With
complex dynamic nature, Heat Exchanger Networks (HENs) should
be operated successfully throughout the whole time horizon even facing
the stochastic and the time-varying disturbances. In current studies,
overdesigning HENs is a commonly adopted strategy to deal with the
stochastic disturbances and also the flexible design. However, it
is not a good choice to find the tradeoff between the dynamic flexibility
and the total annual cost of HENs. In this paper, a new optimization-based
framework for designing dynamic flexible HENs is presented. The key
idea is to consider the ranges of variations in stream output temperatures
to explore such tradeoff. This allows a HEN to work under the stochastic
and the time-varying disturbances without losing stream temperature
targets while keeping the economically optimal energy integration.
This work begins with the multiple disturbances, and then dynamic
flexibility analysis is employed to determine the Generalized Critical
Operating Points (GCOPs) that are proposed to indicate the bottleneck
of dynamic flexibility. As for each GCOP, the HEN retrofit is performed
for the capability of accommodating the stochastic and the time-varying
disturbances. These are formulated as a superstructure-based Mixed
Integer Nonlinear Programming model with the objective of minimizing
the total annual cost. Three cases are given to demonstrate the application
of the proposed framework. Dynamic simulation and quantitative measures
show the overall economic performance and the capability of accommodating
the multiple disturbances.
Improving mass transfer in gas diffusion layers is critical to achieving high-performance proton-exchange membrane fuel cells (PEMFCs). Leaks through the interface between the gas and the membrane electrode assembly frame have been widely investigated, and the controllability of the cathode gas diffusion has not been achieved in most studies. In this study, we develop a structural parameter to investigate the controllability of the gas diffusion mechanism in the cathode in order to improve upon the design and performance of PEMFCs. This parameter accounts for the cathode gas diffusion layer porosity and carbon loading inside the catalyst layer. It is comprehensively calculated to relax the two segments’ distribution along three directions of the coordinate axis. The experimental and simulation results show that the obtained values of the parameter vary and change during voltage stabilization. According to the results, regardless of the materials in the cathode gas diffusion layer, the same steady-state voltage is obtained when the parameter is fixed. The cell could be controllably operated for a wide range of diffusion layer thicknesses by selecting the optimal parameter.
This paper presents a flexible heat exchanger network (HEN) synthesis methodology to cope with (i) the fluctuations of uncertain process parameters and (ii) the gradually accumulated deposit, which have never been simultaneously considered yet. First, a called all-cycle flexibility analysis method is developed to perform the network evaluation throughout entire operation horizon. Then at Part (I) of the methodology, an optimization-based sequential framework is proposed for the design of flexible HENs. The three major steps are (i) initial HEN synthesis; (ii) all-cycle flexibility analysis; and (iii) network improvement toward qualified flexibility and cost expectation via a mixed-integer nonlinear programming (MINLP) model-based iteration between network optimization and flexibility analysis. Furthermore, the optimization of the cleaning schedule cooperates with that of the heat transfer areas in Part (II), striving for lower network redundancy and total cost. At last, two cases are studied to demonstrate the application of the methodology. Results indicate that the fouling in heat exchangers and the parametric fluctuations in operation act synergistically on the flexibility of HEN. And the flexible HEN solutions with and without periodic cleaning both can be obtained by employing the proposed framework with sufficient flexibility guaranteed.
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