- Email: [email protected]
New type of sorption composite for chemical heat pump and refrigeration systems
Applied Thermal Engineering 30 (2010) 1455e1460
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
New type of sorption composite for chemical heat pump and refrigeration systems b _ Bartosz Zajaczkowski a, *, Zbigniew Królicki a, Andrzej Jezowski a b
Institute of Power Engineering and Fluid Mechanics, Wroclaw University of Technology, Wyb. Wyspianskiego 23, 50-370 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences in Wroclaw, Okolna 2, 50-422 Wroclaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2009 Accepted 3 March 2010 Available online 10 March 2010
This paper describes new type of solid sorption composite based on expanded graphite (EXG) and carbon fibers (CF). Process of production (including preparation of the components and molding) of the composite, as well as measurements of its heat transfer coefficient has been presented. Introduction of carbon fibers allowed to obtain thermal conductivity l of about 13e15 W/mK in comparison to 6.5e9.2 W/mK for expanded graphite based only composites described in available literature. Thermal conductivity of the composite has been measured in cryostat using the axial stationary heat flow method. Temperature profiles for various thermal conductivities (ranging 0.3e16 W/mK) were numerically calculated and compared. The composite has been introduced in experimental single stage sorption heat pump system using CaCl2 e NH3 as working pair. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Sorption composite Expanded graphite Carbon fibers Thermal conductivity
1. Introduction Adsorption process is divided into two categories, which differ in the principle of operation: physical and chemical [1,2]. The cause of physical adsorption is van der Walls forces acting between the molecules of solid and gas. In case of chemical adsorption (or chemisorption) the reaction occurs between adsorbent and molecules of gas. The type determines the selection of active components. Typical solid sorption system contains the reactor/adsorber filled with a reactive component. It is connected to a condenser/evaporator or another adsorber containing different active compound. Useful cooling is an effect of the evaporation/desorption of the refrigerant. The solid/vapor sorption systems directly use heat as their energy source, therefore are capable of waste heat or solar energy utilization. It makes them a good alternative to classical vapor-compression for heating and air-conditioning purposes. The most important efficiency limiting factors of sorption systems are: the heat and mass transfer capabilities of the adsorption bed. Various salts being used in chemisorption adsorbers are typically characterized by very low thermal conductivity (0.2e0.4 W/mK) and tend to significantly expand their volume during adsorption (due to changes of their crystalline structure). The latter leads to significantly reduced permeability and make it impossible to repeat the cycle. Thus the continuous operation of the * Corresponding author. Tel.: þ48 71 320 3673; fax: þ48 71 328 3818. E-mail address: [email protected] (B. Zajaczkowski). 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.03.005
device is not possible. These limitations can be overcome through application of consolidated sorbents. The importance of high conductive heat transfer can be easily presented using equation describing conductive heat transfer in adsorbent bed with appropriate source term I0 [1]. Assuming that heat transfer can occur only in radial direction, it can be written as following:
vT vT div l$ þ I0 ¼ cp vx vt
(1)
While the source term I0:
dN I0 ¼ n$ $DHR dt
(2)
Where boundary and initial conditions are:
x ¼ 0 x ¼ R t ¼ 0
vT ¼ 0 vx vT ¼ kðT0 TðxÞÞ l$ vx TðxÞ ¼ T0
dN ¼ 0 dt
Solution leads to radial temperature profiles [1]: Lepinasse et al. [2] obtained similar profiles for sorption system using MnCl2, constraint temperature Tc ¼ 353 K and radius of the adsorbent bed r ¼ 10 mm.
1456
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
T[K] 310
Nomenclature Symbols: cp COPHP Io k mads r t T
l DHR
1min 5min
specific heat, J/kgK coefficient of performance of heat pump source term heat transfer coefficient, W/m2K mass of adsorbent, kg diameter of adsorbent bed, m time, s temperature, K thermal conductivity, W/mK heat of adsorption, J
10min
305 20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
Fig. 1. Calculated temperature profiles during adsorption in the adsorbent bed (CaCl2) with equivalent thermal conductivity l ¼ 0.3 W/mK and constraint temperature Tc ¼ 293 K.
Acronyms: GWP Global Warming Potential ODP Ozone Depletion Potential consolidated sorbent is determined by application, which influences the type of chosen material [6]: 2. Consolidated sorbents Various researchers approached the problem in different ways. The analysis of available literature lead to conclusion that there are three improvement methods of heat and mass transfer capabilities of adsorbent bed, which may be introduced in solid sorption systems [3,4]: Grainy adsorbent e where the active component is in the form of grains. However, this solution is applicable in physical sorption based systems, due to swelling of active salts, which is observed during chemical reaction. Coated adsorbent e where the active component is impregnated on the surface of heat exchanger. It results in significant increase of heat transfer coefficient between adsorbent and the wall; Consolidated adsorbent (composite sorbent) e where the active component is either mixed with or impregnated/intercalated on inert binder. The last one seems to be the most promising. The concept of using consolidated composite adsorbent was introduced about twenty years ago [5] and since then has been studied by various researchers. Such adsorbents are made of compressed porous inert binder and reactive component (mixed with or impregnated on the binder). Materials used as a binder are usually carbon based e.g. active carbon, expanded graphite, and carbon fibers. Increasing popularity of consolidated sorbents can be explained by simplicity of its production and application. The type of
Consolidated sorbents using porous metal hydrides or metal matrix alloys containing Ni, Fe, La, Al, H, with or without subsequent sintering; Consolidated sorbents using metal foams as porous matrices; Consolidated materials using carbon-based materials as inert binder. Introduction of inert binder allowed to overcome several problems which are typical for sorption systems, especially low coefficient of heat transfer. Carbon-based materials are characterized by significantly higher (at least one order of magnitude in comparison with pure salt) heat transfer coefficient [7,8]. However, process of consolidation causes limitations to the mass transfer. Reduced apparent density of porous material influences gas diffusion. Therefore, sorption composite can only be applied in the systems with considerably high working pressures. The research of Lu and Mazet [8] showed that permeability of consolidated sorbents k > 1013 m2 has little effect of global advancement when pressure higher than 0.4 MPa. There are several kinds of composites proposed in the literature. One of the most popular is IMPEX (IMPregnated blocks of recompressed EXpanded graphite) e an expanded graphite (EXG) composite developed by Mauran and his coworkers [5,9,10], patented and manufactured by La Carbone Lorraine. It is characterized by strong anisotropic (radial) heat and mass transfer capabilities. The layered structure of compressed EXG improves gas diffusion in direction perpendicular to that of compression [9,11]. Experimentally obtained values of heat transfer coefficients are l ¼ 0.2e40 W/mK [12]. Mazet et al. [13] analyzed heat transfer
Fig. 2. Expanded graphite (a) and carbon fibers (b).
