Study on heat transfer and cooling performance of copper foams cured MIL-101 adsorption unit tube

Journal Pre-proof Study on heat transfer and cooling performance of copper foams cured MIL-101 adsorption unit tube Zhou Xu, Yu Yin, Junpeng Shao, Yerong Liu, Lin Zhang, Qun Cui, Haiyan Wang PII:

S0360-5442(19)31997-8

DOI:

https://doi.org/10.1016/j.energy.2019.116302

Reference:

EGY 116302

To appear in:

Energy

Received Date: 10 May 2019 Revised Date:

19 August 2019

Accepted Date: 7 October 2019

Please cite this article as: Xu Z, Yin Y, Shao J, Liu Y, Zhang L, Cui Q, Wang H, Study on heat transfer and cooling performance of copper foams cured MIL-101 adsorption unit tube, Energy (2019), doi: https://doi.org/10.1016/j.energy.2019.116302. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Study on heat transfer and cooling performance of copper foams cured MIL-101 adsorption unit tube Zhou Xu†, Yu Yin†, Junpeng Shao, Yerong Liu, Lin Zhang, Qun Cui*, Haiyan Wang** College of Chemical Engineering, Nanjing Tech University, Xinmofan Road No.5, Nanjing, China, 210009 E-mail addresses: *[email protected] **

[email protected] m

refrigerant mass [kg]

T

temperature [°C]

Nomenclature

SCP

specific cooling power [W/kg]

t

time [s]

COP

coefficient of performance

Φ

external diameter [mm]

VCP

volumetric cooling power [kW/m3]

Ψ

unit volume of adsorbents [kg/m3]

ACS

adsorption cooling system Subscripts

CF

copper foams

CFCM

copper foams cured MIL-101

a

adsorption or adsorbent

Qa

adsorption heat [kJ/kg]

d

desorption

Qc

refrigerating capacity [kJ/kg]

ev

evaporation

∆He

latent heat of refrigerant [kJ/kg]

con

condenser

φa

volume fraction of MIL-101 [%]

hw

hot water

M

mass [kg]

cw

cold water

V

volume [m3]

b

bed

0. Abstract A composite copper foams cured MIL-101 (CFCM) prepared by a binderless dip-coating method was exploited. A CFCM-95 adsorption unit tube was developed and the heat transfer performance of CFCM-95 adsorber was studied. Parameters optimization of CFCM-95/isobutane working pair were determined. Results show that MIL-101 is uniformly cured in the three-dimensional dense pores of copper foams, and the maximum thermal conductivity of CFCM reaches 0.86 W/(m·K), which is 14 times higher than that of MIL-101 powder. The fluctuating heating time of unit volume of adsorbents in CFCM-95 adsorber (from 30 °C to 50 °C) is 1.56 s·m3/kg, which is half that of MIL-101 fixed bed adsorber. Moreover, the cooling rate of the CFCM-95 adsorber (from 62 °C to 30 °C) is 1.1 °C/s, which is 3.5 times faster than that of MIL-101 fixed bed adsorber. The cooling capacity, specific cooling power (SCP) and volumetric cooling power (VCP) is 1.8, 2.6 and 4 times that of the adsorption cooling system (ACS) with MIL-101/isobutane working pair, respectively, and among them, VCP is up to 4.442 kW/m3. These results are of great significance for reducing the volume of adsorption chillers and promoting the applications of adsorption cooling technology.

Keywords: Adsorption cooling; Copper foams cured MIL-101; Heat transfer; Adsorption unit tube; Isobutane

1. Introduction Adsorption cooling technology [1, 2] has the advantages of using environmental refrigerants and directly utilizing industrial waste heat or solar energy [3-6]. However, it still faces challenges of low specific cooling power (SCP), inferior heat transfer of adsorbents and large equipment volume in the process of commercializing [7]. In recent years, the researchers have made significant progress in the development of efficient working pairs [8, 9], heat and mass transfer enhancement of the adsorbers [10] and optimizations of the adsorption cooling system (ACS) [11]. Metal-organic frameworks (MOFs) meet a great feasibility of applications in adsorption chillers due to their rich variety of topologies and decoration with functional moieties. Henninger [12] found that the circulating adsorption capacity of ISE-1 for water was 0.21 kg/kg at the evaporation temperature of 10 °C, condenser temperature of 30 °C and desorption temperature of 140 °C. Then, several promising MOFs like MIL-101 [13], CAU-10-H [14], CuBTC [15], MOF-801 [16] and MOF-74 [17] had been widely studied for the potential applications for adsorption cooling technology. Among these MOFs, MIL-101 has advantages of large adsorption capacity and low regeneration temperature. Henninger [18] measured that the adsorption capacity of MIL-101 for water was 1 kg/kg at desorption temperature of 140 °C. Saha [19] determined that the equilibrium adsorption capacity of ethanol on MIL-101 was 1.1 kg/kg at adsorption temperature of 30 °C. Solovyeva [20] measured that adsorption capacity of MIL-101 for methanol was 0.27-0.31 kg/kg at desorption temperature of 80-90 °C. The author’s group [21, 22] studied the adsorption cooling

