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Today, non-energy use (e.g. production of plastics or fertilizers) represents about 10% of the final energy consumed worldwide and is dominated by petroleum products, natural gas and coal. However, due to complex flows of materials and ambiguities between net and gross energy in non-energy data sets, this sector is generally misunderstood. The defossilisation of this sector requires to properly define the lever of actions: can we use renewable alternatives to the currently used fossil fuels as direct inputs to this sector? Can the use of these alternatives in the non-energy demand be more efficient and lead to synergies with the other energy sectors (i.e. electricity, heat and mobility)? This paper focuses on the case-study of Belgium and its non-energy demand, mostly from chemical and petrochemical industries (20% of Belgium final energy consumption). It investigates the required molecules of the end-use demand, i.e. High-Value Chemicals (HVC), methanol, and ammonia, as well as the related mature conversion technologies. A transition is analysed to switch the production of the final molecules from fossil fuels to renewable resources as feedstock (e.g. HVC from naphtha-cracking to biomass gasification or methanol-to-olefins). To fully assess the role of these energy carriers, we used and customised Energy Scope TD, a whole-energy system model which optimises the design and hourly operation of an energy system to minimise its costs and emissions. With the aim of carbon-neutrality, this work emphasises the major role of biomass and methanol as the cornerstones to defossilise the non-energy demand, mostly for HVC. To do so, when reaching high global warming potential reductions, biomass is preferably converted into methanol and, subsequently, into HVC. Finally, given the limited availability of local renewables in Belgium, this study highlights the predominant impact of imports like ammonia and methanol.

Humanity is at the beginning of its third energy revolution, after coal and then oil, we are now moving towards the massive use of renewable energies that we knew since the dawn of time. This change will solve several problems, including climate change, as the concentration of greenhouse gases will stabilise. The success of fossil fuels resides in their storability. Indeed, these energies have been stored in the earth for millions of years and can be extracted whenever humans need energy. Their costs of storage are insignificant compared to their costs of extraction. On the contrary, the main renewable energies are intermittent, i.e. wind and solar energy, and thus cannot be stored. However, these energy can be converted into electricity, heat or another form of energy that can be stored. But storing these energy carriers has a significant cost and thus, redefine the design of the energy system. Hence, the shift from fossil fuels to renewable energies - mainly intermittent - requires a complete rethinking of the energy systemsupply chain from resources to demand while including conversion and storage technologies. In particular, to understand what will be the role of storage technologies in this energy system throughout the energy transition period and after. To address this issue, the thesis addresses the following research question: What is the role of energy storage technologies for the energy transition? Focusing on the role of storage requires the inclusion of its environment, i.e. the energy system. Focusing on the energy transition requires a time horizon fromtoday to about 2050. Thus, answering the research question requires a model that represents the energy systemduring the transition. These has motivated the following five contributions of the thesis. First, it is necessary to review the existing energy system models in the literature. Second, one of these models is improved to account for the complexities of energy storage. This involves, among others, t, (FSA - Sciences de l'ingénieur) -- UCL, 2021

Literature review of the use of "electrofuels", "biofuels" and "synthetic fuels" and proposal of a new taxonomy based on classification provided by official agencies (e.g., Food and Agriculture Organization of the United Nations, International Organization for Standardization). In the urging necessity for an energy transition and the persisting need for fuels, technologies to convert primary resources into end-use fuels are rapidly multiplying. In this context, we suggest comprehensive terminology and classification to characterise the fuels. Given the limited insight provided by the widely-used term “synthetic fuels,” we recommend to use more specific terms like “biofuels”, “electrofuels,” or even hybridised terms (e.g. “bio-electrofuels”). These allow to highlight the contribution of biomass and electricity in the energy balance of the fuel.

