New Mars Forums

I would counter and say that, although different stages of settlement certainly will involve different configurations and technologies, artificial light for bulk farming is not one of them. 

I mischaracterized what you said about satellite dishes, and for that I apologize.

Here's a general thought:

There's a huge difference between an organization or project with large resources and one with unlimited resources.

An organization with large resources will invest them to improve itself and have more resources and capabilities in the future.

An organization with unlimited resources won't: When it encounters a problem it will simply throw more money and manpower at it until it goes away.

One thing louis likes to say when faced with a critique of one of his plans (for example: artificial lighting for greenhouses consumes an obscene amount of power) is that he believes there will be an abundance of electrical energy on early Mars.  This is a good example of "unlimited resource" thinking.  "Build a bigger satellite dish" is another example.

In louis's defense, he does not claim specific technical knowledge that he doesn't have.  His broader point, that it's possible to get higher data rates, is not wrong, although it's also not particularly insightful.

I'm thinking primarily about early to not-so-early settlement phase, more than the first mission, but I guess in principle it could be used for either.

Power production would be in the range of ~500 W or more (by comparison to the output of a very small internal combustion engine).  There's no real upper limit and the system will get more efficient the bigger it gets.  I estimated above that a LCO2 tank will mass 25% of what's inside it, but for a settlement you might be able to get some savings by burying it deep underground.  For reference, a 10 atm pressure corresponds to burying a tank roughly 70 m below the surface of Mars.  Seems like a lot to me but I suppose it's not impossible if you're working at a very large scale.  I suppose you'll get temperature stability if you're buried that deep.

I'm not sure how much the refrigeration, power production, and water heating/storage would weigh.  I think this is the kind of system that makes more sense for a time when you're building things in situ than for an exploration mission.  But I also don't exactly understand the problems you believe there to be with batteries (presuming the initial mission is solar powered, which is far from settled but definitely possible). 

Here's how the system would work:

1. Freeze Carbon Dioxide out of the atmosphere:  This involves removing about 600 kJ/kg of energy from the CO2.  Depending how cold your hot side is, this requires an input of about 150 kJ/kg of work plus inefficiency losses.  If you get clever with your hot side, you can maybe do a little better.  If you're feeling really clever maybe you can find something to freeze at night and then use as your hot side during the day to improve your efficiency.

2. Sponge up some heat.  What I have in mind is using solar energy to heat water up to room-temperature or so.  You might even use it as the "cold side" of your main generator (which, by the way, if we're talking about solar power made on Mars I'm a big fan of the solar power tower).  If you don't get to cheat by getting your waste heat from somewhere it's trivially easy to build a solar power collection system that can heat water to 20 C on Mars.

3. Use some waste heat from somewhere (such as the heat rejected at the hot side of your refrigerator unit) to melt the dry ice into Liquid CO2 and pressurize it.  The reason why I would say to freeze it and then melt rather than going straight from gas to liquid is that it saves you the difficulty of pressurizing gas from 0.007 atm to 5 atm (an increase of over 700 times) which would probably cost much more energy.

4. Store this liquid CO2 at an appropriate pressure and temperature (somewhere around 5-10 atm, -55 C to -40 C probably) until nighttime

5. Use lukewarm water to boil the CO2 and expand it through 4-5 reheat cycles (As described above) at night to extract useful work, eventually ending up with a block of ice and releasing the CO2 to the atmosphere to generate work.

In the ideal case, using the calculations in my post above (post #26 in this topic), the system would consume 145 kJ/kg of work in refrigeration, 650 kJ/kg of heat energy (Which corresponds to 2 kg of lukewarm water to each 1 kg of CO2), and would output 166 kJ/kg of useful work.  The actual refrigeration work will be higher and work output lower, unfortunately.

In principle, the technology that could be used is what Zubrin likes to call "gaslight era".  Seeing as we do not live in that era there's all sorts of electrical and electronic sensors, valves, etc. that can improve the efficiency and reduce the labor required for operations, but it's not required for the system to work.

A funny, steampunk-style image that crosses my mind is workers in spacesuits shoveling dry ice like coal into a tank.

Yup, there it is!  Would that version be high-res enough to use?

Well that's the thing, eh?  99.8% of the time you don't get burned. All you've got to do is not touch the hot pan with your hand. Common sense, right?

Anyway we agree so there's no need to beat this horse any further.

How do you feel about using liquid CO2 and liquid water for heating for nighttime energy storage?  It seems to me like a really good option (highly efficient, reliable, easily manufactured in situ) but I'm interested in your take.

I don't think I ever understood wildfires living back on the East Coast. 

I think I understand them a little better now.

I live in Washington, between the Cascades and the Rockies.  These are some tall mountains, so tall in fact that they dominate the weather in this portion of the country.  The rain shadow from the Rockies goes as far East as the Mississippi river in some places.  Despite Western Washington's reputation for rainy weather, Central Washington is a dry desert.  It didn't rain at all from June to October.  Big portions of Western states are protected parks or forests.  When they dry out, they burn.  There was a solid week when the sky turned a gray-orange Martian color and the air smelled like ash, and there wasn't even a big fire within 100 miles of me.