performance of MIL-101 (shaped MIL-101 pellets at 3 MPa) with water and ethanol working pair, and results showed that adsorption capacity of water and ethanol on MIL-101 were 0.95 kg/kg and 0.74 kg/kg at 25 °C, respectively. The cooling capacity of the ACS with MIL-101/water working pair was 1059 kJ/kg at the evaporation temperature of 10 °C, which was 2.24 times that of silica/water working pair. When the desorption temperature was 80 °C, the cooling capacity of the ACS with MIL-101/ethanol working pair was 2.2 times that of activated carbon/ethanol working pair. However, the above ACS with negative-pressure working pairs need to keep high vacuum [23]. To improve the cycle operation stability of the ACS, the author’s group [24] constructed a positive-pressure adsorption chiller with MIL-101/isobutane working pair. Results showed that the adsorption capacity of MIL-101 for isobutane was up to 0.455 kg/ kg at 30 °C, and the cooling capacity of the ACS was 45.7 kJ/kg at the hot water temperature of 95 °C, adsorption temperature of 30 °C and desorption time of 30 min. Pinto [25] developed an adsorption chiller with activated carbon/R134a working pair with the SCP of 430 W at desorption temperature of 80 °C. Simulated ACS with working pairs comprised of activated carbon (Maxsorb III) and a refrigerant like propane, n-butane, HFC-134a, R-32 or R507a was reported by Ismail [26], and results showed that propane is preferred at higher ambient temperature. Nevertheless, these positive-pressure systems suffered from poor heat transfer performance. In order to enhance the heat transfer performance of the adsorber, Henninger

[14, 27, 28] studied SAPO-34, MIL-101, HKUST-1 and CAU-10-H coating, in which the CAU-10-H coating was applied to a full-scale finned flat-tube exchanger with the SCP of 1.369 W/g. Freni [29] tested the fin-tube heat exchanger coated with a compact layer of SWS-1L (CaCl2 in mesoporous silica gel). Results showed that the SCP of SWS-1L/water working pair was up to 150-200 W/kg. Bu [30] showed that the optimal SCP reached up to 801.7 W/kg when the expanded graphite content in the wood chips-CaCl2 was 30 wt% and the adsorption time was 10 min. Aziz [31] found that the addition of small-sized aluminum sheets in the activated carbon fixed bed could simultaneously enhance the heat transfer and adsorption rate of water. The COP and SCP of ACS with activated carbon/water working pair increase by 13.5% and 92.8%, respectively. Bahrami [32] indicated that the content of flake graphite in the CaCl2-silica gel adsorbents was up to 15% and the equilibrium adsorption time on CaCl2-silica gel/flake graphite composite for water reduced from 70 min to 30-50 min, while the corresponding SCP of CaCl2-silica gel/flake graphite composite/water working pair increased by 67%. Hu [33] prepared a composite zeolite/foam aluminum by percolation method, and the thermal conductivity of the composite was up to 2.89 W/(m·K), which was about 30 times more than that of zeolite particles. In addition, the SCP of the simulated adsorption chiller reached 158.6 W/kg, which was 1.8 times that of the zeolite particle adsorber. Freni [34] directly sintered highly porous copper foams on the external surface of copper pipes, and coated the foams with several layers of zeolite 4A through in-situ hydrothermal synthesis. Simulation results showed that the volumetric cooling power (VCP) was up to 103-214 kW/m3.

From above, most studies on the applications of MOFs for adsorption refrigeration process focus on the determination of adsorption equilibrium and the simulation of adsorption refrigeration performance. Experimental studies on the thermal enhancement of MOFs, the structures of adsorber and adsorption refrigeration system are rarely reported, and no literature on copper foams cured MIL-101 have been reported. In this work, a novel composite copper foams cured MIL-101 (CFCM) was prepared by a binderless dip-coating method for enhancing thermal conductivity of MIL-101. Thermal conductivities of the CFCM with different porosities had been measured through hot disk method. A CFCM-95 adsorption unit tube was designed and a positive-pressure adsorption cooling system (ACS) with CFCM-95/isobutane working pair was built. The heat transfer and adsorption cooling performance of the ACS with CFCM-95 adsorber and MIL-101 fixed bed adsorber were compared. It provides the basic data for the development and application of adsorption cooling technology.

