Chr Lamnatou

R Vaillon

S Parola

D Chemisana
email: daniel.chemisana@udl.cat

Photovoltaic/thermal systems based on concentrating and nonconcentrating technologies: Working fluids at low, medium and high temperatures

Keywords: Photovoltaic/Thermal (PVT) systems, Low-temperature applications, Medium-temperature applications, High-temperature applications, Technologies with/without solar concentration, Buildings, industry, power plants HIT Heterojunction

    Photovoltaic/thermal systems based on concentrating and non-concentrating technologies: Working fluids at low, medium and high temperatures Chr. Lamnatou, Rodolphe Vaillon, S. Parola, D.

INTRODUCTION

Photovoltaic (PV) cells absorb and convert solar radiation into electricity [START_REF] Tripanagnostopoulos | Aspects and improvements of hybrid photovoltaic/thermal solar energy systems[END_REF].

However, the main part of the collected solar radiation is transformed into heat, increasing PV-cell temperature and, therefore, reducing PV-cell efficiency [START_REF] Dupré | Thermal Behavior of Photovoltaic Devices[END_REF]. This undesirable phenomenon can be avoided by using a heat extraction device with fluid circulation. In this way, PV panels can work at a more satisfactory temperature. PV systems which combine PV modules with thermal units (involving the circulation of a fluid, e.g. air or water) are known as hybrid Photovoltaic/Thermal (PVT) systems and produce both electrical and thermal energy [START_REF] Tripanagnostopoulos | Aspects and improvements of hybrid photovoltaic/thermal solar energy systems[END_REF]. In other words, PVT installations provide higher total energy output (in comparison to PV modules which generate only electrical energy) and environmental advantages [START_REF] Tripanagnostopoulos | Energy, Cost and LCA Results of PV and Hybrid PV/T Solar Systems[END_REF][START_REF] Tripanagnostopoulos | Performance, Cost and Life-cycle Assessment Study of Hybrid PVT/AIR Solar Systems[END_REF].

PVT systems are appropriate for different kinds of applications (domestic, industrial, etc.). By placing emphasis on the working fluid, water-cooled PVT (PVT/water) and air-cooled PVT (PVT/air) systems are commonly used for water and indoor space heating, respectively [START_REF] Tripanagnostopoulos | Aspects and improvements of hybrid photovoltaic/thermal solar energy systems[END_REF].

The temperature of the working fluid plays a pivotal role and is associated with the type of application [START_REF] Tripanagnostopoulos | New designs of building integrated solar energy systems[END_REF]. PVT systems appropriate for different temperatures/applications were developed: domestic hot water production, indoor space heating/cooling, desalination, pool heating, crop drying, industrial process heating and so on [START_REF] Joshi | Photovoltaic -Thermal systems (PVT): Technology review and future trends[END_REF].

In the case of PVT applications in the building sector, it can be noted that there are two basic categories: 1) Building-Added Photovoltaic/Thermal (BA PVT) (for instance, these systems are mounted on the roof of a building), 2) Building-Integrated Photovoltaic/Thermal (BIPVT) (these systems form part of the building structure itself, e.g. façade-or roof-integrated configurations) [START_REF] Chr | Photovoltaic/thermal (PVT) systems: A review with emphasis on environmental issues[END_REF].

There are articles that include reviews about PVT systems. In Table 1, the major references (with emphasis on review papers about PVT) are presented. It can be noticed that the majority of these studies focus on issues such as PVT design, performance, working fluids, applications, temperature effect and PV cooling. Table 1. References about PVT systems with emphasis on review papers.

PV cells and thermal collector: Materials Heat-transfer fluid PV-cell temperature Heat-extraction system Joshi and Dhoble (2018) [START_REF] Joshi | Photovoltaic -Thermal systems (PVT): Technology review and future trends[END_REF] PVT concept PVT systems: Classification Non-concentrating PVT systems CPVT systems Spectral-beam-splitting PVT systems Sultan and Ervina Efzan (2018) [START_REF] Sultan | Review on recent Photovoltaic/Thermal (PV/T) technology advances and applications[END_REF] Types of PVT modules Solar greenhouses with PVT modules -Type of study (experimental, modelling, both experimental/modelling).

-Working fluid (air, water, nanofluids, etc.).

-Solar energy collection by means of concentrating or non-concentrating devices.

-Type of PV cells (mono-Si, poly-Si, etc.).

-Systems with or without thermoelectric elements.

-Efficiencies (electrical, thermal, etc.).

-Type of application (buildings, industry, drying, desalination, etc.).

-Location/country where a certain system was evaluated.

In light of the issues mentioned above, it can be noted that the proposed framework provides the basis for identifying key challenges (in terms of materials, technologies, efficiencies and so on) that PVT systems are facing.

LITERATURE REVIEW

PVT SYSTEMS FOR LOW-TEMPERATURE APPLICATIONS

The present section deals with PVT systems with working fluids at low temperatures (less than 60 o C). In the literature on PVT, there is a whole host of studies on this kind of systems and, therefore, three subcategories have been considered: 1) PVT/air, 2) PVT/water, 3) PVT with both air and water, PVT with nanofluids, etc.

In Table 2, selected references about PVT/air systems for low-temperature use are presented. The results show that:

• The majority of the references are about modelling or experimental/modelling studies on systems without solar concentration.

• Most of the systems do not have thermoelectric elements.

• A large number of investigations include mono-Si or poly-Si PV cells.

• In most cases, the maximum temperature of the working fluid (air) ranges from around 30 to 55 o C.

• Thermal efficiencies range from 10 to 90%. Moreover, many systems show PV conversion efficiencies around 6-16%. In certain cases, high overall efficiencies (80% or higher) were found.

• Regarding the type of application, a large number of investigations are about building-integrated configurations whereas there are a few studies on dryers.

• A large majority of the references are about systems with PCM appropriate for BI applications.

• In most cases, the systems were investigated for the climatic conditions of Europe and India.

In terms of the achievement of high efficiencies, it is worth mentioning the study by Nazri et al. [START_REF] Nazri | Energy economic analysis of photovoltaic-thermal-thermoelectric (PVT-TE) air collectors[END_REF] (PVT/air with a thermoelectric element) on the grounds that a maximum thermal efficiency of 84% was found. It was noted that mass flow rate considerably influences thermal-efficiency results. It is also worth mentioning the study by Kamthania and Tiwari [START_REF] Kamthania | Energy metrics analysis of semi-transparent hybrid PVT double pass facade considering various silicon and non-silicon based PV module Hyphen is accepted[END_REF]. Multiple silicon-based and non-silicon-based BIPVT/air modules were investigated and the findings show that the Heterojunction with Intrinsic Thin Layer (HIT) system has the highest: i) module efficiency, ii) net annual electrical energy, iii) overall annual thermal energy, iv) exergy output.

Regarding BIPVT, Assoa and Ménézo [START_REF] Assoa | Dynamic study of a new concept of photovoltaic-thermal hybrid collector[END_REF] investigated a roof-integrated PVT/air system. A model was developed and validated with data of an experimental setup near Lyon, in France. It was found that forced ventilation offers advantages from a thermalproduction point of view. However, natural ventilation provides adequate cooling of the PV panels. In Figure 1, the experimental building with the prototype roof-integrated PVT/air system is illustrated.

Figure 1.

The roof-integrated PVT/air system studied by Assoa and Ménézo [START_REF] Assoa | Dynamic study of a new concept of photovoltaic-thermal hybrid collector[END_REF]. An experimental building at TENESOL company site. Source: Assoa and Ménézo [START_REF] Assoa | Dynamic study of a new concept of photovoltaic-thermal hybrid collector[END_REF]. Table 2. PVT/air systems: working fluid at low-temperatures.

Study / year

Type of study

Solar concentration

Type of PV cells

Thermoelect ric element

Temperature of the working fluid: Low

Efficiencies

Type of application:

-Buildings -Etc. In Table 3, selected references about PVT/water systems for low-temperature applications are presented. The results demonstrate that:

Location of the study

Candan

• A large number of investigations include both modelling and experiments on systems without solar concentration.

• In most cases, the systems do not include thermoelectric components.

• The majority of the studies about concentrating systems refer to CRs less than 10X and include a whole host of configurations (with Compound Parabolic Concentrators (CPCs), Fresnel lenses, micro-concentrators, parabolic-trough concentrators, etc.).

• In most cases, mono-Si and poly-Si PV cells were used.

• The majority of thermal efficiencies are around 50-85% and electrical/PV efficiencies are approximately 12-15%.

• Many references are about BI configurations and, in general, about systems for buildings and domestic use whereas there are a few studies about agricultural applications (e.g. greenhouses, dryers of agricultural products).

• There are a few investigations about PVT systems with PCMs, integratedcollector-storage, heat pipes, chillers, desiccants and polymer PVT collectors.

• Most of the systems were evaluated for the climatic conditions of Europe and China, operating at around 50-60 o C working-fluid temperatures.

Regarding the achievement of high efficiencies, Alves et al. [START_REF] Maadi | Coupled thermal-optical numerical modeling of PV/T module -Combining CFD approach and two-band radiation DO model[END_REF] investigated a PVT/water with a Solarus reflector (CR: 1.7X) and mono-Si PV cells. The system is appropriate for domestic water heating and, in the case of Sweden, showed an electrical efficiency of 19.1%. Alves et al. [START_REF] Maadi | Coupled thermal-optical numerical modeling of PV/T module -Combining CFD approach and two-band radiation DO model[END_REF] noted that by increasing the water flow rate, more heat (from the receiver) is removed. This means that there is a reduction in PV-cell temperature and increase in PV efficiency. Moreover, Khelifa et al. [START_REF] Khelifa | Modeling and detailed study of hybrid photovoltaic thermal (PV/T) solar collector[END_REF] studied a PVT/water system (without solar concentration). In certain cases, high thermal efficiencies (around 94%: maximum) were found and, in relation to these high efficiencies, it was highlighted that the collector is perfectly insulated and covered by a flat glass [START_REF] Khelifa | Modeling and detailed study of hybrid photovoltaic thermal (PV/T) solar collector[END_REF]. In Table 4, studies about different kinds of low-temperature PVT systems (bifluid configurations or systems with fluids different to those included in Tables 2 and3), are presented. The results show that:

• The majority of the references are experimental/modelling investigations without involving solar concentration and thermoelectric devices.

