Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

An experimental investigation on the effect of pH variation of MWCNT–H2O nanofluid on the efficiency of a flat-plate solar collector

The effect of pH variation of MWCNT–H₂O nanofluid on the efficiency of a flat-plate solar collector was investigated experimentally. The experiments were carried out using 0.2 wt.% MWCNT with various pH values, 3.5, 6.5, and 9.5, and Triton X-100 as additive. The procedure of ASHRAE standard was used for testing the thermal performance of flat-plate solar collector. Results show that by increasing or decreasing the pH values with respect to the pH of isoelectric point, the positive effect of nanofluid on the efficiency of solar collector is increased.     

Experimental Study on Laminar Forced Convection of Al2O3/Water and Multiwall Carbon Nanotubes/Water Nanofluid of Varied Particle Concentration with Helical Twisted Tape Inserts in Pipe Flow

In this study an experimental investigation has been carried out to analyze the laminar forced convection of Al₂O₃/water and multiwall carbon nanotubes (MWCNT)/water nanofluids through uniformly heated horizontal circular pipe with helical twisted tape inserts. Tests were conducted for varied range of nanoparticle volume concentration (0.15%, 0.45%, 0.60%, and 1%) and helical tape inserts of twist ratios of 1.5, 2.5, and 3. The heat transfer enhancement and the increase of friction factor of nanofluids with helical inserts are compared with that of pure water results with plain tube without inserts. The Nusselt number is found to increase with the increase in Peclet number and nanofluid concentration. The MWCNT/water nanofluids with helical screw tape inserts exhibits higher thermal performance compared to Al₂O₃/water nanofluid. The maximum thermal performance factor was found to be 1.79 and 1.99 for Al₂O₃/water and MWCNT/water nanofluids with helical twisted tape inserts, respectively. The pressure drop for Al₂O₃ nanofluid is found to be higher compared to the MWCNT nanofluid for all the twist ratio of helical screw tape inserts.     

Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of thermosyphon solar water heater system fitted with helical twisted tape of various twist ratios has been performed and presented. The helical twisted tape induces swirl flow inside the riser tubes, which increases the heat transfer and pressure drop. The empirical correlations developed for Nusselt number and friction factor with various twist ratios (Y = 3, 4, 5, 6) are fitted with the experimental data with a discrepancy of less than ±4.54% and ±6.13% respectively. The results are compared with a plain tube collector at the same operating conditions. Conclusions made from the results show that heat transfer enhancement in twisted tape collector is higher than the plain tube collector with minimum twist ratio and gradually decreases with increase in twist ratio. The overall thermal performance of twisted tape collector is found to increase with increase in solar intensity.     

Experimental studies on heat transfer and thermal performance characteristics of thermosyphon solar water heating system with helical and Left–Right twisted tapes

Experimental investigation of heat transfer, friction factor and thermal performance of thermosyphon solar water heater system fitted with helical and Left–Right twist of twist ratio 3 has been performed and presented. The helical twisted tape induces swirl flow inside the riser tubes unidirectional over the length. But, in Left–Right system the swirl flow is bidirectional which increases the heat transfer and pressure drop when compared to the helical system. The experimental heat transfer and friction factors characteristics are validated with theoretical equations and the deviation falls with in the acceptable limits. The results show that heat transfer enhancement in twisted tape collector is higher than the plain tube collector. Compared to helical and Left–Right twisted tape system of same twist ratio 3, maximum thermal performance is obtained for Left–Right twisted tape collector with increase in solar intensity.     

Optimal design of a forced circulation solar water heating system for a residential unit in cold climate using TRNSYS

An indirect forced circulation solar water heating systems using a flat-plate collector is modeled for domestic hot water requirements of a single-family residential unit in Montreal, Canada. All necessary design parameters are studied and the optimum values are determined using TRNSYS simulation program. The solar fraction of the entire system is used as the optimization parameter. Design parameters of both the system and the collector were optimized that include collector area, fluid type, collector mass flow rate, storage tank volume and height, heat exchanger effectiveness, size and length of connecting pipes, absorber plate material and thickness, number and size of the riser tubes, tube spacing, and the collector’s aspect ratio. The results show that by utilizing solar energy, the designed system could provide 83-97% and 30-62% of the hot water demands in summer and winter, respectively. It is also determined that even a locally made non-selective-coated collector can supply about 54% of the annual water heating energy requirement by solar energy.     

Performance Analysis of Flat-Plate Solar Collector Having Silver Nanofluid as a Working Fluid

A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m × 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m² and the inlet temperatures were controlled in the range of 35–65°C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fluid. In our experiments, for 10,000 ppm concentration of silver nanoparticles, the optical characteristic and the thermal loss characteristic of the solar collector, under steady-state condition with a mass flux of 1.2 kg/min-m², were 0.691 and 4.869 W/m²-K, compared with 0.684 and 7.178 W/m²-K, respectively for 1,000 ppm concentration and 0.702 and 8.318 W/m²-K for water. When the flow rate was different from the standard value, the solar thermal characteristics were also improved with the nanofluid.     

