Theoretical study and performance evaluation of hydrogen production by 200 W solid oxide electrolyzer stack

High temperature steam electrolyzers, taking advantage of high temperature heat, can produce more hydrogen by using less electrical energy than low temperature electrolyzers. This paper presents an experimental study on hydrogen production by using a 200 W solid oxide stack working in reverse mode. A thermodynamic study of the process was performed by measuring the heat and mass balance of stack at different operating conditions. Different definitions of efficiency were used to highlight the limit and potential of the process. The I–V curve, the flow rate measurements and the GC analysis on outlet flows were used to calculate the hydrogen and oxygen productions. In addition, the influence of steam dilution, water utilization and operating temperature on conversion efficiency and stack's thermal balance was evaluated. With this aim, the tests were performed at three operating temperature (700 °C, 750 °C and 800 °C) over a range of steam inlet concentration from 50% to 90% and water utilization up to 70%. The hydrogen and oxygen flows produced by electrolysis, at different loads, were directly measured after water condensation: net flows up to 2.4 ml/(min cm²) of hydrogen and 1.2 ml/(min cm²) of oxygen were measured and compared to the theoretical ones, showing a good agreement.     

A critical review on cathode materials for steam electrolysis in solid oxide electrolysis

The major technologies being considered for the green hydrogen production are polymer electrolyte membrane (PEM) and solid oxide electrolysis (SOE). While PEM electrolysis technology is nearing commercialisation with units now being globally installed at tens of MW scale, SOE technology is still under development with units available only at 100s of kW scale and at much higher costs per kW. SOE due to its high operating temperatures (close to 800 °C) has the potential to reduce the electric energy input by up to 30% for the hydrogen production per tonne by using the low-cost thermal energy input available from the industrial or downstream synthesis processes. The SOE cathode, where steam electrolysis occurs, plays a crucial role in dictating the cell voltage losses and the stability of the cell operation that eventually has a large impact on the SOE efficiency and lifetime. The current state-of-the-art cathode materials based on Ni-YSZ pose many challenges. There is, therefore, a global effort to find alternative cathode materials suitable for steam electrolysis in SOE. This review critically reviews novel nanoengineered cathode materials and points to the fact that such materials synthesized using infiltration and exsolution techniques, in combination with advanced materials characterisation like high-temperature scanning probe microscopy and in situ Raman spectroscopy can be a right approach to find the suitable cathode materials for steam electrolysis in SOE. This, however, may need to be combined with a techno-economic analysis to provide the technical and economic viability of these materials for the SOE commercialisation.     

Solar power tower as heat and electricity source for a solid oxide electrolyzer: a case study

High‐temperature steam electrolysis (HTSE) consists of the splitting of steam into hydrogen and oxygen at high temperature in solid oxide electrolyzers. Performing the electrolysis process at high temperatures offers the advantage of achieving higher efficiencies as compared to the conventional water electrolysis. Furthermore, this allows the direct use of process heat to generate steam. This paper is related to the FCH JU (Fuel Cells and Hydrogen Joint Undertaking) project ADEL (ADvanced ELectrolyser For hydrogen Production with Renewable Energy Sources), which investigates different carbon‐free energy sources to be coupled to the HTSE process. Renewable energy sources are able to provide the high‐temperature steam electrolysis (HTSE) process with heat and power. This paper investigates the capability of Concentrating Solar Power (CSP) technologies to provide the HTSE process with the necessary energy demand. The layout of the plant is shown in the following figure. The design of commercial‐scale high‐temperature steam electrolysis has been carried out. The HTSE plant is coupled to an air cooled solar tower. The configuration and the operating parameters of the solar tower are based on those of the solar tower of Jülich (Germany), which is operated by DLR. This paper presents the results of process analysis performed to evaluate the hydrogen production from a HTSE plant coupled to an 80MWth air solar tower. Additionally, the dynamic behavior of the system during energy fluctuations has been analyzed. The receiver‐to‐hydrogen efficiency (based on the Higher Heating Value of hydrogen) is 26% at a hydrogen production rate of 680 kg/h in steady‐state operation. The overall solar‐to‐hydrogen efficiency is calculated to be at 18%. Moreover, the analysis under transient conditions shows that a steady‐state operation of the plant is only possible for 8 h. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrogen production performance of 3-cell flat-tubular solid oxide electrolysis stack

