Ejector-Based Air Conditioner Utilizing Natural Refrigerants

Transcription

1 Purdue University Purdue e-pubs International Refrieration and Air Conditionin Conference School of Mechanical Enineerin 010 Ejector-Based Air Conditioner Utilizin Natural Refrierants Dariusz Butryowicz Institute of Fluid-Flow Machinery Mark J. Berander Manetic Developent Kail Sierciew Institute of Fluid-Flow Machinery Jaroslaw Karwacki Institute of Fluid-Flow Machinery Follow this and additional works at: Butryowicz, Dariusz; Berander, Mark J.; Sierciew, Kail; and Karwacki, Jaroslaw, "Ejector-Based Air Conditioner Utilizin Natural Refrierants" (010). International Refrieration and Air Conditionin Conference. Paper This docuent has been ade available throuh Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional inforation. Coplete proceedins ay be acquired in print and on CD-ROM directly fro the Ray W. Herrick Laboratories at Herrick/Events/orderlit.htl

2 10, Pae 1 Ejector-Based Air Conditioner Utilizin Natural Refrierants Dariusz BUTRYMOWICZ 1 *, Mark J. BERGANDER, Kail SMIERCIEW 1, Jaroslaw KARWACKI 1 1 Institute of Fluid-Flow Machinery, Polish Acadey of Sciences, Gdansk, Poland Phone: , fax: , butry@ip.da.pl Manetic Developent, Inc., Madison, CT, USA Phone: , ark@dienery.co * Correspondin Author ABSTRACT The paper deals with odelin of the ejection cycle, especially for solar air-conditionin. The presented approach for ejector analysis has been proposed, based on forulation of perforance curve of the installation and perforance curve of an ejector. The operation point of the whole syste is found at the interception of these two curves. The exeplary perforance curve of the ejector has been showed experientally and nuerically. 1. INTRODUCTION Residential and coercial air-conditionin consues over 15% of all electric enery enerated in the USA and creates two sources of environental pollution: 1) the ozone-depletion effect of traditional refrierants belonin to CFC and HCFC roups, and ) the eission of reenhouse ases connected with the eneration of electricity. Both sources are contributin sinificantly to the lobal warin effect. Additionally, with enery cost risin constantly, industry is lookin to reduce electricity expenses as a eans of lowerin their fixed costs in order to stay copetitive. This paper presents the developent of air-conditionin technoloy that copletely eliinates the ozone depletion effect by usin natural refrierants and also draatically reduces the need for electric power. This is accoplished by usin free or inexpensive heat either solar or waste heat, as the ain source of enery instead of electricity. The described syste is a odification of a well-known vapor copression cycle (VCC) and it uses our previously developed ejector device for non-echanical copression. Instead of pressurizin the refrierant by a echanical copressor, a pup copresses the liquefied refrierant, then heat is added to evaporate it and finally the refrierant is re-copressed in an ejector without any echanical enery spent. The ain difference between this cycle and the conventional refrieration cycle (reverse Rankine cycle), besides eliination of a copressor, is that it requires three heat sources at different teperatures rather than two, naely at the enerator level, which is the teperature of the solar or waste heat source, at a condensin level, which is the abient teperature (actually this is a heat sink) and the evaporator teperature required for coolin effect. The theoretical analysis of the cycle cobined with laboratory experients conducted on a specially constructed test-stand is described here. A new ethod of analysis was proposed based on the set of two perforance characteristics: the first for an ejector and the second one for the rest of the refrieration syste. The operatin paraeters of the whole syste are deterined as the intersection of these two characteristic lines. This is a very siilar ethod as in the case of the well-known liquid echanics procedure for pup selection for a iven pipeline syste. The ain objective of nuerical calculations and experiental evaluations was to deterine the ejector perforance, which is necessary to find the operatin point of the syste. The analysis presented in this paper deonstrates that the proposed syste is feasible fro a therodynaic standpoint and, furtherore, that the International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

