Development and recent trends in greenhouse dryer: A review

Published Date
Renewable and Sustainable Energy Reviews
November 2016, Vol.65:1048–1064, doi:10.1016/j.rser.2016.07.070

• Author 

• Sumit Tiwari ^a,,
• G.N. Tiwari ^a
• I.M. Al-Helal ^b

• aCentre for Energy Studies, Indian Institute of Technology Delhi, Hauzkhas, New Delhi 110016, India
• bDepartment of Agricultural Engineering, College of Food & Agriculture Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia

Received 7 September 2015. Revised 26 June 2016. Accepted 21 July 2016. Available online 29 July 2016. 

Abstract

Food is necessity for human being. As the world population is increasing, it is very difficult to fulfil everyone's need of food. One of the alternatives of this problem is the preservation of crops, vegetable and fruits when it is available in abundant amount. Drying is one of the best methods to preserve agricultural products for long time but it requires lot of energy. As availability of electricity per capita in developing and under developed countries is very less, thus the electricity uses for heating purpose cannot be economically and environmentally justified option. So entrapment of thermal energy from solar radiation may be the best option for drying. Solar energy can be utilized for drying in different ways namely open sun drying and closed drying (direct and indirect). Open sun drying have various disadvantages like contamination of dust particles, bacteria in crop, decolouration of the product after drying etc. To overcome these problems greenhouse drying or closed drying has been developed. This review is an attempt to explore different types of drying systems was developed across the world. Further different thermal modelling, mathematical modelling and performance evaluation on the basis of characteristic curve have been discussed. One of the thermal modelling has been discussed in detail to evaluate heat transfer coefficient, heat absorbed and moisture evaporate with experimental validation to give practical exposure to the researchers.

Keywords

Deep bed drying

Direct solar dryer

Greenhouse dryer

Indirect solar dryer

Thin layer drying

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Nomenclature

useful energy collected

absorptivity of the absorber plate

overall heat loss coefficient

ambient temperature

air flow rate

specific heat

temperature in mixing chamber

effective total radiation

surface area of collector

fraction of solar radiation on north wall

absorptivity of jaggery

area of greenhouse wall

mass of jaggery

jaggery temperature

room temperature

vapour pressure at jaggery temperature

relative humidity in greenhouse

ground absorptivity

ground temperature

ground area

area of vent

gravity

difference in vapour pressure

L_c

characteristic length

P(T_p)

partial vapour pressure of moist air at crop surface temperature

relative humidity of moist air above crop surface

A_t

area of tray

MR

moisture ratio

a, b, c

constant

µ_v

dynmic viscosity

V_v

wind velocity

available solar energy falling on the collector,

transitivity of the glazing

average fluid temperature in the collector

collector efficiency

temperature rise

convective heat transfer coefficient on the inner layer

initial temperature

mean temperature in bottom compartment

outer convective heat transfer coefficient

fraction of solar radiation on jaggery

intensity on greenhouse wall

transitivity of greenhouse wall

specific heat of jaggery

convective heat transfer coefficient of crop

area of jaggery

vapour pressure at greenhouse air temperature

convective heat transfer coefficient greenhouse floor to ground,

temperature of surface of floor of greenhouse,

convective heat transfer coefficient greenhouse floor to room

coefficient of discharge

difference in pressure head

overall heat loss from greenhouse wall

h_c

convective mass transfer coefficient due to moisture evaporation in crop drying

K_v

thermal conductivity of humid air

P(T_e)

partial vapour pressure of moist air above crop surface

latent heat of vaporization

time interval

k, n

drying coefficient or empirical coefficient

β'

coefficient of thermal expansion=(1/(T+273))

ρ_v

density

T_fi

initial fluid temperature

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 Table 1

Table 1.

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 Table 2

Table 2.

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Fig. 10.

 Table 3

Table 3.

 Table 4

Table 4.

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