Philippe Morin

Stéphane Pitois

Julien Fatome
email: jfatome@u-bourgogne.fr

Simultaneous polarization attraction and Raman amplification of a light beam in optical fibers

Keywords:

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Simultaneous polarization attraction and Raman amplification of a light beam in optical fibers

Introduction

With the development of ultra-high-bit-rate and transparent optical communication systems and their associated complex modulation formats and signal processing issues, several research groups have focused their attention on the conception of novel devices allowing a fast and alloptical control of the state of polarization (SOP) of a light beam. Indeed, in order to overcome electronic bottleneck and to go beyond the limit of current electronic feedback systems [START_REF] Martinelli | Polarization stabilization in optical communications systems[END_REF]- [START_REF] Koch | Record 59-krad/s Polarization Tracking in 112-Gb/s 640-km PDM-RZ-DQPSK Transmission[END_REF], nonlinear effects taking place in optical fibers have received much attention as a possible way to develop ultrafast polarization processing. For instance, Martinelli et al. have reported in refs. [START_REF] Martinelli | Evidence of Ramaninduced polarization pulling[END_REF]- [START_REF] Ferrario | Effective All-Optical Polarization Control Induced by Raman Nonlinear Amplification[END_REF] an original effect of polarization pulling in a fiber Raman amplifier based on polarization dependent gain of the amplification process [START_REF] Martinelli | Evidence of Ramaninduced polarization pulling[END_REF]- [START_REF] Sergeyev | Activated polarization pulling and de-correlation of signal and pump states of polarization in a fiber Raman amplifier[END_REF] whereas Thevenaz et al. have demonstrated in ref. [START_REF] Thevenaz | All-optical polarization control through Brillouin amplification[END_REF] that an all-optical polarization control can be obtained through stimulated Brillouin back-scattering occurring in optical fibers [START_REF] Thevenaz | All-optical polarization control through Brillouin amplification[END_REF]- [START_REF] Zadok | Stimulated Brillouin scattering slow light in optical fibers[END_REF]. Recently, it has also been reported that an all-optical polarization stabilization of a telecom signal can be achieved thanks to a counterpropagating four-wave-mixing (FWM) process taking place in a low-birefringence, i.e. low polarization mode dispersion (PMD), optical fiber [START_REF] Pitois | Polarization attraction using counter-propagating waves in optical fiber at telecommunication wavelengths[END_REF]- [START_REF] Morin | All-optical nonlinear processing of both polarization state and intensity profile for 40 Gbit/s regeneration applications[END_REF]. This nonlinear polarizer was experimentally observed by simultaneously injecting an arbitrary polarized signal wave and a polarized control pump beam via a counter-propagating scheme. More precisely, we have shown in these previous works that, thanks to the FWM process, the pump wave induces a unidirectional exchange of energy between the orthogonal polarization components of the signal wave, thus leading to the so-called polarization attraction or "funnel" effect [START_REF] Kozlov | Theory of lossless polarization attraction in telecommunication fibers[END_REF]- [START_REF] Pitois | Polarization and modal attractors in conservative counterpropagating four-wave interaction[END_REF]. Here, an important feature of this phenomenon is that the polarization attraction process is supposed to act even though the pump and signal waves have different frequencies [START_REF] Pitois | Nonlinear polarization dynamics of counterpropagating waves in an isotropic optical fiber: theory and experiments[END_REF]- [START_REF] Pitois | Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers[END_REF], basically because the four-wave mixing effect remains constantly phase-matched whatever the frequency detuning between the two waves [START_REF] Pitois | Nonlinear polarization dynamics of counterpropagating waves in an isotropic optical fiber: theory and experiments[END_REF]- [START_REF] Pitois | Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers[END_REF].

Based on this last property, the aim of this work is thus to provide a proof-of-principle demonstrating that it is possible to combine both polarization attraction and Raman amplification in a unique segment of optical fiber thanks to a polarized frequency-shifted counter-propagating optical pump wave. We would like to emphasize that the possibility of coupling this two fundamental optical functions in such a device has already been reported in a previous work, in which pioneering experiments were performed in the visible domain by means of kW power nanosecond pulses injected in a few meters of isotropic optical fiber [START_REF] Pitois | Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers[END_REF]. In this novel work, we focus our attention on the possibility to achieve such a process in the C-band around 1.55 µm, with both continuous waves and telecommunication signals, thus demonstrating the full potential of this device for practical applications. The first part of the manuscript is devoted to theoretical considerations dealing with Raman amplification and polarization attraction process occurring in an optical fiber pumped by two counter-propagating optical waves. In next section, we describe the experimental set-up used to observe these two combine effects. Finally, third and fourth parts of the paper are devoted to experimental results obtained either with a continuous optical signal and a 10 Gbit/s Return-to-Zero (RZ) telecom signal, respectively.

Theoretical Considerations

Basically, the device presented in this paper and combining both Raman amplification and polarization stabilization, is based on the nonlinear interaction between two counter-propagating waves occurring in an isotropic or low PMD optical fiber (see Fig . 1). For sake of simplicity, and without loss of generality, we will consider in this theoretical part the case of a perfectly isotropic fiber. To this aim, let us first consider two laser beams, a forward signal beam and a backward pump beam that counter-propagate with the same wavelength ( s =  p ) in an isotropic optical fiber with arbitrary polarizations. The propagation of these two waves can be modeled by the following set of nonlinear coupled equations [START_REF] Pitois | Nonlinear polarization dynamics of counterpropagating waves in an isotropic optical fiber: theory and experiments[END_REF]- [START_REF] Pitois | Polarization and modal attractors in conservative counterpropagating four-wave interaction[END_REF].

      v v u u v u u v u i z u t u * 2 2 2 2 1 1 2 2 2 2 v 3 2 v            (1a)       u v u v v u v u v i z v t v 2 2 2 2 v 3 2 v * 2 2 2 2 1 1            (1b)       v v u u v u u v u i z u t u * 2 2 2 2 2 2 2 2 2 2 v 3 2 v            (1c)       u v u v v u v u v i z v t v 2 2 2 2 v 3 2 v * 2 2 2 2 2 2            (1d)
where u and v are the left and right circular polarizations of the signal beam whereas u and v are the circular polarizations of the pump beam, respectively.  is the usual nonlinear Kerr coefficient of the fiber while v 1 and v 2 represent the group-velocities of the signal and pump waves, respectively. The first four terms on the right-hand side of each equation describe selfphase and cross-phase modulation effects. The last term describes the four wave mixing process responsible for energy exchanges between the two circular polarization components of each wave. As it has been demonstrated in previous works, this exchange of energy can lead to a polarization attraction effect providing that the pump wave is injected into the fiber with a circular polarization. In that case, the signal wave emerges from the fiber with a fixed circular polarization opposed to the pump, independently of its initial input SOP [START_REF] Pitois | Nonlinear polarization dynamics of counterpropagating waves in an isotropic optical fiber: theory and experiments[END_REF]- [START_REF] Pitois | Polarization and modal attractors in conservative counterpropagating four-wave interaction[END_REF].

As can be seen from equations (1), the nonlinear terms of the right-hand side are basically invariant with respect to a frequency shift of the backward wave, i.e. if u and v are changed into u e -it+iz and v e -it+iz , respectively. In fact, as the frequency difference between the two waves increases ( s >  p ), one should essentially modify these equations to include the Raman contribution to the Kerr nonlinearity. The resulting equations can thus be written as [START_REF] Pitois | Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers[END_REF].

* 1 2 2 2 2 1 1 ) 0 ( ) ( 3 4 v ) ( ) 0 ( 3 4 ) ( ) 0 ( 3
4 ) 0 ( ) 0 ( 3
4 ) 0 ( 3 2 v v v v u g g i u v g g u g g v g g u g i z u t u b a b a a a b a a                                                                                       (2a) * 1 2 2 2 2 1 1 ) 0 ( ) ( 3 4 v ) ( ) 0 ( 3 4 ) ( ) 0 ( 3
4 ) 0 ( ) 0 ( 3
4 ) 0 ( 3 2 v v u u v g g i v u g g v g g u g g v g i z v t v b a b a a a b a a                                                                                       (2b)
where  is the nonlinear Kerr coefficient,  is the fractional Raman contribution to the total nonlinearity,  = (1- and is the pump-signal frequency detuning. Similar equations are obtained for the circular components of the pump wave simply by changing z into -z, u into u and v into v . In these equations, g a and g b represent the Raman gains associated with co-rotating and counter-rotating circularly polarized waves and are given by :

              0 ) exp( 2 ) ( ) exp( ) ( ) ( dt t i t b t i t a g a (3a)      0 ) exp( ) ( ) ( dt t i t b g b (3b)
where the two Raman response functions a(t) and b(t) can be approximated by [START_REF] Trillo | Parametric and Raman amplification in birefringent fibers[END_REF] :

) / sin( ) / exp( ) ( ) ( 1 2 2 2 1 2 2 2 1       t t t b t a     (4a) ) / exp( 2 ) ( 2 2   t r t b   (4b)
In these equations,  1 =32 fs,  2 =12 fs and r represents the relative values of the parallel and orthogonal Raman gains in the linear basis components (r ≈ 0.2).

