Page 160 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 145

Overflowed energy through receiver aperture due to natural
convection is
P CONV ¼ a AP ðT w T a ÞA 1 (3.23)

We substitute Eq. (3.22) into Eq. (3.23),
P CONV ¼ a AP ðT w T a ÞA 1

0:18 1:12 0:982 l AP

1 T w 2:47 d AP L l (3.24)
¼ 0:088Gr 3 cos q
T a L L
ðT w T a ÞA 1
in which l is the air thermal conductivity under ambient air
temperature, W/(m$K).
4. Conductive heat loss P COND . Conductive heat loss of receiver
depends on the thermal radiation through the receiver wall
surface and thermal-insulating materials, values of which are
mainly determined by its thermal-insulating performance.
T w T a
P COND ¼ (3.25)
R
Provided that absorber surface temperature T w and ambient air
temperature T a have been obtained, heat loss is transmitted
through absorber to the thermal-insulating material, and further
through thermal-insulating material to receiver exterior wall
surface; the exterior wall surface exchanges thermal through natural
convection and dissipates it into the atmosphere. Thermal resistance
of this process has been calculated as follows:
1 2pkH
z þ h wb pðr AP þ dÞH (3.26)
R r AP þ d
ln
r AP
in which h wb refers to the nature convective heat-transfer coefficient
2
of receiver exterior wall surface, W/(m $ C); d refers to the thickness
of thermal insulating material, m; r AP refers to the radius of receiver
aperture, m; and H refers to the axial length of receiver, m.
Due to the nonuniform internal temperature of receiver and the
installation angle of receiver, Nusselt distribution for heat
exchange between receiver exterior surface and the air is also
complex. Here, the horizontally placed long cylinder is used to
simplify the calculation of Nusselt number through natural