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Thermal Ice Storage Systems

30 October 2004 | Link | by John Herbert

Smart Design

In theory, Thermal Storage Systems (TSS), often called Ice Storage Systems (ISS) are simple.thermal storage, ice storage, ice solutions, time of use tariffs, energy conservation choices

Create a supply of ice when the energy rates are low, and later use that supply of ice for air-conditioning or process purposes when the energy rates are high. However, like so many engineering strategies, the details matter.

How much ice?, Which is the most effective?, How much money will I save?, What is the capital cost and payback? all are common questions.

Economics

Plainly thermal storage is economics driven, offering operational cost savings whilst using off-peak or time of use tariffs to provide chilled water or ice and then used for cooling purposes during peak hours. Without a differentiation between the day/night tariff there would be no operational benefit.

The TSS are most likely to be cost-effective in situations where:

1. A facility's maximum cooling load is much greater than the average load

2. The utility rate structure has high demand charges, or a high differential between on and off peak energy rates

3. An existing cooling system is being expanded

4. An existing tank is available

5. Limited electric power is available at the site

6. Backup cooling capacity is desirable

7. Colder distribution would be advantageous

It's difficult to generalise when a TSS application will be cost-effective, but if your systems meet one or more of the above criteria, it may be worth doing a detailed analysis.

For air conditioning applications ice storage also offers that lower temperature medium reduce plant capacities adding to the savings.

ice storage thermal storage hong kong china energy saving

In retrofit applications lower chilled water temperatures offer the opportunity to increase system capacity without expenditure.

In addition to the direct benefits, lower energy consumption equates to lower emissions and reduced environmental impact.

Tariffs

Typically, the lowest cost tariffs, called time of use (TOU) tariff or off-peak tariff, occur over-night.

For example, presently China Light and Power (CLP) in Hong Kong offer an off-peak tariff priced at HK$ 0.619/Kwhr plus demand charge.

Additionally, the power shortage crisis in China has witnessed the introduction of time-of-use tariffs. The goal, to encourage industry to operate overnight offers the opportunity to consider thermal storage options. For example the 2005 rate in Guangzhou were 2/3 of the daytime rate, offering the chance to significantly lower the operating cost.

Storage Medium

The storage medium determines how large the storage tank will be and the size and configuration of the HVAC system and components.

The options include chilled water, ice, and eutectic salts. Ice systems offer the densest storage capacity but the most complex charge and discharge equipment. Water systems offer the lowest storage density but are the least complex. Eutectic salts systems fall somewhere between both.

Chilled Water

Chilled-water storage systems use the sensible heat capacity of water 1 Btu per pound (lb) per degree Fahrenheit (F)—to store cooling capacity. They operate at temperature ranges compatible with standard chiller systems and are most economical for systems greater than 2,000 ton-hours in capacity.

The capacity of a chilled-water thermal energy storage (TES) system is increased by storing the coldest water possible and by extracting as much heat from the chilled water as practical (thus raising the temperature of the return water). For a given tank volume, increasing the temperature differential from 10° to 20°F will double the cooling capacity. Ice. Ice thermal storage systems use the latent heat of fusion of water 144 Btu/lb to store cooling capacity.

Storing energy at the temperature of ice requires refrigeration equipment that can cool the charging fluid (typically, a water/glycol mixture) to temperatures below the normal operating range of conventional air-conditioning equipment. Special ice-making equipment or standard chillers modified for low-temperature service are used. When ice thermal storage is incorporated into a new building system (or a major retrofit) the low temperatures of the chilled-water supply allow the use of low-temperature air distribution (usually calling for Fahrenheit temperatures in the mid-40s, versus the mid-50s for conventional systems), meaning smaller fans and ducts are needed. When ice is the storage medium, there are several technologies available for charging (creating ice) and discharging (using the ice to cool circulated fluid) storage:

Ice harvesting systems feature an evaporator surface on which ice is formed; it is then periodically released into a storage tank that is partially filled with water.

External melt ice-on-coil systems use submerged pipes through which a refrigerant or secondary coolant is circulated, causing ice to accumulate on the outside of the pipes.

Storage is discharged by circulating the warm return water over the pipes, melting the ice from the outside. Internal melt ice-on-coil systems also feature submerged pipes on which ice is formed. Storage is discharged by circulating warm coolant through the pipes, melting the ice from the inside. The now-cold coolant is then pumped through the building cooling system or used to cool a secondary coolant that goes through the building's cooling system.

Pre-engineered tanks that can be easily configured for different applications are available from several manufacturers, including Calmac, Baltimore Air Coil, and FAFCO. Tanks are most commonly available in capacities ranging from 50 to 500 ton-hours (Calmac has the low side of this market, between 50 and 150 ton-hours); multiple tanks are used to meet the required cooling load. One advantage of multiple tanks is flexible location, particularly for retrofit projects where space is limited tanks can be spread throughout available space in parking structures, mechanical rooms, or other locations. The tanks are then piped together to form a single cooling system. Ice slurry systems store water or water/glycol solutions in a slurry state—a partially frozen mixture of liquid and ice crystals that looks much like a frozen fruit smoothie.

To meet cooling demand, the slurry may be pumped directly to the load or to a heat exchanger that cools a secondary fluid that circulates through the building's chilled-water system. Internal melt ice-on-coil systems are the most commonly used type of ice storage technology in commercial applications.

External melt and ice harvesting systems are more common in industrial applications, although they can also be applied in commercial buildings and district cooling systems. Ice slurry systems have not been widely used in commercial applications.

Eutectics

Eutectic salts, also known as phase-change materials, use a combination of inorganic salts, water, and other elements to create a mixture that freezes at a desired temperature. The material is encapsulated in plastic containers that are stacked in a storage tank. Water is circulated through the tank for charging and discharging.

The most commonly used mixture for ice storage systems freezes at approx. 8°C (47°F), which allows the use of standard chilling equipment to charge the store.

Operating Strategies

Several strategies are available for charging and discharging storage to meet cooling demand during peak hours.

Full storage

A full-storage strategy, also called load shifting, shifts the entire on-peak cooling load to off-peak hours.

The system is typically designed to operate at full capacity during all non-peak hours to charge storage on the hottest anticipated days. This strategy is most attractive where on-peak demand charges are high or the on-peak period is short.

Partial storage

In the partial-storage approach, the chiller plant operates to provide part of the peak period cooling load, the balance provided from storage. Partial storage systems may be operated as load-levelling or demand-limiting dependant on the project economics.

In a load-levelling system the chiller would be sized to operate at its full capacity for 24 hours on the hottest days. The strategy is most effective where the peak cooling load is much higher than the average load.

In a demand limiting system, the chiller operates at a reduced capacity during on-peak hours and is often controlled to limit the facility's peak maximum demand charge. Demand savings and equipment costs are higher than they would be for a load-levelling system and lower than for a full storage system.

Further Information

For further information regarding Kelcroft's services, call today, our Hong Kong office telephone: +(852) 2335 9830, send us a fax +(852) 2335 9862, or send us an email message



About the Author

Mr John Herbert is the Managing Director at Kelcroft. He was educated and trained in the United Kingdom, has managed and engineered solutions for more than twenty fours years, advising clients across three continents, whilst retaining a litigation free record.

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