Grid Connected Photovoltaics

System Optimisation

Conclusions

We carried out a system optimisation analysis based in the improvement of the overall efficiency of the AC-DC conversion, i.e. the inverter. In order to analyse the two proposed designs and their combinations, we run a Simulation and we got hourly output data for PV panels at 30 and 90-degree tilt. We manipulated this data in a spreadsheet and ordered it, getting the power exceedence curves plotted in Fig. 1.

Fig. 1. Power Exceedence curves.

We define a ratio relating the installed PV Peak Power and the Nominal Power of the Inverter:

Undersizing Ratio (UR) = Inverter Pn / PV Array Pp

Methodology

We set up a model for the inverter efficiency, by dividing the inverter load in 4 regions and assigning a different expression to each one.

Fig. 2. Inverter Efficiency vs. Inverter Load.

Inverter load is calculated from PV Output depending on undersizing ratio and number of working inverters.
Hourly efficiency is calculated applying the expressions defined in Fig. 2 to the different Inverter Load ranges.
The overall efficiency is calculated:

Overall Efficiency = Average Inverter Output / Average PV Output

We assumed the inverter gives its Nominal Power when the PV Output is higher than Pn Inverter, i.e. IL > 100%. In fact, inverters are able to stand current levels above their nominal output for a short period of time, so that they could deliver more energy that we are taken into account. When current over the nominal is maintained long enough, the inverter changes the current to its nominal value to avoid overheating. All this features have to be checked for the specific inverter.

Table. 1. and 2.contain a summary of the method and some example values of the analysis.

Table. 1.Undersized Single Inverter Analysis.

TILT = 30 | UR (Undersizing Ratio) = 0.83 | SINGLE INVERTER

Table. 1.Undersized Multiple(3) Inverter Analysis.

TILT = 30 | UR (Undersizing Ratio) = 0.83 | MULTIPLE(3) INVERTER

Results

The overall efficiency of the inverter throughout a year is plotted for Undersized Single and Multiple (3) Inverters. We have considered South facing orientation for all this calculations and the results are especific for the used climate dataset.

Fig. 4. Inverter Efficiency vs. Undersizing Ratio.

Conclusions

The best PV Array/Inverter configuration for an optimum energy efficiency is multiple inverters in parallel, and operation of only the minimum of inverters on line such that all inverters operate on or as near as possible from their peak efficiency. Overall efficiency improvements up to 4 % can be achieved this way. Efficiencies as high as 92.5% from a highest possible of 93% are achieved with only 3 inverters. On the other hand this system design criterion implies the implementation of procedures to sense the available PV power and control the on-off switching of the inverters.

However, if a single inverter configuration is desired, as would be in small systems (<5 kVA), the inverter power rating should be optimised by undersizing it. An undersizing of the inverter ranging from 50 to 80 % of the PV installed capacity, depending on the tilt of the PV Array, can lead to improvements as high as 2 % in system efficiency. In this case, is necessary to check that the inverter can stand the imposed working conditions.

Multiple Inverter configurations show low sensitivity to undersizing. Therefore, it will probably be possible to use smaller and cheaper inverters without loosing much efficiency. Further analysis is needed to cover these aspects.