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Working pairs for resorption refrigerator
Applied Thermal Engineering 31 (2011) 3015e3021
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Working pairs for resorption refrigerator H.S. Bao a, R.G. Oliveira a, b, R.Z. Wang a, *, L.W. Wang a, Z.W. Ma a a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Universidade Federal de Santa Catarina, Campus Araranguá, 88900-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2010 Accepted 26 May 2011 Available online 2 June 2011
The performances of three chemisorption working pairs operating under the resorption cycle were studied gravimetrically by comparing the desorbed and adsorbed mass of refrigerant in different operation conditions. All pairs used NH3 as refrigerant and MnCl2 in the main reactor, but each one used a different salt for the cooling effect production in the secondary reactor. These salts were NH4Cl or NaBr or BaCl2. The experimental results indicated that the degree of conversion in reaction between the NH3 and BaCl2 was inferior to 25% during cooling production at 0 C or below, whereas the reactions with the other salts had conversions of at least 80%. When the systems operated with heat source temperature for the main reactor at 155 C, heat sink temperature for both reactors at 30 C, and cooling effect production temperature at 0 C the coefficient of performance (COP) of the system using NH4Cl and the system using NaBr were similar and around 0.30; however, the former system had a specific cooling power (SCP) 5% higher than that of the latter system. Because the reaction in the system with NH4Cl was practically halted in a period much shorter than that used in the experiments, it is possible to expect that if the period of the cooling period was shortened, the difference between the SCP of those systems would be much higher. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Resorption Refrigeration Adsorption Ammoniates Comparison
1. Introduction Solid sorption refrigerators can be driven by low-grade heat sources, like industrial waste heat or solar energy. Furthermore, they may help reducing global warming and depletion of the ozone layer because the refrigerants normally used are water, ammonia or methanol [1]. In conventional solid sorption refrigerators, the cooling effect is obtained during the vaporization of refrigerant in the evaporator, but alternatively, the cooling effect could be obtained during the desorption process inside a secondary reactor. Sorption systems employing a secondary reactor in lieu of the evaporator and the condenser are called resorption systems or thermo-transformers. Under the same heat source temperature, resorption systems may produce higher coefficient of performance (COP) than that of conventional solid sorption systems, because the desorption heat of the sorbent inside the secondary reactor is higher than the vaporization enthalpy of the refrigerant [2,3]. In addition, a resorption system can be installed in any position, or in ambient with no gravity because there is no liquid inside the reactors.
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.05.035
Usually, the reactors of the resorption system have different sorbent salts, which under the same pressure have different equilibrium temperatures. Thus, the salt in the main reactor is known as high temperature salt (HTS), whereas the salt in the secondary reactor is named as low temperature salt (LTS). The changes of temperature and pressure during a single stage resorption cycle can be followed in the Clapeyron diagram shown in Fig. 1. The high-pressure period of the cycle involves endothermic decomposition of the complex HTS-refrigerant, inside the main reactor, and an exothermal synthesis of the complex LTSrefrigerant, inside the secondary reactor. During the high-pressure period, high temperature heat (Qdes,HTS) is supplied at Th to the main reactor, while middle temperature heat (Qads,LTS) is released at ambient temperature (Tm), by the secondary reactor. The cooling effect is obtained during the low-pressure period, when the reaction is reversed. Heat (Qdes,LTS) at low temperature (Tl) is consumed during the decomposition of the complex LTS-refrigerant, while middle temperature heat (Qads,HTS) is rejected to the ambient by the main reactor. Goetz et al. [4] demonstrated the feasibility of cooling production at 0 C, with a resorption system that had NiCl2 and BaCl2 as HTS and LTS, respectively. Later, Lepinasse et al. [5] studied a resorption system with MnCl2 and PbCl2, which was used to keep an 88 L air-filled box at a temperature below 0 C for 2 h.
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Nomenclature CP COP COPA
mRe m Ph Pl Qads Qdes S.S. Th Tl Tm
specific heat (J g1 K1) coefficient of performance, i.e. efficiency in cold production coefficient of performance and amplification, i.e. efficiency for combined heat and cold production mass of the reactor (g) the average mass of ammonia inside the reactor (g) high pressure (Pa) low pressure (Pa) adsorption heat (J) desorption heat (J) stainless steel high temperature (K) low temperature (K) middle temperature (K)
Greek letters DH reaction enthalpy (J g1) DS entropy of transformation (J g1 K1)
Oliveira et al. [6] studied a resorption system for simultaneous heating and cooling effect productions that had NH4Cl as LTS and MnCl2 as HTS. The cooling and heating effects were obtained at 5 C and 50 C, respectively. In another resorption system for simultaneous heating and cooling effect productions [7], NaBr was used as LTS, instead of NH4Cl. Heating and cooling productions occurred at 70 C and 10 C, respectively, and the combined coefficient of performance and amplification (COPA) was 1.11. The resorption cycle may be also combined with the conventional sorption cycle to provide higher COP. Li et al. [8] proposed such a combined cycle, which used four reactors, one condenser and one evaporator. The adsorption heat released by the HTS in one of the four reactors was used to promote the decomposition of the complex HTS-refrigerant inside another reactor at an intermediate temperature level. In this system, four cooling effects
could occur, at the expense of only one external heat input at high temperature. In this work, we compared experimentally the performance of three different working pair operating in a different temperature conditions for a resorption cycle. The LTS of the different working pairs were NaBr, BaCl2 and NH4Cl. The ammoniate of PbCl2 was not compared due to its low maximum adsorption capacity (0.29 kg kg1), which would lead to relatively low cooling capacity per unit mass of the LTS. All working pairs had MnCl2 as HTS. The ammoniate of NiCl2 was not used as HTS due to the relative high decomposition temperature. The machines were designed to achieve cooling production below 0 C, and the performances of the machines were described in terms of COP and specific cooling power (SCP). 2. Materials and method 2.1. Experimental prototype The experiments were performed with three bench-scale resorption systems. Each of them comprised two cylindrical reactors (internal diameter 54 mm) with a 20 mm gas channel in the middle. The reactors were connected through a pipe, as presented in Fig. 2. A piezoelectric pressure sensor with uncertainty of 2.5 kPa was installed in the pipe that connected the HTS reactor to the LTS reactor. The temperature at two radial positions of the sorbent blocks was measured with thermoresistances Pt1000. One sensor was installed close to the gas channel in the center of the reactor and the other one was placed near the reactor wall. The uncertainty in the temperature measurement was 0.3 C. Three thermostatic baths were used to simulate the heat source, the heat sink and the cooling effect production, at Th, Tm and Tl, respectively. The temperatures inside the reactor and inside the baths, and the pressure in the systems were monitored and recorded every 10 s with a data-logger. 2.2. Composite sorbent A consolidated composite sorbent made with salt impregnated in expanded graphite (EG) was used in each reactor (Fig. 2). MnCl2 was the HTS in the main reactor of all systems, whereas NaBr, BaCl2
Fig. 1. Clapeyron diagram for a single stage resorption cycle.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
Fig. 2. Scheme of the experimental test rig.
