Welcome, from sunny Australia!
The Geothermal Power Plant in more detail

Quick link to -
- Ok, how the whole thing is supposed to work ...
- Construction
- How much power?

I sat down a short while ago, (as I write this) and decided to figure out just how to make the turbine method of power generation actually work, rather than just give a general description on the previous page. I thought it wouldn't be that hard to do, but after a fair amount of thought I realised that there are difficulties with having a closed system and also getting a decent pressure differential to make the turbine work well. (enough)
So, I had to have a bit of a re-think, and this is what I came up with ...
The main shaft would be the same diameter, but would be divided into two main sections, both vertically and horizontally. The shaft would have a smaller shaft one one side, for reasons that I'll explain shortly. The main shaft would also be divided up into the lower, power generating section, and the upper, water condensor/collector section.
The basic principle is exactly the same, but the way that the waste steam from the turbine is ducted is quite different - It now would travel up that secondary shaft, still picking up a little heat from the outside rock, until it finally reaches a point above the power generator section. I'll also explain why it works like this shortly.
The power section would be made from a number of small generator modules. Each module would have a large water tank, that would soak up the heat from the surrounding rock to the point where it would be giving off large amounts of steam. The 'boiler' would be sealed until it reaches a minimum of about 1,000psi, so that the steam pressure would run the turbine at high speed.
This is a simplified schematic of the power generator module, showing the main components -

(Note that this picture has the generator turned on its side to make it easier to fit onto this page)

1 - The electrical generator. This would be specially designed to spin at high speed. The weight/inertia of this wouldn't be too important as once the device had spun up, it would continue to spin for years to come. I'm basing the electrical power output of each module on this generator being able to make at least 500kw of power. (read the paragraph on the turbine as to why I reckon this is about right)
2. By means of a shaft, the generator is connected to the turbine. (see later picture/paragraph for details) Just above the turbine exhaust you can see how the housing bends across to the side to exit into the secondary ducting, exhausting upwards. Just below the turbine is the main water pump, which serves two purposes. The main purpose is to keep the boiler topped up with cold water, and the secondary reason is for turbine lubrication & cooling.
3. This is the main regulator valve, and it does two things - Regulates the steam gas flow to control the turbine's speed, and also keeps the boiler pressure to a minimum of 1,000psi.
4. The boiler. This is one of the very few metal components (steel, coated to stop corrosion) in the module. I'm not sure how big the boiler would have to be, but I'd guess about 8 metres long x 1.5 metres wide, which would make it about 33³m in volume. There'd be an air gap at the top, so there'd only be about 30³m odd of water, thus, about 30 tonnes of weight there. Some experimentation would be needed here to find out just how large the boiler would need to be, to make sure that the water supply coming into the boiler is just enough to keep the water level constant as the water steams up. It may well end up a lot smaller at the lower levels due to the higher rock temperatures, but the modules above would need larger volumes of water, I'd guess, to allow for the longer boiling times.
All the machinery would be supported from the base of the boiler, with just the single drive shaft from the turbine to the generator being able to move around with thermal creep. (by using a splined 'quill shaft') The boiler itself would be held it place with aluminium brackets, as aluminium conducts heat like you just wouldn't believe.
5. Secondary ducting. This is where turbine exhaust flows upwards, to the water collector/condensor section. On either side of this ducting is the cool water downflow pipes - This is the return path for the water from the collector section to the power modules.

Ok, the turbine is the heart of the module, so here it is in a bit more detail -

Things to note in that diagram -
- Unlike an aircraft's turbine, this one fans 'out' instead of wasting inwards. Aircraft ones do that to save space, reduce inertia,  and improve the gas flow through the exhaust. To get the maximum torque from this turbine, however, I've arranged the blades as far from the rotating axis as possible as none of the factors that affect aircraft are important 5km below the ground! Torque is all important when it comes to spinning up the big electrical generator, so I believe that this is the best configuration.
- I've marked the letters 'S' and 'R'; these stand for Stator and Rotor, respectively. If you are unfamiliar with these terms, they mean that the stators don't move (they straighten the gas flow onto the next rotor stage) say still, and the rotors rotate. I've used three stages (3 x rotor & 3 x stator) as this is typically what is used in a small-ish aircraft turbine to make over 3,000hp.
- The inlet manifold comes in at 90°, but with good design the steam can be turned into the turbine first stage without too much loss in pressure. It's the only practical way to get the steam into the turbine, unfortunately.
- The entire turbine is made from ceramic material. This is not to reduce weight & inertia, but more to eliminate corrosion for long life. I also thought that by using ceramics it would be possible to use water as a lubricant for the ceramic bearings! This would negate the need for a separate oil system, thus making it all that much less complex and reliable.
- Off the bottom of the turbine is the main water pump. This pump is a gear pump, as they require the least amount of power to drive compared to other types of pumps. It spins at the same speed as the turbine, and it's main purpose is to keep the boiler full of water. It would have a bleed-off on the outlet side to lubricate the turbine's bearings.

