Page 165 - Design of Solar Thermal Power Plants

150 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

The above heat loss has been calculated without considering the
influences of wind speed and direction. For a commercial power
plant, the receiver is normally mounted inside a sealed absorption
space, in which the influence of wind on conductive heat loss can
be neglected; yet its influences on convection loss shall be
considered.

3.3 THERMAL PERFORMANCE OF PARABOLIC
TROUGH COLLECTOR

Calculation of the efficiency of parabolic trough collector is compara-
tively complex, which is related to the solar irradiance, axial layout of
concentrator, optical performance of concentrator, working temperature
of heat-transfer medium, ambient air temperature, wind speed, and
concentration field features; thus it is very difficult to ensure the precision
of calculation. Normally, an efficiency calculation formula shall be offered
by the equipment manufacturer.

3.3.1 Parabolic Trough Receiver Tube Heat Loss Parameters
As variable properties of vacuum, transparent glass tubes and coatings
against temperature variation are involved, the heat-transfer theory
calculation on evacuated tube heat loss is comparatively difficult, which is
normally obtained through experimental measurements. The test data of
heat loss coefficient of the evacuated tube from SCHOTT of Germany is
shown below, which is taken as an example to demonstrate the approx-
imate range of heat loss coefficient of the evacuated tube.
Table 3.3 has listed data corresponding to Fig. 3.20. The unit of heat loss
coefficient is the heat loss power along unit length of axial line of the
evacuated tube: W/m.
In Fig. 3.20, the lateral axis is the difference of evacuated tube tem-
perature and ambient air temperature, whereas the vertical axis is the heat
loss coefficient of the evacuated tube (W/m). When the temperature
difference is 293 C, the respective heat loss coefficient is about 113 W/m;

when the temperature difference is 393 C, the respective heat loss
coefficient is about 257 W/m. Presently, there has been no China state
standard on the testing of the evacuated tube heat loss coefficient.
When using various unit measured values, it is necessary to carefully
check the testing approaches and conditions described in the testing
report. The heat loss coefficient is related to the thermal charging
mode, materials of heater, steady state conditions during temperature
measurement (ambient air temperature, heater temperature), position of