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Solar Barn Update

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Posted by Gary on August 15, 2004, 6:01 pm
I added an update to the solar barn collector that has been discussed
here before.

Changes include:
Construction revision to better control glazing panel thermal deflections.

A rundown and test of several summer overheat protection schemes.

A test on how many layers of window screen make the best absorber.

The update is on the few pages.

Its at:

http://users.montanadsl.net/~reysa/     -- then click on the first link
  -- its a 430KB pdf.

Let me know if you have any trouble with the link.

Thanks again for the help from this group on the project!


Posted by Toby Anderson on August 16, 2004, 3:41 pm
Greetings Gary.

If your goal is to cheaply get heat into the garage on a sunny day,
then you've done a great job.

During the 16 hours of winter night(twice as long as an 8 hour winter
day), however, you could come up with a better design. You wrote:

3) there is no insulation, since you want to encourage heat
transfer out the back of collector, not discourage it,
The R value for the collector wall is about 2.5, which is poor
compared to the R 13 or more of an insulated wall.

Putting 2 layers of 1 inch foam board in the garage side will provide
this insulation.... it would also retard heat from entering the garage
during the day. So, you could add vents at the top and the bottom. As
the sun heats up the air in your collector, it will rise out the top
vent and draw it in the bottom vent.


Posted by Gary on August 17, 2004, 3:46 am
 Toby Anderson wrote:

Hi Toby,
The part you quote above is from the 2nd pdf file, and is just a
concept -- the actual barn collector (in the first pdf) does have high
and low vents, and this is how the bulk of the collected heat gets to
the barn.  A bit passes through the uninsulated collector wall when
the sun is shinning.  Its the first pdf that I put the updated
material in -- I should have made this more clear in the post.

The barn loss is about 170 BTU/F-hr not counting the collector wall.
The collector wall at R2.5 would be (160ft^2/2.5 ft^2-F-hr/BTU) = 64
BTU/F-hr -- so it is a bit over a quarter of the barn total loss.  For
an average night time delta T of 20F (starts at 30F and ends at 10F),
this would be (64 BTU/hr-F)(16 hr) (20F) = 20K BTU loss overnight.  If
the effective mass of the garage is 4000 lb of water, this would
amount to 20K/4K = 5F warmer in the morning -- this does seem like a
worthwhile gain, so I probably should insulate this wall as you suggest.


I am working on trying to do some solar heating for my house now.
This is a challenge, since it is very poorly oriented for solar.  I am
adding the outer door/collectors to my garage door opening (as
mentioned here before) to provide some daytime heat to the house
without any thermal storage.  In addition to this, I am thinking about
using the rest of my South facing wall (about 200 ft^2) to add solar
collectors that are somewhat similar to the barn collector, but with
integrated storage -- it ends up looking a bit like a flattened
version of Nick's solar closet concept.  South to North the collector
has: a glazing layer, a screen or shade cloth absorber, an insulated
partition with high and low vents, a chamber North of the partition
with water containers to store heat.  During a sunny day, air
circulates through the absorber chamber (heating up), and then into
the storage chamber (heating the water).  Poly film check valves
prevent reverse flow at night.  The back wall of the storage chamber
would actually be the existing house wall.  A duct/fan would extract
heated air from the storage chamber when needed for the house.  My aim
is to store enough heat during a sunny day to carry the house through
to the next morning.  I'm not attempting to cover no sun days.
Any thoughts on this?
I think its kind of an interesting concept -- if carefully integrated
into a new construction wall, it might add less than a foot of thickness.

Just to get a feel for how well this might work I am converting one 4
ft bays of the barn collector to this configuration.  I obtained 40
one gallon jugs of water, and some 2 inch foam board.  I plan to hang
the water jugs just North of the collector bay, and enclose around the
back of them with the foam board.  So, air is heated by the collector,
circulates out the top vent, down past the jugs, and back into the
lower vent.  I'll record the temperature of a top, middle and bottom
just and see what percentage of incident solar energy gets into the
water storage as the day progresses.  I am wondering if natural
convection is going to be good enough to get the job done?

