Performance analysis and improvement for CC-OTEC system

Transcription

1 Journal o Mehanial Siene and Tehnology 22 (2008) 1977~1983 Journal o Mehanial Siene and Tehnology DOI /s Perormane analysis and improvement or CC-OTEC system ANG Tong *, DING Liang, GU Chuangang and YANG Bo Shool o Mehanial Engineering, Shanghai Jiao Tong University 800 Donghuan Road, Shanghai, , China (Manusript Reeived April 16, 2008; Revised June 11, 2008; Aepted July 23, 2008) Abstrat Oean Thermal Energy Conversion (OTEC) is a way to generate eletriity by the temperature dierene o seawater rom the upper surae to dierent depths. Closed Cyle o OTEC (CC-OTEC) pumps the working luid vapor to high pressure to propel the turbine-generator and produe eletriity. However, low temperature dierene o seawater is the largest onstraint to the utilization ompared to the onventional power plants. Solar energy reheated power yle is proposed to improve the pratiability. Instead o traditional eiieny, net yle eiieny (NCE) is applied to assess the perormane o CC-OTEC system. The ators that inluene the perormane, suh as working luid and orresponding evaporating pressure, superheating temperature and turbine outlet pressure, are disussed. Considered the engineering appliation, the appropriate net output power should be at least 50kw. Keywords: CC-OTEC; Thermal eiieny; Perormane; Solar energy Introdution This paper was presented at the 9 th Asian International Conerene on Fluid Mahinery (AICFM9), Jeju, Korea, Otober 16-19, * Corresponding author. Tel.: , Fax.: address: twang@sjtu.edu.n KSME & Springer 2008 The yle o Oean Thermal Energy Conversion (OTEC) is a tehnique that generates eletriity by the temperature dierene between surae seawater and deep seawater. In the oean, the temperature o the water dereases with an inrease in depth. It is generally onsidered that as long as the temperature dierene between the warm oean surae and the oean deep exeeds 20, the OTEC system an be propelled to generate eletriity. In the Closed Cyle o Oean Thermal Energy Conversion (CC-OTEC) system, warm surae seawater and old seawater are used to vaporize and ondense a working luid separately, suh as anhydrous ammonia, whih drives a turbine-generator in a losed loop produing eletriity. As early as in 1881, Frenh physiist D Arsonval proposed the idea o OTEC by ammonia as the working luid, whih was irstly demonstrated by Laude. A small OTEC plant (Mini-OTEC) mounted on a barge o Hawaii was built in 1979 by the state o Hawaii, whih produed more than 50 k o gross power, with a net output o up to 18 k. Subsequently, U.S. and Japan have perormed extensive studies on OTEC and built several researh OTEC plants, with a growing power one by one. In 1980 Taiwan power ompany o China planed to put the power o OTEC plant together with the residual heat rom 3rd an 4th nulear plant. Later in 1985 Guangzhou Institute o Energy Conversion o Chinese Aademy o Siene studied a Droplets Upgrade Cyle method used in OTEC system to improve seawater potential energy and built two Mini-OTEC experimental benhes o 10 and 60 in Tanner (1995) ame to a onlusion in his projet that Taiwan represents the loset it between tehnology and market with regards to OTEC as an energy soure in the near uture. Ignoring other ators, u C. (1998) hose the speii power as the objetive untion in the design o an OTEC Rankine power plant. Odum (2000) made an energy analysis o a landbased OTEC system by energy evaluation methods.

