Effect of porous media on the performance of the double-pass flat plate solar air heater

The heat transfer characteristics and performance of the double-pass flat plate solar air heater with and without porous media are studied numerically. The mathematical models described the heat transfer characteristics of the double-pass flat plate solar air heater are derived from the energy conservation equations. The implicit method of finite-difference scheme is employed to solve these models. The effect of the thermal conductivity of the porous media on the heat transfer characteristics and performance is considered. The results obtained from the model are validated by comparison with experimental data of previous researchers. There is reasonable agreement between the present model and experiment.     

Heat-transfer enhancement in double-pass flat-plate solar air heaters with recycle

This work theoretically and experimentally investigates a device for inserting an absorbing plate into the double-pass channel in a flat-plate solar air heater with recycle. This method substantially improves the collector efficiency by increasing the fluid velocity. The results are represented graphically and compared with a downward-type single-pass solar air heater. Considerable improvement in heat transfer is obtainable by employing recycle-type double-pass devices instead of single-pass devices or a conventional double-pass heater with the same flow rate. The absorbing plate location influence on heat-transfer efficiency enhancement and the hydraulic dissipated power increment is also discussed.     

Collector efficiency of upward-type double-pass solar air heaters with fins attached

The collector efficiency of upward-type double-pass flat plate solar air heaters with fins attached and external recycle is investigated theoretically. The double-pass device was constructed by inserting the absorbing plate into the air conduit to divide it into two channels (the upper and lower channels). The double-pass device introduced here was designed for creating a solar collector with heat transfer area double as well as the extended area of fins between the absorbing plate and heated air. Moreover, the advantage of external recycle application to solar air heaters is the enhancement of forced heat convection strength, resulting in considerable device heat transfer performance improvement. This advantage may compensate for the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference).     

Diurnal response of solar air heaters

A time-dependent heat transfer model has been developed to predict the diurnal performance of a conventional solar air heater which consists of a flat passage between two metallic plates through which the heating fluid (air) is made to pass. The effect of various parameters such as the heat capacities of the glass cover, the absorbing plate and the air streams, as well as the heat transfer coefficient from the absorbing plate to the air stream, have been investigated. It is found that the heat capacities of the glass cover and the absorber do not alter the performance characteristics of the solar air heater while the heat capacity of the air stream significantly influences both the rise in temperature of the fluid and the shift in phase. However, the effective heat transfer coefficient from the absorbing plate to the air stream significantly affects the diurnal response of a solar air heater.     

Transient analysis of a solar air heater of the second kind

This paper presents a theoretical analysis along with the experimental validation study of a solar air heater of the second kind. The heater consists of a flat passage between two metallic plates through which the heat transfer fluid air is made to pass by some auxiliary means. Study of the periodic response of different parameters of this solar air heater is attempted. The heat balance equations governing the behaviour of the system are solved explicitly. The results obtained from the analytical expressions for the transient variation of outlet air temperature compare well with experimental data. Predictions are also made regarding effects of different performance parameters of the air heater with variations of air mass flow rate and plate emissivity with the hope of optimizing the collector configuration.     

Investigation on double-glazed solar air heater connected in series with rock bed solar collector-cum-storage system

This paper presents an experimental investigation along with a theoretical model of a double-glazed flat plate solar air heater connected in series with an integrated rock bed collector-cumstorage unit. Predictions are made regarding the effects of heat transfer coefficient and number of glazings on the performance of the air heater. The model fits quite well with the experimental observations.     

Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters

In this paper, the double pass flat and v-corrugated plate solar air heaters are investigated theoretically and experimentally. Analytical models for the air heater with flat and v-corrugated plates are presented. Numerical calculations have been performed under Tanta (latitude, 30^° 47^′ N) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, output power and overall heat losses of the flat and v-corrugated plate solar air heaters are also presented. The effect of mass flow rates of air on pressure drop, thermal and thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are also investigated. The results showed that the double pass v-corrugated plate solar air heater is 11-14% more efficient compared to the double pass flat plate solar air heater. It is also indicated that the peak values of the thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are obtained when the mass flow rate of the flowing air is 0.02 kg/s.     

