#1 - Thermal Energy Storage - Cold Chiller Water
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Underground Water Storage Tanks
THERMAL ENERGY STORAGE - Darco underground fiberglass tanks can be used for storing energy in the form of Cold or Hot water. Cold water created by evaporative coolers or refrigeration compressors may be efficiently stored in an uninsulated underground cistern due to the constant cool soil temperature 10 feet below grade. Hot water is collected from solar panels or another heat source and stored in an insulated fiberglass tank either below or above ground.
Cold water "Chiller Thermal Storage Tanks" commonly use diffuser manifolds and transfer water directly into and out of the storage tank without the use of heat exchangers. Since the ΔT is usually about 25 degrees, the water volume flowing through the tank as well as the overall storage capacity must be large. Chiller systems can use a single naturally stratified tank or a 2-tank warm / cool storage system as shown in Illustration #1.
Compressors run during off peak evening hours when electricity costs are lower. Cold water is then circulated to provide cooling during the following day while the compressors are at rest. Choosing the compressor units, sizing storage tank water volume, while looking at the weekly load cycle is an interesting engineering project. In certain parts of the country, evaporative cooling can assist or even replace mechanical compressors in mass air chilled water systems. The beauty of cold water storage is it's simplicity and freedom from maintenance. Chilled water tanks are not normally insulated since the average sub-surface ground temperature at 10 feet stays about 55 degrees F.
Hot water "Solar Thermal Storage Tanks" may include internal submerged heat exchanger tubes or rely on external heat exchangers. Generally, Solar Tanks hold water at approximately 150 degrees F. When sufficient hot water inflow is available, mechanical destratification is used to blend the higher temperature surface water into the rest of the tank volume. The key to efficiency is maintaining the highest practical water temperature and volume, while using natural stratification to provide the highest discharge water temperature. External plate type heat exchangers may provide greater design flexibility in certain applications. Darco underground Solar Storage Tanks are insulated in most applications and are available with or without internal tube style exchangers or diffusers as shown in Illustration #2.