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ARTICLE IN PRESSG ModelUSION-7428; No. of Pages 4

Fusion Engineering and Design xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Fusion Engineering and Design

jo ur nal home p age: www.elsev ier .com/ locate / fusengdes

nhanced arrangement for recuperators in supercritical CO2 Braytonower cycle for energy conversion in fusion reactors

.P. Serrano, J.I. Linares ∗, A. Cantizano, B.Y. Moratillaafael Marino Chair on New Energy Technologies, Comillas Pontifical University, Alberto Aguilera, 25, 28015 Madrid, Spain

i g h l i g h t s

We propose an enhanced power conversion system layout for a Model C fusion reactor.Proposed layout is based on a modified recompression supercritical CO2 Brayton cycle.New arrangement in recuperators regards to classical cycle is used.High efficiency is achieved, comparable with the best obtained in complex solutions.

r t i c l e i n f o

rticle history:eceived 23 August 2013eceived in revised form 21 March 2014ccepted 31 March 2014vailable online xxx

eywords:

a b s t r a c t

A domestic research program called TECNO FUS was launched in Spain in 2009 to support technologicaldevelopments related to a dual coolant breeding blanket concept for fusion reactors. This concept ofblanket uses Helium (300 ◦C/400 ◦C) to cool part of it and a liquid metal (480 ◦C/700 ◦C) to cool the rest; italso includes high temperature (700 ◦C/800 ◦C) and medium temperature (566 ◦C/700 ◦C) Helium coolingcircuits for divertor. This paper proposes a new layout of the classical recompression supercritical CO2

Brayton cycle which replaces one of the recuperators (the one with the highest temperature) by another

upercritical CO2 cyclerayton cycleual coolant blanketecompression Brayton cycleower conversion system

which by-passes the low temperature blanket source. This arrangement allows reaching high turbineinlet temperatures (around 600 ◦C) with medium pressures (around 225 bar) and achieving high cycleefficiencies (close to 46.5%). So, the proposed cycle reveals as a promising design because it integrates allthe available thermal sources in a compact layout achieving high efficiencies with the usual parametersprescribed in classical recompression supercritical CO2 Brayton cycles.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

A domestic research program called TECNO FUS [1] wasaunched in Spain in 2009 to support technological developmentselated to a dual coolant (He/Pb–Li) breeding blanket design con-ept for fusion reactors, based on Model C configuration for fusionower Plant Concept (PPCS) [2]. Several authors have proposed theupercritical CO2 cycle (S-CO2) as a power conversion system foreneration IV fission reactors [3,4]. In [5] a complete explanationf the cycle, including layout and economic considerations, can be

Please cite this article in press as: I.P. Serrano, et al., Enhanced arrangfor energy conversion in fusion reactors, Fusion Eng. Des. (2014), http

ound. In the case of fusion reactors, many researchers have alsoonsidered this cycle as a promising one [6]. This cycle, which haseen already analyzed by Angelino [7] is a recuperative Brayton

∗ Corresponding author. Tel.: +34 91 542 28 00; fax: +34 91 559 65 69.E-mail addresses: linares@dim.icai.upcomillas.es, linares@upcomillas.es

J.I. Linares).

ttp://dx.doi.org/10.1016/j.fusengdes.2014.03.083920-3796/© 2014 Elsevier B.V. All rights reserved.

cycle using CO2 as working fluid. The key of the cycle is the useof the working fluid at pressures higher than the critical, with thecompression process taking place close to the critical point. Thisfact entails to a low compression work, being the compressor con-sumptions around 20% of the turbine power output, which is muchlower than in a Helium Brayton cycle. However, the operation inthe proximity of the critical point affects the heat recovery process.This problem is usually solved with the so-called recompressioncycle, but also other arrangements are possible [8].

The reactor considered in TECNO FUS program exhibits fourthermal sources with different temperature levels: two from thedivertor cooling and two from the blanket cooling. Several powerconversion systems that integrated all of the thermal sources wereformerly analyzed in [9]. Other complex layouts including Helium

ement for recuperators in supercritical CO2 Brayton power cycle://dx.doi.org/10.1016/j.fusengdes.2014.03.083

and S-CO2 Brayton cycles were analyzed in [10]. These layoutsincluded combined cycles with bottoming Rankine cycles (organicfluid for Helium Brayton and steam for S-CO2) and even separated S-CO2 (dual cycles), one for the low temperature blanket cooling and

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Table 1Thermal sources.

