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The global growth in electricity consumption and the shortcomings of renewable electricity generation technologies are some of the reasons why it is still relevant to evaluate the performance of power conversion technologies that are used in fossil fuel power stations. The power conversion technology that is widely used in fossil fuel power stations is the Rankine cycle. The goal of this study was to determine if the efficiency of a typical Rankine cycle can be improved by adding a heat pump as a bottoming cycle. Three simulation models were developed to perform this evaluation. The first is a simulation model of a Rankine cycle. A quite detailed Rankine cycle configuration was evaluated. The simulation model was used to determine the heating requirements of the heat pump cycle as well as its operating temperature ranges. The efficiency of this Rankine cycle was calculated as 43.05 %. A basic vapour compression cycle configuration was selected as the heat pump of the combined cycle. A simulation model of the vapour compression cycle and the interfaces with the Rankine cycle was developed as the second simulation model. Working fluids that are typically used in vapour compression cycles cannot be used for this application, due to temperature limitations. The vapour compression cycle’s simulation model was therefore also used to calculate the coefficient of performance (COP) for various working fluids in order to select a suitable working fluid. The best cycle COP (3.015 heating) was obtained with ethanol as working fluid. These simulation models were combined to form the simulation model of the Rankine-heat pump combined cycle. This model was used to evaluate the performance of the combined cycle for two different compressor power sources. This study showed that the concept of using steam turbine or electrical power to drive a compressor driven vapour compression cycle in the configuration proposed here does not improve the overall efficiency of the cycle. The reasons for this were discovered and warrant future investigation. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

The global growth in electricity consumption and the shortcomings of renewable electricity generation technologies are some of the reasons why it is still relevant to evaluate the performance of power conversion technologies that are used in fossil fuel power stations. The power conversion technology that is widely used in fossil fuel power stations is the Rankine cycle. The goal of this study was to determine if the efficiency of a typical Rankine cycle can be improved by adding a heat pump as a bottoming cycle. Three simulation models were developed to perform this evaluation. The first is a simulation model of a Rankine cycle. A quite detailed Rankine cycle configuration was evaluated. The simulation model was used to determine the heating requirements of the heat pump cycle as well as its operating temperature ranges. The efficiency of this Rankine cycle was calculated as 43.05 %. A basic vapour compression cycle configuration was selected as the heat pump of the combined cycle. A simulation model of the vapour compression cycle and the interfaces with the Rankine cycle was developed as the second simulation model. Working fluids that are typically used in vapour compression cycles cannot be used for this application, due to temperature limitations. The vapour compression cycle’s simulation model was therefore also used to calculate the coefficient of performance (COP) for various working fluids in order to select a suitable working fluid. The best cycle COP (3.015 heating) was obtained with ethanol as working fluid. These simulation models were combined to form the simulation model of the Rankine-heat pump combined cycle. This model was used to evaluate the performance of the combined cycle for two different compressor power sources. This study showed that the concept of using steam turbine or electrical power to drive a compressor driven vapour compression cycle in the configuration proposed here does not improve the overall efficiency of the cycle. The reasons for this were discovered and warrant future investigation. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

The Organic Rankine Cycle (ORC) may be the superior cycle for power generation using low temperature and low power heat sources due to the utilization of high molecular mass fluids with low boiling points. They are flexible, simple, easy to operate and maintain, and offer many possible areas of applications including waste heat recovery and power generation from biomass, geothermal and even solar energy. Therefore, they may prove to be of significant importance in reducing global greenhouse gas emission and in the mitigation of climate change. In this thesis the technical feasibility and economic profitability of implementing an ORC in a district heating boiler station is investigated. A model of ORC connected to the hot water circuit of one of the biomass boilers of the boiler station is simulated. The achieved evaporation temperature is estimated to 135 degrees C and the condensation temperature is found to vary in the range of about 70-100 degrees C. The results show that it is both possible and profitable to implement an ORC in the studied boiler station. A maximum net present value of 2.3 MSEK is achieved for a 400 kW system and a maximum internal rate of return of 8.5%, equivalent to a payback period of 9.5 years, is achieved for a 300 kW system. Furthermore, the investment is found to be most sensitive to changes in the electricity price, net electric efficiency and capital expenditure cost.

No description available.

