Development of Lithium Bromide-Water Absorption Heat Pump System for Simultaneous Production of Heated-up Air and Steam From Waste Heat

Transcription

1 HEFT214 1 t International Conference on Heat Transfer, Fluid Mecanics and Termodynamics July 214 Orlando, Florida Development of Litium Bromide-Water bsorption Heat Pump System for Simultaneous Production of Heated-up ir and Steam From Waste Heat Marumo K., Kobayasi N. and Itaya Y.* *utor for correspondence Department of Mecanical Engineering, ifu University, ifu , Japan, yitaya@gifu-u.ac.jp BSTRCT bsorption eat pumps (HPs operate by refrigeration tecniques tat use eat witout requiring a compressor. In tis study, an innovative HP system is proposed tat uses waste eat at 8 C to produce ot air (at least 12 C, wic can be used for applications suc as drying, and to simultaneously generate steam of C. ir is eated directly by eat excange in te absorber working in te eating mode of a LiBr/H 2 O HP system. Steam is ten produced by eat excange wit te absorption solution, wic is still at ig temperature. Te performance of a benc-scale HP was evaluated by examination of running in a continuous mode. Te tested HP acieved a ot air temperature of more tan 12 C at te outlet of te absorber, and steam up to 115 C was simultaneously generated by recovering eat from 8 C ot water. Te coefficient of performance, wic is defined as te ratio of eat generated to te power consumed for pumping fluid, exceeded 2. Te eat transfer rate in te absorber was dominated by an air stream troug a bundle of, but temperature in te evaporator was a significantly sensitive factor for te increasing temperature in tis proposed HP system. INTRODUCTION 9]. However, te need for steam wit pressures as low as.2 MPa is not strong in industry. NOMENCLTURE Te petrocemical and steel industries are te most energyintensive manufacturing industries by a significant margin. Waste eat at a temperature level of 6 9 C is generated by te industrial processes of tese industries in large quantity. Researc and development of te recovery of suc waste energy is essential and important for effective utilization of energy. bsorption eat pumps (HPs operate by refrigeration tecniques tat use eat witout requiring a compressor [1 4]. HPs ave been recently applied to steam generation and increasing eat levels of ot water by recovering waste eat [5- [m 2 ] Heat transfer surface area COP [-] Coefficient of performance C p [J/(kg K] Specific eat D o [m] Outside diameter of tube c [kg/s] Mass flow rate of air H [J/kg] Specific entalpy &M [kg/s] Mass flow rate &m [kg- Rate of absorption or evaporation H 2 /(m 2 s] &Q [W] Heat transfer rate Re [-] Reynolds number T [K] Temperature U [W/m 2 K] Overall eat transfer coefficient w [W] Input power for pumps T [K] Temperature difference z [m] Cartesian axis direction Special caracters [wt%] Concentration of LiBr in solution µ [Pa s] Viscosity of air Subscripts 1 Inlet of fluid 2 Outlet of fluid bsorber or air C Condenser E Evaporator 1 E2 Evaporator 2 enerator 256

2 g i o p S W as pase (air or steam Input eat base Inside of tube Outside of tube Input pump power base bsorption solution Water novel steam generation system as been proposed to recover waste ot water (8 C and to yield steam otter tan 15 C by using a water-zeolite adsorption eat pump [1]. Te performance of tat system as been evaluated for a bencscale system [11]. Te system introduces direct contact of water to zeolite to yield ig temperature compressed steam in te steam generation step as well as direct drying of zeolite by a ot air stream in te bed to reduce te desorption time by means of efficient eating in te regeneration step. Tis requires dry air otter tan 12 C to sufficiently regenerate zeolite saturated wit water. In recent years, CO2 ot blast eat pump wic generates 12 air was developed. Compared wit an oil-fired boiler, tis new eat pump can reduce CO2 emissions and running costs by about 41% eac year. In tis study, an innovative LiBr/water HP system is suggested for eating waste eat from 8 C and simultaneously producing ot air at temperatures over 12 C, wic would ten be available for te zeolite regeneration, and steam of C for preeating te bed of zeolite due to its adsorption prior to te steam generation step. ir is eated directly by eat excange in te absorber working in te eating mode of an HP. Steam is ten generated by recovering eat in te absorption solution wic is still at a ig temperature. Tis system can be driven by termal energy, consuming only a little electricity for pumping fluid. However, eat transfer between air and a eating surface is generally muc lower tan eating or boiling water streaming on te surface. To address tis, a LiBr/water solution is supplied so tat it flows down and forms a film along te inside wall of spiral set vertically wile air flows among te bundle of in te absorber. Tis structure can enlarge te eat transfer surface area and allows directly enancing te eat transfer rate by increasing te number of and introducing a turbulence promotion effect on te surface of te spiral. Te present work examines te performance of a benc-scale HP system to determine te eat transfer parameters necessary to design a larger-scale system. In te case wic generated only 12 air by te innovative HP system, compared wit an oil-fired boiler, tis new eat pump can reduce CO2 emissions and running costs by about 98% eac year. EXPERIMENT Figure 1 sows a scematic diagram of flow in te experimental apparatus. Te main structure of te present HP system was composed of an evaporator (evaporator 1, an absorber, a regenerator, a condenser, an evaporator(evaporator 2,a solution eat excanger, solution pumps, and water pumps. Tubes of te eat excanger employed in te HP system are of spiral and bare types (Figure 2; te specifications are listed in Table 1. Te absorbent used in te HP was an aqueous solution of litium bromide (LiBr. ll operational processes took place in a closed system under sealed evacuated atmospere. Te absorbent solution condensed in te regenerator was fed to te absorber by a pump troug te T WE2 WE Evaporator 1 bsorber Evaporator 2 enerator Condenser ge H ge T 2 T S1 g S1 ge2 M & S1 T S1 S1 T WC2 T WC1 T W2 Cooling water W g T WE1 Hot water T* ge Water vapor T 1 ir T ge2 H ge2 Water vapor WE21 WE2 T WE21 T W1 Hot water H g Water vapor T* gc S2 T S2 S2 T S2 T S2 S2 Solution eat excanger water Figure 1Scematic diagram of flow in experimental apparatus 257

