advanced energy conversion to power ecm

8/13/2019 Advanced Energy Conversion to Power Ecm

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ergamonEnergy Convers MgmtVol. 38, No. 10-13, pp. 941-955, 1997

1997 Elsevier Science LtdAll rights reserved. Printed in Great Britain

P l I : S01 96.4 190 4(96 )00 125 -2 0196-8904/97 1%00 + 0 .00

A D V A N C E D E N E R G Y C O N V E R S I O N T O P O W E R

N O A M L I O RD e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g a n d A p p l i e d M e c h a n i c s , U n i v e r s it y o f P e n n s yl v a n i a ,

Phi la delp hia , PA 19104-6315, U.S.A .

A b s t r a c t - - T h i s p a p e r r e v i e w s s o m e l e a d i n g n o v e l e n e rg y c o n v e r s i o n a p p r o a c h e s w h i c h a r e a i m e d a ti m p r o v i n g p o w e r g e n e r a t i o n e f f ic i en c y a n d / o r r e d u c i n g h a r m f u l e m i s s i o n s. S o m e o f t h e c o n c e p t s u s e d f o rc y c l e i m p r o v e m e n t a r e h i g h e r t o p t e m p e r a t u r e s , i m p r o v e d c o m b u s t i o n s y s t e m s , e v a s i o n o f t h e C a r n o t l i m i tb y i n t e g r a t i o n w i t h f u e l c e l l s a n d d i r e c t n u c l e a r e n e rg y c o n v e r s i o n t o p o w e r ( t h e n u c l e a r g e n e r a t o r ) ,r e d u c t i o n o f e x e rg y d e s t r u c t i o n b y s t a g i n g a n d t h e u s e o f e x e rg y -e f fi c ie n t c o m b u s t i o n p r o c e s s e s , t h e u s eo f l o w e r t e m p e r a t u r e h e a t s i n k s a n d t h e u s e o f r e n e w a b l e , e n v i r o n m e n t a U y - b e n i g n e n e rg y s o u r c e s. S o m eo f t h e s y s t e m s d e s c ri b e d i n t h i s p a p e r a r e h y b r i d m u l t i - t e m p e r a t u r e s o u r c e c y c l e s ( i n c lu d i n g t h e h y b r i ds o l a r- po w ered / fue l - a s s i s t ed Rank ine cyc l e ), h igh - t em pe ra tu r e chem ica l ga s t u rb ine cyc le s , f ue l - ce l l - t opped

R a n k i n e c y c le s , h ig h t e m p e r a t u r e e j e c t o r - t o p p i n g p o w e r c y c l e s a n d h y b r i d n u c l e a r / fo s s i l fu e l p o w e rg e n e r a t i o n s y s t e m s . T h e u s e o f s p a c e ( t h e e x t r a - t e rr e s t ri a l e n v i r o n m e n t ) f o r e n e rg y c o n v e r s i o nimp ro v e men t i s a l so d i s cus sed . 1997 E l s ev i e r Sc i ence L td .

P o w e r g e n e r a t i o n E n e rg y c o n v e r s i o n F u e l c e ll s N u c l e a r p o w e r R e n e w a b l e e n e rg yP o l l u t i o n C o m b u s t i o n S p a c e p o w e r

N O M E N C L A T U R E

A = Exe rgy (k J /kgmole )g = G i b b s f u n c t i o n ( k J / k g m o l e )P = P re s su re ( kPa )

= P roces s r a t e ( kgmole / s )= U n i v e r s a l g a s c o n s t a n t ( k J / ( k g m o l e . K ) )

S = E n t r o p y ( k J / (k g r a o l e . K ) )t = Time (s )

T = Te m p e r a t u r e ( K )

Greek symbols2 - - Reac t i on a f f i n i t y ( k J /kgmole )/ z - - Chemica l po t en t i a l ( k J /kgmo le )t = H ea t f l ux (k J /m2s )

V/c = Ca rno t pow er cyc l e e f f i c i encyX - M o l e f r a c t i o n

Subscriptsc = Co ldd = D e s t r u c t i o nf = F uel

h = Ho tI = O f spec i e s Io = Re fe r e nce s t a t ep = P r o d u c t i o n

Superscripts = P e r un i t t ime

1 . I N T R O D U C T I O N

1 . 1 . S c o p e

E n e rg y c o n v e r s i o n s y s t e m sare advanced i f they improve upo n convent iona l ones Theimp rovem ents ma y be in one o r severa l ca tegor ies : h igher energy and /or exergy e ff ic iency lower

energy and /or spec ies em iss ions lower cap i ta l cos t lower opera t ing cos t s o r requ i red exper t i se andh i g h e r r e li a b il it y. O b v i o u s l y i m p r o v e m e n t s i n s o m e c a t e g o r i e s s h o u l d n o t c a u s e u n a c c e p t a b l e

94


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942 LIOR: ADVANCED ENERGY CONVERSION TO POWER

dete r io ra t ion in o thers , and synerg i s t i c improvements where an improvement in one ca tegoryresul ts a lso in improvements in other categories are the most desirable . For example, increasingeff ic iency also reduces energy emissions to the environment .

Th is paper rev iews some lead ing nove l energy convers ion approaches which a re a imed a timproving power generat ion eff ic iency and/or reducing harmful emissions, in large par t based onwo rk by the au tho r a nd h i s co-workers . Some o f the concep ts used fo r cyc le improvem ent a reh igher top t empera tu res , improved c om bus t ion sys tems , evasion o f the Carn o t l imi t by in tegra t ionwith fuel cel ls and direct nuclear energy conversion to power ( the nuclear generator) , reduct ionof exergy destruct ion b y s taging and the use of exergy-eff ic ient com bust io n processes , the use oflower t empera tu re hea t s inks and the use o f renewable , env i ronmenta l ly -ben ign energy sources .

1.2. ner gy conversion efficiencyPo we r generat ion termino logy typical ly def ines the pro cess eff ic iency in terms of energy, par t ia l ly

due to t radi t ional reasons a nd pa r t ia l ly because process inputs , such as fuel , are s t il l costed base don their energy (usua l ly heat) value. At the sam e t ime i t is increasingly real ized that the t rue m easureof pow er gene rat ion eff ic iency is the exergy eff ic iency, which relates the w ork ou tpu t to the m aximaluseful work value of the input [1 , 2] .

Fo r a g iven the rmodyna mic s ta te o f a sys tem and g iven energy or exergy inpu ts dur ing a p rocessin which one o r m ore cons t ra in t s on the sys tem were removed , the maximal a mo unt o f work wi llbe produced in a revers ible process . Such processes are character ized by inf ini tes imal ly smallthermodynamic dr iving forces and negl igible diss ipat ion ( internal entropy generat ion) , andtherefore lead, unfortunately, to e i ther inf ini tes imal ly low process ( including power generat ion)rates , or inf ini te ly large process hardware requirements . Pract ical i ty, therefore , a l lows someirreversibi l i ty in the process and thermodynamical ly maximal energy or exergy conversioneff ic iencies cann ot be at ta ined, but R D has, is and should be con duc ted to app roa ch them closerand closer. Detai led exergy (or second-law) analysis indeed serves wel l in that effor t since it a lonecan identify the specific irreversibilit ies.

The max imal efficiency qc of energy conv ersion proc esses which occ ur due to hea t f low is def inedby the Carno t l imi t

T~. c = 1 - Z 1 )

where T and Th a re the abso lu te t empera tu res o f the co ld hea t s ink and the ho t hea t source fo rthe po we r cycle , respect ively. This l imit is s imply the e xpression o f revers ibil i ty in a therm al energyconversion process . Although only a guide for pract ical power generat ion eff ic iency, equat ion (1)clear ly indicates the desirabi l ity of increasing the heat source temp erature and dec reasing that o fthe heat sink.

Energy convers ion p rocesses which do no t occur due to hea t t r ans fe r a re no t l imi ted by theCarnot efficiency, but by process irreversibilit ies.

