Comparison of energy and exergy analysis of fossil plant,ground and air source heat pump building heating system

The energy and exergy flow for a space heating systems of a typical residential building of natural ventilation system with different heat generation plants have been modeled and compared. The aim of this comparison is to demonstrate which system leads to an efficient conversion and supply of energy/exergy within a building system.The analysis of a fossil plant heating system has been done with a typical building simulation software IDA–ICE. A zone model of a building with natural ventilation is considered and heat is being supplied by condensing boiler. The same zone model is applied for other cases of building heating systems where power generation plants are considered as ground and air source heat pumps at different operating conditions. Since there is no inbuilt simulation model for heat pumps in IDA–ICE, different COP curves of the earlier studies of heat pumps are taken into account for the evaluation of the heat pump input and output energy.The outcome of the energy and exergy flow analysis revealed that the ground source heat pump heating system is better than air source heat pump or conventional heating system. The realistic and efficient system in this study “ground source heat pump with condenser inlet temperature 30 °C and varying evaporator inlet temperature” has roughly 25% less demand of absolute primary energy and exergy whereas about 50% high overall primary coefficient of performance and overall primary exergy efficiency than base case (conventional system). The consequence of low absolute energy and exergy demands and high efficiencies lead to a sustainable building heating system.     

Performance analysis of a heat pump assisted drying system

The paper describes a simulation model developed to predict the performance of drying systems assisted by vapour-compression heat pumps. The heat is used to preheat the air stream before it enters the drying chamber. Energy consumption is thus reduced, as the heat pump is capable of delivering more energy as heat than it in fact consumes as input work. Ambient air provides the heat source. A computer program, based on simplified modelling of components (compressor, heat exchangers and drying chamber) has been developed. Results have been produced for a typical application, revealing that a considerable reduction in energy consumption can be obtained with the use of a heat pump. The effect of air flow rate on system performance is also studied.     

Energy and exergy analysis of fossil plant and heat pump building heating system at two different dead-state temperatures

In this paper, we deal with the energy and exergy analysis of a fossil plant and ground and air source heat pump building heating system at two different dead-state temperatures. A zone model of a building with natural ventilation is considered and heat is being supplied by condensing boiler. The same zone model is applied for heat pump building heating system. Since energy and exergy demand are key parameters to see which system is efficient at what reference temperature, we did a study on the influence of energy and exergy efficiencies. In this regard, a commercial software package IDA-ICE program is used for calculation of fossil plant heating system, however, there is no inbuilt simulation model for heat pumps in IDA-ICE, different COP (coefficient of performance) curves of the earlier studies of heat pumps are taken into account for the evaluation of the heat pump input and output energy. The outcome of the energy and exergy flow analysis at two different dead-state temperatures revealed that the ground source heat pumps with ambient reference have better performance against all ground reference systems as well as fossil plant (conventional system) and air source heat pumps with ambient reference.     

Exergy analysis of solar energy applications

Solar energy is a clean, abundant and easily available renewable energy. Usage of solar energy in different kinds of systems provides scope for several studies on exergy analysis. In the present work, a comprehensive literature review has been carried out on exergy analysis of various solar energy systems. The systems considered under study are solar photovoltaic, solar heating devices, solar water desalination system, solar air conditioning and refrigerators, solar drying process and solar power generation. The summary of exergy analysis and exergetic efficiencies is presented along with the exergy destruction sources.     

An exergy analysis on the performance of a counterflow wet cooling tower

Cooling towers are used to extract waste heat from water to atmospheric air. An energy analysis is usually used to investigate the performance characteristics of cooling tower. However, the energy concept alone is insufficient to describe some important viewpoints on energy utilization. In this study, an exergy analysis is used to indicate exergy and exergy destruction of water and air flowing through the cooling tower. Mathematical model based on heat and mass transfer principle is developed to find the properties of water and air, which will be further used in exergy analysis. The model is validated against experimental data. It is noted from the results that the amount of exergy supplied by water is larger than that absorbed by air, because the system produces entropy. To depict the utilizable exergy between water and air, exergy of each working fluid along the tower are presented. The results show that water exergy decreases continuously from top to bottom. On the other hand, air exergy is expressed in terms of convective and evaporative heat transfer. Exergy of air via convective heat transfer initially loses at inlet and slightly recovers along the flow before leaving the tower. However, exergy of air via evaporative heat transfer is generally high and able to consume exergy supplied by water. Exergy destruction is defined as the difference between water exergy change and air exergy change. It reveals that the cooling processes due to thermodynamics irreversibility perform poorly at bottom and gradually improve along the height of the tower. The results show that the lowest exergy destruction is located at the top of the tower.     

