The rate of transition toward sustainable energy supply for all sectors is constrained not by the resource potential of renewable energy, which is many times current demand, but by the quantity which can be stored.
Currently the penetration of renewable energy is limited by its intermittency so that it is necessary to continue to provide base-load power generation and back-up capacity using fossil fuels. By storing off-peak wind or solar energy in the form of carbon-neutral gaseous and liquid energy carriers, the viable proportion of renewable energy in the grid can be increased to 100%. An integrated sustainable energy system is possible, supplying the power, heat, and transport sectors, facilitated by the ‘Renewable Power Methane’ concept proposed by Sterner et al. Transport plays a central role in such a system and we show here how a contiguous transition process to carbon-neutral vehicles is possible which will provide affordable sustainable mobility.
The drivers towards change
Over 90% of road transport uses petroleum oil-based fuels whilst air transport is 100% dependent on such products. The International Energy Agency (IEA) predict that by 2020 the majority of new cars will be sold outside the OECD and that all of the rise in oil demand to 2035 will originate from the transport sector in countries with developing economies. By this time China will become the largest importer of oil (and coal) and the global vehicle fleet will double to 1.7 billion vehicles China and India both have consumption-to-reserves ratios which are similar to the EU and the US. Concerns regarding security of energy supply in the transport sector are thus acute.
The financial implications are huge: at an average oil price of $100/barrel the EU and the US each transfer $350 billion (every year) outside their borders in order to satiate their thirst for oil. This situation is exacerbated by the on-going reduction in resource diversity. Up to the early 1970s Western investor-owned oil companies controlled – directly or indirectly – almost all of the world’s oil production and reserves. In 2006 companies owned or claimed by their national governments controlled 80% of global oil reserves, with a further 14% controlled by Russian companies and joint ventures between Western and national oil companies. Western investor-owned companies controlled only the remaining 6% outright.
Current global production of 87 million barrels per day (mb/d) in existing fields is declining to the extent that by 2035 a gross capacity increase of 47 mb/d, equal to twice the current total production of the middle-eastern OPEC counties, is required simply to stand still. These considerations of securing future supply should alone suffice to incentivise the development of an integrated sustainable energy system in which transport plays a key role. When combined with the imperative of reducing greenhouse gas emissions, the motivations to seek solutions based on 100% renewable energy are compelling.
Fig. 1(b) represents a generic open-cycle process in which CO2 is captured from the flue gases of industrial plant, e.g. power stations, aluminium plants, or cement factories, and is combined with renewable hydrogen to synthesize fuel. By combining the hydrogen with CO2 it is chemically liquefied into a high energy density hydrocarbon fuel. Clearly, if the captured CO2 stems from the combustion of fossil energy resources this approach is not renewable and will still result in an increase in atmospheric CO2 concentration. Rather than a re-cycling process it amounts to CO2 re-use and offers the potential of a notional reduction in emissions of approximately 50%.
Fig. 1(c) illustrates a closed-cycle fuel production process in which, ideally, there is no net release of non-renewable CO2. The hydrogen generation process in both Figs 1(b) and 1(c) is likely to be via the electrolysis of water and this represents by far the greatest energy input to the process, as shown later. For this reason the fuels produced in this way may be referred to as ‘electrofuels’ as they are essentially vectors for the storage and distribution of electricity generated from renewable energy. When the feed stocks are water and CO2 from the atmosphere the fuel production and use cycle is materially closed and therefore sustainable.
Such a cycle offers security of feed stock supply on a par with that of the ‘hydrogen economy’ since the time scale for mixing of CO2 in the atmosphere is sufficiently short to ensure a homogeneous distribution. With access to sufficient water and renewable energy, the process has the potential to provide fuel from indigenous resources. As the oil price escalates the provision of carbon neutral liquid fuels can ultimately be financed by the elimination of the wealth transfer involved in the purchase of oil. The recycling of the CO2, rather than sequestrating it, after it has been removed from the atmosphere cannot result in any net greenhouse gas (GHG) reduction. Its inclusion in a closed carbon cycle to make transport fuels, however, can potentially have the effect of rendering carbon neutral the fastest growing GHG emissions sector.
