Θερμοδυναμική ανάλυση θερμαντλιών Vuilleumier

Vuilleumier heat pumps are machines that operate using heat at high temperature as energy input and they pump heat from a low to a moderate temperature level. The pressure fluctuation of the working gas is realized without the use of work, but thermally. They can be used for building heating and cooling or as refrigerators at very low (cryogenic) temperatures. The development of the Vuilleumier machines has been an ongoing process since the first patent of Rudolph Vuilleumier in 1918, aiming at the design of efficiently operating heat pumps and cryogenic refrigerators. At first, the research was performed in USA for the production of cryogenic refrigerators for space use. Later, the research was focused on Vuilleumier heat pumps for buildings and many experimental units were built and tested. Although many prototypes have been designed during all these years, Vuilleumier machines still exhibit low volumetric specific heat power due to inherent thermodynamic and aerodynamic losses. An accurate description and understanding of these complicated phenomena is required in order build competitive machines for modern applications. In the present PhD thesis, the fundamental parts of regenerative machines, the thermal and mechanical compressor are presented, which coupled form a Stirling heat pump. Then, the operation of the same Stirling machine as a prime mover, as a heat pump or as a refrigerator is explained and it follows a presentation of the connection between two Stirling machines to form a heat actuated heat pump like a Vuilleumier machine. Several configurations and types of the Vuilleumier machines are presented together with their operation and their benefits and disadvantages.Furthermore, an extensive presentation of the evolution of Vuilleumier machines from their first patent until now is conducted. All available studies, patents, books and reports are cited for both heat pumps and cryogenic refrigerators and the work of each researcher is briefly discussed. The theoretical or experimental performance of various Vuilleumier cryogenic refrigerators and heat pumps is summarized in corresponding tables and also all the Vuilleumier related US patents.Next, the types of existing models that are used for the prediction of the performance of Stirling engines are presented together with one-dimensional thermodynamic models generated, in the context of this dissertation, especially for Vuilleumier machines. At first, the development of the ideal isothermal model is presented. It follows the ideal adiabatic model and the thermodynamic segmentation of the machine into work-heat converting control volumes. Then the application of this model on real machines is discussed and the results of a comparison between the two ideal models are also presented. It is included in addition, the description of several losses inherent in a Vuilleumier machine and the physics behind the derivation of equations that calculate their values. Finally, a validation of the developed computer codes according to the thermodynamic models is conducted by comparison with experimental data.Moreover, there is a comprehensive three-dimensional Computational Fluid Dynamics (CFD) simulation of a Vuilleumier machine that was designed for this purpose at three different operating speeds. At first, existing CFD studies on Stirling engines are presented and then there is an analytical description of the designed machine, the boundary conditions applied, the computational grid used and all the equations that were utilized for the generation of this numerical model. For the simulation, a commercial CFD software was used. The results from the simulation are presented next, providing details about the thermodynamic and fluid mechanics quantities distribution in every space of the machine and the interaction between them. Useful 3D illustrations of the temperature, the pressure and the velocity are given too. Moreover, calculation of heat and work transfer between the machine and the surrounding is performed for all three speeds. Then, heat transfer coefficients are derived in relationship with the Reynolds number of the flow. Finally, the efficiency of the designed Vuilleumier machine is calculated and it is compared against experimental data.