mansilla, rivera-tinoco, werkoff, bouakou

8/18/2019 Mansilla, Rivera-Tinoco, Werkoff, Bouakou

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Hydrogen production by high temperatureelectrolysis coupled with an EPR, SFR or HTR:techno-economic study and coupling possibilities

Rodrigo Rivera-TinocoCentre Energétique et Procédés, Ecole Nationale Supérieuredes Mines de Paris, 60 bd. Saint Michel 75006 Paris, Franceand CEA/SACLAY-DEN/DANS/I tésé Bât 460-91191Gif sur Yvette CEDEX, FranceE-mail: [email protected]

Christine MansillaCEA/SACLAY-DEN/DANS/I tésé Bât 460-91191Gif sur Yvette CEDEX, FranceE-mail: [email protected]

Chakib Bouallou*

Centre Energétique et Procédés, Ecole Nationale Supérieure desMines de Paris, 60 bd. Saint Michel 75006 Paris, FranceE-mail: [email protected] *Corresponding author

François Werkoff Association Française de l’Hydrogène 28, rue Saint Dominique75007 Paris, FranceE-mail: [email protected]

Abstract: Hydrogen production by high temperature electrolysis coupled withthree nuclear reactors (the European pressurised reactor, the sodium-cooledfast reactor and the very high temperature reactor) was studied in terms of perspectives and hydrogen production costs. Firstly, we present the featuresof producing water steam by using the three nuclear reactors. Secondly,we present the hydrogen production cost for the HTE process coupled witheach type of nuclear reactor. These costs are optimal values of the hydrogen

production cost for the mentioned couplings and they were estimated by usinga genetic algorithm procedure. High potentiality for these HTE couplings wasassessed and contrary to steam source temperatures, the electricity price appearedto be a key parameter for low hydrogen production costs.

Keywords: EPR; high temperature electrolysis; hydrogen; nuclear; SFR;techno-economic; VHTR.

Reference to this paper should be made as follows: Rivera-Tinoco, R.,Mansilla, C., Bouallou, C. and Werkoff, F. (2008) ‘Hydrogen production by hightemperature electrolysis coupled with an EPR, SFR or HTR: techno-economicstudy and coupling possibilities’, Int. J. Nuclear Hydrogen Production and Applications, Vol. 1, No. 3, pp.249–266.

Int. J. Nuclear Hydrogen Production and Applications, Vol. 1, No. 3, 2008 249

Copyright © 2008 Inderscience Enterprises Ltd.


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Biographical notes: Rodrigo Rivera Tinoco is a PhD student in ProcessEngineering at Ecole des Mines de Paris, France. He has worked in the field of techno-economics of hydrogen production by High Temperature Electrolysiscoupled to different kinds of energy sources. From previous degrees, hespecialised on petrochemistry, polymer transformation and process engineering.

Christine Mansilla received a PhD in Industrial Engineering in 2006 andworks in techno-economic evaluations and optimisations, especially related tomassive hydrogen production from advanced high temperature processes (hightemperature electrolysis, thermochemical cycles).

Chakib Bouallou received a PhD in Heat Transfer from INSA-Lyon, France, in1989, and was a Senior Scientist at the Center for Energy and Processes (CEP)at Ecole des Mines de Paris. He leads the gas–liquid transfer team in closecollaboration with contractors. His research is focused on a limited array of

topics (Steam generation, CO2 capture, H2 production), mainly on transport phenomena, including chemical kinetics. The methods are experimentalobservation and modelling of complex phenomena with the aim of developingsimulation tools for processes.

François Werkoff has been a French ‘Docteur d’État’ since 1973 and workedat CEA, the French Atomic Energy Commission from 1968 to 2007. He has been involved since 2001 in Techno-Economical assessments of hydrogen production by using various primary energies: nuclear, reforming from fossilfuels, and geothermics. Since the beginning of 2008, he has been an activemember of the AFH2: ‘Association Française de l’Hydrogène’.

