Energy and exergy utilizations of the U.S. manufacturing sector

The energy and exergy utilizations in the U.S. manufacturing sector are analyzed by considering the energy and exergy flows for the year 2002. Detailed end-use models for fourteen intensive industries are established using scattered data from the Manufacturing Energy Consumption Survey (MECS). Since the MECS data exhibit many gaps, data from other sources are used, as well as a number of assumptions are made to complete the models. The methodology applied and the assumptions made are clearly described so that the methods can be readily modified to fit different needs. The end-use models provide a starting point to estimate the site and embodied energy and exergy efficiencies. The average site energy and exergy efficiencies of the manufacturing sector are estimated as 63.5% and 38.8% respectively, while the embodied energy and exergy efficiencies are estimated as 52.7% and 32.1% respectively. The low efficiency values suggest that many opportunities for better industrial energy utilization still exist.     

An estimation of the energy and exergy efficiencies for the energy resources consumption in the transportation sector in Malaysia

The purpose of this work is to apply the useful energy and exergy analysis models for different modes of transport in Malaysia and to compare the result with a few countries. In this paper, energy and exergy efficiencies of the various sub-sectors are presented by considering the energy and exergy flows from 1995 to 2003. Respective flow diagrams to find the overall energy and exergy efficiencies of Malaysian transportation sector are also presented. The estimated overall energy efficiency ranges from 22.74% (1999) to 22.98% (1998) with a mean of 22.82±0.06%$22.82\pm 0.06\%$ and that of overall exergy efficiency ranges from 22.44% (2000) to 22.82% (1998) with a mean of 22.55±0.12%$22.55\pm 0.12\%$. The results are compared with respect to present energy and exergy efficiencies in each sub-sector. The transportation sector used about 40% of the total energy consumed in 2002. Therefore, it is important to identify the energy and exergy flows and the pertinent losses. The road sub-sector has appeared to be the most efficient one compared to the air and marine sub-sectors. Also found that the energy and exergy efficiencies of Malaysian transportation sector are lower than that of Turkey but higher than Norway.     

Society exergy analysis: a comparison of different societies

Data from exergy analyses for a number of different countries found in the literature are compared and the differences are discussed. In studies of Sweden, Ghana, Japan, Italy and Norway, the exergy in material flows had been considered, in addition to the flows of energy carriers. In other studies, the use of energy carriers was analyzed for the USA, Finland, Canada, Brazil, Turkey, the Organization for Economic Co-operation and Development (OECD) countries and the World. The exergy of material flows in these societies was estimated. The total annual exergy input per capita to the societies ranged over one order of magnitude. The total exergy efficiency varied from approximately 0.1 to 0.3, whereas the end-use exergy efficiency in general was slightly higher. It was found that different investigators had made somewhat different assumptions on exergy efficiencies in specific sectors, and these assumptions are discussed. However, the structure of the energy system appeared to be more important for the total exergy efficiency than the assumptions on the sectors. In particular, the residential–commercial sector represents major irreversibilities in many societies. In countries where electricity from thermal power plants has a significant contribution to the end use, this also caused large irreversibilities. Finally, the method of society exergy analysis is discussed. It is pointed out that, because of structural dissimilarities, different countries should be compared with care. However, the development within each society can be evaluated using exergy analyses. Furthermore, such analyses can be used as a means to increase the awareness of the notion of energy quality and degradation.     

Exergy analysis of hydrogen production from steam gasification of biomass: A review

Exergy analysis of hydrogen production from steam gasification of biomass was reviewed in this study. The effects of the main parameters (biomass characteristics, particle size, gasification temperature, steam/biomass ratio, steam flow rate, reaction catalyst, and residence time) on the exergy efficiency were presented and discussed. The results show that the exergy efficiency of hydrogen production from steam gasification of biomass is mainly determined by the H₂ yield and the chemical exergy of biomass. Increases in gasification temperatures improve the exergy efficiency whereas increases in particle sizes generally decrease the exergy efficiency. Generally, both steam/biomass ratio and steam flow rate initially increases and finally decreases the exergy efficiency. A reaction catalyst may have positive, negative or negligible effect on the exergy efficiency, whereas residence time generally has slight effect on the exergy efficiency.     

