Exergetic analysis of an LPG evaporator/regulator with thermal storage

In this study, the efficiency of an LPG evaporator/regulator (E/R) is investigated on both energy and exergy concept. The E/R, which is a key part of LPG conversion systems that enables gasoline engines to be operated on LPG when desired, has been transformed to a thermal energy storage (TES) system using an adequate amount of phase change material (PCM) to be able to store waste energy of the engine coolant, so that the engine can be operated on LPG rather than gasoline even at cold start to decrease fuel consumption and exhaust emissions. The engine has been tested at idle speed at 4 °C environment temperature. The PCM application provided the engine to be operated on LPG at cold start and increased the efficiency of the E/R in a considerable extent on both energy and exergy bases. It was observed that using PCM in the E/R for thermal storage, the net efficiency differences of the E/R with PCM application reach to the values of 20% and 10%, respectively for the energy and exergy calculations. The net 2nd law efficiencies were lower than the ones that of 1st law with the values of about 11% for gasoline operation, and 8% for LPG operation of the engine.     

Exergetic assessment of EAHEs for building heating in Turkey: A greenhouse case study

The present study undertakes an exergy analysis of earth to air heat exchanger (EAHE) and applies to a local one in Turkey. Namely, the exergy performance of an EAHE has been evaluated in a demonstration in Solar Energy Institute of Ege University, Izmir, Turkey. Exergetic efficiencies of the system components are determined as an attempt to assess their individual performances. The daily maximum heating coefficient of performance (COP) value for the system is obtained to be 6.18. The total average COP in the experimental period is found to be 4.74.     

Exergetic assessment of solar hydrogen production methods

Hydrogen is a sustainable fuel option and one of the potential solutions for the current energy and environmental problems. Its eco-friendly production is really crucial for better environment and sustainable development. In this paper, various types of hydrogen production methods namely solar thermal (high temperature and low temperature), photovoltaic, photoelecrtolysis, biophotolysis etc are discussed. A brief study of various hydrogen production processes have been carried out. Various solar-based hydrogen production processes are assessed and compared for their merits and demerits in terms of exergy efficiency and sustainability factor. For a case study the exergy efficiency of hydrogen production process and the hydrogen system is discussed in terms of sustainability.     

Exergetic comparison of efficiency indicators for combined heat and power (CHP)

Legislative regulations in favor of combined heat and power (CHP) production have been implemented in many countries. Although these regulations put different emphasis on power production vs. process heat production, they are based on energy quantities and not on exergy. In order to analyze and compare the exergetic consequences of the various legislations, a relative avoided irreversibility (RAI) is defined. This can be regarded as the exergy loss that is avoided when reference plants with separate production are replaced by an actual CHP plant. Some series of industrial and district heating CHP plants, under varying operational conditions, are used as test cases. It is seen that some, but not all, CHP cases are exergetically beneficial to separate generation. Comparison with the RAI allows a quantitative assessment of the various performance indicators. It is seen that exergetic improvements were only captured to a limited degree by the various energy-based efficiency indicators. Some legislatively defined indicators even appear to discourage thermodynamic improvements.     

Exergetic life cycle assessment of hydrogen production from renewables

Life cycle assessment is extended to exergetic life cycle assessment and used to evaluate the exergy efficiency, economic effectiveness and environmental impact of producing hydrogen using wind and solar energy in place of fossil fuels. The product hydrogen is considered a fuel for fuel cell vehicles and a substitute for gasoline. Fossil fuel technologies for producing hydrogen from natural gas and gasoline from crude oil are contrasted with options using renewable energy.     

Retrofit of distillation columns in biodiesel production plants

Column grand composite curves and the exergy loss profiles produced by the Column-Targeting Tool of the Aspen Plus simulator are used to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a biodiesel production plant. Effectiveness of the retrofits is assessed by means of thermodynamics and economic improvements. We have considered a biodiesel plant utilizing three distillation columns to purify biodiesel (fatty acid methyl ester) and byproduct glycerol as well as reduce the waste. The assessments of the base case simulation have indicated the need for modifications for the distillation columns. For column T202, the retrofits consisting of a feed preheating and reflux ratio modification have reduced the total exergy loss by 47%, while T301 and T302 columns exergy losses decreased by 61% and 52%, respectively. After the retrofits, the overall exergy loss for the three columns has decreased from 7491.86 kW to 3627.97 kW. The retrofits required a fixed capital cost of approximately $239,900 and saved approximately $1,900,000/year worth of electricity. The retrofits have reduced the consumption of energy considerably, and leaded to a more environmentally friendly operation for the biodiesel plant considered.     

