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Solid oxide fuel cell (SOFC)–gas turbine (GT) hybrid systems can produce power at high electrical efficiencies while emitting virtually zero criteria pollutants (e.g., ozone, carbon monoxide, oxides of nitrogen and sulfur, and particulate matters). This study presents new insights into renewable hydrogen (RH2)-powered SOFC–GT hybrid systems with respect to their system configuration and techno-economic analysis motivated by the need for clean on-demand power. First, three system configurations are thermodynamically assessed: (I) a reference case with no SOFC off-gas recirculation, (II) a case with cathode off-gas recirculation, and (III) a case with anode off-gas recirculation. While these configurations have been studied in isolation, here we provide a detailed performance comparison. Moreover, a techno-economic analysis is conducted to study the economic competitiveness of RH2-fueled hybrid systems and the economies of scale by offering a comparison to natural gas (NG)-fueled systems. Results show that the case with anode off-gas recirculation, with 68.50%-lower heating value (LHV) at a 10 MW scale, has the highest efficiency among the studied scenarios. When moving from 10 MW to 50 MW, the efficiency increases to 70.22%-LHV. These high efficiency values make SOFC–GT hybrid systems highly attractive in the context of a circular economy as they outcompete most other power generation technologies. The cost-of-electricity (COE) is reduced by about 10% when moving from 10 MW to 50 MW, from USD 1976/kW to USD 1668/kW, respectively. Renewable H2 is expected to be economically competitive with NG by 2030, when the U.S. Department of Energy’s target of USD 1/kg RH2 is reached.

Batteries are important for promoting renewable energy, but, like most engineered products, they contain multiple hazardous materials. The purpose of this study is to evaluate industrial-scale batteries using GreenScreen® for Safer Chemicals, an established chemical hazard assessment (CHA) framework, and to develop a systematic, transparent methodology to quantify the CHA results, harmonize them, and aggregate them into single-value hazard scores, which can facilitate quantitative comparison and a robust evaluation of data gaps, inconsistencies, and uncertainty through the implementation of carefully selected scenarios and stochastic multicriteria acceptability analysis (SMAA). Using multiple authoritative toxicity data sources, six battery products are evaluated: three lithium-ion batteries (lithium iron phosphate, lithium nickel cobalt manganese hydroxide, and lithium manganese oxide), and three redox flow batteries (vanadium redox, zinc-bromine, and all-iron). The CHA results indicate that many materials in these batteries, including reagents and intermediates, inherently exhibit high hazard; therefore, safer materials should be identified and considered in future designs. The scenario analysis and SMAA, combined, provide a quantitative evaluation framework to support the decision-making needed to compare alternative technologies. Thus, this study highlights specific strategies to reduce the use of hazardous materials in complex engineered products before they are widely used in this rapidly-expanding industry sector.

Energy storage systems are critical for enabling the environmental benefits associated with capturing renewable energy to displace fossil fuel-based generation, yet producing these systems also contributes to environmental impacts through their materials use and manufacturing. As energy storage capacity is scaled up to support increasingly renewable grids, the environmental benefits from their use may scale at different rates than the environmental impacts from their production. This implies the existence of capacity thresholds beyond which installing additional storage capacity may be environmentally detrimental. Identifying such thresholds are important for ensuring that energy storage capacity selection in future grids are consistent with net emissions reduction goals, but such thresholds have not been studied in the present literature. To identify such thresholds, here we combine electric grid dispatch modeling with life cycle analysis to compare how the emissions reductions from deploying three different flow battery energy storage types on a future California grid (>80% wind and solar) compare with emissions contributions from producing such batteries as total battery capacity installed on the grid increases. Depending on the type of battery and environmental impact indicator (greenhouse gas or particulate matter emissions), we find that the marginal environmental benefits of storage begin to diminish at deployed capacities of 38–76% of the mean daily renewable generation (256–512 GWh in our California scenarios) and reach zero at 105–284% of mean daily renewable generation (700–1810 GWh). Such storage capacities are conceivable, but upstream impacts of storage must be assessed in evaluating the environmental benefits of large-scale storage deployment, or they could negate the environmental benefits of regional electricity system decarbonization.

