Control-and-energy-simulation-of-variable-refrigerant-flow-air-conditioning-system-combined

Control and energy simulation of variable refrigerant?ow air conditioning system combined with outdoor air processing unit Yonghua Zhu,Xinqiao Jin*,Zhimin Du,Xing Fang,Bo Fan

School of Mechanical Engineering,Shanghai Jiao Tong University,Shanghai200240,PR China

h i g h l i g h t s

A VRF and ventilation combined system is proposed to take merits of both parts.

A simulation platform is developed for the combined system.

The combined system could maintain thermal comfort and indoor air quality.

Characteristics of system energy ef?ciency are revealed for further optimal control.

a r t i c l e i n f o

Article history:

Received1September2013 Accepted28December2013 Available online6January2014

Keywords:

Combined air conditioning system Variable refrigerant?ow

Outdoor air processing

Indoor air quality

Part load ratio a b s t r a c t

A variable refrigerant?ow(VRF)unit and outdoor air(OA)processing unit combined air conditioning system is proposed as a solution for ventilation problems in VRF systems.System structure and control strategies are addressed.Simulation platform is established based on component and sub-system models developed.Control and energy performances analysis are then put forward under various conditions.It is found that the combined system could maintain all the zones at their speci?c set-points within small errors no matter set-points of the zones are the same or different.In addition,indoor air quality can be ensured.Energy ef?ciency characteristics of the combined system are greatly affected by the OA supply temperature of the OA processing unit,which opens up the opportunity of minimizing the energy consumption of the combined system through optimal control strategy.Results reveal that the best OA supply temperature can be obtained through optimizing part load ratio of the OA processing unit to a range in which the system operates with high ef?ciency.

1.Introduction

It is estimated by recent simulation studies and?eld surveys that heating,ventilating and air-conditioning(HVAC)systems consume as much as40%of the total electricity use in the of?ce buildings[1],which arouses worldwide attention to study the methods for reducing energy use of the air conditioning systems. Conventional HVAC systems,such as all-air variable air volume (VAV)systems,have been found several de?ciencies along with the wide application in many types of buildings.These de?ciencies [2,3]including occupying much space,in?exibility in maintenance and commissioning,energy wasting for offsetting the extra cooling by reheating,etc.The variable refrigerant?ow(VRF)system,?rst introduced to the market in Japan,now has earned a wide application in both residential and commercial buildings,due mostly to high energy ef?ciency under part load condition and individualized thermal comfort capability[4e7].In addition,duct losses in the VRF systems can be almost ignored due to the in-space location of the indoor units[3].

Up to date,there have been many literature about experimental and numerical studying of the VRF systems.Xia et al.[8]developed a test rig for a three-pipe VRF system having?ve indoor units.It was found that the COP of the system did not vary too much ac-cording to the part load ratio.Aynur et al.[9,10]evaluated the performance of VRF systems by experimental and simulation ap-proaches.Choi and Kim[11]investigated the capacity modulation method by varying the indoor loads,the electronic expansion valve (EEV)opening and the compressor speed.Shi et al.[12]and Shao et al.[13]developed a?uid network model to simulate the per-formance of the three-pipe VRF system.Zhou et al.[4]developed a simulation module for the VRF system based on EnergyPlus by curve-?tting method for predicting and evaluating energy perfor-mances of the VRF systems and hence for making comparisons with

*Corresponding author.Institute of Refrigeration and Cryogenics,Shanghai Jiao Tong University,No.800,Rd.Dong-Chuan,Dist.Min-Hang,Shanghai200240,PR China.Tel./fax:t862134206774.

E-mail address:xqjin@https://www.360docs.net/doc/d418073601.html,(X.

Jin).

Contents lists available at ScienceDirect Applied Thermal Engineering

journal ho mepage:www.elsevier.co m/lo

cate/apthermeng

https://www.360docs.net/doc/d418073601.html,/10.1016/j.applthermaleng.2013.12.076

Applied Thermal Engineering64(2014)385e395

other types of HVAC systems.Similar research can be found in Ref.

