# Tag Info

15

Not everything scales linearly. In particular, the cross-sectional area of supports required scales faster than height of a structure, all else held constant. This explains why ants have tiny thin legs compared to elephants. An ant linearly scaled up to elephant size would not be able to stand, or would snap its legs trying. The same thing happens to ...

9

It's not a simple relationship. First let's deal with the kinetic energy of the wind passing through the rotor. The mass of air passing through it in one second, $m$, is equal to the the density of the air ($\rho$), times the surface area of the rotor (${\pi}r^2$), times the velocity of the air ($v$). i.e. ${\rho}{\pi}r^2v$. The kinetic energy of that air ...

9

As it happens, I just recently went through that calculation myself for a different site. Given the following facts from a quick web search, it isn't difficult to work out the numbers. The maximum efficiency of a (large) windmill is about 40%. The density of air is 1.225 kg/m3 You need about 50 mW (10 mA at 5V) to light up an LED First, we'll need about ...

6

The fundamental problem in this case is that the turbine in not powered by 'wind' as such but by the relative motion of the car through the air. This is not 'free' energy, because the turbine must do work to generate energy it must also exert a net force opposing the motion of the car ie drag. So any energy you generate with the turbine must ultimately be ...

5

The cost of a wind turbine is not proportional to its size - like everything that needs to be manufactured, the cost exponentially increases as manufacturing becomes more complex, beyond what has been done. In terms of real estate on the ground, building a giant wind turbine may be more efficient (maybe not in fact, because stability issues may require the ...

5

A 2-bladed wind turbine is less stable mechanically than 3 (or more) blades. Because the two blades are in line, it is much easier to twist the hub of the turbine in the direction along the line of the blades than to twist it at right angles to the blades. If the turbine is being rotated because the wind direction changes, this will cause an unbalanced ...

5

Given that water is roughly 1000× as dense as air, and incompressible besides, it should be fairly obvious: A wind turbine would be destroyed by the forces that water would put on it, and a water turbine wouldn't even notice the forces created by wind.

5

The math doesn't make any engineering sense. A 5MW turbine typically rotates at about 10 RPM, near enough 1 radian per second. Assuming you had reasonably big sprockets on the chain drive, say 2 meters diameter (!!) the linear speed of the chain would be 1 meter /second, and the tension in the chain would be about 5MN or about 500 tons force. Something ...

4

You need to push air through the turbine at 11 m/s, or just above. That's what the rated windspeed means: it's the lowest windspeed at which the turbine hits rated power, at standard atmospheric pressure. The volume of air you need is determined by the area of the rotor. Let's say it's $\alpha m^2$. So you know you need to push through $11\alpha m^3/s$. So ...

3

The two main considerations concerning wind power generation is the speed of the air and the duration of wind (air flow). The more of both, the better. Trying to harness wind flow within a building will be difficult because the air flow will most likely be low (slow speeds) and the duration of flow may also be short, except on very windy days. Even a wind ...

3

Where does the power for a wind turbine come from? Answer - the wind. So how do you get 15kW from the wind rather than 11kW? Answer - you either need a location with a higher wind speed, or you need to pick up more wind, i.e. the blades need to be longer. How does this affect cost? Answer - a longer blade needs more material for its extra length. But then ...

3

One other reason is that the propeller-type turbine allows it to be rotated to avoid damage in high winds. When the air is flowing transverse to the axis of the propeller it encounters relatively little drag. It would be hard to imagine an orientation that would allow the anemometer-type design to avoid this.

3

Because the back of the cups create a lot of drag thereby reducing the power produced. The blade profile used in classic 3-bladed HAWTs is also used in VAWTs and they look like "egg beaters" needing top and bottom bearings. The engineering complications of VAWTs make the 3-bladed HAWT much more popular also due to the power output, rotating mass and visual ...

3

One of the advantages of two blades over three blades is less materials are required to make the blades. The disadvantage of a two bladed rotor compared to a three bladed rotor is due to the interruption to airflow and additional turbulence that the pole, that the wind turbine sits on, produces. This becomes apparent when one of the blades is directly in ...

3

I don't think you really designed a 1300 W wind turbine. You assumed your generator would be able to produce the necessary current but to be able to produce 1300 W you have to build a wind turbine big enough, given its efficiency. At $C_p=0.3$ and $v=10\ \mathrm{m/s}$, you simply need an area of: $$A=\frac{2P_T}{C_pv^3\rho}=7.07\ \mathrm{m^2}$$ \...

