Imène Ferroukhi

Cécile Bord

René Lavigne

Christophe Chassard

Julie Mardon
email: julie.mardon@vetagro-sup.fr

Exploring alternative salting methods to reduce sodium content in blueveined cheeses

Keywords: salt reduction, blue cheese, food microbiota, calcium lactate, alternative methods

Reducing sodium content in food is a major public health challenge, especially for cheeses. This study investigates different salt reduction methods in surface dry-salted blue cheese and their effect on biochemical, physicochemical, microbiological, rheological and sensory characteristics of cheese. The produced cheeses were salted with two granulometries and various methods. All methods decreased sodium content in cheese core (-11% to -67%) and induced higher water activity. Sprinkling (-45% NaCl content) and calcium lactate substitution (-10% NaCl content) were technically feasible. The sanitary quality and cheeses' microbiota were not impacted except for a reduction of yeasts and moulds. Higher indexes of richness and diversity was found on salt-reduced cheeses with a dominance of Streptococcus, Lactococcus and Leuconostoc mesenteroides. Texture and appearance of the cheeses were also affected. Finally, this research identified the limits of salt reduction in blue-veined cheeses. Sprinkling and partial substitution with calcium lactate should be further studied.

Introduction

The sodium content of processed foods is considered a public health concern. There is strong evidence for a causal relationship between salt intake and blood pressure, leading to cardiovascular disease [START_REF] Doyle | Sodium Reduction and Its Effect on Food Safety, Food Quality, and Human Health[END_REF]. The World Health Organisation has suggested reducing sodium intake by 30% in order to obtain the WHO guideline of 2 g/day (i.e., 5 g of salt/day) by 2025 [START_REF] Who | Development of a Yogurt-type Product from Saccharified Rice[END_REF]. It was estimated that 75% of sodium intake comes from processed foods [START_REF] Brown | Salt intakes around the world: Implications for public health[END_REF]. In France, bread, meat products, soups and cheese are the main contributors to sodium chloride (NaCl) intake in the population [START_REF] Anses ; Bansal | Reduced-sodium cheeses: Implications of reducing sodium chloride on cheese quality and safety[END_REF].

Blue-veined cheeses appear as the saltiest variety of cheeses with salt content up to 4% [START_REF] Anses ; Bansal | Reduced-sodium cheeses: Implications of reducing sodium chloride on cheese quality and safety[END_REF]. Despite its nutritional value [START_REF] Ferroukhi | Functional changes in Bleu d'Auvergne cheese during ripening[END_REF], one serving of blue cheese may contribute up to 40% of the recommended dietary Na intake [START_REF] Guinee | 16-Reducing salt in cheese and dairy spreads[END_REF]. Thus, reducing NaCl content of blue cheese is necessary to help improve health outcomes. However, the reduction of salt is difficult to achieve because of its major functions in cheese: it modulates the physico-chemical and biochemical properties of the cheese during ripening, maintains a low a w which controls the development of micro-organisms, and is involved in the sensory characteristics of finished products [START_REF] Fox | Sensory character of cheese and its evaluation[END_REF][START_REF] Guinee | Salting and the role of salt in cheese[END_REF].

Many varieties of blue cheese are salted with an excess of salt by repeated surface application of dry NaC1 while others are brine-salted [START_REF] Guinee | Salting and the role of salt in cheese[END_REF]. Surface dry-salting process is complex and poorly studied. Dry salt is dissolved on the surface and diffuses slowly into the matrix, creating a counterflow of moisture from the centre to the outside [START_REF] Guinee | Salting and the role of salt in cheese[END_REF]. Salt diffusion depends on the intrinsic properties of the cheese, the quantity of salt added, the degree of salt rehydration on the surface and the salt granulometry [START_REF] Guinee | Chapter 13 -Salt in Cheese: Physical, Chemical and Biological Aspects[END_REF]. Consequently, studying salt reduction strategies in such salted products implies to investigate these parameters.

One of the salt reduction strategies is based on the simple reduction of the level of added NaCl (or direct decrease of NaCl) as already performed in soft and semi-hard cheeses, Feta and Cheddar cheese [START_REF] Aly | An attempt for producing low-sodium Feta-type cheese[END_REF][START_REF] Dugat-Bony | Effect of sodium chloride reduction or partial substitution with potassium chloride on the microbiological, biochemical and sensory characteristics of semi-hard and soft cheeses[END_REF][START_REF] Møller | Erratum to: Manufacture and biochemical characteristics during ripening of Cheddar cheese with variable NaCl and equal moisture content[END_REF]. [START_REF] Dugat-Bony | Effect of sodium chloride reduction or partial substitution with potassium chloride on the microbiological, biochemical and sensory characteristics of semi-hard and soft cheeses[END_REF] reported that salt reduction in soft cheese induced more sensory changes than in semi-hard cheese. In contrast, salt reduction increased the growth of spoilage agent in semi-hard cheese. Nevertheless, there is still a lack of information on reducing the salt content of other cheeses such as dry-salted cheeses. The direct reduction of salt quantities has so far been applied in cheese technologies where salting is done in coagulum and the common quantity of salt used is known. This is different from dry surface salting of blue-veined cheeses where an uncontrolled quantity of dry salt is rubbed and retained on the cheese surface.

The second strategy focuses on the total or partial replacement of NaCl by other salts.

In Cheddar, Mozzarella and Kefalograviera cheeses it has been shown that potassium chloride (KCl) can be used successfully to achieve a large reduction in sodium without adverse effect on the quality of cheeses [START_REF] Chavhan | Effect of potassium-based emulsifying salts on sensory, textural, and functional attributes of low-sodium processed Mozzarella cheese[END_REF][START_REF] Grummer | Manufacture of reduced-sodium Cheddar-style cheese with mineral salt replacers[END_REF][START_REF] Grummer | Use of potassium chloride and flavor enhancers in low sodium Cheddar cheese[END_REF][START_REF] Katsiari | Manufacture of Kefalograviera cheese with less sodium by partial replacement of NaCl with KCl[END_REF]. In contrast, other authors have reported that substitution of salt by KCl increases the bitterness of Cheddar cheese and disrupt the biochemical reactions of the cheese such as lipolysis and proteolysis [START_REF] Fitzgerald | Effect of Total and Partial Substitution of Sodium Chloride on the Quality of Cheddar Cheese[END_REF][START_REF] Lindsay | Effect of Sodium/Potassium (1:1) Chloride and Low Sodium Chloride Concentrations on Quality of Cheddar Cheese1[END_REF][START_REF] Rulikowska | The impact of reduced sodium chloride content on Cheddar cheese quality[END_REF]. However, because of hyperkalemia risks, KCl-substituted products should be consumed with caution by consumers with renal disease [START_REF] Berthet | Nutrition et insuffisance rénale chronique[END_REF]. The substitution of NaCl by MgCl 2 or CaCl 2 has been reported imparting off flavours (sour, bitter, metallic, and soapy) and decreases Cheddar cheese acceptability [START_REF] Fitzgerald | Effect of Total and Partial Substitution of Sodium Chloride on the Quality of Cheddar Cheese[END_REF][START_REF] Grummer | Manufacture of reduced-sodium Cheddar-style cheese with mineral salt replacers[END_REF]. In addition, from a nutritional standpoint MgCl 2 and CaCl 2 are acid-forming compounds [START_REF] Remer | Influence of nutrition on acid-base balance-Metabolic aspects[END_REF]. Little research has been done on the salt substitution in blueveined cheeses. One recent study used organic calcium salts to reduce the acid-forming potential of blue-veined cheeses [START_REF] Gore | Calcium lactate as an attractive compound to partly replace salt in blue-veined cheese[END_REF]. This research revealed that a partial NaCl replacement by calcium lactate induced a reduction of Na content by 19% in Fourme d'Ambert. Except for Gore et al. study (2019), calcium lactate has been used so far in calcium fortification of milk (39%), cottage cheese and yogurt (36%) [START_REF] Reykdal | Validation of chemical measures of calcium with bioassay of calciumfortified cottage cheese[END_REF][START_REF] Singh | Influence of calcium fortification on sensory, physical and rheological characteristics of fruit yogurt[END_REF]Yonis et al., 2013). Therefore, this substitute could be of nutritional interest.

In this context, it's mandatory to further explore salt reduction strategies in surface dry-salted products to provide relevant alternative process and allow blue cheeses to better fulfil nutritional guidelines. Thus, the objective of this study was to investigate the relevance of alternative salting methods to reduce the salt level in a surface dry salted blue-veined cheese. Various salting processes and salt granulometries were tested through two kinds of reduction strategies: simple reduction of NaCl quantities and partial substitution of NaCl by calcium lactate. The different effects of these salt reductions on physicochemical, biochemical, microbiological, rheological and sensory parameters of finished products were monitored.

Materials and methods

Cheese Making

Cheesemaking of a blue-veined cheese 'Bleu d'Auvergne' was carried out according to the PDO specifications and under industrial conditions (Figure S 1) [START_REF] Inao | Institut national de l'origine et de la qualité[END_REF]. Raw cow milk (about 1000 L) was pasteurised (72 °C, 30 s) and standardised for fat content and calcium chloride. Milk then was matured (36 °C, 45 min) and added with a classical starter culture (Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis, Streptococcus thermophilus) (Danisco, Danemark), hétérofermentative bacteria (Leuconostoc mesenteroides subsp. Mesenteroides) (Lallemand, France) and Penicillium roqueforti (SAS LIP, France). A quantity (30mL/100L) of liquid rennet extract (Caglificio Clerici, Italy) was also introduced shortly thereafter at 34,6°C. After that, the coagulum was cut vertically and horizontally and then stirred to form 'the cap' of the coagulum grains (3 cm 3 ). The syneresis was carried out in moulds to obtain drained curds. After the moulding, the cheese curds were dry-salted according to the various salting treatments tested. Six days after the manufacture, the cheeses were pricked. This step aims to create air channels in the core of the cheese to enable the Penicillium roqueforti to grow and colonize it. The ripening was carried out at 8 °C and 95% RH over a period of 28 days.

Salting Treatments and Sampling

Surface dry-salting of Bleu d'Auvergne are described in Table 1. To obtain anhydrous salts, all the salts were previously dried (102 °C, 24h). Two control treatments (Cc and Cf) varying in the salt granulometry (coarse salt (0.9-3.15mm) or fine salt (0.22 mm)) were applied according to the traditional method by rubbing the cheese surface with excess of salt.

