Currently, about 22% of global electricity is being supplemented by different renewable sources. Wind energy is one of the most abundant forms of renewable energy available in the atmospheric environment due to different air-currents spread over the troposphere and stratosphere. The demand of modern wind energy conversion system (WECS) has increased to achieve a suitable alternate renewable energy source. In this paper, after a brief introduction, the classification of WECS is reviewed with attractive illustrations. The various mechanical materials and electrical components of WECS are discussed. The flow of power in WECS and its control strategies are also been described. The wind energy conversion is carried out with a suitable controlling mechanism for power grid integration. A maximum power-point tracking controller is an effective controlling method to extract the maximum possible power from the turbines. The present trends in WECS and the scope for improvement and future prospects are discussed. The materials used for both the blade and generator have been found to be key elements of wind turbines. Recycling of the polymer matrix composite materials are found to be a great threat to wind power plants, as well as to their supply chain industries.  The demand of energy over the globe is unremittingly growing owing to the growing of population, energy consumption per capita and huge industrialization. The availability of conventional fossil fuels is limited . Thus, eThe early usage of turbines was found in the form of a carousel, like the vertical axis turbines in Persia used for pumping water and grinding food grains in very small scale, around the 10th century. Horizontal axis turbines were adopted in England, which had to be manually oriented according to the direction of the wind currents, thus giving it the name post mill . HoweverWind energy has been the most abundant form of energy available in the atmospheric environment with varying air currents spread over the troposphere and stratosphere. Wind energy has been used for a multitude of purposes for thousands of years across the globe. From the use of sails in boats and rafts, to wind driven mills for grinding grains, wind energy has found its use throughout history. Wind energy has been a staple for transportation in oceans until as recently as the 19th century. The early usage of turbines was found in the form of carousel like vertical axis turbines in Persia for pumping water and grinding food grains in a very small scale, around the 10th century . HorizonThe invention of lead acid batteries by Camille Faure in 1881 provided a viable option for researchers to advance their studies for harnessing wind energy through turbines, since lead acid batteries were lighter and smaller. In the following years, prominent researchers such as Professor James Blyth, and Charles Brush successfully built some forms of horizontal axis turbines that were capable of charging up the accumulators available and illuminate a single mansion or real estate ,20. ThisThe demand for the modern wind energy conversion system (WECS) has increased to achieve a suitable alternate renewable energy source. In this review paper, the various mechanical and electrical components of WECS are discussed. The flow of power in WECS and its control strategies are also described. We elaborate on wind energy conversion with a suitable controlling mechanism for power grid integration. A maximum power-point tracking controller has been found as an effective controlling method to extract the maximum possible power from the turbines. The present trends in WECS and the scope for improvement and future prospects are also discussed. The typical materials used for the major components are discussed with neat illustrations. This review article will make the researchers’ job of selecting the materials best suited for the blade and generator easier. The WECSs fall into various categories depending on different factors. The most common classification of WECS is the one based upon the axis of rotation of the system or rotational axis, turbine, power control, and rotational speed control criteria.Horizontal Axis Wind Turbines: The shaft of the turbine and the shaft of the generator are placed parallel to the plane of the ground and mounted to a certain height to face the required wind speed depending on the geography. The elevated height of the turbine also facilitates the turbine to rotate with enough clearance to the ground. The mechanical components of the turbine are enclosed in a housing, which is shaped aerodynamically called a nacelle. The nacelle in this case is directly behind the rotor. According to the placement of the nacelle with respect to the turbine and tower, there are upwind and downwind HAWTs as shown in Vertical Axis Wind Turbines: This type of turbine has the turbine mounted on the shaft, which doubles up as the tower of the turbine structure and the rotor’s blades are vertically placed aerofoil sections. However, various designs of VAWTs are available, like Darrious and Savonious rotor turbines . The insThere are two basic axes of rotation possible for a turbine with respect to the ground, such as the horizontal axis wind turbine (HAWT), which rotates in an axis parallel to the ground, and the vertical axis wind turbine (VAWT), which rotates in an axis perpendicular to the ground. The function of both types of turbine systems is to produce kinetic energy through wind pressure generated by the turbine and convert this into electrical energy.Low Power turbines: These are turbine systems capable of producing on average a maximum of 30 kW. These systems find use in remote locations for household electricity demands and charging up of batteries. They are also used in the case of an emergency to reduce dependence on primary power sources;Medium Power Turbines: Turbines pushing out in the spectrum of 30 kW to 300 kW fall in this segment. However, they are mostly used to power up the homes in small localities. They are used along with other sources of renewable energy or other power storage devices;High Power Turbines: These refer to the systems where a large production of electricity is undertaken. These are incorporated in the large size wind farms that are associated to the power grids responsible for transporting the energy throughout cities.Turbines are also categorized based on their electrical output. The output power determines the size of the wind farm. Based on this, the prevailing turbine system technologies can be divided into three categories which are:A passive stall controller of WECS controls the input power of the wind by virtue of the aerodynamics of the blades and does not include sensors or actuators to stop the motion of the blades. In such WECS, the blades are fixed so that there is no relative motion with the hub, generally incorporating bolted or riveted connection. The design poses a stall when the wind flowing through the rotor exceeds a certain speed. This takes place since the first blade that comes in contact with the wind turns the flow into a turbulent one and the other blades continue to rotate within the wake of the first blade, thus significantly reducing the lift force experienced by those blades, stalling the turbine. Mostly used in fixed type fixed speed WECS, these are the cheapest and simpler designs than other types of WECS;An active stall control system is similar to the passive system; however, it uses sensors to predict the wind speeds and changes the pitch of the blades by means of motors or hydraulic actuators to interfere with the wind in a certain manner that induces a high turbulence and thus creating a stall. Active stall control systems are observed in high power turbines;Another type of WECS in this category uses similar mechanism as the active stall-controlled design, but the action is slightly different. Unlike the active stall controller, the pitch controller changes the pitch of the blades so that they don’t produce lift by reducing interaction with the flow of wind. This is different than the previous type which depended on increased turbulence, which still utilizes aerodynamics of the blade. This control system produces faster response times and thus, it is used extensively at present in the high-power turbine systems.Power control ability plays an important part in the design of turbines, since they are completely dependent on wind flow for generating power. As mentioned earlier, each turbine is rated for producing certain power. This power gets overshot when the turbine is subjected to stronger winds than they were designed to withstand. While, this produces more power, there are possibilities of mechanical failures due to increased vibrations and resulting unbalanced loads. Thus, power production is restricted under increased wind speeds, which can be achieved by three controlling methods, namely passive stall, active stall and pitch controllers. The other control methods include grid side controller and phase lock loop controller. Besides, a crow bar circuit is also used to act as protection for the whole electrical circuitry . To obtaTr) is rotating at an angular velocity (Tω rad/s) and the velocity of wind (wν) acting upon it, the tip speed ratio (λ) can be expressed by Equation (1) [λ and the design parameters of the turbine.pC) of the turbine. The pC of a turbine is a unitless constant, particular to the design of the turbine. It is a ratio of power captured by the turbine and the available power in the wind due to the flow. The mathematical representation of the power coefficient is expressed in Equation (2) [Fixed Speed WECS: This type of turbine system composes the first generation of commercially set-up grid connected turbines. They are well proven over the ages and have reliable and simple construction. However, the simple nature of these turbines makes them operable only at a certain rated wind velocity, thus the name fixed-speed WECS. Winds at a faster velocity reduce the power coefficient (tion (2) ,32.wP is the power of wind flowing at wv and can be expressed by wP = ½wρAv3.The criteria of rotational speed control classify the grid connected with wind energy conversion devices into fixed- and variable- speed systems. Here, tip speed ratio is a required concept to explain the classification effectively. In a wind turbine, the tip speed ratio is defined as the ratio of tangential velocity of the blade-tip to that of the wind driving speed of the turbine. When the turbine of radius (tion (1) , which iP = τ · ω) captured by a rotating object is defined as a product of torque (τ) and angular velocity (ω). Here, the pC can also be expressed as Equation (3).Now, the mechanical power (ρ) and velocity (νw) of wind, area of turbine rotor (A) and diameter of transmission gears (d), and can be expressed by Equation (4). Therefore, the power coefficient (pC) again can be given by Equation (5) [Torque applied by wind is a function of density (tion (5) .(4)τ=12pC is directly proportional to the ω and inversely to the velocity of wind stream (νw). This implies that at a constant wind stream velocity, higher turbine speed yields higher coefficient of power. This equation demonstrates that with increasing wind velocity the coefficient of performance decreases. Since power generated by the turbine is a function of pC, the power produced decreases as well. Thus, the drivetrain experienced an increased mechanical stress, while the power conversion efficiency falls significantly, nullifying the use of fixed speed turbines in commercial purposes.b.