Research Paper4E analysis and multi-objective optimization of a CCHP cycle based on gas turbine and ejector refrigeration
Introduction
Nowadays, not only how much energy resources used to generate electricity is important, but also the way this production impacts the environment is crucial. For instance, greenhouse gases have been an important matter for humanity since the date it was considered as one of the contributors to global warming. Hence, it is necessary to seek a new source of energies which is clean, free and widely available for future. Today we get most of our energy from coal, oil and natural gas also known as fossil fuels which produce a huge amount of emission to the environment. One of the potential solutions to reduce emission is to use integrated energy systems. Multi-generation energy systems are an example of integrated energy systems where the topping cycle waste heat is being recovered to produce useful output such as heating, cooling, hot water, fresh water and even hydrogen. Multi-generation cycles can produce several useful outputs simultaneously. Among these outputs, heating, cooling, and electricity are the pervasive human being needs to sustain life. Combined cooling, heating and power (CCHP) is one of the well-known multi-generation energy systems. These systems have gained a growing deal of attention from researchers and manufacturers who have further considered them form an economic point of view in recent years, because of their importance in saving energy resources and reducing environmental pollutions. Many aspects of this technology are still under development [1], [2], [3], [4], [5].
Ahmadi et al. [6] investigated the exergo-environmental evaluation of a CCHP cycle equipped with a Rankine cycle (RC) together with a micro gas turbine (GT). The energy, exergy, environmental analysis, and parametric study were performed and the sustainability and environmental impacts assessment were investigated. In another work, Ahmadi et al. [7] applied an evolutionary optimization method to optimize a multi-generation energy system equipped with a gas turbine as the prime mover. Ameri et al. [8] evaluated the performance of a novel configuration of a micro gas turbine cogeneration with a heat recovery steam generator (HRSG) and a steam ejector refrigeration cycle (ERC). Also, Ahmadi et al. [9] provided a comprehensive multi-objective optimization based on an absorption chiller, a micro gas turbine, a domestic water heater (DWH), HRSG, an ERC, and an electrolyzer to produce several products namely hot water, cooling, power, heating, and hydrogen. Ebrahimi and Ahookhosh [1] combined an ORC cycle, a micro gas turbine, and an ERC as a multi-generation system. Parametric study and sensitivity analyses were performed to assess effects of design variables on cycle performance. Furthermore, genetic algorithm (GA) optimizes the objective function based on a combination of exergy and energy efficiencies. Alelyani et al. [10] investigated a novel combined cooling and power (CCHP) system based on utilization of heat rejected by the Rankine cycle via its condenser. The cycle consists of a Rankine, gas refrigeration (reverse Brayton), liquid desiccant, ejector, and evaporative cooling cycles. The results showed that the cycle is capable of generating 103 kW of electrical power, fresh water at 2.7 m3/h capacity, 181 kW of sensible cooling, and 1631 kW of latent cooling capacity at a fixed heat source input of about 2.4 MW at 210 °C.
Sadeghi et al. [11] studied an ERC via thermodynamic modeling, exergo-economic analysis, and optimization. For this purpose, the product unit cost of the system along with exergy efficiency were taken as the objective functions and a multi-objective optimization was carried out to find optimal values of design parameters (e.g. condenser, generator, and evaporator temperature). The optimization findings were as a set of optimum points based on which the Pareto front was obtained. Wang et al. [12] evaluated a new solar-based CCHP system and studied the performance of the system with variation of the slope angle and the hour angle of the aperture plane of the corresponding solar collectors. In later research, Wang et al. [13] parametrically analyzed a new CCHP system equipped with a solar-driven transcritical CO2 power and refrigeration cycle into which a transcritical CO2 refrigeration cycle and a Brayton cycle (BC) with ejector-expansion were incorporated. The system employed solar energy to lower the fossil fuels consumption and associated environmental problems. Soltani et al. [14] used GA for multi-objective optimization of a CGAM problem. Taking into account product cost and exergy efficiency as objective functions and major parameters as decision variables, the optimal point was indicated based on the Pareto front plot. Xia et al. [15] proposed a novel combined cooling and power system based on a Brayton Cycle (BC), an ORC and an ejector refrigeration cycle. The waste heat from the internal combustion engine was utilized to drive the system. They optimized their system by genetic algorithm from viewpoint of exergoeconomics with considered five key variables (compressor pressure ratio, compressor inlet temperature, BC turbine inlet temperature, ORC turbine inlet pressure and the ejector primary flow pressure). Boyaghchi et al. [16] presented a novel micro solar CCHP cycle with an integrated ORC under two scenarios: consumption in winter and summer. To have a continuous and stable operation, a thermal storage tank was set to balance the inconsistency between the demand for thermal energy by the CCHP subsystem and solar energy supply. Further, thermo-economic analyses of the cycle were performed. Yao et al. [17] presented a compressed air energy storage-based CCHP cycle to achieve an exergy efficiency of around 51%. In this cycle, compressed air storage was utilized to save surplus energy and discharging it during peak power consumption period. Anvari et al. [18] investigated a multi-generation cycle made up of an organic Rankine cycle, a HRSG, a gas turbine, and an absorption refrigeration cycle. Adding ORC and absorption refrigeration cycle to the gas turbine and HRSG led to 2.5% and 0.75% improvement in exergy efficiency of the whole cycle, respectively.
