Text in Afrikaans / This thesis deals with the nature and operation of liens in South African law. Whereas enrichment liens are classified as real rights, debtor creditor liens are not (sometimes they are referred to as personal rights). The South African law of lien is principally founded on Roman Dutch law and is similar to Dutch law before the enactment of the current Burgerlijk Wetboek (BW). In Dutch law the BW specifies which persons have a lien and under what circumstances. In South African law a creditor can establish a lien against an owner who has a duty to perform in terms of an agreement between the parties or, in the absence of such an agreement, on the basis of unjustified enrichment. My research has shown that a lien is not a subjective right but a capacity vested in a creditor by operation of law. It empowers him to retain an owner’s (debtor) thing until the creditor’s claim against him has been discharged. Extra-judicially the lien holder can rely on his lien or he can raise it as a defence against the owner’s rei vindicatio. The categorisation of liens as either enrichment liens, or debtor creditor liens is at the most an indication of the different sources of the creditor’s right to which the lien is accessory. In Dutch law liens are classified as verhaalsrechten on the owner’s estate. These rights (verhaalsrechten) are further classified as specific opschortingsrechten. A lien is therefore a capacity granted to certain creditors by law to retain an owner’s (debtor) thing. Regarding the operation of liens against third parties (derdenwerking), Dutch law distinguishes between a third party with an older right to the thing and a third party with a later right to the thing. In South African case law derdewerking (real operation) of liens is often used to explain the fact that the lienholder may, in the absence of an agreement with the owner, retain the owner’s thing until his claim against the owner has been discharged. In South African law a lien is a defence to the owner’s rei vindicatio. Reliance on real operation (derdewerking) is often a means of explaining that a lien exists against an owner with whom the creditor had no agreement. Real operation, however, refers to the question against whom other than the owner the lien may be enforced once its existence has been established. A lien can be enforced against the creditors of the owner (debtor), the curator of the debtor’s insolvent estate, heirs, mortgagees and servitude holders. In Dutch law the BW grants preference to liens. Owing to the particular provisions of the Insolvency Act 24 of 1936 liens in South African law enjoy preference above other secured creditors of the insolvent owner (debtor). / Hierdie proefskrif handel oor die aard en werking van retensieregte in die Suid- Afrikaanse reg. Verrykingsretensieregte word algemeen as saaklike regte bestempel en skuldeiser-skuldenaar-retensieregte nie. Laasgenoemde word soms as persoonlike regte tipeer. Die Suid-Afrikaanse reg insake retensieregte is hoofsaaklik op die Romeins-Hollandse reg geskoei en stem in ‘n groot mate ooreen met die posisie in die Nederlandse reg voor die inwerkingtreding van die huidige Burgerlijk Wetboek (BW). In die Nederlandse reg bepaal die BW uitdruklik watter persone in watter omstandighede ‘n retensiereg het. In die Suid-Afrikaanse reg kan ‘n skuldeiser ‘n retensiereg vestig teen ‘n eienaar wat prestasiepligtig is op grond van ‘n ooreenkoms met die skuldeiser of, in die afwesigheid van ‘n ooreenkoms, op grond van verryking. My navorsing toon dat ‘n retensiereg nie ‘n subjektiewe reg is nie, maar ‘n terughoudingsbevoegdheid wat deur regswerking totstandkom. Dit stel die skuldeiser in staat om die eienaar (skuldenaar) se saak te hou totdat die eienaar voldoen aan die vorderingsreg wat die skuldeiser teen hom het. Die