Going Solar...the best solution for Mars.

SpaceNut · 2017-06-11 19:34:29

Results and Equipment for the production of 1kg/s of Oxygen [7]
CO2    0.638 kg/day
Water    0.618 kg/day
Total Electric Power    185 Watts
Compressors 1 Unit    0.42 Watts
Pumps)    2 Units    2.17E-04 Watts
Heat Exchangers 3 Units    59.7 Watts
Valves    2 Units
Condenser 1 Unit    16.1 Watts
Reactors    1 Unit    33.1 Watts
Electrolyzer    183 Watts
Heaters 1 Unit    1.07 Watts

Antius · 2017-06-11 22:38:59

The problem with titanium and other non-ferrous metals like aluminium and magnesium, is that they do not have an infinite stress cycle. Assuming you are talking about pressure vessels and not cryogenic storage tanks, there would be a finite number of pressure cycles before the tank became unsafe. Pressure vessels for repeated use are usually carbon steel or ductile low-alloy steel.

Flexible bladders are not a good idea. They would suffer low energy density due to the low density of the stored gas and at typical Martian temperatures, most elastomers are beneath their glass transition temperature. A steel pressure vessel pressurised to ~200bar is a better idea. Exactly what the original authors of your thin-film study recommended.

Also, there would appear to be little benefit to converting hydrogen into methane. Every new chemical transition you undertake wastes more energy. The more energy you waste the more panels needed, the more heavy cabling and transformer units required and the more EVA times needed to assemble all this stuff. Electrolysis is already an inefficient way of storing energy. Assuming you are electrolysing water to produce hydrogen and then burning the hydrogen in an engine, total efficiency is 25-30%. Adding methane synthesis to the set of reaction steps cuts that down to ~20%.

This is why Li-ion batteries are better. Most of what you put in, you get out the other end. They are heavier for sure, but realistically the solar power system will not be as light as 0.1kg/m2 when transmission is factored in. So efficiency is important in a mass optimized system.

Last edited by Antius (2017-06-11 22:52:49)

kbd512 · 2017-06-11 23:34:21

If the continuous output requirements are kept to 2kWe or less, then PV power and batteries becomes mass-competitive with other competing solutions. The Cygnus spacecraft apparently requires a maximum of 850We of power. I expect life support systems to add another 872We (672We for MOXIE, 100We for CAMRAS, 50We for IWP) to the avionics and thermal control requirements. A permanent battery could feasibly provide that amount of power and could be located within the habitat module since it's non-toxic, non-flammable, and performs better at elevated temperatures.

The methalox plant idea is not mass-favorable, as a function of the power required to achieve a given level of output, no matter what the power source is. In order to produce any methane, water from Mars or hydrogen from Earth is required. If you obtain water from the regolith, it's better to store the water for life support. Methane becomes important after science determines that .38g produces no adverse health effects.

louis · 2017-06-12 02:47:42

I think that with the timeline set out under the solar energy option, the need for the pressure tanks to operate is only a couple of years so I am not sure about this problem you have highlighted about repeated pressure cycles. The following site suggests:

Titanium Vessel are highly cost-effective over the entire lifecycle of the equipment. Properly maintained, TITAN Titanium Pressure Vessels can operate for decades, making them a very economical material selection over other available metals.

http://www.titanmf.com/products/pressur … e-vessels/

And no mention of problems here:

https://www.honiron.com/choosing-right- … re-vessel/

In terms of a two year timeline, I can't see much of a problem. I don't think this will be a scenario where the tank is emptied and then refilled frequently. It's really more of an insurance policy. There will be some usage for industrial experiments requiring additional power. On most sols, during the hours of low or no light, the two tonnes of chemical batteries (capable of storing 400 KWehs) can provide sufficient power (remembering that under the solar option most of the "heavy lifting" involved in life support - such as oxygen production, cooking, water processing and heating of water - will occur during the 8 hours or so of maximum insolation and thus be powered by the solar panels). There may be times it is needed to maintain supplies, such as during an autumnal dust storm but that will be exceptional.

The key problem with the solar energy option for Mission One is simply the lack of a failsafe option if you arrive in some freak weather scenario with a huge dust storm that continues for months. It won't be a problem once we are well established on Mars and have created a large and secure reserve of methane and chemical battery storage. But up until that point (probably after the first six months or so) we need that methane storage that comes from the pre-landing period.

Converting intermittent energy into a permanent and dependable store of energy is not my definition of "wasting energy". It's a system gain.

The equivalent chemical battery mass required to match the amount of methane/oxygen powered electrical energy available on landing (4950 Kwehs) would be about 25 tonnes, whereas the methane/oxygen plant and storage would be no more than about 5 tonnes.