BNK LM LDIV HDIV

Inlet temperature (◦C) 300 480 566 700

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Outlet temperature (◦C) 400 700 700 800Thermal power (MW) 793 1976 329 248

he other for the rest of thermal sources. Cycle efficiencies achievedy supercritical CO2 Brayton cycles under these enhanced arrange-ents ranged from 41% to nearly 47%. The authors proposed a novel

rrangement, denoted as REC3 (three RECuperators), which added new recuperator that by-passes the lower temperature blanketource. It also avoided the use of a bottoming cycle in order to reachfficiencies around 47% [11]. At the present paper, authors analyzen evolution of the REC3 cycle, denoted as REC2 (two RECuper-tors), which proposes the suppression of one of the three heatecoveries in REC3 due to its low thermal load at certain turbinenlet design pressures. Comparison with the REC3 cycle at differenturbine inlet conditions is performed and a final design is proposed,hich is more technically feasible than the REC3 layout.

. Methodology

.1. Thermal sources

The thermal sources of the dual coolant fusion reactor are givenn Table 1. They have been taken from European postulated config-rations for a fusion Power Plant Concept (PPCS) known as dualoolant (He/Pb–Li) concept described in [2]. The heat from theivertor is removed by Helium, both in the bulk (LDIV) and in thearget plate (HDIV). Blanket cooling is carried out by Helium (BNK)nd Pb–Li (LM).

.2. Layouts assessed

Fig. 1 shows a classical recompression S-CO2 Brayton cycle3,5–7] with an additional recuperator by-passing the BNK sourceBy-pass Blanket Recuperator, BBR). The recompression cycle faceshe problem of different specific heat values of CO2 at the recuper-tor by dividing the heat recovery process into two units, one forigh temperature (HTR) and the other for low temperature (LTR).ownstream the LTR, the low pressure flow is split so that a fractiony-passes the heat exchanger where heat is released to the envi-onment, designated as precooler (PC), and the main compressor.

Please cite this article in press as: I.P. Serrano, et al., Enhanced arrangfor energy conversion in fusion reactors, Fusion Eng. Des. (2014), http

his fraction is compressed in an auxiliary compressor and joinshe rest of the flow downstream the LTR high pressure outlet. Withhis arrangement a lower mass flow rate through the high pressuretream of LTR is obtained, compensating its higher specific heat.

PC

Maincompressor

Auxiliarycompressor Turbine

LDIV LM

BNK

BBRREC2 & REC3

no in classical layoutLTR

Generator

HDIV

HTR(no in REC2)

Fig. 1. Layout of the REC2 and REC3 cycles.

PRESS and Design xxx (2014) xxx–xxx

The additional heat exchanger BBR shown in Fig. 1 is necessaryto achieve higher turbine inlet temperatures without an excess ofthe allowable BNK inlet temperature in the CO2 stream, which islimited by the Helium returning temperature of 300 ◦C. So, remov-ing enough thermal energy from the CO2 leaving the turbine, itis possible to achieve a high degree of thermal recovery in bothHTR and LTR. This cycle, denoted as REC3 was previously analyzedby the authors [11], achieving cycle efficiencies between 46% and47%, depending on the turbine inlet pressure. The replacement ofthe BBR by a heat recovery steam generator which supplies heatto a Rankine cycle was studied in [10], achieving cycle efficienciesaround 46.7%.

The variations of the classical recompression S-CO2 cycle pre-viously described allow achieving high efficiencies, but combiningtwo power plants (S-CO2 and Rankine) or adding an additional recu-perator and operating at high turbine inlet pressures in the REC3cycle. It was found in [11] that HTR works with low thermal loadfor some pressure ranges. This fact has lead to investigate a newalternative in the present paper, designated as REC2 in which HTRhas been suppressed.

2.3. Main assumptions

The turbomachinery has been modeled by isentropic efficien-cies, assuming 93% for the turbine and 88% for the compressors[12,13]. Pressure drops in each side of every heat exchanger havebeen set to 40 kPa [10]. Pressure drops in ducts have been neglected.An electrical efficiency of 97% in the generator has been considered.Proximity to the CO2 critical point in the precooler causes sharpvariations in the CO2 properties which entails to sharp increases inheat rejection, producing an inefficient use of the heat transfer area[14]. In order to avoid this problem, values of 30 ◦C and 85 bar atthe precooler outlet have been considered. The minimum approachtemperatures have been set to 7.5 ◦C in LTR and 5 ◦C in both BBRand BNK. It has been checked that thermal effectiveness in eachheat exchanger is according to the range of the maximum valuesachieved in Printed Circuit Heat Exchangers (PCHE), i.e. from 92% to98.7% [12,15]. The outlet temperature at LM and LDIV has been setto the same value, determining the mass flow rate fraction at LDIV.In both REC2 and REC3 layouts, the outlet temperature in the lowpressure stream of BBR has been optimized in order to maximizethe cycle efficiency (temperatures lower than 200 ◦C have not beenallowed in the REC3 cycle to keep a minimum heat load at HTR).The mass flow rate fractions at BBR and auxiliary compressor areleft free, being determined by the solution of the model.

