Wednesday, July 19, 2017

Kuiper Belt Navigation and Mining.


How might we steer objects from the Kuiper belt to achieve objectives in the inner orbits of the solar system? This question arose from my proposal to alter the rotation of the planet Venus for purposes of exploitation and habitation. That proposal is discussed in the article "Small Fetters" in which I express doubt that humanity has the right stuff to achieve any such objective.

Some critics denied that it could be practicable to achieve anything of the kind as a realistic engineering project at all, even in principle. Some simply said that the scale of the project was too large ever to be done in principle. Some said that it could not be worth doing even if it were possible. Some of the more open-minded suggested that, assuming it were done when 'tis done, then 'twere well I should explain how it were to be done and why.

In a thread I pointed out that Kuiper Belt objects, that authorities assume to number in their millions — some estimates suggest billions, should bear kinetic energy more than adequate for practical exploitation. I did not at that time bother to discuss the detailed engineering, taking it for granted that navigating rocks from trans-Pluto orbit to intercept Venus suitably was not intellectually challenging in principle, however challenging the project might appear from political or engineering perspectives.

However, contrary to my expectations, some participants in the thread impugned this, and since some aspects of the topic are of intrinsic interest, I here discuss approaches to obtaining and navigating comets, asteroids, and assorted Kuiper Belt meteorites as required.

I still skirt currently irrelevant engineering issues of course.

Several other ideas and approaches might prove relevant, but here I concentrate on projects such as the engineering of Venus and Mercury and assume that the scale of the project would involve something of the order of 100,000 Kuiper Belt objects of perhaps 10 km diameter on average. Please do not bother to point out that not all Kuiper Belt objects of that order of magnitude are spherical; this is strictly a spherical cow exercise.

Obviously there are many types of approach to such potential projects, and in this discussion I ignore all that are not aimed at a long-term commitment (probably 1000 years or more, possibly even several thousand years; gaining a new planet with all its resources would be worth a good deal more than that in any rational scheme of things). The discussion also has nothing to do with steering any single object, of either gigantic or trivial size, nor bringing it to some sort of relative stop or storage trajectory at some arbitrary position in space. Instead I consider perhaps tens to hundreds of thousands of objects of worthwhile size, small enough to steer economically, and large enough to achieve particular objectives.

Nothing of the kind would be worth discussion without an appropriate infrastructure, but at such a distant remove, details of that infrastructure are hardly worth discussion. Anyone who doubts this should read Arthur C. Clarke's original proposal for communications satellites; he proposed that they should be manned! His proposal was in no way stupid; in fact, given our current applications of communications satellites, if they really had to be manned, they still would have been worth it; it is just our luck that our advances in technology since World War II free us from any requirement for such an extravagance. Similarly, it is practically certain that anything I propose here would seem ludicrous to engineers of the 22nd century.

Instead I merely brush over some plausible requirements of such an infrastructure in discussing principles.

Thousand-Year Projects


It is well not to be too casual about beginning a highly technological 1000-year project. I suggest that for reasons of navigation and communication we should start by installing at the very least a few hundred permanent unmanned satellites in strategic orbits in the solar system. That no such project has yet been undertaken is a blot on our record already. However, the solar system equivalent of a GPS system plus communication relay system plus near space (meaning solar system in this context) astronomic and cosmological observation system, would be necessary as the first step in the project. Call such craft the Relay Satellites.

Relay Satellite Infrastructure


We should be able to install a profitable, workable and worthwhile foundation within a few decades, but the planned lifetime of the Relay Satellite system would be measured in millennia rather than years and there would be no question of committing to naive uniformity of design during such a period. That infrastructure would be an indefinite project, adapting to our needs and technologies for the foreseeable future. In practice of course, certain apparently arbitrary standards might remain constant, but such are merely practical details that are not immediately relevant to us. The principle is well understood and tolerated and there is a fair discussion of a traditional example at:

'Nuff said on that point.

As I see it the Relay Satellites should have modest navigational capabilities, just about enough for maintaining station and attitude in their appropriate trajectories indefinitely. Some of them in near solar orbit might use solar power of one sort or another, but by far the majority would occupy orbits beyond Mars, and some perhaps beyond Pluto. Whether to power them with beamed energy, isotope energy, or on-board fission or fusion power generators is an example of a question I leave to future generations. They should need considerable power for communication at least, plus very sensitive reception equipment, because they would be communicating partly with tiny craft that could not carry giant antennae to capture faint signals. Whether there would be relatively few multifunction satellites or relatively many specialist function satellites, and whether a satellite that outlives its fuel supply would be parked and catalogued, or scrapped, refurbished, recycled, or destroyed, I also leave to future interests. Personally I like the idea of large-scale, modular satellites to be serviced and upgraded by specialist unmanned craft, but I do not insist upon that.

Note that there is no suggestion that such a fleet of Relay Satellites should be dedicated to the Kuiper Belt object navigation project. There would be plenty of function for it without that. It is quite possible that a Kuiper Belt initiative would affect the scale and details of parts of the fleet, but that is not especially relevant here.

Prospecting Craft


The second class of craft, prospecting craft, would in contrast be specialised for ranges of function dealing with exploration and prospecting in the Kuiper belt. To this end they would be capable of extremely long range, long-term navigation beyond the orbit of Pluto. Their communication and navigation capabilities would be powerful, but specialised for their role. They would rely on the permanent Relay Satellites for most of such functions, and partly for keeping track of the prospecting craft. Of course, each satellite would have its own intelligence and a very large memory, probably petabytes rather than terabytes, enough not only to manage the data that it accumulates, but the parameters of the infrastructural system. They would have sufficient intelligence for routine tasks, including some fairly complex ones, because apart from the question of how far robotics would have advanced in the next century or two, such tasks would be, if not highly stereotyped, at least confined to a small universe of discourse. They also would have great redundancy of function and capacity to ensure resilience in the face of predictable radiation and unpredictable accident. None of your single-drive hard disks and the like!

Prospecting craft would be exceptional in the amount of reaction mass and energy that they would have to carry, because they not only would have to reach the Kuiper Belt, but would have to do considerable amounts of unpredictable navigation within it. At such a distance from the sun, solar power would be practically worthless, probably including solar wind power. It also would not be practical for such craft to rely on isotopic thermal energy; they would need fission power generators at least, which is the good news; the bad news is that they also would need large amounts of reaction mass, even if they used ion thrusters. Much as they would rely on the relay satellites for control and communication, they might have to rely on rendezvous with tugs and maintenance vehicles for refuelling and upgrading.

Their function would be to locate and characterise as many Kuiper Belt bodies as possible, determining their nature, mass, trajectories and the like by any practical means, whether optical, radar, infrared, gravitational, theoretical or generic, to name but four... err... or so... Bodies that either pose a threat to the inner solar system, or that seem to be potentially valuable to the main project, probably would be visited physically to obtain all relevant information. For example, bodies that are rich in ice or ammonia might either be particularly valuable or not usable for the purposes of the project. Similarly, bodies that amount to aggregations of gravel might be valuable if their trajectories were particularly suitable for gentle manoeuvring, but hopelessly dangerous to use otherwise unless they could be cemented, say by combination with an iceberg or ammonia-berg.

