Thermodynamic performance assessment of vacuum membrane-based dehumidification and air carrying energy radiant air-conditioning system (VMD-ACERS)

Liang Chun,Guangcai Gong,Xi Fang,Pei Peng

Key Laboratory of Building Safety and Energy Efficiency Ministry of Education College of Civil Engineering,Hunan University,Changsha 410082,China

Keywords: Fan coil unit cooling system Temperature and humidity independent control ACERS Vacuum membrane-based dehumidification COP

ABSTRACT Temperature and humidity independent control (THIC) air-conditioning system is a promising technology.In this work,a novel temperature and humidity independent control (THIC) system is proposed,namely VMD-ACERS,which integrates vacuum membrane-based dehumidification and air carrying energy radiant air-conditioning system.This work establishes a novel coefficient of performance (COP)model of VMD-ACERS.The main parameters affecting the COP of conventional fan coil unit cooling system (FCUCS) and VMD-ACERS are investigated.The performance of FCUCS and VMD-ACERS are compared,and the energy-saving potential of VMD-ACERS is proved.Results indicate that,for FCUCS,the importance ranking of parameters is basically stable.However,for VMD-ACERS,the importance ranking will be affected by FCU and refrigerant.The most important parameters of VMD-ACERS are condensation temperature and permeate side pressure.On the contrary,superheating,subcooling are relatively less important parameters.For VMD-ACERS,it is not necessary to pursue the membrane with very high selectivity,because the selectivity of membrane would also be a less important parameter when it reaches 500.The COP of VMD-ACERS is higher than that of FCUCS when the permeate side pressure is higher than 8 kPa.The VMD-ACERS solves two technical problems about power-saving and thermal comfort of conventional THIC,and can extend the application of THIC air-conditioning system.

1.Introduction

At present,the air-conditioning electricity consumption accounts for 15% of total electricity consumption in China,and the average annual growth rate of this ratio is about 20%[1].In fact,not only in China,but in most countries of the world,the demand of air-conditioning cooling has been constantly increasing [2].Therefore,it is of great significance to develop an efficient refrigeration and air-conditioning system.

Temperature and humidity independent control (THIC) is a promising air-conditioning technology that decouples the temperature and humidity control process [3].THIC has at least two advantages.The first one[4]is that THIC can realize precise control of temperature and humidity.The second one [3]is that THIC can reduce the load of chiller and increase the evaporation temperature,thereby increasing the energy efficiency of chiller.At present,THIC is mainly applied in industry[5],such as lithium battery factories,food production factories and electronic chip factories,etc.The main dehumidification methods of THIC are wheel dehumidification and liquid dehumidification.Two major technical issues limit the further application of THIC [6].The first one is that the conventional dehumidification ways require heating regeneration for continuous dehumidification.The regeneration process will consume a lot of energy if there is not enough waste heat.The second one is that increasing evaporation temperature will reduce the heat transfer temperature difference between chilled water and air.In order to ensure the cooling capacity,it is generally necessary to increase the flow rate of supply-air or the heat exchange area,which are serious challenges to thermal comfort and initial investment.

Vacuum membrane-base dehumidification (VMD) is a new dehumidification technology,which realizes dehumidification by the cooperation of vacuum pump and selectively separating membrane[7–9].VMD does not require heating regeneration,so it is small and flexible [10].VMD is different from wheel dehumidification and solution dehumidification [6],and it does not need solid or liquid desiccant,so this technology is especially suitable for occasions where low-grade heat sources are lacking.Air carrying energy radiant air-conditioning system (ACERS) is a new type of radiant air-conditioning system recently proposed by Gonget al.[11].ACERS improves the radiation heat transfer while reducing the indoor wind velocity by a metal orifice plate.ACERS has been applied in a balance room of Changde institute for food and drug control for more than 5 years.Relevant results have proven that ACERS has a better thermal comfort and energy saving potential [11,12].

