Vincent Lonij

Absolute and ratio measurements of the polarizability of Na, K, and Rb with an atom interferometer

We measured the ground-state electric-dipole polarizability of sodium, potassium, and rubidium using a Mach-Zehnder atom interferometer with an electric-field gradient. We find {alpha}{sub Na}=24.11(2){sub stat}(18){sub sys}x10{sup -24}cm{sup 3}, {alpha}{sub K}=43.06(14)(33), and {alpha}{sub Rb}=47.24(12)(42). Since these measurements were all performed in the same apparatus and subject to the same systematic errors, we can present polarizability ratios with 0.3% uncertainty. We find {alpha}{sub Rb}/{alpha}{sub Na}=1.959(5), {alpha}{sub K}/{alpha}{sub Na}=1.786(6), and {alpha}{sub Rb}/{alpha}{sub K}=1.097(5). We combine our ratio measurements with the higher-precision measurement of sodium polarizability by Ekstrom et al. [Phys. Rev. A 51, 3883 (1995)] to find {alpha}{sub K}=43.06(21) and {alpha}{sub Rb}=47.24(21).

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.

Dispersive atom interferometry phase shifts due to atom-surface interactions

We used the Toulouse atom interferometer to study how Van der Waals (VdW) interactions between atoms and surfaces cause velocity-dependent phase shifts for atomic de Broglie waves. By introducing a thin nano-grating in one branch of this interferometer, we observed a phase shift that depends on velocity to the power −0.49. This dispersion serves to measure both the strength and the position dependence of the atom-surface potential in the range from 5 to 10 nm from the surface, and it can also set new limits on non-Newtonian gravity in the 2 nm range.

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.

Cover slip external cavity diode laser

A 671 nm diode laser with a mode-hop-free tuning range of 40 GHz is described. This long tuning range is achieved by simultaneously ramping the external cavity length with the laser injection current. The laser output pointing remains fixed, independent of its frequency because of the cover slip cavity design. This system is simple, economical, robust, and easy to use for spectroscopy, as we demonstrate with lithium vapor and lithium atom beam experiments.

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.

Can atom-surface potential measurements test atomic structure models?

van der Waals (vdW) atom-surface potentials can be excellent benchmarks for atomic structure calculations. This is especially true if measurements are made with two different types of atoms interacting with the same surface sample. Here we show theoretically how ratios of vdW potential strengths (e.g., C₃(K)/C₃(Na)) depend sensitively on the properties of each atom, yet these ratios are relatively insensitive to properties of the surface. We discuss how C₃ ratios depend on atomic core electrons by using a two-oscillator model to represent the contribution from atomic valence electrons and core electrons separately. We explain why certain pairs of atoms are preferable to study for future experimental tests of atomic structure calculations. A well chosen pair of atoms (e.g., K and Na) will have a C₃ ratio that is insensitive to the permittivity of the surface, whereas a poorly chosen pair (e.g., K and He) will have a ratio of C₃ values that depends more strongly on the permittivity of the surface.