Matthew Muller

An Investigation into Spectral Parameters as they Impact CPV Module Performance

The CPV industry is well aware that performance of triple junction cells depends on spectral conditions but there is a lack of data quantifying this spectral dependence at the module level. This paper explores the impact of precipitable water vapor, aerosol optical depth (AOD), and optical air mass on multiple CPV module technologies on‐sun in Golden, CO.

Determining Outdoor CPV Cell Temperature

An accurate method is needed for determining cell temperature when measuring CPV modules outdoors. It has been suggested that cell temperature can be calculated through a procedure that shutters sunlight to the cells while measuring the transients in open‐circuit voltage (Voc) and heat sink temperature. This paper documents application of this shutter procedure to multiple CPV modules at NREL. The challenges and limitations are presented along with an alternate approach to measuring CPV cell operating temperature.

Considerations for How to Rate CPV

The concentrator photovoltaic (CPV) industry is introducing multiple products into the marketplace, but, as yet, the; community has not embraced a unified method for assessing a nameplate rating. The choices of whether to use 850,; 900, or 1000 W/m2 for the direct-normal irradiance and whether to link the rating to ambient or cell temperature will; affect how CPV modules are rated and compared with other technologies. This paper explores the qualitative and; quantitative ramifications of these choices using data from two multi-junction CPV modules and two flat-plate; modules.

Performance of CPV system using three types of III-V multi-junction solar cells

Performance of III-V multi-junction solar cells depends on spectral conditions according to which junction limits the photocurrent. Specifically, the response of concentrating multi-junction solar cells depends on the illumination at the cell surface. Because the illumination condition depends on alignment, it is important to characterize the CPV performance not only for a mono-module but also for a system or an array. In this paper the spectral effect on the CPV system and the mono-module consisting of III-V multi-junction solar cells from three different manufactures will be discussed.

Minimizing Variation In Outdoor CPV Power Ratings

The CPV community has agreed to have both indoor and outdoor module power ratings. The indoor rating provides a repeatable measurement off the factory line while the outdoor rating provides a measure of true on‐sun performance. The challenge with an outdoor rating is that conditions that impact the measurement such as the spectrum, temperature, wind speed, etc are constantly in flux. This work examines methodologies for determining the outdoor power rating with the goal of minimizing variation even if data are collected under changing meteorological conditions.

Procedural considerations for CPV outdoor power ratings per IEC 62670

The IEC Working Group 7 (WG7) is in the process of developing a draft procedure for an outdoor concentrating photovoltaic (CPV) module power rating at Concentrator Standard Operating Conditions (CSOC). WG7 recently achieved some consensus that using component reference cells to monitor/limit spectral variation is the preferred path for the outdoor power rating. To build on this consensus, the community must quantify these spectral limits and select a procedure for calculating and reporting a power rating. This work focuses on statistically comparing several procedures the community is considering in context with monitoring/limiting spectral variation.

Evaluating the IEC 61215 Ed.3 NMOT procedure against the existing NOCT procedure with PV modules in a side-by-side configuration

Nominal operating cell temperature (NOCT) is a simple parameter to distinguish the thermal performance of one PV module design from another. Recently, the National Renewable Energy Laboratory (NREL) participated in NOCT round-robin testing designed to quantify the reproducibility of NOCT values between eight different test laboratories. This work expands on the round-robin testing by further examining NOCT results produced at NREL. Heat transfer modeling suggests that similarly constructed/packaged modules should not have the widely varying NOCT values that are publicly reported. In order to test this premise, a side-by-side NOCT comparison is presented for three glass/silicon/plastic modules that represent the extreme range of reported NOCT values. A glass/silicon/glass module is also included in the side-by-side comparison to gauge the impact of changing a packaging parameter. Working group 2 of the International Electrotechnical Commission Technical Committee 82 has recently drafted a replacement for NOCT that is titled “Nominal Module Operating Temperature” (NMOT). With this change in progress, NREL data are also used to compare NOCT to NMOT.

One year NOCT Round-Robin testing per IEC 61215 standard

The nominal operating cell temperature (NOCT) was developed as a reference characterization test procedure to quantify the module cell temperature for different module designs in a standard reference environment (SRE) of 20 °C ambient temperature, 800 W/m2 irradiance and 1 m/s wind speed [1]. The NOCT value is a key performance parameter to be measured as per the IEC 61215 (Edition 2) standard, and it is required to be independently measured and reported to qualify for the incentive programs of various agencies including California Energy Commission. Ideally, the NOCT value of a specific module design should be identical irrespective of testing laboratory, location, month or season. The objectives of this NOCT Round-Robin testing were: (i) to identify if NOCT values are significantly influenced by the testing approach or site specific test conditions experienced by different labs in the world (referred to as reproducibility); (ii) to identify if NOCT values are dependent on the month or season within any single laboratory (referred to as repeatability), and (iii) to identify if NOCT values are significantly influenced by the type (thermocouple or RTD) and position (backsheet or cell) of thermal sensor used by different labs in the world. A total of eight polycrystalline silicon modules with a nameplate rating of 217 watts were continuously tested over a year in eight different test laboratories around the world, one at each participating laboratory. All test samples are of the same model, were supplied by a single manufacturer, were received as a batch and were assumed to be identical with no variability in manufacturing. The test laboratories were not supplied with any specific testing procedure and they were asked to perform the testing as per their established standard operating procedures (SOP).

A scalable method for extracting soiling rates from PV production data

We present a method for analyzing time series production data from photovoltaic systems to extract the rate at which energy yield is affected by the accumulation of dust, dirt, and other forms of soiling. We describe an approach that is based on prevailing methods, which consider the change in energy production during dry periods. The method described here builds upon these methods by considering a statistical sample of soiling intervals from each site under consideration and utilizing the robust Theil-Sen estimator for slope extraction from these intervals. The method enables straightforward application to a large number of sites with minimal parameterization or data-filtering requirements. Furthermore, it enables statistical confidence intervals and comparisons between sites.

Optical cell temperature measurements of multiple CPV technologies in outdoor conditions

It is well known that photovoltaic performance is dependent on cell temperature. Although various methods have been explored to determine outdoor concentrating photovoltaic (CPV) cell temperature, no method has proven to work across all module technologies and result in desirable uncertainties. Menard (2012) has recently published results claiming accurate measurements of cell temperature using the wavelength shift of light emitted from the sub-cells of a Semprius CPV module. This work focuses on efforts to verify Menards results using additional CPV technologies that are on-sun at NREL. Baseline electro-luminescence emission is recorded for modules under a low level forward bias and under isothermal conditions using thermal chambers. The same modules or sister modules are then placed on NRELs high accuracy two-axis tracker for outdoor measurements. Photo-luminescence emission peaks are measured for multiple modules at stable wind and irradiance conditions. Emission results from the sub-cells are compared to what is documented in the literature for the given semiconductor material. The signal to background ratio is analyzed and the possible broad applicability of this procedure is discussed.