and NH4Cl were the LTS in the secondary reactors. These salts reacted with NH3, according to Eqs. (1e4).
MnCl2 $2NH3 þ 4NH3 4MnCl2 $6NH3 þ 4DHMnCl2
(1)
NaBr þ 5:25NH3 4NaBr$5:25NH3 þ 5:25DHNaBr
(2)
BaCl2 þ 8NH3 4BaCl2 $8NH3 þ 8DHBaCl2
(3)
NH4 Cl þ 3NH3 4NH4 Cl$3NH3 þ 3DHNH4 Cl
(4)
Expanded graphite was used as inert material in the composite sorbent to avoid salt agglomeration, and to enhance the heat and mass transfer properties of the adsorbent beds [9e12]. The composite consolidated sorbents were prepared as previously described [12]. The procedure mainly included soaking EG in a salt solution, and drying the mixture. Then, the composite powders were inserted in the reactors, and compressed to form blocks. The mass ratio of salt to EG was 13:7, the density of the block with MnCl2 was 0.31 g/cm3 whereas the density of the blocks with the other salts were 0.30 g/cm3. 2.3. Experimental procedures 2.3.1. Identification of the degree of conversion in the low- and high-pressure periods A resorption cycle has a high-pressure (HP) period and lowpressure (LP) period, and the degrees of conversion for the reactions presented in Eqs. (1)e(4) were assessed for each period of the cycle, and under the operation conditions presented in Table 1.
Table 1 Operation conditions of HP period and LP period. HP period
LP period
Heat source ( C)
Heat sink ( C)
Heat source ( C)
Heat sink ( C)
145 155 165 145 155 165
30 30 30 35 35 35
0 5a, 5b 0 5a, 10b
30 30 35 35a, 30b
a b
MnCl2/NaBr and MnCl2/NH4Cl. MnCl2/BaCl2.
3017
The HP and LP periods lasted 30 and 50 min, respectively, and the reactors were immersed in the thermostatic baths during these periods. The time of pre-heating and pre-cooling before the beginning of each period of the cycle were between 5 and 10 min. Before and after each period of the cycle, the reactors were disconnected from the gas pipe, and the mass of the reactors was measured with an electronic balance (Sartorius AG BS2202S), with an uncertainty of 0.01 g. The amount of ammonia desorbed or adsorbed was assumed as the difference of mass between two consecutive measurements of the same reactor. The initial mass of refrigerant in the reactors undergoing synthesis was 0.0 0.3 g, and in the reactors undergoing decomposition was 30 0.3 g. Regardless the conditions of temperature in the low-pressure period, the reactors had always the same mass at the beginning of each experiment because the experiments were preceded by a standard high-pressure period, in which the temperatures of the heat sink and heat source were set to 20 C and 180 C, respectively. For the same reason, the reactors had always the same mass at the beginning of the experiments in the high-pressure period, because they were preceded by a standard low-pressure period, where the heat sink and heat source temperatures were 20 C and 30 C, respectively. The degree of conversion (x) was obtained with Eq. (5).
x ¼
mc mc;max
(5)
Where mc was the mass of refrigerant desorbed or adsorbed in each period, mc,max was the theoretical maximum mass of refrigerant that each reactor could react during each period, and which was designed as 30 0.3 g, for all systems tested in this work. Therefore, the mass amount of each salt prepared for experiment was calculated from this given mass of refrigerant and the stoichiometric ratio of each reaction in Eqs. (1e4). Each experimental condition was repeated four times, and the results presented in this paper are the mean values. The relative difference of the four measurements were smaller or equal to 3%. 2.3.2. Identification of COP and SCP Once the experiments described in the previous section were finished, we selected standard working conditions to test the promising working pairs (MnCl2/NaBr and MnCl2/NH4Cl) in continuous-cycle operations, and we calculated the COP and SCP for each pair. The high temperature heat source was set to 155 C, the heat sink temperature was 30 C, and the cooling effect production occurred at 0 C. All the other experimental procedures are similar to those described in Section 2.3.1. At the beginning of each experimental condition, the degrees of conversion of the reactions with the LTS and the HTS were respectively 0.0 0.01 and 1.00 0.01. Then, the experiments in this condition were repeated until the difference between the mass desorbed and mass adsorbed by of the same reactor was smaller than 1%. The useful cooling production (QU,LTS) was obtained with the energy balance presented in Eq. (6), and the input energy at high temperature during the high-pressure period (Qinput,HTS) was obtained with Eq. (7). Here the heat exchange with the ambient was assumed negligible.
QR;LTS þ QS;LTS ¼ QU;LTS
(6)
QR;HTS þ QS;HTS ¼ Qinput;HTS
(7)
Where QR was the reaction heat and QS was the sensible heat of all components of the reactors. The sensible heat QS was calculated with the following equations:
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Table 2 Parameters used in the calculation of the COP.
