Graphene Device Sops Up Sunlight, Heats to 160 Degrees Celsius in Seconds
spectrum.ieee.org
spectrum.ieee.org
Edit: I recommend a look at the original paper instead [0]. The significant feature appears to be the ability to absorb light at a wide range of angles, rather than simply the absolute absorptivity.
> suitable for a wide variety of uses, including desalination of seawater, color displays, photodetectors, and optical components for communication devices.
So: time will tell!
It could make for a great solar shower lining though...
> 90-nm ultra-thin
I don't know the density, but let's say 2g/cm³. That means a 1cm² piece of this film weighs 18 micrograms.
> 30°C to 150°C in 30 seconds
If by "sunlight" they mean 1000 W/m², which is generous, then our 1×1 piece would catch 0.1 W. Over 30 seconds, that makes 3 joules.
3 joules to heat 18 micrograms by 120 degrees – that's a specific heat capacity of 1.388 joules per gram and K (a bit more if the density is lower than 2, a bit less if it's higher). So: a bit more than typical metals, probably, but not by much.
[0]http://www.alternative-energy-tutorials.com/solar-hot-water/...
In contrast, absorber plates tend to be large and heavy. 1lb per square foot isn't uncommon. They are also often produced with (relatively) expensive copper.
This might not be a breakthrough in terms of new capabilities, but it could allow a scale of use not currently practical.
Though also consider their applications weren't limited to desalination and other "macro" applications, but also electronics, sensors and such. It sounds like this might be significantly cheaper than current monolayer graphene on substrate.
Though the prospect of sending a coffee mug that only works in the sun to some geeks I know, does bring with it some wry humour opportunities.
As for other uses, agreed, much potential if it has an edge over alternative offerings.
Are dark objects dark just because the light they emit are in an invisible frequency range?
"Are dark objects dark just because the light they emit are in an invisible frequency range?" the light they reflect back may well encompass the non visible spectrum, after all that's how radar works and the drive for radar absorbing materials by the military. So not all objects that for us in out limited light spectrum visual senses are equal, even if they look the same - black. May be one reflects back UV, one may absorb it. So for a true picture you would need to see an absorption graph of the full spectrum (visible and non visible).
Crux is - objects we see via refracted light - we are seeing in negative. So whilst we see green grass, it's true colour is everything but green.
I've never liked this phrasing. In what sense is the "true" color not green if we perceive green? It is the color it reflects, it's just a little counter-intuitive when you first learn of the physics behind it.
For me, it gets down to physics and how we perceive things. From a human perception aspect - the true colour of grass is green. But when you add the science aspect, it is not that clear cut. Somewhat gets down to context.
For me - if an object emits light - that's it's true colour. Equally how it refracts light is the perceived colour. Objects that emit light have a true and perceived colour the same. Though there is always other factors like whats in between the object and the light (be that refracted or emitted). The sun and sky being good example of that at play. But equally even at a small scale, the air will have an impact, even if we can not perceive it.
But yes, we're all human and with that, you are right in that the true colour from our perception shows grass is green. So I'll try and refer to grass as its true colour of green and it's physics colour as red/blue.
Which feels perhaps a better way of phrasing and one I'll try and run with until something better comes along.
Is it better for the inside of a fridge to be black or white?
Discuss.
A fridge body is a pure insulator so the answer has to be white. Also it barely matters because convection will cool the surface anyway. Super straightforward with no math to do or conflict to resolve, unless I'm missing something.
I’ve always thought that we should just all move to the refrigerator-as-a-bunch-of-independent-drawers design. I think it would be easier to browse efficiently and easier to design a kitchen around.
Now, if you look at the back of the fridge where is expels/radiates the heat - that is black. For me, I've often thought that heat could be utilised. Be is pre warming up water designated for hot water use (which would also aid in cooling fridge and increase its efficiency and life expectancy as well). Or some peltier/sterling type energy recovery.
But good question and in the same spirit:
Why is thermal paste and pads white?
It's a specific use case, but this exhaust heat is very useful.
Use the top side of the refrigerator as a delicate drier for damp things that cannot be tumble dried. Soggy umbrellas. Leather gloves (even shoes). Hats made sweaty from work or exercise.
Put them on the top of the fridge overnight and IME by morning they are just about perfectly dried out.
Offhand, it doesn't seem like there would be much thermodynamic advantage in the heated surfaces of a fridge being black. If there were, wouldn't you expect that car radiators and such would be black anodised too? I'd imagine conduction is going to be a much much bigger effect than radiation - other concerns like cost, durability, thermal conductivity of the paint, and aesthetics will likely be more important in deciding what colour paint to use.
That question already assumes that specific heat and thermal conductivity are different properties.
At that point it's competing with any number of readily-available black coatings that are suitable the use.
Focus on (1) is great, but (2) is the breaker of chains. Solving (2) at enough efficiency should largely obviate the need for (3). Store and ship massively efficient solid state "batteries" instead of building (ie, owning) transmission infra
If this is accurate that has great value and utility
For that matter, it doesn't seem like they specify under what conditions this heating is taking place. Fire a high-powered laser at something and you might see that behavior in a variety of materials.
It's like how defect-free glass is 2-4 times stronger than steel. Actual glass is obviously nowhere near that strong and nobody pretends it is.
Edit: https://phys.org/news/2019-03-cambridge-spin-out-graphene-co...
Peltier devices are useless for most applications because of its poor efficiency.
Vampire skin!
Whilst it can absorb energy in the range specified - how efficient it is along that range is another question.
Another aspect, by not being black, I'd expect it to radiate heat less. But then, like many, so many question, so little data.
Gets down to if the absorption rate is liner - which it almost definitely is not going to be the case here and as I suspect, an absorption bias in the infrared spectrum is at play.
Carnot efficiency is dictated by the hot-cold side temperature delta. For internal combustion engines, 20-45% is fairly typical. In electrical generation, thermal energy is generally reckoned at 3x electrical generation.
PV efficiency has a maximum of 37% for single-layer PV, up to a theortical maximum slightly north of 80% for an "infinite" layer cell. (Additional layers provide a diminishing return.) In practice, 40-50% is a likely maximum achievable, and at reasonable costs, 20% is frequently assumed.
Additional considerations are capacity factor, spacing factor, panel angle, and inverter losses.
And PV has a practical effective lifetime of about 20 years, due to numerous degradation mechanisms.
See:
https://old.reddit.com/r/dredmorbius/comments/28cvgv/calcula...
Thermal materials such as this are likely best suited to thermal capture and direct-heating systems, as low-grade space or water (pre)heating.