In winter, we sleep under thick blankets, but in summer we only need a thin sheet. However, the building envelope provides the same level of insulation throughout the year. Bart de Boer from Efficiency & Infrastructure is exploring the possibilities of developing a dynamic building envelope: “It is possible to reduce the demand for energy for heating and cooling by a factor of 10 whilst maintaining the same level of comfort.”

An energy technologist who looks at a structure sees an interior and exterior climate separated by a building envelope. Whereas the exterior climate varies in time (day/night, summer/winter), the interior climate must be fairly constant and comfortable. The building envelope must ensure this with the help of boilers, artificial light, ventilators and air conditioning. This is how it works in practice, but the price is a high energy bill and that is painful. ECN employee Bart de Boer from Efficiency & Infrastructure thought something could be done to improve this. And with like-minded others, he switched the static envelope during his studies for a so-called dynamic building envelope.
In modern construction, the building envelope (collective term for floor, roof and facade) is static from a physical perspective. Its insulation value and window surface area, for example, are fixed, and identical in summer and winter while conditions outside differ significantly. De Boer and co were wondering whether it would be possible to have a building envelope with variable properties. An envelope that is closed during the winter nights providing effective insulation but allows sufficient light to penetrate during the day and at the same time an envelope that blocks solar heat in summer whilst enabling the home to cool down easily during the night. De Boer: “In various projects and studies together with TNO (the Netherlands’ Organisation for Applied Scientific Research), we have been already been working on this for some time with Climate Adaptive Building Shells (CABS), but on a fairly small scale. In order to work thoroughly on a coherent building concept, we submitted the research project FACET together with TNO, TU/e, TU Delft and CHRI. As of October 2009, FACET (Facade als Adaptief Comfortverhogend en Energiebesparend Toekomstconcept/Facade as Adaptive Comfort-enhancing and Energy-saving Future concept) has been approved as a three-year EOS-LT project.”

Inverse simulation approach
Many research teams around the world are focusing on the idea of a dynamic building envelope. There is even an international working group called Climate Responsive Building Elements, within which CHRI is represented. But the FACET researchers use a unique starting point in which the envelope is not fixed but variable. They reason from inside to outside, while those who follow the classical approach use a static envelope as their point of departure and reason from outside to inside. De Boer: “During building simulations, you usually use data from the outside climate and examine whether you can use a particular building envelope to create an interior climate with an as low as possible demand for heating, cooling, ventilation, lighting. We reverse this by asking “Which envelope properties are ideally required to remain within specific limits of interior comfort without auxiliary energy?”

The adjustable FACET-envelope optimally help the desired interior climate and the offered outside climate to adapt (illustration: ECN).

The interior climate must stay between 19 and 21 °C the whole year round, for example, and provide sufficient daylight, views and good air quality. The question is therefore what properties the building envelope will need in order to achieve this. This is a revolutionary way of thinking and simulating, which means that the computer model you use to calculate concepts must be organised differently.”
FACET researchers assume the building envelope is a ‘black box’ that is practically entirely variable in the extent to which it excludes, mitigates, filters, stores or lets through light, air, solar heat, solar radiation, etc. The FACET building envelope is assigned a series of tasks that it performs like a kind of chameleon, according to what is required to harmonise the interior climate demand and outside climate on every time scale. De Boer: “A few of these tasks are related. If you block solar heat, you also mitigate some of the sunlight. The manner in which you incorporate this relationship in the model also has various approaches in turn. In this respect, fellow researchers from TU Eindhoven are working hard on a Multi-Objective Optimisation method in which the inverse approach is accessed as best as possible by simulating an almost infinite number of variants and then determining optimal combinations of building envelope parameters.”

User aspects
Technical aspects play a key role within FACET. But no matter how structurally sound the future building envelope concept is, its success depends largely on what users think of it if it is incorporated in their home, school or office. This part is the responsibility of researchers at TU Delft. Thanks to psychological research into thermal comfort, they know, for example, that people like it if the temperature varies somewhat over the course of the day. It is even said that this is conducive to one’s health. De Boer: "This is good to know since such a variation can be included as a precondition in the model. And if the requirements change, for example, if a building once used as an office is assigned a second purpose as an apartment building for the elderly, such a dynamic building envelope is sufficiently flexible to switch from office comfort to a comfort plan for an old people’s home.”

Technology development
Researchers search for concepts with FACET. It is up to industry, i.e. the market, to elaborate the concepts so that they can be implemented in reality. Contours can be used, though, to express what is feasible: “For the realisation of the concepts, you can consider ‘low-tech’ solutions such as insulating shutters for windows, but also technologies yet to be developed such as controllable vacuum insulation and switchable glass.” De Boer has compared the results with a reference building that complies with the current standard for non-residential buildings with an EPC (Energy Performance Coefficient) of 1.1. “Good insulation, effective sun blinds and additional ventilation help enormously, but a dynamic building envelope makes it possible to reduce the demand for heat to virtually zero, so no heating supply is required in winter. And in summer we can reduce the demand for cooling by 50%. Annually, this means that we achieve a reduction in the demand for cooling and heating of around 90% (excluding any additional energy required for ventilation) compared to a standard building, whilst maintaining the same level of comfort for users.” Heating and cooling are the main energy guzzlers, incidentally. However, power for lighting can also be saved by using natural lighting efficiently, combined with energy-efficient lighting with daylight control – something that TNO is studying in this project. Another 50-60% reduction in lighting costs can be achieved this way, according to De Boer.

Feedback board
Although FACET researchers enjoy exploring the boundaries of building technology for the long term, they understand that their research must not lose sight of what is required in practice. In order to gain market feedback, a special consultative group has been established as a feedback board, including representatives from all sectors of construction.

Contact
Bart de Boer
Efficiency & Infrastructure / Energy Technology in Built Environment
Tel.: +31 (0)22 456 4901
E-mail: Bart de Boer  

Info
For more information about the FACET project, visit www.eosfacet.nl or e-mail FACET for further details.

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