In a previous study published in 2004 [START_REF] Pitois | Simultaneous achievement of polarization attraction and Raman amplification in isotropic optical fibers[END_REF], we have demonstrated numerically from these equations and experimentally in the visible domain the possibility of combining Raman amplification and polarization attraction in the same optical fiber. In fact, it has been shown that the counter-propagating polarized pump wave can act both as a polarization attractor and a source of energy for the signal wave. These pioneering experiments were conducted using a short piece of perfectly isotropic fiber with very high peak power nanosecond pulses whose wavelengths were in the visible domain, around 575 nm. In this present paper, our approach is radically novel since we now consider a much longer standard optical fiber, having a low polarization mode dispersion, which allows the device to be compatible with continuous wave, pulse train or telecom processing around 1.55 µm and involving average powers below 1 W. Even though the residual birefringence of such a fiber is very small, it cannot rigorously be considered as perfectly isotropic. Nevertheless, it was shown in a recent work [START_REF] Fatome | Observation of light-by-light polarization control and stabilization in optical fibre for telecommunication applications[END_REF]- [START_REF] Kozlov | Theoretical study of polarization attraction in high birefringence and spun fibers[END_REF] that such a fiber can be seen as a concatenation of short pieces of isotropic fibers, each acting as a polarization attractor for the signal wave, so that an effective polarization attraction effect can occur at the fiber output thanks to an average phenomenon. This basic approach was confirmed by Sugny et al. in refs. [START_REF] Assémat | Complete nonlinear polarization control in an optical fiber system[END_REF]- [START_REF] Assémat | Hamiltonian tools for the analysis of optical polarization control[END_REF].

In fact, one of the most important consequence is that the possibility to use such a long optical fiber results in a significant increase of the effective nonlinear interaction length between the counter-propagating waves, thus involving beam powers compatible with telecom applications.

Finally, it is also important to note that the counter-propagating Raman amplification effect is fundamentally not affected by the presence of a weak fiber birefringence. As a consequence, one can expect that such a long low-PMD fiber pumped by an adequate counter-propagating beam could act both as a polarization attractor and a Raman amplifier for the signal wave, thanks to an average process similar to that observed in refs. [START_REF] Fatome | Observation of light-by-light polarization control and stabilization in optical fibre for telecommunication applications[END_REF], [START_REF] Kozlov | Theory of lossless polarization attraction in telecommunication fibers[END_REF]. In other words, we believe that our system can be considered as a succession of short pieces of isotropic fibers in which the signal is simultaneously repolarized and amplified as described by equations [START_REF] Koch | Record 59-krad/s Polarization Tracking in 112-Gb/s 640-km PDM-RZ-DQPSK Transmission[END_REF]. This assumption will be confirmed through the experimental evidences reported in the section below.

Experimental set-up

Figure 2 illustrates the experimental set-up used to investigate the combined effects of Raman amplification and polarization attraction. At the transmitter side, pulses with full-width at half maximum (FWHM) of 2.5 ps and repetition rate of 10-GHz are first generated using a modelocked fiber laser (MLFL), operating at a wavelength of 1545 nm. The pulses are then broadened to 45 ps by an optical band pass filter (OBPF) and data encoded by means of an intensity Mach-Zehnder modulator (MZM) driven by a 10-Gb/s 2 7 -1 pseudo-random bit sequence (PRBS). A polarization scrambler is inserted to introduce random polarization fluctuations at a rate of 0.625 kHz. Before injection into the optical fiber, an Erbium doped fiber amplifier (EDFA) was used to reach the suitable average power of 24 dBm (250 mW). The fiber under-test is a 20-km long nonzero dispersion shifted fiber (NZDSF). The dispersion, dispersion slope, nonlinear coefficient and polarization mode dispersion (PMD) are 0.2 ps/nm/km, 0.09 ps/nm 2 /km, 1.7 (W.km) -1 and 0.03 ps/km 1/2 , respectively. The fiber is inserted between two WDM couplers so as to inject and monitor the signal wave. The counter-propagating Raman pump beam, which allows to control the polarization as well as to amplify the signal, was a linearly polarized incoherent wave having a spectral bandwidth of 40-GHz centered around of 1480 nm and with an average power which can be adjusted between 0 and 1 W. At the receiver, the SOP of the signal is analyzed onto the Poincaré sphere by means a commercially available polarization analyzer. Finally, a 30-GHz bandwidth oscilloscope associated with a bit error rate (BER) analyzer allowed to monitor in real-time the intensity profile of the outcoming pulses and transmission quality.

Experimental results: continuous optical signal

Experimental evidences of the combined effects of polarization attraction and Raman amplification were first achieved in the continuous wave (cw) regime at 1548 nm. The state of polarization (SOP) of the signal wave was measured at the output of the fiber as a function of the Raman pump power, for a fixed signal power of 250 mW. Results are shown in Fig. 3 for different pump powers ranging from 0 to 850 mW. In absence of Raman pump (Fig. 3a), and due to the initial polarization scrambling, the different points, representing the signal polarization states, are uniformly distributed onto the Poincaré sphere. As can be seen in Figs. 3b to 3d, when the pump power is gradually increased from 0 to 850 mW, all the signal polarizations asymptotically converges towards the same output state of polarization thanks to the polarization attraction effect (Fig. 3d). Simultaneously, a signal intensity on/off gain of 6.7 dB was measured at the output of the fiber for a pump power of 850 mW, thanks to the counter-propagating Raman amplification effect. Further information can be obtained by monitoring the output signal wave in the temporal domain thanks to a low-bandwidth oscilloscope. A linear polarizer was inserted just before the photodiode in order to simulate a polarization dependent component, i.e. so that all polarization fluctuations are turned into intensity fluctuations. The oscilloscope was used in persistence mode to superimpose the different waveforms. The results are shown in Fig. 4 for a signal power fixed to 250 mW, without (Fig. 4a) and with (Fig. 4b) the 850-mW counter-propagating Raman pump wave. When the pump wave is turned off (Fig. 4a), the temporal trace shows different levels of intensity, resulting from the different states of polarization induced by the initial polarization scrambling. When the pump wave is turned on (Fig. 4b), the initial power fluctuations induced by the polarization scrambling are significantly reduced, leading to a unique level of intensity at the output of the system and thus demonstrating the ability of our system to reduce polarization fluctuations without any polarization-dependent losses (PDL).

Experimental results: 10-Gbit/s optical signal

In this section, and in order to underline the practical compatibility of our device with telecommunication applications, we have replaced the initial continuous wave by a 10 Gbit/s PRBS signal wave at 1545 nm. Let us recall that thanks to the counter-propagating configuration of the system, both the polarization attraction and the Raman amplification effects are only sensitive to the average intensity profiles of the signal and pump waves, rather than on the short scale intensity profiles. In fact, the main issue which should be considered when using picosecond pulse trains is the temporal broadening induced by the group-velocity dispersion of the fiber and Kerr nonlinearity. Nevertheless, with the fiber parameters used in our experiment, the excess of temporal broadening observed after propagation in the optical fiber is kept below 10 ps. Figure 5 shows on the Poincaré sphere the SOP of the 10 Gb/s signal at the output of the system as a function of the pump power and for a fixed signal power of 24 dBm (250 mW). As can be seen, all signal polarizations asymptotically converged towards the same output state of polarization when pump power was increased from 0 mW (Fig. 5a), 200 mW (Fig. 5b), 400 mW (Fig. 5c) and finally 850 mW (Fig. 5d). Note that an on/off Raman gain of 6.7 dB was simultaneously achieved when pump power is increased up to 850 mW. 850 mW. The signal power was fixed to 24 dBm. A 6.7 dB on/off gain was achieved for a pump power of 850 mW.

