The world is currently facing a situation where growing energy demand meets shrinking resources for fossil fuels. Improvements in energy efficiency make it possible to reduce the growth of the demand, but in the long term, a gap between the demand and availability of resources will become unavoidable. To close this gap, the use of alternative energy resources is necessary. For some forms of renewable energy a conversion into electricity or mechanical work is possible on demand. Power plants that run on biogas for example could be used as production sites for electric power with variable capacity. However, the efficiency of the energy production per unit area for biomass is not sufficient to cover the total energy demand of the world′s population.1 To overcome this low areal efficiency a more direct use of the energy derived from the sun should be employed, for example, by utilizing photovoltaics or wind power. These energy sources suffer from the severe disadvantage that power generation does not happen on demand but more or less randomly with only limited predictability. Consequently, medium- and long-term energy storage units will become crucial elements of the energy systems of the future.
Many potential storage technologies are either already available or under development today. However, only chemical energy storage seems appropriate for large-scale, long- and medium-term storage, because other options suffer from low storage densities, limited availability, or losses during the storage period.2 To store electrical energy, hydrogen can be produced from electrolysis and subsequent power generation can be performed by re-oxidation of the hydrogen to water. The main challenge in this concept is the storage of the hydrogen. The gravimetric storage density of hydrogen is excellent because of the low molar mass, but the volumetric storage density at ambient conditions is unacceptably small. To increase the volumetric density, measures must be taken to reduce the specific volume, which can be achieved by either cooling or pressurizing. A temperature difference of approximately 270 K is created if hydrogen is cooled down to liquefaction. Heat transport into the tank therefore is unavoidable even in the case of very good insulation. To prevent the build-up of pressure, a permanent release of evaporated hydrogen is necessary, which causes continuous losses of hydrogen during the storage period. The alternative of high-pressure storage only reaches lower volumetric storage densities, and the heavy, pressure-stable tanks significantly lower the overall gravimetric storage density.
To overcome these drawbacks, it is reasonable to convert the hydrogen into another chemical form. This type of conversion can be differentiated into two principal types: reversible and irreversible conversion (comparison in Figure 1). In the case of reversible conversion, an unsaturated organic compound is hydrogenated and subsequently dehydrogenated when energy is needed, making the carrier compound effectively recyclable. These compounds are called liquid organic hydrogen carriers (LOHC) and are a subclass of energy carrying compound. A number of different compounds are under research for use as LOHCs such as toluene,3 azaborines4, 5 and N-ethylcarbazole.2, 6 For irreversible conversion the hydrogen reacts with a carbon source (usually carbon dioxide) to form a fuel that is combusted without having previously released the hydrogen. Typical products of such conversions are methane,7 methanol,8 and long-chain alkanes.9 A border case is the reforming of methane or methanol before power generation to regain the hydrogen, and a significant difference to the LOHC concept is that the produced carbon dioxide is not recycled but instead released to the atmosphere. The concentrated carbon dioxide stream that was used for the conversion of the hydrogen is therefore lost.
In this study, we compare the two general approaches for chemical energy storage using thermodynamic modeling. We chose hydrogen storage using N-ethylcarbazole6 as a model compound for the reversible conversion and the production of liquid alkanes (Fischer–Tropsch synthesis) as a model process for irreversible conversion. It has to be noted that renewable-energy-based Fischer–Tropsch processes can also be based on biomass conversion, as has been demonstrated by various research groups.10–13 The use of biomass-derived synthesis gas has some advantages over reacting hydrogen (produced from electrolysis) with carbon dioxide, such as eliminating the need for the reverse water–gas-shift (RWGS) reaction. Nevertheless, we focused on the later approach, since only this is an option for the storage of electrical energy.