Wind and solar energies present a time and space disparity that generally leads to a mismatch between the demand and the supply. To harvest their maximum potentials, one of the main challenges is the storage and transport of these energies. This challenge can be tackled by electrofuels, such as hydrogen, methane, and methanol. They offer three main advantages: compatibility with existing distribution networks or technologies of conversion, economical storage solution for high capacity, and ability to couple sectors (i.e., electricity to transport, to heat, or to industry). However, the level of contribution of electric-energy carriers is unknown. To assess their role in the future, we used whole-energy system modelling (EnergyScope Typical Days) to study the case of Belgium in 2050. This model is multi-energy and multi-sector. It optimises the design of the overall system to minimise its costs and emissions. Such a model relies on many parameters (e.g., price of natural gas, efficiency of heat pump) to represent as closely as possible the future energy system. However, these parameters can be highly uncertain, especially for long-term planning. Consequently, this work uses the polynomial chaos expansion method to integrate a global sensitivity analysis in order to highlight the influence of the parameters on the total cost of the system. The outcome of this analysis points out that, compared to the deterministic cost-optimum situation, the system cost, accounting for uncertainties, becomes higher (+17%) and twice more uncertain at carbon neutrality and that electrofuels are a major contribution to the uncertainty (up to 53% in the variation of the costs) due to their importance in the energy system and their high uncertainties, their higher price, and uncertainty.

Integration of a high share of intermittent renewable energy sources is essential to reach a fossil free energy system. In the southern European countries, solar energy appears as an ideal candidate. Due to the spatial and temporal disparity of such an energy source, it faces two main challenges: (i) the seasonal pattern of solar irradiation induces a need for long-term storage; (ii) the locations with the biggest solar potential are not close to the major places of consumption. Many studies have shown the interest of the electro-fuels (e.g. hydrogen) as long-term storage and energy carriers. However, their integration in a whole and multi-regional energy system is rarely studied. This work addresses the following two questions: What will be the contribution of the electro-fuels as energy carriers and longterm storage? How can the different regions collaborate to reach a fossil free energy system at the lowest cost? We applied a whole and multi-regional energy model, EnergyScope Multi-Cell, to the Italian energy system of 2050. Italy is divided into 3 interconnected regions or cells: the North, the Centre, and the South. The country has a large solar potential in the South and high energy demand in the North. In the modelled scenario, Italy does not reach a self-sufficient renewable energy system. It imports 7.7% of its primary energy as electricity and 1.4% as green hydrogen. The North is responsible for 88% of those imports. The overall system relies on high electrification of the demand, sector-coupling, and exchanges between regions to integrate high shares of renewable energy sources (RES). The disparity of RES potentials and end-use demand among regions drives exchanges of electricity (58 TWh/year), synthetic gas (20 TWh/year), and woody biomass (7 TWh/year) between them. In particular, the South produces an excess of energy and export it to the two other regions.

On the road to a carbon-neutral continent, Europe will have to implement major changes to its energy system. The following transformations are needed: massive integration of renewable energies, from which intermittent ones; electrification of the heat and mobility sectors; use of renewable fuels (e.g. biogas and hydrogen); and use of storage technologies. As a result, the energy system of tomorrow will be very different from that of today. Recently, technologies based on intermittent renewable announced cheaper production costs than their equivalent fossils. However, these announced costs usually do not account for the adaptation of the energy system to implement back-up technologies or reinforce the grid. The levelised costs of electricity (LCOE), a common metric to compare electricity generation technologies, is extended in this work to compare dispatchable technologies with intermittent ones. By applying an energy model (EnergyScope TD) that optimises the system design and operation, it computes the integration costs of photovoltaic energy and therefore its system LCOE. We find that accounting for the integration costs almost double the system LCOE of the photovoltaic compared to a LCOE calculated on the technology only. Another key finding is the importance of the electrification among sectors, which facilitates the integration of additional intermittent renewables and at a cheaper cost.

Wind and solar energies present a time and space disparity that generally leads to a mismatch between the demand and the supply. To harvest their maximum potentials, one of the main challenges is the storage and transport of these energies. This challenge can be tackled by electrofuels, such as hydrogen, methane, and methanol. They offer three main advantages: compatibility with existing distribution networks or technologies of conversion, economical storage solution for high capacity, and ability to couple sectors (i.e. electricity to transport, to heat, or to industry). However, the level of contribution of electric-energy carriers is unknown. To assess their role in the future, we used whole-energy system modelling (EnergyScope Typical Days) to study the case of Belgium in 2050. This model is multi-energy and multi-sector. It optimises the design of the overall system to minimise its costs and emissions. Such a model relies on many parameters (e.g. price of natural gas, efficiency of heat pump) to represent as closely as possible the future energy system. However, these parameters can be highly uncertain, especially for long-term planning. Consequently, this work uses the polynomial chaos expansion method to integrate a global sensitivity analysis in order to highlight the influence of the parameters on the total cost of the system. The outcome of this analysis points out that, compared to the deterministic cost-optimum situation, the system cost, accounting for uncertainties, becomes higher (+17%) and twice more uncertain at carbon neutrality and that electrofuels are a major contribution to the uncertainty (up to 53% in the variation of the costs) due to their importance in the energy system and their high uncertainties, their higher price, and uncertainty., SCOPUS: ar.j, info:eu-repo/semantics/published

