Thermal Storage for Big Spaces

A relatively new secondary refrigerant is being successfully applied in large-scale cooling applications, both as a heat transfer fluid and as a thermal energy storage (TES) medium for low-temperature stratified TES. Not only can this aqueous fluid be formulated in various concentrations to meet desired system operating temperatures, but it also exhibits useful properties that make it an attractive alternative to traditional brines and refrigerants used for large applications.

This refrigerant is a water-based solution whose primary dissolved component is a common corrosion inhibitor. It is generally employed at relatively low concentrations of total dissolved solids (TDS), with typical concentrations from 3- to 7-weight-percent TDS, though concentrations to 15-weight-percent TDS or more are also practical. Due to relatively low dissolved-solids concentrations—where the fluid is primarily water—the refrigerant exhibits thermophysical properties that are relatively close to those of pure water, and therefore, generally quite attractive. Table 1 (p. 50) is a comparison of properties for 3- and 7-weight-percent concentrations of this refrigerant with those for plain water and for typical minimum concentrations (20- and 30-volume-percent) of ethylene-glycol in water.

Table 1 illustrates that at its typical operating concentrations, the fluid provides less freezing point depression than do ethylene-glycol/water solutions at their typical operating concentrations. Consequently, its formulations are not appropriate for applications that require significant freezing point depression, i.e., below approximately 10

Nevertheless, the fluid has found increased application in the past 10 years. A comparison of all fluid properties—except freezing point—listed in Table 1 demonstrates the advantages of this secondary refrigerant. For example, it exhibits slightly less rise in specific gravity, somewhat higher specific heat and markedly lower viscosity, especially at reduced operating temperatures. And there is much higher thermal conductivity, essentially equal to that of plain water, with implications for improvements in chiller and heat exchanger performance and cost.

Table 2 (p. 52) compares corrosion rates for common materials exposed to various fluids. Generally, corrosion rates below 1.0 mil per year are considered acceptable. The corrosion rates for the 7-weight-percent secondary refrigerant fluid have been confirmed via ongoing corrosion coupon testing in an actual commercial installation over a 10-year period from 1994 through 2003.

The use of this fluid at Chicago’s McCormick Place Exposition Center since early 1994 provides the data that attests to its long-term performance. Reported data include not only the fluid’s corrosion coupon results, but also its chemical stability and microbiological activity. But first, a brief description of the McCormick Place system is in order.

Over the long haul

For 10 cooling seasons, a district energy system in Chicago has served the 5-million-sq.-ft. McCormick Place, the adjacent 800-room Hyatt Regency Hotel and other nearby facilities. A 123,000 ton-hour, 8.5 million gal., above-ground, welded-steel, stratified TES tank stores and delivers the fluid at supply and return temperatures of 30guration. The TES tank minimizes electric energy costs while also providing a levelized cooling demand to better match the plant’s trigeneration system: combined cooling, heating and power. The TES tank can deliver instantaneous cooling at rates up to 25,000 tons. Connected cooling customers are listed in Table 3 (p. 52).

The low 30°F supply temperature was an integral part of McCormick Place’s South Hall (opposite page) design for low temperature air distribution, which resulted in a seven-figure capital savings associated with the air-side system. The large Delta T—24°F—resulted in a relatively compact TES tank with a footprint of only 0.5 sq. ft. per ton, much less than that of a conventional chiller plant. Moreover, the large Delta T combined with an economical above-ground welded-steel tank and affordable low-temperature fluid resulted in a capital cost of under $200 per ton of peak capacity for the fully installed tank and fluid, far below the $1,000 to $1,400 per ton that is common for a conventional chiller plant capacity.

Currently, a peak connected cooling load of 21,000 tons is redundantly served using only 16,800 tons of chillers plus the TES tank. It is anticipated that the future West Hall of McCormick Place—to be similar in size to the South Hall—will also be interconnected to the existing low-temperature TES system. Through the 10 years of service to date, there has been no need for chemical replacement or ongoing water treatment—either for microbial or corrosion control—other than a single replenishment of tolytriazole, which is maintained at five to 10 parts per million (ppm) to control yellow metal corrosion, which is common in chilled water systems.

As mentioned above, data taken at McCormick Place attests to the long-term performance of this secondary refrigerant, with respect to corrosion inhibition, chemical stability and microbial control:

Chemical stability. An important aspect in the selection of any thermal storage medium is the chemical stability of that medium over time and over the repeated thermal cycling that it encounters during operation. The results of independent analyses of fluid samples taken during the McCormick Place TES system’s initial six years of operation indicate the long-term operational stability of the fluid, not only over an extended period of time, but also over a series of many hundreds of operating thermal cycles.

Corrosion inhibition . The fluid provides inherent inhibition against ferrous metal corrosion. The only supplementary additive is the tolytriazole for corrosion protection of copper and its alloys. Recounted below are typical excerpts from reports of the periodic testing of water chemistry and corrosion coupons, analyzed by an independent laboratory:

• June 1995: “The results were nothing less than exemplary … Corrosion monitoring data coming from all test methods on the primary loop indicate perfect control … The corrosion coupon data that we just collected confirms the superiority of the program in place. Specifically, metal loss rates were less than 0.01 mils per year for mild steel, 304 stainless steel and copper.”

• August 1995: “Corrosion control in this system can be summed up as being superior. Copper, mild steel, and 304 stainless steel all had metal loss rates &0.01 mils per year. That’s as good as it gets.”

