Polyhouse-Style Solar Drying Tunnel

A polyhouse-style solar tunnel dryer is a passive or hybrid drying structure built with a curved transparent UV-stabilised polythene cover over multi-rack drying trays. It is used to dry chillies, turmeric, coriander, ginger, fish, coconut, papad, fruits and herbs at higher and more uniform throughput than open sun drying.

Function

Ambient air enters through an inlet vent, is solar-heated as it passes over a black absorber plate and through the canopy of trays inside the tunnel, picks up moisture from the product, and exhausts through outlet vents (natural draft) or fans (forced draft). The tunnel both raises drying temperature and excludes rain, dust, insects, rodents and birds, which open sun drying does not.

Design and specifications

ICAR-CIPHET Ludhiana (Central Institute of Post-Harvest Engineering and Technology, established 3 October 1989) has developed a smart microcontroller-controlled solar tunnel dryer that integrates phase-change material thermal storage, load cells for continuous weight monitoring, and DHT22 humidity sensors. The controller switches between recirculation (when outside air is wetter than tunnel air) and exhaust (when outside air is drier), reducing drying time and improving product quality.

Operation

Product is loaded onto stainless-steel-mesh trays in single layers. Inlet and outlet vents are adjusted manually in passive units, or controlled automatically by the microcontroller in smart variants. Drying capacity ranges from a few hundred kilograms to a few tonnes per batch depending on tunnel size.

Subsidy and adoption

The technology is supported by the Ministry of New and Renewable Energy (MNRE) and by state horticulture missions through capital subsidy. Adoption is heavy in spice-growing regions of Andhra Pradesh, Telangana, Karnataka, Tamil Nadu, Kerala and the North-East.

References

Salient Technologies Developed by ICAR-CIPHET. ICAR-CIPHET.
Microcontroller-Based Solar Tunnel Dryer. Plant Science Today.