Reducing the Energy Carbon Content - Energy and Climate: How to Achieve a Successful Energy Transition - page 78

However, while the conversion efficiency of monocrystalline silicon
cells is approximately 16-17%, the figure drops below 10% with multi-
crystalline silicon thin layers. The efficiency of amorphous silicon cells is
even less, which accounts for the fact that their market share has become
very low. Mixed structures combining crystalline and amorphous silicon
seem to offer a potentially interesting option.
Studies are being conducted on other materials such as copper-indium
diselenide, from the family of chalcopyrites, which offers the possibility of
high efficiencies, nearly 20%. Semiconducting organic materials may also
be used. They are easy to implement; they can be used in the form of
flexible sheets but, for the time being, their efficiency is at best about 5%
and their lifetime is insufficient [62-64].
As prices continue to drop, photovoltaic electricity production should
eventually become competitive.
Before we can expect to see widespread development in the use of
photovoltaic cells, progress is required in the field of energy storage to
overcome the problem of intermittent energy supply.
Advantages and limitations of geothermal energy
The principle of geothermal energy is based on extraction of heat from the
subsoil.
Low-temperature geothermal energy is easier to obtain: it is used as an
additional energy source when heating buildings, generally being associ-
ated with heat pumps. The energy supplied for heating purposes reached
3Mtoe in 2006.
Geothermal energy can also be used for the production of electrical
energy in countries with a high geothermal gradient, such as Iceland.
Provided that the drilling costs can be reduced sufficiently and the
necessary materials are available, this type of energy production could
be extended to other countries by implementing deep geothermy.
In 2006, electricity production represented the supply of 59 TWh, for
an installed power of about 9GW.
Compared with solar and wind, geothermal energy has the great
advantage of being supplied on a continuous basis. Expertise and
knowledge of the subsoil are required, however, hindering its develop-
ment. The necessary infrastructures must be paid back over relatively
long durations, with the risk of a temperature rise of the geothermal
source after a long period of operation, since the heat removed is only
renewed very slowly.