Novel Technique for Power Generation in HAWP Systems...a motor to lift the complete airborne system. When the airborne system attains the desired altitude, where high-speed wind flows,

Novel Technique for Power Generation in HAWP

Systems

A. Sheryl Arulini, Dept. of EEE, BIE,

Anna University

Sardar Patel Road, Guindy,

Chennai, Tamil Nadu 600025

Abstract— This paper presents a power generation technique

using high altitude wind power generating system buoyed by a

aerostat filled with light gas by which electrical energy is

extracted with the help of high-altitude streamlined wind. The

best generation and transmission mechanisms that provide the

right power-to-weight (P/W) ratio and efficiency of the overall

system are examined. The differences in weight and total losses

with deviations in generation voltage and pole-pair number

(frequency) of the permanent magnet synchronous generator

have been examined. Also, the design of the tether that

transports electrical power to the ground-based station is

presented. To find the optimal weight of the tether, AC and DC

transmission mechanisms using conductors that use

aluminum/copper are studied and compared. It is found that

aluminum conductor offers better P/W ratio than using the

copper conductor. By means of the detailed analysis of

generation and transmission mechanisms, it is determined that

the optimal electrical power architecture is medium voltage

(MV) AC generation and also transmission. It reveals better

P/W ratio and efficiency in contrast with low-voltage AC

generation and MV DC transmission. The designated electrical

design simplifies the electric system by transporting the power

electronic converter from the aerial unit to the ground base

station and thus the overall P/W ratio is increased by a factor of

7% approximately.

Index Terms— AC Transmission, Dc Transmission, High-

Altitude Wind Power (HAWP), Low Voltage (LV), Medium

Voltage (MV), Permanent Magnet Synchronous Generator

(PMSG), Power-To- Weight (P/W) Ratio, Tether.

I. INTRODUCTION

Solar and wind have emerged as two major sources of renewable energy in the last two decades [1]. Solar power generating system has a lower power density (150–250 W/m2 ) as compared with the power density of conventional thermal power generating system (1000–1200 W/m2 ). Whereas a conventional wind power generating system requires huge civil constructions and suffers from low capacity factor (30%–35%) [2] (capacity factor is defined as the ratio of actual output energy over a period of time to potential output energy, if it were possible for it to operate at the rated power indefinitely) [3]. Due to these reasons, the penetration of renewable energy sources have not significantly increased in present power market [1]. However, true potential of wind power could be extracted using high-altitude wind. The speed of wind increases with the increase in the altitude from the ground surface [4], as expressed in (1). In addition, at higher altitudes, the wind flow is streamlined and consistent in

nature. Since the wind power is proportional to the cube of the wind speed and directly proportional to the turbine area, AT , as mentioned in (2), a large amount of electrical power can be extracted with reduced turbine size.

(1)

(2)

where ρa is density of air, Prated is rated power

of high-altitude wind power (HAWP) generating system, AT

is swept area of rotor blade in m2 , υ0 is the known velocity

of wind in meter per second at earth surface, υ(h) is the speed of wind in meter per second at an altitude h in meter above sea level, υ0 is the known wind speed in meter per second at a known altitude h0 above sea level in meter, CP (λ) is the coefficient of power extraction by the turbine, and α is the Hellman’s coefficient of the surface that depends on the terrain.

Various concepts of harnessing HAWP have been explained

in [5]–[10]. HAWP generation system based on the motor–

generator concept is discussed in [5] and [6]. The turbine

rotor acts as a propeller and electrical machine acts as

a motor to lift the complete airborne system. When the

airborne system attains the desired altitude, where high-

speed wind flows, and electrical machine operates as a

generator and extracts the cross wind power. Consequently,

the extracted wind power is sent back to the ground station

using tether cables. The concept of an airborne wind

turbine (AWT)-cum-generator supported by buoyancy

provided by light gas filled blimp/aerostat is framed in

[7] and [8]. At high altitude above the earth’s surface, a

stationary AWT extracts wind power and sends it to the

ground using suitable power conversion and transmission

mechanisms. The airborne unit can be actuated to move up–

down and sideways to orient itself in the direction of wind

for maximum power extraction. This concept of generating

wind power without using cross wind can generate a power

of two to three times higher than the power generated by the

conventional wind turbine [9]. The electric system for

generating HAWP consists of generation, conversion, and

transmission system in the airborne unit. This paper focuses

on determining the optimal solution for power generation,

International Journal of Engineering Research & Technology (IJERT)

ISSN: 2278-0181

www.ijert.orgIJERTV4IS110408

(This work is licensed under a Creative Commons Attribution 4.0 International License.)

Vol. 4 Issue 11, November-2015

424

conversion, and transmission methods for a HAWP

generating system supported by light filled blimp/aerostat.

Fig.1.Conceptualized and animated figure of blimp supported HAWP

generating system developed by Altaeros Energy [11].

HAWP generating system supported by blimp, as shown in

Fig. 1, can be deployed easily without any grid connection.

Hence, this system can be used for remote power supply,

emergency power requirements, as well as for grid-

connected power supply [8], [9].

