Performance improvement of the combined cycle power plant by intake air cooling using an absorption chiller

This paper studies how to improve the capacity of the combined cycle (CC) power plant which has been operated for 8 years. The most popular way is to lower intake air temperature to around 15 °C (ISO) and 100% RH before entering the air compressor of a gas turbine (GT). Thailand has 3 seasons: winter, summer and rainy season. According to 2003 Bangkok monthly weather data, all year ambient temperature is higher than 15 °C. This research proposes a steam absorption chiller (AC) to cool intake air to the desired temperature level. It could increase the power output of a GT by about 10.6% and the CC power plant by around 6.24% annually. In economic analysis, the payback period will be about 3.81 years, internal rate of return 40%, and net present value 19.44 MUS$.     

Combining the nuclear power plant steam cycle with gas turbines

Nuclear steam power plants (NPP) are characterized by low efficiency, compared to steam power plants using fossil fuels. This is due to the relatively low temperature and pressure-throttling conditions of the NPP compared to those using fossil fuel. The light water pressurized water reactor (LW PWR) commercially known as AP600 was suggested for Kuwait cogeneration power desalting plant (CPDP). It has 600 MW nominal power capacity and 33% overall efficiency. Meanwhile, the Kuwaiti Ministry of Electricity and Water (MEW) installed plenty of gas turbines (GTs) to cover the drastic increase in the peak electrical load during the summer season. Combining some of these GTs with the AP600 can increase the capacity and efficiency of the combined plant, compared to either the GT open cycle or the NPP separate plants. This paper investigates the feasibility of utilizing the hot gases leaving the GT to superheat the steam leaving the steam generator of the AP600 NPP, as well as heating the feed water returning to the steam generator of the NPP condenser. This drastically increases the power output and the efficiency of the NPP. Detailed modifications to the NPP power cycle and the resulting enhancement of its performance are presented.     

Power system optimization

Long-term gas purchase contracts usually determine delivery and payment for gas on the regular hourly basis, independently of demand side consumption. In order to use fuel gas in an economically viable way, optimization of gas distribution for covering consumption must be introduced. In this paper, a mathematical model of the electric utility system which is used for optimization of gas distribution over electric generators is presented. The utility system comprises installed capacity of 1500 MW of thermal power plants, 400 MW of combined heat and power plants, 330 MW of a nuclear power plant and 1600 MW of hydro power plants. Based on known demand curve the optimization model selects plants according to the prescribed criteria. Firstly it engages run-of-river hydro plants, then the public cogeneration plants, the nuclear plant and thermal power plants. Storage hydro plants are used for covering peak load consumption. In case of shortage of installed capacity, the cross-border purchase is allowed. Usage of dual fuel equipment (gas–oil), which is available in some thermal plants, is also controlled by the optimization procedure. It is shown that by using such a model it is possible to properly plan the amount of fuel gas which will be contracted. The contracted amount can easily be distributed over generators efficiently and without losses (no breaks in delivery). The model helps in optimizing of fuel gas–oil ratio for plants with combined burners and enables planning of power plants overhauls over a year in a viable and efficient way.     

Thermo-economic modeling of an indirectly coupled solid oxide fuel cell/gas turbine hybrid power plant

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency, in order to improve system efficiencies and economics. The SOFC system is indirectly coupled to the gas turbine power plant, paying careful attention to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 20.6 MW at 49.9% efficiency. The model also predicts a break-even per-unit energy cost of USD 4.65 ¢ kWh⁻¹ for the hybrid system based on futuristic mass generation SOFC costs. This shows that SOFCs may be indirectly integrated into existing GT power systems to improve their thermodynamic and economic performance.     

