Alkali Metal Thermal to Electric Energy Converter MAK Lodhi, University of The Punjab, Lahore, Pakistan, and Texas Tech University, Lubbock, TX, USA & 2009 Elsevier B.V. All rights reserved. The alkali metal thermoelectric converter (AMTEC)- System is actually a thermal energy converter. However, due to its close relationship to the sodium high tem- perature batteries, we have included this article in the Secondary Batteries–High Temperature systems section. Introduction The conversion of thermal and/or mechanical energy to electrical energy has been historically the mainstay for power systems. With the advent of Faraday’s law of electromagnetic induction and the steam engine, dynamic conversion systems evolved rapidly and reached their zeniths with engineering details mostly for terres- trial and aircraft devices. As potential alternatives to dynamic conversion systems, the static thermal to electric conversion systems have been investigated, particularly for space power systems. With the discovery of Seebeck effect, the thermoelectromotive force, which occurs in materials under the effect of temperature gradient, has been the driving force for static thermoelectric gener- ators. A large number of static devices have thus been evolved and are being studied. The goal for static con- version systems has been to develop static converters with efficiencies comparable to those of dynamic systems. Although the static conversion systems are in existence for quite some time, they have received more attention and gained importance in recent times because of newer applications of generating and using electricity such as in spacecraft, hybrid electric vehicles, military uses, and domestic purposes. Alkali metal thermoelectric converter (AMTEC) is one of the many new static energy con- version systems that are being considered for these purposes (Table 1). Alkali metal thermoelectric converter is relatively a new type of conversion device but the history of its technology can be said to date back to 1916 when Rankin and Merwin described a new compound called ‘allotropic a-alumina’. It clearly had a different structure than a-alumina but had similar characteristics, so they named it ‘b-alumina’. In 1936, Ridgway and a group of scientists showed that sodium was present in the allotropic a-alumina. In subsequent years, development continued at a slow pace until Yamaguchi and Suzuki discovered a closely related com- pound called b 00 -alumina in 1943. In 1967, Yao and Kummer reported that this material exhibited the ability to rapidly diffuse sodium ions and conduct them easily. This information aroused interest in many researchers to develop an applicable battery using these materials as electrolyte. The reason why the b-alumina family had at- tracted the attention of scientists was because it exhibited the unique ability to conduct ions through its structure and showed great resistance to the passage of electrons. Exploiting this property, Ford Scientific Laboratory in Dearborn, MI in 1968 was the first to conceive a prototype design of AMTEC in a sodium heat engine using a specific electrolyte of this family, thus adding a new static direct heat to electricity conversion system. Soon after, NASA, followed by the US Air Force, became interested in this technology as a possible power generation device in space exploration, especially when it involved the more remote areas of space where solar power is negligible. Space ex- ploration then became its primary target where many space technology companies and research institutions sought to improve its design and function. Alkali metal thermoelectric converter technology evolved rapidly from a laboratory concept to a technology with demonstrated feasibility within about 5 years. It progressed from demonstration of essential characteristics and functions to a need for dem- onstration of AMTEC cells with high reliability, high effi- ciency, and integrability with heat sources in a viable space power system design. The potential superiority of AMTEC performance over other contemporary static conversion technologies is so significant that once the space-related issues are resolved, AMTEC would quickly come under consideration for flight qualification phases. Alkali metal thermoelectric converter is a thermally regenerative electrochemical device for the direct con- version of heat into electrical power. The electrochemical process involved in the working of AMTEC is the ion- ization of the alkali metal atoms. The electrons produced flow through the external load thus doing the work, and finally recombine with the ions to form neutral atoms for recycling. Physical Description of Alkali Metal Thermoelectric Converter Initially, AMTEC was developed as a liquid anode cycle but soon the vapor anode cycle system took over. Pres- ently, the use of the vapor-fed AMTEC cycle system has been in vogue and is being investigated. A typical AMTEC unit is sealed and housed in a sealed container (Figure 1). The working fluid should have a high nega- tive potential, lightweight, and abundant availability for 334
Transcript
Alkali Metal Thermal to Electric
33
Energy Converter
MAK Lodhi, University of The Punjab, Lahore, Pakistan, and Texas Tech University, Lubbock, TX, USA
& 2009 Elsevier B.V. All rights reserved.
The alkali metal thermoelectric converter (AMTEC)-System is actually a thermal energy converter. However,due to its close relationship to the sodium high tem-perature batteries, we have included this article in theSecondary Batteries–High Temperature systems section.
Introduction
The conversion of thermal and/or mechanical energyto electrical energy has been historically the mainstayfor power systems. With the advent of Faraday’s lawof electromagnetic induction and the steam engine,dynamic conversion systems evolved rapidly and reachedtheir zeniths with engineering details mostly for terres-trial and aircraft devices. As potential alternatives todynamic conversion systems, the static thermal to electricconversion systems have been investigated, particularlyfor space power systems. With the discovery of Seebeckeffect, the thermoelectromotive force, which occurs inmaterials under the effect of temperature gradient, hasbeen the driving force for static thermoelectric gener-ators. A large number of static devices have thus beenevolved and are being studied. The goal for static con-version systems has been to develop static converterswith efficiencies comparable to those of dynamic systems.Although the static conversion systems are in existencefor quite some time, they have received more attentionand gained importance in recent times because of newerapplications of generating and using electricity such asin spacecraft, hybrid electric vehicles, military uses, anddomestic purposes. Alkali metal thermoelectric converter(AMTEC) is one of the many new static energy con-version systems that are being considered for thesepurposes (Table 1).
Alkali metal thermoelectric converter is relatively a newtype of conversion device but the history of its technologycan be said to date back to 1916 when Rankin and Merwindescribed a new compound called ‘allotropic a-alumina’. Itclearly had a different structure than a-alumina but hadsimilar characteristics, so they named it ‘b-alumina’. In1936, Ridgway and a group of scientists showed that sodiumwas present in the allotropic a-alumina. In subsequentyears, development continued at a slow pace untilYamaguchi and Suzuki discovered a closely related com-pound called b00-alumina in 1943. In 1967, Yao andKummer reported that this material exhibited the abilityto rapidly diffuse sodium ions and conduct them easily.
