Heliostats in the solar field concentrate sunlight up to the receiver atop the tower in Tower CSP, to heat molten salts. All twenty-four Tower CSP plants currently operating or under construction in China’s 1 GW renewable energy parks utilize molten salts to transfer the solar heat.
Nearly all the Tower-type Concentrated Solar Power (CSP) plants in operation use molten salts to transfer heat to produce electricity. This thermal form of solar energy has value because it incorporates stored thermal energy, which enables grid benefits like rolling reserves.
With their relatively low temperature limit of 565°C, these molten salts operate well within the requirements for electricity production in a Rankine steam cycle, which is typically used to generate electricity in thermal power plants, whether fueled by coal, nuclear, or solar thermal energy.
CSP research into higher temperature operation
However, the cutting edge of CSP research involves higher temperature concentrated solar technologies. High-temperature solar receivers or solid-state absorbers can be used to supply the heat to produce green hydrogen or sustainable aviation fuels through thermochemical reactions, processes that require temperatures as high as 1500°C.
Among these higher-temperature concentrated solar technologies, solid-state solar absorbers that heat a gas, such as air or CO2, are much investigated, and some designs, like Synhelion’s, are nearing first commercialization in Europe.
But gases are not dense, so transferring heat within this type of solar absorber is relatively complex. Design is critical, and researchers are trying various ceramics with internal geometries to facilitate heat transfer using gases.
In their recent paper Multi-objective optimization on thermo-structural performance of honeycomb absorbers for concentrated solar power systems published at Case Studies in Thermal Engineering, Masoud Behzad and his team investigate how to reduce thermal stress in a basic honeycomb shape solar absorber.
In commercial operation, such a receiver would be several meters thick. Each square hole in the honeycomb would be about 2.5 mm X 2.5 mm in cross section, but reaches to the back of the receiver over a length of several centimeters. The heliostats in the solar field would focus sunlight to heat the gas or ambient air at the front of the honeycomb.
3-D printing has enabled more complex internal geometries
Advanced 3-D printing is being used to precisely customize the internal geometry of these solar absorbers, enabling them to trap and efficiently transfer the heated gas.
“There are many new investigations, especially in Germany, Spain, and Italy, in various honeycomb-shaped materials,” said Masoud.
“It could be metal, it could be ceramics, usually for most experiments, they are using silicon carbide. There are several shapes of these channels, and many porous shapes are being investigated. And nowadays, because of 3D printing, for the shapes of these channels, you can find honeycomb or rectangular cross-section.”
This solar-heated gas or air needs to penetrate to the back from the irradiated front surface of the honeycomb without thermal stress.
Suction pulls the hot air through the honeycomb
“Ventilators provide negative pressure so the air can be sucked inside these channels,” he said.
“So it’s a kind of suction, a fan to provide negative pressure behind the receiver, to suck air through the channels. Other designs use porous materials, which are nice in volumetric effect, so most of the heat stays inside the volume of the absorber. But they have a higher pressure drop.”
Masoud’s team optimizes the factor of safety
Ceramic materials are excellent for high-temperature operations and can withstand temperatures up to 1500°C. However, they can be very brittle, so in cases like this, where temperatures fluctuate, this could damage the structure.
For their investigation, Masoud’s team chose a readily available basic square honeycomb that has already been established in previous studies in Germany and is simple to manufacture. By trying different radiation or velocities, they instead focus on estimating thermal stress.
“So we didn’t change the design of the honeycomb,” he explained.
“Because there is not that much information on the thermal stress inside the solid absorbers. Most studies focus solely on heat transfer or the fluid dynamics of the air inside the channels. We want to know how exactly these materials behave in high radiation environment, If we cannot have a uniform heat absorption inside of the channels, this could impose a high thermal stress to the system, their useful life cycle, and how they react to the variation of temperature, to night and day.”
The paper states: “We adopted a multi-objective optimization approach by integrating computational fluid dynamics, heat transfer, and thermal stress analysis. To streamline computational efforts, the Taguchi method was employed, reducing the number of required simulations while maintaining a relative error below 5 %. A critical mass flow to absorbed power ratio of 5 × 10− 6 (kg/s)/W was identified, beyond which thermal efficiency stabilizes, providing practical guidance for operational optimization. The optimal configuration achieved a thermal efficiency of 89.3 % and a factor of safety of 87.3 %, with a channel width of 3 mm, a thickness of 0.3 mm, an outlet static pressure of − 70 Pa, and a radiation flux of 650 kW/m2. These findings establish a robust framework for optimizing honeycomb receivers, addressing thermal and structural performance while maintaining simplicity in manufacturing processes.”
3D-printed ceria is a game-changer to increase solar fuel efficiency
3D-Printed solar receiver of honeycomb mesh to spread heat evenly
Innovation in its volumetric absorber takes OVR efficiency to 90%
