Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 245

3.5

3.0

2.5

Measured I-V curve 1 at 887 W

2.0

Measured I-V curve 2 at 896 W

Measured I-V curve 3 at 897 W

1.5

Two-diode fit to all measured curves

1.0

Two-diode curve at STC

0.5

One-diode curve calculated from
measured I-V curve transferred to STC

0.0

0 5 10 15 20 25 30 35 40 45

Voltage / V

Figure 6.43 Measured I - V curves, two-diode model fitted to the I - V curves, transformation of two-
diode curve to STC, and calculation of one-diode parameters from STC conditions

The whole system was then simulated online using the irradiance on the tilted
plane and module temperature only as an input. As the process of data transfer and
simulation takes only milliseconds, simulations were first carried out on a second
basis. However, the deviation between measurement and simulation was rather high,
as the measured irradiance information from the pyranometer has a slow time constant
of about 20 seconds. For fast-moving clouds, a sharp drop in irradiance leads to only a
slow reduction of the measured pyranometer value, whereas the PV plant immediately
reduces power. The simulation value is based on the pyranometer measurement and
thus overestimates the simulated power. Conversely, in the case of a rapid rise in
irradiance, the pyranometer measures too lowa value and the PVmodel underestimates
the simulated power, so that the error can be 10% or higher (see Figure 6.44).
If themonitored data is averaged over an interval of 1minute , the difference between
simulation and measurement is nearly zero, because the pyranometer time constant is
no longer significant (see Figure 6.45).
The simulation data fits the measured power data excellently even on a day with
wildly fluctuating meteorological conditions. The difference in energy output over the
measurement period of 3 hours is 0.5%. The mean arithmetic error between measure-
ment and simulation is 2%.

6.3.4 Summary of Renewable Energy Plant Online Simulation

In this work, a methodology was proposed and demonstrated to simulate grid-
connected photovoltaic power plants not only in the planning stage, but also online