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CLOSE THIS BOOKSolar Drying in Morocco (GTZ)
3. Drying techniques
3.1 Drying of fruits and vegetables
VIEW THE DOCUMENT3.1.1 Principle of operation
VIEW THE DOCUMENT3.1.2 Drying apricots
VIEW THE DOCUMENT3.1.3 Drying of grapes
3.2 Drying of medicinal and spice plants
VIEW THE DOCUMENT3.2.1 Principle of operation
VIEW THE DOCUMENT3.2.2 Drying of spearmint

Solar Drying in Morocco (GTZ)

3. Drying techniques

3.1 Drying of fruits and vegetables

3.1.1 Principle of operation

The solar dryer of the Marrakesh type converts solar energy into heat and electricity, exhibiting typical operational characteristics that are described in the following on the basis of selected examples.

Figure 8 shows the daily pattern of global radiation, ambient temperature, and temperature of the drying air at the dryer intake and outlet while the drying system is empty, on a typical cloudless summer day (June 20, 1991).

Total daily global radiation reached a value of 7.85 kWh/m² When the sun was at its zenith, a maximum radiation intensity of 973 W/m² was measured.

The global radiation is absorbed by the collector and the dryer, converted into heat energy, and then convectively emitted to the drying air. The temperature of the drying air increases with the global radiation, reaching 65°C at the dryer intake about 45 minutes after zenith. This yields a maximum temperature rise in the collector of 29 K. Inside the dryer, the temperature increases by another 14 K; the daily maximum at the dryer outlet is 79°C. During the night, the system radiates heat and the drying air cools to a temperature that is 4 K below ambient on average.


Figure 8: Daily pattern of global radiation, ambient temperature, and temperature of the drying air at the dryer intake and outlet in Marrakesh on June 20, 1991 (V = 730 m³/h)

When the system is being used for drying, evaporation of the moisture content of the produce causes the air inside the dryer to cool. If the produce contains a great deal of moisture, the resulting cooling effect is correspondingly great; with a low moisture content, by contrast, absorbed radiation completely offsets the cooling effect and the temperature even continues to rise toward the dryer outlet. Consequently, during operation a drying zone forms and gradually migrates from the intake to the outlet.

The temperature rise in the collector is proportional to the global radiation (see Figure 9). While air is flowing through the collector, the supply voltage applied to the fans—and consequently the air flow rate—is inversely proportional to the temperature reached. At a radiation intensity of 1000 W/m², the solar dryer of the Marrakesh type heats up by l9 K at an applied voltage of 220 V (air flow = 1300 m³/h), and by 29 K at 100 V (air flow = 730 m³/h). The point where the regression line intersects the x-axis indicates the radiation level at which the collector begins supplying heat energy. At an air throughput of 1300 m³/h, nearly 200 W/m² global radiation is needed in order to compensate for the system's heat losses.


Figure 9: Temperature rise in the collector at supply voltages of 100 and 220 V, as a function of global radiation

The PV-powered version has the advantage that the supply voltage powering the fans, and thus also the air flow volume, increases with the intensity of radiation. This prevents the produce from being damaged by excessive temperatures. Over the course of the day, the operating air flow volume varies between 370 and 930 m³/h. At a radiation intensity of 1000 W/m², the solar module generates 16.9 V. If the voltage drops below 6 V, the fans come to a halt. Consequently, they cannot operate at night.

Figure 10 shows the collector's effective output as a function of global radiation.

As long as the air flow rate remains constant, there is a linear relationship between useful output and global radiation.

The collector requires a certain minimal radiation intensity in order for the system to begin supplying useful energy. This minimum is higher at greater temperature differentials, that is, at lower air flow rates. This is because of better transmission of heat between the absorber and the air current on the one hand, and the smaller heat losses at faster air speeds. All radiation above this level is converted into useful output with a constant efficiency factor.

Figure 11 shows the regression line in mains-powered operation. The daily efficiency of the collector in each case can be read off the slope of the regression lines. It is 41% at a supply voltage of 100 V and 45% at 220 V.

In photovoltaic-driven operation, the effective output also increases linearly with the global radiation, starting at a radiation intensity of 300 W/m², At 1000 W/m², the air flow rate is 930 m /h; at this point the generated useable output is about 450 W per m² of collector surface area.


Figure 10: Effective collector output as a function of global radiation (100 V = 730 m³/h. 220 V = 1300 m³/h.


