Elementary question on Increasing Solar Heat Gain thru a Window

[buck]

If I want to collect as much solar heat through a window as possible, and
don't care about insulating values will I be better off with a single pane
or a double pane glass? Where I live in the southwest we get a lot of south
sun through some small fixed upper windows and I would like to maximize the
heat during the day to heat the house for the colder winter night........ I
probably should know the answer but I am not sure. Or would a double pane
save more of the heat loss at night and therefor it would be a wash???
-TIA

 

[Ed Sebesta]

I would would think that you need the double pane glass to avoid thermal
loss due to conduction, both during the night, and also during the day
during the winter. The sun loss due to double pane would be a reflection
from the inner glass to the exterior which is going to be fairly minor.
Window glass is clear and the absorption would not be very small. The
thermal loss through glass is high. Glass is a good conductor of heat, and
the insulation of single pane of glass is the non-moving and slowly moving
layers of air on the surface.

If you could have shutters or blinds this would optimize your energy
situation, since you could close them at night, and during the summer during
the day if your house got too warm.

If you change your glass go for the new titanium dioxide coated windows
which are self-cleaning, they will increase transmission since your window
won't be dirty.

I am a chemical engineer.

Ed

 

[buck]

Hey Ed...... So which allows more sun heat to come in during the
daytime?...... single pane glass or double pane glass? Could I generate
more head coming in if I installed a fresnel lens or some kind of window
film in the window or is that totally not going to heat anything??

-also thanks for your initial response.... much appreciated!

 

[[email protected]]

Could I generate
more head coming in if I installed a fresnel lens or some kind of window
film in the window or is that totally not going to heat anything??

Why do I think this could be more appropriate in a group devoted to
the red light district of Amsterdam?

 

[[email protected]]

Double or triple-pane windows have the best insulating value. It
matters what kind of gas is used in multi-pane windows: atomically
heavier gases (e.g. krypton) move slower and thus have higher
insulating value. There are also some coatings (e.g. low-E) that'll
reduce the amount of infrared radiation produced, so you'll want to
pay attention to that, too.

I really wonder, though, whether having a window that would allow
infrared radiation through would ever pay off over the course of the
year, even in your location. Unless you effectively screen the window
during the summer you'll be allowing a large amount of infrared
radiation into the house, generating heat that you'd have to somehow
either block or redirect.

My house has a large single pane picture window that faces south so I
tried a simple experiement by building a solar collector composed of
black 1-1/2" ABS tubing and elbows with a 110v muffin fan at one end
to move air through the assembly. On sunny days air going into the
collector measured 66-68 degrees came out at 120-130 degrees. However,
I don't think there was enough volume to make it worthwhile and here
in Oregon there not that many sunny days to begin with. Not rigorously
emperical, true, but good enough that I came to the conclusion I'd be
better off replacing the single-pane window with a double-pane widow
filled with krypton gas and low-E coating to provide better insulation
during the winter and lower infrared radiation transmissivity during
the summer.

 

[buck]

Thanks to All for your comments..... Much appreciated!

 

[[email protected]]

If I want to collect as much solar heat through a window as possible, and
don't care about insulating values will I be better off with a single pane
or a double pane glass?

Single, or none

... would a double pane save more of the heat loss at night and therefor
it would be a wash???

Double is likely better, for a window on a living space that's 70 F for
24 hours per day. Where I live near Phila, 1000 Btu/ft^2 falls on a south
wall on a 30 F average January day. If each pane has 90% transmission and
adds R1, one would transmit 900 Btu/day and lose 24h(70-30)1ft^2/R1, for
a net gain of -60 Btu/day. Two would have a 330 Btu/day gain, 3 would have
409, 4 would have 416, and more than 4 would have less net gain...

