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DEHYDRATION OF SALT HYDRATES FOR STORAGE OF LOW TEMPERATURE HEAT: SELECTION OF PROMISING REACTIONS AND NANOTAILORING OF INNOVATIVE MATERIALS
I. A. Simonova, Yu. I. Aristov
Boreskov Institute of Catalysis SB RAS pr-t Ak. Lavrentieva, 5, Novosibirsk, Russia, 630090 Phone:/Fax: +7 (383) 3309573; e-mail: sia@catalysis.ru, aristov@catalysis.ru
v- Г
Simonova Irina Aleksandrovna
Information: Ph. D. student of Boreskov Institute of Catalysis SB RAS (Novosibirsk), Department «Physical chemistry».
Education: Section «Chemistry» of Department of Natural Sciences, Novosibirsk State University, Chair «Environmental Chemistry» (bachelor's degree, 2003), Chair «Chemistry» (master's degree, 2005). Fields of scientific interests: adsorption, composite water sorbents, heat pumps, transformation of solar energy, alternative energy sources.
Publications: 1 paper, 7 conference's abstracts.
Information: Head of the Laboratory of Energy Accumulating Materials and Processes in the Boreskov Institute of Catalysis. Sc. D. and Professor of Physical Chemistry.
Education: Moscow Physical Technical Institute (1977), physicist-engineer on fast chemical processes.
Fields of his scientific interests: adsorption, chemical kinetics, chemical and adsorption methods of heat transformation, heat pumps, rational use of energy, synthesis and study of composite adsorbents.
Publications: 127 papers, 1 monograph
Aristov Yurii Ivanovich
Hydration/dehydration reactions suitable for chemical conversion of low grade heat were selected. Parameters of the Van't Hoff equation for more than 20 reactions were calculated from experimental data or taken from handbooks. A reversible cycle of heat storage unit was plotted and its boundary temperatures were determined for three typical climatic zones. Lithium and calcium nitrates were put in the silica pores of various size to obtain novel nanocomposite materials for heat storage in the moderate and warm climates. Their hydration temperature increased in smaller pores that can be used for its fine adjustment to consumer demands. Sorption kinetics was controlled by the Knudsen diffusion.
Introduction
Fulfillment of the Kyoto protocol requires the replacement of fossil fuels with renewable energy sources and rational use of heat in industry, transport and dwellings (re-use of exhaust heat, energy storage, etc.). These new heat sources have lower temperature potential that opens a niche for applying chemical and adsorption technologies for energy transformation and storage which are necessary to adjust the energy production output with the heat consumption input. The reversible formation/ decomposition of crystalline hydrates of inorganic salts are considered as promising for thermochemical conversion of low-grade heat [1-3]. The basic cycle of a chemical heat storage unit (CHSU) is plotted on Fig. 1.
Line VL presents the vapor-liquid equilibrium while line VS determines the equilibrium vapor pressure over a salt hydrate. Typical CHSU operates between three thermostats (I, II h III) maintained at high (Tg), middle (Tc) and low (Te) temperatures (Fig. 2). Such a three temperature (3T) CHSU consists of an evaporator E at temperature Te, a condenser C at temperature Tc and a reactor R connected with the appropriate thermostats (Fig. 2). The CHSU transforms heat under two modes, namely, heat storage (charging) and heat release (discharging).
Under the heat storage mode the reactor is connected to the condenser maintained at temperature Tc that is commonly the ambient temperature. The reactor consumes the heat of an external heat source (the high temperature thermostat
Статья поступила в редакцию 10.01.2008 г. Ред. per. № 210.
The article has entered in publishing office 10.01.2008. Ed. reg. No. 210.
Fig.1. Basic cycle of a 3T chemical heat storage unit Рис. 1. Рабочий цикл трехтемпературного химического аккумулятора теплоты
20 hydration/dehydration reactions. For each VS line the reversible 3T heat storage cycle was plotted and the boundary temperatures Te and Td were determined for preset values of Tc. Preliminary selection of promising reactions was done for three geographical zones with conventionally moderate, warm and hot climates.
