Development Trends of Air Flow Velocity Measurement Methods and Devices in Renewable Energy

Abstract

This article presents an overview of airflow velocity measurement methods applied to renewable energy. Basic measurement methods used in this field are discussed: tachometric, ultrasonic, and calorimetric anemometry. The principle of operation and basic properties of anemometers are presented, and based on publications from the last decade, a systematic review of development directions and trends in this field is made. The aim of the article is to familiarize people dealing with renewable energy problems, in particular wind energy, with the current state of knowledge in the field of anemometric measurements, properties of individual types of measuring devices, as well as directions of development of measurement tools. This will allow for the optimization of processes in the field of wind energy, in particular in the selection of the location of the energy facility, implementation of investments and control, diagnostics, and monitoring during operation. The selection of metrological tools adequate to the problem also allows for ensuring an appropriate level of work safety and environmental and ecological harmony and supporting the process of sustainable development.

1. Introduction

Moving away from greenhouse gas-generating energy based on the combustion of fossil fuels such as coal [1,2], oil, and natural gas [3,4] is one of the key challenges facing our generation. The only alternative is the production of useful energy based on the use of nuclear reactions. This applies not only to nuclear power plants [5,6,7,8] but, above all, to the development of renewable energy, which is based on the thermonuclear reaction that constantly occurs on our nearest star—the Sun [9]. One of the forms of energy used in renewable energy, the primary source of which is the Sun, is the kinetic energy of wind [10]. Nobel Prize winner, Bob Dylan, sang in 1962: “The answer is blowing in the wind”. Wind energy currently provides about 8% of the total global demand for useful energy [11]. We live on Earth constantly immersed in fluid, although this fact does not particularly occupy our attention on a daily basis. This fluid is, of course, the atmosphere, and its movement is the wind. The mass of the atmosphere is over 5 × 1018 kg, which is only about one-millionth of the mass of the Earth [12]. However, the Earth’s possession of an atmosphere makes our planet unique, at least in the immediate astronomical vicinity. The atmosphere is a mixture of gases and aerosols, i.e., air. Air is essential for the existence of life on Earth, but it also performs a huge number of useful functions in our lives. One class of such functions is related to energy, and what is especially important today is renewable energy.

Air masses, like any physical body, moving at a certain velocity, have kinetic energy. This energy has long been used, for example, to power sailing ships or windmills [13,14]. The kinetic energy of the wind currently plays one of the key roles in renewable energy. Wind turbines [15,16,17], which convert the kinetic energy of the wind into electrical energy, can be observed in practically all parts of our globe [18,19]. In addition to the directed movement of air masses in the atmosphere that drives wind turbines [20,21], turbulent movement also occurs. It can be defined as a disordered movement, where the physical quantities describing it are random in both time and space. A feature of turbulent air movement is the occurrence of eddies on a wide range of time and space scales. Kinetic energy is also associated with turbulent movement. However, this form of energy is mostly irreversibly dissipated. Attempts are being made to convert it into useful energy, but this is a difficult and complex issue [22,23].

In renewable energy, there is a need to store energy. This results from the fact that many sources produce energy periodically, only when the original form of energy, solar radiation or wind, is available. The air that is omnipresent on Earth is used to store potential energy in the form of compressed gas, which can then be used in pneumatic systems [24,25]. Compressed air is one of the forms of energy storage in renewable energy and making it available as required [26,27,28,29,30,31].

Air also plays a key role in many energy processes because it is a source of oxygen, which is necessary in combustion processes. This also applies to renewable energy when the burned raw material can be relatively quickly regenerated in a natural process and does not increase the amount of carbon dioxide in circulation. This type of renewable energy can be associated with the use of biogas [32,33], biomass [34,35], and biofuels [36,37].

The air component, oxygen, is also necessary in all energy processes related to the functioning of living beings. External respiration is a set of physiological processes of exchanging respiratory gases with the environment. It is the basis of cellular respiration, which is a multi-stage, biochemical process of oxidation of organic compounds aimed at obtaining metabolically useful energy [38,39]. Respiration is associated with the production of carbon dioxide, but thanks to photosynthesis in plants, oxygen returns to the atmosphere. In the case of equilibrium, energy transformations in the biological cycle can be classified as renewable.

The processes of converting energy associated with air into useful energy require support from technologies for measuring air movement parameters [40,41]. In all the described processes, we are dealing with the use of air as an energy carrier. In order to monitor, control, and optimize these processes, it is necessary to obtain information on the values of parameters related to the state and course of the energy phenomena taking place. One of the key parameters here is the airflow velocity. Depending on the phenomenon being studied, we may be dealing with the problem of measuring velocity as a scalar or vector value. It may be required to measure the instantaneous or average velocity, with averaging in time or space. Another problem may be the need to determine the flow velocity field in a given measurement area.

Measuring devices designed to measure the flow velocity of fluids, especially air, are called anemometers. This common term covers a whole range of various tools, devices, and measuring systems using different principles of operation based on many physical phenomena. A historical review of the development of anemometric methods, techniques, and tools can be found in the article [42]. Our article is a specialist review of the development and improvement of anemometric methods and devices in application to renewable energy. Its aim is to indicate, based on the latest selected literature, the most important development trends in this field and to support the process of forecasting potential further research directions. The article is to enable people involved in the development of renewable energy to familiarize themselves with innovative anemometric methods, new technologies and original application solutions used in this field of measurements. Such knowledge will contribute to the optimization of the measurement process and improvement of the efficiency of renewable energy facilities. So far, there is no review of the development of anemometric measurement methods in this context in the literature.