Οι αντλίες θερμότητας Vuilleumier είναι μηχανές οι οποίες λειτουργούν χρησιμοποιώντας θερμότητα σε υψηλή θερμοκρασία σαν πηγή ενέργειας και αντλούν θερμότητα από χαμηλή θερμοκρασία προς μία μέτρια. Η διακύμανση της πίεσης του εργαζόμενου αερίου πραγματοποιείται χωρίς την κατανάλωση έργου, αλλά θερμικά. Οι μηχανές αυτές μπορούν να χρησιμοποιηθούν για θέρμανση και ψύξη κτηρίων ή για ψύξη σε πολύ χαμηλές (κρυογονικές) θερμοκρασίες.Η εξέλιξη των μηχανών Vuilleumier γίνεται διαρκώς από την πρώτη πατέντα του Rudolph Vuilleumier το 1918 και σκοπό έχει την σχεδίαση αποδοτικών αντλιών θερμότητας και κρυογονικών ψυκτών. Αρχικά, η έρευνα πραγματοποιούταν στις ΗΠΑ για την παραγωγή κρυογονικών ψυκτών για διαστημικές εφαρμογές. Αργότερα, η έρευνα επικεντρώθηκε σε θερμαντλίες Vuilleumier για κτήρια και πολλές πειραματικές μονάδες κατασκευάστηκαν και δοκιμάστηκαν. Παρόλο που πολλά πρωτότυπα έχουν σχεδιαστεί όλα αυτά τα χρόνια, οι μηχανές Vuilleumier ακόμα παρουσιάζουν χαμηλή θερμική ισχύ για τον όγκο τους λόγω έμφυτων θερμοδυναμικών και αεροδυναμικών απωλειών. Είναι απαραίτητη μία ακριβής περιγραφή και η κατανόηση αυτών των πολύπλοκων φαινομένων προκειμένου να κατασκευαστούν ανταγωνιστικές μηχανές για σύγχρονες εφαρμογές. Στην παρούσα διατριβή παρουσιάζονται τα θεμελειώδη τμήματα των αναγεννητικών μηχανών, ο θερμικός και ο μηχανικός συμπιεστής οι οποίοι αν συνδυαστούν σχηματίζουν μια αντλία θερμότητας Stirling. Έπειτα, επεξηγείται η λειτουργία της ίδιας μηχανής Stirling σαν εργοπαραγωγός, σαν θερμαντλία ή σαν ψύκτης και ακολουθεί παρουσίαση της ένωσης μεταξύ δύο μηχανών Stirling για τον σχηματισμό θερμαντλίας που λειτουργεί με θερμότητα, παρόμοια με την μηχανή Vuilleumier. Παρουσιάζονται επιπλέον αρκετές διαμορφώσεις και τύποι μηχανών Vuilleumier, μαζί με την λειτουργία τους και τα πλεονεκτήματα και μειονεκτήματα τους.Ακόμα, πραγματοποιήθηκε μία εκτενής παρουσίαση της εξέλιξης των μηχανών Vuilleumier από την πρώτη πατέντα μέχρι σήμερα. Αναφέρονται όλες οι διαθέσιμες εργασίες, πατέντες, βιβλία και αναφορές τόσο για θερμαντλίες όσο και για κρυογονικούς ψύκτες. Το έργο κάθε ερευνητή συζητείται σύντομα. Η θεωρητική ή πειραματική επίδοση διαφόρων κρυογονικών ψυκτών και θερμαντλιών Vuilleumier συνοψίζεται σε πίνακες καθώς επίσης και όλες οι σχετικές με μηχανές Vuilleumier αμερικανικές πατέντες.Έπειτα, παρουσιάζονται οι τύποι υπαρχόντων μοντέλων που χρησιμοποιούνται για την πρόβλεψη των επιδόσεων κινητήρων Stirling μαζί με μονοδιάστατα θερμοδυναμικά μοντέλα που δημιουργήθηκαν στα πλαίσια αυτής της διατριβής ειδικά για μηχανές Vuilleumier. Στην αρχή, παρουσιάζεται η ανάπτυξη του ιδανικού ισοθερμοκρασιακού μοντέλου. Ακολουθεί, το ιδανικό αδιαβατικό μοντέλο και ο θερμοδυναμικός χωρισμός της μηχανής σε όγκους ελέγχου όπου έργο μετατρέπεται σε θερμότητα και αντίστροφα. Στην συνέχεια συζητείται η εφαρμογή αυτού του μοντέλου σε πραγματικές μηχανές καθώς και τα αποτελέσματα από μία σύγκριση μεταξύ των δύο ιδανικών μοντέλων. Περιλαμβάνεται επίσης η περιγραφή πολλών απωλειών που εμφανίζονται σε μηχανές Vuilleumier και η φυσική πίσω από την παραγωγή εξισώσεων που υπολογίζουν τις τιμές των απωλειών. Τέλος, πραγματοποιήθηκε μία τεκμηρίωση του κώδικα υπολογιστή που αναπτύχθηκε σύμφωνα με τα θερμοδυναμικά μοντέλα μέσω σύγκρισης με πειραματικά δεδομένα.Επιπλέον, έγινε μία πλήρης τριδιάστατη CFD (Computational Fluid Dynamics) προσομοίωση μίας μηχανής Vuilleumier που σχεδιάστηκε για αυτόν τον σκοπό, σε τρεις διαφορετικές ταχύτητες λειτουργίας. Αρχικά, παρουσιάζονται οι υπάρχουσες CFD εργασίες πάνω σε κινητήρες Stirling και έπειτα μία αναλυτική περιγραφή της σχεδιασμένης μηχανής, των συνοριακών συνθηκών που εφαρμόστηκαν, του υπολογιστικού πλέγματος και όλων των εξισώσεων που χρειάσθηκαν για την δημιουργία του αριθμητικού μοντέλου. Για την προσομοίωση χρησιμοποιήθηκε ένα εμπορικό λογισμικό CFD. Τα αποτελέσματα της προσομοίωσης παρουσιάζονται στην συνέχεια και παρέχουν λεπτομέρειες για την κατανομή θερμοδυναμικών και ρευστομηχανικών μεγεθών σε κάθε χώρο της μηχανής καθώς και της αλληλεπίδρασης μεταξύ τους. Δίνονται ακόμα χρήσιμες τριδιάστατες αναπαραστάσεις της θερμοκρασίας, της πίεσης και της ταχύτητας. Επιπλέον, πραγματοποιήθηκε υπολογισμός της μεταφοράς θερμότητας και έργου μεταξύ της μηχανής και του περιβάλλοντος και για τις τρεις ταχύτητες λειτουργίας. Στην συνέχεια, παρήχθησαν συντελεστές μεταφοράς θερμότητας σε συνάρτηση με τον αριθμό Reynolds της ροής. Τέλος, η απόδοση της σχεδιασμένης μηχανής Vuilleumier υπολογίστηκε και συγκρίθηκε με πειραματικά δεδομένα

