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Rice University researchers propose a way to boost solar efficiency

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Re: Rice University researchers propose a way to boost solar efficiency

#101

Post by tim333 »

skosch wrote: post #75
p1esk wrote: post #66

Earlier quoted context omitted.

Sorry for a potentially stupid question: why do they need to convert heat into visible light? Why can't we convert heat directly into electricity at the same time as we convert visible light into electricity?

You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing. As a rule, everything around you glows – in other words, it emits photons. Some infrared ones, some visible ones, some ultraviolet (or even higher-frequency) ones. How much of each depends on the temperature of the material. Most things (e.g. you) are too co…

>This heats up the nanotubes, which then emit visible photons

I'm a bit skeptical that that would work. Otherwise you could build a perpetual motion machine by having the nanotubes in a hot room and running your perpetual motor off a solar cell powered by the light.

My guess is when the article says emit light they mean emit infra red light / radiation rather than visible. The article doesn't really make sense to me - I imagine someone got confused by the physics rather than it being an actual breakthrough in energy generation. I note the article says "propose a way to build" rather than having actually built something that works.

Re: Rice University researchers propose a way to boost solar efficiency

#102

Post by ThomPete »

lukev wrote: post #74
ThomPete wrote: post #53

Earlier quoted context omitted.

Oh, it's definitely amazing relative to where it was, just not to where it needs to be to compete. Solar and wind is less than 1% of world energy consumption and is not expected to be more than 3-4% in 2040. Energy is not a product/market problem it's a physics problem and we haven't had any major breakthroughs in that space for a long long time. https://www.iea.org/weo/?fbclid=IwAR3eH1AFcRSPSEit8JINLCMvE_...

This... doesn't have to be the case. Some markets already have way higher renewable penetration. Solar and wind represented about 20% of all generation in California in 2018 ( https://ww2.energy.ca.gov/almanac/electricity_data/total_sys... ). In the UK this year, solar and wind are almost 25% of generated electricity ( https://assets.publishing.service.gov.uk/government/uploads/... ) Incremental improvement and econo…

You are talking about electricity, not energy.

Electricity is only ~20% of the entire energy usage as far as I remember.

Re: Rice University researchers propose a way to boost solar efficiency

#103

Post by fengazer »

p1esk wrote: post #96
skosch wrote: post #75

Earlier quoted context omitted.

You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing. As a rule, everything around you glows – in other words, it emits photons. Some infrared ones, some visible ones, some ultraviolet (or even higher-frequency) ones. How much of each depends on the temperature of the material. Most things (e.g. you) are too co…

I see, thanks! Why are visible light photons "stronger" than infrared photons?

Visible light photons have more energy each. The shorter the wavelength, the higher the energy of the photon.

Light with a short enough wavelength (UV-B, for example) has enough energy to damage the DNA in our skin and increase our risk of skin cancer.

Re: Rice University researchers propose a way to boost solar efficiency

#104

Post by marshray »

dreamcompiler wrote: post #77
marshray wrote: post #67

Earlier quoted context omitted.

If you want to get pedantic, you can't "make electricity" either because 'electricity' isn't a defined concept. Usually when people use words such as "make electricity", they actually mean to refer to energy or power transferred using electromagnetic fields interacting with a conductor. But a purely chemical reaction that charges a battery could be said to "make electricity" without even involving that. More info: ht…

I was going for a simpler idea: Light is photons, electricity is moving electrons. And yeah, if you're talking about electromagnetic fields we're back to photons again. It just struck me as a sentence that was very misleading to lay people.

It's only simpler because "electricity is moving electrons" is not a meaningful claim.

What is the physical unit of 'electricity'? There's not one, because it's not a thing.

Re: Rice University researchers propose a way to boost solar efficiency

#105

Post by p1esk »

fengazer wrote: post #103
p1esk wrote: post #96

Earlier quoted context omitted.

I see, thanks! Why are visible light photons "stronger" than infrared photons?

Visible light photons have more energy each. The shorter the wavelength, the higher the energy of the photon. Light with a short enough wavelength (UV-B, for example) has enough energy to damage the DNA in our skin and increase our risk of skin cancer.

If that's the case, how is it possible that low energy photons absorbed by carbon nanotubes can generate high energy photons?

Re: Rice University researchers propose a way to boost solar efficiency

#106

Post by quote »

anewguy9000 wrote: post #46

ive always wondered why leaves all generally evolved to be green instead of black. wouldnt black be more efficient?

Because other wavelengths were probably already taken. https://en.m.wikipedia.org/wiki/Purple_Earth_hypothesis (Black is not a wavelength but rather the absence of visible light, e. g. it has all been absorbed)

Re: Rice University researchers propose a way to boost solar efficiency

#107

Post by sdenton4 »

skosch wrote: post #75
p1esk wrote: post #66

Earlier quoted context omitted.

