Numerical Study and Experimental Comparison of CO2 Reciprocating Compressors for Small Cooling and/or Freezing Capacity Applications

Transcription

1 Purdue Unversty Purdue e-pubs Internatonal Compressor Engneerng Conference School of Mechancal Engneerng 2006 Numercal Study and Expermental Comparson of CO2 Recprocatng Compressors for Small Coolng and/or Freezng Capacty Applcatons Joaqum Rgola Unverstat Poltecnca de Catalunya G. Raush Unverstat Poltecnca de Catalunya Carlos D. Perez-Segarra Unverstat Poltecnca de Catalunya Assens Olva Unverstat Poltecnca de Catalunya Follow ths and addtonal works at: Rgola, Joaqum; Raush, G.; Perez-Segarra, Carlos D.; and Olva, Assens, "Numercal Study and Expermental Comparson of CO2 Recprocatng Compressors for Small Coolng and/or Freezng Capacty Applcatons" (2006). Internatonal Compressor Engneerng Conference. Paper Ths document has been made avalable through Purdue e-pubs, a servce of the Purdue Unversty Lbrares. Please contact epubs@purdue.edu for addtonal nformaton. Complete proceedngs may be acqured n prnt and on CD-ROM drectly from the Ray W. Herrck Laboratores at Herrck/Events/orderlt.html

2 C116, Page 1 Numercal Study and Expermental Comparson of CO 2 Recprocatng Compressors for Small Coolng and/or Freezng Capacty Applcatons. J. RIGOLA, G. RAUSH, C.D. PÉREZ-SEGARRA, A. OLIVA Centre Tecnològc de Transferènca de Calor (CTTC), Unverstat Poltécnca de Catalunya (UPC) ETSEIAT, C/Colon, 11, 08222, Terrassa, (Barcelona), Span Tel ; FAX: e-mal: cttc@cttc.upc.edu; ABSTRACT The present paper s a numercal and expermental comparatve study of hermetc and sem-hermetc recprocatng compressors workng wth carbon doxde as flud refrgerant aganst the conventonal R134a hermetc compressors. When coolng condtons for one-stage compressor type (.e., evaporaton temperatures between 7.2ºC and 10ºC) have been consdered, the numercal and expermental results obtaned have shown a reasonable good agreement, whle the comparatve global values have ndcated very smlar effcences between the carbon doxde compressor and the conventonal ones. In all these cases the CO 2 sem-hermetc compressor presents hgher mass flow rate and better effcency than the CO 2 hermetc compressor. However, when freezng condtons are analysed (.e., evaporaton temperature between 10ºC and 40ºC) the onestage CO 2 compressor shows lower COPs n comparson wth R134a conventonal compressor. In these cases a numercal study of the avalablty of two-stage sem-hermetc recprocatng compressors s presented n order to obtan comparatve or even better effcences aganst conventonal hermetc recprocatng compressors. 1. INTRODUCTION Refrgeratng systems are mostly based on cycles usng H(C)FCs fluds. The Montreal protocol (UNEP, 1987) stpulated the phasng out of H(C)FCs as refrgerants that deplete the ozone layer (ODP), whle the Kyoto protocol (UNFCCC, 1997) encouraged promoton of polces for sustanable development and reducton of global warmng potental (GWP). The EU oblges to reduce the greenhouse gases emsson by 8% n 2010, followng nternatonal Kyoto protocol. Thus, nvestgaton and use of new and natural refrgerants s an mportant goal. Durng the last decade, the nvestgaton ndcates that carbon doxde has an mportant nterest as a natural flud refrgerant. The use of CO 2 s the only refrgerant replacement wth the securty of not beng harmful to the envronment, non-toxc and non-flammable. Durng the last decade, the nvestgaton ndcates that the use of carbon doxde has an mportant nterest as a natural flud refrgerant (Lorentzen, 1994), (Jakobsen, 1998), (Kruse, 1999) and (Flemng, 2003). After 1992 the use of trans-crtcal carbon doxde cycles as a natural heatng and coolng alternatve systems have had a revval. Durng last two years, t has been an mportant ncrease of revews (Km, 2004), semnars (Semnaro Europeo sugl mpant a CO 2, 2005) and specal ssue journals (IJR, 2005) about fundamental process and system desgn ssues n carbon doxde systems, where the mportant trends n CO 2 technology n refrgeraton, ar-condtonng and heat pumps applcatons are provded.