2.4. Fluid modeling

Helium has been modeled as an ideal gas with a constant spe-cific heat at constant pressure (5.19 kJ/kg K); liquid metal (Li–Pb)has been modeled as an incompressible liquid with a constant spe-cific heat equal to 0.195 kJ/kg K [16]. CO2 has been considered asa pure substance and its properties have been taken from the cor-relations given in [17] and have been implemented in EngineeringEquation Solver [18], which has been used to develop the wholecode.

3. Results

Fig. 2 compares the cycle efficiency between REC2 and REC3cycles at different turbine inlet conditions. As in any Brayton cycle

ement for recuperators in supercritical CO2 Brayton power cycle://dx.doi.org/10.1016/j.fusengdes.2014.03.083

the higher the turbine inlet temperature, the higher the cycle effi-ciency is. However, the turbine inlet temperature is limited by thelowest temperature of the thermal source (helium returning toBNK, 300 ◦C). This problem has been overcome with the inclusion of

ARTICLE IN PRESSG ModelFUSION-7428; No. of Pages 4

I.P. Serrano et al. / Fusion Engineering and Design xxx (2014) xxx–xxx 3

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ig. 2. Cycle efficiency comparison between the REC3 and the REC2 cycles, at rep-esentative turbine inlet temperatures.

BR in both REC2 and REC3 cycles, as previously explained. Com-aring REC2 and REC3 curves it is seen that at the same turbine

nlet temperature, the efficiency achieved in REC2 is always higherhan in REC3 cycle, with the advantage of reaching the maximumfficiency at lower pressures. It can be also seen that at high tem-erature (650 ◦C in the shown case) is not possible to operate with

ow pressure because the output temperature of the mixed flowownstream from BNK and BBR heat exchangers excesses the liq-id metal inlet temperature (480 ◦C). In the curves at 550 ◦C and00 ◦C of Fig. 2 two zones can be appreciated, each one with itswn pressure range. In order to achieve the maximum cycle effi-iency, the optimal BBR outlet temperature in the low pressure sideas searched. So, in a first zone of low pressures, the turbine outlet

emperature is high enough to require a large heat removal at BBR,eing the behavior of the curve similar to a recuperative Braytonycle with high efficiencies at low pressure ratios; in a second zonef high pressures, the turbine outlet temperature is reduced so theeat recovery is less important, being the behavior of the curveore similar to a non recuperative Brayton cycle, with better effi-

iencies at higher pressure ratios. The pressure window dependsn the turbine inlet temperature, so at 650 ◦C the second zone doesot appear in the pressure range covered by Fig. 2.

Figs. 3 and 4 explain the behavior of the REC2 cycle. So, Fig. 3hows the outlet temperature of BBR in the REC2 cycle is similar

Please cite this article in press as: I.P. Serrano, et al., Enhanced arrangfor energy conversion in fusion reactors, Fusion Eng. Des. (2014), http

o the inlet temperature of LTR in the REC3 cycle, below 425 bar ofhe turbine inlet pressure. In the same way, Fig. 4 shows the heatxchanged in LTR is similar in both the REC2 and the REC3 cycles

ig. 3. Low pressure side temperatures at some recuperators with 600 ◦C of turbinenlet temperature.

Fig. 4. Heat exchanged in recuperators with 600 ◦C of turbine inlet temperature.

in the same range of pressures. It is also shown in Fig. 4 that heatexchanged in HTR in the REC3 cycle is added to BBR in the REC2cycle, also below 425 bar. Fig. 3 shows a clear change in BBR outlettemperature above 425 bar in the REC2 cycle, being similar to HTRinlet temperature in the REC3 cycle. In the same way, Fig. 4 showsa clear decrease in the heat exchanged in BBR in the REC2 cycle,adding the heat exchanged in HTR in the REC3 cycle to LTR in theREC2 cycle. These sharp variations confirm the previous explana-tions about the two pressure ranges in the cycle efficiency plots(Fig. 2). So, below 425 bar the amount of heat exchanged by theBBR in REC2 is large enough to reduce the LTR inlet temperaturein the low pressure stream, and consequently the LTR outlet tem-perature in the high pressure stream to a compatible value withthe BNK inlet conditions; above 425 bar, the turbine outlet tem-perature allows reducing the heat recovery requirement, achievingthe highest cycle efficiency by unloading the BBR which entails toincrease the BBR outlet temperature in the low pressure stream.Thus, it can be seen that the role of the suppressed HTR in theREC2 cycle is assumed by BBR at low pressures and by LTR at highpressures. The amount of heat recovered decreases when pressureincreases.

