Solar Energy, Vol. 20, pp, 289-291. PergamonPress1978. PrintedinGreatBritain

TECHNICAL NOTE

Solar heated fruit dehydrator

H. R. BOLIN, A. E. STAFFORD and C. C. HUXSOLL

Western Regional Research Center, Agricultural Research Service, U.S.D.A., Berkeley, CA 94710, U.S.A.

(Received 24 April 1977; in revised form 22 July 1977)

1. INTRODUCTION Heat from the sun has been used since antiquity to dry foods. This method of preservation has been especially used in con- junction with certain fruits, such as grapes, figs, etc. In recent years, some fruit drying methods, such as with prunes, have changed from sun-drying to hot-air dehydration, because the fruit dries faster and has better quality. With the possibility of energy restrictions in the future, natural gas for hot-air dehydration may be reduced. If this occurs, an alternate energy source will be needed for heating air. Since fruits are grown in hot sunny areas, and also since they are harvested and dehydrated during the time of the year that solar radiation is abundant, solar heating looks attractive. In determining the feasibility of using solar energy, numerous commercially available solar heating panels were in- vestigated. These heating panels were expensive, running up to $30 per square meter. Recently, Foster and Peart [1] used inflated polyethylene tubes as solar collectors to warm air for grain drying. Since polyethylene sheets cost only about ten cents per square meter, this appeared to be an economical device for solar heating of air. These experiments were designed to determine the amount of heat that could be supplied by such collectors, con- structed from clear and black polyethylene, for fruit dehydration.

\ 2. HEAT COLLECTOR

The heat collectors for these experiments were prepared from either 4 or 6 rail polyethylene material 3 and 3.4 m wide by 24 m long. Tube configuration consisted of a black heat collector tube inside a large diameter clear tube which acted as an insulator. The sun's rays pass through the clear tube and are absorbed on the black tube. The black heat collector tube was prepared by folding over a 3 m wide sheet of polyethlene and heat sealing. This produced a 0.gm dia. tube 24m long. The transparent insulating outer tube was prepared by wrapping a 3.4 m wide clear polyethlene sheet around the black tube and sealing the edges. This provided a l m dia. outer tube, giving a 5 cm air insulation layer between the tubes. Tube orientation, because of existing structures, was north-south. Air pressure and flow was provided by a 30 cm vane blower, powered with a I h.p. electric motor, blowing through a 14cm hole into the polyethylene tube, which was in turn connected to a dehydrator 0.75 x 1.5 x I m. The dehydrator held twelve trays, 0.6 m 2, each containing approxi- mately 3 kg fruit. Air velocity over the trays was 90--130 m per min.

3. DEHYDRATION

Two types of drying procedures were investigated, cross-flow without air recirculation and cross-flow with partial recirculation. In nonrecirculation drying, air only makes one pass through the dryer. The maximum temperature rise through the collector (A T) obtained by this procedure was 15°C above ambient, realizing a collector thermal efficiency of 13 per cent. In drying the fruit, dryer efficiency was found to be low, because the exiting air still contained considerable heat. This could be alleviated by using a longer dehydrator that would hold more fruit. Different tube configurations were tested to determine their efficiency in this type of drying. One such variation was to seal the black tube together in the middle down the length of the tube, forming multiple channels. This tube held less volume of air, but its

usefulness was limited because the air flow rate had to be reduced or the rib sections would rip. In dehydration, a rapid air flow rate is needed to obtain good drying.

The major emphasis using this twelve tray dehydrator was on partial air recirculation since a higher total temperature was realized, causing the fruit to dry faster. To dry by this method, the 24 m tube was either bent to form a U so some air from the dehydrator could be pulled into the fan air inlet, or a tube was prepared by cutting and taping to provide three 90 ° bends, Fig. 1. The amount of fresh air drawn into the system, and hot air exhausted, was regulated to obtain a maximum recirculation without allowing the humidity to rise appreciably. In this series of studies, cut apricots were dried using the former bent tube collector configuration, and prunes by the latter taped tube solar collector. Apricots dehydrated by this procedure dried about 38 per cent faster in the solar heated dehydrator, compared to sundrying, Fig. 2, even though the solar dehydrator was shut down overnight.