The conceptualization and optimization of the designed electric power system for AWT based on generator–motor concept is carried out in [5] and [12]. In [5], the airborne wind power generating system consists of eight generators that work in both generator and motor modes and the complete system is rated for 100 kW. The generation-side and the transmission-side voltage levels are determined for rated 100-kW application. In [13], multi objective optimization of brushless dc motor has been accomplished for solar airplane application. Optimization of permanent magnet synchronous generator (PMSG) for hydraulic lifting system is presented in [14]. The complete design and optimization of the generator for direct-driven wind turbine has been discussed in [15]. The size and weight of direct-driven wind power system has been assessed in [16] for scaling up the power level. Design and optimization of tether for dc transmission have been conducted in [5], but ac transmission mechanism is not included in its study. However, the study of various mechanisms of HAWP generation and transmission system to find the optimal solution for designing a complete electric system has not been investigated yet. This paper finds the optimal solution for generation and transmission of power and decides the requirement of power electronic converters for the airborne unit.

HAWP generating system, which floats at an altitude of 1 km from ground surface, of 10-, 50-, and 100-kW power ratings are considered in this paper. The turbine rotor radii and rotational speeds are determined so as to obtain optimal tip speed ratio (TSR) for maximum power extraction. Furthermore, this paper evaluates the weight, efficiency, and power-to-weight (P/W) ratio of the generator with respect to generation voltage and frequency. At different power levels, optimal generation voltage and frequency are calculated to attain a better compromise between P/W ratio and efficiency of the generator. A tether cable, which serves as a power cable and provides mechanical strength, has been designed for both dc and ac transmissions. In addition, a comparison between ac and dc transmission systems is performed with respect to P/W ratio and overall weight of the tether. Optimal transmission voltage at the

desired transmission efficiency and weight of tether, for various power levels, are calculated and presented in this paper. Depending on the power generation and transmission mechanisms, two different electric power architectures for harvesting HAWP have been proposed and compared. In [5], an electrical system is designed for flying

Fig. 2. Variation of wind power extraction coefficient with respect to TSR.

Electric generator to harvest HAWP and it had an overall efficiency >90% at rated power. In this paper, an electrical architecture for blimp supported HAWP generating system is designed for minimum weight of the overall airborne system with an overall system efficiency >90% at rated power.

Section II calculates optimal rotational speed of rotor to

extract maximum power for three different power levels.

The optimal rotational speed is used to calculate the

optimal number of poles of the generator in Section III.

The section also elaborates on the selection and

optimization of generation mechanism. The design of

electromechanical tether is presented in Section IV. In

addition, the determination of optimal transmission voltage

is carried out in Section IV. Comparison between two

proposed electric systems is explained in Section V.

II. OPTIMAL TSR FOR MAXIMUM POWER

EXTRACTION The wind speed at an altitude h, measured in meter, above

earth surface is given by (1). Considering wind speed of 10 m/s at the ground level, the wind speed υ (h) calculated at an altitude of 1 km is 24 m/s (surface roughness α, 0.3). The power extracted by the AWT as a function of high-altitude wind speed, turbine area, and power extraction coefficient is conveyed by (2). Power extraction coefficient, C P, is not a constant parameter and varies with TSR, λ (maximum limit of 59%). The power extraction coefficient as a function of TSR, λ, is given in [17].

(3)

Fig. 2 shows the variation of power extraction coefficient with respect to variation in TSR. For the proposed HAWP systems, optimum power extraction coefficient is found to be 0.442 at a TSR of 7.1. High rotational speed of the rotor may lead to lessen the power extraction if optimal TSR is not matched. For the rated power, the optimal turbine rotational speed can be obtained using


ISSN: 2278-0181




425

(4)

TABLE I

OPT IM A L ROTAT I O NA L SP EED OF TURBINE ROTOR FOR MAXIMUM POWER EXTRACTION

Power Level (kW)

Rotor Radius (m) Optimal Turbine Rotational Speed (rpm)

10 0.98 1690

50 2.2 755

100 3.1 535

Where R is rotor diameter in meter and wopt is optimal turbine rotational speed in radian per second. For three different power levels, the desired optimal turbine rotational speed is listed in Table I. Three different optimal rotational speeds of the turbine are used to calculate the optimal generation frequencies in Section III. The use of gearbox in the drive train is eliminated due to sufficient rotor speed to drive the generator.

III. POWER GENERATION SYSTEM FOR HAWP

Existing wind power generation systems use induction

genorators (IGs), doubly fed IGs, SGs, and PMSGs

depending on the system requirements. As explained in

[16]–[18], PMSG exhibits better efficiency and P/W ratio

than other types of machines. IG has higher reliability

(due to brushless con- figuration and rugged rotor

design) than PMSG, but IG is not preferred for direct-

driven wind power generating system with variable speed

operation. In addition, PMSG (a brushless machine)

requires less maintenance and, therefore, provides higher

reliability as compared with other brushed synchronous

machines. Hence, PMSG is preferred choice as an airborne

electric generator for HAWP generating system.

Comparative studies of different PMSGs are presented

In [19] and [20]. Radial flux machine (magnets made up of

NdFeB) exhibits better efficiency and low cost than the

axial flux machine [20]. In addition, radial flux machine

allows better cooling, as stated in [13]. Hence, three-phase

radial flux machines with concentrated winding are used as

an electric generator in HAWP generating system.

A. Weight Modelling of PMSG Generator

Optimization of various parameters of PMSG has been carried out in [13] and [14] for several industrial applications. For a HAWP generating system, an efficient and low weight machine is required in order to reduce the weight of the airborne system. Hence, the overall weight and losses in the machine are expressed mathematically to evaluate the efficiency and P/W ratio of the machine.

The overall weight of the machine depends on the

generation voltage and operating frequency (number of

poles in the machine) of the generated power. Total

approximate weight of the PMSG is the sum of weight of

copper, copper insulation, rotor iron, stator iron, and

permanent magnets (weights of external housing and

cooling setup have been excluded from the calculation).

