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  • 1.
    Adsten, Monika
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Materials Science.
    Perers, Bengt
    Wäckelgård, Ewa
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences.
    The influence of climate and location on collector performance2002In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 25, no 4, p. 499-509Article in journal (Refereed)
    Abstract [en]

    The influence of annual climate variations on the performance of solar thermal collectors in the northern part of Europe has been investigated. The annual solar collector energy output has been calculated with the MINSUN simulation program using hourly, measured climatic data for the years 1983–98 for three cities situated in the south (Lund), central (Stockholm) and north (Luleå) of Sweden. A synthetic year created with the Meteonorm weather simulation program was also used in the simulations. Two solar thermal collectors were modelled: a flat plate solar collector and a tubular vacuum collector, both of commercial standard.

    The thermal energy output is strongly correlated to the annual global irradiation at a horizontal surface. The annual average energy delivered from the flat plate collector was 337 kWh/m2 for Stockholm (337 for Lund and 298 for Luleå), and from the vacuum tube collector 668 kWh/m2 for Stockholm (675 for Lund and 631 for Luleå) at an operating temperature of T=50°C. Maximum deviations from the average value for this 16-year period are around 20% for the flat plate and 15% for the vacuum tube collector, at T=50°C.

    The relation between global irradiation on a horizontal surface and the annually collected thermal energy at a constant operating temperature could be fitted to a linear equation: qu=aG(0°)+bT, where qu is the energy output from the collector, G(0°) the global irradiation at a horizontal surface, T the average temperature of the collector fluid, and a and b fitting parameters in a double linear regression analysis.

  • 2.
    Asmuth, Henrik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    Navarro Diaz, Gonzalo
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    Madsen, Helge Aagard
    DTU Wind Energy, Technical University of Denmark, Frederiksborgvej 399, 4000, Roskilde, Denmark.
    Branlard, Emmanuel
    National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, 80401, USA.
    Meyer Forsting, Alexander R.
    DTU Wind Energy, Technical University of Denmark, Frederiksborgvej 399, 4000, Roskilde, Denmark.
    Nilsson, Karl
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    Jonkman, Jason
    National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO, 80401, USA.
    Ivanell, Stefan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    Wind Turbine Response in Waked Inflow: A Modelling Benchmark Against Full-Scale Measurements2022In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 191, p. 868-887Article in journal (Refereed)
    Abstract [en]

    Predicting the power and loads of wind turbines in waked inflow conditions still presents a major modelling challenge. It requires the accurate modelling of the atmospheric flow conditions, wakes of upstream turbines and the response of the turbine of interest. Rigorous validations of model frameworks against measurements of utility-scale wind turbines in such scenarios remain limited to date. In this study, six models of different fidelity are compared against measurements from the DanAero experiment. The two benchmark cases feature a full-wake and partial-wake scenario, respectively. The simulations are compared against local pressure forces and inflow velocities measured on several blade sections of the downstream turbine, as well as met mast measurements and standard SCADA data. Regardless of the model fidelity, reasonable agreements are found in terms of the wake characteristics and turbine response. For instance, the azimuth variation of the mean aerodynamic forces acting on the blade was captured with a mean relative error of 15–20%. While various model-specific deficiencies could be identified, the study highlights the need for further full-scale measurement campaigns with even more extensive instrumentation. Furthermore, it is concluded that validations should not be limited to integrated and/or time-averaged quantities that conceal characteristic spatial or temporal variations.

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  • 3.
    Ayob, Mohd Nasir
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Malaysia Perlis, Sch Mechatron Engn, Arau 02600, Perlis, Malaysia.
    Castellucci, Valeria
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Wave energy potential and 1-50 TWh scenarios for the Nordic synchronous grid2017In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 101, p. 462-466Article in journal (Refereed)
    Abstract [en]

    This study estimates the wave energy potential along the coasts of the Nordic countries with the Nordicsynchronous grid as a chosen boundary. A model for wave farm allocation was developed and applied to achieve annual energy production targets of 1 TWh, 3 TWh, 10 TWh and 50 TWh. The study is based on 10 years of data, from 2005 to 2014, from the European Center for Medium-Range Weather Forecasts. Data from a total of 728 coordinate points along the Nordic countries, with a 0.125° x 0.125° spatial resolution, were considered. An algorithm was developed to generate the scenarios, to estimate the installed capacity of wave farms at different locations along the coasts, and to measure the physical space required by the farms. This analysis of the four energy target scenarios resulted in a required installed capacity of 337 MW, 1.02 GW, 3.42 GW and 17.09 GW, covering a stretch of the total coast of 0.4, 1.2, 3.8 and 18.9% respectively. The total annual wave energy resource for the Nordic countries is determined at 590 TWh, most of which is available along the Norwegian coast.

