| How it works
A wind turbine is designed to extract mechanical energy from the displacement of air (wind) and convert it into electrical energy. As air flows through the rotor blades they rotate and this drives an induction generator which produces electricity. The power output is proportional to the cube of the speed so power output increases faster than wind speed.
A wind turbine cannot extract all the wind kinetic energy as wind still has a residual speed after it passes through it. The maximum amount of energy that can be extracted is about 59%, this is known as the Betz limit. The ratio between the energy extracted and the wind energy available is referred to as the power coefficient (Cp).
The power output can be calculated with the formula: P = 0.5.ρ.A.Cp.V3 where P is the air density, A is the rotor area and V is the wind speed before going through the rotor.
Wind turbines rotate when the wind has a minimum value (cut-in) and will stop if the wind speed is too high (cut-off). Turbines are rated at a certain speed (and corresponding power), meaning that when the wind speed goes beyond the rated speed, the power output remains constant. The maximum amount of energy that can be produced in a year is therefore equal to the rated power x 8760 hrs (1 year).
In practical situations the wind does not always blow at the rated speed so the energy produced is a lot less than this. The ‘capacity factor’ is the ratio of actual energy produced to maximum amount at rated power. It is a critical parameter to assess wind potential. While the best reported capacity factors can reach 30-50% in the best areas of Scotland (large turbine) it is generally much lower in most location, in particular for small wind installed near dwellings [17].
More information can be found on the websites [1,2,3] at the bottom (references).
Benefits of wind energy [1,2,3]
- Wind energy is among renewable energy sources with higher potential for Europe
- Target 20% of electricity production in 2020
- Wind turbines produce
- No air, water, or thermal pollution
- No greenhouse gases
- No smog
- Wind installations leave no trace to environment after dismantling a site
- Most activities (agriculture, industrial) can be maintained on a wind farm site
- Competitive energy price + capacity is fast to build/install
- Using wind power offsets pollution that would be generated by the utility company
Definition of small-wind:
- Small wind refers to small size turbines with typical power between 500W and 25kW, which may be connected or not to the grid. The swept area is generally smaller than 200 m2, this corresponds roughly to rotor diameters smaller than 16m.
- The typical rated speed is about 12m/s while cut-in speed is generally around 3m/s. the cut-off speed varies according to different manufacturers
- Most manufacturers recommend a minimum average wind speed of about 4.5 to 5m/s [4,5] to consider a site as potentially viable. We have investigated the various parameters influencing yearly energy yield and found that the knowledge of the average wind speed alone for a site is largely insufficient to forecast the performance of a small-wind installation. For instance Evoco reports pay-back periods of just a few years for small wind installations [5], we believe this is only possible in specific sites which are isolated and benefit from excellent wind resource and in many case with financial help, either in the form of grants or feed-in tariffs.
Why use small wind
- Small-wind generation allows to strengthen the grid.
- Wind is also very attractive due to relatively simple technology, provided the sites have a minimum average wind.
- Wind power is in general reported to be very competitive with solar PV or bio-fuel [1,2,3]. While this is true for large scale wind, the results we have obtained for small wind do not generally agree with this statement, however in specific sites where the wind resource is adequate small wind can be viable.
Potential issues with small-wind:
Technical: Unlike for large farm projects, adequate site studies are not conducted in micro-wind generation, more particularly:
- There is insufficient knowledge of the wind speed profile. Maps are available with average yearly wind speed but this is not sufficient [6,7,8]
- Most dwellings are not isolated and the effect of large objects in the vicinity such as trees or buildings is not easy to evaluate
- These issues will affect the capacity factor of the turbine at a given site and reduce its energy production
- Site selection is therefore a key criteria!
Environmental:
- Visual impact: there are restrictions on where turbines can be installed in various countries and most installations require a permit [9,10,11].
- Noise: according to most reports it is not enough to be found objectionable by most people.
- A typical residential 10kW turbine has a 50-60dB level. This is less noise than the average washing machine.
- Birds death rate is reported to small as compared to total human related numbers. It is much less than house cats, electrical lines or even windows.
- Safety: There is an excellent track record so far, and the probability of a wind turbine installation falling down is reportedly much lower than for most trees!