The weight of the machine can be expressed as

Mgen = Mcu + Mins + Mpm + Mrot + Msta (5)

Where Mgen , Mcu , Mins , Mpm , Mrot , and Msta are

estimated weights (in kilogram) of the generator, copper winding, insulating material, permanent magnets, rotor iron, and stator iron of the machine, respectively.

1. Weight of Copper Used in Stator Windings: For the calculation of copper weight of the machine, total volume of copper used in the stator windings is required. Thus, the radius of the copper strand needs to be estimated to calculate the volume of copper used. The radius of the copper wire is a function of generated phase voltage, copper current density, and rated power. The radius of copper strand, rc , total volume, Vcu , and the weight, Mcu , of the copper used in the windings of the machine are given by [21].

(6)

(7)

(8)

where N is the total number of turns in each phase, r is the radius of the copper winding in meter, Vph

is generated phase voltage (in volts), ρcu is the density of

copper in kg/m3, Jcu, Vcu , and σcu are current density of

copper in A/m2, volume of copper in m3, and electrical rc

=conductivity of copper in seimen/meter, respectively.

2. Weight of Insulations Used in Stator Windings: :

The winding insulation also contributes to the overall

weight of the machine. The weight of insulating material

depends upon the thickness of insulating material and total

length of copper strand. The thickness of the winding

insulation depends on the generated phase voltage of the

machine. The thickness of insulating material required and

the estimated weight of insulation are calculated using [9],

[22]

(9)

(10)

where tins is the thickness of winding insulation in

meter, ρins is the density in kg/m3, and S dielectric

constant in volt per meter of the insulation material.


ISSN: 2278-0181




426

3. Weight of Permanent Magnets: The weight of

permanent magnets is another component in overall

weight of the machine. It depends on the shaft speed,

pole-pair numbers, and properties of magnet. The

mathematical expression for estimation of the weight of

permanent magnets used in the generator is expressed as

[23], [24]

(11)

Where, ρpm is the density of the magnet used in kg/m3, Cv is the coefficient of utilization of permanent magnet, Ns is rotational speed of generator shaft in revolution per minute, Hc is maximum coercive force of the magnet in ampere per meter, and P is total number of pole pair in the generator.

4. Weight of Rotor Iron: The rotor iron adds up to the

overall weight of the machine. The volume of rotor iron

depends mainly on the rotational speed, tangential stress, and

density of iron. The volume, Vrot, and mass of the rotor iron

used, Mrot , are given by [23], [24]

(12)

(13)

where ρiron and σF tan are density in kg/m3 and tangential

stress in pascal of rotor, respectively.

5. Weight of Stator Iron: Stator iron is the heaviest component in the generator. The volume, Vsta, and the weight, Msta , of the stator iron pivot mainly on the rated power of the machine are stated in [23] and [24]. In addition, maximum flux, output coefficient, winding coefficient, current density, and turns of copper windings also affect the volume of stator iron used in the machine. The expression for stator iron volume and weight can be expressed as [23], [24]

(14)

(15)

where ts is the estimated thickness of stator iron in meter,

L in is the axial length, and Din is the inner diameter of the

machine in meter. The ratio of inner diameter to the length of the machine is assumed to be 4:5 [23], [24].

The output coefficient, inner diameter, and axial length of the generator are required to calculate weight of the stator iron. The output coefficient, inner diameter, and

axial length to estimate the weight of stator are given by [23], [24]

(16)

(17)

where σ p is output coefficient, e is armature reaction

factor of the generator, kw is winding coefficient (0.9–1),

and Am is linear current density in ampere per meter of

PMSG.

The required number of turns of the copper winding

determines the weight of copper inside the machine. Since

air-gap flux density of the machine is related to the

turns of the copper required, air-gap flux density needs

to be estimated to calculate the turns ratio of the copper

winding [23], [24]. The air-gap flux density depends on the

magnetic properties of permanent magnets and dimension of

the machine. The air-gap flux density can be expressed as

[23], [24]

(18)

where φg is air-gap flux density of the machine in Wb/m2.

The number of turns required for the machine depends on

the flux, φg , phase voltage, Vph, and operating speed, Ns , in

revolution per minute and can be expressed by [23], [24]

(19)

Finally, from the required turns of the winding using (19),

the weight of copper and insulation can be

estimated by (6)–(10). Similarly, the weight of permanent

magnets, rotor iron, and stator iron are calculated using (11),

(13), and (15). Hence, the overall weight of the machine

as a function of generation voltage and number of pole

pair (operating frequency) for various power ratings, of the

machine can be estimated using (5).

B. Loss Modelling of the generator

For evaluation of total losses of PMSG, losses are divided

into copper loss and iron loss. Copper loss depends on the

volume of copper used and iron loss depends on the

maximum flux, operating frequency, and volume of the stator

iron [25]. Losses of the machine are calculated as [5], [25]

Pcu = 3 I 2 Rph = J 2 Vcu /σcu (20)


ISSN: 2278-0181




427

Fig. 3. Variation of the weight of the generator with respect to generation voltage for 100-kW power level.

Fig. 4. Variation of the weight of the generator with respect to number of pole pair for 100-kW power level.

