the role of solar energy in the drying of vine fruit

THE ROLE OF SOLAR ENERGY IN THE DRYING OF VINE FRUIT

[Manuscript received March 27, 19621

Summary

In Australia grapes are dried almost exclusively by natural energy on tiered racks usually sheltered by an iron roof.

Under these conditions it was shown that there is a close relationship between direct solar radiation and rate of drying. Wavelengths of 0 .7 to 0 .9 p, which are abundant in sunlight, penetrated grape skins, particularly after (cold) dipping treatments, and temperatures within the grapes in excess of 8°C above air temperatures were frequently recorded. The quantity of water evaporated from grapes on a weighed drying rack approximated closely to estimates based on simultaneous measurements of solar radiation.

The efficiency of tiered drying racks was superior to that of two types of low cost air driers in which solar energy absorbers were used.

In Australia grapes are dried almost exclusively by natural energy on tiered racks usually sheltered by an iron roof. Although the physical factors involved in drying have been reported by Martin and Stott (1957), and the effect of preliminary dipping treatments by Penman and Oldham (1954), little has been recorded about the source of energy for drying. The purpose of this investigation was to determine the part played by direct solar radiation, and to consider possible improvements in the racks, or their replacement by some form of solar-heated air drier.

11. EXPERIMENTAL

(a ) Transmission of Radiation through Grape Skins

The skins of sultana grapes were removed and mounted in the light beam of a Hilger "Uvispeck" spectrometer fitted with a tungsten light source.

( b ) Internal Temperatures of Grapes

A "Stantel" type F 2311/300 thermistor covered by a piece of aluminium foil was inserted into the grape through a hole, which was then sealed with molten beeswax. The resistance was measured by the method of Herrington and Handley (1948). By using a number of calibrated thermistors simultaneous measurements were made of temperatures inside grapes exposed to the sun on trays as well as a t a number of points on a drying rack shaded by an iron roof.

* Chemical Engineering Section, C.S.I.R.O., Chemical Research Laboratories, Melbourne.

SOLAR ENERGY IN VINE FRUIT DRYING

(c) Rate of Drying of Grapes on Racks

A special drying rack (Plate 1, Fig. 1) which could be weighed during the drying period without disturbing the fruit was similar in cross section to a standard tiered rack.* The weighing section, which was 6 f t 6 in. long, was suspended from the roof by four large coil springs which were maintained at a constant extension throughout the experiment by transferring water into balance tanks fitted a t the end of the weighing section.

---- ViSlBLE LIGHT INFRARED -------- --- 60 r

0 1 I I 1 I J 0.5 0.6 0.7 0.8 0.9 1.0

WAVELENGTH ( p )

Fig. 1.-Transmission of solar radiation through natural and dipped sultana grape skins. Natural grapes. 0 Emulsion-

dipped grapes.

At the start of each experiment 1000 lb of dipped grapes was distributed on the wire tiers with the balance tanks empty. As the drying proceeded, water was added in weighed amounts to compensate exactly for the loss of water from the fruit. At the same time measurements were made with a Cassella actinograph to assess the total solar radiation from sun and sky, and the degree of shading was estimated by the methods described by Phillips (1951).

* Dimensions of the standard drying rack are 150 ft long, 5 ft wide, and 8 ft high. The racks are constructed by stretching wire netting between steel (or wooden) posts. A roof of corrugated iron 8 f t wide is usually provided. (See Plate 2, Fig. 2.)

664 B. W. WILSON

( d ) Drying Rates in Air Driers Heated by Solar Energy

(i) Closed Cabinet Drier with Internal Absorber.-A solar drier was first constructed in the form of an insulated box with an inclined glass window in the side facing the sun (see Plate 1, Fig. 2). The drier measured 4 f t by 4 f t by 4 f t and the glass window was inclined a t 52'. The absorbing elements consisted of inverted L-shaped steel fins 48 in. long by 2 in. deep by 1 in. wide, welded to a base plate of the same material (16 gauge black mild steel sheet). The exposed surface of the fins was painted with a mat black paint, and the base plate was insulated with a 14 in. thick layer of mineral wool. It was estimated that a maximum air temperature of 75°C would be attained from an inlet temperature of 35°C and incident radiation a t 750 mW/cm2.

Fig. 2.-Temperature inside dipped sultana grapes exposed all day to solar radiation. e Natural grapes. 0 Emulsion-dipped

grapes.

