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ISP x thrust = power. An isp of 4000 and t/w of 30 is a lot of power. In terms of storable propellant, isp 4000 equals a propellant energy density of 270MJ/kg. That's 27 times lox/kerosene. This is beyond the energy density of any known chemical propellant. And it is difficult to design a device that can transfer that sort of thermal energy to a propellant whilst remaining physically intact. Hence the need for bomb driven propulsion.
Another option for SSTO is the sky hook. The vehicle reaches some fraction of orbital velocity and intercepts the end of an orbital tether.
Control rods do not moderate neutrons, they absorb them. All of the space reactor concepts I have seen have been fast reactors, either sodium or lithium cooled.
Solar power does not work well either as a mission power source or a permanent base power source on Mars. As a mission power source, even when stretched to its maximum feasible limits it does not compete with a small fast reactor on a weight basis, even at the most favourable location on Mars. And it takes hundreds of EVA hours to set up. As a base power source, it does not perform well on a mass basis and requires demand side management to work without infeasible levels of storage. Making solar power systems on Mars is not energetically favourable, as energy payback times are at least 1 decade.
Louis has an ideological obsession with solar power. That is why these debates appear to go nowhere. It isn't about the technology or real world practicality, it is about his emotions. We have done enough analysis now to show that this is not a good solution for any stage of the Mars campaign. If one man cannot accept that for personal reasons, so be it. The rest of you should get on with developing practical plans.
So basically, Martian industrial infrastructure has to be shutdown for 75% of the time, as it has to align with an intermittent power supply. By doing that, you are shifting costs elsewhere - lowering storage costs by increasing marginal capital costs in whatever is using the power. Most industry wouldn't stay in business very long if it did that on Earth. On Mars where most infrastructure is imported at enormous cost, it is rather absurd.
We have already examined solar dynamic power on Mars. EROI was ~4, which implies an energy payback time of several years on Mars. That makes for expensive power and low growth rate, as you must wait several years just to recover the energy needed to build the plant in the first place.
Polycrystalline silicon cells have an even higher embodied energy than thin film - about three times greater, but they are 15-20% efficient, so will yield 1.5 - 2 times as much electricity. Even if no energy storage is used, energy payback time is still 4.4 years and EROI would be <5 for a 20 year cell life. If methane/oxygen storage is used, payback time is 13.2 years, which isn't workable at all. Again, the technology suffers from poor energy economics.
One hundred miles per charge is impressive for something over a century old, especially considering it must have been using lead-acid batteries.
Space planes would appear to have questionable usefulness. There is very little lift at sensible speeds above 100,000' and the air resistance heating at high Mach speeds is enormous. At Mach 6 it is above the melting point of aluminium at 100,000'. Mass ratios are inherently poor, the air frame would have limited reusability and the thermal protection would add weight and require refurbishment between flights. The space plane is limited to being a lower stage (out of 2-3 stages) and is a technically difficult one at that.
There does not seem to be much that a space plane can do that a reusable pressure-fed LOX/Propane lower stage could not. For smaller stages a simple drag chute could slow it to acceptable terminal velocity for an ocean splashdown. Tow it back to port, refill and reuse. The trick is to make it rugged and simple.
Also consider that Victorian era folk (in the UK at least) were exposed to much higher toxin levels than we are today. Primary energy production in the UK was no lower in 1900 and most of it came from coal and released hideous amounts of pollution. The health effects of that would have been worse than an entire lifetime of Mars radiation.
My wife has a background in disease epidemiology. One of the things I learned from her that surprised me is that most people get cancer several times a year, but in most cases the immune system suppresses it. It is only when that mechanism fails and the disease proliferates that we become aware of it.
This does suggest that the secret to reducing cancer risk in a Mars population revolves around good nutrition and a physically demanding lifestyle, not hiding from radiation. It is probably better to build houses on the surface under pressure domes, eat and work like Victorians and simply forget about radiation.
According to Wiki, the embodied energy in thin-film solar is 1305MJ/m2.
https://en.wikipedia.org/wiki/Embodied_energy
Full sun at the top of the Martian atmosphere is 590W/m2. Assuming a 70% transmittance of solar energy through the Martian atmosphere (after dust scattering, IR and UV absorption) the RMS power reaching the Martian equator during daylight hours is 292W/m2, or 146W/m2 averaged over the whole day. For 10% efficient thin-film cells, average power output is 14.6W/m2.