The second part of the article presents methods of measuring airflow velocity and indicates methods commonly used in renewable energy. The third part discusses current directions of development, modification, and applications of tachometric anemometers. The fourth part is devoted to new technologies and measurement methods used in ultrasonic anemometers. Part five discusses the development of methods and technological progress in calorimetric thermal anemometry. Part six contains information on innovative solutions for other types of anemometers for renewable energy. Conclusions resulting from the review are included in the last part of the article.

2. Airflow Velocity Measurements in Renewable Energy

The physical quantity, which is the velocity of any object, is defined as the derivative of the object’s position vector with respect to time. In many problems, the velocity of an object can be determined directly from the definition. This results from the fact that both the determination of the object’s position and the measurement of time are possible with high accuracy. In fluid mechanics, however, this issue has a different nature because gases and liquids are generally treated as continuous media. There is, therefore, a fundamental problem with distinguishing the fluid element whose velocity is subject to measurement. Actions aimed at solving this problem have led to the creation of two basic groups of flow velocity measurement methods: indirect methods and marker methods. Indirect methods use various physical phenomena, the course of which is determined by physical parameters, among which the flow velocity occurs. Based on the model of the phenomenon and the measurement of its course parameters, the measured velocity is determined. Physical phenomena such as pressure effects, momentum transfer, ultrasound propagation, heat transfer, and Karman vortices are used here.

Pressure methods involve determining the flow velocity indirectly by measuring the effect of pressure change on a measuring element placed in the flow. Standard measuring elements used in this method are orifices, nozzles, capillaries, Venturi nozzles, Pitot tubes, and Prandtl tubes [43]. Due to the basic nature of the physical phenomena occurring here, these methods are often used as reference methods.

Anemometers operating on the basis of momentum transfer typically use a measuring element in the form of a rotating turbine, the rotational speed of which is a function of the flow velocity [44]. The cup anemometer, similar to those used today, was invented in 1845 by the Reverend Dr. John Thomas Romney Robinson of Armagh Observatory. This anemometer consisted of a horizontal rotor with four semicircular cups. The rotor, driven by the wind, turned on a vertical axis at a speed proportional to the wind velocity. By counting the revolutions in a given time, the velocity of the flow was determined. The advantage of this method is the approximately linear relationship between both quantities. In wind velocity measurements, anemometers with a horizontal or vertical turbine rotational axis are used. Another variant of anemometers with momentum transfer is devices in which the measuring element is a stationary aerodynamic obstacle placed in the flow. The velocity is measured by measuring the force with which the flow affects the obstacle [45].

The propagation of sound waves in the flow is used in ultrasonic anemometers. Sonic anemometers were most likely developed in the early 1950s or late 1940s. This can be assumed based on available publications from that period. Many internet sources state that the sonic anemometer was invented by geologist Dr. Andreas Pflitsch in 1994, but this is hardly credible information. These devices have a set of one or more ultrasonic transmitters and receivers. The flow velocity is determined by measuring the propagation time of the wave between the source and the detector [46]. With the appropriate spatial arrangement of ultrasonic transmitters and receivers, it is possible to measure the velocity vector in space.

Another group of anemometers is calorimetric anemometers, which use the phenomenon of heat transfer between the flow and the measuring element of the anemometer. This element typically is a conductor or semiconductor heated by electric current. The heat flux transferred to the flow is a measure of the velocity. The heat flux is measured by measuring the power of the heating and the temperature of the measuring element. These anemometers require calibration in a reference flow [47]. Fundamental to the creation of the foundations of calorimetric anemometry is the work of L. V. King from 1914 [48]. The author presented the concept and theoretical foundations of this measurement method. He also described a measuring device built on the basis of a heated thin wire connected to a passive resistance bridge.

The formation of Karman vortices behind an obstacle placed in the flow can also be used for anemometric measurements. The frequency of these vortices is a function of the measured flow velocity [49].

Marker methods, on the other hand, consist of measuring the velocity of a naturally existing marker or one introduced into the medium, assuming that the marker propagates at a velocity close to the fluid velocity. Markers are objects whose movement with the fluid can be observed and measured. The most commonly used are optical, mechanical, ionization, radiation, chemical, and thermal markers. In this group of methods, the most popular are laser methods with optical markers. LDA (Laser Doppler Anemometry) [50] is a method of measuring flow velocity by measuring the frequency of flashes caused by markers moving through the area of laser light interference fringes. PIV (Particle Image Velocimetry) [51] consists of illuminating a selected cross-section of the flow with laser light and taking photographs of the marker’s distribution at successive moments of time. Based on the correlation of individual photographs, we obtain the vectors of the movement of marker particles at a known time. This allows us to determine the flow velocity field in the tested area. Both of these methods, as well as other less frequently used marker methods, are utilized mainly in specialized laboratory and calibration measurements and will not be discussed in this article.

Among the discussed methods of measuring the flow rate of fluids, three typically used for measurements in renewable energy were selected for further analysis of progress and development of methods. These are tachometric, ultrasonic, and calorimetric anemometry. Anemometry based on pressure measurement is also used in this field, but since it is a classical method based on the basic equations of flow physics, in this case, no rapid development is noted. Based on our team’s many years of experience in the development of measurement methods, construction of anemometric devices, and implementation of laboratory, technical, and industrial measurements, selected typical features of the measurement devices discussed in the article are presented in

Table 1. Selected typical metrological properties of tachometric, ultrasonic, and calorimetric anemometers.