The efficiency of the polarization attraction process can be also evaluated by calculating the degree of polarization (DOP) of the output signal polarization. The DOP is defined as: 6a). On the contrary, when the 850 mW pump beam is injected into the fiber, thanks to the polarization attraction process, all the output pulses emerge from the fiber with the same state of polarization. As a result, the initial intensity fluctuations linked to the polarization variations are greatly reduced, leading to a significant opening of the eye-diagram (Fig. 6b) and recovering of the data. At the same time, the Raman amplification process leads to an intensity gain of the signal beam of nearly 6.7 dB. These results clearly demonstrate the ability of our system to combine simultaneously an all-optical control and stabilization of the polarization state as well as an amplification of a 10 Gbit/s RZ telecom signal. Finally, we have also measured the corresponding bit-error-rate (BER) of the 10-Gbit/s signal as a function of the average power incoming on the receiver (Fig. 8). The reference is illustrated by the back-to-back configuration (i.e. at the fiber input) in squares. At the output of the system, when the polarization of the signal is scrambled, corresponding to the eye-diagram of Fig. 6a, the BER is limited to 3.10 -3 (stars). When the 850-mW counter-propagating pump wave is injected into the fiber (circles), the quality of the transmission is greatly improved and low BER penalties were obtained on the receiver.

Conclusion

In this paper, we report an all-optical device which allows combining both polarization attraction and amplification of an optical signal in a unique segment of standard optical fiber. The

Fig. 1 :

 1 Fig. 1: Principle of the device allowing both polarization attraction and Raman

Fig. 2 :

 2 Fig. 2: Experimental setup (a) 10 Gb/s RZ transmitter (b) all-optical polarization

Fig. 3 :

 3 Fig. 3: Evolution of the SOP of the output cw signal as a function of the Raman

Fig. 4 :

 4 Fig. 4: Intensity profile (detected behind a polarizer) of the output CW signal,

Fig. 5 :

 5 Fig. 5: Evolution of the SOP of the 10 Gbit/s signal wave at the output of the fiber as

SFig. 6 :

 6 Fig. 6: Evolution of the DOP of the 10-Gbit/s signal wave at the output of the fiber as

Fig. 7 :

 7 Fig. 7: Eye-diagram (detected behind a polarizer) of the output 10 Gbit/s, without (a)

Fig. 8 :

 8 Fig. 8: Evolution of the bit error rate as a function of average power in back-to-back

polarization attraction phenomenon is based on a four-wave mixing process whereas the optical amplification is obtained thanks to stimulated Raman scattering. These two effects were obtained simultaneously in the telecom domain by injecting a polarized counter-propagating pump wave in the optical fiber with a wavelength around 1480 nm. The experimental results were performed with both a continuous signal wave and a 10 Gbit/s PRBS RZ optical signal around 1550 nm.


Thu Thuy Nguyen

Elisabeth Chachaty
email: elisabeth.chachaty@igr.fr.

Clarisse Huy

Carole Cambier

Jean De Gunzburg

France Mentré

Antoine Andremont

Correlation between Fecal Concentrations of Ciprofloxacin and Fecal Counts of Resistant Enterobacteriaceae in Piglets Treated with Ciprofloxacin: toward New Means To Control the Spread of Resistance?

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

3

Increasing resistance to fluoroquinolones in Gram-negative bacilli is of major concern because it compromises their therapeutic use [START_REF] Lee | Impact of discordant empirical therapy on outcome of community-acquired bacteremic acute pyelonephritis[END_REF][START_REF] Mølbak | An outbreak of multidrug-resistant, quinolone-resistant Salmonella enterica serotype typhimurium DT104[END_REF]. Colonic flora is the reservoir of many of the Enterobacteriaceae species with clinical significance [START_REF] Donskey | The role of the intestinal tract as a reservoir and source for transmission of nosocomial pathogens[END_REF] which are exposed to antibiotic residues during fluoroquinolone treatments [START_REF] Pecquet | Faecal excretion of ciprofloxacin after a single oral dose and its effect on faecal bacteria in healthy volunteers[END_REF]. It has been shown that administration of enrofloxacin, a fluoroquinolone used in animals, could promote the emergence of quinolone resistant enterobacteria in fecal flora of pigs [START_REF] Wiuff | The effects of oral and intramuscular administration and dose escalation of enrofloxacin on the selection of quinolone resistance among Salmonella and coliforms in pigs[END_REF]. We also found that administration of oral ciprofloxacin is associated with the emergence of quinolone resistant strains of Enterobacteriaceae in fecal flora from human volunteers (3). However, rates of emergence of resistance were not significantly different although the volunteers were exposed to different antibiotic dosages. Of note, detection of resistance was only qualitative in that study, i.e. absence versus presence of detectable resistant bacteria, and not quantitative, expressing the densities of resistant bacteria in the feces. Indeed, several lines of evidence suggest that quantitative aspects of emergence of resistance should also be analyzed. For instance, in neutropenic patients, Gram-negative bacilli bacteremias are caused by the predominant fecal clone of this type (11). Also, the density of a given Escherichia coli strain in a subject's fecal flora can influence to some extent its ability to cause urinary tract infection [START_REF] Moreno | Relationship between Escherichia coli strains causing acute cystitis in women and the fecal E. coli population of the host[END_REF]. In order to explore further the dynamics of fluoroquinolone resistance in the fecal flora during treatments, we assessed here in a prospective randomized study in piglets the relationship between fecal concentrations of ciprofloxacin and amounts of excreted ciprofloxacin resistant

Enterobacteriaceae.

Twenty-nine piglets from a single farm were included 4 weeks after birth. They were born from sows that were treated at the time of parturition with 2 g oxytetracycline given intramuscularly, but had not received directly any antimicrobials. Piglets were housed in individual boxes for 21 days before the start of treatment (D1) and were randomly assigned to oral treatment with placebo (9 piglets), ciprofloxacin 1.5 mg/kg/day (10 piglets) or 15 mg/kg/day (10 piglets) from D1 to D5. Ciprofloxacin suspension (Ciflox®, Bayer) or mineral water (placebo group) was administered once a day at least 4 hours before food to animals fasted for at least 12 hours. The protocol was approved by the local ethical committee.

Fecal samples were recovered from piglets after anal stimulation on mornings before treatment (at D-1 and D1), during treatment (at D3 and D5, before ciprofloxacin administration), and after treatment (at D7 and D9). Samples were stored frozen at -80°C until microbiological analysis and ciprofloxacin assay were performed blinded of the treatment group. Fecal concentrations of ciprofloxacin were measured by microbiological assay (5) after ten-fold dilution in HCl 0.1N and then further dilutions in phosphate buffer pH 8.0. Counts of total and resistant Enterobacteriaceae were obtained after plating serial dilutions of fecal samples on Drigalski agar (BioRad laboratories, Marnes-la-Coquette, France) supplemented or not with 20 mg/l of nalidixic acid or with 2 mg/l of ciprofloxacin. However, since there was no significant differences in the counts obtained on the two types of media (data not shown), only counts on ciprofloxacin containing agar were used for further statistical analysis.

Individual area under the curve (AUC) from D1 to D9 of ciprofloxacin fecal concentrations (AUC_CIP), of log counts (AUC_RES) and of percentages (AUC_%RES) of ciprofloxacin resistant Enterobacteriaceae above pre-treatment baseline values were computed by trapezoidal approach. These criteria were compared across groups by nonparametric ANOVA and pairwise Wilcoxon tests if global tests were statistically significant (with significance level at 0.05). All statistical analyses were performed using SAS 9.1 (SAS, Cary, NC).

Concentrations of ciprofloxacin increased with the dose administered and peaked at 84.8±57.9 versus 11.6±12.6 µg/g of feces at D5 in piglets treated with 15 and 1.5 mg/kg/day of ciprofloxacin respectively (p<0.001) (Fig. 1A). Fecal antibiotic activity was detectable to some extent in all animals 2 days after cessation of treatment (D7) but no activity was detected in any animal at D9. AUC_CIP was significantly higher for the group treated with 15 mg/kg/day of ciprofloxacin than that for the group treated with 1.5 mg/kg/day (p<0.0005) (Table 1).

Most of the piglets (25/29, 86%) had ciprofloxacin resistant Enterobacteriaceae detected in fecal samples before treatment. Administration of ciprofloxacin promoted the increase of both absolute counts (Fig. 1B) and percentages of ciprofloxacin resistant enterobacteria (Fig. 1C) in the 2 treatment groups when compared with the placebo group. AUC_RES and AUC_%RES were significantly different between the groups treated with 1.5 mg/kg/day and 15 mg/kg/day versus the placebo group (both p<0.01 for AUC_RES and both p<0.0005 for AUC_%RES). AUC_RES and AUC_%RES were also significantly higher for the group treated with 15 mg/kg/day than that for the group treated with 1.5 mg/kg/day (p<0.05 and p<0.001 respectively) (Table 1).

There were significant correlations between AUC_CIP and AUC_RES as well as between AUC_CIP and AUC_%RES (both p<0.0001, Spearman tests). These relationships were where AUC_RES 0 , AUC_RES max and AUC_CIP 50 are respectively the baseline, the maximal effect and the value of AUC_CIP to obtain 50% of the maximal effect (Fig. 2).