Selection of the salts for heat conversion
To select an appropriate inorganic salt for CHSU the following equations were applied. The temperature dependence of equilibrium vapor pressure during the formation/decomposition of salt (S) hydrate:
S nH2O + mH2Ovap ^ S• (n + m)H2O; (1) is expressed by the Van't Hoff equation:
ln Ph,o =-
A rH mRT
A rS mR
(2)
Fig. 2. Schematic of a 3T chemical heat storage unit working at charging (1) and discharging (2) modes. R — denotes reactor, C — condenser, E — evaporator, V — water vapor
Рис. 2. Схема трехтемпературного химического аккумулятора теплоты1, работающего в режимах поглощения (1) и вы1деления (2) тепла. R — реактор, C — конденсатор, E — испаритель, V — пары1 воды1
at Tg) that causes the decomposition of a salt hydrate (point 1). To release the stored heat the reactor switches from the thermostat at Tg and the condenser at Tc to the thermostat at Tr = Tc and the evaporator at Te (discharging mode, Tr = = Tc — point 2).
This 3T cycle is uniquely characterized by the three temperatures: the evaporator temperature Te, the condenser temperature Tc and the temperature of the external heat source T . Conditions of reversible operation impose an unambiguous link between the three boundary temperatures of the CHSU cycle [3, 4]. For instance, if the condenser temperature Tc is set, it determines a) the vapor pressure Pc at the charging mode, hence, the reactor temperature Tr = Tg necessary for dehydration (Fig. 1); b) the reactor temperature at the discharging mode (Th = Tc), hence, the vapor pressure necessary for hydrate formation Pe and the corresponding temperature in the evaporator (dotted lines on Fig. 1). Hence, knowing the position of equilibrium lines for the vapor-liquid (VL) and vapor-solid (VS) transformations and assigning one of these temperatures completely determine the cycle.
In this communication we found parameters of the vapor-solid transformation for more than
where PH O is the equilibrium water vapour pressure, ArH and ArS correspond to the variation of the standard enthalpy and entropy in the course of reaction (1), m is the number of exchanged molecules of water, R is the absolute gas constant, T is the temperature of reaction (1). As there is a large difference in values of ArH and ArS for various salt hydrates, one might expect that transition (1) can occur at quite different temperatures Tr (or Th).
The pressure over liquid water and ice was described by the following equations:
ln Ph,o =-
218171 3806.8
T2
T
+ 11.768, T>0 °C , (3)
ln Pho =-
6135.2 T
+ 17.362, T <0 °C.
(4)
The screening of literature data on the water pressure over various salt hydrates ensured the determination of parameters of simplified equation lnPHO = 6 -b/t (Table 1).
After plotting the equilibrium VS lines for these reactions, we graphically determined temperatures Te and Td which provided the reversible CHSU operation if the heat sink (ambient) temperature Tc = Th had been prefixed. The so defined temperature Te characterizes the minimal temperature that can be obtained in the evaporator, while Td gives the minimal temperature of external heat source necessary to drive the cycle. Both temperatures are presented for selected reactions in Table 2.
Reactions (1-3) require quite high driving temperature of 90-150 °C, consequently, to release the stored heat the evaporator at rather low temperature can be used (Table 2) that provides the temperature lift AT = (Tc - Te) - 60...85 °C. On the contrary, reaction 5 needs only 50-65 °C for the hydrate decomposition with AT - 22-24 °C that can be interesting for moderate climatic zones with
International Scientific Journal for Alternative Energy and Ecology № 10(54) 2007
© 2007 Scientific Technical Centre «TATA»
Table 1
Parameters of equation ln PH O = 6 - b/ T for transition between crystalline hydrate of selected salts (P — water vapor pressure, mbar; T — temperature, K)
Salts Parameters Reference
(n+m)-n 6 в Temperature range, C
BaBr2 2-1 25.51 7131.13 24-108 [5]
1-0 21.60 6976.68 40-130 [5]
BaCl2 2-1 25.71 7092.62 20-102 [5]
27.39 7658.61 34-103 [6]
1-0 29.11 9228.64 40-130 [5]