λ’ is maintained, the system can thus maintain the designed value of pC, which indicates maximum aerodynamic efficiency. It is a relatively complex and costly system, however the high yield of the system compensates for these shortcomings. Present wind energy systems use them on a large scale.Variable Speed WECS: The variable speed WECS were innovated from the older designs, that is the fixed speed WECS, considering the limitations of the latter, as utilization of wind power in power systems started to grow. Components of WECS are depicted in The Equation (5) demonstrates that the value of The WECS is an integrated system comprised of a symphony between the mechanical and electrical components. Among the various designs of wind energy conversion systems previously discussed, some components bind them together. These systems indeed share lots of components, which are able to convert the kinetic energy of wind into electrical energy in order for it to be stored in the cells efficiently. One of the biggest parts of grid connected WECS is the wind turbine assembly itself, which is the biggest tangible component. The wind turbines are compound mechanical machines built up of several small components responsible for feeding the electrical components. The aforementioned Mechanical parts of a wind turbine have a great role in conversion of the energy. The flow of wind has some energy in it due to their kinetic nature. The mechanical regime caters to introduce a ‘work’ in the system by allowing a mass to rotate and transmit it to the desired location to interact with the electrical components. The rotor subassembly is the rotating body, which includes rotor blades, rotor bearings, nose cone, hubs, and the pitch drive system.Rotor: Rotor assembly is the aggregate of the barely distinctive elements that interact with the wind. The assembly consists of the blades, the hub and the nose cone. The blades are the aerodynamic devices responsible to initiate the drive. In HAWTs, they resemble the propeller of aeroplane, which operates on similar principle. The blade of a typical horizontal axis wind turbine generates lift, which turns the blade during wind flow in the axial direction. The same is not true for a vertical axis wind turbine, which can either be a lift type . Braking from maximum speed of the generator shaft can cause high wear of the latter and can also cause fire hazards due to increased temperatures. Hence, they are used after proper actuation of pitch and/or yaw drives, which reduce the initial speed of the rotor.Nacelle: It is the enclosure housing most of the mechanical components. In horizontal axis wind turbines, the nacelle is the structure mounted on the tower behind the rotor. The size and shape of the nacelle is influenced greatly by the design of the gearbox and reduction required. This fiberglass construction is constructed to reduce turbulence to enable less vibration through the structure. Manolesos et al. demonstrated the aerodynamic properties of generic wind turbine nacelle designs . VerticapC. Each blade is fitted with independent pitch control mechanism for this purpose. Likewise, the yaw drive is employed to align the turbine according to the direction of wind flow in order to obtain maximum efficiency. Discussion about control mechanisms and strategies would be covered further with greater details.Pitch and Yaw Drives: Pitch of turbine blades refer to the rotational degree of freedom about the longitudinal axis of the blade and the yaw is the same for the top of the turbine (including the rotor and nacelle) with respect to the tower. With regard to pitch in the classification of WECS, as we have discussed, every turbine is rotated to function at a certain wind speed and produce a designed power. Since the power generation of a turbine depends on the energy captured from wind during higher velocity of operational winds, the blades are “pitched” to alter the value of aerodynamic efficiency Wind Measuring Equipment: As mentioned in regard to the pitch and yaw drives, the turbines operate on complex mechanisms, however the control of those mechanisms is ultimately managed by monitoring environmental conditions. Thus, turbines are further equipped with any kind of mechanical anemometer to measure the wind velocity around the system. An anemometer is a miniscule vertical axis turbine like arrangement with cupped struts, mounted on to the nacelle. Further, wind vanes are utilized in some wind turbines for judging the direction of flow, based on which of the controller signals the yaw drives to align itself to the wind.Some of the major mechanical components that are characteristics of WECS are:Generator: A wind generator is an electromechanical component responsible for converting rotational motion into electrical power, thus converting rotational kinetic energy into electric potential. A typical AC generator works by moving a conductor loop in a static magnetic field based on the electromagnetic induction law of Faraday. Traditionally various types of generator have been in use with the introduction of a squirrel cage induction generator (SCIG) almost 35 years ago ,36 alongPower Converter: The output electric parameters from the generator vary widely according to the wind speed. Thus, each turbine would produce different voltages and frequencies of current at a particular instant, which is not possible to be directly coupled to the grid without refining. A power converter thus acts as an interface for connecting the turbine to the grid. The power converter transforms the output voltage from AC to DC using a rectifier and again the rectified DC to AC, having a constant voltage and frequency in subsequent steps using inverter circuit ,38.Step-up Transformer: The output from the generator in a grid connected MW turbine is rather low, in the range of 400 to 690 volts, which requires it to be stepped up since the grid connects directly to the high-tension lines . The necWind Farm Collection Points or Point of Common Coupling: The point of common coupling (PCC) is a connecting ground of all the turbines of a wind farm. The preferred mode of connection in the wind farms is a parallel connection, which facilitates either maintaining a desired potential difference or having a defined node for connecting more turbines if necessary .Electrical components are separated into two segments, some of which are located inside the nacelle alongside the mechanical components, while others are placed on the ground away from the tower. The chief function of the electrical subsystem is to convert the mechanical energy gathered from the wind as torque and speed into electrical energy. A block diagram of a typical grid-connected WECS is depicted in V, the power (P), which is a rate of change of kinetic energy of wind according to definition can be expressed in Equation (6) [The WECS is a constant speed or constant frequency device, which is fitted with an induction generator, which is operated by a wind turbine without any controller. The power content in a stream of wind is a function of wind flow velocity. When the speed of wind stream is tion (6) ,43.(6)Pm) and square of velocity (V) as given in Equation (7) [Kinetic energy of a system is given as product of mass (tion (7) ,13.(7)Et, V is constant, thus, Inserting the expression of kinetic energy in the Equation (6), we get, tion (8) .(8)P=12A and the air density be ρ. Therefore, the mass flow rate can be written below in Equation (9) [Let the cross-sectional area of the wind column in consideration be tion (9) .(9)m˙=ρTherefore, Equation (8) can be rewritten utilizing the value of mass flow rate from Equation (9). This can be written as given in Equation (10) .(10)P=1V1 and downstream velocity of wind, V2 and the maximum extractible power is called the Betz limit. The wind turbine cannot extract the whole available power due to breaking of wind velocity from V1 to V2, allowing a continuation of wind regime [The turbine has no hub and has an infinite number of blades with no drag from any of the blades ;The flow is non-compressible, that is, the density of the air remains constant throughout;Thrust experienced by the rotor is uniform throughout.The Equation (10) represents the power content in a steam of wind only due to its flow and velocity. However, similar to Carnot heat engine, a turbine system cannot theoretically extract the whole power from the flow. This is governed by Betz Equation, which deals with upstream velocity of wind, d regime . The est1 and S2, respectively.Betz equation further considers the flow to be axial to the turbine. The ideal turbine is in a non-rotational form at the time of placing it in the wind atmosphere. The cross-sectional area of swept by the blade is A while the cross-sectional area upstream and downstream are designated the values SV while the velocity of wind upwind from the rotor is designated to be V1 and the downstream velocity is designated to be V2. By definition, the turbine extracts mechanical energy from the wind by reducing its kinetic energy from the upstream to the downstream. Therefore, V1 would be greater than V2 and subsequently, the cross-sectional area enhances from the upstream to the downstream, and hence, S1 would be lesser than S2. Since the stream of air is considered incompressible from the listed assumptions, the mass flow rate remains constant throughout.The speed of wind interacting with the rotor is taken to be F) can be obtained by applying Euler’s theorem [The force experienced by the rotor of the wind  is the rate of energy flow per unit area. The cross-sectional area is pre-defined to be A, and the rate of energy flow is effectively the power flow, P.Power flux is an essential fluid parameter, which defines the rate of energy flowing through a given cross-sectional area. By definition, power flux (P from Equation (9), we can arrive at the relation given by Equation (11).Using general value of V1 in over a cross-sectional area A, which is demonstrated in Equation (12).The kinetic power content in the air upstream to the rotor is obtained by replacing the velocity term with upstream velocity pC is also known as the aerodynamic efficiency since turbines rotate by the action of aerodynamic properties of the blades. It signifies the available power percentage extracted by a turbine. The pC value can be expressed in terms of P and W and we get a simplified relation as expressed in Equation (13) [We can introduce a dimensionless number obtained as the ratio of the net power extractable from wind, to the total power content of the same wind stream. This designates the coefficient of performance or efficiency of extracting power. The coefficient of performance represented by ion (13) ,44.(13)b as zero in Equation (13), we get pC to be 0.5 and setting b as 1, we get pC to be zero. Thus, the nature of the curve needs to be investigated in this domain. Moreover, putting b = 1 gives V1 = V2, which represents no kinetic energy being extracted from the wind and b = 0 gives V2 = 0, which represents no wind flow beyond the turbine. Between the points b = 0 and b = 1, a point of zero slope would give the maximum value of pC. Therefore, differentiating pC with respect to b whenEquation (13) gives a sensible relation between the coefficient performance and velocities of wind before and after hitting the turbine. Setting the value of ion (14) .(14)dCpb = −1 or b = −1, V2 = V1.The Equation (14) is derived to find a suitable solution. The equation holds for two values of V2 is in positive values of V1. Therefore, this solution should be ignored when pC. This has been validated experimentally in a wind tunnel and mathematically modeled in line with many other studies [pC, which is expressed in Equation (15) [This gives a trivial solution for the Equation (11) since the direction of the wind cannot be reversed and the range of  studies . Therefoion (15) .