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Performance prediction on a resorption cogeneration cycle for power and refrigeration with energy storage
Renewable Energy 83 (2015) 1250e1259
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance prediction on a resorption cogeneration cycle for power and refrigeration with energy storage L. Jiang, L.W. Wang*, X.F. Zhang, C.Z. Liu, R.Z. Wang Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai, 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 10 June 2015 Accepted 12 June 2015 Available online 20 June 2015
Energy conversion technologies, especially for power generation and refrigeration, driven by the low temperature heat source are gathering the momentum recently. This paper presents a novel cogeneration cycle combining power and refrigeration with energy storage function. MnCl2eCaCl2eNH3 is selected as the working pair. Phase change materials of “50 wt% NaNO3 þ 50 wt% KNO3” and “65 mol% capric acid þ 35 mol% lauric acid” are chosen for heat and cold storage, respectively. Heat and mass transfer property of composite adsorbents are investigated, and isentropic efficiency of scroll expander is tested by compressed air. Based on experimental results, a cogeneration system with power of 300 W maximum and cooling power of 2 kW is designed and analyzed. Analysis shows that total energy efficiency of cogeneration system increases from 0.316 to 0.376 and energy efficiency decreases from 0.402 to 0.391 when evaporation temperature increases from 10 to 20 C. Cold releasing process is able to last 91 min with cold storage function. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Resorption Refrigeration Scroll expander Power
1. Introduction In recent years, utilization of low grade heat has become one of the hottest topics due to high energy consumption and severe environmental pollution. Electricity, easy to distribute and supreme to control, plays one major role of high energy demanding [1]. For utilization of low grade heat, Organic Rankine Cycle (ORC) and Kalina cycle are two attractive cycles for power generation. ORC is able to be driven by heat source temperature even lower than 100 C. Many researchers investigated ORC system with different working fluids. Quoilin et al. [2] constructed a numerical model of an ORC by using a prototype with two hot air flows as the heat source and compared with the experimental results. Results showed that when the mean temperatures of the first and second hot air flows were 86.4 C and 163.2 C, respectively, a maximum energy efficiency of 7.4% was achieved. Kaska et al. [3] performed an energy and exergy analysis on an ORC driven by industrial waste heat from the steel industry with the R245fa working fluid. Simulation results showed that the energy efficiency and exergy efficiency of the system were 10.2% and 48.5%, respectively.
* Corresponding author. E-mail address: [email protected] (L.W. Wang). http://dx.doi.org/10.1016/j.renene.2015.06.028 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
However, due to energy consumption of pressure pump, energy efficiency is not as high as 10% in the real system [4]. Kalina Cycle is another cycle for power generation with working fluid of waterammonia [5]. Corman [6] and Martson [7] demonstrated that Kalina cycle had great potential in recovering the middle and low temperature heat source such as solar power and geothermal resource. To improve energy efficiency of utilizing the low grade heat, power generation and refrigeration are considered to be combined. One way is to cascade power generation system with heat driven refrigeration system, which can enhance exergy efficiency of heat source. Wang et al. [8] established and analyzed solar powered cascading cogeneration cycle with ORC and adsorption technology. Results showed that the total exergy efficiency is 0.56e0.74. Jiang et al. [9] investigated cascading cogeneration system of ORC and CaCl2/BaCl2 two-stage adsorption freezer. The cascading cycle improved the exergy efficiency for the heat utilization to 20.4%e29.1%. The other way is to combine power generation with heat driven refrigeration system, which means added power generation apparatus into heat driven refrigeration system. Adsorption and absorption refrigeration are two major way driven by low temperature heat source [10]. One good example is that silica gel-water adsorption cooling system has been considered for application based on the solar radiation data of Tokyo [11]. Based on adsorption refrigeration, scroll expander is added between
L. Jiang et al. / Renewable Energy 83 (2015) 1250e1259
Nomenclature A c COP E f0 HTS h K L LTS m n P Q R SCP S s t T U I
Pre-exponential factor Specific heat (kJ/(kg. C)) Coefficient of performance Reaction activation energy Difference of reaction equilibrium potential High temperature salt heat transfer coefficient (W/m.K) Arrhenius factor Latent heat of vaporization (J/kg) Low temperature salt Mass flowrate (kg/s) Order of reaction Pressure(Pa) Heating power (kW) Gas constant (J/(mol.K)) Specific cooling power (W/K) Factor related to heat source Heat transfer area(m2) Time (s) Temperature (K) Voltage (V) Current (A)
adsorber and condenser. Bao et al. [12] investigated the chemisorption cooling and electric power cogeneration system driven by low grade heat. However, since scroll expander selected was not suitable for the adsorption system, efficiency of the cogeneration system was not high. After that, Bao et al. [13] established the model of scroll expander for analyzing the ideal operation performance for the adsorption-cogeneration system. Results showed that the cogeneration mode can achieve 1000 W power output for optimal working condition. Based on absorption refrigeration, Goswami proposed a combine power and refrigeration cycle, which was expected to improve energy and exergy efficiency [14]. However, although power generation efficiency is reasonable, this cycle enjoys very poor refrigeration performance since it produces cooling effect through one precooler [15]. Both absorption and adsorption refrigeration technologies take two disadvantages if ammonia is performed as the working fluid, which is safety problem and sensitivity to cooling temperature. Actually, resorption refrigeration can match different reaction equilibrium temperature when different alkali metal halides are employed [16]. Since it utilizes decomposition heat for refrigeration [17], an improved refrigeration performance can be guaranteed [18]. Based on resorption refrigeration, Wang et al. [19] proposed a novel resorption cogeneration cycle for power and refrigeration. Compared with the Goswami cycle, total exergy efficiency for power and refrigeration is 0.56e0.74, which is improved by 10%e40%. Nonetheless, there are less experimental simulation and comparison in field of resorption cogeneration. In this paper, a novel resorption cogeneration system of a cycle for 300 W power maximum and 2 kW refrigeration power is designed. The basic parameters for simulation such as heat and mass transfer performance of composite adsorbents and isentropic efficiency of scroll expander are investigated by concerning experiments. In order to have an overall understanding of the novel system, part of simulation results are compared with experimental results for validation.
W x
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Power output of expander (W) Adsorption quantities (kg/kg)
Greek letters Density of water vapor (kg/m3)
r
Subscripts ads Adsorbent c Condensing cool Cooling water chilled Chilled water de Desorption eq Equilibrium ex Exergy eva Evaporator f Fluid h Heat i Inlet out Outlet ref Refrigeration s Isentropic process tot Total w Wall
2. Establishment of the resorption cogeneration cycle The resorption cogeneration cycle is presented in Fig. 1, which mainly include oil tanks, HTS and LTS adsorption beds, high temperature and low temperature PCM energy storage tanks, expander and other auxiliary components such as valves and pumps. The working processes are as follows: (1) Desorption process of HTS bed for power generation. In this process, V1, V3, V4, AV1, AV2 and Pump 1 are open. HTS bed is heated by oil tank 1 which is simulated as low grade heat with certain temperature. Meanwhile V10 and Pump 4 are open, and LTS bed is cooled by the cooling tank which is simulated as environmental heat sink. Desorbed ammonia of HTS bed through AV2 is expanded in the expander and generates power there. The expanded ammonia will be
Fig. 1. Cogeneration cycles for power and refrigeration.