24.93 7859.85 80-170 [6]
Ba(OH)2 8 1 8-1association 25.36 6883.45 10-36 [5]
8-1dissociation 25.30 6865.65 10-36 [5]
CaCl2 6-64 21.20 5778.45 -14-30 [5]
64-2 21.94 6133.42 -14-36 [5]
2-1 20.40 5987.23 5-176 [5]
Ca(NO3)2 3-2 23.75 6710.89 10-65 [5]
2-0 25.07 7154.98 10-60 [5]
CaSO4 2-0 23.89 6303.16 5-90 [5]
2-0.5 27.07 6404.63 69-100 [5]
0.5-0 34.31 11574.63 82-143 [5]
Cd(NO3)2 4-2 22.29 6268.07 81-137 [7]
2-0 17.04 4746.46 109-178 [7]
CUSO4 5-3 24.49 6598.06 6-100 [5]
3-1 25.20 6961.72 6-114 [5]
1-0 25.38 8670.20 24-138 [5]
FeCl2 2-1 24.07 7428.64 90-124 [5]
KOH 1-0 29.18 9525.26 24-30 [5]
LiBr 1-0 27.35 9357.69 80-160 [5]
LiCl 3-2 13.88 5269.67 -29- -17 [5]
LiCl 2-1 55.69 15711.14 0-13 [5]
1-0 27.46 8560.10 30-101 [5]
LiJ 3-2 36.43 11854.16 64-70 [5]
2-1 21.40 6873.34 70-74 [5]
1-0.5 10.39 3310.05 110-124 [5]
0.5-0 10.32 3486.93 124-160 [5]
LiNO3 3-0 23.97 6576.23 30-90 [8]
MgCl2 6-4 26.44 8119.52 41-110 [5]
4-2 18.74 5842.57 10-160 [5]
1-0 17.48 5756.46 100-170 [5]
MgHPO4 3-1 17.18 4423.75 30-144 [5]
Mg(NO3)2 6-2 16.44 7097.31 24-40 [5]
MgSO4 12-7 20.28 5088.28 10-24 [5]
7-6 27.40 7352.14 10-38 [5]
5-4 21.08 5555.47 24-31 [5]
4-1 21.98 5998.16 24-31 [5]
Na2HPO4 12-7 24.74 6421.77 6-27 [5]
7-2 20.48 5261.64 10-24 [5]
2-0 105.05 30572.94 18-24 [5]
NÍSO4 6-0 50.36 16280.42 70-90 [6]
4-1 45.49 16912.29 133-166 [6]
1-0 40.89 21969 318-370 [6]
SrBr2 6-1 23.71 6744.87 20-88 [5]
SrCl2 6-2 24.15 6487.89 20-62 [5]
2-1 24.86 7132.93 24-130 [5]
SrO 1-0association 14.30 5333.46 393-670 [5]
1-0dissociation 14.32 5348.79 396-670 [5]
Sr(OH)2 8-1association 25.55 6773.09 0-31 [5]
8-1dissociation 25.44 6742.56 0-31 [5]
Zn(NO3)2 6-4 22.65 6246.60 10-33 [5]
4-2 52.43 15982.85 20-39 [5]
Table 2
Boundary temperatures of reversible 3T cycle for selected hydration/dehydration reactions
Salt[(m+n) - n] CT ii 1 T T e Td
1. CUSO4 [1-0] 45.0 -38.5 143.2
35.0 -46.1 132.8
25.0 -53.8 122.3
2. BaBr2 [1-0] 45.0 -23.7 136.2
35.0 -30.8 123.8
25.0 -37.8 111.3
3. BaCl2 [1-0] 45.0 -19.3 102.7
35.0 -28.8 94.7
25.0 -38.2 86.5
4. Ca(NO3)2 [2-0] 45.0 11.1 75.6
35.0 0.5 66.8
25.0 -8.7 57.8
5. LiNO3 [3-0] 45.0 22.5 65.5
35.0 11.8 56.5
25.0 1.4 47.3
poor solar radiation. Decomposition of calcium nitrate dihydrate (reaction 4) occurs at 70-75 °C, that can be appropriate for warm climatic zones. This reaction provides quite reasonable value of AT - 35 °C.
Thus, the variety of known salt hydrates allows efficient conversion and storage of heat with temperature potential 50-150 °C. However, application of bulk hydrates can meet severe shortages (salt swelling/expansion, hysteresis between decomposition and synthesis reactions, kinetic limitations due to the formation of core on the salt external surface, corrosion, etc.). The effective way to overcome these complications is a dispersion of a salt by its confinement to pores of a host matrix as it was described in [9].
Based on this concept, we synthesized composite sorbents «Ca(NO3)2/SiO2» and «LiNO3/SiO2» by a dry impregnation of silica gels (pore size 3.5-15 nm) with a saturated aqueous solution of these salts, a volume of the impregnating solution being equal to the pore volume of the silica. The water sorption isobars of the new materials were measured at PHzO = 17mbar. The transition temperatures showed the temperature level of the heat can be released during discharging mode if the reactor is connected to the evaporator maintained at Te =15 °C. We found that the hydration temperature increased with the rise in the pore size (Fig. 3 and 4) that can be used for fine adjustment of the heat release temperature to the consumer demands. This nano-size effect may be attributed to increased contribution of the surface energy to the total Gibbs energy of the system [10]. For these composites the hydration/dehy-dration hysteresis was significantly reduced [11] as well as the hydration kinetics became much faster that for the bulk salts (Fig. 5), so that the rate of gas-solid reaction is commonly controlled
3D -10 50 5D 7D BO 90 1<Ю 110
Temufcralurt, 'C
Fig.3. Water sorption isobars for bulk Ca(NO3)2 and composites «Ca(NO3)2/SiO2» with different silica pore size.