Evaluation of exergy and energy efficiencies of photothermal solar radiation conversion

In this paper, a new thermodynamic model for photothermal solar radiation conversion into mechanical through a heat engines is proposed. The developed equations allow for the energy and exergy contents of solar radiation to be found, as well as the energy and exergy efficiencies corresponding to concentration type solar-thermal heat engines operating under a range of conditions. The calculation method remains accurate to other published models when their assumed conditions are imposed to the newly developed model. The heat flux absorbed by the receiver (which is assumed to be a grey body and is placed in the focal point of the solar concentrator) depends on the hemispherical absorptivity and emissivity, concentration ratio and receiver temperature. The model is used to conduct a parametric study regarding the energy and exergy efficiencies of the system for assessing its performance. The use of a selective grey body receiver (having a reduced emissivity and a high absorptivity) for enhancing the conversion efficiency is also studied. If the absorptivity approaches one and the emissivity is low enough the photothermal conversion efficiency becomes superior to the known black body receiver limit of 0.853. It is found that in the limit of receiver emissivity tending to zero and absorptivity lending to one, the present model gives the exergy content of solar radiation because the work generated reaches its maximum. In this situation the energy efficiency approaches the exergy efficiency at 1-ITTIN0/TINS where T_S and T₀ are the sun and ambient temperatures, respectively. The influence of the ambient temperature on the exergy and energy efficiencies becomes apparent, with effects of up to 15%, particularly for high absorptivity and low emissivity. The heat transfer conductances at sink and source of the heat engine have a considerable impact on the efficiency of solar energy conversion. The present model is developed in line with actual power system operations for better practical acceptance. In addition, some irreversibility parameters (absorptivity, emissivity, heat transfer conductivity, etc.) are studied and discussed to evaluate the possible photothermal solar radiation conversion systems and assess their energy and exergy efficiencies.     

Exergy analysis on combustion and energy conversion processes

When we consider exergy analysis on combustion and thermodynamic processes, we introduce another concept against energy analysis, which is supported by an evaluation of its temperature level. When a higher temperature energy than that an ambient level is taken into consideration, it can be put for some domestic or industrial purpose. A medium temperature energy of 30–60 °C is used for domestic heating, and a high temperature of 200 °C and above is suitable for power generation or process heating. Therefore, we study exergy concept supported by temperature level. When we discuss power generation, a high temperature energy of 1500 °C and above in combined cycle has a higher conversion efficiency than that of 500–600 °C in steam cycle. If we try to apply high temperature air combustion, a preheated air temperature of 1000 °C and above can be produced by exhaust heat recovery from stack gas, which has been developed as a new technology of energy conservation. In this study, the authors present an exergy analysis on combustion and energy conversion processes, which is based on the above-mentioned concept of exergy and energy supported by temperature level. When we discuss high temperature air combustion in furnace, this process shows a higher performance than that of the ambient air combustion. Furthermore, when we discuss the power generation and heat pump processes, the minimum ambient temperature would already be known for each season, and the conversion performance can be estimated by the maximum operating temperature in their cycles. So, the authors attempt to calculate the exergy and energy values for combustion, power generation and heat pump processes.     

Exergetic performance evaluation of heat pump systems having various heat sources

In this study heat pump systems having different heat sources were investigated experimentally. Solar‐assisted heat pump (SAHP), ground source heat pump (GSHP) and air source heat pump (ASHP) systems for domestic heating were tested. Additionally, their combination systems, such as solar‐assisted‐ground source heat pump (SAGSHP), solar‐assisted‐air source heat pump (SAASHP) and ground–air source heat pump (GSASHP) were tested. All the heat pump systems were designed and constructed in a test room with 60 m² floor area in Firat University, Elazig (38.41°N, 39.14°E), Turkey. In evaluating the efficiency of heat pump systems, the most commonly used measure is the energy or the first law efficiency, which is modified to a coefficient of performance for heat pump systems. However, for indicating the possibilities for thermodynamic improvement, inadequate energy analysis and exergy analysis are needed. This study presents an exergetic evaluation of SAHP, GSHP and ASHP and their combination systems. The exergy losses in each of the components of the heat pump systems are determined for average values of experimentally measured parameters. Exergy efficiency in each of the components of the heat pump systems is also determined to assess their performances. The coefficient of performance (COP) of the SAHP, GSHP and ASHP were obtained as 2.95, 2.44 and 2.33, whereas the exergy losses of the refrigerant subsystems were found to be 1.342, 1.705 and 1.942 kW, respectively. The COP of SAGSHP, SAASHP and GSASHP as multiple source heat pump systems were also determined to be 3.36, 2.90 and 2.14, whereas the exergy losses of the refrigerant subsystems were approximately 2.13, 2.996 and 3.113 kW, respectively. In addition, multiple source heat pump systems were compared with single source heat pump systems on the basis of the COP. Exergetic performance coefficient (EPC) is introduced and is applied to the heat pump systems having various heat sources. The results imply that the functional forms of the EPC and first law efficiency are different. Results show that Ex_loss,total becomes a minimum value when EPC has a maximum value. Copyright © 2008 John Wiley & Sons, Ltd.     

