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Geothermal Heating
Geothermal Heating is the direct use of geothermal energy for heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. As of 2007, 28 GW of geothermal heating capacity is installed around the world, satisfying 0.07% of global primary energy consumption. Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter.
Geothermal energy originates from the heat retained within the Earth since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application. However, even cold ground contains heat, below 10 feet or 3 meters, the ground is consistently 12.8 °C (55 °F) in moderate climates, and it may be extracted with a heat pump.
Briefly and simply explained
Ground source heat pumps rely on an energy exchange between the air within the building being heated and the ground. Below ten feet the earth's temperature is fairly constant, generally around ~10 °C (~50 °F). During the summer when the ambient temperature of the building exceeds that of the ground heat pumps are used to pump heat from the building in to the transfer medium (typically water with small amounts of ethanol or glycol) and is subsequently pumped through narrow pipes into the ground so that the heat can be dissipated in the earth. When the ambient temperature falls below the ground temperature the process works in reverse. Heat pumps extract heat from the ground and use it to heat the building.
Applications
| Country | Production PJ/yr | Capacity GW | Capacity Factor | Dominant applications |
|---|---|---|---|---|
| China | 45.38 | 3.69 | 39% | bathing |
| Sweden | 43.2 | 4.2 | 33% | heat pumps |
| USA | 31.24 | 7.82 | 13% | heat pumps |
| Turkey | 24.84 | 1.5 | 53% | district heating |
| Iceland | 24.5 | 1.84 | 42% | district heating |
| Japan | 10.3 | 0.82 | 40% | bathing (onsens) |
| Hungary | 7.94 | 0.69 | 36% | spas/greenhouses |
| Italy | 7.55 | 0.61 | 39% | spas/space heating |
| New Zealand | 7.09 | 0.31 | 73% | industrial uses |
| 63 others | 71 | 6.8 | ||
| Total | 273 | 28 | 31% | space heating |
There are a wide variety of applications for cheap geothermal heat. In 2004 more than half of direct geothermal heat was used for space heating, and a third was used for spas. The remainder was used for a variety of industrial processes, desalination, domestic hot water, and agricultural applications. The cities of Reykjavík and Akureyri pipe hot water from geothermal plants under roads and pavements to melt snow. Geothermal desalination has been demonstrated.
Geothermal systems tend to benefit from economies of scale, so space heating power is often distributed to multiple buildings, sometimes whole communities. This technique, long practiced throughout the world in locations such as Reykjavik, Iceland, Boise, Idaho, and Klamath Falls, Oregon is known as district heating.
Extraction
Main article: Ground-coupled heat exchanger
Some parts of the world, including substantial portions of the western USA, are underlain by relatively shallow geothermal resources. Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. In these areas, water or steam may be captured from natural hot springs and piped directly into radiators or heat exchangers. Alternatively, the heat may come from waste heat supplied by co-generation from a geothermal electrical plant or from deep wells into hot aquifers. Direct geothermal heating is far more efficient than geothermal electricity generation and has less demanding temperature requirements, so it is viable over a large geographical range. If the shallow ground is hot but dry, air or water may be circulated through earth tubes or down hole heat exchangers which act as heat exchangers with the ground.
In areas where the shallow ground is too cold to provide comfort directly, it is still warmer than the winter air. The thermal inertia of the shallow ground retains solar energy accumulated in the summertime, and seasonal variations in ground temperature disappear completely below 10m of depth. That heat can be extracted with a geothermal heat pump more efficiently than it can be generated by conventional furnaces. Geothermal heat pumps are economically viable essentially anywhere in the world. One geothermal district heating system at Drake Landing enhances storage of solar energy in the ground to such an extent that no heat pumps are needed.
Economics
Geothermal energy is a type of renewable energy that encourages conservation of natural resources. According to the U.S. Environmental Protection Agency, Geo-exchange systems save homeowners 30–70 percent in heating costs, and 20–50 percent in cooling costs, compared to conventional systems. Geo-exchange systems also save money because they require much less maintenance. In addition to being highly reliable they are built to last for decades.
Some utilities, such as Kansas City Power and Light, offer special, lower winter rates for geothermal customers, offering even more savings.