Energy Crisis theme 1: Hot bubbles
Posted Dec14-10 at 02:30 PM by Jon Richfield
Drilling in Hot Rock
As I already have mentioned, studies show that existing drilling technology can penetrate far deeper into hot rock than we require. All the same I actually am not currently contemplating conventional forms of drilling, except during specific phases where engineering experts might see it as advantageous.
There are several problems with drilling through deep, hot rock with the conventional drill string. One is the sheer difficulty and cost of dealing with such a long string, and another is removal of the drilled-out material from the hole.
Instead I propose approaches based on drilling by application of heat and pressure. Near the surface this would be costly at least, and might be unpractical in other ways too, but as we go below the zones where the environment is cool enough for humans to work the rock, the shaft sinking would be taken over by automated equipment. At levels where this too becomes unpractical, we can begin to exploit the very heat that had been an obstacle at shallower levels. Whether to use chemical or nuclear energy for drilling is an open question. Possibly first one then the other.
There are two obvious chemical options. One would be to pump down oxygen and a suitable fuel, possibly hydrogen, carbon, or hydrocarbon fuels. The drill head would be a torch nozzle of a suitable diameter, design, and material, and the whole unit would work under pressurised gas (oxygen?).
I rather suspect that an alternative design would be better, based at least partly on solid fuels such as solid carbon. Oxidisers such as nitrates, perchlorates or peroxides could be dumped in blocks, or pumped down, perhaps in slurry form. Other fuels such as aluminium or sodium might get their oxygen directly from the ambient molten rock instead. In general the chemistry of the process would offer an interesting field for study and prudent experiment.
In any case, the mix would be designed to melt its way down as a pool of molten rock continuously enriched with fuel.
The molten pool principle could be especially valuable during the early phases of extending the actual bubble downwards. By continually adding modest amounts of fuel to the molten pool of salts, extra heat could be applied selectively to make sure that the hottest rock and flux would be at the lowest point below the inlet. If working fluid were injected radially to cool the walls, this should increase the tendency of the bubble to grow downwards rather than sideways.
By systematically injecting fuels and maintaining pressure, the whole system could melt its way down. The resulting hole would not be a narrow puncture such as a drill would produce, but perhaps metres in diameter.
An interesting version of the molten pool approach would be to form a pool of molten iron, manganese, or other suitable metal, feeding it with alternating doses, first of oxidisers such as peroxides, oxygen, and perchlorates, and then of reducing agents such as carbon, aluminium, sodium and hydrogen or hydrides. The metal pool not only would be designed to be chemically reactive, but also dense. Properly controlled it should eat and melt its way through most kinds of rock, largely catalytically, because molten slag of various kinds could be reclaimed by feeding in reducing agents such as sodium. Temperatures could be raised by feeding in oxidisers. The high density would concentrate the forces on the bottom of the cavity. Slag of various kinds would float to the surface of the pool, where they could be drawn out onto the sides of the cavity. Some of the fuels, such those that produce volatile oxides, would react to increase the pressure. Others, such as reactive metals, would reduce pressures, yielding carbides and carbon instead of CO and CO2. These reactions could be reversed by adding more oxidisers as required.
Melting rock instead of drilling through it might sound very inefficient, but it has its compensations. Firstly there is hardly any drill string problem, which otherwise could be prohibitive in terms of time and mechanical problems. The heat produced would generally be very effectively contained, stored, and conserved on the way down. It partly be reclaimed from the coolants when they re-emerge, and the cooling would contribute to the congealing of the walls.
Alternatively, at a suitable depth we could begin to melt our way down by installing a high temperature fission reactor that relies on Doppler broadening to reduce neutron absorption for temperature control by negative feedback. Such a reactor would be designed to be of considerably greater density than any molten rock it might encounter. It therefore would sink through any rock that it melted.
If switching the reactor off turned out to be difficult, it could simply be permitted to consume its residual fuel in sinking away from the site of the bubble once we had reached the most desirable depth. Otherwise it might be possible to keep it at the bottom of the bubble, supplying energy to assist in the inflation.
As the heat source melted its way down, the hole would be kept open with pressure maintained from above. Exactly what would happen behind the drill would depend on many factors. A fair amount of experiment would be necessary to establish the best principles, let alone to design actual equipment. I doubt that the actual rock itself would be strong enough to withstand the ambient pressures at the depths desired, even if cooled, but if forming and cooling would do the trick, that would be an attractive option.