2 T H E E N V I R O N M E N T A L R E A C T I O N T O P O W E R G E N E R A T I O N

2.1. nergy conversion species emissionsConve nt iona l pow er genera t ion p rocesses a re accom panied b y the emiss ion o f spec ies in to the

environment , with associated detr imental effects . Such emissions occur during the fuel extract ion,t ranspor ta t ion and convers ion phases and m ay produ ce secondary emiss ions o f thei r own as theydepos i t in the env i ronment o r a re s to red somewhere . The y cons i st o f a va r ie ty o f spec ies , r ang ingf rom rad ioac t ive mate r ia ls , some of which, such as p lu ton ium, have ha l f- lives o f t ens o f thousandsof years, hydroca rbon , inorgan ic and ine rt gases, l iqu id hydroc arbons and so lu tes in wa te r ( suchas coa l mine d ra inage), and var ious com bus t ion p roduc t s . P reva len t amon g the l a t t e r i s CO2, whichunt il r ecen t ly was thought to be harmless and has then b een found to have m ajor e ffec t on g loba lwarming . Perhaps one o f the l eas t ha rmfu l emiss ions i s tha t o f wa te r vapor, which i s the p r imaryemission when H, is used as the fuel .


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LIOR: ADVANCEDENER GY CONVERSION TO POWER 943

Sola r and lunar energy power genera t ion p rocesses p robab ly emi t the l eas t amo unt o f species ,conf ined to em issions assoc ia ted wi th the p rocesses used to m ake the mate r ia ls a nd ma nufac tu rethe sys tem com ponen ts and to chemica l decom pos i t ion o f these mate r ia ls dur ing the ope ra t ion o fthe power genera t ion sys tem.

2.2. nergy conversion energy emissions

An y of the energy inpu t in to a po wer genera t ion sys tem which does no t ge t conver ted in to use fu lw ork end s up as heat , which in a s teady-state process is discarded into the enviro nm ent i f unused.I t is consequent ly important to note that high eff ic iency is not only rewarding from the powerou tpu t s t andpoin t , bu t i t a l so reduces the the rmal burden on the env i ronment .

Al though such energy emiss ions mos t ly end up u l timate ly as heat , they in man y cases have o therenviron me ntal ly detr imental effects during their t ransform ation into heat. Fo r example, a l thoughconst i tut ing o nly a small f ract ion o f the discarded energy, radiat ive emissions from nuclear pow erplants , such as neutrons and ~ par t ic les and ? rays , as wel l as s t rong electr ic f ie lds emit ted byelectr ical pow er generat ion and t ransmission equipm ent , m ay cause harm signif icant ly bey ond theirenergy content .

3 . R A I S I N G T H E T O P T E M P E R AT U R E

Most energy sources are not in pr inciple l imited by the temperature to which they can elevatethe working f luid of the power generat ion cycle . This is operat ional ly obvious in fuel combust ionsystems and nuclear f iss ion or fusion systems, but is even t rue for diffuse sources such as solarenergy : t emp era tu res o f thousands o f degrees have a l ready been a t t a ined wi th ex is t ingconc entrators . Increasing the efficiency of thermal pow er generat ion cycles by raising the toptem peratu re is , thus , not constrained by the potent ia l o f the energy source to do so, bu t isconstrain ed by the abi l i ty of engineer ing mater ia ls and devices to withstand higher temp eratures ,som etimes ac com pan ied also by higher pressure and oth er effects detr imental to the abi l i ty of thedevice to perform i ts funct ion.

M uch progress has been made dur ing the pas t cen tury in ra i sing the top t empera tu re o f thewo rking f luids . This was achieved by a com binat ion of bet ter mater ia ls and m ore ingenious device

engineer ing, such as turbine blade cool ing, intermit tent co mb ust ion acc om panie d by cool ing ininternal com bus t ion engines and m agnet ic f ie ld confinement of plasmas at temp eratures o f mil l ionsdegrees in fus ion pow er exper iments . H ere we g ive an example o f a p roposed pow er cyc le in whichh igher top t empera tu re can be ach ieved by incorpora t ion o f a dev ice which can to le ra te suchtemperatures because i t has no moving par ts , is not subjected to high pressures and can thus beconstructed from avai lable mater ia ls such as graphi te , graphi te composi tes or ceramics .

The p ropo sed sys tem i s the e jec to r- topp ing pow er cyc le p ropose d b y Freedm an and Lior [3] ,descr ibed in Fig. 1 . The hot gases generated in a furnace at temperatures above the tolerance ofava i lab le gas tu rb ines a re used to com press ano ther gas in an e jec to r. They a re coo led the reby toa level acceptable for use in present day turbines by a process which produces compression workof ano ther gas . F rom the second- law v iewpoin t, the s t raigh t coo l ing o f com bus t ion gases from thecombus t ion t empera tu re to tha t accep tab le fo r tu rb ine opera t ion , a s p rac t i ced in conven t iona l

sys tems , des t roys comple te ly the exergy con ta ined be tween these two tempera tu res . H ere the samecool ing is accompl i shed wi th conco mi tan t p roduc t ion o f use fu l work , c lea r ly an impor tan t exerget icimprovement . The e jec to r, a s opposed to tu rb ines , can opera te a t the very h igh t empera tu resbecau se of i ts inherent ly s imple construct ion and absence o f mo ving par ts , which resul t in very lowmechanical s t resses and high rel iabi l i ty.

Alth oug h ejectors -have relatively low efficiencies , the e jector-based to pping cycles m ay have anoveral l higher eff ic iency than that of current turbine-b ased to pping cycles , becau se of two m ajoradvantages: 1) the ejector can tolerate higher temp eratures than a turbine, and 2) i t cou ld usework ing f lu ids which have the rmop hys ica l p roper t ies super io r to those which can be used in tu rb inetoppin g cycles. The f luids chosen in that s tudy were hel ium the secondar y f luid) and s odiumasthe pr im ary f luid) . I t i s bel ieved that hel ium is a goo d choice as a turbine f luid because o f its lowmolecular weight and i ts high specif ic heat ra t io , a l though i t i s harder to compress in the ejector.Sodium was chosen because there is experience in using i t in high-temperatureenergy systemsa n di ts the rmophys ica l p roper t i es seem to be more p rom is ing than those o f o thercandidate materials.


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9 44 L I O R : A D VA N C E D E N E R G Y C O N V E R S IO N T O P O W E R

N E AT I N P U T N g VA P O R( C O M a U S T I O N ) 1 5 0 0 K

~, 11 AT A

f l ; e -I E J E C T O R I_ I M e 5 2 2 K H e +N a

~ ' )- .T C L 4 3 ATA 1 0 2 4 K

I ~ I T U R l l I N E . ~ 0 ATA~

_

~ ) Q ~ STEAU) G E N E R AT O R

N 6 3 K

0 . 7 8 ATA

T ~H E E J E C TO RTO P P IN G C Y C L E C O N V E N T I O N A L I

R A N K I N ECYCL E

Fig. 1 . Flow diagra m of an ejector- top ping cycle [3].

I t is possible that o ther wo rking f luid pairs are bet ter (and safer re la t ive to sodium ) and a th orou ghthermoec onom ic se lec tion s tudy was recomm ended . A po ss ib le mate r ia l o f cho ice fo r the e jec to ris coated or sheathed graphi te , as of ten used in appl icat ions such as re-entry vehicles and rocketnozz les . Advanced ce ramic mate r ia ls , a re becom ing ava ilab le fo r con t inuous op era t ion up to abo ut2000 K.

