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The penetration of intermittent renewable energies requires the development of energy storage technologies. High temperature electrolysis using solid oxide electrolyser cells (SOECs) as a potential energy storage technology, provides the prospect of a cost-effective and energy efficient route to clean hydrogen production. The development of optimal control strategies when SOEC systems are coupled with intermittent renewable energies is discussed. Hydrogen production is examined in relation to energy consumption. Control strategies considered include maximizing hydrogen production, minimizing SOEC energy consumption and minimizing compressor energy consumption. Optimal control trajectories of the operating variables over a given period of time show feasible control for the chosen situations. Temperature control of the SOEC stack is ensured via constraints on the overall temperature difference across the cell and the local temperature gradient within the SOEC stack, to link materials properties with system performance; these constraints are successfully managed. The relative merits of the optimal control strategies are analyzed.

Porous composite electrodes play a critical role in determining the performance and durability of solid oxide fuel cells, which are now emerging as a high efficiency, low emission energy conversion technology for a wide range of applications.In this paper we present work to combine experimental electrochemical and microstructural characterisation with electrochemical simulation to characterise a porous solid oxide fuel cell anode. Using a standard, electrolyte supported, screen printed Ni-YSZ anode, electrochemical impedance spectroscopy has been conducted in a symmetrical cell configuration. The electrode microstructure has been characterised using FIB tomography and the resulting microstructure has been used as the basis for electrochemical simulation. The outputs from this simulation have in turn been compared to the results of the electrochemical experiments.A sample of an SOFC anode of 6.68 mu m x 5.04 mu m x 1.50 mu m in size was imaged in three dimensions using FIB tomography and the total triple phase boundary density was found to be 13 mu m(-2). The extracted length-specific exchange current for hydrogen oxidation (97% H-2, 3% H2O) at a Ni-YSZ anode was found to be 0.94 x 10(-10), 2.14 x 10(-10), and 12.2 x 10(-10) A mu m(-1) at 800, 900 and 1000 degrees C, respectively, consistent with equivalent literature data for length-specific exchange currents for hydrogen at geometrically defined nickel electrodes on YSZ electrolytes. (C) 2010 Elsevier B.V. All rights reserved.

Solid oxide fuel cell (SOFC) based micro combined heat and power (micro-CHP) systems exhibit fundamentally different characteristics from other common micro-CHP technologies. Of particular relevance to this article is that they have a low heat-to-power ratio and may benefit from avoidance of thermal cycling. Existing patterns of residential heat demand in the UK, often characterised by morning and evening heating periods, do not necessarily complement the characteristics of SOFC based micro-CHP in an economic and technical sense because of difficulties in responding to large rapid heat demands (low heat-to-power ratio) and preference for continuous operation (avoidance of thermal cycling). In order to investigate modes of heat delivery that complement SOFC based micro-CHP a number of different heat demand profiles for a typical UK residential dwelling are considered along with a detailed model of SOFC based micro-CHP technical characteristics. Economic and environmental outcomes are modelled for each heat demand profile. A thermal energy store is then added to the analysis and comment is made on changes in economic and environmental parameters, and on the constraints of this option. We find that SOFC-based micro-CHP is best suited to slow space heating demands, where the heating system is on constantly during virtually all of the winter period. Thermal energy storage is less useful where heat demands are slow, but is better suited to cases where decoupling of heat demand and heat supply can result in efficiencies.

Solid oxide fuel cell combined heat and power (CHP) is a promising technology to serve electricity and heat demands. In order to analyse the potential of the technology, a detailed techno-economic energy-cost minimisation model of a micro-CHP system is developed drawing on steady-state and dynamic SOFC stack models and power converter design. This model is applied it to identify minimum costs and optimum stack capacities under various current density change constraints. Firstly, a characterisation of the system electrical efficiency is developed through the combination of stack efficiency profiles and power converter efficiency profiles. Optimisation model constraints are then developed, including a limitation in the change of current density (A cm−2) per minute in the stack. The optimisation model is then presented and further expanded to account for the inability of a stack to respond instantaneously to load changes, resulting in a penalty function being applied to the objective function proportional to the size of load changes being serviced by the stack. Finally, the optimisation model is applied to examine the relative importance, in terms of minimum cost and optimum stack maximum electrical power output capacity, of the limitation on rate of current density change for a UK residential micro-CHP application. It is found that constraints on the rate of change in current density are not an important design parameter from an economic perspective.