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adsorbed by LTS bed through AV1. V9, V11 and Pump 3 are open. Phase change process of low temperature PCM will provide the cooling power, which is transported to the onsite by the thermal fluid of ethanol solution and consumed by chilled tank. Cooling power can be transported under a constant level because phase change process happens under a constant temperature. (2) Cold releasing process of low temperature PCM. In this process, V9, V11 and Pump 3 are open. Phase change of low temperature PCM happens and provides the cooling power, which stores refrigeration from the last cycle. The cooling power is transport through the ethanol solution and consumed by chilled tank. The process accompanies with the power generation process for continuous refrigeration for the resorption cogeneration system. (3) Desorption process of LTS bed for refrigeration. AV1, AV2, V10 close and V6, V7, V8, V9, V12, AV3, Pump 2 and Pump 3 are open. HTS bed is cooled by oil tank 2 that is simulated as environmental heat sink, and it will adsorb ammonia from LTS bed when its pressure is lower than that of LTS bed. Desorption process of LTS bed generates the refrigeration and consumed by the chilled tank. The redundant cooling power will be stored in the low temperature energy storage tank through low temperature PCM. The process accompanies with the refrigeration process of LTS bed for the resorption cogeneration system. (4) Heat storage process of high temperature PCM. After the desorption process, V1, V4 close and V2, V5 open, oil tank 1 will be connected with high temperature PCM energy storage tank, and redundant energy will be stored in the energy storage tank by phase change process of high temperature PCM. Actually there are two different modes of energy storage processes can be used in this system. One is energy stored in the high temperature and low temperature PCM tanks. High temperature PCM is to recover the redundant heat for improving system efficiency. Low temperature PCM is to maintain the continuous refrigeration output. The other mode is the cooling capacity stored in the reactor of LTS if refrigerant valve is closed after the desorption process of LTS by the adsorption/desorption capacity of these two beds. 3. Thermal conductivity and permeability of adsorbent 3.1. Thermal conductivity Thermal conductivity is investigated by the Laser flash measuring method, and the type of instrument was LFA447 produced by Netzsch Company. Development of composite adsorbent and measuring process is according to the reference [20]. Table 1 shows thermal conductivity of consolidated composite CaCl2 and MnCl2 for different density and mass ratio of salts. Results indicate that thermal conductivity increases while mass ratio of salts decreases and density increases. For different composite CaCl2 thermal conductivity ranges from 12.1 to 48.3 W/(m.K). Thermal conductivity of consolidated composite MnCl2 takes the same trend as composite CaCl2 for different density and mass ratio of salts. Thermal conductivity of MnCl2 ranges from 8 to 42.8 W/(m.K). 3.2. Permeability Table 2 shows that permeability of consolidated composite CaCl2 and MnCl2 with different mass ratio of salt and density. Results indicate that for different samples of consolidated composite
CaCl2 permeability ranges from 9.31 1010 to 3.05 1014 m2. Permeability increases when the density decreases and mass ratio of salt increases. Consolidated composite MnCl2 also takes the same trend with permeability. Permeability ranges from 8.02 1011 to 1.95 1014 m2. 4. Investigation on scroll expander Scroll expander is chosen for expansion process of power generation since it is suitable for small type power generation system. Scroll compressor (type ATC0062 C2) is modified as scroll expander and tested. Fig. 2a displays the schematic of test unit, and Fig. 2b displays picture of the system. A piston air compressor first compresses the air. The air is dried and heated by a dryer and heating belt and then air flowrate is measured by a flow meter, after that it expands and generates the power. Two temperature and pressure sensors are set at inlet and outlet of the expander. In the experiments, expander inlet temperature, inlet pressure and outlet pressure are controlled under theoretical calculation conditions of expander of the resorption system. The ideal work output, isentropic efficiency, work to power efficiency as well as the internal efficiency for power generation are calculated and studied. The calculation equations can be referred to reference [21]. Fig. 3a and Fig. 3b show the experimental results. When the pressure of expander inlet varies from 0.6 to 1 MPa, isentropic efficiency and work to power efficiency maintain at about 60% and 80%. 5. Properties of composite PCM Phase change temperature, latent heat of pure nitrate and nitrate/ENG-TSA samples are measured by differential scanning calorimetry (DSC). DSC thermal analysis started from 150 C to 280 C with a heating rate of 5 ºC/min and 280 C to 150 C with a cooling rate of 5 ºC/min. Fig. 4 indicates results of pure nitrate and nitrate/ENG-TSA samples. Phase transition properties of pure nitrate are consistent with values of literature [22]. Phase change temperature of nitrates/ENG-TSA shifts slightly from pure nitrates. Results show that melting temperature of the nitrate/10 wt% ENGTSA and nitrate/20 wt% ENG-TSA shifts from 220.8 C to 221.6 C and 221.9 C, respectively. Freezing temperature of the nitrate/ 10 wt% ENG-TSA and nitrate/20 wt % ENG-TSA shifts from 223.0 C to 223.5 C and 223.6 C, respectively. ENG-TSA, as one additive, inclines to adjust phase change temperature. The main reason is because ENG-TSA provides more micro channels to make PCM and ENG-TSA tighter, thus enhancing the capillary force. 6. Design of the cogeneration system 6.1. System design 3-dimension model of the cogeneration system with resorption refrigeration and power generation is designed as Fig. 5. The cogeneration system is composed of two adsorption beds; each bed is composed of three unit beds. The upper three unit beds are filled by MnCl2 composite adsorbent and compose one adsorption bed for MnCl2 while the other three unit beds are filled with CaCl2 composite adsorbent. Although mass of adsorbent are same as the design of two unit beds, there are two major advantages of this design. One advantage is that the design will avoid becoming pressure vessel and guarantee the system safety. The other advantage is the design of two separated unit beds will reduce metal heat capacity. Two PCM reactors are also fixed at each side of
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Table 1 Thermal conductivity of composite CaCl2 and MnCl2. Density(kg/m3)
Mass ratio of salts
Thermal conductivity of CaCl2 (W/(m.K))
Thermal conductivity of MnCl2 (W/(m.K))
400
50% 67% 75% 80% 83% 50% 67% 75% 80% 83% 50% 67% 75% 80% 83%
31.7 23.3 17.3 14.2 12.1 40.5 27.4 20.8 17.8 14.8 48.3 32.5 27.2 23.0 18.8
28.1 20.2 15.6 11.2 8.0 34.9 24.8 18.6 14.6 11.9 42.8 30.7 25.7 20.4 16.5
450
500
Table 2 Permeability of consolidated composite CaCl2 and MnCl2. Density(kg/m3) 300
400
500
Mass ratio of salts 50% 67% 75% 80% 83% 50% 67% 75% 80% 83% 50% 67% 75% 80% 83%
Permeability of CaCl2 (m2) 7.34 6.34 8.05 9.25 9.31 5.20 4.52 5.20 5.52 7.52 3.05 6.05 1.74 2.54 5.42
14
10 1013 1012 1011 1010 1014 1013 1012 1011 1011 1014 1013 1013 1012 1011
Permeability of MnCl2 (m2) 5.14 7.04 6.44 7.97 8.02 4.20 4.42 4.90 7.52 7.93 1.95 1.85 3.95 3.35 6.21
1014 1013 1012 1012 1011 1014 1013 1012 1011 1011 1014 1013 1012 1011 1011
Fig. 2. Compressed air power generation system (a) Schematic, (b) Photograph.
HTS and LTS adsorption bed. High temperature PCM reactor is located at the lower position of the system while low temperature PCM reactor is located at the higher position. High temperature PCM reactor could collect the excessive heat from heat source. Lower temperature PCM reactor could store redundant cooling power for continuous refrigeration since resorption refrigeration system is characterized as intermittent system. The expansion machine includes the scroll expander, torque meter and generator. Scroll expander is installed between HTS and LTS adsorption bed for generating the power. One torque meter is connected between scroll expander and generator to test the torque. For the better mass transfer performance, five ammonia pipes with small diameter are designed to connect each unit adsorption bed and converged into one ammonia pipe with larger diameter, which leads to uniform
adsorption and desorption process. Actually, one ammonia pipe with larger diameter takes role of connecting the HTS adsorption bed with LTS adsorption bed. 6.2. Simulation Structure of unit tube-shell bed for resorption refrigeration is shown in Fig. 6. In the process of desorption, hot oil goes into HTS adsorption bed via heat flow inlet for heating and flows out from the heat flow outlet. Meanwhile, LTS adsorption bed is cooled by the cooling medium. Ammonia desorbs from HTS adsorption bed and goes to LTS adsorption bed through ammonia pipe. In the process of adsorption, LTS adsorption bed is controlled as evaporation temperature, and HTS adsorption bed is cooled by the
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Fig. 3. Performance of the scroll expander (a) Isentropic efficiency and the ideal work, (b) work to power efficiency and power.
Fig. 6. Schematic of typical tube-shell bed.
Fig. 4. Typical DSC curve of pure nitrate and nitrate/ENG-TSA composite PCMs.
Fig. 7. Equivalent modeling of adsorption bed for simulation.