(15)Cp=This value of d stream . It is td stream ,46,47.Control in Level 1: Transmission system and distribution system operator (TSO/DSO) controlsControl in Level 2: Wind farm centralized controlControl in Level 3: Wind turbine centralized controlAs discussed earlier in This level can be broken down into mechanical and electrical controls.Pitch controlYaw controlMechanical Controls:Reactive power generation (RPG)Fault ride-through (FRT)Electrical Control:Damping controlAncillary servicesElectro-Mechanical Controls:Integration and synchronization of gridMPPT controlControl in Level 4: Grid connection (GC) and maximum power point tracking (MPPT) controlSpeed, torque and power control of generatorVoltage oriented control of gridControls in Level 5: Generator and control of gridCurrent control of generatorDC chopper controlCurrent control of gridControls in Level 6: Power control converterThe whole operation of control works on a system of feedback. In case of faults in the grid, the fault ride-through control of control level 3 generates a fault enable signal, signifying a positive identification for a fault, which is responded to by coordinating for better control performance by mechanical and electrical controls included in control levels 1 to 6. Feedback signals in a traditional wind energy conversion system consist of voltage and current of grid, angular speed along with voltage and current of generator, DC link voltage conditioner, a rotor positioning with respect to mean position and wind speed from the anemometer mounted on the nacelle .Level 1: Transmission system operators or distribution system operators supervisory control.The output from the turbine or an entire wind farm is not uniform; rather it varies with undulations in wind, which cannot be controlled. The coupling of this unstable energy waveform into the existing pool of energy causes reliability issues and the excessive penetration of a large scale WECS into the existing power system. The WECSs are linked to the TSO/DSO dispatch centers and keeps them updated about the active as well as reactive power generation statuses on a continuous basis. The TSO/DSO sends active and reactive signals based on the received status signals to the respective wind farm setup. Over the years, the large-scale wind farms have changed their operation from being passive sources of power to active ones through constant efforts made by the TSO/DSOs.Level 2: Wind farm centralized control.This acts as a communicative medium between the wind farm and the level 6 control. Every turbine in a wind farm is connected to the centralized hub through communication lines. Through it, the status of the WECS is shared to the TSOs and DSOs. The data acquisition and supervisory control systems are responsible for monitoring the operation of the wind farms. The active and reactive power requirements are defined for each specific wind turbine in a farm by the centralized control of that farm, which are transmitted chronologically as per the number of turbines it is in the farm. Since the control is highly reliant on feedback channels, the static compensators of the wind farms, including static synchronous compensator (STATCOM) and static var compensator (SVC), are initiated to meet the reactive power generation (RPG), when the centralized control realizes it is unable to do so normally. Besides all the operations required to meet the references imposed by the higher level, i.e., level 6, the centralized control also checks the aerodynamic interactions.Level 3: Wind Turbine Centralized Control.The level 4 of the control system is a cluster of electrical, mechanical and electro-mechanical systems as we have established in the classification. The pitch and yaw control mechanisms are responsible for changing the physical operating parameters and come in mechanical controls, whereas RPG and FRT are responsible for signal modifications, establishing the electrical controls. The third control includes damping, which can be both mechanical as well as electrical. Mechanical damping is necessary to protect mechanical components from torsional vibrations in the drivetrain and resonances in the tower caused by wind, whereas electrical damping provides protection to electrical counterparts by damping electrical sub-synchronous resonance in grids.The ancillary service acts as an electrical back-up for the mechanical control system and includes the power storage module as well as an uninterrupted power supply (UPS). The energy storage module collects power from idle spinning of the rotors without wind, i.e., rotation due to inertia after wind has fallen below cut-in velocity . As mentLevel 4: Grid connection and MPPT control.MPPT signifies the control loop that is associated with extracting the peak power at all times. The loop works by using a suitable algorithm on control to adjust the phase between the grid current and grid voltage for it to correspond with the grid power factor requirement. Synchronization of grid parameters is possible by different methods such as zero-crossing, grid voltage filtering and phase-locked loop. Among them, the phase-locked loop is preferred due to the robustness of the controller against the voltage harmonics and simplicity of the platform .Optimal TSR (OTSR) Control: The maximum peak point tracking as discussed previously, utilizes algorithms to estimate reference operational speed  based Control: Similar to the previous control method, MPPT strategy utilizes the wind speed for controlling the turbine speed. In this process, the power produced based on wind speed curve is referenced to the power curve provided by the manufacturer of the wind turbine, which is initially tested in controlled wind speeds, which provides the output power curve. Each wind speed and corresponding power, control” .Optimal Torque (OT) Control: The previous methods rely completely on the speed of wind to govern the control of the operational parameters. Therefore, even with the implementation of expensive wind speed measurement equipment, the input information is still incomplete. Thus, for increased reliability, other MPPT control techniques also utilize the rotational speed of the generator rotor. In this, the speed of the generator is used to calculate the reference electromagnetic torque With the levels defined, we can now move on to discuss the strategies undertaken by the WECS. Depending on the type of generator used, the control strategies can be broken down as the generator based WECS. The DFIG exhibits the salient feature of variable operational speed due to the integration of the variable resistor in the circuit. Presently, the widest used generator in WECS is DFIGs. The following controls are therefore in the context of DFIG based WECS.We can conveniently rearrange the expression in Equation (2) to get the power produced by the rotor and by substituting the value of TT) and the turbine speed (ωT) can be stated by Equation (18) and Equation (19), respectively:TP = Power output from turbine, Tω = rotational speed of turbine, mω = rotational speed of generator, gbr = gear ratio of gearbox.The turbine torque (eT) produced by the generator should be equal to the mechanical torque (mT) experienced from the drivetrain. Therefore, at eT = mT, from the above relations, the torque can be written as a function of angular velocity of the generator by Equation (20):The torque experienced by the generator from the drivetrain is defined by The reference electromagnetic torque considers optimal coefficient of power and tip speed ratio. Therefore, the reference torque is given by Equation (21):opK can be treated as a constant for the operation and can be calculated initially with the known design parameters. The torque can be obtained by multiplying this with the angular velocity of the generator. The electromagnetic torque follows a quadratic relation with the speed of the generator evident from the expression. The control in this strategy employs control levels 1 and 2 to equalize the electromagnetic torque to the reference value, by referencing during operation. This method is also used on a large scale in commercial MW-WECS [d.mω of a generator is directly proportional to the vw. Therefore, similar to the power curve used in the wind turbine power curve–based control, this strategy utilizes power versus wind velocity curve to compute the reference power, Power Signal Feedback (PSF) Control: Another MPPT control utilizes the input signal in the form of generator speed. This follows the basic principles of Power Curve based control strategy, however instead of monitoring the wind speed, sensors are employed to monitor the speed of the generator. The Equation (16) signifies that the the wind .e.af) of the generator. The predictions implemented regarding the wind speed category based on the basis of time scales are [Ultra-short-term forecast: These predictions are regarded for wind conditions about an hour ahead.Short-term forecast: This is applicable for forecasts several hours ahead.Medium-term forecast: The prediction of wind conditions in the time span from a few hours to about a week.Long-term forecast: Used for detecting wind conditions more than a week ahead.Speed Sensor-less (SSL) Control: This MPPT control operates without the use of any kind of speed sensing device. Rather, the wind speed is predicted by autoregressive statistical models of WECSs, which are based on the measured frequency ,  MW-WECS .d.Power f.Hill Climb Search (HCS) Control: It is a method of maximum peak point tracking control strategy that involves a disturbance in the reference signal. The inputs in this control are obtained in the form of generator speed and dc-link current. The output is a command signal that is needed for extraction of maximum/peak power. A schematic of the hill climb search control is depicted in The predicted wind speed is used to calculate the reference power, The control methods discussed above are used precisely in a doubly fed induction generator (DFIG) based WECS. Other examples of WECS architecture, such as the permanent magnet synchronous generator (PMSG) and the squirrel cage induction generator (SCIG) based designs, are substantially more expensive than the DFIG. Moreover, the inverter output filters and electromagnetic interference (EMI) filters used in the previous types of WECS are rated for 1 p.u. of output power, which also contributes to the complexity of designing the filter, making it more costly. The WECS employing DFIG at its core uses back to back converters. The primary advantage thus obtained is the reduced converter rating, which is about 30% of the generator rating; while the speed range of variable speed WECS stays at about 33% of the synchronous speed mechanisms .wv as an input parameter. A schematic flow of TSR control is depicted in R is the turbine rotor radius. This speed control command is used further to the control loop responsible for controlling the speed. The PI controller is responsible for processing the command and to limit the speed of the turbine rotor to the desired maximum/peak value of rotor speed obtained by the evaluation.Tip Speed Ratio (TSR) Control: Not unlike the optimal TSR control strategy used in case of a DFIG based WECS, the optimal TSR (OTSR) is evaluated with the velocity of wind, Power Signal Feedback (PSF) Control: The power signal feedback system of control utilizes the information about the rotor speed as input of the controller and power produced as a feedback, directly from the generator. In this system of control, the reference power is evaluated through the algorithm, which is then compared to the produced value of power obtained through feedback loop.A PMSG is a staple in the industry for innovating and incorporating new designs, and has attributed to it a higher efficiency alongside greater power density and an achievable smaller diameter of turbine in direct-drive systems. These are used particularly in older generation turbines to keep the complexity lower. Permanent magnet materials of high energy are also available at quite a reasonable price, making these generators desirable. With increasing wind farms, the focus is concentrated on achieving WECS which is reliable, and has lower maintenance cost and noise while being efficient. This converged to direct drive PSMG based WECS. A conclusive design consisting of three MPPT based controls is applied on available wind turbines.T) and angular velocity (ω). Therefore, using Equation (21), it is easily perceivable that to optimal power (PO) can be obtained given in Equation (23) similarly by multiplying the result with the term of angular velocity, applying minor modifications to it. Since here the rotor speed is monitored, the angular velocity of the generator has to be replaced by the angular velocity of the rotor.We know that the power produced at any instance depends on torque ;fuel cell (FC);flywheel (FW);superconducting magnetic energy storage (SMES);supercapacitor (SC);compressed air energy storage (CAES);pumped hydroelectric energy storage (PHES);water