Optimal solar panel positioning on the surface of Mars

ABSTRACTThis work will calculate the power generated by static solar panels at different elevation angles as a function of location and solar longitude on Mars. The generated power at each latitude will then be compared to the theoretical maximum extractable power from active sun-tracking panels. Solar energy from 200 nm to 2000 nm will be considered using a radiative transfer code that includes atmospheric scattering and gaseous absorption. The results of this analysis will provide information for the optimization of energy generation on future solar-powered Mars surface missions. 1. INTRODUCTIONThe optimization of solar power generation on Mars is important for the planning of future surface missions. Given the low wind and lack of geothermal energy on the surface of Mars, solar radiation is the optimal renewable energy source for use on the surface. Difficulty of transport caused by mass budgets and engineering concerns make traditional fuels a poor choice for use on surface missions. While recent missions have been effectively powered by radioisotope thermoelectric generators, solar power is an option which does not depend on the extremely limited available supply of Pu-238 while also providing a longer lasting stable power source for long duration missions. Our research aims to determine the optimal angular position of static solar panels for various latitudes and compare the loss in available energy compared to using a more complex sun-tracking panel. This will be accomplished using a 1-dimensional radiative transfer model for solar radiation entering the Martian atmosphere at all combinations of latitude and solar position. Since a previous analysis of solar energy available for use carried out in Delgado-Bonal et al. (2016) concluded that solar panel effectiveness is dominated by incoming solar radiation rather than environmental factors such as temperature differentials, determining optimal solar panel placement is the an important aspect of maximizing available energy for future solar-powered Mars missions. This work will provide currently unavailable data on extractable solar energy for non-horizontal panels at any given location on Mars, allowing power generation to be considered alongside science value during mission site selection. 2. METHODSWe employ the 1-dimensional radiative transfer model previously developed by Moores, Smith and Schuerger (2017) and Smith and Moores (2020) to calculate available solar radiation fluxes at the Martian surface. This model builds upon the doubling and adding code of Griffith et al. (2012), having extended wavelength ranges and absorption parameters adapted to the Martian atmosphere. The model considers radiation from both the sky and surface reflections to compute a full hemispherical angular grid of radiances. Fluxes were calculated for wavelengths from 200nm to 2000nm, which account for approximately 93% of the inbound solar flux. Wavelengths below 200nm experience extremely high CO2 absorption and wavelengths longer than 2000nm only account for 6.2% of total solar energy, making these limits an effective choice for computational efficiency (Smith and Moores, 2020). The model includes Rayleigh scattering for the major atmospheric components CO2, N2, Ar, and O2 alongside absorption from CO2 and O2. Absorption for wavelengths outside of 400-1000nm for CO2 and 650-880nm for O2 were ignored due to negligible contributions (Smith and Moores, 2020). Mie scattering from aerosols was considered across the entire wavelength range, with aerosols modelled as cylindrical particles and the Martian surface modelled as a Hapke surface (Smith and Moores, 2020). Outputs integrated across the whole sky are used to determine the available solar energy as a function of Martian latitude and solar longitude.Directional dependence of solar energy will be calculated from sky and ground radiance map outputs of the radiative transfer model. Calculations will first be performed for a static panel at a variety of angles from the horizontal receiving energy from portions of the sky within its line of sight along with reflections from the surface. Topography will be ignored due to the large variance in possible landing sites. Solar flux data will then be adjusted for solar panel efficiencies at each wavelength band in the 200-2000nm range to determine total available energy. Once static panel calculations are complete, a sun-tracking panel with elevation angle always equal to that of the sun will be tested. Optimal static angles for a given latitude will then be compared to the sun-tracking results. 3. RESULTS AND DISCUSSIONCurrently the horizontal panel total solar flux has been calculated and work is under way to model directional dependence of the flux. Once completed, this work will provide direction as to both the best positioning of solar panels and the gains provided by upgrading to a sun-tracking solar panel. The benefit of the active solar panel can then be weighed against engineering and mass budget costs in the planning of future Mars missions. Since the results will be computed across the entirety of the planet, available energy data will be usable during initial site selection for missions instead of requiring a site-specific analysis after selection. This will be particularly relevant for missions seeking to use solar panels at latitudes far from the equator where horizontal solar panels are much less effective, such as would be the case for small Martian meteorological stations. REFERENCES1. C. L. Smith, J. E. Moores, Icarus. 338, 113497 (2020).2. C. A. Griffith et al., Icarus. 218, 975–988 (2012).3. A. Delgado-Bonal et al., Energy. 102, 550–558 (2016).4. J. E. Moores, C. L. Smith, A. C. Schuerger, Planetary and Space Science. 147, 48–60 (2017).

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