Figure 11: Regression lines of the collector's output in mains-powered operation

3.1.2 Drying apricots

In Morocco the Canino variety ripens between late May and early July, depending on the weather. The harvest campaign lasts an average of four weeks. The fruits are medium-sized, deeply furrowed, nearly smooth, and orange-yellow in color with reddish dots. The flesh of the fruit is light-yellow, firm, juicy, aromatic, tart to the taste, and rich in carotene (1.79 g/100 g) and vitamin C (9.40 mg/100 g). Canino apricots are used almost exclusively for canning.

A typical drying batch comprises 640 kg of fresh apricots. After being halved and depitted, 600 kg of apricot halves are left. All of the following steps refer to this processing quantity.

Preparation: Unripe, overripe, rotten, and damaged fruits and foreign material are sorted out, and pesticide residues and dirt are washed off. The apricots are then halved and depitted either using a kitchen knife or, on special work tables, with parabolic-shaped knifes; while the knife is held steady, the fruit is rotated against the. blade, so that it is cut around the seam. The flesh is checked for insect attack, and culled if necessary. The pits are collected separately. Only flawless fruits should be used for drying.

Pretreatment: In order to retain the fruit's color and flavor, prevent loss of vitamin A and C, and prevent the growth of microorganisms, the apricots are treated with sulfur. For this purpose, the apricot halves are submerged for 30 minutes in a 7% solution of sodium disulfite (Na2S2O5) (mixing 7.5 kg of Na2S2O5 per 100 liters of water). The weight ratio of apricot halves to the submersion bath should be about l: 3; in other words, 300 liters of solution are needed to treat 100 kg of apricots. An entire batch can be sulfited in 6 successive submersion operations. Once mixed, the same solution can continue to be used throughout the apricot season. After treating a drying batch, the submersion container is refilled with water up to its original level, adding a corresponding amount of Na2S2O5 plus the amount absorbed by the apricots, which is 2.1 kg of Na2S2O5. The submersion container must then be sealed air-tight by covering it with a plastic sheet or a lid until it is time to treat the next batch. In order to rule out health hazards, in Europe the SO2 content in dried apricots is not permitted to exceed 2000 mg/kg. The submersion treatment described remains within this limit.

Drying: The fruits are spread out in a single layer on the grating of the dryer, in a shingled pattern with the cut surface facing up (application density = 15 kg/m²). The dryer is then closed and placed in operation. In order to prevent damage to the fruits, the temperature of the drying air must not exceed 65°C. In mains-powered operation, the built-in temperature control system ensures this (the permissible maximum temperature must be manually set); in PV-driven operation, it is automatically ensured. The way to test whether the fruits are dried is to pinch one between the thumb and index finger; if none of the fruit flesh is squeezed out, the final moisture content of 25% has been reached and the dried apricots (120 kg) may be removed from the dryer. Under cloudless weather conditions, drying takes about two days.

Storage: The dried apricots are placed in cardboard boxes lined with plastic sheet, and stored in a cool, dark place. The storage room should be aerated and clean and kept free of insects and other pests. From time to time, the condition of the merchandise must be checked.

Time requirements: 50 worker-hours are needed to prepare and treat one batch with sulfur. Another 16 worker-hours are required to load the dryer, and 0.6 worker hour to unload and package the dried apricots.

Work schedule: If, for example, 8 workers are available, the work can be organized as follows:

9:00 a.m.

12:00 a.m.

Preparation

12:00 a.m.

1:30 p.m.

Break

1:30 p.m.

4:45 p.m.

Preparation and sulfur treatment

4:45 p.m.

4:50 p.m.

Unloading of the dryer

4:50 p.m.

6:50 p.m.

Loading of the dryer

If more workers than this are available, the dryer can also be loaded earlier. It is not possible to speed up the 2-day duration of drying, however.


Figure 12: Drying of apricot halves in the mains-powered solar dryer, expressed as moisture content over time

Because the temperature of the drying air depends on global radiation, the drying pattern of the solar dryer of the Marrakech type is strongly influenced by the time of day. To illustrate this, Figure 12 depicts drying of apricots in the mains-powered solar dryer as recorded on June 21-23, 1992. The system was loaded with 600 kg (15 kg/m²) sulfur-treated apricot halves and placed in operation at 6:00 p.m., at an air throughput rate of 1000 m³/h. During the first night the moisture content of the fruits dropped from the original level of 85% to 82%. At 6:00 p.m. in the evening of the second day the moisture content had fallen to 65%. The desired final moisture content of 25% was reached at 5:00 p.m. on the following day, after a total drying time of 47 hours. 120 kg of dried apricots were then unloaded (see Table 6).

In PV-powered operation, the fans do not work at night. During the day, however, the air throughput rate increases with global radiation, so that the temperature remains high at all times. Despite the lack of ventilation during the night, drying does not take any longer than in the mains-powered system.