Climate change and increased electrification of space and water heating in buildings can significantly affect future electricity demand and hourly demand profiles, which has implications for electric grid greenhouse gas emissions and capacity requirements. We use EnergyPlus to quantify building energy demand under historical and under several climate change projections of 32 kinds of building prototypes in 16 different climate zones of California and imposed these impacts on a year 2050 electric grid configuration by simulation in the Holistic Grid Resource Integration and Deployment (HIGRID) model. We find that climate change only prompted modest increases in grid resource capacity and negligible difference in greenhouse gas emissions since the additional electric load generally occurred during times with available renewable generation. Heating electrification, however, prompted a 30-40% reduction in greenhouse gas emissions but required significant grid resource capacity increases, due to the higher magnitude of load increases and lack of readily available renewable generation during the times when electrified heating loads occurred. Overall, this study translates climate change and electrification impacts to system-wide endpoint impacts on future electric grid configurations and highlights the complexities associated with translating building-level impacts to electric system-wide impacts.

The hybridization of gas turbine technology with high temperature fuel cells represents an ultra-high efficiency, ultra-low emission, fuel flexible power generation platform. The performance of past prototypes has been limited by marginal compatibility of the two primary sub-systems. This paper addresses the challenge of selecting compatible hardware by presenting a simple and robust method for bespoke hybrid system design and off-the-shelf component integration. This is the first application of detailed, spatially resolved, physical models capable of resolving off-design performance to the integration analysis of FC-GT hybrids. Static maps are produced for both turbine and fuel cell sub-systems that readily evaluate the compatibility and hybrid performance. Molten carbonate and solid oxide fuel cells are considered for hybridization with recuperated micro-turbines and larger axial flow gas turbine systems. Current state-of-the-art molten carbonate technology is shown to pair well with present micro-turbine technology in an FC bottoming cycle design achieving 74.4% LHV efficiency. Solid oxide technology demonstrates remarkable potential for integration with larger scale axial turbo-machinery to achieve greater than 75% LHV efficiency. This performance map technique closely matches results from detailed integrated hybrid system analyses, and enables quick determination of performance requirements for balance of plant design and optimization. © 2014 Elsevier B.V. All rights reserved.

To evaluate the dynamic operation and feasibility of designing and operating a self-sustainable hydrogen fueling station using renewable energy sources, system models for a hydrogen fueling station using a proton exchange membrane (PEM) electrolyzer and fuel cell have been developed to simulate the renewable sources and fueling dynamics together with hydrogen production and station operation. Theoretical models have been integrated to simulate station performance when subjected to measured power and fueling demand dynamics from a public fueling station and measured renewable energy supply dynamics. The theoretical models that are integrated into various self-sustainable station design configurations include a Proton Exchange Membrane (PEM) electrolyzer and PEM fuel cell, hydrogen compressor, and storage tank. The fueling dynamics and power consumption dynamics were obtained from an operating public hydrogen fueling station and implemented in the system model. Various control strategies are simulated and the station performance is determined to depend upon the way renewable power is utilized in the station. Due to the round trip efficiency penalty associated with converting electricity to hydrogen (in an electrolyzer) and vice versa (in a fuel cell), the results suggest that the station operation power should be supplied by the renewable sources directly whenever possible, and that the hydrogen fuel cell should provide power only when there is no renewable power available (the third control strategy tested in this paper). The simulated hydrogen fueling station powered by 200 kW wind turbines or 360 kW solar PV were determined to successfully operate in a self-sustainable manner while dispensing ∼25 kg of hydrogen per day. This study provides insights regarding the sizing of the station components such as renewable energy conversion devices, electrolyzer and fuel cell, and storage tank. The cost of the hydrogen was determined to be $8.01 per kg when the station is powered by 200 kW of wind turbines and operated using control strategy 3, while it increased to $20.22 per kg when the station is powered by 360 kW of PV array and operated using control strategy 3. This study provides a basis for achieving self-sustainable renewable hydrogen fueling stations. With further optimization and development, these self-sustainable renewable hydrogen fueling stations could provide valuable interconnections (especially in remote locations) throughout the hydrogen infrastructure network and further support the integration of renewable sources for vehicle fuels.

