Thermal performance analysis of natural convection solar dryers under no load condition: experimental investigation and numerical simulation

ABSTRACT

The present work evaluates the thermal performance of direct, indirect, and mixed mode passive solar dryers under no load condition in terms of efficiency and convective heat transfer coefficient between absorber plate and moving air (h_c,p-a). The temperature measurements of various components of the collector and drying chamber of solar dryers were monitored at regular time intervals throughout the experimental period. Based on the energy balance equations of different parts of the dryer and experimentally observed temperatures, the h_c,p-a was determined. The no load efficiency was found to be 31.40%, 27.55%, and 41.43% in direct, indirect, and mixed mode solar dryers, respectively. The efficiency was observed to be a function of temperature difference between absorber plate and air and the radiation intensity. The average values of h_c,p-a was found to be 16.31, 14.92, and 23.81 W/m²°C in direct, indirect, and mixed mode solar dryers, respectively. Finite element (FE) model was developed using COMSOL Multiphysics software to study the temperature distribution of air inside the solar dryers. The lower values of statistical errors indicated a close agreement between the experimental and predicted air temperature inside the dryers.

KEYWORDS:

• Thermal performance
• no load
• solar dryer
• COMSOL Multiphysics
• convective heat transfer coefficient

Nomenclature

A	=	Absorber area, m2
Cp	=	Heat capacity of air, kJ kg−1 K−1
hc,a-g	=	Convective heat transfer coefficient between hot air and glass cover, W m−2K−1
hc,p-a	=	Convective heat transfer coefficient between absorber plate and air, W m−2 K−1
hr,p-g	=	Radiative heat transfer between absorber plate and glass cover, W m−2 K−1
hw	=	Wind heat transfer coefficient, W m−2 K−1
I	=	Solar radiation intensity, W m−2
Lins	=	Insulation thickness
ṁ	=	Air mass flow rate at collector inlet, kg s−1
Qu	=	Useful thermal energy, W
Ta-i	=	Temperature of air at collector inlet, K
Tamb	=	Ambient air temperature, K
Ta-o	=	Temperature of air at drying chamber outlet, K
Ts	=	Sky temperature, K
${\bar{T}}_g$	=	Average glass cover temperature, K
${\bar{T}}_a$	=	Average hot air temperature, K
${\bar{T}}_p$	=	Average absorber plate temperature, K
Ub	=	Bottom heat loss coefficient, W m−2 K−1
Ut	=	Top heat loss coefficient, W m−2 K−1
Nu	=	Nusselt number
Ra	=	Rayleigh number
K	=	Thermal conductivity of air, W m−1 K−1
ν	=	Velocity of air at the inlet of the dryer, m s−1
ρ	=	Density of air, kg m−3
ε	=	Emissivity of glass
σ	=	Stefan-Boltzmann constant, W m−2 K−4