2 1978 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~1983 Considering the parameters o pipe length, pipe diameter, seawater depth, and low rate o seawater, Yeh R. H. (2005) studied the eets o the temperature and low rate o old seawater on the net output o an OTEC plant. Due to the low temperature dierene, the power generated by the turbine is not suiient to support persistent operation o OTEC system. Gérard (2007) proposed 1D model or temperature distribution to get that worldwide power soures whih ould be extrated rom the operation o OTEC plants might be limited. In order to inrease the easibility o OTEC system, the other kinds energy should be introdued to raise the temperature dierene, suh as solar energy, geothermal heat and some industrial exhaust heat, et. In the distrits with rih oean thermal energy, there is also abundant solar energy. It an be as the additional heat soure to enlarge the temperature dierene between heat soures. As or CC-OTEC system, the power yle is improved as solar energy reheated power yle. 2. Solar energy reheated power yle A sketh o CC-OTEC system with solar olletor is shown in Fig. 1. Reheated by solar energy, the working luid at state point 1 is at higher pressure and superheating status ompared with the original system. Superheated vapor expands in the turbine and propels the turbine to rotate, and then the turbine an drive the motor to generate eletriity. Flowing out o the turbine, the working luid gets to state point 2 with inevitable losses in expanding proess. Cooled by the old seawater in ondenser, the working luid gets to state point 3 with lowing loss in ondensation proess and under superooled liquid status. Then it is pumped to higher pressure state point 4. Preheated by the warm seawater and reheated by solar energy, it passes state point 5 and returns to state point 1. The status o working luid varies proess in p-h diagram is shown in Fig. 2 by the solid line orrespondingly. The ideal proess without any losses is represented by the dashed line During the ideal proess, the working luid at state point 1 expands adiabatially in turbine to propel it and gets to state point 2, and the heat exhanging proesses are at onstant pressure. In the designed irulation, the temperature dierene by heat exhange and pressure losses must be onsidered. At state point 4 the working luid is normally preheated to 20~25 by the surae warm water, at state point 3 it is ondensed to 8~12 by the old seawater. Generally, the ordinary solar olletors an at least raise the temperature o thermal soure to 70. Compared with the temperature o oean surae, the pressure and temperature o working luid an be greatly inreased by the solar olletor, i.e. the temperature dierene between the old and thermal soure is enlarged. Vega (2002) suggested that the parameters at dierent status in CC-OTEC system ould be expressed as ollowings. ith the internal and mehanial loss in turbine speii enthalpy at state point 2 is omputed as h2' = h1 ( h1 h2) η t (1) Considering the head o old seawater pump, atual pressure at state point 3 an be expressed as p 4' 5' ' 3 2 2' h Fig. 1. Sketh o CC-OTEC system with solar olletor. Fig. 2. p-h diagram o CC-OTEC system.

3 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~ p ' = p p (2) 3 2 ond n = t p p (10) Correspondingly, pressure at state point 4 is expressed as p = p + p + p (3) 4' 1 preh evap here, p preh is the pressure loss o working luid in preheater by the oean surae water, p evap is the pressure loss o working luid in solar olletor. The onsumed work o working luid pump is: p ( p 4 p 3) v 3M = (4) η p Atual enthalpy at state point 4 : p 4 = h 3 + (5) M h Mass o old seawater an be get by heat exhange balane, M = ( h2 h3) M Cp t (6) In CC-OTEC system, long adiabati duts must be ixed to transer the old seawater rom the oean deep. There exists loal ritional resistane and on way ritional resistane. Yeh (2005) omputed the work o old seawater pump by the method. For simpliity only head o old seawater pump L is provided here to get the work: M gl p = (7) ηp Compared with that o warm seawater to preheater, p is muh greater. So only p is onsidered here. In the solar olletors the absorption heat is Q = h h M (8) ( 1 5) Turbine output work is always expressed as = ( h h ) M (9) t 1 2 Then the net power o the yle is Vega (2005) expressed the traditional thermal eiieny as n η tr = ( h h ') M orking luid seletion (11) One the solar olletor is introdued, the temperature o heat soure an be at least inreased to 70, the temperature o working luid in CC-OTEC varies between 10 ~ 65, whih is near the normal atmospheri temperature. The low boiling point working luids an be applied. The most ommonly used working luids in CC-OTEC are ammonia, Freon, water and a ew kinds o hydroarbon. Several aspets should be onsidered under the seletion proess o working luid, suh as the harateristi, lowing losses, heat exhange, stability, orrosivity, saety and easibility o lab and engineering simulations. Taking into aount these ators, we propose the ollowing priniple or the system. 1 Considered the environmental protetion problem, non- materials whose ODP=0 is demanded. It should have low GP value, narrow two-phase zone and proper pressure and temperature intervals, low visosity, big thermal ondutivity, stable hemial property. Also, it should be non-poisonous and nonombustible. 2 Considering rom the engineering easibility, we demand the low seawater amount to guarantee proper seawater pipe diameter and length. 3 Considering rom the equipment design, we demand low evaporation pressure to redue the intensity requirement o turbine, evaporator and venting pipe. O ourse, seleting a working luid to satisy all the requirements above, it s atually a little bit diiult. Aording to the above priniples and ormer experiene, the perormanes o NH 3,, R11, R502, R12 are ompared. Provided that the system onditions are as the ollowings: ondensation temperature is 10, evaporation temperature is 40. Pressure loss o ondenser, preheater, solar olletor is 0.05bar, 0.05bar, 0.03bar, respetively. Head o old seawater pump is 30m. Eiieny o turbine, seawater pump, and working luid pump is 0.63, 0.72 and 0.72, respetively.