Antius wrote:

The problem with titanium and other non-ferrous metals like aluminium and magnesium, is that they do not have an infinite stress cycle. Assuming you are talking about pressure vessels and not cryogenic storage tanks, there would be a finite number of pressure cycles before the tank became unsafe. Pressure vessels for repeated use are usually carbon steel or ductile low-alloy steel.
Flexible bladders are not a good idea. They would suffer low energy density due to the low density of the stored gas and at typical Martian temperatures, most elastomers are beneath their glass transition temperature. A steel pressure vessel pressurised to ~200bar is a better idea. Exactly what the original authors of your thin-film study recommended.
Also, there would appear to be little benefit to converting hydrogen into methane. Every new chemical transition you undertake wastes more energy. The more energy you waste the more panels needed, the more heavy cabling and transformer units required and the more EVA times needed to assemble all this stuff. Electrolysis is already an inefficient way of storing energy. Assuming you are electrolysing water to produce hydrogen and then burning the hydrogen in an engine, total efficiency is 25-30%. Adding methane synthesis to the set of reaction steps cuts that down to ~20%.
This is why Li-ion batteries are better. Most of what you put in, you get out the other end. They are heavier for sure, but realistically the solar power system will not be as light as 0.1kg/m2 when transmission is factored in. So efficiency is important in a mass optimized system.

louis · 2017-06-12 03:06:57

Looks like very low energy usage to produce several tonnes of oxygen over the sort of pre-landing period I am suggesting. I find that encouraging. I am assuming the s stands for sol...is that right? As in 1 kg per sol.

SpaceNut wrote:

An ISRU Propellant Production System to Fully Fuel a Mars Ascent Vehicle
Dry Reforming: A Unique Flowsheet for Fuel Production on Mars
MARS Return Fuel Production Using Table Top Electrolysis & Sabatier Reaction
http://voitlab.com/courses/thermodynami … en_on_Mars
Results and Equipment for the production of 1kg/s of Oxygen [7]
CO2    0.638 kg/day
Water    0.618 kg/day
Total Electric Power    185 Watts
Compressors 1 Unit    0.42 Watts
Pumps)    2 Units    2.17E-04 Watts
Heat Exchangers 3 Units    59.7 Watts
Valves    2 Units
Condenser 1 Unit    16.1 Watts
Reactors    1 Unit    33.1 Watts
Electrolyzer    183 Watts
Heaters 1 Unit    1.07 Watts

Antius · 2017-06-12 04:19:05

1kg of oxygen is 13.1MJ of chemical energy, which requires about 20MJ of input to an electrolysis cell. That's 5.6kWh/kg. 5.6kWh divided by 24 hours is 233 watts continuous. Of course, if you are only powering that cell during peak sun, the actual power requirement is 1120 watts for 5 hours. And that cell must be larger, more massive and you will need to expend some energy heating it up each day before you can use it. Entropy is expensive.

louis · 2017-06-12 05:37:51

Or to put it another way - 233 watts would be about 2.5 kgs of Megaflex at the Mars surface...not exactly a deal-breaker.

Antius wrote:

1kg of oxygen is 13.1MJ of chemical energy, which requires about 20MJ of input to an electrolysis cell. That's 5.6kWh/kg. 5.6kWh divided by 24 hours is 233 watts continuous. Of course, if you are only powering that cell during peak sun, the actual power requirement is 1120 watts for 5 hours. And that cell must be larger, more massive and you will need to expend some energy heating it up each day before you can use it. Entropy is expensive.

Antius · 2017-06-12 08:53:14

I have discovered what may be a problem with the thin-film PV mission concept.

As I stated previously, 100grams/m2 is the weight of individual cells tested in the lab, not the mass of the whole system. The thickness of vapour deposition amorphous silicon photoactive layer is ~1 micrometre. At this thickness, the breakdown voltage of the silicon is 30volts. Applying a reasonable safety factor, panel output voltage cannot much exceed 12volts.

For 10,000m2 of solar panels, divided into rolls 1m wide, I calculated the mass of copper cabling needed to transmit power from each roll of cells back to the edge of the array with no more than a 2V voltage drop. The answer is 113.613 tonnes.

I have not calculated the voltage drop across the panels themselves. Nor have I attempted to assess the energy loss due to the inverter and transformer, or the weight of these items.

For relatively small arrays deployed in space, where the transmission distance back to the inverter and transformer are just a few metres, the voltage drop is less of a problem. But larger systems like this would require that either additional mass budget is allowed for cabling, or that mass budget is allocated to local inverters and transformers which steps up the voltage for every 10m2 of cells. Another alternative would be to increase the thickness of the silicon layer and use a thicker backing material. But that adds mass to the panels.

My conclusion is that as a power source for a manned mars mission, thin-film PV is in its early concept stage and we do not yet have a good understanding of how a developed system would work. Planning around an assumed weight of the system is therefore premature.

Last edited by Antius (2017-06-12 09:13:41)

louis · 2017-06-12 09:43:47

That's interesting. Intuitively 113 tonnes doesn't sound right to me.

Do you have a link for the figures you are giving?

How are you laying out the array?

