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  • 301. Abu-Odeh, A.
    et al.
    Galvan, E.
    Kirk, T.
    Mao, Huahai
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Q.
    Mason, P.
    Malak, R.
    Arróyave, R.
    Efficient exploration of the High Entropy Alloy composition-phase space2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 152, p. 41-57Article in journal (Refereed)
    Abstract [en]

    High Entropy Alloys (HEAs), Multi-principal Component Alloys (MCA), or Compositionally Complex Alloys (CCAs) are alloys that contain multiple principal alloying elements. While many HEAs have been shown to have unique properties, their discovery has been largely done through costly and time-consuming trial-and-error approaches, with only an infinitesimally small fraction of the entire possible composition space having been explored. In this work, the exploration of the HEA composition space is framed as a Continuous Constraint Satisfaction Problem (CCSP) and solved using a novel Constraint Satisfaction Algorithm (CSA) for the rapid and robust exploration of alloy thermodynamic spaces. The algorithm is used to discover regions in the HEA Composition-Temperature space that satisfy desired phase constitution requirements. The algorithm is demonstrated against a new (TCHEA1) CALPHAD HEA thermodynamic database. The database is first validated by comparing phase stability predictions against experiments and then the CSA is deployed and tested against design tasks consisting of identifying not only single phase solid solution regions in ternary, quaternary and quinary composition spaces but also the identification of regions that are likely to yield precipitation-strengthened HEAs.

  • 302. Abu-Rmileh, Amjad
    et al.
    Garcia-Gabin, Winston
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Detection and Prevention of Hypoglycemia in Automated Insulin Delivery Systems for Type 1 Diabetes Patients2012In: Advances in Medicine and Biology / [ed] Leon V. Berhardt, Nova Science Publishers, Inc., 2012, p. 249-266Chapter in book (Refereed)
  • 303. Abu-Rmileh, Amjad
    et al.
    Garcia-Gabin, Winston
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Hypoglycemia prevention in closed-loop artificial pancreas for patients with type 1 diabetes2011In: Diabetes: Damages and treatments / [ed] Everlon Cid Rigobelo, IN-TECH, 2011, p. 207-226Chapter in book (Refereed)
  • 304.
    Abu-Rmileh, Amjad
    et al.
    Universidad de Gerona.
    Garcia-Gabin, Winston
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Smith Predictor Sliding Mode Closed-loop Glucose Controller in Type 1 Diabetes2011In: Proceedings of the 18th IFAC World Congress, 2011, 2011Conference paper (Refereed)
    Abstract [en]

    Type 1 diabetic patients depend on external insulin delivery to keep their blood glucose within near-normal ranges. In this work, two robust closed-loop controllers for blood glucose control are developed to prevent the life-threatening hypoglycemia, as well as to avoid extended hyperglycemia. The proposed controllers are designed by using the sliding mode control technique in a Smith predictor structure. To improve meal disturbance rejection, a simple feedforward controller is added to inject meal-time insulin bolus. Simulation studies were used to test the controllers, and shown the controllers ability to regulate the blood glucose within the safe limits in the presence of errors in measurements, modeling, and meal estimation.

  • 305. Abu-Rmileh, Amjad
    et al.
    Garcia-Gabin, Winston
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Wiener sliding-mode control for artificial pancreas: A new nonlinear approach to glucose regulation2012In: Computer Methods and Programs in Biomedicine, ISSN 0169-2607, E-ISSN 1872-7565, ISSN 0169-2607, Vol. 107, no 2, p. 327-340Article in journal (Refereed)
    Abstract [en]

    Type 1 diabetic patients need insulin therapy to keep their blood glucose close to normal. In this paper an attempt is made to show how nonlinear control-oriented model may be used to improve the performance of closed-loop control of blood glucose in diabetic patients. The nonlinear Wiener model is used as a novel modeling approach to be applied to the glucose control problem. The identified Wiener model is used in the design of a robust nonlinear sliding mode control strategy. Two configurations of the nonlinear controller are tested and compared to a controller designed with a linear model. The controllers are designed in a Smith predictor structure to reduce the effect of system time delay. To improve the meal compensation features, the controllers are provided with a simple feedforward controller to inject an insulin bolus at meal time. Different simulation scenarios have been used to evaluate the proposed controllers. The obtained results show that the new approach out-performs the linear control scheme, and regulates the glucose level within safe limits in the presence of measurement and modeling errors, meal uncertainty and patient variations.

  • 306. Abu-Shaban, Z.
    et al.
    Bhavani Shankar Mysore, R.
    Mehrpouyan, H.
    Ottersten, Björn
    SnT - Univ. of Luxembourg, Luxembourg, Luxembourg.
    Enhanced List-based Group-wise overloaded receiver with application to satellite reception2014In: 2014 IEEE International Conference on Communications (ICC), IEEE conference proceedings, 2014, p. 5616-5621Conference paper (Refereed)
    Abstract [en]

    The market trends towards the use of smaller dish antennas for TV satellite receivers, as well as the growing density of broadcasting satellites in orbit require the application of robust adjacent satellite interference (ASI) cancellation algorithms at the receivers. The wider beamwidth of a small size dish and the growing number of satellites in orbit impose an overloaded scenario, i.e., a scenario where the number of transmitting satellites exceeds the number of receiving antennas. For such a scenario, we present a two stage receiver to enhance signal detection from the satellite of interest, i.e., the satellite that the dish is pointing to, while reducing interference from neighboring satellites. Towards this objective, we propose an enhanced List-based Group-wise Search Detection (LGSD) receiver architecture that takes into account the spatially correlated additive noise and uses the signal-to-interference-plus-noise ratio (SINR) maximization criterion to improve detection performance. Simulations show that the proposed receiver structure enhances the performance of satellite systems in the presence of ASI when compared to existing methods.

