Thermal solar water heater with H2O-Al2O3 nano-fluid in forced convection: experimental investigation

This paper describes laboratory experiments with a thermal solar water heater consisting of a flat-plate solar collector and helical coil heat exchanger using Al₂O₃ nano-particles dispersed in water as a working. The experiments were carried out for various nano-particle concentrations, from 0% to 3% (by volume), through forced convection cooling. The experiments were carried out under the climatic conditions of Tanta University, Egypt. The laboratory work has been carried out in actual thermal environment in August 2013. The experiments have an emphasis on the main parameters with impact on the water production temperature. These parameters include the solar radiation, the feed water mass flow rate and the nano-particle volume fraction. The main conclusion is that considerable improvement in the daily solar collector efficiency is obtained with increasing the nano-particle concentration up to 11% for concentration 3% with; this increase in efficiency is bounded by ±10% uncertainty. The outlet water temperature is increased with increasing of nano-particle concentration by 5.46% for concentration 2%. The helical heat exchanger effectiveness is increased by 4.25% for a concentration of 1% with ±13% uncertainty. The helical heat exchanger effectiveness and solar collector efficiency are increased with decreasing the working fluid mass flow rate.     

Numerical analysis on heat transfer characteristics of ionic liquids in a tubular heat exchanger

ABSTRACT

The efficiency of the solar thermal system depends on the performance of the solar collectors. There is a need to operate solar collector at higher possible temperature to attain maximum efficiency limits. However, the performance of the collector system is limited by the heat transfer characteristics of the working fluid called as the heat transfer fluid (HTF). Water is widely used as the HTF in the solar collector, but the major problem of using water as the HTF is its limited operating temperature. The objective of the work is to investigate the heat transfer characteristics of ionic liquids in tubular heat exchanger (HE) suitable for the solar thermal application. The HE was designed for a heat duty of 1?kW based on the thermal transport property of available ionic liquids and the computational fluid dynamics (CFD) analysis was performed. The results indicate that there is only minimal deviation between the assumed and CFD data.     

Experimental investigation of the solar cooker during sunshine and off-sunshine hours using the thermal energy storage unit based on a parabolic trough collector

A solar cooker based on a parabolic trough collector with thermal energy storage (TES) was investigated. In this experimental set-up, solar radiations were focused on the absorber tube and the collected heat was transferred to the solar cooker by natural circulation (thermosiphon) of the working fluid. The water and thermal oil (engine oil) were used separately as working fluids. Acetanilide was used as the TES material in the solar cooker. In day time, the phase change material (PCM) stored heat as well as transferred it to the cooking pot. In evening time, the stored energy by PCM was used to cook the food. The cooking process was carried out with different foods and with variation in the quantity of food. It was found that the temperature of thermal oil was 10–24°C higher than water as the working fluid. The system was able to cook the food twice a day and the rate of evening cooking was higher as compared with noon cooking. Using thermal oil as the working fluid, the quantity of heat stored by PCM was increased by an amount of 19.45–30.38% as compared with water.     

Thermal performance analysis of evacuated tubes solar air collector in Indian climate conditions

The thermal performance of one-ended evacuated tubes solar air collector is experimentally investigated during the winter season at NIT Kurukshetra, India [29 ° 58^′(latitude) North and 76 ° 53 ^′ (longitude) East]. The collector consists of 15 one-ended evacuated tubes with different lengths of directional inner aluminium tubes (inserted tubes) and a manifold channel, with air used as a working fluid. The inlet air flows through the directional inner aluminium tubes as a result of forced convection. In this experiment, evacuated tubes are used for producing hot air corresponding to different lengths of directional aluminium tubes without using any intermediate fluid. The temperature of the outlet air depends on the air flow rate, length of the directional aluminium tube and solar intensity. The maximum temperature difference between outlet air and inlet air at solar intensity 904 W/m² was found to be 72.7 °C with a flow rate of 5.06 kg/h and length of 0.83 m.     

