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This is by far the most common question in the solar thermal industry, and strangely enough, many people have begun to take sides. Like a Red Sox vs Yankees type of rivalry, this is a heated debate. In the end, the real answer is that it depends on the job you’re working on–the climate, the roof, your budget, and the type of system you’re working on. For example, if you want to install a drain-back system, evacuated tubes simply will not work. Bob Ramlow recently wrote some helpful tips on selecting proper solar thermal equipment but I wanted to follow this up with a specific discussion on solar thermal technology.
I’ll share with you an awesome comparison. Bob Nape had the same question, so he decided to install two systems side by side and monitor them in real time on the Internet. Check it out with the below link. IT IS AWESOME.
There are several design parameters to consider while looking into flat plates or evacuated tubes. So we’ll go over a few:
Flat plate collectors tend to be cheaper than evacuated tubes because they are a simpler design and easier to manufacture. This analysis however, is on a collector-to-collector level. Flat plates are cheaper collector-to-collector and also on BTU per $$ level. Evacuated tubes tend to be 10-15% more expensive than flat plates, but their processing costs are dropping.
Generally speaking, you want to design these systems to last 40 years, and they should be able to. In the past, I’ve spoken with contractors who have said the flat plate collectors will tend to have longer warranties–a 20-year warranty as opposed to a 10-year warranty for evacuated tubes. This is presumably because of the level of sophistication with the technology. Be sure to check with the distributor and manufacture of the collector to see how long the warranty is.
Flat plat collectors are heavier, take up more room, and can be cumbersome to install on certain roofs. Evacuated tubes tend to have lighter components and are easier to manage on the roof. On the other side, evacuated tubes tend to be more fragile than flat plates.
Flat plate collectors can only heat water up to 170-180 degrees Fahrenheit, which means there is very little risk of overheating. Evacuated tubes, on the other hand, can heat water to well over 250 degrees. For this reason, they are much more likely to overheat than flat plates, and you need to be more concise with your design. If you’re using evacuated tubes, it’s always better to oversize your storage tank rather than under-size it for this reason. Evacuated tubes are also used more in colder climates because they are more efficient than flat plates in extremely cold temperatures. In extremely warm climates, evacuated tubes have a very high chance of overheating, so be careful if you’re in the southern states.
In determining the best collectors to use, it will depend on what you’re going to use the water for and how much you’re going to use. Evacuated tubes can heat large amounts of water very quickly and can get the water above 180 degrees. So, if you have a sizable load, such as in commercial or space heating situations, you may want to look into evacuated tubes. Flat plates work best with domestic water. Their temperate range fits well within code for hot water usage. They can also be used for space heating in low-heat hydronic applications, but you’ll need to size accordingly.
If you’re in the Northeast or Midwest, snow loading and production during the winter is a large factor. Generally, evacuated tubes are not good at shedding snow. This is because the tubes are such a good vacuum that they do not shed much snow. Flat plate, on the other hand, can shed snow easily with just a little sunlight.
Structural Wind Loading:
If you’re working on a job where the rafters are questionable, and you’re in an area with significant wind loading, evacuate tubes tend to have the advantage. They are lighter in general because less water runs through the system, but they also have less wind resistance because the wind can pass through the collectors.
If both types of collectors are placed side by side on the same roof, the performance is based on the difference between the entering water temperature that you’re heating and the ambient temperature. In other words, the performance depends on the temperature of the water coming from the storage tank compared to the ambient temperature, i.e. the temperature of the surroundings. As the variance increases (i.e. you’re in colder temperatures), evacuated tubes become more efficient.
See this graphic as an example:
So the conclusion is that it depends. To be honest, they both work. If designed correctly, you’re not going to see a huge difference between one or the other in most cases. It’s not like an evacuated tube produces 100% hotter water at 50% less cost–we’re talking about incremental performance considerations here. Just look at Bob Nape’s side-by-side comparison. They both work.
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