  • 307. Abu-Shaban, Z.
    et al.
    Mehrpouyan, H.
    Grotz, J.
    Ottersten, Björn
    SnT - University of Luxembourg, Luxembourg .
    Overloaded satellite receiver using SIC with hybrid beamforming and ML detection2013In: 2013 IEEE 14th Workshop on Signal Processing Advances in Wireless Communications (SPAWC), New York: IEEE , 2013, p. 450-454Conference paper (Refereed)
    Abstract [en]

    In this paper, a new receiver structure that is intended to detect the signals from multiple adjacent satellites in the presence of other interfering satellites is proposed. We tackle the worst case interference conditions, i.e., it is assumed that uncoded signals that fully overlap in frequency arrive at a multiple-element small-size parabolic antenna in a spatially correlated noise environment. The proposed successive interference cancellation (SIC) receiver, denoted by SIC Hy/ML, employs hybrid beamforming and disjoint maximum likelihood (ML) detection. Depending on the individual signals spatial position, the proposed SIC Hy/ML scheme takes advantage of two types of beamformers: a maximum ratio combining (MRC) beamformer and a compromised array response (CAR) beamformer. The performance of the proposed receiver is compared to an SIC receiver that uses only MRC beamforming scheme with ML detection for all signals, a joint ML detector, and a minimum mean square error detector. It is found that SIC Hy/ML outperforms the other schemes by a large margin.

  • 308.
    Abusubaih, Murad
    et al.
    Telecommun. Networks Group, Technische Univ. Berlin.
    Gross, James
    Telecommun. Networks Group, Technische Univ. Berlin.
    Wiethoelter, Sven
    Telecommun. Networks Group, Technische Univ. Berlin.
    Wolisz, Adam
    Telecommun. Networks Group, Technische Univ. Berlin.
    On Access Point Selection in IEEE 802.11 Wireless Local Area Networks2006In: 31st IEEE Conference on Local Computer Networks, Proceedings 2006, IEEE conference proceedings, 2006, p. 879-886Conference paper (Refereed)
    Abstract [en]

    In wireless local area networks often a station can potentially associate with more than one access point. Therefore, a relevant question is which access point to select "best" from a list of candidate ones. In IEEE 802.11, the user simply associates to the access point with the strongest received signal strength. However, this may result in a significant load imbalance between several access points, as some accommodate a large number of stations while others are lightly loaded or even idle. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of "best" access point selection for IEEE 802.11 systems. In detail, we first derive a decision metric the selection can be based on. Using this metric we propose two new selection mechanisms which are decentralized in the sense that the decision is performed by each station, given appropriate status information of each access point. In fact, only few bytes of status information have to be added to the beacon and probe response frames which does not impose significant overhead. In addition, we show that our mechanism improves station quality of service and better utilizes network resources compared to the conventional one implemented today in IEEE 802.11 devices.

  • 309.
    Abusubaih, Murad
    et al.
    Technische Universit¨at Berlin.
    Gross, James
    Technische Universit¨at Berlin.
    Wolisz, Adam
    Technische Universit¨at Berlin.
    An Inter-Access Point Coordination Protocol for Dynamic Channel Selection in IEEE802.11 Wireless LANs2007Conference paper (Refereed)
    Abstract [en]

    This paper presents and evaluates an Inter-Access Point Coordination protocol for dynamic channel selection in IEEE 802.11 WLANs. It addresses an open issue for the implementation of many distributed and centralized dynamic channel selection policies proposed to mitigate interference problems in Wireless LANs (WLANs). The presented protocol provides services to a wide range of policies that require different levels of coordination among APs by enabling them to actively communicate and exchange information. An Intra-Cell protocol that enables interaction between the AP and its accommodated stations to handle channel switching within the same cell is also presented.

  • 310.
    Abusubaih, Murad
    et al.
    TU Berlin.
    Wiethoelter, Sven
    TU Berlin.
    Gross, James
    Wolisz, Adam
    TU Berlin.
    A New Access Point Selection Policy for Multi-Rate IEEE 802.11 WLANs2008In: International Journal of Parallel, Emergent and Distributed Systems, ISSN 1744-5760, E-ISSN 1744-5779, Vol. 23, no 4, p. 291-307Article in journal (Refereed)
    Abstract [en]

    In wireless local area networks, often a station can potentially associate with more than one access point (AP). Therefore, a relevant question is which AP to select 'best' from a list of candidate ones. In IEEE 802.11, the user simply associates to the AP with the strongest received signal strength. However, this may result in a significant load imbalance between several APs. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of 'best' AP selection for IEEE 802.11 systems. In detail, we first derive a new decision metric which can be used for AP selection. Using this metric, we propose two new selection mechanisms which are decentralised in the sense that the decision is performed by each station, given appropriate status information of each AP. In fact, only few bytes of status information have to be added to the Beacon and Probe Response frames which does not impose significant overhead. We show that our mechanism improves mean quality of service of all stations and better utilises network resources compared to the conventional one implemented today in IEEE 802.11 devices. Also, the schemes are appealing in terms of stability and provide their performance improvement even for denser or lighter network configurations.

  • 311.
    Acar, Yalda
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Jingstål, Pontus
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Influence of the Non-linear Effects in the Design of Viscous Dampers for Bridge Cables2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this master thesis the performance of external viscous dampers attached to cables in cable-stayed bridges have been studied. A comparison has been performed between a linear and a non-linear cable model. The comparison was carried out for two bridge cables, one from the Dubrovnik Bridge and the other from the Normandie Bridge. The performance of the dampers have been measured in terms of maximum achieved damping ratio and minimum amplitude of vibration.

    The analysis was performed using the finite element method. The damping ratio was measured using both the half-power bandwidth method and by calculating the loss factor. The half-power bandwidth method can only be applied to a linear system. Therefore, the loss factor was evaluated for the linear model and compared to the results obtained using the half-power bandwidth method. From the comparison, it was concluded that the damping ratio evaluated using the loss factor was similar to the results obtained when using the half-power bandwidth method. However, when calculating the loss factor, it was of great importance that the resonance frequency of the system was accurately determined. The loss factor was then calculated for the non-linear model and compared to the results obtained for the linear model. Since the loss factor measures the energy dissipated in a system, it could be utilised for the non-linear model. When computing the strain energy for the non-linear model an approximate method was used to take into consideration the strain energy caused by the static deformation of the cable.