Fig. 5. 3-dimension model of the cogeneration system.
cooling medium. Ammonia is adsorbed by HTS adsorption bed through LTS adsorption bed, producing the cooling effect. On the basis of working process above, the related modeling is applied for numeral simulation, which is shown in the Fig. 7. Energy and mass conversation are two major concerns of adsorption bed in the adsorption and desorption process. For establishing the equation, mass of adsorbent is equivalent to six adsorption beds and
heat transfer area of six tube-shell beds are equivalent to one tubeshell bed. By utilizing the finite element method, thermodynamic parameter is separated into discrete points for the simulation. That can reflect the influence of heat and mass transfer performance of the materials on the performance of the cogeneration system.
6.3. Mathematic model 6.3.1. The dynamic equation of thermal chemisorption reaction Mathematical model of the cogeneration system is established, considering the variables such as permeability, thermal conductivity, and reactor parameters. Performance of the system is simulated, and main assumptions are as follows:
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(1) Both temperature and pressure are uniform throughout the whole adsorption bed, and the refrigerant is adsorbed uniformly inside the adsorbent. (2) Heat and mass transfer of adsorbent only happen through the radial direction. (3) There is no temperature difference between adsorbent and refrigerant. (4) Temperature variation in radial direction is neglected between heat transfer fluid with tube. (5) Internal efficiency of power generation is based on the testing results. The adsorption kinetic equation is established based on the following equations from 1 to 5. Basic chemical adsorption kinetic model expression:
v¼
dx ¼ KðP; TÞ$f ðxÞ dt
(1)
where x is adsorption quantity, f(x) is related to the variety of the reaction, which can be expressed as:
f ðxÞ ¼ ð1 xÞn
(2)
where n is order of the reaction. K (P, T) is Arrhenius factor based on the Arrhenius theory, which reflects the difference between non-equilibrium and equilibrium working condition, and it can be expressed as:
KðP; TÞ ¼ kf 0 ðP; TÞ
(3)
E k ¼ A$exp RT
(4)
where A is pre-exponential factor and E is reaction activation energy, f0 (P,T) is the difference of reaction equilibrium potential, which is related to the difference between constrained pressure and equilibrium pressure, and its expression is:
m Pc Peq ðTÞ $exp T 273 f 0 ðP; TÞ ¼ To 273 Pc
(5)
where Peq is the equilibrium pressure, which is obtained by Clausius-Clapeyron equation, m is parameter of the reaction
Peq
DHr DS ¼ exp þ R RTc
(6)
where DHr is reaction enthalpy between adsorbent and gas, DS is entropy of chemical reaction, Tc is the constrained external temperature of the adsorbent in the process of chemical reaction.
6.3.2. Establishment of mathematical equations Mathematical model is established based on the physical model as mentioned above. Energy conservation equation is established on the basis of heat transfer of the fluid as well as heat conduction inside composite adsorbent. Dynamic equation of the chemical reaction rate is set up based on adsorption/desorption process. The physical properties changes of the adsorbent in the reaction processes are also considered. Through comprehensive analysis of heat and mass transfer performance, the optimal compound adsorbent is chosen for simulation as the bulk density of 400 kg/m3 and 80% mass ratio of CaCl2. As metal container is cylindrical, two-dimensional heat conduction equations are established as the cylindrical coordinate
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since the anisotropic characteristic of composite adsorbent [23]:
rcp
vT v vT 1 v vT ¼ lz þ rlr þS vt vz vz r vr vr
(7)
where lz and lr are thermal conductivity in the axial and radial direction; S is the factor related to the heat source, which is expressed as:
S¼
nNH3 dx DH V dt
(8)
where nNH3 is mass transfer quantity of the ammonia, V is volume of composite adsorbent，dx/dt is the rate of the adsorption reaction, DH is the enthalpy of the reaction. Heat loss of the system is assumed to be neglected since the bed is wrapped in insulation materials. Therefore, its boundary condition is assumed as the adiabatic boundary. The third category boundary is applied because boundary inside exchange heat with heat fluid:
lr
vT ¼ 0 r ¼ ±rw vr
(9)
lz
vT ¼ 0 z ¼ 0; zh vr
(10)
lr
vT ¼ h T Tf r ¼ ±ri vr
(11)
Heat transfer fluid exchanges the heat with metal wall from top to bottom. The energy conservation equation of the fluid is as follow:
rf cpf
vTf vTf v2 Tf þ rf ucpf ¼ lf 2 þ Sf vt vz vz
(12)
where lf is thermal conductivity of heat exchange fluid, u is velocity of heat transfer flow, Sf is the heat exchanged between the flow and the metal shell, which can be expressed as.
Sf ¼ s$h$ T Tf
(13)
where s is heat transfer area of unit control body, Tf is the temperature of heat transfer fluid, T is metal wall temperature of adsorption bed. when inlet temperature of heat exchange flow is constant, outlet temperature gradient tends to zero, so the boundary condition is:
Tf ¼ constant z ¼ 0
(14)
vTf ¼ 0 z ¼ zh vz
(15)
The adsorption kinetic equation is established from Eqs. (1)e(5). According to experimental results in Ref. [24], suitable parameters of composite adsorbent CaCl2 are able to be provided for the adsorption process to simulate chemical reaction. The adsorption kinetic equation can be expressed as follows:
Pc Peq ðTÞ T 273 4 dx E $ ¼ A$ð1 xÞ2 $exp $ T 273 dt RT Pc c
(16)
where x is the adsorption quantity, if x is defined as the quantity of desorption, then use (1-x) instead of x. Data of the parameters of equation (15) are listed in Table 3.
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Table 3 Parameters of the chemical reaction rate equation.
Desorption Adsorption
Table 4 Parameters for calculation of tube shell bed.
A
E/J mol1
Parameter
Value
0.0005 0.00204
2385 1383.24
Inner radius of adsorbent Outside radius of adsorbent Overall height of adsorbent Volume of adsorbent Flowrate of heat transfer fluid Thermal conductivity of heat transfer fluid Heat transfer coefficient before adsorption Heat transfer coefficient after adsorption Heat transfer area Radius of heat transfer fluid Mass transfer quantity of ammonia Mass ratio of salt Reaction enthalpy of MnCl2 Reaction entropy of MnCl2 Reaction enthalpy of CaCl2 Reaction entropy of CaCl2
12.5 mm 23.5 mm 1.02 m 1.24 L 0.2 L/min 0.54 W/m.k 150 W/m2.k 550 W/m2.k 0.04 m2 12.5 mm 4 mol/mol 4:1 47416 J/mol 227.9 J/(mol$K) 42269 J/mol 229.7 J/(mol$K)
The physical properties will change accordingly due to the adsorption quantity for adsorbent in the process of adsorption. After testing, adsorption and desorption reaction has little influence on the permeability of composite adsorbent, so the constant permeability is applied in the simulation. In contrast, the adsorption and desorption reactions have larger influence on the thermal conductivity of composite adsorbent. Thermal conductivity are considered as linear change according to the reference [24], and then average thermal conductivity lz and lr for radical and axial direction in the reaction process can be calculated. Specific heat capacity and thermal conductivity of the adsorbent can be fitted by the following equation:
mg m xM2 m ð1 xÞM1 þ cp2 salt þ cp1 salt V V V
rcp ¼ cpg
l ¼ l2 x þ l1 ð1 xÞ
(17) (18)
where x is the reaction degree in adsorption process, subscripts 1 and 2 are represented the parameter before and after the adsorption process. The heating power of the system：
Z
tcycle
0
Qh ¼ ¼
cw $mw $ THTS;in TLTS;out dt tcycle
(19)
where tcycle is the cycle time (s), cw is specific heat of water (J/ (kg.K)), THTS,in and TLTS,out are the inlet and outlet temperature of hot water (K). Cooling power of the system:
Z Qref ¼
tcycle 0
cw $mw $ TLTS;in TLTS;out dt tcycle
(20)
7. Simulation results 7.1. Temperature Fig. 8 shows the experimental and simulation results of temperature variation of LTS adsorbent under the condition of different evaporation temperature, which ranges from 10 C to 10 C. For simulation, results shows when evaporation temperature is lower than 0 C, temperature of LTS adsorbent decreases in few minutes at the beginning and then increases gradually. When evaporation temperature is higher than or equals to 0 C, temperature of LTS adsorbent decreases even faster. For evaporation temperature as high as 10 C, temperature of LTS adsorbent takes minimum 3.56 min to reach the lowest value. Comparably, for evaporation temperature as low as 10 C, temperature of LTS adsorbent takes maximum 4.4 min to reach the lowest. The decline of evaporation temperature of LTS adsorbent becomes sharper when the evaporation temperature increases from 10 to 10 C. Comparing simulation data with experiments, it indicates that simulation data agree with experimental data well with increment of evaporation temperature. The smallest temperature difference is about 0.43 C for evaporation temperature of 10 C while the largest temperature difference is about 2.75 C. This is mainly because properties such as thermal conductivity and permeability are tested for
where mw,eva is the mas flowrate of chilled water in the evaporator (kg/s); TLTS,in and TLTS,out are the temperature of chilled water inlet and outlet (K). COP of the system is:
COP ¼
Qref Qh
(21)
where Qref is the cooling amount, Qh is the heating amount for the system. The SCP per kilogram adsorbent can be expressed as:
SCP ¼
Wref mtot
(22)
where mtot is total mass of composite adsorbent (kg), i.e. both the mass of chlorides and expanded natural graphite are considered. The whole controlled body is simulated by the internal nodal method. The finite element analysis is performed in a cylindrical coordinate, and software MATLAB is used for simulation. All the parameters used in the mathematic model are listed in Table 4.
