Effects of irradiance and other factors on PV temperature coefficients

Validation of a Photovoltaic Module Energy Ratings Procedure at NREL

“…Pacific Gas and Electric Co. and others [3] found that most PV modules mounted outdoors rarely, if ever, produced their rated power [4]. This discrepancy is due, in large part, because under irradiance conditions greater than 500 W/m 2 , PV modules typically operate much hotter that the 25°C cell temperature specified by SRC.…”

Photovoltaic module and array performance characterization methods for all system operating conditions

“…Photovoltaic array (module) performance for an arbitrary operating condition can be described by Equations (1)(2)(3)(4)(5). The variables defining the operating condition are irradiance, cell temperature, absolute air mass, and solar angle-of-Portions of this document may be iilqible in electronic image products.…”

“…The ASTM method doesn't translate well to other operating conditions, doesn't address all factors involved in outdoor performance ratings, and is often considered no better than &lo% accurate when applied in the field to large photovoltaic arrays. The limited versatility of the ASTM method led utilities to define what they considered a more realistic procedure for specifying system performance based on a month-long evaluation period with measurements translated to a specified solar irradiance, ambient temperature, and wind speed (2,3). This procedure gave performance at a realistic operating temperature, but resulted in regression analyses that were less accurate than desired and limited in their ability to distinguish the interactive influences of solar irradiance, solar spectrum, solar angle-of-incidence, temperature coefficients, degree of thermal equilibrium, ambient temperature, and wind speed.…”

What is Happening with Regards to Thin-Film Photovoltaics?

“…Typically for a-Si:H modules, T-coefficients become less negative, sometimes even positive after prolonged exposure. A detailed study how degradation affects the amount of degradation that is observed has been published (Whitaker et al 1991). There was a resurgence of a-Si:H activities after the year 2005 when big companies entered the a-Si module arena by making or adapting deposition lines for a-Si:H-based PV modules.…”

“…Clearly, there are changes in all technologies, and as modules age or are stressed, transient behavior, power and temperature coefficients will change (Whitaker et al, 1991, del Cueto & von Roedern, 2006, Wohlgemuth, 2010. I sense some reluctance in the testing community to specify accelerated stress conditions, because not all effects and mechanisms for module degradation or failure are known, and because some people in that community hold out the hope that there would be better accelerated stress conditions that would better predict real world performance of modules.…”