A review of the performance of double pass solar air heater

Improvement of the thermal performance of a solar air heater can be obtained by enhancing the rate of heat transfer. The thermal efficiency of double pass solar air heater is higher in comparison to single pass with the concept involved of doubling the heat transfer area without increase in the system cost. Numbers of studies have been carried out on the performance analysis of double pass solar air heater provided with heat transfer augmentation techniques viz. using extended surfaces, packed bed, corrugated absorber were reported in the literature and found more increase in the thermal efficiency in comparison to conventional double duct solar air heater. These studies includes the design of double pass solar air heater, heat transfer enhancement, flow phenomenon and pressure drop in duct. This paper presents an extensive study of the research carried out on double pass solar air heater. Based on the literature review, it is concluded that most of the studies carried out on double pass solar air heater integrated with porous media and extended surfaces. Few studies were carried out with corrugated absorber. Further no study has been reported so far on double pass solar air heater with absorber plate artificially roughened from both the sides. Mathematical models based on energy analysis of some configurations of solar air heater have been discussed.     

Thermal performance assessment of recyclic double-pass flat and V-corrugated plate solar air heaters

Double-pass solar air heaters occupy an important place among solar air heating systems, because of minimal heat loss and maximum thermal efficiency with marginal heater size and cost. In the present work, investigations related to the thermal performance predictions have been carried out for double-pass flat and V-corrugated absorber plate solar air heaters under recycle operation. The mathematical models proposed herein are solved using an analytical approach that uses an iterative solution procedure. Furthermore, based on simulation results obtained from the analytical study, the optimum value of the recycle ratio, the mass flow rate, the absorptivity and the emissivity at which the heaters yield the maximum value of the thermal efficiency have been identified and presented using response surface methodology (RSM). The results of RSM revealed that the mathematical models are significant. In addition, results of the present study are validated and compared with previous studies. A reasonable agreement and significant improvement have been achieved.     

Performance of suspended flat plate air heater

In this communication, a heat transfer model to predict the transient behaviour of a suspended flat plate solar collector with constant flow of fluid (air) above the absorber has been presented. A reflecting sheet with an air gap between the absorber plate and bottom insulation to reduce heat loss has been used. The effect, on performance of the air heater, of the parameters viz, spacing between cover and plate, heat capacity of air and absorber plate, flow rate of fluid and collector length have been studied. The effect of changing the averaging inlet temperature with varying collector length has also been studied.     

Optimization of a smooth flat plate solar air heater using stochastic iterative perturbation technique

Due to low heat transfer capability, the thermal efficiency of solar collectors is very low and various techniques are implemented to increase the performance of solar air heaters. There is a need for optimization of design and operating parameters for maximizing the thermal gain from the solar air heating systems. In this paper a stochastic iterative perturbation technique (SIPT) is implemented to obtain the optimized set of different system and operating parameters i.e. the number of glass cover plate, emissivity of the plate, mean plate temperature, rise in temperature, tilt angle and solar radiation intensity for different Reynolds number. The results obtained have also been compared with the results obtained from genetic algorithm and random search global optimization technique for smooth flat plate solar air heater.     

Optimal Porosity in an Air Heater Conduit Filled with a Porous Matrix

In the present study, flow and forced convective heat transfer in an air heater conduit filled with a porous matrix with a uniform constant solar heat flux has been investigated analytically, based on minimal entropy generation principle. While trying to decrease entropy generation due to heat transfer, pressure loss entropy generation increases, which indicates that an optimal porosity value exists. The influence of Reynolds number, fluid properties, constant uniform heat flux, flow, and geometry of the system on the optimum matrix porosity has been investigated. It was revealed that optimum matrix porosity values increase as Reynolds number increases. In the range of the present study, a correlation predicting optimal matrix porosity was proposed using least squares analysis.     