• In many cases, silicon-based PV cells were used.

• Most of the investigations about concentrating systems refer to CRs up to 20X, involving configurations such as CPCs and parabolic-trough concentrators.

• The majority of the PV efficiencies are 13-15% (approximately). Moreover, in many cases, thermal efficiencies reach up to around 70%.

• Different types of applications were presented: by way of illustration, BI systems and configurations with PCM as storage material.

• A large majority of the studies are based on systems with working fluid temperatures around 40-55 o C.

• The systems were investigated for different climatic conditions.

PVT SYSTEMS FOR MEDIUM-TEMPERATURE APPLICATIONS

In the literature about PVT systems with working-fluid temperatures between 60 and 90 o C, there is a more balanced number of cases with/without solar concentration in comparison to the literature about PVT systems with low-temperature working fluids.

Table 5 presents studies about PVT systems without involving solar concentration and with using working fluids at medium temperatures. The results indicate that:

• A large number of references include experimental/modelling and modelling studies about systems without thermoelectric components.

• In most cases, silicon-based PV cells were utilised.

• The majority of the thermal efficiencies are around 55-60% and electrical/PV efficiencies are approximately 13-15%.

• A large majority of the investigations are about PVT/water and PVT/air configurations, for industrial and domestic applications (including BI systems).

• There are a few references about configurations with PCMs, heat pumps, ethylene-tetrafluoroethylene cushions and polymeric PVT collectors.

• In the vast majority of cases, the working-fluid temperatures are approximately 60-70 o C.

• The systems were evaluated for different climatic conditions.

Ren et al. [START_REF] Ren | Feasibility of an innovative amorphous silicon photovoltaic/thermal system for medium temperature applications[END_REF] investigated an amorphous silicon PVT/water configuration appropriate for medium-temperature applications, verifying that this kind of PV technology is feasible from a technical/thermodynamic point of view (for mediumtemperature applications). The experiments were conducted in Hefei, in China. In Figure 3, the PVT system and experimental/numerical results are illustrated. In terms of CPVT, Haiping et al. [START_REF] Haiping | Experimental study on a flash tank integrated with low concentrating PV/T (FT-LCPVT) hybrid system for desalination[END_REF] developed and tested a flash tank integrated with a low-concentrating PVT system with CPCs (geometric CR: 4X; Figure 4), demonstrating the feasibility of this system for cogeneration of fresh water and electricity.

The PVT panel is based on mono-Si PV cells, produces electricity and preheats saline In Table 6, references about PVT systems involving solar concentration and using working fluids at medium temperatures are presented. The results suggest that:

• The majority of the references are about modelling and experimental/modelling investigations on PVT/water systems without thermoelectric elements.

• In most cases, the CRs are up to 190X and the working-fluid temperatures are around 62-90 o C.

• In most of the investigations, mono-Si and multi-/triple-junction PV cells were used.

• The majority of thermal efficiencies are approximately 50-68%. In most cases, PV efficiencies are around 10-19% and 35-38%, depending on the PV cells (single-junction vs. multi-junction, etc.).

• A large number of investigations are about domestic applications and desalination whereas there are a few studies on greenhouses.

• The systems were examined for different climatic conditions.

PVT SYSTEMS FOR HIGH-TEMPERATURE APPLICATIONS

In the field of PVT systems with the working fluids at high temperatures (higher than 90 o C), one challenge is to cope with the reduction in PV-cell efficiency at high temperatures. Crisostomo et al. [START_REF] Crisostomo | Experimental testing of SiNx/SiO2 thin film filters for a concentrating solar hybrid PV/T collector[END_REF] noted that spectral beam splitting is a solution that allows PV cells to work at low temperatures whereas thermal receiver operates at high temperatures. However, in this case, the fraction of solar irradiation that without beam splitting would be converted into heat by the PV cell is lost. SiNx/SiO2 multilayer thinfilm filters were used as beam splitting devices in a linear Fresnel mirror-based CPVT system with CR 10X. Indoor testing was conducted (involving filters, silicon-based PV cells, thermal sensor and solid-state plasma light source). The experiments revealed that the PV cells, illuminated by the light reflected by the filters, show 9.2% (absolute) higher efficiency in comparison to the same PV cells without the filters [START_REF] Crisostomo | Experimental testing of SiNx/SiO2 thin film filters for a concentrating solar hybrid PV/T collector[END_REF]. Widyolar et al.

[170] investigated a spectrum splitting hybrid concentrating solar power/CPV collector (design, simulations, testing; geometric CR: 50X; Figure 5). Double-junction InGaP/GaAs solar cells were used. Experiments using a fluid with suspended solid In Table 7, references about PVT systems with solar concentration for hightemperature use are presented. The analysis indicates that:

• The majority of the references are about modelling studies, suggesting that experimental developments are still in their early stages.

• In most cases: i) the systems do not include thermoelectric elements, ii) CRs are up to 1000X, iii) the working fluid is water, iv) temperature spans are from around 100 to 250 o C.

• There are a few investigations with working fluids such as oils, salts and nanofluids.

• Most of the systems have triple-junction or silicon-based PV cells.

• The majority of thermal efficiencies are around 50-60% and electrical/PV efficiencies are approximately 20-30%.

• Regarding applications, a large majority of the investigations refer to domestic installations, desalination, polygeneration and large-scale systems.

• A vast majority of the references are about configurations with parabolic concentrators.

• The systems were examined for different climatic conditions.

Riggs et al. [START_REF] Riggs | Technoeconomic analysis of hybrid PV/T systems for process heat using electricity to subsidize the cost of heat[END_REF] presented a study about high-temperature PVT systems (Figure 6: schematics), highlighting the fact that concentrating solar systems offer multiple applications such as industrial and commercial. Moreover, it was found that the energy

CHALLENGES AND FUTURE PROSPECTS

In light of the issues discussed in section 2, PVT systems for low-and mediumtemperature applications pose the following challenges:

-Development of PVT systems which are based on multiple kinds of PV cells (apart from the typical configurations with Si-based PV cells) and different types of working fluids (nanofluids, bi-fluid systems and so on).

-Design of innovative systems with concentrators and/or thermoelectric components, aiming at the achievement of high electrical and thermal efficiencies for a wide range of applications (residential, agricultural, industrial, etc.).

-Development of burgeoning systems which form part of the building structure itself (BI solar systems).

On the other hand, high-temperature PVT systems pose challenges such as:

-Utilisation of PV-cell materials and concentrators suitable for high CRs.

-Design of PVT configurations with CRs higher than 1000X and working-fluid temperatures higher than 300 o C, offering a whole host of applications (industrial, largescale power generation and so on).

-Testing (under real and experimental conditions).

In the literature on PVT systems, there are studies which place emphasis on CPVT technologies, highlighting the fact that systems with solar concentration offer a raft of solutions ranging from domestic hot water production to organic-Rankine configurations.

In this case, an additional challenge is related to heat-transfer components/medium.

Elements such as fins, micro-channels, storage devices, heat exchangers and PCMs were evaluated with a view to examining issues such as heat-transfer performance and thermal/electrical efficiencies. Moreover, the performance of the heat-transfer fluid depends on multiple factors, namely viscosity, density, temperature, thermal mass, time and mass flow rate. Furthermore, the systems should be feasible from an economic point of view. To this end, parameters such as feed-in tariff and carbon credit should be considered [START_REF] George | Concentrated photovoltaic thermal systems: A component-by-component view on the developments in the design, heat transfer medium and applications[END_REF].

Additional CPVT challenges are associated with the evaluation of the systems from different angles (experimental, numerical and so on), investigation of the impact of the concentrators (CR, temperature, etc.) and the role of the COP. The results of the literature on CPVT show that these systems are promising and, in the short run, are expected to be competitive with conventional energy systems [START_REF] Daneshazarian | Concentrating photovoltaic thermal (CPVT) collectors and systems: Theory, performance assessment and applications[END_REF].

CONCLUSION

The present article provides a thorough review of PVT systems from a specific viewpoint. In particular, the PVT configurations have been categorised by the temperature of the working fluid. In each case, useful information has been extracted: namely working fluid, PV-cell types, solar concentration ratio, fluid temperature, electrical and thermal efficiencies, type of application and location of the study.

Analyses of the main features of the PVT systems (on the basis of three workingfluid temperature ranges: low, medium and high) have revealed the major trends of the research that has hitherto been conducted. Accordingly, gaps remaining to be filled have been identified (simultaneous improvement of electrical and thermal efficiencies and so on).

By way of illustration, more diverse PV technologies need to be used and developed, especially in the case of systems operating at high temperatures. To this end, thermal systems should be optimised, in line with different levels of solar concentration and resulting temperatures. On the other hand, hybridisation with thermoelectric elements is a hitherto-immature technology and, therefore, further development is needed. The remaining challenges are more numerous for the high-temperature applications, in terms of working fluid, heat transfer components and heat storage. Much as modelling and simulations provide solid design tools, tests under real operating conditions are absolutely required. Additional factors such as costs, payback times, materials and environmental impacts (in comparison to conventional energy systems) should also be taken into account.

Fudholi

  et al. (2019) [27] Monofacial and bifacial PV cells Efficiency of bifacial PV modules PVT/air systems with monofacial PV cells PVT/air systems with bifacial PV cells Vaishak and Bhale (2019) [28] PVT systems PVT systems with heat pumps PVT systems with heat pumps: Details about the components Research and development Future prospects The aforementioned issues indicate that in the literature about PVT there is a lack of review papers which place emphasis on working-fluid temperature. The present article provides an overview about different kinds of PVT systems, appropriate for buildings, industry, etc. Its main contribution is the classification of the PVT configurations according to the temperature of the working fluid. The classification criteria are as follows: -Low-temperature systems: Working-fluid temperatures < 60 o C -Medium-temperature systems: 60 o C ≤ working-fluid temperatures ≤ 90 o C -High-temperature systems: Working-fluid temperatures > 90 o C Furthermore, the PVT systems are categorised by taking into account additional criteria:

Figure 2 .

 2 Figure 2. The low-temperature CPVT system studied by Wu et al. [104]: a) system with a thermoelectric component, b) nanofluid cooling tubes. Source: Wu et al. [104].