Water-in-glass evacuated tube solar water heaters

Evacuated tube solar collectors have better performance than flat-plate solar collectors, in particular for high temperature operations. A number of heat extraction methods from all-glass evacuated tubes have been developed and the water-in-glass concept has been found to be the most successful due to its simplicity and low manufacturing cost. In this paper, the performance of a water-in-glass evacuated tube solar pre-heater is investigated using the International Standard test method ISO 9459-2 for a range of locations. Factors influencing the operation of water-in-glass collector tubes are discussed and a numerical study of water circulation through long single-ended thermosyphon tubes is presented. Preliminary numerical simulations have shown the existence of inactive region near the sealed end of the tube which might influence the performance of the collector.     

Study of the possibilities of thermal performance enhancement of flat plate solar water collectors by using of nanofluids as heat transfer fluid

The current work represents the simulation results of the thermal performances of flat-plate solar collector with heat transfer fluid–nanofluid (SiO₂ + water with 5% concentration) which is obtained experimentally [1]. The dependence of the outlet temperature and gained useful energy of heat transfer fluid (nanofluid) on the flow rates (10, 15 and 20 L/h) at different ranges of incident solar radiation (500–1000 W/m²) was obtained.     

全玻璃真空太阳集热管光—热性能

从能量平衡议程导出全玻璃真空太阳集热管的3个光－热性能参数,即热损系数UTL、闷晒太阳曝辐量H和空晒性能参数Y与真空热管理的罩管玻璃的太阳透射比τ、选择性吸收表面的太阳吸收比a、发射比ε、和集热管内的气体压强p,太阳辐照度、环境温度和真空太阳集热管的几何尺寸等函数关系。理论计算的真空太阳集热管的3个热性能参数与实验的结果比较一致。上述全玻璃真空太阳集热管的光－热性能参数已经被国家标准《全玻璃真空太阳集热管》（GB／T17049－1997）采用。     

Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis

Heat transfer, friction and thermal performance characteristics of CuO/water nanofluid have been experimentally investigated. The nanofluid was employed in a circular tube equipped with modified twisted tape with alternate axis (TA). The concentration of nanofluid was varied from 0.3 to 0.7% by volume while the twisted ratio (y/W) of TA was kept constant at 3. The experiments were performed in laminar regime (Reynolds number spanned 830 ≤ Re ≤ 1990). The uses of nanofluid together with typical twisted tape (TT), TA alone and TT alone were also examined. To evaluate heat transfer enhancement and the increase of friction factor, the Nusselt number and friction factor of the base fluid in the plain tube were employed as reference data. The obtained results reveal that Nusselt number increases with increasing Reynolds number and nanofluid concentration. By the individual uses of TA and TT, Nusselt numbers increase up to 12.8 and 7.2 times of the plain tube, respectively. The simultaneous use of nanofluid and TA improves Nusselt number up to 13.8 times of the plain tube. Over the range investigated, the maximum thermal performance factor of 5.53 is found with the simultaneous employment of the CuO/water nanofluid at 0.7% volume and the TA at Reynolds number of 1990. In addition, the empirical correlations for heat transfer coefficient, friction factor and thermal performance factor are also developed and reported.     

Comparative study of three thermosyphon solar water heaters made of flat-plate collectors with different absorber configurations

The paper presents a comparative study of three solar water heaters made of flat-plate collectors with different absorber configurations. The performance of the three solar water heaters is assessed under the same conditions. The collectors have the same surface area and are glazed. The theoretical model for each collector type, with the transient effects taken into account, is based on a control volume. By considering a small element of the collector in each case and the storage tank, six partial differential equations were developed for each solar water heater and were solved numerically for a cloudy day. This study shows that the thermal performance obtained with the solar water heater using the absorber-pipe lower bond configuration in the solar collector is always greater than the two others. These results showed that the solar water heater made of the absorber-pipe lower bond configuration is more efficient than the other systems.     

Performance of a flat-plate solar thermal collector using supercritical carbon dioxide as heat transfer fluid

Energetic and exergetic performance analyses of flat-plate solar collector using supercritical CO₂ have been done in this study. To take care of the sharp change in thermophysical properties in near critical region, the discretisation technique has been used. Effects of zonal ambient temperature and solar radiation, fluid mass flow rate and collector geometry on heat transfer rate, collector efficiency, heat removal factor, irreversibility and second law efficiency are presented. The optimum operating pressure correlation has been established to yield maximum heat transfer coefficient of CO₂ for a certain operating temperature. Effect of metrological condition on heat transfer rate and collector efficiency is significant and that on heat removal factor is negligible. Improvement of heat transfer rate is more predominant than increase in irreversibility by using CO₂. For the studied ranges, the maximum performance improvement of flat-plate solar collector by using CO₂ as the heat transfer fluid was evaluated as 18%.     

