Ultra deep geothermal power plant

This is a proposal to exploit the geothermal heat source in full to generate electricity in the range of Gig watts deep down and for the way to get there. Instead of vertical drilling a sloped tunnel is used to get to deeper layers of the earth. Advanced tunnelling machines as used for the 50 km long Gotthard Tunnel are able to reach lengths and depths never possible before. The necessary deepness (about 10 – 20 km) is determent by the temperature of the rocks. The hotter rocks provide a higher efficiency of the Carnot process. An array of smaller tunnels reaches out to a great volume of hot rocks providing a huge flow of heat.

The installation of the steam turbines together with generators under ground prevents energy losses of the steam on the long way up. After passing the turbines the steam rises up in a second tunnel as in a chimney, cools down, condensate to water, runs down in pipes and starts the entire process again. The high gravitational pressure (~1000bar) of the water may not only be used to press it into the rocks, but also for high pressure water turbines, to drive tunnel-drilling machines along with any machinery with water hydraulic motors. The gravitational pressure gain of any fluid may also be used for a huge cooling system – all free of charge.

The high building costs will be rewarded by the greatest sustainable power plant with the smallest impact on the environment. It can be build in any country making them more independent from oil.

Today’s power plants utilizing geothermal heat are placed on the surface of the earth and utilize 2 small holes for water downwards and steam upwards, only about 3 km deep. To do this that close under the surface is not only inefficient, but may also result in deformations of the surface by changing the pressure down there in a greater volume lifting or lowering the earth surface or even both on different places, which not hydraulically communicate.

The drilling of small holes has big disadvantages; the drill head must be attached to the drilling rig on the surface and the torque must be transmitted over a very long distance. The bore head can only be repaired by taking it out all the way. There are also not yet shaft drilling machines for vertical drilling to 10 – 20 km.

The temperatures are not everywhere high enough for a good efficiency of the Carnot process. The temperature rises about 20-30°C every kilometre down, somewhere even less. There are not accurate numbers available. The greater the temperature and pressure of the steam the greater is the efficiency of the turbines, therefore we have to go deeper. Down there the turbines will be also more powerful, because of a greater volume of steam generated. The surface of the hot rocks can be made larger and the temperature gradient is higher; subsequently the flow of heat is higher. To prevent heat losses the turbines must be installed near the heat source down there.

Furthermore the turbines on the surface of the earth can’t utilize the gravitational energy of the water running down in pipes to great depths reaching a pressure of 1000 bar at 10 km.

To reach greater depths advanced tunnel-drilling machines with water hydraulic motors are utilized like the ones for the Gotthard tunnel with a length of more than 50 km. They are able to make tunnels of any given length and are already able to work at an angle of 30° downwards. The same tunnel with the slope of 30° will do – even with a smaller diameter as long the big steam turbines fit in. The new deep drilling technology is practical already there, only never used to go deep in the earth. The 30° slope is not necessary for the function of the deep power plant, but it is convenient to drive down on a robust rack railway providing transportation to and from the power plant. 2 parallel tunnels as the Gotthard tunnel has, are advisable, one for humans and one for the steam. The deeper the stronger must be the armor of the tunnel. The drilling will be stopped if the temperature is high enough to transform water into steam of about 500°C or if the tunnel machine melt’s down – a sure sign that it is hot enough.

In a higher and cooler region, just tolerable for humans due to a huge cooling system, a horizontal tunnel is drilled. High-pressure water turbines, steam turbines with generators, transformers etc are installed – even hotels with a view to a small see – the deepest on earth.

It would be a hydroelectric power plant with a drop height of about 10 km! Only the volume of the water is limited.

The sloped tunnel goes further in deeper, very hot regions, possible with a smaller diameter. The tunnelling machine must work remote controlled without any humans present. It will be driven by water-hydraulic motors (Wolfhart principle). Everything is heavenly water-cooled. Drilling in such deepness cost no energy any more. This machine could even work under water and remain on the ground of the tunnel.

