HEAT TRANSFER PROBLEMS FOR THE PRODUCTION OF HYDROGEN FROM GEOTHERMAL ENERGY

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1 Heat SET 2005 Heat Transfer n Components and Systems for Sustanable Energy Technologes 5-7 Aprl 2005, Grenoble, France HEAT TRANSFER PROBLEMS FOR THE PRODUCTION OF HYDROGEN FROM GEOTHERMAL ENERGY J. Sgurvnsson 1*, C. Manslla 1, B. Arnason 2, A. Bontemps 3,4, A. Maréchal 3, T. I. Sgfusson 2, F. Werkoff 1 1 CEA/DEN/DM2S/SERMA/LTED CEA/SACLAY (Bat. 470)-91191, GIF-SUR-YVETTE CEDEX FRANCE * to whom correspondence must be addressed (JS206070@AQUILON.CEA.FR) 2 UNIVERSITY OF ICELAND, SCIENCE INSTITUTE, Dunhag 3,107, REYKJAVIK, ICELAND 3 UNIVERSITE JOSEPH FOURIER, BP N GRENOBLE CEDEX 9, FRANCE 4 CEA/DRT/DTEN/GRETH 11, Rue des Martyrs-38054, GRENOBLE CEDEX 9, FRANCE ABSTRACT Electrolyss at low temperature s currently used to produce Hydrogen. From a thermodynamc pont of vew, t s possble to mprove the performance of electrolyss whle functonng at hgh temperature (Hgh Temperature Electrolyss: HTE). That makes t possble to reduce consumpton but requres a part of the energy necessary for the dssocaton of water to be n the form of thermal energy. A collaboraton between France and Iceland ams at studyng and then valdatng the possbltes of producng hydrogen wth HTE coupled wth a geothermal source. The nfluence of the ext temperature on the cost of energy consumpton of the drllng well s detaled. To vaporze the water to the electrolyser, t should be possble to use the same technology currently used n the Icelandc geothermal context for producng electrcty by usng a steam turbne cycle. For heatng the steam up to the temperature needed at the entrance of the electrolyser three knds of heat exchangers could be used, accordng to specfc temperature ntervals. INTRODUCTION The use of hydrogen as a substtute to hydrocarbons, s currently the object of many research and development tasks n the world. To be sustanable, a hydrogen producton process must be carred out wthout consumpton of raw materals other than water and drven by energy produced wthout greenhouse gas emssons. Electrolyss at low temperature carred out usng sustanably produced electrcty satsfes these constrants and s currently used to produce Hydrogen. However, today t s more expensve than the process of producng hydrogen by steam reformng of natural gas, whch presents a double dsadvantage snce t consumes natural gas and rejects carbon doxde. From a thermodynamc pont of vew, t s possble to mprove the performance of electrolyss whle functonng at hgh temperature (HTE). That makes t possble to reduce consumpton but requres a part of the energy necessary for the dssocaton of water to be n the form of thermal energy. Recent HTE research programs have profted from new fnancngs, manly wthn the Generaton IV Internatonal Forum framework for developng long term nuclear reactors (Generaton IV, 2002). The forum consders the possbltes of usng nuclear energy, partcularly hgh temperature helum cooled reactors (HTR) whch nclude the possblty of producng hydrogen. A collaboraton between France and Iceland ams at studyng and then valdatng the possbltes of producng hydrogen wth HTE coupled wth a geothermal source. Its prncpal objectve s the producton of hydrogen by HTE coupled wth a geothermal source n the Icelandc context. The goal s to buld a 5 kwe prototype on the ste of Nesjavellr, whch s 20 km from Reykjavk (GEYSER, 2004). Iceland brngs ts competences and ts experence n the feld of geothermal drllng, the explotaton, the transport and the transformaton (n partcular n electrc power) of steam extracted at 1

