On-site Performance Evaluation and Monitoring of Grid Connected Solar PV Plant

Better performance is required in each solar PV plant to utilize the energy and quickly recover the invested money. However, performance should not be deceived by the wrong measurements and sensing hardware. Performance monitoring is significantly affected by the quality of the measurement hardware and the professionalism of the installation. Hence, measurement error should be very minimum in evaluating the plant, this is assured by good hardware and sensor.


India is blessed with tremendous solar energy resources. It has the potential to be amongst the top three countries globally in the next five years in terms of installed capacity of renewable energy, thereby attracting foreign investment, creating millions of new jobs, reducing poverty and achieving sustained socioeconomic development. Accordingly, enhancing the share of renewable in the overall energy basket of the country is a priority for the present government. With solar power at the centre stage, Government has ambitious programme to install 100 GW of solar electricity by 2022. Overall Government of India announced a target of installing 175 GW of Renewable Energy Capacity Various initiatives is being taken by the government for creating conducive policy environment for development of Renewable Energy in the country. However, there is a need to undertake innovative and determined efforts with a view to reducing the cost of finance and cost of innovative RE technologies for speedy deployment of competitive solar power installations in the country. In each of the mentioned effort, there are hurdles associated, which also needs to be dealt with.
India is situated in hot climatic zone of the globe and blessed with tremendous solar irradiance almost all part of the country. With about 300 clear, sunny days in a year, India's theoretical solar power reception, on only its land area, is about 5000 Peta-watt-hours per year (PWh/yr) (i.e. 5,000 trillion kWh/yr or about 600,000 GW). The daily average solar energy incident over India varies from 4 to 7 kWh/m2 with about 1,500-2,000 sunshine hours per year (depending upon location), which is far more than current total energy consumption.
Recently there has been large number of solar PV plant in MW scale installed in various parts of the country and supporting the conventional grid to optimize the energy needs of the country. The success of these MW solar PV plant is its simplicity and its modular nature. Further, it doesn't require frequent maintenance.
Solar PV projects are always thought to be simplest power projects to develop; people often does not give proper attention to the Operation & Maintenance and execution work . It is often seen that many of solar PV power plants are not performing as desired due to improper monitoring and O&M. Finally developer sees the dip in expected project cash flows.
Accurate measurement of these performance index further depends on various factor like measurement methodology, accuracy and precision of sensor used for the measurement etc and discusses the methodology and various factor associated with measurement of performance index which is the reflection of the performance of plant. To measure the performance of the plant there are few index as follow:

Performance index & CUF/PLF
There are many factors which define the performance of the SPV plant which are Performance ratio (PR), CUF and PLF etc. PR is widely used in Europe and US for performance assessment of Solar Projects while CUF/PLF is used in India. PR is a measure for the performance of a PV system taking into account environmental factors (temperature, irradiation, etc.) and on the other hand CUF that completely ignores all these factors and also the de-rating or degradation of the panels. PR is used as a tool to compare different solar PV systems with each other - even if they are located at different locations since all environmental factors will be taken into account.image
PR take into account the availability of the grid, irradiance needed to generate electrical energy and irradiance levels at a given period of time whereas CUF doesn't take into account all these factors. Every investor who is in solar industry is worried about the performance of the solar PV plant. Performance ratio (PR) is the parameter which gives insights how efficiently the plant converted the solar energy into electrical energy. This is the important parameter to compare various solar PV plant even globally situated in different climatic zones to assess the performance. Before actually installing a plant , designer run the simulation and calculate the PR , if there is a deviation in actual and simulated PR , then it indicates a fault or problem in solar PV plant.
As PR depends upon various climatic parameters, foremost is the irradiation and followed by temperature, soiling etc. parameter which doesn't change also plays critical role in calculating the PR E.g. the active surface of the panels and the efficiency of the panels. Both the information can be found in the datasheets of the panel manufacturer.

Following are the important things required for calculating the Performance Ratio:

  • Define the analysis period in advance. The optimum analysis period is one year.
  • Generator area of the PV plant: The active area of the PV system is known (or can be calculated).
  • PV panel efficiency (Manufacturer gives this detail in datasheet) Measured plant output-Energy Output (ABT meter installed at grid feed side and it doesn't include the in-house energy consumption).
  • Calculated, nominal plant output: This value will be calculated separately.
  • In-plane irradiance data: Value as given by the irradiance sensor (preferred a Pyranometer)
While calculating the PR, there may be measurement error in irradiance, it will wrongly calculate the PR. Hence, there are few parameters on which the PR calculation depends:

Environmental factors
  • Improper positioning of Temperature sensor on PV module.
  • Shading and/or Soiling of Pyranometer
Other factors
  • Recording period.
  • Conduction/cabling losses.
  • Efficiency of the PV modules.
  • Efficiency of the inverter.
Impact of Calibration
  • Proper calibration of equipment is required ,failure to do so impacts system rating uncertainty and degradation rates.
  • Don't blindly trust data sheets.
  • Often, the manufacturer calibration is valid for only one year.
  • A complete end-to-end calibration, when possible, limits systematic errors.
  • Periodic Pyranometer calibrations (such as NREL's BORCAL) are essential
  • 24 month schedule is normal for periodic inspection / cleaning of sensors.
Higher temperature adversely affect the performance of the PV module, at higher temperature because of the Voc reduction, power gets reduced. Solar module covered with dust causes hot spot on the module result in pre-mature rupture of the solar module. Frequent cleaning is required to avoid this problem. Energy modeled depends upon the irradiance value, and sometime the sensor is covered with the dust. If the measurement period is too short (i.e. less than 1 month), chances are that "non- typical" events like rainfalls, cloudy weather, etc. will influence the measurements. A longer time frame of data acquisition brings more reliable PR results. Along with the geographic parameter system loss also plays major role in reduction of PR , greater the system loss , less the PR.The degradation of solar cells with time can also result in lower PR values. Solar pyranometer should be cleaned at regular interval to avoid the deposition of dust on the sensor. Dust causes inaccurate measurement of the solar irradiance which give faulty result while calculating the Performance ratio.