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
1457
Fig. 3. Molding of the EXG/CF composite.
coefficient of IMPEX blocks with apparent density 200 kg/m3 and obtained l ¼ 16 W/mK. Han and Lee measured effective heat transfer of a composite based on calcium chloride, barium chloride and manganese chloride. The results were l ¼ 10e49 W/mK and showed that effective thermal conductivity strongly depends on the bulk density, weight fraction of graphite and the ammoniated state of salt [14]. Han et al. [15] has analyzed the influence of acid treatment (intercalation with sulfuric acid at different temperatures) on the porosity and chemical composition of expanded graphite, obtaining higher values (range 1012e1015 m2) at lower treatment temperatures. Radial thermal conductivity was in range 4.6e42.3 W/mK. Bou et al. [16] introduced layered, foliated graphite blocks, where each sheet is characterized by different apparent density, and the active agent is dispersed in it. This solution leads to noticeably increased reaction rates (for t 40 min). Lee et al. [17] introduced non-uniform reaction blocks, where apparent density changes with radius direction 165, 222, 279, 337, 394 kg/m3 gradually), which lead to increased transfer capabilities and reaction rates. 3. EXG-CF composite Since discovery of carbon fibers, enormous amount of research is conducted in order to explore potential application, including their usage as fillers in composite materials to improve the thermal, electrical and mechanical performances of various systems.
The new composite material described in this paper has been developed and patented [18]. It is based on the composition of expanded graphite (EXG) and carbon fibers (CF) (Fig. 2). High heat transfer capabilities of carbon fibers allow the conclusion that using them, as an additive in sorption systems, should result in obtaining sorption composites with higher transfer capabilities than other solutions described in available literature. The concept proposed in this paper is to combine both EXG and CF and mold them together to obtain desired shape and apparent density. The graphite not only ensures heat transfer but also allows forming of new composite into desired shape. In this case single ring-shaped sorption module has the diameter 10 cm, height between 0.7 and 1.5 cm (Fig. 3b). Diameter of centrally located gas diffuser is 1 cm (Fig. 3c). To ensure repeatability and uniform characteristics of several modules, preparation and mixing of the materials was conducted in carefully predefined conditions. Controlled parameters were: mass of graphite to mass of fibers ratio, mass of metallic salt, mixer’s rotational speed as well as the force applied during molding. Different compositions of the materials were prepared and tested (CF/EXG ratio 0.25e1.0, inert materials to salt ratio 0.25e0.5, molding force 500e900 kN). Experiments revealed the important mechanical limitation of carbon fibers. Too much pressure causes fibers to loose its structure and turn into carbon dust, which makes it impossible to be formed into usable solid shape. Proper determination of CF/EXG ratio (which has to be estimated experimentally) allows introducing higher molding forces.
Fig. 4. Thermal conductivity measurements of solid sorption EXG-CF composite.
1458
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
λ [W/mK]
15
14
13
Composition (% wt): 42,5% - calcium chloride 42,5% - expanded graphite (EXG) 15% - carbon fiber (CF)
12
260
270
280
290
300
310
320
330 T(K)
Fig. 5. Example values of heat transfer coefficient of the EXG-CF sorption composite (42.5%wt. of CaCl2, 42.5%wt. of EXG, 15%wt. of CF).
Table 1 Comparison of EXG-CF composite heat transfer coefficient with other composites available in the literature. No.
Method
l [W/mK]
Source
1
Active component with consolidated expanded graphite and carbon fibers (EXG-CF). Active component with consolidated expanded graphite Active component with consolidated expanded graphite Consolidated expanded graphite Consolidated expanded graphite with active carbon in weight proportions 10e30% Consolidated mixture of graphite and zeolite Consolidated mixture of graphite and silica-gel
13.5e14.5
e
6.5e9.5
[19]
10e49
[14]
2e20 2e10
[21] [22]
2e10 10e20
[23] [24]
2 3 4 5 6 7
The axial expansion of composites after molding, due to resilience of the fibers, has been observed. Noted increase of height was about 1e3 mm (w6e20% increase of volume) depending on the CF/ EXG ratio. Although actual increase of porosity wasn’t measured experimentally and it is planned in the future analyses.
