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Flat Plate vs. Evacuated Tube Collectors

Three Bosch Solar Thermal Collectors installed by APEX Thermal Services, Olivebridge, NY, Ulster CountyThree Bosch Solar Thermal Collectors installed by APEX Thermal Services, Olivebridge, NY, Ulster CountyThis is a Comparison of Solar Thermal Flat Plate Collectors versus Evacuated Tube Collectors - I so love this comparison.  So many folks out there are making huge claims about the efficiency of their Solar Hot Water collectors, particularly evacuated tubes. 

So what solar hot water technology is actually the best, you ask?  Well . . . "It depends," is the correct answer!  Your specific situation should actually be determining what technology is best, not text from someone's sales literature.  Cost, aesthetics, efficiency, desired loads, etc. should determine what technology is best for your institution, home or business.  We strongly suggest working with an experienced solar professional to assist you in determining what will perform best in your particular situation.

Solar Thermal Flat Plate Collectors come in several different configurations; Parallel tubes with headers, and serpentine are the most common types.  There are also parallel tubes with a pinched header, and collectors with a drain port in the center (most recently).  Basically they use a painted or specially coated metal collector surface, with a tubing configuration to carry away the collected heat.  All this is housed within an aluminum frame, insulated behind and on the sides, and it has a glass cover.  Flat Plate Solar Hot Water Collectors haven't really changed much over the past thirty years, except for some specialized selective coatings, any insulation performance improvements, and some different tubing configurations.

Evacuated Tube Collectors 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 assembly with attached absorber fins, dependent 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 heat transfer fluid remains up in the manifold, and the heat pipe transfers solar energy up into the manifold which heats the 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 U-tube diameters 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.  However, I see their negative as the intense heat being delivered to a relatively small surface area, reaching into the manifold, and the longevity of seals and all of the components that will see those temperatures.

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, rather than upstate NY.  The evacuated tube's insulation is better than an insulated aluminum box, with a large glass cover, so the flat plates may not achieve the output temperatures that evacuated tubes may achieve on a very cold day.  This type of performance can also be expected during windy conditions.  Flat plate solar collectors have small vent holes to reduce internal condensation.  Cold wind will cause some drafts inside the flat plates, while the evacuated tubes would be significantly less prone such losses.  However, the flat plates have a lot of collector plate surface area, when compared to the evacuated tubes small fins, so even though flat plates may produce water at a lower temperature, they will likely be providing a higher volume of water.  Please note that the volume of water heated and the input and output temperatures of the collectors will effect the overall BTU output calculation.  This winds up being the proper way to determine who's collector is best, not just output temperature.  So. . . High output temperature alone does not dictate who's collectors are more efficient.  Larger volumes of water at a ten degree lower output temperature might deliver more BTUs and greater dollar savings!

Let me shed some light on things worthy of consideration when selecting Solar Tharmal System collectors:

Snow Shedding: Sorry, the evacuated tubes have some serious issues here.  Tip your flat plates up to about 45 to 55 degrees+ and they will shed snow very well.  Less angle, and you will need some melting to assist the shedding.  With evacuated tubes, the snow can get wrapped 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, rather 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.

Three-Collector Solar Thermal Drainback System, South Salem, NY, Westchester Co NYThree-Collector Solar Thermal Drainback System, South Salem, NY, Westchester Co NYCost: 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 flat plate collectors.  Before you purchase 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.  One former sales rep told me to expect evacuated tubes to fail within 5-10 years.  Most evacuated tubes 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.  Better yet, hire a trained experienced professional to design and install your solar hot water system.

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 flat plate collectors, especially with warmer outdoor temperatures.

Hybrid Arrays: Certainly, a robust and well-designed combination of both solar thermal flat plates and evacuated tubes is an attractive opportunity, in my opinion.  The 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 potentially being the ultimate space heating setup.  It would also provide some economies, compared to purchasing all evacuated tubes.

Sixteen-Collector Solar Hot Water, Space Heating and Pool Heating System, Hilllsdale, NY, Columbia CountySixteen-Collector Solar Hot Water, Space Heating and Pool Heating System, Hilllsdale, NY, Columbia CountySolar 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, very well when the sun shines.  During November of 2012, in the Hudson Valley, it seemed like we had no sun for the entire month.  In contrast, February is usually cold, yet it is a particularly good month for solar energy generation, due to rather clear skies.  My 1,100 square foot pole barn has solar assisted radiant slab with an oil boiler.  It had 128 square feet of flat plate collectors mounted vertically, on a South-facing wall (Summer of 2013 I added another 63 square feet of collectors).  I considerd this original array rather 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.  Tests after adding three more (20+ year old used collectors) indicated I will be getting about a 10 degree temperature increase, with the added collector surface area.  If the sun stays out, my boiler is disabled, and solar heats the concrete all day long.  The original arrangement reduced my oil costs by somewhere around 50% the first year.  I can't wait to see what adding 50% more collector area does this season!

We typically recommend a minimum of four to six solar thermal collectors 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 irregular 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 collectors, and nowhere to use all of the available heat (unless you have a swimming pool). 

Remember that you will 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 heating and radiant radiators are also possibilities.  Radiant radiators can be quite costly.  Their BTU output must also be de-rated if you plan on heating with lower than rated temperature water.  Radiant heating 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 solar energy 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.  Expanding solar hot water systems over time is an option, but requires thoughtful decisions early on.  If you are considering this, the lineset pipe diameter should be made larger to accommodate the larger flow required by more collectors.  Additional solar storage tanks can be added upon system expansion.  With an entry level system, at least you can get about 75% of your domestic hot water heated for free.  Larger solar heating systems can start to take a bite out of your space heating costs.  At present, in the Hudson Valley, most folks with modest, average sized homes can be spending between $2,500 and $4,000 per year for space heating.  Remember, your best financial investment will not be a system that handles 100% of your heating energy needs, due to equipment costs, space constraints, etc.

Contact APEX Thermal Services, a premiere solar heating service provider serving the Hudson Valley, to design and install your Solar Hot Water, Radiant Heating, Boiler or Water Heating System.  Apex Thermal Services can assist you if you are located in: Westchester County, Putnam County, Orange County, Dutchess County, Ulster County, Sullivan County, Greene County, Columbia County, Albany County, Schenectady County, or Rensselaer County.