Novel micro solar collector for portable hydrogen production

摘要

A novel concept that utilizes solar thermal energy at a small scale in order to produce a portable power source is presented. Methanol is reformed in a micro-reactor to obtain hydrogen, which can be subsequently used to produce electricity through a proton exchange membrane fuel cell (PEMFC). The methanol reforming process is an endothermic reaction that occurs between 220°C to 300°C. The present work shows that the power required for reaching and maintaining this temperature can be supplied by the free solar radiation to substitute other parasitic heat sources previously used (e.g., electrical heaters or exothermic reactions), thus increasing the overall efficiency of the reforming process.A small flat plate solar collector coated with a solar selective surface and insulated using a vacuum layer is used to absorb the energy from the sun. Due to the low flow rate required for the chemical reaction to take place, it is shown that no concentration of sunrays is necessary to reach the above mentioned range of temperature, allowing for a simpler design. A numerical model was used to prove the feasibility of the solar collector. This model was validated against experimental results obtained using a 60mm by 80mm prototype installed in a vacuum chamber and illuminated with a solar simulator lamp. It is shown that two physical parameters are critical to guarantee the system performance: a) the thermal emittance of the absorbing surface which must be kept below approximately 8% at high temperatures, and b) the vacuum quality of the insulation layer which should not exceed 10^-4 mbar. A further investigation of the heat transfer in a rarefied environment was conducted, to measure the temperature as a function of pressure from a horizontal and a vertical flat plate in the range 10^-5 mbar to 10^3 mbar.From the results obtained in this study, it is envisaged that the solar powered micro-reactor will produce hydrogen with a higher overall efficiency than the present reactors by taking advantage of the solar radiation.