Alternatively, as the shaft progresses, it could be expanded by pressure to somewhat wider than its desired diameter. Then perhaps a molten refractory lining could be shaped and cooled in place. The wall of the lining could formed to contain any channels required for carrying down coolants or extracting hot fluids.
Another option would be to install voussoirs forming the tunnel wall together with any incorporated structures or mechanisms. The rock around the voussoirs could be allowed to settle in behind them while still soft, holding them in place as it cooled. Coolant entering the walls would keep the surrounding rock stronger than the rest of the rock in the region.
A still simpler, but possibly more attractive approach would be simply to stop melting the way down once reaching rock of a suitable temperature, but instead just apply pressure to expand both the walls and the floor. Pause the process while installing the next set of voussoirs, then apply cooling to the walls. Once the voussoirs were set in place, apply pressure till the floor once again began to sink and make room for the next set of voussoirs. Ultimately this process would end in blowing the power bubble itself, installing no more voussoirs, but only cooling the walls enough to prevent expansion of the bubbles from creating buoyancy problems.
Impossible Pressures
One of the most rational objections concerns the difficulty of applying the necessary pressures for working at such depths at all, let alone for forcing bubbles into the soft rock tens of km down.
Well, I never said it would be easy. I am as aware as anyone that forcing such pressures down a tube in the field is not at all the same thing as playing tricks in the laboratory.
One might of course dismiss that objection with hand-waving reassurances: after all, we have overcome problems in the past that at first had seemed not just equally challenging, but equally absurd. And yet, many such technologies are nowadays routine, not only taken for granted, but regarded as boring by that minority of the couch potato population as have so much as heard of them. I am tempted to cite Clarke's law of technologies: that any sufficiently advanced technology is indistinguishable from magic, except that a far more powerful influence is that any effective technology soon seems far more tedious than marvellous to the smuggest and most petulant sector of humanity: the Great Uneducated and Uneducable.
However, although a direct assault on multi-megapascal pressures might well be forbidding, multistage hydraulic devices of modest performance could be stacked at intervals on the way down the shaft, such that both the mechanisms and the surrounding rocks could handle individual levels of forces with comfortable safety margins.
Another class of technique could also contribute to the effort. Filling the hot parts of the shaft with molten salts or similar substances could partly or completely counter the surrounding hydrostatic forces.
Challenging in general? Definitely. And yet, I regard the challenge of achieving of profitable nuclear fusion as far more challenging yet.
As I already have mentioned, studies show that existing drilling technology can penetrate far deeper into hot rock than we require. All the same I actually am not currently contemplating conventional forms of drilling, except during specific phases where engineering experts might see it as advantageous.
There are several problems with drilling through deep, hot rock with the conventional drill string. One is the sheer difficulty and cost of dealing with such a long string, and another is removal of the drilled-out material from the hole.
Instead I propose approaches based on drilling by application of heat and pressure. Near the surface this would be costly at least, and might be unpractical in other ways too, but as we go below the zones where the environment is cool enough for humans to work the rock, the shaft sinking would be taken over by automated equipment. At levels where this too becomes unpractical, we can begin to exploit the very heat that had been an obstacle at shallower levels. Whether to use chemical or nuclear energy for drilling is an open question. Possibly first one then the other.
There are two obvious chemical options. One would be to pump down oxygen and a suitable fuel, possibly hydrogen, carbon, or hydrocarbon fuels. The drill head would be a torch nozzle of a suitable diameter, design, and material, and the whole unit would work under pressurised gas (oxygen?).
I rather suspect that an alternative design would be better, based at least partly on solid fuels such as solid carbon. Oxidisers such as nitrates, perchlorates or peroxides could be dumped in blocks, or pumped down, perhaps in slurry form. Other fuels such as aluminium or sodium might get their oxygen directly from the ambient molten rock instead. In general the chemistry of the process would offer an interesting field for study and prudent experiment.
In any case, the mix would be designed to melt its way down as a pool of molten rock continuously enriched with fuel.
The molten pool principle could be especially valuable during the early phases of extending the actual bubble downwards. By continually adding modest amounts of fuel to the molten pool of salts, extra heat could be applied selectively to make sure that the hottest rock and flux would be at the lowest point below the inlet. If working fluid were injected radially to cool the walls, this should increase the tendency of the bubble to grow downwards rather than sideways.
By systematically injecting fuels and maintaining pressure, the whole system could melt its way down. The resulting hole would not be a narrow puncture such as a drill would produce, but perhaps metres in diameter.
