Is Time Magazine Misrepresenting How Energy and Power Are Measured?

[russ_watters]

I like to point out bad journalism, and here's one I saw this weekend in Time magazine:

the $16,000 Swift wind turbine can generate 1.5 kilowatts (kW) an hour, i.e., enough to power the average lightbulb for 15 hours.

http://www.time.com/time/magazine/article/0,9171,1860920,00.html

kW is power and is already in units of energy per unit time: there is no such thing as a kW per hour. You can actually plug ten 100W light bulbs into it and run them for 1 hour, 2 hours, 15 hours, or however long the wind lasts.

Ironically, I didn't see this article, I saw a response letter to the editor - the response talked about how little power (or energy, can't remember which) that was (so we should do nuclear power), but didn't catch the error.

 

[russ_watters]

I sent them this email:

Your "Got Wind" article contains an elementary science error, confusing power and energy: "...the $16,000 Swift wind turbine can generate 1.5 kilowatts (kW) an hour, i.e., enough to power the average lightbulb for 15 hours."

http://www.time.com/time/magazine/article/0,9171,1860920,00.html

In fact, kilowatts is alreay a rate (1000 Joules per second) and there is no such thing as a "kilowatt an hour". You could say it generates 1.5 kilowatt hours per hour, which would indeed power a light bulb for 15 hours, but that is an awfully cumbersome way to describe the capacity of a wind turbine. It would be better to say that 'it generates 1.5 kW of power: enough to power 10 average light bulbs.' It can power them all for 1 hour, 15 hours, or until the wind stops.

This article - and all too common shoddy science writing in the media in general - is both a symptom and a cause of the scientific illiteracy of the general public.

Russ Watters
Trappe, PA
Moderator, www.physicsforums.com

 

[franznietzsche]

As I'm always explaining to people, if you read it in a news article, it was probably run through a blender somewhere between reality and the printer.

 

[JasonRox]

You sure got them.

 

[Cyrus]

You sure got them.

You sure got him.

 

[JasonRox]

You sure got him.

I sure did.

 

[Proton Soup]

i think they probably mean 1.5 kW-hours/day, which makes sense given a 100W light bulb for their example. it's a good example of why energy policy opinions of people with liberal arts degrees should be taken with a grain of salt, but overall, it's not a huge screwup.

 

[Gokul43201]

I don't understand this...

You could say it generates 1.5 kilowatt hours per hour, which would indeed power a light bulb for 15 hours

...or this...

i think they probably mean 1.5 kW-hours/day, which makes sense given a 100W light bulb for their example.

It takes 1.5 kW-hours of ENERGY to power a (100W) lightbulb for 15 hours.

"1.5 kW-hours per hour" and "1.5 kW-hours/day" are both amounts of power, not energy.

 

[russ_watters]

[response to Proton Soup] No, it really is a 1.5 kW turbine. http://www.swiftwindturbine.com/specifications.php

1.5 kWh/day would be less than 100 W and wouldn't be worth having.

I probably wouldn't have sent them an email, but the fact that they printed the same error twice without correcting it implies they still don't know they made the error.

 

[russ_watters]

I don't understand this...
...or this...It takes 1.5 kW-hours of ENERGY to power a (100W) lightbulb for 15 hours.

"1.5 kW-hours per hour" and "1.5 kW-hours/day" are both amounts of power, not energy.

Yes, that's why I said it would be cumbersome - it is a non-standard power unit.

But think of it this way (by my units): every hour it generates enough energy to light a 100 W bulb for 15 hours.

 

[Proton Soup]

[response to Proton Soup] No, it really is a 1.5 kW turbine. http://www.swiftwindturbine.com/specifications.php

1.5 kWh/day would be less than 100 W and wouldn't be worth having.

I probably wouldn't have sent them an email, but the fact that they printed the same error twice without correcting it implies they still don't know they made the error.

that's 1.5kW with a 14m/s(31mph) wind, which isn't realistic for most locations if you're talking averages. if you're buying a windmill, you'd want to know what the average power production is at your location to determine if the unit is economically viable. if this is the max power output, it's valuable to me as an engineer, but for the public at large, it just seems intentionally confusing and makes it sound like it's more powerful than it is.

now the next spec is valuable and should have been what the writer focused on:

Annual Power Supplied: up to 2000 kWh

in my http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_a.html" , this works out to $217.80/year. in Doug Morrell's state of Michigan, $236/year. and all of this at a price tag of $16,000.00. for that, it'd better last a very long time with zero maintenance. if you could manage 5% a year on a $16000 investment, that'd be $800 a year and you get to keep not only the interest but reinvest most of the principle.

federal subsidies aside, it's a really bad investment. and i think that's what makes the journalism bad.

and who wants 5 windmills in their yard?

