H. Mortezapour - Department of Mechanics of Agricultural Machinery, Faculty of Agriculture, Tarbiat Modares University, Islamic Republic of Iran.
B. Ghobadian - Department of Mechanics of Agricultural Machinery, Faculty of Agriculture, Tarbiat Modares University, Islamic Republic of Iran.
M. H. Khoshtaghaza - Department of Mechanics of Agricultural Machinery, Faculty of Agriculture, Tarbiat Modares University, Islamic Republic of Iran.
S. Minaei - Department of Mechanics of Agricultural Machinery, Faculty of Agriculture, Tarbiat Modares University, Islamic Republic of Iran.

In the present study, saffron was dried using a heat pump-assisted hybrid photovoltaic-thermal solar dryer. The effect of different drying air temperatures at three levels (۴۰, ۵۰, and ۶۰oC) and two different modes of the dryer (with and without heat pump system) were investigated on drying behaviour of saffron. After collecting the pertinent data, eleven drying models were used to describe drying characteristics of saffron. Quality characteristics of the dried products (including: colouring, aromatic strength and bitterness) were also evaluated. The results indicated that drying time decreased by ۶۲% with increasing air temperature from ۴۰ to ۶۰oC. Moreover, applying heat pump with the dryer reduced RH of drying air and, consequently, enhanced drying rate and shortened drying period by ۴۰%. A two-term drying model presented a relatively higher R۲ and lower, MBE, and RMSE values at both modes of drying and, therefore, was selected to explain drying behaviour of saffron among the other models. The results of saffron quality evaluation showed that colouring characteristics of saffron improved with drying temperature and heat pump system. Meanwhile, aromatic strength of saffron increased with increasing air temperature. But, no significant change in bitterness was observed at different levels of temperature and heat pump system.

PV-T dryer, Mathematical modelling, Quality drying, Two-term model