2 the geothermal sources. France brngs ts scentfc knowledge and ts ndustral experence on the thermal problems of heat transfers for varous ranges of temperatures, as well as the results of the research and development on Sold Oxde Fuel Cells, whch works n reverse of the electrolysers at hgh temperature. Hydrogen from geothermal energy. The economcally harnessable geothermal energy n Iceland has been estmated at approxmately 200 TWh/year, of whch only 1% has been harnessed up to now. Assumng the same technology used n Iceland today to produce electrcty from geothermal steam, the 200 TWh/year could be used for the producton of 20 TWh/year of electrcty, or 17 TWh/year of hydrogen. (Arnason and Sgfusson, 2000) The Jules Verne Project. The Jules Verne project whch was the subject of a conventon on behalf of the two states, marks the frst stage of the collaboraton between France and Iceland. Its prncpal objectve s a techno-economc study of the producton of hydrogen by HTE coupled wth a geothermal source n Iceland. The estmated duraton of the project s from 2003 to The followng stage of collaboraton wll carry on nto a start up program, GEYSER, dated around 2009 to buld a 5 kwe prototype on the ste of Nesjavellr. In secton 2, we wll present the present status of the Nesjavellr plant, and the present heat exchangers used there. Secton 3 wll be devoted to a dscusson on the nfluence of the heat source temperature (HTR or geothermc) and the presentaton of the prospects of deeper drllng. In secton 4 requrements for the heat exchanger network for HTE wll be provded, takng nto account the partculartes of the GEYSER project. PRESENT STATUS OF THE NESJAVELLIR PLANT The geothermal plant n Nesjavellr (fg. 1) currently produces 90 MW of electrc energy and 400MW of thermal energy. The thermal energy s used to heat up water through a heat exchanger after whch t s dstrbuted for house heatng and general hot water consumpton. The heat exchanger s a ppe counter current exchanger (fg 2). There s no need for specal ant-corroson technques because of the low levels of oxygen n the geothermal steam. Because of hgh volcanc actvty, the strata of rock under Nesjavellr are relatvely young. Rock temperature s hghest next to volcanc fractures. At sea level, the temperature there s approxmately 100 C. At two klometres down, t exceeds 350 C. In Iceland the drllng wells are usually from 1-3 km deep wth pressures from 3-7 MPa. Fg. 1 The Nesjavellr plant For electrcty producton n Nesjavellr, steam s extracted at 3 MPa and vaporzed to 1.3 MPa at a temperature of about 200 C. Before enterng the turbnes steam and water are separated, wth mnerals extng wth the water and ncompressble gases 0.4% of the mass, contnung wth the steam. In Iceland, geothermal energy accounts for 39% of the total consumpton, ol accounts for 38%, hydro 19% and fnally coal 4%. (Arnason and Sgfusson, 2000) Ths percentage s so large because all house heatng and consumer hot water comes from geothermal resources. The two man processes for producng electrcty are the hydro and geothermal energy processes. The electrcty producton capacty of the natonal electrcty company Landsvrkjun n Iceland s over 1200 MW today and wll ncrease by 600 MW when the newest hydro electrc power plant at Karahnjukar wll be ready. Whle most of the electrcty s produced by hydro energy, around 10% s produced by geothermal energy. Hydrogen has been produced largely from alkalne electrolyss n Iceland for over 50 years, manly for ammona used n fertlzer producton. A novelty n utlsaton happened wth the ntroducton of the ECTOS (ECTOS, 2003) project n 2003, where part of the publc transport system n Iceland s run on hydrogen. The hydrogen for the ECTOS project s produced by alkalne electrolysers technology from Norsk Hydro.. Fg. 2 Heatng water heat exchanger at Nesjavellr 2

