© Strojni{ki vestnik 46(2000)8,517-524 ISSN 0039-2480 UDK 621.574.013:620.92 Pregledni znanstveni ~lanek (1.02) © Journal of Mechanical Engineering 46(2000)8,517-524 ISSN 0039-2480 UDC 621.574.013:620.92 Review scientific paper (1.02) Analiza eksergijskih tokov absorpcijske hladilne naprave An Analysis of Exergy Flows in an Absorption Chiller Vasilije Vasi} - Jurij Krope - Darko Gori~anec V prispevku je podan postopek analize eksergijskih in anergijskih tokov v enostopenjski absorpcijski hladilni napravi z delovnim medijem LiBr/HO ter postopek izračuna eksergijskega izkoristka v odvisnosti od stopnje uporabe naprave. Nakazana je prednost absorpcijske hladilne naprave pred kompresijsko hladilno napravo, ki se izkazuje v možnosti uporabe eksergijsko revne odpadne toplote in v rabi alternativnih energetskih virov. © 2000 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: naprave hladilne, naprave absorpcijske, tok energijski, izkoristek eksergijski) This paper presents our analysis of the exergy and anergy flows and exergy efficiency of an absorption chiller which has been calculated for all areas of the devices exploitation. This analysis has been implemented on a single-stage absorption chiller with LiBr/HO as a working media. We wish to show the advantages of sorption chillers, in our case absorption chillers, in comparison to compressor chillers in utilizing low exergy, rejected heat and in the use of alternative energy sources. © 2000 Journal of Mechanical Engineering. All rights reserved. (Keywords: thermodynamics, absorption chiller, exergy flow, efficiency) 0 UVOD Potreba po hlajenju postaja v sodobnem svetu vedno večja zaradi večjih zahtev po bivalnem ugodju, kar pa je povezano z zahtevnejšimi tehnološkimi procesi. Hlad tako postaja enakovreden produkt električni energiji in toploti. Hlad se pridobiva najpogosteje s kompresorskimi hladilnimi napravami, katerim postajajo vse bolj konkurenčne absorpcijske hladilne naprave in namesto mehanske (električne) energije uporablja toploto. Absorpcijska hladilna naprava (sl. 1) je sestavljena iz dveh obtokov - gretja in hlajenja, ki sta med seboj povezana. Posebnost naprave je toplotni kompresor, medtem ko so elementi hladilnega obtoka enaki kakor pri kompresorski hladilni napravi. Delovne snovi absorpcijske hladilne naprave so okolju precej bolj prijazne od tistih pri kompresijski hladilni napravi. Najpogosteje se uporabljata delovni snovi - binarna zmes LiBr/HO ali H2O/NH3. 0 INTRODUCTION The demand for cooling is growing as peoples’ expectations of a more comfortable life, sur-rounded by technology continues to increase. Cool-ing is becoming a product equivalent to electricity and heat. The cooling process is most frequently carried out compressor chillers, of which absorption chillers are becoming increasingly significant. In contrast to the compressor chillers, absorption chillers are driven by heat rather than electricity. An absorption chiller (Figure 1) works us-ing two interconnected cycles - heating and cooling. The device’s speciality is its thermal compressor, while the cooling cycle is undertaken using the same components found in compressor chillers. The working media of absorption chillers are environmentaly friendly compared to those used by compressor chillers because the most frequently used working media for absorption chillers are bi-nary mixtures of LiBr/H2O and H2O/NH3. gfin^OtJJlMlSCSD 00-8 stran 517 |^BSSITIMIGC V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows 1 DELOVANJE NAPRAVE Delovanje absorpcijske hladilne naprave poteka, prikazano posplošeno, na dveh tlačnih nivojih, treh temperaturnih nivojih in treh nivojih koncentracije hladiva [1]. Generatorju in uparjalniku se toplota dovaja na najvišjem oz. najnižjem temperaturnem nivoju, medtem ko se okolici toplota predaja na srednjem temperaturnem nivoju iz kondenzatorja in absorberja (sl. 1). Vezava zunanjega obtoka kondenzatorja in absorberja je lahko ločena (vzporedna vezava) ali pa povezana (serijska vezava). Učinkovitejši način je serijska vezava [1], ki je uporabljena v primeru analize eksergijskih tokov. Izhodišče vsake termodinamične analize procesa