3 (a Spiral (b Bare Figure 2Heat transfer used in HP system Table 1Tubes of eat excanger of HP Element Evaporator 1 bsorber enerator Condenser Evaporator 2 Material Copper Copper Copper Copper Copper Number of Lengt (mm Inner diameter (mm Sape Vertical spiral Vertical spiral Vertical spiral U-saped bare Vertical bare Evaporator 1 bsorber Evaporator 2 Hot water bsorbent solution Water vapor bsorbent solution enerator Hot water bsorbent solution Condenser Cooling water Figure 3Structures of te upper part of eac apparatus 258

4 Temperature [ C] Outlet temp. of air Inlet temp. of ot water Mass flow rate of air Inlet temp. of cooling water : 1: 2: 3: 4: 5: 6: 7: Time [] solution eat excanger. film of te solution flowing down te inside wall of te vertical was formed in te absorber to absorb water vapor from te evaporator. In te evaporator, vapor was evaporated on a pure water film flowing on te inside wall of te eated by ot water at 8 C, wic flowed around te. Te detailed structures are sown in Figure 3. Ten, te solution was eated during downward flow due to an increase of exotermic dilution eat in te solution and te latent eat by water vapor absorption. ir streamed in a counter flow around te in te absorber, and eat excange took place to eat te air troug te from te solution. Te absorbent solution diluted by absorbing water vapor was passing troug te solution eat excanger and an evaporator (evaporator 2 for eat recovery prior to returning into te generator. In evaporator 2, steam was ten produced from a film of pure water flowing on te outside wall of te eated by absorption liquid, wic still ad a ig temperature. In te generator, te absorbent solution flowed down, forming a film on te inside wall of te, and was eated by ot water (8 C, wic flowed around te. Te solution was condensed during downward flow by evaporation of water vapor. Te evaporated water vapor transferred into te condenser, and was condensed on te surface of te, wic were cooled by water fed into te inside. Te water in te condenser was returned to te evaporator. Te performance of te HP was monitored continuously during its operation. Wen it was confirmed tat te operation acieved a sufficiently steady state, te temperatures of te fluids were measured at te inlet and outlet of eac device. Te absorbent solution was sampled at te inlet and outlet of te absorber to determine te concentration of LiBr by using an infrared moisture analyzer (Simadzu MOC-12H. (a (b (c (d Inlet temp. of air Figure 4Time variation of temperature in absorber of HP Hot water mass flow rate: 9.9 kg/s Cooling water mass flow rate: 7.2 kg/s RESULTS ND DISCUSSION Heating Beavior of Hot ir in bsorber Figure 4 sows te time beavior of te outlet temperature of ot air eated in te absorber of te HP. Te mass flow rates of ot water supplied to te evaporator and cooling water to te condenser were fixed at 9.9 kg/s and 7.2 kg/s, respectively. Inlet temperatures of te ot water and te cooling water are also marked in te figure. Te outlet temperature of te air in te absorber was eated more tan 12 C witout significantly influencing te mass flow rate of air, wic was fed in te range of.8 to.5 kg/s. Te air temperature was influenced, rater, by ot water temperature in te evaporator; te air temperature reaced 13 C wen te ot water temperature ad risen by only 5 C. Te overall eat transfer coefficient U in te absorber was estimated from te experimental results by U = Q & T (, (1 were te eat transfer rate & Q and mean temperature difference T are determined as sown in Table 2. Specific entalpy of te solution can be found as a function of temperature and te concentration by consulting te references [2, 3]. Figure 5 sows te overall eat transfer coefficient in te absorber against te Reynolds number for te air stream flowing troug te bundle of, wic is given by Re = D o c /µ, (2 were D o is te outside diameter of te, c is te maximum mass flow rate of air troug a bundle of, and µ is air viscosity. Te symbols (a, (b, (c, and (d in Figure 5 correspond to te conditions labeled in Figure 4. Te overall eat transfer coefficient rose wit an increase in Re or te air flow rate. Tis Mass flow rate of air [kg/s] 259