Figure 3 .

 3 Figure 3. The amorphous silicon PVT/water system studied by Ren et al. [133]: a) experimental setup, b) electrical efficiency (experimental results), c) thermal efficiency (experimental results), d) temperature distribution of the absorber plate (Tb) (numerical results; different inlet temperatures (Tin)). Source: Ren et al. [133].

Figure 4 .

 4 Figure 4. The medium-temperature CPVT studied by Haiping et al: a) The Low-Concentrating Photovoltaic/Thermal -Solar Thermal Collector (LCPVT-STC) experimental set-up, b) details about the CPVT module. Source: Haiping et al. [136].

(Figure 5 .

 5 Figure 5. The high-temperature CPVT system studied by Widyolar et al. [170]: a) details about the secondary reflector, b) hybrid Heat Collection Element (HCE), c) primary mirror, Normal Incidence Pyranometer (NIP) sensor, shadow tracking, etc. Source: Widyolar et al. [170].

Figure 6 .

 6 Figure 6. Schematics related to the study by Riggs et al. [205] about CPVT systems for high-temperature applications: a) energy conversion (hybrid receiver), b) CAD model of a dish-based CPVT system with 2-axis tracker. Source: Riggs et al. [205].

Table 3 .

 3 PVT/water systems: working fluid at low-temperatures.

	Study

/ year Type of study Solar concentration Type of PV cells Thermoelect ric element Temperature of the working fluid: Low Efficiencies Type of application: -Buildings -Etc. Location of the study

	Yin et a (2016) Li et	E-M E-M	No Yes	Mono-Si Si-based	No (but it is No	Equilibrium temperature = (psychrometric Maximum	At 25 o C, PV (maximum): efficiency-ratio In one of the	BIPVT (roof-premises: PVT for	USA Spain;
	al. (2014) [88] al.		(CR: 3.6X)		noted that a	temperature 35 o C) chart) temperature:	cell efficiencies: around 59% (for of 12.8 was cases: optical	integrated) with Integration on buildings: CPCs	Scotland;
	(2013) [78] (2017)				case with	with water flow: around 41 o C at	15.1% and inlet found (this ratio efficiency of the	PCM concrete roofs vs. flat modules	UK
	[69] [98]				thermoelectri	around 40 o C the inlet of the	11.9%, for 850 temperature = is 50.7 if PV CPCs 84%
					c element	first heat	and 1100 W/m 2 , 20 o C) generation is
					could be	exchanger of	respectively; taken into
	Kroiß	E	No	Poly-Si	examined No	Fluid outlet the CPVT;	Thermal Electrical (PVT account)	Desalination by	Germany
	et al.				(future	temperature: rooftop (case:	efficiency: system): 14%	using polymer
	Ibrahi (2014) Jouhar	M E	No No	Poly-Si Not	No prospect)) No	Water around 22 o C PVT module Spain)	Thermal: 53.9% (1100 (standard test Energy	Domestic PVT collectors BIPVT (roof-	Not directly UK
	m et al. [79] a et al.			directly		temperature (maximum) temperature:	around 50% W/m 2 , 66 conditions); conversion	applications integrated)	stated
	(2009) [61] (2016) Soltani [89] et al.	M	Yes (CR: 8X)	stated Crystalline silicon	Yes	(outlet): around between 28 and Maximum 31 o C 33 o C temperatures of	(maximum); PV ml/min water Mean thermal efficiency: 50% Thermal: cell: around flow) (PVT system): for the PVT around 57%	PVT with parabolic-	Not directly stated
	(2018)					(maximum) the two sides of	12% 69% configuration (maximum);	trough
	Amriza [99]	E-M	Yes (PVT: 2 lines	Crystalline	No	Maximum the	(maximum) PV cell: 14.3% electrical	Buildings collectors	Spain
	l et al. He et Wang	E-M E	of 26 series No No	silicon Not Poly-Si	Yes (cooling No	outlet Water Tank water thermoelectric	(standard Electrical Daily average (thermoelectric	Building Heat-pipe	China China
	da (2013) al. et al.	M	No connected PV	Mono-Si directly	No and heating	Maximum tank temperature: temperature: temperature generator:	Global annual conditions) (of the heat-pipe thermal, generator):	Domestic hot applications BIPVT	Portugal
	Silva and [70] (2014) (2016a [80] ) [90]		cells optimised for concentrating	stated	systems)	temperature: around 53 o C 21 o C (average) maximum: 89.97 and around 50 o C 47.2 o C 45 °C	energy: 24% PVT): electrical and about 0.5% (15% thermal; approximately total (maximum)	water with heat-pipe production PVT systems
	Fernan		systems)			(simulated:900	9% electrical) 16.7%; Thermal efficiencies of
	des Yang	E-M	Yes	Mono-Si	No	W/m 2 ; water Glass-cover-	(of the system): the system: Overall (CPVT	PVT with quasi-	China
	(2010) He et et al.	E-M	No (CR: 8X)	Not	Yes (cooling	Maximum flow rate 600 plate	System COP: around 23.5% 61.1%, 7.8% system): 55.6-	Domestic parabolic	China
	[62] al. (2018)			directly	and heating	water L/h) temperature:	higher and 68.9%, 59%; Electrical	applications concentrators
	(2013) Kim et [100]	E	No	stated Mono-Si	system) No	temperature in Outlet around 60 o C	than 0.45; Thermal and respectively efficiencies	BIPVT (roof-	Korea
	Chemis [71] al.	E-M	Yes	Crystalline	No	Maximum the storage tank: temperature: (maximum)	Thermal: Thermal electrical (simulated:900 (system): 16.6-	BIPVT integrated)	Spain
	ana et (2014)		(CRs: 7-10X)	silicon		water around 32-34 o C around 44 o C	around 65% (experimental): efficiencies W/m 2 ; water 20%	applications
	al. (2011) [81]					temperature: around 33 o C	(maximum) 12.06%; (average) of the flow rate 600 (experimental); Electrical BIPVT L/h) Optical	with Fresnel linear
	[63]					(Re = 125;	(experimental): collector: 30% efficiency and	concentrators
	Wang	E	No	Poly-Si	No	channel Maximum tank-	10.27% and 17%, Maximum mirror	Heat-pipe	China
	et al.					height/width = water	respectively thermal: 44% utilisation	BIPVT;
	Shan et (2016b	M	No	Not	No	2.43; module Maximum temperature:	PV: around (simulated:300 (concentrator):	Appropriate for potential of	China
	al. Ziapou ) [91]	M	No	directly Poly-Si	No	length = 1 m) outlet For 120 kg of 53.83 o C	12.5% For 120 kg of W/m 2 ; water 94.4%	BIPVT Domestic building	Different
	(2013) r et al.			stated		temperature: water in the (simulated:900	(maximum) water in the flow rate 200	applications applications integration:	cases were
	Sonnev [72] (2014) Fayaz	E E-M	Yes No	Mono-Si Poly-Si	No No	Incoming water 56.3 °C tank and W/m 2 ; water Maximum	Thermal: 56%; tank and L/h); Maximum Maximum	Greenhouses with integrated-walls, roofs, PVT with	Netherlands examined Malaysia
	eld et al. Vivar (2019) [82] et al.	E	(CR: 25X) Yes	Mono-Si	No	temperature: Fluid 1.5 m 2 : tank L/h) temperature 22.4 o C; collector area flow rate 200 output	The combined 1.5 m 2 : system): 81% Electrical: 11% collector area electrical: 7.9% thermal (PVT	lenses Domestic storage solar with Fresnel collector-balconies PCMs	Australia
	(2011) et al. [101]		(CR: 38X)			outgoing water temperature: not water (PVT): 57.4 o C	efficiency of the electrical and 77.36%	(cultivation of applications water heater
	[64] (2013) Chemis [73] ana et	E	Yes (geometric CR: 10.32X)	Crystalline silicon	No	temperature: exceeding 60 o C temperature Outlet water and 56 o C 25.8 o C; flow: 45.9 o C temperature: (numerical and	system can (overall) Mean optical (numerical and exceed 12.5%, thermal efficiency: 51% experimental	ornamental with PVT with BIPVT (façade-plants) micro-integrated) with	Spain
	al.					18.6 kg/s; 52 o C experimental	70%; Average (overall) 36.1% results,	concentrators Fresnel
	(2016)					module length: results,	electrical: 8%; respectively);	(Fresnel reflectors
	Brown [92]	E	No	Mono-Si	No	1.55 m Temperature at respectively)	Average Not directly Electrical:	mirrors) Buildings: PVT	Ireland
	e et al.					the outlet pipes:	thermal: 50% stated 12.75%	applications
	Cristof (2016) Karimi	M E	No Yes	Poly-Si Mono-Si	No No	PV-cell around 24 o C Water-	Average For the CPVT improved	Domestic hot with PCM Buildings: PVT	Mediterran China
	ari et El-[83] et al.	E	Yes (CR: 5.85X)	Mono-Si	No	temperature for Hot water (maximum) temperature in	efficiencies Not directly system (one of experimentally	water PVT with Fresnel	ean Egypt
	al. Seesy (2017)		(geometric			the PVT: average the tank (for the	(PVT without stated the cases): for the PVT-	production by applications lenses
	(2012) [65] and Khelifa [93] Khalil et al. (2014) (2016) Maadi [74] [84] Jaaz et (2019) et al.	E-M M E	CR: 4X) No No Yes	Not directly stated Poly-Si Poly-Si	No No No	30.6 °C (maximum) temperature: Outlet CPVT system): 53 °C temperature: 33.1 o C (final), 39 o C 15 o C (initial) Output (maximum) Maximum PV-around 43 o C temperature:	air layer): 29% for the thermal Thermal: Thermal 40.8% PCM around 94% and electrical and 14% for the (maximum) 14.7% If the PV Electrical: effect is greenhouse	using polymer with low-cost Water heating, PVT collectors mirrors drying, air Buildings conditioning, PVT with CPCs etc.	Algeria Not directly Malaysia stated
	Redpat h et al. (2012) [66] Ibrahi Adeilla al. [102] m et al. et al. (2017) (2014) (2016) [94] [75] [85] Buker Asaee et al. et al. He et (2017) (2014) al. [95] Alves	E E E E-M M E-M E-M	Yes (CR: 1.8X; a configuration without concentration was also studied) No Yes No No Yes (CR less than 2X) Yes	Poly-Si Poly-Si Not directly stated Mono-Si Crystalline silicon Silicon-based Mono-Si	No No No No No No No	Outlet temperature: around 57 o C (maximum) for the PVT with CPC Outlet Output cell temperature (considering the temperature: temperature: (PVT with greenhouse around 50 o C around 36 o C CPC): 67 o C effect) (maximum) (maximum) Water Maximum water temperature: Maximum hot-temperature: around 35 o C water 55 o C Test 2: water	Thermal: around 50-60%, depending on the system; Electrical: around 8-10%, PV: 10.4-Electrical: 17%; 14.5% considered, 11.3%; Thermal: 83% (maximum) there is an Thermal: increase of 12% 44-51% in thermal System thermal: User-defined efficiency of the efficiency (case: 300 W/m 2 ) depending on around 20% Not directly PV panels: the system (maximum); stated 11.7% For Sweden,	PVT BIPVT BIPVT with/without CPCs for domestic and industrial applications BIPVT (roof-BIPVT (roof-integrated) integrated) PVT applications PVT with	UK Malaysia Indoor testing by means of a solar-UK simulator Canada China Sweden;
	Gang et al. (2012) [67] Chávez Urbiola and Vorobi ev [97] [68] (2017) (2013) et al. [87] mano (2016) Buono Buker et al. (2015) [77] Rosa-Clot et al. [96] en (2017) m and Andres [103] [86] Gauta (2019) [76] (2016) et al.	M E-M M E-M E M	No Yes (CR: around 52X) No No No No (CR: 1.7X)	Mono-Si Not directly stated Poly-Si Mono-Si Poly-Si directly stated Not	No Yes (thermoelectri c generators) No No No No	Water temperature: 45 o C (or higher) l/min) 45 o C Thermal energy stored in water at a temperature of 50 o C (approximately) flow rate: 6 the case temperature: 30.25 o C (mass depending on water set point temperature of 47 and 55 o C, Domestic-hot-One of the case studies: roof with an average water Maximum daily water temperatures: pneumophila (modelled) growth of Legionella 48.7 o C (experimental), concentrator: 58 o C tank: 55 o C to prevent the 49 o C (average): the PVT with At the top of the temperature (maximum) temperature for outlet	Standard electrical efficiency (PV module): 15% (maximum) respectively 9% Electrical efficiency 20% and 13.2%, winter: around 50% system): around and B: 8.8% 12%; PVT 5%; Thermal: efficiency (PVT systems A PV (reference): of the system: PV: around 15% (maximum); Overall thermal Average electrical efficiencies of (maximum) around 63% the BIPVT in Spain) Thermal: 18.9%; (maximum) around 11.4% (maximum, for electrical: Portugal, 15% Electrical: 19.1%; For PV: around electrical:	Domestic PVT with heat chiller integrated) with BIPVT (roof-Domestic applications BIPVT (roof-integrated) with desiccant Buildings and applications industrial absorber (boxed) pipes concentrators integrated) with direct flow water heating applications: reflection BIPVT (façade-trator; Domestic with diffuse-reflector/concen	Italy Mexico UK Italy Spain Denmark; China Portugal
	Evola Finocc	M E-M	No No	Poly-Si Not	No No	Outlet Maximum	Electrical: Under cooling	Industrial BA PVT	Italy Italy
	and hiaro et			directly		temperature: 40 temperature:	around 13.5%; conditions an	combined with
	Marlett al.			stated		o C (inlet around 58 o C	Thermal energy-	desiccant

