Te Exploration on te Energy Saving Potential of an Innovative Dual-temperature Air Conditioner and te Mecanism of te Teoretical Mixed Refrigeration Cycle Zao Lei, Zao Xijin, Hu Andu Professor, graduate student, graduate student (Scool of Environ & Muni Engi, Xi an University of Arcitecture and Tecnology, Xi an, Saanxi, 710055, PRCina) Abstract A conventional room air conditioner can cause unpleasant draft sensations and to produce poor indoor air quality A typical energy-saving radiant cooling air conditioning system makes use of cilled water for cooling It may also incur energy losses due to secondary eat transfer existing in te wole process To overcome tese disadvantages, an innovative dual-temperature air conditioning system and its corresponding teoretical mixed refrigeration cycle are proposed Tis consists of a separate air andling unit and a metal radiation panel as te dual-temperature evaporators, a compressor, a condenser, two termal expansion valves and an ejector Mass and energy conservation equations are establised for te air andling process and te teoretical mixed refrigeration cycle is analyzed Te state properties in te termal processes and system performance are determined and compared wit tose of te conventional air conditioner wit fres air It is found tat te coefficient of performance(cop)of te teoretical dual-temperature refrigeration cycle improves by 1335% to 747 Keywords dual-temperature air conditioner; mixed-refrigeration cycle; COP; ejector; metal radiation panel Zao, Lei, PD, Professor, Xi an University of Arcitecture & Tecnology leizao0308@otmailcom,+86-9-80169 Researc fields: Caracteristics of HVAC and refrigeration systems, Innovative tecnologies to reduce building energy consumption In a room wit a conventional air conditioner, te supplied air is not evenly distributed and tere may be a draft sensation, wic may cause air-conditioning symptoms, suc as eadace, cest distress, dizziness, etc [1] In comparison, te indoor air temperature distribution in rooms wit radiant cooling is relatively even and te termal comfort sensation can be improved Te energy consumption can be reduced to some extent as well If te radiant cooling can be supplemented by a fres air supplying system, it may even improve te indoor air quality [ 3] However, radiant cooling usually uses cilled water as te medium to supply cooling, tat is, te cilled water flows troug te pipes embedded on te panel wic cools te surface of te panel by eat convection and conduction Ten te panel cools te indoor air and walls by eat convection and radiation to acieve te desired air-conditioning effect It can reac termal balance and meet te requirement of building cooling load [4 5] In general, low temperature water needs to be used to deumidify te air to offset te indoor umidity generation Tus, condensation on te panel surface may occur and te evaporation temperature of te refrigeration cycle is relatively low as well Terefore, te COP of te refrigeration cycle remains relatively low In addition, energy losses caused by secondary eat transfer leave more room for te wole system performance to be improved if tey Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
can be avoided For te above reasons, an innovative dual-temperature air-conditioner and te corresponding teoretical refrigeration cycle were proposed,wic can provide cooling as well as fres air Te operational states at te design condition were determined and te performance of te teoretical refrigeration cycle was analyzed and compared wit tose of te conventional air conditioner wit fres air andling 1 MECHANISM AND SYSTEM PATTERN OF THE INNOVATIVE DUAL-TEMPETURE ROOM AIR CONDITIONER Te system scematic and te corresponding refrigeration cycle of an innovative air-conditioner are sown in Fig1 (a) and (b) Refrigerant vapor at low temperature (state 1a) is sucked into te compressor and compressed to be supereated vapor (state ) and ten leaves te compressor at te condensation pressure It is cooled and condensed to be te saturated refrigerant liquid (state 3) in te condenser and is discarged into te suction-line excanger, were it excanges eat wit te refrigerant vapor leaving te ejector to become sub-cooled liquid (state 3a) Te sub-cooled liquid at te condensation pressure flows troug two termal expansion valves trottled to be wet vapors at two different evaporation pressures (state 4 and 5) Ten tey enter te radiant panel and te air andling unit, respectively, and absorb eat to evaporate Te radiant panel acts as te ig temperature evaporator of te dual-temperature air conditioner On te top of te panel are embedded pipes of small diameter, in wic te refrigerant flows and becomes a saturated vapor (state 6) troug eat excange wit room air and walls by eat convection and eat radiation