ali postroja je energijska bilanca (prvi glavni zakon termodinamike). V apsorbcijski hladilni napravi 1 THE PERFORMANCE OF THE ABSORPTION CHILLER The functioning of an absorption chiller can be described simply as a process between two pres-sure levels, three temperature levels and three levels of coolant concentration [1]. Heat is provided to the generator and evapo-rator at either the highest or lowest temperature lev-els, respectively. In the mean time, heat is delivered from the absorber and the condenser to the surround-ings at the medium temperature level (Figure 1). The external-flow connection to the condenser and the absorber can be either independent (parallel flow) or connected (serial flow). Serial connection has proved to be the more efficient method [1] and has been adopted when presenting our analysis of exergy flows. At the beginning of each thermodynamic analysis of thermal processes or a thermal plant we use the energy balance (First Law of Thermodynam- 7 H-----* Generator Generator 11 r Porabniki tople sanitarne vode (ali hladilni stolp) Consumer of hot domestic water (or cooling tower) Prenosnik toplote Heat Exchanger Vir toplotne energije Driving Heat Source Toplotni kompresor Thermal Compressor Sl. 1. Shema absorpcijske hladilne naprave Fig. 1. Model of the absorption chiller 00-8 VH^tTPsDDIK stran 518 V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows imamo različne oblike energij (npr. toplotna, električna), katerih vrednost se kaže v stopnji zmožnosti za pretvarjanje v druge oblike energij. Popolnoma spremenljiv del energije v druge oblike energij se imenuje eksergija, nespremenljiv del pa anergija. Vse energije preračunamo na enako osnovo - eksergijo. Analiza eksergijskih tokov, ki temelji na drugem glavnem zakonu termodinamike, ima nalogo določiti: mesta nepovračljivosti, velikost nepovračlji-vosti, smer odvijanja procesov in celotno učinkovitost naprave. Pri analizi eksergijskih tokov se ne omejimo samo na analizo popolnosti procesov v napravi, temveč tudi na procese s toplotno menjavo delovne snovi s toplotnimi prejemniki in medsebojnimi vplivi na okolje [2]. Velikost nepovračljivosti - anergijskih tokov je praviloma odvisna od vrste opreme, delovne snovi in pogojev obratovanja. Osnovni vzroki nepovračljivosti energijskih procesov v absorpcijski hladilni napravi so [3]: - ohlajanje pare s temperature v generatorju na temperaturo kondenzacije, - dušenje hladilnega sredstva z dušilnim ventilom s tlaka kondenzacije na tlak uparjanja, - segrevanje pare z uparjalne temperature na temperaturo absorpcije, - prenos toplote v prenosniku toplote termičnega kompresorja, - segrevanje s hladivom bogate raztopine, - dušenje s hladivom revne raztopine za prenosnikom toplote in - prenos toplote na zunanje nosilce toplote v zunanjih obtokih. Kljub želji po zmanjšanju ali celo preprečitvi nastanka nepovračljivosti procesov imamo v dejanski absorpcijski hladilni napravi na nekatere nepovračljivosti zelo malo ali povsem nobenega vpliva. 2 PRERAČUN EKSERGIJSKIH TOKOV Preračun eksergijskih tokov je izveden na podlagi podatkov s preglednice 1 in literature ([1] in [4]) z določitvijo specifične eksergije snovnega toka, v karakterističnih točkah naprave (sl. 1 in 2). Specifična eksergija snovnega toka pove, koliko dela pridobi enota masnega toka pri povračljivem medsebojnem delovanju z okoljem [2]. Specifična eksergija snovnega toka v karakterističnih točkah naprave v hladilnem in zunanjem tokokrogu določimo z enačbo: ics). In the absorption chiller different forms of energy occur (e.g. heat, electricity), their values are re-flected through their ability to convert to other forms of energy. The fully convertible part of energy as another form of energy is called exergy (availability) and the unconvertible is called anergy. We therefore calculate all energy forms on the same basis – exergy. The analysis of exergy flows, according to the Second Law of Thermodynamics, is the a task of determining: places of irreversibility, the direction of occurred processes and the effective-ness of the whole device. When performing the analysis of exergy flows we did not only analyze the