Efficiency assessment of an integrated gasifier/boiler system for hydrogen production with different biomass types

In this study, we utilize some experimental data taken from the literature, especially on the air-blown gasification characteristics of six different biomass fuels, namely almond shell (ASF), walnut pruning (WPF), rice straw (RSF), whole tree wood chips (WWF), sludge (SLF) and non-recyclable waste paper (NPF) in order to study the thermodynamic performance of an integrated gasifier–boiler power system for its hydrogen production. In this regard, both energy and exergy efficiencies of the system are investigated. The exergy contents of different biomass fuels are calculated to be ranging from 15.89 to 22.07 MJ/kg, respectively. The hydrogen concentrations based on the stack gases at the cyclone exit are determined to be between 7 and 18 (%v/v) for NPF and ASF. Also, percentages of combustible vary from 30% to 46%. The stack gas has physical and chemical exergies. The total specific exergy rates are calculated and illustrated. These values change from 3.54 to 6.41 MJ/kg. Then, two types of exergy efficiencies are calculated, such as that exergy efficiency 1 is examined via all system powers, exergy and efficiency 2 is calculated according to specific exergy rates of biomass fuels and product gases. While the exergy efficiencies 1 change between 4.33% and 11.89%, exergy efficiencies 2 vary from 18.33% to 39.64%. Also, irreversibilities range from 9.76 to 18.02 MJ/kg. Finally, we investigate how nitrogen contents of biomass fuels affect on energy and exergy efficiencies. The SLF has the highest amount of nitrogen content as 5.64% db while the NPF has the lowest one as 0.14% db. The minimum and maximum exergetic efficiencies belong to the same fuels. Obviously, the higher the nitrogen content the lower the efficiency based on an inverse ratio between exergy efficiency and nitrogen content.     

More efficient biomass gasification via torrefaction

Wood torrefaction is a mild pyrolysis process that improves the fuel properties of wood. At temperatures between 230 and 300 °C, the hemicellulose fraction of the wood decomposes, so that torrefied wood and volatiles are formed. Mass and energy balances for torrefaction experiments at 250 and 300 °C are presented. Advantages of torrefaction as a pre-treatment prior to gasification are demonstrated. Three concepts are compared: air-blown gasification of wood, air-blown gasification of torrefied wood (both at a temperature of 950 °C in a circulating fluidized bed) and oxygen-blown gasification of torrefied wood (at a temperature of 1200 °C in an entrained flow gasifier), all at atmospheric pressure. The overall exergetic efficiency of air-blown gasification of torrefied wood was found to be lower than that of wood, because the volatiles produced in the torrefaction step are not utilized. For the entrained flow gasifier, the volatiles can be introduced into the hot product gas stream as a ‘chemical quench’. The overall efficiency of such a process scheme is comparable to direct gasification of wood, but more exergy is conserved in as chemical exergy in the product gas (72.6% versus 68.6%). This novel method to improve the efficiency of biomass gasification is promising; therefore, practical demonstration is recommended.     

