2. Theory

A WT converts kinetic energy from air flow to electrical energy through subassemblies including rotor blades, drivetrain, generator, and electronic control systems, as well as other auxiliary components. As the kinetic energy is extracted, the air flow that passes through the turbine rotor must slow down. Assuming there is a boundary surface that contains the affected air flow inside, a long stream tube extended far from the upstream and to the downstream with varied cross sections is often used to study the thermodynamics of horizontal-axis WTs [6, 7] (Figure 1). The wind speed, pressure, and temperature at the inlet of the stream tube are represented by V1, P1, and T1, respectively. Their counterparts at the outlet are V2, P2, and T<sup>2</sup> and at the rotor are Vave, Pave, and Tave. Here a constant specific humidity ratio is assumed in the stream tube for a short-period time (e.g., 10 minutes or 1 hour). The following sections explain the theory of WT thermodynamics in two aspects, energy analysis and exergy analysis, which both apply the meteorological variables such as wind speed, air density, atmospheric pressure, temperature, and humidity. The use of energy and exergy efficiencies considering a comprehensive set of meteorological variables can enable us to accurately evaluate the efficiency performance of WTs.

Thermodynamic Analysis of Wind Energy Systems DOI: http://dx.doi.org/10.5772/intechopen.85067

Figure 1. A schematic plot of WT stream tube for thermodynamic analysis.
