Evaluation of conversion efficiency of light to hydrogen energy by Anabaena variabilis

Cyanobacteria provide an efficient system for producing _H2${\mathrm{H}}_2$ from water using solar energy. The energy conversion efficiency can be defined by the ratio of _H2${\mathrm{H}}_2$ produced to the light energy absorbed. An IR and opalescent plate method was used to measure the light energy absorbed. Since cyanobacteria absorb light in the visible range but not in the infrared range, the net amount of light energy absorbed by the cells can be estimated by measuring the IR and visible light intensities transmitted through the biochamber. A rectangular biochamber was used for measuring the conversion efficiency from light energy to _H2${\mathrm{H}}_2$ energy. A quantum meter and radiometer were used to measure the light intensity transmitted through the chamber. Anabaena variabilis was cultured in a BG11 medium with 3.6 mM NaNO₃${}_3$ and the light intensity was 40–50 μmol/m²/s$\mathrm{\mu }\mathrm{mol}/{\mathrm{m}}^2/\mathrm{s}$ in the growth phase and 120–140 μmol/m²/s$\mathrm{\mu }\mathrm{mol}/{\mathrm{m}}^2/\mathrm{s}$ in the _H2${\mathrm{H}}_2$ production phase. The maximum _H2${\mathrm{H}}_2$ production was 50 ml for 40 h and cell density was 1.2 g/l. The _H2${\mathrm{H}}_2$ production rate was 4.1 ml _H2/g${\mathrm{H}}_2/\mathrm{g}$ dry cell weight/h. Based on the light absorbed in the _H2${\mathrm{H}}_2$ production phase, the energy conversion efficiency from light to _H2${\mathrm{H}}_2$ was 1.5% on average and 3.9% at the maximum. Based on the light energy absorbed in the cell growth and _H2${\mathrm{H}}_2$ production phases, the energy conversion efficiency was 1.1% on average.