Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 92

geological old continental crustal shields (e.g. Canada, India, South Africa) lower
temperature gradients can be observed (e.g. 10 K/km). In contrast, much higher
gradients are measured in tectonically active, young crust areas, e.g. at the
boundaries of lithosphere plates (Fig. 2.55; e.g. in Iceland, in Larderello in Italy
(approximately 200 K/km)) or in rift regions (e.g. up to 100 K/km in the Rhine
rift) (see /2-4/).

Eurasian plate

EURASISCHE PLATTE

North American plate

N.AMERIKANISCHE PLATTE N.AMERIKANISCHE PLATTE

Eurasian plate

EURASISCHE PLATTE

Hawaii Hawaii

African plate

AFRIKANISCHE PLATTE AFRIKANISCHE PLATTE

Pacific plate

PAZIFIK - PLATTE

Nazca NAZCA -

Indian-Australian
plate

INDISCH-
AUSTRALISCHE
PLATTE

plate PLATTE

S-American plate

AMERIKANISCHE PLATTE S.AMERIKANISCHE PLATTE

Antarctic plate

ANTARKTISCHE PLATTE

zur Elektroenergiegewinnung genutzte geothermische Felder
bekannte Hoch-Temperatur-Felder
Mittelozeanische Rücken/Transformstörungen
Subduktionszonen

Geothermal field used for electricity generation
Known high temperature geothermal field
Transform fault
Subduction zone

Fig. 2.55 Important lithospheric plates on earth (see /2-4/)

The temperature gradient in the earth's mantle can be estimated from its geo-
physical properties. The temperature has to be below the melting point of the
mantle's siliceous rocks, even taking into consideration the pressure dependence
of the melting temperatures. The maximum temperature gradient in the earth's
mantle is therefore estimated to be in the order of 1 K/km.

Heat content and distribution of sources. The temperature profile in the earth's
mantle is limited by the melting temperature for iron and nickel in the core of the
earth. According to that estimate, temperatures of around 1,000 °C predominate in
the upper mantle. For the earth's interior, maximum temperatures of 3,000 to
5,000 °C can be assumed (Table 2.6).
Assuming a mean specific heat of 1 kJ/(kg K) and a mean density of the earth
of around 5.5 kg/dm 3 , the heat content of the earth can be estimated to be at about
12 to 24 10 30 J. The heat content of the outer earth crust up to a depth of 10,000 m
is approximately 10 26 J.
The heat stored within the earth results firstly from the gravitational energy at
the time of the formation of the earth approximately 4.5 billion years ago due to
contractions of gas, dust and rocks. Secondly there is a probably still existing
primordial heat. Thirdly the heat existing within our planet has been produced