1 Introduction

Nowadays, hydrogen is essentially used in the chemical and petrochemical industry andit is mainly produced through Steam Methane Reforming (SMR), which is a high CO2emission process. However, considering the importance of global warming and reductionof greenhouse gas emissions, alternative massive hydrogen production processes arenecessary; firstly to avoid the fossil CO2 accumulation in the atmosphere and secondly,to fulfil the hydrogen needs for industrial processes, especially petrochemical and refining,which will present an annual increase between 4 and 5% for the next 20 years (Bugat,2006). Also, hydrogen demand will be amplified over the following years because of itsincreasing applications as an energy carrier and for industrial processes that have starteda sustainable and low CO2 motion, such as the metallurgical industry. Among severalfuture alternative processes for hydrogen production that are being studied throughout the

world, the process of High Temperature Electrolysis (HTE) coupled with nuclear reactorsseems feasible and competitive to produce hydrogen on a large scale. Results also presenta wide range of coupling possibilities (Forsberg, 2005).

Regarding CO2 emissions, Utgikar and Thiesen (2006) carried out a Life CycleAssesment (LCA) of hydrogen production by HTE-nuclear coupling, considering a VeryHigh Temperature Reactor (VHTR) of 600 MWth with 45% efficiency, and they foundthat CO2 emissions could be smaller than almost any other hydrogen production process.Assuming that hydrogen production by HTE coupled with any nuclear reactor allows amassive hydrogen production with low CO2 emissions, we propose and study the HTE process of water steam coupled with three nuclear reactors; the European PressurisedReactor (EPR), Sodium-cooled Fast Reactor (SFR) and VHTR.

250 R. Rivera-Tinoco, C. Mansilla, C. Bouallou and F. Werkoff


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No published literature dealing with the HTE process coupled with an EPR or a SFR for hydrogen production has been found. However, hydrogen production by differentkinds of high temperature nuclear reactors has been evaluated. Harvego et al. (2006)studied the coupling of a Modular Helium Reactor (MHR) of 600 MWth coupled with theHTE process. They considered that 90% of the MHR heat (with a Brayton cycle) wouldserve electricity production and 10% steam generation. They studied the heat exchange between the secondary helium loop and a steam generator, avoiding all tritium transfer to the steam circuit. Verfondern and von Lensa (2005) proposed the usage of a HighTemperature Gas Reactor (HTGR) for a hydrogen European economy context. Finally,Yildiz and Kazimi (2006) studied several nuclear reactors and they proposed differentnuclear applications depending on the reactor type (Figure 1).

Hydrogen production by high temperature electrolysis 251

Figure 1 Technology options for nuclear hydrogen production

Source: Yildiz and Kazimi, 2006.

In this work, we firstly describe the HTE process. Then, the proposals for water steamgeneration by the nuclear reactors studied are presented followed by the evaluations of the hydrogen production cost for HTE-nuclear reactor couplings. For the hydrogen production cost, a techno-economic study was carried out. An optimisation method, basedon genetic algorithms was used to estimate the lowest hydrogen production cost for eachsystem proposed.


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2 High temperature electrolysis process

The HTE processing of steam produces hydrogen at temperatures over 1073 K andaccording to the HTE simplified configuration presented in Sigurvinsson et al. (2007),two principal parts are involved in this process: a heat exchanger network that recoversthe heat of the hydrogen and oxygen produced by the electrolysis; and the electrolyser cells, which are solid oxide electrolysis cells modelled based on the Nernst equation.Regarding the heat exchanger network, the steam coming from external sources enters thenetwork composed of two parallel series of three counter-current heat exchangers classed by operation temperature. The first class of heat exchangers (low temperature – LT) goup to 923 K, the second ones (medium temperature – MT) between 923 and 1123 K, andfinally the high temperature (HT) exchangers, which go over 1123 K (Mansilla et al.,

2007). The overheated steam that exits these last exchangers enters the electrolyser. Theelectrolyser product streams (hydrogen-steam and oxygen) are used to heat the water steam in the heat exchangers. The initial source of the steam could be geothermal steamgenerators (Sigurvinsson et al., 2007) or nuclear reactors, which are the focus of this work.

Concerning the electrolyser, solid oxide electrolysis cells could be used by imposingthe current density to produce a certain quantity of hydrogen by two possible operatingmodes; the allothermal and autothermal (Rodriguez and Pinteaux, 2003). The currentdensity is the variable that mostly determines the operation mode, because it is closelylinked to the energy losses by the Joule effect in the electrolyser cells. In the allothermalmode, low current density leads to a low energy loss by the Joule effect as heat, which atthe same time is not able to supply enough energy corresponding to a part of the reactionenthalpy (T S ). In order to complete the energy needed by the electrolysis reactionenthalpy, reactors, such as the VHTR need to supply heat to the cells. This operation

mode corresponds to the best energy efficiency but worst production (Rodriguez andPinteaux, 2003). This kind of operation mode will not be investigated in this study.