Antius wrote:

I have discovered what may be a problem with the thin-film PV mission concept.
As I stated previously, 100grams/m2 is the weight of individual cells tested in the lab, not the mass of the whole system. The thickness of vapour deposition amorphous silicon photoactive layer is ~1 micrometre. At this thickness, the breakdown voltage of the silicon is 30volts. Applying a reasonable safety factor, panel output voltage cannot much exceed 12volts.
For 10,000m2 of solar panels, divided into rolls 1m wide, I calculated the mass of copper cabling needed to transmit power from each roll of cells back to the edge of the array with no more than a 2V voltage drop. The answer is 113.613 tonnes.
I have not calculated the voltage drop across the panels themselves. Nor have I attempted to assess the energy loss due to the inverter and transformer, or the weight of these items.
For relatively small arrays deployed in space, where the transmission distance back to the inverter and transformer are just a few metres, the voltage drop is less of a problem. But larger systems like this would require that either additional mass budget is allowed for cabling, or that mass budget is allocated to local inverters and transformers which steps up the voltage for every 10m2 of cells. Another alternative would be to increase the thickness of the silicon layer and use a thicker backing material. But that adds mass to the panels.
My conclusion is that as a power source for a manned mars mission, thin-film PV is in its early concept stage and we do not yet have a good understanding of how a developed system would work. Planning around an assumed weight of the system is therefore premature.

kbd512 · 2017-06-12 11:46:59

Antius,

That mass figure you quoted seems incredibly high. No current production thin film technology requires that kind of copper mass for power transmission. I haven't seen any thin film PV technologies that come close to ATK's MegaFlex or DSS's MegaROSA, as it relates to array power density. There may be some sort of graphene-based thin film technology in a lab somewhere, but in the real world MegaFlex and MegaROSA win the power density argument. ATK's UltraFlex has flown and will fly in the near future on real robotic missions to Mars. Incidentally, a number of solar power companies have started integrating the power inverters into their arrays so that less in the way of power conversion equipment is required and losses are reduced.

Louis,

I did make a mistake in my calculations of what the array would weigh. Detailed information about UltraFlex and MegaFlex has not been easy to come by. The kW/kg figures posted by ATK are accurate, but to that number you add the mass of the PMAD (Power Management And Distribution) equipment. A 250We/kg array would therefore weigh 1kg for 250We at 1AU, but to that number you then have to add the mass of the PMAD equipment. For high-output systems, PMAD mass increases. There is apparently a rule-of-thumb that dictates about 11.4kg/kW for regulated high-output arrays.

A 100kW MegaFlex array itself at 1AU would be 400kg. To that, the mass of the PMAD equipment must be added (11.4kg/kW * 100kW = 1,140kg). As you can see, the mass of the PMAD greatly exceeds the mass of the arrays themselves. Multiply the array mass figure by 2.74 to get an array with equivalent output on Mars. So, a 100kWe peak output MegaFlex array weighs 1,096kg on Mars. The mass of the PMAD would increase as a function of the copper required, but should not increase substantially from the 1,140kg figure as a function of array power.

The mass of drive mechanisms for Sun tracking and ground mounts must also be added for each array. If the arrays were 10kWe (1AU) or thereabouts, then there's no reason why a pair of astronaut could not deploy the arrays by hand as a function of the low masses involved. There would be a minimum of 28 10kWe arrays required to generate a peak output of 100kWe. Assuming an hour of deployment time per array, this would mean 4 days to deploy all arrays. The deployment mechanisms could be eliminated from the arrays to simplify and reduce mass.

To deploy an array that could produce 100kWe continuously, irrespective of surface conditions, would take a few weeks because many more arrays would be required. The mass would greatly exceed the mass of a fission reactor providing equivalent power, but it could be done without any tools or machinery to move a heavy reactor.

As Oldfart1939 has stated previously, we seem to be going to a lot of trouble to avoid using fission reactors. Right now, PV and batteries are not mass-competitive with simple and inefficient fission reactors for producing continuous output. NASA estimates that a LOX-only ISRU demonstrator mission would easily cost a couple billion dollars. Methane production would almost certainly double that cost estimate. The fact that an equivalent mass of PV panels can produce more peak output than equivalent fission reactors ignores the fact that peak output is 1/6 of a day. Then there's the other 2/6's of the day where output declines precipitously and for at least 3/6's of the day you get no output at all.

Your methalox plant concept that stores 1,000kg of CH4 requires about 3,500kg of LOX for an optimal burn. NASA estimates that that kind of LOX output requires more than 900 days to produce using day-only PV power. All that for a couple weeks of power output? Why not just send more PV panels and batteries? At least that way, you know you're getting some amount of guaranteed power output.

louis · 2017-06-12 13:16:00

kbd,

You'll see I did make an allowance of 1 tonne for additional power equipment (cabling, inverters, transformers on a one third rule of thumb) in mass analysis of the solar energy system. Given my 3 tonnes for the solar panels is probably an over-estimate (I was referencing 150 w/kg at 1AU, rather than the highter 250 w/kg), I probably have some leeway there.