3 THE INFLUENCE OF THE TEMPERATURE OF THE HEAT SOURCE Hgh-temperature electrolyss (HTE) s an alternatve to the conventonal electrolyss process. Some of the energy requred to splt the water s provded as heat nstead of electrcty, thus reducng the overall energy requred and mprovng process effcency. Because the converson effcency of heat to electrcty s low compared to usng the heat drectly, the energy effcency can be mproved by provdng the energy to the system n the form of heat rather than electrcty. In Iceland the cost of extractng thermal energy from a geothermal source s only about 10% of the cost of electrcty produced. kwh ( th geo ) =. 1kWh( e geo ) 0 (1) For HTR the expected effcency of electrcty producton s 50% hence we can assume that the cost of thermal energy for HTR s 50% of the prce of electrcty. kwh ( th HTR ) =. 5 kwh( e HTR ) 0 (2) Thermal energy from a geothermal source s very nexpensve compared to thermal energy from a HTR. In the Icelandc context steam could be suppled at 200 C. Accordng to V.K.Jonsson et al only 3.8 kwh (e) /Nm 3 H 2 s needed wth a thermal nput of 200 C, compared wth about 4.5 kwh (e) /Nm 3 H 2 n conventonal electrolysers. Electrcty prce to ndustry n Iceland s approxmately /kwh compared to /kwh (DGEMP-DIDEME, 2003) for mddle term electrcty produced by nuclear reactors n France. When ths prce dfference s taken nto account the dfference between producng hydrogen by HTE n Iceland and France s evdent, regardng only the cost of vaporzng and heatng water to the operatng temperature of the electrolyser. (fg. 3) 0,35 Cost of Geothermal and HTR suppled energy Table 1 Energy prces, Geothermal/HTR /kwh(e) Iceland 0,014 /kwh(e) France 0,0284 /kwh(th) Iceland 0,0014 /kwh(th) France 0,0142 Although only 200 C of thermal nput s possble today, that could change. Recent research performed by Landsvrkjun on deep drllng n Iceland show a possblty of extractng C steam at a depth of 4-5 km. At ths tme deep drllng s purely expermental but could become a possblty wthn the next 10 years. The nfluence of the ext temperature on the cost of the energy consumpton of the drllng well s obvous. A recent evoluton of the HTR concept s the Very Hgh Temperature Reactor (VHTR) for whch the helum temperature at the outlet of the reactor core would be up to C. Wth ths knd of heat source the endothermal soluton, dscussed n the next secton, would be possble wth no extra heatng by electrcty. THE HEAT EXCHANGER NETWORK Operatng modes for HTE There are three possble operatng modes for HTE dependng on the energy balance at the electrolyser level: Endothermal, sothermal and exothermal. Endothermal. The temperature of the steam decreases from the nput of the electrolyser to the output. Ths corresponds to the best energy effcency but worst producton cost because an endothermal electrolyser s much more expensve than an exothermal one. Isothermal. The temperature of the steam s the same at the nput as at the output. The energy effcency s better than n the exothermal case but the electrolyser cost stll outweghs better effcency. Cost /kg [H2] 0,3 0,25 0,2 0,15 0,1 0, Suppled thermal energy [ C] 850 Geothermal, Iceland HTR, France Exothermal. The temperature of the steam ncreases from the nput of the electrolyser to the output. Ths corresponds to the worst energy effcency but best producton cost because an exothermal electrolyser nvestment cost s the lowest of the three possbltes (T. Pnteaux 2002). The exothermal mode s best suted for the geothermal context snce the nput temperature s only 200 C. Fg. 3 Cost of vaporzng and heatng water to the requred temperature for the electrolyser 3

