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Global renewable energy resources and use in 2050 229

Because of its intermittency, some researchers have put forward ambitious
schemes to avoid this problem. One, first proposed in the mid-1970s, is solar satel-
lite power (SSP). The proposal would involve a fleet of satellites, placed so as to
receive 24-h insolation. Each would carry an array of PV cells on a lightweight frame.
The electricity generated would be converted to microwave energy, beamed to Earth
receiving stations, and then converted back to electricity. The costs of satellite place-
ment would be high, and since energy conversion losses would occur at each stage
from insolation through to electric power generation on Earth, EROEI values could
be low.
Other proposals would see vast solar farms installed in each of the world’s deserts,
so that both seasonal and diurnal variations in insolation could be circumvented. Evi-
dently this scheme would require a worldwide interconnected grid, the building of
which would be the greater part of this hugely expensive Scheme [30]. A recent
variant—the Desertrec scheme—would build large solar and wind farms in the deserts
of north Africa and the Middle East to supply European as well as local energy needs
[31]. It is doubtful, however, whether Europe would wish to become largely dependent
for its electricity supply on distant countries. At present, very little electricity is
exported across borders [4]. Furthermore, the project would still not solve the inter-
mittency problem.
Solar electricity, particularly from PV cells, is different from other RE sources in
several ways. Not only is its resource base orders of magnitude higher, but the mon-
etary costs of PV cells of a given rated capacity have decreased exponentially with
time, because of continuous improvement in the materials used. Further break-
throughs are promised [32]. But there are several factors that need consideration.
The first is that the expected life of PV units may be much lower than
anticipated—closer to 17years rather than 30years [33]. Second, the development
of higher efficiency PV cells may rely on exotic materials, which have a low global
resource base, and will prove increasingly costly to extract in monetary, energy, and
environment terms. We may thus be substituting one environmental problem for
another [34,35]. Third, PV cells are only part of the total system costs, and the other
costs, particularly for supporting structures in solar farms, are less likely to see further
cost reductions per installed watt.

6.6 Geothermal energy

6.6.1 Introduction

Geothermal energy derives both from the interior heat of the Earth, left over from its
violent birth, and from radioactive decay of various isotopes in both Earth’s interior
and crust [2]. Total annual heat output at the planetary surface is estimated at some
1300EJ, of which only about 280EJ is from terrestrial surfaces. The first geothermal
electric plant was built in Italy over a century ago, although the use of geothermal heat
has a long history. The global technical potential for electricity production is thought
to be small, with most estimates in the 1–3EJ range, because field temperatures of
above 150–200°C are needed [2].