  • 4.
    Bahaj, A. S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    James, P. A. B.
    Direct and indirect benefits of grid connected photovoltaics in low energy social housing2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 5.
    Bahaj, A.S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    James, P. A. B.
    Braid, R.
    Multiple string inverters for PV systems < 5kWp: When is this justified?2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 6.
    Bernhoff, Hans
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Sjöstedt, Elisabeth
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Wave energy resources in sheltered sea areas: A case study of the Baltic Sea2006In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 31, no 13, p. 2164-2170Article in journal (Refereed)
    Abstract [en]

    Wave energy is a renewable source, which has not yet been exploited to a large extent. So far the main focus of wave energy conversion has been on the large wave energy resources of the great oceans on northern latitudes. However, large portions of the world potential wave energy resources are found in sheltered waters and calmer seas, which often exhibit a milder, but still steady wave climate. Examples are the Baltic Sea, the Mediterranean and the North Sea in Europe, and ocean areas closer to the equator. Many of the various schemes in the past consist of large mechanical structures, often located near the sea surface. In the present work we instead focus on wave power plants consisting of a number of small wave energy converters, forming large arrays. In this context, we look at advantageous arrangements of point absorbers, and discuss the potential of the Baltic Sea as a case study.

  • 7. Blunden, L. S.
    et al.
    Bahaj, A. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Aziz, N. S.
    Tidal current power for Indonesia? An initial resource estimation for the Alas Strait2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 49, no 0, p. 137-142Article in journal (Refereed)
    Abstract [en]

    Indonesia – with its many narrow straits and significant tidal range – might be expected to have considerable potential for tidal current power generation. A particularly promising site is the Alas Strait, with shallow depth, high tidal current velocity and location far from major shipping lanes. In this study, a hydrodynamic numerical model of the Alas Strait was run and validated against some tidal current velocity measurements. The results of the model were then used to estimate the practically exploitable energy resources in the strait, using a development of a method used in previous resource estimations. In this method, the incident velocity on each row of an array was extracted from the model and then attenuated based on the direction of the flow and the number of rows deep into the array. In this way, the effects of practical array shape can be simulated without the need to include computationally expensive parameterization of turbines in the model. Two scenarios were considered, with and without a maximum depth limitation of 40 m. The first scenario gave an estimate of the practically exploitable annual energy yield from the Alas Strait of 330 GW h with the second scenario nearly double at 640 GW h.

  • 8.
    Boström, Cecilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ekergård, Boel
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Mikael
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Linear generator connected to a resonance circuitIn: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 9.
    Boström, Cecilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Lejerskog, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Stålberg, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Thorburn, Karin
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Experimental results of rectification and filtration from an offshore wave energy system2009In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 34, no 5, p. 1381-1387Article in journal (Refereed)
    Abstract [en]

    The present paper presents results from a wave energy conversion that is based on a direct drive linear generator. The linear generator is placed on the seabed and connected to a buoy via a rope. Thereby, the natural wave motion is transferred to the translator by the buoy motion. When using direct drive generators, voltage and current output will have varying frequency and varying amplitude and the power must be converted before a grid connection. The electrical system is therefore an important part to study in the complete conversion system from wave energy to grid connected power. This paper will bring up the first steps in the conversion: rectification and filtration of the power. Both simulation studies and offshore experiments have been made. The results indicate that this kind of system works in a satisfactory way and a smooth DC power can be achieved with one linear generator.

  • 10.
    Breton, Simon-Philippe
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Nilsson, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Olivares-Espinosa, H.
    Masson, C.
    Dufresne, L.
    Ivanell, Stefan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 70, p. 153-163Article in journal (Refereed)
    Abstract [en]

    The influence of imposed turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier-Stokes equations in their incompressible form using a finite volume approach. In this code, the Large-Eddy Simulation technique is used for modeling turbulence, and the wind turbine rotors are represented as actuator discs whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today's offshore wind farms. Turbulence is pre-generated with the Mann model, with imposed turbulence intensity levels of 4.5% and 8.9%. The aim with this study is to investigate features of the flow that depend solely on imposed turbulence and the presence of wind turbine rotors. For this reason, the turbines are isolated from their environment, and no effect from the presence of the atmospheric boundary layer is modeled, i.e., a non-sheared inflow is used. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of standard deviation of the velocity components, turbulence kinetic energy, mean velocity, and power spectra of the axial velocity fluctuations. The mean power production along the row of turbines is also used as an indicator. Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the flow conditions in which they operate. The results obtained for the standard deviation of the velocity components, turbulence kinetic energy, power and mean velocity as functions of downstream distance show that an asymptotic wake state seems close to be reached, in the conditions tested, near the end of the 10 turbine row. Significant changes towards this state are seen to happen faster when imposing turbulence in the domain. Power spectra of the axial velocity fluctuations are shown to provide interesting information about the turbulence in the flow, but are found not to be useful in determining if an asymptotic wake state is reached.

  • 11.
    Bülow, Fredrik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Sandra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    No-load core loss prediction of PM generator at low electrical frequency2012In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 43, p. 389-392Article in journal (Refereed)
    Abstract [en]

    A method for measurement of frequency dependent electromagnetic core loss of a permanent magnet generator is presented. Core loss of a PM generator is measured at electrical frequencies ranging from 4 to 14 Hz. Core loss in the same interval is simulated using the finite element method and frequency domain loss separation. The specific loss is both extrapolated from specific loss at 50 Hz and measured directly at 4, 8, 12 and 16 Hz. Core loss simulations based on extrapolated specific loss are 38–53% smaller than measured loss. Core loss simulations based on specific loss measured at 4, 8, 12 and 16 Hz are 19–23% smaller than measured loss.

