Analyses of thermodynamic performance for the endoreversible Otto cycle with the concepts of entropy generation and entransy

In this paper,power the the and endoreversible the Otto cycle is analyzed with the entropy generation minimization objectives,and the the entransy theory.of The output power,the heat-work conversion efficiency are taken as the optimization rate,and relationships the output heat-work conversion efficiency,entransy the entropy generation and the entropy generation rate numbers,the work entransy are loss rate,The entransy loss of coefficient,the dissipation rate the entransy variation associated with discussed.applicability entropy the entropy generation minimization and the entransy theory while to the analyses is also analyzed.It is found do that smaller generation rate does not always lead to larger output our power,smaller entropy entransy generation loss numbers and not always lead to larger heat-work conversion efficiency,either.larger the In calculations,power,both larger larger rate larger entransy variation heat-work rate associated with work correspond also to that output while entransy is loss coefficient suitable results the in larger conversion developed efficiency.It is found concept of entransy dissipation not always for analyses because it was for heat transfer.     

Heat transfer performance evaluation of one-stream heat exchangers and one-stream heat exchanger networks

One-stream heat exchangers and one-stream heat exchanger networks are widely used in engineering. In this paper, the heat transfer performance evaluation of one-stream heat exchangers and one-stream heat exchanger networks is analyzed with the concepts of entropy generation rate, entropy generation number, revised entropy generation number, entropy resistance, entransy dissipation rate, entransy dissipation number and generalized thermal resistance. For the analyzed one-stream heat exchangers, our numerical results show that the extremum value of the entransy dissipation rate and the minimum values of the entropy resistance and the generalized thermal resistance always lead to the largest heat transfer rate or the lowest temperature of the cooled object,while the minimum values of the other parameters do not always. For the analyzed one-stream heat exchanger networks, the minimizations of entransy dissipation rate, entransy dissipation number and generalized thermal resistance always correspond to the lowest average temperature of the cooled objects, while the minimizations of the other parameters do not. Therefore, only the extremum entransy dissipation principle and the minimum generalized thermal resistance principle are always applicable for the heat transfer performance evaluation of the systems in this paper, while the applicability of the other parameters is conditional.     

Equivalent thermal resistance minimization for a circular disc heat sink with reverting microchannels based on constructal theory and entransy theory

Thermal designs for microchannel heat sinks with laminar flow are conducted numerically by combining constructal theory and entransy theory. Three types of 3-D circular disc heat sink models, i.e. without collection microchannels, with center collection microchannels, and with edge collection microchannels, are established respectively. Compared with the entransy equivalent thermal resistances of circular disc heat sink without collection microchannels and circular disc heat sink with edge collection microchannels, that of circular disc heat sink with center collection microchannels is the minimum, so the overall heat transfer performance of circular disc heat sink with center collection microchannels has obvious advantages. Furthermore, the effects of microchannel branch number on maximum thermal resistance and entransy equivalent thermal resistance of circular disc heat sink with center collection microchannels are investigated under different mass flow rates and heat fluxes. With the mass flow rate increasing, both the maximum thermal resistances and the entransy equivalent thermal resistances of heat sinks with respective fixed microchannel branch number all gradually decrease. With the heat flux increasing, the maximum thermal resistances and the entransy equivalent thermal resistances of heat sinks with respective fixed microchannel branch number remain almost unchanged. With the same mass flow rate and heat flux, the larger the microchannel branch number, the smaller the maximum thermal resistance. While the optimal microchannel branch number corresponding to minimum entransy equivalent thermal resistance is 6.     