Fuel 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, P-I 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. © 2014 Elsevier B.V. All rights reserved.

This work builds upon previous high temperature fuel cell (HT-FC) modeling studies, capturing both steady state performance and transient behavior of HT-FC stacks by merging simplified dimensional aspects of a planar fuel cell stack with first principles physical modeling. Dynamic simulations are developed that spatially resolve fluctuations in temperature, pressure and concentration distributions during transient operation. A significant portion of the heat transfer occurs prior to and after the air passes over the electrochemically active portions of the cell, justifying additional heat transfer pathways from the stack to the air in order to accurately characterize the thermal transients and temperature distributions in the HT-FC stack. The highly configurable MatLab-Simulink® model developed can simulate both solid oxide and molten carbonate fuel cells utilizing either direct or indirect internal reforming. The perturbation response characteristics of the dynamic model to load, fuel flow, air flow and composition perturbations are discussed, and control strategies are introduced that minimize temperature fluctuations. Analysis indicates air flow and inlet temperature controls are sufficient to control average temperature and average internal temperature gradients. Internal heat transfer dynamics substantially change the spatial temperature distribution and local temperature gradients during typical operating conditions and perturbations. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.

Hybrid 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. © 2012 Elsevier B.V. All rights reserved.

This study compares the production of hydrogen with high temperature fuel cells (HTFCs) that tri-generate power, heat and hydrogen to distributed and centralized steam methane reformation (SMR) supply chains. The defined supply chain steps of hydrogen production include: production, treatment, distribution, storage, dispensing and use. Different technologies for each step in the supply chain have been analyzed from an energy standpoint, resulting in ten different supply chain scenarios. Results show that liquefaction of hydrogen is the most energy intensive of all the treatment processes and that it is only effective for long delivery distances. When the energy required for the hydrogen treatment (i.e., liquefaction, compression) is included, it is shown that compressed gas hydrogen at 200 bar is the least energy intensive for delivery distances shorter than 84 km if transported by diesel truck. For distances longer than 84 km, 500 bar compressed hydrogen is more efficiently transported than at 200 bar compressed hydrogen. For distances larger than 550 km, liquefied hydrogen is more efficiently distributed than compressed hydrogen at 500 bar. Results show that the highest supply chain efficiency corresponds to distributed hydrogen production via tri-generating HTFC (∼76%) followed by centralized SMR with 500 bar compressed hydrogen distribution (∼71%). The lowest supply chain efficiency values correspond to distributed SMR plants (∼60%) and centralized SMR with transportation of hydrogen in liquid form (

The use of a high temperature fuel cell (HTFC) to continuously and simultaneously polygenerate hydrogen in combination with electricity and heat represents a promising technology as a source of fuel for fuel cell vehicles. Different configurations of polygenerating HTFC, including different designs with internal and external reforming are options to polygenerate electricity, hydrogen and heat. The current study analyzes and compares six different configurations based on solid oxide technology. Efficiency results based upon the Supplemental Input Method demonstrate that internal reforming configurations achieve higher performance than when hydrogen product is produced in an external reformer. The overall efficiency and the efficiency in the generation of each product are used as the basis for comparison. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

High temperature fuel cells can be designed and operated to poly-generate electricity, heat, and useful chemicals (e.g., hydrogen) in a variety of configurations. The highly integrated and synergistic nature of poly-generating high temperature fuel cells, however, precludes a simple definition of efficiency for analysis and comparison of performance to traditional methods. There is a need to develop and define a methodology to calculate each of the co-product efficiencies that is useful for comparative analyses. Methodologies for calculating poly-generation efficiencies are defined and discussed. The methodologies are applied to analysis of a Hydrogen Energy Station (H 2ES) showing that high conversion efficiency can be achieved for poly-generation of electricity and hydrogen. © 2010 Elsevier B.V.

