Thermodynamic analyses of different integrated energy generating systems using renewable and waste heat sources

Abstract

Global warming, alteration in the climate, depletion of the ozone layer, acidic rain, increase in airborne toxins, waste disposal concerns, and contamination of water have all had a significant impact on the earth in recent decades. Moreover, according to the International Energy Agency 2021 report, the global demand for energy/power is rapidly expanding, with a projected 21% increase by the year 2040. Research on cooling demand under future climate change scenarios reveals that cooling electricity consumption might double in most European nations, reaching an increase of 15-126% even in heating-dominated areas like Canada. By 2030, India's cooling demand is expected to climb by 45%, with the city of Mumbai alone accounting for about 24% of the entire cooling demand in the United States. All of these projected forecasts raise worries regarding environmental issues, as 81% of energy demand is still fulfilled by the consumption of fossil fuels, which are major source of carbon dioxide (CO2) emissions. Such CO2 emissions alone provide 74.4 % of the total, making them by far the most important contributor to global warming and the accompanying damage to the environment, even if other greenhouse gas emissions also play a role. As a result, the average global temperature of earth continues to rise, resulting in climate change. According to the Global Climate Risk Index 2021, India is the 7th most affected nation by climate change. To address the aforementioned global concern, almost 200 countries joined the 2021 United Nations climate change summit/conference in Glasgow, which approved a global agreement to combat climate change and support vulnerable countries. The summit aims to reduce CO2 emissions by 45% by 2030 compared to 2010 levels, with the goal of reaching net zero by 2050 to limit the average global temperature rise to 1.5 oC. In connection to the above summit, India has set a timeline for installing 500 GW of non-fossil fuel capacity (nuclear, hydro, solar, wind and biomass) by 2030. Also, India established four aims, including (a) meeting 50% of its energy needs through renewables by 2030, (b) India would lower its overall estimated carbon emissions by one billion tonnes from 2021 to 2030, (c) India aims to lower its economy's carbon intensity by more than 45% by 2030, and (d) India would reach the goal of net-zero by 2070. Based on the above discussion, it is evident that using renewable heat energy sources, recovering the waste heat, and enhancing the processes and energy efficiency can reduce the fossil fuel dependency of several residential/industrial energy applications. Renewable and waste heat have a low-grade enthalpic level and should be combined with other technologies to bring it to a practical level. Over the past decades, scientist and engineers have tried various ways to develop high-efficient integrated energy systems to directly transfer low-grade heat sources (e.g., waste heat, solar energy, biomass, etc.) to electricity, cooling, and space heating. Among them, the ammonia-water based combined cooling, heating, and power systems have drawn keen interest of researchers through the globe. These systems are known as energy system distributors and provide following benefits: (a) simultaneous electricity, heating, and cooling production with high efficiency, (b) significant reduction in greenhouse gas emissions, (c) fuel and energy demand with low costs, (d) high reliability, and (e) lower electricity usage during peak summer demand. This work aims to develop ammonia-water based integrated systems operating on waste heat, solar and biomass based energy resources. Feasibility investigation of these systems are studied based on thermodynamic balance equations for performance assessment of thermal systems in terms of energy output, energy efficiency, exergy efficiency, and overall exergy destruction. In India, around 200 million people in India lack access to electricity, despite India being the world’s third-largest producer and consumers of power. As a result, alternative energy generation at the micro and decentralized levels is important for addressing this deficit. In this regard, the development of power/cooling/heating sources from waste heat and renewable energy is a rapidly growing and attractive segment of the energy market. Accordingly, a literature review was conducted to learn about the current advancements in the ammonia-water based integrated systems. Based on the research gaps identified in the literature survey, the present work has been accomplished in the following steps. In the first study (chapter 3), an inverse analysis of a single stage refrigeration system using ammonia and water absorption is presented. A method for selecting an objective function is shown, with the knowledge that the selection of the goal has no bearing on the inverse analysis's result. Reducing the overall exergy destruction based inverse function allows for the simultaneous retrieval of input parameters, all of which satisfying the same objective. It is found that the overall minima of exergy destruction coincide with the maximum of coefficient of performance and exergy coefficient of performance for absorption refrigeration system operating under a fixed cooling load. Therefore, the choice of objective function has no