With respect to PVT based on nanofluids, Wu et al.

[START_REF] Wu | Performance analysis of photovoltaic-thermoelectric hybrid system with and without glass cover[END_REF]

investigated a CPVT system with thermoelectric elements (figure

2

). Glazed and unglazed configurations were

Table 4 .

 4 PVT systems: working fluids (nanofluids, air/water, etc.) at low-temperatures.

	Study /	Type	Working	Solar	Type of	Thermoelect	Temperature	Efficiencies	Type of	Location
	year	of	fluid	concentration	PV cells	ric element	of the working		application:	of the
		study					fluid: Low		-Buildings	study
									-Etc.
	Wu et	M	Nanofluid	Yes	Not	Yes	PV cell: around	Thermal: 54.3%	Engineering	Not directly
	al.			(CRs up to 5X)	directly		53 o C	(maximum;	applications	stated
	(2015)				stated		(maximum)	theoretical)
	[104]
	Oruc et	M	Water	No	Silicon-	No	Average	Energetic	Water and/or	USA
	al.		electrolyze		based		temperature of	efficiencies of	room heating;
	(2016)		r				the electrolyte	the PVT	Powering fuel
	[105]						in the chamber:	system: 56-	cell-based
							53 o C	59%; Electrical:	cars
								around 15%
								(maximum)
	Su et	M	Bifluid	No	Not	No	Water-water	Water-water	PVT with dual	China;
	al.		(water-		directly		PVT: hot water	PVT: electrical	channels for	India
	(2016)		water or		stated		at 39.4 o C (0.1	and overall	different fluids
	[106]		air-water				kg/s); air-air	efficiencies:
			or water-				PVT: hot air at	7.8% and
			air or air-				45.6 o C (0.1	84.2%,
			air)				kg/s)	respectively
								(0.15 kg/s)
	Jarimi	E-M	Bifluid	No	Not	No	With both fluids	Electrical:	PVT-bifluid	Malaysia
	et al.		(air; water)		directly		operating (at the	around 4-4.5%,	applications
	(2016)				stated		same time),	depending on
	[107]						mean PV-cell	the case
							temperature:
							51.4 o C (air
							flow rate:
							0.0074 kg/s)
	Othma	E	Air;	No	Transparen	No	Outlet	Optimum	PVT with air	Malaysia
	n et al.		Water		t		temperature	results: 0.05	and water
	(2016)						44.6 o C (0.01	kg/s air mass	heating systems
	[108]						kg/s air mass	flow rate; 0.02
							flow rate; 350	kg/s water mass
							W/m 2 ); Outlet	flow rate; total
							temperature	thermal
							32.2 o C (0.02	efficiency
							kg/s water mass	(combi system):
							flow rate; 350	76%; average
							W/m 2 )	outlet
								temperature:
								27.4 o C;
								electrical
								efficiency: 17%
								(800 W/m 2 )
	Yazdan	M	Water;	Yes	GaAs	No	Outlet water	Total energy	PVT with	Not directly
	ifard et		TiO2/water	(CRs: 4X, 6X,			temperature:	efficiency 62%	parabolic-	stated
	al.		nanofluid	8X, 10X, 12X,			37.4 o C (0.008	(in the case

Table 5 .

 5 PVT systems without involving solar concentration: working fluids at medium temperatures.

	Study /	Type	Working	Type of	Thermoelect	Temperature	Efficiencies	Type of	Location
	year	of	fluid	PV cells	ric element	of the working		application:	of the
		study				fluid: Medium		-Buildings	study
								-Etc.
	Cristof	E-M	Water	Poly-Si	No	Solar-collector	Average: 55.5%	Polymeric PVT	France
	ari et					output: 62 o C	(thermal),	for domestic
	al.					(maximum)	12.7% (PV)	applications
	(2009)
	[115]
	Rajoria	M	Air	Not	No	Outlet air	Electrical	System	India
	et al.			directly		temperature:	(average):	appropriate for
	(2012)			stated		around 70 o C	11.3%	BI applications
	[116]					(maximum;
						case III)
	Kalogir	M	Air	Poly-Si	No	Air-gap	PV cell: 15%	BIPVT	Cyprus
	ou et					temperature
	al.					around 77 o C
	(2014)					(maximum): for
	[117]					an air gap of
						0.02 m and a
						steady flow
						velocity of 0.05
						m/s
	Herran	M	Water	Mono-Si	No	Constant hot	PV module:	Domestic	UK
	do et					water supply	15.4%	applications
	al.					temperature:	(maximum)
	(2014)					60 o C
	[118]
	Canelli	M	Water	Not	No	Temperature of	For the PVT	Buildings: PVT	Italy;
	et al.			directly		the fluid for the	system: Not	with chiller	Canada
	(2015)			stated		end user: set at	directly stated
	[119]					60 o C

Table 6 .

 6 PVT systems involving solar concentration: working fluids at medium temperatures.

	Study /	Type	Working	Concentration	Type of	Thermoelect	Temperature	Efficiencies	Type of	Location
	year	of	fluid	ratio (CR)	PV cells	ric element	of the working		application:	of the
		study					fluid: Medium		-Buildings	study
									-Etc.
	Sun	M	Air	2X	Not	No	Temperature of	Combined	PVT with CPCs	Not directly
	and Shi				directly		the PVT	efficiency		stated
	(2009)				stated		system: around	(system with
	[137]						92 o C	CPC): around
							(maximum)	75%
								(maximum)
	Cui et	E	Water	Theoretical	Mono-Si	No	PV cell: 87.7 o C	PV cell: 5.7%	PVT with CPCs	China
	al.			maximum CR:			(907 W/m 2 )	(907 W/m 2 );
	(2010)			5.1X				total efficiency
	[138]							of the CPVT
								system: higher
								than 70%
	Bernar	E-M	Water	Geometric CR:	Mono-Si	No	Water outlet	Measured	Domestic hot	Sweden;
	do et			7.8X			temperature:	electrical: 6.4%	water	Portugal;
	al.						around 77 o C	at 25 o C water	production;	Zambia
	(2011)						(maximum)	outlet	pool heating
	[139]							temperature
	Kerzm	M	Water	80X	Multi-	No	Bulk fluid	Multi-junction	PVT with	USA
	ann				junction		temperature:	cell (average):	Fresnel lenses
	and						around 62 o C	34.7%
	Schaef						(maximum;
	er						July)
	(2012)
	[140]
	Ong et	E-M	Water	CR up to 5000X	Low-	No	Deionised	Optical: around	Desalination	Switzerland
	al.				thermal		water, pre-	85%		; Egypt
	(2012)				resistance		heated to
	[141]				multi PV		around 80 °C:
					chip		evaporator
					receiver
					package

Table 7 .

