With the increasing push for renewable energy, the number of installations of photovoltaic solar plants is rising steeply. While a solar power plant essentially captures the freely and abundantly available solar energy, large one time and on-going costs are associated with commissioning and maintaining a solar power plant. This technical note elaborates on the working of PV plants, some weather parameters which affect the performance of the plant and role of a weather monitoring station in maintaining and improving the plant performance.
Working Of A PV Solar Plant
To begin with, let us have a quick look at the block diagram and basic functioning of a PV solar plant.
At the heart of any PV plant are solar panels which may or may not have a solar tracking system. The solar tracking system facilitates panel movement to ensure that the sun rays fall normally to the solar panels for most of the time and thus improves the energy output. The DC current generated from the solar panel goes to a voltage regulator and through it to the control unit. Since the PV panels generate DC power, an inverter is required to convert DC to AC. MPPT unit is also usually installed to ensure that the system operates at the maximum power point.
Parameters Affecting The Performance Of A PV Plant
Performance of a solar plant is measured in terms of its Performance Ratio (PR). The performance ratio (PR) for a solar power plant is defined in IEC 61724  and is a metric commonly used to measure solar photovoltaic (PV) plant performance for acceptance and operations testing. The PR measures how effectively the plant converts sunlight collected by the PV panels into AC energy Performance ratio is the ratio of actual electricity generated by the plant to the electricity that would have been generated if the plant consistently converted sunlight to electricity at the level expected from the rated DC power. PR plays a crucial role for financiers as well as plant operators as the profitability of the entire plant is very closely linked to the PR.
The PR depends not only on the system parameters (efficiency of panels, system design, inverter efficiency, cell mismatch, wiring etc.) but is also greatly affected by numerous weather parameters like elevated PV module temperatures, a reflection of the sunlight from panel surface etc. In the daily operation of the plant, weather-related parameters play a major role. For instance, as the solar radiation increases, the plant output increases; but the increased radiation increases the temperature of the PV modules which reduces their efficiency and hence, brings down the PR of the plant. A study conducted shows that a 3 Deg. C. rise in annual temperature brings down the plant performance by 0.9%. The graph below shows the effect of an increase in irradiance on the DC efficiency
Fluctuations in ambient temperature also greatly affect the performance of the solar plant. Everything else remaining constant, fluctuations in ambient temperature from 25 Deg.C. to 45 Deg. C. can deteriorate plant performance by 5 to 10%.
Relation Between Temperature And DC Efficiency
Apart from irradiation and module temperature, wind also significantly affects the performance of a PV plant. Usually, higher wind speed results in cooler modules and hence higher efficiency. A wind velocity which is 3 m/s more than usual wind velocity will enhance the DC efficiency by roughly 2%. The graph below shows the effect of wind velocity on panel temperature for Silicon panels.
Role Of A WMS In Performance Monitoring
A weather monitoring station can be immensely helpful in monitoring the efficiency and performance of any solar power plant. The data from the WMS (Weather Monitoring Station) can be used to get many insights about the plant operation and possible avenues to increase the plant output. A WMS cannot change the weather parameters, but it helps to analyze and predict them so as to get maximum output from the solar plant. Typically, a weather monitoring station monitors parameters such as solar radiation, other weather conditions such as wind velocity, wind direction, humidity, module temperature etc. WMS also calculates the performance ratio of the solar power plant using all the captured data. The adjoining figure shows construction and various components used in a WMS.
Role Of Sensors And DataLogger
As it can be seen, at the core of any WMS is a data logger which captures data from various sensors like pyranometer, anemometer, module temperature sensor, rain gauge etc. Even a slight error in the measurement of any parameter can drastically impact the calculated PR values. For instance, the typical scale factor of a solar radiation sensor is generally 10uV/Watt/m.sq. Imagine a 100 MW plant which can generate up to 100,000 KW power. This translates into up to 100,000 KWhr of energy generated in an hour. Let us assume that the solar radiation level is 500 W/m^2 at the moment. If the sensitivity of the pyranometer used is 10 UV/W/m^2 and the error in the measurement of irradiance due to data logger is 50 microvolts (1%) which is equivalent to 5 W/m^2, there will be an error of 5/500 in irradiation measurement translating in an error of
(5/500) * 100,000 = 1000 KWhr error in the measurement of energy per hour.
At a rate of INR 4 / unit (KWhr), it will translate to INR 4000 per hour. So, if the plant is generating lesser energy than as per reading of radiation level shown by data logger due to dust on panels, actionable data for cleaning the panel and generating more energy may be lost, on the other hand if the data logger is showing lower reading than the energy available and therefore generated, the plant operator may not be able to report factual data. This apparently minor error in the measurement of solar radiation will affect the calculation of efficiency and in a 100 MW plant, will result in far from realistic energy values.
Apart from good quality sensors, a high-resolution data logger is a must to capture the data accurately. High-resolution 24-bit data loggers from Aeron can yield much better data as compared to data loggers with a resolution of 13-16 bits which are commonly available in the market.
Performance of a PV plant is highly dependent on weather parameters like solar radiation, module temperature, wind speed etc. A weather monitoring station equipped with high-quality sensors and high-resolution data logger will help in monitoring the plant performance and take corrective actions.