4
This information aroused interest in many researchersto develop an applicable battery using these materials aselectrolyte. The reason why the b-alumina family had at-tracted the attention of scientists was because it exhibitedthe unique ability to conduct ions through its structureand showed great resistance to the passage of electrons.Exploiting this property, Ford Scientific Laboratory inDearborn, MI in 1968 was the first to conceive a prototypedesign of AMTEC in a sodium heat engine using a specificelectrolyte of this family, thus adding a new static directheat to electricity conversion system. Soon after, NASA,followed by the US Air Force, became interested in thistechnology as a possible power generation device in spaceexploration, especially when it involved the more remoteareas of space where solar power is negligible. Space ex-ploration then became its primary target where many spacetechnology companies and research institutions sought toimprove its design and function. Alkali metal thermoelectricconverter technology evolved rapidly from a laboratoryconcept to a technology with demonstrated feasibilitywithin about 5 years. It progressed from demonstration ofessential characteristics and functions to a need for dem-onstration of AMTEC cells with high reliability, high effi-ciency, and integrability with heat sources in a viable spacepower system design. The potential superiority of AMTECperformance over other contemporary static conversiontechnologies is so significant that once the space-relatedissues are resolved, AMTEC would quickly come underconsideration for flight qualification phases.
Alkali metal thermoelectric converter is a thermallyregenerative electrochemical device for the direct con-version of heat into electrical power. The electrochemicalprocess involved in the working of AMTEC is the ion-ization of the alkali metal atoms. The electrons producedflow through the external load thus doing the work, andfinally recombine with the ions to form neutral atoms forrecycling.
Physical Description of Alkali MetalThermoelectric Converter
Initially, AMTEC was developed as a liquid anode cyclebut soon the vapor anode cycle system took over. Pres-ently, the use of the vapor-fed AMTEC cycle system hasbeen in vogue and is being investigated. A typicalAMTEC unit is sealed and housed in a sealed container(Figure 1). The working fluid should have a high nega-tive potential, lightweight, and abundant availability for
Table
1S
om
esta
tic
energ
yconvers
ion
for
space
pow
er
No.
Devic
eH
eat
sourc
eT
em
p.
(K)
Work
ing
fluid
Pow
er
(W)
Speci
fic
pow
er
(Wkg�
1)
Pow
er
densi
ty
(Wcm�
2)
Unit
cost
($W�
1)
Lifetim
e
(Y)
Effi
cie
ncy
(%)
Technolo
gy
sta
tus
1T
IEC
–1600–2000
Vapor
––
B8
–4
5,o
217
–
2T
OE
CR
TG
800–2000
None
(SiG
e)
––
––
47
7V
oyager
flig
ht,
relia
ble
well
pro
ven
3T
PV
Sola
rC
onc,
RT
GG
PH
S
1470–2400
None
–6–10
5–
7–14
(30)
Explo
rato
rysta
te
4A
MT
EC
Sola
r,nucle
ar,
any
900–1300
Na,
Kvapor
and
liquid
W-M
W13–20
for
o1
kW
45
B1000
15
(pro
j)13–19,
40
(pro
j)
Most
advanced
sta
ge
leadin
gto
space
flig
ht
test
5H
YT
EC
Sola
r,nucle
ar
900
Li,
LiN
a,
liquid
H
–Lig
ht
Wt
––
20
(pro
j)–
Losses
at
pow
er-
pro
ducin
gste
pare
yet
tobe
dete
rmin
ed
6T
AP
CS
ola
r,
com
bustion
1100
Gas,
Li,
He,
Na,
Li
1kW
––
––
15
Early
sta
teof
develo
pm
ent
7LM
MH
DR
eacto
r,R
TG
1123–1673
Mix
ture
of
liquid
and
vapor
100
kW
–
100
MW
––
––
o10
Explo
rato
rysta
te
AM
TE
C,
alk
ali
meta
lth
erm
oele
ctr
icconve
rter;
GP
HS
,genera
l-p
urp
ose
heat
sourc
e;
HY
TE
C,
hyd
rogen
therm
oele
ctr
ic;
LM
MH
D,
liquid
meta
lm
agneto
hyd
rod
ynam
ics;
RTG
,ra
dio
isoto
pe
therm
oele
ctr
ic
genera
tor;
TAP
C,
therm
oacoust
icp
ow
er
conve
rter;
TIE
C,
therm
ionic
energ
yconve
rter;
TO
EC
,th
erm
oele
ctr
ic;
TP
V,th
erm
op
hoto
volta
ic.
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 335
TerminalExternal load
Porouselectrode
Condenser
Sodium return artery
Sodium flow (blue arrows)
Low-pressure sodium vapor
Liquid−vapor interface(evaporator)
BASE tubes
High-pressure sodium vapor
Plenum plate
Hot plate Power feed through
ElectrodesCurrent collector
BASE
B B
A
A
Section A−A Section B−B
Na Na
Na
Na
Na
NaNa+
Na+
Na+e−
e−
e−e−
e−
e−
Figure 1 Photograph of a sealed alkali metal thermoelectric converter (AMTEC) with its interior parts. BASE, beta aluminum solid
electrolyte. Courtesy of Michael Schuller.
Table 2 Properties of some elements having negative potential
336 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
cost effectiveness. An alkali metal with relatively lowmelting point would satisfy these qualities. Out ofa number of possible samples (see Table 2), sodium ischosen for this task. At the heart of AMTEC lies the betaaluminum solid electrolyte (BASE), made of a densemicrocrystalline sintered ceramic material (Na5/3Li1/3
Al32/3O17) in the form of individual tubes connectedin series as shown in Figure 2. The BASE separatestwo regions of alkali (sodium in this case) vapor intro-duced: a high-temperature (900–1300 K) high-pressure(20–100 kPa) region and a low-temperature (400–700 K)low-pressure (o100 Pa) region (Figure 2). From thephysical dimensions of this type of AMTEC cell given inTable 3, it looks like a D-size cell. Two thin porous
electrodes are placed on the inside and outside walls ofthe BASE, as shown schematically in Figure 3. Theelectrode mounted on the inner side of the BASE acts asanode and the one on the outer side of the BASE acts ascathode. The inner and outer electrodes are each con-nected with current collectors, of which one collectselectrons at the anode (high-pressure side of BASE) andother conducts them through an external load. Electronsfrom the external load are brought back to the cell at thecathode (low-pressure side of the BASE) to recombinewith sodium ions. The requisite length of leads con-necting the two electrodes carries the electric currentthrough the load. The working fluid, initially in the liquidstate stored in the condenser at one of the ends of the
Figure 2 Components of a typical alkali metal thermoelectric converter (AMTEC) cell. BASE, beta aluminum solid electrolyte.
