Solar Energy Perspectives - IEA

Solar Energy Perspectives: Testing the limits
and transportation. To be clearly distinguished from electric TWh (or “TWh e ”), they are all
designated below as TWh thermal (TWh th ), as for both direct use as heat and for transportation
purposes they represent the calorific value of the fuel, and not the kinetic energy of transport.
Estimates of the amounts of biomass available for energy purposes vary widely, in relation to
land and water needs of agriculture and food production (for opposite views, see e.g. Delucchi
and Jacobson, 2011a and b, and Singer, 2011). The IEA Technology Roadmap: Biofuels for
Transport states that by 2050 it should be possible to provide 9 000 TWh th of biofuels, and
about 19 000 TWh th of biomass for heat and electricity from residues and wastes, along with
sustainably grown energy crops (IEA, 2011g). This division of the estimated total sustainable
biomass, however, rests on a model that foresees much less solar and wind generation than
this publication, and thus requires larger amounts of biomass to decarbonise the power
sector. Biomass might be best employed in the transport sector when lowest possible CO 2
emissions are sought. From the same feedstock, increased by 10% by the use of solar heat in
the manufacturing process, one could provide 18 000 TWh th of biofuels, leaving about
8 500 TWh th for heat and power, which is only slightly more than the quantity the industry
could absorb by 2050 according to Taibi et al. (2010).
Assuming that electricity, with 10 000 TWh e in transport, displaces three times more
combustible fuels, the remaining needs in transport would be about 25 000 TWh th , of which
18 000 TWht th will be biofuels, leaving a need for fossil fuels of 7 000 TWh th , mostly if not
exclusively oil products blended with biofuels for specific quality requirements.
The other 25 000 TWh th would meet heating needs in buildings and especially industry, not
covered by electricity and ambient energy through heat pumps. Direct solar heat could likely
provide 20% of the total, mostly to heat water and low-temperature processes. This would
thus represent 5 000 TWh th .
Assuming a capacity factor for solar thermal systems of 1 000 hour per year, 5 000 TWh th of
solar heat production would require a thermal capacity of 5 000 GW th . An efficiency of 70%
and peak solar irradiance of 1 kW/m 2 lead to a required surface area of 7 150 square
kilometers, i.e. a little less than 1 square meter per inhabitant - an almost trivial figure compared
to the 500 000 km 2 required by solar electricity generation (and partially included through
hybrid PV-Thermal panels). Direct solar heat would add little to solar energy’s footprint.
Other renewables would provide significant contributions to heat requirements. The IEA
Technology Roadmap: Geothermal energy suggests by 2050 a contribution from geothermal
heat of 1 600 TWh th
, which adds to its generation of electricity (IEA, 2011b).
In total, biomass, direct solar heat and geothermal heat would provide about 15 000 TWh th ,
leaving room for 10 000 TWh th of fossil fuels, mostly natural gas and coal. Figure 11.9 shows
the resulting subdivision of total energy by sources.
CO 2 emissions and variants
CO 2
emissions resulting from this combination can now be assessed. They would result from
the generation of 1 000 TWh e
from natural gas in balancing plants, the combustion of
7 000 TWh th
of oil products for transport, and, unless CCS is available on a large scale,
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© OECD/IEA, 2011