3 C116, Page 2 The possbltes that one-stage hermetc recprocatng compressors offer for commercal refrgeraton applcatons of small coolng capacty (.e. vendng machnes, dsplay cabnets, etc.) usng carbon doxde as flud refrgerant under trans-crtcal cycle condtons have been prevously demonstrated (Rgola, 2005). In these cases the evaporaton temperature ranges around 10.0ºC for coolng capactes of 600W approxmately. The results along ths range have shown promsng perspectves although wth the necessty of more efforts. The present paper presents an mproved hermetc recprocatng compressor than (Rgola, 2005) and a new verson of sem-hermetc recprocatng compressor, both studed numercally and compared expermentally. Ths last one presents an almost comparable COP between the conventonal R134a sub-crtcal and the new carbon doxde trans-crtcal refrgeraton systems desgn when coolng condtons are consdered. Unfortunately, the effcences obtaned when freezng condtons (from 23.3ºC to 35ºC of evaporaton temperature) are taken nto account decreases aganst the conventonal R134a system. In ths case, an alternatve presented, s the use of two-stage sem-hermetc recprocatng compressor, passng the mass flow through two compresson chambers n seres wth the possblty to reduce the motor sze. The present paper have also evaluated numercally these cases, showng the possblty to obtan comparable or even better COP aganst the conventonal R134a sub-crtcal cycle for freezng applcatons. In the feld of two-stage recprocatng compressors, only (Neksa, 2000) presented a sem-hermetc recprocatng compressor type n the refrgeraton and freezng felds, although for medum and hgh coolng capacty ranges wth compresson volumes from 15 to 60 cc. No works have been found consderng a two-stage sem-hermetc recprocatng compressor for freezng applcatons and small/medum coolng capactes. In the same way, (Yamasak, 2004) presented a comparable work n the refrgeraton and freezng felds wth a comparable compresson chambers dsplacement of 1.28 and 0.83 cc for frst and second stages respectvely although consderng a rollng pston type nstead of recprocatng ones. In concluson, the present paper ams wth the dea to present the last news on one-stage hermetc and sem-hermetc carbon doxde compressors numercally compared and expermentally valdated for small coolng applcatons wth almost comparable COP aganst R134a sub-crtcal conventonal systems, together wth a numercal study of twostage sem-hermetc CO2 recprocatng compressors n order to show the avalablty of these ones for small freezng applcatons wth smlar or even better COP than R134a conventonal sub-crtcal cycles. 2. NUMERICAL SIMULATION MODEL OF RECIPROCATING COMPRESSORS The numercal smulaton model of hermetc recprocatng compressors (Pérez-Segarra, 2003), used to obtan the numercal results presented, s based on the dvson of the whole doman (tubes and chambers where the gas s flowng, shell, crankcase, ol, etc.) nto strategc control volumes (CVs). The chambers and the shell consttute sngle CVs, the rest of the parts can be dvded nto an arbtrary number of CVs. For each CV a set of algebrac equatons s obtaned by means of the ntegraton of the conservaton governng equatons (contnuty, momentum, and energy), n a one dmensonal and transent form. For the flud flow, pressure, temperature, and densty are obtaned at the centre of each CV from the contnuty equaton, energy equaton, and state equaton, respectvely. Veloctes are determned from the momentum equaton at the faces of the man CV usng a staggered grd. Ths procedure allows the evaluaton of elements such as resonators, parallel paths, etc. For any of the CVs n whch the refrgerant s flowng, the general governng equatons can be wrtten n terms of the local averaged flud varables, neglectng body forces, and both axal shear stresses and axal heat conducton, n the followng form: m + m& o m& = 0 (1) t m ( h e t c ) + mv + t m& v m& v = F (2) o o m& o ( ho eco ) m& ( h ec ) = V + s ~ p t Q& w (3)

