Solar Water Heating

Figure 1. Solahart passive 302J Free Heat (80 Gal) solar water heater

In a passive system, the natural Thermosiphon effect is used to transfer heat into the domestic water. Cooler fluid descends and warmer fluid rises. The warmer fluid heated by the sun rises in the collectors and then to the tank where it transfers its heat to the household water. As the heat exchange fluid then cools, it descends again to the bottom of the collector to go through the entire process again. The process is self-sustaining and requires no electronics, pumps or controllers. The water storage tank is also fitted with an electrical backup element for when weather conditions don’t allow the sun to heat the water to a desired temperature. Solahart (owned by Rheem) makes an outstanding system in the passive category.

It’s easy to identify a passive system. The water storage tank has to be positioned above the collectors. If the water storage tank is positioned below the collectors, pumps, controllers and some additional equipment are required to circulate the heated fluids from the collector/s to the tank/s. This is the principle of active solar water heating systems.

Just the collectors are mounted on the roof. The tank/s can be positioned almost anywhere in the house or office building. The temperature on the roof is compared with the temperature of the water or heat exchange fluid. When necessary, a controller activates one or more pumps to circulate the heat exchange fluid to the collectors and back to a heat exchanger where the heat is transferred to the domestic water. Of the various types of active systems on the market, Solar Texas recommends the active drainback system. Solar Texas markets SunEarth and AET products in this category.

In a drainback system, when the heat exchange fluid is not required, it drains back from the collectors to a separate storage tank. This has a number of advantages over a standard active system:

• First, the drainback system facilitates a longer time between services because the heat exchange fluid is only used when it is required and not in the collectors 24/7.
• Second, should a pump or controller ever fail or the power go out, the fluid in the collectors simply drains back rather than remaining in place and suffering stagnation issues with high temperatures. 

Comparison of Passive vs Active

It’s really up to the homeowner as to which system to choose. Both are very good and outstanding products. The active system is a little more expensive in acquisition costs, but both systems are subject to the 30% Federal tax credit in 2007.

The initial expense is more than that required for traditionally powered and instantaneous water heaters, but the benefits are that up to 95% of subsequent hot water needs are provided free.  Good quality solar water heating systems are constructed for long life. The solar water heaters that Solar Texas markets can be expected to outlast two, three or more traditional systems. 

To considerably reduce installation cost, it is best to have these systems on hand as a new house or business structure is built. The solar water heater, correctly sized, will provide a complete water heating system on its own. All the systems marketed by Solar Texas have backup heating as standard.

Contact Solar Texas and find out how a solar water heater can benefit you.