Guillaume Rozenberg

Brigitte Sebastia

est destinée au dépôt

From one Generation to another. Ruptures and continuities in 'traditional' knowledge transmission within a Southern Indian community

Alexis Avdeeff

To cite this version:

Alexis Avdeeff. From one Generation to another. Ruptures and continuities in 'traditional' knowledge transmission within a Southern Indian community: Ruptures and continuities in 'traditional' knowledge transmission within a Southern Indian community. Marine Carrin; Harald Tambs-Lyche; Dominique Blanc. Transfer of Knowledge and Children's Agency : Reconstructing the Paradigm of Socialization, Primus Books, pp. 94-108, 2016, 978-9384082635. halshs-01819087 as astrologers and doctors, and are often consulted by all classes of people' (thurston 1909[V]: 305). in all of tamil nadu, Val • l • uvar practitioners are renowned for their skill, seen almost as innate, in forecasting and diagnosing. their reputation-by virtue of their supposed expertise-is such in some rural areas, that, very often, the name Val • l • uvar is synonymous with co ¯ciyan ¯, astrologer, and to a lesser extent with maruttuvan ¯, indigenous physician. nowadays, though some among the Val • l • uvar's younger generation have continued the traditional occupation of astrology, very few of them pursue the practice of indigenous medicine. indeed, the ways of transmitting caste knowledge is changing, and consequently caste occupation is changing too. in this chapter, then, i shall show how ruptures and continuities in the transmission of one caste's knowledge reshape the occupation of the new generation, and lead us to rethink, more generally, our conceptions of caste through its contemporary evolutions. to do so, i shall expound, in the first part of this essay, the nature and content of the caste knowledge and the traditional occupations related to it. then, in the second part, i shall explore the hereditary transmission of this knowledge within the family group, from one generation to another (paramparai). Finally, in the third part, i shall analyse the trajectory of change [START_REF] Barth | an anthropology of Knowledge[END_REF] in this hereditary transmission, by looking at endogenous and exogenous factors that modify the intergenerational transmission process. val • l • uvar 'traditional' knowledge: nature and content in this case study, the notion i use of a caste's knowledge overlaps in fact with multiple kinds of knowledge. Knowledge is not a monolithic concept related only to explicit and organized knowledge like astrological and medical corpora are. Following mahias ' terminological reflections (mahias 2011: 17-18), the concept of knowledge used in this essay encompasses abilities, knowledge-based actions and experiential knowledge enabled by individual practice. Likewise, when i talk about Val • l • uvar's knowledge, i don't intend to indicate that this body of knowledge is neatly delimited and static. as a body of knowledge passed on from generation to generation, it changes and evolves continuously. indeed, within one lineage, paramparai, each generation adds its own innovations (mainly in the field of indigenous medicine and the preparation of remedies). each generation must make its own the knowledge passed down from the previous one. thus, the process of transmission transforms and reshapes what is transmitted. transmission is not transferring (Debray 1997: 49). Val • l • uvar's caste knowledge is the result of an ever-evolving transmission process, and in that sense it may be argued that this 'traditional' knowledge is in fact contemporary, and even more so in its practical aspects. in this essay, i focus only on the knowledge related to the Valluvar's practice of astrology and indigenous medicine. 7 in order to draw the contours of this knowledge, i have tried to collect information from several Val • l • uvar practising astrology and indigenous medicine, in villages of various districts in tamil nadu (Viluppuram, Cuddalore, nagapattinam and thanjavur), and in a few villages of Puducherry. i have observed a number of consultations, both astrological and medical, and conducted in-depth interviews with some practitioners and their family. thus, i have looked for empirical regularities in order to highlight the shared features on which their caste's knowledge is based.

the analysis of the data from this multi-sited ethnography reveals that Val • l • uvar's knowledge, as i have tried to define it is, in fact, composed of a multitude of local representations as well as personal innovations, preventing me from establishing a clear model. however, as a gross simplification, i shall argue that Val • l • uvar possess both collective and individual knowledge. indeed, a part of their knowledge is common and corresponds to that of indigenous people: knowledge related to the environment, local beliefs, cosmological representations, etc. most knowledge is specific to their traditional occupation, and consequently shared among community members: knowledge, both theoretical and practical, about astrology and indigenous medicine (how to cast a horoscope, how to take the pulse, etc.). 8 But some knowledge may belong specifically to some families-we may say lineages-or even to an individual. For example, some elder Val • l • uvar will, because of their own experience, be the only source of certain types of knowledge, like knowing how to read old palm-leaf bundles, using some specific mantras, particular divination technics, or personal medicine recipes, etc. Following this assessment, it may be argued that the Val • l • uvar's knowledge is quite heterogeneous, both in its sources and in its practical application. in fact, what is 'traditional' in the emic discourse is not so much knowledge, but rather the caste occupation.

in spite of this heterogeneity of knowledge and practices, there do exist common features that we may observe among practitioners. First, they share what we may call a group knowledge, or in other words cultural knowledge, encompassing norms, beliefs, rituals and language. Concerning this cultural knowledge, i noticed that all my informants were literate in tamil, some of them in english, and the elders in old tamil. this fact is quite unusual in the context of rural untouchables, especially for the older generation. the Val • l • uvar, unlike other ex-untouchable rural communities have a high literacy rate (avdeeff 2011: 254-6). indeed, literacy is necessary to practice astrology, perhaps even more than to practice indigenous medicine. 9 Val • l • uvar practitioners need to have a good command of tamil, both spoken and written as all the steps of the elaboration of the horoscope necessitate reading and writing. the astrologer must first calculate the planetary positions, then he has to refer to the almanac, and often he finds it necessary to read some astrological treatises to refine his interpretation. then, he has to write the ja ¯takam, birth chart, on palm-leaves (see Plate 7.1) or, currently, in a booklet (see Plate 7.2).

regarding the language used during astrological consultations, some astrologers-mainly the older ones-may use a technical vocabulary borrowed from an old form of tamil, which is no longer used by the new generations, as explained by anbushelvan, one of my informants: 'the language we use for astrology is cutta-tamil ¯ which means pure form of tamil. it is the oldest form of tamil . . . you can find this language in the old dictionaries.' For example, to say 'star', instead of using the widespread Sanskrit term naks • atram or its tamilized form nat • cattiram, they would use the old tamil form mı ¯n ¯ (star).

PLate 7.1. Detail of the first palm leaf of an individual's ja ¯takam.

Courtesy: alexis avdeeff.

PLate 7.2. Blank form of a ja ¯takam, from a printed booklet.

Courtesy: alexis avdeeff.

in the same way, for the planet Venus, instead of using s ´ukra or cukkiran ¯, they would use the term vel • l

• i-mı ¯n ¯ 'the silver star' as well as other tamil metaphorical terms as vet • i or vit • i-vel • l

• i, which mean 'the morning star'. another relevant point related to speech is that Val • l • uvar have a secret spoken language, a paripa ¯s • ai. this secret language is a kind of trade slang mostly used in 'business' situations-not to say during consultations-so that patients do not understand what the Val • l • uvar say. i came to know about it, by chance, during one of my first interviews with Gurujeevan:

We have a secret language. When we don't want others to know what we are talking about, we use this language. it is not a different language. it is just code words, which we use to communicate between us. and we don't use these code words with people from other communities. We just use this to maintain some secrets within our community . . . but, we are less using it now. in the olden days, since they [Val • l • uvar] were in the profession of seeing astrology and practicing medicine, they [Val • l • uvar] needed this language to maintain some secrets. But nowadays most of our community people are engaged in government jobs or some other jobs in the private sector, so . . . 10 as pointed out by Gurujeevan, and confirmed by other informants, this secret language is nowadays less and less known by the new generations, and, therefore, decreasingly used.

Concerning the practice of indigenous medicine-whether the practitioner presents himself as a citta-vaittiyan ¯ (Siddha doctor), as an na ¯t • u-maruttuvan ¯ (indigenous practitioner), or modestly as a pacu-maruntu-vaittiyan ¯11 -all informants have the knowledge of the local Materia Medica. they know the medicinal properties of several plants, herbs, fresh leaves (paccilai), and roots (mu ¯likai), which they apply directly on the body or use to prepare elaborate remedies. in order to diagnose the illness, all the Val • l • uvar physicians first do the na ¯t • ipparı ¯ks • ai, the examination of the pulse (na ¯t • i), before asking for other details from the patient. he will then confirm and refine the pulse symptoms by taking other symptoms into account. they also share the same taxonomic schemes about diseases whose causes are frequently attributed to malevolent actions, violent deaths, or bad planetary positions (Carrin-Bouez 1980: 85). regarding the concrete practice of astrology, all the practitioners use a pañca ¯n. kam, an almanac. they refer to it in order to cast the horoscope (ja ¯takam pa ¯kka) by calculating the planetary positions at birth. they also use it to know the auspicious and inauspicious times for a particular period, which is a central concern of the daily life of the village.

paramparai: hereditary knowledge transmission as we have seen, Val • l • uvar's knowledge is heterogeneous and multifaceted. relying on fieldwork data and following ingold's reflections on enculturation and enskilment (ingold 2000: 36-8), i assume three distinguishable aspects of knowledge frame Val • l • uvar caste's knowledge: a cultural, a theoretical and a practical one.

