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.   

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