The grapes were loaded onto wire mesh trays, which were arranged in tiers in the cabinet behind the absorber. During daylight warm air was circulated by a small electric fan upwards through the finned solar heater and down through the fruit on the trays. The moisture was removed partly by condensation on the steel floor of the drier and partly by the admission of fresh air and release of humid air.

(ii) Air Driers with Separate Solar Absorbers.-A separate solar absorber was used to raise the inlet air temperature before i t was passed over the drying fruit and then exhausted to the atmosphere. Because of the very large amount of air to be heated, i t was necessary for the solar absorber to be very large and the cost very low. It was

SOLAR ENERGY IN VINE FRUIT DRYING 665

found that this requirement could be met by making a simple absorber of hessian (burlap) material painted with black water-emulsion paint. In the first pilot model, which is illustrated in Plate 2, Figure 1, the material was mounted over a wooden tray measuring 8 f t by 4 ft. Air was drawn through the blackened cloth a t 250 cu. ft/min by a small ventilation fan and then expelled through the fruit, which was arranged on wire trays in a series of nesting boxes. The best results were obtained with only a single layer of material.

Fig. 3.-Temperature inside sultana grapes on the east side of a drying rack. w Tier No. 8, 24 in. below roof. Tier No. 3,

24 in. above ground.

Plate 2, Figure 2 , shows a similar system of drying which was installed on a much larger scale alongside one of the coilventional drying racks, which was covered with black plastic curtains to form a drying chamber. In this experiment the absorber had an area of 1100 sq. f t and was constructed directly on the ground. Posts were driven and the blackened jute cloth was supported on rails approximately 3 f t above the ground. The sides of the absorber were sealed with polyethylene sheet.

The solar energy absorber was connected to the drying chamber by a short steel duct containing an axial flow fan driven by a 2.5 h.p. electric motor. Air was drawn from the solar energy absorber a t 1200 cu. ft/min and blown through the drying chamber in a single pass. The loss of moisture was checked by weighing a number of trays of fruit a t different positions in the drier.

666 B. W. WILSON

(a ) Transmission of Radiation through the Grape Skins

The transmission of wavelengths typical of solar radiation through the skins of sultana grapes is shown in Figure 1. For untreated grapes and emulsion-dipped grapes,* the maximum transmission was obtained in the range 0.70 to 0.90 p

(i.e. short infrared radiation) and the transmission of visible light was apparently much less effective. The low values for visible light could have been the result of considerable scattering of light at the surface of the skin.

Fig. 4.-Temperature inside sultana grapes on the west side of a drying rack. A Tier No. 8, 24 in. below roof. A Tier No. 2,


Figure 1 shows that the treatment of sultana grapes with cold-dipping emulsion has a favourable effect on the transmission of radiation in addition to increasing the loss of moisture through the skin.

( b ) Internal Temperature of Grapes during Drying

The results of measurements of temperatures inside grapes exposed in the open air to direct sunlight are given in Figure 2. During most of the day the temperature inside untreated grapes increased to 6.0-8.0°C above the ambient air temperatures, but the rate of drying was extremely slow. By contrast, the temperatures inside emulsion-dipped grapes were seldom in excess of 4.0°C above air temperature, owing to the more rapid loss of moisture.

* Dipping solution is 2.0% "Mobil" Dipping Oil and 2 . 5 % potassium carbonate.


Figures 3-5 show the temperatures inside the grapes on the second day of drying on tiered racks aligned north and south. On the roofed racks the grape temperatures responded rapidly to low angle solar radiation in the early morning and late afternoon. Temperatures above the ambient air temperature were observed on all levels, which showed that the solar radiation must have been absorbed by the fruit and converted to heat at a rate in excess of that required to provide the latent heat for the evaporation of water and heat losses by convection to the air.

Fig. 5.-Temperature inside sultana grapes in the middle of a drying rack. Tier No. 8 , 24 in. below roof. 0 Tier No. 3,


Towards the middle of the day, the tiers were shaded by the roof of the rack, and the temperature inside the grapes fell below air temperature. During this period some of the solar energy absorbed earlier as sensible heat was released as latent heat of evaporation. A similar situation existed a t night, when a rapid fall in the air temperatures freed solar energy stored up as sensible heat during the late afternoon.

During the middle of the day the onset of air drying was clearly indicated by temperatures falling below the air temperature; and direct radiation ceased to be a source of latent heat for the evaporation of moisture. In clear weather, conditions by this time were ideal for air drying with high air temperatures, low air humidities, and an occasional gust of thermally generated wind passing through the racks-all tending to facilitate the loss of moisture from the grapes. Some later observations showed that the change from one source of heat to another was not accompanied by a noticeable change in the rate of drying, which proceeded a t a remarkably uniform rate from sunrise to sunset.

