Solar power and solar cells

At noon, the solar power on Mars is estimated at 430 [W/m2] during spring and summer at latitude lower than 20°N. For the same condition on Earth, we assume 950 [W/m2] during a sunny day. According to the variable position of the sun in the sky, the solar power function can be approximated by a cosine.

To retrieve this energy, solar cells cover the wing (around 0.512 [m2] for the 3.2 m wingspan airplane). In terms of efficiency, the better choice would have led us to GaAs Triple Junction cells with efficiencies of 27-28 [%]. However, those cells have a mass density of around 90 [mg/cm2]. As weight has a major impact on the required power for leveled flight, the better choice is silicon cells (e.g. RWE-32 cells) that reach 16-18 [%] efficiency with only 32 [mg/cm2]. Furthermore, the flexibility of those thin cells will also be an advantage for their integration on the wing.

Based on the characteristic of a single cells, simulation have been achieved to optimize the placement of the cells on the wing. The critical moment is when the sun angle is low because solar cells in the shadow impose a low current to all other cells in the serie.

Finallly, a total of 216 solar cells will be installed on the demonstrator plane. They will be divided in 3 modules connected in parallel, each of them composed of 2 series of 36 cells. The cells will be encapsulated using a mechanically favorable symmetrical laminate combined with a fiber glass reinforced plastic coating. This encapsulation is non-reflective. Thus, we will obtain a flexible arrangement easily integrable on the plane and connectable to the power circuit. At maximum sun conditions, the available power will be 28 [W] for each module, which makes a total of 84 [W].