The reason I am not advocating more chemical batteries at the pre-landing stage is because 7.5 tonnes worth of solar panels, plant, storage tanks and generators can deliver the equivalent amount of electric energy contained in 27 tonnes of solar panels plus chemical batteries, before you land, but can also go on producing energy once you are there on the planet. Once you land you can use ISRU to create more methane storage and also more chemical batteries. By the end of two years, we should have quite substantial energy storage in place for the following mission.

I am not proposing sun tracking. Sun tracking is not so important on Mars owing to the higher levels of ambient light. I think as long as we assemble the arrays to even out production over the sol, that will be OK. Solar tracking would probably bring in less than 10% additional output - while adding substantial mass.

I know you've set out some mass estimates for a nuclear power solution, but perhaps you should set on a separate thread your proposal in more detail. I don't really accept your claim about nuclear power mass being lower - but it might be easier to address the issues on a separate thread.

kbd512 wrote:

Louis,
I did make a mistake in my calculations of what the array would weigh. Detailed information about UltraFlex and MegaFlex has not been easy to come by. The kW/kg figures posted by ATK are accurate, but to that number you add the mass of the PMAD (Power Management And Distribution) equipment. A 250We/kg array would therefore weigh 1kg for 250We at 1AU, but to that number you then have to add the mass of the PMAD equipment. For high-output systems, PMAD mass increases. There is apparently a rule-of-thumb that dictates about 11.4kg/kW for regulated high-output arrays.
A 100kW MegaFlex array itself at 1AU would be 400kg. To that, the mass of the PMAD equipment must be added (11.4kg/kW * 100kW = 1,140kg). As you can see, the mass of the PMAD greatly exceeds the mass of the arrays themselves. Multiply the array mass figure by 2.74 to get an array with equivalent output on Mars. So, a 100kWe peak output MegaFlex array weighs 1,096kg on Mars. The mass of the PMAD would increase as a function of the copper required, but should not increase substantially from the 1,140kg figure as a function of array power.
The mass of drive mechanisms for Sun tracking and ground mounts must also be added for each array. If the arrays were 10kWe (1AU) or thereabouts, then there's no reason why a pair of astronaut could not deploy the arrays by hand as a function of the low masses involved. There would be a minimum of 28 10kWe arrays required to generate a peak output of 100kWe. Assuming an hour of deployment time per array, this would mean 4 days to deploy all arrays. The deployment mechanisms could be eliminated from the arrays to simplify and reduce mass.
To deploy an array that could produce 100kWe continuously, irrespective of surface conditions, would take a few weeks because many more arrays would be required. The mass would greatly exceed the mass of a fission reactor providing equivalent power, but it could be done without any tools or machinery to move a heavy reactor.
As Oldfart1939 has stated previously, we seem to be going to a lot of trouble to avoid using fission reactors. Right now, PV and batteries are not mass-competitive with simple and inefficient fission reactors for producing continuous output. NASA estimates that a LOX-only ISRU demonstrator mission would easily cost a couple billion dollars. Methane production would almost certainly double that cost estimate. The fact that an equivalent mass of PV panels can produce more peak output than equivalent fission reactors ignores the fact that peak output is 1/6 of a day. Then there's the other 2/6's of the day where output declines precipitously and for at least 3/6's of the day you get no output at all.
Your methalox plant concept that stores 1,000kg of CH4 requires about 3,500kg of LOX for an optimal burn. NASA estimates that that kind of LOX output requires more than 900 days to produce using day-only PV power. All that for a couple weeks of power output? Why not just send more PV panels and batteries? At least that way, you know you're getting some amount of guaranteed power output.

Antius · 2017-06-12 14:21:02

Here is a link to the tech spec for Alta Devices thin-film PV. This is a California based company that specializes in producing lightweight PV cells for applications like electric aircraft.

http://www.altadevices.com/wp-content/u … -brief.pdf

Take a look at the 5x cell module. This has an open circuit voltage of 5.47 volts. Under normal circumstances, no one would produce a panel with an output voltage so low. You cannot use voltage that low. I would strongly suspect that the reason for this is the simple fact that a higher voltage (i.e. putting more cells in series) would fry the life out of PV films that thin.

Last edited by Antius (2017-06-12 14:23:19)

Antius · 2017-06-12 14:38:38

kbd512 wrote:

Antius,
That mass figure you quoted seems incredibly high. No current production thin film technology requires that kind of copper mass for power transmission. I haven't seen any thin film PV technologies that come close to ATK's MegaFlex or DSS's MegaROSA, as it relates to array power density. There may be some sort of graphene-based thin film technology in a lab somewhere, but in the real world MegaFlex and MegaROSA win the power density argument. ATK's UltraFlex has flown and will fly in the near future on real robotic missions to Mars. Incidentally, a number of solar power companies have started integrating the power inverters into their arrays so that less in the way of power conversion equipment is required and losses are reduced.