Then, five methods of net reduction of NaCl quantities were tested by changing salt granulometry and application procedure (STc, SRc, STf, SRf and SP). STc and STf consisted of rolling cheese on a layer of coarse or fine salt spread on a tray. SRc and SRf were to remove the excess of salt from the surface by rubbing. The SP method was to sprinkle 100g of fine salt over the whole surface of the cheese. One method of partial substitution of NaCl with calcium lactate (S30) was carried out. The substitution (S30) was carried out using fine salt replaced with 30% of calcium lactate (0.162 mm) (Merck KGaA, Germany), in order to have a homogeneous grain size mixture. The mixture of NaCl-calcium lactate was homogenised in a mixer and placed in sealed containers until applications.

Eight blocks of cheese (2 ±0.5kg) were sampled at 28 days of ripening. The rind was cut to 5mm using a cutting wire and the core was cut into pieces and ground in a blender (Moulinex, France). In order to limit the variability due to the heterogeneity of blue cheese matrix, the core samples were homogenised by mechanical agitation and frozen at -20 °C in sealed bags until analysed. For each salting modality, one ripened cheese was held in cold storage at 1°C prior to sensory analysis. A commercial cheese (Std) was included to compare the produced cheeses to a reference, particularly for sensory evaluation. This cheese was only studied for physicochemical and sensory analysis.

Physicochemical and Biochemical Analyses

The physicochemical composition of the cheeses was analysed in compliance with ISO standards. Cheese pH was measured using a penetration pH electrode CG 840 (Schott, Mainz, Germany). Dry matter (DM) was analysed by desiccation. In compliance with the standard NF ISO 5534:2004, a weighed test portion mixed with sand was dried by heating it for 24 hours at 102 °C. The dried test portion was then weighed to determine the mass loss.

Fat content was determined using the HEISS method according to the procedure described in standard NF V 04-287:2019. In a cheese butyrometer, the proteins were first dissolved with a combination of acetic acid and perchloric acid and then separated from the fat by centrifugation. The fat content was read directly on the butyrometric scale with correction equation. Water activity (a w ) was measured in ≈ 5 g grated cheese samples at 20 °C, as per the manufacturer's instructions (HYGROLAB C1, Rotronic, Bassersdorf, Schweiz). Ratio of fat in dry matter (FDM) was calculated. Total nitrogen matter (TNM) was determined by the Kjeldahl method in compliance with the standard NF ISO 8968-1:2014. Total Nitrogen content was converted to crude protein by a factor of 6.38. Calcium (Ca) and sodium (Na) contents were measured by inductively coupled plasma optical emission spectroscopy according to NF EN 16943:2017. The samples were first digested with nitric acid and hydrochloric acid. After nebulisation, the aerosol was directed to a high frequency induced argon plasma, in which the elements were atomised and excited for irradiation. Sodium content was converted into salt percent. All measurements were performed in triplicate.

Microbiological analyses

Cheese samples (25 g) were homogenised in a stomacher for 2 min with 250mL of peptone water (BagMixer CC, France). Serial dilutions (10 -7 ) and the sowing were made with spiral method (EasySpiral Dilute, interscience, France). Total microflora was counted according to NF EN ISO 4833-2:2013. Leuconostocs were enumerated on Mayeux, Sandine and Elliker agar (MSE), Lactococcus on M17 agar and Lactobacillus on MRS agar according to NF EN 15787.Yeasts and moulds were counted with NF V 08-059. Enterobacteriaceae were also screened on VRBG agar. Cell counts on media were performed in triplicate.

In order to investigate microbial communities in blue cheese, a metagenomic analysis was performed on cheeses after 28 days of ripening. DNA was extracted from the cheeses (Penicillium-free zone) using the FastDNA® SPIN Kit for Soil (MP Biomedicals, Illkirch, France). DNA was quantified using Qubit Fluorometric Quantitation (Invitrogen, Thermo Fisher Scientific, Waltham, MA) method. Amplification and sequencing were carried out in the V3-V4 region of the 16S rDNA gene. Amplicons were sequenced with Illumina technology in 2x250bp. Amplicon data from high-throughput sequencing was analysed using the rANOMALY pipeline [START_REF] Theil | rANOMALY: AmplicoN wOrkflow for Microbial community AnaLYsis[END_REF], which relies on the dada2 R package to produce amplicon sequence variants (ASVs) as taxonomic units. A decontamination step was carried out based on prevalence of contaminant ASVs, as identified in the blank samples, and on DNA concentration, as described by the decontam package included in the rANOMALY pipeline. Taxonomic assignment of bacterial sequences was based on two databases, i.e. DAIRYdb v2.0 and SILVA 138, keeping the assignment with the highest confidence or the deepest taxonomic rank [START_REF] Meola | DAIRYdb: A manually curated reference database for improved taxonomy annotation of 16S rRNA gene sequences from dairy products[END_REF][START_REF] Quast | The SILVA ribosomal RNA gene database project: Improved data processing and web-based tools[END_REF]. The filtered ASVs count table was used to perform statistical analyses.

Sensory Analyses

A panel composed of 8 assessors from the sensory analysis laboratory of higher education and research institute VetAgro Sup was selected according to standard ISO 8586-1:2012 guidelines. These panellists had previous experience in the evaluation of dairy products. In order to identify the differences between cheeses and to classify the product groups in a sensory space, a flash profile was conducted. It is a fast descriptive method, based on the selection of a combination of free choice terms and a comparative ranking evaluation [START_REF] Dairou | A Comparison of 14 Jams Characterized by Conventional Profile and a Quick Original Method, the Flash Profile[END_REF][START_REF] Delarue | Rapid Sensory Profiling Techniques: Applications in New Product Development and Consumer Research[END_REF]). It's a method to study the degree of similarity between samples and provides a relative sensory positioning (mapping) of the samples.

The evaluation was conducted in two sessions. A slice of cheese (50 g) was presented on plates coded with random 3-digit numbers. All cheeses were served at 20 ± 1 °C, presented monadically and distributed according to a Williams Latin Square designed to take into account the first effect of order and carry-over based on the simultaneous presentation of the whole samples set. During the first session (2.5-hr), subjects selected their own terms to describe and evaluate 9 blue-cheeses simultaneously: Eight cheeses salted with various processes presented in Table 1 and one commercial blue cheese as a standard. Between each sample, panellists were asked to rinse their mouth successively with unsalted crackers and tepid water to remove fatty residual. At the end of this session, a discussion with panellists allowed to finalise the list of descriptors generated by each panellist. During the second session, all samples were presented again to the assessors who were asked to rank each product on a ranking scale (rank 1 = least intense, rank 9 = most intense). For the final session, assessors evaluated the cheeses again and ranked them, using the generated list of descriptors. All evaluation sessions were led in computerised booth according to ISO standard 8589:2010. Data was collected using Tastel software® (version 2011; ABT Informatique, Rouvroy-sur-Marne, France).

Texture Measurement

The texture parameters Hardness (N ), adhesiveness ( N.s ) , cohesiveness and gumminess were determined using a rheometer (Kinexus pro+, rSpace for Kinexus 1.61 Software, Malvern Instruments, Malvern, UK), by carrying out the texture profile analysis (TPA) method described by [START_REF] Nishinari | Parameters of Texture Profile Analysis[END_REF]. Three cylindrical samples (2 cm of diameter) were randomly collected from each cheese. Each sample was held at 20 °C for 1 h before analysis and was maintained at 20 °C during measurement with the Peltier heating element. Samples were compressed to 20% of their original height, using two compression cycles at a constant crosshead velocity of 50 mm/s². The texture parameters were calculated according to [START_REF] Henneberry | Interactive effects of salt and fat reduction on composition, rheology and functional properties of mozzarella-style cheese[END_REF].

Statistical Analyses

Statistical analysis of all data was performed using XLSTAT software 2020 (Addinsoft, Paris, France). Results are reported as a means ± standard deviation. Shapiro-Wilk test (p <0.05) was used to check the normality of the data. A non-parametric test (Kruskal-Wallis) and a post-hoc comparison (Conover-Iman procedure) were applied to all data (Except for sensory data) as it did not have a normal distribution. Differences between mean values were considered significant at P <0.05. Regarding sensory data, the Generalised Procrustes Analysis (GPA) was used for the configuration of the consensus between the judges' sensory maps. GPA calculates a consensus from data matrices of an experiment in sensory profiling and allows comparing the proximity between the attributes that are generated by panellists. For the Flash profile, one data matrix corresponded to each judge.

Results and Discussion

As salt intake is a major public health problem, it is essential to implement actions to reduce salt in foods [START_REF] Anses ; Bansal | Reduced-sodium cheeses: Implications of reducing sodium chloride on cheese quality and safety[END_REF]. Blue-veined cheeses represent an important source of salt due to their specific salting methods usually performed in an excess of salt. For the first time, this work investigates the relevance of different surface dry salting methods to reduce salt content in blue-veined cheeses. In other words, this research was done in order to screen a high number of salting methods to identify the most relevant alternatives that will be considered in blue-veined cheese technology.

Effect of Reducing Salt Content on Physicochemical and Biochemical Composition

The physico-chemical and biochemical composition of blue-veined cheese cores salted by different methods is presented in Table 2. Applied salt reduction methods have impacted all parameters of the ripened cheeses (P <0.05), except for mean fat and total nitrogen content of 28 and 19 g/100g respectively (data not shown).

The pH obtained for all the cheeses in this study was comparable to the pH found in Bleu d'Auvergne [START_REF] Bord | Impact of heating on sensory properties of French Protected Designation of Origin (PDO) blue cheeses. Relationships with physicochemical parameters[END_REF][START_REF] Duval | Microbial dynamics in industrial blue veined cheeses in different packaging[END_REF][START_REF] Ferroukhi | Functional changes in Bleu d'Auvergne cheese during ripening[END_REF]. pH values did not greatly change in the coarse salted cheeses. In contrast, Cf and S30 fine salted cheeses had a higher pH than others with pH of 6.08 and 6.28, respectively. Among the tested salting processes, a significant difference was observed in the a w values. Cf cheese showed the lowest a w (0.954) compared to the salt-reduced and the coarse-salted cheeses. A high a w was also observed in salt-reduced Cheddar cheese [START_REF] Mccarthy | Effect of salt and fat reduction on the composition, lactose metabolism, water activity and microbiology of Cheddar cheese[END_REF][START_REF] Rulikowska | The impact of reduced sodium chloride content on Cheddar cheese quality[END_REF]. The dry matter of the studied cheeses did not vary with salt reduction or granulometry. However, it was observed that SRc cheese had a low dry matter content (51.89%), related to high a w . However, the commercial cheese Std showed a higher dry matter than other cheeses and a low a w , probably related to a high salt content.

