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#1 2005-08-10 01:03:12

John Creighton
Member
From: Nova Scotia, Canada
Registered: 2001-09-04
Posts: 2,401
Website

Re: C02 as a working fluid.

I am interested in seeing what kinds of thermodynamic cycles can be driven on mars. I found the phase diagram of C02.
http://en.wikipedia.org/wiki/Image:Phas … of_CO2.png
I have a little trouble reading it since it is a logarithmic plot but it looks like the critical point is at 80 Atmospheres and 30 degrees Celsius and the triple point is at 6 atmospheres and –60 degrees calculus. Probably during the day by passive means we could get the temperature at the condenser down to –20 degrees 20 Atmospheres. At night time we could evacuate the co2 further as the temperature will fall to –60. If the greenhouse is at 30 degrees Celsius, that gives a temperature difference of 50 degrees to run the thermodynamic cycle during the day. I will work out the cannot efficiency of this later

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#2 2005-10-30 03:34:18

MarsDog
Member
From: vancouver canada
Registered: 2004-03-24
Posts: 852

Re: C02 as a working fluid.

Good starting point is the greenhouse, with a large pond, acting as a heat storage, sink or source. During dust storm power shortages, the greenhouse could allow emergency power to be generated.

Temperatures may drop to -135°C, but CO2 freezes out around -100°C, so another working fluid is preferred. The greenhouse could warm up the CO2 and it would  freeze outside after powering a Stirling engine, generating power ?

CO2 is only liquid at high pressures, and a high pressure closed cycle machine could be made, somehow transporting the solid CO2. There is similarity in that you do not want to design a steam engine with water ice on Earth.

Hydrogen and Argon may be the gasses of choice.

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#3 2005-10-30 03:57:47

noosfractal
Member
From: Biosphere 1
Registered: 2005-10-04
Posts: 824
Website

Re: C02 as a working fluid.

I believe hydrogen is the working fluid of choice for Sterling engines.
.


Fan of Red Oasis

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#4 2005-10-31 00:48:10

MarsDog
Member
From: vancouver canada
Registered: 2004-03-24
Posts: 852

Re: C02 as a working fluid.

Looking at:
http://answers.google.com/answers/threadview?id=578288
http://hyperphysics.phy-astr.gsu.edu/hb … rcond.html
http://hyperphysics.phy-astr.gsu.edu/hb … cn.html#c1

and actual numbers

H2
http://en.wikipedia.org/wiki/Hydrogen
(300 K) 180.5 mW/(m·K)
(25 °C) (H2)28.836 J/(mol·K)

He
http://en.wikipedia.org/wiki/Helium
(300 K) 151.3 mW/(m·K)
(25 °C) 20.786 J/(mol·K)

Argon
http://en.wikipedia.org/wiki/Argon
(300 K) 17.72 mW/(m·K)
(25 °C) 20.786 J/(mol·K)

CO2
http://www.airliquide.com/en/business/p … p?gasid=26
Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 14.65 mW/(m.K)
Heat capacity at constant volume (Cv) (1.013 bar and 25 °C (77 °F)) : 0.028 kJ/(mol.K)

===========================

H2 is best gas for both Heat conductivity and heat capacity.
CO2 and Argon have low heat capacity per weight, and do not conduct heat well.

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#5 2005-10-31 07:17:03

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: C02 as a working fluid.

But since the plan is to create fuels from the available CO2 will the hydrogen that you would bring you must then proceed to using the next best instead as a compromise to the plan.

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#6 2005-10-31 11:49:48

MarsDog
Member
From: vancouver canada
Registered: 2004-03-24
Posts: 852

Re: C02 as a working fluid.

Solid CO2 is convenient to store on Mars, gather the morning frost.
Mix it with water and instant power from a pneumatic motor.

How far could you go on a liter of water assuming 4 miles per KWH ?

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#7 2005-10-31 12:00:50

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: C02 as a working fluid.

Also if you add a little heat for greater expansion in the chamber it will give a little more kick to its movement.

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#8 2005-10-31 13:49:20

MarsDog
Member
From: vancouver canada
Registered: 2004-03-24
Posts: 852

Re: C02 as a working fluid.

If the crew had a large coffe maker, 
4 liter (2 large coke bottles, or gallon milk volume equivalent) near boiling point.

The coffe tasted terrible. Don't waste the heat.
Pour it into the Dry ice chamber. Instant power.

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#9 2021-04-16 22:12:56

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: C02 as a working fluid.

bump

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#10 2021-04-17 07:49:05

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 5,831

Re: C02 as a working fluid.

For SpaceNut ... thanks for bringing this topic back into view!

It contains a high proportion of fact to opinion ... I noted that MarsDog is/was from Vancouver, Canada, and that is where katkam.ca fetches images of the English Bay, every five minutes during daylight hours.  I've never ventured that far NorthWest, but feel as though I am a native because of watching the traffic on the bay for years.

One of the posters made an observation that caught my attention ... he compared the dry ice to pneumatic tool scenario to trying to make a steam engine with water ice as input.  I am still pondering taking the risk of calling a US steam engine boiler manufacturer.  The company is still in business, serving a small and highly specialized market.

The immediately obvious difference between feeding a steam engine with ice is that the melted ice becomes a liquid at 1 ATM, while CO2 ice becomes a gas at 1 ATM, and throughout the pressure range of interest for a pneumatic tool.

The topic just renewed contains heat data that parallels data provided by kbd512 and Caliban more recently in the Pneumatic Tools topic.

