Energy Efficiency MeasuresTHERMAL STORAGEThermal Storage may be a money saver in situations when:
Thermal Storage uses “stored” energy, usually in the form of chilled water or ice to provide cooling when electric rates are high or when cooling demands are high. For example, an office building might have a large cooling load during the day and have a time-of-day electric rate. During the night when electricity is cheaper, chillers create chilled water or ice stored in large tanks. During the day when the cooling is needed, chilled water is circulated throughout the building “depleting” the thermal storage that was built up during the night. The graphics show how the cooling load can be redistributed to minimize peaks. Figure 1 shows the cooling demand during a typical day in an example office building. The cooling demand results from people, equipment, and other thermal and solar loads. Figure 1 Figure 2 shows the peak that can be redistributed to the night or off-peak time. Figure 2 Figure 3 shows the load redistributed. In this scenario, the total cooling of the system is the same, but the peak has been drastically reduced. Figure 3 This pattern is particularly important when demand costs are higher than energy costs, a common circumstance for both commercial and industrial buildings in Utah. Thermal energy can be stored in a variety of ways. Chilled water can be stored in tanks. Ice can also be stored in tanks in several different configurations: refrigerant or other coolant can be circulated in submerged piping, causing ice to build up on the outside of the piping. It is also possible to engineer the thermal energy storage system so that, ice can be formed in the piping while refrigerant circulates around the outside. A third method, called encapsulated ice, uses plastic containers of water submerged in a tank with refrigerant or coolant passing over the containers. In any of these methods, to retrieve the “stored” thermal energy, the process is reversed. Water is also frequently used as a medium for storing heat. A large insulated tank can be used to store hot water from a boiler then used to supplement heat from the boiler when demand exceed the boiler’s capacity. This enables downsizing the boiler and enhances overall systems efficiency under some conditions. Thermal Storage can have a large first cost and is not a solution for every situation. A detailed analysis including hourly building cooling loads, system performance data, and the applicable utility rate schedules is critical in evaluating the economics for such a system. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has a handbook chapter dedicated to Thermal Storage that provides more detailed, technical information, 2003 ASHRAE Handbook, HVAC Applications, Chapter 34, Thermal Storage. ASHRAE also has published a guideline with a long list of additional resources, “ASHRAE Design Guide for Cool Thermal Storage” available for purchase on www.ashrae.org. Click on the following link to read a U.S. Department of Energy case study of how thermal storage was used to reduce the operating costs of a Veterans Administration medical center facility in Dallas, Texas: http://www.eere.energy.gov/femp/pdfs/uesc_cs_3.pdf. |
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