Washington is not known for its fires because they're worse both to the South and the East.  In total, the US has about 1,000,000 square kilometers of protected federal land (IE National Park + National Forests).  Much of this is in fire-prone areas of the West.  There's also a smaller amount of state land and lots of open grassland, which I believe can burn at times.  Canada has even more, I believe, and fires don't stop at international borders.  Neither does the smoke and ash they generate.

1,000,000 km^2 is 4 times the area of the entire United Kingdom, and most of this land is very sparsely populated, which is why fires so rarely cause human casualties.

Those are some cool pictures but in general I think wheels (possibly with treads) are going to be a better way to do ground transport

Hey kbd512,

My high-level take on radioisotope power is that it's good under the following circumstances:

1. Relatively low power (below ~1 kW)

2. Not used for primary generation

3. Reliability is critical

4. Cost is not a major concern

5. Alternatives are inferior to the point that it's worth getting into a political fight over nuclear power in space

A corollary of points 2 and 4 is that it's being used for a niche application.

It seems to me that 1 through 4 are the case in this application.  Whether or not 5 is true is up to the organization running the mission, and in my opinion this is relevant whether that organization is public, private, or somewhere between the two.

To speak briefly on the politics of nuclear power more broadly: It is my opinion that nuclear power is a great source of electrical energy, particularly if you want to reduce CO2 emissions associated with electricity production.  However, there is always going to be some tiny chance (no matter how good your engineers and procedures are) that some series of mistakes, equipment failures, laxness, and bad luck that will result in a release of radioactivity to the environment.  The average person does not spend much time thinking about energy policy and the average person is certainly not an engineer.  Radiation is scary but also a genuine threat to your health in large quantities that can neither be avoided nor repaired.  I guess what I'm saying is that, to a certain extent, I get why nuclear is unpopular even if I personally think it should be deployed more widely.

On the topic of Pu-238 vs. Sr-90: I say go for whichever is more available, basically.  I would like to see investment in the facilities to produce this because our need isn't going to go away.  In the context of a serious exploration/settlement program $50 million isn't that much money and it's a great investment if it improves mission capacity or reliability. 

As far as the question of whether to remove the outer layer of covering from the Pu-238 bricks: I wouldn't.  There's some benefits if you're using a gas turbine or something, but really 811 K is really quite hot, especially against a background of 230 K.  For carnot efficiency purposes, 811 K against a background of 230 K will be comparably efficient as 1023 K against a background of 290 K.  There's no way the CO2 will get to a temperature even close to that and I don't think heat transfer will be a huge issue.  Even if it does reduce effective power somewhat, bringing 6 RHUs isn't meaningfully more difficult than bringing 4.

On the other hand, a criticality incident (no matter how unlikely) is an avoidable mission-killer.  The best case is that you've introduced another task that's potentially hazardous and requires specific attention to details.  This is true no matter how simple it would be to avoid a criticality incident--it's best for it to be a fun piece of trivia rather than a real procedural consideration for the astronauts.  The worst case is killing or stranding one or more crew members.

I've already said it plenty but I want to reiterate for the crowd that I agree that a criticality incident is both extremely unlikely and extremely avoidable.

Seeing as the efficiency of lasers is generally on the order of 10% it's hard for me to imagine that it's going very well

This is a nice idea in the abstract but there definitely are some problems.  What if 60% of the settlement wants to be a wretched hive of villainy and scum, but the other 40% would prefer to operate as a peaceful and friendly village where due process and property rights are respected?  This is a good example of how democracy can be in conflict with freedom and justice.  I don't know how aggressive the central government should be about enforcing the four principles I laid out above, but there are certain things we know about how societies should work.  Among them, rule by criminal mobs is bad.

I want to follow up the previous post by talking about how different cycles and functions are different from the steam turbine cycle (Technically known as the Rankine Cycle) described above.

The first modification I want to talk about is the use of pistons instead of turbines.  Pistons are simpler and work better at lower power, but they're also less efficient: While a turbine might achieve 85% of the isentropic (ideal) work, a piston is probably closer to 70%. 

The second is a rocket engine, which is technically a thermodynamic engine.  In this case, the working fluid heats itself via combustion.  The work input comes from tapping the output directly, to power a turbopump, and the work output comes in the form of kinetic energy in the exhaust.  There is no cooling stage; instead, the fuel cools as it mixes with the atmosphere.

The third is an internal combustion engine.  In an ICE, the compression happens in part by mechanical compression by the piston, but a lot more compression happens because of the combustion, which produces extra gas and massively increases the temperature of the gas inside the fixed (or, quasi-fixed, relative to the speed of the piston) volume of the cylinder.  Like a rocket engine there's no cooling phase because the gases are vented to the environment and allowed to cool there.

The fourth is a heat pump.  Most often, a heat pump is used to remove heat from a volume, which is called refrigeration.  For a heat pump, you start with a gas and heat it by compressing it (By applying work).  You allow this heat to leave the gas, then allow the gas to expand while recovering some work.  Then, the gas picks up heat from its environment, after which you restart the process.