QS ¼
Reactor
HTS (MnCl2)
LTS (NH4Cl)
LTS (NaBr)
mE.G. (g) mSalt (g) mRe (g) CP,Salt (J g1 C1) CP;NH3 (J g1 C1) CP,E.G. (J g1 C1) CP,S.S.(J g1 C1) DH (J g1) DmNH3 i (g g1salt)
29.88 55.50 732.42 0.600a 4.59c 0.61d 0.46e 2924f e
16.94 31.46 482.03 1.602b 4.59 0.61 0.46 1628g 0.745 0.002
18.63 34.59 515.98 0.505b 4.59 0.61 0.46 1790h 0.646 0.003
a b c d e f g h i
Average value between 30 and 165 C [16]. Average value between 15 and 35 C [16]. Average value of saturated liquid between 0.33 and 14.24 bar [16]. Graphite [17]. Stainless steel 304 [17]. [18],[19]. [6]. [7]. Experimental data of the continuous-cycle operations.
Fig. 3. The conversion of each pair under different conditions. (a) HP period. (b) LP period.
4 X
Qi
(8)
i¼1
Q1 ¼ ðm$CP $DTÞSalt
(9)
Q2 ¼ ðm$CP $DTÞE:G:
(10)
Q3 ¼ ðm$CP $DTÞNH3
(11)
Q4 ¼ ðmRe $CP $DTÞS:S:
(12)
The reaction heat QR was calculated with Eq. (13).
QR ¼ DH$DmNH3
(13)
In the calculation of QS, it was assumed that all the elements of the reactive block (salts and E.G.) and NH3 inside the reactor had the same temperature, and which were measured as described in Section 2.1. The temperature of the reactors wall was measured with two thermoresistances Pt1000, which were tied at two
Fig. 4. The reacted amount of each pair under different temperature conditions. (a) HP period. (b) LP period.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
3. Results and discussion
Table 3 Mass of ammonia that reacted during each cycle (g g1salt).
3.1. Influence of the operation conditions on the degree of conversion
NH4Cl/MnCl2
NaBr/MnCl2 Cycle tests number
HP period
LP period
HP period
LP period
1 2 3 4
0.819 0.651 0.654 0.649
0.679 0.639 0.642 0.643
0.915 0.743 0.748 0.746
0.751 0.741 0.745 0.743
Fig. 3a shows that in the HP period, the operation conditions had similar influence on the degrees of conversion of the reaction with NH4Cl and with BaCl2, and the reaction with NaBr had the smallest degrees of conversion. The uncertainty in the degrees of conversion plotted in Fig. 3 were within 4.5%. In all the systems studied, the variation of the degree of conversion with temperature was more pronounced until the heat source temperature was 155 C. When the heat sink was 30 C, and the heat source was between 145 C and 155 C, the degree of conversion increased 0.013 to 0.018 per C. Above 155 C, the degree of conversion increased only 0.005 to 0.006 per C. Thus, heat source temperatures higher than 155 C should only be used if one intends to shorten the HP period, but not to obtain considerable gain in conversion. The influence of the heat sink temperature in the degree of conversion decreased with the heat source temperature. When the heat source was 165 C, the degrees of conversion obtained with heat sink temperature at 30 C and at 35 C were similar, but when the heat source was 145 C, the degrees of conversion were 0.08e0.11 lower at the heat sink temperature of 35 C than at 30 C. Fig. 3b shows that the degree of conversion in the reaction with BaCl2 during the LP period was lower than 0.25. Contrarily, the reaction with NH4Cl had almost complete conversion (about 88%), at any heat sink temperature, or at any temperature of cooling production effect. The degrees of conversion of the reaction with NaBr were at most 0.1 smaller than that of the reaction with NH4Cl, and the maximum difference occurred when the heat sink temperature was 35 C.
different angular positions on the external surface of the wall, and DT was the difference between the mean temperature values obtained at the beginning and at the end of each period of an experimental condition. Table 2 shows the parameters used in Eqs. (8)e(13). The coefficient of performance for refrigeration, COPideal and COP, can be calculated respectively as
COPideal ¼
COP ¼
DHLTS DHHTS
(14)
QU;LTS Qinput;HTS
(15)
The specific cooling power SCP was calculated with Eq. (16).
SCP ¼
QU;LTS t$mLTS
(16)
Where t denotes the reaction time of the LP period, and mLTS is the mass of the LTS.
a1 10.0
b110.0
9.5
9.5
9.0
9.0 8.5
8.0 7.5
1
7.0
23
1
6.5
0
5
8.0 7.5
24
10
15
t (min)
20
25
6.0
30
b2
1 0
5
10
15
t (min)
20
25
1 30
1.0
1.8
0.9
1.6
0.8
1.4
0.7 P(Bar)
P(Bar)
3
3
6.5
2.0
1.2
0.6
1.0
0.5
0.8
1
0.6 0.4
4 2
7.0
4
234
6.0
a2
P(Bar)
P(Bar)
8.5
5.5
3019
3 0
4
0.4
2 4
0.3 10
20 30 t (min)
40
50
0.2
1 0
2 10
3 20 30 t (min)
40
50
Fig. 5. Variation of pressure. (a) NH4Cl/MnCl2. (b) NaBr/MnCl2. (1) HP period. (2) LP period. (1, 2, 3, 4 ¼ consecutive cycles).
