Techno-economic Assessment of Solid Oxide Fuel Cells and Fuel-assisted Electrolysis Cells in Future Energy Systems

The Master's programme Industrial Ecology is jointly organised by Leiden University and Delft University of Technology - There is an increasing awareness in society of the problems and risks associated with climate change. In response, a transition from fossil to renewable energy sources is taking place. Meanwhile, there is an ongoing trend of increasing electricity demand for various applications. There is a growing number of electric appliances, as well as the electrification of heating, cooling, and transportation services. These changes provide a challenge when it comes to matching electricity production and consumption. The intermittent nature of wind and solar energy, combined with the large loads of heat pumps and BEVs, reduces the flexibility and robustness of the system in the future. Solid oxide cells (SOCs) can contribute to solving the balancing problem of the electricity grid. SOCs perform electrochemical conversions and are developed for decentral energy conversion applications. A solid oxide fuel cell (SOFC) uses a fuel to produce electricity, hydrogen, and heat. Solid oxide fuel-assisted electrolysis cells (SOFECs) consume electricity and a fuel while producing hydrogen and heat. Gaseous fuels like natural gas, hydrogen, and biogas can be used. SOFCs have been under investigation for decades, and are mainly applied as backup power source. SOFEC technology, on the other hand, is rather unknown. Only a dozen of articles has been published, dedicated to theoretical models and small-scale lab tests. These SOCs are seen as promising technologies, because of the high energy efficiency, flexible operation, and ability to produce hydrogen. The aim of this research is to assess whether it is feasible to introduce SOFCs and SOFECs in the (future) Dutch energy system. This research takes a broad perspective, by assessing the technological components in relation to the energy system as a whole. In addition, multiple aspects that determine the feasibility are assessed. These aspects are the technical possibilities and limitations of SOCs, their cost-effectiveness under different assumptions, the environmental implications, and societal drivers and barriers for the introduction in niche markets. Existing literature shows that the commercial competitiveness of SOCs is currently limited because of high investment costs and a limited lifetime. The manufacturing costs can be decreased by learning through research and development, and higher production volumes. To prolong the service life, degradation of the electrodes at microscopic level should be minimised. Recent research indicates that regular switching between fuel cell and (fuel assisted) electrolysis mode can also prevent degradation. It is therefore recommended to further investigate how the service life is prolonged for a SOFC that is operated regularly as SOFEC. A thermodynamic 1-D cell model was used to describe the inputs and outputs of a SOFC and a SOFEC. Based on this model, non-linear sets of equations with two degrees of freedom were derived for both technologies. These equations approximate the energy conversion by SOCs for different current densities and fuel utilisations. The level of complexity of these equations was suitable for the further analysis of the operation of the technologies. The derived equations were used to implement SOFC and SOFEC technologies in oemof, an energy system optimisation framework. The framework was then used to model the energy system of the Netherlands, including all major energy carriers, i.e. fossil fuels, electricity, heat, biomass, and hydrogen. In the model, energy production, conversion, and consumption technologies are included, as well as corresponding hour-by-hour patterns. An optimisation algorithm is used to find the optimal combination of technologies and their optimal operation that results in a system that most effectively fulfils demand. The objective is to minimise the total system costs, excluding subsidies or taxes. Costs related to GHG emissions, carbon tax or emission certificates, are included. By doing so, it is the first optimisation model of its kind for the Netherlands. Not all aspects of the real energy system are represented in the model. Most notably, energy transport and demand response are not included. The results clearly indicate that SOFCs can become an important part of the Dutch energy system if the costs decrease sufficiently and a longer lifetime of the stacks is achieved. The cost-effectiveness stems from the efficient co-production of three energy carriers and the low resource use. High GHG emission taxes are favourable for the diffusion of SOFCs. Their installation results in an emission reduction of around 5.2% (or 8 MtCO2-eq./a) compared to a system without fuel cells. No evidence was found for the influence of SOFCs on the competitiveness of renewable electricity sources. Under some conditions, SOFCs can in fact squeeze solar and wind out of the electricity market. If the costs of operating SOFCs remain high, then the installed capacity is limited to an