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I've been struggling to find a definition of what exactly w/kg means in relation to solar panels like Megaflex.
In relation to Megaflex we have seen the w/kg figure mentioned frequently.
It seems it could refer to:-
(a) peak capacity
(b) an average ouput over 24 hours, assuming 12 hours of darkness in every 24, probably based on an orbital satellite.
(c) an average output whilst operational (ie when directly exposed to the sun in orbit at 1AU).
Something Antius posted caused me to query what the figure related to.
Can others help by giving any links or give their opinions.
I had been assuming (b) on a 24 hour night and day cycle. But (a) seems unlikely given the ATK panels are now about 29% efficient and that should mean they are getting 377 watts all the time when facing the Sun at 1AU, so a peak of 150 or even 250 seems unlikely).
However, I am now wondering whether (c) might be the case.
Does anyone have a definitive answer?
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Spectrolab: Solar Panel
This has data for Improved Triple Junction (ITJ), Ultra Triple Junction (UTJ), and neXt Triple Junction (XTJ), but not "XTJ Prime". Notice it provides data for 28°C, Beginning Of Life in W/m². It also provides mass with either 3 mil or 6 mil thick cover slide. Substrate is not included. "Substrate" means the backing the cells are adhered to. Mass is in kg/m². From those numbers you can calculate W/kg. This will give you power produced in full sunlight in Earth orbit. And Beginning Of Life, perpendicular to the Sun. Exposure to radiation will degrade the cell, reducing power produced. You would have to look up the particular cell to see power produced End Of Life, and expected life span. Then extrapolate between BOL and EOL for the fraction of life you want. Or just design for EOL to ensure it will produce enough power.
Spectrolab space products: cells
These guys produced the solar cells for Spirit and Opportunity, so I think it's an appropriate link.
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From what you say, that implies that the ATK w/kg figure relates to when the panel is face on to the sun in space.
Spectrolab: Solar Panel
This has data for Improved Triple Junction (ITJ), Ultra Triple Junction (UTJ), and neXt Triple Junction (XTJ), but not "XTJ Prime". Notice it provides data for 28°C, Beginning Of Life in W/m². It also provides mass with either 3 mil or 6 mil thick cover slide. Substrate is not included. "Substrate" means the backing the cells are adhered to. Mass is in kg/m². From those numbers you can calculate W/kg. This will give you power produced in full sunlight in Earth orbit. And Beginning Of Life, perpendicular to the Sun. Exposure to radiation will degrade the cell, reducing power produced. You would have to look up the particular cell to see power produced End Of Life, and expected life span. Then extrapolate between BOL and EOL for the fraction of life you want. Or just design for EOL to ensure it will produce enough power.
Spectrolab space products: cells
These guys produced the solar cells for Spirit and Opportunity, so I think it's an appropriate link.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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If it's exposed to constant sunlight, peak output is average output.
Use what is abundant and build to last
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The answer will basically determine which Mars mission architecture is practical at this time.
If, as would seem likely, the world's space agency's fail to get their arses into gear and develop a space nuclear reactor, future Mars missions will need to depend on Solar.
If whole-system power density comes in at >15W/kg for a baseload supply (i.e. including storage) then future missions could proceed with Mars Direct style missions using solar power. If whole system mass is closer to 5W/kg, then it makes more sense to build a solar power system at a single base and have all new missions go to that base. Long range expeditions to distant parts of the planet would be dispatched using long range rovers.
Last edited by Antius (2017-06-13 10:31:19)
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I explained what the W/kg figure means in your "Going Solar..." thread, Louis. Unless I read the documentation wrong or missed something, it's the mass figure that corresponds with the maximum output of the PV array when it's pointed directly at the Sun, at 1AU distance, in space.
If you get closer to the Sun, you get more power until you run into the breakdown voltage of the PV cells, thermally degrade the cells, or run into a PMAD limitation, whichever comes first. If you get further from the Sun, you get less power. The inverse square of the flux density determines how much incident radiation strikes a panel with a given area when it's pointed directly at the Sun. Any reflected (off another object like a planet, for example) or concentrated (using a lens) light received would add to the output, up to a point. Any shadow or condition where the panel is not pointed at the Sun would subtract from the output.
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OK, but then I am still a bit surprised because this allegedly "ultra lightweight" panel must weigh about 2.2 Kgs per square metre...odd...
I think this still requires a bit more research. I also think Antius is underestimating the power output on the Mars surface. I am currently looking into that.
I explained what the W/kg figure means in your "Going Solar..." thread, Louis. Unless I read the documentation wrong or missed something, it's the mass figure that corresponds with the maximum output of the PV array when it's pointed directly at the Sun, at 1AU distance, in space.
If you get closer to the Sun, you get more power until you run into the breakdown voltage of the PV cells, thermally degrade the cells, or run into a PMAD limitation, whichever comes first. If you get further from the Sun, you get less power. The inverse square of the flux density determines how much incident radiation strikes a panel with a given area when it's pointed directly at the Sun. Any reflected (off another object like a planet, for example) or concentrated (using a lens) light received would add to the output, up to a point. Any shadow or condition where the panel is not pointed at the Sun would subtract from the output.
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There many parts to solar and having panels of cells is only the beginning to which the question of w/kg would only yield on that part of the total system to make solar useable day and night. Sure you could direct attach some load to the panels but they may not function as intended depending on what you are providing the power too.
Many panels use supercaps to help with that initial current voltage swing caused by the load attached to the cells. that inital for a battery storage is a charging unit that conditions the current going into the batteries which does not stay steady during the charging cycle. So that gets a 2nd and 3rd number being created by the next partsof the solar panel to battery values for use in w/kg again to which the panels in the 1st is different.
Sure we could direct connect to the batteries but thats only good for a direct current device so most AC devices would no function partially or at all. Not to mention the batteries would drain fairly quickly. That said we would make use of dc to ac invertors and convertors to be able to provide the correct voltages to the loads that we would attach. So now we have another the 4th w/kg value when solving to what we can do with solar panels.
Value of solar energy getting to mars will not only depend on the season but whether it is near or far from the sun on its eliptical trial around the sun. Which is all the more reason to have the panels tracking to get the most out of them even at the lowest level of solar getting to mars surface.
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You do have to provide voltage and frequency control for any electrical system apart from simple self contained units (garden light type), where the batteries are connected to a solar panel just through a diode and there is only one load. A similar, unregulated power consumer is a little Peltier fan used to circulate hot air. These don't even have a battery.
The regulators, rectifiers, inverters and or transformers, along with switchgear, busbars and cabling would apply to any scheme using electrical power.
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I understand that and made an allowance for such equipment being one third of the panels's mass in my solar energy proposal, which seemed to match experience on Earth.
But w/kg figure for Megaflex is normally given in relation to the panels, not the overall system. The problem would appear to be that there is not available a proper breakdown of the mass of the Megaflex into its constituent parts.
You do have to provide voltage and frequency control for any electrical system apart from simple self contained units (garden light type), where the batteries are connected to a solar panel just through a diode and there is only one load. A similar, unregulated power consumer is a little Peltier fan used to circulate hot air. These don't even have a battery.
The regulators, rectifiers, inverters and or transformers, along with switchgear, busbars and cabling would apply to any scheme using electrical power.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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