• October 1995: “Negligible corrosion of stainless steel or mild steel based on both corrosion coupons and corrator results … Tolytriazole residual of 7.6 ppm protects all yellow metals … The results of corrosion … control of the primary and secondary systems have been consistently excellent.”

• July 1996: “Corrosion control in the primary system continues to be exceptional for carbon steel, stainless steel, and copper. Metal loss rates did not exceed 0.04 mils per year. We typically consider rates less than 1.0 mils per year to be outstanding. Furthermore, there was no pitting evident on any of the metal samples. Secondly, the water quality analysis supports the findings we are observing. The pH and tolytriazole are at levels necessary to provide complete passivation of ferrous and nonferrous metals.”

Nine years of independent corrosion coupon testing from the Chicago installation confirms results that are two orders of magnitude better than what is typically considered acceptable.

Microbiological activity control. In the McCormick Place system’s 10 years of operation, there have been no additions of the fluid chemicals or of any biocide chemicals. Recounted here are typical excerpts from reports of the periodic testing of water chemistry and microbiology, analyzed by an independent laboratory. On all dates tested, the system showed no signs of microbial growth or deterioration of system piping or loss of heat transfer at the heat exchangers is expected.”

Further independent microbiology testing was conducted in October 1999 on fluid samples drawn from the system, confirming that the 7-weight-percent solution still inhibits microbiological growth.

Benefits beget more applications

The ability of this alternative fluid to allow lower supply temperatures and larger supply-to-return Delta Ts provides numerous benefits in large cooling applications, both in the distribution system and in TES applications. The lower supply temperature can provide improved humidity and comfort control. It can also provide capital and operating cost savings when coupled with low-temperature air distribution. The larger Delta T reduces the size, capital cost and operating cost of distribution pumps and piping. And it reduces the volume, footprint and capital cost of TES tanks by as much as 30% to 40%, relative to conventional chilled water TES. The fluid also allows the use of efficient and economical stratified TES at supply temperatures below the 39°F limit associated with plain water. Furthermore, it provides an attractive option for phased TES growth through the conversion of a conventional chilled-water TES system to a low-temp fluid TES system, with an increase in storage and discharge capacities of 40% to 70%—with no increase in storage volume. Finally, operating experience has shown that use of the fluid essentially eliminates the need for any ongoing water treatment either for microbial or corrosion control.

In the past few years, numerous large chilled-water TES systems have been dual-specified. That is, they initially enter operation as conventional chilled-water TES systems. However, the tanks are pre-designed for future conversion to low-temp fluid service at a reduced supply temperature (28er than the one in Chicago.

In fact, the Orange County Convention Center in Orlando, Fla., recently doubled in size to become the largest such facility in North America. Rather than invest in a new 10,000-ton chiller plant addition, the county contracted with a local district cooling system developer to purchase chilled water for cooling needs. The developer has ownership and operating responsibility for the existing chiller plants at the convention center and at a large manufacturing facility two miles away.

Rather than add a new chiller plant, the developer instead interconnected the two existing facilities via two miles of 36-in.-dia. chilled-water supply and return mains and installed the world’s largest stratified TES system, which is recharged using otherwise idle nighttime capacity from the two existing chiller plants. When compared with the cost of a new chiller plant, this approach saved more than $5 million in capital cost and more than $500,000 per year in operating energy costs. The 160,000 ton-hour, 17.6 million gallon, above-ground welded-steel TES tank is designed to meet peak cooling loads of 20,000 tons and was fully installed at a cost of only $150 per ton. The system entered service during 2003. Pre-designed for potential future conversion from chilled water service at 40he district cooling network.

TES has its large-scale applications, and new options in secondary refrigerants are helping to make this so.

Comparison of Fluid Properties

	Plain Water	SoCool aqueous fluid 3% (wgt)	7%(wgt)	Ethylene-Glycol in water 20% (vol)	30%(vol)
Freezing Point (°F)	32.0	28.7	24.4	17.6	6.7
Properties at 60°F
Specific Gravity	1.00	1.02	1.05	1.03	1.05
Specific Heat (BTU/lb-°F)	1.00	0.97	0.93	0.91	0.87
Viscosity (cP)	1.12	1.22	1.35	1.86	2.49
Thermal Conductivity
BTU/hr-ft-°F)	0.34	0.34	0.34	0.28	0.26
Properties at 30°F
Specific Gravity	n.a.	1.02	1.05	1.04	1.05
Specific Heat (BTU/lb-°F)	n.a.	0.98	0.94	0.90	0.86
Viscosity (cP)	n.a.	1.9	2.05	3.14	4.33

Comparison of Corrosion Rates

	UNINHIBITED		INHIBITED
Corrosion Rate (mils per year)	Plain Water	30% (vol) Eth-Glycol	30% (vol) Eth-Glycol	7% (wgt) SoCool
Mild (Carbon) Steel	9.69	44.5	0.04	0.01
Copper	0.08	0.16	0.12	0.01

Cooling Load Growth of the Low Temp TES System in Chicago

Initial Service	Connected Cooling Customer	Approximate Cooling Peak
1994	McCormick Place Exposition Center, East Hall	4,000 tons
1994	McCormick Place Exposition Center, North Hall	4,000 tons
1997	McCormick Place Exposition Center, South Hall	8,000 tons
1998	Hyatt Regency McCormick Place Hotel	1,000 tons
2000	Private “Internet hotel” computer facility	3,000 tons
2000	Chicago MPEA (Metro Pier & Expo Authority) offices	1,000 tons

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