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Methanol is considered as an essential base chemical, which is widely known for its versatility and broad potential use contexts in future chemical industries and energy storage applications. Methanol production pathways powered by renewable electricity sources, also known as power-to-methanol processes, are characterized by low specific life-cycle emissions and are therefore of paramount interest. One possible renewable process chain features proton-conducting high temperature electrolyzers combined with a direct hydrogenation of CO2. In this paper, a techno-economic forecast study of this process chain is presented and specific production costs of renewable methanol under different electricity supply scenarios are determined and discussed for the years 2030 and 2050. The studies showed that flexible grid-supported scenarios through direct spot-market participation and renewable scenarios enable the largest production cost reduction potential in the upcoming decades.

Renewable energy sources are the major pathway to reduce the carbon dioxide emission in the energy sector. However, the fluctuating electricity generation of solar or wind power plants encounter mismatches between energy generation and demand as well as possible grid instability issues. For that reason, large-scale thermal energy storage (TES) systems are one of the promising solutions to overcome these issues and to allow the operation of a sustainable and reliable energy system based on renewable energy sources. A TES system converts electrical energy into thermal energy during power surplus. The thermal energy is stored and reconverted to electricity during power demand by a supercritical carbon dioxide (sCO2) power cycle. TES has many advantages, such as simplicity, low cost and reliability over alternative storage technologies. The sCO2 power cycles on the other hand offer a variety of advantages, such as a high conversion efficiency, a compact turbomachinery and a temperature glide that fits well with sensible thermal energy storages. To further develop and assess this approach, the EU-project CEEGS (CO2 based electrothermal energy and geological storage system) deals with the development of a high-efficient, cost-effective and scalable energy storage technology. Here, transcritical CO2 cycles are integrated with underground energy storage to achieve, simultaneously, long-term CO2 sequestration and, potentially, geothermal heat extraction. During periods of excess electricity generation, the charging cycle is in operation. Here, CO2 is received from a stationary source and an electric motor drives a compressor of a heat pump system to increase the CO2 pressure from approximately 35 bar to 200 bar, while increasing the temperature from approximately 15 °C to 150 °C. The sCO2 heats up a hot-water storage while reducing its temperature to approximately 45 °C before entering an expansion turbine During the expansion the temperature and pressure are reduced to 0°C

The importance of methanol as a basic building block of the chemical industry and as a means of chemical energy storage of renewable energy sources (e.g. wind & PV) will steadily increase in the upcoming years. Based on renewable electricity and through the coupling of a proton-conducting steam electrolyzer for the generation of pure H2 with a heterogeneously catalyzed direct synthesis of methanol from anthropogenic CO2, an attractive method for the production of methanol can be provided. To enable an efficient and economic application of these so-called power-to-methanol processes, high system efficiencies as well as suitable concepts for system control as well as system and heat integration for alternating operating conditions are of particular importance. In this work, a transient and real-time capable system model of a power-to-methanol process based on tubular proton-conducting high temperature electrolyzers is presented. The obtained stationary simulation results reveal beneficial operational windows and system efficiencies (0.488 to 0.617) with respect to the chosen process design and heat integration concept. The power-to-methanol process model also incorporates a multitude of feedback control loops or controllers, to manipulate relevant operating parameters of all employed sub-processes in case of fluctuating power inputs. The presented studies assess the transient responses of the modeled power-to-methanol system to defined step changes of the apparent cell voltage under negative feedback control of crucial operational parameters.

Methanol is a crucial commodity in the chemical industry and is employed as precursor for many products. It can be used to store fluctuating renewable energy, specifically benefiting from its liquid state at ambient temperatures. As the demand for green, renewable methanol is projected to soar in the next decades, environmentally friendly and sustainable pathways for its production have to be provided. Through the combination of proton-conducting high temperature electrolysis for the provision of dry H2 with a heterogeneously catalyzed hydrogenation of CO2, efficient and simple power-to-methanol production processes can be established. Here, a novel power-to-methanol system model capable of real-time transient simulation is presented and viable operating windows are determined for different key operating parameters of the respective main process stages. A techno-economic assessment is carried out to determine the specific production costs of renewable methanol. Specific methanol production costs of 2419 €∙t-1MeOH for small-scale applications (1.12 MW) were retrieved, which corresponds to a more than fourfold increase over the current market price of conventionally produced methanol. Increases in system scale are found to decrease the methanol production costs due to economy-of-scale effects. The sensitivity of the process economics is assessed with regards to crucial operational and capital characteristics.