To produce a baseload energy supply using synthetic propellant production to store energy, we must gather 3 units of energy for every 1 unit of baseload power. That is because energy storage in chemical fuels is no more than 25% efficient in the real world. So after buffering, the power output of 1m2 of solar panel is 4.87W. The total payback time for the solar cells alone is therefore 8.5 years. It is questionable whether thin-film cells would survive that long in the high-UV environment on Mars. And payback period doesn't account for the continuous decline in cell efficiency, the energy cost of power transmission or the embedded energy used in making the energy storage system. Include that and payback time will be above 10 years (remember the energy storage system is basically a whole other power plant).
Unless the energy investment required to build these thin film solar cells is substantially lower than traditional thin film technologies (I mean more than an order of magnitude lower) it would be difficult to get excited about the prospects for thin-film silicon PV on Mars. It is close to being an energy sink.
I don't know about other places, but in the UK, power consumption at the lowest point of night is about 35% lower than the peak value in day-time. Many service industries may work 9-5, and loads will be lower outside of working hours. Domestic consumption is lower (though by no means zero) at night. Most industries and a good chunk of services, work 24/7, so their power consumption is flat. The reason is simple: if you have invested in capital intensive equipment, you don't want it sitting idle for half or two-thirds of the time. Anything that you ship to Mars is capital intensive. The mugs back home are paying for it, but there is a limit to what they are capable of paying for.
Storing large amounts of power in synthetic fuels from a PV system to cover night time loads is very inefficient. For a non-tracking PV system, that means storing at least two-thirds of what you use for a base load system. Storage efficiency is about 25%. So to produce 2400kWh across the entire day (i.e. a constant 100KWe) some 7200kWh of power must be gathered by the PV system. Average power during the day is the RMS of a sine wave (0.707), multiplied by peak power. So a system that gathers 7200kWe over 12 hours has average power of 600kWe during daylight hours and a peak power of 0.85MWe. That would be equal to a 2MWe-peak system here on Earth, to produce a baseload power of 100KWe on Mars, using methane oxygen storage.
Encapsulated organic cells have lifetimes of a few thousand hours on Earth, after which UV damage fatally degrades them.
On Mars, the UV damage rate to photosystems (i.e. chlorophyll) is about 1000 times higher than on Earth (See Table 1 in the link below).
http://www.atmos.washington.edu/~davidc … ng2000.pdf
How long will those organic cells last in an environment like that? How long would the polyethylene backing and encapsulation last? Even if these things are very cheap, they aren't much good if you have to replace them on a timescale of weeks.
Energy payback time / EROI is doubly important on Mars, especially if ISRU is being used to expand the energy supply. Moving to Mars does not obviate energy economics, if anything it accentuates it.
You have paid billions of dollars to ship infrastructure and people to Mars with the objective of building native energy sources. You need to pay those people while they are there. If EROI is 80, then those people and assets are generating 20 times more output than if EROI is 4. But it gets worse. Remember, a quarter of that EROI must be reinvested just to maintain the energy supply. So an EROI of 4 actually provides 3 units of surplus energy.
That is a problem because even in a non-growing economy, there is still non-energy infrastructure that must be operated and maintained using energy. If the EROI is only 4, there is very little surplus to build anything else or expand the energy supply. This makes basic living very expensive and growth very difficult.
If EROI is 80, it is clearly very easy to spare enough energy to maintain the energy supply and there is likely to be a huge surplus available after maintaining and operating existing systems, a surplus that can be invested in new growth. If it takes 1 unit to maintain the energy supply, 2 units to run and maintain non-energy infrastructure, then an EROI of 80 gives 77 surplus units that can be reinvested in new infrastructure, energy sources, research into new technology, etc, or just consumer products to be enjoyed. If just 10 of those units are invested in expanding energy supply at an EROI of 80, then at the next iteration there will be 880 units of energy available, 11 of which must be reinvested to maintain the energy supply.