Here is a picture of the partially completed test collector:


Posted by nicksanspam on August 20, 2004, 1:00 pm

For overnight heat storage, you might do better just using the thermal
mass of the house... It has a lot more surface, so the air near the
collector glazing could be cooler, with less heat loss to the outdoors.
If your house has 400 Btu/h-F of conductance and 4K Btu/F of capacitance,
RC = C/G = 10 hours. If it's 70 F at dusk and 30 F outdoors, with no
internal heat gain, it would cool to 30+(70-30)e^(-16/10) = 38 F 16 hours
later. Brr. Making it 60 F at dawn would require that RC = -16/ln(30/40)
= 56 hours, ie C = 56x400 = 22,246 Btu/F, eg 2671 1-gallon jugs :-)

If you heat a ceiling capacitance C near the air heater outlet to 100 F
by dusk and keep the house 50 at night, the house needs 16(50-30)400
= 128K Btu over 16 hours, and C = 128K/(100-50) = 2560 Btu/F, eg 307
ceiling jugs or a 30" diam x 36' greenhouse poly film duct filled with
4" of water, over some welded-wire fence or 1/2" plywood with a layer
of foil underneath.

In my experience, those cross-linked polyethylene milk jugs will leak
if hung like that.


Posted by Gary on August 20, 2004, 11:13 pm
 Hi Nick,

nicksanspam@ece.villanova.edu wrote:

I have a simulation that includes the house (modeled as equivalent to
6k lb of water), and the garage door collectors (with no storage).  I
use the TMY2 weather data for Billings to drive the simulation.
In the simulation, I let the garage door collector put heat into the
house during the day until the house temperature gets up to 75F.  Once
the house has reached 75F, I dump any additional heat from the garage
door collector -- this  happens fairly often in the shoulder months.
At night I let the house temperature cool to 60F before turning the
furnace on -- this means I am getting 6000lb * 1BTU/lb-F *(75F - 60F)
= 90K BTU out of house as thermal storage?   This is good for about 6
hrs -- 30F + (75F - 30F)e^(-6/16) = 61F?
(I plan to check the 6K house mass (which I have no real justification
for :-) by measuring actual cool down rate and comparing it to
predicted with 6k mass and 400 BTU/F-hr loss rate -- this should work?)

I can add about 180 ft^2 more collector area to the South wall of the
house, but it seems like there is not a lot of point to doing this
without adding some new thermal storage that would allow the collected
heat to be held over for the night.

With its heat going to storage, the added collector might make another
110K BTU of stored heat available on the night after a sunny day,
which would probably get me most of the rest of the way through a cold
night after a sunny day?

I have been thinking about some schemes for collector + storage:
1) add 180 ft^2 of air collectors to the south wall of house, route
the heated air to an insulated thermal storage enclosure (water?) in
the conditioned crawl space that is right behind the collector wall,
and draw heat from the storage at night.  This seems a bit complex,
having ducts, fans, controllers, and two separate things to construct
(collector and storage)?  But, maybe its more efficient than method 2?

2) Do a collector with integrated storage, which might be a refined
version of the one I am testing in the barn (described at top of this
post and at the "Test of Integrated Storage Collector" link at
http://users.montanadsl.net/~reysa/ ).  The collected heat would be
removed from the storage chamber with a duct and fan that delivers
heated air to the house.  The storage containers might be 4 inch to 6
diameter lengths of plastic pipe a bit like the ones that Steve Baer
showed at the ASES Workshop?

3) Do a collector with integrated storage in which the storage
containers are located directly behind the collector glazing such that
the sun directly heats the containers.  At night automatically deploy
an insulated panel between the glazing and the water containers to
reduce night losses.  It seems like this would be more efficient as a
collector during the day than scheme 2, but the design and
construction of the movable (roll up?) insulated panel would be difficult.

3) Take a look at your suggestions on adding ceiling thermal mass.

