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Solar Water Heating
Quality First Home Improvement Newsletter
Sept 2009 - Issue 101
Basically, there are two types of solar water heating systems - Compact systems (passive systems) and active systems.
Simply put, active systems use a pump, and passive systems don't.
Passive systems
There are two types of passive solar energy water heater systems thermo siphon systems, and batch heaters, neither require a pump. Below is a photo showing collector plates and the storage tank of a thermo siphon system.
A thermo siphon system has the storage tank located above the solar collectors.This allows it to take advantage of a principle known as natural convection. Natural convection is based on the fact that warm water rises and cold water sinks. This phenomenon occurs due to the difference in the density of the warm water and the cold water. The cold water has a higher density, or is heavier, so it sinks.
Natural convection creates a flow of water through the system. As the cold water sinks to the bottom of the storage tank, it flows back to the solar collectors to be warmed up. Once warmed up, it naturally rises, flowing back to the storage tank to be used by the household or recirculated to the solar collector if it cools down before being used.
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Author - Roland G. Ludlow
Batch heaters
A batch heater consists of a water storage tank, usually painted black, that is placed inside of an insulated collector box that has a glazed glass cover to allow sunlight to hit the water tank.
The system is hooked up to a home's hot water heater. Valves are used to direct the flow of water to the batch heater, or divert the flow when sun is unavailable. When hot water is used in the home, water pressure from the home's plumbing moves the water through the system. Because the storage tank is inside the solar collector, these systems are also known as integrated collector storage (ICS) systems.
Typically, passive systems are not as energy efficient as active systems that use a pump.
Active Systems
Active solar water heating systems use pumps to circulate the water or heat-transfer fluid through the system. They are considerably more efficient than passive system. There are two types of active solar energy water heater systems, closed-loop and open-loop systems.
Closed-Loop System
In a closed-loop system, a heat transfer fluid is used.
The heat transfer fluid is usually a water-antifreeze mixture, most often GLYCOL. After the fluid is heated in the solar collectors, it is pumped to a storage tank where a heat-exchanger transfers the heat from the fluid to the household water. A closed-loop system is also known as an "indirect" system since the household water is heated by the heat transfer fluid and not directly in the solar collector. Below is an example of a Glycol closed loop system.
Open-loop system
In an open-loop system, household water is heated in the solar collector and pumped to a storage tank. The heated water is then piped to faucets for use by the household.
An open-loop system is also known as a "direct" system since the household water is heated directly in the solar collectors. Since this system uses regular household water directly in the collector, it should only be used in areas that do not experience freezing temperatures.