In this work,a novel THIC system combining VMD and ACERS is proposed.The novel THIC system is named vacuum membranebased dehumidification and air carrying energy radiant airconditioning system (VMD-ACERS).The system could solve the second technical problem that traditional THIC needs to overcome,that is,the thermal comfort decreases as the flow rate of supply-air increases.This work establishes a coefficient of performance(COP)model to investigate the performance of VMD-ACERS.

2.Prior Work

Vacuum membrane-based dehumidification (VMD) technology has attracted lots of interest in recent years [13–17],and it can achieve isothermal dehumidification [7].VMD employs a vacuum pump to drive dehumidification.Selectively separating membrane allows water vapor to pass easily under the effect of pressure difference,while other gases are difficult to pass.Combining VMD and evaporative cooling is an interesting work [18,19].This technology not only does not require refrigerant,but also extends the application of evaporative cooling technology.El-Dessoukyet al.[19]proposed a system in which a VMD device was installed before evaporative cooling unit.In the system,the wet air is first dehumidified by the VMD equipment and then cooled by the evaporative cooling unit.Therefore,the system can be applied to high temperature and high humidity climatic environments.In addition,the system can save 86%of energy compared to conventional mechanical vapor compression system.Dais-Analytic [20]proposed a membrane heat pump system that combines VMD and evaporative cooling technology.The system employs a vacuum pump to promote the effect of evaporative cooling and dehumidification.Besides,in the system,evaporative cooling and dehumidification are both based on selectivity separating membrane.Relevant results [18]indicate that the COP of the system is about 5.15.The above systems are efficient,especially in humid environments.

Selectively separating membrane is one of the most important components of VMD.In general,the membrane contains many hydrophilic substances.Recently,various dehumidification membranes have been developed,such as recyclable beta-cyclodextrin membrane [21],Aqueous salt solution liquid membrane [22],amorphous cellulose composite membrane [23],Pebax (R) 1657/Graphene oxide composite membrane [24],Triethylene glycol/polyvinyl alcohol membrane [25],Polyvinyl alcohol/LiCl membrane [8]and ionic liquid membrane [26],etc.Permeance and selectivity are two important performance indicators of selectively separating membrane [7].The permeance indicates the membrane’s penetration of water vapor.The selectivity is the ratio of water vapor permeance to air permeance.It is always desirable that both of the permeance and selectivity are high.However,the permeance and selectivity of all the above selectively separating membranes are inversely related.Buiet al.[16,27]investigated the coefficient of performance (COP) of VMD.They found that the selectivity has an effect on COP of VMD,but the permeance has no effect on COP of VMD.

Optimizing VMD system can effectively improve the dehumidification effect and COP.Lianget al.[15]found that the relative humidity can decrease from 80%to 35%when the flow rate of feed side is 400 ml·s-1and the permeate side pressure is 20 kPa.The dehumidification effect is fully satisfactory for the dehumidifying requirement of general office buildings.Zhaoet al.[28]proposed a system that can realize dehumidification and water harvest.The system saves 24.3% energy compared to conventional condensing dehumidification device.In addition,Zhaoet al.[17]also proposed a thermally induced membrane dehumidification system.In the system,a heat source is installed on the permeate side.When the heat source temperature is 75°C and the pressure of permeate side is only 8.54 mbar lower than the feed side pressure,the system can reduce the content of water vapor in air by 12.4%.The results indicate that the permeate side pressure of VMD can be further elevated under the aid of waste heat source.Other than that,by means of a 150-hour test,it is found that the dehumidification effect of VMD is significant when the permeate side pressure is 78 kPa[29].The above experimental results prove that VMD is also feasible when the permeate side pressure is high.However,Woods[9]suggests that the permeate side pressure of water vapor should be less than 1.25 kPa.It is difficult to be realized,and so low permeate side pressure may also lead to a very low COP of VMD.Buiet al.[16,27]evaluated the effect of permeate side pressure on COP of VMD.From their results,COP of 2–3 [16]is achievable and the theoretical limit for the COP is 180 [27].Thuet al.[30]responded with an investigation about COP of the coupling system of mechanical vapor compression and dehumidifier.They found that the breakeven COP of dehumidifier needs to reach 4.3–6.8(Ideal cycle)to ensure that the coupling system can compete with current condensing dehumidification air-conditioning system.The result points out the minimum COP requirement of dehumidifier.Chenet al.[31]investigated the energy performance of nextgeneration dedicated outdoor air cooling systems,and found that membrane cooling systems achieve higher energy efficiencies than baseline chiller system.According to the above literature review,at present,there are few systems similar to VMD-ACERS.

