Θερμο - οικονομική βελτιστοποίηση κύκλου rankine οργανικού μέσου σε πολλαπλές κλίμακες ισχύος και αξιολόγηση μεθόδων αύξησης της ενεργειακής και εξεργειακής του απόδοσης

The scope of the present thesis encompasses a number of strategies for enhancing the cost competitiveness of Organic Rankine Cycle (ORC) energy systems. These strategies include: 1) the thermo-economic optimization of the standard ORC (SORC) (chapter 3) and 2) the theoretical investigation of innovative, advanced cycle configurations and concepts for improving its first and second law efficiency (chapters 4, 5 and 6). Finally, in chapter 7 the development and construction of an innovative domestic scale biomass-fueled trigeneration system based on the combined operation of a supercritical ORC and a vapour compression cycle (VCC) is presented. In the first chapter, an overview of the historical growth and current technological and commercialization status of the ORC technology is presented, including an analysis of the market share of the largest companies which are active in the field and their main target applications. The potential and state-of-the-art of ORC systems oriented towards the utilization of different energy sources (geothermal, waste heat recovery, biomass, solar and domestic/micro-CHP) are briefly discussed, while the main advantages and barriers are outlined in each case. The chapter closes with a more detailed analysis of the motivation and goals that have driven the work undertaken throughout this thesis.In the second chapter, the fundamental principles of ORC technology and the general methodological guidelines followed in the thesis are presented. Firstly, the advantages of ORCs against the conventional water-steam Rankine cycle are summarized. Secondly, the properties of organic working fluids, which constitute a critical component of the design of ORC applications, are analyzed. The main advantages and potential drawbacks of hydrocarbon working fluids, which are considered in the present thesis, are highlighted. Subsequently, the main principles governing the design of ORC systems are introduced and the main aspects of the methodology followed in the subsequent chapters of the thesis are presented and discussed. Furthermore, the main thermodynamic performance evaluation indexes used in the thesis are defined and explained. The following section of the chapter involves a brief introduction into some commonly considered efficiency improvement strategies. In the last part of the chapter, a list of the working fluids that are examined is given and their main properties are presented. The third chapter is focused on the development of an integrated, holistic methodology aiming at the thermo-economic optimization of ORCs, oriented primarily towards waste heat recovery. Within the investigated scale range (1-700 kWe), the specific investment cost (SIC) of ORC modules varies from a minimum of approximately 2000 €/kWe at medium/higher scales to a maximum of over 8000 €/kWe at very low scales. More specifically, at very low (<10 kWe) capacities, which correspond to micro ORC systems, the SIC is very high, ranging from approximately 4000 €/kWe to 8000 €/kWe. At 50 kWe, the SIC ranges from 3000 €/kWe to 6000-7000 €/kWe, while at 100 kWe it typically ranges from 2500 €/kWe to 6000 €/kWe. There is a considerable economy of scale effect for power capacities of up to 50-100 kWe. For installed power capacities higher than 100 kWe, the economy of scale effect becomes progressively weaker and practically disappears at capacities above 300 kWe, as the SIC variation for higher power outputs becomes nearly flat, being stabilized at around 2000 €/kWe. Due to the increased contribution of the expanders into the total equipment cost, the expander type selection and configuration greatly influence the overall capital expenditure. For low critical temperature fluids, screw expanders are more cost-effective than turbines, due to their higher vapour densities. On the other hand, for high critical temperature fluids turbines are more favourable from a techno-economic viewpoint. The most critical design variables are the evaporation pressure as well as the condensation temperature of the cycle, followed by the superheating degree, which can play a major role in reducing the costs of expansion machines. Furthermore, the evaporator pinch point is a neutral variable, having a minor influence on the total costs, with the exception of low-temperature applications. Lastly, it was determined that while having the potential to increase the power output of ORCs, the addition recuperation is generally not favourable from a thermo-economic standpoint in waste heat recovery applications.In the fourth chapter, the concept of double stage ORCs (DSORCs) is investigated. More specifically, a DSORC is