Geothermal

Technology Outline

• Types of geothermal resources
• Distribution of geothermal resources world wide
• Major applications of geothermal energy
• Geothermal heat pumps (geoexchange systems)

Geothermal energy is the natural heat of the earth. Geologically, the earth can be divided, from the surface to the interior, into four layers - the crust (5 to 40 km in thickness, temperature from 15 oC to 500 oC), the mantle (2885 km in thickness, temperature from 700 to 3,000 oC), the outer core (2270 km in thickness, temperature at about 5000 oC) and the inner core (radius of 1216 km, temperature at about 7000 oC). Heat is continually generated by the decay of radioactive elements from the molten interior and flows outward to the surface of the earth.

The heat energy inside the earth's crust can be utilized in the form of geothermal energy, at locations where a heat carrier (water in the liquid phase or steam) is present to transfer the heat from the deep hot zones to or near the surface in the form of geysers or spas, or where a heat carrier can be used to extract the heat from hot dry rocks or magma. These forms of geothermal energy are not available in Hong Kong.

"Geothermal heat pumps" (also called ground-source heat pumps or geoexchange systems) use the earth as a heat source in winter and a heat sink in summer. Although not strictly conforming to the definition of "geothermal energy" as given above, this type of systems may find applications at certain locations in Hong Kong. However, geothermal heat pump is generally not considered a renewable energy technology.