The necessity of energy independency, political and climatic concerns is driving the global attention towards sustainable energy sources. The European commission targets carbon-neutrality with net zero greenhouse gas emission by 2050 [1], which results in a decarbonization of the energy sector by renewable energy sources. Nevertheless, a significant challenge arises from the fluctuating power infeed of wind and solar based plants, which may result in grid instability, overstrain and mismatch in supply-demand. Thus, energy storage systems (ESS) are required to decouple electricity generation from the demand and to enhance the reliability of the energy system. Furthermore, a successful implementation of such a storage technology on a large scale requires cost-effective cycles for energy storage and reconversion. Therefore, a Power-to-Heat-to-Power system is proposed, based on electrical charged high temperature ceramics and a supercritical CO2 (sCO2) power cycle for discharge. The high temperature sensible thermal energy storage is manufactured from waste material of the aluminum production, which is a low-cost storage solution. As a result, such a thermal storage combines an important contribution to the circular economy with an economic solution for large scale, location-independent energy storage. CO2 becomes supercritical when temperature and pressure are higher than 31 °C and 74 bar. In this phase the viscosity is low and the density as well as the heat capacity are high. Hence, the installed components, in particular the turbomachinery of the sCO2 power cycle are highly compact, which results in low investment costs and low thermal losses. Furthermore, high temperatures up to 600 °C can be utilized to generate electricity at high thermal efficiency. An inhouse code was used to study various heat transfer fluids, materials and designs for the thermal storage vessel during the charge and discharge cycle. Furthermore, the heat exchanger transferring the heat from t

Large-scale energy storage plants based on power-to-gas-to-power (PtG-GtP) technologies incorporating high temperature electrolysis, catalytic methanation for the provision of synthetic natural gas (SNG) and novel, highly efficient SNG-fired Allam reconversion cycles allow for a confined and circular use of CO2/CH4 and thus an emission-free storage of intermittent renewable energy. This study features a thorough technology assessment for large-scale PtG-GtP storage plants based on highly efficient sCO2 power cycles combined with subsurface CO2 storage. The Allam cycle employs supercritical CO2 as working fluid as well as an oxy-combustion process to reach high efficiencies of up to 66 %. The entire PtG-GtP process chain assessed in this study is expected to reach maximum roundtrip efficiencies of 54.2 % (with dedicated and sufficient O2 storage) or 49.0 % (with a dedicated air separation unit). The implementation of said energy storage systems into existing national energy grids will pose a major challenge, since they will require far-reaching infrastructural changes to the respective systems itself, such as extensive installations of renewable generation and electrolysis capacities as well as sufficient subsurface storage capacities for both CO2 and CH4. Therefore, this study incorporates an assessment of the present subsurface storage potential for CO2 and CH4 in Germany. Furthermore, a basic forecast study for the German energy system with an assumed mass deployment of the proposed SNG-based PtG-GtP energy storage system for the year 2050 is conducted. In case of a fully circular use of CO2/CH4, when electricity is solely generated by renewable energy sources, 736 GW of renewables, 234 GW of electrolysis and 62 GW of gas-to-power capacities are required in the best case scenario in 2050. The total storage volume on the national scale of Germany for both CO2 and CH4 was determined to be 7.8 billion Nm3, respectively, leading to a CH4 storage capacity of 54.5 TWh.

The conversion of renewable energy into hydrogen (H2) by power-to-gas technologies involving electrolysis is seen today as a key element in the transition to a renewable energy sector. Wastewater treatment plants (WWTP) could make use of oxygen (O2) produced alongside H2 in biological treatment steps, however, production costs of electrolysis O2 should be competitive with respect to those of conventional O2 production processes. In this study, mathematical models of a polymer electrolyte membrane electrolyser (PEME) plant and the WWTP of the Benchmark Simulation Model Nr. 2 (BSM2) were used to simulate electrolysis O2 supply to an activated sludge (AS) system and estimate net costs of production (NCP) via a techno-economic assessment (TEA). Assuming that produced H2 is sold to a nearby industry, NCPs for O2 were calculated for two scenarios regarding PEME plant dimensions, which correspond to average (scenario 1) and optimal (scenario 2) electricity availability, i.e. medium and high number of full load hours per year. For each scenario, estimations were made for four alternatives for electricity supply and costs, namely conventional sources, photovoltaic (PV), and on/off shore wind energy, and using sets of optimistic, neutral and pessimistic data regarding system performance and market conditions. The analyses were done for 2020 as reference year and again for 2030 based on forecasts of relevant data. The results of the dimensioning of the PEME plant in scenario 1 show that a 6.4 MW PEME operated for 4,073 full load hours per year is able to cover the O2 demand of the AS system during more than 99% of the simulated period. The same is true for a 4.8 MW PEME operated for 6,259 full load hours per year in scenario 2. The TEA shows that investment costs for the PEME stacks and the operational costs for electricity make up most of the NCP of electrolysis O2. Although NCP for electrolysis O2 are always higher than those of conventional O2 sources for the year 2020, in

Input files and simulation results for the related publication.