 7 PVT systems appropriate for high-temperature applications.

	Study

/ year Type of study Working fluid Solar concentration Type of PV cells Thermoelect ric element Temperature of the working fluid: High Efficiencies Type of application: -Buildings -etc. Location of the study

	Zhao et (2015) Tripath	M M	Water glycol; Water;	Not directly Not directly	Crystalline Not	No No	Working-fluid high-Outlet fluid	Electrical: 22% at CR around 20.6% PV module:	PVTs with with micro-PVT with CPCs	Not directly India
	al. [191] i and Moale	M	Ethylene Molten salt Water-	stated stated 15X	silicon directly Mono-Si	No	temperature at temperature temperature: Output fluid	10.4% about 400X; (CR: 1X; 25 o C) around 12.5% Electrical:	direct concentrators Buildings;	stated Iran
	(2010) Tiwari man et		glycol ammonia		stated		the outlet (higher than 150 190 o C from the panels:	(maximum); Optical (maximum) 12.8% (average	absorption industry
	[173] (2016) al.		absorption				of the thermal °C) liquid (maximum) around	Thermal: 66% (measured): annual);
	[199] (2018)		refrigeratio				unit: 108 o C channel 130-140 o C	86.5% Thermal: 58%
	[210]		n cycle				(maximum) (maximum;	(average
	Helmer Otanic Tripath	M M M	Water Working Water	Geometric CRs: Ranging from Not directly	Multi-The values Not	No No No	At CRs higher Maximum heat-Outlet water June, July)	For CRs higher For outlet fluid Electrical: annual)	Industrial Applications PVT with CPCs	Not directly Not directly India
	Jiang s et al. ar et al. i et al.	M	Heat-fluid	Geometric CRs 10-2000X 40X to 240X or stated	Silicon-junction selected directly	No	Heat energy of than 300X, transfer-fluid temperature:	Overall optical: than 300X, temperature around 13%	Parabolic-with spectral	Not directly stated stated
	et al. (2014) (2015) (2016) Ben	M	transfer Water	up to around 9X from 50X to 10X, 20X	based for the stated Triple-	No	about thermal losses outlet around 195 o C Maximum	76.4% system overall 300 o C: thermal (maximum) Electrical: 21%	trough PVTwith filtering and Textile-industry	stated Tunisia
	(2010) [182] [192] [200] Yousse		fluid	(evaluation 600X, depending	bandgap junction		250-300 o C can are almost temperature: (maximum; outlet hot water	(maximum) efficiency: around 75% (maximum);	beam-splitting high-applications;
	[174] f et al.			parameter: 7X) on the case	refer to: Si, InGaP/InG		be achieved negligible at 600 o C mass flow rate temperature:	with the 75%; Maximum (maximum), Thermal: 62%	filter and temperature PV PVT with
	(2018)				GaAs, aAs/Ge		mean fluid 0.002 kg/s; 220 °C	optimised electrical: electrical (maximum)	evacuated-tube parabolic-
	[211]				CdTe,		temperatures up January) (midday)	splitting coating around 29% around 28%	collectors trough
					InGaP		to 160 o C	(maximum)	concentrators
	Al-Hassan	M M	Water Water;	Not directly 1X, 2X, 3X, 4X,	Not Mono-Si	No No	Tank Maximum	The COP of the One of the	PVT with Domestic	United Not directly
	Alili et Calise Renno i et al. Vossier	M M M	Water Water -Ag/water Oil	stated Not directly 100-900X 5X, 6X, 7X, 8X, For line-focus:	directly Triple-Triple-Single-	No No No	temperature CPVT outlet Maximum fluid working-fluid Maximum	hybrid solar CPVT thermal: PVT system: cases: electrical Maximum	desiccant; Desalination; Domestic Power plants	Arab Italy Italy stated USA
	al. et al. and (2016) et al.		glycol nanofluids	stated 9X, 10X 25-50X; for	stated junction junction junction		(average): temperature: outlet temperatures: temperatures:	air conditioner up to 60%; electrical 12.25%, efficiency for	domestic-hot-polygeneration applications	Emirates
	(2012) (2014a Petito [201] (2018)			point-focus:	InGaP/InG		around 99 o C 100 o C (or temperature around 100 o C 393 o C, 550 o C	is higher than electrical: around 25% thermal the high-	water with point-focus
	[175] ) [183] (2015) [212]			1000-2000X	aAs/Ge		(maximum) higher) (summer):	that of a around 20% (maximum),ther exergetic temperature PV	production parabolic-
	[193]						around 118 o C	vapour-mal around 70% 1.72% cell (for normal	concentrator
	Crisost	E	Water	10X	Front	No	Maximum	compressor-Thermal: 75%; (maximum) global solar	PVT with	Australia
	omo et Abdelh	E-M	Oil	60X	junction Thin-film	No	temperature Maximum	cycle (with Maximum PV-At the radiation):	Fresnel mirrors Power plants	USA
	al. Crisost amid et	M	Water	10X	n-type Si, GaAs, single-	No	(thermal Hybrid-outlet	PVs; solar cell (hybrid Optical: 71% maximum outlet 42.3%	for commercial Domestic or	Not directly
	(2014) omo et al.				silicon GaInP/Ga junction		output): it is collector goal: temperature:	absorption system with (maximum); temperature	and industrial industrial	stated
	[169] al. (2016) Alayi	M	Heat-	Not directly	As GaAs Not	No	expected to pressurised 365 o C Maximum	cycle) filter): 29.1% Thermal: 80% (365 o C): For the	buildings applications; CPVT with	Iran
	(2015) [202] et al.		transfer	stated	directly		reach 150 o C water at around output	(maximum) experimental simulations:	PVT with parabolic-
	Chávez [194] (2019)	E	Water fluid	55X, 95X	Crystalline stated	Yes (Bi2Te3)	Hot-plate 140 o C temperature of	Efficiencies of thermal heat exchanger	Domestic (or spectral beam trough	Mexico
	-Gomes [213]	E-M	Water	1.5X	silicon; Mono-Si	No	temperature: Collector the CPVT	the PV panel: 15% with 60%	other) Asymmetric splitting and concentrators	Sweden
	Urbiola et al.				amorphous		200 o C stagnation collector:	thermoelectric-(average) about efficiency;	applications; CPVT Fresnel mirrors for buildings
	et al. (2014)				silicon		temperature: 150 o C	generator 37%, maximum Collector fin	CPVT with collectors
	(2012) [184] Abdelk	M	Water	100X, 300X,	Triple-	No	about 120 ºC Maximum	concentrator Thermal: electrical efficiency	parabolic PVT with	Libya
	[176] ader et			500X, 700X,	junction		output fluid	systems around 59% around 8% factor: 0.7	mirrors parabolic mirror
	DeJarn al.	M	Fluid with	Not directly 900X	GaAs InGaP/InG	No	Goal: to achieve temperature:	(maximum Exergetic: (maximum);	PVT with concentrators	Not directly
	ette et (2015) Radwa Widyol	M E-M	plasmonic Water;	stated Up to 40X Geometric CR	aAs/Ge Poly-Si Double-	No No	an operating around 94 o C For CR 40X The system was	irradiance): higher than 40% Electrical: Electrical 16-Simulations:	spectral fluid PVT with point-Power plants;	stated Egypt USA
	al. [195] n et al. ar et al.		nanoparticl Heat-	50X	junction		temperature of (CR: 900X) and Re = 5, tested at 600 °C	4-17%, (at 300 o C) around 22% 19.5%, thermal Optical 64%;	filters and focus Fresnel-domestic or
	(2014) (2016) (2019)		es transfer		InGaP/Ga		300 o C outlet water (maximum)	depending on (maximum) 60.5-63%, Thermal 52%	plasmonic lens commercial hot-
	[185] [203] [170]		fluid with		As		temperature	the case depending on (at 600°C);	nanoparticles concentrators water
	Atheay	M	Water particles	Not directly	Not	No	Outlet fluid 90.2 o C	Electrical: the case (CR, Efficiency of	PVT with CPCs production	India
	Buono Liu et a et al.	M E-M	Heat-Not	Not directly Geometric CRs stated	Triple-Crystalline directly	No No	Assumption: the Operating temperature:	Electrical (in Efficiency of around 13.2% Re) the back-	PVT with Heat engine;	Not directly China
	mano et al. al. (2016) (2014) [196] Singh	M	transfer fluid directly stated Water	stated from around 3X to 30X Not directly	junction silicon stated Not	No	system operates temperature of around 130 o C up to 180 o C the thermal (maximum; Outlet water	one of the the system: (maximum; reflecting CPV cases): 19-25%; 26.5% (for solar case: partially Electrical: subsystem: 6%	parabolic dish CPVT system for indoor space with beam PVT with CPCs	stated India
	(2013a [186] and			(approximately) stated	directly		receiver: 300 o C case: glazed temperature:	optical: slightly cell operating at covered around 12%	heating/cooling, splitter and for water
	) [177] Tiwari Al	M	Organic	Not directly	stated Not	No	inverted around 110 o C Outlet	lower than 60% 25 o C) horizontal PVT-(maximum); Based on the	domestic hot linear Fresnel distillation PVT based on	Jordan
	(2017) Nimr		fluid (solar	stated	directly		absorber (maximum) temperature at	CPC); annual designed	water, reflector organic Rankine
	[204] and Al		cells		stated		partially the exit of the	instantaneous cogeneration: configuration:	electricity cycle; PVT with
	Mojiri Ammar	M	Water; immersed	CRs: from 10X to	Crystalline	No	Heat-transfer covered PVT-absorber tube:	Not directly thermal: around 22.55% PV-module	PVT rooftop parabolic-	Not directly
	Buono et al. i	M	Water Propylene in the	Geometric CR 20X	Triple-silicon	No	Maximum liquids for CPC) 130 o C	PV cell: around stated 59% efficiency	PVT with applications trough	Italy stated
	mano (2014) Riggs (2019)	M	glycol; Water fluid)	500X Up to 1800X	junction Multi-	No	operating temperatures Temperatures (maximum)	35% (maximum; PV cell: 27-around 12%;	parabolic dish with micro-Domestic hot concentrators	USA
	et al. (2013b [187] et al. [214] (2017)		Ethylene glycol		junction GaAs		temperature: higher than 100 up to 650 °C at 100 o C o C the surface of	case: glazed 33% absorber inverted efficiency	for indoor space water heating; concentrators heating/cooling, desalination;
	) [178] [205]						the thermal	absorber 64-75%	domestic hot food processing,
	Calise	M	Water	Not directly	Triple-	No	The PVT can receiver	One specific partially	water, Desalination; etc.	Italy
	et al. Valiza	M	Water	stated Not directly	junction Not	No	operate at Outlet fluid	case: electrical covered PVT-By increasing	electricity trigeneration Domestic and	Not directly
	(2014b Widyol deh et	E-M	Heat-	Geometric CR stated	Single-directly	No	100 o C Maximum temperature	20%, thermal CPC) Thermal the length of the	system; PVT PVT with CPCs industrial	USA stated
	Wu et ) [188] ar et al. al.	E	Distilled transfer	Geometric CRs: around 60X	Not junction stated	No	Aim of the (maximum) outlet (maximum):	Not directly 50% (based on (experimental): collector from 3	High-with parabolic-applications	Australia
	Mittel al. Sharaf (2017) (2019)	M M	Water water; Pure fluid	200X 10-30X Up to 2000X	Triple-directly Multi-GaAs	No Yes	Coolant outlet project: One of the temperature: around 95 o C	For coolant stated beam radiation) Multi-junction approximately to 90 m:	Desalination temperature trough PVT systems with linear	Not directly Not directly
	man et al. (2013) and [206] [215] [179] Orhan		Propylene water; (Therminol glycol; Al2O3/wate VP-1)		junction stated junction InGaP/		temperature: working fluid cases: heat-365 o C (0.1 m/s) around 210 o C up to 150 °C transfer-fluid	outlet PV cells 37%; Maximum thermal temperature: eliminate the electrical: about efficiency	applications with densely-parabolic-collectors packed trough	stated stated
	(2009) Looser (2016)	E-M	Silicone Various r	Not directly	Silicon-InGaAs/Ge	No	(maximum) for Goal: heat outlet	around 210 o C Goal: negative impact 8% 62.5% and	Beam-splitting receivers concentrators	Australia
	[171] et al. [197]		oil; commercia nanofluid;	stated	based		CR 200X production at temperature:	and CR 200X: enhancement of of high 60%; Electrical:	applications;
	(2014) [189] Lamnat ou et	M	Mineral oil l heat-transfer Al2O3/synt Water hetic oil	Geometric CR around 13X	Poly-Si	No	135 o C (approximately) around Heat 120-125 o C production: at	thermal around 59%, electrical system efficiency temperature on Annual around 20-22% conversion production: (maximum)	Buildings, etc.; PVT with micro-PV modules	France
	Calise al.	M	Water fluids nanofluid	Not directly	Triple-	No	Maximum about 140 o C:	around 19% Based on the and reduction in efficiency 996 kWh	PVT with concentrators with	Italy
	et al. (2018)			stated	junction		output Lecoeuvre et al.	beam radiation: lifetime energy (for high CRs) electricity, 2190	parabolic dish concentrating
	Otanic ar et al. (2013) [180] [207] Stanley	M E	Heat-transfer Water	Up to 200X Geometric CR	GaAs; Silicon-Silicon-	No No	Peak fluid temperature: temperature: (2018) [208] around 175 o C Receiver:	Maximum combined thermal 50%, kWh thermal costs electrical Total (for the	CPVT coupled electrothermal for indoor space solar thermal heating/cooling, Spectral beam unit	Not directly stated Australia
	(2010) [172] Calise et al. et al. (2016) Gomaa	M M	fluid Water Water	Not directly stated 42X From around 5X	based Triple-junction based Silicon-	No No	around 250 o C (summer); set point: 170 o C The system includes one-primary thermal absorber inlet Operation at	efficiency: around 32.3% 20.8% Exergy efficiency system; at 120 o C receiver One specific	model domestic hot water, Desalination; polygeneration; splitting PVT with linear Buildings;	Italy Not directly
	(2015) [190] [198] (2018) et al.			(approximately) to 46X	based		axis tracking and can operate temperature up appropriate for to 200 o C 150 o C is	for solar concentrations (summer day): around 20% 50%; Electrical: electrical, 62% temperature): case: 18%	electricity PVT with parabolic-industrial concentrators; industry	stated
	Polons ky et [209]	E	Water	Around 400-450X Triple-junction	No	PV cell: around (maximum) absorption 120 o C at 100 o C double-effect	10-50 and a band-gap 1.5-efficiency (for (relative to the Maximum about 3.8% thermal	parabolic dish collectors PVT with trough applications	Germany Israel;
	al.				GaInP/GaI		(maximum) refrigeration	2.0 eV the module): total incident	concentrators
	(2013) Mojiri	M	Water;	20X	nAs/Ge Crystalline	No	The hybrid and organic	(approximately) around PV cell power)	Rooftop	Not directly
	[181] et al.		Propylene		silicon		receiver has a Rankine cycle	(baseline):	applications	stated

