CPC内聚光式热管集热管温度特性研究

为提高太阳能热水系统的输出温度,将CPC聚光技术应用于热管式真空集热管中,开发了一种新型的CPC内聚光式热管集热管。对该集热管建立数学模型,模拟计算其传热过程,获得了导热肋片温度、热管冷凝段温度等参数随太阳辐射强度的变化规律,并通过试验验证了数学模型的可靠性;与常规热管式真空管集热管传热特性相对比,证实了该集热管可大幅提高太阳能热水器输出温度。     

真空玻璃盖板热管平板式太阳能热水器的研制

介绍了研究成功的采用真空玻璃盖板的热管平板式太阳能热水器,测试了其性能,并与全玻璃真空管太阳能热水器和蜂窝热管太阳能热水器进行了比较。实测得到,真空玻璃盖板热管平板式热水器的日平均效率比后两分别大13．3％和6．5％,平均热损系数比后两分别小52．5％和21．5％。真空玻璃盖板平板式太阳能热水器性能优越,有很好的应用前景。     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.     

An experimental heat transfer study for periodically varying-curvature curved-pipe

Experiments were conducted for water flowing through a varying-curvature curved-pipe inside a larger diameter straight pipe to form a double-pipe heat exchanger with water as the working medium. The heat transfer coefficients were obtained using the Wilson plot method. The effects of the Dean, Prandtl, Reynolds number and the curvature ratio on the average heat transfer coefficients and the friction factors are presented. A higher Dean number results in a higher heat transfer rate. It is found that the heat transfer rate may be increased by up to 100%, as compared with a straight pipe, while the friction coefficient increased by less than 40%. Therefore, it is promising to use S-shaped pipes instead of straight pipes for the performance enhancement for a heat exchanger such as a solar collector.     

Theoretical investigation of the performance of a novel loop heat pipe solar water heating system for use in Beijing,China

A novel loop heat pipe (LHP) solar water heating system for typical apartment buildings in Beijing was designed to enable effective collection of solar heat, distance transport, and efficient conversion of solar heat into hot water. Taking consideration of the heat balances occurring in various parts of the loop, such as the solar absorber, heat pipe loop, heat exchanger and storage tank, a computer model was developed to investigate the thermal performance of the system. With the specified system structure, the efficiency of the solar system was found to be a function of its operational characteristics - working temperature of the loop heat pipe, water flow rate across the heat exchanger, and external parameters, including ambient temperature, temperature of water across the exchanger and solar radiation. The relationship between the efficiency of the system and these parameters was established, analysed and discussed in detail. The study suggested that the loop heat pipe should be operated at around 72 °C and the water across the heat exchanger should be maintained at 5.1 l/min. Any variation in system structure, i.e., glazing cover and height difference between the absorber and heat exchanger, would lead to different system performance. The glazing covers could be made using either borosilicate or polycarbonate, but borosilicate is to be preferred as it performs better and achieves higher efficiency at higher temperature operation. The height difference between the absorber and heat exchanger in the design was 1.9 m which is an adequate distance causing no constraint to heat pipe heat transfer. These simulation results were validated with the primary testing results.     

Performance analysis on a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation

In China, because of the emergence of a large number of high‐rise buildings, the solar hot water heater system often uses the balcony wall‐mounted method for installation. The thermoelectric energy converter is proposed as one of the possible technologies to incorporate solar water heater to produce electricity for building application. In this paper, the conceptual development and theoretical analysis of a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation are all proposed. The new system takes into account many advantages, including the high heat transfer, low convective heat loss, and low contact thermal resistance. The exergy analysis method based on the second law of thermodynamics is also introduced to evaluate the performance of this system. The results show that a novel micro‐channel heat pipe evacuated tube solar collector‐incorporated thermoelectric generation has a high thermal performance with addition of electricity production. Copyright © 2016 John Wiley & Sons, Ltd.     

聚光太阳能吸热管的吸热传热特性

基于太阳能选择性吸收涂层的辐射性能,建立聚光太阳能吸热管光热耦合传输的数理模型,理论研究聚光太阳能吸热管的吸热传热特性。研究表明,吸热管壁温度随着聚光能流密度增加而线性升高,而吸热效率在中等聚光能流密度时达最大值。太阳能选择性吸收涂层性能对吸热传热有重要影响,具有低红外发射系数涂层的系统吸热效率明显较高,而红外辐射能量损失率则在中等聚光能流密度时最小。管内强迫对流可以显著提高吸热管效能,吸热效率随流速增加而提高,而管壁温度则显著下降。     

Fabrication and testing of a non-glass vacuum-tube collector for solar energy utilization

An evacuated tubular solar collector was fabricated from acrylics for improved resistance to shattering. A plasmatron was employed to apply a thin gas-barrier coating to the surfaces of the plastic tube to prevent/alleviate gas infiltration. Experiments were conducted to investigate the effect of vacuum level on the performance of the non-glass vacuum-tube solar collector. Inserted in the evacuated tube was a finned heat pipe for solar energy collection and heat transfer to a water tank. Time variations of temperatures on the heat pipe surface and in the water tank were recorded and analyzed for different degrees of vacuum in the collector. The steady-state temperature of the non-glass collector was compared to that of a commercial glass vacuum-tube collector to assess the feasibility of the use of evacuated plastic tubes for solar energy collection. A simple analytical model was also developed to assist in understanding and analyzing the transient behavior and heat losses of the vacuum-tube solar collector.     