to better than 3 mK. The temperature was measured using a constantanemanganin thermocouple, with the liquid nitrogen and liquid helium temperatures as reference points. The temperature difference along the sample, established by a small electric heater glued to the specimen, was determined by means of a differential thermocouple (Fig. 4b). The temperature gradient observed along the sample was 0.2e0.3 K. To prevent heat transfer between the sample and the environment, the measurements were performed under high vacuum and four shields were mounted around the sample to reduce the heat losses due to radiation at finite temperature. The temperature gradient on the innermost of these shields was maintained close to that on the sample via an extra heater held by a second temperature controller. All current and voltage leads were thermally anchored to this shield. The maximum experimental systematic error was below 15% (caused mainly by the uncertainty of the sample geometry) and the spurious errors estimated from the point scattering did not exceed 2%. Obtained values are in the range 13e15 W/mK (Fig. 5) in comparison to 6.5e9.5 W/mK registered for expanded graphite only based sorbent [19]. Huang et al. [20] showed, that pursue of higher values of thermal conductivity than 14e15 W/mK will not significantly influence sorption system efficiency. However obtaining a higher l, will often result in decreased porosity, which prevent mass flow. Such analysis has been performed by Han et al. [14] and Mazet et al. [21]. They obtained higher values of thermal conductivities, however at the cost of significantly reduced permeability. Also, the composition of expanded graphite and active carbon or graphite and zeolite didn’t result in significant increase in heat transfer coefficient. Mixing graphite with silica-gel showed increased heat transfer coefficient but this mixture operates as a physical adsorber not chemical one. Comparison of the values of l in available literature and results of EXG-CF composite (see Table 1) validates assumption that introduction of carbon fibers increased heat transfer capabilities of the latter. Calculated profiles (Fig. 6) for experimentally measured thermal conductivity (Fig. 4) show the temperature gradients in the bed about DT z 2 K in comparison to pure salt where DT z 9 K (Fig. 1). Also numerical simulations of temperature profiles (based on equations (1) and (2)) for higher conductivities (>15 W/mK) show only very small improvement.
4. Thermal conductivity of adsorbent bed 5. Experimental setup and verification Thermal conductivity measurements of the composite were carried out by the axial stationary heat flow method in the temperature range 5 to 54 C (263e328 K) on platelets, which were kept cold by anchoring one end of the sample onto a thick copper panel mounted on the heat key of the cryostat (Fig. 4a). The samples were cut out of prepared composite modules along its diagonal in order to measure thermal conductivity in radial direction. The temperature of the measurement chamber was stabilized
T[K] 310
a 1min
The composite has been later applied in the sorption system constructed [25] and modeled [26] at the Institute of Power Engineering and Fluid Mechanics (Fig. 7). Ammonia NH3 and calcium chloride CaCl2 have been chosen as a working pair. Both components are environmentally neutral (GWP and ODP coefficients of ammonia are zero), easily obtainable and react with each other in temperature/pressure ranges suitable for
b
T[K] 310
1min
5min
5min 10min
10min
305
305
20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
Fig. 6. Calculated temperature profiles (CaCl2 with constraint temperature Tc ¼ 293 K): (a) EXG composite (l ¼ 8 W/mK), (b) CF-EXG composite (l ¼ 14 W/mK).
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
1459
Fig. 7. The scheme and photograph (rear view) of sorption system and its adsorber.
Fig. 8. Evolution of the temperature of adsorbent during adsorption and desorption of ammonia on EXG-CF-CaCl2 composite.
Fig. 9. Change of temperature and pressure in adsorber in relation to mass of reacted adsorbent mads.
standard refrigeration/air-conditioning/heat pumping purposes. Experimental analysis confirmed suitability of adsorber containing EXG-CF composite with calcium chloride as reacting component to operate as a heat pump or refrigeration system. Fig. 8 shows the
evolution of the temperature and pressure inside adsorber during adsorption and desorption, as well as intermediate heating and cooling phases. The system was allowed to completely cool down and stabilize naturally, hence the long time between cycles.
1460
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
Tests in a heat pump mode were carried under evaporation pressure Pe ¼ 3.5e4 bar and condensation pressure Pcond ¼ 9e10 bar. It allowed to obtain COPHP w 0.48e0.96 (in various operating conditions and controlled factors) in comparison to modeled w1.1. The experimentally obtained value was mainly decreased by the influence of thermal masses of system components. Fig. 9 shows registered variation of temperature in relation to the mass of reacted adsorbent, as well as corresponding variation of pressure registered in the adsorbent during the process. 6. Conclusion The introduction of carbon fibers in sorption composites resulted in increase of heat transfer coefficient of consolidated solid sorption composite from about 0.1 W/mK for a pure salt, up to the level that is considered optimal (w15 W/mK) [20]. The value has been measured in a cryostat by the axial stationary heat flow method in the temperature range 5 to 54 C (263e328 K). Temperature profiles (Fig. 6) calculated based on experimentally obtained values of thermal conductivity (Fig. 5) allow concluding that application of EXG-CF composite should remove the heat transfer in the bed from the list of limiting factors and therefore increase overall working efficiency of the sorption system. Moreover, resilience of the fibers increased porosity of the bed, due to post-compression expansion. The exact order of magnitude of this increase (in correspondence to expansion) is yet to be determined. Further development of EXG-CF composite will require closer analysis of the mass transfer in its porous structure in order to determine optimal working conditions for given permeability and pressure. The composite has been then introduced in the experimental sorption heat pump and used to successfully operate sorption cycle. The experiment proves repeatability of the working cycle. Obtained COPHP (which was mainly decreased by thermal mass of the adsorber) was about 0.48e0.96. References [1] B. Zaja˛ czkowski, Przenoszenie ciep1a i masy w reaktorze sorpcyjnej chemicznej pompy ciep1a, PhD thesis, 2008, Faculty of Mechanical and Power Engineering, Wroclaw University of Technology. [2] E. Lepinasse, V. Goetz, G. Crozat, Modeling and experimental investigation of a new type of thermochemical transformer based on the coupling of two solidegas reactions. Chemical Engineering and Processing 33 (3) (1994) 125e134. [3] W. Wongsuwan, S. Kumar, P. Neveu, F. Meunier, A review of chemical heat pump technology and applications,. Applied Thermal Engineering 21 (15) (2001) 1489e1519. [4] F. Meunier, Solid sorption: an alternative to CFCs. Heat Recovery Systems and CHP 13 (4) (1993) 289e295. [5] S. Mauran, M. Lebrun, P. Prades, M. Moreau, B. Spinner, C. Drapier. Active composite and its use as reaction medium, United States Patent 5283219; 1994.