3.Principle of Vacuum Membrane-Based Dehumidification and air Carrying Energy Radiant Air-Conditioning System (VMDACERS)

The VMD-ACERS is a novel cooling system proposed in this work.Fig.1 shows the basic schematic of the VMD-ACERS.The indoor space can be divided into two zones by VMD-ACERS.The first one is high velocity low temperature buffer storage zone and the other one is low velocity air-conditioning zone.In VMDACERS,air is cooled by fan coil unit (FCU) firstly and then enters the buffer storage zone.The FCU in VMD-ACERS only needs to bear sensible heat load,so it is different from traditional fan coil unit cooling system (FCUCS).In VMD-ACERS,the latent heat load is borne by the vacuum membrane-based dehumidifier.

From the principle of VMD-ACERS,it is certain that the FCU does not have the task of dehumidification,so the temperature of chilled water can be set higher.Since higher temperature of chilled water is beneficial to increasing COP of chiller [32],VMD-ACERS has the potential to improve the COP of chiller.However,increasing the temperature of chilled water will reduce the heat transfer temperature difference between water and air.Therefore,the VMD-ACERS needs to increase the air flow rate of FCU to ensure that the cooling capacity can meet the cooling load.Because there is a metal orifice plate in VMD-ACERS,increasing the air flow rate of FCU will not bring a noticeable draft sensation in air conditioning zone.

Fig.1.Schematic of VMD-ACERS.

The above is the advantages of VMD-ACERS,but VMD-ACERS may also increase energy consumption.The new energy consumption of VMD-ACERS consists two parts,one is the energy consumed by VMD,and the other is the energy consumption caused by the increase in flow rate of supply-air.VMD-ACERS will be inefficient if the total energy consumption of the two parts exceeds the energy saved by chiller.The issue will be analyzed and discussed in this work.

4.Methodology

4.1.Model development of COP of cooling systems

4.1.1.COP of conventional vapor compression refrigeration system

Fig.2 shows a typical temperature-entropy diagram for a vapor compression refrigeration cycle.According to the information in Fig.2,the COP of traditional vapor compression refrigeration unit can be calculated by Eq.(1).

In order to distinguish it from the convective heat transfer coefficient (h),htis used to represent the specific enthalpy,and the subscripts A,C and D correspond to the points in the Fig.2.Other items are described in Nomenclature.Eq.(1) indicates that the enthalpies of three points of A,C,D need to be known to calculate the value of COP.

Fig.2.Temperature-Entropy diagram for a traditional vapor compression refrigeration cycle.

The enthalpies of points A and C are easy to determine when the evaporation temperature,condensation temperature,superheating and subcooling are determined.However,the enthalpy of point D is difficult to determine.Therefore,this work provides an equation to get the enthalpy of point D.A novel and mathematically more rigorous proof process is shown in Supplement A,and the equations presented here consider both compressor superheating at inlet and compressor superheating at outlet.In Supplement A,some reasonable and necessary assumptions are made as follows:

(1) Line EB is approximately a straight line.

(2) Line BC is parallel to line ED.

(3) The compression process (CD) is isentropic.

Eq.(1)and Eq.(2)can be utilized to calculate COP of traditional vapor compression model by several simple physical variables,such as evaporation temperature,condensation temperature,superheating and subcooling.

4.1.2.COP model of FCUCS

For traditional fan coil unit cooling system(FCUCS),the COP can be calculated by Eq.(3).It is worth noting that the power of the pump is not taken into account because it is not the focus of this paper.

