Compressed Air Windmills

One of the things that I always keep an eye out for are ways in which to utilize the slow rotation/high torque of windmills without the need for expensive and high maintenance gear boxes.  The most common type of electric generating wind turbines rely on such a gear box to spin their generator’s rotor at the required speed, and to control the angle of the turbine blades for survivability and to provide an optimal power flow to the generator rotor.  I would like to propose a windmill design that requires at most one or two gears.  This windmill would be similar in appearance to common electric wind turbines with a tall mast with wind harvesting blades attached to a nacell.  However, instead of a gear box within the nacell there is a low speed/high torque air compressor which takes in filtered ambient air and compresses it.  This compressed air is fed into a buried storage reservoir for energy storage during windy periods.  The compressed air reservoir would then be tapped to directly supply compressed air for tooling use, to drive an electric generating turbine, and possibly to drive hirsch vortex tubes if local heating and cooling is needed.  Finally, and this is probably the most interesting aspect of the design, the angle of the blades of the turbine would not be mechanically rotated for control as they are in standard turbines, instead, the shape of the blades would be controlled pneumatically.  In other words, the compressed air generated by the turbine would be used to inflate control bladders on the blades to control what the blades do with the incident wind.  I think that with carefully chosen shape control bladders, these blades would be able to fail safely in a way that the current rigid blades could not.  In a high wind storm, for instance, a stuck mechanically controlled blade could cause a wind turbine to be torn apart, but with a shape controlled blade, a failed blade could be designed to simply deflate to a shape that diverts wind safely.

Update 10-14-2016

I had mentioned that Hirsch vortex tubes could be used to supply local heating and cooling, but I feel that I should expand on this and explain how it could contribute to the overall efficacy of the system.  The air compressor will naturally squeeze a great deal of heat out of the air it compresses when the wind is blowing.  Since the wind will be blowing, a primary air cooling system (a radiator design) is recommended for survivability.  In other words, the compressor should be able to survive its own heat generation as long as that air cooling system is functional.  However, a secondary heat exchange system could provide some added efficiency.  It is presumed that one of the more consistent uses of the compressed air generated by the windmill will be an electric generator.  This generator will naturally require heat to operate because expanding air absorbs ambient heat.  Therefore, to prevent the electric generator from icing up when the wind is blowing, a heat exchange system can be employed to bring heat from the compressor to that generator.  But, recall that the intent of the compressed air storage is to keep the electric generator running even after the wind stops blowing.  During such times the compressor will not be able to supply heat and another source is required.  Here is where the Hirsch vortex tubes can be utilized to keep the generator heated and operating at capacity.

In regards to the air compressor itself, I declared that a low speed-high torque air compressor would be employed, but I spoke little about the design of such a thing.  To make it low speed/high torque, the compressor will need to act upon a larger volume of air then common air compressor designs ever would, and it would be desirous for it to operate at a constant torque rather than a time varying torque to keep the blades from jerking in an alarming and mechanically dangerous way as it operates.  To achieve this constant torque, something akin to a multiple cylinder engine might be required, such that one of the cells is always in the compression phase as the others are intaking ambient air or exhausting compressed air.  A design similar to the Wankel engine except without the need for the ignition portion of the cycle, and copied several times to insure constant torque might accomplish these criteria in a scalable way.

Update 10-15-2016

The air compressor would require a means by which to manage its power output to match the rate at which the wind is blowing, because a pleasant zephyr simply cannot compress as much air as a gale.  For this, and in keeping with the proposed multi-chamber semi-Wankel engine, the compressor would employ a means by which to “turn off” the individual compression chambers.  This would not require any additional gearing, but would instead work by unsealing the compression chambers, allowing those unsealed cells to keep spinning without adding any additional load to the shaft except for its friction.  So, if the compressor has say, twenty chambers, then the number of chambers which are sealed at any given time could be selected from 0, 8, 10, 12, 13, 14, 15, 16, etc.  Note that 1-7 cannot be selected because a minimum number of chambers is required to keep the torque constant and prevent blade jerking.

Update 10-17-2016

The blades of a wind turbine are a crucial aspect of the design, and the existing blade designs are engineered to accomplish many tasks simultaneously.  A typical wind turbine blade is a solid skin of light strong material that is given a perfect aerodynamic shape.  It is a balance of mechanical strength and utility, and as a testament to them, one of their greatest drawbacks seems to be that they are too large for standard shipping.

I have said above that I would want to eliminate the need to rotate the blades to control how they harvest the wind power.  This blade control shares some of the goals of the variable compressor loading that I described above.  The idea is that there is a mechanically optimal range of rotation speeds and drag forces for the turbine blades, and the blades are kept within those ranges by varying the amount of shaft load and by controlling the tilt of the turbine blades.

