The experimental study showed that a very economical solar drying does not involve any energy expenditure and no loss of fruit. Applying the drying of green apples by solar collectors shows no pollution and we find no economic loss and drying time and very fast. Application of Solar Panels for low thermal drying is the level of exchange with the air in the use of the dynamic thermal vein panel, to improve the heat transfer. Fin Improve the relationship between temperature and thermal efficiency of a solar panel system and air drying to reduce the drying time of the green apple.

Solar energy, Simulation, An air heating plane solar panel, Temperature and thermal the solar panel, Green apple.

The experimental study showed that a very economical solar drying does not involve any energy expenditure and no loss of fruit. Applying the drying of green apples by solar collectors shows no pollution and we find no economic loss and drying time and very fast. Application of Solar Panels for low thermal drying is the level of exchange with the air in the use of the dynamic thermal vein panel, to improve the heat transfer. Fin Improve the relationship between temperature and thermal efficiency of a solar panel system and air drying to reduce the drying time of the green apple.

Solar energy, Simulation, An air heating plane solar panel, Temperature and thermal the solar panel, Green apple.

To improve the performance of solar panels, a wise choice of their components enables thermal losses between the absorber and the environment to be limited. Recent research studies have focussed more particularly on the circulation of the coolant fluid as a means of optimising performances and in which several methods have been proposed to deal with this objective. Zugary and Vulliere [1] sought to limit the losses near the fore part of the solar panel; two other researchers [2,3] centred their work more on the absorber {the active part of the solar panel} while yet other papers [2-8] have shown that placing baffles in the dynamic air vein of the solar panel enables a turbulent air flow to be created which in turn boosts the interchange of thermal convection between the air and the absorber.

The baffles have to be placed very carefully. They can be fixed either onto the insulating material or under the absorber or indeed in both positions. In all three cases, results are improved because of the reduction of the hydraulic diameter (D_h) when compared with the performances of solar panels without baffles. In the air flow vein, the Reynolds number is calculated by starting from the maximum speed (V_m) corresponding to the minimum air flow section of the duct (S_min) and is expressed by:

D_h: the hydraulic diameter of the duct

b’: the blockage coefficient in the air vein

The coefficient of thermal convection interchange (h_ccf) between the absorber and the coolant fluid is dependent on the Reynolds number; it is an increasing Re function, otherwise h does not increase. It can therefore be deduced that when b’ increases, both Re, and consequently h_ccf, increase. The minimum air flow section of the duct (S_min) is dependent on the shape of the baffles, their dimensions and their layout in relation one to another. The following three positional fixings of the baffles have been studied:

Given the above findings and to continue with the experiments, it was decided to fix the baffles onto the insulating material. When choosing the geometrical shape of the baffles to be used, certain criteria have to be satisfied. Indeed, both the layout and the shape of the baffles influence air flow during its trajectory. The baffles ensure that the absorber is well irrigated, reduce the zones of inertia, create turbulence and lengthen the course of the air by increasing the time it remains in the solar panel. A meticulous and systematic study was then undertaken of several different methods of arrangement of the air flow veins in the solar panels.

The first part of this paper deals with a comparison of the results obtained, initially using solar panels without baffles (SC) and then with baffles. Of the latter, two types were selected, namely Delta-shaped Curved Longitudinally (DCL1) and Ogival-shaped Curved Longitudinally (OCL1) baffles. The second part concerns the results obtained when firstly using solar panels without baffles and then with DCL1 baffles for drying GINGER. In addition, to conduct experiments that would highlight the effects of baffles even further, GINGER were dried by using a solar panel provided with Transversal-Longitudinal ones (TL) of the same type as those already studied [4]. A comparison of the results obtained shows that a solar panel provided with baffles is far more efficient than one without them.

Ex Experimental device is shown in Figure 1.

The solar panel with a single passage of air (Figure 2) consists of:

Too, the shape of the inlet (details A) and outlet (details B) of air of the solar panel have to be carefully arranged so as to avoid heating any dead zones. The baffles are fixed on a hardboard sheet just above the polystyrene plate. The experiments took place at Valenciennes in North-East France, the co-ordinates of which are : latitude : φ =50.3°; altitude: Z=60m; longitude: L=3.5°, and on a day in July which was considered typical of mean solar time flux (Figure 3) and which corresponds with the average for the years 1999 and 2000 [9].