(21)

Ploss = Pcu + Pir (22)

where Pcu, Pir, and Ploss are copper, iron, and total losses (in

watts) of PMSG, respectively, Iph and Rph are the phase

current in ampere and phase resistance in ohm of PMSG

machine, respectively, Kh and Ke are hysteresis and eddy

current constants of the permanent magnet, respectively, and

Bmax, β, ωs, and Vsta are maximum flux density in W/m2,

Steinmetz constant, generated frequency in radian per

second, and the total volume of stator iron in m3,

respectively. The efficiency ηg (which depends on the

overall losses of the machine) and P/W ratio (which depends

on the overall losses and weight of the machine) for PMSG

are expressed as

(23)

(24) Using (23) and (24), the variation of generator’s efficiency

and P/W ratio with respect to the variations of generated phase voltage and number of pole pair (operating frequency) at different power levels can be assessed.

C. Optimal generation voltage and Operating frequency of

PMSG

The constraints taken into account to find the

optimal operating point for the generator are as follows:

Fig. 5. Weight variation of 100-kW generator with respect to generating

voltage and number of pole pair.

1) generator efficiency should be >95% that allows sufficient forced air cooling of the generator without thermal runaway.

2) Total number of pole pairs should not exceed 30 [5]. Higher number of pole pair increases complexity in manufacturing and cost of PMSG. In this section, generator’s weight, losses, and P/W ratio are evaluated against the variation of pole-pair number and generation voltage using [5]-[24]. The major equations involved for the generation of plots are as follows:

1) equations (5), (8), (10), (11), (13), and (15) are involved

for evaluating weight;

2) equations (20)–(22) are involved for evaluating losses;

3) equations (5) and (22)–(24) are involved for evaluating

P/W ratio.

The total weight of the generator is mainly decided by the generating voltage and the operating frequency (pole pairs). The additional requirements of the insulations with an increase in the generation voltage increase the weight of the generator. However, increment in the weight of the generator due to increase in the generation voltage level is not significant

<5000 V, as shown in Fig. 3. The percentage increase in the generator weight is ∼20% when generation voltage increases from 500 to 5000 V.

Fig. 4 shows the variation of generator weight with respect to the number of pole pair. Increase in the pole-pair number increases the operating frequency of the machine, which decreases the weight of the required stator iron, rotor iron, and permanent magnets of the machines. However, the rate of reduction in the weight of the generator due to increase in pole pairs is small at pole-pair number >30, as shown in Fig. 4. When pole-pair number increases from 5 to 30, the weight of generator decreases by 80%. However, the increase in pole pairs from 30 to 40 gives reduction on the generator weight by 4% only. The deviation in the generator weight with respect to pole pair and generation voltage is shown in Fig. 5. The generator exhibits the least weight at low generation voltage and high pole-pair number.


ISSN: 2278-0181




428

Fig. 6. Variation of the generator losses with respect to pole pairs for 100-

kW power level.

Fig. 7. Variation of generator losses with respect to generation voltage

for 100-kW power level.

The core loss of the generator depends on the volume of stator iron and operating frequency. Increase in the pole number of the machine increases the operating frequency, which leads to substantial increase in core loss of the machine. However, copper loss of the machine is independent of pole number of the machine. Equations (20)–(22) are employed to study the loss variation of generator with respect to pole-pair number and generation voltage. The effects of pole number on core loss, copper loss, and total losses of the machine are shown in Fig. 6.

The copper loss of the machine depends on the volume of copper used in the windings, as expressed in (20). At low generation voltage, thicker copper wire is used, which has a lower number of turns. In contrast, at high generation voltage, thinner copper wire is used, but it requires a large number of turns. Hence, at any generation voltage, volume of copper used is constant, resulting in constant copper loss of the machine. The core loss of the machine has no relation with the generation voltage. Fig. 7 shows the relation of various losses with respect to generation voltage. Fig. 8 shows the total losses of the machine with respect to generation voltage and pole-pair number.

Efficiency and P/W ratio of the machine are calculated using (23) and (24). Fig. 9 shows the decreasing trend of the P/W ratio of the machine with the increase in generation voltage. However, the rate of decrease in the P/W ratio with the increase in generating voltage is minimal up to 5000 V. The effect of pole-pair number on the P/W ratio of the machine is shown in Fig. 10. At very low pole number, P/W ratio of

Fig. 8. Total losses of the generator as the function of generating voltage and number of pole pair for 100-kW power level.

Fig. 9. Variation of P/W of the generator with respect to generation voltage for 100-kW power level.

Fig. 10. Variation of P/W of the generator with respect to pole pairs

for 100 kW power level.

the machine is very low; but it increases sharply and saturates, maintaining nearly constant P/W ratio in the order of 800 W/kg at pole-pair number >30. Higher pole-pair number leads to higher operating frequency that reduces the size of rotor, stator, and permanent magnets in the machine. However, higher pole pair number increases the core loss of the machine that limits the increment in P/W ratio.

The change in P/W ratio of the machine for three different power levels of HAWP system is shown in Figs. 11–13. It i s clear t h a t l o w -voltage ( LV) m a c h i n e g i v e s b e t t e r P/W ratio than medium voltage (MV) machine. Nonetheless, the operating voltage of the machine can be noticeably increased with slight compromise in P/W ratio of the machine, as shown in Figs. 11–13.


ISSN: 2278-0181




429

Fig. 11. P/W ratio variation of 100-kW generator with respect to generating

voltage and pole pair.

Fig. 12. P/W ratio variation of 50-kW generator with respect to generating

voltage and pole pairs.

Table II shows operating points of HAWP generating

systems without constraints. The P/W ratio depends on the

total losses and the weight of the machine. Total losses are

subjected to the volume of iron, volume of copper and

operating frequency. The gross weight of the machine relies

on the weight of iron, permanent magnets, copper, and

insulating material. The optimal operating points for different

power level HAWP generating systems are listed in Table II.

