International solar thermal energy researchers have successfully tested CONTISOL, a solar reactor that runs on air, able to make any solar fuel like hydrogen and to run day or night – because it uses concentrated solar power (CSP) which can include thermal energy storage.
The promise of solar fuels is that we could have zero carbon fuels like hydrogen without the climate-damaging carbon emissions it takes to make hydrogen from natural gas today, so perfecting solar reactors is key to a 100% clean energy future.
Instead of burning a fossil fuel for the heat needed to drive the thermal chemistry process, for chemical reactions like splitting H2 (hydrogen) from H2O, scientists have been testing various kinds of reactors heated by the thermal form of solar, CSP, which uses mirrors to concentrate solar flux on a receiver.
To attain zero-carbon heat for thermochemical reactions – which can operate at temperatures as high as 1,500 C – experts view the direct heat of CSP as a more efficient clean energy source than electricity from PV or wind.
There will be an unlimited supply of sunlight over centuries, and no climate consequences when thermochemistry is driven by solar energy. The only disadvantage compared to burning fossil energy, is that the sun goes down at night.
Night solar through CSP
Now, a group of scientists at the German Aerospace Center (DLR) supported by the Aerosol and Particle Technology Laboratory of CPERI/CERTH Greece have built and tested a new solar reactor design that includes storage so it can provide round the-clock heat like the current fossil-fired method, but without the emissions.
“Solar reactors in the past have had the problem of what you do at night when you don’t have sun, or even when clouds go by,” said the paper’s lead author, Justin Lapp, formerly of DLR, and now Assistant Professor of Mechanical Engineering at the University of Maine.
Lapp explained that when the temperature drops, the reaction could need to be halted or the flow rate of the reactants slowed, reducing the amount of products that you get out. If the reactor shuts down at night it cools off, not just wasting residual heat, but starting over from nothing next morning.
How it works
“So the main idea of CONTISOL was to build two reactors together,” he said. “One where sunlight is directly doing chemical processing.The other side for storing energy. In the chemical channels the high temperatures of the material drive the chemical reaction and you get a change from reactants to products within those channels, and in the air channels cooler air goes in the front and hotter air comes out the back.”
By combining storage capabilities with a direct solar thermochemical reactor, they get the best of both worlds, stable temperatures round the clock but also the most efficient heat source to perform reactions because it’s direct, so “you don’t have as many losses with multiple steps between the sunlight and the chemistry that’s happening.”
CONTISOL uses an open air receiver, based on the volumetric air receiver operated at its test solar tower at Julich by DLR (Deutsches Zentrumfür Luft- und Raumfahrt), which can heat air to 1,100 C. There an open air receiver takes air from the atmosphere and pulls it through small channels in a monolithic material.
“Ours is a volumetric air receiver like this one,” said Lapp. “The center is an extruded monolith; a large cylinder with many smaller rectangle channels. Every other row of channels is used for chemistry or for passing air through the monolith. These channels are open to the front to allow sunlight to go in and heat up this monolithic material.” The original test used silicon carbide for the multi channel receiver, but the scientists plan to try Inconel, a harder metal alloy for the receiver. “Silicon carbide is a little bit difficult to manufacture because it cannot be machined as well as a metal. So getting very tight tolerances can be tough. It’s not too expensive but it is not the easiest material to work with in manufacturing,” he related. Temperatures between 800-900 C are needed to rearrange water or hydrocarbon molecules into most solar fuels, so that was the temperature goal. The prototype reactor successfully operated at 850 C at lab-scale: 5kW.
CONTISOL was tested at Cologne, Germany using simulated ‘suns’, rather than an actual solar field, and the storage and heat exchanger was also simulated, as the reactor itself is the innovation.
“This scale is a scientific prototype simply for us to understand how to control it. It wouldn’t be commercialized at 5 kW,”he said. “Commercially, 1-5 MW would be about the smallest for industrial-scale reactors, and they could scale to 100 MW or even larger.” “In our case we are doing methane reforming as an example. But it’s not tied to methane, it could make any number of solar fuels. One interesting one is hydrogen production from sulphuric acid as a cycling material. When you evaporate sulphuric acid at about 400 C into steam and SO3, it is not corrosive, so you can even use stainless steel components.”