• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1203
  • 747
  • Tagged with
  • 1950
  • 1914
  • 1883
  • 189
  • 169
  • 131
  • 123
  • 117
  • 115
  • 111
  • 103
  • 102
  • 101
  • 92
  • 83
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
61

Förutsättningar för säsongslagring av värme i Luleå Energis fjärrvärmenät

Vikman, Elin January 2018 (has links)
Värmelagret för detta arbete undersöks med syftet att minska oljeanvändning för ett kraftvärmeverk i Luleå. I arbetet utreds två olika alternativ för borrhålslagring. Det första alternativet är ett decentraliserat lager ute i fjärrvärmenätet och det andra alternativet är ett centraliserat lager nära kraftvärmeverket. Borrhålslagret ska leverera värme till fjärrvärmenätets framledning under vintertid. Eftersom temperaturen från lagret är lägre än fjärrvärmenätets framledningstemperatur krävs en uppgradering av värmen från lagret för att höja temperaturen. I arbetet utreds två olika metoder för uppgradering. För det decentraliserade lagret sker temperaturhöjningen genom en kompressionsvärmepump och motsvarade för det centrala lagret är en biobränslepanna. För att utföra arbetet har litteraturstudier inom värmelagring, simulering i programmet DST (Duct storage systems), samt beräkningar i Microsoft Excel 2013 varit de övergripande metoderna. Vidare har ekonomiska förutsättningar för investering av ett borrhålslager utretts och även den miljömässiga påverkan genom beräkning av minskade koldioxid-utsläpp. De viktigaste resultaten från arbetet är att ett system med ett decentraliserat lager och en kompressionsvärmepump med en installerad effekt på 50 MW kan reducera oljeanvändningen med 28 GWh under ett år, vilket motsvarar en minskning på 43 %. Ett sådant system har en återbetalningstid på 21 år och bidrar till en minskning av koldioxid-utsläpp på 2034 ton per år. För ett centralt lager med en biobränslepanna som uppgradering och en installerad effekt på 50 MW uppgår återbetalningstiden till 23 år och koldioxid-utsläppen minskar med 2838 ton per år. Slutsatserna från arbetet är att energin i dagsläget är för billig för att det ska vara ekonomiskt motiverat att lagra överskottsenergi. Dock framstår det som intressant att undersöka vidare i ett borrhålslager eftersom det finns potential att minska oljeanvändningen och därmed få en mer hållbar värmeproduktion. / The heat storage for this project is being investigated with the aim of reducing the oil consumption for a combined heat and power plant in Luleå. Two different options for a borehole storage are investigated. The first option is a decentralized storage in the district heating network, and the second option is a centralized storage near the power plant. The borehole storage will supply heat to the district heating network during the winter. Since the temperature from the storage is lower than the district heating temperature, an upgrade of the heat from the storage is required to increase the temperature. Two different methods for upgrading are being investigated. For the decentralized storage, the temperature increase is achieved by a compression heat pump and corresponding to the central storage is a biofuel boiler. To carry out the project, literature studies concerning heat storage theory, simulation in the computer program DST (Duct storage systems), and calculation in Microsoft Excel 2013 have been the overall methods. Furthermore, the economic conditions for investment of a borehole storage are assessed and also the environmental impact by calculating reduced carbon dioxide emissions. The main results of the project are that a decentralized storage and a compression heat pump with an installed power of 50 MW can reduce the oil consumption by 28 GWh over a year, which corresponds to a reduction of 43 %. Such a system has a payback period of 21 years and the reduced carbon dioxide emissions are 2034 tonnes in a year. For a central storage with a biofuel boiler for the upgrade and an installed power of 50 MW, the payback time is 23 years and the carbon dioxide emissions are decreased by 2838 tonnes in a year. The conclusions from the study are that the energy is currently too cheap for it to be economically justified to store surplus energy. However, it appears interesting to further investigate a borehole storage because of the potential of reducing the oil consumption and thus obtain a more sustainable heat production.
62

HFAB har smak för celler på sina tak : Kartläggning av potentialen att installera solceller på HFAB:s fastighetstak

