Tag Archives: geothermal

Private Sector must be the Answer

In Al Gore’s award winning movie “An Inconvenient Truth” he used the old saw to depict a real problem with global warming. If you put a frog in hot water it will immediately jump out. Put a frog in cold water but very slowly warm it up and the frog will stay until it is too late and be boiled alive.

That is a nice analogy for the dilemma we face with with global warming. The process is slow. Another analogy would be to call it glacially slow, but glaciers are moving, and melting, at a fairly rapid pace these days. Humans and a number of animals evolved to react to rapidly occurring threats – the snap of a twig in the brush, the glint of light from an eye, and we are ready to fight or flee.

Global warming is a decades to centuries change that threatens us now, and many just don’t see the threat, a threat not to us individually, but to our future. Some are so insensitive to the risk that even if they believe it to be true, won’t react because it doesn’t matter to them personally. If the majority of us hold this opinion, we are doomed as a species.

Some governments are beginning to react with policies that favor carbon free energy strategies, but the steps are often small and can be more costly than simple business as usual burning of fossil fuels. Hey, it’s on face value cheaper and we know how it works.

On a more hopeful note is the fact that technology got us into this problem, but technology and the private sector, hold the potential to get us out. Obviously we need to stop burning fossil fuels, especially coal and oil. Natural Gas, essentially methane, is does not produce as much pollution as the others, but ultimately its use must be curtailed also.

There two ways to replace the fossil fuels, use less through efficiency and replace energy production with non-carbon sources such as wind, solar and geothermal. Of the three, wind is the most developed. We currently get about 4 % of our electric energy production from wind, entirely land based. The potential for off shore wind, especially on the east coast affords considerable potential but currently is more expensive to exploit than wind resources in the midwest. Currently the cost of wind generated power is as cheap as that from a modern coal fired plant. And the costs continue to decline, the opposite of the cost for producing power from coal.

Solar Photovoltaic systems (solar panels) are sprouting up everywhere, especially since the price has dropped by half in just the last few years. Not only are homeowners adding panels to their roofs but utility scale systems are being installed. Entergy recently announced that they intend to build a 500 acre solar farm near Stuttgart. For perspective, a square mile covers 640 acres.

Until the intermittent energy sources of wind and solar penetrate to about 30% of total production, no additional back up power is needed. Essentially there is enough existing reserve power to keep the lights on after dark when the wind isn’t blowing. Beyond that, battery backup will be needed. Development and deployment of utility scale battery production will surely follow the demand.

Geothermal Heat Pumps

The term geothermal when applied to heat pump technology means that the ground is used as a heat exchange medium, rather than the air. Heat pumps are nothing more than reversible devices to heat and cool a home.

The technology is the same as refrigeration. When a gas expands, it absorbs energy from the air, thus cooling the surroundings. Refrigeration works by using a pump to compress a gas, called the working fluid. The compressed gas, now a fluid, is moved to the area to be cooled and then allowed to expand. The heat being moved by a heat pump is expelled away from the area to be cooled. For most systems, “away” is the air outside the house.

The hotter it is outside in the summer, the harder a heat pump has to work to cool your home. This is where the ground comes into play. Geothermal heat pumps use the ground as “away”. The heat exchanger portion of a geothermal heat pump is connected via wells drilled or lateral lines on the property to water or some other liquid which transfers the heat to the much cooler ground, rather than the much warmer air. This process is more efficient at moving heat, and therefore summer cooling costs are lower.

The process is reversed in the winter. Compressing a gas inside a home produces heat, then the compressed gas is moved out of doors and allowed to expand and cool out of doors. Heat pumps are quite efficient at heating in the winter as long as the temperature is not too low. The colder it gets the less efficient the system. For a geothermal heat pump, it doesn’t matter what the air temperature is because the heat exchange is with the ground which is about fifty to sixty degrees Fahrenheit year round. In the winter the ground is warmer than the air so geothermal heat pumps are more effective than traditional air source systems.

Overall geothermal heat pumps are more efficient than normal air source heat pumps, particularly during the temperature extremes of summer and winter. In a study at Fort Polk Louisiana, heating costs during winter days below freezing were forty per cent lower. During the summer days with the temperature over ninety degrees, the costs of cooling were also about forty percent lower with the ground source heat pumps compared to air source heat pumps.

Heat pumps work best when the difference between the outside and inside temperatures are not great. Geothermal heat pumps take advantage of the more stable ground temperature, keeping the difference lower than for air source heat pumps.geothermal_heat_pump2

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Geothermal systems require the drilling of wells or laying lateral lines to create the ground contact so the systems are inherently more expensive that simple air source systems, but because of their greater efficiency, usually have payback periods on the order of five to ten years.

Geothermal Energy

The term geothermal has come to be associated with two technologies which are only tangentially related; first, power can be produced by drilling into the ground to a depth where the rock is hot enough to boil water. The other use of the term geothermal is associated with ground source heat pumps which need only drill down a few feet to a temperature of fifty to sixty degrees Fahrenheit.
 
Utility scale power can be produced by drilling into the ground to a depth where the rock is hot enough to boil water to produce steam. The steam is then used to drive a turbine to generate electricity just as a nuclear reactor or a coal fired power plant produces steam to turn turbines. Electricity production from geothermal heat requires drilling several kilometers into the earth and is consequently very expensive, but in certain locations heat is near enough to the surface to make its utilization practical.
 
Heat at the core of the earth is approximately 6000 degrees Celsius, hence a temperature gradient exists: twenty five degrees C per kilometer. The heat is due to at least two factors, residual heat from the accretion of the planet over four billion years ago and radioactive decay of certain elements such as Uranium and Thorium.
 
To economically produce power, hot rock must be within three or four kilometers of the surface. This only occurs in geologically active regions, such as areas with earthquakes and/or volcanoes. In these locations fissures in the earth’s crust allow movement of magma near enough to the surface to be exploited for power production.
 
The simplest design for a geothermal power plant takes advantage of hydrothermal convection. Cool water from the surface seeps underground, is heated and then rises back to the surface. The heated water, now steam, is obtained by drilling wells to capture the steam and directing it to turbines for energy production. The water from the condensed steam can then returned to continue the cycle.
 
Although the heat is essentially free, the cost of drilling and maintenance of equipment can be high. Subterranean steam extracts caustic materials which corrode even the most inert metals. A limiting factor for energy production can be the rate of heat transfer through rock. As heat is extracted from rock surrounding the well site, heat must be transferred through the rock, limiting the rate of heat extraction.
 
The United States leads the world in geothermal electric capacity. The US has about 2.7 GigaWatts installed, a quarter of world capacity. Twenty six plants in one location called the geysers,

geysers north of San Francisco, accounts for three quarters of the total US production. For comparison, one nuclear reactor has a capacity of just under one GW.
 
Parts of Alaska, Washington, Oregon, California, and much of Nevada and Hawaii have potential for geothermal electricity production and much of the Rocky Mountain area could extract useful heat for direct uses such as space heat for apartment buildings, schools, and other large facilities.