The highest temperature differential was obtained at around 1 p.m, in Suisun, California, on 14 July while drying apricots, Fig. 3, where AT of 30°C above ambient was obtained. Air flow through the dehydrator was about 130 m per min. Approximately five-sixths of the air was recirculated, with one-sixth fresh air being drawn into the system. Approximately 0.3kg of dried apricots were produced per square meter of collector area, which would translate to 1.0 kg of fruit per square meter of land area, since the collectors are cylindrical. Regular sun drying produced about 2.3 kg of fruit per square meter of land area, but a longer drying time was required.

The shaped tube, used to dry French prunes, was about 5 m shorter than the regular 24 m tube, giving an area exposed to the sun of about 22 m 2 compared to 30m 2. This dehydrator configuration did not reach as high a AT as the former dryer; however, it did reach a higher total temperature, because of a higher ambient temperature. This dryer was operated for 3 con- secutive days, being turned off at sundown and back on at sunrise. The 24kg load of prunes dried faster in the solar dehydrator than by sun-drying, with prunes dried by the former method containing 35 per cent moisture, and by the latter 45 per

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Fig. 1. Solar heated dehydrator utilizing partial air recirculation: A-dehydrator, B-fresh air inlet, C-fan, D-polyethylene, E-Air

exhaust.

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290 Technical Note

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DRYING TIME - HOURS

Fig. 2. Apricot drying curves. 80

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Fig. 3. Temperature during apricot dehydration.

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cent moisture, at the end of the third day. No mechanical problems were experienced, even though the polyethylene was in the 65°C range for about 4-6 h every day.

A temperature gradient was found to exist within the polyethylene heating tube, with the air near the top of the tube being hotter than near the bottom. This would be expected since only half of the tube is exposed to the sun at any given time and a laminar flow would be experienced which would result in minimal mixing. The gradient was only appreciable in the last third of the tube. Air deflectors could be placed inside the tube to set up turbulence.

A through-flow polyethylene heating unit could easily be used as a supplemental heat source, Fig. 4. In this method it could preheat the air going into a dryer so less gas would have to be used to raise the air to the desired temperature. The polyethylene heating tubes could be strapped on top of the dehydrator because of their light weight, or extended out into an adjoining field.

Off-season storage would be a minor problem because when deflated the tube can be folded and put into a storage box, No extended tests were run to determine the life of the tubes; however, because of their low cost they could be changed periodically. Normal dust did not reduce the heating capacity of the experimental units.

4. PRODUCT

The solar dehydrated apricots had a different appearance than the sundried fruit, with the former having a variable yellow- orange color and the latter a uniform bright orange appearance. This visual difference has been noted by other workers [2]. Sulfur dioxide retention was greater in the solar dehydrated apricots, which contained 4200 ppm SOz, comparing to 2000 ppm in the 20 per cent moisture sundried products. Also, dehydrated apricots contain more vitamin A because they have not been exposed to sunlight[3]. No difference was noted in the dried prunes.

Technical Note 291

O sundrying greater retention of vitamin A and, also, the fruit is not exposed to the elements during drying, thus minimizing damage from inclement rains. In this particular solar dehydrator design the collector and dehydrator etticiencies are low. These efficien- cies could probably be increased by increasing the length of the polyethylene tubes and, also, using multiple insulation layers. At the same same time, the dehydrator length would need to be increased so sufficient fruit contact time would be obtained to absorb a larger percentage of the heat from the air before it was exhausted.

Fig. 4. Dehydrator heated by combination gas and solar energy: A-dehydrator, B-gas flame, C-fan, D-polyethylene solar tubes,

E-cars of drying product.

These preliminary studies have shown the feasibility of using low cost polyethylene tubes to heat air for fruit dehydration. Dehydration offers the advantage of faster drying than regular

REFERENCES

1. G. H. Foster and R. M. Peart. Solar grain drying, Agric. Information Bulletin No. 401 (1976).

2. T. A. Schwarz and Fred S. Nury. How apricots react to dehydration. Canner/Packer 130, 56A (1961).

3. H. R. Bolin and A. E. Stafford. Effect of processing and storage on provitamin A and vitamin C in apricots. J. Food Sci 30, 1034 (1974).