Thermodynamic Analysis Of Solar Thermochemical Hydrogen Production

The rational and efficient utilization of sustainable,low-carbon and clean energy is an important measure for countries to optimize and transform the energy development structures against the background of the depletion of energy resources,environmental pollution and climate change.As a strategic energy source for renewable and sustainable development,ubiquitous solar energy occupies an important position in energy development and is the most suitable clean energy for energy structure transformation.Hydrogen energy can be used as clean and efficient secondary energy source,flexible and intelligent energy carrier,so it is known as "future energy".The usage of solar energy for hydrogen production,that is,solar hydrogen production,realizes the production of clean energy with clean energy,is in line with the trend of clean energy development,as well as has the potential to meet the world's energy demand.Therefore,solar hydrogen production has attracted extensive attention from governments and scholars.In the context,the paper carried out thermodynamic research on solar thermochemical hydrogen production,aiming to theoretically indicate direction for the future development of solar thermochemical hydrogen production and look for solar hydrogen production technologies that can be applied on large scaleFirstly,the enthalpy change(?H),entropy change(AS),and Gibbs free energy change(?G)of the reactions were used to analyze the selected 10 solar thermochemical hydrogen production processes of 3 types(thermochemical cycle,solar-driven carbonaceous feed,and hydrogen sulfide decomposition).Then,the difficulty of matching the process with solar energy was evaluated according to Tg(temperature at ?G=0)of the endothermic reaction.The results are as follows:for thermochemical cycle,the more the number of reaction steps,the lower the Tg,the easier it is to match with solar energy;for the processes using carbonaceous feed as the hydrogen sources,the difficulty order for matching with solar energy is alkane>alcohol>acid;for the carbonaceous feed having same funtional group,the more C atoms contained in feed,the process more difficult matching with solar energy;the processes using carbonaceous feed(the number of C atom<3)as the hydrogen sources are easier to match with solar energy than the thermochemical cycle(the number of reaction step?3);the decomposition of formic acid is most easily matched with solar energy,and the decomposition of hydrogen sulfide is the most difficult to match with solar energy in all selected processesSecondly,the theoretical energy efficiency of different processes were calculated It was found that the order of energy efficiency from high to low is:solar-driven carbonaceous feed,thermochemical cycle,and hydrogen sulfide decomposition.Among these processes,the decomposition of formic acid shows highest efficiency,the efficiency of sulfur iodine(S-I)cycle is also higherFinally,the hydrogen production cost and exergy efficiency of the above three types and the most mature solar hydrogen production processes were summarized,and the sustainability index(SI)was further calculated according to the exergy efficiency.It showed that the most economical process is solar-driven carbonaceous feed,followed by hydrogen sulfide decomposition,solar energy coupled with water electrolysis is centered,and thermochemical cycle.The SI of solar-driven carbonaceous feed is the highest,the thermochemical cycle and hydrogen sulfide decomposition are centered,and solar energy coupled with water electrolysis is the lowest.