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re: a simple solar collector
2 aug 2004
gary   wrote:

>i have an attached garage that i use as a shop (no cars allowed).  the
>garage door is 8ft by 18ft and faces south --it constitutes about half
>of my total available south exposure.

nice. i have an 8'x9' south garage door...

>i have been thinking about ways to make this garage door opening work
>to collect solar energy for heating the house.  i have worked out one
>scheme that seems promising to me...
>add a fixed layer of glazing (e.g. polycarbonate panels) about 4 to 6
>inches south of the existing garage door.  mount the glazing panels
>inside of the existing garage door framing.  the new glazing panels
>would be removable (perhaps 4ft wide by 8ft high). they would remain 
>in place for the heating season.  the glazed panels would be attached 
>to the garage door framing in such a way as to allow little 
>infiltration (unlike my current garage door :-).

i saw an arrangement like this at an auto repair place in brooklyn,
but the panels stopped 2' short of the ceiling. polycarbonate expands
with temperature...

>just north of the new glazing add a layer of black metal window
>screening. the screening could be supported off the back surface of
>the glazing panels, but would be spaced off the glazing by about 2 to
>3 inches.  when the garage door is down, the screen would be about
>midway between the new glazing panels and the door.
>paint the south face of the garage door a darker (but not too ugly)
>color to increase its solar absorption.
>when the sun is shinning, raise the garage door about 10 inches.  this
>makes a 10 inch gap at the bottom of the door, but also
>opens a horizontal slot at the top of the door as the top panel
>starts to get pulled north along the horizontal part of the door
>track.  the gap is about 3.5 inches wide.  with the door in this
>position cool garage air near the floor enters the gap below the door,
>makes it way up through the space between the new glazing and the
>garage door, and picks up heat from the door and from the screening.

it's simpler and more efficient to open the door all the way. i'm reluctant
to do that automatically because of burglars. a house alarm system might
prevent the door opening if nobody's home, but then we don't get the heat.

last winter, i insulated my garage walls and ceiling and put a layer
of greenhouse poly film over the outside of the door. i was pleased with
the light and heat when the door was open, but it was more difficult to
work on large objects, since everything had to go through the man door on
the opposite wall. poly film is very transparent to ir, but it's cheap
and it comes in large pieces. the view is cloudy...

>based on experience with the barn heater, i think this would do a
>good job of heating (probably overheating) the garage space,
>but i would really like the heat to go to the house, not the garage.
>so, add the following components to transfer most of the collected
>heat to the house:
>add a 12 inch tall vertical divider that runs east-west across the
>garage ceiling about 18 inches north of the plane of the garage door.
>the top of the divider is in continuous contact with the ceiling.

would that interfere with opening the door all the way?

>this makes a cavity that traps the heated air (which is buoyant)in the
>space just above the 3.5 inch top gap.   the top panel of the door
>also helps to guide air into this cavity area...
>add a duct with an inline electric fan that connects from the cavity
>to the house space.  the fan would have a simple thermostatic control
>that would turn the fan on when the air in cavity reaches (say) 80f.

how many cfm? 

>a manual override switch would allow the fan to be turned off when heat
>is not wanted... 

or a $15 line voltage thermostat.

>a simple return grill in the garage/house wall would allow return air
>to enter the garage.

i suppose you will want to prevent reverse thermosyphoning at night.

>i suppose that the opening and closing of the garage door could be
>automated based on a sun sensor, but this seems like overkill in our
>case where the garage door opener is very handy to the house.

i would definitely want to do that. perhaps duane johnson can design
and sell a kit that provides a pulse (like a finger on a push button)
in the morning and evening, with a door position sensor to make sure
the door is open during the day, vs vice-versa.
>here is an attempt at a nick-ulation of the performance for billings, mt.:
>net collector area = 90% of 144 ft^2 = 130 ft^2
>transparency of pc glazing = 0.95

maybe 0.9.

>absorption = 0.95

as i recall, black aluminum window screen is about 0.5, so 25% of the sun
would find its way back out the glazing, with a reflective door... 0.95
would require a door reflectance rho, where 0.25rho = 0.05, ie rho = 0.2. 
green is 0.5. flat black is 0.04...