Table 2 compares the performance of the REC2 and the REC3cycles in selected points. Turbine inlet temperature of 600 ◦C hasbeen chosen in both cases for comparison because it produces ahigh efficiency in REC2 at 225 bar, value in accordance with most

ement for recuperators in supercritical CO2 Brayton power cycle://dx.doi.org/10.1016/j.fusengdes.2014.03.083

of S-CO2 cycles analysis [3,5,6,9,12,13]. Cycle efficiency is similarin both cases (higher in the REC2 indeed), but the lower maximumpressure and the suppression of the HTR make the REC2 designmuch more attractive. Table 3 shows pressures, temperatures and

Table 2Performance of the REC2 and the REC3 cycles.

REC3 REC2

Turbine inlet pressure (bar) 315 225Turbine inlet temperature (◦C) 600 600Turbine mass flow rate (kg/s) 11,372 14,757Main compressor (MW) 307.97 214.28Auxiliary compressor (MW) 175.02 231.34Turbine (MW) 2075 2049Generator (MW) 1544 1555LTR (MW) 1285 1571LTR effectiveness (%) 91.68 90.45HTR (MW) 633.2 –HTR effectiveness (%) 82.69 –BBR (MW) 3091 5933BBR effectiveness (%) 98.09 98.58BNK effectiveness (%) 97.78 98.18Precooler (MW) 1754 1742Efficiency (%) 46.15 46.48

ARTICLE ING ModelFUSION-7428; No. of Pages 4

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Table 3Main CO2 thermal-hydraulic data in REC2 and REC3 cycles.

REC3 REC2

LTR temperaturesHP inlet (◦C) 62.05 51.83HP outlet (◦C) 132.15 118.22LP inlet (◦C) 152.2 130.4LP outlet (◦C) 69.55 59.33

HTR temperaturesHP inlet (◦C) 142.22 −HP outlet (◦C) 174.29 −LP inlet (◦C) 200 −LP outlet (◦C) 152.22 −BBR temperaturesHP inlet (◦C) 174.29 125.40HP outlet (◦C) 431.69 472.30LP inlet (◦C) 436.69 477.30LP outlet (◦C) 200 130.40

Thermal sources temperaturesBNK inlet/outlet (◦C) 174.29/395 125.40/395LM inlet/outlet (◦C) 423.30/582.93 461.2/586.64LDIV inlet/outlet (◦C) 423.30/582.93 461.2/586.64HDIV inlet/outlet (◦C) 582.93/600 586.64/600

Mass flow rate fractionAuxiliary compressor 0.1855 0.3024BBR 0.7717 0.8563LDIV 0.8573 0.8573

Turbomachinery pressuresMain compressor LP/HP (bar) 85/317 85/226.6Auxiliary compressor LP/HP (bar) 85.4/316.6 85.4/226.2

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. Conclusions

A new layout for S-CO2 recompression Brayton cycle has beenroposed, denoted as REC2 cycle. It has been developed from aormer layout, designated as REC3 cycle, which achieved high effi-iency for a dual coolant blanket fusion reactor with two rangesf temperature in its thermal sources. The REC3 design added oneore recuperator to the classical recompression cycle, although

voided the use of combined cycles or split cycles for low andigh temperature thermal sources. The REC2 design maintainshe same number of recuperators of the classical recompressionycle, although they are connected differently. This cycle achieveslightly higher efficiencies than the REC3 cycle with a lower tur-ine inlet pressure. The possibility of reduction of the turbine

nlet pressure makes the REC2 cycle more feasible than the REC3

Please cite this article in press as: I.P. Serrano, et al., Enhanced arrangfor energy conversion in fusion reactors, Fusion Eng. Des. (2014), http

Also the suppression of one recuperator reduces the investmentnd the footprint of the layout. The efficiency reached is close to6.5% with 225 bar and 600 ◦C as turbine inlet conditions (315 barequired in REC3 cycle).

[

PRESS and Design xxx (2014) xxx–xxx

Acknowledgement

TECNO FUS is funded by the Ministry for Science and Innovationof the Spanish Government through CONSOLIDER-INGENIO 2010Programme (CSD2008 079).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.fusengdes.2014.03.083.

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