[5]for a water-cooled VRF system.Zhu et al.[14,15]developed generic simulation models for the VRF system both in cooling and heating modes,where the generality means the model is component-number independent.Control performance of the VRF systems is another research focus.Masuda et al.[16]developed a control method for a VRF system with two indoor units.Chen et al.[17]investigated the control performance of a triple-evaporator system in which a relative simple model by black-box method was developed ?rst.The simulation results show good control performance in the evaporator temperature and superheat.Shah et al.[18]used the mean void fraction and moving boundary approach to develop the VRF system model for designing advanced closed-loop controllers.Lin and Yeh [19]addressed the feedback control design for a triple-evaporator air conditioner through sys-tematic identi ?cation.The identi ?cation procedure produces a low-order,linear model suitable for the controller design.The studies suggested that the EEV and the compressor speed should be controlled simultaneously to make the system operate in high ef ?ciency.

Besides the research endeavors that have been made for ?eld testing,energy and control simulation of the VRF system,combining VRF systems with other systems are also studied aiming at taking advantages of different systems.Aynur et al.[20,21]studied the performances of integration of VRF and heat pump desiccant both in cooling and heating modes.The integration was found to be promising in terms of energy saving and better indoor thermal comfort.Karunakaran and Parameshwaran [22,23]devel-oped a simulation model and a test rig for joint control of the VRF system and ventilation devices.The system actually is a variant of VAV system that the air is processed by two evaporators (in cooling mode)of the VRF laid in series.Jiang et al.[24]proposed a solid desiccant heat pump and VRF combined air conditioning system to form a temperature and humidity independent control system.

Literatures [3,20,21]also pointed out that the shortcoming of the VRF system lacking ventilation ability has not been solved thor-oughly.The VRF system can hardly ensure the indoor air quality (IAQ)alone without additional ventilating devices.In most cir-cumstances,an energy recovery ventilator [25,26]will be installed accompanying with the VRF system for inducing ef ?cient amount of ventilation air while recovering energy from the exhaust air stream in order to reduce the OA load.However,the energy recovery ventilator usually is not controllable for the outdoor air ?ow [27],thus ?xed rather than suitable outdoor air ?ow is supplied to the air conditioning zones,and it often results in energy wasting,which

will be worse in the case of applications with varying occupancy.Inadequate combination of VRF with the ventilation system (such as the energy recovery ventilator)either results in poor indoor thermal comfort,or more energy consumption due to the additional OA load [28].That ’s why the combination of the VRF system and the venti-lation system gains importance in practical applications.Unfortu-nately,researchers in the VRF ?eld focus their interests mostly on features of the equipment itself [6e 9,17e 19,29,30],e.g.,simulation or experiment research of the variable speed compressor and the electronic expansion valve (EEV)and novel control logic,etc.Still very few attentions [15,25,26]are paid to the combination of VRF system with ventilation system.With above considerations,a new air conditioning system combining VRF unit and OA processing unit is proposed aiming to take advantages of both parts.The OA pro-cessing unit cools and dehumidi ?es OA through an inherent direct expansion (DX)unit,and supplies the processed OA with low hu-midity to the indoor as the supply air in cooling mode.And the parallel VRF system accommodates the remaining loads,including the indoor loads and possible OA load.

In current research,the combined air conditioning system (noted as combined system hereinafter)is intended to be investi-gated in terms of control strategy,energy performance,thermal comfort and IAQ in cooling mode.Based on the system and sub-system models of VRF unit and other related components devel-oped and validated in previous researches [14,15,31,32],a TRNSYS [33]based simulation platform of the combined air conditioning system is developed ?rst.Simulation results are displayed and discussed following.

The remainder of the paper is organized as follows:Section 2describes simulation methodology of the combined system after the structure and control strategies are illustrated.Section 3dis-plays results and discussions of the combined system under the control strategies.Energy and ef ?ciency characteristics of the combined system are discussed in Section 4.And Section 5con-cludes this study.

