Exergy analysis of fluidized bed drying of moist particles

Energy and exergy analyses are conducted of the fluidized bed drying of moist materials for optimizing the operating conditions and the quality of the products. In this regard, energy and exergy models are developed to evaluate energy and exergy efficiencies, and are then verified with experimental data (for the product, wheat) taken from the literature. The effects of inlet air temperature, fluidization velocity, and initial moisture content on both energy and exergy efficiencies are studied. Furthermore, the hydrodynamic aspects, e.g., the bed hold up, are also studied. The results show that exergy efficiencies are less than energy efficiencies due to irreversibilities which are not taken into consideration in energy analysis, and that both energy and exergy efficiencies decrease with increasing drying time.     

First and second law analyses of laser cutting process in relation to the end product quality

Thermal analysis of laser cutting process is carried out, and the first and second law efficiencies of the cutting process are formulated. Thermal efficiencies are predicted for various laser scanning speeds and laser output power levels. The experiment is conducted to examine the resulting cutting sections. The F-test is conducted to assess the end product (cutting section) quality and, later, the thermal efficiencies are related to the end product quality. It is found that increasing laser output power lowers the first and second law efficiencies of cutting, which is more pronounced with reducing the laser scanning speeds. The end product quality improves for low laser output power levels and high laser scanning speeds similar to the thermal efficiencies. Copyright © 2007 John Wiley & Sons, Ltd.     

Understanding energy and exergy efficiencies for improved energy management in power plants

An extensive overview is provided of various energy- and exergy-based efficiencies used in the analysis of power cycles. Vapor and gas power cycles, cogeneration cycles and geothermal power cycles are examined, and consideration is given to different cycle designs. The many approaches that can be used to define efficiencies are provided and their implications discussed. Improvements of the management of energy in power plants that stem from understanding the efficiencies better are described. Examples are given to illustrate the efficiencies and their differences, with the results presented using combined energy and exergy diagrams. It is anticipated that the results will provide a convenient and practical tool for engineers and researchers dealing with the analysis, design, optimization and improvement of power cycles.     

Assessment of the energy and exergy utilization efficiencies in the Turkish agricultural sector

This study deals with evaluating the energy and exergy utilization efficiencies in the Turkish agricultural sector over a 12‐year period from 1990 to 2001. In the energy and exergy analyses, two main energy sources, namely fuels and electricity, are taken into consideration, while the sectoral energy and exergy efficiencies are compared for this period. These main energy sources include diesel for tractors and other vehicles, and electricity for pumps. Overall energy utilization efficiencies are obtained to vary between 29.1 and 41.1%, while overall exergy utilization efficiencies are found to range from 27.9 to 37.4% in the analysed years, respectively. It may be concluded that the present technique proposed here may be used as a useful tool in analysing and evaluating the energy and exergy utilization efficiencies, identifying energy efficiency and/or energy conservation opportunities and dictating the energy strategies of countries. Copyright © 2005 John Wiley & Sons, Ltd.     

Energy and exergy efficiencies in Turkish transportation sector, 1988–2004

This study aims at examining energy and exergy efficiencies in Turkish transportation sector. Unlike the previous studies, historical data is used to investigate the development of efficiencies of 17 years period from 1988 to 2004. The energy consumption values in tons-of-oil equivalent for eight transport modes of four transportation subsectors of the Turkish transportation sector, including hard coal, lignite, oil, and electricity for railways, oil for seaways and airways, and oil and natural gas for highways, are used. The weighted mean energy and exergy efficiencies are calculated for each mode of transport by multiplying weighting factors with efficiency values of that mode. They are then summed up to calculate the weighted mean overall efficiencies for a particular year. Although the energy and exergy efficiencies in Turkish transport sector are slightly improved from 1988 to 2004, the historical pattern is cyclic. The energy efficieny is found to range from 22.16% (2002) to 22.62% (1998 and 2004) with a mean of 22.42±0.14% and exergy efficiency to range from 22.39% (2002) to 22.85% (1998 and 2004) with a mean of 22.65±0.15%. Overall energy and exergy efficiencies of the transport sector consist mostly of energy and exergy efficiencies of the highways subsector in percentages varying from 81.5% in 2004 to 91.7% in 2002. The rest of them are consisted of other subsectors such as railways, seaways, and airways. The overall efficiency patterns are basically controlled by the fuel consumption in airways in spite of this subsector's consisting only a small fraction of total. The major reasons for this are that airways efficiencies and the rate of change in fuel consumption in airways are greater than those of the others. This study shows that airway transportation should be increased to improve the energy and exergy efficiencies of the Turkish transport sectors. However, it should also be noted that no innovations and other advances in transport technologies are included in the calculations. The future studies including such details will certainly help energy analysts and policy makers more than our study.     