A conserva t ive analys is fo r a 50 M W ne t ou tpu t p lan t wi th a sod ium top t emp era tu re o f 1500 Kand pressure of 1 .08 M Pa ( in the boi ler) with the othe r condi t ions s how n in Fig. 1 , has exhibi tedthat this cycle has an efficiency 6.4 higher than a conv ent ional s team R ankin e cycle with outtopp ing . The improveme nt was seen to inc rease to 11 fo r top sod ium condi tions o f 2000 K,5 MP a. The b o t tomin g cyc le in bo th cases was a R ankine cyc le wi th ex it s team c ondi t ions o f 810 K,

24 M Pa. N o a t t emp t w as made as ye t to op t imize the f lu ids o r opera t ing condi t ions and i t i sexpected that even bet ter eff ic iency improvements can be at ta ined.Other im provemen ts were no ted in tha t the h igh t empera tu re opera t ion a l lows the use o f a muc h

smaller furnace ( the sodium boi ler) with pract ical ly no ash accumulat ion or emission problems.This holds good promise for coal ut i l izat ion.

4 . I M P R O V I N G L O W T E M P E R AT U R E C Y C L E E F F I C I E N C Y B YH I G H - T E M P E R AT U R E S U P E R H E AT

There of ten exis t s i tuat ions where the top cycle temperature is wel l below the tolerance l imit ofconven t iona l mate r ia l s and dev ices . Some of the examples a re so la r powe r sys tems which usef la t-p late o r m odera tdy -conce n t ra t ing co l lec to rs , geo thermal sys tems , was te -hea t opera ted sys temsand wate r-coo led nuc lea r reac to rs where the l imi ta t ion i s due to the accom panying s team pressureand the combined pressure and tempera tu re e ffec t s on fue l - rod in tegr ity.

The key to efficiency impro vem ent o f such cycles is the wel l -know n fact that the isobars in anenthalpy--entropy (Moll ier) diagram (Fig. 2) diverge drastical ly as the f luid changes from l iquid tovap or and cont inue diverging as the vap or is increasingly superheated. This divergence is the reaso nwhy Rankine cyc les have an ex t remely favorab le backw ork ra t io , r equ i r ing very l it tl e en tha lpyincrease in the compress ion o f wa te r re la t ive to the s team en tha lpy d ro p dur ing the expans ionbe tween the sam e two i sobars in the tu rb ine . B ased on th i s phenomen on , severa l cyc les have beenproposed where the s team genera ted by the p r imary, low tempera tu re , energy source i s fu r the rsuperhea ted by some o ther means and then expanded th rough a tu rb ine to make work .

One example o f th is i s the hybr id so la r-powered / fue l-ass is ted Rank ine cyc le s tud ied b y L ior andco-w orkers [4 , 5] and others [6 , 7]. As sho wn in Figs 2 an d 3, the lower tem peratu re (here 102C)s team genera ted b y so la r energy i s superhea ted by passage th rough an in te rna l hea t - recovery hea t


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L I O R : A D V A N C E D E N E R G Y C O N V E R S I O N T O P O W E R 9 45

>

d

X

zo

xternalSuperhea t

Torom Ilegene rotor / ~. r q e n e r o t o r

SolarEnergy

From I~cenomizer

ENTROPY

Fig . 2 . M ol l i e r d iagram of the so la r powered fue l superhea ted R ank ine cyc le [4] .

exchanger ( the regenera tor ) and then hea ted fur ther by a fue l - f i red superhea te r to i t s toptemp era ture of 600C (cor responding to top tempera tures used in convent iona l s team po wer p lan ts,bu t here a t jus t a tmosph er ic pressure). The s team i s then expan ded thro ugh a low-pressure s teamturb ine to produce power and i s cooled to condensa t ion tempera ture by in te rna l hea t exchangeand recovery. The co ndensed s team i s p rehea ted by in te rna l hea t recovery and then re turned tothe solar boi ler.

Analys i s o f the cyc le has show n tha t i t s efficiency a t the above condi t ions is about 18-20 , m orethan doub le tha t o f a pow er cyc le opera t ing a t the so la r-genera ted s team tempe ra ture of about100C, impressive ly accompl i shed by the addi t ion f rom the fue l source of on ly about 20 of theto ta l energy. A p ro to type cyc le and a 30 hp counter- ro ta t ing tu rb ine wi th an e ff ic iency of 75 weredes igned and bui l t by the au thor and h is co-workers [4 , 5 ]. Al thoug h i t requi res fur ther p ro of inla rger p lan ts , econo mic ana lysi s has pred ic ted a c lear advantag e o f such hybr id p lan ts ov er thoseopera t ing wi th the lower tempera ture hea t source on ly (al so see com men ts in Sec t ion 7 abou t thes imi la r Luz so la r power g enera t ion sys tem) . I t is no tewo r thy tha t so la r energy can a l so be usedto superhea t the s team, by em ploying so la r concent ra tors , thus avoid ing the n eed for fue l.

S imi la r cyc les were a l so prop osed for use w i th geotherm al sources [8] and wi th au tomo t iveengines [9] . W ater-cooled nuc lear reac tors a re l imi ted in the i r top s team tempera ture an d pressureto ab ou t 285C, 6.9 MP a, s ignif icant ly lower than the 600C, 30 MP a l imits of advanc ed fossi l -fuelf ired s team p lan ts . F ur therm ore , nuc lear power p lan ts do not p rovide superhea ted s team, whi lefossil fuel plants d o. Th e efficiency of such nuc lear pow er plants is therefo re l imited to 29-35 ,up to ab out 1 /3 lower than tha t o f advanced foss il fue l pow er p lan ts. A com para t ive energy andexergy ana lys is o f an opera t ing B W R pow er p lan t [10] has co nc luded tha t e ff ic iency can improveby incorpo ra t ion of a fossi l- fue l -f ired economizer, superhea te r and rehea te r, ups t ream and


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946 LIOR: ADVA NCED ENERG Y CONVERSION TO POW ER

dow ns t ream of the reac to r vesse l, r espect ive ly. In fac t, the Conso l ida ted E dison C om pany of NewYo rk cons t ruc ted a P W R nuc lea r s ta t ion ( Ind ian Po in t ) which incorpora ted a separa te o i l -f ir edsuperheater, w ith a resul ting 21 gain in efficiency [11]. The cost of produ ced electr ic i ty was foun dto d rop by about one- th i rd when compared to conven t iona l nuc lea r power p lan t s wi thou t fue lsuperheat . Another external ly-superheated nuclear plant using oi l for superheat ing was bui l t nearLingen, Germ any , achieving an efficiency imp rovem ent of near ly 33 [12] . Ne i ther plant is,however, in opera t ion any longer.

5 . L O W E R I N G T H E B O T T O M T E M P E R A T U R E

Therm al po we r plant eff ic iency increases as the bot to m tem peratu re is lowe red. In thetempera tu re range o f ambien t coo lan ts , an effic iency improvem ent o f up to abou t 1 /2 i s ob ta inedfrom each C by w hich Tc is lowered. I t i s thus de sirable to seek ways to d o so an d a few aredescr ibed below.

One w el l -t rodden pa th i s the improvem ent o f hea t t r ans fe r in the hea t re jec t ion equ ipment , m os tprominen t ly in the power p lan t condenser. Th is lowers the condensa t ion t em pera tu re and pressureof the s team by br ing ing i t s t empera tu re c lose r to tha t o f the coo lan t . At the same t ime th i sapproach inc reases cap i ta l cos t s and pumping energy use .

A m ost intui t ively obvio us way to that en d is the f inding and use of colder coolants . Since powe rstat ion locat ion is present ly dictated in large par t by the need for some proximity to the users andby env ironm ental constraints , the select ion f lexibil i ty as wel l as the exis t ing differences betwe en theavai lable convent ional coolant sources are ra ther small . At the same t ime there potent ia l ly exis ta t l eas t th ree low tempera tu re hea t s inks fo r the rmal power p lan t s which dese rve cons idera t ion :the co ld wate r in the dep ths o f the oceans th rough out the wor ld , the co ld a i r, wa te r a nd ice inthe polar regions, and space.

