A mathematical model of thermal performance of a solar air heater with slats

A mathematical model for computing the thermal performance of a single pass flat-plate solar air collector is presented. Air channels were formed by providing metal slats running along the circulated air passage linking the absorber plate by the bottom one in an endeavor to enhance the thermal efficiency of the solar air collector. A mathematical model, therefore, is developed by which the influence of the addition of the metal slats on the efficiency of the solar collector is studied. A computer code that employs an iterative solution procedure is constructed to solve for the governing energy equations to estimate the mean temperatures of the collector. The effect of volume airflow rate, collector length, and spacing between the absorber and bottom plates on the thermal performance of the present solar air heater was investigated. Furthermore, a numerical comparison of the present design with the most common type of solar air heaters is conducted. The results of the comparison have indicated that better thermal performance was obtained by the modified system.     

The thermal and economic characteristics of solar air collectors with different delta turbulators arrangement

The benefits of using a solar air collector with different array delta turbulators absorber plate are experimentally investigated to assess this type of collector under western and middle of Iraq climate conditions. The solar collector was investigated with four different absorber plate in which flat plate, in‐line delta turbulators, staggered delta turbulators and inclined staggered delta turbulators with different mass airflow rate. The economic characteristics and overall thermal performance of the collector are compared with other heating systems. The major findings show that the delta turbulators enhance the economic characteristics and the overall thermal performance of collectors due to vortex generation and damping the development of the thermal boundary layer in the direction of airflow. A substantial enhancement is observed in lowering both life‐cycle cost and increasing energy saving with delta turbulators. This study will likewise provide a new direction to the work trend in western and middle of Iraq climate conditions during winter days.     

Thermal performance of an air solar collector with an absorber plate made of recyclable aluminum cans

The present paper describes the development and testing of an efficient single-glass air solar collector with an absorber plate made of recyclable aluminum cans (RAC). This collector was designed as a proposal to use recycle recyclable materials to build absorber plates of air solar collectors at an acceptable cost. The absorber plate of the collector consisted of eight circular cross section air flow channels of 128 recyclable aluminium cans. Each channel was built with 16 recyclable cans blackened with common opaque black paint of 0.903 absorptance and 0.097 reflectance. The design parameters to determine the size of the collector were obtained by implementing a simulation model for double flow air solar collectors. Also, to determine the appropriate configuration for a uniform air flow distribution inside the eight RAC air channels, a hydrodynamic numerical study was carried out. The RAC air solar collector designed and built was tested outdoors following the ASHRAE 93-86 standard to determine the time constant, the thermal efficiency and the incidence angle modifier. Comparison between the predicted theoretical temperatures and the measured ones were in good agreement. Comparison between the thermal efficiency of the RAC air solar collector with the ones reported in the literature is presented.     

Heat transfer dynamics in an inflatable-tunnel solar air heater

A mathematical model that describes the dynamics of the heat transfer in an inflatable-tunnel solar collector for air heating is proposed and validated. The model is distributed-parameters, one-dimensional and unsteady-state. It considers the thermal inertia of a pebble bed acting as the absorber surface and is constituted by three equations that describe the temperature distributions of the three system components: polyethylene cover, transfer fluid (air) and absorber surface.To solve the governing equations, a novel numerical scheme that differs from the standard method of finite differences in the form of generating the discretization equations is proposed. In this scheme, the dimensionless versions of the equations are reduced to linear canonical forms of first order and then are solved analytically in small spatial domains to produce discretization equations in an explicit form.To validate the quality of the present model, some experimental tests in a 50 m long inflatable-tunnel solar collector were carried out. Results of the model compare favorably with experimental results.     

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.     

Development and testing of low cost solar energy collectors for heating air

Solar Energy Collector designs utilising plastic films for the absorber and the glazing are developed for air heating applications. The objective is the cost reduction of such collectors with reasonable performance and acceptable life time. The developed collectors were of nonporous as well as porous type, the latter having a special synthetic textile as the absorber. An experimental facility is described to study the performance of the collectors as a function of typical physical parameters and the experimental results are discussed. The collector, with a black porous textile absorber, was found to be much superior to the collector of nonporous type. The thermal performance of the former gets, however, severely affected if the porous absorber is not suitably optimized and if the back of the collector is not properly insulated. Considering life time of a collector as a parameter and taking into account the investment cost, interest rate and the useful energy available from the collector, a simple expression has been developed for rating various collectors. A comparison of the numerical data for the present porous collector and commercially available conventional collector for air heating shows that the present collectors have a better economic potential even if one assigns a short life time to them.     

