A simple solar collector

[Gary]

Hi,

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.

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.

Here's how it works:
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 .

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.

There is an attempt at an ascii sketch at the bottom of this message.

Note: The garage door consists of 5 horizontal panels, each 19 inches
tall and 18 ft wide -- they roll up on a set of tracks. The panels
are metal with about R5ish insulation embedded. There is weather
striping all the way around, but its hard to seal something so big and
flexible very well.

Operation as a solar collector:
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.
This is a bit like the free convection solar heater that I use on my
barn (described at http://users.montanadsl.net/~reysa/)

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.
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. I have a similar
(unintended) cavity on the barn collector that is formed by the
ceiling joists that tends to confirm that this approach might work).

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.
A manual override switch would allow the fan to be turned off when heat
is not wanted. Or, perhaps, a PV panel driven fan? A simple return
grill in the garage/house wall would allow return air to enter the garage.

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.

When the sun is not shinning, just shut the garage door. In this
night time position, There is less heat loss then there was because
the added glazing decreases infiltration and increases R value.

If heat is desired in the garage instead of in the house, just open
the garage all the way. If things are getting too warm, just close
the garage door all the way. I suppose some thermal storage in the
form of water barrels could be added just North of the garage door if
some carryover through the night is desired. If I leave the glazing
up in the spring and fall, I can open the garage door, and have a shop
with a view

PERFORMANCE:
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
Absorption = 0.95

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
Ucol is loss coef for single glazed col, including rad

Net gain = 128K - 69K = 59K,
or 59000/(90000 * 0.85) = 0.8 gals of propane at 85% efic

or, (30 days)(0.8gals/day) = 24 gal/month
or, (24 gal/mo)($1.30 /gal) = $31 per month


If you do all the heating months from Oct -> March in the same way,
the totals come to 149 gallons of propane per year. The savings are
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

These seem a bit conservative to me in that no credit is taken for the
decrease in nighttime losses from the garage due to the new glazing,
and some heating occurs in other months.


PRO/CON:
Here is my first cut at pros and cons.

Pros:
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
investment.

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.

Cons:
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.

I am a bit concerned about making this look OK from outside. Any ideas?

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.

--
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 ??


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 --------------

 

[Gordon Richmond]

I have done almost exactly what you describe to my garage. I have a 22
X 24 garage with a double-wide rollup door that I use as a shop to
work on cars. I don't have to have daily entry-egress for the cars,
though.

I bought some light lumber, 1X4s and 2X4s, and made a light frame that
is basically a friction fit into the framing around the outside of the
door. Framing is made in three panels, with the center section readily
removable so a car can enter the shop, and for summer use. Covered the
framing with 6 mil poly plastic.

Observations: no hard numbers; shop is definitely warmer, as air
infiltration around the uninsulated garage door has nearly ceased. On
a sunny winter day, you can stand near the inside of the garage door,
and feel the heat radiating off the wood panels (unpainted natural
wood color outside, BTW).

I have yet to compare gas consumption for this year and last year, but
I can't see how this addition could have increased consumption. The
garage is heated by in-floor hot water heating.

The "solar collector" isn't too pretty to look at, but i live in a
rural area, and I'm the only one who has to see it. About 400-odd
miles north of Billings, actually.

Gordon Richmond

 

[[email protected]]

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.

PERFORMANCE:
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

PRO/CON:
Here is my first cut at pros and cons.

Pros:
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
investment.
Nice.

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.

Cons:
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 --------------

Nick

 

[Roger_Nickel]

Cons:
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.

An 18ft opening is wide for hinged doors, you will end up with accordion
folding doors on a sliding door track if you go for vertical hingeing, a
lot of extra trouble and expense.

I am a bit concerned about making this look OK from outside. Any ideas?

Using the existing framing makes it tough. If you can fit in a porch
around the door you may be able to find in your local auction rooms a
set of glass folding doors that would suit.If you can live with access
to only on side of the garage, an ordinary sliding gass door would do
the job.

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.

Easy enough to frame up a hinged porthole to let long boards through

--
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 ??

If starting from new it would probably be easier to use double glazed
instead of solid panels in the door and use a concrete floor slab in the
garage for heat storage. Hot air will still rise to the ceiling and can
be circulated by fan. A porch above the door can be arranged to let
winter sun in and keep summer sun out. Grow grapes on it if you can and
you will have summer shade and fruit as well.

