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CH-12-C063
Applications of Low Temperature Fluid (LTF) in Thermally Stratified Thermally Stratified Thermal Energy Energy Storage (TES)
John S. Andrepont
ASHRAE Member
Abstract Thermally-Stratified Chilled Water (CHW) Thermal Energy Storage (TES) is a commonly used technology in large TES applications for peak load management of cooling or air-conditioning loads. However, CHW TES requires a large unit storage volume (proportional to the TES capacity and inversely proportional to the CHW supply-to-return temperature differential); also, the minimum operating supply temperature for thermally-stratified CHW TES is approximately 39 °F (4 °C), the temperature at which water exhibits its maximum density. This paper presents the use of an aqueous fluid (water with dissolved chemicals) as an alternative storage medium in thermally-stratified TES. Such Low Temperature Fluid (LTF) TES, compared to conventional CHW TES, can operate with a larger supply-to-return Delta T (for a smaller unit storage volume) and a colder operating supply temperature. The LTF TES technology has been employed commercially in a wide range of TES applications since 1994, in systems with individual TES capacities ranging from very small (60 ton-hrs or 210 kW-hrs) to very large (123,000 ton-hrs or 432,500 kW-hrs), and operating with supply temperatures ranging from 30 °F (-1 °C) to 38 °F (3 °C). The paper presents: a discussion of the desirable characteristics of a LTF, a description of the LTF chemistry, thermo-physical properties of the LTF at various combinations of chemical concentration and temperature, examples of operating installations, including end-user types, ages, capacities, and temperatures, a representative example of thermal stratification operating data, data related to the long-term chemical stability of the LTF, data related to the long-term control, by the LTF, of corrosion and microbiological activity, and a discussion of the comparative typical advantages and limitations of LTF TES versus those of the traditional technologies of CHW TES and ice TES.
Presented to the American Society of Heating, Refrigerating and Air-conditioning Engineers ASHRAE Winter Meeting – Chicago, Illinois, USA - January, 2012
John S. Andrepont is the founder and president of The Cool Solutions Company, Lisle, Illinois, USA.
© 2012 ASHRAE 487
INTRODUCTION AND BACKGROUND A unique, low temperature fluid (LTF) has been developed and put into commercial use in applications of thermallystratified sensible heat Thermal Energy Storage (TES). But first, let‟s briefly review traditional cool TES technologies, th eir inherent characteristics and limitations, and the drivers for the development and use of LTF TES. Two distinct families of TES have seen substantial and successful commercial application for many decades: Latent heat TES, typically ice storage, in which energy is stored as a change in phase of the storage medium, and Sensible heat TES, typically chilled water (CHW) storage, in which energy is stored as a change in temperature of the storage medium. Thermally Stratified Chilled Water TES The various historical methods that have been employed in chilled water storage systems to maintain the desirable separation between warm (return) water and chilled (supply) water include: multiple tanks or the “empty tank method”; compartmentalized tanks, tanks in series, or baffle-and-weir tanks; internal diaphragms or membranes; and thermally stratified tanks (in which the density difference between the two temperature zones is employed to float the warm water on top of the cool water, thus maintaining thermal separation). During the 1980s and ‟90s, the use of thermal stratification grew to dominate the market for large chilled water (CHW) TES tanks. There are a number of significant advantages to employing thermally stratified storage, compared with the earlier alternative methods of chilled water TES, including the use of a single storage tank with no internal moving parts, leading to such benefits as: reduced capital cost; reduced operating cost; simplicity; reliability; energy efficiency; and proven performance (including the availability of turnkey equipment supply inclusive of guaranteed thermal performance). However, there are some inherent limitations to the application of stratified storage, notably: difficulty in designing for excessively rapid discharge (or charge) rates; high volumetric space requirements; lack of modularity; and a lower limit on the storage and delivery supply temperature of approximately 39.4 F (4.1 C), dictated by the temperature at which the maximum density of pure water occurs. Due to these inherent limitations, alternative media for thermal stratification have been, and continue to be, of interest for some specific applications. ALTERNATIVE MEDIA FOR THERMALLY STRATIFIED TES The numerous characteristics desired in an ideal thermal stratification medium are that it: is highly available and economical; has well-known and attractive thermophysical properties; is highly compatible with other materials; and is safe. Water provides the vast majority of these characteristics extremely well. It is unequaled in its virtually universal availability and low capital cost, as well as having excellent thermophysical properties. When somewhat lower than normal operating temperatures have been needed for heat transfer fluid applications (typically for freeze protection), the most common choices have been aqueous solutions, which provide properties that in most cases are similar to (though usually not as attractive as) those of pure water. The most common fluids have traditionally been aqueous salt solutions and water-glycol solutions. Each could also be used for low temperature thermal stratification. However, the corrosivity of sodium chloride and calcium chloride aqueous salt solutions, and the thermophysical properties and high capital costs of ethylene-glycol and propylene-glycol solutions, generally eliminate their consideration for use as low temperature stratified TES media. An alternative fluid exhibiting the desired characteristics of an ideal LTF was identified, developed, and placed in commercial service in multiple applications, the first being in 1994. This particular LTF is an aqueous solution in which the dissolved chemicals are primarily Sodium Nitrite (a corrosion inhibitor commonly used in chilled water systems) and Sodium Nitrate (an agricultural fertilizer commonly used in direct contact with soil), plus some buffer chemicals to maintain pH in the range of 9 to 10, and trace amounts of tolytriazole for corrosion control of copper and copper alloys. The LTF exhibits attractive thermophysical properties, only slightly less favorable than those of plain water, as listed in Table 1.
