Exergy Principles Research Articles

A solar based system for integrated production of power, heat, hot water and cooling

Renewable energy sources have gained great importance in meeting the increasing energy demands in today's world and reducing the environmental impacts, such as greenhouse gas emissions, particularly caused by traditional fossil fuel sources. In this study, a unique integrated energy system driven by solar power is proposed. In order to achieve poly-generation, a concentrated solar power tower system is integrated with a Rankine cycle, an organic Rankine cycle, an absorption cooling system, greenhouses, and a water heating system. The proposed system is designed to generate power, hot water, cooling, and heating for residential buildings and greenhouses. The system analysis is performed in terms of thermodynamics using energy and exergy principles, and then both energy and exergy efficiencies are used to assess the system's overall performance. Through parametric studies, the effects of changing various operating parameters and conditions on performance are examined. The energy and the exergy efficiencies for the overall system are found to be 22.64% and 21.8%, respectively.

EXERGY-BASED SUSTAINABILITY ANALYSIS OF FOOD PRODUCTION SYSTEMS

As the global population continues to grow, the sustainability of food production systems is increasingly critical, coupled with escalating environmental concerns. Traditional sustainability assessments primarily focus on resource consumption and waste generation, often overlooking the overall efficiency and quality of energy and matter flows within these systems. This comprehensive review paper explores the potential of the exergy concept as a holistic and comprehensive approach to assessing the sustainability of food production systems. Exergy analysis offers valuable insights into the thermodynamic efficiency, resource utilization, and environmental impacts of these systems. By incorporating exergy principles into sustainability assessments, researchers and policymakers can better understand the strengths, weaknesses, and opportunities for improvement in food production systems. This paper highlights key studies and applications that have utilized the exergy concept, discussing its benefits and limitations.

A critical review of the thermo-hydraulic performance of vortex generators using the field synergy and exergy principles

A critical review of the thermo-hydraulic performance of vortex generators using the field synergy and exergy principles

Multi-objective optimization of preheating system of natural gas pressure reduction station with turbo-expander through the application of waste heat recovery system

• Proposing waste heat recovery configuration in natural gas-pressure reduction stations. • All of the components in the proposed and conventional preheating system are sized and designed. • Applying multi-criteria decision-making methods to find the optimal design. • A reduction in the total cost compared with conventional configuration is up to 98%. • A decease in exergy destruction compared with conventional configuration is 78%. The preheating system is regarded as an integral part of a natural-gas pressure reduction station (NGPRS) when a turbo-expander is substituted with regulators. In this paper, two configurations are compared for optimal designing preheating system used in NGPRS with the turbo-expander system. The first case uses water for a hot fluid that is warmed in the gas-fired heater (GFH), and the second is a Waste Heat Recovery (WHR) configuration. Multi-objective optimization and multi-criteria decision-making (MCDM) methods are implemented to identify the optimal design of mentioned preheating systems and to acquire the ability for the designer to select the optimal solution readily. The total cost and modified exergy efficiency (MEE) are selected as the objective functions. Moreover, sensitivity analyses of design parameters and their effects on objective functions are carried out for both configurations. Results show that the optimal designs suggested by MCDM methods in the WHR configuration provide a substantial reduction not only in total cost up to 98% but also in exergy destruction up to 78%. In addition, MEE increases by as much as 5 percent compared with the conventional configuration. Based on the aforementioned points, the WHR configuration is advantageous in both economic and exergy principles.

High performance buildings with optimized energy systems based on exergy principles

Optimising buildings and energy systems with a focus on mitigation of greenhouse gas emission is not an easy task: first, principles are necessary to be established and then the steps to be followed. Exergy efficiency can show the best issues: building envelope insulation and air tightening, low-valued energy sources, low-exergy heating and cooling systems, avoiding destructive exergy-processes like boilers burning fossil fuels, systems and grids for low-temperature heating and for high temperature cooling, renewable energy sources, daylight, and passive solar energy use supported by control strategies of building systems can contribute to a better future when considering global warming and comfort.