[6] L.W. Wang, R.Z. Wang, R.G. Oliveira, A review of adsorption working pairs for refrigeration. Renewable and Sustainable Energy Reviews 13 (3) (2009) 518e534. [7] S. Wang, J. Wu, X. Li, R. Wang, Developments of consolidated adsorbent and experimental study of consolidated activated carbon blocks. Taiyangneng Xuebao/Acta Energiae Solaris Sinica 25 (3) (2004) 283e287. [8] H.-B. Lu, N. Mazet, B. Spinner, Modeling of gasesolid reaction e coupling of heat and mass transfer with chemical reaction. Chemical Engineering Science 51 (15) (1996) 3829e3845. [9] C. Coste, G. Crozat, S. Mauran, GaseouseSolid Reaction, United States Patent 4595774; 1986. [10] S. Mauran, P. Prades, F. L’Haridon, Heat and mass transfer in consolidated reacting beds for thermochemical systems. Heat Recovery Systems and CHP 13 (4) (1993) 315e319. [11] B. Spinner, Ammonia-based thermochemical transformers. Heat Recovery Systems and CHP 13 (4) (1993) 301e307. [12] N. Mazet, M. Amouroux, Analysis of heat transfer in a non-isothermal solidegas reacting medium. Chemical Engineering Communications 99 (1991) 175e200. [13] N. Mazet, H.-B. Lu, Improving the performance of the reactor under unfavorable operating conditions of low pressure. Applied Thermal Engineering 18 (9e10) (1998) 819e835. [14] J.H. Han, K.-H. Lee, H. Kim, Effective thermal conductivity of graphite-metallic salt complex for chemical heat pumps. Journal of Thermophysics and Heat Transfer 13 (4) (1999) 481e488. [15] J.H. Han, K.W. Cho, K.H. Lee, H. Kim, Porous graphite matrix for chemical heat pumps. Carbon 36 (12) (1998) 1801e1810. [16] P. Bou, M. Moreau, P. Prades, Active composite with foliated structure and its use as reaction medium, United States Patent 5861207, 1999. [17] C.H. Lee, S.H. Park, S.H. Choi, Y.S. Kim, S.H. Kim, Characteristics of non-uniform reaction blocks for chemical heat pump. Chemical Engineering Science 60 (5) (2005) 1401e1409. [18] B. Zaja˛ czkowski, Z. Królicki, (Patent Application) Multifunction active component carrier, and active component carrier production method. Wielofunkcyjny formowany nosnik czynnika aktywnego i sposób wytwarzania wielofunkcyjnego formowanego czynnika aktywnego, Polish Patent Application P382966, 2007. [19] K. Wang, J.Y. Wu, R.Z. Wang, L.W. Wang, Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats. International Journal of Refrigeration 29 (2) (2006) 199e210. [20] H.-J. Huang, G.-B. Wu, J. Yang, Y.-C. Dai, W.-K. Yuan, H.-B. Lu, Modeling of gasesolid chemisorption in chemical heat pumps. Separation and Purification Technology 34 (1e3) (2004) 191e200. [21] N. Mazet, P. Meyer, P. Neveu, B. Spinner, Concept and study of a double effect refrigeration machine based on the sorption of solid and ammonia gas and controlled by heat pipes, in: International Absorption Heat Pump Conference. ASME, New York, NY, USA: New Orleans, LA, USA, 1994, pp. 407e412. [22] S. Biloe, V. Goetz, S. Mauran, Characterization of adsorbent composite blocks for methane storage. Carbon 39 (11) (2001) 1653e1662. [23] J.J. Guilleminot, J.B. Chalfen, A. Choisier, Heat and mass transfer characteristics of composites for adsorption heat pumps, in: International Absorption Heat Pump Conference. ASME, 1993, pp. 401e406. [24] T.-H. Eun, H.-K. Song, J.H. Han, K.-H. Lee, J.-N. Kim, Enhancement of heat and mass transfer in silicaeexpanded graphite composite blocks for adsorption heat pumps: part I. Characterization of the composite blocks. International Journal of Refrigeration 23 (1) (2000) 64e73. [25] B. Zaja˛ czkowski, Z. Królicki, B. Bia1ko, T. Wilk, Physical and chemical adsorption as a new trend in obtaining refrigeration effect, in International Confer , Poland 87e94. ence of IIR, Comm. B1, B2, C2, 2008: Poznan [26] Z. Królicki, B. Zaja˛ czkowski, T. Wilk, Selected aspects of thermodynamic modeling of solar assisted chemical ice maker with ammonia and chloride salt, in: 61st ATI National Congress e International Session. Solar Heating and Cooling, Perugia, Italy, 2006, pp. 95e100.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
New type of sorption composite for chemical heat pump and refrigeration systems b _ Bartosz Zajaczkowski a, *, Zbigniew Królicki a, Andrzej Jezowski a b
Institute of Power Engineering and Fluid Mechanics, Wroclaw University of Technology, Wyb. Wyspianskiego 23, 50-370 Wroclaw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences in Wroclaw, Okolna 2, 50-422 Wroclaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2009 Accepted 3 March 2010 Available online 10 March 2010
This paper describes new type of solid sorption composite based on expanded graphite (EXG) and carbon fibers (CF). Process of production (including preparation of the components and molding) of the composite, as well as measurements of its heat transfer coefficient has been presented. Introduction of carbon fibers allowed to obtain thermal conductivity l of about 13e15 W/mK in comparison to 6.5e9.2 W/mK for expanded graphite based only composites described in available literature. Thermal conductivity of the composite has been measured in cryostat using the axial stationary heat flow method. Temperature profiles for various thermal conductivities (ranging 0.3e16 W/mK) were numerically calculated and compared. The composite has been introduced in experimental single stage sorption heat pump system using CaCl2 e NH3 as working pair. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Sorption composite Expanded graphite Carbon fibers Thermal conductivity