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H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
The reaction between BaCl2 and NH3 has the highest enthalpy among the reactions of the low temperature salts studied. According to described in the literature [13e15] the activation energy is directly related to the enthalpy of reaction for a number of chemisorption reactions. Thus if the activation energy is also related to the enthalpy of the reactions studied in this work, the poorest performance of the system with BaCl2 could be explained with the aid of the Arrhenius Law that states that the reaction kinetics decreases with the activation energy. The relative mass of ammonia adsorbed and desorbed at different heat source and heat sink temperatures are presented in Fig. 4a and Fig. 4b, respectively, and the errors were within 4.5%. At any condition studied, the system with NH4Cl had the highest amount of ammonia adsorbed and desorbed per unit mass of LTS, whereas the system with BaCl2 system had the lowest amount. Both the molar mass (M) and the stoichiometric coefficients (n) of the reactions are important factors in the selection of LTS. The reaction with BaCl2 has the highest stoichiometric coefficient, but the ration between M and n is the lowest one among those of the three LTS reactions studied. Thus, for the same maximum adsorption and desorption quantity, the system containing BaCl2 will need the largest amount of salt.
a
0 -2 -4
T(oC)
-6 -8
1*
-10
2* 3* 4* 1#
-12 -14
2# 3#
-16 -18
4#
0
10
20 30 t (min)
40
50
20 30 t (min)
40
50
b 0
2* 3*,4* 1*
T (o C )
-2 -4 -6 -8 -10 -12 -14 -16 -18 -20
4# 3# 2#
Table 4 Performance of the resorption systems.
NH4Cl NaBr
COPideal
COP
QU,LTS (kJ g1salt)
SCP (W kg1salt)
0.56 0.61
0.30 0.31
1.21 1.16
403 385
3.2. Performance of the systems The experiments to assess the COP and the SCP of resorption systems were focused only on the systems that had NH4Cl and NaBr, because the reactions in the system with BaCl2 had little conversions, and the mass desorbed during the low-pressure period was the smallest one. Table 3 shows that after two cycles, the mass of ammonia desorbed by the HTS during the HP period became approximately equal to the mass of ammonia desorbed by the LTS during the LP period. Furthermore, the mass of refrigerant transferred ðDmNH3 Þ became almost constant, and the differences of the final uptake between successive cycles were less than 0.005 (g g1salt). In the system with NaBr, the mass of ammonia that reacted during each period of the cycle was 0.646 0.003 (g g1salt), whereas in the system with NH4Cl, the reacted mass of ammonia was 0.745 0.002 (g g1salt). The pressure (Fig. 5) and the temperature (Fig. 6) in the system with NaBr and with NH4Cl presented similar variation after the second cycle, which together with the constant value of adsorbed and desorbed mass was another indication that the systems had already reached a steady-in-the-mean operation after the second cycle. Thus, the COP and the SCP presented in Table 4, and which were calculated using the values of adsorbed and desorbed mass in the forth cycle of each system, could represent the values to be expected in a continuous operation of the machines. In Fig. 6 it is possible to see that the temperature inside the reactor with NH4Cl was nearly constant after 30 min of the reaction, whereas in the reactor with NaBr, the temperature varied during the whole 50 min of the LP period. The variation of the temperature inside the reactor during the LP period is related to the energy consumed during the reaction and the energy absorbed from the heat source; thus, a nearly constant temperature close to the heat source temperature, as observed in the reactor with NH4Cl is an indication that the reaction had almost halted before the end of the LP period, and this period for the system using NH4Cl could be shortened. The reaction of ammonia with NH4Cl has smaller enthalpy than the reaction of ammonia with NaBr, thus as mentioned in Section 3.1, the activation energy of the former reaction was smaller, and the kinetics was faster. Both systems achieved minimal temperatures of 16 C during the LP period, and had similar COP; however, the system with NH4Cl had a higher SCP than that of the system with NaBr. Although the SCP presented in Table 4 of the both systems were calculated based on a cooling period of 50 min, the reaction and the cold production in the system with NH4Cl was practically finished in 30 min, as discussed above. Hence, it is reasonable to expect that the system with this salt would present higher SCP than that presented in Table 4, if a shorter LP period was chosen.
1#
0
10
Fig. 6. Temperature variation in the composite block during the LP period of consecutive experiments. (a) NH4Cl/MnCl2 system. (b) NaBr/MnCl2 system. (1, 2, 3, 4: consecutive cycles. #: T at the gas channel; * ¼ T at inner wall of the reactor wall).
4. Conclusion The comparison of three different working pairs operating under the resorption cycle for cooling production below 0 C was conducted with a gravimetric method, where the mass of refrigerant that reacted in different situations were compared.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
The system with NH4Cl had the highest amount of ammonia adsorbed and desorbed per unit mass of low temperature salt either in the low-pressure (LP) period or in the high-pressure (HP) period. Although in the HP period, the reaction inside all systems had similar degree of conversion, the reaction in the system with BaCl2 had a very small degree of conversion during the LP period, and thus, this salt was inadequate for cooling production at or below 0 C, when used in pair with MnCl2. When the heat sink temperature was between 30 C and 35 C, the heat source temperature should be around 155 C to ensure that at least 85% of the reaction was completed, in a period of 30 min. The mean mass of refrigerant that reacted during a cycle was 0.745 0.002 g g1salt in the system that used NH4Cl, and 0.646 0.003 g g1salt in the system with NaBr. The systems with each of these salts reached a minimum temperature of 16 C and had similar COP, but the system with the former salt had an SCP 5% higher than that of the system with NaBr. However, because during the low-pressure period the reactions in the system with NH4Cl were almost completed in a time much shorter than that of the system with NaBr, it is expected that the difference between the SCP of these two systems would be much higher, if a shorter length for the LP period was chosen. Therefore, among the LTS studied, NH4Cl is the most suitable to be used in a resorption machine with MnCl2 as high temperature salt, if the system is designed to produce cooling effect equal or below 0 C. Acknowledgements This work was supported by the Key project of the Natural Science Foundation of China under the contract No. 50736004. References [1] R.Z. Wang, R.G. Oliveira, Adsorption refrigerationean efficient way to make good use of waste heat and solar energy, Progress in Energy and Combustion Science 32 (2006) 424e458. [2] E. Lepinasse, B. Spinner, G. Crozat, Modelling and experimental investigating of a new type of thermochemical transformer based on the coupling of two solid-gas reactions, Chemical Engineering and Processing 33 (1994) 125e134.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Working pairs for resorption refrigerator H.S. Bao a, R.G. Oliveira a, b, R.Z. Wang a, *, L.W. Wang a, Z.W. Ma a a b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Universidade Federal de Santa Catarina, Campus Araranguá, 88900-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 January 2010 Accepted 26 May 2011 Available online 2 June 2011
