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Geothermal Energy-Current Status and Future Possibilities


Savanna Sampson, Carli Fenimore, and Colby Bernanrdo. Tandon School of Engineering- New York University


Abstract


The geology of our planet possesses an almost unlimited supply of energy that can be captured and used. The intense heat derived from the Earth – geothermal energy – is theoreti- cally more than adequate to supply humanity’s energy needs. Its widespread use would promote national security and help the United States and other countries become less dependent on fuels from often unreliable foreign markets. Geothermal energy has negligible environmental impacts. The concern over the rising level of Greenhouse gases in the atmosphere is driving industrial development away from reliance on fossil fuels and is providing an incentive for the use of renewable energy.


There are several limiting factors that inhibit widespread reliance on geothermal energy as a major player in world- wide energy production. Plant sites need to be placed where heat is most accessible: where the crust is thin – near plate boundaries – or at hot spots, places proximal to the surface and near existing energy infrastructure. The unique nature of each potential geothermal plant site poses a challenge to financing and cost projections.


The ongoing development of flexible geothermal power gen- eration and storage systems will allow geothermal plants to more efficiently capture the Earth’s heat and take advantage of the excess energy generated in off-peak hours. The disad- vantages do pose a meaningful threat to the more widespread use of geothermal energy. Regardless, current research and energy growth projections strongly indicate that within the next 50 to 100 years, geothermal energy will become a more substantial contributor to the world’s energy supply.


Introduction


Over the past 100 years the energy industry has fashioned innovative and creative alternative solutions for power sup- ply. Many of these developing technologies rely on natural renewable sources such as wind and solar. However, these and other potential new energy sources are not limited to those on or outside the Earth’s surface. Internally, the geology of our planet possesses an almost unlimited supply of energy (in the form of heat) that can be captured and used. This intense heat derived from the Earth – geothermal energy – is theoretically more than adequate to supply humanity’s energy needs. The reason why geothermal energy is such a tempting alternate power supply is because of its abundance and presence in many areas around the world.


Geothermal energy is ubiquitous because of the nature of the Earth’s crust and the geothermal gradient. Magma in the lower asthenosphere and mantle underlies the relatively brittle surface of the continents and oceans. At an average


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Figure 1. Global Geothermal Use.


temperature of 300 degrees Celsius (C), magma is capable of generating an enormous amount of heat. This heat is dissi- pated towards the surface at an average rate of 30 degrees C per km. So at relatively shallow depths (within 5 km of the surface) this heat can be captured and used in power generation. The most widely used method of geothermal energy exploitation is hydrothermal convection. This system is a basic loop where cool water is injected into the crust, then heats up to steam and rises to the surface where it is captured. The steam then is used in a pneumatic system to provide pressure to operate (turn) turbines which then generate electricity. The subse- quent condensate is then returned to the system and re-used.


How it Works


There are three distinct methods to capture geothermal energy: dry steam, flash, and binary (Goldstein and Hiriart). Dry steam is a technology where the steam is extracted directly via heating from a subsurface geothermal reservoir in order to power a generating turbine. The spent steam is condensed and re-used.


The flash steam method generates heat by varying pres- sures. If the pressure of liquid in the system - water at the boiling point - is lowered, the heat energy in the water is reduced to a level equal to the final pressure. The energy - or enthalpy - made available when pressure is reduced will evaporate some of the water, producing flash steam. However, only a portion of the condensate evaporates as flash steam. How much depends on the enthalpy in the condensate at the initial and the final pressures. Typically, significant amounts of make-up water need to be added to this type of system.


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