4 C116, Page 3 Momentum eq. (2) s specfcally characterzed when the flud refrgerant s flowng through sudden enlargements or contractons (contract coeffcent), through valves (effectve force and flow area), or between pston and cylnder (leakage). The governng equatons of the flow are dscretsed by means of an mplct control volume formulaton, the convectve terms are numercally approxmated usng the frst order upwnd numercal scheme, whle transent terms allow numercal approxmaton usng second and thrd order numercal schemes. The effectve flow area s evaluated together wth momentum equaton consderng a multdmensonal model based on modal analyss of flud sold nteracton through the valves. The equaton to obtan the valve poston s teratvely solved usng a fully mplct frst order Heun method. The nstantaneous compresson chamber volume s evaluated at each tme-step, dependng on nstantaneous pston poston, whch s obtaned from the force balance among gas pressure n the compresson chamber, electrcal motor torque and crankshaft connectng rod mechancal system. The resultng algebrac system of equatons s solved usng a standard drect lower upper decomposton (LU) method. The sold thermal behavour s based on heat global balances for each one of the macro-volumes consdered. The dscretsed set of energy equatons for the sold elements and ol s lnearsed and drectly solved by an nverse matrx system LU resoluton. The followng fully mplct dscretsed energy equaton can be appled, consderng convecton between sold k and flud : ρ n 1 k n n 1 n n c pk ( Tk Tk ) Vk Tk T j conv n rad n = Akj + Q&,, + Q& kj t R k (4) j kj The ntal condtons are the gas flow pressure, velocty, and temperature dstrbuton along all compressor CVs, together wth both sold elements and ol temperatures. The nlet tme-average gas pressure and temperature, and the outlet tme-average gas pressure, must be specfed at each tme step as boundary condtons. The whole set of dscretsed equatons of the flud flow are teratvely solved at each tme-step, by means of a segregated algorthm. A smple-lke algorthm, extended to compressble flow, has been used to solve the flud flow equatons. The complete set of dscretsed momentum, energy and pressure correcton equatons n the whole compressor doman s solved at each tme step by the Gauss-Sedel plus TDMA method. Parallel crcut and extra elements (such us double orfces, resonators, parallel paths, etc.) are consdered n the formulaton. The nstantaneous valve poston and compresson chamber volume at each tme-step consdered are necessary to solve the momentum equaton. Sold walls and ol temperatures can be evaluated usng the same gas flow tme-steps, although ths s not necessary due to the large dfferences on tme scales of these phenomena. The process runs step by step n the tme drecton gvng the transtory evoluton of the dfferent varables. For each tme-step, several teratons have to be carred out untl convergence s reached. After a complete cycle s run, sold wall and ol temperatures are updated. It s worthwhle to menton that, when the motor torque balance s mplemented, real mean frequency s not an nput data but an output data of the model. The present numercal smulaton model has also been extensvely valdated (Rgola, 2003)(Rgola, 2004) for a range of compressor capactes (between 7.5 and 16 cc), at varous compresson ratos (evaporaton temperatures from -35 to 7.2ºC), and workng wth several refrgerant fluds (R134a, R600a or R404A). 3. EXPERIMENTAL SET-UP DESCRIPTION FOR ONE-STAGE COMPRESSORS An expermental set-up has been specally desgned to evaluate the thermal and flud dynamc behavour of carbon doxde trans-crtcal cycles and to valdate the numercal smulaton results of the one-stage recprocatng compressors. The expermental unt s made up of the followng elements: a one stage carbon doxde compressor prototype, dual heat transfer col gas cooler and evaporator together wth a meterng valve. The auxlary flud used n the gas cooler and the evaporator annul s water. Two thermostatc heatng and coolng unts control the nlet auxlary water temperature n the condenser and evaporator auxlary crcuts, respectvely. The volumetrc flow n these secondary crcuts s controlled by two modulatng solenod valves and measured by means of two magnetc flow-meters, wth an accuracy of ± 0.01 l/mn. from 0 to 2.5 l/mn., and ± 0.5 % F.S. from 2.5 l/mn. to 25 l/mn. More detals about nstrumentaton accuracy and components geometry are referenced n Rgola (2004)(2005). j