We showed a significant positive correlation between the AUC of fecal ciprofloxacin concentrations and the AUC of log ciprofloxacin resistant Enterobacteriaceae counts from D1 of treatment to D9 post treatment. Rapid increase of ciprofloxacin resistant enterobacteria from the beginning of ciprofloxacin administration probably resulted from the selection of resistant bacteria which were already present in low counts before treatment in the fecal flora of most (86%) of our piglets. Indeed, in a previous study in human volunteers (3), only 6 of 48 (12.5%) had quinolone resistant enterobacteria detected before treatment and the increased prevalence of resistance was only observed after the end of the treatment without significant dose relationship. The piglet study described here was much more fitted to evidence pharmacokinetic/pharmacodynamic relationship since the doses varied by a factor of 10, starting from a very low dose to a clinical dose (from 1.5 to 15 mg/kg/day) whereas they only varied by a factor of 3 (250 mg twice/day to 750 mg twice/day) in the volunteers. Median ciprofloxacin fecal concentrations at steady state were also much lower in piglet than in human volunteers groups of treatment (7 and 73 versus 845 and 1938 µg/g of feces respectively

). An increase in the percentage of ciprofloxacin resistant Bacteroides fragilis strains in feces of human-flora-associated mice treated with various doses of ciprofloxacin has been also reported as dose-related [START_REF] Perrin-Guyomard | Impact of residual and therapeutic doses of ciprofloxacin in the human-flora-associated mice model[END_REF]. However, our results differ from the absence of correlation between antibiotic dosage and the percentages of fecal quinolone resistant enterobacteria during treatment that was observed by others in pigs receiving several doses of enrofloxacin varying in a range of 1 to 6 [START_REF] Wiuff | The effects of oral and intramuscular administration and dose escalation of enrofloxacin on the selection of quinolone resistance among Salmonella and coliforms in pigs[END_REF]. Dynamic of emergence of resistant bacteria is dependant of several parameters among which the concentrations of free and active antibiotic reaching the colonic flora, MICs of bacterial populations and barrier effects exerted in the colonic ecosystem. This may account for the nonlinear correlation between fecal antibiotic concentrations and level of resistant bacteria we observed and, also, for differences in results from similar studies.

We used AUCs to characterize the fecal pharmacokinetic/pharmacodynamic relationship for ciprofloxacin, taking into consideration the whole time-course of concentrations and bacteria counts. We believe that AUCs of fecal counts are the most relevant end-points because they describe, better than single time point values, the total amount of resistant bacteria excreted,

which is actually what one wants to decrease to control the spread of resistance.

Indeed, the link we showed between intestinal concentration of ciprofloxacin and the amount of ciprofloxacin-resistant enterobacteria excreted by the animals may be of upmost importance in the current context of the spread of bacterial resistance. It has been shown in a rat model that the administration of a compound consisting of activated charcoal entrapped within zinc-pectinate beads could remove residual colonic ciprofloxacin (4). Thus, if such a removal could be obtained in animals or humans receiving therapeutic doses of quinolones without impairing the pharmacokinetics of the drug in plasma, this would open new avenues within the scope of the recently signaled eco-evo drugs (1) to help to control of the emergence and spread of quinolone resistance in gut-originating Gram-negative bacteria.

This work was conducted in part with a grant from Da Volterra and the authors would like to thank the Professor Pascal Gustin, Service of Pharmacology, University of Liège, Belgium for his scientific advices.

11FIG. 1 . 12 FIG. 2 .

 1122 FIG. 1. Mean (and 95% confidence intervals) of ciprofloxacin concentrations (Panel A) and of log counts of ciprofloxacin resistant Enterobacteriaceae (Panel B) as well as of percentages of Enterobacteriaceae resistant to ciprofloxacin (Panel C) in fecal samples from piglets treated with placebo (n = 9), 1.5 mg/kg/day (n = 10) and 15 mg/kg/day (n = 10) of oral ciprofloxacin from day 1 to day 9.

TABLE 1 .

 1 Descriptive statistics by treatment group on AUC of ciprofloxacin concentrations (AUC_CIP), AUC of log counts of ciprofloxacin resistant Enterobacteriaceae above baseline (AUC_RES) a and AUC of percentages of Enterobacteriaceae resistant to ciprofloxacin (AUC_%RES).

			Mean±SD
	Treatment group	AUC_CIP	Median (Range) AUC_RES	AUC_%RES
		(µgday/g)	(log CFUday/g)	(%day)
	Placebo	0	-1.3±5.3	4.5±6.1
	(n=9)	0	0.5 (-13.5-5.1)	0.9 (0.1-15.6)
	Ciprofloxacin 1.5mg/kg/day	44.0±32.2	7.6±6.2	284.9±124.8
	(n=10)	30.6 (10.1-113.5)	7.1 (-0.2-21.5)	261.8 (86.5-504.8)
	Ciprofloxacin 15mg/kg/day	362.5±179.8	14.7±7.6	567.1±98.2
	(n=10)	299.3 (191.2-711.5)	15.0 (-1.0-26.0)	568.0 (403.9-700.0)

a Some values of AUC_RES are negative because of the normalisation by the baseline.

This work was supported by a grant from DaVolterra.


P Audigane

S Brown

Le Gallo

Pearce J 10

Spiers C 11

ULTimateCO2 : A FP7 European Project dedicated to the understanding of the long term fate of geologically stored CO2

Although the technical feasibility of CCS has been proven with the development of smallscale pilot sites that draw on oil and gas industry experience, the EC Directive on the Geological Storage of CO2 requires operators to demonstrate that the long-term fate of the CO2 in the reservoir will ensure permanent containment. Other stakeholders, notably the general public and their representatives, seek answers to questions on the behaviour and impact of the injected CO2: "What will happen to the CO2?", "Will it leak from the chosen reservoir?", "Will it stay underground?", "For how long?". Such questions, at whatever level, can only be answered convincingly through a better understanding of a chain of complex physical and chemical processes. This requires a significant increase in scientific knowledge beyond the state-of-the-art since, unlike other domains, we do not currently have experience to draw on for this new technology.

The European Commission has selected the ULTimateCO2 project for funding under its FP7 R&D 2011 program. ULTimateCO2 will assess the long-term CO2 storage behaviour in terms of efficiency and security. The project is dedicated to studying the main physical processes needed to develop a better, quantitative understanding of the long-term geological storage of CO2, namely: (i) reservoir trapping mechanisms, (ii) sealing integrity of fractured and faulted caprock, and (iii) leakage through interfaces in the well vicinity. The research programme will comprise laboratory experiments, numerical modelling application and development, field data implementation and review of natural and industrial analogue evidence. Close collaboration with the NER300 candidates and EEPR demonstration sites will underpin all investigation with relevant supply of data industrial context. ULTimateCO2 will define a set of recommendations that will enable both regulators and operators to demonstrate that site specific long-term site performance will lead to permanent and safe CO2 containment. This will enable robust storage and allow clear conditions to be established for post-closure liability transfer.

The four-year ULTimateCO2 programme comprises seven work packages (see Figure )   WP1 concentrates on overall coordination of the project, ensuring the link between the consortium and the European Commission, overseeing collaboration with the other associated participants (Advisory Board), monitoring progress and deliverables, managing cooperation and data exchange between participants and reporting to the EC.

WPs 3, 4 and 5 are focused on understanding the three main aspects determining CO2 long term confinement efficiency and impacts on surrounding areas.

WP3 improves the understanding of fundamental long-term physical and geochemical processes. This include the rates and relative importance of SDRM (structural, dissolution, residual and mineral) trapping at reservoir scale, and addresses the influence of minor constituents that may be contained within the CO2 stream. This is evaluated through experimental and modelling applications at planned CO2 storage demonstration sites. Static geological modelling and dynamic reservoir simulation is conducted to underpin much of the consideration of long-term reservoir containment. Experiments are conducted to assess (i) the long-term residual trapping efficiency in sandstone samples containing residual concentrations of CO2 (ii) the potential impact of impurities such as SOX, NOX, H2S, O2 and (iii) the Fe release in sandstone reservoir rocks, as induced by acid gases co-injected with CO2.

WP4 focuses on the long-term caprock sealing integrity, including the effects of discontinuities, such as faults or fracture networks and considering the influence of fluid-rock interactions. In order to ascertain the sealing capacity of potential geologically "weak points" in the caprock, a number of factors must be considered: i) fault and fracture network characteristics and their thermo-hydro-mechanical behaviour, ii) effects of chemical degradation on the mechanical and transport properties of intact and faulted caprock, iii) sealing efficiency of caprocks in response to long-term changes in basin conditions, iv) consequences of long-term coupling between these processes. As such, this work package incorporates a combined approach of observation, experimental testing and modelling to deal with each of these identified aspects.