 

[Gary]

Hi Nick -- thanks for the comments -- a couple questions below if you
have time. -- Gary

It's simpler and more efficient to open the door all the way. I'm reluctant
to do that automatically because of burglars.

I agree that if I only want to heat the garage, it would be better to
just open the door all the way, but if I want to get the heat to the
house via the duct/fan arrangement described below it seems like I
have to do something like the partial door opening scheme above to
collect the heat for the fan intake -- or is there a better way?

I think that if I just opened the garage door all the way, and had
(say) a fan that would blow air into the house that the garage would
overheat, and also that I would not get as much heat to the house?

A house alarm system might

prevent the door opening if nobody's home, but then we don't get the heat.

I guess one advantage of rural MT is that there aren't many burglars

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...

I am leaning toward adding a column in the middle of the 18 ft span.
This makes two 9 ft wide openings. I think I could then make the
glazing panels look something like garage doors, and have one (ir
nire) of them actually hinged on a vertical hinge line to allow easier
access.

Would that interfere with opening the door all the way?

The 12 inches is as far down as the divider could go without
interfering with the door. When the door is raised 10 inches, the top
of garage door matches up pretty well with the bottom of the divider
-- I think this will help to channel hot air into the cavity formed by
the divider, and then into the duct.

How many cfm?

If I were to use the barn collector as a guide it has (10
vents)(0.5ft^2) = 5 ft^2, and a typical sunny day vent velocity is
140 fpm -- this would give
(5 ft^2) (140 ft/min) = 700 cfm
or, if scaled down by the collector area (144/160) (700) = 630 cfm ??

Or a $15 line voltage thermostat. yes


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

Yes -- I was thinking about a motorized damper that is activated with
the fan. Or maybe just the poly film "check valve".

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.




Maybe 0.9.

Yes, 0.9 -- I thought that SunTuf quoted 0.95, but when I went back
and looked it is 0.9.

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...

I like the way that you can see through one layer of window screen --
it seems to me it makes it look better from outside, and also gives
you a view from inside.
If I am understanding what you are saying, half of the incoming light
goes through the screen, and part of that is reflected back by the
door, and half of the reflected part gets back through the screen --
for the green this would be: (0.5)(0.5)(0.5) = 12%lost?? I think
something just a bit better than green at 0.5 would be OK with me.

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

I did some hour by hour simulations for Billings using the TMY2 data,
and the assumption of 120F for the collector temperature hurts the
performance quite a bit when outside temperatures are very low (high
losses), or solar gain is not high (eg partly cloudy) -- ie the model
calcs losses with a 120F collector temperature that would probably
never be reached on a partly cloudy day.
I guess I need some way to vary the collector temperature according to
the actual conditions?
I suppose a more complex control system and a variable speed fan would
also help?

About 10K Btu/h.

The hourly simulation actually shows some peaks above 20K BTU/h

A 1000 cfm fan would have a 10 F temperature rise.
?? not sure what this means ??

I like Grainger's 4TM66 $73 5850 cfm 90 W reversible 16" window fan,
with 3 speeds and a thermostat.

I'll take a look at that.

Nice

I may have been a bit hasty assuming all of the heat that is generated
could be used -- the hourly simulation shows times during mid day on
warm days when the supply exceeds the demand by quite a bit --
especially in the warmer months.

Another pro: I really enjoyed the light with the door open.
Much better than working under the single bulb in the garage.

Yes, I open the big door sometimes in the summer now, but its not
something you want to do when it 10F outside

It may not raise the cost much...




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 have rebuilt my barn collector so that the corrugated PC panels are
supported every 2.5 ft perpendicular to the corrugations -- this makes
an amazing difference -- there is almost no buckling or deflection
now. This is true even thou the wood supports are only 3/4 by 3/4 wood.

 

[Gary]

An 18ft opening is wide for hinged doors, you will end up with accordion
folding doors on a sliding door track if you go for vertical hingeing, a
lot of extra trouble and expense.

I am leaning toward adding a column in the middle of the 18 ft span
that would divide it into two 9- ft spans -- then I could make the
glazing panels look something like the older style outward opening
garage doors that used to be common.

Using the existing framing makes it tough. If you can fit in a porch
around the door you may be able to find in your local auction rooms a
set of glass folding doors that would suit.If you can live with access
to only on side of the garage, an ordinary sliding gass door would do
the job.