Whereas the adoption of battery electric or hydrogen fuel cell vehicles requires paradigm shifts in the costs of the vehicles themselves or their fuel distribution infrastructure, or both, the development of carbon-neutral liquid fuels enables a contiguous transition to sustainable transport. Drop-in fuels such as gasoline, diesel, and kerosene can be produced from CO2 (via CO) and H2 via Fischer-Tropsch (FT) synthesis but the simplest and most efficient liquid fuel to make is methanol. Indeed the option to make gasoline is retained even if methanol is produced initially since the former can be made via the Exxon-Mobil methanol-to-gasoline process. In addition to being the simplest fuel to synthesize from CO2 and water feed stocks, methanol provides much greater biomass feed stock diversity since it can be made from anything which is (or ever was) a plant.
Although ethanol is currently the most familiar alcohol fuel used in the transport sector there is also much experience of methanol and it has been successfully used in large-scale fleet trials over a period of 15 years in the 1980s and 1990s. With widespread availability of methanol to extend the biomass limit of sustainable ethanol, dedicated vehicles with high-compression-ratio engines, optimised to exploit the high octane index and high heat of vaporization of the alcohol fuels would evolve. This would reduce the magnitude of the upstream renewable energy required to make carbon-neutral fuels. Importantly, such engines can still operate on gasoline at lower power output, circumventing range anxiety issues during the evolution of the supply infrastructure.
To aid the transition to electrofuels based on methanol it is possible to make a relatively conventional vehicle operate on any combination of methanol, ethanol, and gasoline with the aid of an alcohol fuel sensor and modified engine management software. A more immediate application for methanol is in blends of up to 3% by volume in European gasoline. Additionally, as described previously in proActive, it is possible to formulate ternary blends of methanol, ethanol, and gasoline which have the same stoichiometric air-fuel ratio and volumetric energy concentration as any binary ethanol-gasoline blend. In the form of E85 substitutes it has been shown that these ternary blends can act as drop-in fuels for flex fuel vehicles (FFVs), (of which there are over 8 million in the U.S. alone) and, in addition to serving as a market pull for methanol synthesized from CO2, can act to extend the use of the limited amount of ethanol produced as a sustainable biofuel. The ability of synthesized methanol to extend the biomass limit of ethanol prevents biofuels being regarded as a dead-end as a future transport energy vector.
The need for large scale energy storage
The movement towards high levels of renewable energy is encumbered by its intermittent supply. In areas of high wind penetration in the US, prime on-shore wind energy is claimed to be cost competitive with coal, with a levelized cost of energy (LCOE) as low as $45/MWh.The global annual growth rate of over 20% which wind energy has sustained over the part 15 years is likely to stall in the future due to the difficulties of dealing with off-peak generation. Large-scale energy storage solutions are the key to unlocking this problem.
The options are limited by the requirements of scale, as evidenced by the fact that underground pumped storage is being advocated.
One possibility for large-scale energy storage is to use off-peak renewable energy to synthesize chemical energy carriers. Chemical energy storage systems, based on the conversion of renewable energy into a gaseous or liquid energy carrier, enable the stored energy to be either re-used for power generation or transferred to other energy sectors such as transport, where the de-carbonization issue is more problematic, and there is an ever-present demand to supply a high-value energy carrier. In the case of liquid fuels the demand to fuel the vehicles is already in place and is ever present.
Here we propose to extend the scheme of Sterner to include the use of liquid fuels made from renewable energy and re-cycled CO2. The general approach is to store renewable energy first in hydrogen via the electrolysis of water but then, for an additional small energy penalty, this hydrogen is reacted with CO2 to form an infrastructure-compatible hydrocarbon energy carrier. In this way hydrogen is used in the fuel rather than as the fuel. Importantly, if all processes are powered with carbon-free energy and the CO2 used to make the fuel is captured directly from the atmosphere, then the combustion of this fuel would result in zero net increase in the atmospheric CO2 concentration. Methane can be made in this way using surplus renewable energy and fed into the gas grid for storage producing a large buffer with essentially no time limits for storage using existing infrastructure in developed countries. This concept, which integrates the gas and electricity grids, is called Renewable Power Methane by Sterner. The combined existing storage and pipeline capacity of the German natural gas network is about 200 TWhth, enough to satisfy consumption for several months, compared with the existing pumped hydro storage capacity of 0.04 TWhth in the power grid.
Energy-dense liquid fuels can also be synthesized by this approach, resulting in an energy carrier which is easy to store, distribute, and utilize in other sectors. In the form of liquid hydrocarbon fuels they can be used in the transport sector and can supplement or extend the use of biofuels. They also have the potential to completely replace fossil-fuels since they are not feed-stock-limited. Methanol, with its H/C ratio of 4, is particularly well suited to this task, being the simplest organic hydrogen carrier which is liquid at normal ambient conditions. Gasoline, diesel, and kerosene can also be synthesized as drop-in fuels at a higher energy penalty and using more complex plant than that required for methanol production.
