<p><strong></strong>The aim of this Project is to study the influence of high return temperatures in district heating on the costs for heat and power production in a CHP plant.When the temperatures of the water coming back to the heating plant are so high, the overall performance of heat and power production is decreased and, consequently, also the production costs. Along the project, the influence of this temperature on the different parts of a CHP plant are analysed as well as the economical impact it has. At the same time, some general impacts on the entire network are mentioned.</p><p> </p><p>A real network is used in this project, and it is the net of district heating in Gävle (Sweden), and the most of the study is focus in its bigger combined heat and power production plant (CHP), called Johannes.</p>

<p><strong>Abstract</strong></p><p>The behavior for MoSi₂ based high temperature heating elements for resistive heating has been examined in elevated temperature and low oxygen content environment. MoSi₂ spontaneously forms a protective SiO₂ scale at high temperature if the amount of oxygen in the ambient atmosphere is sufficient according to the following reaction:</p><p>5MoSi₂ + 7O₂(g)  7SiO₂ + Mo₅Si₃</p><p>If the oxygen content at a specific temperature is too low, SiO(g) is more stable than SiO₂ and the following reaction will occur instead:</p><p>2SiO₂  2SiO(g) + O₂(g)</p><p>Then surface will be Si-deplated and finally, the base material will be exposed. Si and Mo will oxidize and degas from the surface as SiO and MoO₃ with severe diameter reduction of the heating element as a result. It is therefore of high interest to find the relationship between the maximum element temperature and the oxygen content in the ambient atmosphere to be able to fully exploit the potential of the heating elements and also to aid and help diagnose customer complaints.</p><p> </p><p>After 14 full scale tests in a custom made atmospheric furnace, the following equation could be calculated:</p><p>p(O₂) = 1.748·10^{0.01677·T·log(e)-10}</p><p>The equation gives the minimum oxygen content at a specified temperature. The equation is based on 100 hours tests at atmospheric pressure, gas flow rate of 4 liter per minute, varying temperature and varying oxygen content. Nitrogen has been used as carrier gas for the oxygen.</p>

Two thermal convection problems of geophysical interest are examined, theoretically. First, convection in the earth's mantle is treated on the basis of a one-dimensional 'strip model'. This model results from further simplification of the well known 'Rayleigh model'. For homogeneous, Newtonian fluids, the strip model yields results similar to those obtained by the Rayleigh method. The strip model is used to determine the critical Rayleigh number for convection in an internally heated two-phase fluid. The critical number depends on the parameters of the phase transition, the physical properties of the fluid, and the depth of the fluid layer. Depending on these factors, a univariant phase transformation may either enhance or hinder convective instability. For the olivine-spinel and spinel-oxides transitions in (MgFe)₂SiO₄ which are thought to take place in the upper mantle, it is shown that the critical Rayleigh number is altered only slightly from the critical number for convection in a fluid with one phase. This result holds both for convection in the entire mantle or convection restricted to the upper mantle. Hence the phase changes are of minor importance regarding the existence of mantle convection in general. A method for estimating the order of magnitude of the displacement of the phase surface as a function of Rayleigh number is outlined for a fluid with only one phase transition. The strip model is also used to treat convection in non-Newtonian fluids obeying a power law rheological equation. If the mantle is governed by a flow law of this type, it appears that convection can take place. Lastly, the procedure for applying the strip model to fluids with variable viscosity and thermal conductivity is outlined. The second convection problem concerns some aspects of convection of fluids in thin vertical fractures in the crust. A steady state model is developed to estimate the magnitude of the mass flow as a function of fracture thickness. It is shown that fractures of the order of a millimeter thick or greater can carry a measurable convective flow. A time dependent model is used to estimate the rate of decay of the mass flow with time. The results indicate that in fractures of the order of a centimeter thick, a measurable decrease of the mass flow takes place after a period of the order of a day. This rapid decay rate suggests that the principal effect of sea water convection in extensive fracture systems which are expected on mid-ocean ridge crests is to cool a volume of crustal rock in the vicinity of the fractures. Circulation of sea water in vertical fractures in the upper crust may provide an explanation of 1) the relatively low conductive heat flow measured at some locations on ocean ridge axes and 2) the very 'noisy' data obtained in the axial zone. / Graduation date: 1972

Graduation date: 1963

A steady state, axial flow thermal conductivity test apparatus was designed and constructed to operate between room temperature and approximately 4 Kelvin, and to be compatible with existing electronic instrumentation and a continuous flow cryostat. The test design included a radiation shield that had its temperature profile matched to that of the sample to minimize radiation heat transfer losses. The cryostat was used to provide the controllable, low temperature test environment in which the test apparatus would operate. A special wiring bundle was constructed to ensure proper connection of the test device to the required electronic instrumentation, which was controlled from a computer by custom written software. Once assembled, the thermal conductivity of a high purity copper sample was measured over the temperature range from 45 to 300 Kelvin and compared to literature recommended values. The test was performed a second time to check repeatability of the measurements over a range of temperature. Next, the thermal conductivity of a high purity niobium sample was measured and compared to literature recommended values. This test was also performed twice. When completed, these tests had demonstrated the accuracy and repeatability of the measurement of thermal conductivity by the test apparatus over the range of temperatures specified and over a range of conductivities. Finally, the thermal conductivity of a sample of the bulk metallic glass Vitreloy 1 was measured over the same temperature range. As far as was known, this was the first time the thermal conductivity of this particular material had been tested below 400 Kelvin. / Graduation date: 2003

Graduation date: 2000

Graduation date: 1997

Transition edge sensors are the detectors of choice for a wide range of applications; from dark matter search, neutrino search, to cosmic radiation detection from near infrared to millimeter wavelengths. We are developing transition edge sensors using superconducting iridium thin films and we are proposing their use for future dark matter and neutrino search experiments. Our Ir films are deposited using an radio frequency (RF) magnetron sputtering and photolithographic techniques and measured using an adiabatic refrigerator capable of reaching temperatures of a few tens of mK. This thesis presents a detailed description of superconducting iridium thin films from the fabrication process to the characterization of the film properties at room temperature and low temperature. Alternative options for the bias circuit used to read out the TES signals will be discussed, we are proposing the use of RLC resonant circuits and transformers instead of SQUIDS.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2005. / Restricted until May 2006. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 258-283).