What is thermal energy storage in a district cooling system?
Thermal energy storage, in the context of district cooling, is the process of producing and storing cooling energy during periods of low demand. The stored cool energy is then discharged to meet cooling requirements during periods of high demand. Depending on the type of thermal energy storage technology, the cool storage medium can be in the form of chilled water, ice, or some other form of phase change media.
How does a thermal energy storage system work?
Thermal energy storage effectively decouples cooling energy generation from cooling demand. As shown in the graph above, chillers are operated continuously during off-peak hours, from 2200 to 0800 hours, even when cooling demand is less than the chiller production capacity. The excess cooling energy from the chillers is stored in the thermal energy storage system. During peak hours, from 0800 to 2200 hours, cooling demand exceeds chiller operating capacity, and the stored energy is discharged to supplement chiller production capacity in meeting the higher cooling demand.
How does a thermal energy storage system help to reduce the installed capacity of a district cooling plant?
In a traditional central chiller plant system (without thermal energy storage), the chiller operating capacity must be chosen to match the maximum cooling load on the design day. Using the cooling load profile shown in the chart above as an example, the chiller operating capacity must match the peak cooling load of 16,975RT, which occurs only once per 24-hour cycle. Every other hour, the chiller plant will run at a lower part load. This is not conducive to the efficient use of production assets.
In the case of Thermal Energy Storage, the total chiller operating capacity does not have to match the maximum design day cooling demand. Furthermore, the chiller operation can be decoupled from the end-user cooling demand, allowing the total chiller operating capacity to be sized considerably smaller than the maximum cooling load. As shown in the chart above, proper sizing of the chiller operating capacity allows the chillers to run continuously throughout the 24-hour cycle. The thermal energy storage system is charged during off-peak hours and then discharged during peak hours to supplement the chillers’ chilled water production.
Thermal Energy Storage and Chiller Plant Efficiency
The incorporation of thermal energy storage allows the district cooling plant to have a smaller installed chiller capacity. At the same time, the chillers will be able to operate at a higher and more constant load continuously throughout the day. This leads to improved asset utilization efficiency and higher average chiller COP.
Thermal energy storage enables more chillers to operate at night when the ambient wet-bulb temperature is lower which allows for the lower cooling water temperature to be supplied to the chiller condenser. The resulting lower compressor lift will increase chiller COP and improve overall chiller plant efficiency.