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Energy and Buildings 36 2004 690 695 Combined air conditioning and tap water heating plant using CO2as refrigerant Willy Adriansyah Thermodynamic Research Laboratory Inter University Research Center Institut Teknologi Bandung 40132 Bandung West Java Indonesia Abstract A combined air conditioning and tap water heating plant using carbon dioxide CO2 as refrigerant has been investigated theoretically and experimentally The system is suitable for countries with year around cooling demand such as Indonesia or Singapore and a need for hot tap water A unique CO2transcritical cycle characteristic for heating process can afford an improvement to a CO2air conditioning system when rejected heat from the system is recovered Some parameters affecting performance of the combined system are discussed 2004 Elsevier B V All rights reserved Keywords Combined ac and water heating Carbon dioxide Transcritical cycle Optimum pressure 1 Introduction The use of natural refrigerant after CFC era has been attracting many research institutions and related industries Among common substances that can be used as refrig erant such as air NH3 carbon dioxide CO2 or R 744 hydro carbon and water CO2has unique characteristics and almost fulfi ll all required properties to be used as re frigerant It has zero ODP and negligible GWP high heat transfer coeffi cient excellent availability compatible with material of refrigeration system very low cost and the most important free from refrigerant market monopoly 1 However there is one short of drawback The operating pressure is high around eight times compared to that of conventional refrigerants This needs completely new design of system components and with current construction and production technology the components can be produced 2 Various applications of CO2as refrigerant in transcrit ical cycle have been investigated and some of them show promising result especially for heat pump 3 For air conditioning application the performance of CO2is com parable to that of R 22 or R 134a 4 The other interesting potential of CO2is for producing simultaneous cooling and heating Such system could yield superior total system effi ciency ratio of cooling load and heating load to compressor power consumption because of the nature of transcritical cycle which absorbs energy at constant temperature and rejects energy at gliding temperature This type of applica E mail address willy termo pauir itb ac id W Adriansyah tion is suitable in tropical countries such as Indonesia and Singapore where cooling is needed year around In a build ing which needs air conditioning and water heating use of rejecting heat from an air conditioning system to produce hot water could conserve energy Depending on the type of buildings and occupant culture recovered energy from the air conditioning system could be quite high In this paper a combined air conditioning and tap water heating system operates in transcritical cycle will be discussed 2 Transcritical cycle The word transcritical comes from the characteristic of this cycle Heat is absorbed at constant temperature below the critical temperature while heat rejection process takes place at pressure above the critical pressure The reason why CO2should be operated in this cycle is that the critical temperature of CO2is very low i e 31 C and for most air conditioning system run in hot climate most of outdoor air temperature is around or above this value Even when the air conditioning system could be run below the critical point refrigerating capacity will be very low Fig 1 shows transcritical cycle on Phdiagram on which effect of heat rejection pressure is depicted The corresponding fl ow sheet is shown to the right on the fi gure Since heat is rejected at supercritical region pressure and temperature is independence and for a specifi ed tempera ture the pressure can be changed There is no condensa tion process in this cycle and heat rejecting device is named gas cooler As can be seen on Fig 1 for the same outlet 0378 7788 see front matter 2004 Elsevier B V All rights reserved doi 10 1016 j enbuild 2004 01 014 W Adriansyah Energy and Buildings 36 2004 690 695691 Fig 1 Transcritical cycle on Ph diagram and the corresponding fl ow sheet temperature of the gas cooler point 3 or 3 compressor discharge pressure can be changed from pressure at point 2 to pressure at point 2 It can also be observed that specifi c refrigerating capac ity will change as discharge pressure changes It means the refrigerating capacity can be controlled by changing discharge pressure This capacity control cannot be applied in a conventional refrigeration cycle because all processes occur below critical point Since altering discharge pressure will change both specifi c refrigerating capacity and com pressor power coeffi cient of performance COP will vary with the discharge pressure Fig 2 shows variation of COP with discharge pressure where qe means specifi c refriger ating capacity wc means specifi c compressor power and cop means coeffi cient of performance Fig 2 also shows that specifi c refrigerating capacity in creases as discharge pressure increases The increase in specifi c refrigerating capacity is more than the increase in specifi c compressor power consumption until it reaches a point where the increase is slower Meanwhile com pressor power consumption increases more or less linear with discharge pressure Hence COP fi rst increases until a point and then decreases The pressure where COP reaches maximum value is called optimum pressure Under normal operation transcritical