From a global perspective, encompassing the three aspects mentioned earlier, the transmission process of knowledge among the Val • l • uvar is twofold: diachronic and politic (Debray 1997: 15-24). indeed, it is a process over time, linking generations by establishing relationships between elders and youth. But hereditary transmission is also a community obligation, in the sense that it directly concerns the perpetuation of Val • l • uvar group culture and identity. thus, by its twofold nature, hereditary knowledge transmission is a long learning process for each new generation, beginning in early childhood, and spreading over years. this process is hardly directly observable in the fieldwork situation by ethnographers who have mainly worked, instead, on data collected from informants, usually old (Sigaut 1991: 33-4). thus, this long-term transmission process is difficult to comprehend in its whole complexity, as pointed out by ruddle: '. . . almost without exception, most ethnographers, if they discuss childhood at all, have little to say about how traditional knowledge of specific skills is transmitted' (ruddle 1993: 17). in this essay, i focus on the main features framing this hereditary transmission process rather than on the detailed description of a specific skill transmission.

thus, in order to highlight the features of this transmission process within the Val • l • uvar community, i shall examine how informants, who have been taught in the traditional way, reflected upon their own apprenticeship process.

Cultural aspect of Val • l • uvar caste's knowledge equals, in other words, the group culture of Val • l • uvar. it includes local beliefs (most of them are shared with other local communities), but also community rituals, norms and values as well as language, especially the old technical tamil related to the practice of astrology and the paripa ¯s • ai. the acquisition process of this kind of knowledge, as i have defined it, is enculturation. this process is mainly unconscious and informal since children acquire this group culture by immersion in the community. they learn unconsciously through day to day situations and informally through the observation of behaviour that prevails in their community culture. For example, paripa ¯s • ai learning is informal and unconscious and implies this continuous immersion within the elder group, as illustrated by Gurujeevan: 'From our childhood we have been hearing our grandparents and other ancestors using some of these code words and only like that we learnt.' this enculturation process is fundamental since it endows children with the fundamentals of Val • l • uvar culture; by affording them keys to decode the theoretical knowledge as well as a basic framework for their future practice.

acquisition of theoretical knowledge is primarily a matter of rote learning, be it related to astrology or indigenous medicine. the learner has to memorize oral and textual materials. it is mainly a formal and conscious process, though the learned knowledge may not be fully understood. indeed, some parts of this theoretical knowledge will become meaningful only through individual practice. For the young learner, the acquisition process of theoretical knowledge can be both formal and informal as well as conscious and unconscious as recalled by Sudarsan:

While he [his father] was preparing a medicine, he was chanting the preparation methods and the name of the ingredients. those people were used to chant in order to prepare the recipes properly and also because chanting avoids feeling the tiredness. While they were chanting, we were hearing it unknowingly. and that stuck to our mind. Whenever we have some doubts, we often refer to the old books and also to the new modern books to clarify the doubt. ' [. . .]. in the evening time he was clearing our doubts. When he was free at evening time, he was asking us:

-today we have ground a medicine, what is the name of this medicine? -how do you analyse the pulse? this is how he did to teach us.

this example is significant in several ways. First, it reveals the use of mnemotechnics by putting recipes in the form of songs in order to memorize them easier and to facilitate the preparation of medicines. and this procedure has a direct impact on the youth who surround the practitioner. indeed, they memorize the songs unconsciously and indirectly learn the way ahead. this sensitizes the youth to theoretical knowledge which they can then deepen by referring to textual resources. it highlights the primordial role of orality as far as codified knowledge is concerned, at least at the beginning of the learning process (and of course during the pre-literate time of early childhood). But it also appears that orality plays a continuous role in a more formal and conscious manner of learning as shown by the way the father questions his children by the end of the day. Finally, this sequence suggests that learning is collective since Sudarsan is never referring to himself alone but to his peer group ('us', 'we') and even when he talks about his father, he talks about 'they', stressing that transmission is intergenerational. But transmission of knowledge is not only a matter of orality, as Sudarsan revealed, later in our conversation:

it [astrological knowledge] comes hereditarily as we possess old palm scriptures which give details about everything [related to astrology]. For the past seven generations, our family has been practicing astrology, and we are maintaining these palm scriptures for the coming generations.

. . . there are separate palm scriptures written for maruntu. But this one [Sudarsan showed me a bundle on his desk] gives details only about astrology. '. . . i only know few things about maruntu and paccilai-mu ¯likai used for treatment, but my father used to practice medicine and he had a lot of palm scriptures on maruntu. he also had the palm scriptures that give details on the medicine written by the famous 18 Siddhars, the Siddha medicine.

thus, transmission of theoretical knowledge also implies the transmission of text corpora related to astrology and indigenous medicine, formerly engraved on palm-leaves bundles, o ¯lai cuvat • i. and as far as written corpora are involved in this process, literacy is necessary to decode and acquire the organized knowledge, as highlighted by maiyil Vahanan, one of my informants who practices both astrology and indigenous medicine:

Siddhars have written the medicines and the recipes in the form of poems . . . For a person who doesn't know how to scan verses, and thus identify the true meaning of words, then it will be difficult. a person who does not know grammar cannot understand. the third kind of knowledge i have defined-the practical one-is closely related to the previous two. indeed, though practical knowledge results from individual practice, it is framed by the group culture and supplied by theoretical inputs. acquisition of practical knowledge is, thus, a matter of practice and enskilment (ingold 2000: 36-8). But we can distinguish two distinct steps in this learning process: apprenticeship and individual practice. the first is related to enskilment through practice under the supervision of a knowledgeable instructor while the second is enskilment through individual practice. thus, personal enskilment through individual practice is clearly independent from the traditional transmission of knowledge as it pertains exclusively to an individual. i shall not detail this process here since i am concerned by vertical transmission.

apprenticeship goes through observational learning and imitation. Children learn by imitating and repeating an observed behaviour. the main situation from which a Val • l • uvar child can learn that way is during father's consultations, as illustrated by Gopalakrishnan: i started learning astrology from my father when i was 12 years old. . . . i learnt from my father only. From my childhood, i used to sit with him and listen to all what he was saying. Later when i was little bigger, i started to ask questions and he answered and cleared my doubts. Like this i started learning this [astrology].

Sometimes, the child assists his father by filling the birth chart booklets of a patient, learning, in that way, how to cast the horoscope. Women have also a role to play in this apprenticeship process. indeed, the grandson of an old Val • l • uvar astrologer, who was fifteen years old when i met him, told me that he learnt how to use and prepare some medicines through observing his mother and his aunts in the kitchen. as endogamy prevails in Val • l • uvar community, women are generally daughters of practitioners, and are aware of the caste's knowledge. then as wives of a practitioner, they help their husbands in the preparation of remedies. otherwise, as far as apprenticeship of indigenous medicine is concerned, transmission is not only a matter of observation and imitation, since, in some cases, practitioners may transmit personal notes on their own practice to their descendants, as revealed by maiyil Vahanan:

Concerning mu ¯likai, there are several things to know. each mu ¯likai has its unique properties to cure some diseases, but the same mu ¯likai, when mixed with two or three others, cures other diseases. By knowing these techniques coming from the Siddhars and transmitted by his ascendant, he [father of maiyil Vahanan] himself tested the medicine by giving them to the sick who were approaching him. then he was able to identify the good compositions of medicines and adjust them. then he wrote those recipes in notebooks. i learn those things by reading his notebooks and practicing myself maruttuvam. as following hereditary lines, we are practicing medicine. it is called paramparai an ¯upavam vaittiyam. . . . By reading the available books on Siddha medicine, my father's writings on medicine, and through my own experience, i am making medicines. thus, practical knowledge is foremost a matter of transmission of personal experience, which implies relationships between more and less experienced Val • l • uvar practitioners. Besides, each generation brings its own innovations, which are passed down to the next, reshaping, in that manner, the knowledge transmitted.

when a tradition of knowledge changes Val • l • uvar caste knowledge-as we have seen it in its three interconnected aspects-is in fact an ever-evolving body of knowledge and practices, shaped by the internal processes of its own transmission. But nowadays, due to endogenous and exogenous factors, hereditary transmission is deeply changing, thus affecting both knowledge and practice.

