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Geothermal systems
and heat exchangers

a briefing document

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Geothermal systems and heat exchangers is a sub-document to Distributed energy systems and micro-generation, and part of a series of briefing documents on the problems of power consumption, posed by the steady depletion of fossil fuels and most particularly of pumpable oil.
One of a grouping of documents on global concerns at abelard.org.
on energy on global warming
sustainable futures briefing documents

On housing and making living systems ecological

Tectonics: tectonic plates - floating on the surface of a cauldron

Geothermal systems and heat exchangers
geothermal basics
in praise of a geothermal system
the benefits of a geothermal system
on heat exchangers
on heat pumps
some sources of geothermal heat
using volcanic activity to generate hydrogen from water
geothermal hot spots for energy generation

site map

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geothermal basics

  1. Geothermal means extracting heat from the Earth.
  2. The heat is then transferred to the place of work.
  3. Sometimes that heat is concentrated by heat exchangers.
  4. The ‘work’ to be done by the heat varies.
  5. The heat comes from two directions
    1. from the sun (a fusion source)
    2. from the fission at the centre of the Earth
  6. Temperatures in the ground, and the heat available, vary with depth.
    Temperature further differs according to subsurface/rock conditions, as well as surface conditions such as closeness to the equator (latitude). For instance, the below-surface heat can be a few feet down in temperate zones, or thousands of feet down in permafrost zones. Wheras, in Iceland, the heat from the depths of the earth can be bubbling to the surface.

The term ‘geothermal’ is sometimes used in a confusing manner, because there are two common approaches to heat extraction from the Earth in a geothermal system:

  • Local, private systems;
  • community generation.

Geothermal heat can be extracted from deep bores, or with horizontal systems that are only a few feet below the ground’s surface. In this briefing document, we are most interested in shallow-level systems with heat exchangers (heat concentrators). If there is enough room, the pipes, often in coils, are spread out in shallow trenches as described in the article at Parris puts in a heat pump.

in praise of a geothermal system

“Well, the news is that it works. The system is simple to operate, reliable, responsive, quiet and efficient, the house is warm as toast, and the electricity bill (I used to have storage heaters) is plummeting. But the pump cost about £7,000 and the groundworks cost as much again. The planning bureaucracy was irksome and the mess horrendous. It was not strictly an economic decision. I did it out of interest, and because it's good to be a pioneer.”

Marker at abelard.org

In situations where land area is limited, an alternative solution is to bore down a few hundred feet, as in following project:

the benefits of a geothermal system - recommended reading

“None of the other options I looked at compare to geothermal. A geo system is so efficient that for each unit of energy put in, it gets out three to five units of energy! Magic! It gets out five times as much energy as a conventional high-efficiency furnace! Putting geo into a typical home is, from a carbon perspective, equivalent to planting an acre of trees, or taking two cars off the road! What technology can compete with that?! The holy grail of heating and cooling, I say.”

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heat exchangers

Heat exchanger systems can also be used with an air exchange unit, and are often reversable to provide cooling in hot weather. These are similar to the boxes often seen hanging outside blocks of flats and offices in warmer areas. Other heat exchangers use bodies of water, even swimming pools or specially built underground systems as heat sources.

Heat exchangers usually provide between three and five units of heat for one unit of input electricity, according to efficiency, and require more energy the greater the temperature differential to be achieved.click to return to index on Geothermal systems and heat exchangers briefing document

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on heat pumps

“Any appliance that takes heat from one area and moves it to another is a heat pump. [A refrigerator is a type of heat pump.]

“Most heat pump installations involve what is called a split system. The outdoor unit contains the compressor and a heat exchanger, called a coil. The indoor unit contains another coil, a fan that blows air through your duct system, grille, and electric heating elements.

“The outdoor and indoor units are connected by copper tubes that move a gas refrigerant (such as Freon) between the indoor and outdoor coils. This refrigerant has the ability to absorb heat from the air, even at very low temperatures.

“In the winter, the refrigerant absorbs heat from outdoor air drawn across the outdoor coil. The refrigerant becomes hot but is made even hotter (in excess of 140 degrees F) by going through the compressor.

“This hot gas travels through a copper tube to the indoor coil. The fan draws air through your return grille and pushes the air across the indoor coil. The hot gas transfers its heat to the air blown across the coil and into the duct system.”

heat pump efficiency

The performance of a heat pump is often shown as the COP or Coefficient of Performance. This relates to the amount of energy that is extracted for each unit of energy used to run the pump.

A heat pump could provide between 3 and 5 kW of heat for each kW invested. The return is effected by the temperature achieved from the source and the heat or cooling required from the appliance.

For example, if 3kW is used by a electrically driven heat pump and 9kW is produced by a ground source for heating, the COP would be 3.

click to return to index on Geothermal systems and heat exchangers briefing document

using volcanic activity to generate hydrogen from water

“Engineers on the project have calculated that increasing the temperature by 200 degrees and the pressure by 200 Bar will mean that, for the same flow rate, the energy extracted from such a borehole will go up from 5MW to 50MW.

“Power station manager Albert Albertsson predicts that, by the end of the century, "Iceland could become the Kuwait of the North". The vision is to use this cheap and carbon-free energy to split water, to yield hydrogen that could be despatched around the world in tankers.”

“Some 90% of all homes in Iceland are heated by geothermal energy; and a number of power stations are also producing electricity from steam at around 240C, extracted from boreholes between 600 and 1,000m deep.”

Remember that the centre of the Earth is hot, due to nuclear activity.

See also using nuclear power to generate hydrogen from water.

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geothermal hot spots for energy generation

“[...] the ratio of helium isotopes, can be used to identify areas with high resource potential for geothermal energy [...]”

Geo-thermic map of the USA, based on helium isotope measurements. Source: treehugger.com

click to return to index on Geothermal systems and heat exchangers briefing document


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the address for this document is https://www.abelard.org/briefings/geothermal.php

950 words
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