AECL-5512

ATOMIC ENERGY & S & L'ENERGIE ATOMIOUEOF CANADA LIMITED V j & j F DU CANADA LIMITEE

PEAK POWER AND HEAVY WATER PRODUCTION FROM

ELECTROLYTIC H 2 AND 0 2 USING CANDU REACTORS

by

M.HAMMERIJ, W.H. STEVENS, W.J. BRADLEY and J.P. BUTLER

Based on a paper presented at the 1st World Hydrogen Energy Conference,

March 1-3, 1976, Miami Beach, Florida

Chalk River Nuclear Laboratories

Chalk River, Ontario

April 1976

PEAK POWER AND HEAVY WATER PRODUCTION FROM

ELECTROLYTIC H2 AND O2 USING CANDU REACTORS*

M. Hammerli, W.il. Stevens, W.J. Bradley and J.P. Butler

*BaA<id on a Pape.K Vftz&zntud at the.

lit Wo Kid HydKogtn EnnKgy Con&e.ie.nce,,

1-3 ttoLKch 1976, Miami Beach, Tlonida

Atomic Energy of Canada LimitedChalk River Nuclear Laboratories

Chalk River, Ontario, CanadaKOJ 1J0

April 1976AECL-5512

Production d'eau lourde e t d ' é l e c t r i c i t é de pointe à p a r t i r de H2 e t 02

é]ectrolytiques au moyen de réac teurs CANDU*

par

Hammerli, W.H. Stevens, W.J. Bradley e t J .P . B u t l e r

* D'après une communication présentée aupremier Congrès mondial de l ' é n e r g i e

e r -t i r é e de l ' hyd rogène , tenu du 1 au 6mars 1976 à Miami Beach en F l o r i d e .

Résumé

On présente un système combiné de production d'eau lourde et destockage d'énergie. L'énergie nucléaire hors pointe est stockée sousforme de H2 (et 0z) é lect ro ly t ique dont le deuterium a été transféréen grande part ie à de l 'eau dans une colonne catalyt ique d'échange dedeuterium H2/H2O. Les principales propriétés et les avantages du procédémixte "électrolyse-échange catalyt ique de D20" font l ' ob j e t de commentaires.D'importantes quantités de D20 pourraient être produites économiquementsi les rapports entre le coût de l 'énergie de base et celui de l 'énergiede pointe étaient raisonnables. De 30 à 40% de l 'énergie primaire devraientêtre disponibles pour l 'énergie de pointe so i t par Lurbines gaz-vapeurso i t par cel lu les chaudes.

L'Energie Atomique du Canada, LimitéeLaboratoires Nucléaires de Chalk River

Chalk River, Ontario

Avr i l 1976 AECL-5512

PEAK POWER AND HEAVY IVATER PRODUCTION FROM

ELECTROLYTIC Il2 AND O2 USING CANDU REACTORS*

M. Hammerli, IV.II. Stevens, W.J. Bradley and J.P. Butler

ABSTRACT

A combined energy storage - heavy water production systemis presented. Off-peak nuclear energy is stored in theform of electrolytic H2 (and O2) from which a largefraction of the deuterium has been transferred to waterin an H2/H2O deuterium exchange catalytic column. Themr-in features and advantages of the combined electro-lysis - catalytic exchange D20 process are discussed.Significant quantities of D20 could be produced econo-mically at reasonable peak to base power cost ratios.Thirty to forty percent of the primary electric energyshould be available for peak energy via either gas-steamturbines or fuel cells.

*Ba&e.d on a Pape.fi PA.e-6 untcd at the.

Hi Wo Kid Hydiogzn Enznqy Conhzno.net,

1-3 Hafich 1976, Miami Beach, Florida

Atomic Energy of Canada LimitedChalk River Nuclear LaboratoriesChalk River, Ontario, Canada

KOJ 1J0April 2976

AECL-5512

PEAK POWER AND HEAVY WATER PRODUCTION FROMELECTROLYTIC H2 W'D O2 USING CANDU* REACTORS

M. Haminerli, W.H. Stevens, W.J. Bradley and J.P. Butler

Atomic Energy of Canada LimitedChalk River Nuclear Laboratories

Chalk River, Ontario, Canada KOJ 1J0

INTRODUCTION

Ontario Hydro, the public utility that supplies electricityto the Province of Ontario, is the major producer of nuclearpower in Canada. The utility has 2,222 MWe**of installednuclear power, 5,000 MWe under construction, and a further5,000 MWe in the planning stage. Nuclear generation presentlyaccounts for 1S% of Ontario's electric power and by 1990 itis expected [1] to reach 60-65%. The nuclear plants were de-signed for base load since the CANDU pressurized heavy waterreactors were expected to have a lower Total Unit Energy Cost(TUEC) than base loaded fossil fuelled therm?l generatingstations. In actual operation the CANDU stations do producethe least expensive thermally generated electricity in Ontario.In May 1973 before fossil fuel prices escalated so dramatically,the maturity TUEC curves for the Pickering nuclear station andthe Lambton coal fired station of the same size and vintagecrossed at a Net Capacity Factor (NCF) of about 60% [1]. InMarch 1975, the maturity TUEC curves, based on actual costexperience for the two stations, crossed at a NCF of about 221.Furthermore, at 801 NCF, the Pickering TUEC is 7.03 $/MWh andthe Lambton TUEC is 16.18 $/MWh, based on cu.rrent costs ofnew low sulphur coal [1].

