Flat Plate Solar Air Heater Research Articles

Thermal and effective efficiency based analysis of discrete V-down rib-roughened solar air heaters

A parametric investigation of discrete V-down rib-roughness geometry in the absorber plate of solar air heater has been carried out and results obtained are compared with those of conventional flat plate solar air heater. The thermal and thermohydraulic performance evaluation has been carried out for various values of Reynolds number and roughness parameters of discrete V-down rib-roughness geometry in the absorber plate of solar air heater duct. The performance curves are plotted in terms of Reynolds number and temperature rise parameter. The discrete V-down rib-roughened solar air heater performs better as per thermal efficiency or heat energy gain criteria for the investigated range of Reynolds number and rib-roughness parameters. The thermal efficiency is high for discrete V-down rib-roughened solar air heater due to higher turbulence; however, the pumping power required for air flow also increases. The thermohydraulic performance on the basis of effective efficiency is a more suitable criterion as it takes into account both the heat collected and the pump work required. For Reynolds number range (or temperature rise parameter) generally employed in solar air heaters, the discrete V-down rib-roughened solar air heater gives higher effective efficiency. It was found that there exist optimum roughness parameters of discrete V-down rib for a given Reynolds number (or temperature rise parameter) at which the effective efficiency is the highest. Curves of optimum roughness parameters based on effective efficiency criterion are also plotted.

Evaluation of thermal performance of wire mesh solar air heater

Solar energy is an economical alternative to the today's energy demand. Solar air heater is used for heating the air economically but produces low thermal efficiency. In this study, an attempt has been made to increase the thermal efficiency of solar air heater by using a wire mesh to enhance the heat transfer, thereby increasing the efficiency. The matrix solar air heater with a wire mesh produces higher thermal efficiency over the conventional flat plate solar air heater.

EXTERNAL RECYCLE ON UPWARD-TYPE DOUBLE-PASS SOLAR AIR HEATERS

The external recycle effect on collector efficiency in an upward-type double-pass flat-plate solar air heater is investigated theoretically. Application of recycle to a solar air heater has two conflicting effects: the desirable effect of increasing the heat transfer coefficient and the undesirable effect of decreasing the heat transfer driving force (temperature difference). The theoretical predictions show that more than 150% improvement in collector efficiency is obtainable if external recycling is carried out in a way that the advantage effect compensates for the disadvantage effect. The collector performance improvement increases with increasing recycling ratio, especially when operating at a lower airflow rate with higher inlet air temperature. The collector efficiency obtained with a double-pass device with recycling improves the efficiency obtained with a single-pass device of the same size with recycling.

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

Abstract 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.

Downward-type solar air heaters with internal recycle

The effect of internal-recycle operation on the collector efficiency in flat-plate solar air heaters has been investigated theoretically. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with an internal recycle, where the desirable effect of increasing fluid velocity to decrease the heat transfer resistance compensates for the undesirable effect of decreasing the driving force (temperature difference) of heat transfer, due to the remixing effect at the inlet by recycle operation. The enhancement increases with increasing reflux ratio, especially for operating at lower air flow rate, as well as with higher solar radiation incident and inlet air temperature. Further, the performance in a solar air heater operated with internal recycle overcomes that in the same-size device operated with external recycle.

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).

Performance evaluation of a natural-convection solar air-heater with a rectangular-finned absorber plate

This paper deals with an experimental investigation to evaluate different thermal characteristics of a natural-convection flat-plate solar air-heater with longitudinal rectangular fins array. Having determined the thermal performance of the system a Nusselt number correlation is presented for such finned duct devices. In the presented empirical model which may have industrial applications, solar radiation and ambient temperature have been considered as independent parameters. Other characteristics of the system such as different dimensionless variables, plates and outflow temperatures, efficiency, and mass flow rate have been empirically modeled based on these variables. The particular difference in this study in comparison with the other similar studies is the presentation of an empirical model for rectangular-finned solar air-heaters. This model proposes design concepts and rules of thumb, and demonstrates the calculations of the design parameters. Based on the order of magnitude analysis, solar radiation has been found to be the main parameter which characterizes the thermal behavior of the system. Besides, exergy analysis has been carried out, and optimum conditions in which the system has the highest performance have been determined.

Energy and exergy analysis of a new flat-plate solar air heater having different obstacles on absorber plates

This study experimentally investigates performance analysis of a new flat-plate solar air heater (SAH) with several obstacles (Type I, Type II, Type III) and without obstacles (Type IV). Experiments were performed for two air mass flow rates of 0.0074 and 0.0052 kg/s. The first and second laws of efficiencies were determined for SAHs and comparisons were made among them. The values of first law efficiency varied between 20% and 82%. The values of second law efficiency changed from 8.32% to 44.00%. The highest efficiency were determined for the SAH with Type II absorbent plate in flow channel duct for all operating conditions, whereas the lowest values were obtained for the SAH without obstacles (Type IV). The results showed that the efficiency of the solar air collectors depends significantly on the solar radiation, surface geometry of the collectors and extension of the air flow line. The largest irreversibility was occurring at the SAH without obstacles (Type IV) collector in which collector efficiency is smallest. At the end of this study, the energy and exergy relationships are delivered for different SAHs.

