Page 271 - Managing Global Warming

228 Managing Global Warming

electricity output per turbine of a given rating. Various proposals include turbines
borne aloft by tethered balloons and tethered self-propelled turbines. Another pro-
posed concept would be based on airborne kites [27].
As with ground-based turbines with an average rated output of, for example, 5MW,
many thousands would need to be deployed for significant electricity production.
Great care would need to be taken to lessen the dangers to aircraft, birds, and even
people on the ground from either falling cable or the turbines themselves. If wind
energy continues to rise at only a linear rate, land and shallow sea locations will be
ample, and air-based turbines are unlikely to be more than novelties, even in 2050.

6.5 Solar energy

6.5.1 Introduction
Solar energyistheonlyREsourcewith vastpotential comparedwithanypossible human
energy consumption level, with annual global insolation estimated at 5600ZJ. The inso-
2
lation level at any location (in Wm ) varies in a predictable way with latitude, season,
and time of day, but also with cloud cover, which is harder to predict. The result is that
solar energy technical potential per unit horizontal area is far greater for some countries
than others, and that, like wind power, is only intermittently available. In 2015, only
253TWh of solar electricity was produced, or slightly <1% of total electric output
[5,28]. Most of this output was from photovoltaic (PV) cells with installations in a great
manycountries,andtherestfromsolarelectricityenergyconversion(STEC)plantsinthe
United States and Spain [21]. Solar collectors are devices for heating water, and
6
2
652 10 m of such devices have been installed worldwide by the end of 2016 [29];
the heat so generated (375TWh in 2016) is included in RE statistics.
Not all solar energy needs to be converted by some energy device. Passive solar
energy has been used for millennia, and its potential is vast. It can be used for both
heating buildings and for natural lighting. Passive solar is most effective if buildings
are designed with its use in mind, but existing buildings can be retrofitted. It is often
very difficult to determine the difference between passive solar energy utilization and
energy conservation, and passive solar energy is not presently included in RE produc-
6
tion statistics. If the Earth’s land and oceans did not annually absorb 39 10 EJ inso-
lation from the sun, its temperature would be close to absolute zero, rather than its
present average of 15–16°C. In one sense, solar energy already provides all our energy
needs, except for a few hundred exajoules from fossil-fuel combustion, which makes
any calculation of the contribution of passive solar energy difficult to interpret.

6.5.2 Solar energy in 2050

Recent growth in global solar electric production has been exponential, and given the
possibility of technical breakthroughs, any predictions of future output are unlikely to
be useful. It should be remembered that wind energy also experienced an exponential
growth phase, before slowing to linear growth over the past decade.