Maryline Nasr

Roman Viter

Cynthia Eid

Fabienne Warmont

Roland Habchi

Philippe Miele

Mikhael Bechelany
email: mikhaelbechelany@univ-montp2.fr.

Synthesis of Novel ZnO/ZnAl 2 O 4 multi co-centric nanotubes and their long-term stability in photocatalytic application

Based on the Kirkendall effect, novel double, triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4 have been successfully fabricated by combining the two techniques of Electrospinning and Atomic Layer Deposition. The as-prepared samples were annealed at 900°C under air. Their morphological, structural and optical properties were studied by Scanning Electron Microscopy (SEM), Transmission electron microscopy (TEM), X-ray Diffraction (XRD), Energy-Dispersive X-ray spectroscopy (EDX), UV-Visible spectrophotometry, Raman spectroscopy, Photoluminescence (PL) and reflectance emission. The performances and long-term stability of these multi co-centric nanotubes for photocatalytic applications have been evaluated under the same conditions. As result, in the photodegradation of methyl orange (MO) under UV irradiation, the triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4 exhibit a higher photodegradation efficiency (94% and 99%, respectively) in repeated and long-term applications compared to the pure ZnO which has very low long-term photocatalytic stability. Thus, the fact of coupling these two semiconductors ensured a high photocatalytic activity and long term stability.

Introduction

The serious water pollution caused by industrial production is increasingly being addressed by photocatalysis which is a less expensive and more efficient treatment method 1 . The development of semiconductor photocatalysts like titanium oxide (TiO2) 2 and zinc oxide (ZnO) 3 was regarded as one effective way to partly solve the energy and environmental problems. However, the photocatalytic efficiency of the singlecomponent semiconductor ZnO was seriously impeded due to its poor long term stability in photocatalysis and to the fast recombination rate of photogenerated electron-hole pairs in the semiconductor 4 . Therefore, many efforts have been developed to increase the separation efficiency of photogenerated electron-hole pairs and the long term stability of ZnO nanocatalysts [START_REF] Zhang | ACS applied materials & interfaces[END_REF] . Previous studies indicated that combining ZnO with another semiconductor, e.g., In2O3 6 , SnO2 7 and NiO 8 has been proved to be an effective method to improve its photocatalytic activity. Zinc aluminate (ZnAl2O4) is a spinel type oxide, which has high chemical and thermal stability, low surface acidity and high mechanical resistance as a bulk 9,10 . Zinc aluminate was considered as an attractive material for different applications, such as optical coating or host matrix, high temperature ceramic material, catalyst and catalyst support 11,12 . ZnAl2O4 is a naturally available mineral commonly called gahnite with a normal spinel structure having all zinc cations in the tetrahedral and all aluminum cations in the octahedral sites of the cubic face-centered lattice of oxygen anions. 13 ZnAl2O4 is also a semiconductor material suitable for ultraviolet (UV) photoeletronic application due it wide energy bandgap (about 3.8 eV) 14 . Few studies involving the degradation of organic dyes using ZnO/ZnAl2O4 as a photocatalyst are reported in the literature 12,15,16 . Zhang et al. 17 prepared for instance ZnO/ZnAl2O4 composite hollow microspheres using hydrothermal method. Zhao et al. 18 dispersed the ZnAl2O4 nanoparticles inside a network of ZnO. They showed that the effect of coupling both ZnO and ZnAl2O4 phase in the ZnO/ZnAl2O4 nanocomposite ensured the efficient separation of photogenerated e -and h + pairs, which was prerequisite for the enhancement of the photocatalytic performance. Therefore, in order to enhance the photocatalytic activity and the long term stability, novel ZnO/ZnAl2O4 multi co-centric nanotubes were synthesized by combining the two techniques: electrospinning and atomic layer deposition (ALD). Electrospinning is an attractive and widely used technique for the preparation of nanofibers. Starting from a polymer solution, sub-micronic fibers can be synthesized using this technique. The electrostatic field applied between a syringe and a collector disk provides the transformation of the droplet to a Taylor cone. The droplet is stretched under electrostatic field to form a sub-micron fiber after the evaporation of the solvent 19,20 . As for the Atomic Layer Deposition (ALD) technique is based on two separate chemical reactions limited on the surface of the sample. This deposition mechanism makes the ALD an outstanding technique for 3D deposition with high aspect ratio. ALD was widely used for nanotube fabrication using different templates such as polycarbonate membrane, PAN and PVC electrospun nanotubes 21,22 . No other thin film technique can approach the conformity achieved by the ALD on high aspect ratio structures. In the present work, we report the fabrication of a novel morphology of multi cocentric nanotubes of ZnO/ZnAl2O4 and we study for the first time their long term stability in photocatalytic performance. Herein, we combine the electrospinning and ALD techniques to elaborate double, triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4 based on the kirkendall effect between aluminum oxide and zinc oxide 23 . Electrospun PAN was used as template for the multi co-centric nanotubes of ZnO/ZnAl2O4. The structural, morphological and optical properties of the prepared samples were analyzed. The performances of these materials for long term stability and the photocatalytic degradation of methyl orange (MO) under UV have also been evaluated.

Experimental 1. Chemicals and Materials

Diethyl Zinc {(DEZ), Zn (CH2CH3)2, 95%}, Trimethylaluminum {(TMA), Al (CH3)3, 97%}, N, N-Dimethylformamide (DMF; 99.8%), Polyacrylonitrile (PAN; MW= 500 000) and methyl orange (MO) were purchased from Sigma Aldrich. All chemicals were used without any further purification.

Preparation of PAN nanofibers

Electrospinning solution was prepared by dissolving 10 wt. % of polyacrilonitrile in dimethylformamide 24 . The solution was maintained under agitation for 4 hours. The polymer solution was loaded into a plastic syringe having a stainless steel needle with a diameter of 0.7 mm at a constant flow rate of 1 ml/h. The electrospinning process was performed at 38 ± 5°C in an ambient atmosphere under an applied voltage of 25 kV. Nanofibers were collected on a rotating coil covered with an aluminum foil with a rotation speed of 400 rpm. The distance between the tip of the needle and the aluminum foil was maintained at 20 cm.

ALD sequences

The ALD was performed using a homemade reactor 25 . All the ALD depositions were performed at 60°c using the following sequence mentioned in Table 1. All precursor pulses were coupled with 25 Sccm Ar flow as gas vector; purge was performed with 100 Sccm Ar flow as gas vector. After ALD deposition PAN fibers were heated at 450°C with a heating rate of 1°C.min -1 for 8 h in air to eliminate the core of carbon, and then calcined at 900°C for 12 h in order to obtain ZnO/ZnAl2O4 co-centric nanotubes. As shown in Table 2, double, triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4 were obtained by alternating the deposition sequences of Al2O3 and ZnO cycles on the PAN nanofibers.