    From the comparison between the linear and non-linear cable models, it was concluded that the optimal damper coefficients obtained by both models are not significantly different. However, there is an uncertainty in the results due to the fact that an approximate method was used when calculating the strain energy for the nonlinear model. It was also observed that a very accurate evaluation of the system’s resonance frequency was needed to calculate the loss factor. It was also observed that the variation in amplitude of vibration for varying damper coefficient was small for all modes of vibration for the Dubrovnik Bridge Cable as well as for the first mode of vibration for the Normandie Bridge Cable. The difference in the results between the two bridge cables needs to be investigated further in order to get a better understanding of the results.

  • 312.
    Acar, Yalda
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Jingstål, Pontus
    KTH, School of Architecture and the Built Environment (ABE).
    Influence of the Non-linear Effects in the Design of Viscous Dampers for Bridge Cables2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this master thesis the performance of external viscous dampers attached to cables in cable-stayed bridges have been studied. A comparison has been performed between a linear and a non-linear cable model. The comparison was carried out for two bridge cables, one from the Dubrovnik Bridge and the other from the Normandie Bridge. The performance of the dampers have been measured in terms of maximum achieved damping ratio and minimum amplitude of vibration.

    The analysis was performed using the finite element method. The damping ratio was measured using both the half-power bandwidth method and by calculating the loss factor. The half-power bandwidth method can only be applied to a linear system. Therefore, the loss factor was evaluated for the linear model and compared to the results obtained using the half-power bandwidth method. From the comparison, it was concluded that the damping ratio evaluated using the loss factor was similar to the results obtained when using the half-power bandwidth method. However, when calculating the loss factor, it was of great importance that the resonance frequency of the system was accurately determined. The loss factor was then calculated for the non-linear model and compared to the results obtained for the linear model. Since the loss factor measures the energy dissipated in a system, it could be utilised for the non-linear model. When computing the strain energy for the non-linear model an approximate method was used to take into consideration the strain energy caused by the static deformation of the cable.

    From the comparison between the linear and non-linear cable models, it was concluded that the optimal damper coefficients obtained by both models are not significantly different. However, there is an uncertainty in the results due to the fact that an approximate method was used when calculating the strain energy for the nonlinear model. It was also observed that a very accurate evaluation of the system’s resonance frequency was needed to calculate the loss factor. It was also observed that the variation in amplitude of vibration for varying damper coefficient was small for all modes of vibration for the Dubrovnik Bridge Cable as well as for the first mode of vibration for the Normandie Bridge Cable. The difference in the results between the two bridge cables needs to be investigated further in order to get a better understanding of the results.

  • 313.
    Acar, Yalda
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology.
    Skrobic, Karina
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Technology.
    Småhuset – val av bergvärme, fjärrvärme eller pellets2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This thesis describes the new construction of a small house and it includes studies of three heating systems: geothermal heating, district heating and pellet. The purpose of this thesis is to project a small house which can manage Building Regulations, BBR conditions and then examine three possible heating systems for the small house. This thesis is a literature study which gives an understanding of the heating systems technical structure, properties and costs. An empirical study completes the literature study by giving a deeper understanding of district heating costs and pellet costs. The empirical study contains two interviews, one of which was conducted in a district heating corporation and the other interview was conducted in a pellet corporation. The interview persons have answered structured questions in investment, maintenance and operating costs. The studies shows that the three heating system are different from each other for example the investment costs for district heating is much cheaper than the investments cost for pellets and geothermal heating. Finally, settles a comparison in the three heating systems properties and costs settles through a value analysis. The value analysis was an effective method when we based on literature studies and empirical studies made a valuation of the heating systems properties, investment costs, maintenance and operating costs. A weighting showed that district heating as heating system is the most optimal solution for the small house located in Borlänge.

  • 314.
    Accili, Alessia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Natural ventilation strategies for nearly – Zero Energy Sports Halls2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In line with the article 9 of the EPDB, member states shall ensure that all new public properties are nearly zero energy buildings (n-ZEB) by December 31, 2018. Sports buildings account for a significant share of the European building stock consumption. More than half of their current energy needs are related to lighting, and a relevant energy use is due to domestic hot water. This work aims to test different energy measures to design nearly zero energy sports halls in Mediterranean climates. Under a holistic approach, the design of the base case sports hall includes the implementation of passive strategies in combination with renewable energy and energy efficient systems in order to meet the n-ZEB conditions. However, a special focus is put on the study of the sports hall ventilation requirements. A natural ventilation system is proposed as an alternative to a traditional mechanical one. The effectiveness of the analyzed ventilation strategies is validated using TRNSYS, a dynamic simulation tool. Therefore, natural ventilation impact on thermal comfort, air quality and energy needs is estimated. A cost effective evaluation is done following the methodology proposed by the European Directive. Additionally, the study is complemented with a short period of measurements in a selected existing facility according to which poor indoor air quality is the main cause of users discomfort during period of maximum occupancy. The obtained results show that the combination of reduction of thermal transmittance of the envelope, optimization of the windows surfaces, façades orientation, introduction of shading devices, installation of energy efficiency systems as LED lamps and use of natural and night ventilation, are advantageous for the reduction of heating, cooling and artificial lighting demand. Overall, consisted primary energy savings are achieved. Moreover, the described strategies ensure indoor thermal comfort, minimizing the period of overcooling and overheating, and provide good air quality conditions for most of the occupied time along one year simulation. Finally, it is verified that the PV system integration positively affects the sports hall performance toward n-ZEB standards.

  • 315. Accoto, Nadia
    et al.
    Rydén, Tobias
    Lund University.
    Secchi, Paolo
    Bayesian Hidden Markov Models for Performance-Based Regulation of Continuity of Electricity Supply2010In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 25, no 3, p. 1236-1249Article in journal (Refereed)
    Abstract [en]

    A fundamental aspect in the regulation of the continuity of electricity supply is the identification of faults that could be caused by an exceptional event and, therefore, that are outside the utility control and responsibility. Different methods have been proposed during the years: the interpretation of the observed faults as a signal of an underlying system naturally leads to the analysis of the problem by means of a hidden Markov model. Thesemodels, in fact, are widely used for introducing dependence in data and/or for modeling observed phenomena depending on hidden processes. The application of this method shows that the model is able to identify exceptional events; moreover, the study of the estimated model parameters gives rise to reality-linked considerations.

