Is a wind turbine suitable for my needs?

Martyn1981

zeupater said:

Hi

Whatever the efficiency of the turbine design there's a theoretical limit to the power available to harvest ...

As a quick rule of thumb you could work on 5W per sqm of swept area at 2m/s (~4.5mph), so at 4m/s (~9mph) it would be 40W (8x5), at 8m/s (~18mph) 320W (8x40), 16m/s (~36mph) 2560W (320x8) with wind speeds much above this normally resulting in some type of protective action (braking/closing/furling etc) .... 

... the next consideration is the location ... if you're in a low wind area then whatever comes next is practically irrelevant as no/little wind = no power ...

... there's then the efficiency of the turbine itself which likely ranges from 10% to 50%, which in itself is subject to the   application of the Betz limit (max ~59% of the theoretical power), and this is before considering the effect of cut in/out speeds ...

... Ah, then there's the realisation that everything you've just calculated depends of laminar air flow with the really big issue of any turbulent airflow resulting from any obstruction within a considerable distance from the turbine site causing a major reduction in apparent efficiencies .... although VAWTs claim to cope with turbulence better than HAWTs, there's the effects of practically halving the swept area and energy losses from always rotating some collecting area against the wind to consider .... 

Finally, there's the efficiency losses related to cables/inverters etc to consider or the electrons are practically useless ...  

All confusing, but the best place to start is looking at the rule of thumb element to just see what the theoretical power limit is at whatever wind speed you like, then multiply that by 0.59 (Betz) before scaling up the swept area to whatever the manufacturer specs claim ... this just gives a feeling as to whether the specs are anywhere near realistic ....

HTH - Z

Gonna need a big VAWT.  

My negativity about domestic WT's, is partly born out of bitterness that I'd really like one, but they simply won't work for me. As I learned a decade or so back chatting on the sustainable energy sites.

As far as I've seen, nothing has changed for HAWT's, they simply can't overcome the problems of turbulence. Whilst VAWT's can handle turbulence, their lower efficiency, requires a far larger size, in fact vastly larger than all the hyped hope offerings we tend to see.

I try to keep my hopes up, that a VAWT design can one day meet the challenge, but in reality, the size needed would be impractical, and most likely uneconomic.

Edit - And I should add, that I think the Ripple investment model for WT's probably beats anything that a domestic WT could manage, even in the unlikley event some solution(s) are found. [The same applying to some people with very small and/or shaded roofs for PV, who may find the Ripple PV investments better.]



Edit 2 - Actually Z, if you have the time and some knowledge, would you be able to provide a guess. Extremely roughly, at the sort of size a VAWT would need to be for say a moderate wind speed suburban area, with average turbulence (bad turbulence). I'm thinking ~500W, but my brain couldn't cope, and I'm sure it would be well past the size of a shed. [Damn, now my silly brain is imagining a combined VAWT shed, but the Waltzer ride always made me feel sick.]

Obviously I appreciate this is a 'how long is a piece of string' question, but I'm wondering if it's just 'silly' big, or well into ridiculous/insane?

QrizB

A 1000 watt nominal WT with a capacity factor of 25% (which you might possibly achieve in an urban or suburban environment) would generate 2190kWh a year.

People generally don't like to live on mountain ridges, and from memory, (sub)urban wind speeds are typically around the 4-6m/s level.

At 4m/s we've said you're looking at 40W/sq.m of swept area, which gives a minimum swept area (ignoring efficiency or thw Betz limit) for a 1kW turbine of 25 square metres.

I'm practice you'll need to double that as a minimum for a HAWT, and double again for a VAWT. So that's 50 sq.m (8m diameter) for a HAWT of 100 sq.m. (10x10m) for a VAWT.

Which seems quite big!

Martyn1981

Cheers QrizB, but I have to say, where's your spirit of adventure? How about a VAWT detached house, what could possibly go wrong?

QrizB

Martyn1981 said:

Cheers QrizB, but I have to say, where's your spirit of adventure? How about a VAWT detached house, what could possibly go wrong?

You could have a VAWT semi, one rotor each!

I might've been a little conservative in my calcs, designing for mean wind speed and then applying a capacity factor. But even if we assume a CF of 100% we're still looking at a 4 metre diameter HAWT or 5x5m VAWT.

zeupater

Martyn1981 said:

Edit 2 - Actually Z, if you have the time and some knowledge, would you be able to provide a guess. Extremely roughly, at the sort of size a VAWT would need to be for say a moderate wind speed suburban area, with average turbulence (bad turbulence). I'm thinking ~500W, but my brain couldn't cope, and I'm sure it would be well past the size of a shed. [Damn, now my silly brain is imagining a combined VAWT shed, but the Waltzer ride always made me feel sick.]

Obviously I appreciate this is a 'how long is a piece of string' question, but I'm wondering if it's just 'silly' big, or well into ridiculous/insane?

Hi

Looked at this some time ago when researching VAWTs and the result effectively said that where we are it's a non-starter ... unless you can build one for yourself using really cheap materials ... I did end up working on a basic design for a hexagonal garden pergola based system (think mini bandstand!) with baffles accelerating airflow into a bladed enclosed VAWT setup (approx 2x2m) as well as reducing the effect of spinning against the wind, but design improvements became never ending resulting in more/better materials and higher expense so never committed to build the damn thing !!!! ...    

The issue really becomes one of wind speed where you live - you can't use average wind speeds as they're pretty meaningless, what you need to do is model the historical wind speed exceedence profile. This gives a prediction of the percentage of the year that airflow will be within a range of speeds therefore you can estimate the generation within those ranges ... add up the totals and you have something to work on .... problem is that the profile will probably show (from memory) that ~80% of the year the wind (at 10m) is below 4m/s whilst only ~0.5% of the time it's useful in the 16m/s range .... this is the real killer because the cut in speed of whatever professional small turbine you can get is likely around 4m/s and the rated output is normally at ~16m/s - even forgetting turbulence, over-speed cutout and relative performance altogether, a 500W rated turbine could only generate at ~500W for ~0.5% of the time, so ~22kWh/year (365(d)x24(h)x0.5(kW)x0.005(%)) .... repeat this exercise for each decile in the wind speed exceedence profile for your area, take cut-in and cut-out speeds into consideration and you'll get an idea of what's really possible, before  turbulence etc .... 

... alternatively, if you don't live on a hilltop in a high wind area, either buy one and expect to moan about it's performance, or build one as something to do  ...

The wind here is currently 21.1km/h, so ~6m/s, so a 'light breeze' probably just about enough to start a turbine spinning ... I'd 'guess' (  )  that a 500W turbine on a 10m pole with no turbulence would be generating ~20W, which considering it's almost sunset and there's a line of clouds between us and the sun and is visibly darkening outside, is still around 20W lower than the solar on the roof and <2% of what we were generating about 1 hour ago .... and all this in the middle of winter .... ...

HTH - Z

QrizB

Some links from a previous thread on home wind turbines:

Met Office page with mean wind speed map (a knot is more-or-less 0.5 metres per second)

RENSmart map with NOABL data

Global Wind Atlas with mean wind speed and power density layers

And an interesting post from almost 20 years ago. Starts well (says solar PV isn't yet economic but might be in 10-20 years time), recommends solar water heating (at the time, a fair suggestion) but then predicts a mass adoption of domestic wind (which kinda flopped, for all the reasons discussed in this thread).

Utility-scale wind generation, on the other hand, is now the UK's second-biggest single source of electricity. Electric Insights only goes back to 2009 but then, wind provided only 2.73% of the UK's electricity - 1/10th as much as coal. Wind is now bigger than coal was then, and coal is pretty much extinct.