Power-to-X (alsoP2X andP2Y) areelectricityconversion,energy storage, and reconversion pathways from surplusrenewable energy.^[1][2] By linking the power sector to other energy sectors, power-to-X conversion technologies offer the possibilities to exploit synergies across the whole energy as intended with the concept ofsector coupling and fully integrated smart energy systems.^[3]

TheX in the terminology can refer to one of the following: power-to-ammonia, power-to-chemicals,power-to-fuel,^[4]power-to-gas (power-to-hydrogen, power-to-methane) power-to-liquid (synthetic fuel, particularlycarbon-neutral fuel), power to food,^[5]power-to-heat. Electric vehicle charging, space heating and cooling, and water heating can be shifted in time to match generation, forms ofdemand response that can be called power-to-mobility and power-to-heat.

Collectively power-to-X schemes which use surplus power fall under the heading of flexibility measures and are particularly useful in energy systems with high shares of renewable generation and/or with strongdecarbonization targets.^[1][2] A large number of pathways and technologies are encompassed by the term. In 2016 the German government funded a €30 million first-phase research project into power-to-X options.^[6]

Surplus electric power can be converted to gasfuel energy for storage and reconversion.^{[7][8][9][10]} Direct currentelectrolysis of water (efficiency 80–85% at best) can be used to producehydrogen which can, in turn, be converted tomethane (CH₄) viamethanation.^[7][11] Another possibility is converting the hydrogen, along with CO₂ to methanol.^[12] Both these fuels can be stored and used to produce electricity again, hours to months later.

Hydrogen and methane can be used as downstreamfuels, fed into thenatural gas grid, or used to makesynthetic fuel.^[13][14] Alternatively they can be used as a chemicalfeedstock, as canammonia (NH₃).

Reconversion technologies includegas turbines,combined cycle plants,reciprocating engines andfuel cells. Power-to-power refers to the round-trip reconversion efficiency.^[7] For hydrogen storage, the round-trip efficiency remains limited at 35–50%.^[2] Electrolysis is expensive and power-to-gas processes need substantial full-load hours to be economic.^[1] However, while round-trip conversion efficiency of power-to-power is lower than with batteries and electrolysis can be expensive, storage of the fuels themselves is quite inexpensive.^{[citation needed]} This means that large amounts of energy can be stored for long periods of time with power-to-power, which is ideal for seasonal storage. This could be particularly useful for systems with highvariable renewable energy penetration, since many areas have significant seasonal variability of solar, wind, andrun-of-the-river-hydroelectric generation.

Despite it also being based fundamentally onelectrolytic chemical reactions,battery storage is not normally considered a power-to-fuel concept.

The purpose of power-to-heat systems is to utilize excess electricity generated by renewable energy sources which would otherwise be wasted. Depending on the context, the power-to-heat can either be stored as heat, or delivered as heat to meet a need.^[15]

In contrast to simple electric heating systems such as night storage heating which covers the complete heating requirements, power-to-heat systems are hybrid systems, which additionally have traditional heating systems using chemical fuels like wood or natural gas.^[16]: 124 When there is excess energy the heat production can result from electric energy otherwise the traditional heating system will be used. In order to increase flexibility power-to-heat systems are often coupled with heat accumulators. The power supply occurs for the most part in the local and district heating networks. Power-to-heat systems are also able to supply buildings or industrial systems with heat.^[17]

Power-to-heat involves contributing to the heat sector, either byresistance heating or via aheat pump. Resistance heaters have unity efficiency, and the correspondingcoefficient of performance (COP) of heat pumps is 2–5.^[7] Back-up immersion heating of bothdomestic hot water anddistrict heating offers a cheap way of using surplus renewable energy and will often displacecarbon-intensive fossil fuels for the task.^[1] Large-scale heat pumps in district heating systems with thermal energy storage are an especially attractive option for power-to-heat: they offer exceptionally high efficiency for balancing excess wind and solar power, and they can be profitable investments.^[18][19]

Power-to-mobility refers to the charging ofbattery electric vehicles (BEV). Given the expected uptake of EVs, dedicated dispatch will be required. As vehicles are idle for most of the time, shifting the charging time can offer considerable flexibility: the charging window is a relatively long 8–12 hours, whereas the charging duration is around 90 minutes.^[2] The EV batteries can also bedischarged to the grid to make them work as electricity storage devices, but this may cause additional wear to the battery.^[2][20]

According to the concept of sector coupling interconnecting all the energy-using sectors will require the digitalisation and automation of numerous processes to synchronise supply and demand.^[21]

A 2023 study examined to role that power‑to‑X could play in a highly‑renewable future energy system forJapan. The P2X technologies considered includewater electrolysis,methanation,Fischer–Tropsch synthesis, andHaber–Bosch synthesis and the study usedlinear programming to determine least‑cost system structure and operation. Results indicate that these various P2X technologies can effectively shift electricity loads and reducecurtailment by 80% or more.^[22]

• Grid energy storage
• Flywheel
• Electrofuel

• Energy policy
• Energy policy of Germany
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