Direct and Indirect Systems
There are both direct and indirect systems. Direct systems generally use water that enters the building as a heat transfer fluid. One example would be a pool heating system where the water flows directly through the collector and into the pool with heat being directly transferred to the designated source.
More common are the indirect systems that circulate heating fluids through separate tubing within the water heater to heat or preheat the water in the tank. This fluid could be water or a glycol-based mixture. In either instance, the fluid does not come in direct contact with the potable water. In these systems, there is typicallyÂ solar-based heat-exchangeÂ tubing located in theÂ lower portionÂ of the hot water tank and a backup element from the electric or other system located in the upper part of the tank. The solar heats the water to a range of 120 - 140 degrees and the backup kicks in if the temperature falls below 120. If the temperature exceeds 140, the transfer fluid pump shuts down to prevent overheating. Pumps and controls work to keep the system at the desired temperature. Mixed valves also regulate the temperature to the tap so no one gets scalded.
Passive DHW Systems
When evaluating domestic hot water (DHW) systems, there are two basic types to consider: passive and active. In a passive system such as thermo-siphon, the water is heated and stored in a collector on the rooftop. These systems use gravity to supply the hot water as needed. The cooler water is denser and sinks to the bottom of the panel naturally. In turn, the heated water rises creating a flow without the use of pumps. These systems are best suited for coastal and moderate climates as freezing can be a serious limitation.
Active DHW Systems
Systems that require pumps come in a variety of forms. The main variation lies in how they deal with freezing and overheating issues. Fully Filled Systems pump water into the collector at all times, suitable for warmer climates where freezing is not an issue. Drain-Back Systems are more common and control the water flow depending on temperature. If the water gets too hot, the pump stops and allows the water to flow back from the solar collector into a holding tank. If the water gets too cold, the pump also shuts down and allows the water to drain back. This “drain-back” feature prevents freezing that could damage the collector or other parts making it suitable in colder climates.
A number of closed loop systems use an anti-freeze mixture instead of water. The major benefit of this is that the systems can handle more extreme temperatures. The solution is similar to the radiator fluid in your car, except this glycol mixture is not poisonous. Even so, when glycol is used, it is in a closed loop and does not mix with water. The glycol flows through a tubing to heat the water via heat exchange. The glazed collectors are mounted on the roof and look similar to PV solar panels but are usually a little larger. Most homes only require two collectors to meet their hot water needs.
Pool Heating Systems
Pool heating systems generally utilize unglazed black plastic. The new plastics are much more UV resistant and have greater durability than past materials. These systems are usually roof mounted and allow circulating hot water to warm a swimming pool. In extreme climates such as the Southwest, the water can even be pumped at night to dissipate heat and actually cool the pool when it becomes too warm.
Solar Thermal ROI
Currently natural gas is still reasonably priced. Yet, many rely on propane or electricity to heat their water. These people can expect a payback of about 6 years on an installed system. To shorten that return on investment (ROI), there are various incentives available for U.S. residents via state rebates and a 30% federal tax credit on the amount not covered by the rebates. Here is California rebate information: http://www.gosolarcalifornia.org/solarwater/