Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 30

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.

Figure 3.9: Mean collector efficiency ratings

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, the flat plate collectors are more efficient when the inlet/ambient temperature difference is between 10°F and about 70°F. After that point, the evacuated tube collectors become more efficient. The efficiency of the collector 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°F–140°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°F outside and the fluid returning to your collector is 100°F. In this condition, you would look to the point at 50°F on the graph. Flat plate collectors are about 40 percent efficient, and evacuated tube collectors are 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°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, flat plate collectors will outperform evacuated tubes. Now, if you needed really high temperatures, say higher than 160°F, then evacuated tubes 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 evacuated tube collectors is that they are better collectors 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 flat plate collectors 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.