Trade-off designs and comparative exergy evaluation of solid-oxide electrolyzer based power-to-methane plants

Solid-oxide electrolyzer (SOE) based power-to-methane (PtM) system can efficiently store surplus renewable power into synthesis natural gas by electrolysis and methanation. The system performance depends on the operating point of the electrolyzer and system design, particularly the heat exchanger network. In this paper, we investigate a SOE based PtM plant with a fixed-bed catalytic methanator and a membrane module for methane upgrading. A top-down approach is first employed to derive optimal system designs step by step from the system concept, to optimal conceptual designs with the trade-off between system efficiency and methane yield, to design-point selection and heat exchanger network design. Then, exergy evaluation with the exergy calculated into thermal - mechanical - non-reactive - reactive parts is applied to the derived four specific system designs to understand how exergy dissipation and performance of the overall system and each component vary from one to another. The results show that the system efficiency can reach between 80 and 85% (HHV) or 75-80% (LHV) when operating SOE with an inlet temperature of 700 degrees C and a utilization factor over 60%, above which electrical steam generation can be avoided and the steam can be generated by the heat from methanation reaction (around 80-85%) and anode outlet (15-18%). The system's exergy efficiency can achieve around 75-80% with the input exergy mainly destructed within the SOE (25-35%), methanator (25-35%) and heat exchangers (10-17%). However, exergy efficiencies of the SOE and methanator are high, over 90%. Depending on the temperature level of the cold stream and the temperature difference, heat exchangers generally have an exergy efficiency of over 50-80%. The electrical steam generator can only achieve an efficiency of around 20% and leads to a significant drop of system efficiency if employed; however, small electrical heating to reach the desired SOE inlet temperature, although bad, is acceptable. Therefore, one preliminary design guideline for such systems should be the avoidance of electrical steam generation. (C) 2018 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Power-to-fuels via solid-oxide electrolyzer: Operating window and techno-economics

Stefan Diethelm, Tzu-En Lin, François Maréchal, Jan Van Herle, Ligang Wang

Power-to-fuel systems via solid-oxide electrolysis are promising for storing excess renewable electricity by efficient electrolysis of steam (or co-electrolysis of steam and CO2) into hydrogen (or syngas), which can be further converted into synthetic fuels with plant-wise thermal integration. Electrolysis stack performance and durability determine the system design, performance, and long-term operating strategy; thus, solid-oxide electrolyzer based power-to-fuels were investigated from the stack to system levels. At the stack level, the data from a 6000-h stack testing under laboratory isothermal conditions were used to calibrate a quasi-2D model, which enables to predict practical, isothermal stack performance with reasonable accuracy. Feasible stack operating windows meeting various design specifications (e.g., specific syngas composition) were further generated to support the selection of operating points. At the system level, with the chosen similar stack operating points, various power-to-fuel systems, including power-to-hydrogen, power-to-methane, power-to-methanol (dimethyl ether) and power-to-gasoline, were compared techno-economically considering system-level heat integration. Several operating strategies of the stack were compared to address the increase in stack temperature due to degradation. The modeling results show that the system efficiency for producing H2, methane, methanol/dimethyl ether and gasoline decreases sequentially from 94% (power-to-H2) to 64% (power-to-gasoline), based on a higher heating value. Co-electrolysis, which allows better heat integration, can improve the efficiency of the systems with less exothermic fuel-synthesis processes (e.g., methanol/dimethyl ether) but offers limited advantages for power-to-methane and power-to-gasoline systems. In a likely future scenario, where the growing amount of electricity from renewable sources results in increasing periods of a negative electricity price, solid oxide electrolyser based power-to-fuel sys

2019

Thermo-economic Optimization of a Solid Oxide Fuel Cell, Gas Turbine Hybrid System

Nordahl Autissier, Daniel Favrat, François Maréchal, Francesca Palazzi, Jan Van Herle

Large scale power production benefits from the high efficiency of gas-steam combined cycles. In the lower power range, fuel cells are a good candidate to combine with gas turbines. Such systems can achieve efficiencies exceeding 60%. High temperature Solid Oxide Fuel Cells (SOFC) offer good opportunities for this coupling. In this paper, a systematic method to select a design according to user specifications is presented. The most attractive configurations of this technology coupling are identified using a thermo-economic multi-objective optimization approach. The SOFC model includes detailed computation of losses of the electrodes and thermal management. The system is integrated using pinch based methods. A thermo- economic approach is then used to compute the integrated system performances, size and cost. This allows to perform the optimization of the system with regard to two objectives: minimize the specific cost and maximize the efficiency. Optimization results prove the existence of designs with costs from 2400

/kW for a 44% efficiency to 6700

/kW for a 70% efficiency. Several design options are analysed regarding, among others, fuel processing, pressure ratio or turbine inlet temperature. The model of a pressurized SOFC-microGT hybrid cycle combines a state-of- the-art planar SOFC with a high speed micro gas turbine sustained by air bearings.

2007

Optimal design of solid-oxide electrolyzer based power-to-methane systems: A comprehensive comparison between steam electrolysis and co-electrolysis

Stefan Diethelm, Hossein Madi, François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang

Power-to-methane technologies have been regarded as a promising alternative to offer small or large-scale, long-timescale (daily/weekly/seasonal) energy storage as well as the opportunity of utilizing CO2. The performance of the core component, the electrolyzer, largely determines how well a power-to-methane system can perform, making high-temperature solid-oxide electrolysis attractive because of its inherent high electrical ef- ficiency. More importantly, solid oxide electrolysis uniquely allows co-electrolysis of steam and CO2 for producing syngas, the composition of which can be readily, flexibly adjusted to synthesize different hydrocarbon fuels. In this paper, for both steam and co-electrolysis, we comprehensively and comparatively investigate several critical design issues of a solid oxide electrolyzer based power-to-methane system with fixed bed methanation reactor and membrane-based methane upgrading: (1) system level heat integration, (2) the impacts of operating variables (e.g., operating voltage, reactant utilization, anode/cathode feed ratio, and operating pressure of the methanation reactor and membrane) on system performances, (3) the competitiveness of the electrolyzer operation with pure oxygen production, and (4) the possibility of avoiding electrical heating, which is necessary for thermoneutral operation to heat up the electrolyzer feeds to the required temperature. To achieve this target, a multi-objective optimization platform with integrated heat cascade calculation is employed with experimentally calibrated component models. The results show that, for both steam and co-electrolysis, there is a trade-off between system efficiency and methane yield: pursuing a higher efficiency generally reduces the methane yield, which is a consequence of electrochemistry, stack cooling and system-level heat integration. Instead of sweep air, pure oxygen production is preferred only at small current density, which delivers the highest system efficiency but the lowest methane yield. When the electrolyzer operates exothermically, methane production and the total power consumption can vary in much wider ranges than those with the electrolyzer operating under thermoneutral mode, which leads to potential enhancement of operation flexibility and reliability. The co-electrolysis coupling with strongly-exothermic syngas methanation, in general, offers better heat- integration opportunity with sweep air, but less with pure oxygen production. In addition, several design heuristics, e.g., the operating pressure of the electrolyzer and methanation reactor, are concluded to potentially guide practical applications.

2018