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.
The question now should be, where does my situation fit into this? For most domestic water and space heating applications, we are trying to get our fluid up to 120–140 degrees F. Let’s consider an example in which you have a system design to heat your domestic hot water. Let’s say it is 50 degrees F outside, and the fluid returning to your collector is 100 degrees F. In this condition, you would look to the point at 50 degrees F on the graph. A flat plate collector is about 40 percent efficient, and an evacuated tube solar collector is around 34 percent. That’s quite a difference in performance, even on a relatively cold day. If you are properly dumping the heat, the inlet temperature on most residential applications is usually 100–110 degrees F at most. However, at that point, you will not need much more to reach your desired temperature.
We have found that, for most residential water and space heating conditions, a flat plate collector will outperform an evacuated tube solar collector. Now, if you needed really high temperatures, say higher than 160 degrees F, then evacuated solar tube might be the right collector for the job. Like we said before, it all depends on where and how it is being used. Pick the right tool for the job.
The second claim made for the evacuated tube solar collector is that it is better during cloudy conditions. Figure 3.10 graphs the efficiency ratings for all three SRCC conditions, including cloudy, low-sun weather. As you can see, the point where the collectors’ efficiency ratings cross is less than the average, signaling an increased efficiency. However, they are still not more efficient than a flat plate collector in most temperatures.
Additionally, you need to consider the value of a system being better at harvesting a decreased resource. If there isn’t much solar radiation to gather in the first place, being slightly better doesn’t amount to a whole lot of Btu. More of a little bit is still only a little bit.
Although much of the industry debate has centered around efficiency, as it is the obvious selling point, we have tended to focus more on the long-term performance of collectors than on the short-term. The system you install today should be designed to last at least 40 years. There is no substitute for quality.
An easy indicator of the quality of the collector is the warranty. Most come with a 10-year warranty, although some have stretched this to 15 years. You might also want to check into the manufacturer. How long has the company been in business? A few companies have been making since the ’70s and ’80s and have continued to put out long-lasting products. You may also want to find out where the manufacturer is from. We have several companies here in the Midwest that we like to deal with because they are local. This not only cuts down on shipping and transportation costs, it also allows us some flexibility if we need something in a hurry.
Climate and Location
In some climates, such as ours, snow is a significant factor in solar collector performance. Snow can accumulate on the solar collector and diminish the solar resource. A flat plate collector will have a distinct advantage over other types as it will shed snow very well when installed in climates that experience significant snowfall. The large pane of glass on the front will lose heat, causing the snow to slough off. An issue with the evacuated tube solar collector is that it does not shed snow. Because the evacuated tubes are such good insulators, little heat escapes, and the snow that accumulates on the tubes can stick for a long time. Their surface is also irregular, so snow packs between the tubes as well. We have seen instances where roof-mounted evacuated tube collector arrays got packed with snow in the early winter and stayed that way till spring, which rendered them completely useless for a good portion of the year. The lesson here is to always mount an evacuated tube solar collector at a significant angle if used in a climate that experiences snow. Also, never flush mount an evacuated tube solar collector in a climate that experiences snow.
Absorber coatings will influence the performance of a solar thermal collector, and the amount of this effect may be more critical depending on the climate the solar collector will be placed in. Some types of absorber coatings have a higher performance rating than others. In climates that experience a high number of sunny days per year and also experience relatively consistent warm temperatures, solar collectors can perform well with less efficient absorber coatings, whereas in climates with fewer sunny days per year and colder temperatures, solar collector coatings with higher efficiencies work best.
You may also need to consider the density of the absorber area compared to the overall footprint of the solar collector. Manufacturers and certification agencies use the terms gross collector area, net aperture area and absorber area. Gross collector is the entire area including the frame; aperture area is the size of the glass; and absorber area is the amount of surface that will actually absorb solar radiation. In a flat plate collector, virtually the whole collector area equals the absorber area, with only about an inch around the edge being frame and not absorber. In an evacuated tube solar collector, there is space between each tube that is not absorber area. In most instances, a flat plate collector will take more than 25 percent less space than an evacuated tube solar collector that has an equivalent absorber area. This may be an important factor because a common limiting factor in siting solar collectors is the amount of available mounting space. In other words, you want to have enough space for the amount of solar collector area you want; so to get as much heat as you can, you want to have a type of solar collector that has the best gross-to-net absorber ratio.
Note that the amount of solar energy that falls on a square foot of the Earth is a constant and cannot be changed by the type of solar collector used. The primary way to increase the amount of energy collected is to increase the absorber area.
By now it has probably become clear that we are not completely impartial when it comes to solar collector selection. It is difficult to remain unbiased while still trying to provide the knowledge we have gathered through years of experience installing, maintaining and designing these systems. We have seen the best performance from flat plate collectors, and we want you to have the same success. We have no doubt that evacuated tube solar collectors have a secure place in the solar thermal industry, especially in high-temperature applications. For instance, the emerging solar thermal-powered air conditioning systems that use evacuated tube solar collectors to drive single- or double-effect chillers hold great promise. However, for the majority of domestic water and space heating applications, the flat plate collector has a proven track record.
Learn more about how to get the most from solar energy in How to Position Your Solar Energy System for Maximum Output.
This excerpt has been reprinted with permission from Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems by Bob Ramlow and Benjamin Nusz, published by New Society Publishers, 2010.