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Pick the Best Solar Water Heater for Your Home

Solar hot water heaters are simple, cost-effective and have been around almost as long as mankind itself.

| May 2016

  • The true measure of a solar water heater's efficiency is in how it performs.
    Photo by Christian Delbert/Fotolia
  • Figure 3.9: Mean solar collector efficiency rating.
    Graph courtesy New Society Publishers
  • Figure 3.10: Solar collector efficiency ratings over all conditions.
    Graph courtesy New Society Publishers
  • "Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems" by Bob Ramlow and Benjamin Nusz is an introduction to modern solar energy systems.
    Cover courtesy New Society Publishers

Of all the renewable energy options open to us, solar hot water heaters have been around the longest. There are several designs to choose from, and selecting the best solar collector can be confusing. Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems, by Bob Ramlow and Benjamin Nusz (New Society Publishers, 2010), is the definitive guide to this clean reliable technology. In this excerpt, the authors explain the efficiency, durability and climate considerations for different solar collector designs including the flat plate solar collector and evacuated tube solar collector designs.

You can purchase this book from the MOTHER EARTH NEWS store: Solar Water Heating.

Solar Collector Efficiency

No matter which manufacturers you talk to, they will probably brag about their solar collector efficiency being the best. The truth of the matter is that they are all wrong. There is no such thing as the “most efficient” collector. For instance, consider evacuated tube solar collectors. Since their conception, much has been said about their performance as they are commonly heralded around the solar industry as a more efficient collector. If you were to look just at the collector design, that’s an easy assumption to make. Normally, if you reduce heat loss, in this case by the vacuum of the tubes, you would increase efficiency. So it may sound counterintuitive if we tell you that pool collectors, which are simply plastic tubes placed out in the sun, are actually more efficient for some applications even though they are not as well-insulated. It takes more than a clever design to produce hot water. It takes a system that is appropriately suited to what you want it to do.

The measure of solar collector efficiency should really be how it performs when put to use. The best way to measure that is through an independent testing organization. One of the good things that came out of the late ’70s solar boom was the Solar Rating and Certification Corporation (SRCC). The SRCC rates and certifies many of the collectors on the market today. It is the most common and reliable source in the United States for independent information about solar collector efficiency. The SRCC does not perform the required tests on the collectors. The test was developed by the American Society of Heating, Refrigeration and Air-Conditioning Engineers, and is performed at accredited testing facilities. The SRCC uses the test results when rating the collectors. We strongly suggest buying collectors that they have certified. Not only does the test calculate solar collector efficiency and performance, it also tests for durability and reliability. Both are critical for determining the value of a collector. The results are free to the public and can easily be accessed online. Using the SRCC gives us good solid standardized data for comparing collector performance.

When rating a collector, the test measures the amount of heat, in Btu, that it will produce, based on a certain amount of radiation that shines on the collector. The testing facility usually does this with big lights to ensure consistency between tests, but some facilities conduct the tests outdoors using real sunlight. Because the solar resource is inconsistent, three conditions are considered: clear day (2,000 Btu/ft2/day), mildly cloudy (1,500 Btu/ft2/day), cloudy day (1,000 Btu/ft2/day). The conditions mimic how the amount of sun will vary depending on location and climate. As a second variable, the test will alter the temperature at the site. This is actually the difference of the temperature of the fluid going into the collector (inlet temperature) and the temperature outside (ambient temperature). This gives you a measure of how hot the fluid is that you are trying to heat and how cold it is outside. Figure 3.9 graphs the ability of each type of collector to convert sunlight into usable Btu for all of the temperature variables. The data is an average of all three sun conditions and was taken from a sample of ten manufacturers of each type of collector to provide a measure of overall performance.

As you can see, when there is very little difference between the inlet temperature and the ambient temperature, the pool collectors are significantly more efficient than both flat plate and evacuated tube collectors. Does this make them the most efficient collector? No. It simply means that they are better during some conditions. Similarly, a flat plate collector is more efficient when the inlet/ambient temperature difference is between 10 degrees F and about 70 degrees F. After that point, an evacuated tube solar collector becomes more efficient. Solar collector efficiency is entirely contingent on where and how it is being used.

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