2. Experimental 2.1 Preparation method of CFCM Copper foams (China YiYang Foametal New Material Co., Ltd) with porosity of 92%, 95% and 98% were named as CF-92, CF-95 and CF-98, respectively. MIL-101 powder was scale up synthesized by a hydrothermal method without HF [24]. Preparation of CFCM adsorbents was as follows. The suspension was prepared by dispersing the powderous MIL-101 (<120 mesh) in deionized water with a mass ratio of 1:3 and stirring at 500 r/min for 30 min. Prior to coating, copper foam billets

were washed by trichloroethylene (Shanghai Shenbo Chemical Co., Ltd, ≥99 wt%) in the ultrasonic bath for 5 min and rinsed with deionized water for 3~4 times, followed by a treatment in ethanol (Wuxi Yasheng Chemical Co., Ltd, ≥99.7 wt%) for 5 min in the ultrasonic bath and dried at 60 °C. Pretreated copper foam billets including CF-92, CF-95 and CF-98 were immersed in the suspension of MIL-101 for 60 s, then the composites were dried at 60 °C for 6 h and programmed up to 120 °C for 3 h. The composites prepared from CF-92, CF-95 and CF-98 were named as CFCM-92, CFCM-95 and CFCM-98, respectively.

2.2 Thermal conductivity of CFCM CF-92, CF-95 and CF-98 with a dimension of 30 mm×30 mm×9 mm were chosen to the determination of thermal conductivity. Thermal conductivities of CFCM-92, CFCM-95 and CFCM-98 were tested by a thermal constants analyzer (TPS 2500S, Sweden Uppsala Hot Disk Co., Ltd.) at 25 °C in accordance with ISO22007-2-2015 (relative error of 3%).

2.3 Morphology analysis of CFCM CFCM-95 with a dimension of 4 mm × 4 mm × 2 mm was examined by using a field emission scanning electron microscope (TM3000, Hitachi). The preparation method of CFCM-95 was presented in 2.1.

3 Results and discussion 3.1 Preparation and thermal conductivity of CFCM 3.1.1 Images of CFCM Copper foams with the porosity of 92%, 95 and 98% were impregnated by

MIL-101 by the binderless dip-coating method, optical images of CF-92, CF-95, CF-98, CFCM-92, CFCM-95 and CFCM-98 are shown in Fig. 1 (a)-(f).

Fig. 1 Photographs of (a) CF-92, (b) CF-95, (c) CF-98, (d) CFCM-92, (e) CFCM-95 and (f) CFCM-98.

From Fig. 1 (d)-(f), the pore volume of copper foams is fully used to cure MIL-101 and the volume fraction of MIL-101 is significantly improved. The volume fraction of MIL-101 in CFCM-95 and CFCM-98 exceeds 90%, which is higher than that in the zeolite/aluminum foam composite prepared by the percolation method (about 80%) reported in the literature [33]. The preparation process of CFCM in this work is simple and operational. Without a binder, MIL-101 powder can be uniformly and fully deposited in the three-dimensional dense pores of copper foams within 60 s. 3.1.2 Morphology analysis SEM images of CF-95 and CFCM-95 are shown in Fig. 2 (a)-(d). It can be seen from Fig. 2 (a) that CF-95 is a three-dimensional porous material with a pore size of 0.8~1 mm. Fig. 4 (b)-(d) are SEM images of CFCM-95 with magnification of 50, 200 and 500 times, respectively. From Fig. 4 (b)-(d), the three-dimensional mesh cavities of CF-95 are filled with MIL-101, and MIL-101 is tightly contacted with the skeletons

of copper foams, which can effectively reduce the contact thermal resistance and strengthen the heat transfer between MIL-101 and copper foams. Results show that MIL-101 powder can be uniformly cured in the three-dimensional dense pores of copper foams without binders.

(a)

(b)

(c) (d) Fig. 2 SEM images of (a) CF-95 and (b-d) CFCM-95.

3.1.3 Thermal conductivity The three-dimensional skeleton structure of the CF-92/CF-95/CF-98 promotes the formation of three-dimensional heat conduction network in the MIL-101 matrix, provides a better heat conduction path for heat transfer of MIL-101. Thermal conductivities of CFCM-92, CFCM-95 and CFCM-98 are shown in Table 1. The volume fraction of MIL-101 in the CFCM is evaluated as equation (1).