The autothermal mode means that the energy of the reaction enthalpy needed for theelectrolysis of steam is completely supplied by the current density, providing the electric potential (G ) and the heat (T S ) by the Joule effect in the cells. It is the worst modefor energy efficiency, but early studies present the best production cost for this mode(Rodriguez and Pinteaux, 2003). The modelling of the electrolyser in our paper wascarried out as in work by Sigurvinsson et al. (2007).

3 Steam production by nuclear reactors: EPR, SFR and VHTR

3.1 The European Pressurised Reactor

For the EPR, two possible ways of obtaining water steam are proposed. Firstly, thedrawing off of water steam from the secondary circuit. Secondly, full water steam production with an EPR secondary loop was considered. Then steam coming from whatis drawn off or from the secondary consecrated loop would be sent to the HTE process.For both options, current discussions about operation pressure could lead to a low operationHTE pressure in order to protect the high temperature electrolyser from damage. Evenif specific pressure values have not been decided, in this study and for this reactor we propose an isenthalpic pressure drop to an operation pressure under 5 MPa.



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3.1.1 Operating conditions of the reactor Considering that the EPR is one of the third generation Pressurised Water Reactors(PWR) (Bittermann et al., 2001), we considered the EPR design as a reference (Areva,2006), and for further technical details we studied the PWR-1300 design (EDF, 1986).This reactor presents four loops each composed by a primary loop with water at high pressure (~15.5 MPa) and a secondary loop at a lower pressure. At the steam generatorsfunctioning with the primary and secondary loops, primary circuit temperatures are between ~568 and 596 K. Heat is transferred to the secondary circuit to evaporate water at a pressure going from 6.0 to 8.0 MPa and temperatures up to 569 K. The globalefficiency for an EPR is expected to be 36% and its availability at 92%.

3.1.2 Water steam drawing off from secondary circuit

Figure 2 shows the secondary circuit drawing off proposal. Mass and energy balanceshave been carried out for one secondary EPR loop, taking into account the design heat production at 1125 MWth. Considering that the secondary circuit pressure of 7.8 MPacould damage the electrolyser cells, three operation pressures were studied for thedrawing off of water steam: 4.05, 3.04 and 1.52 MPa. We propose an isenthalpic pressuredrop of the drawn off water steam, which would be performed after the outlet stream of the steam generator. The proposed pressure drop could lead to a slight condensation of the drawn off steam. Based on the enthalpy balance, we present the theoretical propertiesof the steam in Table 1.


Table 1 Properties of drawing off stream after pressure drop

Pressure (MPa)

4.05 3.04 1.52

Outlet temperature (K) 523 506 471kg steam/(kg liquidkg steam) 0.978 0.978 0.986

Figure 2 EPR secondary circuit drawing off schema


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From a potentiality point of view, an EPR secondary loop at 100% capacity exploitationwould generate 708 kg/s of steam. Then based on the average of French reactorsexploitation presented in ELECNUC (2002–2006), we estimated that an EPR working at96.8% and dropping off 1.5% of the steam of one secondary loop, could produce enoughelectricity for the power grid (in which the average electricity demand would require the88.54% of the EPR capacity), and 1.23 kg/s of hydrogen per loop, calculated by usingEquation (1), at steam total conversion in the HTE process. This would lead to an annualhydrogen production of 1.42108 kg per reactor that represents ~2% of Europeanhydrogen production (E4Tech, 2005). Moreover, independently of electrical need, the factof fixing a nuclear exploitation peak could allow the EPR a steady state exploitation,which corresponds to the ideas exposed by Forsberg (2005) about hydrogen production by steady state nuclear reactor operation and the use of reversible fuel cells in order tofulfil the electricity peak demands.


