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A new approach of retrofit design methodology in cogeneration heat and power systems based on constructal theory is presented in this paper. A cogeneration system may consist of different turbines, steam levels and steam generators. The steam demand of each level is determined and should be supplied. The purpose of this paper is to retrofit the existing total site heat and power cogeneration system utilizing the concepts of constructal theory. Developing constructal theory to total site cogeneration systems may lead to divide the total site into different constructs. In this paper the total site cogeneration system will be divided into three constructs: turbines, turbine array between each two levels and steam generators array. Using constructal theory simplifies the total site complex system to a simpler system that can be solved easily by a simple search and sort method. The best configuration of the total site would have the minimum operating cost. Using constructal theory would simplify the optimization procedure of cogeneration systems in addition to reach better conceptual design especially in more sophisticated systems. The methodology is applied to a sophisticated total site heat and power cogeneration system as case study from literatures. The constructal retrofit results 14.1% and 14.3% reduction in operating cost and fuel consumption respectively.

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In the present work, constructal design of annular finned tube has been studied. Geometrical parameters include fin diameter, fin thickness, fin pitch, tube outer diameter, tube length while physical parameters involve pressure drop number, Stanton number, fin-to-air conductivity ratio, and in-tube fluid-to- air conductivity ratio. The aim of this study is to enhance heat transfer by letting the geometrical degrees of freedom to morph. It was observed that at certain flow conditions, there exist optimal geometry and fin number for the finned tube construct in which its thermal resistance is minimum. Fin efficiency and tube-side convective heat transfer coefficient are higher at low pressure drops and Stanton numbers. In these conditions, analytical relationships were proposed to predict optimal heat transfer, optimal fin number and optimal geometry. It was seen that the optimal fin thickness-to-fin pitch ratio is merely dependent on the fin volume fraction; and it rises with the increase in fin volume fraction. Moreover, the optimum fin number is directly proportional to fin spacing – to- fin pitch ratio and inversely proportional to Stanton number. Furthermore, it was seen that in the range of parameters considered in this study, the tube with 3400 fins and aspect ratio of 0.63 has the most heat transfer rate.

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Optimum design and performance improvement of the Heat Recovery Steam Generator (HRSG) have noticeable effects on the thermal efficiency of the combined cycle power plants. Therefore, HRSG must be designed in such way that maximizes the heat recovery and improves the overall performance of the plant.
In this paper, a method for design and optimization of a triple pressure HRSG is proposed. It is shown how to simultaneously optimize the operating and geometric design parameters of the HRSG by using the constructal theory. Considering the minimum total entropy generation as the objective function, the optimum parameters in the HRSG unit are derived by using the genetic algorithm method under the fixed total volume condition. Optimized total volume is derived by converting the exergy destruction to cost of entropy generation in order to compare with the capital cost and the results show that there is a trade-off between them. Also, aspect ratios of the units, the heat transfer area for each component of the HRSG and thermodynamic properties are significant features of the flow configuration inducted by the Constructal design. Furthermore, the effects of changing in the temperature and flow rate of hot gas on the optimal values of the total volume, power and steam production are determined.