Courtesy of Michael Schuller.
Table 3 Dimensions the of parameters of a typical AMTEC
cell
Component Dimension/
material/shape
Cell diameter (mm) 31.75
Cell height (mm) 101.6
Evaporator type Deep cone
Evaporator elevation (mm) 5.18
Evaporator standoff thickness (mm) 0.71
Evaporator standoff material SS
Standoff rings (mm) 1.1
Ring material Ni
Stud area (mm2) 38
Stud material SS
Number of BASE tubes 5
Tube length (mm) 32
Electrode/tube area (mm2) 600
Tube braze material TiNi
Current collector 60-mesh Mo
Feedthrough braze TiCuNi
Radiation shield type Circular
Shield material SS
Condenser type Creare
Hot side SS
Cell wall SS
AMTEC, alkali metal thermoelectric converter; BASE, beta aluminum
solid electrolyte.
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 337
cell, is carried by a capillary tube, called sodium returnartery, into the evaporator placed at the other end of thecell, where it is heated and turned into vapor (Figure 2).The evaporator thus maintains liquid–vapor interface
during the operation of the cell. The evaporator side endcarries a hot plate, which is heated from an external heatsource. The far end of the cell, where the condenseris housed, releases the heat of sodium condensation. Thecell contains a radiation shield, laid against the cell wallsabove the BASE tubes to reduce the parasitic heat lossesthrough the cell walls. In addition to radiation shield, achevron radiation shield system consisting of a requirednumber of chevrons at desired angles is placed above theBASE tubes when needed for maximizing the poweroutput and efficiency. The cell is operated in vacuum.
Working Principle of Alkali MetalThermoelectric Converter
On applying heat at one end of the cell where the innercontainer is mounted, electric current is produced at theother end of the cell. The sodium vapor pressure at theanode/BASE interface (the high-pressure side) is equalto the saturation pressure at the evaporator (where liquidsodium is converted into vapor) temperature. The pres-sure differential between the two sides of the BASE isassociated with potential energy, which can be convertedinto useful work through sodium. As the liquid sodium atcondenser temperature enters the evaporator in the hotregion of the cell, it starts absorbing the externally sup-plied heat until it vaporizes and reaches the desiredtemperature of the inside BASE tube. The inside BASE
Figure 3 The beta aluminum solid electrolyte (BASE) and
electrodes in detail. Reproduced with permission from Lodhi MAK
and Briggs JB (2007) Temperature effect on lifetimes of AMTEC
electrodes. Journal of Power Sources 168(2): 537–545.
338 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
tube temperature is kept slightly higher than that ofthe evaporator to prevent condensation of sodium in theanode cavity and the potential electric shorting of thecell. Because of the high pressure, sodium vapor tries toexpand. The b00-alumina is impermeable to neutral atomsand also to electrons. Thus, the only way for the pressureto be released (in other words, for the vapor to expand) isfor neutral sodium atoms to ionize, allowing the sodiumions to pass through the BASE wall. The sodium atoms,hence, ionize, producing sodium ions and free electrons.Owing to the thermodynamic pressure across the BASE,ionization of sodium metal vapor occurs in the hot regionat the interface of anode and the BASE according to thefollowing electrochemical reaction:
Na ðvaporÞ-e� þNaþ
The BASE permits the sodium ions to pass through itsmaterial and the pressure differential causes this move-ment of the ions. The positive sodium ions accumulateon the low-pressure side. The electrons collect on the
current collector at the high-pressure side, resulting in anelectrical potential, which balances the pressure differ-ential and prevents further flow of sodium ions. Withappropriate electrodes, this electrical potential can beutilized to drive an electrical current when a load isplaced on the system (i.e., connecting the inner and outerelectrodes with a lightbulb, CD player, or deep spaceprobe for example). The electrons flow from the anodethrough the lead and the external load to the cathode onthe outer (low-pressure) side of the BASE. Sodium ionsthat have passed through the BASE reach the interfacebetween the BASE and the cathode, where they re-combine with the electrons to form neutral sodium atomsagain as they were to begin with. At the interface of thecathode and the BASE, the following reaction takes place:
e� þNaþ-Na ðvaporÞ
The neutral sodium atoms escape from the interface ofthe BASE and the cathode in the form of sodium vapor atlow pressure in the outer low-temperature low-pressureregion. The low-pressure sodium vapor flows to thecondenser where it releases its heat of condensation andcondenses to the liquid state. Nearly entire temperaturedrop occurs in the low-pressure vapor state. After con-densation, the condensed liquid sodium reaches the wickannulus to the inlet of a direct current (DC) electro-magnetic pump or a porous capillary wick. This system isused to return the liquid sodium to the evaporator in thehigh-pressure region where it is converted into high-pressure vapor. In this manner, the sodium continues torecycle and maintains the cell operation. A typical cellcontains approximately 3 g of sodium circulating throughthe loop at a rate of up to 0.3 g min�1.
The overall functioning of AMTEC is based on fourprinciples dealing with (1) thermal, (2) vapor pressureloss, (3) electrochemical, and (4) electrical models. Theseprinciples or models and their functions could notbe described in detail in the space provided here: theirdescription involves a large number of mathematicalderivations and expressions. However, for the self-consistency of this article, a very brief description isgiven here.
(1) Thermal model: This model determines the heatconduction in the cell structure and thermal radi-ation exchanges among all surfaces within the cellcavities. The model calculates radiation and con-duction heat fluxes between different elements ofthe cell. Thermal losses between the heat sourcesand heat sink contribute greatly in reducing theconversion efficiencies of the device. This modelminimizes them. As the BASE is covered with elec-trode and current collectors, its rough surface can betreated close to a black body for temperatures above1000 K with emissivity constant equal to 0.9, whereas
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 339
at the condenser surface it is only 0.02 particularlywhen the liquid sodium coats that surface.
