An initial look at braking systems and blade design on wind turbines
1/18/2012
Summary
It is theoretically possible to power a small office using 3 small wind turbines and a storage battery. The battery should be fairly large to guard against long bouts of windless weather; at this time I think something like a 130 Ah, Group 30 Trojan deep cycle battery should be suitable, for under $200. As for braking, I haven’t yet seen an ideal breaking solution, but comparing dynamic and frictional braking techniques, I think a dynamic braking resistor would be helpful. I have seen examples of software used to design ideal aerodynamic surfaces (like HyperSizer for very large turbines), but not yet come up with an idea to improve the blade design.
Problem
A wind generator will produce power continuously, but demand is not necessarily continuous, so a storage battery is desirable to accumulate charge. The SolAir wind generator we are using can provide 12 V or 24 V. We have chosen to use a 12 V to 120 V inverter (producing AC from DC) so we should get a 12 V battery which is capable of being heavily charged and discharged.
I found that the braking system on a wind turbine is crucial for all but the most trivial applications. Large wind turbines have extremely long blades (perhaps 50 m diameter) and although the rotational velocity might be small (20 rpm) the tip speed at the edge of the blade might be 200 mph. A small wind turbine (like the SolAir generator that we are using) has blades about 2 ft (61 cm) in length, the potential rotational velocity is much higher (manufacturer says that in high winds, it could exceed 1000 rpm, although this might be exaggeration). Furthermore, the blade is made of aluminum and has a very flat edge. When this blade is at speed, it is very capable of cutting off children’s limbs or killing birds that fly into it.
Although the behavior of birds is hard to predict, it is desirable that even a small wind turbine have braking capability, even if mounted high above where children can reach it, if only so that when the turbine must be serviced, it can be done safely.
Most large wind turbines achieve maximum efficiency at higher windspeeds; 33 mph (15 m/s) is common. Most wind power stations cap the speed allowed at 45 mph to stop the blades moving so fast that they overcharge the battery.
Because 33 mph is a pretty stiff wind, it is probable that the wind turbine will operate mostly at less than max power output. The blade design should not be a weak point. It should catch as much productive wind as possible.
Theoretical Limits
The theoretical maximum efficiency of a wind turbine is 59.3% as calculated by Albert Betz in 1919. That is, 59.3% of the kinetic energy of the wind can be turned into kinetic energy of a plane incident to it (the spinning blades). Since the electric generator is very efficient (over 90%) the max real efficiency is still over 50%, which would be enormous. The finality of the Betz limit (59.3%) casts doubt on the company's claim that this device is capable of converting up to 70% of the wind’s energy to electricity. But perhaps that figure includes the energy provided by the supplemental photovoltaic cells.
Unfortunately, the wind is constantly changing direction and magnitude, so it’s extremely difficult to find the actual kinetic energy present in a quantity of air. What is more often done is list the capacity factor, which is a metric normally reserved for power plants to describe what percentage of their namesake power (say, a 1000 MW power plant) is actually produced in a given period of time. For baseload power plants, the percentage is generally above 90% as the demand for their power is continuous and predictable. For wind power plants, the capacity is generally much lower, at 25%, owing to the fickleness of the wind. So a wind generator which lists a possible 2 MW could only produce in the neighborhood of 250 kW in sustained operation.
If we take 25% as a reasonable benchmark, then three 800W solar generators produce a net 600W continuous output. The demands of our office (one desktop computer, one server, fluorescent lighting, and perhaps other rarely-used testing equipment) will not be static, but vary throughout the day. Theoretically, if the server and computer were both at max power (400W apiece, say) they would be consuming more power than the wind generator could deliver. But unless engaged in heavy computing, they would not be using anywhere near that amount of power. For moments of peak demand, the battery would simply discharge slightly until demand once again was reduced. Assuming that the electric generator on the turbine and the inverter to get AC power from the battery are both very efficient, (>90% efficiency) the wind turbines would theoretically be able to power the office.