Te tube-fin eat excanger in te air-andling unit (AHU) acts as te low temperature evaporator, in wic te refrigerant evaporates to saturated vapor at low evaporation pressure (State 7) by absorbing eat from te fres air supplied as required to provide good indoor air quality Te refrigerant vapor leaving te ig temperature evaporator flows into te ejector and induces te refrigerant leaving te low temperature evaporator into te ejector Tese two streams of refrigerant mix and diffuse to become state 1 and enter te suction-line eat excanger, in wic it excanges eat wit te refrigerant leaving te condenser Ten it becomes supereated vapor (state 1a) and returns to te compressor Tus, te cycle is completed [6] Te evaporation temperature of te refrigerant in te low temperature evaporator is comparatively low and is used to deumidify te fres air, wic offsets te latent cooling load Te amount of te air andled reduces, comparing wit tat in te conventional air conditioner Tus, te size of te tube-fin eat excanger can be reduced appropriately Te fres air flows troug a specifically designed nozzle, inducing indoor air to mix and reac a proper temperature, wic is supplied to te occupied zone Te condensing water formed on te surface of te tube-fin eat excanger can be easily collected and discarged to outdoor troug a pipe [7] Te refrigerant flowing troug te radiant panel evaporates at a comparatively ig evaporation temperature, only meeting te sensible cooling load Tus, condensation can be avoided from occurring on te panel And te improved evaporation temperature elps to improve te COP of te refrigeration cycle Te cilled radiation may elp to better te indoor termal comfort as well 1Compressor Condenser 3a/3bTermal expansion valve 4Metal radiant panel 5Air-andling unit 6Ejector 7Suction-line eat excanger 8 Room (a) Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
(b) Fig1 Scematic of te dual-temperature air conditioner and te corresponding teoretical refrigeration cycle OPETIONAL STATES AND PERFORMANCE ANALYSIS ON THE DUAL-TEMPETURE AIR CONDITIONER Mass and energy conservation equations were establised for te teoretical refrigeration cycle and te air-andling process of te conventional and te dual-temperature air-conditioner in order to find te energy saving potential of te new system proposed Te operational states and performance were analyzed 1 Determination on te Ratio of te Load Sared by Two Evaporators and te Supply Air Parameters How te cooling load is sared by te radiant panel and te AHU in te dual-temperature air conditioner not only determines te indoor termal comfort and te indoor air quality, but also determines te mass flow rates troug te two evaporators, te cooling capacity and te COP of te wole system [8-11] Tis is mainly decided by te sensible and latent cooling loads An office of 40 m in Xi an was selected as an example Te design calculation was conducted for te dual-temperature air conditioning system to determine te ratio of cooling load sared by te radiant panel and AHU Tere is a west-facing exterior wall and two windows of size m 18m eac facing westward Te adjacent rooms are all air-conditioned, and considered adiabatic Suppose tat tere are four persons, from wom te sensible and latent eat emissions are 605W/( p) and 733W/( p), respectively, wit te moisture emission rate of 109g/( p) Te eat emission rate of te illumination and office equipment are 160W and 00W, respectively Te design air-conditioning outdoor dry-bulb and wet-bulb temperature are 35 and 58 Te indoor design temperature and relative umidity are 6 and 50% To acieve te desired indoor termal comfort and to ensure satisfactory indoor air quality, te amount of te fres air to be supplied is cosen to be 30m 3 /( p), tat is, 10 m 3 / in total Tis is 83 L/s-p or 4 cfm/p Tere is a small air outlet installed in te office to ensure indoor pressure balance and mass conservation Te office design cooling load was calculated to be 5031W, using te metod recommended by te local code and andbook [1,13] Te dual-temperature air conditioner makes use of te radiant panel and te air-andling unit as te evaporators Te structure of te AHU and te air andling process are scematically sown in (a) and (b) Te outdoor fres air is cooled and deumidified in te AHU and induces te indoor air to mix and reac te feasible supply air state SA Ten it is supplied into te office Tat is, te fres air supplied offsets te indoor moisture generation Tus, te umidity ratio and te dew-point SA1 of te supplied fres air can be obtained by calculations using eq (1)~(4) 9 Outdoor air inlet 10Return air inlet 11Supply air outlet 1Condensate drain plate 13Condensate drain pipe 14 Coil of evaporator 15Fan 16Induction nozzle (a) Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