Investigations on the behaviour of 2 kW natural gas fuel processor

In this paper, the first experimental investigations on a pre-commercial natural gas steam reformer have been presented. The fuel processor unit contains the elements as follows: desulfurizer, steam reformer reactor, CO shift converter, CO preferential oxidation (PROX) reactor, steam generator, burner and heat exchangers.The fuel processor produces 45 Nl/min of syngas in which the hydrogen concentration is about 75 vol.% and the other chemical species are nitrogen, carbon dioxide and methane. The CO concentration is below 1 ppmv, so that this reforming system is suitable for the integration with a PEM fuel cell stack.The experimental activity has been conducted in a test station, properly designed to measure the behaviour of the fuel processor. The laboratory test facility is equipped by a National Instruments Compact DAQ real-time data acquisition and control system running Labview™ software. Several measurement instruments and controlling devices have been installed. Furthermore, a gas chromatograph is used to measure the product gas composition during the tests.The aim of this work has been to analyze the behaviour of this pre-commercial steam reforming unit during its operation cycle in different operating conditions (full and partial loads) in order to study its integration with a PEM fuel cell for developing a high efficiency microcogeneration system for residential applications.     

Thermo-economic modeling of a solid oxide fuel cell/gas turbine power plant with semi-direct coupling and anode recycling

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency in order to improve system efficiencies and economics. The SOFC system is semi-directly coupled to the gas turbine power plant, with careful attention paid to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 21.6 MW at 49.2% efficiency. The model also predicts a breakeven per-unit energy cost of USD 4.70 ¢/kWh for the hybrid system based on futuristic mass generation SOFC costs. Results show that SOFCs can be semi-directly integrated into existing GT power systems to improve their thermodynamic and economic performance.     

Thermo-economic optimization of an indirectly coupled solid oxide fuel cell/gas turbine hybrid power plant

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10-MW GT power plant, operating at 30% efficiency, in order to improve system efficiencies and economics. The SOFC system is indirectly coupled to the GT, in order to minimize the disruption to the GT operation. A thermo-economic model is developed to simulate the hybrid power plant and to optimize its performance using the method of Lagrange Multipliers. It predicts an optimized power output of 18.9 MW at 48.5% efficiency, and a breakeven per-unit energy cost of USD 4.54 ¢ kW h⁻¹ for the hybrid system based on futuristic mass generation SOFC costs.     

Enhancing the efficiency and power of the triple-pressure reheat combined cycle by means of gas reheat, gas recuperation, and reduction of the irreversibility in the heat recovery steam generator

The main methods for improving the efficiency or power of the combined cycle are: increasing the inlet temperature of the gas turbine (TIT), inlet air-cooling, applying gas reheat, steam or water injection into the gas turbine (GT), and reducing the irreversibility of the heat recovery steam generator (HRSG). In this paper, gas reheat with recuperation was applied to the regular triple-pressure steam-reheat combined cycle (the Regular cycle) by replacing the GT unit with a recuperated gas-reheat GT unit (requires two gas turbines, gas recuperator, and two combustion chambers). The Regular cycle with gas-reheat and gas-recuperation (the Regular Gas Reheat cycle) was modeled including detailed modeling of the combustion and GT cooling processes and a feasible technique to reduce the irreversibility of its HRSG was introduced. The Regular Gas Reheat cycle and the Regular Gas Reheat cycle with reduced-irreversibility HRSG (the Reduced Irreversibility cycle) were compared with the Regular cycle, which is the typical design for a commercial combined cycle. The effects of varying the TIT on the performances of all cycles were presented and discussed. The results indicate that the Reduced Irreversibility cycle is 1.9–2.15 percentage points higher in efficiency and 3.5% higher in the total specific work than the Regular Gas Reheat cycle, which is 3.3–3.6 percentage points higher in efficiency and 22–26% higher in the total specific work than the Regular cycle. The Regular Gas Reheat and Reduced Irreversibility cycles are 1.18 and 3.16 percentage points; respectively, higher in efficiency than the most efficient commercially-available combined cycle at the same value of TIT. Economic analysis was performed and showed that applying gas reheat with recuperation to the Regular cycle could result in an annual saving of 10.2 to 11.2 million US dollars for a 339 MW to 348 MW generating unit using the Regular cycle and that reducing the irreversibility of the HRSG of the Regular Gas Reheat cycle could result in an additional annual saving of 11.8 million US dollars for a 439 MW generating unit using the Regular Gas Reheat cycle.     