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Table 1. Concentration In Water (weight %) 0.0 0.0 0.0 0.0 0.0 3.0 3.0 3.0 7.0 7.0 7.0 15.0 15.0 15.0 15.0
Approximate LTF Thermophysical Properties (CB&I, 2005) Specific Heat (Btu/lb-F) 1.000 1.001 1.001 1.004 1.006 0.969 0.971 0.978 0.928 0.931 0.943 Freezing Point (F) 32.0 32.0 32.0 32.0 32.0 28.7 28.7 28.7 24.4 24.4 24.4 17.6 17.6 17.6 17.6 Nominal Minimum Thermal Storage Supply Temp (F) 39.4 39.4 39.4 39.4 39.4 35.0 35.0 35.0 30.0 30.0 30.0 20.0 20.0 20.0 20.0
Temp (F) 60.0 54.0 50.0 40.0 32.0 60.0 54.0 35.0 60.0 54.0 30.0 60.0 54.0 30.0 20.0
Specific Gravity 1.0000 1.0004 1.0006 1.0013 1.0007 1.020 1.021 1.023 1.047 1.049 1.053 1.103 1.108 1.111 1.120
Viscosity (cP) 1.12 1.23 1.30 1.53 1.75 1.22 1.32 1.78 1.35 1.43 2.05 1.65 2.33
0.888
The Thermal Conductivity of the 7.0 weight % concentration, at 68 F, is 0.343 Btu/hr-ft-F versus 0.345 for plain water. LOW TEMPERATURE FLUID (LTF) TES APPLICATIONS Some examples of thermally stratified TES installations using this LTF are presented in Table 2. These examples illustrate the broad applicability of the technology, with a wide range of end-use types, a wide range of locations and climates, TES capacities ranging from a low of 60 ton-hrs (210 kW-hrs) to a high of 123,000 ton-hrs (432,500 kW-hrs), and operating TES supply temperatures ranging from 30 °F (-1.1 °C) to 36 °F (+2.2 °C). The largest application has also been in continuous operation since 1994. Table 2. Examples of Low Temperature Fluid (LTF) TES Applications TES Capacity TES Supply/Return Temperatures 123,000 ton-hrs (432,500 kW-hrs) 30/54 °F (-1.1/12.2 °C) 90,000 ton-hrs (316,400 kW-hrs) 36/60 °F (2.2/15.6 °C) 40,000 ton-hrs (140,600 kW-hrs) 32/56 °F (0.0/13.3 °C) 60 ton-hrs (210 kW-hrs) 36/53 °F (2.2/11.7 °C) 3,292 ton-hrs (11,570 kW-hrs) 33/58 °F (0.6/14.4 °C) 9,300 ton-hrs (32,700 kW-hrs) 31/51 °F (-0.6/10.6 °C) 1,820 ton-hrs (6,400 kW-hrs) 34/60 °F (1.1/15.6 °C)
Application Type Location (Year of Initial Operation) Convention Center District Energy System Chicago, Illinois, USA (1994) International Airport Dallas/Fort Worth, Texas, USA (2004) University Campus (and Turbine Inlet Cooling) Princeton, New Jersey, USA (2005) Biotechnology Research and Manufacturing Berkeley, California, USA (2006) Manufacturing and Testing Facility Mexicali, Baja California Norte, Mexico (2007) Government Medical Center Detroit, Michigan, USA (2008) Commercial Building San Ramon, California, USA (2009)
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AN EXAMPLE ILLUSTRATES THE PERFORMANCE OF THE LOW TEMPERATURE FLUID A brief review of an actual representative LTF TES installation illustrates the initial and long-term performance of the low temperature fluid (Andrepont, 2000). The LTF performance has been documented through many years of successful, continuous, commercial operation in a 123,000 ton-hr (432,500 kW-hr), 8.5 million gallon (32,000 m3) thermal storage system in Chicago, Illinois. The TES system, which is part of a District Energy (Cooling and Heating) system, is capable of serving peak cooling loads of 25,000 tons (88,000 kW), utilizing chilled fluid supply and return temperatures of 30 and 54 F (-1.1 and +12.2 C). The fluid has been in operation since early 1994, exhibiting chemical stability plus control over corrosion and microbiological activity throughout its operating life, without chemical additions since initial system commissioning (other than modest incremental additions – less than 1% of the tank volume – at times of volumetric expansion of the interconnected piping network, as necessary to maintain the desired chemical concentration). The project earned the host convention and exposition facility the 1995 Energy User News “Energy Efficient Building Award” in the Thermal Storage category. The Low Temp Stratification Additive Various options were evaluated for the project, including no TES, ice TES, and alternative thermal stratification media such as aqueous chloride brines and water-glycol solutions. None were fully satisfactory or economical. An accelerated research and development program resulted in the identification, testing and use of a new thermal stratification fluid. The selected medium was the aqueous solution of Sodium Nitrate and Sodium Nitrite. It was formulated and installed in a 7 weight % concentration, appropriate to allow thermal stratification at a supply temperature of 30 F in the TES tank. The fluid‟s thermophysical properties are very nearly as attractive as t hose of pure water. For example, its specific heat is only slightly less than that of water, while its higher specific gravity combines with the lower specific heat to yield nearly unchanged storage capacity per unit volume. Viscosity is only slightly higher than that of pure water, even at the reduced operating temperatures. It needs only trace additives to provide the necessary levels of corrosion control. Most importantly, its capital cost is a fraction of that which would have been incurred with the use of water-glycol solutions. The method of utilizing this fluid as low temperature