A means of extracting the maximum of heat stored in the absorber is to place baffles in the mobile vein of air; they can be fixed either on the underside of the absorber or onto the insulating material, or indeed on both places. The objective is thereby to raise the temperature at the solar panel outlet (T_SC), i.e. increasing thermal efficiency, and to reduce charge losses to a minimum [2-8]. Results have been obtained using the solar panel without baffles and subsequently provided with baffles in two stages, firstly with the DCL1 type and then with the OCL1 (Supplementary File).

The baffles selected for use from the range of shapes already experimented with [10] are formed by bending the otherwise straight delta and ogival wings (Figure 5) [10-12, 27, 29], and fixing them onto the insulating material (Figure 4). The apex angle β of these baffles is 45° (Preferential angle) [13].

Figure 4: Solar panel equipped with baffles

The index (1) referring to the DCL1 and OCL1 types of baffles indicates that the air flow takes place near their tips (Figure 5).

Experiments carried out in a wind tunnel [14] have shown that the increase in incidence enables swirling fragmentation to progress continuously on the upper surface of the curved wing. The flow ends in a total disorganisation of the swirling systems at the leak edge of the wing which promotes the creation of a flow of considerable turbulence and, consequently, a better convective thermal interchange, which in turn improves the ratio between temperature and thermal efficiency. Total fragmentation of the swirls takes place at an incidence higher than 65°. The nature of the flow obtained as observed in the wind tunnel has been highlighted (Supplementary File). Other such visualisations of other shapes of wings [15,26,28] have confirmed results of these differing observations concerning the progressive fragmentation of swirls (Table 1).

Prior to a presentation of the results obtained in this first part, an explanation of the mathematical expression used to calculate thermal efficiency is called for. The Letz model [16] has been used as it is one of the most recent and complete formulae, being so because it takes into account not only the relative humidity of the air and the leak-flow of air as it is sucked into the sensor by the ventilator but also the temperatures at the inlet and outlet points of the solar panel. According to prior enthalpy assessment, made by the authors of this paper, of the different modes selected for the experiments, thermal efficiency (η) is determined by:

where P(Z)/PO = (0.88)^Z and for Valenciennes P(Z)/PO ≈ 1

ρ_o = 1.293 Kg/m3 and C_P = 1005J/Kg.K.

The captive surface AC is 1.28m². In our case, thermal efficiency (η) is given for a constant global solar flow (I_GS) of 1063.5W/m² corresponding to solar midday. Therefore, for a specific flow of 35m³/h.m², 54% thermal efficiency was obtained in the case of the solar panel provided with DCL1 baffles. By increasing the flow to 70m³/h.m², 80% thermal efficiency was achieved. In the case of the solar panel without baffles, the respective thermal efficiency percentages obtained with each of the two flows were only 35% and 55% (Figure 6) for temperatures at the solar panel outlet (T_WB) of 61.7°C and 53.3°C as opposed to 82.8°C and 66.3°C in the case of a solar panel provided with DCL1 baffles. However, under the same conditions as above, but using OCL1 baffles, slightly lower results were obtained than those with DCL1 ones. This can be explained by the fact that the air flow section differs and so, and in particular, the progressive fragmentation of swirls occurs a little earlier. Indeed, for the same flows, the respective thermal efficiency percentages are 54% and 70% and correspond to temperatures (T_WB) of 71.3°C and 64.8°C. With these configurations using DCL1 and OCL1 type baffles, 50% thermal efficiency is obtained respectively with flows of 32m³/h.m² and 33m³/h.m² as opposed to 58m³/h.m² when using an SC solar panel where relative flow reductions are respectively 44.8% and 43.1%. The respective temperatures (T_WB) are 84.5°C and 82°C as opposed to 55°C. The latter correspond to respective temperature rises (ΔT=T_WB–T_e) of 50.1°C and 61°C as opposed to 27.2°C. In both cases, the amounts of charge losses are acceptable.

However, the blocking effect of TL baffles enables a very turbulent flow to be created and, consequently, provides a very good level of thermal interchange. It is worth noting that the resultant charge losses are very high because the air flow through the duct is very weak compared with that attained with other types of baffles. A thermal efficiency of 50% obtained with a specific flow of 23m³/h.m² corresponds to a temperature (T_WB) of 104°C, i.e. an improvement in temperature (T_e) of 75°C at the solar panel inlet. These results are decidedly better than those obtained when using DCL1 or OCL1 type baffles. The lengthening of the distance covered by the air in the solar panel duct results in an even better interchange of heat between the coolant air and the absorber.