Energy and exergy performance assessment of a novel solar-based integrated system with hydrogen production

In this study, a new solar power assisted multigeneration system designed and thermodynamically analyzed. In this system, it is designed to perform heating, cooling, drying, hydrogen and power generation with a single energy input. The proposed study consists of seven sub-parts which are namely parabolic dish solar collector, Rankine cycle, organic Rankine cycle, PEM-electrolyzer, double effect absorption cooling, dryer and heat pump. The effects of varying reference temperature, solar irradiation, input and output pressure of high-pressure turbine and pinch point temperature heat recovery steam generator are investigated on the energetic and exergetic performance of integration system. Thermodynamic analysis result outputs show that the energy and exergy performance of overall study are computed as 48.19% and 43.57%, respectively. Moreover, the highest rate of irreversibility has the parabolic dish collector with 24,750 kW, while the lowest rate of irreversibility is calculated as 5745 kW in dryer. In addition, the main contribution of this study is that the solar-assisted multi-generation systems have good potential in terms of energy and exergy efficiency.     

Application of exergy analysis in designing helium liquefiers

Exergy has proved to be a useful tool to analyze and optimize the design and operation of many systems. Some studies on helium refrigerators and liquefiers based on exergy are available in literature. In this paper, systematic evaluation of important operating and geometric parameters has been done to determine the exergy destructions in components as well as in the entire cycle of Collins helium liquefiers. Grassmann diagram of exergy flow has been shown to be of immense help in understanding relative importance of different components used in the system. Compressor pressure, expander flow rates, heat exchanger surface area are some of the parameters optimized considering both presence and absence of pressure drop in the heat exchangers. Non-dimensionalization of parameters makes the results applicable to plants of any capacity. Exergy-analysis based on Second Law proves to be far superior to the First Law based energy analysis in designing of the helium plant as the former is holistic in approach and capable of deriving some additional conclusions. Results derived on Collins cycle may be applicable in large-scale helium liquefiers by providing basic understanding of the influence of components on the plant performance and reasonable initial guess values in their design and simulation.     

Energy and Exergy Analysis on Drying of Banana Using Indirect Type Natural Convection Solar Dryer

Abstract

Energy and exergy analysis, in the thermodynamics, is an important tool used to predict the performance of drying system. In this work, energy and exergy analyses are made during the drying process of banana using an indirect type passive solar dryer. Solar flat plate air collector is used to heat the air. Banana gets sufficiently dried at temperatures between 28 and 82?°C. Solar radiation is measured and it is ranged from 335 to 1210?W/m². Using the first law of thermodynamics, energy analysis was carried out to estimate the amounts of energy gained from solar air heater. Also, applying the second law of thermodynamics, exergy analysis was carried out to determine exergy losses during the drying process. The exergy losses varied from 3.36 to 25.21?kJ/kg. In particular, the exergy efficiency values vary from 7.4 to 45.32%.     

热泵干燥系统运行特性的有效能研究

热泵干燥能源消耗少，环境污染小，干燥品质高，适用范围广，是一种性能优良的种子干燥机械。从热力学第一、第二定律两方面分析了热泵干燥系统的节能优势所在，得出采用辅助冷凝器是闭式热泵干燥系统的理想方式；而且热泵干燥系统存在一个最佳的蒸发温度，可使系统除湿率和效率最大。     

太阳能-空气源热泵联合干燥系统设计及干燥枸杞的实验研究

为缩短枸杞干燥时间,提高干制枸杞的质量,减少能源消耗,本文提出了一种新型太阳能–空气源热泵联合干燥系统。该系统主要由太阳能集热器和空气源热泵机组等设备组成,可以实现太阳能单独干燥、热泵单独干燥和太阳能–空气源热泵联合干燥三种工作模式。本文根据枸杞的干燥特性,分段设定最佳的干燥温度,进行了热泵单独运行和太阳能–热泵联合运行两种工作模式下干燥枸杞的对比实验。结果表明,干燥50 kg枸杞,太阳能–热泵联合运行比热泵单独运行节省了2.9 kW?h电能,若同时除去系统本身的耗能,节省的电能占热泵单独运行耗电量的29.5%。同时,与太阳能单独干燥相比,太阳能–热泵联合干燥具有较高的除湿能耗比,两者最大差值为0.71 kg/(kW?h)。本文提出的太阳能–热泵联合干燥系统具有提高干燥产品的品质、缩短干燥时间和节约干燥成本等优点,适宜推广。     