2.Simulation of the combined system and its controls 2.1.Description of the combined system and control designing A schematic diagram of the proposed combined system is shown in Fig.1.The system can be divided into two parts struc-turally,VRF part and OA processing part.The VRF part consists of one outdoor unit and multiple indoor units (IDU),varying the refrigerant ?ow rate with the help of the variable speed

compressor

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in the outdoor unit and the electronic expansion valve (EEV)located in each IDU to match the cooling/heating load in order to maintain the zone air temperature at the indoor set-point.The OA processing part consists of a DX unit and VAV boxes.The OA conditioned by DX unit is supplied into zones by the supply fan through the VAV boxes.The VAV box regulates the conditioned OA ?ow corresponding to CO 2contaminant concentration for main-taining IAQ,since many studies [1,31]have found a worsening of IAQ outcomes at higher CO 2concentrations.In addition,there is an enthalpy wheel between the exhausted air and the OA to recover energy.

The control strategies of the combined system are also shown in Fig.1.The control strategies are designed for three targets:a)maintaining thermal comfort of the multi-zone,b)maintaining acceptable indoor air quality,and c)ensuring reliable operating of the VRF unit and the OA processing unit.

For VRF part,two control loops are set for accommodating the varying loads.The zone temperature control loop samples the return air temperature of each zone to regulate the opening of EEV in the IDU.The VRF capacity control loop samples superheated degree of the overall suction vapor to regulate the speed of the compressor.Mass ?ow rate of the refrigerant supplied to the evaporators are controlled by the two loops.

For OA processing part,four control loops are set for pro-cessing and regulating suf ?cient OA for ventilation.OA ?ow rate control is accomplished by combination of VAV box control and supply fan control based on concept of demanded control venti-lation (DCV)[31].When dampers of the zones are regulated,supply air static pressure in the air duct may change,and the supply fan control loop modulates the fan speed to maintain

constant supply static pressure.In addition,economizer cycle technique [34]is also incorporated and it works when the OA enthalpy is smaller than EA enthalpy in cooling mode.The other two control loops are designed for conditioning OA.The OA supply temperature is maintained by the EEV opening control loop through regulating the opening of the EEV.The DX capacity control loop samples the superheat degree to adjust the compressor speed.

2.2.Simulation of the combined system

The simulation of the proposed combined system is setup based on authors ’previous works [14,15,31,32].The developed simulator integrates the individual sub-system or component models into a complete system by use of TRNSYS software.It should be noted here that the ready-made multi-zone building model provided by TRNSYS,i.e.,Type56,is used for simulating thermal performances of each zone.However,parameters and settings of the building,including location,materials,dimensions,etc.,are provided accordingly,which will be detailed discussed and modeled in the following sub-section.

Both the VRF unit and the DX unit are air cooled refrigeration systems.In addition,the DX unit can be treated as a VRF unit with only one evaporator.It is reasonable to develop generic models (i.e.,independent of component numbers)for the VRF unit.In authors ’previous work [14],several simulation models for the VRF unit were developed and validated.All the models were proved of fast computing and evaporator-number independence,and showed good ability for control analysis.One of the VRF models is used in the simulation of the combined

system.

Fig.1.Schematic and control designing of the combined system.

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2.3.Building description and air conditioned zone model

An of?ce building is selected to accommodate the combined system.The building has six?oors in total above ground.Each?oor of the building is divided into six conditioned thermal zones,cor-responding to four outdoor exposures(east,west,south and north), an interior zone(including a ring-shaped corridor)and a center core,as shown in Fig.2.The center core is space for elevators and stairs.The air conditioned zones are numbered from zone1to zone 25for conveniently identifying their locations.A summary of the key parameters of the building is listed in Table1.

By considering each zone as a control volume,ordinary differ-ential equations[31,32],which express the transient behaviors of temperature,humidity and CO2concentration,are derived and described as follows.It is noted here that balances of the sensible load and the latent load are separately considered for having an insight into their variations and in accordance with the modeling methodology of TRNSYS.