5 C116, Page 4 4. RESULTS The results shown n the present paper are dvded n three man subsectons. The frst subsecton s a global comparatve study or expermental valdaton of the numercal smulaton model for a GLY80 conventonal subcrtcal hermetc recprocatng compressor workng wth R134a as flud refrgerant and two versons of one-stage carbon doxde hermetc recprocatng compressor HCL15(1) and (2). After the extended valdaton, the second subsecton s a global expermental comparatve study between the GLY80 conventonal compressor for small coolng applcatons aganst both last mproved versons of HCL15(2) and SHCL15(2), last prototypes of hermetc and sem-hermetc compressors, respectvely. Fnally, the thrd subsecton s a numercal study of the mprovements obtaned when one-stage carbon doxde compressor s numercally adapted to a two-stage compressor for small freezng applcatons. These last numercal results are obtaned usng the numercal smulaton tool of secton 2 consderng a hermetc compressor, although the expermental prototypes can be desgned as sem-hermetc type. 4.1 Global numercal comparson and expermental valdaton All compressors presented below have been numercally evaluated wth the numercal smulaton model of secton 2. The conventonal compressor has been expermentally tested n a calormeter set-up, whle the carbon doxde compressors have been valdated n the specfc expermental unt of secton 3, specally desgned and bult to analyse hgh-pressure sngle stage vapour compresson trans-crtcal refrgeratng equpments. Fgure 1 shows the numercal and expermental comparson of mass flow rate, power consumpton and COP of the conventonal R134a compressor (GLY80) and the one-stage hermetc recprocatng compressors (HCL15(1) and HCL15(2)). A descrpton of the compressors geometry presented s detaled n (Rgola, 2005). The comparatve results of the carbon doxde trans-crtcal cycle are presented under dfferent evaporaton temperatures, wth an nlet flud compressor and outlet gas cooler temperatures of 32ºC and a gas cooler pressure of 85 bars, n all cases. Numercal mass flow rate (kg/h) % -10% Expermental mass flow rate (kg/h) Numercal power consumpton (W) % -10% Expermental power consumpton (W) 3 +10% Numercal COP % Expermental COP Fgure 1. Numercal and expermental comparatve results of mass flow rate, power consumpton an COP for dfferent conventonal compressors and CO 2 prototypes at dfferent workng condtons.

6 C116, Page 5 The results of Table 1 show the reasonable good agreement between the expermental data and the numercal ones. Dfferences on mass flow rate are lower than 6%, whle dfferences on power consumpton are lower than 4% n all studed cases. The man dfferences take place on COP, wth a maxmum of 10% and lower of 8% n the majorty of the studed cases. 4.2 Expermental comparatve analyss between one-stage CO 2 compressors Once dfferent carbon doxde hermetc recprocatng compressors have been numercally analysed and optmsed, bult and expermentally tested, sem-hermetc recprocatng compressors have been buld followng the same strategy, takng the advantage to desgn a drect soluton and a cylnder head n contact wth the ambent. Both changes and some news mprovements on geometry after the frst SHCL15(1) verson have allowed to obtan ths second and mproved SHCL15(2) sem-hermetc compressor descrbed on (Raush, 2005) wth some mnor changes. Table 1 shows the expermental global comparatve results between the hermetc HCL15(2) and sem-hermetc SHCL15(2) carbon doxde compressors workng n the trans-crtcal set-up cycle descrbed on secton 3 under the condtons of subsecton 4.1, aganst R134a commercal ACC hermetc recprocatng compressor workng n a subcrtcal conventonal cycle followng ISO 917, under the workng condtons of nlet compressor temperature 35ºC, condenser temperature 55ºC and outlet flud temperature 46ºC. Table 1: Global expermental comparatve results of a conventonal R134a compressor vs. hermetc and sem-hermetc carbon doxde compressors. Tevap m& ηv W & ηsme Q & evap COP (C) (kg/h) (%) (W) (%) (W) GLY HCL15(2) ,0 333,8 53,0 463, SHCL15(2) GLY HCL15(2) ,1 353,5 59,1 681, SHCL15(2) ,5 374,4 64,6 788, GLY HCL15(2) ,7 365,2 61,1 841, SHCL15(2) ,6 353,6 72,4 964, e The results of Table 2 ndcate that hermetc and sem-hermetc mproved carbon doxde compressor prototypes of 1.5 cc are able to produce a coolng capacty of 400, 600 and 800 W at 10, 0 and 7.2ºC of evaporaton temperatures, respectvely. The mass flow rate of CO 2 hermetc compressor s qute smlar than GLY80 conventonal R134a hermetc one. However, the sem-hermetc CO 2 presents a hgher mass flow rates around 16, 14 and 10% for the evaporaton temperatures of 10, 0 and 7.2ºC, respectvely. Despte of ths, the sem-hermetc compressor stll shows a 10% COP lower aganst conventonal R134a compressor, although the same sem-hermetc compressor presents a hgher COP around 3% and 6% n comparson wth R134a conventonal compressor at 0ºC and 7.2ºC of evaporaton temperatures, respectvely. In all studed cases, the CO 2 sem-hermetc compressor ndcates a better COP around 4, 8 and 15% n comparson wth the CO 2 hermetc compressor at 10, 0 and 7.2ºC, respectvely.

