Analysis of Propane Thermal State in Underground Tanks Taking Into Consideration Design Issues

The paper presents an analysis of annual variations of temperature and the corresponding saturation pressure of saturated liquid propane in a mounded cylindrical tank used for liquefied petroleum gas (LPG) storage. A mathematical model has been established based on a finite volume method. Boundary and initial conditions have been assumed for the Croatian climate. Simulations have been performed using the commercial CFD (Computational Fluid Dynamic) code on the representative numerical example. Analysis results have been presented through diagrams. It has been pointed out that the present regulation and practice for such tanks defines too high a design pressure of 1.64 MPa for the Croatian climate and that pressure of 1.2 MPa is sufficient to ensure proper design of such vessels. Significant simplifications and savings in production process can be achieved by changing the unnecessarily high design pressure

The techno-economic justification for the changing of a system for separate heating/cooling and electrical energy production with a CCHP system with a micro-turbine and absorption refrigeration unit

Sustav odvojene proizvodnje toplinske energije iz kotla, energije za hlađenje iz kompresijskoga rashladnog uređaja i električne energije iz vanjske mreže može se zamijeniti CCHP sustavom koji se temelji na radu plinske turbine i apsorpcijskoga rashladnog uređaja. Za tu su namjenu napravljeni su modeli sustava primjenom objektno orijentiranog programskoga jezika Modelica, na temelju kojih su vršene simulacije za stacionarna stanja i izvedena tehno-ekonomska analiza te analiza osjetljivosti sustava u ovisnosti o promjeni parametara kao što je faktor hlađenja, stupanj iskoristivosti kotla, omjer električnog opterećenja, cijena električne energije, cijena goriva i trošak investicije. Razvijeni model sustava koristiti će se u istraživanjima koja slijede za dinamičke simulacije s uvedenim dinamičkim rubnim uvjetima.The system of separate production of heat energy from a boiler and cooling energy from a compression refrigeration unit powered with electrical energy from an external network can be replaced with a CCHP system based on the operation of a gas turbine and absorption refrigeration unit. For this purpose, models of the system were derived through the use of the object-oriented programming language Modelica, on the basis of which steady-state simulations were carried out, as well as technical-economic and the sensitivity analysis of the system, in relation to the changes of influential parameters such as coefficient of performance, the boiler efficiency, the ratio of the electric load, costs of electrical energy, fuel prices and investment costs. Developed system model is intended to be used in further research for dynamic simulations with implemented dynamic boundary conditions

Investigation on possibilities for biogas production from organic waste on the Croatian island of Krk

Predmet ovog rada je analiza mogućnosti korištenja biogenog dijela komunalnog i poljoprivrednog otpada u bioplinskom energetskom postrojenju na otoku Krku. Karakteristike ove vrste otpada su različiti sastav i velike sezonalne varijacije u količinama otpada. Ukupna količina komunalnog otpada na najvećem hrvatskom otoku Krku je relativno mala u usporedbi s gradskim središtima, iako postoje značajne oscilacije u količini otpada tijekom godine uslijed turističke aktivnosti tijekom ljeta. Na temelju dostupnih količina organskog otpada tijekom godine analiziran je energetski potencijal dobivenog bioplina s obzirom na dostupna tehnička rješenja za energetsko iskorištavanje istog. Bioplinsko postrojenje je predloženo za anaerobnu fermentaciju otpada i obradu bioplina. Simulacija efekata primjene bioplinskog i kogeneracijskog postrojenja za razdoblje od jedne godine je provedena kako bi se optimiziralo za maksimalnu energetsku efikasnost i proizvodnju električne i toplinske energije. Uspoređeni su scenariji sa korištenjem jedne ili dvije kogeneracijske jedinice. Rezultati pokazuju da je u slučaju veoma varijabilne proizvodnje bioplina tijekom godine energetski efikasnije korištenje dvije kogeneracijske jedinice. Emisije stakleničkih plinova su negativne.The subject of this paper is the analysis of the possibilities of using biodegradable municipal and agricultural waste in a biogas energy production facility on the island of Krk. The main characteristic of this type of waste is the variable composition. The total amount of municipal waste on the largest Croatian island of Krk is relatively small in comparison to cities. Particularly significant is the high variability of feedstock volumes depending on the period in the year. As a result, the biogas production is unstable and varies on a monthly basis. To overcome that situation a scenario with two cogeneration units was compared to a scenario when only one unit is used. The approach of two units, that operate based on the available biogas inflow, has shown to be more efficient than using a single unit. Net GHG emissions are negative

