Improvement of direct absorption solar collectors (DASCs) performance by using nanofluids
- Autores: Jorge Burgos Rodríguez
- Directores de la Tesis: Leonor Hernández López (dir. tes.), Rosa Mondragón Cazorla (dir. tes.)
- Lectura: En la Universitat Jaume I ( España ) en 2024
- Idioma: inglés
- Número de páginas: 200
- Tribunal Calificador de la Tesis: M. Elena Navarro (presid.), Gladys Mínguez Vega (secret.), Nuria Navarrete Argilés (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías Industriales y Materiales por la Universidad Jaume I de Castellón
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
This Thesis presents a set of experimental studies related to the application of solar nanofluids (NFs) and hybrid emulsions with phase change materials (PCMs) in direct absorption solar collectors (DASCs). The main objective is to develop novel materials capable of improving the absorption, storage and heat transfer properties of the conventional fluids used in direct absorption solar technologies.
Current technologies to produce energy from renewable energy sources allow climate change challenges to be faced to, thus, reduce greenhouse gas emissions from the production of fossil fuel-based energy. By paying attention to solar energy, the renewable energy transition helps to meet energy neutrality challenges, while also making these new technologies available worldwide.
Solar thermal energy is one of the most important renewable technologies used to deal with the transition of the energy system due to the unlimited and immense capacity of the energy provided by the Sun. Solar thermal collectors are capable of absorbing radiation from the Sun and converting it into heat for further uses. These systems use fluids, such as water, molten salts, thermal oils, etc., that are capable of transporting the heat transferred from an absorber surface. These fluids are known as heat transfer fluids (HTFs). However, conventional collector systems involve the drawback of heat losses during the absorption process in different stages, which is a disadvantage that can be solved by developing DASC systems. In this new solar collectors configuration, HTFs can be used to directly absorb solar radiation, and the resulting thermal energy can be transferred by HTFs, or even stored.
This Thesis focuses on developing new HTFs by incorporating nanoparticles (NPs), known as solar nanofluids and hybrid emulsions with PCMs. They present enhanced optical properties and higher thermal energy storage capacity than conventional fluids. To fulfill this purpose, in the present work water-based gold NFs (Au NFs) of different particle sizes (5 and 20 nm) and concentrations (5.1, 28.2 and 51.3 ppm), and hybrid carbon-paraffin/water emulsions with 5 wt. % of PCM (paraffin wax) and carbon black NPs at low concentrations (0.01 wt. %), were proposed and produced as enhanced HTFs for the DASC application.
The optical behavior of both HTFs was characterized by analyzing different optical parameters with the help of spectrophotometric techniques. For the Au solar NFs, the influence of particle size and concentration on the extinction coefficient was analyzed, and the surface plasmon resonance characteristic of metallic particles, such as Au, was observed. For hybrid emulsions, due to the opacity of samples, spectral reflectance and transmittance were measured and, thus, spectral absorptance was obtained. An improvement in the absorption capacity of both new HTFs was made thanks to the addition of Au and carbon black NPs, respectively.
Besides optical properties, thermal properties were measured to evaluate the effect of NPs and paraffin on storage and heat transfer capacity. The addition of NPs to the base fluid did not bring about a remarkable change in the final thermal conductivity or the specific heat capacity of both the new HTFs due to the low concentration of NPs. Notwithstanding, it was proven that the total thermal energy storage density of HTFs can be increased by using hybrid emulsions thanks to the contribution of the latent heat of the paraffin PCM.
Finally, new experimental facilities were developed using simulated and natural sunlight to study the photothermal conversion efficiency (PTE) of the new Au solar NFs and hybrid emulsions to evaluate their ability to convert solar radiation into thermal energy. For this purpose, facilities with and without sunlight concentration were developed, and differing scenarios were assumed to calculate PTE, including thermal losses and phase change. It was demonstrated that low concentrations of NPs in both HTFs, together with the latent heat contribution of paraffin in hybrid emulsions, increase PTE and enhance the performance of direct absorption solar technologies.
