Adaptation of Cu(In, Ga)Se2 photovoltaics for full unbiased photocharge of integrated solar vanadium redox flow batteries

The integration of photovoltaics and vanadium redox flow batteries (VRFBs) is a promising alternative for the direct conversion and storage of solar energy in a single device, considering their inherent higher energy density versus other redox pairs. However, this integration is not seamless unless the photovoltaic system is customized to the voltage needs of the battery, which unlike artificial photosynthesis, continuously increase with the state-of-charge. We have developed an integrated solar VRFB with adapted low-cost Cu(In, Ga)Se-2 modules of 3 and 4 series-connected cells (solar efficiency of mini-solar module 8.1%), and considering the voltage requirements (1.3-1.6 V), we have evaluated the influence of the photovoltaic operation region on the final efficiency of the solar VRFB. Full unbiased photocharge under 1 Sun illumination has been achieved resulting in high energy (77%), solar-to-charge (7.5%) and overall round trip energy conversion efficiencies (5.0%) exceeding the values reported in the literature for other solar VRFBs, thus demonstrating the feasibility and intrinsic potential of adapting low-cost commercial photovoltaics to such energy storage systems.