Heating water with the sun

Dual Collectors with Sunnovations

Example of a solar hot water system.

Solar water heating (SWH) systems capture the sun’s heat energy and use it to heat water, or other liquids. The system collects the sun’s energy using a solar collector. That energy is then transferred into a water storage tank, similar to those used by conventional water heaters. Systems have a backup heater to ensure that you will always have hot water, even if the sun isn’t shining.

The payback time for solar water heating systems depends on whether you heat water with gas or electricity, but a 5-7 year payback time is a good rough estimate.

System Components

Diagram of how solar thermal systems provide hot water to homes. Source: U.S. Department of Energy.

Diagram of how solar thermal systems provide hot water to homes. Source: U.S. Department of Energy.

Collectors
There are two common types of solar collectors used for residential and small commercial applications: flat plate and evacuated tube collectors.

Flat plate collectors are the most common and cost effective type of collector for solar hot water systems. These collectors are insulated, weatherproof boxes that hold an absorber plate. The sun heats this plate and transfers the heat to a heat transfer fluid flowing through tubes in or near the plate. Flat plate collectors can typically produce temperatures up to 180ºF and in sunny, warm conditions can have efficiencies in the 65% to 78% range, but lose effectiveness when it gets extremely cold or cloudy.

Evacuated tube collectors are a parallel series of rows of transparent glass tubes that surround rows of absorbers. The air in the tubes is then removed (evacuated) to form a vacuum. This reduces heat loss and increases efficiency and, as a result, evacuated tube collectors can reach temperatures of up to about 300ºF. Evacuated tube collectors will typically have top efficiencies in the 40% to 55% range, but retain their effectiveness better in extremely cold weather. This type of collector is more expensive than flat plate collectors, but is also more efficient.

Storage tanks

In addition to the collectors, SWH systems require hot water storage tanks or tank. In a two-tank system, the water is preheated in the solar tank using heat exchangers before entering the original hot water heater. In a one-tank system, the solar heat exchanger is built into a single tank. A circulating pump circulates the heat transfer fluid from the collectors on the roof via insulated pipes (typically copper) and additional plumbing connectors.

Temperature sensors

Temperature sensors are usually installed in the collector and water tank. These are connected to a controller, which turns on the pump when fluid in the solar collector is approximately 15ºF hotter than the water in the tank. Systems may also contain an expansion tank, a safety measure that can relieve excess pressure in the system should it get too hot. In addition, some controllers can measure the amount of energy being produced by the system.

Operations and Maintenance

Once installed, solar water heating systems require minimal maintenance. Over the 30+ year lifetime of a solar hot water installation, you may need to replace the water pump after 10-15 years. This costs is for this is $100-$300. Additionally, excess heat can cause the glycol mixture in indirect systems to deteriorate and corrode system components. You should have your glycol checked and changed (if necessary) every 3-5 years. This typically costs about $100-$200 per visit.

*Excerpt from Guidebook for Solar Water Heating Projects,” December 2011.