Dating from the early research work of Lewis in 1921 [17] and Sherwood in 1929 [18], techniques of drying have been the subject of many scientific publications and continues to be a priority field of research, especially in respect of countries where traditional methods remain in use and are essential for want of better. As it is readily available at little or no running costs compared with other sources of energy such as electric resistors [19], solar energy is obviously an alternative. At Valenciennes (France), simulated solar energy was used to carry out experiments applied to drying pre-dried green apple; the simulator was conceived to provide conditions of a typical July day.

For solar energy to be harnessed effectively, certain difficulties have first to be overcome and to be achieved with the help of technically viable and economically profitable systems. The choice of the type of dryer is conditioned by whether or not the product in question can withstand solar radiation; it also has to be made between direct or indirect dryers and depends, too, on the commercial value of the product. Since the performances of absorbers are higher than the thermal conversion capacity of the product, the use of an indirect dryer is the more effective. The system under study is therefore an indirect solar dryer functioning by thermal forced convection.

The construction of a drying installation is very complex and requires the taking into account of a number of parameters and the mastering of many phenomena before it can be devised. For it to work efficiently, it is necessary, first of all, to estimate the quantities of products to be treated and then to carry out a thorough study of the design of the system. What is important, from the thermal point of view and to ensure that the components of the installation are optimised, is to evaluate the various modes of transfer and to assess the energy-giving potential while taking into account the coupling between the warm air generator and the drying unit with a view to its dimensional set-up. In our experiment, the device (Figure 1), being constructed with only one plane solar panel and a “drying cupboard” holding four trays {i.e. a simplified version of a sort of kiln}, has been designed to treat small quantities of products and, consequently, equipping it also with means for storing energy; with an auxiliary heating system and with a device for recycling air is therefore unnecessary.

The quantities of heat (Q_u) recovered by the coolant fluid, as far as the absorber is concerned, depends on the efficiency of the solar panel used. Given that these quantities are proportional to the variations in temperature between the inlet and the outlet of the solar panel, the results presented above show that a solar panel provided with baffles functions more efficiently and so baffles are essential fittings because they reduce drying times.

In our experimental work, the objective is to carry out drying by a simulation of solar energy. However, for a given air flow, we wanted to study the variations in certain parameters of the drying process as at different times during the typical day under consideration. In view of the considerable expenditure that could be involved in setting up a real-life operation, the use of thermal forced convection would seem to be less suitable in applying the findings of our smallscale experiment to a large-scale situation. Nevertheless, it would be profitable to take advantage of natural convection in a solar chimney. Its application is, of course, all the more valid in geographical zones deprived of electrical power. The choice between forced and natural convection depends on several factors, in particular on the quantity of the product to be treated, on the capillary structure of that product and its nutritional value while not neglecting the financial budget available. Drying time is indeed of paramount importance. As regards large-scale {industrial} concerns, an external source of energy is required. Where electrical power is available, even if weak but at an affordable rate, it is logical to make use of it to actuate the ventilators, blowers or other devices necessary to increase the efficiency of the system. Where a system functions with natural convection in a solar chimney, the driving force of gravity is created by differences in the density of air between the exterior {ambient conditions} and the interior of the chimney. The height of the chimney, which influences the efficiency of extracting air, is a factor that has therefore to be adequately investigated. Pasumarthi and Sherif [20] have shown that for a given height and an increasing solar flux, the temperature at one and the same given point in the chimney also increases. Heat interchange improves but the total charge losses of the system, which are proportional to the height of the chimney and to the differences in air density, increase considerably.

Prior to setting out the findings of our experiments, a brief description of the type of TL baffles used is called for. The height of the large (transversal) baffle is 2.5 cm and that of the small (longitudinal) one is 2 cm (Figure 7). The surface A_C is 1.28 m². [Kge/KgMS: kilograms of water per kilograms of dried mass of the product]