Entransy analyses of the thermodynamic cycle in a turbojet engine

The analysis and the design of turbojet engines are of great importance to the improvement of the system performance.Many researchers focus on these topics,and many important and interesting results have been obtained.In this paper,the thermodynamic cycle in a turbojet engine is analyzed with the entransy theory and the T-Q diagram.The ideal thermodynamic cycle in which there is no inner irreversibility is analyzed,as well as the influences from some inner irreversible factors,such as the heat transfer process,the change of the component of the working fluid and the viscosity of the working fluid.For the discussed cases,it is shown that larger entransy loss rate always results in larger output power,while smaller entropy generation rate does not always.The corresponding T-Q diagrams are also presented,with which the change tendencies of the entransy loss rate and the output power can be shown very intuitively.It is shown that the entransy theory is applicable for analyzing the inner irreversible thermodynamic cycles discussed in this paper.Compared with the concept of entropy generation,the concept of entransy loss and the corresponding T-Q diagram are more suitable for describing the change of the output power of the analyzed turbojet engine no matter if the inner irreversible factors are considered.     

T-Q diagram analyses and entransy optimization of the organic flash cycle(OFC)

The electrical power generation from low temperature heat source attracts more and more attentions but the temperature mismatching between the heat sources and working medium in the organic Rankine cycle(ORC)becomes an issue.The organic flash cycle(OFC)is an effective solution to this issue.In this paper,the OFC is analyzed by the concept of entransy loss and the T-Q(temperature-heat flow rate)diagram for the heat-work conversion.The equations for cycles of the basic OFC and the OFC whose heat source is the exhaust gas of the turbine in a Brayton cycle(the combined cycle)are derived theoretically and the results indicate that larger entransy loss rate leads to larger output power with prescribed inlet parameters of the hot stream in the discussed cases,which is displayed by the T-Qdiagram intuitively.Two numerical examples demonstrate that the optimal mass flow rate of the working medium for the maximum entransy loss rate is the same as that for the maximum output power.The T-Qdiagram analyses is in accordance with the numerical results.The concept of entransy loss can be used as the criteria for the OFC optimization.     

通风型光伏/光热建筑一体化系统的实验研究

以铜铟镓硒光伏组件为光电单元,通过构建对比实验组研究通风流速和气候条件对通风型光伏/光热建筑一体化系统的光热和光电性能的影响.结果表明,系统得热功率和发电功率均与辐照强度呈正相关,受环境温度影响,变化规律并非完全一致.当通风流速为2.2 m/s时,太阳能综合利用率达到65.4%,提出平均得热效率和相对发电量用于表征系统的光热及光电性能,获得与通风流速的定量关系;当通风流速由0.4 m/s增大到2.2 m/s时,系统平均得热效率提高了130.0%,发电效率仅增加4.0%左右.在考虑阻力耗能的情况下,通风流速是系统高效运行的关键,通过不同地域的重复实验,验证了定量关系的普适性.     

热声热机换热器性能的分析

热声换热器热量传递的速率与效率直接影响着热声热机的性能. 　　耗散理论能更好地揭示换热器的传热优化特性,在热声换热器研究中引入　　耗散理论,针对顺流和逆流两种情况,计算了热声换热器的　　耗散热阻,并和最小熵产原理的结果进行了对比分析. 结果表明,在一定条件下,顺流比逆流情况下的不可逆损失要大;当换热器低温端流体的热容量小于高温端流体热容量时不可逆损失较小,结果最优.     

Numerical study on optical and electric-thermal performance for solar concentrating PV/T air system

Hybrid photovoltaic/thermal (PV/T) system with solar concentrator is an effective way to improve solar energy conversion efficiency. In this work, a single-pass PV/T air system with a three-trough compound parabolic concentrator (CPC) of concentration ratio 2.0 is designed and the solar incident distributions at the solar cell surface are calculated by ray tracing method. Based on energy balance, the heat transfer models of all main components in this system are developed. The effects of some main designing and operational parameters on the electric-thermal performance of the system are analyzed. The results show that the solar radiation intensity can be higher than 1200 W/m² at most area of the cell surface. The temperature of the air and cell surface increases along the length of the system. Thus the system efficiency of the CPC is higher than that of the system without the CPC. The thermal efficiency, exergy and electrical efficiency of this CPC system increase with increasing of the air mass flow rate and the length of the system. With increasing packing fraction the electrical efficiency increases, but the thermal efficiency decreases. The exergy efficiency increases slightly with the packing fraction rising. The data obtained in this work are valuable for the design and operation for this kind of solar concentrating PV/T systems.     

