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Solar energy storage by the reversible reaction: N2O4⇄2NO2. Theoretical and experimental results
Solar Energy,Vol. 29, No. 6, pp. 535-540, 1982 Printed in Great Britain.
0038~92X1821120535~650300/0 PergamonPress Ltd.
SOLAR ENERGY STORAGE BY THE REVERSIBLE REACTION: N204~2NO2. THEORETICAL AND EXPERIMENTAL RESULTSt VITTORIO RAGAINI
Istituto di Chimica Fisica, Universit~ di Sassari, Via Vienna 2-07100 Sassari, Italy (Received 13 July 1982; revision accepted 4 March 1982) Abstract--The reversible reaction of N204~2N02 has been experimentally studied at temperatures between 60 and 140°C in the gas phase, in a recirculating system including the decomposition reactor for N204, and the recombination apparatus for NOv Calculated thermal balances of heat exchanged in different experimental conditions agree well with experimental data. For the reaction to be carried out in the liquid phase, under pressure, some comparisons have been made among heat storage capacities (HSC) with respect to different processes. An hypotetical plant based upon the reversible reaction has HSC from 1.7 to 3 times greater than one employing direct heating of water; the latter being based upon a AT of 30-100°C, The investigated reaction has one of the lowest turning temperatures (about 60°C) among those useful for the storage of solar energy by means of flat collectors. These characteristics joined with a maximum HSC of 200 kcal-1 -~ (for the liquid-phase reaction) makes the above-mentioned reaction worthy of further studies.
INTRODUCTION
The reversible reactions for storage of solar energy[l10] have considered thermal decomposition and/or dehydration of inorganic salts, hydroxides, oxides, mainly of groups IA and IIA, many organic and inorganic catalytic reactions (for example the dissociation of cyclohexane in benzene and hydrogen and the dissociation of SO3), and thermochemical generation of hydrogen through multiple-step reactions. In the past few years some interesting photochemical reactions have been proposed [11 a-f] and reviewed [11 d, f] for use as solar energy storage systems. Some of the proposed reactions show a very high Heat Storage Capacity (HSC), for instance, for norbornadienequadricyclene interconversion, HSC = 1.05 kJ-cc ~ (or 250kcal-l-l)[12]. Unfortunately some disadvantages are presented [l la, b, f, h] which might reduce the practical applicability of this class of reactions. It is known[l,4,5] that many characteristics are required for thermal reactions to be useful for energy storage: (i) complete reversibility of the cycle after an infinite length of time; (ii) high energy density per unit volume (high HSC); (iii) easy separation of product; (iv) high thermodynamic efficiency, etc. For short-term energy storage (day-night, or periods of 1-2 weeks), and to obtain domestic hot water, the necessity of the easy separation of products is less important, but two characteristics are relevant: (i) high HSC; (ii) the relatively low temperature at which the endothermic reaction takes place. The latter characteristic allows in fact the employment of low-cost solar collectors. The decomtCommunication partly presented at the 18th International Conference (COMPLES Conference): Solar Energy, New Prospects. Milano, Italy, 23-27 September 1979 535
position of the chemical substance storing energy, should take place at temperatures not higher than 100°C. The equilibria N204~2NOz~2NO + 02 and storage by the first reaction. The title reaction belongs to the class of reactions which do not involve the separation of products; this reaction is particularly useful for short term heat storage because it satisfies the last two requirements mentioned in the preceeding section. However, nitrogen dioxide is toxic and in the presence of water it forms nitric acid. These characteristics imply many safety problems which may be partly overcome by using nitrogen dioxide in anhydrous form and in stainless steel apparatuses. The title reaction, however, should be considered as an example of reversible reaction useful for short term heat storage using inexpensive solar collectors. In the two consecutive equilibria:
(1)
N204~2NO2~2NO + 02
the dissociation of -~O4 is complete at 140°C, while in the second reactiou NO~ is totally dissociated at 619°C [14]. NO2 is rarely dissociated below 150°C and at 200°C the dissociation is about 0.5 per cent[14]. Below 20°C the gas N204 + NO2 condenses to an orange-yellow liquid. The normal boiling point of N204 + NO2 mixture is 21.04°C[14]. The vapour pressure of the liquid is well described by the following equation[14]: 1753
l o g PmmHg = - - ' ~
+
9.00436 - 11.8078 x 10-4 × T + 2.0954
×
10-6 × T 2
(2)
The enthalpy of the reaction N204--o 2NO2 AH, is only
536
V. RAGAINI
lightly dependent on the temperature T(K), as shown in upper part of Fig. 1. In the lowest part of this figure the degree of dissociation of N:O4(x) and of NO2(y) are reported as a function of temperature (°C). In Table 1 some numerical values of x[14], vapour pressures[15] and densities of N204+NO2 mixture[14] are reported. Implicitly in Ref.[14], the degree of dissociation (above 21°C) is referred to the gas phase equilibrium. In the Appendix it is shown why the degree of dissociation of N204 is about the same for both the gaseous and liquid equilibria N2On~2NO2. The total heat storage capacity (HSC), AH/V, where V is the volume of the storage container, is easily calculated for each temperature range of the cycle by
Iy
summing the heat exchanged due to the chemical reaction 2NO2 ~ N204, to the sensible heat exchanged. Table 2 shows a comparison of the heat storage capacity, k c a l - l - J , between systems employing the heat exchanged by the reversible reaction N204 (liquid)---~2NO2(liquid), and those employing a fluid, with density 1 and specific heat 1 kcal - (kg °C) t (for example water), which exchange only sensible heat. Table 3 shows the HSC of some energy storing processes and the turning temperature (T*) of the thermal decomposition reactions. Such temperature has been introduced[l] to give a measure of the reversibility of a reaction. It corresponds to the temperature at which the equilibrium constant is equal to 1.
