Page 185 - Design of Solar Thermal Power Plants

170 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

heat-transfer fluid; T a refers to the ambient air temperature; A a refers to
the aperture area of parabolic trough collector; A f refers to the heat
exchange area between metal evacuated tube and heat-transfer fluid; A am
refers to the heat exchange area between metal evacuated tube and the
environment; U bf refers to the heat-transfer coefficient between metal tube
and heat-transfer fluid; U ba refers to the comprehensive heat-transfer
coefficient between metal evacuated tube and the environment; srefers
to time; S refers to the part absorbed by the exterior wall surface of metal
evacuated tube when solar DNI is perpendicular to the aperture of
parabolic trough collector. The last one to the right of the equation is to
express the heat exchange process between metal evacuated tube and the
neighboring environment, which is normally within the glazed shield
tube.
Similarly, the energy balance equation can be established for the heat-
transfer fluid within the metal tube of parabolic trough collector, which
can be expressed as

dT f
C f ds ¼ A U ðT T Þ _ mc ðT T Þ (3.39)
f
b
f
bf
fi
fo
f
in which C f refers to the thermal power of heat-transfer fluid; c f refers to
the specific thermal capacity of heat-transfer fluid; T fi refers to the inlet
temperature of the tube; T fo refers to the outlet temperature of the tube; _ m
refers to the mass flow.
1/A f U bf and 1/A am U ba are substituted with thermal resistance R bf and
R ba separately, then Eq. (3.38) and Eq. (3.39) can be once again expressed
as:
T T
dT b b f T b T a
C b ds ¼ SA a R bf R ba (3.40)
And
dT f T b T f
C f ¼ _ mc f ðT fo T fi Þ (3.41)
ds R bf
Then Eq. (3.41) is reorganized into

T b dT f T f
¼ C f þ þ _ mc f ðT fo T fi Þ (3.42)
R bf ds R bf
On both sides of the above equation with time s, the derivative is taken,
then
2
1 dT b d T f 1 dT f dT fo dT fi
¼ C f þ þ _ mc f (3.43)
R bf ds ds 2 R bf ds ds ds