Production of hot air with quasi uniform temperature using concentrated solar radiation

The design of a hot air solar generator for different uses has been simulated while investigating the flow induced by a hot disc placed at the entrance of the open ended vertical cylinder. Ambient air (Pr = 0.7) enters the bottom of the cylinder with constant velocity and temperature, and flows up through the cylinder as a result of natural convection. The cylindrical wall is heated by thermal radiation emitted by the disc. The pressure drop due to acceleration of the flow to the cylinder-inlet causes the appearance of thermosyphon effect around the thermal plume. At the top part of the cylinder, the flow exploration shows the full development of the turbulence and the uniformity of thermal and hydrodynamic fields. The study of the thermal spectral density indicates that the turbulent structures seem to be sufficiently small not to be sensitive to viscosity, but large enough to be sensitive to Archimedes effects.     

太阳能驱动的膜式加热加湿系统性能实验研究

中空纤维膜加湿系统能从根本上解决空气加湿过程中气液夹带的问题.通过搭建太阳能驱动的中空纤维膜加热加湿系统试验台并在冬季进行实验测试,分析出太阳能辐射量、空气体积流量和热水体积流量对系统加热加湿性能的影响.研究发现提高太阳能辐射量和空气体积流量对系统的加湿能力和热性能系数均有积极影响,而前者的影响更为显著.为了获得最好的...     

无盖板渗透型太阳能空气集热器热性能的实验研究

无盖板渗透型太阳能空气集热器是一种易于实现建筑一体化的高效新风预热及干燥装置。为了掌握无盖板渗透型集热器在实际工况下的热性能,建造了集热面积为2.2 m2的太阳能空气加热系统实验台,并在2009年2月至2009年4月对其热性能进行了户外瞬态实验研究。实验结果表明出口空气温度随辐射强度的增加而升高,太阳辐射强度是影响集热器出口空气温度的最重要因素,而室外空气温度的影响极小。空气温升随风量的增加而减小,集热器热效率随风量的增加而增大。在三个测试日中集热器的平均热效率分别为58%、63%和72%,高于普通平板太阳能空气集热器。     

逆流开式发生器内部的热、质传递规律

针对以太阳能加热的空气为携热介质，以LiBr溶液为工作介质的填料塔型开式发生器，建立热质传递数学模型。对2种系统结构形式进行比较，并分别研究太阳能集热板温度、液气比、环境相对湿度以及填料层高度对溶液再生的影响，以揭示此类发生器内热、质传递的规律，为产品开发、设计提供理论帮助。     

Effect of obstacles with different opening means on thermal stratification in hot water storage tanks

为研究具有内置隔板的太阳能蓄热水箱隔板开孔尺寸及位置对其内部热分层效果的影响,对9种隔板开孔位置的太阳能蓄热水箱内温度场进行了数值分析,结果显示：在相同的流动参数及开孔面积条件下,隔板中心开1个圆孔的水箱热分层效果最好。对于多开孔的水箱,开孔位置对水箱内热分层影响不大,但对蓄热量影响显著。对于隔板中心开1个圆孔的水箱,在不同流动参数条件下,冷、热水出口温差随着冷水入口流速的增大呈先增后减的趋势,当冷水入口流速大于0.9 m/s时,减弱了热分层的稳定性。     

Energy and exergy analysis of a new solar air heater with latent storage energy

In this paper, we propose a new solar air heater with a packed-bed latent storage energy system using PCM spherical capsules. At daytime, the solar heating system stored the thermal solar energy as sensible and latent heat, however, at night it restored. Some parameters, such as the global solar radiation and the mass flow rate are varied to investigate their effect on the absorbed, used, and recovered heat from the system. An optimization study using the first and second laws of thermodynamics is also carried out to obtain the energy and exergy efficiencies. The experimental study was conducted, designed, and realized in the Research and Technology Center of Energy (CRTEn) in Tunisia. The experimentally obtained results are used to analyze the performance of the system, based on temperature distribution in different parts of the collectors, absorbed, instantaneous stored and used thermal energy. The daily energy efficiency varied between 32% and 45%. While the daily exergy efficiency varied between 13% and 25%. The effect of the mass flow rate of air on the outlet temperature of the solar air heater is examined.     

Performance analysis of solar chimney using mathematical and experimental approaches

A mathematical model based on one‐dimensional energy and mass balance across the solar chimney has been developed. The air flow characteristics such as exit velocity and temperature are evaluated with respect to the collector inclination angle, hourly solar radiation, ambient temperature, and wind speed. The model is validated by comparing the performance parameters obtained, with the experimental results and also with the experimental data of different geometrical range and environmental conditions from the literature. An average deviation of 8% for exit air velocity and 1.35% for exit air temperature is obtained for the solar chimney with absorber inclination angle 30°, collector area 0.41 m², and chimney height 0.24 m. The experimental daily average and maximum exit air velocity during the month of April are 0.5 and 0.88 m/s, respectively. The predicted optimum operating conditions are 75° inclination angle, 0.63 m² absorber area, and 0.48‐m chimney height. The maximum average exit air velocity and temperature numerically obtained are 0.64 m/s and 331 K, respectively, when operating with optimum conditions. It is observed that the exit air velocity increases 33% by increasing the absorber area from 0.5 to 3 m² for a solar chimney with 0.5 m height. An increase in exit air velocity of 52% was obtained by increasing the chimney height from 0.5 to 3 m for a solar chimney with 0.64 m² absorber area. A reduction in exit air velocity of 4% was observed for the increment in wind flow over the glass cover from 1.5 to 3 m/s. These results confirm that the solar chimney could be designed based on the predicted monthly performance by the present model.     

