PV System Energy Performance Evaluations

To demonstrate that a large-scale PV system is installed and operating properly, run an energy test to verify its performance across the full range of site conditions.

APV system energy test, or energy performance evaluation, documents the energy yield of a PV system over a period of time, leading to an assessment of whether the system is performing according to the model used to estimate energy production. In this article, we define test objectives and explain the theoretical approach. We describe the test calculation philosophy and methodology. Finally, we discuss the process for validating data, running test calculations and reporting results.

Verifying PV System Performance

To meet investors’ financial expectations, PV systems must perform predictably over many years. In “PV System Performance Guarantees” (SolarPro magazine, June/July 2011), Mat Taylor and David Williams explain: “To finance and construct a large-scale solar project, there has to be a risk-mitigating mechanism in place to reassure investors, which include large banks and institutional investors. A PV performance guarantee contract is the tool used to give the at-risk owner confidence that the system and investment will perform as expected.”

A performance guarantee is, at its root, a simple concept: a seller promises a buyer that a PV system will produce a certain amount of energy. However, it is customary for the buyer to assume the weather risk associated with the guarantee. The guaranteed energy is not a fixed quantity, but instead varies based on the measured weather conditions during the guarantee period.

Unlike a short-term performance ratio test or a capacity test, which establishes the power rating of a PV system under very specific environmental conditions, a long-term energy test verifies PV system performance over the entire range of environmental conditions at a given site over a calendar year. An energy test can provide greater confidence that a PV system is installed and operating properly.

Capacity test. The capacity test determines a PV power plant’s generation capacity or effective power rating. This test is often conducted prior to the plant’s commercial operation date, typically during system commissioning, and compares the facility’s expected capacity with its measured capacity. As such, a capacity test is a critical step in the process of verifying that a PV system is properly designed and installed. Different approaches to capacity tests are outlined in standards published by the ASTM International—including ASTM E2848, “Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance,” and ASTM E2939, “Standard Practice for Determining Reporting Conditions and Expected Capacity for Photovoltaic (PV) Non-Concentrator Systems”—as well as industry technical reports.

It is relatively easy to incorporate a capacity test into project commissioning activities, in part because you can perform the test over a period of days or weeks. However, the results of a capacity test are of limited value for making long-term predictions about system performance. The authors of the National Renewable Energy Laboratory (NREL) Technical Report “Analysis of Photovoltaic System Energy Performance Evaluation Method” (see Resources) explain: “The power generation of a PV system may be documented by a capacity test that quantifies the power output of the system at set conditions, such as an irradiance of 1,000 W/m2, an ambient temperature of 20°C, and a wind speed of 1 m/s. A longer test must be used to verify the system performance under a range of conditions.”

Short-term performance ratio test. You can also describe a PV power plant qualitatively according to its performance ratio, expressed as a percentage. This ratio compares a plant’s actual energy production to its theoretical energy-generating potential and describes how efficient a PV power plant is in converting sunlight incident on the PV array into ac energy delivered to the utility grid. International standard IEC 61724, “Photovoltaic System Performance Monitoring—Guidelines for Measurement, Data Exchange and Analysis,” published by the International Electrotechnical Commission, defines this performance metric.

You can use performance ratios to compare PV power plants across different locations around the world, which makes these ratios popular with some companies and financers. Unfortunately, performance ratio test results depend on weather, which makes them susceptible to seasonal bias. Depending on the environmental conditions at the time of testing, the results may be artificially high or low, as shown in Figures 1a and 1b. Unless you correct the performance ratio for a PV system to neutralize weather bias, tests may obtain substantially different values in the summer versus the winter. The authors of the NREL Technical Report “Weather-Corrected Performance Ratio” (see Resources) elaborate: “[A] colder site will provide a higher [performance ratio], implying more electricity generation if everything else is equal. Unfortunately, associated with this dependence on the weather is a bias error in the metric that introduces unnecessary risk in contractual acceptance testing.”

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