3 10, Pae aount of heat required to produce a iven coolin load is reasonable and achievable fro the solar or waste heat sources.. AIR-CONDITIONING CYCLE The scheatics of the siplest ejection refrieration syste toether with its therodynaics cycle in lo(p)-h coordinates are presented in Fi. 1. Liquid refrierant is passed throuh the pup to the enerator (point 8). The liquid is then heated in the enerator by either enery fro solar panels or waste heat. First stae of heatin produces saturated vapor, which is then heated further and leaves the enerator in a superheated condition (point 1). The deree of vapor superheat is the function of the enerator capacity as well as ass flow rate. Such superheated vapor enters the otive nozzle of the ejector and underoes an expansion fro the enerator pressure p to the lower pressure, which is an evaporation pressure p e (point ). The ejector sucks in vapor fro the evaporator (point 7), and ixes it with expanded vapor (point ) and in consequence, the ixed vapor in state 3 is obtained. The pressure of the workin fluid initially rises slihtly as a result of the oentu exchane, and then it rises ore in the diffuser up to the point 4, achievin the level of the condensation pressure p c. Copressed vapor enters the condenser, where it condenses and ay also subcool dependin on the conditions of the coolin in the condenser. The workin fluid leaves the condenser in the liquid state (point 5). It is then divided to two parts: one part flows to the enerator throuh the circulatin pup, eanwhile the reainin part flows to the evaporator throuh the expansion (throttlin) valve, in which it is throttled to the evaporation pressure of p e, achievin the condition of wet vapor (point 6). Throuh the boilin in the evaporator, the workin fluid absorbs coolin capacity Q o fro the refrierated ediu, which ay be the air circulatin in the air-conditioned roo, or the ice water, which in the turn will cool the air-conditioned roo. Fiure 1: Scheatic diara of ejector device and ejector cycle in lo(p)-h co-ordinates The ain difference between this cycle and the conventional refrieration cycle (reverse Rankine cycle) besides eliination of a copressor, is that it requires three heat sources at different teperatures rather than two, naely at the enerator level (8-1) which is the teperature of the solar source, C, a condensin level (4-5), which is the abient teperature of 5-35 C and the evaporator teperature (6-7) at approx. 5-8 C. 3. APPROACH TO CYCLE ANALYSIS In order to analyze the above syste, we have proposed our own approach for odelin of ejection cycles. The ain concept of this ethod is to split the cycle and separate clean therodynaic relationship fro the paraeters characterizin the operation of ejector (Fi.1). This will allow to separate the investiation of the flow in an ejector, which is rather coplicated fro the operation of the entire syste, which on the other hand can be described by relatively siple equations. The applications of existin odels based on the conservations equations only, requires the assuption of a drivin pressure. However, both, the condensation pressure and the evaporation pressure are fixed paraeters in coolin devices. They depend upon the theral capacity of condenser, the teperature and the flow rate of coolin water, flow conditions throuh the condenser, etc. In addition, the perforance of the condenser is effected by external conditions, such as abient teperature. Siilarly, the evaporation pressure is a function of the coolin capacity of the evaporator, as well as the required coolin teperature. As far as prediction of the drivin (otive) pressure in an ejector, it was shown (Sierciew K. et. al, 008) that it was necessary to odel the relationship between the copression ratio and ass entrainent ratio Π = f(u) for the entire ejection process. The operatin point of the International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