Improvement of part load efficiency of a combined cycle power plant provisioning ancillary services

According to the type of ancillary service provisioned, operation mode of a power plant may change to part load operation. In this contribution, part load operation is understood as delivering a lower power output than possible at given ambient temperature because of gas turbine power output control. If it is economically justified, a power plant may operate in the part load mode for longer time. Part load performance of a newly built 80 MW combined cycle in Slovakia was studied in order to assess the possibilities for fuel savings. Based on online monitoring data three possibilities were identified: condensate preheating by activation of the currently idle hot water section; change in steam condensing pressure regulation strategy; and the most important gas turbine inlet air preheating. It may seem to be in contradiction with the well proven concept of gas turbine inlet air cooling, which has however been developed for boosting the gas turbine cycles in full load operation. On the contrary, in a combined cycle in the part load operation mode, elevated inlet air temperature does not affect the part load operation of gas turbines but it causes more high pressure steam to be raised in HRSG, which leads to higher steam turbine power output. As a result, less fuel needs to be combusted in gas turbines in order to achieve the requested combined cycle’s power output. By simultaneous application of all three proposals, more than a 2% decrease in the power plant’s natural gas consumption can be achieved with only minor capital expenses needed.     

The effect of elevated inlet air temperature and relative humidity on cogeneration system

The effect of elevated inlet air temperature and relative humidity on a gas turbine (GT) cogeneration system performance was investigated. The analysis was carried out on a GT of a capacity 171 MW at ISO condition, which is integrated with a dual pressure heat recovery steam generator (HRSG), the cogeneration system had been tested under Kuwait summer climate conditions. A computational model was developed and solved using engineering equation solver professional package to investigate the performance of a dual pressure GT‐HRSG system. The suggested HRSG is capable of producing high‐pressure superheated steam at 150 bar and 510°C to operate a power generation steam turbine cycle, and a medium pressure saturated steam at 15 bar to run a thermal vapor compression (TVC) desalination system. In this research, the influence of elevated inlet air temperature and relative humidity on the energy assessment of the suggested cogeneration system was thoroughly investigated. Results indicated that operating GT under elevated values of inlet air temperatures is characterized by low values of net power and thermal efficiency. At elevated inlet air temperatures, increasing relative humidity has a small positive impact on GT cycle net power and thermal efficiency. Integrating the GT with HRSG to generate steam for power generation and process heat tends to increase energy utilization factor of the system at elevated inlet air temperatures. Increasing inlet air temperature plays a negative impact on power to heat ratio (PHR), while relative humidity has no effect on PHR. Copyright © 2009 John Wiley & Sons, Ltd.     

Synergistic integration of a gas turbine and solid oxide fuel cell for improved transient capability

A theoretical solid oxide fuel cell–gas turbine hybrid system has been designed using a Capstone 60 kW micro-gas turbine. Through simulation it is demonstrated that the hybrid system can be controlled to achieve transient capability greater than the Capstone 60 kW recuperated gas turbine alone. The Capstone 60 kW gas turbine transient capability is limited because in order to maintain combustor, turbine and heat exchangers temperatures within operating requirements, the Capstone combustor fuel-to-air ratio must be maintained. Potentially fast fuel flow rate changes, must be limited to the slower, inertia limited, turbo machinery air response. This limits a 60 kW recuperated gas turbine to transient response rates of approximately 1 kW s⁻¹. However, in the SOFC/GT hybrid system, the combustor temperature can be controlled, by manipulating the fuel cell current, to regulate the amount of fuel sent to the combustor. By using such control pairing, the fuel flow rate does not have to be constrained by the air flow in SOFC/GT hybrid systems. This makes it possible to use the rotational inertia of the gas turbine, to buffer the fuel cell power response, during fuel cell fuel flow transients that otherwise limit fuel cell system transient capability. Such synergistic integration improves the transient response capability of the integrated SOFC gas turbine hybrid system. Through simulation it has been demonstrated that SOFC/GT hybrid system can be developed to have excellent transient capability.     