Low-grade heat sources, here defined as below 300 ºC, are abundantly available as industrial waste heat, solar thermal, and geothermal, to name a few. However, they are under-exploited for conversion to power because of the low efficiency of conversion. The utilization of low-grade heat is advantageous for many reasons. Technologies that allow the efficient conversion of low-grade heat into mechanical or electrical power are very important to develop. This work investigates the potential of supercritical Rankine cycles in the conversion of low-grade heat into power. The performance of supercritical Rankine cycles is studied using ChemCAD linked with customized excel macros written in Visual Basic and programs written in C++. The selection of working fluids for a supercritical Rankine cycle is of key importance. A rigorous investigation into the potential working fluids is carried out, and more than 30 substances are screened out from all the available fluid candidates. Zeotropic mixtures are innovatively proposed to be used in supercritical Rankine cycles to improve the system efficiency. Supercritical Rankine cycles and organic Rankine cycles with pure working fluids as well as zeotropic mixtures are studied to optimize the conversion of lowgrade heat into power. The results show that it is theoretically possible to extract and convert more energy from such heat sources using the cycle developed in this research than the conventional organic Rankine cycles. A theory on the selection of appropriate working fluids for different heat source and heat sink profiles is developed to customize and maximize the thermodynamic cycle performance. The outcomes of this research will eventually contribute to the utilization of low-grade waste heat more efficiently.

New technologies to converting heat into usable energy are constantly being developed for renewable use. This means that more interactions between different energy grid will be applied, such as utilizing low thermal waste heat to convert its energy to electricity. With high electricity price, such technology is quite attractive at applications that develop low waste heat. In the case of excess heat in district heating (DH) grid and the electricity price are high, the waste heat can be converted to electricity, which can bring a huge profit for DH companies. Candidate technologies are many and the focus in this degree rapport is on the so-called Organic Rankine Cycle (ORC) that belongs to the steam Rankine cycle. Instead of using water as a working fluid, organic working fluid is being used because of its ability to boil at lower temperature. Because this technique is available, it also needs to be optimized, developed, etc. to achieve the highest appropriate efficiency. This can be done, for example, by modeling different layouts, analyzing functionality, performance and / or do a simulation of various suitable working fluids.  This is the purpose of this degree project and the research parts are to select working fluids suitable at low temperatures (70-120) °C, the difference analysis between the selected fluids and identification of the parameters that most affect the performance. There are many suitable methods to apply to achieve desired results. The method used in this rapport degree is commercial software such as Mini REFPROP, CoolPack, Excel but the most important part is simulation with AspenPlus. The selected and suitable working fluids between the chosen temperature interval are R236ea, R600, R245fa and n-hexane. Three common layouts were investigated, and they are The Basic ORC, ORC with an internal heat exchanger (IHE) and regenerative ORC. The results show that in comparison between 120°C and 70°C as a temperature source and without an internal heat exchanger (IHE), R600 at 70°C, has the highest efficiency about 13.55%. At 110°C n-hexane has the highest efficiency about 18.10%. R236ea has the lowest efficiency 13.16% at 70°C and 16.29% at 110°C. R236ea kept its low efficiency through all results. Without an IHE and a source range from 70 °C up to almost 90 °C, R600 has the highest efficiency and at 90°C n-hexane has the highest efficiency. With an IHE and between (70-90) °C R245fa still has the highest efficiency. With or without IHE and a heat source of 110 °C n-hexane has the highest efficiency 18.10% and 18.40%. R236ea gets the greatest increase 5.2% in efficiency but remains with the lowest efficiency. With Regenerative ORC, n-hexane had an optimal middle pressure about 0.76 bar. The optimal pressure corresponds to a thermal efficiency of 17.52%. The most important identified parameters are the fluid characteristics such as higher critical temperature, temperature source, heat sink, application placement and component performance.         The current simulations have been run at some fixed data input such as isentropic efficiencies, no pressure drops, adiabatic conditions etc. It was therefore expected that the same efficiency curve would repeat itself. This efficiency pattern would differ with less or higher values depending on the layout performance. However, this pattern was up to 90 degrees Celsius and gets a very noticeable change by the change of the efficiency for n-hexane. Therefore n-hexane is chosen with Regenerative ORC because it had the highest efficiency at the highest temperature source tested. This is due definitive to the fluid properties like its high critical temperature compared to the other selected fluids. R236ea remains the worst and that’s also related to the fluid properties. It is also important to note that these efficiencies are only from a thermodynamic perspective and may differ when combining both thermal and economic perspectives as well as application placement. These high efficiencies will certainly be lower at more advanced or real processes due to various factors that affect performance. Factors such as component´s efficiency and selection, pipe type and size, etc. To maintain a constant temperature when it’s not, flow regulation is then necessary and that’s also affects the performance.   The conclusion is that the basic ORC which does not have an IHE and from 70 up to 90 degrees Celsius, R600 has the highest efficiency. Higher temperature gives n-hexane the highest efficiency. With an IHE and between (70-90) °C R254fa has the highest efficiency. At higher temperature source n-hexane has the highest efficiency. ORC with an IHE has the best performance. The R236ea has the worst performance through all results. With regenerative ORC, an optimal meddle-pressure for n-hexane is 0.76 bar. Important parameters are The properties of the fluid, temperature source, heatsink, Application placement and component performance. / Nej

