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分析了功、热、能和[火用]的物理意义以及与热力学定律的关系,做功和传热是能和[火用]传递与转换的两种途径,从热力学第一定律定义的能量只有相对意义。[火用]是系统相对于环境所具有的做最大有用功的能力,相对于选定的环境,[火用]是系统的状态参量。常规的[火用]计算式是从热力学第一和第二定律导出的结果,从动力学的角度讨论了[火用]及其普遍化表达式的物理含义。[火用]起源于系统与环境的不平衡,如果系统与环境之间存在着某种(或几种)强度量差,在强度量差的推动下系统可能自动地变化到与环境相平衡的状态(寂态),在这样的过程中系统可以对外做功,这种做最大有用功的能力就是系统的[火用]。在能量公设的基础上,[火用]的微分被普遍地表示为强度量差与其共轭的广延量微分的乘积。[火用]的普遍化表达式完整地反映了[火用]的物理含义及其动力学特征,利用能量和[火用]的普遍化表达式导出了[火用]损失的普遍化表达式。 相似文献
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煤炭一直以来都是中国最主要的一次性能源,相应地,燃煤锅炉也占有电力市场绝大部分份额。燃煤锅炉存在诸多能量损失途径,能量转换效率较低。系统地分析燃煤锅炉的热力性能非常必要。是热力学第二定律中的一个重要概念,它不仅能反映能量的数量,更能反映能量的品质。基于概念,对某600MW超临界燃煤锅炉模型进行了详细的分析,综合考虑物理和化学,计算了系统的损失、耗散等参数,对锅炉的设计、优化提供了可靠依据。 相似文献
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建立一种不可逆的四温度位吸收式热泵模型,导出其最小传热面积与四热源熵变化率的关系并得到了热力学第二定律的类比表达式,获得了最佳供热率、性能系数和传热面积之间的优化关系,所得结论可为四热源吸收式热泵的优化设计和最佳工况选择提供新的理论途径。 相似文献
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《太阳能学报》2014,(2)
以60m2空调测试房间安装的地源热泵空调系统为研究对象,利用热力学第一定律能量分析与第二定律分析相结合的方法,对该实验系统进行能量及的平衡与效率分析。研究结果表明:整个系统的能量效率与效率分别为61%、4.8%。系统部件中地埋管换热器(Borehole heat exchanger,简称BHE)的平均损失为9.993kW,对应该系统的制冷性能系数为3.02。压缩机、冷凝器、蒸发器、BHE及循环泵的效率分别为47.1%、24.4%、42.2%、7.2%、47.7%。同时数据表明系统部件中BHE的平均能量效率最高,达到96.1%,但其效率最低只有7.2%;BHE中热量损失相对最少,可用能损失相对最多,当BHE中换热量与进口温度不变时降低循环介质流量可以有效提高系统效率。 相似文献
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从热力学第二定律角度分析透平膨胀过程中降的构成,对管输天然气做功能力进行理论分析,得出了温度、压力、化学的计算方法和透平膨胀输出轴功极限能力的评价因子。在理论分析的基础上,进一步给出了现有的基于冷电联产的联合循环方式,从机电一体化角度提出了该领域基于总能系统理论的多学科的研究思路。 相似文献
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《Applied Energy》2007,84(7-8):701-718
In previous research, the author developed a general abstract framework for the exergy content of a system of finite objects [Grubbström RW. Towards a generalized exergy concept. In: van Gool W, Bruggink JJC, editors. Energy and time in the economic and physical sciences. Amsterdam: North-Holland; 1985. p. 41–56]. Each such object is characterised by its initial extensive properties and has an inner energy written as a function of these properties. It was shown that if these objects were allowed to interact, there is a maximum amount of work that can be extracted from the system as a whole, and a general formula for this potential was provided. It was also shown that if one of the objects was allowed to be of infinite magnitude initially, taking on the role as an environment having constant intensive properties, then the formula provided took on the same form as the classical expression for exergy.As a side result, the theoretical considerations demonstrated that the second law of thermodynamics could be interpreted as the inner energy function being a (weakly) convex function of its arguments, when these are chosen as the extensive properties.Since exergy considerations are based on the principle that total entropy is conserved when extracting work, these processes would take an infinite time to complete. In the current paper, instead, a differential-equation approach is introduced to describe the interaction in finite time between given finite objects of a system. Differences in intensive properties between the objects provide a force enabling an exchange of energy and matter. An example of such an interaction is heat conduction. The resulting considerations explain how the power extracted from the system will be limited by the processes being required to perform within finite-time constraints.Applying finite-time processes, in which entropy necessarily is generated, leads to formulating a theory for a maximal power output from the system. It is shown that such a theory is possible to develop, and the resulting equilibrium conditions are compared with to those of the exergetic equilibrium. 相似文献
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S. E. Wright 《Renewable Energy》2004,29(2):179-195