If EROI is 4, and 1 unit is reinvested in maintaining energy supply, 2 units are invested in running and maintaining non-energy infrastructure, then only 1 unit is left for both reinvestment and consumer products. If 10% of that unit is invested in new energy at EROI of 4, then at the next iteration there will be 4.4 units available, 1.1 of which must be reinvested to maintain the existing energy supply.
This is why human living standards did not improve on Earth until we started mining high EROI fossil fuels. High EROI means both high wealth per capita and the potential for high rates of growth. It will be very difficult to achieve high EROI on either Earth or Mars using diffuse renewable energy sources. The declining EROI of Earth bound energy sources is the primary reason behind the huge debt bubble, financial crisis and squeeze on living standards in recent years.
The average surface dose rate measured by curiosity is 22.5 microsieverts per hour.
http://www.mars-one.com/faq/health-and- … exposed-to
For an 1800 hour working year in a greenhouse, total dose would be 40.5mSv. Over a 20 year career, total dose would be 0.81Sv. That is enough to increase fatal cancer risk later in life from 20% to 24%. All else being equal, it would knock about a year off the average life expectancy.
Then again, we can all reduce our cancer risk by at least half just by fasting two days a week. Combine that with a diet heavy in micronutrients and risk goes down further. This is why the Victorians rarely suffered from cancer, whereas today we drop like flies because of it.
If we are looking at ISRU, I would point out that it should be possible to construct heat exchangers, boilers and crude steam generating systems on Mars, much as Louis has described for his solar power system. The bulk material needed is low carbon steel, which can be moulded into bulk components using the carbonyl process. Only the 512kg reactor core itself would need to be shipped from Earth under this scenario.
If we can manufacture steam engines sets that are 15% efficient, then a 400kWth reactor core would generate 60kWe. The power density of the from-Earth component would be 117W/kg. If uranium represents most of the weight of the core and burn-up is 10% say at end of life, then the core would produce 54million kWh over its lifetime. If it costs $2000/kg to ship stuff from Earth under colonisation phase, the cost of importing the reactor would add 1.87 cents to the cost of generating a unit of electric power on Mars. That really is lost in the noise.
I looked earlier at the possibility of building small Magnox reactors on Mars, using Martian natural uranium and reduced carbon as a moderator. Calder Hall took several years to build and the workforce ran into many hundreds of people. Each reactor weighed 33,000 tonnes (most of which was shielding) and produced 50MWe. So that's 660Kg/KWe, maybe ~100Kg/kWe without shielding. That compares to 8.53Kg/kWe for the SAFE-400 core. So, until the population on Mars reaches a level of thousands, it will make more sense importing fast reactor cores from Earth and building the secondary systems on Mars.
You can claim this is a belief rather than the product of analysis all you like.
Your analysis completely ignores the fact that the Mars settlement will have huge per capita productive capacity. Why? Because we will (if we have any sense) send them there with an array of machines which they can use to exploit Mars In Situ resources, rather than
just sending them as dumb actors. All the Mars resources - the iron ore, the land, the water and so on - are freely available, as freely as air on Earth.With all that productive capacity available they can afford to be less efficient in producing energy than utility companies on Earth (which in case are normally competing with other energy providers).
But as I have explained you have to compare Mars ISRU with the full cost of importing energy from Earth. A ten tonne 100 Kwe nuclear reactor might cost, at $20,000 per Kg, $200 million to ship to Mars (never mind the cost of developing and building it) and would displace 10 tonnes that could be used for other projects that could develop ISRU
False. It is not necessarily true that you cannot build nuclear reactors using ISRU on Mars. There is nothing very magic about a nuclear reactor. It is a steel pot with uranium in it. No different to a boiler really. The only thing that really would be extremely difficult on Mars is enriching uranium. But HEU is so energy dense, that we could import it from Earth without seriously impacting reactor economics on Mars. And there is always the option of carbon moderated, natural uranium reactors. We were building those back in 1942. Even if fast reactor cores were imported from Earth, the pressure vessels, heat exchangers and generating equipment could be built on Mars just as easily as solar dynamic power plants. It is essentially the same kit, only the heat source is different.
When you have to start building energy systems on Mars using energy systems built on Mars, then the energy payback of those systems becomes extremely important. A low EROI energy source would effectively be a head wind that prevents growth of the colony, because so much of your workforce and resources would go into just maintaining the energy supply you have.
