Advanced Building Simulation

72 Degelman
The procedure to simulate average daily dew-point temperatures is identical to
the dry-bulb temperature simulation method presented in the previous portions of
this section. Standard deviations for dew-point are seldom in any publications,
so they can simply be set equal to the standard deviations for average daily temperatures.
The ultimate control on dew-point temperature has to also be programmed
into the software, that is, the dew-point can never exceed the dry-bulb temperature
in any one hour or in any one day. This final control usually results in a dew-point
simulation that obeys nature’s laws and the historical record.
3.5 Model for solar radiation
3.5.1 Introduction
In this section, we will illustrate a model that derives the solar variables. The most
significant variables are the sun’s position in the sky and the amount of solar radiation
impinging on numerous building surfaces, passing through windows, etc. Much of this
model can be directly computed by well-known equations. However, since the amount
of solar radiation penetrating the earth’s atmosphere is dependent on sky conditions,
a modeling tool has to be developed to statistically predict cloud cover or other turbidity
aspects of the atmosphere. In the latter regard, this model has some similarities
to the temperature sequence prediction model in that it follows a stochastic process
that is bounded by certain physical laws.
3.5.2 Earth–sun geometry
Predicting solar energy incident on any surface at any time is not difficult if the local
sky conditions are known. First, the sun’s position is determined by two angles: the
altitude angle, �, and the bearing angle, �z. These angles are shown in Figure 3.6.
� z = Zenith angle
� = Altitude angle
Ψ z = Azimuth angle
N
Figure 3.6 Sun position angles.
W
Ψ z
Normal line
� z
�
E
Sun path
Sun
Vertical
plane
S