PV system planning for stand-alone requires the over sizing of the PV field and of the battery pack capacitance in order to guarantee sufficient electrical power for the operation of electrical load in the periods of low solar irradiation, but this leads to an increase of the cost of the kWh as regards the one produced by a grid-connected PV system.

The size of the PV systems for island applications is calculated balancing the electrical power absorbed from loads with solar energy incident on the PV modules converted in electrical power.

(1) Ec = Agmin x H x ng

Ec = daily electrical power absorbed from consumers in Wh/day

Agmin = minimal area of the PV field in m2 H = daily global solar irradiation in Wh/m2/day ng = efficiency of the PV plant

For traditional stand-alone PV plants, a tilt of 60° of the plane module array is the best choice because in this condition the daily solar irradiation is more constant during the year.

Global Solar Irradiation

Instead, for the hybrid PV-Fuel system, a tilt of 40° of the plane module array is the best compromise in terms of yearly availability of global solar irradiation and of constancy of daily global solar irradiation; in fact we have that:

— yearly availability of global solar irradiation is quite close to the one with the Average monthly of daily global solar irradiation modules array tilted of 30°; in Manfredonia, calculated for different tilt planes

— in autumn and winter the

daily global solar irradiation is quite close to the one with the modules array tilted of 60°.

To determine PV field size for island applications, in (1) it has been placed:

— for classic PV system, H = lowest value, corresponding to the month of December;

— for hybrid PV-Diesel system, H = annual medium value.

The load has been chosen so that the maximum engaged power does not exceed too much the 3 kW and so that the daily medium consumption of energy is of approximately 7 kWh, approximately corresponding to 2500^2600 kWh/year, standard value for typical domestic user.

The capacitance of the battery pack is calculated by the expression:

(2)

Qb = Ecmax x Nga / nbx DOD

Ecmax = maximum daily energy absorbed from electrical load in Wh/day nb = battery efficiency, typical value is 80%;

DOD = Dept Of Discharge, maximum discharge level to avoid battery damaging, typical value is 80%;

Nga = lack of sun irradiation in days.

Daily Electric Power Demand W 2.500 — 2.000 — 1.500 — 1.000 — 500 0 0.

To determine the battery capacitance, it has been placed in (2):

—

ҐГ К T 00 6.00 12.00 18.00 time

for classic PV system Nga = 5 days;

— for hybrid PV-Fuel system Nga = 1 day.

Distribution of electrical consumers power demand in t weekdays

On the basis of the local global solar irradiation and typical daily domestic user power demand, it has been planned and realized a hybrid PV-Fuel system in Manfredonia with:

— two PV systems of 20 PV modules each for a total installed nominal power of 2 x 1024 Wp, angle of tilt 40°, two inverter Sunny Boy 850 of the SMA;

— bi-directional converter Sunny Island 3300 of the SMA;

— battery pack of 30 storage elements with capacitance of 250 Ah/2 V each;

— electrical generator of 5 kVA, one phase, gasoline feeding;

—

Architecture of the hybrid PV-Fuel system realized in the ENEA Monte Aquilone Test Site

data acquisition and plant monitoring with Sunny Boy Control Plus of the SMA.

Using the electrical generator as a backup power supply to balance the difference between electrical consumers demand and monthly average of daily solar irradiation, it has been obtained in comparison with a classic system:

— a reduction of approximately 38% in the size of the PV field;

— a reduction of 80% in the capacitance of the battery pack;

— an increase of the electric power continuity in presence of numerous and uninterrupted low solar irradiation days.

The architecture of the system, represented in the previous figure, allows us to shape it in two ways:

— hybrid PV-Fuel system in stand-alone: the Sunny Island has the task to support and to control the grid for the operation of the PV systems and manages electrical consumers, the battery and the external power supply;

— PV system in grid connect: in lack of mains voltage, the Sunny Island has still the task to support and to control the grid for the operation of the PV systems and behaves also as backup power supply.

All the devices, inverter, power supply and electrical consumers, for the presence of the Sunny Island, interface directly on the stand-alone to industrial voltage and frequency increasing flexibility in comparison with a classical system.

The experimental activities performed on the realized system have produced numerous data useful to optimize the system sizing as a function of fuel cost, and to confirm cost reduction of installation and management estimate in planning.