Thermal storage systems can save operating costs by using off-peak electricity to produce chilled water or ice for cooling during peak hours. The storage systems are most likely to be cost-effective in situations where:
- Your facility’s maximum cooling load is much greater than the average load
- The utility rate structure has high demand charges, ratchet charges, or a substantial difference between on- and off-peak energy rates
- You’re expanding an existing cooling system
- An existing tank is available
- Limited electric power is available at the site
- Backup cooling capacity is desirable
- Cold-air distribution would be advantageous
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.
What are the options?
If you’re considering a cool storage system, you’ll need to decide which medium and tank you’ll use for storage and which operating strategy you’ll use for charging (creating ice) and discharging (using the ice to cool the circulated fluid) storage.
The storage medium determines how large the storage tank needs to be and the size and configuration of the HVAC system and its components. Options include chilled water, ice, and phase-change materials (PCMs) (figure 1). PCMs work the same as ice—they’re frozen to a certain temperature and provide cooling as they melt. Overall, ice systems offer the densest storage capacity (allowing for the smallest use of space) but require complex charge and discharge equipment. Water systems offer the lowest storage density but need simpler equipment. PCMs fall somewhere in between.
Chilled water. Chilled-water storage systems use the sensible heat capacity of water—1 Btu per pound 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. You can increase the capacity of a chilled-water thermal energy storage system 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 per pound—to store cooling capacity. Storing energy at the temperature of ice requires refrigeration equipment that can cool the charging fluid (typically, a water and 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 you incorporate ice thermal storage into a new building system (or a major retrofit), you need to use smaller fans and ducts. This is because the low temperatures of the chilled-water supply allow the use of low-temperature air distribution (usually Fahrenheit temperatures in the mid-40s, versus the mid-50s for conventional systems).
You have several options for charging and discharging storage when ice is the medium:
- Ice-harvesting systems feature an evaporator surface that ice forms on; the ice is periodically released into a storage tank that’s partially filled with water.
- External-melt ice-on-coil systems circulate refrigerant or secondary coolant through submerged pipes, causing ice to accumulate on the outside of the pipes. Circulating the warm return water over the pipes melts the ice from the outside, discharging the storage.
- Internal-melt ice-on-coil systems use ice that forms on submerged pipes. Warm coolant circulates through the pipes, melting the ice from the inside and discharging the storage. The now-cold coolant runs through the building’s cooling system or cools a secondary coolant that goes through the building’s 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 like a frozen fruit smoothie. The system may pump the slurry directly to the load (or to a heat exchanger that cools a secondary fluid that circulates through the building’s chilled-water system) to meet cooling demand.
- There are alternative ice on coil internal melt systems that are compatible with prepackaged direct expansion units. They can shift up to 96% 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, an evaporating refrigerant or a water/glycol solution flows through the coils, and the ice that forms on the heat-transfer surface freezes up to 95% 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.
Internal-melt ice-on-coil systems are the most common ice-storage technology used in commercial applications. External-melt and ice-harvesting systems are more common in industrial applications, although the systems are also used in commercial buildings and district cooling systems. Ice slurry systems haven’t been widely used in commercial applications.
PCMs PCMs, also known as eutectic salts, use a combination of inorganic salts, water, and other compounds to create a mixture that freezes at a desired temperature. Plastic containers holding the material are stacked in a storage tank, and water circulates through it.
The most commonly used mixture for thermal storage freezes at 47°F, which means you can use standard chilling equipment to charge storage. However, PCMs lead to higher discharge temperatures, limiting your operating strategies. For example, you may only use PCMs in full-storage operation if dehumidification requirements are low.
Storage tanks must be strong enough to withstand the pressure of the storage medium and need to be watertight and corrosion-resistant. Aboveground outdoor tanks must be weather-resistant. Buried tanks need to withstand the weight of the soil and any other loads that might be above the tank, such as parked cars. You can also insulate tanks to minimize external condensation and thermal losses, which typically run 1% to 5% per day. Options for tank materials include steel, concrete, and plastic.