Poon, Jackie, Ayoub, Ali January 2018 (has links)
No description available.
63

Experimental assessment of fluid hammer prediction algorithms

Kihlström, Jessica January 2018 (has links)
Fluid hammer is a problem, which may lead to catastrophic consequences and many engineers have a deficiency in competence when it comes to its prediction. This study´s objective is to assess various fluid hammer prediction models, to evaluate how substantial pressure thrusts would affect a system and to verify the analytical results toward experimental data. The experimental data are collected from a novel experimental rig constituted of tubes, valves and data acquisition (DAQ) equipment. The literature review reveals several algorithms, analytical calculations and items of transient numerical analysis software (simulation software) for this purpose. In this study, the fluid hammer prediction is performed by analytical and numerical analyses using the commercial mathematic software suite Maple 2017. Assessments reveal that the results differed between analytical calculation, simulation and experimental data. The experimental activities yield the lowest pressure thrust magnitude and transient analysis predicted the highest. For the sake of conservative prediction, use of simulation software is recommended. / Vätskeslag är ett problem som kan ge katastrofala följder och många ingenjörer saknar kompetens när det handlar om att förutspå det. Målet med denna studie är att utvärdera olika metoder för att utvärdera hur betydande tryckstötarna blir i ett system samt att verifiera analytiska data gentemot ett experiment. Datan för experimentet fås från en testrigg som består av tuber, ventiler och instrumentering. Av litteraturstudien framgår att det finns flertalet algoritmer, analytiska beräkningar och transienta numeriska mjukvaror (simuleringsprogram) till detta syfte. I den här studien värderas och jämförs vätskeslag genom analytiska och numeriska analyser med hjälp av mjukvarupaketet Maple. 2017 Utvärdering av resultaten visar att de varierar med de olika metoderna. Experimentet gav de lägsta tryckstötarna och simuleringarna gav de högsta. Därför rekommenderar jag att för säkerhets skull använda simuleringsprogram.
64

Camera and laser based diagnostics relevant for entrained flow gasifiers

Ögren, Yngve January 2018 (has links)
No description available.
65

And then they lived sustainably ever after? : Part I, experiences from rural electrification in Tanzania, Zambia and Kenya

Ilskog, Elisabeth January 2005 (has links)
Access to basic, clean energy services is essential for sustainable development and poverty eradication. Still, many people worldwide lack access to modern energy, such as electricity. In Africa, the lack of electricity services is affecting in particular rural areas. Support to rural electrification is therefore given high priority by the national governments and donor organisations. There is an international trend towards deregulation and privatisation of electricity supply, which is also affecting the way to organise rural electrification services in developing countries. The objective of the work presented in this thesis has been to reach increased knowledge of the impact from organisational and institutional factors on project sustainability. The thesis presents the first part of the on-going study, and is focused on the development aspects and factors that are of direct importance for the sustainability of the electricity services. The result presented will contribute to the understanding of the critical factors, and provide input to the second part of the study, which will focus on the broader sustainability aspects of rural electrification in developing countries. From a study of six projects in Tanzania, Zambia and Kenya, using different organisational set-ups and supply technologies, findings show that with sufficient economic, managerial and educational support in the beginning of an organisations life, an electrification project can manage to survive and develop. The projects have many strengths and weaknesses in common, indicating that there is no single most appropriate organisation form for electricity services in rural areas. The present trend with a greater emphasis on privatisation and private sector involvement however implies a risk of marginalising other forms of organisations, such as community-based organisations, government utilities and co-operatives. Among many stakeholders there are a number of "concepts-taken-for-granted" on rural electrification. These are however not supported by the findings from the study. The observed deviations between expectations and realities can obstruct the development since leading decision-makers may have unrealistic expectations when planning for new electrification activities. Future efforts should therefore be concentrated on additional evaluations of experiences from implemented electrification projects, where the opportunities and constraints of rural electrification can be tackled with the empirical reality in mind. By doing so the ambiguities, complexities and all the paradoxes of rural electrification can hopefully be better managed. The findings show that electricity by it self does not lead to sustainable development. Productive uses are mainly constituted by low-load uses, such as lighting for extension of working hours for shops and bars, and to enable lighter forms of income-generating activities in households. Additional findings show that there are still many constraints that likely will affect foreign investors to remain scarce in the rural electricity sector in many countries in Africa, at least for the next few years. The work presented in the thesis has been performed through funding from The Swedish International Development Agency, Department for Research Cooperation (SAREC). Additional financing has been received from Ångpanneföreningen's Foundation for Research and Development (ÅFORSK). The project is part of a research cooperation programme between Makerere University in Kampala, Uganda and Luleå University of Technology. / <p>Godkänd; 2005; 20070103 (haneit)</p>
66