The theoretical and experimental investigation of the phase-change solar thermosyphon

The paper is concerned with the study of two-phase solar thermosyphon for domestic hot water (SDHW). Due to a combination of ideas of the heat pipe and classical thermosyphon, the heat transfer in the proposed installation is to be realised by the liquid - gas phase change, what should increase significantly both the heat efficiency of the system and the system applicability.     

Development of heat pipes for solar water heaters

Numerous heat pipes were designed, manufactured, and filled on a specially developed filling rig. Each heat pipe was incorporated into a prototype solar water heater developed for this purpose, and was tested under actual insolation condition. An extensive testing program lasting for more than a year revealed that the heat pipes perform satisfactorily as heat transfer elements in solar water heaters.A special heat pipe featuring a compact and effective condenser configuration was also tested. It was observed to likewise exhibit isothermal behavior and hence promised potential for large scale solar applications.     

Solar flux distribution study in heat pipe cavity receiver integrated with biomass gasifier

Technological advances have taken place in the field of solar receivers, gasifiers, and heat pipes, however, the integration of these technologies is not significantly available. In this paper, the conceptual design of a novel biomass gasifier is presented. The system facilitates the solar capture and gasification process separately. It is fitted with a heat pipe arrangement to transfer heat from the solar receiver zone to the gasifier zone. Collection of heat pipes comprised of few straight tubes and few innovative semi-‘S’ shaped tubes. Solar receiver geometry is modified to semi-cylindrical shape and the evaporator section of heat pipes is arranged circumferentially inside the solar receiver. The conventional gasifier is modified with an arrangement to distribute uniform solar heat throughout the fixed bed of biomass feedstock. This paper aims to present the optical analysis of the proposed heat pipe embedded solar receiver. Heat pipe disposition inside the cavity, receiver positioning on focal planes and slope error are varied to perform optical performance of the proposed solar reactor. Average solar flux is found to increase up to 1.1-fold to 1.7-fold placing cavity receiver below focal height by 16 and 32 mm respectively. Also, the magnitude and flux profiles incident on surfaces are affected with concentrator slope error. Average flux reduces up to 21.7% with 4 mrad as compared to 2 mrad error.     

Optimization of a wickless heat pipe flat plate solar collector

The transient thermal behaviour of wickless heat pipe flat plate solar collectors has been analyzed with regard to various parameters. These parameters include global solar radiation intensity, inlet cooling water temperature, absorber plate material and thickness, ratio of pitch distance to wickless heat pipe diameter and ratio of condenser section length to total wickless heat pipe length. The results show that the selection of an absorber plate having a high value of conduction heat transfer rate per unit temperature difference is limited by the pitch distance.     

新型拐角式整体针翅回转热管设计与试验

提出了一种新型拐角式整体针翅回转热管,对该热管进行了详细的理论分析与设计,并对其传热性能进行了试验测试。结果表明:拐角式整体针翅回转热管的轴向温度从蒸发段到冷凝段逐渐降低,最大轴向温差随着转速的增大而减小;回转热管蒸发段的管壁温度沿圆周方向上的温差随转速的增大而变大,冷凝段的管壁温度沿圆周方向上的温差随转速增大而变小;回转热管的整体传热功率随转速的提高而增大;当充液率约为15%时回转热管的热阻最小,传热性能最好;吸液芯对热管传热性能的影响不占主导地位,在低转速工况下吸液芯提高热管传热能力,在高转速下则起阻碍作用;与平行轴回转热管相比,拐角式回转热管传热性能提高了5倍。     

Experimental performance analysis and optimisation of medium temperature solar thermal collectors with silicon oil as a heat transfer fluid

