Heat Engine

[vansig]

This thread assumes otherwise wasted x-ray energy is to be captured and converted with a heat engine. See the topic “the onion” for an alternative method. As a point of comparison, both methods assume 15 kJ of x-rays are generated each pinch, and there are ~333 pinches/s, giving 5 MW to convert.

The theoretical limit on efficiency for a heat engine depends on the difference in operating temperatures for the working fluids. The greater this difference, the better. But practicalities prevent operating the system at temperatures above the creep limit of structural materials. Steel is the common choice, at 810 kelvin upper bound for the hot side. If steam turbines are used, and water condensers, then the cool side will be around 373 kelvin. Theoretical efficiency for this case tops out at (810 – 373)/810 = 54%.

What is the size and cost?

[Aeronaut]

What’s the creep temperature for a generic stainless steel?

[QuantumDot]

there is a design for a thermo photo voltaic device that in theory could achieve 85 percent efficiency, from what i have read it hasn’t come close to actually achieving that but there is hope that it will.

[Aeronaut]

QuantumDot wrote: there is a design for a thermo photo voltaic device that in theory could achieve 85 percent efficiency, from what i have read it hasn’t come close to actually achieving that but there is hope that it will.

Sounds like inconel, along with the associated prices of machining it. I’d guess that an off-the-shelf SS would provide enough efficiency for most apps, just like CPU chip pricing.

[nemmart]

QuantumDot wrote: there is a design for a thermo photo voltaic device that in theory could achieve 85 percent efficiency, from what i have read it hasn’t come close to actually achieving that but there is hope that it will.

This sounds like Power Chips (www.powerchips.gi). Not sure if it’s snake oil or if it’s real. Anyone know the scoop on these guys?

[zapkitty]

nemmart wrote:

there is a design for a thermo photo voltaic device that in theory could achieve 85 percent efficiency, from what i have read it hasn’t come close to actually achieving that but there is hope that it will.

This sounds like Power Chips (www.powerchips.gi). Not sure if it’s snake oil or if it’s real. Anyone know the scoop on these guys?

Dunno about that group but the basic principle is known: micron-gap thermovoltaic tunneling.

In fact one company was incubated at MIT and took those initials for its name: mtpv corporation.
http://www.mtpvcorp.com

… as I said the principle is known… making it into salable products remains to be seen but it would be tremendously useful if they succeed…

[vansig]

Aeronaut wrote: What’s the creep temperature for a generic stainless steel?

this 1972 westinghouse patent seems like a good place to compare different steels: http://www.freepatentsonline.com/3635769.html

interestingly,
operating temperature for the hot side of the heat engine in a CANDU fission reactor is said to be 290 °C. That leads to ~33% efficiency.
— http://www.wordiq.com/definition/CANDU_reactor

Apparently the biggest problem with using steam is boiler system corrosion. — http://www.gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jsp

[Aeronaut]

vansig wrote:

What’s the creep temperature for a generic stainless steel?

this 1972 westinghouse patent seems like a good place to compare different steels: http://www.freepatentsonline.com/3635769.html

interestingly,
operating temperature for the hot side of the heat engine in a CANDU fission reactor is said to be 290 °C. That leads to ~33% efficiency.
— http://www.wordiq.com/definition/CANDU_reactor

Apparently the biggest problem with using steam is boiler system corrosion. — http://www.gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jsp

Do we really need to boil the working fluid to get high efficiency?

[MTd2]

Why not tin? Going almost to boiling temperature and lowing it to melting, it gives an ideal of 83% efficiency. It has a reasonably high density and atomic weight, so it can capture a lot of x-rays.

[vansig]

MTd2 wrote: Why not tin? Going almost to boiling temperature and lowing it to melting, it gives an ideal of 83% efficiency. It has a reasonably high density and atomic weight, so it can capture a lot of x-rays.

i’m not worried about capturing the xrays; what’s a worry is, how do you contain the hot fluid? what is your container made from? how do you cold-start it?
what is the engine’s principle of operation? how close to ideal does it’s efficiency get?

[vansig]

Aeronaut wrote:
Do we really need to boil the working fluid to get high efficiency?

Turbines employ the pressure differential between the vapour and the condensed liquid, but other ways are conceivable, each with its own constraints. Thermal photovoltaics, suggested above, would work by spontaneous emission and capture within a narrow band of wavelengths.
Following are the blackbody charts for two different temperatures…

Attached files

[MTd2]

vansig wrote: i’m not worried about capturing the xrays; what’s a worry is, how do you contain the hot fluid? what is your container made from? how do you cold-start it?
what is the engine’s principle of operation? how close to ideal does it’s efficiency get?

Use tungsten as the container, which has extremely high boiling points and high tensile strength. Cool the tin by immersing the tungsten, molded as a coil, in water.The vapor will move helices. Cold start the system it using tungsten as a resistor until it melts tin, and so, turn on the machine. For efficiency, I have no idea, but the differences of temperature are huge. The only element better than tin is gallium, but it is very rare.

[psupine]

Might be a good time to draw attention to turbines based on supercritical CO2.

A quick google give a short summary which also links to the main report (which is 6.5MB):

http://nextbigfuture.com/2009/06/supercritical-co2-recompression-cycle.html
http://web.mit.edu/jessiek/MacData/afs.course.lockers/22/22.33/www/dostal.pdf

The advantages of CO2 compared to steam, are that the turbine is much smaller and the operating temperature is lower for the same efficiency (53%). The main disadvantage is that the pressures involved are higher, but with a lower temperature, the metal will have less tendency to creep.

The other alternative is Helium, and if we are already cooling electrodes with helium then running the turbine on that would bypass the need for heat transfer to another working fluid. Helium could also be run through any old X-ray to bulk heat converter mass. Pb is cheap.

[Author]

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