Performance Investigation of Various Carbon Allotropes Based Single-phase and Hybrid Nanofluids for High Pressure and Temperature Solar Thermal Systems

The energy sector (transportation, industry, electricity, and heat production) contributes approximately 70% of greenhouse gas (GHG) emissions. From the available data and global situation, it is evident that the energy industry needs to be decarbonized urgently to meet the global targets of limiting the temperature rise to 1.5 °C and mitigating climate change's adverse effects. The need of the hour is to develop commercial technology which can be sustainable, cost-competitive, environmentally friendly, or GHG emission-free and reliable to meet the global energy needs at the base load, which present renewable technologies are unable to offer. Solar energy is the most abundantly available resource on the earth. Furthermore, solar thermal technology is an underdeveloped and untapped resource with the mammoth potential to meet global energy needs by fulfilling the criteria discussed above. Solar thermal technology faces problems of inefficient heat transfer fluids (HTFs) and higher capital expenditure (CAPEX) of the systems, which ultimately yields higher energy prices, thus making it difficult to adopt. The present research developed a methodology that focuses on solving these two problems associated with solar thermal technologies. A complete solution that includes technical and policy measures has been developed to make solar thermal technologies competitive with other renewable energy technologies. The efficient heat transfer fluid is designed using various carbon allotropes, which directly impacts the reduction in the prices of energy produced. Besides, policy measures have also been suggested to reduce equipment costs, and the concept of cyclic economy model-based integrated solar thermal technology has been introduced. These concepts combined can support the objective of reliable, GHG emission-free, cost-competitive, and sustainable energy technology. The research methodology included the basis of finalizing the path of the thesis or design of the experiment, a literature review to identify the recent developments in the field of solar thermal energy and heat transfer fluids, and based on the set criteria, further developments are made to develop efficient heat transfer fluids for high pressure and temperature solar thermal systems. The generated heat transfer fluid is characterized for various heat transfer properties at high pressure (up to 70 bar) and temperature (up to 300 °C). Using those properties, a parabolic trough collector system has been investigated for energy generation and associated costs of energy produced. Furthermore, policy measures have been suggested based on past developments in other fields of renewable energy, and a mathematical model for an integrated system based on solar thermal technology has been developed. The research contributes significantly to the existing literature. The contribution includes but is not limited to identifying the best-suited heat transfer fluid for the solar thermal systems, characterizing the nanofluids prepared using major allotropes of Carbon for thermophysical properties at high pressure and temperature, developing a mathematical model for exhibiting the impact of integrated cyclic economy model for reducing the price of energy produced using solar thermal systems and carrying out the risk assessment and Environmental Impact Assessment (EIA) for application of nanofluids in commercial-scale solar thermal projects and more. The experimental analysis yielded promising results. The experimental results from the thesis work reported maximum enhancement in thermal conductivity up to 39.5% for single-phase nanofluids and up to 103.72% for hybrid nanofluids. The maximum enhancement in heat capacity was observed to be 12.4%. The zeta potential experiments exhibited values as high as 65 mV for the nanofluids. These thermophysical properties and simulation yielded a reduced Levelized cost of energy (LCOE) by up to 13% approximately, and energy enhancement of up to 4.1% was observed. The research lays the foundation for decision-makers to develop a real commercial-scale solar thermal technology-based integrated system to meet the world's energy needs reliably, sustainably, and competitively.