Fig. 8. Temperature variation of LTS adsorbent.
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environmental temperature. When temperature is lower than 0 C, permeability will decrease and mass transfer become poor in the adsorption process, which leads to the slower reaction rate and smaller temperature decline. 7.2. Dynamic adsorption characteristic Fig. 9 shows desorption and adsorption rate change of LTS adsorbent under the condition of 130 C heat source temperature, 30 C cooling temperature and 0 C evaporation temperature. Results indicate that it takes about 20 min to finish the desorption process. However, it takes 5 min to finish about 60% of the whole adsorption process, which is the main reason for the phenomenon of Fig. 8. Global conversion for desorption changes from 1 to 0.4. Simultaneously, global conversion for adsorption changes from 0.4 to 1. It also demonstrates adsorption process is a little faster than the desorption process, which takes about 4.8 min to finish 60% of the desorption process. The reason is that the adsorption process of LTS enjoys larger temperature differences. Temperature differences cause the higher driven pressure to precede the process.
Fig. 10. COP and SCP changing with the cycle time.
7.3. Refrigeration performance Fig. 10 shows COP and SCP change with cycle time under the condition of 130 C heat source temperature, 30 C cooling temperature and 0 C. Results demonstrate that both COP and SCP increase rapidly in the first five minutes then decrease slowly. The maximum COP is about 0.31 which happens in 3.8 min. Simultaneously, the maximum SCP is about 161 W/kg in 3.6 min. Fig. 11 shows the trends of COP and SCP when evaporation temperature changes from 10 C to 10 C. For simulation, results demonstrate that both COP and SCP increase with the increment of evaporation temperature. For the evaporation temperature from 10 C to 10 C, COP ranges from 0.22 to 0.256. Simultaneously, SCP ranges from 126 W/kg to 143.7 W/kg. It indicates that the difference between simulation and experimental data fit well when refrigeration temperature is above 0 C, which is consistent with the results of temperature in Fig. 8. The largest gap between simulation and experiment for COP and SCP are 0.02 and 5.9 W/kg while the smallest gap are 0.002 and 0.27 W/kg, respectively. 7.4. Power generation performance In the simulation of the cogeneration system, desorbed ammonia flow is held on for a while with ammonia valve AV2
Fig. 9. The desorption and adsorption rate of LTS.
Fig. 11. COP and SCP vary with evaporation temperature.
closed and then releasing it at 10th minute in the desorption process. That leads to the flow rate behaving like pulses lasted for a very short time as shown in Fig. 12, which depicts both power generation and refrigeration for the cogeneration system. Results also indicate the maximum power output is nearly 375 W at
Fig. 12. Power generation and cooling power vary with cycle time.
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Fig. 13. Temperature of high temperature PCM for different working process, (a) Heat charging process; (b) Heat discharging process.
Fig. 14. Temperature of low temperature PCM for different working process, (a) Cold storage process; (b) Cold releasing process.
10.16 th minute in desorption process while the maximum refrigeration power is 2.24 kW at 3rd minute in adsorption process. Power generation happens in every 30 min while the cooling power lasts for 26.5 min.
process takes 80 min and temperature difference between enthanol solution inlet and outlet is 7.8 C. Fig. 14b indicates temperature change of low temperature PCM in cold releasing process. It can be displayed that PCM is heated from 11 to 18 C in the cold releasing process. Cold releasing process takes 91 min and the temperature difference between oil inlet and outlet is 4.1 C.
7.5. Performance of energy storage Fig. 13 shows temperature variation of high temperature PCM for different working process. Fig. 13a indicates temperature variation of high temperature PCM in heat charging process. It can be seen that high temperature PCM is heated from 213 C to 232 C in heat charging process. It includes heating process from 213 C to 223 C, melting process at 221 C, and heating process from 221 C to 232 C. The maximum temperature difference is 11 C. The heat charging process takes 73 min and temperature difference between oil inlet and outlet is 8.1 C. Fig. 13b indicates temperature variation of high temperature PCM in heat discharging process. It can be seen that PCM is cooled from 232 to 212 C in the heat charging process. The maximum temperature is 10 C. Heat discharging process takes 91 min and temperature difference between oil inlet and outlet is 5.1 C. Fig. 14 shows temperature variation of low temperature PCM for different working process. Fig. 14a indicates temperature variation of low temperature PCM in cold storage process. It can be indicated that low temperature PCM is heated from 18 to 12.5 C in cold storage process. It includes cooling process from 18 to 15 C, melting process at 15 C, and cooling process from 15 to 12.5 C. The maximum temperature difference is 5.5 C. The cold storage
7.6. The analysis on the cascading cycle Total energy efficiency and exergy efficiency of the cascading cycle can be calculated as follows:
htotal:energy ¼
htotal;exergy ¼
Qref þ W Qh Eex;Qref þ W EORC þ Eex;Qh
(23)
(24)
Fig. 15 shows energy efficiency and exergy efficiency varies with evaporating temperature. When heat source temperature is 130 C and evaporating temperature varies from 10 C to 20 C, total energy efficiency increases from 0.316 to 0.376 as well as exergy efficiency decreases from 0.402 to 0.391. 8. Conclusions A novel cogeneration cycle of power and refrigeration with energy storage was presented and the concerning system was
L. Jiang et al. / Renewable Energy 83 (2015) 1250e1259
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11160706000, Program for New Century Excellent Talents in University by the Ministry of Education, China (NCET-11-0333).
References
Fig. 15. Total energy efficiency and exergy efficiency vs. evaporating temperature.
