Technology Overview: Chilled Water TES Systems

October 16, 2013
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Chilled Water systems are one of the most popular options for thermal energy storage (TES). Chilled water systems are appealing for many applications because they are less complex and have lower operating costs than alternative technologies, but require more physical space as they offer lower storage density than other options. Most chilled water systems are designed for large commercial projects over 500,000 square feet with sufficient space available to accommodate a chilled water storage tank. TES chilled water storage tanks are made of steel or concrete and can be located either above or below ground. Smaller systems typically use prefabricated steel tanks, while larger systems often use reinforced concrete reservoirs.

Image: TES tank serving large commercial/retail buildings.

Basic Function of Chilled Water Storage System

In this type of TES system, chillers produce and store chilled water in collection tanks at night during periods of off-peak electrical demand, and use the chilled water during daytime peak demand hours for cooling. During off-peak hours, when electrical rates are low, chillers produce 40°F water that is pumped into the storage tank (“charging” mode). When cooling is required during daytime hours when electric rates are higher, the stored chilled water (40-42°F) is pumped through the cooling system’s chilled water circuit to cooling coils at the air handling units (“discharging” mode). With a properly sized collection tank, a facility’s cooling needs can be met with minimal electrical usage during peak hours.

Types of Chilled Water Storage Systems

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Chilled water 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).

To ensure chilled water storage system efficiency, the chilled water must be kept from mixing with the warmer return water inside the tanks. There are several different types of storage systems that are distinguished by the way they keep the chilled and warm water from mixing, including stratified tank/nozzle matrix, multiple tank systems, and diaphragm systems.

The most common method is the stratified tank or nozzle matrix method. This design uses diffusers/nozzles located at the top and bottom of the storage tank fill and disperses warm and chilled water during operation. These diffusers/nozzles are designed to provide a stable, sharply defined transition layer, or "thermocline", thus allowing for the natural stratification of warm water at the top of the tank and chilled water at the bottom. In multiple tank systems, one tank is always left empty to dispense or receive water which keeps the cold supply water and warm return water from mixing. In diaphragm systems, chilled water is separated in a single tank by vertical or horizontal moving diaphragms as the volume of chilled water in the tank changes.


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