,
.8
,j
4
~_- 2N02 ~
J 2NO* 02
IX
O
l
I
I
I
200
300
400
500
%
Fig. 1. (Top) Enthalpyof the reaction N204~ 2NO2 as a function of the absolute temperature T. (Ref. 13). (Bottom) Degree of dissociationof N204(x) and of NO2(y) as a function of the temperature (°C). (Ref. 14).
Table 1. Physical and thermodynamicdata on the system N:O4-NO2 Temperature
Fraction
(oC)
of
N204
dissociated
Density the
Pressure
of
liquid
of
the
vapour-iiquid equLlibrium(atm)
N204 --NO 2 (g-cm -3 ) Ref,
(14)
Ref,(14)
Ref.(15)
40
0,2944
1,3984
2,74
60
0°5266
1.3493
5,00
80
0,7589
1.2995
90
0.8269
1o2745
14,8
100
0,8920
1,2495
19,2
120
0.9588
1.1996
42.4
140
1.000
1,1505
66.6
Critical
Temp.
Average
Specific
Conversions:
= 157.8 Heat
1 g-cm
(°C) of
-3
1 cal-(g.°c)
the
103 -1
(Ref.
11.7
15 )
liquid
= 0.477
Kg-m-3; = 4.187
cal-(g.oC)
1 atm = 101325 K J - ( K g . Oc) -1
-1
Pa
(Ref.
14
)
Solar energy storage by the reversible reaction:
537
Table 2. Comparisonof the Heat Storage Capacity (HSC) in kcal- 1-I between systems employingthe reversible (liquid~2NO2 (liquid) and those employing the heat exchanger only by the sensible heat of a fluid with density 1 and specific heat 1 kcal-(kg.°C)-1 (for example water)
reactionN204
HSC
Temperatures of the
-1
(kcal-1
cycle N204
(l)-----.~,~
( ~H
)
2 NO2
Ratio
) Water
(1)
or
(HSC)
(oC) •
max
T
min
V
=( &H Total
)
V
+
( &H
Chem
V
N204-NO 2
similar
) Sens
fluids
(HSC)
water
80
40
119.9
=
94.2
+
25.7
40
3
100
40
156.6
= 118.7
+
37.9
60
2.6
2.2 2.0
120
40
179.0
= 129.4
+
49.6
80
140
40
195.7
= 134.9
+
60.8
1 O0
90
60
77.9
=
59.1
+
18.8
30
2.6
120
60
119.1
=
82.6
+
36.5
60
2.0
140
60
136.4
= 88.7
+
47.7
80
1.7
EXPERIMENTAL The cycle with nitrogen dioxide has been studied experimentally using the substances in the gas phase. The solar collector was simulated by means of a heated chemical reactor. The experimental apparatus, with the recirculating pump (P), the decomposition reactor (E), the gas-chromatograph (G) for the analysis of the reacting mixture (nitrogen and NO2 + N204), the apparatuses to exchange heat and to measure the heat exchanged (H, L, M), is shown in Fig. 2. The apparatus is firstly evacuated from C and then, before the admission of N2Oa+N02 mixture, it is
flushed with pure nitrogen. The gaseous mixture N204 + NO2, coming from the cylinder A, passes through valve B and flow-meter D. After the measurement of the temperature, T', the mixture feeds the decomposition reactor E and the substances, sampled by coil F, are analyzed by the gas-chromatograph G. The gas, containing mainly NO2 and nitrogen, after measurement of the temperature, T, passes through the recombination apparatus H, where the reaction 2NO2 ~ N204 takes place. Outside the coils of the apparatus H, acetone or dichlomethane, fed from /, is maintained at the boiling point (56 and 40°C, respectively at 1 arm). Then the heat
i D
7
EB
c
M N204-N02
Fig. 2. Schematic view of the experimental apparatus. A, cylinder with mixture; B, C. valves; P, recirculating pump; D, flow meter; T, T', Thermometers;E, decompositionreactor; G, F, gas chromatographand sampling loop respectively; L reserve of the liquid; H, heat exchanger; L, M, apparatuses to condense and to measure, respectively, the liquid distillate in the heat exchanger.
538
V. RAGAINI
Table 3. Heat storage capacity (HSC) and turningTemperature (T*) for some energy storing reactions lkcal-I l=4187kJ-m-3 l c a l - g l = 4 1 8 7 J - k g t Reaction
Sensible Water Rock
HSC K c a L _ l - l ( c a l - g -1 )
(Iron,
~T
~T
Photochemical NO C1 (g)
50(50)
= 50°C)
(40% v o i d ,
= 50°C)
= 50°C)
17(10)
5-b
45(5.5)
5-b
Reactions
--~ q - NO (g)
(Q = 11%)
Norbornadiene
+ 1/2
C12(g)
0.365(137)
11-d
250(285)
11-d
(b) ~
(Q + 17%) Thermal
Ref.s
Heat S t o r a g e (~T
Metal
TW(a) (°C)
Quadricyclene (b)
decomposi£ion
Reactions
A) C a t a l y z e d 2S03(g)~-~_2SO2(g)
+ 02(g)
OH4(g)+H20(g)~__CO(g)+3H2(g) C6H12(g)~C6H6(g)+3H2(g) B) G a s / S o l i d (c) II M (OH)2(s)-÷MIIo(s)+H20(g) NH4X(S)~NH3(g)+HX(g) C) L i q u i d
1,4 1,4
600(d)
295
4,5-a
330-580
250-730
1,4
460-640
230-380
1,4
465
1,4
400(300)
10
Liquid/liquid N204(1)~_2
T*
688
740
3 g)
H2SO4(98%)~_H2SO4(60%)+H20(g)
a)
762
30(d)
/gas
NH4H S 0 4 ( 1 ) ~ N H 3 ( g ) + H 2 0 ( g ) + S O
0)
110(d)
is
defined
modynamic (change
in
80-200(65-160)
N02(1)
[ l]as
the
equilibrium the
temperature
constant
heat
giving
K = i;
capacity
between
efficiency
[11-d]
&G°
therefore, products
60
= 0,
or
T h i s work
giving
assuming
and r e a g e n t s ) ,
the
&Cp it
the~
= 0 derives
T* = & H # ^ ~ l & o . L~m $298 b) Q = e n e r g y 0)
II M
d)
SO3; S02;
storage
= alkaline H20;
earth C6H12;
element;
X = halogen.
