# When is thermal building simulation useful?

Static calculations are often sufficient for the energy design of buildings. But in some cases, dynamic thermal building simulations are of advantage.

Lina Rosenthal

|

September 30, 2021

WÖHR + BAUER

## What is a thermal building simulation?

In building simulation, a digital model of the building is created in a simulation program. Here, the physical interactions and all important influences on the thermal, energetic and plant behavior of the building should be mapped as realistically as possible. The thermal building simulation focuses on the thermal behavior of the building. It is mainly about the temperature curves and the energy consumption for heating and cooling. The plant systems are usually not modeled in detail. Rather, it is determined which services must be provided by the plant technology (e.g. heating and cooling load) in order to maintain certain temperatures in the building.
Which aspects are particularly important and what level of detail is required depends on the building and the questions that are to be answered with the help of the building simulation.

## Differences between thermal building simulation and static calculation methods

In principle, thermal building simulation involves dynamic calculations, i.e. the temporal progression of various variables is considered. This is why it is called dynamic simulation. In building simulation, for example, reference weather data is read into the simulation program for every hour of the year and temperatures in the interior are determined by means of balance equations. The state of the building is recalculated again and again by solving the balance equations and the result of the previous calculation step is the initial state for the next calculation step. How many calculation steps take place can be set depending on the program. For example, it is also taken into account that a solid wall with high heat capacity stores thermal energy. This leads to the fact that the temperature in the wall and thus also the interior temperature in the thermal exchange is more stable and fluctuates less strongly with the outside temperature. The result of a dynamic thermal building simulation is the temporal course of a variable such as temperature and power.

Due to the static calculation methods according to german norms, for example, reference values for different building types and weather conditions are used in tabulated form to issue the energy certificate (according to DIN 18599). The formulas for determining the energy demand are also based on balance equations, but these are not solved anew for each time step. Instead, correction factors are used, such as effective heat capacity, to account for storage effects. Due to the lack of temporal resolution in the static calculation, these effects would otherwise be completely lost.
It is also not possible to deal with each building individually; instead, categorization is used. For example, in the energy certificate that must be submitted with the building application, a distinction is made between residential and non-residential buildings, and there are numerous reference values that address different types of use, site conditions (a lot of/low wind), and building types (e.g., high/low mass) that are included in the calculation, but of course, not all variants can be covered. The reference values are average values that have proven to be a good approximation for many buildings in the past. For example, the annual energy demand, as well as heating and cooling load, can be estimated based on the building data and the calculation method according to the standard. Dynamic interactions are not directly included in the calculation. As a result of the static calculation, characteristic values are obtained, but not temporally resolved results such as the temperature curve over a year.

The static calculation method differs considerably from the dynamic simulation and can sometimes lead to different results.

## Three questions that can be answered with the help of thermal building simulation:

1. How can the building envelope be optimized to achieve the highest possible thermal comfort while maintaining high energy efficiency?
Window sizes/ orientation, shading and insulation are particularly in focus here.
2. What is the heating/cooling load and heating/cooling energy demand, and consequently how large do I need to design my heating and cooling system?
Heating and cooling load is the peak power that must be applied to maintain specified temperatures. Heating and cooling demand is the energy that heating and cooling require in a year.
3. Is summer thermal protection met or how can it be met?
Summer thermal insulation is prescribed in the german Building Energy Saving Ordinance (Gebäudeenergiesparverordnung GEG) and DIN 4108-2 and is intended to prevent overheating in summer with the help of structural measures.

## When is thermal building simulation particularly useful?

A thermal building simulation can be useful especially for buildings that do not comply with the standard. If it is a largely conventional building, the legal requirements ensure a minimum standard for energy efficiency and compliance with summer thermal protection.

