Page 322 - A Comprehensive Guide to Solar Energy Systems

Chapter 15 • Integration of PV Generated Electricity into National Grids 327

Progression” scenarios (these are two of four scenarios used by National Grid to look at
future gas and electricity demand and supply—in consumer power an empowered con-
sumer makes choices and drives activity whilst in slow progression change is slow because
of a lack of direction and ability to make investment choices). The results highlight the
increased influence of distributed resources, which are not visible over time. In 2016/17
the highest total output from generators not visible is 17 GW. By 2025/26 it is predicted that
this will rise to 34 GW in consumer power and 27 GW in slow progression.
Not only does this generated electricity need to go somewhere (someone needs to want
to make use of it at the time it’s produced), but it also has to be balanced with the output
of other sources as described earlier. This means, we could either see a desire to limit the
output of solar PV directly when it is particularly sunny and windy, or a growth in the flex-
ibility which balances its output. We could find that much of the output is exported to the
European continent. But it could also be used to charge batteries or other forms of energy
storage. Of course, solar PV operators may start to limit their own output if they find that
the price they receive is not satisfactory.
however, it is possible to overcome this shortcoming we have highlighted. After all, you
know that solar PV output will be high if the sun is shining, and won’t if it’s not.
To improve the way PV generation output was forecast in the United Kingdom, a pilot
project was started with Sheffield Solar [4]. The data has helped grid operators to better
manage flows on the electricity grid network. To obtain the maximum solar PV output, it
is necessary to estimate the availability of the solar resource. In the United Kingdom this
is being done by the Met office at a number of sample sites, the radiance observations are
being linked to solar power generation, and forecasting models are being developed.
This exercise becomes more challenging if you don’t know the level of capacity which
is behind the connection. Some designers size the panels and the converter rating dif-
ferently, with the panels being oversized. Fig. 15.6 (see Fig. 5.2 in [6]) demonstrates the
behavior of two solar installations with the same export capacity but with different gen-
erating capacities. Storage would add further complexities to the use of solar PV power as
illustrated in Fig. 15.6 (see Fig. 5.2 of reference [6]). Equally, the additional storage could
provide the flexibility required to compensate for some of the other issues touched on in
this chapter (see reference [12]).

15.6 ‘Nonsynchronous’ Inverter Type Generators Supporting
the Network

Solar PV generators are linked to the grid by inverters which convert dC electrical power
from panels into AC power suitable for injecting into the grid. Properly configured, a grid
tie inverter (GTI) enables a home owner to use a power generation system such as so-
lar system without extensive rewiring and without needing batteries to fill in peaks and
troughs in output. If the power being produced is insufficient, the deficit will be sourced
from the electricity grid.