4.9 Solar drying of agricultural produce in Israel present experience and future prospects
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- I.J. Kopelman
The purpose of the following document is to outline the major current activities in solar drying of fruits, vegetables, herbs etc. in Israel. The document is divided into two main parts; an overall view followed by a detailed description of the activities.
Israel has a large commercial vegetable and herbs solar drier plus a few similar semi-commercial installations. The units are operated by indirect air-heated flat surface solar roofs backed up by fossil-fuel heating. Substantial activities in the country are also related to the development of solar systems hardware (collectors' glazing components, storage of heat, etc.) that can support, among other things' a drying operation. Such development involve: (a) simple and practical maintenance for small driers; (b) total systems for low temperature air heated flat surface solar roofs; (c) parabolic solar collector systems for steam generators. The Government encourages the utilization of solar energy in commercial plants by subsidizing (up to 50%) the capital cost of the solar installations. Experience indicates that the economic feasibility of solar drying is at present of a limited nature. It depends upon climate; type of product and length of season; cost of alternative sources of heat (coal' fuel oil, diesel oil? gas); cost of capital, etc. However, efforts are continuously being made to improve the durability of the solar drier components. A breakthrough in technology of storage of solar heat will also be required for economical large scale solar drying operations.
PRESENT SITUATION OF DRYING UNITS
SMALL DRYING UNITS
Description of the large drying unite is as follows:
A solar roof with an area of 1 100 sq.m has been operating for the past few years in the "Maon Spices" plant, at the Western Negev. The solar roof is being used as a substantial energy source for the dehydration process of the agricultural products: parsley, paprika and wild marjoram. The herbs and plants being used as raw material for the production of spices are harvested from the fields of the Maon region settlements. They arrive at the plant with a water content of 80%-85% and after being dehydrated down to a water content of 6%-10%, in a drying chamber on a total area of 300 sq.m of perforated moving belt.
Rock wool insulation sheets 25 mm thick, were installed on the roof support beams' on top of which black painted corrugated steel plates were incorporated. At a few centimeters from the steel plates wee put a transparent rigid plastic layer UV resistant (Qualex) which is used for glazing.
Fresh air enters through adjustable openings at the upper end of the roof. The air flows between the glazing layer and the steel plates and is heated as it proceeds through 30 cm wide ducts.
Solar energy is used for preheating of air used in a spray-drying operation in the Hills-Koors Algae Production Ltd. The solar roof is of approximately 150 sq.m. It consists of prefabricated panels composed of 4050 mm cast-foam insulation' selective-surface coating and tough, nonreflective solar film glazing. The panels (marketed by Energy Systems and Engineering under the name SOLIRAC) are highly thermal efficient; however, the current price (approximately 90$ per sq.m of panel) make the unit somewhat expensive. Nevertheless, the system could be found to be costeffective in cases where the climate is particularly favourable for solar heating and particularly in new structures where the panels can be installed in place of regular roof components.
(Under construction - scheduled to go into operation - May 1984) It is a modern drying plant (approximately 1 ton per hour throughput) for herbs and spices (primarily oregano, basil and sage). Although the prime source of energy for drying could be from fossil fuels, a 15-20% saving of energy is expected by means of simple and inexpensive solar heating. The single storey plant is constructed from prefabricated metal sheets (Behlen structural systems) resulting in a structure having a 24 m span, 33 m long convex metal roof with an approximately 3 200 cubic meter of space. The roof will be painted with black lacquer. Insulation matresses will be placed upon the upper ceiling panels.
Such simple modifications will yield an approximately 950 sq.m maintenance free solar roof with practically no added costs to the structure and with an expected total energy saving of 15% to 20%.
Further possibilities for modifications of the solar roof will be studied during the full operation of the plant. These will include the feasibility of adding glazing material and selective painting of the roof resulting in further saving of energy.
SMALL DRYING UNITS
The Electra Company has been engaged in the last several years in a development program for solar air driers. The program has been concentrating on domestic size scale units with emphasis on simplicity and low cost per unit.
Other small scale units for solar drying have been evaluated in different research institutes primarily by the Agricultural Research Organization at the Volcani Center.
SCHEMATIC DIAGRAM OF THE AFULA DRYING - BEHLEN TYPE CONVEX ROOF (by INTRADCO LTD.)
SCHEMATIC DIAGRAM OF THE MAON SOLAR ROOF DRYING INSTALLATION
SCHEMATIC DIAGRAM OF THE SOLIRAC INSTALLATION (by ENERGY - SYSTEMS AND ENGINEERING LTD.)
ELECTRA (ISRAEL) LTD.. FOOD DRYING BY SOLAR ENERGY DOMESTIC DRYING UNIT
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