WP5 is dedicated to the understanding of the chemical and mechanical processes affecting the long-term sealing integrity of the near-wellbore zone. The damaging due to drilling of the near-well caprock zone, as well as the quality of the placement of the cement plug and sheath can play a major role in determining the available migration pathways. The loss of bonding between different materials can cause annular pathways along the interface between casing, cement and caprock. Furthermore, uncertainties prevail concerning the range of permeability for cement and caprock degraded by chemical fluid-rock interactions and the effects of impurities in the fluid phase, as well as how porosity and permeability evolve over long time scales.

WP2 is aims at integrating the results of the detailed study of these abovementioned three main aspects (WPs 3, 4 and 5) to assess the long-term impacts of geological storage at the basin or regional scale. The value of such modelling is to study (i) brine displacement induced by CO2 injection into a deep saline formation, (ii) fault reactivation induced by CO2 injection into a compartmentalized depleted gas reservoir, and (iii) CO2 leakage from deep saline formations to fresh groundwater through abandoned wells. Modelling will integrate field data from CO2 storage demonstration sites to provide test cases.

WP6 is dedicated to uncertainty assessment, and supports all the other work packages by providing them with a framework for addressing the confidence that can be placed on the long-term extrapolation of identified processes, and numerical simulation results.

WP7 will disseminate the project results to various selected target audiences. Clear recommendations will be established on the basis of the scientific results and these will be made available for storage projects, thus enabling site-specific evaluation of the long-term fate of geologically stored CO2.

Figure 1 :

 1 Figure 1 : Graphical presentation of the ULTimateCO2 project


Paul Byrne
email: paul.byrne@univ-rennes1.fr

Jacques Miriel

Yves Lenat

Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 1: Basic concepts and performance verification

Keywords: heat pump, heating, cooling, thermosiphon, defrosting

may come     Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 1: Basic concepts and performance verification

Context and objectives

Energy efficiency of buildings is continuously improving thanks to more and more stringent thermal regulations.

Besides, comfort requirements demand more and more energy. The demand for cooling is rising to compensate internal heat gains caused by more and more household electrical equipment whilst domestic hot water (DHW) demands continue to increase. A better thermal envelope implies less energy for heating and more for cooling.

Therefore thermal needs of new buildings follow the trend of more balanced energy demands between heating and cooling. ECBCS Annex 48 of the International Energy Agency deals with this issue [START_REF] Stabat | IEA ECBCS Annex 48 Subtask 1: Analysis of heating and cooling demands and equipment performances[END_REF]. An answer to a simultaneous energy demand in heating and cooling is the heat pump, since it has simultaneously a heating capacity at the condenser and a cooling capacity at the evaporator.

This study presents the design of a heat pump that can satisfy fluctuating needs, simultaneous or not, in heating and cooling. This heat pump with heat recovery is named HPS (Heat Pump for Simultaneous heating and cooling). It is suited to buildings such as hotels where DHW demands are high, small commercial units like groceries and petrol stations or glass-fronted north-south oriented office buildings where simultaneous needs in space heating and cooling are frequent.

The first objective of the HPS is to produce, as often as possible, heating and cooling energies using the same electric energy input at the compressor. Chua et al. [START_REF] Chua | Advances in heat pumps systems: A review[END_REF] evocate a significant reduction in carbon footprint using improved heat pumps for heating and cooling. Many authors work on such systems for simultaneous production in various applications. For example, Liu and Hong [START_REF] Liu | A Comparison of energy efficiency between variable refrigerant flow systems and ground source heat pumps[END_REF] compare ground source heat pumps to variable refrigerant flow systems, both able to provide heating and cooling energies to four perimeter zones and one core zone of a simulated small office building. White et al. [START_REF] White | A heat pump for simultaneous refrigeration and water heating[END_REF] expose the advantageous performance of a transcritical CO 2 heat pump for simultaneous refrigeration and water heating. Gong et al. [START_REF] Gong | A new heat recovery technique for air-conditioning/heat-pump system[END_REF] present an air-conditioning/heatpump system recovering heat for domestic hot water production. All these heat pumps demand more research and development but consume less energy than conventional systems.

The second objective is to propose an answer to reduce the performance loss of air-source heat pumps during winter. This aspect depends on the dynamic behaviour of the HPS which is dealt with in part 2 of this article [START_REF] Byrne | Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 2: Dynamic behaviour and two-phase thermosiphon defrosting technique[END_REF].

A HPS prototype was built and tested. The aim of the first part of the study is to verify that the performance in each mode of operation (heating, cooling and simultaneous modes) is correct.

HPS concepts

Components

The HPS prototype (Fig. 1) produces hot and chilled water using plate heat exchangers. A balancing air coil works either as a condenser for heat rejection in a cooling mode or as an evaporator for heat suction in a heating mode. The air evaporator and the air condenser are never used at the same time. These functions have been assembled in the same three-fluid air coil (air, high pressure refrigerant and low pressure refrigerant) in order to decrease the finned surface area compared to separate air condenser and evaporator. When the tubes of the air evaporator are used the surface of the fins near the tubes of the air condenser are also used and vice versa. A subcooler is connected to the cold water loop to carry out a short-time heat storage during winter sequences.

Depending on the mode of operation, the electric components (compressor, fan and electronic valves named Evr) are managed automatically by a programmable controller or manually by the operator. The thermostatic expansion valves are named TEV1 (connected to the water evaporator) and TEV2 (connected to the air evaporator). Table 1 shows the general specifications of the components. The chosen refrigerant is R407C, which is widely used in heat pumping technology. Non-return valves named Nrv1 and Nrv2 are placed at the outlets of the air and water condensers.

Operating modes

Three operating modes can be run.

• The simultaneous mode produces hot and cold water using the water condenser and the water evaporator (electronic valves Evr1 and Evr3 are open).

• The heating mode produces hot water using the water condenser, the air evaporator (electronic valves Evr2

and Evr3 are open) and also the subcooler to store the subcooling energy in the cold water tank.

• The cooling mode only produces cold water using the water evaporator and the air condenser (electronic valves Evr1 and Evr4 are open).

A production ratio r p can be defined following Eq. ( 1) using heating and cooling capacities. For a simultaneous production mode this ratio is close to 1.3.

c h p Q Q r & & = (1)
A demand ratio r d can be defined following Eq. ( 2) using integrated heating and cooling demands.

demand cooling demand heating r d = (2)
Depending on the building's demand ratio r d , two sequences can be run:

• if r d > r p , heating demand is higher than cooling demand, the sequence starts in the simultaneous mode and continues in the heating mode when the cooling demand is satisfied;

• if r d < r p , cooling demand is higher than heating demand, the sequence starts in the simultaneous mode and continues in the cooling mode when the heating demand is satisfied.

The variability of operating times in each mode enables the control system to adapt hot and chilled water production to heating and cooling demands.

High pressure control

The high pressure control system is based on the temperature control of refrigerant in a liquid receiver. High pressure is controlled depending on the hot source temperature. The whole operation is described in the second article on this work concerning the dynamic behaviour [START_REF] Byrne | Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 2: Dynamic behaviour and two-phase thermosiphon defrosting technique[END_REF].

Comparison between the HPS and a standard reversible heat pump

The HPS is able to produce simultaneously hot and cold water whereas the standard reversible heat pump cannot. Over a year, if the simultaneous demand in heating and cooling is important, using a heat pump with heat recovery such as the HPS enables to achieve substantial energy savings. If not, the reversible heat pump would be the best techno-economical choice.

The HPS heat exchangers and connexions are sized and optimized, each of them for a unique application. For example, the water evaporator will never be used as a water condenser. This is not the case for standard reversible heat pumps: heat exchangers efficiencies and thus overall performance usually decrease in the reversed mode [START_REF] Rajapaksha | Influence of reversing methods on the performance of a reversible water-to-water heat pump[END_REF].

Because the HPS components are dedicated either to evaporation or condensation, pressures vary in a smooth manner during transitions between modes. The longevity of the HPS compressor is increased compared to the one of the reversible heat pump subjected to important mechanical strains during switches of the reversing valve.

The operations of the HPS and a standard reversible air-to-water heat pump installed in a hotel of 45 bedrooms situated in Rennes were analyzed in yearly simulations using Trnsys software [START_REF]TRNSYS, A Transient Simulation Program[END_REF][START_REF] Byrne | Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO 2 as a working fluid[END_REF]. Both plants were equipped with electric auxiliary heaters and produced heating and cooling energy for space heating, space cooling and DHW production. The results showed an annual electric energy saving of 19.8%. The annual performance improvement of the HPS reached 16.6%. These interesting results were obtained thanks to the simultaneous mode of production, to the use of the cold water tank as a short-time heat storage during winter sequences for performance enhancement and to the two-phase thermosiphon defrosting technique.