I don't understand this...
...or this...It takes 1.5 kW-hours of ENERGY to power a (100W) lightbulb for 15 hours.

"1.5 kW-hours per hour" and "1.5 kW-hours/day" are both amounts of power, not energy.

yes, i know that kW-hours are energy. and i honestly didn't realize you'd have problems interpreting that the 15 hours @ 100W is also on a per day basis.

 

[WarPhalange]

[response to Proton Soup] No, it really is a 1.5 kW turbine. http://www.swiftwindturbine.com/specifications.php

Wow. My computer takes about 500W, not counting the monitor, modem, or router. And how much does a kWh of power cost? Like 7 cents or something? I think I'll pass on this.

 

[Cyrus]

If you want to kill some time. Google windmill failure.

When the blades start flapping they can do some serious damage. The same thing happens on helicopters. If the helicopter blade flaps at excessive angles it cuts off the tail boom (just like this windmill cut off the tower) or cuts into the cockpit killing the pilots.



A windmill is simply a helicopter blade oriented vertically.

 

[Topher925]

kW is power and is already in units of energy per unit time: there is no such thing as a kW per hour

The article didn't state kilowatts PER hour, it said kilowatts AN hour. I think what he was going for was kilowatts for an hour. I've seen that kind of unit of notation used before in some technical publications as kwh doesn't make much sense to people. Its kind of a stupid unit for energy anyway.

A windmill is simply a helicopter blade oriented vertically.

This is completely and utterly wrong. A helicopter rotor and a wind turbine are both turbomachinery that operate in an open fluid but that's about all that they have in common. Blade flapping on helicopters is caused by a difference in relative velocity at the blades due to the helicopters forward motion. The blade flapping is always normal to the fuselage, not the parallel to it. If a helicopter has a boom strike or a cockpit strike (I don't think this has ever happened) its because of a poor rotor head or blade design, not because of flapping.

Flapping on wind turbines is caused by shadowing from the mast or supporting structural column of the turbine. The failure in the video wasn't caused by blade flapping or a column strike. It was caused by a structural failure near the blade root. That wind turbine should have been shut down at such high wind speeds.

http://inlinethumb45.webshots.com/44844/2310005410056081071S600x600Q85.jpg

 

[mgb_phys]

1.5 kW-hours/day

Does make some sense - if the generator is 1.5kW and on average runs for an hour a day then it makes more sense to say it generate 1.5KWh, a unit people recognise and which electricity is priced in, every day.
Better than saying it is a 1.5Kw generator with a load capability factor of 6.25%

 

[mgb_phys]

That wind turbine should have been shut down at such high wind speeds.

It should - the fault was caused by the gearbox/braking system failing to automatically stop the blades when it went overspeed.
The montiors did warn the company that there was a fault and the evacuated an area - the surprise was how far the failed blades went.

 

[Cyrus]

This is completely and utterly wrong. A helicopter rotor and a wind turbine are both turbomachinery that operate in an open fluid but that's about all that they have in common. Blade flapping on helicopters is caused by a difference in relative velocity at the blades due to the helicopters forward motion. The blade flapping is always normal to the fuselage, not the parallel to it. If a helicopter has a boom strike or a cockpit strike (I don't think this has ever happened) its because of a poor rotor head or blade design, not because of flapping.

Unfortunately, it has happened. My professor who wrote a book on helicopter aerodynamics, told us so. I also wrote something about this in my paper for the class too. See:

http://www.ntsb.gov/ntsb/GenPDF.asp?id=MIA00FA102&rpt=fi

Examination of the tail boom assembly revealed four main rotor blade strikes in addition to the fracture that resulted in the separation of the tail boom.

When the rotational RPM of the helicopter rotor becomes to low, excessive blade flapping occurs. You are correct that the blades flap in the sense of a teetering rotor (which is what your thinking of), but fully articulated blades are able to lead/lag and bend up/down. Down to the point that they have to put stops so they don't droop down and hit the tarmac when at rest.

Flapping on wind turbines is caused by shadowing from the mast or supporting structural column of the turbine. The failure in the video wasn't caused by blade flapping or a column strike. It was caused by a structural failure near the blade root. That wind turbine should have been shut down at such high wind speeds.

It is basic aerodynamic knowledge that blade flapping is a result of a balance between centrigual forces from rotation and lift from the airfoil section that defines the coning angle of the blade.

A wind turbine is a helicopter in autorotation. Although a helicopter is probably more in the vortex ring state than a wind turbine is (though I could be wrong on that one).

