ELECTRICITY GENERATION WITHOUT DYNAMOS: A REVIEW OF RECENT
DEVELOPMENTS
by
AC Britten
ERID, Corporate Services Division, Eskom
tony.britten@eskom.co.za
Synopsis
This paper reviews the basic principles and applications of four lesser known electric power generation
techniques, namely, thermoelectric, thermionic, magnetohydrodynamic (MHD) and solar powered satellites.
Whilst all are of technical interest, only thermoelectric and thermionic generators have been applied
operationally (mainly in niche areas), and intrinsic MHD and solar power satellites not all. There appears to
be some scope for retro-fitted MHD Topping to increase the efficiencies of coal and gas-fired plant, but todate, this technology has not been applied operationally. The optimism expressed (about efficiencies) in the
mid-1990s has not been realised, and intrinsic MHD and MHD Topping are today not seen as viable
technologies for utility applications. Solar power satellites, though apparently judged to be technically
feasible, are severely constrained by cost, security and environmental impact considerations, and are
unlikely to be applied in the next 50 years, if ever.
1.
INTRODUCTION
This paper reviews the present status of a group of electric power generation techniques
which were being actively researched some 40 years ago; these methods seemed
promising then, but have not been nearly as widely applied as anticipated by the author
of [1], and certainly not in power utilities [1]. It is nonetheless felt that it would be
interesting and informative to see what their present state of application is, and what can
be learned from them; indeed the question could be asked of them, have we missed
anything?!
The techniques and technologies are:
•
Thermoelectric generation
•
Thermionic generation
•
Magnetohydrodynamic generation (MHD)
•
Fuel cells
Fuel cells are today a practical technology for some specialised utility applications, and
as such, are described elsewhere at this conference.
The solar power satellite and microwave power transmission concept is not linked
historically to the above technologies – it is much more recent – nor is it technically
related to them. However, what is common to all of them is the absence of conventional
electromechanical generators.
In each case, pertinent technical background is given. This is to allow the reader to gain
an understanding of the relevant technical principles.
1
2.
THERMOELECTRIC GENERATION
Background
The principle on which thermoelectric generation is based was discovered by J Seebeck
in 1821. It was not until the discovery of the electron in the late 1800s that its principle of
operation could be fully understood. Since the 1950s, the application of semiconductor
technology has resulted in the development of practical thermoelectric generators for
space applications.
Principle
A thermoelectric power generator is a solid state heat engine which converts thermal
energy into electrical energy by means of the application of the Seebeck Effect [2] .This
effect occurs when two dissimilar metals, which have been joined to form an electric
circuit, are heated at one junction and cooled at the other, so causing a small dc current
to flow in the circuit. See figure 1.
Figure 1: Basic Seebeck Effect (Reproduced from [2])
The voltage driving the current through the galvanometer, or the electrical load, is known
as the Seebeck voltage, and is directly proportional to the temperature difference
between the hot and cold junctions. For metals, as distinct from semiconductors (to be
discussed), typical Seebeck voltages lie in the range 0,4 to 0,6 mV/100 ºC, at least up to
1000 ºC. How in simple terms is the Seebeck voltage generated?
Heating of a metallic conductor at one end causes the migration of free elections to the
cold end. If the two ends of the conductor are joined to a second dissimilar electrical
conductor, a net electric current will flow in the circuit. The net current flow is the result
of the fact that the accumulation of free electrons in each conductor is not the same; this
causes a net concentration of electrons in the circuit, and hence, current flow. If similar
metals are used, there will be no net current flow.
2
The circuit in figure 1 is known as a thermocouple. To generate Seebeck voltages in the
range of volts requires a large number of thermocouples to be connected in series and
the application of temperature differences of at least 500 to 1000 ºC. Such a
configuration is known as a thermopile. Though quite widely used as voltage sources in
the mid 1900s, thermopiles were found to be inefficient (less than 3 %) as generators of
electric power. One of the reasons for the low efficiency was the dissipation of energy in
its relatively high internal resistance. It was only much later (in the 1950s) when
semiconductors were used as the basic thermocouple (instead of dissimilar metals) that
thermopiles became a more practical source of energy. What is the technical basis for
this?
If the dissimilar metals of figure 1 are replaced by p and n type semiconductors (bismuth
telluride, for example), Seebeck voltages of about 40 to 50 times those for dissimilar
metals can be generated. This property, together with their moderate thermal
conductivity compared with that of metals, has led to the modern form of the thermopile.
Application and present status
A sketch of the modern form of the thermoelectric generator is shown in figure 2.