Thermo-hydraulic performance of solar air heaters having integral chamfered rib roughness on absorber plates

This paper presents results of an experimental investigation of the performance of solar air heaters with chamfered repeated rib-roughness on the airflow side of the absorber plates. The roughened elements have a relative roughness pitch of 4.58 and 7.09 while the rib chamfer angle is fixed at 15°. For the airflow duct depths of 21.8, 21.5 and 16 mm, the relative roughness heights for the three roughened plates used are 0.0197, 0.0256 and 0.0441, respectively. The airflow rate per unit area of absorber plate has been varied between 0.024 to 0.102 kgs⁻¹ m⁻² (flow Reynolds number ranges from 3750 to 16 350). The study shows substantial enhancement in thermal efficiency (10 to 40%) over solar air heaters with smooth absorber plates due to the enhancement in the Nusselt number (50% to 120%). The thermal efficiency enhancement is also accompanied by a considerable enhancement in the pumping power requirement due to the increase in the friction factor (80% to 290%). At low flow rates, corresponding to applications requiring air at a high temperature, the solar air heater with roughness elements having a high relative roughness height, yields a better performance. However, at high flow rates the increase in the pumping power is greater than the relative gain in the energy collection for a greater relative roughness height and, hence, the net gain is higher for smaller roughness heights. At still higher flow rates, the smooth duct air heater has better effective efficiency. A mathematical model for thermal performance prediction of solar air heaters with absorber plate having integral chamfered rib-roughness has been presented. The experimental and predicted values of thermal efficiency lie within ±7% with a standard deviation of ±5.8%.     

Design and optimization of an air heating solar collector with integrated phase change material energy storage for use in humidification–dehumidification desalination

Compared to solar water heaters, high-temperature solar air heaters have received relatively little investigation and have resulted in few commercial products. However, in the context of a humidification–dehumidification (HD) desalination cycle, air heating offers significant performance gains for the cycle. Heating at a constant temperature and constant heat output is also important for HD cycle performance. The use of built in phase change material (PCM) storage is found to produce consistent air outlet temperatures throughout the day or night. In this study, the PCM has been implemented directly below the absorber plate. Using a two dimensional transient finite element model, it is found that a PCM layer of 8 cm below the absorber plate is sufficient to produce a consistent output temperature close to the PCM melting temperature with a time-averaged collector thermal efficiency of 35%. An experimental energy storage collector with an 8 cm thick PCM layer was built and tested in a variety of weather and operating conditions. Experimental results show strong agreement with model in all cases.     

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。     

Performance of plastic solar air heating collectors with a porous absorber

The thermal performance of solar air heaters consisting of a porous textile absorber between two PVC foils has been investigated. The efficiency of the heaters depends strongly on the characteristics of the textile forming the absorber and on the back insulation. For an incident solar radiation of 687 W/m² at the collector's surface, a temperature rise of 16-6°C in the air flowing through the solar collector at a rate of 800 m³/h, was achieved, thus yielding an efficiency of nearly 71 percent. Further it was found that the linear approximation for the Hottel-Bliss equation leads to erroneous estimations for the collector's parameters when the absorber is porous; for the same type of collector with a denser textile as absorber, however, such an approximation yields, as usual, correct numerical values for the characteristic parameters of the collector.     

Technical note Comparative study on the thermalperformances of solar air heater collectors with selectiveand nonselective absorber-plate

The use of a selective absorber in a solar air collector permits the lowering of theforward heat loss by radiation to the environment. This technique improves the thermal heatperformance of the collector, but it raises the cost price of the installation. To increaseperformance, it is essential to reduce the absorber temperature and in consequence the forwardheat loss. To this effect, using finned systems, a fully developed turbulent flow is created, thatcools the absorber and raises the air temperature. That remedies the low thermophysicalproperties of the air and at the same time shows that the use of any black-painted plate, as anonselective absorber, permits the attainment of high thermal efficiencies with low cost.     