Also: I agree with your analysis as to the cause of failure on the windturbine though. I also don't argue about what you said in terms of the mast causing flapping in the case of a wind turbine.

 

[Topher925]

Unfortunately, it has happened.

I know helicopters have had boom strikes and many have had fuel probe strikes. My comment in parentheses was regarding cockpit strikes. Is your professor Gordon Leishman?

It is basic aerodynamic knowledge that blade flapping is a result of a balance between centrigual forces from rotation and lift from the airfoil section that defines the coning angle of the blade.

Do you have a source for this? What is causing the oscillation if the blade is in balance?

A wind turbine is a helicopter in autorotation. Although a helicopter is probably more in the vortex ring state than a wind turbine is

I don't think so. Autorotation refers to a balance of induced inner and outer profile torque from an inflow through the rotor essentially creating a half turbine and half propeller rotor. I believe the autorotation you are referring to is not actually autorotation (the word is commonly misused) but "windmilling" where the rotor acts as a turbine in order to keep it spinning in order to prevent crashing from power loss. BTW, a rotor can not be in autorotation and in VRS at the same time. Also, wind turbines typically never enter a VRS and in the worst case operate in a turbulent wake state.

Sorry to hijack your thread russ.

 

[Cyrus]

I know helicopters have had boom strikes and many have had fuel probe strikes. My comment in parentheses was regarding cockpit strikes. Is your professor Gordon Leishman?

Yes.

Do you have a source for this? What is causing the oscillation if the blade is in balance?

When the blades rotate they are always subjected to perturbations from unsteady air loads, wind gusts, etc. Because of the low rotational RPM, the centrigual force can't keep the blades straight out, so any distrubance will make the tip path plane start to oscillate.

http://books.google.com/books?id=nM...&hl=en&sa=X&oi=book_result&resnum=1&ct=result

Autorotation refers to a balance of induced inner and outer profile torque from an inflow through the rotor essentially creating a half turbine and half propeller rotor.

Yes, that's correct.

I believe the autorotation you are referring to is not actually autorotation (the word is commonly misused) but "windmilling" where the rotor acts as a turbine in order to keep it spinning in order to prevent crashing from power loss.

http://books.google.com/books?id=nMV-TkaX-9cC&pg=PA88&dq=leishman+power+ratio+for+climb

The point is autorotation. It's the point where the rotor is extract exactly enough energy to overcome the drag to slow it down. A helicopter never wants to act like a turbine (i.e extract power from the air), or it will overspeed the rotor and cause it to explode (exactly like in the video). I would agree with the word windmilling in the sense that you are using it.

BTW, a rotor can not be in autorotation and in VRS at the same time. Also, wind turbines typically never enter a VRS and in the worst case operate in a turbulent wake state.

See the graph in the second link above.

You know a good deal about wind turbines. Do you work with them?

 

[Topher925]

When the blades rotate they are always...

I wouldn't call blade coning flapping, but I understand your point about flapping at low rotor head speeds.

The point is autorotation. It's the point where the rotor is extract exactly enough energy to overcome the drag to slow it down.

I guess this is just formalities. If the rotor shaft has drag, it must be absorbing power to keep it spinning which would put the point below the power line on the graph which I used in so many power point presentations. If you neglect friction, then yes the rotor is not absorbing any power. I just like to be clear about this as I used to easily confuse people at my previous place of employment.

See the graph in the second link above.

Again, formalities. Notice it says "Point of IDEAL autorotation" and not practical autorotation. If you can design an autogyro that can operate in VRS at steady state, your a better man than me. Also, I think that text your referencing is a fantastic book and IMO the best modern and up to date reference for helicopter aerodynamic design, you can tell your professor I said this. It was one of the only textbooks that always stayed on my desk at work because I used all the time. Although I would have liked to see some mention of china weights in there.

You know a good deal about wind turbines. Do you work with them?

I've never actually worked with windmills or helicopters before but I have worked with something similar. Between undergrad and grad school I worked for a company on a venture capital project. Which if you watch this http://ctngreen.com/energy/" you will quickly see why I had to know so much about these things.

 

[Pythagorean]

There is no such thing as a "kilowatt per hour"

While I see your point in the context they were using it, is there really no use for a "change in power"?

I mean, technically, isn't there such thing as a "kilowatt per time" if the power is changing (much like acceleration to velocity). Perhaps you'd have to call it "change in power over time"...

Just saying, if you correct them on one thing, but mislead them about another, isn't that like one step forward one step back?

 

[WarPhalange]

While I see your point in the context they were using it, is there really no use for a "change in power"?

I mean, technically, isn't there such thing as a "kilowatt per time" if the power is changing (much like acceleration to velocity). Perhaps you'd have to call it "change in power over time"...

