Design of a Pilot SOFC System for the Combined Production of Hydrogen and Electricity under Refueling Station Requirements

The objective of the current work is to support the design of a pilot hydrogen and electricity producing plant that uses natural gas (or biomethane) as raw material, as a transition option towards a 100% renewable transportation system. The plant, with a solid oxide fuel cell (SOFC) as principal technology, is intended to be the main unit of an electric vehicle station. The refueling station has to work at different operation periods characterized by the hydrogen demand and the electricity needed for supply and self-consumption. The same set of heat exchangers has to satisfy the heating and cooling needs of the different operation periods. In order to optimize the operating variables of the pilot plant and to provide the best heat exchanger network, the applied methodology follows a systematic procedure for multi-objective, i.e. maximum plant efficiency and minimum number of heat exchanger matches, and multi-period optimization. The solving strategy combines process flow modeling in steady state, superstructure-based mathematical programming and the use of an evolutionary-based algorithm for optimization. The results show that the plant can reach a daily weighted efficiency exceeding 60%, up to 80% when considering heat utilization.

Small scale SOFC systems

Nordahl Autissier

The increasing cost of carbon based fuels pushes the electricity generation market towards more efficient electricity generation technologies. In this context, fuel cells offer the opportunity to increase efficiency and lower CO2 and NOx emissions, in particular with co-generation units. The complexity, as well as the number and different combinations of components involved with a SOFC stack make their analysis impossible without dedicated tools. The simultaneous consideration of economic and thermodynamic considerations is a key challenge for engineers and decision makers. The present work contributes to the development of design tools for fuel cells and energy systems. A detailed model of fuel cell stack and a model of a complete system were developed. Based on a commercial CFD code, additional routines were implemented to model the electro-chemical behavior of a fuel cell as well as the reforming reaction schemes. The model is able to predict cell behavior under various fuel feeds. The detailed 3-D model of stack repeat unit allowed to identify weaknesses of a first stack geometry. With the combination of experimental observations and modeling activities, specific problems of the existing concept were identified and a new geometry designed, together with the corresponding stack concept. A new stack design is presented with successful experiments up to 1 kWe. Besides the stack, a co-generation fuel cell system is composed of multiple components such as heat exchangers, reformer, recycle loops... The number, the type and size of those components influence the system costs but also its efficiencies (thermal and electrical). A multi-objective thermo-economic optimization methodology was defined, allowing to optimize at the same time the configuration and operating parameters of a system. This method was applied to two sizes of co-generators : a 1 kW unit and a 30 kW unit. HTceramix SA (Yverdon, Switzerland) is developing part of a co-generation unit. The work indicates the potential improvement of their prototype. The 1 kW unit was optimized in a first phase on thermal and electrical efficiencies, and in a second phase on electrical efficiency and installed costs. Those two results indicate the influence of the objective function selection on the results. The high operating temperature of Solid Oxide Fuel Cells (SOFC) offers good opportunity for coupling with a gas turbine. A simple concept of pressurized SOFC-µGT was studied and optimized, for an net electrical output of 30 kW. The simple layout described in this work allows present technical feasibility of the concept. Moreover, the costs of electricity, lower than 0.20$/kWh, makes it economically viable solutions. The methodology and the tools developed allow the engineer to explore the solution space through optimal configurations, in order to select those fitting with its priorities and technical constraints.

EPFL2008

The role of process integration techniques in the thermo-economic optimization of energy conversion systems

François Maréchal, Laurence Tock

Within the goal of reducing greenhouse gas emissions and supplying sustainable energy, complex integrated energy conversion systems have to be designed by taking into account thermodynamic, economic and environmental considerations simultaneously. The process performance highly depends on the quality of the process integration and on the energy conversion, in particular, on how the heat requirements are balanced and how the combined heat and power generation is integrated. In chemical processes, the heat requirements after heat recovery are typically satisfied by the energy conversion system, while for fuel and electricity producing processes, the requirements are balanced by waste and process streams available on-site. The optimal internal heat recovery and utility integration can be computed by applying heat cascade models that are solved by using mathematical programming techniques. The mass and energy balances of the process unit operations are therefore satisfied by the optimal energy conversion system integration without having to explicitly model the heat exchanger network. The presented thermo-economic optimization approach combines flowsheeting and process integration techniques with economic evaluation and life-cycle assessment in a multi-objective optimization framework. The advantage of including the process integration model in the design process is that the influence of the process design and operation is reflected on the thermo-environomic performance of an energy balanced system in which the heat supplied from energy resources is integrated. The methodology has been successfully applied to study CO2 capture concepts in power plants using natural gas, coal or biomass resources, to analyse thermo-chemical H2 production processes and to evaluate different biofuel processes producing Fischer-Tropsch fuel, methanol, dimethylether and H2 by biomass gasification. The potential performance improvement through process integration is revealed for each case. In particular, it is highlighted how the optimization of the combined cycle valorizing the excess heat and of heat pumping options improve the process efficiency by simultaneously maximizing the combined production of fuel, captured CO2, heat and power. This shows that this computer-aided process design strategy combining flowsheeting, energy integration, cost assessment and multi-objective optimization is a powerful tool to systematically generate different process options, assess trade-offs and reveal process improvements through process integration.

2013

Triple-mode grid-balancing plants via biomass gasification and reversible solid-oxide cell stack: Concept and thermodynamic performance

Chengzhou Li, Tzu-En Lin, François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang, Yumeng Zhang

Biomass-to-electricity or -chemical via power-to-x can be potential flexibility means for future electrical grid with high penetration of variable renewable power. However, biomass-to-electricity will not be dispatched frequently and becomes less economically-beneficial due to low annual operating hours. This issue can be addressed by integrating biomass-to-electricity and -chemical via ‘‘reversible’’ solid-oxide cell stacks to form a triple-mode grid-balancing plant, which could flexibly switch among power generation, power storage and power neutral (with chemical production) modes. This paper investigates the optimal designs of such a plantconcept with a multi-time heat and mass integration platform considering different technology combinationsand multiple objective functions to obtain a variety of design alternatives. The results show that increasing plant efficiencies will increase the total cell area needed for a given biomass feed. The efficiency difference among different technology combinations with the same gasifier type is less than 5% points. The efficiency reaches up to 50%–60% for power generation mode, 72%–76% for power storage mode and 47%–55% for power neutral mode. When penalizing the syngas not converted in the stacks, the optimal plant designs interact with the electrical and gas grids in a limited range. Steam turbine network can recover 0.21–0.24 kW electricity per kW dry biomass energy (lower heating value), corresponding to an efficiency enhancement of up to 20%points. The difference in the amounts of heat transferred in different modes challenges the design of a common heat exchange network.