In the real world, panels would need local transformers as well as inverters to step up voltage. But as I noted, if transmission distances are a few metres or less, it isn't an issue. When systems get to be 100m across, transmission requires increasing voltage. That wouldn't be an issue in most applications. But transformers are heavy.

kbd512 · 2017-06-12 15:28:34

louis wrote:

kbd,
You'll see I did make an allowance of 1 tonne for additional power equipment (cabling, inverters, transformers on a one third rule of thumb) in mass analysis of the solar energy system. Given my 3 tonnes for the solar panels is probably an over-estimate (I was referencing 150 w/kg at 1AU, rather than the highter 250 w/kg), I probably have some leeway there.

The 11.4kg/kW figure comes from NASA, but you'll see it all over the place in space power distribution systems design. This has traditionally been a relatively accurate estimate of what most PMAD systems for high-output applications end up weighing. IIRC, the 7kg/kW figure I used previously came from the PMAD for the DS-1 probe. However, that was a much lower output application.

louis wrote:

The reason I am not advocating more chemical batteries at the pre-landing stage is because 7.5 tonnes worth of solar panels, plant, storage tanks and generators can deliver the equivalent amount of electric energy contained in 27 tonnes of solar panels plus chemical batteries, before you land, but can also go on producing energy once you are there on the planet. Once you land you can use ISRU to create more methane storage and also more chemical batteries. By the end of two years, we should have quite substantial energy storage in place for the following mission.

The intermittency problem is not an issue with the batteries, it's an issue with the surface environment. Apart from complete battery failure, which would only be caused by destruction of the batteries (apart from failure of the charge controller's voltage regulator, highly unlikely, and NASA build batteries with redundant voltage regulation), I can't think of another issue that would require much more capacity than what is required to maintain continuous output when the Sun doesn't shine. If there's a complete failure of all batteries, you're still just as dead, but death would occur some number of days or weeks later.

7,500kg / 4kg/kW = 1,875kW at 1AU or between a peak of 675kW and 983kW on the surface of Mars. A dust storm can reduce that to 10% of the normal output. That's 188 arrays to deploy, so it'd take approximately 25 days to deploy them all. Assuming all six astronauts work on the project, then that's just over a week. The minimum amount of power required to sustain life support would be available in the first 48 hours.

louis wrote:

I am not proposing sun tracking. Sun tracking is not so important on Mars owing to the higher levels of ambient light. I think as long as we assemble the arrays to even out production over the sol, that will be OK. Solar tracking would probably bring in less than 10% additional output - while adding substantial mass.

Sun tracking maximizes array output. Even if the tracking motors fail, which is unlikely since they're completely sealed, the motors don't weigh that much and therefore don't cost that much. A 10kWe (1AU) array weighs 40kg on Earth and 15.2kg on Mars. A stepper motor required to move a 15.2kg panel is not very heavy. Since the array mass required is already so large, adding this feature is not a deal breaker. It might add another couple hundred kilograms or so to the array size you're planning.

louis wrote:

I know you've set out some mass estimates for a nuclear power solution, but perhaps you should set on a separate thread your proposal in more detail. I don't really accept your claim about nuclear power mass being lower - but it might be easier to address the issues on a separate thread.

You stated that you think solar power is the best solution for providing power on Mars and now you're getting feedback on competing viewpoints with mass figures for what things actually weigh or traditionally have weighed on past missions. If you think some sort of mass requirements have been hidden or not represented for the nuclear solution, then simply state what you think those are instead of saying that complete mass breakdowns haven't been provided.

Antius · 2017-06-12 16:04:05

kbd512 wrote:

The 11.4kg/kW figure comes from NASA, but you'll see it all over the place in space power distribution systems design. This has traditionally been a relatively accurate estimate of what most PMAD systems for high-output applications end up weighing. IIRC, the 7kg/kW figure I used previously came from the PMAD for the DS-1 probe. However, that was a much lower output application.

88W/kg is still impressive. On Mars that would still give power output of 26.5W/kg for the panels in full sun and 9.4W/kg on a time averaged basis.

If a 33% efficiency is assumed for LOX-hydrogen electrolysis/ fuel cell energy storage, and 75% of energy is stored, then effective power density of the panels drops to 3.76W/kg. If the energy storage mechanism is 200Whr/kg, then total mass required is 9000kg. The total mass of the panels is 26,596kg. So total system mass is 35,596kg. If the panel mass is halved, them total system mass is 22,298kg, or 4.5W/kg.

By comparison, the SP-100 with stirling cycle would yield 19W/kg. But the SP-100 would suck up a billion dollars of development funds and would face endless regulatory project delays. So maybe that extra 25 tonnes of power supply weight for a solar power system is something we can live with. What matters is total project cost after all.

