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- Gustafsson, Bertil, et al.
(författare)
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Time Compact Difference Methods for Wave Propagation in Discontinuous Media
- 2003
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In an earlier paper by Gustafsson and Mossberg, a fourth order one-step method was constructed for the solution of wave propagation problems. The method is based on the first order system form of the PDE, and uses a staggered grid both in space and time. The method was also applied with good results to a problem with discontinuous coefficients without using any special procedure across the discontinuity. In this paper we will analyze a model problem from acoustics, and demonstrate the theoretical foundation for this behavior. Furthermore, we shall present more detailed numerical experiments which confirm the theoretical results.
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- Tornberg, Anna-Karin, et al.
(författare)
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A new type of boundary treatment for wave propagation
- 2006
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Ingår i: BIT Numerical Mathematics. - : Springer Science and Business Media LLC. - 0006-3835 .- 1572-9125. ; 46, s. S145-S170
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Tidskriftsartikel (refereegranskat)abstract
- Numerical approximation of wave propagation can be done very efficiently on uniform grids. The Yee scheme is a good example. A serious problem with uniform grids is the approximation of boundary conditions at a boundary not aligned with the grid. In this paper, boundary conditions are introduced by modifying appropriate material coefficients at a few grid points close to the embedded boundary. This procedure is applied to the Yee scheme and the resulting method is proven to be L2-stable in one space dimension. Depending on the boundary approximation technique it is of first or second order accuracy even if the boundary is located at an arbitrary point relative to the grid. This boundary treatment is applied also to a higher order discretization resulting in a third order accurate method. All algorithms have the same staggered grid structure in the interior as well as across the boundaries for efficiency. A numerical example with the extension to two space dimensions is included.
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- Wahlund, Bertil, et al.
(författare)
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Increasing biomass utilisation in energy systems : A comparative study of CO2 reduction and cost for different bioenergy processing options
- 2004
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Ingår i: Biomass and Bioenergy. - : Elsevier BV. - 0961-9534 .- 1873-2909. ; 26:6, s. 531-544
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Tidskriftsartikel (refereegranskat)abstract
- Emissions of greenhouse gases, such as CO2, need to be greatly reduced to avoid the risk of a harmful climate change. One powerful way to mitigate emissions is to switch fuels from fossil fuels to renewable energy, such as biomass. In this paper, we systematically investigate several bioenergy processing options, quantify the reduction rate and calculate the specific cost of reduction. This paper addresses the issue of which option Sweden should concentrate on to achieve the largest CO2 reduction at the lowest cost. The results show that the largest and most long-term sustainable CO2 reduction would be achieved by refining the woody biomass to fuel pellets for coal substitution, which have been done in Sweden. Refining to motor fuels, such as methanol, DME and ethanol, gives only half of the reduction and furthermore at a higher specific cost. Biomass refining into pellets enables transportation over long distances and seasonal storage, which is crucial for further utilisation of the woody biomass potential.
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- Wahlund, Bertil, 1968-
(författare)
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Rational bioenergy utilisation in energy systems and impacts on CO2emissions
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increased concentration of greenhouse gases in theatmosphere, in particular CO2, is changing the Earth’s climate. Accordingto the Kyoto protocol, where the international community agreedon binding emission targets, developed countries are committedto reduce their greenhouse gas emissions. The increased use ofbiomass in energy systems is an important strategy to reduce CO2emissions. The purpose of this thesis has been toanalyse the opportunities for Sweden to further reduce CO2emissions in the energy system, by rationallyutilising woody biomass energy. The characteristics of currentcommercially operating biofuel-based CHP plants in Sweden aresurveyed and systematically presented. A consistent andtransparent comprehensive reference base for system comparisonsis given. Furthermore, the fuel effectiveness and contributionto CO2reduction is calculated. The governmentalsubsidies of the CHP plants’investment, expressed as costof specific CO2reduction, appears to be low.The competitiveness of biomass-fuelled energy production inrelation to fossil-based production with carbon capture isanalysed, showing that the biomass-fuelled systems provide acompetitive option, in terms of cost of electricity andefficiencies. The remaining Swedish woody biofuel potential ofat least 100 PJ/yr is principally available in regions with abiomass surplus. Transportation is therefore required to enableits utilisation in a further national and international market.Refining the biofuel feedstock to pellets, or even furtherrefining to motor fuels (DME, methanol or ethanol) or power,could facilitate this transport. Different options for fuelrefining are studied and compared. The entire fuel chain, fromfuel feedstock to end users, is considered and CO2emissions are quantified. Substituting fuelpellets for coal appears to be the most costeffectivealternative and shows the largest CO2reduction per energy unit biofuel. Motor fuelsappear more costly and give about half the CO2reduction. Transportation of the upgraded biofuelpellets is highly feasible from CO2emissions point of view and does not constitute ahindrance for further utilisation, i.e. the pellets can betransported over long distances efficiently with only limitedemissions of CO2.Bioenergy utilisation has additional features forenvironmental improvement, apart from the CO2aspect. Waste heat from biofuel-based CHP can becost-effectively used in conjunction with sewage treatment. Theincoming sewage water to the nitrification process can bepreheated with the waste heat, and thereby substantiallyenhance the nitrification and the reduction of ammoniumnitrogen during the winter season.Keywords:CO2reduction, energy system, biofuel, CHP, refining,fuel pellets, ethanol, methanol, DME, fuel substitution, sewagewater, nitrification.
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