M i c p d T i

d s

?m s;VRF;i c p

T s;VRF;iàT i

tm s;OA;i c p

T s;OAàT i

tQ i

(1)

M i dw i

d s

?m s;VRF;i

w s;VRF;iàw i

tm s;OA;i

w s;OAàw i

tD i(2)

V i d C i

d s

?v s;i

C s;iàC i

tG$Occp i(3)

where,M is the mass of the air,V is the volume of the zone,T is the indoor temperature,m s is the mass?ow rate of supply air,T s is the temperature of supply air,Q is the internal load,w is the indoor humidity ratio,w s is the humidity ratio of supply air,D is the in-ternal humidity load,C is the indoor CO2concentration,C s is the CO2concentration of supply air(360ppm is used),v s is the volume ?ow rate of supply air,Occp is the number of occupants and G is the amount of CO2emission rate of people(5?10à6m3/s is used).The subscript VRF,OA and i represent VRF part,OA processing part and i th zone,respectively.

The internal load(Q)and the internal humidity load(D), generated by occupant,lighting and equipment,are simulated by offering operation schedules to mimic the practical variations. Other factors,including solar radiation,heat transfer through window and envelops are not manually interacted.Table2de-scribes the methodology of heat gain calculation for occupant, lighting and equipment.LightMax,EqMax and OccMax denote designed lighting load per square,designed equipment load and designed occupancy number,respectively.LSchedule,ESchedule and OccSchedule denote possibility of maximum lighting,number of operating equipment and occupancy density,respectively.OHeat denotes heat generation rate per

person.

Fig.2.Typical?oor plan of the building(unit in drawing:meter).

Table1

Critical information of the building.

Item Description

Building location Shanghai,China

Building type and storeys Of?ce building,6-story

above ground

Gross?oor area

(air conditioned area)

4700m2

Typical?oor plan Area?28m?28m,

?oor-to-?oor height?3.5m

Zone height(m) 2.7m

Solid wood door size Width?height?1.0m?2.0m

Windows and shading Low-e double pane glazing.

Window width?height?1.8m

?1.5m;sill height?0.80m;

no shading device

External wall Aerated concrete wall,thickness?310mm

Internal wall Concrete blocks,thickness?200mm

Floor Reinforced concrete,thickness?120mm

Table2

Settings and calculation method of the heat gain.

Item Description

Lighting Lightload?LightMax?LSchedule?ZoneArea

LightMax?20W/m2;LSchedule(0e1)

Equipment Equipload?EqMax?ESchedule

EqMax?120W;ESchedule(0e6)

Occupant Occpload?OccMax?OccSchedule?OHeat

OccMax?6;Seated,very light activity,OHeat?65

Fig.3.Base occupant density and schedules of equipment and lighting.

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3.Control performances of the combined system 3.1.Test conditions

Zones 7to 12as shown in Fig.2are selected for the study because of good representing of internal and external factors such as orientations,radiations,etc.Heat generated by lighting and equipment are designed the same for each zone.Schedules of lighting and equipment are shown in Fig.3.The pro ?le of occu-pancy in each zone is not the same and is simply distinguished by multiplying a different occupant factor onto a base occupant den-sity,which is shown in Fig.3as well.The occupant factors for zone 7to 12are 0.95,1.0,0.8,1.05,1.10and 0.90,respectively.The light schedule (0e 1)describes the possibility of maximum lighting to mimic those of ?ces with intensive light that one can turn (some of)them on and off conveniently.The corridor adjacent to these zones is assumed to have an independent air conditioner,which has a thermostat set to 24 C in cooling mode.It is assumed that there is no air exchange between different ?oors in this paper.30th July of Shanghai,whose TMY (Typical Meteorological Year)weather pro-?le is shown in Fig.4,is chosen for representing the typical cooling conditions.The combined system is supposed to be operated dur-ing 8a.m.e 20

p.m.

Fig.4.TMY weather pro ?le of Shanghai in 30th

July.

Fig.5.Control performances of the VRF part in the same set-point case:(a)temperature,(b)relative humidity,(c)EEV opening,(d)compressor speed.