4 10, Pae 3 syste was then deterined as an intersection of the perforance curve of an ejector and the syste capacity curve. Based on the real perforance curve of the ejector, the required otive pressure could be predicted for a iven coolin capacity, eoetry of the ejector, and assued operatin paraeters of the syste. Therefore, the ain task in the proposed ethod for the copression cycle analysis will be to prepare two characteristics (one for the ejector and another for the entire syste), describin the dependence of diensionless copression ratio Π on the entrainent ratio U. The knowlede of such perforance curves for the syste and for the ejector will allow to find the operatin point of the syste. This is a very siilar ethod as in the case of the well-known procedure fro liquid echanics for pup selection for a iven pipeline syste. The copression ratio is iven by the followin equation: pc pe p pe (1) The operation of the syste is described by the enery conservation equation only. Because p and U are unknown paraeters in ejection systes, and there is no available inforation about the p value in the enery conservation equation, there is a need to seek other relations in order to deterine the value of the drivin vapor pressure. For every ejector the individual operation curve can be prepared. Preparin the perforance curve is rather a difficult task in an absence of experiental investiations, and the only way is the nuerical odelin. The study of an exact odel is coplicated due to various processes occurrin in an ejector such as: ixin of the streas, ass, oentu and enery transfer, eddies, separation of the strea fro boundaries, shock waves, etc. Because of all these processes the odel of ejector operation becoes coplex. However, results received fro experients carried over on siilar ejectors under siilar operatin conditions, can be applied to siplify the entire odelin task. 3.1 Perforance of the syste The perforance of the syste results fro the enery balance and does not contain any inforation about the ejector, therefore it treats the ejector as a black box. The enery conservation law for the entire coolin syste can be written as Q c Qe Q P p () where Q c is capacity of the condenser, enerator, and P p Q e is the coolin capacity of the evaporator, Q is enery input to the P in coparison with reainin P is power of the pup. Because of the very sall value of P p coponents in forula (), it can be nelected. Now, the individual theral capacity of each device can be expressed as follows: after suitable transforations we can obtain: Q ( h h ) (3) e e 7 6 Q ( h h ) (4) 1 8 e ( 4 5 Q ( h h ) (5) c e h 1 eh 7 ( e ) 4 h1 h7 h (6) For a refrieratin plant, the quantity of transferred heat has to be known, therefore the coolin capacity of the evaporator Q e is also known. Fro Eq.(3) the ass flow of refrierant flowin throuh the evaporator can be deterined. The enthalpy h 7 is a function of pressure and teperature of vaporization: h 7 = h(p e, t e ) and t e = t s (p e )+ΔT e, where t s (p e ) is saturation teperature and ΔT e is the vapor superheat. Then, the enthalpy h 4 can be calculated fro the followin equation: International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

5 10, Pae 4 h( p ) Uh h4 ( p, U) 1U 1 7 (7) In order to describe the operation of the coolin syste, the processes occurrin inside of the ejector are not considered. However, the condition of refrierant at the ejector outlet has to be known. This leads to the assuptions that ixin takes place at constant pressure and processes of expansion and copression are isentropic. If the pressure and the aount of superheat of drivin fluid are known, then teperature of fluid is: t = t s (p )+ΔT, and the enthalpy h 1 can be obtained as a function h 1 = h(p, t ). Assuin an initial entrainent ratio U, the enthalpy at the ejector outlet can be found fro Eq. (7). Enery conservations equation between inlet and outlet of the drivin nozzle can be written as: 1 h1( p ) h w (8) And enery equation for a ixin chaber: 1 1 h w U h7 (1 U) h3 w3 (9) Further assuin zero velocity of vapor after ixin process (state 3), expressin the outlet velocity fro the otive nozzle usin the difference of enthalpies w =(h 1 -h ), and akin a few other transforation, ives: h( p ) h h( p ) Uh h (1 U) (1 U) (10) Thus, the enthalpy h 3 is a function of p and entrainent ratio U can be expressed as follows: where h( p ) Uh h( p ) h p U h h (1 U) (1 U) h ( p ) h 1 ( p, U) (1 U) h (, ) h ( p, U) (11) (1) is the kinetic enery chaned at the diffuser resultin in static pressure rise. It needs to note that the above result of h 3 is not a total enthalpy but a static enthalpy. The total enthalpy is: h ( p, U) h ( p, U) h h (13) 3' 3 4 Calculatin the entropy at this state, pressure at the outlet fro the ejector can be defined as p 4 = p(s 3=4, h 4 ). This pressure has to be copared with assued p c, and in case of a sinificant difference, the entrainent ratio U should be recalculated. These calculations should be repeated with different U values, until the difference between p 4 and p c is not sinificant. As a result, one point of the characteristic Π = f(u i ) is deterined. In order to obtain the entire characteristics of the syste, the above calculations should be repeated for different values of the drivin vapor pressure and data points plotted. 3. Perforance of the ejector The odel describin the work of isentropic ejector was developed for the purpose of this project. For a iven ejector eoetry, specific paraeters describin the processes inside the ejector are: p e, T e, p c, T. Additionally, the drivin vapor pressure p, was initially assued. The enthalpy and density of drivin vapor was calculated as functions: h 1 = h(p, t ) and ρ 1 = ρ(p, t ). As the consequence of expandin in the nozzle, vapor achieves pressure p = p 7 = p e. The reainin paraeter of expanded vapor can be found as: h = h(p, s 1= ), t = t(p, s ), ρ = ρ (p, t ). Constant value of specific heat was assued. The critical velocity in the nozzle throat is iven by: International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