	Tachometric Anemometers	Ultrasonic Anemometers	Calorimetric Anemometers
typical applications	one-dimensional or two-dimensional wind velocity measurements, open flow measurements, ventilation measurements	three-dimensional wind speed measurements, industrial measurements in pipelines, measurements of gas flow with variable composition	one-dimensional or omnidirectional air velocity measurements, velocity profile measurements, laboratory measurements, ventilation measurements
lower range limitations of velocity measurement	friction force in rotor bearings, dimensions, and inertia of the rotor	flow disturbance through the measuring sensor	natural convection caused by the sensor temperature
upper range limitations of velocity measurement	durability of the sensor structure	acoustic interference caused by the interaction of the sensor with the flow	sensor durability and sensitivity decrease with increasing velocity
typical disadvantages of anemometers	measurement averaging, velocity field disturbance, overestimation of readings in fluctuating flows, moving mechanical elements	measurement averaging, large sensor size, complex anemometer construction, high price	sensitivity of the measurement to the composition and temperature of the medium, need for calibration, delicate structure of the sensor
typical advantages of anemometers	simple construction, linearity of readings, low sensitivity to temperature and medium composition, moderate price	absolute measurement, low sensitivity to temperature and medium composition, low measurement uncertainty, no moving parts	ability to measure fast-changing velocity fluctuations, low invasiveness of measurement, close-to-point measurement, no moving elements
main directions of development	new construction materials, 3D printing, magnetic rotor bearing, fiber optic signal reading, autonomous power supply, and wireless signal transmission	new sensor design technologies, application of AI methods in signal processing, positioning of sensors in the measurement space using drones, reduction in costs, and widespread use	sensors in microelectromechanical and nanoelectromechanical technology, multi-sensor arrays, sensors for extremely low or extremely high temperatures, intelligent sensors, sensor networks

. This table can be a simplified guide to the selection of anemometric methods in renewable energy.

Table 2. Parameters of selected commercial tachometric, ultrasonic, and calorimetric anemometers.

	Anemometer Type	Type of Measurement	Velocity Measurement Range	Velocity Measurement Uncertainty	Flow Direction Measurement	Temperature Range	Estimated Price
tachometric anemometer	05108 RM Young Company Traverse City, MI, USA	2D velocity vector measurement	1 to 100 m/s	±0.3 m/s or ±1%	azimuth 0 to 355°	−50° to +60 °C	1500 $
ultrasonic anemometer	Aeolus 3 Senseca Italy Srl Selvazzano Dentro, Italy	3D velocity vector measurement	0.01 to 85 m/s	±0.2 m/s or ±2%	azimuth 0 to 360° elevation −60° to 60°	−40 to +60 °C	5000 $
calorimetric anemometer	Fluke 923 Fluke Corporation Everett, WA, USA	1D velocity measurement	0.2 to 20 m/s	5%	no direction measurement	−20 to +60 °C	500 $

presents sample parameters of three selected commercial anemometers operating based on the above methods. Based on the data in this table, it is possible to compare the parameters of the devices presented here based on different physical principles of operation. Of course, the number of commercially available measuring devices in this field is huge, and Table 2 is only an illustration.

The following chapters will discuss the development trends of selected types of anemometers used in air flow measurements in the field of renewable energy.

3. Current Directions of Development, Modification, and Application of Anemometers with a Rotating Measuring Element

Anemometers with a rotating measuring element, also called tachometric anemometers, are an important class of devices designed to measure airflow velocity in renewable energy. In this group of devices, there are two main types of device construction. The first type is vane and propeller anemometers, in which the measuring element rotates in a plane perpendicular to the measured velocity. The second type is cup anemometers, in which the measuring element rotates in a plane parallel to the measured flow velocity. Figure 1 shows a schematic construction of typical anemometers with a rotating measuring element.

Anemometers with a rotating measuring element operate based on the phenomenon of momentum exchange between the tested flow and the rotor. These anemometers are widely used in flow metrology, but their main application area is energy, meteorological and ventilation measurements. They have many advantageous metrological properties. These are approximately linear characteristics, a wide range of measured velocities, properties that average the measurement result, and limited sensitivity to changes in the physical parameters of the medium. However, measurements in time-varying flows and with a non-uniform velocity field may be burdened with a significant error. This is due to the relatively large dimensions and large mechanical inertia of the rotating measuring element. In such cases, a complex interpretation of the measurement result is required.

A simplified dynamic mathematical model for anemometers with a rotating measuring element can be written in the form of equations [52]:

$$a{\displaystyle \frac{d\omega }{dt}}=-{\omega }^2+{\displaystyle \frac{v^2}{b^2}}$$

(1)

$$V=b\omega ,$$

(2)

where:

v—actual flow velocity,

ω—angular velocity of the rotating measuring element,

V—measure of flow velocity,

t—time,

a, b—model parameters.

Equation (1) is the equation of state of the rotor in the flow recorded based on the energy balance, while Equation (2) is the anemometer output equation [52]. Parameter a depends on the rotor and medium parameters and concerns the dynamic parameters of the anemometer, while parameter b is a function of the rotor parameters and concerns the static parameters. These parameters can be calculated analytically but are more often obtained in the process of anemometer calibration. The issue of calibration and control of anemometer parameters is the subject of continuous development and improvement.