designed and predicted. Heat and mass transfer properties of composite adsorbent are tested to predict the refrigeration performance. Scroll expander is also investigated by the compressed air testing units to simulate power generation performance. Properties of PCM are tested by DSC for thermal storage. Conclusions were yielded as follows: For different CaCl2 composite adsorbent, thermal conductivity ranges from 12.1 to 48.3 W/(m.K). Thermal conductivity of MnCl2 composite adsorbent ranges from 8 to 42.8 W/(m.K), which takes the same trend as the composite CaCl2 for different density and mass ratio of salts. Permeability for different samples of CaCl2 and MnCl2 composite adsorbent range from 9.31 1010 to 3.05 1014 m2 and from 8.02 1011 to 1.95 1014 m2,respectively. Compressed air is used to evaluate the performance of the scroll expander. When the pressure of expander inlet varies from 0.6 to 1 MPa, isentropic efficiency and work to power efficiency maintain at about 60% and 80%. Internal efficiency for power generation is 53% when pressure of expander inlet is 1 MPa. Phase change temperature of nitrates/ENG-TSA shifts slightly from pure nitrates. Melting temperature of the nitrate/10 wt% ENGTSA composite and nitrate/20 wt% ENG-TSA shift from 220.8 C to 221.6 C and 221.9 C, respectively. Freezing temperature of the nitrate/10 wt% ENG-TSA and nitrate/20 wt % ENG-TSA shift from 223.0 C to 223.5 C and 223.6 C, respectively. When heat source temperature is 130 C and the evaporating temperature varies from 10 C to 20 C the total energy efficiency of the cogeneration system increases from 0.316 to 0.376 and exergy efficiency decreases from 0.402 to 0.391. Acknowledgments This research was supported by the National Science Foundation of China for Excellent Young Scholars under contract number 51222601, Foundation of Science and Technology Commission of Shanghai Municipality, P.R. China, under contract number
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance prediction on a resorption cogeneration cycle for power and refrigeration with energy storage L. Jiang, L.W. Wang*, X.F. Zhang, C.Z. Liu, R.Z. Wang Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai, 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 10 June 2015 Accepted 12 June 2015 Available online 20 June 2015
Energy conversion technologies, especially for power generation and refrigeration, driven by the low temperature heat source are gathering the momentum recently. This paper presents a novel cogeneration cycle combining power and refrigeration with energy storage function. MnCl2eCaCl2eNH3 is selected as the working pair. Phase change materials of “50 wt% NaNO3 þ 50 wt% KNO3” and “65 mol% capric acid þ 35 mol% lauric acid” are chosen for heat and cold storage, respectively. Heat and mass transfer property of composite adsorbents are investigated, and isentropic efficiency of scroll expander is tested by compressed air. Based on experimental results, a cogeneration system with power of 300 W maximum and cooling power of 2 kW is designed and analyzed. Analysis shows that total energy efficiency of cogeneration system increases from 0.316 to 0.376 and energy efficiency decreases from 0.402 to 0.391 when evaporation temperature increases from 10 to 20 C. Cold releasing process is able to last 91 min with cold storage function. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Resorption Refrigeration Scroll expander Power
1. Introduction In recent years, utilization of low grade heat has become one of the hottest topics due to high energy consumption and severe environmental pollution. Electricity, easy to distribute and supreme to control, plays one major role of high energy demanding [1]. For utilization of low grade heat, Organic Rankine Cycle (ORC) and Kalina cycle are two attractive cycles for power generation. ORC is able to be driven by heat source temperature even lower than 100 C. Many researchers investigated ORC system with different working fluids. Quoilin et al. [2] constructed a numerical model of an ORC by using a prototype with two hot air flows as the heat source and compared with the experimental results. Results showed that when the mean temperatures of the first and second hot air flows were 86.4 C and 163.2 C, respectively, a maximum energy efficiency of 7.4% was achieved. Kaska et al. [3] performed an energy and exergy analysis on an ORC driven by industrial waste heat from the steel industry with the R245fa working fluid. Simulation results showed that the energy efficiency and exergy efficiency of the system were 10.2% and 48.5%, respectively.
* Corresponding author. E-mail address: [email protected] (L.W. Wang). http://dx.doi.org/10.1016/j.renene.2015.06.028 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
However, due to energy consumption of pressure pump, energy efficiency is not as high as 10% in the real system [4]. Kalina Cycle is another cycle for power generation with working fluid of waterammonia [5]. Corman [6] and Martson [7] demonstrated that Kalina cycle had great potential in recovering the middle and low temperature heat source such as solar power and geothermal resource. To improve energy efficiency of utilizing the low grade heat, power generation and refrigeration are considered to be combined. One way is to cascade power generation system with heat driven refrigeration system, which can enhance exergy efficiency of heat source. Wang et al. [8] established and analyzed solar powered cascading cogeneration cycle with ORC and adsorption technology. Results showed that the total exergy efficiency is 0.56e0.74. Jiang et al. [9] investigated cascading cogeneration system of ORC and CaCl2/BaCl2 two-stage adsorption freezer. The cascading cycle improved the exergy efficiency for the heat utilization to 20.4%e29.1%. The other way is to combine power generation with heat driven refrigeration system, which means added power generation apparatus into heat driven refrigeration system. Adsorption and absorption refrigeration are two major way driven by low temperature heat source [10]. One good example is that silica gel-water adsorption cooling system has been considered for application based on the solar radiation data of Tokyo [11]. Based on adsorption refrigeration, scroll expander is added between
L. Jiang et al. / Renewable Energy 83 (2015) 1250e1259
Nomenclature A c COP E f0 HTS h K L LTS m n P Q R SCP S s t T U I
Pre-exponential factor Specific heat (kJ/(kg. C)) Coefficient of performance Reaction activation energy Difference of reaction equilibrium potential High temperature salt heat transfer coefficient (W/m.K) Arrhenius factor Latent heat of vaporization (J/kg) Low temperature salt Mass flowrate (kg/s) Order of reaction Pressure(Pa) Heating power (kW) Gas constant (J/(mol.K)) Specific cooling power (W/K) Factor related to heat source Heat transfer area(m2) Time (s) Temperature (K) Voltage (V) Current (A)
adsorber and condenser. Bao et al. [12] investigated the chemisorption cooling and electric power cogeneration system driven by low grade heat. However, since scroll expander selected was not suitable for the adsorption system, efficiency of the cogeneration system was not high. After that, Bao et al. [13] established the model of scroll expander for analyzing the ideal operation performance for the adsorption-cogeneration system. Results showed that the cogeneration mode can achieve 1000 W power output for optimal working condition. Based on absorption refrigeration, Goswami proposed a combine power and refrigeration cycle, which was expected to improve energy and exergy efficiency [14]. However, although power generation efficiency is reasonable, this cycle enjoys very poor refrigeration performance since it produces cooling effect through one precooler [15]. Both absorption and adsorption refrigeration technologies take two disadvantages if ammonia is performed as the working fluid, which is safety problem and sensitivity to cooling temperature. Actually, resorption refrigeration can match different reaction equilibrium temperature when different alkali metal halides are employed [16]. Since it utilizes decomposition heat for refrigeration [17], an improved refrigeration performance can be guaranteed [18]. Based on resorption refrigeration, Wang et al. [19] proposed a novel resorption cogeneration cycle for power and refrigeration. Compared with the Goswami cycle, total exergy efficiency for power and refrigeration is 0.56e0.74, which is improved by 10%e40%. Nonetheless, there are less experimental simulation and comparison in field of resorption cogeneration. In this paper, a novel resorption cogeneration system of a cycle for 300 W power maximum and 2 kW refrigeration power is designed. The basic parameters for simulation such as heat and mass transfer performance of composite adsorbents and isentropic efficiency of scroll expander are investigated by concerning experiments. In order to have an overall understanding of the novel system, part of simulation results are compared with experimental results for validation.
W x
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Power output of expander (W) Adsorption quantities (kg/kg)
Greek letters Density of water vapor (kg/m3)
r
Subscripts ads Adsorbent c Condensing cool Cooling water chilled Chilled water de Desorption eq Equilibrium ex Exergy eva Evaporator f Fluid h Heat i Inlet out Outlet ref Refrigeration s Isentropic process tot Total w Wall
2. Establishment of the resorption cogeneration cycle The resorption cogeneration cycle is presented in Fig. 1, which mainly include oil tanks, HTS and LTS adsorption beds, high temperature and low temperature PCM energy storage tanks, expander and other auxiliary components such as valves and pumps. The working processes are as follows: (1) Desorption process of HTS bed for power generation. In this process, V1, V3, V4, AV1, AV2 and Pump 1 are open. HTS bed is heated by oil tank 1 which is simulated as low grade heat with certain temperature. Meanwhile V10 and Pump 4 are open, and LTS bed is cooled by the cooling tank which is simulated as environmental heat sink. Desorbed ammonia of HTS bed through AV2 is expanded in the expander and generates power there. The expanded ammonia will be
Fig. 1. Cogeneration cycles for power and refrigeration.