5 implies tat te eat transfer between te LiBr aqueous solution and air may be dominated by convective eat transfer into air on te outside walls of te in te absorber. Te overall eat transfer coefficient was compared wit eat transfer coefficients predicted for bot sides of te by previously reported empirical correlations to verify te eat transfer mecanism. Heat transfer in a liquid film flowing down a vertical tube wall occurs according to a formula of Wilke [12]. Te convective eat transfer coefficient for air flowing troug a bundle of was predicted by Bell [13]. Te predicted convective eat transfer coefficient between air and te outside walls of te and te overall eat transfer coefficient, determined from conduction troug tube walls and convection on bot sides of te, are also plotted in Figure5. Te overall eat transfer coefficient was almost te same as te convective eat transfer coefficient on te outside of te. Tese results reveal tat convective eat transfer between te air and te dominates te overall eat transfer, as noted above. Te dependency of te Reynolds number on te overall eat transfer coefficient obtained by examination was somewat different tan te predicted dependency because te eat transfer mecanism troug spiral in te absorber is more complicated tan te analogous problem for bare, wic can ignore te exotermic reaction in te solution. However, te magnitudes became reasonably close at Re=4 wit respect to te average temperature difference, defined by T in Table 2. Te coefficients of performance, COP p and COP, of te HP, wic were calculated by te equation in Table 2, are plotted in Figure 6. COP p based on te electric power used to pump fluid was enanced by increasing Re and was greater tan 2 wen Re was greater tan 3. COP corresponds to termal efficiency, and is based on te total input energy. COP reaced as ig as 3% for Re above 3. Figure 5 Effect of te mass flow rate of air on eat transfer coefficient (for te case sown in Figure 4 Simultaneous Hot ir Heating and Steam eneration Te simultaneous eating of air and generation of steam was evaluated. Figure 7 sows te time beaviors of temperatures in te absorber and evaporator 2 of te HP. Hot air (8 Cat inlet was eated to 125 C in te absorber and saturated steam at 1 C was simultaneously generated at a rate of.47 kg/s under atmosperic pressure in evaporator 1. Wen steam generation is regulated to.25 kg/s, te steam temperature reaced 115 C and te ot air temperature remained at 125 C = (Q& + Q& E2 / w COP = (Q& + Q& /(Q& + Q& E Re Figure 6Effect of te mass flow rate of air on COP Figure 8 sows te influence of steam generation temperature on te total COP. COP p for only ot air obtained witout steam generation was 17; under te same operating conditions in combination wit steam generation, tis improved COP p to over 2 altoug, te magnitude was reduced at increasing steam temperatures. CONCLUSION n innovative LiBr/water HP system was developed to use waste eat at 8 C to simultaneously produce air otter tan12 C and steam. Te results obtained by examination of a benc-scale system and te numerical analysis can be summarized as by te following points: 1. ir was successfully eated to over 12 C by direct eat excange in te absorber. Te ot air temperature was significantly influenced by temperature in te evaporator, rater tan by te mass flow rate of air. 2. Te coefficient of performance, defined as te ratio of ot air energy to power consumed for pumping fluid, improved wit an increase in air flow rate or Reynolds number and exceeded 15 for Re more tan Simultaneous ot air over 12 C and steam up to 115 C were obtained, as desired, and te coefficient of performance was greater tan 2.Higer steam temperatures reduced te performance. CKNOWLEDMENT Te autors acknowledge financial support by te Researc and Development Program for Innovative Energy Efficiency Tecnology under te New Energy and Industrial Tecnology Development Organization (NEDO project based on a grant from te Japanese Ministry of Economy, Trade and Industry. E COP 26

