In a hybrid solar dryer, solar energy with a conventional or some auxiliary source of energy such as electricity, bio-mass, etc. can be used in combined or single mode. It combines both the actions, direct solar radiation heating as well as preheating of air using an auxiliary energy source. 
Ferreira et al. Developed a hybrid dryer for banana drying. Thermal characteristics of the dryer were observed better than open and artificial dryer. The energy economy of the dryer was reported to be 38% with respect to the artificial dryer. The drying time obtained in hybrid solar dryer was observed lower than open and artificial drying for the same outlet air temperatures. Prasad et al. presented an experimental study for ginger drying and compared the results with solar biomass hybrid drying and open sun drying. The drying rate of hybrid dryer was reported more than open sun drying. The overall drying efficiency of drier was observed to be 18% and 13% under summer and winter climatic conditions respectively. Based on the annuities of expenditure, the increase in benefits using this dryer was found to be 104 US$ with the payback period of 1 year. The schematic view of hybrid solar dryer is shown in Fig (1).
Fig.(1). Hybrid solar dryer
Thanaraj et al. Studied copra drying having different compositions by using solar hybrid drier and suggested processing of high quality white copra in a solar hybrid dryer. The thermal efficiency was reported to be 15.5%. Juraev and Akhatov developed a mixed mode forced convection solar drier that can be used as a greenhouse in winter time and as a solar drier during rest of the year. The overall drying cost of the dryer could be considerably high due to the use of solar panel to run the air fan for forced convection. The schematic view of mixed mode forced convection solar drier is shown in Fig (2).
Fig. (2). Mixed mode force convection solar drier
Bolaji and Olalusi evaluated the performance of a mixed mode solar dryer for yam chips drying. The drying rate, collector efficiency, and percentage of moisture removed (dry basis) obtained were 0.62 kg/h, 57.5% and 85.4% respectively. Madhlopa and Ngwalo designed a convective solar dryer with a biomass backup heater having three modes of operation (solar, biomass, and solar-biomass) for drying pineapple. The solar mode yielded the highest thermal efficiency whereas the biomass mode exhibited lowest efficiency. However, the dryer performed most satisfactorily with a backup heater with overall efficiency of 13±2%. Hossain et al. developed a hybrid solar dryer for tomato drying having drying efficiency varying from 17 to 29% depending on operating conditions. Plane reflector used in the dryer reported 10% increase in the collector efficiency. The schematic view of hybrid solar dryer integrated with auxiliary heating and drying unit is shown in Fig (3).
Fig. (3) Hybrid solar dryer integrated with auxiliary heating
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In today’s world of advanced technology, hybrid solar dryers are the best alternative device available for fast solar drying of products with required product quality. The average collector efficiency in these dryers is observed to be 62.5%. The hybrid solar dryer efficiency is observed to vary from 17 to 29% which can be increased by employing certain techniques like plane reflector, glazing effects, recirculation of drying air, etc. The drying efficiency can also be improved by reducing the thermal losses and by making homogeneous product drying.
References
- Hossain, B. Amer, K. Gottschalk
Hybrid solar dryer for quality dried tomato
Int J Dry Energy, 26 (12) (2008), pp. 1561–1601
- Ferreira, A. Charbel, R. Pires, J. Silva, C. Maia
Experimental analysis of a hybrid dryer
Therm Eng, 6 (2) (2007), pp. 03–07
- Prasad, A. Prasad, V. Vijay
Studies on the drying characteristics of zingiber officinale under open sun and solar biomass (Hybrid) drying
Int J Green Energy, 3 (2006), pp. 79–89
- Thanaraj, N. Dharmasena, U. Samarajeewa
Comparison of drying behaviour, quality and yield of copra processed in either a solar hybrid dryer on in an improved copra kiln
Int J Food Sci Technol, 42 (2007), pp. 125–132
- Juraev, J. Akhatov
Study of mixed-mode forced convection solar dryer
Appl Sol Energy, 45 (1) (2009), pp. 61–64
Bolaji B, Olalusi A.
Performance evaluation of a mixed mode solar dryer.
Technical Report, vol. 11; 2008. p. 225–31
Madhlopa A, Ngwalo G.
Convective solar dryer with a wood waste backup heater for dehydration of food.
University of Malawi – The polytechnic, Private Bag 303, Chichiri, Blantyre 3; 2005
Written by .
Syed Mudabbar Hussain Shah
The Author is final year student of B.Sc (Hons.) in Food Engineering Department of Food Engineering University of Agriculture, Faisalabad