Here,the power of chiller is calculated by Eq.(4).

Here,w2is the power of fan coil unit(FCU).The basic data ofw2is provided by SEIYEA air conditioning equipment Co.,Ltd in China.According to these basic data,the relationships between power and flow rate of different FCUs are plotted,as shown in Fig.3.It can be found that the power and flow are almost linearly related.Consequently,linear equation can be utilized to represent the relationship between power and flow,as shown in Table 1.For concision,the relationship between power and flow rate of FCU is expressed as Eq.(5).

Table 1Fitted linear equations about power and flow relationship

Finally,the COP of FCUCS can be calculated by Eq.(6).

Fig.3.Power versus flow rate of FCU.

4.1.3.COP model of VMD-ACERS

For VMD-ACERS,the value of COP can be calculated by Eq.(7).It is worth noting that the power of fan in VMD and the power of pump in cooling system are ignored because they are relatively stable and small.

Here,w1is the power of chiller when the chiller only bears sensible heat load.It can be calculated by Eq.(8).w3is the power of vacuum membrane-based dehumidifier,and it can be calculated by Eq.(10).

In order to ensure that the dehumidification capacity can meet the requirement of latent heat load,the flux of water vapor through membrane can be calculated by Eq.(11).It is noticed that the area,permeance and driving force of membrane system should be designed or selected through Eq.(11).The flux of air through membrane can be calculated by Eq.(12).

Substituting Eq.(11),Eq.(12)and Eq.(13)into Eq.(10)leads to Eq.(14).

Finally,the COP of FCUCS is obtained,as shown in Eq.(15).

4.1.4.Relationship between flow rate of supply-air and evaporation temperature in VMD-ACERS

For VMD-ACERS,sensible heat load and latent heat load are borne by two different devices.As described in Section 2,elevating the evaporation temperature will cause an increase of flow rate of supply-air.In this section,the relationship between evaporation temperature and the flow rate of supply-air will be established.The details are as follows.

Total load and sensible heat load can be calculated by Eq.(16)and Eq.(17) respectively.

Similarly,the total load and the sensible heat load can also be calculated by Eq.(18) and Eq.(19) respectively.

Here,the subscriptiindicates the case where the temperature and humidity independent control (THIC) technology is not adopted.The average heat transfer temperature difference in Eq.(19) can be calculated by Eq.(20).Additionally,Eq.(21) is a constraint.

When the VMD-ACERS is adopted,the temperature and humidity are independently controlled.In the case,FCU only needs to bear the sensible heat load.Consequently,the sensible heat load can be calculated by Eq.(22).

Here,the subscriptjindicates the case where the temperature and humidity independent control technology is adopted.In the case,the average heat transfer temperature difference in Eq.(22)can be calculated by Eq.(23).Additionally,Eq.(24)is a constraint.

The Eq.(25) can be obtained from Eq.(19) and Eq.(22).

Since the heat exchange area of FCU has not changed,Eq.(25)is equivalent to Eq.(26)

The convective heat transfer coefficient between air and heat exchanger in FCU can be calculated by a famous Zhukauskas equation [33],namely Eq.(27).Since the temperature change of air in FCU is not too obvious,the influence of the change of physical properties of air on heat transfer is neglected.Therefore,the relationship between the heat transfer coefficient before adopting VMD-ACERS and after adopting VMD-ACERS is satisfied with Eq.(28).

The Eq.(29) indicates the relationship between the heat transfer temperature difference and flow rate of supply-air.In VMDACERS,the sensible heat load can be calculated by Eq.(30).

Substituting Eq.(18) and Eq.(30) into Eq.(9) leads to Eq.(31).Additionally,Eq.(32) indicates that the temperature difference between evaporation temperature and supply-water temperature is 2.5°C.