Thermodynamic Analysis of Vuilleumier Heat Pumps

It has been 101 years since a Swiss-American engineer called Rudolph Vuilleumier conceived an idea on how to generate a heat pump device that would operate using heat at high temperature as energy input. The device that he invented, the Vuilleumier machine, utilized working gas expansion and compression at three variable volume spaces in order to pump heat from a low to a moderate temperature level. The interesting characteristic of the Vuilleumier machine is that the induced volume variations are realized without the use of work, but thermally. This is the reason why it has a potential to operate at modern applications where the pollution of the environment is not a choice. It is a perfect candidate for such applications, as it consists only of metallic parts and inert gas. Using these units for heating and cooling buildings, large energy savings can be accomplished as they can be operated at small scale in common buildings or at large scale providing heat power to entire building blocks without using fossil fuels. The use of Vuilleumier machines for industrial applications or inside vehicles is also a feasible option. Another field where these machines have already been involved is the cryogenics, as they are also able to provide refrigeration at very low temperatures like the very similar and well-known Stirling refrigerators. The possible uses of Vuilleumier machines can only be confined be the limits of imagination of the people being involved with them. The development of the Vuilleumier machines has been an ongoing process since the first patent of Rudolph Vuilleumier in 1918, aiming at the design of efficiently operating heat pumps and cryogenic refrigerators. However, the research on this type of regenerative machines is not yet very extensive and analytical. The present dissertation, aims at the knowledge advancement on Vuilleumier machines, providing an additional effort to broaden the research on this field. In the past, several industries and universities have been engaged with the development of these machines. At first, the research was performed in USA for the production of cryogenic refrigerators for space use. The US Army, the US Air Force and NASA collaborated with companies such as R.C.A., Philips, AiResearch, Hughes Aircraft and Kinergetics and managed to realize space missions with Vuilleumier cryogenic refrigerators. Later cryogenic refrigerator research initiated in Japan and Korea also, where heat pumps for heating buildings were also developed for companies such as Mitsubishi, Daikin, Sanyo and Kawasaki. In Europe, mainly in Germany and Denmark, two decades after the cryogenic refrigerators development in USA, universities and companies focused their research on Vuilleumier heat pumps and many experimental units were built and tested. The Technical University of Munich, the Technical University of Denmark, Dortmund University and the collaboration BVE-Thermolift are among them. Nowadays, the research is still active in USA and Europe for heat pumps and in Japan and China for cryogenic refrigerators mostly. Although many prototypes have been produced during all these years, Vuilleumier machines still exhibit low performance and efficiency due to inherent thermodynamic and aerodynamic losses. An accurate description and understanding of these complicated phenomena is required in order build competitive machines for modern applications. Considering the heat pumps, the volume specific heat power has to be increased compared to other heat pump devices. Vuilleumier machines are bulky because of the low pressure ratio accomplished. In addition, the efficiency has a potential of further improvement with the design of more effective heat exchangers and regenerators and the reduction of flow losses. For the cryogenic refrigerators, the efficiency is insignificant, but the cooling capacity and the refrigeration temperature are of great importance and there are still a lot of improvements that can be done, such as applying new materials or reducing the thermal losses. In the present dissertation in Chapter 1 the fundamental parts of regenerative machines, the thermal and mechanical compressor are presented, which coupled form a Stirling heat pump. Then, the operation of the same Stirling machine as a prime mover, as a heat pump or as a refrigerator is explained and it follows a presentation of the connection between two Stirling machines to form a heat actuated heat pump like a Vuilleumier machine. Several configurations and types of the Vuilleumier machines are presented together with their operation and their benefits and disadvantages. An extensive presentation of the evolution of Vuilleumier machines from their first patent until now is conducted in Chapter 2. All available studies, patents, books and reports are cited for both heat pumps and cryogenic refrigerators and the work of each researcher is briefly discussed. The theoretical or experimental performance of various Vuilleumier cryogenic refrigerators and heat pumps is summarized in corresponding tables and also all the Vuilleumier related US patents. In Chapter 3 the types of existing models that are used for the prediction of the performance of Stirling engines are presented together with one-dimensional thermodynamic models generated, in the context of this dissertation, especially for Vuilleumier machines. At first, the construction of the ideal isothermal model is presented. It follows the ideal adiabatic model and the thermodynamic segmentation of the machine into work-heat converting control volumes. Then the application of this model on real machines is discussed and the results of a comparison between the two ideal models are also presented. Chapter 3 includes in addition the description of several losses inherent in a Vuilleumier machine and the physics behind the derivation of equations that calculate their values. Finally, a validation of the developed computer codes according to the thermodynamic models is conducted by comparison with experimental data. In Chapter 4 there is a comprehensive three-dimensional Computational Fluid Dynamics (CFD) simulation of a Vuilleumier machine that was designed for this purpose at three different operating speeds. At first, existing CFD studies on Stirling engines are presented and then there is an analytical description of the designed machine, the boundary conditions applied, the computational grid used and all the equations that were utilized for the generation of this numerical model. For the simulation, a commercial CFD software was used. The results from the simulation are presented next, providing details about the thermodynamic and fluid mechanics quantities distribution in every space of the machine and the interaction between them. Useful 3D illustrations of the temperature, the pressure and the velocity are given too. Moreover, calculation of heat and work transfer between the machine and the surrounding is performed for all three speeds. Then, heat transfer coefficients are derived in relationship with the Reynolds number of the flow. Finally, the efficiency of the designed Vuilleumier machine is calculated and it is compared against experimental data

