An investigation of double-glass-covered trapezoidal salt-gradient solar pond coupled with reflector

ABSTRACT

In the present study, a trapezoidal salt-gradient solar pond (TSGSP) has been investigated experimentally. The top surface of solar pond has been covered with double-glass cover in order to reduce the evaporative and convective losses from the top. This results in increase of temperature even in the top zone of the solar pond and leads to more volume utilization for heat storage in the pond. A reflector made of aluminium sheet has been used to enhance the solar intensity on the solar pond during sunny hours. A procedure, to determine optimum tilt angle of reflector in order to utilize maximum amount of solar energy at noon, has been proposed. The use of reflector enhanced the average solar intensity on the top surface of solar pond by 22%. The maximum average temperature of trapezoidal solar pond with glass cover and reflector has been observed to be 70.5°C. The thermal efficiencies of LCZ, NCZ and UCZ for the trapezoidal solar pond with double-glass cover and reflector have been estimated to be 32.73%, 23.22% and 5.30%, respectively. In addition to experimental investigation, the sunny area ratio of TSGSP has been theoretically computed and compared with the cuboid solar pond having same top surface area and depth in order to see the effect of pond shape on sunny area ratio. The average yearly sunny area ratio of trapezoidal solar pond has been determined to be 11% higher than that of cuboid one.

KEYWORDS:

• Trapezoidal solar pond
• glass cover
• reflector cover
• shading area
• temperature distribution

Nomenclature

$A_{sh}$	=	Shading area of the solar pond, m2
$A_{su}$	=	Sunny area of solar pond, m2
$A_{sur}$	=	Surface area of solar pond, m2
$C$	=	Concentration of water, kg/m3
$C_P$	=	Specific heat capacity, J/kg K
$E$	=	Total solar energy on the top surface of solar per unit area, W/m2
$h$	=	Local solar time
$i$	=	Incidence angle
$k$	=	Thermal conductivity of salt water, W/m K
$l$	=	Length of the reflector, m
LCZ	=	Lower convective zone
$L_W$	=	Width of the square solar pond, m
$L_{WI}$	=	Width of Ith layer for TSGSP, m
$n$	=	Day of the year
NCZ	=	Non-convective zone
$Q_{Solar}$	=	Net solar energy reached to the surface of solar pond, W
$s$	=	Salinity, g/kg
$S{h}_I$	=	Shading length of the Ith layer in the solar pond, m
SSP	=	Square solar pond
$S_I$	=	Decrease the length of a solar pond due to the trapezoidal shape of the solar pond
$T$	=	Temperature of saline water, °C
TSGSP	=	Trapezoidal salt-gradient solar pond
TSP	=	Trapezoidal solar pond
$T_f$	=	Final temperature of saline water, °C
$T_i$	=	Initial temperature of saline water, °C
$T_{NCZ}$	=	Temperature of NCZ, °C
$T_{UCZ}$t	=	Temperature of UCZ, °C Time, sec
UCZ	=	Upper convective zone
$V$	=	Volume of solar pond for particular zone, m3
$x$	=	Depth of water, m
$\mathrm{\Delta }x$	=	Thickness of each layer, m
$\mathrm{\Delta }{Z}_{LCZ}$	=	Thickness of LCZ
$\mathrm{\Delta }{Z}_{UCZ}$	=	Thickness of UCZ

Greek letters

$\alpha$	=	Elevation angle
$\beta$	=	Fraction of radiation that enters in the solar pond
$\gamma$	=	Angle between the reflected sun rays and the horizontal surface of solar pond
$\theta$	=	Inclination angle of reflector
${\theta }_i$	=	Incidence angle of solar radiation with the normal of solar pond surface
${\theta }_{rf1}$	=	Refraction angle of sun rays passing through air to glass
${\theta }_{rf2}$	=	Refraction angle of sun rays passing through air to water
${\theta }_z$	=	Zenith angle
${\theta }_h$	=	Hour angle
$\phi$	=	Latitude angle
${\delta }_d$	=	Declination angle
$\rho$	=	Density of saline water, kg/m3
$\tau$	=	Transmittance