Self-tensioning pneumatic tower technology for wind turbines and cell towers

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Pochari Technologies has recently invented a new type of highly novel and highly useful structure for use for wind turbine and cell tower support structures. There are many interchangeable definitions or names for this technology, a few proposed so far are “hydrostatic tower”, “pneumatic tower” and “autogenous tower”, but the term “energetic tower” or “active structure tower” are suitable as well. The concept of using pressure to generate rigidity itself is not new, inflatable domes make use of it. In 2009, Melvin L. Prueitt [1] patented an inflatable structure drawing its rigidity from compressed air using low compressive strength fibers. In 1984, Jack G Bitterly [2] patented a hoop-stress-loaded hydraulic column to bear vertical loads. Pochari Technologies’ design differs considerably from these prior concepts in configuration, but not in the principle of being a fully hydrostatic structure. Firstly, our tower self-tensions its guy cable, since it’s designed for tower applications. Secondly, our design unlike Pruiett’s, uses a single cylindrical tube for simplicity. To this date, there is not one terrestrial structure that uses this brilliantly novel concept of bearing a load using the internal pressure of a cylindrical pressure vessel as opposed to transferring it into the wall of the structure. While a conventional structure derives its rigidity from the intrinsic stiffness of a particular material, such as concrete or steel, the autogenous tower does not use the stiffness of a material to bear weight, but uses solely hydrostatic force to bear a load, therefore the structure derives its stiffness from energy, so the term “energetic tower” is apt. But let us escape semantics and turn to the important details. A hydrostatic tower falls under the category of an active structure that employs the energy from a compressed medium, this can be gas or liquid, to impart linear force onto a piston which causes the self-tensioning of guy cables. The structure becomes fully rigid as a consequence. While “hydrostatic” force denotes a liquid, it need not be a liquid, any pressurized medium imparts hydrostatic forces onto its surrounding, the atmosphere for example is considered to produce “hydrostatic” forces. The principle of hydrostatic force is exploited in virtually all engines. An internal combustion engine uses the force of rapidly expanding gas to force a piston down, a hydraulic ram generates immense linear force by highly compressed oil, not to mention the power generated from large streams of water in dams. The power of hydrostatic force is often overlooked and has rarely been applied to structural engineering. In some instances, structural engineering has been able to make use of pressurized mediums, such as inflatable domes, automobile tires, and basketballs, all of these structures make use of hydrostatic force to function and attain stiffness. While the strict definition would be “pneumatic” as they make use of gas, pneumatic fails to emphasize the energetic and forceful phenomena at play in this structure. The autogenous tower at its core is a pneumatic structure, which exploits the constant action of tightly squeezed gas molecules to suspend great masses and generate huge upward forces yielding a “self-tensioned” structure. The crucial design element to emphasize is the use of hoop stress-loaded components, which is the main pressure-bearing tube.

There are four main components to the autogenous tower.

#1 Cylinder/top-mount assembly

#2 Pneumatic containment structure (composite pipe)

#3 Guy cable assembly

#4 Foundation pad with a pressure modulation system.

The tube is filled with pressurized gas, this ideally is non-reactive such as argon or nitrogen, but hydrogen can also be used due to its light weight and very high heat capacity, which minimizes density changes. As the open-ended cylinder is filled, the piston is pushed to the end of the cylinder until it exits the end. Guy cables are fastened to the piston, preventing the piston from exiting, thereby generating tension on the cables. The tube is placed vertically in the air, with the piston all the way at the top of the tube just near the end, the bottom of the tube is attached to a foundation pad, bearing the weight of the hydrostatic force as well as the weight of the tube. The pressure-bearing cylinder is subject only to hoop stress, the cylinder bears none of the weight that can be placed atop the piston. This allows the tube to be designed as an ultra-slender member thanks to the fact we eliminated lateral and compressive loads. The working principle of the invention is the hydrostatic cylinder, employing a pressurized medium, preferably gas, the column no longer becomes a load-bearing member. At the base of the column, a concrete pad with an air compressor carries the weight of the column plus the force of the pressure. The column is a slender cylinder, designed to withstand the internal pressure of the gas only, the column derives its lateral stability from a series of guys, much like a classic guyed communication tower. At the end of the cylinder, the piston is able to reciprocate up and down freely, transferring one of its linear forces to the walls of the tube. As the column is filled with a pressurized medium, the piston is subject to a force equal to the pressure times the area. For example, if the cylinder is 250mm in diameter and carries 10 MPa of pressure, the forcing acting on the cylinder is 49,900 kg, or nearly 50 tons. This force would result in the piston being lifted with great speed until it exits the end of the cylinder, the guy cables carry all of this force and transfer it to the foundation pads on the ground. The top of the piston contains four or more connections where a low-elasticity cable is fastened, the cable can be constructed out of ultra-high molecular weight polyethylene for maximum performance. One of the most elegant aspects of this structural technology is its exploitation of the tremendous tensile capacity of Dyneema, the ultimate tensile strength of Dyneema is 3500 MPa, or 508,000 psi, while this material can withstand immense loading, it cannot carry an ounce of compressive loading, this is why it is rarely used for any conventional structural applications. In the autogenous tower, as the piston is subject to upward force, it rises immediately tensioning the cables in the process until the structure becomes utterly rigid, unable to flex whatsoever. The tube bears none of this linear force, as it is subject only to hoop stress, therefore the tube can be as slender as possible minimizing its weight to the absolute minimum.