(2) Vapor pressure loss: The vapor pressure losses at theanode side are negligible compared to the sodiumpressure at the evaporator temperature greater than900 K. The vapor pressure at the anode side is equalto the sodium saturation pressure at the evaporatortemperature. However, at a condenser temperaturebelow 50 K, sodium vapor pressure below 100 Pa atthe cathode side is 2–3 orders of magnitude lowerthan that of the anode side. The actual pressure ofsodium vapor at the cathode and BASE interfacedepends on the sodium saturation pressure at thecondenser temperature and the pressure losses owingto sodium vapor flow between the BASE and thecondenser. The vapor pressure loss model determinesthe sodium vapor pressure at the surface of cathodeand BASE as a function of temperature and sodiumvapor flow rate.
(3) Electrochemical model: This model is utilized indetermining the voltage differential across the BASEas a function of cell electric current, external loadresistance, BASE temperature, condenser tempera-ture, and sodium vapor pressure differential across theBASE. The cell potential is the equilibrium potentialminus ohmic losses and the current-dependent volt-age losses. Ohmic losses include ionic resistance of theBASE, contact losses between BASE, electrodes, andcurrent collectors, and losses in the collectors.
(4) Electrical model: The main purpose of this model isto keep the electrical losses to a minimum. It deter-mines the electrical resistance of BASE and currentcollectors, and total electrical current as a function ofload resistance. The internal electrical losses are dueto ionic resistance of the BASE, contact resistance ofelectrodes, sheet resistance on the plane of the elec-trodes, resistance of the current collectors, bus wires,and conductor leads to the load, and charge exchangepolarization losses at the BASE/electrodes interfaces.The most significant contributions to the internalelectrical losses are BASE ionic resistance and chargeexchange polarization loss on the cathode side.
It should be borne in mind that the aforementionedprinciples do not work independent of each other. In-stead, they are closely coupled to each other. Therefore, amethod is developed to ensure good coupling to eachother, thus allowing minimum losses overall and in-creasing the efficiency of the cell.
Advantages and Uses of Alkali MetalThermoelectric Converter Cells
Alkali metal thermoelectric converter converts the workdone during the nearly isothermal expansion of sodium
vapor to produce a high-current and low-voltage elec-tron flow. Owing to its principle of working, it has manyinherent advantages over other conventional generators.Some of those salient features are briefly mentionedbelow:
1. High efficiency: This, probably, is one of the mostimportant advantages of AMTECs. These devicesare capable of achieving high efficiencies at relativelylow operating temperatures. For instance, an opti-mized AMTEC can potentially provide a theoreticalconversion efficiency between 20% and 40% whenoperated at hot-side temperatures in the range of1000–1200 K and at cold-side temperatures in therange of 400–700 K. Alkali metal thermoelectricconverters are capable of achieving high efficiency,close to Carnot efficiency. Other conventional de-vices cannot achieve such high efficiencies.
2. High power density: Alkali metal thermoelectricconverters inherently have high power densities.Power densities of up to 1 W cm�2 were achievedas early as 1978. This means that the size of thecell is small relative to power output. This attributeis closely related to efficiency. Some designs ofAMTEC cells have calculated energy conversionefficiencies of up to 23.5% with power densities of19.8 W kg�1. The power density value can go as highas 0.5 kW kg�1 of system mass in AMTECs withoptimized design.
3. Closed-loop design: There is no transfer or flow ofmatter either into or outside of the cell system. Asa result, there are no problems of leakage, mal-functioning of valves, meters, flow-regulating de-vices, etc., that are possible if the working fluid flowsinto and out of the system.
4. Absence of moving parts: This is an importantcharacteristic of AMTECs although some otherconverters such as thermocouples also have thisfeature. The absence of moving parts eliminatesseveral associated problems. Moving parts areinvariably associated with problems of wear and tear,leakage, and friction. Although these can be reducedwith proper lubrication, there is virtually no way thatthey can be eliminated. Moreover, lubrication comeswith its attendant problems such as oil leaks, sealantproblems, and possibly undesirable chemical inter-action with other components. Besides, moving partsand lubricant movement (if liquid) will give riseto problems related to the dynamic stability of thestructure especially if it is a spacecraft. Noise andvibration are virtually nonexistent in AMTECs.
5. Reliability: AMTECs are extremely reliable partlybecause of the absence of moving parts and thebasic functional technology – AMTEC technologydoes not basically involve chemical reactions between
340 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
different substances, thus avoiding any unpredictablefailure of the chemical reactions. It only involvesan electrochemical reaction (ionization) of a singleelement.
6. Maintenance-free operation: This is a result of manyof the features that have been associated with respectto its working principle. Once the system is startedand a steady state is reached, no further externalintervention is required. The cell will run for a verylong time without requiring any maintenance work.This feature too makes AMTECs well suited forlong-duration space and terrestrial applications.
7. Competitive production costs: AMTEC technologyutilizes materials that are commonly or easily avail-able for most of its components. Even the criticalsolid electrode can be produced at competitivecosts. Fabrication of the cell, including deposition ofelectrodes on the solid electrolyte and wick manu-facturing, can be achieved at very economical costs.The manufacture of AMTEC cells is thus avery economically viable venture. Competitive pro-duction costs of AMTEC technology are because itutilizes materials that are commonly or easily avail-able for most of its components.
8. Working temperatures: AMTECs have relativelyhigh heat-rejection temperature (about 600 K) andlow heat source temperature (about 1200 K). Theheat-rejection temperature has to be kept at a levelabove the phase change temperature (371 K) ofsodium in order to maintain sodium in liquid state.Besides, for the optimum performance of the cell, thetemperature of the condenser has to be kept at thatlevel. The low heat source temperature (about900–1200 K) of the AMTEC allows it to reuse therejected heat of converters working at high tem-peratures, such as the thermionic energy converter(TIEC) that rejects heat at high temperatures. Suchan arrangement, called cascading, thermally con-necting the heat-rejecting end of the other devicewith the source end of the AMTEC. With thischaracteristic of AMTEC, it can be used as a meansof recycling the heat rejected by other devices, thusreducing the thermal pollution while converting theheat into electrical energy.
9. Flexibility of heat source: Heat input to AMTECcells can be from many different sources (nuclearreactor, nuclear radio isotopes, combustion, solarpower) or any form. This flexibility allows AMTECtechnology to be applied to many fields such asspace, military, terrestrial, and domestic/residentialuses.