4 Heat exchangers The enterng temperature of the electrolyser could be between 750 C and 900 C. To be effectve from a thermodynamc pont of vew, the HTE requres a recovery of the heat contaned at the ext from the electrolyser. Heat needs to be recovered n parallel from oxygen and from the hydrogen-steam mxture, n order to heat the steam n contact wth the geothermal source, up to the desred temperature at the entry of the electrolyser. When desgnng a heat exchanger network for hgh temperature steam electrolyss, the results should not be the same f the heat source s geothermal or f t s heat produced by a hgh temperature reactor. Ths s due to the fact that physcal propertes are dfferent: the heat s not suppled at the same temperature; and several economc parameters are dfferent too. Indeed, the cost of thermal energy s much lower n the case of geothermal, and the dscount rate should not be the same. The hgh temperature heat exchanger network has been optmsed wth a techno-economc method usng genetc algorthms (Manslla et al., 2005). The prncple of the method s mnmzng an objectve functon: the total cost, whch ncludes nvestment costs as well as operatng costs. Fgure 4 shows the heat exchanger network for an exothermal HTE coupled wth an HTR. The heat exchanger network adapted to geothermal heat (fg. 8) wll dffer from HTR at the thermal source wth the addton of more types of exchangers specfed for dfferent temperature levels. The heat exchangers can be classfed nto 3 categores accordng to the ranges of temperatures Fg. 4 Exothermal heat exchanger network coupled wth an HTR Fg. 5 Medum temperature exchanger Fg. 6 Hgh temperature exchanger Fg. 7 Very hgh temperature exchanger Snce we have a wde range of temperatures, we need dfferent heat exchangers and materals. All of the suggested heat exchangers are counter-current flow and prmary surface heat exchangers. The temperature ntervals are defned as follows: Medum temperature: < 650 C, Stanless steel heat exchanger capable of 7 MPa up to 300 C. (fg. 5). Hgh temperature: 650 C < T < 850 C, nckel based heat exchanger capable of 1 2 MPa at 650 C, currently under testng. (fg. 6). Very hgh temperature: > 850 C, ceramc based heat exchanger capable of MPa at 850 C, more nvestgaton on what materal and whch heat exchanger to use for ths temperature level s needed. (fg. 7) The heat exchanger suggested for the > 850 C temperature level s stll beng tested and further detal wll be avalable soon. However t could also be possble to operate the electrolyser at lower than 850 C temperatures usng exstng technology. In assumng that plate heat exchangers wth prmary surfaces are used to nsure a hgh effectveness, the overall heat transfer coeffcent can be derved as follows, neglectng the conducton thermal resstance: 1 U = (3) h h 1 2 4

5 where h 1 and h 2 are the heat transfer coeffcents for the hot (1) and cold (2) sde respectvely and gven by Nu λ h =,= 1,2 d h (4) where d h = 2 e s the hydraulc dameter and where Nu s the Nusselt number whch can be calculated for specfc plates by (GRETh, 1999) /3 Nu = Re Pr (5) Pr beng the Prandtl number and Re beng the Reynolds number calculated from 2 m& Re = (6) µ B where B s the plate wdth, m& the mass flow rate and µ the dynamc vscosty of the flud n the hot (=1) and n the cold (=2) sde respectvely. These here above equatons allow us to determne U and to calculate ether the heat flow rate Q & or the heat exchange area A usng the logarthmc mean temperature dfference classcal equaton T ml Tml followng the Q& = U A (7) Heat exchanger network The geothermal heat exchanger network s presented n fgure 8. Wrtng the heat and mass balance equatons for each heat exchanger and relatng t to