Evaluating a prototype compact thermal energy storage tank using paraffin-based phase change material for domestic hot water production

A full-scale thermal energy storage system using phase change materials (PCM) is experimentally investigated for solar and geothermal applications. The system consists of a rectangular tank filled with PCM and a staggered fin tube heat exchanger (HE). The system is designed for the production of Domestic Hot Water (DHW) based on the EU Commission Regulation No 814/2013 [1] requirements. The characteristics that are studied are the stored energy density of the system, the heat transfer rate through the HE during the charging and discharging processes, the adequacy of produced hot water amount and the storage efficiency of the tank. The results of the experiments confirmed the potential of the system to meet several prerequisites of a DHW installation and in addition to make the operation of the coupled solar collector or ground heat pump efficient

Computational Approach of Charging and Discharging Phases in a Novel Compact Solar Collector with Integrated Thermal Energy Storage Tank: Study of Different Phase Change Materials

A numerical study was carried out to investigate charging and discharging processes of different phase change materials (PCMs) used for thermal storage in an innovative solar collector, targeting domestic hot water (DHW) requirements. The aim was to study PCMs that meet all application requirements, considering their thermal performance in terms of stored and retrieved energy, outlet temperatures, and water flow rate. Work was carried out for three flat-plate solar panels of different sizes. For each panel, a PCM tank with a heat exchanger was attached on the back plate. Simulations were conducted on a 2D domain using the enthalpy–porosity technique. Three paraffin-based PCMs were studied, two (A53, P53) with phase-change temperatures of approximately 53 °C and one of approximately 58 °C (A58H). Results showed that, during charging, A58H can store the most energy and A53 the least (12.30 kWh and 10.54 kWh, respectively, for the biggest unit). However, the biggest unit, A58H, takes the most time to be fully charged, i.e., 6.43 h for the fastest feed rate, while the A53 unit charges the fastest, at 4.25 h. The behavior of P53 lies in between A53 and A58H, considering stored energy and charging time. During discharging, all PCMs could provide an adequate DHW amount, even in the worst case, that is, a small unit with a high hot water consumption rate. The A58H unit provides hot water above 40 °C for 10 min, P53 for 11 min, and A53 for 12 min. The DHW production duration increased if a bigger unit was used or if the consumption rate was lower