10. Modular design: AMTECs are amenable to amodular design. Alkali metal thermoelectric con-verters typically generate high current (thousands ofamps) at low voltage (hundreds of millivolts) because
of the use of large electrode areas electrically inparallel rather than in series. Therefore, the modularfeature of AMTECs means that powerful systemscan be built by simply connecting smaller cells inseries together.
Because of these advantages, AMTECs are importantin many applications ranging from space and military todomestic uses. These converters can be used for hybridelectric vehicle systems, independent and portable powergeneration units for military uses, micro-cogeneration,residential power generation either in conjunction withor independent of the electric grid, power generationfor recreational vehicles, power for air-conditioningand lighting in cross-country transportation, chargingrechargeable batteries, and residential self-powered fur-naces. However, it is in space applications that thistechnology may find the greatest use and importance.It offers a viable alternative to current energy conversionsystems for use in spacecraft with significant advantages.For instance, a coupled general-purpose heat source(GPHS) (which would provide the heat energy for powergeneration) and AMTEC system working at 15–20%efficiency will require only one-fourth the mass of theradioisotope (plutonium-238) required as heat source inthe present thermoelectric conversion system workingat 4% efficiency, resulting in large savings in mass, fuel,and cost.
Problem Identification in Alkali MetalThermoelectric Converter Technology
There is always room for improvement of any device orapparatus. In spite of the many advantages of AMTECtechnology, its current designs are not totally free fromsome sore points. There are a couple of drawbacks stilllinked with AMTEC, although removable or at leastreducible to a great extent. One is with respect to steady-state efficiency and the other is related to its time-dependent performance.
Alkali metal thermoelectric converters can theoreti-cally perform at efficiencies close to Carnot efficiency.However, in practice, current designs have only providedefficiencies in the range of 10–15%. This indicates thatcurrent designs have not yet taken care of high parasiticlosses. Notwithstanding, it is a design problem, and notinherent to the AMTEC cell technology. Hopefully, itcan be corrected by improving design. The other prob-lem of the AMTEC cell is with the power versus timecharacteristic, observed experimentally in an AMTECmodel cell. The power–time characteristic of this modelcell shows that its maximum power output decreases withtime from 2.45 to 1.27 W in 18 000 h. The drop in poweroutput also means that the efficiency of the cell alsodecreases. This characteristic may preclude the use of the
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 341
cell for many applications that require operation of thecell for long durations of time, unless it is corrected. Asboth these problems cause the cell to work at a reducedperformance level, it is important to look for their causes.With so many advantages, these are some aspects ofAMTEC that may be addressed to make it work withbetter performance level, as it is capable of working closeto Carnot (ideal) efficiency. The current designs ofAMTEC are under investigation for further improve-ment. The above drawbacks may be due to the followingreasons (a few are listed):
1) The products of chemical reactions between thesodium vapor and the materials of the container maydeposit on the surfaces of sensitive components suchas BASE, electrodes, and current collectors, thus re-tarding the conductivity of ions and electrons.
2) They may also enter the material structure anddeposit on the grain boundaries increasing grainboundary resistance to ionic motion in the BASE, orreplace some Naþ ions in the structure. All of thesewill increase the ionic resistance of the BASE. In thesame manner, similar phenomenon may be attributedto electrodes.
3) Loss of sodium from the BASE can increase its ionicresistance.
4) The electrode performance may be affected by thelong duration of cell operation at high temperatures.
From these drawbacks, it appears that BASE and elec-trodes are mainly responsible for the aforementioneddrawbacks. Therefore the performance of AMTEC withrespect to these major components is examined in thenext section.
The Effect of BASE and Electrodes onPower Degradation
The BASE and electrodes may undergo several changesas a result of working at temperatures about 1000–1200 Kin the high-pressure, high-temperature sodium vapor en-vironment, which is highly caustic. These changes can bebroadly classified as chemical contamination and thermalbreakdown. In the case of chemical contamination, theproducts of chemical reactions between the sodium vaporand the materials of the container, such as stainless steel,may deposit on the surface of the BASE and electrodes,thus obstructing the flow of ions through the BASE andelectrons through electrodes. They may also enter thematerial structure and deposit on the grain boundaries,increasing grain boundary resistance to ionic motion, orreplace some Naþ ions in the structure of BASE. By thesame token, the same products of chemical reactions mayenter the electrode material structure and deposit on thegrain boundaries, thus increasing grain boundary
resistance to electron motion in the electrodes. All of thesewill increase the ionic resistance of the BASE and electronresistance in electrodes. Thermal breakdown manifestsitself in the form of loss of sodium from the BASE, for-mation of molten dendrites in the BASE, and crack for-mation or changes in the microstructure. Loss of sodiumfrom the BASE can increase its ionic resistance. Themolten sodium dendrites, propagated through the entirethickness of the BASE, can create an electrical short,causing electrons at the anode/BASE interface to flowthrough the dendrite to the BASE/cathode interfacedirectly, without flowing through the external load, thusreducing output power. Cracks in the BASE material canpropagate through the entire thickness, causing high-pressure sodium to flow to the low-pressure side withouthaving ionized at the anode/BASE interface. This willreduce the number of electrons, produced as a result ofreduced ionization, which would have been available tothe external load, causing a decrease in the power output.Changes in the microstructure, especially grain growth,will increase resistance to ions flowing through the BASEmaterial. It can thus be seen that changes in the propertiesand/or structure of the BASE as well as electrodes maysignificantly affect power output. Alternatively, when thesedefects are removed, there would be an improved level ofperformance.
Further Improvements
It is not within the scope of this article to go into thetechnical details of overcoming those drawbacks. Never-theless, some ideas may be thrown to help understandhow those needed improvements can be achieved in theAMTEC technology. These improvements are made pri-marily with the changes in the BASE and electrodes andtheir contamination, as discussed in the previous section.
The contamination of the BASE by-products ofreactions between the sodium vapor and stainless steelin the cell at high temperatures adversely affects theproperties of the BASE. As a means to reduce this con-tamination, ionic or other kinds of filters may be used. Inthis manner, contamination may be reduced or the con-taminants may even be prevented from reaching theBASE to affect its performance efficiently. This may befeasible certainly for terrestrial applications of AMTECtechnology where the filter can be replaced as and whenneeded. The use of filters, however, is not practical forspace applications where frequent replacement of filteris not possible. For that and also terrestrial purposes aswell, one should look at the problem from a differentangle, say the use of a different material for the BASE.
