MANUFACTURING I THERMAL
Lane 1 Lane 2
Table 4: Cross Profile
1 Empty oven 2 Partially loaded oven 3 Fully loaded oven
These three load variants are intended to reflect various possible production statuses in the field of solar cell manufacturing. Empty means that the oven was not loaded with cells for a period of 10 minutes, after which it was profiled. Partial loading of the oven occurs during the production process when continuous loading of the oven cannot be assured due to the upstream silk-screening process (e.g. due to removal of broken cells).
Figure 8: Comparison of heat absorption at various solar wafers
This status was simulated by not loading the oven for a period of 8 minutes and then releasing 50 wafers into the oven. 30 seconds after the last wafer entered the oven, the oven was profiled with the help of the test setup. In order to record the temperature profile in the fully loaded state, the oven was loaded with 50 cells and the test setup was transported into the oven immediately thereafter. 50 more cells were fed to the oven directly after the test board.
Evaluation of the resulting data reveals that maximum temperature in the peak zone can be assumed to be nearly normally distributed for each loading status. Figure 10 shows the probability grid of the left-hand thermocouple in the fully loaded state as an example.
Figure 9: Cross profile of a dual lane firing system Field results
Machine capability analyses were conducted on an RFS-D-250 single-lane firing oven manufactured by Rehm Thermal Systems GmbH with unchanging oven settings (temperature, conveyor speed, exhaust air, supply air). A calibrated test board was used to this end consisting of a defined metal sheet (see figure 7b) with 2 type K thermocouples. A Datapaq Q18 data logger was used to record absolute temperature data. 50 oven temperature profiles were recorded for each of the following three loading statuses with this test setup. Maximum
absolute temperature Tmax in the firing zone was determined with the help of Datapaq InsightSolar Tracker software.
Cmk >1.67 for Left Thermocouple Right Thermocouple Empty oven
± 8.5 K
Partially loaded oven ± 7.5 K Fully loaded oven
± 8.5 K Table 5: Ascertained tolerance widths 26
www.solar-international.net I Issue IV 2014
± 8.5 K ± 9.5 K ± 7.5 K
Figure 10: Probability grid for temperature measured at the left thermocouple
Under these conditions, it’s possible to calculate as of which tolerance width a machine capability of Cmk > 1.67 can be achieved for the various loading statuses. The overview in table 5 shows the calculated tolerance widths of the thermocouples for the various loading statuses.
It was possible to substantiate that the firing system fulfills
Min Temp. in K 921.3 918.0
Max Temp. in K 928.1 926.0
Mean Value in K 925.1 922.0
Standard Deviation in K 1.4 1.8
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