Plasma Energy Conversion System for Electric Power Generation

A.O Ayeleso, M.T.E Kahn and A.K. Raji

Abstract-In the conventional conversion system, a large amount of energy (about 60%) is required to generate electric power for industrial applications and commercial usage. As a result, there is a need for more energy conversion systems which can be used to produce reliable and efficient electrical power. In this paper, the

present study focuses on the direct energy conversion systems such

as magnetohydrodynamics (MHD) and plasmadynamic (PDC). In these systems, a plasma source is directly converted into electrical energy without the use of any mechanical energy. Furthennore, the electrical power generated from these systems is very efficient and large loss of energy is greatly minimised. The objective of the

present study is to develop an improved MHD energy conversion system based on the principle of Faraday's Law of electromagnet­

ism and plasma physics. The testing of this system will be explored using the available plasma sources in the Western Cape, South Africa. These sources may include gas discharge fluorescent light, flames, gas laser, solar wind, aurora and earth's ionosphere. Another objective of the study is to use numerical simulations (1-dimensional and 2-dirnensional MHD models) to study the

dynamics of plasma fluid flowing through a rectangular MHD generator channel and a conducting magnetic field.

Index Terms- Conventional conversion systems, electric

power, Faraday's Law of electromagnetism, plasma sources

Magnetohydrodynamics, Plasmadynamic, plasma physics.

1 INTRODUCTION

Power consumption is becoming one of the major

challenges affecting many industrial and commercial users.

The fossil fuel and other conventional resources that are currently being used for power generation are not sufficient for industrial applications and commercial usage [1],[2],[3].

Furthermore, in the conventional energy conversion system

(hydroelectric and thermodynamics), a large amount of

energy (about 60%) is required when converting thermal energy into mechanical energy. Therefore, there is a need

for more research to discover new technologies which can

be used for converting different forms of energy into

electrical power. One of the new technologies is the direct

energy conversion system (the magnetohydrodynamics (MHD) and plasmadynamic (PDC». The MHD and PDC

systems uses plasma energy to generate efficient electric

power [1],[4]. These systems convert plasma energy

(primary energy) directly into electrical energy without passing through any mechanical energy stage [1],[5]. The

focus of the present study is to develop an efficient system

Mr. A. O. Ayeleso, Electrical, Electronic & Computer Engineering, Cape Peninsula University of Technology, P O Box 1906, Bellville 7535, Cape Town , South Africa (e-mail: 211014788@mycput.ac.za).

Prof. M.T.E. Kahn, director, Energy Research Centre, Electrical, Electronic & Computer Engineering, Cape Peninsula University of Technology, Bellville campus, (e-mail: khant@cput.ac.za).

Dr. A. K. Raji, Electrical, Electronic & Computer Engineering, Cape Peninsula University of Technology, Bellville campus, (e-mail: rajia@cput.ac.za).

that would convert plasma energy into electric power. The

electric power generated from the plasma can be used in

vehicles, houses, satellites and industries. Fig. 1 depicts the

stages of the direct energy conversion system.

Exolhermic Energy Convers�n

Primary Energy -+ Chemical Reaclion f-+ Thermal Energy -+ Syslem

(MHD/PDe)

�

Electrical Energy

Fig. I. The stages of the direct energy conversion system

2 DIRECT ENERGY CONVERSION SYSTEM

The direct energy conversion system can be grouped into two categories: The PDC and MHD conversion systems.

2.1 Plasmadynamic conversion system

The PDC conversion system is a method of converting

thermal energy of plasma into electric power without plasma flow. This method begins when two floating conducting

electrodes are immersed into high temperature plasma. One

of the electrodes is magnetised while the other is unmagnetised. By measuring the potential difference

between the two electrodes, the output voltage can be

measured. In addition, the potential difference between the

magnetised and unmagnetised electrodes drive a current to

an extemal load where electric power is measured [4].

2.2 Magnetohydrodynamics conversion system

The MHD conversion system is a distinctive method used for the generation of electric power based on the principle of

Faraday's Law of electromagnetism and plasma physics, as

shown in Fig. 2. In this method, a plasma (ionised gas)

flowing with high velocity is required [3],[5],[6],[7].

