Working fluids for heat transformers

A large amount of thermal energy, in the temperature range of 50–90°C, released to the atmosphere by many commercial installations such as agrofeed, paper mills, dairies and process industries, can be upgraded making possible its use in various forms. Many attempts have benn made in most industrial sectors to recovery this energy by heat pumps, organic Rankine cycles and heat transformers. Among various possibilities, heat transformers present an attractive solution as low level heat can be transformed into a higher temperature with minimum consumption of external energy. Further transformers do not require high maintenance and operating cost.The theoretical performance characteristics of single stage heat transformers using various promising binary mixtures as working fluids have been discussed in this paper. The coefficient of performance, energy efficiencies and mass circulation ratio have been analysed as a function of heat delivery temperature. The comparison of working fluids has also been discussed.     

Thermodynamic study of advanced absorption heat transformers—II. Double absorption configurations

A thermodynamic analysis has been carried out to study the performance of double absorption heat transformers (DAHT) assuming water/lithium bromide as the working fluid. The performance of single (SSHT) and two stage heat transformers (TSHT) analyzed in Part I, was compared with the performance of double absorption heat transformers (DAHT) under the same operating conditions. The results showed that single stage heat transformers (SSHT) were the simplest and most efficient. Greater absorber temperatures were reached with two stage heat transformers (TSHT). However, these systems were in general less efficient than the others and technically the most complex. Double absorption heat transformers (DAHT) were technically simpler than two stage heat transformers (TSHT) and may reach absorber temperature as high as these systems.     

Thermodynamic study of advanced absorption heat transformers—I. Single and two stage configurations with heat exchangers

A thermodynamic analysis was carried out to study the effect of heat exchanger effectiveness (EF) on the performance of single stage heat transformers (SSHT). Moreover, an analysis of three different arrangements of two stage heat transformers was performed using a mathematical model assuming water/lithium bromide as the working fluid. An increase in the solution heat exchanger effectiveness (EF) greatly improved the performance of absorption heat transformers when the absorber temperature was at least 40°C higher than the temperature of the heat supplied to the system. In two stage heat transformers (TSHT), higher absorber temperatures were obtained by coupling the absorber of the first stage to the evaporator of the second. However, higher performance coefficients were obtained in general by coupling the absorber of the first stage to the generator of the second.     

Thermodynamic modelling of absorption-resorption heating cycles with some new working pairs

This paper presents the results of a thermodynamic cycle analysis of single stage resorption heat pump (RHP) and resorption heat transformer (RHT) cycles with the new working pairs R22-NMP and R22-DMA. The coefficients of performance (COP) are correlated with the low grade source temperature, temperature at which useful heat is obtained and ambient temperature. The COPs are in the range 1.20–1.60 for the RHP mode and 0.25–0.45 for the RHT mode. Absorber temperatures (useful temperatures) as high as 50°C in the RHP mode and 87°C in the RHT mode have been obtained. It is observed that absorption-resorption systems are inflexible in their range of operating temperature and necessitate a higher pump work as compared with simple single-stage absorption heating systems. However, single stage RHTs show higher temperature boosts than simple absorption heat transformers.     

Coupling a two-stage absorption heat transformer with finite heat sources and sinks

The coupling among a two-stage heat transformer (TSHT) and finite heat sources and sinks is considered. A novel configuration is proposed, in which the evaporator of the second stage is fed both by the absorber of the first stage and by the external source. A lumped-parameter mathematical model is used to compute the performance of the proposed TSHT. It has been found that the range of applicability is widened, when compared with the usual configuration. Moreover, the use of a two-stage apparatus allows one to noticeably increase the performance compared with that obtainable with a simpler single-stage transformer. Thus, it is possible to increase the output heat and/or the user temperature, to reduce the discharge temperature, or to use lower quality energy sources. The results also allow one to draw some more general conclusions on the applicability of absorption heat transformers of different configurations, when different heat sources/sinks are considered.     

On the performance of advanced absorption heat transformers—I. The two stage configuration

High performance absorption heat transformers based on improved configurations can be proposed with the aim of widening the range of operation of the single stage heat transformer (SSHT). In this paper a two stage heat transformer arranged by coupling the absorber of the first stage to the evaporator of the second stage (TSHT) is analyzed by means of a lumped-parameter mathematical model, written with reference to the water-sulphuric acid system.The computed values of four different indexes of performance show that the proposed TSHT allows one to noticeably increase the gross temperature lift obtainable with a SSHT, while saving a large fraction of the input energy.     

