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Accepted Manuscript
Analysis of a Hybrid System of Liquid Desiccant and CO2
Transcritical Cycles
Xiangyu Chen , Yijian He , Yi Wang , Guangming Chen
PII:
DOI:
Reference:
S0140-7007(18)30285-8
https://doi.org/10.1016/j.ijrefrig.2018.07.035
JIJR 4065
To appear in:
International Journal of Refrigeration
Received date:
Revised date:
Accepted date:
30 December 2017
3 July 2018
31 July 2018
Please cite this article as: Xiangyu Chen , Yijian He , Yi Wang , Guangming Chen , Analysis of a
Hybrid System of Liquid Desiccant and CO2 Transcritical Cycles , International Journal of Refrigeration
(2018), doi: https://doi.org/10.1016/j.ijrefrig.2018.07.035
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Highlights
 A system is proposed by efficiently cascade between CO2 and desiccant cycles.
 Dissipated heat of the CO2 cycle could be utilized to 50℃.
 COPhs’ is obviously increased, at least 15%, on an operational mode of ICTH.
 Energy consumption of CO2 cycle could be reduced over 26.3%.
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Analysis of a Hybrid System of Liquid Desiccant
and CO2 Transcritical Cycles
Xiangyu CHEN, Yijian HE*, Yi WANG, Guangming CHEN
Institute of Refrigeration and Cryogenic, Zhejiang University; Key Laboratory of the Refrigeration
and Cryogenic Technology of Zhejiang Province,Hangzhou, China
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ABSTRACT:A novel hybrid system of liquid desiccant and CO2 transcritical cycles is
proposed, where LiCl solutions are used as desiccants. Dissipated heat of the transcritical
cycle is used to regenerate LiCl solutions, and the heat could be utilized efficiently with large
temperature decrease. From insight of thermodynamics, model of the hybrid system is built.
Based on the calculation results, temperature of the utilized heat could range from 120℃ to
50℃. In addition, temperature and humidity independence control (THIC) could be conducted
by the hybrid system. Under the same dehumidification capacity, its evaporation temperature
could be obviously lifted. For the hybrid system, compression work (W) of the CO2
transcritical cycle could be reduced from 15% to 26%. Futher more, compared with cooling
dehumidification at 7℃ by a conventional CO2 transcritical cycle, total power consumption of
the hybrid system could be reduced by 13.7% at an evaporation temperature of 12℃. More
notablely, dehumidification efficiency of the hybrid system (COPhs’) could be increased from
15% to 30% compared with dehumidification efficiency of cooling dehumidification by CO2
transcritical cycle (COPcs).
KEYWORDS: Transcritical Cycle; Liquid Desiccant System; Hybrid System; Performance
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1. Introduction
A liquid desiccant system can use low-grade heat and renewable energy. With the challenges
of energy shortage, liquid desiccant technique has obtained great attentions [1-3]. Comparing
the economic and environmental performance of the dehumidification system driven by
natural gas boilers, electric heat pumps, solar energy and solar heat pump, respectively,
Abdel-Salam et al. [4] pointed out that the use of solar energy could reduce annual operating
costs and reduce CO2 emissions. A liquid desiccant system coupled with vapor compression
refrigeration, which becomes popular as one promising method of temperature and humidity
independence control (THIC), has been investigated by many researchers [5-7]. Su et al. [8]
proposed a hybrid compression-absorption air-conditioning (AC) system combined with
liquid desiccant dehumidification. The condensing heat from the condenser of the absorption
refrigeration system could be used to regenerate the diluted liquid desiccant solution. The
energy efficiency of Su’s system is 34.97% higher than that of a traditional absorption AC
system under the same operation condition. Li et al. [9] developed a new liquid desiccantvapor compression hybrid system by adopting an auxiliary regenerator, in which could the
cooling capacity loss could be cut down about 1.5% to 8.2%. Due to its advantages of
environment protection and energy conservation, a liquid desiccant system, driven by lowgrade heat such as waste heat, solar energy and so forth, could be widely used in many areas
[10-13].
Due to the leakage of the refrigerants in a traditional vapor compression refrigeration system,
the refrigerants will produce harmful effects to environment. A transcritical refrigeration
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cycle, using CO2 as the refrigerant, offers the potential for environment friendly refrigeration
and air conditioning in applications, without the ozone depletion and global warming
problems associated with conventional refrigerants [14]. In addition, temperature required for
regeneration of solutions in a liquid desiccant system are generally over 50℃. The dissipated
heat of conventional compression refrigeration systems is difficult to meet such requirements.
The dissipated heat temperature of CO2 transcritical refrigeration cycles range up to 120℃.