Regarding salt content and salt/moisture, there was no significant difference between Cc (1.70% of NaCl) and Cf (1.68% of NaCl) control cheeses salted with two different salt grain sizes. The opposite observations have been made in Fourme d'Ambert cheese, where cheeses salted with fine salt had a higher salt content than cheeses salted with coarse salt [START_REF] Gore | Calcium lactate as an attractive compound to partly replace salt in blue-veined cheese[END_REF]. This reflects that the change in salt granulometry did not induce a change in sodium content in the cheese core, i.e. coarse and fine salt penetrated the cheese in the same way. Nevertheless, there was more variability in cheeses salted with coarse salt than in cheeses salted with fine salt. This suggests the difficulty of controlling dry surface salting with coarse salt. The commercial cheese (Std) contained a high salt content (3.5% in salt and 6.4% in salt/moisture) compared to other cheeses. This elevated salt value is probably due to a double dry-salting process which is a common practice in Bleu d'Auvergne cheese making, explaining the difference reported in salt content. In order to assess the impact of new applied salting methods and limit the variability, cheeses from this research were salted only once.

In coarse salted cheeses, tray salting (STc) and rubbing surface salting (SRc) processes induced a high reduction of sodium contents (28 and 67%, respectively) compared to control (Cc). Rubbing off the excess salt on the surface produced a significant reduction in salt, particularly with coarse salt SRc, indicating that coarse salt is greatly removed if it is rubbed off. However, it has been observed that rubbing the wet surface of the cheese may deteriorate the rind and heterogeneously remove a significant salt quantity. For cheeses salted with fine salt, STf and SRf cheeses were also reduced by 21 and 17% salt compared to control cheese Cf. The tray salting method (STc and STf), which aimed to control deposition of salt on each side of cheese, gave a close reduction of salt for both granulometries. The disadvantage of this technique (STc or STf) was the time required to salt the full cheese surface. The objective was to find solutions for the cheese industry, this method could control and reduce the amount of salt but may be difficult for a large-scale production. The sprinkling treatment also led to a significant salt reduction (-45% of salt) compared to control (Cf). Calcium lactate substitution reduced sodium content by 10% (P<0.0001). A significant decrease (-19%) in sodium has been reported by [START_REF] Gore | Calcium lactate as an attractive compound to partly replace salt in blue-veined cheese[END_REF] in Fourme d'Ambert cheese upon the substitution of salt with 75% calcium lactate. For the same substitution ratio of salt, other authors obtained a higher reduction using KCl in Coalho and Akkawi cheeses [START_REF] Costa | Manufacture of reduced-sodium Coalho cheese by partial replacement of NaCl with KCl[END_REF][START_REF] Kamleh | The effect of partial substitution of NaCl with KCl on the physicochemical, microbiological and sensory properties of Akkawi cheese[END_REF]. In the dry-salted Feta cheese, the substitution of NaCl by KCl (3:1 and 1:1) did not affect the salt content [START_REF] Katsiari | Reduction of sodium content in Feta cheese by partial substitution of NaCl by KCl[END_REF]. We hypothesised that the low salt reduction resulting from calcium lactate substitution in this study is probably due to the heterogeneity of the cheese. The second salting process, as commonly used in the salting of blue cheese, could probably increase the salt reduction level. Nevertheless, only one study previously tested salt replacement in dry salting on the surface of blue-veined cheeses [START_REF] Gore | Calcium lactate as an attractive compound to partly replace salt in blue-veined cheese[END_REF].

Salt reductions influenced Ca content of cheese cores (P<0.0001). Cheeses salted with fine salt had a relatively higher amount of calcium than cheeses salted with coarse salt. Blueveined cheeses are a category with a complex matrix resulting in the variability of composition parameters such as salt [START_REF] Ferroukhi | Functional changes in Bleu d'Auvergne cheese during ripening[END_REF][START_REF] Gkatzionis | Diversity and activities of yeasts from different parts of a Stilton cheese[END_REF].

Difference in Ca content might be related to the heterogeneity of the matrix which causes variable mineral migration during ripening.

The calcium lactate-substituted cheese had the highest calcium content (606.6 mg/kg) and this was 21 and 13% higher than the control Cf and the standard Std cheese. Considering the Ca level brought by S30 treatment, a 40g serving of cheese would attain 25.5% of recommended daily calcium intake [START_REF] Anses ; Bansal | Reduced-sodium cheeses: Implications of reducing sodium chloride on cheese quality and safety[END_REF]. This salt has already been studied to fortify Cottage cheese and yoghurt with calcium [START_REF] Reykdal | Validation of chemical measures of calcium with bioassay of calciumfortified cottage cheese[END_REF][START_REF] Who | Development of a Yogurt-type Product from Saccharified Rice[END_REF]Yonis et al., 2013). This may highlight the nutritional interest of this salt in cheese making.

The level of calcium in the cheese depends on the Ca content of the milk but also on mineral exchange after salting. It was reported that the salt on the surface enters inside the cheese, causing a counterflow of whey which exists towards the surface loaded with minerals such as calcium (Y. [START_REF] Graet | Migration des macro et oligo-éléments dans un fromage à pâte molle de type Camembert[END_REF][START_REF] Graet | Migration du calcium et des phosphates inorganiques dans les fromages à pâte molle de type Camembert au cours de l'affinage[END_REF]. However, salt diffusion into the cheese matrix (and conversely Na and Cl migration) is limited by some cheese characteristics, such as free water, viscosity of the aqueous phase, tortuosity and porosity [START_REF] Guinee | Salting and the role of salt in cheese[END_REF][START_REF] Guinee | Sodium chloride and moisture changes in Romano-type cheese during salting[END_REF] This might be the reason for the difference in calcium content in obtained cheeses.

Effect of Reducing Salt Content on the Microbial Communities

Table 3 shows the counts of total microorganisms, lactic acid bacteria (LAB), yeasts and moulds for cheeses salted with different salting methods. The microbial composition of cheeses was significantly impacted by the salting processes (P<0.05) (Table 3). Total counts were not different in the salt-reduced cheeses. A comparable effect was observed in a dry salted cheese (São João cheese) reduced in salt content [START_REF] Soares | The effect of lowering salt on the physicochemical, microbiological and sensory properties of São João cheese of Pico Island[END_REF].

As opposed to other studies of salt reduction in Cheddar [START_REF] Rulikowska | The impact of reduced sodium chloride content on Cheddar cheese quality[END_REF][START_REF] Schroeder | Reduction of Sodium Chloride in Cheddar Cheese: Effect on Sensory, Microbiological, and Chemical Properties1[END_REF], there were no marked effects of salt treatment among all experimental Bleu d'Auvergne cheeses in the total bacterial count. Aerobic mesophilic bacteria should mainly correspond to lactic acid bacteria (LAB) added after milk pasteurisation, as expected levels of total aerobic mesophilic bacteria were found in experimental and control cheeses.

Nevertheless, the Lactococcus counts were lower in the Cf and STf cheeses. This might be due to test or sample variability during the analysis. In this study, Enterobacteriaceae were not found (below the required count) suggesting that the salt reductions implemented had no impact on the sanitary aspect of the cheeses. A similar result has been reported in salt-reduced cheese [START_REF] Soares | The effect of lowering salt on the physicochemical, microbiological and sensory properties of São João cheese of Pico Island[END_REF].

Yeasts and moulds (YM) were also affected by salting methods. In the coarse-salted cheeses, the salt-reduced cheeses contained a lower YM counts than control cheeses which had 9.00 log cfu/g. Cheeses salted with fine salt showed a lower YM counts than coarse salted cheeses except for S30 which had the highest level of YM (9.26 log cfu/g). A large proportion of cheeses contained a lower YM count than found in Bleu d'Auvergne cheese [START_REF] Duval | Microbial dynamics in industrial blue veined cheeses in different packaging[END_REF]. Moreover, opposite observations were reported on Halloumi and Cheddar cheeses substituted with up to 50% KCl, which had no impact on yeast and mould counts [START_REF] Kamleh | The effect of substitution of sodium chloride with potassium chloride on the physicochemical, microbiological, and sensory properties of Halloumi cheese[END_REF][START_REF] Reddy | Microflora of Cheddar Cheese Made with Sodium Chloride, Potassium Chloride, or Mixtures of Sodium and Potassium Chloride[END_REF]. The development of the distinctive flavours of blue cheese is dominated by metabolism of moulds during ripening. Salt in the moisture and opening of the matrix by Leuconostocs, which produces CO 2 through the citrate metabolism, lead to the growth of Penicilium roqueforti in the cheese [START_REF] Marth | Applied Dairy Microbiology[END_REF]. The low salt levels obtained in this study could be responsible for the low growth of yeasts and moulds.

A total of 354620 16S rRNA (V3-V4 regions) sequencing reads were generated from the 8 cheese samples, covering different salting methods. The reads were assigned to 36 ASVs for which taxonomic assignment was possible down to species level (or group of species sharing almost identical sequences) in most cases. These ASVs belonged to 21 different bacterial species (Figure 1) and four different bacterial phyla (Firmicutes, Actinobacteriota, Proteobacteria and Bacteroidetes). In all cheeses analysed, Streptococcus, Lactococcus and Leuconostoc mesenteroides species were dominant with a raw abundance sum of 123966, 127126 and 27923 reads, respectively. In a previous study, a dominance of these starter culture species was observed in Bleu d'Auvergne cheese [START_REF] Ferroukhi | Functional changes in Bleu d'Auvergne cheese during ripening[END_REF] and other blue-cheeses [START_REF] Caron | Strong effect of Penicillium roqueforti populations on volatile and metabolic compounds responsible for aromas, flavor and texture in blue cheeses[END_REF][START_REF] Flórez | Microbial diversity and succession during the manufacture and ripening of traditional, Spanish, blue-veined Cabrales cheese, as determined by PCR-DGGE[END_REF]. This bacterial dominance could explain why the bacterial abundance of non-starter LAB strains was extremely low, as these bacteria occur at higher levels only in later stages of ripening [START_REF] Blaya | Symposium review: Interaction of starter cultures and nonstarter lactic acid bacteria in the cheese environment1[END_REF]. Also species of Brevibacterium and Brachybacterium genus were present with a relative abundance compared to the others of 0.1 and 3%, respectively (Table S 1). Enterobacteriaceae species were detected in all samples with a relative abundance of less than 0.5%. In line with the microbial count, the low percentage of Enterobacteriaceae indicates that the different salting methods did not negatively impact the sanitary quality of the products. Lactococcus and Leuconostoc species showed a higher percentage in the saltreduced cheeses, especially in the SP and S30 cheeses. This is supported by the results obtained with microbial enumeration. In contrast, Streptococcus was lower in the salt-reduced cheeses. Another Leuconostoc species was detected only in SP cheese with 0.011% relative abundance.

