Learning Objectives - After this course, you should be able to:
- Discuss the key principles of ice-based thermal energy storage (TES).
- Understand why TES can reduce air pollution in some situations.
- Explain how TES can reduce mechanical equipment costs in commercial buildings.
- Explain the two primary ways in which TES can reduce operating costs in commercial buildings.
Somehow, cooling buildings with ice seems primitive-harkening back to the days before refrigerant-cycle air-conditioning when we cooled food and sometimes buildings with blocks of ice. But using ice today is one of the most advanced, smartest ways to cool buildings, and the practice is growing.
Ice-based thermal energy storage is finding its way into more and more high-tech green buildings, including the Durst Organization's recently completed Bank of America building at One Bryant Park in New York City. The growth in thermal energy storage is no surprise once one understands how it works, why it makes economic sense, and why it's a great environmental solution.
Understanding thermal energy storage (TES)
The basic principle of ice-based thermal energy storage for cooling is simple. Ice is produced at night when electricity is cheap, and that ice becomes the source of cooling energy during the day. Ice works so well because a great deal of heat is absorbed and released during freezing and melting (referred to as latent heat). Materials can store heat both as sensible heat and as latent heat. Sensible heat is stored as the temperature of a solid or liquid is changed. Latent heat is stored when there is a change in phase-in this case between solid and liquid.
The latent heat of ice is 144 Btu per pound, meaning that melting or freezing one pound of ice at 32 degrees Fahrenheit absorbs or releases 144 Btus of heat. By comparison, water only stores one Btu per pound for every degree Fahrenheit difference in temperature. So if chilled water is used for TES rather than ice, a lot more volume is required (assuming temperature cycling of 20 degrees Fahrenheit, you would need seven times as much chilled water as ice to provide the same amount of cooling). Other phase-change materials, such as paraffins and eutectic salts, can also be used in TES systems, but these are far less common than ice or chilled water, and their heat-storage effectiveness may drop over time.