The collective results of the long-term research campaign on cup anemometers are presented in [53]. Such anemometers, dedicated to measuring air flow velocity in wind energy, require continuous control and standardization. As part of the described studies, a large series of calibrations and analyses of anemometer parameters were carried out, examining the effect of various factors on the measurement results. The variability of rotor parameters, climatic conditions, and the aging process of anemometer components were taken into account. The result of this work is the development of a procedure for detecting operational disturbances and failures of anemometers in the measurement process.

The disadvantage of mechanical anemometers with a rotating measuring element is poor dynamic properties. The time it takes for the rotor to settle after changing the flow velocity is significant, and this time is shorter when accelerating the rotor and longer when decelerating. In these devices, in studies of turbulent flows, there is a phenomenon of overestimating the average velocity measurement related to the rotor inertia. Optimization of the dynamics of the measurement process and estimation and minimization of measurement uncertainty are possible based on the mathematical model of the anemometer (1). In [54], a concept of a method for optimizing the dynamic properties of an anemometer with a rotating measuring element was proposed, and the results of model tests were presented. This method consists of equipping the device with additional electronic rotor acceleration and braking systems. These systems are controlled by a derivative of the instantaneous value of the angular speed. The conducted studies show that the use of the optimization system allows for an approximately tenfold reduction in the error in measuring the average velocity in fluctuating flows.

In the article [55], the authors proposed an innovative design of a cup anemometer, which was entirely manufactured using 3D printing technology. The anemometer was manufactured using additive manufacturing of a hexagonal mechanical structure made of ABS material. The prototype of the anemometer was tested in a wind tunnel in the velocity range of up to 12 m/s. This technology does not allow for high precision of measurements, but it can be used to produce a small batch of anemometers with a design and dimensions selected for a specific, individual measurement problem.

An interesting and original solution for the design of a vane anemometer is presented in the article [56]. A conventional vane anemometer is distinguished by the fact that it has a maintenance-free, autonomous power supply. Such solutions, combined with wireless transmission of measurement data, enable the deployment of such devices in hard-to-reach places and monitoring of the airflow velocity field, for example, in wind farms. A triboelectric nanogenerator driven by the rotational energy of the measuring rotor was used to power the anemometer. The nanogenerator is made of polymer cylinders rolling inside a ring containing copper electrodes. The triboelectric phenomenon is used here, consisting of inducing charges during contact with appropriately selected pairs of materials. The charges accumulated on the electrodes are collected and converted into electric current powering the device. The authors state that the power supply is activated at an air velocity below 2 m/s. Thanks to the integration of the velocity sensor and the power supply system, the device is a complete, autonomous, intelligent, and wireless element of the measurement system for monitoring the environment, for example, in wind energy systems.

A conventional vane anemometer with an original system for measuring rotor speed and transmitting data is presented in [57]. The measuring element of the anemometer is a rotor placed in the flow, and its vanes reflect light transmitted to them by optical fiber. The reflected signal returns to the measuring device using the same optical fiber. The advantage of the device is that the elements placed in the measuring area do not contain power supply elements or electrical circuits. The measuring signal can be transmitted over long distances. The construction material of the measuring element can also be adapted to the working environment. The system was developed especially for measuring wind velocity in difficult environmental conditions, such as high electromagnetic interference. It can also be used in facilities requiring intrinsic safety. This solution for measuring wind velocity and transmitting data is particularly interesting in terms of the possibilities of application in power plants.

The article [58] presents an original concept and pilot studies of an innovative method and algorithm for correcting the dynamic error of anemometers with a rotating measuring element. The method consists of precise measurement of the rotor rotational speed and correction of the measured air velocity, taking into account the dynamics of the device in accordance with the developed algorithm. The correction algorithm is based on the dynamic model of the anemometer and allows for extending the measurement frequency bandwidth and reducing the dynamic error. The algorithm requires measurement of the rotor angular speed with high accuracy and high time resolution and calculation of the derivative of the angular velocity. The studies achieved a significant reduction in the dynamic error and extension of the measurement frequency bandwidth of the vane anemometer by about 40 times for the velocity of 2.5 m/s and 10 times for the velocity of 12.5 m/s.

The problem of ice layer deposition and contamination in cup anemometers was analyzed in the article [59]. The research team conducted theoretical and experimental studies of the cup anemometer under conditions of additional mass loading, simulating icing, or contamination. The results of the analyses led to the conclusion that the anemometer readings in conditions of 50% coverage of the cup with sediments may be underestimated by 10%. Therefore, the possibility of ice or contamination deposition on the anemometer rotor should be taken into account both in the anemometer design and construction process and during measurements.

A vane anemometer designed to be placed on a flying drone—an unmanned aerial vehicle—is presented in the article [60]. In the design of the vane anemometer, the authors were inspired by the profile of an owl’s wings, while a triboelectric generator was used to process the rotor’s rotation. The generator consists of rotating kapton strips sliding on a circumferential copper commutator set. The electric signal from the commutators allows for measuring the rotor’s rotational speed and for powering an electronic system with low power consumption. In order to measure the spatial velocity field around tower energy facilities, the authors propose placing four such anemometers on a flying drone. The anemometer’s measurement range is 1.6–10.7 m/s, with a measurement resolution of 0.057 m/s.