Table 6: Parameters for drying apricot halves in the mains-powered solar dryer

Parameter


Value

Dryer surface area

40

Batch weight when loaded

kg

600

Duration of drying

h

47

Extracted water

kg

480

Throughput

kg/d

308

Final quantify produced

kg

120

Electric power requirements

kWh

1.4

3.1.3 Drying of grapes

Depending on the weather, the King's Ruby variety ripens in Morocco sometime between late August and late October. The harvest campaign lasts an average of nine weeks. The grapes are small, seedless, and very sweet; on any given vine, they may vary in color from white to dark red. King's Ruby grapes were originally planted in Morocco for syrup production, although the factory was never built. King's Ruby is not suitable either for consumption as fresh fruit or for making wine, and consequently today this variety is exclusively processed into raisins in industrial drying facilities.

A drying batch normally comprises 800 kg of grapes. All of the following steps refer to this processing quantity.

Preparation: Unripe, dry, rotten, and damaged grapes are carefully removed, and pesticide residues and dirt are washed off Large clusters are divided in half by using the nails of the thumb and index finger to make a slit in the end of the stem, which is then pulled apart into two halves. Only flawless grapes should be used for drying.

Pretreatment: In order to remove the waxy outer coating of the grapes—thus increasing water vapor diffusion and helping to control microorganisms—they are treated with an alkaline solution. For this purpose, the grapes are submerged for 3 minutes in an aqueous emulsion consisting of 7% potassium carbonate (K2CO3) and 0.2% olive oil (100 liters of water are mixed with 7.5 kg of K2CO3 and 0.215 liter of olive oil), vigorously moving them up and down the whole time. In terms of weight, the ratio of grapes to solution should be about l: 3—in other words, if 30 kg of grapes are to be treated at once, there should be 90 liters of solution. The entire batch can then be treated in 27 successive submersion operations. Because the olive oil tends to become rancid, the solution should not be used for any longer than 3 weeks. While treating a drying batch, the solution should be regularly freshened by adding small amounts of olive oil. After treating a drying batch, it must then be topped up to the original level with water, and an appropriate amount of K2CO3 added. Afterwards the treatment container should be covered by a plastic sheet or lid air-tight until it is time to treat the next batch.

Drying: The grapes are placed in the dryer and spread evenly on the grating (application density = 20 kg/m²). The dryer is then closed and placed in operation. In order to avoid damaging the fruits, the temperature of the drying air must not exceed 65°C. In mains-powered operation, the built-in temperature control system ensures this; it is only necessary to manually set the maximum permissible temperature. In PV-powered operation, the same effect is automatically achieved. The way to tell whether the fruit is ready is to squeeze a grape between the thumb and index finger. If no fruit flesh is squeezed out, then the final desired moisture content of about 20% has been reached, and the dried grapes (approx. 200 kg) are ready to be taken out of the dryer. In all, the drying process takes about six days on average.

Follow-up treatment: The raisins are rubbed between the palms of the hands to separate them from the coarse stems. They are then placed in a sieve and shaken back and forth to get rid of the fine stems. Any remaining stems are removed by hand.

Storage: The raisins are placed in cardboard boxes lined with plastic sheet and stored in a cool, dark place. The storage room should be aerated and clean, and kept free of insects and other pests. From time to time the condition of the merchandise should be checked.

Time requirements: Depending on the quality of the raw fruit, up to 38 worker-hours can be needed to prepare and pre-treat one drying batch. Another 2 worker-hours are needed to load the drier and 8 worker-hours for unloading, follow-up treatment, and packaging.

Work schedule: f, for example, six workers are available, the work can be organized as follows:

- 8:00 a.m. - 12:00 a.m.

Preparation

- 12:00 a.m. - 1:30 p.m.

Break

- 1:30 p.m. - 3:50 p.m.

Preparation and pretreatment

- 3:50 p.m. - 3:55 p.m.

Unloading of the dryer

- 3:55 p.m. - 4:15 p.m.

Loading of the dryer

- 4:15 p.m. - 5:30 p.m.

Follow-up treatment and packaging

Figure 17 shows, by way of example, drying of raisins in the mains-powered solar dryer as recorded on September 21, 1991.


Figure 17: Drying of grapes in the mains-powered solar dryer, expressed as moisture content over time

The system was loaded with 800 kg (20 kg/m²) of pretreated grapes and placed in operation at 4:00 p.m.; the air throughput rate was 1000 m /h. The initial moisture content of the grapes was about 80%. Following a total drying time of 140 hours, the desired final moisture content of 20% was reached at 12:00 a.m. on the sixth day. 200 kg of raisins were then unloaded. After the fourth day, the fan must be switched off at night to prevent the raisins from reabsorbing moisture. This is not necessary in the PV-powered system (see Table 7).

