0410Chill Factor

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Continuing Education

Use the follow ing learning objectives to focusyour s tudy w hile reading this months ContinuingEducation article.

Learning Objectives - After reading this article,you w ill be able to:

1. Discuss the key principles of ice-basedthermal energy storage (TES).

2. Understand w hy TES can reduce air pollution in some situations.

3. Explain how TES can reduce mechanicalequipment costs in commercial buildings.

4. Explain the tw o primary w ays in w hichTES can reduce operating costs incommercial buildings.

Chill Factor A number of structurally innovative towers defy convention, and gravity, by getting bigger as they gettaller April 2010

Alex Wilson

Somehow, cooling buildings with ice seems primitiveharkeningback to the days before refrigerant-cycle air-conditioning when wecooled food and sometimes buildings with blocks of ice. But usingice today is one of the most advanced, sm artest ways to coolbuildings, and the practice is growing.

Ice-based thermal energy storage is finding its way into more andmore high-tech green buildings, including the Durst Organizationsrecently completed Bank of America building at One Bryant Park inNew York City. The growth in thermal energy storage is no surpriseonce one understands how it works, why it makes economic sense,and why its a great environmental solution.

Understanding thermal energy storage (TES)

The basic principle of ice-based thermal energy storage for coolingis s imple. Ice is produced at night when electricity is cheap, and thatice becomes the s ource of cooling energy during the day. Ice worksso well because a great deal of heat is absorbed and releasedduring freezing and melting (referred to as latent heat). Materials canstore heat both as sensible heat and as latent heat. Sensible heat isstored as the temperature of a solid or liquid is changed. Latent heat

is s tored when there is a change in phasein this case between solid and liquid.The latent heat of ice is 144 Btu per pound, meaning that melting or freezing one pound of ice at 32 degrees Fahrenheitabsorbs or releases 144 Btus of heat. By comparis on, water only stores one Btu per pound for every degree Fahrenheitdifference in temperature. So if chilled water is used for TES rather than ice, a lot more volume is required (assum ingtemperature cycling of 20 degrees Fahrenheit, you would need seven times as m uch chilled water as ice to provide thesame amount of cooling). Other phase-change materials, such as paraffins and eutectic salts, can also be used in TESsystems , but these are far less comm on than ice or chilled water, and their heat-storage effectiveness may drop over time.

11.04.2010 McGraw-Hill Construction - Continuin

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Photo: HUNTERGATHERER




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The iconic Bank of America building at One Bryant Parkrelies on thermal-energy storage for about one-quarter of its cooling.

Photo: David Sundberg/Esto

There are a number of different types of ice-based TES systems in use today. The most common for buildings (asopposed to district-cooling systems ) is internal-melt ice-on-coil. Ice is made by circulating a working fluid (a glycol-water solution) cooled by a chiller through tubing embedded in water tanks. This occurs at night us ing off-peak electricity.During the day, the glycol solution is circulated through the ice tank and chilled by the ice. As the working fluid is chilledduring the day, it melts the ice from the inner surface outward. Either the chilled working fluid is used directly for cooling,or that glycol-water solution pass es through a heat exchanger to transfer cooling energy to water that is circulatedthrough the building.

Internal-melt TES systems can be very large with single tanks, such as s ystems produced by Baltimore Aircoil and

Evapco; others are fairly small and modular, such as systems produced by CALMAC Manufacturing and FAFCO. IceEnergy makes such a system adapted to packaged air-conditioning systems.

The benefits of us ing off-peak electricity

TES shifts electricity use from daytime hours, when mos t cooling loads occur, to nighttime, when electricity demand islower and costs are usually less. Most utility companies offer time-of-use pricing for commercial and indus trialcustomers; some also offer this pricing for residential cus tomers . The discounts in off-peak electricity pricing vary widely,according to Doug Reindl, a professor of mechanical engineering at the University of Wisconsin and director of theHVAC&R Center (previously the Thermal Storage Applica-tions Research Center), but are often 30 to 50 percent and canbe even greater.