The P/W ratio of the machine increases with the increase in

the rated power up to 50 kW. However, in the range of 50–

100 kW, increase in rated power does not have significant

effect on P/W ratio. The operating points indicated in Table II

with bold letters violate the constraints, i.e., efficiency <95%

and pole-pair number >30.

Table III shows the optimal operating voltage and frequency for three different output power levels, when generator is subjected to specified constraints. As a consequence, the P/W ratio is reduced and it does not have a linear relation with the rated power. The P/W ratio increases with increase in power rating up to 50 kW and then saturates, thus maintaining

Fig. 13. P/W ratio variation of 10-kW generator with respect to generating voltage and pole pairs.

TABLE II OPTIMAL GENERATION FOR VARIOUS POWER LEVELS HAWP SYSTEMS

WITHOUT CONSTRAINTS

Power Level

Generator Weight

P/W ratio

Overall loss

Generator 𝝶

10 22.5 414 680 92.8

50 43 1053 4700 90.6

100 86 1062 8700 91.3

TABLE III OPTIMAL GENERATION PARAMETERS FOR VARIOUS POWER LEVELS HAWP

SYSTEMS WITH CONSTRAINTS (EFFICIENCY OF 95%)

Power Level Generator Weight

P/W ratio Generated frequency

10 23 413 86

50 45 1006 106

100 119 800 85

constant P/W ratio irrespective to power rating. The

variation of P/W ratio with power levels, with and without

constraints, is shown in Fig. 14. The detailed study of

PMSG in this section leads to the following conclusions:

1) three-phase LV generation using PMSG gives better

P/W ratio for the machine;

2) higher number of pole pair reduces the weight

but increases the losses of the machine;

3) Generation voltage can be significantly increased

with slight compromise in P/W ratio of the machine.

Fig. 14. P/W ratio variation of the generator with respect to power rating of the machine.


ISSN: 2278-0181




430

Fig. 15. Designed tether cable for (a) dc transmission and (b) ac

transmission.

IV. TRANSMISSION IN HAWP GENERATING SYSTEM

The tether serves as power transmission cable as

well as it provides mechanical strength that holds the

airborne unit. In addition, the tether is used to deploy and

move the blimp to the required location. Hence, it should

have enough mechanical strength to support the blimp,

better electrical efficiency for efficient power transmission,

and should be able to withstand adverse environmental

conditions.

A tether designed for the blimp supported HAWP

generating system consists of four major components:

1) Core to provide tensile strength;

2) Conductors to transmit power to ground base station 3) Insulator to insulate the power cables;

4) Outer jacket layer for physical protection.

The use of coaxial cable forms a kink due to continuous

stress and strain [5]. Therefore, tether geometry, as

shown in Fig. 15(a) and (b), is used for dc and ac

transmission.

A. Tensile Strength of the Core

Aramid (Kevlar-49) and high modulus poly-

ethylene (HMPE) are two options that can be used for high

tensile strength at low specific weight. Kevlar-49 has a

Fig. 16. Decrease in conductor radius with respect to increase in the transmission voltage for Al and Cu conductors for 100-kW power level.

Fig. 17. Increase in insulation thickness with respect to increase in the

transmission voltage for 100-kW power level.

tensile strength of 3620 Mpas at a specific weight of 1.44

and HMPE has a tensile strength of 2400 Mpas at a specific

weight of 0.97. Kevlar core exhibits better strength-to-weight

ratio as compared with HMPE, henceforth, Kevlar core is

used. The required diameter of the core and the weight of the

Kevlar core are expressed as

(25)

(26)

(27)

(28)

(29)

where Fx and Fy are the horizontal force and vertical force

in N acting on the blimp due to wind gusts, Ab is the cross-

sectional area of blimp in m2 , T is the tension acting on the

tether cable, a is the intended upward acceleration of the

blimp in m/s2 , me is the total mass of nongaseous items in

the blimp in kilogram, m g is the mass of gas inside blimp

in kilogram, ρg is the density of filled gas in kg/m3, and T

is the tension on the tether in N . Ts , ρKev , dKev , and WKev

are the tensile strength in Pa , density in kg/m3, diameter in

meter, and the overall weight in kilogram of Kevlar core

used.


ISSN: 2278-0181




431

B. Design of Transmission Cable

Conductor cables are either made up of aluminum or

copper material, which are insulated using cross linked

Fig. 18. Variation in P/W ratio of tether with respect to transmission

efficiency and transmission voltage for dc transmission with Al/Cu

conductor for 100-kW power level.

Polyethene. Extra protection for the cables is provided with

outer jacket layer made up of elastomer and synthetic fibers

[5]. Transmission voltage determines the weight of the

conductor. Higher transmission voltage reduces the conductor

volume, as shown in Fig. 16. However, higher transmission

voltage requires thicker insulation, as shown in Fig. 17.

Therefore, optimal transmission voltage needs to be

determined that gives a better P/W ratio and desirable

transmission efficiency for both ac and dc transmission

system.