  • 12.
    Castellucci, Valeria
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Markus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Tidal effect compensation system for point absorbing wave energy converters2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 51, p. 247-254Article in journal (Refereed)
    Abstract [en]

    Recent studies show that there is a correlation between water level and energy absorption values for the studied wave energy converters: the absorption decreases when the water levels deviate from average. The situation appears during tides when the water level changes significantly. The main objective of the paper is to present a first attempt to increase the energy absorption during tides by designing and realizing a small-scale model of a point absorber equipped with a device that is able to adjust the length of the rope connected to the generator. The adjustment is achieved by a screw that moves upwards in the presence of low tides and downwards in the presence of high tides. Numerical results as well as experimental tests suggest that the solution adopted to minimize the tidal effect on the power generation shows potential for further development.

  • 13.
    Ekergård, Boel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Sandra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Boström, Cecilia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Supplementing rare-earth metal with ferrite in wave power generatorIn: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 14.
    Ekström, Rickard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Kurupath, Venugopalan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Svensson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Measurement system design and implementation for grid-connected marine substation2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 55, p. 338-346Article in journal (Refereed)
    Abstract [en]

    A marine substation for a grid-connected wave power plant has been designed and constructed at Uppsala University. The measurement system for the substation has been developed from basics, and the procedure is described in this article. Subsea electrical installations set high constraints on the control and data acquisition systems used, and traditional GSM networks and GPS time synchronization are difficult and inefficient. These circumstances exclude many well-proven methods to be utilized. A compactRIO-based system, with integrated real-time controller and Field-Programmable Gate Array chip, is chosen as most suitable for this task. The system is designed to meet the special requirements encountered in this unusual application, including large data sampling, grid connection control and protection systems. The system is communicating to shore via a subsea copper cable and single-pair high-speed digital subscriber line modems. The design was successful and full-fills all the system requirements. The aim of the article is to provide future researchers with a good design and implementation procedure for setting up large measurement systems.

  • 15.
    Eriksson, Sandra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bergkvist, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Design of a unique direct driven PM generator adapted for a telecom tower wind turbine2012In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 44, p. 453-456Article in journal (Refereed)
    Abstract [en]

    A vertical axis wind turbine has been designed to electrify a novel kind of telecommunication tower. This paper presents the design of a generator for this purpose. The generator is a permanent magnet generator rated at 10 kW. It has an unusually large diameter to fit on the outside of the telecommunication tower. The generator has been designed by using a two-dimensional FEM model. Simulations show that the generator has high efficiency through the whole operational interval. Furthermore, the generator has a high overload capability enabling electric control of the turbine. The generator has been built and the design shown feasible. Preliminary experimental results show that the induced voltage is lower than expected from simulations indicating insufficient modelling of three-dimensional effects, which are particularly large in a generator with these unusual dimensions.

  • 16.
    Eriksson, Sandra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Solum, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Simulations and experiments on a 12 kW direct driven PM synchronous generator for wind power2008In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 33, no 4, p. 674-681Article in journal (Refereed)
    Abstract [en]

    A direct driven permanent magnet (PM) synchronous generator has been designed and constructed and results from the first experimental tests are presented. The generator has been designed using the finite element method (FEM) and dynamic simulations have been performed to study the generator. The simulations are performed by using an electromagnetic model, which is described by a combined field and circuit equation model and is solved in a finite element environment. The stator winding of the generator consists of circular cables and the rotor has surface mounted, arched PMs. A complete experimental setup has been constructed consisting of a motor, a frequency converter, a gearbox and electrical loads. Oscilloscopes are used to measure the voltage and the current for each phase. Measurements have been performed for both full load and no load at rated speed. The harmonic content of the voltage is analyzed and compared to results from simulations. Furthermore, the generated electric power has been calculated from knowing the voltage and current and is compared to the simulated power. The agreement between experimental results and results from simulations based on finite element calculations is very high.. especially considering harmonics. Several sources of error are suggested that could cause the small differences between the simulated results and the measured data for the constructed generator.

  • 17.
    Forsting, Alexander R. Meyer
    et al.
    Tech Univ Denmark, DTU Wind & Energy Syst, Frederiksborgvej 399, DK-4000 Roskilde, Denmark..
    Navarro Diaz, Gonzalo
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    Segalini, Antonio
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Andersen, Soren J.
    Tech Univ Denmark, DTU Wind & Energy Syst, Anker Engelunds Vej 1, DK-2800 Lyngby, Denmark..
    Ivanell, Stefan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Wind Energy.
    On the accuracy of predicting wind-farm blockage2023In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 214, p. 114-129Article in journal (Refereed)
    Abstract [en]

    To assess the uncertainty in blockage quantification, this study proposes a comparison of farm blockage predictions from wind-tunnel experiments, Reynolds Averaged Navier-Stokes based simulations using multiple numerical setups, and analytical models. The influence of the numerical setup is demonstrated to be small if a consistent definition of blockage (able to sort out systematic errors) is used. The effect of domain confinement and turbulence intensity is investigated assessing their range of variability. Different analytical models performed similarly in comparison to the numerical data, demonstrating the best accuracy for realistic spacing between the turbines and supporting their use as reliable engineering tools.