基于微通道的太阳能集热器及其性能模拟

针对传统的平板型太阳能集热器集热效率较低的问题,本文设计了一款微通道集热器,采用数值模拟方法研究了微通道集热器的工作状况,并分析了传统平板型集热器的管中心距在稳态传热条件下对集热器的效率影响。仿真结果表明:相同条件下,平板型集热器的管间距越小,集热效率越高;微通道集热器的平均集热效率比最佳管间距的平板型集热器高9.3%,比常用的两种平板型集热器分别高20.6%、30.6%。该结果有利于优化平板型集热器的设计参数,为微通道集热器的实际应用提供了依据。     

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。     

Numerical investigation on the thermohydraulic performance of a shell-and-double concentric tube heat exchanger using nanofluid under the turbulent flow regime

The heat transfer characteristics of water-based Al₂O₃ nanofluid flowing through the annulus-side of a shell-and-double concentric tube heat exchanger (SDCTHEX) are investigated numerically. The temperature-dependent thermophysical properties of the nanofluid and pure water were used. The heat exchanger is analyzed considering conjugate heat transfer from hot oil flowing in the shell and the inner tube to the nanofluid flowing in the annulus formed between the concentric tubes. The overall performance is assessed based on the thermohydraulic performance. The overall thermohydraulic performance of the SDCTHEX, expressed in terms of the ratio of the overall heat transfer rate to the overall pressure drop with the nanofluid flowing in the annulus, is lower than that obtained with water when compared at constant hot fluid mass flow rates and at different inner tube diameters.     

Performance studies on solar collector with grooved absorber tube configuration using aqueous ZnO–ethylene glycol nanofluids

Solar flat plate collector (SFPC) is regarded as one of the best renewable energy devices to acquire hot water for domestic usage. Though its theoretical efficiency is projected at higher scale, its conversion efficiency is observed to be low due to various collector losses. Hence, to achieve improved performance of collector different techniques for heat transfer augmentation in circular pipes can be adopted. Among passive and active techniques available, passive is preferred over active due to economics and saving in exergy. Hence, in the present study absorber tube configuration is modified with internal grooves in the collector setup to enhance the rate of heat transfer. Also, fluids dispersed with metal oxide particles provide an increase in thermal conductivity such that ZnO-based aqueous EG mixture nanofluid is used as working medium to analyze the performance of collector setup. Also, the change in heat transfer rate and temperature profile of grooved tubes under forced laminar conditions for different working fluids are compared and reported.     

Experimental study of exergetic performance of Al2O3 nanofluid in a double‐pipe heat exchanger fitted by a new modified twisted tape

The use of nanofluids and surface enhancers today are among the new technologies used to increase heat transfer. In this study, heat transfer phenomena in heat exchanger were investigated using Al₂O₃ nanoparticles and modified spiral band as flow turbulator. Results are verified with well‐known correlations. The results show that the tube with cross‐hollow twisted tape inserts has the best exergetic performance for different hollow widths of the tape. Clearance, which is defined as the width between the tube and twisted tape, also affects the heat transfer performance. The smaller the clearance, the better is the exergetic performance. The tube can achieve the best exergetic performance when the number of unilateral twisted tapes is four. The results showed that increasing nanofluid concentration improves exergetic performance.     

太阳能-空气复合热源热泵热水器的性能模拟与分析

介绍了一种新型太阳能—空气复合热源热泵热水装置(SAS-HPWH)。该装置通过使用独特设计的螺旋翅片蒸发管的平板型集热/蒸发器,可以在不同的天气情况下切换运行太阳能热源热泵模式、太阳能与空气双热源热泵模式和空气源热泵模式,制取生活热水。论文主要针对自行设计的一台150L的SAS-HPWH,建立系统的数学模型,并以太阳能输入比例为准则研究系统的运行模式与特性。模拟结果显示该热水器在不同天气特征情况下可高效率地制造55℃热水。论文还分析了太阳辐射、环境温度以及压缩机的容量对系统特性的影响,提出使用变频压缩机,根据不同的天气情况调节制冷剂流量,进一步提高系统的整体性能。     

Performance analysis of solar water heater combined with heat pump using refrigerant mixture

In the research presented in this paper the thermal performance of a solar water heater combined with a heat pump is studied. A solar collector was modified from corrugated metal roofing with a copper tube attached beneath. The performance of the solar water heater was tested, and models for the collector efficiency and storage tank were developed and used for the evaluation of their performance when combined with a heat pump system.