Chemical and structural Characterization

X-ray diffraction (XRD) measurements were carried out using a PANAlytical Xpert-PRO diffractometer equipped with an X'celerator detector using Ni-filtered Cu-radiation (λ= 1.54 ˚A). Scanning electron microscopy (SEM) images were taken with a Hitachi S4800, Japan. Transmission electron microscopy (TEM) images were taken with a PHILIPS-CM 20. The UV-Vis absorbance spectra of methyl orange were recorded by a Jasco V-570 UV-VIS-NIR spectrophotometer. Raman spectra have been obtained with a Horiba spectrometer, λ= 659 nm. Energy-Dispersive X-ray spectroscopy analysis (EDX) was taken with Zeiss EVO ED15 microscope coupled with an Oxford X-MaxN EDX detector. Photoluminescence (PL) and reflectance measurements have been performed using fiber optic technique measurement system. Nitrogen laser (340 nm, 0.4 mW, 100 Hz) has been used as excitation source. The emission spectra have been measured with Ocean Optic spectrometer HR2000+, equipped with optical fiber and collecting lenses. The SpectraSuit software was used for collecting spectra in MSExcel files. Reflectance spectra were measured using reflectance Y-probe and UV-Vis Ocean Optics light source (DT-MINI-2-GS). The PL and reflectance spectra were recorded in the range of 360-750 nm and 380-800 nm, respectively.

Photocatalytic activity measurement

Methyl orange (MO) was used as a reference for organic pollutant to evaluate the photocatalytic activity of the ZnO/ZnAl2O4. The photocatalytic performance of the samples was evaluated by analyzing the decrease in concentration of the MO during exposure to UV light irradiation. The reaction temperature was kept constant at 25 ± 0.2 °C by circulating water in a cylindrical tube surrounding the photo-reactor during the entire experiment. The decomposition was carried out in many beakers containing a suspension of 10 mg of each sample in 25 ml of MO solution (10 mg/L) under UV light irradiation (obtained from a 400 W light source, emission wavelength 300 -400 nm). The distance between the lamp and the dye solution was maintained at 10 cm. The solution was irradiated with UV light for 3 hours. Each 1 hour, 3 ml of the sample solution was taken out and centrifuged to remove the catalyst. The centrifuged solutions were analyzed by a UV/Vis spectrometer. The absorbance spectra of MO (major absorption band around 462 nm) were recorded to measure the change in the concentration of MO [START_REF] Dhakal | Device and Materials Reliability[END_REF] . After irradiation, the photocatalytic degradation efficiency has been calculated using equation 1: 27 Photodegradation (%) = (C0 -C) /C0 × 100 (Equation 1)

Where C0 is the initial concentration and C is the final concentration of dye before and after photo-irradiation respectively. This equation shows the dye photocatalyst degradation percentage. 1, the new morphology of ZnO/ZnAl2O4 double (3 sequences), triple (5 sequences) and quadruple (7 sequences) co-centric nanotubes is well-defined. The nanotubes are separated by voids due to the Kirkendall effect and surface diffusion as will be explained later in the section dedicated to the mechanism. The nanotubes thickness was measured from the SEM images using image analysis software (Image J1.29X). The average thickness was measured on 50 randomly chosen nanotubes of each sample (Figure 2). After calcination, we measured an average thicknesses of (100 ± 5 nm), (150 ± 7 nm) and (200 ± 9 nm) for double, triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4, respectively. Also a uniform thickness of (50 ± 5) nm for each individual nanotube in all samples was measured. Therefore, we can assume that the increase of the thickness may cause an increase of the surface area.A larger surface area provides more surface active sites for the adsorption of reactants molecules, making the photocatalytic process more efficient. An energy dispersive X-ray spectrum (EDX) of all multi co-centric nanotubes recorded along with elemental analysis is presented in Table 3. The EDX analysis reports the Al, Zn and O presence without any residual carbon confirming the ALD complete reaction and the polymer removal with the setup mentioned above in Table 1. The crystal structure was examined by X-ray diffraction. Figure 2 (A-B 29 . The average crystalline size of ZnO and ZnAl2O4 in the four samples was estimated by the Scherrer equation 19 . The high grain fineness numbers of both ZnO and ZnAl2O4 in all samples are clearly seen in Table 4. Thus, XRD analysis confirmed the successful synthesis and the good crystalline structure of ZnO/ZnAl2O4 multi co-centric nanotubes by combining electrospinning and ALD. Figure S1 (Supporting information) illustrates the TEM images of double, triple and quadruple cocentric nanotubes of ZnO/ZnAl2O4. These images reveal that both ZnO and ZnAl2O4 phase are highly crystallized and all the nanotubes were composed of nanosized grains. The grain size was measured using ImageJ, a Java-based image processing program. It was found to be (30 ± 3 nm), (40 ± 5nm) and (50 ± 5 nm) for the double, triple and quadruple co-centric nanotubes of ZnO/ZnAl2O4, respectively. It is well known that Raman spectroscopy is a characterization method to measure the frequencies of the long-wavelength lattice vibrations (phonons). Raman spectroscopy can provide useful information on crystal symmetry, impurity, grain size, residual stress and disorder 29 . The Raman spectra shown in Figure 3 were measured at room temperature. Raman signals are sensitive to the crystal structures and the defects. For all the samples, we observed 3 peaks corresponding to the active modes of ZnO. A dominant sharp peak was observed at 432 cm -1 corresponding to the Raman active optical phonon mode E2 which is the characteristic of hexagonal wurtzite ZnO. Other small peaks were also observed at 327 cm -1 and 578 cm -1 . The peak at 327 cm -1 (E2H-2L) is due to multiple phonons scattering, and the small peak at 578 cm -1 (E1L) is observed due to structural defects like oxygen deficiency 3 . The intensity of the peak at 432 cm -1 as compared to other peaks is very high which indicates high crystal quality for all samples. In particular, it can be seen that the intensity of this peak is increasing going from double (a) to quadruple tubes (c) which is consistent with the XRD results presented above. As shown in Figure 3 (a to c) corresponding to the Raman spectra of ZnO/ZnAl2O4 double, triple and quadruple co-centric nanotubes respectively, two peaks located at 414 and 654 cm -1 are clearly observed. Based on our knowledge, there is no previous study on the Raman spectroscopy of ZnAl2O4. Thus, these two peaks may correspond to the active modes of ZnAl2O4 since they are not observed in the previous study of the pure ZnO. 31 According to Abou Chaaya at al. 22 , the observed results point to high crystalline structure and high emission rate, due to the lower rate of non radiative recombination through surface states and other defects. Development of double, triple and quadruple nanotubes resulted in significant quenching of photoluminescence. It is known that the band gap of ZnAl2O4 is over 3.77 eV and it cannot be seen using the excitation source, applied in the present work. 32 The first obvious reason is a reduction of ZnO ratio and therefore, the emission intensity. However, due to low laser power the most significant factor could be a charge separation due to complex structure on the the surface. Chen et al. 32 has reported that the decrease of UV peak in ZnO/ZnAl2O4 could be due to additional forming defects on the interface which provide non radiative recombination. Ratio of intensities of visible and UV emissions is shown in Table 5. It is clearly seen that with the increase of the number of ZnO/ZnAl2O4 layers, the concentration of defects increased similarly as described by Abou Chaaya et al. 22 Thus, the forming of double, triple and quadruple ZnO/ZnAl2O4 nanotubes results in defect formation, stimulation of non radiative recombination and/or surface charge transfer, what is significantly improved the photocatalytic applications.