The hot region will be utilized to transform large amounts of water into steam, which is collected in the lower part of the tunnel hold down by a strong cover plate hold down by an adjustable water column over the cover plate. A 2 km water column will balance 200bar steam pressure. The over heated high pressure steam goes though a pipe to the steam turbines in a giga watt range. After passing the turbines the steam rises upwards in a big chimney or sloped tunnel respectively a long way where it cools down and condenses on the end to water again. The long tunnel has its own weather system. Most of the water will be collected under a condenser and flows downwards in a long pipe, wherein the pressure gradually rises. Gravity generates enough pressure for injection it in the hot rocks. There is no water pump necessary. There is a second reason for it, because in contrast to a small bore hole filled with water the tunnel brings the low air pressure to the rocks below – for instance only 200 bar instead of 2000bar pressure. A tunnel is building up a low-pressure zone in the rocks surrounding the tunnel. Cracks accelerate the process. But the likelihood of generating small earthquakes by Fracking is smaller in this deepness, because of the greater distance to the surface of the earth and the greater softness of the rocks.

A greater temperature gradient in a larger volume will generate more steam. Several smaller tunnels and fracking may help to get enough steam. The hot zone with smaller tunnels looks like a root system of a giant tree. The catchments area for the heat is practical unlimited. The amount of generated electrical energy depends on the amount of steam generated per second. It depends on the dimensions of the volume of the reachable pores hot rocks, the temperature gradient and the heat conductivity of the rocks itself. The rocks may have an un-cooled temperature of about 800° C what falls in a more manageable temperature if cooled by water. The hotter the rocks are, the better is the entire process. The deeper the tunnelling machines can go, the better will be the power plant.

This power plant is even more powerful than nuclear power plants. In principle it is also nuclear energy, but made in the core of the earth – ergo not harmful for humans. It needs nothing from the surface of the earth, except water to start the process. It is a circulation of water/steam between hot rocks and colder regions. The deeper it is, the greater the difference in temperature, the better the efficiency.

If all steam condenses to water no new water is needed for the turbines. Depending of the collecting high the water pressure will be different. The best scenario is to condense all steam short under the surface with help of a pipe coil heat exchanger. This would generate the highest water pressure down there. In deeper levels condense water will also be collected. Every collecting inlet has it’s own pipe. Even only 2 km over a turbine the incoming water has a pressure of 200bar.

The described water/steam/water circulation is sustainable for a very long time, even during an ice age. It needs only a cold region over a hot one to generate electricity. That’s all.

1 – main tunnel; 2 – 3 – horizontal tunnel; 4 – smaller tunnel; 5 – screen; 6 – river; 7 – main water pipe; 8 – water turbine; 9 – nozzle; 10 – cooling grid; 11 – surface of the earth; 12 – steam turbine; 13 – hot rocks; 14 – water pipe to the rocks; 15 – small see; 16 – cooling pipes; 17 – hermetic door; 18 – steam pipe; 19 – cover plate; 20, 21 – hotel; 22, 23 – drilling machines; 24 - pump

Referring to Fig 1, there is shown a schematic cross section of the entire creation, but the sketch is not true to scale.

There is a 50 km long tunnel 1 like the Gotthard tunnel, but with a slope of 30°.

At a deepness of 10 - 20 km – depending on the local temperature – is a power plant with many turbines in horizontal tunnels 2 and 3. Even deeper is a smaller branch 4 of the main tunnel made by remote controlled tunnel machines. The region 13 of the pores hot rocks is un-cooled over 800° C , but will get cooler if pressure water is injected through the pipe 14 and an array of nozzles 9. The hot pressurized steam is collected in the lower portion of the tunnels 1 and 4. A heavy cover plate 19 separates the hot steam from the rest of the tunnels. Water will fill up a portion of the tunnel over the cover plate. This helps to hold the cover plate in place against hot steam of about 200bar pressure. At a thickness of 2 km water would be a balance of all forces. There is a pipe connecting both deep sees for spill over. To lower the water level there is an extra pump 24 to press the water into the hot pressurized region back in the circulation. This pump runs only if needed. The power plant could go on generating electricity only with the water in the own deep sees. The condense water from above has enough pressure to penetrate the hot rocks even with the detour over the water turbines 8. If the condensation in the long tunnel is complete, no water from above is necessary any more. Harmful things from the rocks go back where they are coming from.