Experimental analysis of thermal performance of evacuated tube solar air collector with phase change material for sunshine and off-sunshine hours

An experimental investigation of an evacuated tube solar air collector coupled to a latent thermal energy store for generating hot air when no solar radiation is incident was undertaken. Acetamide was used as a phase change material (PCM). The latent thermal energy store was integrated with the manifold of the solar collector and water was used as the working fluid transferring solar gain to the air being heated. The maximum measured temperature differential between the heated air and the ambient air was 37°C and 20.2°C during conditions of incident and non-incident solar radiation, respectively. This occurred using a circular fin configuration at a flow rate of 0.018?kg?s^?1. The efficiency at low (0.018?kg?s^?1) air flow rates was 0.05–0.50 times less as compared to high (0.035?kg?s^?1) air flow rates. This system has advantages over systems using sensible storage as it can be used after sunset due to better heat storing capacity of the PCM.     

Year-round numerical simulation of a parabolic solar collector under Lebanese conditions: Beirut case study

A detailed thermal and optical numerical model is developed to simulate the performance of a small-scale parabolic collector having an evacuated receiver line with selective coating, taking into account different energy balances and interactions with the surrounding. An analytical model is developed to estimate the direct, diffuse and global solar radiation intensities on inclined surfaces. The collector performance model was validated using published experimental data. A year-round dynamic simulation for the collector performance under Beirut climatic conditions was carried out with an economic and environmental analysis. The outlet water temperature could reach a maximum of 114°C in July and 52°C in December by employing a collector of about 6 m² aperture area with 0.01 kg/s water flow rate. The maximum daily thermal energy production is attained in July with 22.267 kWh while January exhibits the lowest thermal energy production with 6.704 kWh per day with a maximum thermal efficiency of 72%.     

太阳能平板集热器关键参数对其热性能的影响

为了提升太阳能在建筑能源供给体系中的比重,形式多样的太阳能集热设备及其系统在建筑领域得到了广泛应用。基于机理分析法,建立了具有单层玻璃盖板的管板式太阳能平板集热器的稳态传热模型。并且针对集热管间距、集热管内径、工质入口温度和工质质量流量等关键参数对集热器集热效率的影响特性进行了数值模拟与分析。结果表明,建立的该稳态传热模型是可行的;此外,在其余参数值保持不变的情况下,减小集热管间距或增加集热管内径均可使集热器瞬时效率增大;增大工质入口温度会导致集热器瞬时效率下降;而增大工质质量流量会提升集热器瞬时效率。这些结论对于太阳能平板集热器在太阳能建筑一体化的实际应用中,具有一定的参考作用。     

Performance evaluation of evacuated tube solar domestic hot water systems in Hong Kong

Evacuated tube solar water heaters are increasingly in use in Hong Kong because of their good thermal efficiency and high water temperature achievable as compared to the flat-plate solar water heaters. But so far their thermal performance has not been systematically evaluated and therefore not well known to the users. This paper reports our experimental and numerical works on evaluating the performance of the two common types of evacuated tube solar water heaters for domestic hot-water applications. These are the single-phase open thermosyphon system and the two-phase closed thermosyphon system. Our results show that the daily and annual thermal performance of the two-phase closed thermosyphon solar collector is slightly better than the single-phase open thermosyphon design. But the payback periods of the two are relatively the same because of the higher initial costs of the two-phase closed thermosyphon collector system. Although economically they are less attractive than the flat-plate type collector system, they are suitable for applications in advanced systems with higher temperature demands.     

槽式太阳能集热器聚光面能流分布及传热特性

本文运用蒙特卡罗光线追踪法模拟了LS-2型槽式太阳能集热系统的聚光特性,并以此为边界条件,进一步研究传热工质为THERMINOL55合成导热油时该集热器内管壁和管内流体的温度分布特性和传热特性。结果表明,管壁和管内流体温度分布十分不均匀。并考察了不同导热油以及导热油的流速对传热效率的影响工质流速对管壁温度分布影响较大,当太阳直射辐照为1000 W/m^2,导热油入口温度为160℃,流速为0.05 m/s时,吸热管圆周方向最大温差为235℃左右,当流速增加到0.05 m/s时,最大温差减小到142℃左右。     

Dynamic performances of solar heat storage system with packed bed using myristic acid as phase change material

