If children are sitting in front of a bunch of lego bricks planning to build a house, this can be a very creative process. Each kid will have an idea they want to realize and, probably after some short discussion, the action will start. Once the little architects start quarrelling, parts of the building can be rapidly torn down to be re-built immediately, with more components being added. In the worst case, the structure will be demolished and the kids will concentrate on another project. In the real world, working on this "Trial and Error" principle would be quite expensive, and most probably end up in chaos and strife.
The construction of a building involves many stakeholders: the builder, various planners and architects, subcontractors and very different trades need to find common solutions to cope with the ever-growing requirements in the building sector. Nowadays the focus is on energy issues relating to buildings and districts, the development of efficient energy supply concepts, and on measures to minimize the energy demand by including the use of renewable sources of energy. This process goes hand in hand with identifying criteria for an indoor climate that takes account of the users‘ needs and the specific usage of the indoor spaces. Computer-aided simulation tools are used to optimize and adapt the complex technical relations within a building (such as thermodynamic systems and technical plants) as early as in the planning stage. For many years, scientists of the Thermal Comfort, Models and Simulation and Design Tools working groups in the Department Energy Efficiency and Indoor Climate at Fraunhofer IBP have been concerned with developing and maintaining a comprehensive set of different computer-aided planning tools. Regarding the assessment of buildings and districts in terms of energy and lighting performance, an increasing demand for calculation tools has emerged both nationally and internationally, due to the increased requirements on energy performance and to recently introduced assessment methods. Many new approaches, processes, and tools for efficient and effective planning and building are currently discussed under the heading of Building Information Modeling (BIM). "There is no such thing as the ONE tool that collects all relevant information and can be used by all persons involved. Requirements are simply too diverse for such a tool", explain Sebastian Stratbücker and Simon Wössner, the two working groups managers. "This is why we concentrate on rendering individual software modules compatible and on interconnecting them through interfaces."
Speaking a common language
An engineer involved in a construction project deals with technical parameters and calculation rules, an architect handles scales, dimensions and construction plans, a planner of technical building systems is concerned with dimensioning and arranging the building services systems. The building industry faces increasingly complex tasks and rising demands. Builders should be able to make critical investment decisions already in an early phase of the construction process, as these decisions not only affect the construction costs but - to a much greater extent - also influence the operation of buildings. In this context, planning security requires that different building services systems, such as heating, ventilation, cooling, lighting or solar shading systems are adapted to one another to become functionally integrated into the overall system called "Building". Last but not least, all of these systems should be validated in their entireness. To implement this requirement, instruments are needed, which enable all stakeholders to acquire knowledge and understanding for the project and which allow interrelating the effects and requirements of the individual sectors. "This is why a ›common language‹ is needed", says Stratbücker. As a promising trend, developments were identified that rely on the equation-based and object-oriented modelling language Modelica. To begin with, a physical model is formulated, which is subsequently transferred into a system of equations by means of an automated translation algorithm. Finally, the model is efficiently calculated using different numerical solution methods. A particular advantage of the Modelica technology is the option of interdisciplinary collaboration for scientists and engineers, who can use validated model libraries to represent systems with the necessary degree of complexity – without compromising in terms of transparency and plausibility.
Modular simulation promotes interdisciplinary cooperation
"The modular structure of the simulations enables us to accurately represent the complexity of present and future building systems and by this to effectively support planners", Stratbücker explains. With this aim, the scientists of Fraunhofer IBP concentrate on developing and implementing new calculation algorithms, which describe complex physical relationships. On the other hand, user-friendly information tools and advisory tools are created, so that engineers, planners, and consultants can access the latest findings in building research. These tools are applied in everyday planning practice and in evaluating special issues relating to indoor environments, energy and lighting technology. "In this context, a particular focus is on the solutions for calculating the energy needs, delivered energy and primary energy for heating, cooling, ventilation, domestic hot water and lighting in buildings (DIN V 18599) which are developed by my colleague Simon Wössner. In thousands of cases, software manufacturers have integrated these solutions into their software products for energy consultants", the IBP scientist adds.