I think the premise of the topic (CO2 as a "working fluid") may explain it's short life earlier... the material simply is ** not ** liquid at a pressure that makes any sense (to me at least).

It is possible that Calliban may have in mind a nuclear reactor design in which CO2 would work fine as a working fluid, but even there it would become a gas if temperature and pressure exceed the values shown in the phase diagram.  Perhaps at those temperatures and pressures it doesn't matter.

(th)

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#11 2021-04-17 17:36:54

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 882

Re: C02 as a working fluid.

What generally happens in the S-CO2 cycle, is that CO2 remains a gas throughout the cycle.  However, following its exit from the turbine, the gas passes through a recouperator that removes as much heat as possible, reducing its temperature beneath critical point.  This also reduces the back pressure on the turbine, increasing its work output.  The CO2 is now at low pressure and low temperature, beneath its critical temperature.  At this point, it is compressed for reinjection into the boiler or reactor.  The amount of work needed to compress the CO2 is much lower than would be the case for an ideal gas, as the temperature is beneath critical temperature.  Before reinjecting into the boiler, the CO2 passes through the recouperator, which preheats it.  Cycle efficiency of 50% is achievable at boiler temperatures of 600+°C.  This is largely because of compressor work savings accrued by compressing CO2 beneath its critical point (31°C).  It would not be possible for air or helium  as their criticalmpoints are cryogenic.

What may be different about CO2 cycles on Mars, is that they may be open loop.  On Mars, CO2 can be found as a solid and possibly as a liquid.  If liquid CO2 can be injected into a boiler at -50°C, say, the amount of work needed to do so would be small even at boiler pressures in the hundreds of bar range because of the incompressible nature of liquids and their low specific volume.  If the CO2 is then heated above its critical point in a very enclosed volume, it may raise pressure of several hundred bars.  If it expands back to 10mbar, before being vented into the Martian atmosphere, then each litre of liquid CO2 could produce as much as 500KJ of mechanical work in the turbine.  The turbine itself would be very compact due to the high molar mass of CO2.  No recouperators are needed, as waste CO2 can be returned to the atmosphere.
Hence, abundant liquid CO2 can support very power-dense generation cycles.  The system would be compact and easy to build.  There does need to be a heat source of course.  Enter the Aqueous Homogenous Reactor (AHR).  A very cheap and simple reactor, coupled to a simple and power-dense generation cycle could generate power very cheaply.


Interested in space science, engineering and technology.

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#12 2021-04-17 19:08:50

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 5,831

Re: C02 as a working fluid.

For Calliban re #11

SearchTerm:CO2 cycle nuclear reactor mechanical energy delivery Post #11

One issue that SpaceNut has raised when the open loop is considered is the energy cost of cleaning the natural "product"

Is there an energy cost to simply planning to use solid CO2 as the input?

Is there a way to "open cycle" to the environment while avoiding contamination with dust?

Or can the system you're describing be run with the natural "product" and not impacted by the occasional iron oxide particle?

(th)

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#13 2021-04-18 09:04:13

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 21,910

Re: C02 as a working fluid.

repost

Calliban wrote:
louis wrote:

I'd forgotten about this. It does seem that my original thought, that you could capture CO and oxygen from the atmosphere   and have a ready made energy store when humans landed a couple of years later is not so crazy and could work. Whether it's worth going to that trouble compared with just loading 30 tons of methane and oxygen on a Starship is another matter. I guess boil off over two months might be an issue?

If I read this correctly, Louis is suggesting the capture of CO and O2 already in the atmosphere on Mars.

It could be done, but it is no trivial undertaking.  The Martian atmosphere is 95% CO2, 2% Argon, 2.8% Nitrogen, 0.174% O2 and 0.0747% CO.  For this to work, you must first separate the CO2 from the other gases.  The CO2 in the Martian atmosphere is close to its triple point temperature, so relatively little compressor work would be needed to increase its density relative to the other components.  The remaining gases would be 40% Argon, 56% N2, 3.48% O2 and 1.49% CO, by volume.  The CO would be about 1% by mass.  On this basis, the calorific value of the non-CO2 content of the Martian atmosphere is about 100KJ/kg ( CO heat of combustion is 10MJ/kg).  At 1bar, the mixture would have volumetric energy density of 150KJ/m3.  At 300bar, it would be 45MJ/m3.  Because the CO and O2 are present in low concentration, you could probably store the mixture in a single tank, without risk of explosion.  To burn it, you would may need to pass it through a heated catalyst bed, as the concentration of fuel and oxidiser is too low to support flaming combustion, even under extreme compression.

In terms of total work output, one needs to compress 19kg of CO2 for every 1kg of stored gas mixture, which would release 100KJ of energy when released.  Put another way, the chemical energy content of the Martian atmosphere is 5KJ/kg or 65J per cubic metre at 6.1mbar.  This is almost certainly too little for any release of net energy from compressing and combustion of the Martian atmosphere.  None the less, it may be interesting to consider this as an energy storage mechanism.  The compressed CO2 has its own value as a CAES working fluid.  Heating it above 31°C would produce high pressure gas that could drive a compact gas turbine.  The stored compressed residual CO containing mixture has chemical energy density 100KJ/kg.  Taking the Cv of the gas to be 1KJ/Kg.K, complete combustion would raise the temperature of the gas by 100°C.  So along with its internal pressure energy, it could be a useful energy storage mechanism for short range vehicles and compressed air tools.  It is worthy of further investigation, I think.
.

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