Large-scale energy storage plants based on power-to-gas-to-power technologies incorporating high temperature electrolysis, catalytic methanation of H₂ and CO₂ and novel, highly efficient methane-fired Allam reconversion cycles allow for a confined and circular use of CO₂ and thus an emission-free storage of intermittent renewable energy. The Allam power cycle is considered as a beneficial power plant concept, which employs supercritical CO₂ as working fluid as well as an oxy-combustion process to reach high efficiencies of up to 66%. The combination of said process chain could reach a maximum roundtrip efficiency of 54.2 % assuming the presence of sufficient storage capacities for all relevant technical gases. In a technically feasible scenario, paired with a separate air separation unit instead of stationary O₂ storages, roundtrip efficiencies of 49.0 % were determined.. The implementation of said energy storage systems into existing national energy systems will pose a major challenge, since they will require far-reaching infrastructural changes to the respective systems itself, such as extensive installations of renewable generation and electrolysis capacities as well as sufficient subsurface storage capacities for both CO₂ and CH₂. Furthermore, an exemplary energy system forecast for Germany for the year of 2050 is presented to show the viability of the energy storage concept. In case of a fully circular use of CO₂, when electricity is solely generated by renewable energy sources (RES), 736 GW of RES, 234 GW of electrolysis and 62 GW of gas-to-power capacities are required. The total storage volume on the national scale of Germany for both CO₂ and CH₄ was determined to be 7.8 billion Nm³, respectively, leading to a CH₄ storage capacity of 54.5 TWh. The present investigation illustrates the feasibility of large-scale energy storage systems for renewable electricity based on high temperature electrolysis, catalytic methanation and Allam power cycles paired with lar

Die Speicherung von überschüssiger Elektroenergie aus regenerativen Energiequellen in relevanten Größenordnungen stellt eine wesentliche technologische Hürde der Energiewende in der Bundes-republik Deutschland dar. Der nationale Strombedarf in Zeiten unzureichender regenerativer Er-zeugung kann zukünftig mit Speicherkraftwerken auf Basis von aus CO2 und Elektrolysewasser-stoff produziertem synthetischen Methan (Power-to-Gas) und methangefeuerten, hocheffizienten Kraftkreisläufen (Gas-to-Power) basierend auf dem Allam-Kreislauf gedeckt werden. Durch die Nutzung von überkritischem CO2 als Kreislaufmedium lassen sich Wirkungsgrade von bis zu 66% für den Gas-to-Power-Pfad und Gesamtwirkungsgrade des Speicherzyklus von >50% erzielen. Zur Umsetzung dieses Konzeptes sind ausreichende Speicherkapazitäten für CO2, CH4 und O2 sowie regenerative Erzeugungskapazitäten vorzuhalten. Der vorliegende Beitrag untersucht den vorge-stellten Energiespeicherzyklus unter Annahme eines geschlossenen CO2-Kreislaufs gekoppelt mit geologischer Untergrundspeicherung von CO2 und CH4 im Sinne des Carbon Capture, Utilization and Storage (CCUS). Zur technischen Bewertung des Konzeptes wurde das Potential der Speiche-rung von CO2 in Aquiferen in Deutschland analysiert und eine Studie des deutschen Energiesys-tems im Jahre 2050 für verschiedene Orientierungsszenarien durchgeführt. Mit Hilfe der erstellten Szenarien konnten die zur Erlangung einer CO2-neutralen Energiespeicherung benötigen Kapazitä-ten der regenerativen Stromerzeugung, elektrolytischen Wasserstoffbereitstellung und Rekonversi-on von CH4 sowie Speicherkapazitäten für CO2, CH4 und O2 abgeleitet werden.