Secondary working fluids (secondary refrigerants, heat transfer fluids, antifreezes, brines) have long been used in various indirect re-frigeration and heat pump systems. Aqueous solutions (water solu-tions) have long been used as single phase (liquid only) secondary working fluids for cooling in supermarkets, ice rinks, heat recovery systems, heat pumps and other applications. However, aqueous solutions are increasingly used also for freezers in supermarkets and other applications in low temperature refrigeration. Of importance when comparing different secondary working fluids for indirect systems are the following basic thermophysical properties: freezing point, density, viscosity, specific heat, and thermal conductivity. Reliable data of these properties are needed to choose suitable fluid and to make technical calculations of an indirect refrigeration or heat pump system. The main intention of this work has been to select thermophysical property data with good or acceptable technical accuracy of a number of aqueous solutions that can be used by the refrigeration and heat pump industry, rather than focusing on a limited number of property values or scientifically very accurate measuring techniques. A thorough literature search was in view of this made to find the most reliable property values of aqueous solutions. Detailed literature references are given for thermo-physical properties of the following aqueous solutions, without other additives: Ethylene and propylene glycol, ethyl and methyl alcohol, glycerol, ammonia, potassium carbonate, calcium, lithium, magnesium and sodium chlorides as well as potassium acetate and potassium formate. Some laboratory measurements were made of most of the fluid types when literature values were incomplete or deemed unreliable. Methods used are briefly described and results are given. Much of the work was reported on in the Engineering Licentiate Thesis: Thermophysical properties of liquid secondary refrigerants, A Critical Review on Literature References and Laboratory Measure-ments (Melinder 1998a). That material forms the basis for the charts and tables used in the IIR-publication Thermophysical properties of liquid secondary refrigerants (Melinder, 1997). The present thesis reports on an update made since 1998, including re-view work done on two additional fluids not covered in Melinder (1998a). The thesis describes how the selection of property values results in tables and charts intended for the industry. Coefficients for poly-nomial equations are generated from these property values using a Matlab program and this material is intended as a useful tool for computer treatment. Aqueous solution of ethyl alcohol is used as example to see how this process is made. This choice of fluid can also be seen as a test of this method, as the basic thermophysical properties of aqueous solutions of ethyl alcohol present more chal-lenges than the other fluids examined. A comparison is made of a few types of aqueous solutions used as secondary working fluids for two types of applications. The first example is bedrock heat pumps and the second is cooling cabinets in a supermarket. An effort is made to see how the additive con-centration affects the thermal performance. Most aqueous solutions used as single-phase secondary fluids can also be used as ice slurry, a fluid consisting of liquid and ice where small ice crystals are produced, usually with some type of ice generator. The ice crystals are then transported to the cooling object from which heat is removed when ice crystals melt. This results in less temperature change in the cooling object and makes it also possible to reduce the volume flow rate and to use smaller pipe dimensions in the system. In order to choose a secondary fluid for ice slurry use and to make correct technical calculations of the ice slurry system there is a need to examine and evaluate thermo-physical properties and other aspects of ice and of the aqueous solution used. For dimensioning purposes it is of interest to estimate ice mass fraction and enthalpy values and enthalpy-phase diagrams can serve that purpose. This thesis presents enthalpy-phase diagrams made by author that besides isotherms contain lines with ice fraction and lines connecting enthalpies at freezing point and 1, 2, etc. to 10 K below the freezing point curve. / QC 20100609