10 FOR P=1 TO 10'number of panes
20 SUN=1000*.9^P'sun passing through window (Btu/ft^2)
30 LOSS=24*(70-30)/P'24-hour heat loss through window (Btu/ft^2)
40 PRINT P,SUN-LOSS
50 NEXT

# panes net gain (Btu/day)

1 -60
2 330
3 408.9999
4 416.0999
5 398.4899
6 371.4409
7 341.1539
8 310.4671
9 280.7537
10 252.6783

If S Btu/ft^2 falls on the wall on an average T (F) temp day, the net
gain = S0.9^P-24h(70-T)/P; dgain/dp = 0 makes 24(70-T) = -P^2ln0.9x0.9^PS,
eg P = e^(P+21)/19 for Phila. Plugging in P = 2 on the right makes P = 3.4
on the left, then 3.6, 3.64, and 3.65...

A single pane window (or a single layer of polycarbonate plastic) on a low-
thermal mass sunspace that gets cold at night might gain 900 Btu/day and lose
6h(70-30)1ft^2/R1 = 240, for a net gain of 660. Two panes might gain 810 and
lose 120, for a net gain of 690... not much more, but condensation where
moist house air touches the indoor glazing surface would be less likely.

Nick

 

[[email protected]]

If I want to collect as much solar heat through a window as possible, and
don't care about insulating values will I be better off with a single pane
or a double pane glass? Where I live in the southwest we get a lot of south
sun through some small fixed upper windows and I would like to maximize the
heat during the day to heat the house for the colder winter night........ I
probably should know the answer but I am not sure. Or would a double pane
save more of the heat loss at night and therefor it would be a wash???
-TIA

Far more significant than comparitive window pane gains, you need to
look into storage. You need thermal mass to store the heat during the
day and release it slowly at night. Insulated curtains over your
windows at night would help immensely, but I do not think you will
have any significant difference in solar gain between dual or single
pane. Of course the duals would lose less at night, but again you
need thermal mass. If you don't care about appearance 55 gallon steel
drums painted black and filled with water are ideal (water is an
outstanding thermal mass). Get food grade plastic drums and you can
can make them doulble as emergency water supply. Stone walls. Dark
slate if you have living space you want to look nice. Google "Trombe
Wall" for more info.

Hal

 

[[email protected]]

... Google "Trombe Wall" for more info.

Yuck.

Nick

 

[[email protected]]

Yuck.

why is that a "yuck"? Trombe walls are an excellent representation of
the principle of thermal mass storage.

Hal

 

[[email protected]]

why is that a "yuck"? Trombe walls are an excellent representation of
the principle of thermal mass storage.

And very poor solar collectors, since they store and lose heat back through
the glazing all night, like a direct gain (aka "direct loss" house.

Nick

 

[Nick Hull]

If you change your glass go for the new titanium dioxide coated windows
which are self-cleaning, they will increase transmission since your window
won't be dirty.

I would love to hear from anyone with experience with self-cleaning
windows; I am considering getting some and don't know if they are worth
it.

 

[[email protected]]

... If you want to try a very simply experiment for inexpensive solar
thermal gain, hang a black sheet in the window. You will be amazed at
how much heat the room gains throughout the day.

Less than without the sheet

Nick

 

[[email protected]]

Please explain... I'm listening...

The sheet doesn't increase the amount of sun that passes through the window,
but with no sheet, the air and materials near the window could be close to
70 F. With the sheet, the air and materials would be warmer, so there would
be more heat loss to the outdoors.

Nick

 

[[email protected]]

The temperature in the room definitely goes higher with a black sheet
hanging in the window.

Not so high as without the sheet.

Nick

 

[[email protected]]

Nick, I'd like to hear more... I've experienced just the opposite.

Maybe you stood near the black sheet. You might try measuring the central
room air temp with a thermometer, with and without the black sheet. With
no sheet, the air near the window might be 70 F. The sheet and the air near
the window would be warmer, with less room airflow near the window, and
the window pane would be warmer by radiation from the warm sheet, raising
the heat loss to the outdoors.

