- Email: [email protected]
Electrochemical solar ponds hybrid thermal and electric storage
Solar & Wind Technology Vol. 4, No. 4, pp. 447-450, 1987 Printed in Great Britain.
0741--983X/87 $3.00+.00 Pergamon Journals Ltd.
ELECTROCHEMICAL SOLAR PONDS HYBRID T H E R M A L A N D ELECTRIC S T O R A G E J. DORIA, M. C. DE ANDRf/S and C. ARMENTA Grupo de Energia Solar, Departamento de Fisica Fundamental, Facultad de Ciencias Fisicas, Universidad Complutense, 28040 Madrid, Spain (Received 4 December 1986; accepted 15 January 1987) Abstraet--A hybrid thermal and electric energy collection and storage system for solar applications is proposed. Hybrid storage is achieved by transforming a conventional solar pond into a secondary activity cell, where the inert electrolyte (NaC1 solution) has been partially replaced by active ones totally soluble on both oxidated and reduced phases, which produce highly reversible reactions. The tested REDOX pairs were : Pb/Cr ++, Cr +++, HaO+//Fe +++, Fe ++, HaO+/C Pb/Cr ++, Cr +++, HaO+//Cr2OT, Cr +++, H30+/C. The suggested hybrid system has been designed for static operation and large storage capacity. Changing the solar pond into an electrical accumulator improves thermal efficiencyof the pond and does not increase building expenses; furthermore solar pond high temperature is advantageous for the accumulator operation and helps for current extraction.
1. INTRODUCTION Solar applications have not expanded as expected 10 years ago, largely due to difficulties in storing the converted energy either by thermal or photovoltaic means. Concerning thermal storage, only large sized seasonal storage systems are suitable and within this group, solar ponds are the most cost effective. Acid or alkaline electrical accumulators present serious drawbacks when coupled to stationary photovoltaic arrays : electrolyte stratification, limited number of charge-discharge cycles, low effective capacity (usually less 20% of their nominal capacity) and poor current supply at low ambient temperature. Studies were begun in 1980, restricted to thermal storage in solar ponds and analyzing the physical phenomena which take place in them [1-5]. The cost effectiveness of solar ponds as solar energy collection and storage devices has already been shown. However, no attention has been paid to a particular characteristic of the solutions existing in the pond: their ionic nature. This property will allow its transformation into either a concentration or an activity cell for electric energy production or storage with only slight modifications to the pond. The aim of this work was to consider the viability of electrical energy production or storage by electrochemical means, without disturbing thermal performance.
Our first tests were devoted to solar pond conversion into a primary activity cell with mass transport. The results were encouraging: output voltage stability, high performance and large current density [6]. Besides, thermal conversion is improved because salt mass flow through the non-convective zone (NCZ) is decreased due to the existing electric field in the mentioned zone [7]. Later on, solar pond conversion into an electrical accumulator was considered. Initial tests, trying to adapt the operation of the pond to commercially available accumulators gave negative results. Small inert electrolyte concentrations in active electrolytes produce self-discharge processes and voltage decay was rapid and continuous. Finally, the experiments performed with highly soluble electrolytes in both oxidated and reduced phases, showed the viability of solar pond utilization as a hybrid and electrical system for solar applications.
2. TOPICS In an electrical accumulator for photovoltaic applications the following conditions must be observed : (i) High reversibility in the oxo-redox reactions, which can only be achieved by extremely soluble materials in both oxidated and reduced phases. (ii) Chemically inert electrodes to all electrolytes
447
448
J. DORIA et al.
Non-convective
Etectrode
~ ....
1]"
"
zone
-,c,.ve
. . . . .
H
etec,roty'ce ~
Etectrode . . . .
-,c;,v;
....
etectrot,,e
'
']l
Fig. 1. Solar pond as an electrical accumulator.
contained in the system capable of providing a suitable overpotential discharge. (iii) In a static system, active ion transport must be enlarged by enforced convection. As a result, current density is increased. (iv) The ionic bridge between cells must avoid active specimen mixture, which would produce selfdischarge and show as low ionic compensation resistance as possible. Figure 1 shows how a solar pond can be converted into an electrical accumulator that fulfils all the above conditions. The lower inert electrolyte is replaced by two active and highly soluble ones until typical density values are obtained. The active electrolytes can flow through the inert electrodes which may be placed off the system. The non-convective zone acts as an ionic bridge, thus allowing for large current densities to be extracted.