(b) Fig Structure of te induction type fan coil and te -d diagram of te air andling process V were ρ(d d ) (1) V SA1 D denotes te volumetric flow rate of te outdoor fres air, m 3 /; denotes te air density, kg/m 3 ;d represents te umidity ratio of te return air, kg/kg;d SA1 represents te umidity ratio of te air at apparatus dew point, kg/kg; indoor moisture generation rate, kg/ were ( V V V ρ V ρ d SA1 ρ) d V SA ρ d D represents te represents te volumetric flow rate of () te return air, m 3 /; d SA represents te umidity ratio of te supply air, kg/kg ( V ρ V ρ) SA (3) V ρ V ρ SA1 were SA1 is te entalpy of te air at apparatus dew point, kj/kg; SA is te entalpy of te supply air, kj/kg; is te entalpy of te return air(kj/kg) SA 101 t SA ( 501 186 t 0 001d SA ) SA ( 4 ) were t SA is te supply air temperature, Taking te supply air temperature as 16,te parameters of eac state can be obtained Results are listed in Table 1 Since te amount of te fres air is 10m 3 /,te energy consumed in processing te fres air to te state SA1, te apparatus dew point, can be calculated by: Q V ( ) 031kW Te SA1 oter part of te cooling load, tat is, Q 1 3kW, will be andled by te radiant panel For te office studied, te cooling capacity ratio of te cilled panel and te AHU sould be 3: at te design condition To make te supply air temperature reac 16, some of te indoor air will be induced to mix wit te fres air after andling Te amount of te indoor air to be induced is calculated to be V 7531 m 3, and te relative umidity is 763% All te state parameters experienced in te air andling processes are illustrated in Table 1 Table 1 Te air parameters of eac state t/ t wet/ φ/% / kj/kg t dew / d/g/kg 35 580 4841 79435 486 1718 6 1864 50 5301 14796 10495 SA1 968 968 100 8587 968 7467 SA 16 1350 763 38009 11503 8635 Analysis on te Teoretical Refrigeration Cycle of te Dual-Temperature Air-Conditioner In order to analyze an idealized refrigeration cycle, te following assumptions were taken: (1) te pressure loss along flow pipes, in te condenser and evaporators, etc are neglected () Te refrigerant leaving te condenser and te evaporator are deemed as saturated liquid and saturated vapor, respectively (3) Te refrigerant is in quasi-equilibrium state and te processes occurring in te ejector are idealized neglecting various losses Te acceleration and te diffusion process in te ejector are deemed as isentropic, wic can make use of te energy to its advantages (4) Te pressure of te refrigerant leaving te nozzle in te ejector drops to te low evaporation pressure and mixes wit te refrigerant induced from te evaporator of low evaporation pressure at constant pressure (5) Te kinetic energies of te refrigerant entering te nozzle, leaving te evaporators and te ejector are neglected 1 Determination on te refrigerant mass flow rates of te two evaporators Te refrigerant mass flow rates troug te ig and low temperature evaporators at teir individual evaporation pressures are crucial parameters for te dual-temperature refrigeration cycle As sown in Fig1 (b), te refrigerant leaving te condenser flowing troug te suction-line eat excanger excanges eat wit te refrigerant from te ejector Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
and becomes sub-cooled, ten it is divided into two parts One part flows troug te termal expansion valve 3a, reacing te state 4 and enters te radiant panel, in wic it evaporates by absorbing eat from te indoor air and surfaces troug eat convection and eat radiation and reaces te state 6 Te oter part flows troug te termal expansion valve 3b, reacing te state 5, and enters te air-andling unit, in wic it evaporates by absorbing te energy from te fres air to reac te state 7 Assume te condensing temperature to be 40, te ig evaporating temperature to be 15 to avoid condensation from occurring on te radiant panel, and te low evaporating temperature to be 5 so tat te fres air can be deumidified Two streams of refrigerants mix in te ejector and flow into te suction-line eat excanger absorbing eat from te refrigerant leaving te condenser to become supereated vapor Assume te supereated temperature is 15 Taking R as te refrigerant, te state properties of state 3 3a 4 5 6 and 7 can be calculated by using te software REFROP80, as sown in Table Since te energy te refrigerant absorbs in te two evaporators are 30kW and 031kW,te mass flow rates of te refrigerant flowing troug tem are 17950 g/s and 1399 g/s, Q1 1 6 4 respectively, in terms of m ( ) and Q 7 5 m ( ) Determination on te suction pressure of te teoretical refrigeration cycle Te evaporating pressures of te refrigerant in two evaporators are different Te refrigerant leaving te ig temperature evaporator enters te ejector as te main stream accelerating troug an isentropic process in te nozzle to reac state 6s Ten it induces