Flat-tubular solid oxide electrolysis cells have been manufactured with a ceramic interconnector in a body to minimize the stack volume and eliminate metallic components. The NiO-YSZ supports are prepared by the extrusion method, and the other cell components, which included the electrolyte, air electrode, and ceramic interconnector, are fabricated by slurry coating methods. The active area of a single cell is 30 cm², and the gas tightness of the stack is checked in the range below 2 bars. The effects of the operating conditions on the performance of a solid oxide electrolysis stack are investigated using electrochemical impedance spectroscopy and the I-V test. Consequently, it is confirmed that sufficient steam content stimulates the electrochemical splitting of water and decreases the activation energy for water electrolysis at a high temperature. In our 3-cell stack test, the hydrogen production rate is 4.1 lh⁻¹, and the total hydrogen production was 144.32 l during 37.1 h of operation.     

High temperature electrolysis of hydrogen bromide gas for hydrogen production using solid oxide membrane electrolyzer

Using solid oxide membrane, this paper presents the theoretical modeling of the high temperature electrolysis of hydrogen bromide gas for hydrogen production. The electrolysis of hydrogen halides such as hydrogen bromide is an attractive process, which can be coupled to hybrid thermochemical cycles. The high temperature electrolyzer model developed in the present study includes concentration, ohmic, and activation losses. Exergy efficiency, as well as energy efficiency parameters, are used to express the thermodynamic performance of the electrolyzer. Moreover, a detailed parametric study is performed to observe the effects of various parameters such as current density and operating temperature on the overall system behavior. The results show that in order to produce 1 mol of hydrogen, 1.1 V of the applied potential is required, which is approximately 0.8 V less compared to high temperature steam electrolysis under same conditions (current density of 1000 A/m² and temperature of 1073 K). Furthermore, it is found that with the use of the presented electrolyzer, one can achieve energy and exergy efficiencies of about 56.7% and 53.8%, respectively. The results presented in this study suggest that, by employing the proposed electrolyzer, two-step thermochemical cycle for hydrogen production may become more attractive especially for nuclear- and concentrated solar-to-hydrogen conversion applications.     

80,000 current on/off cycles in a one year long steam electrolysis test with a solid oxide cell

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm⁻² in the ON mode (−0.07 Acm⁻² in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm²/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.     

Efficient STEP (solar thermal electrochemical photo) production of hydrogen – an economic assessment

A consideration of the economic viability of hydrogen fuel production is important in the STEP (Solar Thermal Electrochemical Photo) production of hydrogen fuel. STEP is an innovative way to decrease costs and increase the efficiency of hydrogen fuel production, which is a synergistic process that can use concentrating photovoltaics (CPV) and solar thermal energy to drive a high temperature, low voltage, electrolysis (water-splitting), resulting in H₂ at decreased energy and higher solar efficiency. This study provides evidence that the STEP system is an economically viable solution for the production of hydrogen. STEP occurs at both higher electrolysis and solar conversion efficiencies than conventional room temperature photovoltaic (PV) generation of hydrogen. This paper probes the economic viability of this process, by comparing four different systems: (1) 10% or (2) 14% flat plate PV driven aqueous alkaline electrolysis H₂ production, (3) 25% CPV driven molten electrolysis H₂ production, and (4) 35% CPV driven solid oxide electrolysis H₂ production. The molten and solid oxide electrolysers are high temperature systems that can make use of light, normally discarded, for heating. This significantly increases system efficiency. Using levelized cost analysis, this study shows significant cost reduction using the STEP system. The total price per kg of hydrogen is shown to decrease from $5.74 to $4.96 to $3.01 to $2.61 with the four alternative systems. The advanced STEP plant requires less than one seventh of the land area of the 10% flat cell plant. To generate the 216 million kg H₂/year required by 1 million fuel cell vehicles, the 35% CPV driven solid oxide electrolysis requires a plant only 9.6 mi² in area. While PV and electrolysis components dominate the cost of conventional PV generated hydrogen, they do not dominate the cost of the STEP-generated hydrogen. The lower cost of STEP hydrogen is driven by residual distribution and gate costs.     

