Mikael Bergkvist

Net load variability in Nordic countries with a highly or fully renewable power system

Increasing the share of intermittent renewable energy (IRE) resources such as solar, wind, wave and tidal energy in a power system poses a challenge in terms of increased net load variability. Full ...

Anomalies in integrating sphere measurements on structured samples

Anomalies are reported in total and diffuse reflectance values obtained from measurements on structured samples with a double-beam integrating sphere. It is demonstrated that these anomalies are caused by an interplay between the sample structure and geometric sphere imperfections. The sample structure causes a confinement of the diffuse reflectance which may suffer port losses as well as erroneously high signal levels. This can lead to deviations in the recorded reflectance values as high as 50%.

Intensity distribution in the collector plane from structured booster reflectors with rolling grooves and corrugations

Abstract While testing different reflector materials for external reflectors for solar collector arrays, it was found that standard rolled aluminium and corrugated aluminium materials could perform almost as well as mirror-like materials. A ray tracing model was developed to calculate the intensity in the collector plane for solar radiation from reflector materials with grooves or corrugations. Laboratory measurements, for reflector samples, with a specially designed spectral scatterometer were used to determine the angular intensity distribution of the reflected radiation. Calculations with the model using measured intensity distributions show that the scatter from aluminium materials with rolling grooves will be directed close to the specular direction and along an almost circular arc in the collector plane. The intensity in the collector plane will be redistributed slightly upward or downward depending on the season and time of day; therefore, both an increase and decrease in average intensity can occur during the year relative to a mirror-like material with the same total reflectance. For rolled aluminium, a small performance improvement can be achieved compared to a mirror reflector with equal total reflectance. Corrugated surfaces will yield a significant increase in average intensity onto the collector aperture at times when the radiation from a mirror-like reflector would otherwise be lost above the collector.

Observation of diffuse interference in reflectance from oxide-coated metals☆

Abstract Experimental results on the diffuse reflectance from oxide-coated metals are reported. Previous observations of interference-like structure in the diffuse spectra are confirmed. For Cu 2 O on copper and TiO 2 on titanium the structure is opposite in phase to the specular interference variation. The diffuse out-of-phase interference is absent for tandems of Cu 2 O on stainless steel and for the thinnest layers of TiO 2 . The appearance of out-of-phase diffuse interference appears to be connected to roughness at the oxide-metal interface. It is speculated that diffuse reflectance spectroscopy might be a valuable tool for studying the oxidation of metals.

Optical scattering from oxidized metals. 2: Model verification for oxidized copper.

A model for the calculation of diffuse reflectance spectra for oxidized metals is applied to thermally oxidized copper films and compared with experiments. The model calculations reproduce the spectral structure observed in the experiments. It is demonstrated that the air-oxide interface roughness dominates the scattering for wavelengths shorter than the absorption threshold of the oxide, and the oxide-metal interface roughness dominates for longer wavelengths. Using the model for fitting calculations the rms roughness values for the two interfaces are determined independently. The roughness values agree with the results from high accuracy stylus profiling of the oxide front surface as well as stylus profiling and total integrated scattering of He-Ne light from the bare metal surface, obtained after etching away the oxide. The good overall agreement between the calculated and experimental diffuse reflectance spectra, as well as the rms roughness values determined with different techniques, confirms the applicability of the model.

Determination of the SiO 2 /Si interface roughness by diffuse reflectance measurements

The problem of determining the roughness of the SiO(2)/Si interface is treated. A model is used based on the Fresnel formalism and scalar scattering theory. The resulting formulas express the diffuse reflectance as a function of the optical constants of the two materials, the oxide thickness and the rms roughness of the interfaces. Using the roughness values as adjustable parameters, quantitative information about the interface roughness is obtained from the diffuse reflectance spectra for an SiO(2)/Si double layer. Excellent agreement between calculated and experimental spectra is obtained for an rms roughness of 9.0 nm at the front surface and 2.2 nm at the oxide substrate interface for the case of a low-pressure low-temperature CVD film of SiO(2) on Si.

Determination of interface roughness by using a spectroscopic total-integrated-scatter instrument

A spectroscopic total-integrated-scatter instrument has been constructed. It uses a Coblentz sphere for the collection of the scattered light and a lamp with a monochromator as a light source. It can be used to measure diffuse reflectance as well as transmittance. The instrument has been used to measure diffuse reflectance of thermally and chemical-vapor-deposition oxidized silicon wafers. Comparisons are made with measurements by using a spectrophotometer with an integrating sphere. The data have been interpreted with a parameterized model for light scattering from a double layer, to obtain rms surfaceroughness values for the two interfaces of the oxide film.

Quantitative interface roughness studies of copper oxide on copper

Abstract Interference in the optical reflectance from oxidized metals has been studied. The diffuse component exhibits an interference-like wavelength variation, oppositely phased to the regular interference structure for the specular reflectance. The scattering originates from the oxidation-induced roughness at the oxide-metal interface. With a simple model based on the Fresnel formalism and scalar scattering theory, it is shown that quantitative information about the roughness of the interface can be obtained from the diffuse reflectance spectra. The results are compared with roughness values obtained both from surface profilometry and total integrated scatter measurements of the metal surface after removing the oxide.

Wind Energy Converters and Photovoltaics for Generation of Electricity after Natural Disasters

Abstract During recovery and reconstruction after a natural disaster, an autonomous power supply might be needed for an extended period of time. In this work, the feasibility of using small‐scale wind power and battery storage for power supply is evaluated and compared with systems containing photovoltaics. The investment cost per yearly produced and for an optimized energy system supplying small loads (2 or 20 KW peak) has been calculated for 32 sites, predominantly in frica and the iddle ast. The sites represent foreign activities of the wedish ivil ontingencies gency at the end of 2012. Since wind speed measurement series often have a lot of missing data, autoregressive moving average models were trained and used to generate hourly time series of wind speed. This methodology proved robust, even when data availability was very low or when measurements were only taken every third hour. The results of the simulations show that photovoltaic/battery systems outperform wind/battery systems at all evaluated sites. This can be explained by lower investment cost per yearly produced and smoother daily/weekly power output over the year for the photovoltaic system. The proportion of wind power for optimized systems comprising wind, photovoltaics and battery bank is generally very low and the system cost is almost identical to the corresponding photovoltaic/battery systems. In conclusion, at lower latitudes and with little time for a proper wind measurement campaign, photovoltaics should be the primary candidate for replacing or complementing conventional diesel generators.

Using the MIUU Model for Prediction of Mean Wind Speed at Low Height

This paper presents an evaluation of using the MIUU mesoscale model to predict mean wind speed at 10 meters above ground. This was done by comparing the model output to 128 meteorological measurements in Sweden. Although some terrain-dependent bias was seen, the model worked surprisingly well. Mean absolute error (MAE) was 0.58 m/s or 0.52 m/s if a few measurements taken in highly complex terrain were excluded. By using a multiple regression model, taking into account the terrain at site, the latter value was lowered to 0.39 m/s. The model described in this paper can be useful for a first evaluation of the wind conditions for small-scale wind power or when measurements are not feasible.