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Benefits
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The Benefits of Ice Thermal Storage for Air Conditioning Projects

Ice can provide a system that yields:

Benefits

Lowest First Cost Design

Air conditioning systems with ice storage can be installed at the same or less first cost than traditional conventional systems if the system takes advantage of 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.

Smaller Chillers and Cooling Towers

Small Chiller

By designing the system around 24-hour per day chiller operation, the size of the chillers and cooling towers 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% of the peak cooling load. The balance of the cooling requirement is provided from the ice storage system.

Reduced Pump and Pipe Sizes

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 18°F (10°C) instead of the more traditional 10°F (5.5°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

Air Handling

When the air distribution is designed with lower supply air temperature, the size of the ductwork, fans, and fan motors are reduced.

Reduced Electrical Distribution

Electrical Demand Comparison

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.

The savings associated in a properly designed ice storage system are substantial. The cost of the thermal storage equipment is more than offset by the savings described above.

Case Studies

To demonstrate ice storage’s cost and energy savings, studies comparing traditional cooling systems and ice storage cooling systems were prepared for BAC by a highly regarded consulting engineering firm located in Baltimore, Maryland. The studies include buildings designed with central, decentralized, and roof mounted packaged air conditioning systems.

The studies clearly show that for systems of equal quality, ice storage systems are not only competitive, but are often lower in first cost than traditional systems. First cost savings of more than $1.00 per ft2 ($10.76 per m2) are achievable in most cases. Peak demand and energy consumption, key components of overall operating costs, also are reduced significantly. Construction costs, equipment sizes, and energy usage will vary with location; accordingly, these items are relative and savings are possible everywhere.

Case Study Criteria

The building models used are 20-story, 10-story, and 4-story office buildings, each with 20,000 ft2 (1,860 m2) per floor. Floor-to-floor and floor-to-ceiling dimensions are 12 ft (3.7m) and 9 ft (2.7 m), respectively. Exterior walls are medium weight construction with a U value of 0.10. Twenty-five percent of the perimeter wall have fixed, double-glazed glass with medium colored, horizontal venetian blinds on the interior. The roof U value is 0.07. Lighting is 2.67 w/Ft2 (28.73 w/m2) of building area and recessed, fluorescent, air-troffer lighting fixtures are used throughout with heat transfer to ceiling spaces for central air systems. General electric equipment loads are calculated at 0.25 w/Ft2 (2.69 w/ m2). No special loads, such as computers, are included. All floor levels have suspended ceilings. Ceiling spaces are used for returning air, which is collected, adjacent to the centrally located equipment shaft and returned through ductwork.

The building cooling and heating loads are based on a summer outdoor design of 91°F (33°C) dry bulb temperature and 55% relative humidity, and winter design temperature of 12°F(-11°C). Heat rejection equipment is sized on a summer outdoor design of 95°F (35°C) dry bulb and 78°F (26°C) wet bulb.

Construction costs (mechanical, electrical, and space) are estimated using costs obtained from various equipment manufacturers, Means cost estimating data, and other similar references. Space costs cover the costs of constructing equipment enclosures and do not include real estate costs or loss of revenue due to reduced rental space.

Example

10 Story, 400 Ton (1,400 kW), 200,000 ft2 (18,580 m2) Office Building

400 Ton Peak Load

The building load profile shown is typical for an office operation. The air-conditioning system is started up at 7 am to bring the building down to temperature before the majority of the office workers arrive around 8 am. The system is shut off around 7 pm after everyone has left.

Chillers

The chillers required for an ice thermal storage system are significantly less than for a traditional system, generally 50 to 60% of the peak cooling load. For this comparison:

Traditional System

(2) 200 Nominal Ton (704 kW each) Chillers

Ice Thermal Storage System

(1) 200 Nominal Ton (704 kW) Chiller

Savings at $314/Ton ($89.20/kW) installed is $62,800

Cooling Towers

The size of the cooling towers will also be reduced significantly.