The next problem deals with the structural stabilityof the BASE. Polycrystalline b00-alumina was used as thesolid electrolyte in the model AMTEC for the power
342 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
measurement experiment because it has higher ionicconductivity than polycrystalline b-alumina. However,b00-alumina, is inherently unstable. Doping with lithiumor magnesium oxides stabilizes it. These dopants, however,are detrimental to the life of the electrolyte. On the con-trary, the single-crystal b-alumina may be used instead,for better conductivity and stability than polycrystallineb00-alumina although it costs much less. Single-crystalb-alumina is stable even without doping and has higherionic conductivity than polycrystalline b00-alumina. Poly-crystalline b-alumina may also be used rather than single-crystal b-alumina when compared in terms of economy andquality control. Its conductivity can be increased by theprocess of sintering and annealing. The order of materialsin terms of increasing ionic conductivity is polycrystallineb-alumina, polycrystalline b00-alumina, single-crystal b-alumina, and single-crystal b00-alumina. However, the high-performance materials degrade more rapidly than thelow-performance materials. Thus, for long-term use withlimited access to the electrolyte, the choice of material hasto be optimized with respect to these parameters for aspecific requirement.
The current density affects the degradation of BASE.There is a critical value of current density below whichno degradation will occur. This critical value depends onthe amount of lithium oxide (Li2O) content in the BASE.Therefore, the current density in the BASE should bekept below the critical value to eliminate degradationowing to current density. The higher the current densityabove the critical value, the higher the rate of degrad-ation. If it is not possible to maintain the current densitybelow the critical value, it should be kept as close to thatvalue as possible to reduce the rate of degradation.
Loss of sodium from the BASE as sodium oxide affectsnot only its ionic conductivity but also the stability of theBASE. The range of sodium oxide content for which theBASE is stable is dependent on its stabilizer content.Accordingly, the stabilizer content should be adjusted sothat the anticipated loss of sodium oxide during theoperating life of the cell still maintains the sodium oxidecontent within the range of stability corresponding tothat stabilizer level. Although there are several metalstabilizers for the BASE, such as magnesium and lithium,lithium is more commonly used.
Further investigation with respect to the time-dependentpower characteristic of AMTEC seems to be undertaken.This can be optimized with computer simulation. The grainsize analysis needs a thorough investigation for the choice ofthe material for the BASE and electrodes.
Ongoing Research
Efforts in improving the performance of AMTEC are inprogress: both experimental and theoretical aspects are
considered with respect to the few problems mentionedabove as well as others. It may not be possible to describeall the ongoing research efforts within the limited spaceof this article. However, a very brief account of someof the ongoing aspects of AMTEC research and theirpreliminary findings are presented here. The problemsarising from the long and continuous use of AMTEC aremainly the degradation of its power output and loss ofefficiency. Recent studies show that BASE and electrodesare two of the most power-degrading components in theAMTEC. The electrode performance strongly dependson its material. The electrical resistance, thermal ex-pansion coefficient, vapor pressure, and surface self-diffusion coefficients are the most desired parameters inthe choice of the material for electrode construction. Theelectrodes must have suitable properties and be stableunder operating temperatures for the desired period oftime. They should be able to provide a site for thethermoelectrochemical reactions for sodium ionizationand recombination of sodium ions and electrons. For that,the electrode materials are expected to have some pre-requisite characteristics such as
1. The electrode material in contact with the BASEmaterial should have a high tolerance to thermalexpansion. Ceramic electrodes have a lower toler-ance compared to the metal electrodes in general.
2. The negative Gibbs free energy should be largeenough to prevent material loss owing to dissociation.
3. The electrode should be thermoelectrochemicallycompatible with the BASE and other components, toprevent corrosion.
4. The sodium ions should be able to easily migratefrom the high-pressure anode side to the low-pres-sure cathode side through the BASE.
5. The electrode should not be a barrier to sodium ions.6. The electrons should be able to leave the anode
site with least resistance and travel rapidly to andthrough the load and recombine at the cathode–BASE interface to complete the circuit.
7. The electrical conductivity of electrodes should behigh enough to allow the electrons to move throughthe external circuit.
8. The electrical resistance and the surface self-diffusion should be low for an ideal electrode.
9. It is important that the electrode does not alter itsphysical morphology during the long period ofoperation.
10. Electrode materials should have high melting point toallow a lower surface diffusion coefficient. This aspectcorresponds to low sintering of the electrode grains.
11. The grains of the electrode material should coalescevery slowly in order to prevent the appearing ofvoids, as the voids open up the electrical conduct-ivity decreases.
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 343
The triple phase boundary involves the electrode,BASE, and sodium interaction. A longer triple phaseboundary is the result of a finer grain size. Also thecharge transfer process occurs at the triple phaseboundary. If the length of the triple phase boundaryincreases, the overvoltage decreases. The electrode witha finer grain size will yield higher current density. Theelectrical resistance and the surface self-diffusion shouldbe low for an ideal electrode. Also, the thermal expansioncoefficient of the electrode should be very similar to thatof the BASE material. The lifetime of AMTEC elec-trodes is dependent on the sintering rate of the material,which in turn is dependent on the operating temperatureof the cell and inherent properties of the material. Thetime for which the performance of the electrode issatisfactory to produce adequate power output for theAMTEC system is defined as the electrode lifetime. Ifthe electrode grains grow to a diameter of 1000 nm (somelimit to 500 nm), then they cease to function adequately.The sintering rate of the electrode material is a functionof the operating temperature of the cell; thus, the oper-able lifetime of an AMTEC electrode is dependent onthe temperature. As the operating temperature increases,the sintering rate of the electrode also increases. Asgrains sinter, they coalesce, resulting in an increase inthe grain volume. The coalescing of grains increases theporosity of the electrode material and causes the contactsurface area of the electrodes with the BASE to decrease.The performance of the electrode is related to the con-tact between electrode and the BASE, which is measuredby a parameter called temperature-independent ex-change current or just exchange current. The exchangecurrent depends on the operating fluid pressure atthe interface of the BASE and electrode. The pressureat the interface is the sum of the pressure caused by thecondensation of metal vapor from the condenser, thepressure owing to the metal vapor leaving the exteriorsurface of the electrode, and the pressure drop fromthe electrode–BASE interface to the electrode surface.Therefore, temperature-independent exchange currentis a sensitive measure of the nature of contact betweenelectrode and BASE interface. It can also be relatedwith the operable lifetime of the electrode throughgrain size.