Right hand rule of

generator

Fig. 2. Principle of Magneto hydrodynamics

From Fig. 2, when plasma flows through a strong magnetic

field, jj, placed between two conducting electrodes

(cathode and anode), an electric current, 7, is generated

from the voltage, Vo, measured between the electrodes, as

shown in Fig. 3 [3],[5],[6],[8]. Furthermore, the conducting

electrodes must be placed on the sides at 9Cf to the induced

magnetic field in order to create a retarding force (Lorentz force) which is perpendicular to the direction of the field

and the measured current. The MHD conversion system can be categorised into different groups: Faraday's generator

with continuous electrodes, Faraday's generator with

segmented electrodes, Hull generator, disc generator and

series connected generator with discontinuous electrodes.

Each group varies according to the way the electrodes are

connected with the load.

2.2.1 Faraday's generator with continuous electrodes The Faraday's generator can be used with continuous

electrodes, as shown in Fig. 3. In this generator, the conducting electrodes and other materials such as load (R) and magnet are simple to construct. However, because the electrodes have the same potential, the fluid circuit is

vertical to the flow of plasma. Consequently, the circuit

components experience more long direction which causes large output losses. In addition, the transverse component of

the circuit is reduced as well [3],[6],[9:41-43].

1 .... \�x B v

Plasma (ionized gas l-

Output Voltage

l<--- @'---_

Fig. 3. Faraday's generator with continuous electrodes

2

From Fig. 3, in order to obtain good output voltage, the

electrical conductivity, (J, of the plasma must be above the

temperature range of 2000 K [2],[8]. This temperature is

usually maintained in the Faraday's generator walls.

Furthermore, when the plasma is in motion with velocity, v, a magnetic field, jj, is applied transverse to its motion.

Consequently, the charged particles inside the plasma experience an induced electric field, current and retarding

force [1 ],[3],[5],

E=vxjj , (1)

where E is the induced electric field, v is the particle

velocity and jj is the induced magnetic field.

The induced current is given by,

1= (JXE, (2)

where (J is the electrical conductivity of the plasma.

The retarding force on the conducting electrodes is given by,

F=IxB, (3)

where J is the induced current.

2.2.2 Faraday's generator with segmented electrodes The Faraday's generator can also be used with segmented

electrodes, as shown in Fig. 4. In this generator, the

electrodes and other materials such as insulators, loads and

magnet are simple to construct. Furthermore, since each

segmented electrode is connected to an external load, large

losses are greatly minimised. Hence, the potential difference

of each of the circuit is differently separated [3],[6],[9:41-

43].

1 y <;J-x

B V

Plasma (ionized 9as)_

____ I + Cathode wall

Magnetic field coil

Fig. 4. Faraday's generator with segmented electrodes

2.2.3 Hull generator The Hull generator design is shown in Fig. 5. In this generator, the electrode's edges are shortly connected

together and the load is connected to the first and the last

electrodes [6],[9:41-43]. If an electric potential (loads) is connected at different electrode points in the channel, an

equipotential will flow diagonally across the insulator walls.

Consequently, the electrodes might wait for some period of

time before it matches the equipotential. The output power

that can be obtained from a hall generator is driven by the

electric field circuit. Furthermore, this generator produces

fewer losses than the Faraday's generator because the resulting output voltage is very high and shorting of the

induced current is significantly reduced. However, due to the high velocity of plasma, the hull generator is less

efficient in terms of capturing the output current unlike the

Faraday's generator, which is very sensitive to plasma flow [3].

i y

�x B v

Plasma (ionized 9as)_

Load

Magnetic field coil

Fig. 5. Hull generator

-> -- v

2.2.4 Series connected generator with discontinuous electrodes

The Series connected generator with discontinuous electrodes is shown in Fig. 6. In this generator, the

electrodes are connected diagonally and the load is

connected to the first and the last electrodes. Furthermore, if the insulator wall of this generator is arranged in the

direction of electric field, more loads can be connected

[6],[9:43-44].

1 y

-S-x B v

Plasma (ionized gasl-

Load

....

2.2.5 Disc generator In this generator, the plasma flows between the centre and edge of the disc. Also, a circular Helmholtz coil is placed

above and under the disc to create magnetic excitations,

which produce Faraday's current around the edge of the

disc. The benefit of this generator is that it is compact and the size of the magnet is very small [3]. The advantage of

the above mentioned MHD conversion systems over other energy conversion systems is that they have high efficiency,

full power generation and cost effectiveness. In the present

study, a MHD generator and plasma with high temperature

will be investigated in order to produce reliable and efficient electric power. Furthermore, this research will help to

improve on the existing MHD conversion systems.

