Selasa, 25 Agustus 2009

WATER tURBIN TEORI DARRIUS

 

Turbine Theory

 

For those familiar with wind power the governing equation for water current turbines is identical. Since the density of water is about 850 times larger than that of air, the area and velocity required to produce the same amount of power can be drastically reduced. The available power is dependent on a number of factors, the primary ones being the swept rotor area and the velocity. The result being,

P = K x½ xA xV³

where for the Darrieus turbine the area, A is equal to diameter times the height. The other symbols are as follows: P - Power produced, K - performance coefficient, ρ - density of water, and V - the average velocity of the water. The velocity is the most important variable in determining site applicability, a slight increase in the velocity will have a great effect on the power produced.

The performance coefficient, K, takes a variety of factors into account. Each turbine has a different maximum value of K, which indicates the efficiency levels for that turbine. A recent study by J. Vocadlo cited the K for undershot wheels at a maximum of .06, while the Darrieus was rated at a maximum of .42. As a result the Darrieus would be 7 times more efficient than the traditional undershot wheel if both were operating at peak efficiency. In practice however, other losses are incurred in the transmission of the power to the generator and in the generator itself. A more realistic coefficient of 0.17 may be utilized in order to determine the output power of a Darrieus turbine and PM Generator. Simplifying the first equation by substituting the values for density and K results in,

P = 125 xA xV³

where A is in m², V is in m/s and P is in Watts. For those more familiar with Imperial Units the equation becomes,

P = 0.329 xA xV³

where A is in ft², V is in ft/s, and P is in Watts.
 

Velocity and Force Diagram

 

These sorts of diagrams are commonplace in Engineering. They help the viewer quickly understand the causes and results of the different physical things acting on the object. Often we would call them Free Body Diagrams (FBDs).

Here V1 represents the speed of the turbine blade in the water, while V2 represents the speed of the water alone. The resultant vector here, V3 is the speed of the turbine with respect to the water. This is the actual direction of the airfoil in the water if the water were not moving. This vector V3 results in F1, the lift force of the airfoil.

This vector F1 then must be changed so that it reflects the physical frame of reference of the turbine. This results in F2 being the load carried by the arm, while F3 is the resultant force forward. There is a drag force also here, but for the simplicity of the diagram it was left out as it is not significant. As the turbine moves around its circular path the forces generated will be constantly varying. As a result, only this one section of the airfoil rotation has been portrayed here and it is not to scale.

Development of Darrieus Turbines

 

The turbine's namesake Mr. G. J. M. Darrieus was issued a French patent in 1926 for a "Turbine with Cross-flow axis of Rotation" and was later issued a US patent in 1931. The next time period when there was considerable interest in this style of turbine was in the early 1970's. This renewed interest brought about the development of the Darrieus turbine as commercial wind power generators. Introduction of this technology to water turbines has been slower than that of wind. A number of groups have constructed turbines with shaft power from a few Watts for model tests to 5kW for one of the final freestream prototypes.

Alternative Hydro Solutions Ltd. has taken these concepts and modified them to be more suitable for smaller rivers. A number of design simplifications have been incorporated over the previous designs while maintaining the turbine efficiency.

Our History

 

Alternative Hydro Solutions Ltd. began in 1991 with Mr. Gregory being employed in the Hydrogenerator field and having read an article on the Darrieus turbine at work. Slow progress was made until the use of some aluminium extrusions was started. At this point some preliminary testing was performed while towing a boat mounted version behind a powerboat. Further testing led to a site on the Mississippi river near Ottawa, Ontario (see pictures below) and a contract with the government (Natural Resources Canada) to carry out further testing. After this testing was complete a short hiatus was taken until the production of a couple of units for sale were undertaken. Another short break resulted in the pursuit of this opportunity full time by Mr. Gregory.

 


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A retrofit of an original unit

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A site on the Mississippi river near Ottawa.
 


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Turbine Sites

 

With the Darrieus turbine the water levels remain effectively the same and the lack of civil structures required make this an environmentally friendly device. Typically these units have been mounted on a pontoon, barge, or small boat, however, for smaller streams other methodologies may be more cost effective. These could include a built-in support beam extending either fully or partially over the river.

The amount of debris flowing down the river is also a factor in siting a turbine. If there is a constant flow of heavy debris some protection for the turbine will be required. A "trash rack" to deflect the heavier pieces would provide a safer operating environment. For this reason removal of the turbine during a heavy spring runoff would also be advised. This is very site dependant; in some locations, e.g. downstream of a dam, few trash problems would be encountered.

Interference with fish should not be viewed as a problem when using a Darrieus turbine. The motion of the blades will tend to deter any fish from going near the turbine and typically the small width of the turbine compared to the large width of the river will allow plenty of area for the fish to migrate without passing through the turbine. Should a fish stray and go through the turbine the low speed of rotation, the blunt leading edge, the openness of the turbine, and the lack of any walls or ducting for the fish to be trapped against would prevent any damage from being done.

