a b s t r a c tHybrid fuel cell gas turbine sensitivity to ambient perturbations is analyzed using experimental and dynamic simulation results. Experimental data gathered from the world's first pressurized hybrid SOFC-GT system tested at the University of California, Irvine, capture performance variations due to diurnal temperature oscillations. A dynamic modeling methodology demonstrates accuracy, robustness, and clearly identifies critical system sensitivities that require additional control systems development. Simulation results compare favorably with dynamic experimental responses. Predictions of component temperatures, pressures, voltage and system power exhibited 5• C, 2 kPa, 2 mV, and 0.5% error respectively. Moderate ambient temperature fluctuations, 15• C, caused variations in stack temperature of 30 • C, and system power of 5 kW. Small to moderate changes in fuel composition produced 30• C shifts in stack temperature and 25% changes in system power. Simple control loops manipulating fuel cell air flow through SOFC bypass and inlet temperature through recuperator bypass are shown to effectively mitigate internal temperature transients at the expense of reduced system output. The observed temperature fluctuations resulting from typical environmental perturbations are of concern for performance loss and diminished longevity. Experiments and dynamic simulation results indicate the importance of integrated control systems development for hybrid fuel cell gas turbine systems.

A high performance multi-input multi-output feedback controller has been developed to minimize solid oxide fuel cell (SOFC) spatial temperature variation during load following. Cathode flow rate and its inlet temperature are used to minimize spatial temperature variations in the SOFC electrode electrolyte assembly for significant load perturbations. We focus on control design in the presence of nonideal actuation. This includes the effects of fuel processing delays, cathode inlet thermal delays, and parasitic power associated with the blower supplying air to the cathode. The controller, based on energy-to-peak minimization synthesis, is applied to a dynamic model of an anode-supported coflow planar SOFC stack. The results indicate that many of the problems associated with realistic and imperfect actuation can be addressed with relatively standard control synthesis modifications, but fuel flow delays can compromise power following significantly. Finally, a strategy that relies primarily on partial internal reformation for power following addresses many of the difficulties associated with reformer delays.

IntroductionThe U.S. Department of Energy has devoted significant effort towards technological breakthroughs for highly efficient low emission electricity production [1e4]. Rising oil prices, the possibility of carbon taxation, and unnerving dependence on foreign energy sources stimulated the federal government's interest in clean energy sources that can meet new greenhouse gas emission targets. Fuel cells promise the dual benefits of high efficiency energy conversion and extremely low pollutant emissions, but adoption of the technology has been limited by high initial capital costs and lack of market acceptance. The challenges include poor economies of scale in manufacturing due to the relatively high materials costs and surface-area scaling characteristics, the complexity of the balance of plant and control systems, a lack of proven system durability, and operations and maintenance costs. Available online at www.sciencedirect.com journal h om epa ge: www.elsev ier.com/locate/he i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 7 9 3 5 e7 9 4 6

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h i g h l i g h t sDynamics and controls development for fuel cell gas turbine (FC-GT) hybrid systems. Molten carbonate hybrid achieves 2:1 turndown at 66% efficiency (LHV) and 1.5 MW. Solid oxide hybrid achieves 4:1 turndown at 71% % efficiency (LHV) and 100 MW. Spatial temperature variation and surge margin are maintained during transients. Cascaded PeI controllers with feed-forward are utilized for hybrid system control. a r t i c l e i n f o a b s t r a c tFuel cell gas turbine hybrid systems have achieved ultra-high efficiency and ultra-low emissions at small scales, but have yet to demonstrate effective dynamic responsiveness or base-load cost savings. Fuel cell systems and hybrid prototypes have not utilized controls to address thermal cycling during load following operation, and have thus been relegated to the less valuable base-load and peak shaving power market. Additionally, pressurized hybrid topping cycles have exhibited increased stall/surge characteristics particularly during off-design operation. This paper evaluates additional control actuators with simple control methods capable of mitigating spatial temperature variation and stall/surge risk during load following operation of hybrid fuel cell systems. The novel use of detailed, spatially resolved, physical fuel cell and turbine models in an integrated system simulation enables the development and evaluation of these additional control methods. It is shown that the hybrid system can achieve greater dynamic response over a larger operating envelope than either individual sub-system; the fuel cell or gas turbine. Results indicate that a combined feed-forward, PeI and cascade control strategy is capable of handling moderate perturbations and achieving a 2:1 (MCFC) or 4:1 (SOFC) turndown ratio while retaining >65% fuel-to-electricity efficiency, while maintaining an acceptable stack temperature profile and stall/surge margin.