In concentrated solar power (CSP), how well the working fluid transfers heat through a receiver can make or break system efficiency.
For solar-heated thermochemistry, solar receivers need to operate at very high temperatures, up to 1400°C. Gases like air are cheap, nontoxic, and have no temperature limitations, so researchers are working on gas-based solar receivers that move heated gases like air, helium, or nitrogen.
Though these gas-based receivers can operate at high temperatures, gases don’t transfer heat as well as liquids like molten salts. So, how can the best receiver for high-temperature operation improve its heat transfer abilities?
A research team has experimentally tested different internal flow channel designs for solid state gas receivers, aiming to find the sweet spot between maximizing heat transfer while minimizing pressure losses.
The study
David D’Souza at IMDEA Energy and Universidad Nacional de Educación a Distancia (UNED) and team looked at how the actual internal geometry of a pressurized gas solar receiver could affect its performance. They reported their results in Experimental assessment of different compact flow channel geometries on pressurised gas solar receivers, published at Applied Thermal Engineering.
Following an extensive review and numerical analysis of previous studies, the team reported the results of an experimental test of the effect of different internal channel shapes on heat transfer.
Their experimental campaign evaluated the effects of different internal flow channel geometries on absorber thermal performance.
“In our previous study, the literature review and numerical analysis, we modeled a very wide range of different geometries,” said D’Souza.
“We’ve conducted this analysis with the overarching objective being to optimize the system to achieve high thermal efficiencies and simultaneously reduce pressure drops.”
The test bed
The tests were performed in a specially designed test bed using air pressurized from 4 to 12 bar. The team made a micro-channel receiver prototype in stainless steel and Inconel 625, a popular solar receiver material, because it can withstand high-temperature, corrosive environments and the thermal shocks of daily cycling common to solar thermal technologies.
Inside, they tried various internal shapes for the flow channels, essentially varying the aspect ratio of the rectangular channels as well as the channel wall thickness and number of channels. They varied mass flow rate, inlet pressure level and incident radiation flux to experimentally characterize each absorber variant. Each offered its trade-offs.
The team compared how the geometry of the internal flow channels influences heat transfer, pressure drop, and thermal efficiency under pressurized air flow.
Their numerical modelling analyzed six flow channel geometries commonly used in compact heat exchangers and additionally varied four common geometrical parameters, varying the number of channels and each channel’s breadth, height, and wall thickness. They found that the right geometry can make quite a difference.
D’Souza explained it in this way: “In typical systems, especially using gas like air or s-CO2, but also other heat exchanger fluids, you find that higher thermal efficiencies tend to correlate with increased pressure drops. We would also like to reduce the pressure drops because that means higher pumping costs and higher parasitic losses in the system. So our goal was to try to couple high thermal efficiencies with low pressure drops. And we found in our numerical analysis that, yes, these kinds of designs of internal geometries existed that basically optimize for these two factors.”
Their modeling had predicted that taller and narrower channels would perform better. The experimental results proved that the modeling was correct. The taller channels performed the best.
The test confirmed that the geometry of flow channels significantly affects the thermal efficiency and pressure drop in pressurized gas solar receivers.
“This behaviour stems from the relative thermal resistances offered by the heat transfer fluid flowing through the flow channels and the channel walls of the absorber,” D’Souza explained.
“Taller channels, with the height being the dimension in line with the incident radiation, increased the channel wetted surface area and decreased the fluid velocity in each channel for the same mass flow rate. This reduced the pressure drop, which is directly related to the channel velocity, without penalizing the heat transfer from the incident surface through the bulk of the absorber given the relatively lower thermal resistance offered by the solid channel walls of the absorber as against the heat transfer fluid.”
Why does this matter?
In solar receivers that use air or other gases to transfer heat, energy loss occurs not only when the solar heat is not transferred but also when more power is needed to push the hot gas through the system to prevent pressure drop.
So improving the geometry of solar receivers designed for high temperatures is crucial for scaling up solar thermal technology, particularly for industrial heat applications and solar fuels that use high temperatures.
The team’s extensive review of previous works aimed at analyzing problems and finding solutions to demonstrate the potential of pressurized gas receivers with optimized compact flow channel geometries for concentrated solar thermal applications.
“Our ultimate goal was to identify, if there existed, the ideal geometry for thermal efficiency and lower pressure drop,” he said.
“And from our numerical analysis, we found that yes, there actually are, depending on the internal geometry, specific shapes that have better thermal efficiencies, with lower pressure drops.”
What their test found
“In our numerical and experimental investigation, what we discovered was that taller, narrower channels have the best overall performance,” D’Souza said.
“If you solely would like to maximize thermal efficiency, then there’s no question, the smaller the channels, the more thermally efficient the receiver, because you’ll have the fluid inside your channels at the maximum velocity, so they’ll have very high heat transfer. On the other hand, if you have this very high thermal efficiency, you will suffer from very high pressure drops. However, from our numerical study and now, in what has been confirmed in our experimental works, it was found that the receivers with taller channels have better performance in terms of simultaneously having good pressure drop as well as having good heat transfer.“
More reading:
[1] D. D’Souza, ‘Application of compact flow channel geometries to pressurised solar receivers: a numerical and experimental analysis’, Universidad Nacional de Educación a Distancia, 2023. doi: https://rgdoi.net/10.13140/RG.2.2.17832.55041
[2] D. D’Souza, M. J. Montes, M. Romero, and J. González-Aguilar, ‘Energy and exergy analysis of microchannel central solar receivers for pressurised fluids’, Applied Thermal Engineering, vol. 219, p. 119638, Jan. 2023, doi: 10.1016/j.applthermaleng.2022.119638.
[3] D. D’Souza, M. J. Montes, M. Romero, and J. González-Aguilar, ‘Experimental assessment of different compact flow channel geometries on pressurised gas solar receivers’, Applied Thermal Engineering, vol. 266, p. 125634, May 2025, doi: 10.1016/j.applthermaleng.2025.125634.
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