Heat pump assisted distillation. VII: A feasibility study on heat transformer assisted distillation systems

The problems in matching a heat driven heat transformer to a distillation process in heat pump assisted distillation are discussed. The performance of such a system is a function of the temperature difference across the distillation column. Plots are given of the calculated coefficients of performance against the temperatures of the top and bottom products. In addition plots are given of the calculated coefficients of performance against the generator temperature when the generator is independently heated. The percentage of energy saved is also plotted against coefficient of performance.     

Comments on derivation of an index for evaluating economics of cogeneration systems and its applications

Industrial cogeneration systems usually must satisfy a power load and heat loads at different temperatures. The limitations of the economic index proposed by Pak and Suzuki for such cogeneration systems is discussed in this paper. The importance of a rational exergetic basis for evaluation of different grades of energy is emphasised. Thermodynamic criteria, e.g. the exergetic efficiency, relative fuel savings and fuel chargeable to power, are shown to provide useful information regarding cogeneration options. Any assessment scheme for cogeneration schemes must incorporate thermodynamic criteria in addition to economic criteria.     

Heat pump assisted distillation. V: A feasibility study on absorption heat pump assisted distillation systems

The problems in matching a heat driven absorption heat pump to a distillation process in heat pump assisted distillation are discussed. The performance of an absorption system is a function of the temperatures in the evaporator, the condenser, the absorber and the generator and the ratio of the mass flow rate in the secondary circuit to the mass flow rate in the primary circuit. In absorption systems design choices are limited by the Gibbs phase rule. Plots are given of the coefficient of performance against the temperatures of the top and bottom products and also against the energy saved.     

A new and applicable method to calculate mass and heat transfer coefficients and efficiency of industrial distillation columns containing structured packings

Most of the methods developed for efficiency estimation of distillation columns were based on the empirical mass transfer and hydraulic relations correlated to laboratory data. Therefore, these methods cannot estimate efficiency of industrial columns with sufficient accuracy. In this paper, a new and applicable method was developed for calculation of efficiency (and mass and heat transfer coefficients) of distillation columns containing structured packings. This method has potential advantages; e.g., it can calculate efficiency without using any empirical mass transfer and hydraulic correlations and models, and without the need to estimate the operational and hydraulic parameters of column. Therefore, it will be free of errors, limitations, and complexities of such empirical items. In addition, precision of the method does not decrease with increasing complexity of operating conditions and design parameters of column. The method can be used for efficiency calculation of any structured packing, including new ones, in distillation columns.     

An integrated system for ammonia production from renewable hydrogen: A case study

This study presents an analysis and assessment study of an integrated system which consists of cryogenic air separation unit, polymer electrolyte membrane electrolyzer and reactor to produce ammonia for a selected case study application in Istanbul, Turkey. A thermodynamic analysis of the proposed system illustrates that electricity consumption of PEM electrolyzer is 3410 kW while 585.4 kW heat is released from ammonia reactor. The maximum energy and exergy efficiencies of the ammonia production system which are observed at daily average irradiance of 200 W/m² are found as 26.08% and 30.17%, respectively. The parametric works are utilized to find out the impacts of inlet air conditions and solar radiation intensity on system performance. An increase in the solar radiation intensity results in a decrease of the efficiencies due to higher potential of solar influx. Moreover, the mass flow rate of inlet air has a substantial effect on ammonia production concerning the variation of generated nitrogen. The system has a capacity of 0.22 kg/s ammonia production which is synthesized by 0.04 kg/s H₂ from PEM electrolyzer and 0.18 kg/s N₂ from a cryogenic air separation unit. The highest exergy destruction rate belongs to PEM electrolyzer as 736.2 kW while the lowest destruction rate is calculated as 3.4 kW for the separation column.     

Exergetic analysis of the refrigeration system in ethylene and propylene production process

The exergetic analysis is a tool that has been used successfully in many studies aiming a more rational energy use reducing the cost of the processes. With this analysis it is possible to perform an evaluation of the overall process, locating and quantifying the degradation of exergy. In this context, the present work aimed the exergetic analysis of the refrigeration cycles in ethylene and propylene production process, calculating the loss of exergy, in order to propose changes in the operational variables of the cycles used, trying to reduce the rate of destroyed exergy in the process. The commercial simulator Hysys^© (version 3.2) was used to obtain thermodynamic properties of the process streams and to perform mass and energy balances. The application of new operational conditions in these cycles resulted in a reduction of about 13% of the losses of exergy for the refrigeration system of the process.     