I propose that the same type of control which comes from changing the angle of attack of the turbine blades and an even greater level of control can be achieved using fixed blades that change their shape.  For this new blade deisgn I envision a central structural beam around which is constructed a light rigid frame which forms a wider circular column around the central beam.  This frame is then covered in a flexible and elastic wind-breaking material.  Spaced around the central beam at the openings in the rigid frame, are gas bladders or pneumatic piston actuators that, when inflated or actuated, press out against the elastic material to change its shape.  In order to “fail safe” the actuators or gas bladders would be spring returned or deflated such that if pneumatic control power is lost, the blade returns to its cylindrical shape and stops harvesting wind.  This design has two advantages over tilt blades.  Firstly, the active surface area of the blade can be finely tuned by changing how much of the blade is cylindrical as well as the angle of attack.  This means that for any wind situation or angle of attack, the controller or automated control program can decide if the blades are 100%, 75%, 27.3%, or 10% effectve area, whereas with a tilt blade, those percentages have a fixed dependence upon the angle of attack.  Secondly, with the central beam design and the flexible wind-breaking covering, it becomes possible to create blades that are shipped in pieces and quickly assembled on site at the time of construction.

There are a few extra things which need to be taken into account for such a design to work.  Firstly, I have said that the compressed air generated by the compressor could be used to power the blade pneumatic controls, this requires the use of some sort of device which allows the pressurized air to transition from the non-rotating tower column into the rotating blades, without losing pressure.  This device would be a special bearing.  Additionally, either electric power and control signals would need to be transferred through the bearing or an isolated pneumatic line for each actuator or bladder.  The minimal way to accomplish this is by putting electrical power through another special bearing (needing only two terminals), and then to transmit control signals wirelessly from a transmitter in the tower to receivers in the blades.  Those receivers then use a small amount of the electric power to switch the pneumatic control power to the desired actuators.

Next there is likely to be a need to quickly and easily maintain or replace the outer skin of the blades.  For this, it seems most reasonable to add some attachment points and other apparatus on the tower’s column to allow maintainance personelle to anchor one of the blades in its downward position, then cut off the old skin, and roll on a new skin from the tip of the blade to its base, not unlike an enlarged nylon stocking.

Update 10-20-2016

In regards to the compressed air energy storage, it is likely that the machines/tools operated by the compressed air will have an optimal operating pressure.  However, the pressure in the storage tank will have to be variable as the amount of energy stored will be a function of the pressure in that tank.  Therefore, it will be best to design the storage tank such that it is rated for pressures far in excess of the optimal operating pressures of the electric turbine and other devices.  Also, devices with relatively low optimal operating pressures should be preferred because the maximum pressure will be limited by the capabilitites of the compressor in addition to the rating of the tank.  Then, these lower operating pressure devices will be fed through dedicated pressure regulated secondary storage tanks.  These tanks will maintain the devices’ optimal operating pressures by means of a regulating valve that draws air from the higher variable pressure storage tank if and only if the pressure seems to be dropping below optimal within the secondary tank.  By this means, the electric turbines and pneumatic tools can be designed for operation at a single regulated pressure, rather than for the widely varying pressures of the storage tank.  This design will also mean that there will be a threshold pressure within the storage tank below which nothing will be able to operate. The energy remaining at this cutoff pressure would be unusable unless some means to up-regulate the pressure were to be employed.  Such an up-regulating device would, I think, have to be a compressed air driven air compressor, which may not end up being economically efficient enough to pay for itself.

Second Update 10-20-2016

This basic pressure down-regulating system should work reliably, but there is still a lot of inefficiency that seems like it should be eliminable.  Specifically, the energy stored in the pressure difference between the storage tank and the secondary tank is mostly lost.  In electronics, when you have a DC voltage that is too high, one of the efficient tricks used is to pulse that high voltage for short intervals through reactive devices that smooth it down to a constant lower voltage.  Unfortunately, when one is talking about mechanical compressed air valves rather than transistors, there are mechanical limitations to how fast switching can occur, and valves wear out at a much faster rate than transistors.  So, there is a better solution.  The compressed air driven electric turbine can be designed to operate in stages when the storage tank pressure is at high levels.  Each stage is its own self contained turbine, which operates at a single pressure differential and all of these stages power the same shaft.  The important part is that the outlet of each stage can be switched to drain into a pressure stroage vessel rather than to open air, and the inlet to each stage can be switched to any of these other storage vessels.  So, for instance, if each stage was designed to convert a 20 psi pressure differential into work and there were four stages and the main storage tank pressure was at 100 psi, then the device would be switched such that the first stage would get regulated 95 psi input and output air at about 75 psi into a storage container, that air would be down-regulated to 70 psi and fed into the second stage which outputs air at about 50 psi into another storage vessel which then gets down-regulated to 45 psi and fed into the third stage of the turbine which outputs about 25psi, and finally is regulated and fed into the fourth turbine.  Once the main storage tank pressure drops below 95 psi, the stages switch from a stack of four into two stacks of two, and when the pressure in the main storage tank drops below 50 psi, the stages switch to four parallel stacks of one.  This method allows for the preservation and use of more of the energy stored in the compressed air, at the cost of additional storage tanks, regulators and valves.