I am thinking that the amount of thermal storage for the new collector
should be about equivalent to what the new collector can pick up
in one sunny winter day.  There does not seem to be any point in
trying to store more than this, because the loss rate of my house is
high enough to use all of the stored heat on the night of the sunny day?

To store the heat from one sunny day I need ABOUT:
Heat to store = (1450 BTU/day-ft^2)(40%)
               = 600 BTU/ft^2 of collector

Storage mass  = (600 BTU/ft^2)/ (120F - 75F) (1BTU/lb-F) =
               = 13 lb/ft^2

Where 40% is kind of a target rate for getting incident solar energy
into the water storage -- it has to account for collector losses and
other losses in getting heat transfered to the water -- reasonable?
And, 120F is max temp of stored water, and 75F in minimum useful temp
of stored water.
And, 1450 BTU/day-ft^2 is avg sunny day solar energy on vertical panel
in mid-winter at 45 lat.

This would be 1.6 gal/ft^2, or 50 gal per 4X8 panel, or 320 gals for
the full 200 ft^2.  Thus the 40 one gallon jug 4X8 test collector.

I am also thinking about using the same system for domestic water
preheat, since this would allow the collector to "earn its keep"
during the summer (at the cost of some winter house heating performance).

I realize (and the simulation reminds me) that even in MT, not all
days are sunny, and, in the end, this setup will save less than 50% on
my heat bill, but a 30% to 50% saving is much better than the 0% I'm
getting now :-)

I'll think about how I could accomplish something like this.  I have
to say that having a lot of water over my head in poly film ducts is a
bit unsettling.  I guess the poly ducts could be over the ceiling
sheet rock with insulation over them?  Perhaps with a safety container
of heavy poly sheeting?  I'm trying to think how I sell my wife on the
Alum foil ceiling :-)

Good to know.
I used the gallon jugs in this little test collector because they
were only 50 cents each (including the distilled water :-), so it
was a cheap/easy way to get an feel for how well this kind of
collector might work. I have it set up in the barn now and I am
recording temperatures at the top and bottom vents and top, middle,
and lowest row of jugs, and ambient.  I have only got about one day of
data, which looks roughly like this around mid afternoon:

    Ambient:          72F
    Top vent:          142F
    Bottom vent:         93F
    Top row bottles:    112F
    Mid row bottles:     94F
    Bot row bottles:     81F
    Air velocity in vent: 30fpm

I plan to refine the setup a bit (fix heat leaks etc), and get a few
more days data, and also check on cool down rate at night, but
tentatively, it seems like more air velocity would lower the air
temperature some, and reduce collector losses, and would also transfer
heat to the water more rapidly?  As the water heats up, the air
temperature would come up, and heat the water to a higher temperature,
but at a lower efficiency? Judging by the strong vertical
stratification in jug temperatures, and the relatively low lower vent
temperature, I am inclined to say that heat transfer from the air to
the bottles is fairly efficient?
I'm not at all sure that this is the way to go, but I plan to tinker
with venting, maybe add a fan, or? ... just to see what kind of
performance it could yield.
It seems to me that a refined version of this integrated storage wall
collector could be an interesting idea for new construction for folks
planning houses having quite a bit of South wall, but who don't care
for the acres of windows and bare concrete floors approach?? It would
look pretty much like a normal wall, only 6 inches or so thicker with
some nice looking vents?  Windows could be mixed into the wall for
light and views.  I think that its overall performance would be better
than lots of window area with thermal mass  in the floor, because the
window losses are so high at night?  I've also noticed that the low
winter sun, and strong reflection off snow cover, results in a lot of
glare, and that people tend to draw the curtains to cut the glare --
which somewhat defeats the heat collection in floor mass.

Any ideas or suggestions for improvements would be appreciated.


As an aside, I have been working on reducing the heat loss of the
house.  I am using the book "Insulate and Weatherize", Bruce Harley,
Taunton Press as a guide.  I find this book to be very helpful and
highly recommend it.

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