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| 2. |
- Auger, A. -T, et al.
(författare)
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Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from OSIRIS observations
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 583
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Tidskriftsartikel (refereegranskat)abstract
- Context. Since August 2014, the OSIRIS Narrow Angle Camera (NAC) onboard the Rosetta spacecraft has acquired high spatial resolution images of the nucleus of comet 67P/Churyumov-Gerasimenko, down to the decimeter scale. This paper focuses on the Imhotep region, located on the largest lobe of the nucleus, near the equator. Aims. We map, inventory, and describe the geomorphology of the Imhotep region. We propose and discuss some processes to explain the formation and ongoing evolution of this region. Methods. We used OSIRIS NAC images, gravitational heights and slopes, and digital terrain models to map and measure the morphologies of Imhotep. Results. The Imhotep region presents a wide variety of terrains and morphologies: smooth and rocky terrains, bright areas, linear features, roundish features, and boulders. Gravity processes such as mass wasting and collapse play a significant role in the geomorphological evolution of this region. Cometary processes initiate erosion and are responsible for the formation of degassing conduits that are revealed by elevated roundish features on the surface. We also propose a scenario for the formation and evolution of the Imhotep region; this implies the presence of large primordial voids inside the nucleus, resulting from its formation process.
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| 3. |
- Berggren, Björn, et al.
(författare)
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Innovative solution for heat recovery of ventilation air in older apartment buildings - with low intervention affecting the residents
- 2013
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Ingår i: ; , s. 428-438
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Konferensbidrag (refereegranskat)abstract
- Buildings today account for 40% of the world’s primary energy use and 24% of the greenhouse gas emissions. Reducing the energy use in buildings is an important measures to reduce energy dependency and greenhouse gas emissions. In Sweden, there are roughly 2 500 000 dwellings in multifamily houses and single family houses. There is a need to address the energy use within the existing dwellings; especially within multifamily buildings constructed 1960-1975, which accounts for almost one million dwellings or roughly 40 % of the dwelling stock in Sweden. There are good examples of energy renovations of buildings, reducing the energy use by more than 50 %. To reach the energy savings; balanced ventilation with heat recovery was a necessary measure in all projects. However, in all projects; loss of ceiling height in parts of the apartments is a consequence of the measure. Furthermore, it required the residents to temporary move during installment in the majority of the projects. The purpose of this study is to investigate the potential of using a heat exchanger, suitable for natural ventilation, exhaust air ventilation and hybrid ventilation, for installment in the existing building stock in Sweden. A qualitative and quantitative comparison is carried out; comparing mechanical exhaust air ventilation, balanced mechanical ventilation with heat recovery and the heat exchanger suitable for natural ventilation, exhaust air ventilation and hybrid ventilation. The qualitative analysis compares the advantages and disadvantages of the different ventilation system. The quantitative part investigates the potential energy savings and a potential quantity of dwellings where heat recovery may be installed. By installing heat recovery, energy used for space heating may be reduced by roughly 25%. Combining installment of heat recovery with improvement of the building envelope may reduce the energy demand for heating by roughly 75 %. Heat recovery should be able to install in roughly 80 % of the dwellings in the existing dwelling stock, built before 1980.
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| 4. |
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| 5. |
- Berggren, Björn, et al.
(författare)
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Normalisation of Measured Energy Use in Buildings — Need for a Review of the Swedish Regulations
- 2019
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Ingår i: Cold Climate HVAC 2018. CCC 2018. - Cham : Springer International Publishing. - 9783030006617 - 9783030006624 ; , s. 995-995
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Konferensbidrag (refereegranskat)abstract
- Normalisation of measured energy use in buildings is important in order to verify their performance in user phase. Two methods for normalisation have been presented in Sweden, static and dynamic normalisation. The static normalisation considers deviating hot water use, indoor temperature, internal loads and external climate. The dynamic normalisation is based on repeated simulation, meaning that the initial simulation, carried out during the design phase, is repeated with updated conditions regarding actual use of the building and exterior climate. The ratio between the first and second simulation is used as a factor for normalisation. A pre-study has been initiated in Sweden to enable further development of the two methods. This paper present the two methods, the initiated pre-study, and some early findings. The early findings show that there is need for further development of the methods presented.
