Page 326 - Advances in Renewable Energies and Power Technologies

2. Classification of Photovoltaic Irrigation Systems 299

2.1.2 Grid-Connected Plants
Grid-connected plants can draw power from the grid during the hours when the PV
generator cannot produce enough energy to satisfy the needs of the consumer. On the
contrary, when the PV system produces more energy than required, the surplus can
be injected into the grid, which, in turn, operates as a large accumulator. Therefore,
in most cases, grid-connected systems do not need to use accumulator banks.
Furthermore, such plants offer the advantage of distributed rather than centralized
generation.
We can classify grid-connected plants into two categories depending on the type
of energy management.
2.1.2.1 Net Metering
Net metering is an energy management model that allows the energy produced to be
injected into the grid but not immediately consumed. In this model, the energy is not
sold, but the grid is used as power “storage” facility. Excess power that is produced,
but not self-consumed, can be transferred to the grid, and the consumer may make
use of the grid when the PV plant does not generate enough power to meet their
demands.
An annual energy balance is required to account for the energy injected into and
drawn from the grid. The following data must be taken into account:
• The amount of energy exchanged with the grid
• The equivalent value, in Euros, of the electrical energy supplied to the grid
• The value, in Euros, of the rates paid for the energy drawn from the grid, divided
into energy charges and service charges
Net metering is an advantageous system for the consumer because the annual
payment is based on monetary value rather than on energy. Thus, both the prices
of the energy injected into and drawn from the network and the cost paid by the
user to access the grid must be considered.
2.1.2.2 Self-consumption
In this type of system, the generated PV energy can only be consumed if the de-
mand is simultaneous with energy production. However, such systems cannot
use the public grid as a power storage facility and, therefore, the energy surplus
produced is lost.

2.1.3 Hybrid Systems
Other systems combine PV power plants with other sources of energy to cope with
the problem of energy supply security [26]. Carroquino et al. [27] used a hybrid PV-
Diesel system to irrigate Mediterranean crops. Panayiotu et al. [28] compared both a
stand-alone PV system and a hybrid PV-Wind installation for a household applica-
tion. They concluded that neither of them was preferable to the other, as the appro-
priate type of renewable energy system was dependent on the specific location and
climate.