Course Syllabus (Contents Part 8) Geothermal Energy

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1 Course Syllabus (Contents Part 8) PART 8 GEOTHERMAL ENERGY Geothermal energy Typical geothermal gradient Global geothermal resource Plate boundaries & hot spots Major geothermal regions US geothermal resources Alaska s geothermal resource British Columbia s geothermal potential New Zealand s geothermal resource Iceland s geothermal resource World installed capacity & potential History of geothermal energy Physics of geothermal energy Aquifer-type geothermal reservoir Volcano-related heat sources Magma geothermal energy Geothermal well head Geothermal power plant Geothermal turbine Turbine installation Dry steam system Flash steam system Binary steam system Example power plants District heating District heating equipment Street & sidewalk heating Environmental considerations Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 197 Geothermal Energy The earth is still cooling from 4,600 million years ago when it originally formed, while decay of naturally occurring radioactive elements (thorium 232, uranium 238 and potassium 40) at the earth s core continues to generate heat equal to 50% of the losses through the crust. Geyser Hot spring Boiling mud The net result is that deep below the surface is a vast reservoir of molten rock at temperatures up to 12,500 F. This resource is spectacularly evident when magma erupts volcanically through a flaw in the earth s crust, or wherever hot water and steam emerge as hot springs and geysers. Whakarewarewa Thermal Reserve in New Zealand s North Island contains many hot springs Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 198 1

2 Typical Geothermal Gradient To explore a potential geothermal resource, a core hole is drilled into the lithosphere to different depths where temperature measurements are taken. Exploratory core holes are usually limited to a depth of around 3000 feet (1/2 mile), but geothermal extraction wells can go as deep as 1 ¼ mi. The huge difference between surface and interior temperatures drives heat flow out of the earth. The interior temperature gradient is fairly uniform (linear) because of convective mixing, but as the surface is approached the cooler rocks become too stiff to convect - heat is then transported by conduction, with the upper 2000 feet forming a fairly effective insulating layer. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 199 Global Geothermal Resource The thermal energy flowing through the crust is about 60 mw/m 2 as a global mean, but it is not uniform everywhere. As can be seen from the global heat flux map below, significant regions exist where the heat flow is higher than the mean, and visa versa. While small scale local variations in heat flux are undoubtedly due to rock chemistry and underlying strata, the large scale effects observed on the global map result from heat leakage through fissures and faults. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 200 2

3 Plate Boundaries & Hot Spots The earth s crust is like a thin, solid shell enclosing the fluid interior. However, this shell is cracked in several places, thereby forming plates, which are in motion relative to each other (1 10 cm/yr). Slip boundary Divergent boundary Convergent boundary Plate motion & speed Current major geothermal sites At the boundaries between these plates, heat is able to escape to the surface easier, resulting in hot spots and volcanic activity. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 201 Major Geothermal Regions The world s geothermal regions correspond to where tectonic plate boundaries coincide with the land masses. Although many countries possess geothermal resources, Iceland, New Zealand, USA (California, Nevada, Alaska & Hawaii) & Canada (British Columbia) have pushed ahead with installing significant geothermal energy conversion facilities. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 202 3

4 US (48 States) Geothermal Resources The west coast boundary between the North American and Pacific plates is "sliding" along the San Andreas fault from the Gulf of California up to northern California, producing frictional heating and fissures where geothermal heat flux is concentrated. Click here for all 50 states There is also a volcanic hot spot under Yellowstone and intraplate extension with hot springs in the Great Basin. California currently generates the most geothermal electricity in the world. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 203 Alaska s Geothermal Resource The North American and Pacific plates are subducting from the Cascade volcanoes north through the Aleutians, providing Alaska with many geothermal springs. Unfortunately the majority of the high temperature hot springs suitable for direct electricity generation are located along the Aleutian archipelago, which is sparsely populated and remote. However, even the lower temperature geothermal resources of Alaska are well suited to heating homes a resource that the native Inuit and Aleut population has exploited for centuries. Fairbanks is well served by numerous hot springs in the interior. Spring water temps at surface: < 200 F > 200 F Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 204 4

5 British Columbia s Geothermal Potential - Hot Spring Hot water or steam in excess of 200 C directly convertible to electricity. Gradient heat up to 200 C with temperature increasing at about C for each 1000 m depth temperatures suitable for indirect conversion to electricity Gradient heat up to 100 C with temperature increasing at about C for each 1000 m depth temperatures suitable for directly heating homes British Columbia s hottest geothermal region in the south is well placed to serve the major metropolitan areas of Vancouver, and Seattle. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 205 New Zealand s Geothermal Resource New Zealand is an island country in the southern Pacific Ocean southeast of Australia. Discovered by Abel Tasman in 1642, the islands were explored by Capt. James Cook four times between 1769 and The first settlements were made in This small Pacific "ring of fire" country lies along the southeast subducting boundary of the Pacific plate. As a consequence it has many hot-spring areas and several active volcanoes. In the early 1950s, with no oil and little hydropower on the North Island, geothermal development was started at the Wairakei field. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 206 5

6 Iceland s Geothermal Resource Iceland is an island on the Mid-Atlantic Ridge rift zone. Iceland sits squarely where the Eurasian and North American crustal plates are born in volcanic fire. From the earliest settlement (ninth century) crops were planted in naturally-heated ground for rapid growth and early harvest. Iceland has many high and low temperature geothermal systems, which are used to produce over 50% of the county s electrical needs. Iceland is also fortunate to possess abundant hydropower (providing 20% electricity) Geology & Geothermal Resources of Iceland High temperature fields Main low temperature fields Neovolcanic zone Rocks 1-3 million years Rocks older than 3 million years Holocene sediments Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson Stored heat X J The youngest rocks appear in the middle, and as the plates move apart the oldest rocks end up at the eastern and western ends of the island. In addition to volcanoes and geysers, Iceland has glaciers and ice caps World Installed Capacity & Potential Press a to get text details about that region or for graphical statistics. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 208 6

7 History of Geothermal Energy The exploitation of geothermal resources dates back to Roman times, when spa towns clustered around natural hot springs using advanced plumbing for their time years before the arrival of Europeans, the Polynesian settlers in New Zealand depended on geothermal sources for cooking, heating, bathing and therapeutic purposes. Europeans developed the Wairakei field in the 1950 s, but this was only the second in the world.generation of electrical power started in Italy in 1904, fostered by Prince Piero Ginori Conti, resulting in the arrival of the Larderello power plant in Larderello plant at its early stage (Left) and its modern day form (Right) Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 209 The Physics of Geothermal Energy Geothermal resources become exploitable when they form some type of accessible heat reservoir. In the example shown, rain water is able to get below an impermeable cap rock layer by running down natural fissures in the rock strata. The water is heated by a subterranean hot spot, causing it to rise up under pressure. Fall 2005 Alternative & Renewable Energy USF Class Notes - Stuart Wilkinson 210 7