Thermal energy storage (TES) involves adding heat (thermal) energy to a storage medium, and then removing it from that medium for use at some other time. This may involve storing thermal energy at high temperatures (heat storage) or at low temperatures (cool storage).
In HVAC applications, the most-common storage media used for cool thermal storage are ice and water. A chilled-water storage system uses the sensible-heat capacity of a large volume of water to store thermal energy. A chiller is used to lower the temperature of water, and this cool water is stored in a large tank for use at another time. An ice storage system, however, uses the latent capacity of water, associated with changing phase from a solid (ice) to a liquid (water), to store thermal energy.
Several ice storage technologies have been introduced, flourished for a short period of time, and subsequently left the marketplace. Glycol-based ice storage systems continue to be very popular because they are simple and are similar to conventional chilled-water systems. Any application that is suitable for a chilled-water system is a candidate for glycol-based ice storage.
This type of ice storage system uses a chiller to cool a heat-transfer fluid, often a mixture of water and antifreeze (such as glycol), to a temperature below the freezing point of water. This fluid is pumped through one or more ice storage tanks, where heat is transferred from the water inside the tank to the heat-transfer fluid. This causes the water inside the tank to freeze.
When the thermal energy is needed at a later time, the heat-transfer fluid is again pumped through the storage tank, but now at a temperature above the freezing point of water. Heat is transferred from the heat-transfer fluid to the ice stored inside the tank, causing the ice to melt.
Adding ice storage to an HVAC system can reduce the utility costs associated with cooling by shifting the operation of the chiller from times of high-cost electricity to times of low-cost electricity.
Figure above shows a design-day cooling load profile for an example building. Between midnight and 6 a.m., the building is unoccupied and there is no cooling load. At 6 a.m., the building begins to be occupied, and the cooling load increases. The cooling load is highest between 11 a.m. and 4 p.m., and then decreases dramatically after 5 p.m. as people leave the building. There is a small cooling load that continues throughout the evening hours, before going away at midnight.
Most electric utility companies experience the greatest demand for electricity during the daytime hours, with some even facing capacity shortages. To encourage the reduction of electricity use during these times, many electric utility companies have established time-of-day rates that create time windows for higher-cost electricity during these periods of high demand. The hours when the cost of electricity is high are often referred to as the “on-peak” period. On the other hand, the “off-peak” period refers to the hours when the cost of electricity is lower.
For this same example building, noon to 8 p.m. is defined as the on-peak period. All other hours are defined as the off-peak period.
Another common component of the electric utility rate is a demand charge. This is a fee based on the highest power (kW) draw, or demand, used by the building during a specified time frame. Typically, either the demand charge only applies to the on-peak period, or the on-peak demand charge is significantly higher than the off-peak demand charge.
Ice storage systems lower monthly utility costs by melting ice to satisfy building cooling loads during the on-peak period. This avoids, or significantly reduces, the electricity required to operate the chiller during that time frame. The operation of the chiller is shifted to the off-peak period, during which the cost of electricity is lower and the demand charge is lower or non-existent. The chiller is used during that period to freeze the water inside the storage tanks, storing the thermal energy until the on-peak period.
In this example, the building cooling loads that occur during the on-peak period, which occurs between noon and 8 p.m., are satisfied by melting the stored ice, and the chiller is turned off.
This type of system, often called a “full-storage system,” is only possible if the storage capacity of the tanks is large enough to satisfy the on-peak cooling loads for the given day.
The installed cost of a full-storage system, however, may not be feasible. Many ice storage systems have enough capacity to satisfy only a portion of the on-peak cooling loads. This type of system is often called a “partial-storage system.”
In this example partial-storage system, the cooling loads that occur during the on-peak period are satisfied by melting ice and operating the chiller. The chiller operates at a reduced capacity, consumes less energy, and draws less power. Cooling loads greater than the capacity provided by the chiller are satisfied by melting the stored ice.
Turning off the chiller, or significantly reducing its capacity, during the on-peak period reduces the consumption of this higher-priced electricity and reduces the on-peak electrical demand. Both can result in lower monthly utility bills.
At first glance, it might appear that an ice storage system designed to reduce on-peak electrical demand (kW) is the same as a system designed to reduce on-peak electrical consumption (kWh). Which of the two is most important, however, can significantly change how the system is designed and/or controlled.
To reduce the on-peak demand, the system should melt ice only when the electrical demand of the building is highest. It is perfectly acceptable to have ice remaining inside the tank at the end of the day. This approach, called “peak shaving,” is commonly used when the on-peak electrical demand (kW) rate is high, but the electrical consumption (kWh) rates are nearly equal from off-peak to on-peak periods. Peak shaving attempts to find the optimum balance between reducing on-peak electrical demand (by melting ice and operating the chiller at reduced capacity) and avoiding significantly increasing off-peak electrical consumption (which happens when the chiller needs to operate in the ice-making mode).
Alternatively, to reduce on-peak electrical consumption, the system should melt as much ice as possible every day. This approach, called “load shifting,” is commonly used when the on-peak electrical consumption (kWh) rate is significantly higher than the off-peak consumption rate. Load shifting attempts to reduce on-peak electrical consumption as much as possible by melting all of the ice during the on-peak period, and shifting chiller operation to the off-peak period.
While it is possible that a system designed for peak shaving may have the same ice storage capacity as a system designed for load shifting, these two systems are controlled differently.
In addition to lowering monthly utility costs, another potential benefit of ice storage is to reduce the size and capacity of mechanical cooling equipment.
When ice storage is used to satisfy all or part of the design (or worst-case) cooling load, the chiller may be able to be downsized as long as the downsized chiller has sufficient time to re-freeze the water inside the tanks.
Smaller, electrically driven chillers may also result in smaller electrical service to the building, which can also reduce installed cost.
Potential Benefits
- Lower utility costs
- Lower on-peak electrical consumption (kWh)
- Lower on-peak electrical demand (kW)
- Smaller equipment size
- Smaller chiller
- Smaller electrical service (A)
- Reduced installed cost
- May qualify for utility rebates or other incentives
While the ice storage tanks add to the installed cost of the system, the impact of downsizing the mechanical cooling equipment may offset some (or all) of this added cost. Additionally, some electric utility companies offer rebates or other incentives when ice storage is used to reduce on-peak electrical demand. When these incentives are available, adding ice storage may even reduce the overall installed cost of the system.
In some installations, each of these benefits might be realized. In other installations, however, one or more may not occur. For example, adding ice storage may lower utility costs, but the time available to re-freeze the water inside the tanks may be so short that the chiller must remain the same size in order to freeze the water fast enough.
Reference
TRC019-EN Ice Storage Systems