A review of the applications of heat pipe heat exchangers for heat recovery

The waste heat recovery by heat pipes is accepted as an excellent way of saving energy and preventing global warming. This paper is a literature review of the application of heat pipes heat exchangers for the heat recovery that is focused on the energy saving and the enhanced effectiveness of the conventional heat pipe (CHP), two-phase closed thermosyphon (TPCT) and oscillating heat pipe (OHP) heat exchangers. The relevant papers were allocated into three main categories, and the experimental studies were summarized. These research papers were analyzed to support future works. Finally, the parameters of effectiveness of the CHP, TPCT and OHP heat exchangers were described. This review article provides additional information for the design of heat pipe heat exchangers with optimum conditions in the heat recovery system.     

Heat pipe heat exchanger for heat recovery in air conditioning

The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air in air conditioning applications. Two streams of fresh and return air have been connected with heat pipe heat exchanger to investigate the thermal performance and effectiveness of heat recovery system. Ratios of mass flow rate between return and fresh air of 1, 1.5 and 2.3 have been adapted to validate the heat transfer and the temperature change of fresh air. Fresh air inlet temperature of 32–40 °C has been controlled, while the inlet return air temperature is kept constant at about 26 °C. The results showed that the temperature changes of fresh and return air are increased with the increase of inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections are also increased to about 48%, when the inlet fresh air temperature is increased to 40 °C. The effect of mass flow rate ratio on effectiveness is positive for evaporator side and negative for condenser side. The enthalpy ratio between the heat recovery and conventional air mixing is increased to about 85% with increasing fresh air inlet temperature. The optimum effectiveness of heat pipe heat exchanger is estimated and compared with the present experimental data. The results showed that the effectiveness is close to the optimum effectiveness at fresh air inlet temperature near the fluid operating temperature of heat pipes.     

热管热回收装置在空调系统中的应用研究

增加新风量可明显改善室内空气质量(IAQ)，但亦意味着空调系统耗能的增加。与传统的室内热回收装置相比，以热管为传热元件的热回收装置具有节能、高效、紧凑、投资少的优良特征，可提高室内空气质量并降低系统运行维护费用。     

真空管集热器在太阳能空调系统中的应用

一 前言 从节能和环保的角度考虑,用太阳能替代或部分替代常规能源来驱动空调系统,正日益受到世界各国的重视.太阳能空调的最大优点在于季节适应性好,太阳能空调系统的制冷能力随太阳辐射能量的增加而提高,这与夏季人们对空调的迫切要求相匹配.     

Analysis of horizontal mantle heat exchangers in solar water heating systems

The characteristics of horizontal mantle heat exchangers are investigated for application in thermosyphon solar water heaters. An experimental model of a horizontal mantle heat exchanger was used to evaluate the flow patterns in the annular passageways and the heat transfer into the inner tank. Flow visualisation was used to investigate the flow structure, and the heat transfer was measured for isothermal inner tank conditions. A numerical model of the flow and heat transfer in the annular passageway was developed and used to evaluate the heat flux distribution over the surface of the inner tank. The numerical results indicate that configurations of mantle heat exchangers used in current solar water heater applications degrade thermal stratification in the inner tank. The effects of inlet flow rate, temperature and connecting port location are quantified.     

蓄热式热泵在空调系统中的应用

讨论了蓄热式热泵的工作原理、特点及设计方法 ,同时介绍了一个实际工程。指出使用蓄热式热泵既可充分合理利用能源 ,降低用户运行费用 ,又为电网提供了一种调荷避峰的好方式。     

Numerical study of heat pipe application in heat recovery systems

Heat pipes are two-phase heat transfer devices with extremely high effective thermal conductivity. They can be cylindrical or planar in structure. Heat pipes can be embedded in a metal cooling plate, which is attached to the heat source, and can also be assembled with a fin stack for fluid heat transfer. Due to the high heat transport capacity, heat exchangers with heat pipes have become much smaller than traditional heat exchangers in handling high heat fluxes. With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectronics, energy and other industrial sectors.Utilisation of a heat pipe fin stack in the drying cycle of domestic appliances for heat recovery may lead to a significant energy saving in the domestic sector. However, the design of the heat pipe heat exchanger will meet a number of challenges. This paper presents a design method by using CFD simulation of the dehumidification process with heat pipe heat exchangers. The strategies of simulating the process with heat pipes are presented. The calculated results show that the method can be further used to optimise the design of the heat pipe fin stack. The study suggests that CFD modelling is able to predict thermal performance of the dehumidification solution with heat pipe heat exchangers.     