1. Introduction Adsorption process is divided into two categories, which differ in the principle of operation: physical and chemical [1,2]. The cause of physical adsorption is van der Walls forces acting between the molecules of solid and gas. In case of chemical adsorption (or chemisorption) the reaction occurs between adsorbent and molecules of gas. The type determines the selection of active components. Typical solid sorption system contains the reactor/adsorber filled with a reactive component. It is connected to a condenser/evaporator or another adsorber containing different active compound. Useful cooling is an effect of the evaporation/desorption of the refrigerant. The solid/vapor sorption systems directly use heat as their energy source, therefore are capable of waste heat or solar energy utilization. It makes them a good alternative to classical vapor-compression for heating and air-conditioning purposes. The most important efficiency limiting factors of sorption systems are: the heat and mass transfer capabilities of the adsorption bed. Various salts being used in chemisorption adsorbers are typically characterized by very low thermal conductivity (0.2e0.4 W/mK) and tend to significantly expand their volume during adsorption (due to changes of their crystalline structure). The latter leads to significantly reduced permeability and make it impossible to repeat the cycle. Thus the continuous operation of the * Corresponding author. Tel.: þ48 71 320 3673; fax: þ48 71 328 3818. E-mail address: [email protected] (B. Zajaczkowski). 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.03.005
device is not possible. These limitations can be overcome through application of consolidated sorbents. The importance of high conductive heat transfer can be easily presented using equation describing conductive heat transfer in adsorbent bed with appropriate source term I0 [1]. Assuming that heat transfer can occur only in radial direction, it can be written as following:
vT vT div l$ þ I0 ¼ cp vx vt
(1)
While the source term I0:
dN I0 ¼ n$ $DHR dt
(2)
Where boundary and initial conditions are:
x ¼ 0 x ¼ R t ¼ 0
vT ¼ 0 vx vT ¼ kðT0 TðxÞÞ l$ vx TðxÞ ¼ T0
dN ¼ 0 dt
Solution leads to radial temperature profiles [1]: Lepinasse et al. [2] obtained similar profiles for sorption system using MnCl2, constraint temperature Tc ¼ 353 K and radius of the adsorbent bed r ¼ 10 mm.
1456
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
T[K] 310
Nomenclature Symbols: cp COPHP Io k mads r t T
l DHR
1min 5min
specific heat, J/kgK coefficient of performance of heat pump source term heat transfer coefficient, W/m2K mass of adsorbent, kg diameter of adsorbent bed, m time, s temperature, K thermal conductivity, W/mK heat of adsorption, J
10min
305 20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
Fig. 1. Calculated temperature profiles during adsorption in the adsorbent bed (CaCl2) with equivalent thermal conductivity l ¼ 0.3 W/mK and constraint temperature Tc ¼ 293 K.
Acronyms: GWP Global Warming Potential ODP Ozone Depletion Potential consolidated sorbent is determined by application, which influences the type of chosen material [6]: 2. Consolidated sorbents Various researchers approached the problem in different ways. The analysis of available literature lead to conclusion that there are three improvement methods of heat and mass transfer capabilities of adsorbent bed, which may be introduced in solid sorption systems [3,4]: Grainy adsorbent e where the active component is in the form of grains. However, this solution is applicable in physical sorption based systems, due to swelling of active salts, which is observed during chemical reaction. Coated adsorbent e where the active component is impregnated on the surface of heat exchanger. It results in significant increase of heat transfer coefficient between adsorbent and the wall; Consolidated adsorbent (composite sorbent) e where the active component is either mixed with or impregnated/intercalated on inert binder. The last one seems to be the most promising. The concept of using consolidated composite adsorbent was introduced about twenty years ago [5] and since then has been studied by various researchers. Such adsorbents are made of compressed porous inert binder and reactive component (mixed with or impregnated on the binder). Materials used as a binder are usually carbon based e.g. active carbon, expanded graphite, and carbon fibers. Increasing popularity of consolidated sorbents can be explained by simplicity of its production and application. The type of
Consolidated sorbents using porous metal hydrides or metal matrix alloys containing Ni, Fe, La, Al, H, with or without subsequent sintering; Consolidated sorbents using metal foams as porous matrices; Consolidated materials using carbon-based materials as inert binder. Introduction of inert binder allowed to overcome several problems which are typical for sorption systems, especially low coefficient of heat transfer. Carbon-based materials are characterized by significantly higher (at least one order of magnitude in comparison with pure salt) heat transfer coefficient [7,8]. However, process of consolidation causes limitations to the mass transfer. Reduced apparent density of porous material influences gas diffusion. Therefore, sorption composite can only be applied in the systems with considerably high working pressures. The research of Lu and Mazet [8] showed that permeability of consolidated sorbents k > 1013 m2 has little effect of global advancement when pressure higher than 0.4 MPa. There are several kinds of composites proposed in the literature. One of the most popular is IMPEX (IMPregnated blocks of recompressed EXpanded graphite) e an expanded graphite (EXG) composite developed by Mauran and his coworkers [5,9,10], patented and manufactured by La Carbone Lorraine. It is characterized by strong anisotropic (radial) heat and mass transfer capabilities. The layered structure of compressed EXG improves gas diffusion in direction perpendicular to that of compression [9,11]. Experimentally obtained values of heat transfer coefficients are l ¼ 0.2e40 W/mK [12]. Mazet et al. [13] analyzed heat transfer
Fig. 2. Expanded graphite (a) and carbon fibers (b).