the most salient endogenous factor affecting the hereditary transmission is the loss of knowledge concerning the way to write and read palm-leaves. indeed, today, books have replaced palm leaves and consequently their use has been lost. only few of my older informants were handling inherited palm leaves, and only one of them-who was more than eighty years old-was able to write in the old manner. in many cases i observed, Val • l • uvar of the younger generation part with the palm leaves inherited from their father or grandfather because they are not able to read them. i remember that Gurujeevan, as we were talking about palm leaves, asked me to follow him behind his house near a small stream, to show me the place he threw all the palm-leaves of his father. i then saw a small mound of illegible palm leaves decaying into dust. this example is not unique, and illustrates sharply the change in the transmission process. Palm leaves have been replaced by books in modern tamil, and for many Val • l • uvar these palm leaves are no longer useful, as explained by maiyil Vahanan:

[my father] had old scriptures. i am using other books, which are available in the shops; because in the old scriptures the language used is different from current tamil, while in those printed books the text is in modern tamil. So, i am using those books. Furthermore, in those days measuring system was totally different from the modern metric one. those things were converted and modified in the new books. if we go and ask the shopkeeper [the apothecary] with those old measures, he wouldn't be able to understand. in these modern books, we can find the new measures. it is helpful for us. Siddhars have written the medicines and the recipes as poems. those old songs are now available in functional tamil, so that everyone could understand.

to this endogenous factor-the loss of old tamil language used in palm leaves-may be added an exogenous one. indeed, as revealed by maiyil Vahanan, the diffusion of printed books in modern tamil seems to have accentuated this situation. as a result of these changes, we are currently witnessing a standardization of theoretical knowledge, a process that has begun several decades ago. indeed, printed books in modern tamil, on astrology and indigenous medicine, are available to all, in bookshops. obviously, orality still plays an important role in the transmission of theoretical knowledge within the Val • l • uvar community, but as for the acquisition of this knowledge through written materials, the hereditary transmission lineage seems broken. the theoretical written materials are now standardized and come from outside the community.

the Val • l • uvar tradition of knowledge is also strongly affected by other exogenous factors. Since the first half of twentieth century, institutionalization of indigenous medicines is a factor which deeply transforms the modalities of medical knowledge transmission. Unlike the practice of astrology, the practice of medicine is supervised by the government through a strict policy of registration. traditional practitioners are also subject to this system, though in tamil nadu, the state government seems to be more benevolent towards indigenous practitioners than the central one (Sébastia 2009: 6-8). in other words, this means that if a traditional practitioner wants to be recognized, he has to provide a medical qualification awarded by a state-recognized institution (in our case, a Siddha college). my informants regularly point out this fact as a cause of abandon of the medical indigenous practice within the Val • l • uvar community. indeed, a Val • l • uvar who would like to pursue the traditional practice of maruntu, has to leave the village and go to a Siddha college for his training, in order to become a 'Siddha doctor' recognized officially. that way, replication system of Val • l • uvar tradition of knowledge based relationships between more and less experienced Val • l • uvar practitioners is broken. and as stressed by ingold, '. . . it is on the reproduction of these relationships, . . . that the continuity of a technical tradition depends' (ingold 2000: 37). at the time i was doing my fieldwork, in reaction to this institutionalization, the Val • l • uvar Sangam, viz., the international Valluvan Community Federation Welfare trust, was planning to set up training centres in astrology and Siddha medicine, in order to train young Val • l • uvar in their own tradition, thus following the example of traditional practitioners' associations [START_REF] Sébastia | Governmental institutions vs. associations. the multifaceted expression of Siddha medicine in tamil nadu[END_REF]). Yet, if the institutionalization of medicine has deeply transformed the practice of indigenous medicine, the recent emergence of astrology in indian universities does not seem to have really changed the way of transmitting the astrological knowledge. at most, these new curricula allow some Val • l • uvar astrologers to complete their traditional training, and gain an official degree.

other exogenous factors like changes in the socio-economic context also affect the Val • l • uvar tradition of knowledge indirectly. indexed as Scheduled Castes in tamil nadu as well as in Puducherry, Val • l • uvar benefit from the reservation policy. indeed, the indian government has set up a quota system whereby a part of positions are reserved in government services and in public sector units as well as state-run and sponsored educational institutions. moreover, according to the article 46 of the Constitution of india, 12 state governments must provide access to education for Scheduled Castes. this system, which has improved the standard of living of some ex-untouchable communities by giving them access to education, formations and jobs, has at the same time caused ruptures in the hereditary transmission process. as observed during fieldwork, young Val • l • uvar children are going to school, and so they are not as much immerged in the group culture as they were before. they are in contact with a different tradition of knowledge, that of the modern indian state, which shape their minds in another way. Furthermore, going to school everyday prevents Val • l • uvar children from assisting their father during consultations, thus breaking the observational learning process. nevertheless, i observed that in some cases, children and teenagers assist their father after school or during their day off. But according to many of my informants, this 'part-time apprenticeship' is not enough to transmit their personal experience properly. and mostly, these new generations of Val • l • uvar, educated in schools and universities, prefer working in educational institutions or in the government jobs they can get through the reservation policy system. From a financial point of view, it is more advantageous to work in those sectors than to pursue the traditional occupation of the group. members of the Val • l • uvar families i met (both males and females) assume such positions at the expense of their traditional occupation. Generally, the only practitioner (generally the father, an uncle or the grandfather of the family) is also the last who knows how to practice. nephews, children and grandchildren are usually engaged in a more 'modern life' as they put it. For this new generation, working as astrologer or as indigenous physician, is generally seen as oldfashioned. these observations confirm the general trend highlighted by B. Sébastia concerning tamil indigenous practitioners: 'the lack of interest in siddha in the education policy as well as the registration have incited many children of practitioners to abandon the profession of their forefathers and to take jobs in the indian administration or, for the richest, to work as biomedical doctor, engineer, teacher, etc.' (Sébastia 2009: 8).

conclusion: caste occupation and identity

While, from a general point of view, the Val • l • uvar's traditional occupations change deeply under the influence of exogenous socio-economic factors, their practice of astrology seems less affected by the other exogenous factor changing the modalities of knowledge transmission, than their practice of indigenous medicine. though we are witnessing a standardization as well as an institutionalization of their astrological and medical knowledge-through the emergence of new modalities of knowledge acquisition-the Val • l • uvar practice of astrology remains. thus, knowledge of the new generation of Val • l • uvar is no longer the result of a unique hereditary transmission process, but rather the mix of various acquisition processes external to the community.

occupational change is the main consequence of these tremendous transformations. and the Val • l • uvar's case is not unique; other communities throughout india see their traditional occupation changing (heuzé 2011; mahias 2011; Prévot 2011). in a system where professional specialization is strongly linked to community identity, one might expect the splitting of the ja ¯ti's identities, and at the same time the collapse of the caste system. But this is not happening. the strength of the caste system lies in its ability to integrate changes. traditional occupations are evolving and changing, but the communities remain and re-invent themselves. as for the Val • l • uvar, their sense of belonging has never been so strong, in spite of these community structural changes, as illustrated by the self-assertion movement on the political scene, and led by the Val • l • uvar Sangam (avdeeff 2011; avdeeff and tambs-Lyche 2011). this self-assertion process relies on the valorization of their traditional caste occupation, and the exaltation of their 'inherited' expertise.

identities are, thus, moving, adapting themselves to new social configurations. they are glorified, exalted, and even re-invented as needed. thus, communities remain, even reinforcing themselves in the system. re-invention of tradition and commercialization of local knowledge may be the keys to understand these adaptation processes. and in our case, it is perhaps in a small tamil city that the future of Val • l • uvar community is at stake. indeed, the last decade has seen the development of several Na ¯d • ı ¯13 astrological centres in the small town of Vaitheeswaran Koil. Na ¯d • ı ¯ astrology is presented as a particular form of astrology, based on the belief that lives of all human beings have been foreseen and written down on palm leaves by the sages of ancient time. Seekers come from around the world to have a (costly) appointment with a Val • l • uvar astrologer in order to find their palm leaf, and receive a lecture of their past, present and future life. But according to all my informants, this Na ¯d • ı ¯ astrology's tradition has nothing to do with the astrology traditionally practised by Val • l • uvar. this spectacular reactivation, appropriation and commercialization of a local practice by some individuals, encourage a lot of young Val • l • uvar to learn Na ¯d • ı ¯ astrology, which is more lucrative than 'traditional' astrology, in training centres like Vaitheeswaran Koil at the expense of the astrology practised by their forefathers. here, the invented tradition is killing the older one off. notes 1. a common translation for this term is 'community of birth'. a ja ¯ti is an endogamous group, and is generally associated with a traditional occupation. each linguistic region has its own ja ¯ti, even if we can distinguish recurrent features from one region to another. the roman transliteration of the tamil term should be ca ¯ti, but for the sake of clarity, the pan-indian term ja ¯ti will be used in this essay. 2. this essay is based on data collected during three years of fieldwork in Puducherry andin rural tamil nadu, between 2006 and2008. 3. the transliteration of tamil terms used in the present essay follows that of the Tamil Lexicon, University of madras. Tol ¯il is a term usually used in tamil to designate an occupation or a professional duty and, by extension, the traditional duty of a ja ¯ti. 4. my informants are using both expressions: na ¯t • u-maruttuvam and na ¯t • u-maruntu.