Beräkning av potentialen för styrning av värmelaster i enskilda byggnader : Utveckling av en metod som baseras på historisk elanvändning och utomhustemperatur

Sjökvist, Anton January 2018 (has links)
I Sverige ställs allt större effektrelaterade krav på både distribution och produktion av el, krav som förväntas öka i framtiden. Genom att kunderna är flexibla i sin efterfrågan kan den effekt som elsystemet är dimensionerat för minskas, vilket sänker kostnaderna för investeringar i bland annat elnät och spetslast. Dessutom kan utnyttjandegraden av variabla förnyelsebara energikällor öka, vilket har positiva ekonomiska och miljömässiga effekter. Den största potentialen för flexibel efterfrågan på el i Sverige finns i småhus med elberoende uppvärmning. Genom att styra uppvärmningen smart kan stora samhällsekonomiska vinster göras utan att inomhuskomforten påverkas. Beroende av byggnadens tekniska egenskaper och uppvärmningssystemets typ, skiljer sig den tillgängliga styrbara effekten, och tiden en styrning kan pågå mellan olika byggnader. Energimarknadsinspektionen anser att ett hinder mot ökad efterfrågeflexibilitet i Sverige är att kunskapen om den tillgängliga potentialen för varje slutkund är låg. Detta gör det svårare för både slutkunder, energitjänsteföretag och elnätsbolag att beräkna de nyttor som finns med att installera styrutrustning av värmesystem hos specifika kunder. Denna uppsats syftar därför till att undersöka möjligheterna till att uppskatta en byggnads potential för styrning av värmelaster, utifrån data på elförbrukning och utomhustemperatur. För att uppfylla syftet studerades först relevant teori inom området och tidigare arbeten med att, utifrån elförbrukning, uppskatta slutkunders möjlighet till efterfrågeflexibilitet. Därefter togs en egen metod fram för att uppskatta en byggnads potential för styrning av värmelaster, utifrån data på elförbrukning och utomhustemperatur. Slutligen analyserades 172 slutkunder i Trondheimsregionen, vilka var kategoriserade som hushåll, utifrån den framtagna metoden. Arbetet resulterade i en metod som utgår från en så kallad energisignatur, vilken grundas på sambandet mellan utomhustemperatur och elförbrukning. Genom detta ges en god uppfattning om effektbehovet för uppvärmning vid olika utomhustemperaturer vilket alltså gör det möjligt att beräkna den tillgängliga styrbara effekten. Denna metod gör det också möjligt att bestämma om en byggnad har elberoende uppvärmning eller ej. Tiden som en styrning är möjlig att genomföra är beroende av hur stort energiunderskott som kan skapas i byggnaden utan att inomhuskomforten sjunker. Detta beror av hur mycket termisk energi som kan lagras i byggnaden och hur välisolerad den är, vilket beskrivs som byggnadens värmetröghet. Intressanta samband, som skulle kunna användas för att bestämma värmetrögheten hos byggnader identifierades, då tidigare dags utomhustemperatur konstaterades ha stor påverkan på uppvärmningsbehovet hos flera av de analyserade byggnaderna. En byggnad med stor värmetröghet ska i teorin ha en stor påverkan av tidigare dags utomhustemperatur. Detta samband kunde dock inte valideras eftersom den data som analyserats var anonymiserad, varför de beräknade nyckeltalen inte kunde kopplas till olika byggnaders tekniska egenskaper. Inte heller byggnadens typ av uppvärmningssystem och värmedistributionssystem gick att bestämma i analysen. Detta kan påverka både potentialen för styrning av värmelaster, då vissa system är mer lämpade för styrning än andra, samt kostnaderna för att installera styrutrustning. En annan felkälla i metoden är att uppvärmningen behöver vara i linje med det faktiska uppvärmningsbehovet för att det ska vara möjligt att dra korrekta slutsatser om påverkan från tidigare dags utomhustemperatur. Av de 172 analyserade byggnaderna beräknades den styrbara effekten för 79 stycken, då övriga byggnader antingen saknade elberoende uppvärmning eller ansågs ha ett för osäkert samband mellan utomhustemperatur och elanvändning för att kunna inkluderas i beräkningarna. Det kunde konstateras att 15% av de analyserade byggnaderna stod för över hälften av den tillgängliga styrbara effekten i byggnadsbeståndet. Detta visar på vikten av att identifiera de kunder som har störst potential.
67

Co-gasification of black liquor and pyrolysis oil: Fuel conversion and activity of alkali compounds

