Energize Staffwriter
Despite the focus on developing better batteries for electric vehicles, liquid fuels will still feature significantly in the transport and traction industry, and most major vehicle manufacturers are developing liquid fuel vehicle options for the future, based on “green” fuels.
There are a number of sectors in the transport and traction industry where battery power is considered impractical, such as vehicles for long distance heavy transport, marine and air transport, as well as in vehicles for heavy duty mine, construction and agricultural applications.
Liquid fuels have several advantages over battery energy storage systems:
- Higher energy density, meaning that longer trips are possible between refuelling
- Fuels can be bulk transported to the point of usage without permanent connections, such as transmission lines
- Refuelling is quick
- Liquid fuels can be stored for long periods in a variety of simple containers.
Higher energy density means a lower mass of fuel for a given distance of travel, or a greater travel distance for a given mass of fuel.
Hydrogen
Hydrogen is the basic component of all fuels considered here and is used either as a source of combustion on its own, or as fuel cell feedstock, or in combination with other elements. So-called ‘green’ hydrogen is produced by electrolysis of water using renewable energy (RE) sources. The oversizing of RE plants often leads to over-production causing the price of renewables-sourced electricity to become very low or even go negative. This surplus production causes real problems.
In the UK, power purchase agreements include a take-or-pay clause which dictates that where total electricity production exceeds demand, wind farms are paid to curtail production. In 2021, these curtailment payments were estimated to exceed ₤100 million (about R2,2 billion).
These payments represent a huge amount of surplus electricity generation (>2,4 TWh), which could have been used by wind farm operators to generate hydrogen at a discounted price. The availability of cheap electricity from renewable sources is driving the development of hydrogen-based fuels.
Direct combustion
Hydrogen can be combusted directly in specially adapted internal combustion engines. There are already many models on the market which combust hydrogen directly. Manufacturers such as Tata are focusing on the hydrogen combustion engine, and expect it to reach commercialisation before hydrogen fuel cell vehicles will.
Hydrogen fuel cell EV
In this application hydrogen is used to feed a fuel cell which provides electricity to drive the electric motor.
The end-to-end efficiency is of the order of 25 to 35%. In other words, 25% of the electricity supplied by the RE source is converted to energy at the motor. The overall efficiency is very heavily influenced by the efficiencies of the electrolyser and the fuel cell, and there is much research going into improving the performance of these two items.
The hydrogen fuel cell electric vehicle (HFCEV) option is being pursued seriously by several major manufacturers, including BMW, which has a HFCEV model at production stage, and which intends to offer these vehicles as part of their future range, alongside battery-electric vehicles (BEVs). Some manufacturers see the EV market being overtaken by HFCEVs in the future, and anticipate that the BEV “craze” will die out.
Hydrogen carriers: ammonia
Several hydrogen carriers have been considered, but ammonia is the only carrier that contains no carbon. Ammonia has several advantages over hydrogen. Ammonia has nine times the energy density of Li-ion batteries, and three times that of compressed hydrogen, making it very attractive as a potential carbon-free energy carrier. Ammonia liquefies at a relatively high temperature of -33°C, and at a pressure of 7,5 Bar at 20°C.
e-Fuels
So-called ‘e-fuels’ (electro-fuels) represent somewhat of an anomaly in the carbon neutral liquid fuel market in that they consist of hydrocarbons and when burned release carbon dioxide and other elements into the atmosphere. The carbon neutrality comes from the fact that recycled CO2 is used to synthesize the fuels. e-Fuels are synthetic fuels produced by synthesising renewably produced hydrogen with captured carbon to create a new hydro-carbon fuel. e-Fuels have a high energy density and can be ‘dropped-in’ as a direct replacement for existing petroleum fuels used in shipping and aviation and, in effect, offer a chemically identical low carbon alterative to replace oil-derived fuels. Liquid hydrocarbons are the most effective energy carriers of all liquid fuels because they are:
- Relatively high in energy density compared to other fuels and hydrogen carriers
- Liquid at normal temperature and atmospheric pressure, and hence easy and safe to transport and store.
- Volatile enough to vaporise easily
- Useable in a number of different energy converters, such as internal combustion engines, turbines, fuel cells.
- Drop in replacements for existing hydrocarbon fuels
While their ability to perform and act in the same conditions as conventional fuels means they offer an attractive solution in the short term, it does come at a cost. This is a quandary that industry consumers and producers must address: whether they want to accept the higher unit cost but utilise carbon neutral fuels that enable the use of a global pre-existing network, or take a lower unit cost of hydrogen and face the higher capital costs of constructing or repurposing a hydrogen supply network.
Why liquid fuels?
As liquid fuels are produced using electricity, one could ask whether it would not be cheaper to use the electricity directly in an EV, rather than convert it to liquid fuels. The answer lies in the cost and availability of green hydrogen. Charging an EV relies on grid power being available, and the cost is based on the tariff at the point of connection. This tariff is based on the cost of all generation sources, and the tariff structure. In Gauteng, an inclining block tariff is used, and as the EV charging comes on top of other consumption, it will be charged the highest block tariff. Highway charging stations are even more expensive. Surplus power is not always available when required for EV charging, whereas hydrogen can be generated when power is available at the right price, and stored for later use. It is estimated that increased production of green hydrogen will bring the price down to the point where the cost of running a liquid-fuel vehicle becomes competitive with EVs.
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