cycle should be run at the optimum Fig 2 Performance variations at various discharge pressures pressure When more cooling effect is needed for short time such as at pulling down period discharge pressure could be increased beyond the optimum pressure by sacrifi cing COP 3 Combined air conditioning and tap water heating Transcritical cycle is ideal for heating process Rejecting heat at supercritical region means heat transfer occurs by sensible cooling in the gas cooler Temperature of CO2will decrease continuously from discharge temperature to tem perature out of the gas cooler Because there is no condensa tion high hot water temperature can be achieved Hot water up to 90 C can be produce easily while heating COP still high 3 Suchhighhotwatertemperaturecannotbeachieved by a conventional heat pump system without running the system at very low heating COP Fig 3 shows two heat rejection processes in a transcritical cycle using CO2and in a conventional refrigeration cycle using R 22 Note that pinch temperature the smallest temperature difference in a heat exchanger occurs somewhere inside heat exchanger in conventional cycle due to condensation process while it can occur at the cold end of heat exchanger in transcritical cycle Temperature different at the cold end of heat rejection process is very important factor which affects COP This temperature different is named temperature approach For the same cooling medium temperature the lower tempera ture approach the higher the COP will be On Fig 3 it can be seen that the temperature approach is lower in CO2sys tem than in R 22 system because pinch temperature occurs at the cold end of heat exchanger From experimental evidence it is found that CO2has ex cellent performance in a hot water heat pump system while rather inferior in an air conditioning system compared to conventional cycle transcritical cycle operated as a com bined air conditioning and heat pump would be an interest ing application in areas where there is a need for cooling and heating simultaneously This combined system offers both saving in energy consumption for producing hot water 692W Adriansyah Energy and Buildings 36 2004 690 695 Fig 3 Heat rejection process in trancritical cycle left and conventional cycle right Fig 4 Ideal combined system with one gas coolers and improvement of the performance of the air conditioning system Fig 4 shows a schematic of an ideal combined system All rejected heat is utilized for water heating process and of course it will have the highest effi ciency In application where not all rejected heat is captured and in fact this is most of the case an additional gas cooler is needed to reject the rest of heat as shown in Figs 5 and 6 There can be two possible arrangements of these gas coolers series Fig 5 and parallel Fig 6 The parallel arrangement gives fl exibility in controlling capacity of the gas cooler for producing hot water The ca pacity can be controlled by regulating distribution of CO2 fl owing to both gas coolers Another advantage of the par allel confi guration is a higher improvement of the air con ditioning performance CO2temperature leaving the gas coolers will become lower as percentage of heat recov ery increases at condition where inlet water temperature is lowerthancoolingmediumtemperature Asthistemperature becomes lower the air conditioning performance becomes higher On the other hand the capacity is fi xed for operating con dition in the series arrangement Cooling COP will be dic tated by cooling medium temperature and enhancement of the air conditioning side is negligible Despite a minor en hancement there is an advantage of the series arrangement Fig 5 Combined system with series confi guration W Adriansyah Energy and Buildings 36 2004 690 695693 Fig 6 Combined system with parallel confi guration Heat transfer area of the additional gas cooler for the same hot water load is smaller compared to the parallel arrange ment This is because the entire mass of CO2from the com pressor fl ows through the additional gas cooler 4 Prototype of combined air conditioning and water heating system Test rig of the combined air conditioning and tap water heating system is a modifi ed version of hot water heat pump system developed by Zakeri et al 5 with an addition of a water cooled gas cooler and its water loop The original version of the test rig was designed for heat pump water heater using CO2with heating capacity of 50kW at 0 C evaporation temperature 7 C inlet water temperature and 60 C hot water temperature The new water loop together with a water cooled gas cooler is designed and built which is intended to simulate different heat sink conditions The two water cooled gas coolers in the test rig can be arranged in series or parallel by switching CO2 fl ows in the pipelineinordertoobservethecharacteristicofthesystemin different gas coolers confi guration Fig 7 shows photograph of the test rig Fig 7 Prototype of the combined air conditioning and water heating system Table 1 Relative uncertainties for two different discharge pressures ParameterPh 85bar Ph 110bar Compressor power 0 17 0 15 Air cooled gas cooler capacity 4 16 3 73 Evaporator capacity 4 66 3 88 Refrigerant fl ow rates 4 35 3 79 Cooling COP 4 66 3 88 Isentropic effi ciency 4 69 4 07 Volumetric effi ciency 4 37 3 81 5 Instrumentation and measurement accuracy All temperature measurements are carried out by type T thermocouple with accuracy 0 5 C All thermocouples are connected to a data logger that converts voltage signal from the thermocouples to temperature DRUCK pressure transducers are used to measure abso lute