retensiereghouer kan buitegeregtelik daarop steun, of dit as ‘n verweer teen die eienaar se rei vindicatio aanwend. Die kategorisering van retensieregte in skuldeiser-skuldenaar-retensieregte en verrykingsretensieregte is hoogstens aanduidend van die verskillende ontstaansbronne van die vorderingsreg waartoe die retensiereg aksessoor is. In die Nederlandse reg word retensieregte in die BW as verhaalsregte op die eienaar se boedel geklassifiseer. Hierdie verhaalsregte word uitdruklik as besondere opskortingsregte getipeer. ‘n Retensiereg is dus ‘n terughoudingsbevoegdheid wat deur die objektiewe reg aan sekere skuldeisers verleen word. Ten aansien van die derdewerking van retensieregte tref die Nederlandse reg ‘n onderskeid tussen ‘n derde met ‘n later reg op die saak en ‘n derde met ‘n ouer reg op die saak. In die Suid-Afrikaanse regspraak word “derdewerking” dikwels aangewend om te verklaar waarom die retensiereghouer ‘n eienaar se saak mag terughou totdat hy vergoed is vir uitgawes wat hy aan die eienaar se saak aangebring het, terwyl hy geen ooreenkoms met die eienaar gehad het nie. In die Suid-Afrikaanse reg is ‘n retensiereg ‘n verweer wat teen die eienaar se rei vindicatio geopper kan word. Die beroep op “derdewerking” is dus dikwels die kapstok om te bepaal of ‘n retensiereg teen die eienaar geopper kan word, terwyl dit eintlik verwys na die afdwingbaarheid van ‘n bestaande retensiereg teen ander persone as die eienaar. ‘n Retensiereg kan teen skuldeisers van die skuldenaar, die kurator van die skuldenaar se insolvente boedel, erfgename, verbandhouers en serwituutgeregtigdes afgedwing word. In die Nederlandse reg verleen die BW voorkeur aan retensieregte. Vanweë die besondere bepalings in die Insolvensiewet 24 van 1936 geniet retensieregte in die Suid-Afrikaanse reg ook voorkeur bo ander versekerde skuldeisers van die insolvente eienaar (skuldenaar). / Private Law / LL.D.

Text in Afrikaans / This thesis deals with the nature and operation of liens in South African law. Whereas enrichment liens are classified as real rights, debtor creditor liens are not (sometimes they are referred to as personal rights). The South African law of lien is principally founded on Roman Dutch law and is similar to Dutch law before the enactment of the current Burgerlijk Wetboek (BW). In Dutch law the BW specifies which persons have a lien and under what circumstances. In South African law a creditor can establish a lien against an owner who has a duty to perform in terms of an agreement between the parties or, in the absence of such an agreement, on the basis of unjustified enrichment. My research has shown that a lien is not a subjective right but a capacity vested in a creditor by operation of law. It empowers him to retain an owner’s (debtor) thing until the creditor’s claim against him has been discharged. Extra-judicially the lien holder can rely on his lien or he can raise it as a defence against the owner’s rei vindicatio. The categorisation of liens as either enrichment liens, or debtor creditor liens is at the most an indication of the different sources of the creditor’s right to which the lien is accessory. In Dutch law liens are classified as verhaalsrechten on the owner’s estate. These rights (verhaalsrechten) are further classified as specific opschortingsrechten. A lien is therefore a capacity granted to certain creditors by law to retain an owner’s (debtor) thing. Regarding the operation of liens against third parties (derdenwerking), Dutch law distinguishes between a third party with an older right to the thing and a third party with a later right to the thing. In South African case law derdewerking (real operation) of liens is often used to explain the fact that the lienholder may, in the absence of an agreement with the owner, retain the owner’s thing