[peppercorn (OP)]

Latent Heat of Sublimation (Dry Ice)  246 BTU/lb. (-110° F)

"Solid carbon dioxide (dry ice) sublimates readily at atmospheric pressure at -78.5°C (197.5 K, −104.2 °F), while liquid CO2 can be obtained at pressures and temperatures above the triple point (5.2 atm, -56.4°C)." - Wikipedia

If a heat pump is used to lower the temp of CO2 (perhaps separated from a exhaust stream) until dry-ice is formed, is the amount of expansive work available in subliming the dry-ice of a large enough energy density to make a worthwhile store of power?

How does it compare with, say, molten-salt reservoirs for energy density?

Could the combination of the Dry-ice for cold side of a heat engine and Molten-salt as the hot side lead to a better cycle efficiency?

Is there any historical examples of using DI-to-gaseous-CO2 in a machine?

[Geezer]

Nitrogen works better, but the efficiency is still pretty lousy.

[peppercorn (OP)]

Nitrogen works better, but the efficiency is still pretty lousy.

You've got to get Nitrogen a lot colder though.

Heat pumps (condensing) are fairly efficient are they not?

[rosy]

Isentropic:
http://www.isentropic.co.uk/
.. are doing pumped-heat energy storage things at 72-80 % round trip efficiency using air as a working fluid, with a view to providing smoothing to the national grid (in the UK and presumably elsewhere).
TNS interviewed them a few weeks back:
http://www.thenakedscientists.com/HTML/content/interviews/interview/1501/

That's as against hydrogen electrolysis/fuel cells (round trip 50-60 % efficient), or pumped water (about 75 %) according to wikipedia.. molten salt has a theoretical round trip of 99 %:
http://en.wikipedia.org/wiki/Grid_energy_storage

I'd think the big problem with dry ice might be that it's easier to handle fluids (move them about and so on). Tho' I guess if you're working at elevated pressure you might be able to use the liquid.

[Geezer]

This is the paper on LN2 work that was done at U of W.

http://frontierzero.com/references/Articles/Isothermal%20LN2%20Engine.pdf

[SeanB]

You can use CO2 as a refrigerant, but the pressures involved are very high, and are orders of magnitude above what you most commonly find in your daily life. This means any flaws or poor assembly shows up rather explosively, but an advantage is that the working fluid is not toxic, though the oils it carries with for lubrication can cause problems if it catches fire during a fault causing the release of gas.

[CliffordK]

I've always had troubles maintaining uniform high volume sublimation of CO₂, although I haven't played with it too much.

Mix it with water, and it tends to ice over.

You could crush it.  Otherwise, moving it might be cumbersome.  It couldn't be injected directly into an engine.

The thermal expansion of a liquid vs a solid should be similar.

Are you planning a closed-loop or open-loop system.  CO₂ is only 0.04% in the atmosphere, although available from other more concentrated sources.  A closed-loop system would require lots of storage, but then would work with any "refrigerant".  including ammonia or propane. 

With Nitrogen, you have the expansion from liquid to gas phase.  But, you also have all the expansion from -195 °C up to room temperature, assuming you had efficient heating.

Are you designing a zero-em vehicle (noting, of course, that you may have CO₂ emissions, but a cycle of zero net emissions).  Or..  supplementing an internal combustion engine in which case you have a heat source, and could use other fuels.  For example, some people have tried water injection into an internal combustion engine which tends to give the same expansion effect and an associated moderate efficiency gain.

I've always wondered why one couldn't couple a steam generation system to an internal combustion engine, although I suppose the gains might only be moderate, but much of the wasted energy is in the form of heat.

[Geezer]

I've always wondered why one couldn't couple a steam generation system to an internal combustion engine, although I suppose the gains might only be moderate, but much of the wasted energy is in the form of heat.

BMW did that quite recently. Unfortunately, the amount of power recovered is pretty meagre. I would not be surprised if they have given up on the idea.

[kornbredrsqar]

what would be the impact on society if someone had an invention that could cut the volume of steam needed to generate electricity in half,
we could use less coal,natural gas, uranium, and oil. and this same technology could be used in hydroelectric dams conserving millians of gallons of water, and in industry ,where air and hydrolic motors are the main source of mechanical power,the posibilities are hard to emagine, if you had invented such a device what would you do??

[Geezer]

if you had invented such a device what would you do??

If I was convinced it really worked, I'd file a patent before I talked to anyone about it. If I was less sure, I'd find an expert in the subject and give them 50% of the ownership of the idea before filing any patents.

[Bored chemist]

what would be the impact on society if someone had an invention that could cut the volume of steam needed to generate electricity in half,
we could use less coal,natural gas, uranium, and oil. and this same technology could be used in hydroelectric dams conserving millians of gallons of water, and in industry ,where air and hydrolic motors are the main source of mechanical power,the posibilities are hard to emagine, if you had invented such a device what would you do??

I might have to start by rewriting the laws of thermodynamics.

[kornbredrsqar]

I wouoldn't throw uot any books away just yet but after building acrude prototype i went to work on gtting a patent wich has been filed and have a more robust prototype that is about 95% complete.
but this all takes time and money both of wich are in short supply around these parts. and the woods are full of theives looking to sell you a pig in a poke.

[Geezer]

If you've filed a patent, it will become public knowledge anyway, so there is no downside in explaining how it works. Even if someone discovers a flaw, it's better to find sooner rather than later.

[kornbredrsqar]

you would be suprised how hard it has been just trying to find someone who will even listen that is who actualy understands what i'm talking about, but the patent was just filed last week so i have been pretty quiet about it until now. the crooks i reffered to earlier are the invention promotion scammers that seem to be the only people you come in contct with when looking for help in this area. i look foward to discussing my designs with someone with experiance in rhe field but am not sure i want to do so in a public setting just yet.

[Geezer]

you would be suprised how hard it has been just trying to find someone who will even listen that is who actualy understands what i'm talking about,

I wouldn't. I've filed patents myself  []

[peppercorn (OP)]

Are you planning a closed-loop or open-loop system.  CO₂ is only 0.04% in the atmosphere, although available from other more concentrated sources.  A closed-loop system would require lots of storage, but then would work with any "refrigerant".  including ammonia or propane.

Yes, I had envisaged a static (non-vehicular) system that could be open-loop, where the CO2 in the exhaust gas (most simply recovered from burning, say charcoal) could be captured and chilled (using some of the heat engine's mechanical work to run an efficient heat-pump) to dry-ice state.
I realise the dry-ice's energy density would be quite low (and maybe far too much would be needed), but it could be used in conjunction with a large thermal mass (a big-vat of hot water - reject heat from the steam engine or even crude solar) to exaggerate the expansion in the piston (or other external heat engine - steam turbine).

Being open loop does of course have efficiency advantages, especially if the CO2 has a fairly low-energy route to be separated from the exhaust and cooled to ambient.


Also - if, rather than air, almost pure oxygen (see pressure-swing concentrator) was used in combustion there would be practically no gas separation necessary in the exhaust stream.