A listing of when thermal building simulation is particularly worthwhile:

• For all individual buildings that deviate from the standard, e.g. large glass surfaces, intelligent building automation, high thermal mass of the building
• Comparison of different scenarios e.g. different heating and cooling systems
• Simulation of extreme situations, e.g. extreme weather (future weather)
• Finding limits of usability/plant technology, e.g. too many people in the conference room in midsummer
• Summer thermal protection can be demonstrated more easily, if necessary, since dynamic temperature-reducing factors can be included in the calculation (e.g. intelligent control of window shading, night ventilation)
• Storage effects should be taken into account, e.g. when designing plant technology in conjunction with an ice storage system
• Especially for large innovative projects, a simulation can help to uncover planning errors

The building simulation usually offers more realistic predictions and thus more planning reliability. In this way, the building can be better optimized in advance.
However, this usually involves higher costs and more time than simply performing static calculations. The increased costs and time spent in the planning phase can pay off many times over in later building operations.
For the certification of buildings concerning sustainability and energy efficiency, many criteria must be fulfilled. The evidence for this can be provided using simulation. In many cases, static calculation methods are not sufficient. Well-known certification systems are DGNB and LEED.
A more precise design of the plant technology with the help of simulation can save costs since such a large safety factor does not have to be added. Here, there is a need for reliable models for the plant technology and/or measurement data so that the behavior can be mapped correctly.
The simulation model can be used to compare different control strategies and thus select the most efficient one. However, the feasibility of control strategies must be coordinated with different trades. The simulation model can later be used for monitoring to find errors and optimize building operation. For this, however, interfaces (e.g. to the building control system) must be set up for the transfer of all necessary measurement data.

The disadvantages of building simulation could also be understood as challenges. If simulation becomes common practice, the effort for its implementation will also decrease in the future. A functioning exchange of information between the trades is particularly important here, so that simulation and building practice are intertwined.

## Six goals of thermal building simulation:

1. Optimization of building envelope and design
2. Improvement of comfort
3. Increase of energy efficiency
4. Ensure functionality under different usage/environmental conditions
5. Gain planning reliability and avoid planning errors
6. Saving costs through more efficient system technology and regulation

## What kind of building simulation do I need?

This depends on the questions you would like to answer with the simulation. If you only need the proof of the summer thermal insulation by means of simulation, then ask an energy consultant, here there are meanwhile extensions for existing "energy consultant software".
If you are interested in dynamically designing and optimizing your building, the question is what you want to optimize. It may make sense to consider the building only thermally or also in terms of plant technology (plant simulation). If you want to optimize the building envelope (e.g. wall structures, windows, roof), a thermal simulation should be sufficient. The quality of the building envelope can be easily assessed after construction with the help of a thermal imaging camera (see cover picture Tim Reckmann | ccnull.de).
If you are interested in the interaction of building and plant technology with a special focus on efficient design and control, a coupled plant and building simulation makes sense. This makes sense especially in the industrial sector, for example if cooling is needed in one area and heat in the other. There is often a great potential for energy savings through intelligent planning.
The ideal is a functioning import of all data from the BIM model of your building into the simulation software. This is only possible when a finished CAD model and all necessary data are available. This procedure is therefore not suitable for carrying out simulations as early as the pre-planning stage.

## Simulate only what you want to know: Building Simulation with Modelica

It is not always necessary to simulate the whole building including the system technology, it depends much more on what you want to know.
If, for example, you want to compare different cooling systems, then in many cases it is sufficient to simulate only one room. If you model the room that overheats particularly quickly in the building and specify realistic boundary conditions, the different cooling systems can be evaluated very well in terms of energy efficiency and comfort.
In this case, it is not necessary to model the entire building with all its details, which of course saves time and money.
In our daily work, we use Modelica to model a wide variety of systems in the fields of thermodynamics, process and energy engineering. The Modelica language allows great flexibility. It allows us to adapt existing models at will or to create completely new ones. This makes it possible to simulate new innovative ideas. If measurement data are available, parameters can be adjusted and models validated.

With the specialists for sustainable and digital buildings from SBC Frankfurt, we have founded the joint venture SBC.sim. Together, we use building simulation as an important tool for innovative, resource-saving buildings, while also keeping an eye on the ESG sustainability criteria.

We are eager to hear your innovative ideas and would be happy to consider together whether a building simulation could be useful in your project. Just get in touch with us.

M.Sc.

Lina Rosenthal

PR & Office Management

TLK Energy

Lina Rosenthal completed her bachelor's degree in physics at RWTH Aachen University. She then studied building energy systems at TU Berlin and wrote her master's thesis in 2018 in cooperation with TLK Energy. Since then, she has been part of the team. As an all-rounder, she advises on simulation software, takes care of marketing activities and organizes the Modelica and TIL training.