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
1457
Fig. 3. Molding of the EXG/CF composite.
coefficient of IMPEX blocks with apparent density 200 kg/m3 and obtained l ¼ 16 W/mK. Han and Lee measured effective heat transfer of a composite based on calcium chloride, barium chloride and manganese chloride. The results were l ¼ 10e49 W/mK and showed that effective thermal conductivity strongly depends on the bulk density, weight fraction of graphite and the ammoniated state of salt [14]. Han et al. [15] has analyzed the influence of acid treatment (intercalation with sulfuric acid at different temperatures) on the porosity and chemical composition of expanded graphite, obtaining higher values (range 1012e1015 m2) at lower treatment temperatures. Radial thermal conductivity was in range 4.6e42.3 W/mK. Bou et al. [16] introduced layered, foliated graphite blocks, where each sheet is characterized by different apparent density, and the active agent is dispersed in it. This solution leads to noticeably increased reaction rates (for t 40 min). Lee et al. [17] introduced non-uniform reaction blocks, where apparent density changes with radius direction 165, 222, 279, 337, 394 kg/m3 gradually), which lead to increased transfer capabilities and reaction rates. 3. EXG-CF composite Since discovery of carbon fibers, enormous amount of research is conducted in order to explore potential application, including their usage as fillers in composite materials to improve the thermal, electrical and mechanical performances of various systems.
The new composite material described in this paper has been developed and patented [18]. It is based on the composition of expanded graphite (EXG) and carbon fibers (CF) (Fig. 2). High heat transfer capabilities of carbon fibers allow the conclusion that using them, as an additive in sorption systems, should result in obtaining sorption composites with higher transfer capabilities than other solutions described in available literature. The concept proposed in this paper is to combine both EXG and CF and mold them together to obtain desired shape and apparent density. The graphite not only ensures heat transfer but also allows forming of new composite into desired shape. In this case single ring-shaped sorption module has the diameter 10 cm, height between 0.7 and 1.5 cm (Fig. 3b). Diameter of centrally located gas diffuser is 1 cm (Fig. 3c). To ensure repeatability and uniform characteristics of several modules, preparation and mixing of the materials was conducted in carefully predefined conditions. Controlled parameters were: mass of graphite to mass of fibers ratio, mass of metallic salt, mixer’s rotational speed as well as the force applied during molding. Different compositions of the materials were prepared and tested (CF/EXG ratio 0.25e1.0, inert materials to salt ratio 0.25e0.5, molding force 500e900 kN). Experiments revealed the important mechanical limitation of carbon fibers. Too much pressure causes fibers to loose its structure and turn into carbon dust, which makes it impossible to be formed into usable solid shape. Proper determination of CF/EXG ratio (which has to be estimated experimentally) allows introducing higher molding forces.
Fig. 4. Thermal conductivity measurements of solid sorption EXG-CF composite.
1458
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
λ [W/mK]
15
14
13
Composition (% wt): 42,5% - calcium chloride 42,5% - expanded graphite (EXG) 15% - carbon fiber (CF)
12
260
270
280
290
300
310
320
330 T(K)
Fig. 5. Example values of heat transfer coefficient of the EXG-CF sorption composite (42.5%wt. of CaCl2, 42.5%wt. of EXG, 15%wt. of CF).
Table 1 Comparison of EXG-CF composite heat transfer coefficient with other composites available in the literature. No.
Method
l [W/mK]
Source
1
Active component with consolidated expanded graphite and carbon fibers (EXG-CF). Active component with consolidated expanded graphite Active component with consolidated expanded graphite Consolidated expanded graphite Consolidated expanded graphite with active carbon in weight proportions 10e30% Consolidated mixture of graphite and zeolite Consolidated mixture of graphite and silica-gel
13.5e14.5
e
6.5e9.5
[19]
10e49
[14]
2e20 2e10
[21] [22]
2e10 10e20
[23] [24]
2 3 4 5 6 7
The axial expansion of composites after molding, due to resilience of the fibers, has been observed. Noted increase of height was about 1e3 mm (w6e20% increase of volume) depending on the CF/ EXG ratio. Although actual increase of porosity wasn’t measured experimentally and it is planned in the future analyses.