The process is very simple. Firstly clean water from a river 6 is getting through a screen 5 and runs without any pump through an about 50 km long pipe 7 gaining gravitational pressure of about 1000bar at a deepness of 10 km feeding the high pressure water turbines 8 or water hydraulic motors (Wolfhart Principle). With a rest pressure of about 250bar the water is running through the pipe 14 and is injected with nozzles 9 into hot rocks 13 where the water is changed into steam and collected under the cover plate 19. A pipe 18 feeds the steam turbines 12 with about 170bar at 580° C - each generating about 250MW electricity like the SST900 from Siemens. They are 11 m wide and fit in the tunnels. They drive down on a rack railway very slowly. Good brakes are essential, otherwise the heavy load will run down in the hot rocks and never seen again.

After passing the steam turbines the steam rises up in the tunnel and cools down. At last it will condense in a heat exchange system 10 close under the surface of the earth 11. The condense water will be collected and runs down again a long pipe and feeds first the water turbines again and then it is changed into steam again. In the best-case scenario no water from the river is necessary any more. It is a closed sustainable circulation as in nature. The deep tunnel has it’s own weather system. There are no big pumps necessary.

Not 100% of the water can be recaptured under the heat exchanger 10, but in different heights other water collecting stations can be installed.

A big fan, driven by pressure-water may push the steam out faster and sucks simultaneously fresh air in through the other tunnel, what lowers also the air pressure down there.

The workers or intranauts respectively must work in the beginning under an elevated air pressure and in a hot environment. They look like astronauts in their cooling suits.

After completion strong hermetic doors 17 close the horizontal tunnels up. (There is only a small tunnel connecting both tunnels for the intranauts going around the sloped tunnel. It could be made also only one horizontal tunnel, but 2 are better for redundancy.) A comfortable climate is made behind these doors. Temperature is lowered to 28°C and pressure to 1bar normal air pressure.

Walls may be cooled by fresh water in pipes 16 from above or/and thermal isolated with heat tiles like the ones from the space shuttle. The electrical power cables are placed inside the cool water pipes for cooling purposes in the hot regions. A lot of ice may be used as for south Africa’s deepest mining operation. A strong cooling system could be made with the gravitational power for every fluid coming down the long pipes from above. It works like a big pump in an air conditioning unit, but free of charge.

A small see 15 holds fluctuations in temperature down. With fresh water it is even for swimming suitable. There is even a small beach in front of a kind of hotel “Deep View” 20 for intranauts – contrary to astronauts - with a view of the bright illuminated small see. In the other tunnel is another “hotel” called “hot rock” 21. It is also a shelter for the workers in an emergency with all live supporting systems. There will be only very few workers down there in normal times. They may stay over night in the hotels. It can be extra hermetic closed like in submarines. For redundancy every important installation is doubled. In future times most work will be done by robots or remotely.

To drill even deeper in the hot layers there are special tunnel drilling machines necessary. Water hydraulic motors drive them and everything is heavenly water-cooled. Even under water seems drilling possible. The tunnelling machines 22 and 23 remain on the grounds of the tunnels and go further if needed - if still running.

This technology will work already at much lower deepness, but not that efficient.

The simplest way would be to let the fresh water from the river run down permanently and let the steam come up to the power station on the surface of the earth as it is done in smaller versions without tunnelling. Some heat loss will occur for the steam coming up the long tunnel.

The USA produces now 100 000GW/h per year on geothermic energy, but they could produce 3.2 Trillion with this new technology what means 32 millions times more.