PSM高压电源干式变压器的热分析计算

多绕组干式整流变压器作为托卡马克装置加热系统中高压电源的一个重要设备,其性能优良与否直接关系到高压电源的输出品质。开展变压器的功率损耗与散热分析研究,减少变压器因温度上升造成的性能影响,对保证变压器的良好工作状态是十分重要的。通过MATLAB对干空气下的比热容、传热系数等热物理性质与温度的关系进行拟合分析,得到相关的关系方程,对变压器的对流传热与热辐射进行分析计算,得到仅在空气自然对流和热辐射的情况下,不能使变压器的温度控制在满足性能的温度范围之内。在强迫空气对流的情况下,实现了变压器良好的散热。进一步利用ANSYS对变压器的温度场分布进行分析,利用温度场的分布趋势图观察强迫风冷相对于自然冷却的优点,强迫风冷降低了整体的温度,使热量更快地散发到周围大气中,减少了变压器受高温的危害。     

超导电力技术新进展及其未来发展的思考

近年来高温超导材料研究取得很大进展,它在电力领域的应用研究已受到广泛关注,一些示范样机,诸如高温超导输电电缆、变压器、故障电流限制器、电机和储能装置已经研制成功并投入示范性试验.超导技术是21世纪具有战略经济意义的高新技术,文章将介绍高温超导电力应用研究的新进展及其未来发展的思考。     

Selection of alloys and their influence on the operational characteristics of a two-stage metal hydride heat transformer

A heat transformer can upgrade heat to a higher temperature. A two-stage heat transformer has a greater temperature upgrading potential than a single-stage heat transformer, e.g. heat can be upgraded from a level of about 130–140°C to temperatures of about 200°C. A practical method to select suitable hydrides to be used in a two-stage heat transformer is presented. The example discussed shows that the selected alloys result in a reasonable operation of the two-stage heat transformer. Three different evaluation criteria viz. coefficient of performance, alloy output and temperature output, are introduced to compare the operational characteristics of heat transformers with different alloys; the influence of some metal hydride properties on the operational characteristics is also discussed.     

Thermodynamic design data for absorption heat transformers—part III. Operating on water-lithium iodide

Derived thermodynamic design data, including Carnot coefficients of performance, enthalpy-based coefficients of performance with and without an economizer heat exchanger and flow ratios for possible combinations of operating temperatures, are calculated for absorption heat transformers operating on a system in which water is used as the working fluid and aqueous lithium iodide solution as absorbent. The effects of operating temperatures as well as economizer heat exchanger on the performance of the absorption heat transformer are illustrated graphically.     

Thermodynamic design data for absorption heat transformers. Part six: Operating on water-carrol

The Gibbs phase rule and thermodynamic properties of the working pair limit the choice of operating temperatures. For any combination of temperatures, the concentrations in the absorber and the generator and hence the flow ratios are fixed. For any particular working pair, the coefficient of performance is related to the flow ratio.Tables of possible combinations of operating temperatures, including flow ratios, Carnot coefficients of performance nd enthalpy based coefficients of performance have been presented for absorption heat transformers operating on water-carrol (lithium bromide-ethylene glycol). The interaction of operating temperatures has been illustrated graphically.     

中低温余热与甲醇化学间冷相结合热力循环研究

本文基于探讨中低温余热与清洁合成燃料间接燃烧相结合的能量释放新思路,揭示同时降低燃烧过程品位损失和提高低品位能的功能力的机理。采用图象佣分析方法,明确地指出甲醇间接燃烧佣损失减少的原因。从系统集成角度,探讨燃气轮机循环中利用压气机间冷的低温热与甲醇吸热裂解相结合的热力循环,并研究和揭示该循环热力特性规律。本文为能量释放新机理的研究和构思新颖中低温余热的热力循环提供基础依据。     

地源热泵地下换热系统的实验研究

地源热泵地下换热系统对于地源热泵系统的稳定运行和地源热泵系统的投入成本起着关键的作用.为了对地下换热系统换热效果和周围土壤的温度场进行实验研究,我们在北京工业大学高科技能源楼建立了一套包含60套不同结构地下换热系统的实验台.利用温度及流量测试装置获得运行过程中温度变化并计算换热量,探求不同结构地下换热系统的换热情况.本实验台还可以收集系统运行过程中地下换热系统的传热温度扩散半径,实验系统不仅为地源热泵的设计提供了数据,而且为地源热泵的深入研究提供了平台.本文给出的部分实验结果证明,根据当地地质情况、负荷需求及系统运行模式配置能源井是至关重要的.     