Mechanism of ZnO/ZnAl2O4 formation

Based on the Kirkendall effect, multi co-centric nanotubes of ZnO/ZnAl2O4 were elaborated. All the steps for the ZnO/ZnAl2O4 elaboration were described in Figure 6-A. The Kirkendall effect is a classical phenomenon in metallurgy 33 . It basically refers to a nonequilibrium mutual diffusion process through an interface of two metals A and B (in our case A and B refer to ZnO and Al2O3, respectively) so that vacancy diffusion occurs to compensate for the unequal material flow 34,35 . In planar metallic bi-layers, this effect can give rise to void formation near the bond interface and within the fast-diffusion side, thus deteriorating the bonding strength of the interface 23 . Figure 6-B shows schematically the generalized model for hollow structure formation based on the kirkendall effect and surface diffusion. For the diffusion flux, we assume JA > JB. In the initial stage, Kirkendall voids are generated near the A/AB interface during vacancy assisted exchange of material via bulk inter-diffusion. The voids are the sinks for subsequent inward flux of vacancies (JV = JA -JB) and thus grow in size. The voids coalesce into bigger ones and touch the compound layer AB, in one sense breaking the connection for lattice diffusion and in the other sense establishing new bridges as fast transport paths for the remaining material A. 36,37,38 Herein, we synthesized photocatalysts multi cocentric nanotubes of ZnO/ZnAl2O4 with relatively high surface area. The hollow nanotubes allow multiple reflections of UV light within the interior cavity that facilitates the efficient use of the light source. Moreover, the void in the ZnO/ZnAl2O4 co-centric nanotubes allows more efficient transport for the reactant molecules to get to the active sites and inhibiting charge recombination, hence enhancing the efficiency of photocatalysis. The photocatalytic activities of the ZnO/ZnAl2O4 multi cocentric nanotubes were evaluated by the photodegradation of methyl orange (MO) under UV light (wavelength < 360 nm). MO was chosen as a model organic pollutant. Figure 7 shows the dye adsorption without catalysts under UV irradiation and photocatalytic degradation of organic dye in the presence of catalysts under UV irradiation at different periods of time (0, 60, 120, 180 min). The major absorption peak of MO is positioned at 462 nm. As shown in Figure 7a, the concentration of MO without any catalysts almost does not change for every measurement under UV irradiation. Thus, light irradiation in the absence of any catalysts does not work in the photocatalysis of MO, indicating that the photodegradation of MO can be neglected. As shown in Figure 7 (b to d), after 3 hours and under identical experimental conditions, MO was degraded up to 70%, 94% and 99% in the presence of ZnO/ZnAl2O4 double, triple and quadruple cocentric nanotubes respectively (Figure 8). When ZnO and ZnAl2O4 are coupled together, photons may be absorbed in both ZnO and ZnAl2O4 and form the electron-hole pairs. The electrons at the bottom of the Conduction Band (CB) of ZnAl2O4 would migrate to that of the ZnO; whereas holes at the Valence Band (VB) at the top of ZnAl2O4 would remain there. On the other hand, the holes at the VB at the top of the ZnO would migrate to that of ZnAl2O4, with electrons at the CB bottom of ZnO remaining there 28,18 . Such process, which is energetically favorable and reduces the probability of the electron-hole recombination, is regarded as the key factor for the enhancement of photocatalytic activities of the ZnO/ZnAl2O4 nanotubes compared to the pure ZnO 17 . In addition, according to the EDX results presented above, in the case of double co-centric nanotubes (photodegradation: 70%), the content of ZnO is the lowest (Zn: 20 at. %). The low content of ZnO will reduce the catalytic efficiency of ZnO/ZnAl2O4 co-centric nanotubes, which is attributed to complete or sparse coverage of ZnO on the ZnAl2O4. This affected the photocatalytic activity of ZnO or ZnAl2O4 itself, which can only use about 3-5% of UV light in sunlight. 40 Also, it was well known that the photocatalytic activity is mainly governed by crystalline phase, light absorption capacity and active surface area 41 . For comparison, the ZnO/ZnAl2O4 quadruple and triple co-centric nanotubes exhibit the higher photocatalytic activity (99% and 94%, respectively) compared to the ZnO/ZnAl2O4 double co-centric nanotubes (70%). From SEM results, we assume a bigger active surface area with a bigger number of layers so S Quadruple > S Triple > S Double. From the XRD analysis, the grain fineness number of both ZnO and ZnAl2O4 is increasing as follow D Quadruple > D Triple > D Double. As confirmed previously by photoluminescence, the recombination of e -and h + is inhibited thus, more photoelectrons were available compared to pure ZnO. These results indicate that ZnO/ZnAl2O4 triple and quadruple cocentric nanotubes have excellent absorption capacity of UV light, large active surface area, high crystallite sizes and lower recombination rate of electron-hole pair. Figure 9 shows the different kinetic linear curves. The photodegradation reactions follow a Langmuir-Hinshelwood first-order kinetics model. The explanation is described as follows: 42 r = dC/dt = kKC/1+KC (equation 4)

Where r is the degradation rate of MO (mg (L min) -1 ), C is the concentration of the MO solution (mg l -1 ), t is the irradiation time, k is the reaction rate constant (mg (L min) -1 ), and K is the adsorption coefficient of MO (mg L -1 ). Since the initial concentrations (C0 = 10 mg L -1 ) of the MO solutions are very low in the present experiments, the relationship between ln(C0/C) and reaction time t shows that the decomposition of MO with different photocatalysts accords with a pseudo first order kinetic 42 : ln (C0/C) = kKt = kat (equation 5)

Where ka is the apparent first-order rate constant (min -1 ) and C is the concentration at time t. ka obtained from the linear dependence between ln (C0/C) and time are reported in Table 5. The increasing order of the rate constants in the samples is: ZnO/ZnAl2O4 quadruple > ZnO/ZnAl2O4 triple > ZnO/ZnAl2O4 double > MO. The rate constant exhibits a maximum of 0.0253 min -1 for ZnO/ZnAl2O4 quadruple co-centric nanotube, which is 3.7 and 1.5 times higher than that of ZnO/ZnAl2O4 double and ZnO/ZnAl2O4 triple co-centric nanotubes, respectively. Thus, compared with other samples, the ZnO/ZnAl2O4 quadruple cocentric nanotubes show higher photocatalytic activity under UV light, which is represented by larger value of ka. All these results indicate that compared with single phase ZnO material reported in previous studies 17 , the coupling effect between ZnO and ZnAl2O4 increases the life time of charge carriers which leads to the enhancement of its photocatalytic activity. The re-use of catalysts is very important. However to our knowledge, there is no study concerning the long term stability of ZnO/ZnAl2O4.Therefore, we performed for the first time the photocatalytic repeatability test of each catalyst. Repetitive degradation of MO by Double, Triple and Quadruple co-centric nanotubes of ZnO/ZnAl2O4 under UV light irradiation was conducted to study their long-term stability and the results are shown in Figure 10. After each cycle, the catalyst was separated from the solution by centrifugation. From Table 6, it can be seen that after 180 min of UV light irritation and in the presence of the same ZnO/ZnAl2O4 samples, MO has been degraded in four repeated cycles. The loss of efficiency was only 6% after the fourth cycle, which can be attributed to the loss of catalyst during centrifugation after each cycle. In addition, the used nanotubes show the same off-white color as the newly prepared catalyst. These results suggest that the multi co-centric nanotubes of ZnO/ZnAl2O4 have higher photodegradation efficiency in repeated and long-term applications compared to the pure ZnO which has very low long-term photocatalytic stability 4 .

Figure 1 .

 1 Figure 1. Scanning Electron Microscope images of ZnO/ZnAl2O4 double, triple and quadruple co-centric nanotubes annealed in air at 900°C.

Figure 2 .

