Maria Brogren

Titanium–aluminum–nitride coatings for satellite temperature control

Abstract Intense solar irradiation, radiative cooling to outer space, and internal heat generation determine the equilibrium temperature of a spacecraft. The balance between the solar absorption and thermal emittance of the surface is therefore crucial, in particular for autonomous parts directly exposed to the solar radiation and thermally insulated from the main thermal mass of the spacecraft. The material composition but also the coating thickness are found to influence the equilibrium temperature of an object in space. In this paper we report on a systematic search for a suitable composition and thickness of TixAlyNz alloy coatings prepared by reactive, unbalanced magnetron sputtering from targets consisting of differently sized titanium and aluminum sectors. The films were deposited on glass, glassy carbon, aluminum sheet metal, and on sputtered aluminum and TixAl(1−x) films on glass. The stoichiometry and sheet resistance of the films was determined with Rutherford backscattering and four-point probe measurements respectively. Reflectance spectra for the visible and infrared spectral ranges were used to obtain average solar absorptance and thermal emittance values used in model calculations of the equilibrium temperature. Neglecting internal heat contributions, the lowest calculated equilibrium temperature in orbit around the Earth, 32.5°C, was obtained for a 505-nm-thick Ti0.14Al0.47N0.40-film.

Design of concentrating elements with CIS thin-film solar cells for façade integration

A concentrating photovoltaic wall element, including Cu(In, Ga)Se-2-based modules, aluminum reflectors, and an insulation for building integration, has been developed and evaluated. The geometric concentration ratio of the system is about 3 x. However, the measured maximum electric power from the modules is only 1.9 times that of identical vertical modules without reflectors, due to optical losses and a decrease in fill-factor from 0.6 to 0.5 under concentrated light. The optical efficiency of the system is 76% at an effective solar height of 40degrees

Low-concentrating water-cooled PV-thermal hybrid systems for high latitudes

This paper treats the design and evaluation of low-concentrating, water-cooled photovoltaic-thermal hybrid systems with stationary parabolic reflectors for groundbased applications or facade-integration of photovoltaics at high latitudes. The use of photovoltaic-thermal hybrid string modules in combination with cheap aluminum reflectors has a potential to significantly increase power production from a given solar cell area and reduce the cost of photovoltaic electricity.

Hammarby Sjöstad–an interdisciplinary case study of the integration of photovoltaics in a new ecologically sustainable residential area in Stockholm

The integration of photovoltaic (PV) systems in apartment buildings in a new residential area, Hammarby Sjostad in Stockholm, has been studied using an interdisciplinary approach including e.g. interviews with actors and modelling of PV systems in PVSYST. Four of the ten construction companies represented in the area will install PV systems. The yearly electricity production from these systems has been estimated to be 63 MWh or equal to an electricity demand of 38 (out of 2300) households in the area. Interviews reveal that obstacles for the integration of PV in buildings are e.g. perceived expense and a lack of knowledge. The choice of PV technology is based more on economy, aesthetic appearance, and a wish to demonstrate environmental concern, than on optimal system performance. By integrating renewable energy technologies in the buildings, the construction companies will lay a ground for an ecologically sustainable living, but how these opportunities are utilised by future residents, managers, and caretakers of the buildings will be of decisive importance for the final outcome.

TixAlyN coatings for temperature control of spacecraft

Thermal control is an important issue in small-scale satellite design, and thin film coatings suit the limited mass and volume constraints. Group IVB transition metal nitride films meet the criteria that the satellite surface must be mechanically and chemically stable, and electrically conducting. Thin film TixAlyNz coatings have been investigated and tailored for temperature control. The films were deposited by reactive sputtering on aluminum substrates in N2/Ar-atmosphere. The solar absorptance, (alpha) , and thermal emittance, (epsilon) , were calculated from spectral reflectance measurements. It was found that an optimization of film composition leads to a reduced equilibrium temperature. The composition temperature. The composition Ti0.16Al0.41N0.43 has a flatter reflectance curve than TiN, and was found to be close to optimal. By varying the film thickness, interference effects could further reduce the equilibrium temperature. A 650 nm Ti0.16Al0.41N0.43 film showed a reflectance interference minimum positioned at the maximum of the blackbody spectrum, resulting in an increased emittance. Neglecting internal heat contributions, the lowest calculated equilibrium temperature was 34.6 degrees C for this film.