Energy consumption by multi-stage flash and reverse osmosis desalters

Kuwait and most of the Gulf countries, depend mainly on desalted water from the sea for satisfying their fresh water needs. These countries are using the multi-stage flash (MSF) desalting system, as the ‘work horse’ for their water production. This system is less efficient in energy consumption as compared to the reverse osmosis (RO) system. Moreover, large units based on the MSF system have to be combined with steam or gas turbines power plants for better utilization of steam supplied to the MSF units at moderately low temperature and pressure (as compared to steam produced by large steam generators). The value and the cost of the thermal energy supplied to the MSF desalting system depends on the method of supplying this energy. This steam can be supplied directly from a fuel operated boiler or heat recovery steam generator associated with a gas turbine. It can also be supplied from the exhaust of a steam back pressure turbine or bled from condensed extraction steam turbine at a pressure suitable for the desalting process. Any energy comparison should be based on simple criteria, either how much fuel energy is consumed to produce this energy or how much mechanical energy is needed per unit product. The energy consumed in the light of the practice used in most Gulf countries are discussed here. In this study, reference desalting and power plants are used for comparison purposes. This study shows that shifting from MSF desalting system to the RO system can save up to 66% of the fuel energy used to desalt seawater.     

Development of high intensity CDC combustor for gas turbine engines

Colorless distributed combustion (CDC) has been demonstrated to provide ultra-low emission of NOx and CO, improved pattern factor and reduced combustion noise in high intensity gas turbine combustors. The key feature to achieve CDC is the controlled flow distribution, reduce ignition delay, and high speed injection of air and fuel jets and their controlled mixing to promote distributed reaction zone in the entire combustion volume without any flame stabilizer. Large gas recirculation and high turbulent mixing rates are desirable to achieve distributed reactions thus avoiding hot spot zones in the flame. The high temperature air combustion (HiTAC) technology has been successfully demonstrated in industrial furnaces which inherently possess low heat release intensity. However, gas turbine combustors operate at high heat release intensity and this result in many challenges for combustor design, which include lower residence time, high flow velocity and difficulty to contain the flame within a given volume. The focus here is on colorless distributed combustion for stationary gas turbine applications. In the first part of investigation effect of fuel injection diameter and air injection diameter is investigated in detail to elucidate the effect fuel/air mixing and gas recirculation on characteristics of CDC at relatively lower heat release intensity of 5 MW/m³ atm. Based on favorable conditions at lower heat release intensity the effect of confinement size (reduction in combustor volume at same heat load) is investigated to examine heat release intensity up to 40 MW/m³ atm. Three confinement sizes with same length and different diameters resulting in heat release intensity of 20 MW/m³ atm, 30 MW/m³ atm and 40 MW/m³ atm have been investigated. Both non-premixed and premixed modes were examined for the range of heat release intensities. The heat load for the combustor was 25 kW with methane fuel. The air and fuel injection temperature was at normal 300 K. The combustor was operated at 1 atm pressure. The results were evaluated for flow field, fuel/air mixing and gas recirculation from numerical simulations and global flame images, and emissions of NO, CO from experiments. It was observed that the larger air injection diameter resulted in significantly higher levels of NO and CO whereas increase in fuel injection diameter had minimal effect on the NO and resulted in small increase of CO emissions. Increase in heat release intensity had minimal effect on NO emissions, however it resulted in significantly higher CO emissions. The premixed combustion mode resulted in ultra-low NO levels (<1 ppm) and NO emission as low as 5 ppm was obtained with the non-premixed flame mode.     

Exergy analysis of the recuperative auto thermal reforming (R-ATR) and recuperative reforming (R-REF) power cycles with CO2 removal

Two relatively innovative gas turbine (GT) based power cycles with high CO₂ removal potential have been proposed and discussed in terms of exergy analysis. Fuel decarbonisation is applied by the means of auto thermal reforming (R-ATR) and simple reforming (R-REF), in order to convert the primary natural gas into a highly H₂ and CO₂ concentrated fuel. Thus, CO₂ is captured with amine chemical absorption into a specific unit and, finally, the decarbonised fuel is sent to the GT combustion chamber. No bottoming steam cycle is included, which should promote the size flexibility of the powerplant. The heat content of GT exhausts is employed partially to sustain the endothermic reforming reactions and partially for cycle recuperation. Moreover, the possibility of steam blade cooling has been investigated.The efficiency is optimised at low pressure ratios (7–10) in the steam cooled R-ATR, whereas higher values have been found in air cooled version (16–17). Generally, the R-ATR solution shows higher efficiency levels, mainly due to the reduced combustion chamber and CO₂ capture exergy destruction and higher cycle recuperation degree.The exergy analysis showed a relatively limited influence of combustion chamber losses on the primary fuel exergy input (20–23%). The relative loss of CO₂ removal unit is limited as well (5–7%) when compared with values of semi-closed GT configurations. The exergy destruction of R-ATR and R-REF CO₂ removal sections is greatly reduced if steam blade cooling is adopted. Generally, all the proposed cycles showed satisfactory values of efficiency (43–46% under optimised conditions) taking into account that they do not involve combined power plants.     