An entransy dissipation-based optimization principle for solar power tower plants

The entransy theory,which can be used to optimize the heat transfer network of a solar power tower system(SPTS)and improve its energy efficiency,was introduced in this paper.Firstly,the irreversibility of the heat transfer processes in a SPTS was analyzed and the total entransy dissipation equation of a SPTS was derived.Then,two types of optimization problems(reducing the total circulating flow rate or the total heat-exchanging area)of a SPTS were solved with conditional extremum model based on the formulas of total entransy dissipation.Finally,the entransy dissipation-based optimization principle was applied to a simple SPTS without re-heater and a complex SPTS with a re-heater.The results showed that under the chosen calculation conditions the minimum total thermal conductance was 19306.03 W K?1 for a SPTS without re-heater when the total heat capacity rate of heat transfer fluid(HTF)was 3200 W K?1.The minimum total thermal conductance was about 7.9%lower than the value predicted based on the typical outlet temperature of a receiver.This meant that the total heat exchange area or initial investment could be effectively reduced under the prescribed total HTF circulating flow rate.We also studied the variation trends of the two optimized results including minimum total HTF heat capacity rate and minimum total thermal conductance.The minimum total HTF heat capacity rate decreased with the given total thermal conductance,the minimum total thermal conductance decreased first and then increased with the given total HTF heat capacity rate.We also found that for a SPTS with a re-heater,the mixing temperature and the mixing position of HTF had significant effects on the two types of optimization problems.     

多结太阳能电池聚光的应用分析

GaInP/GaInAs/Ge多结太阳能电池在聚光的条件下最高转换效率可以在40%以上,这种多结太阳能电池性能优异,对温度的影响相对较弱,且随着加工技术的改进其单片价格逐渐降低.采用菲涅尔镜聚焦的方式,可以提高单位面积的辐射量,从而降低发电成本.通过对高倍菲涅尔镜聚光发电进行研究分析,得出了电池性能提高的数据,同时研究聚光误差所产生的影响,对定位提出了±1°的误差要求.     

太阳能冷热电联供系统在城市建筑中的适用性分析

以太阳能冷热电联供集成系统为研究对象,设计了光伏组件、太阳能集热器与冷热电联供集成系统。以传统分供系统为参照对象,通过"以电定热"和"以热定电"两种运行方式,提出了一定区域内光伏组件和太阳能集热器的功率公式,并以节能减排率作为该系统的评价指标来评价太阳能在3种典型城市建筑(办公建筑、商业建筑、住宅建筑)中的合理使用方式。结果表明,以热定电模式运行时,太阳能联供系统在办公建筑中存在太阳能的最佳耦合方式;以电定热模式运行时,太阳能联供系统在不同建筑中均以光伏形式耦合。     

Work entransy and its applications

The entransy theory has been applied to the analyses of heat-work conversion systems. The physical meaning and the applications of work entransy are analyzed and discussed in this paper. Work entransy, which is clarified to be a process dependent quantity, is not the entransy of work, but the system entransy change accompanying work transfer. The relationship between the work entransy and the output work is set up. When the application preconditions are satisfied, larger work entransy leads to larger output work. Entransy loss, which was proposed and applied to heat work conversion processes with irreversible heat transfer, is the net entransy flow into the system and the summation of work entransy and entransy dissipation. The application preconditions of entransy loss are also discussed.     

