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New energies 24/06/2020

The molecules of the energy transition

Whatever the path to a zero-carbon world, electricity will not be the sole source of energy: molecules such as hydrogen, methane and ethanol will be required for a long time to come.

Hydrogen or carbon-based molecules will be indispensable for a long time to come if the energy transition is to become a reality.

The virtues of each of these molecules are often the most apparent when considering the transport sector.

Ronnie Belmans & Jan Mertens

For a long time, we have considered the decarbonisation of electricity as a key part of the energy transition, in particular by means of a massive increase in the use of renewable energy sources to generate electricity. Despite the intermittency of renewables, the development of information and data management technologies should ensure that the power system operates in a stable and reliable manner. Supply will have to adapt to fluctuating demand, in particular by using batteries to store surplus energy. At the end of the day, customers will be able to rely on electricity to provide the best energy services, however, even if this “electric model” will certainly be at the heart of the future energy system, there are nevertheless certain questions and concerns. 

What Share For Electricity

First of all, a clear distinction must be made between the energy system as a whole and electricity generation, which only accounts for a small part of it. For example in Belgium in 2016, 81.4 terawatt hours of electricity were supplied (for a production of 85.4 TWh). It should be noted that with the increase in decentralized production, i.e. local production such as cogeneration (combined heat and power), wind turbines and photovoltaics, these figures are becoming less and less accurate.

The total final energy consumption (electricity, gas, oil, coal, biomass, waste) was 489 terawatt hours. In addition, primary energy consumption (which measures a country’s total energy demand) was 657 terawatt hours.

The difference can be explained by the energy consumed by the chemical, iron, steel and nuclear industries etc, however this figure does not include energy used by maritime transport and international aviation. At the end of the day, it is estimated that electricity accounts for 16.6% of final energy consumption in Belgium. If we consider all of Europe, this figure rises to 17.9% (3,255 terawatt hours of electricity compared to 18,154 TWh of primary energy).

According to the second law of thermodynamics, the direct use of electrical energy is always preferable because transforming electricity into chemical or thermal energy reduces its quality. Energy quality is measured in terms of its exergy, which represents its capacity to do physical work. The exergy of electricity is 100%, however depending on what services are required it is sometimes difficult to replace a given energy source by electricity from renewable sources. This is where molecules can complement or even become a substitute for the electrons.

What molecules are we talking about? Hydrogen, methane, methanol, ethane and ethanol amongst others. Whatever we use them for, it is vital that we make these molecules using carbon free electricity or biogas, otherwise the energy transition will fail. Only two methods are carbon neutral: using green energy to make molecules from water and CO2 or from biomass, which we will not discuss. An alternative, or rather an intermediate solution, would be carbon capture and storage, or even reusing CO2.

In heavy, energy-intensive industries, molecules are needed as energy carriers, but above all as raw materials. To make a molecule of methane (CH4) or methanol (CH3OH) from CO2 and H2O requires much more energy than the (reusable) energy contained in the molecule itself. Consequently, this need for molecules will greatly increase total energy demand. Certain sectors of industry that require molecules for specific applications, for example specialised chemistry, will have to be studied on a case-by-case basis.

A good example of the contribution of molecules to a successful energy transition is during the “Dunkelflaute”, a German term for a long period without sun and wind when energy production is impossible. Hydrogen and “clean” molecules consisting of one (methane and methanol) or two carbon atoms (ethane C2H6 and ethanol C2H5OH) will help make up for the lack of solar and wind energy. It will then be a matter of choosing the most suitable one.

Transport And Molecules

The virtues of each of these molecules are often the most apparent when considering the transport sector, although we must be careful not to generalize. Two-wheeled vehicles, from the increasingly popular electric bikes to the growing market for electric motorcycles, do not need molecules. The same goes for private electric cars. For all these vehicles, batteries are more advantageous in terms of cost than hydrogen-based solutions. They can be charged easily using the existing infrastructure and they boast much better energy efficiency compared, for example, to fuel cells.

The future of road freight transport is still unclear, although it is true that battery powered electric trucks are already on the road.  Siemens is taking a different approach: it has equipped one lane of a motorway with catenaries so that hybrid trucks can run on electricity, whilst simultaneously charging their batteries, which are indispensable for the first and last kilometers of their journey after leaving the eHighway. Here again, the need for molecules will be low or even zero.

Trains, buses and local transport will increasingly be electric. Inland water transport will probably also be powered by electricity, at least for part of its activity, however we do not know yet whether the batteries’ energy density will allow boats to cover long distances without them taking up too much space on board or increasing weight significantly. Molecules will certainly have a role to play in this sector, especially for maritime transport where energy needs are much greater. The same is true for aviation, although the first electric aircraft have already begun making short-range flights. As for the drones that will play such an important role in future mobility solutions (delivery services, taxis in large cities etc.), they are already electric.

Without expressing a preference, let us just note at this stage that different molecules are available. Since each has its advantages and disadvantages, it is important to take into account the energy required for their production and the fact that this energy must come from a surplus of renewable electricity.

Hydrogen is a very promising molecule, but not necessarily as an energy carrier. There are two kinds of low-carbon hydrogen, hydrogen produced by the electrolysis of water using renewable electricity (green hydrogen) and hydrogen produced from fossil fuels, but whose CO2 is captured (blue hydrogen). This low-carbon hydrogen, along with synthetic fuels, will be essential for decarbonising large sectors of the world economy including the chemical, petrochemical, steel, cement and paper industries and will therefore be a valuable aid in our efforts to limit global warming to less than 2°C.

