The portability of the wind energy has been until recently, a major knock. However, the energy niche is quickly growing with development as small portable wind turbine generators (Kleinschmidt, 2008). The gadget is a wind power harnessing device that is small enough or can be taken down, packed up and moved with easily. The generator is powerful enough to provide energy for charging smartphones and other USB devices (Kleinschmidt, 2008). The operation principle for portable wind turbine generator is less the same that of large wind turbine generator.
The device comprises of a drive train that has the main shaft, a hub, rotor blades, and some have in-built batteries for storing charge (Brown, 1997). The operation principle constitutes two processes. The first process is the conversion of kinetic of the moving air into mechanical energy when the blades are rotated by wind (Abea, 2005). The second process is electromagnetic conversion of this mechanical energy through a generator that is then transmitted to the external appliances or stored internally. Wind turbines are classified either by their speed control or by their mechanical power control and comes with several design variations. The designs vary in economics, performance and optimum locations. Vertical axis turbines are typically best for portable wind turbine generator due to their lower vibration, lower maintenance, and the ability to locate or to have a device at the ground level (Skajaquoda, 2014).
The figure below shows a typical portable wind turbine generator.
Figure 1: A typical portable wind turbine generator.
Portable Wind turbines generator capture wind, but essentially the energy comes from the sun. The Earth’s atmosphere cooling and the differential heating of the sun cause density differences. The resulting buoyancy forces eventually cause wind.
Figure 2: Buoyancy forces
Once this moving wind exist it is possible to convert its kinetic energy into electrical energy. Physical laws govern the performance the turbine generator (Abea, 2005). The kinetic energy for moving per volume is
KE = 12ᵨv2
Where u is the velocity of air, and ᵨ is the air density. For a stationary turbine with area (A) swept by it rotor blades, the amount of kinetic energy per unit time entering are A is given by;
pair = 12ᵨv2 * A * u = 12ᵨu3A
The equation gives the air stream total power through the area the rotor blades sweeps. However, not all this power can be extracted. Normally the velocity of the air coming out of the turbine blades is never zero and this why it is not possible to extract all the energy. No additional air can flow through the turbine to bring more energy if the existing air is not moving. Hence, there exists some limiting balance between the air flow-rate in the turbines blades that bring additional energy, and the percentage power extracted (Brown, 1997). Betz limit quantifies this relationship with an assumption an average velocity through the turbine. The figure below be demonstrated this relationship (Abea, 2005)
Figure 3: Upstream and downstream flow relationship.
The cross-sectional area of the air stream changes due to capture of the air kinetic energy. Due to relatively constant air pressure and the conservation of its mass, the cross-sectional area increases. The average velocity through the turbine is
uu = uu+ ud2
Hence, the mass flow rate through the blades area is
M = ᵨA (uu+ ud2)
And the power extracted from the moving air by the blades is given by the difference in kinetic energy of the air behind and front of the turbine.
pextracted = m2(uu 2 - ud2)
Substituting the equation for the mass flow rate
pextracted = pa4 (uu 2 - ud2)( uu+ ud)
The efficiency can be obtained by comparing the airstream kinetic energy, and the power extracted.
pextractedpair = 12(1+(u duu)1)- (u duu)2-(u duu)3)
The plot of this power ratio yields a curve as shown below
This show that the maximum power extraction from any given air stream happens when the air is slowed down by a third. The maximum efficiency from the graph is approximately 59.26percent
This ratio is referred to as Betz limit (Brown, 1977). Betz limit or Criterion applies in portable wind turbine generator designs. It is the theoretical power ratio that can be obtained from an ideal wind stream. Modern machines operate at lower efficiencies typically 20 to 40% due to mechanical imperfections, roughness of the blade surface and friction.
Optimal tip speed ratio is another important concept in designing portable wind turbine generator. It is the ratio between the rotor tip speed and the free wind stream speed. If the rotor rotates very slowly the Betz limits is limited as it does not extract much energy as it could since it allows too much wind to pass through (Calvert, 1979). On the other hand if the blade rotates too fast it creates so much drag as it appears as a disc to the wind. The tip speed depends on the number of blades used blade air-foil profile and the type of the wind turbine. The TPS can be obtained as shown below, and it is a dimensionless quantity (Abea, 2005)).
TSR = rotor tip speedwind speed = ᵤV = ᵚrV
Where: ᵚ = angular speed
V = speed of the wind
r = radius of the rotor
Abea K, Nishidab M, Sukuraia A. et al (2005), Experimental and numerical investigations of flow fields behind a small wind turbine with a flanged diffuser. Journal of Wind Engineering and Industrial Aerodynamics Volume 93, Issue 12, Pages 951-970
Brown, J. E., Brown, A. E., “Harness the Wind, the Story of Windmills,” Dodd, Mead and Company, New York, 1977.
Calvert, N. G., “Windpower Principles: Their Application on the Small Scale,” John Wiley and Sons, 1979.
Kleinschmidt, K. A. (2008, 06). Modern portable power generators -small, sleek and super stable! QST, 92, 45-48. Retrieved from http://search.proquest.com/docview/228477694?accountid=1611
Skajaquoda; introducing trinity, a portable wind turbine power station for charging USB devices. (2014). Resource Week, , 146. Retrieved from http://search.proquest.com/docview/1518700295?accountid=1611