perfection of the processes in the device, but we also focussed on the processes which consider heat interaction with the working media and the heat recipients as well as the device’s mutual interaction with the environment [2]. How big are the irreversibilities – anergy flows usually depend on the type of equipment, the working fluid and the operating conditions. The basic causes of irreversibilities of the energy processes in absorption chillers are [3]: - steam cooling from the temperature in the generator at the condensing temperature; - throttling the cooling media with a throttle valve from the condensing pressure to the evaporation pressure; - warming up the vapor from the evaporation temperature up to the absorption temperature; - warming up with coolant-rich solution; - throttling with coolant-poor solution behind the solution heat exchanger; - heat exchange on the working media in external circulation loops. Despite the desire for a reduction of or even prevention of irreversibilities occurring in the proc-ess, in the real absorption chiller there is avery small or no influence on the irreversibilities. 2 CALCULATION OF EXERGY FLOWS The calculation of exergy flows is performed using the data from Table 1 and any available literature ([1] and [4]) with a determination of specific flow exergy in labelled positions of the chiller (Fig. 1 and 2). The specific flow exergy tells us how much work has been produced with the unit of mass flow by reversible and mutual interaction with the environment [2]. The specific flow exergy in the device’s labelled positions, in the cooling and heating cycle is determined by the equation: e = h – h - T .(s - s) (1). Za določitev vrednosti specifične eksergije v toplotnem kompresorju običajno uporabimo eksergijski diagram na sliki 3. Usually we use the exergy diagram (Fig. 3) to determine values of specific flow exergy in the thermal compressor. stran 519 V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows Preglednica 1. Osnovni podatki absorpcijske hladilne naprave Table 1. Basic data for the absorption chiller Osnovni podatki prenosnikov toplote Basic data for the heat exchangers k-AEV = k.AC = 11,9 kW/K 17,9 kW/K k.AGen = 8,5 kW/K masni pretok zunanjega obtoka mass flow in the external circuits: k.AA = _______ 2,0 kW/K 6,1 kW/K kondenzator in absorber condenser and absorber uparjalnik evaporator generator generator koncentrirana raztopina with coolant-rich solution qm,k = qm,a = 4,2 kg/s qm,up = 2,3 kg/s qm,gen = 3,2 kg/s qm,rr = qm,1 = 0,45 kg/s (a) Kondenzator (a) Condenser (b) Generator (b) Generator (c) Uparjalnik (c) Evaporator (d) Absorber (d) Absorber (e) Izmenjevalnik toplote (e) Solution heat exchanger (f) Dušilni ventil 2 (f) Throttle valve 2 (g) Črpalka raztopine (g) Solution pump (h) Dušilni ventil 1 (h) Throttle valve 1 Sl. 2. Diagram pretoka eksergij v absorpcijski hladilni napravi Fig. 2. Exergy flow diagram in the absorption chiller 00-8 VBgfFMK stran 520 V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows Sl. 3. Diagram ex,x delovne snovi LiBr/H2O [5] Fig. 3. ex,x diagram for working media LiBr/H2O [5] Negativni predznak specifične eksergije dobimo v primeru, ko se povečuje entropija toka snovi pri hkratnem padcu tlaka pod tlak okolice [2]. To se pojavi, v analiziranem primeru, za stanja v točki 9 in 10 (slika 1). Rezultati preračuna eksergijskih tokov absorpcijske hladilne naprave so podani v preglednicah 2 in 3, kjer oznake posameznih veličin ustrezajo oznakam na sliki 1 in diagramu eksergijskih tokov na sliki 2. A negative sign for specific flow exergy was obtained for the case where the entropy of flow increased with a simultaneous pressure drop below the value of the environment’s pressure [2]. This occured, in our case, for positions 9 and 10 (Fig. 1). The calculated results of exergy flows in the absorption chiller are given in Table 1 and Table 2, where the thermodynamic properties correspond to those named in Figure 1 and exergy flow diagram in Figure 2. Preglednica 2. Vrednosti specifične eksergije, masnega pretoka in temperature v posameznih točkah naprave Table 2. Values of mass flow, temperature and