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Global renewable energy resources and use in 2050 233

increase the demand for both agricultural and urban land. Already the growing urban-
ization of the world is leading to reductions in good farmland and increased need for
water resources. It also, of course, will increase the demand for primary energy.
As production of RE continues its inevitable rise (see Fig. 6.1), the quality of the
remaining resources will generally fall. Wind speeds for new wind farms will be lower
than those being built today, new geothermal electricity plants will need to cope with
lower field temperatures, and hydrosites with less favorable locations. Even for solar
energy, with its near-inexhaustible resource base, high insolation areas will often be
remote from load centers. Shortages of scarce minerals needed for PV cell and wind
turbine production could also hinder RE output growth [46]. The result will be
declines in the EROEI for RE, and hence rising costs per kWh produced.
Ongoing climate change will have mixed impacts on the fortunes of RE. Geother-
mal and tidal energy will not be directly affected, because they are independent of
climate change. The most affected will be bioenergy, for two reasons. First, yield
increases for key crops such as grain could stagnate or even reverse in some regions
because of adverse growing conditions brought about by climate change [47], even
with better crop selection to match the changing climate. Second, rising demand
for food and biomaterials, the biomass-based products that compete with bioenergy,
will likely lower the future global technical potential for bioenergy.
A countertrend will be advances in technology, particularly for solar energy. Sev-
eral RE sources have been utilized for a century or more, and steady improvement
rather than technical breakthroughs, are all that can be expected. Wind turbines have
undergone great advances in materials, design, and output compared with those com-
mon in the 1920s in the rural areas of the United States, but no new breakthroughs are
on the horizon, unless turbines can dispense with their earth-bound supporting tower
and take to the skies (Section 6.4.2).
An additional consideration is the need for energy storage, as intermittent energy
sources—mainly solar, but also wind and perhaps wave energy—together come to
dominate the global energy supply. A variety of options are available for energy stor-
age, ranging from the already firmly established pumped water storage, compressed
air (or combustible gases) in underground caverns, batteries of various types including
flow batteries, and even conversion of electricity to some other energy carrier, such as
hydrogen [48,49]. Pumped water storage has high energy recovery, but the global pos-
sibilities for its expansion are limited. In contrast, underground storage of gas (either
air or methane, for example) while lower in energy recovery, has perhaps the greatest
potential.
There is at present no general agreement on the extent of energy storage necessary
to support large-scale RE, with estimates examined for example ranging from 6% [50]
to 35% [51] of annual supply. What is clear is that the storage method adopted will
likely depend on the RE energy mix, grid connectivity, and the effect storage has on
EROEI values [50–52]. For some energy sources, the storage problem is not so acute.
For example, heat storage is possible for STEC because it is a thermal system, thus
allowing some electricity production at times of low or zero insolation. In calculating
EROEI, each storage method requires its own energy inputs for production and oper-
ation. Not only must inputs be factored into any assessment of RE supply, they must