3 Experimental study

Experimental setup

A HPS prototype was built and connected to a water distribution system (Figs. 2 and3). The water system is used to limit the temperature variation of hot and cold water. It is composed of two water tanks, a circulation pump and four fan coil units (FCU) that dissipate the heating or cooling energy produced by the heat pump. The heating and cooling nominal power of the FCU is respectively 1.85 kW (50°C/40°C) and 1.5 kW (7/12°C).

Temperature and humidity of the air circulating through the AHX are controlled, namely in order to obtain frosting conditions at the air coil.

Type K thermocouples were placed on every inlet and outlet of the refrigeration cycle components on refrigerant, water and air sides. All thermocouples, (for example the ones giving T hot gas , T aEv and T wEv , shown in Fig. 1) were placed in contact with the copper tubes and recovered by a lagging tape. The uncertainty on these measurements is ± 0.5 K. High and low pressure sensors come from the P299 series of Johnson Controls. Their accuracy is ± 1%. The sensors were connected to an acquisition unit using a time base of 1 second and to the control computer through which were also managed the different electric devices. Electric energy consumption was given by a pulse electric meter. Water mass flow rates were calculated using water meters.

Performance verification

The performance of the HPS in terms of instantaneous COP was assessed during operations in heating, cooling and simultaneous modes. The COP is defined by the ratio of thermal capacity over electric power input at the compressor. In the simultaneous mode, heating and cooling capacities are summed. Refrigerant superheating is taken into account. Thermal capacities are calculated for a 0 K subcooling. Tables containing the results of COP calculations displayed in Figs 4 to 6 are available in the appendix for each test in the different modes. For each measurement, these tables also present the corresponding evaporating temperature, condensing temperature and superheat.

Figs. 4 to 6 show the COPs of the HPS prototype in heating, cooling and simultaneous modes in proportion to modelled COPs. The modelled COPs are calculated using the selection software of the compressor manufacturer depending on condensing and evaporating temperatures, useful superheat (in the evaporator) and subcooling.

Minimum suction superheat admitted by the software is 4 K. This suction superheat value was taken for the model. The uncertainty of the model is 5%. The sensibility of sensors and counters produce uncertainties on the measurements of thermal capacity and electric power of respectively 3.72% and 0.33%. However, results show discrepancies up to 15% between experimental and modelled values for the lower values of COPs. The lower values of COPs appear for the higher differences between the temperatures of hot and cold sources. This means that the difference between operating temperatures and the ambient temperature of the laboratory is higher. In spite of the attention paid to setting up the sensors and isolating them thermally, the metrology only guarantee an uncertainty interval of 15%. Nevertheless, it is sufficient to state that the special tubing of the circuit does not affect the heat pumping performance.

The variation of the composition of the refrigerant mixture R407C influences the system performance. The presence of a liquid receiver can cause a more important modification of the refrigerant composition and an increased COP reduction [START_REF] Rajapaksha | Influence of liquid receiver on the performance of reversible heat pumps using refrigerant mixtures[END_REF]. Figs. 4 to 6 show that this effect is not significant for the HPS.

Conclusions

This article presents the design of an air-source heat pump for simultaneous heating and cooling (HPS) that offers improved performance compared to standard reversible heat pumps. Apart from producing simultaneously hot and cold water, the configuration enables to switch from the air evaporator to a water evaporator during winter sequences. This machine proposes an answer to reduce the performance loss of air-to-water heat pumps under low ambient temperatures by the use of operating sequences alternating between an air evaporator and a water evaporator. Moreover, while standard air-source reversible heat pumps are penalized by defrosting (break in the heat production, decrease in COP), the HPS carries out defrosting using a two-phase thermosiphon with increased heat pump performance. Thanks to the special management of produced energies, simulations showed an annual performance increase of 16.6% when comparing the HPS to a standard reversible heat pump [START_REF] Byrne | Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO 2 as a working fluid[END_REF].

The HPS performance was verified in the three operating modes: heating only, cooling only and simultaneous heating and cooling, by comparison with the selection software of the compressor manufacturer.

In part 2 of this article [START_REF] Byrne | Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 2: Dynamic behaviour and two-phase thermosiphon defrosting technique[END_REF], the phenomenology of the HPS operation is studied on a dynamic point of view. The concepts of high pressure control, transitions between modes and the two-phase thermosiphon defrosting technique are analysed and validated experimentally.

Liste of figures

Fig. 1 Fig. 2 .Fig. 4 .Fig. 5 .Fig. 6 .

 12456 Fig. 1. HPS circuit

List of tables


Paul Byrne
email: paul.byrne@univ-rennes1.fr

Jacques Miriel

Yves Lenat

Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 2: Dynamic behaviour and two-phase thermosiphon defrosting technique

Keywords: heat pump, heating, cooling, thermosiphon, defrosting

. Its performance on defined operating conditions corresponds to the data given by the selection software of the compressor manufacturer. In the present part of this article, the operation of the high pressure control system, the transitions between heating, cooling and simultaneous modes and the defrosting sequence are analysed and validated experimentally.

NOMENCLATURE

Context and objectives

Energy efficiency of buildings is continuously improving thanks to more and more stringent thermal regulations.

Besides, comfort requirements demand more and more energy. The demand for cooling is rising to compensate internal heat gains caused by more and more household electrical equipment whilst domestic hot water (DHW) demands continue to increase. A better thermal envelope implies less energy for heating and more for cooling.

Therefore thermal needs of new buildings follow the trend of more balanced energy demands between heating and cooling. ECBCS Annex 48 of the International Energy Agency deals with this issue [START_REF] Stabat | IEA ECBCS Annex 48 Subtask 1: Analysis of heating and cooling demands and equipment performances[END_REF]. An answer to a simultaneous energy demand in heating and cooling is the heat pump, since it has simultaneously a heating capacity at the condenser and a cooling capacity at the evaporator.

This study presents the design of a heat pump that can satisfy fluctuating needs, simultaneous or not, in heating and cooling. This heat pump with heat recovery is named HPS (Heat Pump for Simultaneous heating and cooling). It is suited to buildings such as hotels where DHW demands are high, small commercial units like groceries and petrol stations or glass-fronted north-south oriented office buildings where simultaneous needs in space heating and cooling are frequent.

Part 1 of this article [START_REF] Byrne | Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 1: Basic concepts and performance verification[END_REF] presents the basic concepts and the performance verification of a R407C HPS prototype.

This second article validates the dynamic operation of the HPS, namely the transitions between the different modes, the high pressure control and the two-phase thermosiphon defrosting technique.

The first objective of the HPS is to produce, as often as possible, heating and cooling energies using the same electric energy input at the compressor.

The second objective of the HPS is to reduce the performance loss that characterizes air-source heat pumps during winter due to low source temperature and frosting [START_REF] Xia | Frost, defrost, and refrost and its impact on the air-side thermal-hydraulic performance of louvered-fin, flat-tube heat exchangers[END_REF][START_REF] Shao | Comparison of heat pump performance using fin-and-tube and microchannel heat exchangers under frost conditions[END_REF]. Huang et al. [START_REF] Huang | Comparison between hot-gas bypass defrosting and reverse-cycle defrosting methods on an air-to-water heat pump[END_REF] compared the most common defrosting methods: reversed-cycle defrosting and hot-gas bypass defrosting. When using the reversed-cycle defrosting technique, reversible air-to-water heat pumps record a performance loss because of a break in the heat production and a drawing of energy from the heat stock previously produced. The air and water heat exchangers have to carry out alternatively condensation at high pressure and evaporation at low pressure. When a change of mode occurs, the reversing valve switches and the previously high pressure section of the refrigeration circuit becomes the low pressure section. The opposite phenomenon occurs in the previously low pressure section. The pressure ratio changes nearly instantaneously from a high value to a very low value. This leads to high mechanical strains on the compressor that impact on its longevity. The hot-gas bypass defrosting system avoids the shocks induced by the switch of the reversing valve and the heat loss in the hot water tank that would occur in a standard reversible heat pump. However a performance loss remains because of the bypass of hot vapour from the compressor discharge. Liang et al. [START_REF] Liang | Control strategy and experimental study on a novel defrosting method for air-source heat pump[END_REF] detailed another defrosting method: hot gas from the discharge line is bypassed, throttled and injected into the evaporator to provide sensible heat as a defrosting energy.