Performance analysis of a double-pass thermoelectric solar air collector

The thermoelectric (TE) solar air collector, sometimes known as the hybrid solar collector, generates both thermal and electrical energies simultaneously. A double-pass TE solar air collector has been developed and tested. The TE solar collector was composed of transparent glass, air gap, an absorber plate, thermoelectric modules and rectangular fin heat sink. The incident solar radiation heats up the absorber plate so that a temperature difference is created between the thermoelectric modules that generates a direct current. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the absorber plate. The ambient air flows through the heat sink located in the lower channel to gain heat. The heated air then flows to the upper channel where it receives additional heating from the absorber plate. Improvements to the thermal and overall efficiencies of the system can be achieved by the use of the double-pass collector system and TE technology. Results show that the thermal efficiency increases as the air flow rate increases. Meanwhile, the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold side of the TE modules. At a temperature difference of 22.8 °C, the unit achieved a power output of 2.13 W and the conversion efficiency of 6.17%. Therefore, the proposed TE solar collector concept is anticipated to contribute to wider applications of the TE hybrid systems due to the increased overall efficiency.     

Heat transfer correlations for vertical mantle heat exchangers

This paper investigates heat transfer in vertical mantle heat exchangers for application in low flow solar domestic hot water systems. Two new heat transfer correlations for vertical mantle heat exchangers with top entry port and bottom exit ports are developed. The correlations are based on computational fluid dynamic modelling of whole vertical mantle tanks. The correlations are combined with a heat storage model in a simulation program that predicts the yearly thermal performance of low flow solar domestic hot water systems based on mantle tanks. The model predictions of energy gains and temperatures are compared with outdoor measurements and the model is found to give reliable results.     

整体式太阳能空气集热器传热性能分析

研究了整体式太阳能空气集热器的传热性能,建立了稳态假设下的热量传递物理数学模型;通过CFD软件计算,分析了进口空气流速、空气温度、太阳辐照强度对集热器传热性能的影响;计算结果表明:集热器的集热效率随工质流速的增大而增大、随进口温度的升高而下降、随太阳辐照度的波动很小,受太阳辐射强度的影响不大。     

DSG太阳能槽式集热器动态特性

采用数值模拟方法,分析了以水,水蒸气为工质的DSG槽式集热器的动态流动与传热特性.首先建立了管内流体的一维多相流动与传热模型,并利用差分法对该模型进行求解,计算结果与现有文献数据吻合较好.分析了稳态条件下,集热器出口流体工质参数受太阳辐射强度、流体质量流量、人口温度和入口压力的影响规律.在动态分析中,研究了辐射强度变化所导致的出口参数变化特性.从阶跃响应和脉冲响应的分析中得出,虽然热惯性存在,但短期的辐射强度波动对出口温度仍有较大影响,但对出口压力的影响较小.辐射波动将对一次直通DSG系统出口温度产生很大波动.     

Terrific effect of H2 on 3D free convection MHD flow of C2H6O2H2O hybrid base fluid to dissolve Cu nanoparticles in a porous space considering the thermal radiation and nanoparticle shapes effects

In this article, the physical aspects of natural convection magnetohydrodynamic flow of Cu/Ethylene glycol-water nanofluid past a porosity vertical stretching sheet under impact of thermal radiation, shape and slip factor and suction/injection process has been analyzed using Runge- Kutta Fehlberg fifth order (RKF 5) numerical method. The influence of variable, different parameters such as nanoparticles shape factor, named hexahedron and Lamina on temperature and velocity profiles are exemplified quantitatively through graphs. Outputs demonstrate thermal radiation impact causes to produce heat and increase the temperature profile by increasing nanofluid molecules energy. Lamina shape nanoparticle has a greater effect on increasing Nusselt number (Nu) compared to hexahedron.     

Theoretical analysis of some configurations of double exposure solar air heaters

We present a theoretical analysis of three different configurations of double exposure solar air heaters. The heat-balance equations are written in each case and are solved explicitly. The periodic responses have been studied for different combinations of parameters. Measured values of solar radiation in the plane of the collector and the ambient temperature are used in the analysis. The effects of collector length, reflectance and flow rate have been studied.The outlet air temperature increases with an increase in plate length but the instantaneous and average efficiency decrease. The outlet air temperature is approximately the same, for a given model and combinations of parameters, when the ratio of the length of the air heater to the mass flow rate is constant. A double exposure solar air heater performs better than a single exposure air heater if the reflectance for solar radiation is greater than 10%.     