Heat in
Conductor
P-type
N-type
Heat sink
Heat out
I
Electrical load
-
+
V=IR
Figure 2: Basic form of modern thermoelectric generator (Adapted from [3])
Several hundred p-n thermocouples or pairs may be connected in series/parallel
configurations to create a thermopile or thermoelectric generator. Heat is provided by the
burning of fossil fuels, the decay of long life isotopes and waste heat from nuclear
reactors [1,3]. Temperature differences of 100 to 1000 ºC are typically used; the range
of efficiencies achieved is about 5 to 20 %; costs are very dependent on fuel charges,
conversion efficiency and the power density achieved; the use of waste heat from other
processes tends to minimise the average energy cost [3].
The present range of applications is quite diverse, from nanowatt level generators used
for powering wrist watches (with body heat as the thermal source), the use of exhaust
heat in automobiles to augment the vehicle’s electric power system and proposed
100 kW systems for orbiting space stations [3].
3
The low ratings of thermoelectric generators show that they are not suitable, or indeed
economic, as a means of augmenting or supporting conventional utility electric power
supply systems. The competitive costs of kW level generators and their suitability for
exploiting waste heat, however, suggest that special applications, such as dip proofing in
power systems, might be viable [3].
THERMIONIC GENERATION
Background
Thermionic emission, which is the basis of the thermionic generator, was discovered
accidentally by Edison in 1883. Thermionic generation has not been widely applied, but
its relative simplicity and lack of moving parts has resulted in various specialised space
applications [1,3]. Much research was done in the 1950s and 60s on the development of
plasma diodes in which nuclear fission provides the heat for the cathode.
Principle
The ρhenomenon of thermionic emission refers to the “boiling off” of electrons from the
heated cathode of a vacuum diode. The resulting flow of electrons creates a low voltage
source which can deliver power to a load.
The principles of thermionic emission are explained in simple terms with reference to the
basic diagrams shown in figures 3 and 4.
Vacuum
Electrons
Space charge barrier
Vsp
ө
ө
Cathode
+
Va
ө
ө
work function
of cathode
Vc
Work function
of anode
Vo (output
voltage)
Anode
-
dc output to external circuit
Figure 3: Basic thermionic
generator/convertor
(adapted from [1])
Figure 4: Potential profile for ideal
convertor
In essence, it is the temperature difference between the cathode and the anode that
drives the electron current through electrical load. At room temperature, the energy of
free electrons at and near the surface of the cathode is not sufficient to allow them to
overcome the potential barrier or work function of the material; hence, these electrons
cannot escape from its surface. When the cathode is heated (typically in the range of
1600 to 2500 °K), some electrons acquire enough energy to escape from the surface
and so enter the inter-electrode space between the cathode (emitter) and anode
(collector). When an electron reaches the collector it “falls down” the potential barrier,
which represents the work function of the anode. This is being accompanied by the
release of energy that is converted into heat at the anode.
4
If the anode work function is less than that of the cathode, the net potential difference Vc
–Va (see figure 4) can drive current through an electrical load.
The deleterious effect of the space charge potential Vsp is to reduce the net output
voltage Vo given by Vo = Vc – (Va + Vsp). This influence may be reduced by using small
anode-to-cathode spacings, 0.01 mm or less, and adding low pressure cesium vapour to
the inter-electrode space. Cesium vapour improves the performance of the generator by
lowering the work functions of the electrodes and neutralising the space charge.
Practical thermionic generators are characterised by low output voltages (about 0,5 V
dc/convertor), high current densities, namely 5-10 amps/cm2, low power-to-weight ratios,
and conversion efficiencies of 10-15 % [1,3]. The basic thermionic convertor is shown in
figure 5. Ratings of 1 kW at 1000 amps and 1 volt are believed to have been
demonstrated.
Applications and Status
Reference [3] states that the thermal and electrical characteristics of thermionic
generators make them “attractive” and suitable for use in space with nuclear or radio
isotope heat sources. It is furthermore stated in [3] that the waste heat produced in
intermediate-temperature industrial processes can be used to generate the high
temperatures required by the cathode. This could facilitate the co-generation of
electricity in some industrial processes. Another application is their use as topping units
in steam power plants, but no details of specific applications have been found. It must
be concluded, therefore, that thermionic generation, though useful, is not a significant
technology in large scale power generation.
insulator
Hot cathode
Heat in
ө
ө
ө
Insulator
Cool anode
Heat out
Injection of cesium vapour
Electrical load
Figure 5: Quasi-practical thermionic generator (Adapted from [3])
5
3.
MAGNETOHYDRODYNAMIC GENERATION
Background
Magnetohydrodynamic (MHD) generation is a process by which the kinetic energy of an
electrically conductive fluid flowing against a magnetic field is converted into electrical
energy. The principle is explained in the next section.
The underlying principle of MHD was first suggested and demonstrated, though not
convincingly, by Faraday in 1832. He used the voltage measured across a flowing river
to show that a conductive fluid moving at right angles to the earth’s magnetic field could
generate small amounts of electric power [1,4].