The performances of three chemisorption working pairs operating under the resorption cycle were studied gravimetrically by comparing the desorbed and adsorbed mass of refrigerant in different operation conditions. All pairs used NH3 as refrigerant and MnCl2 in the main reactor, but each one used a different salt for the cooling effect production in the secondary reactor. These salts were NH4Cl or NaBr or BaCl2. The experimental results indicated that the degree of conversion in reaction between the NH3 and BaCl2 was inferior to 25% during cooling production at 0 C or below, whereas the reactions with the other salts had conversions of at least 80%. When the systems operated with heat source temperature for the main reactor at 155 C, heat sink temperature for both reactors at 30 C, and cooling effect production temperature at 0 C the coefficient of performance (COP) of the system using NH4Cl and the system using NaBr were similar and around 0.30; however, the former system had a specific cooling power (SCP) 5% higher than that of the latter system. Because the reaction in the system with NH4Cl was practically halted in a period much shorter than that used in the experiments, it is possible to expect that if the period of the cooling period was shortened, the difference between the SCP of those systems would be much higher. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Resorption Refrigeration Adsorption Ammoniates Comparison
1. Introduction Solid sorption refrigerators can be driven by low-grade heat sources, like industrial waste heat or solar energy. Furthermore, they may help reducing global warming and depletion of the ozone layer because the refrigerants normally used are water, ammonia or methanol [1]. In conventional solid sorption refrigerators, the cooling effect is obtained during the vaporization of refrigerant in the evaporator, but alternatively, the cooling effect could be obtained during the desorption process inside a secondary reactor. Sorption systems employing a secondary reactor in lieu of the evaporator and the condenser are called resorption systems or thermo-transformers. Under the same heat source temperature, resorption systems may produce higher coefficient of performance (COP) than that of conventional solid sorption systems, because the desorption heat of the sorbent inside the secondary reactor is higher than the vaporization enthalpy of the refrigerant [2,3]. In addition, a resorption system can be installed in any position, or in ambient with no gravity because there is no liquid inside the reactors.
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.05.035
Usually, the reactors of the resorption system have different sorbent salts, which under the same pressure have different equilibrium temperatures. Thus, the salt in the main reactor is known as high temperature salt (HTS), whereas the salt in the secondary reactor is named as low temperature salt (LTS). The changes of temperature and pressure during a single stage resorption cycle can be followed in the Clapeyron diagram shown in Fig. 1. The high-pressure period of the cycle involves endothermic decomposition of the complex HTS-refrigerant, inside the main reactor, and an exothermal synthesis of the complex LTSrefrigerant, inside the secondary reactor. During the high-pressure period, high temperature heat (Qdes,HTS) is supplied at Th to the main reactor, while middle temperature heat (Qads,LTS) is released at ambient temperature (Tm), by the secondary reactor. The cooling effect is obtained during the low-pressure period, when the reaction is reversed. Heat (Qdes,LTS) at low temperature (Tl) is consumed during the decomposition of the complex LTS-refrigerant, while middle temperature heat (Qads,HTS) is rejected to the ambient by the main reactor. Goetz et al. [4] demonstrated the feasibility of cooling production at 0 C, with a resorption system that had NiCl2 and BaCl2 as HTS and LTS, respectively. Later, Lepinasse et al. [5] studied a resorption system with MnCl2 and PbCl2, which was used to keep an 88 L air-filled box at a temperature below 0 C for 2 h.
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Nomenclature CP COP COPA
mRe m Ph Pl Qads Qdes S.S. Th Tl Tm
specific heat (J g1 K1) coefficient of performance, i.e. efficiency in cold production coefficient of performance and amplification, i.e. efficiency for combined heat and cold production mass of the reactor (g) the average mass of ammonia inside the reactor (g) high pressure (Pa) low pressure (Pa) adsorption heat (J) desorption heat (J) stainless steel high temperature (K) low temperature (K) middle temperature (K)
Greek letters DH reaction enthalpy (J g1) DS entropy of transformation (J g1 K1)
Oliveira et al. [6] studied a resorption system for simultaneous heating and cooling effect productions that had NH4Cl as LTS and MnCl2 as HTS. The cooling and heating effects were obtained at 5 C and 50 C, respectively. In another resorption system for simultaneous heating and cooling effect productions [7], NaBr was used as LTS, instead of NH4Cl. Heating and cooling productions occurred at 70 C and 10 C, respectively, and the combined coefficient of performance and amplification (COPA) was 1.11. The resorption cycle may be also combined with the conventional sorption cycle to provide higher COP. Li et al. [8] proposed such a combined cycle, which used four reactors, one condenser and one evaporator. The adsorption heat released by the HTS in one of the four reactors was used to promote the decomposition of the complex HTS-refrigerant inside another reactor at an intermediate temperature level. In this system, four cooling effects
could occur, at the expense of only one external heat input at high temperature. In this work, we compared experimentally the performance of three different working pair operating in a different temperature conditions for a resorption cycle. The LTS of the different working pairs were NaBr, BaCl2 and NH4Cl. The ammoniate of PbCl2 was not compared due to its low maximum adsorption capacity (0.29 kg kg1), which would lead to relatively low cooling capacity per unit mass of the LTS. All working pairs had MnCl2 as HTS. The ammoniate of NiCl2 was not used as HTS due to the relative high decomposition temperature. The machines were designed to achieve cooling production below 0 C, and the performances of the machines were described in terms of COP and specific cooling power (SCP). 2. Materials and method 2.1. Experimental prototype The experiments were performed with three bench-scale resorption systems. Each of them comprised two cylindrical reactors (internal diameter 54 mm) with a 20 mm gas channel in the middle. The reactors were connected through a pipe, as presented in Fig. 2. A piezoelectric pressure sensor with uncertainty of 2.5 kPa was installed in the pipe that connected the HTS reactor to the LTS reactor. The temperature at two radial positions of the sorbent blocks was measured with thermoresistances Pt1000. One sensor was installed close to the gas channel in the center of the reactor and the other one was placed near the reactor wall. The uncertainty in the temperature measurement was 0.3 C. Three thermostatic baths were used to simulate the heat source, the heat sink and the cooling effect production, at Th, Tm and Tl, respectively. The temperatures inside the reactor and inside the baths, and the pressure in the systems were monitored and recorded every 10 s with a data-logger. 2.2. Composite sorbent A consolidated composite sorbent made with salt impregnated in expanded graphite (EG) was used in each reactor (Fig. 2). MnCl2 was the HTS in the main reactor of all systems, whereas NaBr, BaCl2
Fig. 1. Clapeyron diagram for a single stage resorption cycle.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
Fig. 2. Scheme of the experimental test rig.
