Page 143 - Design of Solar Thermal Power Plants

128 3. GENERAL DESIGN OF A SOLAR THERMAL POWER PLANT

40000
35000 Cleaning expense
Feed-in tariff loss /YUAN 25000 Feed-in tariffloss, 0.8 YUAN/kWh
30000
Feed-in tariffloss, 1 YUAN/kWh

20000
15000
10000
5000
0
1 6 11 16 21 26 31 36 41
Time/day
FIGURE 3.4 Defining the mirror cleaning time.

reflective surface may drop even faster. In the case of cleaning the mirror
twice a month, it is more appropriate to take a value of about 83% as
annual mean reflectance.
The ratio of cleaning times, energy loss, and paid cleaning expense
shall be calculated. Currently, the cleaning expense for each square meter
of mirror is about 2 yuan. For a heliostat concentration field of 10,000
square meters, the respective cleaning expense would be 20,000 yuan. The
daily loss of reflectance of the mirror is 0.8%. When the expense of power
loss exceeds the concentration field cleaning expense, the cleaning process
shall start. Fig. 3.4 indicates the cleaning time defined by the intersection
of the mirror cleaning expense and economic loss caused by power
attenuation from dust accumulation loss.
Assuming the feed-in tariff of an CSP plant is 1 yuan/kWh, the
cleaning time indicated in Fig. 3.4 approximates 27 days. Obviously this
value changes according to the cleaning costs, daily dust accumulation
status, and feed-in tariff of the power plant. After the mirrors are cleaned,
reflectance bounces back to 94% from 72.4%.
Assuming the feed-in tariff of the CSP plant is 0.8 yuan/kWh, the
cleaning time indicated in Fig. 3.4 approximates 30 days.

3.2.5 Atmospheric Transmittance Analysis
Atmospheric transmittance is related to the degree of local sand blow.
Due to the short focal distance of a parabolic trough collector, it is not
necessary to consider transmittance. However, for a large-scale power
tower plant, this factor must be considered.
During transmission of optical waves in the atmosphere, absorption
and scattering of atmospheric gas molecules and aerosols may result in