Sun to liquid, an alternative way to green fuel

One disadvantage of e-fuel production is their high demand for electrical energy. In times where Europe face a serious risk of electricity shortages some search for alternative production methods. Sun-to-liquid or Solar fuels could be one solution.

The term solar fuel does not refer to e-fuels produced from solar power. Even if the actual fuel products are chemically identical and often come from the same synthesis routes, they differ significantly at the beginning of the production chain. In the case of e-fuels, in a first step hydrogen is produced by an electrolysis process, which requires a lot of electrical energy. In the case of solar fuels, the energy of sunlight is used directly for this step. Synhelion, a product from the Swiss federal institute of technology developed and successfully tested a reactor system that produces fuels just with Sunlight, water, and CO2.


The basic idea

The principle of solar fuels is, in simple terms, the inversion of combustion. If a liquid fuel is burned, a lot of energy in the form of heat is produced in addition to CO2 and water vapor. If, conversely, a lot of high-temperature heat is available, a liquid fuel can be produced from CO2 and water.


The technology

A large array of controllable mirrors reflects solar radiation and focuses it onto a focal point at the top of a tower. There the solar receiver is mounted, which can generate process heat of over 1500 °C from the focused sunlight. This heat feeds a thermochemical reactor, which can produce syngas from water and CO2 from various sources. This syngas, a mixture of H2 and CO, is then converted to liquid fuels using standard gas-to-liquid technologies such as Fischer-Tropsch synthesis. 

© Synhelion

© Synhelion

The advantages

Since this technology does not require the sunlight to be converted into electricity by photovoltaics, the efficiency of the system is increased. In addition, a Thermal Energy Storage (TES) developed for this purpose makes it possible to store the absorbed heat from the sunny hours, and thus to be able to feed the operation of the reactor even at night when the sun is not shining. This enables the reactor to operate 24 hours a day.

The concept of the plant is also very flexible and can therefore be adapted to the conditions at the site. On the input side, different CO2 sources can be integrated, such as biomass, industrial processes, or direct air capturing. As with almost all XtL technologies, almost any type of fuel can be generated as a system output, depending on the connected process chain. 

synhelion process

© Synhelion

The Roadmap

After the success of their medium-scale demonstration field tests, Synhelion is now constructing an industrial-scale solar fuel plant in Germany, with a planned annual production capacity of 10,000 liters solar kerosene. Although they can produce any kind of liquid fuel Synhelion focuses on sustainable aviation fuel, as SWISS and the Lufthansa Group started a collaboration with the pioneers and will use the green solar fuel in their aircraft.

The next step is a first commercial-scale plant with a target output of 500’000 liters by 2025. The following years they want to increase their capacity and build more solar fuel plants to reach a production of 875 million liters per year in 2030.

This can not solve all the energy problems, but it will help to decarbonize different sectors.  

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