and NH4Cl were the LTS in the secondary reactors. These salts reacted with NH3, according to Eqs. (1e4).
MnCl2 $2NH3 þ 4NH3 4MnCl2 $6NH3 þ 4DHMnCl2
(1)
NaBr þ 5:25NH3 4NaBr$5:25NH3 þ 5:25DHNaBr
(2)
BaCl2 þ 8NH3 4BaCl2 $8NH3 þ 8DHBaCl2
(3)
NH4 Cl þ 3NH3 4NH4 Cl$3NH3 þ 3DHNH4 Cl
(4)
Expanded graphite was used as inert material in the composite sorbent to avoid salt agglomeration, and to enhance the heat and mass transfer properties of the adsorbent beds [9e12]. The composite consolidated sorbents were prepared as previously described [12]. The procedure mainly included soaking EG in a salt solution, and drying the mixture. Then, the composite powders were inserted in the reactors, and compressed to form blocks. The mass ratio of salt to EG was 13:7, the density of the block with MnCl2 was 0.31 g/cm3 whereas the density of the blocks with the other salts were 0.30 g/cm3. 2.3. Experimental procedures 2.3.1. Identification of the degree of conversion in the low- and high-pressure periods A resorption cycle has a high-pressure (HP) period and lowpressure (LP) period, and the degrees of conversion for the reactions presented in Eqs. (1)e(4) were assessed for each period of the cycle, and under the operation conditions presented in Table 1.
Table 1 Operation conditions of HP period and LP period. HP period
LP period
Heat source ( C)
Heat sink ( C)
Heat source ( C)
Heat sink ( C)
145 155 165 145 155 165
30 30 30 35 35 35
0 5a, 5b 0 5a, 10b
30 30 35 35a, 30b
a b
MnCl2/NaBr and MnCl2/NH4Cl. MnCl2/BaCl2.
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The HP and LP periods lasted 30 and 50 min, respectively, and the reactors were immersed in the thermostatic baths during these periods. The time of pre-heating and pre-cooling before the beginning of each period of the cycle were between 5 and 10 min. Before and after each period of the cycle, the reactors were disconnected from the gas pipe, and the mass of the reactors was measured with an electronic balance (Sartorius AG BS2202S), with an uncertainty of 0.01 g. The amount of ammonia desorbed or adsorbed was assumed as the difference of mass between two consecutive measurements of the same reactor. The initial mass of refrigerant in the reactors undergoing synthesis was 0.0 0.3 g, and in the reactors undergoing decomposition was 30 0.3 g. Regardless the conditions of temperature in the low-pressure period, the reactors had always the same mass at the beginning of each experiment because the experiments were preceded by a standard high-pressure period, in which the temperatures of the heat sink and heat source were set to 20 C and 180 C, respectively. For the same reason, the reactors had always the same mass at the beginning of the experiments in the high-pressure period, because they were preceded by a standard low-pressure period, where the heat sink and heat source temperatures were 20 C and 30 C, respectively. The degree of conversion (x) was obtained with Eq. (5).
x ¼
mc mc;max
(5)
Where mc was the mass of refrigerant desorbed or adsorbed in each period, mc,max was the theoretical maximum mass of refrigerant that each reactor could react during each period, and which was designed as 30 0.3 g, for all systems tested in this work. Therefore, the mass amount of each salt prepared for experiment was calculated from this given mass of refrigerant and the stoichiometric ratio of each reaction in Eqs. (1e4). Each experimental condition was repeated four times, and the results presented in this paper are the mean values. The relative difference of the four measurements were smaller or equal to 3%. 2.3.2. Identification of COP and SCP Once the experiments described in the previous section were finished, we selected standard working conditions to test the promising working pairs (MnCl2/NaBr and MnCl2/NH4Cl) in continuous-cycle operations, and we calculated the COP and SCP for each pair. The high temperature heat source was set to 155 C, the heat sink temperature was 30 C, and the cooling effect production occurred at 0 C. All the other experimental procedures are similar to those described in Section 2.3.1. At the beginning of each experimental condition, the degrees of conversion of the reactions with the LTS and the HTS were respectively 0.0 0.01 and 1.00 0.01. Then, the experiments in this condition were repeated until the difference between the mass desorbed and mass adsorbed by of the same reactor was smaller than 1%. The useful cooling production (QU,LTS) was obtained with the energy balance presented in Eq. (6), and the input energy at high temperature during the high-pressure period (Qinput,HTS) was obtained with Eq. (7). Here the heat exchange with the ambient was assumed negligible.
QR;LTS þ QS;LTS ¼ QU;LTS
(6)
QR;HTS þ QS;HTS ¼ Qinput;HTS
(7)
Where QR was the reaction heat and QS was the sensible heat of all components of the reactors. The sensible heat QS was calculated with the following equations:
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Table 2 Parameters used in the calculation of the COP.
