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Solar at Home

Solar at Home


The trials, tribulations and rewards of going solar
Solar at Home Home

Will solar thermal heat up again?


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Editor’s Note: Scientific American‘s George Musser will be chronicling his experiences installing solar panels in Solar at Home (formerly 60-Second Solar). Read his introduction here and see all posts here.

It used to be that the term "solar panel" connoted a solar thermal panel, which uses sunlight to heat your house or tap water, as opposed to a photovoltaic (PV) panel, which produces electric power. These days, though, attention (not to mention sunlight) focuses on PV. Many people assume that solar hot water heaters are all well and good for, say, Israel, but ill-suited to high-latitude, cloudy, snowy climes such as the U.S. Northeast. But Scott Wilson of Olney, Md., begs to differ.

Responding to my call for stories about solar installations, Wilson sent me a description of his thermal system. His blog provides even more details and makes the case for installing thermal instead of photovoltaic panels:

"I don’t have a grid-tied PV system, but I do have a solar thermal hot water system. As great as PV is, based on cost-efficiency, solar hot water should probably be most people’s first contact with solar. I was on our local tour of solar homes this year, after saving up 1.5 years worth of performance data.

"Our system is a 16 tube evacuated array, with an equivalent power collecting ability of about 1.5 kW. It has a pressurized (34 psi) propylene glycol/water mixture that circulates through a manifold at the top of the array, picking up heat from a ‘hot finger’ that extends from the top of the individual tubes. That fluid travels to a heat exchanger mounted in the bottom of the tank, where it dumps the heat to the surrounding cold water, then travels back up to the roof. The water itself never travels to the roof, unlike the ‘drain-back’ flat plate systems of the ’70s and ’80s, which suffered, I’m told, from inferior quality issues, and gave solar domestic hot water a bad name.

"There are temperature sensors on the collector, which measures the maximum temperature directly from the tubes, and in the water at the bottom of the water tank. An electronic controller turns on a pump, which circulates the glycol fluid, when the temperature difference between the collector and the bottom of the tank exceeds a given amount, currently 10 degrees F. When the difference goes below 3 degrees F, the pump shuts off.

"The system basically preheats the water on the bottom of the water tank, so that the electrical element, mounted near the top of the tank, works less. In the summer, the water in the tank can be 120 degrees F at the top and as high as 110 degrees on the bottom, heated by the sun. I also added a small PV powered battery system for running the pump, so from about May to September, we turn off the electrical element at the breaker panel, and our entire hot water supply is off-grid.

"As to the cost, we paid $10,093.68 for the tubes, plumbing, pump, controller, data logger, new 120-gallon tank and installation. We received a $2,000 federal tax credit and a $2,000 Maryland state rebate. If you are in Montgomery County, Md., you also get a property tax credit of $1,500. So our net cost was $4,593.68. Our monthly savings, averaged over winter and summer for the past one-and-one-half years has been $30 per month. Our payoff will be 12.7 years. If you bought the same system now, you would still get a $2,000 state rebate, and a straight 30% federal tax credit, since the $2,000 cap has been lifted. Your net cost would be $3,565.58, including the Montgomery County credit. Your payoff period would be 9.9 years.

"As for our latitude, we bought the evacuated tube system over a flat plate system, since it is freeze-tolerant, and more efficient. But flat plat systems are sold here that also use glycol and are therefore freeze-tolerant also. An evacuated tube system like ours shouldn’t really be limited by anything, other than available sun. On clear 20 degree days in winter, the tubes will get up to 70-75 degrees. And, unlike PV, shade doesn’t completely destroy performance. I did a study comparing the energy collection in the fall and spring when the sun was at the same elevation, and the day length was the same. The only difference was leaves on the branches in the fall, and no leaves on branches in the spring. Leaves give about a 30% reduction versus no leaves, but I still collect energy when the sun penetrates leafless branches."

Some installers combine both types of panels in hybrid photovoltaic/thermal (PVT) systems, but they’re not common. Indeed, some companies that used to install thermal systems, such as Borrego Solar, no longer do. Chris Anderson, Borrego’s chief technology officer, says that thermal and PV systems require contractors with different skills and juggling the two would be tricky. That said, his own house has both. Someday, maybe all of ours will, too.

Scott Wilson’s solar thermal array, courtesy of Scott Wilson





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Comments 24 Comments

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  1. 1. craigbrown 12:38 am 11/18/2009

    I think a 10-12 year payoff will be too long for the majority at the moment.

    Link to this
  2. 2. mackerirl 8:53 am 11/18/2009

    We installed a similar system (Dublin, Ireland) . We are actually more northerly than Vancover, though we do benifit from the gulf stream. System was 3 panels with 12 tubes each. The system can get our 300Ltr tank up to 70C easily in summer, with the circulating mixture going as high as 130C. Even cold clear winter days I can see the system circulating, warming the cold 5C water to perhaps 30-40C and the heating system does the rest.