3

The Betz limit will give you a good theoretical maximum from your wind velocity and swept area. Your real world turbine will fall somewhere below this number based on its efficiency. For measuring power production over a period of time, you need wind data for that particular area. Wind data can be iterated and summed for a particular turbine, but generally ...

3

The design looks like a Savonius turbine, but horizontally mounted, instead of the typical vertical mounting that you see with this design. I'm really not sure why this would be mounted this way either, as one of the key benefits of the Savonius design is that it's never not facing the wind. The bottom of the Wikipedia article mentions that the horizontal ...

3

Because in the real world, all else is never equal, but let's start there analytically. What does "all else being equal" actually mean? With respect to a flat swept disk perpendicular to an ideal fluid with a uniform free stream flow field, from Momentum Theory, the induced flow field associated with optimum performance has slowly been specified in ever ...

3

You are looking for a “vertical axis wind turbine”. See below for some design examples:

3

TL;DR:IMHO, the driver for keeping the electrical generation closer to the hub is for primarily for eliminating -as much as possible- moving parts for safety concerns. The secondary benefit is the reduced losses. IMHO the problem is the distance and the complexity. For the following examples I will use as an example a Wind turbine with the following ...

3

The arch has been designed to resist wind loads but just the wind load its 630 feet arch can cause. The motto of not over designing the structure to reduce the weight of the structure to a manageable weight has lead to even reducing the viewing openings on top to 7 by 27 inches. Over 500 tons of pressure was used to jack the legs of the Arch apart for the ...

2

Try to cut a few from balsa wood to see what you get. Experimentation is probably the best method on such a small scale, and the math won't get you very far without an extensive knowledge of the materials (i.e. the cheaper the material, the less likely the blade will look like the big ones). I worked summer activities at my university using materials from ...

2

When used as a generator, a motor can generate approximately the same power that it requires for operation as a motor at rated speed and torque. In general, the voltage produced by a generator is directly proportional to speed. Power is proportional to voltage multiplied by current. The maximum current that can be drawn from a generator is generally a fixed ...

2

The theoretical maximum efficiency of a wind turbine is the Betz Limit, which is $\frac{16}{27}$ ( ~ 59.3%). The result dates back to 1919. The Betz limit is, as you note, speed invariant. To get an intuition as to why, consider the system speeded up or slowed down. This changes the fluid's velocity, but does not change any of the issues around the ...

2

The constant rotational speed is the first excitation frequency, mostly referred to as 1P. The second excitation frequency is the rotor blade passing frequency: NbP in which Nb is the number of rotor blades: 2P for a turbine equipped with two rotor blades, 3P for a three bladed rotor. source: https://ocw.tudelft.nl/wp-content/uploads/...

2

Very basically in modern wind turbines which act as a wing and get lift, if you add too many blades because of the wake interference you will get kind of a stall in lift because you will develope a big ellipsoid of air rotating with the blades as opposed to going through and imparting lift on it. In jet engines they counter this by adding stator blades and ...

2

The Betz limit is based on an extremely crude model for an actuator disk. If you want to compare the performance of the product of a billion dollar industrial development program, let's at least use a respectable benchmark. Image lifted from: Ideal Optimum Performance of Propellers, Lifting Rotors and Wind Turbines - https://openscholarship.wustl.edu/cgi/...

1

Part of the answer to this will be about considering how one optimizes the locations of wind farms and bridges. One of them likes to have a lot of wind. The other (well, the trucks on it) prefers to avoid this. So co-locating the two would necessarily involve compromises. That doesn't mean it's impossible. More pragmatically, a large wind turbine develops ...

1

One huge safety issue would be protection against turbine blade failure. There is no realistic way to catch and restrain a broken piece of blade, and apart from any loss of life during the incident itself, there would be a risk of the bridge being unusable for a long time while repairs to the damage were carried out. Accident statistics for UK wind farms ...

1

I think you may be confusing vertical axis wind turbine based on lift mechanism as in an airplane wing, with the low efficiency models that are used to show the wind speed on rooftops as part of weather vane. For example in a sail ship as opposed to intuition, the best wind is not the back wind which pushes the ship forward. it at best will push the ship ...

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