Technology Overview: Ice Storage TES Systems

October 16, 2013
 
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Ice storage thermal energy storage (TES) systems are similar to chilled water storage systems, but rely on ice instead of chilled water as a storage medium. Ice storage systems are less often used than chilled water systems due to being more complex and less efficient, but offer the densest storage capacity. Ice storage tanks are about 75% smaller than chilled water tanks and as a result are most commonly used in small commercial buildings with limited space. Ice storage systems can also produce lower temperature chilled water, resulting in smaller chilled water pumps, piping, and coils; however, ice storage systems use more energy to make ice, are more complex to design, and are often more expensive to install and operate. The choice of TES technology is often dependent on factors such as desired system operating temperature, existing chiller plant equipment, available space, cost-effectiveness, and the preferences of the facility’s designer..

Image: Multiple ice storge small TES tanks with chilled water lines at Chabot-Los Positas Community College District. DOE-Sandia

Basic Function of Ice Storage Systems

During off-peak hours, an ice storage system’s chiller creates ice or freezes the storage medium (“charging” mode).  An ice storage system chiller operates at a higher kW/ton rating than a conventional AC chiller because it requires more energy to create ice than to chill water. When there is cooling load during the daytime on-peak periods, the ice in the storage tank is allowed to melt and the chilled water is pumped from the storage tank at 32°F and circulated through the cooling system’s chilled water lines (“discharging” mode). The chilled water is circulated to the air handling system’s existing chilled water coils to cool the space.

Types of Ice Storage Systems

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Ice storage is one type of Thermal Energy Storage (TES) technology. TES is the most popular way to implement the demand-side energy management strategy known as 'Permanent Load Shifting' (PLS).

There are two types of ice storage systems: ice building systems (static systems) and ice harvesting systems (dynamic systems). Static systems in general are more compact and less costly than dynamic systems. In static ice building systems, ice is produced around multiple coils or tubes submerged in a storage tank. The coils contain a fluid with a lower freezing point temperature than water, such as a water/ethylene glycol solution. Since the temperature of the fluid is lower than the freezing point of water, ice is formed at the surface of the coils.

Another approach to static ice storage uses glycol and encapsulated containers placed inside the storage tank. This system is a closed loop, pressurized and is not open to the atmosphere. A glycol/brine solution and water mixture (at about 25°F) is pumped from the chiller through a tank and around the container balls and freezing them.

In dynamic ice harvesting systems, ice is produced on evaporator surface plates, and as the thickness of the ice reaches a set target, the system warms the plates to release the ice into a tank below that is partially filled with water. By repeating this process, a large quantity of ice water can be generated and stored for the discharging mode. During discharge, cold water is pumped from the bottom of the ice water tank and the warmed return water is distributed over the ice at the top of the tank.

 

 
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