ther heat transfer rates calculated wth the equaton (9), the characterstcs of the heat exchanger network can be optmsed to mnmze total cost. In ths calculus, a recyclng of the gases must be taken nto account. The output of the electrolyser s frstly oxygen and secondly a mxture of hydrogen and steam. Ths mxture s dependent on the recyclng rato r. Not all the water s electrolysed and the number r ndcates ths rato of unelectrolysed water. The mxture of water and hydrogen s defned as follows: M & 1 = 1 & & (8) H 2 m, H 2 ( r ) mh 20 + r mh 2O M H 2O ( 1 r ) & O2 1,O 2 = m& H 2O (9) 2 M H 2O m The man characterstcs of the heat exchanger network are presented n table 2 and fgure 8 when t s optmsed to mnmze the total cost. M Table 2 Heat exchanger characterstcs m& H2O (kg/s) 15.4 m& 1 H2 (kg/s) 6.2 m& 2 H2 (kg/s) 12.1 m& 1 O2 (kg/s) 9.2 m& 2 O2 (kg/s) 3.3 Q H2 HT (kw) 6049 Q H2 MT (kw) 6323 Q H2 LT (kw) 6895 Q O2 HT (kw) 1587 Q O2 MT (kw) 2193 Q O2 LT (kw) 1404 Q total (kw) ε H2 HT ε O2 HT ε H2 MT ε O2 MT ε H2 LT ε O2 LT < Re < *10-4 < Pr < 9.73*10-3 U ~ 1(kW/m 2 C) In fg 8 the heat exchangers are labelled as follows: Hgh Temperature (HT), Medum Temperature (MT), Low Temperature (LT). 950 C 912 C 950 C 912 C 714 C 509 C 404 C 705 C 480 C HT 742 C MT 442 C LT 230 C 296 C 230 C H 2 H 2 O O 2 H 2 O Fg. 8 Geothermal heat exchanger network 710 C 420 C 5

6 CONCLUSION The producton of Hydrogen by Hgh Temperature Electrolyss appears to be very promsng manly n the Icelandc geothermal context. One key to the HTE effcency s the recuperaton of heat at the outlet of the electrolyser by heat exchangers. The needed heat exchangers are under test for medum and low temperatures but over 850 C they stll need further developng. NOMENCLATURE A: Heat exchange surface area (m 2 ) B: Wdth of the plate exchanger (m) d h : Hydraulc dameter (m) m& : Mass flow rate (kg/s) µ : Dynamc vscosty (kg/m.s) k: Thermal conductvty (W/m. C) Pr: Prandtl number h: Heat transfer coeffcent (W/m 2. C) U: Overall heat transfer coeffcent (W/m 2. C) e: Heght of the channel (m) Q & : Power, heat flow rate (kw) ε: Effectveness of the heat exchanger Re: Reynolds number Pr: Prandtl number M: Molar mass r: Recyclng rato electrolyss 18th Internatonal Conference on Effcency, Cost, Optmzaton, Smulaton and Envronmental Impact of Energy Systems Pnteaux, T., 2002, Analyse des potentaltés de producton d hydrogéne par électrolyse de la vapeur d eau á haute température en couplage avec un réacteur nucléare de type Hgh Temperature Reactor (HTR). DRT, Unversté Joseph Fourer. GENERATION IV, 2002, Generaton IV Roadmap Crosscuttng Economcs R&D Scope Report. GIF , December2002: onomcs_r-d_scope_report.pdf DGEMP-DIDEME, Coûts de référence de la producton électrque, GEYSER, 2004, Hgh temperature electrolyss at CEA and comparson wth other hydrogen producton technologes, IEA Workshop on hgh temperature electrolyse,, San Antono. GRETh, 1999, Pertes de presson et transfert de chaleur dans les echangeurs a plaques en smple phase, Manuel technque du GRETh. SUBSCRIPTS 1: Steam n 1 2: Steam n 2 3: Steam n 3 4: Steam n 4 HT: Hgh temperature MT: Medum temperature LT: Low temperature H2: Propertes of hydrogen O2: Propertes of oxygen H2O: Propertes of water REFERENCES Arnason Brag and Sgfusson T.I., 2000, Iceland as a future Hydrogen Economy, Internatonal Journal for Hydrogen Energy 25. Pp Jonsson V., Gudmundsson R.L., Arnason B. and Sgfusson T.I., 1992, The Feasblty n Usng Geothermal Energy n Hydrogen Producton, Geothermcs, Vol. 21 No 5/6 pp ECTOS, , Ecologcal Cty Transport System. Manslla, C., Sgurvnsson J., Bontemps A., Maréchal A., Werkoff F., 2005, Heat management for hydrogen producton by hgh temperature steam 6