Enhanced coal gasification heated by unmixed combustion integrated with an hybrid system of SOFC/GT

For clean utilization of coal, enhanced gasification by in situ CO₂ capture has the advantage that hydrogen production efficiency is increased while no energy is required for CO₂ separation. The unmixed fuel process uses a sorbent material as CO₂ carrier and consists of three coupled reactors: a coal gasifier where CO₂ is captured generating a H₂-rich gas that can be utilized in fuel cells, a sorbent regenerator where CO₂ is released by sorbent calcination and it is ready for capture and a reactor to oxidize the oxygen transfer material which produces a high temperature/pressure vitiated air. This technology has the potential to eliminate the need for the air separation unit using an oxygen transfer material. Reactors' temperatures range from 750 °C to 1550 °C and the process operates at pressure around 7.0 bar. This paper presents a global thermodynamic model of the fuel processing concept for hydrogen production and CO₂ capture combined with fuel and residual heat usage. Hydrogen is directly fed to a solid oxide fuel cell and exhaust streams are used in a gas turbine expander and in a heat recovery steam generator. This paper analyzes the influence of steam to carbon ratio in gasifier and regeneration reactor, pressure of the system, temperature for oxygen transfer material oxidation, purge percentage in calciner, average sorbent activity and oxidant utilization in fuel cell. Electrical efficiency up to 73% is reached under optimal conditions and CO₂ capture efficiencies near 96% ensure a good performance for GHG's climate change mitigation targets.     

Targeting for energy efficiency and improved energy collaboration between different companies using total site analysis (TSA)

Rising fuel prices, increasing costs associated with emissions of green house gases and the threat of global warming make efficient use of energy more and more important. Industrial clusters have the potential to significantly increase energy efficiency by energy collaboration. In this paper Sweden’s largest chemical cluster is analysed using the total site analysis (TSA) method. TSA delivers targets for the amount of utility consumed and generated through excess energy recovery by the different processes. The method enables investigation of opportunities to deliver waste heat from one process to another using a common utility system.The cluster consists of 5 chemical companies producing a variety of products, including polyethylene (PE), polyvinyl chloride (PVC), amines, ethylene, oxygen/nitrogen and plasticisers. The companies already work together by exchanging material streams. In this study the potential for energy collaboration is analysed in order to reach an industrial symbiosis. The overall heating and cooling demands of the site are around 442 MW and 953 MW, respectively. 122 MW of heat is produced in boilers and delivered to the processes.TSA is used to stepwise design a site-wide utility system which improves energy efficiency. It is shown that heat recovery in the cluster can be increased by 129 MW, i.e. the current utility demand could be completely eliminated and further 7 MW excess steam can be made available. The proposed retrofitted utility system involves the introduction of a site-wide hot water circuit, increased recovery of low pressure steam and shifting of heating steam pressure to lower levels in a number heat exchangers when possible. Qualitative evaluation of the suggested measures shows that 60 MW of the savings potential could to be achieved with moderate changes to the process utility system corresponding to 50% of the heat produced from purchased fuel in the boilers of the cluster.Further analysis showed that after implementation of the suggested energy efficiency measures there is still a large excess of heat at temperatures of up to 137 °C.     