More serious experimental work began in the USA in 1938, and was based on an
analysis which showed that power levels up to the 500 MW range were feasible [4].
Despite intensive research since the 1960s, the initial promise of MHD for commercial
generation has not so far been realised. Several major research programmes were
ended or curtailed in the mid 1990s, and since then, interest in MHD technology has
waned.
Principle
What follows here is a much simplified description of the principles of MHD generation,
but enough basic detail is being given to allow the important fundamentals to be
understood.
In essence, MHD generation is the process in which the kinetic energy of an ionised gas
flowing at right angles at a velocity v to a static magnetic field B is converted into an
orthogonal voltage E, as shown in figure 6(a). Figure 6(b) shows a simplified equivalent
electric circuit.
The induced dc electric field E in volts per unit length (anode to cathode spacing) of the
convertor is given by the vector equation
E=vxB
where:
v = velocity of flow of the gas in m/s
B = magnetic flux density in Tesla
Thus, for a hot plasma flowing at v = 1000 m/s and B = 5 T
│E│= 1000 X 5
= 5000 V/m
The electrical output power Pe, for maximum power transfer is
Pe =(E/2)2 x G watts/m3
where G is the source conductivity, ie, plasma conductivity in siemens.
If the conductivity of the gas is taken to be 10 siemens, then the output power in this
case will be (5000/2)2 x 10 = 62,5 MW/m3.
6
The movement of the hot plasma requires the application of mechanical “push power”
which in this case will be 125 MW/m3. For higher output power levels, say, in the
500 MW range, the length and cross-sectional area (i.e., the volume) of the MHD
convertor will have to be increased correspondingly. Gas temperatures and velocities
will have to be in the range of 2700 ºC and 1000 m/s respectively. The gases used are
mainly argon and helium. To achieve the high flux densities required, superconducting
magnets are used. Seeding of the gas with sodium or potassium vapour is used to
increase its conductivity at manageable temperatures [4].
Rsource
Cathode
B
E
Gas flow
Electrical
load
Induced
voltage
Electrical
load
Anode
Figure 6 (a): Basic principle of
MHD generation
generation
Figure 6 (b): Simplified
equivalent circuit of
an MHD generator
Application and Status
This section attempts to put MHD in today’s electric power generation context and to see
where it might be important to investigate possible applications, if any, in a large utility
such an Eskom.
The key point to appreciate here is that MHD is not a stand-alone source of energy; in
the words of Professor Hugo Messerle, “it is a conversion process that makes it flexible
for use with any heat source as long as it produces a hot enough plasma” [4]. What
follows here is a brief summary of some of the key claims made in Messerle’s book.
It is claimed that the application of MHD as topping plant, to coal fired power plant with
an efficiency of about 40 %, would increase the overall efficiency to 47 % or higher
(values as high as 60 % have been suggested) [4]. This would reduce losses and hence
fuel costs, and appears to be cost-effective [5]. MHD topping can be applied as topping
plant to new coal or gas fired generation, or retrofitted to coal-fired systems.
The application of MHD topping as a retrofit to coal fired steam plant is shown in
figure 7. The electrical convertor technology for coupling the output of the MHD source
to the grid is mature and well understood, and does not appear to be a technical
challenge.
According to [4,5], MHD topping plant has been applied in a number of proof-of-concept
and pilot studies; examples are:
•
An Australian feasibility study of an MHD topping retrofit to an existing 200 MW
coal fired plant
•
Planned demonstration of MHD retrofit to natural gas plant at Brindisi, Italy.
7
•
Plans formulated in 1995 to apply MHD topping to various coal and gas-fired plants
in China, Russia and Japan
Figure 7: Concept of retrofitting of MHD Topping to steam plant (Reproduced from [4])
The puzzling aspect concerning the activities mentioned above (which go back to 1995)
and benefits claimed for MHD topping is, why is it not being promoted commercially?
Why is so little apparently known about it? The claimed improvements in efficiency alone
would seem to justify serious interest in MHD topping, as well as the reductions in CO2
emissions.
In a recent communication from EPRI to the author [6], it was confirmed that no major
research has been done since about 1996, apart from Russian studies on a coal-fired
facility. EPRI is also unaware of any present commercial activities in MHD [6].
The predictions made about intrinsic MHD and MHD topping have not been realised. It
is unlikely, therefore that these will be significant technologies for utility applications in
the foreseeable future.
4.
BEAMED MICROWAVE POWER GENERATION AND TRANSMISSION: SOLAR
POWER SATELLITE (SPS)
Background
The Solar Power Satellite (SPS) is the concept in which a satellite in geostationary orbit
converts solar energy into microwave power which in turn is transmitted over the
distance of 36 000 km to a terrestrial receiving site. Delivered power levels of 1 GW
have been mooted [7,8]. Although SPS is very controversial, it has been partly assessed
by the International Union of Radio Scientists (URSI) thereby gaining a degree of
credibility.
