An Engineering Extension energy feature
An Engineering Extension energy feature
By Tom Logan
Ordinarily we think of ice as something to chill drinks, reduce swelling, or provide recreation for skaters and hockey
players. Today we are using ice to cool large buildings. The process is called cool storage and it involves production of
chilled water or ice at off-peak electrical and cooling conditions and storing it to use during peak times.
There are several advantages to cool storage. One is the reduction in electrical demand.
Demand, as the electrical industry uses the term, is the maximum average electrical use over any 15-30 minute period. By leveling electrical demands, facilities can reduce electrical costs. This is done by distributing energy usage over a period of time instead of having peaks and valleys. The total electrical consumption remains the same, but is spread over the entire day, not just a short period when demand is high -- and so are rates.
Another advantage is the savings gained from lower electric rates. Utilities may offer lower rates at night. This enables them to better use generating equipment. Ice or chilled water is produced at night when rates are low. The stored "cool" is then used during the day. Large short-duration loads are also ideal for cool storage.
Examples would be sports arenas, churches, and theaters. At these types of facilities, smaller equipment can store cooling over a day and use it to meet a short-term load.
A cool storage system generally costs more to construct than a conventional cooling system, but it makes up for this by taking advantage of off-peak electrical rates and reduced demand charges. There are a number of different types of systems which produce and store cold water and ice. They all use cold-generating equipment such as chillers, storage tanks or basins, and pumps and piping to deliver cooling to the building at a later time.
Regardless of the type of system used to generate the coolness, it must be operated correctly to maximize energy efficiency. There are several approaches to this.
Just as a football team must have a game plan, a cool storage system must have an operating strategy. Whatever the system is, it operates on one of three strategies -- full storage, partial storage, and demand limiting.
Normally, a system uses several chillers to produce the cooling for storage. They can be operated singly or in combination with the other chillers, depending on the load. Full storage systems operate the chillers only in the off-peak hours, and cool from storage during peak hours. A timer turns the chillers on and off.
Partial storage systems operate their chillers through the entire period. They store additional cooling during the off hours when building cooling requirements are less than chiller capacity. During peak hours, the stored ice or cold water supplements the cooling system.
A demand limiting system operates similarly to the partial storage system, because chillers store cooling in off-peak hours for later use. The difference is that as building electrical demand starts to climb during business hours, chillers shut off in stages to level the building's electrical demand, and cooling is drawn from storage. After normal working hours, when electrical demand falls, chillers come back on line in stages as the level of electrical demand permits.
As for control, a full-storage system is the easiest to operate because the chiller is either on or off. In partial storage and demand limiting systems, operation is more complicated, but computer monitoring simplifies this.
It is important that operators of cool storage systems are trained and understand how their equipment works. A storage system won't save energy and money if you operate it improperly. For example, if you use the stored ice sooner or in greater quantity than required, your system suffers because the cooling equipment may need to be brought back on line at full capacity. This may not be sufficient to handle the instantaneous load. A high demand charge may also result.