storage.Advancements in energy storage technologies are at a peak as well . Even th2) as cathode and lead (Pb) as anode. During the discharge process, chemical reactions in the cell lead to the synthesis of lead sulfate, whereas, during charging, this reaction reverses and Pb gets deposited back onto the electrodes. Recent developments have demonstrated that introducing carbon into the cell can help with the prevention of detrimental crystallization during charging and discharging cycles [Advanced lead-acid battery: Regular lead–acid batteries have been a staple for a variety of applications in every corner of industry, as well as in an automotive and domestic context. Though, most prominent and specific applications include frequency regulation, bulk storage of energy in case of integration of variable renewable energy sources and distributed energy storage systems. As a result of versatile and vast usage of the Pb–acid batteries, these have been an interesting focus for further enhancements. A conventional Pb–acid battery comprises lead–dioxide  on all sides. This is placed inside a sodium sulfate surrounding. The advantage of sodium sulphur battery is a high energy density, making it suitable for places of application that demand short yet potent bursts of energy. These batteries are used in rather functional contexts in energy distribution grid applications, wind power integration, in ancillary services as UPS batteries and much more.Lithium ion battery: As the name points out, these batteries incorporate positively charged lithium ions , that flow through electrolyte and promotes flow of electrons across the connected outer circuit. From time to time, lithium ion batteries have found a place in almost all electronic applications, alongside medical uses, owing to the far superior energy density values. At present, lithium ion batteries are being tried and tested for application in automotive industries, prompting its upcoming uses in fields of renewable energy industry . CurrentFlow batteries: A flow battery technology stores energy by the virtue of chemical energy in the form of two electrolyte reservoirs inside the body of the battery. The chemical energy is converted into electrical energy when one liquid is displaced slowly and steadily from one tank to another. The displacement of the liquid when reversed acts to charge the batteries. Since they are the function of the fluid motion, they are termed as flow batteries. The energy storing capacity of these batteries is solely governed by the size of the tanks and the energy density is dependent on the volume flow rate from anode to cathode. The primary advantage, for which they are considered as a potential candidate for evolving battery technologies, comes from the fact that these batteries can have very high-power ratings, and the electrolytes can be stored and replaced easily, more conveniently than typical cell electrolytes, coupled with adjustable and replaceable flow properties according to the application.In contrast to flywheels that store energy by virtue of mechanics , FC and The water storage is an unused generated energy, which is trying to be converted into potential energy from the water bring pumped from a lower container to a higher one. In the near future, its performance might be compared with chemical batteries .Adaption of newer technologies has always been a lengthy process. Usually, a new technology is expensive to begin with, making them undesirable at this early stage. However, with lots of scope for improvement, those who take up the idea and make it the focus of further studies and increase understanding make them cheaper and more acceptable, besides making them superior with regard to existing technologies . In thisTechnologies based on supply and conversion of wind energy;A technical overhaul, with new fundamentals, which cannot be a result of incremental research on prevalent technologies; andA technology which is in its early developmental stages.Since wind energy is a relatively newer source of energy, most of the resources that capture the energies have focused on the development of turbines and tower designs, as well as methods to increase the efficiency of existing systems. However, possibilities for the evolution of technologies are endless. However, a future emerging technology in this field of study would consider the following criteria to be accepted :TechnoloWhile some of the technologies enlisted below have been used in practice, they may not have necessarily been used in wind energy systems and thus have ample space for improvements to be made usable. Even if not a completely radical innovation, large scale adaptation of such technologies might be considered as an emerging technology.AWE systems are a collective group of devices that harness energy in a similar form as a regular WECS set-up, instead having the external features of an unmanned aircraft or an autonomous kite. An airborne wind turbine is schematically shown in The two broad categories of the AWE system involve ground-gen and fly-gen systems. The ground-gen concept aims at converting the mechanical power into electrical power at the ground level, while fly-gen systems do so in the airborne unit itself. The fly-gen systems are further classified into crosswind and non-crosswind, out of which the crosswind turbines have been observed to produce more power . BesidesHowever, there are many technical challenges associated that are yet to be overcome for making this usable, such as reliability and durability of flexible components and erosion by wind and water. Recent trends in the research of this system consist of drone concepts, aerodynamic modeling, electronics and sensor technologies for autonomous control of the tethered devices, and controls for autonomous take-off and landing.This concept differs from traditional offshore concepts by the method of foundation. While the latter employs a fixed foundation to the floor of the ocean, this innovative design is a free-floating structure related to the semi-submersible tension leg or the spar platforms. The floating is controlled by various anchoring systems. Since it is a diversion from the existing fixed off-shore designs, there high potential for the exploration of newer techniques to integrate the tower to the floating base. Moreover, the impact of changing turbine parameters including TSR, blade designs and number of blades on the floating platform can be investigated .Recent progress in this topic has led to evolution of three main types of floating off-shore foundations including spar-buoy, tension leg platform, and spar-submersible, which are depicted in Spar-buoy: Stability in this set-up is provided by a large cylindrical buoy filled with ballast under the tower structure. This helps in lowering the center of gravity (CG) in the system below the level of the center of buoyancy and increases the stability. Apart from the heavy buoy under the water, the hollow structure above helps to keep the center of buoyancy high.Tension Leg Platform: This design incorporates the buoyant component to be semi-submerged into the ocean surface, which is then attached to the ocean floor by means of tension mooring lines, providing stability to the tower above it. The buoyant structure is maintained; submerged by balancing out the upward buoyant force. it experiences by the mooring lines’ tension.Spar-Submersible: Also called the semi-submersible design, this concept incorporates multiple columns that are linked by bracings or submersible pontoons. The columns provide hydrostatic stability to the structure, whereas the pontoons provide additional buoyancy to it. This system also uses catenary mooring lines to keep it in place [in place .This concept of wind turbine system integrates high speed, low torque rotors at the tip of the main blades. Therefore, the generator can be coupled directly without the necessity of torque reduction gearbox arrangements for these rotors. The efficiency of these rotors to extract wind energy can go beyond the Betz limit, since it is actually the thrust of these tip-rotors that provides reaction torque to the main rotor of the turbine . The rotReplacing the large size of a rotor with multiple smaller rotors is helpful to reduce the overall loads on the structure of the turbine. This concept made it possible to install a large power system of about 20-MW at a single site. A company like, Vestas, has adapted this concept to develop a 900 kW four rotor design, illustrated in The modelling of multi-rotor system needs more development owing to its challenging aspect, as well as aerodynamic studies regarding effects of wake produced by each rotor on adjacent ones need to be looked further, since the design tools for simulating aerodynamic behavior required to visualize a multi-rotor system and turbulent winds are in their elementary stages. The overall design of the setup is also worth looking into for improving the reliability, which is curtailed by the increased static loads and dynamic vibration of increased number of components .Diffuser Augmented Wind Turbines, chiefly abbreviated as DAWT is a modified design of normal horizontal axis wind turbines, with the addition of a diffuser structure around the rotor. The diffuser is encased around with a diffuser that resembles the shape of a funnel, used to concentrate the approaching stream of wind. Modification can be introduced to this structure by adding a rim around the exit side of the diffuser and an inlet shroud at the entrance side (upwind). The diffuser has been proven to increase the generated power compared to traditional HAWTs . A schemThe concept of DAWTs is being developed in Japan at a semi-commercial level. The rating of these is in a range of ten kW at present , with thWith a more complex design than its counterpart, it may not be a cheap alternative. However, if it is used properly, it would be suitable in certain situations. At present, diffuser augmented turbines face the biggest challenges of structural loading. Though this design has sound potential, they are best suited for small scale applications.In wind energy conversion systems, both mechanical and electrical components play a vital role. Therefore, the materials used for mechanical and electrical components in the WECS are also extremely crucial. Specifically, the materials used for mechanical parts help to get ‘work’ of the system using a mass to rotate the turbine blade and the materials used for electrical parts help to transmit the mass to a desired location. Typically, a wind turbine is constructed of nearly 25,000 elements, which are grouped into several major components including, tower, nacelle and rotor . The rotIn a rotor subassembly, the rotating body comprises rotor blades, rotor bearings, nose cone, hubs, and the pitch drive system. Therefore, the major mechanical components of the WECS, for which the selection of material is very important, are rotor, main shaft, gearbox, mechanical brakes, nacelle, pitch and yaw drives, and wind measuring equipment. On the other hand, the major electrical components of the WECS, for which the selection of material is very important, are generator, power converter, step-up transformer, and so on. The electrical subsystems convert the mechanical energy into electrical energy from the wind. The most useful components employed for the wind turbine energy supply chains from the necessary raw materials, processed materials, components and up to the final products are depicted in In the WECS, the wind turbine with efficient wing construction is extremely important to the material scientists. The various components that are used in a wind turbine for renewable energy generation are supportive materials of the turbine, struts, shaft, bearings, chain, sprockets, gear, turbine blades, motor components, tower and so on. In this aspect, NASA invented many turbines with various materials and different technologies, which were mainly aimed at improving energy conversion. The most recent research on wind turbine focused on composite blade materials, steel tube towers, variable-speed generators, and dynamic components including aerodynamics of the blades