4 1980 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~1983 η tr (%) NH 3 R12 R502 R t sup Fig. 3. η tr o dierent working luid. V 1 (L/s) NH 3 R12 R502 R t sup Fig. 4. Volume low rate at state point 1 o dierent working luid. Dierent working luid yle perormanes are plotted in Fig. 3 to Fig. 5. Absissa denotes t sup, i.e. superheating temperature. Ordinate denotes traditional thermal eiieny η tr, and volume low rate o working luid at state point 1 V 1. η tr is a diret riterion when hoosing the working luid; V 1 determines the diameter o seawater pipe. All o parameters are important ators during the proess o seleting the working luid. As shown in the igures, ammonia has a pretty good quality. Under the same ondition, ompared with the others, ammonia has higher yle eiieny and lower absorbed energy rom solar olletors. No doubt that ammonia is the best hoie just aording to yle perormane. However, ammonia must guarantee some degrees o superheating temperature, otherwise at state point 2 it an easily enter into two-phase zone. It an be ound rom the igures that ammonia only has the data over 20 o superheating temperature. It results in high demand or solar olletor and inreases the investment ost. Besides, ammonia is easily dissolved in water and has ertain toxiity, not satisied with prinipal 1. One it is leaked out, it will have a bad eet on halobios. Consequently ammonia is not applied here. R11 relatively has a ine perormane, but with a nonzero number o ODP, it is not satisied with prinipal 1, either. As speii volume o R11 is muh more, rom Fig. 4 the inlet volume low rate o turbine is greater than the others, whih results in a very large omponent requirement. As a result, is seleted as the working luid, whih has a satisying quality aording to parameters and nearly satisies all the three prinipals presented above. 4. Cyle eiieny disussion Compared with the traditional steam power yle, there is no additional mineral resoures onsumption in the solar energy reheated power yle. hatever the oean thermal energy or the solar energy in the CC-OTEC system, both o them are primary energy soure, whih are abundant, lean and ree. Aordingly it s not so proper to judge the perormane o CC-OTEC system by the traditional thermal eiieny η tr. Based on the deinition o yle eiieny, net yle eiieny (NCE) should be deined as the ollowing expression. n NCE = (12) t Supposed all o the system onditions are the same as above, NCE o dierent working luid yle is plotted in Fig. 5. It nearly has the same tendeny as η tr in Fig. 3, but the absolute data is dierent. The system an t be applied in the industry beause o the too low eiieny. One NCE is used as the evaluating indiator, it has the same magnitude as that o the normal steam power yle. So NCE is more suitable than η tr as the evaluating indiator or perormane o CC-OTEC system.

5 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~ NCE (%) 35 NCE (%) NH 3 R12 R502 R t sup o C 60 o C 55 o C 50 o C t evap Fig. 5. NCE o dierent working luid. η tr (%) o C 1 60 o C 55 o C 50 o C t evap Fig. 6. η tr o dierent turbine inlet temperature. There are three methods to improve thermal eiieny or the traditional steam power yle: inrease o inlet temperature or pressure o turbine and redution o the outlet pressure o turbine. In terms o the results o alulation, these three methods an also raise NCE. The eet o inreasing the inlet pressure o turbine is equivalent to that o the addition o evaporation temperature. Assumed that system onditions are the same as the ormer disussed and is taken as the working luid, the eiienies under dierent turbine inlet temperature are shown in Fig 5-6, where 65, 60,55,50 represents the varying inlet temperature o turbine. The absissa denotes evaporation temperature; the ordinate denotes η tr and NCE, respetively. Fig. 7. NCE o dierent turbine inlet temperature. Compared with them in Fig. 6 and Fig. 7, the inluene o inrement rate o evaporation temperature on the eiieny is muh greater than that o superheated temperature. This is to say, inrement o evaporation temperature is more eetive than that o superheated temperature in inreasing the CC-OTEC eiieny. For the subjet investigated in our problem, the outlet pressure is restrited by the temperature o old seawater, whih an be onsidered as onstant. Consequently, under the ompromise o no additional load o exhangers and assurane o appropriate working luid s quality, inreasing the turbine inlet pressure, i.e. evaporation temperature, is the most eetive way to improve OTEC perormane. Besides, there is a very important message in Fig. 6 and Fig. 7. η tr inreases with t evap linearly, while NCE does not. It signiies that it is unneessary to add the evaporation temperature to improve the perormane o CC- OTEC system. Thereore, NCE is more suitable or assessing the perormane o the system. 5. Engineering example Aording to all the relations obtained above, the CC-OTEC design onditions are provided in Table 1 and the status at eah state point is displayed in Table 2. The parameters o low rate, pumps work and eiienies are alulated and demonstrated in Table 3. Instead o η tr, NCE is omparative to the eiieny o traditional power plant. Consequently the CC- OTEC system with solar energy reheated has the great market in pratial appliation.