modeled, consisting of a high (HT) and a low-temperature (LT) stage. Configurations including the same and different working fluids in the two stages have been examined. An optimization routine was developed to calculate the optimal values of the evaporation pressures of the two stages, as well as the pinch point in the evaporator and the condensation temperature of the HT stage in order to maximize the total power output of the system within a wide range of heat source temperatures. The main design variables affecting the optimization of DSORCs are primarily the evaporation pressures in each stage and, at a lesser extent, the pinch point value in the evaporator of the HT stage. It was shown that the optimal selection of the evaporation pressures in the two stages and the pinch point in the HT stage evaporator is largely influenced by the relation among the heat source inlet temperature and the critical temperatures of the working fluids of the DSORC. In principle, low evaporation pressures for both stages should be chosen when the heat source temperature is very low. In this case, low pinch point values should be set for the HT stage evaporator. When, on the other hand, the heat source temperature is very high, high and low pressures should be selected for the evaporators of the HT and LT stage respectively, while the pinch point value in the HT stage evaporator should be increased. Lastly, when the heat source temperature is located between the critical temperatures of the working fluids used in the DSORC stages, both evaporation pressures should be moderately increased, while the pinch point in the HT stage evaporator should be occasionally given high values. DSORCs are favourable compared to single stage cycles operating with the two respective working fluids used in their stages mainly when the heat source temperature is either far lower than the critical temperatures of these fluids, or, secondarily, within a specific range between them. The maximum relative exergetic efficiency benefits can be as high as about around 20 %. DSORCs can be an appealing option, especially for low-temperature waste heat recovery applications, when working fluids that could produce more efficient single stage cycles (under the given heat source temperatures) are unavailable for technical, economic, environmental or safety reasons. In this case, less expensive or more easily available alternative high critical temperature fluids can be coupled with lower critical temperature fluids.In the fifth chapter, binary zeotropic mixtures of 7 natural hydrocarbons at variable molar concentration ratios corresponding to a total of 196 different working fluid combinations are evaluated as working fluids of ORC systems. The relative exergetic efficiency improvement compared to optimized pure fluid ORCs (PORCs) ranges from a minimum of 3.6 % to up to 48.3 %. In principle, the highest exergetic efficiency benefits for most mixtures are attained at very low heat source temperatures, ranging from 100 ºC to 140 ºC. On the other hand, at heat source temperatures higher than 200-260 ºC (depending on the mixture), the performance advantage of ZORCs compared to PORCs becomes insignificant. This happens because the increase of the heat source utilization efficiency attained by ZORCs is relatively more significant at lower heat source temperatures and becomes irrelevant for high grade heat sources. Considering a specific binary mixture, ZORCs are mostly favourable compared to the PORCs of its components when a) the heat source temperature is lower than the critical temperature of the low critical temperature component (LTC) and, at a lesser extent, b) within a region between the critical high critical temperature component (HTC) and LTC critical temperatures. ZORCs could be especially attractive within the low Ths region between 100-140 °C, in which they have significantly improved performance and similar rotational speeds and size parameters compared to PORC. Furthermore, they could be economically feasible within the temperature region from 180 to 240 °C, in which despite their lower efficiency improvement potential, the turbine cost savings could make up for the increased cost of heat exchangers. In the sixth chapter, the application of regeneration for increasing the energetic and exergetic performance of ORCs is investigated. Three non-recuperative and recuperative regenerative ORC (RGORC) configurations, the O-ORC, the CB-ORC and the CF-ORC, were investigated, optimized and compared to the standard cycle. The recuperative SORC always performs better than the non-recuperative regenerative cycles. As a result, non-recuperative regenerative configurations are not recommended as alternatives to recuperative SORC configurations since they lead to lower efficiency at increased costs, due to the