Experiments of a solar flat plate hybrid system with heating and cooling

Based upon the prior research of the solar hybrid water heater and refrigerator, a new flat plate solar hybrid system with heating and cooling was proposed and experimental prototype device was constructed. With this new hybrid system, the heat and mass transfer can be improved effectively both in desorption process and adsorption process. The conventional flat plate solar water heater collector absorber is immersed inside adsorbent bed in the new hybrid system. The experimental results show that not only the cooling effect can be obtained, but also both the sensible heat of the adsorbent bed and the adsorption heat can be recovered effectively to produce hot water for domestic use. The COP of this new flat plate hybrid system can reach 0.11 and the heat efficiency is about 0.45, this achievement has demonstrated an efficient way of the application of solar energy.     

Thermal performance of V-trough solar air heater with the thermal storage for drying applications

It was analysed the thermal performance of V-trough solar air heater with the thermal storage for drying applications. An active V trough solar air heater having concentration ratio of 1.5 with the thermal energy storage is designed and fabricated by using the flat plate non-selective surface embedded by paraffin wax based thermal energy storage. The developed system is tested by thermal performance test at site having the latitude of 12.97° N and longitude of 77.57° E over the period of time with and without the thermal storage. It was found that the incorporation of thermal storage is gives 10% higher thermal performance than without the thermal storage and other existing flat plate air heater. The total cost of the system is INR.30834/- and it saves conventional fuels and saves environment. It was concluded that the proposed system will be more economical if it was hybridized with utilization of the renewable energy sources of heat or waste heat.     

A review on roughness geometry used in solar air heaters

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. Number of geometries of roughness elements has been investigated on the heat transfer and friction characteristics of solar air heater ducts. In this paper an attempt has been made to review on element geometries used as artificial roughness in solar air heaters in order to improve the heat transfer capability of solar air heater ducts. The correlations developed for heat transfer and friction factor in roughened ducts of solar air heaters by various investigators have been reviewed and presented.     

Performance studies on flat plate solar air heater subjected to various flow patterns

The performance of flat plate solar air heater subjected to various flow patterns (over flow, under flow and double pass) were experimentally investigated. An aluminium flat plate of 1.2. × 0.7 m² size was employed as absorber plate. Two glass plates of similar size were used as protection for heat loss from absorber plate to atmosphere. Performance of the solar air heater at various mass flow rates (0.014, 0.0279 and 0.042 kg/s) was also reported. Thermal efficiency is recorded higher during double pass experimental conditions. Heat gained by air is found higher at experimental conditions having double pass when compared with over flow and under flow.     

A review on methodology of artificial roughness used in duct of solar air heaters

In order to enhance rate of heat transfer to flowing air in the duct of a solar air heater, artificially roughened surface of absorber plate is considered to be an effective technique. Investigators reported various roughness geometries in literature for studying heat transfer and friction characteristics of an artificially roughened duct of solar air heaters. In the present paper an attempt has been made to categorize and review the reported roughness geometries used for creating artificial roughness. Heat transfer coefficient and friction factor correlations developed by various investigators for roughened ducts of solar air heaters have also been reported in the present paper.     

蜂窝热管平板式太阳能热水器的实验研究

把蜂窝技术和热管技术结合起来应用于平板式太阳能热水器，可以大大减少热损失，提高日平均效率。经多次实验证明，蜂窝热管平板式太阳能热水器的日平均效率可以比真空管式热水器的高，热损系数则要小很多，具有推广应用价值。     

A review of turbulence promoters used in solar thermal systems

In order to enhance rate of heat transfer to flowing air in the duct of solar air heater and in heat exchanger or in cooling of turbine blade various turbulence generators viz. ribs, baffles and delta winglets are considered as an effective technique. Investigators reported various turbulence generators in literature for studying heat transfer, friction characteristics and flow pattern in heat exchanger and in solar air heater. In the present article an attempt has been made to categorize and review the reported turbulence generators used for heat transfer enhancement in heat exchanger and solar air heater and gives an approach for further research for forced convection from the surfaces with large scale roughness.