Synhelion’s solar receiver achieves a temperature of 1,500°C and higher. That’s unprecedented. Even the Gen3 particle-based solar receiver being pilot-scale tested now at Sandia – and the DLR CentRec receiver licensed to Heliogen – operate at the (relatively!) lower temperature of 1,000°C.
One of Synhelion’s solar receivers is on the same tower as the 1,000°C CentRec particle receiver at DLR‘s test site in Jülich. Mirrors called heliostats in front of the tower concentrate and reflect multiple “suns” from these mirrors up to the receiver to generate heat.
As a test site, this solar field of heliostats is already much smaller than a commercial CSP project solar field at a typical 100 MW. Yet the Synhelion receiver is not even using the whole solar field at Jülich, according to Carmen Murer, speaking for the firm.
“The DLR solar field has something like 20,000 square meters of heliostats,” said Murer. “But we actually only use about a quarter of it to heat our receiver.”
That makes this even more of an achievement, to achieve this much higher temperature even from just a portion of a small test-site solar field, a real breakthrough in CST technologies.
“The solar receiver is inspired by nature, by the greenhouse gas effect, to reach ultra-high temperatures,” Murer added. “The receiver cavity is filled with a greenhouse gas flowing from the aperture toward the back of the cavity. Typical gases employed are water vapor and CO2; both very potent greenhouse gases. Solar radiation enters the cavity through a window and passes with minimal absorption through the gas. The black surface of the cavity absorbs the heat, thermalizes, and re-radiates it back into the cavity. The greenhouse gas absorbs the thermal radiation and is heated up, acting as heat transfer fluid.”
Here’s how the receiver achieves the high temperature at this comparatively low solar concentration.
How the greenhouse effect works in Synhelion’s gas-based solar receiver
In a call from Switzerland, CEO Gianluca Ambrosetti explained how it works. The CO2 and water vapor gas comes in cold and gets heated up as it moves through the receiver. The hotter gases inside create a shield that keeps the hotter interior heat from getting back out, the same way that CO2 in the greenhouse effect traps heat on Earth.
“The main way a solar receiver loses heat and becomes inefficient is because that very high temperature goes out the window,” Ambrosetti explained. “This is not happening here because the cold layers of the gas are now a greenhouse gas field of CO2 and water vapor shielding the aperture from the heat radiating from the back of the cavity. The receiver is the central part of our technology. And, ironically, it works by using the same greenhouse effect that we are trying to mitigate as an industry.”
The solar thermochemistry Synhelion performs to make its hydrocarbon solar fuel is then carried out in an adjacent reactor. Typically a little heat is lost in transferring heat from the solar receiver to the reactor. Ambrosetti said it is maybe 50 to 100 degrees. So the reactor would then be able to perform thermochemistry at 1,450°C or 1,400°C degrees.
Last year, Synhelion’s gas-based solar receiver supplied this high-temperature heat to a world leader in traditional reactor technology; Wood. Synhelion and Wood sited their receiver and reactor together on the tower at the Jülich testing site. They successfully produced the world’s first syngas using sunlight to create heat directly. (Synhelion produces first solar syngas in Wood’s industrial-scale reforming reactor )
Solar receiver future: Gas, Liquid, or Solid?
This kind of gas-based solar receiver design comprised one route to higher temperature Gen-3 CSP competing for funding by the US DOE. Each needed to use materials that won’t degrade at a higher temperature than the 565°C of today’s commercial molten salts, as DOE Industrial Decarbonization Director Avi Shultz explained in 2018. The contenders were gases like air, liquids like sodium, and solids like sand.
Sandia won that award with their bauxite-sand-based falling particle receiver and is now constructing the pilot plant at their solar research test site. Synhelion continues its work with clinker production for CEMEX at Sandia and produced the world’s first solar synfuel in the Wood’s reactor on the Jülich tower. However, it is not using either particle receiver at Sandia or Jülich. Their proprietary receiver is sited below DLR’s particle receiver on that tower.
“We don’t work with particles at all,” Ambosetti affirmed. “Our proprietary receiver is our core development. Our completely new receiver technology, that works with greenhouse gases as heat transfer fluid, enables us to generate solar process heat above 1,500 degrees.”
Market focus on solar fuels
Synhelion, which began as a spin-off from the ETH Zürich solar research laboratory, does not intend its high-temperature receiver to be used in a typical commercial 100 MW tower CSP project with a full circle solar field. These only need to generate heat at 500-600°C. That’s sufficient for electric power production; even 400°C is enough.
But Synhelion is focused on a higher temperature CST market – solar thermochemistry for producing solar fuel to decarbonize transportation or direct solar heat to decarbonize industries like cement that need very high temperatures for industrial processes.
The DOE has also funded Synhelion to provide their gas-based receiver for a second experiment at the University of Florida, this time on perovskites for solar fuels. The first tested the doping of ceria with nickel to make solar syngas. Can a version of their receiver work as efficiently at just a 5 kW scale?
“It is possible to scale the receiver down,” Ambrosetti explained. “One way to scale it down without losing efficiencies is to increase the pressure because you will need a certain amount of molecules in the volume to absorb.”
For the new Florida experiment to make solar hydrogen, Synhelion will modify their smallest version, a 50 kW receiver operating at the Very High Concentration Solar Tower of IMDEA Energy located in Móstoles, Spain.
“For the Florida experiments, we need to scale our receiver a bit down,” he said. “This is not a problem as we do have an architecture for small receivers. At the same time, we are also scaling up our receiver. On the DLR tower in Jülich, we installed a 250 kW receiver. For our first industrial-scale solar fuel plant DAWN, which we are currently building, we have developed a 600 kW receiver.”
So the Synhelion solar receiver is trialing several routes simultaneously: green hydrogen using solar thermochemistry in the US for the DOE, and increasing the efficiency of solar clinker production with CEMEX at the Sandia pilot, and supplying the solar heat to Wood’s thermochemistry reactor to make Synhelion’s own solar aviation fuels in Germany.
“It’s a big puzzle. The university collaborations are very relevant for us. There are many elements, and we need to look at different things.” said Ambrosetti.
With all these projects and partnerships worldwide, their highly efficient high-temperature solar receiver is getting a real workout. As the decarbonization of the many high-temperature industrial processes comes to the fore, Synhelion may become not only the world’s first solar aviation fuel business but also supply their highly efficient solar receiver to an entirely new industry in the 21st century, solar thermochemistry.