specific flow exergy in the labelled position of the absorption chiller Masni pretok Mass flow Temperatura Temperature Specifična eksergija Specific exergy qm.i kg/s t °C ex.i kJ/kg 1 0,4500 41,9 48,3000 2 0,4500 41,9 48,3000 3 0,4500 71,1 120,7000 4 0,4216 85,7 131,0500 5 0,4216 52,5 69,0000 6 0,4216 50,3 62,1000 7 0,0287 76,2 102,6000 8 0,0287 31,5 0,8369 9 0,0287 2,1 - 5,1740 10 11 12 13 14 15 16 17 18 Masni pretok Mass flow qm.i kg/s 0,0287 3,2000 3,2000 4,2000 4,2000 4,2000 4,2000 2,3000 Temperatura Temperature t____°_C 2,1 29,3 34,5 25,0 29,3 12,0 2,3000 5,0 1,7700 Specifična eksergija Specific exergy ex.i kJ/kg -155,1000 95,0 34,6500 88,3 29,1200 0,5079 1,4550 0,0737 0,5079 0,4333 stran 521 glTMDDC V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows Preglednica 3. Vrednosti eksergijskih in anergijskih tokov enostopenjske absorpcijske hladilne naprave Table 3. Values of exergy and anergy flows in the single-stage absorption chiller Sestavina naprave Component of device kondenzator condenser absorber -85,87 absorber 67,7200 prenosnik toplote heat exchanger generator uparjalnik evaporator dušilni ventil - 1 throttle valve - 1 dušilni ventil - 2 throttle valve - 2 črpalka raztopine solution pump Eksergijski tok Exergy flow Sestavina Component . Exi kW 3,1200 6,6100 generator 13,5700 3,7900 0,001033 Anergijski tok Anergy flow . An kW -68,47 71,41 Zunanji obtok External loop Eksergijski tok Exergy flow D . Exi kW 1,54 3,59 6,77 - - 75,48 -18,4 75,48 3,05 Anergijski tok Anergy flow D . An kW 69,73 82,98 71,41 0,11 - - 0,85 - - 2.1 Razlaga rezultatov Splošno je znano, da se pri termodinamični analizi toplotnih procesov odločamo za eksergijsko bolj varčne naprave. Med naštetimi vrstami nepovračljivosti, ki se pojavljajo v napravi (sl. 2), se izkaže, da se največje nepovračljivosti pojavljajo v generatorju in absorberju (pregl. 3), kjer se namreč poleg nepovračljivosti pri prenosu toplote pojavljajo še nepovračljivosti zaradi mešanja delovne snovi. Na sliki 1 opažamo nasprotno smer toplotnega in eksergijskega toka v uparjalniku. Ta pojav je specifičen v tem, da se telesom, katerih temperatura je pod temperaturo okolice in se jim toplota odvaja, vrednost eksergije veča in nasprotno [6]. Prav zaradi tega se lahko utemeljeno sklepa, da lahko termodinamično pravilno zapišemo izkoristek vložene energije v napravo, v kateri hkrati potekata gretje in hlajenje, samo z eksergijskim izkoristkom. Za absorpcijsko hladilno napravo, pri kateri izkoriščamo samo hlad, lahko določimo eksergijski izkoristek z enačbo: DExUP yA AHN DE xGEN +Pp 2.1 Comments on the results It is a well-known fact, that with a thermo-dynamic analysis of thermal processes we are con-centrating on the more exergy-saving type of plants. The named forms of irreversibilities, which occurred in the device – Figure 2, become obvious according to the presented analysis, because the greatest irreversibilities occured in the generator and absorber (Table 3). Besides the irreversibilities in the heat transfer, irreversibilities due to the mixture processes were also present. In Figure 1 we can see the opposite directions of energy and exergy flows by evaporator. This phenomenon is specific for the systems (bodies) whose temperature are below the surrounding’s temperature and whilst they are rejecting heat (being cooled), their exergy value is growing and vice versa [6]. Therefore, this brings us to the using conclusion that thermodynamics is the only correct way to express the efficiency of consumed energy in the device where heating and cooling are simultaneously performed only with exergy efficiency. For the absorption chiller, where we utilize only cooling, we can determine the exergy efficiency using the following equation: 3, 05 (2). 18,4 0,166 V primeru, da poleg hladu izkoriščamo še oddano toploto kondenzatorja in absorberja (npr. segrevanje sanitarne vode), lahko eksergijski izkoristek absorpcijske hladilne naprave določimo z enačbo: yA . .. DEx +DEx +DEx 3,59+1,54+3,05 In the case where we also utilize rejected heat from the absorber and the condenser, we can determine the exergy efficiency with the following equation: AHN DE xGEN +Pp 18,4 0, 445 (3). 