In Table S 2, community richness and diversity were estimated for each cheese using four alpha diversity parameters (Observed, Chao1, Shannon and Simpson). The values of richness indicators were higher in the salt-reduced cheeses. SP and S30 cheeses had Observed and Chao1 indices of 27 and 28, respectively, for both indicators (P<0.05). The same observations were made on bacterial diversity with Shannon and Simpson's index (Table S 2).

The control cheeses had the same richness and diversity values. Species dissimilarity between cheeses (Beta-diversity) was assessed using the Bray-Curtis method. The results showed that species present in the cheeses were similar (p>0.05) and the salt reduction did not affect the type of species present in the cheese.

Effect of Reducing Salt Content on Rheological and Sensory Parameters

Table 4 shows the means for the TPA parameters studied in Bleu d'Auvergne cheese at 28 days of ripening. Hardness, cohesiveness and gumminess parameters were not significantly affected by the salting methods. This finding is consistent with other salt reduction investigations in Cheddar and Feta cheeses [START_REF] Fitzgerald | Effect of Total and Partial Substitution of Sodium Chloride on the Quality of Cheddar Cheese[END_REF][START_REF] Katsiari | Reduction of sodium content in Feta cheese by partial substitution of NaCl by KCl[END_REF]. However, SRc and STf cheeses tended to have a lower hardness value. The variability in the matrix of blue-veined cheeses could account for the non-significant effect on the hardness of cheeses. Adhesiveness was significantly lower in control cheeses Cc and Cf than in salt-reduced cheeses. STc, SRc and STf cheeses had the highest adhesiveness (-0.539, -0.472 and -0.501, respectively) SP and S30 had a lower adhesiveness value than the other salt-reduced cheeses. Similarly, an increase in adhesiveness due to salt reduction has been reported by [START_REF] Henneberry | Interactive effects of salt and fat reduction on composition, rheology and functional properties of mozzarella-style cheese[END_REF] in Mozzarella cheese. These authors explained that the increase in adhesiveness during ripening is related to the increased hydration of caseins which should reduce the cohesive (attractive) forces with the calcium phosphate para-casein network.

The sensory analysis was carried out using a flash profile in order to assess the sensory differences between different salt reductions applied to blue cheeses compared to control cheeses and a standard commercial cheese. The panellist used different terms to characterise the cheeses according to appearance, texture, aroma, and taste. The number of attributes varied between 8 and 12, with an average of 10 attributes for each participant and a total of 74 terms of differentiation (Figure S 2). Most of the sensory attributes obtained in this study were also generated in other blue-veined cheeses [START_REF] Gkatzionis | Effect of Yarrowia lipolytica on blue cheese odour development: Flash profile sensory evaluation of microbiological models and cheeses[END_REF] and Camembert cheese [START_REF] Galli | Lactobacillus rhamnosus GG improves the sensorial profile of Camembert-type cheese: An approach through flash-profile and CATA[END_REF] analysed by flash profile. The consensus index (Rc) was 0.639 (63.9%) with a high residual percentage (>100%). Std cheese had the lowest residual percentage (83.4%), this could suggest that the commercial cheese had a complex descriptors from other cheeses and a less homogeneous consensus among the panel for this product. This was different from the consensus and higher total variance reported by [START_REF] Gkatzionis | Effect of Yarrowia lipolytica on blue cheese odour development: Flash profile sensory evaluation of microbiological models and cheeses[END_REF].

The low consensus among the testers demonstrated in the test may be due to the difficulty in ranking of the product caused by the high number of cheeses and the variability of their matrix.

The Generalised Procrustes Analysis (GPA) analysis of the Flash profile data provided the relative positioning of the samples based on generated attributes. The plots defined by the first two factors of the GPA analysis (Figure 2) explained a medium percentage of the total variance (50.48%; 29.10% and 21.38% for F1 and F2 respectively). The distribution of products in the Factorial Map was heterogeneous (Figure 2 (a)). Generally, good discrimination between cheeses was observed. The first axis discriminated the coarse control cheese Cc and commercial cheese Std with sprinkled cheese SP on one side and the fine salt control cheese Cf, the substituted cheese S30 and rubbed cheese SRc with other reduced salt cheeses (STc, SRf and STf) on the other side.

Looking at the attribute map (Figure 2 (b)), among the 74 descriptors generated, the attributes of creamy, marbling and odour intensity were found by many judges and were grouped on the biplot. Bitterness, salty and firmness also appeared in some panellists.

The results showed that texture was impacted by salt reductions. The control cheese Cc and the standard commercial cheese Std were characterised by a firm texture whereas the salt reduced cheeses STc, STf and SRf were characterised by a creamy texture. This result was in agreement with the hardness and adhesiveness index previously found in the texture of these cheeses. A number of studies have demonstrated the impact of salt on the structure and texture of cheeses [START_REF] Pastorino | Effect of Salt on Structure-Function Relationships of Cheese1[END_REF][START_REF] Schroeder | Reduction of Sodium Chloride in Cheddar Cheese: Effect on Sensory, Microbiological, and Chemical Properties1[END_REF]. Regarding the marbling and odour intensity descriptors, it seemed that this character was more present in SP and Std cheeses. This indicated that marbling quantity was impacted by salt reduction (in agreement with microbiological results), but also related to the heterogeneity of cheese slices distributed to panellists. A within-product variability, due to blue-veined heterogeneity distribution, was already observed during the sensory analysis of this type of cheese [START_REF] Bord | Impact of heating on sensory properties of French Protected Designation of Origin (PDO) blue cheeses. Relationships with physicochemical parameters[END_REF].

Bitterness was found in the S30 and both the Cc and Cf cheeses controls but this was not representative of many judges. Bitterness in cheese was often related to salt levels. [START_REF] Mistry | Influence of Salt on the Quality of Reduced Fat Cheddar Cheese1[END_REF] showed a decrease in bitterness with increasing salt in Cheddar cheese.

However, the bitterness found in our study was not necessarily associated with salt (-10% of sodium (g/100g) in S30) but instead, with a higher pH in these cheeses, especially in S30 (6.28). [START_REF] Lee | Preparative Methods of Isolating Bitter Peptides from Cheddar Cheese[END_REF] have reported a high pH in the bitterest cheeses.

Furthermore, a significant level of calcium (such as in S30 cheese) contributes to bitterness [START_REF] Engel | Taste Active Compounds in a Goat Cheese Water-Soluble Extract. 2. Determination of the Relative Impact of Water-Soluble Extract Components on Its Taste Using Omission Tests[END_REF]. Contrary to other salt substitutions with MgCl 2 or CaCl 2 used in the salting of Cheddar cheese [START_REF] Grummer | Manufacture of reduced-sodium Cheddar-style cheese with mineral salt replacers[END_REF], the impact of the calcium lactate salt substitution on off-flavour was not found in this study.

During this investigation, salty taste was generated only by five panelists and did not characterise the commercial cheese Std. It has been reported that food structures impacts the sensory perception of salt, through changes in salt release rate and availability [START_REF] Busch | Sodium reduction: Optimizing product composition and structure towards increasing saltiness perception[END_REF][START_REF] Tournier | The impact of mastication, salivation and food bolus formation on salt release during bread consumption[END_REF]. An important relationship between food texture and masticatory behaviour of subjects was also established [START_REF] Kohyama | Effects of Sample Thickness on Bite Force for Raw Carrots and Fish Gels[END_REF]. For example, the quantity of salt released from a soft cheese is higher than that released from a hard cheese [START_REF] Phan | In vivo sodium release related to salty perception during eating model cheeses of different textures[END_REF]. Distribution of other sensory attributes was relatively variable and heterogeneous between judges, suggesting that salting methods have impacted sensory quality of products (notably texture and marbling development) and this effect was differentially perceived between panellists. In summary, in this study, the salting methods had a clear effect on cheese texture with a higher adhesiveness index and a creamy appearance but had little effect on other sensory aspects.

Conclusion

We investigated for the first time the relevance of different alternative salting processes to reduce the salt level in a surface dry-salted blue cheese. All the salting methods applied significantly reduced the salt content without altering the sanitary quality of the cheeses. However, several tested methods showed their technical non feasibility, especially the application of coarse salt which increases variability in blue cheeses. Calcium lactate appeared to be a good salt substitute with a salt reduction. Calcium lactate has proven to be a substitute for salt with a 10% reduction in salt and a 21% increase in calcium. Given the variability and heterogeneity of blue-veined cheeses, it would be interesting to validate the effect of calcium lactate as a salt substitute on the quality of these cheeses. Regarding the sensory characteristics, a good discrimination between cheese samples and the identification of the most salient sensory attributes of all samples was achieved. Texture, appearance and certain sensory characteristics of the cheese, such as odour, were highly influenced compared to the salty perception. The bitterness was not representative and there was no off-flavour in this result. This exploratory study clearly demonstrated the technical feasibility of various salt reduction methods and their impact on the quality of blue cheese. Sprinkling and calcium lactate substitution methods are a practicable technique that could be improved and applied at large scale. Finally, this research has screened a large number of salting methods in order to identify the most relevant alternatives that can be further investigated in the salting of blue cheeses. Table 1. Salting methods

Tables

Figure 1 .

 1 Figure 1. Microbial composition of produced cheeses. The vertical bars indicate the percentage of relative abundance of the corresponding bacterial sequences at species level. A: Total bacterial composition in each cheese, B: Bacterial composition of the subdominant population (without Streptococcus species, Lactococcus species and Leuconostoc mesenteroides) in each cheese.

  Figures

FiguresFigure S 1 .

 1 Figures

Figure S 2 .