stratified cool storage in a thermal storage tank is protected by a United States Patent and by patents in various other countries (CB&I, 2005). The Results An extremely high design TES discharge rate (25,000 tons or 88,000 kW of instantaneous cooling load) was able to be accommodated, without undue complexity or cost in the stratification flow diffusers, through the use of a large operating supply-to-return fluid Delta T of 24 F (13.3 C) and the resultant decrease in the design flow rate. Storage tank volumetric space requirements were reduced to only 9 ft 3/ton-hour (0.07 m3/kW-hr), roughly half of the requirement for conventional chilled water TES. Modularity for incremental capacity expansions was addressed by sizing the storage to meet the initially smaller loads, while operating at conventional Delta Ts. The increased subsequent capacity was met simply by operating at larger Delta Ts. Also, the larger Delta T facilitated sizing the TES installation for future system load growth, while working within the physical limitations of the site, and at an attractive capital cost premium. Stable, predictable stratification and thermal performance have been achieved down to the design supply temperature of 30 F. (Even lower temperatures are achievable, if desired.) Total turnkey installed capital cost for the TES storage tank, inclusive of the low temperature stratification fluid, was approximately $40/ton-hour ($11/kW-hr), in 1994. Rated at its design discharge capacity, this represents an installed cost for chiller plant peaking capacity of approximately $200/ton ($57/kW), in 1994, which is a small fraction of the capital cost for equivalent conventional chiller plant capacity.
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Figure 1
123,000 ton-hr (432,500 kW-hr) LTF TES serving the Convention District in Chicago, Illinois.
LONG-TERM PERFORMANCE Multiple years of data from the initial 123,000 ton-hour (432,500 kW-hr), 8.53 million gallon (32,300 m3) application in Chicago, as well as data from other applications, confirm the performance of the low temperature thermal stratification fluid in several key areas, based on independent testing of fluid samples, including: 1. chemical stability over time and over hundreds or thousands of thermal cycles (Table 3), 2. corrosion control of various system materials, at extremely low rates (Table 4), and 3. control of microbiological activity, for LTF concentrations ranging from 3.0 to 7.0 weight % dissolved solids (Table 5). Chemical Stability Other than the initial charge of fluid in early 1994, no additions of the LTF chemicals have been made at the Chicago installation throughout the entire seventeen year period from start-up through the writing of this paper (other than following the expansion of the piping network, when a volume of water was added to the system, which diluted the chemical concentration, necessitating a modest chemical addition to maintain the desired concentration).
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Table 3.
Fluid Sample (date) Apr „94 May „94 Jun „94 Aug „94 Sep „94 Oct „94 Nov „94 Apr „95 Jun „95 Aug „95 Nov „95 Dec „96 Jul „97 Feb „99 Chem A (wgt %) 4.95 5.08 5.07 5.04 5.06 5.04 5.02 5.02 5.03 5.03 5.07 5.08 4.96 5.04 Chem B (wgt %) 1.81 1.92 1.85 1.80 1.79 1.78 1.83 1.72 1.73 1.79 1.73 1.76 1.76 1.76
Chemical Stability over Time and Thermal Cycling
Chem C (wgt %) 0.10 0.20 0.16 0.11 0.11 0.10 0.10 0.11 0.11 0.10 0.09 0.09 0.08 0.11 Chem D (wgt %) 0.10 n.a. 0.08 0.07 0.07 0.07 0.08 0.07 0.06 0.07 0.07 0.11 0.11 0.11 pH at 60 oF 9.38 9.60 9.39 9.39 9.37 9.37 9.38 9.35 9.38 9.35 9.32 9.38 9.40 9.26 Spec Gravity at 60 oF 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.048 1.047 1.048 Visc (cP) at 60 oF 1.36 1.36 1.33 1.34 1.35 1.36 n.a. n.a. n.a. 1.34 n.a. n.a. n.a. n.a. Freeze Point (oF) 25.7 25.7 n.a. n.a. n.a. 25.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
“n.a.” indicates that data is not available, due to testing not having been conducted. Corrosion Control The fluid provides inherent inhibition against ferrous metal (steel and iron) corrosion. The only supplementary additive is tolytriazole, maintained at approximately 5 to 10 parts per million for corrosion protection of cuprous metal (copper and copper alloys). Periodic testing of corrosion coupons and fluid chemistry at the Chicago installation were analyzed by an independent laboratory, throughout multiple years of system operation, illustrating exemplary results. Note that the measured corrosion rates are two orders of magnitude better than what is typically considered to be acceptable. Table 4. May 1995 Corrosion Coupon Analysis (corrosion rate in mils per year) Mild Steel <0.01 304 Stainless Steel <0.01 Copper <0.01 May 1994 Water Analysis pH Total Iron as Fe (ppm) Tolytriazole (mg/l) 9.40 n.a. 14.6 Corrosion Control Jun 1996 0.04 0.01 0.02 May 1995 9.40 n.a. 8.1 Sep 1997 - Sep 1999 <0.01 n.a. 0.01 Sep 1995 n.a. n.a. 7.6 Jun 1996 n.a. <0.05 8.0
Sep 1995 0.02 <0.01 n.a. Aug 1994 9.70 <1.20 7.3
“n.a.” indicates that data is not available, due to testing not having been conducted.