This is a large subject, not worth exploring at this point in any depth. Suffice to say, some such functional craft would be needed to locate and evaluate the objects to be selected for navigation in the project.

Tugs and Maintenance Craft


The third class of craft would be the tugs and maintenance craft. They would be a job lot, and I do not discuss their design, which would be variable at all stages of the project. All the other craft would require updating, modification, refuelling, repair, and possibly even retrieval. The Relay Satellites might well require being transported to their stations as well. No one tug or maintenance craft would be suitable for all such functions. However, each one probably would be versatile and each one would have powerful thrusters of appropriate kinds. However, they might need less fuel than the prospectors, because they would have shorter missions, and more closely defined.

Rockrider Craft


How many different classes of craft would be needed, I cannot say; the only other one worth discussing here would be the Kuiper Belt object navigation craft. Let’s call it the Rockrider craft. It is the one at the cutting edge, or the coalface if you prefer. It's job would be to rendezvous with a selected object, prepare it for transport, and steer it to the objective.

What possibly, just possibly, could be simpler?

A lot of things of course, but not many are as worthwhile.

Manned and Unmanned Craft in Such Projects


Very well. Notice that I have said practically nothing so far about manned craft. I do not say there would be none such, but for the purposes so far discussed, I cannot see any being required, and frankly I cannot in the short term see any manned craft being practical for transient applications beyond say, the orbit of Mars. We are after all speaking of Rockrider craft undertaking voyages of decades at least, and commonly of centuries. Even if we condemned convicts to such voyages, it is hard to imagine what we would want them to do out there in space, even if we could trust them there. I leave such distasteful speculations to those with the appropriate distastes.

Target Objects and Objectives


Now, each Rockrider craft would have the task of rendezvous with a nominated object that had been identified, located, and characterised by the prospector craft. Apart from a few prototypes, probably none would be launched before some thousands of target bodies had been selected as having suitable masses, constitutions, neighbours, and trajectories for the project. Many, possibly the majority of such Kuiper Belt objects, might be perfect for launching in a few hundred thousand years, but useless for short-term projects of 1000 years or so. However, there are assumed to be many millions of bodies out there, so I do not feel too defensive about assuming that we would be spoilt for choice of suitable objects.

A suitable object would have to be one that could profitably be adjusted in its attitude and trajectory, for a Rockrider craft to manipulate and navigate it down from say 40 astronomic units, to rendezvous as required with Venus or Mercury at less than 1 AU. For example, if an otherwise suitable 100 gigatonne object were spinning at a rate of several hertz, the very task of de-spinning it strikes me as discouraging; I would rather go on to look for something friendlier. Nor would we be interested in 10-tonne or peta-tonne objects, or at least that is what I assume.  Again, we would prefer to deal with objects whose orbit we could adjust most economically in terms of energy and time. Exactly which variables would be most important in a given case, I do not much speculate upon.

I suspect that very circular orbits would be expensive to adjust, whereas elliptical orbits that approach, or could be coaxed near to Neptune, could be adjusted drastically at modest cost. Difficult decisions would be the business of the human orbital engineers, but generally most decisions could be handled programmatically. Possibly one could use nuclear explosions for crude preliminary adjustment of some kinds of orbits of suitable bodies, or even to persuade some bodies to collide usefully.

Using collisions might seem a bit optimistic, given that even millions of bodies so far out would have a considerable mean separation, but it’s just an example of the kind of consideration that might arise.

The point in general is that we would select bodies with orbits that could be adjusted with minimal investment of energy and material, whether by bombs or by thrust.

However, we could afford a reasonable investment, bearing in mind that a typical energetic profit for dropping from say 40 AU to the orbit of Venus would be about sixty-fold, and to Mercury, 100-fold. Those already are attractive figures. And if we could gain useful momentum by slingshotting past the major planets, we could increase that profit dramatically.

Slingshotting would be important for more than just the increased yield of energy; it would be vital for steering large bodies. Any adjustment of the trajectory of a billion- to trillion-tonne mass would be so expensive that we would care less about the factor of profit, than whether we could afford the project at all. As a result we might well be happy to work at a trajectory for a few centuries to get a finally profitable result by nibbling at the gravitational field of one planet after another. The computing load would be heavy, but routine. Much of it would be done Earthside many years in advance. There would be plenty of time to seek out the most obscure scenarios for each Kuiper Belt object, where each major improvement in handling a single rock would be worth billions of dollars.

Slingshotting would be important in two different ways: energetic gain, as mentioned, and steering. Energetic gain would work essentially by parasitising the orbital momentum of a planet. This is no novelty; it already is a routine technique of long standing in spacecraft navigation.

Steering is another aspect. Obviously passing a sufficiently large planet in a suitable trajectory can change the course of a body almost arbitrarily within the ecliptic. What is more, by passing the planet at greater or lesser distance, one can affect the angle of change practically as much or as little as one likes. In passing close to the planet, one is in a position to adjust one’s exit direction greatly by adjusting one’s incoming course by only a few tens or hundreds of kilometres. To achieve such a difference would require only the gentlest of nudges or persistent pressure a few years in advance. However, with such a delicate requirement one can see why we would want such an elaborate infrastructure of navigation satellites.

The other steering requirement that slingshotting offers, is one’s position relative to the ecliptic. By adjusting one’s position so that we pass to the north or south of a planet, we can adjust our approach so as to move out of the ecliptic. We could for example hit the north or south pole of Mercury practically vertically, or either limb of Venus grazingly, with an enormous bang in either case. We would deliver many times more energy and momentum than we had invested. It would achieve either excavation or adjustment of rotation as required.  

Riding the Rocks


Well then, we know that if we can steer a planet into a good starting position and apply a bit of adjustment at critical points, and can stop our rock against a target instead of having to stop it in space, we have it made.

This is critically important, please note. If we got no more energy out of a rock than we had put into it, we might as well save ourselves the trouble and go and shove at the planet directly.  And if we did that, we could not nearly afford the energy. This whole exercise is predicated on the idea that we might manage to get away with three or four orders of magnitude less energy, by application of a little brainwork, commitment and patience. And being apes rather than termites, we can easily manage that can’t we? 

The way we always do?

All the same, there still is the requirement to apply that fraction of a percent of energy, or nothing special will happen.

Let us then consider a hypothetical project. Imagine a typically peanut-shaped, teratonne, predominantly rocky body, spinning about a short axis, but not so fast that any part of it is travelling much faster than escape velocity for this rock. OK?

Still, the spin is unacceptably high. Our Rockrider selects a suitable spot, based on the prospector’s information, instruction from Earth, and its own calculations, lands there, using tethers as necessary, checks the details, and drills into the body of the rock near one end, probably using plasma or laser drilling for the most part. Together with adjustments calculated in the light of what it finds on the way down, it carefully places a multi-megatonne nuclear bomb ordered in advance in the light of the prospecting report decades before, covers the hole nicely like a cat, retreats to a safe distance a few hundred kilometres away in space and on the sheltered side of the rock, and when the attitude and position are right, it blasts a tidy slice off one end of the rock. The resultant vector of the blast both kills the rotation or very nearly, and accelerates the rock into an improved, more elliptical, trajectory. You see, the depth of the bomb was such that several thousand tonnes of material were blasted off at a modest velocity, imparting a really efficient delta-V in the desired direction.