7 C116, Page Numercal comparatve analyss of dfferent two-stage CO 2 hermetc compressors. The dea of consder a two-stage recprocatng compressor n a trans-crtcal cycle when carbon doxde s used as flud refrgerant, specally focussed when low evaporaton temperatures are consdered (freezng condtons), s based on take advantage of reduce the pressure rato n the compresson chamber, obtan a reduced outlet gas compressor temperature on the second stage compressor chamber, usng an ntercooler between the frst and the second compressor chambers, and mprove the compressor electrcal motor nstead of one-stage compressors. The next cases presented are obtaned consderng the outlet evaporator temperature of 32ºC, nlet pressure at three dfferent condtons of 10, and 35ºC evaporaton temperatures, outlet second stage compressor pressure of 85 bars and an nlet second stage compressor temperature of 32 ºC. Both frst and second compressor chambers dsplacement are teratvely consdered snce both mass flow rate are almost equal, then ntermedate pressure s obtaned varable. Tables 2, 3 and 4 show the numercal results of dfferent cases presentng the optmal choce at the three dfferent evaporaton temperatures, presentng an mprovement on power consumpton and COP n comparson wth the one-stage conventonal cases on the frst row of each Table. Table 2. Carbon doxde trans-crtcal cycle comparatve results at 10ºC. Tevap V 1 V 2 p ev p nt p gc m& W & e W & Q & e evap T out COP (C) (cc) (cc) (bar) (bar) (bar) (kg/h) (W) (W) (W) (-) Table 3. Carbon doxde trans-crtcal cycle comparatve results at 23.3ºC. Tevap V 1 V 2 p ev p nt p gc m& W & e W & Q & e evap T out COP (C) (cc) (cc) (bar) (bar) (bar) (kg/h) (W) (W) (W) (C) (-) Table 4. Carbon doxde trans-crtcal cycle comparatve results at 35ºC. Tevap V 1 V 2 p ev p nt p gc m& W & e W & Q & e evap T out COP (C) (cc) (cc) (bar) (bar) (bar) (kg/h) (W) (W) (W) (C) (-)

8 C116, Page 7 Numercal results of all cases concludes that two-stage recprocatng compressors present a reducton on power consumpton due to the decrease on pressure ratos of both compresson chambers n comparson wth one-stage compressor, an mportant outlet gas compressor decrease and fnally a COP mprovement from 20% to 15% at 35ºC and 10ºC of evaporaton temperatures, respectvely. The same numercal results show that an ntermedate pressure around bars as consequence of the compresson chambers dsplacement selected presents the optmum COP n all cases. Table 2 presents two cases wth the same ntermedate pressure of 40 bars, changng the volume dsplacements. The second one present lower COP although havng a hgher low compresson volume, due to the mass flow rate s lmted from the hgh compresson volume, whch s the same n both cases. Table 3 shows that the case wth an ntermedate pressure of 50 bars present lower COP than the others and a hgher outlet compressor temperature. Fnally, Table 4 shows that numercal results at 35.0ºC of evaporaton temperature presents a better mprovement n comparson wth the other Tables. In the same Table the volumes that determne an ntermedate pressure of 60 bars presents a smlar COP than one-stage compressor. 5. CONCLUSIONS A numercal smulaton for the thermal and flud dynamc behavor of hermetc recprocatng compressors s descrbed and used to obtan carbon doxde compressors optmzaton n order to assure the better effcency of these compressor type aganst the conventonal hermetc compressors for small coolng and/or freezng capactes. An expermental setup has been desgned and bult to expermentally valdate the numercal results presented wth a reasonable good agreement and dfferences lower than 10% n mass flow rate, power consumpton and COP. The numercal and expermental results presented has concluded the possbltes of one-stage carbon