Last edited by Antius (2017-06-12 16:24:42)

louis · 2017-06-12 16:27:58

Well I don't really understand your point...you're saying it's used with electric aircraft...Well something like the Solar Impulse 2 uses up to 52 Kwes of power. The power plant (which includes heavy batteries) is about 450 kgs. The plane's panels can generate 340 KWehs per day. There's no mention of transformers or heavy cabling. How's all that working? Some of the panels are 40 metres away from the power plant.

https://en.wikipedia.org/wiki/Solar_Imp … ifications

According to the graph linked to below, regarding Megaflex, with a single array diameter being 17 metres - the double array's area is 450 sq. metres. That weighs 1000 Kg and produces 120 Kwes at 1AU (I know - seems lower than some other quotes but let's run with that at the moment) .

https://2.bp.blogspot.com/-VFpo6AoMFFI/ … gaflex.png

Anyway, the Megaflex seems to draw power into a central point. For an array massing 1000 kgs, we would be producing about 45Kwes average. Let's assume we go to 180 Kwe average, to allow for conversion inefficiencies, battery charging and dust storm conditions. That would mean we needed 4 tonnes for the system. But we are only talking 4 double arrays for the whole system. We should be able to locate those very close to the main hab in a 3/4 circle. I doubt any array would be further away than about 20 metres. So maybe 80-100 metres of cable taking the electric power to the main power plant. 130 tonnes? I don't think so.

I am not suggesting we need all that (as other evidence suggests Megaflex is more efficient than 120 w per kg) but that is just a thought experiment that might be helpful to you.

Antius wrote:

Here is a link to the tech spec for Alta Devices thin-film PV. This is a California based company that specializes in producing lightweight PV cells for applications like electric aircraft.
http://www.altadevices.com/wp-content/u … -brief.pdf
Take a look at the 5x cell module. This has an open circuit voltage of 5.47 volts. Under normal circumstances, no one would produce a panel with an output voltage so low. You cannot use voltage that low. I would strongly suspect that the reason for this is the simple fact that a higher voltage (i.e. putting more cells in series) would fry the life out of PV films that thin.

louis · 2017-06-12 16:53:14

1. See my response above.

2. The SP 100 doesn't exist and was discontinued as a project decades ago. I don't think you should be quoting it.

3. Why would a solar energy system that doesn't make propellant (my proposal) be using 75% of energy for storage. It makes much more sense just to do a lot of the work during the day (make oxygen, heat water and so on). Your figure play is getting a bit desperate.

4. My assumption has been that when people quote a watts per kg figure, that is the average figure. 9.4/kg would be an absurdly low figure for Megflex on Mars as an average. Can you give a link to justify that figure?

5. Please note that we are not now talking about 10,000 sq metre system using v. low efficiency PV if we are referencing Megaflex. Probably 1800 sq. metres of Megaflex will be sufficient. Four double arrays, no more than 20 metres from the hab.

Antius wrote:

kbd512 wrote:
The 11.4kg/kW figure comes from NASA, but you'll see it all over the place in space power distribution systems design. This has traditionally been a relatively accurate estimate of what most PMAD systems for high-output applications end up weighing. IIRC, the 7kg/kW figure I used previously came from the PMAD for the DS-1 probe. However, that was a much lower output application.
88W/kg is still impressive. On Mars that would still give power output of 26.5W/kg for the panels in full sun and 9.4W/kg on a time averaged basis.
If a 33% efficiency is assumed for LOX-hydrogen electrolysis/ fuel cell energy storage, and 75% of energy is stored, then effective power density of the panels drops to 3.76W/kg. If the energy storage mechanism is 200Whr/kg, then total mass required is 9000kg. The total mass of the panels is 26,596kg. So total system mass is 35,596kg. If the panel mass is halved, them total system mass is 22,298kg, or 4.5W/kg.
By comparison, the SP-100 with stirling cycle would yield 19W/kg. But the SP-100 would suck up a billion dollars of development funds and would face endless regulatory project delays. So maybe that extra 25 tonnes of power supply weight for a solar power system is something we can live with. What matters is total project cost after all.

kbd512 · 2017-06-12 17:06:30

Antius and Louis,

What you two seem to be discussing is how Ohm's Law applies to an electrical circuit. You want to maximize pressure (voltage) and minimize flow (current), to minimize resistive losses (power or watts lost to resistance) in the conductor (copper wire). If you step the voltage up to 24V from 12V, the mass of the copper required to carry a given amount of current goes down because losses from resistance are lower.

This concept is commonly applied in aircraft electrical power systems. Jet aircraft typically use 24V+ electrical buses, whereas general aviation aircraft typically still use 12V electrical buses. As a function of all the wiring for the electronics in the jet, lower voltages would require a greater mass of copper and thus mass (this concept applies to any other electrical power application, for that matter) to carry the same current.

