Regenerative EMF Cells : Regenerative ... - ACS Publications

1 Regenerative Electrochemical Systems: An Introduction HERMAN A. LIEBHAFSKY

Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0064.ch001

General Electric Research and Development Center, Schenectady, Ν. Y.

Among the increasingly complex and sophisticated methods of energy conversion needed today, regenerative electrochemical systems occupy an important place. In this introduction, the author attempts to mitigate the confusion that exists in the naming of these systems, to discuss their thermodynamics on the basis of simple examples, and to show that these systems may be regarded as foreshadowed by the work of Grove (1839). If ex perience with fuel batteries is a valid guide, the development of practical regenerative electrochemical systems will encounter many difficult engineering problems, and the difficulty of de veloping such systems will increase with the complexity of the transport problems involved.

'Tphe systems, terrestrial and extra-terrestrial, we need for energy conversion today grow increasingly complex and sophisticated. Describing these systems is difficult and entails a growing risk of confusion. Consider a patent {10) for fuel cells to fit between the axles of automobiles: these cells happen to be plastic fuel containers, and the story goes that electro chemical fuel cells benefited from mistaken identity when newspapers carried notices of a large contract for the fuel containers. Figure 1 shows the names used for some of the systems in the ACS symposium held on regenerative cells in 1965. A short, general, and reasonably precise name for the systems under discussion is regenerative electrochemical system. A l l our complete systems seem to contain at least one electrochemical cell, a class in which "fuel cell" is included. The system regenerates a working substance to make a regenerated working substance; here we stretch a term associated with steam engines to include the two components fed separately to the anode and to the cathode of the fuel cell. For example, if A B is the working 1

Crouthamel and Recht; Regenerative EMF Cells Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

REGENERATIVE EMF CELLS

2

Regenerative emf cells Regenerative fuel cell system Regenerative emf system Thermally regenerative system Regenerative bimetallic cell Thermally regenerative galvanic system Thermally regenerative emf cell systems


Figure 1. Partial list of names in symposium programs The term, Regenerative Electrochemical Systems, is suggested to replace the above names

substance for a regenerative electrochemical system, then A and Β make up the regenerated working substance. The regenerative electrochemical system is thus a "black box" (Fig ure 2) into which only energy enters and from which only energy departs; it is an energy conversion system closed with respect to mass. Of course, the three-word name is too short to be completely descriptive and may need occasional amplification; for example, it might be taken to include black boxes (Figure 3), not closed with respect to mass—important in the field of fuel cells (7). Thermodynamics, having no curiosity, finds black boxes to its liking. Given the input and the net useful output for a regenerative electrochem ical system, one can calculate its efficiency without knowing what the black box contains. One must distinguish here between net useful output and total output, for the latter includes rejected energy that is degraded and heat attributable to irreversibility. The first two laws of thermo dynamics must both be considered. To simplify this discussion, it will be assumed that the other energy inputs are converted either into heat or into electrical energy before they enter the black box. This simplification will include most systems near practical realization; it excludes, for example, the direct photochemical regeneration of the working substance. Electrical energy (direct current) as input is the simpler case. As the output is energy of the same kind, one is entitled to ask what purpose the

REPRESENTATIVE INPUTS

OUTPUTS ELECTRICAL ENERGY(USEFUL OUTPUT)

ATOMIC ENERGY

REJECTED ENERGY (USUALLY HEAT)

HEAT ENERGY

[FROM REVERSIBLE OPERATION

LIGHT ENERGY

I ADD LOSSES CAUSED BY [IRREVERSIBILITY

ELECTRICAL ENERGY

Figure 2. Outputs of a regenerative electrochemical system with representative inputs


1.

