 Solution

Take an explicitly illustrative European annual mean ground-level irradiance of $100\mathrm{W}{\mathrm{m}}^{-2}$, the stated conversion fraction $0.005$, and dry carbohydrate heating value $16\mathrm{MJ}\mathrm{k}{\mathrm{g}}^{-1}$. One square kilometre then producesHence $\boxed{M\sim 10^3\mathrm{tonnes}\mathrm{k}{\mathrm{m}}^{-2}\mathrm{yea}{\mathrm{r}}^{-1}}$, or about $10\mathrm{tonnes}\mathrm{h}{\mathrm{a}}^{-1}\mathrm{yea}{\mathrm{r}}^{-1}$, as a dry-mass order-of-magnitude estimate. The assumed irradiance is not a site-specific value supplied by the paper. The result scales linearly with irradiance and conversion efficiency and inversely with heating value; wet harvested mass would additionally depend on water content.