bearing on the optimization process as long as the conditional statement of continuous cooling load is satisfied. Consequently, an inverse analysis is carried out with a goal of achieving the lowest possible value of overall exergy destruction (53.50 kW at 150 °C generator temperature) for the absorption refrigeration system operating at fixed cooling load. Multiple optimized outcomes with generator temperatures as low as 127.34 °C are produced via inverse analysis, all satisfying the same objective. The second study (chapter 4) introduced a novel combined power and cooling cycle developed by integrating modified Kalina and Goswami cycles that share a common absorber. Unlike most of the conventional studies which were aimed at minimizing the overall exergy destruction of the cycle, this work clarifies that such a practice did not ensure the optimized attainment of total turbine work output, cooling output and exergy efficiency of the cycle. Therefore, this conditional nature of overall exergy destruction is addressed here through the optimization of an integrated objective function addressing each of the desired performance parameters using a dual-mode dragonfly algorithm. The new optimization function shows 1.84, 6.74 and 1.33 times improvement in cycle’s total turbine output, cooling output and exergy efficiency, respectively with respect to the general practice of overall exergy destruction minimization. Furthermore, this theme of collective optimization is extended for a multi-generation system, by putting forth forward problem-based and inverse problem-based optimization functions. It is seen that there is no single standalone objective which generates maximum possible exergy efficiency of 27.39% and also have overall exergy destruction close to 88.97 kW (minimum possible value). So, these standalone objectives are clubbed to form two equally weighted forward and inverse objective functions, which are also minimized by dragonfly algorithm. The results show that the value of exergy efficiency for the forward (28.05%) and inverse (28.43%) based objectives are found to be higher than any of their standalone objective. Also, the overall exergy destruction for these forward (89.73 kW) and inverse (92.09 kW) based objectives are quite less than for the standalone case when exergy efficiency is 27.39% (113.62 kW). In the third study (chapter 5), an effective performance framework based on distillation technique is suggested as a substitute for the separator and flashing method for linking the two cogeneration cycles. This helps mitigate the primary flaw in the flashing process, which is the significant pressure energy lost during the process, which has an adverse effect on the amount of additional outputs that are produced. For a coupled solar based tri-generation cycle, substituting the distillation unit for the flashing unit results in an increase of 1.76 times and 1.11 times in the total turbine output and total cooling output, respectively. Subsequently, the distillation unit is employed to enhance the Goswami cycle outputs, which is the most basic combined power and cooling cycle. The addition of distillation unit in the conventional Goswami cycle improved the total output/exergy efficiency from 38.50 kW/31.37% to 43.55 kW/32.68%, respectively. After that, the distillation unit is used in addressing the main weakness of the conventional Goswami cycle, namely that its cooling output is rather low in comparison to its ability to produce power. A scheme is suggested which incorporates a distillation unit assisted absorption chiller setup to the Goswami cycle, with primary goal of producing auxiliary cooling without any pressure losses and extra work input. The present modification has increased the conventional Goswami cycle’s power and cooling output from 25.25 kW to 29.83 kW and 6.03 kW to 55.52 kW, respectively. In the fourth/last study (chapter 6), the thermochemical route of energy/bioenergy production like the biomass assisted gasification combined system are discussed. Initially, a modified equilibrium model which is based on a single global gasification reaction is presented which operates at different gasification/syngas temperature and moisture content of the biomass. The model is used to study the variation in syngas composition, equivalence ratio, lower heating value of syngas and coldgas efficiency for different input conditions. After that, an integrated system is presented that produces and burns syngas completely into fluegas, and then uses the latter as a thermal input in a triple-pressure cogeneration cycle after first being used for a gas turbine cycle. The study focuses on monitoring the outputs under parameters that are supposed to change frequently while the system is running in a near-real working environment. The primary variables that fluctuate are the temperature of syngas during gasification, the moisture content of biomass, the temperature of fluegas after combustion, and the concentration of ammonia in the cogeneration cycle. Under variable operating conditions, the maximum recorded output from the gas turbine cycle is 12699.49 kW, while that from cogeneration cycle is 1106.16 kW (= turbine output) and 833.73 kW (= cooling output). Finally, an experimental study is conducted on the gasification of rice straw, with the main objective of computing the molar formula of bio-char and bio-oil and use the same in the modelling process.