Ground heat exchangers—A review of systems,models and applications

The temperature at a certain depth in the ground remains nearly constant throughout the year and the ground capacitance is regarded as a passive means of heating and cooling of buildings. To exploit effectively the heat capacity of the ground, a heat-exchanger system has to be constructed. This is usually an array of buried pipes running along the length of a building, a nearby field or buried vertically into the ground. A circulating medium (water or air) is used in summer to extract heat from the hot environment of the building and dump it to the ground and vice versa in winter. A heat pump may also be coupled to the ground heat exchanger to increase its efficiency. In the literature, several calculation models are found for ground heat exchangers. The main input data are the geometrical characteristics of the system, the thermal characteristics of the ground, the thermal characteristics of the pipe and the undisturbed ground temperature during the operation of the system. During the first stages of the geothermal systems study, one-dimensional models were devised which were replaced by two-dimensional models during the 1990s and three-dimensional systems during recent years. The present models are further refined and can accommodate for any type of grid geometry that may give greater detail of the temperature variation around the pipes and in the ground. Monitoring systems have been set up to test various prototype constructions with satisfactory results.     

A review of heat pipe systems for heat recovery and renewable energy applications

Advancements into the computational studies have increased the development of heat pipe arrangements, displaying multiphase flow regimes and highlighting the broad scope of the respective technology for utilization in passive and active applications. The purpose of this review is to evaluate current heat pipe systems for heat recovery and renewable applications utility. Basic features and limitations are outlined and theoretical comparisons are drawn with respect to the operating temperature profiles for the reviewed industrial systems. Working fluids are compared on the basis of the figure of merit for the range of temperatures. The review established that standard tubular heat pipe systems present the largest operating temperature range in comparison to other systems and therefore offer viable potential for optimization and integration into renewable energy systems.     

A review on microchannel heat exchangers and potential applications

Energy conversion and utilization are continuous but ever increasing processes for sustainability and economic development. Environmental concerns, such as thermal and air pollution, have dictated the practices of energy conservation and recovery, as well as the implementation of clean energy sources. Heat exchangers are an important component for processes where energy conservation is achieved through enhanced heat transfer. Such issues as increased energy demands, space limitations, and materials savings have highlighted the necessity for miniaturized light‐weight heat exchangers, which provide high heat transfer for a given heat duty. However, while traditional heat exchangers employ conventional tubes (?6 mm) with various cross‐sections, orientations, and even the enhanced surface textures, the technology is nearing its limits. Microchannels (broadly ?1 mm) represent the next step in heat exchanger development. They are a particular target of research due to their higher heat transfer and reduced weight as well as their space, energy, and materials savings potential over regular tube counterparts. In contrast to traditional tube heat exchangers, the heat transfer and fluid flow correlations, and the systematic design procedures are not yet well established for microchannels. It remains to be established whether the classical fluid flow and heat transfer theories and correlations are valid for microchannels. Numerous investigations are underway with researchers consolidating evidence on both sides of this question. This paper surveys the published literature on the status and potential of microchannels, and it identifies research needs, and defines the scope for long‐term research. Based on results from the review, an air‐to‐liquid crossflow experimental infrastructure has been developed and commissioned. It will be used to investigate the heat transfer and fluid flow for a variety of working fluids in different microchannel test specimens. Further information and the heat balance status of the developed test facility are also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

Application of nanofluids in heat exchangers: A review

The purpose of this review summarizes the important published articles on the enhancement of the convection heat transfer in heat exchangers using nanofluids on two topics. The first section focuses on presenting the theoretical and experimental results for the effective thermal conductivity, viscosity and the Nusselt number reported by several authors. The second section concentrates on application of nanofluids in various types of heat exchangers: plate heat exchangers, shell and tube heat exchangers, compact heat exchangers and double pipe heat exchangers.     

Analysis and application of variable conductance heat pipe air preheater

The heat transfer analysis of variable conductance heat pipe air preheater was carried out. The temperature trans-fer matrix was obtained for the air preheater that comprises several discrete heat transfer units with same or different heat transfer surface area in a parallel or counter flow mode. By using the temperature transfer matrix, the outlet fluid temperatures could be easily calculated for a given air preheater and inlet fluid temperatures. The active length of condenser in a variable conductance heat pipe is determined according to the flat interface model. With the same initial conditions, the comparisons between variable conductance heat-pipe air preheater and regular heat pipe air preheater has been analyzed and tested in terms of heat pipe wall temperature, heat transfer surface area and outlet fluid temperatures. Based on the real industrial applications, it has been confirmed that the variable conductance heat pipe air preheater has excellent performance of anti-corrosion and anti-ash-deposition especially at the variable working condition and the sulfur coal (5%-6% mass fraction of sulfur) condition.     