A techno-economic appraisal of hydrogen generation and the case for solid oxide electrolyser cells

There is significant interest in alternatives to fossil fuels in order to reduce carbon dioxide emissions. One option is the use of hydrogen in applications such as fuel cells. There are various routes to the production of hydrogen, one being via the electrolysis of water. Water electrolysers are already operational within industry on a small-scale, accounting for 4% of world hydrogen production. These electrolysers operate at low temperatures and require electrical power input that has been shown to be costly due to the limited efficiency of the electrolysis process. However, the use of high temperature solid oxide electrolyser cells (SOECs) has the potential to generate hydrogen with a higher electrical efficiency which may allow electrolysis to become cost competitive with steam methane reforming (SMR), depending on where the heat and electrical power to service the SOEC comes from.This paper examines the various routes to hydrogen production and, in particular electrolysis technologies. The cost of hydrogen production is examined in the context of the source of the electricity and the efficiency of the electrolysis process compared to SMR generation. It is found that to become cost competitive with SMR, the lowest cost electricity is required, sourced either from nuclear or combined cycle gas turbine plants with electrolysis efficiency as high as possible, meaning that SOEC technology is particularly attractive.     

Efficiency of hydrogen production systems using alternative nuclear energy technologies

Nuclear energy can be used as the primary energy source in centralized hydrogen production through high-temperature thermochemical processes, water electrolysis, or high-temperature steam electrolysis. Energy efficiency is important in providing hydrogen economically and in a climate friendly manner. High operating temperatures are needed for more efficient thermochemical and electrochemical hydrogen production using nuclear energy. Therefore, high-temperature reactors, such as the gas-cooled, molten-salt-cooled and liquid-metal-cooled reactor technologies, are the candidates for use in hydrogen production. Several candidate technologies that span the range from well developed to conceptual are compared in our analysis. Among these alternatives, high-temperature steam electrolysis (HTSE) coupled to an advanced gas reactor cooled by supercritical CO₂ (S-CO₂) and equipped with a supercritical CO₂ power conversion cycle has the potential to provide higher energy efficiency at a lower temperature range than the other alternatives.     

Efficient utilization of waste heat from molten carbonate fuel cell in parabolic trough power plant for electricity and hydrogen coproduction

Direct steam generating parabolic trough power plant is an important technology to match future electric energy demand. One of the problems related to its emergence is energy storage. Solar-to-hydrogen is a promising technology for solar energy storage. Electrolysis is among the most processes of hydrogen production recently investigated. High temperature steam electrolysis is a clean process to efficiently produce hydrogen. In this paper, steam electrolysis process using solar energy is used to produce hydrogen. A heat recovery steam generator generates high temperature steam thanks to the molten carbonate fuel cell's waste heat. The analytical study investigates the energy efficiency of solar power plant, molten carbonate fuel cell and electrolyser. The impact of waste heat utilization on electricity and hydrogen generation is analysed. The results of calculations done with MATLAB software show that fuel cell produces 7.73 MWth of thermal energy at design conditions. 73.37 tonnes of hydrogen and 14.26 GWh of electricity are yearly produced. The annual energy efficiency of electrolyser is 70% while the annual mean electric efficiency of solar power plant is 18.30%.The proposed configuration based on the yearly electricity production and hydrogen generation has presented a good performance.     

Marketability analysis of green hydrogen production in Denmark: Scale-up effects on grid-connected electrolysis

Green hydrogen is produced through different methods in the lab but only a few technologies are commercialized. Cost reduction is widely expected to compete with the existing carbon-emitting alternatives. This paper compares alkaline, proton exchange membrane, and solid oxide electrolysis cells as the dominant technologies. Economic analyses with scale-up effects show meaningful differences between PEM and alkaline electrolyzers as relatively settled methods and solid oxide as an immature technology. Monte Carlo simulations on grid-connected electrolysis using the Danish electricity market confirm that both PEM and alkaline electrolyzers can already produce hydrogen with less than 3 €/Kg if taxes and levies are removed. The price may even drop below 2 €/Kg after the mass adoption of all three technologies. Furthermore, if electricity is delivered at half prices, the levelized cost of hydrogen falls around 1 €/Kg. The capabilities for cost reduction after scaling-up are 33%, 34%, and 50% in alkaline, PEM, and solid oxide electrolyzers respectively while they could get intensified with subsidization to 56%, 59%, and 70%. The results indicate that solid oxide electrolyzers can be as economical as alkaline and PEM ones. However, grey hydrogen seems to remain unbeatable without subsidized electricity and/or carbon tax adjustments.     