P„ = 17 mbar
2
Puc. 3. H3o6apu cop6u,uu napoe eodu MaccueuuM Ca(NO3)2 u K0Mn03umaMu «Ca(NO3)2/SiO2» c pa3nuuuuM pa3MepoM nop cunuKazena. PHO = 17M6ap
30 40 50 60 70 80 SO 100 110
Temperature, °C
Fig. 4. Water sorption isobars for bulk LiNO3 and composites «LiNO3/SiO2» with different silica pore size. PH O = 17 mbar Puc. 4. H3o6apu cop6u,uu napoe eodu MaccueuuM LiNO3 u KoMno3umaMu «LiNO3/SiO2» c pa3nuuuuM pa3MepoM nop cunuKazena. PH O = 17 M6ap
n -,-.-1-.-<-
t :u -iii if #t 1» liine, stt"
Fig. 5. Hydration kinetics for composite «Ca(NO3)2/SiO2» (1) and bulk Ca(NO3)2 (2)
Puc. 5. KuuemuKu zudpamaiuu KoMno3uma «Ca(NO3)2/SiO2» (1) u Maccueuozo Ca(NO3)2 (2)
q International Scientific Journal for Alternative Energy and Ecology № 10(54) 2007
DO © 2007 Scientific Technical Centre «TATA»
by intrapartical water diffusion. The initial part of the uptake curve can be described by the Fickian diffusion model. This allowed determination of the efficient water diffusivity in the composite pores De = (1.2±0.5)10-6 m2/s that is close to the Knud-sen diffusivity in pores of 15 nm size corrected by the porosity and tortuosity of the composite.
Possible thermodynamic cycle of the adsorp-tive heat storage unit based on «Ca(NO3)2/SiO2» is displayed in Fig. 6. This material can be charged at Tg = 75 °C by removing the adsorbed water down to the final uptake 0.04 g/g, the desorbed water being condensed at Tc = 30 °C that is quite easy to reach at warm climatic zones. To get the stored heat back it is sufficient to connect the reactor with the evaporator at Te =5 °C. The temperature of the released heat changes between 48 and 32 °C, the majority of the heat is liberated at T >40 °C (Fig. 6). In this cycle 1 g of the composite exchanges 0.15 g H2O that corresponds to the energy storage capacity app. 0.4 kJ/g. This material is suitable for storing just low temperature heat: at Tg = 80 °C the amount of the adsorbed water increases to only a small extent (up to 0.17 g/g, Fig. 6).
oJ---- I ^ . ... *
20 40 60 80 100 120 140
Temperature, °C
Fig. 6. Cycle of adsorptive heat storage unit with «Ca(NO3)2/SiO2» as an adsorbent. Solid lines — the isobar of water adsorption at PH O = 9 mbar (•) and the isobar of water desorption at PH O =45 mbar (P). Dashed lines - the sorption isosters corresponding to the water uptake 0.02, 0.04 and 0.19 g/g
Рис. 6. Цикл адсорбционного теплоаккумулятора на основе композита «Ca(NO3)2/SiO2». Сплошные линии -изобары сорбции паров воды при PH O = 9 мбар (•) и PH O = = 45 мбар (P ). Пунктирные линии — изостеры сорбции паров воды, соответствующие содержанию воды 0,02; 0,04 и 0,19 г/г
Conclusions
In this communication we collected parameters of the vapor-solid transformation for more than 20 hydration/dehydration reactions. For each reaction the reversible 3T cooling cycle was plotted and the boundary temperatures Te and Td were determined for preset values of Tc. Preliminary selection of promising reactions was done for three geographical zones with conventionally moderate, warm and hot climates. Two novel composite ad-
sorbents were synthesized by the dry impregnation of the silica pores with calcium and lithium nitrates. Their sorption properties were found to be promising for transformation of low temperature heat. It is shown that the variation of the pore size of host matrix and the nature of confined salt ensures the nanotailoring of a set of innovative materials which would suit all important heat transformation cycles in various climatic zones.
The Russian Foundation for Basic Researches (grants 05-02-16953, 05-03-34762 and 07-08-13620) and the Siberian Branch of the Russian Academy of Sciences (Integration project №11) are gratefully acknowledged for partial financial support.
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