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  • 18. Galloway, Pascal W.
    et al.
    Myers, Luke E.
    Bahaj, AbuBakr S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Quantifying wave and yaw effects on a scale tidal stream turbine2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 63, no 0, p. 297-307Article in journal (Refereed)
    Abstract [en]

    The behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently unclear. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi-directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in device capital cost. Therefore it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure. The experiments presented in this paper were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque whilst one blade was instrumented to acquire blade root strain, azimuthal position and rotational speed all at high frequency. The maximum out-of-plane bending moment was found to be as much as 9.5 times the in-plane bending moment. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero rotor yaw, scaled wave height of 2 m and intrinsic wave period of 12.8 s. A new tidal turbine-specific Blade-Element Momentum (BEM) numerical model has been developed to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was also included in the BEM model. Steady loading on an individual blade at positive yaw angles was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust. The inclusion of steady yaw effects (using the often-applied skewed axial inflow correction) in a BEM model should be neglected when waves are present or will result in poor load prediction reflected by increased loading amplitude in the 1P (once per revolution) phase.

  • 19.
    Goude, Anders
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bülow, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Aerodynamic and electrical evaluation of a VAWT farm control system with passive rectifiers and mutual DC-bus2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 60, p. 284-292Article in journal (Refereed)
    Abstract [en]

    A wind farm with a simple electrical topology with passive rectifiers and a single inverter (mutual topology) is compared to a more complex topology where each turbine has a separate inverter (separate topology).In both cases, the turbines are controlled electrically by varying the extracted power with the rotational velocity as control signal.These two electrical topologies are evaluated with respect to absorbed power for a farm of four turbines placed either on a line or in a square formation.The evaluation is done with an aerodynamic vortex model coupled with an electrical system model.Simulations predict that individual control is beneficial for aerodynamically independent turbines if flow velocities differ significantly between turbines. If the differences in flow velocities are caused by one turbine operating in the wake of another, the deviations in power output between the topologies are less prominent.The mutual topology even deliver more power than the separate topology when one turbine is in the wake of another turbine if the wind speed changes rapidly.

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  • 20.
    Goude, Anders
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bülow, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Robust VAWT control system evaluation by coupled aerodynamic and electrical simulations2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 59, p. 193-201Article in journal (Refereed)
    Abstract [en]

    A control system for a vertical axis wind turbine is presented.The control strategy is to determine the electric load solely from the rotational velocity and the characteristics of the turbine, thus measurement of the incoming wind velocity is not required.The control system is evaluated with an aerodynamic vortex model coupled with an electrical model.Three different sets of control system parameters are tested, due to the trade off between high power absorption and achieving a fast system with high stability.The control systems are tested against a reference strategy where the wind speed is known.The simulations show that the three control systems provide a similar power absorption as the reference case.For dynamic cases, with fast changes in wind speed, the fast control strategies are beneficial.All control strategies are stable throughout the simulations when proper power absorption characteristics of the turbine are used. It is also shown that if peak power absorption is estimated at a too low tip speed ratio, the control strategies may inadvertently stop the turbine.

  • 21.
    Goude, Anders
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ågren, Olov
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Simulations of a vertical axis turbine in a channel2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 63, p. 477-485Article in journal (Refereed)
    Abstract [en]

    The power coefficient of a turbine increases according to the predictions from streamtube theory for sites with a confined fluid flow. Here, a vertical axis turbine (optimized for free flow) has been simulated by a two-dimensional vortex method, both in a channel and in free flow. The first part of the study concerns the numerical parameters of channel simulations. It is found that for free flow and wide channels, a large number of revolutions is required for convergence (around 100 at the optimal tip speed ratio and increasing with higher tip speed ratio), while for smaller channels, the required number of revolutions decreases. The second part analyses changes in turbine performance by the channel boundaries. The turbine performance increases when the channel width is decreased, although the results are below the predictions from streamtube theory, and this difference increases with decreasing channel width. It is also observed that the optimal tip speed ratio increases with decreasing channel width. By increasing the chord, which decreases the optimal tip speed ratio, the power coefficient can be increased somewhat.

  • 22.
    Grandell, Leena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Thorenz, Andrea
    Silver supply risk analysis for the solar sector2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 69, p. 157-165Article in journal (Refereed)
    Abstract [en]

    The anthropogenic silver cycle shows the global material flows of silver on an annual basis. Beginning with mine supply the silver flows in various end use sectors. It is either stored as part of the above ground silver reserves or gets consumed and recycled or dissipates on landfills or with sewage. The article discusses supply demand scenarios of silver. The study is methodically based on an analysis of resource specific factors like exploration rates, reserves and resources and regional distribution of exploration areas. Among the numerous application areas for silver the one with the greatest growth potential is discussed in detail. Solar energy sector is a fast growing area which is analysed quantitatively in detail to obtain information on potential future bottlenecks in material availability. Due to its' high electrical conductivity silver is used in silicon photovoltaic technology and Gratzel cells to form the electrical contacts. In concentrated solar power applications silver is the material of choice as the coating of the mirror because of its' high optical reflectivity. Both concerns about climate change as well as questions related to energy resources and energy security stress the importance of renewable energy technologies. This has resulted in several future scenarios with partly very ambitious goals for the construction of the new energy infrastructure. The scenarios are discussed in the light of known silver resources. 

