Wind farms vs everything else

Thanks @wxman, this deserves a dedicated thread:

Before doing a more critical review, I need to read the sources cited in these papers. I noticed when looking for the paper, my mangled search suggests @wxman may have unique insights about wind boundary effects. For example, do electrical transmission lines and tower induce similar thermal effects?

Later,
Bob Wilson

 

These are recently published papers which bring new eyes to the renewable energy. The first one is an open paper (aka., free) that claims wind generators are a low power per square meter source:

• 0.3-47 W/sq meter - wind
• 10-120 W/sq meter - solar

Now the wind number has me really scratching my head. To me, the wind turbine land use area should the the foot-print of the tower. Suspended above the ground, the land remains viable for agriculture. But to get such a low power density suggests the outer boundary of the wind farm is being used. My first reading of the article doesn't clear this up but they reference five sources "(Smil 1984, 2015, MacMay 2009, 2013a, 2013b)". Yet they also site:

• 6.0% - wind generated US electricity
• 1.8% - solar generated US electricity

So I think we need to take another look at their sources and definitions with these understandings:

• Terrestrial wind farms remain multi-use in rural areas. You don't want to put a housing development under them although I suspect a factory or business might work with precautions to deal with blade failure. There are several mountain ranges that would also work for ridge-line sited wind farms.
• Roof top solar makes every urban area an excellent choice but limits their multi-use. In effect the solar farm becomes a shaded area which is highly desirable in some areas including roofs.

The second paper is even more interesting because it claims there are microclimate heat effects associated with the wind turbines and mixing the boundary layer, especially at night. A laminar flow above the ground, especially without cloud cover, allows the lowest layers to become cooler than the moving air above. Is this the mechanism?

Especially important is if there is any heat injected into this microclimate by the wind turbines, a different effect from redistribution. Air moving through a turbine gives up kinetic energy to the generator which should cool the flow. There can be some heat generated from friction and blade tip losses but it should pale compared to the energy taken from the wind. We really need to understand the thermal dynamics. We would also expect to see similar effects for other structures tall enough to pierce the boundary layer like broadcast towers and tall chimneys.

We would also expect that if there were a light snow cover, the heat effects should be seen as early melting around the wind farm. I haven't seen photos showing this effect.

Bob Wilson

 

An MIT report: Wide-scale US wind power could cause significant warming - MIT Technology Review

. . .
The Harvard researchers said their findings closely matched directly observed effects from hundreds of US wind farms.

Keith, an outspoken proponent of clean energy to combat global warming, says he’s sure the paper will be misinterpreted or misrepresented by some to argue against the rollout of wind power.

“But it would be unethical for the research community to hide impacts from renewables because we think they should be promoted,” he said.​

There in lies the challenge, field observations that validate their model.

As for "misinterpreted or misrepresented" the Daily Mail will do:
Large-scale wind farms 'could WARM the planet' | Daily Mail Online

Another credible source: https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-12-00376.1



Bob Wilson

 

Now I'm getting a clue: National Wind Watch | Size of Industrial Wind Turbines

The ground area is defined by a circle whose minimum radius is the blade length because the turbine should rotate to face the wind. Now if there is a second, near-by turbine, there needs to be a gap large enough so within a reasonable range, the turbines won't be in the other's down stream turbulence. So the area increases to handle variable wind direction. Now I understand the low W per meter square.

Within a limited range, the parallel axis relative to the wind can be +/- 5-10 degrees. This avoids inter-turbine wind turbulence.​

Now the blade sweeps a circle and looks like a perforated disk to the wind. This means there is a tear-drop shaped, high pressure field in front of the bade-circle that has the effect of distorting the incoming wind around what for all effective purposes is a pressure nose cone. This will divert some of the warmer air down, to the ground with a maximum effect at the disk of the rotors. Then the blade tip vortex comes into play.

It may be possible to build a circular, earth dam with possibly a recessed dish in the center. This has the effect of pinching and increasing the ground velocity. It would like a duct increase the power produced in that part of the rotor rotation. It would not reduce the surface temperature effect as much as improving rotor power generation.​

Trailing off of each blade tip is a vortex that entraps air in a miniature tornado. This vortex drags air from the upper, warmer streams closer to the ground where another effect occurs. A vortex is attracted to a surface and thus extends further down stream from the turbine.

There are wing tip air foils that reduce the vortex losses. The wind turbine becomes more efficient and the down-stream vortex mixing effects are reduce. ​

Now there is one thermodynamic effect not addressed, compression and expansion of the stream. In front of the rotor disk, the air slows down due to higher pressure and is compressed. This heats the air stream. After passing through the rotor disk, the pressure is lower, cooling the flow. The effect is most pronounced at the hub but this cooled air is not tossed to the ground. So there would be temperature difference even if not measurable on the ground.

Now if we measure the air mass through the disk, the temperature difference upstream and downstream can be used to measure the loss of energy. This energy loss should be equal to the power generated plus other losses that are not dumped into the stream.

Now I'm a happy camper ... I always appreciate the conservation of energy.

Bob Wilson

 

Those were my recommendations to mitigate the surface heating 'problem.' The winglets allow the blades to be smaller for the same power.

Personally, I like the vertical axis wind turbines as they can be built with a layered, wedding cake structure. The highest rotors would be the smallest diameter followed by the next layer, a larger diameter and the bottom layer the largest diameter. I would probably have a flattened donut shape between each layer to all but eliminate tip vortexes and hold the generators. The rotors would turn in alternating directions to reduce generator sizes and all but the top would have a one-way ratchet so they only turn in one direction.

Bob Wilson

 

I was thinking:

Here you see a single level with 'end plates'. I see it on a larger scale with each stage sized for the wind velocity during night laminar flow:

I'll try to sketch my concept later.

Bob Wilson

 

Concept of Operation​

• First install and anchor the central tower/support
• Build each layer on the ground and raise to mounting position
• Each layer can be tested before adding the next
• Alternating turbine rotation nulls out generator torque on central support
• Guy wires are optional to support increased hight and minimize bending load on central support
• Induction generators support power and frequency management
• Aero shaped edge of generator disks increases wind velocity and avoid tip vortexes
• Wide diameter generator minimizes mechanical stress
• 'Tree shape' minimizes upper layers being directed to surface
• Lower turbines provide additional power in turbulent, day-time wind

Bob Wilson

 

ROM height: ~328 ft (BOE: National Wind Watch | Size of Industrial Wind Turbines)

The remaining dimensions require test articles, electrical, aerodynamic, and structural modeling. For example, I've show counter rotating wind turbine wheels at each layer to avoid adding a cumulative torque to the center 'tower.' However, it may be only alternating rotation at each level which reduces the parts count. This is just one of a large number of design decisions that have to be worked out. But let's not get too far from the original goals:

• smaller land use foot print than a bladed wind turbine
• reduce local thermal effects at the surface (note: Florida orange growers may want that effect)

Bob Wilson