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Microbiological Activity Periodic testing of the LTF chemistry and microbiology at the Chicago installation was conducted and analyzed by an independent laboratory, including periodic comprehensive microbiological assays of fluid samples. Virtually no microbiological activity has been detected; and the detectable rates were orders of magnitude below levels of concern. Table 5. Microbiological Analysis Microscopic Observations Algae, green Algae, blue-green Algae, diatoms Bacteria, iron Bacteria, sulfur Fungi, mold spores Fungi, mold hyphae Fungi, yeast Mucilaginous substance Extraneous (particulate) matter Decomposed organic matter Protozoa Microbiological Assays Total aerobic bacteria (cfu/ml) Slime-forming bacteria (%) Nitrite-reducing bacteria May 1994 nil nil nil nil nil nil nil nil nil few nil nil 100 0 n.a. Microbiological Activity Aug 1994 nil nil nil nil nil nil nil nil nil rare nil nil <100 0 n.a. May 1995 nil nil nil nil nil nil nil nil nil few nil nil 100 0 negative Sep 1995 nil nil nil nil nil nil nil nil rare few nil nil <100 0 negative Jun 1996 - Sep 1999 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
“n.a.” indicates that data is not available, due to testing not having been conducted. Further independent microbiology testing was conducted on fluid samples drawn from the system. Results confirmed that the original 7.0 weight % solution, as well as even a notably lower concentration fluid (diluted to only 3.5 weight %), inhibited microbiological growth, even when microbes were introduced into the fluid samples. COMPARISON OF TES TEHNOLOGIES Table 6. Inherent Characteristics of TES Technologies (typical) Ice TES good good excellent poor fair good fair fair fair poor poor CHW TES poor fair poor excellent excellent poor excellent good excellent excellent excellent LTF TES fair good good good good excellent good good good excellent poor
Volume Footprint Modularity Economy-of-Scale Energy Efficiency Low Temp Capability Ease of Retrofit Rapid Discharge Capability Simplicity and Reliability Site Remotely from Chillers Dual-Use as Fire Protection
No single TES technology is best for all applications. Some rules-of-thumb are presented in Table 6 to contrast the
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typical inherent characteristic advantages and limitations of Ice TES, Chilled Water TES, and LTF TES (recognizing that these are only generalizations and that exceptions do occur). Note that each technology exhibits some poor and some excellent characteristics; thus, it is critical to understand the requirements of an application when selecting a TES technology. CONCLUSIONS AND RECOMMENDATIONS An additive used to produce an aqueous, low temperature stratification fluid has been identified and demonstrated through successful commercial applications. It has been in service since 1994, including in an extremely large TES installation in a District Cooling system. Numerous other installations have been in service in diverse applications and locations, in large and small capacities, and at various operating supply temperatures. The LTF storage medium has demonstrated the ability to mitigate the limitations inherent to conventional stratified chilled water TES: 1. Rapid discharge (or charge) rates - achieved by the use of higher fluid supply-to-return Delta Ts, 2. Volumetric space - reduced, inversely proportionately to the Delta T, 3. Lack of modularity - now addressed by incrementally increasing Delta T, and 4. Lower limit on storage and delivery temperature - now able to achieve 30 F supply temperature, or lower. Furthermore, seventeen years of operational experience and test data have demonstrated the fluid‟s: 1. Chemical stability over time and over hundreds and thousands of thermal cycles, 2. Corrosion control of typical system materials (at extremely low rates), and 3. Exemplary experience with microbiological activity (even with the fluid diluted to half of the initial concentration). When used as a low temperature heat transfer fluid to serve distributed cooling loads, the fluid reduces distribution pipe size and cost, and also reduces pumping capital and operating energy costs. Perhaps most significantly, by reducing the storage volume (by as much as half or more), system capital cost for new systems is substantially reduced. Also, retrofitting the low temperature fluid into existing chilled water storage systems dramatically adds capacity, economically, and without the need to increase the storage volume. Additional applications of this alternative, low temperature, thermal stratification fluid should be considered, including both new and retrofit applications. It will be most beneficial wherever low supply temperatures and/or high supply-to-return Delta Ts are desirable or necessary, or where higher thermal storage capacities per unit volume are desired. The LTF can provide a means of expanding the capacity of an existing chilled water TES system. And the LTF offers an enhancement to water treatment challenges for control of both corrosion and microbiological activity. LTF TES should be evaluated as an alternative to traditional Ice TES and CHW TES technologies during the initial planning and conceptual design stages of cooling systems for which TES is being considered. ACKNOWLEDGMENTS The author recognizes Mr. Mohammad Rantissi, Director of Energy Plant Operations with the Chicago MPEA (formerly with the Trigen Energy Corporation) for sharing the long-term performance data from their LTF installation. The author also recognizes the Chicago Bridge & Iron Company for the kind use of the data in Table 1 and the photo in Figure 1. REFERENCES Andrepont, J.S. 2000. Long-term performance of a low temperature fluid in thermal storage and distribution applications. Proceedings of the 13th Annual IDEA College/University Conference. CB&I. 2005. SoCool® low temperature thermal energy storage stratification fluid. Chicago Bridge & Iron Company document B7232-2.