Heat transfer fluids (HTFs) play an essential role in solar water heating systems by transferring collected energy from the collector, perhaps via a heat exchanger to the store. If the store is at a much higher temperature than the fluid, the store acts as a heat source, whereas the fluid acts as a coolant, thus reversing the collection process. This action must be avoided through good controls. Experimental performance analysis and comparison of three different types of solar collectors; a non‐concentrating evacuated tube heat pipe and two concentrating single‐sided and double‐sided coated evacuated tube heat pipes collectors are installed and tested using Dow‐corning 550® silicon oil as an HTF under the same operating in‐door control conditions, and results are presented in this paper. The performance of these solar collectors was determined from the overall increase in inlet and outlet fluid temperatures, overall fluid temperature differential, energy collection rate, optical efficiencies, and thermal performances. Temperature differential, energy, and collection efficiency diagrams plotted against time were used to represent and compare the solar collectors. Finally, a comparative analysis of these solar collectors using either pressurised water or Dow‐corning 550 silicon oil as HTF is presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Heat-pipe enhanced solar-assisted heat pump water heater

A heat-pipe enhanced solar-assisted heat pump water heater (HPSAHP) is studied. HPSAHP is a heat pump with dual heat sources that combines the performance of conventional heat pump and solar heat pipe collector. HPSAHP operates in heat-pump mode when solar radiation is low and in heat-pipe mode without electricity consumption when solar radiation is high. HPSAHP can thus achieve high energy efficiency. A prototype was designed and built in the present study. An outdoor test for a HPSAHP in the present study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as compared to the heat-pump mode COP (2.58).     

Heat transfer performance and exergetic optimization for solar receiver pipe

The basic physical model of solar receiver pipe with solar selective coating is established, and associated heat transfer and exergetic performances are analyzed and optimized. Because of the heat losses of natural convection and infrared radiation, the energy absorption efficiency has a maximum at optimal incident energy flux. As the pipe radius decreases or flow velocity rises, the wall temperature drops for higher heat transfer coefficient, while the heat absorption efficiency increases. Along the flow direction, the heat absorption efficiency almost linearly decreases, while the exergetic efficiency will first increase and then decrease. As the inlet temperature rises, the heat absorption efficiency of the solar receiver pipe decreases, while the exergetic efficiency of absorbed energy obviously increases, so the exergetic efficiency of incident energy will reach maximum at the optimal inlet temperature. Additionally, the maximum exergetic efficiency of incident energy and optimal inlet temperature both increase with flow velocity.     

氧化石墨烯对脉动热管传热性能的影响

实验选用外径为4mm、内径为2mm的铜质脉动热管研究了氧化石墨烯对以去离子水和体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响。实验分别采用加有少量氧化石墨烯的去离子水溶液(简称氧化石墨烯水溶液)和体积分数为50%的乙醇溶液(简称氧化石墨烯乙醇溶液),氧化石墨烯质量分数均为0.03%。实验发现：氧化石墨烯对以去离子水为工质的脉动热管传热性能具有强化作用,对以体积分数为50%的乙醇溶液为工质的脉动热管传热性能的影响较差,但都和脉动热管的加热功率密切相关。对于以去离子水为工质的脉动热管,在加热功率低于20W时,氧化石墨烯对脉动热管的强化作用较弱；当加热功率在30～60W之间时,氧化石墨烯对脉动热管的强化作用较强,在3.71～11.33%之间,且强化作用随加热功率的增大呈逐渐增强趋势；但随着功率继续增大,氧化石墨烯的强化作用逐渐减弱,当加热功率达到80W后,热管传热性能减弱,原因可能是氧化石墨烯颗粒出现了沉降现象。     

Al-Si合金与高温热管的传热性能试验

提出了一种基于太阳能碟式聚光器的Al-Si合金储能锅炉的构想,搭建了Al-Si合金与高温热管传热的实验平台。试验结果表明,Al-Si合金与高温热管之间的传热密度为54.4 kJ/m2。对Al-Si合金的传热温度分布进行模拟,以热流密度为54.4 kJ/m2,换热系数为200 W/(m2.K),空气温度25℃的对流边界条件时,模拟结果和试验测试结果比较吻合,为Al-Si合金储能锅炉设计提供了依据。     

基于相变微胶囊悬浮液的太阳能光伏热系统的研究进展

在太阳能光伏热系统中,光伏电池温度过高会导致太阳能发电效率下降。相变微胶囊悬浮液(MEPCMS)是一种潜热型功能性流体,将其作为冷却介质用于太阳能光伏热系统可以有效降低光伏电池温度,提高系统的能量利用率。针对相变微胶囊易泄露、导热性差等问题提出了改性方法,使其具有光热转换功能并提升了综合性能。基于性能评价指标分析了太阳能光伏热系统性能的影响因素。结果发现,流速、浓度和太阳辐照量是影响MEPCMS在太阳能光伏热系统中换热性能的关键因素。适当增加MEPCMS浓度和流速能提高工质的换热性能,在降低光伏板温度的同时增加太阳辐照量和系统热电产量,但需结合太阳辐照量大小合理匹配工质的浓度和流速。未来研究方向可集中在提升MEPCMS在太阳能光伏热系统中的换热性能、探究运行参数和太阳辐照量之间的匹配关系、优化集热器结构、利用其蓄热性解决太阳能间歇性等方面。