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[en] In order to study the effect of buoyancy on the performance of solar air collector, the theoretical analysis and experimental tests of four solar air collectors with different structures under natural convection and mixed convection are carried out. The results show that the air temperature rise of the protrusion-corrugated plate air collector is the highest in the natural convection, which is 9.17 Chigher than that of the flat plate collector, and the air outlet velocity is 0.19 m/s, increasing by 16.88% than that of the flat plate collector. Observing the effects on the heat transfer performance of mixed convection, it can be found, in addition to the protrusion-corrugated plate air collector, the buoyancy plays a positive role on the other three solar air collectors in the upward flow, while the buoyancy plays a negative role on the other three solar air collectors in the downward flow, and the enhanced degree of the buoyancy to the corrugated plate air collector is the largest, while the enhancement degree of the flat plate collector is the least.
[en] A solar tunnel drier was used to dry red hot pepper under the tropical weather conditions of Bangladesh in order to investigate its performance and the quality of the drier product. The drier comprises a plastic sheet-covered flat plate collector and a drying tunnel. The drier is arranged to supply hot air to the drying tunnel using two small fans powered by a 40 watt PV module. Fresh red pepper was water blanched before drying. In each drying batch in the solar tunnel drier, 20 kg of dried red pepper with 4 to 6% moisture content (wb) was obtained from 80 kg of fresh red pepper with initial moisture content of 73 to 75% (wb) in 20 to 22 hours of drying while it took 32 to 34 hours to bring down the moisture content of similar sample to 8 to 10% (wb) in sun drying methods. The pepper dried in the solar tunnel drier was completely protected from dust, dirt, rain, insects, birds, rodents and microorganisms and it was a quality-dried product in term of colour and pungency. The solar tunnel drier is recommended for drying of pepper as well as vegetables and fruits in developing countries especially in Bangladesh
[en] Highlights: • Monthly optimum tilt angle is predicted for North latitudes between 0° and 80°. • From practical considerations, an optimum fixed value all along the year is derived. • Three solar radiation models are compared from the point of view of total incident solar radiation density prediction. - Abstract: The paper presents a theoretical study on the optimum tilt angle for flat plate collectors at different geographical locations and different time moments over a year. The solar radiation density is estimated based on three analysis models, namely Hottel and Woertz model (the simplest), Isotropic diffuse model (Liu and Jordan model) and HDKR model (Hay–Davis–Klucker–Reindl), passing from the simplest to the most complex one. Further, the absorbed solar radiation density is computed and the optimum tilt angle is found for different geographical locations (covering the Globe latitudes). The optimum value for the tilt angle is researched for maximum incident solar radiation and also for maximum absorbed one. The results indicate that the same angle is obtained when looking for maximum incident and for maximum absorbed solar radiation. When comparing the three above mentioned models, different tilt angles are supplied by applying the Hottel and Woertz model and respectively the others two (both of them providing the same angle). Although, the predicted absorbed solar radiation is the same in the summertime for all three models, and probably overestimated by the last model in the springtime. The obtained results give an overview over the whole year operation at different geographical latitudes. When using the Hottel and Woertz model for estimating the incident solar radiation, the optimum tilt angle for a flat plate collector should be computed as simplest as βopt = φ − δ function on the latitude and declination. If a fixed value is easier to be used over a specific operation period, an optimum value is provided by this research. The novelty consists in finding this global optimum instead of “local” ones for which monthly adjustment is required. The results are compared to the approximation given in the technical literature according to which the optimum tilt angle should be local latitude plus or minus (for winter or summer seasons, respectively) 10° or 15° (with small variations as it is further presented)
[en] Experimental investigation to study the effect of coupling a flat plate solar collector on the productivity of solar stills was carried out. Other different parameters (i.e. water depth, direction of still, solar radiation) to enhance the productivity were also studied. Single slope solar still with mirrors fixed to its interior sides was coupled with a flat plate collector. It has been found that coupling of a solar collector with a still has increased the productivity by 56%. Also the increase of water depth has decreased the productivity, while the still productivity is found to be proportional to the solar radiation intensity.(Author)
[en] A theoretical analysis of different intervals of intermittent two-axis tracking of the sun, on the amount of annual energy received by flat-plate collectors, has been carried out. The analysis was done for Ipoh, a city near the university at a latitude of 40 34 North in Malaysia. For the analysis, a computer program was developed to calculate the solar insulation according to the interval settings, considering ASHRAE Standard Sky assumption. Both direct and diffused components of solar radiation have been considered. The tracking system was targeted for flat plate collectors where the degree of tracking accuracy would be much lower Hence, the tracking mechanism will be much simpler and lower in costs. Results showed that by a 3-hour intermittent tracking, a flat-plate collector could get as much as 35% more annual energy than a fixed one. The 3-hour interval tracking greatly simplifies the gear mechanism from the motor to the solar collector. (Author)
[en] Highlights: ► Photothermal testing was done before and after degradation by using flat plate collector. ► AC and reverse periodic plating techniques are used to prepare the selective absorber samples. ► Absorptivity, emissivity, collective flux and conversion efficiency before and after degradation were calculated. ► Degradation testing carried out in air at different temperatures: 100, 200, 250 and 300 °C. ► Degradation process leads to a decrease in the solar absorptance and increase in the thermal emittance of the films. - Abstract: The alternate and reverse periodic plating techniques were used to prepare the selective absorber samples. The glazed type-flat plate collector was used for photothermal testing. The calculated absorptivity of the selective absorbers (before degradation) falls in the range 92.0–97.0% and after degradation falls in the range 88.3–92.2%; while emissivity (before degradation) falls in the range 11.0–22.5% and after degradation falls in the range 11.6–23.4%. The maximum calculated net collective flux (before degradation) is in the range 590–699 W/m−2 and after degradation is in the range 566.4–664.1 W/m−2. The conversion efficiency (before degradation) is in the range 0.73–0.78 and after degradation falls in the range 0.70–0.74. Degradation testing carried out in air at different temperatures: 100 °C, 200 °C, 250 °C, and 300 °C. The degradation process leads to a decrease in the solar absorptivity and increase in the thermal emissivity of the films especially at 300 °C. Different parameters are discussed concerning the photothermal and the degradation testing.