Here on Earth, for millennia before the industrial revolution, human numbers and living standards remained more or less constant. Most energy came from biomass and a large proportion of human work was spent maintaining that energy supply, producing food and fodda. There wasn't enough spare energy to divert into investments that could generate much growth. Total human numbers were about 500 million in Alexander the Great's day and were still only about 700 million when Elizabeth 1 sat on the English thrown, nearly 2000 years later. Around 1800, human beings started mining fossil fuels and burning them in machines. Suddenly, they had access to a high EROI energy source. Within a century, human living standards and human numbers had exploded.
If you want to build a civilisation on Mars, you need to develop native high EROI energy sources.
I haven't ignored the additional combustion requirement on Mars. But as I understand it, perhaps you can correct me, nuclear reactors operating in space or on Mars will also have added mass because they won't be water or gas cooled. Isn't that right? All this proves is that operating on Mars is not as easy when it comes to steam engines and nuclear reactors. Solar panels, I would say, are relatively unaffected, though we have to cope with the low temperatures and
No. We have already examined the mass budgets for nuclear reactors deployed on Mars. Radiators were part of that mass budget. We are going over and over the same topics again.
What combustion requirements? Combustion may be used to power vehicles on Mars and maybe even as an energy storage medium. It will not be an energy source as there is no natural oxidizer on Mars and (so far as we know) no natural fuels. I thought that was well understood by everyone here.
Louis, thank you for the links. I have built solar heating systems in my workshop, though I have never attempted a steam engine. Looking at your links, I am tempted to have a go at it.
I do not doubt for a minute that building small solar powered steam engines is possible on Mars, if we ship machinery and tools to a small base and devote enough man-power to it. I am sceptical it will be an economical venture. Here is why:
http://large.stanford.edu/courses/2015/ph240/kumar2/
1. The EROI of solar thermal power systems in Earth’s deserts with 2.3x the solar constant of Mars is 19 before buffering (i.e. energy storage) and ~9 after buffering. In this analysis, buffering is provided by pumped storage which is efficient but capital and energy intensive to build. On Mars, a solar thermal system would almost certainly buffer by storing heat in rock bodies, which would provide better energy density and lower embedded energy than pumped storage, but there will still be energy cost involved in building the store and energy losses in using it. EROI will naturally be lower because sunlight intensity is only 43% of Earths.
2. Steam engines are quite inefficient, historically <10% efficient for locomotive steam engines. With multiple stages and a condenser cycle, efficiency can increase to between 10-20%, but the mass and complexity of the engine increase progressively. Efficiency problems occur due to the relatively low input temperatures and pressures into the expanders, heat transfer into the cylinder and pistons, pumping losses in the engine and for open cycles, loss of enthalpy still stored in the vented, saturated steam. Some steam engines include multiple reheat cycles to get around the final problem, but again, you are adding mass and complexity to the device. Smaller units are generally less efficient than larger units (due to heat transfer and pumping losses). To minimise both thermal losses and pumping losses, it is advantageous to keep the power profile of the engine as flat as possible, i.e. run it at 100% 24/7. In other words, for better EROI, build it big and run it flat out. Is it a coincidence that this also improves plant economics?
The EROI graph referenced above comes from the Weissbach study and is referenced within the link. Let’s make a few adjustments for a system on Mars: (1) Solar intensity is only 43% of Earth’s; (2) Small steam engines are used, rather than large turbine plants. Let’s be generous and assume turbine efficiency was 25% and steam engine efficiency is 15%. Piston steam engines are a lot heavier per kW than turbo-generator sets, but let’s ignore that for now; (3) Let us assume that solar thermal energy storage is a more efficient means of buffering than pumped storage. Let’s say a power plant using this method on Earth would achieve an EROI of 12 rather than 9. Results:
EROI = 12 x 0.43 x (15/25) = 3.1
On the plus side, we do not need to connect to a long-distance transmission system, which improves EROI by about 30%. So final EROI is about 4. The study also notes that the replacement rate of solar concentrators is an optimistic assumption, but let’s ignore that too.