- Small wind turbines do not interfere with TV reception
Small wind is more expensive than large size turbines on a ‘per kW’ basis
- Maintenance cost is ‘reportedly’ smaller than for large turbines (this may not be true for all manufacturers!)
- The capacity factor must therefore be higher to achieve a reasonable pay-back period
- Feed-in tariffs are in place in various countries in order to develop small-wind where the resources are sufficient.
- The UK feed-in tariffs [12] are dependant upon the power of the turbine, for small-wind power range we have:
- 34.5p for power less than 1.5kW
- 26.7p for power between 1.5kW and 15kW
- 24.1p for power between 15kW and 100kW
- We have looked at the financial viability of small wind based upon feed-in tariff in place and the capacity factor which depends on the location
UK regulations key points:
For detailed information, the standards are available from BWEA [10]
There are requirements on:
- Performance
- Noise levels
- Strength and safety
- Duration test
Manufacturers have to:
- Compile checklist
- Submit a certification by accredited body
- Provide adequate label (mostly the yearly energy assuming 100% availability and wind average speed)
Planning permission may be needed depending on mast height and depending upon the specific countries legislation.
- Certain sites are restricted due to visual impact
- Specific documentation must be provided to customer
Electrical requirements
- Specify the maximum values for the voltage and currents between 0-50m/s speed range
- Cable rating
- Voltage drop in cable normally < 4%
- Isolator for installation and maintenance
- Earth to turbine and building are separated
- Lightning and surge protection
- Metering of instant power and energy transmitted
Mechanical requirements
- Withstand 35m/s sustained speed
- Resist to 50m/s speed gusts
- Tower must be resistant to continuous vibrations and corrosion
- Foundations are submitted to specific regulations
- Moving parts must be >3m away from people or animals
- Designed to prevent climbing!
The US has standard requirements as well [9]
Site location:
- Most institutions and manufacturers recommend a minimum average wind speed at the site of 4.5m/s to 5m/s for wind to be viable [4].
- The wind distribution also important. The same average speed does not mean the same energy output yearly! This is due to the fact that the power is proportional to the cube of the wind speed so higher speeds allow to get more energy output (up to the rated speed).
- The area must be clear of obstacles and well exposed. Obstacles will reduce the effective wind speed and/or cause turbulence. We have found that this is a particularly important issue in urban or residential environment where the capacity factor is reduced a great deal.
- The turbine should be set at optimum height. If the hub is higher the wind speed will be also higher and more energy will be produced but at the same time the installation will have more constraints
- The distance to the supply should be kept relatively small to minimise installation costs and losses. Usually within about 500m.
- The turbine must also be at a minimum distance from any building or even areas where people may often gather. This is driven by safety requirements
Standard costs
- Turbines cost about £2-3 per W. Installed cost is >£3 per W [5]
- Larger turbines (>3kW) are cheaper and more effective. For a 6kW compared with 600W, the ratio of the cost to energy produced can be as much as one third.
- Obviously, the larger the turbine the more constraints there may be regarding installation and authorisation.
- Side equipment like inverter or batteries can double (or more) the installation cost
- Grid connection can be expensive. For instance the cost of a 0.5-1mile cable can be up to 3 times the turbine installation cost
Manufacturers:
- The technology is well advanced and optimised. Turbines of various power ratings are available commercially from many companies.
- The main manufacturers include for instance Proven [4], Ecovo [5] or Ampair. Proven has among the longest experience in the field and their turbines have been installed in various parts of the world.
- There are already a large number of manufacturers worldwide [13,15,16], who either make turbines or just specific parts, as well as related equipment (electrical, etc…)
- The “Wind Power Conference” is to take place in Dallas on May 2010, there are over 1000 exhibitors just for the small wind (<100kW) section!
- Not all manufacturers are necessarily reliable, the problem is the product quality and longevity is not easy to assess!
- Key differentiating parameters between manufacturers include:
- Robustness, longevity
- Blade flexibility
- Material (composites) and design, which influence the maximum operating speed and turbine resistance to strong sustained winds [4]
- Amount of maintenance needed
|