Analysis and assessment of a novel hydrogen liquefaction process

In the present study, a novel supercritical hydrogen liquefaction process based on helium cooled hydrogen liquefaction cycles to produce liquid hydrogen is thermodynamically analyzed and assessed. The exergy analysis approach is used to study the exergy destruction rates in each component and the process efficiency. The energy and exergy efficiencies of liquefaction process are found to be 70.12% and 57.13%, respectively. In addition, to investigate the process efficiency more comprehensively to see how it is affected by varying process parameters and operating conditions, some parametric studies are undertaken to examine the impacts of different design variables on the energy efficiency, exergy efficiency and exergy destruction rates of the hydrogen liquefaction process. The results show that the increases in the cycle pressure of hydrogen and helium result in increasing hydrogen liquefaction process exergy efficiency and providing a smaller pinch point temperature difference of catalyst beds related with the heat transfer surface area and more efficiently process.     

Exergetic analysis of various types of geothermal power plants

Based on available surveys, it has been shown that Iran has substantial geothermal potential in the north and north-western provinces, where in some places the temperature reaches 240 °C. In order to better exploit these renewable resources, it is necessary to study this area. Thus, the aim of this paper is a comparative study of the different geothermal power plant concepts, based on the exergy analysis for high-temperature geothermal resources. The considered cycles for this study are a binary geothermal power plant using a simple organic Rankine cycle (ORC), a binary geothermal power plant using an ORC with an internal heat exchanger (IHE), a binary cycle with a regenerative ORC, a binary cycle with a regenerative ORC with an IHE, a single-flash geothermal power plant, a double-flash geothermal power plant and a combined flash-binary power plant. With respect to each cycle, a thermodynamic model had to be developed. Model validation was undertaken using available data from the literature. Based on the exergy analysis, a comparative study was done to clarify the best cycle configuration. The performance of each cycle has been discussed in terms of the second-law efficiency, exergy destruction rate, and first-law efficiency. Comparisons between the different geothermal power plant concepts as well as many approaches to define efficiencies have been presented. The maximum first-law efficiency was found to be related to the ORC with an IHE with R123 as the working fluid and was calculated to be 7.65%. In contrast, the first-law efficiency based on the energy input into the ORC revealed that the binary cycle with the regenerative ORC with an IHE and R123 as the working fluid has the highest efficiency (15.35%). Also, the maximum first-law efficiency was shown to be given by the flash-binary with R123 as the working fluid and was calculated to be 11.81%.     

Exergetic evaluation of methods for improving power generation efficiency of a gas turbine cogeneration system

A cogeneration system generating both heat and power for district heating and cooling is required to be more efficient to improve its economy. In this paper, three typical methods for improving the power generation efficiency of a gas turbine cogeneration system are evaluated by examining exergy flow at various points of the system. The three methods investigated are: (a) to raise turbine inlet temperature, (b) to incorporate a regenerative cycle, and (c) to introduce a dual-fluid cycle. Exergy flows at various points of each cogeneration system have been evaluated. It has been shown through quantitve analyses of exergy flows (1) what kind of energy loss of the system can be reduced by introducing each efficiency-improving method, (2) that the method of incorporating a regenerative cycle is highly useful in improving exergy efficiency of the cogeneration system. © 1997 by John Wiley & Sons, Ltd.     

Exergetic evaluation of biomass gasification

Biomass has great potential as a clean, renewable feedstock for producing modern energy carriers. This paper focuses on the process of biomass gasification, where the synthesis gas may subsequently be used for the production of electricity, fuels and chemicals. The gasifier is one of the least-efficient unit operations in the whole biomass-to-energy technology chain and an analysis of the efficiency of the gasifier alone can substantially contribute to the efficiency improvement of this chain. The purpose of this paper is to compare different types of biofuels for their gasification efficiency and benchmark this against gasification of coal. In order to quantify the real value of the gasification process exergy-based efficiencies, defined as the ratio of chemical and physical exergy of the synthesis gas to chemical exergy of a biofuel, are proposed in this paper. Biofuels considered include various types of wood, vegetable oil, sludge, and manure. In this study, exergetic efficiencies are evaluated for an idealized gasifier in which chemical equilibrium is reached, ashes are not considered and heat losses are neglected. The gasification efficiencies are evaluated at the carbon-boundary point, where exactly enough air is added to avoid carbon formation and achieve complete gasification. The cold-gas efficiency of biofuels was found to be comparable to that of coal. It is shown that the exergy efficiencies of biofuels are lower than the corresponding energetic efficiencies. For liquid biofuels, such as sludge and manure, gasification at the optimum point is not possible, and exergy efficiency can be improved by drying the biomass using the enthalpy of synthesis gas.     