This utility's peak power demands are presently met mainlywith fossil fuel fired plants, with some hydro, and some pumped

CANada Deuterium Uranium**MWe - megawatts of electrical capacity

- 1 -

water storage contributing [2]. However, for economic reasonsoutlined above, and since the costs of fossil fuels of alltypes are increasing faster than the cost of nuclear fuelbased on natural uranium for the CANDU family of reactors,Ontario HycTro IT already considering nuclear reactors forpeaking duty in the future.

An alternative approach to designing a CANDU reactor specifi-cally for load following would be to concentrate on an energystorage system. Such a system could eventually reduce in-stalled generating capacity from that reauired tc meet peakpower demand to more nearly that required for the yearlyaverage power demand.

In this paper we consider storing off-peak nuclear energy inthe form of electrolytic hydrogen and oxygen, with reconversionto electric power as required for peaking. This concept isnot new [3-9]. We propose an added feature, however, whichmakes this concept considerably more attractive, particularlyin the overall CANDU power reactor system. By means of ournewly developed catalytic system for exchanging hydrogenisotopes between hydrogen and water [10,11], the deuteriumconcentration process, inherent in the electrolysis, would bemarkedly enhanced and more of the valuable heavy water by-product stream produced. Depending on how the credit is taken,either the peak power cost is lowered, or the cost for heavywater is lowered. The system is shown schematically in Fig. 1.Potential uses of off-peak nuclear hydrogen, and oxygen [e.g.Ref. 12] other than generating electricity are included.

This scheme becomes attractive at reasonable peak to basepower cost ratios not only because deuterium is produced as avaluable' by-product, but also because this peaking operationis pollution free and the chemical source for energy storage,namely H20, is cheap, abundant and renewable in the cycle. Inpassing, it is worth emphasizing that deuterium recovery fromany large hydrogen production facility should be seriously con-sidered since it can be a valuable by-product.

The text consists of an overall description of the deuteriumextraction process followed by a fairly detailed evaluation ofthe deuteiium exchange catalyst column since this aspect willprobably be new to most readers. An outline of the varioustypes of conventional electrolysis cells is presented next,followed by a discussion of the parameters for the electrolysisplant. Two short sections are included dealing with theadvantages of the combined electrolytic-catalytic deuteriumextraction process, and with D20 production as a by-productof energy storage via hydrogen. Finally, H2-O2 gas-steamturbine cycles are briefly discussed from the viewpoint of—Present world price set by the USA is ^$125/kg

- 2 -

high pressure electrolysis.

The size of the peaking operation that will be considered isone which could produce 75 Mg O80 tons) n20 (99.8%) per yearand assumes a 50% availability of base power for energystorage. The power for this D20 production (̂ 940 MWe)represents about 5% of Ontario Hydro's 1975 installed capacity,which is ^18,5 50 MWe [13].

All deuterium concentrations in tbis paper are expressed eitherin parts per million (ppm) of D/(D+H) or as atomic percent.

A COMBINED ELECTROLYTIC-CATALYTIC EXCHANGE HEAVY WATER PROCESS

Efficient concentration of deuterium is accomplished by acombined electrolysis-catalytic exchange process, henceforthsimply referred to as the combined process, shown schemati-cally in Fig. 2. Basically, hydrogen gas, produced inelectrolysis cells, which is already depleted in deuteriumrelative to the electrolyte, is further stripped of deuteriumby being passed counter current to feed water through a columnin which catalytic exchange of hydrogen isotopes occursbetween gaseous hydrogen and liquid water. The oxygen and thehydrogen that is highly depleted of deuterium go tostorage and a small stream of water, enriched in deuterium, istaken from the elect!nlysis unit.

In more detail, purified feed water is fed to a dehumidifierwhich serves to reduce the water content of the hydrogen gasand improves recovery of the deuterium in the water vapour en-trained in the hvdro"en gas. A similar enuilibrat ion functionis performed by a scrubber between the electrolysis cells andthe bottom of the exchange column, and in addition preventsany entrained electrolyte in the hydrogen from contacting theexchange catalyst. The dehumidifier and the scrubber couldbe readily incorporated into the top and bottom sections ofthe exchange column itself, each consisting of inert columnpacking.

The exchange column contains a hydrophobic Pt catalyst [10]which remains active in the presence of liquid water andcatalyzes the exchange of deuterium between water and hydrogengas. The column is operated as a trickle bed reactor. Labora-tory tests, while continuing, indicate that the necessarycatalytic activity and reasonable catalyst lifetimes for thismonothermal mode of hydrogen isotope exchange are obtainable.Although platinum \\s the active metal, commercial catalystshave already been produced with sufficiently low concentra-tions that the platinum cost is no longer the main cost inproducing the catalyst.

Tn the exchange column, deuterium is transferred from theelectrolytic hydrogen gas to the vatcr. The final deuteriumconcentration in the hydrogen gas at the top of the columndepends primarily on the operating temperature . f the column,the degree of "pinching" (95% towards equilibrium is quitefeasible), and the natural abundance of the feed water. Thelatter can vary from about 131 ppm to 149 ppm in Canadianfresh waters [14]. The Great Lakes have about a 148 ppmdeuterium content and this is the value chosen for our cal-culat ions.

An operating temperature of 00°C hns been chosen as a com-promise between the equilibrium separation factor*, ac, 'mdthe overall reaction rate. At 60°C, ar for the reaction

img + n 2 o £ t if2 + nno £ ( l )

is 3.2 so that the equilibrium value of deuterium at the topof the column is l-̂-§ =• 46.3 ppm while the working value chosenis 50 ppm (pinching to 92%).

The deuterium, recovery for these conditions is therefore

where Dp and Dt are the deuterium concentrations in ppm inthe feed water and the hydrogen gas at the top of the exchangecolumn, respectively.