ACKNOWLEDGMENTS

The authors would like to thank ''Ministerio de Economía y Competitividad'' of Spain for the funding (grant reference ENE2016-81040-R). D. Chemisana thanks ''Institució Catalana de Recerca i Estudis Avançats (ICREA)'' for the ICREA Acadèmia award. Chr. Lamnatou is Lecturer of the Serra Húnter programme. Figures 123456: reproduced with permission.


domain: sdu phys

text:
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The excitation of NH 2 in the interstellar medium

Keywords:

Accurate estimation of the abundance of the NH 2 radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH 2 by both ortho-and para-H 2 are presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the M j randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho-and para-NH 2 in collisions with ortho-and para-H 2 . The new data were then employed in radiative transfer calculations to simulate the excitation of NH 2 in typical star-forming regions such as W31C. We compared the excitation and brightness temperatures for different NH 2 transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities. As a consequence, NH 2 abundance derived from the observations will be significantly reduced by the use of the present rate coefficients.

INTRODUCTION

Observations of nitrogen hydrides are crucial to constrain the nitrogen chemistry in space since they are at the root of the nitrogen chemical network. Among nitrogen hydrides, NH2 radical is one of the key molecules to understand such chemistry. Indeed, NH2 is a light and floppy molecule thought to be an important reactant intermediate in the production/destruction of nitrogen-bearing molecules such as ammonia (NH3).

Interstellar NH2 was first detected by van Dishoeck et al. (1993) in absorption towards Sgr B2. Further, absorption lines of NH2 were observed through the use of the long-wavelength spectrometer aboard ISO (Cernicharo et al. 2000;Goicoechea et al. 2004;Polehampton et al. 2007). More recently, the Herschel Space Observatory (Pilbratt et al. 2010) with the Heterodyne Instrument for the Far-Infrared (HIFI, De Graauw et al. 2010)) offered the possibility to observe the ground-state rotational transitions with resolved hyperfine structure of NH2. NH2 is also well known in comets (Magee-Sauer et al. 1989;Swings et al. 1943), and have been observed in stellar photospheres via it's electronic, vibrationrotation, and high rotational transitions.

Moreover, NH2 exhibit spin symmetry states whose relative abundances are sensitive to the H2 ortho-to-para ratio (OPR) in the gas phase. NH2 observations may then also be able to put constraints on the H2 OPR in dense gas.

The absorption lines detected were employed to derive NH:NH2:NH3 abundance ratios of ∼ 2:1:1 in lukewarm diffuse clouds (Persson et al. 2010) and ∼ 3:1:20 in the cold envelope of low-mass protostars (Le Gal et al. 2014). Emission lines were also detected more recently in denser and hotter interstellar regions (Persson et al. 2016). Anomalous (i.e. non statistical) ortho-to-para ratios of NH2 and NH3 were derived in the diffuse gas (Persson et al. 2012;Persson et al. 2016). These ratios were however successfully reproduced by gas-phase models including a rigorous nuclear-spin chemistry (Faure et al. 2013;Le Gal et al. 2014), suggesting that NH2, just like NH3, is mainly formed in the gas-phase via a series of successive hydrogen abstraction NH + n + H2 (n = 0 -3) reactions followed by electronic dissociation recombination. A recent work has also emphasized the importance of the H-exchange reaction NH2 + H in the ortho-para conversion of NH2 (Le Gal et al. 2016).

Due to the low density of the interstellar medium (ISM) ( 10 2 -10 6 cm3), the energy levels of the NH2 molecule are expected to be out local thermodynamical equilibrium (LTE). Hence, deriving NH2 abundance from absorption and emission spectral lines requires excitation calculations that include collisional rate coefficients for transitions between the molecular energy levels induced by the most abundant ISM species, such H2. As no collisional data were available for the NH2 molecule and in order to perform non-LTE analysis of the NH2 observations, Persson et al. (2016) crudely estimated rate coefficients assuming a quenching rate coefficient of 5 × 10-11 cm 3 s -1 and state-specific downward rate coefficients for radiatively allowed transitions that scale in proportion to radiative line strengths.

However, it has been shown that the degree of excitation of molecules depends crucially on the set of collisional rate coefficients used (Roueff & Lique 2013). Consequently, inaccurate rates may induce important errors in the determination of the molecular abundance in molecular clouds. Hence, considering the sensitivity of the radiative transfer models to the collisional rate coefficients, it is very important to provide accurate values for the NH2 collisional rate coefficients.

Recently, we presented quantum close-coupling calculations for the pure rotational excitation of NH2 due to collisions with H2 molecules (Bouhafs et al. 2017, hereafter Paper I). The calculations were based on a recent, high-accuracy full-dimensional NH4 potential energy surface (PES) adapted for rigid-rotor scattering calculations. Collisional cross sections were obtained for all transitions among the first 15 energy levels of both ortho-and para-NH2 and for total energies up to 1500 cm -1 .

In this paper, we generate the first realistic set of rate coefficients of NH2 in collisions with H2 from the collisional cross sections of Paper I. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the Mj randomizing approximation for temperatures ranging from 5 to 150 K.

The paper is organized as follows. In Section 2, we briefly present theoretical approach used to generate the collisional data. Section 3 presents rate coefficients for selected rotational and hyperfine-resolved transitions in NH2 induced by collisions with ortho-and para-H2. Finally, section 4 is devoted to an astrophysical applications and the conclusions of this work are drawn in Section 5.