according to Tamil Lexicon, the meaning of maruttuvam is the practice of medicine and maruntu is medicine. the prefix na ¯t • u means country and comes to reinforce the notion of indigeneity. thus, na ¯t • u-maruttuvam can be translated by 'practice of indigenous medicine' and na ¯t • u-maruntu by 'indigenous medicine' or 'local medicine'. 5. Scheduled Castes are indian population groups recognized by the State, mainly the ex-untouchable communities. they benefit from the policy of positive discrimination established by the post-independent State. 6. Paraiyars are the majority among the Scheduled Castes of tamil nadu; see [START_REF] Deliège | Les Paraiyars du Tamil Nadu[END_REF]. 7. indeed, regarding their recognized traditional occupation, Val • l • uvar are also seen as the priests of the Paraiyars. 8. astrological and medical knowledge could be also shared, to some extent, with other practitioners from various communities, like for example Brahmins. 9. Unlike casting the horoscope, one does not need to be literate to master medical practice, since it rests on the practitioner's perception of his patient's body (pulse, symptoms, etc.) in order to establish a diagnosis, which remains oral.


A.-L Perrier
email: anne-laure.perrier@grenoble.cnrs.fr

J.-M Duchamp

P Ferrari

A MINIATURIZED THREE-PORT DIVIDER/COMBINER

Keywords: output isolation, harmonic rejection, miniaturized device, power divider

We propose a new topology based on a semi-lumped complex impedance transformer for the design of miniaturized power dividers. Our design is based on an impedance transformer in the input branch and a loop in the output branches. The input impedance transformer is constituted by a transmission line loaded with a capacitor. A prototype has been realized using hybrid technology for a 1 GHz operating frequency. The electrical length of the device is 26.4°. This leads to a miniaturized device, more than three times shorter than a typical Wilkinson power divider. This divider exhibits insertion loss lower than 0.3 dB, return loss better than 20 dB and the isolation between the two output ports above 20 dB. Moreover, the low-pass behavior of the input impedance transformer leads to a good rejection of the third harmonic.

INTRODUCTION

The technical maturity of power dividers dates from the sixties when Wilkinson published a paper on a N-port divider (with N even or odd) [START_REF] Wilkinson | An N-way hybrid power divider[END_REF]. The main aim of this kind of divider is to uniformly spread the input power P into N output ports. All outputs are in phase and have a P/N power. Another significant property is the good isolation between output ports. Wilkinson's dividers are designed with quarter-wave transmission lines.

In general, power dividers can be classified in two groups: three-port and four-port dividers. Since 1960, several researches on the three-port dividers have been carried out, with main objectives to increase the bandwidth by increasing the number of sections [START_REF] Cohn | A class of broadband three-port TEM-mode hybrids[END_REF], to distribute a nonuniform power to each output [START_REF] Parad | Split-tee power divider[END_REF], to decrease the size of the devices using lumped elements [4 -6] and to eliminate the spurious harmonics [START_REF] Yi | Modified Wilkinson power divider for nth harmonic suppression[END_REF]. All these researches are based on real or artificial quarter-wave transmission lines, used as impedance transformers to change a resistive load into another one. In parallel, researches were performed on complex impedance transformers, not based on quarter-wave transmission lines. A principle was shown in 1967 [START_REF] Hamid | On the design of stepped transmission-line transformers[END_REF].

In this paper, we show a compact topology of power dividers having the same properties as the Wilkinson power dividers. They are based on a complex impedance transformation. From the output to the input: the output branches transform a resistive load into a complex load using a small-length transmission line; this complex load is then transformed to a resistive load thanks to a small-length complex impedance transformer. This topology, proposed for a tunable power divider in 2006, leads to miniaturized and fixed power dividers with a harmonic rejection [START_REF] Perrier | A semilumped miniaturized spurious less frequency tunable three-port dividercombiner with 20 dB isolation between output ports[END_REF]. The complex impedance transformer is realized by a transmission line loaded by a capacitor, leading to a slow-wave and low-pass behavior.

The miniaturized power divider is described in Section 2, design equations are developed in Section 3 and design charts are carried in Section 4, whilst Section 5 shows simulated and mea-sured results carried out on a hybrid prototype. Conclusion draws out the agreement between simulated and measured results.

PRINCIPLE

A classical three-port Wilkinson power divider is constituted by a T junction and two output impedance transformers (see Fig. 1). A resistor between the output ports is necessary to achieve output ports isolation.

Figure 2 shows the topology of the power divider proposed in this article. As shown in [START_REF] Perrier | A semilumped miniaturized spurious less frequency tunable three-port dividercombiner with 20 dB isolation between output ports[END_REF], this topology can be divided in two blocs: an "input transformer" and an "output loop." This loop is necessary to achieve isolation between the two output ports (ports 2 and 3). i is the electrical length and Z i the characteristic impedance of the transmission line "i." Contrary to the Wilkinson power divider, the impedance transformer is split between the input port and the output loop. The input impedance at the junction point (J), seen from ports 2 and 3, is not necessarily equal to the matching input impedance (usually 50 ⍀). The input transformer differs from [START_REF] Perrier | A semilumped miniaturized spurious less frequency tunable three-port dividercombiner with 20 dB isolation between output ports[END_REF], it is just composed by one capacitor and one transmission line.

DESIGN EQUATIONS

To extract design equations, the even and odd mode approach has been used.

Even Mode

In the even mode, the symmetrical axis of the divider can be replaced by a magnetic wall, i.e., an open circuit. In this case, the capacitor and the resistor of the output loop can be removed. The input impedance Z in seen at the input of the output loop (J) can be calculated: Relation [START_REF] Cohn | A class of broadband three-port TEM-mode hybrids[END_REF] shows that the Z in impedance can resume itself as the transformation of Z 0 /2 through a transmission line with Z 2 /2 characteristic impedance and 2 electrical length. Hence the two transmission lines and the two output ports of the output loop can be replaced by one transmission line and one port as described in Figure 3.

Z in ϭ ͩ Z 2 Z 0 ϩ jZ 2 tan͑ 2 ͒ Z 2 ϩ jZ 0 tan͑ 2 ͒ ͪ 2 2Z 2 Z 0 ϩ jZ 2 tan͑ 2 ͒ Z 2 ϩ jZ 0 tan͑ 2 ͒ , ( 1
)
Z in ϭ Z 2 2 Z 0 2 ϩ j Z 2 2 tan͑ 2 ͒ Z 2 2 ϩ j Z 0 2 tan͑ 2 ͒ . ( 2
)
Figure 4 shows the equivalent electrical circuit of the power divider for the even mode.

The impedances Z e , ZЈ e and ZЉ e seen along the divider are given by:

Z Љ e ϭ Z 0 1 ϩ jZ 0 C 1 , (3)
Z e Ј ϭ Z 1 ZЉ e ϩ jZ 1 tan͑ 1 ͒ Z 1 ϩ jZЉ e tan͑ 1 ͒ , ( 4
)
Z e ϭ Z 2 2 ZЈ e ϩ j Z 2 2 tan͑ 2 ͒ Z 2 2 ϩ jZЈ e tan͑ 2 ͒ . ( 5
)
When the port impedances are purely real, two equations can be defined to realize the power divider output matching:

Re͓Z e ͑Z 1 ,Z 2 , 1 , 2 ,C 1 ͔͒ ϭ Z 0 2 (6) Im͓Z e ͑Z 1 ,Z 2 , 1 , 2 ,C 1 ͔͒ ϭ 0 (7)
These two equations with five unknowns lead to an infinity of solutions. They will be displayed on several charts in Section 4.

Odd Mode

Whilst using the odd mode, the symmetrical axis of the power divider can be replaced by an electrical wall, i.e., a short circuit. In that case, the input transmission line and the first capacitor are short circuited. The equivalent electrical circuit of a half power divider is given in Figure 5 for the odd mode.

