Daniel Cormode

Emergence of superlattice Dirac points in graphene on hexagonal boron nitride

It is well known that graphene deposited on hexagonal boron nitride produces moire patterns in scanning tunnelling microscopy images. The interaction that produces this pattern also produces a commensurate periodic potential that generates a set of Dirac points that are different from those of the graphene lattice itself.

Response of graphene to femtosecond high-intensity laser irradiation

We study the response of graphene to high-intensity, 50-femtosecond laser pulse excitation. We establish that graphene has a high (∼3 × 1012 Wcm−2) single-shot damage threshold. Above this threshold, a single laser pulse cleanly ablates graphene, leaving microscopically defined edges. Below this threshold, we observe laser-induced defect formation leading to degradation of the lattice over multiple exposures. We identify the lattice modification processes through in-situ Raman microscopy. The effective lifetime of chemical vapor deposition grown graphene under femtosecond near-infrared irradiation and its dependence on laser intensity is determined. These results also define the limits of non-linear applications of graphene in femtosecond high-intensity regime.

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.

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.

Measuring degradation rates without irradiance data

A method to report photovoltaic (PV) system degradation rates without using irradiance data is demonstrated. First, a set of relative degradation rates are determined by comparing daily AC final yields from a group of PV systems relative to the average final yield of all the PV systems. Then, the difference between relative and absolute degradation rates is found using a Bayesian statistical analysis. This approach is verified by comparing to methods that utilize irradiance data. This approach is significant because PV systems are often deployed without irradiance sensors, so the analysis method described here may enable measurements of degradation using data that were previously thought to be unsuitable for degradation studies.

Optical characterization of electron-phonon interactions at the saddle point in graphene.

The role of many-body interactions is experimentally and theoretically investigated near the saddle point absorption peak of graphene. The time and energy-resolved differential optical transmission measurements reveal the dominant role played by electron-acoustic phonon coupling in band structure renormalization. Using a Born approximation for electron-phonon coupling and experimental estimates of the dynamic lattice temperature, we compute the differential transmission line shape. Comparing the numerical and experimental line shapes, we deduce the effective acoustic deformation potential to be Deff(ac)≃5  eV. This value is in accord with recent theoretical predictions but differs from those extracted using electrical transport measurements.

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 economic value of forecasts for optimal curtailment strategies to comply with ramp rate rules

We present a method to calculate the economic value of forecasts, based on the use of forecasts to optimize curtailment strategies in scenarios with a ramp rate rule. We consider how and when to limit PV power output in order to comply with a ramp rate rule to avoid penalties, but also calculate how curtailment will reduce revenue from energy yields. This framework provides a way to assess the value of forecasts.

Probing Electron-Phonon Interactions at the Saddle Point in Graphene

High frequency differential transmission spectroscopy of graphene, probing near the M-point, is performed and analyzed theoretically. Electron-phonon coupling is identified as the chief mechanism for renormalization with an effective acoustic deformation potential of approximately 5eV.