Stand-alone system procedure
This page gives a step-by-step procedure when defining a stand-alone system in PVsyst.
First step
As for any PVsyst system, you have to specify the orientation of the PV array.
NB: It is now possible to define several subarrays, and therefore several orientations with Stand-alone systems.
Second step
You are asked to define the user's needs.
By default for little systems, this is proposed as a list of domestic appliance and the details of their use (may be seasonal or monthly).
For industrial or bigger systems, you can press Other profiles. This gives many possibilities of defining a load profile, including by a CSV file of hourly values.
Third step
Click the button System.
Pay attention to the Pre-sizing tool (top frame) which gives advice about the required Battery bank and PV array power.
This is a rough pre-calculation, based on your weather data file and above definitions. You should define:
- the required autonomy (usually around 4 days).
- the acceptable probability that the needs are not covered by the system (PLOL for "probability of Loss of Load),
- the nominal voltage of the battery bank.
The program will then perform a system pre-sizing. Please be aware that the battery capacity is calculated here in such a way that it covers the full requested autonomy, without PV production at all. This should indeed account for a PV production corresponding to the worst n-days sequence in the year. This will be done in a next version.
You also have a button Detailed pre-sizing for a more refined sizing study, for different battery sizes, different weather data distributions, or according to the PLOL parameter.
Very important: This pre-sizing tool is independent on your system design. It doens't transmit its values to your system, except for some defaults. The values in the system (battery pack definition, PV power) are always according to your choice.
Fourth step
Define the battery pack, by choosing a battery model ("Storage" page).
=> The program will propose the number of batteries in series and in parallel, according to the suggestions obtained in the previous pre-sizing tool.
You should also define the operating temperature conditions for the batteries, according to your system implementation.
NB: When choosing lead-acid batteries, you can have a look on some technology advices.
Fifth step
Define the array configuration and control strategy ("PV array" page)
- Acknowledge the pre-sizing propositions (planned power or available area),
- Choose a PV module model in the database,
- Choose the control strategy (direct coupling, MPPT or DCDC converter).
- In a first step, you are advised to choose the "Universal controller", to get rid of the specific control conditions.
- => the program determines the number of modules in series and parallel, according to the battery voltage.or MPPT conditions and required PV power.
NB: With the "Direct coupling" strategy, the PV module voltage should match the battery voltage in operation. Any mismatch will result in "Pmpp losses".
The best suited modules are those with 36 cells per 12V battery. In cold climates, modules of 33 cells/12V (66 cells/24V) may also be used.
But 60 or 48 cells modules are not suited for direct coupling: with such modules you have to use a MPPT or DCDC power conditioning.
During this first stage, you can do some studies by varying the operating conditions, namely the operating thresholds for the Charging / Discharging commands. You can modify these thresholds within the "Universal controller".
Only during a second phase of your project, when the system configuration will be well established and will give globally the expected results in terms of PLOL or unused energy, you will choose a specific commercial model of controller.
NB: you can create your own controller model, suited for your system (for example for parametric studies) by simply saving the Universal model in its actual configuration. In this case, please give it its manufacturer's and model's name, and save it using the convention "Manufacturer_Model.RLT" in your own database.
Sixth step
In the "Back-up" page, you can specify a Diesel Genset, and adjust its operating (charging) power.
Battery inverter
In the present time, you cannot define an inverter with stand-alone systems: the user's needs are expressed in terms of Energy, whatever the DC or AC use. If you have an inverter you should increase the user's needs, in order to account for its efficiency. Stand-alone inverters are completely different as usual Solar Inverters connected to the grid. They will be implemented in a future version.
Seventh step
Pass to the button Detailed Losses for the definition of all system losses, which are defined at reasonable default values for your first simulations.
Finally
Run the simulation, and analyse the results on the report.
You can now play with these parameters, and retrieve the automatic proposed values by clicking on the associated default checkbox at any time.
Manual optimizations
Begin with a sufficient battery size (for example close to the value proposed by the pre-sizing, which is over-evaluated).
If you have a big unused energy, your PV array is oversized. If you have a big Loss of load, it is undersized.
You can have boths cases simultaneously, as for some hours of the year the battery may be fully charged, and other hours it may be empty. This problem is particularly important at high latitudes, where the summer and the winter PV yields are very different.
Now when you have found a satisfactory PV power, you may diminish the battery size and observe the effect. The size will become reasonable when the unused energy and the loss of load will increase in a similar way.
Remember that the battery cost is very important. It is usually preferrable to oversize the PV array rather than choosing a too big battery. However these choices are strongly dependent on your requirements (namely the reliability of the energy availability). The battery size may be significantly reduced when using a Genset.
Remember also that the PLOL may be different from one year to another one, as it depends on the worst sequence of bad days in your weather data.
Error messages
Warnings will be displayed if there are some incompatibilities between the chosen parameters. Red warnings are not acceptable (simulation cannot be performed) and orange warnings are indicative. These colours will be thrown back on the System LED's button.