Ok, how the whole thing is supposed to work ...
... or, "a day in the life of a drop of water!"  :)
Let's start in the boiler - The outside rock temperature is up around 350°C, and so since the module would have been heat soaked so that entire device would also be at around that temperature, thus heating the mass of water quite well to boiling point. All around the skin of the boiler, large amounts of steam would be bubbling off, rising to the top where the pressure regulator would be keeping the steam to at least 1,000psi on it's way to the turbine. The turbine would be spun up to around 30,000rpm, making about 1,000hp in power, spinning the electrical generator so it makes at least 500kw of power. The steam, now having lost a fair bit of temperature and pressure, exits the module at a a temperature somewhat less than the outside rock but still at high speed and with a relatively large volume, thus making rise up the secondary shaft.
Since the steam would remain as steam until the rock temperature is less than about 100°C, it would rise for a couple of kilometres, up where the water condensor/collector section is. This section is dead simple, and looks a little like this -

The flaps sticking out overlap so that the water that collects on them cannot fall straight back down the shaft. At the bottom of each flap is a small pipe that connects to the return water pipe, which ducts the water all the way back down to the power section. The top of the shaft would be open and so a lot of hot air would be coming out - If you really wanted to get every last little of energy out of the shaft, I reckon that a series of of small horizontal windmills to spin in the upwards flowing air would get a bit more power from the shaft for very little cost.

Anyway, back to the water flow - the down pipes would go down all the way to the bottom of the main shaft. However, only one pipe would be full to the bottom, (this one being only full half way) and the other one only being full to the halfway point down the power section. The reason for this is so as to not have a huge pressure differential along the length of each pipe into each power module - There would be at least 100 modules over a single kilometre, so the difference in water pressure from top to bottom would be quite large. I figure the solution would be to have one pipe feeding the lower half of the modules and the other pipe feeding the top half, thus minimising the pressure differential.
In any case, the pipes would be full enough so that the pressure at the bottom of each pipe would be just lower than the boiler pressure. This is to make sure that the water pump on the bottom-most module doesn't have to constantly regulate 'backwards' against the water pressure. The down pipes would have to be well insulated against the high rock temperature, so as to deliver the water below 100°C. (Note that as the depth increases, so does the outside air pressure, and so does the boiling point of water; roughly 1°C per 300 metres.)
The water pump would then pump liquid water back into the bottom of the boiler, whilst regulated to keep the water level to a pre-set point to keep a good air gap at the top of the boiler.
Thus the cycle starts again.
It'd also be very easy to get the system working after it's been built - Simply throw a hose over the edge of the hole and turn the tap on for a while ... ;)  It would take a while for the boilers all to settle down to whatever level they're happy working at, but power would start being generated as soon as the first (lowest) boiler made steam.

I've thought about what material would be best for the construction of the main shaft, module framework, etc. What is needed is a material that's cheap, easy to shape, and resistant to ~400°C. I figure that plain ol' concrete would do the trick!
Since the main shaft would have to be dug then lined with concrete, I picture the best way to do this would be to cast concrete 'shells', each one making up 1/3 of the circumference of the shaft and designed so that they can slip together from being transported down the centre of the main shaft. They'd be locked in place with a few steel pins hammered into the rock.
The power modules framework would also be best made from cast concrete. With the cast concrete sections, I think that it would be best if they were cured after casting in an autoclave, under high pressure and temperature so they would be stabilised for the conditions they'd be experiencing for quite some time afterwards.
As mentioned before, the turbine would best be made from ceramics, to give the longest life and maintenance free service.
The electrical generator would be a bit of a problem, as conventional soldering couldn't be used as the ambient temperature would melt any of the usual joints, so, mechanical joints would have to be used. That makes life a little more difficult, but far from impossible.
The top of each power module would match up with the bottom of the one above, with small electrical motors driving the seals for the water, electrical cables, and data & controller cables.
There are two computer controlled valves, one for the boiler outlet and the other for the water flow into the boiler. Both would need very reliable electric motors to drive the valves.
All the computer power to control each module would be up on the surface, so no special heat resistant boxes would be needed for them. (each module would be controlled be means of a discreet code, so there would need to be a very basic chip at each valve, along with temp & press sensors, but they are very basic devices that're quite rugged compared to 'number cruncher' computer chips)
There's really only about four moving parts, so I believe that the modules would be extremely reliable in the long term ... which is good, because it would be rather time consuming to get the very bottom one out of the shaft! ;)

How much power?
I am making some very rough assumptions, based on my experiences with aircraft jet engines, but I hope that you'll agree with my reasoning.
One aircraft I flew had a Garret TPE-331-10UA turboprop engine. I've seen the turbine section out of the aircraft on a work bench, and I could easily hold it in one hand - They make about 3,000hp, (~2,200kw) of which about 2,000hp is used to drive the compressor and accessories, leaving about 1,000hp to drive the propellor. In my mind's eye, I picture the turbine used in this geothermal power system to be about three times larger than the Garret one, but the power that the steam has would be quite a bit less than the power of the combustors of the Garret, however, this is well and truly compensated for by the fact that the load on the ceramic turbine is a LOT less than on the Garret, and I also think that using the trick of making the turbine wider than the Garret's the power output would not be too far off the aircraft's 2,200kw, but to be terribly pessimistic I'll call it 500kw.
I'd say that each power module would be about 10 metres long, so 100 of them could be fitted every kilometre of the shaft. If you only use the last one kilometre of the shaft, then that's at least 50 megawatts of power for basically nothing!
If we are a little more optimistic with some of the assumptions, say, 1,000kw for each module, and have 250 per shaft, then you're up to a good 250 megawatts.
I'd say that the practical power figure lies somewhere around ... 100 megawatts?

Of course, that's just the one shaft - They would only take up the size of a small house on the surface, so there's no reason why a new shaft couldn't be dug every kilometre or so, thus having a relatively small amount of land generate a lot of power.

One thing that I dug up (pun intended!) that supports the amount of heat below the surface is the picture below.

If you look carefully at the enlarged image, you can see the outline of Australia

And here's a similar view, but easier to see.

It's from here, and it clearly shows the huge amount of energy available, especially just down the road from where I live in Australia.

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