leads to the "conservation of resources,” even though this doubles the sinking cost and substantiality increases operating costs (pumps). We may note that this energy — rarely used except in heating as this relatively salt-charged water rarely exceeds a temperature of 80 to 100 °C — is not of solar origin, in the common sense of the word; like coal or oil, it comes from the earth’s crust but the two former substances constitute potential energy (a chemical reaction is necessary to extract energy from them), while an operating geothermal layer brings energy directly to the earth’s surface. We can also remark that geothermal deposits exist just about everywhere, in varying degrees, and are relatively well distributed in zones of human activity. Wind energy seems quite appealing at first glance (the maximum usable power is shown by the relation W = 0.38S-V3; W in kW, S in m2, V in m/s). For winds of approximately 15 m/s, a windmill equipped with good vanes (estimated output per vane: 60%) produces as much power per square meter as thermal collectors (estimated output 15% to 20%) in a country near the 45th parallel. But, as is often the case in the study of new forms of energy, these figures are deceiving as, 1 may add, are all averages. Figure 2 shows that, for countries situated between the
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