Waste not, want not! Eat your heart out, Saturn V!

Having checked how well the blast had worked, perhaps while it waited for the blast site to cool, the Rockrider lands again, possibly on the blast site, and anchors itself nicely. It begins to run its nuclear generator and plasma drills and to excavate more material in the form of vaporised rocky or sooty material that it condenses as an impalpable powder in very intense atmospheres of energetic electrons and ions in separate  chambers. The cooled particles become powerfully charged microscopic electrets. An electret wouldn't have to retain its charge for more than a few seconds, but in practice probably would do so for years or indefinitely.

Meanwhile the Rockrider has unlimbered its main thrusters, which are specialised twin (or multiple?) linear accelerators, whether electrodynamic, electrostatic or laser. Details, details... It feeds them charged dust particles that they accelerate to modest velocities, roughly two thirds of the delta-V of the whole system as calculated for the entire trip. Pretty well optimal for the energy utilisation, which is one of the limiting factors for the project. In principle such electret propulsion could be far more efficient than ion thrusters. The reaction mass is cheap. In navigating a teratonne rock we could afford to use thousands of tonnes as charged reaction mass without serious regret.

Right. The Rockrider and its Earth support have calculated not only the best things to do, but the best ways to adjust the intensity and direction of the acceleration in feedback to the response of the rock to the thrust. That is what we call steering, right?

Now it gets boring. That Rockrider is going to sit there for a long time, rendezvousing with a few planets for slingshot purposes during the next few centuries. Possibly it gets a few more charges of fuel from visiting tugs and maintenance craft. “Oh, it’s you again is it?” “Yeah chatterbox. Who did you expect? Goldilox?” A few days before impact, the Rockrider kisses its mount goodbye, and goes off for some maintenance and its next trip, which had been chosen for it before it even started on this one.

Coasting Rockriders could kill time by acting as incidental observers of conditions and events wherever they pass or pause, or by relaying signals wherever convenient.

Notice that there are major differences between any viable options for this kind of navigation and the Buck Rogers stories. There is no question of fast turns and dramatic accelerations (except for the occasional nuclear blast of course.) Everything is worked out years or centuries in advance and gently nudged for centuries en route. We cannot afford the energy or the risks of abrupt manoeuvres, but we can trade energy for time, which we have plenty of if we are to aspire to the dignity of termites rather than apes. 

Another objection that might occur to cavillers is that if it is going to take us hundreds of years to ride a single rock home, and we need 100000 rocks, we will take a lot of millions of years for the project. But that is a blinkered point of view. We would have thousands of Rockriders, all working in parallel, and sometimes in teams. It might be centuries before more than a few of the first rocks began to splash down, but then it would be a flood of hundreds per year. Although it would generally be the intention that each Rockrider would bring in more than one rock, even one typical rock would pay a generous profit for its Rockrider.

A really serious objection for a race of monkeys is that we would be labouring for human benefits thousands or millions of years after we were multiply recycled dust and dregs, and monkeys don't do that sort of profitable venture; they want it fast, cheap, and now. But all is not lost. Though the ultimate bull market benefits are far in the future, there is plenty of bull for the developers en route. Whole dynasties of companies could profit hugely from running the projects, improving the technology, applying the information gained much as we have profited immeasurably from satellite communications, weather observations, and Earth science and mapping, even while moaning bitterly and persistently about the costs of space technology and research.

Preventing a single dino-killer collision with Earth (never mind turning it into a profitable collision with Venus or Mercury) would pay for the whole initiative.

What could be simpler? Or easier?

Or more gratifying?

Saturday, April 1, 2017

Heavier Duty Banking -- Appendix & Supplement

Heavier Duty Banking

Appendix & Supplement

Shortly before commencing to write this article I wrote on the topic of energy storage by means of suspending masses that could release usable power as they yielded up their potential energy, which in all cases amounted to a maximum of mass times height.

The topic of storage of potential energy was well worn, and I only got into thinking it over during a discussion in which the idea of suspending huge pistons in fluid-filled cylinders sprang to mind. In my previous article on the topic a considerable range of options and variations emerged. Subsequently a friend showed me that the idea was not as novel as I had imagined, and in fact online exploration revealed that some companies had already been floated to implement some of the ideas I had mentioned.

Oh well, whatever has been original before can be original again...

In itself this congruence of great minds was nothing to be astonished at and I am sure that the items I saw were the merest samples of what is being explored in practice. This essay is just an appendix to my previous effort; to refer to the major premises I promote it is necessary to read that essay as well, preferably before continuing to read this one. The text below is not intended to supplement my previous ideas with suggestions from external sources, which would be pointless anyway; it is to add some thoughts in the light of the sheer scale of some of the proposals I have seen and to emphasise some points and proposals that to me seem to have been neglected elsewhere.

Let us begin by recapitulating some of the essential features of my original suggestions, and developing a few more principles.

  1. The fundamental principle is to use pistons in vertical cylinders as masses to be raised by fluid pressure as a medium for storing energy. Letting down the masses to drive the fluid through power generators would deliver the power on demand.

  2. The extent to which the cylinders are to be built above or below ground level is not essential to the principle of the device; for any scale of unit and choice of  materials the economic output is a function of height/depth; the longer the path up and down the cylinder and the greater the mass to follow that path, the better. Accordingly the ideal cylinder in any realistic situation should be determined by the relative costs of the piston material, and the of above-ground and subterranean construction at various depths and heights. Each of these costs is significant in calculating the trade-offs and the general economics of any such project. This point of cost justification will be taken largely for granted in the following discussion; specific figures would be too speculative at this point.

  3. The pistons are to be functionally “dumb”, inert in themselves, with no internal mechanism. They are raised and lowered by fluid pressure control through ports in the cylinder wall, and detents retractable into the cylinder wall hold them suspended at given heights when they are neither charging nor discharging their stored energy. This differs radically from some other designs.

  4. The choice of working fluid is a matter of choice to suit local needs and approaches; I like the idea of oil where that may be practical, but obviously water, possibly brine, has its own advantages, partly depending on the design.

  5. Notionally the pistons should be monolithic in function and as dense as may be practical. I still see lead as the most desirable material, followed by various forms of iron, though I have seen alternative suggestions such as concrete, which to me seem inferior in several respects. As I make clear below, the fact that the pistons are monolithic in function need not imply that they are monolithic in structure.

  6. One problem with lead is that it is soft and deformable enough to be vulnerable to damage in accidents during installation. Such damage could compromise a piston’s precision and options for sealing the contact between the piston and the cylinder wall.

  7. Another problem with the piston idea is that the mass would need to be enormous. Various designs assume piston masses of hundreds of tonnes or even hundreds of thousands of tons. Installation and handling of such large objects and great masses as indivisible units could be inadvisable if there is any practical alternative.

  8. Accordingly instead of simple, solid slugs of lead, each piston should be jacketed with a suitable material. The most obvious design might be a hollow box to be filled effectively solidly with suitably packable lead segments. Originally I assumed a steel box, but I am increasingly interested in the possibility of polymer jackets, possibly with steel-reinforced floors designed to receive the segments and rest on detents without harm.

