M.R.I. Ramadan

Experimental investigation of an indirect type natural convection solar dryer

Abstract An indirect type natural convection solar dryer is designed, constructed and investigated experimentally under Tanta prevailing weather conditions. The system consists of a flat plate solar air heater connected to a cabinet acting as a drying chamber. The air heater is designed to be able to insert various storage materials under the absorber plate in order to improve the drying process. Sand is used as the storage material. Drying experiments have been conducted with and without storage materials for different spherical fruits, such as seedless grapes, figs and apples, as well as vegetables, such as green peas, tomatoes and onions. The solar irradiance, temperature distribution in different parts of the system, ambient temperature and relative humidity of the inlet and outlet drying air have been recorded. The equilibrium moisture content M e for seedless grapes is reached after 60 and 72 h when the system is used with and without storage material, respectively. Therefore, the storage material reduces the drying process by 12 h. In order to accelerate the drying process, the drying products are divided into pieces and then chemically treated by dipping the samples into boiling water containing 0.4% olive oil and 0.3% NaOH for 60 s. However, the required time to achieve M e for the chemically treated seedless grapes, when the system is used with sand as a storage material, is drastically reduced to 8 h. Moreover, we found that the storage and chemical pretreatment have caused significant decreases of the drying time for all the investigated crops. The present system is capable of drying 10 kg of chemically treated grapes or green peas during 20 h of sunshine.

Empirical correlations for drying kinetics of some fruits and vegetables

An indirect type natural convection locally made solar dryer, consisting of a flat-plate solar air heater connected to a drying chamber, has been investigated experimentally and theoretically. The system is used for drying fruits and vegetables such as seedless grapes, figs, green peas, tomatoes and onions. The experimental data are used to calculate the drying constants k for the selected products. The obtained values of k are then correlated with the drying product temperature Tdp. Linear correlations between k and Tdp are found to satisfactorily describe the drying curves of the materials under study. Furthermore, the characteristics constants c and n of Henderson’s equation are determined for the selected crops using the experimentally measured relative humidity of the drying air. A simple mathematical model is presented for the drying chamber based on the energy and mass balance equations and employing the proposed correlations for k as well as the values of the constants c and n. Numerical calculations are performed on typical summer and winter days in Tanta. Comparisons between experimental and theoretical results showed that good agreement has been achieved. The effect of the mass of the drying product mdp on the dryer performance is also investigated by computer simulation.

Parametric study of a solar air heater with and without thermal storage for solar drying applications

A transient analytical model is presented for a flat-plate solar air heater with and without thermal storage. The flowing air temperature is assumed to vary with time and space coordinates. Analytical expressions are obtained for various temperatures of the air heater elements as well as for the temperature of the storage material. The performance of the air heater is investigated by computer simulation using the climatic conditions of Tanta (Lat. 30° 47′ N, Egypt). Effects of design parameters of the air heater such as length (L), width (b), gap spacing between the absorber plate and glass cover (df), mass flow rate (m) and thickness and type of the storage material (sand, granite and water) on the outlet and average temperatures of the flowing air are studied. It is found that as L and b increase the average temperature of flowing air (Tfav) increases up to typical values for these parameters. Typical values for L and b are obtained as 3 and 2 m, respectively. The outlet temperature (Tfo) of flowing air is found to decrease with increasing gap spacing and mass flow rate of air. Improvements in the heater performance with storage have been achieved at the optimum thickness (0.12 m) of the storage material. Therefore, the air heater can be used as a heat source for drying agricultural products and the drying process will continue during night, instead of re-absorption of moisture from the surrounding air. Comparisons between experimental and theoretical results indicated that the proposed mathematical model can be used for estimating the thermal performance of flat-plate solar air heaters with reasonable accuracy.