Discharge processes Reactions :
Anode Cathode
Charge processes Hydrogen is produced in a secondary reaction in charge processes. As we are dealing with an acid medium, charge reactions can be written : Anode Cathode
Preliminary tests for suitable electrolyte and electrode material selection were performed in a small electrochemical cell consisting of a cylindrical tank. The two half-elements were joined together by a double porous wall filled with a NaC1 solution. The selected accumulator, based on data, was : Pb/Cr + +, Cr + + +, HsO+//Fe + + +, Fe + +, H3O+/C. The normal potential of this accumulator is 1.18 V and its nominal capacity if 26.8 Ah per 1-M solution. The experimental prototype used is shown in Fig. 2. The tank containing the two active electrolytes and the inert one can be seen in the middle. The electrodes placed on both sides of tank are ionically connected with it by circulating magnetic pumps.
Cr + + + + e Fe ++
The outer discharge element is of 2 ~ resistance. Open circuit voltage time evolution vs charge state was obtained from chemical analyses of samples periodically extracted. The results are plotted in Fig. 3. It can be observed that voltage is hardly influenced by charge state. Its value is kept at 1 V for capacities of about 10% of the nominal value. From the data and nominal capacity value, it is concluded that electrochemical conversion efficiency is 87% (average current density is 100 A m2).
Reactions:
3. TESTING DEVICES: EXPERIMENTAL DATA AND RESULTS
Cr + + Fe +++ + c
Cr++++e H20+e Fe ++
Cr ++ ½Hz+OH Fe++++e .
Hydrogen production can be negligible in a single cycle, but its effect on cathodic reaction inefficiency is increasingly severe as more cycles are performed. This problem can be solved by suitable overcharge with some sacrifice in performance, or by hydrogen addition on Fe + +/Fe + + + half-element. The progressive system capacity loss, due to hydrogen production, can be seen in Fig. 4. The charging process is repeated under identical conditions in each cycle, supplying the same charge amount and beginning at the same discharge level. After 20 cycles, the charged system capacity has come down to 65% of the initial capacity. On the other hand, hydrogen addition on iron cell allows for total initial capacity recovery. The results moved us to modify the hybrid accumulator pond as shown on Fig. 5. Cells are detached
Electrochemical solar ponds hybrid thermal and electric storage
449
Fig. 2. A view of the experimental device. Table 1. Initial and discharge conditions
Anode CrC13 Cathode FeC12
Vol. (1)
HC1
(g cm -3)
Inert Elec.
Q(1 h -I)
t(°C)
9.75 (1 M) 9.75 (1 M)
1M 1M
1.3 1.3
NaC1 NaC1
40 40
60 60
1.6 --
IOO~ o
ILl
U "x
1.2
0.8
o
o
"x 80_ ,,,
r| "-,
so-
[ Ha
Q. -~
0.4
o
o 0
I
I
I
I
20
40
60
80
I IO 0
Q. o
Injection
40--
20--
Disch0rge Lever, D (%)
Fig. 3. Open circuit voltage-time evolution vs charge state.
0
I
I
I
1
I
I0
20
30
40
50
Number of cycles (N)
from each other by a substance like bentonite, which allows for active specimen diffusion control as well as molecular hydrogen flow that reduces Fe +÷+ placed on a Cr ÷ + +/Cr + + half-cell.
Fig. 4. Accumulator capacity evolution.
present, this kind of accumulator is being studied and there are no definitive results yet.
Other accumulators A p a r t from the accumulators described the following has been tested : Pb/Cr + + + Cr + +, H30+//Cr2OT, Cr + + +H~O+/C. First results seem to show that the suggested system has the good properties o f the F e - C r one, but with an important advantage: Cr diffusion between cells would not produce material contamination. A t
4. CONCLUSIONS
From the experimental results it can be stated that the suggested hybrid system can solve thermal and electric energy storage difficulties linked to thermal and photovoltaic solar energy conversion. Transforming a solar pond into an electrical accumulator does not increase cost and helps pond
450
J,
Non-convective
al.
et
DORIA
zone
m
Fe+÷+ + / Cr +*
e-
~
Bentonite --
C r ++-~
Fe++
-Etectrode~
/ / +e-
Fig. 5. Schematic representation of a hybrid thermal and electrical accumulator.