Attention is especially paid on the fuzzy logic regulation strategy. Electric heating defrosting can also be used indirectly by heating the air flowing throughout the outdoor coil [START_REF] Kwak | A study on the performance enhancement of heat pump using electric heater under the frosting condition[END_REF]. Another technique described by Liu et al.

[8] consists in re-circulating exhaust air in the air evaporator in order to reduce the frost growth and to delay the defrosting sequence. All conventional defrosting methods consume energy. The HPS proposes a special circuit design that creates a two-phase thermosiphon used for defrosting. In refrigeration plants, thermosiphons are usually cancelled out by non-return valves because they represent risks of liquid refrigerant trapping in the circuit. The HPS takes advantage of this phenomenon to carry out defrosting. Thermosiphons are known to be an efficient means of heat transfer [START_REF] Lee | Experimental study on the double-evaporator thermosiphon for cooling HTS (high temperature superconductor) system[END_REF][START_REF] Hakeem | Prediction of temperature profiles using artificial neural networks in a vertical thermosiphon reboiler[END_REF]. Dobson [START_REF] Dobson | Simulation of the two-phase flow in a thermosyphon using an inclined transparent tube with the lowerend closed and the upper-end open[END_REF] showed that thermosiphon effects were difficult to model.

He also observed that the flows were oscillatory.

The aim of this phenomenological study is to verify that the high pressure control system is efficient, that transitions between modes are sufficiently smooth and to validate the innovative defrosting technique using a two-phase thermosiphon.

Further in the HPS concepts

The HPS prototype scheme is shown in Fig. 1. The different components of the circuit and the operating modes Switching from one mode to another is simply carried out by simultaneously shutting and opening the appropriate electronic valves without stopping the compressor. A high pressure control system, described hereafter, is necessary to ensure the proper operation of the HPS.

Summer sequence

During summer, the sequence alternates between cooling and simultaneous modes. A cooling demand is detected by a cold water temperature higher than the set temperature. A summer sequence is then launched, beginning by the simultaneous mode in case of possible demand in heating. When the heating demand is satisfied, the HPS switches to the cooling mode and the air condenser is used instead of the water condenser. The HPS stops when the cooling demand is satisfied or switches back to the simultaneous mode if a heating demand is detected by the measurement of a low hot water temperature.

Classic winter sequence

During winter, the sequence alternates between heating and simultaneous modes. The cold water tank is used as a short-time heat storage. If a heating demand is detected, the sequence begins by the heating mode engaging the water condenser, the air evaporator and the subcooler. While producing hot water using the water condenser, the cold water tank is heated, usually from 5 to 15°C, by the refrigerant subcooling energy. When the temperature of 15°C is reached, the HPS regulation switches to the simultaneous mode and uses the energy previously stored in the cold water tank as a cold source for the water evaporator. The cold water tank temperature decreases from 15 to 5°C.

In the simultaneous mode, the evaporating temperature is higher than in the heating mode from the moment that ambient air is colder than the cold water tank. Therefore, using the simultaneous mode for a certain time during the winter sequence enables to produce hot water continuously with improved average system performance compared to standard air-source heat pumps. Besides, in the simultaneous mode, the air evaporator is not used for evaporation and can be defrosted using a two-phase thermosiphon detailed hereunder.

Winter sequence with defrosting

In the heating mode, under cold outside air temperatures, the fins of the air evaporator get frosted. Before frost thickness becomes critical, the cold tank temperature has raised 15°C and the simultaneous mode has been engaged. In this mode, the air coil is automatically defrosted by a two-phase thermosiphon formed between the two evaporators. A supplementary flow of vapour comes out of the water evaporator and migrates towards the air evaporator where the temperature, and thus the pressure, is lower. At the outlet of the water evaporator, the refrigerant vapour temperature is between 10°C and 5°C depending on the cold water temperature. The refrigerant exchanges latent heat with the frosted fins, condenses and flows back to the water evaporator or to the suction accumulator by gravity.

The major advantage of this sequence is to carry out defrosting without stopping heat production. Frost thickness can thus be minimized and mean convection heat transfer coefficients at the evaporator can be maximized. The average heat pump efficiency under frosting conditions is improved compared to the performance of standard air-source heat pumps that use hot gas or reversed cycle defrosting methods.

High pressure control

As pressures and temperatures are linked during condensation, high pressure control can ensure that condensation occurs in the appropriate heat exchanger. Moreover it is able to control the condensation temperature and thus the heating capacity. A special liquid receiver is placed on the liquid line. It is connected to the compressor discharge line and the inlet of the air evaporator by copper tubes of smaller diameter on which normally closed electronic on-off valves are placed (EvrHP and EvrLP on Fig. 1).

If the chosen mode is the heating mode, condensation occurs in the water condenser. The controller calculates the set point for high pressure, function of the hot water inlet temperature at the water condenser (Eq. ( 1)) and a temperature discrepancy characterized experimentally. The equation is a curve fit of data extracted from tables published by the International Institute of Refrigeration [START_REF]IIF/IIR, R407C, Propriétés thermophysiques, Tables et diagrammes pour l'industrie du froid[END_REF]. The discrepancy ∆T CD takes into account the pinch difference between hot water inlet temperature and bubble temperature of refrigerant, a slight subcooling because of a possible heterogeneity of phase in the tube at the location where the temperature sensor is placed and a security in relation to the control deadband. For condensation at the air coil in the cooling mode, the set point for high pressure is a function of the air inlet temperature and another temperature discrepancy proper to the heat exchanger.

+ ∆ + ⋅ + ∆ + ⋅ + ∆ + ⋅ = - - - CD in hw CD in hw CD in hw Set T T T T T T HP (1)
The objective of the high pressure control is to carry out a complete condensation in the water condenser.

Without this system the thermostatic expansion valve would have "naturally" controlled the system so that condensation would finish in the subcooler. Some latent heat from condensation would have not been directly transferred to the hot water.

The high pressure control system indirectly controls the volume of liquid in the receiver. The volume of liquid in the different condensers depends on the mode. If the high pressure is below the set point, the electronic valve EvrHP is opened. The receiver is filled up with gas coming from the compressor discharge line at a pressure higher than the pressure in the receiver until the set pressure is reached. The gas entering the receiver drives the liquid towards the evaporator. The non-return valve closes because pressure becomes higher at the outlet than at the inlet. The subcooling heat exchanger and the bottom part of the water condenser are filled up with more liquid until the appropriate level of liquid is reached. If however the chosen mode is the cooling mode, the condenser becomes the air heat exchanger. The set point for pressure is then the lowest possible. The electronic valve EvrBP is opened. The pressure is reduced by driving the vapour out of the top part of the receiver towards the inlet of the air evaporator. The refrigerant in a liquid phase is sucked out of the water condenser and the subcooler and enters the receiver.

The proper operation of the control system depends upon a special liquid receiver being designed quite high and quite narrow with the main objective to enhance temperature stratification and to limit as far as possible thermal transfer between the gas and the liquid. When the gas is injected, part of it condenses. When gas is rejected to the low pressure, part of the liquid evaporates. These phenomena reduce the liquid variation induced by a pressure variation in the receiver. Although it delays the response in terms of high pressure, it ends by stabilizing the control system. The receiver is also thermally insulated to reduce the heat transfer towards the ambience.

3 Experimental study

Experimental setup

The experimental setup is described in the first part of this article [START_REF] Byrne | Experimental study of an air-source heat pump for simultaneous heating and cooling -Part 1: Basic concepts and performance verification[END_REF].

Dynamic behaviour

The operating sequences were tested, verifying the operation of the high pressure control method and the transitions between modes. Fig. 2 presents the evolution of temperatures of water and air sources during a 10minute sequence engaging first the heating mode, followed by the simultaneous mode. The acceptable operation of the high pressure control system is also shown in this figure by the measurement of hot gas temperature at the top of the liquid receiver. The set pressure increases following Eq. ( 1) depending on the hot water temperature at the entrance of the condenser. The hot gas temperature increases when the electronic valve EvrHP opens and lets hot gas enter the receiver and decreases with thermal exchange with the environment. Hot gas injection frequency is higher during the heating mode than during the simultaneous mode because of the position of the liquid receiver in the system: the receiver was placed in the air flow. During the heating mode the air evaporator and the fan are in function. Thermal exchange between the liquid receiver and the environment is enhanced during this mode. During the simultaneous mode the air circulation is stopped. Less thermal losses are thus observed. Fig. 2 highlights that the prototype has to be modified to reduce thermal losses of the liquid receiver and that the injection frequency is too high (between 25 and 40 seconds) to ensure a satisfactory longevity to the electronic valve EvrHP.

