5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS
Figure 7 presents the on-board fuel storage costs related to the shipping sector, with inputs from automotive-related studies to illustrate an example of the cost differences.
The costs refer to current and near-term estimates towards the year 2030 and specify the investment cost solely in terms of storage tanks and not the storage system, which may incur additional costs with auxiliary equipment. This illustration aims not to point to a specific cost level for any of the fuel storages, as the literature estimates are very diverse, but rather to show the differences between the different types of fuel categories and related storages.
Several key observations can be made using this figure. First, one can observe several orders of a magnitude cost difference between liquid fuels and high-pressure compressed and liquefied fuels, with the median estimates at a 10-20 times difference favouring the former. Second, the range of uncertainty for compressed and liquefied fuels is much broader than for the category of liquid or moderately compressed fuels.
Third, the fuels that entail significant energy consumption for compression or liquefaction also have high storage costs.
Such cost differences are also reflected in the cost of fuel distribution and fuelling infrastructure. On the shipping topic, Taljegård et al. [122] estimated the investment cost of fuelling infrastructure in ports at 100-200 $/kW for liquid fuels and between 1600 and 2100 $/kW for LMG and LH2. Helgeson and Peter [35] also estimate that even towards the year 2050, both compressed and liquefied hydrogen infrastructure for road transport will remain ten times more expensive than gasoline or diesel. These are essential aspects to consider in the future choice of fuels.
5.5.1 POWER AND HEAT PRODUCTION
In the power and heat production sectors, biomass is already a common fuel, often used in retrofitting old coal plants. While this solution is often in easy reach for slashing CO2 emissions quickly, existing research [126] has found that the direct use of biomass in power plants is neither very efficient nor very flexible. Future energy systems will thus require robust units that can deal with variable wind and solar production. While combined cycle gas turbines (CCGT) can be suitable for this task due to their high efficiency and potential for flexible operation, other technologies such as fuel cell CHPs may also fill this role, but without the same level of flexibility [68]. In both cases, any gaseous fuels may be used for this purpose, ranging from biogas, syngas, and methane to hydrogen.
5.5.2 INDUSTRY
The industrial sector may use the same fuels as power and heat production, although this may require a lower level of flexibility due to the nature of the demands. Large segments of the fuel consumption in the industry cannot be included in balancing the supply and demand but instead operate on pre-determined schedules or even continuously.
Fuel quality can be another consideration for the industry that may eliminate some fuel options such as biogas or syngas from direct usage. This means that more refined fuels such as methane or hydrogen would supply the demands, which is also the assumption in the most ambitious scenarios of the EU’s long term strategic vision [34]. Methane can replace natural gas in the same applications as today, provided that electrification and energy savings reduce the renewable fuel demand sufficiently. On the other hand, hydrogen may also be an alternative fuel and is already deployed in the iron and steel industry in Sweden [127].
5.5.3 TRANSPORT
Liquid fuels are typically used in the transport sector, but it is also common to use compressed or liquefied gaseous fuels. These require internal combustion engines (ICE) or fuel cells (FC). ICEs combust the fuel in spark or compression engines for road and shipping, while aviation uses mostly jet turbines. FCs generate electricity through a thermochemical reaction, which in turn powers an electric motor.
Road transport has the widest variety of renewable fuel options, and apart from battery-electrification, which has the best potential in this sector [51], vehicles can use diesel, gasoline, HVO, methanol, DME, ethanol, methane or hydrogen in four-stroke ICEs. Some of these fuels, namely methanol, methane or hydrogen, can also combine with polymer membrane fuel cell (PEMFC), but methanol and methane will also require a reformer to convert the hydrocarbon to hydrogen. High-temperature PEMFC
5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS
is the most mature technology among the existing FCs in transport, but other options exist, such as solid oxide fuel cells (SOFC), which do not need reformers but are less tolerant to load changes and are technologically less developed [128].
The majority of the fuels used in road transport are also suitable for shipping, except for gasoline or ethanol. Conversely, ammonia is suitable in shipping, being the only transport sector that can handle this fuel due to its high toxicity. ICEs are the primary type of propulsion today and are split into four-stroke and two-stroke engines. Four-stroke engines are often found in smaller ships on short distances and where onboard space is limited, while two-stroke engines are usually combined with large ships due to their higher fuel efficiency. Apart from ICEs, future ships may also operate with high-temperature PEMFC or SOFC, similar to road transport.
Aviation requires a specific range of fuels and propulsion systems. The only currently viable replacement for fossil jet fuel is renewable jet fuel, which has a similar chemical composition as its fossil counterpart. The quality requirements for jet fuels are very high, and only a few renewable jet fuels are certified. Those certified are HEFA and FT jet fuels, exclusively sourced from biomass, but other pathways may soon receive certification [94].
6 THE THREE STUDIES
Chapter 5 described the components for renewable fuel pathways, including the challenges arising with the choice of various technologies. It also demonstrated that a variety of technologies must be considered when designing future renewable fuel pathways. In addition to the literature review performed in Chapter 1, this can be summed up as a part conclusion: that no single technology or fuel pathway can solve the issue of replacing fossil fuels efficiently and within biomass constraints; rather, a mix of multiple solutions will be necessary. This chapter aims to determine which are the feasible renewable fuel pathways that can integrate with future sustainable energy systems. The three research articles included in this dissertation combine the components defined in the previous chapter to identify the role of technologies in a variety of scenarios and sensitivity analyses.
The first two research articles [1,2] use energy system analysis and deal primarily with the first part of the pathways, namely resources, primary and secondary treatment, with an overview of storage and utilisation. They inquire into the roles of different types of biomass and biomass-based fuels in the context of resource limitations and costs for all energy sectors while also incorporating the role of CO2-electrofuels. The third study [3] deals primarily with the other end of the pathways, namely the primary and secondary conversion of resources, storage and utilisation. This study focuses on the maritime transport sector. An illustration of the contribution from the three studies is shown in Figure 8. The three studies differ in terms of their geographical focus, with Study 1 dealing solely with the Danish context, Study 2 addressing both Denmark and the EU, while Study 3 takes a global approach, albeit still with a Nordic perspective.
Figure 8: Illustration of the value chain links analysed throughout the three studies.
The three studies are further attached in this chapter on pages 44, 56 and 75.
6.1 STUDY 1 - THE ROLE OF BIOGAS AND BIOGAS-DERIVED