Thermal storage systems offer building owners the potential for substantial operating cost savings by using off-peak electricity to produce chilled water or ice for use in cooling during peak hours. The storage systems are most likely to be cost-effective in situations where
It's difficult to generalize about when cool storage systems will be cost-effective, but if you meet one or more of the above criteria, it may be worth doing a detailed analysis.
If you're considering a cool storage system, you'll need to make choices: which medium and tank you'll use for storage and what strategy you'll use to dispatch the system.
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 (see Table 1). Overall, 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 fall somewhere in between.
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:
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.
Another type of ice-on-coil system has emerged in recent years that offers compatibility with prepackaged direct expansion units and is capable of shifting up to 96 percent of peak load by charging the ice-storage system during cooler nighttime temperatures and meeting the entire daytime cooling load by discharging the stored ice. These systems store cooling energy using the internal ice-on-coil method, where heat transfer coils made of plastic or copper tubing are arranged in an insulated storage tank and surrounded by water. In charging mode, ice forms on the heat-transfer surface as either evaporating refrigerant or a water/glycol solution flows through the coils, freezing up to 95 percent of the water. In discharge mode, condensing refrigerant or warm water/glycol solution flows through the coils, melting the ice and delivering cooling to the distribution unit.
Eutectic salts. 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 through which water is circulated. The most commonly used mixture for thermal storage freezes at 47°F, which allows the use of standard chilling equipment to charge storage, but leads to higher discharge temperatures. That in turn limits the operating strategies that may be applied. For example, eutectic salts may only be used in full storage operation if dehumidification requirements are low.
Storage tanks must have the strength to withstand the pressure of the storage medium, and they must be watertight and corrosion-resistant. Aboveground outdoor tanks must be weather-resistant. Buried tanks must withstand the weight of their soil covering and any other loads that might occur above the tank, such as parked cars. Tanks may also be insulated to minimize external condensation and thermal losses, which typically run 1 to 5 percent per day. Options for tank materials include steel, concrete, and plastic.
Steel. Large steel tanks, with capacity of up to several million gallons, are typically cylindrical in shape and field-erected of welded plate steel. They are then tightly wrapped in taut steel cable to pre-stress the tank walls. Some kind of corrosion protection, such as an epoxy coating, is usually required to protect the tank interior. Cylindrical pressurized tanks are generally used to hold between 3,000 and 56,000 gallons.
Concrete. Concrete tanks may be precast or cast in place. Precast tanks are most economical in sizes of one million gallons or more. Cast-in-place tanks can often be integrated with building foundations to reduce costs, but cast-in-place tanks are more sensitive to thermal shock. Large tanks are usually cylindrical in shape, while smaller tanks may be rectangular or cylindrical.
Plastic. Plastic tanks are typically delivered as prefabricated modular units. Plastic tanks that are used outdoors require ultraviolet (UV) stabilizers or an opaque covering to protect against the UV radiation in sunlight. Cylindrical tanks come in sizes as small as six feet in diameter, enabling them to be located in congested building spaces. Rectangular tanks are commonly available in sizes up to 8 x 8 x 20 feet.
Steel and concrete are the most commonly used types of tanks for chilled-water storage. Most ice harvesting systems use site-built concrete, external-melt systems usually use concrete or steel tanks, internal melt systems usually use plastic or steel, and eutectic salt systems commonly use concrete tanks with polyurethane.
Several strategies are available for charging and discharging storage to meet cooling demand during peak hours. These are:
Full storage. A full-storage strategy, also called load shifting, shifts the entire on-peak cooling load to off-peak hours (see Figure 1). The system is typically designed to operate at full capacity during all nonpeak 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 runs to meet part of the peak period cooling load, and the remainder is met by drawing from storage. The chiller is sized at a smaller capacity than the design load. Partial storage systems may be run as load-leveling or demand-limiting operations.
In a load-leveling system (see Figure 2), the chiller is sized to run 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 runs at reduced capacity during on-peak hours and is often controlled to limit the facility's peak demand charge (see Figure 3). Demand savings and equipment costs are higher than they would be for a load-leveling system and lower than for a full-storage system.
Perform a detailed feasibility study. The analysis required is involved, and it is best accomplished by following an established procedure. A good source for feasibility analysis is the Design Guide for Cool Thermal Storage, published by ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers—order it at www.ashrae.org). To perform the study, you'll need the following information:
Storage equipment manufacturers will provide simulations of storage performance for a given load profile and chiller temperature.
Some manufacturers are offering freeze-point depressants, which lower the temperature at which the cooling medium will freeze. This enables chilled-water thermal storage systems to provide greater cooling capacity by lowering the freezing point and by improving low-temperature fluid stratification. Pure water reaches maximum density at 39.4°F, so it will not stratify at lower temperatures, which reduces the cooling capacity that can be extracted from a charged TES tank. The freeze-point depressant is added to the water in the TES loop, so the TES tank design must account for the presence of this additive to prevent corrosion (the product contains sodium nitrate and sodium nitrite, along with other additives).
Ice slurry systems are a priority research area for ASHRAE, so look for new developments in that technology. The primary advantage of ice slurries comes from their high latent cooling capacity. A slurry that contains about 20 percent ice can triple the cooling capacity of a conventional 40°F supply/55°F return chilled-water distribution system. The higher cooling capacity, in turn, leads to significant reductions in the cost of piping and in the pumping energy required. The challenge lies in controlling the behavior of the slurry—as pumps push it around the piping system, ice can congeal and block flow at valves, joints, and pumps.