From Eq.(16) to Eq.(32),the relationship between flow rate of supply-air and evaporation temperature in VMD-ACERS is established.The calculation process of the relationship is shown in Fig.4.The input parameters include flow rate of supply-air (Q),temperature of supply-water,return-water,return-airand supply-air.Besides,the relative humidity of return-air(60%)and supply-air(95%)are also two important input parameters.In Fig.4,the flow rate of supply-air(Q（i）)and the temperature and humidity of supply-air and return-air are provided for Eq.(16) and Eq.(17) to calculate the total load and sensible heat load.Simultaneously,the temperature of supply-water,return-water,return-airand supply-airare provided for Eq.(20) to calculate the average temperature difference of heat exchange when VMD-ACERS is not adopted.Conversely,when VMD-ACERS is adopted,the average temperature difference can be calculated by Eq.(29) under the condition that new flow rate of supply-air (Q（j）) is known.The temperature of supply-air,supply-waterand return watercan be calculated by Eqs.(23),(30)and (31).Finally,the evaporation temperature (T1（j）) of adopting VMD-ACERS can be obtained by solving Eq.(32).

Fig.4.Calculation process of relationship between flow rate of supply-air and evaporation temperature.

4.2.Full factorial numerical experiment method

The COP models for cooling systems has been established in Section 4.1.In order to investigate the key parameters affecting the performance of the cooling systems,some full factorial numerical experiments are designed.

When ACERS is not adopted,the cooling system is FCUCS.Both the sensible heat load and latent heat load are borne by FCU.The constant parameters are as follows.

(1) The temperature of supply-air is 16°C and relative humidity is 95%.

(2) The temperature of supply-water is 7°C and the temperature of return-water is 12°C.

(3) The return-air temperature is 26°C and the return-air relative humidity is 60%.

The variables are shown in Table 2.Here,the flow rate of FCUs are their lowest flow rate.The properties of the selected refrigerants are illustrated in Table 3 [34].

For VMD-ACERS,the sensible heat load and latent heat load are equal to the sensible heat load and the latent heat load in FCUCS,respectively.In addition,the return-air temperature and relative humidity of VMD-ACERS are consistent with that of FCUCS.The variables are shown in Table 4.

Table 2Ranges of variables for full factorial numerical experiment of FCUCS

Table 3The properties of the selected refrigerants

Analysis of range (ANORA) is employed to process results data of this work.This method is suitable to the importance ranking of parameters in COP models.For thexth variable and theyth level,the average result is calculated by Eq.(33).Here,zindicates the number of experimental results under the condition of thexth variable and theyth level.Here,Kindicates the result of numerical experiment.

The value of range at the condition of thexth variable is calculated by Eq.(34).In ANORA,the larger the value is,the more important thexth variable is.

4.3.The performance comparison of FCUCS and VMD-ACERS

For FCUCS,it is a reference object of VMS-ACERS,so the parameters are fixed values,as follows.

(1) The temperature of supply-air is 16°C and relative humidity is 95%.

(2) The temperature of supply-water is 7°C and the temperature of return-water is 12°C.

(3) The return-air temperature of is 26°C and the return-air relative humidity is 60%.

(4) The condensation temperature is 50°C.

(5) The subcooling is 10°C and the superheating is 5°C.

(6) The refrigerant is R134a.

(7) The FCU is FP136 and the flow rate of supply-air is 680 m3·h-1.

For VMD-ACERS,the sensible heat load and latent heat load are equal to the sensible heat load and the latent heat load in FCUCS,respectively.In addition,the return-air temperature and relative humidity of VMD-ACERS are consistent with that of FCUCS.The refrigerant is R134a.The temperature of permeate side is 25°C.The selectivity of membrane is 500.The variables are shown in Table 5.

Table 4Ranges of variables for full factorial numerical experiment of VMD-ACERS

Table 5Ranges of variables of VMD-ACERS for comparison with FCUCS

5.Results and Discussion

5.1.Model validation

The COP model of traditional vapor compression refrigeration unit is a key part in this work.In order to verify the reliability of this work,the results of COP model of traditional vapor compression refrigeration unit are compared with the latest explicit COP model proposed by Maet al.[32].Fig.5 shows the relationships between COP and evaporation temperature from Ma’s work [32]and this work.Similarly,the relationships between COP and condensation temperature from Ma’s work [32]and this work are shown in Fig.6.The results indicate that the COP from this work is enough close to the COP of literature [32].