Bach Oller, Albert January 2016 (has links)
Gasification using entrained flow reactors generates syngas that can be upgraded tochemicals with little gas cleaning. Black liquor (BL) is a by-product from pulping industrythat consists of residual wood constituents and spent pulping chemicals. Currently, it iscombusted to recycle the pulping chemicals and at the same time generate steam and power.Alternatively, BL is one of the most attractive fuels for entrained flow gasification due to thecatalytic activity of alkali compounds inherent in BL, possibility for pressurized feeding andthe shared logistics with the pulping plant. However, the high content of ash in BL is also anenergy penalty. Therefore the efficiency of BL gasification can be improved by co-gasifyingit with more energy rich fuels.The current work investigates the gasification characteristics of BL and pyrolysis oil(PO) blends by means of laboratory experiments. Experiments with varying BL/PO blendingratios were conducted using three different devices. An isothermal thermogravimetric reactorwas used to measure the reactivity of char under varying temperature and gas compositions. Asingle particle reactor was used to investigate the conversion of single droplets when exposedto high temperature reactive gas flow using lean, stoichiometric and rich CH4-air flames.Finally, a drop tube furnace was used to study the effect of temperature, gas composition andparticle size on gas, tar, and gasification residues at high temperature (800-1400 °C).Char reactivity of mixture samples was more than 30 times that of PO and comparableto that of pure BL, thereby indicating that catalytic activity was still very high after theaddition of PO. High temperatures enhanced alkali release in the gas phase; however, theconcentration of alkali left in the particles remained high at any temperature and for anymixing ratio. Additionally the blends showed better carbon conversion than pure BL. Theconversion rate of large particles (500-630μm) was controlled by mass diffusion and completecarbon conversion was never reached even at T =1400 °C. In comparison with pine-wood thatwas used as a reference, BL-based samples showed much lower tar concentrations in thesyngas. The difference was attributed to alkali elements. Remarkably, the addition of PO toBL further promoted tar reforming in the presence of CO2. The addition of PO alsosignificantly increased the yields of CH4 and CO.
68

Commissioning of Entrained Flow Reactor for Combustion and Gasification Research.

Hägglund, Philip January 2017 (has links)
No description available.
69

Integrating Heat Pumps into Smart Grids

Fischer, David January 2017 (has links)
Transforming our energy system towards 100% renewable energy sources requires radical changes across all energy sectors. Heat pumps as efficient heat generation technology link the electricity and heat sector. From 2010 to 2015 between 750,000 and 800,000 heat pumps have been sold every year across Europe. Those heat pumps, when connected to thermal storage or using the buildings’ thermal inertia, have the potential to offer demand flexibility to the power system. In a renewable, interconnected and to a large extend decentralised energy system - the smart grid - heat pumps can be operated according to the need of the electric power system. This might impact heat pump system design, controls and operation, which is investigated in this thesis. The main objective of this thesis is to add knowledge and suggest methods to facilitate the transition towards a renewable and smart energy system, in which heat pump systems and their flexibility are used and designed in an optimal way. Therefore this thesis investigates the integration of heat pumps in a smart grid on three different system boundary levels. On each boundary level the focus is on different aspects and different methods are used. On the highest boundary level, the integration of heat pumps into a smart grid and the resulting requirements for heat pump system design are studied. Results of a literature study show, that currently discussed applications of heat pumps in a smart grid focus on the provision of ancillary services, the integration of renewable energy sources, and the operation under time variable electricity prices. Integrating heat pumps into the power system can be achieved by direct, indirect and agent based control strategies. The next level of investigation covers the aggregation of heat pumps into pools. For this purpose a stochastic bottom-up model for heat pump pools has been developed. This model accounts for the diversity of buildings, heat pump systems and occupants. Results of a simulation study of a heat pump pool highlight the fact that flexibility of heat pumps is not constant and is changing during the course of the day and year. A characteristic response of a heat pump pool towards direct load control signals is identified and shows three characteristic phases 1) charging/ activation phase, 2) steady state phase, 3) discharging/regeneration phase. It is found that the duration of the control signal and the load shift strategy implemented in the heat pump systems are decisive for flexibility. Further it is shown that flexibility might come at the cost of efficiency of the local heat pump systems. On the level of individual buildings this thesis explores to which extent the sizing of heat pumps, storage and back-up heater as well as system controls have to be adjusted when integrating heat pumps into a smart grid. Results of a structural optimisation study, targeting to minimise total cost of ownership, show that sizing of the heat pump unit and the electric back-up heater remain almost unchanged when PV and time variable electricity prices are introduced. However an increase in storage capacity is beneficial to profit from time variable prices or onsite photovoltaic (PV). It showed that the ways heat pumps and storages are sized in Germany today provided sufficient storage capacity for most of the investigated scenarios. Furthermore increasing storage leads to diminishing returns as investment costs and system losses increase with increasing storage size. This leads to the conclusion that local heat pump system efficiency as well as flexibility requirements of the power system should be considered, when designing heat pump systems. Improving the controls shows great potential for increasing heat pump system efficiency, reducing operation cost and scheduling heat pump operation along to match the requirements from the power system. A dynamic building simulation study, where rule-based, predictive rule-based and model predictive control approaches were compared, reveals that the use of model predictive controls can reduce annual electricity cost and increase PV self-consumption significantly, compared to tailored rule-based and predictive rule-based control approaches. When deciding upon a control strategy the following should be taken into account: complexity of design, robustness against changes in external conditions and computational resources. It is shown that operating heat pumps in a smart grid changes operating hours, temperatures, on/off cycles and seasonal performance compared to today’s heatdriven operation. It is shown that the goals to reduce operating cost, maximise system efficiency or increase PV self-consumption can be conflicting and are often impossible to achieve simultaneously. Not necessarily will operation in a smart grid increase the efficiency of individual systems, rather offers the possibility to increase efficiency of the overall energy system. It is found that sizing, controls and use-case are interconnected and should be considered simultaneously in the design process of heat pump systems. A goal for future research should be the design of optimum flexible heat pump systems, where the heat pump unit, the building, the hydraulic system, heat distribution, storage and controls are designed optimally for the flexibility requirements of both the end-users and the power system.
70

