Cheap material converts heat to electricity • Hacker News Board

Re: Cheap material converts heat to electricity

#91

Post by SigmundA » Wed, Aug 04, 2021, 1:40 AM UTC

bernulli wrote: post #69

grishka wrote: post #68

Earlier quoted context omitted.

> In this universe the laws of thermodynamics specifically forbid destroying heat, or converting it into something else. But wait, what does a Stirling engine run on? Or a steam engine? You clearly can get some energy out of heat. Also, objects that are hotter than absolute zero radiate infrared light — and you, in principle, could convert light, regardless of its spectrum, into electricity.

They run on temperature differences, that's how you get usable energy out of it. Google Carnot engine, there's even a super simple way to find the maximum efficiency just from the temperatures!

Temperature difference is caused by heat. A heat engine converts heat into mechanical work. A Carnot heat engine is an idealized form of heat engine vs say a Diesel heat engine.

You can most certainly convert heat into something else. It's called the law of conservation of energy, not heat, which is just one form of energy.

Re: Cheap material converts heat to electricity

#92

Post by blueblisters » Wed, Aug 04, 2021, 5:26 AM UTC

davidinosauro wrote: post #84

blueblisters wrote: post #67

Earlier quoted context omitted.

> I thought about many ways to get rid of heat without having to dump it somewhere, but thermodynamics is such a terrible thing. This is likely a stupid question but temperature is a statistical property describing the energy of molecules right? Is there no way to tap the energy of the molecules for useful work without having a temperature difference?

You might want to read about Maxwell's daemon [1]. I came across this a few weeks back, two links away from the HN homepage. I found it intreaguing and read more about it but I can't quite umderstand the proposed solutions, likely due to my lack of background in thermodynamics. [1] https://en.m.wikipedia.org/wiki/Maxwell's_demon

Clearly, Maxwell was much better (and funnier) in articulating what I was asking. Looks Maxwell's demon can work some of the time at a molecular level, but on average, you need to do work to extract useful heat from matter at a macroscopic level. The hypothesis being that you need to do work just to identify hot/energetic molecules from the cold/low-energy molecules.

Re: Cheap material converts heat to electricity

#93

Post by yetihehe » Wed, Aug 04, 2021, 10:47 AM UTC

dhajzhz wrote: post #79

marktangotango wrote: post #48

Earlier quoted context omitted.

Isn't the minimum exhaust temperature the same as the temperature of the heated water? How can it be less?

If the exhaust is too cold the water vapor condenses out left with a puddle of acid (the acid is from the trace sulfur in the Nat gas) Also, cold exhaust doesn't exhaust (since it cant rise through a density change). So, if your exhaust is too cold, it will collect in your basement (CO2 should be a bit denser than air, so absent any drafts it'll collect) The previous owners of my house installed my high efficiency st…

> Also, cold exhaust doesn't exhaust

Unless you have a fan blowing smoke into flue. Without fan, my furnace would not work, exhaust is only slightly above water temperature and furnace is designed so that condensate pools in special container, which you have to empty or connect to sewage.

Re: Cheap material converts heat to electricity

#94

Post by darksaints » Wed, Aug 04, 2021, 1:53 PM UTC

XorNot wrote: post #30

marksbrown wrote: post #13

Current peltier chips are barely a few percent efficient, break easily and require high temperature differentials. They're used in stove-top fans and RTGs. The latter should be Sterling engines to be ignorant mind. I don't doubt there are some interesting things to be accomplished by shaping the flow of heat conduction but I sincerely doubt it'll be energy generation directly. Thermodynamics isn't something you can j…

The history of moving parts in space missions (RTGs) is why Kepler's mission is degraded: keeping reaction wheels and other spinning things lubricated and moving when you can't get in there and service them is hard.

Consequently, NASA is often at the forefront of technologies that reduce moving parts. For example:

https://technology.nasa.gov/patent/LEW-TOPS-80

https://technology.nasa.gov/patent/LEW-TOPS-106

Re: Cheap material converts heat to electricity

#95

Post by kwhitefoot » Thu, Aug 05, 2021, 4:45 PM UTC

dhajzhz wrote: post #79

marktangotango wrote: post #48

Earlier quoted context omitted.

Isn't the minimum exhaust temperature the same as the temperature of the heated water? How can it be less?

If the exhaust is too cold the water vapor condenses out left with a puddle of acid (the acid is from the trace sulfur in the Nat gas) Also, cold exhaust doesn't exhaust (since it cant rise through a density change). So, if your exhaust is too cold, it will collect in your basement (CO2 should be a bit denser than air, so absent any drafts it'll collect) The previous owners of my house installed my high efficiency st…

Most new and recently installed domestic gas boilers in the UK use balanced flues not chimneys and are condensing boilers. The exhaust is fan assisted and the temperature is very low.

See https://en.wikipedia.org/wiki/Condensing_boiler#Exhaust

Re: Cheap material converts heat to electricity

#96

Post by bernulli » Thu, Aug 12, 2021, 6:02 PM UTC

SigmundA wrote: post #91

bernulli wrote: post #69

Earlier quoted context omitted.

They run on temperature differences, that's how you get usable energy out of it. Google Carnot engine, there's even a super simple way to find the maximum efficiency just from the temperatures!

Temperature difference is caused by heat. A heat engine converts heat into mechanical work. A Carnot heat engine is an idealized form of heat engine vs say a Diesel heat engine. You can most certainly convert heat into something else. It's called the law of conservation of energy, not heat, which is just one form of energy.

But the limits to these transformations are governed by the 2nd law of thermodynamics, not the first. You can't arbitrarily convert heat into other forms of energy.

E.g., if your machine relies on all energy in my room to spontaneously gather in my coffee mug to heat it up, while simultaneously cooling my warm room, this would be perfectly fine with conservation of energy and super useful. Unfortunately, it would also violate the 2nd law and it's thus something that doesn't happen in our universe.