In the combined process any conventional electrolytic cellwith a suitable separator between the anode and cathode com-partments may be used. From the point of view of deuteriumseparation in the cell, however, the operating temperatureshould be as low as possible since the electrolytic H/D separa-tion factorf, CXE> is inversely proportional to temperature [15]A value for ag = 6.0 was chosen for the more detailed cal-culations. This would be readily attainable on mild steelcathodes at 60°C [16] and on many other metals at lower tem-peratures. The effect of changing aE on catalyst requirementswill be discussed later.

The steady-state deuterium concentration in the cell can beselected at will simply by changing the ratio of cell productflow to feed water flow and adjusting the reflux. The higher

aC is defined as the D/H ratio in the water divided by theD/H ratio in the hydrogen gas at C° celsius

1ag is defined as the H/D ratio in the evolved gas divided bythe H/D ratio in the electrolyte

- 4 -

the steady state deuterium concentration in the cells however,the larger is the number of transfer units' (defined later)required in the exchange column since the electrolytic H2 willhave a correspondingly higher deuterium concentration.

Because the abundance of deuterium in natural water is so low,it is prudent to cascade [17] to 2 or more electrolysis-exchange staecs and to limit the deuterium concentration intiie 1st stage cells. Cascading reduces the total catalystvolume at the expense of more transfer units [17].

Since the electrolyte must be removed from the product waterbetween each stage because catalyst performance deterioratesin the presence of KOH, the optimum number of stages anddeuterium concentration in each stage product must take thecost of electrolyte removal into account. Partial evaporationis the most likely method for electrolyte removal. This stepcould, however, be eliminated by using a cell incorporating asolid electrolyte such as a solid polymer. The ultimatechoice of basic cell would be made on the basis of economicstaking into account the factors mentioned above.

In calculating the material balances in our paper, thedeuterium concentrations between phases have been calculatedaccording to the exact relations:

.'iJld

Y =

X =

X + a(l-X)

aY

Y(a-l) + 1

(3a)

(3b)

where X and Y are the atom fractions in the water and hydrogengas respectively, and a is the appropriate separation factor.

The number of transfer units, N.T.U., has been calculatedfrom

N. T . U. =1-J £n 1-J + J (4)

where J = mG/L in which m is the slope of the equilibrium lineover the concentration range of interest, and G and L are thegas and liquid molar flows; Y and X are the atom fractions of

A

The concept of "number of transfer units" in a packed columnis closely related to that of "number of theoretical plates"in a column with sieve or other type of plates.

- 5 -

deuterium in the gas and liquid phase respectively, and thesubscripts t and b refer to the top and bottom of the exchangecolumn. For small deuterium concentrations the slope of theequilibrium line, m, is proportional to 1/oc but this approxi-mation introduces a significant error when the deuterium con-centration of interest is greater than about 10?». Graphicaland analytical methods were used to calculate N.T.U. at thehigher concentrations.

1-XCHANGE COLUMN PARAMETERS

Preliminary cost estimates indicate that a 3-stage processis the most economic configuration Tor the combined processand that the optimum 1st stage concentration is of the orderof 0.3%.

A complete material and deuterium balance, including humiditycorrections, is presented in Fig. 3 for a£ = 6 and for 1stand 2nd stage products of 0.3% and 101, respectively. Forthese conditions, the 2nd and 3rd stage watei flows are 4.9%and 0.15% of the 1st stage flow respectively.

The relatively large 1st stage flows lead to the first stagecontributing the major costs (see Fig. 4). Although the datain Fig. 4 are for a commercial GS (Girdler-Sulphide) plantbased on the dual temperature H2S-H2O exchange reaction, theprinciple depicted applies to all H/D separation processes.Even for the hypothetical perfect separation nrocess, about7000 atoms of hydrogen must be handled or treated in somemanner in order to extract the one deuterium atom present atthe natural concentration.

The relative catalyst volumes for the 1st and 2nd stages areshown in Fig. 5 as a function of different 1st stage enrich-ments for the same 2nd and 3rd stage parameters as in Fig. 3.The 3rd stage catalyst volume represents such a small per-centage, even for a small 1st stage enrichment factor, it isnot shown in Fig. S.

The effect of ag on the 1st stage catalyst volume was cal-culated for a 1st stage deuterium product of 0.3%, ignoringthe small humidity corrections. For ag = 1, 3, and 6, theincrease in the catalyst volume would be 57.51, 31.61 and11.4% re~spectively relative to a = 9.

The total catalyst volume recmired for the assumed 75 MgD2O/a based on 4000 h of operation increases continuouslyas the 1st stage enrichment increases (Fig. 6). As Fig. 6shows, the minimum required total catalyst volume isapproached by making the 1st stage enrichment as small aspossible, even less than 600 ppm being indicated.

- 6 -

However, as Fig. 6 also illustrates, the electric power forelectrolysis decreases as the 1st stage enrichment increases.This re f 1 ec ts ~tTTe fTTcT that the total amount of reflux, whichmust be elcctrolyzed again, also decreases as the 1st stageenrichment increases. For example, at 600 ppm the total re-flux is 25% while at 10,000 ppm it is only 1.5% of the feedwater flow. The optimum deuterium concentration in the 1ststage product will therefore depend on the relative totalcosts of the II/D exchange catalyst column and the electrolysisplant.