Application of CCHPs in a centralized domestic heating, cooling and power network—Thermodynamic and economic implications

The main objective of the present study is to utilize the waste heat and low-medium temperature geothermal heat sources in a centralized domestic heating, cooling and electricity network. Both geothermal and waste heat (from cement industry) are utilized to run domestic-scale combined cooling, heating and power (CCHP) units to meet the thermal and electrical demands of a residential area as the case study. Energy and exergy principles are applied to the designed CCHPs and the employed components, while exergoeconomic analysis is developed to show the economic feasibility of the proposed distributed energy systems. It is observed that the designed CCHPs not only meet the local energy demand in a sustainable way, but also deliver surplus thermal and electrical energy to the main grids. The thermodynamic analysis shows that under the base condition, the geothermal and waste heat driven CCHPs operate with sustainability index of 1.985 and 2.747, respectively. The economic evaluations demonstrated that, by using the proposed local CCHPs instead of the main grids to supply energy demand of the case study, it is possible to gain a benefit of 15687 € per year. This results in a lower energy payment of the end-users.

Real-life implementation of a linear model predictive control in a building energy system

Recent legislation on the reduction of carbon dioxide emission encourages the improvement of building energy systems’ efficiency as well as minimizing the usage of primary energy resources and damaging impacts on the environment. This fact has attracted the attention of energy system engineers to the importance of developing control strategies to use energy resources more efficiently. Moreover, the development of new energy efficient components and complex energy concepts in last decades has heightened the need to design advanced control strategies to reach the full potential of the recently-developed complex energy systems even more. The objectives of the advanced control strategies that have been developed to be energy efficient are obtained from energy analysis. However, an energy analysis is unable to provide information on the quality of energy streams flowing through a system. In this study, exergy, which is the maximum available work that can be extracted from a system during a process that brings the system into equilibrium with its environment, is considered as the objective of the optimization. The main objective of this research is to develop an exergy-based control strategy for building energy systems and investigate its applicability in real life. The control strategy is supposed to regulate a building energy system in a way that the most exergy efficient operation of the system in question is achieved. To reach this goal, a Model Predictive Control (MPC) for building energy systems using the exergy principles is developed. Model predictive control, which uses a system model to predict the future states of the system, offers a promising solution to the need for more advanced control strategies for complex building energy systems. In order to keep the calculation times as short as possible, the approach of Mixed Integer Linear Programming (MILP) is used in the development of model predictive controller. To demonstrate the functionality of the linear model predictive controller, it is implemented in a building energy system. The use case of this research is the energy supply system of the E.ON Energy Research Center's main building in Aachen, Germany. During an experiment, the exergy-based mixed integer linear MPC made meaningful decisions and demonstrated that it is able to control the forward flow temperature of the energy system with only minor deviations from the specified set point range.

Exergoeconomic analysis and optimization of a hybrid system based on multi-objective generation system in Iran: a case study

The present work aims to study a multi-objective generation system based on renewable solar and wind energy that has been selected according to the climatic conditions of the area under study, i.e. Bandar Abbas, Iran. In general, the system includes six different sections such as solar cycle, single effect absorption chiller, Rankine cycle with ammonia–water fluid, wind turbine, electrolyzer, and multi-stage flushing desalination. First, the system is investigated in terms of thermodynamic and exergy principles. Then, exergoeconomic analysis with specific exergy costing method (SPECO) is carried out based on the concepts of product, fuel, and cost balancing. The sensitivity analysis investigates the effect of decision parameters on energy and exergy efficiencies. Finally, single-objective optimization is analyzed using conjugate method and the multi-objective optimization is analyzed using genetic algorithm. Results show that, based on multi-objective optimization, isentropic efficiency of turbine is 81%, isentropic efficiency of pump is 77%, pressure ratio of Rankine cycle is 40.32, solar cycle mass flow is 0.8 k/s, outlet temperature of boiler is 113.62°C, exergy efficiency is 32.88%, and operating cost rate is 8.45 dollars per hour. Using multi-objective optimization, operating cost rate decreases by 22.6% and exergy efficiency increases by about 5.31%.