te refrigerant leaving te low temperature evaporator into te ejector and mixes togeter to reac state 8 And ten, it is diffused to reac state 1 in te diffuse section in te ejector Leaving te ejector, it flows into te suction-line eat excanger and is supereated to be state 1a, wic is te suction state of te compressor Te states and velocity of te refrigerant flowing troug te ejector are calculated as following Define te eject coefficient as te ratio of te mass flow rate of te induced fluid to tat of te main stream fluid, tat is m / m1 For te dual-temperature refrigeration cycle, te eject coefficient is te ratio of te mass flow rate of te refrigerant leaving te radiant panel to tat of te refrigerant leaving te air-andling unit Based on te energy conservation principle, te refrigerant of mass flow rate m 1 flows troug te nozzle wit its pressure decreased to te low evaporation pressure, and it is accelerated to te speed, c6 ( ) (5) s 6 6s Ten it mixes wit te induced refrigerant vapor of m leaving te low temperature evaporator at constant pressure In terms of te momentum conservation equation, te speed of te mixed refrigerant reaces c c (6 6 ) 6 s c7 c6s s 1 1 1 mix (6) Tus, te entalpy of te refrigeration at tis point becomes, 8 ( 6s 1 c6s ) 1 7 mix c (7) Te refrigerant is diffused in te ejector during an isentropic compression process, tat is s = s 8 1 reacing te inlet state of te suction-line eat excanger It can be expressed as, 1 8 c 67 1 mix ( 6s 1 6cs ) 1 7, ( 8 ) In te suction-line eat excanger, refrigerant vapor at suction pressure absorbs eat from te refrigerant liquid leaving te condenser to reac te state point Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
1a at 15 Ten it enters te compressor and experiences an isentropic compression process to reac state point, tat is, s1 s Te properties of te caracteristic states in te refrigeration cycle are listed in Table Te suction vapor is supereated by 66 Te compressor suction vapor pressure and discarge temperature are 064833MPa and 15336MPa, respectively And te pressure ratio of te compression process is 365 31 Determine te Air Supply State Te teoretical refrigeration cycle and air-andling process of te conventional air-conditioner wit fres air andling are illustrated in Fig3 (a) and 3(b) Table Refrigerant properties of te caracteristic states in te teoretical dual temperature refrigeration cycle State Point Pressure /MPa Temp / Entalpy /kj/kg Entropy kj/(kg k) 1 064833 94 40881 17418 1a 064833 15 4131 17569 15336 603 4351 17569 3 15336 40 4965 11665 3a 15336 3675 4533 1156 4 078931 15 4533 1158 5 058411 5 4533 1169 6 078931 15 41016 1730 6s 058411 5 4031 1730 7 058411 5 40685 17436 8 058411 5 40634 17418 3 Performance analysis on te teoretical dual-temperature refrigeration cycle Te teoretical energy consumption of te compressor can be calculated as W 1 1 (m m)( ) =0668 kw (9) Since te cooling capacities at two evaporation temperatures are 031 kw and 30 kw, respectively, for te dual-temperature refrigeration cycle, te COP Q1 Q is, COP W, it may reac 754 ideally 3 OPETIONAL STATES AND PERFORMANCE ANALYSIS OF A SIMILAR CONVENTIONAL AIR-CONDITIONER (a) (b) Fig3 Teoretical refrigeration cycle of te conventional air conditioner wit fres air and te -d diagram of te air andling process Te return air and fres air mix and flow across te evaporator and reject eat to te refrigerant inside Te air is supplied into te room to offset te cooling load and umidity generation In terms of te eat and umidity balance and te supply air is disposed to be saturated air, we can write ( V V ) (d dsa ( V SA ) D (10) Q V ) ( V V were is te entalpy of te outdoor fres air, ) (11) kj/kg; is te entalpy of te return air, kj/kg; represents te eat load of te room, kw Te supply air state can be determined, tat is, d o 1019g/kg, 0 3998kJ/k g,and te required Q Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
amount of te return air can also be obtained to be 877m 3 / All te state properties in te air-andling process are listed in Table 3 Table 3 State properties involved in air andling process of conventional air conditioner wit fres air t/ t wet / t dew indoor However, te flow rate of te air to be φ /% d /g/kg / /kj/kg / andled by te conventional air-conditioner is about 35 58 484 79435 486 1718 99m 3 /, wic consists of 10 m 3 / fres air and 6 1864 50 5301 14796 10495 87 m 3 / circulating air Tus, te size of te SA 144 144 100 3998 144 1019 air-andling unit and te required fan power can bot be reduced significantly In addition, te supply air 3 Analysis on Teoretical Refrigeration Cycle of Conventional Air-Conditioner Taking te rated refrigeration condition of te conventional air-conditioner as an example, tat is, supposing te condensing temperature to be 40, te evaporating