  9. Each segment of lead inside a piston should be suitably shaped and coded with a unique, machine-readable identification number so as to be suitable for installation and removal on site by intelligent gantries. There are so many possible alternative designs for such segments that I do not discuss the details here, beyond remarking that any design needs be easy to place into a stable and precise configuration, probably in an oil medium. The essential effect is that the jacket then could be designed for assembly and manipulation on site by gantry, installed and packed with lead, with no requirement for handling the total piston mass by any means other than fluid pressure.

  10. The lead segments might be designed so that once installed, their weight could exert some outward force on the piston walls to enforce proper sealing against the cylinder walls, though this is not an essential feature. One way to achieve that would be by fitting the slabs of lead at an angle of probably less than one degree from the horizontal on an oil surface, thereby resting a slight fraction of their mass against the outer wall of the jacket.

  11. The jacket itself, if of steel and of really large scale design, must also be modular. In sizes up to say, two metres diameter and ten metres long, the jacket fairly routinely might be transportable and installable, but some of the designs I have seen online seemed to imply sizes of about ten metres diameter and 125 metres long (and eight of those above each other in a single cylinder...)  Though I do not here deny the feasibility of such units, nor even their possible desirability once installed, I do not think it would be worth trying either to transport or install them as finished units, whether empty or with their internal mass installed. The very notion of assembling the jackets on installation is challenging and intriguing, bearing in mind the many technologies for doing so, ranging from welding to bolting and gluing; it is as sobering as the challenge of the required precision on the required scale. The whole idea strikes me as a charming example of an engineering project in its own right.
    But not trivial.

  12. Nowadays we have options other than steel jacketing. Some modern polymers, probably fibre-reinforced, might be equally suitable for the jacketing, either as a one-piece structure, or floored with steel, or with just a steel rim around the bottom to accommodate detents that might be designed for extension inward from the cylinder wall to hold the pistons stationary where necessary. One advantage of such jackets over steel, is that they could be cast or welded in place from the bottom up, creating an essentially perfect fit with the cylinder wall. Depending on the nature of the polymer, they could be cured with the aid of ultraviolet or gamma ray sources as they are fabricated in place on site, though for my part I rather favour thermoplastics instead of thermosets.
    But those are details that one should not force on the polymer engineers in advance.

  13. Brakes or detents of some sort clearly are necessary for a number of purposes. Pumping fluid beneath or between pistons would achieve nearly all required positive movement, either raising pistons, usually to store energy, or lowering them, usually to generate power. However, when it is necessary for a piston to remain in one position indefinitely, such as while the energy reservoir is full and the power demand is very low, then under constant pressure one must expect undesirable leakage past the piston. It then would be desirable to apply positive static control. Or such controls might become necessary in dealing with a damaged piston. Some schemes proposed in other discussions suggest brakes for holding the piston in place, but I reject that idea partly because of the constraints it would place on the piston's density and complexity, and partly because of the tendency to damage the cylinder wall, not to mention the problem of brakes slipping on the wall lining, very likely damaging the wall in the process and jeopardising the integrity of the seal.

  14. Instead I prefer the use of detents. There are many design options, but what I like offhand is the idea of detents that fit into gaps in regions in which fluid can be pumped into or out of the spaces between stacked pistons. The detents could be cantilever bars recessed into the cylinder walls. They could be deployed by control machinery when no piston either is in the way or needs to pass. The detent assemblies might perhaps include some sort of shock-absorbing mechanism; even at the immensely slow speeds in question, one does not simply say "whoa!" to a 10000-tonne mass. These details too are for the engineers to decide. Still, valving the fluid flow should offer very fine control indeed, so it might be possible to rely on direct control rather than shock absorbers, An attractive idea is to use bi-stable detents designed to remain passively engaged or retracted until once again activated.

  15. At the same level as each ring of detents there would be one or more input/output ports in the cylinder wall, through which fluid could be forced in by pumps or drawn off either to generate power or to lower a piston.

  16. The seal between the piston and the wall would be of self-lubricating polymer on either surface or both. The cylinder's internal wall might be polished steel or lined with hard silicone or other appropriate polymer surface. In a steel cylinder jacket the seal could be cylinder rings extending entirely round the piston without any gap, and made of solid, self-lubricating polymer. If the piston's entire jacket were itself of polymer, it could be fabricated in place to fit the cylinder, providing its own seal with no piston rings. To exploit the flexibility of the polymer in such a jacket, lead segments inside the piston could be designed to exert some of their weight against the piston wall, forcing it snugly against the cylinder wall. In either case the width of the sealing surfaces should exceed the width of any interruptions in the wall, such as inlet-outlet openings or detent recesses, so that they would not present any seepage problems whenever the piston passes over.

  17. The bottom edge of the otherwise cylindrical piston should be chamfered into a recessed rectangular rabbet all round the edge, deep enough to accommodate the detents, and high enough to enable the matching input-output ducts to work at full capacity even if the head of the piston immediately below is in actual contact. See figure. In the design of large pistons this might demand that at least the floor of the piston be of steel.

  18. The foregoing designs would place all the fluid- and energy-handling resources and also all the controls outside the pistons, and in fact outside the cylinders as well. Ideally, once the cylinder and pistons are constructed and installed, they should never need maintenance apart from occasional inspection. All the rest of the attention could be devoted to the piping, the fluid, the pumps etc.

  19. Some of the designs described by other parties online put multiple pistons into a single cylinder. This entails both advantages and disadvantages. Most importantly it makes it possible to limit the size of any mass to be handled as a unit at any point in an operation; it also reduces the pressure that the pumps must work against and so on.

  20. Multi-piston cylinders do introduce complications, such as the need for fluid to bypass pistons, and they complicate the design of semi-open units that either have mushroom-headed pistons, or that elevate large fractions of the working mass above the cylinder. But for very large systems multiple piston designs probably are unavoidable for this approach at least.

  21. In the multi-piston approach considered in this essay the pressure pipe or pipes run up the outside of at least one side of each cylinder, with inlets or outlets for the fluid at least at each level where the chamfered rabbet around the bottom of a cylinder may be brought to rest. At each such level, there also should be a set of detents.

  22. Assuming that we use the multi-piston approach, I propose that the workable pressures within the system be at least somewhat greater than twice the pressure exerted by any one piston.

  23. In withdrawing power from a multi-piston cylinder, first insert the detents into bottom of the gap through which the lowest piston selected to deliver power must pass. Then insert enough fluid beneath the pistons selected for immediate power delivery, to remove the load from their detents. When the pressure on the detents stops, retract the appropriate detents. Thereafter begin to remove the fluid supporting the selected pistons via the selected power turbines, re-inserting their fluid into the space above the highest of the sinking pistons.  As each piston delivering power comes close to the end of its intended course, the detents at the bottom of that gap already having been inserted, the next piston up gets released if required, and the procedure continues.

  24. Cylinders can be daisy-chained either to maximise output pressure in series, or to maximise power output in parallel.

  25. In any closed or semi-closed system in which the fluid is driven back up above the moving piston or pistons as they sink, the mass of the fluid itself does not contribute to the output power, because it has to be raised during power output.