Possible Solutions
Car batteries are generally “starting” batteries, with very high cranking amps but very low tolerance for discharge. They are a poor choice for storage and are suitable only when continuously kept at >80% charge.
A better option is “deep cycle” batteries, which are designed to be discharged more completely. They do have less cranking amps than a starting battery, and are costlier, but they are a decent choice for energy storage. A popular measure for storage capacity is “reserve capacity”, which states how long the battery can be discharged at a certain current drain. A more general one is “amp-hours”, which states the product of current and time for which the battery can discharge. Deep cycle batteries with 25-50 Ah are available for less than $100. On balance, I think going for a bigger battery would be better than building up from smaller batteries if the need increases. A 130 Ah Group 30 deep cycle battery from a maker called Trojan is available for less than $200.
There exist many methods of causing the blades to stop. They can broadly be defined as frictional braking and dynamic braking.
Physical braking is possible by including a disc (or rotor) rotating with the rotor shaft of the blades, which would be stopped by friction surfaces pressed into it by hydraulic cylinders. This is exactly the principle of what stops a car. Low-cost off-the-shelf small disc assemblies are available by many tool and hardware companies for under $100 but most are designed for trailers, and are relatively heavy. The best option here would be mechanical disc brakes, designed for very light vehicles such as power chairs and scooters. I note that there are such calipers (with pads already in place) for about $25 on websites. The rotors themselves are sold separately for as little as $36.50, meaning that a complete braking solution could cost $60 per turbine. The provided mounting hardware might not be suitable for fitting to a wind turbine, as these were meant for wheels, but if needed we could fabricate a mount from bolts and plates available at any local hardware store for little cost. Although I have not yet done any calculations to the effect, I believe that if these little brakes can stop a scooter with a 150-lb occupant, they can stop an all-aluminum turbine.
If it is found to be cheaper to use drum brakes than discs, these could also be mounted on the shaft in the same way, and achieve braking in the same way using a wheel cylinder. But unlike in passenger cars (where drum brakes are sometimes still used because they are cheaper), it seems that drums are not any cheaper. In my experience (I worked in a brake shop for four months), drum brakes are also significantly more difficult to service.
I have some worries about using physical brakes, since any additional weight on the shaft could cause imbalance and would increase the inertia of the spinning shaft, making the precious little energy available from the wind become harder to catch. Although we would mount the caliper on a stationary bracket, the disc would have to be spinning with the shaft. The same issue would creep up for drum brakes as well. An issue with all mechanical brakes is that they need to be physically engaged by a user, which might not be feasible from a distance. A hydraulic brake could achieve the action at a distance by pressing a button or pedal of some kind, but if a hydraulic caliper is needed, it will be even heavier and yet more expensive.
Dynamic braking is based on electrical principles. Freak gusts can cause the blades to easily spin too quickly for the generator to allow. To slow down the turbine when the battery is full, the power can be redirected to a dynamic braking resistor which will absorb the electricity being produced by the kinetic energy of the blades as heat. This is a safe way to restrict speed and is widely used in the industry. In order to bring a small wind turbine to a sudden halt, it is possible to simply disconnect the battery and short the terminals of the generator, permitting no voltage between them and bringing the shaft to a big halt. I have read mixed reports on the success of this. Some say that it is perfectly safe for small wind turbines, but some say it is always damaging to the generator and should never be attempted. Microlog Technologies produces an “electric brake panel” that does exactly this kind of braking. Dynamic braking has been described as more reliable than frictional braking, which has moving parts that can fail and need to be replaced.
Some helpful links:
http://www.gokartsupply.com/discbr.htm Disc brake assemblies
http://www.micromediaplus.com/microlog_wind_brake.html Microlog Technologies: electric brake panel
http://www.windmission.dk/workshop/BonusTurbine.pdf Very broad article on wind turbine operation and construction
http://www.reo.co.uk/files/dynamic_braking_resistors_02-08_engl.pdf REO dynamic braking resistors
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