Performance and scalability of isolated DC-DC converter topologies in low voltage, high current applications

Fuel cells are a promising alternative for clean and efficient energy production. A fuel cell is probably the most demanding of all distributed generation power sources. It resembles a solar cell in many ways, but sets strict limits to current ripple, common mode voltages and load variations. The typically low output voltage from the fuel cell stack needs to be boosted to a higher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell, there is a need for high efficiency power converters, and in the case of low voltage, high current and galvanic isolation, the implementation of such converters is not a trivial task. This thesis presents galvanically isolated DC-DC converter topologies that have favorable characteristics for fuel cell usage and reviews the topologies from the viewpoint of electrical efficiency and cost efficiency. The focus is on evaluating the design issues when considering a single converter module having large current stresses. The dominating loss mechanism in low voltage, high current applications is conduction losses. In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but in the case of diodes, the effectiveness of paralleling depends strongly on the semiconductor material, diode parameters and output configuration. The transformer winding losses can be a major source of losses if the windings are not optimized according to the topology and the operating conditions. Transformer prototyping can be expensive and time consuming, and thus it is preferable to utilize various calculation methods during the design process in order to evaluate the performance of the transformer. This thesis reviews calculation methods for solid wire, litz wire and copper foil winding losses, and in order to evaluate the applicability of the methods, the calculations are compared against measurements and FEM simulations. By selecting a proper calculation method for each winding type, the winding losses can be predicted quite accurately before actually constructing the transformer. The transformer leakage inductance, the amount of which can also be calculated with reasonable accuracy, has a significant impact on the semiconductor switching losses. Therefore, the leakage inductance effects should also be taken into account when considering the overall efficiency of the converter. It is demonstrated in this thesis that although there are some distinctive differences in the loss distributions between the converter topologies, the differences in the overall efficiency can remain within a range of a few percentage points. However, the optimization effort required in order to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topology selection can become crucial.

Structural and operational optimisation : applications in energy systems

In this work mathematical programming models for structural and operational optimisation of energy systems are developed and applied to a selection of energy technology problems. The studied cases are taken from industrial processes and from large regional energy distribution systems. The models are based on Mixed Integer Linear Programming (MILP), Mixed Integer Non-Linear Programming (MINLP) and on a hybrid approach of a combination of Non-Linear Programming (NLP) and Genetic Algorithms (GA). The optimisation of the structure and operation of energy systems in urban regions is treated in the work. Firstly, distributed energy systems (DES) with different energy conversion units and annual variations of consumer heating and electricity demands are considered. Secondly, district cooling systems (DCS) with cooling demands for a large number of consumers are studied, with respect to a long term planning perspective regarding to given predictions of the consumer cooling demand development in a region. The work comprises also the development of applications for heat recovery systems (HRS), where paper machine dryer section HRS is taken as an illustrative example. The heat sources in these systems are moist air streams. Models are developed for different types of equipment price functions. The approach is based on partitioning of the overall temperature range of the system into a number of temperature intervals in order to take into account the strong nonlinearities due to condensation in the heat recovery exchangers. The influence of parameter variations on the solutions of heat recovery systems is analysed firstly by varying cost factors and secondly by varying process parameters. Point-optimal solutions by a fixed parameter approach are compared to robust solutions with given parameter variation ranges. In the work enhanced utilisation of excess heat in heat recovery systems with impingement drying, electricity generation with low grade excess heat and the use of absorption heat transformers to elevate a stream temperature above the excess heat temperature are also studied.

Recovery of carboxylic acids from aqueous streams by extraction and back extraction

