THERMODYNAMIC ANALYSIS OF SOLAR BASED POLYGENERATION SYSTEMS FOR A RESIDENTIAL COMMUNITY

Abstract

Electricity generation using renewable energy helps in combating the emissions produced by fossil fuel-based units. One such renewable energy source which can be utilized more effectively is solar energy and it is one of the finest accessible choices for fighting the issues of increasing electricity use, fossil fuel depletion, and global warming because it is available freely and abundant. However, there are some limitations with solar energy-based systems and one of them is their intermittency in operation as solar energy is not accessible throughout the day and does not remain the same throughout the year. Utilization of solar energy to generate electricity, cooling, and freshwater, hydrogen (as an energy source) in remote areas in a sustainable way still poses lots of challenges to researchers because of poorer conversion efficiencies. The goal of the current work is to design new solar-operated systems to produce electricity, cooling, hydrogen, and fresh water in remote areas. Thus, in this thesis, three solar energy-based systems (solar heliostat system, parabolic trough collector (PTC) operated polygeneration system, and solar operated trigeneration system) for the self-sustained community are presented and evaluated by thermodynamic principles using energy and exergy analyses for technical feasibility. Parametric study of each system individually would help to understand the impact of design factors on the systems performance. System 1 is a solar heliostat system (based on molten salt) that can meet out the electricity demand, hydrogen (for the refuelling of the vehicles) and cooling load in a community in remote areas. Steam Rankine cycle is utilized to feed the electrical power demand while some of the steam is bleed out to operate the two-stage ammonia water-based absorption system for cooling application. The result of the System 1 shows that with a heliostat area of 6000 m2 , 372 kW of electricity, 610 kW of cooling capacity, and 7.2 kg/h of hydrogen is generated. Furthermore, results of exergy study reveals that the significant exergy is being destroyed in the central receiver (1170 kW) followed by heliostat (980 kW). The performance evaluation of the presented system is made via exergy and energy efficiencies and estimated as 17.7%, and 38.9% respectively. Effects of some crucial parameters such as direct normal irradiance, evaporator temperature, the bleeding ratio etc. have been studied on the overall system performance. It has been found that 55% of useful exergy is being destroyed in the central receiver and heliostat iv field. On an average DNI of 700 W/m2 , for a 6 hour day the designed system can provide a cooling capacity of 3 kW to each house with an electrical load of 2 kW. Furthermore, the produced hydrogen can fuel, 100 vehicles with an average range of 100 km per day. Parametric analysis reveals that the central receiver efficiencies increase with an increase in DNI. System 2 is a new parabolic trough collector (PTC) operated polygeneration system that is used to produce freshwater, hydrogen, cooling, and electricity in a residential society. Dowtherm A (a heat transfer fluid) is utilized for transferring the heat from PTC to ORC, which is used to produce electrical power. The produced electrical power is utilized in three different ways, namely, to run the home appliances, to generate hydrogen (using water electrolysis), and to produce cooling (through a vapor compression cycle). Vapor compression cycle supplied the cooling to preserve the food, milk, and to operate the freezing desalination process. Effects of several factors, such as direct normal irradiance, evaporator temperature, and seawater inlet temperature, have been analyzed on the overall system behaviour. Thermodynamic study results show that for a PTC area of 2000 m2 , an electrical output obtained is 72 kW, the cooling rate is 112 kW, and the amount of fresh water obtained is 18.4 l/day and 10.8 kg/day of hydrogen for mean solar irradiation of 700 W/m2 . The systems’ energy efficiency is computed as 17.5%, and systems’ exergy efficiency as evaluated as 10.9%. Simulation results show that on a typical summer day, in India (environment temperature of 35℃, direct normal irradiation of 700 W/m2 ), the presented system will give electrical power to a housing society of around 60 houses/apartment (having 250 people) in a sustainable way. Furthermore, the proposed system delivers a cooling rate of 2 kW per house. The analysis also reveals that the rate of exergy destroyed in the parabolic trough collector is extreme leading to poor overall efficiencies of the system. System 3 is a new solar-operated trigeneration system to provide cooling, electricity, and fresh water using PTCs in remote areas. Electrical power is generated by using ORC, and cooling and fresh water (using freezing desalination technique) is obtained by two-stage NH3-H2O vapor absorption system run by solar energy. Simulation results show that for PTC arrays of 200 m2 , an electrical output obtained is 3.3 kW, cooling rate is 20.4 kW, and rate of freshwater produced is 36 kg/h for an average solar irradiance of 700 W/m2 . Additionally, sensitivity analysis is conducted by varying the parameter like solar irradiation, evaporator temperature, seawater inlet temperature etc. and their effect on performance characteristics of the overall setup is v investigated. Analysis reveals that maximum exergy is being wasted in the parabolic trough collectors pursued by HRVG. Simulation results show that the systems’ energy efficiency is 18.8% and the systems’ exergy efficiency is 4.7%. Additionally, PTCs’ exergy and energy efficiencies are calculated as 29.8% and 70%, respectively. Analysis also reveals that PTC has an exergy destruction rate of 85.0 kW. In overall, each system has its own merits and demerits and could provide a potential option for solar-dominated remote areas to obtain cooling, freshwater, hydrogen, and electrical power in an environmentally safer manner. It has been observed that System 1 is suitable for the location where sufficient sunshine is available to produce hydrogen, electricity and cooling for a residential community of 200 houses (800 people). However, System 2 can be installed or recommended for the location having sufficient sunshine and scarcity of potable drinking water. The designed system produced four useful outputs namely, electricity, cooling, freshwater and hydrogen for the community of 250 people. While System 3 can be implemented in the location where there is a need of cooling, electricity and freshwater with an abundance of sunshine and source of saline water. Additionally, the results in this thesis clearly shows the importance of hydrogen production in any solar energy system. The result presented in the current thesis may help the designers/researcher to create a self-sustained community in a more environmental and benign manner.