As with many other alternative energy sources, solar thermal reforming suffers from intermittency problems. Tatsuya Kodama and his colleges have come up with a new way to reduce the impact temporary disruptions–such as clouds drifting across the sun–have on the efficiency of a solar thermal reformer. They lined the outer tube of a double-walled tubular reformer with a mixture of Na2CO3– a salt–and MgO. While molten salt has a high heat and latent heat capacity, it conducts heat poorly. Mixing it with Mg– which has high conductivity–allows the heat from the salt to transfer to the methane in the reactor efficiently. The integration of Na2CO3 and MgO into a solar thermal reformer allowed it to continue running at high efficiency–methane conversion of 90% or more–for 22 minutes longer than without the salts. —Tim Fine
Kodama, T., Gokon, N., Inuta, S., Yamashita, S., 2009 Molten-Salt Tubular Absorber/Reformer (MoSTAR) Project: The Thermal Storage Media of Na2CO3–MgO Composite Materials. Journal of Solar Energy Engineering 131.4 041013.
Kodama and colleges at Niigata University’s Department of Chemistry and Chemical Engineering tested the effects of integrating Na2CO3 and MgO into a reformer. The intent of the experiment was to examine the feasibility of this approach for solar thermal reforming. The experiment used a conventional reformer reactor. The tubes were heated until the catalyst bed reached 920ºC. After 100% methane conversion was observed the power to the reactor was intermittently turned on and off to simulate clouds drifting over the sun.
Clouds drifting over the sun cause a greater decrease in the efficiency in solar thermal reforming than in traditional photovoltaic panels. Solar thermal reformers have to re-heat to the temperature necessary to breakdown their feedstock. Using molten Na2CO3 as an energy reservoir can circumvent this problem by providing heat when the solar energy is temporarily disrupted. This study tested double walled tubes lined with Na2CO3, 90% Na2CO3 and 10% MgO, and 80% Na2CO3 and 20% MgO. All three combinations retained heat more efficiently when the power was off than tubes without Na2CO3. The tube containing 90% Na2CO3 and 10% MgO proved most effective at maintaining the reactor temperature after the power supply had been interrupted. Thirty minutes after powering off the reactor, the temperature had dropped to 770ºC: the methane conversion rate had only marginally decreased to 95%. The findings presented in this study could greatly reduce the problems caused by solar radiation in solar reformers, so long as they can be replicated in field tests.