There a re many loca t ions a round the wor ld , even near the equa tor, where ocean wate rtempera tu res a re down to abo ut 5C a t dep ths be low abo ut 500 m. A f te r cons ider ing pumpinglosses , the use of this water for cool ing the power plant condenser is expected to ra ise the planteff ic iency by at least 10 . Several experime nts of ocean -therm al energy conversion (OT EC ) havedem ons t ra ted tha t the cons t ruc t ion o f the p ip ing sys tem to these dep ths , and the pumping of thecold water to the surface, are feasible and within reasonable cost . Remaining issues to be moredefini t ively resolved are environmental impact , including effects on the ocean f lora and fauna,

~ . 1 : ~ / ~ . . . . . . . . . . . . . . ~_ . . . . . . . . .

~ ~ / / o o

, l c , ' - . I,~. c I I~lo ;t c c

_

i R O R W T E R OOLI O

Fig. 3. Flow diagram of the solar-powered/fuel-superheated power system with typical operatingcond itions [5].


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LIOR: ADVANCEDENER GY CONVERSION TO POWER 947

tempera tu re inc rease o f the ocean wate r, r e leases o f CO2 f rom the wate r ra ised to lower p ressuresur roundings and s tab il i ty o f the p ip ing sys tem under s to rm condi t ions .

Pow er genera t ion eff ic iency co uld be s ignif icantly increased by u sing the cold air or ice of thepo la r reg ions as coo lan t. T o take fu l l advan tage o f coo lan t t em pera tu res m uch be low the f reez ingpoin t o f wa te r, o ther w ork ing f luids wou ld have to be used . In cons idering th is approach , someof the majo r obstac les are environmen tal im pact and, i f fossil fuels are used, the problem s oft ransport ing them to such s i tes .

The u se of space as the heat s ink for pow er plant is very appeal ing. Being at near a bsolute zerotemp erature , i t i s indeed the lowest a t ta inable te mp erature h eat s ink. Ha ving immense, and e verexpanding, s ize , i t would be affected negl igibly by any heat addi t ion from the ear th . Diversion ofthe power-generat ion related energy emissions from the terrestr ia l s inks to space would serve wel lin heal ing our environment and prevent ing i ts fur ther deter iorat ion. The most direct way to affectcool ing is by radiat ive heat t ransfer f rom the power plant coolant into space. This is a l ready donein satell i te po we r plants an d requires the placem ent o f the pow er s ta t ion in space. The difficult iesof doing so are obv ious, but even the current tech nology is sui table for this purpo se and aside-benefit is the remov al of pow er s ta t ions an d their detr imental effects f rom terrestr ia l locat ions.

Du e to po wer p lan t s it ing requ i rements , po wer t ransmiss ion to use r s it e s i s a p rob lem com mo nto a ll o f the above-descr ibed ways fo r reduc ing the bo t tom tempera tu re . S uper-conduc t ingtransmission l ines, pow er t ransmission by m icrowaves, or on-si te man ufactur ing of easi lyt ranspor tab le fue l s fo r e lec t rochemica l o r combus t ive energy convers ion a re some of thetechnolog ies to be exp lored and advanced fo r tha t purpose .

6 . C L E A N E R C O M B U S T I O N

6.1. PrefaceMany methods a re be ing pursued to ob ta in combus t ion which p roduces lower emiss ions ,

espec ia l ly o f the spec ies mo re haza rdous to hea l th . Descr ibed here a re two approaches in whichthe au thor and h i s co-workers a re engaged .

6.2. The radiatively-conductively stabilized combu stor RC SC )In most combust ion devices the energy necessary to heat the fuel to the point of igni t ion is

suppl ied by back-mixing ei ther by molecular or turbulent diffusion. The back-mixing produces anex tended reac t ion zone , con tac t be tween the fue l-a ir mix ture and the p roduc t s o f com bus t ion andosci l la t ions. These three s ide-effects are kn ow n to en hance NOx formation.

A me thod fo r s t ab il iz ing com bus t ion by rad ia t ion and conduc t ion was exp lored ana ly ti ca lly andexperim ental ly by C hurchi l l, Lior a nd co-w orkers for m any years [13, 14] , with gaseous, vo lat ileoi l and propa ne/pulve r ized-coa l m ixture fuels . In their s tudies using f luid fuels , Churchi l l , L ior andco-worke rs have demons t ra ted tha t ex t remely low leve ls o f NOx 5-100 ppm ) were genera ted areduc t ion o f abo ut an o rder o f magni tude as comp ared w i th conven t iona l burners) , mos t ly because

of the rap id hea t ing o f the fue l and min imal back-mix ing of the p roduc t s o f com bus t ion in suchburners . This f lame s tabi l izat ion is shown schematical ly in Fig. 4 , where the thermal feedbackoccurs by rad ia t ion f rom the ho t dow ns t ream reg ion of the burner to the co lder ups t ream reg ionand also by longi tudinal conduct ion through the tube wal l in the same direct ion. The cold,unbu rned gases en te r ing the tube a re hea ted to the ign it ion t empera tu re p r imar i ly b y convec t ionfrom the tu be wal l but a lso by conve ct ion from the radiant ly-heated par t ic les whe n sol id fuels areused) . The hot , burn ed gases , in turn, he at the downs tream wall by convect ion.

W hen the fuel was pulver ized coal , the f lame zone was foun d to be thicker than with f luid fuels ,but thin, f ro m a bo ut 30 cm near the w ai ls to ab ou t 50 cm at the center, re la t ive to conven t ionalpulver ized coal com busto rs . C oal res idence t imes were thus o nly 0.125 to 0 .6 s , smal ler than ther e s i d e n e t imes of abo ut 1 s in comm ercial com busto rs using recirculat ion for f lame s tabi lizat ion,e v e n though the RC SC ana lyzed here had gas ve loc i ti e s which a re about tw o orders o f magni tudes lower than those in such comm erc ia l combus tors . Th is has the p rom ise o f s igni fi can t reduc t ionin NOx produc t ion .


8/15

948 LIOR: ADVANCED ENERGY CONVERSION TO POWER

p u l v a i z a l c o si /s ir m i z t mlow

Fig. 4. The thermal feedback mechanism in the radiatively/conductively stabilized combustor [13 14].

6.3. The chemical gas turbine topping cycle

To a t t a i n h igh t op t empe ra tu r e s and ye t l ow NOx emi s s ions , Ara iet al. [ 15] have p ropo sed acom b i ned B ray to n -R ank ine cyc le (F ig . 5 ), in wh ich fue l -r i ch com bus t i on o f a m i x t u r e o fcom pre s s e d a i r and fue l t ake s p l ace i n t he f i r s t bu rne r, t he ho t ga s con t a in ing unbu rne d fu e lexpa n d s t h r o ugh t he h igh t emp e ra tu r e t u rb ine , is t hen r ecom pres sed and pa s sed on t o a s e co n d ,l e a n m i x t u r e bu rne r. Th e ga s is t hen expan ded t h rou gh a l ower t empe ra tu r e t u rb ine and t he n u s edto ge n e ra t e s t e am in t he bo i l e r o f t he Rank ine cyc l e , The fue l - r i ch combus t i on a t t he h i g he s tt em p e ra t u r e s a l l ows good con t ro l o f NOx p roduc t i on i n t ha t r educ ing a tmosphe re . Be in g be lo wthe s t o i ch i ome t r i c conce n t r a t i on a t w h ich NOx p rodu c t i on i s max im a l , t he fuel - le an comb us t i o nin t he s eco nd comb us to r a l so p roduces on ly sma l l amo un t s o f NOx . B ecause o f t he pos sib il i ty o f

r e a c t i on c o n t ro l i n t he t opp ing pa r t o f t he cyc le , t he au tho r s ha ve ca l led it a chem ica l g a s t u r b i n ecycle.

