Small Wind Industry moves towards higher kilowatts

Power generation for the small wind industry is experiencing a change. The demand for higher power turbines is increasing in many markets all around the world. Demand is moving from less than 1kw to 5kw and higher. These are generally for home purposes or businesses. Typically the speed of these 5kw horizontal wind turbines range from 200 rpm to 450 rpm at the rated output for the permanent magnet generator. Producing more power than required lets you store the excess generation in batteries which can be used whenever required or even sell the excess generation back to the state grid in countries which allow it. This is an innovative way to generate and store energy which can have low payback periods through PMGL’s wind permanent magnet generators.

Pension Funds: Latest investors in the energy sector

Wind energy companies seek to develop more and more operational wind energy projects and farms every year. Many nations have now pledged to get a major portion of their power from renewable sources of energy like wind, water etc. but funds for these projects are huge and large companies are relying less on loans from banks.

Developers of large renewable energy projects are now targeting investments from pension funds. The expansion plans of many companies and nations have clearly laid down the need of development of renewable energy projects in coming decades. For achieving these goals, pension funds look to be a good investment source since they have a lower threshold for return
on invested capital.

Latest news of Swedish wind power, Arise Windpower, indulging in such new investment plans for upcoming projects show the eagerness of investors to see renewable energy projects as a source of stable and consisting cash flow. If these institutional investors are successfully tapped, then the inflow of capital will get a boost and a long term growth trajectory can be drawn for this industry.

MPPT: Maximum Power Point Tracking 

Maximizing Power output of wind permanent magnet generators with MPPT

Maximum Power Point Tracking or MPPT is an electronic system that allows grid tie inverters and battery chargers to get maximum power from the electricity source. These sources can be solar panels or other renewable sources like wind using permanent magnet alternators. MPPT is a technique through which maximum power is obtained. For wind generators, a controller compares the output of the pm alternator and compares it with the battery voltage. MPPT changes the electrical output voltage and current levels. This available power is provided to the battery in the form of increased current. MPPT detects the optimal power that can be obtained from the system and thus makes changes in the voltage.

Example

Imagine a conventional controller (non MPPT) that connects the alternator (75W) to the battery (12V) to charge a discharged battery. This makes the permanent magnet alternator operate at the battery voltage which may not be the ideal voltage at which the module is capable of producing. The maximum power may be obtained at some other voltage point. By limiting the voltage to 12V, the 75W wind generator system is able to produce power limited to a 53W module. Hence potential is not reached.

With a MPPT system attached arrangement, the controller will detect the voltage at which the module gives its maximum possible voltage which in this example is of 17V. The MPPT system then operates the module at 17V and extracts complete 75W of power. This is irrespective of the current battery voltage.

Wind Turbine for High Speed Winds

Aviation technology has developed designs to harness wind energy at higher speeds. Modern wind turbines are developed with the motive of utilizing more and more wind that strikes the blades of the wind mill. While wind is certainly a renewable source of energy, main problem with wind is its erratic nature. Power per square meter is the cube of the wind velocity.

Wind Energy measurements:

 Kinetic energy of the wind is half of (mass * velocity squared)

Amount of energy moving past a point on the wind turbine therefore depends on the velocity.So the power per unit area comes out to be Kinetic Energy * velocity = MV³ 

Having power being related to the cube of velocity creates a large difference in generation potential for higher wind speeds.

Imagine wind blowing at 60mph gives 27 times more power than wind at 20mph.  

Wind mill efficiency: Windmills cannot have 100% efficiency since the structure itself impedes the flow of the wind. Many times the efficiency depends on the actual wind speed, in correlation to the wind speed design.

Offshore wind:

Wind energy is the most popular renewable source of energy worldwide which can meet electricity demands in a sustainable and clean way. Offshore wind is attractive since it has minimal environmental effects. Also the wind speeds at offshore areas is greater which means increased production as power is related to the cube of the wind velocity. On an average the wind speed increases by 10-20% at off shore wind farms.  But higher energy yield has to compensate the additional maintenance and installation cost. Hence the needs of modern wind turbines which can withstand higher wind speeds and perform efficiently are in great              demand. 

High Speed Wind Permanent Magnet Generator  

There is a demand for wind turbines which start producing at low wind speeds and withstand

high wind speeds too. To continually run at high speeds, foundation, tower (mount), blades, and the permanent magnet generator should be designed properly so that they are strong enough to handle the greater mechanical force and electrical output.

The energy in the wind is proportional to the cube of the wind speed, so a wind turbine operating at very high speed will be under a huge amount of stress but also have the opportunity to generate exponentially. Many manufacturers are using fewer blades for a wind turbine which are longer and can withstand higher speeds. Also windings of the permanent magnet generators should not overheat under this stress. Hence permanent magnet alternators are built to withstand higher wind speeds so that the efficiency of the wind mills is not affected and no damage is done to the internal parts of the generator. All these measures lead to the development of a turbine which withstands speeds which are higher than average. These models are essential since the power obtained from these high speed winds is enormous and should not be wasted. Power

densities are increased if we successfully utilize these high speed winds which are in abundance at many off shore and high latitude areas.