  26. To raise any combination of multiple pistons for accumulating energy, first raise the top piston to just above its highest intended detents, probably inserting intermediate detents as the piston passes as a precaution against emergencies. Stop just above the top detents and  engage them. Then proceed downwards, raising the next pistons in turn.

Note that this essay discusses just one class of design options. The choice of design detail would depend on many variables such as the scale, the materials, the desired duty cycles, the respective costs of available materials and so on.   

Wednesday, February 22, 2017

Heavy Duty Energy Banking

Some facts and implications
The Principles  
More practical Alternatives and Elaborations  
If a little is good, then a lot...  
Getting More Serious About Practical Problems  
Mole's eye view  
If it still don't work, I gets a bigger 'ammer  
What's your tipple?  
Illustrations of some possibly useful configurations  


A few years ago, eheu fugaces, I wrote on the topic of energy storage in submarine tents of compressed air. One’s mind tends to congeal around preconceptions (mine does anyway) so it was a long time before I seriously considered alternatives, and I tended to sneer at ideas concerning say, energy storage in the form of elevated heavy masses; they struck me as limiting and inelegantly mechanical, bulky and small of scale.

Bear in mind incidentally, that it is important not to confuse the concept of energy storage with that of energy generation; that is a perennial hazard in trying to convey the value of energy storage. The assumption is not that your storage device will produce energy, any more than your flask will produce wine, or even produce water to turn into wine. It does however leave room for recognising that as long as you can get the water or wine in the first place, a flask can be more useful for storing that water (or wine) than say, cupping it in your hands till you need it. Similarly, whereas a wind turbine can generate power while the wind blows, or a solar cell can do so while it is in sunlight, either or both would be useless in a calm in the dark, and if one needs a constant power supply, then it is well to have a means of storing the necessary energy against power-hungry intervals of still air at night.

Recently however, an on-line discussion stimulated me to think a little more flexibly, and as I now see things, it seems that there is room for reflection. Most such schemes take the form of raising water and letting it drive power turbines when necessary. Increasingly however, there is a trend towards the storage of electricity in the form of batteries. And there are other schemes, such as flywheel storage.

All those storage media have their points, and I do not pretend that the principle I describe will supersede all of them, but it does have points of interest and in special circumstances it might possibly be of practical value. The necessary structures would have very few working parts and should last indefinitely with minimal maintenance.

Oh yes, you ask, then why don't we raise water for all our energy storage problems? And if it isn't in fact such a valuable way to store energy, what possesses us to use it in all our great power generating dams?

The answer is that we take advantage of the fact that until recently we have had great tracts of land available to cover with water, and huge amounts of water to cover them with, and people didn't fuss much about the consequences of repeatedly flooding and exposing large areas of dam floor. But nowadays, both monetarily and ecologically, the costs of land and water are everywhere rising catastrophically.

So maybe there is no harm to keeping an eye open for future alternatives to water storage dams for energy, especially if those alternatives have particular advantages such as:

  • not destroying or consuming resources such as land and water on a large scale;
  • not causing pollution as fossils fuels and emptied dams do;
  • not decaying in storage or in use as batteries do; and
  • not seriously reducing the intensity or quality of the power they yield as they approach the end of their store of energy.
In this last attribute, dams, most forms of compressed air, flywheels, and batteries fall short.

So much for the sales talk.

Some facts and implications

Now, in this discussion, I will be cavalier with my arithmetic and my engineering assumptions, and without apology; if the argument were dependent on great precision, it would not be of much practical use, because most practical engineering has to deal with dirty, noisy, approximate conditions and we can’t afford systems that go haywire every time a passer-by sneezes. For example, for convenience I choose figures so rough as to be nearly fictitious, not least because I don’t want to bog down the discussion with pointless recourse to calculators. And some parameters I hardly even consider, such as the price of digging deep holes for construction; the devils in details vary with circumstances, and an engineer who cannot make allowances for such considerations in practice is not worth his salt. (Yes, I realise that some engineers are women, but I generally assume that a woman who can make it in the engineering world probably is very much worth her salt.)

Anyway, for convenience I assume for example that the density of lead is 10, whereas I fully realise that it is in fact closer to 11, and I take other liberties with the facts too when I reckon that they are close enough for jazz. But generally I am overly conservative, so you need not think that I am sneaking in unjustified assumptions under a cloak of innumeracy.

How did lead get into this for example, you ask? Isn’t lead that nasty, poisonous stuff? Yes, but that is one of its advantages. Its main use in bygone years was in high-octane fuel, which rightly was discontinued, causing a slump in the price of lead, which nowadays is used mainly in lead-acid batteries and building materials such as roofing. And in my opinion, its use in batteries is likely to wane in the next decade or two anyway.

For us it has other advantages too; technologically it is well understood; properly used, it is easy to fabricate, and chemically fairly inert, which takes care of a lot of safety concerns. Also, it is not flammable in bulk.

And it is dense.

Proverbially dense in fact.

There are denser materials, some practically twice as dense, such as depleted uranium, tungsten, osmium and so on, but all those are far more expensive and not nearly as plentiful as lead. Mercury is nice, but much too expensive, and even more poisonous than lead. In fact among common dense materials lead is unique in its combination of favourable features at affordable prices. Iron probably would rank next, and some of the suggestions could have been based on iron instead, which has a density nearer to 8. But I’ll assume lead for most purposes.

The figure 10 for the density is convenient because if we take a tonne of lead as 1 cubic metre, then we can take the pressure exerted by a column of lead one metre high as exerting a downward pressure of about one bar. And why do I use a deprecated unit like bar? It simply is convenient; a bar is about 1 atmosphere and accordingly easy to visualise, and it is 100000 Pascal, for those who prefer to think in Si units.

Accordingly a ten-metre column of lead exerts a pressure of ten atmospheres, and so on. Roughly speaking, without making allowance for the cost of heavier-duty machinery and structures, the higher the pressure, the greater the efficiency and the higher the amount of energy that can be stored.

The Principles

First principle: any time you can build a place high enough in which to store a sufficiently dense fluid, you can in principle use it as an energy store by constructing a suitable generator at its bottom outlet, and filling it with that fluid. In this essay I'll assume an electricity generator, which is general enough for our purposes. Suit yourself about thinking in terms of alternative forms of energy.

The denser the fluid and the higher the drop and the higher the pressure, the larger the energy capacity. Furthermore, such combinations potentially increase both the energy density and the efficiency.

So far we have achieved the notional engineering sophistication of the backyard mechanic who understands three principles.

  • I reckon it's working, so it aint broke, so don't fix it, and certainly don't even think of trying to improve it.
  • If it really, really won't work then look for someone who can lend you a hammer, and explain to you which way round to hold it.
  • If it still don't work, look for someone with a bigger hammer.

An example of a device to store energy according to such principles might be a cylindrical tower (yes, yes, I know, but other shapes such as cones or mushrooms have cons as well as pros, and are less scalable in our context. Our towers will be prismatic in principle and cylindrical in practice; just watch this space). 

Pump a fluid, say water, into the tower and when it is full enough, we can withdraw energy by using it to drive our turbine or other hydrodynamic generator; just open its tap to drive the turbine, and close the tap or drive the turbine in reverse when we want to store more energy.