Separation of carboxylic acids from aqueous streams is an important part of their manufacturing process. The aqueous solutions are usually dilute containing less than 10 % acids. Separation by distillation is difficult as the boiling points of acids are only marginally higher than that of water. Because of this distillation is not only difficult but also expensive due to the evaporation of large amounts of water. Carboxylic acids have traditionally been precipitated as calcium salts. The yields of these processes are usually relatively low and the chemical costs high. Especially the decomposition of calcium salts with sulfuric acid produces large amounts of calcium sulfate sludge. Solvent extraction has been studied as an alternative method for recovery of carboxylic acids. Solvent extraction is based on mixing of two immiscible liquids and the transfer of the wanted components form one liquid to another due to equilibrium difference. In the case of carboxylic acids, the acids are transferred from aqueous phase to organic solvent due to physical and chemical interactions. The acids and the extractant form complexes which are soluble in the organic phase. The extraction efficiency is affected by many factors, for instance initial acid concentration, type and concentration of the extractant, pH, temperature and extraction time. In this paper, the effects of initial acid concentration, type of extractant and temperature on extraction efficiency were studied. As carboxylic acids are usually the products of the processes, they are wanted to be recovered. Hence the acids have to be removed from the organic phase after the extraction. The removal of acids from the organic phase also regenerates the extractant which can be then recycled in the process. The regeneration of the extractant was studied by back-extracting i.e. stripping the acids form the organic solution into diluent sodium hydroxide solution. In the solvent regeneration, the regenerability of different extractants and the effect of initial acid concentration and temperature were studied.

Hydrometallurgical recovery of valuable metals from secondary raw materials

Wastes and side streams in the mining industry and different anthropogenic wastes often contain valuable metals in such concentrations their recovery may be economically viable. These raw materials are collectively called secondary raw materials. The recovery of metals from these materials is also environmentally favorable, since many of the metals, for example heavy metals, are hazardous to the environment. This has been noticed in legislative bodies, and strict regulations for handling both mining and anthropogenic wastes have been developed, mainly in the last decade. In the mining and metallurgy industry, important secondary raw materials include, for example, steelmaking dusts (recoverable metals e.g. Zn and Mo), zinc plant residues (Ag, Au, Ga, Ge, In) and waste slurry from Bayer process alumina production (Ga, REE, Ti, V). From anthropogenic wastes, waste electrical and electronic equipment (WEEE), among them LCD screens and fluorescent lamps, are clearly the most important from a metals recovery point of view. Metals that are commonly recovered from WEEE include, for example, Ag, Au, Cu, Pd and Pt. In LCD screens indium, and in fluorescent lamps, REEs, are possible target metals. Hydrometallurgical processing routes are highly suitable for the treatment of complex and/or low grade raw materials, as secondary raw materials often are. These solid or liquid raw materials often contain large amounts of base metals, for example. Thus, in order to recover valuable metals, with small concentrations, highly selective separation methods, such as hydrometallurgical routes, are needed. In addition, hydrometallurgical processes are also seen as more environmental friendly, and they have lower energy consumption, when compared to pyrometallurgical processes. In this thesis, solvent extraction and ion exchange are the most important hydrometallurgical separation methods studied. Solvent extraction is a mainstream unit operation in the metallurgical industry for all kinds of metals, but for ion exchange, practical applications are not as widespread. However, ion exchange is known to be particularly suitable for dilute feed solutions and complex separation tasks, which makes it a viable option, especially for processing secondary raw materials. Recovering valuable metals was studied with five different raw materials, which included liquid and solid side streams from metallurgical industries and WEEE. Recovery of high purity (99.7%) In, from LCD screens, was achieved by leaching with H2SO4, extracting In and Sn to D2EHPA, and selectively stripping In to HCl. In was also concentrated in the solvent extraction stage from 44 mg/L to 6.5 g/L. Ge was recovered as a side product from two different base metal process liquors with Nmethylglucamine functional chelating ion exchange resin (IRA-743). Based on equilibrium and dynamic modeling, a mechanism for this moderately complex adsorption process was suggested. Eu and Y were leached with high yields (91 and 83%) by 2 M H2SO4 from a fluorescent lamp precipitate of waste treatment plant. The waste also contained significant amounts of other REEs such as Gd and Tb, but these were not leached with common mineral acids in ambient conditions. Zn was selectively leached over Fe from steelmaking dusts with a controlled acidic leaching method, in which the pH did not go below, but was held close as possible to, 3. Mo was also present in the other studied dust, and was leached with pure water more effectively than with the acidic methods. Good yield and selectivity in the solvent extraction of Zn was achieved by D2EHPA. However, Fe needs to be eliminated in advance, either by the controlled leaching method or, for example, by precipitation. 100% Pure Mo/Cr product was achieved with quaternary ammonium salt (Aliquat 336) directly from the water leachate, without pH adjustment (pH 13.7). A Mo/Cr mixture was also obtained from H2SO4 leachates with hydroxyoxime LIX 84-I and trioctylamine (TOA), but the purities were 70% at most. However with Aliquat 336, again an over 99% pure mixture was obtained. High selectivity for Mo over Cr was not achieved with any of the studied reagents. Ag-NaCl solution was purified from divalent impurity metals by aminomethylphosphonium functional Lewatit TP-260 ion exchange resin. A novel preconditioning method, named controlled partial neutralization, with conjugate bases of weak organic acids, was used to control the pH in the column to avoid capacity losses or precipitations. Counter-current SMB was shown to be a better process configuration than either batch column operation or the cross-current operation conventionally used in the metallurgical industry. The raw materials used in this thesis were also evaluated from an economic point of view, and the precipitate from a waste fluorescent lamp treatment process was clearly shown to be the most promising.

