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Contract No. DE-AC36-08GO28308
Photovoltaic Degradation
Rates
— An Analytical Review
Dirk C. Jordan
and Sarah R. Kurtz
To be p
ublished in Progress in Photovoltaics: Research
and
Applications
Journal Article
NREL/
JA-5200-51664
June
2012
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Abstract
As photovoltaic penetration of the power grid increases, accurate predictions of return on
investment require accurate prediction of decreased power output over time. Degradation rates
must be known in order to predict power delivery. This article reviews degradation rates of flat-
plate terrestrial modules and systems reported in published literature from field testing
throughout the last 40 years. Nearly 2000 degradation rates, measured on individual modules or
entire systems, have been assembled from the literature, showing a median value of 0.5%/year.
The review consists of three parts: a brief historical outline, an analytical summary of
degradation rates, and a detailed bibliography partitioned by technology.
Keywords: Photovoltaic modules, photovoltaic systems, performance, outdoor testing, field
testing, degradation rates
1. Introduction
The ability to accurately predict power delivery over the course of time is of vital importance to
the growth of the photovoltaic (PV) industry. Two key cost drivers are the efficiency with which
sunlight is converted into power and how this relationship changes over time. An accurate
quantification of power decline over time, also known as degradation rate, is essential to all
stakeholders—utility companies, integrators, investors, and researchers alike. Financially,
degradation of a PV module or system is equally important, because a higher degradation rate
translates directly into less power produced and, therefore, reduces future cash flows [1].
Furthermore, inaccuracies in determined degradation rates lead directly to increased financial
risk [2]. Technically, degradation mechanisms are important to understand because they may
eventually lead to failure [3]. Typically, a 20% decline is considered a failure, but there is no
consensus on the definition of failure, because a high-efficiency module degraded by 50% may
still have a higher efficiency than a non-degraded module from a less efficient technology. The
identification of the underlying degradation mechanism through experiments and modeling can
lead directly to lifetime improvements. Outdoor field testing has played a vital role in
quantifying long-term behavior and lifetime for at least two reasons: it is the typical operating
environment for PV systems, and it is the only way to correlate indoor accelerated testing to
outdoor results to forecast field performance.
Although every reference included in this paper contains a brief to slightly extensive summary of
degradation rate literature, a comprehensive review could not be found. This article aims to
provide such a summary by reviewing degradation rates reported globally from field testing
throughout the last 40 years. After a brief historical outline, it presents a synopsis of reported
degradation rates to identify statistically significant trends. Although this review is intended to be
comprehensive, it is possible that a small percentage of the literature may not have been
included.
1
2. Historical Overview
Figure 1 shows a map with degradation rates reported in publications discussed in this article.
The size of each circle is indicative of the number of degradation rates reported at a given
location. The four major regions prior to the year 2000 wherein long-term field observations
have taken place are the USA, Europe, Japan, and Australia. These four regions are discussed
within their historical context, as understanding the PV history for terrestrial applications
elucidates time and place of degradation rate field observations. After 2000, a large number of
observations have been reported with equal diversity in technology and geography.
2.1. USA
The modern era of PV technology could be claimed to have started in the 1950s at Bell
Telephone Laboratories [4, 5]. When the Space Age officially started with the launch of the
Russian Sputnik satellite in 1957, PV technology and satellites were ideally suited for each other.
The first satellites such as Vanguard I required only moderate power, and the weight of the solar
panels was low. Reliability was ensured by protecting the cells with a quartz or sapphire cover
sheet from energetic particles outside the atmosphere and by using n-on-p type cells [6]. The oil
crisis of 1973 changed the focus of PV from space to terrestrial applications, particularly
applications in remote locations. Major oil companies were among the first to provide PV a
terrestrial market in the form of supervisory controls, cathodic well corrosion protection, buoys,
oil platform lights, and horns [7] that were much more economical than traditional battery-
powered solutions for remote locations on land and water [8]. However, with an environment
drastically different from space applications, the long-term reliability of PV modules faced vastly
different challenges. These were addressed starting in 1975 through the Flat-Plate Solar Array
project under the auspices of the Energy Research and Development Administration, which in
1977 was integrated into the U.S. Department of Energy [9-11]. Because of its PV experience in
space, the National Aeronautics and Space Administration was involved through two
laboratories, the Jet Propulsion Laboratory (JPL) in California and the Lewis Research Center in
Ohio. JPL conducted a block buy program, procuring state-of-the-art modules and testing
(a)
Figure 1. Geographical distribution of degradation rates reported in publications, (a)
worldwide and (b) a large part of Europe. The size of the circle is indicative of the
number of data points from a given location.
