4 ms·

Perovskite-Based Thermochromic Windows Reduce Load and CO2 Emission in Buildings

40 points by ph0rque 4y ago

mikeyouse 4y ago
This is pretty exciting research -- it would be great if we could add more glass to our buildings without the energy costs associated with heating and cooling. I think people underestimate how much energy loss is caused by windows.. If you have a wall assembly that is 25% R-2 glass and 75% R-20 insulated wall -- the total R-Value of that assembly is only R-6.15. Say you doubled the wall insulation and superinsulated 75% of the assembly to R-40, that would surely make a big impact, right? Nope, total R-value of the assembly only increases to R-6.95, an improvement of only 13%. If on the other hand, you merely increased the R-2 glass to R-3, the total wall assembly would improve to R-8.3. An improvement of nearly 35%.
- bialpio 4y ago
  Naive question: is reduction in energy loss linear with R-value or is there a different relationship? Improvement of R-value by 35% tells me nothing if the function to calculate energy loss does something strange with the R-value afterwards... :)
  - mikeyouse 4y ago
    So R-Value refers to the material's ability to resist heat flux and depends on the temperature difference between either side of the assembly. The general formula is Heat Loss = (surface area x temperature difference) / R-Value So as you rightly hypothesize, the "first" bit of insulation is more important than the next bit. Going from R-1 to R-2 decreases energy loss by 50%, going from R-2 to R-3 decreases it by a further 30%, R-3 to R-4 by a further 20%, etc. So for my example with a 40ºF difference between inside and outside, if that wall assembly were 120sq ft, the original wall with R-6.15 would be losing 780btu/hr. The 13% improvement in R-Value from the first example would reduce energy costs by 10% (700btu/hr of energy loss), the 35% improvement would reduce costs by 25% (578btu/hr). It's strictly better to increase r-value but the highest leverage impacts come from the bottom.
  - waldothedog 4y ago
    I believe Ufactor is a more holistic/thoughtful figure for thinking about energy transmission through building envelopes. Might be of interest https://www.greenbuildingadvisor.com/article/all-about-u-factor https://www.greenbuildingadvisor.com/article/all-about-u-fac...
    - mikeyouse 4y ago
      In a sense that's true - it's more natural to consider in the same way that MPG is less obvious compared to gallons per hundred miles. But they're representing the same quality, U-Factor is just the reciprocal of R-Value. So R-Value represents the thermal resistance and U-Value the transmittance but you can easily take the reciprocal of one to get the other (e.g. my R-2 window is just a U-0.5).
      - bialpio 4y ago
        First off, thanks for the explanation in your other post! The main point I had is that there are gotchas when talking about improvements in R-value in terms of percentages that we the uninitiated won't be aware of. U-value doesn't seem to have that problem if it's just a simple reciprocal of R-value (IIUC, halving U-value halves heat loss, doubling it doubles the heat loss - that's a way easier relationship to understand to me).
        mikeyouse 4y ago
        Yep, that's a good point and a good way to think about it. My MPG reference was an eye-opening moment for me -- the US uses MPG which is really backwards from what you care about (in the same way that R-value and U-value are). So increasing your mileage from 10mpg to 12mpg saves exactly as much gasoline as increasing your mileage from 30mpg to 60mpg. This is plainly obvious when you consider gallons per 100 miles, [10->8.3] & [3.3->1.7].
- tguvot 4y ago
  If you are talking R-values - you are right. But if we get a bit more practical, walls have a bunch of thermal mass (drywall on the inside, stucco on the outside, at least in my case ). During the summer, i have walls that get to ~30C on the inside. So even when it's evening/night, and temperature outside drops to 15C from 35C during the da, I open windows and run whole house fan, those walls keep on heating the air inside while lowly R rated glasses shed the temperature quickly. So in my case doubling insulation in wall (in order to prevent drywall from heating up) likely to provide more effect than bumping R value of the glass.
doodlebugging 4y ago
This sounds great. They still have work to do though before this can be a commercial product if they only get 200 cycles out of it. Once they solve the durability problem this should be a great product for upgrading windows in commercial and residential uses.
twawaaay 4y ago
This sounds great but there are alternatives. One nice trick I have seen is windows offset so that the light penetrates deep into rooms during winter but does not penetrate in at all during summer. And then the floors acting as thermal mass and made to distribute the heat throughout the house. And so on. I wish I could find this youtube video again. The house was built probably a hundred years ago or so by an American architect for himself. There is a lot that can be done with mundane materials with a little bit of thoughtful design.
- ph0rque 4y ago
  There's a neat overhang calculator that I used to size my window and door overhangs on the south side: https://www.susdesign.com/overhang/ https://www.susdesign.com/overhang/
- breischl 4y ago
  You can do something similar with an appropriately sized & angled awning/overhang. eg the visitor center at Zion NP does that, along with a number of other clever things. PDF: https://www.nps.gov/zion/learn/nature/upload/DOE%20Brochure.pdf https://www.nps.gov/zion/learn/nature/upload/DOE%20Brochure.... But for commercial buildings where they're trying to squeeze out every square foot of rentable space, and probably are not allowed to overhang their property lines, it's likely not an option.
  - twawaaay 4y ago
    The overhangs don't have to take away from rentable space. It is in effect just a sun shade sticking out of the house horizontally blocking the sun without blocking the view like a baseball cap. I am looking to buy a property right now and I have resigned myself that if I want a nicely designed house I will probably have to do it myself. A well designed house does not have to be more expensive, it is just engineering -- and engineering is about knowledge, experience and ability to use both to make tradeoffs for a better result. The knowledge is there but very few seem to be making use of it. Everybody tries to squeeze a lot of new tech but forget about old lessons -- just like in software development...