Blade and rod coating of low solid dispersions

Työn aiheena oli tehdä ohut barrierkalvo terä- tai sauvapäällystys menetelmällä. Erilaisissa elintarvikepakkauksissa käytetään hyviä barrier-ominaisuuksia omaavia ohuita päällysteitä. Elintarvikepakkauksen tehtävä on suojata pakattua tuotetta ympäristöltä, mahdollistaa helppo kuljetus ja säilytys sekä antaa tarvittavat tiedot tuotteesta tuotteen käsittelijöille ja loppukäyttäjille. Diplomityön teoriaosuudessa keskityttiin barrierpäällystykseen, eri päällystysmenetelmiin, niiden erityisvaatimuksiin ja ominaisuuksiin. Teoriaosuudessa käsiteltiin myös vaadittavia barrier-ominaisuuksia ja haasteita niiden saavuttamisessa. Kirjallisuuden perusteella haasteiksi nousivat helposti muodostuvat mikroreiät. Kokeellinen osa jakautui kahteen osakokonaisuuteen: laboratoriokokeisiin ja pilot-koeajoon. Laboratoriokokeita tehtiin ennen pilot-ajoa, jotta pilot-koeajoon voitiin valita parhaat päällystereseptit. Pilot-koeajonäytteiden päällystemäärät osoittautuivat liian pieniksi ja siksi laboratoriossa tehtiin jatkotutkimuksia riittävän päällystemäärän saavuttamiseksi. Tämän työnperusteella pohjakartongin ominaisuuksilla, erityisesti karheudella, on merkittävä vaikutus päällystyksen onnistumisessa ja yksinkertaisilla resepteillä ja päällystysmenetelmillä ei saada tarpeeksi laadukasta kalvoa.

Purification of Hydrocarbon Waste Streams

Purification of hydrocarbon waste streams is needed to recycle valuable hydrocarbon products, reduce hazardous impacts on environment, and save energy. To obtain these goals, research must be focused on the search of effective and feasible purification and re-refining technologies. Hydrocarbon waste streams can contain both deliberately added additives to original product and during operation cycle accumulated undesired contaminants. Compounds may have degenerated or cross-reacted. Thus, the presence of unknown species cause additional challenges for the purification process. Adsorption process is most suitable to reduce impurities to very low concentrations. Main advantages are availability of selective commercial adsorbents and the regeneration option to recycle used separation material. Used hydrocarbon fraction was purified with various separation materials in the experimental part. First screening of suitable materials was done. In the second stage, temperature dependence and adsorption kinetics were studied. Finally, one fixed bed experiment was done with the most suitable material. Additionally, FTIR-measurements of hydrocarbon samples were carried out to develop a model to monitor the concentrations of three target impurities based on spectral data. Adsorption capacities of the tested separation materials were observed to be low to achieve high enough removal efficiencies for target impurities. Based on the obtained data, batch process would be more suitable than a fixed bed process and operation at high temperatures is favorable. Additional pretreatment step is recommended to improve removal efficiency. The FTIR-measurement was proven to be a reliable and fast analysis method for challenging hydrocarbon samples.