(b)
2
them [12]. Based on field experience and failure analysis of degraded modules, each of the five
block buys placed more and more stringent accelerated stress tests on the modules, providing
valuable information toward later standards such as module qualification standard IEC
61215 [13, 14]. Field tests were conducted via installation at various sites including the Lewis
Research Center and the Lincoln Laboratory at MIT, constituting the first systematic outdoor
testing [15]. While Block I modules did not experience high failure rates in the field, they
exhibited high degradation rates and provided insights into the various types of outdoor
degradation mechanisms [16-18]. Roesler et al. also reported high degradation rates for pre-
Block V modules in a 60-kW plant at the Mt. Laguna Air Force Station; these were probably
caused by hot spot problems (Wohlgemuth, private communication) [19].
From 1983 to 1985, the Atlantic Richfield Oil company constructed the first large PV site at
what is known today as the Carrizo Plain National Monument in central California. The
produced electricity was sold to the Pacific Gas and Electric Company, which also supervised
the data monitoring. The Carissa Plains project, as it was known at the time, used mirror
enhancement resulting in high module temperature and ultraviolet exposure. The rapid power
decline and maintenance experience at this site were initially attributed to the significant
encapsulant browning [20-22]. Wohlgemuth and Petersen later demonstrated that much of the
power loss in these modules was due to bad solder bonds, not ethylene vinyl acetate (EVA)
browning [23]. In 1986, the Photovoltaics for Utility Scale Application (PVUSA) was initiated, a
cost-sharing collaboration between private companies and government [24]. The project was
designed to bridge the gap between large utility companies unfamiliar with PV technology and
the small PV industry unfamiliar with the requirements of large utility companies [25]. The main
PVUSA sites were Davis, CA, USA, and Maui, HI, USA [26]. In addition to valuable hands-on
experience and detailed knowledge about maintenance costs, PVUSA also provided a new rating
methodology that is still used today [27, 28]. The long-term performance of the main sites can be
found in the PVUSA progress reports [29, 30]. The PVUSA project required qualification to tests
developed at the National Renewable Energy Laboratory (NREL), Sandia National Laboratories,
and JPL[30]. In an extensive field survey of systems consisting of pre- and Block V modules,
Rosenthal et al. found that the failure rates decreased significantly from 45% for pre-Block V to
less than 0·1% for Block V modules [31]. In addition, degradation rates for 10 selected systems
were found to be larger than 1%/year. Atmaram et al. reported on Block IV and V
monocrystalline Si systems deployed in Florida and found degradation rates well below
1%/year [32].
In 1977, the Department of Energy established the Solar Energy Research Institute in Golden,
Colorado. In 1991, it was renamed as the NREL. Outdoor testing of modules and submodules
started at the Solar Energy Research Institute in 1982. When amorphous silicon (a-Si) modules
first became commercially available, NREL began to report degradation rates that were
substantially higher than 1%/year for single and tandem junction modules although the
continuous testing time rarely exceeded 1 year, implying that some of this was the initial light-
induced degradation [33, 34]. Pratt and Burdick reported on the multiyear progress of a 4-kW a-
Si array commissioned in Michigan in 1987. Although the degradation rate was found to be
much lower, it still exceeded 1%/year in the first year of operation and then was stable between
years 2 and 3 [35-37]. Kroposki and Hansen showed similar results (initial light-induced
3