Simulation Model of Dynamic Behaviour of Refrigerated Products in a Ship’s Refrigerated Container

Na temelju razvijenoga matematičkog modela, te konstrukcijskih karakteristika ispitivanoga rashladnog kontejnera, njegova rashladnog agregata i karakteristika hlađenoga tereta izrađen je simulacijski model dinamičkog ponašanja hlađenoga tereta. Prezentirani model dio je dinamičkog modela za cjelokupni sustav brodskoga rashladnog kontejnera koji, uz hlađeni teret, uključuje: kontejnerske stijenke, vlažni zrak u kontejneru, kondenzacijsku jedinicu, isparivač, kontejnersku opremu, ventilator isparivača i grijač odmrzivača. Ispitivanje je provedeno na 12,2-metarskom (40- -stopnom) rashladnom kontejneru. Za modeliranje i simulaciju koristilo se metodom sistemske dinamike (System Dynamics) i simulacijskim programom Powersim. Dinamički model hlađenoga tereta u brodskome rashladnom kontejneru verificiran je na temelju usporedbe rezultata simulacije s empirijski očekivanim kvalitativnim podacima.Simulation model of dynamic behaviour of refrigerated products has ben based on the developed mathematical model, constructional characteristics of the tested container, its refrigeration plant and characteristics of refrigerated products. The model presented is a part of dynamic model of the complete ship’s refrigerated container system, whose individual subsystems are refrigerated products, container wall, refrigerated humid air in the container, condensation unit, evaporator, container outfit, evaporator fan and defroster heater. The research has been conducted on a 12,2-metre (40-foot) refrigerated container. The method of System Dynamics and Powersim simulation program have been used for modelling and simulation. The dynamic model of the refrigerated products in a ship’s refrigerated container has been verified and conformed by the comparison of simulation results with empirical qualitative data

Numerička studija strujanja zraka i prijenosa topline unutar sportske dvorane

Numerical analyses, performed as a part of a redesign project for an air- conditioning system of a sport hall, have been presented. The existing and redesigned air-duct arrangements as well as different types of air supply outlets have been analyzed using three dimensional steady-state numerical simulations of turbulent air flow, taking into account buoyancy effects. Commercial CFD software Star-CCM+ has been used for fluid flow simulations based on the finite volume method. It was determined that the use of special diffuser type with variable outlet flow patterns for winter and summer operation produced better thermal environmental conditions in air- conditioned space, as compared to existing air grilles. The application of computational fluid dynamics (CFD) methods for air distribution analysis has proven to be a very useful tool in design of modern air distribution systems.U radu su prikazane numeričke analize provedene u okviru izrade projekta rekonstrukcije sustava klimatizacije sportske dvorane. Korištenjem numeričkih simulacija trodimenzijskog stacionarnog turbulentnog toka zraka, uz uvažavanje utjecaja prirodne konvekcije, analizirani su postojeći i novoprojektirani sustavi distribucije zraka i različiti tipovi dovodnih otvora za zrak. Komercijalni CFD (CFD - Računalna dinamika fluida) softver Star-CCM+ korišten je za simulaciju toka fluida zasnovanu na metodi konačnih volumena. Utvrđeno je da se korištenjem posebnih difuzora s različitim načinom istrujavanja za zimski i ljetni rad postižu bolji uvjeti lagodnosti u klimatiziranom prostoru u usporedbi s postojećim ventilacijskim rešetkama. Primjena računalne dinamike fluida (CFD) za analizu strujanja zraka pokazuje se kao koristan alat u projektiranju suvremenih sustava distribucije zraka