in horizontal axis turbines, pitch and span control mechanisms. Typically, the weight sharing of an onshore wind turbine plant consists of 75% foundations, 23% turbine, and 2% in cite cables. The turbine part comprises the 59 wt% tower, 22 wt% nacelle, and 19 wt% rotor. The mass distribution of each important elements of wind turbine plant is illustrated in For several years, scientists have been trying to find suitable materials for rotor blades. Earlier the rotor blades were made from heavier metals. These high density or higher mass metals are resistant to environmental conditions such as high turbulent wind, however are rendered not helpful as a result of their large weight. In addition, the different mass concentrations away from the center create larger rotational moment of inertia. This causes a reduction in the acceleration of the rotor. This difficulty led the focus of the material towards composites, which have high rigidity without increasing the mass. Therefore, the advantages, disadvantages, scopes and challenges of the conventional as well as natural fiber reinforced composite materials used for the wind turbine blades are depicted in In addition, different advanced smart materials such as, piezoelectric materials are trying to use for “wind tree” in order to produce energy or power from the wind flow. The soft flexible materials are used as a leaf element to generate maximum power. The trunk parts of the wind tree-like structure are made of hard materials such as steel. The steel trunks stem into tinier branches, which are attached with many leaf-shaped wind turbines. These wind trees have recently been exploited to all types of wind, from gentle breezes to powerful gusts, in both urban and rural areas .A long way has been traversed, with an array of technologies implemented in building the modern-day wind energy conversion systems. However, it has long list of prospects in the upcoming future. Shifting the focus away from the carbon-based fossil fuel reserves has opened up a multitude of opportunities waiting to be explored. This review paper discussed in detail the mechanical and electrical components of conventional WECSs, along with the various alterations in the designs, which have helped in categorization of the systems. This review paid much attention to the ecological applications of WECS and wind power plants. It is also to be informed that the hydropower plant could be another alternative sustainable power source since wind flow is a less stable power source. For wind power generation, the water consumption is lowest. It has the lowest relative greenhouse gas emission. Thus, it has the most favorable social impacts and is considered as one of the most sustainable renewable energy sources, followed by hydropower, photovoltaic, and geothermal.Most commercial applications have adopted HAWT designs for WECS over VAWTs due to the former’s compatibility with large scale power production. Different control techniques and strategies were discussed. A majority of wind turbines in the present context operate on doubly fed induction generators, the control strategies of which have been tabulated expressing the salient features of each design. In the context of the present scenario, the evolving technologies for harnessing power from wind have been highlighted, and the future emerging technologies have also been addressed. The reliability of wind energy has increased exponentially over the last few decades. The expansion of wind energy along with other renewable energy sources will continue to grow in the near future as well.We have found that materials for the blade and generator are the most critical elements of wind turbines. Since many components of wind power plants use synthetic polymer matrix composite materials, the recycling of composite is a cross-sector challenge. This huge challenge is not only a problem for the wind power plants, but also to its supply chain industries. Therefore, all the sectors connected to synthetic polymer matrix composite materials must look into the matter together in order to develop cost-effective solutions and value chains for the combined volume of composite waste.


This article dwells upon the additive manufacturing of high-energy materials (HEM) with regards to the problems of this technology’s development. This work is aimed at identifying and describing the main problems currently arising in the use of AM for nanostructured high-energy materials and gives an idea of the valuable opportunities that it provides in the hope of promoting further development in this area. Original approaches are proposed for solving one of the main problems in the production of nanostructured HEM—safety and viscosity reduction of the polymer-nanopowder system. Studies have shown an almost complete degree of deagglomeration of microencapsulated aluminum powders. Such powders have the potential to create new systems for safe 3D printing using high-energy materials. In recent years, there has been explosive growth in interest toward additive manufacturing in many scientific research areas. This can be seen from the increase in the number of publications on this topic , as wellAdditive manufacturing technologies have been developing for over 30 years. While at the onset, they consisted of relatively simple 3D printing of polymer prototypes, now it is possible to use AM to obtain metallic or non-metallic prototypes or functional products that do not require mechanical post-processing. One of the examples is additive manufacturing by means of layer-by-layer laser fusion of powder ,5.The main direction of AM application today is the production of functional products for aerospace and automotive  industriDespite significant advances in AM in recent years, some areas remain problematic. These include AM of high-energy materials (HEM).Recently, interest has arisen in high-energy nanocomposites. Those make it possible to create miniature energy systems based on nanomaterials with more powerful oxidation kinetics than macroscale materials. Such composites are called “reactive materials” ,8, “metaConventional technologies such as pressing or casting are used for manufacturing products from high-energy materials. These technologies also suffer from imperfections: resulting products may contain pores and cracks , and somAdditive manufacturing technologies are the most promising field for the fabrication of products from high-energy nanocomposites since they are commonly and widely used for the creation of nanocomposite products ,14. AddiMethods for creating planar (2D) structures from reactive materials are outside the scope of this review (unless they are a variation of 3D), and they are considered in detail in .In this research, we rely on the results of the review  and earlMaterial extrusion is the first and, currently, the most popular additive manufacturing process for a wide range of products in terms of affordability and reliability. Material extrusion is a 3D inkjet printing process whereby the material is fed through a nozzle or jet. This principle of operation is mostly used in inexpensive desktop 3D printers. The requirement for materials for the fabrication of items is the ability to push the material through the nozzle. Any pasty material (sometimes preheated) can be used for “drawing” 2D cross-sections of a certain 3D model.A variation of the process under consideration is the creation of flat structures, for example, screen printing for pyrotechnic applications  and DoctThe extrusion process is also used for the formation of more complex 3D structures. To this end, the “ink” is applied layer by layer through the hole in the 3D printer head . The resThe pen-type technique is a variation of this method. Here, a specially prepared ink containing a dispersed phase  and a binder are used. The ink fills the syringe, and the extrusion process is actuated by pressurized air. The adjustment of air pressure allows for controlling the printing speed and setting up the technology in accordance with the ink viscosity. However, manufacturing articles with a solid content of over 90%, using the pen-type technique, is still a problem. This is because high pressure is required, which also results in the slurry stratification phenomenon, i.e., the separation of the binder from the solids .To overcome the limitations associated with high ink viscosity, an ultrasonic technique with high frequency and amplitude oscillations is used   ,29. TherIn general, DIW direct printing technology achieves a relatively high slurry-solid content and appropriate viscosity of the material using special techniques (ultrasound) and careful ink formulation designs, individually designed for each application.One of the important advantages of this process, as applied to high-energy materials, is the relatively low operating temperatures, which reduce the probability of ignition. The operating temperatures are determined by the physical and chemical characteristics of the material.FDM is a widely used additive manufacturing technology. It is based on the use of a plastic filament which is deposited in layers through a heated extruder onto a substrate . TypicalToday, the FDM printing process has matured to an industrial level, and inexpensive commercial printers are used in the manufacturing process. However, there is little data on the fabrication of high-energy composites based on this technology. The reason for this is obvious: FDM involves a rather high degree of material heating to increase its fluidity. This limits the range of materials that can be printed with FDM to those with a melting point significantly lower than the reaction temperature ,31.Another approach that still does not completely eliminate ignition during printing consists of filling a thermoplastic polymer filament with solid HEM particles .The main difference between FDM and DIW is the required feedstock viscosity  (PVDF), which can serve both as a filament backbone in printing and as an oxidizing agent during the reaction ,35,36,37Summarizing the research findings obtained on the use of the FDM method for the additive manufacturing of HEM, it can be assumed that this technology has high potential. The main disadvantages of FDM are the limited surface finishing capabilities and precision as well as poor mechanical properties of the products . BalanciVat Photopolymerization is a 3D printing process that implements a light source to cure liquid photocurable resins (photopolymers). The key difference between the two technologies lies in the type of light source used for curing the material: in SLA , it is uA 3D printer involved in the implementation of vat photopolymerization is equipped with a container with photopolymer resin, which is cured by the light source. SLA printers use a vat with liquid photopolymer resin that is cured by UV radiation. Laser beam processing is done layer by layer. After one layer has been treated, the printer platform is shifted by a distance equal to the layer thickness (0.05–0.15 mm). The resin-filled plate then passes over the cross-section of the item and re-coats it with new material. The next layer is formed on this liquid surface and is joined with the previous one. Thus, an entire 3-dimensional object is formed.DLP uses visible light projectors such as arc lamps. In this case, each layer of the object being fabricated is projected into a vat of liquid resin, which hardens layer by layer as the platform is raised or lowered. Thanks to this treatment method, DLP is faster than most 3D printing methods.There are a number of works devoted to SLA application in energy product manufacturing: fuel pellets for hybrid engines , gun pow-All the components must be chemically compatible.-The viscosity of the material should be sufficiently low, preferably below 20 Pa⋅s .-The material must be sufficiently stable over time, i.e., sedimentation must not occur for several hours.-The material must be sensitive to the wavelength range of the light source used.-The material must be promptly cured, preferably within a few seconds.-Light penetration into the material (including light scattering) must be limited to provide sufficient resolution in all directions.