A review on biomass‐based hydrogen production and potential applications

In this paper, a detailed review is presented to discuss biomass‐based hydrogen production systems and their applications. Some optimum hydrogen production and operating conditions are studied through a comprehensive sensitivity analysis on the hydrogen yield from steam biomass gasification. In addition, a hybrid system, which combines a biomass‐based hydrogen production system and a solid oxide fuel cell unit is considered for performance assessment. A comparative thermodynamic study also is undertaken to investigate various operational aspects through energy and exergy efficiencies. The results of this study show that there are various key parameters affecting the hydrogen production process and system performance. They also indicate that it is possible to increase the hydrogen yield from 70 to 107 g H₂ per kg of sawdust wood. By studying the energy and exergy efficiencies, the performance assessment shows the potential to produce hydrogen from steam biomass gasification. The study further reveals a strong potential of this system as it utilizes steam biomass gasification for hydrogen production. To evaluate the system performance, the efficiencies are calculated at particular pressures, temperatures, current densities, and fuel utilization factors. It is found that there is a strong potential in the gasification temperature range 1023–1423 K to increase energy efficiency with a hydrogen yield from 45 to 55% and the exergy efficiency with hydrogen yield from 22 to 32%, respectively, whereas the exergy efficiency of electricity production decreases from 56 to 49.4%. Hydrogen production by steam sawdust gasification appears to be an ultimate option for hydrogen production based on the parametric studies and performance assessments that were carried out through energy and exergy efficiencies. Finally, the system integration is an attractive option for better performance. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermodynamic methodology to support the selection of feedstocks for decentralised downdraft gasification power plants

Thermodynamic criteria as a feedstock selection tool for decentralised downdraft gasifiers coupled to spark-ignition engines are presented in this work. The methodology consists of an energy and exergy analysis of gasification process. The analysis is carried out by computational modelling of the gasification process as a function of biomass type (ultimate analysis, moisture content and heating value) and fuel/air ratio. Considering a system operating with different wood species, analysed parameters are gas heating value, energy and exergy efficiencies and engine fuel quality (EFQ). With a fixed fuel/air ratio (2.6) and moisture content (20%wt), it is highlighted that as the carbon-oxygen molar ratio of wood decreases from 2.0 to 1.78 as model input, reaction temperature increases by 9%, energy and exergy efficiencies diminish by 1.8% and 4.2%, respectively, while EFQ increases by 3.2%. Therefore, for decentralised power plants, biomass should be selected to produce higher EFQ.     

A comparative investigation of various greenhouse heating options using exergy analysis method

This study deals with modeling and analyzing the performance of greenhouses from the power plant through the heating system to the greenhouse envelope using exergy analysis method, the so-called low exergy or LowEx approach, which has been and still being successfully used in sustainable buildings design, for the first time to the best of the author’s knowledge. For the heating applications, three options are studied with (i) a solar assisted vertical ground-source heat pump greenhouse heating system, (ii) a wood biomass boiler, and (iii) a natural gas boiler, which are driven by renewable and non-renewable energy sources. In this regard, two various greenhouses, the so-called small greenhouse and large greenhouse, considered have heat load rates of 4.15 kW and 7.5 MW with net floor areas of 11.5 m² and 7.5 ha, respectively. The overall exergy efficiency values for Cases 1–3 (solar assisted vertical ground-source heat pump, natural gas boiler and wood biomass boiler) of the small greenhouse system decrease from 3.33% to 0.83%, 11.5% to 2.90% and 3.15% to 0.79% at varying reference state temperatures of 0 to 15 °C while those for Cases 1 and 2 (wood biomass and natural gas boilers) of the large greenhouse system decrease from 2.74% to 0.11% and 4.75% to 0.18% at varying reference state temperatures of −10% to 15 °C. The energetic renewability ratio values for Cases 1 and 3 of the small greenhouse as well as Case 1 of the large greenhouse are obtained to be 0.28, 0.69 and 0.39, while the corresponding exergetic renewability ratio values are found to be 0.02, 0.64 and 0.29, respectively.     

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.     

Experimental investigation of energy and exergy efficiency of masonry-type solar cooker for animal feed

The performance of a masonry animal feed solar cooker was evaluated in terms of energy and exergy. It is a low-cost cooker made of cement, bricks, glass covers and a mild steel absorber plate. The energy and exergy efficiencies of the animal feed solar cooker were experimentally evaluated. The energy output of this cooker ranges from 1.89 to 49.4 kJ, whereas the exergy output ranges from 0.11 to 2.72 kJ during the same time interval. The energy efficiency of the cooker varies between 1.12% and 29.78%, while the exergy efficiency varies between 0.07% and 1.52 % during the same period.     

