William E. Boyson

Analysis of factors influencing the annual energy production of photovoltaic systems

The most relevant basis for designing photovoltaic systems is their annual energy production, which is also the best metric for monitoring their long-term performance. An accurate array performance model based on established testing procedures is required to confidently predict energy available from the array. This model, coupled with the performance characteristics of other balance-of-system components, provides the tool necessary to calculate expected system performance and to compare actual versus expected energy production. Using such a tool, this paper quantifies the effect of the primary factors influencing the DC-energy available from different photovoltaic module technologies, and contrasts these influences with other system-level factors that often result in significantly less AC-energy delivered to the load than the array is capable of providing. Annual as well as seasonal energy production is discussed in the context of both grid-tied and stand-alone photovoltaic systems.

Performance Model for Grid-Connected Photovoltaic Inverters

This document provides an empirically based performance model for grid-connected photovoltaic inverters used for system performance (energy) modeling and for continuous monitoring of inverter performance during system operation. The versatility and accuracy of the model were validated for a variety of both residential and commercial size inverters. Default parameters for the model can be obtained from manufacturers specification sheets, and the accuracy of the model can be further refined using measurements from either well-instrumented field measurements in operational systems or using detailed measurements from a recognized testing laboratory. An initial database of inverter performance parameters was developed based on measurements conducted at Sandia National Laboratories and at laboratories supporting the solar programs of the California Energy Commission.

Comparison of PV system performance-model predictions with measured PV system performance

The U.S. Department of Energy has supported development of the Solar Advisor Model (SAM) to provide a common platform for evaluation of the solar energy technologies being developed with the support of the Department. This report describes a detailed comparison of performance-model calculations within SAM to actual measured PV system performance in order to evaluate the ability of the models to accurately predict PV system energy production. This was accomplished by using measured meteorological and irradiance data as an input to the models, and then comparing model predictions of solar and PV system parameters to measured values from co-located PV arrays. The submodels within SAM which were examined include four radiation models, three module performance models, and an inverter model. The PVWATTS and PVMod models were also evaluated.

Stabilization and performance characteristics of commercial amorphous-silicon PV modules

The successful commercialization of any new photovoltaic technology is difficult. Understanding the products performance and aging characteristics is a prerequisite for the manufacturer. Amorphous-silicon thin-film modules are now in commercial production, and their market penetration is being limited to some degree by a lack of understanding of environmental influences that impact system design and operation. This paper summarizes our detailed performance characterization of multiple modules from four different manufacturers over several years of continuous outdoor exposure in Albuquerque, NM. Common stabilization characteristics have been observed for both tandem and triple-junction modules, and the influences of solar spectrum and seasonal (thermal) annealing have been clearly identified. Implications for system performance modelling are presented.

Array Performance Characterization and Modeling for Real-Time Performance Analysis of Photovoltaic Systems

Improvements in the methods used for photovoltaic (PV) system design, performance rating, and long-term monitoring are needed by the rapidly growing industry, as well as by the U.S. Department of Energy in evaluating progress by solar technology development initiatives. This paper describes an improved model for rating and monitoring PV array performance, discusses initial results from an outdoor laboratory designed to assist industry in optimizing system components and integration, and provides a brief discussion of the system performance metrics currently being used by the PV community

Long-term performance and reliability assessment of 8 PV arrays at Sandia National Laboratories

In the last decade, c-Si module degradation rates of ≪1%/year have been reported [1–3]. It is unclear if this degradation rate extends directly to the string level and what is the expected statistical spread of degradation rates. Nine photovoltaic (PV) arrays totaling nearly 100 kW at Standard Reporting Conditions are currently being used at Sandia National Laboratories (SNL) primarily for inverter testing. The measured power degradation of these arrays at the string level varied from no change over three years within measurement error to greater than 25% in three years. This paper outlines the methodology used to test the DC output, outlines analysis techniques used to evaluate the array performance, provides a current reliability assessment, presents the comparative data for up to five years of use and exposure, and discusses the methods used to track down the causes of unexpected string-level degradation.

Comparison of module performance characterization methods

The rating and modeling of photovoltaic (PV) module performance has been of concern to manufacturers and system designers for over 20 years. Both the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL) have developed methodologies to predict module and array performance under actual operating conditions. This paper compares the two methods of determining the performance of PV modules. The methods translate module performance to actual or reference conditions using slightly different approaches. The accuracy of both methods is compared for both hourly, daily, and annual energy production over a year of data recorded at NREL in Golden, CO, USA. The comparison of the two methods is presented for five different PV module technologies.

Experimental optimization of the performance and reliability of stand-alone photovoltaic systems

Stand-alone photovoltaic systems are deceptively complex. Optimizing the performance and reliability of these systems requires a complete understanding of their behavior as a function of site-dependent environmental conditions. Individual component specifications provide useful design information. However, to fully understand the interactions between components, it is necessary to simultaneously characterize the performance of the system and its separate components under actual operating conditions. This paper describes how a new 30-day outdoor testing procedure was coupled with array performance modeling to accomplish this objective. The procedure measures battery capacity, determines appropriate set-points for charging, and based on daily intervals quantifies DC-energy available from the array, charge-controller efficiency, battery efficiency, inverter efficiency, overall system efficiency, days of autonomy, and AC-energy available by month.

Investigation of factors influencing the accuracy of pyrheliometer calibrations

The accuracy of solar cells calibrated as primary reference cells is directly dependent on the accuracy of the pyrheliometer used to measure the direct beam solar irradiance on the cell. Pyrheliometers are also used in measuring performance of concentrating photovoltaic modules. In order to reduce errors in photovoltaic performance measurements, we have investigated the calibration uncertainties for pyrheliometers from two manufacturers. Our calibration comparisons are relative to an absolute cavity radiometer traceable to the World Radiometric Reference. This paper quantifies the effects of aging, temperature, time-rate-of-change of temperature, wind, solar spectral shifts, linearity, window transmission, and solar tracking on pyrheliometer calibrations. Uncertainty remaining after accounting for these factors is 0.8% at the 2-sigma level.

Successful transfer of Sandia National Laboratories' outdoor test technology to TÜV Rheinland Photovoltaic Testing Laboratory

Sandia National Laboratories (Sandia) has observed an increased demand for high accuracy outdoor photovoltaic (PV) module characterization using Sandias Photovoltaic Array Performance Model [1]. To meet this demand, Sandia entered into a competitively-bid agreement in May 2009 with TÜV Rheinland Photovoltaic Testing Laboratory (TÜV-PTL) to transfer Sandias capability to fully characterize standard, commercial-scale PV modules. Sandia and TÜV-PTL worked closely on two round-robin experiments and months of subsequent work and discussions that resulted in module performance output calculations agreeing to within +/−2.5%.