The Sun can provide an unlimited amount of energy, but there’s one major downside – night. For roughly half the day facilities and devices that rely on solar power have to shut down or store extra energy, and the process is particularly disruptive to concentrated solar power (CSP) plants which work on heat. Now, scientists at the German Aerospace Center have tested a solar reactor concept known as CONTISOL, which also contains a thermal energy storage system to allow it to run day or night.
While photovoltaic systems harvest energy from light, CSP systems capture the heat from the Sun instead. This most often takes the form of facilities made up of an array of mirrors reflecting sunlight towards receivers on a central tower, which uses the heat to trigger thermochemical reactions. That might involve converting water into steam to run a turbine, or in the case of CONTISOL, producing hydrogen by splitting water steam into its constituents.
To do so, the reactor needs to reach temperatures of between 800° C and 900° C (1,472° F and 1,652° F). That’s all well and good when the Sun is shining, but with 12 hours or so of darkness every night the system cools off, wasting time and heat as the reactor has to warm back up every morning.
The CONTISOL concept was designed to solve that problem. During the day, the sunlight heats up two sets of chambers full of air pulled in from outside. Once hot, the air in one chamber is piped into the reactor to trigger the thermochemistry reaction to produce hydrogen, while the other chamber simply stores the energy. Once the Sun goes down, the system can draw hot air out of the storage chamber and continue using that in the reactor.
“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,” says Justin Lapp, lead author of a paper describing the CONTISOL test run. “So the main idea of CONTISOL was to build two reactors together. 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.”
To test the system, a team from the German Aerospace Center set up a small-scale prototype under the heat of simulated suns. The device managed to successfully operate at a temperature of 850° C and a power of 5 kW. That’s pretty low overall, but the goal was to test the viability of the reactor, and that would be scaled up for commercial use.
“This scale is a scientific prototype simply for us to understand how to control it,” says Lapp. “It wouldn’t be commercialized at 5 kW. Commercially, 1 to 5 MW would be about the smallest for industrial-scale reactors, and they could scale to 100 MW or even larger.”
Before that happens, the team plans to continue tweaking the design, materials used and the chemical processes involved.
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