Open Loop & Closed Loop Systems
Direct & Indirect Solar Water Heaters
The overall concepts of solar water heating are relatively straightforward. Solar water heating systems have solar collectors which harness the sun's energy to heat water and store it. The two types of solar water heating systems are direct (open loop solar water heater systems) and indirect (closed loop solar water heater systems).
Closed Loop Solar Water Heating Systems
Image: U.S. Department of EnergyClosed loop solar water heater systems, or indirect systems, circulate a non-freezing heat transfer fluid (such as propylene glycol solar antifreeze) through the solar collectors, where it is heated by the sun, and then into a storage tank where a heat exchanger transfers the heat from the fluid to the water.
A closed loop solar system works well in areas that experience freezing temperatures. In contrast, open loop solar water heater systems, or direct systems, should not be used for cold climates. This is because in an open loop solar water heater system, the home's water is directly heated in the solar collectors, then pumped to a storage tank or used up directly. No heat transfer fluid is used. Because the water is passed outside the home into the solar collectors, it should not be used in cold climates due to the possibility of freezing and damage to the collectors.
Open Loop Solar Water Heating Systems
Image: U.S. Department of EnergyOpen loop solar water heater systems are generally less expensive to install than closed loop solar heaters but can have drawbacks, including the aforementioned freeze potential. Also, open loop solar water heater systems can overheat. For example, in areas with hard water, scaling can accumulate in the collectors of the open loop solar water heater causing damage and reducing performance.
Solar water heaters (both open loop solar water heater and closed loop solar systems) are often used as preheaters to augment conventional gas or electrical water heaters. This guarantees a supply of hot water, even in periods of limited sunshine. A solar water heating system will increase your home's value, reduce energy costs, and minimize environmental impacts for decades.