 2 Figure S 2. Number of sensory descriptors generated by the judges

Table S 2

 S Number of reads and diversity indices obtained from blue-cheese cores. 767

	Cc		STc	SRc	Cf	STf	SRf	SP	S30
	Reads	36011	33303	36738	35558	31761	33554	35227	41566
	Observed ASV's	20	24	22	22	22	23	27	28
	Chao1	20	24	22	22	22	23	27	28
	Shannon	1.09	1.28	1.25	1.09	1.2	1.21	1.37	1.3
	Simpson	0.57	0.64	0.64	0.57	0.61	0.62	0.64	0.65
	P-Value	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01

Acknowledgements

We thank S. Alvarez, K. Fayolle and D. Guerinon (UMR 545 Fromage, Vetagro Sup), for their valuable technical assistance. We also thank B. Desserre for biomolecular analysis help and S. Theil for the bioinformatics support. This work was supported by FEDER (European Regional Development Funds) in the framework of the call for proposals Pack Ambition Recherche 2018 in the Auvergne-Rhône-Alpes region (France). It was also co-financed by the Syndicat Interprofessionnel Régional du Bleu d'Auvergne (SIRBA) et de la Fourme d'Ambert (SIFAM). We also thank the Pôle Fromager AOP Massif Central for its assistance.

Yonis, A. a. M., Elzamzamy, F. M., & Elmorsi, S. A. (2013). FORTIFICATION OF BANANA STIRRED YOGURT WITH CALCIUM. Journal of Food and Dairy Sciences, 4(5),[183][184][185][186][187][188][189][190][191][192] https://doi.org/10.21608/jfds.2013.71826

STf 517.5 ± 1.5 e 535.9 ± 5.5 d 1.3 ± 0.0 d 2.5 ± 0.1 cd 5.4 ± 0.0 a 53.1 ± 0.6 a 0.966 ± 0.003 c SRf 484.5 ± 2.3 d 559.9 ± 5.3 d 1.4 ± 0.0 d 2.7 ± 0.0 d 5.8 ± 0.0 de 52.5 ± 0.0 a 0.968 ± 0.003 c SP 467.1 ± 2.5 cd 369.5 ± 1.6 b 0.9 ± 0.0 b 1.8 ± 0.0 b 5.9 ± 0.0 f 52.4 ± 0.1 a 0.964 ± 0.006 c S30 606.6 ± 3.5 f 609.5 ± 25.8 e 1.5 ± 0.1 e 2.9 ± 0.1 e 6.3 ± 0.0 h 52.7 ± 0.3 a 0.980 ± 0.001 d Std 523.5 ± 28.5 e 1415.0 ± 15.0 g 3.5 ± 0.0 g 6.4 ± 0.0 g 5.8 ± 0.0 ef 55.


Sonja Kallio

Monica Siroux
email: monica.siroux@insa-strasbourg.fr

Exergy optimization of a multi-stage solar micro-cogeneration system

This paper proposes a dynamic model of a solar-based micro-cogeneration system called photovoltaic-thermal (PVT) collector to perform a design optimization of the multi-stage PVT system. The parametric study reveals the most important design parameters influencing the water-based flat-plate PVT system performance. The analysis also shows an existing trade-off between thermal and electrical efficiencies during the PVT operation. A novel exergy-based multi-objective design optimization method is demonstrated to find a trade-off design solution of the multi-stage PVT collector which compromises between the electrical and thermal exergy efficiencies under different weather conditions. The electrical and thermal exergy efficiencies are defined to be the objective functions of the Matlab gamultiobj-function, which is a multi-objective evolutionary algorithm using nondominated sorting genetic algorithm-II (NSGA-II). As a result of the algorithm, the Pareto optimal sets were derived that revealed the optimal solutions taking into account the tradeoff nature of the optimization problem. The decision-making method called an ideal point method was used in the decision-making process to find the final optimal solutions for different weather conditions. The results revealed that the optimal number of the PVT collectors in series depended on the weather conditions and decreased from 3 to 2 if the conditions got cooler.

INTRODUCTION

The building sector has a high impact on greenhouse gas emissions and final energy consumption in the European Union (EU) by being the largest energy end-use sector with a share of 41% [START_REF] Doroudchi | Approaching net zero energy housing through integrated EV[END_REF]. Renewable energy production is getting a stronger role in energy production. Micro combined heat and power (micro-CHP), or co-generation, systems are operating in the building sector to produce simultaneously from a single fuel source heat and power at high efficiency and close to the consumption point. The systems are used to produce on-site energy for space heating, domestic hot water (DHW) and electricity demand. The European Parliament has defined the micro-cogeneration to be the units up to an electrical output of 50 kW [START_REF] Martinez | Numerical investigation of energy potential and performance of a residential building-integrated solar-chp system conference: System simulation in buildings[END_REF]. However, micro-CHP commonly refers to units up to 15 kW of electrical power [START_REF] Murugan | A review of micro combined heat and power systems for residential applications[END_REF]. The renewable energy-based micro-CHP systems are in a key role in reaching the primary energy and pollutant emissions reduction targets of the EU [START_REF] González-Pino | Modelling and experimental characterization of a Stirling engine-based domestic micro-CHP device[END_REF].

Solar energy can be counted to be an inexhaustible source of energy, and its use does not depend on the energy markets. At the moment, solar energy is mostly deployed in the building sector in terms of photovoltaic (PV) panels and thermal collectors [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]. This separated production of electricity and heat can be combined by the technology called photovoltaic-thermal (PVT). The PVT collector is a renewable solar-based microcogeneration system that produces electricity by the PV module and useful heat by cooling the PV module with a coolant circulation. That leads to increased overall system efficiency but also an increasing electrical efficiency due to the decreased operation temperature of the PV module [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]. Zhang et al. [START_REF] Zhang | Review of R&D progress and practical application of the solar photovoltaic/thermal (PV/T) technologies[END_REF] argued that the most commonly used and installed PVT technologies are air-and water-based collectors. The popularity of the waterbased PVT collector is increasing due to its ability to increase the electrical efficiency and better thermal energy utilization in DHW and space heating. The highly dynamic and weather dependent operation of a single PVT collector is widely studied in the literature, and a range of theoretical PVT models has been introduced and validated in [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF][START_REF] Calise | Experimental and numerical analyses of a flat plate photovoltaic/thermal solar collector[END_REF][START_REF] Chow | Performance analysis of photovoltaic-thermal collector by explicit dynamic model[END_REF][START_REF] Bhattarai | Simulation and model validation of sheet and tube type photovoltaic thermal solar system and conventional solar collecting system in transient states[END_REF][START_REF] Kallio | Energy analysis and exergy optimization of photovoltaic-thermal collector[END_REF][START_REF] Khelifa | Approach for the modelling of hybrid photovoltaic-thermal solar collector[END_REF]. However, the N number of the PVT collectors can be connected in series to increase the coolant outlet temperature and produce more thermal energy at a higher quality than from a single collector. This kind of multistage PVT collector can be used in several applications. In the building sector, the multi-stage PVT collector can be integrated with energy storages, such as thermal storage, battery systems and electrical vehicles (EV). On the other hand, the multi-stage PVT collector is useful for other applications as well. Sahota and Tiwari [START_REF] Sahota | Review on series connected photovoltaic thermal (PVT) systems: Analytical and experimental studies[END_REF] conducted a review on N series-connected PVT systems. They explored different PVT applications, such as flat plate PVT collectors with solar still and greenhouse dryer. Liang et al. [START_REF] Liang | Performance evaluation of sheetand-tube hybrid photovoltaic/thermal (PVT) collectors connected in series[END_REF] studied the performance of PVT collectors connected in series. They performed a numerical and experimental study, in which six PVT collectors were connected in series resulting in a total amount of 36 collectors. They analysed coolant fluid outlet temperature, PV module temperature, and electrical and thermal efficiencies. Dubey and Tiwari [START_REF] Dubey | Analysis of PV/T flat plate water collectors connected in series[END_REF] studied flat plate water PVT collectors in series and calculated energy and exergy yield by varying the number of collectors under four different weather conditions in India. They discovered that increasing the mass flow and the number of collectors increased thermal energy gain but only until a certain point.

The performance of the multi-stage PVT system can be analysed in terms of energy and exergy analysis. The energy analysis concerns only the quantity aspects related to the energy production of the system neglecting the quality aspect. For this reason, the exergy method is used to complete the energy assessment of the system. The exergy method takes into account the quality of the produced energy as an ability to work at the temperature, at which a certain energy flow is available. Evola and Marletta [START_REF] Evola | Exergy and thermoeconomic optimization of a water-cooled glazed hybrid photovoltaic/thermal (PVT) collector[END_REF] performed an exergy-based optimization of a flat plate water-based PVT system. They concentrated on optimizing the coolant inlet temperature in order to maximize the overall exergy efficiency of the system at any operating condition. They discovered that low inlet temperature resulted in very high overall energy efficiency, but in terms of exergy, this is not recommended. The optimized inlet temperature and overall exergy efficiency varied between 30-42 • C and 14-15%, respectively, depending on the mass flow rate and the weather conditions. Tiwari et al. [START_REF] Tiwari | Exergy analysis of N-photovoltaic thermal-compound parabolic concentrator (N-PVT-CPC) collector for constant collection temperature for vapor absorption refrigeration (VAR) system[END_REF] conducted an overall exergy analysis of the N-PVT compound parabolic concentrator collector for constant collection temperature. The system was applicable to the absorption refrigeration system. They concluded that using an exergy approach in design optimization is highly recommended.

In recent years, the conventional correlation-and simulationbased design optimization methods have got alongside new design methods using nature-inspired artificial intelligence methods, such as genetic algorithm (GA), evolutionary algorithm and particle swarm optimization (PSO). The multiobjective optimization techniques are used to solve optimization problems where the objectives have a trade-off with each other. In terms of the PVT collector performance, the electrical efficiency suffers if the thermal efficiency is maximized and the other way round. The energy-based multi-objective optimization of the water-based PVT collector performance parame-ters with the non-dominated sorting genetic algorithm (NSGA-II) was performed in [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF][START_REF] Tamayo | Multi-objective optimization of hybrid photovoltaic-thermal collectors integrated in a DHW heating system[END_REF]. The Pareto front was obtained for different optimization problems to illustrate the trade-off between solutions. The multi-objective optimization of the airbased PVT system with the NSGA-II algorithm was investigated in [START_REF] Ouhsiane | Multi-objective optimization of hybrid PVT solar panels[END_REF]. The single objective optimization of the PVT collector with the genetic algorithm was performed in [START_REF] Sobhnamayan | Optimization of a solar photovoltaic thermal (PV/T) water collector based on exergy concept[END_REF] to maximize the overall exergy efficiency.

Within the given framework, there is a lack of studies considering the multi-objective optimization of the number of collectors in a PVT series based on the genetic algorithm and exergy approach. Additionally, there is a research question if the optimal number of the PVT collectors varies according to the weather conditions.