stability. Besides, p o n d t e m p e r a t u r e improves accum u l a t o r operation. Inert electrolytes do n o t produce self-discharge a n d the n o n - c o n v e c t i v e zone, acting as an ionic bridge, allows for a quick ionic c o m p e n s a t i o n . F o r c e d circulation o f electrolytes avoids stratification in the lower convective zone a n d improves electrode reaction kinetics because o f t u r b u l e n t flow of active electrolytes. System overcharge is unnecessary if h y d r o g e n p r o d u c e d d u r i n g a c c u m u l a t o r charging is reinjected into the anode. Finally, a l t h o u g h the suggested system can be used with or w i t h o u t a solar p o n d , b o t h systems together present very attractive possibilities in solar energy applications.
REFERENCES 1. M. C. de Andr6s, Estudio experimental y anal6gico de un
2. 3.
4. 5. 6.
7.
estanque solar no convectivo con gradiente de concentracibn salina. Tesis doctoral, Facultad de Fisicas, Universidad Complutense, Madrid (1982). M. C. de Andros and J. Doria, An~ilisis de los fen6menos de difusi6n salina en un estanque solar no convectivo para el control de la estabilidad. An. Fis., B, 79, 182 (1983). M. C. de AndrOs, J. Doria and J. M. Garcia, Estudio de ias funciones de transmisibn de la radiaci6n en un estanque solar no convectivo para la resoluci6n de los modelos de transferencia de calor en el mismo. An. Fis., B, 79, 259 (1984). M. C. de Andr6s and J. Doria, Determinaci6n experimental de los par~metros de transferencia t6rmica en un estanque solar no convectivo. An. Fis., B, 79, 194 (1983). M. C. de AndrOs and J. Doria, Modelo anal6gico termoel6ctrico para la simulaci6n de un estanque solar. An. Fis., B, 79, 272 (1983). J. Doria, M. C. de Andros and C. Armenta, Modificaciones de un estanque solar no convectivo para su aprovechamiento t6rmico y electroquimico. Fundamentos. An. Fis., B, 79, 267 ('1983). C. Armenta, Estanques solares electroquimicos. Tesis doctoral, Facultad de Fisicas, Universidad Complutense, Madrid (1984).
0741--983X/87 $3.00+.00 Pergamon Journals Ltd.
ELECTROCHEMICAL SOLAR PONDS HYBRID T H E R M A L A N D ELECTRIC S T O R A G E J. DORIA, M. C. DE ANDRf/S and C. ARMENTA Grupo de Energia Solar, Departamento de Fisica Fundamental, Facultad de Ciencias Fisicas, Universidad Complutense, 28040 Madrid, Spain (Received 4 December 1986; accepted 15 January 1987) Abstraet--A hybrid thermal and electric energy collection and storage system for solar applications is proposed. Hybrid storage is achieved by transforming a conventional solar pond into a secondary activity cell, where the inert electrolyte (NaC1 solution) has been partially replaced by active ones totally soluble on both oxidated and reduced phases, which produce highly reversible reactions. The tested REDOX pairs were : Pb/Cr ++, Cr +++, HaO+//Fe +++, Fe ++, HaO+/C Pb/Cr ++, Cr +++, HaO+//Cr2OT, Cr +++, H30+/C. The suggested hybrid system has been designed for static operation and large storage capacity. Changing the solar pond into an electrical accumulator improves thermal efficiencyof the pond and does not increase building expenses; furthermore solar pond high temperature is advantageous for the accumulator operation and helps for current extraction.
1. INTRODUCTION Solar applications have not expanded as expected 10 years ago, largely due to difficulties in storing the converted energy either by thermal or photovoltaic means. Concerning thermal storage, only large sized seasonal storage systems are suitable and within this group, solar ponds are the most cost effective. Acid or alkaline electrical accumulators present serious drawbacks when coupled to stationary photovoltaic arrays : electrolyte stratification, limited number of charge-discharge cycles, low effective capacity (usually less 20% of their nominal capacity) and poor current supply at low ambient temperature. Studies were begun in 1980, restricted to thermal storage in solar ponds and analyzing the physical phenomena which take place in them [1-5]. The cost effectiveness of solar ponds as solar energy collection and storage devices has already been shown. However, no attention has been paid to a particular characteristic of the solutions existing in the pond: their ionic nature. This property will allow its transformation into either a concentration or an activity cell for electric energy production or storage with only slight modifications to the pond. The aim of this work was to consider the viability of electrical energy production or storage by electrochemical means, without disturbing thermal performance.
