The proposed small-sized parabolic trough solar collector (PTC)
consisted of a sheet of reflective mirror with parabolic shape and a
receiver tube located at the focal line of the concentrator. The
reflective mirror, whose reflectance could be 0.93, was a kind of
tempered glass with silver plating on the back. Different from traditional
one-way metal–glass evacuated tube, the receiver of this
new small PTC was a non-evacuated imitated U-type receiver tube.
As is shown in Fig. 1(a), the special solar receiver consisted of a
finned aluminum tube, a double glazing evacuated tube, supports
and insulation stuffing. There was a metal sheet dividing the aluminum
tube into two halves, forming an imitated U-type. Metal
fins, with black absorbing coating on their surfaces, have been
fixed on the outside of the aluminum tube. Due to multiple reflection
and absorption of the fins, an artificial blackbody would be
formed, which improved the absorptivity of the receiver
significantly. The double glazing evacuated tube was composed
of two layers of glass, the space between which was kept vacuum,
and a metal elastic support was placed in the space at the sealing
Nomenclature
A aperture area of the collector (m2)
c specific heat of the fluid (J/kg C)
d inner diameter of circular tube (m)
d0 equivalent diameter of the absorber tube (m)
ID direct normal solar radiation intensity (W/m2)
Ieff effective solar radiation intensity (W/m2)
L length of the collector (m)
m_ mass flow rate of the fluid (kg/s)
Q gained heat of the fluid (W)
T fluid temperature (C)
V_ volume flow rate of the fluid (m3/h)
W width of the aperture (m)
Greek symbols
g thermal efficiency
k thermal conductivity of the fluid (W/m C)
m kinematic viscosity (m2/s)
h solar incident angle ()
q fluid density (kg/m3)
t/t0 fluid flow velocity (m/s)
Subscripts
c collector
f fluid (thermal oil)
i inlet
o outlet
Abbreviations
ETC/ETCs evacuated tube solar collector
FPC/FPCs flat plate solar collector
HTF heat transfer fluid
PTC/PTCs parabolic trough solar collector
Re/Re0 Reynolds number
SWH solar water heater