C6H 6 s t o r e d
of the reaction 2NO2--, N204 and the sensible heat of the gaseous mixture produce the distillation of acetone or dichloromethane. The vapour, distilled from H, is condensed in L; its quantity is measured in the graduated cylinder M. The gaseous mixture leaving H is recirculated by pump P. The quantity V' (cc-h -1) of acetone or dichloromethane distillated in H, and recovered in the cylinder M, gives an experimental measure of the heat exchanged in H between the gases flowing inside the coils (N2, NO2, N204) and the liquid outside the coils. The value Vexp(cc-h- i), obtained after subtracting from V' the quantity of liquid spontaneously evaporated, can be compared with the theoretical value V, (cc-h i) cal-
as l i q u i d s .
culated by a thermal balance. Such balance takes into account, at the different fluxes, P (Nl-h ~), and composition, x (per cent molar N204), of the gas feeding the reactor (E), the heat exchanged: (i) owing to the reaction (2) NO2 ~ N204; (ii) as sensible heat, due to the different temperatures of the gas between inlet (Ti) and outlet (To) in the exchange apparatus H; (iii) to increase the temperature of the liquid to its boiling point; (iv) necessary to distill the quantity V, of liquid. The comparison between Vexp and Vt is reported in Table 4 at different experimental values of the parameters T;, To, P, x. DISCUSSIONANDCONCLUSIONS The results reported in Table 2 show that using the
Solar energy storage by the reversible reaction:
539
Table 4. Comparison between the volume of liquid distillated from H(Fig. 2), Vexp,and the one calculated by a thermal balance, Vt Substance
Ti
T
Distillated
(°C)
(°C)
(Nl-h
143
58
90
25
144
58
60
99
56
70
137
57
116
56
D&chloeomethane
107
at
112
at
Tb
Tb=56
Tb=40
°C
°C
Plant for solar water
( m o l %)
exp
(cc-h -1)
V
t
(cc-h -1)
61
33
54
49
29
40
39
130
21
79
79
40
32
31
29
41
90
41
82
95
41
60
50
72
76
130
43
40
52
50
56
114
41
30
60
45
47
2NOa
(SCHEMATIC VIEW)
- Maximum storage time T-10days -Heat capacity O0-200kcaH4 in the range g0-140'C V
M
V~r
Cold water
II
N2o, ~,)
)
V
58
heater by the reversible reaction
~O4 ~
Hot
XN204
-1
(°C)
Acetone at
P
o
mally insulated st~e container
I' t ~o2 (,
(+NO2) I o=t.~s ]
I (+N20~) L-m-tls
lar collector __
]
valves thermometer magnetically driven pump manometer
Fig. 3. Schematic view of a plant for solar water heater for domestic necessities using chemical energy stored in the insulated container.
system with nitrogen dioxide, the heat storage capacity ranges from 3 to 1.7 times greater than water or similar fluids. This means, for example, that employing cold water at 15°C, heated to 55°C for domestic necessities, a
plant with a storage container of 101., operating between 90 and 60°C, can produce 201. of hot water at 55°C; the same plant produces 7.51. of the same hot water using an intermediate fluid which operates only by sensible heat. The calculated values V, of Table 4 fit the experimental ones, Vexo, on the average of 93 per cent. This correspondance is satisfactory considering the experimental apparatus employed, Fig. 2, and the hypothesis of the equilibrium between N204 and NO2 in the gas leaving the heat apparatus H. This agreement and the results of Table 2 confirm the possibility of using the liquid system N204-NO2 to store and to exchange heat and should encourage the study of a plant for general application like that shown in Fig. 3. A storage container with inside and outside temperatures of 90 and 20°C, respectively, with volume of 1 m 3, insulated with a layer 20 cm thick of rockwool, dissipates 42kcal-h ~. Therefore, in one week the heat dissipated (7000 kcal), is less than 10 per cent of the total heat capacity of the plant. Clearly the plant, shown in Fig. 3, may be integrated with conventional sources of energy when hot water above 55°C is required. The data reported in Table 3 shows that the reaction N204~2NO2, with the components in the liquid state, has high HSC and one of the lowest turning temperature T*, in a range useful for domestic application of solar energy. The difference in the density between N204 and NO2 in the liquid state facilitates the recirculation of these components (Fig. 3). Some reactions, attractive for high HSC, have low efficiency or a high value of the turning temperature (Table 3 and Refs. 1, lid). The results of the present work and the above-mentioned properties makes the investigated reaction attractive for further studies.
Acknowledgements--The author thanks Dr. Antonio Lozio for his valuable help in the experimental work.
