THERMODYNAMIC AND KINETIC LIMITATIONS ON THE CONVERSION OF SOLAR ENERGY INTO STORABLE CHEMICAL FREE-ENERGY

Abstract— Energy-storing reactions in a system after light excitation transform the system into a thermo-dynamically metastable state. The effects of the microreversible back-reaction on the net yield of the storage process and on the stability of the products are discussed. It is shown that the inevitable losses of photon energy in the conversion process may be estimated and considered as roughly independent of the threshold frequency of the system. The best attainable conversion efficiency as a function of the threshold absorption wavelength may then be estimated. Rough calculations show an ultimate efficiency of about 20% with the optimum threshold wavelength in the range 600–800 nm.
Photochemical production of fuel, like hydrogen from water, requires that more than one photon of the solar spectrum is used per product molecule formed. The low photon density of sunlight leads to troublesome stability requirements on the intermediates in the reaction mechanism. Some features of consecutive two-photon mechanisms in organized systems and homogenous solutions are discussed.