The effect of temperature uncertainty on Proton Exchange Fuel Cell (PEFC) performance

The temperature of operation is a key parameter in determining the performance and durability of a polymer electrolyte fuel cell (PEFC). Controlling temperature and understanding its distribution and dynamic response is vital for effective operation and design of better systems. The sensitivity to temperature means that uncertainty in this parameter leads to variable response and can mask other factors affecting performance. It is important to be able to determine the impact of temperature uncertainly and quantify how much PEFC operation is influenced under different operating conditions. Here, a simple lumped mathematical model is used to describe PEFC performance under temperature uncertainty. An analytical approach gives a measure of the sensitivity of performance to temperature at different nominal operating temperatures and electrical loadings. Whereas a statistical approach, using Monte Carlo stochastic sampling, provides a ’probability map’ of PEFC polarisation behaviour. As such, a polarisation ’area’ or ’band’ is considered as opposed to a polarisation ’curve’. Results show that temperature variation has the greatest effect at higher currents and lower nominal operating temperatures. Thermal imaging of a commercial air-cooled stack is included to illustrate the temporal and spatial temperature variation exerienced in real systems

Combined current and temperature mapping in an air-cooled, open-cathode polymer electrolyte fuel cell under steady-state and dynamic conditions

In situ diagnostic techniques provide a means of understanding the internal workings of fuel cells so that improved designs and operating regimes can be identified. Here, for the first time, a combined current density and temperature distributed measurement system is used to generate an electro-thermal performance map of an air-cooled, air-breathing polymer electrolyte fuel cell stack operating in an air/hydrogen cross-flow configuration. Analysis is performed in low- and high-current regimes and a complex relationship between localised current density, temperature and reactant supply is identified that describes the way in which the system enters limiting performance conditions. Spatiotemporal analysis was carried out to characterise transient operations in dead-ended anode/purge mode which revealed extensive current density and temperature gradients

Effect of temperature uncertainty on polymer electrolyte fuel cell performance

The temperature of operation is a key parameter in determining the performance and durability of a polymer electrolyte fuel cell (PEFC). Controlling temperature and understanding its distribution and dynamic response is vital for effective operation and design of better systems. The sensitivity to temperature means that uncertainty in this parameter leads to variable response and can mask other factors affecting performance. It is important to be able to determine the impact of temperature uncertainly and quantify how much PEFC operation is influenced under different operating conditions. Here, a simple lumped mathematical model is used to describe PEFC performance under temperature uncertainty. An analytical approach gives a measure of the sensitivity of performance to temperature at different nominal operating temperatures and electrical loadings. Whereas a statistical approach, using Monte Carlo stochastic sampling, provides a ‘probability map’ of PEFC polarisation behaviour. As such, a polarisation ‘area’ or ‘band’ is considered as opposed to a polarisation ‘curve’. Results show that temperature variation has the greatest effect at higher currents and lower nominal operating temperatures. Thermal imaging of a commercial air-cooled stack is included to illustrate the temporal and spatial temperature variation experienced in real systems

Simultaneous thermal and visual imaging of liquid water of the PEM fuel cell flow channels

Water flooding and membrane dry-out are two major issues that could be very detrimental to the performance and/or durability of the proton exchange membrane (PEM) fuel cells. The above two phenomena are well-related to the distributions of and the interaction between the water saturation and temperature within the membrane electrode assembly (MEA). To obtain further insights into the relation between water saturation and temperature, the distributions of liquid water and temperature within a transparent PEM fuel cell have been imaged using high-resolution digital and thermal cameras. A parametric study, in which the air flow rate has been incrementally changed, has been conducted to explore the viability of the proposed experimental procedure to correlate the relation between the distribution of liquid water and temperature along the MEA of the fuel cell. The results have shown that, for the investigated fuel cell, more liquid water and more uniform temperature distribution along MEA at the cathode side are obtained as the air flow rate decreases. Further, the fuel cell performance was found to increase with decreasing air flow rate. All the above results have been discussed

Secure and Robust Compressed-Domain Video Watermarking for H.264

The objective of this thesis is to present a robust watermarking algorithm for H.264 and to address challenges in compressed-domain video watermarking. To embed a perceptually invisible watermark in highly compressed H.264 video, we use a human visual model. We extend Watson's human visual model developed for 8x8 DCT block to the 4x4 block used in H.264. In addition, we use P-frames to increase the watermark payload. The challenge in embedding the watermark in P-frames is that the video bit rate can increase significantly. By using the structure of the encoder, we significantly reduce the increase in video bit rate due to watermarking. Our method also exploits both temporal and texture masking. We build a theoretical framework for watermark detection using a likelihood ratio test. This framework is used to develop two different video watermark detection algorithms; one detects the watermark only from watermarked coefficients and one detects the watermark from all the ac coefficients in the video. These algorithms can be used in different video watermark detection applications where the detector knows and does not know the precise location of watermarked coefficients. Both watermark detection schemes obtain video watermark detection with controllable detection performance. Furthermore, control of the detector's performance lies completely with the detector and does not place any burden on the watermark embedding system. Therefore, if the video has been attacked, the detector can maintain the same detection performance by using more frames to obtain its detection response. This is not the case with images, since there is a limited number of coefficients that can be watermarked in each image before the watermark is visible.Ph.D.Committee Chair: Mersereau, Russell; Committee Member: Jayant, Nikil; Committee Member: Lanterman, Aaron; Committee Member: McLaughlin, Steven; Committee Member: Rossignac, Jare

Two sides of one tapestry : efficient implementation of the wavelet cryptosystem and a fast correlation attack via LDPC codes

M.S.Faramarz Fekr

Execution backtracking using reverse execution of machine code

Execution backtracking is the process of restoring the state of a program to an arbitrary point earlier in its execution history. It is used to facilitate program debugging. In this thesis, a novel execution backtracking approach is developed and implemented to assist the task of debugging software. The approach is demonstrated for structured C programs and exploits backtracking the program at machine code level. The approach has a lower run-time overhead than the existing approaches. The execution backtracking approach is integrated with a diagnosis test bed that consists of a symbolic debugger, a C cross-compiler, a virtual machine and a symbolic reasoner. A symbolic debugger is used to debug the target executable which is instrumented with a C cross-compiler. Relative advantages of the approach is that it is language independent, it is able to backtrack in presence of pointer operations, and it supports true execution replay and dynamic slicing. A relative disadvantage of the approach is that it is only able to partially restore a program's data state.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat