Review of experimental research on supercritical and transcritical thermodynamic cycles designed for heat recovery application

Supercritical operation is considered a main technique to achieve higher cycle efficiency in various thermodynamic systems. The present paper is a review of experimental investigations on supercritical operation considering both heat-to-upgraded heat and heat-to-power systems. Experimental works are reported and subsequently analyzed. Main findings can be summarized as: steam Rankine cycles does not show much studies in the literature, transcritical organic Rankine cycles are intensely investigated and few plants are already online, carbon dioxide is considered as a promising fluid for closed Brayton and Rankine cycles but its unique properties call for a new thinking in designing cycle components. Transcritical heat pumps are extensively used in domestic and industrial applications, but supercritical heat pumps with a working fluid other than CO2 are scarce. To increase the adoption rate of supercritical thermodynamic systems further research is needed on the heat transfer behavior and the optimal design of compressors and expanders with special attention to the mechanical integrity

Modeling of an Integrated Renewable-Energy-Based System for Heating, Cooling, and Electricity for Buildings

An integrated numerical model that describes the operation of a renewable-energy-based system for a building’s heating, cooling, and domestic hot water needs is described in this study. The examined energy system includes a vapor compression multi-source heat pump, PVT collectors, borehole thermal energy storage, and water tanks. Energy balance equations for the collectors and the tanks are coupled with correlations for the heat pump and the piping losses within a thermal network approach. The non-linear system of equations that arises is solved by employing in-house software developed in Python v. 3.7.3. The performance of the numerical tool is validated against measurements collected during the pilot operation of such a system installed in Athens (Greece) for two 5-day periods (summer and winter). It is shown that the proposed model can predict, both qualitatively and quantitatively, the building’s energy system performance, whereas limited deviations from the experimental findings are mostly observed when highly transient phenomena occur. The numerical tool is designed with flexibility in mind and can be easily adapted to accommodate additional energy-system configurations and operational modes. Thus, it can be utilized as a supporting decision tool for new energy systems’ designs and the optimization of existing ones

EVALUATING THE FACTORS AFFECTING THE BREAK-EVEN COST OF ON-SITE PV GENERATION AT INDUSTRIAL UNITS

This paper studies the way in which the regulatory framework and market rules affect the feasibility of on-site PV generation for large industrial units. In most European markets net metering and feed-in tariffs for selfconsumed electricity are not possible or are being phased out, providing an incentive to the industry for becoming more flexible in the way electricity is consumed in order to maximise the percentage of the variable electricity generated on-site that is self-consumed. The electricity cost for the industry is the benchmark for PV or any other onsite generation technology and in general on-site solar energy is competitive with that. However, as the regulation develops further, the exemptions of paying the regulated charges for the electricity that is self-consumed are phased out. Also the cost of flexibility required to self-consume all variable on-site generation has to be added to the LCOE of solar electricity, moving it further away from the competitiveness benchmark. Still, as the LCOE of solar electricity reduces continuously mostly due to the reduction of PV system costs, it becomes competitive for more and more users in more and more target markets

Development of a Computational Fluid Dynamics Model for the Simulation of Transport Phenomena and their Effect on the Operation and Emissions of Hydrogen-Fuelled Internal Combustion Engines

372 σ.Η παρούσα Διδακτορική Διατριβή εκπονήθηκε στο εργαστήριο Μηχανών Εσωτερικής Καύσης του Εθνικού Μετσόβιου Πολυτεχνείου, όπου δεν υπήρχε προηγούμενη εμπειρία σε ρευστομηχανικά μοντέλα προσομοίωσης. Στα πλαίσια της Διατριβής αυτής κατασκευάστηκε ένας ρευστομηχανικός κώδικας, με σκοπό να αναπτυχθεί η αντίστοιχη τεχνογνωσία και να υπάρξει εμπειρία σε τέτοιου είδους μοντέλα προσομοίωσης σε καθαρά ερευνητικό επίπεδο. Για τους λόγους αυτούς προτιμήθηκε η ανάπτυξη ενός κώδικα από την αρχή, παρά να εφαρμοστεί κάποιος εμπορικός κώδικας. Πιο συγκεκριμένα, αναπτύχθηκε εξ αρχής ένα ρευστομηχανικό μοντέλο προσομοίωσης σε καμπυλόγραμμες συντεταγμένες, στις τρεις διαστάσεις, για τον υπολογισμό των φαινομένων μεταφοράς στο εσωτερικό του κυλίνδρου εμβολοφόρων μηχανών εσωτερικής καύσης (ΜΕΚ) και την περιγραφή της προαναμεμιγμένης καύσης του υδρογόνου σε κινητήρα Otto και της παραγωγής των εκπεμπόμενων ρύπων μονοξειδίου του αζώτου. Παράλληλα, αναπτύχθηκαν ορισμένα μοντέλα, τα οποία ενσωματώθηκαν στο ρευστομηχανικό κώδικα και συντελούν σημαντικά στον αξιόπιστο υπολογισμό των διάφορων ιδιοτήτων του αερίου στο εσωτερικό των κυλίνδρων εμβολοφόρων ΜΕΚ. Συγκεκριμένα, αναπτύχθηκε ένα μοντέλο μεταφοράς θερμότητας, το οποίο βασίζεται σε συναρτήσεις τοιχώματος και περιλαμβάνει έναν όρο πίεσης και καύσης, με σκοπό τον αξιόπιστο υπολογισμό των τοπικών ροών θερμότητας. Επιπλέον, αναπτύχθηκε ένα απλό μοντέλο διακένων του εμβόλου, με το οποίο υπολογίζεται η μεταβολή της παγιδευμένης μάζας του αερίου εντός του κυλίνδρου σε κάθε χρονική στιγμή, η αποθήκευση αερίου στο εσωτερικό των διακένων και η απώλεια μάζας που διαφεύγει στο στροφαλοθάλαμο. Με τη χρήση των δύο αυτών μοντέλων προσομοιώνεται πιο ρεαλιστικά η λειτουργία εμβολοφόρων ΜΕΚ, αφού λαμβάνονται υπόψιν περισσότερες διεργασίες, ενώ δεν απαιτείται η ρύθμιση κάποιου διορθωτικού συντελεστή. Τέλος, αναπτύχθηκε εξ αρχής ένα μοντέλο διάδοσης της φλόγας κατά την προαναμεμιγμένη καύση του υδρογόνου σε κινητήρα Otto, το οποίο ενσωματώθηκε στο ρευστομηχανικό κώδικα. Με τη χρήση του μοντέλου αυτού προβλέπεται η διάδοση της φλόγας κατά την περίοδο της έναυσης και της τυρβώδους ανάπτυξης της φλόγας, ώστε κατόπιν να υπολογιστεί ο ρυθμός αντίδρασης των συστατικών στα υπολογιστικά κελιά, που βρίσκονται εντός της φλόγας, καθώς και οι εκπεμπόμενοι ρύποι μονοξειδίου του αζώτου. Ο βασικός σκοπός της Διδακτορικής Διατριβής είναι η αξιόπιστη και ρεαλιστική περιγραφή των φαινομένων μεταφοράς που λαμβάνουν χώρα στο εσωτερικό του κυλίνδρου εμβολοφόρων ΜΕΚ, τόσο σε συνθήκες ετεροκίνησης (σε κινητήρες Otto, Diesel και HCCI) όσο και σε συνθήκες με καύση (σε κινητήρα Otto με καύσιμο το υδρογόνο), καθώς επίσης και ο υπολογισμός των εκπεμπόμενων ρύπων μονοξειδίου του αζώτου. Ταυτόχρονα, είναι επιθυμητή η προσομοίωση διάφορων κινητήρων, χωρίς κανέναν περιορισμό ως προς τις γεωμετρίες του θαλάμου καύσης που συναντώνται. Από τη διερεύνηση και προσομοίωση διάφορων περιπτώσεων, φάνηκε ότι τόσο το μέσο πεδίο ροής του αερίου στο εσωτερικό του κυλίνδρου, όσο και το τοπικό, υπολογίζονται ικανοποιητικά, σε σύγκριση με υπολογισμένα αποτελέσματα και μετρήσεις αντίστοιχα. Επιπλέον, η ανάμιξη αέρα/υδρογόνου σε απλή γεωμετρία οχετού-εγχυτήρα και στον οχετό εισαγωγής ενός κινητήρα Otto προβλέπεται ποιοτικά σωστά, σε σύγκριση με εύρη μετρήσεων και υπολογιστικά αποτελέσματα αντίστοιχα. Σχετικά με την αξιολόγηση του νέου μοντέλου μεταφοράς θερμότητας που αναπτύχθηκε, διενεργήθηκε εκτενής μελέτη, αρχικά σε συνθήκες ετεροκίνησης, και εν συνεχεία σε συνθήκες με καύση. Το γενικό συμπέρασμα που προκύπτει είναι ότι οι τοπικές ροές θερμότητας με τη χρήση του μοντέλου που αναπτύχθηκε, προβλέπονται με αρκετή ακρίβεια κατά τη διάρκεια του κλειστού κύκλου λειτουργίας (κυρίως κατά τη συμπίεση), ενώ η γενική του επίδοση είναι καλύτερη από τα περισσότερα αντίστοιχα μοντέλα που μπορούν να βρεθούν στη βιβλιογραφία. Το μοντέλο διακένων του εμβόλου που αναπτύχθηκε, δοκιμάστηκε σε διάφορους κινητήρες και σημεία λειτουργίας (σε συνθήκες ετεροκίνησης και με καύση), ενώ για την αξιολόγησή του χρησιμοποιήθηκε κυρίως το δυναμοδεικτικό διάγραμμα, αφού η πίεση αποτελεί μια καλή ένδειξη της μάζας γομώσεως του κυλίνδρου. Με τη χρήση του μοντέλου αυτού φάνηκε ότι γίνεται πιο ρεαλιστικός ο υπολογισμός της παγιδευμένης μάζας του αερίου του κυλίνδρου σε κάθε χρονική στιγμή, ενώ επιπλέον η πίεση του κυλίνδρου βρίσκεται πιο κοντά στη μετρημένη σε κάθε περίπτωση που εξετάστηκε (ειδικά η μέγιστη πίεση). Στη συνέχεια αξιολογήθηκε το μοντέλο προαναμεμιγμένης καύσης υδρογόνου που αναπτύχθηκε σε κινητήρα Otto με καύσιμο το υδρογόνο, για τον οποίο είναι διαθέσιμα πειραματικά δεδομένα. Αρχικά, επιλέχθηκαν οι εκφράσεις της στρωτής και τυρβώδους ταχύτητας της φλόγας και ρυθμίστηκε ο μοναδικός διορθωτικός συντελεστής που χρησιμοποιήθηκε. Κατόπιν, συγκρίθηκαν τα υπολογιστικά αποτελέσματα με πειραματικά δεδομένα κατά τη μεταβολή του λόγου ισοδυναμίας, χρονισμού έναυσης και βαθμού συμπίεσης, όπου διαφάνηκε η αξιοπιστία των προβλέψεων του μοντέλου, που σχετίζονται με μεγέθη απόδοσης (π.χ. πίεση κυλίνδρου, ενδεικνύμενο έργο) και εκπεμπόμενων ρύπων μονοξειδίου του αζώτου. Στη συνέχεια έγινε μια λεπτομερής διερεύνηση των φαινομένων μεταφοράς στον ίδιο κινητήρα Otto με καύσιμο το υδρογόνο, αλλά σε διαφορετικά σημεία λειτουργίας. Συγκεκριμένα, υπολογίστηκε η απόδοση της καύσης και οι κύριοι μηχανισμοί απωλειών της διαθέσιμης χημικής ενέργειας του καυσίμου (διάκενα, άκαυστο καύσιμο, διάσταση προϊόντων), όπως επίσης και οι απώλειες θερμότητας του αερίου προς τα τοιχώματα του κυλίνδρου, κατά τη μεταβολή του φορτίου και του βαθμού συμπίεσης. Επιπλέον, φάνηκαν οι δυνατότητες του μοντέλου που αναπτύχθηκε, με το να παρέχει τις τοπικές κατανομές της θερμοκρασίας του αερίου και του κλάσματος μάζας του μονοξειδίου του αζώτου στο εσωτερικό του κυλίνδρου κατά τη διάρκεια της καύσης και της αποτόνωσης.The current Ph.D. Thesis has been conducted in the Laboratory of Internal Combustion Engines of the National Technical University of Athens (NTUA), where a computational fluid dynamics (CFD) model has been developed, in order to gain the know-how in such numerical tools. For this reason, a dedicated computer model has been originally developed. More precisely, a three-dimensional CFD model has been developed at curvilinear coordinates, for the calculation of transport phenomena in the cylinders of internal combustion engines. Moreover, the combustion phenomena have been simulated, concerning a hydrogen-fuelled spark-ignition engine, where additionally the exhaust nitric oxide emissions have been calculated. In order for more reliable simulations to occur, critical sub-models have been also developed and incorporated in the CFD code. Specifically, a heat transfer model has been developed, which is actually a wall-function, it includes a pressure and combustion term and is based on the compressible version of the law-of-the-wall. Additionally, a piston-crevice phenomenological model has been developed, with which the variation of the trapped gas mass can be calculated, together with the flow rates at every crevice region and the gas that is lost to the crankcase (blow-by). With the use of these two sub-models, the simulations become more realistic and reliable, since the mass transport phenomena is taken into consideration, and the local heat fluxes are predicted more adequately. It should be noted that no calibration constant is used in these two sub-models. For the simulation of combustion phenomena, a sub-model for the prediction of the premixed flame front has been developed, and incorporated to the CFD code. This sub-model can simulate the hydrogen flame development during the ignition and the turbulent development phases, in order for the reaction rates to be then calculated at the computational cells that have been swept (fully or partially) by the flame front. Also, exhaust nitric oxide emissions are calculated using the extended Zeldovich mechanism. In this sub-model only one calibration constant is used, existing in the expression of the turbulent flame speed. The main scope of this Ph.D. Thesis is the reliable and realistic simulation of transport phenomena, occurring in the cylinders of reciprocating internal combustion engines, at motoring conditions (in spark-ignition, Diesel and HCCI engines), and at fired engines (in hydrogen-fuelled spark-ignition engine), together with the calculation of nitric oxide exhaust emissions. From the investigation of various simulation cases it has been revealed that the mean and local gas flow-field is calculated adequately, and the turbulent mixing of air with hydrogen is predicted qualitatively correct, in comparison to a range of measured data and computational results of other relevant numerical tools. Concerning the evaluation of the developed heat transfer model, an extended investigation has been conducted in a large variety of engines and different operating conditions, both at motoring and firing conditions. The general conclusion is that with the use of this sub-model the local heat fluxes can be calculated with higher accuracy, in comparison to measured data, relevant to their peak values and their trend during the compression and expansion stroke. Additionally, this sub-model performs much better than the most widely ones, used in commercial/research CFD codes. The developed crevice sub-model has been tested at different engines and operating conditions. The cylinder pressure has been used for its evaluation, since it is a good indication of the trapped gas mass at every time-instant during the closed engine cycle. With the use of this sub-model the variation of the trapped gas mass is calculated, together with the gas that is lost to the crankcase, while the predicted cylinder pressure matches in good terms the measured one, both at motoring and firing conditions. Afterwards, the developed hydrogen combustion model has been also evaluated. A hydrogen-fuelled spark-ignition engine has been simulated, for which an extensive database of measurements is available, provided by another research team (Univ. of Ghent, Belgium). The expressions of the hydrogen laminar and turbulent flame speed have been first selected, together with the adjustment of the sole calibration constant used in the CFD model. Then, an extended evaluation took place, where the calculated results were compared to measured data, when varying the equivalence ratio, compression ratio and spark-timing. These results concern the cylinder pressure, gross heat release rate, gross indicated work and nitric oxide exhaust emissions. In every simulation case has been revealed that the combustion model can adequately calculate such demanding combustion phenomena with good accuracy, in comparison to measured data. Then a very detailed investigation took place, concerning the same hydrogen-fuelled spark-ignition engine, but at different operating conditions. This investigation deals with the quantification of the hydrogen combustion efficiency and the main loss mechanisms that have been identified (hydrogen loss to crankcase, unburned hydrogen at the exhaust, combustion products dissociation). Moreover, the heat loss to the cylinder walls has been calculated, together with the available thermal energy of the exhaust gas. A more detailed investigation followed, describing the local mechanism of nitric oxide production pattern for various equivalence ratios. This investigation provided the dependence of the nitric oxide emissions from the local gas temperature, and the interaction with the prevailing flow-field, since combustion products are mixed at mid/low engine loads, due to the weak swirling flow.Γεώργιος Μ. Κοσμαδάκη