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| 6. |
- Bergman, Svante
(författare)
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Bit loading and precoding for MIMO communication systems
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis considers the joint design of bit loading, precoding and receive filters for a multiple-input multiple-output (MIMO) digital communication system. Both the transmitter and the receiver are assumed to know the channel matrix perfectly. It is well known that, for linear MIMO transceivers, orthogonal transmission (i.e., diagonalization of the channel matrix) is optimal for some criteria such as maximum mutual information. It has been shown that if the receiver uses the linear minimum mean squared error (MMSE) detector, the optimal transmission strategy is to perform bit loading on orthogonal subchannels. In the first part of the thesis, we consider the problem of designing the transceiver in order to minimize the probability of error given maximum likelihood (ML) detection. A joint bit loading and linear precoder design is proposed that outperforms the optimal orthogonal transmission. The design uses lattice invariant operations to transform the channel matrix into a lattice generator matrix with large minimum distance separation at a low price in terms of transmit power. With appropriate approximations, it is shown that this corresponds to selecting lattices with good sphere-packing properties. An algorithm for this power minimization is presented along with a lower bound on the optimization. Apparently, given the optimal ML detector, orthogonal subchannels are (in general) suboptimal. The ML detector may suffer from high computational complexity, which motivates the use of the suboptimal but less complex MMSE detector. An intermediate detector in terms of complexity and performance is the decision feedback (DF) detector. In the second part of the thesis, we consider the problem of joint bit loading and precoding assuming the DF detector. The main result shows that for a DF MIMO transceiver where the bit loading is jointly optimized with the transceiver filters, orthogonal transmission is optimal. As a consequence, inter-symbol interference is eliminated and the DF part of the receiver is actually not required, only the linear part is needed. The proof is based on a relaxation of the discrete set of available bit rates on the individual subchannels to the set of positive real numbers. In practice, the signal constellations are discrete and the optimal relaxed bit loading has to be rounded. It is shown that the loss due to rounding is small, and an upper bound on the maximum loss is derived. Numerical results are presented that confirm the theoretical results and demonstrate that orthogonal transmission and the truly optimal DF design perform almost equally well. An algorithm that makes the filter design problem especially easy to solve is presented. As a byproduct from the work on decision feedback detectors we also present some work on the problem of optimizing a Schur-convex objective under a linearly shifted, or skewed, majorization constraint. Similar to the case with a regular majorization constraint, the solution is found to be the same for the entire class of cost functions. Furthermore, it is shown that the problem is equivalent to identifying the convex hull under a simple polygon defined by the constraint parameters. This leads to an algorithm that produces the exact optimum with linear computational complexity. As applications, two unitary precoder designs for MIMO communication systems that use heterogenous signal constellations and employ DF detection at the receiver are presented.
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| 7. |
- Bernardo, Ricardo, et al.
(författare)
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Measurements of the Electrical Incidence Angle Modifiers of an Asymmetrical Photovoltaic/Thermal Compound Parabolic Concentrating-Collector
- 2013
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Ingår i: Engineering. - : Scientific Research Publishing, Inc.. - 1947-3931 .- 1947-394X. ; 5, s. 37-43
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Tidskriftsartikel (refereegranskat)abstract
- Reflector edges, sharp acceptance angles and by-pass diodes introduce large variations in the electrical performance of asymmetrical concentrating photovoltaic/thermal modules over a short incidence angle interval. It is therefore important to quantify these impacts precisely. The impact on the electrical performance of the optical properties of an asymmet-rical photovoltaic/thermal CPC-collector was measured in Maputo, Mozambique. The measurements were carried out with the focus on attaining a high resolution incidence angle modifier in both the longitudinal and transversal directions, since large variations were expected over small angle intervals. A detailed analysis of the contribution of the diffuse radiation to the total output was also carried out. The solar cells have an electrical efficiency of 18% while the maxi-mum measured electrical efficiency of the collector was 13.9 % per active glazed area and 20.9 % per active cell area, at 25 °C. Such data make it possible to quantify not only the electrical performance for different climatic and operating conditions but also to determine potential improvements to the collector design. The electrical output can be increased by a number of different measures, e.g. removing the outermost cells, turning the edge cells 90°, dividing each receiver side into three or four parts and directing the tracking, when used, along a north-south axis.