Impressive Efficiency Results obtained during tests conducted by PM Generators Ltd.

PM Generators Ltd. is best known for its world class permanent magnet alternators and best quality products. PM Generators Ltd. has further taken steps to maintain their competency in the industry by conducting efficiency tests the results of which continue to prove that PMGL manufactures the most efficient products in the industry.

Need of the tests

PM Generators Ltd.  makes sure that only the best products are manufactured at its facility. For this purpose it conducts research and development of newer designs with increased efficiencies and better outputs. Similar tests were conducted to test the efficiencies obtained at varying speeds for a recently built permanent magnet alternator. These alternators are perceived as the future of renewable energy generation. Hence it is all the more important to develop more efficiency in this field.

How were the tests conducted?

We know that efficiency is output by input in terms of percentage. For a permanent magnet alternator, the output is in the form of electrical energy and the input is mechanical energy. So we basically needed to measure these two results, the input and the output to calculate the efficiency. Since the output is electrical, we measure the loaded voltage and current. The input side is mechanical energy which is generally provided by a motor. The factors to be considered here are speed and torque. All these quantities help us calculate the efficiency of the machine.

Results

The alternator tested was rated at 750rpm. We required the efficiency not only at the rated speed but also at half speed and the customer's cut-in speed. At the rated rpm, the PMGL alternator achieved 95% efficiency. At 400 rpm it was over 90% efficient and at 200rpm, it was 74%.

  The efficiency of more than 90% at less than the rated speed is worth noting. All alternator models at PM Generators Ltd. give very high efficiencies between 85% and 95%. 

Materials for Wind Permanent Magnet Generator

 Winding Wires

In electric industries, winding wires play a very important role. Their role spreads from generators to transformers, control devices and even pumps. Many materials can be used to make wires. The most popular material used is copper. It is the most accepted material. 

Copper has many advantages which makes it a number one choice. It offers very little resistance. It can be either solid or stranded. Materials which make the water impervious to water, grease, chemicals and oil are needed at this stage. Hence insulation used is of tough grade polymer compounds.

Winding wires are either enameled or coated with various types of varnish and other insulating materials. These wires are used as semi finished material for generating electromagnetic fields and transforming electrical energy.

Varnishing of Winding Wires
A layer of insulation is provided on the wires by varnishing. One important feature of these varnishes is that they must resist high temperatures. Wires are mainly made of conductive material; copper is the most popular as said earlier. Varnishes are made of copolymer which is made from thermoplastic of thermosetting resin. The conductor can be covered with one or more layer of varnishing. These layers can withstand temperatures up to 200°C. The resin is made of any of the followings materials like

Importance and need of Varnishing
Varnishing is done to give extra strength and stability to the copper winding wires. Varnishing increases the ability of these wires to withstand partial discharge and voltage peaks at high temperatures. Thus these varnished insulated windings are used to make coils for permanent magnet alternators.

Bearings

To get the correct efficiency we have to make sure all the necessary measures are taken, like using the correct type of bearings. Sealed ball bearings are commonly used , which should be correct in size and should be able to withstand axial loads.

Sealed Ball Bearings

This type of bearing covers the balls and provides a greater degree of protection because the shield on the sealed ball bearing actually comes into contact with the inner race. But this comes with a price; the high contact pressure of the seal against the rotating inner race increases the    friction and torque losses and can cause a buildup of heat as well.

Magnets

Grade: The grade of a magnet refers to the maximum energy product of the material that composes the magnet. In simple terms it is to tell how strong the magnet is. The higher the grade the stronger the magnet. Unit of energy product is Gauss. Sintered Nd-Fe-B will remain indefinitely; they experience a minute reduction in flux density over time. As long as their physical properties remain intact, they can work well with same efficiency for 100 years.

Magnets will start losing their properties if we heat them above their rate temperatures which is why you cannot overload a permanent magnet generator.

We coat the surfaces of the magnets to protect them from external environment and this will not affect the magnetic properties of the magnets. This increases the air gap between the magnets and the device it is held to. The coatings are very thin and hence the air gap change is negligent.

Precision manufacturing

Bearing Housing: These are very important for safe operation and durability of industrial bearings. These are used to mount the bearings safely.

There are many types of bearing housings available. The possibility of relubrication should be kept in mind while choosing the type. Housings which require relubrication are called relube type others are called maintenance free bearing housing. One other type can be split type and unsplit type. 

