Perfomance analysis of hybrid systems based on externally heayed closed-cycle engines
Abstract
This work presents a comparative analysis of hybrid systems that make use of closed-cycle externally heated bottoming systems. Two options are considered: reciprocating (Stirling) engines and supercritical carbon dioxide turbines. These engines share the common feature of working on closed cycles with optimised fluids (H2 and CO2 respectively). However, they differ in their internal structure: Stirling engines make use of volumetric machinery whereas the SCO2 system is composed by turbomachinery. In both cases, the working fluid is subjected to very high pressure and temperature in the range of 50-200 bar and 40-650 ºC. A brief description of both bottoming systems is provided in the article along with the expected performance of each case in on-design and off-design (part load) conditions. The analysis is therefore split into two stages. First, a comparison is shown for on-design operation aiming to evaluate the maximum efficiency attainable by the proposed systems. Second, a preliminary analysis of off-design operation is presented. The paper concludes that hybrid systems based on atmospheric fuel cells and externally heated closed-cycle bottoming engines have the potential to outperform conventional pressurised fuel cells and gas turbines hybrids while preserving the topping system from the demanding operating conditions of the latter configuration
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P oceedings o EFC2011
Eu opean Fuel Cell - Pie o Lunghi Con e ence & Exhibi ion
Decembe 14-16, 2011, Rome, I aly
EFC11064
PERFORMANCE ANALYSIS OF HYBRID SYSTEMS BASED ON EXTERNALLY
HEATED CLOSED-CYCLE ENGINES
D. Sánchez, J. Muñoz de Escalona, R. Chaca egui, T. Sánchez
The mal Powe G oup
School o Enginee ing, Uni e si y o Se ille
Se ille, Spain
ABSTRACT
This wo k p esen s a compa a i e analysis o hyb id
sys ems ha make use o closed-cycle ex e nally hea ed
bo oming sys ems. Two op ions a e conside ed:
ecip oca ing (S i ling) engines and supe c i ical ca bon
dioxide u bines. These engines sha e he common
ea u e o wo king on closed cycles wi h op imised luids
(H2 and CO2 espec i ely). Howe e , hey di e in hei
in e nal s uc u e: S i ling engines make use o olume ic
machine y whe eas he SCO2 sys em is composed by
u bomachine y. In bo h cases, he wo king luid is
subjec ed o e y high p essu e and empe a u e in he
ange o 50-200 ba and 40-650 ºC.
A b ie desc ip ion o bo h bo oming sys ems is
p o ided in he a icle along wi h he expec ed
pe o mance o each case in on-design and o -design
(pa load) condi ions. The analysis is he e o e spli in o
wo s ages. Fi s , a compa ison is shown o on-design
ope a ion aiming o e alua e he maximum e iciency
a ainable by he p oposed sys ems. Second, a
p elimina y analysis o o -design ope a ion is p esen ed.
The pape concludes ha hyb id sys ems based on
a mosphe ic uel cells and ex e nally hea ed closed-cycle
bo oming engines ha e he po en ial o ou pe o m
con en ional p essu ised uel cells and gas u bines
hyb ids while p ese ing he opping sys em om he
demanding ope a ing condi ions o he la e con igu a ion
INTRODUCTION
The aim o his wo k is o de elop a new concep o
hyb id sys em based on a opping mol en ca bona e uel
cell and a bo oming ex e nally hea ed hea engine. The
in eg a ion is o he indi ec ype ( he e is hea exchange
bu no mass ans e ) wha b ings abou a highe li e
expec ancy o he uel cell due o i s a mosphe ic
ope a ion. O he in e es ing ea u es om a global
s andpoin a e ease o ope a ion and load con ol and
possibili y o ope a e he uel cell in s and-alone mode.
The i s hea engine conside ed is a closed-cycle gas
u bine ope a ing wi h supe c i ical ca bon dioxide. This is
a con igu a ion s udied ex ensi ely so a by he au ho s
who epo a be e pe o mance o such sys em wi h
espec o con en ional hyb id sys ems (wi h ei he
indi ec o di ec in eg a ion) bo h in on-design and o -
design condi ions [1,2]. The ad an age o supe c i ical
luids is he e y low wo k equi ed o ele a e he luid’s
p essu e a he comp esso and hence he highe ne
wo k o he sys em. The in e es o ca bon dioxide lies in
he close o ambien c i ical empe a u e (30.98 ºC) and
low c i ical p essu e (71 ba ). These p ope ies make i
possible o de elop supe c i ical cycles wi h con en ional
cooling echniques and economical piping.
The second hea engine unde conside a ion is a
S i ling engine. I is also a closed-cycle engine bu makes
use o a ecip oca ing con igu a ion. The wo king luid o a
S i ling engine can be hyd ogen, helium, ni ogen o
simply ai ; in he p esen pape , hyd ogen is selec ed o i
yields he bes pe o mance [3].
A S i ling engine comp ises: (i) a pai o cylinde s
whe e he luid is comp essed a low empe a u e and
expanded a high empe a u e, (ii) a hea e and a coole
o add and ejec hea o and om he engine
espec i ely, (iii) a egene a o placed in he middle o
hese hea exchange s in o de o inc ease he e iciency.