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.     

Exergy analysis of industrial pasta drying process

In this study we present an energy and exergy modelling of industrial final macaroni (pasta) drying process for its system analysis, performance evaluation and optimization. Using actual system data, a performance assessment of the industrial macaroni drying process through energy and exergy efficiencies and system exergy destructions is conducted. The heat losses to the surroundings and exergy destructions in the overall system are quantified and illustrated using energy and exergy flow diagrams. The total energy rate input to system is 316.25 kW. The evaporation rate is 72 kg h^?1 (0.02 kg s^?1) and energy consumption rate is found as 4.38 kW for 1 kg water evaporation from product. Humidity product rate is 792 kg h^?1 (0.22 kg s^?1) and energy consumption rate is found about 0.4 kW for 1 kg short cut pasta product. The energy efficiencies of the pasta drying process and the overall system are found to be as 7.55–77.09% and 68.63%. The exergy efficiency of pasta drying process is obtained to be as 72.98–82.15%. For the actual system that is presented the system exergy efficiency vary between 41.90 and 70.94%. Copyright © 2006 John Wiley & Sons, Ltd.     

Exergetic analysis of a desiccant cooling system: searching for performance improvement opportunities

The current increase of the energy consumption of buildings requires new approaches to solve economic, environmental and regulatory issues. Exergy methods are thermodynamic tools searching for sources of inefficiencies in energy conversion systems that the current energy techniques may not identify. Desiccant cooling systems (DCS) are equipments applied to dehumidifying and cooling air streams, which may provide reductions of primary energy demand relatively to conventional air‐conditioning units. In this study, a detailed thermodynamic analysis of open‐cycle DCS is presented. It aims to assess the overall energy and exergy performance of the plant and identify its most inefficient sub‐components, associated to higher sources of irreversibilities. The main limitations of the energy methods are highlighted, and the opportunities given by exergy approach for improving the system performance are properly identified. As case study, using a pre‐calibrated TRNSYS model, the overall energy and exergy efficiency of the plant were found as 32.2% and 11.8%, respectively, for a summer week in Mediterranean climate. The exergy efficiency defect identified the boiler (69.0%) and the chiller (12.3%) as the most inefficient components of the plant, so their replacement by high efficient systems is the most rational approach for improving its performance. As alternative heating system to the boiler, a set of different technologies and integration of renewables were proposed and evaluated applying the indicators: primary energy ratio (PER) and exergy efficiency. The heating system fuelled by wood was found as having the best primary energy performance (PER = 109.6%), although the related exergy efficiency is only 11.4%. The highest exergy performance option corresponds to heat pump technology with coefficient of performance (COP) = 4, having a PER of 50.6% and exergy efficiency of 28.2%. Additionally, the parametric analyses conducted for different operating conditions indicate that the overall irreversibility rate increases moderately for larger cooling effects and more significant for higher dehumidification rates. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermodynamic analysis of a multigeneration system using solid oxide cells for renewable power-to-X conversion

A hybrid renewable-based integrated energy system for power-to-X conversion is designed and analyzed. The system produces several valuable commodities: Hydrogen, electricity, heat, ammonia, urea, and synthetic natural gas (SNG). Hydrogen is produced and stored for power generation from solar energy by utilizing solid oxide electrolyzers and fuel cells. Ammonia, urea, and synthetic natural gas are produced to mitigate hydrogen transportation and storage complexities and act as energy carriers or valuable chemical products. The system is analyzed from a thermodynamic perspective, the exergy destruction rates are compared, and the effects of different parameters are evaluated. The overall system's energy efficiency is 56%, while the exergy efficiency is 14%. The highest exergy destruction occurs in the Rankine cycle with 48 MW. The mass flow rates of the produced chemicals are 0.064, 0.088, and 0.048 kg/s for ammonia, urea, and SNG, respectively.     

