Geothermal energy comes from two sources:

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• Geothermal energy comes from two sources
• radioactive decay in the crust of the earth,
  (e.g. the uranium, thorium, and potassium content
  of rocks like granite), and
• conduction of heat trickling through the mantle
  from the earths core.
• The heat in the core is there because the earth
  used to be red-hot, and its still cooling down
  and solidifying
• Geothermal is an attractive renewable because it
  is always on, independent of weather or surface
  conditions
• Geothermal power stations can be switched on and
  off to follow demand.

core
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wikipedia.com
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• But how much geothermal power is available?
• Geothermal power varies between
• an ordinary location on the earths crust
• special hot spots like Iceland (figure 16.3).
• Hot spots are obvious places, but what about
  ordinary locations?
• The difficulty with making sustainable geothermal
  power is that the conduction of heat is very
  slow. If you try and extract the heat too quickly
  it will cool down the rock before the heat can be
  replenished from below.
• If you stick a pipe down a 15-km hole in the
  earth, it is easily hot enough to boil water
• If you could stick two pipes down, pump cold
  water down one pipe and withdraw hot water (or
  steam) from the other
• But after a while, you will reduce the
  temperature of the rock, and the heating effect
  will diminish.
• You now have a long wait before the rock at the
  bottom of your pipe warms up again.

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Geothermal power that would be sustainable forever
In a typical continent, the heat flow from the
center coming through the mantle is about 10
mW/m2. The heat flow at the surface is 50 mW/m2
0.05 W/m2 (compared to solar radiation of 100 W
/ m2) so the radioactive decay has added an extra
40 mW/m2 to the heat flow from the centre. At a
typical location, the maximum power we can get
per unit area is 50 mW/m2. But that power is
not high-grade power, its low-grade heat thats
trickling through at the ambient temperature up
here. To make electricity, we must drill down
and use a source that is at a higher temperature
than the ambient temperature.
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Geothermal power that would be sustainable forever
The temperature increases with depth as shown
in figure 16.4, reaching a temperature of about
500 ?C at a depth of 40 km. Between depths of 0
km where the heat flow is biggest but the rock
temperature is too low, and 40 km, where the
rocks are hottest but the heat flow is 5 times
smaller (because were missing out on all the
heat generated from radioactive decay) there is
an optimal depth at which we should put a pipe.
The optimal depth depends on what sort of power
station machinery we use. The maximum
sustainable power is fixed by finding the optimal
depth assuming that we have an ideal engine for
turning heat into electricity, and that drilling
to any depth is free.
radioactivity
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Maximum Geothermal Power For the temperature
profile shown in figure 16.4, DM calculated that
the optimal depth is about 15 km. Under these
conditions, an ideal heat engine would deliver
17mW/m2. At the world population density of 43
people per square km, thats 10 kWh per person
per day, if all land area were used. In the UK,
the population density is 5 times greater, so
wide-scale geothermal power of this
sustainable-forever variety could offer at most 2
kWh per person per day. This is the
sustainable-forever figure, ignoring hot spots,
assuming perfect power stations, assuming every
square meter of continent is exploited, and
assuming that drilling is free. And that it is
possible to drill 15-kmdeep holes.
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• Geothermal power as mining
• In enhanced geothermal extraction from hot dry
  rocks (figure 16.5), the steps are
• drill down to a depth of 5 or 10 km, and fracture
  the rocks by pumping in water. (This step may
  create earthquakes, which dont go down well with
  the locals.)
• drill a second well into the fracture zone.
• pump water down one well and extract superheated
  water or steam from the other.
• This steam can be used to make electricity or to
  deliver heat.
• The biggest estimate of the hot dry rock resource
  in the UK could conceivably contribute 1.1 kWh
  per day per person of electricity for about 800
  years.

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Practical example Southampton Geothermal
District Heating Scheme was, in 2004 at least,
the only geothermal heating scheme in the UK.
It provides the city with a supply of hot
water. The geothermal well is part of a combined
heat, power, and cooling system that delivers hot
and chilled water to customers, and sells
electricity to the grid. Geothermal energy
contributes about 15 of the 70GWh of heat per
year delivered by this system. The population
of Southampton at the last census was 217 445, so
the geothermal power being delivered there is
0.13kWh/d per person in Southampton.
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World and US Geothermal Power
Geothermal electric power plants have been
limited to the edges of tectonic plates until
recently. In the US this means the West coast
(California to Washington).
wikipedia
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• Hydrothermal Power Systems
• There are three geothermal power plant
  technologies being used to convert hydrothermal
  fluids to electricity.
• The conversion technologies are
• dry steam,
• flash, and
• binary cycle.
• The type of conversion used depends on the state
  of the fluid (whether steam or water) and its
  temperature.

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Dry steam power plants systems were the first
type of geothermal power generation plants built.
They use the steam from the geothermal
reservoir as it comes from wells, and route it
directly through turbine/generator units to
produce electricity. Flash steam plants are
the most common type of geothermal power
generation plants in operation today. They use
water at temperatures greater than 360F (182C)
that is pumped under high pressure to the
generation equipment at the surface. Binary
cycle geothermal power generation plants differ
from Dry Steam and Flash Steam systems in that
the water or steam from the geothermal reservoir
never comes in contact with the turbine/generator
units.
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In 1960, Pacific Gas and Electric began operation
of the first successful geothermal electric power
plant in the United States at The Geysers in
California. The original turbine lasted for more
than 30 years and produced 11 MW net power. In
2008, the field supported 15 plants, all owned by
Calpine, with a total generating capacity of
725 MW, making them the largest geothermal
development in the world
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• A geothermal heat pump or ground source heat pump
  (GSHP)
• This is not the same as geothermal power itself
  (which is generally for deeper systems).
• Refers to a central heating and/or cooling system
  that pumps heat to or from the ground.
• the Earth is a heat source (in the winter) or a
  heat sink (in the summer).
• moderate temperatures in the ground boost
  efficiency and reduce the operational costs of
  heating and cooling systems, and may be combined
  with solar heating to form a geosolar system with
  even greater efficiency.
• the core of the heat pump is a loop of
  refrigerant pumped through a vapor-compression
  refrigeration cycle that moves heat.
• seasonal variations drop off with depth and
  disappear below seven meters due to thermal
  inertia (in Missouri, the frost depth is about 2
  meters).
• even shallow ground temperature is warmer than
  the air above during the winter and cooler than
  the air in the summer.
• These systems are covered in Chapter 21 Smarter
  Heating

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Ground source heat pumps work in a similar manner
as air source heat pumps, minus the high cost. A
typical household can save 1500 a year or more.
This can give most systems a payback period of
three to five years. GSHP's are more than three
times as efficient as the most efficient fossil
fuel furnace. They deliver three units of energy
for every one unit used to power the heat-pump
system.
Typical horizontal-loop geothermal installation
includes a heat pump in conjunction with a
forced-air system and water heater. Piping loops
in the ground draw latent heat to the house in
winter. In summer, the system carries excess
house heat to the ground. (Popular Mechanics,
1998)
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