Adria E. Brooks

Comparing ramp rates from large and small PV systems, and selection of batteries for ramp rate control

We compare the AC power fluctuations from a 1.6 MW and a 2 kW photovoltaic (PV) system. Both of these PV generating stations exhibit fluctuations exceeding 50% of their rated capacity in under 10 seconds. The smaller system can fluctuate more rapidly, exhibiting 50% dropouts in 3 seconds. Although the MW-scale system covers 4000 times as much ground area, the bandwidth of the fluctuations is remarkably similar. We explore explanations for this observation, and we discuss the impact of this on battery sizing.

Forecasts of PV power output using power measurements of 80 residential PV installs

We describe a new method to forecast the power output from photovoltaic (PV) systems under cloudy skies. We forecast cloud-induced fluctuations in output power by using measurements from 80 residential rooftop PV systems as an input to a forecasting algorithm described here. We compare the performance of our new method to results from our numerical weather model, and also to forecasts based on our images from a ground-based sun-tracking sky camera. Our numerical weather model provides forecasts of irradiance up to several days in advance. In comparison, our network of PV systems can forecast output up to an hour in advance. Our sky-camera image analysis algorithms can be used to forecast 10 minutes in advance. We also show how hybrid methods can improve the accuracy of hour-ahead forecasts.

Analysis of 80 rooftop PV systems in the Tucson, AZ area

We present field performance measurements of 80 rooftop systems in the Tucson, AZ region. We describe a framework that enables identification the causes of system performance variations. We can distinguish between shading, module orientation, outages, and weather conditions from the performance data alone, i.e., without physical inspection of the system. The modules used in the systems that we study are predominantly from 2 manufacturers: Sunpower and Schott, with two types of modules from each manufacturer. We show that the variation in system performance due to these four module types is smaller than the variations due to other system-level effects. We discuss the distribution of de-ratings due to shading, and de-ratings due to clouds, as well as the significance and duration of outages. We also examine the effect of system age on annual final yields.

Performance reviews from the Tucson Electric Power solar test yard

At the Tucson Electric Power (TEP) solar test yard, over 20 different grid-connected PV systems are being tested. Most of these PV systems are in the 1 to 2 kW range. We present measured conversion efficiencies, final yields, performance ratios, temperature de-ratings and degradation rates for 20 PV systems. The final yields are also compared to predictions from PVwatts and PVsyst. The systems include flat plate PV modules from Sunpower, Sharp, BP, Uni-solar Sanyo, Shell, Astropower, Solarex, and Evergreen Solar. The performance of several of these flat plate PV systems has been recorded starting in 2003. Since then, several types of concentrating PV modules have been also deployed at the Tucson Electric Power solar test yard, including modules from Solyndra, Prism Solar Technologies Inc., Skyline Inc., and Semprius Inc. At the same location Solon Corporation is testing several flat plate modules with 1-axis and 2-axis trackers.

I–V curves and visual inspection of 250 PV modules deployed over 2 years in tucson

To study degradation of PV systems, we measured current-voltage (I-V) curves from 250 modules that have been deployed in grid-tied systems for 2 to 12 years in Arizona. We also documented visual signs of weathering. The I-V curves exhibit a variety of imperfections and considerable dispersion of Pmax values. Although many of these modules appear weathered, we found few significant correlations between visible signs of weathering and lower Pmax values, and the most degraded modules have no correlation between visual defects and performance. This indicates that important forms of degradation can remain hidden to the eye.

PV system power loss and module damage due to partial shade and bypass diode failure depend on cell behavior in reverse bias

Power loss due to partial shade was compared for two types of commercial photovoltaic modules. We also tested both types of modules with bypass diodes removed to simulate diode failure. Modules with uniformly low reverse bias voltage (VBR) cells (of -4V) lost significantly less power compared to modules with high VBR cells (of -15V) when subjected to partial shade. Differences in power loss were even larger when bypass diodes were removed. Associated with higher power loss, we observed several types of degradation on modules with high-VBR cells. A model to simulate power loss in an array due to partial shading that uses VBR as the main parameter was found to be in good agreement with measured power losses in the field.

The consequence of soiling on PV system performance in Arizona; Comparing three study methods

We study the consequence of soiling on different flat-plate photovoltaic modules in Tucson, Arizona. We compare three different methods of study. These methods include (1) a comparison of energy yields between strings of PV modules, held at MPP, that are regularly cleaned and naturally soiled, (2) measurement I-V curves from individual PV modules in the field before and after cleaning, and (3) measurement I-V curves from individual PV modules in the laboratory before and after cleaning using a flash tester. All methods indicate an approximate 1% performance enhancement from cleaning. Laboratory I-V curve measurements provide the most precise results.

String-Level (kW-scale) IV curves from different module types under partial shade

Partial shade is known to cause significant de-rating for most photovoltaic (PV) systems. However, the specific de-rating due to partial shade from differently shaped objects like overhead wires is not well known. We present current-voltage (I-V) curves from three different strings containing 4 to 9 PV modules while applying various types of partial shade, including shade from electrical cables in front of the modules. We show that a 1-kW string of nine poly-Silicon modules manufactured by Sharp can have a power loss of 13% due to shading just 1% of its surface area. We verify that blocking columns of cells in a module has less impact on power loss than blocking rows. This demonstrates how the non-linear response to partial shade can depend on the placement of bypass diodes that isolate different sub-strings within each module. Furthermore, we quantify the reduction in power and I-V parameters caused by shade from wires (or PVC pipes in our experiments).

Holographic CPV field tests at the Tucson Electric Power solar test yard

Holographic concentrators incorporated into PV modules were used to build a 1600 W grid-tied PV system at the Tucson Electric Power solar test yard. Holograms in concentrating photovoltaic (CPV) modules diffract light to increase irradiance on PV cells within each module. No tracking is needed for low concentration ratios, and the holographic elements are significantly less expensive than the PV cells. Additional advantages include bi-facial acceptance of light, reduced operating temperature, and increased cell efficiency. These benefits are expected to result in higher energy yields [kwh] per unit cost. Field tests of the holographic concentrator system are reported here. A performance ratio greater than 1 was observed. The field tests include comparison with other flat plate non-tracking PV systems at the same test yard. Predicted yields are also compared with the data.

Characterization and Use of a Sinton FMT-350 Flash Tester at the Tucson Electric Power Solar Test Yard

We characterize a Sinton Instruments FMT-350 module I-V flash tester using NREL primary calibration reference modules. We use the FMT-350 to measure the effect of soiling and light induced degradation on PV modules.