40 points by ph0rque 4y ago

This is pretty exciting research -- it would be great if we could add more glass to our buildings without the energy costs associated with heating and cooling. I think people underestimate how much energy loss is caused by windows.. If you have a wall assembly that is 25% R-2 glass and 75% R-20 insulated wall -- the total R-Value of that assembly is only R-6.15. Say you doubled the wall insulation and superinsulated 75% of the assembly to R-40, that would surely make a big impact, right? Nope, total R-value of the assembly only increases to R-6.95, an improvement of only 13%. If on the other hand, you merely increased the R-2 glass to R-3, the total wall assembly would improve to R-8.3. An improvement of nearly 35%.

bialpio 4y ago
Naive question: is reduction in energy loss linear with R-value or is there a different relationship? Improvement of R-value by 35% tells me nothing if the function to calculate energy loss does something strange with the R-value afterwards... :)
- mikeyouse 4y ago
  So R-Value refers to the material's ability to resist heat flux and depends on the temperature difference between either side of the assembly. The general formula is Heat Loss = (surface area x temperature difference) / R-Value So as you rightly hypothesize, the "first" bit of insulation is more important than the next bit. Going from R-1 to R-2 decreases energy loss by 50%, going from R-2 to R-3 decreases it by a further 30%, R-3 to R-4 by a further 20%, etc. So for my example with a 40ºF difference between inside and outside, if that wall assembly were 120sq ft, the original wall with R-6.15 would be losing 780btu/hr. The 13% improvement in R-Value from the first example would reduce energy costs by 10% (700btu/hr of energy loss), the 35% improvement would reduce costs by 25% (578btu/hr). It's strictly better to increase r-value but the highest leverage impacts come from the bottom.
- waldothedog 4y ago
  I believe Ufactor is a more holistic/thoughtful figure for thinking about energy transmission through building envelopes. Might be of interest https://www.greenbuildingadvisor.com/article/all-about-u-factor https://www.greenbuildingadvisor.com/article/all-about-u-fac...
  - mikeyouse 4y ago
    In a sense that's true - it's more natural to consider in the same way that MPG is less obvious compared to gallons per hundred miles. But they're representing the same quality, U-Factor is just the reciprocal of R-Value. So R-Value represents the thermal resistance and U-Value the transmittance but you can easily take the reciprocal of one to get the other (e.g. my R-2 window is just a U-0.5).
    - bialpio 4y ago
      First off, thanks for the explanation in your other post! The main point I had is that there are gotchas when talking about improvements in R-value in terms of percentages that we the uninitiated won't be aware of. U-value doesn't seem to have that problem if it's just a simple reciprocal of R-value (IIUC, halving U-value halves heat loss, doubling it doubles the heat loss - that's a way easier relationship to understand to me).
      - mikeyouse 4y ago
        Yep, that's a good point and a good way to think about it. My MPG reference was an eye-opening moment for me -- the US uses MPG which is really backwards from what you care about (in the same way that R-value and U-value are). So increasing your mileage from 10mpg to 12mpg saves exactly as much gasoline as increasing your mileage from 30mpg to 60mpg. This is plainly obvious when you consider gallons per 100 miles, [10->8.3] & [3.3->1.7].
tguvot 4y ago
If you are talking R-values - you are right. But if we get a bit more practical, walls have a bunch of thermal mass (drywall on the inside, stucco on the outside, at least in my case ). During the summer, i have walls that get to ~30C on the inside. So even when it's evening/night, and temperature outside drops to 15C from 35C during the da, I open windows and run whole house fan, those walls keep on heating the air inside while lowly R rated glasses shed the temperature quickly. So in my case doubling insulation in wall (in order to prevent drywall from heating up) likely to provide more effect than bumping R value of the glass.

doodlebugging 4y ago

This sounds great. They still have work to do though before this can be a commercial product if they only get 200 cycles out of it. Once they solve the durability problem this should be a great product for upgrading windows in commercial and residential uses.

twawaaay 4y ago

This sounds great but there are alternatives. One nice trick I have seen is windows offset so that the light penetrates deep into rooms during winter but does not penetrate in at all during summer. And then the floors acting as thermal mass and made to distribute the heat throughout the house. And so on. I wish I could find this youtube video again. The house was built probably a hundred years ago or so by an American architect for himself. There is a lot that can be done with mundane materials with a little bit of thoughtful design.

ph0rque 4y ago
There's a neat overhang calculator that I used to size my window and door overhangs on the south side: https://www.susdesign.com/overhang/ https://www.susdesign.com/overhang/
breischl 4y ago
You can do something similar with an appropriately sized & angled awning/overhang. eg the visitor center at Zion NP does that, along with a number of other clever things. PDF: https://www.nps.gov/zion/learn/nature/upload/DOE%20Brochure.pdf https://www.nps.gov/zion/learn/nature/upload/DOE%20Brochure.... But for commercial buildings where they're trying to squeeze out every square foot of rentable space, and probably are not allowed to overhang their property lines, it's likely not an option.
- twawaaay 4y ago
  The overhangs don't have to take away from rentable space. It is in effect just a sun shade sticking out of the house horizontally blocking the sun without blocking the view like a baseball cap. I am looking to buy a property right now and I have resigned myself that if I want a nicely designed house I will probably have to do it myself. A well designed house does not have to be more expensive, it is just engineering -- and engineering is about knowledge, experience and ability to use both to make tradeoffs for a better result. The knowledge is there but very few seem to be making use of it. Everybody tries to squeeze a lot of new tech but forget about old lessons -- just like in software development...