The effect of gas compositions on the performance and durability of solid oxide electrolysis cells

High-temperature electrolysis with various gas compositions has been performed to investigate the effects of the hydrogen partial pressure and the humidity generated by the steam electrode on the performance and durability of solid oxide electrolysis cells. The power density of the button cell used in this research is 0.48 W cm⁻² at 750 °C, and the flow rates of the air and humidified hydrogen are 100 cc min⁻¹. By changing the flow ratio of H₂:Ar:H₂O(g) from 10:0:4 to 1:9:4, the cell's OCV decreases from 0.973 V to 0.877 V, and the charge transfer resistance increases from 1.126 Ω cm² to 1.645 Ω cm². The close relationship between the conversion efficiency of high-temperature electrolysis and steam composition is evident in the increase in the cell's charge transfer resistance from 0.381 Ω cm² to 1.056 Ω cm² as the steam content changed from 40 vol% to 3 vol%. Although the electrochemical splitting of water is stimulated in the short term by excessive steam flow, the Ni-YSZ electrodes have been damaged by the steam electrode's low H₂ partial pressure. Consequently, the steam electrode's gas composition must be optimized in the long-term because of the trade-off between performance and durability, which depends on the water concentration of the steam electrodes.     

Life cycle assessment of H2 generation with high temperature electrolysis

Water electrolysis is a well-established process for hydrogen production but requires efficiency improvements to reduce costs. High temperature electrolysis (HTE) as a means to higher efficiency was advanced in the EU project RelHY. Through Life Cycle Assessment (LCA), also the environmental performance of five HTE-based hydrogen production systems was evaluated: operation with power and steam from a nuclear plant, continuous and intermittent operation with wind power and water, intermittent operation with natural gas or biogas reforming as back-up. Large scale natural gas reforming (NGR) was used as a reference. The LCA aims to identifying environmental hotspots of HTE plants and comparing their operation. The results show that stack manufacturing has the strongest impact during construction of the HTE plant while the impacts during H₂ production are largely due to power supply. All HTE variants studied lead to less life cycle CO₂-equivalent emissions than NGR. However, only the wind powered HTE variants without back-up use less energy than NGR. The other impacts and flows show different patterns. The results and limitations of the study are discussed.     

Comprehensive assessment of system efficiency and competitiveness of nuclear power plants in combination with hydrogen complex

The strategy provides construction and commissioning of a number of new nuclear power units for the development of nuclear energy in Russia. The share of nuclear power plants increase in the energy systems of Russia is predicted from 19 to 22% in the future, up to 2050. Nuclear power plants planned to involve in the primary frequency control at the same time. All these circumstances exacerbate the problem of providing nuclear power plants with a basic electrical load in the night period, including during the daily period. The energy strategy of Russia provides for the production of hydrogen by low-carbon methods, one of which is water electrolysis using nuclear power. Hydrogen production is included in the development strategy of the at operating Russian NPPs. Hydrogen production planned at the Kola NPP by water electrolysis. Thus, the article provides a rationale for the effectiveness of combining nuclear power plants with a hydrogen complex based on the production of hydrogen by electrolysis of water. The effectiveness substantiated of the new principle of combination with overheating of the working fluid steam turbine cycle of the NPP taking into account the safety of handling hydrogen. A new system proposed for the combustion of hydrogen in oxygen, which makes it possible to overheat the working fluid of the NPP steam turbine cycle with undissociated steam, which significantly reduces the content of unreacted hydrogen in the working fluid flow. In addition, a system was developed and proposed for removing unreacted hydrogen and oxygen from the steam phase of the working fluid of the NPP steam turbine cycle. Thermodynamic and technical-economic new estimates are presented and analyzed of the efficiency of combining NPP with a hydrogen complex.     

Life cycle assessment of high temperature electrolysis for hydrogen production via nuclear energy

A life cycle assessment (LCA) of one proposed method of hydrogen production—the high temperature electrolysis of water vapor—is presented in this paper. High temperature electrolysis offers an advantage of higher energy efficiency over the conventional low-temperature alkaline electrolysis due to reduced cell potential and consequent electrical energy requirements. The primary energy source for the electrolysis will be advanced nuclear reactors operating at temperatures corresponding to those required for the high temperature electrolysis. The LCA examines the environmental impact of the combined advanced nuclear-high temperature electrolysis plant, focusing upon quantifying the emissions of carbon dioxide, sulfur dioxide, and nitrogen oxides per kilogram of hydrogen produced. The results are presented in terms of the global warming potential (GWP) and the acidification potential (AP) of the system. The GWP for the system is 2000 g carbon dioxide equivalent and the AP, 0.15 g equivalents of hydrogen ion equivalent per kilogram of hydrogen produced. The GWP and AP of this process are one-sixth and one-third, respectively, of those for the hydrogen production by steam reforming of natural gas, and are comparable to producing hydrogen from wind- or hydro-electricity powered conventional electrolysis.     