 2 Figure 2. (A-B) XRD patterns of (a) Double, (b) Triple and (c) Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 3 .

 3 Figure 3. Raman Shift of (a) Double, (b) Triple and (c) Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 4 .FTable 5 .Figure 5 .

 455 Figure 4. (A) Reflectance spectra and (B) Band gap calculation from reflectance spectra of ZnO nanotubes and Double, Triple, Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 6 .

 6 Figure 6. A) Different steps of ZnO/ZnAl2O4 nanotubes formation. B) Kirkendall effect and surface diffusion process, where JA, JB, and Jv are diffuse flux of metal A, B, and void, respectively

39

 39

3 .Figure 7 .

 37 Figure 7. UV-Vis spectra of the photocatalytic degradation of MO under UV light: (a) MO without catalyst, (b) Double, (c) Triple and (d) Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 8 .

 8 Figure 8. Photodegradation of MO by Double, Triple and Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 9 .

 9 Figure 9. Kinetics of Methyl orange degradation by Double, Triple and Quadruple co-centric nanotubes of ZnO/ZnAl2O4

Figure 10 .

 10 Figure 10. Long-term catalytic stability of Double, Triple and Quadruple co-centric nanotubes of ZnO/ZnAl2O4 in four repeated cycles of MO degradation under UV irradiation.

Table 1 .

 1 Step time investigation of Al2O3 and ZnO deposition on PAN nanofibers

		Pulse Exposure Purge Water Pulse Exposure Purge
					Al2O3 (TMA)
	Time	0.1s	30s	40s	2s ZnO (DEZ)	30s	40s
		0.2s	20s	40s	2s	30s	40s

Table 2 .

 2 Deposition sequences of Al2O3 and ZnO cycles on the PAN nanofibers

	ZnO/ZnAl2O4 co-centric nanotubes	Deposition sequences
	Double	100Al2O3/200ZnO/100Al2O3

Results and discussion 1. Morphological, Structural and optical properties of ZnO/ZnAl2O4 multi co-centric nanotubes

  SEM has been used to insure the feasibility of multi co-centric nanotubes of ZnO/ZnAl2O4 by combining the two techniques, Electrospinning and ALD. As shown in Figure

Table 3 .

 3 EDX

			Atomic percentages
	Samples	Zn	Al	O
	Double	20.52	30.4	49.08
	Triple	22.1	29.06	48.84
	Quadruple	22.77	27.96	49.27

data showing the composition of multi co-centric nanotubes of ZnO/ZnAl2O4 annealed under air

Table 4 .

 4 Grain size of ZnO and ZnAl2O4 multi co-centric nanotubes

Double Triple Quadruple Grain size (nm)

	ZnO	29	35	47
	ZnAl2O4	31	42	50

Table 6 .

 6 Kinetic parameters for photocatalytic activities of ZnO/ZnAl2O4 double, triple and quadruple co-centric nanotubes

		MO	Double	Triple	Quadruple
	ka (min -1 ) 0.00035 0.0067 0.0167	0.0253
	R 2	0.9538	0.9113 0.9436	0.9386

Table 6 .

 6 MO photodegradation percentages by Double, Triple and Quadruple co-centric nanotubes of ZnO/ZnAl2O4 in four repeated cycles.

		Double Triple Quadruple
	First cycle	70%	94%	99%
	Second cycle	67%	93%	98%
	Third cycle	65%	90%	96%
	Fourth cycle	64%	89%	95%

Acknowledgements

M. Nasr would like to thank the Lebanese University for the PhD funding and the financial support of the 'Elaboration of metal oxide nanofibers and membranes for photocatalytic applications' project. This study was partially supported by the ANR project ANR-14-CE07-0011 "BONALD".

Graphical Abstract


David Chèze

Alessandra Cuneo

Carlo Macciò

Matteo Porta

Giuseppe Dino

Andrea Frazzica

Andrea Gabaldón

FOUR INNOVATIVE SOLAR COUPLED HEAT PUMP SOLUTIONS FOR BUILDING HEATING AND COOLING

Keywords: solar panels, heat pumps, heating and cooling, buildings, electricity self-consumption, grid feed-in

Despite the huge primary energy consumption associated with heating and cooling (H&C) demand in EU building stock, the share of installed renewable H&C solutions is still marginal (i.e. 5%). In order to speed up a transition towards the widespread application of renewable H&C in buildings, innovative solutions are designed to compete against traditional solutions. SunHorizon project aims to demonstrate the potential for a user-friendly and costeffective solution based on an optimized design and combination of commercial innovative solar technologies (thermal or/and PV) and Heat Pumps (HP). This paper presents how different technologies (two solar panels, three HPs and thermal storage) have been coupled in four different Technology Packages (TPs) to satisfy H&C demand of both residential and tertiary buildings. Preliminary thermo-economic TRNSYS simulation results are presented about four demo sites in Latvia, Germany and Spain, to demonstrate such innovative solutions able to cover up to 80% of H&C demand with a reduction of greenhouse gas (GHG) emissions up to 70%.

Introduction

Buildings are responsible for 41% of EU energy consumption and H&C equals 55% of the total EU energy demand [START_REF] Odyssee | Energy efficiency trends in buildings[END_REF]. This demand is mostly met by fossil fuels, with natural gas having the main share, while renewable energy sources (RES) remain marginal (5%). However, according to the EC's H&C Strategy, the H&C sector still accounts for 59% of total EU gas consumption, and almost half of EU's buildings have boilers with an efficiency rate below 60% (IEA. 2018). Nevertheless, RES are becoming increasingly common and socially accepted. Therefore, to speed up a transition towards RES-based H&C in buildings, innovative solutions are designed to compete against traditional solutions with equivalent or lower investment costs and payback times, while guaranteeing similar or better levels of comfort. Both residential and tertiary new buildings types are expected to become highly efficient. Currently, solar panels and Heat Pump (HP) are the most common and socially accepted RES Based H&C solutions. Bringing together existing and mature HP and solar technologies guarantees a simple energy system nevertheless efforts are still required to refine their coupling, as they are used to be designed and operated separately. Solar and HP can form an integrated solution with several key advantages: high RES share and energy efficiency, low electricity and primary-energy demand, low GHG/CO2 emission. HP are well suited to low-temperature needs as for well-insulated residential and tertiary buildings. In parallel solar thermal and PV reduces also the thermal and electricity demand in non-renovated envelopes. The current work is performed in the context of the SunHorizon project (H2020 Project GA818329) which objective is to analyze the performance of building-integrated solar and HPs solution specifics and their individual challenges towards saving Non-Renewable Primary energy (PEnren) via increased Renewable Energy Ratio (RER), cost reduction (optimized size, installation cost reduction etc.) and reliability (lifetime and reduced maintenance). Innovative components as the BH20 unique thermal compressor heat pump from BOOSTHEAT (BH), the Hybrid adsorption/compression chiller from FAHRENHEIT (FAHR), the high-vacuum flat thermal panels from TVP SOLAR (TVP) and the hybrid PVT panels from DualSun (DS) are integrated in four TPs including thermal energy water storage from RATIOTHERM (RT) and A-/B-WHP (reversible Air/Brine source to Water HP) from BDR THERMEA (BDR). Details on each specific technology could be found in SunHorizon website. Each TP aims at covering space heating (SH), domestic hot water (DHW) and/or space cooling (SC) demand of eight different demo sites (small and large residential buildings as well as tertiary buildings) across EU climates, local energy mix and costs in Germany, Spain, Belgium and Latvia [START_REF] Cea | Report on baseline and boundary conditions of SunHorizon demo sites, including monitoring aspects[END_REF]. A quick outlook of the composition of the four TPs and the demo sites locations is illustrated in Tab. 1. In Section 2, the key aspects of the preliminary design and TRNSYS modelling with respect to TP1, TP2, TP3, TP4, in particular experience in the elaboration of compliant control rules between the manufacturers of the solar and HP system parts has been presented. Then, Section 3 essentially emphasizes noticeable characteristics and the respective estimated energy and costs savings through the analysis of the simulation results, referring to Key Performance Indicators (KPI) commonly used in solar and HP contexts [START_REF] Cartif | BDR THERMEA, Ratiotherm, Fahrenheit, Dualsun[END_REF].