Mathematical Modelling of Dynamic Behaviour of Refrigerated Products in a Ship’s Refrigerated Container

U radu je opisan razvoj matematičkog modela koji omogućuje izračunavanje promjena temperature i vlažnosti hlađenog tereta u brodskomu rashladnom kontejneru, tj. simulaciju njegova dinamičkog ponašanja. Vlažni zrak koji opstrujava oko hlađenog tereta u kontejneru u stalnom je dodiru s površinom tereta. Na taj način dolazi do procesa izmjene topline i mase između hlađenog tereta i vlažnog zraka. Pri tome se misli na pojave ishlapljivanja na površinama tereta koje su pristupačne zračnoj struji, čime se suši samo površinski sloj tereta. Ovo je razmatranje ograničeno na sušenje samo grube vlažnosti, a ne i higroskopne. Razvijeni matematički model, koji opisuje dinamičke promjene temperature i vlažnosti hlađenog tereta ovisno o različitim utjecajnim čimbenicima, zasnovan je na masenim i energijskim bilancama. Postavljeni model modificiran je radi opisa faznih promjena, pa omogućuje simulaciju dinamičkog ponašanja hlađenog tereta i u temperaturnom području ispod 0 °C (uključujući 0 °C).The presented paper describes the development of mathematical model suitable for calculating temperature and humidity response and for simulation of dynamic behaviour of refrigerated products in a ship\u27s refrigerated container. Moist air, circulating around refrigerated products within refrigerated container is continuously in direct contact with surface of the goods. Simultaneous heat and mass exchange occurs between refrigerated goods and moist air. During such process mass exchange is considered as evaporation at the product surface exposed to air circulation. Only top surface of the product is therefore drying. This paper is limited to consideration of surface drying moisture, but not the hygroscopic moisture. Developed mathematical model, describing the dynamic changes of temperature and humidity of refrigerated goods caused by multiple influential factors, is based on mass and energy balances. Such model provides simulation of dynamic behaviour of refrigerated product between temperature ranges of above and below 0°C (including 0°C)

Dinamičke pojave i učinak izvora topline kod sušenja piljenog drva

High costs, irrational consumption of thermal energy and low efficiency are often characteristics of sawn timber drying by warm humid air in chamber drying kilns. A weak link between technological planning with the sawn timber mass dynamics in the systems with more drying kilns is the most common reason that influences considerably the economy of timber thermal processing and the heat source pre-dimensioning. Lack of experimental research and usage of general relations are common reasons for insufficient practical compliance and connection of drying thermal needs and the heat source capacity. Air heater thermal load has been researched during the sawn oak timber drying, 25 mm thick, as well as 32 mm and 50 mm thick beech, by applying of appropriate regimes during all the stages in more cycles of convective drying. Current thermal needs have been measured and registered in relation to maximal values during the characteristic intervals of each drying cycle. The exact values have been shown, while the defined correlation polynome has been used to describe a graphic and mathematical model of thermal load in all intervals of tested sawn timber drying cycles. An analytical procedure for determination of total maximal thermal needs of more drying kilns has been suggested based on timber moisture decreasing and technological drying planning. Using a characteristic example, its applicability and contribution to an optimal thermal source dimensioning has been shown.Viši troškovi, neracionalna potrošnja toplinske energije i mala učinkovitost česte su značajke sušenja piljenog drva toplim vlažnim zrakom u komornim sušarama. Slaba povezanost tehnološkog planiranja s dinamikom sušenja piljene drvne mase u sustavima s više sušara najčešće značajno utječe na ekonomičnost toplinske obrade drva i predimenzioniranje izvora topline. Nedostatak eksperimentalnih istraživanja i korištenjem općih relacija česti su razlozi nedovoljne praktične usklađenosti i povezanosti toplinskih potreba sušenja i kapaciteta izvora topline. Provedena su istraživanja toplinskog opterećenja grijača zraka pri sušenju piljenog drva hrasta debljine 25 mm te bukve debljine 32 mm i 50 mm, primjenom odgovarajućih režima tijekom svih faza u više ciklusa konvektivnog sušenja. Izmjerene su i registrirane trenutne toplinske potrebe u odnosu na maksimalne vrijednosti tijekom karakterističnih intervala svakog ciklusa sušenja. Prikazane su egzaktne vrijednosti, a pridruženim korelacijskim polinomom opisan je grafički i matematički model toplinskog opterećenja u svim intervalima ispitivanih ciklusa sušenja piljenog drva. Predložen je analitički postupak za određivanje ukupnih maksimalnih toplinskih potreba više sušara na osnovu dinamike snižavanja vlage drva i tehnološkog planiranja sušenja. Karakterističnim primjerom je prikazana njegova primjenjivost i doprinos optimalnom dimenzioniranju izvora topline