Our first tests were devoted to solar pond conversion into a primary activity cell with mass transport. The results were encouraging: output voltage stability, high performance and large current density [6]. Besides, thermal conversion is improved because salt mass flow through the non-convective zone (NCZ) is decreased due to the existing electric field in the mentioned zone [7]. Later on, solar pond conversion into an electrical accumulator was considered. Initial tests, trying to adapt the operation of the pond to commercially available accumulators gave negative results. Small inert electrolyte concentrations in active electrolytes produce self-discharge processes and voltage decay was rapid and continuous. Finally, the experiments performed with highly soluble electrolytes in both oxidated and reduced phases, showed the viability of solar pond utilization as a hybrid and electrical system for solar applications.
2. TOPICS In an electrical accumulator for photovoltaic applications the following conditions must be observed : (i) High reversibility in the oxo-redox reactions, which can only be achieved by extremely soluble materials in both oxidated and reduced phases. (ii) Chemically inert electrodes to all electrolytes
447
448
J. DORIA et al.
Non-convective
Etectrode
~ ....
1]"
"
zone
-,c,.ve
. . . . .
H
etec,roty'ce ~
Etectrode . . . .
-,c;,v;
....
etectrot,,e
'
']l
Fig. 1. Solar pond as an electrical accumulator.
contained in the system capable of providing a suitable overpotential discharge. (iii) In a static system, active ion transport must be enlarged by enforced convection. As a result, current density is increased. (iv) The ionic bridge between cells must avoid active specimen mixture, which would produce selfdischarge and show as low ionic compensation resistance as possible. Figure 1 shows how a solar pond can be converted into an electrical accumulator that fulfils all the above conditions. The lower inert electrolyte is replaced by two active and highly soluble ones until typical density values are obtained. The active electrolytes can flow through the inert electrodes which may be placed off the system. The non-convective zone acts as an ionic bridge, thus allowing for large current densities to be extracted.
Discharge processes Reactions :
Anode Cathode
Charge processes Hydrogen is produced in a secondary reaction in charge processes. As we are dealing with an acid medium, charge reactions can be written : Anode Cathode
Preliminary tests for suitable electrolyte and electrode material selection were performed in a small electrochemical cell consisting of a cylindrical tank. The two half-elements were joined together by a double porous wall filled with a NaC1 solution. The selected accumulator, based on data, was : Pb/Cr + +, Cr + + +, HsO+//Fe + + +, Fe + +, H3O+/C. The normal potential of this accumulator is 1.18 V and its nominal capacity if 26.8 Ah per 1-M solution. The experimental prototype used is shown in Fig. 2. The tank containing the two active electrolytes and the inert one can be seen in the middle. The electrodes placed on both sides of tank are ionically connected with it by circulating magnetic pumps.
Cr + + + + e Fe ++
The outer discharge element is of 2 ~ resistance. Open circuit voltage time evolution vs charge state was obtained from chemical analyses of samples periodically extracted. The results are plotted in Fig. 3. It can be observed that voltage is hardly influenced by charge state. Its value is kept at 1 V for capacities of about 10% of the nominal value. From the data and nominal capacity value, it is concluded that electrochemical conversion efficiency is 87% (average current density is 100 A m2).
Reactions:
3. TESTING DEVICES: EXPERIMENTAL DATA AND RESULTS
Cr + + Fe +++ + c
Cr++++e H20+e Fe ++
Cr ++ ½Hz+OH Fe++++e .
Hydrogen production can be negligible in a single cycle, but its effect on cathodic reaction inefficiency is increasingly severe as more cycles are performed. This problem can be solved by suitable overcharge with some sacrifice in performance, or by hydrogen addition on Fe + +/Fe + + + half-element. The progressive system capacity loss, due to hydrogen production, can be seen in Fig. 4. The charging process is repeated under identical conditions in each cycle, supplying the same charge amount and beginning at the same discharge level. After 20 cycles, the charged system capacity has come down to 65% of the initial capacity. On the other hand, hydrogen addition on iron cell allows for total initial capacity recovery. The results moved us to modify the hybrid accumulator pond as shown on Fig. 5. Cells are detached
Electrochemical solar ponds hybrid thermal and electric storage
449
Fig. 2. A view of the experimental device. Table 1. Initial and discharge conditions
Anode CrC13 Cathode FeC12
Vol. (1)
HC1
(g cm -3)
Inert Elec.