increased equipment components and construction complexity. However, they could constitute an appealing option when the implementation of recuperative standard ORCs is not possible, which is the case for low Tcrit fluids. On the other hand, recuperative RGORCs are superior compared to the recuperative SORC, while their efficiency improvement potential is negatively correlated to the superheating degree. When a superheating degree of 40 K is assumed, the highest relative energetic efficiency increase is observed in the case of the CF-ORC, ranging from 2.59 % to 7.93 %, followed by the O-ORC, ranging from 1.29 % to 6.45 %. When a superheating degree of 10 K is assumed, the relative energetic efficiency increase in the case of the CF-ORC, ranges from 2.78 % to 9.47 %, followed by the O-ORC, ranging from 1.29 % to 14.40 %. The CB-ORC shows the lowest performance improvement, reaching a maximum relative energetic efficiency increase of 1.35 % compared to the recuperative SORC. The application of regenerative and recuperative cycles for improving the exergetic performance of standard ORC systems in waste heat recovery-type applications is not recommended, since the energetic efficiency improvement of regenerative ORCs fails to outbalance the increased exergy losses occurring due to the higher temperature of the working fluid at the evaporator inlet. Regardless, the implementation of RGORCs could be justified if higher Tcrit fluids that could be thermodynamically superior are unavailable. In the seventh chapter, the simulation, design and development of an innovative domestic scale biomass-powered trigeneration unit based on the interconnection of an ORC and a VCC is presented. The sizing and selection of the various mechanical and electrical equipment components is discussed. The system will be tested in the future to demonstrate its technical feasibility and actual performance potential and to determine efficiency improvement enhancements and modifications. In the eighth chapter, the main conclusions of the present thesis are summarized along with proposals for future work.Το αντικείμενο της παρούσας διατριβής αφορά στρατηγικές για την αύξηση της οικονομικής αποδοτικότητας των ενεργειακών συστημάτων κύκλου Rankine οργανικού μέσου (ORC). Οι στρατηγικές αυτές περιλαμβάνουν: 1) τη θερμο-οικονομική βελτιστοποίηση του συμβατικού ORC (SORC) (κεφάλαιο 3) και 2) τη θεωρητική διερεύνηση καινοτόμων προηγμένων διατάξεων του κύκλου και μεθόδων για τη βελτίωση του ενεργειακού και εξεργειακού του βαθμού απόδοσης (κεφάλαια 4, 5 και 6). Τέλος, στο κεφάλαιο 7 παρουσιάζεται η ανάπτυξη και η κατασκευή ενός καινοτόμου, οικιακής κλίμακας συστήματος τριπαραγωγής που τροφοδοτείται από βιομάζα και βασίζεται στη συνδυασμένη λειτουργία ενός υπερκρίσιμου ORC και ενός κύκλου συμπίεσης ατμού (VCC).Το πρώτο κεφάλαιο περιέχει μια επισκόπηση της ιστορικής εξέλιξης και της τρέχουσας τεχνολογικής και εμπορικής κατάστασης της τεχνολογίας ORC, ενώ αναλύεται το μερίδιο αγοράς των μεγαλύτερων κατασκευαστών και οι βασικές εφαρμογές τους. Οι δυνατότητες και οι προοπτικές των συστημάτων ORC που προσανατολίζονται στην αξιοποίηση διαφορετικών ενεργειακών πηγών (γεωθερμία, απορριπτόμενη θερμότητα, βιομάζα, ηλιακή ενέργεια και οικιακά συστήματα μικροσυμπαραγωγής) παρουσιάζονται εν συντομία, ενώ εκτίθενται τα κυριότερα πλεονεκτήματα και εμπόδια ανά περίπτωση. Το κεφάλαιο κλείνει μια πιο λεπτομερή ανάλυση του στόχου της διδακτορικής διατριβής και την παρουσίαση της δομής της. Στο δεύτερο κεφάλαιο παρουσιάζονται οι βασικές αρχές της τεχνολογίας ORC και η γενική μεθοδολογία που ακολουθείται στα κεφάλαια που ακολουθούν. Κατ’ αρχάς, αναλύονται τα πλεονεκτήματα της τεχνολογίας σε σχέση με τη συμβατική τεχνολογία κύκλου Rankine νερού-ατμού. Έπειτα, συνοψίζονται οι ιδιότητες διαφόρων οργανικών ουσιών, που αποτελούν κύριο στοιχείο σχεδιασμού συστημάτων ORC, ενώ επισημαίνονται τα βασικά πλεονεκτήματα και μειονεκτήματα των υδρογονανθράκων ως εργαζόμενων μέσων. Στη συνέχεια παρουσιάζονται οι σημαντικότερες αρχές καθώς και η μεθοδολογία σχεδιασμού συστημάτων ORC που ακολουθείται στα επόμενα κεφάλαια της εργασίας. Επιπλέον, ορίζονται οι χρησιμοποιούμενοι δείκτες θερμοδυναμικής απόδοσης, ενώ γίνεται σύντομη αναφορά σε βασικές στρατηγικές αύξησής της. Στο τέλος του κεφαλαίου δίνεται η λίστα των εργαζόμενων μέσων που εξετάζονται στην εργασία με τις βασικές τους ιδιότητες.Το τρίτο κεφάλαιο εστιάζει στην ανάπτυξη μιας ολοκληρωμένης μεθοδολογίας με στόχο τη θερμο-οικονομική βελτιστοποίηση συστημάτων ORC προοριζόμενων κυρίως για αξιοποίηση απορριπτόμενης θερμότητας. Εντός της εξεταζόμενης κλίμακας ισχύος (1-700 kWe), το ειδικό κόστος επένδυσης (ΕΚΕ) των συστημάτων κυμαίνεται από κατ’ ελάχιστον 2000 €/kWe σε μεσαίου και μεγάλης κλίμακας εφαρμογές έως και 12000 €/kWe σε μικρής κλίμακες ισχύος. Συγκεκριμένα, σε πολύ μικρές (