Page 275 - Managing Global Warming

232 Managing Global Warming

Another method of extracting energy from the ocean is through OTEC, which uses
the temperature difference between the surface tropical oceans and the colder water
1 km down to run a heat engine to generate electricity. Since the deep cold waters are
really part of the return flow of the major ocean currents, the global OTEC potential
cannot be considered separately from that for energy extraction from surface (ocean)
currents. Because the temperature difference is at best only about 20–25°C, electricity
is generated at low efficiency, and very large volumes of water must be brought to the
surface for each kWh produced. EROEI values will therefore be low.
Further, while small shore-based OTEC plants avoid this problem (and can also be
used to coproduce fresh water), for globally significant production, OTEC plants will
need to be ship-based and continuously move to maintain the necessary temperature
difference. The electricity produced will have to be converted to some other energy
form such as ammonia or hydrogen, and periodically shipped back to shore. The
energy costs of water pumping and ship fuel, and of building the plant and the ship,
and the energy losses in energy conversion (and reconversion if needed) will mean a
very low EROEI value for OTEC electricity [43].
3
Each year 35,000–40,000km of low salt content fresh water enters the world’s
oceans [44], which have an average salt content of about 3.5%. The resulting
osmotic pressure at the interface is theoretically capable of generating 95EJ of
energy. [45]. At present this energy source is untapped, and there appear to be no
plans for its development, possibly because of its technical difficulty, and its likely
high environmental costs.

6.8 Discussion

In the year 2050, we have argued that RE will be operating under very different con-
ditions than those that prevail in 2018. A high global carbon price is likely to be in
place, with the price having progressively risen over time. In 2016, fossil-fuel use
was still increasing globally. If further high output continues, it is likely that by
2050, the remaining fossil fuels will be much more costly to extract than at present,
and will carry higher GHG and general environmental costs. Despite much research
and even limited field trials, it is probable that in 2050 geoengineering will only be
deployed locally. Despite its relatively low estimated monetary costs, it carries the
risk of adverse environmental effects, particularly on regional precipitation, and hence
lacks global political consensus. By 2050, carbon sequestration is likely to have been
implemented on a small scale, but will probably be regarded as at best a minor tech-
nique for climate mitigation. All these factors will generally favor RE sources over
fossil fuels, enabling RE output to steadily expand. On the other hand, over the next
decade or so, it is possible that most carbon reductions will come from energy con-
servation and energy efficiency measures. The resulting spare capacity in fossil-fuel
power stations could inhibit growth of RE output.
But as we have shown, ongoing climate, land use, and environmental changes will
also adversely affect both the economics and technical potential for RE in 2050. The
first factor is the expected growth of global population, which will, ceteris paribus,