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Applications of Low Temperature Fluid (LTF) in Thermally Stratified Thermally Stratified Thermal Energy Energy Storage (TES)
John S. Andrepont
ASHRAE Member
Abstract Thermally-Stratified Chilled Water (CHW) Thermal Energy Storage (TES) is a commonly used technology in large TES applications for peak load management of cooling or air-conditioning loads. However, CHW TES requires a large unit storage volume (proportional to the TES capacity and inversely proportional to the CHW supply-to-return temperature differential); also, the minimum operating supply temperature for thermally-stratified CHW TES is approximately 39 °F (4 °C), the temperature at which water exhibits its maximum density. This paper presents the use of an aqueous fluid (water with dissolved chemicals) as an alternative storage medium in thermally-stratified TES. Such Low Temperature Fluid (LTF) TES, compared to conventional CHW TES, can operate with a larger supply-to-return Delta T (for a smaller unit storage volume) and a colder operating supply temperature. The LTF TES technology has been employed commercially in a wide range of TES applications since 1994, in systems with individual TES capacities ranging from very small (60 ton-hrs or 210 kW-hrs) to very large (123,000 ton-hrs or 432,500 kW-hrs), and operating with supply temperatures ranging from 30 °F (-1 °C) to 38 °F (3 °C). The paper presents: a discussion of the desirable characteristics of a LTF, a description of the LTF chemistry, thermo-physical properties of the LTF at various combinations of chemical concentration and temperature, examples of operating installations, including end-user types, ages, capacities, and temperatures, a representative example of thermal stratification operating data, data related to the long-term chemical stability of the LTF, data related to the long-term control, by the LTF, of corrosion and microbiological activity, and a discussion of the comparative typical advantages and limitations of LTF TES versus those of the traditional technologies of CHW TES and ice TES.
Presented to the American Society of Heating, Refrigerating and Air-conditioning Engineers ASHRAE Winter Meeting – Chicago, Illinois, USA - January, 2012
John S. Andrepont is the founder and president of The Cool Solutions Company, Lisle, Illinois, USA.
© 2012 ASHRAE 487
INTRODUCTION AND BACKGROUND A unique, low temperature fluid (LTF) has been developed and put into commercial use in applications of thermallystratified sensible heat Thermal Energy Storage (TES). But first, let‟s briefly review traditional cool TES technologies, th eir inherent characteristics and limitations, and the drivers for the development and use of LTF TES. Two distinct families of TES have seen substantial and successful commercial application for many decades: Latent heat TES, typically ice storage, in which energy is stored as a change in phase of the storage medium, and Sensible heat TES, typically chilled water (CHW) storage, in which energy is stored as a change in temperature of the storage medium. Thermally Stratified Chilled Water TES The various historical methods that have been employed in chilled water storage systems to maintain the desirable separation between warm (return) water and chilled (supply) water include: multiple tanks or the “empty tank method”; compartmentalized tanks, tanks in series, or baffle-and-weir tanks; internal diaphragms or membranes; and thermally stratified tanks (in which the density difference between the two temperature zones is employed to float the warm water on top of the cool water, thus maintaining thermal separation). During the 1980s and ‟90s, the use of thermal stratification grew to dominate the market for large chilled water (CHW) TES tanks. There are a number of significant advantages to employing thermally stratified storage, compared with the earlier alternative methods of chilled water TES, including the use of a single storage tank with no internal moving parts, leading to such benefits as: reduced capital cost; reduced operating cost; simplicity; reliability; energy efficiency; and proven performance (including the availability of turnkey equipment supply inclusive of guaranteed thermal performance). However, there are some inherent limitations to the application of stratified storage, notably: difficulty in designing for excessively rapid discharge (or charge) rates; high volumetric space requirements; lack of modularity; and a lower limit on the storage and delivery supply temperature of approximately 39.4 F (4.1 C), dictated by the temperature at which the maximum density of pure water occurs. Due to these inherent limitations, alternative media for thermal stratification have been, and continue to be, of interest for some specific applications. ALTERNATIVE MEDIA FOR THERMALLY STRATIFIED TES The numerous characteristics desired in an ideal thermal stratification medium are that it: is highly available and economical; has well-known and attractive thermophysical properties; is highly compatible with other materials; and is safe. Water provides the vast majority of these characteristics extremely well. It is unequaled in its virtually universal availability and low capital cost, as well as having excellent thermophysical properties. When somewhat lower than normal operating temperatures have been needed for heat transfer fluid applications (typically for freeze protection), the most common choices have been aqueous solutions, which provide properties that in most cases are similar to (though usually not as attractive as) those of pure water. The most common fluids have traditionally been aqueous salt solutions and water-glycol solutions. Each could also be used for low temperature thermal stratification. However, the corrosivity of sodium chloride and calcium chloride aqueous salt solutions, and the thermophysical properties and high capital costs of ethylene-glycol and propylene-glycol solutions, generally eliminate their consideration for use as low temperature stratified TES media. An alternative fluid exhibiting the desired characteristics of an ideal LTF was identified, developed, and placed in commercial service in multiple applications, the first being in 1994. This particular LTF is an aqueous solution in which the dissolved chemicals are primarily Sodium Nitrite (a corrosion inhibitor commonly used in chilled water systems) and Sodium Nitrate (an agricultural fertilizer commonly used in direct contact with soil), plus some buffer chemicals to maintain pH in the range of 9 to 10, and trace amounts of tolytriazole for corrosion control of copper and copper alloys. The LTF exhibits attractive thermophysical properties, only slightly less favorable than those of plain water, as listed in Table 1.