540
V. RAGAIN[ REFERENCES
1. W. E. Wentworth and E. Chen, Simple thermal decomposition relations for storage of solar thermal energy. Solar Energy 18, 205 (1976). 2. E. G. Kovach (Editor), Thermal Energy Storage. Report of a NATO Science Committee Conference, Turnberry, Scotland, 1-5 March 1976. Particularly: Working Group A (High Temperature Thermal Energy Storage); Working Group B (Low Temperature Thermal Energy Storage). 3. T. A.Chubb, (a) Analysis of gas dissociation solar thermal power system. Solar Energy 17, 129 (1975); (b) Characteristics of CO2-CH4 reforming-methanation cycle relevant to solchem thermochemical power system. Solar Energy 24, 341 (1980). 4. R. W. Mar and T. T. Bramlette, Thermochemical Storage Systems, Chap. 26 of Solar Energy Technology HandbookPart A (Edited by W. C. Dickinson and P. N. Cheremisinoff) Marcel Dekker, New York (1980). 5. (a) N. Giordano, Thermodynamic analysis of solar energy storage through thermochemical reversible reactions. (b) A. Di Vecchia, V. Rosselli and D. Ruggi, Low and medium temperature heat storage for solar applications. Proc. 18th Int. Conf. ( COMPLES Conf.), Milano Italy 23-27 September 1979. 6. A. E. Fouda, G. J. G. Despault, J. B. Taylor and C. E. Capes, Solar storage systems using salt hydrate latent heat and direct contact heat exchange--I. Preliminary desigh consideration. Solar Energy 25, 437 (1980). 7. H. W. Prengle Jr., J. C. Hunt, C. E. Mauk and E. C. -H. Sun, Solar energy with chemical storage for cogeneration of electric power and heat. Solar Energy 24, 373 (1980). 8. W. M. Raldow and W. E. Wentworthl Chemical heat pumps--a basic thermodynamic analysis. Solar Energy 23, 75 (1979). 9. R. G. Bowrey and J. Jutsen, Energy storage by reversible oxidation of barium oxide. Solar Energy 21,523 (1978). 10. E. C. Clark and C. C. Hiller, Sulfuric acid-water chemical heat pump/energy storage system demonstration. ASME, Annual Meeting-Energy Storage Session (1978). 11. (a) R. R. Hautala, J. Little and E. Sweet, The use of functionalized polymers as photosensitizers in an energy storage reaction. Solar Energy 19, 503 (1977). (b) C. Kutal, D. P. Schwendiman and P. Grutsch, Use of transition metal compounds to sensitize a photochemical energy storage reaction. Solar Energy 19, 651 (1977). (c) J. R. Bolton, Photochemical storage of solar energy. Solar Energy 20, 181 (1978).(d) G. Jones, II, T. E. Reinhardt and W. R. Bergmark, Photon energy storage in organic materials--the case of linked anthracenes. Solar Energy 20, 241 (1978). (e) B. Carlsson and G. Wettermark, The photochemical heat pipe. Solar Energy 20, 87 (1978). (f) H. D. Scharf, J. Fleischhauer, H. Leismann, I. Ressler, W. Schleker and R. Weitz, Criteria for the efficiency, stability and capacity of abiotic photochemical solar energy storage systems. Angew. Chem. Int. Ed. Engl. lg, 652 (1979). (g) M. S. Chan and J, R. Bolton, Structures, reduction potentials and absorption maxima of synthetic dyes of interest in photochemical solar-energy storage studies. Solar Energy 24, 561 (1980). (h) M. A. Fox and N. J. Singletary, Solar energy utilization by carbanion photolysis. Solar Energy 25, 225 (1980). 12. D. S. Kabakoff et al., Enthalpy and kinetics of isomerization of quadricyclene to norbornadiene. Strain energy of quadricyclene. J. Am. Chem. Soc. 97, 1510 (1975). 13. Handbook of Chemistry and Physics, 59th Edn. p. D--45. The Rubber Co., New York (1978).
14, P. Pascal, Nouveau Trait~ de Chimie Min&ale. Tome X, pp. 364-391. Masson, Paris (1956). 15. Landolt-B6rnstein, Zuhlenwerte und Funktionen. SpringerVerlag, Berlin (1960). APPENDIX
If ideal behavior is assumed for the liquid, (1), and vapor, (v), phases in the equilibria: (N204)1 ~ (N204)~, tl ~'~ 2(N02)1 ~,~2(NO2)~ then it is well known that the following equation holds: x~p°(T) = y p
(A 1)
where x~ and yi are the molar fractions of the i species in the liquid and vapor phases, respectively, p°(T) the vapor pressure of i liquid, P the total pressure. Applying eqn (A1) to the species i and j it derives: _ yi/xi Pi°(T) a'i - ~ - pfi(T)
(A2)
where a# is the relative volatility of i-j substances. From eqn (A2) for a binary equilibrium: Yi = -
aox~ -
I + (aii -
(A3)
l)xi
If aii = 1, then y~ = x i. Even in the most important Refs.[14, 15], the separate vapour pressures of N20~ and NO2 are not reported. However if eqn (2) is applied at temperatures of 100-120°C, where the degree of dissociation of N204 is elevated (90-95 per cent), then we can consider the results as the vapour pressure of NO2. On the other hand if we apply a Clausius-Clapeyron equation to the data of Ref.[14] for the vapour pressure of N204-NO2 system between -10°C, +0.2°C (where the degree of dissociation of N20~ is less than 2 per cent) then we can consider this equation so derived as the one of pure N204. This equation is: log PmrnHg
1838.28 T + 9.15172
(A4)
with an absolute value of the correlation coefficient equal to 0.99946 for five experimental points. Equations (2) and (A4) applied at 100 and 120°C give the following results of p(mmHg; 1 mmHg = 133.3 Pa): t(°C) species NO~ (eqn (2)) N204 (eqn (A4)) aNo~N2o4
100 p = 14377
120 p = 25419
p = 16801 p = 29919 0.856
0.850
The values of aNO:-S:o, are high enough to consider Yi = xi; for example for a =0.850: x=0.1, y=0.09; x=0.5, y=0.46; x = 0.9, y = 0.87.