  • 316.
    Aceituno Chavez, David
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Autocorrelation analysis on a 3D Ising model near the phase transition2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
  • 317.
    Acevedo Gomez, Yasna
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Lindbergh, Göran
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Lagergren, Carina
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
    Reformate from biogas used as fuel in a PEM fuel cell2013In: EFC 2013 - Proceedings of the 5th European Fuel Cell Piero Lunghi Conference, 2013, p. 163-164Conference paper (Refereed)
    Abstract [en]

    The performance of a PEM fuel cell can be easily degraded by introducing impurities in the fuel gas. Since reformate of biogas from olive mill wastes will contain at least one third of carbon dioxide, its influence was studied on a PtRu catalyst. A clean reformate gas for the anode (67% H2 and 33% CO2) without any traces of other compounds was used and electrochemical measurements showed that the performance of the fuel cell was hardly affected. However, diluting the hydrogen with higher amounts of CO2 will reduce the performance remarkably.

  • 318.
    Acharya, Rutvika
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fluid Dynamics of Phonation.2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This thesis aims at presenting the studies conducted using computational modeling for understanding physiology of glottis and mechanism of phonation. The process of phonation occurs in the larynx, commonly called the voice box, due to the self-sustained vibrations induced in the vocal folds by the airflow. The physiology of glottis can be understood using fluid dynamics which is a vital process in developing and discovering voice disorder treatments.

    Simulations have been performed on a simplified two-dimensional version of the glottis to study the behavior of the vocal folds with help of fluid structure interaction. Fluid and structure interact in a two-way coupling and the flow is computed by solving 2D compressible Navier-Stokes equations. This report will present the modeling approach, solver characteristics and outcome of the three studies conducted; glottal gap study, Reynolds number study and elasticity study.

  • 319.
    Acharya, Rutvika
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Investigation of Differences in Ansys Solvers CFX and Fluent2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This thesis aims at presenting Computational Fluid Dynamics studies conducted on an axisymmetric model of the Siemens SGT-800 burner using Ansys Fluent, Ansys CFX and Ansys ICEM. The goal is to perform a mesh study and turbulence model study for isothermal flow. The result will show the differences observed while using the two solvers by Ansys, Fluent and CFX. Two different meshes, A, coarse and B, optimal have been used for the mesh study. This will reveal the mesh dependency of the different parameters and if any differences are observed between the solver’s convergence and mesh independency performance. To further validate the mesh independency, a simplified test case is simulated for turbulent flow for 32 different cases testing the numerical algorithms and spatial discretization available in Ansys Fluent and finding the optimal method to achieve convergence and reliable results. Turbulence model study has been performed where k-ε, k-ω and k-ω Shear Stress Transport (SST) model have been simulated and the results between solvers and models are compared to see if the solvers’ way of handling the different models varies.Studies from this thesis suggest that both solvers implement the turbulence models differently. Out of the three models compared, k-ω SST is the model with least differences between solvers. The solution looks alike and therefore it could be suggested to use this model, whenever possible, for future studies when both solvers are used. For the models k-ε and k-ω significant differences were found between the two solvers when comparing velocity, pressure and turbulence kinetic energy. Different reasons for its occurrence are discussed in the thesis and also attempts have been made to rule out few of the reasons to narrow down the possible causes. One of the goals of the thesis was to also discuss the differences in user-interface and solver capabilities which have been presented in the conclusions and discussions section of the report. Questions that still remain unanswered after the thesis are why these differences are present between solvers and which of the solvers’ results are more reliable when these differences have been found.

  • 320.
    Acheampong, Isaac
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Land and Water Resources Engineering.
    Urban biodiversity; a global perspective.2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    A majority of the world’s cities are situated in or near areas of high biodiversity. Rise in global urban population resulting in rapid urban expansions (larger cities) is a threat to urban biodiversity, which has implications for the ecological health and general well being of humans. The study exploits consistent global land use data to compare 102 cities across the globe on a measure of urban biodiversity, within 15 km and 30 km from the approximate centres of the cities. Cities with high population and higher percentage of land use dedicated to artificial infrastructure recorded lower percentage size reserved for natural habitat, and vice versa. Further testing in regression analysis with birds and plants species as response variables shows a relation with urban extent and size of natural habitat which seeks to promote sustaining ecosystems services. Since urban biodiversity has implications for human ecological health, its indicators must be constantly measured and monitored, while adhering to best practices that conserve nature.

  • 321.
    Acheampong, Josephine
    KTH, School of Industrial Engineering and Management (ITM), Industrial Economics and Management (Dept.), Entrepreneurship and innovation.
    Green Financing: Financing Circular Economy Companies: Case Studies of Ragn-Sellsföretagen AB and Inrego AB2016Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The circular economy (CE) has been identified as a catalyst in sustainable development and economic growth that has the potential to move society from the traditional linear model of resource consumption in the form of take-make-waste to an innovative circular model in the form of reduce-reuse-recycle.

    Transitioning from the linear economy to the CE requires changes in four areas: material and product design, business models, global reverse networks and enabling business environments. This study considers the financing needs of CE companies as a result of business model changes.

    Through the case studies of Ragn-Sellsföretagen AB and Inrego AB, analysed with secondary data from ING Bank and primary data collected through semi-structured interviews with the case companies, this research sheds more light on the financing needs of circular economy companies and how they are financed.

    Findings from this research suggest that the financing needs of circular economy companies depend on the value proposition of the company. In accordance with the pecking order of capital structure, all financing needs of the companies studied are financed from internal sources, particularly retained earnings before external debt financing is accessed. Findings indicate the willingness of banks to finance circular economy companies.

    The results of this research suggest that the circular economy companies studied do not need financial support from the government or its agencies to succeed even though favourable laws are welcomed. They report that their long-term success depends on their ability to remain innovative in their business models, aligning with Schumpeter’s creative destruction model.