The impedances Z o and ZЈ o seen along the half power divider can be calculated as:

ZЈ o ϭ jZ 2 tan͑ 2 ͒, (8)
1 Z o ϭ 2 R ϩ j2C 2 ϩ 1 ZЈ o . ( 9
)
When the port impedances are purely real, like for the even-mode, two other equations should be verified to realize the power divider output matching: (10)

Re͓Z o ͔ ϭ 2RZ 2 2 tan 2 ͑ 2 ͒ ͑R Ϫ 2RC 2 Z 2 tan͑ 2 ͒͒ 2 ϩ ͑2Z 2 tan͑ 2 ͒͒ 2 ϭ Z 0
Im͓Z o ͔ ϭ Ϫ j RZ 2 tan͑ 2 ͓͒R Ϫ 2RC 2 Z 2 tan͑ 2 ͔͒ ͑R Ϫ 2RC 2 Z 2 tan͑ 2 ͒͒ 2 ϩ ͑2Z 2 tan͑ 2 ͒͒ 2 ϭ 0 (11)
Considering only the physical solution [R Ϫ 2RC 2 Z 2 tan ( 2 )] ϭ 0, R and C 2 can be extracted from the two Eqs. ( 10) and (11), leading to:

R ϭ 2Z o (12) C 2 ϭ 1 2Z 2 tan͑ 2 ͒ (13)

DESIGN CHARTS

Design charts have been derived from the design Eqs. ( 6), ( 7), (12), and (13) obtained from the even-odd mode theory. The design charts of Figure 6 give the characteristic impedances Z 1 and Z 2 and the reactances 1/C 1 and 1/C 2 versus the total electrical length tot ϭ 1 ϩ 2 . The ratios x ϭ Z 1 /Z 2 and y ϭ 1 / 2 are used as design parameters.

These charts allow synthesizing various power dividers. In practice, the technology and the substrate used limit the choice of the characteristic impedances and capacitors values.

REALIZATION AND RESULTS

To illustrate the new concept developed for the design of miniaturized power dividers, a prototype has been realized in a hybrid technology using surface mounted capacitors. For simplicity, the design has been done for a 1 GHz operating frequency. We choose x ϭ 1 and y ϭ 1 and the characteristic impedance of the transmission lines is fixed to 85 ⍀. These choices lead to tot ϭ 26.4Њ as shown in Figure 6, so 1 ϭ 2 ϭ 13.2°.

The device was designed in microstrip technology on a Rogers™ RO4003 substrate (relative permittivity r ϭ 3.36, The transmission lines lengths are 7 mm to reach the 13.2°electrical length, for each transmission line. We noted that the total electrical length is 39.6°(3 ϫ 13.2), leading to a miniaturized power divider compared to the 180°(2 ϫ 90°) of the Wilkinson power divider. The total length is divided by 4.5.

The values C 1 ϭ 2.8 pF and C 2 ϭ 4 pF given by the charts (see Fig. 6) are not normalized surface mounted capacitors, they have been replaced by C 1 ϭ 3 pF and C 2 ϭ 3.9 pF. The photography of the power divider is shown in Figure 7. The first capacitor is connected to the ground plane with a via and the second capacitor is connected between the two output ports.

Simulated and measured results are shown in Figure 8. Simulations were carried out with ADS [START_REF]Advanced Design System (ADS), Version 2004A[END_REF]. Measurements were carried out with a WILTRON ™ 360 Vector Network Analyzer and a standard Short-Open-Load-Thru (SOLT) calibration procedure.

The agreement between these results is correct at the operating frequency. S 11 , S 22 , and S 32 minima are centered on 1 GHz as predicted in simulation. The measured bandwidth at Ϫ20 dB is 20% for S 11 , 10% for S 22 , and 10% for S 32 . On the whole bandwidth, S 21 varies between Ϫ 3.1 dB and Ϫ3.4 dB, leading to less than 0.4 dB insertion loss. Insertion loss is minimum (0.1 dB) at 1 GHz.

However, variations are important, especially on S 21 , for frequencies higher than 2 GHz. Measured results show a transmission This transmission zero can be very useful in the case of the use of the power divider for small bandwidth applications where the third harmonic (3f 0 ) is undesirable [START_REF] Yi | Modified Wilkinson power divider for nth harmonic suppression[END_REF]. An additional inductance can be added in practice to adjust the frequency of the transmission zero. This new topology allows realization of power divider more compact than device presented by Ahn and Wolff [START_REF] Ahn | General design equations, small-sized impedance transformers, and their application to small-sized three-port 3-dB power dividers[END_REF] with the same transmission line characteristic impedance. In their publication [START_REF] Ahn | General design equations, small-sized impedance transformers, and their application to small-sized three-port 3-dB power dividers[END_REF], a power divider constituted by two 15.3°electrical length transmission lines is proposed with characteristic impedance of 195.9 ⍀. For the same characteristic impedance, our topology, with x ϭ 1 and y ϭ 1 (see Fig. 6), allows to conceive a power divider constituted by three 5°electrical length transmission lines with tot ϭ 10°. So our new topology is more compact in term of length and in term of surface.

CONCLUSION

A new topology of miniaturized equal three-port power dividers has been proposed. Based on an input impedance transformer and a small length output loop, it is not based on the classical quarter wave length transmission lines.

Design equations and charts have been carried out with the even-odd mode theory. A prototype has been realized in a hybrid technology with surface mounted capacitors, to validate the approach. The prototype was realized to operate at 1 GHz. The total electrical length of the device is only 39.6°compared with 180°for a classical Wilkinson power divider. A 10% bandwidth, sufficient for small bandwidth applications, is obtained at Ϫ20 dB on S 11 , S 22 , and S 32 parameters. The insertion loss is low as 0.1 dB at the center frequency and limited to 0.4 dB for the whole bandwidth.

An additional inductance can be added in series with the capacitor of the input impedance transformer to reject an undesired harmonic.

These performances lead to a possible integration for a device in a monolithic microwave integrated circuit (MMIC) technology.

Figure 1 Figure 2

 12 Figure 1 Wilkinson power divider

Figure 3 Figure 4

 34 Figure 3 Output loop equivalent electrical circuit for the even mode

Figure 5

 5 Figure 5 Equationuivalent electrical circuit of an half divider for the odd mode

Figure 6 Z 1 ,Figure 7

 617 Figure 6 Z 1 , Z 2 , and 1/C 1 , 1/C 2 versus x, y, and tot

ACKNOWLEDGMENTS

The authors thank the Re ´gion Rho ˆne-Alpes which has supported this work (EMERGENCE program).


Pierre Garcia
email: pierre.garcia@promes.cnrs.fr

Alain Ferriere

Jean-Jacques Bezian

Codes for solar flux calculation dedicated to central receiver system applications: A comparative review

Keywords: Solar flux calculation, Central receiver systems, Optical modeling, Ray-tracing, Cone optics

come

Codes for solar flux calculation dedicated to central receiver system applications: A comparative review

Introduction

To optimize and design a CRS it is essential to know the performances of the subsystem formed by the tower and the heliostat field. Experimental work in this field has shown the necessity to master user-friendly modeling tools to design, simulate and optimize solar components of tower plants. Unluckily a lot of work is generally needed to adapt the codes for concentrated solar flux calculation to specific features and specific needs of each project. None of such codes can be considered as a standard tool for research or industry applications. Our purpose is not to develop another tool -many of them are available and effective -but to screen existing codes, trying to determine which one should be used depending on the objectives of the simulation, and which results can be expected. Such comparison had already been initiated in the frame of SolarPACES (solar power and chemical energy systems, IEA implementing agreement) to validate optical tools on a standard test case [START_REF] Pitz-Paal | Software developments for system analysis and optimization[END_REF], but to our knowledge this work was not continued.

Concerning heliostat fields, two kinds of problems can be distinguished. The first one is an optimization problem: ''What is the best heliostat field layout to maximize the collected solar energy or to minimize the cost of that energy?''; the second one is a performance calculation: ''What is the power reflected by the field and arriving on the receiver aperture?'' The PROMES laboratory is interested in both problems. Indeed, one goal is to be able to optimize the design of any CRS project at any location on economic criteria such as cost of the produced electricity. An additional goal is to assess the performances of the solar field for the PEGASE [START_REF] Garcia | The PEGASE project: design and performance evaluation of a hybrid solar/fossil gas turbine system for electricity generation[END_REF] project located at THEMIS (Targasonne, France, see Fig. 1). To answer this double objective, the needed tool must be able to determine flux maps on receiver aperture, to establish efficiency matrixes of the solar field, to optimize the field layout on costs criteria (cost of produced kilowatt-hour, cost of installed kilowatt), and to calculate instantaneous or annual performances of the concentrating system.

