The efficiency of PV solar plants is a major concern for the plant owners as well as the financing agency. A slight variation in the efficiency of a PV plant will significantly affect the energy generation.
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 [1] 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 sun’s radiation 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.
Even a slight degradation in the PV plant output will greatly affect the profitability and return on investment of the plant. It is therefore very crucial to monitor in real time the efficiency of the PV plant along with individual weather parameters which affect the efficiency of the solar plant.
Out of these weather parameters, we will discuss how solar irradiance is measured and its effect on PV efficiency. Before we discuss how solar irradiance is measured, let us have a look at how these weather parameters, including solar irradiance, affect the performance of any PV plant.
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 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 effects of weather parameters on the performance of a PV solar plant increases in irradiance on the DC efficiency.
Fluctuations in ambient temperature also greatly affect the performance of the solar plant. If everything else remains constant, fluctuations in ambient temperature from 25 deg. C to 45 Deg. C. can deteriorate plant performance by 5 to 10%. Wind velocity and direction have a critical role and affect the ambient temperature which can again impact the efficiency of the PV modules.
As it is clear from the above discussion, solar irradiance is a major parameter which affects the efficiency of the PV plant. Any standard Weather Monitoring Station will have provision to measure solar irradiance as well along with other weather parameters. Solar irradiance is defined as radiant flux received by a surface (PV module in this case) per unit area, irradiance is measured in W/m2. The sensor which is used to measure solar irradiance is called a pyranometer.
The solar irradiance which reaches the earth is spread over 300 nm to 2800 nm. The pyranometer should ideally have a flat spectral response over this range. In Large scale plants, Direct Normal Irradiance, Diffuse Horizontal Irradiance, and Global Horizontal Irradiance all are required to be measured. Commonly used pyranometers consist of a thermopile. In this type, the heat sensing element is partially exposed to sunlight and a part of it is isolated from the sun. As a result, the two parts of the pyranometers are at different temperatures. Based on the temperature difference, a proportional current signal is generated which is measured and used to monitor solar irradiance. Other types of commercially available pyranometers are Photodiode based pyranometers or irradiance sensors which are not as accurate and wideband as thermopile pyranometer. The thermopile pyranometer is generally a passive sensor with output in micro-volts which requires highly accurate analog to digital data capturing.
Monitoring solar irradiance helps you to find the Performance Ratio of the plant which helps the plant owner to find out the gap between the actual plant output and the possible maximum output.
This PV performance data can then be used to search for issues in the solar plant such as faulty PV panels, faulty string combiner boxes, faulty wiring, soiling of panels (dust on panels) etc. In many plants, especially the large-scale plants, multiple WMS are used. The comparison between the PR of these individual sections and respective weather parameters will lead to finding out faulty or underperforming panels. Replacing these panels, correcting wiring issues and cleaning of panels can improve the plant performance.
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