6 10, Pae 5 w cr p ( 1)/ 1 (14) The ass flow rate of otive vapor can be calculated usin the followin equation: 1/ 1 p At 1 (15) T R 1 where A t is the cross-section area of the throat of the nozzle, κ is heat capacity ratio κ = c p /c v, β is the critical pressure ratio. Because two streas with different teperatures enter the ixin chaber separately, their averae teperature was deterined. The teperature of a ixture at the end of the cylindrical part of ejector is iven by an isentropic relationship: ( 1)/ t Ut 7 p ( U) t( p, U) (16) 1U p In the above equation p (U), pressure of the ixture is an unknown paraeter that depends on the entrainent ratio U. Usin a siilar isentropic relationship as in eqn. (16), the density of the ixture ρ (p ) and the outlet teperature t 4 = t c can be calculated. Velocity of the ixture can be calculated fro ass balance equation and velocity at the ejector outlet can be obtained fro the continuity equation: 11 U w( p, U) (17) A ( p ) 11 U w4 ( p, U) A d 4 (18) where A d is the cross-section area of the of diffuser, and ρ 4 = ρ(p 4, t 4 ) is the density of the ixture at the outlet of the diffuser. Velocity achieved by the secondary vapor at the inlet to the ixin chaber is iven by the followin equation: U w7 ( U) A 7 7 (19) where, area for secondary fluid at the ixin chaber inlet A 7 = A - A, A is the ixin chaber area, A drivin nozzle outlet area and ρ 7 = ρ(p e, t e ) ρ. Values of p and U are unknown, it is therefore necessary to coe up with an additional relationship in order to find the solution. This can be a oentu conservation equation, written separately for a ixin chaber Eq. (0) and a diffuser Eq. (1): A ( p p ) (1 U ) w w U w 7 0 (0) 1 Ap Ad Ap p4ap d 4 e w 4 w (1) Equations (0) and (1) are siultaneous equations with two unknowns: ixture pressure p and entrainent ratio U., which now can be deterined. The assuptions, taken for writin the above oentu conservation equations should be noted. Specifically, friction forces were nelected and shock wave were not considered. While it is International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

7 10, Pae 6 enerally reconized that the pressure rise in the ejector is caused by the shock wave, our approach assues that the copression is the result of oentu exchane only. 4. EXPERIMENTAL RESULTS The operatin point of the ejector should be known in order to develop a ethodoloy for its desin. It can be found either theoretically by odelin or experientally. Within this project, we considered both approaches for isobutene used as a refrierant. For the experients, the testin ri was erected accordin to the scheatics shown in Fi.. The ejector has been initially desined, based on the above presented theory and its approxiate eoetry is shown in Fi. 3. The syste was instruented with teperature and pressure sensors, ass floweters for refrierant and coolant. Separate heat load syste was built for an evaporator and a heat rejection syste for a condenser. Both were filled with a lycol solution. 1 vapor enerator evaporator 3 secondary fluid ass floweter 4 priary fluid ass floweter 5 condenser 6 liquid refrierant storae 7 lycol pup 8 refrierant pup 9 lycol ass floweter 10 throttlin valve 11 ejector 1 solar collector Fiure : Scheatic of the testin ri. Fiure 3: Axi-syetric cross-section of tested ejector The ain source of heat was a solar syste consistin of 1 vacuu solar collectors installed on the roof of the laboratory. The axiu aount of solar heat was approxiately 0 kw. Additionally, electric heaters were installed to provide an auxiliary heat source. Preliinary tests have confired the stable operation of the syste under air-conditionin conditions suitable for ceilin-coolin. Systeatic experiental investiations were carried out for a wide rane of operation paraeters of the ejector includin entrainent ratio, pressure rise, as well as pressure and teperature profiles in the ixin chaber and the diffuser. An exeplary ejector perforance is presented in Fi. 4. International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