Adriansyah [15] studied an air conditioning system using CO2 as refrigerant coupled with a
water heating subsystem. From his analysis, such a system could fully meet the needs for the
countries with around cooling demand and hot water. Wang et al. [16] conducted a
thermodynamic analysis of a compression-absorption refrigeration air-conditioning system
coupled with liquid desiccant dehumidification. The results show that the system can improve
the performance of CO2 transcritical cycle significantly and might meet the requirements for
air conditioning all year round.
Up to now, study on a liquid desiccant cycle, which is driven by dissipated heat of a CO2
transcritical refrigeration cycle, is rarely reported. A hybrid system of liquid desiccant and
CO2 transcritical cycles is proposed in this context. Thermodynamic performance is
investigated on this hybrid system, and comparisons are conducted between the hybrid
system and a CO2 transcritical cycle.
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2. System descriptions
Figure 1 Descriptions for a hybrid system
A hybrid system consisting of CO2 transcritical and liquid desiccant cycles is illustrated as
Figure 1. In the liquid desiccant cycle, the concentrated LiCl solution (state 6) from the
regenerator is sent to the solution heat exchanger, where it is precooled by the dilute LiCl
solution from the bottom of the dehumidifier (state 2), and then the precooled solution (state
7) is further cooled in the cooling device. The low temperature solution (state 1) flows into
the liquid distribution device on the top of the dehumidifier. Due to the vapor pressure
difference between the humid air and the surface of concentrated LiCl solution, the air is
dehumidified and the solution is diluted (state 2). The dilute LiCl solution is preheated in the
solution heat exchanger (state 3), and then it is sent to the gas cooler by the solution pump
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and further heated by the superheat CO2. Finally, the high temperature solution (state 5)
flows into the regenerator. An air heat exchanger is used, in which the ambient air (state 12)
is preheated by the exhaust air (state 14) before flowing into the regenerator (state 13). In the
CO2 transcritical cycle, the high temperature refrigerant (state a) enters the gas cooler,
regenerating the dilute LiCl solutions. The refrigerant is further cooled in the internal heat
exchanger (state c), and becomes lower pressure (state d) by the throttle valve. Subsequently,
the refrigerant flows into the evaporator (state e) and cool the dehumidified air. Then, the
refrigerant vapor is preheated in the internal heat exchanger (state f) and sent back to the
compressor.
The liquid desiccant cycle is used to bear the latent heat load of the indoor air. The CO2
transcritical cycle is to provide regeneration heat for the liquid desiccant cycle and to bear the
sensible heat load of the dehumidified air. Therefore, independent control of temperature and
humidity (THIC) could be conducted by the hybrid system.
3. Thermodynamic model
3.1 System assumptions
For thermodynamic model of the proposed hybrid system, it is assumed to be in steady state.
Detailed assumptions are presented as following:
(1) Dehumidifier and regenerator are adiabatic;
(2) Heat and mass transfer resistance in liquid phase is negligible;
(3) Filler is fully infiltrated, and heat transfer areas are equal to mass transfer one;
(4) No axial diffusion in dehumidifier and regenerator;
(5) The outlet temperature of refrigerant (state b) is 3℃ higher than that of the inlet solution
(state 4) in gas cooler;
(6) Heat losses and pressure losses in the pipes are neglected.
3.2 Liquid desiccant cycle model
The fundamental energy and mass equations to all components of the liquid desiccant cycle
are given in Table 1. represents mass flow rate (kg·s-1); represents air moisture content
(kg·kg-1); represents solution concentration;
represents the temperature (℃);
represents the cooling capacity if the cold water (kW);
represents available condensation
heat (kW). The state points are illustrated as Figure 1.
Component
Mass conversion equation
(
(
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Dehumidifier
Regenerator
Solution H.X.
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Table 1 Mass and energy conversation equations of liquid desiccant cycle
)
)
(
(
Energy conversion equation
)
)
(
(
(
)
)
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(
(
Cooling device
(
)
(
Gas cooler
Air H.X.
(
(
(
(
)
)
)
(
)
(
)
)
)
)
)
(
)
In this context, NTU-ε model is used to analyze processes of the dehumidifier and
regenerator [17-19]. Define the number of heat transfer units
as following:
(1)
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where, is the heat transfer coefficient, kJ·(m2·℃)-1;
represents the specific surface area
of filler, m2·m3;
represents volume of filler, m3;
represents the mass flow rate of air,
kg·s-1 and
is the heat capacity at constant pressure of the air, kJ·(kg·℃)-1.