Also problematic; the higher the tower, the higher the cost, and it would take a big tower to store any energy. There are some inconvenient facts.

Devices to store energy by raising weights require some sophisticated engineering, and in most circumstances they might be expensive to build to supply more than a few megajoules. The fundamental problem is that it takes a lot of mass suspended at a considerable height to store many megajoules.

Oh. What are megajoules?

A convenient measure of energy in any useful form.

Consider: 3.6 megajoules (MJ) equal 1 kilowatt-hour (kWh).

You might consume more energy than 1 kWh just in roasting a joint of meat for a family meal, and yet, just 1 MJ is how much energy it takes to raise a 10 Tonne mass 10 metres. That sounds energy-cheap of course, but unfortunately, the other side of the coin is that from a ten tonne mass raised ten metres, you can barely get enough energy to prepare a meal.

So it is hard to imagine the billions of tonnes of water that power utilities need for schemes to store power in elevated storage dams.

One tonne of water occupies about 1 cubic metre, and a ten-metre column of water exerts a pressure roughly the same as atmospheric pressure, and not surprisingly, as already mentioned, if we work with molten lead instead of water, then it takes a column about 1 metre high to exert a pressure equal to one atmosphere.

And the same goes for a block of solid lead 1 metre high.

But how does one pump solid lead, why not just forget it and stick to molten lead?

Well, molten lead presents its own difficulties, and there are other advantages to solid lead apart from coolness and inertness.


Consider a cylinder, say 100 metres, roughly as high as a thirty-story building. Expensive, but not enormously so. Now you pump it full of water and let it empty through a turbine. Suppose it has a diameter of about 3.568 metres; that rather eccentric figure I chose because it implies a  cross sectional area of about ten square metres. Then water to fill it weighs 1000 tonnes. And the energy you get out of it varies with the height of the water column; full power from the top, half power from water halfway up, and so on. The total output is the same as if you had dropped 1000 tonnes through 50 metres, not counting the fact that the last ten tonnes or so would yield practically no energy. (Hardly any pressure, see?) At 1 megajoule per ten metres per ten tonnes, that cylinder could yield something like 500 megajoules at best.

But suppose that instead of a 100 metre column of water, you had had a 10 metre high lump of lead in the top of the cylinder, and dropped that. There would be one awful crash of course, but this is just an illustration. The lead, all 1000 tonnes of it, would drop 90 metres, with a possible yield of some 900 megajoules. If we designed the cylinder so that the lead could stick 10 metres out of the top, we could get a neat, round 1000 megajoules.

But what about a more useful arrangement, though less exciting? We could dangle the lead slug on a chain wound onto a reel, so that it could wind down peacefully, spinning a dynamo to generate electricity.

Much nicer of course, but we must try to do better. That arrangement would need a lot of moving parts with lots of wear and maintenance, and it is no joke lifting and controlling massive weights like 1000 tonnes even once. The gearing alone would be horrendous. In practice we might need to cycle the system a few times daily, and each cycle would stress the system drastically.

Not very practical. Get a bigger hammer.

More practical Alternatives and Elaborations

We most certainly are nowhere near to a practical design yet, but let's see whether we can improve matters.

Notice that the dangling mass certainly would have some tempting points. For a start, unlike the draining fluid, it would yield essentially full power from the word go and continue doing so right up to the end; the pressure would not peter out the way that the fluid pressure would. That simplifies a lot of the design and permits more economical installation and running.

But what about all that wasted space? The slug only occupies 10% of the tower. The fluid exploited the full volume, and unlike the slug, it smoothly distributed the stresses that it exerted on the wall and base of the cylinder.

So. Retain the fluid. Make the cylinder smooth on the inside. Manufacture the slug to fit into the cylinder with a sliding seal like a piston, but no chains, no suspension, unless it seemed advisable to be able to stop the mass at the top with an adequate set of actuated lugs that could keep it parked when fully charged. That should not be a very demanding challenge.

The system could pump fluid in at the bottom, raising the slug by fluid pressure. A ten-metre-long slug would require only about a ten-bar pressure to raise it smoothly; ten bars however would be the pressure only at the lower face of the slug; as the fluid level rose, its final mass, concentrated over a ten square-metre area, would add its thousand tonnes or so to the store, giving us a total pressure of about twenty bars. 

That is enough pressure to be efficiently useful, without being too high for realistic engineering. (I emphasise again that, being inspired by the example of the wall of Saint Donald Trump, I am working with convenient figures, not trying to pre-empt any professional's design.)

Now, this notional design, fluid and all, could yield about 1400 or 1500 megajoules, whereas even doubling the tower height without including the lead would only yield a total of about 1000 megajoules; and you could buy a lot of lead for the money saved by building a 100 metre tower instead of 200 metres. Furthermore, the presence of a 10-metre lead slug floating on top of the column of fluid would have a dramatically beneficial effect: it would guarantee a minimum pressure of 10 bars even as the last few litres flowed out, a pressure as good as the best that the fluid alone could yield with the tower chock full.

Starting to sound better, right?

Of course, it does assume some resources apart from the lead and the tower; the fluid for one thing. If the structure stood by the seaside or even by a perennial freshwater source, one simply could pump water in and run it out indefinitely. But in most places water is not free and we are becoming used to thinking of it in terms of responsibility. To waste 1000 tonnes of water per cycle would not be responsible. But there are alternatives. Most obviously there could be a reservoir into which the turbine discharged and from which it would draw water to recharge the tower. With a some filtration and topping up and a little disinfectant, it could stay good indefinitely.

If a little is good, then a lot...

One of the attractions of the power cylinder units is that n of them can be linked into the equivalent of n times the height or n times the cross sectional area of the cylinder, according to preference. Ten cylinders accommodating ten 10-metre-long pistons would in many ways be equivalent to one much higher cylinder with a single 100-metre-long piston, but without the forbidding pressure problems.

In short, many of the desirable attributes of such devices are linearly scalable over wide ranges of sizes, and they offer pretty fair economies of scale too, before the diseconomies of scale begin to bite.

Such principles of scaling could be exploited in many ways and should improve both the versatility and the maintainability of a large  system.

Such a battery of cylinders could be designed easily to accommodate a million tonnes of lead in say a square kilometre of land, and to offer building space on top. In cylinders with a working height of 100 metres it should be possible to store something like a million megajoules, something like 300 gigawatt-hours, enough to run quite a large city for a week or two.

After all, hubris is one of humanity's major virtues, not so?

More to the point such stored energy could buffer realistic generation shortfalls for months of routine operation perhaps. For that sort of function a square kilometre is pretty compact, especially when the area above the battery need not be wasted.

Getting More Serious About Practical Problems

The devil is nowhere more present in the details, than in simplicity. Let us consider a few complications.

To begin with, you object, if I think of suspending 1000-tonne lead weights at the tops of towers 100 to 200 metres high and deep, I surely should be running for president and building walls, instead of wasting your time with such nonsense.

Even constructing such a lump on the ground would be a non-trivial task, let alone manipulating it. It would not be just a scaled-up version of casting fishing sinkers, even if one tried to do it in situ. And what about the necessary precision? If leakage past the piston were not completely trivial, the system could hardly store the output of a hamster wheel for a day.