ϕa =

Ma × 100% VCFCM ⋅ ρb

(1)

Where φa is the volume fraction of MIL-101, %; Ma is the mass of MIL-101, kg; VCFCM is the volume of the CFCM, m3; ρb is the bulk density of MIL-101, 0.28 g/cm3. Table 1 Thermal conductivities of CFCM-92, CFCM-95 and CFCM-98 sample

Thermal conductivity (W/(m·K))

φa (%)

CFCM-92

0.86

76

CFCM-95

0.73

93

CFCM-98

0.38

95

In Table 1, thermal conductivities of CFCM-92, CFCM-95 and CFCM-98 are 0.86 W/(m·K), 0.73 W/(m·K) and 0.38 W/(m·K), respectively, in which the thermal conductivity of CFCM-92 is nearly 14 times higher than that of MIL-101 powder (0.06 W/(m·K)). As the porosity of copper foams increases, the thermal conductivity of CFCM decreases because the copper content is reduced, but the corresponding volume fraction of MIL-101 increases. When the porosity of copper foams rises from 92% to 95%, the volume fraction of cured MIL-101 increases by 22%, and the thermal conductivity decreases by 15%. The thermal conductivity is significantly degraded with further increase of porosity to 98%. For adsorption cooling process, the amounts of MIL-101 should be increased as much as possible while improving the thermal conductivity of the CFCM composite. Therefore, the CFCM-95 are selected to the subsequent ACS.

3.2 Development of CFCM-95 adsorber and construction of ACS 3.2.1 CFCM-95 adsorption unit tube A certain size of CF-95 is selected and wrapped around the outer wall of a heat exchange copper pipe with leaving a certain gap by a thermal conducting adhesive (Shenzhen Junye Adhesive Technology Company), and the "unit tube" of the

CFCM-95 adsorber is shown in Fig. 3 (a). MIL-101 is cured in the three-dimensional network pores of CF-95 on the unit tube by the dip-coating method proposed in 2.1, and the structure of the CFCM-95 adsorption unit tube is shown in Fig. 3 (b). CFCM-95 outside the unit tube can be heated or cooled by hot media or cold media inside the unit tube. Different materials (copper, aluminum or stainless steel) and various sizes of metal tubes may be selected to prepare adsorption unit tubes. The CFCM-95 adsorber can be assembled with multiple CFCM adsorption unit tubes.

(a) (b) Fig. 3 Structure diagram of the CFCM-95 adsorption unit tube: (a) before curing and (b) after curing.

3.2.2 System description An absorber of the CFCM-95 adsorption unit tube (Referred to as the CFCM-95 adsorber) was presented in section 3.2.1, a positive-pressure adsorption cooling system with CFCM-95/isobutane working pair is shown in Fig. 4. Seen from Fig. 4, the system mainly consists of an adsorber, a condenser, an evaporator, two water tanks and a level gauge (0-300 mm, accuracy of 0.1mm). Temperatures in the ACS were measured by Pt 100-type thermistors (-30~200 °C, accuracy of 0.1 °C). The

Monitor and Control Generated System (MCGS) can accurately capture the required temperature and pressure (CYB-15 pressure transducer, 0~1500 kPa, accuracy of ~1 kPa) of the evaporator, adsorber and condenser.

Fig. 4 Schematic diagram of the positive-pressure adsorption cooling system. VP-vacuum pump, A-adsorber, C-condenser; E-evaporator, L-liquidometer, FM-flowmeter, T-temperature sensor, P-pressure sensor, Number 1-10-ball valves V1-V10, Chin-chilled ethanol inlet, Chout-chilled ethanol outlet.

The experimental procedures in this work are the same as reported in the literature [24]. Experimental procedures of the positive-pressure adsorption cooling system with CFCM-95/isobutane working pair are as follows. (1) Adsorption process. Open V4 and V5, and the adsorberof the CFCM-95 adsorption unit tube is circulated by cold water. When the temperature drops from desorption temperature (Tdes) to adsorption temperature (Tads) as well as the pressure of the adsorber being reduced from condensation pressure (Pcon) to evaporation pressure (Pev), open V7 between the adsorber and the evaporator. This process can be seen as an isometric adsorption process.