Testing the performance of a prototype thermal energy storage tank working with organic phase change material for space heating application conditions

A prototype Latent Heat Thermal Energy Storage (LHTES) unit has been designed, constructed, and experimentally analysed for its thermal storage performance under different operational conditions considering heating application and exploiting solar and geothermal energy. The system consists of a rectangular tank filled with Phase Change Material (PCM) and a finned tube staggered Heat Exchanger (HE) while water is used as Heat Transfer Fluid (HTF). Different HTF inlet temperatures and flow rates were tested to find out their effects on LHTES performance. Thermal quantities such as HTF outlet temperature, heat transfer rate, stored energy, were evaluated as a function of the conditions studied. Two commercial organic PCMs were tested A44 and A46. Results indicate that A44 is more efficient during the charging period, taking into account the two energy sources, solar and heat pump. During the discharging process, it exhibits higher storage capacity than A46. Concluding, the developed methodology can be applied to study different PCMs and building applications

Computational Approach of Charging and Discharging Phases in a Novel Compact Solar Collector with Integrated Thermal Energy Storage Tank: Study of Different Phase Change Materials

A numerical study was carried out to investigate charging and discharging processes of different phase change materials (PCMs) used for thermal storage in an innovative solar collector, targeting domestic hot water (DHW) requirements. The aim was to study PCMs that meet all application requirements, considering their thermal performance in terms of stored and retrieved energy, outlet temperatures, and water flow rate. Work was carried out for three flat-plate solar panels of different sizes. For each panel, a PCM tank with a heat exchanger was attached on the back plate. Simulations were conducted on a 2D domain using the enthalpy–porosity technique. Three paraffin-based PCMs were studied, two (A53, P53) with phase-change temperatures of approximately 53 °C and one of approximately 58 °C (A58H). Results showed that, during charging, A58H can store the most energy and A53 the least (12.30 kWh and 10.54 kWh, respectively, for the biggest unit). However, the biggest unit, A58H, takes the most time to be fully charged, i.e., 6.43 h for the fastest feed rate, while the A53 unit charges the fastest, at 4.25 h. The behavior of P53 lies in between A53 and A58H, considering stored energy and charging time. During discharging, all PCMs could provide an adequate DHW amount, even in the worst case, that is, a small unit with a high hot water consumption rate. The A58H unit provides hot water above 40 °C for 10 min, P53 for 11 min, and A53 for 12 min. The DHW production duration increased if a bigger unit was used or if the consumption rate was lower

Latent Thermal Energy Storage Application in a Residential Building at a Mediterranean Climate

An innovative thermal energy storage system (TESSe2b) was retrofitted in a residential building in Cyprus with a typical Mediterranean climate. The system comprises flat-plate solar collectors, thermal energy storage tanks filled with organic phase change material, a geothermal installation consisting of borehole heat exchangers with and without phase change material and a ground source heat pump, an advanced self-learning control system, backup devices and several other auxiliary components. The thermal energy storage tanks cover the building’s needs at certain temperature ranges (10–17 °C for cooling, 38–45 °C for heating and 50–60 °C for domestic hot water). A performance evaluation was conducted by comparing the TESSe2b system with the existing conventional heating and cooling system. The systems were simulated using commercial software, and the performance of the systems and the building’s energy needs were calculated. Based on the energy quantities, an economic analysis followed. The equivalent annual primary energy consumption with the conventional system resulted in being 43335 kWh, while for the storage system, it was only 8398 kWh. The payback period for the storage system was calculated to be equal to 9.76 years. The operation of the installed storage system provided data for calculations of the seasonal performance factor and storage performance. The seasonal performance factor values were very high during June, July and August, since the TESSe2b system works very efficiently in cooling mode due to the very high temperatures that dominate in Cyprus. The measured stored thermal energy for cooling, heating and domestic hot water resulted in being 14.5, 21.9 and 6.2 kWh, respectively. Moreover, the total volume of the phase change material thermal energy storage tanks for heating and domestic hot water was calculated to be roughly several times smaller than the volume of a tank with water as a storage medium