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Simulation process: stand alone system

The simulation process simultaneously manages the PV Array production, the Battery, a possible Back-up production, and the user consumption. At the meeting point (battery terminals), all voltages (DC) are the same and the simulation has to perform a current balance.

Battery current

For each component, the current is a complex function of the voltage:

  • PV-array: For controllers with MPPT converter, the current is the available PV energy divided by the voltage, For direct coupling, we have to search the operating point on the I/V array characteristics (irradiation and temperature already known), paying attention that ohmic, module quality and mismatch losses have an action on the actual current, for a given voltage,
  • Battery: the voltage characteristics of the battery model depends on the state of charge (SOC), temperature and current,
  • Load: Given the energy, establishes the current as function of the voltage,
  • Back-up generator: Given the energy, establishes the current as function of the voltage.

Therefore the evaluation of the current balance within the battery (charging or discharging) has to be achieved by performing successive iterations, adjusting each of these currents at each step.

Once the currents are determined, PVsyst evaluates the SOC evolution corresponding to this current in the battery. The SOC and battery voltage are calculated in the same way for the end of the time interval.

Regulation state

The system behaviour depends on the regulation state. These states could be:

  • PV-array disconnected when the battery is full,
  • Load disconnected in case of deep battery discharge,
  • Back-up generator (if defined in the system) possibly running when the SOC attains a low limit.

All these disconnexions should be followed by reconnexions with an hysteresis on the voltage or SOC.

Due to battery voltage evolution, these operating conditions may change during the time step. In this case the program determines the exact time when a regulator threshold condition is met, evaluates the energies for this hour fraction, and starts again a balance loop until the end of the hour according to the new operating conditions.

The duration of the charging and discharging states is accumulated in the results. Although the charging and discharging states are exclusive at a given time (the battery current is either positive or negative), you may have complementary charging and discharging periods within an hour if the state has changed.

Several subarrays

It is now possible to define the stand-alone simulation with several subarrays, and therefore several orientations. However there is one only battery pack, defined for the whole system.

In this case during a time step, the calculation is made separately within each subarray, with its own control device, and a battery share corresponding to the PNom share of the subarray within the system. At the end of the hourly step, the central battery evolution is evaluated according to the charging/discharging energies cumulated in each subarray. At the beginning of the next step, the battery state is transferred to the subarrays, which each apply their own control conditions for the next step.

This is an approximation, quite acceptable if the control conditions are not too different.

Results

Several variables are computed during and after this process: array running characteristics, battery storage and ageing, load an used energies, etc.

The final results should be analysed, mainly using the loss diagram. Besides the main result ESolar (the energy delivered to the user from the sun), an interesting information is the energy directly delivered (direct use), with respect to the energy which has been stored in the battery. You may try to optimize the direct use by a better "demand side management" (DSM), i.e. try to minimize the night consumption, and transfer a maximun of the needs during the day.