LiEBHAFSKY

3

Introduction

ΛΜΛΛ

ΛΛΛΛΛ -

ÇQJ C0/0 CELL FHEAT Γ I0

CA) CELL 2

2

Ί

>C0 ANO TAS 2

2


ΝΛΛΛΛ WATER CO GAS REACTOR H HEAT 2

llix FUEL CELL

-CO,

AND

TAS

Id

WAT ΕI CO; GAS REACTOR Η, FHEAT

FUEL CELL

>C0 AND TAS 2

Figure 3. Black boxés containing fuel cells (7) All boxes have the same ideal efficiency. Note that two of the boxes are regenerative with respect to water

system serves. The answer is that it stores energy. If the regenerated working substance is A and Β (dissociated AB), one may regenerate it from A B during time h; one may withdraw electrical energy during time U by allowing A and Β to be consumed in the electrochemical cell. Time ti could be night, the time of off-peak load in a terrestrial power plant; U might then be the time of peak load. For a power plant on a satellite, h could be orbital day with a silicon converter of solar energy in operation, and h could be orbital night. In either case, the advantage gained by storing energy needs to be great enough to outweigh the disadvantages associated with the black box. To simplify further discussion, we shall take A B to be water and the electrochemical reactions to be as shown at the top of the following page. This kind of regeneration was uppermost in Grove's mind when he invented the fuel cell. In a postscript dated January 1839 to his paper of a month earlier (5), Grove says of an experiment with a hydrogen/oxygen cell: "I hope by repeating this experiment in series to effect decomposition



4

Electrochemical Reactions Anode: H » 2H+ + 2 electrons Cathode: * 0 + 2H+ + 2 electrons - H 0 Sum: H + *0 = H 0

(1) (2) (3)

Regenerative Reaction Reverse of (3) : H 0 = H + *0

(4)

2

2

2

2

2


2

2

2

2

of water by means of its composition"—i.e., to electrolyze water with electrical energy from a hydrogen/oxygen battery (italics supplied). He did this with a battery of 26 cells in series; four of these cells are shown in Figure 4. ox

Ay

Figure 4. An approach to regenerative electrochemical systems; the work of Grove (1839)—showing only four cells of fuel battery

To change Grove's system into our black box, we supply electrical energy for electrolysis from the outside, remove from the box the electrical energy produced, and arrange for the necessary mass transport within. We then have a regenerative electrochemical system (Figure 5) with water as the working substance. It is easy to imagine a simplification (Figure 6) in which the same device functions part-time as an electrolysis cell or battery, and part-time as a fuel cell or battery. Hereafter we shall use "cell" to include "battery," unless a distinction must be made.



1.

LiEBHÀFSKY

5

Introduction

In principle, the cell of Figure 6 could be operated reversibly and isothermally without having heat rejected by the system. There would, of course, have to be provision for storing and exchanging within the system the small amounts of heat TdS associated with reversible isothermal operation. The ideal cycle efficiency would be unity: the fuel cell and the electrolyzer would each operate at E° = 1.23 volts under standard conditions and 298.1°K, and the current efficiency would be 100%. A current efficiency of less than 100% means that side reactions are occurring. These will almost never be reversible. When they are not, the system cannot be returned to its original state; the state of the system then depends upon its history so that the system cannot qualify as an invariant regenerative system. Side reactions include corrosion reactions, and these are often the greatest threat to invariance. The cycle efficiency includes the efficiency of the fuel cell and of the electrolyzers, and each of these individual efficiencies is the product of a voltage efficiency and a current efficiency. With water as working fluid, it is usually possible to find conditions where the current efficiencies are virtually 100%. Consequently, we shall use only the voltage efficiencies here. At useful current densities optimistic values for these voltage efficiencies are: 08 123 Fuel Cell: »? "= ^ 3 Electrolyzer: i j (5) E

TC

whence the cycle efficiency (the product) is: vc

-

re

m

5

( 6 )

°·

Actual efficiencies will be lower—partly because actual systems will contain auxiliary equipment with parasitic power requirements. An account of early work (1) on the system of Figure 6 with an ionexchange membrane as electrolyte contains the opinion: "it is likely that a more stable membrane can be found that will be suitable for regenerative operation." At the Direct Energy Conversion Operation, General Electric Co., Lynn, Mass., new membranes have been found that appear suitable.

