Investigations of pit thermal energy storages in district heating systems

In recent years, there has been a growing interest in the use of pit thermal energy storages (PTES), which is mainly attributable to their low cost and high efficiencies. However, the existing knowledge derives primarily from short-term performance monitoring of seasonal storages and a small number of scientific studies. Specifically, the effect of storage geometry, soil conditions, and storage operation remains largely uninvestigated, although they potentially have a large impact on performance. Thus, this thesis aimed to investigate methods for assessing storage performance, determining the effect of storage design and soil conditions, and quantifying the economic impact of PTES.In order to assess the performance of PTES systems, a thorough review of key performance indicators regarding efficiency and stratification was conducted. Initially, the most common energy efficiency expressions were compared, and a recommendation for handling the internal energy change was provided. Moreover, exergy efficiency was found to be the only efficiency indicator that accounted for heat losses and mixing in the storage. Regarding stratification, exergy destruction was recommended as an indicator that can evaluate stratification without being biased by differences in heat losses. The performance of PTES was further investigated by assessing the impact of the geometry in terms of the slope of the storage sides and the aspect ratio of the lid. It was found that, due to a smaller surface area, changing the slope of the side walls from 26° (typical PTES side-wall slope) to 44° could reduce the total heat losses by 20%. Additionally, a square-shaped PTES had 9% less heat loss compared to a rectangular one. Moreover, the impact of groundwater on PTES performance was investigated by simulating the heat transfer in the soil domain. It was found that groundwater could increase heat losses toward the ground by up to 60%, compared to a case without groundwater. Furthermore, the increase in groundwater temperature was investigated since it is subject to regulation in many countries. It was found that if the groundwater table was at a depth of at least 25 m, the groundwater temperature could be maintained below 20 °C for a seasonal PTES. However, this was not feasible for the short-term PTES operation. The economic impact of PTES was investigated using the Danish city of Viborg as a case study. Since it has become common practice to install electricity-based generation technologies in distinct heating grids, a system using an air-to-water heat pump, an electric boiler, and a PTES was compared to a system without a PTES. It was found that the PTES could reduce the levelized cost of heat by 10%, with a payback period of 5.1 years. Furthermore, if the PTES charge temperature was reduced from 90 °C to 80 °C, the cost of heat could be decreased by an additional 4%. Furthermore, the impact of thermal energy storage (particularly PTES) was investigated at a country level, with a focus on Denmark. It was found that heat storage enabled higher installation of renewable technologies (i.e., 35% higher PV capacity and 10% higher wind capacity) compared to scenarios without heat storage. In parallel, heat storages allowed for utilizing excess electricity (through power-to-heat technologies), leading to 53% lower curtailment levels and ultimately to a 2.4 €/MWh lower average heat price. Last, it was demonstrated that only by utilizing heat storage systems could carbon neutrality be achieved for the energy system by 2050

Modeling, Validation, and Analysis of a Concentrating Solar Collector Field Integrated with a District Heating Network

In recent years, concentrating solar collectors have been integrated with several district heating systems with the aim of taking advantage of their low heat losses. The present study investigates the Brønderslev combined heat and power plant, which consists of a 16.6 MW parabolic trough collector field, two biomass boilers, and an organic Rankine cycle system. The study focuses on the solar collector field performance and integration with the district heating network. An in situ characterization of the parabolic solar collector field using the quasi-dynamic test method found that the field had a peak efficiency of 72.7%. Furthermore, a control strategy for supplying a constant outlet temperature to the district heating network was presented and implemented in a TRNSYS simulation model of the solar collector field. The developed simulation model was validated by comparison to measurement data. Subsequently, the simulation model was used to conduct a sensitivity analysis of the influence of the collector row spacing and tracking axis orientation. The results showed that the current suboptimal tracking axis rotation, made necessary by the geography of the location, only reduced the annual power output by 1% compared to the optimal configuration. Additionally, there were only minor improvements in the annual heat output when the row spacing was increased past 15 m (ground cover ratio of 0.38)

