The tubing that absorbs the solar heat reflected off the parabolic trough-shaped solar collector in Trough CSP is normally a metal. Researchers at PROMES-CNRS have instead trialed glass tubing filled with graphene-infused nanoparticles suspended in water, to improve performance.
In concentrated solar thermal (CST) energy collection using parabolic trough-shaped mirrors, the heat from the sun is captured by aiming its reflection from a parabolic trough-shaped mirror onto a thin tube at its central focal point.
This thin tube contains a fluid that transfers solar heat to where it can be used by industrial processes, replacing heat from fossil fuels.
Solar researchers have previously shown that adding nanoparticles to this fluid improves its ability to absorb heat. But which nanoparticles will work best? And could there be a way to make a new heat transfer fluid that can absorb light as well?
Now, a team of solar researchers led by Miguel Sainz-Mañas at PROMES-CNRS have tested the use of graphene nanoparticles dispersed in water, and carried in glass tubing instead of metal so it is able to absorb both heat and light. Because of its two-dimensional nature, graphene’s high surface-to-volume ratio and stability at high temperatures make it ideal for absorbing incident radiation uniformly.
Nanofluid with graphene particles enhances solar absorption IMAGE ©PROMES-CNRS
“The 2-D aspect of graphene is what is interesting, in comparison with spherical particles,” explained Sainz-Mañas in a call from France.
“Graphene allows us to have a higher surface-to-volume ratio, which entails a higher absorptance with less nanoparticle mass added to the fluid. Absorptance similar to current selective absorbing surfaces can be obtained. This allows a direct absorption of the incident radiation by the transfer fluid, replacing the absorbing surface by a semi-transparent material results in a volumetric absorption. This is due to a higher absorption cross-section: projected surface area absorbing the incident radiation. ”
With rising interest in industrial sector decarbonization and carbon capture and reuse, carbon nanoparticles like graphene are drawing attention because of their excellent light absorbing and heat transfer properties.
“The graphene dispersion used is very stable at even temperatures close to the boiling point of the base fluid, which is not the case for other nanofluids,” he explained.
However, little research has been done on using graphene nanoparticles in concentrated solar thermal applications.
Other studies investigated graphene nanoparticles in oil or water for solar applications, but the PROMES-CNRS study is the first to try them under concentrated solar radiation. Traditionally, trough-based CSP (for power) has used oil to transfer heat, but water avoids oil spill risks and is well suited to supply the temperatures needed by many industrial processes.
“We are using graphene, a particle that hasn’t been used in water in parabolic trough collectors, to our knowledge,” said Sainz-Mañas.
“The idea is that these graphene particles are going to be stable at high temperatures, and therefore, we can use the collector at high pressure and have water in a liquid state at high temperatures.”
The PROMES team first performed a study to determine the effectiveness of the graphene-based nanofluid they used. They theorized that graphene nanoparticles should be particularly effective because their flat 2D shape gives them a large surface area that helps absorb light. First, they characterized it optically using a spectrophotometer to measure the transmitted and scattered light by the nanofluid at different particle concentrations. Then, a numerical analysis using the Discrete Dipole Approximation (DDA) allowed a deep study of the particle morphology influence on the optical response of the particle dispersion.
“This part is mainly to understand how the particle volume fraction and its morphology (size and shape) will affect the optical properties,” he said.
“For example, if I have 0.1 grams per liter, how much will I absorb, and how will this absorption vary with the graphene concentration? And we found that with very low particle concentrations, around 0.2 gram per liter, we’re going to be able to absorb over 96 % of the incident radiation with a receiver of similar dimensions than current solar thermal collectors. Thus, showing high potential for its use in parabolic collectors.”
Next, they want to determine the stability of the particles.
“If you want to use this type of nanofluid in commercial solar thermal collectors, the particles must be stable for long-term, several months, without agglomeration nor sedimentation.” Sainz-Mañas explained.
“If your particles are agglomerated, the absorption decreases as the absorbing surface per particle mass is reduced. With this study, we’re able to understand how the stability of particles evolves with time, and therefore validate its use in the experimental pilot under solar concentration.”
The researchers conducted extensive studies to determine the optimal concentration of graphene nanoparticles. They found that a concentration of 0.2 grams per liter provided a light absorption of 88% for their specific collector dimensions (33.6 mm inner diameter).
With this knowledge on the particles’ stability and their optical properties, they designed the experimental pilot to study and verify the feasibility of this type of collector.
The PROMES team then built a small-scale parabolic trough solar collector to test this graphene nanofluid in real-world conditions. Two additional innovations improved performance: Two-axis tracking enabled the trough collector to maximize its solar potential. Then, instead of using the standard opaque metal tube, they built transparent tubing comprising two layers of glass. This enables the absorption of light as well as heat.
Glass tubing facilitates both heat and light absorption
The team innovated tubing made from a double layer of clear glass instead of the standard metal tubing in today’s trough CST. This has two benefits: it gives sunlight direct access to the entire volume of the graphene nanoparticle dispersion rather than heating only the surface, as in the metal tube. Glass tubing could also have mechanical advantages.
“With glass tubing, we can have an entirely glass trough system. This avoids the existing glass-to-steel connections mechanical constraints. However, the double-glass tubing may entail new constraints related to the pressure of the hydraulic circuit or temperature differences between the inner and outer tubes. Experimental test must be done to answer these questions,” he noted.
“But to industrialize this type of collector, dealing with the nanoparticles might be challenging because they might interfere with the pumps or cause problems in the hydraulic system. Or they could interfere with either the heating system or the heat transfer mechanism. This is something that could happen, but for now, in my prototype, I haven’t seen them.”
The commercial application
Sainz-Mañas sees this technology being commercialized for supplying heat at 200 °C (They know the upper limit. Above 250 °C, the graphene nanoparticles would not remain stable; they would agglomerate). However, temperatures up to 200 °C are used in processing most foods and in many other industrial processes.
The real-world test enabled them to verify the collector’s optical and thermal efficiency. The information it yielded on temperature and pressure validated the model and also showed them where to change several parameters to better understand how they impact the collector’s performance: fluid velocity, inlet temperature, and particle concentration.
The test was conducted at just 50 °C to validate the concept and uncover any issues.
“It did work at 50 °C, but we saw an impact on the pH of the nanofluid affecting the particle stability with a concentration of 0.2 grams per liter” Sainz-Mañas commented.
“We observed a direct dependance of the particle stability with the pH of the sample. By controlling the pH, good stability was achieved using a 0.3 grams per liter graphene aqueous dispersion. This pilot allowed us to verify the feasibility of the collector and find the key points affecting its performance. Total collector efficiencies of 74 % were obtained using 0.3 grams per liter graphene dispersions. . In a further study, we will increase pressure and temperature because the idea is not to work at 50 °C, but to operate as it would commercially, at 200 °C.”