(2) Desorption process. When the adsorption process is over, close the valves V4, V5 and V7. Open V2 and V3, and the adsorber is heated by hot water as well as the circulating water of the condenser is turned on. When the system pressure is from Pev to Pcon, open V6 and the isobutane is then condensed in the condenser. After the desorption process is completed, close V2, V3, V6 and open V4. The liquid of isobutane enters into the evaporator under differential pressure. Repeat the steps (1) and (2). Cyclic capacity of isobutane is stable after 3-4 cycles. Ignoring the heat loss and variation of sensible heat, Qc, SCP and VCP of the ACS are calculated by the equation (2), (3) and (4). Qc = ∆m d × ∆H e / M a

(2)

SCP = Qc / tcyc

(3)

VCP = SCP × M a / Vb

(4)

Where Qc is the cooling capacity, kJ/kg; ∆md is the amount of desorbed isobutane, kg; ∆He is the latent heat of vaporization of isobutane at the evaporation temperature, kJ/kg; Ma is the mass of MIL-101, kg; SCP is the specific cooling power, W/kg; tcycle is the cycle time, s; VCP is the volumetric cooling power, kW/m3; Vb is the volume of the adsorber, m3. The main uncertainty of the calculated Qc, SCP and VCP comes from the liquidometer (shown in Fig. 4). Refer to the literature [32], relative error of the results in this work was calculated as follows,

∆y = y

2

 ∂y   ∆vi     ∑  i =1  ∂vi   y  k

2

(5)

Where y is a result calculated from the measured data, y means (v1, v2, …, vk), vi is measured data, ∆ vi represents the measurement error calculated based on precisions of sensors and meters.

3.3 System study Heat transfer performance of the CFCM-95 adsorber was evaluated by determining the temperature variation during the desorption/adsorption processes in the positive-pressure adsorption chiller. Moreover, parameters optimization and cycle stability of the ACS with CFCM-95/isobutane working pair were studied. 3.3.1 Heating time of unit tube adsorber Fluctuating heating process of the CFCM-95 adsorber and MIL-101 fixed bed adsorber [24] was compared at hot water temperature of 85 °C with a flow rate of 20 L/h, as shown in Fig. 5. Seen from Fig. 5, the heating rate of the adsorber in relatively low temperature ranging from 30 °C to 50 °C is affected by the mass ratio of adsorber and MIL-101. It takes 113 s for the CFCM-95 adsorber to rise from 30 °C to 50 °C, and the heating time of unit volume of adsorbents in the CFCM-95 adsorber (adsorber volume of 0.69 L and MIL-101 mass of 50 g) is 1.56 s·m3/kg, which is about half that of the MIL-101 fixed bed (adsorber volume of 0.64 L and MIL-101 mass of 30 g), indicating that the structure of CFCM-95 unit tube can efficiently improve the heat transfer performance of the adsorber.

Temperature of adsorber(

)

55

50

45

40

35

CFCM-95 adsorber MIL-101 fixed bed adsorber

30 0

20

40

60

80

100

120

140

160

Desorption time(s)

Fig. 5 Fluctuating heating curves of the CFCM-95 adsorber and MIL-101 fixed bed adsorber from 30 °C to 50 °C (desorption process).

3.3.2 Cooling process of unit tube adsorber Water temperature variation at the outlet of CFCM-95 adsorber during the cooling and adsorption process was compared with the MIL-101 fixed bed adsorber at cooling water temperature of 22-24 °C with a flow rate of 50 L/h, as shown in Fig. 6. It can be seen from Fig. 6 that the cooling water temperature at the outlet of two adsorbers has the same trend via time, including the process of rapid cooling, slight increase and then decrease. Hot CFCM-95 adsorber, which has just completed the desorption process, are cooled and the water temperature at the outlet drops from 83 °C to 45 °C within 20 s with a cooling rate of 1.9 °C/s. When CFCM-95 adsorber drops to a lower temperature, MIL-101 in the CFCM-95 adsorber begins to adsorb adsorbates and produces greater adsorption heat, which leads to water temperature at the outlet rising from 45 °C to a peak value of 62 °C within 10 s (from 20 s to 30 s). As the adsorption process progressing, the decrease of adsorption rate and adsorption capacity results in the decrease of adsorption heat (Qa). Cooling water temperature at the outlet of the CFCM-95 adsorber drops from 62 °C to 30 °C within 30 s with a

cooling rate of 1.1 °C/s. Water temperature at the outlet of the MIL-101 fixed bed adsorber in the cooling process is reduced from 83 °C to 53 °C within 22 s with a cooling rate of 1.5 °C/s. In its adsorption process, water temperature at the outlet rise from 53 °C to a peak value of 57 °C within 10 s (from 22 s to 32 s), then drops from 57 °C to 30 °C within 88 s (from 32 s to 120 s) with a cooling rate of 0.3 °C/s. Temperature of adsorber outlet(? )

90 CFCM-95 adsorber MIL-101 fixed bed adsorber

80 70 60 50 40 30 20 0

20

40

60

80

100 120 140

160

180 200

Time(s)

Fig. 6 Variation curves of cooling water temperature at the outlet of CFCM-95 adsorber and MIL-101 fixed bed adsorber (adsorption process).

