Steel wind tower resistance to sunlight

ezintegral

Hello,

For my Mechanical Engineering project, I'm working on a wind tower. One of the specifications we have to respect is a 50 years resistance to sunlight.

We've chosen inox steel as the primary material (after calculating the various constraints due to the wind the the rotor up on the tower, and determining the deflection of the neutral axis of the beam, etc)

The problem is that I have no idea how to determine whether a type of steel will resist to sunlight or not. The solar constant is 1.367 kJ/s on average, and 50 years is roughly 25 years of daylight and 25 years of night (a lot of approximations in there), so we calculated the total amount of energy that the tower will receive in 25 years.. it's a lot of energy, and I know a lot of it is dissipated (how?)

Anyway, that's why I'm here: Is there any way to determine how will a steel (or any other material) will resist to sunlight provided we know it thermal properties (transmittance, resistivity, etc..)?

Thank you

SteamKing

Unless the sun goes nova, I wouldn't worry about how well the steel holds up when exposed to sunlight.

How do you think the energy received by the steel will be dissipated? Remember, steel is not capable of trapping heat very well. Apply thermodynamics to answer your question.

ezintegral

That's what I think too, but I see no scientific argument to defend that position. I'm not supposed to say in front of my professors "I wouldn't worry" if I don't have a strong number in hand to prove it.

Well the first idea that comes to mind is that the heat will be transferred to the earth on which the tower is built-in.

SteamKing

Well you aren't building your wind tower in a vacuum. Some of the heat absorbed by the wind tower is going to be dumped back into the atmosphere, especially at night. Since you are presumably going to construct the wind tower in a region which has a lot of wind available, then you should research heat transfer from the steel to the surrounding air.

256bits

Is that a material specification or structural specification that has to be met? Does the specification specifically state resistance to sunlight or resistance to a sunlit environment, in which case other conditions may be of concern, such as chemical or abrasive corrosion.

You could investigate how certain materials have a chemical change brought about under sunlight. Carbon black is added to hinder decompostion for certain rubbers and plastics under ultaviolet light from the sun. Protection of structural members and body of a structure can also be achieved by application of a coating such as paint.

As a structure, when under heating from the sun, one side of the stucture would aquire more thermal energy from the sun than the other, producing elongations in members and a corresponding amount of stress in the frame. Will the structural members be under the allowable limit and also not fail due to fatigue over 50 years, both brought on by thermal stress and strain.

You have mentioned the choice of stainless steel, and you should, if asked, be able to validate your choice over a less expensive material from a strength, corrosion resistance, life duration and maintenance arguement, which could include material cost, maintenance cost, and assembly and fabrication cost. Certainly we do not see too many bridges, if any, built from stainless steel, nor wind milltowers built from concrete.

I just thought I would add in some of the details worth considering when 'engineering' something. Whether these are, should be, or will be part of your analysis in your project depends upon time constraints and whether you are producing a 5 page or 50 page report.

Go well.

Studiot

First what do you mean by 'inox steel' ?

Do you mean Corten or other weathering grade steel?

Or do you in fact mean stainless steel?
In general there would be little point building of stainless with a design life of only 50 years.
The structural thicknesses required for a tower would likely last that anyway.

Whilst discussing structural grades, you do realise these must be ductile to cope with the flexing and twisting strains that will be imposed over the service life of the tower?
Many grades of stainless are too brittle for general structural use.

UK design codes require a life of 120 years for bridges and I have never built an entire one using stainless, though many component parts may well be so fabricated.

Your main enemy is not sunlight but salt in the air. So if your towers are in a near marine environment you will have various corrosion issues to address.

For stainless you have to choose the correct compositions or the parts will corrode
You need to avoid dis-similar metal junction corrosion at connections with other materials.

Sunlight does seriously degrade coating protection systems (paint, polymeric coatings etc) This is why some systems include a layer of mica flakes, whose large flat surfaces reflect the harmfuls rays.

Fabrication using stainless also presents extra difficulties. Are your towers welded?

This sounds an interesting project, but you should take real world conditions into account and not just use a magic word (inox) to avoid real issues.

go well

SteamKing

inox steel = stainless steel

Daniel Kopial

Hi, I wouldn't worry about sun hitting the steel frame. Steel has a very high critical temperature point. I would suggest you to look at the material manufacturer's data sheet/mechanical properties. I believe the critical temperature is somewhere in there.