Fig.5.COP of traditional vapor compression refrigeration unit versus evaporation temperature.

5.2.Importance ranking of parameters in FCUCS

Since the influence of these parameters on FCUCS can be found in many literatures,it will not be discussed in this work.Parameter importance ranking is the focus of this study.The results of ANORA are shown in Fig.7.It can be seen that the type of refrigerant has little effect on the importance ranking of parameters.By means of comparison,it can be found that the importance ranking of these parameters satisfies Eq.(35).

5.3.Parameter sensitivity analysis and importance ranking of VMDACERS

Fig.6.COP of traditional vapor compression refrigeration unit versus condensation temperature.

Fig.7.ANORA of COP of FCUCS with different fan coil units and refrigerants.

Since the COP model of VMD-ACERS is established by this work,it is necessary to study the sensitivity of these parameters before analyzing the importance of the parameters.Fig.8 shows the COP of VMD-ACERS as a function of FCU and flow rate of supply-air.For FP68 and FP136,the COP increases with the increasing of flow rate of supply-air firstly,and then decreases with the increasing of the flow rate of supply-air.The phenomenon indicates that increasing the flow rate of supply-air can increase the COP of VMD-ACERS,but when the flow rate of supply-air is too high,the COP will decrease.That is because high flow rate of supply-air brings more energy consumption of VMD-ACERS.For FP238,although COP is always increase with the increasing of flow rate of supply-air of FCU,it is obvious that the COP gradually became flat in Fig.8.Therefore,the way to improve the performance of VMD-ACERS by increasing flow rate of supply-air is only effective within a certain range of flow rate of supply-air.

Fig.8.COP of VMD-ACERS with different fan coil units versus flow of air supply.

In Fig.9,the COP of VMD-ACERS decreases with the increasing of permeate side temperature.However,increasing the permeate side temperature can effectively improve the dehumidification efficiency[17].Therefore,the feasibility of heating on the vacuum side deserves further study.Fig.10 shows the effect of selectivity of membrane on COP of VMD-ACERS.From Fig.10,it is noticed that selectivity of membrane has no obvious impact on COP of VMDACERS.Therefore,for VMD-ACERS,it is not necessary to pursue the membrane with very high selectivity.However,it does not mean that the selectivity of membrane is not important.It just means that it is relatively less important for VMD-ACERS when the selectivity of membrane reaches 500.This result is consistent with the analysis in the literature[16].In addition,we have investigated many reports in recent years,and found it is easy to achieve a selectivity of 500,as shown in Table 6.

Table 6The characteristics of recently developed membrane

Fig.9.COP of VMD-ACERS with different fan coil units versus temperature of permeate side.

Fig.10.COP of VMD-ACERS with different fan coil units versus selectivity of membrane.

Fig.11 shows the relationships between COP of VMD-ACERS and permeate side pressure under different FCUs.The COP always increases with the elevation of permeate side pressure.Woods [9]suggests that the permeate water vapor pressure should be lessthan 1.25 kPa,which is very difficult to realize in HVAC.However,many experimental results have proved that high permeate side pressure can also produce a certain degree of dehumidification effect.Especially,when the permeate side pressure is about 20 kPa,the relative humidity can be reduced from 80%to 35%after being treated by VMD [15].From the results illustrated in Fig.11,higher COP of VMD-ACERS can be realized by elevating permeate side pressure.Therefore,it is valuable and feasible to develop a VMD-ACERS with higher permeate side pressure.

Fig.11.COP of VMD-ACERS with different fan coil units versus pressure of permeate side.