But once again, what might be in the details?

Let's start with the concept of precision. It shouldn't be too bad a problem. Given that the engineers will have designed the towers to take the necessary pressure, I suspect that a pre-stressed concrete or a steel cylinder, lined with steel sheet and either polished or lined with a suitable, carefully smoothed polymer, possibly epoxy, would do very well. Then the slug — it need only fit to within a mm or two for a thoroughly manageable manufacturing job. But supply it with the equivalent of piston rings of suitable polymer all the way up, and the fit could be very tight and resilient. I suspect that for wear resistance and low friction self-lubrication,  piston rings of ultra-high-molecular weight polyethylene (UHMWPE) would last indefinitely. With piston rings of such materials, no gap, such as we have in the typical metal piston ring, should be necessary.

But again, those are details.

And yet, you suspect that I still am skirting the problem of handling thousand-tonne slugs, am I?


But divide and conquer, say I.  Fabricate the floor, walls, and top of the piston of suitable steel. Whether our steel be stainless, or coated with a polymer such as a suitable epoxy, or both, I do not mind. It should supplied with a suitable lid that is proof against leakage of the cylinder's working fluid, and it should be installed empty, but complete with piston rings. Such a unit shouldn't weigh more than a few tonnes. Once installed in the cylinder, it then could be floated to a comfortable depth for the operation and filled with lead.

Better never want the completed piston taken out for maintenance you say? Maybe, but that was the good news. Such a piston filled with a thousand tonne slug of lead would be a long term disaster waiting to happen, maintenance or no maintenance.

Well, in my opinion lead (or just possibly cast iron) would in fact be the right material, but we need not think in terms of using it as a single slug. There are two obvious ways of filling the piston without having to destroy it to remove the content: either pack it with lead segments, or fill it with lead pellets or rods, then fill the spaces with a suitable grease, soft wax, or heavy oil.

Now, the lead pellet idea is not so attractive because of the low density to be expected, though a suitable choice of combinations of pellet diameters would have attractions if the pellets were packed with vibration to settle them closely under the oil filler.

Instead of the pellets and their attractions, I prefer the idea of lead segments. A notionally perfect packing would be possible, with segments of shaped lead of convenient size for handling, say a tonne or perhaps just 50 kg each. Prisms could be cheaply extruded, possibly on the spot, though that is a detail, and installed in vertical arrangement in the steel piston. Special sections could be installed to fill in peripheral gaps. Oil could fill the spaces, ease the fitting and remain to protect the metal.

My own preference however, would be for pieces in the shape of sectors of circular slabs, of a size to fit into the inner space of the piston like pie slices. In narrower cylinders entire disks might be better. Units might be contoured for sophisticated fitting, though I doubt that would be necessary or desirable. If a central vertical passage were to accommodate a closed pressure tube, it would be possible to expand such a tube to force the sectors against the steel piston wall to force a good contact between the piston rings and the cylinder wall. Whether that would be important in practice, I cannot guess in advance. It might be better instead to fill the packed piston's ullage with oil or grease under pressure. But in any case such lead segments should give the piston a density practically as good as solid lead. 

The thickness of the pie-slice sectors would depend on the most convenient mass to handle; I suspect that pie slices weighing several tonnes each would be convenient, conveyed by a crane and positioned by hand. The upper face of each slice would have a hollow above its centre of mass to engage the lifting cable; for the rest it would be as smooth as practical. As the best design would not have the upper and lower face of equal shape, only the upper face has such a recess, thereby rendering incorrect installation less likely. Given a gap of diameter 3.568 metres in the piston into which the lead must fit, each metre of thickness would weigh one hundred tonnes. A layer 20 cm thick would weigh twenty tonnes. Split that into ten slices of 36 degrees, and each slice would weigh two tonnes; a convenient figure for a man to push around for fine positioning at the end of the lowering cable, but if desired, the thickness and angle of the slices could be adjusted either up or down.

But, still assuming two-tonne slices, five layers of ten slices would be fifty slices in a 1-metre layer of lead, weighing a total of 100 tonnes, and a thousand-tonne ten-metre piston would require 500 slices. Tedious, but not forbidding. Installation of the lead, or removing it for maintenance, should take less than a week I reckon. For really large installations with multiple cylinders and pistons, robot gantries instead of human controlled ballast handling probably would be more economical and safer, both for installation and for maintenance.

The speed of laying the slices, plus the precision of placement and stability of each layer could be improved by so contouring and lubricating the upper and lower surfaces of the slices that they would mate and instantly settle into position as soon as they were put down anywhere within several cm of the proper position, and that position should alternate in angle between layers so that the slices in each layer would be held firmly in place without slipping or sliding about and each slice would lie above the contact line between the slices immediately beneath, and not immediately above any single slice. The options for suitable patterns are very wide and trivial to design, so I'll not discuss them here, except to remark that the base of the piston container should match the underside contour of the slices.

The steel jacket could be proof against accidents that would ruin a solid lead piston.

Once installed in the cylinder on top of the working fluid, the piston should never have anything to do but float up and down, passively, slowly and smoothly without much friction to speak of. Its working life without maintenance should be indefinite, even if the fluid needed occasional replacement.  

A possible exception might be when the piston must be held in position either for some maintenance work or for static storage of energy when there is no room to store more and there is no demand. It seems to me that the best way to handle this would be with sets of static detents in the form of say, rectangular steel beams recessed radially into the cylinder walls at appropriate heights. At need they could be projected some tens of cm into the cylinder. Two detents in each set would be adequate in theory, though three would be better, and six or eight better still, but again, let the engineers decide. The faces of the detent beams would be contoured to match the inner surface of the cylinder to let the piston pass harmlessly when they are not in use. Their vertical dimension would be far less than the length of the piston, and preferably less than the width of the piston ring face, so that either damage or the vertical escape of fluid past the recess should be minimal.

Mole's eye view

Mass and height are crucial to the function of the units, but in other respects most of the system is very simple. But even simplicity entails complexity in unexpected ways.

To begin with, we are speaking of a lump of lead reciprocating in a smooth cylinder, than which not many ideas could be simpler. Still, we must recognise that one complication already touched on is that of scale. To store and deliver interesting amounts of energy we need a big lump of lead and a big cylinder, both in diameter and height. And the cylinder also needs to stand up to sizeable internal pressures. Even at modest pressures such as twenty bars, one would not like to be too near a cylinder a few metres across if that wall ruptured.

But large, precise, rupture-proof walls are expensive.

One way to reduce the risk would be to drill vertical shafts into hard ground to accommodate the lower regions of the cylinder, preventing any dramatic ruptures. And the deeper the shaft, the more energy could be stored. However, shaft sinking is an expensive activity, so one would not wish to dig more deeply than necessary. And no matter how deep the shaft, the higher one could extend the cylinder economically above ground, the greater the economy and usable scale. And one could increase the practical height by retaining the rocky spoil from the shaft to pack round the base of the above-ground part of the cylinder. And if we were to increase the scale by constructing a battery of daisy-chained cylinders in a fairly close array, with perhaps five-or-ten metre spacing, we could manage some serious height, say fifty or 100 metres down and up.