3 PLASMA ENERGY SOURCE

Plasma energy can be used in the MHD power generator to

produce a reliable and efficient power. This type of energy are obtained from electrically conducting ionised gas

(plasma) [10:3],[11:3],[12],[13],[14]. The electrons inside

the ionised gas display collective behaviour when they are separated from their atoms, [15: 1],[16],[17]. In addition, the

plasma sources that are used in Faraday's generators (MHD)

are mostly created by thermal ionisation, where high gas

temperatures are required. These types of plasma include gas discharge fluorescent light, gas laser, solar wind, aurora

and earth's ionosphere [18],[19]. However, some of these plasma sources contain high temperatures which can be

lowered significantly by seeding with alkali metals such as

potassium nitrate, rubidium, caesium, sodium and lithium

[1],[3]. The alkali metals are required because they can

ionise easily at lower temperatures. Fig. 7 depicts the

temperature and number density of typical plasmas [20].

- .... v Fig. 7. The temperature and number density of typical plasmas

Magnetic field coil

Fig. 6. Series connected generator with discontinuous electrodes

3

4 PLASMA DYNAMICS MODELLING

Plasma behaviour can be analysed through two different

models: Fluid and Kinetic models. The fluid model is

considered in this paper because plasma particles can be

treated as a single charged fluid (I-dimensional, MHD).

These particles (electrons, ions and neutral gas) move with

the same velocity and their dynamics are governed by the

combination of Maxwell's and Navier-Stokes equations

(differential approximations). In addition, the Maxwell's and Navier-Stokes equations are usually used to describe

the MHD model of magnetized plasma and the related

electric and magnetic fields inside a medium. These

equations are thus given by [21 :9],[22],

a). MHD plasma description:

• Mass continuity:

• Momentum:

• Ohm's law:

ap +v.(pv)=O at (4)

(5)

(6)

where 17 = meVe) n/ is the plasma or spritzer resistivity,

me is the electron mass, ne is the electron density and ne 2 is the electron-ion collision frequency.

b). Maxwell's equations for MHD:

• Faraday's law:

However, in the presence of electron-ion collisions, particles

resistivity and magnetic fields exists in the plasma [22]. The conditions that are required for using the above 1-

dimensional MHD equations can be summed up to three assumptions [23].

• The radius of the ion gyro must be very small compared

to the characteristic length scale of MHD conditions. •

• The ion and electron temperatures must be equal so that

collision between them is large and viscous effects must also be ignored.

•

• The plasma or spritzer resistivity should be small to

avoid resistive diffusion.

In the subsequent studies, the study of plasma dynamics

with respect to the 2-dimensional MHD model will be

investigated using computational fluid dynamics (CFD)

software such as ansys fluent and Comsot.

5 PAST AND PRESENT RESEARCH ON PLASMA

TECHNOLOGY

Plasma dynamics and behaviours have been investigated through developing and sending of probing instruments to

space. One of these instruments is a high frequency

Langmuir probe on-board QSA T satellite which was

developed by the engineers at the laboratory of spacecraft

environment interaction, Kyushu Institute of Technology, Japan. The main objectives of the QSA T satellite are to

investigate the plasma within the Aurora region, and to have

a better understanding of the spacecraft charging caused by energetic particles in this region [24]. The present study also

reviewed some other experiments in order to understand the characteristics of different types of MHD generators. These

experiments are given as below,

• A segmented MHD generator was developed at the

A vco-Everett Research Laboratory, Massachusetts. The aB --=-VxE at

• Charge continuity:

(7) plasma source used inside this generator was a Jet Propellant

(JP4) and the seeded gases used were potassium carbonate

(K2C03) and potassium hydroxide (KOH). An output power

of about 93 kW was obtained from each single electrode of

the generator. The outcomes of this experiment showed that

VXJ=O

• Ampere's law:

(8) gases from the conventional heat sources can be made with

adequate conductors of electricity in MHD generator,

provider that a small amount of easily ionisable seeded

impurities is added to the gas. In addition, the use of

segmented electrodes help to reduce hall current and also

(9) encourages further studies on the power distributed across

the segmented electrode channels of the generator [25].

From equations (4) to (9), the plasma state is assumed to be

in collisionless because magnetic fields lines of force seem to be frozen inside the plasma. Hence, the plasma resistivity

for an ideal plasma state condition is equal to zero, 17 = 0 .

4

• Another experiment was also conducted by A YCO

Mark 11 in the United State. The focus of their research was

to build a segmented electrode MHD generator that

produces 1.5 MW as electrical power for short time duration

(10 seconds). The plasma source used inside the generator

was Methycyclohexane (3000° K) and the seeded gas used

was potassium hydroxide (KOH) [26].