On the basis of so-called model libraries, Fraunhofer IBP currently does research in the scope of a project funded by the German Federal Ministry for Economic Affairs and Energy, which aims at establishing future building simulations following an object-oriented approach. In this way it will be possible to provide new application programs for special client requirements using pre-defined interfaces according to a modular principle. With the aid of the Functional Mockup Interface (FMI) it is now possible to get several planners and tools directly involved in creating a simulation model. "This actually means that many planners with their own special know-how join together to work on a solution, which will integrate all specific models into one", Sebastian Stratbücker explains the principle. Interdependencies or effects that modifications within one trade will have on other trades can be immediately identified and accounted for. Another major benefit of the modular simulation technology in combination with the FMI standard is the option of examining technical innovations in detailed simulations - evaluating not only their energy demand, but also their impacts on the indoor climate.
Modular simulation tools
For different expert areas like indoor air quality, thermal comfort, energy efficiency, or lighting technology, expert planners in future will rely on their specific models.
The building’s performance is planned and simulated on the basis of user models. As societal structures and hence the daily routines of large sections of society have changed during the last decades, Fraunhofer IBP scientists are continuously doing research on static and dynamic models. For instance, they use static models from building measurement projects as a whole system. In the case of plus-energy buildings, for instance, the average difference between static calculation and measurement was found to be only marginal, even in such high-performance buildings. To represent not only the behaviour of the average type of user, but also that of several different groups, the tools are based on empirical models using different data. Among other parameters, the focus is on users‘ heating and ventilating patterns and on the question which conditions (like solar radiation or outdoor temperatures) could influence the building system. "Of course, we always concentrate on the users and their needs, because automated systems such as solar shading or lighting are designed to optimally reconcile a comfortable indoor climate with energy performance aspects, not to have a negative impact on the user. For instance, if the solar shading system continues to go up and down on a partly cloudy day, this can be quite a nuisance for the occupants", Stratbücker says. "In the final analysis, the accuracy of both static and dynamic models increases along with a continuously improved knowledge of the boundary conditions."
At present, however, he and his team are focussing their work on zonal models, which are already available in Modelica as a so-called VEPZO library. Here, the space is divided into several zones, which allows making detailed predictions concerning indoor airflows, wall temperatures, and temperature stratification, the impact of heat sources and air outlets, or surface condensation on cold surfaces. The researchers examine the effects of non-steady state building operation with regard to systems and user acceptance, thus extending the options such as the choice of appropriate spatial and temporal model scales or a refinement, for instance in the case of high local air velocities. Zonal flow simulations prove to usefully complement hygrothermal or energy-related simulations during the building design stage; by integrating these, it is also possible to determine the distribution of temperature and humidity in an air volume, in addition to the usual processes of energy and moisture transfer through structural components. For some buildings, it is rather difficult to predict the prevailing indoor climate, or the situation cannot be represented by simplified calculation procedures; in these cases, zonal models provide the appropriate tool for ensuring high-quality planning. Examples of such buildings are open-plan offices, factory buildings, or atria. Due to the height of their spaces, the latter can be characterized by considerable temperature differences. Previously, CFD simulations were used in these cases. However, the effort required for conducting such a precise simulation is not to be equated with the benefit obtained. "It is true that the zonal approach yields a less detailed spatial resolution than CFD simulation, but they take only a fraction of the time. Besides, many building issues do not require such a high resolution as achieved by CFD, and a large part of the boundary conditions is only vaguely known, anyway", Stratbücker clarifies. Further benefits of the zonal VEPZO models are that the type and position of emission/ control systems for heating and venting and the supply and exhaust air openings can be explicitly examined and evaluated with regard to the resultant indoor climate. This means that ideal airflow patterns can be planned for complex spatial situations; besides, the effect of local influences (like solar radiation) on the indoor climate can be represented. It is also possible to evaluate thermal comfort and the efficiency of ventilation in advance. In contrast to current methods, the components are here no longer understood as a mere source of internal heating or cooling loads. Rather, they now yield detailed information on their direct physical effect on the locally different indoor climate, using parameters such as the surface temperatures of radiators or the recirculated air volume of convectors. Conventional simulation environments cannot provide this information due to their insufficient level of detail. "We have found that these tools yield an appropriate spatial resolution - particularly in the field of buildings - while requiring only a limited effort for creating and analysing the respective spatial situation", the IBP researcher sums up. "Moreover, we are able to reliably predict the indoor air quality by determining the indoor distribution of pollutants like CO2 (on the basis of air flow calculations), depending on the position."