The number of the mass transfer units
and Lewis number are defined as Eqs. (2)
and Eqs. (3), respectively:
(2)
(3)
(
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where,
is the mass transfer coefficient, kJ·(m2·℃)-1 and
represents the air density,
kg·m3; Lewis number is taken as 1. And, the differential equations of mass transfer and heat
transfer are defined as Eqs. (4) and Eqs. (5), respectively:
(
)
(4)
)
(5)
represents the enthalpy of air side at gas-liquid interface, kJ·kg-1;
represents the moisture content of air side at gas-liquid interface, kJ·kg-1 and
is
-1
the heat capacity at constant pressure of the solution, kJ·(kg·℃) .
COP of the liquid desiccant cycle is defined as:
⁄
(6)
(
)
(7)
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where,
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where,
equivalents cooling capacity required to remove moisture from the air in the
dehumidifier at the inlet temperature (state 1), kW; represents required regeneration heat,
kW.
As for a traditional cooling dehumidification system, the evaluation criterion, COPcs, is the
ratio of cooling capacity by mechanical to mechanical work. However, there are no uniform
evaluation criteria for a hybrid dehumidification system driving by both low-grade heat and
mechanical work. Based on the criterion used by He [20],
is defined to evaluate the
dehumidification efficiency of the hybrid system in this context, the formula is as follow:
(
)⁄
(8)
where,
is cooling capacity in evaporator of transcritical cycle, kW; W is compression work
of transcritical cycle, kW.
If the extra power caused by pumps and fans of the hybrid system is considered, the
dehumidification efficiency of the hybrid system is defined as:
(
)⁄(
)
(9)
where,
is extra power caused by pumps and fans, kW.
Available dissipated heat of the hybrid system is defined as
(
)
(10)
where, m r represents mass flow rate of refrigerant; and
represent the inlet and outlet
temperature on the refrigerant side at the gas cooler, respectively.
3.3 Model validation
In this context, the theoretical model is validated by using experimental data [21]. For two
kind of operational conditions of the liquid desiccant cycle, the comparison results are shown
in Table 2, where Tai, Wai, Tao and Wao represent the inlet temperature, outlet one and
humidity content of the air, respectively. Tsi, Xsi, Tso and Xso represent the inlet temperature,
outlet one and solution content of the LiCl solution, respectively. The deviations between the
calculation results by the model developed in this context and experimental results are lower
than 6.05%.
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-1
Wai(g·kg )
17.6
Table 2 Model verification by comparing results with Ref. [21]
Parameter
Ref. [21]
Present work
Deviation (%)
-1
Wao(g·kg )
14.51
15.07
3.86
Xso(g·kg-1)
24.95
24.85
0.40
Tao(℃)
29.4
29.6
0.68
Tso(℃)
30.1
29.9
0.66
Wao(g·kg-1)
14.88
15.78
6.05
-1
Xso(g·kg )
24.93
24.78
0.60
Tao(℃)
29.6
29.7
0.34
Tso(℃)
30.7
30.5
0.65
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4. Results and discussions
The set conditions of the hybrid system are shown in table 3 [22]. The inlet humid air of
humidifier is mixed air and the inlet ambient air of the air heat exchanger is outdoor air. The
mixed air consists of 90% indoor air and 10% outdoor air. Based on the theoretical model and
set conditions, a simulation case of the liquid desiccant cycle is conducted under the
following parameters:
%;
kg·h-1; =32℃; =58℃; the liquid-gas ratio of
the dehumidifier and regenerator is 0.5 and 2.5, respectively; dehumidification capacity of the
system is 2.05kg·h-1. The height of the dehumidifier and regenerator are 0.48m and 0.50m,
respectively. The simulation results are shown in Table 4.
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Indoor
Mixed air
Supply air
Table 3 Design conditions of the hybrid system
Dry bulb temperature
Wet bulb temperature
Air moisture content
℃
℃
g·kg-1
35.7
28.5
22.192
26.0
19.5
11.700
27.0
20.5
12.750
17.0
15.9
11.017
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Table 4 The parameters of the liquid desiccant cycle
state Temperature LiCl
Air moisture
Mass flow
-1
(℃)
concentration (%) content (g·kg )
rate (kg·h-1)
1
32.0
35
/
600
2
34.1
34.67
/
600
3
51.6
34.67
/
600
4
51.6
34.67
/
600
5
58.0
34.67
/
600
a
116.2
/
/
0.48
b
54.6
/
/
0.48
6
53.6
35
/
600
7
36.2
35
/
600
8
36.2
35
/
600
9
27.0
/
12.75
1200
10
29.3
/
11.02
1200
11
17.0
/
11.02
1200
12
35.7
/
22.192
240
13
42.0
/
22.192
240
14
52.2
/
30.59
240
Enthalpy
(kJ·kg-1)
112.1
116.5
165.7
167
187.4
20.8
-100.5
172.5
123.8
123.8
59.8
57.6
45
92.9
99.5
132
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46.0
30.0
30.5
/
/
/
30.59
/
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3342.9
3342.9
125.4
125.7
127.8
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4.1 Performance of the liquid desiccant cycle
In this section, the influence on the performance of the liquid desiccant cycle without
utilizing dissipated heat of CO2 transcritical cycle is discussed from the cooling temperature
and the ambient air moisture content. Inlet parameters of the dehumidifier are: Tai=27℃,
Xsi=0.35. Inlet air temperature of the regenerator is 35.7℃. And, the dehumidification
capacity of the system is 2.05kg·h-1.