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Table 1. Concentration In Water (weight %) 0.0 0.0 0.0 0.0 0.0 3.0 3.0 3.0 7.0 7.0 7.0 15.0 15.0 15.0 15.0
Approximate LTF Thermophysical Properties (CB&I, 2005) Specific Heat (Btu/lb-F) 1.000 1.001 1.001 1.004 1.006 0.969 0.971 0.978 0.928 0.931 0.943 Freezing Point (F) 32.0 32.0 32.0 32.0 32.0 28.7 28.7 28.7 24.4 24.4 24.4 17.6 17.6 17.6 17.6 Nominal Minimum Thermal Storage Supply Temp (F) 39.4 39.4 39.4 39.4 39.4 35.0 35.0 35.0 30.0 30.0 30.0 20.0 20.0 20.0 20.0
Temp (F) 60.0 54.0 50.0 40.0 32.0 60.0 54.0 35.0 60.0 54.0 30.0 60.0 54.0 30.0 20.0
Specific Gravity 1.0000 1.0004 1.0006 1.0013 1.0007 1.020 1.021 1.023 1.047 1.049 1.053 1.103 1.108 1.111 1.120
Viscosity (cP) 1.12 1.23 1.30 1.53 1.75 1.22 1.32 1.78 1.35 1.43 2.05 1.65 2.33
0.888
The Thermal Conductivity of the 7.0 weight % concentration, at 68 F, is 0.343 Btu/hr-ft-F versus 0.345 for plain water. LOW TEMPERATURE FLUID (LTF) TES APPLICATIONS Some examples of thermally stratified TES installations using this LTF are presented in Table 2. These examples illustrate the broad applicability of the technology, with a wide range of end-use types, a wide range of locations and climates, TES capacities ranging from a low of 60 ton-hrs (210 kW-hrs) to a high of 123,000 ton-hrs (432,500 kW-hrs), and operating TES supply temperatures ranging from 30 °F (-1.1 °C) to 36 °F (+2.2 °C). The largest application has also been in continuous operation since 1994. Table 2. Examples of Low Temperature Fluid (LTF) TES Applications TES Capacity TES Supply/Return Temperatures 123,000 ton-hrs (432,500 kW-hrs) 30/54 °F (-1.1/12.2 °C) 90,000 ton-hrs (316,400 kW-hrs) 36/60 °F (2.2/15.6 °C) 40,000 ton-hrs (140,600 kW-hrs) 32/56 °F (0.0/13.3 °C) 60 ton-hrs (210 kW-hrs) 36/53 °F (2.2/11.7 °C) 3,292 ton-hrs (11,570 kW-hrs) 33/58 °F (0.6/14.4 °C) 9,300 ton-hrs (32,700 kW-hrs) 31/51 °F (-0.6/10.6 °C) 1,820 ton-hrs (6,400 kW-hrs) 34/60 °F (1.1/15.6 °C)
Application Type Location (Year of Initial Operation) Convention Center District Energy System Chicago, Illinois, USA (1994) International Airport Dallas/Fort Worth, Texas, USA (2004) University Campus (and Turbine Inlet Cooling) Princeton, New Jersey, USA (2005) Biotechnology Research and Manufacturing Berkeley, California, USA (2006) Manufacturing and Testing Facility Mexicali, Baja California Norte, Mexico (2007) Government Medical Center Detroit, Michigan, USA (2008) Commercial Building San Ramon, California, USA (2009)
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AN EXAMPLE ILLUSTRATES THE PERFORMANCE OF THE LOW TEMPERATURE FLUID A brief review of an actual representative LTF TES installation illustrates the initial and long-term performance of the low temperature fluid (Andrepont, 2000). The LTF performance has been documented through many years of successful, continuous, commercial operation in a 123,000 ton-hr (432,500 kW-hr), 8.5 million gallon (32,000 m3) thermal storage system in Chicago, Illinois. The TES system, which is part of a District Energy (Cooling and Heating) system, is capable of serving peak cooling loads of 25,000 tons (88,000 kW), utilizing chilled fluid supply and return temperatures of 30 and 54 F (-1.1 and +12.2 C). The fluid has been in operation since early 1994, exhibiting chemical stability plus control over corrosion and microbiological activity throughout its operating life, without chemical additions since initial system commissioning (other than modest incremental additions – less than 1% of the tank volume – at times of volumetric expansion of the interconnected piping network, as necessary to maintain the desired chemical concentration). The project earned the host convention and exposition facility the 1995 Energy User News “Energy Efficient Building Award” in the Thermal Storage category. The Low Temp Stratification Additive Various options were evaluated for the project, including no TES, ice TES, and alternative thermal stratification media such as aqueous chloride brines and water-glycol solutions. None were fully satisfactory or economical. An accelerated research and development program resulted in the identification, testing and use of a new thermal stratification fluid. The selected medium was the aqueous solution of Sodium Nitrate and Sodium Nitrite. It was formulated and installed in a 7 weight % concentration, appropriate to allow thermal stratification at a supply temperature of 30 F in the TES tank. The fluid‟s thermophysical properties are very nearly as attractive as t hose of pure water. For example, its specific heat is only slightly less than that of water, while its higher specific gravity combines with the lower specific heat to yield nearly unchanged storage capacity per unit volume. Viscosity is only slightly higher than that of pure water, even at the reduced operating temperatures. It needs only trace additives to provide the necessary levels of corrosion control. Most importantly, its capital cost is a fraction of that which would have been incurred with the use of water-glycol solutions. The method of utilizing this fluid as low temperature