Periodic Reporting for period 1 - ONESTEP (Optimized Nanofluids for Efficient Solar Thermal Energy Production)
Période du rapport: 2023-05-01 au 2025-10-31
Climate change is the greatest existential threat to the world. To overcome this challenge, the EU has set substantial decarbonization goals through the European Green Deal. Renewable energy production is among the main drivers of emission reduction and solar energy covers over half of today’s green energy generation. The most efficient solar technology is solar thermal energy capture: taking in radiation from the sun and using it directly as heat or to generate electricity through steam turbines.
To improve the viability of solar thermal energy capture, water-based nanofluids (suspensions of dark, nanometric particles) are gaining traction because their photothermal (PT) boiling generates more steam than conventional technologies. However, upscaling is not currently feasible due to very little theoretical understanding of PT boiling and a lack of tools to improve this understanding.
This project aimed to expand our knowledge of PT boiling and facilitate the development of novel nanotechnology for solar thermal applications. To that end, we (1) explored the material characteristics needed for boiling optimization, (2) endeavoured to develop innovative techniques to examine PT boiling using acoustics and x-rays, and (3) performed boiling tests to elucidate the mechanisms behind the PT boiling phenomenon.
To improve the viability of solar thermal energy capture, water-based nanofluids (suspensions of dark, nanometric particles) are gaining traction because their photothermal (PT) boiling generates more steam than conventional technologies. However, upscaling is not currently feasible due to very little theoretical understanding of PT boiling and a lack of tools to improve this understanding.
This project aimed to expand our knowledge of PT boiling and facilitate the development of novel nanotechnology for solar thermal applications. To that end, we (1) explored the material characteristics needed for boiling optimization, (2) endeavoured to develop innovative techniques to examine PT boiling using acoustics and x-rays, and (3) performed boiling tests to elucidate the mechanisms behind the PT boiling phenomenon.
Through a combined effort between researchers in Norway and France, we examined the characteristics of two candidate nanoparticles, coffee and carbon black. Using different concentrations, temperatures, and dispersion methods, both thermal properties and stability were measured. These were essential to the development of the PT boiling measurement methodologies and the analysis of the boiling tests, contributing to the two publications and five conference presentations that emerged from this project.
Two new PT boiling measurement techniques were created. The first used acoustics, where frequencies are inversely proportional to bubble size and amplitudes are directly proportional to the number of bubbles. Initially, we desired to explore the smallest possible bubbles at several 100 kilohertz frequencies. However, we discovered that, at the highest frequencies, there was substantial attenuation (reduction in amplitude) and the acoustic signals were not distinguishable. Therefore, in furtherance of the technique development, we completed a study looking at how the nanofluids were attenuating the acoustic signals at higher frequencies. We then developed an experimental rig to capture clear, distinguishable acoustic signals in the auditory range (up to 20 kHz).
The second measurement technique used x-rays. Specifically, a CT-scanner into which the rig developed for acoustic studies was inserted. Here, we examined different imaging agent and nanoparticle concentrations and found the ideal combination (with the addition of post-processing) to image the bubbles during PT boiling. This worked down to a minimum diameter of 0.8 mm.
Understanding the limitations of the new techniques, we performed boiling tests at different concentrations of carbon black. This produced novel acoustic spectra that we could compare to understand how the nanoparticles were affecting boiling and the mechanism by which steam generation was enhanced.
Two new PT boiling measurement techniques were created. The first used acoustics, where frequencies are inversely proportional to bubble size and amplitudes are directly proportional to the number of bubbles. Initially, we desired to explore the smallest possible bubbles at several 100 kilohertz frequencies. However, we discovered that, at the highest frequencies, there was substantial attenuation (reduction in amplitude) and the acoustic signals were not distinguishable. Therefore, in furtherance of the technique development, we completed a study looking at how the nanofluids were attenuating the acoustic signals at higher frequencies. We then developed an experimental rig to capture clear, distinguishable acoustic signals in the auditory range (up to 20 kHz).
The second measurement technique used x-rays. Specifically, a CT-scanner into which the rig developed for acoustic studies was inserted. Here, we examined different imaging agent and nanoparticle concentrations and found the ideal combination (with the addition of post-processing) to image the bubbles during PT boiling. This worked down to a minimum diameter of 0.8 mm.
Understanding the limitations of the new techniques, we performed boiling tests at different concentrations of carbon black. This produced novel acoustic spectra that we could compare to understand how the nanoparticles were affecting boiling and the mechanism by which steam generation was enhanced.
This project’s primary scientific impacts relate to advancing our knowledge of the physics behind photothermal boiling and creating new methodologies for investigating further. This was a significant, multi-dimensional scientific impact that will persist beyond its immediate scope. First, it created new, high-quality knowledge about photothermal boiling in nanofluids, on which future research can be based. In time, this can lead to improved solar energy capture technologies as there will be a refined understanding of the underlying process. The related, open-access data generated over the course of the project can also be directly used for training machine learning models for future data analysis. A new understanding of acoustic attenuation in nanofluids will also help with the design of experiments where sound waves pass through nanofluids. Second, it resulted in novel, more accurate interdisciplinary techniques for measuring (acoustic) and imaging (CT) the photothermal boiling process, allowing future researchers worldwide to produce better data on nanofluid boiling than has previously been available. This will strengthen the human capital in green energy and related fields (like chemical engineering and medicine) in research and, eventually, innovation through better steam-generating prototype assessment strategies. In addition, by distributing the project results through open-access databases and conferences aimed at the scientific community we purposefully fostered the diffusion of knowledge and a culture of open access within the scientific community. These were precisely the impacts expected from the project.