Thermal Energy Storage Classification
Thermal energy storage technologies commonly used in the district cooling industry can be classified according to the form of energy stored in the system. Cool energy can be stored either in the form of sensible heat or latent heat.
Sensible Heat Storage
In a sensible heat storage system, the energy is stored as sensible heat associated with the change in temperature of the storage media. The storage media does not undergo a phase change. The amount of energy stored in a sensible heat storage system is dependent on the sensible heat capacity of the media and the degree of temperature change during the charging process. In district cooling systems, the most popular form of sensible heat storage is the chilled water storage system.
Latent Heat Storage
In a latent heat storage system, the energy is stored as latent heat as the storage media undergoes a phase change, transitioning from liquid to solid form. The amount of energy stored in a latent heat storage system is dependent on the latent heat of fusion of the media.
In district cooling systems, the most popular form of latent heat storage is the ice storage system.
Chilled Water Storage System
A chilled water storage system utilizes the specific heat of water (4.18 kJ/kgOC) for storing cool energy. The volume of water required is determined by the temperature difference between the chilled water supply and the chilled water return.
The storage capacity or amount of energy which can be stored is given by the following equation:
In district cooling systems chilled water is typically stored at temperatures ranging from 4OC to 4.4OC. These temperatures are suitable for standard water chillers without the need for brine chillers. To maximize usable storage, the chilled water temperature differential must also be maximized.
At the lower end, the practical limit is 4OC at which water density is highest. The limitation on the higher chilled water return temperature is determined by cooling coil performance and dehumidification requirement. For comfort cooling purposes, the practical limit for this temperature difference is around 10OC ~ 11OC.
The table above shows the chilled water storage densities that can be achieved with different differential temperature values. (Assumption: Storage efficiency of 100%)
The efficiency of a chilled water storage system is also determined by the degree of separation between the cold water and the warm water in the tank. Various kinds of methods have been devised to achieve an acceptable degree of separation. The figure below shows the different kinds of chilled water storage systems commonly used.
By far the most popular chilled water storage technology of choice in district cooling systems is the thermal stratification system which offers significant advantages such as excellent economy of scale, relatively high storage efficiency, and simplicity.
Ice Storage System
Ice thermal storage makes use of the latent heat of fusion of water (335 kJ/kg) for storing cool energy. The storage volume is determined by the final proportion of ice to water in a fully charged tank and is typically in the range of 9.47 RTH/m3 to 14.21 RTH/m3, depending on the ice storage technology.
As ice thermal storage requires the formation of ice, the required charging temperature of the secondary coolant ranges from -6OC to -9OC. This is below the normal operating range of conventional cooling equipment for air-conditioning applications. Depending on the ice storage technology, special ice-making equipment or brine chillers are used for low-temperature service. The heat transfer fluid for ice production may be a refrigerant or a secondary coolant (usually Ethylene Glycol solution).
The two types of ice storage technology commonly found in district cooling applications are the encapsulated ice system and the ice on coil system.
Encapsulated ice system
The encapsulated ice system consists of plastic capsules containing water with a nucleating agent. The plastic capsules are immersed in a tank of Ethylene Glycol solution (also known as brine solution). During charging, cold ethylene glycol is circulated through the tank, and in the process, ice is formed in the plastic capsules. During discharging, the process is reversed, with warm glycol being circulated through the tank to melt the ice in the capsules, thereby releasing the stored cold energy.
Ice on Coil System
The ice on coil system consists of heat exchange tube/coil bundles immersed in a tank filled with water. Ethylene Glycol solution (Brine solution) is circulated through the tubes. During the charging process, cool energy is stored through the formation of ice around the circumference of the heat exchange tubes. The process is reversed during the discharging process by melting the ice on the tubes, thereby releasing the stored cool energy.
The ice on coil system can be further categorized into the internal melt and the external melt systems. The main difference between these two technologies can be narrowed down to the way in which the ice is melted during the discharge cycle.
The internal melt ice on coil system reverses the flow of the brine solution during the discharge cycle to melt the ice “internally” i.e., from the inside out, hence the term “internal melt”.
The external melt ice on coil system circulates warm chilled water through the tank during the discharge cycle to melt the ice “externally” i.e., from the outside in, hence the term “external melt”.