As a first approximation we assume the total cost for theexchange column and electrolysis plants are proportional tothe total catalyst volume and the electrolysis power, respect-ively. Preliminary cost estimates show that electrolysiscosts are likely to be higher than catalyst costs by about afactor of 2 to 4. Thus, total relative costs have been cal-culated n ••• a function of 1st stage enrichment for electrolysiscost to catalyst cost ratios, CfjCc, of 2, 3 and 4.

From this analysis, depicted in Fig. 7, it is obvious that,while the optimum deuterium concentration in the 1st stageproduct increases as Cn/Crj increases, this effect is small, andthat a range of 0.15$ to 5% appears suitable. Fig. 7 alsoshows that the increase in the catalyst cost at higher 1ststage enrichments than the optimum is more gradual comparedto the increase in the electrolysis cost at lower 1st stageenrichments. Furthermore, the total relative cost, changes byabout 25« when C]]/CQ is either increased or decreased by one,and is nearly independent of the 1st stage enrichment.

From the foregoing it is clear that a 1st stage enrichment to0.3% deuterium is a reasonable first choice. For this case,the total volume of catalyst required for the chosen pro-duction rate, namely,

75 x 103

D2O Production Rate = = 18.75 kg D20/h, (5)4000

is about 200 m3 (7000 cu. ft.) of which 94.51, 5.31 and 0.2%are required in the 1st, 2nd, and 3rd stage exchange columns,respect Ively.

For a given catalyst volume and activity, the actual dia-meter and height of the catalyst section of the column dependon the superficial (linear) gas velocity. To a good firstapproximation, the height equivalent to a transfer unit,H.T.U., is given by

H.T.U. = FH2 at S.T.P./K a (6)

- 7 -

where Fj{2 is the super f i (".i al hydrogen pas velocity and Kya is

the effective mass transfer coefficient. Column sizes aretabulated in Table I for two superficial gas velocities, 3.05m s~l (10 ft. s""") and 10 m s~J , both at S.T.P. Experimentsindicate the minimum superficial gas velocity at pressure isof the order of 0.3 m s"1 for good overall performance. Tnthis calculation, we have assumed that Kva is independent ofgas flow rate. Experiments indicate this is a close approxi-mation for our exchange columns over the limited range ofpractical superficial gas ATlocities considered.

To put the size of the 1st stage catalyst columns for thecombined process into perspective, the volume of the 1st stagetowers in the dual temperature H2S-H2O process would be aboutan order of magnitude larger for the san;: 1st stage productand production rate.

ELECTROLYSIS: CELLS AND ELECTROLYTES

As already mentioned, any conventional electrolysis cell witha suitable separator between the anode and cathode compart-ments may be used for the combined process. The basicelectrochemical reactions for both acidic and basicelectrolytes are tabulated in Table TIA per four faradays*ofcharge passed through the cell.

For liquid electrolytes such as H 2SO U and KCiH the elementalelectromigrat ion reactions involve the transport numbers *: +

and t" of both the cation and anion respectively, but onlyone ion takes part in the electrode reactions. Therefore, inaddition to the electrolysis of water, the net cell reactionsalso involve the transfer of electrolyte from one side of theseparator to the other as shown in Table ITB. The amounttransferred per faraday of charge depends on the transportnumber of the ion which is not directly involved in theelectrode reactions. To prevent a concentration gradientbetween the two halves of the cell, which would of courseresult in a "back emf", the nnolyte and catholyte should bemixed external to the cell continuously. Electrolyte cir-culation is necesssry in any case for proper temperature con-trol in the cell at practical current densities.

In the case of a cell incorporating either a solid polymerelectrolyte or a solid o/ide electrolyte, all the electriccurrent is carried in the electrolyte by the same ion in-volved in the electrode reactions, that is, its transportnumber is unity. Therefore, no electrolyte is transferred.

Solid electrolytes in the form of cation (acidic) or anion(alkaline) exchange membranes offer several advantages toliquid electrolytes as follows:

faraday = 96.4 89 kC

- 8 -

(1) handling of liquid electrolytes which may be verycorrosive 15 eliminated since pure, deionized water is theonly liquid required;

(2) since the membranes are thin (-0.3 mm) and the electrodemetals may be applied directly onto the membranes' sur-faces, the total unit cell thickness is only of the orderof 2-3 mm;

(3) th Mnbranes with metal screen current collectors on bothsid':s "an withstand differential pressures of the order of6 MPa ,^900 psi);

(4) electrolyte recovery from the product stream is eliminated.

However, solid electrolytes in the form of ion exchange mem-branes do have some disadvantages, namely:

(1) their specific electrical resistance is generally higherthan that of the corresponding strong acid or baseelectrolyte;

(2) they are relatively expensive;

(3) they require deionized water as feed to maintain theirconduct ivity.

The solid polymer electrolyte technology in the cationicform (sulfonic acid type) is being actively pursued by theGeneral Electric Co. [18] for electrolytic hydrogen and oxygengeneration. Laboratory scale (150 cm3 at S.T.P. of H2)electrolyzers are already commercially available and largerscale prototypes have been built, e.g. an 0 2 generator for asix man space station for NASA. Some of the G.E. cells havebeen tested [18] up to 2.2 A cm'2 (2000 A/sq. ft.) at about353 K. For long-term stability 1.1 A cm"2 seems to be a morerealistic maximum current density.

If fuel cells were to be used for reconverting the evolvedgases into electricity, the electrolysis cells need not behigh pressure cells. But, if a gas turbine were to be em-ployed, it would probably be advantageous to compress thegases electrochemically since the Nernst potential requiredfor a gas pressure of 2 MPa (̂ 20 atm) is less than 50 mV at60°C. Electrolyzers operating at 2 MPa are already availablecommercially [19], and a G.E. prototype solid polymerelectrolyte cell has also passed pressure tests in this range.