分离式热管在空调系统中的应用现状及前景分析

简要介绍了国内分离式热管技术的研究概况及传热原理。概述了分离式热管换热器用于热泵干燥机的原理及实践中应注意的问题。由于空调系统新风、排风的距离较远 ,应用传统的能量回收装置需要建造较长的气体输送管道使得冷、热气体汇聚到同一地点进行热交换 ,这不仅是不经济的 ,且在实际工程中几乎是不可行的。据此 ,本文提出分离式热管用于空调系统排风的能量回收 ,巧妙利用分离式热管的特点 ,既可避免大流量气体迁移导致的复杂管路设计 ,又能有效回收排风中的低品位能量 ,减少了制冷 /热设备的制冷 /热量 ,从而达到节能的目的。针对空调工程中热源在上、冷源在下及冷、热源相距较远时的热量传递问题 ,提出采用一种新型分离式热管—泵或风机驱动的动力型分离式热管。对分离式热管用于空调系统能量回收作了简要的应用前景分析     

Analysis of enthalpy change with/without a heat pipe heat exchanger in a tropical air conditioning system

In an earlier paper (Yau, 2006. Application of a heat pipe heat exchanger to dehumidification enhancement in tropical HVAC systems—a baseline performance characteristics study. Int. J. Thermal Sci., accepted for publication), the baseline performance characteristics of the 8‐row wickless heat pipe heat exchanger (HPHX) were established for it being used in a vertical configuration under tropical climate conditions. The present paper covers the tests and simulation conducted on the same experimental HVAC system without the HPHX installed, thereby determining the enthalpy change for the air passing through the chilled water coil (CWC) alone (i.e. without the pre‐cooling or reheating effect of the HPHX). These experimental results, in comparison with those already obtained, would also allow an examination of how the reheat recovery with the 8‐row HPHX installed was influenced by the same key inlet parameters. The final results show that the enthalpy change with a HPHX installed for all cases examined are significantly higher than enthalpy change without a HPHX installed, demonstrating that the cooling capability of the CWC was enhanced by the HPHX. Copyright © 2006 John Wiley & Sons, Ltd.     

热管和内螺纹管在低温空气预热器中的应用

结合目前低温空气预热器存在的问题和实际应用的结果，通过对光管、内螺纹管和热管的传热，流动阻力以及壁温（低温腐蚀）性能进行对比分析，阐述了内螺纹管和热管换热器作为空气预热器的优点，并说明了它们的适用条件。     

Improving the performance of air conditioning systems by using phase change materials: A review

This paper reviews the application of phase change materials (PCMs) for improving the performance of air conditioning systems. The different methods of integrating PCMs into air conditioning systems are presented. Moreover, the effects of PCM geometry, flow, and heat transfer characteristics on the performance of air conditioning systems and the potential use of PCMs in increasing the energy savings and coefficient of performance of air conditioning systems are also discussed. Recent studies on the thermodynamic (energy and exergy), economic, and environmental benefits of integrating PCMs into air conditioning systems are reviewed. Several methods for the preparation and optimal selection of PCMs are proposed to improve the performance of air conditioning systems, and then the challenges relating to PCM properties, optimal thickness, and PCM containers are highlighted. The economic aspects, humidity effect, life cycle assessment, and use of solid‐solid PCMs are cited as potentially important topics for future research.     

A comparison of solar absorption air conditioning systems

A computer simulation of solar powered absorption air conditioning systems is discussed. The results of simulations of various systems composed of conventional flat plate or evacuated tube collectors, wet or dry cooling towers, lithium bromide-water or aqua-ammonia working fluids and hot water, chilled water or refrigerant storage alternatives are obtained over a common operating cycle. Performance of the lithium bromide-water working fluid is shown to be superior to aqua-ammonia. Relative performance gains realized with the evacuated tube collector and relative performance losses associated with the dry cooling tower are presented. Chilled water storage is shown to be advantageous for an evacuated collector, dry cooling tower, lithium bromide-water system.     

Various criteria in optimization of a geothermal air conditioning system with a horizontal ground heat exchanger

Thermodynamic and thermoeconomic optimization of a horizontal geothermal air conditioning system has been performed. A model based on energy and exergy analysis is presented here. An economic model of the system is developed according to the Total Revenue Requirement (TRR) method. The objective functions based on the thermodynamic and thermoeconomic analysis are developed. An artificial intelligence technique known as evolutionary algorithm has been utilized for optimization. This approach has been applied to minimize either the total levelized cost of the system product or the exergy destruction of the system. Three levels of optimization including thermodynamic single objective optimization, thermoeconomic single objective optimization and multi‐objective optimization (with simultaneous optimization of thermodynamic and thermoeconomic objectives) are performed. In multi‐objective optimization, both thermodynamics and thermoeconomic objectives are considered, simultaneously. In the case of multi‐objective optimization, an example of decision‐making process for selection of the final solution from available optimal points on Pareto front is presented here. The results obtained using the various optimization approaches are compared and discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental studies on condensation heat transfer of the moist air outside the horizontal circular pipe with a porous layer

INTRODUCTIONWiththedevelopmentoflargespacecraftlikespaceshuttle,spacestation,heatdissipahonofequipmentonboardincreasesrapidly.ThermalcontrolsystemwithliquidloopisnecessarytOregulatetemperatureandheatexchangerhasbeenanindispensableequipmentinthespacecraft.Formannedflight,condensingheatexchangeristheessentialelementtocontrolthetemperatoreandthehumidityinthehabitationmodule.Themainfunchonofthecondensingheatexchangeristoremovetheredundantheatandsteaminthemodule.Therefore,thecondensingheatexc…