1. Introduction

Global installed wind power capacity has been tremendously increased over the last 15 years from 23,900 MW in 2001 to 486,790 MW in 2016 [1]. More than 314,000 WTs are now operating around the world, which accounts for more than 4.3% of 2015 global electricity demand. Yet it is still far from ambitious targets, e.g., increasing wind energy's contribution to 20% of US electricity supply by 2030 [2]. To approach that, it is of critical importance to accurately evaluate the WT performance considering realistic environmental conditions.

The most common factors that are considered when planning a wind farm include substantial wind resources, landowner and community support, feasible permitting, compatible land use, nearby access to electrical grid, appropriate site conditions for access during construction and operations, aviation compatibility, and favorable electricity market [3]. However, the influences of meteorological variables (e.g., pressure, temperature, and humidity) are often neglected which

could cause inaccurate evaluation of WT performance. For example, a dry air assumption (i.e., constant air density) does not really consider the moisture changeability. Baskut et al. discussed the effects of several meteorological variables including air density, pressure difference, humidity, and ambient temperature on exergy efficiency and suggested that neglecting these meteorological variables while planning wind farms could cause important errors in energy calculations [3].

The efficiency performance of a WT can be studied in two aspects, energy and exergy efficiencies. The former is calculated as the ratio of produced electricity to the total wind potential within the swept area of the rotor. Thus, only the kinetic energy of the air flow is considered in the energy efficiency calculation, while other meteorological variables such as pressure and temperature are often neglected. The latter considers the maximum useful work that can be obtained by a system interacting with an environment in thermodynamic equilibrium state [4]. The exergy efficiency along with availability and capacity factor of a small WT (rated power 1.5 kW) has been studied in Izmir, Turkey, to assess the WT system performance [5]. Sahin et al. developed an improved approach for the thermodynamic analysis of wind energy using energy and exergy, which provided a physical basis for understanding, refining, and predicting the wind energy variations [6]. According to [7], exergies are suggested as the most appropriate link between the second law of thermodynamics and the environmental impact, in part because it measures the deviation between the states of the system and the environment.

2.1 Energy analysis

A schematic plot of WT stream tube for thermodynamic analysis.

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

Figure 1.

that is calculated as

results in

5

rotor power can be calculated as

can be further expressed as

The energy analysis of WT systems stems from the air flow's kinetic energy Ek

where m and V are the mass and speed of the air flow, respectively. The mass m

where ρ is the air density, A is the rotor swept area perpendicular to the flow, and t is the time that the flow passing through the swept area with speed V. By applying the simple momentum theory, the rate of momentum change is equal to

where V<sup>1</sup> and V<sup>2</sup> are the wind speeds at the inlet and outlet, respectively, of the stream tube (Figure 1). The rate of momentum change is also equal to the resulting

where Vave is the average flow speed at rotor. On the other hand, the rate of

Based on the conservation of energy, Eqs. (4) and (5) should be equal which

Hence, the retardation of the wind before the rotor ð Þ V<sup>1</sup> � Vave is equal to the retardation of the wind after the rotor ð Þ Vave � V<sup>2</sup> . By Eqs. (2), (4), and (6), the

<sup>1</sup> � <sup>V</sup><sup>2</sup> 2

the overall change of velocity times the mass flow rate m\_ , i.e.,

thrust force. Thus, the power absorbed by the WT is calculated as

<sup>E</sup>\_ <sup>k</sup> <sup>¼</sup> <sup>1</sup> 2 m V \_ <sup>2</sup>

Vave <sup>¼</sup> <sup>1</sup> 2

kinetic energy change of the flow can be calculated as

mV<sup>2</sup> (1)

m ¼ ρAVt (2)

<sup>M</sup>\_ <sup>¼</sup> m V \_ ð Þ <sup>1</sup> � <sup>V</sup><sup>2</sup> (3)

P ¼ m V \_ ð Þ <sup>1</sup> � V<sup>2</sup> Vave (4)

(5)

ð Þ V<sup>1</sup> þ V<sup>2</sup> (6)

Ek <sup>¼</sup> <sup>1</sup> 2

This brief précis thus illustrates the importance of energy and exergy analyses for wind energy systems considering meteorological variables and provides a motivation for the thermodynamic analysis conducted herein. The chapter presents the methods and results of thermodynamic analysis of a 1.5 MW WT, which is assumed to be deployed in the northeastern United States, experiencing meteorological reanalysis data retrieved from the NASA's MERRA-2 data set. Matlab scripts are developed to calculate the energy and exergy efficiencies using the MERRA-2 data set. Section 2 provides the fundamental theory of thermodynamic analysis, particularly in derivations of energy and exergy efficiencies. The studied site, meteorological data, and the selected WT are explained in Section 3, which is followed by results and discussion in Section 4. Concluding remarks are provided in Section 5.