-Once illuminated, the material must have sufficient mechanical strength to withstand transportation, treatment and use.The use of photopolymerization processes in a vat imposes a number of requirements for the materials :-All the-The energy resin must not decompose when exposed to the light source used for curing.-The energy properties  must be sufficient for the intended use.Additional requirements largely depend on the intended use of the printed product. In the case of energy cells, it can be said that the following requirements must be met:The main advantages of the photopolymerization methods are high resolution and high performance. The light-induced process eliminates the use of toxic hardeners. The main disadvantage is the need to use polymers that are curable by UV or visible light. This significantly limits the list of materials available for printing.Two materials are used in printing with this technology: a base material in the form of a powder and a liquid binder. The binder is sprayed by the printer head, and the powder particles stick together to form a solid 3D model . The powThe difficulty of using this technology with high-energy materials lies primarily in the fact that post-processing is required to eliminate mechanical defects and increase strength . Apart fThere are other additive manufacturing technologies that are less suitable for handling reactive materials . However, literary sources describe attempts to use them.SLS or Selective Laser Sintering is an additive manufacturing technology based on layer-by-layer sintering of powder materials using a laser beam . The lasThe advantages of the SLS technology are high fabrication precision for products of complex geometric shapes, high speed and performance, excellent mechanical properties of products and the absence of wastes. The disadvantages include the high cost of equipment.The research work  investigThe last two decades have seen remarkable developments in the field of energetic materials. There has been a gradual transition from the use of nitrocarbon energetic materials, such as TNT, RDX and CL-20, to microstructural composites and nanothermites . Thermit2/g), a lower ignition temperature, a higher energy density (up to 50 MJ/kg), a shorter ignition time, a higher burning rate [In contrast to micro-sized energetic materials, nanothermites have a higher specific surface area  find their applications in ammunition, pyrotechnics and microcircuits of microelectromechanical systems (MEMS). Very high reaction rates (often in the form of detonation) are useful in the above applications. However, due to the high sensitivity, there is a certain hazard during their production and handling.In addition to high sensitivity, there are other factors limiting the use of nanothermites , includiIn addition to the reactive material, energetic inks contain a binder. The main role of the binder is to improve the mechanical properties of the composite . The binAn integral part of the ink used in most additive manufacturing technologies is a solvent or dispersion medium. The use of liquid media increases safety by reducing the sensitivity to electrostatic discharge as compared to the use of dry powders .The suspension used in additive manufacturing  must be properly designed to meet the following requirements: uniform distribution of particles, fast solidification kinetics and suitable rheology for substance flow and shape retention . The rheDespite the advantages of nanothermite systems, safety concerns hinder the production and use of nano-energetic materials to a certain extent.The problems of additive manufacturing of HEMs can be divided into two groups: problems associated with the use of materials in the production process ; the quality issues of the final products .-Providing a high reaction rate ;-Providing a low controlled reaction rate and the absence of gas formation ;-Releasing heat during the reaction ;-Generating gases .Reactive material products from the additive manufacturing process have been proposed for a wide variety of applications. Different applications may require very different, sometimes oppositional, properties of reactive materials:-Choose an adequate 3D printing method;-Investigate the feasibility of the processes pertinent to the preliminary preparation of materials, printing and post-processing while ensuring their compliance with safety requirements;-Achieve the required mechanical and energy properties of the final product.The design of processes for the additive manufacturing of products from high-energy nanocomposites is based on the requirements of a specific engineering task and is an example of engineering art. Here, it is necessary to:The research methods may include mathematical modeling or physical modeling, including the use of simulators of explosives.Mathematical modeling of 3D printing processes is complex and multi-faceted. It must take into account many problems pertinent to mechanical engineering, heat and mass transfer, as well as chemical kinetics. Currently, there are many works devoted to the mathematical modeling of various additive manufacturing processes. The authors of  propose The advantage of additive manufacturing of high-energy products lies not only in the ability to control the properties of products by changing the physicochemical parameters of the feedstock . The ability to control geometric 3D shapes on a micro-scale is also valuable since the rate of combustion and detonation depends on them . This opThus, it can be assumed that the most important challenges in obtaining high-energy nanocomposites adapted for 3D printing technologies are: (i) materials safety and (ii) the viscosity of the binder-nanopowder system. Here, high viscosity is a result of the agglomeration of nanoparticles and their high specific surface area. One of the approaches to solving these problems lies in the deagglomeration and encapsulation of aluminum nanoparticles.Increased reactivity of aluminum nanopowder is known to be one of the key characteristics that contribute to its extensive research and application in the composition of energy systems ,86,87,88Another disadvantage is the increased tendency of nanoparticles to agglomerate. This significantly reduces the viscosity of the powder-polymer system and complicates the preparation of materials for 3D printing. There are different approaches to solving this problem. For example, in Ref. , it was To solve this problem, we propose a new technology. It aims to modify the surface of aluminum nanopowder by creating a protective coating on it ,93. The 2/g. Nanoparticles form micron-sized agglomerates  should be created on nanoparticle surfaces to prevent contact between them until the moment of their active oxidation. To apply such films to nanoparticle surfaces, agglomerates of nanoparticles must first be disintegrated.The disintegration of nanoparticle agglomerates occurs during the intensive processing of powders exposed to the effect of the liquid organic solvent flow containing chemical surface modifiers. The additional agglomerate disintegration effect is caused by the interaction of modifier molecules with the surface of nanoparticles resulting in the formation of a film on the nanoparticle surfaces.The substances presented in This research has shown that in samples of nanopowders treated with 8-hydroxyquinoline, the content of the fine fraction relative to the initial aluminum nanopowder slightly increases. The share of nanoparticles less than 100 nm in size is 23% of their total number.In the samples of nanopowders modified with pyrocatechol, the proportion of nanoparticles with a size less than 100 nm is 32% of their total number, while the difference between the fractions becomes less pronounced. Somewhat better disaggregation of nanopowders is achieved when using stearic acid. The fraction of particles less than 100 nm is 36% of their total number.The best dispersion is achieved when using acetylacetone, whereby the fraction of particles less than 100 nm is 56%. The data obtained show that the deagglomeration effect is determined by the bond strength between the modifier and the particle surface, as well as the difference in the intermolecular interaction of the modifiers used. Stearic acid forms unstable covalent compounds with surface aluminum cations, so its efficiency as a surface modifier is low. Pyrocatechol and 8-hydroxyquinoline readily polarize, which enhances the interaction of protective layers formed by these modifiers on the surface of nanoparticles with each other. Acetylacetone forms strong covalent bonds with superficial aluminum cations. Furthermore, acetylacetone molecules are characterized by low polarizability and strong intermolecular repulsion. A protective layer of acetylacetone on the surface of nanoparticles protects (shields) the surface of nanoparticles, thereby reducing the level of interactions between them. Thus, it was shown that acetylacetone has the highest efficiency as a surface modifier. This ensues from the data obtained that the creation of protective layers (films) that prevent aluminum nanoparticle agglomeration requires surface modifiers in order to form bonds with superficial metal atoms. These modifiers make it possible to reduce the surface energy of the dispersed phase.For microencapsulation, the best methods are colloidal methods of material fabrication. Those rely on the deagglomeration of nanoparticles in a liquid medium in the presence of a polymer, sorption of the polymer on nanoparticle surfaces followed by solvent removal. These methods are simple and technologically mature, while allowing for the achievement of almost complete deagglomeration and, therefore, uniform coating of nanoparticles with a polymer layer.HTPB—hydroxyl-terminated polybutadiene with low molecular weight.N-allyl-5-vinyltetrazole and acrylonitrile.MPVT is a copolymer of 1-methyl-5-vinyltetrazole, 2-methyl-5-vinyltetrazole, In this work, the following components were used as polymers for surface modification:Petroleum ether (70/100) was used as a solvent for the HTPB polymer, and dimethylformamide and ethyl acetate (butyl acetate) were used for the MPVT polymer.It has been experimentally established that the highest efficiency of the microencapsulation process is achieved in units with periodic interruptions in the liquid medium flow. Periodic liquid medium flow interruptions lead to multiple discontinuities in the liquid medium flow (the effect of acoustic cavitation). Accordingly, ultrasonic vibrations are generated in a wide frequency range, which is very effective in the case of nanoparticles. Under the effect of the generated liquid medium flows, the aggregates of nanoparticles are disintegrated, and the mass transfer of reagents is intensified.Microencapsulation of nanoparticles was performed by the following technique:Solvent (300 mL), 20.00 g of powder and 100 mg of polymer were placed in a round-bottom flask with a capacity of 500 mL. The mixture was processed in an HG-15D high-speed homogenizer over 30 min at a stirring speed of 5000 rpm. The solvent was removed on an IKA 10 RV rotary evaporator, and the resulting powder material was dried at a pressure of 1 Torr over 16 h.Additive manufacturing technologies used with high-energy materials have been considered based on the analysis of literary sources.The advantages and disadvantages of these technologies from the point of view of using high-energy materials for printing have been shown.Requirements for the preparation of materials for printing by using each of the technologies under consideration have been formulated.It has been shown that the chemical reactivity of HEMs significantly complicates the problem of additive manufacturing, development and optimization of 3D printing methods. In addition to customary complications in the development of these methods , there are specific problems associated with the nature of the reactive substances. First and foremost, it is the possibility of initiating reactive materials in the 3D printing process.This paper proposes a method for the microencapsulation of nanosized aluminum powders with polymeric materials of different chemical compositions. It has been shown that polymers form a continuous homogeneous layer (film) on the surface of nanoparticles. The research has demonstrated an almost complete degree of deagglomeration of microencapsulated aluminum powders. Such powders open the potential for creating new systems for safe 3D printing using high-energy materials.The development of new paste formulations for the 3D printing of HEMs is the subject of further research, which will be published in a separate paper.


