What is an transient building simulation?
The transient dynamic building simulation is the balancing of all relevant energy flows of a building at selected times. Especially the transmission heat flows, the enthalpy flows, the change of the internal energy by storage processes and also electrical performances are balanced. The balancing is usually carried out over fixed time steps, often at intervals of 1 hour over a certain period, usually a whole year or even only over the period of the heating or cooling period.
For this purpose, the object to be calculated, such as a building, is divided into zones. For these zones, e.g. individual rooms, an energy balance, or mass balance, can then be drawn up. If zones, rooms or the building services are in interaction, an equation system is created in this way, which the simulation has to solve numerically. In contrast to the flow simulation, which uses spatial discretization, the energetic building simulation only requires precisely defined zones. For this reason, one likes to speak of the method of zone balancing.
The temporal resolution through defined time steps must be sufficient for the physical process. In building services engineering, the main influencing factor is the weather. In order to be able to take the weather into account as accurately as possible in a simulation, so-called annual test reference data (TRY - Test Reference Year) is usually used, which contain the most important averaged weather data, such as air temperature, wind, humidity and solar radiation from previous recordings of past years, and provide them in a temporal resolution of 1 h for different regions.
While the use of averaged weather data enables the most exact possible result with regard to expected energy flows and thus energy efficiency, extreme climatic years are often used to design the building services. In this way, the dimensioning of the building services can be carried out without the usual safety surcharges and thus without significant overdimensioning.
The accuracy of standard demand forecasts, energy certificates based on simplified regulations, is limited. This becomes particularly clear in the following example, which shows the heat transfer coefficient for a selected double glazing insulation window and thus the transmission losses as a function of wind conditions. It becomes clear that, depending on the wind speed, very different heat flows pass through the window and thus contribute to significantly different heat and cold loads. This is particularly important for low opaque buildings.