The transition towards more sustainable, fossil-free energy systems is interlinked with a high penetration of stochastic renewables, such as wind and solar. In this context, energy storage technologies of different kinds - namely electrical, thermal and chemical - are commonly expected to play a major role; however, this role is seldom quantified using whole energy system models. In this paper, our goal is to assess and quantify the role of these different storage technologies in low carbon energy systems. To do this, we apply EnergyScope TD, a novel open-source energy planning model, to the real case study of the national energy system of Switzerland; concretely, we optimise the Swiss energy system for a target future year with an hourly resolution. The results indicate the following trends: (i) Thermal storage in combination with heat pumping becomes the main source of flexibility in the electricity sector; (ii) electrification allows a high penetration of renewables in the transportation sector, and the remaining share of mobility is supplied by synthetic fuels; (iii) a mix of storage technologies is needed for different applications at different timescales, such as synthetic fuels for long-term and for mobility demand, and thermal storage for short term (day-week) and for heat demand. Overall, it emerges that storage technologies - such as thermal storage, vehicle to grid and synthetic fuel storage – can help decrease cost, decarbonise the mobility and heating sectors, and reduce dependency on fossil fuels.

The transition towards more sustainable, fossil-free energy systems is interlinked with a high penetration of stochastic renewables, such as wind and solar. Integrating these new energy resources and technologies will lead to profound structural changes in energy systems, such as an increasing need for storage and a radical electrification of the heating and mobility sectors. To capture the increasing complexity of such future energy systems, new flexible and open-source optimization modelling tools are needed. This paper presents EnergyScope TD, a novel open-source model for the strategic energy planning of urban and regional energy systems. EnergyScope TD optimises both the investment and operating strategy of an entire energy system (including electricity, heating and mobility). Additionally, its hourly resolution (using typical days) makes the model suitable for the integration of intermittent renewables, and its concise mathematical formulation and computational efficiency are appropriate for uncertainty applications.

To face climate changes and energy dependency, governments encourage their industries and communities to increase the share of renewable energy (RE). However, the RE production is mostly inflexible. The risk of unmatching electricity market grows. Tools such as power plant flexibility, import/export, demand management, storage and RE curtailment are developed to handle this problem. This study focuses on the energy storage mix required for the energy transition of the electricity system to high RE shares. An hour based model is developed in order to optimise the renewable energy and storage assets by maximising the energy return on investment (EROI) while respecting power fluxes constraints. The model is used to quantify the storage needs for the energy transition of Belgium. An in-depth analysis is performed for four scenarios. Depending on the RE deployment and nuclear share, EROI between 5 and 10.5 are obtained. Large scale storage is required as soon as the energy mix has more than 30% of RE. With more than 75% of RE, power to gas becomes unavoidable. This study highlights that curtailment can be limited to less than 5% of RE production. These values are the result of the optimum between increasing storage, RE capacity and curtailment.

The document summarizes the discussions held during a workshop on the energy transition, organized by several Belgian academic and industrial experts. All the questions raised above are addressed in a general way, trying to express and justify the different points of view, starting from works published in the literature.

The world has experienced several energy transitions. During the end of the XIX century, coal became the first energy resource before biomass. After WWII, the thermodynamic quality of hydrocarbons made them the first primary energy resource. Finally, today due to global warming threats, humanity starts a decarbonising energy transition from fossil fuels to renewable energy (RE) sources. This study focuses on the energy storage mix required for the energy transition of the electricity system to high RE shares illustrated by the case of Belgium. An hour-based model is used in order to yearly optimise the country RE energy return on investment (EROI). The storage dynamics is studied through its monthly needs. For an 80% RE Belgium, curtailment reaches 3.4%. It induces as much losses as the batteries. At high RE shares, Power to Gas (PtG) becomes unavoidable even if the amount of energy involved is less than 2% of the overall yearly consumption.