Once fully pressurized, the cylinder is unable to move downward unless a force greater than the force acting upon is it produced. In the case of a wind turbine, the piston carries the dead weight of the nacelle and blades plus the lateral forces resulting from static pressure acting on the turbine blades and nacelle in addition to the lateral loads acting on the tube from wind friction. The pressure bearing tube, while not subject to the gravimetric load of the nacelle, is nonetheless still subject to the static force of the wind which causes it to bend, this bending motion is prevented from occurring with the guy wires, but in the autogenous tower, unlike a classic guyed tower, the guy wires ultimately transfer this lateral load into compressive load on the column structure by connecting it to the piston. The upward force of the piston allows the column to be tensioned thereby reducing the compressive loads it must withstand. As mentioned, the tube wants to bend from the force of the wind, the guy cables may prevent this, but this lateral force is simply transferred or converted directly into compressive loading, a classic guy tower fails in compression, not in bending. As the tower wants to bend, the cables must pivot since they cannot stretch, as a result, the only way for lateral movement to occur is by shortening the tower, that is for the tower to sag. This places very strong compressive loads on the lattice structure of the conventional guyed tower. In the autogenous tower, this is completely prevented by running a cable vertically from the piston to the intermediate tube stabilizing guy cables. As the tube wants to bend from the wind, the lateral guy prevents it by attempting to compress the tube, but rather than this compressive load being transferred to the tube, it is transferred to the piston by the vertical cables. Therefore, the tube itself is simply standing there totally idle, with entirely of the exogenous structural loads carried by the piston, and the tube bearing only the pressure of a gas at 5-10 MPa. The four Dyneema cables experience some degree of stretching when loaded, at a load factor of 10% of breaking strength, Dyneema (ultrahigh molecular weight polyethylene) will stretch approximately 0.5%, this increased length is then retracted using a winch mounted on the guy mooring sites. In order to maintain a constant force at the piston, a compressor and relief valve are installed at the base of the tower. When the temperature is high during the day, the density of the gas decreases, and the volume it occupies increases, resulting in greater pressure. The converse happens when the temperature falls. The pressurized tube can be constructed out of either a rigid or flexible material, as long as it can withstand the internal pressure. The material can be composite, fiberglass, kevlar/aramid, carbon fiber, or metals, aluminum, steel, titanium, magnesium etc. This provides a brief overview of the technology, applications include not only wind power, but pile drivers, novel high elevation structures for human habitation, communication towers, cell, radio etc, and potentially stationary gantry cranes. Due to our philosophy, we have decided to make the invention open source to promote development and experimentation ultimately leading to low-cost energy for mankind. What’s astounding about this wind turbine technology is the impressive EROI.

