Fuel storage is an important consideration when replacing established fossil liquid and gaseous fuels with new fuels with different chemical and physical properties. Storage can take place both on-board vehicles or on land, for instance, in proximity to the production site or at large transport nodes such as ports or airports.
Fossil fuels have high volumetric and gravimetric densities, making them cheap to store, particularly in a liquid state. Gaseous fossil fuels such as methane are slightly more expensive to store because they require pressurised storage. In any case, both liquid and gaseous fuel storage methods are currently used to store refined oil products and natural gas. Nevertheless, the discussion regarding on-land storage must extend to the infrastructure requirements to supply these fuels and the transport modes that will be using them.
The production of new fuels should account for existing infrastructure, the deployment of new infrastructure, all at the lowest possible cost and in close synergy with the other parts of the energy system [108]. The available types of fuels that may be produced in the future can be categorised into four types, based on the type of storage they use:
• Liquid fuels
• Low-pressure compressed gaseous fuels
• High-pressure compressed gaseous fuels
• Liquefied gaseous fuels 5.4.1 LIQUID FUELS
The liquid fuel category includes all fuels produced as liquids and remain in a liquid state at standard temperature and pressure (STP). This category includes methanol, FT liquids, jet fuel, bio-oils, ethanol and bio-diesel. For all these fuels, the storage requirements are similar to existing fossil liquid fuels, such as oil, diesel or petrol, as they only require steel tanks. The infrastructure for delivering and fuelling vehicles can, in many cases, be adapted from existing fossil fuels at low cost [109,110]. Many of these fuels are also suitable for blending with fossil fuels, which may be a path for introducing renewable fuels into the transport sector.
5. COMPONENTS FOR RENEWABLE FUEL PATHWAYS
5.4.2 LOW-PRESSURE COMPRESSED GASEOUS FUELS
Low-pressure compressed gaseous fuels include methane, biogas, syngas and transport fuels such as DME and ammonia. Biogas and methane are suitable for stationary applications and require several bars of pressure for storage in tanks.
Methane can also be stored in the natural gas grid or underground storage. The natural gas network and underground storage facilities are relatively widespread in Europe, making methane transport and distribution a cost-effective solution [111]. Biogas storage is similar to methane storage, except that it poses difficulties for underground storage due to CO2 contamination, acidifying underground water sources [112].
However, biogas may be stored in low-pressure tanks suitable for use in connection with power and heat production [113].
Syngas (also known as producer gas) is the fuel produced from the thermal gasification of biomass and can be used in stationary applications with 10-20 bars of pressure. It requires specialised types of storage as it contains hydrogen, which may cause metal embrittlement. Due to the low energy content per volume caused by the CO and CO2 presence (as in the case of biogas), it also needs larger storages than methane and may pose difficulties for underground storage [114].
The storage requirements for transport fuels are slightly different. DME and ammonia are gaseous fuels at STP but require a moderate pressure increase to be stored as liquids. These become liquid at low pressures of up to 10 bars, e.g. similar to fossil LPG (liquefied petroleum gas) [115]. Depending on the ambient temperature, it is common to increase the pressure to higher values to keep the fuel in liquid form.
Unlike DME, ammonia is toxic, so that additional handling costs may arise for this type of fuel. However, ammonia is a commodity handled for many years, and thus, there exists considerable experience and knowledge.
5.4.3 HIGH-PRESSURE COMPRESSED GASEOUS FUELS
The category of high-pressure compressed gaseous fuels includes methane and hydrogen. Methane can be stored at low pressures for use in stationary applications, but its transport use requires a new fuelling infrastructure capable of storing and compressing methane at the demand location. The electricity input for compressing the methane can be estimated at 0.025 kWhel/kWhCMG [20], and compressed methane gas (CMG) requires approximately 200 bars of pressure when stored on-board vehicles [116]. The high pressure is necessary to increase its volumetric density, even though this still leaves it one quarter as dense as petrol and half as dense as methanol.
Hydrogen is another fuel that requires high pressures in both stationary and on-board storages, albeit at different levels. For use in automotive applications, the pressure varies between 350 and 700 bars due to the low volumetric density [117], also implying a significant electricity consumption, between 0.03 to 0.04 kWhel/kWhH2.
Hydrogen transport and fuelling infrastructure may pose other challenges, even though there has been some discussion of converting the natural gas grid to a hydrogen grid [118]. However, this solution faces significant technological limitations, which makes the cost significantly more uncertain.
5.4.4 LIQUEFIED GASEOUS FUELS
The category of liquefied gaseous fuels also includes methane and hydrogen. The storage requirements are different from the other liquefied fuels such as ammonia or DME due to the significantly higher energy requirements for liquefying methane and hydrogen. Electricity consumption for liquefaction reaches 0.06 kWhel/kWhLMG and 0.25 kWhel/kWhH2 [35], with considerable associated investment costs [119].
Liquefaction of such fuels is necessary to improve their volumetric energy density in applications such as heavy-duty long-distance road transport, shipping or aviation.
Even liquefied, liquefied hydrogen (LH2) remains half as dense as methanol, while LMG is one third as dense as LH2.
5.4.5 SUMMARY
The order in which the five types of storages are represented in this chapter is also, in most cases, their cost order: liquid fuels have the lowest cost while high-pressure compressed gaseous fuels and liquefied fuels have the highest cost.
Figure 7: Overview of current and near-term fuel storage estimates from the literature mainly connected with shipping [120–125].
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.