Fuel cells have decided advantages including compatibility with renewable fuels such as hydrogen, methanol and methane. It is often claimed that they have greater potential for efficient operation than heat engines because they are not restricted by the Carnot limitation. However, in this paper a generalized (exergy analysis) approach is utilized to clarify the comparison of the theoretical performance potential of heat engines and fuel cells, in particular, to show that fuel cell conversion is restricted by the second law of thermodynamics in the same way as heat engines. The Carnot efficiency is simply a manifestation of the second law for the heat engine excluding the combustion process. It is shown that the maximum work obtainable from the conversion device is related to the change in flow exergy between reactants and products, that is in general, not equivalent to the change in Gibbs free energy. For equivalent reactant and product temperatures, the difference between the change in Gibbs free energy and the change in flow exergy is equal to the exergy flux of heat transfer that must be rejected by the device due to absorption of entropy from the reactant-product flow. The importance of exergetic (second-law) efficiencies for evaluating performance is demonstrated. Also, exergy analysis is utilized to resolve a number of efficiency related issues for endothermic reactions. 相似文献
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Matjaz Prek 《Energy》2006
Traditional methods of human thermal comfort analysis are based on the first law of thermodynamics. These methods use an energy balance of the human body to determine heat transfer between the body and its environment. By contrast, the second law of thermodynamics introduces the useful concept of exergy. It enables the determination of the exergy consumption within the human body dependent on human and environmental factors. Human body exergy consumption varies with the combination of environmental (room) conditions. This process is related to human thermal comfort in connection with temperature, heat, and mass transfer. In this paper a thermodynamic analysis of human heat and mass transfer based on the 2nd law of thermodynamics in presented. It is shown that the human body's exergy consumption in relation to selected human parameters exhibits a minimal value at certain combinations of environmental parameters. The expected thermal sensation also shows that there is a correlation between exergy consumption and thermal sensation. Thus, our analysis represents an improvement in human thermal modelling and gives more information about the environmental impact on expected human thermal sensation. 相似文献
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A method to determine stratification efficiency of thermal energy storage processes independently from storage heat losses 总被引:1,自引:0,他引:1
A new method for the calculation of a stratification efficiency of thermal energy storages based on the second law of thermodynamics is presented. The biasing influence of heat losses is studied theoretically and experimentally. Theoretically, it does not make a difference if the stratification efficiency is calculated based on entropy balances or based on exergy balances. In practice, however, exergy balances are less affected by measurement uncertainties, whereas entropy balances can not be recommended if measurement uncertainties are not corrected in a way that the energy balance of the storage process is in agreement with the first law of thermodynamics. A comparison of the stratification efficiencies obtained from experimental results of charging, standby, and discharging processes gives meaningful insights into the different mixing behaviors of a storage tank that is charged and discharged directly, and a tank-in-tank system whose outer tank is charged and the inner tank is discharged thereafter. The new method has a great potential for the comparison of the stratification efficiencies of thermal energy storages and storage components such as stratifying devices. 相似文献
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AbstractEnergy 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%. 相似文献
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Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector 总被引:2,自引:0,他引:2
Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl2·6H2O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively. 相似文献