W e h a v e pe r f o rm ed an ana ly si s o f t h i s cyc le and foun d t ha t t he ove ra l l e f fi ci ency f o r t h eco n d i t i o n s l i s ted i n F ig . 5 i s 62% and t he exe rgy e ff ec ti venes s i s 76%. A t op t ima l c i r cums t ance sa n d t he s a m e t op t emp e ra tu r e o f 1773 K the e f fi ci ency was fo und t o r e ach 66% wi th an e xe rg yeff ic iency of 81% .

7 . SOLAR AND LUNAR ENERGY

S o l a r r a d i a t i on can be u sed fo r d i r ec t conve r s ion t o e l e c t r i c i t y, u s ing pho tovo l t a l c c e l l s , f o rt h e rm a l p l a n t pow er gen e ra t i on by con ve r t i ng it i n t o hea t , o r by i ts c a t a ly t i c e ff e ct in va r i o u spho toche m ic a l r e ac t i ons , such a s pho to syn the s i s , w i th subsequen t conve r s ion o f t he b i o m a s s t oh e a t . Th e so l a r ene rgy i npu t t o e a r t h a l so g ive s r is e t o o the r e f fec ts wh ich can be exp lo i t e d f o r powe r


9/15

LIOR: ADV ANC ED ENERGY CONVERSION TO POWER 949

genera t ion such as wind ocean cur ren ts and the ocean tempera tu re d i ffe rences. The m oonproduces t ides and con t r ibu tes to wa ve fo rma t ion a l lowing the use o f t ida l and w ave energy.

In the con tex t o f advance d energy convers ion the so la r and lunar sources excel pa r t i cu la r ly intwo areas: they are pract ical ly inexhaust ible pro duc e minimal species emissions and d o not a l tert h e g l o b a l h e a t b a l a n c e . U n l e s s u s e d i n t h e r m a l p o w e r c y c l es o r a s c o m b u s t i o n f u e ls , th e y a l s o h a v em i n i m a l l o c a l t h e r m a l e m i s s i o n s . A t t h e s a m e t i m e , t h e s e s o u r c e s h a v e a l o w e n e rg y fl u x a n dt h e r e f o r e r e q u i r e l a rg e a r e a s a n d a l arg e q u a n t i t y o f m a t e r i a l f o r t h e i r us e . T h e c o m p o n e n t s o f th ep l a n t i t s e lf b e c o m e t h e n a s i g n if i c an t c o n s u m e r o f e n e rg y a n d s o u r c e o f e m i s s i o n s i n t h e p r o c e s so f t h e i r p r o d u c t i o n a n d u s e [ 16 ].

P r o b a b l y t h e m o s t r a p i d p r o g r e s s i n s o l a r e n e rg y c o n v e r s i o n t e c h n o l o g y i s s e e n i n p h o t o v o l t a i c s ,w h e r e s i n g le c el l e f fi c ie n c ie s h a v e e x c e e d e d 3 0 % i n t h e l a b o r a t o r y . T h e s e n u m b e r s a r e a p p r o a c h i n gp o w e r g e n e r a t i o n e f fi c ie n c ie s i n c o n v e n t i o n a l n u c l e a r p o w e r p l a n t s . I m p r o v e d e f fi c ie n c y, r e d u c e dc o s t a n d b e t t e r l o n g - t e r m s t a b i l it y o f t h e c e ll s a r e t h e p r i m a r y o b j e c t i v e s o f R & D i n t h a t f i e ld .

S o l a r t h e rm a l p o w e r g e n e r a t io n w a s a l r ea d y p r o v e n u n d e r c e r t ai n e c o n o m i c c ir c u m s t a n c e s t ob e c o m m e r c i a l l y c o m p e t i t i v e in s m a l l f u e l - s u p e r h e a t e d p l a n t s , s u c h a s th o s e m a n u f a c t u r e d a n do p e r a t e d b y L u z C o . [1 7] , w h i c h h a v e p r o d u c e d e l e c tr i c it y a t a n e f fi c ie n c y o f u p t o 3 8 % a t a c o s to f 8 c / k W h a n d r e l ia b l e h i g h -e f f ic i e nc y o p e r a t i o n w a s a c h i e v e d b y a n u m b e r o f l a rg e s c a l e c e n t r a lr e c e i v e r p o w e r p l a n t s .

8 . B E Y O N D C A R N O T

8.1, refaceP o w e r g e n e r a t i o n s c h e m e s w h i c h d o n o t n e e d h e a t a s a p r i m a r y i n p u t a r e n o t s u b j e c t t o t h e

C a r n o t e f f ic i e n cy l i m i t a ti o n s , t h u s r e l i e v in g t h e o b s t a c l e s a s s o c i a t e d w i t h t r y i n g t o a t t a i n h i g h t o pa n d l o w b o t t o m t e m p e r a t u r e s i n o r d e r t o i n c r e a s e e ff i ci e nc y. C l a s s i c al e x a m p l e s o f s u c h d e v i c e s

xhaust

98.41kPa

r Steamc y c l e8 7 3 K ~

9 5 2 1 d P a~ J . N . 7 ~ r a ~

L - -

12SSK =

I 1 9 6 8 .2 8 k P a ( ~ r -I F u e l ~

I 8 8 K ~

I G a s c y c l e 1t

C o m p r e s s o r e f f i c ie n c y8Genera to r e ff i c i ency95

Flow rates; CH 4 143.092 Tons/Hr (2.247619776 kmol/s)Air 285907956 Tons/HrEquiva lenc e ra t io : ( ) 2 .86Rankinecyc le ma ss f low ra te :200 kg/sTotal work output 176Mw 63% -1s t l awEFF3

G a s c y c l e 2 g ' ~ 8 8 9 K

I 4 1 3 3 K " " ~ ' y / 1 0 1 .3 6 k P a

I ) 98.~,1~aP 2 - 7 1 0 7 3 K

I - - - - T ' - i f'~2317 .71kPa ~

I I cB2h l l f (~) ' ' v ' 2 / 1 9 1 / . 2 7 k P a~ ' I I [ 8 7 3 K_ J L - - - -~_ 2015.37kP a _

9 8 . 4 k P a /I ~ 8 7 3 KEff.=.9 ~ 2015.37kPa ~ 981.8K

s73K 1917 .2~a a I O n ~2 3 1 7 . 7 1 k P a ~

. I

-1

_1

Fig. 5. The RAN chemical gas turbine cycle.


10/15

950 LIOR: ADVANCED ENE RGY CONVERSION TO POWER

are hydro -pow er p lan ts and water-cur ren t o r wind turb ines. M ore nove l dev ices a re photovo l ta iccel ls , fuel-cel ls , and bat tery-l ike devices which generate energy due to solut ion concentrat iondi ffe rences across a semi-permeable me mb rane [18]. The encouraging an d rap id deve lopm ent ofphotovo l ta ic energy convers ion was men t ioned above . S ignif ican t p rogress is a l so be ing m ade inthe de velop men t and use of fuel cells [19] and a top ping cycle using the m [20, 21] is described below.

8.2. u e l c e l l sFuel cells conv ert chem ical energy o f fuel direct ly into electrici ty. In fuel-cel l chem ical react ions

the repos i t ion ing of the assoc ia ted e lec t rons i s ach ieved wi th g rea te r cont ro l than in com bus t ion .In th e process, a po rt ion of the electro--chemical energ y of electron bond ing is extracted electr ical lyra ther than be ing to ta lly d i ss ipa ted in to therm al energy (i .e . in to rand om mo t ion of the reac t ioncomp onents ) as in comb us t ion . Thus , there i s l ess assoc ia ted en t ropy prod uct ion th an in o rd inarycombu s t ion , where e lec tron energy i s no t explo i ted and the am ou nt o f en t rop y produc t ion i s le f tuncons t ra ined .