  • 23. Haralambopoulos, Dias
    et al.
    Polatidis, Heracles
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Renewable energy projects: structuring a multi-criteria group decision-making framework2003In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 24. Henfridsson, Urban
    et al.
    Neimane, Viktoria
    Strand, Kerstin
    Kapper, Robert
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Danielsson, Oskar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Sundberg, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Thorburn, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Ericsson, Ellerth
    Bergman, Karl
    Wave energy potential in the Baltic Sea and the Danish part of the North Sea, with reflections on the Skagerrak2007In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 32, no 12, p. 2069-2084Article in journal (Refereed)
    Abstract [en]

    Wave power, along with renewable energy-generating sources like tides and streams, is underestimated considering its advantageous physical properties and predictability. This paper examines possible examples of wave power installations in the Baltic Sea and the Danish part of the North Sea. Hindcasting data is used allowing estimations of wave energy generated and results show promising areas in the North Sea, but also several parts of the Baltic Sea are of interest. The study is based upon linear generator technique, placed on the seabed using point-absorbers arranged in arrays of up to several thousand units. The study aims at showing the physical possibilities of wave energy, including economical feasibility and environmental advantages of wave energy even in moderate wave climates. With discussion from two examples in the Baltic Sea, one in the Danish North Sea and a new pilot study site in the Swedish part of Skagerrak, this study show feasible illustrations of wave energy takeouts. Project examples vary in size due to distance to grid, grid voltage, and may thus be economically feasible. Examples also show considerations in societal and nature conservation matters, including aspects such as industrial and military interests, archaeological or marine reserves and local geology. The authors conclude that wave energy electric conversion is an option that needs more attention and which has several advantages compared to conventional renewable sources. Sound engineering, in combination with producer, consumer and broad societal perspective is advised for a sustainable development of wave energy conversion.

  • 25. Jentsch, Mark F.
    et al.
    James, Patrick A. B.
    Bourikas, Leonidas
    Bahaj, AbuBakr S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Transforming existing weather data for worldwide locations to enable energy and building performance simulation under future climates2013In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 55, no 0, p. 514-524Article in journal (Refereed)
    Abstract [en]

    Building performance and solar energy system simulations are typically undertaken with standardised weather files which do not generally consider future climate predictions. This paper investigates the generation of climate change adapted simulation weather data for locations worldwide from readily available data sets. An approach is presented for ‘morphing’ existing EnergyPlus/ESP-r Weather (EPW) data with UK Met Office Hadley Centre general circulation model (GCM) predictions for a ‘medium–high’ emissions scenario (A2). It was found that, for the United Kingdom (UK), the GCM ‘morphed’ data shows a smoothing effect relative to data generated from the corresponding regional climate model (RCM) outputs. This is confirmed by building performance simulations of a naturally ventilated UK office building which highlight a consistent temperature distribution profile between GCM and RCM ‘morphed’ data, yet with a shift in the distribution. It is demonstrated that, until more detailed RCM data becomes available globally, ‘morphing’ with GCM data can be considered as a viable interim approach to generating climate change adapted weather data.

  • 26.
    Kjellin, Jon
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bülow, Fredrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Eriksson, Sandra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Deglaire, Paul
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Power coefficient measurement on a 12 kW straight bladed vertical axis wind turbine2011In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 36, no 11, p. 3050-3053Article in journal (Refereed)
    Abstract [en]

    A 12 kW vertical axis H-rotor type wind turbine has been designed and constructed at Uppsala University. A measurement campaign has been performed to collect data to calculate the power coefficient using the method of bins. The measurement was performed at different constant rotational speeds on the turbine during varying wind speeds to observe the power coefficients dependence on tip speed ratio. The power coefficient peaked at 0.29 for a tip speed ratio equal to 3.3.

  • 27.
    Lalander, Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    In-stream energy converters in a river: Effects on upstream hydropower station2011In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 36, no 1, p. 399-404Article in journal (Refereed)
    Abstract [en]

    The use of in-stream energy converters in rivers is an area of research that is still in its preliminary stages. The driving force of river flows is the potential energy the water gains when it precipitates on mountainsides, and this energy is traditionally converted by hydropower stations, where dams are used to create a larger head. Using an in-stream energy converter would be advantageous in areas restricted by regulation. In this paper the effects of using these converters on the upstream water level in a river are studied. This has been done both with an analytical model and with a numerical model. The analytical model described the water level increase due to energy capture to depend on how large fraction of the channel that is blocked by the turbine. It was also shown that as the converter induces drag on the flow, and as energy is lost in wake mixing, the total head loss will be a sum of energy capture and energy losses. The losses correspond to a considerable fraction of the total head drop. The numerical model was used to evaluate these results. The model used was the 3D numerical model MIKE from the DHI Group in Sweden. Turbines were modelled with an inbuilt function in the program. The results from the model did not correspond to the analytical results, as the energy capture was equal to the head drop in the program. (c) 2010 Elsevier Ltd. All rights reserved.

