Ongoing volcanic activity in many parts of the world suggests a potential abundance of geothermal energy. There are locations where geothermal energy produces steam that directly and indirectly drives turbines and electrical generation equipment. The Top Energy group in New Zealand flows geothermal steam through heat exchangers to vaporize pentane that drives the turbines. The Raser group has developed a heat exchange technology to convert geothermal heat at near the boiling point of water to electric power. There is an evolving technology that shows the potential to convert even lower-grade geothermal energy into electric power.

Research undertaken by Dr. Jorg Schlaich in Germany revolves around using a physically very large technology that will operate at relatively low efficiency while being able to convert renewable thermal energy to electric power at very competitive cost per megawatt. His research forms the basis in the development of solar towers, a scale model prototype of which is being demonstrated as proof of concept near Seville in the south of Spain. That research and its proof of concept indicates that it may be adapted to generate large amounts of electric power from low-grade geothermal energy where the temperature levels are well below that of the boiling point of water.

Much of that low-grade geothermal energy can be found at deep in the earth in the permeable and porous rock of exhausted natural gas wells and exhausted oil wells. The rock temperature at the bottom of these wells can vary between 50 degrees C and 120 degrees C (120 degrees F to 248 degrees F) and that heat is accessible because in many parts of the world water is pumped into the wells to displace the residual oil or gas. The water becomes the medium by which to transfer heat to the surface using wells that had originally been drilled to extract gas or oil. The cooled water may then be returned to the deep porous rock via other nearby wells.

The Green Tower Group of Germany and South Africa as well as the Enviromission group of Australia and the United States propose to operate their solar towers 24 hours per day. They intend to do so by storing some daytime thermal energy in tanks and troughs of water. To access geothermal energy, the center of the tower would be built directly above a deep-drilled well of an exhausted deposit of natural gas. The air intake skirt of the tower would extend to include the return well located some 2 miles or 3 kilometers away. The water reservoir in the porous rock could be several thousand meters below ground surface.

Geothermally heated water at 60 degrees C to 80 degrees C would be pumped up one well and flow through a spiral pipe placed on the ground under the intake skirt before being flowed into the return well. The water would have over 845 times the density and over 4-times the specific heat of the intake air flowing under the skirt and toward the tower. The spiral pipe would heat the air current at a comparable rate of effectiveness as a cross-flow heat exchanger and becomes the trigger that sustains the air current in the tower.

The geothermal heat under the tower and its intake skirt would serve a similar purpose as the control valve at a hydroelectric dam, the trigger of a thyristor or the button of an aerosol can that requires very little energy to initiate the movement of a vast amount of energy. There would be much latent heat on the ground and on large bodies of water for several kilometers surrounding the air intake of the tower skirt. The tower would become the conduit through which air currents carry low-grade thermal energy to higher altitude.

Outside of tropical regions solar towers may operate on solar energy during summer and on other sources of renewable thermal energy during cooler weather, thereby improving the tower's cost competitiveness per unit output. During winter air at 10 degrees C or 50 degrees F could flow into the intake skirt and cool the water in spiral heater pipe from 80 degrees C or 176 degrees F down to 20 degrees C or 68 degrees F. The temperature or the air approaching the turbines built into the base of the tower could rise to over 40 degrees C or 104 degrees F.

Heating the air inside the tower would increase both the updraft and the air velocity through the turbines. Tower technology would allow greater extraction of thermal energy from saturated geothermal steam that is slightly above the boiling point of water. All of the latent heat of condensation would be transferred to the incoming air as the steam in the pipe system cools and is transformed into liquid water. A geothermal tower could operate on geothermal steam for the first several years after which it may then operate on low-grade geothermal energy for the next several decades.

The depth and extent of the porous rock in the earth below the tower could determine as to whether to extract only geothermal energy or whether to use the porous rock as a means of seasonal thermal storage. During summer heat from concentrated solar thermal energy or from large-scale air conditioning systems may be pumped into the porous rock using water pipe systems. This option would be viable if insufficient heat from molten magma deep in the earth is transferred into the porous rock during summer when the tower operates mainly on solar energy.

Low-grade geothermal energy could become available from closely spaced test wells that may be uncapped. Modern drilling technology may be used to drill an interconnecting lateral well at great depth through which water may flow between pair of wells that are located near a river or ocean. The difference in temperature to between the surface water and the deep water may energize a closed cycle heat engine that operates on a refrigerant such as ammonia or R138a as the working fluid. Such technology would be comparable to ocean thermal energy conversion technology.

Conclusions

An abundance of capped deep test wells exist worldwide and offer the opportunity to access geothermal energy at low cost. Most were drilled into the earth during an earlier period of oil exploration. Other similar deep wells were drilled into productive deposits of oil and natural gas that have since become exhausted. These wells also hold potential to provide abundant low-grade geothermal energy.

Solar towers, vortex engines and geothermal towers only need heat the air at the base of the towers to a few degrees above the prevailing ambient to generate a convection current of air that will flow up the tower. Despite low thermal efficiency, large towers can be designed to operate on a combination of geothermal and solar heat to improve the cost competitive per unit of power output against other renewable thermal technologies.