Easy enough to frame up a hinged porthole to let long boards through



If starting from new it would probably be easier to use double glazed
instead of solid panels in the door and use a concrete floor slab in the
garage for heat storage. Hot air will still rise to the ceiling and can
be circulated by fan. A porch above the door can be arranged to let
winter sun in and keep summer sun out. Grow grapes on it if you can and
you will have summer shade and fruit as well.

I guess my problem is to get the heat from the garage (where I don't
need much) to the house -- that was the reason for the vertical
divider along the ceiling and the duct/fan. Maybe there is a simpler way?

Thanks for the comments -- Gary

 

[Gary]

I have done almost exactly what you describe to my garage. I have a 22
X 24 garage with a double-wide rollup door that I use as a shop to
work on cars. I don't have to have daily entry-egress for the cars,
though.

I bought some light lumber, 1X4s and 2X4s, and made a light frame that
is basically a friction fit into the framing around the outside of the
door. Framing is made in three panels, with the center section readily
removable so a car can enter the shop, and for summer use. Covered the
framing with 6 mil poly plastic.

Observations: no hard numbers; shop is definitely warmer, as air
infiltration around the uninsulated garage door has nearly ceased. On
a sunny winter day, you can stand near the inside of the garage door,
and feel the heat radiating off the wood panels (unpainted natural
wood color outside, BTW).

So, you don't even have to open the door to get the radiant inside --
nice!


Thanks for the comments -- Gary

 

[[email protected]]

I agree that if I only want to heat the garage, it would be better to
just open the door all the way, but if I want to get the heat to the
house via the duct/fan arrangement described below it seems like I
have to do something like the partial door opening scheme above to
collect the heat for the fan intake -- or is there a better way?

Warm air rises, but it seems to me you still need the screen to avoid
storing sun in the concrete floor, and maybe to reduce the amount of
light and radiant heat when you are working in the garage.

I think that if I just opened the garage door all the way, and had
(say) a fan that would blow air into the house that the garage would
overheat, and also that I would not get as much heat to the house?

Maybe not. The tradeoff is keeping the temperature and daytime loss
through the glazing low. In full sun, 0.9x250x130ft^2 = 29.3K Btu/h
passes through the glazing. If the air near the glazing has temp T (F),
(T-40)130 is lost through the glazing. The 3.5"x18' slot has 5.25 ft^2
of area. With 70 F air in the bottom slot and a 7' height diff between
top and bottom slots, Q = 16.6x5.25xsqrt(7(T-70)) cfm will flow into the
garage, carrying I = (T-70)Q Btu/h of heat. So we have something like
this, viewed in a fixed font:

T
--- | 1/130
|---|-->|---*---www--- 40
--- |
29.3K Btu/h | I -->
---www--- 70

29.3K Btu/h = (T-40)130 + 231(T-70)^1.5, or T = 70 + ((265-T)/1.78)^0.666

Plugging T = 100 F into the right side makes T = 182 on the left.
Plugging T = 182 in on the right makes T = 82.9 on the left.
Repeating makes T = 91.8, then 91.1... Not too hot.
You might collect 22.4K Btu/h of useful heat
with a 76% efficiency.

The 12 inches is as far down as the divider could go without
interfering with the door. When the door is raised 10 inches, the top
of garage door matches up pretty well with the bottom of the divider
-- I think this will help to channel hot air into the cavity formed by
the divider, and then into the duct.

You may not need it, if the garage ceiling is insulated. You might add
thermal mass to the ceiling.

You may not need a duct, if the fan is near the ceiling.

If the house temp varies much, you might also use a differential thermostat.

We might make this from standard parts, eg a thermostat in a glazed box.

I like the way that you can see through one layer of window screen --
it seems to me it makes it look better from outside, and also gives
you a view from inside.
If I am understanding what you are saying, half of the incoming light
goes through the screen, and part of that is reflected back by the
door, and half of the reflected part gets back through the screen --
for the green this would be: (0.5)(0.5)(0.5) = 12%lost?? I think
something just a bit better than green at 0.5 would be OK with me.

If rho = 0.2 = 0.5g + 0.04(1-g), you might mix 35% green with 65% black.

I did some hour by hour simulations for Billings using the TMY2 data,
and the assumption of 120F for the collector temperature hurts the
performance quite a bit when outside temperatures are very low (high
losses), or solar gain is not high (eg partly cloudy) -- ie the model
calcs losses with a 120F collector temperature that would probably
never be reached on a partly cloudy day.

You might redo the calcs as above.