>nrel says that for the month of nov (kind of an average performance 
>winter month), billings sees 1080 btu/ft^2 per day of radiation on a 
>south facing vertical surface.  average temperatures are 35.1f 
>average, 44.5f maximum, and 25.6f minimum.  so, average day 
>temperature = 39.8 and average night temperature is 30.4f.
>for the about 6 hours of collection time,
>collector gain = (acol)(trans)(absorb)(daily rad)
>                 =(130 ft^2)(0.95)(0.95)(1080 btu/day)
>                 = 128k btu/collection period
>collector loss = (acol)(ucol)(tcol - tamb)(hours door is open)
>                = (130 ft^2) (1.1 btu/ft^2-f-hr)(120f - 39.8f)(6hr) =
>                = 69k btu/collection period
>		tcol is a guess at the average collector temp

that depends on the upper door air gap and the fan cfm. 

>		ucol is loss coef for single glazed col, including rad
>net gain = 128k - 69k = 59k,

about 10k btu/h. a 1000 cfm fan would have a 10 f temperature rise.
i like grainger's 4tm66 $73 5850 cfm 90 w reversible 16" window fan,
with 3 speeds and a thermostat.

>if you do all the heating months from oct -> march in the same way, 
>the totals come to 149 gallons of propane per year.

nice :-)

>...largest in the shoulder months, and smaller in dec and jan (due to 
>less  sun and colder temperature).  i did assume that i could use all 
>the heat for these months, which depends on the house heat loss, but 
>this is (unfortunately) not a problem for my house :-)
>here is my first cut at pros and cons.
>the system should generate heat equivalent to about burning about 150
>gallons of propane a year at 85% efficiency for a saving of $200 in
>our area (and going up each year).
>the cost of the system is low -- i estimate about $340 for materials.
>  the payback on materials would be less than two years.  or, one
>could look on it as an inflation protected, tax free 57% return on


>i like the way the system makes multiple new uses of the existing
>garage door to: 1) provide movable insulation, 2) provide the
>collector vents, 3) provide part of the absorber, and 4) provide a big
>window/collector when opened all the way.
>the system should have a long life with little maintenance.
>the garage should be more comfortable in the winter.  some of the
>collector heat that is intended for the house will "leak" into the
>garage.  also, the heat losses from the garage should be reduced, 
>which should keep it warmer.
>the heat loss from the house should be reduced a bit, because the
>garage shares two walls and a ceiling with the house.
>i like the simple way in which heat can be directed to either the
>garage or the house.
>it seems like a nice simple system that anyone can understand and use.

another pro: i really enjoyed the light with the door open.
much better than working under the single bulb in the garage. 
>you lose easy access to the garage through the large door (e.g. for
>cars).  this does not matter to me, but would be a problem if you
>insist on putting your car in the garage :-).  one potential way
>around this is to integrate the glazing panels into a 2nd set of
>doors that open outward on vertical hinge lines, but this does make it
>a more complex carpentry project.

it may not raise the cost much...

>i am a bit concerned about making this look ok from outside.  any ideas?

a 4'x8' flat polycarb panel probably needs wood on 2' centers to avoid
too much oilcanning. can the wood act as a spring to prevent it entirely? 

>i need to work out some way to be able to rip very long boards on my
>table saw -- now, i just open the garage doors to do this.

hinges or easily removed panels.

>a final thought -- it seems like there ought to be some way to
>integrate this whole concept into a new garage door design (one that
>preserves the ability to open the door as well as collect solar
>energy) -- if widely applied, this might be worth:
>     (10^2 heating days/yr)(10^5 btu saved/day)(10^6 houses?)
>      = 10^13 btu per year  ??

sounds good.

>any comments would welcome -- gary
>     garage ceiling
>---------------------------------+    |
>   <- duct toward house <-- |     |    | <-- south wall
>                            |     |    |
>               3.5in gap ----> \  +----+
>                                \   . |
>                                 \  . |<-- new glazing
>                                 +  . |
>                                 |  . |
>                  garage door -> |  . |
>                                 +  . |
>                                 |  .<------ screen absorber
>                                 |  . |
>                                 +  . |
>        garage                   |  . |
>                                 |  . |
>                                 +  . |
>                                    . |
>                         gap -->    . |
>--------floor ---------------------------- driveway --------------


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