extent for which high capacity utilisations can be achieved. The production of hydrogen is maximised to make expensive SOFCs profitable, indicating that this energy carrier is most important for a cost-effective introduction. If costs can be reduced, SOFCs prove to be a suitable technology for electricity production during periods of low renewable electricity production, while also responding to fluctuating H2 demand. It becomes viable to operate the fuel cells with more flexibility when the technology is less expensive. The influence of the demand on SOFC operation is also considerable. In a scenario with a diverse demand for both heat, hydrogen, and electricity, a wide range of settings is used. In other scenarios, different operating strategies were more optimal. Only two or three operating points were used frequently, while other settings were applied seldom. For the requirements for SOFC technology, this implies that the preferred technical specifications depend on how the energy demand will develop. The role that SOFECs can play in the Dutch energy system is much more limited than that of SOFCs. The conditions for cost-effectiveness are also stricter. A high level of renewable electricity production and/or high GHG emission taxes are required, on top of large cost reductions for SOC technology. Furthermore, its applicability is limited by the amount of biogas available. If SOFCs and SOFECs with the same costs are both available, a SOFC is often preferred, since it allows for much larger system cost reductions. The optimal operating strategy of SOFECs depends on their costs: if the technology becomes less expensive, it is cost-effective to operate at lower current densities with higher efficiency. Furthermore, SOFECs show a limited variation in the production rate over time. Unlike regular electrolysers, SOFECs are hardly operated to prevent electricity excesses. By operating almost continuously, a large fraction of biogas is utilised. A SOFEC profits from high GHG taxes, as it is (almost) carbon neutral. The societal aspects of the development and potential of SOC technology were assessed using the multi-level perspective. This framework conceptualises a niche innovation as being nested structure within a socio-technical regime and a socio-technical landscape. Within the dominant regime of energy production and consumption, there is no fully developed solution to address the challenge of increased fluctuations of electricity demand and intermittent production. Considerable research efforts in the field of SOC technology are undertaken. SOFC have received most attention, resulting in several pilot projects and growing production volumes. Few activities are taking place around SOFECs. A possible strategy is to focus on SOFC applications first, suspending efforts related to SOFECs. When lower electricity prices and lower SOC costs are achieved, it is more viable to introduce a SOFEC.Actors involved in the development of SOFCs are active, with several demonstration projects and market introductions in the area of micro-CHP applications. These dynamics could result in an uptake in the existing regime structure without exploiting and exploring the capability of SOFCs to co-produce hydrogen. Yet this study has shown the advantage of H2 production for cost-effective SOFC applications. It is deemed crucial that a user or market for the energy products of the SOFC or SOFEC is found. Therefore, the energy carrier markets for inputs and outputs form the starting point for identifying market niches, in which the SOC technology can be introduced first. Examples of identified niche applications include a farmer who uses biogas from manure digestion and supplies H2 to trucks, or a neighbourhood community that invests in a common installation. A SOC provides heat to households in a neighbourhood, while H2 is used for fuel cell vehicles or to provide electricity at peak demand. To convince potential users, these perceived barriers can be addressed or the advantages of SOCs can be emphasised. The high efficiency, low emissions, and importance for local grid balancing of SOFCs and SOFECs are arguments in favour of these technologies. In addition, the commitment of other stakeholders in pilot projects or through Green Deal agreements can stimulate the adoption. Highlights • SOC costs should decrease significantly to become an attractive addition to the (future) Dutch energy system. • Co-production of hydrogen is important for the cost-effectiveness of SOFCs. • SOFCs directly reduce GHG emissions and increase resource efficiency. • SOFCs compete with renewable electricity sources and do not facilitate their introduction. • SOFECs are economically less attractive than SOFCs, and their potential is limited by the availability of biogas. • Ideal operating strategies for SOFC and SOFEC installations in the context of an energy system were assessed, and shown to depend on the demand composition and technology costs. • Markets or consumers for heat and hydrogen are a key factor for first applications of SOC technologies.Technology, Policy and ManagementValues Technology and InnovationIndustrial Ecology Joint degree Leiden & Delf