  • 28.
    Leijon, Mats
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Danielsson, Oskar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Eriksson, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Thorburn, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Isberg, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Sundberg, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Ivanova, Irina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Sjöstedt, Elisabet
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Ågren, Olov
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Karlsson, Karl Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Wolfbrandt, Arne
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    An electrical approach to wave energy conversion2006In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, no 31, p. 1309-1319Article in journal (Refereed)
    Abstract [en]

    Motions in nature, for example ocean waves, can play a significant role in tomorrow's electricity production, but the constructions require adaptations to its media. Engineers planning hydropower plants have always taken natural conditions, such as fall height, speed of flow, and geometry, as basic design parameters and constraints in the design. The present paper describes a novel approach for electric power conversion of the vast ocean wave energy. The suggested linear electric energy converter is adapted to the natural wave motion using straightforward technology. Extensive simulations of the wave energy concept are presented, along with results from the experimental setup of a multisided permanent magnet linear generator. The prototype is designed through systematic electromagnetic field calculations. The experimental results are used for the verification of measurements in the design process of future full-scale direct wave energy converters. The present paper, describes the energy conversion concept from a system perspective, and also discusses the economical and some environmental considerations for the project.

  • 29.
    Leijon, Mats
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Skoglund, Annika
    Dep. of Industrial Economics, Royal Institute of Technology, Stockholm.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rehn, Alf
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Industrial Engineering & Management.
    Lindahl, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Industrial Engineering & Management.
    On the physics of power, energy and economics of renewable electric energy sources - Part I2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 35, no 8, p. 1729-1734Article in journal (Refereed)
    Abstract [en]

    Environmental concerns have increasingly led to the installation of Renewable Energy Technologies (RETs) despite the fact that they are recognized as expensive. Innovative efforts within the area are beset with difficulties [1], and they are at risk of producing misdirected or insignificant improvements in terms of the cost effectiveness of total energy conversion systems. This paper investigates how RETs can be evaluated, in terms of economy and engineering solutions, by studying the fundamental physics of renewable energy sources and how it matches with the RETs. This match is described by the "Degree of Utilization". The findings indicate that new innovations should focus on the possible number of full loading hours. RETs that are correctly matched to their energy source generate a higher amount of electric energy and have a higher potential of becoming more competitive. In cases where this aspect has been ignored, leading to relatively small degrees of utilization, it can be understood as an engineering mismatch between installed power, converted energy, and the fundamental physics of the renewable energy sources. Since there is a strong and possibly biased support for so-called mature RETs and already existing solutions, a clarification of how fundamental physical laws affect the cost of investments and payback of investments is needed. The present paper is part I out of II and it focuses on the difference between power and energy and the physics of different energy sources and their utilization.

  • 30.
    Lejerskog, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Boström, Cecilia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hai, Ling
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Experimental results on power absorption from a wave energy converter at the Lysekil wave energy research site2015In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 77, p. 9-14Article in journal (Refereed)
    Abstract [en]

    Power generation from wave power has a large potential to contribute to our electric energy production, and today, many wave power projects are close to be commercialized. However, one key issue to solve for many projects is to decrease the cost per installed kW. One way to do this is to investigate which parameters that have a significant impact on the wave energy converters (WEC) performance. In this paper, experimental results on power absorption from a directly driven point absorbing WEC are presented. The experiments have been carried out at the Lysekil research site in Sweden. To investigate the performance of the WEC, the absorbed power and the speed of the translator are compared. The result confirms that the buoy size and the translator weight have a large impact on the power absorption from the generator. By optimizing the buoy size and translator weight, the WEC is believed to produce power more evenly over the upward and downward cycle. Moreover, to predict the maximum power limit during normal operation, a simulation model has been derived. The results correlates well with experimental data during normal operation. 

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  • 31. Leybourne, Mark
    et al.
    Batten, William M. J.
    Bahaj, AbuBakr S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Minns, Ned
    O'Nians, Jamie
    Preliminary design of the OWEL wave energy converter pre-commercial demonstrator2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 61, no 0, p. 51-56Article in journal (Refereed)
    Abstract [en]

    The consortium responsible for the next stage of development of the OWEL wave energy converter will construct and test a large scale, pre-commercial demonstrator. It is expected that this will be installed at Wave Hub during 2013 and grid connected for a testing period lasting around 12 months. This paper reports on the preliminary design work being undertaken in the development of the marine demonstration device. This concentrates primarily on producing a fully costed design by detailing the hydraulic design and aspects of stability as well as providing insight into various design features such as the power take-off, naval architecture, moorings and control. The design is being largely informed by the results of a 15 month research project funded by the South West Regional Development Agency (SWRDA) in which the performance was determined and a detailed techno-economic model for a large scale OWEL device was generated.

  • 32.
    Lindahl, Johan
    et al.
    Chalmers Univ Technol, Dept Technol Management & Econ, SE-41296 Gothenburg, Sweden..
    Lingfors, David
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Civil and Industrial Engineering, Civil Engineering and Built Environment.
    Elmqvist, Åsa
    Univ Gävle, Fac Engn & Sustainable Dev, SE-80176 Gävle, Sweden..
    Mignon, Ingrid
    Chalmers Univ Technol, Dept Technol Management & Econ, SE-41296 Gothenburg, Sweden..
    Economic analysis of the early market of centralized photovoltaic parks in Sweden2022In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 185, p. 1192-1208Article in journal (Refereed)
    Abstract [en]

    Sweden are one of the countries that experience growing installation volumes of Solar photovoltaic. Traditionally, in Sweden, most of the solar photovoltaic investments and policy incentives have focused on distributed photovoltaic systems. Yet, despite limited policy incentives and pessimistic forecasts, an increasing number of centralized photovoltaic parks have been commissioned and plans for substantial new capacities are communicated. Hence, the current paper investigates why. Detailed information about the underlying costs of six PV parks commissioned in 2019 and 2020 in Sweden were obtained by in-depth interviews with stakeholders and were analysed through levelized cost of electricity calculations. We conclude that the unsubsidised levelized cost of electricity ranged from 27.37 to 49.39 euro /MWh, with an average of 40.79 euro /MWh. This is lower than what are assessed for photovoltaic parks in some recent Swedish electricity system scenario studies. The main reason for the discrepancy is identified to be the assumed interest rates in the system scenario studies and the actual cost of capital experienced in the market. Comparing the levelized cost of electricity values with the market value of solar photovoltaic electricity on the spot market show that four of the six studied parks would be profitable under a merchant business model with the last years spot prices. If the downward price trend continues, Sweden may face an unexpected expansion of photovoltaic parks.