To study the influence of the flow of drying air on drying time, it was considered of value to use two flows, one of 31.3m³/h.m², the other of 60m³/h.m². This adjustment is made with the help of a ventilator (solar simulator). The air flow is measured using a “Jules et Richard” anemometer with a 10 cm diameter propeller. With a flow of 31.3m³/h. m², drying times at the first (bottom) tray (Figure 1) take up to 9 hours as regards the solar panel fitted with DCL1 baffles, 6 hours 35 minutes for that with TL baffles while the longest time taken was with the WB solar panel. There are therefore reductions of drying time of 49% and 60% in comparison with the WB solar panel. The final content of water collected in the WB solar panel is only attained after 14 hours 10 minutes of drying time (Figure 8). The air coming from the level of the first tray is still heavy with moisture and, consequently, for this same air flow, drying time at the level of the fourth (top) tray takes longer for all three types of solar panels used. By increasing the flow to 70m³/h.m², drying times decrease in each of the three solar panels. As drying is brought about by force of speed of the flow, this faster flow results in a more rapid evacuation of the moist air.

By increasing the flow from 31.3m³/h.m² to 60m³/h.m², and as regards the solar panel with DCL1 baffles, the drying time at the first tray is reduced by one hour, i.e. a relative reduction of 15 % whereas a relative reduction of 13.8% is attained using a solar panel with TL baffles. The drying times at the level of the fourth tray are respectively 10 hours (DCL1 baffles) and 8 hours (TL baffles). Comparing these results with the performance of the solar panel without baffles, and with a flow of 60m³/h.m², the reductions in drying times at the first tray are respectively 27% and 49.5%.

A graph (Figure 9) plots the evolution of the loss of mass (ΔM) at each hour, for each of the two flows and for each of the types of solar panel used. Figure 10 shows the evolution in temperature of the product (T_Pr) in relation to the passage of time during the drying process. It is to be noted that for every type of solar panel used, drying takes place at temperatures that vary in accordance with the solar time flux particular to the day on which the experiment is conducted. In every case, a constant phase of drying cannot therefore exist (Figure 11).

Analysis of the findings relative to the WB solar panel (without baffles), reveals that functioning with a low air flow is considerably more efficient because of the reduction in drying time. The mechanical power consumed (P_mc) by the ventilator is proportional to charge losses and to the air flow in the dynamic air vein of the panel.

This same power is expressed by:

where ζ is the factor of friction, characteristic of artificial rough places (baffles).

As the relationship between the two flows is 2.24, the power is therefore increased by a factor of 11.24, a fact which further highlights preference for using a low flow. In spite of the recommendations made by some research workers not to exceed a drying air temperature of 55°C, higher temperatures were used in our experiments (at around solar midday). However, at temperatures above 70°C, reddening spots (i.e. signs of burning) appeared on the products. Indeed, the quality, colour, savour and nutritional value of the product are all closely subjected to conditioning by the thermal process. Consequently, to create ideal drying conditions at temperatures lower than those recommended for the product in question, some precautions can be taken such as :

The quantities of heat available for use and reclaimed at the solar panel outlet are much higher when using solar panels equipped with TL baffles than those with DCL1 baffles. Variations in those quantities (Q_u), in global quantities of drying heat (Q_s) and their differences (ΔQ = Q_u – Q_s) are shown in Figure 12 (for WB solar panels without baffles) and Figure 13 (solar panels equipped with DCL1 baffles). Worthy of note is the fact that the quantities of heat available for use are increased by a factor of approximately 1.65 as regards the performance of the SC solar panel. The differences in quantities (ΔQ) are of some consequence because they are, in fact, surplus to normal requirements for the drying process and can therefore be stored and made available for use, for example, during the night or on days when sunlight is mediocre [22-25]. This excess of heating needs can be kept in underground ducts and thus ready for use when needed.

We observe the effectiveness of the drying of the pommmr green fruit and very effective. The drying time and very fast but no energy expenditure and the performance of the drying and very important and also finacierement the realization of the instalation of the sensors and very simple and less expensive and no pollution, it can be deduced from the results of our experiments using different types of solar panels that the placement of baffles in the air stream is a very important factor that improves the performance of a given solar panel. Of course, several determinants must be taken into account to include the shapes and dimensions of baffles, the number of rows and their layout. The study showed that a solar panel equipped with baffles not only significantly improves the ratio between temperature and thermal efficiency, but also reduces the drying time of the product. It should also be noted that a reduction in the transverse (E) and longitudinal (E) spaces contributes considerably to the quality of the results. In addition, an increase in angle (Δi) results in even better results. However, certain constraints imposed by the nature of the finished product, such as its quality, flavor, color and nutritional value, must be taken into account in determining what constitutes the ideal temperature of the drying air.