The main ad an ages o his engine a e compac ness,
low ib a ions, low noise and good pa load pe o mance.
Du abili y and e iciency a e also o be expec ed o hese
depend on he design ope a ing condi ions [4].
DESCRIPTION OF HYBRID SYSTEMS
A gene al layou o he hyb id sys em p oposed is
shown in Fig. 1. A Mol en Ca bona e Fuel Cell is ed wi h
p ehea ed ai and a mix u e o wa e s eam and na u al
gas ha is indi ec ly e o med in e nally. The necessa y
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s eam o he e o ming p ocess is gene a ed ou side o
he cell in a dedica ed s eam gene a o ha ecupe a es a
ac ion o he was e hea om he sys em. A ca aly ic
bu ne is loca ed in he cell exhaus o bu n he ine i able
excess uel and inc ease he empe a u e o he hea
sou ce used o ope a e he bo oming engine (le i be
no ed ha a minimum empe a u e o 700ºC is equi ed i
he hea engine is o achie e high e iciency).
A ac ion o he gas lea ing he hea exchange ha
ans e s hea om he opping o he bo oming sys em
(HX4 in Fig. 1) is eci cula ed o p o ide he ca hode o
he cell wi h he ca bon dioxide necessa y o a oid
ca bona e s a a ion. The emaining gas is used o
p ehea he uel and ai s eams in o he cell.
Figu e 1. Hyb id sys em layou .
HYBRID SYSTEM PERFORMANCE
Table 1 shows he pe o mances o he wo hyb ids
conside ed along wi h ha o a con en ional sys em using
a ho ai u bine. In all cases he cell ope a es a
a mosphe ic p essu e. Rega ding he S i ling engine, he
a ed ope a ing condi ions a e 150 ba mean p essu e
and 650 ºC head empe a u e (wall empe a u e o he
pipes ha o m he hea e ). The sha speed is 1500 pm.
Figu e 2. Pa -load pe o mance (e iciency and con ibu ion
o he bo oming cycle o o al powe ).
The pa -load pe o mance o he sys ems is shown in
Fig. 2 whe e i is assumed ha bo h closed engines adop
an in en o y con ol sys em. Fo he sake o cla i y, he
con en ional sys em has no been included in he igu e.
A low cu en densi ies, he uel cell exhaus mass
low a e dec eases and so does he a ailable hea o he
bo oming cycle (which hen ope a es wi h lowe
e iciency a pa -load). Ne e heless, due o he highe
e iciency o he cell a pa -load, he global e iciency o
he sys em is highe . In his ega d, he eason why he
S i ling engine expe iences a s eepe d op in e iciency is
he lowe in e nal p essu e a pa -load.
Pa ame e
Ai
SCO2
S i ling
MCFC
Cu en densi y [ A m-2]
1100
STCR [-]
3
Tempe a u e [K]
923
Fuel/CO2 u iliza ion [%]
75/70
E iciency [%]
48.94
CYCLE
Comp esso inle [°C/ba ] 25/1.01 35/75 650 °C
150 ba
Tu bine inle [°C/ba ] 650/2.88 650/216.1
E iciency [%]
26.6
39.9
34.5
HS
Ne e iciency [%]
53.1
57
55
Ne powe [kW]
521.2
553
539.8
Hea engine ac ion [%]
14.7
20.4
18.2
Table 1. Ra ed pe o mance.
CONCLUSIONS
The esul s shown in his wo k allow d awing he
ollowing conclusions:
Hyb id sys ems based on a closed-cycle ex e nally-
i ed hea engines achie e highe e iciencies han
con en ional sys ems unde a ed ope a ing
condi ions. Fo a e e ence case, i is expec ed ha
he supe c i ical ca bon dioxide u bine be close o
60% whe eas he con en ional sys em using a ho ai
u bine does no e en each 55%. This la e igu e
can be expec ed om he S i ling-based hyb id.
The hyb id sys ems p oposed exhibi an excellen
pa -load e iciency. A medium o high loads, he
SCO2 u bine seems o be he mos .
Globally, he in e es o he p oposed sys ems can be
con i med by looking a he con ibu ion o he
bo oming engines o he powe p oduced by he
sys em. While his ac ion ypically decays as wi h
load o con en ional hyb ids, i emains a high alues
o bo h he SCO2 u bine and he S i ling engine.
Finally, i is wo h no ing ha he S i ling engine is
mo e a ac i e in e ms o echnology eadiness.
REFERENCES
(1) D. Sánchez e al., In e na ional Jou nal o Hyd ogen
Ene gy 36 (2011) 10327-10336.
(2) D. Sánchez e al., Jou nal o Powe Sou ces 196
(2011) 4347-4354.
(3) H. Ka abulu e al, Renewable Ene gy 35 (2010) 138-
143.
(4) B. Kong agool e al, Renewable and Sus ainable
Ene gy Re iews 7 (2003) 131-154.