In order to combat corrosion issues, several studies on progressively replacing steel reinforcement elements with composite ones have been conducted in recent years. Hybrid steel–concrete thin slabs in which the steel acts as formwork are also candidates for update in the coming years. Achieving a reliable connection between fiber-reinforced polymer (FRP) and cast-in-place concrete is key to promoting this technology. This study analyzed different connection systems and proposes the novel approach of embedding a flexible fiber fabric as a superficially distributed connector between concrete and FRP. Eight specimens with four different connection strategies were tested using an experimental modal analysis and a quasi-static three-point bending test. The impact of the connection system on the vibrational response, flexural ultimate load, moment response, neutral axis position, shear and dissipated energy was obtained and compared. The results show that the use of an embedded mesh increases the frictional mechanism and produces the best performance in terms of load-bearing capacity and ductility. Traditional reinforced concrete (RC) structures exhibit aging problems related to the corrosion of the steel reinforcement, and there have been many recent studies into strengthening and retrofitting these structures using composite materials. Recently, the FRP–concrete concept was used in manufacturing hybrid slab , column There are two main typologies of FRP–concrete hybrid structures. In the first type, FRP is used as a tubular or box envelope for concrete, whereas in the second and more studied type, FRP is presented as a girder or a pultruded profile connected to a top concrete section to aid bending resistance. Regarding the first approach, several researchers have studied the bending resistance design of hybrid structures based on the box system. Yang et al.  performeFar more research can be found regarding the second hybrid FRP–concrete typology, which is based on connecting FRP pultruded profiles to upper concrete sections to generate hybrid beams. One of the pioneering works on hybrid composite single beams is by Hulatt et al. , in whicRegarding more specific tests on the connection between FRP profiles and concrete parts, Gonilha et al. 2014)  performe  performAs can be observed from the literature review, most of the developments in hybrid FRP–concrete structures have been oriented toward implementing beam-like solutions. There are few contributions to research on plane slab typology, in which FRP can even be used as formwork. On this topic, the work by Gai et al.  has to bIn addition, it can be seen that the connection systems between FRP and concrete always rely on epoxy-bonded solutions for previously cast concrete, or on mechanical punctual connectors for cast-in-place concrete. Accordingly, a novel connection system based on embedding a flexible fiber fabric bonded to FRP and mechanically interlocking it with concrete is proposed, tested and analyzed. This is an entirely new technology, to the best of our knowledge, with the closest research considering fiber fabrics together with FRP profiles being found in the work by Sutter et al. , who tesThe current work focuses on the description and mechanical characterization of the structural response of a novel hybrid slab system made with a thin CFRP sheet, which is also the formwork, and a concrete block that is connected through a flexible fiber fabric. An additional connection using particle-based frictional enhancement is also considered and compared. Among the mechanical characterization tasks, the experimental modal analysis is aimed at qualitatively relating the vibrational response with the particular studied typologies of FRP–concrete connection systems. In addition, three-point bending tests are presented to analyze the load-bearing capacity, strain distribution, evolution of the position of the neutral axis and the dissipated energy, among other parameters. The study presented herein analyses, for the first time, the effect of a flexible fabric mesh as a connector between CFRP sheet and concrete, for producing hybrid structures aimed to support bending efforts. The potential benefits of this study include (i) opening the possibility of replacing thin steel sheets with FRP sheets, avoiding the corrosion problems typical of aggressive environments; (ii) characterizing a connection technology based on distributing the load-transferring mechanism to be more compatible with FRP materials whose mechanical connection is always controversial; and (iii) providing the order of magnitude of the load-bearing capacity of thin slabs produced with this novel connection based on flexible fiber fabric. The materials and the manufacturing process can be seen in 3 of Portland cement with continuous 0 to 12 mm of limestone aggregates and plasticizer, and mixed with a water/cement ratio of 0.6 reaching a compressive strength below 25 MPa. This intended low compressive strength was designed to represent a poor cast-in-place material with the future aim of being combined with a natural composite with a lower performance than the CFRP sheets ) was used to connect the CFRP sheet with the concrete in the proposed novel approach. This mesh had a weight of 225 g/m 45 kN/m a. The fi 45 kN/m g and theSolid particles of different sizes were used to increase the friction interface between the CFRP sheet and the concrete c,d. The Eight hybrid slabs were manufactured with four different connection systems used to study the flexural performance of the slabs. In particular, the research was oriented to comparatively investigate the performance of: (i) mechanical connection provided by aggregates with different sizes bonded to the inner surface of CFRP modifying the roughness of the CFRP–concrete interface; (ii) the mechanical connection provided by a flexible fiber FRP mesh that was bonded to the CFRP sheet and that interlocked concrete into their gaps; and (iii) the orientation of the connection fiber mesh and its relationship with load transfer mechanisms. To provide comprehensive data according with these study aims, specimens were defined so to be able to compare the impact of including (G-S-SM) or not including (G-SM) small-size aggregate particles. It was also possible to study the impact of using (G-SM) or not using (G) the proposed novel connection based on flexible fiber fabric. Finally, it was also possible to analyze the impact of the fabric orientation by comparing specimens with straight mesh (G-S-SM) and inclined mesh (G-S-IM). Other parameters like the number of fabric layers, the material of the fabric, the shape of the thin FRP sheet or the type of adhesive used to bond aggregates and fabric with CFRP were not considered in this first study aimed to provide a global assessment of the novel use of a flexible fabric to connect FRP and concrete. All these parameters, among others, should be analyzed in future works. The labeling of the samples describes the connection system and the numbering of the specimens. Numbers at the end of the labeling (1 or 2) define the specimen repetition. Letters define the connection system: G- for bonded gravel particles; G-SM for the combination of bonded gravel particles and straight (0°) oriented glass fiber mesh; G-S-SM for the same as G-SM with the addition of sand particles to the gravel ones; and G-S-IM for the analogous case but with the mesh oriented at 45° with respect to the CFRP sheet nerves. The variety of specimens is defined in In order to study the different connection systems and to assess the novel proposal of using a flexible fiber fabric as a FRP–concrete connector, two types of tests were defined. First of all, experimental modal analysis was aimed to assess the performance of the studied connections in a non-destructive method with the final goal of providing a qualitative comparison. In addition, destructive bending tests were also conducted to provide quantitative data to compare the different studied connection systems. Direct results of these bending tests such as force–displacement curves, CFRP–concrete relative sliding and strain distribution are aimed to describe the mechanical response and the failure mechanism from a global point of view. In addition, some calculated variables are used to provide clear evidences of the failure mechanism , practical information of the equivalent shear transferring capability  or to assess the ductility of the studied connections . Experimental methods are described in detail in this section.From the hypothesis that the connection system affects the shear transmission between the concrete block and the CFRP sheet, it was expected that the vibrational information would also depend on this connection. Experimental modal analysis (EMA) tests were performed to estimate the vibrational modes, frequencies and damping ratios of the specimens with the different proposed connection systems. The main aim was to qualitatively detect the differences between the connection systems on the vibrational responses. This information could be used to pave the way for future research on the non-destructive assessment of the connection systems of thin hybrid CFRP–concrete slabs, although it is outside the scope of the current research.Specimens were placed as a simply supported beams with a free span of 1800 mm. Tests were conducted using impulse excitation based on ISO standard 7626-5 . An impaThree-point bending tests were performed on simply supported specimens to assess their performance with regard to bending and shear concomitance. The modal shapes and associated frequencies (ω) and damping ratios (ζ) were experimentally obtained via modal analysis for each specimen. Different mechanisms for load transfer are supposed to be developed depending on the analyzed specimen. First of all, aggregates (gravel and sand) bonded to the inner surface of the bottom plates of the CFRP sheets are providing a rigid mechanical interlocking with the concrete. This connection system allowed a full load transfer between concrete and CFRP sheet at the beginning of the test up to the failure of this particular connection. The fragile failure of this load transfer mechanism started at the central section (greatest bending moment) and extended to the extremes. Hence, there was little contribution of the aggregates to any partial connection mechanism. Additionally, it has to be noticed that sand particles might not increase the mechanical interlocking as efficiently as particles of larger size such as gravels. According to the empirical evidences of the tests, the little size of sand particles (less than 4 mm) might determine a weaker sliding plane in the CFRP sheet–concrete interface compared to gravel aggregates. Regarding the load transfer mechanism of the fiber mesh, two cases have to be distinguished. Firstly, for straight mesh (G-SM and G-S-SM specimens), the grid geometry provided a flexible interlocking mechanism based on the shear deformation of the transversal fiber tows. The mesh was bonded to the CFRP sheet every 170 mm (top hat) and mechanically connected with concrete on every grid gap. The failure mechanism was based, in this case, in the shear progressive breaking of the transversal fiber tows, which started at the central section (between top hats) and progressed faster to the extreme edges (bonded). Full FRP mesh–concrete interaction and load transfer capability was observed up to this first breaking point. After shear breaking started, vertical deformation of bending specimens increased but load experienced a fall drop. Load was kept constant along the time while the breaking process moved from the mid