Application of exergy balances for evaluation of process configurations for biological hydrogen production

Exergy analysis was applied to a novel process for biological production of hydrogen from biomass employing thermophilic and photo-heterotrophic bacteria. The exergy content of the process streams is calculated using a MS-Excel spreadsheet.The scrutinized process incurs an exergy loss of 7–9% of the total exergy input. The efficiency based on chemical exergy of biomass feed and produced pure hydrogen refers to 36–45% depending on the configuration of the overall process. The results presented in the paper underline the strong dependence of obtained exergetic efficiency from definition of products and shows options for process improvement and optimization.     

Energy and exergy utilization efficiencies in the Japanese residential/commercial sectors

Unlike the manufacturing sector, the residential/commercial sectors of Japan struggle to meet their environmental requirements. For instance, their CO₂ emission levels have increased tremendously since 1990. This research estimates energy and ‘exergy (available energy)’ efficiencies in Japan's residential/commercial sectors during the period 1990–2006. Since an exergy analysis reveals ‘available energy losses’, it is an effective tool to achieve sustainable societies. The primary objective of this paper is to examine the potential for advancing the ‘true’ energy efficiency in Japan's residential/commercial sectors—by observing energy and exergy efficiency disparities. The results show large differences between the overall energy and exergy efficiencies in the residential (60.12%, 6.33%)/commercial sectors (51.78%, 5.74%) in 2006. This implies great potential for energy savings in both sectors. Furthermore, this research suggests that the residential sector may face more difficulties than the commercial sector, although the latter appears to be less energy-efficient, according to recent statistics. This is because the disparity between energy and exergy efficiencies has expanded in the residential sector since 2000. This study illustrates the importance of exergy analyses in promoting sustainable energy policies and new adaptation strategies.     

Design and thermodynamic assessment of a biomass gasification plant integrated with Brayton cycle and solid oxide steam electrolyzer for compressed hydrogen production

In this paper, a comprehensive thermodynamic evaluation of an integrated plant with biomass is investigated, according to thermodynamic laws. The modeled multi-generation plant works with biogas produced from demolition wood biomass. The plant mainly consists of a biomass gasifier cycle, clean water production system, hydrogen production, hydrogen compression, gas turbine sub-plant, and Rankine cycle. The useful outputs of this plant are hydrogen, electricity, heating and clean water. The hydrogen generation is obtained from high-temperature steam electrolyzer sub-plant. Moreover, the membrane distillation unit is used for freshwater production, and also, the hydrogen compression unit with two compressors is used for compressed hydrogen storage. On the other hand, energy and exergy analyses, as well as irreversibilities, are examined according to various factors for examining the efficiency of the examined integrated plant and sub-plants. The results demonstrate that the total energy and exergy efficiencies of the designed plant are determined as 52.84% and 46.59%. Furthermore, the whole irreversibility rate of the designed cycle is to be 37,743 kW, and the highest irreversibility rate is determined in the biomass gasification unit with 12,685 kW.     

Energy and exergy analysis of syngas production from different biomasses through air-steam gasification

Gasification is a thermo-chemical reaction which converts biomass into fuel gases in a reactor. The efficiency of conversion depends on the effective working of the gasifier. The first step in the conversion process is the selection of a suitable feedstock capable of generating more gaseous fuels. This paper analyses the performance of different biomasses during gasification through energy and exergy analysis. A quasi-equilibrium model is developed to simulate and compare the feasibility of different biomass materials as gasifier feedstock. Parametric studies are conducted to analyze the effect of temperature, steam to biomass ratio and equivalence ratio on energy and exergy efficiencies. Of the biomasses considered, sawdust has the highest energy and exergy efficiencies and lowest irreversibility. At a gasification temperature of 1000 K, the steam to biomass ratio of unity and the equivalence ratio of 0.25, the energy efficiency, exergy efficiency and irreversibility of sawdust are 35.62%, 36.98% and 10.62 MJ/kg, respectively. It is also inferred that the biomass with lower ash content and higher carbon content contributes to maximum energy and exergy efficiencies.     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.