10 Introduction
1.2 The renewables - what is the challenge?
As mentioned above, 86 % of the worlds primary energy consumption in 2014
was provided by the traditional fossil fuel resources, oil, natural gas and coal.
The remaining 14 % consists of nuclear, hydro and other renewables. The
other renewable resources include wind, solar, geothermal and biomass which
combined provide 2.5 % of the worlds primary energy 3. The big question is
whether it is possibly to increase this value signicantly. In 2014 an estimated
250 billion US dollars were invested globally in renewable energy sources, of
which photovoltaics and onshore windturbines received the most 27. While
this investment seems a staggering amount, the installed renewables are not
likely to meet the targets of the 2 oC plan from IEA, which aim for 10225
TWh power generated in 2025 from renewables, an increase of approximately
100 % compared to 2013 27. Many countries are still struggling with making
large integration of renewables feasible and it is clear that regional and national
subsidy schemes are key to further implementation. However, one aspect is
the market and public willingness to invest in these technologies, another is
to evaluate the potential power reserve available. As mentioned above for oil,
gas and coal the reserves are investigated yearly but are not considered to be
very accurate. For renewables the ultimate reserve is the highly useful fusion
generator located approximately 150 billion meters above us, a star known as
the Sun. In fact, on earth we receive approximately 170000 TW of radiation
from the sun, of which close to a third is reected back 28. Assuming we can
harvest that energy with 10 % eciency, we would have to cover approximately
0.016 % of the earth's land surface with solar panels to produce 20 TW 28.
It is also worth noting that solar and wind power are potentially much more
distributed resources compared to fossil fuels. With widespread installation of
wind turbines and solar panels it is possible that the future energy landscape
will contribute to a markedly dierent and more equal political environment,
compared to today.
A key factor to consider is the intermittent nature of solar and wind power,
which can not be turned on and o in the same manner as fossil fuel based plants.
When the sun shines or the wind blows power generation is possible, but when
it is dark or there is no wind there is nothing to gain from these sources. This
intermittency is exemplied in gure 1.11, where Danish electricity demand and
wind power generation is shown for a two-week period in May 2015. The demand
uctuates between 2.5 and 4.7 GW, but the wind power uctuates between close
to zero and 4 GW. There are also times when the wind power generation exceeds
the demand, which means the excess power must be exported. Denmark has an
ambitious plan to expand the electricity generation from wind power up to 50 %
of the national consumption. In 2014 a new record level was reached, where 39.1
% of the national electricity consumption was covered by wind turbines 29. In
gure 1.11 an example is given for doubling the wind power production. With
such a doubling there are longer periods where the wind power exceeds the