In as m uch as the ra te o f en t ropy prod uct ion (Sp) in a process is

Sp = -~ .[d riv in g force(s)] (2)

where T i s the abso lu te tempera ture and/~ i s the process ra te , to reduce en t ropy product ion fora f ixed process ra te one mus t e i ther increase the loca l t empera ture or reduce the re levanttherm od yn am ic driving force(s) . In turn, the rate of useful energy destruct ion, -4d, is directlypropor t iona l to the en t ropy product ion ra te

/id----- To ~ p. (3)

By reducin g process i rreversibi l it ies , device and system efficiencies are improv ed.Let us co mp are the cxerge t ic aspec ts o f o rd inary com bus t ion a nd fue l cel l reac t ions . In ord ina ry

comb us t ion , a fue l i s b roug ht in d i rec t contac t wi th ox ygen to reac t and p roduce oxida t ionproducts . The resu l t i s a convers ion of chemica l energy of the fue l to thermal energy of theproducts [22], in which 20-30% of the fue l exergy i s des t royed and approxim ate ly 80% of thecomb us t ion i rrevers ib il ity occurs dur ing the in te rna l thermal energy exchange subprocess.

W hen a fue l i s burned in a i r a t the ra te /~f the dr iv ing force for the reac t ion i s the d i ffe rencebetwee n the chemical potent ials ( /~) of the reactan ts and prod ucts , w hich is the chem ical aff inity(2) of the reac t ion . The ra te of usefu l pow er consum pt ion by fue l ox ida t ion i s

Ad = To~v To To-~Rr2 = -~Rf(/~ru~t+ #oxy~, -/~produo~). (4)

Fue l cells lower the re act ion aff ini ty by f i rs t passing ions th rou gh an electrolyte . Fo r exam ple,sol id oxide fuel cel ls operate with oxygen ions migrat ing through a sol id electrolyte . By passingoxy gen throu gh the sol id electrolyte prior to fuel oxida t ion, su ch a fuel cell lowers #o,ym, which,in tu rn , lowers the pow er consu mp t ion o f the ox ida t ion reac t ion [equa t ion (4) ] , i .e . thee lec trochemica l po ten t ia l o f oxy gen a t the anode (where the ox ida t ion occurs ) is lower than theva lue sensed in ord ina ry combu s t ion , nam ely the va lue in the a i r f ree s t ream on the ca tho de s ideof the electrolyte .

Up on going throug h the e lec t ro ly te and dropping in po ten t ia l , the oxyg en ions y ie ld e lec t ronsa t a h igher po ten t ia l (at the ano de) than the po ten t ia l a t w hich they were acqui red (a t the ca thode) .The cel l thus del ivers net power, e lectr ical ly. Therefore, af ter passing oxygen through theelectrolyte , the fuel oxidat io n is less violent ( less dissipat ive, less i r reversible) inasm uch as the forcedriving the react ion ~. is reduced.

When hea t i s t ransfer red , the ra te of usefu l power consumpt ion by hea t t ransfer i s

T o

wh ere e is the therm al energ y f lux. By ex tract ing electr ical energ y during th e ov eral l react ion, th eenergy of the reac t ion products i s reduced . In tu rn , the temp era ture grad ien ts be tween the reac t ion


11/15

L I O R : A D V A N C E D E N E R G Y C O N V E R S I O N T O P O W E R 951

F u i l - c o l l I

s y s t e m i ~

i ~ ) - t i I

W ~ m

J h i l t

c h a m b e r e x c h a n g e r

S y m e m1

I i

uel

T ol U l e k

J

O e m

Fig . 6 . The p lan t conf igura t ion o f a fue l-ce ll topp ing pow er genera t ion sys tem [20].

zone and the neighboring zones is lower than that sensed in ordinary combust ion. Thus, re la t ivelyless exergy is destro yed during the internal therm al energy exchange [equ at ion 5)] .

Altho ugh fuel-cel l techn ology has been s tud ied extensively, the best ways to e mp loy fuel-cell uni tsfo r the genera t ion o f e lec t ri cal pow er remain to be de te rmined . A num ber o f fue l -ce l l /power-p lan tconfigurat ions are possible for that pu rpose. O ne possible configurat ion, pro pose d in [20, 21] and

show n in Fig. 6 , is the ut i lizat ion o f a fuel cell as a topping uni t to an exis ting or future con vent ionalpow er plant . In this configurat ion, h ot fuel and oxidan t wo uld f irs t be passed throug h the fuel cellwhich w ould thus p roduc e par t o f the overa ll e lec t ri cal ou tpu t o f the p lan t . Af te r the gases emergefrom the fuel cel ls , s t i l l a t re la t ively high temperature , they would be mixed and oxidat ion wouldbe comple ted by com bus t ion ; the p roduc t s wou ld be used to genera te s team for power ing a Rank inecycle plant which pro duce s the rem ainder of the electr ical energy.

An addi t ional benefi t of fuel-cel l toppin g systems is the reduct ion of exergy con sum ption insubsequen t com bus t ion , dow ns t ream of the fue l-ce ll un i t in the bo i le r combu s t ion chamber. Th isreduc t ion i s a consequence o f a reduc t ion o f the average chemica l po ten t ia ls o f oxygen and fue lbecause they are more di lute af ter par t ia l oxidat ion in the fuel cel ls .

We cons ider the re la t ionsh ip

I .t ~/ T = g i T, P ) / T +9 ~ l n z i 6)

for ideal gases , whe re Zi is the mole frac t ion o f com pon ent I and ~ is the universal gas constant .I t can be seen that a t a given T, as Z~ is reduce d for reac tants and increased for produ cts , theirgilT values are reduced and increased, respect ively, with the effect of reducing the value of2ITSo, i f par t of the fuel oxidat ion has b een acco mp lished in fuel cel ls , thereby decreasing the X~ ofthe fuel and o xyge n and increasing the Zi of the products , the value of2/T a t the onse t o f thesubsequen t comb us t ion in the bo i le r is lowered . S ince2/T goes from the ini t ia l value to zero asthe com bus t ion p roceeds , the e ffec t i s then to reduce i t s average m agni tude dur ing com bus t ion and ,from eq uat ion 5) , to reduce the exergy destruct ion. This conclusion is based on the assump tiontha t the t empera tu re o f the reac tan t s p r io r to combus t ion i s e ssen t ia l ly the same as in o rd inaryboi ler combust ion. I t can be seen from the schematic diagram of Fig. 6 that this wil l be the case,as wil l be confirmed quant i ta t ively below.


12/15

9 5 L I O R: A D VA N C E D E N E R G Y C O N V E R S IO N TO PO W E R

Based on the discussion above, this type of configurat ion reduces the investment in fuel-cel lsbecause they a re thereby used only whi le the chemica l d r iv ing forces a re s t i l l h igh . Ins tead ofcont inu ing the ox ida t ion process wi th increas ing ly d i lu ted reac tan ts , wh ich produces con com i tan t lydecreas ing power y ie ld, the d i lu ted reac tan ts a re fed to the com bus tor, w here they combine mo reeff iciently. I t was no t im plied that this plant con figurat ion is ei ther the mo st eff icient or mo steconom ica l , bu t i t is a s imple example which serves to i l lus t ra te the improv emen t to therm ody nam iceff iciency wh en inc orpo rat ing fuel-cel l uni ts into electr ical pow er-gen erat ing or c ogen erat ing plants .

This power p lan t cons is t s o f : (1) th ree hea t exchangers (prehea te r 1 , p rehea te r ~ 2 and thepow er-cycle hea t exchanger) ; (2) a fuel-cel l uni t ; (3) a com bu st ion ch amb er; and (4) the s team p ow ercyc le of an ex is ting 300 M W pow er p lan t . H ydro gen i s fed to prehea te r @2 a t am bien t p ressureand temperature, to raise i ts temperature to the level needed for operat ing the fuel-ceU unit .Am bient a i r i s passed throug h prehea te rs 1 and 2 for the same purpose .