I suppose a more complex control system and a variable speed fan would
also help?

Maybe. You could put that into the simulation.

A 1000 cfm fan would have a 10 F temperature rise.
?? not sure what this means ??

Moving 10K Btu/h from the garage to the house with
a 1000 cfm fan requires a 10K/1K = 10 F temp rise.

-- the hourly simulation shows times during mid day on warm days when
the supply exceeds the demand by quite a bit -- especially in the
warmer months.

More thermal mass in the house or the garage ceiling might help.

Yes, I open the big door sometimes in the summer now, but its not
something you want to do when it 10F outside

Should be fine, with the glazing and screen in place.

I have rebuilt my barn collector so that the corrugated PC panels are
supported every 2.5 ft perpendicular to the corrugations -- this makes
an amazing difference -- there is almost no buckling or deflection
now. This is true even thou the wood supports are only 3/4 by 3/4 wood.

You can see through Dynaglas, with distortions... Flat polycarbonate
(eg Lexan rolls from GE) is more windowlike, but subject to scratching.

Nick

 

[Gary]

....

Warm air rises, but it seems to me you still need the screen to avoid
storing sun in the concrete floor, and maybe to reduce the amount of
light and radiant heat when you are working in the garage.

Your idea is that the screen would absorb some of the light, heat up,
and cause air around to rise to the ceiling where it could be fanned
into the house?

I wonder if the screen could roll up like a window shade? Then the
screen might be able to basically control whether heat went mostly
into the garage (screen up), or mostly into the house (screen down)?

Maybe not. The tradeoff is keeping the temperature and daytime loss
through the glazing low. In full sun, 0.9x250x130ft^2 = 29.3K Btu/h
passes through the glazing. If the air near the glazing has temp T (F),
(T-40)130 is lost through the glazing. The 3.5"x18' slot has 5.25 ft^2
of area. With 70 F air in the bottom slot and a 7' height diff between
top and bottom slots, Q = 16.6x5.25xsqrt(7(T-70)) cfm will flow into the
garage, carrying I = (T-70)Q Btu/h of heat. So we have something like
this, viewed in a fixed font:

T
--- | 1/130
|---|-->|---*---www--- 40
--- |
29.3K Btu/h | I -->
---www--- 70

29.3K Btu/h = (T-40)130 + 231(T-70)^1.5, or T = 70 + ((265-T)/1.78)^0.666

Plugging T = 100 F into the right side makes T = 182 on the left.
Plugging T = 182 in on the right makes T = 82.9 on the left.
Repeating makes T = 91.8, then 91.1... Not too hot.
You might collect 22.4K Btu/h of useful heat
with a 76% efficiency.


....


You might redo the calcs as above.

I did this, and it looks pretty good -- Tcol varys in sensible ways
with the sun input, and ambient temperature.

Your equation: Q = 16.6x5.25xsqrt(7(T-70)) cfm
Is this for a chimney? Is the 16.6 empirical or?

I guess that on the actual system, I could vary the 5.25 ft^2 gap
area, and/or introduce flow restrictions if I wanted somewhat higher
Tcol at the cost of lower efficiency?

....

On cabinet doors that have a central panel surrounded by a frame, the
central panel expands across the grain, while the frame expands with
the grain. There is much more expansion across the grain, and to keep
the panel from buckling or cracking, it floats in a dado in the frame.
I wonder if the same technique could be used for these garage door
glazed panels -- i.e. the PC glazing panel would float in a dado in
the frame, perhaps with light caulking to prevent infiltration?
I am thinking of the frame breaking the glazing up into panels that
are probably no more than 2 ft by 4ft ish.

Not sure what you meant by using the wood to act as a spring?

Anyway, it seems like I could go ahead and build the glazed
panels/doors, and then experiment with gaps, screens, dividers, ducts
and fans.

You can see through Dynaglas, with distortions... Flat polycarbonate
(eg Lexan rolls from GE) is more windowlike, but subject to scratching.

Nick

I ordered one of the Testo 405's that mentioned in another post --
looks like it should make velocity measurements a lot easier.

 

[[email protected]]

Your idea is that the screen would absorb some of the light, heat up,
and cause air around to rise to the ceiling where it could be fanned
into the house?