Third Update 10-20-2016

Finally, in regards to the main storage tank, it could be possible to make a constant pressure design rather than the variable pressure design that I have presumed above.  To accomplish this, one would take advantage of the existing tower construction of the windmill.  With this concept, a water tower is added to the windmill nacell.  This will contain a fixed volume of water that pushes down on a large compressed air storage piston in the base of the tower.  This piston will then have a constant pressure as determined by the weight of the water pressing down on it divided by the cylinder cross sectional area, and the amount of energy stored will be proportional to the volume of air within the piston.  This allows for a much more efficient extraction of the stored energy, at the cost of additional structural strength to support the water and the additional cost of the massive sealed piston design.  This water, could of course, be replaced with any other heavy material such as steel, dirt, scrap metal, or stones, but the water tower function could be tied to other local systems to ensure water pressure in local residences.  Indeed, this complex installation that I have envisioned seems like it could lend itself to a remote farming/other installation that could utilize nearly every aspect of the design:  The electricity from a compressed air turbine for normal houshold appliances, the compressed air for tooling, farming equipment, and water pumping, and the water tower for reliable water pressure.

Update 10-21-2016

A small but important improvement to the constant pressure storage tank piston concept:  Instead of a single massive piston, it would be better to use multiple smaller pistons which can be designed to be shipped preassembled.  With these multiple pistons designed to operate in parallel, any single piston could be removed from service for repairs or maintenance without taking the whole system offline.

Update 10-23-2016

Regarding the special bearing needed to take a pneumatic line line from a non-rotating frame to a rotating frame, care should be taken to avoid inadvertantly creating a tesla turbine or pump.  There will always be some torque exerted by this bearing on the rotor shaft with respect to the stator as a consequence of having to accelerate the flowing air from stationary to rotating or vice versa.  This torque will act to decelerate the shaft in a manner similar to friction, and it should be taken into consideration if high flow rates are required.  That is unchangeable, but the forces which the design can reduce are boundary layer forces.  These forces are exerted by a fluid traveling through a confined space and they act to pull the walls of that confined space along with the fluid.  This effect is especially prevalent in confined spaces, and can be reduced by giving the fluid more free volume to flow through.  Therefore, this bearing should be designed with a wide, sealed toroidal chamber with inflow evenly distributed at points around its outer circumference and with outflow evenly distributed at points around the rotating center shaft.  This causes the majority of flowing air to move radially with respect to the shaft, and therefore minimizes the amount of boundary layer torque.  Additionally, in order to minimize any reverse centrifugal pump action, the outlet holes should be at a minimal radius from the center of the shaft, and the inlet holes should be as close to the central side of the toroid as possible.  Imagine a donut, the inlet holes would be oriented closer to the donut hole side of the donut toroid then to the outermost radius of the donut.

Update 10-24-2016

The pneumatic control system within the blades of the windmill will always be subjected to additional pressure created by centrifugal forces that are proportional to the rotational speed of the rotor and the distance away from the center of rotation.  Because of these forces, each component must be able to resist misoperation when subjected to pressures outside of their normal operating range.  To quantify the problem, one must calculate the structural failure speed of the blades.  This will be the maximum design speed (with an added margin if desired), and is used to calculate the worst possible overpressures.  Each component must then be designed to operate properly at pressures up to and including that overpressure.


Update 10-25-2016

I’ve come up with an alternative blade design with the same adjustability, but without the outer covering which I foresee to be problematic.  It would still be a wide tapered structural column.  Within a section of this column would be arranged pneumatic pistons that can be extended.  Attached to the extensible end of these pistons would be the corner of a durable sail cloth material.  Within the column, the other two ends of the cloth would be wound around a spring returned spindle.  So, when the piston extends, a triangular section of sail cloth appears.  When the piston retracts, the cloth is retracted as well.  This design would be modular with each blade having two or three of these sails.  Each section would have at least four of these sails, arranged evenly around its circumference such that during normal operation, only two would be deployed.  Also, since pressurized pistons aren’t typically designed to handle transverse loading, the piston can be used to extend a sturdier structural arm such that that arm bears the wind load and the piston only needs to be designed for compression.  Finally, the extensible arms should be spring returned such that the blades return to a safe tapered cylindrical shape in the event of loss of control power.


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