Low-cost automatic activity data recording system

We describe a low-cost, high quality device capable of monitoring indirect activity by detecting touch-release events on a conducting surface, i.e., the animal's cage cover. In addition to the detecting sensor itself, the system includes an IBM PC interface for prompt data storage. The hardware/software design, while serving for other purposes, is used to record the circadian activity rhythm pattern of rats with time in an automated computerized fashion using minimal cost computer equipment (IBM PC XT). Once the sensor detects a touch-release action of the rat in the upper portion of the cage, the interface sends a command to the PC which records the time (hours-minutes-seconds) when the activity occurred. As a result, the computer builds up several files (one per detector/sensor) containing a time list of all recorded events. Data can be visualized in terms of actograms, indicating the number of detections per hour, and analyzed by mathematical tools such as Fast Fourier Transform (FFT) or cosinor. In order to demonstrate method validation, an experiment was conducted on 8 Wistar rats under 12/12-h light/dark cycle conditions (lights on at 7:00 a.m.). Results show a biological validation of the method since it detected the presence of circadian activity rhythm patterns in the behavior of the rats

Electrocardiographic changes during low-dose, short-term therapy of cutaneous leishmaniasis with the pentavalent antimonial meglumine

The pentavalent antimonial (Sb5+) meglumine is the drug of choice for the treatment of cutaneous leishmaniasis (CL) in Brazil. Although the cardiotoxicity of high-dose, long-term Sb5+ therapy is well known, the use of low-dose, short-term meglumine has been considered to be safe and relatively free from significant cardiac effects. In order to investigate the cardiotoxicity of low-dose, short-term therapy with meglumine in cutaneous leishmaniasis, 62 CL patients treated with meglumine were studied. A standard ECG was obtained before and immediately after the first cycle of treatment (15 mg Sb5+ kg-1 day-1). The electrocardiographic interpretation was carried out blindly by two investigators using the Minnesota Code. There were no significant differences in qualitative ECG variables before and after meglumine treatment. However, the corrected QT interval was clearly prolonged after antimonial therapy (420.0 vs 429.3 ms, P<10-6). QTc augmentation exceeded 40 ms in 12 patients, 7 of whom developed marked QTc interval enlargement (500 ms) after meglumine therapy. This previously unrecognized cardiac toxicity induced by short-term, low-dose antimonial therapy has potentially important clinical implications. Since sudden death has been related to QTc prolongation over 500 ms induced by high-dose antimonial therapy, routine electrocardiographic monitoring is probably indicated even in CL patients treated with short-term, low-dose meglumine schedules. Until further studies are conducted to establish the interactions between pentavalent antimonials and other drugs, special care is recommended when using meglumine in combination with other medications, in particular with drugs that also increase the QTc interval.

Working fluid selection and design of small-scale waste heat recovery systems based on organic Rankine cycles

Demand for the use of energy systems, entailing high efficiency as well as availability to harness renewable energy sources, is a key issue in order to tackling the threat of global warming and saving natural resources. Organic Rankine cycle (ORC) technology has been identified as one of the most promising technologies in recovering low-grade heat sources and in harnessing renewable energy sources that cannot be efficiently utilized by means of more conventional power systems. The ORC is based on the working principle of Rankine process, but an organic working fluid is adopted in the cycle instead of steam. This thesis presents numerical and experimental results of the study on the design of small-scale ORCs. Two main applications were selected for the thesis: waste heat re- covery from small-scale diesel engines concentrating on the utilization of the exhaust gas heat and waste heat recovery in large industrial-scale engine power plants considering the utilization of both the high and low temperature heat sources. The main objective of this work was to identify suitable working fluid candidates and to study the process and turbine design methods that can be applied when power plants based on the use of non-conventional working fluids are considered. The computational work included the use of thermodynamic analysis methods and turbine design methods that were based on the use of highly accurate fluid properties. In addition, the design and loss mechanisms in supersonic ORC turbines were studied by means of computational fluid dynamics. The results indicated that the design of ORC is highly influenced by the selection of the working fluid and cycle operational conditions. The results for the turbine designs in- dicated that the working fluid selection should not be based only on the thermodynamic analysis, but requires also considerations on the turbine design. The turbines tend to be fast rotating, entailing small blade heights at the turbine rotor inlet and highly supersonic flow in the turbine flow passages, especially when power systems with low power outputs are designed. The results indicated that the ORC is a potential solution in utilizing waste heat streams both at high and low temperatures and both in micro and larger scale appli- cations.