In the article [61], the research team described the use of a cup anemometer to measure wind velocity in a stratospheric balloon mission. The results of measurements taken during such missions are of significant importance for predicting the state and key parameters of the atmosphere, as well as in application to renewable energy. The aim of the research was to check the possibility of using cup anemometers to measure relative wind velocity in such missions. For the purposes of the measurement experiment, the geometry of the anemometer rotor was optimized. The wind velocity was measured as the balloon was rising to a height of 18 km, with the readings corrected for changes in air density. The research confirmed the possibility of using this type of anemometer in conditions of changing atmospheric density.

4. New Technologies and Measurement Methods in Ultrasonic Anemometers

Ultrasonic anemometers use the dependence of ultrasonic wave propagation on airflow velocity. In the simplest case, such an anemometer has an ultrasonic wave transmitter and receiver placed in the flow at a known distance from each other. Knowing the velocity of wave propagation in a stationary medium and measuring the signal flight time, the flow velocity can be theoretically determined. However, in practice, such a simplified method is not used because the wave velocity in a stationary medium must be known. Therefore, combined transmitter and receiver transducers are used in anemometers. By placing two such transducers in the flow at a known distance and measuring the signal flight time in both directions, the flow velocity component in the direction of the axis on which the transducers are placed can be determined from the relationship [47]:

$$V={\displaystyle \frac{1}{2}}\left({\displaystyle \frac{d}{t_1}}-{\displaystyle \frac{d}{t_2}}\right)$$

(3)

where:

V—measure of flow velocity,

d—transducer distance,

t₁, t₂—flight times of signals in both directions.

Additionally, based on Equation (4):

$$c={\displaystyle \frac{1}{2}}\left({\displaystyle \frac{d}{t_1}}+{\displaystyle \frac{d}{t_2}}\right)=\sqrt{{\displaystyle \frac{\gamma kT}{m}}}$$

(4)

where:

c—velocity of sound in the medium,

γ—adiabatic index,

k—Boltzmann’s constant,

T—temperature of medium,

m—molecular mass,

We can determine the temperature of the medium by knowing the velocity of sound or vice versa [47]. The schematic construction of ultrasonic anemometers is shown in Figure 2.

By using appropriate systems of multiple transducers, we can determine the components of the two-dimensional or three-dimensional flow velocity vector based on the measurement of the time of flight of the wave between the transducers. The presented measurement model (3) is simplified. In a real anemometer, many additional factors must be taken into account, such as the problem of accurate measurement of the time of flight of the signal, the flight of the sound wave through the transducer housings, the inhomogeneity of the medium, the aerodynamic shadows of the transducers, and deposited impurities or noise interfering with the measurement. Although the measurement method has the characteristics of an absolute method, ultrasonic anemometers can be subject to calibration. Due to the low measurement uncertainty, wide measurement range, small influence of the medium parameters, and the lack of moving elements, ultrasonic anemometers are ideal for measurements in renewable energy. However, they are generally much more expensive than anemometers with a rotating measuring element.

Measurements of velocity fields around large-sized objects can be made by placing many sensors at selected measurement points of the object. Another measurement method was presented in the article [62]. The authors placed an ultrasonic anemometer sensor on a flying drone—a six-rotor unmanned aerial vehicle. In this way, it is possible to study wind velocity fields, for example, around wind turbines, power plant chimneys, or mining towers. The authors tested such a measurement system in a wind tunnel. The influence of the rotors and the position of the drone’s hull on the measurement error were studied. Appropriate procedures and correction factors were developed. Experimental measurements of the velocity field around a real object were also performed, and the results were largely consistent with other measurement methods. The error did not exceed 5%. Similar concepts and research results are presented in articles [63,64,65]. This measurement technology seems to be promising in measuring velocity fields around high-rising energy objects.

Airflows in rooms and building installations not only provide air exchange for breathing but also transport heat energy and pollutants. Therefore, it is important to measure air flows and flow velocity fields in locations that are key to the ventilation process. The article [66] describes an ultrasonic anemometer designed for monitoring the environment and energy consumption in buildings. The anemometer uses microelectromechanical MEMS sensors as transmitters and detectors of the measurement signal. The sensors are placed in space in the arrangement of the corners of a regular tetrahedron. The anemometer has a resolution and initial threshold of 0.01 m/s. An absolute air velocity error of 0.05 m/s at a given orientation was obtained. The studies showed angle errors of 3.1° and a velocity of 0.11 m/s at an azimuthal rotation of 360° and angle errors of 3.5° and a velocity of 0.07 m/s at 135° vertical declination. Additionally, the device allows for measuring air temperature using the phenomenon of change in the velocity of sound wave propagation (4). The device is equipped with a magnetic and gyroscopic sensor, which allows for determining the three-dimensional velocity vector in the global reference system, regardless of the anemometer’s position.

In the article [67], the authors described an anemometer built from easily accessible and cheap electronic modules of sound transducers and microcontrollers. The anemometer housing, which is the structure of the spatial arrangement of transducers, was made using 3D printing technology. The Arduino prototype platform was used to implement the anemometer, which allows for easy and quick modification of both the hardware and software parts. The developed anemometer allows for measuring wind velocity and direction, and the measurement data are transferred to the cloud on the Internet. Such an anemometer can, in some cases, be an alternative to commercial devices, in particular in complex metrological problems requiring a non-standard arrangement of measuring transducers.

