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Thursday, 27 June 2013 14:32

Flat Plate vs. Evacuated Tube Collectors

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Flat Plate Panels come in two basic types, either parallel tubes with headers, or serpentine. Basically they use a painted or specially coated metal surface, with a tubing configuration to carry away the collected heat. All this is housed in an aluminum frame, insulated behind and on the sides, and it has a glass cover.

Evacuated Tubes have several designs. We will not go in too deep here, because of the many proprietary approaches. Basically, the glass tube is shaped like a test tube. Now imagine it double walled, and still open on one end. The gap between the double wall construction is evacuated of all air molecules. This places a very good insulation layer between the collector fins (in the center of the tube) and the outside air temperatures. In the center of the tube could be a heat pipe or a U-tube arrangement, dependant on make and model.

With the U-tube design, the solar heat transfer fluid actually flows down through the U-tube, which is inside the insulated evacuated tube. With the heat pipe design the fluid remains up in the manifold, and the heat pipe transfers solar energy up into the manifold and the passing heat transfer fluid. From a design perspective, I give the heat pipe version the nod. It has some mechanical advantages, in my opinion. However, the U-tube models are typically a bit more efficient at collecting energy. My issue with the U-tubes is that the tubes are small, and, over time, they can collect sediment in the bottom, and could eventually clog, so, from a maintenance perspective, I like the heat pipes better.

Extremely Cold Climates OK, so this is where the evacuated tubes have an opportunity to beat flat plates hands down. However, extremely cold climates might be more like Canada or Alaska, than upstate NY. The evacuated tube's insulation is better than an insulated box with a large glass cover, so the flat plates won't achieve the temperatures that evacuated tubes will on a very cold day. This performance is also the same in windy conditions. Flat plates have vent holes to reduce internal condensation. Cold wind will cause drafts inside the flat plates, steal energy and decrease overall efficiency, while the evacuated tubes would be significantly less prone to these losses.

Snow Shedding Sorry, the evacuated tubes have some serious issues here. Tip your flat plates up to about 45-50 degrees and they will shed snow well.

Less angle, and you will need some melting to assist the shedding. With evacuated tubes, the snow can get rapped around the tubes and also attached to the mounting frame and the roof, so the snow won't slide very readily. The improved insulation quality of the evacuated tube also reduces the chance of energy radiating out, to melt snow, so you will likely have more downtime with snow covered evacuated tubes, than with flat plates.

Mounting I give flat plates the nod here. Several of the evacuated tubes I have seen come with some pretty crappy frames, somewhat light duty, in my opinion. Please share any brands of evacuated tubes that come with robust mounting frames, as I have yet to come across any.

Aesthetics On this one, I prefer the flat plates again. They basically look like skylights when flush mounted. When elevated, you could argue that they are both equally unattractive.

Cost Flat plates typically cost less than most evacuated tubes. Beware of low cost evacuated tubes, as they will likely be less efficient at collecting energy than the flat plates. Before you buy your solar collectors be sure to visit the SRCC Rating page to verify their performance. You can compare them to other offerings (and you can compare flat plates to evacuated tubes there as well). The higher quality evacuated tubes can cost considerably more than flat plates. This cost may be justified by the desired end use of the energy, but again, the cost premium can be very steep.

Maintenance Again, flat plates are very simple, and not at all demanding. They can last 30 years plus with 5-7 year antifreeze change intervals. Evacuated tubes are known to go bad. Most have an internal coating that changes color when the vacuum seal is lost. This necessitates replacement of the tube. In some cases, especially with U-tubes, that may require the entire system to be drained. Long-term availability of the tubes should also be a concern; what if your model is discontinued?

Overall energy output Higher quality evacuated tubes have the capability to provide higher water temperatures than flat plate collectors. I do find this to be true. However, it is my opinion, and backed by some research data that I am privy to, that they may achieve this at an expense of water volume (a gallon-per-minute flow rate that is lower than the flat plates). So the actual BTUs produced by each may be quite comparable, if flow rate is also included in the equation. Also, be careful if you are going the evacuated tube route. Some manufacturers manifolds are extremely constricted and may not support series connections of more than one or two sets of tubes. Do your homework before selecting a product.

Cloudy Conditions Evacuated tubes perform better than flat plate collectors during cloudy conditions and during early morning and late evening. I find this to be true, but for the majority of a long sunny stretch, they may be strongly outperformed by the flat plates, especially with warmer outdoor temperatures.

Hybrid Arrays Certainly, a robust and well-designed combination of both flat plates and evacuated tubes is an attractive opportunity, in my opinion. Flat plates could lift the temperatures up to their limit, while the finishing stage uses some evacuated tubes, to further elevate temperatures. I have been involved with some testing of such arrangements, but with some lower-quality evacuated tubes. I am not closed to this being the ultimate space heating setup. It would also provide some economies, compared to purchasing all evacuated tubes.

Solar Space Heating Solar space heating is a viable approach to lowering your heating fuel costs. However, It requires a good, sound design, and you will still require another form of space heating, in cases of long stretches of cloudy weather and storms. Does it work? Yes, when the sun shines. This past November,in the Hudson Valley, it seemed like we had no sun for the entire month. In contrast, February is a particularly good month for solar, even though it is cold, due to rather clear skies. My 1,100 square foot pole barn has solar assisted radiant slab with an oil boiler. It has 128 square feet of flat plate collectors mounted vertically, on a South-facing wall. I consider this setup undersized for its application. However, I have seen 95-100 degrees F coming in from the panels in February, and it gets circulated right through the slab. If the sun stays out, my boiler is disabled, and solar heats the concrete all day. This arrangement has reduced my oil costs by somewhere around 50%.

I typically recommend a minimum of four to six panels for space heating applications. Eight to twelve would be awesome. You could likely go beyond 20 panels, and still need some additional heat sources, based on the sun's availability. It is not recommended to try to go for eliminating 100% of your heating with solar, because in non-heating months, you will have all those panels, and nowhere to use all of the heat that the system could provide (unless you have a pool). Remember that you would also need a heat delivery method that can make use of solar system's typical winter output temperatures (maybe 90 to 110 degrees F, which is far less than 180F sent through most hot water baseboard systems). Radiant heating is the answer here, with slabs and Gyp-crete pours on upper floors being best. Above sub-floor radiant and radiant radiators are also possibilities. Radiant radiators are quite costly. Their BTU output must also be de-rated if you plan on heating with lower temperature water. Radiant zones with tubing installed below sub floors may not match up with solar as well, due to the higher design temperatures required for these systems (typically about 140 degrees F.

Very Important: Do not expect too much from your system! Rome was not built in a day. In any case, installing a solar energy system is the right thing to do and is a step in the right direction. With an entry level system, at least you can get 75% of your domestic hot water heated for free. Remember, the best financial investment will not be a system that handles 100% of your heating energy needs, due to equipment costs, space constraints, etc.

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