You might think the sheet absorbs more sun and reflects less back out the
window than the room does, but that's only true if the room is very shallow
with very light walls, as in your white sheet. Otherwise, sun that enters
the window has a hard time escaping via the window before it reflects upon
and is absorbed by other room surfaces.

Nick

 

[Derek Broughton]

The temperature in the room definitely goes higher with a black sheet
hanging in the window. I've done the experiment plenty of times to
have personally witnessed it. If I were to hang a white sheet in the
window, the room stays cooler. I know that we have had this discussion
before. The basic principle of a radiometer works in much the same
way.

That's not at all the same thing. Hanging a black or white sheet _in the
window_ makes a difference to the amount of energy reflected back out
through the window. Hanging a black sheet in the window will cause an
increase in heat _radiated_ out of the window. Furnishing and painting the
room all black will keep both the reflection and radiation to a minimum.

 

[Tom Peel]

Maybe you stood near the black sheet. You might try measuring the central
room air temp with a thermometer, with and without the black sheet. With
no sheet, the air near the window might be 70 F. The sheet and the air near
the window would be warmer, with less room airflow near the window, and
the window pane would be warmer by radiation from the warm sheet, raising
the heat loss to the outdoors.

You might think the sheet absorbs more sun and reflects less back out the
window than the room does, but that's only true if the room is very shallow
with very light walls, as in your white sheet. Otherwise, sun that enters
the window has a hard time escaping via the window before it reflects upon
and is absorbed by other room surfaces.

Nick

I suspect that the effect can only work if sufficient heat is carried
off the sheet into the room through convection, while at the same time
blocking light reflected off the room surfaces from being radiated back
through the window.
Let's put some numbers into the equations. Let's suppose that 1kW is
coming in through the window,and that the room temperature is in
equilibrium. Without a sheet, suppose that 90% will be absorbed by the
room surfaces and that 10% goes back out the window.
That means that 100W is going out the window, the remaining 900W is
being lost from the room through other heat sinks- if they weren't then
the room temp would not be in equilibrium.
Now put the sheet in, and suppose it has NO effect. The sheet absorbs
all 1000W, PLUS- if the room stays the same temperature - it gets the
100W that the room walls are radiating back out the window!
Now the sheet is heating up nicely, and has to get back in equilibrium
at some temperature by emitting energy. Some of this will go off as
convective heat, the rest as radiation. Let's call the convective heat
x, and the radiation y. The radiation y goes off both sides of the
sheet, half goes out the window, the rest goes back in the room, so we have:
Energy entering the sheet = 1100W = x + y = energy lost from sheet
Energy entering room = x + y/2 = 900W = heat loss from room
We can solve the equation and conclude that for the sheet to have NO
effect on the room temperature, y= 400W and x, the convection, is 700W.
Now, if the sheet emitted no radiation and all the heating was all
convective, then all the 1100W would enter the room and the temperature
would rise. If however there was no convection, the sheet would simply
absorb all the 1100W and radiate half of this out of the window, meaning
the room temperature would fall.
Obviously, the 10% assumption we make is crucial, but there is clearly
a breakeven depending on the size of the windows in relation to the
room, and the effectiveness of convective vs. radiative cooling of the
sheet..

BTW I have a degree in physics.

T.

 

[Eeyore]

I work the night shift and during the day, I hang a black sheet in the
window,
so that the sun won't shine in my room and I can get some much desired
sleep.

Normally, the room would only reach whatever temperature the thermostat
is set to.
Unfortunately, I often wake up because of the excessive temperature in
the room.

You need to hang some aluminised melinex or similar in the window.

Graham

 

[[email protected]]

I suspect that the effect can only work if sufficient heat is carried
off the sheet into the room through convection, while at the same time
blocking light reflected off the room surfaces from being radiated back
through the window.