    Link to this
  3. 3. syed2247 3:04 pm 11/18/2009

    Good article. you do not need to use evacuated tube collectors in most areas of the United States. Flat plate collectors with propylene glycol are perfectly fine for water heating applications. The coat as mentioned, at over $10,000 is bit much and should,mbe more like $5,000 to $6,000 for a modular flat plate colector system. this will give a more reaonable pay back than 10 or 12 years.

    Link to this
  4. 4. rockjohny 4:17 pm 11/18/2009

    Pricing like that should bring in the competition and get more reasonable…that’s just crazy high. All ‘evacuated’ means is it has a vacuum in the tubes to insulate the heating element from environmental temperatures. Chinese companies make simple glass tubed devices with vacuums a LOT cheaper. This article makes it sound like brain surgery to be able to install these, they want to protect their outrageous markup HA!

    Link to this
  5. 5. ormondotvos 5:53 pm 11/18/2009

    You’re making it waaaay too complicated. Heliodyne sells a much cheaper system perfectly adequate for household uses for around $3k. A very simple differential controller kit with about twenty parts total is $49 from http://www.jc-solarhomes.com (I just put mine in operation. Piece of cake.) Grundfos pump $150, panel $800, copper pipe, insulation, tempering valve for safety. Look into it. Use 5200 sealant on the roof piercings. Talk to your contractor buddies. You can also use your existing tank.

    Link to this
  6. 6. fbcooper 11:15 pm 11/18/2009

    I really like looking into much simpler thermal solutions than the one shown here. My intertie PV system is ‘simpler’ in that it doesn’t have so many moving parts nor the batteries of an off-grid system. It uses a series of PV panels and a single ‘box’ that does all the work (a Sunny Boy)- and a couple of cut off switches as required by the utility company. Not difficult to understand. If the transformer fails, it won’t work. Hopefully, it will continue to work for a long time.

    I also don’t like the idea of glycol as a heat exchanger any more than I like the idea of lead acid batteries – I like the idea of intertie as a giant offsite battery – and I prefer water for storage – in California, near the ocean, freezing isn’t a huge concern.

    I am reading more on air heating system to help take the chill from the house using just air as a heat exchange. I also like the idea of solar water heating using much simpler and cheaper components such as shown on jc-solarhomes.com.
    It seems to have freezing issues solved.

    Cheaper also means that when it breaks, it doesn’t cost as much to fix either – the skillset required is working with basic PVC plumbing and simple DC motor pumps.

    Link to this
  7. 7. gilgamesh05 1:33 pm 11/22/2009

    with out getting too technical. Systems such as described above are being installed in syria (where I am originally from)
    for $ 500 US dolars. I know it is on a better latitude and gets better efficiency but numbers like 10,000 dolars are exhorbantly high. is it the thermal solar system or the high cost of labor???

    Link to this
  8. 8. disdaniel 2:36 pm 11/22/2009

    gilgahesh05: it is almost certainly the lack of competition in Maryland. The hardware can’t cost more than $2k, and the labor shouldn’t cost more than another $1k ($2k if you got PhD’s doing the install) 2 guys at $50/hr x 10 hours (???) gets you to $1000.

    craigbrown says: "I think a 10-12 year payoff will be too long for the majority at the moment."
    This is a common attitude that baffles me…10 year payoff means a 10% annual return (guranteed!)–and 12 year is an 8.25% return.
    Much better than the 5% "risk-free" government bond yields on offer–yet many people think like craig. Completely irrational.

    Link to this
  9. 9. Lampasas 6:16 pm 11/22/2009

    at the full cost, this is equal to just over $54,000/kW. Payoff is just over 35 years. Installation looks great, the website is quite good. Cost has to come down to be commercial.

    Link to this
  10. 10. Michael Hanlon 10:29 pm 11/28/2009

    Is this where I should put this? It’s about wind, not solar. But there isn’t a wind blog so , I apologize before hand but will intrude none-the-less.
    Take any of those small one piston engines we use on our lawn mowers, leaf blowers and chainsaws, remove the piston fron the crankshaft, and you are left with a generator that puts out ~~1KV per rotation. Step down the voltage and increase the current output and you’ve recycled what we all throw away into a quite well manufactured base for a generator system. Add a pinwheel to the flywheel or the other end crankshaft , mount it on a pole and generate free electrons from the wisps of the breeze.

    Link to this
  11. 11. Lampasas 8:10 am 11/29/2009

    Rockjohny – this installation does use Chinese tubes – I found this type selling for just over $2200 retail in Canada. My guess is that the installer made a killing. comparable PV systems in capacity are cheaper so not sure why anyone would pay this cost to heat water.

    Link to this
  12. 12. Lampasas 10:32 am 11/29/2009

    calculating the IRR yields a -9.2% return over a fifteen year period based on the $10093 all-in cost – if calced on basis of the fully subsidized cost of $3566, the fifteen year IRR is +2.4% — pretty low returns in my opinion — no wonder there are so few installations

    Link to this
  13. 13. Lampasas 10:34 am 11/29/2009

    what was your system cost/ and what has the month by month performance been?