668 B. W. WILSON

During shading at midday the grape temperature was 6-0°C below air temperature on the first day, but as the fruit dried out during the following days the temperature difference decreased to 2 .O°C.

( c ) Eflect of Solar Radiation on Drying Rate

Tables 1 and 2 show the results of observations on two separate samples made on special weighing racks to compare the rate of loss of moisture with the theoretical drying rate based on the assumption that all the radiant energy was used in evapor- ating water from the grapes. Both sets of observations were made over a number of days of very clear weather when the amount of diff~~se sky radiation would be comparatively small and could be safely neglected.

TABLE 1

RATE OB LOSS O F WATER FROM DIPPED SULTANAS ON A WEIGHED

DRYING RACK

Date

Fob. 20 Feb. 21 Fob. 22 Feb. 23 Fob. 24 Feb. 25 Fob. 26 Peb. 27 Feb. 28 a . 1 Mar. 2

Solar Insolation

(W/cm2)

Water Evaporated per Metre of Rack (kg)

Calculated Observed

Over the first 6 days of the drying cycle, when the bulk of the water is removed from the grapes, the observed rate of drying per unit length of rack was very closely in agreement with the calculated values based on separate solar energy measurements and estimated shade angles. The calculated values were exceeded only in the first series of results (Table l), where the fruit was picked early in the harvest and was known to contain more moisture than usual.

After 6 days the drying rate decreased so rapidly that the incident solar energy appeared to be more than enough to account for all drying requirements. One explanation for the sharp decline in the rate of drying at this point is that there is such a rapid contraction in volume of the grapes towards the end of drying that most of the solar radiation passes through the tiers of the racks without being intercepted by the grapes.


The close agreement between the solar energy observations and the rate of evaporation from the grapes on the tiered racks recalls a similar situation which occurs when water evaporates from the free water surface of a deep tank. This observation is expected to be useful in predicting the rates of loss of moisture from grapes on racks and the contribution of natural energy to drying under specific conditions. If this can be achieved it should be possible to distinguish other factors which are not related to the availability of natural energy but which may nevertheless lead to delays in drying (e.g. faulty dipping procedures).

TABLE 2

RATE OF LOSS OE WSTER FROM DIPPED SULTANAS O N A WEIGHED

DRYING RACK

Mar. 16 Mar. 17 Mar. 18 Mar. 19 Mar. 20 Mar. 21 Mar. 22 Mar. 23 Mar. 24

Mar. 25-27

Water Evaporated per Metre of Rack (kg)

Calculated Observed

Table 3 shows radiation data collected a t Merbein during the harvest season of 1960, which can be regarded as quite favourable for the use of drying racks. On the basis of these figures the optimum conditions for rack drying occur from mid January to mid March. However, even under ideal conditions the vine fruit crop seldom reaches maturity before mid February, and only the latter part of this favourable period can be utilized to advantage. Furthermore, the slow rate of harvesting means that a week or more may elapse, even after the picking starts, before all the drying racks are fully loaded. After the middle of March the daily solar radiation decreases noticeably, and the closely related conditions of air temperature and humidity become steadily less favourable for drying.

If brief delays are experienced in February and early March, a substantial amount of drying capacity may be lost and the drying season drawn out into April and May when conditions are much less favourable. The most obvious method of utilizing solar energy to better advantage in this industry is to organize harvesting conditions to make better use of the favourable conditions in February and early March.

670 B. W. wnsoN

The present investigations did not proceed to the point where it was possible to distinguish between the two sources of energy for drying vine fruits on racks; but i t was evident that very good drying rates are achieved when there is a proper

TABLE 3 MEASUREMENT OF TOTAL INSOLATION (FROM SUN AND SKY) ON A HORIZONTAL SURFACE MADE WITH

A OASSELLA ACTINOGRAPH STATIONED AT MERBEIN, VIC., 1960

Date Jan.

* Clear d a y s .

Total Insolation (mW/cm2)

Peb. March April June

balance between radiant drying and air drying, and that this balance has already been achieved by practical experience over a number of years. Some years ago, when hot dipping treatments were in vogue, the balance was swung more in favour of air drying because exposure to direct radiation tended to darken the product.

SOLAR ENERGY IN VINE FRUIT DRYING 67 1

However, with the introduction of the modern cold-dipping procedures for sultanas little damage due to exposure to solar radiation occurs, and the balance can now be swung in favour of more solar radiation on the drying racks.

