HVAC ContractingTechnical

The Benefits of Ice Thermal Storage

Cooling with ice thermal storage can be the most cost-effective, reliable system approach to cooling offices, schools, hospitals, malls, and other buildings, and provide a steady source of low temperature fluids for process cooling applications. These systems are also environmentally friendly because they help lower energy consumption and reduce greenhouse gas emissions.

Office Load Profile: Chiller Size Without Ice and Chiller Size With Ice Storage. (Click on the chart for an enlarged view.)

LOWEST FIRST COST

Systems with ice thermal storage can be installed at the same or lower first cost than traditional systems when designed with the colder supply water available from ice. The savings that result from the use of smaller chillers and cooling towers, reduced pump and pipe sizes, and less connected horsepower, offset the cost of the ice thermal storage equipment. Additional savings can be found when using lower temperature air distribution, which allows reduced ductwork and fan sizes.

Smaller Chillers and Heat Rejection Equipment: By designing the system around 24-hour per day chiller operation, the size of the chillers and cooling towers or air-cooled condensers required for an ice system is significantly reduced, when compared to conventional chillers and heat rejection equipment sized for the instantaneous peak load. A typical thermal storage design includes chillers that provide 50 to 60 percent of the peak cooling load. The balance of the cooling requirement is provided from the ice storage system.

Reduced Pump and Pipe Sizes: Pump and pipe sizes are also reduced in a properly designed ice storage system. Substantial savings in the chilled water distribution loop are realized when the system design incorporates reduced flow rates that result from using a larger temperature range in the water loop. Use of a larger temperature range, for example 18Â°F (10Â°C) instead of the more traditional 10Â°F (5.5Â°C) temperature range results in a reduction of pipe size. Condenser water pipe sizes are reduced due to lower flow requirements for the smaller chiller. Pump savings due to reduced chilled water and condenser water flow rates are also realized.

Reduced Cooling Coil and Supply Air Fan Sizes: Cooling coils sized using lower supply water temperatures and traditional supply air temperatures are generally smaller due to fewer rows. The reduction in rows leads to lower supply fan hp (kW).

Reduced Air Handling Equipment: When the air distribution is designed with lower supply air temperatures, the size of the ductwork, fans, and fan motors are reduced.

Reduced Electrical Distribution: Smaller chillers, heat rejection equipment, and pumps require less horsepower than a traditional system, which results in smaller transformers, switchgear, wire sizes, and starter panels.

Reduced Generator Size: If a facility has a generator for daily or back-up power, the size of the generator will be significantly reduced when the peak electrical load of the facility is reduced using ice storage.

Typical Building Electrical Demand Profile. (Click on the chart for an enlarged view.)

REDUCED ENERGY COST

An ice thermal storage system reduces peak demand, shifts energy usage to non-peak hours, saves energy, and reduces energy costs.

Reduces Peak Demand and Shifts Energy Usage: With less connected horsepower, ice storage can lower peak electrical demand for the HVAC or process cooling system by 50 percent or more. Since most electrical rates include demand charges during peak demand times and/or higher day versus night kWh charges, savings on electrical bills can be substantial. Peak electrical demand rates of $15 to $18 per kW are not uncommon. In areas with "real-time pricing," where the electric rate varies hour by hour based on the market price of electricity, day to night kWh costs can vary by 500 to 1,000 percent. The use of electricity at night versus peak daytime hours can lead to large savings on energy bills.

Saves Energy: In addition, total annual kilowatt-hours used are less when the system is designed taking advantage of the low supply water temperature available from the ice storage system. Lower kWh consumption is possible for five reasons:

1. Although making ice requires more energy than producing chilled water, the efficiency penalty is not as large since the ice is made at night when condensing temperatures are lower, increasing the efficiency of the chiller.

2. Ice systems typically operate the chiller at full load. Chillers are inefficient when run with low loads during the spring and fall. A typical chiller will operate at less than 30 percent capacity for half the year.

3. Reduced pumping horsepower.

4. Reduced fan horsepower due to lower air pressure drop across the cooling coil. A higher chilled water temperature differential across the cooling coil usually results in fewer rows and therefore a lower pressure drop.

5. The ability to recover waste heat from the chiller for heating water both night and day.

Additional kWh savings are possible if the air distribution is designed to take advantage of the low temperatures available from the ice storage system. As the electric industry continues to deregulate, and time-of-use rates, real-time pricing schedules, and negotiated power prices become standard, ice storage can provide even greater future savings in operating costs.

VARIABLE CAPACITY

The ice thermal storage system will maintain a constant supply temperature regardless of the variations in instantaneous cooling demand. The flow and entering water temperature set the instantaneous capacity.

Percent Capacity Available If One Chiller Is Unavailable. (Click on the chart for an enlarged view.)

IMPROVED SYSTEM RELIABILITY

Ice storage systems provide the reliability necessary to ensure air conditioning is available. With traditional systems, installing multiple chillers provides redundancy. In the event of a mechanical failure of one chiller, the second chiller provides limited cooling capacity. The maximum available cooling for the traditional system would only be 50 percent on a design day.

Most ice storage systems utilize two chillers in addition to the ice storage equipment. Two chillers are designed to provide approximately 60 percent of the required cooling on a design day while the ice storage provides the remaining 40 percent of the cooling capacity. In the event only one chiller is available to provide cooling during the day, up to 70 percent of the cooling capacity is available. The one operable chiller provides 30 percent of the cooling requirement while the ice provides up to 40 percent. Based on typical HVAC load profiles and ASHRAE weather data, 70 percent of the cooling capacity would meet the total daily cooling requirements 85 percent of the time.

REDUCED MAINTENANCE

The ice thermal storage coils have no moving parts so very little maintenance is required. Because the chillers, pumps, and heat rejection equipment are smaller, ice storage systems will have less maintenance than a traditional system. The ice thermal storage system also allows a chiller to undergo routine maintenance during the day when the ice storage can handle the system load.

ENVIRONMENTALLY FRIENDLY

Reducing energy consumption and using electricity at night helps reduce global warming. Electricity generated at night generally has a lower heat rate (lower fuel use per power output), and therefore lower carbon dioxide and greenhouse gas emissions resulting in less global warming. The California Energy Commission concluded that the use of electricity at night created a 31 percent reduction in air emissions over the use of electricity during the day.

With smaller chillers, an ice thermal storage system reduces the amount of refrigerant in a system. Using smaller amounts of refrigerant also helps to reduce global warming.

PROVEN TECHNOLOGY

Ice thermal storage has been successfully applied to thousands of installations worldwide. For example, Baltimore Aircoil Company (BAC) has supplied its Ice ChillerÂ® thermal storage products for projects that range in size from 90 to 125,000 ton-hours (0.3 to 441.3 MWh). Installations include office buildings, hospitals, manufacturing processes, schools, universities, sports arenas, produce storage facilities, hotels, and district cooling applications.

Products include factory-assembled units. For large applications, where space is limited or factory-assembled units are not cost effective, thermal storage coils are available for installation in field-erected tanks. Ice can be built using various refrigerants or glycols on steel coils and is used to provide either chilled water or glycol to the cooling system. This flexibility allows a unit to be provided that meets the user's specific requirements.

Excerpted and reprinted from the Baltimore Aircoil Company (BAC) Product & Application Handbook. For more information, visit www.baltimoreaircoil.com.

Publication date: 09/18/2006

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