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  • 33.
    Lundh, Magdalena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Dalenbäck, Jan-Olof
    CIT Energy Management AB.
    Swedish solar heated residentialarea with seasonal storage in rock: Initial evaluation2008In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 33, no 4, p. 703-711Article in journal (Refereed)
    Abstract [en]

    A partly solar heated building area comprising 50 residential units has been built in Anneberg, Sweden. The system includes low-temperature space heating with seasonal ground storage of solar heat. Heating is supplied by 2400 m(2) solar collectors and individual electrical heaters for supplementary heating. The ground storage comprises about 60,000 m(3) of crystalline rock with 100 boreholes drilled to 65 m depth and fitted with double U-pipes. The collectors will have favourable working conditions but the store is rather small, the estimated heat loss from the beat store is about 40% of stored solar heat and the average solar fraction is estimated to 70% after 3-5 years of operation. An initial evaluation after 2 years of operation shows that, although problems have occurred and several parts seem to work less efficient than expected, the overall system idea works as intended.

  • 34. Mackay, Edward B. L.
    et al.
    Bahaj, AbuBakr S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Challenor, Peter G.
    Uncertainty in wave energy resource assessment. Part 1: Historic data2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 35, no 8, p. 1792-1808Article in journal (Refereed)
    Abstract [en]

    The uncertainty in estimates of the energy yield from a wave energy converter (WEC) is considered. The study is presented in two articles. This first article deals with the accuracy of the historic data and the second article considers the uncertainty which arises from variability in the wave climate. Estimates of the historic resource for a specific site are usually calculated from wave model data calibrated against in-situ measurements. Both the calibration of model data and estimation of confidence bounds are made difficult by the complex structure of errors in model data. Errors in parameters from wave models exhibit non-linear dependence on multiple factors, seasonal and interannual changes in bias and short-term temporal correlation. An example is given using two hindcasts for the European Marine Energy Centre in Orkney. Before calibration, estimates of the long-term mean WEC power from the two hindcasts differ by around 20%. The difference is reduced to 5% after calibration. The short-term temporal evolution of errors in WEC power is represented using ARMA models. It is shown that this is sufficient to model the long-term uncertainty in estimated WEC yield from one hindcast. However, seasonal and interannual changes in model biases in the other hindcast cause the uncertainty in estimated long-term WEC yield to exceed that predicted by the ARMA model.

  • 35. Mackay, Edward B. L.
    et al.
    Bahaj, AbuBakr S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Challenor, Peter G.
    Uncertainty in wave energy resource assessment. Part 2: Variability and predictability2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 35, no 8, p. 1809-1819Article in journal (Refereed)
    Abstract [en]

    The uncertainty in estimates of the energy yield from a wave energy converter (WEC) is considered. The study is presented in two articles. The first article considered the accuracy of the historic data and the second article, presented here, considers the uncertainty which arises from variability in the wave climate. Mean wave conditions exhibit high levels of interannual variability. Moreover, many previous studies have demonstrated longer-term decadal changes in wave climate. The effect of interannual and climatic changes in wave climate on the predictability of long-term mean WEC power is examined for an area off the north coast of Scotland. In this location anomalies in mean WEC power are strongly correlated with the North Atlantic Oscillation (NAO) index. This link enables the results of many previous studies on the variability of the NAO and its sensitivity to climate change to be applied to WEC power levels. It is shown that the variability in 5, 10 and 20 year mean power levels is greater than if annual power anomalies were uncorrelated noise. It is also shown that the change in wave climate from anthropogenic climate change over the life time of a wave farm is likely to be small in comparison to the natural level of variability. Finally, it is shown that despite the uncertainty related to variability in the wave climate, improvements in the accuracy of historic data will improve the accuracy of predictions of future WEC yield.

  • 36. Myers, L. E.
    et al.
    Bahaj, A. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    An experimental investigation simulating flow effects in first generation marine current energy converter arrays2012In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 37, no 1, p. 28-36Article in journal (Refereed)
    Abstract [en]

    At present a small number of full-scale marine current energy converters are undergoing sea trials to demonstrate commercial viability of the technology. In order to provide meaningful quantities of electrical power to the grid, the next phase in the development of the technology will be the installation and operation of farms or arrays composed of multiple devices. As most tidal current sites are bi-directional and with bathymetry constraints, array layouts will necessarily take the form of highly optimized geometric configurations with reduced lateral inter-device spacing. This work discusses the concept of array layouts and proposes an appropriate and clear classification that can aid developers in understanding how arrays operate. This classification is supported by experimental studies conducted using several arrangements of multiple actuator disks to simulate early generation marine current energy converter arrays. The work presents quantification of the flow field around a 2-row array, device/device interaction as well as a study of the structure of the far wake region where subsequent devices could be installed. The results highlight an optimal lateral spacing between devices where, under certain conditions flow can be accelerated between a pair of rotor disks. For the work presented here this accelerated region of flow possessed 22% more kinetic energy than the flow far upstream with no measurable negative effect upon the 2 actuator disks. This enhanced flow speed gives rise to the counterintuitive notion of a downstream row of devices producing more power than the upstream row. This will lead to a synergistic effect whereby an array of devices can generate more power than an equivalent number of isolated machines.

