DOMESTIC WIND MILL
THIS WHITE PAPER PROVIDES THE BASIC INFORMATION ABOUT DOMESTIC WIND MILL
Jytotindra Killedar
M.S. (MECH.ENG); MBA; F.I.E
This is the study I did to prepare my own domestic windmill based on the information on various websites and technical articles
11/24/2011
DOMESTIC WIND MILL
How winds are generated?
Winds are created due to the heat from the sun. The earth’s surface gets heated up due to solar radiation. Different surfaces—sand, water, stone and various types of soil—absorb, retain, reflect and release heat at different rates, and the Earth generally gets warmer during daylight hours and cooler at night. As a result, the air above the Earth’s surface also warms and cools at different rates. Hot air rises, reducing the atmospheric pressure near the Earth’s surface, which draws in cooler air to replace it. That movement of air is what we call wind.
Wind Power is Versatile
Winds are created by movement of air and because of movement it has kinetic energy. The kinetic energy is developed whenever a known mass is made to move. The wind turbine is used to capture this kinetic energy possessed by wind. The energy captured by wind turbine is converted to electrical power. Sometimes the power developed by wind turbine (mechanical energy) may be used directly to drive a water pump. This power is called wind power.
Wind power is pollution free.
Wind power generation uses a natural and virtually inexhaustible source of power—the wind—to produce electricity. Wind power generation is clean as it does not burn any fuel or exhaust any gases into atmosphere. This is very important to maintain the environment pollution free. Hence the wind power is considered as clean power.
Every modern home needs electricity for running modern amenities such as refrigerator, blender, water heater, air conditioner. Practically electricity can be harness to run the any gazette in the house which makes the human life more comfortable. To generate the required electricity a Wind power is one of the most important sources.
How much power (electricity) can be generated by a domestic wind turbine?
The size of the wind turbine is main governing factor in deciding the power output of the wind mill. The second important factor is wind speed.
The power out put is given by the following formula:
P(W)=1/2 ρ.A.(V)^3.η
Where:
P_W=power generated by wind mill in watts
ρ=density of air in kg/m^3
A=Swept area in m^2
V_W=wind velocity in m/sec
η=Efficiency of the entire wind mill system in %
Example:
How much power will be generated by a 3 meter diameter wind turbine rotor with a wind velocity of 4.5 m/sec. Assume efficiency to 35%
Given
1. Rotor diameter 3 meter
2. Wind velocity 4.5 m/s
Solution:
Lete us calculate the swept area
A=π/4 D^2
Where, D= is rotor diameter.
A= 7.0695 m2
Now let us calculate the power developed by wind turbine
P_W=1/2 ρ.A.〖〖(V〗_W)〗^3.η
P_W=1/2 (1.23)(7.0695)(〖4.5)〗^3 (0.35)
P_W=138.66 watts
Factors governing rotor size of domestic wind mill
Open area around the wind mill
The rotor size is also dependant on how much wind is directly coming on the blades. This amount of air is governed by the open space around the domestic wind mill. The length of the blades selected should not interfere with any stationary object. Sometimes stationary objects which can not be moved away will force to reduce the bale length during the design stage itself thereby reducing the output power of the installation under consideration.
Wind speed.
Wind speed is also a governing factor. If the wind speed is doubled then power output increases approximately 8 times. For a given speed of wind for a particular location one needs to find out the optimum size of rotor blade length.
Wind turbulence and shielding due to buildings and trees inhibits sustained strong, gust free wind flow for most of the time, the wind speed will more likely be towards the lower end of the performance specification at 4 m/s (9 mph) that is a light breeze. At this speed the power output of the system will be very low which may not be enough to power a single light bulb. For much of the time the power generated could be less than the quiescent power drain of the inverter.
Practical Power and Conversion Efficiency
German aerodynamicist Albert Betz showed that a maximum of only 59.3% of the theoretical power can be extracted from the wind, no matter how good the wind turbine is; otherwise the wind would stop when it hit the blades. This is also known as Betz’s co-efficient. He demonstrated mathematically that the optimum occurs when the rotor reduces the wind speed by one third. After inefficiencies in the design and frictional losses are taken into account the practical power available from the wind will rarely exceed 40% of the theoretical power.