The concept of synthesizing fuel from feed stocks of CO2 and water was first proposed in the 1970s by Steinberg and there have been many other proposals in the meantime. Three broad generic schemes for incorporating CO2 into fuels can be envisaged as shown in Fig. 1(a)-(c).
The most familiar manifestation of renewable liquid fuels is in the form of biofuels, depicted in Fig. 1(a). Biofuels recycle CO2 by extracting it from the atmosphere as part of the photosynthesis process which forms plants, algae, or cyanobacteria. The feed stocks are CO2 and H2O which are combined using chlorophyll to absorb the energy in sunlight and transform it into chemical energy in the form of carbohydrates in the resulting biomass material.
Fig. 2 shows a schematic representation of the various chemical reactions and processes involving CO2, hydrogen, oxygen, methanol, and carbonates. CO2 is, with water, the end product of any combustion process involving materials containing carbon and hydrogen. Further reactions to form carbonates are exothermic processes. The capture of CO2 in inorganic carbonates and other media is a burgeoning area of research.
Once hydrogen and CO2 are available the simplest and most direct route to producing a high quality liquid fuel is the catalytic hydrogenation of CO2 to methanol via the reaction in figure 3 which shows that, in producing methanol via the direct hydrogenation of CO2, by far the largest component of the process energy requirement is the hydrogen production. This is true of any electrofuel using hydrogen as an intermediate or final energy carrier.
An 80% electrolyser efficiency has been assumed together with a nominal CO2 extraction energy of 250 kJ/mol.CO2 (representing about a 10% rational thermodynamic efficiency relative to the minimum thermodynamic work requirement). This gives a higher heating value (HHV) ‘electricity-to-liquid’ efficiency of 46%, including multi-pass synthesis of the methanol and re-compression of the unconverted reactants. It has also been assumed that the heat of reaction generated in forming the methanol can be used elsewhere in the process, e.g. to offset the distillation energy.
Fig. 4 shows the estimated sensitivity of the process efficiency to the energy requirement for CO2 extraction. Almost 15 years ago Specht et al. measured total-process CO2 capture energy levels of 430 kJ/mol. in a demonstration plant using an electrodialysis process to recover the absorbed CO2. This represents a rational efficiency of less than 5%.
Despite this low CO2 capture and concentration energy the measured overall fuel production efficiency was 38%. This matches well with the corresponding value given by the simplified analysis shown in Fig. 4.
Without policy intervention the intermittent use of alkaline electrolyzers, due to their limited current densities, is likely to be too expensive to produce fuel under present market economics. Improvements on this technology are at an advanced state of development and other promising technologies are emerging. Graves describes the use of high temperature co-electrolysis of CO2 and H2O giving close to 100% electricity-to-syngas efficiency for use in conventional FT reactors. This ultra-efficient high temperature electrolysis process using solid oxide cells combined with a claimed CO2 capture energy (from atmospheric air) as low as 50 kJ/mol. leads to a prediction of an electricity-to-liquid efficiency of 70% (HHV basis). With a constant power supply this high overall efficiency enables the production of synthetic gasoline at $2/gallon ($0.53/litre) using electricity available at around $0.03/kWh. Doty states that off-peak wind energy in areas of high wind penetration in the US averaged $0.0164/kWh in 2009 and the lowest 6 hours of the day averaged $0.0071/kWh. With more pessimistic values for the cost of CO2 capture such as the $1000/tonne quoted by House et al. the gasoline cost component due to the supply of the carbon feedstock alone might be as high as $7.5/gallon (about €1.30/litre). With 20% electrolyser capacity this cost of fuel synthesis could be as high as $4/gallon at $0.03/kWh electricity (higher current density electrolysers could reduce this to $2.2/gallon). For perspective, a cost of $11.5/gallon is around €2.05/litre). Currently gasoline retail costs in the EU range from €1.14/litre to €1.67/litre including duties and taxes (which can be as high as €0.6/litre). In a system which based fuel duty and taxation was based on non-renewable life cycle carbon intensity, a fuel made from air-extracted CO2 and water might be commercially attractive in the medium term.