SunHorizon technologies architecture and modelling

TP1 development and modelling

The challenge of TP1 is to combine each other the TVP solar panels, BH HP and RT thermal storage separate technologies and controllers, while meeting each manufacturer objectives and pursuing replication perspective within SunHorizon and beyond in the future. Fig. 1 shows the hydraulic layout agreed by the manufacturers that ensures parallel operation of TVP and BH on the SH and DHW demand. It enables direct use of TVP heat at possibly high temperature above the cold return line from the radiator space heating SH circuit, and DHW solar preheating from accumulated solar heat in RT tank, before BH20 gas burner ensures water is heated up to the desired temperature. It also mutualizes one outdoor fan coil to operate the BH20 unit during the heating season and to dissipate excess heat from the TVP collector. Apart from the new component for BH thermal compressor heat pump BH20, existing TRNSYS components have been used in the TP1 assembly system model. It runs annual simulation at 3 minutes time step to investigate dynamic component interactions and usual solar combisytem control rules summarized in Tab.2. Solar circuit: The quasi-dynamic solar thermal collector model Type 832 maintained by SPF [START_REF] Haller | TRNSYS Type832v5.00 Dynamic Collector Model[END_REF]) is selected for the solar flat panels performance modelling. The Type 832 parameters for TVP thermal panels are set regarding the coefficients from EN ISO 9806 terminology η0,b = 0.737; c1 = 0.504 W/(m².K); c2 =0.006 W/(m².K²); c3, c4, c6 =0 / Kd= 0.957. Default flat plate coefficients are assumed for IAM b0=0.18 and thermal capacitance 7000 J/K.m². The collector tilt is 60° to match the slope of the roof in Berlin demo site and to reduce the overheating risk in summer when possible no DHW consumption during holidays. The solar tank is Type 340 model [START_REF] Drück | Stratified fluid storage tank with four internal heat exchangers, ten connections for direct charge and discharge and an internal electrical heater -Type 340[END_REF], the solar brine/water and DHW solar pre-heating heat exchangers are Type 5 models.

SH and DHW, heat demand models:

The reference building connected to the TP1 is an historical house (beginning XX th century) renovated in 2006, 284 m² living area occupied by 7 peoples in three levels and thermal zones, under Berlin climate, modelled by multi-zone building TYPE56. The DHWcalc software [START_REF] Jordan | DHWcalc: Program to generate domestic hot water profiles with statistical means for user defined conditions[END_REF] was used to generate a synthetic yearly profile that matches the overall annual estimated consumption with realistic distribution of the consumption, statistical day-to-day variations. The configuration is reflecting a multifamily building, split into 2 households and average total daily consumption 300L at 45°C.

BH20 model:

The BH unit is including the thermal gas fired compressor CO2 heat pump and the conventional condensing gas burner for DHW preparation (also used as backup of thermodynamic cycle on space heating complementary capacity request). The model is mainly relying on user Type 5837 and simulates the physical behavior including internal controller (extern sensors/actuators management) and issues performance outputs in steady state under the variable operating conditions: temperature at the cold (-10/20°C) and hot (30/55°C in SH, 10/85°C in DHW) sides of the heat pump, full or part load operation request (25-100% nominal capacity 20kW in SH operation). The performance outputs rely on continuous interpolation of the performance tables shared by BH from internal tests of the BH20 unit in space heating operation, shown in Fig. 1 (left). The maximum BH20 capacity parameter is variable from the 20kW nominal capacity. The BH20 unit is linked to an outdoor fan coil, modelled by Type 91 heat air/brine exchanger. The air flow rate is controlled by Type5837 SFan 0-100% output fan speed signal that is adjusting the nominal air flow rate to the current heat demand. The electricity consumption of the fan is approximated as a parabolic curve depending on the requested air flow-sfan signal, fitted from the BH20 manufacturer data. On the other hand, the BH Type 5837 is connected to a 65L DHW cylindrical insulated tank modelled by Type 340.

Tab. 2 TP1 overall control strategy TVP solar thermal supply PCOL1 and PCOL2 are flowing solar heat into solar tank when TCOL is 5K higher than the tank bottom temperature TSOL. When RT max temperature is 85°C and TVP max outlet temperature is 120°C, outdoor fan coil, diverter VCOLEX and solar pump PCOL1 are dissipating excess solar heat. DHW DHW heat exchanger is heating first from solar tank the sanitary cold water, assuming tank side flow identical to the water draw then it flows into the Boosheat backup heated DHW tank. Space heating SH radiator demand signal and flow are raised by thermostat THM with 20°C setpoint, differential controller with +1K deadband (radiator circuit heating curve with 70°C at -12°C and 40°C at 15°C). VRADSOL diverter is switching the return flow from radiator circuit to the solar pre-heating tank when the mid-tank temperature is 5K above otherwise the solar tank is bypassed. BoostHeat CO2 thermodynamic cycle activated for space heating with support of auxiliary gas burner to ensure the heating curve flow temperature up to 20kW. The DHW boostheat tank set point temperature is 55°C, heated up by auxiliary gas burner.

TP2 development and modelling

The second TP is focused in the combination of BH HP with hybrid-PVT panels from DualSun together with a stratified thermal storage manufacturer by RT to be installed in a small residential building in Riga. The building is a privately owned single-family residential house built in 2013, with a heated area of 234.8 m2 distributed into two floors. The existing system (Gas boiler) will be integrated with the new system as a back-up service (e.g. during first operational tests, etc.). A model of the SunHorizon system has been produced in TRNSYS 18 in order to preliminary assess its performance.

Fig. 2 -TP2 Schematics for Riga demosite

The Fig. 2 shows the SunHorizon TP2 schematics. Next, a description of the devised concept is presented according to the control strategy description in Tab. 2. The DualSun PVT panels (1) can heat up the stratified tank provided by RT (10) through a direct heat exchanger (4) when the outlet temperature from PVT panels is higher than the RT tank (S3). When the temperature is lower, a three ways valve is switched to connect the PVT panels to the glycol tank (3) in order to run the evaporator of the BH heat pump unit (5). PV production from ( 1) is mainly used to cover electricity demand from the building. When the electrical production exceeds the building's demand, then the smart energy electrical heater (2) can be used to heat up the RT tank (10) till the maximum temperature set for the tank is reached. This way, electricity is almost always self-consumed. The evaporator of the BH unit can be driven by ( 1)+(3) or by an outdoor dry cooler (6). BH unit heats up the flow in two phases. First, the water is heated up in the condenser (up to 50º). Second, if the desired temperature is higher than the condenser outlet, a secondary burner heats up the flow up to 70ºC. DHW demand will be preheated by heat exchanger (7) with the RT tank and later will flow to the DHW tank inside the BH unit to be heated up by the BH unit (5). The radiator stream will be preheated first with the RT tank through a three-way valve (11), and then by the BH unit (5). The BH unit is modelled in TRNSYS in a similar way as in TP1. DualSun PVT panels are modelled in TRNSYS using Type 816, which constitutes a dedicated simulation module with a nominal photovoltaic power of 310W and coefficients from EN ISO 9806 terminology of η0,b = 0.559; c1 = 15.8 W/(m².K); c2 =0 W/(m².K²); c3, c4, c6 =0. The baseline consumption of the building has been evaluated considering weather profile and users profiles. The users' profiles in terms of DHW, space heating and electricity consumption has been modelled using CRESTmodel (McKenna and Thomson, 2016) and calibrated according to the total annual consumption derived from the bills.

