Power-to-X (P2X) is a concept for using surplus renewable power to produce fuels and chemicals. P2X has received a lot of traction in recent years as an energy storage solution and a tool to combat global warming. With the increased availability of low-cost renewable power, P2X is considered a key solution to produce carbon-neutral fuels. For example, the transport sector, including road, air and waterborne transport, adds more than 20% of the global emissions of greenhouse gases (GHGs), per a 2015 International Energy Agency report. Replacing part of these fuels with sustainably produced synthetic fuels will help lower the GHG impact of the transport sector.

There is an increasing trend of renewable energy generation from wind turbines and solar (photovoltaics or concentrated solar power). Power generation from both these technologies is variable and is not always in sync with the grid load. For example, solar produces maximum power during peak sunlight hours and the output from the wind turbine is dependent on the speed of the wind. The intermittent nature of these technologies results in surplus electricity during off-peak hours that require innovative solutions for electricity storage. Battery technology is one solution for the storage of surplus power. However, it lacks the ability to support a wide range of activities that are currently supported by fossil fuels, for example, air and the long haul transport sector, medium- to long-term energy storage, and energy trade and transportation from one region to another. Here, P2X offers a viable solution by producing synthetic fuels, both gas and liquid, that can be directly fed into the existing network of fossil fuels. Synthetic fuels offer a promising alternative to fossil fuels as they have a high energy density that is important for energy storage media; they can be stored cheaply over long periods; and they can be fed into the vast existing infrastructure for energy storage, transmission and use. Methane, methanol and dimethyl ether (DME) are some of the fuels that are at a different level of development under the P2X. Replacing the market share of fossil fuels with the sustainably produced synthetic fuels and chemicals will lower the net CO₂ emissions and provide a tool to combat climate change.

P2X is further classified as power-to-gas (P2G) and power-to-liquid (P2L). In P2G, the end product is a gas, for example, hydrogen and methane. The surplus power can be used to produce hydrogen via water electrolysis. The hydrogen can then further be used to either produce methane by the reduction of CO₂. In P2L, the end product is a liquid fuel or chemical, examples of which are methanol and dimethyl. Both can be used as a chemical energy source, for example as fuel or as raw material to produce further chemicals.

Making sustainable fuels from CO₂

CO₂ is the most oxidized form of carbon and any fuel or energy-containing form of carbon includes hydrogen connected with the carbon (for instance, the reduced form of carbon). Reduction of the CO₂ is an energy-intensive process, which makes sustainable fuels derived from CO₂ challenging. The reduction of CO₂ can be carried out by the addition of electron/hydrogen to CO₂. Overall, the reduction process is an energy addition process that attaches hydrogen atoms with CO₂, which increases the chain length, and, consequently, results in a compound with relatively high heating value (a fuel). There are existing industrial processes that reduce CO₂ to methane and methanol, but they use hydrogen derived from natural gas. Introducing hydrogen derived from renewable power will turn the whole process into carbon negative. As a result, the utilization of fuels produced from such a process can make the whole value chain carbon neutral.

The reduction of CO₂ can be carried out via either of the following technological pathways:

• Electrochemical
• Thermochemical
• Biochemical

Electrochemical pathway

The key process to produce renewable fuels from power is the splitting of water into hydrogen and oxygen atoms via electrolysis. The hydrogen in turn can be directly used as an energy carrier (P2G) or it can be further used as material input to reduce the CO₂ to produce other P2G and P2L fuels and chemicals. Electrolysis is the key step in terms of the economics of the P2X as it represents the two most important factors affecting the production costs that are the capital cost of the electrolyzer and the electricity price (the hydrogen production cost). Two important parameters affecting the capital cost of the electrolyzer are the capacity factor of the electrolyzer unit and the life span of the electrolyzer.

Two other important electrochemical processes are the direct reduction of CO₂ and N2 to produce hydrocarbons, oxygenates and ammonia. However, these require a highly active and selective catalyst that is currently not available to be economically feasible. Especially, the direct reduction of CO₂ requires very large overpotentials to make (longer) hydrocarbon or alcohol products that are compatible with the current energy sector and the chemical industry. For the N2 reduction, both activity and selectivity are significantly worse than nitrogenase enzymes and produce quantities of ammonia that are often below the detection limits of conventional techniques. As such, for both the direct reduction reactions, significantly more active and selective catalysts are needed, which is an area of active research.

Thermochemical pathway

The thermochemical processes are the most developed now and are applied at an industrial scale. The most common are the CO₂ reduction to methane known as Sabatier reaction, a process used for chemical methanation of CO₂ and N2 reduction to ammonia for the fertilizer production known as the Haber–Bosch process. These thermochemical reduction processes are carried out at relatively higher temperatures and pressures and are conversions supported by a catalyst. Generally, the industrial processes run at steady-state using hydrogen derived from natural gas. In P2X, the hydrogen can be derived from water electrolysis powered by renewable power; and a predominant challenge is to make these processes compatible with the variable and renewable H2 feedstocks. This shift will likely require decentralized or intermittent operation at lower temperatures and pressures, necessitating the development of more active and selective catalysts.

Biochemical pathway

There are naturally occurring microorganisms that carry out the reduction of CO₂ and N2, for example, the methanogenic archaea that naturally produce methane during the anaerobic digestion process and N2 reduction to ammonia by nitrogenase enzyme produced by certain bacteria, such as cyanobacteria (blue-green bacteria). In the context of P2X, the challenge is to engineer these microorganisms to either directly use electricity as an energy source for the reduction process known as bio-electro synthesis or uptake an external source of H2 derived from renewable energy. Both of these approaches are being developed with the prime advantage that the biochemical processes are carried out with low temperatures and pressures that will help to link them with de-centralized and small-scale renewable power sources.

Conclusion

P2X is emerging as an alternative for producing sustainable fuels and chemicals, with the thermochemical pathway mature as compared to the underdevelopment of electrochemical and biochemical pathways. Whether P2X products will be cost-effective — as future transport fuel relative to alternatives other than biofuels — the costs for distribution, propulsion and storage systems need to be considered in addition to the large-scale availability of excess renewable power.