Stand-alone system preliminary design

See also: Sizing of a stand-alone PV system: general considerations.

After defining the "Location", the System button displays a first screen where you can define the plane orientation.

NB: the Show optimization button opens a tool that shows winter yield based on plane orientation. For stand-alone systems, plane orientation should be optimized for the worst conditions—that is, winter irradiance.

Pressing "Next" opens a second screen to define user needs from a domestic use perspective. You must specify each anticipated appliance, its consumption, and operating conditions.

Now you can open the "Results" where you specify

• the required autonomy in absence of sun - which determines the battery pack capacity,
• the required "Loss-of-Load probability" (P LOL),
• the planned system voltage.

These parameters determine the array nominal power (that is, the installed STC power according to manufacturer specifications) and the battery pack capacity.

The first result graph shows the potentially available solar energy alongside user needs.

The second graph shows the average battery state of charge (low values can lead to faster battery deterioration) and the monthly PLOL distribution.

The table contains all monthly values, including required backup energy.

Finally, the rough economic evaluation provides estimates of investment and energy cost.

You can now play with the parameters and immediately see the results.

You can print a report or store graphs and tables on the clipboard to export them to another software.

You can also save your project and load another one for immediate comparisons.

Computation

The evaluation of the available irradiance on the collector plane uses the Monthly Meteo tool algorithms, which calculate monthly average irradiation based on instantaneous data from one day per month.

This is insufficient for managing day-to-day storage balance evolution and effective use of solar incident energy. Therefore, the program generates a random sequence of 365 days following the Collares-Pereira algorithms¹, renormalized to monthly sums, and calculates the daily battery balance for three time intervals (morning, day, and evening).

The accuracy is of the order of 10 - 20% (worst case for very tilted installations).

If necessary, the general parameters (array overall efficiency, battery efficiency, battery low charge threshold) are user-modifiable through the menu option "Preferences"/"Edit Hidden Parameter".