An integrated system
To achieve a fully integrated system based on the use of renewable energy requires large-scale storage of an energy carrier which can be readily accessed for power generation. To provide long term energy storage capable of covering the contingency of extreme meteorological events a system based on the integration of the electricity and gas infrastructure would be a key component. Such a system could be based on the synthesis of renewable methane as an energy vector from CO2 and H2 (using the Sabatier process) and in many countries could use the capacity of the existing gas network for storage and subsequent re-use in the power generation and heat sectors. The synthesized and stored methane is thus readily retrievable to smooth out the supply of renewable energy. Sterner describes such a concept in detail and has modelled its operation within a renewable energy system based on wind, solar, and biomass over a period of 1 week on a 1 hour resolution based on a winter load demand. The renewable-power-to-methane synthesis efficiency is predicted to be 48% using measured energy values for capture and concentration of CO2 from air of 430 kJ/mol. giving an overall electricity-to-methane-to-electricity efficiency of 28%.
The production of renewable electricity and renewable methane for power generation back up and use in the heat sector could be integrated with the synthesis of liquid fuels for use directly in transport. A schematic representation of such a system combining the power, heat, and transport sectors is shown in
Fig. 5 where the renewable liquid fuels are represented by methanol, ethanol (C2H5OH) and drop-in hydrocarbon fuels. The production of liquid fuels also offers an energy storage option. The use of stored energy for transport via this route gives a similar efficiency to the ‘round-trip’ efficiency of producing renewable methane and reconverting it for use in charging electric vehicles but allows the use of significantly cheaper vehicles. Sustainable biomass can also be integrated into this system.
The production of carbon-neutral liquid fuels is proposed as a route to the continued provision of compatible, affordable, and sustainable transport. This approach retains the use of low-cost internal combustion engines and liquid fuel systems. These powertrain systems have high power densities, energy storage densities, and low embedded manufacturing and materials extraction energies. They also have considerable potential for further efficiency improvement, especially using highly boosted small (‘downsized’) engines exploiting the superior qualities of alcohol fuels.
The replacement of fossil fuels with carbon-neutral liquid fuels would not compromise current levels of mobility and would enable transport to remain globally compatible. Low-carbon number alcohols can be used for personal mobility and light-duty applications, and synthetic hydrocarbons for applications where maximum energy density is crucial. The technology to enable the transition from the current vehicle fleet to equivalent-cost vehicles capable of using sustainable methanol has been described. This takes the form of either tri-flex-fuel vehicles capable of running on any combination of gasoline, ethanol, or methanol, or current flex-fuel vehicles which can run on specific pre-blended mixtures of these three fuels. All transport energy can be supplied using biofuels up to the biomass limit, and beyond it using carbon-neutral liquid fuels made using renewable energy and CO2 from the atmosphere. The role of biofuels in this transitional route and end-game prevents them being regarded as a dead-end by vehicle manufacturers. The potential to synthesize methanol from anthropogenically re-cycled CO2 and water in large quantities ‘rules in’ the use of some of the limited amount of truly sustainable biomass as a transport fuel, rather than ruling it out.
In addition to minimizing the environmental impact of the rapid growth of transport-related CO2 emissions, the use of atmospheric CO2 and water as feed stocks for renewable energy carriers offers potential freedom from dependency on imported oil and a concomitant reduction in associated financial transfers.
A broader integrated system is proposed here where renewable energy is stored in the form of synthetic methane in the gas grid for supply to the power generation and heat sectors while carbon-neutral methanol and drop-in hydrocarbon fuels are supplied to the high-value transport fuel sector which is difficult to de-carbonize. The liquid fuels also offer an energy storage option, increasing the flexibility of the system. In this scenario both the gaseous energy storage medium and the liquid fuel energy carriers are compatible with existing infrastructures, enabling a soft start to their adoption.
CO2 re-cycled from the atmosphere may impose a short-term cost and energy penalty over flue-gas captured CO2 but directs the necessarily large investment toward leveraging sustainable resources, avoiding further lock-in to technologies based on ‘depletables’ with their ever escalating costs. In any case, the well-to-wheel GHG reduction level of approximately 50% for fuels made from fossil fuel flue gas CO2 is not sufficient to meet long-term EU targets. Making a value-added product such as transport fuel from the extracted CO2 has the potential to accelerate the commercialization of air capture technology and provides a more progressive concept for investors than the sequestration option. The re-cycling of CO2 from the atmosphere to make fuels does not produce the reduction in CO2 concentration which is possible from air capture and sequestration. Making a value-added product form the CO2 may be the most viable way to commercialize direct air capture of CO2 and, by neutralizing the most rapidly growing emissions sector, may be the most pragmatic route to minimizing the eventual stabilized CO2 levels.
Authors: Richard Pearson, Jamie Turner and Peter Edwards