Das Lernsystem Thermopr@ctice stellt eine internetgestützte Lernumgebung für das Berechnen von Übungsaufgaben mit dem Computer-Algebrasystem Mathcad dar. Die veränderte Arbeitsweise des Lernenden besteht darin, das herkömmliche Arbeitsblatt durch den Mathcad-Arbeitsbildschirm zu ersetzen und die Aufgabenlösung unmittelbar auf diesem zu erledigen. Dem Lernenden werden die Übungsaufgaben – hier im Fach Technische Thermodynamik – in individuellen Varianten und mit individuellen Zahlenwerten im Mathcad-Format über Internet bereitgestellt. Die Lösung erfolgt in betreuten Übungen oder am heimischen PC. Für die Lösung benötigte Stoffwerte können der Stoffwertsammlung im Internet entnommen bzw. mit Programmbibliotheken, die an Mathcad angeschlossen sind, berechnet werden. Ergänzend wird eine Formelsammlung angeboten, aus der wichtige Formeln auf den Arbeitsbildschirm gezogen werden können. Nach der Berechnung jeder Teilaufgabe sendet der Lernende das Ergebnis an Thermopr@ctice. Im Fehlerfall werden Zwischenergebnisse angefordert. Realisiert wird die Lernumgebung über PHP-Skripte in Verbindung mit einer MySQL-Datenbank. Durch das Lernsystem werden die Studierenden an moderne Arbeitsweisen unter Nutzung eines Computer-Algebrasystems und fachbezogener Programmbibliotheken herangeführt. Da das System zum Selbststudium konzipiert ist, eignet es sich auch für die Weiterbildung und das Fernstudium. Es kann auf alle Lehrfächer übertragen werden, in denen die Aneignung oder Festigung von Wissen über das Berechnen von Übungsaufgaben erfolgt. Thermopr@ctice wurde im Rahmen des Verbundprojektes „Bildungsportal Sachsen“ des Sächsischen Staatsministeriums für Wissenschaft und Kunst entwickelt. Neue Technologien in der Energietechnik, insbesondere Verfahren mit CO2-Abscheidung, bedingen veränderte Arbeitsfluide. Neben reinen Stoffen kommen zunehmend fluide Stoffgemische zum Einsatz, deren thermodynamische Eigenschaften berechnet werden müssen. Dies betrifft feuchte Verbrennungsgasgemische einschließlich CO2/H2O-Gemische sowie feuchte Luft, auch bei hohen Drücken. Daneben sind die Eigenschaften von gasförmigen, flüssigen und festem Kohlendioxid und Mischungen mit verbliebenen Gasen zu berechnen. Auf Grund der Nutzung von Abwärme sind Absorptionskältemaschinen mit Ammoniak-Wasser-Gemischen und Wasser- Lithiumbromid-Gemischen nach wie vor von Interesse. Die Eigenschaften von Ammoniak/Wasser- Gemischen werden auch für die Berechnung des Kalina-Prozesses benötigt. Zur Konzipierung von ORC-Prozessen müssen die Eigenschaften von Silikonölen und Kohlenwasserstoffen berechenbar sein. Für die Optimierung von fortschrittlichen Dampfkraftwerken sind extrem schnelle Algorithmen für die Berechnung der thermodynamischen Eigenschaften von Wasser und Wasserdampf Voraussetzung. Die weltweit an Bedeutung gewinnende Meerwasserentsalzung bedingt eine immer genauere Modellierung der Verfahren, wofür die Eigenschaften von Meerwasser berechenbar sein müssen. Für die Berechnung solcher Prozesse wurden benutzerfreundliche Programmbibliotheken zur Ermittlung der thermodynamischen Zustandsgrößen einschl. Umkehrfunktionen und Transporteigenschaften der Arbeitsfluide erarbeitet. Zur komfortablen Nutzung der Stoffwert-Bibliotheken steht das Add-On FluidMAT für Mathcad® zur Verfügung. Versionen für Studierende der wichtigsten Programme sind verfügbar.

Thesis (M.S.); Submitted to Central Missouri State University, Warrensburg, MO (US); 17 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. Kerr, Kent. NNSA Kansas City Site Office (US) NNSA Kansas City Site Office. 12/17/2004. Report is also available in paper and microfiche from NTIS.