The radius of the conductor cable, thickness of insulation,

weight of single cable, and overall weight of tether

for dc transmission are expressed in [8], [9] and [22]

(30)

(31)

(32)

(33)

Similarly, for ac transmission radius of the conductor cable,

thickness of insulation, weight of single cable, and

overall weight of tether for ac transmission are

expressed in [8], [9] and [22]

(34)

(35)

(36)

(37)

where rc is conductor radius in meter, L is length of

tether in meter, n is opted transmission efficiency, σc is

conductivity of the conductor in seimen per meter, V

is transmission voltage level in volts, S is dielectric

strength of insulator in volt per meter, tins is thickness of

insulator in meter, ρcon is density of conductor in kg/m3,

ρin is density of insulator in kg/m3, and Wcab and Wjack

are the weight of single cable and outer jacket layer.

Using (25)–(37), different transmission mechanisms using

copper and aluminum conductors are thoroughly

studied in Section IV-C.

C. Tether Weight Optimization The weight of the transmission cable is determined by the required transmission efficiency and optimal transmission voltage. The variation of P/W ratio of aluminum and copper conductor tethers with respect to transmission efficiency and voltage is shown in Fig. 18. It shows that aluminum conductor exhibits a slightly better P/W ratio than that of the copper conductor for power transmission. Hence, aluminum conductor, which is cheaper than copper conductor, is a better choice for power transmission of HAWP generating system. The weight of single cable and the optimal transmission voltage is less in the case of ac transmission system than dc

transmission system, as shown in Fig. 19. DC transmission

system utilizes two p o w e r c a b l e s , wh e r e a s ac

t r a n s m i s s i o n s y s t e m u s e s three power cables for

transmission. Therefore, ac transmission system requires

extra power cable with insulation and increased tether jacket

volume. In consequence, dc transmission system exhibits

slightly better P/W ratio and low weight than ac transmission

system, as shown in Fig. 20.

The conductors inside the tether cable have polyethene

insulation and elastomer external jacket for physical

protection. In addition, no internal cooling mechanism is

present for the power cables. Therefore, transmission

efficiency >97% is preferred in HAWP generating system

[8], [9] that allows sufficient cooling of the power cable to

prevent thermal meltdown. The optimal transmission

efficiency at the best P/W ratio of tether is ∼88%;

however, optimization of tether cable is carried out at an

efficiency >97%. The P/W ratio of tether is compromised

to allow for sufficient temperature rise in tether cable.

Tables IV and V depict the comparison between aluminum and copper conductor in terms of P/W ratio, tether

weight, and optimal transmission voltage at different

transmission efficiencies, for ac and dc transmission

systems. From these tables, it can be concluded that the use

of aluminum conductor gives slightly. Better P/W ratio than

copper and DC transmission shows slight edge over ac

transmission in terms of P/W ratio but at higher

transmission voltage. From the detailed study of tether

optimization, it can be concluded that:


ISSN: 2278-0181




432

1) Aluminum conductor exhibits better P/W ratio than

copper conductor

2) DC transmission system exhibits better P/W ratio than

AC transmission system;

3) optimal transmission voltage for dc transmission

system is higher than ac transmission system.

Fig. 19. Variation in tether weight with respect to transmission voltage

for dc/ac transmission with aluminum conductor for 100-kW power level.

Fig. 20. Variation in P/W ratio of tether with respect to transmission

efficiency and transmission voltage for dc/ac transmission with aluminum conductor for 100-kW power level.

Table Iv

Opt Im A L Generat I O N Voltage And T Ether W Ei Ght F O R A luminum

And Co Ppe R Conductor F Or Ac T Rans Mi S S I O N

Fo R 100-Kw Pow E R Level

Transmission efficiency

Optimal Voltage

Tether weight P/W ratio

Al Cu Al Cu

97 4400 148 160 655 605

98 4800 160 175 608 560

98.5 5200 172 189 572 520

Fig. 21. Electric system of HAWP system with LV-ac generation and MV-

dc transmission.

Table V

Opt Im A L Generat I O N Voltage And T Ether W Ei Ght F O R Alumi Num

And Co Ppe R Conducto R F Or Dc T Rans Mi Ssi On

Fo R 100-Kw Power Level

Transmission

efficiency

Optimal

Voltage

Tether Weight

Al Cu

P/W ratio

Al Cu

97 6800 140 151 689 639

98 7600 154 167 638 586

98.5 8100 162 178 605 552

Table Vi Opt Im A L Generat I O N Voltage And T Ether W Ei Ght F O R A lumi Num

And Co Ppe R Conducto R F Or Ac T Rans Mi S S I O N

Fo R 100-Kw Pow E R L Evel

DC (Transmission efficiency 98.5%)

Power level

Optimal

Transmission

Voltage

Tether Weight

P/W ratio

10 4600 83 117

50 6800 129 380

100 8100 162 572

V. SELECTION OF OPTIMAL ELECTRIC SYSTEM

FOR HAWP GENERAT ING SYSTEM

As calculated in earlier sections, three-phase LV ac (LV-

ac) generation gives maximum P/W ratio of the generation

system and MV dc (MV-dc) transmission gives maximum

P/W ratio for transmission system. However, an airborne

power electronic conversion (PEC) system is required to

transform LV- ac power into MV-dc power. This

additional PEC increases the weight of the airborne

system and reduces the overall efficiency of the system.

Based on the studies performed in the previous sections,

the proposed electrical architectures are:

1) Electrical system with LV-ac generation and MV-

dc transmission;

2) Electrical system with MV ac (MV-ac) generation

and MV-ac transmission.

A. Electrical System With LV-AC Generation

and MV-DC Transmission

The schematic of electric system of the HAWP system with

LV-ac generation and MV-dc transmission is shown in Fig.

21. It consists of an additional airborne PEC that converts

LV-ac power into MV-dc power efficiently. This type of

electrical


ISSN: 2278-0181




433

Fig. 23. Increment in the weight of generator with respect to

generating voltage for 10-, 50-, and 100-kW power level.