to better than 3 mK. The temperature was measured using a constantanemanganin thermocouple, with the liquid nitrogen and liquid helium temperatures as reference points. The temperature difference along the sample, established by a small electric heater glued to the specimen, was determined by means of a differential thermocouple (Fig. 4b). The temperature gradient observed along the sample was 0.2e0.3 K. To prevent heat transfer between the sample and the environment, the measurements were performed under high vacuum and four shields were mounted around the sample to reduce the heat losses due to radiation at finite temperature. The temperature gradient on the innermost of these shields was maintained close to that on the sample via an extra heater held by a second temperature controller. All current and voltage leads were thermally anchored to this shield. The maximum experimental systematic error was below 15% (caused mainly by the uncertainty of the sample geometry) and the spurious errors estimated from the point scattering did not exceed 2%. Obtained values are in the range 13e15 W/mK (Fig. 5) in comparison to 6.5e9.5 W/mK registered for expanded graphite only based sorbent [19]. Huang et al. [20] showed, that pursue of higher values of thermal conductivity than 14e15 W/mK will not significantly influence sorption system efficiency. However obtaining a higher l, will often result in decreased porosity, which prevent mass flow. Such analysis has been performed by Han et al. [14] and Mazet et al. [21]. They obtained higher values of thermal conductivities, however at the cost of significantly reduced permeability. Also, the composition of expanded graphite and active carbon or graphite and zeolite didn’t result in significant increase in heat transfer coefficient. Mixing graphite with silica-gel showed increased heat transfer coefficient but this mixture operates as a physical adsorber not chemical one. Comparison of the values of l in available literature and results of EXG-CF composite (see Table 1) validates assumption that introduction of carbon fibers increased heat transfer capabilities of the latter. Calculated profiles (Fig. 6) for experimentally measured thermal conductivity (Fig. 4) show the temperature gradients in the bed about DT z 2 K in comparison to pure salt where DT z 9 K (Fig. 1). Also numerical simulations of temperature profiles (based on equations (1) and (2)) for higher conductivities (>15 W/mK) show only very small improvement.
4. Thermal conductivity of adsorbent bed 5. Experimental setup and verification Thermal conductivity measurements of the composite were carried out by the axial stationary heat flow method in the temperature range 5 to 54 C (263e328 K) on platelets, which were kept cold by anchoring one end of the sample onto a thick copper panel mounted on the heat key of the cryostat (Fig. 4a). The samples were cut out of prepared composite modules along its diagonal in order to measure thermal conductivity in radial direction. The temperature of the measurement chamber was stabilized
T[K] 310
a 1min
The composite has been later applied in the sorption system constructed [25] and modeled [26] at the Institute of Power Engineering and Fluid Mechanics (Fig. 7). Ammonia NH3 and calcium chloride CaCl2 have been chosen as a working pair. Both components are environmentally neutral (GWP and ODP coefficients of ammonia are zero), easily obtainable and react with each other in temperature/pressure ranges suitable for
b
T[K] 310
1min
5min
5min 10min
10min
305
305
20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
20min
300 0
5
10
15
20
25
30
35
40
45 x[mm]
Fig. 6. Calculated temperature profiles (CaCl2 with constraint temperature Tc ¼ 293 K): (a) EXG composite (l ¼ 8 W/mK), (b) CF-EXG composite (l ¼ 14 W/mK).
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
1459
Fig. 7. The scheme and photograph (rear view) of sorption system and its adsorber.
Fig. 8. Evolution of the temperature of adsorbent during adsorption and desorption of ammonia on EXG-CF-CaCl2 composite.
Fig. 9. Change of temperature and pressure in adsorber in relation to mass of reacted adsorbent mads.
standard refrigeration/air-conditioning/heat pumping purposes. Experimental analysis confirmed suitability of adsorber containing EXG-CF composite with calcium chloride as reacting component to operate as a heat pump or refrigeration system. Fig. 8 shows the
evolution of the temperature and pressure inside adsorber during adsorption and desorption, as well as intermediate heating and cooling phases. The system was allowed to completely cool down and stabilize naturally, hence the long time between cycles.
1460
B. Zajaczkowski et al. / Applied Thermal Engineering 30 (2010) 1455e1460
Tests in a heat pump mode were carried under evaporation pressure Pe ¼ 3.5e4 bar and condensation pressure Pcond ¼ 9e10 bar. It allowed to obtain COPHP w 0.48e0.96 (in various operating conditions and controlled factors) in comparison to modeled w1.1. The experimentally obtained value was mainly decreased by the influence of thermal masses of system components. Fig. 9 shows registered variation of temperature in relation to the mass of reacted adsorbent, as well as corresponding variation of pressure registered in the adsorbent during the process. 6. Conclusion The introduction of carbon fibers in sorption composites resulted in increase of heat transfer coefficient of consolidated solid sorption composite from about 0.1 W/mK for a pure salt, up to the level that is considered optimal (w15 W/mK) [20]. The value has been measured in a cryostat by the axial stationary heat flow method in the temperature range 5 to 54 C (263e328 K). Temperature profiles (Fig. 6) calculated based on experimentally obtained values of thermal conductivity (Fig. 5) allow concluding that application of EXG-CF composite should remove the heat transfer in the bed from the list of limiting factors and therefore increase overall working efficiency of the sorption system. Moreover, resilience of the fibers increased porosity of the bed, due to post-compression expansion. The exact order of magnitude of this increase (in correspondence to expansion) is yet to be determined. Further development of EXG-CF composite will require closer analysis of the mass transfer in its porous structure in order to determine optimal working conditions for given permeability and pressure. The composite has been then introduced in the experimental sorption heat pump and used to successfully operate sorption cycle. The experiment proves repeatability of the working cycle. Obtained COPHP (which was mainly decreased by thermal mass of the adsorber) was about 0.48e0.96. References [1] B. Zaja˛ czkowski, Przenoszenie ciep1a i masy w reaktorze sorpcyjnej chemicznej pompy ciep1a, PhD thesis, 2008, Faculty of Mechanical and Power Engineering, Wroclaw University of Technology. [2] E. Lepinasse, V. Goetz, G. Crozat, Modeling and experimental investigation of a new type of thermochemical transformer based on the coupling of two solidegas reactions. Chemical Engineering and Processing 33 (3) (1994) 125e134. [3] W. Wongsuwan, S. Kumar, P. Neveu, F. Meunier, A review of chemical heat pump technology and applications,. Applied Thermal Engineering 21 (15) (2001) 1489e1519. [4] F. Meunier, Solid sorption: an alternative to CFCs. Heat Recovery Systems and CHP 13 (4) (1993) 289e295. [5] S. Mauran, M. Lebrun, P. Prades, M. Moreau, B. Spinner, C. Drapier. Active composite and its use as reaction medium, United States Patent 5283219; 1994.
