©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 11, November 2012.
By Blake E. Ellis, P.E., Member ASHRAE
About the Author
Blake E. Ellis, P.E., is a principal in the OnSite Energy & Power group at Burns & McDonnell, Kansas City, Mo. Ellis is a member of the Kansas City Chapter of ASHRAE and is the program subcommittee chair for TC 6.2, District Energy.
The Texas Medical Center in Houston is the largest medical center in the world. Thermal Energy Corporation (TECO) provides thermal utilities (chilled water and steam) to many of the buildings in the medical center, and in 2007 TECO had an 80,000 ton (281 MW) chilled water system serving the medical center. Master planning indicated that the cooling load demand for the campus would double to 160,000 tons (563 MW) over the next decade, so TECO sought the most cost-effective way to provide the increased quantity of chilled water to the medical center while maintaining a high level of reliability to serve critical campus needs.
Installing new chilled water production and thermal energy storage (TES) system were evaluated as methods to meet the increasing demand. A life-cycle cost analysis determined that installing TES in a load leveling scheme was the most cost-effective first step to meet the increasing chilled water demand, even without factoring in Electric Reliability Council of Texas’s (ERCOT’s) current pricing strategy (see Real-Time Pricing at right). This resulted in the selection of an 8.8 million gallon (33.3 million L) stratified chilled water storage tank that is 100 ft (30 m) in diameter and 150 ft (46 m) tall. To meet the increasing demand, the TES system was required to be in operation for the 2010 cooling season, followed by the operation of the first phase of the new East Chiller Plant for the 2011 cooling season (Figure 1).
To meet the requirement of being operational a full year before the new east chiller plant and to operate with the existing plants, the TES system had to function independent of the east chiller plant operation, eliminating the possibility of using a primary-secondary chilled water plant pumping scheme where the TES tank is located in the hydraulic bridge between the primary and secondary systems. Finding a way to connect the TES tank to the TECO system would become the most challenging aspect of the project.
The reason this project is unique is twofold. First is the height of tank at 150 ft (46 m), making it the tallest stratified chilled water storage tank in the world. Second is a result of integrating such a tall tank that is open to the atmosphere into a closed chilled water distribution system.
The tall tank height creates 65 psig (448 kPa) of pressure at the bottom of the tank on both the chilled water supply (CHWS) and chilled water return (CHWR) lines connected to the tank. TES tanks normally use a pump on either the CHWS or CHWR line and take advantage of gravity and the pressure differential between the TES tank and the chilled water system to drive water into or out of the tank. Table 1 shows the current system pressures, as well as the results if the system pressures were modified to allow for single direction pumping.
If the chilled water system pressure were raised, the system pressure would greatly exceed the pressure rating of the existing 40-year-old distribution piping, which was estimated to be 115 psig to 125 psig (790 kPa to 860 kPa). If the chilled water system pressure was lowered, the return system would be required to operate at approximately 5 psig initially, and under a vacuum as system loads increase. Therefore, for this installation, as result of the height of the chilled water tank, the connection of the tank into the system and the resulting chilled water system pressures the pumps are required to operate simultaneously (at least in some operating conditions) on both the CHWS and CHWR.
Using simultaneous dual pumping of CHWS and CHWR on a TES tank had been tried previously on other TES systems, but in each instance we studied, it was not successful. To connect this TES tank to the existing chilled water system, the design team needed to accomplish something that hadn’t been previously accomplished.
The system in Figure 2 shows the TES tank, the CHWS and CHWR pumps as well as the chilled water production facilities at the existing central plant and the future east chiller plant. One method of pump control considered was to measure the flow rate into and out of the TES tank. One pump speed would be lead and the other would follow the first pump by matching the flow rate. This is the solution that other installations had unsuccessfully applied in the past.
A reason for failure is the inaccuracy of the flow meters. A small difference of 1% of the flow (160 gpm [10 L/s] in this case) would result in the pumps attempting to push a large volume of water into (or out of) the closed chilled water system (with its fixed volume) to or from the TES tank. This situation would raise (or drop) system pressure, possibly causing pumps to cavitate, the tank to overflow, or pull air into the system. Causing the chilled water system pressures to rise very quickly would raise the system pressure above the rated capacity of the piping system, greatly increasing the chances of pipe failure.
Citation: ASHRAE Journal, vol. 54, no. 11, November 2012
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