The heat generated due to the internal resistance of the cellis most conveniently removed by circulating the cell solu-tion through external heat exchangers. More than enough heatwill likely be generated within a chosen cell design to operate

- 9 -

the catalyst column at 333 K.

Solid oxide electrolyte cells operate at high temperatures,for example up to 1000°C, [20] and therefore they are notlikely to be as suitable for the present application as lowtemperature cells for the following reasons:

(a) the start-up and shut-down times for these high tempera-ture cells will be very much longer relative to the lowtemperature cells and therefore are not suitable fordaily peaking operation;

(b) the electrolytic H/D separation factor, aji, will be veryclose to unity because of the high temperature, thusrequiring a larger number of transfer units in thestripper column for any given enrichment factor.

ELECTROLYSIS PLANT

The size and therefore the capital cost of the electrolysisplant is inversely proportional to the current density for agiven production rate. On the other hand, the operating costsincrease with increasing current density because the individualelectrode overpotentials as well as ohmic losses due to theresistance of the electrolyte are proportional to the currentdensity. The optimum choice of operating parameters willtherefore depend on the relative capital and operating costsfor the particular cell type chosen.

Assuming the same current density for anode and cathode, thegeometric surface area each of anode and cathode required toproduce feed equivalent to 1 kg D2O/h is given by equation (7):

1000 106 n E

Electrode Area = 20.02$ X D~ x R~ x 2 x 9 6 » 4 8 9 x

= 2.68 x 10 1 2 n±V x - r | . n_ n /, ) . (7)

where ng is the number of times the feed water must beelectrolyzed (reflux) , Dp is the deuterium concentration ofthe feed in ppm D/CD+H), R is the fraction of deuterium re-covered, and c.d. is the current density in mA cm"2 based onthe geometric surface area.

Assuming a current density of 500 mA cm"2, the 1 kg D?O/hrequires about B.7 x 103 m2 each of cathode and anode surfacearea for the 0. T5 1st stage product case where ng = 1.05.A 75 Mg plant would therefore require about 10s m2 (>106 sq. ft.)

- 10 -

each of anode and cathode surface area [recall equation

The number of kWh of electrical energy required per kg D20is

Energy /kg D20 = 2.68 x 106 „ b kWh (8)

where Vg is the terminal unit cell voltage in volts. For the3-stage plant under consideration the energy required per kgD20 is 50 MWh and for the plant production the energy requiredis 3.75 x 106 MWh based on an assumed cell voltage of 1.74 V .

A terminal cell voltage of 1.74 V yields an electrical energyefficiency of about 70%f and should easily be attainable withpresent cell designs at moderate temperatures. With more ad-vanced cell designs, efficiencies up to 80% should becomefeasible [21,22] .

The base power required for the assumed electrolysis plant is,therefore,

Power = D20 Production Rate (kg/h) x Energy (MWh)/kg D20

75 x 103

x 50 = 938 MW. (9)4000

This amount of base power represents about 18% of OntarioHydro's projected 1979 nuclear generating capacity. Assumingan overall combined energy efficiency of 401 for electrolysisplus either turbines or fuel cells, then 1.5 x 106 MWh ofpeaking energy would be available. ~~~

ADVANTAGES OF THE COMBINED PROCESS

Advantages of the combined heavy water process as compared toelectrolysis alone are as follows:

(a) For electrolysis alone, operated as an ideal cascade, thetotal number of times the feed must be electrolyzed, ng,is of the order of 1.7 for CCE = 6 and Dp = 148 ppm [23].

*This particular value was chosen because, apart from being areasonable choice at the particular current density, itcorresponds to 50 MWh/kg D2O for the 0.31 first stage en-richment example, a convenient number to remember.

'based on the standard free energy, 1.23 V

- 11 -

For the electrolysis of the coupled electrolysis-chemicalexchange process, ng varies from 1.10 to J . 01 dependingon the 1st stage product deuterium concentration chosen,rrom 1500 ppm to 30,000 ppm. This represents a saving ofat least 60% in the energy costs for electrolysis, whateverthe price of electricity.

(b) Again assuming an ideal cascade .for the conditions mentionedabove, the number of stages reauired for electrolysis alone[23] is of the order of 17. For the combined process, thenumber of stages can easily be reduced to 3 as shown inFir. .3.

D20 PRODUCTION AS A BY-PRODUCT OF ENERGY STORAGE VIA HYDROGEN

The electrical energy cost per kg D20 for electrolysis isshown in Fig. 8 as a function of the peak to base power costratio, Cp/Cg, for different realistic base power energy costs.The net electrical energy cost per kg D:O is zero for Cp/Cj; =2.5 and independent of Cg. If less than 401 of the inputenergy iv'ere recovered this point would be shifted to a higherpeak to base power cost ratio. From Fig. 8 it is also obviousthat for Cp/Cp >2.5, the energy dollar credit per kg D20 in-creases ,-.? CR increases, so that peak power production via theproposed process should become more and more attractive withtime as the alternative peaking operations become relativelymore and more expensive, and as the base power costs increase.At present, Ontario Hydro's peak to base power unit energycost ratio [1] is estimated as greater than 2.5. This ratio isexpected to increase as more and more nuclear stations comeon-line as long as peak demand is met with fossil fired stationsas at present.

Because energy for storage will only be available during off-peak periods, the D2O plant also can only ooerate part of thetime. The resulting extra capital cost per kg D2O would beoffset by the energy dollar credit per kg D20 for any peak tobase power cost ratios greater than about 2.5 (see Fig. 8).

