The Performance Ratio is used frequently to compare the efficiency of grid-connected PV systems at different locations and with different module types and it is one of the most important variables for evaluating the efficiency of a PV plant.
PR is the Ratio of Effective Produced Energy to Ideal Energy Yield at STC Condition. As it’s a Ratio it is Unit less and globally it is being denoted as a Percentage value.
PR = E_Grid / (GlobInc * PnomPV)
where:
E_Gri =Available energy to the grid
GlobInc =Global Irradiation incident on collector plane.
PnomPV= the STC installed power
The closer the Performance ratio value near to 100%,the power plant is more efficiently operating.The 100% performance ratio is not possible due to losses happening in solar power plant during their operation.
While calculating the PR all the losses like Optical losses PV array Losses DC to AC conversion losses and the other System losses are taken into Account.
Types of Performance ratio
- STC Equivalent PR
It is determined by adjusting the power at each recording interval to compensate for difference between the actual PV module temperature and the STC reference temperature. This method normally used when the System PR is to be measured at a short duration of day time. - Annual Temperature corrected PR Here PR is calculated by adjusting the reporting period with respect to power to compensate for the difference between the actual PV module Temperature and the expected annual average module temperature.
- Annual Temperature correction is significant when the Annual PR is required to be Validated for within 15 to 20 days of operation.
Calculation of the performance ratio for an analysis period of 1 year
The following parameters are there for 63kWp power plant.
E_Gri =104971 kWh
GlobInc =2164.3 kWh/M2
PnomPV=63kWp
The PR can be calculated as:
PR = E_Grid / (GlobInc * PnomPV)=104971/(2164.3*63)= 0.7698 OR 78.98%
Which factors influence the performance ratio?
Environmental factors
- Temperature of the PV module:Performance on the temperature of the PV module. At lower temperatures, a PV module is especially efficient. For example, the PV module is cold when the sky is occluded in winter and if full solar irradiation is incident on the cold PV module, then it operates very efficiently. This can generate a high performance ratio value briefly. After a certain time, the PV module heats up and the performance starters decreasing.
- Solar irradiation and power dissipation:The solar radiation is not same through the day.In the morning and evening time,the radiation is low as compare to other part of the day.
- The measuring gauge is in the shade or soiled:This will leads to wrong reading of real time solar radiation measurement.It may cause a PR value more than 100% sometimes.
- PV module in the shade or soiled
Other factors
- Recording period:It can be Day,Week,Month or year.
- Conduction losses:These losses are nothing but DC and AC cable from module to point of connection to the grid.These are depends on the type of cable and conducting material (Copper and Aluminum)of cable.
- Efficiency factor of the PV modules:This has the most influence on the performance ratio of PV plant.Higher the efficiency of PV modules,Higher the Performance ration.The new technological advancement in Solar Module manufacturing like MONO-PERC,Half cut cells etc. played a vital role in overall efficiency increment of.PV module.
- Efficiency factor of the inverter:Normally the efficiency of PV on Grid inverters is more than 97%.
- Degradation of the solar cells:The age-related degradation of the solar cells results in a lower PR value over time. Poly crystalline solar cells age up to 20 % in 25 years but advancement in manufacturing technology of PV modules will decrease this degradation.
- Orientation of the measuring gauge:If the PV plant includes a measuring gauge and this is not correspondingly aligned with the PV modules, this can result in PV values of over 100 % due to different solar irradiation.
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