If the solar collectors are built to gather bulk heat as an end use, rather than electricity, the EROI improves, as the system no longer needs a steam engine (which reduces energy investment) and avoids conversion losses in power generation. So solar thermal systems are much more efficient at gathering bulk heat for end use. EROI for a thermal energy harvesting system could be 30 or more.
Energy density is not the issue on Mars. If it was, we would be shipping coal to Mars.
100M2? You can't mean 100 metres squared (100x100 metres)... so I assume you mean 10x10 metres. What's the problem with that? If that's what it takes to boil a kettle on Mars, so be it. Far less complicated than commissioning, monitoring and maintaining a nuclear reactor.
You still don’t think EROI is important? In a practical sense, the amount of energy and materials embedded in an energy system has a strong bearing on capital cost. That should,’t be surprising. If something is twice the size, you would expect it to be twice as expensive. This is why renewables need subsidies. On Earth, renewable energy systems receive an artificial boost, because fossil fuel energy is available for their manufacture. On Mars things are worse, because you will be using a low EROI energy source to manufacture low EROI energy sources and you need a lot more energy per capita to survive.
One final word on EROI: Why do you think PV system manufacturers are attempting to develop thin-film and printed organic solar cells? Could it perchance have anything to do with reducing the energy invested (and therefore capital cost) of the modules?
I will talk about Mars-built nuclear reactors later on.
Louis, neither your solar turbine nor any realistic nuclear reactor could be built on Mars using ISRU by 100 people. You want to see a solution that you have already chosen for emotional reasons, and you are ignoring the inconvenient facts. No amount of personal enthusiasm can turn a bad solution into a good one.
Have you taken a good look at the brochure that you linked to? Hundreds of curved, polished aluminium mirrors, mounted on individual frames, each one carefully tracking the sun using its own electric motor. That's in addition to the Boiler, the HP steam pipe work, steam drying equipment and HP and LP turbines. This is precision engineering on quite a large scale. Not something that can be built in a hobby workshop.
Then there is the issue of power density. Those polished mirrors within the brochure will have a time averaged power density of about 100 watts per square metre at the Martian equator. After generation losses, that drops to perhaps 20 watts. You would need about 100m2 of those mirrors just to power an electric kettle.
This is the problem with solar power. It is a diffuse energy source even on Earth. Compared to fossil fuels and nuclear power, you need a lot more physical kit to get the same amount of energy out. That makes it a bad deal compared to alternatives in most places.
There has been much hype around Thorium that I think is unwarranted.
Firstly, Th-232 is a fertile material, not a fissile material. One cannot use it as fuel in a reactor, one must first breed U-233 through neutron bombardment. So a thorium reactor must be a breeder reactor. One advantage that U233 has over U235 is that it produces enough neutrons for self-sustaining fission and breeding at thermal energies, hence the Indian thorium breeder is a moderated heavy water reactor. There is nothing very exotic about that - the Canadians have been building heavy water reactors for decades. It is perhaps marginally more economic than a fast reactor, but it lacks the ability to 'fast burn' other heavy actinides, which are not typically fissionable in the thermal spectrum.
I can think of no reason why the use of thorium would eliminate or reduce any safety issues associated with a nuclear reactor. It generates fission products and decay heat just like any other fission fuel. Many modern design water based reactors have very low core damage frequency through the use of passive safety features. But this is a factor of reactor design, it has nothing specifically to do with thorium being used as the source of fuel. U233 is maybe marginally less toxic than plutonium (it is a beta rather than alpha emitter) but it has similar criticality safety margins, which complicates fuel fabrication just as much with U233 as with Pu239. Thorium is marginally more abundant than uranium, but neither metal is rare, especially if a breeding cycle is in use.
Personally I think propellant manufacture for Mision One is unnecessary and a diversion. Of course, I believe in a minimalist Apollo style
lander/ascender, so we won't need to land too much fuel/propellant for the ascent. Last time I looked, I think maybe 5 tonnes of fuel/propellant should do it. NASA seems to overdesign for this sort of thing.
If you are using printed organic solar cells as your principal power source, ISPP is essential because the high UV environment of the Martian surface will degrade the cells within months. If you are stuck on the Martian surface for 2.5 years that leaves you with a bit of a problem. If the propellant plant and power supply are deliberately oversized, then sufficient propellant can be produced to power surface stay before the cells degrade.