Industrial waste heat: Estimation of the technically available resource in the EU per industrial sector, temperature level and country

Industrial waste heat is examined in EU countries, focusing on the amount that can be recovered and exploited, referred to as technical potential of waste heat. An alternative methodology is proposed here, which is based on waste heat fractions derived from a detailed study of the UK industry from the period 2000–2003. These fractions express the part of heat consumption that is wasted and is possible to be recovered. The waste heat fractions have been calculated in this work for each main industrial sector and temperature level. The methodology initially includes the adjustment of waste heat fractions from each industrial sector from the UK industry to the conditions of the different EU countries in the period 2000–2003, in order to account for the different levels of energy efficiency. The second step is to adjust the fractions for the year 2015, using data about the evolution of energy intensity values from 2000 to 2003 to 2015 for each country and sector, resulting to a new set of fractions per country, temperature level and sector. This methodology has enabled the authors to study in detail the waste heat potential per sector and temperature level, using the most recent data. The main outcome is the estimation of waste heat potential for each main industrial sector in the EU, broken down to the amount of waste heat for each temperature range. A similar analysis is conducted for each EU country as well, in order to identify the magnitude of heat recovery opportunities that could exist for every industrial sector at country level. The main result of this analysis is the estimation of the total waste heat potential in EU, which is about 300 TWh/year, with one third corresponding to temperature level below 200 °C, which is often referred to as low-temperature waste heat, another 25% in the range 200–500 °C and the rest above 500 °C (mostly in the range 500–1000 °C). The findings of the current study can be used for assessing the potential of any relevant heat recovery applications, such as heat upgrade and re-use or heat-to-power conversion technologies