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adsorbed by LTS bed through AV1. V9, V11 and Pump 3 are open. Phase change process of low temperature PCM will provide the cooling power, which is transported to the onsite by the thermal fluid of ethanol solution and consumed by chilled tank. Cooling power can be transported under a constant level because phase change process happens under a constant temperature. (2) Cold releasing process of low temperature PCM. In this process, V9, V11 and Pump 3 are open. Phase change of low temperature PCM happens and provides the cooling power, which stores refrigeration from the last cycle. The cooling power is transport through the ethanol solution and consumed by chilled tank. The process accompanies with the power generation process for continuous refrigeration for the resorption cogeneration system. (3) Desorption process of LTS bed for refrigeration. AV1, AV2, V10 close and V6, V7, V8, V9, V12, AV3, Pump 2 and Pump 3 are open. HTS bed is cooled by oil tank 2 that is simulated as environmental heat sink, and it will adsorb ammonia from LTS bed when its pressure is lower than that of LTS bed. Desorption process of LTS bed generates the refrigeration and consumed by the chilled tank. The redundant cooling power will be stored in the low temperature energy storage tank through low temperature PCM. The process accompanies with the refrigeration process of LTS bed for the resorption cogeneration system. (4) Heat storage process of high temperature PCM. After the desorption process, V1, V4 close and V2, V5 open, oil tank 1 will be connected with high temperature PCM energy storage tank, and redundant energy will be stored in the energy storage tank by phase change process of high temperature PCM. Actually there are two different modes of energy storage processes can be used in this system. One is energy stored in the high temperature and low temperature PCM tanks. High temperature PCM is to recover the redundant heat for improving system efficiency. Low temperature PCM is to maintain the continuous refrigeration output. The other mode is the cooling capacity stored in the reactor of LTS if refrigerant valve is closed after the desorption process of LTS by the adsorption/desorption capacity of these two beds. 3. Thermal conductivity and permeability of adsorbent 3.1. Thermal conductivity Thermal conductivity is investigated by the Laser flash measuring method, and the type of instrument was LFA447 produced by Netzsch Company. Development of composite adsorbent and measuring process is according to the reference [20]. Table 1 shows thermal conductivity of consolidated composite CaCl2 and MnCl2 for different density and mass ratio of salts. Results indicate that thermal conductivity increases while mass ratio of salts decreases and density increases. For different composite CaCl2 thermal conductivity ranges from 12.1 to 48.3 W/(m.K). Thermal conductivity of consolidated composite MnCl2 takes the same trend as composite CaCl2 for different density and mass ratio of salts. Thermal conductivity of MnCl2 ranges from 8 to 42.8 W/(m.K). 3.2. Permeability Table 2 shows that permeability of consolidated composite CaCl2 and MnCl2 with different mass ratio of salt and density. Results indicate that for different samples of consolidated composite
CaCl2 permeability ranges from 9.31 1010 to 3.05 1014 m2. Permeability increases when the density decreases and mass ratio of salt increases. Consolidated composite MnCl2 also takes the same trend with permeability. Permeability ranges from 8.02 1011 to 1.95 1014 m2. 4. Investigation on scroll expander Scroll expander is chosen for expansion process of power generation since it is suitable for small type power generation system. Scroll compressor (type ATC0062 C2) is modified as scroll expander and tested. Fig. 2a displays the schematic of test unit, and Fig. 2b displays picture of the system. A piston air compressor first compresses the air. The air is dried and heated by a dryer and heating belt and then air flowrate is measured by a flow meter, after that it expands and generates the power. Two temperature and pressure sensors are set at inlet and outlet of the expander. In the experiments, expander inlet temperature, inlet pressure and outlet pressure are controlled under theoretical calculation conditions of expander of the resorption system. The ideal work output, isentropic efficiency, work to power efficiency as well as the internal efficiency for power generation are calculated and studied. The calculation equations can be referred to reference [21]. Fig. 3a and Fig. 3b show the experimental results. When the pressure of expander inlet varies from 0.6 to 1 MPa, isentropic efficiency and work to power efficiency maintain at about 60% and 80%. 5. Properties of composite PCM Phase change temperature, latent heat of pure nitrate and nitrate/ENG-TSA samples are measured by differential scanning calorimetry (DSC). DSC thermal analysis started from 150 C to 280 C with a heating rate of 5 ºC/min and 280 C to 150 C with a cooling rate of 5 ºC/min. Fig. 4 indicates results of pure nitrate and nitrate/ENG-TSA samples. Phase transition properties of pure nitrate are consistent with values of literature [22]. Phase change temperature of nitrates/ENG-TSA shifts slightly from pure nitrates. Results show that melting temperature of the nitrate/10 wt% ENGTSA and nitrate/20 wt% ENG-TSA shifts from 220.8 C to 221.6 C and 221.9 C, respectively. Freezing temperature of the nitrate/ 10 wt% ENG-TSA and nitrate/20 wt % ENG-TSA shifts from 223.0 C to 223.5 C and 223.6 C, respectively. ENG-TSA, as one additive, inclines to adjust phase change temperature. The main reason is because ENG-TSA provides more micro channels to make PCM and ENG-TSA tighter, thus enhancing the capillary force. 6. Design of the cogeneration system 6.1. System design 3-dimension model of the cogeneration system with resorption refrigeration and power generation is designed as Fig. 5. The cogeneration system is composed of two adsorption beds; each bed is composed of three unit beds. The upper three unit beds are filled by MnCl2 composite adsorbent and compose one adsorption bed for MnCl2 while the other three unit beds are filled with CaCl2 composite adsorbent. Although mass of adsorbent are same as the design of two unit beds, there are two major advantages of this design. One advantage is that the design will avoid becoming pressure vessel and guarantee the system safety. The other advantage is the design of two separated unit beds will reduce metal heat capacity. Two PCM reactors are also fixed at each side of
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Table 1 Thermal conductivity of composite CaCl2 and MnCl2. Density(kg/m3)
Mass ratio of salts
Thermal conductivity of CaCl2 (W/(m.K))
Thermal conductivity of MnCl2 (W/(m.K))
400
50% 67% 75% 80% 83% 50% 67% 75% 80% 83% 50% 67% 75% 80% 83%
31.7 23.3 17.3 14.2 12.1 40.5 27.4 20.8 17.8 14.8 48.3 32.5 27.2 23.0 18.8
28.1 20.2 15.6 11.2 8.0 34.9 24.8 18.6 14.6 11.9 42.8 30.7 25.7 20.4 16.5
450
500
Table 2 Permeability of consolidated composite CaCl2 and MnCl2. Density(kg/m3) 300
400
500
Mass ratio of salts 50% 67% 75% 80% 83% 50% 67% 75% 80% 83% 50% 67% 75% 80% 83%
Permeability of CaCl2 (m2) 7.34 6.34 8.05 9.25 9.31 5.20 4.52 5.20 5.52 7.52 3.05 6.05 1.74 2.54 5.42
14
10 1013 1012 1011 1010 1014 1013 1012 1011 1011 1014 1013 1013 1012 1011
Permeability of MnCl2 (m2) 5.14 7.04 6.44 7.97 8.02 4.20 4.42 4.90 7.52 7.93 1.95 1.85 3.95 3.35 6.21
1014 1013 1012 1012 1011 1014 1013 1012 1011 1011 1014 1013 1012 1011 1011
Fig. 2. Compressed air power generation system (a) Schematic, (b) Photograph.
HTS and LTS adsorption bed. High temperature PCM reactor is located at the lower position of the system while low temperature PCM reactor is located at the higher position. High temperature PCM reactor could collect the excessive heat from heat source. Lower temperature PCM reactor could store redundant cooling power for continuous refrigeration since resorption refrigeration system is characterized as intermittent system. The expansion machine includes the scroll expander, torque meter and generator. Scroll expander is installed between HTS and LTS adsorption bed for generating the power. One torque meter is connected between scroll expander and generator to test the torque. For the better mass transfer performance, five ammonia pipes with small diameter are designed to connect each unit adsorption bed and converged into one ammonia pipe with larger diameter, which leads to uniform
adsorption and desorption process. Actually, one ammonia pipe with larger diameter takes role of connecting the HTS adsorption bed with LTS adsorption bed. 6.2. Simulation Structure of unit tube-shell bed for resorption refrigeration is shown in Fig. 6. In the process of desorption, hot oil goes into HTS adsorption bed via heat flow inlet for heating and flows out from the heat flow outlet. Meanwhile, LTS adsorption bed is cooled by the cooling medium. Ammonia desorbs from HTS adsorption bed and goes to LTS adsorption bed through ammonia pipe. In the process of adsorption, LTS adsorption bed is controlled as evaporation temperature, and HTS adsorption bed is cooled by the
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Fig. 3. Performance of the scroll expander (a) Isentropic efficiency and the ideal work, (b) work to power efficiency and power.
Fig. 6. Schematic of typical tube-shell bed.
Fig. 4. Typical DSC curve of pure nitrate and nitrate/ENG-TSA composite PCMs.
Fig. 7. Equivalent modeling of adsorption bed for simulation.
Fig. 5. 3-dimension model of the cogeneration system.
cooling medium. Ammonia is adsorbed by HTS adsorption bed through LTS adsorption bed, producing the cooling effect. On the basis of working process above, the related modeling is applied for numeral simulation, which is shown in the Fig. 7. Energy and mass conversation are two major concerns of adsorption bed in the adsorption and desorption process. For establishing the equation, mass of adsorbent is equivalent to six adsorption beds and
heat transfer area of six tube-shell beds are equivalent to one tubeshell bed. By utilizing the finite element method, thermodynamic parameter is separated into discrete points for the simulation. That can reflect the influence of heat and mass transfer performance of the materials on the performance of the cogeneration system.