temperature to be 5 and te supereated suction vapor temperature to be 15, te state properties of every state in te teoretical refrigeration cycle are determined, as listed in Table 4 Table 4 Refrigerant states properties in te teoretical refrigeration cycle of a conventional air conditioner State Point Pressure /MPa Temp / Entalpy /kj/kg Entropy /kj/(kgk) 1 058411 5 40685 17436 1a 058411 15 41438 1770 15336 654 43959 1770 3 15336 40 4965 11665 3a 15336 35 4304 1145 4 058411 5 4304 11547 Q 1 4 From m( ),te required refrigerant mass flow rate can be calculated to be m 3071g/s Te pressure ratio of te compress process is 66 and te COP of te teoretical refrigeration cycle is 1 4 COP 6 50 4 COMPARISION OF THE PERFORMANCE 1 OF THE TWO TYPES OF THE AIR-CONDITIONERS Te analysis stated above indicates tat te flow rate of te air to be andled by te AHU in te dual temperature air-conditioner is just 10m 3 /, wic can satisfy te fres air requirement of 4 persons temperature can reac 16 in te newly proposed air-conditioner, in wic te proposed fres air flows troug a nozzle and induces te return air to form te supply air Fres air and return air mix firstly and are cooled to become te supply air, wic is at te dew point, 14, to offset te indoor umid generation Tis indicates te supply air flow rate is reduced apparently Tus, te draft sensation and te noise can be reduced Also te termal comfort can be improved by radiant cooling, compared wit tat produced by te conventional room air conditioner In comparison wit te teoretical refrigeration cycle of te conventional air-conditioner, te suction pressure of te teoretical refrigeration cycle of te dual-temperature air conditioner improves to 06483 MPa from 05481 MPa Te pressure ratio of te compression process reduces by 994% As a result te power consumption decreases from 0770kW to 0677 kw, neglecting te fan power Te COP of te teoretical refrigeration cycle improves by 1373% to be 743 5 CONCLUSION An innovative dual-temperature air-conditioner wit air-andling unit and radiant panel as separate evaporators is proposed Te teoretical refrigeration cycle and its working mecanism are introduced Te operational states and system performance are determined and compared wit tose of a conventional air-conditioner wit fres air for a typical office cooling load Results indicate te COP Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014
of te new system may improve by 1373% to reac 747 wen te required amount of fres air and indoor umid generation are not very large Te structure of te radiant panel and te air-andling unit are being designed and needs furter investigations, wic will be publised soon ACKNOWLEDGEMENT Tis paper is sponsored by te key Saanxi Provincial innovative team project (01KTC-11) and Xi an scientific researc plan project (CX1176-5) REFERENCES [1] Wu Xiping000 Te Volume of Outdoor Air, te Way of Air Distribution and Air Conditioning Diseases Building Energy & Environment 4:67-68 [] Stanley A Mumma001 Ceiling Panel Cooling Systems ASH Journal (43):8-3 [3] Xia Xueying, Zang Xv, Cai Ning,et al 008 Tecnology Analysis and Experimental Researc on Radiant Floor Cooling Combined wit Dedicated 0ut-door Air System Journal of Refrigeration 4:19~3 [4] Yan Zenua, Huang Xiang, Xuan Yong mei008 Application Researc on Air conditioning System of Radiative Cooling by Capillary Tube Refrigeration 7(1): 65-68 [5] Wang Zijie004 Low-temperature radiant eating and radiant cooling Bei Jing: Macinery Industry Pre- ss [6] Liu Jingui, Cen Jiangping, Cen Zijiu006 COP Comparison of Refrigeration Cycles wit Two-Stage Evaporating Temperatures Journal of Applied Sciences 4(5): 538-54 [7] Heating Class of te Civil Engineering Department at Tsingua University Selection Calculation and System Design of Induced Air-conditioner [J] Communication of Building Tecnology (HV&AC), 1976, (): 1-14 [8] Liu Suanqiang, Liu Xiaoua, Jiang Yi 010 Dedicated outdoor air system in te THIC air conditioning system (1): calculation of space moisture load HV&AC 40(1):80~84 [9] Liu Suanqiang, Liu Xiaoua,Jiang Yi010 Dedicated outdoor air system in te THIC air conditioning system (): Determination of Supply air parameters HV&AC 40(1): 85~90 [10] Zao Rongyi, Fan Cunyang, Xue Dianua,et al009 Air Conditioning Bei Jing: Cina Building Industry Press [11] Yin Ping000 Researc of large temperature difference in air conditioning (): Te design metod of a large temperature difference air conditioning supply system HV&AC, 30(5):63-66 [1] Lu Yaoqing007 Practical Design Manual eating and air conditioning Beijing: Cina Building Industry Press [13] 01 Design code for eating ventilation and air conditioning of civil buildings, GB50736-01 Proceedings of te 14t International Conference for Enanced Building Operations, Beijing, Cina, September 14-17, 014