• In the 60's, an experiment was also conducted by

Lindsey at the international research and development company, Newcastle, England. The focus of his research

was to understand what kind of efficiency can be obtained

with the combined use of segmented electrode MHD

generator and a closed loop cycle turbine steam system. The plasma source used was Helium, He, (1500° K - 2500° K) and the seeded gas used was caesium, Cs [26].

• An experiment was also conducted on inductor AC

MHD generator by swift-Hook, Clark and Wright. The

focus of their research was to build an inductor AC MHD

generator which was tested with different gas temperatures

(2000° K, 2500° K and 3000° K). Their conclusions were

that this method requires more energy to produce the needed

magnetic field. As a result, the experiment is not cost­

effective [26],[27].

Considering plasma technologies in South Africa, plasma

are used for treatments of minerals, welding, surface

treatment, silicon etching and metal alloy manufacturing.

Furthermore, plasma is used nowadays as a new system of waste treatments (gasification method). This method

replaces the previously used incineration system. The

plasma assisted gasification is used to produce carbon

synthesis gas, which can be converted into petrol for the generation of electricity [28]. However, with the increasing

demand of electricity in South Africa, the present research

aims to conduct more investigation on plasma dynamics using faraday'S segmented electrodes with a rectangular

duct channel. This investigation will be based on improving

the existing methods of converting plasma energy directly to

electric power.

6 CONCLUSION

The principle of MHD has been reviewed extensively in this

paper and an MHD generator with segmented electrodes is

considered for further investigation and development. The

selection of segmented electrodes with rectangular duct

channel is based on its numerous advantages over other

types of generators. Furthermore, in order to design an

efficient segmented MHD generator, a good knowledge of the electrical conductivity of the working plasma fluid and

its variation with temperature and pressure is required. The conductivity of such plasma gas fluid determines the amount

of power to be generated and also the quantity of magnet to

be used in the segmented MHD generator. Additionally, a superconducting magnet must be used inside the MHD

generator in order to achieve good output power efficiency.

The combustion plasma gas fluid can also be delivered by any heat source with peak temperatures slightly greater than

3000oK. This type of gas is not suitable for use in an MHD

generator unless it is seeded with a small amount of easily ionisable impurity such as potassium salt or caesium. The

use of seeding gas is required because most plasma fluid at

high temperatures do not have sufficient electrical

conductivity .

5

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AUTHORS BIOS AND PHOTOGRAPHS

Ayokunle O. Ayeleso was born in

Nigeria in 1985. He obtained his B.Tech

degree (Physics Electronics) from Federal

University of Technology, Akure in

2008; M.Tech (Electrical) and M.Sc

(Electronics) from Cape Peninsula

University of Technology (CPUT), Cape

Town and Ecole Superieure d'Ingenieurs

en Electronique et Electrotechnique

France, respectively in 2014.

He is currently working toward obtaining the Doctor of

Technology (DTech) degree in Electrical Engineering at CPUT,

Cape Town, South Africa. He is an active member of the Centre

for Distributed Power and Electronics Systems at CPUT.

Mr Ayeleso research interests include plasma physics and

dynamics, electronics and renewable energy systems applications.

Tariq Kahn was born in South Africa. He holds a doctoral degree in electrical engineering from Cape Peninsula University of Technology.

He is currently the head of the Center for Distributed Power and Electronic Systems at the department of Electrical Engineering in CPUT.

Professor Kahn research interests include: optics, MEMS, green energy technology, energy efficiency, distributed generation,

automation and power electronics technology.

6

Raji K. Atanda was born in Nigeria in 1969. He received the B.Eng (Electrical)

from University ofIlorin in 1992; MTech (Electrical) and M.Sc. (Electronics) from

Cape Peninsula University of

Technology (CPUT) and Ecole Superieure d'Ingenieurs en Electronique et Electrotechnique France in 2009

respectively and DTech (Electrical) from CPUT in 2013.

He is currently a lecturer and researcher at the department of electrical in CPUT. He is a member of Power Electronics society of IEEE, Power System society of IEEE and National Energy

Association of South Africa and an active member of the Centre

for Distributed Power and Electronics.

Dr. Raji research interest is in the application of power electronics

technology and control system development for alternative

electricity generation, transmission, distribution and utilization.

Presenting author: The paper will be presented by Ayokunle O. Ayeleso