no longer as attractive as a fixed price. The electric rate changed; however, the need to utilize the TES tank on a daily basis continues. Per Rick Hart, CEM and Energy Manager for the VA North Texas Health Care System, the tank provides cooling during the day to help maintain the electric demand level below 9 MW (9000 kW) and a load factor of > 70%, which is a consideration for electric transmission and delivery charges. Also, due to expansions that have taken place at the campus over the past ten years, the peak cooling load on the chiller plant has increased. Therefore, on the hottest day during the summer, it is advantageous for the TES tank to operate in conjunction with the other chillers in the plant in order to provide dependable cooling for the campus while avoiding a large capital expense for adding more chillers. Also, the TES tank gives the engineering and maintenance staff the flexibility to take the physical plant down for several hours to perform emergency or routine service without interrupting the chilled water supply provided to the campus during high cooling demand periods.
The TES tank at the VA Hospital in Dallas, TX has been operating continuously for the past ten years providing energy cost savings, additional cooling capacity to handle the needs of an expanding campus, and “cooling insurance” by providing chilled water during periods of planned and unplanned downtime of the central plant equipment.