0038~92X1821120535~650300/0 PergamonPress Ltd.
SOLAR ENERGY STORAGE BY THE REVERSIBLE REACTION: N204~2NO2. THEORETICAL AND EXPERIMENTAL RESULTSt VITTORIO RAGAINI
Istituto di Chimica Fisica, Universit~ di Sassari, Via Vienna 2-07100 Sassari, Italy (Received 13 July 1982; revision accepted 4 March 1982) Abstract--The reversible reaction of N204~2N02 has been experimentally studied at temperatures between 60 and 140°C in the gas phase, in a recirculating system including the decomposition reactor for N204, and the recombination apparatus for NOv Calculated thermal balances of heat exchanged in different experimental conditions agree well with experimental data. For the reaction to be carried out in the liquid phase, under pressure, some comparisons have been made among heat storage capacities (HSC) with respect to different processes. An hypotetical plant based upon the reversible reaction has HSC from 1.7 to 3 times greater than one employing direct heating of water; the latter being based upon a AT of 30-100°C, The investigated reaction has one of the lowest turning temperatures (about 60°C) among those useful for the storage of solar energy by means of flat collectors. These characteristics joined with a maximum HSC of 200 kcal-1 -~ (for the liquid-phase reaction) makes the above-mentioned reaction worthy of further studies.
INTRODUCTION
The reversible reactions for storage of solar energy[l10] have considered thermal decomposition and/or dehydration of inorganic salts, hydroxides, oxides, mainly of groups IA and IIA, many organic and inorganic catalytic reactions (for example the dissociation of cyclohexane in benzene and hydrogen and the dissociation of SO3), and thermochemical generation of hydrogen through multiple-step reactions. In the past few years some interesting photochemical reactions have been proposed [11 a-f] and reviewed [11 d, f] for use as solar energy storage systems. Some of the proposed reactions show a very high Heat Storage Capacity (HSC), for instance, for norbornadienequadricyclene interconversion, HSC = 1.05 kJ-cc ~ (or 250kcal-l-l)[12]. Unfortunately some disadvantages are presented [l la, b, f, h] which might reduce the practical applicability of this class of reactions. It is known[l,4,5] that many characteristics are required for thermal reactions to be useful for energy storage: (i) complete reversibility of the cycle after an infinite length of time; (ii) high energy density per unit volume (high HSC); (iii) easy separation of product; (iv) high thermodynamic efficiency, etc. For short-term energy storage (day-night, or periods of 1-2 weeks), and to obtain domestic hot water, the necessity of the easy separation of products is less important, but two characteristics are relevant: (i) high HSC; (ii) the relatively low temperature at which the endothermic reaction takes place. The latter characteristic allows in fact the employment of low-cost solar collectors. The decomtCommunication partly presented at the 18th International Conference (COMPLES Conference): Solar Energy, New Prospects. Milano, Italy, 23-27 September 1979 535
position of the chemical substance storing energy, should take place at temperatures not higher than 100°C. The equilibria N204~2NOz~2NO + 02 and storage by the first reaction. The title reaction belongs to the class of reactions which do not involve the separation of products; this reaction is particularly useful for short term heat storage because it satisfies the last two requirements mentioned in the preceeding section. However, nitrogen dioxide is toxic and in the presence of water it forms nitric acid. These characteristics imply many safety problems which may be partly overcome by using nitrogen dioxide in anhydrous form and in stainless steel apparatuses. The title reaction, however, should be considered as an example of reversible reaction useful for short term heat storage using inexpensive solar collectors. In the two consecutive equilibria:
(1)
N204~2NO2~2NO + 02
the dissociation of -~O4 is complete at 140°C, while in the second reactiou NO~ is totally dissociated at 619°C [14]. NO2 is rarely dissociated below 150°C and at 200°C the dissociation is about 0.5 per cent[14]. Below 20°C the gas N204 + NO2 condenses to an orange-yellow liquid. The normal boiling point of N204 + NO2 mixture is 21.04°C[14]. The vapour pressure of the liquid is well described by the following equation[14]: 1753
l o g PmmHg = - - ' ~
+
9.00436 - 11.8078 x 10-4 × T + 2.0954
×
10-6 × T 2
(2)
The enthalpy of the reaction N204--o 2NO2 AH, is only
536
V. RAGAINI
lightly dependent on the temperature T(K), as shown in upper part of Fig. 1. In the lowest part of this figure the degree of dissociation of N:O4(x) and of NO2(y) are reported as a function of temperature (°C). In Table 1 some numerical values of x[14], vapour pressures[15] and densities of N204+NO2 mixture[14] are reported. Implicitly in Ref.[14], the degree of dissociation (above 21°C) is referred to the gas phase equilibrium. In the Appendix it is shown why the degree of dissociation of N204 is about the same for both the gaseous and liquid equilibria N2On~2NO2. The total heat storage capacity (HSC), AH/V, where V is the volume of the storage container, is easily calculated for each temperature range of the cycle by
Iy
summing the heat exchanged due to the chemical reaction 2NO2 ~ N204, to the sensible heat exchanged. Table 2 shows a comparison of the heat storage capacity, k c a l - l - J , between systems employing the heat exchanged by the reversible reaction N204 (liquid)---~2NO2(liquid), and those employing a fluid, with density 1 and specific heat 1 kcal - (kg °C) t (for example water), which exchange only sensible heat. Table 3 shows the HSC of some energy storing processes and the turning temperature (T*) of the thermal decomposition reactions. Such temperature has been introduced[l] to give a measure of the reversibility of a reaction. It corresponds to the temperature at which the equilibrium constant is equal to 1.
