A review of solar thermochemical processes

Thermodynamic comparison reveals that the processes involving upgrading of carbon feed have energy and exergy efficiency at 50-90% and 46-48% respectively. Multistep thermochemical cycles operating at 900-1200 K have energy efficiency of 34-38%. Metal oxide redox pair based thermochemical cycles operating at 1900-2300 K have energy and exergy efficiencies of 12-32% and 20-36% respectively. Methane reforming and lime production processes have chemical efficiencies of 55% and 35% respectively and have demonstrated better performance than other solar thermochemical processes. A few processes like solar gasification of solid carbon feed have demonstrated lower chemical efficiency of around 10% even at pilot scale. The hydrogen production cost for solar upgrading of fossil fuels is estimated at 3.21-6.10$/kg and is lower than thermochemical cycles at 7.17-19.26$/kg and CSP driven electrolysis at 3.15-10.23$/kg. It is observed that there is limited actual data and significant uncertainty in cost. Under these circumstances, it is recommended that initial screening of processes be done by net energy, material and life cycle analysis.