The program is capable of modelling a number of different system controllers. Most of the control strategies clamp the voltage of the array to the battery voltage. This can be done without regulation, i. e. a system where the array and batteries are simply wired together. It is more common, however, to provide some means of regulating the current provided to the battery. This can be done with either a shunt regulator or a series regulator. Explanations of these two types of regulation may be found in the WATSUN-PV manual.
There are also two different approaches to preventing battery over-charging, trickle charging or voltage limiting. Trickle charging reduces the current to a fixed small value as soon as the battery voltage indicates that it is fully charged. Voltage limiting solves for the array current that will produce no more than the maximum allowable battery voltage once the battery is fully charged. This simulates the decaying off/on cycling of a real controller. In this study, the voltage limiting system was used.
WATSUN-PV also permits the user to simulate maximum power point tracking. With this approach the array and battery voltages are decoupled, so that the array can operate at its maximum power point. The resulting power is adjusted to the battery voltage by the tracker. Once again, both trickle charging and voltage limiting are available to prevent battery over-charging.
System Integration
The WATSUN-PV program uses the array, battery, converter, and invertor I-V models, along with the specified load and regulator system, to determine the operating point for the system for each hour of the simulation period. The operating voltage on the DC side and all the DC currents are then known, so the energy flows to and from the DC components can be calculated for the hour. The current into the battery is used to determine the SOC for the battery at the end of the hour.
This state of charge is then saved in order to determine the operating behaviour of the battery during the next hour.
Once the DC side of the system has been solved for the hour, the status of the converter and hence the generator will be known. Currents and energy flows on the AC side of the system can then he calculated.