Operating strategies of solid oxide fuel cell (SOFC) combined heat and power (CHP) systems are developed and evaluated from a utility, and end-user perspective using a fully integrated SOFC-CHP system dynamic model that resolves the physical states, thermal integration and overall efficiency of the system. The model can be modified for any SOFC-CHP system, but the present analysis is applied to a hotel in southern California based on measured electric and heating loads. Analysis indicates that combined heat and power systems can be operated to benefit both the end-users and the utility, providing more efficient electric generation as well as grid ancillary services, namely dispatchable urban power. Design and operating strategies considered in the paper include optimal sizing of the fuel cell, thermal energy storage to dispatch heat, and operating the fuel cell to provide flexible grid power. Analysis results indicate that with a 13.1% average increase in price-of-electricity (POE), the system can provide the grid with a 50% operating range of dispatchable urban power at an overall thermal efficiency of 80%. This grid-support operating mode increases the operational flexibility of the SOFC-CHP system, which may make the technology an important utility asset for accommodating the increased penetration of intermittent renewable power. © 2009 Elsevier B.V. All rights reserved.

A commercially available natural gas fueled gas turbine engine was operated on hydrogen. Three sets of fuel injectors were developed to facilitate stable operation while generating differing levels of fuel/air premixing. One set was designed to produce near uniform mixing while the others have differing degrees of nonuniformity. The emission performance of the engine over its full range of loads is characterized for each of the injector sets. In addition, the performance is also assessed for the set with near uniform mixing as operated on natural gas. The results show that improved mixing and lower equivalence ratio decrease NO emission levels as expected. However, even with nearly perfect premixing, it is found that the engine, when operated on hydrogen, produces a higher amount of NO than when operated with natural gas. Much of this attributed to the higher equivalence ratios that the engine operates on when firing hydrogen. However, even the lowest equivalence ratios run at low power conditions, higher NO was observed. Analysis of the potential NO formation effects of residence time, kinetic pathways of NO production via NNH, and the kinetics of the dilute combustion strategy used are evaluated. While no one mechanism appears to explain the reasons for the higher NO, it is concluded that each may be contributing to the higher NO emissions observed with hydrogen. In the present configuration with the commercial control system operating normally, it is evident that system level effects are also contributing to the observed NO emission differences between hydrogen and natural gas. © 2009 by ASME.

A dynamic model of an integrated solid oxide fuel cell (SOFC) combined heat and power (CHP) system has been developed. The model was developed by modifying a previously developed generic 5 kW simple-cycle SOFC system. Fuel cell model modifications include changes in methods and constants for estimating over-potentials to better simulate a modern anode-supported planar SOFC. In addition to scaling up and modifying the fuel cell model, a thermal energy storage (TES) tank, exhaust gas duct burner and hot water exhaust gas recuperator model were integrated into the system model. The fully integrated system model can effectively simulate an SOFC-CHP system and evaluate the system performance and efficiency in meeting building electricity and heating demand profiles. For the present effort, dynamic building electricity and heating data from a hotel operated in Orange County, southern California during the months of July and August 2008 were analyzed. Specifically, tradeoffs between SOFC performance and thermal energy storage have been investigated. The simulation results show that the SOFC-CHP system has the ability to follow the dynamic electrical load with appropriate system design and controls. Due to thermal power mismatch during electricity load-following operation, supplementary exhaust gas duct burner heat and/or a TES is required to independently dispatch the fuel cell power and meet the hotel heating demand. However, if the fuel cell is sufficiently sized, the system can achieve greater than 70% efficiency with only a small TES tank and without the need to fire the duct burner. The dynamic model and integrated SOFC-TES concept are shown to be useful for developing integrated CHP systems and to evaluate performance. Copyright © 2009 by ASME.

Adding distributed generation (DG) is a desirable strategy for providing highly efficient and environmentally benign services for electric power, heating, and cooling. The interface between a solid oxide fuel cell (SOFC), typical loads, and the electrical grid is simulated in Matlab/Simulink and analyzed to assess the interactions between DG and the electrical grid. A commercial building load profile is measured during both steady-state and transient conditions. The load data are combined with the following models that are designed to account for physical features: a One-Cycle Control grid-connected inverter, a One-Cycle Control active power filter, an SOFC, and capacitor storage. High penetration of DG without any power filter increases the percentage of undesirable harmonics provided by the grid, but combined use of an inverter and active power filter allows the DG system interconnection to improve the grid tie-line flow by lowering total harmonic distortion and increasing the power factor to unity. © 2008 Elsevier B.V. All rights reserved.