Posted by: Geoff Jones on Dec 14, 2006
About Geothermal Energy
Geothermal energy is basically the generation of electricity or heating/cooling systems that make use of temperatures in the earth itself. The Earth's core lies approximately 6000 km below the surface and reaches temperatures that near 5000ºC (9000ºF). These extreme temperatures are sufficient to heat the rock that surrounds the core (the mantle) causing it to melt. Melted rock in the mantle is known as magma, as often pictured spewing from an active volcano. Liquid magma has a lower density than the solid rock around it, so it tends to move upwards towards the earth's surface. The majority of the time magma stays below the earth's surface and heats up the surrounding rock and any pockets of water that come in contact with it. Geothermal systems typically take advantage of water tables that are heated by the magma.
There are two types of energy that can be obtained from the earth: earth energy and geothermal energy. It is important to note, however, that in many cases Geothermal Energy is the term that is generically used for either type of energy and little or no distinction is drawn between Earth and Geothermal Energy. From my investigations the best distinction between earth energy and geothermal energy is the difference between making use of water that is heated by temperatures in the earth (geothermal) vs. using the temperature of the earth or water, to heat/cool water in a closed system using a heat pump (earth energy).
As mentioned above earth energy uses temperatures found in the earth or water to cool or heat air and water for buildings. Since the temperature of the earth, only a small distance below the surface, remains a relatively constant temperature year round (45-58ºF or approximately 7-13ºC) a heat pump can be used to extract heat from underneath the ground to heat a building. In the summer, the pump can be reversed to provide air conditioning by moving hot air out of the building and down into the ground.
It's more efficient to use earth energy than it is to use a combustion furnace. That's because it requires less energy to move heat from one place to another than it does to convert one kind of energy into another, which is what a furnace does. Earth energy systems can also significantly reduce the emission of greenhouse gases.
Geothermal energy uses steam or hot water in the earth's crust to power turbines to generate electricity or to heat buildings. If the local geography has the right features, geothermal facilities can be installed which are very environmentally friendly. There are basically 3 types of geothermal power generation systems that can be deployed depending on the characteristics of the geothermal site.
The first system is known as a “Dry Steam Reservoir”. A Dry Steam reservoir produces very little water but generates steam which can be piped directly into a steam power plant to provide the necessary power to turn a turbine. One example of a very successful Dry Steam system is known as The Geysers located north of San Francisco. This system has been in production since 1960 and is a great example of Geothermal Dry Steam technology.
The second system is known as a Hot Water Reservoir. A hot water reservoir, which has a water temperature somewhere in the range of 300-700°F, is a pool of hot water located below the earth’s surface which is heated by the transfer of heat from the core of the earth. Hot Water reservoirs are used in what is called a “Flash” power plant. As water is brought up from the hot water reservoir into the plant the change in pressure causes some of the water to “flash” into steam. This steam is then used to turn a turbine and generate electricity. The water that isn’t flashed is then returned back to the hot water reservoir.
The third type of geothermal system is known as a “Binary” power plan. In a binary power plant water that does not reach temperatures sufficient to be classified as a “Hot Water Reservoir” (typically 250-360°F) is brought up from a geothermal reservoir and is passed through a heat exchanger. The heat generated is then used to heat a second “binary” liquid which has a lower boiling point than water and which will subsequently flash into vapor as a result of it’s lower boiling point. This vapor is then used to turn a turbine to generate electricity. The vapor is later condensed and used again. As this is a closed loop system, where none of the gasses from the geothermal reservoir are used directly, there are no emissions in a Binary power plan.
CO2 emissions can vary from plant to plant depending on the characteristics of the reservoir fluid used in the geothermal system and the type of technology used in the power plant. As an example, a typical 100 MW plant will reduce CO2 emissions by 600,000 tons/yr, and NOx and SO2 emissions by 120,000 tons/yr compared to a natural gas plant of equal size1. This reduction would be even higher when compared with the emission levels of a coal fired generating station.
Geothermal energy is used widely in the Philippines, Italy, Indonesia, Mexico, New Zealand, Japan and China. Iceland relies on geysers as its principal source of heat. Several northern communities around the world circulate this type of heated water through pipes under roads to melt ice from the pavement, and the water is also used in aquaculture, car washes and similar applications.
According to information published by the BC Sustainable Energy Association in 2004 at that time there was 9600 MW of geothermal energy produced on a global level. Geothermal energy was commercially viable in 24 countries around the world and US geothermal plants along the West coast produced ~2800 MW of electricity. The Philippines is the second largest producer of geothermal energy worldwide, with 1900 MW. According to the Philippine Department of Energy, an additional eight geothermal power plants are to come on line by 2010, with a further 621 MW of capacity.
Another use of Geothermal energy is known as a “Direct Use”. In direct use systems, geothermal waters typically ranging from around 50-300°F are used directly to provide soothing heat for health spas (i.e. hot springs), to provide ground heat for crop growing and to be pump through buildings for heating. There are even examples of direct systems, like the one in Klamath Falls, Oregon, in which geothermal water is pumped under roads and sidewalks to keep them from freezing over during the cold winter months.
Benefits of Earth and Geothermal Energy
The operating costs of earth energy systems (geothermal heat pumps) are much lower than the cost to operate a combustion furnace with an air conditioning unit. However, the cost to install a complete earth energy system can be higher than the cost to install furnace and air conditioning unit. On average, according to the Canadian Renewable Energy Network (www.canren.gc.ca) an earth energy system can save two-thirds of the cost to heat and cool with electricity.
Earth energy can provide heating in winter, cooling in summer, and year-round hot water for home use. A single system performs all necessary functions and requires only a flick of a switch to reverse the unit for a seasonal change.
Geothermal systems can greatly reduce greenhouse gas emissions compared to similar systems that use fossil fuels.
Earth energy systems can deliver heat to one room and simultaneously provide cooling to an adjacent room. This is extremely useful in institutional buildings such as schools.
According to studies done by the University of Erlangen, geothermal plants can be online an average of 97% in comparison with Nuclear (average online time of 65%) and coal based (average online time of 75%).
1 Western GeoPower Corp, 2003