Since the Kyota Protocol of 2005, global warming caused by the burning of fossil fuels
and the consequent release of carbon dioxide has led to fresh interest in clean, low
emission renewable sources of energy. One of these technologies is SPS.
The idea of SPS was first proposed by Glaser in 1968. It was extensively researched
during the 1980s when the emphasis appears to have been placed on the development
of Klystron power sources in the 100 kW range. These were used in the study of the
transmission of microwave power over short distances (up to few kilometres). Classified
military research was also done, and is probably continuing.
8
Principle
The key operating parameters are summarized in figure 9. The SPS employs microwave
technology for the transmission of power.
Sunlight
Satellite
Microwave power
beam
Site of rectenna
Geostationary Orbit
Earth
Figure 8: Basic topology of geostationary SPS
Impinging solar
radiation
Solar cells
µW Transmission path
Microwave (µW )
power generation
Solar Converter
Efficiency =13 %
µW power
DC Convertor
• 36 000 km @ 2,45 or
5,8 GHz
• Main beam steered to
within 0.0005 degrees
of target
• Atmospheric absorption
about 2% of µW power
transmitted
• Power densities in main
2
beams below 250 W/m
DC output
fed into
power grid
“Rectenna”
1-2 km diameter
Overall efficiency of system, including the
solar converter ≈ 7 %
Figure 9: Main elements of SPS system (Data derived from [8])
9
The above system is based mainly on theoretical studies; no system of gigawatt rating
has been built! Experimental rectenna systems capable to delivering 100 kW of DC
power over a few kilometers, have, however, been demonstrated [7]. There would be
very natural concerns about the effects of high power microwave beams on human
health and the environment. It is stated in [7] that the power densities (at frequencies of
2,45 and 5,8 GHz) at the perimeter of the rectenna will be of the order of 0,1 W/m2;
whilst the present ICNRP limit for continuous public exposure is 10 W/m2; hence, it was
deduced that SPS is safe! This is one example of the earlier biased conclusions, given in
[7], which URSI now does not endorse [8,9].
Application and status
In the view of URSI as given in [8], there are three areas of the technology with which it
[URSI] has grave misgivings: technical complexity, adverse environmental impact and
security. URSI can thus not endorse this technology, and completely dissociates the
organisation from the Draft URSI White Paper of October 2005 [7] in which it was
implied that SPS is a viable technology and that the adverse environmental effects could
be contained [8]. The 2005 White Paper has been retracted, but it nevertheless remains
a record of the thinking of a pro-SPS faction in URSI at the time.
It is understood from [9] that the development of an active SPS programme in Japan is
continuing, despite general misgivings in URSI and elsewhere about the overall impact
and cost of this technology.
SPS and gigawatt-level microwave beam power are intriguing concepts. It is being
presented as a technology that could help alleviate global warming. However, its high
costs; technical complexity; vulnerability to security abuse and severe environmental
impact will probably prevent it from being applied, at least for the foreseeable future, if
ever.
5.
CONCLUDING REMARKS
The aim of this paper has been to review the development of electric generation
technologies which were judged in the 1960s to be promising: the “revolution” predicted
by the author of [1] has not occurred, at least not as far as MHD is concerned.
Thermoelectric and thermionic generation have been able to fill niche applications, and
will continue to do so.
Intrinsic MHD and MHD topping do not appear today to be of any interest as far as utility
applications are concerned. However, research into this technology is continuing, but if
and when any significant breakthroughs are likely to occur, is difficult to say. Solar power
satellites are severely hampered by safety, cost and environmental impact
considerations, and will not be realised in the next 50 to 100 years, if ever.
6.
REFERENCES
1.
2.
3.
4.
5.
J.W. Gardiner, Electricity without Dynamos: the coming revolution in power
generation. Penguin Books, United Kingdom, 1963.
Physics Today, The World Book Encyclopedia of Science, Chicago, 1986.
Thermoelectricity and Thermionic Generation, Encyclopedia of Technology, pp 348
-372.
Hugo K Messerle, Magnetohydrodynamic Electrical Power Generation. John Wiley
and Sons, 1995.
Macmillan Encyclopedia of Energy, Macmillan Reference, Gale Group, USA, 2006.
10
6.
7.
8.
9.
Personal communication with Mr Neville Holt, Technical Fellow, Advanced Coal
Generation Technology, EPRI, 25/1/08.
URSI White Paper on a Solar Power Satellite (SPS), URSI Inter-commission
Working Group on SPS, issued in October 2005.
Revised URSI White Paper on a Solar Power Satellite (SPS), URSI Intercommission Working Group on SPS, issued in September 2006. (Revision of [7]).
Personal communication with Professor Howard Reader, former Chairman (20002006) of URSI Commission E in South Africa, 27/1/08.
.
11