In operation, solid oxide fuel cells ( SOFC s) can be subjected to frequent load changes due to variable power demand. Knowledge of their dynamic behaviour is thus important when looking for suitable control strategies. The present work investigates the open and closed-loop transient response of a co-flow planar anode-supported intermediate-temperature direct internal reforming solid oxide fuel cell to load step-changes. A previously developed dynamic SOFC model, which consists of mass and energy balances and an electrochemical model that relates the fuel and air gas compositions and temperature to voltage, current density, and other relevant fuel cell variables, is used. A master controller that imposes a current density disturbance representing a change in power demand and sets the fuel and air flow rates proportional to that current (keeping the fuel utilisation and air ratio constant) and a typical feedback PID temperature controller that, given the outlet fuel temperature, responds by changing the air ratio around the default set by the master controller, have been implemented. Two distinct control approaches are considered. In the first case, the controller responds to a fixed temperature set-point, while in the second one the set-point is an adjustable parameter that depends on the magnitude of the load change introduced. Open-loop dynamic simulations show that, after a positive/negative load step-change, the overall SOFC temperature increases/decreases and the intermediate period between the disturbance imposed and the new steady-state is characterised by an undershoot/overshoot of the cell potential. Closed-loop simulations when load step-changes from 0.5 to 0.3, 0.4, 0.6, and 0.7 A cm−2 are imposed show that the proposed fixed set-point PID controller can successfully take the outlet fuel temperature to the desired set-point. However, it is also shown that for load changes of higher magnitude, an adjustable set-point control strategy is more effective in avoiding oscillatory control action, which can often lead to operation failure, as well as in preventing potentially damaging temperature gradients that can cause excessive stresses within the SOFC components and lead to cell breakdown.

Mathematical modelling is an essential tool for the design of solid oxide fuel cells (SOFCs). The present paper aims to report on the development of a dynamic anode-supported intermediate temperature direct internal reforming planar solid oxide fuel cell stack model, that allows for both co-flow and counter-flow operation. The developed model consists of mass and energy balances, and an electrochemical model that relates the fuel and air gas composition and temperature to voltage, current density, and other relevant fuel cell variables. The electrochemical performance of the cell is analysed for several temperatures and fuel utilisations, by means of the voltage and power density versus current density curves. The steady-state performance of the cell and the impact of changes in fuel and air inlet temperatures, fuel utilisation, average current density, and flow configuration are studied. For a co-flow SOFC operating on a 10% pre-reformed methane fuel mixture with 75% fuel utilisation, inlet fuel and air temperatures of 1023 K, average current density of 0.5 A cm−2, and an air ratio of 8.5, an output voltage of 0.66 V with a power density of 0.33 W cm−2 and a fuel efficiency of 47%, are predicted. It was found that cathode activation overpotentials represent the major source of voltage loss, followed by anode activation overpotentials and ohmic losses. For the same operating conditions, SOFC operation under counter-flow of the fuel and air gas streams has been shown to lead to steep temperature gradients and uneven current density distributions.

Increasing demand for power and the depletion of fossil fuels are providing opportunities for the development of fuel cells as power generating systems. This paper investigates the integration of a solid oxide fuel cell (SOFC) with biomass gasification for the production of power and heat (combined heat and power (CHP) system). A steady-state model was developed in the gPROMS modelling tool to investigate the integrated system. The system was modelled for two different options, a cold process involving gas cleaning at a reduced temperature and a hot process involving gas cleaning at a high temperature. For each option, the model was used to determine the system efficiency and prospective costs. The electrical efficiency and overall system efficiency for the hot process were found to be 23 and 60% and for the cold process the efficiencies were 21 and 34%, respectively. Superior heat management in the gas cleaning stage of the hot process results in its higher system efficiency. The capital cost for the hot process appears higher than that for the cold process. This differential capital cost may be justified by the income earned from selling the extra heat produced in the hot process. Conversely, the cold process produces a gas stream with lower levels of impurities than the hot process.