Yes, with the fan mounted high on a wall between the house and the garage,
blowing air into the house. The screen might absorb most of the light...
Full sun is 10K FC, but you only need about 50 for working, ie 2% of that.
You might hang one or two layers of 80% (absorbing) black greenhouse
shadecloth inside the garage door (it shrinks about 30% in boiling water.)
Stuppy (800) 733-5025 sells 80% Sundown polypropylene shadecloth in 5, 6,
10 and 12' rolls for about 14 cents/ft^2. You can sew it together with
fishing line and hang it with grommets or "poly clips." Stuppy can make
a nice custom curtain with grommets for another $100 or so.

I wonder if the screen could roll up like a window shade? Then the
screen might be able to basically control whether heat went mostly
into the garage (screen up), or mostly into the house (screen down)?

You could do that, but it seems easier to just slide it along a wire or
turn off the fan or raise its thermostat temp to make the garage warmer.
Thought about storing heat in some pipes or poly film water ducts under
the garage ceiling? They might lie on a layer of foil with welded-wire
fence underneath. The foil would help keep the heat near the ceiling.

I did this, and it looks pretty good -- Tcol varys in sensible ways
with the sun input, and ambient temperature.
Good...

Your equation: Q = 16.6x5.25xsqrt(7(T-70)) cfm
Is this for a chimney? Is the 16.6 empirical or?

Yes. Q = 16.6Asqrt(HdT) cfm is an empirical "stack effect" chimney formula.

I guess that on the actual system, I could vary the 5.25 ft^2 gap area,
and/or introduce flow restrictions if I wanted somewhat higher
Tcol at the cost of lower efficiency?

Yes. Or build another hotter collector inside the outer one, or
collect warmer air on the north side of the mesh and cooler air
on the south side, next to the glazing.

On cabinet doors that have a central panel surrounded by a frame, the
central panel expands across the grain, while the frame expands with
the grain. There is much more expansion across the grain, and to keep
the panel from buckling or cracking, it floats in a dado in the frame.
I wonder if the same technique could be used for these garage door
glazed panels -- i.e. the PC glazing panel would float in a dado in
the frame, perhaps with light caulking to prevent infiltration?

It might easily buckle... 0.020" flat polycarbonate is more flimsy than glass.

I am thinking of the frame breaking the glazing up into panels that
are probably no more than 2 ft by 4ft ish.

Or 2'x2'. More wood and work...

Not sure what you meant by using the wood to act as a spring?

Picture a 4'x8' sheet of flat polycarb in a trampoline frame, always
in tension. If we attach it to a light wood frame on a hot day, it might
make the wood bow inwards when it gets cold, if the fasteners don't tear.
The wood frame might be mounted behind some face boards that let the
inner frame move but don't allow much air to leak around the gaps
formed by the bowing.

Anyway, it seems like I could go ahead and build the glazed panels/doors,
and then experiment with gaps, screens, dividers, ducts and fans.

Sounds like a plan. (Steve Baer would add "Throw away your calculator.")

I ordered one of the Testo 405's that mentioned in another post --
looks like it should make velocity measurements a lot easier.

I ordered one too, altho I already have a $400 Dwyer version that's
larger and less sensitive. I found the Testo in an SBSE "Agents of Change"
tool kit which also contained a "pocket bolometer" (a 30 gallon plastic
trash bag , some Hobos, a compass, a Kestrel 3000 pocket weather meter
(airspeed, temp and RH, $145 from www.forestry-suppliers.com), a Sylvania
light meter, a Raytek MT4 IR thermometer, a solar transit template,
a Pilkington sun angle calculator, a Motorola flicker checker (a spinning
top with a pattern that shows whether fluorescent lamps have magnetic
(120 Hz) or electronic (20 kHz?) ballasts), a "globe thermometer" (a mat
grey ping-pong ball with a hole that slips over a thermocouple, used to
calculate mean radiant temperature), and a long-life plastic soap bubble
solution from Toys-R-Us.

Testo says the hot wire should last for years, but it isn't replaceable,
so I wouldn't use it to stir coffee or to pound nails. Testo considers
this a $150 "disposable instrument"

Nick

 

[Gary]

Still trying to figure out the best way to build the glazed
panels/doors for the garage door solar collector...

....

It might easily buckle... 0.020" flat polycarbonate is more flimsy than glass.

As a test:
I bought a sample sheet of 0.093 thick Acrylic -- it is 30 X 36 inches
(about the size of the panels on the eventual door).
I made a simple wood frame with a rabbet around the inside edge of the
frame to mount the Acrylic in. The rabbet allows is 1/8 inch edge
clearance all the way around. I "glued" the Acrylic into the frame
with silicone caulk (at about 70F).
The theory is that the 0.093 has somewhat more resistance to buckling
than (say) 0.020 Polycarbonate, and the silicone caulk would let it
expand a bit, while still sealing?