Assume it is around 400deg C, there is nothing to worry about...if the sun did able to radiate such high level of heat, I would worry about the wind turbine coil generator meltdown first before the structure even fail. Or even a wildfire started first before the turbine coil generator meltdown.

The only way the structure get heated up to that point is probably several strikes of lighting within a few minutes.

But just a question (like what 256bits asked)...why stainless steel? The poor rate of return for such an expansive windmill is only applicable to the Arab oil Sheikhs who like to drive in those shiny gold/silver plated Ferrari. There are many surface protection technology which is far more cost effective than using stainless steel if your concern is on oxidation. Since the windmill will be stationary (not a ship in the sea etc)...its a lot cheaper & easier to just paint it every 5 years.

telecomguy

Will the tower have a finish on it such as paint? If so, I'd assume the 50yr sunlight requirement would apply to that finish rather than the steel itself. Since the person writing requirements has no idea what materials will be used, they likely are just covering their bases. Also, in your original post you mentioned selecting 'inox steel as the primary material'. This leads me to question what are the secondary materials? These might be susceptible to UV damage as well.

PhanthomJay

Resistance of a material to sunlight is determined primarily by accelerated aging tests in chambers where the material is exposed 24 hours per day to intense ultraviolet radiation for a lengthy period of time. While materials like polymers, rubbers, fiberglass, etc. may be subject to UV deterioration over time, steel, stainless or otherwise, is not. No need to use stainless steel for the tower members or shaft support. As has been mentioned, this is not a good idea for the main structural members. Use 'normal' carbon steel, galvanize it, and paint it every so many years ( from a practical standpoint, consisdering maintenance budgets, painting every 15-20 years should be sufficient).

Daniel Kopial

15-20 years is far too long from where I live (tropical region), but yeah...maybe in the northern region where air is dry (compared to 98% RH here)...15-20 years might be just as good.

While you could also consider galvanized steel, in my place most street lamp mast arms, telecomunications towers & guardrails uses non-painted galvanized steel. The oxide layer formed by the zinc will protect the steel really good.

PhanthomJay

Yes, good point, in my climate northeast USA, galvanized steel will protect the steel against corrosion for a good 15 years (in the absense of heavy salt contamination) before the galvanized coat starts to deteriorate and expose the steel to rusting, after which, painting with a zinc rich paint will protect it for another 10 years or so before it too starts to deteriorate, but often paint cycles are in the order of every 20 years or so. The use of weathering steel ("Corten") eliminates the need for galvanizing and painting and will give you perhaps a life in the order of 75 to 100 years, but while weathering steel is good for mast type structures, there could be concern on lattice towers at overlapping splice points.

Studiot

I have already observed that the oxide coating on Corten steel fails in salt rich atmospheres such as near marine positions.

ezintegral

Thank you all for your answers, I think I'm done with the sunlight criteria.

However, the choice of material might not be the best as some of you said. Using regular steel (instead of the more expensive carbon steel?) and painting it every 10-15 years may be a good solution since my only concern is corrosion. I think we will try galvanized steel with an anti-corrosion painting, since it seems like it's the cheapest way to go.

PhanthomJay

By 'carbon steel' I meant steel with carbon as one of its constituents, which is a common inexpensive structural steel with yield stresses in the 36 to 65 ksi range . Galvanizing it initially and then painting every so many years will make it last for decades . Caution, however, if the steel is in contact with the ground below grade that is subject to varying moist and dry cycles , like in a wetland, special epoxy coatings are required just above and below grade. Buried carbon steel in a salt water tidal marsh can have serious consequences.

Daniel Kopial

Correct me if I'm wrong...but I don't think the steel member will be in direct contact with the ground. It would most likely has some 10-20ft deep reinforced concrete piles, and sitting on top of those piles are the pile-caps using engineering grade grouting. The steel structure will normally just sit on top that grouting fastened with some bolts & nuts or planted few feet into the grouting. For additional surface protection, you could attach some sacrificial anodes to catch some electrons

Daniel Kopial

Hi, since we're already on this topic. Could you kindly share with us how do you plan to cope with the vibrations from the rotating blades & further vibrations/swinging caused by the wind (lateral force)...and perhaps would you look into the area of resonance vibration/swinging which destroyed the Tacoma Narrows Bridge.

A tip: you would look into tuned mass dampers or hydraulic dampers to counter vibrations. Please do share with us which is your choice & why.

Blessed new year everyone!