What's So Hot About Chilled-Water Storage?
By JOHN M. CHANEY, Jr., P.E. , Director, Mechanical Engineering,
Hayes, Seay, Mattern Mattern, Inc., Roanoke, Va.
If power companies around the world are still offering customer incentives to reduce peak demand, when it comes to larger chilled-water cooling systems, why are so many mechanical engineers ignoring the money-saving advantages and other significant benefits of chilled-water storage (CWS)?
Often, new facilities that seem particularly suitable for CWS are being designed to employ conventional, nonstorage cooling systems. Admittedly, there aren't that many papers available documenting the reputed savings, but many happy owners using thermal-energy storage (TES) know that, as compared to their previous cooling systems, they:
Moreover, when adapted properly to the circumstances, CWS also offers competitive front-end and life-cycle costs. Put together, that's a pretty hot case for CWS and TES.
Favorable off-peak, load-leveling power rates that lead to significant operating economies are the advantage most often cited for TES. Nevertheless, some CWS proponents believe that system selection should be determined by a thorough engineering analysis rather than by determining economies based on power company rebates and incentives alone. The same boosters of CWS also feel that the system's future must rely on competing with alternatives on a level economic playing field. More experience may well support those contentions, but existing energy- cost-savings potential-absent any artificial incentives-continues to be a significant near-term inducement in itself.
For example, a 1.5-million-gallon storage tank is estimated to provide an energy demand shift of 1.7 megawatts. With CWS tanks ranging from half a million gallons on the small end to nearly ten million gallons at the high end, the potential for serious money savings can certainly be an attention-getter.
For the hundreds of storage systems in use in the United States today, most of the "selling talk," as previously mentioned, has centered on documented first-year (and subsequent-year) energy savings, some as high as 8 percent to the owner. Again, while that is something to catch an owner's eye, focusing only on energy savings can be a misleading determinant in opting for a CWS cooling system. There are other compelling plusses that designers need to consider. It's useful to consider some of the other benefits CWS can bring to the table, especially over competing ice-storage systems.
CWS systems have consistently proven to offer enhanced operating reliability, greater management flexibility, fewer "bottleneck" situations and spare emergency capacity over ice-storage. Moreover, as hinted earlier, there can be strong capital cost savings and-because of the system's simplicity of operation-favorable life-cycle costs.
Thermal storage systems can reduce the size of heating, ventilation and air-conditioning (HVAC) equipment, further reducing initial capital costs for distribution pumps, piping, fans and ducts. This cost reduction over traditional HVAC componentry-in some cases-can be significant enough to fully offset the additional costs imposed by CWS storage tanks.
Fire safety follows
Often overlooked is the fire- and life-safety aspect of storage systems. Storing large amounts of water on site is an asset for fire protection and, in some CWS designs, the sprinkler-system water is incorporated into the overall CWS system. This can be a serious advantage in many developing industrial parks or sites where infrastructure such as municipal water and sewers are not in place or of sufficient capacity.
Furthermore, a CWS system designed to double as storage for fire-protection system water can reduce the size needed for water lines to a proposed industrial site. This reduction can be enough to merit the project a "go" from planning authorities in those situations where the jurisdiction can't provide water mains of sufficient size to meet site demands. Without question, large water-storage capacity definitely enhances a building or plant's fire-protection capability.
Of course, there are some "cons" to consider. Siting a 500,000-gallon or larger storage tank is unquestionably a challenge in some circumstances. This issue and the attendant construction costs are generally considered the major objections to designs incorporating CWS, whether installation is above or below grade. Working with a creative architect, however, can convert these cons into assets by designing the tank structures to blend in with, or even enhance the particular site.
CWS is not the answer in every large-scale space-conditioning situation, but it definitely is a good answer in a lot more cases than some designers apparently believe. The systems do tend to work better for district-cooling or industrial-process applications where demand exceeds 10,000 ton-hours, but it is also worth considering for retrofits or expansions of some cooling plants. In an expansion situation, CWS can also provide for increased cooling loads without increasing the number of chillers.
Total engineering analysis
Before beginning a design, a total engineering analysis should be conducted to develop a thorough understanding of system requirements. Basic areas to be investigated include:
Further, a detailed economic analysis should look beyond the costs of equipment, installation and construction to the broader picture, including all owning and operating expenses for both primary and secondary systems.
If designers take the time to develop the best solutions, more CWS systems are likely to be specified.
About the author
In addition to his P.E., John M. Chaney, Jr., also holds a registration as a fire protection engineer. In his fifteen years at Hayes, Seay, Mattern & Mattern, Inc., he has been responsible for mechanical/life-safety analyses and designs for research laboratories, education facilities and other campus-oriented structures for government owners and private businesses. Chaney currently serves as director of the mechanical engineering department.