Fuel cell–gas turbine hybrid system design part II: Dynamics and control

Abstract

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.

Advanced control approach for hybrid systems based on solid oxide fuel cells

“…These constraints must be considered not only for steady-state conditions, but also during time-dependent operations [29], such as load variations, ambient temperature changes and start-up/shutdown phases [30]. More specifically, several challenges must be overcome to couple the very fast response of the mGT system (low mechanical inertia of the turbine shaft) with the slow thermal variations of the SOFC stack [31] (high thermal capacitance of fuel cell materials), while the different volume values of SOFC sides generate different time-dependent performance in terms of pressurization/depressurization delays (an important aspect to take into account in order to prevent excessive cathode/anode pressure difference during transient operations [25]). Moreover, the fluid dynamic and chemical responses of the anodic side, important aspects to avoid low STCR values, are usually not in line with the transient behaviour necessary to prevent other failures [32][33][34].…”

Stall/surge dynamics of a multi-stage air compressor in response to a load transient of a hybrid solid oxide fuel cell-gas turbine system

“…McLarty et al studied the dynamic operation of an SOFC-GT topping cycle and showed that the pressurized hybrid topping cycles exhibited increased stall/surge characteristics particularly during off-design operation [26,27]. In another study by McLarty et al controls were utilized to mitigate the spatial temperature variation and the stall risk during load following [27]. The results showed that using the combined feed-forward, PI and cascade control strategy, 4:1 (SOFC) turn-down ratio could be achieved and a 65% efficiency could be maintained.…”

“…The model consists of a compressor, a turbine, a blower, an SOFC, a combustor, three mixers and several bypass valves (one for fuel cell bypass and the others for heater bypass). This model has been previously studied for different applications of SOFC-GT hybrid systems (see, for example [26,27,35,37]). Fig.…”

“…The large volume between the compressor and the turbine causes mass accumulation and produces pressure dynamics during dynamic operation of the hybrid system. The approach is patterned after that which was previously developed at NFCRC [26,27,35,37]. Various control algorithms are applied to the dynamic system model to prevent deep surge in the compressor.…”

Control of two Electrical Plants

“…Strategies for power control are focused in [8] and [9]. Several kinds of controls are proposed in [12][13][14]. Several kinds of controls are proposed in [12][13][14].…”