This paper proposes a dynamic model of a PVT collector with a crystalline silicon roll-bond collector in Matlab/Simulink. The model is extended to describe a multi-stage PVT collector. Energy and exergy analysis is conducted with the simulation model, and the most influencing design variables are identified and evaluated with a parametric study. Due to fact that the optimal design of the multi-stage PVT system cannot be defined with the simulations alone, the multi-objective optimization of the electrical and thermal exergy efficiencies with the non-dominated sorting genetic algorithm (NSGA-II) is conducted and the Pareto optimal set is derived in order to show the set of the optimal solutions taking into account the trade-off between exergy efficiencies.

The rest of the paper is organized as follows. In the second section, the description of a single PVT collector is presented with the geometrical, thermos-physical and optical properties followed by the modelling approach of a single collector. The main governing equations of the model are presented. Additionally, the exergy approach is introduced. The multi-objective optimization method is presented in the third section with the objective functions. In the fourth section, the results of the parametric study and optimization are presented and discussed. Finally, the main conclusion is drawn in the fifth section.

Literature review

In this section, more background on research of the PVT collectors is provided. The current water-based PVT technologies face a challenge in which the electrical efficiency decreases as the operating temperature increases. On the other hand, higher operating temperature increases thermal efficiency. In [START_REF] Han | Investigation of CoSO4-based Ag nanofluids as spectral beam splitters for hybrid PV/T applications[END_REF][START_REF] Walshe | Nanofluid development using silver nanoparticles and organicluminescent molecules for solar-thermal and hybrid photovoltaic-thermal applications[END_REF], novel PVT technologies were investigated to overcome this problem. The solution was to decouple thermally the electrical and thermal units of the collector using an optical coolant fluid layer above the PV modules. This layer worked simultaneously as a spectrum splitter of solar radiation and as a coolant fluid of the system, and its spectral properties were modified using nanofluids. The layer absorbed ultraviolet radiation and part of infrared radiation and transmitted to the PV modules only the useful infrared and visible radiation for electricity production. The results were promising, and the spectrum splitting increased the efficiency of the PVT collector.

The multi-stage PVT collector is useful for multiple applications where distributed micro-scale energy production is required, such as desalination. Giwa et al. [START_REF] Giwa | Polygeneration in desalination by photovoltaic thermal systems: A comprehensive review[END_REF] reviewed comprehensively the polygeneration in different desalination approaches using flat plate and concentrated PVT systems. The integration of the PVT systems with desalination showed increased efficiency. From the economical point of view, the installation costs were higher than PV systems. However, the PVT systems provided cost reduction and reduced environmental impacts in the long term. Anand et al. [START_REF] Anand | A review on solar photovoltaic thermal integrated desalination technologies[END_REF] presented as well a review on PVT integrated desalination technologies. They concluded that the overall performance of the PVT coupled desalination system is better than coupling with separated energy production by PV and solar thermal collectors.

In terms of serial connection, an interesting approach is to connect a flat plate PVT collector in series with another type of solar technology, such as a flat plate solar thermal collector or concentrated PVT to increase thermal and electrical output. Ma et al. [START_REF] Ma | Photovoltaic thermal module solar thermal collector connected in series to produce electricity and high-grade heat simultaneously[END_REF] studied a novel series connection of a PVT and solar thermal collector (PVT-ST) to produce electricity and heat at an even higher quality level. They compared the serial connection to a glazed and unglazed PVT and a PV panel. As a result, the PVT-ST system was competitive in terms of electrical efficiency and had significantly higher thermal and primary energy savings efficiency. Kazemian et al. [START_REF] Kazemian | Optimization of a novel photovoltaic thermal module in series with a solar collector using Taguchi based grey relational analysis[END_REF] conducted a similar study of the PVT-ST system compared to other solar technologies and used the multi-objective Taguchi method to optimize operation variables. They generated 3D-models of each system by SolidWorks software to conduct the study. The results indicated 19% higher daily thermal power of PVT-ST compared to a PVT system but the electrical power was equal.

The exergy analysis is an effective way to investigate the true potential of an energy system. In terms of PVT application, Sarhaddi [START_REF] Sarhaddi | Exergy analysis of a weir-type cascade solar still connected to PV/T collectors[END_REF] conducted an exergy analysis of a PVT connected solar still system. The results of the study showed that two PVT collectors in series improved the most the overall exergy efficiency of the PVT-solar still system.

METHODOLOGY

Description of the PVT collector

A water-based PVT collector with a direct flow geometry and roll bond channels is simulated, analysed and optimized in this study. Figure 1 shows the geometry and cross-section of the PVT collector [START_REF] Barbu | Analysis of a residential photovoltaicthermal (PVT) system in two similar climate conditions[END_REF]. The water-based PVT collector is investigated because they achieve higher overall efficiency than airbased collectors due to the higher heat capacity of water [START_REF] Aste | Design, modeling and performance monitoring of a photovoltaic-thermal (PVT) water collector[END_REF].

High efficient PV cells are still relatively expensive and in this study, the PVT collector has a crystalline silicon PV cell with a reference electrical efficiency (η STC ) of 18.7% at the reference operating temperature (T ref ) of 25 • C. The temperature coefficient (β T ) and packing factor (r c ) are 0.5%/K and 0.95, respectively. The diameter of the tubes is 8 mm and for the initial simulation conditions, the number of tubes is 11. The initial inlet temperature of the coolant fluid is 20 • C and the coolant mass 1 summarizes the main geometrical, thermo-physical, optical properties and the parameters of the single PVT collector used in the simulation and optimization.

The described single PVT collector is multiplied to present a multi-stage PVT collector, in which N number of collectors is connected in series in order to solve a problem of low-temperature heat production of a single PVT collector. The working principle of the multi-stage collector is shown in Figure 2. The coolant mass flow is fed into the first PVT collector, and heat is recovered. The outlet temperature of the first collector is the inlet temperature of the second collector. The coolant temperature is increased by each collector in series, and the thermal exergy efficiency is increased until a certain point.

In reality, the electrical efficiency would be the highest in the collectors at the beginning of the series and the lowest at the end of the series because the temperature of the coolant flow increases towards the end of the series. However, due to simplicity, a global model of the multi-stage PVT collector is used to solve the optimization problem of this study. The model does not calculate the coolant temperature after each PVT or the collector specific electrical efficiency. However, the area, thermal mass and the tube length of the collector array are always increased or decreased by the geometry of the single collector described above.

Modelling approach for energy and exergy analysis

The numerical modelling of a single water-based PVT collector is based on the following main governing equations of the

FIGURE 2

The working principle of the multi-stage PVT collector energy balance in the various layers of the collector. The more detailed derivation of the governing equations and the validation of the model is presented in [START_REF] Kallio | Energy analysis and exergy optimization of photovoltaic-thermal collector[END_REF] where the used heat transfer coefficient are defined as well

The glass cover (g):

m g × c g × d T g dt = h g-e,CV × A × ( T e -T g ) + h g-e,RD × A × ( T sky -T g ) + h g-pv,CV × A × ( T pv -T g ) + h g-pv,RD × A × ( T pv -T g ) + A × 𝛼 g × G irr (1)
The photovoltaic layer (pv):

m pv × c pv × d T pv dt = h g-pv,CV × A × ( T g -T pv ) + h pv-g,RD × A × ( T g -T pv ) + h pv-a,CD × A pv-a × ( T a -T pv ) + h pv-t ,CD × A pv-t × ( T t -T pv ) + G irr × A × (𝛼𝜏) pv -G irr × A × r c × 𝜂 pv(T ) (2)
In which r c is the ration of the cell area to the PVT collector area, in the other words, packing factor. The electrical efficiency η pv(T) depends on the photovoltaic layer temperature T pv . Due to this, the electrical efficiency of the PV depends linearly on the temperature T pv , the temperature coefficient β pv and on the efficiency at standard conditions T ref . The efficiency is calculated according to the following relation:

𝜂 pv(T ) = 𝜂 STC × [ 1 -𝛽 pv × ( T pv -T re f )] (3)
Additionally, the photovoltaic layer temperature T pv depends on the following parameters based on the geometry of the collector [START_REF] Chow | Performance analysis of photovoltaic-thermal collector by explicit dynamic model[END_REF][START_REF] Kallio | Energy analysis and exergy optimization of photovoltaic-thermal collector[END_REF]:

A pv-a = A ( 1 - D o W ) (4) h pv-t ,CD × A pv-t = H pv × L W ×k pv 2 + H adh k adh × H pv D o
(5)

m pv = 𝜌 pv × H pv × A × r c ( 6
)
The absorber layer (a):

m a × c a × d T a dt = h a-pv,CD × A a × ( T pv -T a ) + h a-t × A a-t × (T t -T a ) + h a-i,CD × A a-i × (T i -T a ) (7)
The tube bonding (t):

m t × c t × d T t dt = h t -pv,CD × A pv-t × ( T pv -T t ) + h t -a × A a-t × (T a -T t ) + h t -i,CD × A t -i × (T i -T t ) + h t -f × A t -f × ( T f -T t ) (8)
The insulation layer (i):

m i × c i × d T i dt = h a-i,CD × A a-i × (T a -T i ) + h t -i,CD × A t -i × (T t -T i ) + h i-e,CD+CV × A × (T e -T i ) (9)
The coolant (f):

m f × c f × d T f dt = h t -f × A t -f × ( T a -T f ) + ṁ × c f × ( T f ,in -T f ,out ) (10)
Equations ( 1)-( 7) are implemented into Matlab/Simulink with details to assess the energy and exergy performance of the PVT collector. In the case of multiple PVT collectors, the area, thermal mass of each layer and the tube length of a single panel are multiplied by the number of the collectors in order to assess the performance of the PVT series.

Compared to energy analysis, the exergy analysis is based on the Second Law of Thermodynamics and takes into account the quality of the produced energy.

The exergy balance of the system can be expressed as follows:

∑ Ex in - ∑ (Ex th + Ex el ) = ∑ Ex d ( 11
)
In the case of the PVT collector, the exergy flow to the system Ex in comes from the solar irradiation. However, solar irradiation is not seen as pure exergy and due to this a conversation coefficient is included in the calculation of the PVT incoming exergy [START_REF] Farzanehnia | Exergy and Its Application-Toward Green Energy Production and Sustainable Environment[END_REF]:

Ex in = A × N c × G irr × ( 1 - 4 3
T 0 T sol + 1 3
( T 0 T sol ) 4 ) (12)
In Equation ( 9) T sol is the solar temperature and T 0 is the reference temperature, which according to the literature, should be a constant, for example, T 0 = 20 • C, T 0 = 25 • C or the minimum temperature of the month. In this study, T 0 = 20 • C is selected. N c is the number of collectors.