HYDROGEN AND OXYGEN STORAGE

An analysis of hydrogen and oxygen storage is beyond thescope of this paper. Oxygen storage is easy and hydrogenstorage has been discussed, for example, in reference [7]. Afew comments on storage location may be appropriate, however.

The whole energy storage system could be located anywhere onthe power grid at the expense of electrical transmission losses.For example, decoupling the storage system from the nuclearreactor, except for electron flow, may be desirable if a site

- 12 -

appropriate for underground gas storage were available at.mother location. Decoupling may also be advantageous wherethe reactor is situated some considerable distance away fromthe load centers. By locating the energy storage system neara load center, the major transmission losses would be ab-sorbed at the base rate.

Regardless of the storage method, it is recognized that storageincurs significant losses of useful energy which must be con-sidered in any detailed desipn study of the proposed scheme,

ilYnROGHN-nXY(;i:.N TURBFNHS

During peak power demand the stored hydrogen and oxygen couldin principle be reconverted to electricity by means of ahigh temperature turbine cycle such as that shown in Fig. 9.Hydrogen-oxygen turbine cycles with efficiencies of 55% to70o have been predicted for systems with inlet temperatures upto 1925°C to 2200°C at 10 MPa' for the years 1985 to 2020 [24].In contrast, temperatures as high as 1066°C only are permissibletoday for industrial class tmbines [25]. Thus in actualpractice the temperature will no doubt be limited to 1100°C to1400°C for some years. Electrolyzer pressures of 2 MPa to7 MPa. appear feasible in the near term [26] . Under these con-ditions the turbine conversion efficiency would be expectedto approach 50° as indicated in Fig. 9.

SUMMARY

Rase loaded, natural uranium, heavy water moderated nuclearreactors (the CANDU system) produce the least expensive ther-mally generated electric energy in Ontario. Nuclear powerpresently accounts for fifteen percent of the electric energyin Ontario.

Nuclear plants coupled with some form of energy storage arebeing considered as a means to meet the variable load com-ponent in the future. Several authors have suggestedelectrolytic hydrogen with reconversion to electric energyvia fuel cells, gas turbines or MHD units as a storage system.

In a nuclear system, such as CANDU, that requires heavy water,an additional attraction for electrolytic hydrogen storage isthe possible production of D2O as a by-product from theelectrolysis, which has also already been noted. This schemebecomes even more attractive with an efficient deuteriumextraction method based on a combined electrolysis-catalyticexchange process.

The combined process described is now possible using a newly

- 13 -

developed, wet-proof, hydrogen-water isotope exchange catalyst.The catalyst operates in a monothermal, trickle-bed exchangecolumn ttfith water as feed and electrolysis supplying the re-flux hydrogen. Production of the electrolytic gases inconventional, high pressure [>,2 MPa) cells would aid the ex-change process and would permit gas turbine operation withouta compressor stage. Finally, such a hydrogen-oxygen com-bustion turbine could be coupled to a steam turbine to furtherextract energy and improve efficiency.

With present electrolyzer and turbine technology, 30% energyrecovery is realistic and 40% seems possible in the nearfuture. At 4 0% recovery, the net energy cost for D20 pro-duction would be zero at a peak to base power cost ratio of2,5, a ratio about that for the Ontario Hydro system at present,At a higher ratio, not unexpected in the future as fossil fuelcosts will likely rise faster than nuclear fuel cost, anenergy dollar credit results, and increases as the base loadenergy cost increases. It should be noted that the capitalcost of the electrolysis-exchange equipment, per unit of D2Oproduction, will be higher than expected at first glancesince this plant will not operate during peak load periods.The same situation applies to the steam turbine section ofthe hydrogen-oxygen turbine cycle, which operates only duringpeak load periods. However, the higher the cost of base loadpower, and the greater the spread between variable load andbase load energy costs, the more capital can be afforded forthe system proposed in this paper.

REFERENCES

[1] H.A. Smith, L.W. Woodhead, and L.J. Ingolfsrud, OntarioHydro Report NGD-5, 1975 presented at European NuclearConference, Paris, France April 21-25, 1975

[2] A.J. Harris, Ontario Hydro, private communication

[3] W.L. Hughes, and S.O. Brauser, U.S. Pat. 3,459,953 (1967)

[4] G. Strickland, J.J. Reilly, and R.H. Wiswall in "HydrogenEnergy", N.T. Veziroglu, ed., Plenum Press, N.Y., part A,1974, p. 611

[5] R.G, Murray, ibid., part B, 1974, p. 901

[6] F.J. Salzano, E.A. Chemiavsky, R.J. Isler, andK.C. Hoffman, ibid., part B, 1974, p. 915

[7] C.J. Kippenhan, and R.C. Corlett, ibid., part B, 1974,p. 933

[8] W.R. Parrish, ibid., part B, 1974, p. 949

- 14 -

[9] S.J. Townsend, and E.W. Koziak, ibid., part B, 1974,p. 983

[10] W.H. Stevens, Can. Pat. 907,292 (1972)

[11] W.H. Stevens, U.S. Pat. 3,888,974 (1975)

[12] Environ. Sci. Tech. 9(10) , (1975) 910

[13] R.G. Hamp, Ontario Hydro, private communication

[14] R.M. Brown, E. Robertson, and IV.M. Thurston, AtomicEnergy of Canada Limited Report AECL-3800 (1971)

[15] M. llammerli, J.P. Mislan, and W.J. Olmstead, J. Electro-chem. Soc. 117 (1970) 751 (and references therein)