stratified cool storage in a thermal storage tank is protected by a United States Patent and by patents in various other countries (CB&I, 2005). The Results An extremely high design TES discharge rate (25,000 tons or 88,000 kW of instantaneous cooling load) was able to be accommodated, without undue complexity or cost in the stratification flow diffusers, through the use of a large operating supply-to-return fluid Delta T of 24 F (13.3 C) and the resultant decrease in the design flow rate. Storage tank volumetric space requirements were reduced to only 9 ft 3/ton-hour (0.07 m3/kW-hr), roughly half of the requirement for conventional chilled water TES. Modularity for incremental capacity expansions was addressed by sizing the storage to meet the initially smaller loads, while operating at conventional Delta Ts. The increased subsequent capacity was met simply by operating at larger Delta Ts. Also, the larger Delta T facilitated sizing the TES installation for future system load growth, while working within the physical limitations of the site, and at an attractive capital cost premium. Stable, predictable stratification and thermal performance have been achieved down to the design supply temperature of 30 F. (Even lower temperatures are achievable, if desired.) Total turnkey installed capital cost for the TES storage tank, inclusive of the low temperature stratification fluid, was approximately $40/ton-hour ($11/kW-hr), in 1994. Rated at its design discharge capacity, this represents an installed cost for chiller plant peaking capacity of approximately $200/ton ($57/kW), in 1994, which is a small fraction of the capital cost for equivalent conventional chiller plant capacity.
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Figure 1
123,000 ton-hr (432,500 kW-hr) LTF TES serving the Convention District in Chicago, Illinois.
LONG-TERM PERFORMANCE Multiple years of data from the initial 123,000 ton-hour (432,500 kW-hr), 8.53 million gallon (32,300 m3) application in Chicago, as well as data from other applications, confirm the performance of the low temperature thermal stratification fluid in several key areas, based on independent testing of fluid samples, including: 1. chemical stability over time and over hundreds or thousands of thermal cycles (Table 3), 2. corrosion control of various system materials, at extremely low rates (Table 4), and 3. control of microbiological activity, for LTF concentrations ranging from 3.0 to 7.0 weight % dissolved solids (Table 5). Chemical Stability Other than the initial charge of fluid in early 1994, no additions of the LTF chemicals have been made at the Chicago installation throughout the entire seventeen year period from start-up through the writing of this paper (other than following the expansion of the piping network, when a volume of water was added to the system, which diluted the chemical concentration, necessitating a modest chemical addition to maintain the desired concentration).
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Table 3.
Fluid Sample (date) Apr „94 May „94 Jun „94 Aug „94 Sep „94 Oct „94 Nov „94 Apr „95 Jun „95 Aug „95 Nov „95 Dec „96 Jul „97 Feb „99 Chem A (wgt %) 4.95 5.08 5.07 5.04 5.06 5.04 5.02 5.02 5.03 5.03 5.07 5.08 4.96 5.04 Chem B (wgt %) 1.81 1.92 1.85 1.80 1.79 1.78 1.83 1.72 1.73 1.79 1.73 1.76 1.76 1.76
Chemical Stability over Time and Thermal Cycling
Chem C (wgt %) 0.10 0.20 0.16 0.11 0.11 0.10 0.10 0.11 0.11 0.10 0.09 0.09 0.08 0.11 Chem D (wgt %) 0.10 n.a. 0.08 0.07 0.07 0.07 0.08 0.07 0.06 0.07 0.07 0.11 0.11 0.11 pH at 60 oF 9.38 9.60 9.39 9.39 9.37 9.37 9.38 9.35 9.38 9.35 9.32 9.38 9.40 9.26 Spec Gravity at 60 oF 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.049 1.048 1.047 1.048 Visc (cP) at 60 oF 1.36 1.36 1.33 1.34 1.35 1.36 n.a. n.a. n.a. 1.34 n.a. n.a. n.a. n.a. Freeze Point (oF) 25.7 25.7 n.a. n.a. n.a. 25.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
“n.a.” indicates that data is not available, due to testing not having been conducted. Corrosion Control The fluid provides inherent inhibition against ferrous metal (steel and iron) corrosion. The only supplementary additive is tolytriazole, maintained at approximately 5 to 10 parts per million for corrosion protection of cuprous metal (copper and copper alloys). Periodic testing of corrosion coupons and fluid chemistry at the Chicago installation were analyzed by an independent laboratory, throughout multiple years of system operation, illustrating exemplary results. Note that the measured corrosion rates are two orders of magnitude better than what is typically considered to be acceptable. Table 4. May 1995 Corrosion Coupon Analysis (corrosion rate in mils per year) Mild Steel <0.01 304 Stainless Steel <0.01 Copper <0.01 May 1994 Water Analysis pH Total Iron as Fe (ppm) Tolytriazole (mg/l) 9.40 n.a. 14.6 Corrosion Control Jun 1996 0.04 0.01 0.02 May 1995 9.40 n.a. 8.1 Sep 1997 - Sep 1999 <0.01 n.a. 0.01 Sep 1995 n.a. n.a. 7.6 Jun 1996 n.a. <0.05 8.0
Sep 1995 0.02 <0.01 n.a. Aug 1994 9.70 <1.20 7.3
“n.a.” indicates that data is not available, due to testing not having been conducted.