QS ¼
Reactor
HTS (MnCl2)
LTS (NH4Cl)
LTS (NaBr)
mE.G. (g) mSalt (g) mRe (g) CP,Salt (J g1 C1) CP;NH3 (J g1 C1) CP,E.G. (J g1 C1) CP,S.S.(J g1 C1) DH (J g1) DmNH3 i (g g1salt)
29.88 55.50 732.42 0.600a 4.59c 0.61d 0.46e 2924f e
16.94 31.46 482.03 1.602b 4.59 0.61 0.46 1628g 0.745 0.002
18.63 34.59 515.98 0.505b 4.59 0.61 0.46 1790h 0.646 0.003
a b c d e f g h i
Average value between 30 and 165 C [16]. Average value between 15 and 35 C [16]. Average value of saturated liquid between 0.33 and 14.24 bar [16]. Graphite [17]. Stainless steel 304 [17]. [18],[19]. [6]. [7]. Experimental data of the continuous-cycle operations.
Fig. 3. The conversion of each pair under different conditions. (a) HP period. (b) LP period.
4 X
Qi
(8)
i¼1
Q1 ¼ ðm$CP $DTÞSalt
(9)
Q2 ¼ ðm$CP $DTÞE:G:
(10)
Q3 ¼ ðm$CP $DTÞNH3
(11)
Q4 ¼ ðmRe $CP $DTÞS:S:
(12)
The reaction heat QR was calculated with Eq. (13).
QR ¼ DH$DmNH3
(13)
In the calculation of QS, it was assumed that all the elements of the reactive block (salts and E.G.) and NH3 inside the reactor had the same temperature, and which were measured as described in Section 2.1. The temperature of the reactors wall was measured with two thermoresistances Pt1000, which were tied at two
Fig. 4. The reacted amount of each pair under different temperature conditions. (a) HP period. (b) LP period.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
3. Results and discussion
Table 3 Mass of ammonia that reacted during each cycle (g g1salt).
3.1. Influence of the operation conditions on the degree of conversion
NH4Cl/MnCl2
NaBr/MnCl2 Cycle tests number
HP period
LP period
HP period
LP period
1 2 3 4
0.819 0.651 0.654 0.649
0.679 0.639 0.642 0.643
0.915 0.743 0.748 0.746
0.751 0.741 0.745 0.743
Fig. 3a shows that in the HP period, the operation conditions had similar influence on the degrees of conversion of the reaction with NH4Cl and with BaCl2, and the reaction with NaBr had the smallest degrees of conversion. The uncertainty in the degrees of conversion plotted in Fig. 3 were within 4.5%. In all the systems studied, the variation of the degree of conversion with temperature was more pronounced until the heat source temperature was 155 C. When the heat sink was 30 C, and the heat source was between 145 C and 155 C, the degree of conversion increased 0.013 to 0.018 per C. Above 155 C, the degree of conversion increased only 0.005 to 0.006 per C. Thus, heat source temperatures higher than 155 C should only be used if one intends to shorten the HP period, but not to obtain considerable gain in conversion. The influence of the heat sink temperature in the degree of conversion decreased with the heat source temperature. When the heat source was 165 C, the degrees of conversion obtained with heat sink temperature at 30 C and at 35 C were similar, but when the heat source was 145 C, the degrees of conversion were 0.08e0.11 lower at the heat sink temperature of 35 C than at 30 C. Fig. 3b shows that the degree of conversion in the reaction with BaCl2 during the LP period was lower than 0.25. Contrarily, the reaction with NH4Cl had almost complete conversion (about 88%), at any heat sink temperature, or at any temperature of cooling production effect. The degrees of conversion of the reaction with NaBr were at most 0.1 smaller than that of the reaction with NH4Cl, and the maximum difference occurred when the heat sink temperature was 35 C.
different angular positions on the external surface of the wall, and DT was the difference between the mean temperature values obtained at the beginning and at the end of each period of an experimental condition. Table 2 shows the parameters used in Eqs. (8)e(13). The coefficient of performance for refrigeration, COPideal and COP, can be calculated respectively as
COPideal ¼
COP ¼
DHLTS DHHTS
(14)
QU;LTS Qinput;HTS
(15)
The specific cooling power SCP was calculated with Eq. (16).
SCP ¼
QU;LTS t$mLTS
(16)
Where t denotes the reaction time of the LP period, and mLTS is the mass of the LTS.
a1 10.0
b110.0
9.5
9.5
9.0
9.0 8.5
8.0 7.5
1
7.0
23
1
6.5
0
5
8.0 7.5
24
10
15
t (min)
20
25
6.0
30
b2
1 0
5
10
15
t (min)
20
25
1 30
1.0
1.8
0.9
1.6
0.8
1.4
0.7 P(Bar)
P(Bar)
3
3
6.5
2.0
1.2
0.6
1.0
0.5
0.8
1
0.6 0.4
4 2
7.0
4
234
6.0
a2
P(Bar)
P(Bar)
8.5
5.5
3019
3 0
4
0.4
2 4
0.3 10
20 30 t (min)
40
50
0.2
1 0
2 10
3 20 30 t (min)
40
50
Fig. 5. Variation of pressure. (a) NH4Cl/MnCl2. (b) NaBr/MnCl2. (1) HP period. (2) LP period. (1, 2, 3, 4 ¼ consecutive cycles).
