Chilled Water Storage

 

• Chilled water is normally generated using off-peak energy supply, stored in chilled water storage tanks then distributed for use during peak hours. The economic benefits of chilled water storage systems therefore generally rely on lower off-peak electrical rates.
• Chillers running at lower ambient temperatures which are typical of off-peak use tend to have higher efficiencies.
• It is usually possible to install smaller chillers, heat rejection equipment and pumps when stored chilled water is used to meet peak cooling loads.

Primary-Secondary CHW loops

A typical application of a chilled water storage tank would be on the supply side of a primary chilled water loop in parallel with on or more chillers as shown below.

 

Four-pipe Fan-coil unit air-cooled chillers with chilled water storage

 

Chilled water storage requires a difference in setpoint temperature between the source (charge) side and use side:

 

• Source side (lower): Primary CHW loop setpoint temperature, e.g. 6°C (set in the loop SPM);
• Use side (higher): Chilled water storage setpoint temperature, e.g. 7.5°C (set on the chilled water storage dialog).

 

In the example below the secondary chilled water loop setpoint temperature is set to 10°C, but it could be set lower, down to 6°C.

 

Chilled water storage system temperature settings

 

The chiller provides 6°C chilled water while the chilled water storage tank setpoint is 7.5°C (cut-in temperature is 7.5°C+2.0°C=9.5°C). The secondary chiller SPM temperature should be set the temperature of the chilled water to be supplied to the equipment such as cooling coils etc.

 

Charging is normally done at night-time while tariffs are lower. This can be defined through the chilled water storage source side availability schedule (see screenshot below). Likewise, use side time can be set as required, typically for day-time only, through the use side availability schedule. The use side availability can be set using the On 24/7 schedule in many cases.

 

Chilled water storage setpoint temperature and source side availability schedule

 

Above settings provide tank cooling setpoint temp of 7.5degC, charging at night only and use side available anytime.

 

Plant equipment operation scheme should give a higher priority for chilled water storage, and a lower priority for chiller (screenshot below).

 

Secondary chilled water loop plant equipment operation settings

Single chilled water loop

This section covers use of chilled water storage tanks in a single parallel chiller arrangement as shown in the screenshot below.

 

Chilled water storage in parallel with the chiller

 

As with the primary-secondary loop configuration, the single chilled water loop needs a difference in setpoint temperatures between source (charge) side and use side:

 

• Source side (lower): CHW loop setpoint temperature, e.g. 6°C (through loop SPM);
• Use side (higher): Chilled water storage setpoint temperature, e.g. 7.5°C (set on chilled water storage dialog).

 

For a single chilled water loop configuration, the plant equipment operation needs to set up differently to the primary secondary loops described above. The day-time and night-time operation is defined separately using one item of equipment for each scheme (see screenshot below).

 

Plant equipment operation settings for single chilled water loop arrangement

 

There are currently two types of chilled water storage tank:

 

• 1-Mixed,
• 2-Stratified.

 

The 1-Mixed type simulates a well-mixed, single-node water tank. The 2-Stratified type simulates a stratified, multi-node water tank. Both storage tanks share similar features, such as thermal losses to the zone, however, each type has its advantages which may make one more appropriate than the other depending on the application.

 

Advantages of 1-Mixed type:

 

• requires less input than the stratified tank
• faster execution time than the stratified tank

 

Advantages of 2-Stratified type:

 

• better modelling of thermal storage applications which rely on stratification to improve heat transfer performance.