The customer’s goal from the beginning was to mount solar panels on the roof of a utility building at the Shanghai Flower Port in order to provide electric power for greenhouses to keep them cool in the summer and warm in the winter. So a low-profile horizontal beam tracking 3-sun system was designed. The building block is a 25-kW array as shown in Figure 12.12. It consists of seven beams oriented in the north-south direction driven by a motor and one central drive beam. There are twenty-four 3-sun panels astride each beam.
The system that was designed and built is a nominal 100-kW (PTC) system consisting of four of these building blocks. It therefore contains 4 x 7 x 24 = 672 panels. A photograph of the completed roof-mounted 100-kW array is shown in Figure 12.13. The installation of this system was completed in the middle of November of 2006. An SMA Sunny Central inverter was used along with string monitors to read the outputs from each of the 28 strings corresponding with the 28 beams.
The system performance is documented in Figure 12.14 (left) where the current for one of the typical 28 strings is shown along with the system output
Figure 12.12. Design drawing for 25-kW horizontal beam tracked array consisting of seven beams, each with twenty-four 3-sun modules, and one drive motor and drive beam.
Figure 12.13. Photographs of the completed 100-kW 3-sun array and a series string.
voltage over a day in November of 2006. The system performance is as expected. Since the sun is at 50° off normal toward the south in the wintertime for this horizontal beam system, the max-power current reading of 5 A per string needs to be divided by cos (50) = 0.64 to predict peak summertime operation. From these data, the system peak AC power in the summer will then be (5 x 28/0.64) A x 430V = 94kW. Given an inverter efficiency of 94%, this then equates to 100 kW (PTC), and this is consistent with 672 x 150W = 100.8kW.
Figure 12.14 (right) gives the more recent array power in March of 2008. Given the incidence angle і = 35.6° and cos (35.6) = 0.81, then the summer AC Pmax would be 75.8kW/0.811 = 93.5kW AC і This is nearly the same as for the calculated summer start-up AC power of 94 kW in November of2006. Measurements of string performance were recently made in November of 2008 after cleaning with a projected system power again of 93 kW. There is no indication of system performance degradation.