Transducers used in ultrasonic anemometers are relatively large, which means they introduce an aerodynamic shadow in the tested flow. This causes flow disturbance and introduces an additional source of measurement error. This phenomenon was subjected to research described in [68,69]. Simulation studies were carried out using computer modeling techniques. The studies showed that, depending on the wind velocity and direction, aerodynamic shadows have a different effect on measurement accuracy. The computer simulations were verified using experiments in a wind tunnel. It was confirmed that numerical analyses provide accurate and reliable predictions of the impact of the aerodynamic shadow on the measurement. The results of the work led to the proposal of methods for compensating for this unfavorable phenomenon using machine learning and optimization methods.

Measurements using an ultrasonic anemometer can be disturbed in an environment where strong sonic impulse noise occurs. The article [70] describes the concept of an ultrasonic anemometer for continuous measurement of the two-dimensional wind velocity vector, in which an array of ultrasonic transducers is used. The measurement system consists of a single continuous ultrasonic wave transmitter and a set of receivers arranged in an arc shape. The continuously received signals from many receivers are filtered, and then the flow velocity value and its direction are determined using an algorithm developed by the authors. In the conducted tests, the maximum velocity measurement error was obtained at the level of 1.2%, and the wind direction was determined with an error not exceeding 2°. The simulation tests carried out showed that this method effectively eliminates impulse noise and minimizes measurement uncertainty. This type of anemometer can be used in measurements in power plants where a significant level of sonic interference occurs.

The article [71] presents the results of studies on the effect of the turbulence level in the flow on the readings of an ultrasonic anemometer. The anemometer was calibrated in the velocity range up to 20 m/s with a low turbulence level below 1%, and then the flow was disturbed, obtaining turbulence of 6.1%. It was found that at increased turbulence levels, the ultrasonic anemometer overestimated the readings by 2 to 8%. It was also found that anemometers from different manufacturers react to increased turbulence levels to a different extent. This phenomenon should be taken into account when using this measurement method.

5. Development of Methods and Technological Advances in Calorimetric Thermal Anemometry

Calorimetric thermal anemometry is an indirect method of measuring air flow velocity, which is also often utilized in measurements of renewable energy. Thermal anemometry uses the dependence of the heat flux transferred from the heated measuring element to the flow on the flow velocity. The measuring element is typically thin tungsten or platinum wire or a miniature thermistor heated by electric current. The construction of typical thermal anemometer sensors is shown schematically in Figure 3.

Other elements and heating methods are also encountered. The advantage of this method is that it is a miniature measuring sensor that allows for almost point measurement and disturbs the flow to a small extent. The method allows for measurements of fast-changing velocity fluctuations, which is why it is often used to study turbulent flows. Other advantages include a wide range of measured velocities and the lack of moving sensor elements. Complex measurement systems allow for the measurement of the velocity vector. Orthogonal spatial systems of thermal transducers with anisotropic properties for heat exchange, such as thin platinum wires, are used here. Appropriate thermal transducer systems also allow for the measurement of turbulent stresses in the flow. However, this method is sensitive to changes in the medium parameters and always requires calibration. A simplified dynamic model for measurements using the calorimetric thermal anemometry method can be written in the form of a heat flux balance [48]:

$$P=\left(A+B\sqrt{v}\right)\left(T-T_a\right)+C{\displaystyle \frac{dT}{dt}},$$

(5)

where:

P—power heating the measuring element,

v—actual flow rate,

T—temperature of the heated measuring element,

T_a—medium temperature,

t—time,

A, B, C—model parameters.

The left side of Equation (5) is the heat flux delivered to the measuring element. Most often, it is Joule–Lenz heat generated by electric current. The first component of the right side of Equation (5) is the heat flux transferred to the fluid flow by convection, and the second component is the heat flux stored in the measuring element. The model parameters are determined by calibration in a flow with set parameters in a wind tunnel. Knowing the model parameters, heating power, sensor temperature, and medium temperature, we can determine the flow velocity V in the steady state from Equation (5):

$$V={\left({\displaystyle \frac{1}{B}}\left({\displaystyle \frac{P}{\left(T-T_a\right)}}-A\right)\right)}^2$$

(6)

In real calorimetric anemometer designs, electronic power supply systems are used to maintain selected parameters of the system at a constant level [72]. The most commonly used systems are constant temperature, constant current, and constant voltage. This allows for simplifying the measurement process and obtaining the required metrological parameters.

In measurements using calorimetric thermal anemometers, a significant factor disturbing the measurement is the change in the temperature of the medium. This problem is particularly important in conditions outside the laboratory, which we typically encounter in measurements of renewable energy. Therefore, a number of methods for temperature compensation and correction of thermal anemometers have been developed. A review of these methods, both standard and original studies, is included in the article [73]. These methods use an additional temperature sensor or a specialized signal-processing algorithm. This article is intended for both designers of measuring equipment and people performing measurements.

In many facilities, a critical problem is to ensure an appropriate thermal and respiratory environment evenly throughout the entire cubic capacity of the room. Hence, it is necessary to measure air velocity, which, apart from supplying oxygen, is also a carrier of heat energy, humidity, and biological contaminants. The publication [74] describes an omnidirectional, simple thermal anemometer designed to measure air velocity in the range of 0 to 6 m/s. The sensor used is a commercial thermistor heated by electric current to a temperature higher than the ambient temperature. In the measurements for the proposed system, the uncertainty obtained ranged from 0.11 m/s in the lower measurement range to 0.71 m/s for maximum velocities. The authors propose to place many such sensors in the facility, which will enable multi-point monitoring of spatial and temporal variability of air flows.

