Hot Water Storage

Residential Tanks with Integrated Heat Exchangers by Brian Mehalic

A critical component of a solar thermal system is the storage tank—essentially a "Btu battery" where heat energy is stored. Storage allows the system to provide hot water at any time of day, whether the sun is shining or not, since solar production typically does not coincide with the peak periods of hot water demand, usually mornings and evenings.

The amount of storage needed depends on the number and habits of those using the hot water. For domestic solar hot water production, storage tanks usually range in size from 60 to 120 gallons. And since most people don't want to have to take cold showers during long, cloudy spells, tanks are commonly paired with a backup heat source, either electric or gas.

Integrated Heat Exchangers

There are numerous types of solar hot water systems but most use heat exchangers (see "Solar Hot Water Simplified" in HP107 and "Solar Hot Water: A Primer" in HP84). Usually, and nearly always in climates with the potential for freezing, domestic/potable water does not go through the solar collector loop—instead, a heat transfer fluid (HTF), usually propylene glycol or distilled water, is used in a closed-loop solar circuit. The HTF is pumped through the collectors, where it picks up heat, and then flows through a heat exchanger, where that heat is transferred to the water stored in the tank. Often the heat exchanger is integrated into the storage tank, either as coils submerged in the water, or wrapped around and in contact with exterior of the tank wall (see "Fundamentals of Solar Heat Exchangers" in HP128).

Backup Electric Element:

Covered

Backup Electric Element:

Covered

Solar Heat Exchanger In:

From collector

Solar Heat Exchanger In:

From collector

Solar Heat Exchanger Out:

To collector

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