Reduced concentration polarization and enhanced steam throughput conversion with a solid oxide electrolysis cell supported on an electrode with optimized pore structure

Conversion of steam to hydrogen in solid oxide electrolysis cells (SOECs) is largely limited by concentration polarization occurring in the supporting cathodes. The present study was thus aimed at reducing the concentration polarization and increasing the steam conversion by optimizing the pore structure of the cathodes. Button cells supported on cathodes with straight pore structure showed much improved performance than those with tortuous pore structure especially under steam-lean conditions (1.19 vs. 0.69 A?cm^?2 at 1.3 V, 750 °C and 10% vol. % H₂O). Impedance spectroscopy analysis indicated that the straight pore structure in the cathodes allowed for facile transport of steam, thus effectively mitigating the concentration polarization. The relation between the electrolysis current density and steam concentration determined from the button cells was used to simulate the electrolysis performance of single cells. The simulation predicts that single cells with the straight pore structure in the cathodes can achieve high steam throughput conversion even at high steam feeding rate.     

Economics of hydrogen production and liquefaction by geothermal energy

Seven models are considered for the production and liquefaction of hydrogen by geothermal energy. In these models, we use electrolysis and high-temperature steam electrolysis processes for hydrogen production, a binary power plant for geothermal power production, and a pre-cooled Linde–Hampson cycle for hydrogen liquefaction. Also, an absorption cooling system is used for the pre-cooling of hydrogen before the liquefaction process. A methodology is developed for the economic analysis of the models. It is estimated that the cost of hydrogen production and liquefaction ranges between 0.979 $/kg H₂ and 2.615 $/kg H₂ depending on the model. The effect of geothermal water temperature on the cost of hydrogen production and liquefaction is investigated. The results show that the cost of hydrogen production and liquefaction decreases as the geothermal water temperature increases. Also, capital costs for the models involving hydrogen liquefaction are greater than those for the models involving hydrogen production only.     

Configuration design and performance optimum analysis of a solar-driven high temperature steam electrolysis system for hydrogen production

A new solar-driven high temperature steam electrolysis system for hydrogen production is presented, in which the main energy consumption processes such as steam electrolysis processes, heat transfer processes, and product compression processes are included. The detailed thermodynamic-electrochemical modeling of the solid oxide steam electrolysis (SOSE) is carried out, and consequently, the electrical and thermal energy required by every energy consumption process are determined. The efficiency of the system is derived, from which the effects of some of the important parameters such as the operating temperature, component thickness of the SOSE, leakage resistance, effectiveness of heat exchangers, and inlet rate of water on the performance of the system are discussed. It is found that the efficiency attains its maximum when a proper current density is chosen. The ratio of the required electric energy to the total energy input of the system is calculated, and consequently, the problem how to rationally operate the solar concentrating beam splitting device is investigated. The results obtained will be helpful for further understanding the optimal design and performance improvement of a practical solar-driven high temperature steam electrolysis system for hydrogen production.     

Heat transfer problems for the production of hydrogen from geothermal energy

Electrolysis at low temperature is currently used to produce Hydrogen. From a thermodynamic point of view, it is possible to improve the performance of electrolysis while functioning at high temperature (high temperature electrolysis: HTE). That makes it possible to reduce energy consumption but requires a part of the energy necessary for the dissociation of water to be in the form of thermal energy.

A collaboration between France and Iceland aims at studying and then validating the possibilities of producing hydrogen with HTE coupled with a geothermal source. The influence of the exit temperature on the cost of energy consumption of the drilling well is detailed.

To vaporize the water to the electrolyser, it should be possible to use the same technology currently used in the Icelandic geothermal context for producing electricity by using a steam turbine cycle. For heating the steam up to the temperature needed at the entrance of the electrolyser three kinds of heat exchangers could be used, according to specific temperature intervals.