Architecture is explained in [5] and [8]. In order to

maintain the overall system efficiency >90%, transmission

efficiency Table Ix

Optimal Transmission Parameters For Ac Transmission With Aluminum Conductor

When Transmission Voltage Matches Generation Voltage

AC transmission efficiency of (97%)

Power

Level Optimal

Voltage Tether

Weight Decrement

in tether

weight

P/W

ratio

10

2500

77

6

126

50

3700

118

11

410

100

4400

148

14

655

has to be maintained at 98.5% to compensate for the losses

in PECs used in the airborne system. The optimal

transmission parameters at a transmission efficiency of

98.5% are listed in Table VI. This electrical system uses LV

machine with better P/W ratio than MV machine. However,

the use of airborne PEC and requirement of higher

transmission efficiency reduces the overall P/W ratio of

electrical system. The estimated P/W ratio and efficiency

for airborne PECs are 4 kW/kg and 96%, respectively, [5].

Table VII presents the overall efficiency, weight, and P/W

ratio of complete airborne electrical system that includes

power generation, power conversion, and power transmission

for HAWP generating systems.

B. Electrical System With MV-AC Generation

and MV-AC Transmission

The proposed electric system with MV-ac generation and

MV-ac transmission is shown in Fig. 22. The proposed

Fig. 22. Electric system of HAWP system with three-phase MV-ac transmission.

System does not use any PEC in the airborne system. As

stated in the previous sections, the generator losses

remain constant with increase in the generation voltage.

Generation voltage of the machine can be increased

significantly with a slight increase in the weight of the

machine without any increase in total losses. Hence, MV

generation can be used in HAWP generation system with a

slight compromise in P/W ratio of the machine. The degree

of change in the weight of the generator with respect to

the change in generation voltage at various power levels

is shown in Fig. 23. The generation voltage of the

machine is increased to match the optimal transmission

voltage, which does not surge the weight of generator

substantially, as presented in Table VIII. In addition, the

generation-side voltages and corresponding increase in

weight of the generators for various power levels have been

demonstrated in the same table. For an ac transmission

system, transmission efficiency can be reduced up to 97%.

This increases the P/W ratio and decreases the weight of

tether wire as compared with that required in dc

transmission system (at 98.5% efficiency). In addition, it

reduces the optimal transmission voltage to a lower value

that matches the generation voltage. The increment in the

machine weight due to MV-ac generation is compensated by

the decrement in tether weight. The reduced optimal

transmission voltage and weight of the tether at 97%

transmission efficiency are shown in Table IX. The overall

efficiency and the P/W ratio of the system at different

power levels are presented in Table X. The efficiency and

P/W ratio for both the proposed systems can be compared

from Tables VII and X. The MV-ac generation with MV-ac

transmission exhibits better P/W ratio and efficiency than

LV-ac generation with MV-dc transmission. The P/W ratio

and efficiency are increased by ∼7% and 2%, respectively,

using MV-ac generation as well as transmission instead of

using LV-dc generation and MV-dc transmission, as

m e n t i o n e d in T a b l e s V I I a n d X . Hence, the use

of MV-ac generation and MV-ac transmission in HAWP

generating system provides an edge over previous

system with simple electrical architecture without PEC in the

airborne unit. Therefore, the need for complex design and

control of airborne PEC is not required. For electrical

isolation of HAWP system with the grid, isolated PECs

can be used in the ground-based station.

VI. CONCLUSION

HAWP generating system requires optimal electrical

power architecture for power generation and transmission

mechanism. The reduction in the weight of complete airborne

system reduces the volume of light gas (H2 or He) required

for buoyancy. Optimal turbine rotational speed is determined

for maximum extraction of electrical power from high-

altitude wind. The turbine shaft is directly connected to the

generator shaft without using gearbox. Optimal rotational

speed is used to calculate optimal operating frequency of the

generator. Radial flux PMSG that exhibits better P/W ratio

than for other machines is used as the airborne generator. The

variations of weight and losses of PMSG at various


ISSN: 2278-0181




434

generating voltage and frequency (pole-pair numbers) are

evaluated. Increase in generation voltage decreases the P/W

ratio of the machine. An increase in pole-pair number reduces

the size of generator but increases the core loss of the

machine that limits the P/W ratio. Similarly, tether for ac and

dc transmission are proposed and compared in terms of

overall weight. For desired transmission efficiency, optimal

transmission voltage is determined and corresponding tether

weight is calculated. It is found that aluminum conductor

yields better P/W ratio than copper; hence, it is used as power

transmitting cable for HAWP. Two different electrical

systems are proposed:

1) With MV-ac generation and MV-ac transmission and 2)

with LV-ac generation and MV-dc transmission. The PMSG

exhibits better P/W ratio at LV generation and optimal

pole pair number (as calculated in this paper). Since the

impact of generated voltage level on P/W ratio is not that

substantial, the generation voltage is increased to match the

transmission voltage; the system yields simple electrical

power architecture without PEC in airborne unit. Thus, the

optimal electrical power architecture for HAWP generating

system consists of MV-ac generation and MV-ac

transmission mechanism. This architecture gives the overall

benefits on system efficiency by ∼2% and P/W ratio by 7%

over LV-ac generation and MV-dc transmission mechanism.

REFERENCES

[1] J. Sawin, “Renewables global status report,” Ren21 Steering Committee, Paris, France, Tech. Rep. REN21.2012, 2012.