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Microbiological Activity Periodic testing of the LTF chemistry and microbiology at the Chicago installation was conducted and analyzed by an independent laboratory, including periodic comprehensive microbiological assays of fluid samples. Virtually no microbiological activity has been detected; and the detectable rates were orders of magnitude below levels of concern. Table 5. Microbiological Analysis Microscopic Observations Algae, green Algae, blue-green Algae, diatoms Bacteria, iron Bacteria, sulfur Fungi, mold spores Fungi, mold hyphae Fungi, yeast Mucilaginous substance Extraneous (particulate) matter Decomposed organic matter Protozoa Microbiological Assays Total aerobic bacteria (cfu/ml) Slime-forming bacteria (%) Nitrite-reducing bacteria May 1994 nil nil nil nil nil nil nil nil nil few nil nil 100 0 n.a. Microbiological Activity Aug 1994 nil nil nil nil nil nil nil nil nil rare nil nil <100 0 n.a. May 1995 nil nil nil nil nil nil nil nil nil few nil nil 100 0 negative Sep 1995 nil nil nil nil nil nil nil nil rare few nil nil <100 0 negative Jun 1996 - Sep 1999 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a.
“n.a.” indicates that data is not available, due to testing not having been conducted. Further independent microbiology testing was conducted on fluid samples drawn from the system. Results confirmed that the original 7.0 weight % solution, as well as even a notably lower concentration fluid (diluted to only 3.5 weight %), inhibited microbiological growth, even when microbes were introduced into the fluid samples. COMPARISON OF TES TEHNOLOGIES Table 6. Inherent Characteristics of TES Technologies (typical) Ice TES good good excellent poor fair good fair fair fair poor poor CHW TES poor fair poor excellent excellent poor excellent good excellent excellent excellent LTF TES fair good good good good excellent good good good excellent poor
Volume Footprint Modularity Economy-of-Scale Energy Efficiency Low Temp Capability Ease of Retrofit Rapid Discharge Capability Simplicity and Reliability Site Remotely from Chillers Dual-Use as Fire Protection
No single TES technology is best for all applications. Some rules-of-thumb are presented in Table 6 to contrast the
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typical inherent characteristic advantages and limitations of Ice TES, Chilled Water TES, and LTF TES (recognizing that these are only generalizations and that exceptions do occur). Note that each technology exhibits some poor and some excellent characteristics; thus, it is critical to understand the requirements of an application when selecting a TES technology. CONCLUSIONS AND RECOMMENDATIONS An additive used to produce an aqueous, low temperature stratification fluid has been identified and demonstrated through successful commercial applications. It has been in service since 1994, including in an extremely large TES installation in a District Cooling system. Numerous other installations have been in service in diverse applications and locations, in large and small capacities, and at various operating supply temperatures. The LTF storage medium has demonstrated the ability to mitigate the limitations inherent to conventional stratified chilled water TES: 1. Rapid discharge (or charge) rates - achieved by the use of higher fluid supply-to-return Delta Ts, 2. Volumetric space - reduced, inversely proportionately to the Delta T, 3. Lack of modularity - now addressed by incrementally increasing Delta T, and 4. Lower limit on storage and delivery temperature - now able to achieve 30 F supply temperature, or lower. Furthermore, seventeen years of operational experience and test data have demonstrated the fluid‟s: 1. Chemical stability over time and over hundreds and thousands of thermal cycles, 2. Corrosion control of typical system materials (at extremely low rates), and 3. Exemplary experience with microbiological activity (even with the fluid diluted to half of the initial concentration). When used as a low temperature heat transfer fluid to serve distributed cooling loads, the fluid reduces distribution pipe size and cost, and also reduces pumping capital and operating energy costs. Perhaps most significantly, by reducing the storage volume (by as much as half or more), system capital cost for new systems is substantially reduced. Also, retrofitting the low temperature fluid into existing chilled water storage systems dramatically adds capacity, economically, and without the need to increase the storage volume. Additional applications of this alternative, low temperature, thermal stratification fluid should be considered, including both new and retrofit applications. It will be most beneficial wherever low supply temperatures and/or high supply-to-return Delta Ts are desirable or necessary, or where higher thermal storage capacities per unit volume are desired. The LTF can provide a means of expanding the capacity of an existing chilled water TES system. And the LTF offers an enhancement to water treatment challenges for control of both corrosion and microbiological activity. LTF TES should be evaluated as an alternative to traditional Ice TES and CHW TES technologies during the initial planning and conceptual design stages of cooling systems for which TES is being considered. ACKNOWLEDGMENTS The author recognizes Mr. Mohammad Rantissi, Director of Energy Plant Operations with the Chicago MPEA (formerly with the Trigen Energy Corporation) for sharing the long-term performance data from their LTF installation. The author also recognizes the Chicago Bridge & Iron Company for the kind use of the data in Table 1 and the photo in Figure 1. REFERENCES Andrepont, J.S. 2000. Long-term performance of a low temperature fluid in thermal storage and distribution applications. Proceedings of the 13th Annual IDEA College/University Conference. CB&I. 2005. SoCool® low temperature thermal energy storage stratification fluid. Chicago Bridge & Iron Company document B7232-2.
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