The DC rating of flat-plate PV installations has traditionally been calculated by multiplying the number of PV modules by the peak watt rating for each module. This rating reflects performance at standard test conditions – 1,000 W/square meter direct normal irradiance (DNI) and cell temperature of 25 degrees C (IEC 61215).
However, these conditions are seldom observed in normal operation, because typical module operating temperatures are closer to 45 degrees C.
To better understand typical performance, modules are also characterized under a standard reference environment – 800 W/square meter and ambient temperature of 20 degrees C (IEC 61215), which gives a better estimate of observed performance.
Concentrating photovoltaic (CPV) equipment performance has been quantified based on widely varying conditions. In the past, CPV companies chose between 25 degrees C cell temperature or 20 degrees C ambient temperature – and between 850 W/square meter, 900 W/square meter or 1,000 W/square meter DNI – to determine the power rating(s).
As a result, the meaning of a CPV module's peak watt rating has been difficult to compare, creating confusion, especially when a company neglected to mention the chosen rating conditions.
In order to provide clarity, the International Electrotechnical Commission (IEC) has adopted two sets of conditions that parallel the approach used for flat-plate PV products. The standard test conditions are the same as for flat-plate PV (1,000 W/square meter and cell temperature of 25 degrees C), with the exception that the spectrum is adjusted to be representative of the spectrum that more typically reaches the CPV cells.
In other words, the CPV standard operating conditions are similar to the PV standard reference environment, except that they have a DNI that is representative of typical conditions in CPV target markets.
With this consistency between the approaches used for flat-plate and CPV ratings, the industry can avoid some of the confusion involved with testing.
Nevertheless, in order to take advantage of the new clarity in the ratings, one must check that the manufacturer has switched to the new rating system. It likely will not be long before all manufacturers have made the switch to the new system, converging on standard conditions that facilitate the comparison of all CPV and flat-plate products.
In addition to the definition of the standard reference conditions, the standards committee is developing test methods for how to measure CPV module and system performance. Draft standard IEC 62670-3 describes how to use a simulator or outdoor test platforms to quantify the power output at the new reference conditions.
Very few labs have simulators with the needed collimation of light, as the collimation must be similar to that of natural sunlight, but those labs that do have appropriate simulators can do measurements on any day of the year.
In contrast, outdoor measurements do not require as much special equipment, but they do require a sunny day, which can be hard to come by in some locations. The power ratings measured outdoors or by a simulator will be recorded on the nameplate of the module and will be used for ascribing power plant size.
Why standards matter
Draft standard IEC 62670-2 will provide a yearlong energy test, quantifying the electricity generated relative to the irradiance that was available during the year. CPV modules tend to have lower temperature coefficients than flat-plate modules, but the high-efficiency, multijunction cells are more sensitive to spectrum issues.
The yearlong energy test measures a factor similar to performance ratio, which is easily used for projecting performance at other plants with similar weather. This measurement can also be used to predict the energy expected at sites with different weather if the difference in weather between the two sites can be estimated.
Standards such as IEC 62670-2 are essential to the healthy growth of the industry. Standard procedures define the meaning of performance, and consistent implementation of standards results in increased confidence from customers and investors.
Growth of the flat-plate PV industry has been enabled by the early adoption of useful standards. The implementation of these new CPV standards will build a foundation for the healthy growth of the CPV industry as well.
The IEC Technical Committee 82, Working Group 7 has 84 members from 18 countries. Sarah Kurtz is a principal scientist at the National Renewable Energy Laboratory, where she works with a group to better understand the details of performance and reliability of PV and to convert that knowledge into international standards.
Sandheep Surendran of Surya Design is an independent consultant who is currently leading the development of IEC 62670-3. Hansjorg Lerchenmueller is senior vice president at Soitec, where he is responsible for product strategy.