We’ve done some calculations to estimate the amount of solar power available to our satellite. As you can imagine, it varies greatly with orientation.
There are 8 solar panels, each with 6 solar cells. Each solar cell is 2.277 cm^2 and converts the 1,366 W/m^2 of solar radiation with 27% efficiency. With perfect alignment to the sun, that gives 0.49 W for one of the 8 solar panels. However, perfect alignment is rare and staying perfectly aligned would probably melt the solder holding the solar cells in place. So, we consider several possible orientations in orbit and calculate the second-by-second orientation to the sun.
We consider first a “flat” orientation (aka “bullet” orientation) where the satellite’s nose is toward the sun at one pole, tail to the sun at the other pole, broadside to the sun at the equator, with the satellite long axis aligned to the orbit direction. This orientation takes in 2,042 Joules during its half-orbit facing the sun, for an average of 0.75 W during the half-orbit. Most power comes in near the equator diminishing near the poles.
Next we consider a “radial” orientation where the broadside is to the sun at each pole and the nose to the sun at the equator, always keeping a narrow end pointed toward Earth. This orientation takes in 2,047 J for an average of 0.75 W. No power comes in near the equator with most coming in near the poles.
Last, we consider a “sun seeker” orientation where the broadside is always facing the sun, likely combined with a “BBQ” roll along the long axis for cooling. This orientation brings in 3,208 J, averaging 1.18 W, during its half-orbit. Power is constant throughout.
Conversion of any light reflected from Earth will only increase these numbers. We’ll likely revisit these as we do thermal calculations which must consider Earth visible and IR reflection.
What do these numbers mean for the mission? Basically, with communications and CPU drawing a few watts, it means we can’t run everything at once. We’ll have to run communications infrequently anyway according to international law – 10% duty cycle for transmissions. This takes the 1 W of communication to 0.1 W on average, needing 0.2 W during the sun-side to ensure we’ve banked power for use on the dark side. That leaves 0.15-0.25 W on average available for everything else. We’ll have to design the system to bank energy, waking up for a second or two to check conditions and running experiments only when energy is available. My best estimate right now is 5 minutes of engine operation every 9 hours. We’ll know for sure after we build the onboard power supply.
The full calculations are here: TubeSat Power Estimates