Year-round performance of a modified single-basin solar still with mica plate as a suspended absorber

In a previous study, a single-basin solar still with suspended absorber (SBSSBA) made from aluminium was constructed and investigated experimentally and theoretically. It was found that the daily productivity of the still was about 20% higher than that of the conventional single-basin solar still (SBSS). In this paper, the effect of thermal conductivity of the suspended absorber on the daily productivity of the still is investigated experimentally using aluminium, copper, stainless steel and mica plates as suspended absorbers. The results obtained are compared with those obtained for the SBSS tested under the same climatic conditions of Tanta (lat. 30° 47′ N). The results indicate that it is advisable to use suspended plates made from insulating materials, such as mica, plastic, glass, etc. The daily productivity of the modified still with mica is found to be 42% higher than that of SBSS. Further, the effect of thickness of the suspended absorber on the productivity as well as the year-round performances of both SBSSBA and SBSS for the year 1996 are studied by computer simulation. There is good evidence that the productivity of SBSSBA is less dependent on the thickness of the suspended plate. The annual average productivities of the modified still with mica are found to be 23 and 15.8% higher than those of the conventional still when the basin water masses are 80 and 40 kg, respectively. This indicates that the suspended plate becomes more effective at higher masses of basin water.

A model of an improved low cost-indoor-solar-cooker in Tanta

Abstract A simple flat-plate solar cooker with focusing plane mirrors and energy storage capabilities has been designed and constructed by the locally available materials in Tanta University. The temperature distributions during the test periods through the absorbing plate, the glass covers, the cooking pot, the cooking fluid, the storage medium and the ambient were determined. The diurnal variations of incident global solar insolation were presented. The utilization of sensible heat storage medium such as sand was investigated thoroughly. A jacket of sand ( 1 2 - cm thick) around the cooking pot has improved the cooker performance tremendously. The temperature differences between the heat source and the heat sink during the test time were determined and the results were discussed. Six hour per day of cooking time has been recorded. Approximately three hours per day of indoor cooking has been achieved. An overall energy conversion efficiency up to 28.4% has been obtained which is considered the best among other solar cookers in the literature. The possibility of using a phase change material as a storage medium to obtain longer cooking periods was studied. A thin layer of the salt hydrate Ba(OH)2·8H2O as a jacket around the cooking pot was suggested. The cost of the whole cooker is not more than

Effect of minority carrier lifetime in solar cells

15 including the pot and the labour expenses. A step for commercial production of two-pots-solar-cooker has been taken. Extensive efforts are required to diffuse this technology into the villages of the people who could benefit from it.

Performance investigation of single crystal silicon solar cells fabricated by diffusion technology

Abstract The photoinduced open-circuit voltage decay technique was used to determine the minority carrier lifetime in several single crystal silicon solar cells fabricated by diffusion technology. It is found that the acceptor and donor concentrations (NA and ND, respectively) must be as large as possible in order to obtain high efficiency solar cells. The diffusion length was calculated to be 158 μm at room temperature. The open circuit voltage, Voc, is strongly dependent on ND. The values obtained for Voc at room temperature agree within ± 8% with that given by Green for high performance experimental devices having the same doping level of the base.

Potential barriers and hall mobility of polycrystalline silicon

Abstract Two single crystal silicon solar cells SC1 and SC2 were fabricated by diffusion technology. A p-n junction was established by doping n-type silicon wafers with boron. The fabrication process was described in detail. The I–V characteristics of the solar cells SC1 and SC2 were plotted. Other parameters affecting the performance of the fabricated cells such as incomplete absorption of photons, series resistance, shunt resistance, fill-factor, metallization and heat treatment were carefully examined. Conversion efficiencies of 10.6±0.2% were obtained. These values correspond to effective conversion efficiencies of 12.5±0.2% based on active cell area. Recommendations to fabricate single crystal silicon solar cells with better performance were given.

Thermal performance investigation of double pass-finned plate solar air heater

Abstract Grain boundaries play a dominant role in understanding the properties of polycrystalline silicon and make it difficult to analyse the results. By illuminating the samples with light of approximately solar intensity, the potential barriers at the grain boundaries can be removed and thus approach single crystal behaviour. When the barriers are removed, the mobility between 200 and 400 K is found to vary as T −2 , which is the dependence observed in single crystals for the same order of magnitude of doping. The free carrier concentration of 5 × 10 15 cm −3 is not affected by illumination. The room temperature mobility in 1 mm grain size material after barrier elimination by light is 900 cm 2 V −1 s −1 . The theory of Hall mobility in polycrystalline silicon which explains these observations has been examined.