ABSTRACT The performance of the greenhouse solar heat collector was highly dependent on the solar radiation, ambient temperature and relative humidity. Increase in air temperature inside greenhouse varied from 0.7 to 19.0C, and relative humidity inside greenhouse varied from −16 to 25.7% by air exchange at a flow rate of 6.1 m3/s. The average midday thermal efficiency was around 20.82%. The cured red onion (Allium cepa L.) bulbs were cleaned, outer layer was peeled and trimmed manually, and washed in water to remove dirt and infection. Edible bulb of onion was sliced into 3-mm thick slices by using manual stainless steel slicer. The onion slices were pretreated with sodium chloride, and potassium metabisulphite, for 5 min in three concentration levels (0.25, 0.50 and 0.75%) and drained onion slices were spread in thin layers in trays before keeping them in greenhouse for drying. Onion slices were dried in 19 sunshine hours. Preservatives used as pretreatment in 3-mm sliced onion were found to be significantly different (probability, P < 0.05) from each other. It was found that 0.50% potassium metabisulphite was best. PRACTICAL APPLICATIONS Availability of solar energy is abundant in the Indian subcontinent and drying of agricultural produce using low cost greenhouse is one of the latest developments when crop production is not possible in greenhouse duringsummer season. Low cost greenhouse can be utilized to dry the onion slice asalternative to conventional mechanical dryers. Greenhouse drying can be used for the preparation of shelf-stable dehydrated onion slices for the purpose of use during off-season. The quality of dehydrated onions is good and it can be stored for months together. Dried onion slices can be used same as fresh onion after rehydration, apart from this, it can be added directly into soups, etc., before cooking.