Fuel cell systems for transportation: Status and trends

The U.S. program for the development of direct hydrogen-fueled automotive fuel cell systems has established ambitious performance and cost targets for the 2010 and 2015 time frames. These targets include peak and rated power efficiencies of 60% and 50%, respectively, specific power and power densities of 650 We kg⁻¹ and 650 We L⁻¹, and manufactured costs of $45 and 30 kWe⁻¹ for 80 kWe⁻¹ net systems in the 2010 and 2015 systems, respectively. In this paper, we discuss the use of fuel cell system models to examine the performance and projected manufactured costs of 2005 systems and the improvements needed to meet the 2010 and 2015 system level targets. It appears possible to meet most of the 2010 performance targets with advances such as the nano-structured thin film electrocatalysts and a modified electrolyte membrane capable of operating at up to 95 °C, at least for short periods. To meet the 2015 targets, however, the fuel cell systems may need to operate without pressurization at higher temperatures of up to 120 °C without the need to humidify the fuel gas and air, along with several other improvements in stack and balance-of-plant components. Our simulations provide quantitative estimates of the various performance and cost parameters of the near-term and the advanced systems that can achieve the targets set for automotive fuel cell system development.     

Simulation of an air conditioning absorption refrigeration system in a co-generation process combining a proton exchange membrane fuel cell

In this work, a computer simulation program was developed to determine the optimum operating conditions of an air conditioning system during the co-generation process. A 1 kW PEMFC was considered in this study with a chemical/electrical theoretical efficiency of 40% and a thermal efficiency of 30% applying an electrical load of 100%. A refrigeration-absorption cycle (RAC) operating with monomethylamine–water solutions (MMA–WS), with low vapor generation temperatures (up to 80 °C) is proposed in this work. The computer simulation was based on the refrigeration production capacity at the maximum power capacity of the PEMFC. Heat losses between the fuel cell and the absorption air conditioning system at standard operating conditions were considered to be negligible. The results showed the feasibility of using PEMFC for cooling, increasing the total efficiency of the fuel cell system.     

Waste heat recovery from a landfill gas-fired power plant

Waste treatment and management is a certain challenge especially in areas with high population density. One of the options for waste treatment is landfilling, where the amount of municipal waste also produces landfill gas through anaerobic digestion. The heating value of the landfill gas is high enough to use it as a fuel in combustion processes, e.g. in internal combustion engines (ICEs) to produce electric power.In Ano Liosia, Athens (Greece) up to 6000 tons of waste are landfilled every day and the landfill gas is used in an ICE power station directly at the site of the landfill. The power station consists of 15 ICEs and has an installed capacity of 23.5 MW. The major advantages of using ICE for power generation are the high electrical efficiency of ICEs and their fast load response. However, more than 50% of the landfill gas energy content is still released to the atmosphere as engine waste heat (exhaust gas and engine cooling water).The aim of this paper is to study the possibilities of using this large amount of heat in order to increase the electricity production and efficiency of the Ano Liosia power station. Therefore, a thermodynamic and economic analysis of two different waste heat recovery (WHR) systems is conducted. The water/steam cycle and the Organic Rankine Cycle (ORC) are examined and evaluated by means of thermodynamic cycle simulation and by calculating their specific costs of power generation. Their advantages and disadvantages considering their application in landfillgas-fired ICE power stations are discussed under the consideration of maximal thermodynamic efficiency and minimal costs of power generation.     

Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell

This study investigates the performance of a standard Ni-YSZ anode supported cell under ethanol steam reforming operating conditions. Therefore, the fuel cell was directly operated with a steam/ethanol mixture (3 to 1 molar). Other gas mixtures were also used for comparison to check the conversion of ethanol and of reformate gases (H₂, CO) in the fuel cell. The electrochemical properties of the fuel cell fed with four different fuel compositions were characterized between 710 and 860 °C by I-V and EIS measurements at OCV and under polarization. In order to elucidate the limiting processes, impedance spectra obtained with different gas compositions were compared using the derivative of the real part of the impedance with respect of the natural logarithm of the frequency.Results show that internal steam reforming of ethanol takes place significantly on Ni-YSZ anode only above 760 °C. Comparisons of results obtained with reformate gas showed that the electrochemical cell performance is dominated by the conversion of hydrogen. The conversion of CO also occurs either directly or indirectly through the water-gas shift reaction but has a significant impact on the electrochemical performance only above 760 °C.