As the utility indus try modernizes, the ability of building owners to manage loads internally will become increas ingly

importantand TES provides an excellent means of doing s o. With real-time electricity pricing, for example, the price per kWh can be dram atically higher during certain times of dayoffering a s trong incentive for a company to manage loadsmore precisely. Utility contracts for dispatchable or interruptible loads can provide very attractive dis counts for companiesif they are willing to dis connect chiller loads during peak periods and shift cooling to stored ice.




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Each of the 44 tanks in the building contains 1,600 gallons of water and roughly three milesof plastic tubing through which a glycol solution is circulated to create and then melt ice.

Photo: Gunther Intelmann Photography

TES reduces dem and charges

Most comm ercial and indus trial utility customers pay demand charges in addition to paying for actual consum ption of

electricity. Demand charges are based on the peak electrical demand of a buildingreasoning that the utility companyhas to have that amount of capacity available to the building. Typical dem and charges are $1015/kW of peak demandfor commercial customers, according to Reindl. In some places though, demand charges can be a lot higher; in NewYork City they are as high as $36/kW during peak summ er cooling periods .

It is typical for demand charges to account for about half of a companys monthly electric bill, according to Paul Torcelliniof the National Renewable Energy Laboratory. However, in some cases the demand charges can significantly exceedthe direct electricity consum ption charges. Usually, demand charges are only levied on commercial and indus trialcustomers, though residential customers with very high electricity usage may also have to pay these charges .

In nearly all comm ercial buildings , peak demand occurs during the daytime hours , when lighting, office equipment, andcooling loads are the greatest. Shifting some of the cooling load from daytime to nighttime hours with TES will reduce thepeak electric demand and thus lower demand charges (see figure). The cooling can be shifted almos t entirely to off-

peak hours (full-s torage operation), or just a portion of the load can be shifted to off-peak hours (partial storage). In either case, there will be a significant reduction in demand charges.

First cost savings can pay for equipment

TES systems can reduce first costs in a number of ways. For starters, they allow chillers or packaged air conditioners tobe downsized. Consider this example: If a buildings cooling load is 100 tons, during times of peak afternoon coolingrequirements in the summer it will take as much as 100 tons of cooling to maintain comfort. (We still use ton as ameas ure of coolingit is the rate of cooling provided by one ton, 2,000 pounds , of ice as it melts over a 24-hour day; inmodern terminology, one ton is equivalent to 12,000 Btu/hour of cooling capacity.) Without energy storage, that buildingwould need a 100-ton chiller to meet that peak demand. With energy storage, however, ice is available to meet that peakload so the chillers work can be spread throughout the day and night, meaning that a much smaller chiller can providethe buildings cooling needs. Cooling tonnage can often be reduced by 35 to 50 percent, according to Mark MacCracken,president of CALMAC Manufacturing, yielding cost savings that more than offset the cos t of adding an ice-storagesystem.

With a full-storage system, enough ice is created to meet the buildings entire daytime load, avoiding higher electricrates. By installing a partial-storage TES system (a more common approach), the ice supplements the chillers outputduring the day, which provides less of a reduction in electricity costs but allows the chilleras well as the ice storage




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systemto be downsized.

Data: Ice Energy

Along wi th downsizing chil lers with TES, further first-cost s avings are often realized by decreasing the s ize of ducts,pumps, fans, cooling towers, and power supply infrastructure. The smaller ducting is possible in part because ice-based TES systems typically deliver cooler air. In rare si tuations, these reduced duct sizes can even reduce floor-to-floor height. The Cool Storage Technology Guide, published in 2000 by the Electric Power Research Institute (EPRI),suggests that floor-to- floor height can be reduced by 4 inches when the supply air temperature is reduced from 55degrees to 45 degrees Fahrenheit, as is typical with ice-based TES systems . This height reduction, in turn, reducesstructural and envelope costs , according to EPRI, and can cut total cons truction cos t by 3 percent.

