Technical Application Papers No.10 Photovoltaic plants - ABB

1.6 Intermittence of generation and storage of
the produced power
The PV utilization on a large scale is affected by a technical
limit due to the uncertain intermittency of production.
In fact, the national electrical distribution network can
accept a limited quantity of intermittent input power, after
which serious problems for the stability of the network
can rise. The acceptance limit depends on the network
configuration and on the degree of interconnection with
the contiguous grids.
In particular, in the Italian situation, it is considered dangerous
when the total intermittent power introduced into
the network exceeds a value from 10% to 20% of the total
power of the traditional power generation plants.
As a consequence, the presence of a constraint due to
the intermittency of power generation restricts the real
possibility of giving a significant PV contribution to the
national energy balance and this remark can be extended
to all intermittent renewable sources.
To get round this negative aspect it would be necessary
to store for sufficiently long times the intermittent electric
power thus produced to put it into the network in a more
continuous and stable form. Electric power can be stored
either in big superconducting coils or converting it into
other form of energy: kinetic energy stored in flywheels or
compressed gases, gravitational energy in water basins,
chemical energy in synthesis fuels and electrochemical
energy in electric accumulators (batteries). Through a
technical selection of these options according to the
requirement of maintaining energy efficiently for days
and/or months, two storage systems emerge: that using
batteries and the hydrogen one. At the state of the art
of these two technologies, the electrochemical storage
seems feasible, in the short-medium term, to store the
energy for some hours to some days. Therefore, in relation
to photovoltaics applied to small grid-connected
plants, the insertion of a storage sub-system consisting
in batteries of small dimensions may improve the
situation of the inconveniences due to intermittency,
thus allowing a partial overcoming of the acceptance
limit of the network. As regards the seasonal storage of
the huge quantity of electric power required to replace
petroleum in all the usage sectors, hydrogen seems to
be the most suitable technology for the long term since
it takes advantage of the fact that solar electric productivity
in summer is higher than the winter productivity of
about a factor 3. The exceeding energy stored in summer
could be used to optimize the annual capacity factor of
renewable source power plants, increasing it from the
present value of 1500-1600 hours without storage to a
value nearer to the average one of the conventional power
plants (about 6000 hours). In this case the power from
renewable source could replace the thermoelectric one
in its role, since the acceptance limit of the grid would
be removed.
Photovoltaic plants
17
1 Generalities on photovoltaic (PV) plants