Heating circuit:When the temperature of the RT tank at the center-bottom position within the tank is higher than the temperature of the return heating circuit, the water is preheated at the RT Tank. A mixing valve is used to control the supply temperature of the radiator according to the selected set point taking into account the return temperature. The two radiatiors are connected to the primary heating circuit, since they work with the same water temperature. The pump flow rate is fixed and is equal to the sum of the flow rate of two radiator. Then to provide the right flow rate to the radiators, two diverters are used.

Hot water circuit:

The DHW is first preheated with the RT through a HX and later, it is streamed to the BH dedicated tank. The BH tank is maintained at a given temperature (e.g. 60ºC), in order to ensure the DHW supply temperature requested by the final users. The BH DHW tank is heated by the BH unit, which stops to provide heat to the radiators and heat up the tank. When the tank is heated the BH unit return to provide heat to the radiators. When providing heat to the DHW tank, the BH types turns into the control mode 2, that means that has a behaviour equal to a traditional condensing boiler of 18kW.

Solar circuit:

The solar circuit is activated when the solar irradiation is higher than 540 kJ/hr/m2 and the outlet temperature of DualSun panels is higher than the glycol tank temperature or the RT tank temperature. The heating of the RT tank has the priority. When the outlet temperature is lower than the bottom temperature of the tank, the flow is then switched towards the glycole tank. Due to the local legislation in Latvia, the maximum allowed installed installed capacity of a micro generator used for net metering is 11.1 kW, limiting though the maximum number of panels at 35 (10.9 kWp). Electricity is self-consumed if demand and production are simultaneous. Electricity surplus is supplied to the grid until the total surplus produced reaches the post retrofit electricity consumption. Electricity is provided to smart energy heater once the maximum surplus which can be supplied to the grid is reached.

Evaporator of BH

The BH's evaporator can be driven by an outdoor drycooler or the glycol loop connected to the DualSun panels. Whenever the glycol tank temperature is higher than the outdoor air temperature, the use of the glycol tank as evaporator is prioritized (if BH system is required).

TP3 development and modelling

TP3 illustrated in Fig. 3 will be installed in a tertiary building (civic center) located in Sant Cugat del Vallès (Spain), built in 2006. The heating and cooling installation of the existing system is based on Reversible air to water heat pump: 93.6 kW (nominal cooling capacity) -96.3 kW (nominal heating capacity) and Air handling unit (AHU): 110 kW (cooling capacity) -67.78 kW (heating capacity). The conditioned area and volume are respectively 816 m 2 and 3400 m 3 . A simulation of actual and future HVAC plant was performed by means of two different software: First, the IESVE simulation is run to obtain the heating and cooling (H/C) loads from the building model, then its results are used as input for H/C system plant simulated in TRNSYS18. The TP3 system was modelled in TRNSYS by considering the datasheet of each installed equipment. The AHU was modelled component by component, while the heat pump was modelled by a calculator that implements equation obtained from a statistical analysis of operating data declared in the official datasheet. The non-linear polynomial eq. 1 correlates operating temperatures with real performances of the components. f(T 𝑎𝑖𝑟 , T 𝑤𝑎𝑡𝑒𝑟_𝑜𝑢𝑡 ) = p00 + p10 * T 𝑎𝑖𝑟 + p01 * T water_out + p20 * 𝑇 𝑎𝑖𝑟 2 + p11 * T 𝑎𝑖𝑟 * T 𝑤𝑎𝑡𝑒𝑟_𝑜𝑢𝑡 (eq. 1)

The TP3 system will be integrated with the existing HVAC; TVP panel, FAHR hybrid adsorption chiller and RT stratified tank will be integrated, the new system will be connected in series upstream to the existing heat pump (Fig. 3), the existing primary pump will be changed with a variable speed one. The only heat source is the solar one, TVP panel will produce as much thermal energy as possible with a maximum temperature range between 70°C and 100°C. During winter, the hot water produced by TVP panels will support the existing plant and is directly delivered to the existing heat pump, in summer it supplies the FAHR thermally driven chiller. FAHR Hybrid Chiller (HC) consists of two components: a vapour compression water to water chiller and an adsorption one, they are connected in parallel to a cold buffer tank and then to the existing chiller. The nominal cooling power are 50 kW for the vapour compression chiller and 20 kW for the sorption one. TVP panel surface is 220 m 2 south oriented (azimuth=0) and tilted 30°, the volume of stratified storage is 10 m 3 .

As for the existing reversible HP, the Hybrid Chiller (HC) was modelled by implementing performance equations.

The compression unit was modelled through a performance equation analogous to eq. 1. For the sorption unit a linear model that considers the three operating temperatures (HT-high temperature, MT-recooling temperature, LT-cooling temperature) was implemented. Source data were provided by FAHR and converted in a statistical correlation. In this case the temperatures of water inlet inside the machine were considered, the correlation was calculated both for cooling power and COP. Existing HP (eq. 1) HP 4 power levels: 0% -37% -63% -100%. PID controller based on temperature setpoint of HP water outlet: 40°C for heating and 7°C for cooling A summary of control strategy for each component of the TP3 is reported in Tab. 4. The general controls implemented in the existing HVAC system is only changed regarding the primary pump. The tempering valve mixes the hot water delivered by TVP panel with return warm water from AHU coil during winter operation whenever TVP production is higher than hot water AHU inlet set point (40°C)

Primary pump (variable speed) 60%-100% signal, the intensity of signal is proportional to the difference between the temperature of water outlet from AHU coils and the setpoint 2.4 TP4 development and modelling TP4 couples BDR HPs with DualSun PVT is shown in Fig. 4 applied in Madrid demo case, and consists of DualSun PVT panels (1) driving the evaporator of a Brine Water HP (BWHP) (3), an Air Water HP as back-up ( 6) and two storage tanks from RT: one for space heating and space cooling demand(4), and another one for DHW demand(5). The solar driven heat pump allows to store the solar energy in RT tanks via the BWHP as well as producing PV electric energy to drive both HP and cover partially the electrical demand of the building. A small glycol buffer ( 2) is installed between the BWHP and PVT collectors to de-couple the flow rates. The AWHP one includes an electric resistance (2x6kW) to have the possibility to perform thermal shock as legionella preventive controlin case of need.