Performance analysis of a PEM fuel cell unit in a solar–hydrogen system

In this paper, energy and exergy analyses for a 1.2 kWp Nexa PEM fuel cell unit in a solar-based hydrogen production system is undertaken to investigate the performance of the system for different operating conditions using experimental setup and thermodynamic model. From the model results, it is found that there are reductions in energy and exergy efficiencies (about 14%) with increase in current density. These are consistent with the experimental data for the same operating conditions. A parametric study on the system and its parameters is undertaken to investigate the changes in the efficiencies for variations in temperature, pressure and anode stoichiometry. The energy and exergy efficiencies increase with pressure by 23% and 15%, respectively. No noticeable changes are observed in energy and exergy efficiencies with increase in temperature. The energy and exergy efficiencies decrease with increase in anode stoichiometry by 17% and 14%, respectively. These observations are reported for the given range of current density as 0.047–0.4 A/cm². The results and analyses show that the PEM fuel-cell system has lower exergy efficiencies than the corresponding energy efficiencies due to the irreversibilities that are not considered by energy analysis. In comparison with experimental data, the model is accurate in predicting the performance of the proposed fuel-cell system. The parametric and multivariable analyses show that the option of selecting appropriate set of conditions plays a significant role in improving performance of existing fuel-cell systems.     

Thermal performance of the distributed flow, subatmospheric pressure, flat plate solar collector

Experimental thermal efficiencies for the distributed flow, subatmospheric pressure, flat plate solar collector are reported for a wide range of environmental and operational conditions, and for corrugated Filon and glass covers. Efficiencies for near normal solar incidence are correlated with a parameter formed from the difference between inlet fluid and air temperatures divided by incident solar energy. A mathematical model based on heat transfer concepts applicable to the collector yielded efficiencies which are in close agreement with the corresponding experimental efficiencies when the experimental measurements are inserted into the model.     

Energy analysis applied to food processing

We use first-law and second-law efficiencies on 2 multi-process food processing plants. These efficiencies are 59.6 and 19.4% for a condensed milk plant and 43.0 and 15.5% for the bakery plant. Improvements in both first-law and second-law efficiencies for the bakery can be achieved by reducing the mass of dough moulds and recovery of energy in the bake-oven exhaust to heat the air in the proofing oven. Replacement of the flash-cooling process in the condensed-milk plant by a combination of boiling at 100 °C and cooling via a vapour-compression refrigerator would lead to significant improvements in plant efficiencies.     

Assessment of the Turkish utility sector through energy and exergy analyses

The present study deals with evaluating the utility sector in terms of energetic and exergetic aspects. In this regard, energy and exergy utilization efficiencies in the Turkish utility sector over a wide range of period from 1990 to 2004 are assessed in this study. Energy and exergy analyses are performed for eight power plant modes, while they are based on the actual data over the period studied. Sectoral energy and exergy analyses are conducted to study the variations of energy and exergy efficiencies for each power plants throughout the years, and overall energy and exergy efficiencies are compared for these power plants. The energy utilization efficiencies for the overall Turkish utility sector range from 32.64% to 45.69%, while the exergy utilization efficiencies vary from 32.20% to 46.81% in the analyzed years. Exergetic improvement potential for this sector are also determined to be 332 PJ in 2004. It may be concluded that the methodology used in this study is practical and useful for analyzing sectoral and subsectoral energy and exergy utilization to determine how efficient energy and exergy are used in the sector studied. It is also expected that the results of this study will be helpful in developing highly applicable and productive planning for energy policies.     

