Difference in energy yield between tracking and fixed PV systems

Generally, the analysis of the energy rating of a system is more complicated than the analysis of its power rating [71]. However, system evaluation based on energy output could be considered more robust than a power-based evaluation [81].

As expected, for all the technologies, the tracking system has a higher energy yield than the fixed system, the relative difference depending on the weather conditions. It has been observed that sometimes, at low irradiation levels, the fixed system production is slightly better than for the tracking systems. This leads to a negative energy percentage difference, with the fixed output as the basis. This can be seen in Figures 3.8 and 3.11, which show the percentage difference in output for the three technologies by day and by the irradiation level respectively during October.

In Figure 3.8, it can be observed that all the technologies follow a similar trend on a day-by-day basis. The lines connecting the data points are included to show this trend and do not express any function or correlation between the values. Further, error bars have been added according to the manufacture tolerance in order to show the uncertainty of the energy difference between the fixed and tracking systems. A few

days in each month show a variation in the trend among the technologies and it is observed that this is due to a difference in the output of the tracking system. For example, on 24^th October, the CIGS tracking system yield is greater than that of a-Si and multi-Si tracking systems while the fixed systems have similar energy yields on this day. Particularly, the energy percentage difference of CIGS is around 50% while for the other two technologies is around 20%. On 4^th October, all the technologies have a minus energy percentage difference of around 10%. When a negative percentage difference is observed, this occurs on days with low energy yields and so the percentage difference represents a small change in energy, consistent with a combination of measurement accuracy and manufacturer tolerance on module rating.

This relative increase in energy output from the fixed systems compare to the tracking may also be a result of the omnidirectional nature of diffuse irradiance.

Figure 3.8: Daily percentage energy difference between fixed and tracking systems during October

Table 3.5 shows the specific yield for each technology on these two days. It is observed that on the 4^th October even though the energy percentage difference is around -10% for all the technologies, it is a day with low sunlight levels and the fixed PV systems produce slightly more energy than the tracking systems. On the 24^th October, CIGS has a much greater energy percentage difference than the other two

-15.00 -5.00 5.00 15.00 25.00 35.00 45.00 55.00

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Daily Energy Percentage difference (%)

October (dates) a-Si multi-Si CIGS

technologies, because the CIGS tracking system produces more than the other two tracking systems while the CIGS fixed system has the lowest output compare to the other two fixed systems. This is attributed to shading on the fixed CIGS system by the mono-Si fixed array positioned in front of it and perhaps by the stand-alone system sited on its right side.

Table 3.5: Comparison of specific yield for all the systems on the 4^th and 24^th of October

Specific Yield

October 4th October 24th

a-Si multi-Si CIGS a-Si multi-Si CIGS

kWh/kW kWh/kW kWh/kW kWh/kW kWh/kW kWh/kW

Fixed 0.78 0.82 0.60 4.96 4.75 4.62

Tracking 0.73 0.75 0.54 6.02 5.58 6.94

Figures 3.9 and 3.10 present the specific yield for all the technologies in October for the better understanding of the energy percentage difference values depicted in the Figures 3.8 and 3.11. Again, the lines connecting the data points are included to show the trend of the different technologies.

Figure 3.9: Specific yield for the fixed systems in October 0.00

1.00 2.00 3.00 4.00 5.00 6.00

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Specific Yield (kWh/kW)

October (dates)- Fixed Systems a-Si multi-Si CIGS

Figure 3.10: Specific yield for the tracking systems in October

Figure 3.11 shows that all the technologies tend to increase their energy percentage difference as the irradiation increases, as would be expected since the percentage of direct irradiation and hence the benefit of tracking also increases. However, the three technologies show differences in rate of change shown by the slopes of the linear fits in the figure.

Figure 3.11: Daily percentage energy difference between fixed and tracking structures as a function of daily in-plane irradiation on the fixed structure

(trend line equations for each series are displayed in the respective series colour) 0.00

4.00 4.50 5.00 5.50 6.00 6.50

Daily Energy Percentage difference (%)

In-plane Irradiation (kWh/m²/day)-October 18 days a-Si multi-Si CIGS

The mean monthly percentage differences are shown in Table 3.6. The mean monthly gain across the whole period for all technologies is 24.08%, but there is significant variation between the three technologies.

Table 3.6: Mean monthly differences between the energy yield of the fixed and tracking structures, expressed as the percentage gain of the tracking compared to the fixed structure

Monthly gain between Tracking and Fixed PV Systems (%)

Mean per technology 21.71 18.33 32.21

There are four possible causes for the difference in behaviour between the technologies. Firstly, different module types have different temperature coefficients.

The tracking array will operate at a higher module temperature than the fixed array for most days due to higher irradiance values. The effect on the electrical output will vary depending on the temperature and irradiance. As is observed, the multi-Si array would be expected to have the lowest tracking gain. a-Si would be expected to have a higher gradient of gain with irradiation level, but it is observed to be lower compared to the other technologies, which cannot be explained by the temperature coefficient.

Secondly, the close proximity of the installed systems leads to some shading, particularly in the case of the CIGS fixed system, as has been discussed. This leads to a reduced fixed array output and hence an observed higher gain from the unshaded tracking system. Thirdly, the output measurements include the effect of the inverter matching and efficiency, which may differ between technologies. The a-Si and multi-Si systems have the same inverter model whilst the CIGS system has a different inverter model. This aspect requires further investigation to establish its contribution. Finally, the tolerance in module ratings for the different technologies has been considered. The

values in Table 3.6 assume the nominal rating for all modules in both systems. Using the declared manufacturer tolerances, the range of the possible gains is shown in Table 3.7.

Table 3.7: Variation in mean gain as a result of module rating tolerances

The percentage gains shown by the a-Si and multi-Si systems are lower than would be expected for this location according to a system simulation carried out in PVsyst. The same electrical configuration, similar module types (according to the STC) and the PVGIS CM-SAF solar data for Kanpur have been used for the simulations of the three technologies. The module and inverter specifications of all the simulated systems, as long as the input parameters, which have been used for the simulations are included in Appendix G. The simulations gave a monthly gain of around 25%-40% for the same period of the year depending on the month and the technology. The simulated gain shown in Table 3.7 is the mean for the months considered. The CIGS systems results are in line with the PVsyst simulations, but it is known that there is some shading of the fixed system. The temperature data in the simulations has similar values to the measured data, except for December and March when the PVsyst values were 15%

and 10% lower respectively, thus lowering the relative tracking gain in practice. The PVsyst irradiation values for October, November, February and March (the months for which sufficient solar data were available) were 18.4-35.8% higher than those measured. This would also result in the relative tracking gain being less in practice compared to the simulation.

Mean gain between tracking and fixed PV systems over all 8 months (%) Minimum

Calculated

Nominal Calculated

Maximum Calculated

Nominal Simulated

a-Si 15.91 21.71 27.79 34.20

multi-Si 12.70 18.33 24.25 31.87

CIGS 25.26 31.21 38.83 31.68