Development of Dynamic Model and Control Strategy of Combined Cooling, Heating and Power System Primed with Solid Oxide Fuel Cell-Gas Turbine for Building Application


Student thesis: Doctoral Thesis

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Awarding Institution
Award date16 Mar 2018


Combined cooling, heating and power (CCHP) system has been proven to be an energy-efficient approach against separate energy supplies in a conventional way for buildings. Solid oxide fuel cell (SOFC) is a high-temperature electrochemical conversion device with an electrical efficiency over 40% and operating temperature above 900 °C. Gas-turbine (GT), through expanding the exhaust gas from the SOFC stack, can be used as an effective bottoming cycle to further increase electricity production. In this study, new dynamic models and novel control strategy were developed for the SOFC-GT primed CCHP system for building application.

Firstly, since the SOFC stack is the key equipment unit of the SOFC-GT prime mover set, its electrical and thermal performances have significant effects on the whole CCHP system. The SOFC stack consists of a number of repeating SOFC units connected together. In order to have a complete transient analysis and an in-depth performance evaluation of the SOFC stack, a new two-dimensional (2D) dynamic model of the SOFC unit was built using FORTRAN 6.6. After that, the SOFC unit model was further developed to become the SOFC stack. The developed model is suitable for both hydrogen-fed and methane-fed SOFC. In this 2D dynamic model, electrochemical reaction, chemical reaction, flow field, mass transfer and energy transfer sub-models are coupled to determine its distribution of current density, temperature, pressure and gas composition. A key feature of this SOFC unit model is that fluid properties and cell operating variables are temperature-dependent. Through the developed SOFC unit model, its transient performances, like heat-up and start-up processes, can be evaluated accordingly. Meanwhile, general cell performances, such as current density and electrical efficiency, are able to be assessed under different operating and geometric parameters. Therefore, the appropriate design parameters could be chosen and utilized in the subsequent study for system approach.

Secondly, the SOFC-GT CCHP system contains the highly coupled equipment units for cooling, heating and electricity supplies for building application. In order to have a clear appraisal of system performances and obtain dynamic responses between equipment units, a comprehensive model of the entire SOFC-GT CCHP system was established on the dynamic simulation platform TRNSYS 17 and its component library TESS. To make the system performance closely represent the practical implementation, the effects of ambient air conditions and operating loads on the key equipment units, such as SOFC stack, GT, absorption chillers and compression chillers, were accounted. Furthermore, the SOFC-GT CCHP system was connected to an office building, thus the interaction between the various system energy supplies and the changing building demands could be investigated.

Lastly, due to the complex nature of multiple energy supplies of the SOFC-GT CCHP system and multiple energy demands of the building, a new approach, called multi-supply-multi-demand (MSMD) control strategy, was proposed for the system operation. The MSMD control includes two core algorithms: rolling optimization (RO) and feedback correction (FC). RO algorithm is used to determine the operating schedule of energy supply equipment units based on the forecasting weather information of the next 24 hours. FC algorithm is applied for continual mitigation in case any discrepancy between the actual and predicted energy demands. Through the SOFC-GT CCHP system with energy storages for building application, the effectiveness of the MSMD control strategy was tested. It was found that RO algorithm could determine the operating schedule of the related equipment units at lower primary energy consumption than the conventional mean, while FC algorithm could effectively correct the control variables determined by RO algorithm whenever necessary.

There are three major contributions in this study: the 2D dynamic model for the SOFC unit; the comprehensive model for the SOFC-GT CCHP system; and the MSMD control strategy for the complex system operation. The developed 2D dynamic SOFC model can serve as a valuable tool for dynamic simulation and optimization of SOFC design, no matter on the unit or stack level. The established system model can be adopted for other CCHP system configurations through appropriate modification and adjustment, or even be extended to cogeneration and polygeneration. The proposed MSMD control algorithm can be realized and adopted in a building energy management system for practical control and operation of CCHP system. Meanwhile, it can also apply to systems that are characterized by multiple energy supplies and demands, like building complex and smart grid.

    Research areas

  • Solid oxide fuel cell, Combined cooling heating and power, Control strategy