Fig 1: Comparison of estimated energy losses in photosynthesis (upper green boxes) and photovoltaics (lower blue boxes) based on published data [12, 15, 21]. The efficiency of each individual step is shown in each box. The cumulative efficiency of the entire process is shown in the box below each process. Biological primary photon capture has an upper efficiency bound of 26%  but in practice rarely exceeds 11% .
By contrast, the efficiency of hydrogen production by photovoltaic driven electrolysis is ≈ 14% . A breakdown of the losses in these processes are shown in figure 1. It is estimated that for many plants, light harvesting saturates at only ≈ 13% of the incoming solar flux . Moreover, biological CO2 fixation is limited by the low catalytic rate of RuBisCO. This carboxylating enzyme appears to be already naturally optimized, with little room for improvement -.
The inefficiency of photosynthesis sets the stage for competition between land for agriculture and wilderness and land for energy crops . In order to capture just 1 TW of solar energy (< 1/3rd of US energy use ) at an efficiency of 1%, an area equal to 0.28 times the US cropland would need to be covered with energy crops . Is there a better way to harness solar energy?
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