Weather-Corrected_Performance_Ratio NREL

Weather-Corrected Performance Ratio

Timothy Dierauf and Aaron Growitz SunPower Corporation

Sarah Kurtz National Renewable Energy Laboratory

Jose Luis Becerra Cruz Fichtner

Evan Riley Black & Veatch

Clifford Hansen

Sandia National Laboratories

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. Technical Report NREL/TP-5200-57991 April 2013

Contract No. DE-AC36-08GO28308

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Executive Summary Photovoltaic (PV) system performance depends on both the quality of the system and the weather. One simple way to communicate the system performance is to use the performance ratio (PR): the ratio of the electricity generated to the electricity that would have been generated if the plant consistently converted sunlight to electricity at the level expected from the DC nameplate rating. The annual system yield for flat-plate PV systems is estimated by the product of the annual insolation in the plane of the array, the nameplate rating of the system, and the PR, which provides an attractive way to estimate expected annual system yield. 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.

This technical report defines a way to modify the PR calculation to neutralize biases that may be introduced by variations in the weather, while still reporting a PR that reflects the annual PR at that site given the project design and the project weather file. This resulting weather-corrected PR gives more consistent results throughout the year, enabling its use as a metric for performance guarantees while still retaining the familiarity this metric brings to the industry and the value of its use in predicting actual annual system yield. A testing protocol is also presented to illustrate the use of this new metric with the intent of providing a reference starting point for contractual content.

List of Acronyms AC DC EPC PAC POA PR PV RTD STC TMY alternating current direct current engineering, procurement, and construction provisional acceptance certificate plane of array performance ratio photovoltaic resistance temperature detector standard test conditions typical meteorological year

Table of Contents

Introduction: The Performance Ratio ....................................................................................................... 1 Variability of PR with Weather ................................................................................................................... 2 Theoretical Approach ................................................................................................................................. 3 Average Annual Cell Temperature ............................................................................................................ 6 Calculated Cell Temperature...................................................................................................................... 7 Example Data ............................................................................................................................................... 7 Corrected Performance Ratio from Measured Data ............................................................................... 7 Corrected PR Test Protocol ....................................................................................................................... 9 Purpose ................................................................................................................................................... 9 Parties to the Test and Responsibilities .................................................................................................. 9 Requirements Before the Test .............................................................................................................. 10 Minimum Irradiance Criteria ................................................................................................................ 10 Instrumentation ..................................................................................................................................... 10 Proof of Performance ........................................................................................................................... 11 Calculation Method .............................................................................................................................. 11 1. Operating Cell Temperature ...................................................................................................... 11 2. Average PV Cell Temperature from the Project Weather File .................................................. 13 3. Calculate the Predicted PV Cell Temperature from Measured Meteorological Data ................ 13 4. Temperature-Corrected Theoretical DC Energy Generation ..................................................... 14 5. Determine Corrected Measured PR ........................................................................................... 15 6. Compare with Guaranteed Values ............................................................................................. 15 References ................................................................................................................................................. 16

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The performance ratio (PR) 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 sunlight collected by the PV panels into AC energy delivered to the off-taker relative to what would be expected from the panel nameplate rating. This metric quantifies the overall effect of losses due to: inverter inefficiency, wiring, cell mismatch, elevated PV module temperature, reflection from the module front surface, soiling, system down-time, shading, and component failures. Because many of these factors are indicators of build quality, this metric is popular with some companies and financiers for contractual acceptance testing. However, some of these factors are also weather dependent. Most notably, weather affects the PR by affecting the module temperature. To many financiers, this is an attractive feature of the metric because it helps to understand which locations will provide the most productive plants. For example, a colder site will provide a higher PR, 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. The PV system electrical output changes as weather varies; for example, system output changes with temperature (typically ~ 0.5%/°C), irradiance (typically can vary by as much as 5%–10%, especially for modules with high shunting or series resistance), and spectrum (typically varies by up to ~3%, depending on the difference in responses of the irradiance sensor and the PV module). As shown in Figure 1, the PR can swing radically over a single day. The contract must specify a PR that is representative of the annualized performance at the site, but the season of the measurement is seldom defined, leading to unnecessary risk for both parties. Before proposing a method for reducing this risk, we give some background on PR. PR values for new systems typically range from 0.6 to 0.9 [2-9]. A recent paper summarizing the performance of ~ 100 German PV systems concluded that the cool climates in Germany helped some systems approach, or even exceed, 0.9 [2]. The strong dependence of PR on temperature results in a large seasonal variation in PR, which can be as large as ±10% [3-6, 9]. PR is often corrected to a common temperature of 25°C (standard reporting conditions) [4, 5]. Correction to a cell temperature of 25°C usually results in a higher PR because modules more frequently operate at 45°C. Thus, while correction to 25°C essentially solves the problem of seasonal variations, it may overstate the actual performance and thus does not allow the financier to assess the effect of local climate on the expected performance. Therefore, correction to 25°C is not an acceptable method for removing the seasonal variability in the PR metric because it would change the PR value stated in the contract.