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3.2.Control performances

Two cases,the same and different temperature set-points of the zones are investigated.For the case of the same set-point,the set-points of all the 6zones are 24 C,and the OA supply temperature set-point is also 24 C.For the case of different set-point,the OA supply temperature set-point is 20 C,zone 7and zone 12are 24 C,zones 8is 25 C,zone 9and zone 10are 26 C.In addition,in the case of different set-point,zone 11is not controlled that the IDU is stopped to test how well the combined system adapts to different amount of operating indoor units.

3.2.1.Control performance of VRF part

Figs.5and 6show the control performances of the VRF part in the case of the same set-point and different set-point,respectively.

As shown in Figs.5a and 6a,all the zones can be maintained at their speci ?c set-points,and the control is also stable no matter the set-points change or not.There are tiny peaks and valleys in the temperature pro ?les of the zones in either of the cases,due mostly to the continuously varying OA ?ow rate.For zone 11in the case of different set-point,the temperature changes freely and rationally due to load variation since it is not controlled.The highest tem-perature achieves about 30 C at early afternoon when the cooling load is comparatively large during the day.The zone relative humidity is not directly controlled.Instead,in the process of meeting the sensible load of each zone,dew-point temperature of the air is forced below the tube wall temperature,coincidentally providing dehumidi ?cation as a result.However,as shown in Figs.5b and 6b,relative humidity of all the controlled air condi-tioned zones can meet the thermal comfort requirements as they are all maintained at about 40%e 65%.The separating of the relative humidity in the case of the same set-point is caused by the different occupant condition in each zone.The high temperature and relative humidity of zone 11in the case of different set-point imply the importance of air conditioning for buildings in summer in Shanghai.

Fig.5c and d depict the changing behaviors of the EEVs ’opening and the compressor speed (nominated by dividing the rated value,3000rpm)in the case of the same set-point,respectively.The corresponding variations in the case of different set-point are depicted in Fig.6c and d,respectively.In both cases,all the EEVs except that in zone 11in the case of different set-point quickly respond to the required cooling requirement,which is represented by the difference between the zone air return temperatures and their set-points and the change rates of the zone temperatures.In addition,the compressor speed also changes effectively in response to changes in superheated degree caused by variation of EEVs ’opening so that the compressor capacity can closely match the total

loads.

Fig.6.Control performances of the VRF part in different set-point case:(a)temperature,(b)relative humidity,(c)EEV opening,(d)compressor speed.

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3.2.2.Control performance of OA processing part

The control loops of the OA processing part are designed for ensuring suf ?cient ventilation air in order to reduce CO 2concen-tration,or in other words,to maintain the IAQ.Since CO 2generation is solely related to the number of occupants that it is independent of zone temperature and humidity,OA ?ow rate as well as CO 2concentration in either of the two cases are the same.Therefore,the control performances in OA processing part will be illustrated here only based upon the case of different set-point (Figs.7and 8).

As is shown in Fig.7a,the OA ?ow rate of the combined system varies gradually with the time.It is adjusted according to the variation of the occupants.The rising up and descending down of the OA ?ow rate and the occupancy are almost synchronized as a result.The valley in early afternoon re ?ects rational absence of the occupants for lunch,rest,etc.The least minimum OA mass ?ow rate in this research is set for ensuring basic sanitary condition of the ?oor when occupants are absent.

The OA ?ow rate is a result of the combined activities of the VAV box control and the supply fan control .Fig.7b and c depict the var-iations of the air damper in each VAV box and the supply fan fre-quency.The air dampers are all regulated in rational positions to allow appropriate amount of OA to maintain the zone CO 2con-centration at or below the limit,1000ppm.The supply fan fre-quency is increased or decreased with the openings of the air dampers to maintain the static pressure at the set-point,as shown in Fig.7c.On the other hand,during most of operation time,the openings of the dampers are all smaller than 75%,it means that there is potential for energy-saving optimal control,i.e.,the supply fan frequency could be optimized (reduced)to save energy.

The indoor CO 2concentration as shown in Fig.7d is well controlled at the IAQ-limit during most of the operation time.The reason of the exception of a short time at the beginning can be explained by the hysteresis of the control system.The tiny peaks and valleys of the CO 2concentration pro ?les are almost synchro-nized with the OA ?ow rate,which implies that the more stable the occupancy variation is,the better control of the OA ?ow rate as well as the CO 2concentration will be.