until his claim against the owner has been discharged. In South African law a lien is a defence to the owner’s rei vindicatio. Reliance on real operation (derdewerking) is often a means of explaining that a lien exists against an owner with whom the creditor had no agreement. Real operation, however, refers to the question against whom other than the owner the lien may be enforced once its existence has been established. A lien can be enforced against the creditors of the owner (debtor), the curator of the debtor’s insolvent estate, heirs, mortgagees and servitude holders. In Dutch law the BW grants preference to liens. Owing to the particular provisions of the Insolvency Act 24 of 1936 liens in South African law enjoy preference above other secured creditors of the insolvent owner (debtor). / Hierdie proefskrif handel oor die aard en werking van retensieregte in die Suid- Afrikaanse reg. Verrykingsretensieregte word algemeen as saaklike regte bestempel en skuldeiser-skuldenaar-retensieregte nie. Laasgenoemde word soms as persoonlike regte tipeer. Die Suid-Afrikaanse reg insake retensieregte is hoofsaaklik op die Romeins-Hollandse reg geskoei en stem in ‘n groot mate ooreen met die posisie in die Nederlandse reg voor die inwerkingtreding van die huidige Burgerlijk Wetboek (BW). In die Nederlandse reg bepaal die BW uitdruklik watter persone in watter omstandighede ‘n retensiereg het. In die Suid-Afrikaanse reg kan ‘n skuldeiser ‘n retensiereg vestig teen ‘n eienaar wat prestasiepligtig is op grond van ‘n ooreenkoms met die skuldeiser of, in die afwesigheid van ‘n ooreenkoms, op grond van verryking. My navorsing toon dat ‘n retensiereg nie ‘n subjektiewe reg is nie, maar ‘n terughoudingsbevoegdheid wat deur regswerking totstandkom. Dit stel die skuldeiser in staat om die eienaar (skuldenaar) se saak te hou totdat die eienaar voldoen aan die vorderingsreg wat die skuldeiser teen hom het. Die retensiereghouer kan buitegeregtelik daarop steun, of dit as ‘n verweer teen die eienaar se rei vindicatio aanwend. Die kategorisering van retensieregte in skuldeiser-skuldenaar-retensieregte en verrykingsretensieregte is hoogstens aanduidend van die verskillende ontstaansbronne van die vorderingsreg waartoe die retensiereg aksessoor is. In die Nederlandse reg word retensieregte in die BW as verhaalsregte op die eienaar se boedel geklassifiseer. Hierdie verhaalsregte word uitdruklik as besondere opskortingsregte getipeer. ‘n Retensiereg is dus ‘n terughoudingsbevoegdheid wat deur die objektiewe reg aan sekere skuldeisers verleen word. Ten aansien van die derdewerking van retensieregte tref die Nederlandse reg ‘n onderskeid tussen ‘n derde met ‘n later reg op die saak en ‘n derde met ‘n ouer reg op die saak. In die Suid-Afrikaanse regspraak word “derdewerking” dikwels aangewend om te verklaar waarom die retensiereghouer ‘n eienaar se saak mag terughou totdat hy vergoed is vir uitgawes wat hy aan die eienaar se saak aangebring het, terwyl hy geen ooreenkoms met die eienaar gehad het nie. In die Suid-Afrikaanse reg is ‘n retensiereg ‘n verweer wat teen die eienaar se rei vindicatio geopper kan word. Die beroep op “derdewerking” is dus dikwels die kapstok om te bepaal of ‘n retensiereg teen die eienaar geopper kan word, terwyl dit eintlik verwys na die afdwingbaarheid van ‘n bestaande retensiereg teen ander persone as die eienaar. ‘n Retensiereg kan teen skuldeisers van die skuldenaar, die kurator van die skuldenaar se insolvente boedel, erfgename, verbandhouers en serwituutgeregtigdes afgedwing word. In die Nederlandse reg verleen die BW voorkeur aan retensieregte. Vanweë die besondere bepalings in die Insolvensiewet 24 van 1936 geniet retensieregte in die Suid-Afrikaanse reg ook voorkeur bo ander versekerde skuldeisers van die insolvente eienaar (skuldenaar). / Private Law / LL.D.

