Electrical and thermal performances of photovoltaic / thermal systems with magnetic nanofluids

Abstract

Enhancing solar photovoltaic and thermal conversion performances could contribute to the development of more environmentally friendly hybrid photovoltaic/thermal (PV/T) systems that can be used in applications ranging from household to industrial scales. Due to their enhanced thermal and optical properties, nanofluids have proved to be good candidates for designing PV/T systems with superior performances. The smart behaviour of magnetic nanofluids (MNFs) also known as ferrofluids can further enhance the performances of PV/T systems in an external magnetic field. In this thesis, recent developments in enhancing the electrical and thermal performances of PV/T systems using magnetic nanofluids are extensively reviewed to identify the research gaps that need further attention. In the first part, the recent progress in the heat transfer enhancements by magnetic nanofluids in the absence and presence of magnetic fields is extensively represented and the potential impacts on the PV/T performances were discussed. Various parameters affecting the performances were highlighted, and some areas for further investigation are discussed. The reviewed literature shows that PV/T systems with MNFs were promising. However, their performances need further investigation before they can be used in practical applications. The stability of magnetic nanofluids (MNFs) is a key requirement for their use in engineering applications such as electronic cooling, heat transfer engineering, solar energy engineering, coolants in radiators and biomedical applications such as drug delivery, hyperthermia etc.. In this work, an innovative method for preparing highly stable magnetic nanofluids for heat transfer application is presented. Magnetite nanoparticles (MNPs) were synthesized at room temperature via chemical coprecipitation, coated with citric acid and dispersed in water as a carrier fluid. The phase composition, particle size/morphology and magnetic properties of the synthesized nanoparticles were characterized by X-ray diffractometer (XRD), scanning transmission electron microscope (STEM) and superconducting quantum interference device (SQUID), respectively. The results show that the synthesized nanoparticles are magnetite with a spinel structure without any impurities. The synthesized MNPs exhibit a nearly spherical morphology with a uniform particle size distribution and an average particle size of 11 nm. Based on the analysis of the temperature-dependent magnetic properties at 5-300 K, the coercivity and remanence in the hysteresis loop for the MNPs decrease with the increase in ix

temperature and disappear at room temperature (i.e., 300 K), thus showing a superparamagnetic behaviour. The paramagnetic nanoparticles are of paramount importance in the preparation of highly stable magnetic nanofluids with enhanced magnetic properties. The superparamagnetic behaviour of the synthesized MNPs was analysed using a developed model based on the core-shell theory. The experimental results of magnetization are in reasonable agreement with the calculated data by the developed model. The magnetic susceptibility of the synthesized MNPs increases and the coercivity, exchange bias, remanent and saturation magnetizations decrease with the increase in the temperature. It is indicated that the synthesized MNPs could be used as a promising candidate in various applications such as heat transfer, magnetic hyperthermia and magnetic separation. The stability of prepared MNFs was analysed by UV-Visible spectroscopy and visual observation of the standing sedimentation. The prepared magnetite nanofluids appeared stable for more than 36 days. The results from visual observation were confirmed by UVVisible spectroscopy measurements. The long-term stability was evaluated using a nanofluid destruction rate. The MNFs destruction rate was estimated at 0.03% and 0.13% for the MNFs with particle volume fractions of 0.51% and 1.04 %, respectively confirming their long-term stability. The viscosity of the prepared MNFs was measured in the temperature range of 24oC-80oC at various particle volume fractions. The measured viscosity confirmed the enhancement in the viscosity with the increase of particle volume fraction and its reduction with the rise of temperature. To understand the effects of climatic conditions on photovoltaic systems and the need for PV/T systems, a model for predicting the effects of environmental conditions on the performance of photovoltaic panels was proposed. A one-diode model considering the effects of environmental conditions such as solar irradiance, ambient temperature and wind was developed and simulated in MATLAB/SIMULINK environment. The accuracy of the developed model was confirmed via a comparison of the simulated results with the output characteristics of two polycrystalline photovoltaic (PV) modules from different manufacturers. The effects of various environmental parameters on the cell temperature and output characteristics of PV modules were analysed using the developed model. The results reveal that environmental condition variations have significant effects on the electrical performances of PV modules and should be taken into consideration in PV x system design. The developed model could also be useful in the design of photovoltaic/thermal (PV/T) systems with enhanced performances since it takes into consideration the cooling effects of the wind. Electrical and thermal performances of PV/T systems were analysed using a proposed model. A physical model based on the tube in sheet configuration was designed and the electrical and thermal models were developed and simulated in Maple and the performances of the designed PV/T collector. The performances of the designed PV/T model were assessed for constant climatic conditions and the effects of various climatic conditions (i.e., solar irradiance intensity, temperature and wind speed) were analysed. In addition, the daily and yearly performances of the designed PV/T system were simulated under the climatic conditions of Kigali using the weather data collected from Meteo Rwanda. The results of the simulation revealed that the performances of PV/T collectors varied with the solar irradiance intensity level and the temperature whereas the wind speed showed relatively low. The low influence of the wind speed was due to less heat exchange between the collector and the environment since the studied collector was glazed. The analysis of the performances under the climate of Kigali indicated that waterbased and MNFs-based systems both have high performances. The system with MNFs showed superior performances with enhancements in both energetic and exergetic efficiencies. Due to a lack of appropriate thermal conductivity and viscosity models to predict the performances in a magnetic field, we focused on the systems operating in zero magnetic field.