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A Dual Functional Solid Oxide Fuel Cell for Power Generation and Energy Storage
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Combine energy generation and storage to ensure that networks remain robust as more renewable technologies are adopted, urge John P. Lemmon.
In operation, solid oxide fuel cells (SOFCs) 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.
Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm2 active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 °C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300 K h−1, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 °C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm2 cells) delivering 100 Wel at 790 °C (1 kWel L−1 or 0.34 W cm−2) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.
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.
This study is focused on a hybrid fuel cell/gas turbine (FC/GT) system with an atmospheric pressure solid oxide fuel cell (SOFC). The impact of the gas turbine rotational speed on dynamic performance and controllability of a hybrid system is investigated. The transient response of the FC/GT system to perturbations in the power demand has been investigated. Two operational strategies of gas turbines are compared: (1) fixed speed operation, and (2) variable speed operation. For both operation strategies, a wide range of power production is numerically simulated. The results show that variable speed operation is superior for the FC/GT hybrid configuration studied. Variable speed operation allows a 50% turn down in power with no additional balance of plant equipment required. The system efficiency is maintained above 66% for variable speed operation compared to 53% for fixed speed operation with auxiliary combustion.
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