    Link to this
  14. 14. joewright213 1:06 am 03/10/2010

    I am an installer in California. 10K is on the higher end of things, but not out of the question. The typical installation we do costs the homeowner about $4,500 after incentives.

    I am always surprised when I hear people say how "expensive" these systems are (especially without considering natural gas costs are increasing at 7% avg. annually in CA and this is often the best home improvement for fuel savings and carbon offset). I would urge the consideration of what’s entailed in the typical installation: after an involved marketing process in an undeveloped market, we have an involved permitting process followed by 2 full days of work that involves building envelope penetrations, mounting, flashing, sealing, electrical, plumbing, insulation, painting, disposal, and sometimes drywall, patching, custom fabrication and lots of other tasks and custom work. It is a LOT of hard work. Many people don’t consider the cost of offering a service like this under these market conditions. In any event, the key to the success of the US solar thermal industry is quality, and quality pays you back…

    I urge anyone considering installing solar water heating to understand that it is an investment and a GREAT investment when the project is executed well. If the installation is quality, the homeowner enjoys solar water heating for over 25 years, with a zero-risk return as high as 15%…

    Link to this
  15. 15. solarhotusa 5:04 am 05/4/2010

    Thanks for sharing this type of information with all of us. Its highly motivational to see <a href="http://www.solarhotusa.com&quot; Title=Solar heating systems>Solar Energy Systems</a> efforts stepping up and doing the right thing for the environment and for their budgets. Good Luck.

    Link to this
  16. 16. rai8irm 3:03 am 08/17/2010

    Providence provides portable solar powered water purification systems, solar powered irrigation pumps, alternative energy power systems, and more.

    Our partners have developed "off the grid" solutions that can produce clean water and clean energy virtually anywhere in the world.

    These systems are designed by world-class experts in renewable energy, solar power, electrical engineering, pump design, water filtration, water sanitation, water conservation, energy conservation and other disciplines.
    http://providencetrade.com/

    Link to this
  17. 17. JRHendricks 3:46 pm 01/7/2011

    May of last year we installed a hot water system consisting of an 80 Gallon Electric Drainback System, with one 4′ x 10′ flat-plate collector and a Simple Drainback Tank (single household, family of three). Its a solar hot water tank with an internal 3800 watt electric backup heating element so when there is an insufficient sun (like in the dead of winter) the backup heating element directly provides supplemental heat for hot water. http://simpledrainback.com/NewFiles/80galelectric.html

    Simplest system on the market, and works like a dream (at least with our typ. climate here in Missouri).
    -J

    Link to this
  18. 18. JRHendricks 3:46 pm 01/7/2011

    May of last year we installed a hot water system consisting of an 80 Gallon Electric Drainback System, with one 4′ x 10′ flat-plate collector and a Simple Drainback Tank (single household, family of three). Its a solar hot water tank with an internal 3800 watt electric backup heating element so when there is an insufficient sun (like in the dead of winter) the backup heating element directly provides supplemental heat for hot water. http://simpledrainback.com/NewFiles/80galelectric.html

    Simplest system on the market, and works like a dream (at least with our typ. climate here in Missouri).
    -J

    Link to this
  19. 19. JRHendricks 3:47 pm 01/7/2011

    ay of last year we installed a hot water system consisting of an 80 Gallon Electric Drainback System, with one 4′ x 10′ flat-plate collector and a Simple Drainback Tank (single household, family of three). Its a solar hot water tank with an internal 3800 watt electric backup heating element so when there is an insufficient sun (like in the dead of winter) the backup heating element directly provides supplemental heat for hot water. http://simpledrainback.com/NewFiles/80galelectric.html

    Simplest system on the market, and works like a dream (at least with our typ. climate here in Missouri).
    -J

    Link to this
  20. 20. soniseo 12:06 am 01/27/2011

    Solar powered tank heaters are a perfect solution for the challenges facing us today. We are living in a time where our reliance on cheap oil to power our lives has caused us to harm our environment close to the point of irreperable damage.
    <a href="http://www.solarhotusa.com"&gt;

    Link to this
  21. 21. soniseo 12:07 am 01/27/2011

    Solar powered tank heaters are a perfect solution for the challenges facing us today. We are living in a time where our reliance on cheap oil to power our lives has caused us to harm our environment close to the point of irreperable damage.
    <a href="www.solarhotusa.com"></a>

    Link to this
  22. 22. KWortman 10:38 am 09/20/2012

    I am a huge supporter of solar thermal heat. Why not save money and help the environment? Check out solar thermal in Halifax, Nova Scotia @ http://www.doctorsolar.ca

    Link to this
  23. 23. KWortman 3:41 pm 01/15/2013

    The best things in life are free, including sunshine. Solar thermal heat will certainly help the environment and save you money. Check out solar thermal heating in Nova Scotian @ http://www.doctorsolar.ca or on twitter @doctor_solar

    Link to this
  24. 24. Paul_Lenaerts 1:39 am 02/16/2013

    Solar Thermal is more efficient than PV. See url: http://en.wikipedia.org/wiki/Solar_thermal_energy .

    Paul from Solar Thermal LTD

    Link to this

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