Indirect solar dryer
An indirect solar dryer heats air in a separate collector, then moves it through a covered food chamber. Designs vary in size, materials and airflow.
Equipment · Drying kilns and dryers
Indirect solar collector and drying cupboard
A solar air collector that heats air separately, then passes it through a shaded tray cupboard and out of a chimney. The product never sits in direct sunlight.
What it is
An indirect solar drier splits the job of drying into two parts. A collector heats the air. A separate cupboard holds the product and receives that warm air. The product is shaded, so it never sees direct sunlight. The FAO mission report describes three components and the routes between them, and it treats the indirect drier as a distinct type from the direct cabinet, in which sun falls on the product.
The traditional version described by the FAO is a double-wall wooden cupboard with a separate collector and a small chimney. Its load is four mesh trays of 0.5 square metres each. A later 2014 preliminary report reproduces a dimension drawing credited to a postharvest education source. That drawing is marked not to scale and is used here as a reconstruction aid only.
How the air moves
Sunlight heats the black-painted absorber under a transparent collector cover. The warmed air rises or is drawn into the cupboard through a lower opening, passes up through the mesh trays, and leaves through a chimney or a fan at the top. The FAO report also discusses recycling some of the air. The 2014 drawing labels internal baffles that guide the air in the cupboard, but the baffle sizes are not given.
The collector can be tilted to face the sun and set apart from the cupboard. That separation is what lets the cupboard stay in shade. The sources do not record a flow rate or a pressure drop for the collector, so the balance between collector area and load cannot be checked from them.
Building and testing
A build would start with the collector tray and its black absorber, then seal the transparent cover. The cupboard is built as a double-wall box with insulating space, and the air inlet, tray rails and baffles are fitted. Trays go in, the chimney cap goes on, and the collector is sealed to the cupboard. The order is an interpretation, because the sources give no complete assembly sequence.
Before use, check that air moves through the trays, that the joints do not leak, and that rain cannot enter. Record inlet and outlet conditions over sunny and cloudy days. These checks describe the apparatus only.
Sources for this section
History
An FAO mission report by E. Espiard and S. Mihailov describes the indirect drier from field visits in Sudan, Kenya, Zaire and Upper Volta in August and September 1983. A 2014 preliminary report from the University of California records the traditional FAO design as a double-wall wooden frame with a collector and small chimney.
Equipment details
SizeThe 2014 drawing labels a collector about 150 cm long on a base of about 135 cm and 52 cm wide, with a chimney of about 20 cm diameter. The drawing is marked not to scale, so these are labels, not a build schedule.
PowerSolar The collector runs on sunlight. Some versions add a fan or wind ventilator; the sources describe these as options.
How you get oneBuilt yourself The FAO and the 2014 drawing show the layout. Neither gives a full cut list, so the build depends on measured drawings.
CareClean the collector cover and the trays between seasons and batches. Check the collector-to-cupboard joint for leaks, since a leak lets air bypass the trays.
Made of
- Transparent collector cover — Lets sunlight heat the collector
Glazing type and fixing are not specified by the sources. - Black-painted absorber — Absorbs sunlight and warms the air in the collector
The sources describe a black-painted bottom; the paint and substrate are not specified. - Double-wall wooden cupboard — Holds the trays inside an insulated frame
The traditional FAO cabinet has a double-wall wooden frame. Wall thickness and insulation are not given. - Mesh trays — Carry the product in the warm air stream
Four trays of 0.5 square metres each in the traditional version. The 2014 drawing label of 100 by 50 by 2 cm does not match that area and is not reconciled. - Chimney cap — Lets moist air leave the cupboard
About 20 cm in diameter in the drawing; its height is not specified.
Risks and precautions · Standard risk
Crop-only validation
The traditional indirect cabinet was studied for crops. Using it for other products relies on evidence the sources do not provide.
Hazard sourceBypass leaks
A gap at the collector-to-cupboard joint lets air skip the trays, so product dries unevenly and moisture is not carried away.
Hazard sourceSeal the collector-to-cupboard joint and check it before each season
Control sourceMeasure air temperature and humidity at the inlet and the outlet
Control sourceTest the airflow before loading the cupboard. A cupboard that moves little air dries slowly however much sun the collector gets.
This equipment record makes no food-safety claim. Product safety needs process-specific validation.
Limits
Pressure loss, shadowing and heat loss in the collector set how much air reaches the trays. The sources do not give flow or pressure figures for any version.
Source →The 2014 drawing is marked not to scale and its tray dimensions conflict with the stated tray area. Any cut list has to come from a reconciled source.
Source →The traditional FAO design was studied for crop drying. Its suitability for meat or fish is not established by the sources.
Source →Places
Sudan
Documented use · Country evidenceThe mission visited Sudan in 1983 and reported on indirect driers. The report is not an inventory of the design in this country.
Kenya
Documented use · Country evidenceThe mission visited Kenya in 1983. The report does not inventory the design per country.
Democratic Republic of the Congo
Documented use · Country evidenceThe mission visited the country when it was named Zaire. The report does not inventory the design per country.
Burkina Faso
Documented use · Country evidenceThe mission visited the country when it was named Upper Volta. The report does not inventory the design per country.
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Sources
FAOMethod · History
Report of the second preparatory mission on sundrying techniques in Africa, section Description of an indirect drierAn indirect drier has three components: an external glazed air heater, a drying cupboard holding trays and an exhaust chimney or extractor.
The mission visited Sudan, Kenya, Zaire and Upper Volta in August and September 1983.
Cited in: What it is; How the air moves
University of California Davis, Horticulture Innovation Lab (WFLO)Method · History
Drying and Cooling Technologies preliminary reportThe traditional FAO indirect cabinet is a double-wall wooden frame with a collector and small chimney, with four trays of 0.5 square metres each.
The drawing is marked not to scale; the chimney is about 20 cm in diameter.
Collector labels: 150 cm length, about 135 cm horizontal base, 52 cm width.
Cited in: What it is; How the air moves; Building and testing