This paper is aimed at analyzing the thermal characteristics of packed bed containing spherical capsules, used in a latent heat thermal storage system with a solar heating collector. Myristic acid is selected as phase change material (PCM), and water is used as heat transfer fluid (HTF). The mathematical model based on the energy balance of HTF and PCM is developed to calculate the temperatures of PCM and HTF, solid fraction and heat release rate during the solidifying process. The latent efficiency, which is defined as the ratio between the instantaneous released latent heat and the maximum released heat, is introduced to indicate the thermal performances of the system. The inlet temperature of HTF (50 °C), flow rate of HTF (10 kg/min) and initial temperature of HTF (66 °C) were chosen for studying thermal performances in solar heat storage system. The influences of inlet temperature of HTF, flow rate of HTF and initial temperatures of HTF and PCM on the latent efficiency and heat release rate are also analyzed and discussed.     

Experimental study and analysis of air heating system using a parabolic trough solar collector

An experimental study of air heating system was carried out using a parabolic trough collector with a U-tube aluminium heat exchanger. An evacuated tube placed at the focal length of the parabolic trough collected the solar radiations reflected from the surface of parabolic trough. The air was used as a working fluid, which was heated by passing it through a U-shaped aluminium heat exchanger placed inside the evacuated tube. It was found that efficiency of the parabolic trough collector depends on the mass flow rate, solar intensity and use of fins. It was observed that by using fins at a high mass flow rate of 4.557?kg/h, the maximum temperature of 126°C was achieved which is 13.27% more than the maximum temperature obtained without fins. Furthermore, for a low-mass-flow rate of 1.69?kg/h, the maximum temperature obtained was 149°C.     

小型别墅季节蓄热型太阳能供热技术研究

本文首先根据小型别墅的热水供应和采暖需要,建立了小型别墅季节性蓄热太阳能供热系统流程,进而根据某一特定城市的气象数据,进行全年逐时的计算机模拟。在此基础上,研究了全年热导平衡,各月太阳能供热量,不同蓄热容量对全年太阳能保证率、集热器集热效率、水箱温度的影响。并由此得出结论：蓄热容积是集热而积的3倍左右时,集热器年平均集热效率较高,且在进入冬季时可利用水温差较大;且采用大蓄热容积配合分隔水箱的方式,可以有效提高太阳能在冬季的利用。     

Analysis on the optimum matching of collector and storage size of solar water heating systems in building space heating applications

The overall thermal performance of a solar water heating (SWH) system is significantly affected by the mismatch between the temporal distribution of solar radiation and the heating load. Therefore, a favorable correlation between the collector and storage size should be generated based on the dynamic characteristics of the system. This study focuses on the optimal matching of solar collector area with storage volume for an SWH system (with short-term heat storage capability) for a space heating application. A simplified model of an SWH system based on hourly energy flow is established. System control strategy is integrated into the model in a simple manner without sacrificing computing speed. Based on this model, the combined effect of collector area and storage volume on system thermal performance and economy is analyzed, and a simple procedure for determining the optimal system size is illustrated. A case study showed that for an SWH system utilized for space heating application, the optimized ratio between storage volume and collector area is dependent on the total collector area of the system, and is dominated by the requirement of overheating prevention. The minimum storage volume for a specific collector area that can prevent the storage tank from being overheated can be adopted as the optimum storage volume for that collector area. The optimum ratio between storage volume and collector area increases as the collector area increases. Therefore, a trade-off between heat collection and heat loss has to be made while attempting to increase solar fraction by improving collector area.     

Development and performance test of a low-cost hybrid solar air heater

A hybrid energy system named as SAH embraces the features of solar and thermal devices to generate electrical and thermal energies simultaneously. Recently, some SAH has developed with higher efficiency but less feasible to the developing countries due to higher installation cost. Here, a cheaper SAH with more capability of utilising the solar energy and enhanced efficiency by incorporating fin with the SAH has developed. SAH performances have been analysed experimentally by varying the air mass flow rate from 0.0132 to 0.02166?kg/s for the both fin and without fin attached condition. During mass flow rate of 0.02166?kg/s, maximum efficiency for the collector, photovoltaic cell and the overall efficiency of the system has been recorded as 34.25%, 6.48%, and 35.26% with fin and 28.07%, 4.10%, and 29.37% without fin, respectively. This study would assist the researcher for further improvement of the SAH and guide the consumer to select the appropriate SAH.     