Fig. 4 Integration between existing system and SunHorizon TP4

Modelling approach: In TRNSYS the DualSun PVT panels are modelled using Type 816, using the same coefficients as in TP2. The collector tilt is set to 35º, to optimize the thermal and PV production in Madrid. The small glycol buffer is modelled using Type4a with default values and a volume of 0.2m3. BWHP is modelled using Type 927 using the performance map of GSHP 9 TR-E model from BDR, which has a capacity of 9 kW and a COP of 3.42 (B0/W45, EN14511). The temperature at the entrance of the BWHP is limited to a maximum of 35ºC and minimum of -15ºC with Type 953 (tempering valve) for safety reasons. The AWHP is modelled using 941 using the performance map from HP HPI Evolution model 27 TR-2 from BDR, which has a capacity of 24.4kW and COP 3.94 (A7/W35, EN14511). Heat pump flow rates are designed to supply a temperature difference of 5ºC, whereas the solar loop flow rate is set to 60 kg/h•m2. The building where TP4 is applied consists of a retrofitted social housing building in Madrid. The building demand is estimated using Design Builder resulting in a space heating and space cooling demand of 31.8 kWh/m2 and 9.36 kWh/m2, respectively. Fancoils are modelled using Type 91 to heat or cool the air of the Type 56 building. DHW consumption is estimated in 122 kWh/person using CREST software demand. As baseline case, DHW and SH is provided by a boiler with an efficiency of 80% and cooling is satisfied with individual splits with an EER of 2.5.

Control strategy:

The DualSun solar thermal loop is switch on whenever the temperature at the outlet of the solar collector is five degrees greater than the bottom temperature of the buffer tank, whereas the HP's operation depends on its efficiencies. HP efficiencies are modelled with empirical equations depending on the supply and evaporator's temperature. Tab. 5 control modes are modelled in MATLAB and coupled with TRNSYS via Type155, to assess the efficiency every 5 minutes and switch on the heat pump with the highest efficiency whenever there is DHW, SH or SC request. When there are two demand requests, DHW is supplied with the HP with the highest efficiency. To avoid reversing the mode of the BWHP too many times, the cooling reverse mode is only switched on whenever there is an excess of PV production and no DHW request. When the maximum or minimum temperature at the inlets of the HPs are reached, the control signal is disabled. The MATLAB program takes into account the different operation modes in the different seasons and DHW is always prioritized (See Tab. 5). DHW or SH is requested whenever the top tank temperature is lower than the setpoint temperature (50ºC and 45ºC, respectively). SC is requested whenever the bottom tank temperature is higher than the cooling setpoint temperature (15ºC).

Results and discussion

TP1 results

The parametric study for the preliminary sizing from simulations of solar field and tank regarding the nonrenewable energy and Customer Bill Reduction (CBR) savings is balanced by the amount of solar heat dissipated, since the impact of the solar field overheating protection emerged from industry partners and demo site responsible discussions. The Fig. 5 shows a turning point around 800L solar tank volume for both CBR costs and 10% limitdissipated heat curves that mostly selected the TP1 configuration as 10 m² TVP solar panels, 0.8 m 3 RT tank and 20kW BH unit. The Tab. 6 shows its performance results, considering the existing heating system is gas boiler and SDHW with assumed solar fraction of 50%, consuming 37.2 MWh PE, that means 2.1k€ costs and 7.4 tons CO2,eq emissions per year. The 63% TVP solar useful heat efficiency outperforms by 30% its traditional flat plate competitor (same area) that leads to 3% extra PE savings. The integration of solar thermal TVP and RT tank from BH20 single unit is increasing from 25 to 36% the fsav,PEnren and the GUE from 130 to 132%, mostly by reducing the share of high temperature DHW demand in total BH20 heat supply.

TP2 results

The simulation has been performed considering 35 maximum number of panels (58m²) and an heat pump of 20kW to be installed and varying the dimension of the storage to understand the optimum situation that allow to have a balanced energy savings reduction and customer bill reduction. To calculate the energy cost, the discount obtained on the electricity price thanks to the net metering has been considered: the extra electricity produced and not selfconsumed, can be supplied to the grid. Based on a yearly calculation, a discount of 35% on the energy price is considered for an amount of energy equal to the extra energy supplied. This amount cannot exceed the yearly electricity consumption. From the Fig. 6, the KPIs about reducing energy consumption, greenhouse gases emissions and energy cost show that the tank size has a limited effect (~ 1-2 %) on savings, either fsav or fCBR (39% is about 745€). An on-going optimization of the set of control parameters is aiming to improve further these KPIs.

TP3 results

As detailed in CEA et al. 2020, a tailored hourly weather file for the year 2019 relying on interpolation from Athenium Analytics (an iScan service provided in IESVE) and Meteonorm files was developed to account for realistic user demand in both simulations (building and system). Several analyses performed before the simulation addressed a first sizing of the main components, then, the simulation results assessed the optimal compromise between theoretical optimal size and technical issues. The main technical matters are related to the optimal coupling between solar production and sorption demand, the configuration of hybrid chiller and its consequent sizing, the correct coupling between existing system and TP installation. The hybrid chiller consists in parallel connection of 50 kW vapor compression module and 20 kW sorption unit (nominal cooling power). TVP panel surface is 220 m 2 south oriented and tilted 30°, the volume of stratified storage is 10 m 3 . The main result from Tab. 8 is the reduction of 33.4% of electrical energy consumption and fsol-sh solar fraction in heating season close to one. During summer the hybrid chiller covers the 57.7% of cooling energy supply with an increase of energy efficiency and saving (Fig. 7). Moreover, the sorption unit supplies cooling energy with a very high efficiency due to its low electrical demand and to its renewable source for driving power. The hybrid chiller coupled with a renewable energy source (TVP) let to achieve one of the main goal of Sunhorizon project regarding 33% PEnren savings and 52% renewable energy ratio. The control strategy of hybrid system plays a crucial role to balance the sorption and vapour compression cooling production. Tab. 4 shows different temperature set point for each unit in order to maximise the sorption production and the solar energy supply. The investigation of additional sorption modules installation shown severe increase of auxiliaries' electricity consumption, not balanced by enhanced cooling power. In the sizing of TVP solar field, it shows an overproduction that can't be stored beyond the tank maximum safety temperature, therefore it is dissipated. To minimize the solar energy waste two ways were investigated: reduce the solar field area or increase the stratification tank volume. The first way is not suitable because of the significant decrease of sorption cooling production, from the second, arises technical troubles regarding the installation and not significant benefits in terms of energy storage. The solar energy wasted during summer amounts to 31.9% of production, in winter is 24.1%; during the periods in which there is no heating or cooling demand the solar energy production must be fully wasted since there are no DHW load. Tab. 6 reports the main KPIs estimated for TP3 in Sant Cugat where the relative values are referring to absolute values of PESnren 43994 kWh, GHGsav 7825 kgCO2,eq and CBR 5270€. Furthermore, TP3 achieved better indoor comfort conditions than the existing H/C system by reducing the HCI from 817 to 83 K.h and CCI from 1748 to 1360 K.h.