(1)

(2)

(3)

3.1.3 Full steam production by a secondary loop of the reactor

The EPR will have a total thermal capacity per primary loop of 1125 MWth. The potentialsteam production by each secondary loop would be 708 kg/s, at 7.8 MPa. We propose toadd one pre-heater before the inlet of the steam generator of the secondary loop, whichwould be consecrated to the steam production. This additional equipment is added inorder to minimise the temperature drop, which could influence the reactor’s operation.Recycling part of the steam produced by the pre-heater could allow the temperatureincrease of the feed fresh water from ambient temperature up to 503 K, which is thedesign inlet temperature in the steam generators (Areva, 2006) (cf. Figure 3) Carrying outa mass and energy balance for the pre-heater (condenser type) and the stream mixer M1,solving Equations (2)–(3) and substracting ṁ steam2 from ṁ phst, we observe that a water steam production of 468 kg/s could be reached. This water could represent a hydrogen production of 50 kg/s by the consecrated loop in a total steam conversion into hydrogen.The steam flow sent to the pre-heater (ṁsteam2) would be of 240 kg/s, and the temperature before the pre-heater would be around 285 K. In this case, the electric power neededto perform the electrolysis of water steam would be 2600 MWe (two reactors would beneeded) and it could be reached a global annual hydrogen production up to 1.45109 kg,which represents ~20% of the European hydrogen production (E4Tech, 2005).


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3.2 The SFR

The study of water steam production by a SFR considers a reactor with a tertiary loopwith water as coolant. In this case, supposing that the SFR turbine is a steam condensationtype and not a CO2 type (Srinivasan et al., 2006), we propose a steam drawing off fromthe turbine (Figure 4). Several hypotheses have been taken into account (c.f. Section3.2.2) in order to estimate the pressure and enthalpy of the turbine drawn off steam becauseit is preferable for the global process to be working completely in the gas phase.


Figure 3 Schema of an EPR loop working at full steam production

Figure 4 Schematic representation: steam drawing off from SFR turbine

Source: Based on Srinivasan et al. (2006).

3.2.1 Operating conditions of the reactor

For a first technical approach, a SFR of 40 MWth has been studied in this work basedon the schema of Srinivasan et al. (2006). This reactor presents primary and secondarycooling loops with liquid sodium at 653 K in the inlet stream and 788 K in the outlet


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stream respectively. In the third loop, water steam is produced. The water mass flow inthe third circuit is 19.45 kg/s and the generated steam reaches a temperature of 753 K and a pressure close to 12 MPa. In this reactor consecrated to electricity production, thesteam enters a condensation turbine of 16.4 MWe. Several drawing offs in the turbinestages are currently carried out to preheat the liquid water in the third circuit up to thetemperature needed in the inlet of the steam generators. According to Tucek et al. (2006),the SFR would present low uranium consumption and could lead to a lower electricity production cost; nevertheless discussions about this assumption are still in progress. Theaverage reactor efficiency is estimated at 44.9% (Srinivasan et al., 2006, Zrodnikov et al.,2006) with an annual availability close to 80%.


Figure 5 Schema of an SFR 40 MWth

3.2.2 Water steam drawing off in the turbine

In order to generate water steam for hydrogen production by the HTE process, we studiedthe SFR operation schema presented in Figures 4 and 5. We propose to obtain the water steam by increasing the drawing off from the reactor turbine. Steam entering the turbine presents an enthalpy of 3290 kJ/kg at 753 K and 12.0 MPa, which means that it isan overheated steam. The boiling point of water at this pressure is 601 K. The enthalpy

and pressure decrease as steam is expanded in the different stages of the turbine. UsingEquations (4) and (5), we carried out an estimation of the temperature decrease in theturbine as a function of pressure. The values for the overheated steam and the saturatedvapour properties are shown in Figure 6.

(4)


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A saturated vapour is produced when the turbine pressure is almost diminished to 1.8 MPa.If steam is used for the HTE process, we propose to draw off the steam at the turbine

stage where it is still slightly overheated. A drawing off at 2 MPa could be a feasiblechoice to obtain steam at ~483 K and to avoid steam condensation. Studying the drawingoff proposal and observing the steam behaviour, as well as the total energy recovered by the turbine until the pressure decreases almost to the saturation pressure, at 2.0 MPa,we note that from 504 to 2500 kJ would be recovered from each kilogram of overheatedsteam in the turbine by supposing that no other drawing off has been made until 2.0 MPa.The energy recovery in the turbine would reach 9.8 MWth. A slight decrease of 1% of the steam mass flow could lead to 8.82 MWth and a steam drawing off of 0.195 kg/s. Thiscould allow a 0.022 kg/s hydrogen production, under the assumption that the HTE processworks at 100% conversion of steam.