 

Turbine Construction

 

These turbines are constructed of high quality and durable materials. The turbine blades are custom 6063T5 aluminum extrusions with a solid cross-section in order to provide the required strength. The 6063T5 alloy offers excellent resistance to corrosion and a smooth surface finish. The arms are typically made from the same profile as the blades in order to reduce the losses due to their rotation in the water. This is not true on the larger models as the blades need to be larger for the larger diameter, however, the arms can be made of a smaller airfoil. The mating to the hub is performed with a patentable mechanism, which incorporates a second female extrusion to the arms male. On larger turbines this remains constant, as the arm stays constant between the two diameters of turbines. The shaft is made of stainless steel and is supported by two standard stainless steel pillow block bearings. The frame supporting the two bearings is a standard channel section or flat plate, which may be modified to accommodate a variety of mounting mechanisms. The power is transferred through a flexible coupling to a motor and gearbox combination which allows the motor to run at a higher rpm thereby increasing its efficiency and reducing the torque fluctuations. This turbine is available in 4 diameters presently. A 1.25m diameter as well as a 1.5 m diameter and a 2.5m diameter as well as the largest the 3.0m diameter. All are available in various depths.

A number of electrical options are available depending on site requirements. These include a permanent magnet D.C. generator and a brushless alternator.

 

 

  1. How do I measure the water speed?
    There are a number of methods to measure this. The simplest and least expensive is to use the float method. This involves measuring two spots on the shore about 10m apart. The next step is to throw a floating object into the river upstream of the first marker. When it passes the first marker perpendicular to the flow start the timer. Run downstream and when the float is perpendicular to the second marker stop the timer. Divide the distance traveled by the time to get a figure in meters per second (m/s). Go to the graph and pick out the potential power for your site. Remember that the figures in the graphs are net power out.

 

Power Output

 

As an alternative to the equations the following graph allows a quick comparison of the effects of changing the area and water velocity. To calculate the permissible height for a site take the actual depth and subtract 0.1 m ( 4") for shallow, high speed flows - those greater than 1.3 m/s ( ~ 4ft/s) and 0.3 m (12") for lower speed flows. There are 2 width(diameter) standards, 1.25m and 2.5m. Other sizes may be accomodated at a premuim.

 

Cost

 

Hydro turbines produce power 24 hours per day, every day and as a result can provide a very cost-effective alternative when compared to other forms of renewable energy. The initial capital cost is an important part of the decision to pursue any particular avenue for energy production, as a result Alternative Hydro Solutions Ltd. offers a range of options and sizes. The key element is the turbine itself, a person can fabricate the remainder with some technical ability, and accordingly the turbine is offered separately. From the turbine only to a complete generating system we can supply your needs.

The base turbine consists of a 0.45 m² ( 5.4 ft² ) turbine with a diameter of 1.25m (4.1 ft) and a single support arm. The price of this and any other unit is available on request. Shipping and mounting platforms are not included in these prices.

Further comments on any potential site are available upon request. Some information that would aid in commenting would include the following; water velocity, available depth and width, degree of seasonal variation, and the roughness of the riverbed. Metric hardware is available at a premium.

 

2) How do I measure the diameter and height of the turbine?
These characteristics are governed by a number of power factors.
a) the mounting mechanism plays a role here. If the turbine is to be mounted on a boat several things need to be remembered. The first is the depth of the transom of the boat. The second is the depth of the boat in the water. And the third is the depth of the water in the area swept by the desired turbine.
b) The height is governed in part by the blade stresses. The faster the maximum water speed the lower the distance between blade support arms. Thus for each maximum speed there is a number of support arms which can be safely accommodated.
c) The third factor here is price. The larger the diameter of the turbine, higher

the price, and of course a greater power output.

 

  1. How do I determine the power output of the combined turbine and generator?
    These characteristics are governed by a number of power factors.
    This will be an iterative process. Once you have determined the size and have determined the speed of the water then you can look at the graphs on this site for information. The graphs include the inefficiency of the generator. Thus they are a realistic expectation of the power that you will see at the generator side. Wire losses in transmission are not covered here as they are covered on others sites.

 

  1. How do I mount the unit?
    This will depend on the size of the unit and your waterway size. In some locations a boat may be out of the question, whereas in others a beam across the river will be out. Both of these and other options are viable in some locations. Please see the website for more details.

 

  1. How do I transmit the power?
    The power should be transmitted to shore via a cable. This will depend again on the individual river situation as between the boat mooring and the shore may be a passage for other vessels.

 

  1. ) How often do I need to clean the turbine?
    This answer is determined by the amount of loose debris floating down your waterway. Some places may only need to be cleaned a couple times a season, while others may need to be cleaned several times a day during some seasons.

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