Thermal performance of three solar air heaters

This paper reports the thermal analysis and the results of thermal performance tests of solar air heaters with V-corrugated absorber plates. Six collector units were used to test three collector types: (a) flow over the absorber, (b) flow on both sides of the absorber, and (c) flow under the absorber. Paired simultaneous testing was used for collectors of the same type to check thermal performance differences which might occur due to construction differences or testing variables. Predictions of collector thermal performance, when using temperatures and environmental data from the tests, were in good agreement with the corresponding measured thermal performance. The predicted performance, particularly with solar glass, indicated very good thermal efficiency for vee corrugated absorber plates.     

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 analysis of low thermal conductivity solar energy absorbers

Polymers have been proven to be high potential low-cost materials for the design and mass production not only for ordinary solar water heaters but also for very simple large size, modular solar collectors, suitable for easy erection of large solar heating plants. Their major drawback for solar–thermal conversion applications is their low thermal conductivity, which prohibits their use unless an appropriate absorber design is employed. The low thermal conductivity of polymers has imposed the need of a particular absorber design, which is basically composed of a pair of dark, closely spaced parallel plates at the top of which solar radiation is absorbed, forming a thin channel for the flow of the heat transfer fluid. The aim of the present work is to investigate the particular limitations of this polymer plate absorber design, for a wide range of collector loss and convective heat transfer coefficients between heat transfer fluid and absorber plate. The aim is also to calculate the particular collector efficiency factors and conditions under which the associated collector performance parameters should be modified to account for the finite absorber plate conductance. This conductance was proven to be another decisive absorber design parameter, improper selection of which may probably lead to strong deterioration of the collector efficiency.     

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.     

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。     

Analysis and design of a novel flat plate solar collector

In conventional flat-plate solar collectors, tubes are welded or pressed to a flat-plate. A tubeless flat-plate collector has no tubes welded or pressed to absorber plate. It is merely two-parallel plates made of galvanized steel, and welded to each other. This configuration reduces thermal resistance greatly; a matter which reflects on its efficiency. Those types of collectors proved to be more efficient and economic than conventional flat-plate solar collectors.     

Exergetic optimization of flat plate solar collectors

In this paper, an exergetic optimization of flat plate solar collectors is developed to determine the optimal performance and design parameters of these solar to thermal energy conversion systems. A detailed energy and exergy analysis is carried out for evaluating the thermal and optical performance, exergy flows and losses as well as exergetic efficiency for a typical flat plate solar collector under given operating conditions. In this analysis, the following geometric and operating parameters are considered as variables: the absorber plate area, dimensions of solar collector, pipes' diameter, mass flow rate, fluid inlet, outlet temperature, the overall loss coefficient, etc. A simulation program is developed for the thermal and exergetic calculations. The results of this computational program are in good agreement with the experimental measurements noted in the previous literature. Finally, the exergetic optimization has been carried out under given design and operating conditions and the optimum values of the mass flow rate, the absorber plate area and the maximum exergy efficiency have been found. Thus, more accurate results and beneficial applications of the exergy method in the design of solar collectors have been obtained.     

Theoretical and experimental study of efficiency factor,heat transfer and thermal heat loss coefficients in solar air collectors with selective and nonselective absorbers

The thermal heat performance of a solar air collector depends strongly on the thermal heat loss and the efficiency factor. In order to increase these performances, it is necessary to use a solar air collector which is well insulated and where the fluid flow is fully developed turbulent flow. It needs a high heat transfer between the absorber plate and the fluid to decrease the absorber‐plate temperature and hence the heat loss by radiation from the absorber to the ambient. This increases the efficiency factor. In the present paper, the heat loss and efficiency factor are treated for solar air collectors with selective and nonselective absorber plate. It is shown that the selectivity of the absorber plate cannot play an important role in a well‐insulated solar collector with a fanned system which permits a fully developed turbulent flow and, in consequence, high heat transfer. Copyright © 1999 John Wiley & Sons, Ltd.     

