CHAPTER 2: LITERATURE REVIEW OF RENEWABLE GENERATION
2.5. Types of renewable energy sources
2.5.4. Wind Power sources
37 2.5.3.3. Grid-tied system
These systems are directly coupled to the electric distribution network and do not require battery storage. Figure 2.22 below describes the basic configuration including the grid electric utility. Electric energy is either sold or bought from the local electric utility depending on the local energy load patterns and the solar resource variation during the day. This operation model requires an inverter to convert DC current to AC currents.
Figure 2.22: Diagram of grid-connected photovoltaic system (Libo et al., 2007).
There are many benefits that could be obtained from using grid-tied PV systems instead of the traditional stand-alone schemes.
• Smaller PV arrays can supply the load reliably.
• Less balance of system components are needed.
• Comparable emission reduction potential taking advantage of existing infrastructure.
• Eliminates the need for energy storage and the costs associated to substituting and recycling batteries for individual clients. Storage can be included if desired to enhance reliability for the client.
• Efficient use of available energy. Contributes to the required electrical grid generation while the client‘s demand is below PV output.
38 connected to the grid has become the mainstream, garnering the support of large grid integration.
Wind turbines convert wind energy into electricity. Wind is a highly variable source which cannot be stored, thus, it must be handled according to this characteristic (Blaabjerg & Lonel et al., 2015). A general scheme of a wind turbine is shown in Figure 2.23, where its main components are presented. Figure 2.24 is a schematic representation of a modern power system, which incorporates renewable energy sources, distributed generation, and smart grid functions. Integration is made possible through the extensive use of power electronics. Wind energy is influenced indirectly by the energy of the sun (Burton et al., 2011). Significant quantity of the solar radiation received by the Earth is converted into kinetic energy, the main cause of which is the imbalance between the net outgoing radiation at high latitudes and the net incoming radiation at low latitudes (Burton et al., 2011).
The Earth’s rotation, geographic features and temperature gradients affect the location and nature of the resulting winds (Burton et al., 2011).
Figure 2.23: Power electronics enabled wind turbine energy conversion system (Blaabjerg &
Lonel et al., 2015).
39 Figure 2.24: Schematic diagram of grid connected wind turbine (Burton et al., 2011) The use of wind energy requires that the kinetic energy of moving air be converted to useful energy. As a result, the economics of using wind for electricity supply are highly sensitive to local wind conditions and the ability of wind turbines to reliably extract energy over a wide range of typical wind speeds.
Together with hydroelectric and photovoltaic system, wind generation or the direct conversion of wind into electrical energy is one of the cleanest forms of energy conversion available. When the energy crisis in the seventies was occurring, wind generation became very popular as a prospective replacement for fossil fuel electric generation. But, there are several reasons that wind energy generation has not become a major source of energy.
However the development of wind has been slow due to high cost and the intermittent nature of winds.
2.5.4.1. Operation of wind turbine
The operation of a wind turbine is characterized by two conversion steps. First, the rotor extracts the kinetic energy of the wind, changing it into mechanical torque in the shaft; and in the second step the generation system converts this torque into electricity (Brent, 2013). The power coefficient, Cp gives the fraction of the kinetic energy that is converted into mechanical energy by the wind turbine.
It is a function of the tip-speed ratio and also depends on the blade pitch angle for pitch controlled turbines (Chauhan & Saini, 2014). For the operation of the wind turbine, it is important to take into account the equation of power density. It is used to find which wind site
40 is potentially available to install the wind turbine (Brent, 2013). The expression for kinetic energy in moving air wind is calculated under the following equation:
(2.1) Where P = power of a wind flow;
Ρ = Air density [1,225 kg/m3, under usual conditions];
A = the cross-section area of a wind flow;
V = speed of a wind (m/s).
The useful mechanical power obtained is expressed by means of the power coefficient
C
P(2.2)
The wind velocity suffers retardation due to the power conversion to a speed V3 behind the wind turbine. The velocity in the plane of the moving blades is of average value V2 = (v1 + V3)/2 (Nikolaev et al., 1994)
According to Nikolaev et al., (1994), calculation of useful power is maximum when (V3/V1) = 1/3 and the power coefficient Cp ≈ 0.59.
Wind turbines power coefficient maximum values Cp max = (0.4 - 0.5). This is because of profile loss, tip loss, and wake rotation loss. Tip-speed ratio is an important parameter of wind turbines. It is the ratio of the ratio of the peripheral velocity of the turbine blade tips and the wind speed (Nikolaev et al., 1994).
Figure 2.25: Typical diagram of wind turbine (Mentis et al., 2015)
In present wind systems, the generator system gives an AC output voltage that is dependent on the wind speed. As wind speed is variable, the voltage generated has to be transferred to
V
W A
3
2
V C
P A W3
2
41 DC and back again to AC with the aid of inverters. However, fixed speed wind turbines are directly connected to grid (Chauhan & Saini, 2014).
One of the hottest topics for renewable energy is the generation of electricity by means of wind, but since we cannot control wind, a lot of protections have to be taken into account when having wind generators connected to the grid in order to maintain stability and avoid system collapse (Kishore et al., 2013).