Development of an optimization model for the location of biofuel production plants

Leduc, Sylvain January 2009 (has links)
First generation biofuels have not achieved the expected greenhouse gas emission savings and the production may in some cases compete with food production. Issued from non arable land and certified wood, the production of the second generation biofuels are more adapted to tackle those issues. Very large production plants are however required to reach competitive production costs via economy of scale effects. This may cause large logistical issues as the biomass feedstock often is located on the countryside, while the production plants are situated near harbors to enable boat transports. Moreover negative social and environmental effects may occur due to heavy traffic from the transport of the raw material and the final product, such as road damaging, noise perturbation, pollutant emission increase. To face those intensive logistic issues, the geographical location and size of the plant should be determined optimally with respect to raw material and demand location prior to plant investment and construction. The main aim of this thesis has therefore been to develop a model for optimization of the geographical location of second generation biofuel production plants by minimizing the cost of the complete supply chain, which comprises biomass harvesting, biomass transport, biofuel production, biofuel transport and biofuel distribution. The model is not intended to be applied to maximize the profitability of one single plant, but to minimize the final cost of biofuel for the region's welfare. The development of the model is illustrated via several case studies, where also analysis of critical parameters affecting the fuel production cost and the production plant location has been carried out. The model is a mixed integer program. The production of two liquid biofuels for the transportation sector have been studied, methanol via biomass gasification and ligno-cellulosic ethanol via fermentation. The model has been applied on areas as large as country levels. A set of optimal production plant can be determined to fulfill the biofuel demand of a selected area. It can be applied for different biofuel production processes and take into account the by-products geographically explicitly if required. The model can manage demands, costs and prices that change with time. Existing biomass based industries can be integrated to the model, and thus the competition on the biomass between these plants and possible bioenergy plants can be modeled, giving a better estimation of the available biomass for biofuel production. Biofuel imports from long distances are taken into account and finally policy tools such as carbon tax can be applied to limit the emissions from the transports or as a subsidy to the amount of mitigated fossil fuel emissions from the bioenergy production. The developed model can be applied for any kind of biomass based production plant and feedstock as long as the input data is available. As geographical energy planning is important, the developed model may be a valuable tool for decision makers in order to determine the most suitable strategy regarding locations of new biofuel production plants. / Godkänd; 2009; 20090424 (ysko); DISPUTATION Namn: Sylvain Leduc Ämnesområde: Energiteknik/Energy Engineering Opponent: Docent Pål Börjesson, Lunds universitet Ordförande: Docent Jan Dahl, Luleå tekniska universitet Tid: Torsdag den 4 juni 2009, kl 10.00 Plats: LKAB-salen, Luleå tekniska universitet

Page generated in 0.0403 seconds