With an increase in sintering, grain size increases,which eventually leads to a decrease in the numberof grains. Therefore, contact between BASE and elec-trode also decreases. As the total number of grainsdecreases, the exchange current also decreases, becausethe exchange current is proportional to the number ofgrains in the electrode. With the increase of grainsize, voids within the electrode become larger, whichcauses the electrode conductivity to decrease. Eventually,these voids will grow to a huge size that therewill be no apparent grain-to-grain conduction and the
effective lifetime of the electrode will reach its endeventually.
In brief, when the electrodes are exposed to sodiumunder high pressure and high temperature, they tend tosinter, thus affecting the porosity, resistance, the natureof grains, and the contact of electrodes with the BASE.Voids or porous holes in the electrodes are formed,which allow the sodium to flow through them easily.The voids will increase after an extended period oftime. This reduces the grain-to-grain contact, droppingthe electrical conductivity, and ultimately disablesthe electrodes to function any further. The thermo-chemical role played by electrodes is currently beinginvestigated in order to understand their time-dependentbehavior and thereby that of the AMTEC cell. The time-and temperature-dependent behavior of electrode ishighly correlated to the grain growth function ofits material. Alternatively, the grain size dependenceon time, and thus the power output of electrodes, isdetermined.
The power degradation of AMTEC with respect toelectrode materials at various temperatures for varyingtime durations of operation has been studied theoreti-cally as well as experimentally. As a sample, the results ofgrain size effect on four materials, namely Mo, TiN,RhW, and Rh2W, used for electrode are presented, whichhave been simulated at two different temperatures for aperiod of 15 years. This work indicates the performanceof electrode materials with respect to power output andefficiency in the descending order of Mo, RhW, Rh2W,and TiN when operated at a temperature of 1050 K onthe hot side of AMTEC. However, if the temperature israised to 1150 K, the order is changed to RhW, Rh2W,Mo, and TiN, for both power output and efficiency. Thisleads to the conclusion that the power degradation ishighly sensitive to the operating temperature of the hotside of the AMTEC cell when Mo is used for electrodescompared to the other three materials presented here.Regarding the power degradation and efficiencies attemperatures 1050 and 1150 K molybdenum electrodeshave least power degradation for AMTEC designed forlow-temperature (less than 1100 K) operation. However,their grain growth is too rapid for operation above1100 K. For AMTECs operating in that range of tem-perature, rubidium–tungsten alloys are recommended.The power output of the AMTEC cell is greatly affectedby the choice of electrode materials with respect to grainsize growth. To actually use AMTEC for a long period oftime, thermochemical conditions of electrode materialsshould be investigated to determine which material leastaffects the degradation of power output, when used forabout 15 years. These conditions can be generalizedfor other electrode materials. The power degradation isstrongly coupled with the growth of the grain size of theelectrode materials.
344 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
Recycling of Waste Heat by the TIEC–AMTEC Cascade
Utilization and recycling of waste energy has been themainstay in the recent past along with the conservation ofenergy. With the versatility of the kind of energy used byAMTEC, this problem is being addressed by directlyutilizing the waste energy of a fuel cell as an input powersource for an energy conversion device. The concept maynot be confined just to this situation; it may be extendedto any kind of waste heat, from any source in any form, inprinciple. Otherwise wasted heat can be converted byemploying AMTEC for some practical purposes. Forbetter utilization of rejected heat and improving theefficiency, two appropriate cells are looked for; fortuit-ously, the temperature at which the TIEC rejects its heatis within the range of temperature suitable for AMTECto receive the heat as input. This observation providesthe natural choice for AMTEC to be employed to utilizethe heat rejected by TIEC, i.e., cascading the two cells,the former cell utilizing the rejected heat of the latter.However, the receiving cell, AMTEC vapor anode, is ofdifferent type and efficiency. The algorithm developed issomewhat involved. The problem is divided into twosegments and combined again giving rise to a set ofnonlinear equations to be solved. Alkali metal thermo-electric converter and its working principle have been
EmitterEmitter sleeve, Mo/Re
Sup
Emitter flange,
Mo
Te
Tf
Tc
Collector, Ni
T
Transition piece Trim col
Tb
Figure 4 Schematic diagram of thermonic energy converter (TIEC)
permission from Lodhi MAK and Mustafa A (2006) Use of waste hea
Sources 158(1): 740–746.
described above but not TIEC. For the self-consistency,TIEC is also briefly described here.
Thermionic Energy Converter
Thermionic energy converter is a static energy con-version device. It converts heat energy into electricalenergy. The work on the TIECs began in the 1960s. Thefocus on the TIEC increased greatly because of one of itsgreat advantages – similar to AMTEC, TIEC convertsthermal energy into electrical energy without going intoany intermediate mechanical energy step. Some of itsother attractive properties include high-temperature heatrejection in comparison to other devices for energyconversion and high specific power. As there is no movingpart in the cell, there is no vibration. Its compactnessadds another reason to focus on this cell, particularly forspace use. The primary parts of TIEC are an emitter, anemitter flange, an emitter sleeve, bellows, and a collector(Figure 4). The emitter receives heat from one side andemits electrons from the other side. The rate of theelectron emission from the hot metal surface is a functionof the metal’s work function and temperature. The kin-etic energy of electrons within the bulk metal increaseswith increasing temperature, allowing a higher fraction ofthem to escape from the metal surface. The collector
port rod, Mo
trim
Tcalori
lar, Ni
Calorimetercollar, Ni
Tbel Bellow, Ni
with transition piece for recycling heat. Reproduced with
t of TIEC as the power source for AMTEC. Journal of Power
Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy 345
receives these electrons at a lower temperature. Theemitter acts as a cathode and the collector acts asan anode. The anode and cathode are connected by theelectrical leads, which supply the power to the externalload.
