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.
Geothermal resources are divided into 4 types - hydrothermal, geopressured, hot dry rock and magma.
The formation of hydrothermal resources requires three basic components, a heat source, an aquifer containing accessible water and an impermeable cap rock to seal the aquifer. Hydrothermal resources are made available when hot water or steam is formed in fractured or porous rock at shallow to moderate depths (100m to 4.5km), for example due to intrusion of molten magma from the earth's interior. The energy of a hydrothermal resource is tapped by drilling into the aquifer and extracting the hot water or steam. Hydrothermal resources with high-grade heat (temperature from 180 oC to 350 oC) are usually utilized for electricity generation, while resources with low-grade heat are used for direct heating.
Geopressured resources refer to of hot brine saturated with methane found in large, deep aquifers under high pressure at a depth of about 3 to 6 km. The temperature of the water ranges from 90 to 200 oC. Thermal energy, hydraulic energy (from the pressure) and chemical energy (from burning of methane) can be extracted from geopressured resources.
Under certain geological conditions, the subsurface rocks reach temperatures of 200-300oC within five kilometres of the surface. The energy stored in these hot dry rocks can by harvested by injecting pressurized cold water to cause hydraulic fracture of the rocks and to extract heat from the hot rocks.
Magma is molten rock found at depths of 3 to 10 km and deeper. It has a temperature from 700-1200oC. Since magma is lighter than the surrounding rock, it moves up toward the earth's crust, carrying the heat from below. The heat of the magma can be extracted by circulating a fluid in the top part of the magma chamber. The heat will then be used to power a closed Rankine-cycle machine to generate electricity.
The crust of the earth is made up of huge plates. Geothermal areas are usually located around the boundaries of crustal plates where geological processes allow magma to rise up to the surface. These regions are often associated with volcano activities and earthquakes. As Hong Kong is not located in these regions, geothermal energy is not available here.
Geothermal is the third largest source of renewable energy following biomass and hydropower. According to the Sustainability Report 2005 by Enel (Italy's largest power company), the world's installed capacity of geothermal power, in 24 countries, amounted to 8,933 MWe In January 2005,, with a total annual production of about 56,786 GWh. The installed geothermal power capacities in the ten leading countries were: USA 2564 MW, Philippines 1930 MW, Mexico 953 MW, Indonesia 797 MW, Italy 791 MW, Japan 535 MW, New Zealand 435 MW, Iceland 202 MW, Costa Rica 163 MW, El Salvador 151 MW. In addition to electricity generation, total installed thermal capacity for direct uses of geothermal energy in 71 countries throughout the world was about 27,800 MWt.
Major applications of geothermal energy include direct-use and electricity generation.
Direct-use geothermal systems typically comprise of three major parts: (1) a production facility to deliver the heated water from the geothermal reservoir to the surface, (2) a mechanical system to transport the energy to the direct-use application, and (3) a disposal system to receive and store the cooled fluid.
Direct-uses of geothermal energy include the following:
High-temperature geothermal resources are used for the generation of electricity. The heated water either in the form of steam or liquid, is extracted from the geothermal source to drive a turbine which is coupled with an electric generator. A number of conversion technologies have been developed for geothermal power generation, including dry steam technology, flash steam technology, binary cycle technology and hot dry rock technology.
Geothermal heat pumps (also called ground-source heat pumps or geoexchange system) use the earth as a heat source in winter and a heat sink in summer.
A geothermal heat pump comprises of an external loop buried under the ground to capture heat from the ground or reject heat to the ground, and a circulation system either in the form of open loop or closed loop to keep the heat transfer fluid moving.
In winter the soil is warmer than that of the ambient air. When the working fluid (either air or water) passes through the external loop, it picks up the heat from the surrounding soil and becomes warmer than the ambient air. In summer, it rejects heat to the surrounding soil and becomes cooler than the ambient air. Such systems utilize the relatively constant temperature of the ground for space heating or water heating in winter and for cooling in summer.
Depending on the types of the external loops, geothermal heat pumps can be categorized into open loop type and closed loop type.
In an open loop system, water is used as the working fluid. It enters the system from one end, exchanges heat with the heat pump, and is discharged into a collection reservoir, and will not be re-circulated back to the system. This option is suitable when water is abundant.
In a closed loop system, the heat transfer fluid circulates between the external loop and the heat pump unit in a closed loop manner. There is another type of closed loop system called the "pond closed loop system" in which the external loop is submerged in a water body instead of buried underground. It uses the water body as heat source and heat sink.
Depending on the layout of the external loops, geothermal heat pumps can also be categorized into vertical loop and horizontal loop systems.
Vertical loop systems are systems with their external loop inserted vertically into deep boreholes in the ground. They are applied in locations with limited open space for installing the external loop.
Horizontal loop systems are systems with the external loop laid horizontally in shallow trenches.