Experimental studies on solar flat plate collector with internally grooved tubes using aqueous ethylene glycol

In order to cope up with the increase in energy demand and decline in fossil fuels, it has become imperative to use renewable resources efficiently. Among these renewable resources, solar thermal energy is abundant in nature. Solar water heating systems are one of the most important applications of solar thermal energy. Providing internal fins to absorber tube is the technique to improve heat transfer augmentation. Hence in the present study, experiments were performed on solar flat plate collector with different cross section of absorber tubes (plain tube and internally grooved tubes with different helix angles) and by varying the mass flow rates of the working fluids. This study reports the experimental results of flat plate collector, where the working fluid is water and aqueous ethylene glycol (50 : 50). Temperature profile of grooved absorber tube will be compared with plain tube. Since conversion efficiency of solar devices is low, the present study mainly focuses on improving the efficiency of solar flat plate collector.     

Comparative analysis of single‐ and dual‐purpose corrugated plate solar collector by force convection

An experimental analysis of dual‐purpose corrugated plate solar collector (DPCPSC) is used to heat water and air simultaneously. Three types of corrugated plate solar collectors (CPSCs) were presented: namely single purpose corrugated plate solar collector (SPCPSC), only water heating of aluminum absorber plate (SPCPSC 1) and copper absorber plate (SPCPSC 2) and DPCPSC type water‐air compound heating of aluminum based absorber plate. In this experimental investigation, the comparative analysis of SPCPSC 1 and 2 at mass flow rate (MFR) of water was 0.025 kg/s (case 1 and 2) and 0.04167 kg/s (case 3 and 4), DPCPSC at MFR of air was 0.011 kg/s while MFR of water was 0.025 kg/s (case 5) and 0.04167 kg/s (case 6), respectively, and DPCPSC at MFR of water and air were 0.04167 and 0.023 kg/s, respectively (case 7), are analyzed. The results indicate that the efficiency of the DPCPSC, of case 6 is 16.74% higher than single purpose system that of case 3 and 6.65% also that of case 4. Comparing the DPCPSC, the optimum efficiency of case 7 is 8.64% higher than case 5 and 1.87% also higher than case 6.     

Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans

This study experimentally investigates a device for inserting an absorbing plate made of aluminium cans into the double-pass channel in a flat-plate solar air heater (SAH). This method substantially improves the collector efficiency by increasing the fluid velocity and enhancing the heat-transfer coefficient between the absorber plate and air. These types of collectors had been designed as a proposal to use aluminium materials to build absorber plates of SAHs at a suitable cost. The collector had been covered with a 4-mm single glass plate, in order to reduce convective loses to the atmosphere. Three different absorber plates had been designed and tested for experimental study. In the first type (Type I), cans had been staggered as zigzag on absorber plate, while in Type II they were arranged in order. Type III is a flat plate (without cans). Experiments had been performed for air mass flow rates of 0.03 kg/s and 0.05 kg/s. The highest efficiency had been obtained for Type I at 0.05 kg/s. Also, comparison between the thermal efficiency of the SAH tested in this study with the ones reported in the literature had been presented, and a good agreement had been found.     

Application of folded sheet metal in flat bed solar air collectors

In the present study the chevron pattern of fold structure produced using a recently developed continuous folding technique is considered for the first time in the application of solar air collectors. An experimental study of two types of flat bed solar air collectors, with flat plate and chevron pattern absorbers, is carried out to investigate their performance over a wide range of operating conditions. A theoretical comparison between flat plate, v-grooved and chevron pattern absorbers is also presented. Under the considered configurations and operating conditions, the chevron pattern absorber is found to be the most efficient and that the flat plate absorber the least efficient. The chevron pattern is found to have higher performance, reaching up to 20% improvement in thermal efficiency and an increase of 10 °C in outlet temperature at some ranges of mass flow rates.     

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.     