Fig.8shows the control performances of the DX unit,including the OA supply temperature,opening of the EEV and speed of the compressor.Fig.8a shows that the OA supply temperature can be quickly and well controlled at the set-point,with the help of quickly adjusting of the EEV opening and compressor speed,as shown in Fig.8b.

In conclusion,test results show that the combined system has good ability and performances responding to various conditions.The control strategies are effective and ef ?ciently implemented.In addition,the demand of individual control could also be met by applying this combined

system.

Fig.7.Control performances of OA ?ow:(a)OA ?ow rate,(b)air damper opening,(c)supply fan frequency,(d)CO 2concentration.

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4.Energy ef ?ciency characteristics of the combined system 4.1.Energy consumption under various conditions

Although the OA processing part is designed in basic purpose to supply and process suf ?cient OA for maintaining IAQ,it has ability to accommodate part of the indoor loads when the OA supply temperature is lower enough.In that way,there will be a trade-off between the loads taken by the VRF part and by the OA processing part.However,energy consumption of the combined system will not be the same due to varying ef ?ciencies of the individual parts under different load conditions.From the energy saving point of

view,the operation strategy that can provide suf ?cient cooling but consume the least energy will be preferred.Clearly,energy con-sumption decreases with increasing of COP.Therefore,the energy consumption of the combined system can be reduced or even minimized when both the VRF unit and the OA processing unit operates in the most ef ?cient way.However,it is not easy to make both the VRF unit and the OA processing unit operate with highest COP simultaneously before the characteristics of energy ef ?ciency are completely understood.

To understand the characteristics of energy ef ?ciency of the combined system under different conditions,a group of tests (GP-A)are carried out ?rst.Different OA supply temperature set-point,from 18to 26 C with a 2 C increment,is selected for each test,with all the zones ’set-point 24 C.Other testing conditions,including load settings,zone locations,and weather pro ?le,are not changed.

Figs.9and 10show test results of the cooling capacity and en-ergy consumption of the VRF unit and the OA processing unit in GP-A tests,where the cooling capacity and the energy consumption

are

Fig.8.Control performances of DX unit:(a)OA supply temperature,(b)Compressor speed and EEV

opening.

Fig.9.Cooling capacity of the combined system in GP-A

tests.Fig.10.Energy consumption of the combined system in GP-A

tests.

Fig.11.Energy consumption comparison of the combined system in GP-B tests.

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calculated by integral of the cooling and input power over the operating time,respectively.

As shown in Fig.9,the total cooling capacity of the combined system is the same in all the test cases.The cooling capacity of the OA processing part decreases with the increasing set-point of the OA supply temperature,while the cooling capacity of the VRF part acts in a reverse way.

As shown in Fig.10,the energy consumption of the VRF unit increases with increasing set-point of OA supply temperature for taking more OA loads.However,the energy consumption of the OA processing unit is not monotonely changing with the OA supply temperature.It decreases to the lowest value and then increases with increasing set-point of the OA supply temperature.It means that there is minimum energy consumption for the combined system,i.e.,there is a best OA supply temperature that can mini-mize the energy consumption of the combined system.

On the other hand,another group of tests(GP-B)are further carried out with the OA supply temperature set-point?xed(20 C) and the rated capacity of the OA processing unit(DX unit)varying from50%to150%of that used in GP-A in a25%increment.Other conditions keep the same as GP-A.The results are shown in Fig.11.

As shown in Fig.11,the energy consumption of the OA pro-cessing unit decreases to the lowest value when the current DX capacity(i.e.,the100%case)is used.It then increases with increasing DX capacity.It means that the DX capacity affects the energy consumption of the combined system.We should carefully select DX capacity for practical applications.Actually,the current DX capacity has been determined in GP-A tests for high energy ef?ciency by trial and error method.However,when the DX ca-pacity is determined,the energy consumption of the combined system is mainly affected by the OA supply temperature,as illus-trated in GP-A tests.4.2.Relationship between COP and PLR

The interesting results of the total energy consumption in both GP-A and GP-B can be further explained by the relationships be-tween COP and PLR,where they are calculated as follows.