Σκοπός αυτής της διπλωματικής εργασίας είναι να εμβαθύνουμε στη λειτουργία φωτοβολταϊκού πλαισίου μονοκρυσταλλικού πυριτίου και μέσα από τα αριθμητικά δεδομένα των μετρήσεων και των υπολογισμών, να αποφανθούμε πώς η λειτουργία σε πραγματικές συνθήκες μπορεί να επηρεάσει την παραγόμενη ισχύ του. Στα πλαίσια αυτά, πραγματοποιήθηκαν μετρήσεις ρεύματος και τάσης, στο χώρο του τμήματος των Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών, με φωτοβολταϊκό πλαίσιο μονοκρυσταλλικού πυριτίου ισχύος αιχμής 80 W. Οι μετρήσεις γίνονταν μια φορά την εβδομάδα κατά τη διάρκεια ενός έτους περίπου (2008-2009) και στόχος ήταν να διεξαχθούν μετρήσεις υπό διάφορες συνθήκες ακτινοβολίας και θερμοκρασίας και για αρκετές γωνίες κλίσης ώστε να αποκτήσουμε μια ολοκληρωμένη εικόνα της ενεργειακής του συμπεριφοράς. Στη διάρκεια των μετρήσεων αλλάζαμε την τιμή ενός μεταβλητού φορτίου, για να πάρουμε τη χαρακτηριστική ρεύματος τάσης του συγκεκριμένου πλαισίου και επιπλέον σημειώναμε την ακτινοβολία, τη θερμοκρασία του περιβάλλοντος, του κυττάρου και της πίσω όψης του, καθώς και της κλίσης τοποθέτησης. Ακόμα ελέγχαμε πώς επηρεάζει τη χαρακτηριστική καμπύλη I-V, και κατά συνέπεια την απόδοση, τυχόν φυσική σκίαση από παρακείμενο αντικείμενο. Ο προσανατολισμός των πλαισίων ήταν πάντα προς το Νότο, ώστε να έχουμε περισσότερες ώρες ηλιοφάνειας, μίας και η Ελλάδα είναι χώρα του βόρειου ημισφαιρίου. Κατά την επεξεργασία των μετρήσεων καταλήξαμε στην βέλτιστη κλίση τοποθέτησης του πλαισίου ανά εποχή και είδαμε πως η ακτινοβολία επιδρά θετικά στην απόδοση του σε αντίθεση με τη θερμοκρασία του κυττάρου που τη μειώνει όταν αυτή αυξάνεται. Τέλος, έγινε μια σύγκριση των τιμών που δίνει ο κατασκευαστής σε εργαστηριακό περιβάλλον με τις τιμές των μετρήσεων για να διαπιστώσουμε τις απώλειες που έχουμε όταν το μονοκρυσταλλικό πλαίσιο λειτουργεί σε πραγματικές συνθήκες. / The aim of this diploma thesis is to take a better look at the operation of a monocrystalline silicon photovoltaic module and through the numerical data of measurements and the calculations, to come to a conclusion about how the operation in real conditions can influence his produced power. Measurements of current and tendency have been made in the area of the department of Electrical and Computer Engineering using a monocrystalline silicon photovoltaic module of peak power 80 W. The measurements took place once a week for about a year (2008-2009) and our goal was to obtain measurements under various conditions of radiation and temperature and for some angles of bent so that we acquire a completed picture of its energy behavior. During the measurements we changed a variable load, in order to form the characteristic curve of current and tendency of the module and we also noted down the radiation, the environmental, the cell and the back side temperature of the module, as well as the bent of placement. Moreover, we checked how a possible natural shading from an adjacent object influences the characteristic I-V curve, and as a result the efficiency of the module. The orientation of the module was always South, in order to gain more hours of sunlight, since Greece is a country of the northern hemisphere. While processing the measurements, we found the optimal bent of placement per season for the module and we saw that the radiation affects positively its efficiency contrary to the cell temperature that decreases the efficiency when increased. Finally, we compare the electrical specifications in laboratorial environment that the constructor gives, with the measurements in order to realise the losses that we have when the monocrystalline module functions in real conditions.