Furthermore, an optional feature is to include receiver and thermal cycle models to simulate the whole installation and to analyze its performances, as two of the screened codes do. This global simulation on an annual basis can also be done by integrating the heliostat field and tower subsystem in less specific tools, such as TRNSYS [START_REF] Schwarzbo ¨zl | A TRNSYS model library for solar thermal electric components (STEC), a reference manual release 2.2. IEA-solar power and chemical energy systems, Task III: solar technologies and applications[END_REF], SOLERGY [START_REF] Stoddard | SOLERGY -A Computer Code for Calculating the Annual Energy from Central Receiver Power Plants[END_REF], or ECOSTAR [START_REF] Pitz-Paal | ECOSTAR Road Map Document[END_REF]. The simplified model developed for the ECOSTAR program combines steady-state component efficiencies from experimental or simulated curves. This method is relevant to assess an annual production, but not to simulate the operation of a whole installation on shorter periods when the system dynamics should be considered. TRNSYS may be adapted to model transient behavior of CSP systems, during startup, shut-down or variable weather periods. However this third kind of problem, very useful to define operation strategies of a CRS, is not tackled in this paper.

Abbreviations and glossary

• Abbreviations: In this document the following definitions are used:

• Solar field efficiency: reflected power arriving on the receiver aperture divided by the product of the incident solar power by the total area of mirrors. It includes reflectivity of the mirrors, cosine factor due to the incident angle of the sun on the heliostats, atmospheric attenuation between the heliostats and the receiver, shadowing and blocking effects, and spillage of the flux around the receiver aperture. For the mean annual efficiency, heliostat availability is also taken into account. • Shadowing and blocking effects (S&B): a shadow is a mask (neighboring heliostat, tower) in the incident ray path between the sun and the heliostat surface; a block is a mask (neighboring heliostat) in the reflected ray path between the heliostat surface and the receiver. • Solar field efficiency matrix: bi-dimensional matrix giving the solar field efficiency as a function of sun position (azimuth and elevation). Integrated in tools for performance analysis of CSP systems, such a matrix makes it possible to assess the solar field efficiency all year long. Generally speaking, reflectivity of the mirrors is not included because it may vary during a year.

Some principles of heliostat field modeling

The optical components of a CRS are designed to form an image of the solar disk on a focal plane. However the obtained solar spot has neither the same size nor the same luminance as the sun, because of the following phenomena (He ´nault, 1987):

sun and collector geometry

• size and luminance of the sun, varying every day by diffusion in the atmosphere, • optical aberrations, like heliostat astigmatism, caused by the design of the reflective surfaces, specific defects of solar facilities

• microscopic errors of reflective facets, often considered as negligible, • pointing (or tracking) errors, well known, • curvature and canting errors of facets or modules.

To calculate concentrated solar irradiance, usual approaches are ray-tracing and convolution methods. The principle of ray-tracing methods (or statistical or Monte Carlo methods) is to choose randomly a bundle of rays coming from a surface 1, and then to determine which of them arrive on a surface 2. The irradiance of an elementary surface is proportional to the number of impacting rays [START_REF] Matteı | Rayonnement thermique des mate ´riaux opaques[END_REF]. In the case of a concentrator with one reflection, this algorithm is used twice, first between the sun and the reflective surface with an energetic distribution corresponding to the sun shape, then between the heliostat facet and the receiver with a statistic law for the error distribution related to the defects of the facet. Precision and calculating time increase with the number of rays and the complexity of geometry. In convolution methods (or cone optics), reflected rays from elementary mirrors are considered with error cones calculated by convolutions of normal Gaussian distributions corresponding to each error (sun shape and heliostat errors).

A systematic comparison shows that with the same hypothesis similar results can be reproduced with ray-tracing and convolution methods [START_REF] Pitz-Paal | Software developments for system analysis and optimization[END_REF]. Simulation errors often come from an incomplete description of reflective surfaces and sun shape properties. Nevertheless ray-tracing methods are more flexible and are able to model non-ideal optics (non-imaging concentrators). Indeed they have the advantage of reproducing real interactions between photons and therefore of giving accurate results for small or complex systems but they need higher calculating time and computing power. That is why it is not recommended to use ray-tracing techniques for system optimization.

About the accuracy of codes using convolution/expansion techniques [START_REF] Walzel | A solar flux density calculation for a solar tower concentrator using a two-dimensional hermite function expansion[END_REF] found peak flux error and average absolute error in the range of 1-2%. This is comparable to what can be obtained from a ray-tracing codes like SOLTRACE. [START_REF] Kistler | A user's manual for DELSOL3: a computer code for calculating the optical performance and optimal system design for solar thermal central receiver plants[END_REF] states that annual performances of a solar field can be deduced with accuracies better than 1%. Comparison between first generation codes on a small field (CESA-1) shows very good agreement (about 1% deviation for the power on receiver, 3% for the peak flux). However the accuracy to predict flux and spillages decreases when the slant range decreases with respect to heliostat and receiver size and in particular with high precision heliostat (low canting, focusing and pointing errors). Nevertheless one have to keep in mind that the greatest errors observed on annual performance of a CRS do not come from the optical model but from the other components model (turbine, storage. . .).

First generation codes

First codes designed to calculate solar flux concentration in a CRS came from preliminary studies carried out in the US for Solar One in the late 70s. Among them one may quote HELIOS, MIRVAL and DELSOL, which are still used today. HELIOS was initially developed for analyzing experiments at Sandia's CRTF [START_REF] Biggs | HELIOS: a computational model for solar concentrators[END_REF]. This code can be adapted to all kind of small-sized plants and has become a widespread tool for facility comparison used by many research teams. It can model not only CRS but has also been modified to analyze the optical performances of parabolic troughs (DLR version) or even dishes. This FORTRAN code is based on a cone optics approach: flux density is produced combining error cone of the reflected ray and sun shape by convolution of independent distributions with Fourier transform. Heliostat facets of any shape and any curvature are divided into small elementary mirrors reflecting the sun image on the receiver aperture. HELIOS is considered by [START_REF] Izygon | Mode ´lisation des flux re ´fle ´chis par un champ d'he ´liostats[END_REF] to be the most flexible and sensitive code of its generation because of its low calculation time and its accuracy. However this tool needs a detailed description of heliostats geometry, it is quite complex and hard to use for an inex-perienced user [START_REF] Izygon | Mode ´lisation des flux re ´fle ´chis par un champ d'he ´liostats[END_REF]. It is not adapted to large heliostat fields, it can neither assess annual performances, nor optimize heliostat layout, nor use secondary optics. That is why we decided not to include HELIOS in our comparative study, contrary to MIRVAL and DELSOL.

Simultaneously in France numerous codes were developed before and during experiments at Themis. These are accurate and agree well with US codes from the same period [START_REF] Izygon | Mode ´lisation des flux re ´fle ´chis par un champ d'he ´liostats[END_REF], but they are not dedicated to forecast performances of other CRS facilities [START_REF] He ´nault | Concentration du rayonnement solaire par simple et double re ´flexion: contribution aux me ´thodes de re ´glage et de contro ˆle des surfaces re ´flectrices a `facettes, the `se soutenue a[END_REF]. Moreover when they were written computing techniques and hardware highly differ from current ones, and they have not been updated since the 80s. On a theoretical point of view, advanced mathematical models were used, but none of these tools has a user-friendly interface. The only code that has evolved after the Themis experiment is now called COSAC [START_REF] He ´nault | COSAC Version 2.5, Pre ´sentation Ge ´ne ´rale[END_REF]. This is a ray-tracing calculation tool without approximation, adapted to complex optical systems (spatial optical devices, 3-D spectrometers) but not to heliostat field modeling. Actually the user would need to describe heliostats one by one, to calculate sun position, to add instructions to account for shadowing and blocking. At the time of Themis, flux measurements techniques were still lacking precision, so comparisons between measures and calculations made illusive any improvement in accuracy of simulation codes [START_REF] Izygon | Mode ´lisation des flux re ´fle ´chis par un champ d'he ´liostats[END_REF].

Currently used codes

To compare in an objective and reliable way the features of tools currently used in research centers working on CSP technologies, a questionnaire was sent to developers or heavy users of six codes (UHC, DELSOL, HFLCAL, MIRVAL, FIAT LUX and SOLTRACE). This inquiry contained 23 questions about physical and computational model used, user interface developed, parameters involved, and results obtained.