TP4 results

A parametric study is performed to size the system components, varying the volume of the RT tanks, slope of the DualSun PVT modules and setpoint temperatures. From the main results illustrated in Fig. 8, the system is not sensible with the variation of the volume of the RT tank, but as the volume increases the primary energy savings increases slightly. The lower the tilt inclination, the lower the PV production and the system achieved more PEnren savings between 35-45º (there is not significant variation between this two). By increasing the DHW setpoint to 60ºC in the tank, the savings are reduced in 2.5% points, while by moving the position of the monitored temperature to the middle-top tank (a relative height of 0.7), the savings are reduced in 0.5% (more energy is charged in the tank). By increasing the capacity of the DHW tank, the primary energy savings increase 0.1% as well as the RER. The same happens by increasing the capacity of the SH/SC tank. Nevertheless, due to space limitation in the technical room, the capacity of both tanks are limited to a height of 1.5 meters and a capacity of 1.3 m 3 , and 1m3 respectively, in order to ensure stratification. Furthermore, the current control strategy is compared with a temperature-dependent control strategy that does not consider the efficiencies of the BDR HPs (reference case vs. simpler control in Fig. 7). This simple strategy switches on the BWHP whenever the solar field is warm enough. The minimum working temperature of the BWHP is -15ºC and this control strategy does not allow the BWHP to work when the sun is gone. As a result, during winter time the AWHP is mainly used, increasing the electricity consumption in nearly 1800kWh/year. With a control strategy that allows to compare the efficiency of both HPs, results in 11% more primary energy savings and higher solar thermal production (as the DualSun PVT panels work as evaporator when the sun is gone). This leads to make use of 82.3% of the heat provided by the PVT panels and, by means of the BWHP, provide 66.9% of the heat needed in the system. The optimal cases are obtained with bigger tanks, but due to space limitations option 5 (100 kg/h•panel) with the efficiency-dependent control is chosen, with the following parameters: a flow rate in the DualSun panels of 100 kg/h•panel, solar tilt of 35º, volumes of 0.2 m3 for the buffer tank, 1.3m3 for the DHW tank, and 1 m3 for SH tank, with a DHW setpoint of 50ºC and the position of the sensor at the top of the tank (in heating mode).

Conclusion

The current work managed simulation studies about design and sizing of HP, thermal storage, solar collectors that allow for selecting four appropriate configurations of the TPs for their respective demo sites: small residential building in Berlin, Germany (TP1), small residential building in Riga, Latvia (TP2), tertiary civic center in Saint Cugat, Spain (TP3) and large social-housing apartment in Madrid, Spain (TP4) with annual savings results in terms of non-renewable primary energy consumption, GHG emissions and operating costs. In parallel, other configurations of the four TPs have been selected similarly to end up with the preliminary definition of eight SunHorizon demo pilots. The annual savings are ranging from 33 to 70 % regarding GHG emissions, from 30 to 85% regarding the operation costs. It depends in a large extent on the demo site localization and existing system and verifies the expectation that the larger solar panels area, the higher the savings up to a limit, as illustrated in 1 Most of the savings are obtained by removing the gas contract from the dwellings.

Fig. 5 for TP1. For the TP1, TP3 solar thermal systems, it emphasizes the requirement of excess solar heat dissipation in building context with summer fluctuating heat load, that is facing in the other hand to the urban heat island reduction issue, whenever no detrimental operation cost effects were detected. It shows also rather flat optimal tank size for all TPs, mostly ending with selection of smallest tank in the range to lower the installation costs.

The previous descriptions of control and the simulations analysis showed that evolutions from original single component approach are required by HP controller manufacturer to run efficiently the systems. Indeed, it's the flexible device connected to stable energy network to ensure that the end-user temperature demand is satisfied, to look carefully to the solar REN source state and storage, to optimize overall energy savings sometimes against its pure own performance, or require to run HP internal pump to flow solar in heating circuit, while still considering hardware safety temperature limitation (bypass valve mostly required).

These system models and simulation results are being used for the design of specific tests following the semivirtual approach [START_REF] Chèze | Comparative dynamic performance tests of two real technology packages for buildings heating system retrofit[END_REF] and to compare with the test results before installing the TPs on-site in 2021.

The TRNSYS dynamic simulation models are currently used for the development of advanced control to be demonstrated in Riga and Sant Cugat sites. They will be run again at the end of the demonstration phase for the comparison with the performance monitoring data and the estimation of the savings, and in replication and exploitation case studies where the TPs may compete each other's or against single HP or PV+HP. Beyond project end, these models are the basis of either in-depth system model development with interested industrial partners or simplified system model for sizing tool.

Fig. 1

 1 Fig. 1 TP1 concept for simulation and BH20 core Gas Utilization Efficiency (GUE)

  f(HT 𝑖𝑛 , MT 𝑖𝑛 , LT 𝑖𝑛 , ) = I + a * HT 𝑖𝑛 + b * MT in + c * LT 𝑖𝑛 (eq. 2)

Fig. 3

 3 Fig. 3 Integration between existing system and SunHorizon TP3In Tab. 3, the control strategy considers the behavior of the existing plant deduced from the preliminary data retrieved from the existing monitoring system.

Fig. 5

 5 Fig. 5 Analysis of bill reduction (left), overheating (mid) and system configuration effect (right) Tab. 6 Estimated annual KPIs for TP1 in Berlin demo site

Fig. 6

 6 Fig. 6 Simulated performance of TP2 configurations in Riga demo case Tab. 7 Simulated performance of TP2 selected 1300L configuration in Riga demo case.

Fig. 7 :

 7 Fig. 7: Source distribution for cooling (a) and heating (b) energy supply Tab. 8 TP3 main annual KPIs

Fig. 8

 8 Fig. 8 Parametric Study for TP4-1 in Madrid demo site The estimated TP4 annual KPIs are shown in Tab.9 where the savings are compared to the mostly gas-fueled baseline in Madrid demo case. Considering only AWHP supply for heating and cooling without PVT-driven-BWHP, the simulation estimates 41.3 MWh PEnren consumption, SCOP=2.7 and SEER=4.3. It means the DualSun solar PVT integration with BDR BWHP in TP4 would reach fsav,PEnren =54% and increased SCOP=3.59 compared to AWHP alone since it runs as support in heating mode .

Tab. 5 Control Modes in TP4 for selection of HP supply types to meet the thermal demand types

		Only AWHP on	Only BWHP on	Both on
	DHW request	Mode 1b	Mode 2b	Mode 3b: AWHP>DHW, BWHP>SH
	SH request (heating season)	Mode 1a (heating)	Mode 2a (heating)	Mode 3a: BWHP>DHW, AWHP>SH
	SC request (cooling season)	Mode 1a (cooling)	Mode 2a (cooling). Only if excess of PV production and no DHW request	Mode 3a: BWHP>DHW, AWHP>SC

Tab. 9 Estimated annual KPIs for TP4-1 in Madrid demo site 𝜼 𝑫𝑺,𝟏𝟖°𝑪 𝒈𝒓𝒐𝒔𝒔 fsol,th 𝜼

  𝑫𝑺,𝒆𝒍 𝒈𝒓𝒐𝒔𝒔 𝒇 𝒔𝒆𝒍𝒇,𝒆𝒍 𝑻𝑬𝑹 𝒅𝑻 𝒕𝒂𝒏𝒌,𝑫𝑯𝑾 𝒅𝑻 𝒕𝒂𝒏𝒌,𝑺𝑯/𝑺𝑪 𝑹𝑬𝑹 𝒇 𝒔𝒂𝒗,𝑷𝑬 𝒏𝒓𝒆𝒏 𝒇 𝒔𝒂𝒗,𝑮𝑯𝑮 𝒇 𝑪𝑩𝑹 1

	20.3% 66.9% 16.4% 49.5% 1.2	16K	2K	62.6%	76.1%	70.3% 84.8%

Acknowledgments

The SunHorizon Project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement N. 818329.