[en] This work present a numerical and experimental investigation of the influence of transversal flow deflector on the cooling of a heated block mounted on a flat plate. The deflector is inclined and therefore it guides the air flow to the upper surface of the block. This situation is simulating the air-cooling of a rectangular integrated circuit or a current converter mounted on an electronic board. The electronic component are assumed dissipating a low or medium heat flux (with a density lower than 5000 W/m2), as such the forced convection air cooling without fan or heat sink is still sufficient. The study details the effects of the angle of deflector on the temperature and the heat transfer coefficient along the surface of the block and around it. The results of the numerical simulations and the InfraRed camera measurements show that the deviation caused by deflector may significantly enhance the heat transfer on the top face of block
[en] Highlights: • Single slope solar still coupled with N-identical PVT FPCs has been proposed. • EPBT of the proposed still is found to be 89.90% lower than that of N-PVT-CPC-SS. • An improvement of 27.5% in water production cost over N-PVT-CPC-SS is achieved. • LCCE of N-PVT-FPC-SS is 56.25% higher than that of N-PVT-CPC-SS. - Abstract: This paper communicates the improvement in performance of single slope solar still (SS) by including N identical partially covered photovoltaic thermal (PVT) collectors. Three models have been considered namely, SS included with N identical partially covered PVT flat plate collectors (N-PVT-FPC-SS); SS included with N identical partially covered PVT compound parabolic concentrator collectors (N-PVT-CPC-SS) and conventional SS (CSS). In this work, yearly production of potable water, energy, exergy, energy metrics, cost of distillate output and cogeneration efficiency have been computed for the proposed N-PVT-FPC-SS at 0.14 m water depth under optimized condition for New Delhi, India. Results obtained have been compared with results reported by researchers previously and it has been concluded that exergy based energy payback time is lower by 89.90% and 44.45%; energy production factor is higher by 46.67% and 31.11%; life cycle conversion efficiency is higher by 56.25% and 37.50% and production cost of potable water is lower by 27.05% and 3.20% for the proposed N-PVT-FPC-SS than N-PVT-CPC-SS and CSS respectively. The proposed system can produce potable water on commercial scale and at the same time, it can meet DC electrical power need.
[en] Highlights: • Discriminated dimensional analysis approaches a universal solution of Graetz Problem. • A universal characteristic length abacus is offered; further pipe length is useless. • Universal abacuses of temperature along the pipe is included. • Universal abacuses of local and cumulated heat along the pipe is presented. • Use of abacuses for design purposes is described. - Abstract: A new approach to the heat transmission problem concerning a fully developed laminar flux inside a pipe subject to an abrupt change of temperature in non-reactive conditions is presented. By means of dimensional discriminated analysis, representative dimensionless groups of 2D governing equations and boundary conditions of the problem are managed in order to find a set of dimensionless groups which describe the system’s behavior. The characteristic length of the system, which plays a key role in the formulation, as well as the heat transfer and thermal distribution, are obtained numerically for the range of values of solar thermal flat-plate collectors using the network simulation method. The set of universal abacuses of temperature and heat based on the new dimensionless groups can be used in the design of industrial thermal devices and facilities.
[en] The paper presents a mathematical solution of the one node transient equation for a flat-plate collector by removing approximations like mean fluid temperature and constant initial temperature used in earlier solutions of the equation. Analytical expressions have been obtained for many cases of practical interest. Test procedures for flat-plate collectors often involve transient studies. Deconvolution of these results and extraction of the desired parameters of the collector become easier with the analytical expression obtained here as compared to the numerical methods used earlier. The theory also allows us to predict the performance of the collector under various pumping conditions and can be applied to actual field conditions when storage and load are included