Energy and exergy efficiencies in the Chinese transportation sector, 1980-2009

This paper aims at analyzing energy and exergy efficiencies in the Chinese transportation sector. Historical data is used to investigate the development of efficiencies from 1980 to 2009. Firstly, we calculate energy consumption values in PJ (petajoule) for nine transportation modes of five transportation sub-sectors. Then, the weighted energy and exergy efficiencies for each transportation mode, calculated by multiplying weighting factors with efficiency values of that mode, are summed up to calculate the weighted mean overall efficiencies for a particular year. We find that: (1) In 2009, the energy consumed in transportation sector was 12179.80 PJ, whereas that was 589.25 PJ in 1980. (2) Highways transport was the biggest energy consumer, which consumed 82.0% of total transport energy consumption in 2009. (3) Up to 2009, the oil consumed by transportation accounted for 75.1% of that in the whole country, which is more than the net oil import. (4) The average overall energy and exergy efficiencies are found to be 21.22% and 19.95%, respectively. (5) A comparison with other countries showed that energy and exergy efficiencies of the Chinese transportation sector are slightly lower than those of Jordan, Malaysian, Saudi Arabian and Norwegian, and higher than that incurred in Turkish.     

Performance and parametric investigation of a binary geothermal power plant by exergy

Exergy analysis of a binary geothermal power plant is performed using actual plant data to assess the plant performance and pinpoint sites of primary exergy destruction. Exergy destruction throughout the plant is quantified and illustrated using an exergy diagram, and compared to the energy diagram. The sites with greater exergy destructions include brine reinjection, heat exchanger and condenser losses. Exergetic efficiencies of major plant components are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the plant are 4.5% and 21.7%, respectively, based on the energy and exergy of geothermal water at the heat exchanger inlet. The energy and exergy efficiencies are 10.2% and 33.5%, respectively, based on the heat input and exergy input to the binary Rankine cycle. The effects of turbine inlet pressure and temperature and the condenser pressure on the exergy and energy efficiencies, the net power output and the brine reinjection temperature are investigated and the trends are explained.     

基于压缩空气储能的CCHP系统特性研究

以先进绝热压缩空气储能（AA-CAES）为基础,构建冷热电联产（CCHP）系统,对比4种不同储气室和运行方式方案下的系统特性,并针对关键参数进行敏感性分析。结果表明,采用恒温储气室且滑压运行时系统储能效率和效率最高;采用恒温储气室且恒压运行时系统能量密度最高。第二级换热器损最大,是提高系统性能时的首要优化目标。当换热器效能提高时,储能效率、效率均出现折点。储气室最大压比越大,系统储能效率和效率越低,能量密度越高。采用恒温储气室时,系统不受压缩/膨胀影响;采用恒壁温储气室时,较高的压缩/膨胀功率有利于提高储能效率和效率,但压缩功率升高会降低能量密度。     

A new generation of crystalline silicon solar cells: Simple processing and record efficiencies for industrial-size devices

In contrast to the general opinion that very high efficiencies can only be obtained using complex processing, with the novel technologically simple and environmentally sound obliquely evaporated contact (OECO) type solar cell efficiencies exceeding 21% could be obtained without applying masks or photolithography. Two different approaches of OECO cells using MIS contacts and exclusively Al as metallization are discussed: (i) with a diffused n⁺-emitter (MIS-n⁺p) and (ii) with an inversion layer emitter (MIS-IL). The most important results particularly for industrial production are efficiencies of 19% and 20% for simply to fabricate 10×10 cm² OECO cells on commercial CZ-Si and FZ-Si, respectively. These are the highest efficiencies ever reported for solar cells of industrial size.     