Wind Mill Installation

It is very critical to understand the concept of wind before we think of generating electricity from it. There are many basics like wind blade designs, speed of wind, tower height, site location etc. which should be given due importance before going into windmill construction. In this article we will help you understand these things and take you further toward the completion of windmill designing.  

Height:

Turbines are usually 100-120 m high. Increase in the diameter of the blades will increase the height with it. With more height, the wind power increases and power yielded is more. But the costs increases too and hence a balance is sought when deciding on the height of the tower.

Power:

Power in watts yielded by the wind turbine alternator equals the angular velocity (radians per second = 2 pi RPM/60) multiplied by the torque which is in Newton-meters. In case of a permanent magnet alternator, voltage and current are proportional to the RPM. Hence the power is proportional to the square of the RPM.

A very important thing to note here is that wind speed is not a one to one function of the speed. In fact energy increases by the cube of the wind speed. If we double the wind speed, we get eight times the energy. Hence looking at the maps become all the more important. Even a small difference in wind speed within a given area can have a big impact on the amount of energy a wind turbine can generate. It is also one of the reasons why a taller wind tower can make so much of a difference. You can get a sense of the energy produced by the turbine by the power curve graphs given by the manufacturers along with the turbine.

Speed

§  Start-up Speed – The speed at which the blades begin to rotate.

§  Cut-in Speed – The minimum speed at which the turbines will begin producing electricity.

This factor will depend on the wind permanent magnet alternator.

§  Rated Speed - The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. For example, a "10 kilowatt" wind turbine may not generate 10 kilowatts until wind speeds reach 25 mph. Rated speed for most machines is in the range of 25 to 35 mph.

§  Cut-out Speed - At very high wind speeds, typically between 45 and 80 mph, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways.

Temperature

Utility-scale wind turbine generators have minimum temperature operating limits which apply in areas that experience temperatures below –20 °C. Ice accumulation should always be avoided.

Axis

Vertical axis windturbines (VAWTs) have become popular in recent times. Manufacturers claim that these designs are quiet, efficient, and economical. The rotor shaft in a VAWT is placed perpendicular to the ground. They can be used to generate electricity without getting bothered about placing them in the direction of the wind. These are Omni Directional. Hence they require less parts which makes them less costly and gives ease of maintenance.

They produce a good amount of torque. Its blades have a larger surface area. Hence they can be coupled directly to pumps etc.  Also they are designed to spin at much lower speeds, hence they can function efficiently at higher wind speeds. This decreases noise and vibrations and provides stability.

Horizontal axis wind turbines (HAWTs) are the most common type of wind designs used today. In fact most grid connected commercial wind turbines are today designed with propeller-type rotors mounted on a horizontal axis on top of a vertical tower. These turbines need to be aligned with the direction of the wind, hence allowing the wind to flow parallel to the axis of rotation.

Upwind rotors need a yaw mechanism to keep the axis aligned with wind direction.  Downwind rotors are placed on the lee side of the tower. A great disadvantage in this design is the fluctuations in the wind power due to the rotor passing through the wind shade of the tower which gives rise to more fatigue loads.

Generator torque

With wind permanent magnet alternator, we can set the cut in speeds as well as the cut out speeds. Hence we can generate electricity at a variable speed. Let us take an example. Suppose the turbine speed is 200 RPM at a torque of 40 NM. With permanent magnet generators, the torque generated will be around 90% of 40 NM at same RPM. Other machines are able to provide just around 70% of this torque. 

 

Blade design

The ratio between the speed of the blade and wind speed is called tip speed ratio. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7. Modern wind turbines are designed to rotate at varying speeds. Low inertia is achieved by the use of aluminum and composite materials in the blade design. Hence the blades start rotating quickly.

The speed and torque at which a wind turbine rotates must be controlled for several reasons:

§  For keeping the generator within the speed and torque limits.

§  To optimize the aerodynamic efficiency of the rotor in light winds.

§  To keep the rotor and hub within their centrifugal force limits. The centrifugal force from the spinning rotors increases as the square of the rotation speed, which makes this structure sensitive to over speed.

§  To keep the rotor and tower within their strength limits. Because the power of the wind increases as the cube of the wind speed, turbines have to be built to survive much higher wind loads (such as gusts of wind) than those from which they can practically generate power.

§  To facilitate maintenance. Since it is dangerous to have people working on a wind turbine while it is active, it is sometimes necessary to bring a turbine to a full stop.

§  To reduce noise. As a rule of thumb, the noise from a wind turbine increases with the fifth power of the relative wind speed.

When you gather all these basic information regarding the designing of the wind mill, send us the requirements and the conditions and the skilled and experienced R&D heads at PM generators Ltd. will design an alternator which will match the mechanical power output as per the alternator requirement. PM Generators Ltd. will work on the provided specifications and develop a customized design which will suit your requirements to the fullest.