When the electronic valve EvrLP opens and drives out gas from the receiver towards the entrance of the air evaporator, the high pressure decreases. Fig. 2 does not show this operation that is actually used during transitions between modes having different hot sources.

In order to present each transition fully, figs. 3 and4 show the temperatures of refrigerant and sources during transitions between heating and simultaneous modes and between cooling and simultaneous modes. Globally, temperature variations are smooth for both water and refrigerant at the inlets and the outlets of the water condenser and the water evaporator. Stronger temperature variations appear on the air source and for refrigerant at the air condenser. The latter temperature variation is due to condensation of refrigerant in the air condenser even when it is unused in the simultaneous mode. Temperature and thus pressure decreases while refrigerant is condensing. In this situation, a mechanical shock can occur in the circuit if the electronic valve Evr4 opens. The strategy adopted to avoid this risk is to use the control system to decrease the high pressure to the level of the air heat exchanger side before opening Evr4.

The dynamic behaviour of the HPS is validated because the objectives of an efficient high pressure control system and smooth transitions are satisfied. Oil return has also been checked using the oil light of the compressor: no low level was noticed.

Defrosting technique

Two-phase thermosiphon observation

The two-phase thermosiphon circulates in tubes that have large diameters (represented in green in Figs. 1 and5).

Vapour migrates from the water evaporator towards the air evaporators. Condensed refrigerant returns back by gravity to the water evaporator and the suction accumulator.

Thermographic pictures (Fig. 6) have been taken before and during the defrosting phase at the position of the white square in Fig. 5. Before the defrosting sequence, temperature is homogenous. The tube contains vapour flowing from the air evaporator towards the compressor. When the two-phase thermosiphon defrosting sequence is launched, the thermostatic expansion valve TEV2 does not supply refrigerant to the air evaporator anymore because the electronic valve Evr2 is closed. In Fig. 6 it can be seen that heterogeneity of temperature appears in the tube. The hotter vapour phase flows up the tube in the top part of the section and the colder liquid phase flows down in the bottom part.

Defrosting technique validation

The validation is based on the observation of the frost layer disappearance during a defrosting sequence using the two-phase thermosiphon. The defrosting time is compared to a frost layer disappearance time by thermal exchange with the ambient air surrounding the test bench. In this second case compressor and fans are stopped.

Frosting: The first phase consists in running the heat pump in a heating mode under frosting conditions. Ambient air temperature is controlled around 0°C. Frost progressively appears on the LP sections of the air coils of the prototype. The frost layer thickness increases over a period of 30 minutes.

Defrosting (Fig. 7): After running the heating mode during 30 minutes, the frost layer is assumed to be sufficient to run the defrosting sequence. Fig. 7 shows heterogeneity of the frost layer due to an incorrect tubing of the three-fluid AHX. The electronic valve Evr2 is switched off and the electronic valve Evr1 is switched on.

The water evaporator is used instead of the air evaporator. The simultaneous mode is engaged and the two-phase thermosiphon defrosting starts. The frost layer disappears within 2 minutes. As a comparison, after a 30-minute frosting phase, the prototype was stopped. The defrosting energy was then brought by thermal convective exchange with the ambient air surrounding the test bench. The time of frost disappearance reached 20 minutes.

This comparison confirms that the thermosiphon defrosting technique works very efficiently.

Two-phase thermosiphon characterization

The frost mass is evaluated by the mass of water collected after defrosting. The defrosting energy E df is calculated by Eq. ( 4) using the frost mass m f and the specific latent heat of fusion L fus .

fus f df L m E ⋅ = (4)
The average defrosting capacity is calculated by Eq. ( 5) using the defrosting energy E df and the defrosting time

t df . df df df t E Q = & (5)
The mean refrigerant mass flow rate is given by Eq. ( 6) using the enthalpy difference between saturated vapour and saturated liquid at 0°C.

) 0 ( C h Q m vap df r °∆ = & & (6)
During the defrosting sequence, refrigerant temperatures have been monitored at the outlets of the air evaporators T aEv and the water evaporator T wEv (Fig. 1) Corresponding pressures are approximated using the vapour saturation P-T curve. The vapour mass flow rate at time i depends on the pressure difference using Eq. [START_REF] Kwak | A study on the performance enhancement of heat pump using electric heater under the frosting condition[END_REF].

( )

i i aEv wEv i r k p p m - = & (7)
This equation depends on the pressure drop factor k i . This factor is assumed to be constant during the defrosting sequence because the section of the tube is negligibly reduced by the flow of liquid that returns back from the air evaporator by gravity. Moreover thermal exchange between the two phases, flowing up (vapour phase) and down (liquid phase), has not been studied. Following these assumptions, the refrigerant mass flow rate can be estimated using Eq. ( 8). Defrosting capacity is given by Eq. ( 9). These two equations bring into play the average pressure difference between the outlets of the water evaporator and the air evaporator. (  )

aEv wEv i aEv wEv df i df p p p p Q Q - - ⋅ = & & (9)
Table 1 presents the results for frost mass, defrosting energy and mean values of mass flow rate and defrosting, cooling and heating capacities. Mean cooling and heating capacities are measured on water. These values show that there is no break in the heat production and that the heat source has switched from ambient air to cold water. Fig. 8 shows the evolutions of the refrigerant mass flow rate and the defrosting capacity during the 2-minute thermosiphon defrosting sequence. The defrosting capacity curve decreases from 2.32 to 0.26 kW. The mass flow rate curve decreases from 6.28 to 2.08 g.s -1 . The important variations between 50 s and 100 s are due to the internal pressure control, activated after switching from the heating mode to the simultaneous mode. After 120 s, the temperature difference between the refrigerant temperatures at the outlets of air and water evaporators (respectively T aEv and T wEv in Fig. 1) is around 1°C, which is within the limit of uncertainty of the metrology.

The operating conditions at the beginning of the defrosting sequence are calculated using the software of the compressor manufacturer. They are reported in table 2. According to these values, the supplementary refrigerant mass flow rate to be generated at the water evaporator for the thermosiphon represents 6% of the flow rate at the compressor. Therefore the water evaporator has to be at least oversized in this proportion. The initial defrosting capacity corresponds to more than 12% of the nominal cooling capacity.

Conclusions

This article is the second part of an experimental study of an air-source heat pump for simultaneous heating and cooling (HPS). The first part detailed the basic concepts and the performance validation of the HPS. The present article goes further and deals with the operating sequences alternating between heating, cooling and simultaneous modes. Apart from producing simultaneously hot and cold water, this machine proposes an answer to reduce the performance loss of air-to-water heat pumps under low ambient temperatures by the use of operating sequences alternating between an air evaporator and a water evaporator. Moreover, while standard airsource reversible heat pumps are penalized by defrosting (break in the heat production, decrease in COP), the HPS carries out defrosting using a two-phase thermosiphon with increased heat pump performance and without stopping the heat production. Thanks to the special management of produced energies, simulations showed an annual performance increase of 16.6% when comparing the HPS to a standard reversible heat pump [START_REF] Byrne | Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO 2 as a working fluid[END_REF].

The phenomenology of the HPS operation has first been studied on two points: high pressure control and transitions between modes. The high pressure control is operational but needs more development. Transitions between modes are smooth, thus acceptable.

Then the two-phase thermosiphon has been observed by thermographic pictures of a section of the tube between the two evaporators and validated by a favourable defrosting time in comparison with a situation in which the heat pump was stopped. The characterization of the thermosiphon showed a phenomenon of high capacity.

Nevertheless these encouraging results still need to be comforted by in situ tests. Further research has to be conducted on the two-thermosiphon defrosting technique. More metrology has to be implemented to improve the characterization. The validation has to be conducted under higher levels of frosting using other experimental means such as a climatic room. Finally, R407C is not a good choice for the HPS because the two-phase thermosiphon will provoke a separation of the different components of the refrigerant mixture [START_REF] Rajapaksha | Influence of liquid receiver on the performance of reversible heat pumps using refrigerant mixtures[END_REF]. Eventually, the performance and even the proper operation of the HPS can be altered.

List of figures

(

  heating, cooling and simultaneous modes) are deeply described in the first part of this article [2]. The heating mode uses the air evaporator and the water condenser (Evr2 and Evr3 are open). The cooling mode uses the water evaporator and the air condenser (Evr1 and Evr4 are open). The simultaneous mode uses the water evaporator and the water condenser (Evr1 and Evr3 are open). The present paragraph explains the dynamic behaviour of the HPS depending on the needs in heating and cooling. The operation is based on the alternation of the heating, cooling and simultaneous modes with or without the two-phase thermosiphon defrosting technique.

Fig. 1 Fig. 2 .Fig. 5 .Fig. 6 .Fig. 7 .

 12567 Fig. 1. HPS circuit