Articles » Heat Storage 101

What is heat storage?
Heat storage usually consists of a large volume of water that is heated by a boiler and that heat is stored in a well insulated tank to be used as needed for heat once the boiler has stopped running. These tanks come in different types, shapes, and sizes. They are commercially available, but many choose to build their own or recycle tanks intended for propane or agricultural use.

Open vs. Closed Storage
Heat storage can be pressurized (closed) or non-pressurized (open). Heating systems typically operate with pressurized water to make pumping the heated water to the heat zones easier. Open storage is sometimes called a “pond in a box” because the water is typically stationary in the tank and the heat is transferred in and out via a heat exchanger coil of copper pipe. This creates a separation between the closed heating system and the open storage tank. Alternately, this non-pressurized water can be pumped out of the tank and through a flat plate heat exchanger to create this separation.

Fully closed systems are created by having a pressurized tank to store the heat in. The heated water is pumped in or out of the tank to charge or draw from the tank without the need for a heat exchanger. There are commercially available pressurized tanks designed for heat storage. Alternately people have recycled propane tanks which can be purchased and modified for less money.

Open Storage

Pros:
•Can be constructed on-site in locations that would be impossible to put a large pressurized tank (like in a basement).

Cons:
•These tanks are often lined with EPDM liners which have a high temperature limit of approx. 180° (however for longevity better around 170°). These liners also have only a life of approx. 10 years.
•The need for a heat exchanger (copper coil or flat plate) that is costly.

Closed Storage

Pros:
•Does not require costly heat exchangers.
•No limits on maximum temperature (could be heated even to 200°) allowing for longer times between running the boiler.
•You can get used recycled propane tanks or other types of vessels that are appropriately pressure rated and modify them at a reasonable price.

Cons:
•These tanks tend to be larger and hard to locate in basements. (However, this can be mitigated by using several smaller tanks in series)
•Recycled tanks typically need some welding modifications and either a domestic hot water coil installed or an indirect hot water heater used.
•The larger the volume of pressurized water you have, the larger the expansion tank for your system will need to be. These large expansion tanks can be expensive.

Insulation

No matter what type of tank you decide to build or buy, it will need to be very well insulated. The EPDM lined tanks like the one I built often use polyiso. insulation due to the r-value of 6.5 per inch. Several of the modified propane tanks I have seen used have had a box built around them and then filled with loose cellulose insulation. The desired R-value would likely need to be R20+.

Stratification

Hot water in a tank will naturally want to stratify into layers where the hottest water in near the top. If that stratification can be maintained, there will be hotter usable water near the top of the tank. As a result, when charging the tank water must be drawn from the bottom and returned to the top. When drawing from the tank water is taken from the top and returned to the bottom. (for a more detailed explanation of stratification see: Thermal Energy Storage: Systems and Applications pg.261)