[X] My Assistant

[helvick]

I've been trying to reconcile various crumbs of data regarding the Solar power systems on the MER's and have come to a point where I'm more or less stumped.

I've found the following papers that are repeatedly referenced in the many documents on the Web that evaluate solar power for martian exploration:
"Photovoltaic Array for Martian Surface Power" J. Applebaum, G.A. Landis, 43rd Congress of The International Astronautic Federation August 28-Sep 5 1992.
"Solar Radiation on Mars - Update 1991" J. Applebaum, G.A. Landis. NASA Technical Memorandum 105216

These provide a model for predicting insolation on the Martian surface taking into account Latitude, Tau and Mars orbit (ie LS, declination and distance from the Sun). The latter also gives some a pair of useful models for Tau by date and latitude during a major global storm.

Michael Allisons "Telling Time on Mars" Geophysical Research Letters Vol 24 No 16, Pages 1967-1970, August 15 1997. Gives some handy formulas for calculating time and the relevant orbital elements.

Between the lot I have a model that can replicate the examples provided in a bunch of MER documents (in particular the MER Environmental Requirements Document) and plots out nice daily\annual insolation data for any chosen site.

In order to be a bit more accurate I included the MER-A and MER-B actual Tau values for Sols 1-360 from the MER Analyst Notebook site.

What I want to do is establish what the initial theoretical power output from the panels should have been (to see how accurate the theoretical model is) and for that I need to know the actual surface area of the cells. Rupert Scammels site MER Technical Data says they consist of 55 strings of 20 cells, each measuring 4x2cm (ie 0.88m^2). If the cells are GaInP/GaAs/Ge triple junction cells at ~23.8% efficiency things look a bit low but pretty good (see the chart below). If I look at the rover self portraits though I count 501-505 individual cells and they look bigger than 4x2 to me. I reckon they are 4x6cm which gives a cell area of 1.2m^2, which is a number often listed on NASA sites. Unfortunately that value leads to power values that are waay to high - initial power ~ 1200wHr/sol for Opportunity.

Can anyone give me some definitive data on the efficiency of the cells used and their physical dimensions. Or give me some other explanation for the difference between what my calculations yield and reality.

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[mcaplinger]

http://pdsimg.jpl.nasa.gov/Atlas/MER/documents/insthost.cat says

"The solar panel provides 30 strings of triple junction cells (gallium indium phosphorus, gallium arsenide, and germanium) covering 1.3 square meters, which produced about 800 to 900 W hours per sol at the beginning of the MER mission. Each rover had two reference solar cells, one that measures short circuit current and another that measures open circuit voltage. Due to the change in season from late southern summer to early southern autumn, and the degradation in performance due to dust deposition, the energy produced by this array dropped to about 600 W h per sol, 90 sols after landing."

[helvick]

http://pdsimg.jpl.nasa.gov/Atlas/MER/documents/insthost.cat says

"The solar panel provides 30 strings of triple junction cells (gallium indium phosphorus, gallium arsenide, and germanium) covering 1.3 square meters, which produced about 800 to 900 W hours per sol at the beginning of the MER mission.

I'm still confused. The chart my model produces for Spirt Sol 1 is attached.

My code produces a value for total watts per sq m. If I compare the equivalent charts to the 7 samples I've gotten elsewhere from JPL\Nasa documents my curves match to within about 3% so I'm pretty happy with those as a sanity check now. One of those uses data that is very close to the Spirit at Sol 1 example so the numbers should all tie up.

For Spirit Sol 0, Tau=0.902, Albedo 0.23, Lat -14.57. I get total insolation at the surface of 3779 watt hours/sq m, (1515 beam and 2263 diffuse). Those are Martian watt hours so we need to increase them by a factor of 1.0275 to give us Earth watt hours.

If the actual cell area is 1.21sqm (my best estimate from counting the cells) and the cell efficiency is 23.8% then the total amount of power generated would be 1118 (earth) watt hours. The numbers from JPL above are 20% and 29% lower than that number.

So there must be an efficiency factor that I'm missing. I've been trying to find numbers for the conversion efficiency of the power system which has to be less than 100%. Also the 23.8% efficiency factor of the cells is just the efficiency of the cells themselves and that must be slightly lower on Mars due to the change in spectrum. ESA have an interesting 2002 presentation that gives some idea of the power system efficiency that the MER's should have :Future Space Power Systems. That states that current power management and distribution efficiencies are 80-90%. 80% would do nicely.

Does anyone know what the PMAD efficiency is for MER? I think I can make a stab at the change in cell efficiency due to the spectrum of Martian light on my own.

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[jamescanvin]

If the actual cell area is 1.21sqm (my best estimate from counting the cells) and the cell efficiency is 23.8% then the total amount of power generated would be 1118 (earth) watt hours. The numbers from JPL above are 20% and 29% lower than that number.

When I was thinking about this problem before getting up this morning (I don't know what that says about me!) the only thing I could come up with would be shadowing from all the stuff on the rover deck, particuarly the PMA. I'm sure it's not 20% but it probably makes a significant difference to the total power generated by the arrays.

Cheers,

James.