6 1982 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~1983 Table 1. Design onditions or CC-OTEC system. working luid t evap ( ) 55 t ond ( ) 10 t sup ( ) 10 t o ( ) 2 t ( ) 3 t w ( ) 2 t sol ( ) 5 p ond (bar) 0.05 p preh (bar) 0.05 p evap (bar) 0.03 η p 0.72 η p 0.72 η t 0.63 L (m) 30 n (kw) 50 Table 2. CC-OTEC system status at eah state point. temp. ( ) pressure (bar) speii volume (m 3 /kg) speii enthalpy (kj/kg) Table 3. CC-OTEC system parameters. η tr (%) 4.12 NCE (%) 46.7 V 1 (m 3 /s) V 2 (m 3 /s) Q sol (kw) p (kw) 9.3 p (kw) 37 t (kw) 96.4 Table 4. Size o primary omponents. inlet outlet parameters volume low rate (m 3 /s) diameter o dut (mm) volume low rate (m 3 /s) diameter o dut (mm) turbine medium pump seawater pump Aording to low rate o working luid, the parameters o old seawater low rate and the pipe diameter o turbine are demonstrated in Table 4. As the volume low rate is small, the turbine employed here is not ommon industry equipment or generating eletriity and should be speially designed. As the results o 50kw net power, the size o the equipments and width o pipes just it the onditions above. I the net power redued, the data o eiienies, suh as η p, η p, η t, would redue greatly, and the equipments would not be ound as produts in the market. 6. Conlusion ith the development o industry and eonomy, the energy beame the global problem. Just as other marine energies, OTEC is a lean way to generate eletriity. It does not onsume primary energy soures, and is not subjet to the depletion o energy. Aording to the prinipals presented above, is the more suitable hoie as the working luid. Solar olletor an be introdued into the OTEC system, whih will greatly inrease the temperature dierene and thereore improve the yle perormane and pratial appliation. Net yle eiieny (NCE) is more proper to assess the yle perormane o CC-OTEC system. Under the ompromise o no additional load o exhangers, inrease o the turbine inlet pressure is the most eetive way to improve the system perormane. For the industrial appliation, the appropriate net output power o the system should be at least 50kw. Aknowledgments It was supported by the Projet o Department o Chinese Siene and Tehnology. Nomenlature Cp : Speii heat, kj/(kg. K) g : Aeleration o gravity, m/s 2 h : Ideal speii enthalpy, kj/kg h : Atual speii enthalpy, kj/kg L : Head o old seawater pump, m M : Mass o low rate, kg/s NCE : Net yle eiieny p : Ideal pressure, bar p : Atual pressure, bar Q : Exhanging heat, kw t : Temperature,

7 T. ang et al. / Journal o Mehanial Siene and Tehnology 22 (2008) 1977~ v : Atual speii volume, m 3 /kg V : Volume low rate, m 3 /s : Power, kw : Net power, kw Greek Letters p : Pressure loss, bar t : Temperature dierene, η : Eiieny Subsripts 1,2,3,4,5 : Status s point designation : Cold seawater ond : Condenser or ondensation evap : Evaporator or evaporation : orking luid n : Net amount p : Pump preh : Preheater sup : Superheating t : Turbine tr : Traditional w : arm seawater Reerenes [1] G. C. Nihous, A Preliminary assessment o oean thermal energy onversion resoures, Journal o Energy Resoures Tehnology, 129(1) (2007) [2] H. T. Odum, Emergy evaluation o an OTEC eletrial power system, Energy, 25 (2000) [3] D. Tanner, Oean thermal energy onversion: urrent overview and uture outlook, Renewable Energy, 6(3) (1995) [4] L. A. Vega, Oean thermal energy onversion primer, Marine Tehnology Soiety Journal, 36(4) (2002) [5] C. u and T. J. Burke, Intelligent omputer aided optimization on speii power o an OTEC Rankine power plant, Applied Thermal Engineering, 18(5) (1998) [6] R. H. Yeh, T. Z. Su and M. S. Yang, Maximum output o an OTEC power plant, Oean Engineering, 32(5-6) (2005)