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Research Papers

A Microsolar Collector for Hydrogen Production by Methanol Reforming

[+] Author and Article Information
Raúl Zimmerman, Graham Morrison

School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

Gary Rosengarten

School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australiag.rosengarten@unsw.edu.au

J. Sol. Energy Eng 132(1), 011005 (Nov 09, 2009) (5 pages) doi:10.1115/1.4000354 History: Received January 14, 2009; Revised June 14, 2009; Published November 09, 2009

Proton exchange membrane fuel cells (PEMFCs) are good candidates for portable energy sources with a fast response to load changes, while being compact as a result of their capability to provide a high power density. Hydrogen constitutes the fuel for the PEMFC and can be obtained in situ to avoid transportation and safety problems. An efficient method to produce hydrogen is by methanol steam reforming in a microreactor, an endothermic reaction for which the highest efficiency occurs between 250°C and 300°C. Different methods have been used to reach and maintain these temperatures including electrical heaters and exothermic reactions. We propose to use solar energy to increase the efficiency of the microreactor while taking advantage of a free renewable energy source. The microchannels, where the water-methanol mixture flows, are insulated from the surroundings by a thin vacuum layer coated with a selective material. This coating has a high absorptance for short wavelength incoming radiation and low emittance for infrared radiation, reducing the heat losses. By using these coated insulation layers, the fluid temperature in the microchannels is predicted to be higher than 250°C. Hence, it is expected that the solar-powered microreactor will produce hydrogen with a higher overall efficiency than the present reactors by taking advantage of the solar radiation.

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Figures

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Figure 1

Schematic diagram of a solar-powered fuel cell

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Figure 2

Schematic of flat-plate microsolar collector chip with vacuum insulation layers

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Figure 3

2D schematic of microsolar collector thermal model

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Figure 4

Theoretical stagnation temperature (°C) of the collector, exposed to qsun″=1000 W/m2 and surrounding temperature T∞=25°C as a function of its solar absorptance and thermal emittance (assuming he=10 W/m2 K, τg=0.92, ρg=0.08, εg=0.9, εi=0.02, and εb=0.2)

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Figure 5

Stagnation temperature (°C) of the collector, calculated with a two-dimensional radiation model (assuming T∞=25°C, he=10 W/m2 K, τg=0.92, ρg=0.08, εg=0.9, εs=0.06, αs=0.92, εi=0.02, and εb=0.2)

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Figure 6

Theoretical stagnation temperature (°C) of the collector, exposed to a surrounding temperature T∞=25°C as a function of solar heat flux and the convective heat transfer coefficient (assuming τg=0.92, ρg=0.08, εg=0.9, εs=0.06, αs=0.92, εi=0.02, and εb=0.2)

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Figure 7

Water/methanol mixture temperature (°C) as a function of solar heat flux and volumetric flow rate (assuming ΔH0=0, Ti=T∞=25°C, he=10 W/m2 K, τg=0.92, ρg=0.08, εg=0.9, εs=0.06, αs=0.92, εi=0.02, and εb=0.2)

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Figure 8

Thermal efficiency of the collector for different solar heat fluxes and volumetric flow rates (assuming ΔH0=0, Ti=T∞=25°C, he=10 W/m2 K, τg=0.92, ρg=0.08, εg=0.9, εs=0.06, αs=0.92, εi=0.02, and εb=0.2). The shaded region corresponds to the range of fluid temperature required for methanol reforming.

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