00-8 VH^tTPsDDIK stran 522 - - V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows Moč potrebne črpalke se zaradi doslednosti definicije v enačbah (2) in (3) zapiše, vendar se zaradi majhne vrednosti, (pregl. 3) v računu ne upošteva. 3 SKLEP Prednost sorpcijskih hladilnih naprav, v obravnavanem primeru absorpcijskih hladilnih naprav, pred kompresorskimi hladilnimi napravami je v možnosti uporabe eksergijsko revne odpadne toplote in rabe alternativnih energijskih virov. Kompresorske hladilne naprave so sicer energetsko učinkovitejše in manjše, vendar eksergijsko manj učinkovite in porabljajo čisto eksergijo [7]. Za pogon absorpcijskih hladilnih naprav se lahko uporablja tudi odpadna toplota postroja soproizvodnje. Postroj sočasne proizvodnje električne energije in toplote ter hladu imenujemo trigeneracijski postroj [8]. Pri trigeneracijskih postrojih je zaželeno, da ima absorpcijska hladilna naprava kar se da velik eksergijski izkoristek in s tem dosežemo večji eksergijski izkoristek celotnega trigeneracijskega postroja [8]. We have to consider the power of the pump to obtain the correct definitions, equation (2) and (3), which due to its small value, is unimportant. 3 CONCLUSION The advantage of sorption chillers, in our case absorption chillers, in comparison to compressor chillers is in utilizing low exergy, rejected heat and in the use of alternative energy sources. Compressor chillers have a higher energy efficiency and are more compact, but they are less exergy efficient and utilize pure exergy [7]. To drive the absorption chiller we can also utilize the rejected heat from the cogeneration plant. This kind of plant, with simultaneous production of heat, electricity and cooling is also called the trigeneration system [8]. In the trigeneration plant all the requirements are present, the absorption chiller has the highest possible exergy efficiency, which enables it to achieve higher exergy efficiency in the whole trigeneration plant [8]. 4 OZNAKE 4 SYMBOLS anergijski tok . An W anergy flow specifična eksergija snovnega toka ex J/kg mass flow specific exergy energijski tok Ex W exergy flow specifična entalpija H J/kg specific enthalpy moč P W power masni pretok q kg/s mass flow temperatura T K temperature x % mass ratio eksergijski izkoristek y % exergy efficiency Indeksi: Subscripts: absorber A absorber absorbcijska hladilna naprava AHN absorption chiller generator Gen generator kondenzator K condenser referenčno stanje okolice o reference state črpalka p pump prenosnik toplote PT heat exchanger uparjalnik UP evaporator 5 LITERATURA 5 REFERENCES [1] Herold, E.K., R. Radermacher, S.A. Klein (1996) Absorption chillers and heat pumps. New York, CRC Press. [2] Obersnu, T. (1991) Strojno hlajenje in gretje. Ljubljana, Tehniška založba Slovenije. [3] Bošnjakovič, F. (1986) Nauka o toplini - III dio. Zagreb, Tehnička knjiga. [4] Hellmann, H.-M., F. Ziegler (1998) A simple method for modeling the operating characteristics of absorption chillers, Eurotherm No 59,7/1998, Nancy-France. gfin^OtJJlMlSCSD 00-8 stran 523 |^BSSITIMIGC V. Vasi} - J. Krope - D. Gori~anec: Analiza eksergijskih tokov - Ana Analysis of Exergy Flows [5] Karavan, S.V., I.I. Orehov, E.A. Gavrilov (1986) Ental’pijnnaja i eksergetičeskaja diagrammy vodjanogo rastvora bromistogo litija, No.11,4 K45, Pisčevaja promisljenost’. [6] Voprosy termodinamičeskogo analiza - eksergetičeskij metod (1965) Mir, Moskva. [7] Poredoš, A. (1994) Eksergijska analiza parnih in sorpcijskih hladilnih procesov. Ljubljana, Strojniki vestnik (40), 7+8. [8] Vasic, V, D. Goričanec, D. Kosič (1999) Eksergijska analiza trigeneracionih sistema. Kongres KGH (33), Beograd -Jugoslavija. Naslova avtorjev: mag. Vasilije Vasic prof.dr. Jurij Krope doc.dr. Darko Goričanec Fakulteta za kemijo in kemijsko tehnologijo Univerze v Mariboru Smetanova 17 2000 Maribor Authors’ Address: Mag. Vasilije Vasic Prof.Dr. Jurij Krope Doc.Dr. Darko Goričanec Faculty of Chemistry and Chemical Engineering University of Maribor Smetanova 17 2000 Maribor, Slovenia Prejeto: Received: 15.8.2000 Sprejeto: Accepted: 10.11.2000 00-8 VH^tTPsDDIK stran 524