ABSTRACT
Several natural processes dissipate energy that can be exploited. The abundances, accessibility and consequences of use have been discussed for decades, not always with consistent conclusions. The fundamental considerations emphasised here draw attention to two general principles that appear self-evident when they are recognised. The total energy of any natural process imposes an extreme upper bound on the energy that may be extracted from it, in principle, and the environmental consequences of use increase with the fraction that is used. That fraction will, in any case, be limited by technical considerations. These are inescapable reasons for concentrating on the phenomena with the most abundant energies. Inevitably that means, first and foremost, solar energy. As numbers in Table 25.1 show, wind power ranks second and all the other sources may have particular uses but are also-rans in the quest for power on the scale of the present human use, 16 TW. Harnessing any of these sources has no influence on the energy balance of the Earth, because all of the energy is converted to heat, whether naturally or by diversion to human use. Subsections 25.1.1 to 25.1.8 present comments and explanations of the entries in Table 25.1.
