When using the simplest 1-Standard ground modelling method, heat is transferred between zones and the ground through one-dimensional ground floor surface elements. The 2-D and 3-D effects in the ground heat conduction that occur in reality are not treated by default. EnergyPlus uses ground construction data to model the storage of heat in the ground and it can accurately model heat transfer processes over several days (so it can model the effect of say a warm week followed by a cool week) but it cannot directly model long-term ground thermal storage effects (periods measured in months).
Tip: Other more advanced methods are also provided for more accurate 2D/3D ground modelling.
EnergyPlus applies a constant temperature boundary condition for each month on the lower surfaces of ground adjacent elements.
There are 3 ways to create an adjacency to ground in DesignBuilder:
Monthly ground temperatures are defined at the site level under Site details.
Modellers generally prefer to define the ground temperature just below the slab and exclude the earth layers from the model. This has the advantage of simplicity and clarity and is the approach recommended by EnergyPlus developers.
A contributing factor is that EnergyPlus cannot model very thick constructions so it is necessary to use less thick constructions (2m or smaller) combined with some assumptions about temperatures at about half a metre or less below the floor.
Note: You should not use the ground temperatures in the EnergyPlus weather file header because these are for undisturbed sites. The temperature beneath buildings is significantly affected by the building itself. The EnergyPlus documentation recommends using ground temperatures of 2°C below average internal temperatures for large commercial buildings (where perimeter heat loss is relatively less important). Note that this temperature should be applied directly below the slab and should not include ground material.
This example illustrates how to create a simple 3 storey rectangular building model with:
Draw 3 blocks on the z=0 ground plane. The lowest block will be the basement (completely buried), the middle block will be the ground level block, partially buried and the top block will be the second level. So far only the bottom surface is adjacent to ground because it sits on the z=0 plane.
Draw 2 ground component blocks around the basement building block to give it's surfaces ground adjacency.
Note this step could be avoided by moving the whole building down so that the basement is fully below ground. To do this go to building level, select all the blocks, select the move command, click on one of the bottom corners of the middle block to create a reference point then click on a blank part of the screen to move the reference point down to z=0.
Draw three new ground component blocks to create the sloping ground and the upper ground levels.
The zone of the middle block is shown above. Notice how 3 of the surfaces have been split into external and ground adjacencies (ground adjacencies are flagged by green ground symbols). Note also that the portions of the surfaces touching the ground component block cannot have windows but DesignBuilder attempts to set up the correct % glazing for the remaining external parts of the surface.
The ground component blocks probably do not need to shade this model as they are mainly below all surfaces that can receive solar radiation/light, but if you are modelling reflections you will probably need to include the reflection of solar radiation and light from the ground blocks during simulations. In other cases you may prefer to switch off the generation of the component blocks shading/reflection surfaces to speed simulations. Do this on the Constructions model data tab under Component Block.
The finished model. The texture of Ground component blocks is defined by the material set under Component blocks on the Constructions tab.