The fifth is the system kbd512 and I have been discussing.  In this system, compression happens via freezing CO2 out of the atmosphere and then reheating it to melt and pressurize it.  The work input is the work that goes into operating the freezer.  The heat input comes mostly from the environment (this is the heat that is used to vaporize the LCO2) with additional heat coming either from a Radioisotope Heating Unit or from freezing water.  Work output comes from either a turbine or a piston engine (whichever is chosen) and there is no cooling stage.

As promised, I'm going to go more into detail on this system in this post because I have new information available in the form of thermodynamic data for CO2.

So, as I mentioned previously the heat of sublimation is 571 kJ/kg for CO2.  This is the energy that will need to be removed from the atmosphere in order to freeze it out.  With a COP of 4, this will cost 145 kJ/kg of work input. 

Some of that heat will be returned to the LCO2 to melt and pressurize it.  This heat is "free" because I already accounted for the energy cost of transferring it.

Next, there is the energy required to vaporize the LCO2.  This is to be provided from the environment, which is at a higher temperature than the LCO2.  I do have some concerns about how efficiently this energy can be obtained.  Where does it come from? The ground is generally not a great conductor of heat, and the atmosphere is worse.  It wouldn't be difficult to get to an adequately high temperature using sunlight (put a black surface behind some glass then fill the space with water, or perhaps methanol/ethanol if you're concerned it may freeze), but then you're dependent on sunlight, which means you can only operate during the day.  It'll take some time to warm up, too.  The energy to vaporize the LCO2 is about 330 kJ/kg, by the way.

Then there's the additional energy used to superheat the CO2.  Heating to -20 C will cost you about 30 kJ/kg, while heating to 275 C will cost you about 425 kJ/kg. 

From there, my textbook fails me.  It does not list thermodynamic properties for temperatures below -50 C.  However, by comparison to water vapor I determined that Cp∆T is an acceptable (though imperfect) proxy for the change in enthalpy.

I want to point out that this is a different calculation than the one I was doing earlier.  Earlier, I was approximating the work done using the equation W=(Cv-Cp)∆T based on an incorrect interpretation of the first law of thermodynamics.  I am not entirely certain why it was incorrect but the W=Cp∆T method accords better with the available data and is consistent with the procedure I laid out above.

Anyway, Cp is a little tricky because it varies with temperature.  Luckily, the NIST WebBook provides the coefficients for an equation.  I integrated this equation and put it on a spreadsheet.  Feel free to use the spreadsheet: Here's the link.  All the cells except for Tc and Th are protected, but the "W" cell is the energy output from the turbine.  My recommendation for this spreadsheet: Enter your parameters in the yellow, look at the results in the green, and ignore everything else.

So: For a hot temperature of -20 C and a cold temperature of -75 C, the work done is 41.4 kJ/kg CO2.  For a hot temperature of 275 C and a cold temperature of -75 C, the work done is 318.6 kJ/kg CO2. 

If you wanted to do a reheat cycle with a max temperature of -20 C, you will need to do 4 cycles total, including the first one which I have already described.  The total heating input will be 154 kJ/kg and the total work output will be 166 kJ/kg.

The total efficiency for a hot temperature of -20 C with a single stage is -28% (the system consumes more energy than it produces).  For a hot temperature of -20 C with four stages, the efficiency is 4.3%.  For a hot temperature of 275 C, the efficiency is 23%, which is not terrible.

Having said that, this may be a case where efficiency doesn't matter.  This is fundamentally an energy storage system, after all.  Every battery has a negative efficiency.  If your hot temperature is -20 C, your heat source is so low-grade and simple that you might not care if your efficiency is terrible.  Likewise, if your energy is coming from radioisotopes your rate of power usage is really much more important than the efficiency of the system.

For consistency's sake, I will also give an efficiency number comparable to the one I gave before.  This one ignores the work input to the freezing units and the heat required to vaporize the LCO2 (the former being provided at a different location and the latter obtained for "free" from the surrounding environment).  For -20 C with a single stage, this efficiency is 135% (the system produces more usable energy than is input in the form of heat).  For -20 C with 4 stages this efficiency is 108% (the system still produces more usable energy than heat input, but it's much closer).  For 275 C this efficiency is 75%.

Every efficiency number provided assumes ideal isentropic efficiency for the refrigerator and the turbine/piston, which naturally won't exist in the real world.

So here's the outstanding questions for this system:

• Is it reasonable to try to vaporize CO2 using heat from the environment?

• How does your required power consumption compare to the available power you can get from an RHU?

• What's more important: On-site efficiency or total system efficiency?

• Is it worthwhile to generate jars of LCO2 and carry around or store them for later use?

Hey SpaceNut,

With all due respect this is not even close to right.

While I cannot confidently say what entropy is, I can very confidently say that it is not the same thing as the heat capacity.  The heat capacity measures the amount of energy it takes to raise the temperature of a material.  Entropy is something different, measuring the state of that material.

The coefficient of heat transfer has to do with how well a material conducts heat and it is not related to the other two.