Parametric comparison of a CPVT performance evaluation under standard testing procedures - ISO 9806:2017 and IEC 62108:2016 - for an automated and manual 2-axis tracking solar system stand

Currently, a noticeable lack of literature with respect to a wide-ranging comparison of the precision exhibited by automated and manual two-axis tracking solar systems, particularly within the context of adhering to the standard testing protocols delineated by ISO and IEC. To address this research gap, a symmetrical concentrating Photovoltaic-Thermal solar collector underwent a detailed evaluation encompassing two standard testing procedures such as ISO 9806:2017 and IEC 62108:2016. This comprehensive assessment covered thermal and electrical performance parameters, unfolding across two distinct geographical locations: Athens (Greece) and Gävle (Sweden). Within this experimental framework, an automated two-axis tracking solar system stand was employed at the Greek testing site, while in Sweden it was characterized by the employment of a manual two-axis tracking solar system. The collective peak power performance presented marginal divergence within a narrow range of ± 1% across both testing sites. This culminated in an overall peak power output of 1550 Wpeak, which included an electrical peak capacity of 218 Wpeak and a thermal peak power of approximately 1332 Wpeak. Notably, the most pronounced deviation has been materialized in the transversal and longitudinal Incidence Angle Modifier coefficients, with disparities remaining limited to a threshold of < 5%. These findings underscore the commendable precision hallmarking. In summary, the outcomes presented in this study not only contribute to the extant body of knowledge by bridging the existing gap in literature, but also emphasize the precision inherent to manual two-axis tracking solar systems when compared with automated equivalents.

A Fast CFD-Based Methodology for Determining the Cyclic Variability and Its Effects on Performance and Emissions of Spark-Ignition Engines

A methodology for determining the cyclic variability in spark-ignition (SI) engines has been developed recently, with the use of an in-house computational fluid dynamics (CFD) code. The simulation of a large number of engine cycles is required for the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) to converge, usually more than 50 cycles. This is valid for any CFD methodology applied for this kind of simulation activity. In order to reduce the total computational time, but without reducing the accuracy of the calculations, the methodology is expanded here by simulating just five representative cycles and calculating their main parameters of concern, such as the IMEP, peak pressure, and NO and CO emissions. A regression analysis then follows for producing fitted correlations for each parameter as a function of the key variable that affects cyclic variability as has been identified by the authors so far, namely, the relative location of the local turbulent eddy with the spark plug. The application of these fitted correlations for a large number of engine cycles then leads to a fast estimation of the key parameters. This methodology is applied here for a methane-fueled SI engine, while future activities will examine cyclic variations in SI engines when fueled with different fuels and their mixtures, such as methane/hydrogen blends, and their associated pollutant emissions