6.3. Mathematic model 6.3.1. The dynamic equation of thermal chemisorption reaction Mathematical model of the cogeneration system is established, considering the variables such as permeability, thermal conductivity, and reactor parameters. Performance of the system is simulated, and main assumptions are as follows:
L. Jiang et al. / Renewable Energy 83 (2015) 1250e1259
(1) Both temperature and pressure are uniform throughout the whole adsorption bed, and the refrigerant is adsorbed uniformly inside the adsorbent. (2) Heat and mass transfer of adsorbent only happen through the radial direction. (3) There is no temperature difference between adsorbent and refrigerant. (4) Temperature variation in radial direction is neglected between heat transfer fluid with tube. (5) Internal efficiency of power generation is based on the testing results. The adsorption kinetic equation is established based on the following equations from 1 to 5. Basic chemical adsorption kinetic model expression:
v¼
dx ¼ KðP; TÞ$f ðxÞ dt
(1)
where x is adsorption quantity, f(x) is related to the variety of the reaction, which can be expressed as:
f ðxÞ ¼ ð1 xÞn
(2)
where n is order of the reaction. K (P, T) is Arrhenius factor based on the Arrhenius theory, which reflects the difference between non-equilibrium and equilibrium working condition, and it can be expressed as:
KðP; TÞ ¼ kf 0 ðP; TÞ
(3)
E k ¼ A$exp RT
(4)
where A is pre-exponential factor and E is reaction activation energy, f0 (P,T) is the difference of reaction equilibrium potential, which is related to the difference between constrained pressure and equilibrium pressure, and its expression is:
m Pc Peq ðTÞ $exp T 273 f 0 ðP; TÞ ¼ To 273 Pc
(5)
where Peq is the equilibrium pressure, which is obtained by Clausius-Clapeyron equation, m is parameter of the reaction
Peq
DHr DS ¼ exp þ R RTc
(6)
where DHr is reaction enthalpy between adsorbent and gas, DS is entropy of chemical reaction, Tc is the constrained external temperature of the adsorbent in the process of chemical reaction.
6.3.2. Establishment of mathematical equations Mathematical model is established based on the physical model as mentioned above. Energy conservation equation is established on the basis of heat transfer of the fluid as well as heat conduction inside composite adsorbent. Dynamic equation of the chemical reaction rate is set up based on adsorption/desorption process. The physical properties changes of the adsorbent in the reaction processes are also considered. Through comprehensive analysis of heat and mass transfer performance, the optimal compound adsorbent is chosen for simulation as the bulk density of 400 kg/m3 and 80% mass ratio of CaCl2. As metal container is cylindrical, two-dimensional heat conduction equations are established as the cylindrical coordinate
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since the anisotropic characteristic of composite adsorbent [23]:
rcp
vT v vT 1 v vT ¼ lz þ rlr þS vt vz vz r vr vr
(7)
where lz and lr are thermal conductivity in the axial and radial direction; S is the factor related to the heat source, which is expressed as:
S¼
nNH3 dx DH V dt
(8)
where nNH3 is mass transfer quantity of the ammonia, V is volume of composite adsorbent，dx/dt is the rate of the adsorption reaction, DH is the enthalpy of the reaction. Heat loss of the system is assumed to be neglected since the bed is wrapped in insulation materials. Therefore, its boundary condition is assumed as the adiabatic boundary. The third category boundary is applied because boundary inside exchange heat with heat fluid:
lr
vT ¼ 0 r ¼ ±rw vr
(9)
lz
vT ¼ 0 z ¼ 0; zh vr
(10)
lr
vT ¼ h T Tf r ¼ ±ri vr
(11)
Heat transfer fluid exchanges the heat with metal wall from top to bottom. The energy conservation equation of the fluid is as follow:
rf cpf
vTf vTf v2 Tf þ rf ucpf ¼ lf 2 þ Sf vt vz vz
(12)
where lf is thermal conductivity of heat exchange fluid, u is velocity of heat transfer flow, Sf is the heat exchanged between the flow and the metal shell, which can be expressed as.
Sf ¼ s$h$ T Tf
(13)
where s is heat transfer area of unit control body, Tf is the temperature of heat transfer fluid, T is metal wall temperature of adsorption bed. when inlet temperature of heat exchange flow is constant, outlet temperature gradient tends to zero, so the boundary condition is:
Tf ¼ constant z ¼ 0
(14)
vTf ¼ 0 z ¼ zh vz
(15)
The adsorption kinetic equation is established from Eqs. (1)e(5). According to experimental results in Ref. [24], suitable parameters of composite adsorbent CaCl2 are able to be provided for the adsorption process to simulate chemical reaction. The adsorption kinetic equation can be expressed as follows:
Pc Peq ðTÞ T 273 4 dx E $ ¼ A$ð1 xÞ2 $exp $ T 273 dt RT Pc c
(16)
where x is the adsorption quantity, if x is defined as the quantity of desorption, then use (1-x) instead of x. Data of the parameters of equation (15) are listed in Table 3.
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Table 3 Parameters of the chemical reaction rate equation.
Desorption Adsorption
Table 4 Parameters for calculation of tube shell bed.
A
E/J mol1
Parameter
Value
0.0005 0.00204
2385 1383.24
Inner radius of adsorbent Outside radius of adsorbent Overall height of adsorbent Volume of adsorbent Flowrate of heat transfer fluid Thermal conductivity of heat transfer fluid Heat transfer coefficient before adsorption Heat transfer coefficient after adsorption Heat transfer area Radius of heat transfer fluid Mass transfer quantity of ammonia Mass ratio of salt Reaction enthalpy of MnCl2 Reaction entropy of MnCl2 Reaction enthalpy of CaCl2 Reaction entropy of CaCl2
12.5 mm 23.5 mm 1.02 m 1.24 L 0.2 L/min 0.54 W/m.k 150 W/m2.k 550 W/m2.k 0.04 m2 12.5 mm 4 mol/mol 4:1 47416 J/mol 227.9 J/(mol$K) 42269 J/mol 229.7 J/(mol$K)
The physical properties will change accordingly due to the adsorption quantity for adsorbent in the process of adsorption. After testing, adsorption and desorption reaction has little influence on the permeability of composite adsorbent, so the constant permeability is applied in the simulation. In contrast, the adsorption and desorption reactions have larger influence on the thermal conductivity of composite adsorbent. Thermal conductivity are considered as linear change according to the reference [24], and then average thermal conductivity lz and lr for radical and axial direction in the reaction process can be calculated. Specific heat capacity and thermal conductivity of the adsorbent can be fitted by the following equation:
mg m xM2 m ð1 xÞM1 þ cp2 salt þ cp1 salt V V V
rcp ¼ cpg
l ¼ l2 x þ l1 ð1 xÞ
(17) (18)
where x is the reaction degree in adsorption process, subscripts 1 and 2 are represented the parameter before and after the adsorption process. The heating power of the system：
Z
tcycle
0
Qh ¼ ¼
cw $mw $ THTS;in TLTS;out dt tcycle
(19)
where tcycle is the cycle time (s), cw is specific heat of water (J/ (kg.K)), THTS,in and TLTS,out are the inlet and outlet temperature of hot water (K). Cooling power of the system:
Z Qref ¼
tcycle 0
cw $mw $ TLTS;in TLTS;out dt tcycle
(20)
7. Simulation results 7.1. Temperature Fig. 8 shows the experimental and simulation results of temperature variation of LTS adsorbent under the condition of different evaporation temperature, which ranges from 10 C to 10 C. For simulation, results shows when evaporation temperature is lower than 0 C, temperature of LTS adsorbent decreases in few minutes at the beginning and then increases gradually. When evaporation temperature is higher than or equals to 0 C, temperature of LTS adsorbent decreases even faster. For evaporation temperature as high as 10 C, temperature of LTS adsorbent takes minimum 3.56 min to reach the lowest value. Comparably, for evaporation temperature as low as 10 C, temperature of LTS adsorbent takes maximum 4.4 min to reach the lowest. The decline of evaporation temperature of LTS adsorbent becomes sharper when the evaporation temperature increases from 10 to 10 C. Comparing simulation data with experiments, it indicates that simulation data agree with experimental data well with increment of evaporation temperature. The smallest temperature difference is about 0.43 C for evaporation temperature of 10 C while the largest temperature difference is about 2.75 C. This is mainly because properties such as thermal conductivity and permeability are tested for
where mw,eva is the mas flowrate of chilled water in the evaporator (kg/s); TLTS,in and TLTS,out are the temperature of chilled water inlet and outlet (K). COP of the system is:
COP ¼
Qref Qh
(21)
where Qref is the cooling amount, Qh is the heating amount for the system. The SCP per kilogram adsorbent can be expressed as:
SCP ¼
Wref mtot
(22)
where mtot is total mass of composite adsorbent (kg), i.e. both the mass of chlorides and expanded natural graphite are considered. The whole controlled body is simulated by the internal nodal method. The finite element analysis is performed in a cylindrical coordinate, and software MATLAB is used for simulation. All the parameters used in the mathematic model are listed in Table 4.