Q(1 h -I)
t(°C)
9.75 (1 M) 9.75 (1 M)
1M 1M
1.3 1.3
NaC1 NaC1
40 40
60 60
1.6 --
IOO~ o
ILl
U "x
1.2
0.8
o
o
"x 80_ ,,,
r| "-,
so-
[ Ha
Q. -~
0.4
o
o 0
I
I
I
I
20
40
60
80
I IO 0
Q. o
Injection
40--
20--
Disch0rge Lever, D (%)
Fig. 3. Open circuit voltage-time evolution vs charge state.
0
I
I
I
1
I
I0
20
30
40
50
Number of cycles (N)
from each other by a substance like bentonite, which allows for active specimen diffusion control as well as molecular hydrogen flow that reduces Fe +÷+ placed on a Cr ÷ + +/Cr + + half-cell.
Fig. 4. Accumulator capacity evolution.
present, this kind of accumulator is being studied and there are no definitive results yet.
Other accumulators A p a r t from the accumulators described the following has been tested : Pb/Cr + + + Cr + +, H30+//Cr2OT, Cr + + +H~O+/C. First results seem to show that the suggested system has the good properties o f the F e - C r one, but with an important advantage: Cr diffusion between cells would not produce material contamination. A t
4. CONCLUSIONS
From the experimental results it can be stated that the suggested hybrid system can solve thermal and electric energy storage difficulties linked to thermal and photovoltaic solar energy conversion. Transforming a solar pond into an electrical accumulator does not increase cost and helps pond
450
J,
Non-convective
al.
et
DORIA
zone
m
Fe+÷+ + / Cr +*
e-
~
Bentonite --
C r ++-~
Fe++
-Etectrode~
/ / +e-
Fig. 5. Schematic representation of a hybrid thermal and electrical accumulator.
stability. Besides, p o n d t e m p e r a t u r e improves accum u l a t o r operation. Inert electrolytes do n o t produce self-discharge a n d the n o n - c o n v e c t i v e zone, acting as an ionic bridge, allows for a quick ionic c o m p e n s a t i o n . F o r c e d circulation o f electrolytes avoids stratification in the lower convective zone a n d improves electrode reaction kinetics because o f t u r b u l e n t flow of active electrolytes. System overcharge is unnecessary if h y d r o g e n p r o d u c e d d u r i n g a c c u m u l a t o r charging is reinjected into the anode. Finally, a l t h o u g h the suggested system can be used with or w i t h o u t a solar p o n d , b o t h systems together present very attractive possibilities in solar energy applications.
REFERENCES 1. M. C. de Andr6s, Estudio experimental y anal6gico de un
2. 3.
4. 5. 6.
7.
estanque solar no convectivo con gradiente de concentracibn salina. Tesis doctoral, Facultad de Fisicas, Universidad Complutense, Madrid (1982). M. C. de Andros and J. Doria, An~ilisis de los fen6menos de difusi6n salina en un estanque solar no convectivo para el control de la estabilidad. An. Fis., B, 79, 182 (1983). M. C. de AndrOs, J. Doria and J. M. Garcia, Estudio de ias funciones de transmisibn de la radiaci6n en un estanque solar no convectivo para la resoluci6n de los modelos de transferencia de calor en el mismo. An. Fis., B, 79, 259 (1984). M. C. de Andr6s and J. Doria, Determinaci6n experimental de los par~metros de transferencia t6rmica en un estanque solar no convectivo. An. Fis., B, 79, 194 (1983). M. C. de AndrOs and J. Doria, Modelo anal6gico termoel6ctrico para la simulaci6n de un estanque solar. An. Fis., B, 79, 272 (1983). J. Doria, M. C. de Andros and C. Armenta, Modificaciones de un estanque solar no convectivo para su aprovechamiento t6rmico y electroquimico. Fundamentos. An. Fis., B, 79, 267 ('1983). C. Armenta, Estanques solares electroquimicos. Tesis doctoral, Facultad de Fisicas, Universidad Complutense, Madrid (1984).