Moreover, this drawing off of 1% of the steam flow in a larger size reactor, such assodium reactor BN-1800 (Zrodnikov et al., 2006), would allow an annual hydrogen production of 5.08107 kg, which represents ~1% of the European hydrogen production(E4Tech, 2005). The water steam flow would be of 2.01 kg/s and the power needed in theelectrolyser would be around 20 MWe.

3.3 The VHTR

Concerning the steam produced by the VHTR, we retained the same assumptions exposedin Rodriguez and Pinteaux (2003), Mansilla et al. (2007) and Sigurvinsson et al. (2006)that consider a nuclear reactor with a helium Brayton cycle and a turbine outlet streamtemperature of 983 K. The water is first evaporated and overheated by heat exchange with


Figure 6 Pressure decrease of overheated steam in a 40 MWth SFR turbine

(5)


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the VHTR helium circuit that exits from the turbine and then the steam enters the heatrecovery network of hydrogen and oxygen outlet streams coming from the HTE process.The pressure of the water circuit increases up to 3.0 MPa and the temperature of theoverheated steam would exit the heat exchanger at 783 K, as shown in Figure 7, beforeentering the HTE process.


Figure 7 Heat exchangers network for the VHTR-HTE coupling presented in [16]

4 HTE process coupled to nuclear reactors

As mentioned before, we extend the work of Sigurvinsson et al. (2007) that dealt withusing geothermal energy sources to produce steam at temperatures around 473 K by performing some modifications. Here we deal with the nuclear reactors presented beforeas steam and electricity sources. Figure 8 presents a general flow sheet for the HTE processcoupled to these reactors.

The water steam inlet temperature and pressure for the HTE process depend onthe energy source (reactor) and the pressure drop. Table 2 shows the properties of water steam in the inlet stream of the HTE process for the EPR, SFR and VHTR. The inlettemperature to the electrolyser, before being overheated by the heat exchanger networks,is supposed to be over 1023 K.


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5 Techno-economic evaluation of hydrogen production costs

We present an extension of work by Sigurvinsson et al. (2007) about hydrogen productioncosts (HTE-geothermal) by evaluating the HTE process coupled to nuclear reactors assteam and electricity sources. The optimisation of the systems was performed by usinggenetic algorithms to an objective function composed by investment and operation costsfor the systems.


Figure 8 Schema of HTE process coupled with medium temperature water steam production

Table 2 Inlet steam stream properties depending on the nuclear reactor type: EPR, SFR and VHTR

Reactor EPR SFR VHTR

Inlet steam temperatureand pressure 523 K (4 MPa) 483 K (2 MPa) 743 K (3 MPa)

506 K (3 MPa)471 K (1.5 MPa)

Inlet steam enthalpy (kJ/kg water) 2762.8 2794.0 3400.0


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CTA is the notation for the total cost (!/kg H2). The investment costs are firstly thecapital costs for the heat exchangers (C i,exch) and electrolyser (C i,elec) and the operatingcosts are the thermal energy consumption cost (C o,th) and the electric energy consumptioncost (C o,elec). The total cost is levelled and divided by the hydrogen production discounted(Ht (kg H2/year)). The objective function is expressed with the following equation:


(6)

(7)

where is the discount rate, T e the number of years in use (years), T i the number of yearsof investment (years), and t the year considered. The definition of each cost contributionis as follows: heat exchangers investment (C i,exch): estimated with the exchange surfaceand the material cost per m2 of surface that varies as a function of the operationtemperature range.

Electrolyser investment:

• Is proportional to the surface of the electrolyser.• The operation lifespan of the electrolyser is assumed to be constant for large

ranges of temperatures, voltages and current densities. It has been estimated atarget lifespan value of 5 years.

• The target objective for SOEC cost in Werkoff et al. (2003) presents a 2000 !/kWunit cost for cells with a surface power density of 0.5 W/cm2. We compared thiswith Thijssen’s work (2006), about SOFC cost, which concludes that the actualSOFC cost is a function of manufacturing volume, which is around 1400 !/kWwhen a production volume of 5 MW/year is manufactured. In order to assess thelong-term potentiality of the HTE, the SOEC cost of 2000 !/kW is consideredin this work, even if a reduction in this cost is envisaged in future years.Furthermore, as in Sigurvinsson et al. (2007), the electrolyser investment costis then expressed by:

Thermal energy consumption cost is estimated with the heat needed to evaporate thewater before being introduced in the HTE process.

