Figure 2 shows the measurement data of the Bergamo collector on 2007/07/18, a sunny summer day. During this day only 10 out of 11 mirror rows were operated. Until 14:07 the collector power stays between 35 kW and 40 kW. On this day first tests of the automatic outlet temperature control at a set-temperature of 195°C were performed. By defocusing individual mirror rows, the mirror control software limits the maximum operation temperature. At 14:07 the set-temperature was reached and the collector power was reduced by turning several lines of primary mirrors out of focus. Five out of 10 primary mirror lines were automatically turned out of focus, i. e. instead of 120 m2 only 60 m2 primary mirror area were active, whereupon the collector power dropped to approx. 50% (green line in Figure 2). The collector efficiency refers to the active primary mirror area and therefore stays on the same level (red line in Figure 2). The outlet temperature dropped from approx. 196°C to 191°C, whereupon the inlet temperature dropped with a time delay from approx. 189°C to 186°C as soon as the temperature drop reached the collector inlet.

Shortly later these five primary mirror rows were brought back into focus and subsequently fluid temperature and power were rising again. When half an hour later the outlet temperature reached 199°C two primary mirror lines were taken out of focus, so that the outlet temperature decreased slowly to 195°C. At 15:00 both mirror lines were brought back into focus — power and temperature rose again before the system was turned off completely at 15:10.

1000

900

800

700

600

500

400

300

200

100

0

12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30 15:45 16:00

Time (GMT +1h)

Date: 2008/07/16

1000

900

800

700

600

500

400

300

200

100

0

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Time (GMT +1h)

Figure 3. measurement data of the PSE Fresnel collector in Bergamo, Italy, on July 16th 2008;