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The reaction between BaCl2 and NH3 has the highest enthalpy among the reactions of the low temperature salts studied. According to described in the literature [13e15] the activation energy is directly related to the enthalpy of reaction for a number of chemisorption reactions. Thus if the activation energy is also related to the enthalpy of the reactions studied in this work, the poorest performance of the system with BaCl2 could be explained with the aid of the Arrhenius Law that states that the reaction kinetics decreases with the activation energy. The relative mass of ammonia adsorbed and desorbed at different heat source and heat sink temperatures are presented in Fig. 4a and Fig. 4b, respectively, and the errors were within 4.5%. At any condition studied, the system with NH4Cl had the highest amount of ammonia adsorbed and desorbed per unit mass of LTS, whereas the system with BaCl2 system had the lowest amount. Both the molar mass (M) and the stoichiometric coefficients (n) of the reactions are important factors in the selection of LTS. The reaction with BaCl2 has the highest stoichiometric coefficient, but the ration between M and n is the lowest one among those of the three LTS reactions studied. Thus, for the same maximum adsorption and desorption quantity, the system containing BaCl2 will need the largest amount of salt.
a
0 -2 -4
T(oC)
-6 -8
1*
-10
2* 3* 4* 1#
-12 -14
2# 3#
-16 -18
4#
0
10
20 30 t (min)
40
50
20 30 t (min)
40
50
b 0
2* 3*,4* 1*
T (o C )
-2 -4 -6 -8 -10 -12 -14 -16 -18 -20
4# 3# 2#
Table 4 Performance of the resorption systems.
NH4Cl NaBr
COPideal
COP
QU,LTS (kJ g1salt)
SCP (W kg1salt)
0.56 0.61
0.30 0.31
1.21 1.16
403 385
3.2. Performance of the systems The experiments to assess the COP and the SCP of resorption systems were focused only on the systems that had NH4Cl and NaBr, because the reactions in the system with BaCl2 had little conversions, and the mass desorbed during the low-pressure period was the smallest one. Table 3 shows that after two cycles, the mass of ammonia desorbed by the HTS during the HP period became approximately equal to the mass of ammonia desorbed by the LTS during the LP period. Furthermore, the mass of refrigerant transferred ðDmNH3 Þ became almost constant, and the differences of the final uptake between successive cycles were less than 0.005 (g g1salt). In the system with NaBr, the mass of ammonia that reacted during each period of the cycle was 0.646 0.003 (g g1salt), whereas in the system with NH4Cl, the reacted mass of ammonia was 0.745 0.002 (g g1salt). The pressure (Fig. 5) and the temperature (Fig. 6) in the system with NaBr and with NH4Cl presented similar variation after the second cycle, which together with the constant value of adsorbed and desorbed mass was another indication that the systems had already reached a steady-in-the-mean operation after the second cycle. Thus, the COP and the SCP presented in Table 4, and which were calculated using the values of adsorbed and desorbed mass in the forth cycle of each system, could represent the values to be expected in a continuous operation of the machines. In Fig. 6 it is possible to see that the temperature inside the reactor with NH4Cl was nearly constant after 30 min of the reaction, whereas in the reactor with NaBr, the temperature varied during the whole 50 min of the LP period. The variation of the temperature inside the reactor during the LP period is related to the energy consumed during the reaction and the energy absorbed from the heat source; thus, a nearly constant temperature close to the heat source temperature, as observed in the reactor with NH4Cl is an indication that the reaction had almost halted before the end of the LP period, and this period for the system using NH4Cl could be shortened. The reaction of ammonia with NH4Cl has smaller enthalpy than the reaction of ammonia with NaBr, thus as mentioned in Section 3.1, the activation energy of the former reaction was smaller, and the kinetics was faster. Both systems achieved minimal temperatures of 16 C during the LP period, and had similar COP; however, the system with NH4Cl had a higher SCP than that of the system with NaBr. Although the SCP presented in Table 4 of the both systems were calculated based on a cooling period of 50 min, the reaction and the cold production in the system with NH4Cl was practically finished in 30 min, as discussed above. Hence, it is reasonable to expect that the system with this salt would present higher SCP than that presented in Table 4, if a shorter LP period was chosen.
1#
0
10
Fig. 6. Temperature variation in the composite block during the LP period of consecutive experiments. (a) NH4Cl/MnCl2 system. (b) NaBr/MnCl2 system. (1, 2, 3, 4: consecutive cycles. #: T at the gas channel; * ¼ T at inner wall of the reactor wall).
4. Conclusion The comparison of three different working pairs operating under the resorption cycle for cooling production below 0 C was conducted with a gravimetric method, where the mass of refrigerant that reacted in different situations were compared.
H.S. Bao et al. / Applied Thermal Engineering 31 (2011) 3015e3021
The system with NH4Cl had the highest amount of ammonia adsorbed and desorbed per unit mass of low temperature salt either in the low-pressure (LP) period or in the high-pressure (HP) period. Although in the HP period, the reaction inside all systems had similar degree of conversion, the reaction in the system with BaCl2 had a very small degree of conversion during the LP period, and thus, this salt was inadequate for cooling production at or below 0 C, when used in pair with MnCl2. When the heat sink temperature was between 30 C and 35 C, the heat source temperature should be around 155 C to ensure that at least 85% of the reaction was completed, in a period of 30 min. The mean mass of refrigerant that reacted during a cycle was 0.745 0.002 g g1salt in the system that used NH4Cl, and 0.646 0.003 g g1salt in the system with NaBr. The systems with each of these salts reached a minimum temperature of 16 C and had similar COP, but the system with the former salt had an SCP 5% higher than that of the system with NaBr. However, because during the low-pressure period the reactions in the system with NH4Cl were almost completed in a time much shorter than that of the system with NaBr, it is expected that the difference between the SCP of these two systems would be much higher, if a shorter length for the LP period was chosen. Therefore, among the LTS studied, NH4Cl is the most suitable to be used in a resorption machine with MnCl2 as high temperature salt, if the system is designed to produce cooling effect equal or below 0 C. Acknowledgements This work was supported by the Key project of the Natural Science Foundation of China under the contract No. 50736004. References [1] R.Z. Wang, R.G. Oliveira, Adsorption refrigerationean efficient way to make good use of waste heat and solar energy, Progress in Energy and Combustion Science 32 (2006) 424e458. [2] E. Lepinasse, B. Spinner, G. Crozat, Modelling and experimental investigating of a new type of thermochemical transformer based on the coupling of two solid-gas reactions, Chemical Engineering and Processing 33 (1994) 125e134.
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