Electric energy consumption cost is composed of the electric needs for steamelectrolysis and corresponding at least to the reaction enthalpy ( H r ) under an autothermalmode, as well as the electric pump consumption and an electric overheater placed prior to the electrolyser inlet.

Several costs hypotheses have been taken into account in this optimisation. For theEPR we considered the current cost of electricity produced, corresponding to 33 !/MWhe(DGEMP, 2003). Taking into account that the SFR costs are not still clarified and are


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currently under discussion, in this work we will assume an electricity production costselected from Zrodnikov et al. (2006), for Russian nuclear reactors, in order to enlargethe range for the electricity cost in our sensibility study. The sodium reactor is theBN-1800 reactor, which presents a cost of 80% of EPR electricity production cost. Wewill initially consider a value of 22.7 !/MWhe. However, it seems that the electricity production cost for SFRs would be at least the same as for the EPRs. For the VHTR,the electricity cost would be assumed to be 41.0 !/MWhe, which corresponds to a 25%increase in cost from the EPR electricity production cost due to the need for specialmaterials and alloys to work in extreme conditions. Electrolyser power units of 200 kW,which produce 40,500 kg hydrogen per year, have been evaluated considering that larger scale units would result from just adding electrolyser modules. For all the Nuclear-HTEcouplings, the thermal energy production costs for each reactor were estimated usingtheir design efficiency; the HTE discount rate, the availability, hydrogen production costsensibility to electricity cost up to 50% higher than the electricity production cost. Thesehypotheses and other results are shown in Table 3.

5 Discussion

Besides the evaluation of the possible hydrogen production cost by the HTE processcoupled with nuclear reactors, in this work we present the results for two importantvariables that would enlarge the coupling horizons for this technology; the steamtemperature at the inlet of the HTE process and the electricity cost.

Based on the proposals for steam drawing off from circuits and turbines of theSFR or EPR, we observe inlet temperatures up to 523 K, which is a remote temperature

value from the HTE process operation temperature, over 1000 K. However, studyingthe hydrogen production cost with steam coming from the EPR and entering at 471(1.5 MPa) and 523 K (4 MPa) to the HTE process at the same electricity production cost,a slight variation of 4% is noted. The low contribution of thermal consumption and heatexchanger investment is noted according to Sigurvinsson et al. (2007).

Besides, assuming that the target investment costs of the electrolyser would bereached; the evaluation of the electricity cost influence on the hydrogen production costshows that it is an extremely important variable if lower hydrogen production costs aretargeted (see Table 3). The initial estimations of hydrogen production cost, with the baseelectricity cost for each reactor, show that independently of the steam inlet temperature,the hydrogen cost follows the electricity cost. The SFR electricity cost value, which wasthe lowest among the studied reactors, leads to the lowest hydrogen cost, and the VHTR

electricity cost leads to the highest hydrogen cost. For a difference of 17!

/MWhe betweenthe SFR and VHTR base electricity costs, the hydrogen cost produced with each reactor showed an increase of 35% relative to the SFR reactor, and for the high electricity production cost, evaluated at 1.5 times that of the base, the increase in hydrogen cost was30% relative to the same reference. The influence of the electricity cost increase from the base values to 1.5 times causes an increase of 20% of the hydrogen production cost for each reactor.



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Table 3 Model parameters and results for each nuclear reactor coupled with the HTE Economic assumptions

Te Number of years in use of HTE-incineration coupling 30 yearsTi Number of years of investment 3 yearsAvailability of the process 80%Discount rate 6%

Unitary costs for heat exchangers

Cost of low temperature exchangers 400 !/m2Cost of medium temperature exchangers 800 !/m2Cost of high temperature exchangers 4000 !/m2

Energy costsElectricity cost (co,elec) !/MWhe Base case 1.5 times

the basecase (HE)

EPR 33.0 49.5SFR 22.7 34.0VHTR 40.0 60.0

Thermal energy (co,th) !/MWhth Electricity cost * efficiencyEPR 11.9 17.9SFR 10.2 15.3VHTR 19.2 28.8