Light and heat, ie "long-wavelength light." I'm trying to think of
a situation in which some sort of black sheet would help. Somebody
(Clear Dome Solar?) sells a "solar hot air collector" that fits on
the inside of a window, which seems like a mistake. So maybe we have
a shallow room with a white south wall behind a large south window.

We might blow room air with a fan against the north side of the black
sheet to keep it closer to the room temp, altho that wasn't part of
the original situation. We might use a black sheet with a selective
south surface that radiates less heat to the window pane

Let's put some numbers into the equations. Let's suppose that 1kW is
coming in through the window,and that the room temperature is in
equilibrium.

Maybe the US R2 window is 20 ft^2, and the room air is 70 F.

Without a sheet, suppose that 90% will be absorbed by the room surfaces
and that 10% goes back out the window.

This might mean "10% of the light," since glass does not transmit wavelengths
longer than 3 microns, ie heat, altho it absorbs and reradiates heat.

That means that 100W is going out the window, the remaining 900W is
being lost from the room through other heat sinks...

Here's a formula from page 210 of the 1991 2nd edition of Duffie and
Beckman's Solar Engineering of Thermal Processes:

ALPHAeff = ALPHAi/(ALPHAi+(1-ALPHAi)Aa/Ai), where

ALPHAeff is the effective absorptance of an cavity opening,
ALPHAi is the absorptance of the inner surface of the cavity,
Aa is the area of the aperture of the cavity, and
Ai is the area of the inner surface.

A very large window in front of a shallow room has an Aa/Ai of about 1,
so ALPHAeff is close to Ai. A small window in front of a deep room
(like a crab trap) has Aa/Ai close to zero, so ALPHAeff is close to 1.

A 10' cubical room with a 1 ft^2 solar window (about twice what's needed
for good indoor illumination in full sun) has Aa/Ai = 1/600. If the room
is covered inside with bright white paint with absorptance ALPHAi = 0.2,
the window has an effective absorptance of 0.2/(0.2+(1-0.2)/600) = 0.9934,
so less than 0.7% of the incoming sun is reflected back out of the window.

Now put the sheet in, and suppose it has NO effect. The sheet absorbs
all 1000W, PLUS- if the room stays the same temperature - it gets the
100W that the room walls are radiating back out the window!

The room was radiating (reflecting) light, not heat, no? So we might say
the black sheet gets a total of 1000 W on its south surface, and the room
no longer reflects 100 W of light back out through the window.

Now the sheet is heating up nicely, and has to get back in equilibrium
at some temperature by emitting energy. Some of this will go off as
convective heat, the rest as radiation. Let's call the convective heat
x, and the radiation y. The radiation y goes off both sides of the
sheet, half goes out the window, the rest goes back in the room, so we have:
Energy entering the sheet = 1100W = x + y = energy lost from sheet

How did we get up to 1100 W? In English units, a room at temp Tr (F) might
lose s((Tr+460)^4-(Ts+460)^4) Btu/h-ft^2 to a Ts (F) surface (eg a sheet or
a window pane) by radiation, where s = 0.1714x10^-8. If the surface is
warmer than the room, the room gains heat from the surface. At mean temp Tm
(F), the room and the surface would have a linearized radiation conductance
of 4s(Tm+460)^3 Btu/h-F-ft^2.

We can solve the equation and conclude that for the sheet to have NO
effect on the room temperature, y= 400W and x, the convection, is 700W.

In English units, if the sheet temp is Ts (F), each side might lose about
(Ts-Ta) Btu/h-ft^2 to slow-moving air at Ta (F) by convection. We might
have a thermal chimney between the sheet and the window, with Av ft^2
slots at the top and bottom, H feet apart, and cfm = 16.6Avsqrt(HdT).

Obviously, the 10% assumption we make is crucial, but there is clearly
a breakeven depending on the size of the windows in relation to the room,
and the effectiveness of convective vs. radiative cooling of the sheet..

Got numbers?

Nick