  • 322. Achi, L. M.
    et al.
    Tibert, G.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Hallgren, M.
    Wanted: A larger, different kind of box2016In: IABSE Congress Stockholm, 2016: Challenges in Design and Construction of an Innovative and Sustainable Built Environment, International Association for Bridge and Structural Engineering (IABSE) , 2016, p. 358-367Conference paper (Refereed)
    Abstract [en]

    To have the ability to "think outside the box" is generally regarded as something positive. At a moment in time when resources are scarce, and the problems facing us are many, innovation and professional excellence becomes a requirement, rather than a matter of choice. At the core of our attempts to come up with new, and better solutions are the digital technologies. Within the structural engineering context, the different types of off-the-shelf packages for finite element analysis play a central role. These "black-box" types of software packages exemplify how user friendliness may have harmful consequences within a field where knowledge and the successful mastery of relevant skills is key, and consequently- ignorance may lead to fatal results. These tools make any effort "venturing outside" difficult to achieve. A technical paradigm shift is called for that places learning and creative, informed exploration at the heart of the user experience.

  • 323.
    Achilli, Timothée
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    STATISTICAL STUDY OF THEEARTH'S MAGNETOPAUSEBOUNDARY LAYER PARTICLEPOPULATIONS2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    While double ion populations, with a cold population originating from the solar windand a hotter one from the magnetosphere, are frequently observed in the Earth’s lowlatitudeboundary layers, similar double electron populations are seldom recorded.We performed a statistical study of ion and electron double populations near themagnetopause by using 7 years of THEMIS particle data. After a preliminary study ofmagnetopause crossings characteristics, in particular by determining the typicalenergies of ion and electron populations in regions near the magnetopause, we setup an automated detection algorithm for identifying regions with combined ion andelectron double populations.The statistical study carried out with respect to IMF conditions in the upstream solarwind during and just before the events suggests that such combined ion and electronDouble Population Boundary Layers (DPBL) form preferentially under northward IMFbut with a significant BY component.We interpret this trend as a result of reconnection of the same magnetosheath fieldline in both hemispheres, but with at least one end reconnecting in its hemisphere atlower latitude with a closed magnetospheric field line which already contains a hotelectron source.

  • 324.
    ACHKOUDIR, HOUSSAM
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Hanna, Naowar
    KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).
    Studie av verkningsgrad potentialen för ett vatten baserat Waste Heat Recovery system med kolvexpander2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    AbstractAn evaporator was mounted in the EGR loop of a 12,7 liter Scania Euro V engine (DC1306). A model describing the Rankine cycle was developed with water as refrigerant in the simulation tool GT-Power. The evaporator in the GT-Power model was calibrated with experimental data.The simulations showed that the optimal vapor pressure where the maximum power available from the expander is obtained depends on the EGR temperature. Higher EGR inlet temperature leads to increased optimal vapor pressure. The EGR temperature in case 2 of the ESC cycle is 514 °C for the engine above, this result in an optimal vapor pressure of 120 bar according to the simulation. The optimum level of superheating was analyzed, which means the amount of degrees the vapor temperature is raised at a constant pressure after all the water is evaporated. The simulations show that the highest power in the expander was obtained when the steam was superheated by 10 degrees, i.e. the lowest level of superheating. The steam power after the evaporator is highest at the lowest level of superheating, because of the higher refrigerant flow.Simulations show that the EGR temperature has a bigger impact than the EGR flow. One way to increase the EGR temperature is by supplementary burning, which means injecting fuel into the exhaust pipe. Calculations show that it is more profitable to inject fuel directly into the combustion chamber. Increasing the EGR inlet temperature with 150 °C would result in 2,5 kW higher power output from the expander. Injecting the same fuel flow in the combustion chamber the engine power output increases with 5,2 kW.Operating point 2 in the ESC cycle reduces the fuel consumption with 1,4 % if run at the optimal steam pressure of 120 bar and 10 degrees of superheated vapor. The reduction of the fuel consumption would be 3,41 %, if the power in the exhaust mass flow would be utilized by integrating another evaporator after the turbine.

  • 325.
    Ackeberg, Anders
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Control of Periodic Solutions in Chemical Reactors2003Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 326.
    Ackefelt, Daniel
    et al.
    KTH, School of Industrial Engineering and Management (ITM).
    Weidenbladh, Alfred
    KTH, School of Industrial Engineering and Management (ITM).
    Examining moderators for activity-based working and its consequences2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 327.
    Ackermann, Thomas
    KTH, Superseded Departments, Electrical Systems.
    Distributed Resources in a Re-Regulated Market Environment2004Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    This thesis analyses the value that distributed resources(DR) can contribute to an economically effcient operation of are-regulated electricity market and discusses the relevantchanges in the regulatory framework to allow the appropriatedevelopment of DR whenever DR may increase the economiceffciency of the electricity market.

    Distributed resources thereby combine two aspects:Distributed generation and demand-side resources. Distributedgeneration is defined as generation within the distributionsystem or on the customer side of the meter. Demand-sideresources are those load resources on the customer-side thatcan be used to shift electricity demand from peak to off-peakperiods, or to reduce the overall electricity demand.

    To gain a better understanding of the value that DR mayprovide in a re-regulated market, an analysis of there-regulation approaches in England andWales, Scandinavia,Australia, New Zealand as well as in two regions in the USA isperformed. The key findings of this analysis are that non ofthe re-regulation approaches put special emphasis on DR. Thecomparison also shows that despite very different approachesused in the various markets, the remaining re-regulationproblems are very similar. These key problems are mainlyrelated to market power in the various markets and littlecompetition in network services.

    The thesis shows that DR has the potential to improveoverall economic market effciency by reducing market powerissues and increasing competition in network services. Forinstance, many DR business schemes have no incentive toexercise market power, because withholding generation may havea negative impact on the complex revenue stream from differentincome sources. While this may not allow the general conclusionthat DR will always provide benefits to market operation, it isimportant to consider that not all DR need to provide suchbenefits, as it was also shown that already comparatively smallamounts of demand resources or distributed generation cansignificantlyreduce market power issues. In addition, it wasalso shown that DR can introduce competition into networkservices and therefore can provide incentives for distributionand transmission network companies to operate more costeffectively.

    It was also demonstrated that the potential of DR to improveoverall economic market effciency will only be realized if:

    1. DR is developed in the correct size, i.e. DR should fitinto an existing distribution network and may increase networkutilization rate,2. DR is developed at the correct location in thenetwork, e.g. in areas with congestion problems,3. DR is in operation during the correct times, whenbeneficial to the market, e.g. during times when market powerissues may arise, and4. independent ownership of DR projects isencouraged.