Several manufacturers have pre-engineered coolant tanks that you can easily configure for different applications. Tanks are most commonly available in capacities ranging from 50 to 500 ton-hours; you can use multiple tanks to meet the required cooling load. One advantage of multiple tanks is the ability for flexible locations, particularly for retrofit projects where space is limited. You can place tanks throughout available space in parking structures, mechanical rooms, or other locations and pipe the tanks together to form a single cooling system.
Steel Large steel tanks, with a capacity of up to several million gallons, are typically cylindrical and are so large they must be assembled on site. They’re then tightly wrapped in taut steel cable to prestress the tank walls. 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 1 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 typically come as prefabricated modular units. Plastic tanks for outdoor use require ultraviolet (UV) stabilizers or an opaque covering to protect against the UV radiation in sunlight. Cylindrical tanks come in sizes as small as 6 feet in diameter, perfect for crowded spaces. Rectangular tanks are available in sizes up to 8 by 8 by 20 feet.
Steel and concrete are the most common 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 PCMs commonly use concrete tanks with polyurethane.
There are several strategies for charging and discharging storage to meet cooling demand during peak hours.
Full storage A full-storage (also called load-shifting) strategy switches the entire on-peak cooling load to off-peak hours (figure 2). The system is typically designed to operate at full capacity during all off-peak hours to charge storage on the hottest anticipated days.
Partial storage. In the partial-storage approach, the chiller runs to meet part of the peak-period cooling load, and storage provides the rest. The chiller is sized at a smaller capacity than the design load. You can run partial-storage systems as load-leveling or demand-limiting operations.
In a load-leveling system (figure 3), 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 (figure 4).
How to make the best choice
Perform a detailed feasibility study. The analysis required is involved, and it’s best to follow an established procedure. A good source for feasibility analysis is the Design Guide for Cool Thermal Storage, published by ASHRAE (order it at www.ashrae.org). To perform the study, you’ll need the following information:
- An hourly 24-hour building load profile for the design day
- A description of a baseline nonstorage system, including chiller capacity, operating conditions, and efficiency
- A description of the proposed storage system
- An operating-cost analysis, including demand savings, changes in energy consumption and cost, and a description and justification of assumptions used for annual demand and energy estimates
The description of your proposed storage system should include:
- Sizing basis (full storage, load leveling, or demand limiting)
- Sizing calculations showing chiller and storage capacity, and considering required supply temperature
- An operating profile that shows load, chiller output, and amount added to or taken from storage for each hour of the design day
- Chiller operating conditions while charging storage, and, if applicable, when meeting the load directly
- Chiller efficiency under each operating condition
- Description of the system control strategy for design-day and part-load operation
Storage equipment manufacturers will provide simulations of storage performance for a given load profile and chiller temperature.
What’s on the horizon?
Some manufacturers offer freeze-point depressants, which lower the temperature at which the cooling medium will freeze and thus raise the cooling capacity (effectiveness). The depressants enable chilled-water thermal storage systems to provide greater cooling capacity by lowering the freezing point and improving low-temperature fluid stratification. Pure water reaches maximum density at 39.4°F, so it won’t stratify at lower temperatures, reducing the cooling capacity that can be extracted from a charged thermal energy storage tank. The freeze-point depressant is added to the water in the thermal energy storage loop, so the tank design must account for the additive to prevent corrosion as it contains sodium nitrate and sodium nitrite, and other additives.
ASHRAE is researching ice slurry systems due to their advantage of ice slurries comes from their high latent-cooling capacity. A slurry that contains about 20% ice can triple the cooling capacity of a conventional 40°F supply/55°F return chilled-water distribution system. The higher cooling capacity leads to significant reductions in the cost of piping and the pumping energy required. The challenge lies in controlling the behavior of the slurry—as pumps push it around the piping system, ice can solidify and block flow at valves, joints, and pumps.
Who are the manufacturers?
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