Effects of the cooling temperature (Ts = T1) on the performance of the liquid desiccant cycle
are shown in Figure 2. Where, ms, Tsr (Tsr= T5) and COP represent desiccant mass flow rate,
regeneration temperature and coefficient of performance, respectively.
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(a) Changes on desiccant mass flow rate and regeneration temperature
(b) Changes on regeneration heat and COP
Figure 2 Relationships between performance and cooling temperature
of the liquid desiccant cycle
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It could be found from Figure 2 that desiccant mass flow rate decreases with the decrease of
cooling temperature under the same dehumidification capacity. The change rate of mass flow
is greater when the cooling temperature is higher. The possible reason is that the higher is the
temperature, the larger is the change rate of surface vaper prussure of disiccant. When the
temperature is 32°C, the slight drops in cooling temperature bring a large drop of surface
vapor pressure of disiccant solution, resulting in significant mass flow change.When the mass
flow rate decreases, the solution temperature rises at the outlet of dehumidifier. Therefore,
the regeneration temperature increases. Bacause of the decreases of mass flow rate, the
regeneration heat is reduced. Therefore, the COP of the liquid desiccant cycle increases.
Effects of the ambient air moisture contents (Wai) on the performance of the liquid desiccant
cycle are shown in Figure 3.
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(a) Changes on desiccant mass flow rate and regeneration temperature
(b) Changes on Regeneration heat and COP
Figure 3 Relationships between performance and air moisture contents
of the liquid desiccant cycle
From Figure 3, it is found that, with increase of the ambient air moisture contents, mass flow
rate of desiccant solution and regeneration heat decreases, moreover, regeneration
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temperature and COP increases. Bacause the mass transfer in duhumidifier increases with
increase of the air moisture contents, desiccant mass flow rate is reduced. As a result,
concentration and temperature of desiccant solution at the outlet of dehumidifier becomes
higher, so the regeneration temperature increases. The influence of mass flow rate decrease is
bigger than the influence of regeneration temperature increase, therefore, the regeneration
heat is reduced and COP is increased.
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4.2 Performance of the hybrid system
Operational conditions of the CO2 transcritical cycle are set as: compressor discharged
pressure is 10MPa; gas cooler is cooled by water; the outlet temperature of internal heat
exchanger is 35℃ (state c); outlet temperature of refrigerant (state b) is 3℃ higher than the
inlet temperature of desiccant solution (state 4). Under different operational conditions,
power consumption of pumps and fans changes very slightly, and 0.24kW is used for analysis
in this context.
When the liquid desiccant system coupled with the transcritical cycle, the evaporation
temperature in transcritical cycle rises since the latent load is handled by liquid desiccant
cycle, which improves the performance of the hybrid system. The desiccant cooling
temperature is 32℃, effects of the evaporation temperature (Te) on the performance of the
hybrid system are shown in Figure 4 (W represents the compression work, kW).
Figure 4 Relationships between performance and evaporation
temperature of the hybrid system
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It is found that the CO2 transcritical cycle can fully meet the demand of heat for regeneration
under the evaporation temperature of 10.9℃. When the evaporation temperature increases,
compression work (W) decreases and
increases. As a result,
could range from
3.23 to 3.75. If further decreases the desiccant cooling temperature, evaporation temperature
of CO2 transcritical cycle could be increased. As a result, the compression work could be
reduced more. The refrigerant temperature at the outlet of the compressor ranges from 116.2℃
to 104.2℃ as the evaporation temperature changes from 7℃ to 10.9℃.
When the evaporation temperature of the transcritical cycle is 12℃, effects of the cooling
temperature on the required regeneration heat and available dissipated heat of the hybrid
system are shown in Figure 5. It is found that Q decreases and Qc increases when the cooling
temperature gradually decreases. Because the dehumidification capacity and evaporation
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temperature of the hybrid system are constant,
is almost unchanged. As the cooling
temperature decreases, evaporation temperature of the transcritical cycle could be higher. The
refrigerant temperature at the outlet of the compressor ranges from 114.7℃ to 102.8℃ as the
cooling temperature increased from 27℃ to 31.5℃.