TABLE 4

DRYING OF DIPPED SULTANAS IN A CABINET DRIER HEATED BY AN INTERNAL SOLAR ENERGY

Percentage of Wet Wt. Solar Air

Energy Temp.

(W1cm2)

Drier Temp.

("C)

Thermal Date

Drier Control*

Feb. 27 Feb. 28 Mar. 1 Mar. 2 M r . 3 Mar. 4

Efficiency

(%I

* Trays on the open-tiered racks.

( d ) Field Tests of Solar Driers

Table 4 shows the results of field trials with a closed solar drier with the air heated by the absorption of solar radiation on a blackened metal surface protected by a single layer of glass. Although the temperatures developed inside the drier

TABLE 5

DRYING OF DIPPED SULTANAS IN AN AIR DRIER HEATED BY A SOLAR ENERGY ABSORBER (3.0 m2)

Percentage of Wet Wt.

Date Air

Temp.

("C)

-- --

Solar Energy

(Wlcm2)

Drier Temp.

("C)

Thermal Efficiency

(%)

Feb. 27

--

Drier

590

Control*

Feb. 28 630 Mar. 1 590 Mar. 2 240 Mar. 3 1 570

590

- * See Table 4.

were much higher than the outside air temperature, the grapes dried more slowly than the grapes on the tiered racks, and eventually became infected by moulds and insect pests because of the high humidity of the air.

672 B. W. WILSON

Table 5 shows the results obtained with a small air-drying system in which the grapes were dried in a strong stream of air which had been preheated by passing it through a solar energy absorber made of blackened material. The rate of drying

Feb. 27 Feb. 28 Mar. 1 Mar. 2 Mar. 3 Mar. 4

--- .-

Solar Energy

(W/cm2)

Max. Max. Percentage of Wet Wt.

Control*

Air Temp.

* See Table 4.

.i- N.d., not determined.

Drier Temp. 1-----~-----

in this equipment was almost identical with that observed on the open racks, but the grapes were green when drying was complete.

("C)

TABLE 7

PERFORMANCE OF A 102 m2 SOLAR ENERGY ABSORBER

Solar Air Time Temp.

(hours)

Exit Temp.

("'3

37.2 41.3 41.9 44.1 39.5 39.5 42.8

Heat Absorbed

(kW)

Solar Energy

(kW)

Thermal Efficiency

( % I ----- 32 39 37 45 57 63 80

Table 6 shows the results obtained with a similar treatment on a very large scale. Once again the drying rate was very close to the rate found with open-tiered drying racks. For an average rise in air temperature of 5°C the average thermal efficiency of the absorber was 50% (see Table 7).

PLATE


Aust. J . Agric. Res., Vol. 13, No. 4

SOLAR ENERGY IN VINE BRTjIT DRYING

Aust. J . Agric. Rea., l'ol. 13, X o . 4


IV. CONCLUSIONS

(1) The vertical tiered rack which has been developed by the Australian dried vine fruit industry operates partly by the absorption of direct radiation and partly by natural air circulation.

( 2 ) This system provides a more effective method than solar energy absorbers and a supplementary source of power.

(3) While the rack method is dependent on the weather, the same limitation applies to the solar absorbers, which would be less efficient and a great deal more expensive to operate under unseasonable conditions.

The writer wishes to thank officers and staff of the Commonwealth Research Station, Merbein, for their advice and assistance in carrying out the above investigations.

VI. REFERENCES

HERRINGTON, E. F. G., and HANDLEY, R. ( 1 9 4 8 ) . 4 . Sci. Instmcm. 25: 434. MARTIN, R. J. L., and STOTT, G. L. (1957).-Aust. J. Agric. Res. 8: 444-59. PENMAN, F., and OLDHAM, F. S . (1954).-Food Manuf. 29: 309-13. PHILLIPS, R. 0. (195l).-"Sunshine and Shade in Australasia." Tech. Stud. Exp. Build. Sta.

Aust. No. 23 (D.D.23).

Fig. 1.-Equipment used for measuring the rate of loss of moisture from sultanas on a drying rack.

Fig. 2.-A cabinet type drying chamber heated by an internal solar energy absorber.

Fig. 1.-A pilot model air drier in which a solar absorber is used to preheat the air supplied to chamber for drying sultanas.

Fig. 2.-Field trial of a 1100 sq. f t solar energy absorber to preheat air for drying sultanas on enclosed racks.

the role of solar energy in the drying of vine fruit

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