METHODS

The scattering calculations are based on a five-dimensional (5D) PES determined from the nine-dimensional global PES of NH4 (Li & Guo 2014). Ab initio calculations for the NH2-H2 complex in its ground electronic state were carried out at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [UCCSD(T)-F12a] (Knizia et al. 2009) level of theory with augmented correlation-consistent polarized valence-triple-ζ (aug-cc-pVTZ) basis set (Dunning Jr 1989;Kendall et al. 1992). The collision partners were considered as rigid with geometries corresponding to the ground vibrational state. We refer the reader to Li & Guo (2014) and Paper I for more details about the PES.

In order to solve the close-coupling equations for scattering, the PES was expanded over angular functions using the expression (Phillips et al. 1994) :

V (R, θ, ϕ, θ , ϕ ) = vp 1 q 1 p 2 p(R)tp 1 q 1 p 2 p(θ, ϕ, θ , ϕ ) (1)
where tp 1 q 1 p 2 p(θ, ϕ, θ , ϕ ) is formed from coupled spherical functions Y li,mi (α, β) which are associated with the rotational angular momenta of NH2 and H2.

The global minimum of the 5D PES corresponds to the most stable configuration of the NH2-H2 complex so that H2 is approaching the N atom of the NH2 molecule along the C2v axis with a depth of 213 cm -1 .

The rotational energy levels of NH2 are labelled by three numbers: the angular momentum N1 and the pseudoquantum numbers ka and kc which correspond to the projection of N1 along the axis of the least and greatest moments of inertia, respectively. Two forms (ortho-and para-NH2) caused by the different relative orientations of the hydrogen nuclear spins exist. The para states correspond to ka + kc odd and the ortho states to ka + kc even.

The calculations of pure rotational cross sections (i.e. neglecting the fine and hyperfine structure) that will be used in this paper to generate the rotational and hyperfine rate coefficients were performed with the MOLSCAT program (Hutson & Green 1994) using the close-coupling approach (Green 1975) (Paper I). Calculations were carried out for total energies ranging from 32 to 1500 cm -1 . Transitions among both the 15 lowest levels of ortho-NH2 and para-NH2 were considered. The convergence of the cross-sections with respect to the propagation parameters and with the NH2 and H2 rotational basis has been discussed in details in Paper I.

Here, from the calculated rotational cross-sections, one can obtain the corresponding thermal rate coefficients at temperature T by an average over the collision energy (Ec):

k β→β (T ) = 8 πµk 3 B T 3 1 2 × ∞ 0 σ β→β Ec e -Ec k B T dEc (2)
where σ β→β is the collisional cross section. β ≡ N1α, j2 and β ≡ N 1 α , j 2 , unprimed and primed quantum numbers label initial and final states of the molecules, j2 is the rotational angular momenta of H2, and α specifies the NH2 quantum numbers (e.g. ka, kc). µ is the reduced mass of the system and kB is the Boltzmann's constant. Calculations up to 1500 cm -1 allow determining rotation rate coefficients from 5 to 150 K. The fine/hyperfine structure of the molecule was then considered using a statistical weight approximation that is presented below. The electronic ground state of NH2 is 2 B1 with a net unpaired electronic spin of 1/2, Hence, each rotational level is split by spin-rotation interaction in a fine structure of two sublevels identified by the the total angular momentum J1 with J1 = N1 ± S (where N1 is the rotational angular momentum, and S = 1/2 is the electronic spin). As the nitrogen atoms possess a non-zero nuclear spin (IN = 1), the fine structure levels are further split into hyperfine levels, due to the coupling between the nitrogen nuclear spin and the total angular momentum J1 characterized by the quantum number F1 (F1 = J1 ± IN , IN being the nuclear spin of the first nitrogen atom). The ortho-transitions also have additional splitting due to the the coupling with the net proton spin identified by the quantum number F (F = F1 ± IH , IH = IH 1 + IH 2 being the nuclear spin of the hydrogen atom).

Since NH2 hyperfine resolved rate coefficients can be crucial in detailed models of radiative transfer and in order to obtain a first overview of the impact of collisional rates on the astrophysical models, we have estimated from the rotational collision rates the hyperfine collision rates using the so-called Mj randomizing limit. It is a statistical weight approximation which assumes that the hyperfine de-excitation rate coefficients are proportional to the degeneracy of the final hyperfine level (here, 2F + 1) and completely independent of the initial hyperfine level. Then, the de-excitation rate coefficients between hyperfine structure levels (K RAN

N 1kakcJ 1 F 1 F →N 1ka kc J 1 F 1 F
) are related to the close-coupling rate coefficients between rotational structure levels as follows (Faure & Lique 2012):

K RAN N 1kakcJ 1 F 1 F →N 1ka kc J 1 F 1 F = (2F + 1) (2IH + 1)(2IN + 1)(2S + 1)(2N 1 + 1) K CC N 1kakc →N 1ka kc (3) Were, K CC N 1kakc →N 1ka kc
are the rotational rate coefficients.

IH = H1 + H2 and IN is the nuclear spin of the hydrogen and nitrogen atom, respectively.

As the pure elastic rate coefficients K CC N 1kakc →N 1kakc are one to two orders of magnitude larger than inelastic rate coefficients, the quasi-elastic rate coefficients are set to zero, in order to avoid an overvalue for these transitions. We note also that Eq. ( 1) guarantees that the summed hyperfine rate coefficients are identical to the coupled-channel pure rotational rate coefficients.

RESULTS

We have obtained the rotational rate coefficients of NH2 in collision with both para-H2 and ortho-H2 for the first 15 rotational levels and temperatures between 5 and 150 K.

The variation with temperature of typical NH2-H2 deexcitation rate coefficients is presented in fig. 1 for both para-H2 and ortho-H2. We can notice that the rate coefficients vary smoothly with the temperature especially at temperature higher than 50 K. We also observe that the rate coefficients are generally two to three times smaller for collisions with para-H2(j2 = 0) than for collisions with ortho-H2(j2 = 1). This trend was already observed for several interstellar species like HCl (Lanza et al. 2014), SO2 (Cernicharo et al. 2011) or H2O (Dubernet et al. 2006). Even if ortho-H2 is significantly less abundant than para-H2(j2=0) in the cold ISM, the magnitude of the ortho-H2 rate coefficients makes that this collisional partner has to be included as soon as the ortho-to-para ratio (OPR) of H2 is larger than 0.1.

We can observe that the rate coefficients decrease while ∆N1 increase for both para-and ortho-NH2 in collision with both para-and ortho-H2. Although we can retrieve the predominance of the ∆N1=1 transitions over the ∆N1=2 ones, no rigorous selection rules are defined although transitions with ∆kc = 0 seem to be slightly favored (already found in Paper I).

Using the approximate Mj randomizing method described above, we have obtained inelastic rates coefficients for transitions between the first 249 hyperfine levels of ortho and 88 first hyperfine levels for para-NH2. The hyperfinestructure resolved rate coefficients display the same behavior than the rotationally inelastic rate coefficients. In particular, there is no temperature effect and the rate coefficients for collisions with para-H2 and ortho-H2 differ significantly, wich was expected since the hyperfine de-excitation rate coefficients are proportional to the degeneracy of the final hyperfine state. Therefore, the present results can be considered as a reasonable first estimate of collisional hyperfine rate coefficients, since the statistical method provide hyperfine resolved rate coefficients with an average accuracy, with respect to the memory and CPU cost of full recoupling calculations, and represents a suitable and low-cost alternative for this system. On the other hand, in terms of radiative transfer application, it should be noted that at moderate and high opacities, where the relative hyperfine populations can significantly depart from the statistical weights, the S-IOS method is notably better than the statistical approach (Faure & Lique 2012).

The complete set of (de-)excitation rate coefficients will be available online from the LAMDA (Schöier et al. 2005) and BASECOL (Dubernet et al. 2013) websites.

THE EXCITATION OF NH2

As a first simple application, we have carried out radiative transfer calculations to simulate the excitation of NH2 in interstellar molecular clouds. In particular, we focus on the physical conditions encountered in the massive star-forming region W31C that is a complex and extremely luminous submillimetre and infrared continuum source. The more highly excited ortho-lines are detected in emission from the hot cores, while the lowest ortho-and para-NH2 lines at 953 and 1 444 GHz are in absorption and arise from the envelopes surrounding the hot cores.

Non-LTE calculations were performed with the RADEX code (Van der Tak et al. 2007) using the large velocity gradient (LVG) that simplifies the resolution of the statistical equilibrium equations by assuming that the velocity gradient is large enough to decouple the radiative transfer and the rotational level populations through the introduction of a photon escape probability.

Both collisional and radiative processes are taken into account. Einstein coefficients and frequencies were obtained from the Cologne Data base for Molecular Spectroscopy (Müller et al. 2005).

We include the cosmic microwave background radiation at 2.73 K and we assume a uniform spherical geometry of the cloud. A typical line width of 8.0 km s -1 was considered.

We have chosen different kinetic temperatures (30 and 100 K) for the radiative transfer calculations. We varied the column density of otho-and para-NH2 from 10 12 to 10 14 cm -2 and we consider molecular hydrogen densities in a range from 10 2 to 10 12 cm -3 .

In the radiative transfer calculations, we include all the energy levels for which both radiative and collisional data are available.

Figs. 2 displays the excitation temperatures (Tex) and brightness temperatures (TB) of the observed transition N 1kakc J1 = 1113/2 as a function of molecular hydrogen density for temperatures of 30 and 100 K.

These hyperfine lines display classic behavior as a function of density. The excitation temperatures tend to be equal to the adopted value of background radiation field (2.73 K) at very low volume densities of molecular hydrogen. The excitation temperatures increase to higher values with increasing H2 density, and hence the rate of collisional excitation. At densities greater than a critical value, where the rates of photon and collisional excitation become equal, the excitation temperatures approach the kinetic temperature, and then the LTE approximation becomes valid.