Traditional System

(2) 200 Ton Cooling Towers

Ice Thermal Storage System

(1) 200 Ton (704 kW) Cooling Tower

Savings at $104.5/ton ($29.69/kW) installed is $20,900.

System Distribution Piping

System Distribution Piping

Typically, the distribution piping can be reduced by one pipe size when switching from a typical 10°F (5.5°C) temperature differential in a traditional system to a 14°F (7.8°C) to 20°F (11.1°C) range in an ice storage system. The building is assumed to have 450 linear feet (137m) of piping.

Traditional System

  • 10°F (5.5°C) temperature differential
  • 960 GPM (60.5 lps) requires 8 inch (203 mm) pipe

Ice Thermal Storage System

  • 14°F (7.8°C) temperature differential
  • 685 GPM (43 lps) requires 6 inch (152 mm) pipe

The estimated savings in system distribution piping at $62 per linear foot ($203/m) installed is $27,900. The piping savings per unit of length are less than for the condenser water piping because of the insulation. The low temperature chilled water piping may require thicker insulation than piping with traditional chilled water temperatures.

Condenser Water Piping

With smaller chillers and cooling towers, the condenser piping for an ice thermal storage system is significantly reduced. The condenser water piping is assumed to be 450 linear feet (137 m).

Traditional System

  • 3 GPM per ton (0.06 lps per kW)
  • 1,200 GPM (75.5 lps) requires 8 inch (203 mm) pipe

Ice Thermal Storage System

  • 3 GPM per ton (0.06 lps per kW)
  • 600 GPM (37.8 lps) requires 6 inch (152 mm) pipe

The estimated savings in condenser water piping at $66 per linear foot ($216.5/m) is $29,700.

Pumps

Since the ice thermal storage system requires less flow than a traditional system, it also requires smaller pumps.

Traditional System

  • (2) 15 HP (11 kW) condenser water pumps
  • (2) 10 HP (7.5 kW) primary chilled water pumps
  • (2) 20 HP (15 kW) secondary chilled water pumps

Ice Thermal Storage System

  • (2) 7.5 HP (5.5 kW) condenser water pumps
  • (2) 7.5 HP (5.5 kW) glycol pumps
  • (2) 15 HP (11 kW) chilled water pumps

The estimated pump savings are $9,426 installed.

Electrical

In addition to the first cost savings associated with mechanical equipment, there are also electrical distribution savings. The thermal storage system reduces the total electrical demand of the building by 145 kW. This is a reduction of 46% in the electrical demand of the chilled water side of the air conditioning system. Additional savings are available in the air distribution.

Electrical Demand Comparison

Traditional System Ice System
Chillers (0.6 kW/ton, COP = 5.86) 322 HP (240 kW) 161 HP (120 kW)
Cooling Tower 30 HP (22 kW) 30 HP (22 kW)
Primary Pumps 20 HP (15 kW) 15 HP (11 kW)
Secondary Pumps 40 HP (30 kW) 30 HP (22 kW)
Condenser Water Pumps 30 HP (22 kW) 15 HP (11 kW)
TOTAL HP 442 HP 236 HP
TOTAL kW 329 kW 175 kW

The reduction in total connected horsepower from 442 to 236 (connected kW from 329 to 175) results in savings on transformers, starters and wiring. The estimated electrical savings are $28,000.

Ice Thermal Storage

The ice thermal storage system usually consists of self-contained thermal storage tanks, a concrete slab to put the tanks on, ethylene or propylene glycol in the ice storage loop and a heat exchanger to isolate the glycol from the chilled water coils. The additional costs for these items are:

Additional Costs
1522 ton-hours (5.4 MW) of ice thermal storage units $93,300
Ethylene glycol to charge the system $7,018
Heat exchanger $35,000
Concrete Slab $8,000

Ice Thermal Storage is Lower in First Cost

The savings from smaller chillers, cooling towers, pumps and piping is $178,826. The additional expense for installing the ice thermal storage system is $143,318. A savings of $35,408 or $88.52 per ton ($25.15 per kW)!