Thermodynamic analysis of combined cycle power plant using regasification cold energy from LNG terminal

Being increasingly used clean fuel, liquefied natural gas (LNG) release large amount of cold energy during its regasification process, which can be used for performance augment and energy saving towards regasification of LNG. In this paper a triple pressure Combined Cycle Power Plant (CCPP) with the application of the regasification cold energy is thermodynamically analyzed through energy and exergy principles. Three options were considered to investigate the effects of cold energy on thermodynamic performance of CCPP as a function of ambient temperature. The first option consists of utilization of cold energy for gas turbine (GT) inlet air cooling in the gas cycle (GC), the second option for cooling the condenser circulating water in the steam turbine cycle (STC) and the third option is a combined process of cold energy utilization in both GC and STC. The three presented options highlight the use of two heat exchangers one in GC and second in STC intended for utilization of LNG cold energy in CCPP. It is found that the gain in power output is significant from ambient air temperature 29 °C–45 °C (around 8.5% to 10.5%) and the gain in exergy efficiency is 0.09%–2.2% compared to the base case.

Thermodynamic assessment of zero-emission power, hydrogen and methanol production using captured CO2 from S-Graz oxy-fuel cycle and renewable hydrogen

Thermodynamic analysis of a novel combined system, including geothermal driven dual fluid organic Rankine cycle (ORC), S-Graz cycle, proton exchange membrane electrolyzer (PEME) and methanol synthesis unit (MSU) are carried out using energy and exergy principles. The presented system produces zero emission power and hydrogen using oxy-fuel combustion technology and splitting water into hydrogen and oxygen in the PEME, which consumes the dual fluid ORC produced power. Then, methanol production from captured CO2 and produced H2 is suggested to eliminate the captured CO2 from S-Graz cycle. The energy and exergy efficiencies and sustainability index of 14.7%, 42.43% and 1.737 are obtained for the presented cogeneration system, respectively. Also, results associated with the exergy destruction reveal that most of the destroyed exergy in the system is due to the dual fluid ORC in compare with the other units. Furthermore, it is observed that, within the dual fluid ORC, low pressure evaporator (LPE) is responsible for 33% of exergy destruction. Moreover, the results indicate that increasing higher pressure of the dual fluid ORC improves the system energetic and exergetic performance while an increment in geothermal hot water temperature leads to a reduction in system energy and exergy efficiencies.

Improving the efficiency of gas turbine systems with volumetric solar receivers

The combustion process of gas turbine systems is typically associated with the highest thermodynamic inefficiencies among the system components. A method to increase the efficiency of a combustor and, consequently that of the gas turbine, is to increase the temperature of the entering combustion air. This measure reduces the consumption of fuel and improves the environmental performance of the turbine. This paper studies the incorporation of a volumetric solar receiver into existing gas turbines in order to increase the temperature of the inlet combustion air to 800°C and 1000°C. For the first time, detailed thermodynamic analyses involving both energy and exergy principles of both small-scale and large-scale hybrid (solar-combined cycle) power plants including volumetric receivers are realized. The plants are based on real gas turbine systems, the base operational characteristics of which are derived and reported in detail. It is found that the indications obtained from the energy and exergy analyses differ. The addition of the solar plant achieves an increase in the exergetic efficiency when the conversion of solar radiation into thermal energy (i.e., solar plant efficiency) is not accounted for in the definition of the overall plant efficiency. On the other hand, it is seen that it does not have a significant effect on the energy efficiency. Nevertheless, when the solar efficiency is included in the definition of the overall efficiency of the plants, the addition of the solar receiver always leads to an efficiency reduction. It is found that the exergy efficiency of the combustion chamber depends on the varying air-to-fuel ratio and, in most cases, it is maximized somewhere between the applied inlet combustion air temperatures of 800°C and 1000°C.