Many studies extol the merits of producing electricity in deserts. Not only do they cover large areas of the world, they are also areas of high solar radiation. Research indicates that the Sahara and Australia are both potential areas, but the question is how would we transport this energy from the Sahara to Europe for example? There are two ways: high-voltage direct current (HVDC) lines, or in the form of chemical energy, i.e. molecules. Many specialists talk about hydrogen, but is this molecule the most suitable from an energy point of view and is it the most efficient? It is obvious that the distance between Australia and Europe means that electricity is not an option.

The Advantage Of Methane

Let’s take a closer look at the hydrogen energy chain. Hydrogen is produced by the electrolysis of water using an electrolyser, which is a device that uses electrical energy to split water (H2O) into H2 and O. We will assume an efficiency of 70% for this step. Finding water in the Sahara or in any other desert environment would not be easy, but let’s move on.

The most efficient way of transporting hydrogen to Europe is by ship (liquid hydrogen stored in cryogenic tanks). As the boiling temperature of hydrogen is extremely low (- 252.87°C), its liquefaction is a very energy-intensive process. Different efficiency values exist in the literature, but let’s take 70%.

Energy consumption for transportation, including transporting gas by pipeline to the liquefaction plant on the coast is estimated at 10%, so the efficiency of this third stage is 90%. A further 5% more energy is lost to evaporation. The overall efficiency is therefore approximately 40%. The hydrogen can then be injected directly into the natural gas distribution network and delivered to the final consumer as is.

Hydrogen can be converted into electricity at the point of consumption by a fuel cell (with an efficiency of 60%), therefore avoiding any power losses during transmission as the electricity is produced close to the end consumer. Taking all of these calculations into account, for a production of 1,000 megawatts of electrical energy, 251 megawatts are produced at the end of the chain, i.e. a quarter of the total.

Another solution would be to convert hydrogen into methane using CO2 captured from the air or transported by pipeline, a process with an estimated efficiency of 60%. It is much more efficient to liquefy and transport methane than hydrogen, with respectively 95% efficiency and about 0.1% loss per day (i.e. 3% per trip). In terms of evaporation, we can count on an efficiency of 99%, leading to an overall efficiency at this stage of 54.7%. As far as electricity generation is concerned, a conventional high-efficiency power plant could be used (65%), however as the electricity production is centralised, we would have to take into account line losses (92%), which gives an overall efficiency of 32.7%.

The Right Molecule For The Right Use

Methane appears to be more advantageous in this example: broadly speaking, the choice of the right molecule for a given use is closely linked to its physical properties (see the table on the opposite page).

The most relevant point of comparison is energy density (per unit of mass or volume). The higher the value, the more useful the energy carrier and the easier to transport and store it is, making it, in this case, more suitable for mobile applications. Under normal conditions, the energy density by volume of hydrogen is extremely low, which poses storage and transport problems. We can alleviate these constraints in part by increasing the pressure, but its energy density will remain at best six times lower than that of gasoline, which will always be a major disadvantage in transport.

In comparison, the energy density of liquid methane (LNG) is more than twice that of liquid hydrogen. For the same volume, liquid methane provides twice as much energy. Moreover, as we have seen above, the low boiling temperature of hydrogen leads to significant constraints in terms of the equipment required (tanks, pumps and compressors).

Green hydrogen...or blue?

The conclusion is that there will be a massive need for hydrogen produced from renewable sources in a carbon-free society. This molecule will represent a critical intermediate step in the supply of the specific energy best suited to each need and an essential raw material for industry.

The choice between green hydrogen and its blue counterpart will obviously depend oncost and, more specifically for green hydrogen, on the availability of the renewable energy resources needed to produce it. With regard to blue hydrogen, the question of public opinion and the acceptance of CO2 storage will need to be taken into account.

CO2 capture and storage is best suited to large installations, for example in the chemical or steel industry, as they are already equipped with pipelines and storage facilities. The road to green hydrogen has many twists and turns. Whichever solution is chosen there is still a long way to go, even if some technologies such as electrolysers are already mature.

In any case, hydrogen or carbon-based molecules will be indispensable for a long time to come if the energy transition is to become a reality.

Written by:

Ronnie Belmans, Professor at the Catholic University of Leuven, CEO Energyville and Jan Mertens, Professor at University of Gent, ENGIE Chief Scientific Officer.

The authors would like to thank ENGIE’s Scientific Council for the rich and relevant discussions on the question.

E. Hegnsholt et al., The real promise of hydrogen, Boston Consulting Group,31 July 2019.
A. Scipioni et al. (éd), Hydrogen economy, Academic Press, 2017.
M. Larsson et al., Synthetic fuels from electricity for the Swedish transport sector : comparison of well to wheel energy efficiency and costs, Energy Procedia, vol. 75, pp. 1875-1880, 2015.
J. Van Mierlo et al, Which energy source for road transport in the future ? A comparison of battery, hybrid and fuel cell vehicles, Energy Convers. Manag., vol. 47, pp. 2748-2760, 2006.
U. Vol Bossel, Does a hydrogen economy make sense, proceedings of the IEEE, vol. 94(10), pp. 1826-1837, 2006.

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