Hydrogen production through biomass gasification in supercritical water: A review from exergy aspect

Hydrogen production through supercritical water gasification (SWG) of biomass has been widely studied. This study reviews the main factors from exergy aspect, and these include feedstock characteristics, biomass concentration, gasification temperature, residence time, reaction catalyst, and reactor pressure. The results show that the exergy efficiencies of hydrogen production are mainly in the range of 0.04–42.05%. Biomass feedstock may affect hydrogen production by changing the H₂ yield and the heating value of biomass. Increases in biomass concentrations decrease the exergy efficiencies, increases in gasification temperatures generally increase the exergy efficiencies, and increases in residence times may initially increase and finally decrease the exergy efficiencies. Reaction catalysts also have positive effects on the exergy efficiencies, and the reviewed results show that the effects are followed KOH > K₂CO₃ > NaOH > Na₂CO₃. Reactor pressure may have positive, negative or negligible effects on the exergy efficiencies.     

Ideal thermal efficiency for geothermal binary plants

The Carnot cycle is reviewed as to its appropriateness to serve as the ideal model for geothermal binary power plants. It is shown that the Carnot cycle sets an unrealistically high upper limit on the thermal efficiency of these plants. A more useful model is the triangular (or trilateral) cycle because binary plants operating on geothermal hot water use a non-isothermal heat source. The triangular cycle imposes a lower upper bound on the thermal efficiency and serves as a more meaningful ideal cycle against which to measure the performance of real binary cycles. Carnot and triangular cycle efficiencies are contrasted and the thermal efficiencies of several actual binary cycles are weighed against those of the ideal triangular cycle to determine their relative efficiencies. It is found that actual binary plants can achieve relative efficiencies as high as 85%. The paper briefly discusses cycles using two-phase expanders that in principle come close to the ideal triangular cycle.     

Comparative efficiency assessments for a range of hydrogen production processes

Efficiencies, based on energy and exergy, are comparatively assessed for a wide range of hydrogen production processes, including processes which are

• •• hydrocarbon-based (steam-methane reforming and coal gasification),
• •• non-hydrocarbon-based (water electrolysis and thermochemical water decomposition), and
• •• integrated (steam-methane reforming linked to the non-hydrocarbon-based processes).

A process simulation and analysis computer code is used throughout. Overall efficiencies, based on primary resource inputs, are determined to range widely, from 21% to 86% for energy efficiencies, and from 19% to 78% for exergy efficiencies. Reductions in efficiencies from 100% are found to be attributable only to emissions for energy analysis, and mainly to internal consumptions for exergy analysis. Exergy losses associated with emissions account for a small portion of the total exergy losses.     

First and second law analyses of three brickworks

In this paper, efficiencies based on the first and second laws of thermodynamics are formulated to evaluate the thermal performance of brick manufacturing plants. The results are applied to experimental and production data from three brickworks, having daily production volumes ranging from 50,000 to 200,000 bricks. The analyses show that the larger of the brickworks has higher first and second law efficiencies.     

Orientational relationships for optically non-symmetric solar collectors

More details of collector orientations and incidence angle modifiers are required to specify the instantaneous optical efficiencies of optically non-symmetric solar collectors, such as those using nonimaging concentrators, than to specify the efficiencies of symmetric collectors such as flat plates. In this paper, expressions are presented for the projected incidence angles necessary for evaluating the instantaneous optical efficiencies of non-symmetric collectors. These projected angles are also used when a nearly biaxial incidence angle modifier is approximated by the product of two single-axis incidence angle modifiers. In addition, relationships for incidence angles onto an absorber tube are developed. These differ from the angle onto a cover glazing, and they effect the absorptance of the tube's coating and the transmittance of a cover tube as used in tubular evacuated collectors.     

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.     

First and second law analyses of a typical solar air dryer system: a case study

This communication presents an experimental study based on energy and exergy analyses of a typical solar air dryer. On the basis of the experimental results, the first and the second law efficiencies are calculated with respect to the available solar radiation. It is found that the second law efficiency is much less than the first law efficiency irrespective of the mass flow rate of the circulating fluid. It is also found that both efficiencies are an increasing function with respect to the mass flow rate of the working fluid. For a particular day, it is also found that first and second law efficiencies fluctuate according to radiation. The results obtained in this article are found to be consistent with those obtained by earlier authors as given in the literature.