[6] L.W. Wang, R.Z. Wang, R.G. Oliveira, A review of adsorption working pairs for refrigeration. Renewable and Sustainable Energy Reviews 13 (3) (2009) 518e534. [7] S. Wang, J. Wu, X. Li, R. Wang, Developments of consolidated adsorbent and experimental study of consolidated activated carbon blocks. Taiyangneng Xuebao/Acta Energiae Solaris Sinica 25 (3) (2004) 283e287. [8] H.-B. Lu, N. Mazet, B. Spinner, Modeling of gasesolid reaction e coupling of heat and mass transfer with chemical reaction. Chemical Engineering Science 51 (15) (1996) 3829e3845. [9] C. Coste, G. Crozat, S. Mauran, GaseouseSolid Reaction, United States Patent 4595774; 1986. [10] S. Mauran, P. Prades, F. L’Haridon, Heat and mass transfer in consolidated reacting beds for thermochemical systems. Heat Recovery Systems and CHP 13 (4) (1993) 315e319. [11] B. Spinner, Ammonia-based thermochemical transformers. Heat Recovery Systems and CHP 13 (4) (1993) 301e307. [12] N. Mazet, M. Amouroux, Analysis of heat transfer in a non-isothermal solidegas reacting medium. Chemical Engineering Communications 99 (1991) 175e200. [13] N. Mazet, H.-B. Lu, Improving the performance of the reactor under unfavorable operating conditions of low pressure. Applied Thermal Engineering 18 (9e10) (1998) 819e835. [14] J.H. Han, K.-H. Lee, H. Kim, Effective thermal conductivity of graphite-metallic salt complex for chemical heat pumps. Journal of Thermophysics and Heat Transfer 13 (4) (1999) 481e488. [15] J.H. Han, K.W. Cho, K.H. Lee, H. Kim, Porous graphite matrix for chemical heat pumps. Carbon 36 (12) (1998) 1801e1810. [16] P. Bou, M. Moreau, P. Prades, Active composite with foliated structure and its use as reaction medium, United States Patent 5861207, 1999. [17] C.H. Lee, S.H. Park, S.H. Choi, Y.S. Kim, S.H. Kim, Characteristics of non-uniform reaction blocks for chemical heat pump. Chemical Engineering Science 60 (5) (2005) 1401e1409. [18] B. Zaja˛ czkowski, Z. Królicki, (Patent Application) Multifunction active component carrier, and active component carrier production method. Wielofunkcyjny formowany nosnik czynnika aktywnego i sposób wytwarzania wielofunkcyjnego formowanego czynnika aktywnego, Polish Patent Application P382966, 2007. [19] K. Wang, J.Y. Wu, R.Z. Wang, L.W. Wang, Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats. International Journal of Refrigeration 29 (2) (2006) 199e210. [20] H.-J. Huang, G.-B. Wu, J. Yang, Y.-C. Dai, W.-K. Yuan, H.-B. Lu, Modeling of gasesolid chemisorption in chemical heat pumps. Separation and Purification Technology 34 (1e3) (2004) 191e200. [21] N. Mazet, P. Meyer, P. Neveu, B. Spinner, Concept and study of a double effect refrigeration machine based on the sorption of solid and ammonia gas and controlled by heat pipes, in: International Absorption Heat Pump Conference. ASME, New York, NY, USA: New Orleans, LA, USA, 1994, pp. 407e412. [22] S. Biloe, V. Goetz, S. Mauran, Characterization of adsorbent composite blocks for methane storage. Carbon 39 (11) (2001) 1653e1662. [23] J.J. Guilleminot, J.B. Chalfen, A. Choisier, Heat and mass transfer characteristics of composites for adsorption heat pumps, in: International Absorption Heat Pump Conference. ASME, 1993, pp. 401e406. [24] T.-H. Eun, H.-K. Song, J.H. Han, K.-H. Lee, J.-N. Kim, Enhancement of heat and mass transfer in silicaeexpanded graphite composite blocks for adsorption heat pumps: part I. Characterization of the composite blocks. International Journal of Refrigeration 23 (1) (2000) 64e73. [25] B. Zaja˛ czkowski, Z. Królicki, B. Bia1ko, T. Wilk, Physical and chemical adsorption as a new trend in obtaining refrigeration effect, in International Confer , Poland 87e94. ence of IIR, Comm. B1, B2, C2, 2008: Poznan [26] Z. Królicki, B. Zaja˛ czkowski, T. Wilk, Selected aspects of thermodynamic modeling of solar assisted chemical ice maker with ammonia and chloride salt, in: 61st ATI National Congress e International Session. Solar Heating and Cooling, Perugia, Italy, 2006, pp. 95e100.