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Figure 5 Relationships between performance and cooling
temperature of the hybrid system
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When the cooling temperature of the desiccant is 32℃ and the evaporation temperature of the
transcritical cycle is 10℃, effects of the air moisture contents on the Q and Qc of the hybrid
system are shown in Figure 6.
Figure 6 Relationships between performance and air moisture
contents of the hybrid system
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It is found that both Q and Qc decrease when the air moisture content gradually decreases.
However, the Q decreases slightly at high air moisture contents. From the Figure 3, it could
be found that the desiccant mass flow rate changes slightly at high air moisture contents.
Therefore, the total influence of mass flow rate decreases and the regeneration temperature
increases on Q becomes gentle. Transcritical cycle could fully meet the demand of heat for
regeneration under the air moisture contents of 23g·kg-1. Similarly, because the
dehumidification capacity and evaporation temperature of the hybrid system are constant,
is almost unchanged. The refrigerant temperature at the outlet of the compressor
ranges from 102.5℃ to 106.3℃ as the air moisture contents increased from 18g·kg-1 to
23g·kg-1.
4.3 Comparisons between hybrid and conventional transicritical cycles
Under the same conditions, comparisons of performance between the hybrid system and a
conventional transcritical cycle are shown in Table 5.
Cooling dehumidification
by CO2 transcritical cycle
7
Dehumidification by
hybrid system
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Table 5 Performance comparisons between two cycles under the same conditions
Evaporation
Total power consumption
COPcs
temperature (℃)
(kW)
/COPhs’
1.900
2.895
1.865
3.329
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When cooling dehumidification is directly utilized by a conventional transcritical cycle, the
air moisture content is 11.02g·kg-1, the corresponding dew point temperature is 15℃ at a
standard atmospheric pressure. When the humid air is cooled to the dew point temperature,
the required cooling capacity is 5.5kW. The temperature difference between evaporation
temperature and the temperature of supply air is 8℃, and the evaporation temperature of the
transcritical cycle is 7℃. Based on the thermodynamic model, calculation results show that
1.9kW compression power is consumed, and the dehumidification efficiency of the CO2
transcritical cycle (COPcs) is 2.895.
When the hybrid system is used for dehumidification, the air temperature of the dehumidifier
outlet is 29.3℃ and moisture content is 11.02g·kg-1. The cooling capacity required to remove
moisture from the air in the dehumidifier is 1.333kW. And, the evaporation temperature for
the hybrid system is 7℃. Calculation results show that 1.625kW compression power and
0.24kW extra power by pumps and fans are consumed. The cooling capacity in evaporator is
4.876kW. Therefore,
is 3.329. Compared with cooling dehumidification by the
conventional transcritical cycle, W could be reduced by 14.47% and
could be
increased by 15.0%. If the cooling temperature is further reduced, the evaporation
temperature of the transcritical cycle could be increased to a higher temperature. Based on the
built thermodynamic model, when the evaporation temperature of the hybrid system is 12℃,
the compression work could be reduced to 1.40kW and
could be increased to 3.753.
Compared with cooling dehumidification by transcritical cycle, W is reduced by 26.3%, total
power consumption is reduced by 13.7% and
could be increased by 29.6%.
5. Conclusions
In this study, a hybrid system of liquid desiccant and CO2 transcritical cycles is proposed,
where the dissipated heat with high temperature is used to regenerate desiccants. The effects
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of cooling temperature, air moisture contents, and evaporation temperature on the
performance of liquid desiccant cycle and hybrid system are analyzed in detail. Comparisons
between a conventional CO2 transcritical cycle and the hybrid system are also carried out.
From this investigation, the following conclusion could be obtained:
(1) The dissipated heat with large temperature decrease could be efficiently utilized by the
proposed hybrid system, near to 50℃.
(2) Compression work (W) of the CO2 transcritical cycle for the hybrid system could be
greatly reduced, even more than 26.3%.
(3) Independent control of temperature and humidity (THIC) could be efficiently conducted
by the hybrid system.
(4) For the independent control of temperature and humidity, dehumidification efficiency of
the hybrid system (COPhs’) is obviously increased, at least 15%.
ACKNOWLEDGEMENTS
This work is supported by Cultural Heritage Bureau of Zhejiang Province (Grant
No.2016008) and the National Natural Science Foundation of China (Grant No.51206140 ).
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