The availability of resources and the potential for harnessing them form an essential aspect of future fuel production. The overall aim of reducing and replacing fossil fuels in the energy system with renewable and carbon-neutral energy relies on the replaced fuels originating from renewable sources such as wind, solar photovoltaics, biomass, and carbon and nitrogen resources. Hydropower is another important resource;
however, it is not included in this dissertation due to its geographical limitations but remains an important energy source. In this chapter, VRES, biomass, carbon and nitrogen resources are further discussed.
One key non-energy resource is water, an essential and, in some cases, a limited resource for electrofuel production, which is often excluded in energy system analyses. Water is an indispensable resource for electrolysers, but it has other uses throughout the fuel production value chain, either with carbon capture [60,61] or fuel synthesis [62,63]. For electrolysers, water consumption is significant, at around 9-11.4 kg/kgH2, and is dependent on the type of electrolyser [64–66]. In this dissertation, water is assumed to be available in the three studies, although it is not included in the analyses.
The area necessary for the deployment of renewable energy is another type of non-energy resource. All renewable non-energy sources require space for deployment, although some require more than others. Biomass is the most land-intensive, requiring approximately 500 km2 to produce 1 TWh of energy, while wind and photovoltaics require less than one tenth of this area to produce the same amount of energy [67].
Although these numbers refer to electricity production and not renewable fuel production, these are essential considerations for designing renewable fuel pathways.
5.1.1 VARIABLE RENEWABLE ELECTRICITY SOURCES
VRES are a critical component of all renewable fuels and sustainable energy systems.
The most utilised types of VRES are onshore and offshore wind, solar photovoltaics and solar thermal. While solar thermal is specific to heating systems, the other three VRES can be used in connection with the production of all types of electrofuels.
VRES are impacted by air mass movements and solar radiation, which is reflected in their full load hours. Full load hours are an essential metric in all energy applications as well as electrofuel production as they dictate, to a large extent, the operation of the electrolysers. The Danish Energy Agency [68] estimates that average full load hours will continue to improve and, by 2050, these may reach 1500 hours for photovoltaics, 3800 hours for onshore wind and 4900 hours for offshore wind (values specific for the Nordic area). Offshore wind's advantage in full load hours means that its capacity utilisation is higher, which is reflected in the full load hours of electrolysers. Hence, offshore wind is often linked to the large-scale deployment of electrofuels and Study 1, 2 and 3 are designed with this consideration.
Offshore wind can also be deployed at larger scales than onshore wind in terms of the turbine and aggregated wind farm capacities, as land restrictions are not an issue for offshore infrastructure (although there may be other restrictions). Offshore wind can also address the problem of "NIMBY" ("not in my back yard") attitudes, as the visual and noise impacts occur far from most of the population [68,69]. Even though this incurs higher costs than other VRES, the average offshore wind LCOE in Europe has decreased by 44% in the past ten years, reaching 45-79 €/MWh in 2019 [16] with further reductions in sight [70]. Based on the Danish Energy Agency estimates, these may reach approximately 30 €/MWh in the medium to long term [68].
5.1.2 BIOMASS
Biomass is a broad term that includes a variety of feedstocks originating from forestry, agriculture, some types of waste, and crops grown for the purpose of producing energy. It is a highly valued component for future energy systems due to its carbon neutrality: theoretically, the carbon released in the combustion or conversion of biomass to other fuels has already been captured through photosynthesis from the atmosphere, meaning no new fossil-origin carbon is released. In practice, this is only
5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS
partly true as there is a limit to how much biomass may be deemed sustainable, and the emissions of uncontrolled large quantities of biomass could contribute to the accumulation of CO2 in the atmosphere [71,72].
Due to its very nature, biomass is a limited product with an uneven distribution. Global non-food biomass potential is between 13 and 28 GJ per capita per year [73], while in Europe, it is 15-16.5 GJ per capita per year, depending on the source [74,75]. Current biomass consumption in the EU is just below 6 PJ/year [76], but estimates for the future vary between 8 and 20 EJ [75], depending on the level of exploitation, with the EU’s long-term strategy [34] estimating up to 13 EJ/year by 2050 in the most ambitions scenario. Denmark is estimated to have between 25-35 GJ per capita yearly, depending on which biomass types are included in the estimates. According to various publications [76–78], the Danish potential can reach around 160 PJ/year without including energy crops or algae. With improved straw collection and including energy crops, and incorporating current plans for afforestation [79], these resources could reach over 200 PJ/year [77] based on the more conservative estimates. The estimates for Denmark and Europe are illustrated in Figure 6.
Figure 6: Conservative biomass potential estimates for Denmark and Europe.
Despite its limited potential for energy purposes, biomass is the only renewable resource that can offer the energy system flexibility, similar to that offered by fossil fuels today. Flexibility means that fuel can be stored and then used when needed. For the production of renewable fuels, biomass can be an essential asset as it already contains hydrogen and carbon atoms – the “ingredients” for all hydrocarbons. Apart from the potential to produce biofuels, this is an essential aspect for the production of bio-electrofuels, as this entails reduced electrolytic hydrogen consumption.
5.1.3 CARBON AND NITROGEN
Carbon and nitrogen are abundant in nature. Carbon is found in excess in the atmosphere in the form of CO2 and is the most significant contributor to global temperature increase and climate change. However, CO2 and N2 can also combine with hydrogen to produce a variety of fuels.
Despite its increasing concentration in the atmosphere, CO2 is relatively sparse compared to the most abundant component, N2. However, CO2 can be found in concentrated streams in all combustion processes or other chemical processes. It is not feasible to capture CO2 from all emitters, but it is possible to capture it from large CO2 emitters that do not utilise fossil fuels, a requirement for producing renewable fuels.
Electrofuel production involving hydrocarbons will require reliable sources of carbon with sufficient quantity and flow, which will entail the constant operation of sufficient carbon emitters or ensuring the temporary storage of CO2 until the time of use.
Current large CO2 emitters are power plants or combined power and heat plants.
However, as more VRES take over electricity production and PtH solutions replace fuels in heating, the amount of CO2 emitted will decrease, which also offsets the production timing, making such thermal power and heat production uncertain for large-scale CO2 capture [80]. This may leave the industry as the primary CO2 source for fuel production, but with electrification and changing fuels [81], the options will become fewer. The unavoidable emissions from cement factories may remain a reliable source of CO2, both in quantity and flow, but this will require synergies with on-site fuel production to avoid costly transport and storage.