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| 8. |
- Bernardo, Ricardo, et al.
(författare)
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Measurements of the Electrical Incidence Angle Modifiers of an Asymmetrical Photovoltaic/Thermal Compound Parabolic Concentrating-Collector
- 2013
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Ingår i: Engineering. - 1947-394X. ; 5:1B, s. 37-43
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Tidskriftsartikel (refereegranskat)abstract
- Reflector edges, sharp acceptance angles and by-pass diodes introduce large variations in the electrical performance of asymmetrical concentrating photovoltaic/thermal modules over a short incidence angle interval. It is therefore important to quantify these impacts precisely. The impact on the electrical performance of the optical properties of an asymmetrical photovoltaic/thermal CPC-collector was measured in Maputo, Mozambique. The measurements were carried out with the focus on attaining a high resolution incidence angle modifier in both the longitudinal and transversal directions, since large variations were expected over small angle intervals. A detailed analysis of the contribution of the diffuse radiation to the total output was also carried out. The solar cells have an electrical efficiency of 18% while the maximum measured electrical efficiency of the collector was 13.9 % per active glazed area and 20.9 % per active cell area, at 25 °C. Such data make it possible to quantify not only the electrical performance for different climatic and operating conditions but also to determine potential improvements to the collector design. The electrical output can be increased by a number of different measures, e.g. removing the outermost cells, turning the edge cells 90°, dividing each receiver side into three or four parts and directing the tracking, when used, along a north-south axis.
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| 9. |
- Bernardo, Ricardo, et al.
(författare)
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Performance Evaluation of a High Solar Fraction CPC-Collector System
- 2011
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Ingår i: Journal of Environment and Engineering. - : Japan Society of Mechanical Engineers. - 1880-988X. ; 6:3, s. 680-692
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Tidskriftsartikel (refereegranskat)abstract
- One of the most important goals on solar collector development is to increase the system’s annual performance without increasing overproduction. The studied collector is formed by a compound parabolic reflector which decreases the collector optical efficiency during the summer period. Hence, it is possible to increase the collector area and thus, the annual solar fraction, without increasing the overproduction. Collector measurements were fed into a validated TRNSYS collector model which estimates the solar fraction of the concentrating system and also that of a traditional flat plate collector, both for domestic hot water production. The system design approach aims to maximise the collector area until an annual overproduction limit is reached. This is defined by a new deterioration factor that takes into account the hours and the collector temperature during stagnation periods. Then, the highest solar fraction achieved by both systems was determined. The results show that, at 50° tilt in Lund, Sweden, the concentrating system achieves 71% solar fraction using 17 m2 of collector area compared to 66% solar fraction and 7 m2 of a flat plate collector system. Thus, it is possible to install 2.4 times more collector area and achieve a higher solar fraction using the load adapted collector. However, the summer optical efficiency reduction was proven to be too abrupt. If the reflector geometry is properly design, the load adapted collector can be a competitive solution in the market if produced in an economical way.
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| 10. |
- Bertini, Ivano, et al.
(författare)
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Search for satellites near (21) Lutetia using OSIRIS/Rosetta images
- 2012
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 66:1, s. 64-70
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Tidskriftsartikel (refereegranskat)abstract
- On 2010 July 10 the ESA Rosetta mission flew by the large asteroid (21) Lutetia. One of the scientific goals of the onboard OSIRIS instrument was the search for satellites of the asteroid, with more than 20 images specifically dedicated to this topic. An observational campaign was devised with a selection of filters and exposure times tailored to maximize the possibility of detecting small companions and determining their bound orbits. Data were analyzed with suitable methods to remove cosmic ray hits and known background objects, in order to search for persistent detections of potential interesting flux sources. We found no unambiguous detections of a satellite larger than similar to 160 m inside the entire sphere of gravitational influence. Our search confirmed the absence of bound companions larger than similar to 30 m inside 20 primary radii. These limits are a factor of similar to 30 smaller than the values reported so far from large ground-based telescopes using adaptive optics and from the Hubble Space Telescope.
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