Even without reductions in floor-to- floor height, it is not uncommon for the first-cost savings in chillers and other equipment to entirely pay for the TES system, according to MacCracken, so that the operating cost savings are achievedat very littleor even zeroadditional upfront cost. In retrofit applications, the cost of TES is higher, but the payback of that additional investment wil l be relatively shortthree to s even years, claims MacCracken.

Minimizing air pollution

In some places, shifting cooling loads to off-peak electricity results in less pollution. If baseload power generation isprimari ly hydro or nuclear and peaking plants are foss il-fuel powered, reducing peak demand reduces pollution. In astudy of the environmental impacts of thermal energy storage, the California Energy Commiss ion (CEC), found thatshifting cooling loads from peak to off-peak hours in the Pacific Gas & Electric territory reduces pollution emiss ions by 47percent.

Where peaking power is provided with relatively new gas turbines or hydropower from res ervoirs (where the flow can bereadily controlled) and coal-fired power plants provide baseload power, shifting loads to off-peak hours might not reduceemissions. Emission characteristics will be utility dependent, says Reindl.

Another factor in the environmental benefits of TES is the conversion of pollution emiss ions into smog. Nitrous oxide(NOx) emissions from a given fossil-fuel-fired power plant will result in less smog at night than during the day becausesmog production is a photoreactive processcatalyzed by sunlight. A 2005 report conducted for Ice Energy by E3Ventures of Boulder, Colorado, showed that NOx emiss ions in the Sacramento Municipal Utility District (SMUD) are 56percent lower during off-peak hours than during peak hours, due both to the shift in generation source and the absenceof sunlight.

Ice-Based Thermal Energy StorageEquipment Providers: A Sampler

Baltimore AircoilJessup, Maryland 410-799-6200 baltimoreaircoil.com

Baltimore Aircoil Company (BAC), founded in 1938, is the nations leading m anufacturer of evaporative equipment, including cooling towers , evaporative condens ers, evaporators,and large ice thermal storage systems. Thousands of the companys Ice Chiller internal-melt, ice-on-coil TES systems have been ins talled worldwide in capacities from 90125,000 ton-hours (300440,000 kWh).




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CALMACFair Lawn, New Jersey 201-797-1511 calmac.comCALMAC pioneered modular, internal-melt, ice-on-coil TES in the 1970s and has over 3,500 systems in operation worldwide. Cylindrical IceBank tanks are available in varioussizes with capacities of 41486 ton-hours (1441,710 kWh); tanks are ganged to satisfythe building load. CALMAC tanks are often combined with Trane chillers .

EvapcoTaneytown, Maryland 410-756-2600 evapco.comEvapcos Extra-Pak Ice Coil ice-on-coil TES systems can be configured as either internal-melt or external-melt. Systems typically rely on large field-cons tructed concrete tanks.

FAFCOChico, California 800-944-7652 fafco.comFounded in 1969, FAFCO is the larges t U.S. manufacturer of solar col lectors for poolheating. The company adapted its polymer heat exchanger used for solar pool heating toits IceStor internal-melt ice-on-coil TES in the 1980s and has about 2,000 of these coolingsystems in place.

Ice EnergyWindsor, Colorado 877-542-3232 ice-energy.comIce Energys Ice Bear system is the only TES system designed to work with packaged,direct-expansion air-conditioning equipment. Thus, they can work with smaller commercialbuildings , while other TES systems function with chillers . The Ice Bear can be coupled to a35 ton air conditioner and can store up to 30 ton-hours (100 kWh) of cooling.

Storage of renewable energy

A significant fraction of the nations wind power is produced at night, during off-peak periods. In western Texas, for example, 12 percent of the wind power produced in 2009 was generated during periods when the wholesale price of electricity was worth nothing, because there was no demand for it, and les s than a quarter of the wind outputcorresponds with peak demand. This disconnect between availability and demand will become a bigger problem aswind power becomes a larger contributor to the nations power supply. TES offers a great way to use renewableelectricity produced at night (making ice) to satisfy peak loads (daytime cooling). Another way to look at this is that asmore wind capacity is installed, off-peak power wil l get cheaper (and cleaner), making TES a better and better option.The same arguments will hold true if wave power becomes a significant renewable energy source.