The system will be significantly heavier than an SP-100 reactor when power transmission, over-size propellant plant and storage are factored in. But it avoids the bureaucratic issues of having to build, test and launch a space nuclear reactor. When that is done, you could power the mission by burning the paperwork needed to commission the reactor!
Just a quick poll here: How many posters here have ever raised vegetables in a big garden? How many have ever engaged in farm scale agriculture?
Oh, and on Robert's list, several of the crops require bees for pollination (peas and beans).
Oldfart1939: yes, and yes.
Yes (to the gardening), no to farming.
If we can't store enough oxygen for people to breathe, how do you expect to store enough oxygen for rockets to use? Humans use less than a kilogram of oxygen each day, so we're talking about using ~400 kg per person per year. Such figures are dwarfed by the storage needed for rocketry. There is no problem at all with storing food - and even if you don't store oxygen, storing food and using electrolysis to get oxygen (3 kWh/person-day?) is going to take much less energy than keeping the plants alive.
Quite correct I think. I agree with Robert that a naturally illuminated greenhouse will always be a more desirable means of producing food and oxygen from a systems reliability point of view. The problem is that it would appear to be very difficult to keep it warm without a supplementary heat source for much of the Martian year. The average temperature on Mars is about -55C. If concentrated solar power is used to harvest extra heat, that has reliability issues and costs of its own. If nuclear waste heat is used, that has reliability issues too and the greenhouse needs to be close to the reactor in order for piped heat to be feasible. That complicates reactor shielding. Less of an issue for a long-term base, but not a feasible solution for an expedition.
For early missions, enough stored food should be taken to cover the entire mission. The oxygen needed for the surface stay and return trips should be stored before the crew even arrive. For a long term base, the siting of the base should be chosen to allow natural food production at least in the summer. At high/low latitudes outside of summer, natural illumination may not provide enough heat to keep the greenhouse above freezing. Some modelling needs to be done to understand what the achievable growing season would be at different locations on Mars. One good thing about food - it can be easily stored for years if need be at ambient Martian temperatures. I think the problem with air is not so much providing additional oxygen, but removing excess CO2. If you can't use plants to do this, it must be scrubbed out of the air using an electrically driven fan and chemical reactants.
On Earth in a temperate zone you might be able to grow enough food for one person over a year on 10,000 square metres (100x100 - though that is probably pushing it). To replicate the Sun's effect would require through PV about 40,000 square metres of PV (to generate about 16,000 KwHs per day).
Now, I accept that you can make a lot of "savings" by intense crop rotation but even if you saved let's say 75% of that power input that's still 4000 KwHs per day, or 166 Kw continuous power. Perhaps you can explain how you get to 9.2 Kws which I think is out by a factor of 18!
Of course that's just lighting...haven't mentioned the crop life support which would also require substantial power input in an artificial system.
Also, please note that the more intense the crop rotation, the more intense the nutrition usage, so you would have to have a fully powered up recycling system for human faeces and crop waste or you would have to import additional mass.
Going off subject a bit, but here is a crude approximation.
The average human being consumes about 10MJ of food energy per day. Plant crops are about 1% efficient at converting photons into glucose and starch in Earth sunlight, but only about 40% of that light is photosynthetically available. If you could tailor the input light to the frequencies best suited to photosynthesis (which you can with LEDs), you would need to provide 400MJ of light per day to meet the food needs for one person. The best LEDs are about 50% efficient in the red spectrum. So to produce 400MJ of light, you need 800MJ of electricity, or 222kWh.
Again, an estimation only. Some plants are more efficient, others less so. And I am probably on the optimistic side with LED efficiency. Other energy inputs would likely be dwarfed by the lighting requirement, which is undeniably large. Growing food in this way depends upon cheap and abundant power. It would only make sense if the capital and operating costs of greenhouses on Mars, exceed the cost of power needed to grow crops in higher density spaces.
