Advanced Building Simulation

ackward. First, the average daily horizontal insolation (H) is read from the weather
station’s monthly records. Second, the extraterrestrial horizontal insolation (H o) is
computed outside the atmosphere. The equation for this value is shown here:
H o�(24/�)* I SC* [cos(L)* cos(�)* sin(SRA)�(��SRA)* sin(L)* sin(�)] (3.15)
where H o is the extraterrestrial horizontal daily insolation; I SC, the solar constant;
SRA, the sunrise angle [��* (sunrise time)/12] measured as compass bearing.
The next step is to derive the monthly K – T value by use of the formula:
K T �
H
H o
Ratio K – H Daily total horizontal insolation at the site
T = =
Ho Daily total extraterrestrial horizontal insolation
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Simulation and uncertainty: weather predictions 75
K –
T = 0.7
0
0 0.2 0.4 0.6 0.8 1.0
Fractional of time that total daily radiation < H
(3.16)
The K – T value derived from Equation (3.16) is then used to determine which monthly
K – T-curve to select from the Liu–Jordan graph in Figure 3.8. The K – T-curve defines the
distribution of daily K – T values for all 31 days of a month. The 31 days are evenly distributed
along the horizontal axis (between 0 and 1), and for each day a unique K – T
value is selected. Of course, these days are never entered in a consecutive order; the
0.6
0.5
0.4
Figure 3.8 The generalized K – T curves (from Liu and Jordan 1960).
0.3