1000MW机组回热系统损分布的矩阵方法

基于热力学第二定律的分析法为评价能量转换的"量"和"质"提供了一个统一的尺度,比基于第一定律的能分析法更科学、更合理。根据平衡原理,对于一般的回热系统,建立回热系统中各级加热器的平衡方程,经过严谨的数学推理,获得类似于热力系统汽水分布矩阵的通用的损矩阵方程。该损矩阵方程与回热系统中各级加热器的类型存在一定的对应关系,利用该方程可以方便地得出不同机组回热系统的损分布,为较准确的评价热力系统的热经济性提供有力的依据。     

Thermodynamic analysis of fluidized bed drying of carrot cubes

In this study, the energy and exergy analyses of fluidized bed drying of carrot cubes were investigated. Drying experiments were conducted at inlet air temperatures of 50, 60, and 70 °C, BD (bed depths) of 30, 60, and 90 mm and square-cubed carrot dimensions of 4, 7, and 10 mm. The effects of drying variables on energy utilization, energy utilization ratio, exergy loss and exergy efficiency were studied. The energy utilization and energy utilization ratio varied between 0.105–1.949 kJ/s and 0.074–0.486, respectively. The exergy loss and exergy efficiency were found to be in the range of 0.206–1.612 kJ/s and 0.103–0.707, respectively. The results showed that small particles, deep beds and high inlet air temperatures increased energy utilization, energy utilization ratio, and exergy loss due to high value of heat and mass transfer. Also, the exergy efficiency had maximum value when higher drying air temperature, larger CS (cube size) and shorter BD were used for drying experiment.     

Comparison of energy consumption and specific energy requirements of different methods for drying mushroom slices

Energy consumption for drying of mushroom slices was evaluated using various drying methods including hot air, microwave, vacuum, infrared, microwave-vacuum and hot air-infrared. Results of data analysis showed that the lowest and highest energy consumption levels in drying mushroom slices were associated with microwave and vacuum dryers, respectively. The use of vacuum in conjunction with microwave drying increased energy consumption relative to microwave drying alone. Energy consumption in the hot air dryer showed a downward trend with increasing temperature and an upward trend with increasing air velocity. In drying mushroom using infrared radiation, it was observed that increased air velocity increases drying time and consequently the amount of consumed energy. Using a combination of hot air and infrared drying decreased energy consumption relative to infrared drying alone and increased it relative to hot air drying. In the combined microwave-vacuum dryer, drying time and consequently energy consumption decreased in comparison to the vacuum dryer. Hot air-infrared drying of mushroom slices proved to have the lowest energy consumption.     

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.     

Thermodynamic performance assessment of wind energy systems: An application

In this paper, the performance of wind energy system is assessed thermodynamically, from resource and technology perspectives. The thermodynamic characteristics of wind through energy and exergy analyses are considered and both energetic and exergetic efficiencies are studied. Wind speed is affected by air temperature and pressure and has a subsequent effect on wind turbine performance based on wind reference temperature and Bernoulli’s equation. VESTAS V52 wind turbine is selected for (Sharjah/UAE). Energy and exergy efficiency equations for wind energy systems are further developed for practical applications. The results show that there are noticeable differences between energy and exergy efficiencies and that exergetic efficiency reflects the right/actual performance. Finally, exergy analysis has been proven to be the right tool used in design, simulation, and performance evaluation of all renewable energy systems.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

Exergy analysis on the irreversibility of rotary air preheater in thermal power plant

Energy recovery devices can have a substantial impact on process efficiency and their relevance to the problem of conservation of energy resources is generally recognized to be beyond dispute. One type of such a device, which is commonly used in thermal power plants and air conditioning systems, is the rotary air preheater. A major disadvantage of the rotary air preheater is that there is an unavoidable leakage due to carry over and pressure difference. There are gas streams involved in the heat transfer and mixing processes. There are also irreversibilities, or exergy destruction, due to mixing, pressure losses and temperature gradients. Therefore, the purpose of this research paper is based from the second law of thermodynamics, which is to build up the relationship between the efficiency of the thermal power plant and the total process of irreversibility in the rotary air preheater using exergy analysis. For this, the effects of the variation of the principal design parameters on the rotary air preheater efficiency, the exergy efficiency, and the efficiency of the thermal power plant are examined by changing a number of parameters of rotary air preheater. Furthermore, some conclusions are reached and recommendations are made so as to give insight on designing some optimal parameters.