Synhelion produces solar fuels to decarbonize transportation. They use concentrated solar energy to turn CO2 and water into syngas, which can be converted into solar kerosene, diesel, gasoline, or hydrogen. Their mission is to replace fossil fuels with economically viable synthetic fuels using high-temperature solar heat.
Challenges: jet fuel
Due to its high specific gravimetric energy density, kerosene is widely used as a jet fuel for long-haul aviation. Kerosene jet fuel combustion produces carbon dioxide (CO₂) emission. A round-trip flight from Frankfurt to New York, for example, burns about 156,500 kg of jet fuel. This results in about 570 tonnes of CO₂ or an average of 870 kg CO₂ per passenger. Global aviation is responsible for approximately 5% of the current anthropogenic emissions causing climate change, and this percentage is expected to rise.
Kerosene is produced by fractional distillation of crude oil in an oil refinery. Alternatively, kerosene can be synthesized from syngas—a specific mixture of hydrogen (H₂) and carbon monoxide (CO)—using the well-established Fischer-Tropsch synthesis process. However, the technological challenge is to generate renewable syngas from water (H₂O) and CO₂ using solar energy.
Synhelion develops a solar receiver-reactor that uses concentrated solar energy in a solar tower configuration and reticulated porous ceria redox material to convert CO₂ and water into syngas with a desirable molar ratio of H₂ and CO. The syngas is then used to synthesize kerosene via the established Fischer-Tropsch synthesis process.
The image below shows a solar tower power fuel plant that uses Synhelion’s solar receiver-reactor on the tower to produce solar jet fuel at the IMDEA Energy solar concentrating plant in Madrid (ES).
As schematically depicted in the figure below, the solar tower power fuel plant integrates three subsystems:
- the solar tower concentrating facility,
- the solar receiver-reactor, and
- the gas-to-liquid unit.
The solar tower concentrating facility is equipped with a solar tower and heliostat field. It deflects concentrated sunlight of 2,500 suns to 4,000 suns (1 sun = 1 kW/m²) onto a solar receiver-reactor. The solar receiver-reactor is mounted on top of the solar tower and aimed at the power-weighted center of the heliostat field. It converts CO₂ and H₂O into syngas with a desirable molar ratio of H₂ to CO for Fischer-Tropsch synthesis. The products together with unreacted CO₂ and H₂O are delivered to a gas-to-liquid unit on the ground adjacent to the solar tower. The gas-to-liquid unit uses over 90% of syngas to produce kerosene. Unreacted CO₂ and H₂O are injected back to the solar receiver-reactor.
The most critical subsystem in the solar tower power fuel plant is the Synhelion’s innovative solar receiver-reactor. The diagram below schematically depicts the structure of the solar receiver-reactor.
The solar receiver-reactor consists of a quartz (melting point: 1,650 ºC) glass window, a ceria (CeO₂) absorber, and an absorption area between the quartz window and the ceria absorber. Around the quartz window, inlet nozzles introduce CO₂ and H₂O gas flow into the absorption area. CO₂ and H₂O are heated in this area and converted into CO and H₂ on the hot surface of the ceria. Behind the porous ceria absorber, a central outlet nozzle extracts both product and unreacted gasses.
This innovative design of the solar receiver-reactor improves the energy conversion efficiency. Ceria (melting point: 2,400 ºC) exhibits a high degree of absorption in a slightly reduced state, allowing it to be readily heated by concentrated solar radiation. The high temperature ceria emits corresponding black-body radiation (essentially infra-red radiation) into the absorption area via its surface. The wall of the absorption area reflects black-body radiation. CO₂ and H₂O in the absorption area absorb infra-red radiation effectively and are heated to a high temperature for thermochemical reactions.
If black-body radiation from the ceria surface is not absorbed and emitted through the quartz window, the efficiency of the solar receiver-reactor will be reduced. Thus, the Synhelion’s solar receiver-reactor is ideally suitable for a reactor of greenhouse gasses (infrared absorbing gasses) such as methane (CH₄) and CO₂.
Ceria is the key material of the solar receiver-reactor. Due to its rapid redox kinetics and long-term stability, ceria is currently regarded as the most advanced redox material. The following reduction and oxidation represent the two-step thermochemical redox cycle necessary to produce syngas.
- Reduction :
CeO₂ → CeO2-δ + δ/2 O₂
- Oxidation :
CeO2-δ + δH₂O → CeO₂ + δH₂
CeO2-δ + δCO₂ → CeO₂ + δCO
where δ denotes the nonstoichiometry—a measure of the oxygen exchange capacity and therefore of the syngas yield per cycle. A typical value of δ is 0.04. Thus, the high temperature and removal of oxygen gas (O₂) during the reduction cycle can improve syngas yield.
For typical operating conditions, during the reduction step under vacuum conditions, the ceria temperature increases rapidly to the reduction end temperature of 1,500 ºC. O₂ is released. At the end of the reduction step, the solar input is interrupted (solar input is cut off), and O₂ release rate rapidly decreases to zero, while the ceria naturally cools down to the nominal oxidation start temperature of 900 ºC.
The oxidation of ceria is initiated by simultaneously injecting CO₂ and H₂O, which are full selectivity converted to CO and H₂, respectively. There are no by-products produced. CO and H₂ production rates peak shortly thereafter and decrease monotonically until the ceria is fully oxidized after the oxidation end temperature reaches 654 ºC. The molar ratio of H₂ to CO in syngas is around 2, which is suitable for Fischer-Tropsch synthesis.
The solar-to-syngas energy conversion efficiency of the solar receiver-reactor is defined as the ratio of the calorific value of the syngas produced over the cycle to the sum of solar radiative energy input and additional parasitic energy inputs associated with inert gas consumption and vacuum pumping. The energy conversion efficiency depends primarily on the amount of syngas produced during the oxidation step, compared with the amount of solar energy required to release O₂ during the reduction step. The solar-to-syngas conversion efficiency of the solar receiver-reactor is about 4% at an average solar input of 42 kW. The energy efficiency of the entire solar fuel plant should also consider the optical efficiency of the solar concentrating tower facility and the energy efficiency of the gas-to-liquid unit.
Around 91% of the produced syngas is subsequently processed on-site by the gas-to-liquid unit, yielding a liquid phase containing 16% kerosene and 40% diesel, and a wax phase containing 7% kerosene and 40% diesel.
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The first industrial-scale solar fuel plant, DAWN, is located in Jülich, Germany. The construction began in September 2022.
The DAWN plant consists of a 20-meter-high solar tower and a 1,500 m² mirror surface of the heliostat field. The solar tower houses Synhelion’s solar receiver-reactor and thermal energy storage that enables cost-efficient operation.
The DAWN facility will produce several thousand liters of fuel annually. If the same plant were built in a particularly sunny location and operated around the clock, it could produce about 150,000 liters of fuel annually. The produced fuels (solar kerosene and gasoline) will be used to demonstrate various possible use cases. SWISS will be the first airline to use Synhelion’s solar kerosene.
Synhelion has raised a total of $45.8M in funding over 4 rounds, including a Grant round, a Corporate round, a Series B round, and a Series C round. Their latest funding was raised on Dec 15, 2022 from a Series C round.
Synhelion is funded by 9 investors, including Cemex Ventures, AMAG Automobil und Motoren AG, AMAG Group, Eni, SMS Group, Swiss International Airlines, SMS Concast AG, Swiss KMU Partners, and Federal Ministry for Economic Affairs and Energy (BMWi). AMAG Group and Cemex Ventures are the most recent investors.