Thermonic energy converter converts a gas of freeelectrons in the interelectrode space. These electronsmay thus create a negative space charge between theelectrodes. As a result, a retarding field develops thatdeflects some of the emitted electrons back to the cath-ode. To reduce the space charge effect, the gap betweenthe emitter and the collector is filled with some plasma.The most commonly used substance for this purposeis cesium vapor. Cesium has lower ionization potentialamong candidate substances and its absorption reducesthe surface work function of the electrodes below that ofliquid cesium or any metal.
The high temperature is localized within the emitterand moderate temperature resides elsewhere in thecell. The TIEC cell operates between temperatures of1800 and 2000 K at the emitter end. The cell has beendesigned to work independently of the heat source. Theheat from the emitter zone is transported by means ofradiation and conduction to the other parts of the cell.A portion of the heat is radiated to the surrounding. Theflange receives heat from the emitter through radiationand conduction. The cesium gas, to get ionized and todissipate the electrons, uses some amount of heat aswell. The electrons get accelerated and travel throughthe electrodes toward the collector. Some of the heatenergy is also radiated to the collector. The heat from theflange is conducted to the bellows and radiated to thesurroundings and the collector. The bellows temperatureis considerably less than the temperatures at other nodesbecause the heat is radiated to the surrounding from thebellows. The efficiency of TIEC is calculated as the ratioof the contact potential difference (which is equal to thevoltage on the load) to the sum of the energy carriedaway by the electrons from the cathode and the radiationheat transferred from emitter to collector. The collectortemperature is usually between 1000 and 1250 K. Most ofthe heat energy is given out to the surroundings. Thisheat energy can be used to generate more electricity byadministering it to AMTEC. In order to transport thisheat, one face of a transition piece is attached to thecollector, whereas the other face is attached to the hotside of an AMTEC. In this manner, most of the rejectedheat of the TIECs is conducted to the AMTEC as theinput heat for conversion into electricity as usual. Theemitter converts a portion of heat energy into electricityand rejects the rest from the lower end of the collector,into the transition piece. The heat rejected at the bottomof the collector is picked up by the AMTEC hot plate viathe transition piece. In the case of excess amount of heatsupplied to the transition piece, the heat is passed to the
trim collar. On the contrary, if the transition piecedoes not get enough heat, the trim collar will supply therequired heat to the transition piece. To measure theamount of heat energy passing through the transitionpiece, a calorimeter is used.
The purpose of cascading TIEC and AMTEC is toutilize the waste heat of TIEC by deriving an optimumpower output with minimum heat energy rejected.Cascading of direct energy conversion devices whenthe waste heat of a high-temperature device is used tooperate a low-temperature device would allow thedevelopment of a highly efficient, compact, lightweightpower source and a bimodal system to address futurecivil, defense, and space missions.
Cascading the TIEC and AMTEC is basically done tomaximize the use of input energy source by achieving ahigh efficiency and power density. The cost effectivenessof the cells also depends on the weight of the conversioncells. Cascading the cells also helps to effectively reducethe weight of the converters combined.
Concluding Remarks
The current designs of AMTEC have demonstrated rea-sonably good performance in terms of efficiency and powerdensity although it has great potential for further im-provement in the future and has a great promise for the usein many applications. The design is created using thepassing of isothermal alkali metal vapor through the elec-trolyte, which produces an electric current with low voltageflow. As the functional technology is very simple with lessmoving parts than conventional engines, AMTECs havemany advantages over their counterparts. The drawbacksdiscussed above about the AMTEC cell are more a prob-lem of design than that of the technology. Technologically,it is a sound device for energy conversion. Of course,concentrated effort needs to be directed at improving theBASE and electrode performance in particular. They areresponsible to a significant degree for power production. Inorder to improve the power-time characteristic of AMTEC,efforts maybe directed to consider (i) the use of differentmaterials for the BASE and electrodes, which are least in-fluenced by the contamination, (ii) replacing the poly-crystalline b00-alumina by the single crystal b- or b00-alumina to avoid the adverse effect of the stabilizer used incase of polycrystalline b00-alumina, (iii) maintaining thecurrent density below its critical value, and (iv) keeping thegrain growth level to the minimal.
Alkali metal thermoelectric converter has no movingparts, thus creating no noise or even the vibration.Currently, AMTEC cell efficiencies range from 15% to20%, with prediction for future devices achievingefficiencies up to 40%. The AMTEC cell is very small,lightweight, and highly efficient in relation to its
346 Secondary Batteries – High Temperature Systems | Alkali Metal Thermal to Electric Energy
electrical energy output, making power more portablethan ever. Alkali metal thermoelectric converter is amodular closed-cell, quiet, efficient, compact, and port-able device for the direct conversion of heat from anysource, be it solid, liquid, gas, or radiation, to electricity.The converter being modular allows more powerfulsystems to be constructed by simply adding smaller cellstogether. These qualities make AMTEC a very attractivetechnology for near future for the purpose of spacepower, auxiliary and remote power, portable power, andself-powered appliances.
In an effective way, it has been established thatAMTEC is quite capable of utilizing the rejected energyin the form of heat at high temperature by one device asa potential source of power input. On the one hand, thistechnique provides the efficient use of waste energy, andon the other hand, it suppresses the thermal pollution, asa bonus, considering the fact that the efficiency of thestandalone TIEC used for this demonstration is im-proved by more than one hundred percent when it iscascaded with AMTEC.
Nomenclature
Symbols and Units
Tb
Temperature of the transition piece (K)
Tbel
Temperature of the bellow (K)
Tc
Temperature of the collector (K)
Tcalori
Temperature of the caloimeter (K)
Te
Temperature of the emitter (K)
Tf
Temperature of the flange (K)
Ttrim
Temperature of the trim collar (K)
Abbreviations and Acronyms
AMTEC
alkali metal thermoelectric converter
BASE
beta aluminum solid electrolyte
DC
direct current
GPHS
general-purpose heat source
HYTEC
hydrogen thermoelectric
LMMHD
liquid metal magnetohydrodynamics
RTG
radioisotope thermoelectric generator
TAPC
thermoacoustic power converter
TIEC
thermionic energy converter
TOEC
thermoelectric
TPV
thermophotovoltaic
See also: Applications – Transportation: Aviation: Fuel
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