Experimental investigation of three different solar air heaters: Energy and exergy analyses

The present study aims to compare three different types of designed flat-plate solar air heaters, two having fins (Type II and Type III) and the other without fins (Type I), one of the heater with a fin had single glass cover (Type III) and the others had double glass covers (Type I and Type II). The energy and exergy output rates of the solar air heaters were evaluated for various air flow rates (25, 50 and 100 m 3/m 2 h), tilt angle (0°, 15° and 30°) and temperature conditions versus time. Based on the energy and exergy output rates, heater with double glass covers and fins (Type II) is more effective and the difference between the input and output air temperature is higher than of the others. Besides, it is found that the circulation time of air inside the heater played a role more important than of the number of transparent sheet. Lower air flow rates should be preferred in the applications of which temperature differences is more important.

Effect of selective coating on thermal performance of flat plate solar air heaters

Abstract A transient mathematical model was presented for a single pass flat plate solar air heater. This model was based on an analytical solution of the energy balance equations for various elements of the heater. The flowing air temperature was assumed to vary only in the flow direction. The thermal performance of the heater was investigated by computer simulation using the climatic conditions of Jeddah (lat. 21° 42′ N, long. 39° 11′ E), Saudi Arabia. Effects of solar radiation intensity, mass flow rate of the flowing air ( m ˙ f ) and the length ( L ) and width ( b ) of the absorber plat on the flowing air outlet temperature ( T fo ) and the heater instantaneous ( η inst ) and daily ( η d ) efficiencies were studied. To improve the heater performance, effect of using absorber plates coated with various selective coating materials on the heater performance was also investigated. The best performance was achieved using nickel–tin as a selective coating material with a daily average of the instantaneous efficiency of 0.46. To validate the proposed mathematical model, the simulated results were compared with the measurements that had been performed for the heater with a black painted absorber plate under Tanta, lat. 30° 47′ N (Egypt), weather conditions. It was found that the proposed model is able to predict the T fo accurately with a daily average relative percentage error of 7.7%. It was also inferred that the annual average of η d with a nickel–tin selectively coated absorber is higher than that with a black painted absorber by 29.23%.

Experimental investigation of thermal performance of solar air heater having different obstacles on absorber plates

This paper presents the results of an experimental investigation of the performance for a new flat plate solar air heater (SAH) with several obstacles (Type I, Type II, and Type III) and without obstacles (Type IV). The efficiencies, the heat gain factors and heat loss coefficients are determined for the collectors and comparisons were made among them. The experimental data along with the correlations obtained by linear regression are presented. The optimal value of efficiency was determined for the solar air heater with Type II absorbent plate in flow channel duct for all operating conditions and the collector supplied with obstacles appears significantly better than that without obstacles.

Thermal performance optimization of a flat plate solar air heater using genetic algorithm

Thermal performance of solar air heater is low and different techniques are adopted to increase the performance of solar air heaters, such as: fins, artificial roughness etc. In this paper an attempt has been done to optimize the thermal performance of flat plate solar air heater by considering the different system and operating parameters to obtain maximum thermal performance. Thermal performance is obtained for different Reynolds number, emissivity of the plate, tilt angle and number of glass plates by using genetic algorithm.

Effect of external recycle on upward-type solar air heaters

The external recycle effect on the collector efficiency in upward-type flat-plate solar air heaters is theoretically investigated. External-recycling application to solar air heaters has two conflicting effects due to the forced heat convection strength resulting in considerable device heat transfer performance improvement. The first effect is the advantage of decreasing the heat transfer resistance due to increasing fluid velocity. The second disadvantage effect is the remixing at the inlet which decreases the heat transfer transfer-driving force decrement (temperature difference). The theoretical predictions show that considerable improvement in collector efficiency is obtainable if the operation is carried out with external recycling in which the advantage effect overcomes the disadvantage effect. Device performance enhancement increases with increasing recycling ratio, especially when operating at a lower air flow rate with higher inlet air temperature.

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.

Solar Drying of Peeled Longan Using a Side Loading Type Solar Tunnel Dryer: Experimental and Simulated Performance

This article presents experimental and simulated results of drying of peeled longan in a side-loading solar tunnel dryer. This new type of solar tunnel dryer consists of a flat-plate solar air heater and a drying unit with a provision for loading and unloading from windows at one side of the dryer. These are connected in series and covered with glass plates. A DC fan driven by a 15-W solar cell module supplies hot air in the drying system. To investigate the experimental performance, five full-scale experimental runs were conducted and 100 kg of peeled longan was dried in each experimental run. The drying air temperature varied from 32 to 76°C. The drying time in the solar tunnel dryer was 16 h to dry peeled longan from an initial moisture content of 84% (w.b.) to a final moisture content of 12% (w.b.), whereas it required 16 h of natural sun drying under similar conditions to reach a moisture content of 40% (w.b.). The quality of solar-dried product was also good in comparison to the high-quality product in markets in terms of color, taste, and flavor. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan in this solar tunnel dryer was developed and this system of nonlinear partial differential equations was solved numerically by the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results agreed well with the experimental data for solar drying. This model can be used to provide the design data and it is essential for optimal design of the dryer.