Savings Compared to Traditional System +
Chillers $62,800
Cooling Towers $20,900
Distribution Piping $27,900
Condenser Piping $29,700
Pumps $9,426
Electrical $28,000
TOTAL $178,726
Addition for Ice Thermal Storage System
Ice Thermal Storga Units $93,300
Ethylene Glycol $7,018
Heat Exchanger $35,000
Concrete Slab $8,000
TOTAL $143,318
TOTAL SAVINGS WITH ICE THERMAL STORAGE SYSTEM $35,408

Reduced Energy Cost

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

Reduced Peak Demand

With less connected horsepower, ice storage can lower peak electrical demand for the HVAC system by 50% 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 1000%. 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 very inefficient when run with low loads during the Spring and Fall. A typical chiller will operate at less than 30% 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 airside distribution is designed taking 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.

Reduces Energy Costs

In the 400 TR (1,400 kW) comparison, a peak demand savings of 154 kW and an energy savings of 97,032 kWh were realized with the ice thermal storage system versus the traditional system. The energy savings were as follows:

Savings Compared to Traditional System
Traditional System Ice Thermal Storage System
On-Peak Energy (Noon to 8 pm) 191,263 kWh 73,464 kWh
Mid-Peak (8 am to Noon & 8 pm to Midnight) 175,586 kWh 85,491 kWh
Off-Peak (Midnight to 8 am) 107,450 kWh 218,313 kWh
Total Usage for the Year 473,300 kWh 377,268 kWh
SAVINGS FOR THE YEAR 97,032 kWh

Using rates from a local utility, the electric bill savings for this example would be $28,336 per year. Savings from reduced peak demand and shifting energy usage from peak to off-peak usage is $20,414.

Reduced Maintenance

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

Test Kit

icefluidkit

Controls

Controls

Units Installed

BAC Ice system tanks

Improved System Reliability

Ice storage systems provide the reliability necessary to ensure air-conditioning is available. With conventional 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 conventional system would only be 50% on a design day.

Chiller Capacity

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

Increased Comfort and System Flexibility

Increased comfort comes from a constant, chilled water supply temperature throughout the day without cycling on and off to meet the load requirements.

The lower humidity resulting from lower supply air temperatures provides increased comfort as well as improved air quality and freshness. Typically, in a cold air system with 45°F (7°C) supply air, the space’s relative humidity will be approximately 10% lower than in a similar 55°F (13°C) system. Studies have show that if the space humidity is lowered while the space dry bulb temperature is held constant, occupants feel more comfortable. A more comfortable working environment can also translate into increased employee productivity.

Space Saving

Ice thermal storage saves equipment room and heat rejection space since smaller chillers, pumps, and heat rejection equipment are required. A typical building with ice thermal storage requires one half the chiller and heat rejection capacity of a traditional system. The chilled water pumps will be half the size of a typical 10°F (5.5°C) temperature differential system and the condenser water pumps will be half the size of a traditional system. The plan area required for the equipment room and heat rejection equipment can be cut by one third. The thermal storage units can be located outdoors, in a sub-basement, or under a parking lot; it does not have to be part of the equipment room.

If low temperature air is used, the air handling equipment and ductwork will be smaller. The smaller ductwork can allow a reduction of 3 to 6 inches (76 to 152 mm) in ceiling height, while reducing other building structural costs due to reduced size and weight.

Chilled water storage requires 4 to 10 times the space of ice thermal storage. The use of sensible cooling combined with storage inefficiencies make chilled water storage tanks larger.

Environmentally Friendly

Reducing energy consumption and using electricity at night will 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.

Greenhouse Gas

The California Energy Commission concluded that the use of electricity at night created a 31% 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. Most refrigerants in use today are slated to be banned in the near future under the Montreal Protocol because they contribute to global warming. Using smaller amounts of refrigerant helps to save the ozone and reduce global warming.

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