Page 162 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 147

When Eq. (3.30) is applied, it is necessary to know the mean
temperature T wb of receiver exterior wall surface, which is
calculated as follows:

2 3
2pkH
P COND ¼ 4 5 ðT w T Þ (3.31)
wb
r AP þ d
ln
r AP
Through substituting Eq. (3.27a) into (3.30), and according to Eq.
(3.31) ¼ Eq. (3.30) for energy balance, then T wb can be calculated and
substituted into Eq. (3.31) in order to obtain P COND .
In actual projects, while considering the safety of receiver,
according to the requirement of design norm, receiver surface

temperature shall not exceed 80 C; otherwise, it may result in
fire hazards of cables and other inflammable equipment on
receiver exterior surface. When calculation is conducted through

Eq. (3.31), it is also possible to substitute T wb ¼ 80 C into the
equation, so that

2 3
2pkH
P COND ¼ 4 5 ðT w 80Þ (3.31a)
r AP þ d
ln
r AP
In case of considering the influence of wind, it is also possible to
directly calculate the forced convective heat-transfer coefficient of
receiver exterior surface under forced convection conditions.
5. Receiver heat loss. By substituting the above into Eq. (3.9), heat loss
can be calculated as follows:

P LOSS ¼ P REFCAV þ P RAD þ P CONV þ P COND

2 3 4 4
ε w s T T A 1
a w w g
¼ 1 5 P AP þ
4
A 1 A 2
1 ð1 a w Þ 1 1 ð1 ε w Þ 1
A 2 A 1

0:18 1:12 0:982 d AP

1 T w 2:47 d AP L
þ0:088Gr 3 cos q
T a L
2

l 2pkH
ðT w T a ÞA 1 þ 4
L r AP þ d
ln
r AP
3
2
1

þpl 0:6 þ 0:32Ra 5 ðr AP þ dÞ ðT w T a Þ (3.32)
5
wb