Obese women on a low energy rice and bean diet: effects of leucine, arginine or glycine supplementation on protein turnover

This study examined if leucine, arginine or glycine supplementation in adult obese patients (body mass index of 33 ± 4 kg/m²) consuming a Brazilian low energy and protein diet (4.2 MJ/day and 0.6 g protein/kg) affects protein and amino acid metabolism. After four weeks adaptation to this diet, each subject received supplements of these amino acids (equivalent to 0.2 g protein kg-1 day-1) in random order. On the seventh day of each amino acid supplementation, a single-dose 15N-glycine study was carried out. There were no significant differences in protein flux, synthesis or breakdown. The protein flux (grams of nitrogen, gN/9 h) was 55 ± 24 during the nonsupplemented diet intake and 39 ± 10, 44 ± 22 and 58 ± 35 during the leucine-, glycine- and arginine-supplemented diet intake, respectively; protein synthesis (gN/9 h) was 57 ± 24, 36 ± 10, 41 ± 22 and 56 ± 36, respectively; protein breakdown (gN/9 h) was 51 ± 24, 34 ± 10, 32 ± 28 and 53 ± 35, respectively; kinetic balance (gN/9 h) was 3.2 ± 1.8, 4.1 ± 1.7, 3.4 ± 2.9 and 3.9 ± 1.6. There was no difference in amino acid profiles due to leucine, arginine or glycine supplementation. The present results suggest that 0.6 g/kg of dietary protein is enough to maintain protein turnover in obese women consuming a reduced energy diet and that leucine, arginine or glycine supplementation does not change kinetic balance or protein synthesis.

Humoral response to low molecular weight antigens of Mycobacterium tuberculosis by tuberculosis patients and contacts

Much effort has been devoted to the identification of immunologically important antigens of Mycobacterium tuberculosis and to the combination of target antigens to which antibodies from serum of tuberculous patients could react specifically. We searched for IgG antibodies specific for antigens of 45 to 6 kDa obtained after sonication of the well-characterized wild M. tuberculosis strain in order to detect differences in the antibody response to low molecular weight antigens from M. tuberculosis between patients with pulmonary tuberculosis and contacts. Specific IgG antibodies for these antigens were detected by Western blot analysis of 153 serum samples collected from 51 patients with confirmed pulmonary tuberculosis. Three samples were collected from each patient: before therapy, and after 2 and 6 months of treatment. We also analyzed 25 samples obtained from contacts, as well as 30 samples from healthy individuals with known tuberculin status, 50 samples from patients with other lung diseases and 200 samples from healthy blood donors. The positive predictive value for associated IgG reactivity against the 6-kDa and 16-kDa antigens, 6 and 38 kDa, and 16 and 38 kDa was 100% since simultaneous reactivity for these antigens was absent in healthy individuals and individuals with other lung diseases. This association was observed in 67% of the patients, but in only 8% of the contacts. The humoral response against antigens of 16 and 6 kDa seems to be important for the detection of latent tuberculosis since the associated reactivity to these antigens is mainly present in individuals with active disease.

In vitro modulation of alkaline phosphatase activity of Saccharomyces cerevisiae grown in low or high phosphate medium

Our objective was to characterize the modulation of the activity of Saccharomyces cerevisiae alkaline phosphatases (ALPs) by classic inhibitors of ALP activity, cholesterol and steroid hormones, in order to identify catalytic similarities between yeast and mammalian ALPs. S. cerevisiae expresses two ALPs, coded for by the PHO8 and PHO13 genes. The product of the PHO8 gene is repressible by Pi in the medium. ALP activity from yeast (grown in low or high phosphate medium) homogenates was determined with p-nitrophenylphosphate as substrate, pH 10.4 (lPiALP or hPiALP, respectively). Activation of hPiALP was observed with 5 mM L-amino acids (L-homoarginine _ 186%, L-leucine _ 155% and L-phenylalanine - 168%) and with 1 mM levamisole (122%; percentage values, in comparison to control, of recovered activity). EDTA (5 mM) and vanadate (1 mM) distinctly inhibited hPiALP (2 and 20%, respectively). L-homoarginine (5 mM) had a lower activating effect on lPiALP (166%) and was the strongest hPiALP activator. Corticosterone (5 mM) inhibited hPiALP to 90%, but no effect was observed in low phosphate medium. Cholesterol, ß-estradiol and progesterone also had different effects on lPiALP and hPiALP. A concentration-dependent activation of lPiALP minus hPiALP was evident with all three compounds, most especially with ß-estradiol and cholesterol. These results do not allow us to identify similarities of the behavior of S. cerevisiae ALPs and any of the mammalian ALPs but allow us to raise the hypothesis of differential regulation of S. cerevisiae ALPs by L-homoarginine, ß-estradiol and cholesterol and of using these compounds to discriminate between S. cerevisiae lPiALP and hPiALP.