6 Table 2List of equations for HP Entalpy cange Evaporator 1 Q& E C Pw WE (TWE1 TWE2 Q E = M ge ( H ge H gc bsorber Q& C P (T2 T1 Q& = ge ( H ge H S2 S1( H S1 H S2 enerator Q& C Pw W (TW1 TW2 Q& = g ( H g H S1 + S2( H S2 H S1 Condenser Q& C C Pw WC (TWC2 TWC1 Q C = M g ( H g H gc Evaporator 2 Q E 2 = M ge ( H ge2 HWE2 Q & 2 E2 = H S1H S 1 COP = ( Q& + Q& E /( Q& E + Q& 2 Heat pump = ( Q& + Q& E2 / w w: Input power for pump Mass balance Flow rate M & ge = S2 S1 M & g = g E M S1 S2 M & S2 = S2 = SE 21 Concentration = = = S2 S1 S1 S2 S2 = S 2 S1 S1 Heat driving force Evaporator 1 TE =(TWE1- TWE2 /Ln{(TWE1- T * ge /(TWE2 * ge } bsorber enerator T T = {(T = {(T Sk W1 2 S2 - (T - (T S2 W2 1 Sk } /Ln{(T } /Ln{(T Sk W1 2 Condenser TC =(TWC2 WC1 /Ln{(T * gc WC1 /(T * gc WC2 } S2 /(T Evaporator 2 TE 2 = (T TS1 / Ln{(T - TgE2 /(TS1 - TgE2 } /(T S2 W2 1 Sk } } Mass flow rate of steam:.47 kg/s Mass flow rate of steam:.25 kg/s Temperature [ C] : 1: 2: Time [] Mass flow rate of air [kg/s] Bottom temp. of absorbent Inlet temp. of absorbent in Evaporator 2 Figure 7Time beaviors of temperature in absorber and evaporator 2 of HP. Outlet temp. of absorbent in Evaporator 2 Inlet temp. of air Inlet ot water temp.: 8 C Hot water mass flow rate: 9.9 kg/s Inlet Outlet cooling temp. of water air temp.: 15 C Cooling enerated water mass steam flow temp. rate: 7.2 kg/s Hot water temp. in Evaporator 2 Mass flow rate of air Figure 7Time beaviors of temperature in absorber and evaporator 2 of HP. Inlet ot water temp.: 8 C Hot water mass flow rate: 9.9 kg/s Inlet cooling water temp.: 15 C Cooling water mass flow rate: 7.2 kg/s 261

7 = (Q& + Q& E2 / w = Q& / w COP = (Q& + Q& /(Q& + Q& enerated steam temperature [ C] Figure 8 Effect of te generated steam temperature on COP (air flow rate:.25 kg/s REFERENCES CO P E2 = Q& /( Q& + Q& [1] Florides.., Kalogirou S.., Tassou S.. and Wrobel L. C., Design and construction of a LiBr-water absorption macine, Energy Convers.Manag., Vol. 44, 23, pp [2] Sun D. W., Termodynamic design data and optimum design maps for absorption refrigeration system, ppl. Term. Eng., Vol. 17, 1997, pp [3] Herold K. E., Radermacer R. and Klein S..; dsorption cillers and eat pumps, CRC Press, 1996 [4] Sencan., Yakut K.. and Kalogirou S.., Exergy analysis of litium bromide/water absorption systems, Renewable Energy, Vol. 3, 2, pp E E COP [5] rossman. and Cilds K. W., Computer simulation of a litium bromide-water absorption eat pump for temperature boosting, SHRE Transactions, Vol. 89-1, 1983, pp [6] Fujii T., Development activities of low temperature waste eat recovery appliances using absorption eat pumps,21 International Symposium on Next-generation ir Conditioning and Refrigeration Tecnology,17-19 February, 21, Tokyo, Japan [7] Dong, T., Qian, B., Weng, J., Yu, X., Litium Bromide-Water bsorption Heat Transformer Using 78 C Waste Heat, Proceedings of bsorption Heat Pump Conference,3 September-2 October, 1991, Tokyo, Japan, pp [8] acad, M., Caria, M., bsorption Heat Transformer pplication Refrigerating Macine, Proceedings of te International bsorption Heat Pump Conference,19-21 January, SME ES-Vol.31, 1994, New Orleans, US, pp [9] Bisio,., Pisoni, C., Termodynamic nalysis of Heat Transformers, Proceedings of bsorption Heat Pump Conference,3 September-2 October, 1991, Tokyo, Japan, pp [1] KawakamiY., be Y., Ito K., Marumo K., oyama T., Tanino M., Nakaso K., Nakagawa T., Itaya Y. and Fukai J.,Development of Benc-Scaled dsorption Type Steam Recovery System for enerating Hig Temperature Steam From Hot Waste Water, Proceedings of te213 ICE nnual Meeting, San Francisco, C, US, November 3-8, 213 [11] NakasoK., Nakasima K., Tanaka Y., Iwama Y. and Fukai J.,Study On Performance of te Novel Steam eneration System Using Water-Zeolite Pair, Proceedings of 213 ICE nnual Meeting, San Francisco, C, US, November 3-8, 213 [12] Obana, H., Handbook for design of eat excanger, Kogaku Toso, 2, pp [13] Bell K. J., Excanger design: Based on te Delaware researc report, Petro / Cem., Vol. 32, 196, C26 262