Fig. 8. Temperature variation of LTS adsorbent.
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environmental temperature. When temperature is lower than 0 C, permeability will decrease and mass transfer become poor in the adsorption process, which leads to the slower reaction rate and smaller temperature decline. 7.2. Dynamic adsorption characteristic Fig. 9 shows desorption and adsorption rate change of LTS adsorbent under the condition of 130 C heat source temperature, 30 C cooling temperature and 0 C evaporation temperature. Results indicate that it takes about 20 min to finish the desorption process. However, it takes 5 min to finish about 60% of the whole adsorption process, which is the main reason for the phenomenon of Fig. 8. Global conversion for desorption changes from 1 to 0.4. Simultaneously, global conversion for adsorption changes from 0.4 to 1. It also demonstrates adsorption process is a little faster than the desorption process, which takes about 4.8 min to finish 60% of the desorption process. The reason is that the adsorption process of LTS enjoys larger temperature differences. Temperature differences cause the higher driven pressure to precede the process.
Fig. 10. COP and SCP changing with the cycle time.
7.3. Refrigeration performance Fig. 10 shows COP and SCP change with cycle time under the condition of 130 C heat source temperature, 30 C cooling temperature and 0 C. Results demonstrate that both COP and SCP increase rapidly in the first five minutes then decrease slowly. The maximum COP is about 0.31 which happens in 3.8 min. Simultaneously, the maximum SCP is about 161 W/kg in 3.6 min. Fig. 11 shows the trends of COP and SCP when evaporation temperature changes from 10 C to 10 C. For simulation, results demonstrate that both COP and SCP increase with the increment of evaporation temperature. For the evaporation temperature from 10 C to 10 C, COP ranges from 0.22 to 0.256. Simultaneously, SCP ranges from 126 W/kg to 143.7 W/kg. It indicates that the difference between simulation and experimental data fit well when refrigeration temperature is above 0 C, which is consistent with the results of temperature in Fig. 8. The largest gap between simulation and experiment for COP and SCP are 0.02 and 5.9 W/kg while the smallest gap are 0.002 and 0.27 W/kg, respectively. 7.4. Power generation performance In the simulation of the cogeneration system, desorbed ammonia flow is held on for a while with ammonia valve AV2
Fig. 9. The desorption and adsorption rate of LTS.
Fig. 11. COP and SCP vary with evaporation temperature.
closed and then releasing it at 10th minute in the desorption process. That leads to the flow rate behaving like pulses lasted for a very short time as shown in Fig. 12, which depicts both power generation and refrigeration for the cogeneration system. Results also indicate the maximum power output is nearly 375 W at
Fig. 12. Power generation and cooling power vary with cycle time.
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Fig. 13. Temperature of high temperature PCM for different working process, (a) Heat charging process; (b) Heat discharging process.
Fig. 14. Temperature of low temperature PCM for different working process, (a) Cold storage process; (b) Cold releasing process.
10.16 th minute in desorption process while the maximum refrigeration power is 2.24 kW at 3rd minute in adsorption process. Power generation happens in every 30 min while the cooling power lasts for 26.5 min.
process takes 80 min and temperature difference between enthanol solution inlet and outlet is 7.8 C. Fig. 14b indicates temperature change of low temperature PCM in cold releasing process. It can be displayed that PCM is heated from 11 to 18 C in the cold releasing process. Cold releasing process takes 91 min and the temperature difference between oil inlet and outlet is 4.1 C.
7.5. Performance of energy storage Fig. 13 shows temperature variation of high temperature PCM for different working process. Fig. 13a indicates temperature variation of high temperature PCM in heat charging process. It can be seen that high temperature PCM is heated from 213 C to 232 C in heat charging process. It includes heating process from 213 C to 223 C, melting process at 221 C, and heating process from 221 C to 232 C. The maximum temperature difference is 11 C. The heat charging process takes 73 min and temperature difference between oil inlet and outlet is 8.1 C. Fig. 13b indicates temperature variation of high temperature PCM in heat discharging process. It can be seen that PCM is cooled from 232 to 212 C in the heat charging process. The maximum temperature is 10 C. Heat discharging process takes 91 min and temperature difference between oil inlet and outlet is 5.1 C. Fig. 14 shows temperature variation of low temperature PCM for different working process. Fig. 14a indicates temperature variation of low temperature PCM in cold storage process. It can be indicated that low temperature PCM is heated from 18 to 12.5 C in cold storage process. It includes cooling process from 18 to 15 C, melting process at 15 C, and cooling process from 15 to 12.5 C. The maximum temperature difference is 5.5 C. The cold storage
7.6. The analysis on the cascading cycle Total energy efficiency and exergy efficiency of the cascading cycle can be calculated as follows:
htotal:energy ¼
htotal;exergy ¼
Qref þ W Qh Eex;Qref þ W EORC þ Eex;Qh
(23)
(24)
Fig. 15 shows energy efficiency and exergy efficiency varies with evaporating temperature. When heat source temperature is 130 C and evaporating temperature varies from 10 C to 20 C, total energy efficiency increases from 0.316 to 0.376 as well as exergy efficiency decreases from 0.402 to 0.391. 8. Conclusions A novel cogeneration cycle of power and refrigeration with energy storage was presented and the concerning system was
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11160706000, Program for New Century Excellent Talents in University by the Ministry of Education, China (NCET-11-0333).
References
Fig. 15. Total energy efficiency and exergy efficiency vs. evaporating temperature.
designed and predicted. Heat and mass transfer properties of composite adsorbent are tested to predict the refrigeration performance. Scroll expander is also investigated by the compressed air testing units to simulate power generation performance. Properties of PCM are tested by DSC for thermal storage. Conclusions were yielded as follows: For different CaCl2 composite adsorbent, thermal conductivity ranges from 12.1 to 48.3 W/(m.K). Thermal conductivity of MnCl2 composite adsorbent ranges from 8 to 42.8 W/(m.K), which takes the same trend as the composite CaCl2 for different density and mass ratio of salts. Permeability for different samples of CaCl2 and MnCl2 composite adsorbent range from 9.31 1010 to 3.05 1014 m2 and from 8.02 1011 to 1.95 1014 m2,respectively. Compressed air is used to evaluate the performance of the scroll expander. When the pressure of expander inlet varies from 0.6 to 1 MPa, isentropic efficiency and work to power efficiency maintain at about 60% and 80%. Internal efficiency for power generation is 53% when pressure of expander inlet is 1 MPa. Phase change temperature of nitrates/ENG-TSA shifts slightly from pure nitrates. Melting temperature of the nitrate/10 wt% ENGTSA composite and nitrate/20 wt% ENG-TSA shift from 220.8 C to 221.6 C and 221.9 C, respectively. Freezing temperature of the nitrate/10 wt% ENG-TSA and nitrate/20 wt % ENG-TSA shift from 223.0 C to 223.5 C and 223.6 C, respectively. When heat source temperature is 130 C and the evaporating temperature varies from 10 C to 20 C the total energy efficiency of the cogeneration system increases from 0.316 to 0.376 and exergy efficiency decreases from 0.402 to 0.391. Acknowledgments This research was supported by the National Science Foundation of China for Excellent Young Scholars under contract number 51222601, Foundation of Science and Technology Commission of Shanghai Municipality, P.R. China, under contract number
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