Seen from the cooling rates of two adsorbers, the CFCM-95 adsorber has obvious advantage over MIL-101 fixed bed adsorber. In the Qa removal process, the cooling rate of CFCM-95 adsorber (from 62 °C to 30 °C) is 3.5 times faster than that of MIL-101 fixed bed adsorber (from 57 °C to 30 °C). Results show that the CFCM-95 adsorber has better heat transfer performance than the MIL-101 fixed bed adsorber. From above, for a fixed bed adsorber, there exist high contact thermal resistance inside MIL-101 particles including MIL-101 particles and the heat exchanger (point-to-point, shown in Fig. 7 (a)), which significantly affects the heat transfer

performance of MIL-101. In the CFCM-95 adsorber, MIL-101 is tightly contacted with the skeletons of CF-95 and promotes the heat transfer performance of MIL-101. Moreover, the contact mode between CFCM-95 and the copper tube with the thermal conductive adhesive (Fig. 7 (b)) is surface-to-surface, which reduce low contact thermal resistance and enhance heat transfer. Compared with the MIL-101 fixed bed adsorber, the heat transfer performance of the CFCM-95 adsorber has been effectively improved. It has a higher heating and cooling rate, which will reduce the rising and cooling time of the adsober, accelerate the removal of Qa and improve adsorption cooling efficiency of the ACS.

Fig. 7 Schematic diagram of contact mode between MIL-101 fixed bed adsorber (a) and CFCM-95 adsorber (b).

3.3.3 Effects of cycle parameters Operational parameters of the ACS with CFCM-95/isobutane working pair (Fig. 4) were investigated. Effects of adsorption time, desorption time, evaporation temperature, condenser temperature and hot water temperature on adsorption cooling capacity (Qc) and SCP were tested. According to equation (5), the relative error of Qc, SCP and VCP is 1.2%. (1) Adsorption time

Under the conditions with hot water temperature of 85 °C, cooling water temperature of 22-24 °C, condensation temperature of 30 °C, desorption time of 50 min, evaporation temperature of 15 °C and -5 °C, effects of adsorption time on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair are shown in Fig. 8. From Fig. 8, when the evaporation temperature is 15 °C (Air conditioning conditions), adsorption time is 10 min, the cooling capacity and SCP is 51.45 kJ/kg and 14.10 W/kg, respectively. When the evaporation temperature is -5 °C (Ice making conditions), adsorption time is 15 min, the cooling capacity and SCP is up to 45.95 kJ/kg and 11.26 W/kg, respectively. The adsorption time is obviously reduced with the increase of evaporation temperature because the higher evaporation temperature is corresponding to the higher partial pressure of isobutene (refrigerant). The mass transfer force of isobutane on the surface of adsorbents in the adsorber is enhanced, which is favorable for improving the adsorption rate. 60 14

50 12

45 40

SCP(W/kg)

Cooling capacity(kJ/kg)

55

35 30

Evaporator Temp:15oC

25

Evaporator Temp: -5oC

10 Evaporator Temp:15oC

8

Evaporator Temp: -5oC 6

20 15 10

4

4

6

8

10 12 14 16 Adsorption time(min)

18

20

22

4

6

8

10 12 14 16 Adsorption time(min)

18

20

22

(a) (b) Fig. 8 Effects of adsorption time on (a) cooling capacity and (b) SCP of the ACS with CFCM-95/isobutane working pair. Hot water temperature of 85 °C, cooling water temperature of 22-24 °C, condenser temperature of 30 °C, desorption time of 50 min.