不可逆三源化学势变换器性能的优化分析

建立受传质和质量漏不可逆性影响的一类三源化学势变换器的新循环模型,分析这些不可逆性对化学势变换器性能的影响,导出性能系数与泵能率间的基本优化关系和其它参数的优化表式,同时确定了化学势变换器一些重要参数的最佳工作区域和性能界限。     

Thermodynamic design data for absorption heat transformers—part four. operating on ammonia-lithium nitrate

The Gibbs phase rule and the thermodynamic properties of the working pair limit the choice of operating temperatures. For any combination of temperatures, the concentrations in the absorber and the generator and hence the flow ratios are fixed. For any particular working pair, the coefficient of performance is related to the flow ratio.Tables of possble combinations of operating temperatures and concentrations, including flow ratios, Carnot coefficients of performance and enthalpy based coefficients of performance have beenpresented f for absorption heat transformers operating on ammonia-lithium nitrate. The interaction of operating temperatures has been illustrated graphically.     

Numerical simulation of heat transfer and fluid flow of Water-CuO Nanofluid in a sinusoidal channel with a porous medium

This study has compared the convection heat transfer of Water-based fluid flow with that of Water-Copper oxide (CuO) nanofluid in a sinusoidal channel with a porous medium. The heat flux in the lower and upper walls has been assumed constant, and the flow has been assumed to be two-dimensional, steady, laminar, and incompressible. The governing equations include equations of continuity, momentum, and energy. The assumption of thermal equilibrium has been considered between the porous medium and the fluid. The effects of the parameters, Reynolds number and Darcy number on the thermal performance of the channel, have been investigated. The results of this study show that the presence of a porous medium in a channel, as well as adding nanoparticles to the base fluid, increases the Nusselt number and the convection heat transfer coefficient. Also the results show that As the Reynolds number increases, the temperature gradient increases. In addition, changes in this parameter are greater in the throat of the flow than in convex regions due to changes in the channel geometry. In addition, porous regions reduce the temperature difference, which in turn increases the convective heat transfer coefficient.     

小型通风换热器的实验研究

对自行设计加工的用于回收空调系统中余热的通风换热器进行了实验研究,并在冬季工况下对其工作特性进行了性能测试,得到了空调房间室内外温差和不同迎面风速对换热器的换热性能和流动阻力特性的影响。实验结果表明,该换热器具有换热效率高、阻力小的特点。其换热效率达到64．5％,具有明显的节能效果。     

Optimal choice of the performance parameters of an absorption heat transformer

The optimal performance of an absorption heat transformer, i.e. a type II absorption heat pump, is investigated by using the cyclic model with continuous flow. The effect of thermal resistances between the heat transformer and the heat reservoirs is considered in the model. A general expression related to the rate of heat-pumping, the coefficient of performance and the overall heat transfer area of the heat transformer is derived. The expression is used to optimise the main performance parameters of the heat transformer. The maximum rate of heat-pumping and the corresponding coefficient of performance are calculated. For a given overall heat transfer area of the heat transformer, the optimal relation of the heat transfer areas of the heat exchangers is obtained. The problems concerning the optimal choices of other performance parameters are discussed. The results obtained here can not only enrich the theory of finite time thermodynamics, but also provide some new theoretical bases for the optimal design and operation of real absorption heat transformers.     

变阻抗传输线能量传输效率分析

采用数值方法分析了阻抗各向同性、波速不变的变阻抗传输线（指数型、双曲型和线性型）的电压反射系数,得到了能量传输效率与归一化频率、阻抗变比之间的关系。结果表明,在低频条件下(归一化频率小于2),指数传输线的能量传输效率最高。计算了Z300装置变阻抗传输线的能量传输线效率,指数线为91.6%、双曲线为90.4%,圆锥线为91.2%。     

Thermal modelling and energy conservation studies on freon rankine cycle cooling system with regenerative heat exchanger

This communication presents a thermodynamic analysis and assessment of a Freon fluid Rankine cycle cooling system. The system consists of two subsystems—Rankine engine (RE) power cycle and a vapour compression (V-C) refrigeration cycle. The heat engine subsystem consists of a boiler, turbine, condenser and a feed pump while the cooling subsystem consists of a mechanical compressor, condenser, evaporator and an expansion valve. A number of working fluid combinations for the RE cycle and V-C cycle subsystems have been chosen on the basis of their thermodynamic properties and their suitability judged in terms of the performance parameters, namely, the thermal efficiency of the power cycle and the coefficient of performance (COP) of the refrigeration cycle. A regenerative heat exchanger (RHE) is incorporated in the RE cycle to improve the cycle efficiency and achieve energy conservation.The effects of various operation parameters, namely, component temperatures, adiabatic expansion/compression efficiencies and effectiveness of the RHE on the overall COP have been assessed. It is found that R114 + R22 give the best overall system performance and the presence of the RHE improves the system COP significantly. The effect of V-C cycle condenser temperature is more pronounced as compared to that of the RE cycle condenser and similarly the effect of evaporator temperature in the V-C cycle is more pronounced as compared to that of the boiler in the Rankine cycle subsystem.