Fig.12 shows the results of ANORA of COP of VMD-ACERS.When refrigerant is R134a,it is noticed that the most important parameter is permeate side pressure.This indicates that changing the permeate side pressure has an important impact on COP of VMS-ACERS.The second important parameter is condensation temperature.However,condensation temperature is limited by atmospheric temperature and it is difficult to be changed.Therefore,even if the condensation temperature is important to COP of VMS-ACERS,it is almost impossible to expect to elevate the COP by changing condensation temperature.The third important parameter is the permeate side temperature or the flow rate of supply-air,which depends on the FCU.For instance,when the FCU is FP68,the third important parameter is the permeate side temperature.However,when the FCU is FP238,the third important parameter is the flow rate of supply-air.Different fan coil units bring different flow rate of supply-air,which leads to different evaporation temperature.However,the sensitivity of the COP to evaporation temperature is different under different refrigerants.Therefore,the COP also responds differently to the flow rate of supply-air.It is the main reason for the change in the important ranking of these two parameters,the permeate side temperature and flow rate of supply-air.These two parameters are important optimization parameters for VMD-ACERS because they can be easily adjusted.In addition,superheating,subcooling and selectivity of membrane are relatively less important parameters.For better comparison,their importance is expressed in terms of the number of five-pointed star(★),as shown in the Table 7.The more the number of five-pointed star is,the more important the corresponding parameter is.From Table 7,it is noticed that the importance ranking of some parameters will vary with the refrigerant and FCU.However,the selectivity of membrane and superheating are two parameters whose importance ranking have never changed.

Table 7Importance ranking of parameters in VMD-ACERS

5.4.The performance comparison of FCUCS and VMD-ACERS

Fig.13 presents a more detailed comparison between VMDACERS and FCUCS.Here,FCUCS is utilized as a reference to check if VMD-ACERS is more efficient.In Fig.13,the dotted line represents the COP of the FCUCS when the flow rate of supply-air is 680 m3·h-1.In Fig.13,when the permeate side pressure is a fixed value,the COP of VMD-ACERS increases as the flow rate of supplyair increases.However,if the permeate side pressure is less than 6 kPa,the COP of VMD-ACRES will always be less than the COP of FCUCS.Thus,if the permeate side pressure is too low,the VMDACERS will be difficult to apply in engineering.Conversely,developing a VMD-ACERS with higher permeate side pressure is more promising.It is easy to notice that the COP of VMD-ACERS is higher than that of FCUCS when the permeate side pressure is higher than 8 kPa.It has been reported in the literature[15]that there is a significant dehumidification effect when the permeate side pressure is as high as 20 kPa.Therefore,VMD-ACERS with higher permeate side pressure is achievable.

Fig.12.ANORA of COP of VMD-ACERS with different fan coil units and refrigerants.

Fig.14 shows the evaporation temperature and the supply-air temperature of VMD-ACERS in different flow rate of supply-air.Because the permeate side pressure has no effect on evaporation temperature and supply-air temperature,there is not mark about permeate side pressure in Fig.14.When the flow rate of supply-air increases from 680 m3·h-1to 1360 m3·h-1,the evaporation temperature increases from 5.2°C to 11.98°C and the supply-air temperature increases from 16°C to 21°C.The dotted line indicates the evaporation temperature of FCUCS when flow rate of supplyair is 680 m3·h-1.The evaporation temperature of the VMDACERS is always higher than that of FCUCS.This is the main reason why the COP of the VMD-ACERS may be higher than the that of FCUCS.For ordinary air-conditioning systems,increasing the flow rate of supply-air would cause a noticeable blowing sensation and discomfort.However,VMD-ACERS can relieve this blowing sensation through metal orifice plate,as shown in Fig.1.This is a very important advantage of VMD-ACER.

It is worth noting that when the flow rate of supply-air of VMDACERS and FCUCS are both 680 m3.h-1,the evaporation temperature of VMD-ACERS is still higher than that of FCUCS.It is mainly because the air conditioning load decreases from total load to sensible heat load,and the FCU in VMD-ACERS need not to bear the task of dehumidification.Consequently,the evaporation temperature is higher than that in FCUCS.