I put the panel in the sun at noon with the back closed off.
The inside air temp is running about 135F, ambient is about 80F.
The deflection is quite small -- perhaps 0.1 cm outward bow at mid
panel -- not visible to the eye.

I guess I am wondering if the 0.1 thick Acrylic is a better choice for
this project than the commonly available 0.02 to 0.03 inch thick
Polycarbonate?

Comparing Acrylic and Polycarbonate:
What I have read or remember (quite possibly wrong) is

PC with UV treatment and Acrylic have good UV resistance, and
a 10+ year life.

PC has a higher service temperature, but Acrylic is probably OK for
this application at 170F?

PC has more impact resistance, but Acrylic is still better than glass?

They both have a linear expansion coef of about 3.5 X105 in/in-F.

PC and Acrylic are both susceptible to scratches.

Any thoughts or experience on this?
I guess another possibility would be the dual wall PC panels, which
would provide more stiffness?

Picture a 4'x8' sheet of flat polycarb in a trampoline frame, always
in tension. If we attach it to a light wood frame on a hot day, it might
make the wood bow inwards when it gets cold, if the fasteners don't tear.
The wood frame might be mounted behind some face boards that let the
inner frame move but don't allow much air to leak around the gaps
formed by the bowing.

I may give this a try, but it seems a bit complex. I guess another
possibility would be a laced edge? I have done this for mylar mirrors
with good success.

I ordered one too, altho I already have a $400 Dwyer version that's
larger and less sensitive. I found the Testo in an SBSE "Agents of Change"
tool kit which also contained a "pocket bolometer" (a 30 gallon plastic
trash bag , some Hobos, a compass, a Kestrel 3000 pocket weather meter
(airspeed, temp and RH, $145 from www.forestry-suppliers.com), a Sylvania
light meter, a Raytek MT4 IR thermometer, a solar transit template,
a Pilkington sun angle calculator, a Motorola flicker checker (a spinning
top with a pattern that shows whether fluorescent lamps have magnetic
(120 Hz) or electronic (20 kHz?) ballasts), a "globe thermometer" (a mat
grey ping-pong ball with a hole that slips over a thermocouple, used to
calculate mean radiant temperature), and a long-life plastic soap bubble
solution from Toys-R-Us.

Sounds like Xmas

Gary

 

[[email protected]]

I bought a sample sheet of 0.093 thick Acrylic -- it is 30 X 36 inches
(about the size of the panels on the eventual door).
I made a simple wood frame with a rabbet around the inside edge of the
frame to mount the Acrylic in. The rabbet allows is 1/8 inch edge
clearance all the way around. I "glued" the Acrylic into the frame
with silicone caulk (at about 70F).
The theory is that the 0.093 has somewhat more resistance to buckling
than (say) 0.020 Polycarbonate, and the silicone caulk would let it
expand a bit, while still sealing?

It doesn't need to seal very well, as typical sunspace glazing, if
the main "seal" is plastic film or motorized dampers. It isn't like
a house window. Small air leaks just decrease solar collection
efficiency slightly.

I put the panel in the sun at noon with the back closed off.
The inside air temp is running about 135F, ambient is about 80F.
The deflection is quite small -- perhaps 0.1 cm outward bow at mid
panel -- not visible to the eye.

Nice. But this acrylic is more expensive than polycarb, no?
And smaller pieces mean more mounting labor and wood and
air leaks and fasteners and shading.

I guess I am wondering if the 0.1 thick Acrylic is a better choice for
this project than the commonly available 0.02 to 0.03 inch thick
Polycarbonate?

I'd say no, if we can mount large pieces of pc aesthetically.

I may give this a try, but it seems a bit complex.

You might try making it all with 1x3s... an inner 1x3 frame with the pc
attached to the short edges on 1' centers and an outer 1x3 frame with
the inner one screwed loosely to it at the corners and horizontal and
vertical 1x3s screwed to the outer frame as cap strips. Two inner
frames could share a vertical outer frame and vertical cap strip,
altho that's still lots of labor and wood...

Sounds like Xmas

It was a loan to our "Tool Day" team. We had to give it back...

Our 7/15/04 Portland Brewery Block studies might be on the SBSE web site now.

Nick