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Effciency Related To Thermo-gerneration.

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LAN_403

Patrick

Mon Jun 30 2014, 10:49PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i was wondering what efciency is typical for devices like the TEC1-12706,

and

,d.cGU

Im suspecting that most of the electro-chemical conversion loss would be in the "tail pipe" waste heat, due to the need to carry out combustion products quickly. But what about thermal energy effciency from the heat difference that is present?

keep in mind im willing to eat a lot of chemical-heat loss, since chemical energy is so much denser than battery energy density.

i was thinking about a butane or propane torch heating a heatsink, for the source, and a fan and sink for the cold side.

2Spoons

Mon Jun 30 2014, 11:14PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

The Carnot efficiency limit still applies. Which would be 27% if the hot side was at 138C (limit for that device) and you managed to get the cold side to 25C. Your results will be worse than this.

Patrick

Mon Jun 30 2014, 11:15PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

The Carnot efficiency limit still applies. Which would be 27% if the hot side was at 138C (limit for that device) and you managed to get the cold side to 25C. Your results will be worse than this.

I presumed Carnot would still apply, but still thats pretty bad!
Presuming i can keep the hot-side hot, and cold-side cold, and not buggle it up:

Propane = 50.3 MJ per kg

for 100g fueling my proposed system...
means 50.3 MJ x 0.1 kg = 5.03 MJ available to combust.

At 20% effciecy, 5.03MJ x 0.20 = 1.006 MJ electrically available (minus some more losses like diodes).

converting to watts: J = W x s
therefore, J / W = seconds of power provided.
1.006 MJ / 250 W = 4024 seconds of endurance at 250 watts output.

2Spoons

Mon Jun 30 2014, 11:59PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

what makes it worse is that for your 250W output, you have to get rid of 1kW out the cold side (based on your stated 20%). Not trivial, since keeping the cold side cold is the key to better efficiency.

And it gets worse ... you will not be able to extract all the combustion energy, and keep the flame going : you need a steady exhaust flow, and it will be hot , which means a further loss in efficiency. Thermodynamics sucks a****.

this looks interesting

Hard to know how real it is though.

Patrick

Tue Jul 01 2014, 12:06AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

what makes it worse is that for your 250W output, you have to get rid of 1kW out the cold side (based on your stated 20%). Not trivial, since keeping the cold side cold is the key to better efficiency.

And it gets worse ... you will not be able to extract all the combustion energy, and keep the flame going : you need a steady exhaust flow, and it will be hot , which means a further loss in efficiency. Thermodynamics sucks a****.

Thermodynamics is a tool of the devil.

I expected excessive loss due to "draw-through" (my made up term, air induction and exhaust wasted out the a** end.) But note that i still beat batteries in terms of duration and mass.... even with this terribly comprimised system.

Wastrel

Wed Jul 02 2014, 03:34PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

The common modules use low melting point solders. Modules intended for power generation are more expensive. For efficiency think more like 3 to 5%. One module manufacturer fitted a truck with modules to recover more than a kilowatt and was able to remove the alternator for increased horsepower but the recovery as electrical power was still only a few percent.

Patrick

Wed Jul 02 2014, 07:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

everybody i talk to says less than 10% many say 3% to 8%, so i might abandon this pursuit.

but, the horizon fuel cells are 40% effcient, and in the battle space theres pressure to reduce friendly heat cross sections visible to any potential enemy. so piston-electric generators are supposedly less desirable, but with fuel cells 40% effcient youll need to get rid of 60% waste heat. thats still a huge amount of waste heat. plus the chinese may starve us americans of precious metals in a war.

Carbon_Rod

Thu Jul 03 2014, 12:35AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

You should know a TEG is not a TEC, and the efficiency is related to increasing the temperature gradient.
Unfortunately, almost all publicly accessible piles are very inefficient relative to material cost/size.

Cheers,
Rod

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