Multi-objective design optimization of distributed energy systems through cost and exergy assessments

In recent years, distributed energy systems (DESs) have been recognized as a promising option for sustainable development of future energy systems, and their application has increased rapidly with supportive policies and financial incentives. With growing concerns on global warming and depletion of fossil fuels, design optimization of DESs through economic assessments for short-run benefits only is not sufficient, while application of exergy principles can improve the efficiency in energy resource use for long-run sustainability of energy supply. The innovation of this paper is to investigate exergy in DES design to attain rational use of energy resources including renewables by considering energy qualities of supply and demand. By using low-temperature sources for low-quality thermal demand, the waste of high-quality energy can be reduced, and the overall exergy efficiency can be increased. The goal of the design optimization problem is to determine types, numbers and sizes of energy devices in DESs to reduce the total annual cost and increase the overall exergy efficiency. Based on a pre-established DES superstructure with multiple energy devices such as combined heat and power and PV, a multi-objective linear problem is formulated. In modeling of energy devices, the novelty is that the entire available size ranges and the variation of their efficiencies, capital and operation and maintenance costs with sizes are considered. The operation of energy devices is modeled based on previous work on DES operation optimization. By minimizing a weighted sum of the total annual cost and primary exergy input, the problem is solved by branch-and-cut. Numerical results show that the Pareto frontier provides good balancing solutions for planners based on economic and sustainability priorities. The total annual cost and primary exergy input of DESs with optimized configurations are reduced by 21–36% as compared with conventional energy supply systems, where grid power is used for the electricity demand, and gas-fired boilers and electric chillers fed by grid power for thermal demand. A sensitivity analysis is also carried out to analyze the influence of energy prices and energy demand variation on the optimized DES configurations.

Exergy-based operation optimization of a distributed energy system through the energy-supply chain

Developing sustainable energy systems is crucial in today's world because of the depletion of fossil energy resources and global warming problems. Application of exergy principles in the context of energy supply systems may achieve efficient energy-supply chains and rational use of energy in buildings. This paper presents an exergy-based operation optimization of a distributed energy system by considering the whole energy-supply chain from energy resources to user demands. The problem is challenging in view of the complicated interactions of energy devices and the modeling of exergy losses. To capture these complicated interactions, energy networks are established with exergy losses modeled at the energy conversion step, which accounts for the largest part of the total exergy loss in the whole energy-supply chain. A multi-objective mixed integer programming problem is formulated. The problem is efficiently solved by the novel integration of surrogate Lagrangian relaxation and branch-and-cut. The Pareto frontier, including the best possible trade-offs between the economic and exergetic objectives, is obtained by minimizing a weighted sum of the total energy cost and total exergy loss occurring at the energy conversion step. Results demonstrate that the use of high-quality energy resources is reduced by the reduction of exergy losses, leading to sustainability of energy supply systems.

Operation optimization of a distributed energy system considering energy costs and exergy efficiency

With the growing demand of energy on a worldwide scale, improving the efficiency of energy resource use has become one of the key challenges. Application of exergy principles in the context of building energy supply systems can achieve rational use of energy resources by taking into account the different quality levels of energy resources as well as those of building demands. This paper is on the operation optimization of a Distributed Energy System (DES). The model involves multiple energy devices that convert a set of primary energy carriers with different energy quality levels to meet given time-varying user demands at different energy quality levels. By promoting the usage of low-temperature energy sources to satisfy low-quality thermal energy demands, the waste of high-quality energy resources can be reduced, thereby improving the overall exergy efficiency. To consider the economic factor as well, a multi-objective linear programming problem is formulated. The Pareto frontier, including the best possible trade-offs between the economic and exergetic objectives, is obtained by minimizing a weighted sum of the total energy cost and total primary exergy input using branch-and-cut. The operation strategies of the DES under different weights for the two objectives are discussed. The operators of DESs can choose the operation strategy from the Pareto frontier based on costs, essential in the short run, and sustainability, crucial in the long run. The contribution of each energy device in reducing energy costs and the total exergy input is also analyzed. In addition, results show that the energy cost can be much reduced and the overall exergy efficiency can be significantly improved by the optimized operation of the DES as compared with the conventional energy supply system using the grid power only.