Sorry for a potentially stupid question: why do they need to convert heat into visible light? Why can't we convert heat directly into electricity at the same time as we convert visible light into electricity?

You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing. As a rule, everything around you glows – in other words, it emits photons. Some infrared ones, some visible ones, some ultraviolet (or even higher-frequency) ones. How much of each depends on the temperature of the material. Most things (e.g. you) are too co…

"There is no way to convert heat to electricity directly, unfortunately."

Well, if you have enough heat, you can boil water and then use the steam to turn a turbine... I suppose in some sense this is dependent on a temperature difference, but I wouldn't call the amount of electricity we get by this method 'small.'

Re: Rice University researchers propose a way to boost solar efficiency

#108

Post by TTPrograms »

amluto wrote: post #32

As far as I can tell, this is functionally just a coating with low emittance in the IR outside a narrow band. A matched PV device that is kept cool can, in principle, approach the Carnot efficiency for the temperature difference between the emitter and the PV junction. If the emitter is the sun, then the source temperature is very high and the Carnot efficiency isn’t a major limit. If the source is a solar panel, I’m…

One sketch of such a system is to physically mate an effective high T solar absorber to an effective narrow spectrum photon emitter (as described in this work). Then that can be coupled with a typical solar cell with bandgap matched precisely to the emitter wavelength. So your solar cell is near ideally efficient for the photons it receives. As I understand the emissivity of these materials can exceed blackbody radiation in the near-field but not the far-field (or something like that? Search "Superplanckian emission").

The devices as I am familiar are often called thermophotovoltaic cells: https://en.wikipedia.org/wiki/Thermophotovoltaic

See eg http://xlab.me.berkeley.edu/pdf/259.pdf for a great overview, esp section 3.3.

I saw this video recently that I found accessible from an undergrad physics background and got me interested: https://www.youtube.com/watch?v=XnVVyTD7CzM

Re: Rice University researchers propose a way to boost solar efficiency

#109

Post by ThomPete »

Brakenshire wrote: post #59
ThomPete wrote: post #57

Earlier quoted context omitted.

And you can use them to power your watch perfectly fine too. That doesn't mean it will work at scale for society which is the primary issue. The key thing to look for is energy density as that will be more likely to give society a bang for the buck. Distributing solar cells out with the capacity factor of solar isn't economically feasible for society and neither possible as a main source of energy. Nuclear and Hydro…

These are arbitrary metrics, why does energy density matter for anywhere which has the space? Even in Northern latitudes and high density countries you can generate a lot of electricity with a nominal amount of land. All the more if they can be fitted on rooftops. For sunny places near deserts it can absolutely provide a big percentage, even a dominant percentage of power. The main issue is matching demand to supply,…

Energy density isn't arbitrary. It normally means something can be used in compact form (batteries, oil, gasoline, coal, uranium) and deliver energy at will.

Look at the capacity factor of wind and solar, add to that the huge areas they need and the fact that they are intermittent and you start to get a glimpse of the problem.

This is neither economically nor technically feasible. You can't generate as much as you think and you can't do it at a cost that makes it feasible generally for society not even with lowered cost.

Currently, we are talking 1% of world energy consumption not expected to be much more than 3-4% in 2040 and that's despite huge investments and all the political goodwill you can ask for [1] Keep in mind that the numbers you normally see displayed is for electricity not for energy. Electricity is only a subset of energy.

Furthermore, investments in solar and wind is decreasing especially when you take china out of the equation. [2]

And again. Lab results or theoretically possible advantages most of the times aren't feasible in reality and at scale.

[1] https://www.iea.org/weo/?fbclid=IwAR3eH1AFcRSPSEit8JINLCMvE_...

[2] https://www.globalresearch.ca/growth-renewables-stalled-inve...

Re: Rice University researchers propose a way to boost solar efficiency

#110

Post by Terr_ »

sdenton4 wrote: post #107
skosch wrote: post #75

Earlier quoted context omitted.

You're confused – heat isn't being converted into anything. The article conflates the words "heat" and "infrared radiation", which are not at all the same thing. As a rule, everything around you glows – in other words, it emits photons. Some infrared ones, some visible ones, some ultraviolet (or even higher-frequency) ones. How much of each depends on the temperature of the material. Most things (e.g. you) are too co…

"There is no way to convert heat to electricity directly, unfortunately." Well, if you have enough heat, you can boil water and then use the steam to turn a turbine... I suppose in some sense this is dependent on a temperature difference, but I wouldn't call the amount of electricity we get by this method 'small.'

> I suppose in some sense this is dependent on a temperature difference

It is absolutely dependent on a temperature difference, because it relies on cooler water that isn't already flashing into steam.

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