[16] L.P. Roy, Can. J. Chem. £0 (1962) 1452

[17] M. Benedict, and T.H. Pigford, "Nuclear ChemicalEngineering", McGraw Hill, N.Y., 1957, p. 378

[18] W.A. Titterington, and J.F. Austin, J. Electrochem.Soc, Extended Abstracts, 74-2 (1974) 576

[19] D.J. Hains, Lurgi Canada Ltd., private communication

[20] E.V. Sverdrup, D.H. Archer, and A.D. Glasser, inR.F. Gould, ed., "Fuel Cell Systems - II", Adv. Chem.Ser. 9_0, ACS Pub., 1969, p. 301

[21] J.P. Laskin, in N.T. Veziroglu, ed., "Hydrogen Energy",Plenum Press, N.Y., part A, 1974, p. 405

[22] L.J. Nutall, A.P. Fickett, and W.A. Titterington, ibid,part A, 1974, p. 441

[23] R.J. Munz, M.Sc.Thesis, Univ. of Waterloo, Waterloo, Ont.,1971

[24] W. Hausz, G. Leeth, J). Lueck, and C. Meyer, "HydrogenSystems for Electric Energy", General Electric CompanyCenter for Advanced Studies, Tempo Report 72 T.MP-15,April 1972

[25] A.M. Bueche, "Today's R § D for Tomorrow's Energy",Electric Century, General Electric, Fall 1974

[26] K.A. McCollom, "Use of Energy Storage with UnconventionalEnergy Sources to Aid Developing Countries", paperpresented at the 2nd IECEC, Miami, August 1967

- 15 -

Linear GasVelocity@ S.T.P.m s"1

3.05

10

ColumnParameter

Area

Diameter

Height

Area

Diameter

Height

TABLE I: DIMENSIONS OF THE CATALYST SECTIONS FOR THE STRTPPERCOLUMNS FOR A D2O PRODUCTION RATE OF 18.75 kg h"

1

1st Stage 2nd Stage 3rd Stage

19.5 m2 210 ft2 0.96 m2 10.3 ft2 270 cm2 0.3 ft2

5 m 16.4 ft. 1.1 m 3.6 ft, 18.5 cm 7.3 in.

9.6 m 31.6 ft- 11.1 m 36 ft. 15.8 m 51.8 ft,

5.9 m2 64 ft2 0.29 m2 3.1 ft2 78.5 cm2 12 in2

2.75 m 9.0 ft. 61 cm 2.0 ft. 10 cm 4 in.

33.1 m 109 ft 38 m 125 ft 54 m 177 ft.

TABLE TT\: DIFFERENT TYPES OF IVATER ELECTRiU.YST SCELLS: IXnTVTDUAL RE ACT TOWS Pl-R 4 FARADAYS

Cell Type Anodic Reaction Cathodic RenctionElect romig ration

Reaction

1. Liquid Electrolyte

(a) Acidic e.g. H2SO,,

(b) Basic e.g. KOH

2. Solid PolymerElectrolyte

(a") Acidic: —SO3II"*"

(b) Basic: -P

2H2O>n2+4H +4e

2 I I 2 O 0 2 + 4H +4e

4OH"->O2 + 2H2O+4e

4H +4e+2H2

4H2O+4e-»-2H2

4H +4e->2H2

4H2O+4e+2H2+4OH'

4t Cations+Catholyte4t Anions-»-Anolyte4t* Cations^Catholyte4t Anions^-Anolvte

4H •+ Cathode

40H"-> Anode

3. Solid OxideElectrolytee.g. Zirconia Ceramic

20 +02+4e 4H2O+4e>2H2+2O" 20 •+Anode

The net cell reaction common to each type is: 2H20+0?+2H2

TABLE IIB: NET CELL REACTIONS PER 4 FARADAYS IN ADDITION TO ELECTROLYSIS OF WATER

Cell Type Additional Net Cell Reaction

1. Liquid Electrolyte

(a) Acidic e.g. H2SCU 4t equivalents of electrolyte are transferred fromcatholyte to anolyte e.g. 2I-I2SOi»

(b) Basic e.g. KOH 4t equivalent 3 of electrolyte ave transferred fromanolyte to calholytc e.g. 4KOH

I I

BASEPOWER

NUCLEAR

POWER

PLANTOFF-PEAKPOWER

ANODE CATHODESIDE SIDE

RESERVOIR

H2

RESERVOIR OR

FEED H2O

H 2/H 2O DEUTERIUM

STRIPPER COLUMN

CONVENTIONAL

ELECTROLYSIS PLANT

DEUTERIUM ENRICHEDWATtR TO FINALCONCENTRATION PLANT

SEViAGE TREATMENT,METALLURGICAL PROCESSES. ETC

PEAK POY/ER FROM GAS-STEAMTURBINES OR FUEL CELLS

GAS PIPELINE FOR INDUSTRIALAND DOMESTIC USE

METALLIC HYDRIDES OR LIQUIDH2 FOR TRANSPORTATION SYSTEMS

F i g . 1 . S c h e m a t i c D i a g r a m o f C o m p l e t e S y s t e m f o r P r o d u c i n g ( a ) N u c l e a r H y d r o g e n a n dO x y g e n v i a E l e c t r o l y s i s , ( b ) H e a v y W a t e r , ( c ) P e a k P o w e r v i a Gas T u r b i n e -G e n e r a t o r o r F u e l C e l l s a n d ( d ) O t h e r P o s s i b l e U s e s f o r E i t h e r o r B o t h l l 2 a n d

FEED WATER

(148 ppm)

OXYGEN GAS TOSTORAGE OR ENERGY

CONVERTER ETC. 1

1 °2 h

ELECTROLYS

ANODECOMPARTMENT

HYDROGEN GAS TO

| STORAGE OR ENERGY1 mMVFRTFQ FTP

DEHUMIDIFIER

ff

f 1

H 2 /H 2 O

DEUTERIUM

EXCHANGE

CATALYST

TOWER

>f

SCRUBBER

H2O/

H 2

IS CELLS

CATHODECOMPARTMENT

f

ELECTROLYTESEPARATOR

\

SALT OF

ELECTROLYTE

DEUTERIUM ENRICHED

WATER TO HEAVY WATER PLANT

Fig. 2. Block Diagram of the Combined Electrolytic-CatalyticDeuterium Exchange Heavy Water Process (CECTHWP).