,
.8
,j
4
~_- 2N02 ~
J 2NO* 02
IX
O
l
I
I
I
200
300
400
500
%
Fig. 1. (Top) Enthalpyof the reaction N204~ 2NO2 as a function of the absolute temperature T. (Ref. 13). (Bottom) Degree of dissociationof N204(x) and of NO2(y) as a function of the temperature (°C). (Ref. 14).
Table 1. Physical and thermodynamicdata on the system N:O4-NO2 Temperature
Fraction
(oC)
of
N204
dissociated
Density the
Pressure
of
liquid
of
the
vapour-iiquid equLlibrium(atm)
N204 --NO 2 (g-cm -3 ) Ref,
(14)
Ref,(14)
Ref.(15)
40
0,2944
1,3984
2,74
60
0°5266
1.3493
5,00
80
0,7589
1.2995
90
0.8269
1o2745
14,8
100
0,8920
1,2495
19,2
120
0.9588
1.1996
42.4
140
1.000
1,1505
66.6
Critical
Temp.
Average
Specific
Conversions:
= 157.8 Heat
1 g-cm
(°C) of
-3
1 cal-(g.°c)
the
103 -1
(Ref.
11.7
15 )
liquid
= 0.477
Kg-m-3; = 4.187
cal-(g.oC)
1 atm = 101325 K J - ( K g . Oc) -1
-1
Pa
(Ref.
14
)
Solar energy storage by the reversible reaction:
537
Table 2. Comparisonof the Heat Storage Capacity (HSC) in kcal- 1-I between systems employingthe reversible (liquid~2NO2 (liquid) and those employing the heat exchanger only by the sensible heat of a fluid with density 1 and specific heat 1 kcal-(kg.°C)-1 (for example water)
reactionN204
HSC
Temperatures of the
-1
(kcal-1
cycle N204
(l)-----.~,~
( ~H
)
2 NO2
Ratio
) Water
(1)
or
(HSC)
(oC) •
max
T
min
V
=( &H Total
)
V
+
( &H
Chem
V
N204-NO 2
similar
) Sens
fluids
(HSC)
water
80
40
119.9
=
94.2
+
25.7
40
3
100
40
156.6
= 118.7
+
37.9
60
2.6
2.2 2.0
120
40
179.0
= 129.4
+
49.6
80
140
40
195.7
= 134.9
+
60.8
1 O0
90
60
77.9
=
59.1
+
18.8
30
2.6
120
60
119.1
=
82.6
+
36.5
60
2.0
140
60
136.4
= 88.7
+
47.7
80
1.7
EXPERIMENTAL The cycle with nitrogen dioxide has been studied experimentally using the substances in the gas phase. The solar collector was simulated by means of a heated chemical reactor. The experimental apparatus, with the recirculating pump (P), the decomposition reactor (E), the gas-chromatograph (G) for the analysis of the reacting mixture (nitrogen and NO2 + N204), the apparatuses to exchange heat and to measure the heat exchanged (H, L, M), is shown in Fig. 2. The apparatus is firstly evacuated from C and then, before the admission of N2Oa+N02 mixture, it is
flushed with pure nitrogen. The gaseous mixture N204 + NO2, coming from the cylinder A, passes through valve B and flow-meter D. After the measurement of the temperature, T', the mixture feeds the decomposition reactor E and the substances, sampled by coil F, are analyzed by the gas-chromatograph G. The gas, containing mainly NO2 and nitrogen, after measurement of the temperature, T, passes through the recombination apparatus H, where the reaction 2NO2 ~ N204 takes place. Outside the coils of the apparatus H, acetone or dichlomethane, fed from /, is maintained at the boiling point (56 and 40°C, respectively at 1 arm). Then the heat
i D
7
EB
c
M N204-N02
Fig. 2. Schematic view of the experimental apparatus. A, cylinder with mixture; B, C. valves; P, recirculating pump; D, flow meter; T, T', Thermometers;E, decompositionreactor; G, F, gas chromatographand sampling loop respectively; L reserve of the liquid; H, heat exchanger; L, M, apparatuses to condense and to measure, respectively, the liquid distillate in the heat exchanger.
538
V. RAGAINI
Table 3. Heat storage capacity (HSC) and turningTemperature (T*) for some energy storing reactions lkcal-I l=4187kJ-m-3 l c a l - g l = 4 1 8 7 J - k g t Reaction
Sensible Water Rock
HSC K c a L _ l - l ( c a l - g -1 )
(Iron,
~T
~T
Photochemical NO C1 (g)
50(50)
= 50°C)
(40% v o i d ,
= 50°C)
= 50°C)
17(10)
5-b
45(5.5)
5-b
Reactions
--~ q - NO (g)
(Q = 11%)
Norbornadiene
+ 1/2
C12(g)
0.365(137)
11-d
250(285)
11-d
(b) ~
(Q + 17%) Thermal
Ref.s
Heat S t o r a g e (~T
Metal
TW(a) (°C)
Quadricyclene (b)
decomposi£ion
Reactions
A) C a t a l y z e d 2S03(g)~-~_2SO2(g)
+ 02(g)
OH4(g)+H20(g)~__CO(g)+3H2(g) C6H12(g)~C6H6(g)+3H2(g) B) G a s / S o l i d (c) II M (OH)2(s)-÷MIIo(s)+H20(g) NH4X(S)~NH3(g)+HX(g) C) L i q u i d
1,4 1,4
600(d)
295
4,5-a
330-580
250-730
1,4
460-640
230-380
1,4
465
1,4
400(300)
10
Liquid/liquid N204(1)~_2
T*
688
740
3 g)
H2SO4(98%)~_H2SO4(60%)+H20(g)
a)
762
30(d)
/gas
NH4H S 0 4 ( 1 ) ~ N H 3 ( g ) + H 2 0 ( g ) + S O
0)
110(d)
is
defined
modynamic (change
in
80-200(65-160)
N02(1)
[ l]as
the
equilibrium the
temperature
constant
heat
giving
K = i;
capacity
between
efficiency
[11-d]
&G°
therefore, products
60
= 0,
or
T h i s work
giving
assuming
and r e a g e n t s ) ,
the
&Cp it
the~
= 0 derives
T* = & H # ^ ~ l & o . L~m $298 b) Q = e n e r g y 0)
II M
d)
SO3; S02;
storage
= alkaline H20;
earth C6H12;
element;
X = halogen.