    Market regulations therefore must provide the correctincentives for the correct siting and operation of DR as wellas for independent ownership. The thesis uses examples andempirical data to explain the relevant regulatory aspects indetail and provides suggestions for a regulatory framework thatconsiders the potential of DR to improve overall economiceffciency in an electricity market.

  • 328.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Historical Development and Current Status of Wind Power2005In: Wind Power in Power Systems, John Wiley & Sons, 2005, 1, p. 5-24Chapter in book (Other academic)
  • 329.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Historical Development and Current Status of Wind Power2012In: Wind Power in Power Systems, Second Edition, John Wiley & Sons, 2012, p. 21-24Chapter in book (Refereed)
    Abstract [en]

    The chapter provides an overview of the historical development (mechanical and electrical power generation) of wind power. It also present the current status of wind power world-wide (capacity installed) together with a discussion of the main drivers for the wind power development, e.g. feed-in tariffs, green certificates etc. Furthermore, the chapter briefly discuss the current trends in wind turbine technology, e.g. larger turbines, and projects development, e.g. offshore wind power.

  • 330.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Transmission Systems for Offshore Wind Farms2005In: Wind Power in Power Systems, John Wiley & Sons, 2005, p. 479-503Chapter in book (Other academic)
  • 331.
    Ackermann, Thomas
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Wind Power in Power Systems2005Book (Other academic)
    Abstract [en]

    As environmental concerns have focussed attention on the generation of electricity from clean and renewable sources, wind energy has become the world's fastest growing energy source. The authors draw on substantial practical experience to address the technical, economic and safety issues inherent in the exploitation of wind power in a competitive electricity market. Presenting the reader with all the relevant background information key to understanding the integration of wind power into the power systems, this leading edge text: Presents an international perspective on integrating a high penetration of wind power into the power system Offers broad coverage ranging from basic network interconnection issues to industry deregulation and future concepts for wind turbines and power systems Discusses wind turbine technology, industry standards and regulations along with power quality issues Considers future concepts to increase the penetration of wind power in power systems Presents models for simulating wind turbines in power systems Outlines current research activities Essential reading for power engineers, wind turbine designers, wind project development and wind energy consultants dealing with the integration of wind power systems into distribution and transmission networks, this text would also be of interest to network engineers working for power utility companies dealing with interconnection issues and graduate students and researchers in the field of wind power and power systems.

  • 332.
    Ackermann, Thomas
    KTH.
    Wind Power in Power Systems, Second Edition2012Collection (editor) (Refereed)
    Abstract [en]

    The second edition of the highly acclaimed Wind Power in Power Systems has been thoroughly revised and expanded to reflect the latest challenges associated with increasing wind power penetration levels. Since its first release, practical experiences with high wind power penetration levels have significantly increased. This book presents an overview of the lessons learned in integrating wind power into power systems and provides an outlook of the relevant issues and solutions to allow even higher wind power penetration levels. This includes the development of standard wind turbine simulation models. This extensive update has 23 brand new chapters in cutting-edge areas including offshore wind farms and storage options, performance validation and certification for grid codes, and the provision of reactive power and voltage control from wind power plants. Key features: Offers an international perspective on integrating a high penetration of wind power into the power system, from basic network interconnection to industry deregulation; Outlines the methodology and results of European and North American large-scale grid integration studies; Extensive practical experience from wind power and power system experts and transmission systems operators in Germany, Denmark, Spain, UK, Ireland, USA, China and New Zealand; Presents various wind turbine designs from the electrical perspective and models for their simulation, and discusses industry standards and world-wide grid codes, along with power quality issues; Considers concepts to increase penetration of wind power in power systems, from wind turbine, power plant and power system redesign to smart grid and storage solutions. Carefully edited for a highly coherent structure, this work remains an essential reference for power system engineers, transmission and distribution network operator and planner, wind turbine designers, wind project developers and wind energy consultants dealing with the integration of wind power into the distribution or transmission network. Up-to-date and comprehensive, it is also useful for graduate students, researchers, regulation authorities, and policy makers who work in the area of wind power and need to understand the relevant power system integration issues.

  • 333.
    Ackermann, Thomas
    et al.
    KTH, Superseded Departments.
    Andersson, G.
    Söder, Lennart
    KTH, Superseded Departments.
    Distributed generation: a definition2001In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 57, no 3, p. 195-204Article in journal (Refereed)
  • 334.
    Ackermann, Thomas
    et al.
    KTH, Superseded Departments.
    Andersson, G.
    Söder, Lennart
    KTH, Superseded Departments.
    Overview of government and market driven programs for the promotion of renewable power generation2001In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 22, no 1-3, p. 197-204Article in journal (Refereed)
  • 335.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Andersson, Göran
    Söder, Lennart
    KTH, Superseded Departments.
    Electricity market regulations and their impact on distributed generation2000In: Electric Utility Deregulation and Restructuring and Power Technologies, 2000. Proceedings. DRPT 2000. International Conference on, 2000, p. 608-613Conference paper (Refereed)
    Abstract [en]

    Distributed generation (DG) has attracted a lot of attention recently and might become more important in future power generation systems. As different definitions are used worldwide, the paper briefly discusses the definition of DG. The future development of DG, however, will, to a not insignificant part, depend on the legal framework. As the legal framework can vary significantly for different competitive electricity markets, this paper briefly identifies and analyses some variations in the regulatory approaches, e.g. for power exchanges, balance services and ancillary services, in different countries. It also illustrates the influence of market regulations on the development of distributed power generation. Based on this analysis, it can be concluded that regulatory aspects might decisively influence the development of distributed power generation