We have also found that the column density has almost no impact on the magnitude of the excitation temperature. In fact, the three curves cannot be distinguished in the plots. This can be explained by the fact that NH2 lines are optically thin for the physical conditions considered in this work. At low temperatures (∼ 30 K), the critical density lies at ≈ 10 8 -10 9 cm -3 , while for higher temperatures, the critical density is slightly larger (up to a factor of 2).

Since the rate coefficients do not vary strongly with temperature, the critical densities are relatively independent of the temperature. These critical densities are considerably greater than typical densities in the interstellar medium. Hence, we expect that the NH2 lines are not thermalized, and hence a non-LTE analysis is required even for analysis of absorption spectra, as it was shown in the previous analysis of NH absorption lines in the pre-stellar core 16293N in L1689N (Bacmann et al. 2016). Figs 2 also show the brightness temperatures of the ortho-NH2 lines as a function of molecular hydrogen density. TB increases gradually with the density before being constant at around n(H2)=10 7 -10 8 cm -3 when LTE conditions are reached. Effect of suprathermal excitation is also seen at T kin = 100 K. This effect is due to the selection rules of the radiative and collisional transitions that will overpopulate one level with respect to another lower level and thus cause a sudden increase of the excitation temperature. The two nuclear spin symmetries of NH2 have been observed towards the high-mass star-forming regions W31C using the Herschel-HIFI instrument (Persson et al. 2016). They detected three transitions with fine and hyperfine structure resolved of ortho-NH2 (N 1kakc J1 : 2113/2 → 2023/2, 2025/2 → 1113/2, 4229/2 → 4139/2, 1113/2 → 0001/2) and one of para-NH2 (2125/2 → 1013/2). Persson et al. (2016) modeled the emission line profiles using collision rates of NH2-H2 estimated from an assumed quenching rate coefficient of 5× 10 -11 cm 3 s -1 and statespecific downward rates for radiatively allowed transitions that scale in proportion to radiative line strengths.

We were interested in the impact of the above approximate collisional rates on the excitation temperatures and peak intensity of NH2 hyperfine transitions, we have performed non-LTE radiative transfer calculations with the two sets of rate coefficients, namely the assumed data of Persson et al. (2016) and the present data.

From the observed spectra of W31C, we fix the basic parameters on which depend the LVG computation: the density of the collider H2 = 10 7 cm -3 (with an OPR =1), the kinetic temperature is kept fixed at 80 K (typical of hot core molecular clouds) and the column density of o-NH2 = 2 10 14 cm -2 . The line width being set to 8 km s -1 .

Table 1 presents excitation temperatures (Tex) of the hyperfine lines associated with different NH2 rotational lines, computed with the present set of NH2-H2 and the earlier NH2-H2 rate coefficients of Persson et al. (2016).

In all cases, the excitation temperatures are greater than that of the CMB. Thus, these lines should be seen in emission, in agreement with the observations (Persson et al. 2016). All the lines are found to be optically thin.

It can be observed that the excitation temperatures and peak intensity based on the present rates are increased up to a factor of 30 with respect to those based on the assumed rates of Persson et al. (2016). These results clearly reflect the differences between the corresponding rates and highlight the importance of using computed NH2-H2 rate coefficients.

We conclude that the present rates will significantly improve the accuracy of the NH2 abudances and H2 densities derived from the observations and changes significantly the conclusions about the physical conditions in molecular clouds.

CONCLUSION

We have reported in this paper rate coefficients for the rotational excitation of ortho-, para-NH2 by ortho-and para-H2. The lowest 15 rotational levels of ortho-, para-NH2 were included and kinetic temperatures up to 150 K were considered. Hyperfine resolved rate coefficients were also deduced using the Mj randomizing approximation. These calculations employed the PES for the interaction of NH2 with H2 computed by Li & Guo (2014) using the [UCCSD(T)-F12a] coupled-cluster method.

In order to evaluate the effect of these new data on the astrophysical modelling, we have performed radiative transfer calculations corresponding to typical physical conditions of the ISM where NH2 emissions are detected. The simulated brightness temperatures obtained with our new rate coefficients are significantly larger than the ones obtained with previous rate coefficients assumed by Persson et al. (2016). The new rates lead to an increase by a factor of 1030 of the brightness temperatures, the NH2 abundance derived from the observations will be substantially smaller than the current estimates.

It would be useful to re-analyze the astronomical observations of NH2 rotational intensities to deduce the physical conditions in interstellar molecular clouds. A more complete treatment is now possible due to the availability of accurate NH2-H2 rate coefficients to model all detected lines, in absorption and emission, in order to improve the determination of the abundances and the physical conditions. 2016)

N kakc J 1 F 1 F -→ N ka kc J 1 F 1 F Tex (K) T b 1 (K) Tex (K) T b 2 (K) T b 1 /T b

Figure 1 .

 1 Figure 1. Typical rotational de-excitation rate coefficients of oand p-NH 2 by H 2 (j 2 = 0, 1) as a function of temperature

Figure 2 .

 2 Figure2. Excitation temperature (T Ex , dashed lines), brightness temperature (T B , solid lines) for the lines of NH 2 . The H 2 volume density varies between 10 2 and 10 12 cm -3 and the NH 2 column density from 10 12 to 10 14 cm -2 by a step factor of 10. The linewidth is 8 km s -1 .

Table 1 .

 1 Comparison between excitation temperature and peak intensity for different ortho-NH 2 observed transitions obtained with the assumed collisional rates ofPersson et al. (2016) and those computed in this work.

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c XXXX The Authors

MNRAS 000, 1-5 (XXXX)

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domain: phys spi

text:
Riccardo D'agostino

J M Pouvesle

Azadeh Valinattajomran

Céline Vivien

Sébastien Dozias

Éric Robert

Material processing by atmospheric pressure plasma jets In memory of

Introduction

Plasma Enhanced Chemical Vapour Deposition (PECVD) processes have been used for decades for surface processing in a wide range of industrial applications like semiconductor films, low-k films, barrier diffusion. Thin film deposition is especially of high interest for biomedical applications for the production of protective coatings, adhesion layers, hydrophilic or hydrophobic layers. Up to now, beside DBD type treatment of textiles or food-dedicated films, most of used processes are usually realized at low pressure, as so many time beautifully reported by Riccardo D'Agostino and co-workers [1][2][3]. Actually, there is a great and increasing interest in the development of plasma sources operating at atmospheric pressure. In this talk, we will present results related with plasma polymerization, plasma deposition and plasma surface modification realized with an atmospheric pressure plasma jet (APPJ), the Plasma Gun developed at GREMI [4].

Polymer thin film deposition

The first part of this work deals with plasma polymerisation of TetraMethylDiSilOxane (TMDSO) (C4H14Si2O, 97%, Sigma Aldrich) with the AP-DBD Plasma Gun. Deposited layers are characterized by Fourier Transformed IR spectroscopy, FTIR, and contact angle measurements. The precursor used was introduced in either liquid or gaseous state. Depending on parameters like voltage, frequency, carrier gas and monomer injection, the deposited polymer appears either as a gel-like coating or a transparent film with fringes. The liquid flow rate was regulated by a peristaltic pump, while the vapour flow was ensured by bubbling nitrogen or oxygen with a fixed flow rate of 10 sccm. Microscope slides and polished Silicon wafer (100) were used as substrates. The polymerized coatings have been obtained at frequencies between 500Hz and 4 kHz, for applied voltages between 14 and 20 kV and exposure times from 1 to 10 minutes. The deposition were realized with plasma tube edge-to-substrate distances ranging from 3 to 12 mm. Profilometer measurements revealed thicknesses comprised between 500 nm and 1.5 µm at the middle of the deposit. Deposited films analyses clearly show the efficiency of this atmospheric plasma-type TMDSO polymerisation and their similarity with those usually realized under low pressure RPECVD [START_REF] Vivien | Proc., AVS 62 nd[END_REF]. The most interesting deposited films are obtained when the monomer is introduced under gaseous state, the samples clearly showing a better homogeneity. The influence of the transport gas is not evidenced. It must be stressed that a multi-spot deposition has been obtained from plasma multi-jet delivered by a single Plasma Gun.

Surface modification

Ultra-high-molecular-weight polyethylene (UHMWPE) has been widely used because of its excellent mechanical properties but is often unsuitable for use due to its poor wettability. Plasma treatment enables the surface modification of polymers without affecting the bulk material characteristics. In the present work, the Plasma Gun (PG) was applied to improve the surface properties of UHMWPE films and HDPE tubes. Surface morphology and chemical compositions were performed by means of ATR-FTIR, AFM, XPS, and contact angle measurements with the aim of understanding the surface modification effects. The single plasma jet covers only a few square millimeters, which makes it difficult for large-scale treatment of the film surface. To overcome this challenge, we have considered the use of Plasma Gun arrays consisting of many individual jets placed within close proximity of each other, such structure has a potential to greatly enhance the scale of surface treatment as compared to that of a single plasma jet. FTIR and XPS analyses, indicated that helium plasmas introduced mainly oxygen containing functional groups but also unsaturation has also been observed which has been reported to take place at the same time as surface crosslinking of the UHMWPE. The AFM observations show a surface smoothening with PG at different treatment frequencies. Furthermore, the stability of the surface properties of the UHMWPE was studied by the measure of the water contact angle As shown in Figure 1, the samples treated at higher frequency were less prone to hydrophobic recovery. According to our results, plasma gun has a potential to enhance the surface activation of inner walls of tubes and UHMWPE films. This can be very interesting for biomedical applications such as tissue tolerable polymers, inner walls of vascular grafts and catheters.

Conclusion

From the beginning, Riccardo D'Agostino has been one of the major player and expert in the domain of plasma processing of materials. He contributed to all the main scientific, technological and diagnostic [START_REF] Agostino | [END_REF] developments that led to the present status. He wrote with his team few of the founder papers including plasma etching [7], plasma deposition [1] and plasma functionalization.

In this talk, after few slides dedicated to Riccardo d'Agostino, and at the light of his inspiring work, we will present results on applications of atmospheric plasma jet material processing essentially dedicated to coating and functionalization. They show the great potential of APPJs in that domain that our recent development of large arrays of such systems clearly reinforce.

Fig. 1

 1 Fig. 1 UHMWPE contact angle: aging of plasma untreated and treated at different frequencies (12 kV, He flow 1 slm)

Acknowledgments

This work was supported in part the French network Réseau Plasmas Froids.