C6H 6 s t o r e d
of the reaction 2NO2--, N204 and the sensible heat of the gaseous mixture produce the distillation of acetone or dichloromethane. The vapour, distilled from H, is condensed in L; its quantity is measured in the graduated cylinder M. The gaseous mixture leaving H is recirculated by pump P. The quantity V' (cc-h -1) of acetone or dichloromethane distillated in H, and recovered in the cylinder M, gives an experimental measure of the heat exchanged in H between the gases flowing inside the coils (N2, NO2, N204) and the liquid outside the coils. The value Vexp(cc-h- i), obtained after subtracting from V' the quantity of liquid spontaneously evaporated, can be compared with the theoretical value V, (cc-h i) cal-
as l i q u i d s .
culated by a thermal balance. Such balance takes into account, at the different fluxes, P (Nl-h ~), and composition, x (per cent molar N204), of the gas feeding the reactor (E), the heat exchanged: (i) owing to the reaction (2) NO2 ~ N204; (ii) as sensible heat, due to the different temperatures of the gas between inlet (Ti) and outlet (To) in the exchange apparatus H; (iii) to increase the temperature of the liquid to its boiling point; (iv) necessary to distill the quantity V, of liquid. The comparison between Vexp and Vt is reported in Table 4 at different experimental values of the parameters T;, To, P, x. DISCUSSIONANDCONCLUSIONS The results reported in Table 2 show that using the
Solar energy storage by the reversible reaction:
539
Table 4. Comparison between the volume of liquid distillated from H(Fig. 2), Vexp,and the one calculated by a thermal balance, Vt Substance
Ti
T
Distillated
(°C)
(°C)
(Nl-h
143
58
90
25
144
58
60
99
56
70
137
57
116
56
D&chloeomethane
107
at
112
at
Tb
Tb=56
Tb=40
°C
°C
Plant for solar water
( m o l %)
exp
(cc-h -1)
V
t
(cc-h -1)
61
33
54
49
29
40
39
130
21
79
79
40
32
31
29
41
90
41
82
95
41
60
50
72
76
130
43
40
52
50
56
114
41
30
60
45
47
2NOa
(SCHEMATIC VIEW)
- Maximum storage time T-10days -Heat capacity O0-200kcaH4 in the range g0-140'C V
M
V~r
Cold water
II
N2o, ~,)
)
V
58
heater by the reversible reaction
~O4 ~
Hot
XN204
-1
(°C)
Acetone at
P
o
mally insulated st~e container
I' t ~o2 (,
(+NO2) I o=t.~s ]
I (+N20~) L-m-tls
lar collector __
]
valves thermometer magnetically driven pump manometer
Fig. 3. Schematic view of a plant for solar water heater for domestic necessities using chemical energy stored in the insulated container.
system with nitrogen dioxide, the heat storage capacity ranges from 3 to 1.7 times greater than water or similar fluids. This means, for example, that employing cold water at 15°C, heated to 55°C for domestic necessities, a
plant with a storage container of 101., operating between 90 and 60°C, can produce 201. of hot water at 55°C; the same plant produces 7.51. of the same hot water using an intermediate fluid which operates only by sensible heat. The calculated values V, of Table 4 fit the experimental ones, Vexo, on the average of 93 per cent. This correspondance is satisfactory considering the experimental apparatus employed, Fig. 2, and the hypothesis of the equilibrium between N204 and NO2 in the gas leaving the heat apparatus H. This agreement and the results of Table 2 confirm the possibility of using the liquid system N204-NO2 to store and to exchange heat and should encourage the study of a plant for general application like that shown in Fig. 3. A storage container with inside and outside temperatures of 90 and 20°C, respectively, with volume of 1 m 3, insulated with a layer 20 cm thick of rockwool, dissipates 42kcal-h ~. Therefore, in one week the heat dissipated (7000 kcal), is less than 10 per cent of the total heat capacity of the plant. Clearly the plant, shown in Fig. 3, may be integrated with conventional sources of energy when hot water above 55°C is required. The data reported in Table 3 shows that the reaction N204~2NO2, with the components in the liquid state, has high HSC and one of the lowest turning temperature T*, in a range useful for domestic application of solar energy. The difference in the density between N204 and NO2 in the liquid state facilitates the recirculation of these components (Fig. 3). Some reactions, attractive for high HSC, have low efficiency or a high value of the turning temperature (Table 3 and Refs. 1, lid). The results of the present work and the above-mentioned properties makes the investigated reaction attractive for further studies.
Acknowledgements--The author thanks Dr. Antonio Lozio for his valuable help in the experimental work.
