Perovskite-Based Thermochromic Windows Reduce Load and CO2 Emission in Buildings | Hacker News Reader

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

nrel.gov

40 pointsph0rque22 comments

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%.

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.

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... :)

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.

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-fac...

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).

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).

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].

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.

Using sun angles to design proper overhang is a key insight of passive solar design. Shame it gets overlooked so often!

https://greenpassivesolar.com/passive-solar/building-charact...

https://www.susdesign.com/tools.php

https://youtu.be/OR8EQ0DWpPw

My house is set up this way. Many south facing triple pane windows, very few north. The overhang gives us direct sun into the house in the winter and very little in the summer. We have 11 inch concrete walls with 4 inch styrofoam insulation on each side of our walls (ICF construction). Costs about 40-80 dollars a month natural gas to heat in Alberta Canada.

At your latitude and climate zone those windows are almost certainly either costing your more than the equivalent wall would have or are providing a benefit on the order of less than $20 per year. I've done exhaustive studies at my latitude and climate zone (which is quote similar to yours) with a wide variety of parameters and the numbers just don't pencil out in a significant way. My analysis was exhaustive and included actual weather data and solar irradiance data. In slightly warmer climates this is less the case, though.

Maybe, but the thick walls and styrofoam insulation give it a high r value. The furnace doesn't run much when the sun is out.

And the view is priceless :)

Possibly this one:

https://youtube.com/watch?v=Qq-3cZ0cbws&si=EnSIkaIECMiOmarE

Thank you, sir! That's exactly this one!

You're welcome!

It's amazing what creative designs are dreamt up prior to having AC as a crutch, including examples much older than that. For example, water pools in the centre of Roman courtyards[1], which lowered the temperately passively. Another example are windcatchers common the Middle East[2].

[1] https://en.wikipedia.org/wiki/Impluvium

[2] https://en.wikipedia.org/wiki/Windcatcher

[3] More examples: https://engineeringrome.org/2022/09/26/methods-of-temperatur...

I live in a cold climate so maybe less examples on how to keep your house cool and more about how to keep it warm.

I think I have seen a primitive but very effective heat exchanger in an old house. Normally, when you have a stove and chimney and hot gasses escape the house you also have a lot of cold air rushing in. In this house the hot air was directed around the whole length of the chimney and then through the mass of bricks that formed the stove so that the air coming in was already warm.

This allowed to very efficiently keep the entire house very warm while also nicely ventilated even in very, very cold weather with very little wood needed.

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....

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.

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...

It depends on your building rules. If you're in a zero lot line environment, you can't build an overhang that encroaches on the neighboring lot.

The overhang may also count as your footprint in case of any other setback rules.

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/

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.

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