pressures which send voltage signal to the data logger To obtain more accurate measurement differential pressure transducers are used to measure pressure drop across the heat exchangers Accuracy of these absolute transducers is 0 1 of measured value and accuracy of the differential pressures is 0 04 of measured value Water fl ows through the heat exchangers are measured by turbine fl ow meters The accuracy of the fl ow meters are 0 2 of calibration span times water density CO2 fl ow rates are determined through heat balance cal culation in both gas coolers Three point thermopile was installed to measure temperature different across the gas coolers so that its accuracy becomes 0 3 C However in the fi nal report all uncertainties calculation is based on 0 5 C temperature accuracy A rotation meter and a torque meter are installed through which the compressor power consumption is calculated Ac curacy of the rotation meter is 1 rpm and accuracy of the torque meter is 0 5 of measured value Uncertainties of the experimental data are depending on the discharge pressure especially when running at a pressure around the critical region of CO2at which the uncertainties become higher Most of the optimum conditions are in a range from 85 to 105bar discharge pressure and the relative uncertainties for cooling capacity gas cooler capacity and cooling COP are around 5 Table 1 shows the uncertainties of the experimental results for three discharge pressures at 0 C evaporation temperature and 30 C cooling medium temperature 6 Method of controlling load ratio For parallel confi guration load ratio or percentage of heat recovery xr issetatdesiredvalue Foradischargepressure hot water load Qw can be calculated from the following relationship 694W Adriansyah Energy and Buildings 36 2004 690 695 Qw xr Q0 acbmode where Q0 acmode is total rejected heat of air condition ing without heat recovery at the same discharge pressure Hence for a certain inlet water temperature and hot water temperature the required water mass fl ow rates mw can be calculated through mw Qw Cpw Tw where Cpw is specifi c heat capacity and Twis temperature difference By controlling mass fl ow rates of CO2entering the additional gas cooler hot water temperature can be set to a desired value 7 Experimental results and discussion Fig 8 shows cooling COP at various load ratios It can be seen from the fi gure that at all load ratios the cooling COP trend is the same The main different is the location of the Fig 8 Cooling COP variation at various load ratio Tevap 0 C Tsink 30 C Tw in 20 C Tw out 60 C Fig 9 Cooling COP of the ac only series and parallel confi guration Tevap 0 C Tsink 30 C Tw in 20 C Tw out 60 C optimum pressure An advantage of parallel confi guration is that the cooling COP can be increased by increasing load ratio The optimum cooling COP was increased by 5 7 at 25 heat recovery up to 16 7 at full recovery mode Owing to a large capacity of the additional gas cooler the test rig is not suitable to be run as a combined system with series confi guration To investigate the effect of gas coolers confi guration a verifi ed simulation program are developed and used for the series confi guration The effect of the confi guration on cooling COP is shown in Fig 9 As seen in this fi gure the increase on maximum cooling COP of the series confi guration is negligible com pared to the system without heat recovery However in the parallel confi guration the maximum cooling COP increase from 3 00 to 3 24 at load ratio of 0 25 25 heat recovery and the optimum pressure shifted to a lower value from 90 to 86 2bar Thechangeincoolingcapacityfortheseriesconfi guration is also negligible At the optimum pressure the cooling ca pacity of the series confi guration was 23 6kW as compared to cooling capacity of the system without heat recovery of W Adriansyah Energy and Buildings 36 2004 690 695695 Fig 10 Total COP at various heat recovery ratios Tevap 0 C Tair 30 C Tw in 20 C Tw out 60 C 23 3kW while the cooling capacity of the parallel confi gu ration at optimum discharge pressure and load ratio of 0 25 was 24 6kW In term of overall performance for a combined air con ditioning and tap water heating system the utilization of rejected heat of the air conditioning system should be taken into account when calculating total coeffi cient of perfor mance total COP There will be two boundaries for the total COP the lower boundary when there is no heat recov ery and the upper boundary when all rejected heat is utilized Between these boundaries the total COP will vary depend on the percentage of heat recovery Fig 10 shows total COP as a function of discharge pressure at various heat recovery ratios As can be expected the total COP increases as heat recovery ratio increases It can be seen that the optimum discharge pressure are almost the same as that of the air con ditioning without heat recovery for all load ratios except for full recovery mode where the optimum pressure is higher 8 Summary As in CO2transcritical cycle air conditioning there will be an optimum condition for a CO2combined air condi tioning and tap water heating system at which the system reaches the highest cooling COP The optimum condition is determined by components parameters such as gas coolers confi guration and percentage of heat recovery In the parallel confi guration there is an improvement in air conditioning side as heat recovery increased and the opti mum condition will be depending on the percentage of heat recovery However in the series confi guration the infl uence of heat recovery on the performance of the air conditioning side is