Only thing I could think of is when you are starting up some turbines that generate the power. They can't immediately go full speed, so you could say they increase their power output at 1.5kW per hour or something. But that is such a special case that it's a stretch to try and defend it...

 

[russ_watters]

that's 1.5kW with a 14m/s(31mph) wind, which isn't realistic for most locations if you're talking averages.

Hence the phrase "or until the wind stops".

if you're buying a windmill, you'd want to know what the average power production is at your location to determine if the unit is economically viable. if this is the max power output, it's valuable to me as an engineer, but for the public at large, it just seems intentionally confusing and makes it sound like it's more powerful than it is.

now the next spec is valuable and should have been what the writer focused on:


in my http://www.eia.doe.gov/cneaf/electricity/epm/table5_6_a.html" , this works out to $217.80/year. in Doug Morrell's state of Michigan, $236/year. and all of this at a price tag of $16,000.00. for that, it'd better last a very long time with zero maintenance. if you could manage 5% a year on a $16000 investment, that'd be $800 a year and you get to keep not only the interest but reinvest most of the principle.

federal subsidies aside, it's a really bad investment. and i think that's what makes the journalism bad.

Not exactly what I was going for, but I do tend to agree. Some people would just do such things because they think they are cool, though.

 

[russ_watters]

The article didn't state kilowatts PER hour, it said kilowatts AN hour. I think what he was going for was kilowatts for an hour.

That's a minor wording difference and the numerical result is exactly identical to what I said. "An hour" and "per hour" mean the same thing in this context: "in one hour". Either way, yes, an error in the article.

In any case, yes, that's what their math worked out to: 1.5 kW for an hour is 1.5 kWh, or enough to power 1 light bulb for 15 hours, like they said... Which is a poor way to describe the capacity of that turbine, even if they were describing it that way on purpose, which I don't think they were: you're interpreting one mistake as being a typo that masks the correctness of a calculation that really doesn't work very well. It's a lot more likely that the mistake was just based on a single error in understanding. In other words, one error, not two.

 

[russ_watters]

Does make some sense - if the generator is 1.5kW and on average runs for an hour a day then it makes more sense to say it generate 1.5KWh, a unit people recognise and which electricity is priced in, every day.
Better than saying it is a 1.5Kw generator with a load capability factor of 6.25%

1/24= 4.16%. That would be a pretty awful load factor (here's a site that claims 30% is typical: http://www.bwea.com/ref/faq.html ) and I highly doubt that an advertising bulletin would claim any load factor at all, considering it is different for every installation. And Time certainly was not attempting to calculate the load factor.

[edit] Oh, I was wrong: the manufacturer does list a number utilizing a load factor (sorry, PS - just skimmed your analysis): 2000 kWh per year, which equates to 5.5 kWh per day. That's 15%.

 

[mgb_phys]

In the article it's obviously someone confused about the whole numbers thing.
But kWh/day could make sense as an average generated energy.

 

[FredGarvin]

Blade flapping on helicopters is caused by a difference in relative velocity at the blades due to the helicopters forward motion. The blade flapping is always normal to the fuselage, not the parallel to it. If a helicopter has a boom strike or a cockpit strike (I don't think this has ever happened) its because of a poor rotor head or blade design, not because of flapping.

That's not entirely true. Flapping can happen on the ground as well with no forward motion. Any situation that can cause dissymmetry of lift, i.e. a strong cross wind in a hover. There is also flapping and there is lead/lag. They are the same thing oriented 90 degrees from each other. A fully articulated rotor head will have these movements. A strike can happen none of which are the results of a poor rotor design. Although, I can't think of a scenario that would have a blade going through the cockpit. The geometry just doesn't work that way. You would have to have a rotor head departure for that to happen.

They all are because of a failure of one thing or another though. This has happened a few times that I know of. In my beloved CH-47's there have been droop stop failures that have lead to something like this (notice the nice gash in the fuselage toward the forward pylon):

EDIT: Removed hyperlink and uploaded pic.


And the very cool strikes due to ground resonance. Those videos are all over the net.

 

[Cyrus]

Your image won't allow hyperlink.

 

[Art]

A windmill is simply a helicopter blade oriented vertically.

Seeing as how windmills were around long before helicopters you should say a helicopter is simply a windmill blade oriented horizontally

 

[russ_watters]

But kWh/day could make sense as an average generated energy.

Incidentally, I was writing a report today and one of my bosses flagged me for using the units "BTU-hours/CFM-year" to describe how much energy is added to the air per year by a heater. He said something like 'the client will never understand this non-standard unit'.

 

[mgb_phys]

Next time use eV ?