DOI: 10.20935/acadenergy8467 ISSN: 2998-3665

Relocating the constraint: the physical limits of solar energy conversion from the solar constant to orbit

Emanuel Mango, Rui F. M. Lobo, Mario J. Pinheiro
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Photovoltaic conversion is bounded at every stage by physics rather than by engineering effort, and each strategy devised to circumvent one bound relocates the constraint rather than removing it. This Perspective follows a single quantitative axis—the cascade from the solar constant to the electricity delivered by a deployed module—and examines what the resulting bounds imply for the areal scale of solar deployment. The extraterrestrial irradiance S 0 = 1361 W/m2 is reduced at ground level by atmospheric transmittance, to ≈950–1090 W/m2 for clear skies at low air mass ( τ ≈ 0.7–0.8) and lower still at higher air mass or turbidity. Detailed balance then caps single-junction conversion at 33.7% at the optimal bandgap of 1.34 eV, with sub-bandgap transmission (∼19%), thermalisation (∼33%) and an entropic voltage deficit (∼14%) accounting for the difference from unity. For silicon the radiative detailed-balance value is ≈33%, which intrinsic Auger recombination and free-carrier absorption lower to 29.4%; record laboratory cells reach ∼28% and commercial modules 20–22%. Relaxing the single-junction assumption raises the ceiling without removing it: multi-junction and tandem stacks are bounded by current matching, and multi-exciton generation by its own detailed-balance limit of ∼44%. Propagating these bounds to the field yields an annual average areal power density of order 2–15 W/m2 for utility-scale plants, roughly two orders of magnitude below the solar constant from which the cascade began, with conversion efficiency accounting for only about a factor of five of that reduction. Finally, space-based solar power is analysed as the limiting case in which atmospheric attenuation and the diurnal cycle are removed entirely. We show that the ground-level power density scales as the square of the transmitter aperture, so that the constraint is not eliminated but converted into a requirement for a kilometre-scale phase-coherent aperture in geostationary orbit. We conclude that the invariant governing solar deployment is areal power density, not conversion efficiency alone.

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