Experimental implementation and evaluation video encoding

The object of research of thesis they are the techniques of implementation of coders according to the recommendation MPEG4 - H.264. This recommendation is called to replace the MPEG2 offering better compression and better quality. In continuously more applications it is called to be materialized, from systems television up to handle devices. Having a big breadth of applications, emerges the need for planning of implementation with different needs. Research interest has the design of H.264 building blocks aiming at as smaller, more efficient and more rapid circuits. In the first part of thesis becomes an introduction to the recommendation. Also it presents the basic terms of the operation of coding and decoding. Afterwards is described the operation of the H.264 blocks. In the second part of thesis is presented a software and hardware solution of a H.264 coder that was designed and developed. In the software were implemented the all techniques from the international bibliography as well as the proposed techniques, giving thus the possibility for direct comparison of these. Also, the hardware coder H.264 with size of 266K gates has implemented. The 45% of these gates are occupied by INTRA block. To this high percentage concerning the small compression that can we achieve using intra, led to further study of this. The analysis of operation and the techniques of implementation of INTRA block are presented afterwards. New quality metric are imported base on the table of AC transformation and with decreased number of coefficients. The implementation of quality metric showed that the big circuit of INTRA is owed in the quality metric block leading to the effort to reduce the size of these. The proposed quality metric dct-8 in 116x16 gives light better results compared with proposed from the international bibliography quality metric SATD, decreasing the size at 40%. Instead of this the proposed metrics in 14x4 does not give better results than the quality metric SATD. A new method for 14x4 was developed, in what becomes forecast of number of the bits without using quality metric. The required circuit is 15% smaller than the SATD giving same quality, without use of technical RDO that leaves margins of improvement.Το αντικείμενο έρευνας της διατριβής είναι η πρότυπη υλοποίηση και αξιολόγηση κωδικοποίησης εικονοροών σύμφωνα με το πρότυπο MPEG4 - Η.264. Το πρότυπο αυτό καλείται να αντικαταστήσει το MPEG2 προσφέροντας καλύτερη συμπίεση και καλύτερη ποιότητα. Σε ολοένα και περισσότερες εφαρμογές καλείται να υλοποιηθεί, από συστήματα τηλεόρασης μέχρι σε συσκευές χειρός. Έχοντας ένα μεγάλο εύρος εφαρμογών, ανακύπτει η ανάγκη για σχεδιασμό υλοποιήσεων με διαφορετικές ανάγκες. Ερευνητικό ενδιαφέρον παρουσιάζει η σχεδίαση των επιμέρους κυκλωμάτων έχοντας ως στόχο μικρότερα, αποδοτικότερα και ταχύτερα κυκλώματα. Στο πρώτο μέρος της διατριβής γίνεται μια εισαγωγή στο πρότυπο. Παρουσιάζονται οι βασικές έννοιες και όροι καθώς και η λειτουργία της κωδικοποίησης και αποκωδικοποίησης. Στη συνέχεια περιγράφεται η λειτουργία των επιμέρους τμημάτων ενός κωδικοποιητή. Στο δεύτερο μέρος της διατριβής παρουσιάζεται το λογισμικό και η υλοποίηση σε υλικό του κωδικοποιητή Η.264 που σχεδιάστηκε. Στο λογισμικό ενσωματώθηκαν όλες οι τεχνικές από την διεθνή βιβλιογραφία καθώς και οι προτεινόμενες τεχνικές, δίνοντας έτσι τη δυνατότητα για άμεση σύγκριση αυτών. Επίσης υλοποιήθηκε σε υλικό ένας κωδικοποιητής Η.264 με μέγεθος 266Κ πύλες, εκ των οποίων το 45% καταλαμβάνεται από το κύκλωμα INTRA. Το υψηλό αυτό ποσοστό σε σχέση με τη μικρή συμπίεση που επιτυγχάνεται με τη χρήση του, οδήγησε σε περαιτέρω μελέτη αυτού. Η ανάλυση της λειτουργίας και οι τεχνικές υλοποίησης του INTRA κυκλώματος παρουσιάζονται στη συνέχεια. Νέες μετρικές αποκλίσεις εισάγονται με βάση τον πίνακα μετασχηματισμού των AC συντελεστών και με μειωμένο αριθμό συντελεστών. Η υλοποίηση των μετρικών αποκλίσεων έδειξε ότι το μεγάλο μέγεθος του κύκλωμα του INTRA οφείλεται στις μετρικές αποκλίσεις οδηγώντας στην προσπάθεια μείωσης του μεγέθους αυτών. Η προτεινόμενη μετρική DCT-8 στο τμήμα 116x16 έδωσε ελαφρώς καλύτερα αποτελέσματα συγκρινόμενη με την προτεινόμενη από τη διεθνή βιβλιογραφία μετρική SATD, μειώνοντας το μέγεθος κατά 45%. Η εφαρμογή της προτεινόμενης μετρικής στο τμήμα 14x4 δεν έδωσε καλύτερα αποτελέσματα από τη μετρική SATD. Μια νέα μέθοδος για το τμήμα 14x4 αναπτύχθηκε, στην οποία γίνεται πρόβλεψη του αριθμού των bits χωρίς τη χρήση μετρικών αποκλίσεων. Το απαιτούμενο κύκλωμα είναι 15% μικρότερο σε σχέση με το SATD δίνοντας ίδια ποιότητα, χωρίς χρήση τεχνικής RDO, που αφήνει περιθώρια βελτίωσης

twoaxistracking – a python package for simulating self-shading of two-axis tracking solar collectors

Self-shading in fields of two-axis tracking collectors typically ranges from 1% to 6% of the annual incident irradiation. It is thus essential to account for shading in order to obtain accurate yield estimates and financing for such solar projects. The present study presents the free and open-source Python package twoaxistracking for simulating self-shading in fields of two-axis tracking collectors. The package is freely available at: https://github.com/pvlib/twoaxistracking. The main steps of the method and mathematical formulation are described. Additionally, a demonstration of how to use the package is presented. The shading calculation method excels over previous methods found in the literature in that it can: • Handle arbitrary aperture geometries and distinguish between the total and active areas. • Account for sloped ground and collectors with different heights within the same field. • Reduce computation time by skipping calculations at high solar elevation angles

Evaluation of Stratification in Thermal Energy Storages

Thermal stratification in water-based storages can be destroyed by mixing, heat diffusion, and thermal conduction. For this reason, the evaluation of stratification in water-based thermal energy storages is important for assessing their performance. The most promising indicators were identified and assessed based on their suitability for use in practical applications. The selected stratification indicators were calculated for four simulated storage scenarios comprising a fully stratified, a fully mixed, and two realistic storages. It was found that most indicators had severe limitations in their application. For this reason, a new indicator called internal exergy destruction was proposed, which can be used in combination with the overall exergy efficiency for assessing the performance and stratification of thermal energy storages. The main benefit of internal exergy destruction is that it can be used to compare storages with different heat loss coefficients. In addition, it separates the effects of mixing from the heat losses and is easily applied to real-life storages