Sustainability and policy - Climate Change: Biological and Human Aspects - page 470

Table8.2 Thepotentialcontributionofrenewableenergyto
ourenergysupplyin2025.Source:Cowie,1998a,andchecked
toseeifdataarecompatiblewithIPCCSRESs.Further,the
table'stotallow/highestimatesof1295-1960mtoefor2025
ittheInternationalEnergyAgency's(2004)estimateofsome
2000mtoepossiblefromrenewableenergyfortheyear2030.
Themtoedataarebasedontheassumptionthatthefossilfuel
itdisplaceswouldhavebeenburnedinapowerstationwithan
eiciencyofaround35%

Potential contribution (mtoe)

2025 Energy resource Low estimate High estimate

Existing (2000) HEP 600 600

New HEP 300 500

Photovoltaic cells 5 45

Wave 20 220

Tidal 50 125

Wind 50 90

Geothermal 20 40

Biomass 250 340

Total 1295 1960

Of which is new 695 1360

The idea behind CCS is that instead of fossil fuel consumption short-circuiting
the deep carbon cycle (Chapter 1) through the burning of geological deposits of
carbon-releasing carbon dioxide to the atmosphere, the carbon is captured and stored
geologically. It could also apply to the burning of biofuels, in which case carbon from
the fast carbon cycle is transferred to the deep carbon cycle as opposed to the other way
around. This means that there would be a net reduction in atmospheric carbon dioxide.
There are other storage options but geological storage is both the most viable and
has the least environmental risk (although the technology is currently far from proven).
Other options include precipitating the carbon out as carbonate, so in effect creating
limestone, or pumping it into the deep ocean. The former is not yet commercially
viable. The possible environmental impact of the latter on abyssal ecosystems is
unclear, as is its stability (especially if the oceans warm up).
The capture of carbon dioxide takes place directly after the fossil fuel, or even
biofuel, is burnt, by taking it out of the exhaust flue of a power station and pumping
it into geological formations. The use of depleted oil and gas fields, or alternatively
deep saline aquifers (water-laden porous rock formations), are the principal options.
There are advantages with the former. Primarily, oil and gas fields have a proven
stability and storage ability or tightness, whereas saline aquifers do not. Tightness is
an important factor as the carbon must remain trapped for longer than the time in
which fossil fuel is used at biosphere-affecting levels. Given that this last is likely to be
between four and eight centuries, and given that the half life of carbon dioxide in the
atmosphere is of the order of a couple of centuries, ideally the geological structure's