Experimental analysis of convective heat transfer on tubes using twisted tape inserts,louvered strip inserts and surface treated tube

This paper analyses and compares the effects on outlet water temperature, heat transfer rate and its thermal efficiency of three different types of tubes such as twisted tape inserted tube, louvered strip inserted tube and a surface treated (laser peened) tube are dealt with different operating parameters, viz., solar insolation, water temperature at the inlet and ambient temperature. Non-concentrate collector (flat plate) is used. A plain or normal tube (non-surface treated) was also tested for comparison. Polystyrene and acrylic glasses were used for reduced heat loss. It is observed that the heat transfer rate and its thermal efficiency tend to increase with the increased ambient temperature and solar insolation with the louvered strip inserts had given the best thermal performance in comparison with the other types.     

Experimental and numerical investigation on thermal and electrical performance of a building integrated photovoltaic-thermal collector system

An experimentally validated computational fluid dynamics (CFD) model of a novel building integrated photovoltaic-thermal (BIPV/T) collector is studied to determine the effect of active heat recovery on cell efficiency and to determine the effectiveness of the device as a solar hot water heater. Parametric analysis indicates that cell efficiency can be raised by 5.3% and that water temperatures suitable for domestic hot water use are possible. Thermal and combined (thermal plus electrical) efficiencies reach 19% and 34.9%, respectively. A new correlation is developed relating electrical efficiency to collector inlet water temperature, ambient air temperature and insolation that allows cell efficiency to be calculated directly.     

Thermodynamical analysis of dry cooling tower and solar chimney power plant combination

Solar chimney power plant is one of the rather new technologies that can produce power from solar energy. Its high stack is one of the important parts of the system in which the differential density of air among its top and bottom sections causes air-flow. Dry cooling towers are used in industries for cooling condensing water by utilising this concept and generating air-flow. The structural and conceptual similarity between these two systems cause the feasibility of their combination and usage of waste energy in industries. In this article, a simple and useful analytical thermodynamic model is improved in order to estimate the thermodynamic flow properties for combining both systems. Results show that the stack height and diameter are effective parameters in recovery of power. Moreover, the results show that the collector area does not remarkably affect the system performance.     

可调节喷射泵运行特性的仿真研究

建立喷射泵数学模型,采用面向对象方法,研究用户阻力、热源出水压力喷嘴出口截面积对可调节喷射泵运行特性的影响。用户阻力增大时,混水比减小,压差比基本呈线性增加,工作流体质量流量保持不变。热源出水压力增大时,混水比增大,压差比减小,工作流体质量流量基本呈线性增加。喷嘴出口截面积增大时,混水比先迅速增加后缓慢减小,压差比增大,工作流体质量流量呈线性增加。     

建筑一体化电热冷联产光伏组件能量特性研究

传统太阳能光伏或光热建筑一体化只能为建筑提供单一电能或热能。通过研究一种集成发电、集热、制冷3种功能的建筑一体化电热冷联产光伏组件,对其夏季工况下能量特性进行了实际检测。结果表明:白天,组件集热同时能有效降低光伏电池温度,组件工作温度高于环境温度约8~16℃,发电和集热效率分别为14.1%~13.7%和40.1%~15.7%;晴朗夜间,组件通过对流和辐射两种传热方式进行散热制冷,总制冷功率为26.0~268.5 W/m~2。电热冷联产光伏组件适合与热泵结合,为建筑提供所需能源。     

太阳能辅助地源热泵供热运行特性研究

介绍了太阳能辅助地源热泵,对其各装置性能进行了研究。分析了地下埋管换热器进出口水温及有、无蓄热水箱对太阳能辅助地源热泵性能的影响。太阳能辅助地源热泵制热性能系数随地下埋管换热器进口水温的升高呈下降趋势,随其出口水温的升高呈上升趋势。随地下埋管换热器出口水温升高,蒸发器传热量增大。当太阳能辅助地源热泵中无蓄热水箱时太阳能集热器的瞬时集热效率高于有蓄热水箱时的瞬时集热效率。就总体效果而言,有蓄热水箱要优于无蓄热水箱,这样可使地源热泵运行更加稳定。