Modelling of future integrated energy systems and the potential role of renewable gas

Abstract

Climate change and emissions of greenhouse gases (GHG) have received worldwide attention over the last decades. The Paris Agreement is the global response to mitigate climate changes aiming at limiting the increase of average global surface temperature. The specific aim is to keep temperature increases well below 2°C above pre-industrial levels, and even to pursue efforts to keep the raise to 1.5°C in this century. A substantial transformation of the energy system is needed in the future to reduce GHG emissions from the energy sector and combat climate change. The energy transition will have to head towards clean energy production based on renewable and sustainable energy resources and/or increased use of carbon sinks. The profound and urgent transition of the energy system will be challenging, in particular, in the hard-to-abate sectors, such as high-temperature process heat demand, heavy-duty road transport, shipping, aviation, and peak power demand. Gas, renewable gas and the already-existing gas infrastructure may play a key role in facilitating an effective and cost-efficient energy transition. Gas can: 1) be used in multiple ways in various sectors; 2) be used in conversion technologies to produce liquid biofuels and electrofuels; 3) facilitate system integration by sector coupling; 4) serve as a flexible resource, both in the short- and long-term, and provide flexible generation of electricity and heat in peak-load situations. This PhD study assesses the arising question of the future role that gas, renewable gas and the gas infrastructure may have in the energy transition towards a climate-neutral Danish energy system. This PhD study contributes to the research field within two main themes: i) improved integrated energy systems modelling, and ii) comprehensive assessments of the future role gas may have in the energy system. A central part of the research contribution from this PhD study is the development and improvement of energy systems models and methodologies. The new modelling frameworks include a sufficient representation of the gas system as an integrated part of future energy systems, and thereby allow holistic energy systems assessments. The developments and improvements are primarily implemented in the energy systems optimisation model, Balmorel, and encompasses: 1) Modelling of a comprehensive network from locally distributed available biomass resources, via transportation of the resources to conversion plants, which produce renewable gas and renewable liquid fuels including an extensive representation of electrofuel production pathways. The production of renewable gas and liquid fuels is optimised while taking synergies between the conversion technologies and the electricity and district heating into account; 2) Modelling of energy sectors, where the use of gas may play a role in the future, for example, the industrial sector, while assessing the inherent uncertainty related to the use of gas in the future; 3) Modelling of different investment planning foresight horizons to assess the impacts that could have on the energy system scenarios; 4) Modelling of gas flows in the gas transmission infrastructure, from the production of renewable gas, through compressors, via transport of gas in pipelines, to large-scale storage facilities, short term storage in line-packing, and to the end-consumer. Thereby, the overall modelling frameworks developed in this PhD study enable integrated energy systems modelling with a detailed representation of the chain from renewable gas production, via transport, storage and to the end-use consumer. The novel modelling frameworks developed during this PhD study fill research gaps regarding the modelling of gas as an integrated part of future energy systems and enable comprehensive assessments of the role of gas in a future energy system to be conducted. Results from this PhD study covers the gas supply chain: i) production of renewable gas and liquid fuel; ii) gas flow in the gas transmission infrastructure; iii) end-use sector demands. Results regarding renewable gas production pathways reveal that future Danish gas demands can be covered by renewable gas. The results show that anaerobic co-digestion of a mixed feedstock to produce biogas is the preferred option, where the biogas is upgraded to biomethane using CO2 removal in scenarios with low gas demands. In a scenario with high gas demands, the biogas is upgraded by adding hydrogen in a methanation process. By performing a high spatial energy system analysis, the results reveal that biogas plants would be located in the countryside, as the cost of transporting manure is a determining factor. Results regarding renewable liquid fuel production in a climate-neutral Danish energy system by 2050 show that high liquid fuel demands in the transportation sector would impose high pressure on national biomass resources, and hence a need for electrofuels or more sustainable carbon. Within electrofuels, particularly those using biomass and hydrogen can play a prominent role to fuel future transport sectors. Furthermore, results show that biorefineries would be located near larger cities to benefit from economy-of-scale and to have access to large district heating networks in order to sell excess heat from biorefineries. The excess heat from biorefineries could supply a significant share of the national district heating demand. Results regarding the gas transmission infrastructure indicate that the system is operated without constraints. This result is obtained since capacities in the gas transmission system have been prepared to handle higher gas demands in the past, and are prepared for the addressed energy futures. Moreover, the benefit of using the large-scale storages facilities in Denmark is illustrated through an analysis of a full year. The operation of the storages depends on the scenario, where higher gas demands (domestic or in adjacent markets) yield higher utilisation of the large-scale gas storage facilities. On the contrary, low gas demand scenarios result in limited utilisation of the large-scale storage facilities. This finding opens the possibility for converting one of the storage facilities, for example, to hydrogen storage, which might create value in the future, particularly if high demands, e.g. electrofuel production arise, or if the hydrogen storage can be a part of a new European hydrogen network. Results for a Reference scenario complying with the Danish energy targets for 2030 and 2050 show a decline in gas supply in the energy transition towards a climate-neutral energy system by 2050. Demands decrease from a level today at around 100 PJ to around 55 PJ in 2050. In particular, a decreased use of gas in individual heating in the residential sector, and for power and district heating supply is identified, while the gas used in process heat in the industrial sector may contribute with an increasing share towards 2050. A global sensitivity analysis followed by an uncertainty analysis using Monte Carlo simulations is performed to elucidate the inherent uncertainties related to the future use of gas. The results from the analysis highlight a significant spread in gas consumption levels in a Danish climate-neutral energy system. Finally, the results presented in this PhD thesis show that gas and renewable gas may play a key role in a future climate-neutral energy system. However, the configuration of the remaining energy system and the uncertainties related to competing elements in the energy system yields high uncertainties related to the future use of gas in the Danish energy system. To summarise, this PhD study assesses the arising question of the future role gas may have in an energy transition of the Danish energy system. Extensive modelling frameworks are developed during this PhD study, which enables comprehensive assessments of the Danish energy system. This PhD study provides future pathways for the Danish energy transition towards a climate-neutral system, with a detailed representation of the gas system, i.e. renewable gas supply, transport and consumption, as an integrated part of the future energy system.