Solar Water Heating--Revised & Expanded Edition. Bob Ramlow

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      Figure 3.8: Web-and-tube configuration

      The absorber plate in copolymer collectors is an extruded mat that has many waterways, or risers, that are set close together. Many designs have been produced, but the design that has proven most successful is a web-and-tube configuration. With this design, the tubes are separated by a small web so they are not touching one another. Much like copper absorber plates, these collectors have a header running along the top and bottom of the absorber. The best designs, like web-and-tube absorbers, are made so that each riser is individually attached to the header, which allows the least back pressure within the collector and allows individual connections to be repaired, if needed. Because the risers are situated close together, this type of collector has a large wetted surface, adding to its efficiency. In fact, these collectors can outperform glazed collectors in seasonal pool heating applications.

      Solar pool heating systems that use unglazed plastic collectors are direct systems in which the pool water is circulated through the collectors. This is the most efficient configuration as there are no heat exchanger losses. Most pool water contains additives such as chlorine, which is highly corrosive. This corrosive property of pool water makes copper absorbers or copper pipes noncompatible. This is one of the reasons the collectors are made of plastic, which is not affected by chlorine. The same holds true for the piping, which is typically PVC.

      Plastic pool heating collectors are typically mounted flat on a roof. The collectors are held in place with a set of straps that go over the collectors but are not actually attached to them. The straps are often plastic-coated stainless steel and are threaded through special clips that are bolted to the roof. This method of holding down the collectors allows them to expand and contract on the roof without binding and makes them easy to install.

Comparing Solar Water Heating Collectors

      People often ask us, “What is the best type of collector?” We would like to be able to give you an easy answer, but unfortunately there isn’t one. We have found that many factors come into play when it is time for you to select a collector. It depends on the type of application, the climate of the installation, the amount of room available for the collector array, the budget for the project and other reasons too numerous to mention. What is more important than trying to find the “best” collector is being able to match the characteristics of the collector to the job that needs to be done and the limitations of the site.

       Collector Efficiency

      No matter which manufacturers you talk to, they will probably claim that they offer the most efficient collector. 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 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 a collector’s 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 collectors. The SRCC does not perform the required tests on the collectors. The test was developed by 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 collector performance and efficiency, 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 at solarrating.org. 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.