The terms passive and active in solar thermal systems refer to whether the systems rely on pumps or only thermodynamics to circulate water through the systems.

Passive
The simplest systems are passive solar water heaters, also called batch or breadbox collectors, they are most common in regions that do not experience extensive periods of below freezing temperatures. The water in these solar collectors circulates without the aid of pumps or controls.

Active
Active solar water heaters use pumps to circulate water or an antifreeze solution through heat-absorbing solar thermal collectors.


Direct vs. Indirect
In an indirect system, also known as “closed loop,” a simple pump moves the antifreeze solution through a loop into the solar collector, through the collector’s pipes, and out of the solar collector. The sun-warmed antifreeze solution then flows into a heat-transfer unit where it warms the cool water heading into a conventional hot water tank. The antifreeze solution returns to the pump and again flows into the solar collector without ever mixing with the building’s water. Indirect systems are encouraged in climates with extended periods of below-freezing temperatures.

An important distinction among solar thermal systems is whether they are of direct or indirect design.  In a direct system, the water used by building occupants to wash their clothes or bathe is the same water that is pumped through the solar collector. In an indirect system, an antifreeze solution is pumped through the solar heat collector. This warm solution is then used to heat the water used by building occupants. In this case, water is indirectly heated.

Because office and retail buildings don’t use significantly more or less hot water than homes, the possibilities for solar hot water systems overlap. If the temperature drops below freezing, then we recommend one of the two Indirect Systems below as best options for solar thermal for businesses in the Northeast.

Indirect, Freeze Protection System

• Uses a pressurized loop that circulates a food-grade water-glycol solutions
• Indirectly transfers heat from the water-glycol solution to the potable water entering your hot water tank
• Requires that the glycol solution be changed about every few years
  

Drain-back System (Indirect)

• Uses a non-pressurized, closed loop that circulates a heat-transfer solution—either distilled water, glycol-water or alcohol
• Indirectly transfers heat from the heat-transfer solution to the potable water going into the hot water tank. This heat exchange takes place in a special heat-transfer unit
• Dumps the heat-transfer solution from the solar collector loop into a reservoir tank (thus, drain back system) when outside temperature reaches freezing
• Does not require that the heat-transfer solution be changed