Energy Efficiency in Building Sector in Croatia - Preliminary Energy Studies

U članku je prikazana analiza rezultata odabranih energetskih pregleda i preliminarnih energetskih studija provedenih u sklopu projekta “Poticanje energetske efikasnosti u Hrvatskoj” za građevine javne namjene. Projekt provodi projektni tim UNDP-a (United Nations Development Program). Od početka projekta do danas ukupno je izrađeno više od 1200 energetskih pregleda s izvješćima i preliminarnih energetskih studija. Te studije obuhvaćaju tehničko vrednovanje energetske učinkovitosti komponenti postojećih energetskih sustava građevina, kao što su vanjska ovojnica zgrade, sustavi grijanja, ventilacije i klimatizacije, sustavi za zagrijavanje potrošne vode, rasvjeta i ostale elektroinstalacije ili potrošači električne energije. U okviru studija provodi se vrednovanje potrošnje enegije zgrade u stvarnim uvjetima rada, definiranje opisa mjera za povećanje energetske učinkovitosti (EE mjere) uključujući izračunavanje potencijala uštede, troškova ulaganja i vremena povrata za svaku pojedinu EE mjeru, te planiranje daljnjih postupaka koji će voditi do realizacije EE mjere. Studije također sadrže definiranje potrebnih postupaka za uspostavu kontinuiranog nadzora i poboljšanja potrošnje energije i vode u ispitanim zgradama (uspostava lokalnog sustava gospodarena energijom). Grupa odabranih preliminarnih energetskih studija zgrada javne namjene sa složenim energetskim sustavima analizirana je u cilju procjene trenutnog stanja, te definiranja najučinkovitijih mjera i njihova utjecaja na potrošnju energije i emisije CO2.The paper deals with analysis of results achieved through selected energy audits and preliminary energy studies of public buildings in Croatia, performed as a part of the project “Removal of barriers for energy efficiency in Croatia”. The project is being implemented by the Project Unit of the United Nations Development Program. Over 1200 energy audits and preliminary energy studies have been produced since the beginning of the project. Those studies comprise technical evaluation of energy efficiency of current building system components, such as building envelope, heating, ventilation and air-conditioning system, domestic hot water system, lighting and other electrical installation or consumers. Studies comprise evaluation of building performance, taking into consideration real operation conditions, determination of a list of measures for energy efficiency improvements (EE measures) including calculation of savings potential, investment costs and payback time for each proposed EE measure. The part of each study is also the plan of the further steps that will lead to realization of suggested EE measures, including definition of needed steps to establish continuous control and improvement of energy and water consumption locally in examined building (establishment of local Energy Management System). A group of selected preliminary energy studies for public buildings with the complex energy systems has been analyzed among all performed studies in order to estimate the current state and to define the most efficient measures and their impact on energy consumption and CO2 emission

Matematičko modeliranje termoenergetskih sustava s dizalicama topline pri promjenjivim uvjetima rada : doktorska disertacija

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