EC « ELECTROLYSIS CELL FC • FUEL CELL C * CATHODE A « ANODE (23

8

ELECTROLYTE

DC. OUTPUT

Figure δ. Grove's system (Figure 4) rearranged to make a regenerative electrochemical system



6

ELECTROLYZER OVER TIME t, FUEL CELL OVER

H STORAGE

TIME

2

^^STORAGE

t

2

ELECTROLYTE I

J POROUS ELECTRODES


Figure 6. A regenerative electrochemical system in which the anode (cathode) during electrolysis (overtime h) becomes the cathode (anode) over the fuelcell period (overtime h) Note: More than a year after this manuscript was prepared, M. Klein, Xerox Corp., described the substantial progress that has been made on a system identical in principle with that of Figure 6 (9).

A successful, regenerative, electrochemical storage system with water as the working substance can probably be built for a specialty application. A useful classification of devices to produce electrical energy can be based upon an arbitrary separation of expected advantages into "low energy cost" and "convenience," the latter term including such attributes as low weight, small vol./kw., and silent and clean operation. We then have the following: Type Device Specialty Industrial Central Station

Importance of Low Energy Cost Minor Comparable Overriding

Importance of Convenience Overriding Comparable Minor

In the development of specialty devices, reduced emphasis on cost enlarges freedom of choice, and this advantage will usually outweigh the effect of the stringent requirements these devices must meet. Can devices modeled on Figure 6 compete in the storage of electrical energy on a central-station scale? Not under ordinary conditions. Low cost of the output energy is overriding here, and this cost includes such items as fixed, operating, and maintenance charges. Pumped Storage Until about ten years ago, the central station generating alternating current seemed the most difficult converter for electrochemical devices (fuel batteries) to displace. But it now appears that systems modeled on Figure 6 have even less chance against pumped storage, which relies on pumping water uphill. Friedlander (4) points out that pumped storage in



1.

LiEBHAFSKY

Introduction

7

the United States has grown rapidly from a small beginning in 1950, and that about 10,000 Mw. of pumped-storage capacity is projected or being constructed. Originally, in our language, pumped storage was envisioned as a way of storing low-cost, off-peak energy during time t\ and releasing it as higher-cost peak energy during time fe. Now it is expected (4) that this method of storage will be even more valuable as a "spinning reserve"—a reserve from which energy can be drawn instantly and at low cost to satisfy at any time an increase in demand beyond the capacity of the stations then operating. Friedlander cites convincing data to show that pumped storage can provide stored energy at lower cost than even a conventional steam reheat plant. The better steam plants offer formidable competition to fuel batteries because they achieve comparative thermal efficiencies of about 40%, are highly reliable, and call for capital investment as low as $100/kw. installed. Pumped storage has higher efficiency than such a central station, reliability at least equal, and a capital investment requirement that can be lower even when the cost of transmission line is included {4). (According to a rough but reliable guideline, pumped storage delivers 2 kwh. for each 3 kwh. input. This actual efficiency of § significantly exceeds the 0.5 of Equation 6.) The coming-of-age of the nuclear reactor is sometimes believed to offer an unusual opportunity to fuel batteries and to regenerative electrochemical systems. The argument advanced is that such reactors will provide off-peak electrical energy in large amounts at low cost, and that this energy can be used profitably to generate hydrogen and oxygen for fuel batteries or stored profitably by an electrochemical system. It seems more realistic to assume that there will be keen competition for this energy and that competing alternatives, such as pumped storage, will not lose their advantages. The generation of potential energy by pumping water uphill seems inherently more attractive on a large scale than the electrochemical regeneration of a working substance. The principal problem discussed in the papers to follow is the electrochemical conversion of heat into electrical energy. The thermodynamics of these systems is conveniently approached on the basis of the rearranged Grove system (Figure 5). We have seen that this system can store energy isothermally, provided it operates reversibly. It becomes a heat engine if the working substance absorbs heat during reversible regeneration at the temperature, T and rejects heat during the reversible generation of electrical energy at the lower temperature, TV As thermodynamic conclusions are independent of the means for carrying out a reversible process, electrochemical regeneration of the working substance, as in Figure 5, may properly serve for a general discussion of all methods for reversible regeneration. This kind of system has the great advantage of providing the h