V型多通道平板太阳能集热器的热性能研究

提出一种V型多通道平板太阳能集热器,对其建立稳态传热模型,利用Matlab软件编程进行求解,并进行实验测试验证模型的准确性.利用已验证的传热模型,模拟分析V型多通道平板太阳能集热器的结构和物性参数对其热性能的影响,结果表明当透明盖板和吸热体发射率变大时,集热器的集热效率会呈下降趋势;当V型槽顶角、吸热体长度及空气夹层厚...     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

Thermal performance of an n-pass solar air heater

A mathematical model is developed for the prediction of the thermal performance for an n-pass solar air collector. The thermal efficiency and the temperature rise in teh collector are increased by preheating the air within the collector before it comes into contact with the absorber plate. The thermal losses to the ambient, through the bottom plate, are minimized due to the lower bottom plate temperature. The analysis describes how the governing equations are manipulated with the aid of the boundary conditions to obtain a set of equations which is solved numerically. In order to demonstrate the capability of the model, the analysis is applied to investigate the effect of increasing the number of air passes within the collector on the thermal efficiency and the temperature rise for a range of air mass flow rates. The results indicate that the thermal efficiency of the collector increases rapidly and approaches the optical efficiency of the collector as the mass flow rate is increased. In general, the model can be used to analyze the effect of various design parameters on the thermal performance.     

Experimental study of temperature field in a solar chimney power setup

A pilot experimental solar chimney power setup consisted of an air collector 10 m in diameter and an 8 m tall chimney has been built. The temperature distribution in the solar chimney power setup was measured. Temperature difference between the collector outlet and the ambient usually can reach 24.1 °C, which generates the driving force of airflow in the setup. This is the greenhouse effect produced in the solar collector. It is found that air temperature inversion appears in the latter chimney after sunrise both on a cool day and on a warm day. Air temperature inversion is formed by the increase of solar radiation from the minimum and clears up some time later when the absorber bed is heated to an enough high temperature to make airflow break through the temperature inversion layer and flow through the chimney outlet.     

Exergy analysis of single‐flow solar air collectors with different configurations of absorber plates

In the current study, an experimental analysis of exergy performance for different absorber plates is done. Three types of absorber plates are supplied with different fin arrangements with a variable air mass flow rate. The exergy analysis to evaluate the exergy performance of the solar air heaters uses experimental data for conventional and finned solar air collectors with different arrangements of fins. The main aim of the current study is to compare the exergy performance of the conventional solar air collector with those equipped with fins. The introducing of the fins in different arrangements enhances the absorber surface area, which leads to increased heat transfer. Also, fins induce air turbulence in the flow field, which improves the exergy performance of solar air collector. It is found that the exergy reduces and exergy efficiency enhances with increasing the airflow rate. The traditional flat absorber plate has undesirable exergy loss and exergy efficiency for all ranges of airflow rates. Thus, the flat plate collector presents the most substantial irreversibility, for which the exergy efficiency is the least. However, the results show that the exergy efficiency of inclined staggered turbulators is higher than that of in‐line and staggered turbulators. The optimal value of exergy efficiency is recorded at nearly 77% for the solar air collectors equipped with inclined staggered turbulators compared with other types of configurations.     

Techno-economic optimization of a hybrid collector-cum-storage system

A mathematical model is presented for studying the techno-economic performance of a hybrid solar air heating system. The configuration of the system consists of a collector-cum-storage system for solar heating of air, an adjuster unit for bringing down the higher air temperature to the desired limit by mixing fresh air in it, and an auxiliary energy source to provide the deficient amount of heat to the air to attain the desired temperature whenever required. In order to evaluate the performance of the system, numerical calculations have been made corresponding to the climatic data of Delhi and the cost parameters prevalent in the Indian market. Numerical results show that the cost of the useful energy obtained for optimized values of collector area and water-depth is much less than the cost of electrical heating.     

Numerical and experimental study of a solar equipped with offset rectangular plate fin absorber plate

A mathematical model that allows the determination of the thermal performances of the single-pass solar air collector with offset rectangular plate fin absorber plate is developed. The model can predict the temperature profile of all the components of the collector and of the air stream in the channel duct. Into the latter are introduced the offset rectangular plate fins, which increase the thermal heat transfer between the absorber plate and the fluid. The offset rectangular plate fins, mounted in a staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of the absorber plate. They are characterized by high heat transfer area per unit volume and generate the low pressure losses. The experimental results of the air stream temperature will be compared with the results obtained by the theoretical model suggested.