THERMODYNAMIC ANALYSES OF MULTIPLE EVAPORATORS VAPOR COMPRESSION REFRIGERATION SYSTEMS WITH R410A, R290, R1234YF, R502, R404A, R152A AND R134A

In this paper, comparative thermodynamic analysis of system-1 (multiple evaporators and compressors with individual expansion valves) and system-2 (multiple evaporators and compressors with multiple expansion valves) has been presented which is based on energy and exergy principles. The comparison of systems-1 and -2 using ecofriendly R410A, R290, R1234YF, R502, R404A, R152A and R134A refrigerants was done in terms of COP (energetic efficiency), exergetic efficiency and system defect. Numerical model has been developed for systems-1 and -2 for finding out irreversibility and it was observed that system-2 is better system in comparison with system-1 for selected refrigerants. It was also found that R152a shows better performances than other considered refrigerants for both systems.

Energy Potential Mapping: Visualising Energy Characteristics for the Exergetic Optimisation of the Built Environment

It is difficult to fully satisfy the energy demand of today’s society with renewables. Nevertheless, most of the energy we use is lost as non-functional waste energy, whereas a large part of the built environment’s energy demand is only for low-quality energy, so the initial demand for primary, high-quality energy can be reduced by more effective usage, such as by low-exergy means. Gaining insight into the parameters of energy demands and local renewable and residual energy potentials enables matching energy demand with a fitting potential, not only concerning quantity but taking into account location, temporality and quality as well. The method of Energy Potential Mapping (EPM) aims to visualise the energy potentials and demands by making information of quantity, quality and location of demand and supply accessible. The aspect of quality specifically applies to heat and cold. The methodology of EPM will be described and explained with case studies. The focus specifically lies on mapping heat (and cold), one of the main reasons for energy demand in the built environment. The visualisation of exergy, to be simplified as the quality of energy, becomes an extra parameter in the case of Dutch Heat Maps. These maps can help finding opportunities of practical implementations of exchanging or cascading heat or cold. This way EPM and Heat Mapping (HM) enables application of exergy principles in the built environment. EPM and HM can be seen as a local energy catalogue and can be useful in spatial planning for energy-based urban and rural plans.

The exergy approach for evaluating and developing an energy system for a social dwelling

In this paper the energy and exergy performance of a social dwelling of a multi-family building from the 1960s in Bilbao (Spain) is presented and various improved energy concepts based on exergy principles are proposed and investigated. The aim of this paper is to explore and demonstrate the usefulness of the exergy approach in the assessment and development of an energy system for the dwelling under consideration. The total energy supply system is analysed, including the demand (space heating, domestic hot water and electricity), the system components (for conversion, storage and distribution) and the energy input from energy resources (primary energy and renewable resources). The study includes a comparison of the primary energy input of all cases considered and an analysis of the energy and exergy losses of each system component. The study has shown that the exergy analysis reveals thermodynamic losses that are not revealed using energy analysis and secondly, that the development of an improved energy system based on exergy principles has resulted in a significantly reduced primary energy input compared to the reference situation.

Exergy versus emergy flow in ecosystems: Is there an order in maximizations?

From ecosystems theory, orientors have been derived to give a holistic view of the trend of development of ecosystems themselves. Several principles of maximization, mainly derived from Lotka's maximum power, are connected with these orientors. Other authors (Jorgensen, Patten and coworkers, for example) have shown that among these orientors there are many common aspects, both form a quantitative terms and in principle. We show how H.T. Odum's maximum empower and Jorgensen's maximum exergy principles can be both valid from a practical viewpoint. As suggested by a simple experiment there can be a time order: first the maximization of empower and then the maximization of exergy. This time sequence is also consistent with a maximization trend in the ratio of exergy to empower.

Exergy Analysis: Principles and Practice

The importance of the goal of developing systems that effectively use nonrenewable energy resources such as oil, natural gas, and coal is apparent. The method of exergy analysis is well suited for furthering this goal, for it enables the location, type and true magnitude of waste and loss to be determined. Such information can be used to design new systems and to reduce the inefficiency of existing systems. This paper provides a brief survey of both exergy principles and the current literature of exergy analysis with emphasis on areas of application.