Electrical energy is seen as pure exergy, but the exergy content of thermal energy depends on the temperature at which the heat is made available. In the model, the exergy of thermal (Ex th ) and electrical (Ex el ) energy are calculated as follows, respectively [START_REF] Farzanehnia | Exergy and Its Application-Toward Green Energy Production and Sustainable Environment[END_REF]:

Ex th = ṁ × c f × [ (T out -T in ) -T 0 × ln T out T in ] ( 13
)
Ex el = 𝜂 pv × G irr × r c × A × N c ( 14
)
By Equations ( 8)-( 11) the both thermal (ξ th ) and electrical exergy (ξ el ) efficiencies and the overall exergy efficiency (ξ) of the PVT collector can be calculated as follows:

𝜉 th = Ex th Ex in ( 15
)
𝜉 el = Ex el Ex in [START_REF] Tiwari | Exergy analysis of N-photovoltaic thermal-compound parabolic concentrator (N-PVT-CPC) collector for constant collection temperature for vapor absorption refrigeration (VAR) system[END_REF])

𝜉 = Ex el + Ex th Ex in ( 17
)
The following assumptions are taken into account in the modelling:

• The temperature distribution is uniform in the layers and in the PVT collectors in series. • It is assumed that there are no heat losses through the edges.

• The optical and thermal properties of the materials and fluids are constant. • No surrounding shading or dust is taken into account.

• The thermal resistance between the layers is negligible.

MULTI-OBJECTIVE OPTIMIZATION OF THE PVT COLLECTOR

The energetic and exergetic performance analyses based on the simulations of the thermal PVT model show a trade-off between performance parameters, such as thermal and electrical energy and exergy efficiencies as functions of various decision variables. The simulation results have shown a trade-off between thermal and electrical exergy efficiencies of the PVT collector. The electrical efficiency of the PV panel decreases when the operating temperature of the collector increases. On the other hand, increased operating temperature leads to higher thermal exergy efficiency. This means that both efficiencies cannot be directly maximized at the same time but a solution has to be found, which satisfies both trade-off objectives (electrical and thermal exergy efficiencies) at the same time. The thermal simulations of the PVT system alone do not provide a set of optimal solutions of decision variables in terms of thermal and electrical exergy efficiencies. Instead of simulations alone, the set of optimal solutions can be found by using the multi-objective optimization (MOO) technic using an evolutionary algorithm.

Multi-objective optimization using gamultiobj-function

In this paper, the Matlab function called gamultiobj of Global Optimization Toolbox is used to run the multi-objective optimization of the multi-stage PVT collector. The gamultiobjfunction creates a set of optimal solutions in the space of decision variables on the Pareto front. The function uses a controlled, elitist genetic algorithm, which is a variant of NSGA-II [START_REF] Deb | Multi-Objective Optimization using Evolutionary Algorithms[END_REF]. Generally, the multi-objective optimization problem of two or more trade-off objectives is described mathematically as follows [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]: In this formulation, the objective functions are minimized or maximized subject to certain constraints.

Minimized∕Maximized f m (x) , m = 1, 2, … , M ; Subject to g j (x) ≥ 0, j = 1, 2, … , J ; h k (x) = 0, k = 1, 2, … , K ; x i (L) ≤ x ≤ x i (U ) , i = 1, 2, … , n;
In this work, the overall electrical and thermal exergy efficiencies are two objectives to be optimized. Due to the fact that the NSGA-II is a minimization tool the objective functions are multiplied by (-1) in order to get the maximum values [START_REF] Tamayo | Performance evaluation and multi-objective optimization of hybrid photovoltaic-thermal collectors[END_REF]. The 2-dimensional optimization problem of the exergy efficiencies is formulated as follows:

To minimize:

f 1 (x) = - Ex el Ex in = - 𝜂 STC × [ 1 -𝛽 PV × ( T pv -T re f )] × G irr × A pv × N c A × N c × G irr × ( 1 - 4 3 T 0 T sol + 1 3 ( T 0 T sol ) 4 ) = -𝜉 el (18) f 2 (x) = - Ex th Ex in = - ṁ × c f × [ (T out -T in ) -T 0 × ln T out T in ] A × N c × G irr ( 1 - 4 3
T 0 T sol + 1 3 ( T 0 T sol ) 4 ) = -𝜉 th ( 19
)
where ξ el and ξ th are the electrical and thermal exergy efficiencies, respectively. The electrical exergy efficiency is influenced by the number of the PVT collectors in terms of the thermal mass and tube length according to Equations ( 2)-( 6).

In terms of exergy efficiency, the sensitivity analysis has shown that the most sensible variables of the multi-stage-PVT collector operation are the cooling-water mass flow, inlet temperature and number of collectors. Table 2 presents the decision variables to be optimized and their upper and lower bounds to be used in the optimization.

RESULTS AND DISCUSSION

The simulation-based multi-objective optimization process was conducted with the PVT model in Matlab/Simulink. The main objective of the optimization was to find an optimal number of PVT collectors in series to satisfy the operational purpose of the multi-stage PVT collector. First, the decision variables were varied, and the initial population and score matrixes for the genetic algorithm were generated by the simulation model. These matrices were given for the gamultiobj-function of Global Optimiza- tion Toolbox, which started the iterative process by evaluating the fitness functions with the simulation model and making selection, crossover and mutation of the population. Based on this process, a new population or generation was created. The new generations were created as long as a stopping criterion was met. The criterion was selected to be the number of generations and set to be 100 generations. The population size was 50. The multi-objective optimization process is described in Figure 3.

The optimization was conducted with the weather data concerning three steady-state climate conditions: hot summer day conditions, of which solar irradiation was 1000 W/m 2 , ambient temperature 30 • C and wind speed 1 m/s; medium conditions with 800 W/m 2 , 25 • C and 1 m/s; and cooler summer conditions with 600 W/m 2 , 18 • C and 5 m/s. According to [START_REF] Eborx | European Solar Irradiation[END_REF], these weather conditions can be addressed for different European locations, such as the southern Mediterranean, central and northern Europe, respectively.

Figure 4 presents the simulated temperatures of the different PVT collector layers, ambient temperature and solar irradiation over a summer day. 5,6. The relation of the PVT collectors in series to the efficiencies and outlet water temperature is presented in Figures 789. The mass flow rate had a similar relation to the thermal energy and exergy efficiencies. Both efficiencies decreased when the mass flow increased. However, if the number of the collectors and coolant inlet temperature were increased, the thermal energy and exergy efficiencies behaved differently. This is shown in Figure 8.

Figure 5 shows the trade-off nature of the PVT collector operation. Increasing the mass flow rate of the coolant fluid, increased the electrical exergy efficiency due to the stronger cooling effect. However, at the same time, thermal exergy effi- 2 presents the selected decision variables of the optimization problem.

Figure 6 reveals that increasing the inlet temperature of the coolant fluid increased the thermal exergy efficiency but decreased thermal energy efficiency. Additionally, the electrical efficiency suffers when increasing the inlet temperature because of the higher operating temperature.

Figure 7 presents the increasing coolant outlet temperature and decreasing electrical efficiency when the number of PVT collectors in series was increased. The electrical efficiency was decreased because of the higher operating temperature of the multi-stage PVT collector. Increasing the temperature level of the coolant outlet flow with additional PVT collectors leads to the higher quality of the produced thermal energy. However, there is a certain optimal point to increase the number of collectors although the outlet temperature increases. Figure 8 presents the thermal exergy and energy efficiencies with the additional PVT collectors.

The thermal energy efficiency decreased if the number of collectors was increased due to the increased total collector area and its relation to the gained outlet temperature. However, the exergy approach takes into account the quality of the produced energy and the additional collectors increased the thermal exergy efficiency until a certain point. This point was also influenced by the coolant inlet temperature. Figure 9 presents the thermal exergy efficiency in terms of the inlet temperature and the number of collectors in series.

Figure 9 reveals that the lower coolant inlet temperature required more PVT collectors in series to achieve higher thermal exergy efficiency. If the inlet temperature was increased, the relatively smaller number of the collectors led to higher thermal exergy efficiency.

The parametric study revealed the relations of the decision variables to the performance of the multi-stage PVT system. It revealed the trade-off nature of the PVT operation in terms of thermal and electrical performance. The design of the multistage PVT collector was optimized in terms of the thermal and electrical exergy efficiencies as objective functions in order to maximize the quality of the produced energy. The optimization was performed by using gamultiobj-function of Matlab Global Optimization Toolbox. The function is based on NSGA-II algorithm. The set of optimal solutions called Pareto front was obtained with the generation number of 100 and the population size of 50. Figure 10 presents the obtained Pareto front of the optimal solutions under the medium weather conditions of 800 W/m 2 , 25 • C and 1 m/s.

The decision variable values are shown for three selected solutions. The values of each solution are presented in Table 3. The values of the decision variables show the same behaviour as in the parametric study. This can be seen in Figures 11,12 where each optimal solution of Table 3 is presented in terms of electrical and thermal exergy efficiency.

In Figure 11 the electrical exergy efficiency was the highest in the optimal solutions with the high coolant flow and low number of the PVT collectors. The solutions with the high inlet temperature resulted in the lowest electrical exergy efficiency.

In Figure 12 the thermal exergy efficiency was the highest if the optimal solution had the high inlet temperature and high number of the PVT collectors in series.

The results revealed that the number of collectors in the set of optimal solutions varied from 1 to 9. The higher number of the collectors resulted in the higher thermal exergy efficiency and the lower number to the higher electrical efficiency. The Pareto 3 in terms of the electrical exergy efficiency FIGURE [START_REF] Sahota | Review on series connected photovoltaic thermal (PVT) systems: Analytical and experimental studies[END_REF] The set of optimal solutions from Table 3 in terms of the thermal exergy efficiency front showed a trade-off of the different solutions in which the electrical exergy efficiency varied from 15.95% to 18.8% and the thermal exergy efficiency from 1.2% to 5.5%. If electricity production is the priority of operating the multi-stage PVT system, the decision-maker can select the solution with the higher electrical exergy efficiency by increasing the coolant mass flow rate and decreasing the inlet temperature. On the other hand, the higher thermal exergy efficiency can be achieved by increasing the number of the PVT collectors and increasing the inlet temperature. The Pareto front does not show a single optimal solution but a set of solutions presented in Figures 101112that can be used to help decision-makers to design the optimal multistage PVT system depending on the purpose.