8 10, Pae 7 enerator pressure p = 9.1 bar evaporation pressure p e = var condensin pressure p c =.0 bar ass flow rate = k/h Fiure 4: Experiental copression ratio Π versus ass entrainent ratio U. Coercial CFD code was used for odelin of the isobutene ejector operatin in the superheated vapor reion. A two-diensional axisyetrical odel was applied (Butryowicz et. al, 008) for calculations presented in this paper. The therodynaic properties of the workin fluid are ipleented fro the NIST database. A six-equation turbulence odel was applied, in which Reynolds stresses are calculated directly. The results of the siulation in coparison with experiental results are presented in Fi. 5. Boundary condition used in CFD siulation are shown in Table 1. l = inlet to the ixin chaber Fiure 5: Coparison of calculated static pressure profile with experiental data Table 1: Coparison of CFD results with experiental results priary flow p t p e t e p c t secondary flow e c [bar] [ o C] [bar] [ o C] [bar] [ o [k/h] [k/h] C] EXP CFD EXP CFD The nuerical results show reasonably ood areeent with experiental data, especially in ters of ass flow rate of the otive vapor. However, soe discrepancy for ass flow rate of the secondary fluid e was International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010

9 10, Pae 8 observed. Entrainent ratio U in CFD siulation is hiher (U CFD = 0.56) than easured values (U EXP = 0.435) for tested operation paraeters. It is possible that the above phenoenon is caused by presence of therocouples with the diaeter of 0.5 inside the ixin chaber of the ejector. The easurin caps of these therocouples were located at the center line of the ixin chaber. The presence of the therocouples could be included in nuerical calculations. However, this requires application of the three-diensional odel, which is planned for the future. 5. CONCLUSIONS On the basis of the presented results, the followin was concluded: The odel was forulated for an ejection air-conditionin cycle. It consists of the therodynaic perforance of the syste and the characteristics of the ejector. The perforance of the ejector ay be found by nuerical odelin or by experiental investiations. Both approaches have been presented in this paper. The reasonable areeent between theoretical CFD results and experiental test has been found for isobutene used as a refrierant. Further investiations with natural refrierants are needed to prepare ore eneral inforation and ore accurate odels on relevant ejector paraeters. Based on odelin as well as experientation, the perforance of the ejector can be predicted. The described air-conditioner is intended for residential and coercial buildins, wherever either low-rade waste heat or solar heat is available. However any other applications are possible, for exaple, there is a reat potential of usin the developed technoloy for refrieration and air-conditionin purposes in Third World countries and in reote areas where electric enery is unavailable. Sall solar-based units can be developed for storae of edicines, perishable food, and to cool field clinics to nae just few possibilities. REFERENCES Butryowicz D., Trela M., Karwacki J., Ochryiuk T., Sierciew K., 008, Investiation and odelin of ejector for air-conditionin systes, Archives of Therodynaics, Vol. 9, No. 4: p Sierciew K., Butryowicz D., Karwacki J., 008, Approach of therodynaic analysis of ejection cycles, International Conference Desin and Operation of Environentally Friendly Refrieration and AC Systes, Poznań, Poland: p J. M. Abdulateef, et. Al, Review of Solar-Driven Ejector Refrieration Technoloies, Renewable and Sustainable Enery Reviews 13(009) REFPROP Reference Fluid Therodynaics and Transport Properties, version 8, Leon E.W., McLinden M.O., Huber M.L., National Institute of Standards and Technoloy, Boulder, 006 ACKNOWLEDGEMENT The aterial presented in this paper is based upon work supported by the National Science Foundation under STTR Phase II Award No and by the US Departent of Enery, under SBIR Phase I Award No. SC Additional fundin was provided under Polish Governent Grant No. PB N DISCLAIMER This report was prepared as an account of work sponsored by an aency of the United States Governent. Neither the United States Governent nor any aency thereof, nor any of their eployees, akes any warranty, express or iplied, or assues any leal liability or responsibility for the accuracy, copleteness, or usefulness of any inforation, apparatus, product, or process disclosed, or represents that its use would not infrine privately owned rihts. Reference herein to any specific coercial product, process, or service by trade nae, tradeark, anufacturer, or otherwise does not necessarily constitute or iply its endorseent, recoendation, or favorin by the United States Governent or any aency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Governent or any aency thereof. International Refrieration and Air Conditionin Conference at Purdue, July 1-15, 010