doxde compressors under trans-crtcal cycle for small coolng capacty applcatons, whle the numercal results of the twostage compressors has shown the perspectves of these compressors type for small freezng capacty applcaton under carbon doxde trans-crtcal cycle condtons. NOMENCLATURE COP Coeffcent of Performance (-) T evap evaporaton temperature (C) η sme η v sentropc-mech-elec effcency (%) V compressor chamber volume (cc) volumetrc effcency (%) w cp specfc compresson work (kj/kg) m& mass flow rate (kg/h) power consumpton (W) W & e p ev evaporator pressure (bar) Π pressure rato (-) p gc gas cooler pressure (bar) p nt ntermedate pressure (bar) Subscrpts Q & coolng capacty (W) nlet secton evap T out outlet compressor temperature (C) o outlet secton

9 C116, Page 8 REFERENCES Flemng J, 2003, Carbon doxde as the workng flud n heatng and/or coolng systems. Bulletn of the Internatonal Insttute of Refrgeraton, 83(4):7 15. Internatonal Journal of Refrgeraton, Specal Issue, 2005, CO 2 as Workng Flud Theory and Applcatons, 28:8. Jakobsen A, 1998, Improvng effcency of trans-crtcal CO 2 refrgeraton systems for reefers. In IIF-IIR Commsson D2/3, pages , Cambrdge, UK. Km M, Pettersen J, Bullard CW, 2004, Fundamental process and system desgn ssues n CO 2 vapor compresson systems. Progress n energy and combuston scence, 30: Kruse H, Hedelck R, Süss J, 1999, The applcaton of CO 2 as a refrgerant. Bulletn of the Internatonal Insttute of Refrgeraton, 79(1): Lorentzen G, 1994, Revval of carbon doxde as a refrgerant. Internatonal Journal of Refrgeraton, 17(5): Neksa P, Dorn F, Resktad H, Bredesen A, Development of two-stage sem-hermetc CO 2 -compressors, In IIR Gustav Lorentzen Natural Workng Fluds Conference, pp , Purdue Unversty, USA, Pérez-Segarra CD, Rgola J, Olva A, 2003, Modelng and numercal smulaton of the thermal and flud dynamc behavor of hermetc recprocatng compressors. Part 1: Theoretcal bass. Internatonal Journal of Heatng, Ventlatng, Ar-Condtonng and Refrgeratng Research, 9(2): Raush G, Rgola J, Pérez-Segarra CD, Olva A, Thermal and flud dynamc behavour of a trans-crtcal carbon doxde small coolng system: Expermental nvestgaton. In Internatonal Conference on Compressors and Ther Systems. L15-C639/45 Cty Unversty, London, UK Rgola J, Pérez-Segarra CD, Olva A, 2003, Modelng and numercal smulaton of the thermal and flud dynamc behavor of hermetc recprocatng compressors. Part 2: Expermental nvestgaton. Internatonal Journal of Heatng, Ventlatng, Ar-Condtonng and Refrgeratng Research, 9(2): Rgola J, Raush G, Pérez-Segarra CD, Olva A. 2004, Detaled expermental valdaton of the thermal and flud dynamc behavor of hermetc recprocatng compressors. Internatonal Journal of Heatng, Ventlatng, Ar- Condtonng and Refrgeratng Research,10; Rgola J, Raush G, Pérez-Segarra CD, Olva A, 2005, Numercal smulaton and expermental valdaton of vapour compresson refrgeraton systems. Specal emphass on CO 2 trans-crtcal cycles, Internatonal Journal of Refrgeraton, 28(8): Semnaro Europeo sugl mpant a CO 2, 2005, Component per mpant a CO 2, Fornaser E, Zlo C, Unverstà d Padova, Italy. UNEP 1987, Unted Natons Envronmental Program Montreal Protocol on substances that deplete the ozone layer. UNFCCC 1997, Unted Natons framework conventon on clmate change. Kyoto Protocol. Yamasak H, Yamanaka M, Matsumoto K, Shmada G, Introducton of transcrtcal refrgeraton cycle utlzng CO 2 as workng flud, Internatonal Compressor Engneerng Conference, C090, pp. 1-8, Purdue Unversty, USA, ACKNOWLEDGEMENT The authors gratefully acknowledge the techncal support of R+D ACC Span, S.A. team J. Jover, M. Jornet, J. Pons, and J.M. Serra, the fnancal support provded by ACC Span, S.A. (ref. no. C-04749) (PTR OP) and by the Comsón Intermnsteral de Cenca Tecnología (ref. no. TIC ).