Exergetic analysis of a solar-heated fuel cell system fed by methanol
The present study proposes a combination of solar-powered components (two heaters, an evaporator, and a steam-reformer) with a Proton Exchange Membrane fuel cell to form a powerplant that converts methanol to electricity. The solar radiation heats up the mass flows of methanol-water mixture and air and sustains the endothermic methanol steam-reformer at a sufficient reaction temperature (typically between 220 and 300°C). In order to compare the different types of energy (thermal, chemical, and electrical), an exergetic analysis is applied to the entire system, considering only the useful part of energy that can be converted to work. The effect of the solar radiation intensity and of different operational and geometrical parameters like the total inlet flow rate of methanol-water mixture and the size of the fuel cell on the performance of the entire system is investigated. The results of the exergetic analysis prove that the proposed solar methanol fuel cell system has the potential to be operated with a high efficiency and power density by combining methanol steam-reforming and a PEM fuel cell with solar-powered heating. The chemical exergetic efficiency can be increased by a factor of around 1.6 (e.g. from 36% to 60% for the 1000 W m-2 incoming solar heat flux) by using solar radiation as a heat source instead of any other source (e.g. by burning a chemical fuel). This enhancement of effective exergetic efficiency by using solar power for heating the system amounts to even much higher values of up to above 10 for lower solar heat fluxes and very low flow rates of inlet fuel. The effective exergetic efficiency for the herein presented solar-powered system is significantly higher than any non-solar fuel cell system fed by hydrocarbon or alcoholic fuels. At the same time, an electrical power density per irradiated area of more than 986 W m-2 is obtained for a solar heat flux of 1000 W m-2. Comparable photovoltaic systems would need excessive sunlight-to-electricity efficiencies of more than 98%. Even for decreased intensities of solar radiation as low as 300 W m-2, the system achieves very satisfactory results regarding the solar power density (89.3 W m-2) and the chemical exergetic efficiency (61%). Therefore, the combination of exergy input in form of chemical fuel and solar radiation can be promising to achieve both, a high exergetic efficiency and a high power density per area irradiated by sunlight. © 2010 by ASME.
