Refractive index determination in fuel cells using high-resolution laser heterodyne interferometer Saeed Olyaee*, Mohammad Shams Esfand Abadi, Samaneh Hamedi, Fatemeh Finizadeh Nano-Photonics and Optoelectronics Research Laboratory (NORLab), Faculty of Electrical and Computer Engineering, Shahid Rajaee Teacher Training University (SRTTU), Lavizan 16788-15811, Tehran, Iran article info Article history: Received 20 February 2010 Received in revised form 24 May 2010 Accepted 8 June 2010 Available online 12 August 2010 Keywords: Interferometer Fuel cell Membrane Refractive index Adaptive filter Neural network abstract In this paper, we present an interferometry method for refractive index determination in membranes of fuel cells. This technique is based on the use of an improved laser heterodyne interferometer. The photocurrents of the avalanche photodiodes, resulting from reflected beams of the optical head, are led to the signal conditioner and digital signal processing sections. The optical path difference between the target and reference paths is fixed, and as a result, the phase shift is calculated in terms of the refractive index shift. In addition, nonlinearity of this system is analyzed and modeled with different neural networks and adaptive filter algorithms. For neural networks, the radial basis function (RBF), the multi-layer perceptron (MLP), and the stacked generalization method are simulated. In adaptive filter algorithms, the least mean square (LMS), the normalized least mean square (NLMS), the recursive least squares (RLS), and the affine projection algorithm (APA) are applied. The simulation results indicate that the RLS method is faster and contains minimum mean square error (MSE) compared to the other approaches. Also, comparison between two main approaches shows that the nonline- arity of refractive index determination can be effectively modeled with adaptive filter algorithms. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction The fuel cell is an electrochemical conversion device which converts chemical energy into electricity and heat [1,2]. Fuel cells are among the key technologies that offer clean energy with high conversion efficiency. They are compact, reliable, and clean electrochemical energy conversion devices [3e5]. They are different from electrochemical cell batteries in that they consume reactant from an external source. By contrast, batteries store electrical energy chemically and hence repre- sent a thermodynamically closed system. Fuel cells are used in office buildings, factories, portable electronic devices, transportation, and marine vessels [6e9]. Also, fuel cells are very useful as power sources in remote locations- such as spacecraft, remote weather stations, large parks, rural loca- tions, and in certain military applications. Batteries are not suitable for portable devices like mobile phones and laptops- because they are expensive, heavy weight, and need charging after using a few times [10]. However, two major challenges remain on the path to full commercialization of fuel cell technology: (1) to reduce the cost so it becomes economically competitive with existing power technologies and (2) to * Corresponding author. Tel./fax: þ98 21 22970006. E-mail addresses: s_olyaee@srttu.edu (S. Olyaee), mshams@srttu.edu (M.S. Esfand Abadi), s.hamedi@srttu.edu (S. Hamedi), finizade@ srttu.edu (F. Finizadeh). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 36 (2011) 13255 e13265 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.015