Nomenclature
AC collector area (m2)
CPW water specific heat (kJ/(kgK))
CPS sands specific heat (kJ/(kgK))
g gravitational acceleration (m/s2)
hc1 convection heat transfer coefficient between cover 1 and its upper space (W/(m2 K))
hr1 radiation heat transfer coefficient between covers 1 and 2 (W/(m2 K))
hr2 radiation heat transfer coefficient between cover 2 and ambient (W/(m2 K))
hw convection heat transfer coefficient between cover 2 and ambient (W/(m2 K))
IT incoming radiation in 1-h (kJ/m2)
K thermal conductivity for air (W/(mK))
Ke extinction coefficient (1/m)
L thickness between covers 1 and 2(m)
Kg cover thickness (m)
mW water mass into the tank (kg)
mS sands mass (kg)
n day of year
n1 refractive index in air
n2 refractive index in glass
Nu dimensionless Nusselt number
Q hourly absorbed energy by the water into the storage tank (kJ)
q1 heat flux between cover 1 and its upper space (W/m2)
q2 heat flux between cover 2 and ambient (W/m2)
qT system total heat loss (W/m2)
R1 thermal resistance between absorber and cover 2
((m2 K)/W)
R2 thermal resistance between cover 2 and ambient
((m2 K)/W)
RT thermal resistance between absorber and ambient
((m2 K)/W)
Ra dimensionless Rayleigh number
Ta ambient temperature (C)
TP temperature of absorber (C)
T Glass2 temperature of cover 2(C)
TS sky temperature (C)
DTPC temperature difference between absorber and cover 2
UT total heat loss coefficient(W/(m2 K))
Greek symbols
a thermal diffusivity (m2/s)
b tilt angle of water heating system
b0 volumetric coefficient
m kinematic viscosity (m2/s)
sa transmittance with absorption losses
r Stefan–Boltzmann constant (W/(m2 K4))
d declination
/ latitude
c surface azimuth angle
x hour angle
eP emissivity of absorber plate
eG2 emissivity of cover 2