"For us, the sun shines every day …
– even at night, if we want it to", declares Michael Würth, research fellow at the Fraunhofer Institute for Building Physics IBP, with a twinkle in his eye. This motto was carefully chosen as the focal point of his research work is solar simulation. The researcher’s work in the Hygrothermics department focuses on the performance of building components and structures when exposed to radiation and various climatic influences. His current project, the new National Museum at Oslo, is one of the world’s largest construction projects in the cultural sector, with a budget of more than EUR 600 million. The most characteristic feature of this ensemble of buildings situated immediately at the Oslo harbor water front is the so-called Alabaster Hall, distinguished by a facade extending over several thousands of square meters. Specially designed glass ceramic panels, which are placed in front of the double glass facade, illuminate the special exhibition area with a pleasant, soft light. At the same time, facade-integrated solar protection devices ensure a comfortable indoor climate. Despite this all-glass facade, it is intended to limit the cooling loads inside the hall to the lowest possible level, in particular during the summer months. The cooling load is the amount of heat that needs to be discharged e.g. via an air-conditioning system in order to ensure a comfortable indoor climate. Due to two particularities, the implementation of this requirement presented a special challenge: on the one hand, the facade is exceptionally deep (depth >1 m); on the other hand, extract air flows through the intermediate space between the two glass levels to be discharged from the building. This attached even greater importance to the functional test to be carried out in the laboratory: to begin with, the test set-up required a considerably larger test specimen than usual. With the aid of the solar simulator the IBP scientist was able to meet all the requirements and requests expressed by builders and planners at short notice. Drawing on their comprehensive know-how, the working group on Thermal parameters – Climate simulation created a complex test model, featuring a total of six different climate zones in order to simulate real-life operation.
Practical applications of the solar simulation
Typical applications for solar simulation include accelerated aging of materials, determination of the total energy transmittance, clarification of functionality and service life or so-called thermal load tests, which are performed to examine the spontaneous failure of building components. In addition, emissions that condense as a bright-colored film in the intermediate space of building components, are also an issue. Regarding simulation, solar radiation is one of the most complex climatic elements. Besides determining the appropriate irradiance, also the spectra of the light sources and the optical components of the radiators need to be considered for each requirement. In the case of light-redirecting building components, for instance, relevant factors include the angle of incidence of solar radiation or the shares of direct and diffuse radiation, which depend on the time of day or the season. There are special test facilities for each field of application. When performing aging tests and thermal load tests on larger surfaces, mostly uncomplicated mixed-light sources are used.
A 'radiant' test facility
In the solar simulator, metal halide lamps are used, which emit a near-solar, continuous spectrum in the solar range from 300 to 2500 nm. "In our research work it is an essential criterion that the artificial light source we use is as close as possible to the composition of natural sunlight", Würth explains. "Choosing well-adapted optical components enables us to achieve high-quality directivity. Usually, the irradiances at sample level range between 500 and 1200 W/m²". As a rule, classical accelerated weather tests for organic materials are performed in accordance with tried and tested standard methods using standardized test devices equipped with UV or xenon sources. For special applications such as multiple, superimposed reflections in adjacent buildings, special settings can be selected which allow to realize irradiances of up to 5 kW/m² on partial surfaces. To ensure cost-efficient operation of the test facility, Würth attaches great importance to lamp energy efficiency. Unfortunately, there is currently no satisfactory technical solution available to cover the solar spectrum using LED technology.
To characterize different solar simulation facilities, efficient tools and self-developed methods are at hand, which also allow to perform external measurements on behalf of clients: With the aid of a calibrated spectroradiometer, lamp spectra ranging between 280 and 2500 nm can be determined in steps of 1 nm as absolute irradiances. In addition, this allows to supervise the effectiveness of filters and the aging behavior of the light sources. In the case of highly complex fenestration systems, it is possible to measure spectral transmission (i.e. light transmittance) directly in the solar simulator. To ensure uniform irradiation at the sample plane, pyranometer measurements are supplemented by a camera system combining several hundreds of thousands of reading points.
Customized weathering at the push of a button
In the laboratory, the IBP scientist creates customized weathering conditions by combining selected climatic factors that bear relevance to the task. The climatological data are extracted from databases, which provide data ranging from the desert sun up to locations in Northern Europe. These parameters can be individually programmed. Reproducibility, i.e. the repeatability of the tests under identical conditions, produces reliable results, which enable Michael Würth to compare variant designs or products. "The exposure of building components like walls or roofs to sunlight, namely of full-scale samples sized up to 8 m² positioned in their original installation orientation, in combination with temperature, air humidity or rainfall constitutes a unique feature. Normally, calorimetric measurements of the g-value only require a 1 m² sample of the surface to be examined. For this special purpose, we had some high-performance mobile climatic units built in order to avoid restrictions imposed by climate chamber dimensions", Würth explains. As necessary, the tests can also be carried out in climate chambers with room volumes of up to 250 m³.
Expertise and international networks
"Ankara, Toronto, Milano, Cupertino, Leicester – these are just some of the locations of major construction projects for which we are developing solutions in cooperation with our clients", says Würth who has established the business unit of solar simulation at Fraunhofer IBP in the last few years. "The facade designs are mainly prototypes or breaking new ground, hence classical test methods will not apply in these cases. Besides, the climatic boundary conditions prevailing at the building site also need to be considered", the scientist continues. "We approach the specific circumstances of a construction project in a flexible way, and we want to implement the requirements specified in our clients‘ contract documents always taking the economics of the situation into perspective. If need be, we also choose unconventional methods. For instance, we subdivided a glass roof measuring several square meters into two zones. In parallel operation (but in reverse order), both zones were exposed to alternating four-hour cycles of scorching artificial summer sun, followed by torrential rain and winter temperatures of minus 10 degrees Celsius. These exposure tests lasted for several months."
Circulation areas or roof coverings are also of interest, as the heating of urban surfaces due to climate change is continually gaining importance. A high degree of soil sealing reduces evaporative cooling; at the same time, the buildings themselves store part of the radiated energy. So far, the only valuation standard is the so-called Solar Reflectance-Index (SRI). However, the standard procedure does not consider surfaces with more complex structures or transient storage effects. With the aid of the solar simulator, Würth developed new assessment options for both cases. Due to an integrated sprinkler system and scales it is possible to conduct further hygrothermal tests, such as determining the evaporation performance of building surfaces .
To examine more complex issues in the field of radiation, the Fraunhofer IBP scientists prefer to combine laboratory and field tests. In the lab, artificial weathering in the solar simulator allows checking the most important data of a test set-up, at short notice, in a reproducible manner and independent of the actual weather conditions. Outdoor weathering tests or tests carried out in the calorimetric facade and roof test facility at IBP‘s Holzkirchen site allow to observe performance under realistic, transient climatic boundary conditions. The climatic lab tests, some of which require considerable effort, can be optimized by numerical simulation based on pre-selected appropriate product versions or relevant boundary conditions.
For special tasks involving photometric issues of the solar simulator test setups, Würth cooperates with specialists of IBP’s working group on Lighting technology and passive solar systems.
A comparison of the results obtained from lab tests, field tests and calculations provides valuable insights and creates synergy effects. At Fraunhofer IBP, these scientific disciplines thus optimally interact to complement and stimulate each other, offering clients individual treatment of their specific questions and developing approaches to further optimize their products.