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Understanding Solar Hot Water Systems

What are the different types of solar hot water systems available?
There are five types of solar hot water systems today.  We will explain their operation along with their advantages and disadvantages below.

Open System (the most efficient)
The same water that is used to supply your house, or business, is used in this type of system.  The water from the tank circulates to the collectors, then back to the tank and on to the faucets.  Circulation is accomplished via a water pump (DC or AC).


Closed Antifreeze System
Here, antifreeze circulates from a coil of pipe submerged in the water of the tank to the collector.  The antifreeze coil provides heat to the water reserve in the tank.  This system is protected against freezing weather.

Closed Drainback System (the least efficient)
Here, water in an outer tank is circulated to the collector.  The water level drops back to the tank when the pump stops circulating.  The inner tank holds the pressured water from the house.  There are three efficiency losses present in this system:

  • The loss in the heat transfer between the two tanks.
  • The outer tank with higher temperature looses more heat to the air.
  • The air in the panel loop reduces heat transfer of the water, which will create the need to double the number of panels.



Closed Antifreeze system with internal heat exchanger
Additional losses over 2nd best system (closed antifreeze).  In this system, there are double heat exchangers present with double the losses.  These two tanks lose heat to the air, and demand higher initial costs due to an additional pump that is required.


Closed drainback system with separate heat exchanger tank
Additional losses over the third best system above.  We now have introduced a fourth loss associated with an additional heat exchanger.  This heat exchanger is of course less efficient than the built-in heat exchanger.  This system includes a second tank that is loosing heat to the ambient.  This system also has two additional pumps that will require  maintenance.

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