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.

Tulviin varautuminen Rovaniemellä ja Kittilässä : Ilmastonmuutoksen vaikutukset tulviin

Tulviin varautuminen Rovaniemellä ja Kittilässä, Ilmastonmuutoksen vaikutukset tulviin on koosteraportti tulvista ja tulviin varautumisesta Kemi- ja Ounasjoen vesistöissä Rovaniemen ja Kittilän alueilla. Raportti on tehty kansainväliselle Clim-ATIC -hankkeelle, jossa selvitetään ilmatonmuutoksen vaikutuksia pohjoisilla alueilla sekä laaditaan ilmatonmuutoksen sopeuttamisstrategioita eri kohdealueille. Clim-ATIC -hankkeessa tehdään havainnollisia mallinnuksia ilmastonmuutoksen vaikutuksista, jolloin saadaan konkreettista tietoa, miten ilmastonmuutos vaikuttaa tulvien muuttumiseen ja sääoloihin ja sitä kautta Lapille tärkeille elinkeinoille kuten matkailulle. Raportissa on koottu yhteen Rovaniemen ja Kittilän alueiden ilmastonmuutokseen ja tulvasuojeluun liittyviä tutkimuksia ja selvityksiä. Raportissa käydään läpi Rovaniemellä ja Kittilässä esiintyneet aiemmat tulvatilanteet sekä tutkitaan tulvan leviämistä kerran sadassa vuodessa esiintyvällä tulvalla Lapin ELY-keskuksessa tehtyjen tulvavaarakarttojen avulla. Lisäksi raportissa esitellään Suomen ympäristökeskuksen hydrologian yksikössä tehtyjen ilmastonmuutostutkimusten tulokset. Tuloksissa ilmenee, miten ilmastonmuutos tulee tämän hetkisten arvioiden mukaan vaikuttamaan Rovaniemen ja Kittilän tulviin. Raportin loppuosassa käydään läpi Rovaniemen ja Kittilän alueille suunniteltuja tulvasuojelun vaihtoehtoja.

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Evaluation of Bubble Formation and Break Up in Suppression Pools by Using Pattern Recognition Methods

During a possible loss of coolant accident in BWRs, a large amount of steam will be released from the reactor pressure vessel to the suppression pool. Steam will be condensed into the suppression pool causing dynamic and structural loads to the pool. The formation and break up of bubbles can be measured by visual observation using a suitable pattern recognition algorithm. The aim of this study was to improve the preliminary pattern recognition algorithm, developed by Vesa Tanskanen in his doctoral dissertation, by using MATLAB. Video material from the PPOOLEX test facility, recorded during thermal stratification and mixing experiments, was used as a reference in the development of the algorithm. The developed algorithm consists of two parts: the pattern recognition of the bubbles and the analysis of recognized bubble images. The bubble recognition works well, but some errors will appear due to the complex structure of the pool. The results of the image analysis were reasonable. The volume and the surface area of the bubbles were not evaluated. Chugging frequencies calculated by using FFT fitted well into the results of oscillation frequencies measured in the experiments. The pattern recognition algorithm works in the conditions it is designed for. If the measurement configuration will be changed, some modifications have to be done. Numerous improvements are proposed for the future 3D equipment.

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