A thermal anemometer designed for multi-point measurements of wind velocity and direction in outdoor areas is described in the article [75]. The design of a single sensor, which is a resistive temperature transducer, is based on MEMS (Micro Electro Mechanical System) technology. The measuring element is a polycrystalline silicon beam with dimensions of 0.5 × 2 × 50 μm produced by the vapor deposition method. This technology enables the production of a series of many miniature sensors with repeatable parameters. Sensors placed in the tested airflow are powered by electric current, which heats them. An electronic system with feedback maintains the temperature of the sensors at a given level. Thanks to this, the sensor current is a function of the flow velocity. In the described anemometer, the authors used a set of such sensors, the output signals of which are processed in an artificial neural network (ANN). The network is trained in the calibration process in such a way that the output signal from the anemometer is the wind velocity and direction, and the measurement is temperature compensated. Thanks to their miniature size, low energy consumption, and repeatable technology, the sensors have potential applications in spatial measurements of velocity fields, for example, in wind farms.

An innovative solution for an anemometer without moving parts designed to measure wind velocity in power plants is presented in the article [76]. The anemometer called a thermoelectric anemometer uses a commercial thermoelectric module—a Peltier cell—as a measuring element. One of the cell surfaces is placed on a radiator, and the other is cooled convectively by the wind flow. A set of two cells arranged perpendicularly to each other is used here. Based on the measurement of the thermoelectric voltage generated by the Peltier cell, the wind velocity is determined. The authors tested this device in the velocity range of up to 10 m/s, obtaining high compliance of the results with an ultrasonic anemometer.

The team of authors devoted the works [77,78,79,80,81,82] to various issues related to the use of calorimetric anemometers in the measurement of turbulent flows. The authors proposed innovative methods aimed at extending the measurement capabilities of the devices in the range of high flow fluctuation frequencies. The methods are related to the introduction of adaptive control to the devices, the use of complex measurement algorithms, and the digital processing of measurement signals. An innovative method for testing the dynamic properties of thermal anemometers is also presented. This work has significant implications for measurements in renewable energy, where the analysis of the wind energy spectrum is an important issue.

In the articles [83,84,85,86,87], an innovative concept of a thermal anemometer, different from the standard ones, was presented. In classic calorimetric anemometers, the measuring element is heated by an electric current flowing through a resistive sensor. Joule–Lenz heat is released in the sensor, which causes the temperature of the measuring element to increase. The new type of calorimetric anemometer uses an optical fiber, which is both a measuring element and is used to transmit measurement signals. This measuring system consists of an electronic device placed in the measuring station and an optical fiber connected to the device. The optical fiber is stretched from the measuring station to the tested flow, in which the end of the optical fiber is placed. The measuring element in this anemometer is a specially prepared end of the optical fiber. It is covered on the outside with a thin layer of silver. A strong laser beam is sent from the device through the optical fiber toward its end. The light radiation is absorbed by the silver layer and heats the measuring element. The tested flow absorbs heat from the measuring tip, and the amount of this cooling depends on the flow velocity. The heat flux balance allows the flow velocity to be determined, similarly to classic calorimetric anemometers. Performing this balance requires knowledge of the changes in the temperature of the measuring tip of the optical fiber. For this purpose, a Bragg grating, also called a Bragg filter, is placed in the tip of the optical fiber. It is a segment of the optical fiber several millimeters long, which reflects light of a specific length. The anemometer utilizes the fact that the reflected wavelength is a function of temperature. Therefore, the change in the temperature of the measuring element, dependent on the flow velocity, changes the Bragg wavelength. The reflected light wave returns through the optical fiber to the device, and there, it is subject to appropriate detection in order to determine the temperature of the measuring tip of the optical fiber. Based on the model of the phenomenon, calibration in the wind tunnel, and the measured parameters, the tested flow velocity is determined in the measuring device. In the tests described in the article [83], the measurement range was up to 13.7 m/s. The change in the temperature of the fiber tip in the full measurement range was 92°. Despite the considerable complexity of the measuring equipment, this solution is interesting for many applications in the power industry. The main feature of the device is the fact that the measuring device and its power supply systems can be located at a considerable distance from the tested flow. Only the optical fiber is placed in the measurement area. This feature allows measurements to be taken, for example, in high-pressure flows, areas that are chemically aggressive, or areas that require intrinsic safety. The measuring device can handle many optical fibers and, therefore, conduct multi-point measurements. Due to the small diameter of the optical fiber, the invasiveness of such measurements is negligible. There are also no moving elements in the sensor, which reduces the potential failure rate of the anemometer. Due to the complexity of the equipment, this method does not aspire to widespread use, but it can be an alternative in special applications in which the unique properties of the method determine its selection.

6. Innovative Designs of Other Types of Anemometers for Renewable Energy

The anemometer proposed in the article [88] uses the force with which the wind acts on a vertical rod placed in the flow. This rod from below is mounted on a horizontal, square plate. Four force transducers mounted from the bottom in the corners of the plate are used to attach this plate to the base. Based on the measurements of four force values, the wind velocity and direction are determined. The authors provide the measurement range of the developed anemometer for velocities of 1 to 25.2 m/s with a resolution of 0.1 m/s and for an angle of 0 to 360° with a resolution of 1°. For both values, the measurement uncertainty is estimated at 3%. The article compares the parameters of this anemometer with other commercial devices and states that the device complies with the requirements for technical measurements. The anemometer can also be used in measurements for energy facilities.

