Experiments on solar thermal systems utilizing nanoparticles-laden fluid under stagnation and flow conditions

Abstract

The continuous and increasing usage of fossil fuels is adversely impacting the environment and also presents economical and geo-political challenges for societies around the world. Solar energy is one of the forms of renewable energy resources which is freely and abundantly available on the earths' surface and hence could be a very good potential alternative to fossil fuels. These collectors are used for heating various fluids for various applications ranging from space heating/cooling, process heating, steam generation etc. Such solar thermal collectors are generally are of conventional type (also known as surface absorption-based solar collector) in which solar selective coating is used. Main characteristic of the solar selective coating is that these coatings have high absorptivity within the incident solar spectrum (300-2500 nm) and has low emissivity in the near infrared region (2500-10000nm). Few examples of the selective coatings are TiNOX®, black chrome, LUZ cermet etc.(Kennedy and Price, 2005), but at high incident flux (concentrated solar irradiation) these coatings get damaged. Another issue with the solar selective coatings is that at high incident flux the surface temperature of the coating increases very high, which results in high radiative losses from the surface of the selective coating. These challenges degrade the performance of the surface absorption-based solar thermal collector. One of the alternatives by which these issues can be addressed is by allowing the incident radiation to be absorbed by the working fluid/heat transfer fluid itself. Initially, attempts were made with micron-sized particles (particle size > 100 nm) to absorb the solar irradiation, but there are some challenges with these particles like agglomeration of the particles in the working fluid which may cause abrasion and clogging of pumps and valves. These challenges are undesirable in many heating and cooling applications. In order to overcome the aforementioned challenges with micron-sized particles, researchers started using nanometer-sized particles (particle size ≤ 100 nm) also termed as ‗nanoparticles-laden fluid‘. One of the benefits of the nanoparticles-laden fluid is that the nanoparticles have high stability than micron-sized particles-laden fluid because of strong Brownian motion between the nanoparticles (due to smaller size). Another benefit of using the nanoparticles for harnessing the solar energy is that with the nanoparticles, the absorptivity of the working fluid increases, as most of the available heat transfer fluids (water, oils, molten salts etc.) are transparent within the solar spectrum. The literature suggests that the nanoparticles-laden fluid-based solar collectors have high potential for the harnessing of solar energy and new exploration is needed in this area of research. Thus, the thesis is aimed to understand the optical properties of different nanoparticles-laden fluid and the heat transfer mechanism within the nanoparticles-laden fluid-based solar collector (mono-dispersion and blended nanoparticles) under different working conditions (stagnation condition and flow conditions). In the present work, firstly the performance of nanoparticles-laden fluid-based solar collector has been investigated numerically. After that optical properties of various base fluids and nanoparticles-laden fluids have been investigated. Then heat transfer mechanism of monodispersions and blended nanoparticles-laden fluid under stagnation conditions has been examined with proof-of-concept experimental studies. Finally, the effect of mass fraction and mass flow rate on the temperature rise of nanoparticles-laden fluid under flow conditions has been examined in a laboratory-scale experimental set up. In order to understand, the optical signatures of various base fluids (such as DI water, ethylene glycol, propylene glycol, paraffin oil (heavy), paraffin oil (light), perfluorodecaline and silicon oil) as well as nanoparticles-laden fluids, optical properties were measured within the solar spectrum region (300-2500 nm) using PerkinElmer Lambda 950 spectrophotometer. The optical properties of base fluids show that these fluids are highly transparent and have low solar weighted absorptivity. Thus these fluids are unable to absorb the solar irradiation on their own. The absorption capability of base fluids can be enhanced with the addition of nanoparticles and the optical signature of nanoparticles-laden fluids show that the optical properties depend on factors such as material, shape and the mass fraction of the nanoparticles. The absorptivity increases with increase in mass fraction of the nanoparticles and amorphous carbon nanoparticles show very high absorptivity. Further, to understand the heat transfer mechanism of mono-dispersions and blended nanoparticles-laden fluid, an experimental study was conducted under stagnation conditions. In the mono-dispersion, Co3O4 nanoparticles-laden fluid (base fluid: DI water) was employed as working fluid and in blended-nanoparticles-laden fluid, a blend of Co3O4 and Al2O3 nanoparticles dispersed in DI water were used. The optical characterization of blended nanoparticles-laden fluid shows that the blend has broadband spectrum within 300-1500 nm, thus it is able to absorb the solar irradiation. The performance of both systems was compared with surface absorption-based system, in which working fluid (DI water) was placed in contact with solar selective coating (TiNOX®) copper substrate. All the experimental studies were conducted under identical conditions. The experimental studies with mono-dispersion and blended nanoparticles-laden fluid reveals that under similar working conditions higher temperature is achievable with nanoparticles-laden fluid-based system as compared to surface absorption-based system. Finally, in addition to stagnation conditions, an experimental study was also conducted for the nanoparticles suspension (amorphous carbon dispersed in DI water) under flow conditions. For conducting this experimental study, a spiral-shaped receiver was prepared. For this experimental study, the effect of mass flow rate and mass fraction on temperature rise of the fluid was studied. It was found that under flow conditions, the performance is sensitive to both these parameters (mass fraction and mass flow rate of the fluid). Based on the experimental results, it is concluded that with the addition of nanoparticles to the base fluid the absorptivity of the fluid increases. Further, with the usage of nanoparticles-laden fluid-based system, higher temperature rise can be achieved as compared to surface-absorption based system. Additionally, it is observed that the performance of these systems depend on various factors, such as nanoparticles material, mass fraction, sample thickness and mass flow rate. While harnessing solar energy with nanoparticles-laden fluid-based system, emphasis may be given on those nanoparticles that have high absorption capability at very low mass fractions (such as amorphous carbon), which leads to less agglomeration, high stability of the nanoparticles and overall better performance as compared to surface-absorption based system.