Preliminary design > Stand-alone system presizing > 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 are asked to define the plane orientation.

NB:	the button "Show optimization" opens a little tool which shows the winter yield according to the plane orientation. For stand-alone systems, the plane orientation should usually be optimized according to the worst conditions, i.e., for winter irradiance.

Pressing "Next" gives a second screen to define the user's needs, from a domestic use point of view. You have to specify every foreseen appliance, their consumption and use conditions.

Now you can open the "Results" which asks for

•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 lead to the determination of the array nominal power (i.e. the installed STC power according to the manufacturer specifications), and the battery pack capacity.

The first result graph shows the potentially available solar energy, along with the user's needs.

The second one gives the average state of charge of the battery (low values could lead to a quicker deterioration of the batteries), and PLOL monthly distribution.

The table holds all monthly values, including the needed back-up energy.

Finally the rough economic evaluation gives an idea of the investment and energy price.

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 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 of irradiation on the basis of instantaneous data for one day per month.

This is not sufficient to manage the storage balance evolution from day to day and the effective use of solar incident energy. Therefore, the program generates a random sequence of 365 days, according to the algorithms of Collares-Pereira, renormalised to the monthly sums, and calculates the daily battery balance for three intervals in a day (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".