Electrolyser and process data

Electrolyser power 200 kWHydrogen production 40,500 kg/year Pumps efficiency 80%

Hydrogen production costs – Results

Electricity cost Reactor Pressure Steam Hydrogen cost

(Mpa) temperature (K) !/kg hydrogen

Base case EPR 4.05 523 2.53.02 506 2.51.52 471 2.4

SFR 2.00 483 2.0

VHTR 3.00 743 2.7

High electricity cost (HE) EPR 4.05 523 3.03.02 506 3.01.52 471 2.9

SFR 2.00 483 2.4VHTR 3.00 743 3.5


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In Figure 9, we present the hydrogen production cost compared with the actual hydrogen production cost by SMR. After the SMR, the EPR and the SFR present the best hydrogen production costs. Nevertheless, for these three couplings, the study of minimal securitydistance between the nuclear reactor and the HTE unit should be carried out in order to evaluate the connection cost of the units and the impact of this on the hydrogen production cost.


Figure 9 Hydrogen production cost for different HTE-nuclear reactor couplings: EPR, SFR,VHTR

The assumption that no hazardous materials are passing through from the primary circuitsto the steam line in the nuclear reactors encourages us to propose the drawing off or thechange of usage of steam generators from nuclear reactors, usually focused on electrical power generation (IAEA, 2004; Onufriev, 2006). This operation procedure could agreewith the idea of using nuclear reactors in a steady state and a cogeneration operation,electricity and massive hydrogen production, which could be attractive from a plantoperation point of view, because of the decrease in thermal and mechanic constraints dueto exploitation changes. Moreover, nuclear reactor operation at a quasi-steady state couldlead to an extra economic profit from producing hydrogen during off-peak periods.

6 Conclusion

Hydrogen production by HTE and nuclear coupling, specifically for EPR and SFR presentsa high competitive production cost due to the low electricity cost for both reactors. Letus underline once again that the investment was taken into account as target values (unitinvestment and life expectancy). Varying the electricity cost for the SFR and EPR we


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found that hydrogen could be produced for between 2.0 and 2.4 !/kg and 2.4 to 3.0 !/kg,respectively. As presented in all the results for the three nuclear reactors, the influenceof the electricity cost has been found to be much more important than the inlet steamtemperature in the HTE process. Besides, we found a high potentiality for hydrogen production by using the EPR and SFR steam, even if 1% of their steam flow is used inthe HTE process. In order to include safety considerations in the assessment of thehydrogen production cost as evaluated in the present work, future studies could take intoaccount other constraints such as the safety distance between the nuclear reactor and theHTE process.

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Nomenclature

Av reactor availability (%)

Ap operation time (s/year)

CTA hydrogen production cost (!/kg)

Cpi heat capacity (kJ/kg)

C i,elec electrolyser investment cost (!)

C i,exch heat exchangers investment cost (!)

C o,elec electrolyser operation cost (!/year)

C o,th thermal consumption as an operation cost (!/year)

ci,elec unitary electrolyser investment cost (!/kWhe)ci,exch unitary heat exchangers investment cost (!/m

2)

co,elec unitary electricity cost (!/kWhe)

co,th unitary thermal energy cost (!/kWhth)

H t hydrogen production (kg/year)

j current density (A/cm2)

Lv vaporisation heat kJ/kg



18/18

ṁi mass flow (kg/s) M i molar mass (kg/kmol)

P tot,useful electrolyser power for water dissociation (kW)

P 1 initial pressure, (MPa)

P 2 final pressure (MPa)

R ideal gas constant (8.314 J mol –1K –1)

T i HTE construction time (year)

T e HTE exploitation time (year)

T x temperature after mixing unit (K)

U electrolyser operating voltage of electrolyser (V)

T temperature (K)

Greek symbols

hydrogen conversion efficiency (%)

discount rate (%)

H r reaction enthalpy (kJ/mol)

Subscripts

1 initial

2 final

cs condensed steamfw feed water

H2 hydrogen

H2O water

O2 oxygen

phst steam after mixer

e electric

steam2 steam recycled for preheating

ref reference

t time (year)

th Thermalvap vapour

water water

x mixer outlet stream


mansilla, rivera-tinoco, werkoff, bouakou

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