Converting this wind power into electrical power incurs further losses of 10% or more in the drive train and the generator and another 10% in the inverter and cabling such that finally, the useful output from the wind turbine will be about 30% to 35% of the wind energy available.
Capacity Factor
Electrical generating equipment is usually specified at its rated capacity. This is normally the maximum power or energy output which can be generated in optimal conditions. Since a wind turbine rarely works at its optimal capacity the actual energy output over a year will be much less than its rated capacity. The capacity factor is simply the wind turbine generator's actual energy output for a given period divided by the theoretical energy output if the machine had operated at its rated power output for the same period. Typical capacity factors for wind turbines range from 0.25 to 0.30. Thus a wind turbine rated at 1MW ( Mega Watt) will deliver on average only about 250 KW ( Kilo Watts) of power
The components of a Domestic wind Mill
The wind mill consists of mechanical as well as electrical components.
I. Mechanical components
Wind turbine
Gear box-step-up
Tower/high mast
Tail piece or rudder
II. Electrical components
- Generator
- Rectifier
- Regulator
- Inverter
- Charge Controller
- Battery bank
- Cable
- AC barker panel
I. Mechanical components
Wind turbine
The modern domestic wind turbine is horizontal axis wind turbine and is commonly abbreviated as HWAT. The Following figure shows the general arrangement of horizontal axis wind turbine.
Figure 1: Schematics of Domestic Wind mill
Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted forward into the wind a small amount.
The factors affecting the speed of rotor are as follows;
The surface area, number of, & angle of the rotor blades to the wind direction.
The angular momentum of the rotor, which is dependent on the design & weight distribution of the rotor.
Air resistance to rotation (i.e. drag when the rotors are rotating faster than the wind).
Internal friction & electromagnetic force resistance to magnet rotation in the generator.
Gear box
A gear box is commonly used for stepping up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades into the wind which ceases their rotation, supplemented by brakes.
Tower/High mast
The entire wind mill system is mounted on a high mast or a tall tower to take advantage of the high wind velocities existing at high elevation
Tail piece or rudder
The tail piece helps the wind mill set up to turn itself to face the wind direction. It is important to change the direction of wind mill if wind starts flowing in from different direction. The Tail piece has following important functions.
Maintaining the plane of the rotor perpendicular to the wind direction as it changes, thus maximizing the available wind to power the rotation of the rotor piece.
Balancing the rotor piece so that the net weight of the wind turbine is on top of its support, meaning the structural stress on the support is balanced & thereby minimized i.e. so that the centre of mass is directly over the support or slightly on the rotor side to allow some adjustment to average wind force.
II. Electrical components
Generator
In a typical domestic system the wind turbine is coupled directly to a three phase asynchronous permanent magnet AC generator mounted on the same shaft. To save on capital costs, domestic installations do not have variable pitch rotor blades so the rotor speed varies with the wind speed. The generator output voltage and frequency are proportional to the rotor speed and the current is proportional to the torque on the shaft.
The output from generator is rectified and fed through a regulator to an inverter which generates the required fixed amplitude and frequency AC voltage. The block schematic arrangement is shown in following figure:
Figure 2- Schematics of Electrical System for Wind mill
It is actually a synchronous generator because the frequency of its output is directly synchronized with the rotor speed. In this application however it is called an asynchronous generator because the output frequency of the generator is not synchronized with the mains/utility frequency.
Permanent magnet DC motors work as generators, however, these motors are not designed to work as generators. Therefore a standard permanent magnet Dc motor is not a great choice as generator. When Permanent magnet DC motors used as generators, generally have to be driven far faster than their rated speed to produce their rated voltage. Therefore you should be looking for is a motor that is rated for high DC voltage, low rpm and high current. Steer away from low voltage and/or high rpm motors. You want a motor that will put out over 12 Volts at a fairly low rpm, and a useful level of current. So a motor rated for say 325 rpm at 30 Volts when used as a generator, could be expected to produce 12+ volts at some reasonably low rpm. On the other hand, a motor rated at 7200 rpm at 24 volts probably won't produce 12+ volts as a generator until it is spinning many thousands of rpm, which is way too fast for a wind turbine. So if you are planning for a Permanent magnet DC motors to be used as generator you need to select it wisely. One good alternative is to use the alternators used in automobiles.