COP?

W comtW fan

(4) PLR?

rated

(5)

Where Q and Q rate denote actual and rated cooling of the VRF unit, respectively.W denotes input power,and subscript com and fan denote compressor and supply fan,respectively.It should be noted here that Q and W used here are instantaneous values.

Figs.12and13show the relationships between COP and PLR of the VRF unit and the OA processing unit in GP-A,respectively, where the PLR is calculated as the ratio of the provided cooling to the rated capacity of the individual unit.Generally,for both units, COP decreases with increment of PLR,and vice versa.When PLR locates in the range from0.4to0.7,COP attains its maximum,which is in accordance with the conclusions of experimental studies[35]. COP falls to the lowest when PLR gets its value larger than0.8. However,in cases of high OA supply temperature set-point(e.g., 26 C),COP of OA processing unit increases or decreases in accor-dance with PLR strictly,which is caused by the DX compressor having to operate at its minimum value due to relatively small OA load.PLR and COP of the VRF unit change in a narrower range comparing to that of the OA processing unit.In other words,OA supply temperature has signi?cantly in?uences on operation of the OA processing unit,and the total energy consumption of the combined system is greatly impacted as a

result.

Fig.12.PLR and COP of VRF unit in GP-A

tests.Fig.13.PLR and COP of OA processing unit in GP-A tests.

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The relationship between COP and PLR of the OA processing unit in GP-B is shown in Fig.14.That of the VRF unit is not shown again since the operation of the VRF unit is not affected when the OA supply condition is ?xed.As shown in Fig.14,COP of the OA pro-cessing unit is improved as the DX capacity is decreased during early morning and late afternoon periods when the loads are small.However,the COP is not improved with the decreasing of DX ca-pacity at other times.On the other hand,the COP decreases signi ?cantly as the DX capacity is increased,especially when the capacity is 1.5times larger.And it is the reason that the energy consumption is increased dramatically in these cases.

Speci ?cally,it can be seen from Figs.13b and 14b that,in most of the operation time,PLR of the OA processing unit locates in the range from 0.4to 0.7when the OA supply temperature set-point is 20.So it results in high ef ?ciency operation of the OA processing unit among all the cases.From this point of view,energy con-sumption of the combined system can be reduced by suitable OA supply temperature set-point,i.e.,OA supply temperature set-point could be optimized according to the operation conditions.

In conclusion,results show that the energy ef ?ciency of the VRF unit as well as the OA processing unit changes with variation of load.It reaches the highest when PLR locates in certain range,which indicates that further optimization can be realized by mak-ing both the VRF unit and the OA processing unit operate in the highest ef ?ciency range.

5.Conclusions and future work

Simulations of a new air conditioning system combining VRF with OA processing unit in cooling conditions are presented.The illustration of the combined system has been introduced and its control loops has been designed.The simulation methodology is developed and the tests are carried out to validate the control

performances and energy ef ?ciency characteristics of the combined system.

The simulation tests results show that all the zones of the combined system could be maintained at their speci ?c set-points within a small error after the control is stable no matter the set-points are the same or different,and no matter the amount of operating indoor units changes or not.Besides,the IAQ can be ensured.

The relationship between COP and PLR indicates that further optimization can be realized by making both the VRF unit and the OA processing unit operate in the high ef ?ciency range.The results also show that there is a best OA supply temperature set-point that can optimize the energy consumption of the combined system.As the OA processing unit affects the operation of the combined sys-tem more signi ?cantly than the VRF unit,the optimization can be realized by simply controlling the PLR of the OA processing unit to the most ef ?cient system-operation range.

In this paper,characteristics of the combined system under regular control strategies are simulated and analyzed.Preliminary study about optimal control of the combined system is also carried out.Future work could be put forward in several aspects,especially in online optimization of the OA supply temperature.The results will be presented in subsequent papers.Acknowledgements

The research work presented in this paper is ?nancially sup-ported by National Natural Science Foundation of China (No.51376125)and National Natural Science Foundation of China (No.51376126).References

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