How much energy is lost due to inverter clipping when the power form the PV array far exceeds the inverter output rating? And under what conditions do the losses occur?
Today’s interactive inverters allow high PV to inverter size ratios. An example 7.7kW residential inverter allows a PV array STC input rated up to 12.32kW; that is a 160% oversizing. Some utility scale central inverter design allows up to 200% oversizing of the PV array.
Two issues come to mind with this level of PV array oversizing. How does this affect the life and performance of the inverter and how much of this energy gets clipped (wasted)?
- The answer to the first question is easy. These inverters are transformerless and operate at high efficiencies so there is very little heat created when they are running at full capacity. Clipping is a process of electronic pulse modulation which limits current output and therefore current input; this process eliminates most of the heat that would be generated if the current was converted from input to output.
- The answer to the second question is a little more complicated and depends on many factors but primarily it is the result of irradiance on the array throughout the day. For a fixed PV array, there is only one time of day when the irradiance is highest; for a south facing array, that would be noon; for a SE or SE facing array, that would be around 10:30am and 1:30pm; for an east or west facing array that would be around 9:00am and 3:00pm. With an east-west tracking array, the peak irradiance will be at noon. In the following paragraph we will look at each one of these PV array direction scenarios and examine how much net energy is actually present at the array during its highest irradiance time of day.
First, we will look at the other derating factors. Temperature is going to reduce net energy on average by 5% to 10% depending on the quality of the PV cell technology. PV cell degradation in the first year will be 1% to 3% depending on the quality of the cell technology. Electronics conversion losses will be 2% to 3%. Variances between module output will cause 1% to 3% losses. Soiling will cause 3% to 5% losses depending on the cleaning maintenance cycle. Voltage drop will reduce net energy by 1% to 3%.
- The best quality modules and inverters will net out at around 87% of the solar energy received by the PV array. The lower quality modules and inverters will net out around 73% of the solar energy received the PV array. This is considered the net power conversion efficiency (PCE).
Second, we consider the actual irradiance that the PV array will receive during the peak hours of the day. This is highly variable depending on the location of the PV array; high irradiance locations with consistently clear skies are at the extreme end of the irradiance spectrum such as the Southwestern U.S. deserts; low irradiance locations with variable cloudiness are at the other end of the irradiance spectrum such as New England. Most of the U.S. falls into some range between these two extremes.
- Let’s assume that the highest irradiance occurs in April or May when the temperatures when the skies are clearest and the sun is close to its highest point in the sky. In the SW desert, we may be seeing 1200W/m² for one hour at noon if the array is pointed directly at the sun. (That would be a 10° tilted array facing south.) The net energy output would be 120% higher than STC.
- Using the PCE de-rates with best net of 87% and the worst with 73%, that would bring the net to 104.4% and 87.6% respectively. With the highest rated module and inverter example, that means anything over the rating of the inverter will be clipped. With the lower rated module and inverter, everything over 87.6% of the inverter rating would be clipped. A PV to inverter ratio of 114% would not clip any energy.
- This is an extreme case and this only occurs at one time of year during one part of the day. The average irradiance at this location would be 900W/m² for this month and less for the other months of the year. That gives a net of 87% for the highest rated equipment and 73% for the lower rated equipment. A PV to inverter ratio of 114% and 137% respectively would on-average not clip any energy. This is only for the highest output month of the year.
- In New England, the irradiance at noon may be 1000W/m² if the array is pointed directly at the sun. (That would be 20° tilted array facing south). The net energy output would be 100% of STC.
- Using the PCE de-rates with best net of 87% and the worst with 73%, that would bring the net to 87% and 73% respectively. With the highest rated module and inverter example, that means anything over 87% the rating of the inverter will be clipped. With the lower rated module and inverter, everything over 73% of the inverter rating would be clipped. A PV to inverter ratio of 115% and 137% respectively would not clip any energy.
- This again is an extreme case and this only occurs at one time of year during one part of the day. The average irradiance at this location would be 800W/m² for this month and less for the other months of the year. That gives a net of 70% for the highest rated equipment and 58% for the lower rated equipment. A PV to inverter ratio of 142% and 172% respectively would on-average not clip any energy. This is only for the highest output month of the year.
Most of the time the sun is not at zero incidence angel to the PV array and the skies are not perfectly clear. The average irradiance in most months is anywhere from 20% to 40% less than the peak month. These factors prevent the PV array from producing energy that exceeds the rating of the inverter. Some clipping will occur when the PV array is sized greater than 140% of the inverter rating but only during one or two month of the year and only for an hour or two during some of the days.
The following graph shows the average daily clipping during only one month of the year, April, with a PV to inverter ratio of 160% located near Atlanta GA. The net conversion efficiency is 84% before irradiance variations are factored in below.
Generally speaking, the PV to inverter size ratio for interactive residential systems should not be greater than 140%. The ratio for interactive commercial should not be greater than 160% and the ratio for utility scale systems should not be greater than 180%. You may instead refer to the specific inverter data sheet for the maximum PV array size for the inverter.
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