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Heated windows [longish]
So, I did some testing of a window I purchased [not from the company ME is involved with, ONLY because of shipping across the country] It was pretty interesting, and I have some graphs and such, see below.
Please take these as they are intended, an interesting evaluation by a disinterested party. I am pretty confident of their accuracy, I am very confident that the relative accuracy is very good.
I have an evil plan for my own application that makes it pretty likely that I will use these, the worst case being that I will not always run them.
I think these definitely have a place especially in difficult to heat rooms
K
<a href="http://members.verizon.net/%7Evze4wpr3/68degree.htm">[url=http://members.verizon.net/~vze4wpr3/68degree.htm]http://members.verizon.net/~vze4wpr3/68degree.htm</a>
vertical scale is watts per square foot, horizontal is 'outside'
temperature deg F
This first graph is the most extensive tests I did, and I have great
confidence in the results since some of the test periods ran over 50
hours. I would have left a kink at the left of the graph for the two
warmest temperatures, as this was pretty evident while watching this
that it was a straight linear function at the colder temperatures, but
flattened out as it got warmer. The spreadsheet is a little limited. I would have liked to run some colder
temperatures, but the freezer was not able to pull it any colder. It is
of course possible that the higher power usage numbers are actually
error, but I find it more likely that they represent heat loss to the
room becoming more of a factor. In any event, I don't think that it is
likely that anyone would be running the windows at temperatures any
higher than the mid 40's so it is pretty moot.
<a href="http://members.verizon.net/%7Evze4wpr3/78degree.htm">[url=http://members.verizon.net/~vze4wpr3/78degree.htm]http://members.verizon.net/~vze4wpr3/78degree.htm</a>
again, watts vs outside temp
Running the window at 10 degrees above room temperature. While it is a
limited data set it was again some pretty long run times. My electronic
thermometer died during the warmest run, so I chose not to include those
numbers, but they were consistent.
<a href="http://members.verizon.net/%7Evze4wpr3/Extendeddata.htm">[url=http://members.verizon.net/~vze4wpr3/Extendeddata.htm]http://members.verizon.net/~vze4wpr3/Extendeddata.htm</a>
This is my feeble attempt to integrate these two data sets and answer
the question
"I want to run the windows at this temperature, and it gets this cold
out, how much power will they use?"
It is simply DeltaT[outside]+{deltaT[inside]*2}
Where DeltaT[outside]= Temperature difference from the heated glass
surface to the cold side and deltaT[inside]= Temperature difference from the heated glass surface to the room. I have no reason to think this is accurate other than it looks good, and seems make sense intuitively.
Heated window test result conclusions
With the heatloss design temperature in my area of around 70 degrees F,
100 square ft of windows with an R value of 3[such as the tested window
without power applied] we would have a heatloss of 2310 BTU's, or 677 watts [100/3*70] according to my graph, with the glass surface at 68 degrees F, the
windows would draw about 730 watts, or about 107 percent of the energy
required by the unheated window. In Massachusetts, with electricity
over 17 cents per kWH, this is not very attractive. This lead me to do a
few 'what if' calculations.
If we were to use a triple pane window, with an insulation value of
about R4, the heatloss would be about 1750 BTU's or 513 watts. Next
comes another broad assumption, but one that I think not to far from
accurate: I am going to assume that the center pane in the triple pane
window is thermally the center, and thus at a Delta 70 the center pane
would sit at 35 degrees. I recently had a good friend run an FEA that
confirms that this is a valid assumption, within reason. If we go back
to the graph it shows us that we should use about 400 watts, about 78
percent of the energy required by the unheated window. It should be
noted that my setup could not exceed 9 watts per square foot, due to the
contacts being located on the wide side of a nearly 6 foot pane.
This number starts looking attractive. Although it would take heating
oil at $4.60 a gallon[at 85 percent efficiency] to be literally 'saving'
money, we are now in a place that we can discuss the true comfort of not
having cold windows.
