Thermal Energy Storage--A Primer.

It pays to know the basics

Storing thermal energy for use at a later time is an excellent energy management strategy. Thermal energy storage (TES) systems can store low-cost energy that is generated off-peak as an electrical demand cost-control measure. But TES can also be used to hedge in competitive utility markets for both electricity and gas, to reduce emissions, and to lower energy use.

Frequently, energy is available at one time but needed at another time. TES systems bridge the two times. TES is a mature technology that has been used in a variety of applications ranging from cooling and heating of buildings to cooling of gas turbine inlet air. Some TES systems have been operating continuously and satisfactorily for over 30 years, and some manufacturers and system designers have been in business throughout that period.

A classic TES application collects solar energy during the day for use in heating a building during the night. Recently, it has become common to build cooling reserves during the utility off-peak period for use during the following on-peak period. These applications result in reduced energy cost and, frequently, decreased energy use as well.

When utility energy is used to operate heating or cooling equipment near design capacity and unneeded output is stored for later use, the end user's equipment often runs at a more consistent and efficient rate. The utility may also be able to optimize the use of its equipment. TES operation that smoothes the load profile also reduces energy use, particularly in the case of cooling equipment, because the chillers are operated more at times when they operate more efficiently due to lower ambient wetbulb temperatures.

Alternatively, energy may be available at the discharge of a device or a process at a temperature that is suitable for heating or cooling a space or another process, but the supply does not occur at the same time as the demand. TES provides a means for storing the heating or cooling capacity that might otherwise be wasted and …