6.Conclusions

In this work,a novel THIC system has been proposed,namely VMD-ACER,which integrates vacuum membrane-based dehumidification and air carrying energy radiant air-conditioning.A coefficient of performance (COP) model of VMD-ACERS has been established.The main parameters affecting the performance of conventional fan coil unit cooling system (FCUCS) and VMDACERS have been investigated,respectively.Finally,the performance of FCUCS and VMD-ACERS have been compared to prove the energy-saving potential of VMD-ACERS.Some conclusions can be made as follow:

Fig.13.COP of VMD-ACERS in different pressure in permeate versus flow of air supply.The dotted line indicates the COP of FCUCS.

Fig.14.Evaporation temperature and Supply-air temperature of VMD-ACERS versus flow of air supply.The dotted line indicates the evaporation temperature of FCUCS.

(1) For FCUCS,condensation temperature is the most important parameter.The importance ranking of parameters is basically stable and the choice of fan coil unit (FCU) and refrigerant would not affect the ranking.However,for VMDACERS,the importance ranking will be affected by FCU and refrigerant.In addition,the most important parameters are condensation temperature and permeate side pressure.Conversely,superheating,subcooling are relatively less important parameters.For VMD-ACERS,it is not necessary to pursue the membrane with very high selectivity,because the selectivity of membrane would also be a less important parameter when it reaches 500.

(2) The performance of VMD-ACERS can be improved by increasing the air supply volume in a certain range.The performance of VMD-ACERS can also be increased by increasing permeate side pressure and decreasing permeate side temperature,but these two methods would reduce dehumidification efficiency.

(3) The novel COP models in this work can be utilized to evaluate the comprehensive advantage of VMD-ACERS over FCUCS.By comparison,it is found that the COP of VMDACERS is higher than that of FCUCS when the permeate side pressure is higher than 8 kPa,which indicates that VMDACERS can be more efficient than FCUCS.The main reason for power-saving of VMD-ACERS is the increase of evaporation temperature.When the flow rate of supply-air increases from 680 m3·h-1to 1360 m3·h-1,the evaporation temperature increases from 5.2°C to 11.98°C.

(4) VMD-ACERS can solve two key technical problems about power-saving and thermal comfort of conventional THIC.VMD-ACERS could be more power-efficient than FCUCS,and can avoid the problems of blowing sensation and thermal discomfort in THIC.VMD-ACERS can extend the application of THIC air-conditioning system.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The National Key Technology Support Program(2015BAJ03B01)and the Hunan Provincial Innovation Foundation for Postgraduate Studies(CX20190287)provided financial assistance for this study.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.022.

Nomenclature

cSpecific heat,kJ·kg-1·K-1

FArea of heat exchanger,m2

hConvective heat transfer coefficient,W·m-2·K-1

htSpecific enthalpy,kJ·kg-1

kHeat capacity ratio

MwMolar mass of water,kg·kmol-1

˙mwFlow rate of water,kg·s-1

NuNusselt number

PPressure,Pa

QFlow rate of supply-air,m3·h-1

qsSensible heat load,kW

qtotalTotal heat load,kW

ReReynolds number

SSpecific entropy,kJ·kg-1·K-1

Sw/aSelectivity of membrane

TpTemperature of permeate side,K

TscSubcooling temperature,K

TshSuperheating temperature,K

T1Evaporation temperature,K

T3Condensation temperature,K

VSpecific volume,m3·kg-1

ΔVChange of specific volume of evaporation,m3·kg-1

w1Power of chiller,W

w2Power of Fan,W

w3Power of vacuum pump,W

ε The ratio of sensible heat load to total heat load

θ Heat of evaporation of water,kJ·kg-1

η1Isentropic efficiency of compressor

η2Isentropic efficiency of vacuum pump

Subscripts and Superscripts

aAir

ambFeed side or ambience

inInlet

outOutlet

vacPermeate side

wWater vapor or water