Since 2013, the working group 'Thermal Comfort, Models and Simluation' has been participating in the European research project ECO-SEE, "Eco-innovative, Safe and Energy Efficient wall panels and materials for a healthier indoor environment". The project aims at creating a healthier indoor climate by using innovative and sustainable building materials, while saving 20 to 30 percent of the costs by making use of multifunctional products and their intelligent application in new constructions and in building retrofitting. As a central outcome of the project, an innovative planning and assessment tool will be provided, which will give a quantitative representation of the major benefits for the indoor climate to lay the base for the commercial exploitation of the ECO-SEE-products. In this context, various physical factors shall be transferred into a uniform evaluation scheme, so as to enable users to compare different products and solutions. In addition to recording the hygrothermal properties of wall panels, their acoustical performance characteristics will also be documented and represented in a model. Another essential impact factor is the capability of the materials to actively improve the air quality by absorbing pollutants from the indoor air. This part of the project is led by Fraunhofer IBP, that coordinates an international research team in order to develop a new, model-based planning tool. So far, the partial models for indoor air quality, hygrothermal, and acoustic properties were created and validated on the basis of extensive measurements of the novel ECO-SEE-materials. In the further course of the project, the Fraunhofer IBP team will devise and implement the prototype of a software that integrates all sub-models. On the basis of a building information model (BIM) it shall be demonstrated how a holistic assessment of the ECO-SEE solutions can be conducted in future and how the results can be communicated to the planners.
Building system models are specifically designed to examine the indoor climate. Detailed representations of indoor climate sensors, i.e. measuring sensors, are stored in a model library, which shows zonal impact factors and climatic effects. In future, these models will also reflect any kind of radiation exchange with the environment, whether due to metabolic heat or technical heat sources. One type of sensor reads the equivalent temperature, recording the dry heat emitted from the human body, which is simulated by means of a heated surface. The equivalent temperature is a combined climate index defined in DIN EN ISO 14505-2. It summarizes the air temperature, air velocity and heat radiation. In this way, conditions of the thermal environment can be described by a single figure, which allows the comparative assessment of different climate scenarios. In this context, the researchers employ a measuring system called DressMAN 2.0 , which was developed at Fraunhofer IBP. DressMAN 3.2 uses sensors that are attached to a thermal manikin. These sensors can be distributed across the entire body of the dummy in any desired position and orientation. The sensor models allow the direct comparison between measured and simulated indoor climate parameters. Yet there is another research facility at Fraunhofer IBP that supports the scientists in generating and analysing important data: the so-called IATC (Indoor Air Test Centre). Here, individual emission/ control systems for heating, ventilation, and air conditioning (HVAC) such as convectors, radiators, or ventilation components like swirl diffusers are put into operation and their output characteristics are transferred into a parametric model. "The model library that is going to be generated in this way will include both physical models and the geometrical data of the system, which are required to be known for determining the effective radiant surfaces and for zonal coupling with the surrounding space", Sebastian Stratbücker declares.
Well-equipped, valid libraries and models are fundamental for virtual construction. They enable researchers to digitally examine and optimize the physical and functional characteristics of a structure – before it is actually built. A so-called virtual twin of the building is created, which will be continuously developed throughout its entire life cycle, because it is only during building operation or renovation that the real added value of the digital building information model (BIM) becomes apparent. Today, information is often lacking or has to be provided with great effort - tomorrow it will be easy to refer to databases that contain updated and consistent building data. Stratbücker is convinced: "The R&D that we are conducting today will form the basis for a paradigm shift in practical planning in the near future. Then, virtual construction will mean that all technical and physical performance characteristics of a building can be completely verified by means of the model."