Results of the inquiry

Table 1 sums up the main characteristics of these codes. These tools have features in common. First all of them use exact physical considerations to calculate cosine factor and S&B, except FIATLUX for which this feature is not implemented yet. Likewise all codes can be run in Windows environment on standard computers with user-friendly graphic interfaces like flux maps and 3-D visualization of the installation (Fig. 2) except for MIRVAL. The size of the solar field is not limited. These tools are in the public domain, except FIAT LUX which availability is pending on CIEMAT decision according to software policy, and HFLCAL. However source codes of SOLTRACE and some additional programs (WINDELSOL interface by CIEMAT or DLR extensions for MIRVAL) are not available now.

Two categories of codes

As it can be observed in Table 2, two main categories can be distinguished.

These two categories show many differences because they do not have the same objectives. MIRVAL, FIAT-LUX and SOLTRACE give a detailed description of the reflected power from a heliostat field but include neither thermodynamic conversion nor heliostat field optimization contrary to UHC, DELSOL and HFLCAL. These latter codes contain more approximations in their resolution methods, so they can quickly assess the annual performances of a large-size heliostat field but give less accurate results for small solar fields. To sum up, MIRVAL, FIAT-LUX, SOLTRACE or even HELIOS model the heliostat field and tower subsystem whereas UHC includes the receiver, and DELSOL, and HFLCAL simulate the whole CRS including receiver and power block.

Main features of each code

University of Houston codes

UHC codes (sometimes called RCELL suite) are a suite of interconnected Fortran77 codes, each with a number of optional operating modes, dealing primarily with the optical design of heliostat fields and receivers. Thermal and economic algorithms are incorporated to enable optimization, performance and design studies of the complete plant. Three of these codes are dedicated to the optical subsystem [START_REF] Falcone | A Handbook for Solar Central Receiver Design[END_REF]:

-NS (cellwise performance) provides interception and flux data, diurnal and annual flux data for fixed designs.

The solar field is divided into cells corresponding to regions with uniform heliostat density or fixed number of heliostats, or single heliostats and performance is calculated for a representative heliostat in each cell [START_REF] Lipps | A Cellwise method for the optimization of large central receiver systems[END_REF], -RCELL (cellwise optimization) optimizes solar components (heliostat spacings in field, field boundaries, tower and receiver dimensions) on cost/performance criteria obtaining interception factors for the optimization from a very simple model (for initial trials) or for accurate results from NS outputs (''node files''), -IH (individual heliostat layout and performance code) is a detailed layout processor using RCELL data to specify each heliostat location and can also compute performance for each heliostat or for the whole field.

Input is contained in four modules defining the site and weather, the heliostat design, the receiver design, and the type of run (interception data, field optimization, system optimization, heliostat layout or annual performance). By generating and using data bases (''cosine, shading, and blocking files'' and ''node files'') computing time is saved [START_REF] Falcone | A Handbook for Solar Central Receiver Design[END_REF]. UHC codes have been used to optimize heliostat fields and to evaluate optical performance of a number of CRS, including solar one and solar two.

Interesting features were added during solar two design and operation: an allowable flux constraint was used to optimize the new heliostat layout (Vant-Hull and [START_REF] Vant-Hull | Central receiver system optimization under an allowable flux constraint[END_REF] and a C++ version of UHC incorporating the structure and imaging characteristics of IH helped to compute, in real-time, aiming strategies to control solar flux density on a receiver aperture (protection against excess flux density) (Vant-Hull et al., 1996a).

DELSOL/WINDELSOL

This FORTRAN code was developed to fill the need for an accurate, yet fast, easy to use tool for performance analysis, design and optimization of large and small power systems for electricity and process heat applications [START_REF] Kistler | A user's manual for DELSOL3: a computer code for calculating the optical performance and optimal system design for solar thermal central receiver plants[END_REF]. It typically requires less computer time for perfor-mance calculations than either MIRVAL or HELIOS [START_REF] Falcone | A Handbook for Solar Central Receiver Design[END_REF]. DELSOL is based in part on the performances design approaches developed at the University of Houston (images generated by Hermite polynomials convolution, solar field divided into zones defined by their heliostat density for optimization). DELSOL optimizes tower height, receiver dimensions, and field boundaries on an economic basis, but requires a definition of the heliostat spacings in the field.

WINDELSOL is an adaptation of DELSOL for Windows with new features (user-friendly interface, optimized defined-by-coordinates heliostat field generation. . .) but cannot be used to analyze existing facilities (heliostat coordinates cannot be user-defined). Subdivision of field into sectors and data entry organization make it difficult to introduce new optical parameters. This program seems to be adapted to large systems but, as its developers grant, not accurate for small plants. Its user-friendliness turns it to an interesting tool for preliminary studies of large CRS projects.

HFLCAL

HFLCAL code (for ''Heliostats Field Layout CALculations'') was developed for the GAST hybrid concept (1986), and then applied to other concepts (like Phoebus). It calculates the annual production of a CRS plant depending on its configuration (heliostats, tower, receiver, cycle). The layout and optimization of the global system are designed to maximize electricity production per heliostat or minimize the cost of produced electricity. The annual production is assessed with an error smaller than 5% in comparison with the US codes HELIOS, DELSOL, and UHC [START_REF] Kiera | Heliostat field: computer codes, requirements, comparison of methods[END_REF]. Nowadays it is still updated and used for CRS modeling by DLR researchers (Final Technical Report Solgate, 2005), who consider it easier to adapt than DELSOL.

HFLCAL is able to carry out the hundreds of annual performance calculations needed for determining a costoptimized solar field layout in a reasonable time. It uses a simplified optical model with a flux distribution reflected by each heliostat approached by a circular Gaussian distribution obtained by convolution of three distributions (one accounting for the size and the luminance of the sun, a second for the heliostat error and a third one for tracking errors). Conversely in the 4 other codes described here various mirror errors are detailed (curvature, canting, microscopic. . .) panel by panel. Thus in HFLCAL it is sufficient to consider the central ray of one heliostat and to include these three errors with three standard deviations, according to a model settled by [START_REF] Rabl | Active Solar Concentrators and Their Applications[END_REF]. Consequently this code only does one calculation per heliostat whose focal length is considered constant or equal to slant range.

HFLCAL was used during the SOLGATE project to cost-optimize the solar part of a hybrid CRS plant with pressurized air volumetric receiver. Heliostat field size and layout, tower height, receiver geometry were adapted to the specific needs of HSGT systems for a given capacity at a given location. Comparisons with experimental measures showed that total power on receiver aperture can easily be calculated with high precision, whereas the precision on maximum flux is low and the flux distribution is not accurate (Final Technical Report Solgate, 2005).

MIRVAL

This tool was developed for the rigorous optical performance analysis of the envisaged concepts during Solar One preliminary study. It was used to check flux calculations from DELSOL, HELIOS and UHC. It is quite simple to use and, as it is implemented with Monte Carlo method, ray vectors onto the receiver aperture can be provided, so as to interface with a receiver or secondary concentrator model. For [START_REF] Izygon | Mode ´lisation des flux re ´fle ´chis par un champ d'he ´liostats[END_REF], MIRVAL turned out to be unsuitable for modeling the complex geometry of CETHEL heliostats of the Themis field.

FIAT LUX

Initially designed to validate the optical quality of heliostats, it enables the user to analyze in details existing installations, with a sharp representation of the heliostat geometry. Heliostat orientation can be easily checked before calculation by 3-D visualization of reflected rays from the center of the facets. Flux distribution after calculation is statistically described. FIAT LUX was compared with HELIOS on a Martin-Marietta heliostat, showing good agreement (Monterreal, 2000). Nevertheless this tool is now being developed: a few functionalities are still missing such as S&B effects and user interface.

SOLTRACE

The objective of SOLTRACE is to model complex optical systems for solar power and to analyze their performance [START_REF] Wendelin | SolTRACE: a new optical modeling tool for concentrating solar optics[END_REF]. It is adapted not only to the three main technologies to thermodynamically convert sun power into electricity (CRS, PT and DS) but also to solar furnaces. The optical system is organized in stages in a global coordinates system: stages are sections of the optical geometry that are successively hit by rays in their pathway from the sun to the final receiver. They can be optical ones (physical interactions with rays) or virtual ones (useful to determine positions of rays or flux maps wherever in the system without interacting with them). The reason for this organization is to trace rays in an efficient way in order to limit calculation time. One stage is composed by elements with their own aperture, shape, and optical properties (slope error, specularity error, reflectivity, transmissivity, and refraction). Each element is described in a coordinates system related to its stage.

Contrary to other tools, atmospheric attenuation and tracking errors are not directly included but they can be taken into account, respectively, in the reflectivity of the mirrors and in their slope error. Moreover the complete description of geometry of the elements leads to a cumbersome interface. Indeed for each system all the heliostat field geometry must be built in a spreadsheet including:

-definition of the center, the aperture, the normal direction, and the curvature of each facet of each heliostat in the stage coordinates system; -definition of each stage coordinates system in relation to the global coordinates system.

