Reducing Energy Consumption, while Protecting the Environment - Energy and Climate: How to Achieve a Successful Energy Transition - page 59

which can be recharged by using an external electricity supply source.
Such an option will be discussed in Chapter 6.
Hybrid propulsion is typically a transition technology. It helps to
significantly improve the performance of the present vehicles, while
leading the way for new propulsion systems [42]. It contributes to the
acceleration of further progress in the area of electric batteries and should
thus facilitate the development of electric cars.

Energy storage

Energy storage represents a key technology for adjusting energy supply
and demand, the energy being stored when the availability is higher than
the need and released back in the reverse situation.
It makes it possible to adjust the energy supply to a variable demand
from a source delivering a constant power. Conversely, energy storage
makes it possible to adjust the energy supply to a constant demand froman
intermittent delivery source.
Fossil fuels have the advantage of storing large amounts of energy
within a comparatively small volume. For renewable energy sources this is
not the case, and a large deployment of these renewable energy sources
requires the use of storage systems to compensate for the intermittence of
the supply and also for mobile applications (on board storage).
In the case of electricity production, flexibility can be ensured through
the modulation of the power delivered by a fossil fuel power plant
operating as a back-up.
Thus, a natural gas fired combined cycle can be used to compensate for
the intermittence in electricity production supplied by windmills. Such an
option limits the share of renewable energy sources which can be intro-
duced in the electricity generation system.
Hydraulic storage is the only method currently used on a large scale. In
France for instance, the hydraulic storage of Grand

Maison in the Alps has
a storage capacity of around 400 TWh, handling a peak power amounting
to 1.2GW when storing and 1.8GW during discharge. Unfortunately,
most natural storage sites of this kind are already exploited, at least in
Europe, and it is not easy to extend the storage capacity of such hydraulic
systems. An option to be explored might be the development of new sites
by creating artificial lakes.
Figure 5.4 presents a comparison between different types of storage
systems and illustrates the difficulty in finding a good alternative to liquid
hydrocarbons [47]. The specific energy stored in liquid hydrocarbons