SpaceNut · 2017-06-12 17:23:41

Under estimating at just 10% the value of tracking for output power level an hours that it would collect...

http://www.tapthesun.com/PDF/PSTC%20-%2 … arison.pdf

pg 13 shows just how much more power over the course of a year for the flat seasonal laying on the ground in yellow. draw the imaginary line on the left scale at 10 kwhr/day as that is the 70 percent power line that would be charging the batteries and would be of the working voltage under load. Notice the key on the side for the others to which just about all of them are out performing with simple fixed tilt to using single and double axis tilting or tracking systems.

pg14

The results fall into three categories of
output levels:
•
Fixed flat mounting providing just
over 60% of available power
•
Single-axis horizontal tracking,
adjustable and fixed racks all
hovering in the 75-78% range
•
Single-axis vertical- and polar-
mounted units that offer over 95%
of the available power.

kbd512 · 2017-06-12 19:08:43

Here's the product data sheet for the SolAero (formerly EMCORE) ZTJ solar cells (real space flight qualified PV cells flown on 150+ missions):

ZTJ-Datasheet-Updated-2016.pdf

ZTJM-Datasheet.pdf

The list of data sheets for SolAero products may be found here:

Space Solar Cells / Coverglass Interconnected Cells (CIC)

SolAero is the company under contract from NASA to produce HIHT, LILT, and other forms of PV cell technology for real missions.

ZTJ will power the Mars Insight lander and actually does power the Cygnus spacecraft. After the products (wafers) are delivered to Orbital ATK, ATK ensures that the cells and entire array system meet the quality control specifications dictated by mission requirements. Although SolAero also designs and builds complete PV arrays, ATK is the systems integrator selected by NASA.

Incidentally, SolAero also manufactures 39% efficient arrays for concentrated illumination terrestrial applications. To my knowledge, those arrays are not qualified for space flight.

Incidentally, there is an equation for determining general performance of a PV array:

How to calculate the annual solar energy output of a photovoltaic system?

louis · 2017-06-13 03:02:22

These are earth-based calculations. I have read that on Mars, because of the lack of precipitation, there is much more dust in the atmosphere, which creates much more ambient light...which means the insolation is a lot less directional and so sun tracking produces far less gain. I am not sure tracking - with all the attendant mechanical and deployment issues - would be of value for a Mars Mission if it was saving you mass of say 300 kgs or even 600 kgs out of a 4 tonne solar panel and equipment allowance. Not worth the labour and project effort involved in providing the tracking.

SpaceNut wrote:

Under estimating at just 10% the value of tracking for output power level an hours that it would collect...
http://www.tapthesun.com/PDF/PSTC%20-%2 … arison.pdf
pg 13 shows just how much more power over the course of a year for the flat seasonal laying on the ground in yellow. draw the imaginary line on the left scale at 10 kwhr/day as that is the 70 percent power line that would be charging the batteries and would be of the working voltage under load. Notice the key on the side for the others to which just about all of them are out performing with simple fixed tilt to using single and double axis tilting or tracking systems.
pg14
The results fall into three categories of
output levels:
•
Fixed flat mounting providing just
over 60% of available power
•
Single-axis horizontal tracking,
adjustable and fixed racks all
hovering in the 75-78% range
•
Single-axis vertical- and polar-
mounted units that offer over 95%
of the available power.

louis · 2017-06-13 03:09:46

Fine and dandy kbd...I won't even pretend to understand all that! I have a vague appreciation that, the more power the bigger cable...and the less efficient the solar panel, the more cabling is involved...

But what does that all mean...what would be your guesstimate for the amount of additional equipment and cabling required if we were using say four Megaflex units with 8 x 17 metre diameter panelling giving about 450 sq. metres of panelling? I went with a guesstimate of 1/3 additional mass based on Earth systems experience. But of course, on Mars we can use the lightest possible materials...

kbd512 wrote:

Antius and Louis,
What you two seem to be discussing is how Ohm's Law applies to an electrical circuit. You want to maximize pressure (voltage) and minimize flow (current), to minimize resistive losses (power or watts lost to resistance) in the conductor (copper wire). If you step the voltage up to 24V from 12V, the mass of the copper required to carry a given amount of current goes down because losses from resistance are lower.
This concept is commonly applied in aircraft electrical power systems. Jet aircraft typically use 24V+ electrical buses, whereas general aviation aircraft typically still use 12V electrical buses. As a function of all the wiring for the electronics in the jet, lower voltages would require a greater mass of copper and thus mass (this concept applies to any other electrical power application, for that matter) to carry the same current.

Antius · 2017-06-13 06:09:29

For ultraflex / megaflex, specific power is listed as 150W/kg at 1AU, according to this JPL link.

http://www.lpi.usra.edu/opag/meetings/a … uchamp.pdf.

At Mars, sunlight intensity is 0.43x what it is at 1AU. If atmospheric transmittance is 0.7, and time averaged powr is (root-2)/4 times full sun intensity. That gives a time-average power of 16W/kg.

I am assuming that the goal is to provide 24/7 power, as this is needed for life support and is highly desirable for all functions. This requires that somewhere between 67-75% of power consumed is stored, as the sun is only above the horizon 50% of the time and solar irradiation will follow a sine function of intensity during daytime hours. Lets be generous and assume only 67% of power needs to be stored. If storage efficiency is 33%, then 14,675kg of cells are needed to supply a constant power of 100KWe.

