Third solar collector

Discussion in 'Alternative Energy' started by Ross, Apr 1, 2006.

  1. Ross

    Ross Moderator Staff Member Supporter

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    Sorry for the delay. Here's the final version of collector I recently photographed. Unfortunately I didn't get great pictures of the intake etc. I'll be building one this spring.
    [​IMG]
    If you look carefully you can see the inlet at the bottom right in the collector. It has the same thermostat as his second collector, but this one finally adds in a return air inlet to cylce cool air off his basement floor into the collector. It probably isn't balanced air in and out but its a lot better than over presuring the house!
    [​IMG]
    This is the return back to the far end of the collector. I had more pictures but they didn't work out. All his collectors are only single glazed glass and they aren't even sealed between the panes of glass!! In fact there's a notable gap between the panes. I wonder if a double glazed low E window would work better?
     
  2. Jim-mi

    Jim-mi Well-Known Member

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    No to the low E glass. That in itsself blocks the rays you want heating the absorber plate.
     

  3. Ross

    Ross Moderator Staff Member Supporter

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    I know my picture window is low E filled double glazed and you feel the heat from the sun. The solar gain is from radient heat (light) which passes through and is absorbed onto the steel back and then convectively heats the air which is pumped out. I'm thinking the double glazed window will save more of the heated air than it will block. My first attempt is to duplicate this one with a thermostat and positioning the outlet higher. I do have a double glazed picture window panel kicking around, maybe for a second attempt I try it. Gary had a site with thermal siphons that worked without fans, that would be interesting to try too!
     
  4. SolarGary

    SolarGary Well-Known Member

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    Hi Ross,

    Nice pictures -- thanks!

    My 2 cents on collector design would be along these lines:

    - build as big as you can -- area is really the thing that makes the biggest difference. You can't make up for having a small collector by making it more efficient -- good collectors just don't vary that much in efficiency. If you are heating something like a shop that probably does not have the best insulation, and is a bit leaky, you really can't make the collector to large. My 500 sqft barn/shop has 150 sqft collector wall, and it seems to me thats about right for a cold climate. For a home, if the collector area gets to more than 15 or 20% of the floor area you are heating, then you have to start thinking about providing some heat storage.

    - have a good airpath through the collector that washs all of the absorber plate. Parts of the absorber that don't get airflow over them, just get very hot and loose a lot of heat out the glazing. The link to the book below has some good stuff on this.

    - have enough airflow so that the collector outlet air is not very hot. It may seem like really hot air coming out of the exit vent is good, but its really not. It means that the collector absorber is running really hot, and radiating lots of heat out the glazing. So, if you can, set the airflow through the collector high enough so that the air coming out is hot enough to heat the room and thats all -- it will be more efficient this way, and deliver more heat to the house.

    - On the glazing, if you use glass that came from a window, you want glass that has a high a SHGC (Solar Heat Gain Coeficient) as possible. On newer windows, this is on the window sticker. Low e coatings generally lower the SHGC (there are two types of low e, and one is much worse this way than the other). There is a tradeoff on double glazed vs single glazed -- you lose less heat with the double glazed, but the 2nd layer of glass also absorbs sun, so you get less sun on the absorber. I would not go out of the way to use double glazing, but if thats what you have its fine. Most commercial collectors are single glazed, but also have a selective coating on the absorber that reduces radiation losses.

    - both the collectors that have fan forced air and the thermosyphon collectors that use natural convection to circulate air through the collector can work well. The thermosyphon ones are simplier and don't need fans or thermostats, but they do require more area for the inlet and outlet ducts. If you don't follow the design rules for the thermosyphon collector it just won't get enough air flow through it to work well. The article I did for Home Power has the sizing rules -- the article is a free download here:
    http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
    Fan forced collectors are a bit more forgiving, but you still need to make sure the air washs the whole absorber.


    And, don't forget that a south facing window is just as good at collecting solar energy as a collector is. A window's collection efficiency when the sun is shinning is probably better than most solar collectors. The down side of windows is that they lose a lot of heat at night. This can be overcome with window insulation. My attached garage/shop collector is just a set of glazed doors set just outside the rollup garage door. During the day, I open the rollup door, and let the sun shine in through the glazed doors -- this provides tons of heat and great lighting. Even my wife comments that its the nicest room to be in the house -- warm and light. At night I lower the rollup door, and it insulates the glazed doors that would otherwise lose a lot of heat.
    http://www.builditsolar.com/Projects/SpaceHeating/SolarGarageCollector/garcol.htm
    If you don't believe that this provides heat, take a look at the "How does it do at -20F" link on the same page :)

    Most of the material on different collector designs people have tried is here:
    http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

    The first two on the page are the ones I use on my attached and detached shops.

    The "Solar Air Heating Systems" book that you can download for free has a lot of material on building collectors.

    This book:
    http://www.builditsolar.com/Projects/SolarHomes/PasSolEnergyBk/PSEbook.htm
    (also a free download) has some material on constructing collectors in walls.


    Attached sunspaces can also do a good job of collecting with a thermstatically controled fan to move air from the sunspace to the house.
    Simple sunspaces can be built very very inexpensively.
    For best house heating from a sunspace, you want low thermal in the sunspace so that it heats up quickly and does not absorb heat that you want to go to the house.
    http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm



    Gary
     
  5. SolarGary

    SolarGary Well-Known Member

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    Hi Again,

    I meant to say "you want low thermal MASS in the sunspace ... "
    Gary
     
  6. Jim-mi

    Jim-mi Well-Known Member

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    Ross, I replaced a bedroom window with a big (4'X4') Anderson double casement window unit (true south facing).
    So the day that I put the black chrome collector sheets (2'X2') up to the windows I was big time disapointed at the fact that they barely got warm.
    Lesson learned that the fancy window was blocking all that beautiful winter sun.

    darn it.......