DEPLETEDHYDROGEN[50 ppm]

TO STORAGE99.99 moles

FEEDWATER

OXYGEN ATO

STORAGE

98.89

ANODESICE

SEPARATOR

CATHODESIDE

0.0098Z moles[9S.8 D2Oj

Fig. 3. Complete Mass and Deuterium Balances for the 5-Stage CombinedElectrolytic-Catalytic Heavy Water Process Based on the Follov-ing Assumptions: Dp = 148 ppm, n w a s t e = 50 ppm, D l s t stage

=

0.3%, D2nd Stage = 1Q?°> «li = 6-0, and «c = 5.19. Note that the2nd and 3rd Stage Il2 and O2 are Recombined to Form H2O which isReturned to the Corresponding Previous Stage as Reflux.

O

U-O

o

ai—

100

80

60

40

20

T 1 1 I I I I I 1 1 1 I I I T T ~i—r~prnr

0 . 0

G I R D L E R - S U L F I D E

D U A L T E M P E R A T U R E P R O C E S SWATER ~

D I S T I L L A T I O N

3RD

S T A G E2ND

S T A G E

D E U T E R I U M I N P R O D U C T

Fig. 4. Percent of Total Cost of Heavy Water Production as a Function of theDeuterium Concentration in the Product for the Commercial DualTemperature H2S/K2O Deuterium Exchange Heavy Water Process . Xotetha t Final Enrichment i s via Reduced Pressure D i s t i l l a t i o n of Water.

GO>-

CJ

100 -

80

60

20

i r

BASIS:

= 148 ppm

= 3. 2

RECOVERY = 66;

K v a = 2

2ND STAGE

J I I L J I I L0.2 0.4 0.6 0.8 1.0

DEUTERIUM IN FIRST STAGE PRODUCT

Fig. 5. Percent of Total Catalyst Volume Contained in the 1st and 2nd Stages as aFunction of the Deuterium Concentration in the 1st Stage Product. All OtherAssumptions are the Same as in Fig. 3 Caption.

ro

O

UJ

OQ_

CO

occo_ lLU

1.1

1.0

0.9

0.8

n 7

I I I I i I I i |

- I CATALYSTx-^ -

/ ELECTROLYSIS

• y.1

f

1 1 1 1 1 1 1 1

0

— 240

ro

LU

0.2 0.4 0.6 0.8 1.0

220 §

c/)

200 <C<o

180 gCQ

160

% DEUTERIUM IN FIRST STAGE PRODUCT

Fig. 6. Total Electrolysis Power and Total Catalyst Volume as a Function ofthe Deuterium Concentration in the 1st Stage Product for an AssumedProduction Rate of 18.75 kg D20/h.

1.4

1.3

co , 2o

Ld

yZ I.I<

2O

.0

0.9

0.8 -

0.7

ELECTROLYSIS COSTCATALYST COST

= 4

= 3

0 0.2 0.4 0.6 0.8 1.0

% DEUTERIUM IN FIRST STAGE PRODUCT

Pig. 7. Total Relative Cost for Electrolysis Cost to CatalystCost Ratios of 2, 3, and 4 as a Function of theDeuterium Concentration in the 1st Stage Product. TheArrow Indicates the Assumed Base Case for theseCalculations.

o

aOJQ

ocLLJ

Q_

COo

CD

LU

^ u u

300

200

100

0

100

200

300

•ton

—

—

—

- CB

1

10*

9 V8 V7 VGO

= 5V

CB

|

1

- $

B

I

kIN

/MWASE

I

1 1 'BASIS:

a) 50 M W h / k g D

^) 4 0 % ENERGYRECOVERED

V\ih FORPOWER

1 1 I

1—

20

—

—

Mil

l1

1 1

1 I

1 1

1 1

1I

ll

ll

l

VV7

V8

V9

MO -

11 2 3 4

P E A K T O B A S E P O W E R C O S T R A T I O

Fig. 8. The Net Electrolysis Energy Cost per Kilogram of HeavyWater as a Function of the Peak to Base Power CostRatio, Cp/Cg, for Different Values of CB-

5 .325 k g / s H2 s 119765 k J / k q = ^ 7 , 7 4 8 kW

HYDROGEN-OXYGEN COMBUSTION TURBINE

211 ,400kW

211400 + 101140= 0 . 4 91 2 4 . 7 4 K

3842h667C0 2P

5 07P76 86w

696h1fo6C

161 66w0 152P2979h

260C24 13P3626h649C

36 92i,?4 82P

11 OOh

121 .2wCONDENSER

EXTRACTIONSOMITTEDFOR CLARITY

LEGEND

w = FLOW, k q / s

P = PRESSURE, MPa

h = ENTHALPY, k J / k q

C = TEMPERATURE °C

Fig. P. A Possible Hydrogen-Oxygen Turbine Cycle for theProposed Peaking Plant.

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