(2) Desorption time

Influence of desorption time on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair was studied at hot water temperature of 85 °C, cooling water temperature of 22-24 °C, condenser temperature of 30 °C, adsorption time of 10 min and evaporation temperature of 15 °C, as shown in Fig. 9. From Fig. 9, when the desorption time is 50 min, the cooling capacity of the ACS with CFCM-95/isobutane working pair is 51.45kJ/kg. The SCP reaches 26.43 W/kg at the desorption time of 10 min. The adsorbates on the adsorbents can not effectively desorpted

with

too

short

desorption

time,

which

will

affect

the

next

adsorption/desorption cycle. However, when prolonging desorption time over 10 min, the SCP severely degrades. The appropriate desorption time should be considered. 60 30 25

40

20

30

15

20 Cooling capacity SCP

10 0

0

10

20

30

40

50

SCP(W/kg)

Cooling capacity(kJ/kg)

50

10 5 0

Desorption time(min)

Fig. 9 Effects of desorption time on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair. Hot water temperature of 85 °C, cold water temperature of 22-24 °C, evaporation temperature of 15 °C, condenser temperature of 30 °C, adsorption time of 10 min.

(3) Condenser temperature Fig. 10 shows the effects of condenser temperature on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair at hot water temperature of 85 °C,

cooling water temperature of 22-24 °C, evaporation temperature of 15°C, adsorption and desorption time of 10 min. It can be found from Fig. 10 that the maximum cooling capacity and SCP reaches 73.58 kJ/kg and 61.32 W/kg at condenser temperature of 15 °C, respectively. Lower condenser temperature is beneficial to the condensation of the gas phase refrigerant (isobutane) regenerated in the heating desorption process and increasing the circulating adsorption cooling capacity of the ACS. 70 80

60

70

50

60 40

50

30

40 30

Cooling capacity SCP

20

20 10

10 0

SCP(W/kg)

Cooling capacity(kJ/kg)

90

15

20

25

30

35

0

Condenser temperature(oC)

Fig. 10 Effects of condenser temperature on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair. Hot water temperature of 85 °C, cooling water temperature of 22-24 °C, evaporation temperature of 15 °C, adsorption and desorption time of 10 min.

(4) Hot water temperature Under the conditions as cold water temperature of 22-24 °C, condenser temperature of 30 °C, evaporation temperature of 15 °C, adsorption and desorption time of 10 min, effects of hot water temperature on cooling performance of the ACS with CFCM-95/isobutane working pair are shown in Fig. 11.

45 40 35 30

30 25

20

20 Cooling capacity SCP

10

SCP(W/kg)

Cooling capacity(kJ/kg)

40

15 10 5

0

75

80

85

90

95

0

Hot water temperature(oC)

Fig. 11 Effects of hot water temperature on cooling capacity and SCP of the ACS with CFCM-95/isobutane working pair. Cooling water temperature of 22-24 °C, evaporation temperature of 15 °C, condenser temperature of 30 °C, adsorption and desorption time of 10 min.

Shown in Fig. 11, the maximum cooling capacity and SCP are 39.61 kJ/kg and 32.63 W/kg at the hot water temperature of 95 °C, respectively. When the hot water temperature rises from 85 °C to 95 °C, the cooling capacitiy and SCP increases by 18% and 23%, respectively. Higher hot water temperature can increase the desorption temperature of the adsorber, which is conducive to the desorption of adsorbed refrigerant (isobutane) and increasing the circulating adsorption cooling capacity. From above, in this work, operation parameters of the ACS with CFCM-95/isobutane working pair are evaporation temperature of 15 °C, condenser temperature of 15 °C, adsorption time of 10 min, desorption time of 10 min and hot water temperature of 95 °C. 3.3.4 Cooling capacity and SCP of different adsorbers Operation parameters and cooling performance of the ACS with CFCM-95 adsorber (CFCM-95/isobutene working pair) and MIL-101 fixed bed adsorber (MIL-101/isobutene working pair) were compared, as shown in Table 2.

Table 2 Cooling performance of the ACS with different adsorbers. Adsorber CFCM-95 adsorber MIL-101 fixed bed

Ma (g)

Vb (L)

Ψ (kg/m3)

~50

0.69

72.5

~30

0.64

46.9

Working conditions Thw=85 °C, Te=Tc=15 °C, ta=td=10 min Thw=85 °C, Te=Tc=15 °C, ta=8.3 min, td=30 min

Qc (kJ/kg)

SCP (W/kg)

VCP (kW/m3)

73.6

61.3

4.442

53.8

23.4

1.097

Note: Ma is the mass of MIL-101, g; Vb is the volume of the adsorber, L; Ψ is per unit volume of adsorbents, kg/m3; Thw is hot water temperature, ℃; Te is evaporation temperature, ℃; Tc is condenser temperature, ℃; ta is adsorption time, min; td is desorption time, min.