Counting the points

In the design and construction versions of the LEED Rating System, energy optimi-zation points are tied to theASHRAE/IESNA 90.1 Standard, which calculates s avings based on energy cost. In those rating systems, thermal energystorage ins tallations that save on energy costs will als o garner energy points. In LEED for Existing Buildings : Operationsand Maintenance (LEED-EBOM), however, energy points are linked to federal ENERGY STAR index, which measuresunits of actual energykilowatt hours of electricity and therms of natural gasas opposed to energy cost. Under theENERGY STAR system thermal s torage doesnt fare as well, because it doesnt necessarily reduce overall energy use.In fact, in some cases i t even increases it, due to the losses that happen each time energy is transferred from onemedium to another. (Ice storage systems m ay be eligible for an innovation credit under LEED-EBOM, according toMacCracken, but this is not ass ured.)

Thermal Energy Storage without Chillers

Until recently, ice-based TES systems were only available for large buildings with chiller-based cooling. Since the 1980s, thousands of these systems have been installed in awide range of commercial, institutional, and industrial buildings .According toCommercial Building Energy Consumption Survey (CBECS) data publis hed by the U.S.Department of Energy, chillers provide cooling for 72.6 percent of comm ercial-buildingsquare footage in the U.S. Chillers are also generally used for district-cooling systemsthat provide cooling for 11 percent of U.S. commercial-building square footageand TES

supports most dis trict cooling. But, until recently, TES was not suited to the 12.5 percentof commercial-building square footage that is cooled us ing packaged, direct-expansioncooling.

This may sound like a sm all segment of the market, but that 12.5 percent of commercial-building s pace actually accounts for 95 percent of the nations commercial buildings




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(because most commercial buildings are relatively small). Most of these are too small to justify chillers , though the number also includes many big-box retail stores and factoriesthat are cooled with dozens or even hundreds of rooftop packaged air conditioners . In2003, the Colorado company Ice Energy made TES applicable to these sm aller buildings. Instead of producing chilled water for cooling a building, Ice Energys Ice Bear chills refrigerant that directly expands in the evaporator component of an air conditioner tocool the air. A.W.

Ice Energy's Ice Bear unit incorporates into a building's standard AC system

1 Ice storage section2 CoolData controller 3 Refrigerant management system4 Compressor and condensing unit

Looking ahead

Ice-based thermal energy storage systems s ave building owners money by shifting cooling energy use to off-peakperiods , reducing peak demand, and (with new cons truction) reducing first-costs . As the nations utility grid ismodernized and a sm art grid emerges, TES will become even more important as an energy-management strategy.

Given these benefits of TES, it is indeed s urprising that the practice is not more widely used. Even in green building, TESis uncommon. There are huge opportunities in the commercial-building sector that continue to be missedoftenbecause advantages of TES are not well unders tood. As better tracking and reporting of energy performance becomescommon, and as new utility pricing s tructures incentivize load-shifting, it is likely that thermal energy storage will finallygain the place i t deserves in our palette of HVAC options.

FOR MORE INFORMATION:

"Thermal Energy Storage for Space Cooling," December 2000

Federal Technology AlertFederal Energy Management ProgramU.S. Department of Energy, Washington, D.C.

"Cool Storage Technology Guide," August 2000Electric Power Research Ins titute




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Palo Alto, California

Heating, Ventilating, and Air-Conditioning Systems and Equipment, 2008 (Chapter 50)Heating, Ventilating, and Air-Conditioning Applications, 2007 (Chapter 34)ASHRAEAtlanta, Georgia, 404-636-8400www.ashrae.org

Space Cooling Technology Atlas, 1998(Cool Thermal Storage chapter)

E SourceBoulder, Colorado, 303-444-7788www.esource.com

Originally published in the March/April 2010 issue of GreenSource


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