  • 336.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Centeno-Lopez, Eva
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Grid Issues for Electricity Production Based on Renewable Energy Sources in Spain, Portugal, Germany, and United Kingdom2008Book (Other academic)
  • 337.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Holttinen, H.
    Overview of Integration Studies - Methodologies and Results2012In: Wind Power in Power Systems, Second Edition, John Wiley & Sons, 2012, 2, p. 361-386Chapter in book (Refereed)
  • 338.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems. Energynautics GmbH, Germany; Technical University in Darmstadt (TUD), Germany.
    Morthorst, P. E.
    Economic Aspects of Wind Power in Power Systems2005In: Wind Power in Power Systems, John Wiley & Sons, 2005, p. 383-410Chapter in book (Other academic)
  • 339.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Orths, A.
    Rudion, K.
    Transmission Systems for Offshore Wind Power Plants and Operation Planning Strategies for Offshore Power Systems2012In: Wind Power in Power Systems, John Wiley & Sons, 2012, 2, p. 293-327Chapter in book (Refereed)
    Abstract [en]

    The electric system within an offshore wind power plant and its connection to the main power system pose new challenges to the experts. The best way of interconnecting the wind turbines inside a wind farm has to be found, fulfilling both, redundancy requirements without compromising economic feasibility. The best choice between technologies (HVAC, HVDC - VSC or LCC) for connecting windfarms to shore has to be made, depending on several criteria. The risk of losing this connection versus redundancy has to be economically evaluated. By combining interconnectors and offshore windfarm connections in a modular way, a DC offshore grid can be developed. Anyhow, already during the planning phase the secure operation should be considered thoroughly, because the optimal architecture has to be found, minimizing the necessary assets ensuring secure operation and facilitating later expansion options. The interaction with the onshore grid has to be investigated as well. To enable investigations covering these issues a benchmark offshore test system has been developed which is described in this chapter.

  • 340.
    Ackermann, Thomas
    et al.
    KTH, Superseded Departments.
    Söder, Lennart
    KTH, Superseded Departments.
    An overview of wind energy-status 20022002In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 6, no 1-2, p. 67-128Article, review/survey (Refereed)
  • 341.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    The Value of Wind Power2012In: Wind Power in Power Systems, John Wiley & Sons, 2012, 2, p. 131-155Chapter in book (Refereed)
    Abstract [en]

    The aim of the power plants in a power system is to supply the load in an economical, reliable and environmentally acceptable way. Different power plants can fulfil these requirements in different ways. In order to select the right sources it is important to compare the value of the different sources using an objective approach. The aim of this chapter is describe the different needs of a power system and how these needs can be met with wind power, that is, the value of wind power in a certain system. The values are operating cost value, capacity value, control value, grid loss reduction value and grid investment value. The values can be calculated for different types of power plants, they can be both positive and negative, and they can be calculated both as a physical cost value and a market value.

  • 342.
    Ackermann, Thomas
    et al.
    KTH, Superseded Departments.
    Söder, Lennart
    KTH, Superseded Departments.
    Wind energy technology and current status: a review2000In: Renewable & sustainable energy reviews, ISSN 1364-0321, E-ISSN 1879-0690, Vol. 4, no 4, p. 315-374Article, review/survey (Refereed)
  • 343.
    Ackermann, Thomas
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Tröster, E.
    New Control Concept for Offshore Wind Power Plants: Constant-Speed Turbines on a Grid with Variable Frequency2012In: Wind Power in Power Systems, John Wiley & Sons, 2012, 2, p. 345-359Chapter in book (Refereed)
    Abstract [en]

    By using a permanent magnet induction machine as wind generator, the gearbox and converter can be omitted, and the total number of parts reduced leading to a low maintenance and reliable turbine for offshore application. The rotation speed of the turbine however cannot be matched to the wind speed, reducing the energy yield at part load. To overcome this drawback, a central converter can be used, which adjusts the frequency of the local grid in the wind park; this is the so-called park-variable concept. This concept has been compared with respect to energy yield with constant speed and variable speed turbines. Overall, the differences in energy yield of the investigated concepts are so small that other criteria, such as reliability or cost, may be relevant for the selection of one or the other approach. Above all, the park-variable concept represents an interesting alternative to today's common concepts.

  • 344. Ackland, G. J.
    et al.
    Hepburn, D. J.
    Wallenius, Janne
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Interatomic forces for transition metals including magnetism2010In: 139th Annual Meeting & Exhibition - Supplemental Proceedings, Vol 2: Materials Characterization, Computation And Modeling And Energy, 2010, p. 85-92Conference paper (Refereed)
    Abstract [en]

    We present a formalism for extending the second moment tight-binding model[1], incorporating ferro- and anti-ferromagnetic interaction terms which are needed for the FeCr system. For antiferromagnetic and paramagnetic materials, an explicit additional variable representing the spin is required. In a mean-field approximation this spin can be eliminated. and the potential becomes explicitly temperature dependent. For ferromagnetic interactions, this degree of freedom can be eliminated, and the formalism reduces to the embedded atom method (EAM[2]), and we show the equivalence of existing EAM potentials to "magnetic" Potentials.

  • 345.
    Ackland, Marcus
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
    Wargentin, Robin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematical Statistics.
    How prices of condominiums vary with respect to distance from the city center in 20 major cities in Sweden2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This report will examine how condominium prices vary with respect to the distance from the city center in 20 major cities in Sweden. With regression analysis three models are construct-ed for each city to predict the price of a condominium in the city with a known set of variables such as area, monthly fee and distance from city center. The three models each depend on the distance parameter in different ways; linearly, exponentially, and exponentially with a higher degree polynomial as an exponent. The models are then examined statistically between cities to determine if there is any correlation between price function with regards to distance and population size. Results show that prices do decline substantially when distance to city center increases in all observed cities. There is a significant correlation between price function of distance and population size, but the relation is not enough to, by itself, explain the differ-ences between cities.

  • 346.
    Acquaviva, Alessandro
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Analytical Modeling of Iron Lossesfor a PM Traction Machine2012Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Permanent magnet (PM) machines offer several advantages in traction applications such as high efficiencyand high torque per volume ratio. The iron losses in these machines are estimated mostly with empiricallaws taken from other types of machines or with finite element simulations (FEM). In the first part of thisthesis the objective is to define an accurate analytical model for the stator yoke, teeth and rotor of a PMmotor which should work well enough for all operating point (different loads and frequency).This analytical model is found using an iterative process. After building a loss matrix and flux matrix basedon FEM simulations, it is possible to curve fit each of the lines or the rows of the matrix in order to achievethe best fitting for every operating point. This is a very new approach; it was shown that it gives thepossibility, even with a very limited number of FEM simulations, to achieve an accurate estimation of thelosses.The second part of this report focuses on optimizing this analytical method, comparing it with otherpossibilities, analyzing limits and advantages. Special attention is also given to the effects of the losses onthe temperatures in different parts of the machine. In the last part of the thesis, the analytical model isused to test a new control strategy. Its goal is to reduce the total losses of the motor and optimize the ratiobetween torque and total losses for a given driving cycle.