太阳能空气集热器(Ⅰ型)的数学模型研究

用数学模型化方法研究了太阳能空气集热器的集热传热性能,建立了基于二维假设下的流体流动和热量传递的稳定态数学模型,并用有限差分法进行数值求解,计算得到稳态下太阳能空气集热器的速度、温度分布图以及集热效率与空气流量、入口空气温度、入身太阳辐照度和表面盖板透过率的关系。结果表明,对Ｉ型太阳能空气集热器,集热效率随空气流量和表面盖板透过率的增加而增大,随入口空气温度和入射太阳辐照度的增大而减小。计算结果与     

Exergy analysis of integrated photovoltaic thermal solar water heater under constant flow rate and constant collection temperature modes

In this communication, an analytical expression for the water temperature of an integrated photovoltaic thermal solar (IPVTS) water heater under constant flow rate hot water withdrawal has been obtained. Analysis is based on basic energy balance for hybrid flat plate collector and storage tank, respectively, in the terms of design and climatic parameters. Further, an analysis has also been extended for hot water withdrawal at constant collection temperature. Numerical computations have been carried out for the design and climatic parameters of the system used by Huang et al. [Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Sol Energy 2001; 70(5): 443–8]. It is observed that the daily overall thermal efficiency of IPVTS system increases with increase constant flow rate and decrease with increase of constant collection temperature. The exergy analysis of IPVTS system has also been carried out. It is further to be noted that the overall exergy and thermal efficiency of an integrated photovoltaic thermal solar system (IPVTS) is maximum at the hot water withdrawal flow rate of 0.006 kg/s. The hourly net electrical power available from the system has also been evaluated.     

Thermal modelling of asymmetric overlap roof greenhouse with experimental validation

Global solar radiation availability model and thermal model for newly designed asymmetric overlap roof shape (AORS) greenhouse are presented and experimentally validated. Instantaneous solar radiation flux is utilised in a dynamic thermal model to ascertain the hourly plant and inside air temperature. The AORS is also compared with the previously developed two best greenhouse shapes. An experimental validation of both the models is carried out for the measured instantaneous solar radiation, plant and inside air temperature for a typical day in summer at Ludhiana (31°N and 77°E), Punjab, India. During the experimentation, a tomato crop was grown inside the greenhouse. From the results, it can be inferred that an east–west orientation AORS greenhouse should be preferred due to a lesser solar radiation capture in summer months. The predicted plant and air temperatures are in good agreement with the measured data having a root mean square error of 4.69 and 3.7, respectively.     

Concentration solar dryer water-to-air heat exchanger: Modeling and parametric studies

In this paper we present a modeling and parametric studies of a water-to-air heat exchanger. This exchanger is formed of a fan blowing the air to be heated through a battery of smooth tubes where the hot water—coming from solar concentrators—circulates. The heated air is injected into a thermal room to dry the clay bricks.In the first part, we study the most used models in the estimation of the heat transfer and air flow pressure drop across a tube bundle, and subsequently calculate the required transmitted power to the air.In the second part, we focus on the parametric study of the influence of the different geometric parameters of the exchanger on the heat flow rate, the air outlet temperature, the pressure drop and the requested transferred power to the air. The considered parameters are: The water heat flow rate, the heat exchanger compactness, the rows arrangement, the tube diameter, the transverse pitch, the total number of tubes, the number of rows and the air velocity.Simulations have shown that the heat exchanger performance could be improved essentially throughout the design and manufacturing process by modifying the different geometrical parameters and filling certain conditions.     

日照作用对热源建筑自然通风影响的研究

参照变电站主变压器室，建立了双侧开口热源建筑数值模型，通过改变大气透过率（sunshine fraction）的大小改变日照强度，利用FL U EN T进行求解，考察日照作用对热源建筑自然通风流动的影响。计算结果表明：日照作用影响热源建筑室内外热量传递的大小和方向，当大气透过率为0．15时，室内外两侧传递热量达到平衡。较大的日照强度导致通风量增大，室内气流速度分布更均匀，日照作用下不同高度的气流温度受到的主要影响因素不同。     

Experimental study of flow ratets through inclined open thermosyphons

An experimental set-up has been developed for the investigation of flow inside an inclined cylindrical open thermosyphon. A glass cylinder, whose bottom is sealed and whose top is open to a reservoir, is heated from the sides, generating natural convection currents between itself and the reservoir. The set-up simulates the flow inside evacuated tubular solar absorbers. The cylinder walls are heated in two different ways: (1) uniform wall temperature; and (2) differential wall heating, in which one temperature is applied on the upper surface of the cylinder wall, and another temperature on the lower surface. The second scheme of heating attempts to approximate the manner in which absorbers are usually heated under exposure to solar radiation, with the upper surface normally getting more heat than the lower. Axial velocity measurements are done with a laser Doppler anemometer at the central vertical plane of the orifice. Correlations between the velocity and the independent control parameters (Rayleigh number, aspect ratio and mode of heating) are performed.