Reversible Solid Oxide Cells for Energy Storage in the Netherlands: Thermodynamics and Fuel Selection

Abstract

As a famously flat and low-lying country, the Netherlands has no sites for hydro-power at a large scale, and high-temperature geothermal energy is not easily accessible. Therefore, the Netherlands will need to depend on intermittent renewables such as wind and solar energy for decarbonizing its energy system. This results in a definite need for large scale energy storage in the future Dutch energy landscape. With the lack of potential for pumped hydro storage, but easy access to large salt caverns and gas fields, energy storage in e-fuels (i.e. power-to-X-to-power) seems to be the most likely candidate for energy storage systems in the Netherlands at such a large scale. Among the various technologies available to implement such systems, reversible solid oxide cells (rSOC) are very suitable due to high efficiency and fuel flexibility, in addition to higher capacity utilization made possible by reversible operation.In this study, we investigated three fuels — hydrogen, methane and ammonia — to find the most suitable fuel for use in a standalone rSOC-based energy storage system to balance a future Dutch electrical grid powered entirely by intermittent renewables. In particular, the fuels were compared in terms of the storage capacity and the renewable power capacity that would be required if each of these fuels were used. The effect of a limited use of biomass to reduce energy storage needs was also investigated. Energy from biomass was assumed to stay around the same level as it is at present. Heat from nuclear, concentrated solar, or industrial sources was not considered.Process designs of rSOC systems using each fuel were modelled in Aspen Plus, and the operating points were optimized for round trip efficiency using exergy analysis. The power-fuel flow relationships derived from these models were then used in a model of an energy grid implemented in the programming language R. This model used actual hourly weather data and Dutch energy demand over a period of five years (2014–2018) for supply/demand calculations with a temporal resolution of one day. The required storage capacity and power generation capacity were derived from the results of this energy grid model. Supply-demand imbalances within each day-long interval were assumed to be balanced using batteries, and was outside the scope of this study.From the results of this study, it appears that for reversible SOC systems under the set of assumptions taken, a hydrogen-based system can be more efficient than systems using methane or ammonia. This implies that a grid using hydrogen as its energy storage medium can meet the same demand with a smaller power generation capacity compared to the other fuels. We also found the methane system to be slightly more efficient than the ammonia system.On the other hand, the comparison of storage capacities revealed that the methane system has significantly lower gas storage requirement than the hydrogen system for the same energy demand. The ammonia system also requires larger storage capacity for gases than the methane system, in addition to tanks for liquid ammonia. The use of biomass in winters is found to reduce both the storage and power generation requirements.The results show that the available underground gas storage capacity in the Netherlands is more than required to meet the Dutch electrical demand, with systems using any of the three fuels. However, the renewable power capacity planned for 2030 in the Netherlands will not be sufficient to meet the projected electricity demand in 2030, irrespective of the fuel used for storage. Under these conditions, the system which uses the power capacity most efficiently would be favorable for 2030, which is the hydrogen system.This study compares different fuels from a thermodynamics (exergy) and system sizing perspective. In addition to detailed exergy analysis, further comparative studies will be needed from techno-economic, safety, and social acceptance perspectives before a definitive choice can be made. This study also considers only electrical demand, while the gas storage capacity in the Netherlands might be sufficient to meet other energy demands (heat, transport, industrial feedstock) as well. A study considering other energy demands can be part of future work.With these considerations in mind and with the current design choices, based on current plans for growth of renewables, we find that hydrogen is the most suitable fuel for rSOC-based energy storage in the Netherlands.