  • 347.
    Acuna, José
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Fossa, Marco
    University of Genova.
    Monzó, Patricia
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Palm, Björn
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Numerically generated g-functions for ground coupled heat pump applications2012In: Proceedings of the COMSOL Conference in Milan, 2012Conference paper (Refereed)
    Abstract [en]

    In most ground-coupled heat pump systems, Borehole Heat Exchangers (BHE) represent the typical engineering solution for utilizing renewable energy from the ground. The design of a complex BHE field is a challenging task, due the inherent transient nature of the thermal interaction between the heat exchangers and the surrounding soil. A computation effective method for solving the 3D transient conduction equation describing the ground response to a variable heat load profile is the temporal superposition of pre-calculated temperature response factors or g-functions. In this study Comsol heat conduction models have been developed to calculate g-function values for a borehole field with 64 boreholes. The aim of the investigation is to get an insight on the numerical generation of temperature transfer functions and to some extent provide new information on the Finite Line Source method for analytically generated g-functions as well as on those existing behind existing design software such as EED. The results generally showed a good agreement in lower time ranges. Further in time, the Comsol model revealed to be influenced either by the domain dimensions or the simulation end time.

  • 348.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Bergvärmepumpar Kan Göras Ännu Mer Effektiva2008In: Enegi&Miljö, ISSN 1101-0568, no 3Article in journal (Other (popular science, discussion, etc.))
  • 349.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Characterization and Temperature Measurement Techniques of Energy Wells for Heat Pumps2008Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Ground source heat pumps are a widely used approach to efficiently heat single family houses. In addition to using the ground as a heat source during the winter, it can be used as heat sink and as a free cooling source during the summer. The most common way to carry out the heat exchange with the ground is with the help of energy collectors (borehole heat exchangers) in vertical wells. The quality of the heat exchange depends on the type of collector and on the flow conditions of the circulating fluid. For a complete understanding of the heat transfer performance, it is necessary to carry out careful temperature measurements at research installations and to do a preliminary characterization of the boreholes. These activities might represent a significant cost saving since the system can be optimized based on their outcome. The characterization consists of determining the type of rock and its thermal properties, the groundwater flow at different depths, and the borehole deviation according to the expected position. A comprehensive study about these characterization actions as well as temperature measurement techniques in boreholes using thermocouples and fiber optic technology are described in this report. Study cases from real installations are also presented to exemplify the characterization and measurement methods.

  • 350.
    Acuña, José
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Distributed thermal response tests: New insights on U-pipe and Coaxial heat exchangers in groundwater-filled boreholes2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    U-pipe Borehole Heat Exchangers (BHE) are widely used today in ground source heating and cooling systems in spite of their less than optimal performance. This thesis provides a better understanding on the function of U-pipe BHEs and Investigates alternative methods to reduce the temperature difference between the circulating fluid and the borehole wall, including one thermosyphon and three different types of coaxial BHEs.

    Field tests are performed using distributed temperature measurements along U-pipe and coaxial heat exchangers installed in groundwater filled boreholes. The measurements are carried out during heat injection thermal response tests and during short heat extraction periods using heat pumps. Temperatures are measured inside the secondary fluid path, in the groundwater, and at the borehole wall. These type of temperature measurements were until now missing.

    A new method for testing borehole heat exchangers, Distributed Thermal Response Test (DTRT), has been proposed and demonstrated in U-pipe, pipe-in-pipe, and multi-pipe BHE designs. The method allows the quantification of the BHE performance at a local level.

    The operation of a U-pipe thermosyphon BHE consisting of an insulated down-comer and a larger riser pipe using CO2 as a secondary fluid has been demonstrated in a groundwater filled borehole, 70 m deep. It was found that the CO2 may be sub-cooled at the bottom and that it flows upwards through the riser in liquid state until about 30 m depth, where it starts to evaporate.

    Various power levels and different volumetric flow rates have been imposed to the tested BHEs and used to calculate local ground thermal conductivities and thermal resistances. The local ground thermal conductivities, preferably evaluated at thermal recovery conditions during DTRTs, were found to vary with depth. Local and effective borehole thermal resistances in most heat exchangers have been calculated, and their differences have been discussed in an effort to suggest better methods for interpretation of data from field tests.

    Large thermal shunt flow between down- and up-going flow channels was identified in all heat exchanger types, particularly at low volumetric flow rates, except in a multi-pipe BHE having an insulated central pipe where the thermal contact between down- and up-coming fluid was almost eliminated.

    At relatively high volumetric flow rates, U-pipe BHEs show a nearly even distribution of the heat transfer between the ground and the secondary fluid along the depth. The same applies to all coaxial BHEs as long as the flow travels downwards through the central pipe. In the opposite flow direction, an uneven power distribution was measured in multi-chamber and multi-pipe BHEs.

    Pipe-in-pipe and multi-pipe coaxial heat exchangers show significantly lower local borehole resistances than U-pipes, ranging in average between 0.015 and 0.040 Km/W. These heat exchangers can significantly decrease the temperature difference between the secondary fluid and the ground and may allow the use of plain water as secondary fluid, an alternative to typical antifreeze aqueous solutions. The latter was demonstrated in a pipe-in-pipe BHE having an effective resistance of about 0.030 Km/W.

    Forced convection in the groundwater achieved by injecting nitrogen bubbles was found to reduce the local thermal resistance in U-pipe BHEs by about 30% during heat injection conditions. The temperatures inside the groundwater are homogenized while injecting the N2, and no radial temperature gradients are then identified. The fluid to groundwater thermal resistance during forced convection was measured to be 0.036 Km/W. This resistance varied between this value and 0.072 Km/W during natural convection conditions in the groundwater, being highest during heat pump operation at temperatures close to the water density maximum.

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