Exergy analysis of an experimental heat transformer for water purification

First and second law of thermodynamics have been used to analyze the performance of an experimental heat transformer used for water purification. The pure water is produced in the auxiliary condenser delivering an amount of heat, which is recycled into the heat transformer increasing the heat source temperatures and also the internal, external and exergy coefficients of performance. The theoretical and experimental study was divided into two parts. In the first part, a second law analysis was carried out to the experimental system showing that the absorber and the condenser are the components with the highest irreversibilities. In the second part, with the results obtained from the second law analysis, new test runs were carried out at similar conditions than the former but varying only one selected temperature at the time. Comparing the COP (coefficient of performance) between the old and new test runs, it was shown that higher internal, external and exergy coefficients of performance were obtained in all the new test runs. Also it was shown that the ECOP (exergy coefficient of performance) increases with an increment of the amount of the purified water produced and with the decrease of the flow ratio.     

Energy analysis and exergy utilization in the transportation sector of Jordan

The transport sector is responsible for about 37% of total final energy demand in Jordan, and thus it is considered an important driver for determining future national energy needs. This paper presents energy analysis and exergy utilization in the transportation sector of Jordan by considering the sectoral energy and exergy flows for the last two decades. The transportation sector, in Jordan, is a two-mode system, namely, road, which covers almost all domestic passenger and freight transport and airways. The latter is mainly used for international flights. The average estimated overall energy and exergy efficiencies were found as 23.2% and 22.8%, respectively. This simply indicates that there is large potential for improvement and efficiency enhancement. It is believed that the present technique is practical and useful for analyzing sectoral energy and exergy utilization to determine how efficiently energy and exergy are used in the transportation sector. It is also helpful to establish standards, based on exergy, to facilitate applications in different planning processes such as energy planning. A comparison with other countries showed that energy and exergy efficiencies of the Jordanian transport sector are slightly lower than that of Turkey, and higher than those incurred in Malaysia, Saudi Arabia and Norway. Such difference is inevitable due to dissimilar structure of the transport sector in these countries.     

Exergetic analysis of solid oxide fuel cell and biomass gasification integration with heat pipes

This paper presents an exergetic analysis of a combined heat and power (CHP) system, integrating a near-atmospheric solid oxide fuel cell (SOFC) with an allothermal biomass fluidised bed steam gasification process. The gasification heat requirement is supplied to the fluidised bed from the SOFC stack through high-temperature sodium heat pipes. The CHP system was modelled in AspenPlus™ software including sub-models for the gasification, SOFC, gas cleaning and heat pipes. For an average current density of 3000 A m⁻² the proposed system would consume 90 kg h⁻¹ biomass producing 170 kW_e net power with a system exergetic efficiency of 36%, out of which 34% are electrical.     

Exergetic analysis and optimization of a solar-powered reformed methanol fuel cell micro-powerplant

The present study proposes a combination of solar-powered components (two heaters, an evaporator, and a steam reformer) with a proton exchange membrane fuel cell to form a powerplant that converts methanol to electricity. The solar radiation heats up the mass flows of methanol-water mixture and air and sustains the endothermic methanol steam reformer at a sufficient reaction temperature (typically between 220 and 300 °C). In order to compare the different types of energy (thermal, chemical, and electrical), an exergetic analysis is applied to the entire system, considering only the useful part of energy that can be converted to work. The effect of the solar radiation intensity and of different operational and geometrical parameters like the total inlet flow rate of methanol-water mixture, the size of the fuel cell, and the cell voltage on the performance of the entire system is investigated. The total exergetic efficiency comparing the electrical power output with the exergy input in form of chemical and solar exergy reaches values of up to 35%, while the exergetic efficiency only accounting for the conversion of chemical fuel to electricity (and neglecting the ‘cost-free’ solar input) is increased up to 59%. At the same time, an electrical power density per irradiated area of more than 920 W m⁻² is obtained for a solar heat flux of 1000 W m⁻².