Silicon Microreformer for the Evaluation of Thermal Integration Issues in Microscale Fuel Processing
Keyur Shah, Chemical, Biomedical, Materials Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030 and Ronald S. Besser, Chemical, Biomedical and Materials Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030.
Fuel cell technology is a promising means of transferring chemical energy to electrical energy. Recently, there has been great interest in the development of microscale fuel cell systems for portable power generation. Miniature fuel cells offer promise for the conversion of chemical energy into electrical energy in a highly efficient way, achieving the goals of light weight and low volume. However, the success of fuel cell technology for portable power depends heavily on development of an efficient way of delivering fuel to the cell. The onboard extraction of hydrogen by processing easily stored, high density liquid hydrocarbons seems to be the most promising method of supplying a high purity hydrogen to fuel cells. Because of size and portability, microreactor technology has shown promising results in the field of onboard fuel processing. Thermal management in miniature systems is perhaps the most crucial challenge for microscale fuel processors. The fuel processor needs effective thermal coupling to allow transfer of energy from the heat producing combustor to the endothermic steam reformer. Coupling endothermic and exothermic components of the fuel processor and minimizing losses are crucial to achieving high thermal efficiency. However, such coupling must be accomplished in a manner that permits the maintenance of specific temperatures in the various components and maintains the surface of the package near room temperature. Microreactors generally offer high heat transfer rates mainly because of high surface-to-volume ratio and short conduction paths. This characteristic results in efficient heat extraction but at the same time results in higher heat losses to the ambient. Therefore, thermal management offers a dual challenge of opposing the heat losses from the system that arise from high surface-to-volume ratio in conjunction with maintaining temperature gradients within the system to allow desired conditions in the unit reaction steps. This work is aimed at understanding this critical issue and develops a knowledge base required to rationally design and thermally integrate the microchemical components of a fuel processor. A silicon microreactor-based catalytic methanol steam reforming reactor is designed, fabricated, and demonstrated in the context of complete thermal integration to directly address the heat management issues. The integrated device is made, where vacuum packaging chips, thin-film heater, and temperature sensors are directly embedded with the microreactor to simulate an integrated steam reformer, reflective of an overall fuel processor designed with a system perspective. Detailed experiments are carried out to quantify heat losses through various pathways from the planar microreactor structure. The result provides fundamental insight in understanding of critical thermal transfer issues of an integrated microreactor system such as transfer of heat between reactor components, control of temperature, insulation, and heat losses. Based on the thermal characterization experiments and understanding gained, suggestions are made for scale up of reactor components and a packaging scheme for reduction of convective and radiative losses. An experimental correlation is proposed to predict natural convection heat transfer coefficient from millimeter to submillimeter scale devices. Acquisition of the experimental data help to verify and improve the model used to simulate thermal behavior of an integrated device. This combined experimental/modeling approach then can be used as the quantitative basis for designing an integrated portable fuel processor.