As mentioned above, many CCHP cycles are developed by using absorption refrigeration system. Recently, ejector refrigeration systems have been the focus of attention to produce cooling because of the capability of these systems to utilize low-grade heat sources and also the simplicity of ejector (i.e., no moving parts). Looking at previous studies, the lack of an investigation on an integrated GT, HRSG, RC and ERC as a tri-generation cycle is visible. The present research study tries to comprehensively simulate a CCHP cycle which consists of a Brayton cycle, a dual pressure HRSG, Rankine cycle, an ejector refrigeration cycle and a domestic water heater. In the proposed configuration, waste heat from gas turbine serves as a source of heat to produce heating, power, and cooling. Energy, exergy, economic and environmental impact assessment is applied and the results are presented. In addition, the genetic algorithm as a powerful optimization tool is used to find the best and optimal design variables where the system performance and environmental impacts are optimized.
Section snippets
System description
A combined CCHP system including a Brayton cycle (BC), a Rankine cycle (RC), a dual pressure HRSG, an ejector refrigeration cycle (ERC) and a domestic water heater (DWH) is illustrated in Fig. 1. Air enters the air compressor at point 1 and its pressure is increased to point 2. The compressed air then enters the regenerator and the leaving hot air is guided to the combustion chamber (CC) wherein fuel injection is performed. After that the fuel-air mixture is burnt, the hot flue gases exit the
Mathematical modeling and energy analysis
In order to simulate the CCHP system, thermodynamics laws were employed and the proposed plant was divided into four major parts namely, Bryton cycle, Rankine cycle, ejector refrigeration system and domestic water heater. Each of these subsections includes various components as are presented in following sections:
Exergy analysis
There are different approaches in thermal analyses of the systems, including exergy and energy analyses. However, exergy analysis represents the more comprehensive approach for thermal assessment of systems, because it can helpful to extend strategies for efficient use of energy and determine the magnitude and the locations where destructions occur. If we ignore the kinetic and potential exergy in the system (in fact they are negligible), exergy consists of two major parts: chemical and
Environmental impacts assessment
Assessment of thermal systems in terms of a single parameter (e.g. energy, etc.) is not adequate because the production of electrical energy from fossil fuels leads to CO2 emission which is a primary greenhouse gas. A comprehensive analysis of the thermal systems should involve an environmental assessment to improve the system in terms of CO, CO2 and NOX emission production efficiency. The efficiency of the combustion reaction affects the amounts of CO and NOX depending on different combustion
Economic analysis
Economic analysis is an inherent stage of any feasibility study to assess the applicability of a project with respect to financial consequences. The economic analysis involves the estimation of capital investment cost, total annual cost, capital recovery factor, operation and maintenance cost and some other economic variables, which provide information about the cost and benefits of total investments. The primary output of the economic analysis report is the estimation of the capital cost of
Results and discussion
In this research, a 4E analysis is presented to investigate the exergy, energy, economic aspects, and environmental impacts of the proposed CCHP cycle. In addition, influences of different design variables are investigated on the cycle performance. Hence, a simulation code developed by MATLAB software along with REFPROP 9.0 [36] was used to determine thermodynamic properties of the working fluids. Some assumptions were taken for this purpose. For instance, mole fractions of the inlet air were
Optimization
Since several design parameters have different effects on the system performance, a need for optimization is necessary. In this section, we try to apply an evolutionary algorithm for multi-objective optimization to determine the best design variables of the cycle considering cost and efficiency as two conflicting objective functions.
Conclusion
A CCHP cycle was proposed to recover exhaust gas waste heat from a Brayton cycle. The cycle was made up of a Brayton cycle, a Rankine cycle, a HRSG, an ejector refrigeration cycle and a heater for domestic water supply. Thermal, economic and environmental modeling were carried out. It was observed that combustion chamber acted as the highest cause of exergy loss. A comparison between CCHP and GT cycles revealed that the CCHP cycle provides higher exergy (7%) and energy efficiencies (12%) than
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