Der Großteil elektrischer Energieerzeugung erfolgt durch thermische Kraftkreisläufe, welche durch eine Wärmequelle versorgt werden. Hierbei werden zwei Typen kommerzieller Anlagen betrieben, der offene direkt befeuerte Brayton oder Joule-Kreislauf (bsp. Gasturbinenkreisläufe) und der geschlossene indirekt befeuerte Rankine-Kreislauf (Dampfkreislauf). Neben diesen konventionellen Kraftkreisläufen existiert seit einigen Jahren, basierend auf den Brayton-Kreislauf, ein innovatives Konzept mit superkritischen Kohlenstoffdioxid (sCO2) als Arbeitsmedium. Zahlreiche Studien haben gezeigt, dass sCO2 Kreisläufe das Potential haben, höhere Wirkungsgrade zu erreichen, einen niedrigeren Brennstoff- und Wasserverbrauch zu erzielen (Subbaraman et al., 2011; Kacludis et al., 2012; Shelton et al., 2016). SCO2 kombiniert den Vorteil der hohen Dichte von Flüssigkeiten und die niedrige Viskosität von Gasen, sodass geringere Komponentengrößen, kleinere Footprints der Gesamtanlage und damit niedrigere Kapitalkosten möglich sind. Bei der Prozessführung wird zwischen dem offenen und dem geschlossenen sCO2 Brayton-Kreislauf unterschieden. Bei einem geschlossenen Kreislauf wird sCO2 nahe des kritischen Punktes (ca. 31 °C und 74 bar) verdichtet, in einem Rekuperator vorgewärmt, in einem Erhitzer durch die Wärmequelle aufgeheizt, in einer Expansionsturbine entspannt, durch den Rekuperator auf der Gegenseite geführt und anschließend bis nahe des kritischen Punktes zurückgekühlt, um den Kreislauf erneut zu beginnen. Dieser Prozess kann für unterschiedlichste Wärmequellen, wie fossile Kraftstoffe, Kernenergie, industrielle Abwärme, Geothermie oder Solarstrahlung eingesetzt werden. Der offene Kreislauf adressiert dagegen höhere Temperaturen und wird üblicherweise für eine Sauerstoffverbrennung untersucht. Dabei wird ein Sauerstoff-Methan-Gemisch bei hohen Druck und hoher Temperatur verbrannt. Das Rauchgas wird dann, vermischt mit großen Mengen sCO2, in einer Turbine entspannt. Das sCO2-Wasserdamp

Das Ziel des Teilvorhabens des HZDR war die thermodynamische, strömungstechnische und regelungstechnische Beschreibung und Auslegung des Gesamtsystems des DELTA-Reaktors. Mit Hilfe von geeigneten Simulationsmethoden sollte das Betriebsverhalten des räumlich und thermisch eng gekoppelten Reaktorkonzepts von Hochtemperaturelektrolyse sowie Methanolsynthese untersucht und die durch den Projektpartner TUD-WKET durchgeführten Arbeiten zur konstruktiven Gestaltung des Prozesses begleitet und unterstützt werden. Im Projektverlauf sollte außerdem gemeinsam mit dem Projektpartner TUD-WKET ein Mess- und regelungstechnisches Konzept entwickelt sowie ein geeignetes Analysengerät zur Bestimmung des Produktgasgemisches des Reaktionssystems ausgewählt, beschafft und in die experimentelle Versuchsplattform integriert werden. Darüber hinaus sollte im Rahmen des Teilprojektes ein Simulationsmodell auf Systemebene erstellt und das Verhalten des DELTA-Reaktors unter fluktuierenden Lasten beschrieben werden.

Since the utilization of wind and solar energy is constantly growing, the demand for technologies for temporal and spatial decoupling of energy provision and consumption is steadily increasing. The application of proton-conducting solid oxide electrolysis cells (H-SOECs) has been a main concern of recent research activities since they offer an environmentally friendly and efficient technique for the conversion of excess energy and steam into hydrogen. An appropriate coupling of stationary carbon dioxide (CO2) emitters and consumers offers a promising option for chemical energy storage and the production of valuable chemicals (e.g. methanol). Using electrolytic hydrogen production together with suitable downstream syntheses units as combined power-to-liquid technologies, a promising method for the recycling of carbon dioxide (CCU) can be provided. Since renewables occur intermittently, SOECs have to be capable of withstanding harsh operating conditions and the knowledge of their dynamic behavior is crucial for their future system application. This work studies the transient behavior of a single tubular, proton conducting SOEC during rapid load variations through 2D-FEM single cell and quasi-2D system scale simulations and evaluates the usability of the cell in combination with fluctuating loads.