Application of entransy dissipation based thermal resistance to design optimization of a novel finless evaporator

Entransy has been applied and studied in a broad range of heat transfer optimizations recently. Current study proposes the entransy of evaporators to conduct the optimization of heat exchangers in heating, ventilation, air conditioning and refrigeration systems. A novel finless bare tube heat exchanger was studied using a validated heat exchanger modeling tool. The capacity based optimization and entransy dissipation based thermal resistance were used and compared. The applicability of using entransy dissipation based thermal resistance in this type of heat exchanger optimization has been discussed. It has been demonstrated that minimizing entransy dissipation and maximizing capacity are equivalent to optimizing evaporators with fixed flow rates and different when optimizing evaporators with variable flow rates and the deviation is negligible when heat exchanger capacity is small (～1 kW) and more obvious as heat exchanger capacity increases. Thus entransy dissipation based thermal resistance could be used as an alternative optimization index to capacity for evaporators with fixed flow rate and small capacity evaporators with variable flow rates and should be used individually with capacity as an optimization index for evaporators with large capacity and variable flow rates.     

Experimental study of a micro-scale solar organic Rankine cycle system based on compound cylindrical Fresnel lens solar concentrator

In the present study, a micro-scale solar organic Rankine cycle power generation system was developed. The system comprises of a solar collection system based on compound cylindrical Fresnel lens concentrator and an organic Rankine cycle power generation system integrated with a scroll expander. YD320 and R245 fa were used as the heat transfer fluid and the working fluid, respectively. The effects of the evaporation pressure, the degree of superheat, and the mass flow rate of the working fluid were analyzed to evaluate the solar collection efficiency, the electric power output, the thermal efficiency and exergy efficiency of the system. The results illustrate that both the increasing evaporation pressure and decreasing superheat degree have positive impacts on solar collection efficiency. The electric power increases as the evaporation pressure increases, while the thermal efficiency and the exergy efficiency decrease. However, the system overall efficiency decreases slowly due to the increase of solar collection efficiency. The electric power increases with the increment of the working fluid mass flow rate. The increasing mass flow rate has no visible impact on the thermal and exergy efficiencies of organic Rankine cycle subsystem, whereas a slightly increase of the thermal and exergy efficiencies of the integrated system. The electric power decreases with the increase of the superheat degree, whereas the thermal and the exergy efficiencies of the system increase. The system works more suitably with a higher degree of superheat for the small mass flow rate condition.     

基于MPPT控制策略的光伏电池建模与仿真

太阳能作为当前最具发展潜力之一的可再生能源,将会在今后的能源结构中发挥重要的作用.光伏发电中进行能量转换的最基本单元是光伏电池,通过对光伏电池进行数学建模,并根据其仿真模型分析,能掌握光伏电池的输出特性.光伏电池利用率的高低受各种因素的影响,为了使光伏阵列在任何外界条件下,都能输出当前工况下最多的能量,本文采用最大功率点跟踪策略对其控制.本文推导了光伏电池的数学模型,且选择合适的最大功率跟踪策略对其控制,通过Matlab仿真验证其MPPT控制策略的有效性和正确性.     

20 MW光伏发电系统与电气一次设计

根据20 MW光伏发电系统的工程条件和需求,系统总体方案采用分块发电、集中并网方式,光伏组件选用260 W多晶硅太阳能组件,光伏阵列运行方式为固定式安装式,倾斜角为23°,逆变器为500 k W,光伏方阵的南北间距为5.2 m。对电气一次系统的接入电力系统、电气主接线和电气设备布置进行了设计。根据计算结果可知,该系统的平均年上网电量为17 954 MW·h,年等效满负荷利用时间为896.4 h,容量系数为0.102,节能和环保效益良好。     