Table 7: Parameters for drying grapes in the mains-powered solar dryer

Parameter


Value

Dryer surface area

40

Batch weight when loaded

kg

800

Duration of drying

h

140

Extracted water

kg

600

Throughput

kg/d

33

Final quantity produced

kg

200

Electric power requirements

kWh

4.2

3.2 Drying of medicinal and spice plants

3.2.1 Principle of operation

Figure 22 shows the pattern of global radiation and the temperature of the drying air at the collector and dryer intakes over the course of the day with the drying system empty, as recorded on August 11, 1993.

The total global radiation for this day reached a value of 6.92 kWh/m². When the sun was at its zenith, a maximum radiation intensity of 900 W/m², was measured.

As global radiation increased, so did the temperature of the drying air, reaching 53°C at the dryer intake about 90 minutes after the sun was at its zenith. This corresponds to a maximum temperature rise in the collector of 13 K.


Figure 22: Pattern of global radiation and temperature of the drying air at the collector and dryer intakes over the course of the day, as recorded in Marrakesh on August 11, 1993 (air flow rate = 3850 m³/h)

While the system is being operated, evaporation of the moisture content of the produce cools the air in the dryer. A high moisture content results in a correspondingly great cooling effect. A drying zone forms and migrates vertically in an upward direction through the layer of produce.

Figure 23 shows the effective output of the collector per unit of surface area, expressed as a function of global radiation. Owing to the collector's ability to store heat, during the morning hours a substantial portion of the radiation energy is needed to heat up the collector, whereas during the afternoon and evening the stored heat is re-emitted to the drying air.

The slope of the regression line yields a daily efficiency of the collector of 47%.


Figure 23: Effective collector output as a function of global radiation (220 V = 3850 m³/h.

3.2.2 Drying of spearmint

So far, the solar dryer of the Marrakesh type for medicinal and spice plants has been used to dry spearmint, verbena and sage. Taking the example of spearmint, which is a typical medicinal herb with readily volatile components, the solar drying process involved is depicted in the following.

If planted in the autumn, spearmint can be harvested up to three times the following year. The leaves are dark-green and ovate-lanceolate in shape, with wavy, irregularly serrate edges. The plants grow to a height of 50 to 100 cm, with branching stems. They develop surface runners. The principal constituent of the essential oil is carvone (50-80%), followed by eucalyptol, limonene, linalool, etc. Resins, bitter substances and tanins are also present. Spearmint is mainly consumed fresh or used in dried form to brew tea. Spearmint oil is used in sweets, condiments, and medications.

Right after being cut, the spearmint is placed in the dryer and spread evenly using a pitchfork. The moisture content of the fresh spearmint is about 82%. The dryer can be filled with up to 50 kg/m²). of spearmint; this is equivalent to loading it to a height of about 80 cm. But it is important not to exceed this limit of 80 cm or it will be difficult to turn the spearmint. Consequently, a maximum of 600 kg of fresh spearmint can be loaded into the dryer. The dryer is then closed and placed in operation. To avoid damaging the spearmint, the temperature of the drying air must not exceed 50°C. If this critical value is reached at the dryer intake during the hot months of July and August, the collector must be partially covered with an opaque sheet for a couple of hours. This is not necessary during the other drying months. The spearmint being dried should be turned once in the morning and once in the evening. As soon as the spearmint crumbles when rubbed between the palms of the hands, its final moisture content of about 10% has been reached. Depending on how full the dryer is loaded, drying can take up to four days. The dried spearmint should then be packaged in large, clean paper sacks and stored in a cool, dark place. The storage room should be fresh and clean and kept free of insects and other pests. From time to time the condition of the merchandise should be checked.

By way of example, Figure 24 depicts drying of spearmint in the solar dryer as recorded on August 67, 1993. The system was loaded with 200 kg (16.7 kg/m²) of spearmint and placed in operation at 9:00 a.m.; the air throughput rate was 3800 m³/h. The initial moisture content of the spearmint was about 82%. The desired final moisture content of 10% was reached on 6:00 p.m. on the following day, after a total drying time of 33 hours,. It was then possible to remove 40 kg of dried spearmint from the system (see Table 8).


Figure 24: Drying of spearmint in the mains-powered solar dryer, expressed as moisture content over time.

Table 8: Parameters for drying spearmint in the mains-powered solar dryer

Parameter


Value

Dryer surface area

12

Batch weight when loaded

kg

200

Duration of drying

h

33

Extracted water

kg

160

Throughput

kg/d

24

Final quantity produced

kg

40

Electric power requirements

kWh

17.2

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