Par t ia l ox ida t ion of the fue l takes p lace w i th in the fue l-cel l sys tem. Ha ving de l ivered an am ou ntof electr ical pow er, the p rod uct s t ream s (depleted fuel and air) exi t the fuel-cell uni t a t a h ighertempera ture and , fo l lowing hea t exchang e in prehea te r ~ : 2 , en te r the combus t ion ch amb er wherefue l ox ida t ion i s comple ted . The com bus t ion p rodu ct gas then supplies hea t f i rs t to the pow er cyclean d th en to the in com ing air. W hile imp rovem ents in sol id electrolyte fuel-cells have s ince been

achieved , the fue l -ce ll per form ance charac te r i s tics in th i s s tudy a re assum ed to be those of aWestinghouse Bell-and-Spigot design.Typica l condi t ions for a case in the ana lys is a re show n in F ig . 7 an d the cor responding exergy

and energ y flow diagram in Fig. 8 . The stu dy fo un d tha t th e exerget ic eff iciency of the fuel-cel lun i t ranges f rom 95 .9 to 99 .9%, increas ing wi th decreas ing curren t . Topping conven t iona l Rankinecycle pow er plants w ith fuel cells has been s how n (for a ran ge o f com merc ial fuel cells) to increasethe exerget ic eff iciency of the pla nt b y up to 4 9% , rais ing that eff iciency from th e value o f 41.5%for the conven t iona l pow er p lan t w i thout fue l cel ls to ab out 62% for the fue l -ce l l- topped pow erplant . This improvement s tems from the improved exerget ic eff iciency of fuel oxidat ion in theseproposed topping p ower p lan ts , as co nt ras ted wi th the h igh ly d i ss ipat ive com bus t ion process inconv ention al fuel-f i red ones. Studies o f gas turbine cycles with sol id o xide fuel cel ls were alsorepo rted in Ref. [23].

P I - I i i l l l l

P 4 . gm ~ m i m u i m m m

T, . I m e . c

1" - I I l ' I P C ~ ' ~l r -~m~ll L ~ _ ~ ~ I - ' - -'~ ~ I I l ~ I . . . . . ~ , , ~ a n d H

P l a l i l ll

1 T I " ~ S ' C

~

I T l l m l * I I U q ~

I I ' r , . , m - c I / . , u ,

T . u c I I

Fig. 7. An alyzed con dition s of a fuel. ll topp ing p ower gene ration system [20].


13/15

L I O R : A D V A N C E D E N E R G Y C O N V E R S I O N T O P O W E R 9 53

106.9

\ \... o

sys tem ( too)-1 .9 -4 .6 ~ 4 3

(vl.7100.0 ; I 9.08.7 83.0) 1 23.2)

(e.s)

S y s t e moutpu t .P l

Stacka n OI h e m o s s e s

, / ( e . e )

~ m O u s ~ o n H e a t17 .2 e ~ e r

- 1 9 . 2 1(~-71

(o)

-3.8

To m s y ste mo u t p u t . F '~P z- 4 6 . 4(45 .2 )

0(1.71

COIIdOnsMs 2 .0

-2.0 ~

141.o1

Fans .p u m p s r

I 1.81.8)

O e n e m o r-0 .6

37.7(3~7)

S y s t e mou tpu t .P z

Fig. 8 . Exergy and energy) f low diagram s for a pow er p lant wi th a fuel-ce ll top ping sys tem [20] assumingthat 100 uni ts of exergy and energy are inves ted) .

8.3. The nuclear generatorNu clear power p lan ts op era te a t a thermal e ff ic iency of about 2 9-35 . Therefore , overa ll

e ffic iency of e lec t rica l pow er genera t ion m ay be im proved co ns iderab ly by f i r st und ers tanding a ndthen reduc ing the i r reversib il ity of nuc lear pow er p lan t opera t ion .

Past s tudies of fossi l-fuel po we r s tat ions hav e revealed that ex ergy losses associated w ith boi lerop erat ion are high ly s ignificant . Co nven tional com bus t ion is the most ineff icient process infossi l- fue l p lan ts, consu ming abou t 20-30 of the usefu l energy ( i. e. o f the exergy) of hydroc arbonfue l. He a t t ransfer f rom the h igh- tempera ture produc t gases to lower- tempera ture work ing f lu iddes t roys ano ther 15 of the fue l' s exergy; 5 of the usefu l energy of fue l i s typ ical ly expe l led wi ththe f lue gases . In o ther wo rds , combu s t ion , hea t t ransfer and f lue gas expuls ion wi th in / f rom thesteam gen erato r are responsible for over 83 of the irreversibi l i ty which occurs durin g fossi l-fuelp lan t opera t ion .

In comparison with fossi l -fuel plants , the f iss ion process replaces combust ion to produce therequi red h igh- temp era ture hea t for t ransfer to the work ing m edium o f the s team power cyc le . Inthe case o f nuc lear pow er s ta t ions , there has been l i tt le e ffor t d i rec ted a t the ev a lua t ion o f exergydestru ct ion within these plants . Siegel [24] , emp loying relat ions develop ed b y Prusc hek [25],per form ed a second law ana lysi s on a s team-cooled fas t b reeder reac tor p lan t des igned in G erman y.He fou nd that the largest exergy loss by far occurs in the reac tor i tself .

A secon d law ana lys i s was per forme d by the au thor an d h is co-workers on an opera t ing1145 M W e BW R nuclear pow er s ta t ion to eva lua te p lan t an d subsys tem i rrevers ib il ity [10]. Theresu lt s (F ig . 9 ) d i sc lose tha t ov er 80 of the exergy des t royed dur ing p lan t opera t ion i s a resu l tof the highly-irreversible f iss ion and heat t ransp ort processes within th e reacto r vessel . Planteff iciency and effect iveness are foun d to be 34.4 , wh ich is well below the 40-45 eff iciencies oftypical fossi l -fuel-fi red pow er g enerat ing stat ions.


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9 54 L I O R: A D VA N C E D E N E R G Y C O N V E R S IO N T O P O W E R

74.2_ _ 2 1 3 re

12.2 (37.1) 4.7umps

i o R e a c t o rCoolan t Pum p

~ 1 . 2(11.6)

18713

17&8s . 4 )

F e e d n ~ t H u t e c ,

liT&01 _....,t a e x e

- l , e ~ l t~ .~ ) I_ T tu t~u I - I

1 7 U3.4)

2.0

Geuetstor

O . 4 )

Fig. 9 . Exergy f low diagram for the LaSal le Co unty BW R nuclear power s ta t ion [10] [MW an d ( ofinpu t exergy)].

Based on these wel l -known numb ers an d the resu lt s o f the exergy analysi s , one recomm enda t ionis to give at tent ion once again to the integrat ion of fossi l -fuel-fi red superhe at / reheat uni ts locateddow ns t ream of the reac to r vessel. Th is m odi f ied p lan t conf igura t ion w ould no t on ly improveeff ic iency b y rais ing the top operat ing temp erature , bu t is a lso ant ic ipated to redu ce i r revers ibi l ityassociated with heat t ransfer in the s team generators .