An interesting version of the molten pool approach would be to form a pool of molten iron, manganese, or other suitable metal, feeding it with alternating doses, first of oxidisers such as peroxides, oxygen, and perchlorates, and then of reducing agents such as carbon, aluminium, sodium and hydrogen or hydrides. The metal pool not only would be designed to be chemically reactive, but also dense. Properly controlled it should eat and melt its way through most kinds of rock, largely catalytically, because molten slag of various kinds could be reclaimed by feeding in reducing agents such as sodium. Temperatures could be raised by feeding in oxidisers. The high density would concentrate the forces on the bottom of the cavity. Slag of various kinds would float to the surface of the pool, where they could be drawn out onto the sides of the cavity. Some of the fuels, such those that produce volatile oxides, would react to increase the pressure. Others, such as reactive metals, would reduce pressures, yielding carbides and carbon instead of CO and CO2. These reactions could be reversed by adding more oxidisers as required.
Melting rock instead of drilling through it might sound very inefficient, but it has its compensations. Firstly there is hardly any drill string problem, which otherwise could be prohibitive in terms of time and mechanical problems. The heat produced would generally be very effectively contained, stored, and conserved on the way down. It partly be reclaimed from the coolants when they re-emerge, and the cooling would contribute to the congealing of the walls.
Alternatively, at a suitable depth we could begin to melt our way down by installing a high temperature fission reactor that relies on Doppler broadening to reduce neutron absorption for temperature control by negative feedback. Such a reactor would be designed to be of considerably greater density than any molten rock it might encounter. It therefore would sink through any rock that it melted.
If switching the reactor off turned out to be difficult, it could simply be permitted to consume its residual fuel in sinking away from the site of the bubble once we had reached the most desirable depth. Otherwise it might be possible to keep it at the bottom of the bubble, supplying energy to assist in the inflation.
As the heat source melted its way down, the hole would be kept open with pressure maintained from above. Exactly what would happen behind the drill would depend on many factors. A fair amount of experiment would be necessary to establish the best principles, let alone to design actual equipment. I doubt that the actual rock itself would be strong enough to withstand the ambient pressures at the depths desired, even if cooled, but if forming and cooling would do the trick, that would be an attractive option.
Alternatively, as the shaft progresses, it could be expanded by pressure to somewhat wider than its desired diameter. Then perhaps a molten refractory lining could be shaped and cooled in place. The wall of the lining could formed to contain any channels required for carrying down coolants or extracting hot fluids.
Another option would be to install voussoirs forming the tunnel wall together with any incorporated structures or mechanisms. The rock around the voussoirs could be allowed to settle in behind them while still soft, holding them in place as it cooled. Coolant entering the walls would keep the surrounding rock stronger than the rest of the rock in the region.
A still simpler, but possibly more attractive approach would be simply to stop melting the way down once reaching rock of a suitable temperature, but instead just apply pressure to expand both the walls and the floor. Pause the process while installing the next set of voussoirs, then apply cooling to the walls. Once the voussoirs were set in place, apply pressure till the floor once again began to sink and make room for the next set of voussoirs. Ultimately this process would end in blowing the power bubble itself, installing no more voussoirs, but only cooling the walls enough to prevent expansion of the bubbles from creating buoyancy problems.
Impossible Pressures
One of the most rational objections concerns the difficulty of applying the necessary pressures for working at such depths at all, let alone for forcing bubbles into the soft rock tens of km down.
Well, I never said it would be easy. I am as aware as anyone that forcing such pressures down a tube in the field is not at all the same thing as playing tricks in the laboratory.
One might of course dismiss that objection with hand-waving reassurances: after all, we have overcome problems in the past that at first had seemed not just equally challenging, but equally absurd. And yet, many such technologies are nowadays routine, not only taken for granted, but regarded as boring by that minority of the couch potato population as have so much as heard of them. I am tempted to cite Clarke's law of technologies: that any sufficiently advanced technology is indistinguishable from magic, except that a far more powerful influence is that any effective technology soon seems far more tedious than marvellous to the smuggest and most petulant sector of humanity: the Great Uneducated and Uneducable.
However, although a direct assault on multi-megapascal pressures might well be forbidding, multistage hydraulic devices of modest performance could be stacked at intervals on the way down the shaft, such that both the mechanisms and the surrounding rocks could handle individual levels of forces with comfortable safety margins.
Another class of technique could also contribute to the effort. Filling the hot parts of the shaft with molten salts or similar substances could partly or completely counter the surrounding hydrostatic forces.
Challenging in general? Definitely. And yet, I regard the challenge of achieving of profitable nuclear fusion as far more challenging yet.
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