Photovoltaic plant performance assessment

In our previous article, we described some of the metrics that can be used to monitor photovoltaic plant performance. Those metrics can be evaluated during acceptance tests, or continuously during operation, for a different reason.

Acceptance testing is a set of tests conducted to determine if the requirements of a specification or a contract are met, in the case of photovoltaic plants the technical system specification and the construction contract. Performance evaluation during acceptance tests is done to confirm that the plant has been constructed and operates as per the project plan, or in other words to verify an efficient and complete installation and proper system performance.

The performance of the plant should be continuously monitored even after the acceptance tests are successfully passed, to ensure optimal operation of the system.

Performance ratio (or shortly PR) is one of the widely accepted metrics in the photovoltaic industry that empirically state the efficiency of a given system. This metric can be used in all phases of the lifetime of a system, from a pre-feasibility study to continuous operation, which makes it suitable for use in the contract terms with the construction company that is responsible for building and putting the plant into operation.

Photovoltaic system performance depends on both the quality of the system and the weather. Unfortunately, the PR is, again, a function of both the PV system efficiency and the weather.

If the PR is measured during the winter or during the summer, substantially different values may be obtained,

making this metric insufficient to use as the basis for a performance guarantee when precise confidence intervals are required.

The reason for that is that photovoltaic modules’ efficiency is dependent on the temperature, among other things. Photovoltaic modules are less efficient when the temperature is high, meaning that the performance of the system will be lower in summer. It may seem contra-intuitive, but to clarify things, we are evaluating the ability of the system to convert sunlight into energy, and not the amount of solar energy available.

This bias will result in false high values during the winter months (causing risk for the PV customer because a poor-performing plant might falsely pass the test during this time) and false low values during the summer months (causing risk for the PV installer).

To solve this problem, a weather correct performance ratio has been proposed by NREL (National Renewable Energy Laboratory). The proposed method, mitigates the effect of the temperature on the performance ratio, as can be seen on the simulation presented:

In order to use the methodology, several instruments need to be in place that will measure temperature, irradiation, wind speed as well as energy production from the plant. Appropriate models for simulating energy yield are also needed. It is evident that the methodology is not simple, nor cheap as it requires meteorological instruments, data processing pipelines and simulation methods.

It may seem that this method is economically unfeasible for smaller photovoltaic plants, but luckily, Solar Data offers a service that is based on a recurring payment model, so there is no need upfront cost. A portable meteorological station is deployed, the data from the inverters are automatically collected and processed, resulting in live dashboard that allow continuous monitoring of many metrics, including the weather corrected performance ratio described in this article. Don’t hesitate to contact us if you would like to learn more.