THERMAL ENERGY STORAGE - Darco underground fiberglass tanks can be used for storing energy in the form of Hot or Cold water. Hot water is collected from solar panels and stored in an insulated tank. Cold water created by refrigeration compressors may also be stored in underground tanks. These thermal storage vessels are called either "Solar Tanks" or "Chiller Tanks."

Hot water "Solar Tanks" may include 2 bundles of submerged heat exchanger tubes. One bundle delivers the high temperature solar panel fluid into a properly sized Thermal Storage Tank. The second bundle transfers stored heat energy into the building heating system. Generally, Solar Tanks hold water at approximately 150 degrees F. Exchangers leading into and out of the tank are located at the top of the vessel to take advantage of natural stratification. When sufficient hot water inflow is available, mechanical destratification is used to increase the volume of high temperature surface water. The key to efficiency is maintaining the highest practical water temperature and volume, while using stratification to provide the highest temperature water surrounding the discharge exchanger. External exchangers may provide greater efficiency in certain applications. Darco underground Solar Storage Tanks are insulated to approximately R-50 in most applications and are available with or without internal feed and discharge tube bundles installed.

Cold water "Chiller 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 only about 25 degrees, the volume flowing through the tank as well as the overall storage capacity must be much larger. Chiller systems can use a single naturally stratified tank or a 2 tank warm / cool system as illustrated.

Compressors run during off peak evening hours when electricity costs are lower. Cold water is then circulated to provide air conditioning during the 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. Thermal storage can be accomplished using either cold water or phase change products. The beauty of cold water storage is it's simplicity and freedom from maintenance. Chiller tanks are not normally insulated unless the average sub-surface ground temperature is above 60 degrees.