Examining life-course influences on chronic disease: the importance of birth cohort studies from low- and middle- income countries. An overview

The objectives of this overview are to describe the past and potential contributions of birth cohorts to understanding chronic disease aetiology; advance a justification for the maintenance of birth cohorts from low- and middle-income countries (LMIC); provide an audit of birth cohorts from LMIC; and, finally, offer possible future directions for this sphere of research. While the contribution of birth cohorts from affluent societies to understanding disease aetiology has been considerable, we describe several reasons to anticipate why the results from such studies might not be directly applied to LMIC. More than any other developing country, Brazil has a tradition of establishing, maintaining and exploiting birth cohort studies. The clear need for a broader geographical representation may be precipitated by a greater collaboration worldwide in the sharing of ideas, fieldwork experience, and cross-country cohort data comparisons in order to carry out the best science in the most efficient manner. This requires the involvement of a central overseeing body - such as the World Health Organization - that has the respect of all countries and the capacity to develop strategic plans for `global' life-course epidemiology while addressing such issues as data-sharing. For rapid progress to be made, however, there must be minimal bureaucratic entanglements.

Color degradation kinetics in low-calorie strawberry and guava jellies

The purpose of this study was to follow-up color changes in low-calorie strawberry and guava jellies during storage. To this end, one formulation of each flavor was prepared varying the application of hydrocolloids (pectin and modified starch). The jellies were studied regarding pH, soluble solids, water activity and syneresis. In order to follow-up color changes, the samples remained stored for 180 days in chambers with controlled temperatures of 10 °C (control) and 25 °C (commercial), and color instrumental analyses (L*, a*, and b*) were performed every 30 days. Arrhenius model was applied to reaction speeds (k) at different temperatures, where light strawberry and guava jellies showed greater color changes when stored at 25 °C compared to the samples stored at 10 °C. Activation energy values between 13 and 15 kcal.mol-1 and Q10 values between 2.1 and 2.3 were obtained for light strawberry jelly and light guava jelly, respectively. Therefore, it was concluded that, with respect to color changes, every 10 °C temperature increase reduces light jellies shelf-life by half.

Assessment of lettuce quality during storage at low relative humidity using Global Stability Index methodology

During postharvest, lettuce is usually exposed to adverse conditions (e.g. low relative humidity) that reduce the vegetable quality. In order to evaluate its shelf life, a great number of quality attributes must be analyzed, which requires careful experimental design, and it is time consuming. In this study, the modified Global Stability Index method was applied to estimate the quality of butter lettuce at low relative humidity during storage discriminating three lettuce zones (internal, middle, and external). The results indicated that the most relevant attributes were: the external zone - relative water content, water content , ascorbic acid, and total mesophilic counts; middle zone - relative water content, water content, total chlorophyll, and ascorbic acid; internal zone - relative water content, bound water, water content, and total mesophilic counts. A mathematical model that takes into account the Global Stability Index and overall visual quality for each lettuce zone was proposed. Moreover, the Weibull distribution was applied to estimate the maximum vegetable storage time which was 5, 4, and 3 days for the internal, middle, and external zone, respectively. When analyzing the effect of storage time for each lettuce zone, all the indices evaluated in the external zone of lettuce presented significant differences (p < 0.05). For both, internal and middle zones, the attributes presented significant differences (p < 0.05), except for water content and total chlorophyll.