Public Release: 

In nearly zero energy buildings special attention must be paid to window design

Estonian Research Council

For the last two years TTÜ Nearly Zero Energy Buildings Research Group led by Professor Hendrik Voll (currently holding the position of TTÜ Vice-Rector for Academic Affairs), with support from personal research funding (PUT, i.e. funding for high level research projects), has been seeking solutions for daylight and solar shading for nearly zero energy buildings in cold climates.

In accordance with the EU Energy Efficiency Directive (2010/31/EU/), all new buildings completed after 31 December 2020 must meet the requirements of nearly zero-energy buildings (nZEB). Each EU Member State has the right to determine the precise criteria and principles for designing nZEBs. "The objective of our research project was to provide not only indicative, but fundamental and specific numerical principles for designing nZEBs, their facades, fenestration and solar shading, as well as for preventing the buildings from overheating," Professor Voll says.

The research project "Daylight and solar shading principles for nearly zero energy buildings in cold climates" was divided into three subtopics: the impact of insolation (direct solar access to premises ) on the building's energy consumption and overheating was studied; the impact of static versus dynamic solar shading was analysed and the effect of simplified and detailed window models on the building's energy consumption was investigated.

The studies of the insolation, i.e. direct solar access to premises, indicate that the current requirements set for the minimum insolation hours (at least 2.5 hours of uninterrupted sunshine to the premises) are not justified in designing modern nZEBs. The research demonstrated that following these requirements increases the risk of overheating. The requirements should be based on daylight factor values (insolation characterises direct solar radiation, whereas a daylight factor characterises diffuse radiation, i.e. cloudy weather).

When analysing the different impact of static and dynamic solar shading on nZEBs, the research group developed a tool that the architects and engineers can easily use in software programs in order to facilitate reduction of heat losses already in the designing phase. Dynamic solar shadings (which, unlike static shadings, can be operated automatically as necessary) proved to be more efficient. Hendrik Voll says, "We developed a calculation method for determining the portions of the façades of buildings that receive the required direct solar access in an urban environment . This allows you to determine which portion of the facade needs shading and which does not. In addition, this method can be used to determine whether to select vertical or horizontal shading.

As regards the third subtopic - window models in nZEBs - detailed window models should be preferred to simplified models. "Our study clearly illustrates the importance of different variables, such as accuracy of window models, construction costs, energy prices, interest rate and inflation. The lowest energy use was achieved with large quadruple windows and automated venetian blinds with an advanced control algorithm," says Voll.

The research project based scientists' recommendations to prefer dynamic solar shadings to static ones, detailed window models to standard windows and to design buildings based on the daylight factor, not the solar radiation criteria are taken into account in the EU construction requirements for nZEBs, which will come into force at the end of 2020.

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