This organization implies the calculation of the position of each facet of each heliostat depending on the sun position.

Other codes

The ray-tracing code OPTEC (for ''optimization program for terminal concentrators'', [START_REF] Schoffel | Final report terminal concentrators[END_REF] can handle a solar furnace [START_REF] Neumann | The influence of sunshape on the DLR solar furnace beam[END_REF], a heliostat field or a parabolic dish and add a cone, trumpet or compound parabolic concentrator as terminal concentrator.

Similarly to research centers, industrial groups promoting CSP projects took an interest in calculation of solar field performances through the development of similar tools, often dedicated to design optimization, and not diffused. For example one can note SOLVER from SOLU-CAR (ray tracing for CRS modeling) and SENSOL from SENER (for CRS and PT modeling, [START_REF] Relloso | Solar projects analysis using SENSOL[END_REF].

To complete this overview we shall also quote codes currently in development, intended to replace codes from the 80s, sometimes considered as hard to use and modify, non-adapted to dynamic modeling, lacking documentation, and limited by obsolete computing power constraints [START_REF] Blanco | The TONATIUH software development project: an open source approach to the simulation of solar concentrating systems[END_REF]. These new codes, more modular, will tackle more easily simulation of new CSP systems, such as hybrid concepts, cogeneration plants or with multi-staged receivers. Besides SOLTRACE which may constitute the first code of this generation, TONATIUH [START_REF] Blanco | The TONATIUH software development project: an open source approach to the simulation of solar concentrating systems[END_REF] aims to be a sophisticated software environment for design, optimization and analysis of all CRS, PT, and DS. This open source ray-tracing code will enable users to visualize the installation with state-of-the-art 3-D interface technologies. CIEMAT laboratory is currently developing a series of tools called SCT (Solar Concentration Toolbox) Package, developed under MATLAB software [START_REF] Sanchez | Methodology for generation of heliostat field layout in central receiver systems based on yearly normalized energy surfaces[END_REF]. One tool is dedicated to optical design and optimization of solar receivers, another one to generation of random rays, and the last one to CRS optimization and performance. These codes will use either ray-tracing or convolution techniques. At last the EPFL-LENI laboratory worked to integrate a heliostat-tower model built with RADIANCE in a MATLAB environment and has recently developed a multi-objective evolutionary algorithm for the design of heliostat fields [START_REF] Pelet | Design of heliostat fields using a multi-objective evolutionary algorithm[END_REF].

How to chose an adapted calculation code?

For researchers or engineers that would need to model CRS, it may be difficult to find the right tool that best fit to the question asked. We hope that this study will help them to solve this problem. In the case of an industrial project, an interesting strategy may be to determine first the general layout of the plant (tower height, heliostat field boundary, receiver geometry and technology, storage capacity) from key parameters such as power block characteristics, meteorological data, and load curves. This analysis can be done with tools from the first category. Then a more detailed study including a closer description of heliostats and receiver geometry can be led with tools from the second category to assess solar field performances or to improve pointing and operating strategies.

Another approach to assess global performances of CRS may be the following:

-first to use solar field efficiency matrixes and receiver performances curves from specific codes like the five ones quoted above, -then to integrate these data in TRNSYS with STEC library or in less detailed tools based on a simple energy balance formulation (SOLERGY, ECOSTAR).

With this second approach, one component can be modified more easily without changing the global model. Besides a wider range of concepts can be simulated. However efforts must be made to link components in a consistent way to avoid discrepancies.

Conclusions and perspectives

This study has permitted us to classify tools into two categories, corresponding to two types of problems encountered in studies on CSP technologies:

(1) optimization codes to design installation projects like HFLCAL, UHC-RCELL, or (WIN)DELSOL; (2) performance analysis codes to assess optical performances of a given installation like FIAT LUX, MIR-VAL, UHC-NS or IH, or SOLTRACE.

It is worth noting that other types of modeling tools might be necessary to design and study CRS systems like specific codes for receiver modeling, for heliostat field layout optimization or transient analysis.

For research laboratories or industrial developers, it seems interesting to use one code from each category. Indeed codes from the first category are able to assess the available flux distribution at the top of a tower and thus to size up a receiver. That is why we chose to use SOL-TRACE to model the optical components of an existing facility. This code will be used to study the solar field performances depending on pointing strategy, to describe the flux distribution on receiver module aperture, and to elaborate an accurate efficiency matrix. Besides SOLTRACE can be adapted to other concentrating systems (solar furnaces, dish-Stirling). Tools from the second category, like (WIN)DELSOL, are recommended for techno-economic assessment of CRS technologies. However various additions or modifications must be brought to adapt them to new CRS concepts.

Comparing results of updated codes from both categories on one or various reference test cases should be a challenging work in partnership for research centers involved in CRS or other CSP technologies.

Fig. 1 .

 1 Fig. 1. View of the Themis facility and scheme of the Pegase system.

Fig. 2 .

 2 Fig. 2. Visualization of results from Windelsol and Soltrace for the Pegase facility: (a) annual performances of the solar field from WINDELSOL (view from above); (b) impacting rays on the collector surface from SOLTRACE.

Table 1

 1 Main features of five codes for concentrated solar flux calculation

	Name of code	UHC	DELSOL	HFLCAL	MIRVAL	FIAT LUX	SolTRACE
	Research	University of Houston	SANDIA	GAST project	SANDIA	CIEMAT-PSA	NREL
	team
	Currently	SANDIA, Tietronix	CIEMAT, SANDIA	DLR	DLR,	CIEMAT	NREL,
	used by				SANDIA		CNRS
	Considered	CRS-SF	CRS	CRS	CRS	CRS	CRS-PT-DS-
	technologies						SF
	Development	1974	1978	1986	1978	1999	1999
	start
	Availability	Source and executable version Source and executable	Not available	Source and	?	Executable
			version		executable		version
					version
	Programming	FORTRAN/C++	FORTRAN/Basic	FORTRAN	FORTRAN MATLAB	Delphi5
	language
	Flux	Hermite polynomial	Hermite polynomial	Simplified convolution	Monte	Normally	Monte Carlo
	calculation	expansion/convolution	expansion/convolution	of each heliostat's flux	Carlo ray-	distributed random	ray-tracing
	method				tracing	value of 'slope
						error c
	Receiver type	Flat, cavity or external	Flat, cavity or external	Flat, cylindrical or	Flat, cavity	Flat	Almost any
		cylinder	cylinder	conical	or external		receiver
					cylinder		configuration
	Multiple	Beam-down but not secondary	No	Yes	Yes	No	Yes
	reflections	concentrators
	Contribution	Yes	Yes	Yes	Yes	No	No
	of each loss
	Annual	Yes	Yes	Yes	Yes	No	No
	performances
	Optimized	Heliostats layout and	Heliostat boundary,	Heliostat layout, tower	Heliostat	Not available	Not
	components	boundary, tower height,	layout a , tower height,	height, receiver area	layout b		available
		receiver geometry	receiver size, storage	and orientation
			capacity
	Optimization	Energy or cost criteria with	Cost criteria with optional	Energy or cost criteria Energy	Not available	Not
	criteria and	allowable flux/land constraint	flux/land constraints		criteria b		available
	constraints

a Uses UHC defined spacings.

Table 2

 2

	Two categories of flux calculation codes
		Optimization codes	Performance analysis codes
	Tools	UHC-RCELL, (WIN)DELSOL, HFLCAL	UHC-IH or NS, MIRVAL, FIAT LUX,
			SOLTRACE, HELIOS, (DELSOL)
	Considered subsystems	Overall plant	Optical subsystem
	Data entry	Total ground area of the solar field divided	Detailed heliostat geometry, in text files.
		into zones, in windows
	Calculation method	Simplified convolution	Monte Carlo or similar
	Calculation time	Some seconds for a whole heliostat field	Some seconds to some minutes for one single heliostat
	Accuracy	Increase with field size	Accurate for one heliostat
	Economic calculations	Yes	No

b With DLR additional program only. c For each unitary normal vector to the mirror surface, then calculation of deterministic trajectories for the bundle of reflected rays coming from solar disk with geometric optics laws.

Acknowledgements

The authors wish to thank Allan Lewandowski from NREL, Marcelino Sanchez and Rafael Monterreal from CIEMAT, Mark Schmitz, Peter Schwarzbo ¨zl, and Reiner Buck from DLR, Franc ¸ois He ´nault from CRAL -Observatoire de Lyon, Manuel Blanco from University of Texas, Lorin Vant-Hull from University of Houston and Rafaele Bolliger from EPFL-LENI for their invaluable help.