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Solar heating for barn

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Posted by Gary on February 8, 2004, 12:59 am
I have a detached barn/garage that I plan to add some solar heating to.

The garage is about 24 ft on each side -- about 520 ft^2 inside plus a large

I am in the process of insulating the barn to R19 for walls and loft
The estimated heat loss (after insulating) is 160 BTU/hr-F (this includes
walls, ceilings, doors, and windows, but not the slab).
The climate is SW Montana -- avg Jan high 29F, avg Jan low 6F (but
occasionally -30F :-)

My objective is not to keep the barn toasty warm all the time, but just to
the edge off the night time lows, and make the daytime highs more
comfortable for working (say 50F ish).
Other objectives are: low maintenance, long life, no overheating of barn or
collector in summer, and not ugly.

The collector design I am leaning toward is as follows:

Location:  On the South wall, which has good sun exposure with no
This wall also faces a large reflective snowfield from late Nov to late

Size: Aprox 7 ft high by 16 ft wide (about 103 ft^2 net).

Glazing: SunTuf Polycarbonate corrugated panels (26 inches by 96 inches
    Available locally for $/ft^2, with a "Lifetime Limited Warranty"
The glazing panels will be joined on the collectors vertical support members
to form
one 16 ft wide panel.

The glazing panels are to be mounted 4 inches off the barn siding, and
parallel to the siding, with the bottom of the panels about 18 inches off
the ground.  There will be (non-insulated) closeouts on top, bottom, and
Black, metal window screening will be hung between the barn siding and the
glazing as an absorber.
The flow through the collector is to be by natural convection from the barn
space, into the bottom of the collector, up through the collector, and out
the top of collector back into the barn.  The vents through the barn siding
into the collector are as follows:
One set of 8 vents along the bottom of collector, each 4 inches by 20 inches
(aprox 4% of collector area)
A similar set of 8 vents will be provided along the top of collector.
Flapper valves at the top set of vents will prevent reverse flow at night.

I plan to block some or all of the vents in the summer to prevent
overheating of the barn.

I plan to provide some form of outside ventilation for the collector so it
does not overheat when the vents to the barn are blocked off (not sure what
form this takes).

Performance Guess:
I estimate the heat loss at about 160 BTU/hr-F, or about 3200 BTU/hr with a
20F deg delta temperature (30F outside, 50F inside).

One reference (Principles of Solar Engineering) estimates the sunny day
solar gain per sq ft at 44Nlat for a vertical collector at 1500
Assuming 45% collector efficiency, 103 ft^2 of collector area, and allowing
a factor of 1.5 for reflection from the snow field, I get:
    Heat to barn for day = (1500 BTU/ft^2-day) (103 ft^2)(45%/100)(1.5)
                                = 104k BTU/day (sunny day)
        or roughly 104k/8hr = 13000 BTU/hr (about 4 times the day time
target heat loss/hr)

This would indicate to me that on a sunny day, the barn may heat up fairly
quickly once the sun rises?

To get some idea of the daily temperature fluctuations, I modeled the barn
contents and the internal barn structure as 4000lb of water.  So, when the
heat gain exceeds the loss, the barn's (modeled as water) temperature goes
up, and vice-versa when heat loss exceeds the heat gain.  For any given hour
in this simulation:

        heat loss for the hour  = (160 BTU/hr -F) (Toutside - Tinside)

        heat gain for the hour = (hourly gain) (103 ft^2) (45%) (1.5)

        change in barn temperature = (heat gain - heat loss) /(4000 lb) (1.0

Where "hourly gain", and "Toutside" are table lookups for a sunny day in
January in Bozeman, MT.

Running this model for a few days, the temperatures stabalize at:

   Time           Toutside      Tinside        (heat gain - heat loss)
    6am               6 F            44 F             -4600 BTU
    8am              16               38                +7100
   10am              24              44                 +14400
   12 noon         29               51                 +14500
     2pm             28               57                 +10100
     4pm             20               58                 -2600
     6pm             10               54                 -7400
     12 midnight    6               46                 -6600
     4am               6                38                -5400

So, if you believe this, the minimum garage temperature (7am) is 35F, and
the mid afternoon temperature gets up to the high 50's -- the solar heater
is keeping the inside of barn around 20 to 40F warmer than the outside
temperature.  This would be great (if true).


Are the performance calculations shown above likely to be in the ballpark?
Does the collector seem to be about the right size to meet the objectives?

Air Circulation System:
Will the natural convection vent scheme dramatically reduce the efficiency
of the collector?
(i.e. is there a big payoff for providing forced ventilation?)

Are the vents large enough (4% of collector area) to provide good

Is the black metal window screen an OK absorber?
Should I arrange it such that the air has to flow through the screen in
getting from bottom vents to top vents?  Or, should I arrange it so the air
divides at bottom and flows up behind and in front of the screen?

Flapper Valves (to prevent reverse flow through collector):
I have seen references to the flapper valves in this group, but I don't know
what they are made from, what the geometry is, or how well they work??

Summer Collector Vents (to prevent overheating):
How much summer collector ventilation area should be provided to prevent
overheating of the collector?  Any ideas for thier design?

Any suggestions to improve the geometry, construction, etc. of the system?

Thanks Gary

Posted by Andrew Burgess on February 8, 2004, 6:38 pm

Or maybe shade cloth over the collector in the summer. AFAIK polycarbonate does
degrade in UV, it's coated to reduce this. Shading would extend it's life.
(Though in this application, a little clouding isn't so bad, not like
a window you want to look through, and vents sound easier, and the shade cloth
itself will degrade over time...)

Sounds wonderful. Could you add some simple temperature sensors all over
to really see how it performs?

Posted by Gary on February 11, 2004, 11:11 pm
 Hi Andrew,
I did go ahead and buy a half dozen Dallas Semi "one wire" temperature
sensors and their "TINI" microcomputer with the intent of logging some of
the temperatures -- hope my JAVA skills are up to the job of getting it to
It would be nice if there was some way to also measure and log the velocity
of the air through the collector.

I bought most of the materials for the collector today, and plan to start
building tomorrow.



Posted by Ecnerwal on February 12, 2004, 2:09 am

You have temp sensors. Look up "hot wire anemometer", and Bob's your

Cats, Coffee, Chocolate...vices to live by

Posted by nicksanspam on February 12, 2004, 6:51 am

Or maybe "thermoanemometer." Dwyer makes a $00 version. You might add
a small resistor next to a temp sensor and measure the temp diff with
and without a fixed voltage across the resistor to add some heat (0.1 W?)
to the combination. After a few time constants, smaller temp diffs would
indicate greater air velocities.

You might calibrate it with a 4" diameter x 5' tall thermal chimney. Page
43 of Passive Solar Energy (Bruce Anderson and Malcolm Wells, Brick House,
2nd edition, 1994) says V = 486sqrt(h(To-Ti)/(Ti+460)) feet per minute,
where h is the height difference in feet between intake and outlet vents
and To and Ti are (F) air inlet and outlet temps. For example, h = 5',
Ti = 80, and To = 70 would make V = 486sqrt(5(80-70)/(80+460)) = 148 fpm.
Heating 148Pi(4/12/2)^2 = 13 cfm 10 F takes about 130 Btu/h or 38 watts.

Scientific American (80s?) described how to build one in a 1" steel ball.


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