Artificial neural network and wavelet neural network approaches for modelling of a solar air heater

This paper reports on a modelling study of new solar air heater (SAH) system by using artificial neural network (ANN) and wavelet neural network (WNN) models. In this study, a device for inserting an absorbing plate made of aluminium cans into the double-pass channel in a flat-plate SAH. A SAH system is a multi-variable system that is hard to model by conventional methods. As regards the ANN and WNN methods, it has a superior capability for generalization, and this capability is independent on the dimensionality of the input data’s. In this study, an ANN and WNN based methods were intended to adopt SAH system for efficient modelling. To evaluate prediction capabilities of different types of neural network models (ANN and WNN), their best architecture and effective training parameters should be found. The performance of the proposed methodology was evaluated by using several statistical validation parameters. Comparison between predicted and experimental results indicates that the proposed WNN model can be used for estimating the some parameters of SAHs with reasonable accuracy.

Modelling of a new solar air heater through least-squares support vector machines

This paper reports on a modelling study of new solar air heater (SAH) system efficiency by using least-squares support vector machine (LS-SVM) method. In this study, a device for inserting an absorbing plate made of aluminium cans into the double-pass channel in a flat-plate SAH. A SAH system is a multi-variable system that is hard to model by conventional methods. As regards the LS-SVM, it has a superior capability for generalization, and this capability is independent on the dimensionality of the input data. In this study, a LS-SVM based method was intended to adopt SAH system for efficient modelling. For modelling, different mass flow rates in flow duct and collector types are used and then for obtaining the optimum LS-SVM parameters, such as regularization parameter, and optimum kernel function and parameters, several tests have been carried out. The performance of the proposed methodology was evaluated by using several statistical validation parameters. It is found that root mean squared error (RMSE) value is 0.0024, the coefficient of multiple determinations ( R 2) value is 0.9997 and coefficient of variation (cov) value is 2.1194 for the proposed radial basis function (RBF)-kernel LS-SVM method at 0.03 kg/s air mass flow rate. It is found that RMSE value is 0.0135, R 2 value is 0.9991 and cov value is 2.9868 for the proposed RBF-kernel LS-SVM method at 0.05 kg/s air mass flow rate. Comparison between predicted and experimental results indicates that the proposed LS-SVM model can be used for estimating the efficiency of SAHs with reasonable accuracy.

Potential of roof-integrated solar collectors for preheating air at drying facilities in Northern Thailand

Longan is one of the most widely cropped fruits in Northern Thailand, where a significant amount of the annual harvest is commercially dried and exported as a commodity. Liquefied petroleum gas is generally used as the energy source for heating the drying air, but concern is growing as fuel prices are expected to increase for the foreseeable future. Meanwhile, with the ample solar radiation in Thailand, the roofs of drying facilities could be adapted to serve as solar collectors to preheat the drying air, thus reducing the energy requirement from fossil fuels. In this study, a simulation program for a flat-plate solar air heater was used to estimate the potential to preheat drying air given the conditions of several longan drying facilities. Results showed that solar collectors can replace up to 19.6% of the thermal energy demand during the drying season. Bigger collectors and smaller air channels result in more useful heat, but attention has to be paid to costs and pressure drop, respectively. Annual monetary savings can reach up to THB 56,000 (≈US$ 1800 at US$ 1 = THB 31).

The influences of recycle on performance of baffled double-pass flat-plate solar air heaters with internal fins attached

A new device for inserting an absorber plate to divide a flat-plate channel into two parts with fins attached by baffles and external recycling at the ends is presented. The proposed device substantially improves the heat-transfer efficiency. Experimental and theoretical investigations into the device efficiency are presented. The theoretical prediction agreement with the measured values from the experimental results is good. The experimental and theoretical results are represented graphically and compared with data from the downward-type single-pass solar air heaters of the same size without recycling. Considerable heat-transfer improvement is obtained by employing baffled double-pass operations with external recycling and fin attached over and under the absorber plate. The recycle ratio and absorber plate location influences on the heat-transfer efficiency and on the power consumption increment are also discussed.

NUMERICAL STUDY ON HEAT TRANSFER CHARACTERISTICS AND PERFORMANCE OF DOUBLE-PASS FLAT PLATE SOLAR AIR HEATERS

In this study, the flow and heat transfer characteristics and the performance of double-pass flat-plate solar air heaters are studied numerically. The effects of the total air mass flow rate and the fraction of air mass flow rate in the upper or lower flow channel on the performance of the double-pass flat-plate solar air heaters have also been investigated numerically. The mathematical models that describe the flow and heat transfer characteristics of the double-pass flat-plate solar air heaters are derived from the momentum and energy conservation equations. The developed mathematical model has been solved using a finite volume discrete scheme utilizing the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) pressure-correction scheme combined with QUICK (Quadratic Upwind Interpolation Convective Kinematics) scheme in the frame of staggered system. The results obtained from the numerical model are validated by comparison with experimental data of previous researchers. It has been found that there is a reasonable agreement between the present model results and those obtained by previous investigators.