March-April 2009

From: Fire and Ice

How Ice Storage Works

The working principle of an ice storage system calls for chillers to make ice inside storage tanks during off-peak, nighttime hours when the energy is produced more efficiently, economically, and environmentally. Ice Storage uses the stored “thermal” energy of the ice to cool the buildings during the daytime peak-usage periods. This effectively shifts the electrical load off-peak, while avoiding higher-price energy and demand charges that are imposed by many utilities.

The operation of an Ice Storage System is comprised of two normal modes: the ice charging mode and ice melt/burn mode.

During the ice charging mode, a designated ice-making chiller produces low temperature, 25% glycol solution that freezes the water inside an ice storage tank. This closed glycol loop consists of the ice storage cooling tanks, heat exchangers, and glycol pumps. The ice-charging mode continues until full ice capacity has been reached (usually about eight to 10 hours).

Initiated via an automated process, the ice melt mode begins when the temperature differential increases between the chilled water supply and the chilled water return. During the Ice Melt Mode, the chillers are either turned off or used to supplement the cooling requirements of the system.

The Benefits of Ice Storage Cooling
In many cases, ice storage allows for reducing the size of installed refrigeration capacity. For example, if the installed cost of the chiller is $600 per ton, every avoided ton of refrigeration capacity related to the installed ice storage system capacity saves $600. This assumes that the installation already has supporting chiller equipment in place, such as cooling towers and pumps. For a new installation, this amount can exceed $1,500 per ton, in which case the chiller plant size reduction can often completely offset the cost of storage.