Process Optimization − Digital Innovation Factory

New Procedure for Outdoor Characterization of Façade Collectors and BIPV

© Fraunhofer ISE
Outdoor test Facility for Real-size building Envelope Elements (OFREE).

Building-integrated solar systems (BISS) use the sun to meet some or all of a building’s energy demand, minimizing the consumption of non-regenerative energy. For non-residential buildings with large façade areas, façade elements are pre-fabricated which can be e. g. 3.5 m high and 1.5 m wide. These elements are then successively mounted at the building site. Our new Outdoor test Facility for Real-size building Envelope Elements (OFREE) allows this type of façade element to be measured with regard both to the inward energy flux through the façade and to photovoltaic and solar thermal yields.

The new OFREE outdoor test stand can be used to measure façade elements with a height of up to 3.77 m and a width of 4 m, allowing original façade elements to be used. A further advantage of OFREE is that the test sample is exposed to natural sunlight. In the laboratory, quasi-homogeneous and quasi-parallel irradiation of a larger area is practically impossible. However, this is particularly important for angle-selective façade elements. Thus, properties can be measured for different incidence angles very accurately, without interference from divergent beams.

The new test stand is based on a solar tracker. It has been designed to be mechanically strong enough to support façade elements with a mass of up to 1.5 t. The tracker can be positioned at different angles with respect to the solar radiation, so that many different situations that can occur in reality can be reproduced. In addition to façade elements, roofing elements can of course be measured. A calorimeter is mounted on the tracker. This replaces the interior room and can be set precisely to a temperature between 15 °C and 70 °C. The calorimeter has a black surface to absorb the solar radiation transmitted by the façade element as completely as possible. In addition, heat-flux plates are integrated into the calorimeter to measure the energy flux from the façade to the calorimeter. This allows the energy fluxes on complex, multifunctional façades to be balanced under realistic conditions. A photo of the test stand can be seen in Fig. 1.

The OFREE test stand can be used to characterize buildingintegrated photovoltaic (BIPV), solar thermal (BIST) and photovoltaic thermal (BIPVT) modules, as well as passive façade elements, transparent or opaque. In addition to quasistationary determination of the g value (total solar energy transmittance; solar heat gain coefficient – SHGC), dynamic measurements can be used to validate detailed physical models, with which the façade performance can be simulated for other locations, installation conditions and applications.