540
V. RAGAIN[ REFERENCES
1. W. E. Wentworth and E. Chen, Simple thermal decomposition relations for storage of solar thermal energy. Solar Energy 18, 205 (1976). 2. E. G. Kovach (Editor), Thermal Energy Storage. Report of a NATO Science Committee Conference, Turnberry, Scotland, 1-5 March 1976. Particularly: Working Group A (High Temperature Thermal Energy Storage); Working Group B (Low Temperature Thermal Energy Storage). 3. T. A.Chubb, (a) Analysis of gas dissociation solar thermal power system. Solar Energy 17, 129 (1975); (b) Characteristics of CO2-CH4 reforming-methanation cycle relevant to solchem thermochemical power system. Solar Energy 24, 341 (1980). 4. R. W. Mar and T. T. Bramlette, Thermochemical Storage Systems, Chap. 26 of Solar Energy Technology HandbookPart A (Edited by W. C. Dickinson and P. N. Cheremisinoff) Marcel Dekker, New York (1980). 5. (a) N. Giordano, Thermodynamic analysis of solar energy storage through thermochemical reversible reactions. (b) A. Di Vecchia, V. Rosselli and D. Ruggi, Low and medium temperature heat storage for solar applications. Proc. 18th Int. Conf. ( COMPLES Conf.), Milano Italy 23-27 September 1979. 6. A. E. Fouda, G. J. G. Despault, J. B. Taylor and C. E. Capes, Solar storage systems using salt hydrate latent heat and direct contact heat exchange--I. Preliminary desigh consideration. Solar Energy 25, 437 (1980). 7. H. W. Prengle Jr., J. C. Hunt, C. E. Mauk and E. C. -H. Sun, Solar energy with chemical storage for cogeneration of electric power and heat. Solar Energy 24, 373 (1980). 8. W. M. Raldow and W. E. Wentworthl Chemical heat pumps--a basic thermodynamic analysis. Solar Energy 23, 75 (1979). 9. R. G. Bowrey and J. Jutsen, Energy storage by reversible oxidation of barium oxide. Solar Energy 21,523 (1978). 10. E. C. Clark and C. C. Hiller, Sulfuric acid-water chemical heat pump/energy storage system demonstration. ASME, Annual Meeting-Energy Storage Session (1978). 11. (a) R. R. Hautala, J. Little and E. Sweet, The use of functionalized polymers as photosensitizers in an energy storage reaction. Solar Energy 19, 503 (1977). (b) C. Kutal, D. P. Schwendiman and P. Grutsch, Use of transition metal compounds to sensitize a photochemical energy storage reaction. Solar Energy 19, 651 (1977). (c) J. R. Bolton, Photochemical storage of solar energy. Solar Energy 20, 181 (1978).(d) G. Jones, II, T. E. Reinhardt and W. R. Bergmark, Photon energy storage in organic materials--the case of linked anthracenes. Solar Energy 20, 241 (1978). (e) B. Carlsson and G. Wettermark, The photochemical heat pipe. Solar Energy 20, 87 (1978). (f) H. D. Scharf, J. Fleischhauer, H. Leismann, I. Ressler, W. Schleker and R. Weitz, Criteria for the efficiency, stability and capacity of abiotic photochemical solar energy storage systems. Angew. Chem. Int. Ed. Engl. lg, 652 (1979). (g) M. S. Chan and J, R. Bolton, Structures, reduction potentials and absorption maxima of synthetic dyes of interest in photochemical solar-energy storage studies. Solar Energy 24, 561 (1980). (h) M. A. Fox and N. J. Singletary, Solar energy utilization by carbanion photolysis. Solar Energy 25, 225 (1980). 12. D. S. Kabakoff et al., Enthalpy and kinetics of isomerization of quadricyclene to norbornadiene. Strain energy of quadricyclene. J. Am. Chem. Soc. 97, 1510 (1975). 13. Handbook of Chemistry and Physics, 59th Edn. p. D--45. The Rubber Co., New York (1978).
14, P. Pascal, Nouveau Trait~ de Chimie Min&ale. Tome X, pp. 364-391. Masson, Paris (1956). 15. Landolt-B6rnstein, Zuhlenwerte und Funktionen. SpringerVerlag, Berlin (1960). APPENDIX
If ideal behavior is assumed for the liquid, (1), and vapor, (v), phases in the equilibria: (N204)1 ~ (N204)~, tl ~'~ 2(N02)1 ~,~2(NO2)~ then it is well known that the following equation holds: x~p°(T) = y p
(A 1)
where x~ and yi are the molar fractions of the i species in the liquid and vapor phases, respectively, p°(T) the vapor pressure of i liquid, P the total pressure. Applying eqn (A1) to the species i and j it derives: _ yi/xi Pi°(T) a'i - ~ - pfi(T)
(A2)
where a# is the relative volatility of i-j substances. From eqn (A2) for a binary equilibrium: Yi = -
aox~ -
I + (aii -
(A3)
l)xi
If aii = 1, then y~ = x i. Even in the most important Refs.[14, 15], the separate vapour pressures of N20~ and NO2 are not reported. However if eqn (2) is applied at temperatures of 100-120°C, where the degree of dissociation of N204 is elevated (90-95 per cent), then we can consider the results as the vapour pressure of NO2. On the other hand if we apply a Clausius-Clapeyron equation to the data of Ref.[14] for the vapour pressure of N204-NO2 system between -10°C, +0.2°C (where the degree of dissociation of N20~ is less than 2 per cent) then we can consider this equation so derived as the one of pure N204. This equation is: log PmrnHg
1838.28 T + 9.15172
(A4)
with an absolute value of the correlation coefficient equal to 0.99946 for five experimental points. Equations (2) and (A4) applied at 100 and 120°C give the following results of p(mmHg; 1 mmHg = 133.3 Pa): t(°C) species NO~ (eqn (2)) N204 (eqn (A4)) aNo~N2o4
100 p = 14377
120 p = 25419
p = 16801 p = 29919 0.856
0.850
The values of aNO:-S:o, are high enough to consider Yi = xi; for example for a =0.850: x=0.1, y=0.09; x=0.5, y=0.46; x = 0.9, y = 0.87.