Solar Hot Water System Design

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You can find plenty of water heating systems using solar energy today, including home-built designs that are simple but effective, like the batch heater. This design has no outer jacket or insulation blanket but is painted black and placed in an insulated plywood box.  There is a plastic or glass glazing covering the assembly, which is angled to solar south. Pressurized domestic water goes into the tank’s bottom and heated water comes out from the top to your home’s hot water heater.

Both the glazing and insulation act like a greenhouse, where trapped thermal energy is transferred to the water from the outside of the tank. Heated water decreases in density and floats to the tank’s top for domestic use. However, these water heaters are susceptible to freezing and are best used in warmer climates or drained and not used in the winter.

Commercial batch water heaters are more widely used in developing nations as they are less aesthetically pleasing to North Americans and are not freeze-resistant. (Confirm freezing possibility with your local supplier.) These very simple units are quite affordable and may be ideal for northern environments without harsh winters or homes in warmer climates.

Domestic plumbing systems can use open-loop or direct systems to heat the circulating potable water. These are pump-controlled units with temperature-measuring tools, thermosyphons or the natural convection loop effect.

Currently, the most popular configurations in North America are active drain-back systems and pump-driven, sensor-fitted and electronically-controlled closed-loop designs for capturing and transferring thermal energy. The function of the control system is to monitor the storage tank’s water temperature and check it against the solar collector temperature. If the latter is warmer, the pump will be activated to push water into position to collect the required thermal energy.

The control unit monitors the storage tank’s water temperature to control pump operation; thus the water temperature is kept at a maximum.  Alternatively, the drain-back system features potable water as the main medium for heat transfer between the storage tank and solar collector, with water flowing back to a small storage tank when the pump is off. When the solar collector is drained on cold and cloudy days or during winter nights, the system does not cause the water to freeze. This is also the case if the system remains idle.

Drain-back systems may have a heat exchanger to transfer heat to cooler storage-tank water. This ensures that there is no cross-contamination between the potable household system and the solar collector water loop. 

The preferred configuration in areas where freezing is possible is the closed-loop glycol system. A mixture of water and a food-grade propylene glycol antifreeze stays in the solar collectors, heat exchanger and plumbing lines. A differential electronic controller is connected to the circulation pump (usually using AC) with temperature-sensing devices like those in drainback systems. Alternatively, you can use a DC pump connected to a small PV panel mounted near the solar collectors. Then there is no need for temperature sensors or the differential electronic controller: the circulation pump is activated whenever there is sunshine.