A muc h mo re p rofou nd conc lus ion s tems f rom a fundam enta l exam ina t ion o f the nuc lea rreact ion i tself . M os t of the energy prod uce d during the brea kup of the nucleus in the fiss ionreact ion, an d in the joining o f nuclei in the fusion react ion, is in form of kinet ic energy o f thepro duc ed par t ic les . In a short t ime an d space this valuable m echanical energy, which is pure exergy,is conv erted into h eat a s the par t ic les s low down. Even i f energy is ful ly conserv ed in this s low -dow nprocess, much of the o r ig ina l exergy is des t royed , the m ore so s ince the top t em pera tu res o f thework ing f luid are severely l imited by the safety limits of the fuel rods in the f iss ion reac tor a ndof the fusion system as a w hole in a fusion reac tor. I t i s thus ob viou s that i f the or iginal kinet icenergy of the f iss ion or fusion products could be used direct ly to produce electr ic i ty, akin to ane lec t ro -mechan ica l nuc lea r genera to r, o r p roduce mechan ica l power d i rec t ly, th i s exergydestruct ion w ould be el iminated an d a mu ch m ore eff ic ient conve rsion of the nuclear energy topow er may b e a t t ained . S imi la r to pas t d i scuss ions by the au thors on the reduc t ion o f combus t ionirreversibi l ity [22], one al ternat ive m eans to im prove the exergy efficiency of nuclear react ions a ndhea t t r ans fe r wi th in the reac to r wo uld be to dev i se fi ss ion and fus ion p rocess which w ould inc ludegenera t ion o f use fu l work dur ing , an d as a consequence o f , the par ti c le s low -down process . Forexamp le, i f a system cou ld be devised which w ould o perate as a nuc lear ge nerator (or fuel-cell ),the nuc lea r reac t ion an d reac to r hea t t r anspor t i rrevers ib i li ti e s would be reduced . W ork on athermod ynam ic founda t ion o f nuc lea r reac t ions i s under w ay [26-28] .

9 . S O M E S U G G E S T I O N S

Whi le the con t inu ing improvements in conven t iona l power genera t ion t echnology should no ts top , the 21s t cen tury shou ld see much more devo t ion to unconvent iona l f ron t ie r approaches totha t p rob lem , ob v ious ly wi th p roper a t t en t ion to the acco mpan ying econom ic and soc ieta l is sues .T h e deve lopm ent o f new econom ica l mate ria l s and dev ices wou ld a l low des ign o f the rmal p owe rplants for oper at ion a t higher tempera tures an d eff ic iencies ,but emp hasis should be placed on directenergy conversion, i .e . exergy-efficient processes which are nei ther C arnot- l imited nor acco mp anied


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L I O R : A D VA N C E D E N E R G Y C O N V ER S IO N TO P O W E R 9 55

b y l a r g e th e r m a l a n d s p e ci es e m i s si o n s . A m o n g t h e s e p r o ce s s e s, s o m e o f th e m o s t a p p e a l i n g a tp r e s e n t a r e d i r e c t c o n v e r s i o n o f s o l a r r a d i a t i o n t o e le c t r ic i t y, a n d f u e l ce l ls . D e v i c e s f o r th e s e a r er a p i d l y d e c l in i n g i n p r ic e , h e a d e d f o r c o m p e t i t iv e n e s s w i t h o t h e r p o w e r g e n e r a t i o n s c h e m e s a n da r e i n c re a s i n g l y u s e d in a p p r o p r i a t e c o m m e r c i a l a p p l i c a ti o n s .

D i r e c t c o n v e r s i o n o f f is s io n a n d f u s i o n e n e r g y i n t o e l e ct ri c al o r m e c h a n i c a l p o w e r d e s e r v e s m u c ha t t e n t i o n , e s p e c i a l l y i f t h e n u c l e a r w a s t e p r o b l e m i s r e s o l v e d i n a d e f i n i ti v e l y s a t i s f a c t o r y m a n n e r.

T h e u s e o f s p a c e f o r p o w e r g e n e r a t i o n s e e m s t o b e i n e v it a b le : i t p r o v i d e s t h e b e s t h e a t s i n k a n dr e li ev e s t h e e a r th f r o m t h e p e n a lt i es o f p o w e r g e n e r a t i o n . B o t h t h e c o s ts o f l a u n c h i n g p a y l o a d si n t o s p a c e a n d t h o s e o f e n e r g y t r a n s m i s s i o n a r e d e c li n in g .

I n t h e i n te r i m , m o r e e f f ic ie n t p r o d u c t i o n o f p o w e r f r o m l o w t e m p e r a t u r e s o u r c e s, s u c h a s s o l a ra n d w a s t e h e a t, c o - g e n e r a t i o n , l o w - e m i s s i o n c o m b u s t i o n s y s t e m s , e x p l o r a t i o n o f h y d r o g e n a s f u e la n d s i g n if i ca n t ly s a fe r n u c l e a r p o w e r p r o d u c t i o n m u s t b e p u r s u e d .

S i n ce s o l a r e n e r g y is a n i n e x h a u s ti b l e a n d n o n p o l l u t i n g s o u r c e w h i c h d o e s n o t a l t er t h e g l o b a lt h e r m a l b a l a n c e , c o s t r e d u c t i o n i n s o la r p o w e r p r o d u c t i o n s h o u l d b e p u r s u e d a r d e n t l y.

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Hemisphere, New Y ork, 1988.2. Dunbar, W. R., Lior, N. and Gaggioli, R.,ASME J. Energy Resources Technology 1992, 114, 75.3. Freedman, B. Z. and Lior, N.,ASME J. Engngfor Gas Turbines and Power 1994, 116, 1.4. Lior, N.,Energy Conversion 1977, 16.5. Koai, K., Lior, N. and Yeh, H.,Solar Energy 1984, 32.6. M artin, C. G. and Swen son, P. F., U .S. Patent no. 3 ,950,949, 1976.7. Mannaa, A. R., Ghazi, M. A. and Gaff, H. A.,Energy 1988, 13.8. Ke stin, J., DiPippo, R . and Khalifa, H. E.,Mech. Engng 1978, 10 0(12), 28.9. Bradley, C. E., S AE Technical Paper 860536, Int. Congr. and Exp ., SAE, W arrendale, PA, 1986.

10. Dunbar, W. R., Moody, S. D. and Lior, N.,Energy Conversion and Management 1995, 36, 149.11. M cCorm ick, D. J. and Go rzegno, W. P., The separately fired superheater--a nuclear application at indian point,ASME

Transactions ASME-IEEE National Power Conference. Albany, New York, 1965.12. Deub lein, O., Traub e, K., K ornbilcher, S., Fricke, W ., Schriifer, E., Reinhardt, A ., Jaerscky, R. and Sehu lze, J.,Nuclear

Engng Int. 1968, 13, 929.13. Tang, H., Churchill, S. W. and Lior, N.,Proc . Combustion Inst. 18th Int. Symp. on Combustion. The Combustion

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14. Kim, C. and Lior, N., ASM E Paper 93-WA /HT-37, ASM E W inter Annual Meeting, New Orleans, LA, ASME , N.Y.,1993.15. Arai, N., Teramae, N. and Kobayashi, N.,Energy WorM 1994, 226, 16.16. Lior, N.,Active Solar Systems Chapter 17, ed. G. O. G . L of. MIT Press, Cambridge, MA, 1993, pp. 615-674.17. Lotken, M. and Kearney, D.,Solar Today 1991, 24-26.18. Ohta, T.,Energy Technology Pergamon, Oxford, 1994.19. App leby, A. J.,Fuel-Cells: Trends in Research and Applications. Hemisphere Publishing Co., Washington, D.C. 1987.20. Dunbar, W. R., Lior, N. and Gaggioli, R.,Energy 1991, 16, 1259.21. Dunbar, W. R., Lior, N. and Gaggioli, R.,ASME J. Energy Resources Technology 1993, 115, 105.22. Dunbar, W. R. and Lior, N.,Combust. Sci. Technol. 1994, 103, 41.23. Harvey, S. P. and Richter, H. J.,ASME J. Energy Res. Technol. 1994, 116, 305.24. Siegel, K.,Brennst.-Warme-Kraft 1970, 22, 434.25. Pruschek, V. R.,Brennst.-Warme-Kraft 1970, 22, 429.26. Hatsopoulos, G. N. and Keenan, J. H.,Principles of General Thermodynamics. Krieger, Malabar, FL, 1965.27. Gyftopoulos, E. P. and Beretta, G. P.,Thermodynamics: Foundations and Applications. Macmillan, N.Y., 1991.28. Dunb ar, W . R., Gagg ioli, R. A. and Lior, N.,Int. Syrup. ECOS 92, ed. A. Valero and G. Tsatsaronis, Zarag oza, S pain.

ASM E, New York, 1992, pp. 45-59.

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