Basic concepts of deep geothermal energy

Geothermal energy: basic concepts

Earth’s heat sources:

• „residual” heat of the Earth interior (mantle and
core)
• decay of radioactive isotopes: U238, U235, Th232,
K40 (crust)
• solar radiation (limited 5-25 m below the ground)

Internal heat production: 20TW
Measured heat flow Qs = 44TW

Earth cools down 2X quicker than heat production, however the heat content of the Earth would take over 10⁹ years to exhaust via global terrestrial heat flow (practically inexhaustable on human-scales).

BUT HOW MUCH CAN BE EXTRACTED?

Deep hydrothermal

• Elements: heat source, reservoirs, carrying medium (fluid), recharge
• Fluid convection: heating → thermal expansion of groundwater → rising and replacement by recharging colder meteoric water
• Extraction of heat energy: production of thermal groundwater

Deep stimulated (HDR/EGS)

• Artificially created and enlarged fractures as „heat exchangers” in deep lying hot rock bodies
• Extraction of heat energy: circulating water via production and reinjection wells

Conceptual model of a high-temperature
geothermal system

reference curve for the boiling point of pure waterWhite, 1973

Classification of geothermal resources

(a) Muffler and Cataldi (1978).
(b) Hochstein (1990).
(c) Benderitter and Cormy (1990).
(d) Nicholson (1993).
(e) Axelsson and Gunnlaugsson (2000)