Interestingly, going to quad pane does not get us closer. While it does
use less electricity, percentage wise it is not as good as the triple
pane. This is due to the flattening of the graph at the warmer outside
temperatures. It is my feeling that this flattening is due to some
heatloss to the room, which could be appearing as a constant overlaying
on the graph. Simply, the warmest spots of the window may be radiating,
for instance 1 watt per square foot to the room, no ,matter the outside
temperature, which is not noticeable when the heat loss to the outside
is 6 or 7 watts per square foot, but down below 4 watts is starts
becoming a significant source of consumption.This flattening could also
be error[1], due to the average temp of the window drifting higher at
the warmer outside temps, or the controller wasting energy trying to
keep the temperature too close to the setpoint.
Whatever the reason, most of the reasons I can think of end up being
heatloss to the room, which is generally a 'good thing' although it can
be expensive. This is also reinforced by the much higher electricity
consumption a the higher window temperature test.
With the tested window at 78 degrees F, a Delta T of 70 is going to use
about 1200 watts for that 100 square feet. In a well insulated and tight
house, one would think you would not run this temperature[or higher]
without warming the room enough for the window to cycle on and off
[1]
I could also have incorrectly calculated the temperature on the warmer
end, since I had to use a thermostat with a deadband that the colder
temperatures[with the freezer running flat out] did not have. This last
cannot account for all of the difference, since it would require a shift
to the colder of greater than the possible error.
Please take these as they are intended, an interesting evaluation by a disinterested party. I am pretty confident of their accuracy, I am very confident that the relative accuracy is very good.
I have an evil plan for my own application that makes it pretty likely that I will use these, the worst case being that I will not always run them.
I think these definitely have a place especially in difficult to heat rooms
K
<a href="http://members.verizon.net/%7Evze4wpr3/68degree.htm">[url=http://members.verizon.net/~vze4wpr3/68degree.htm]http://members.verizon.net/~vze4wpr3/68degree.htm</a>
vertical scale is watts per square foot, horizontal is 'outside'
temperature deg F
This first graph is the most extensive tests I did, and I have great
confidence in the results since some of the test periods ran over 50
hours. I would have left a kink at the left of the graph for the two
warmest temperatures, as this was pretty evident while watching this
that it was a straight linear function at the colder temperatures, but
flattened out as it got warmer. The spreadsheet is a little limited. I would have liked to run some colder
temperatures, but the freezer was not able to pull it any colder. It is
of course possible that the higher power usage numbers are actually
error, but I find it more likely that they represent heat loss to the
room becoming more of a factor. In any event, I don't think that it is
likely that anyone would be running the windows at temperatures any
higher than the mid 40's so it is pretty moot.
<a href="http://members.verizon.net/%7Evze4wpr3/78degree.htm">[url=http://members.verizon.net/~vze4wpr3/78degree.htm]http://members.verizon.net/~vze4wpr3/78degree.htm</a>
again, watts vs outside temp
Running the window at 10 degrees above room temperature. While it is a
limited data set it was again some pretty long run times. My electronic
thermometer died during the warmest run, so I chose not to include those
numbers, but they were consistent.
<a href="http://members.verizon.net/%7Evze4wpr3/Extendeddata.htm">[url=http://members.verizon.net/~vze4wpr3/Extendeddata.htm]http://members.verizon.net/~vze4wpr3/Extendeddata.htm</a>
This is my feeble attempt to integrate these two data sets and answer
the question
"I want to run the windows at this temperature, and it gets this cold
out, how much power will they use?"
It is simply DeltaT[outside]+{deltaT[inside]*2}
Where DeltaT[outside]= Temperature difference from the heated glass
surface to the cold side and deltaT[inside]= Temperature difference from the heated glass surface to the room. I have no reason to think this is accurate other than it looks good, and seems make sense intuitively.
Heated window test result conclusions
With the heatloss design temperature in my area of around 70 degrees F,
100 square ft of windows with an R value of 3[such as the tested window
without power applied] we would have a heatloss of 2310 BTU's, or 677 watts [100/3*70] according to my graph, with the glass surface at 68 degrees F, the
windows would draw about 730 watts, or about 107 percent of the energy
required by the unheated window. In Massachusetts, with electricity
over 17 cents per kWH, this is not very attractive. This lead me to do a
few 'what if' calculations.
