Solar thermal systems come in single or multiple tank configurations. Single tank systems combine solar energy heating with backup heating in the same tank. That means solar hot water is always competing with backup heating. We do not believe this can be more efficient or more cost-effective than a multiple tank system.
A multiple tank system pre-heats domestic hot water in a tank (or tanks) which do not have a backup heating source. The pre-heated water is then piped to an additional tank (or tanks) which do have a backup heating source. This eliminates any competition between a solar heating source and any backup or final heating source.
Though single tank systems are appropriate when space is at a premium (limited installation space), they are not the most productive. Multiple tank systems cost more, but they harvest more energy, thus are more attractive.
A simplified schematic is presented below showing an example 2-tank solar thermal system. The tank on the left uses an internal heat exchanger to pre-heat incoming water, while the tank on the right performs any additional heating required, as in winter months when solar thermal systems are less active.