8


regenerated working substance as separated components: here, hydrogen and oxygen. It is thus unnecessary to envision and discuss separation processes. To emphasise that a regenerative electrochemical system of this kind is subject to the Carnot-cycle limitation, the following cycle was pos tulated for A B as working substance (2, 6, 8). Dissociation in Reactor: A B =* A + Β at T Recombination in Fuel Cell: A + Β » A B at T% Heat Exchange (connecting link) between A and Β initially at T and A B initially at T

(I) (II)

x


x

t

(III)

Figure 7 shows that Grove's system can be further changed to accom modate this cycle. Note that dissociation has been assumed to occur at T and recombination only at T%. As the following quotation shows, the possibility of dissociation was considered in the earlier treatment and dismissed: "Instead of behaving like water, A B might be stable in State 2 but unstable in State 1. One might then assume that A B does not dissociate until it is passed over a catalyst at T when it dissociates spontaneously to regenerate A and B " (0). x

h

INPUT

EC « ELECTROLYSIS CELL FC •FUEL

CELL

C > CATHODE A·ANODE HE » HEAT EXCHANGER E2

.ELECTROLYTE

OUTPUT ELECTRICAL ENERGY9 Δ 6 | HEAT REJECTEE) · T ^ S J 1

Figure 7. Grove * system made thermally regenerative (6) The ideal efficiency for the postulated cycle is

and the system has become

an electrochemical heat engine.

To make Reference 6 realistic, calculations were made for water as A B . At Τι = 2000°K, water vapor at 1 atm. is in equilibrium with about 0.0056 atm. hydrogen and half that of oxygen. Within the framework of Reference 6, it seemed logical to ignore the possibility that water might dissociate in the heat exchanger (3). With Ti « 2000°K., and Γ « 500°K., the Carnot efficiency Of this cycle is 75%. With water as working substance, it was shbwn that only 71.7% could be realized in the system of Figure 7 (see Equation 11). The 2


1.

9

Introduction

LiEBHAFSKY

reason for this decrease of 3.3% is that the heat transfer process (III) necessarily had to be irreversible under the conditions assumed because the combined heat capacities of the hydrogen and oxygen regenerated at Τι exceeded the heat capacity of the water consumed in the regeneration. It seems worthwhile to establish whether or not the 3.3% can be easily accounted for by driving reversible heat engines with the rejected heat that gave rise to the 3.3% deficit. This heat is rejected from A and Β to A B during heat transfer, the transfer beginning at T\ and stopping at TV The total heat rejected is: AH - àHi ~ f£ ACpdT (for 1 mole of A B through the cycle) Downloaded by 80.82.77.83 on May 17, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0064.ch001

2

(7)

If the entire heat transfer is to be reversible, the heat must be rejected at a temperature, Γ, that decreases continuously from T\ to TV We pos tulate an infinite number of reversible heat engines operating on this rejected heat, the inlet temperature of each engine differing by dT from that of its neighbor, and the outlet temperature being T% for all engines. We make the unrealistic assumption that ACp is constant, and we continue to neglect the changing dissociation of water between T\ and TV The reversible work then available from these engines is: (8)

w which becomes w

- f* ACpdT + J* ACpT* d In Τ

w

- [AH

(9)

or t

- AHi] +

= -1990 +

I

» X T X In t

fj]

(10)

X 1.379l], (Γ, = 2000°K.; T% = «ΧΤΚ.)

= -1075 cal. From Reference 6, the efficiency with everything but heat transfer reversible was: . Δ

Regenerative EMF Cells : Regenerative ... - ACS Publications

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