All-ceramic solar collector and all-ceramic solar roof

A type of all-ceramic solar collector is introduced. These all-ceramic solar collectors are made from ceramics. The material of absorber coating is V–Ti black ceramic. The solar absorptance of absorber coatings with a reticular formation is in the range of 0.93–0.97, without the attenuation of solar absorptance. The fluid passages are integrated with the absorber plate, which naturally formed in the process of shaping. The integration between fluid passage and absorber plate is good to transfer heat from the absorber plate to the fluid. The thermal efficiency of all-ceramic solar system is more than 50%. The all-ceramic solar system can integrate well with building roof. All-ceramic collector and system are characterized by low cost and long lifetime. Such characteristics reduce the cost of solar energy utilization.     

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.     

Improvement of PV module optical properties for PV-thermal hybrid collector application

Photovoltaic-thermal collectors (or PV-T collector) are hybrid collectors where PV modules are integrated as an absorber of a thermal collector in order to convert solar energy into electricity and usable heat at the same time. In most of the cases, the hybrid collectors are made by the superposition of a PV module on the thermal absorber of a solar collector. In this paper, the approach is different and is to analyze thermal and optical properties related to both PV and solar thermal functions in order to identify an optimum combination leading to a maximum overall efficiency. Indeed, although these two functions do not exploit the same range of radiation wavelengths, thermal and PV functions are not so complementary due to photo-conversion thermal dependency. In this context, an alternative PV cell lamination has been developed with increased optical and thermal performance. The improvements were evaluated around 2 mA/cm² in terms of current density in comparison to a standard module encapsulation. Based on this technique, a real size PV-T module has been built and tested at Fraunhofer solar test facilities. The results show a global efficiency of the PV-T collector above 87% (79% thermal efficiency plus 8.7% electrical efficiency, based on the absorber area).     

Shape optimization for absorber plates of solar air collectors

Solar air heaters can be used for many applications at low and moderate temperatures. There are different factors affecting the solar air heater efficiency, e.g. collector length, collector depth, type of absorber plate, glass cover plate, wind speed, etc. The absorber shape factor is the most important parameter in the design for any type of solar air heater. Increasing the absorber shape area will increase the heat transfer to the flowing air, but on the other hand, will increase the pressure drop in the collector, this increases the required power consumption to pump the air flow crossing the collector. It was most important to find the optimizing angle of the triangular collector. The effect of the change of the absorber shape factor on the collector performance was studied. A theoretical model was constructed for the two types of collectors, taking into account the new parameter, called the absorber shape factor. The results can be used for all types of solar air heaters by changing the value of the absorber shape factor. The optimum angle of the triangular collector was deduced.     

Numerical 3-D heat flux simulations on flat plate solar collectors

A transient 3-D mathematical model for solar flat plate collectors has been developed. The model is based on setting mass and energy balances on finite volumes. The model allows the comparison of different configurations: parallel tubes collectors (PTC), serpentine tube collectors (STC), two parallel plate collectors (TPPC), and other non-usual possibilities like the use of absorbent fluids with semitransparent or transparent plates. Transparent honeycomb insulation between plate and cover can also be modelled. The effect of temperature on the thermal properties of the materials has also been considered. The model has been validated experimentally with a commercial PTC. The model is a useful tool to improve the design of plate solar collectors and to compare different configurations. In order to show the capabilities of the model, the performance of a PTC collector with non-uniformity flow is analysed and compared with experimental data from literature with good agreement.     

Performance modeling of non-metallic flat plate solar collectors

This paper extends the current techniques used in the prediction of flat plate solar collector performance for use in the analysis of non-metallic collectors. An analytical model was developed to study the characteristics of these solar collctors which eliminate the need for metals, glass, and special coatings. Using this model, plate efficiency factors are presented for various common non-metallic absorber plate configurations. A parametric study was conducted with emphasis placed on collector plate thermal conductivity and partial transmittance of glazings to long-wave radiation. From the results of this study, it was shown that it is possible to meet or exceed performance levels of conventional metal tube and fin collectors through the use of non-metallic collectors in the low- to medium-temperature range.