Voltage Rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification
The voltage regulator takes the variable output voltage from power source, either DC as in solar voltaic systems or rectified AC from asynchronous generators such as wind turbines and provides the fixed system reference DC voltage at its output terminals to charge the battery and feed the inverter. Using a fixed DC level to the inverter simplifies the inverter design.
Regulator
The voltage regulator takes the variable output voltage from power source, either DC as in solar voltaic systems or rectified AC from asynchronous generators such as wind turbines and provides the fixed system reference DC voltage at its output terminals to charge the battery and feed the inverter. Using a fixed DC level to the inverter simplifies the inverter design.
Inverter
The inverter is the component which converts DC battery power to AC power at the standard utility supply voltage and frequency. Since the battery voltage is usually quite low, the inverter incorporates a DC - DC converter to transform the low voltage DC input to a higher voltage DC to supply the inverter circuit. The AC output voltage level depends on DC input voltage and unless a stable DC level is supplied by an external regulator, the inverter must also incorporate its own voltage regulator.
The output from an inexpensive inverter could be more like a square wave than a sine wave and for many applications this doesn't matter, however the high harmonic content from such inverters can cause some applications to malfunction. Furthermore, any system designed to supply power back into the grid must meet the utility's very tight tolerances on wave shape, harmonic content, and frequency and voltage stability. This is the function of power conditioning which demands more complex circuit designs and better filtering of the output.
Charge Controller
A wind-electric charge controller´s primary function is to protect your battery bank from overcharging. It does this by monitoring the battery bank— when the bank is fully charged, the controller sends energy from the battery bank to a load bank. Load bank l is an electrical resistance heater, and it must be sized to handle the full generating capacity of the wind generator used. These dump loads can be air or water heaters, and are activated by the charge controller whenever the batteries or the grid cannot accept the energy being produced.
Battery bank
The power handling capacity of the battery must be sufficient to satisfy the peak demand plus a safety factor of about 30%. The energy storage capacity must be sufficient to maintain the electricity supply when local power (sun ,wind or water) might not be available. The period without local power top up could be as long as two or three days and, unless there is some form of load shedding, the battery should be able to supply the full required load for that period. Again a safety factor of at least 30% additional capacity would be prudent however three to six days storage capacity is not unusual.
Note that there is a round trip energy loss inherent in the charging and discharging process of the battery which should also be allowed for. The ratio between the energy removed from the battery during discharge and the energy supplied to charge it is called the columbic efficiency. It is typically around 90% depending on the cell chemistry.
Note also that there will be an additional energy loss of about 10% in the charger used to charge the battery. Battery cycle life is increased if the depth of discharge (DOD) is reduced. Battery life can thus be extended by specifying batteries with a greater capacity than needed so that they are subject to shallower daily depth of discharge
That batteries have a finite life is due to occurrence of the unwanted chemical or physical changes to, or the loss of, the active materials of which they are made. Otherwise they would last indefinitely. These changes are usually irreversible and they affect the electrical performance of the cell. This page describes the factors influencing battery life.
Battery life can usually only be extended by preventing or reducing the cause of the unwanted parasitic chemical effects which occur in the cells. Ways of improving battery life and hence reliability are also considered below.
Cabling
The cabling should be flexile and of proper rating to carry the electrical load generated. The flexibility will help in countering the effects of twisting due to turning of wind turbine when direction of wind changes.
AC barker panel
The AC breaker panel is the point at which all of a home’s electrical wiring meets with the provider of the electricity, whether that’s the grid or a solar-electric system. This wall-mounted panel or box is usually installed in a utility room, basement, garage, or on the exterior of the building. It contains a number of labeled circuit breakers that route electricity to the various rooms throughout a house. These breakers allow electricity to be disconnected for servicing, and also protect the building’s wiring against electrical fires.
References:
1. Electropedia web site
http://www.mpoweruk.com/wind_power.htm
2. Article- “Small wind turbine basics” Published in Energy self sufficiency newsletter August 2005
3. http://www.bukisa.com/articles/339001_the-physics-engineering-of-the-wind-turbine-with-design-hints-for-efficiency-power-generation