span section towards the support edges. In the other, for inclined mesh (G-S-IM specimens), the grid geometry provided two concomitant resisting mechanisms: (i) shear resisting mechanism analogous to the case with straight mesh and (ii) axial resisting mechanism of the tows. Thus, fiber tows were subjected to shear and axial stresses at the same time. Due to the geometry, all tows were inclined and all of them contribute to the locking mechanism . As a result, all these tows of fiber were connected to a larger surface of concrete, in comparison with the straight orientation case, and more fiber quantity was involved in the failure mechanism. The observed failure was more progressive than the previous one, because of the concrete–mesh sliding process and the progressive tensile breaking of the mesh. These two mechanisms were allowed by the axial and shear response of the fiber. The corresponding global failure mode involved a larger FRP mesh–concrete connection area than the just the central section, resulting in higher load transfer capacity between these two phases and a higher load-bearing capacity. Complete interaction was only observed at the very beginning of the tests, whereas partial interaction mostly characterized these specimens’ performance (G-S-IM), even before the peak load.Pictures of the different specimens after failure are included in Plots representing the force–vertical displacement response for all the tested cases are shown in All measurements of CFRP–concrete sliding show precisely the same behavior: the relative CFRP–concrete sliding at the extreme edges of the slabs started at the maximum load. Thus, there is no doubt, according to the experimental results, that the failure of all the specimens was due to the loss of connection between the CFRP thin sheet and the concrete block. It is also noticeable that in all cases, except the G-1 specimen, there is an asymmetric response, so the sliding took place in only one of the halves of the specimens. This is the expected response because random imperfections tend to cause the failure to start on one side or the other one, but not on both of them simultaneously. Anomalous symmetric response of G-1 case may be one of the reasons for its greater load-bearing capacity in comparison with G-2 case.The plots gathering the strain–time results of all tested specimens are displayed in Initially, it should be noted that the vibrational response of mechanical systems is influenced by the supports. As all the tested specimens had the same support, the results obtained from the experimental modal analysis  can be uFirst, it is observed see  that theNon-destructive experimental modal analysis tests showed that there was a certain correlation between the dynamical response and the connection system. The inclusion of a connection mesh led to an increase in the natural frequency . The G sIn conclusion, including small-size particles for the roughness improvement of the CFRP–concrete contact surface (G-SM vs. G-S-SM) reduced the dynamic stiffness of the specimens and it was translated in a 12% lower frequency of vibration of the third bending mode. Thus, it is clear that small-size particles affect CFRP–concrete connection. In contrast, adding a straight mesh caused an increase of 20% of the vibration frequency (comparing G-SM and G cases), proving that the inclusion or not of this connection element may be comparatively assessed by experimental modal analysis. Finally, inclining the mesh has little effect (2.8% of the vibration frequency increase) supporting the idea that the orientation of the mesh has no effect in the lineal elastic response and only plays an important role at failure stage. It should be observed that the obtained results show clear trends, despite the significant variability. The differences between sample results are related to the uncertainty of hand manufacture and the complexity of the interaction in the interfaces for shear transmission. This could present a problem for practitioners that pretend to use it at work, and it means that quality control must be mandatory to produce homogeneous specimens. Therefore, the analysis of the impact of the different connection techniques between the CFRP sheets and concrete is mostly qualitative.Focusing on the overall comparison of the maximum load-bearing capacity, it can be observed that including the mesh connection  causes an average increase in the maximum load of approximately 60%. It can also be seen that placing sand, theoretically intended to increase the frictional response, provides the opposite result, reaching lower load-bearing capacities in comparison with the G-S-SM and G-SM specimens. This result is consistent with the observation from experimental modal analysis. Regarding the applied load, it is clear that incorporating a mesh  allowed the slabs to withstand a residual strength after reaching the maximum peak and the corresponding load drop see . Hence, Regarding the inclination of the mesh, it seems to have a positive effect on the load-bearing capacity of the proposed hybrid slabs (G-S-IM vs. G-S-SM). In addition, this case showed a better connection that translated to less bending cracks developing during tests and the failure mechanism that simultaneously mobilized almost all CFRP–concrete contact surfaces of one half of the slab. However, this case (G-S-IM) showed the second greatest load drop due to connection failure after no mesh (G) cases. Therefore, inclining the mesh helps to distribute the connection mechanism reaching higher loads, but also making the whole system more fragile, as the failure involves the full interface. Strain–time plots see  showed tSummarizing the effect of the key parameters on the direct results of the bending tests, it can be stated that including a flexible fabric connector (G-SM vs. G) increased the load-bearing capacity and brought residual strength. In contrast, including small-size particles in the CFRP–concrete contact (G-SM vs. G-S-SM) reduced the load-bearing capacity but allowed some post-peak strength because of higher frictional effect after contact failure. Finally, inclining the connection mesh (G-S_MS vs. G-S-IM) increased the load-bearing capacity and increased the residual strength but made the failure process more fragile. All these effects are related with the largest quantity of fibers simultaneously contributing (and simultaneously failing) to the connection strength of G-S-IM cases. The neutral axis is referenced from the bottom part of the cross-section. When the neutral axis position was over 35 mm, the CFRP sheet was subjected to tensile stresses, and the concrete below the axis was supposed to be cracked. Conversely, when the axis was below 35 mm, the CFRP sheet was partially compressed. This is shown in The corresponding positions of the neutral axis during the linear-elastic branch . This means that after the failure of the interface, a residual friction between the CFRP, the cracked concrete and the mesh still contributed a little, as was shown in Analyzing the effect of the studied parameters, it was observed that including the mesh (G vs. G-SM) allowed to stabilize the position of the neutral axis between 40 mm and 50 mm for a long period of the test up to the maximum load, whereas the cases without mesh showed a clear downward movement of the position of the neutral axis from the very beginning. Among the cases with meshes, no clear tendency is observed, so the response of the position of the neutral axis seems not to depend on the inclination of the mesh.2). This total compressive/tensile force was calculated from the strain distribution and the corresponding force equilibrium.The overall shape of the plots in The particular analysis of the study variables showed that including a flexible fabric connector (G vs. G-SM) increased the residual shear strength (from 30 kPa to 50 kPa). The same effect is observed when inclining the connection mesh (G-S-SM vs. G-S-IM) that doubled the residual shear strength. Finally, adding small-size particles to the CFRP–concrete interface (G-SM vs. G-S-SM) clearly reduced the shear strength (from 0.37 MPa to 0.11 MPa in average) although slightly increased the residual shear strength .After performing an experimental modal analysis and eight bending tests on CFRP sheet–concrete hybrid slabs with four different connection alternatives, including the novel approach of embedding a flexible fiber mesh, the following conclusions were reached:The structural bending response of this type of slab showed three stages. For small loads (approximately 2 kN in these tests), the specimens performed according Navier’s hypothesis with a linear elastic response of the cross-section. For higher loads, the concrete started to crack and the load–displacement response tended to be non-linear. The existence of interlocking elements  helped to keep the complex shear transmission mechanisms between materials. Finally, a peak load was reached, and a sudden failure occurred due to the loss of the connection between the concrete and CFRP sheet. The load dropped down and both materials were separated leaving a residual shear mechanism from the friction of the rough surfaces. Thus, the structural response of this novel connection system between thin FRP sheets and concrete is qualitatively described. Gravel particles bonded to the CFRP sheet surface provided a superficial connection with concrete. The performance of this connection was fragile and presented the lowest values of load-bearing capacity. This connection system as a stand-alone is not recommended for practical applications.The novel inclusion of a flexible glass-fiber mesh directly bonded onto the CFRP, as a connector between the composite sheet and the concrete, allowed an increase in the load-bearing capacity of the slabs, produced residual strength after connection failure and increased the stiffness of the vibrational response of the slab specimens. A qualitative relationship between the vibrational response obtained from the experimental modal analysis and the load-bearing capacity was observed, although further research is required to confirm this fact and to progress its quantification.The inclusion of sand particles bonded to the inner bottom surface of the CFRP had the unexpected effect of reducing the load-bearing capacity and the cohesion between the concrete and the CFRP sheet, although the post-peak residual strength was slightly increased. The inclination of the mesh clearly contributed to uniformly distributing the shear forces in the interface of the materials. This is likely to be because more fibers were oriented in a direction closer to the shear effort to be resisted, compared to the case of a straight position. Therefore, the inclined mesh seems to be the best choice of the two orientations of this novel connection strategy.All CFRP sheet–concrete hybrid slabs failed when the neutral axis moved into the CFRP area (under 35 mm) causing the top part of the CFRP and the connection of the glass fiber mesh, when existing, to be compressed and to definitively separate from the concrete. Even thin CFRP can suffer buckling, as was observed.The amount of cumulative energy corresponds with a qualitative estimation of the capacity of the slabs to dissipate energy and resist damage. The inclined mesh solution was confirmed to be the most competitive one. In conclusion, the main design lines of thin hybrid FRP–concrete slabs based on flexible fiber fabric connectors are provided as results of this study. Two of them have to be highlighted: using aggregates larger than 5 mm to provide roughness to the inner FRP surface and positioning the fiber fabric in an inclined orientation to distribute the load-transfer mechanism after reaching maximum load along a greater area. Finally, this study opens the door to a wide research field on the characterization of the structural response of these type of structures (or other ones based on the novel proposed flexible distributed connectors), including the study of different materials, geometries or adhesives. These results also put the basis for the development of analytical and numerical models of these systems.