One could of course make use of existing excavations such as abandoned quarries, mine shafts, or open-pit mines that thoughtless conservationists tend to be rabid about remediating instead of re-using. (Have a read in Wikipedia under headings such as "Environmental remediation" and "Remediation of contaminated sites with cement".) 

Such opportunities tend to be in short supply, so they should not be regarded as routine resources, but where they are available, and there in no foreseeable temptation say, to fill them with water as reservoirs or lakes, there also is no reason that they should not accommodate cheap, deep, and very large-scale gravitational energy-storage installations, both underground and above-ground, together with whatever facilities could be established beside or on top.

Even where one just uses rock walls for supporting one side of a bank of cylinders several hundred metres high, that could be extremely valuable for reducing costs. The cost of digging recesses into the walls to accommodate cylinders would be only a fraction of the cost of sinking shafts of comparable size. And many abandoned open-pit mines are several hundred metres deep.

And the scope for batteries of cylinders capable of supporting entire major cities for weeks if necessary, could transform the prospects for exploiting renewable energy resources such as wind, wave, or solar.

Admittedly, though there are major opportunities for similar facilities at sea, I am interested mainly in inland facilities, because I think that submarine tents offer better and less resource-hungry prospects. See for example:

But it is not sensible to omit notional prospects out of hand, so ...

If it still don't work, I gets a bigger 'ammer

The entire principle so far has been to raise the largest possible mass to the greatest possible height (Balloons anyone? Dirigibles?) and dropping it to the greatest possible depth. The hydraulic approach has special advantages for driving turbines, though electromagnetic devices might have their merits; but that is another story.

However, the constraints include the cost of building high, and the cost of digging deep. Also there is the cost of attaining the strength and the diameter of the cylinder.

Other constraints are the mass of fluid, and the size of the piston. So what do engineers do with the parameters during the conceptual design phase? They juggle the parameters and try to optimise the compromises for given applications. If we can increase the mass of the lead sufficiently, we can reduce the height and depth and even the diameter of the cylinder without affecting the output.

Well, why should we have a top on the cylinder at all, beyond what is necessary to keep out weather and dirt? If the piston is allowed to project out of the top, then there is no immediate limit to its length, so there is no reason it should not be as long as the cylinder, filling the cylinder completely when it reaches the bottom. This yields ten times the pressure that a column of water would, and twenty times the energy.  There are of course limits to how high the column could reach without overbalancing, but they would not be very close, and struts to support the column would be relatively cheap.

On the same principle, there is no reason the piston should not be still longer than the cylinder, thereby increasing the mass without having to dig deeper or build higher.

In case one wished to avoid really high structures however, then instead of making the piston simply cylindrical, the part that never descended into the bore could extend out sideways, instead of vertically. Triple the diameter, and it would be nine times as massive as the same diameter of shaft, and easier to balance.

The engineers designing the shaft would of course have to balance the choice of materials and the structures for safety, reliability, durability, and manageability, but that is what engineers are for.

Bless the dears!

What's your tipple?

The choice of fluid in the system is not necessarily automatic. In principle it need not be a liquid, and even some gases would do. I do not deny the possibility of using any gas at all of course, and elsewhere I have agitated for the use of air for underwater power storage. However, gases do have their shortcomings. For our purposes the first one is that they are not very dense, and we really value density in our working materials. Another is that compressed gases tend to be more dangerous than liquids because in the event of a rupture they expand explosively, hurling debris dangerously for long distances. Thirdly, because of their ability to waste power in expansion and contraction, they entail many inefficiencies.

So, suit yourself, but I shall not consider gases seriously in this application.

I have mentioned water from time to time, for example where it is available for the pumping, such as beside the sea or similar water bodies, and water, especially free water, definitely has its attractions. A slightly concentrated brine with a small admixture of biocides such as zinc and boron salts should remain indefinitely clean and safe to handle, but however well you looked after it, it would inevitably present problems of corrosion. It also is not a very attractive heavy-duty lubricant.

So though I do not deny its possible value, I am not enthusiastic about the use of water.

Instead of water, we could use a suitable organic fluid, say diesel fuel or kerosene; it would be only about 0.8 times the density of water, and would cost a good deal more, but it could have its attractions all the same. For example, some such fluids are more mobile than water, requiring less energy to pump. Also, at the end of its period of use, such liquid still would retain much of its original monetary value. And with suitable selection of materials, suitable organic liquids could aid in protecting and lubricating seals and moving parts. It would be interesting to contemplate the use of semi-solids such as greases instead of liquids, but that is a matter of engineering detail that we need not discuss yet.

If we were willing to work with suitable materials under suitable pressures at low enough temperatures, we even could think in terms of liquid SO2 (density 1.4) or the like. However, having had to deal with quite modest quantities of SO2 myself in my murky past, I hesitate to recommend it to anyone in quantities of tens to thousands of tonnes.

Chlorinated organic compounds such as CCl4 and larger molecules, preferably non-volatile, have their own attractions and are comparatively cheap and fireproof, but I do not offhand know of any that is at once cheap enough and otherwise attractive. Also, in spills, they could be very troublesome to clean up in comparison to simple hydrocarbons.

Of course, even oils are destructive to various materials, and they produce vapours that are harmful to workers and present fire hazards, but such problems are well understood and manageable.

Subject to good arguments to the contrary, my favoured option remains something along the lines of a light oil mix that will not present problems of freezing or leaking at ambient working temperatures.  

Some possibly useful configurations

To illustrate some of the principles, I here present some strictly schematic representations of notional designs. They are of course not even vaguely to scale and do not represent viable mechanisms, but I hope they prove helpful to readers who found my wording confusing.

I omit some versions, especially some that I regard as unpromising, such as telescopic compound pistons.


This is a simple minded version in which the weight of the fluid and the lead combine to give a high output of energy per cylinder. It is however not  very compact. Its pressure drops as it empties, but not unacceptably.

Variation on the open circuit, with less constraint on cylinder height and piston mass, which could be any length within reason, filling the cylinder, or longer, even mushroom-shaped, see below. The greater mass of lead raised as high as possible gives very high energy capacity.

This version uses the fluid almost purely to raise the piston; its contribution to the mass is negligible and its volume minimal. The cylinder is as narrow as practical and its wall must be proof against the much higher pressures than other designs achieve. One advantage is that it could greatly reduce the required volume of working fluid.

In some ways the closed circulation piston is more efficient. For one thing, it is more compact in terms of the real estate it occupies, because it requires no reservoir. On the other hand the raised fluid does not contribute to the gross output, but only to the raising of the next batch of fluid. It does however have two  merits: it contributes to the output pressure; and it keeps the working output pressure very constant. It also improves the options for daisy-chaining cylinders by feeding the output of upstream cylinders in above the downstream cylinders (see below). Because the unit is sealed, it is suitable for fluids other than water, such as hydrocarbons.

This is one approach to the design of batteries of cylinders, such as could be used in abandoned open-pit mines. In the configuration  shown here, the cylinders are as deep as may be practical, partly in drilled shafts, and partly covered by spoil from the drilling. They are sealed. The outputs from cylinders may be directed variously to impose pressures in headspaces of downstream cylinders in series, or yield power in parallel with other cylinders.