From Table 2, the volume of the CFCM-95 adsorber is the same as MIL-101 fixed bed adsorber. However, the unit volume of adsorbents in the CFCM-95 adsorber is up to 72.5 kg/m3, which is 1.5 times that of MIL-101 fixed bed adsorber. At the same hot water temperature, evaporation temperature and condenser temperature, desorption time of the CFCM-95 adsorber is one third that of MIL-101 fixed bed adsorber due to the enhancement of heat transfer. Adsorption cooling performance of the ACS with CFCM-95 adsorber is obviously better than that of the ACS with MIL-101 fixed bed adsorber. The cooling capacity, SCP and VCP of the ACS with CFCM-95/isobutene working pair are 73.6 kJ/kg, 61.3 W/kg and 4.442 kW/m3, which are 1.8, 2.6 and 4 times that of the ACS with MIL-101 fixed bed adsorber, respectively. Thus it can be seen, at the same adsorber volume, the adsorber with CFCM-95 adsorption unit tube in this work has good heat transfer performance and can assemble more adsorbents, which significantly improves the VCP of the ACS.

3.4 Cycle stability Adsorption cooling cycle stability of the ACS with CFCM-95/isobutane working pair was investigated under typical air conditioning conditions (evaporation temperature of 15 °C, hot water temperature of 85 °C, condenser temperature of 30 °C,

adsorption and desorption time of 10 min), as shown in Fig. 12. Seen from Fig. 12, after 108 consecutive adsorption/desorption cycles, the cooling capacity of the ACS with CFCM-95/isobutane working pair is stable at 31 kJ/kg~32 kJ/kg without significant attenuation. It indicates that the CFCM-95 adsorbents, adsorption unit tube and the ACS have good operational stability. Compare to the literature [24], the ACS with the MIL-101 fixed bed adsorber has good cycle stability after 500 cycles. It means that the preparation method and binding method do not affect the adsorption properties of MIL-101. The cooling capacity of the

ACS

with

CFCM-95/isobutane

working

pair

and

the

ACS

with

MIL-101/isobutane working pair are both stable. 50

Cooling capacity(kJ/kg)

45 40 35 30 25 20 15 10 5 0

0

10

20

30

40

50

60

70

80

90

100 110

Cycle number

Fig. 12 Cyclic cooling capacity of the ACS with CFCM-95/isobutane working pair. Evaporation temperature of 15 °C, hot water temperature of 85 °C, condenser temperature of 30 °C, adsorption time and desorption time of 10 min.

4 Conclusions (1) A composite copper foams cured MIL-101 (CFCM) is firstly prepared by a binderless dip-coating method in this paper, and the thermal conductivity of CFCM-92 reaches 0.86 W/(m·K), which is nearly 14 times higher than that of the

MIL-101 powder (0.06 W/(m·K)). (2) The development of CFCM-95 adsorption unit tube is simple and convenient. Copper foams can be fixed on a metal tube made of copper, aluminum or stainless steel, and cure adsorbents to make an adsorption unit tube. Moreover, multiple CFCM-95 adsorption unit tubes may be assembled into the adsorbers. (3) The adsorber with CFCM-95 adsorption unit tube shows good heat transfer performance. The heating time of unit volume of adsorbents in the CFCM-95 adsorber is 1.56 s·m3/kg, which is about half that of MIL-101 fixed bed adsorber. The cooling rate of the CFCM-95 adsorber (from 62 °C to 30 °C) is 3.5 times faster than that of MIL-101 fixed bed adsorber. The CFCM-95 adsorber can assemble more adsorbents and the unit volume of adsorbents is up to 72.5 kg/m3, which is 1.5 times that of MIL-101 fixed bed adsorber. (4) The cooling capacity, SCP and VCP of the ACS with CFCM/isobutane working pair is 73.6 kJ/kg, 61.3 W/kg and 4.442 kW/m3, which is 1.8, 2.6 and 4 times that of the ACS with MIL-101 fixed bed adsorber, respectively. In addition, Operational stability of the ACS is successfully verified after 108 consecutive adsorption/desorption cycles. It can be of great significance for reducing the volume of adsorption coolling prototype and promoting the applications of adsorption cooling technology.

Acknowledgements This work was supported by the Project of Natural Science Foundation of China under contract No. 51476074 and the Priority Academic Program Development of

Jiangsu Higher Education Institutions (PAPD).

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1. Copper foams cured MIL-101 (CFCM) was prepared by a binderless dip-coating method. 2. A CFCM adsorption unit tube adsorber was developed. 3. CFCM adsorber reduces heating time by 50% and increases cooling rate by 2.6 times. 4. Specific Cooling Power of CFCM/isobutane working pair improved by 2.6 times.