基于理论的相变储能换热器传热性能分析

在理论成功应用于常规换热器的基础上,将传递效率、耗散数及基于耗散的换热器热阻应用于相变储能换热器的传热性能分析中。定义广义耗散率并由此推导出相变储能换热器蓄热、放热及总过程的传递效率及其瞬时值。确定耗散数及基于耗散的换热器热阻计算中换热量的取法。选取一种相变储能装置作为分析对象,通过理论分析绘制各主要部分温度变化趋势,进一步简化得到硅油、水的出口温度表达式,作为算例分析基础。结果表明, 传递效率的应用范围最广,可用于计算相变储能换热器蓄热、放热及总过程的(瞬时)不可逆热损失,且评价结果与传热性能相符,瞬时传递效率随蓄热时间的增加先增大后不变再增大,随放热时间的增加先减小后不变再减小; 耗散数在蓄热过程和总过程中的评价结果与传递效率一致,瞬时耗散数随蓄热时间的增加先减小后不变再减小,然而在放热过程中的应用受限。基于耗散的换热器热阻的部分评价结果与实际不符,应用限制较大。蓄热过程及总过程中,当蓄热量、取热量与蓄、放热阶段时长同步变化时, 传递效率、耗散数与基于耗散的换热器热阻几乎无变化;当装置传热性能提高时, 传递效率增大, 耗散数减小,基于耗散的换热器热阻减小;放热过程中,设置参数的变化不影响装置传热性能, 传递效率基本无变化。     

Constructal optimization of cylindrical heat sources with forced convection based on entransy dissipation rate minimization

Based on constructal theory and entransy theory,the optimal designs of constant-and variable-cross-sectional cylindrical heat sources are carried out by taking dimensionless equivalent resistance minimization as optimization objective.The effects of the cylindrical height,the cylindrical shape and the ratio of thermal conductivity of the fin to that of the heat source are analyzed.The results show that when the volume of the heat source is fixed,there exists an optimal ratio of the center-to-centre distance of the fin and the heat source to the cylinder radius which leads to the minimum dimensionless equivalent thermal resistance.With the increase in the height of the cylindrical heat source and the ratio of thermal conductivity,the minimum dimensionless equivalent thermal resistance decreases gradually.For the heat source model with inverted variable-cross-sectional cylinder,there exist an optimal ratio of the center-to-centre distance of the fin and the heat source to the cylinder radius and an optimal radius ratio of the smaller and bigger circles of the cylindrical fin which lead to a double minimum dimensionless equivalent thermal resistance.Therefore,the heat transfer performance of the cylindrical heat source is improved by adopting the cylindrical model with variable-cross-section.The optimal constructs of the cylindrical heat source based on the minimizations of dimensionless maximum thermal resistance and dimensionless equivalent thermal resistance are different.When the thermal security is ensured,the optimal construct of the cylindrical heat source based on minimum equivalent thermal resistance can provide a new alternative scheme for the practical design of heat source.The results obtained herein enrich the work of constructal theory and entransy theory in the optimal design field of the heat sources,and they can provide some guidelines for the designs of practical heat source systems.     

太阳能热水系统及光伏光热系统在上海地区年运行性能研究

以上海市的年气象数据为计算依据,综合考虑了太阳辐照、水温、环境温度的影响,在忽略夜间损失的情况下,以天为单位,对太阳能热水系统及太阳能光伏光热系统连续加热模式下的年运行性能进行了分析计算,研究了两种系统在上海地区的应用潜力及节能特性.计算结果表明,太阳能热水系统具有较高的年平均综合效率,同时具有较短的静态回收周期;光伏光热系统一年中仅在7月、9月具有较高的综合效率.     

基于改进扰动法的光伏电池MPPT仿真研究

根据光伏电池的特性,搭建了其仿真模型,能够模拟不同日照和温度条件下电池的输出特性.针对占空比扰动法的不足,提出了一种基于模糊- PI控制的占空比扰动法进行最大功率点跟踪,并在Matlab环境下进行了仿真验证.仿真结果表明,与占空比扰动法相比,该方法在外界环境变化时能够快速跟踪光伏电池的最大功率点,有效提高最大功率点的跟踪精度,具有良好的动态和稳态性能.