[2] G. M. Masters, Renewable and Efficient Electric Power Systems. New York, NY, USA: Wiley, 2004.

[3] UMass. (Aug. 2014). UMass Amherst. [Online]. Available: http://www.umass.edu/windenergy/publications/published/communityWindFact Sheets/RERL_Fact_Sheet_2a_Capacity_Factor.pdf

[4] M. L. Loyd, “Crosswind kite power,” J. Energy, vol. 4, no. 3, 1980, Art. ID 80–4075.

[5] J. W. Kolar et al., “Conceptualization and multiobjective optimization of the electric system of an airborne wind turbine,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 1, no. 2, pp. 73–103, Jun. 2013.

[6] B. W. Roberts et al., “Harnessing high-altitude wind power,” IEEE Trans. Energy Convers., vol. 22, no. 1, pp. 136–144, Mar. 2007.

[7] M. Garcia-Sanz, N. White, and N. Tierno, “A novel approach to airborne wind energy: Overview of the EAGLE system project,” in Proc. IEEE Nat. Aerosp. Electron. Conf. (NAECON), Jul. 2011, pp. 167–172.

[8] J. Adhikari, A. K. Rathore, and S. K. Panda, “Harnessing high altitude wind power using light gas filled blimp,” in Proc. 39th IEEE IECON, Nov. 2013, pp. 7163–7168.

[9] J. Adhikari and S. K. Panda, “Overview of high altitude wind energy harvesting system,” in Proc. 5th IEEE PESA, Dec. 2013, pp. 1–8.

[10] J. Adhikari, A. K. Rathore, and S. K. Panda, “Modular interleaved soft-switching DC-DC converter for high-altitude wind energy application,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 2, no. 4, pp. 727–738, Dec. 2014.

[11] (Aug. 2014). Altaeros Energies. [Online]. Available:http://www.altaerosenergies.com/wind.html.

[12] R. A. Friedemann, F. Krismer, and J. W. Kolar, “Design of a minimum weight dual active bridge converter for an airborne wind turbine system,” in Proc. 27th Annu. IEEE Appl. Power Electron. Conf. Expo. (APEC), Feb. 2012, pp. 509–516.

[13] P. Ragot, M. Markovic, and Y. Perriard, “Optimization of electric motor for a solar airplane application,” IEEE Trans. Ind. Appl., vol. 42, no. 4, pp. 1053–1061, Jul./Aug. 2006.

[14] T. Wang and Q. Wang, “Optimization design of a permanent magnet synchronous generator for a potential energy recovery system,” IEEE Trans. Energy Convers., vol. 27, no. 4, pp. 856–863, Dec. 2012.

[15] R. Vermaak and M. J. Kamper, “Design aspects of a novel topology air- cored permanent magnet linear generator for direct drive wave energy converters,” IEEE Trans. Ind. Electron., vol. 59, no. 5, pp. 2104–2115, May 2012.

[16] G. Shrestha, H. Polinder, and J. A. Ferreira, “Scaling laws for direct drive generators in wind turbines,” in Proc. IEEE Int. Electr. Mach. Drives Conf. (IEMDC), May 2009, pp. 797–803.

[17] R. Melício, V. M. F. Mendes, and J. P. S. Catalão, “Power converter topologies for wind energy conversion systems: Integrated modeling, control strategy and performance simulation,” Renew. Energy, vol. 35, no. 10, pp. 2165–2174, 2010. [Online]. Available:http://www.sciencedirect.com/science/article/pii/S0960148110001114

[18] H. Polinder, F. van der Pijl, G.-J. de Vilder, and P. Tavner, “Comparison of direct-drive and geared generator concepts for wind turbines,” in Proc. IEEE Int. Conf. Electr. Mach. Drives, May 2005, pp. 543–550.

[19] J. Wang, Z. P. Xia, D. Howe, and S. A. Long, “Comparative study of 3-phase permanent magnet brushless machines with concentrated, distributed and modular windings,” in Proc. 3rd IET Int. Conf. PowerElectron., Mach. Drives, Apr. 2006, pp. 489–493.

[20] A. Parviainen, M. Niemela, J. Pyrhonen, and J. Mantere, “Performance comparison between low-speed axial-flux and radial-flux permanent- magnet machines including mechanical constraints,” in Proc. IEEE Int. Conf. Electr. Mach. Drives, May 2005, pp. 1695–1702.

[21] A. Husain, Fundamentals of Electrical Engineering. New Delhi, India: Dhanpat Rai Pub., 2012.

[22] (Jan. 2014). Microwave Information Resource. [Online]. Available: http://www.microwaves101.com/encyclopedia/coax_power.cfm

[23] J. F. Gieras and M. Wing, Permanent Magnet Motor Technology: Design and Applications. New York, NY, USA: Marcel Dekker, 2002.

[24] J. Pyrhonen, T. Jokinen, and V. Hrabovcova, Main Dimensions of a Rotating Machine. New York, NY, USA: Wiley, 2008, pp. 281–300. [Online]. Available: http://dx.doi.org/10.1002/9780470740095.ch6

[25] C. Mi, G. R. Slemon, and R. Bonert, “Modeling of iron losses of permanent-magnet synchronous motors,” IEEE Trans. Ind. Appl., vol. 39, no. 3, pp. 734–742, May/Jun. 2003.


ISSN: 2278-0181




435

Documents

Novel Technique for Power Generation in HAWP Systems...a motor to lift the complete airborne system. When the airborne system attains the desired altitude, where high-speed wind flows,