If we were to use a triple pane window, with an insulation value of
about R4, the heatloss would be about 1750 BTU's or 513 watts. Next
comes another broad assumption, but one that I think not to far from
accurate: I am going to assume that the center pane in the triple pane
window is thermally the center, and thus at a Delta 70 the center pane
would sit at 35 degrees. I recently had a good friend run an FEA that
confirms that this is a valid assumption, within reason. If we go back
to the graph it shows us that we should use about 400 watts, about 78
percent of the energy required by the unheated window. It should be
noted that my setup could not exceed 9 watts per square foot, due to the
contacts being located on the wide side of a nearly 6 foot pane.
This number starts looking attractive. Although it would take heating
oil at $4.60 a gallon[at 85 percent efficiency] to be literally 'saving'
money, we are now in a place that we can discuss the true comfort of not
having cold windows.
Interestingly, going to quad pane does not get us closer. While it does
use less electricity, percentage wise it is not as good as the triple
pane. This is due to the flattening of the graph at the warmer outside
temperatures. It is my feeling that this flattening is due to some
heatloss to the room, which could be appearing as a constant overlaying
on the graph. Simply, the warmest spots of the window may be radiating,
for instance 1 watt per square foot to the room, no ,matter the outside
temperature, which is not noticeable when the heat loss to the outside
is 6 or 7 watts per square foot, but down below 4 watts is starts
becoming a significant source of consumption.This flattening could also
be error[1], due to the average temp of the window drifting higher at
the warmer outside temps, or the controller wasting energy trying to
keep the temperature too close to the setpoint.
Whatever the reason, most of the reasons I can think of end up being
heatloss to the room, which is generally a 'good thing' although it can
be expensive. This is also reinforced by the much higher electricity
consumption a the higher window temperature test.
With the tested window at 78 degrees F, a Delta T of 70 is going to use
about 1200 watts for that 100 square feet. In a well insulated and tight
house, one would think you would not run this temperature[or higher]
without warming the room enough for the window to cycle on and off
[1]
I could also have incorrectly calculated the temperature on the warmer
end, since I had to use a thermostat with a deadband that the colder
temperatures[with the freezer running flat out] did not have. This last
cannot account for all of the difference, since it would require a shift
to the colder of greater than the possible error.
0
Comments
-
re
Thought there would be more interest, probably everyone fell asleep before reaching the end of the post.
I got some fancy color pics if you can see powerpoint files:
members.verizon.net/~vze4wpr3/Window-Report.ppt
A bunch of WAG's in this also, I don't even know what most of it means, but the one that shows the temps of the surfaces[page 10] is what I was after.
Lots of pretty colors tho............
Finally got my password reset, not until convincing Dan that it was my first day on the web...........0 -
I suspect that you and I are the only beings interested in this....
Thanks for the posting Keith.
We are still awaiting the ship to come into port, and in the mean time are continuing doing our own R&D individually.
One of your findings that echo's our findings is that the lower the glass surface temperature, the higher the operating efficiency, I am also finding that the glass really does not have to be that hot in order to significantly increase human comfort. In fact, anything over 85 degrees F gets UN comfortable over a period of time. This is what we call the thermally opaque mode (temps less than 85 but above 70). In other words, the glass surface temp is kept at a temperature that the human bodies skin can't feel it's presence.
Another interesting application is in the elimination of the production of condensation on windows. All we have to do is maintain a surface temperature of 70 degrees F, and we can guarantee NO condensation will form on the glass. This will significantly reduce the physical plant requirements in areas like hospitals and natatoriums where high humidity is fought by moving large volumes of air. The amount of energy consumed to maintain 70 degree glass surface temperatures is significantly less than the parasitic cost of operation of the blowers and fans associated with conventional means currently being employed.
Thanks for the info.
METhere was an error rendering this rich post.
0
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