The cantilever anemometer uses a flexible, rectangular plate placed in the flow and fixed on one side to the base. The effect of wind causes deformation of the plate, which is measured using a piezoresistor placed on the plate. This type of anemometer was subjected to tests described in the article [89]. The measurement range of such an anemometer is limited by the critical velocity at which the plate starts to vibrate. The authors theoretically and experimentally determined the conditions for the occurrence of such vibrations. The results of the tests were used to propose such an anemometer design that effectively suppresses parasitic vibrations. This allowed the measurement range of the device to be extended sevenfold.

The article [90] presents a pressure anemometer in the form of a foil placed on the leading edge of the airfoil. This interesting solution can be used in the monitoring and diagnostics of wind turbine blades, as well as in new forms of air transport, such as autonomous air vehicles. In this solution, the wind velocity is measured by an integrated, two-layer, capacitive pressure sensor with a polyvinylidene fluoride membrane. The characteristics of the anemometer were determined in the tests for air velocity from 4 to 16 m/s. Below 4 m/s, the sensitivity of the anemometer decreases rapidly.

The design of the inertial anemometer was proposed in the article [91]. The anemometer was designed to measure wind velocity and direction on wind farms. It is constructed of a spherical element containing a three-axis accelerometer and gyroscope integrated circuit. The spherical element is mounted in a pendulum-like manner and is deflected by the wind. Its deviation from the vertical is determined based on the readings of the accelerometer and gyroscope. The measurement algorithm uses artificial neural networks.

Natural ventilation in buildings is energy-efficient and allows for ensuring appropriate air quality in rooms. Optimal regulation of the ventilation system requires real-time measurement of air flows. The authors of the article [92] developed an anemometer using a pendulum placed in the tested flow. The deflection of the pendulum depends on the velocity and direction of the flow. In the proposed solution, the angle and direction of deflection are measured using a semiconductor integrated circuit containing a three-axis gyroscope. This system is placed at the lower end of the pendulum. The developed anemometer allowed for measuring small velocities up to 1.2 m/s, as well as determining the flow direction and analyzing its low-frequency fluctuations.

In the article [93], a mechanical–photoelectric anemometer was described. The design of the device uses the phenomenon of generating vibrations of a miniature, flexible beam placed in a rectangular cavity. These vibrations are caused by airflow, and the vibration frequency is a function of the flow velocity. Vibration detection is carried out using photoelectric cells. An integrated light-emitting diode and a photodetector were used here, with the light being reflected from an aluminum foil placed at the end of the vibrating beam. In the developed anemometer, a linear dependence of the vibration frequency on the flow velocity was obtained in the range from 2.4 to 27 m/s. This range allows the anemometer to be used in wind velocity measurements in renewable wind energy.

In the article [94], the authors proposed an interesting solution of an anemometer for measuring wind velocity and direction in areas intended for obtaining renewable energy. The anemometer uses a commercial, integrated, three-axis accelerometer and gyroscope. The wind velocity measurement is performed based on the measurement of the acceleration of a massive measuring element subjected to the wind force. Signal processing and velocity calculation are performed in a processor system using Kalman filtering [95]. The presented anemometer works optimally at wind velocities from 1.2 m/s to 10 m/s, with a measurement uncertainty of 5%. The angular resolution of the wind direction is 3°.

7. Conclusions

This article presents an overview of methods used in renewable energy to measure the flow velocity of gases, especially air. The main types of anemometric methods and the physical basis of their operation are discussed. Three types of devices were selected to analyze the development and progress in the field of anemometric measurements: tachometric, ultrasonic, and calorimetric anemometers. Based on precisely selected scientific publications from the last decade, an overview of innovations and development of measurement methods and anemometric tools used in renewable energy was made. This review, covering almost one hundred scientific articles selected based on the criteria of novelty and impact on the development of metrology, allowed the authors to systematize the main trends and directions for improving anemometric techniques. Based on the review, the dominant development trends of metrological tools used in renewable energy in anemometric measurements can be indicated:

• Improvement of measurement techniques, calibration, standardization, and control of measuring devices, and improvement of their reliability.
• Modification and development of measurement methods, in particular in the field of improving measurement uncertainty and dynamic properties of devices.
• Application of complex measurement signal processing methods and measurement algorithms, including neural networks, machine learning, and other technologies based on Artificial Intelligence.
• Application of controlled aircraft to position equipment in measurement areas, especially around large and high-energy facilities.
• Creation of extensive measurement networks based on wireless data transmission and collection and sharing of data in the clouds.
• Application of micro-electromechanical MEMS and nanoelectromechanical NEMS technologies to build complex measurement sensors.
• Application of integrated and hybrid multi-parametric sensors equipped with signal processing systems and compensation of signals interfering with the measurement.
• Application of integrated semiconductor devices: three-axis accelerators and gyroscopes to position probes.
• Application of 3D printing to build elements of measurement instruments, including dedicated devices developed for a specific metrological problem.
• Application of fiber optic techniques to build sensors, process signals, and transmit data.

The literature studies conducted by the authors of the article in the discussed field, as well as the selection and presentation of the main development trends, should be of interest to designers, contractors, and servicemen of wind energy facilities, as well as teams of scientists involved in research in this field. The presented review can be the basis for the selection of measurement methods in specific metrological issues and indicate the possibilities and directions for improving measurement techniques. Improvement of techniques and development of anemometric measurement systems allow for the optimization of processes in the area of wind energy. These processes concern the selection of the investment location, implementation of the energy facility and control, diagnostics, and monitoring during operation. Anemometric measurements are also important due to the safety of energy facilities and crews working on their operation [96].