Roof Mounted Wind Turbines in Urban Areas

Are roof mounted turbines a wind up?

Roof mounted wind turbines have had a bad press. After the initial excitement died down reports started coming out about how little power they actually generated compared to what the manufacturers stated and how long the pay back periods were. I thought I do a little research and report my findings.

Lets answer the basic questions.

How do wind turbines work?

A mass of moving air has kinetic energy - think about how much energy you exert in walking into a strong wind. Wind turbine blades are designed and angled to extract some of that kinetic energy and convert it into a rotational force. This is converted into electricity using a generator which is like the dynamo on your bike. A dynamo has a magnet and a coil of wire to convert the wheels spinning motion into electricity for your lights. On a wind turbine it's bigger, more efficient and a lot more ‘engineered' but the principle is the same.

How much power can they generate?

The amount of wind power that can be generated at a location is a function of 3 things

Wind Velocity - The power is proportional to the cube of the wind velocity. This means there is 8 times more power available at 6m/s than at 3m/s. Each time the wind speed doubles there is 8 times more power available. Obviously wind speed is the dominant factor.
Blade diameter - The larger the area the wind turbine blades sweep through the more power. A 2m diameter turbine will generate up to four times as much power as a 1m turbine. This is a big factor.
Blade design and internal efficiency - At most 59% of the wind energy can be turned into rotational energy. This is fundamental. All designs will be less than this and most a lot less. Then you have to convert this to electrical energy with efficiency losses. Overall, you multiply the theoretical maximum power by a factor that is between 25-50%. This is important but will not help if the first two points are not OK. Wind turbines are also designed with a cut-in wind speed (below which no power is generated) and a cut-out speed. Generally the cut-in speed is designed so low that it's irrelevant and the cut-out is limited by the peak output power of the turbine.

What effects wind velocity?

Obviously this varies with the daily weather but it is possible to measure the average wind speed at a location and indeed this has been done as shown here on the national wind speed map. This is the average wind speed measured at a height of 10m in a location free from any obstructions. The two caveats here are height and obstructions.

Height

The average house is 6-8m in height and wind speed is increases with height. This would mean between 15-30% less power just due to the lower mounting height of the turbine.

Obstructions

The average turbine will either be mounted on the roof or side of the house. These will act as obstructions to any wind flow - as will neighbouring buildings, trees, hills etc. This means the air speed measured in the national database is likely to be very different to that on the roof of a house in a built up area. Location, location, location!

The average wind velocity itself isn't enough information to work out the power available anyway. You need to know how it varies throughout the year. Rather than an average wind speed you need a weighted average speed as times of higher wind speeds contribute much more power than equal times of lower wind speed. The location has been shown to alter the distribution (from a Rayleigh to a more general Weibull distribution) of wind speeds which can be significant in estimating how much power is available.

How much money could I save per year - in theory?

I'll roughly calculate a graph based on a typical commercial roof mounted wind turbine. It won't be perfect but at least (hopefully) ballpark. Note it will assume a given average wind speed which may not be true in your location. In making this I've assumed a cut off speed of 10m/s and a cut in of 2m/s and a blade diameter of 2m. The turbine has an overall constant conversion efficiency of 33% and the wind speed is a Rayleigh distribution. Assumed 1kWh costs 12p to buy from a utility company.

Ideal wind turbine savings per year

Note that the graph tails off towards the higher average speed values which is due to the fact that some of the time the wind speed is too high and the turbine cuts out - no power generated. This graph also assumes that your load (i.e. how much and when you use electricity) matches the generation or you can store the energy (batteries) or sell it back into the national grid. At 6m/s it would generate about £200 worth of electricity annually but at 4m/s that figure drops to about £80 and only about £35 at 3m/s.

If we then assume a purchase cost of £1500 (as an example) then we can work out a pay back time for this.

Graph showing pay back time for an idea wind turbine versus average wind speed

So, if you mounted this turbine at a location that really does have an average wind speed of 6m/s then it would roughly pay for itself in 7-8 years. However, look at what happens when the average wind speed drops. At 4m/s it is 18-19 years and at 3m/s it is 43-44 years!

Remember these graphs also assume that you either use all the electricity generated, store it for later or sell it. It also does not include any reduction in power consumed in operating the wind turbines.

So, what is the problem?

You may have guessed by now - realistic average wind speed at a location.

A trial in Warwick in 2007-2008 tested 5 different manufacturers turbines on 26 different buildings. The building height ranged from single storey to high rise blocks (45m).

Excluding anytime the turbines had to be serviced or repaired the average turbine would have saved you £27.60 per year in electricity.

The wind turbines that were on single or two storey urban buildings had average wind speeds of between 2-3m/s compared to around 5m/s predicted from national database.

Wind turbines in rural areas performed better but all still had lower than average wind speeds and the distribution of wind speeds didn't match the commonly used Rayleigh model.

If I live in a city, town should I buy one?

Probably not.

Since they first came on the market much work has been done to more accurately predict which locations are best suited to have roof mounted wind turbines. In particular the MET office and the carbon trust have done some interesting modelling of urban environments to study the factors that effect wind speeds.

Most manufacturers have responsibly taken onboard the work that has been done in assessing which locations are best and now recommend a thorough investigation (which they could carry out) to see if a wind turbine is suitable for your property. However, most don't appear to give realistic figures of what energy saving you will make and highlight optimistic savings.

If I live in a rural area should I buy one?

Well its much more likely to actually generate useful amounts of energy.

If you live in a rural area you may want to consider a mast mounted turbine situated away from any buildings, neighbours, trees or other high obstacles. This way you will get maximum benefit from the wind. Rural areas typically suffer less from turbulence effects and boundary layer problems that plague urban locations and hence, higher wind speeds are actually realised. Though its still worth having a survey done first to see how much energy you would save.

Mast mounted turbines are also a lot more money (though potentially generate more as well due to their larger size). Prices are around the £10,000 - £20,000.

Anything else to consider?

You will probably need planning permission.
You need to be very careful about structural damage to your home due to forces transferred from the turbine in strong winds
If the turbine or mast starts vibrating it could become a noise nuisance.

Conclusions on Roof Mounted Wind Turbines

I confess writing this article on roof mounted wind turbines is slightly depressing. I really wanted them to work. I had read press reports about poor performance but as I had never read the studies myself I was open to reason. However, after looking into the fairly extensive studies that have been done it is obvious that most people living in urban environments would not save money in buying and using a roof mounted wind turbine.

Urban Wind Turbines