In theory, a well maintained garden here on Earth can be very efficient. If we apply the same assumptions on plant efficiency as above, and assume an annual insolation of 1000kWh/m2/annum (South East England insolation) it would take 101m2 of land to sustain one person. In reality, that would be extremely difficult as it would require all other conditions to be perfect (i.e. eating all the plant, no weed plants, no unexpected temperature extremes, use of winter insolation, etc). But it does illustrate that a lot of food can be grown in small spaces if necessary. Using artificial light, these sorts of limits can be approached, as we are growing plants in perfectly tailored conditions.
http://www.abc.net.au/news/2017-05-15/p … st/8526868
Looks like we are no longer in the realm of the possible and plausible, but the actual and doable.
"1000m2 of this material would weigh about 100 kilograms."
So 10,000 sq. metres would weigh in at only a tonne.
If efficiency was 3% that might give you about 400 KwHs per sol from 10,000 sq. metres.
If efficiency could be raised to 12% then that would be 1600 KwHs - we'd need less than a couple of tonnes for mission one.
Impressive. Organic solar cells. These ones have been encapsulated in PET to reduce photo-degradation, which would otherwise destroy them after a few hundred hours of irradiation.
I think the problem with printed organics is the relatively short lifetime (a few thousand hours at best) due to photo-degradation. These are Earth surface values, where the ozone layer effectively filters out most UV and all UVC. I am not sure what the effective lifetime would be on Mars. I guess you are talking a few months rather than years. If the cells really are that light, maybe that is enough time to produce the propellant needed for the trip home and to power surface operations. A system generating a time-average power of 400kW running a propellant plant with 50% efficiency, would generate nearly 160 tonnes of methane oxygen bipropellant over 90 days. That is enough to power the surface expedition for 2.5 years and provide propellant for the trip home. The propellant plant and storage tanks would need to be big, much bigger than under the nuclear scenario, because the tanks need to contain twice as much propellant and the propellant plant has to work at a much greater rate, as it only operates during the day and all propellant must be produced before the cells degrade after just a few months.
The solar panels themselves would cover a huge area, but maybe it could be made to work. A 400kWe average power output system, would cover an area of about 30,000m2 at 10% efficiency. If some allowance is made for cell degradation, maybe 40,000m2 (10 acres). Total cell mass would be ~4 tonnes. Power cables and transmission would increase this substantially, as the voltage output of the cells is necessarily low to prevent breakdown of the cells. The mass of the propellant plant would be several times greater, as it must produce twice as much fuel in less than one third as much time. Due to the large area of the panels, it would take a lot of man-hours to deploy the system. So there is a lot of inherent risk here, since you cannot deploy before crew arrival.
I would agree. I calculated previously that meeting 1 person’s calorie requirements using plants grown under artificial light would require some 220kWh per day, that’s equivalent to a continuous power output of 9.2kW. For six people, the energy requirement is about half the mission power supply. Some plants could be grown this way on the first mission, but unless there is a lot of power to spare, it will not be more than a minor compliment to diet. Maybe some high vitamin salad plants.
During the northern summer, solar constant is several kWh per day. A small poly tunnel could be used to compliment crew diet using natural light, provided it was protected by insulation during Martian night.
Fossil methane would be a useful discovery as a reducing agent for metal oxides and a precursor for plastics. As a bulk fuel, one would need to find an oxidising agent as it will not burn in the Mars' CO2 atmosphere.
There seems to be a consensus that you need at least two since it would be arrogant folly to assume they can't malfunction. So without shielding that is 9 tonnes and with shielding it is 13 tonnes.
Many advocates of nuclear say they still want to take small RTGs, PV panelling and chemical batteries to provide needed flexibility. You're probably talking about at least 2 tonnes if you want to build in that back up and flexibility.
Louis, the mission does not necessarily need two reactors. There are many faults that could result in mission failure. Reactor faults are only one category. If the probability of failure is small compared to all other mission related risks, it doesn't make sense to double power supply mass.
Oops. The specific power of Pu-238 is 560W per kg, about 45% more than Sr-90. SrO has about one third the density of PuO2.
So its better than I initially thought. With strontium so cheap and abundant and able to provide 70% of the specific power of Pu-238, I do wonder why plutonium is used at all. There is no global shortage of Strontium-90 - it is one of the most abundant isotopes in fission product waste. Most countries with power reactor programmes would be happy to give it away to a good cause. The UK has about 30 tonnes of it in high level waste stores. France even more.