The main application of this tower, and its invention origin, is wind energy. There is an ever-growing need for inexhaustible and clean energy, one such source is terrestrial wind power. In order to make existing wind turbine technology more competitive, reduce cost, and generate more energy, the ability to practically exploit higher velocity wind is desired. Since the energy from wind is the cube of the velocity, increasing the velocity only slightly has very significant effects on the annual power output of the turbine. Unfortunately, only a select few sites on land feature wind speeds over 9 meters per second. On the other hand, at a height of 200 meters, many onshore sites around the world have wind speeds of up to 11.5 km/s. For example, using a hypothetical site in the Nebraska sandhills, the average wind speed at 100 meters is 8.65 m/s, yet it increases to 10.66 at 200 meters. This difference of only 2.01 m/s would yield an additional 196 kW for a 21-meter diameter turbine. That is with an increase of only 2 meters per second, the power output doubles. At a capacity factor of 0.5, the annual increase in output from doubling the hub height is 860,000 kWh, or a potential revenue stream of $43,000 at a price of 5 cents/kWh. In reality, the capacity factor cannot be held constant, as it increases sharply with mean wind speed. The concept of a capacity factor is effectively a measurement of the variability in mean wind speed, most wind turbines will generate less power than their rated power at the mean wind speed, higher velocity wind regimes tend to feature less variability, whereas as low-speed regions are much more variable, with extended “doldrum” periods where the turbine produces virtually no power. An estimate from the Western U.S places the capacity factor for 7 m/s at just above 35% and over 65% at 11 m/s. This means that not only does the baseline hourly power output double, but the total annual yield increases as well. For the wind speed at 100 meters of 8.65 m/s, the capacity factor is estimated to be 46%, so the annual yield goes up by 34% above and beyond the doubling of power. Since wind speed decays towards the ground due to surface roughness, there is a strong incentive to design a new generation of high-altitude wind turbines. The impetus of the invention is the inability of classic tower technology to facilitate such heights feasibly and cost-effectively. Conventional steel wind turbine towers rarely exceed 100 meters onshore, squandering the vast potential of higher speed above. The principal limitation preventing designers from reaching these higher speed winds at 200 meters or more is the weight and concomitant cost of the conventional steel tower begins to escalate dramatically. Conventional wind turbine towers are constructed from colled-rolled steel drums, this cylindrical column is subject to both compressive loads from the weight of the nacelle as well as tensile loads from the mast bending moment due to static wind loads. In order to achieve a minimum degree of rigidity, for a 1 MW wind turbine, a 200-meter conventional steel tower would weigh over 330 tons, the cost of fabricating and erecting such a heavy tower is prohibitive, hence the current practice of remaining at around 100 meters or less of hub height for onshore turbines.

In light of these limitations, a better solution is called for, the aim of this invention is to facilitate the design of high altitude wind turbines using a lightweight low-cost structure employing the principle of hydrostatic force. Interestingly, by using this elegant structure, the material reduction and subsequent power density of wind energy is improved dramatically. The benchmark for energy density has always been nuclear fission, with deuterium fusion the only conceivable energy technology that surpasses it. But in practice, a fission reactor in a pressurized water configuration has actually a lower power density than diesel or gas turbine powerplants. The average pressurized light water reactor in the U.S uses approximately 50 tons of steel and 120 tons of concrete per megawatt of electrical capacity. Conventional wind turbines use far more than this, but since a large preponderance of this weight is concentrated in the tower structure, when the load-bearing tower structure is eliminated, the steel required is reduced markedly. In fact, the pressurized tube can be constructed out of composites such as fiberglass, lightweight metals such as aluminum or magnesium, or even low compressive strength fibers such as aramid/kevlar. Even if the tube was constructed from steel, it would only weigh 25 tons for a 450 kW turbine, translating into a steel requirement only slightly higher than a nuclear reactor. The ability of a wind energy system, harvesting free terrestrial energy, to achieve a power density almost as high or equal to state-of-the-art nuclear power plants, is nothing short of astonishing and achievable only with Pochari autogenous tower technology. In addition to the promising application for wind technology, the communication tower market is the prime first application for hydrostatic tower technology. Present guyed mast systems have abysmal payload capacities, with most guyed masts in the 400-foot range being able to bear only 100 lbs of antenna weight excluding the weight of a maintenance worker. With Pochari Technologies’s self-tensioning tower technology, the same diameter and weight tower can carry tens of thousands of pounds, many orders of magnitude more than a steel lattice structure. This has the potential to utterly revolutionize the communication tower industry.

Sources:

[1] https://patents.google.com/patent/US20090260301

[2] https://patents.google.com/patent/US4685253A/en?oq=+4685253

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