Figure 13 presents the overall exergy efficiency of each solution in Table 3 according to Equation [START_REF] Tamayo | Multi-objective optimization of hybrid photovoltaic-thermal collectors integrated in a DHW heating system[END_REF].

The overall exergy efficiency varied from 20.03% to 21.87% between the solutions. The results revealed a solution which FIGURE [START_REF] Liang | Performance evaluation of sheetand-tube hybrid photovoltaic/thermal (PVT) collectors connected in series[END_REF] The set of optimal solutions from Table 3 in terms of the overall exergy efficiency maximizes the overall exergy efficiency by having the inlet temperature of 31 • C, 4 collectors in series and the mass flow rate of 0.0269 kg/s. This solution results to electrical, thermal and overall exergy efficiencies of 17.1%, 4.77% and 21.87%, respectively.

The decision-making process is required to select the final optimal solution from the set of optimal solutions. A hypothetical point called an ideal point can be used to help the decisionmakers [START_REF] Fan | A multi-objective design optimisation strategy for hybrid photovoltaic thermal collector (PVT)-solar air heater (SAH) systems with fins[END_REF]. At this point, marked in Figure 10, both objectives have their independent optimal values, but it is not a feasible solution in the multi-objective optimization and not located on the Pareto front. However, the final optimal solution might be the closest point of the Pareto front to the ideal point. In this case, Solution B in Figure 10 would be the final optimal solution if the decision-maker does not have any particular desire to maximize electrical or thermal exergy efficiency within the Pareto front optimal solutions. On the other hand, in the case of PVT collectors, the decision makers can select the final solution which maximizes the overall exergy efficiency showed in Figure 13. However, in this case, the ideal point method in Figure 10 did not result in the same solution but to the solution with only 0.3% lower overall exergy efficiency.

The optimization was performed under three different weather conditions cold, medium and hot to see an impact of the conditions on the Pareto optimal solutions. In Figure 14 are presented three Pareto fronts under different weather conditions.

Figure 14 shows that the set of optimal solutions was different depending on the weather conditions. In terms of exergy efficiency, it seems to be beneficial to select the number of PVT collectors in series depending on the weather conditions of the system location. However, the different Pareto fronts had also an area where they overlap each other. This combined area of the Pareto front rather maximizes the electrical exergy efficiency than thermal exergy efficiency within the objective space. The highest electrical exergy efficiency of 19.1% was reached under cold conditions and the highest thermal exergy efficiency of 7% was reached under hot conditions. The weather conditions had FIGURE [START_REF] Dubey | Analysis of PV/T flat plate water collectors connected in series[END_REF] The Pareto optimal front for the electrical and thermal exergy efficiencies under three different steady-state weather conditions an impact on the range of optimal solutions in the objective space. The cold conditions resulted in the smallest range while the hot conditions had the widest range.

By using the ideal point method of the decision making, the three final optimal solutions of each weather condition were selected and marked in Figure 11. The inlet temperature varied from 24 to 31 • C. The number of PVT collectors was decreased from 3 to 2 if the weather conditions were changed from hot to cold. The selected optimal solutions revealed that under the colder weather conditions the multi-stage PVT collector had better electrical performance compared to the optimal solution under medium and hot conditions. In this case, it can be reasonable to couple the multi-stage PVT collector with electric storage and use heat as a by-product. On the other hand, under hotter weather conditions the reasonable coupling application can be a heat-driven desalination system.

CONCLUSION

In this paper, solar cogeneration based hybrid energy system was modelled into Matlab/Simulink environment. The parametric study was conducted and the optimal sizing problem was solved using NSGA-II algorithm. The ideal point method was used in the decision-making process to select the final optimal solution from the Pareto front for three different weather conditions.

The dynamic model of the water-based multi-stage PVT system was optimized using gamultiobj-function of Matlab Global Optimization Toolbox which is based on the fast elitist multiobjective optimization algorithm NSGA-II. The electrical and thermal exergy efficiencies were used as objective functions because of their trade-off nature in the PVT operation which leads to difficulties for decision-makers to select the electrical and thermal efficiencies and the design variables. In this study, the selected decision variables were the coolant mass flow rate, number of PVT collectors and coolant inlet temperature to the system. The results showed that multi-objective optimization based on the genetic algorithm has a high potential when designing the multi-stage PVT system. The used multi-objective optimization approach finds the Pareto front which is a set of optimal solutions in the objective space. This set can be used by decision-makers to find the final optimal solution.

The main findings of the parametric study and multiobjective optimization were:

• Increasing the inlet temperature of the coolant fluid increased the thermal exergy efficiency but decreased thermal energy efficiency. The exergy approach revealed the hidden efficiency although the energy efficiency indicated inefficiency. • Increasing the coolant mass flow rate increased the cooling effect and electrical exergy efficiency but decreased thermal exergy efficiency. • Increasing the number of PVT collectors decreased the electrical exergy efficiency and thermal energy efficiency. However, the thermal exergy efficiency indicated the efficiency of the collectors in series by increasing until a certain point. • Electrical and thermal exergy efficiencies of the PVT operation cannot be maximized at the same time. The multiobjective optimization based on NSGA-II is a suitable method to optimize PVT and multi-stage PVT system design. • The Pareto optimal front can be used by the decision-makers to select a suitable optimal design of the multi-stage PVT system depending on the purpose and the weather conditions. • The higher number of the PVT collectors in series resulted in the higher thermal exergy efficiency and the lower number to the higher electrical efficiency. • The climate conditions had an impact on the optimal number of the PVT collectors in series. Under the cooler weather conditions, a smaller amount of the PVT collectors is recommended compared to warmer conditions.

In future work, the investigation of using the multi-objective optimization approach to the dynamic hybrid energy system including multi-stage PVT collector, thermal and electrical storage, electrical vehicle and demand-side will be conducted. The multi-stage PVT collector model will be developed to show the outlet temperature of each collector in series. Additionally, analysis and optimization of the flat plate multi-stage PVT collector coupled with concentrated PVT collectors would be interesting for future work.

NOMENCLATURE

FIGURE 1

 1 FIGURE 1The single PVT collector: (a) Direct flow collector geometry, (b) cross section of the PVT collector[START_REF] Barbu | Analysis of a residential photovoltaicthermal (PVT) system in two similar climate conditions[END_REF]

FIGURE 3 FIGURE 4

 34 FIGURE 3The simulation-based multi-objective optimization process

FIGURE 5 FIGURE 6

 56 FIGURE 5 variation of electrical and thermal exergy efficiency with the coolant mass flow rate

FIGURE 7 FIGURE 8 FIGURE 9

 789 FIGURE 7The variation of electrical exergy efficiency and the coolant outlet temperature with the number of the PVT collectors

FIGURE 10

 10 FIGURE 10The Pareto optimal front in the objective space of the electrical and thermal exergy efficiencies

FIGURE 11

 11 FIGURE 11 set of optimal solutions from Table 3 in terms of the electrical exergy efficiency

TABLE 1

 1 The main geometrical, thermo-physical and optical properties of the single PVT collector

			Air		Thermal
	Property	Glass	gap	PV	absorber	Unit
	Emissivity (ε)	0.9	-	0.96	-	-
	Absorbance (α)	0.1	-	0.9	-	-
	Transmittance (τ)	0.93	-	-	-	-
	Thickness (H)	0.004	0.02	0.006	0.001	m
	Area (A)	2	-	1 . 9	2	m 2
	Density (ρ)	2200	-	2330	2699	kg/m 3
	Specific heat (c)	670	-	900	800	J/(kg K)
	Thermal	1.1	-	140	237	W/(mK)
	conductivity (k)
	Property		Fluid	Insulation	Unit
	Thickness (H)		-	0.04		m
	Area (A)		-	2		m 2
	Density (ρ)		1050	16		kg/m 3
	Specific heat (c)		3605	1120	J/(kg K)
	Thermal conductivity (k)	0.615	0.035	W/(mK)

flow 0.017 kg/s (60 kg/h). Table

TABLE 2

 2 Decision variables for multi-objective optimization problem

	Symbol	Decision variable	Bounds	Unit
	ṁ	Mass flow rate	0.008≤ x(1)≤ 0.042	kg/s
	N

c No. of collectors 1 ≤ x(2) ≤ 10 T in Inlet temperature 15 ≤ x(3) ≤ 40 • C

TABLE 3

 3 Decision variables for the multi-objective optimization problem in the Pareto front

ṁ [kg/s] N T in [ • C] ξ el ξ th

	0.0344	9	40	-0.1595	-0.0551
	0.0367	1	21	-0.1883	-0.0120
	0.0367	1	21	-0.1883	-0.0120
	0.0306	1	22	-0.1869	-0.0157
	0.0175	3	25	-0.1728	-0.0446
	0.0294	2	22	-0.1834	-0.0249
	0.0283	2	31	-0.1770	-0.0388
	0.0242	2	27	-0.1786	-0.0355
	0.0344	9	40	-0.1595	-0.0551
	0.0144	1	27	-0.1796	-0.0329
	0.0244	1	22	-0.1860	-0.0180
	0.0269	4	31	-0.1710	-0.0477
	0.0178	3	23	-0.1742	-0.0425
	0.0336	1	21	-0.1880	-0.0127
	0.0269	2	23	-0.1821	-0.0280
	0.0289	1	30	-0.1810	-0.0308
	0.0161	3	24	-0.1724	-0.0451
	0.0311	7	40	-0.1613	-0.0549
	0.0344	9	40	-0.1595	-0.0551
	0.0367	1	21	-0.1883	-0.0120

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2021 The Authors. IET Renewable Power Generation published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology

ACKNOWLEDGEMENTS

The authors would like to thank Interreg V Rhin supérieur ACA-MODES project for their support and funding of this research.

CONFLICT OF INTEREST

The authors declare no conflict of interest.


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Experimental study using infrared thermography on the convective heat transfer of a TGV brake disc in the actual environment,

Monica Siroux, Souad Harmand, Bernard Desmet

To cite this version:

Monica Siroux, Souad Harmand, Bernard Desmet. Experimental study using infrared thermography on the convective heat transfer of a TGV brake disc in the actual environment,. Optical Engineering, 2002, 41 (7), 10.1117/1.1481896. hal-03964180