The energy density of hydrogen-oxygen storage system is taken to be 250Wh/kg. Some 1800KWh must be stored. So storage system mass is 7200kg. Total system mass is therefore 21,875kg. That's a mass power density of 4.57W/kg. That is a little less than one quarter of the mass power density of the SP-100 with stirling power conversion. In this configuration, SP-100 is a less than optimal system as it must operate at part power. Perhaps Louis would like to check my calculations?

On the subject of SP-100: SP-100 was cancelled largely because there was no foreseeable need for it in the late 90s, as NASA had no realistic humans to Mars or moon programme. The Clinton administration were also keen to exterminate all of the US advanced nuclear projects for political reasons. The integral fast reactor programme was eliminated by the same people. It was close to commercialisation at that point.

If we needed such a reactor, the programme could presumably pick up where it was left off. How much it would cost to develop is a subject of some debate. That said, SP-100 was a thing of the 80s and 90s. We could probably improve upon it today, as there have been some advancements since then.

Last edited by Antius (2017-06-13 07:04:06)

louis · 2017-06-13 08:03:53

I think we need to look into that Megaflex figure more - this comes back to the other thread I just started about what w/kg means.

Not sure of the provenance of this graph but it seems to suggest a figure of over 2 kgs per square metre for Megaflex...

https://2.bp.blogspot.com/-VFpo6AoMFFI/ … gaflex.png

Over 2 kgs per square metre is hardly ultralight...are they including the whole caboodle - all the power equipment as well? However, if that figure is correct then it would roughly approximate to 150 w/kg at full power at 1AU.

What should be fairly straightforward is not proving so!

Where did you get the atmospheric transmittance figure from?

I think your figure of 16w/.kg must be too low. Will explain that further.

Antius wrote:

For ultraflex / megaflex, specific power is listed as 150W/kg at 1AU, according to this JPL link.
http://www.lpi.usra.edu/opag/meetings/a … uchamp.pdf.
At Mars, sunlight intensity is 0.43x what it is at 1AU. If atmospheric transmittance is 0.7, and time averaged powr is (root-2)/4 times full sun intensity. That gives a time-average power of 16W/kg.
I am assuming that the goal is to provide 24/7 power, as this is needed for life support and is highly desirable for all functions. This requires that somewhere between 67-75% of power consumed is stored, as the sun is only above the horizon 50% of the time and solar irradiation will follow a sine function of intensity during daytime hours. Lets be generous and assume only 67% of power needs to be stored. If storage efficiency is 33%, then 14,675kg of cells are needed to supply a constant power of 100KWe.
The energy density of hydrogen-oxygen storage system is taken to be 250Wh/kg. Some 1800KWh must be stored. So storage system mass is 7200kg. Total system mass is therefore 21,875kg. That's a mass power density of 4.57W/kg. That is a little less than one quarter of the mass power density of the SP-100 with stirling power conversion. In this configuration, SP-100 is a less than optimal system as it must operate at part power. Perhaps Louis would like to check my calculations?
On the subject of SP-100: SP-100 was cancelled largely because there was no foreseeable need for it in the late 90s, as NASA had no realistic humans to Mars or moon programme. The Clinton administration were also keen to exterminate all of the US advanced nuclear projects for political reasons. The integral fast reactor programme was eliminated by the same people. It was close to commercialisation at that point.
If we needed such a reactor, the programme could presumably pick up where it was left off. How much it would cost to develop is a subject of some debate. That said, SP-100 was a thing of the 80s and 90s. We could probably improve upon it today, as there have been some advancements since then.

Oldfart1939 · 2017-06-13 08:37:36

This will probably be my last comment on this thread, as I've been sitting back watching where this is all going.

The topic has devolved into detail obfuscation, that is, blinding from view the overall complexity of what Louis is promoting. It's exactly this sort of technological complexity which contributed to the stillborn 90 Day Plan! I see it as being overly complicated with far too many components, all of which must operate for a minimum of 3 years without failure or hands-on maintenance. On one hand, a cost-plus contractor would probably love it, but the Mars-nauts on the sharp end of the stick would be cursing the complexity. On the other hand, it violates my basic scientific principle of KISS; Keep. It. Simple. Stupid! What Louis has suggested in order to keep Mars free from nuclear power, he's contrived this somewhat elegant-appearing system of massive solar panel arrays, methane storage, LOX production and storage (all involving a battery of mechanical devices subject to wear out and breakdown), all for a fanciful vision of nuke avoidance.

What really needs done here is setting up an Excel Balance sheet, with everything required from both systems with both the credits on one side and the debits on the other. This means adding up ALL the assets required on one side versus the assets of the other; then do a mass analysis, cost of transporting the system to Mars, include all the engineering expenses, etc. That's exactly how a NASA, SpaceX or Orbital ATK senior administrator would address the issue.

OK, that's my final statement on this thread. Let's get back to discussing HOW we're going to Mars!

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