  • 37. Nguyen, Van-Dang
    et al.
    Jansson, Johan
    KTH Royal Inst Technol, Dept Computat Sci & Technol, Stockholm, Sweden.
    Goude, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hoffman, Johan
    KTH Royal Inst Technol, Dept Computat Sci & Technol, Stockholm, Sweden.
    Direct Finite Element Simulation of the turbulent flow past a vertical axis wind turbine2019In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 135, p. 238-247Article in journal (Refereed)
    Abstract [en]

    There is today a significant interest in harvesting renewable energy, specifically wind energy, in offshore and urban environments. Vertical axis wind turbines get increasing attention since they are able to capture the wind from any direction. They are relatively easy to install and to transport, cheaper to build and maintain, and quite safe for humans and birds. Detailed computer simulations of the fluid dynamics of wind turbines provide an enhanced understanding of the technology and may guide design improvements. In this paper, we simulate the turbulent flow past a vertical axis wind turbine for a range of rotation angles in parked and rotating conditions. We propose the method of Direct Finite Element Simulation in a rotating ALE framework, abbreviated as DFS-ALE. The simulation results are validated against experimental data in the form of force measurements. It is found that the simulation results are stable with respect to mesh refinement and that the general shape of the variation of force measurements over the rotation angles is captured with good agreement.

  • 38.
    Ning, Dezhi
    et al.
    State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China.
    Zhao, Xuanlie
    State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Kang, Haigui
    State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China.
    Hydrodynamic performance of a pile-restrained WEC-type floating breakwater: An experimental study2016In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 95, p. 531-541Article in journal (Refereed)
    Abstract [en]

    In this paper, a system which integrates an oscillating buoy type wave energy converter with a vertical pile-restrained floating breakwater is introduced. A preliminary experimental study on the hydrodynamic performance of the system is carried out in a wave flume under the action of regular waves. A current controller-magnetic powder brake system is used to simulate the power generation system. The design is verified against published results. The power-take off damping characteristics are investigated, and the current controller-magnetic powder brake system can simulate the (approximate) Coulomb damping force very well. The effects of various parameters, including wave period and wave height, dimensions of the system and excitation current, on the hydrodynamic performance are investigated. Results indicate that the power take-off damping force, draft and relative width between the floating breakwater and the wavelength have a significant influence on the hydrodynamic performance of the system. A range can be observed for which the capture width ratio of the system can achieve approximately 24% while transmission coefficient was kept lower than 0.50 with the proper adjustment of power take-off damping force, and the floating breakwater performs in an effective manner. The new concept provides a promising way to utilize wave energy cost-effectively.

  • 39.
    Olauson, Jon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Integration of Renewable Energy Sources Group, KTH Royal Institute of Technology, Stockholm, Sweden.
    ERA5: The new champion of wind power modelling?2018In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 126, p. 322-331Article in journal (Refereed)
    Abstract [en]

    Output from meteorological reanalyses are used extensively in both academia and industry for modelling wind power. Recently, the first batch of the new ERA5 reanalysis was released. The main purpose of this paper is to compare the performance of ERA5 and MERRA-2 (a commonly used reanalysis today) in terms of modelling i) the aggregated wind generation in five different countries and ii) the generation for 1051 individual wind turbines in Sweden. The modelled wind power generation was compared to measurements. In conclusion, ERA5 performs better than MERRA-2 in all analysed aspects; correlations are higher, mean absolute and root mean square errors are in average around 20% lower and distributions of both hourly data and changes in hourly data are more similar to those for measurements. It is also shown that the uncertainty related to long-term correction (using one year of measurements and reanalysis data to predict the energy production during the remaining 1–5 years) is 20% lower for ERA5. In fact, using one year sample data and ERA5 gives slightly more accurate estimates than using two years of sample data and MERRA-2. Additionally, a new metric for quantifying the system size and dispersion of wind farms is proposed.

  • 40.
    Olauson, Jon
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bergkvist, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Modelling the Swedish Wind Power Production Using MERRA Reanalysis Data2015In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 76, no 4, p. 717-725Article in journal (Refereed)
    Abstract [en]

    The variability of wind power will be an increasing challenge for the power system as wind penetration grows and thus needs to be studied. In this paper a model for generation of hourly aggregated wind power time series is described and evaluated. The model is based on MERRA reanalysis data and information on wind energy converters in Sweden. Installed capacity during the studied period (2007–2012) increased from around 600 to over 3500 MW. When comparing with data from the Swedish TSO, the mean absolute error in hourly energy was 2.9% and RMS error was 3.8%. The model was able to adequately capture step changes and also yielded a nicely corresponding distribution of hourly energy. Two key factors explaining the good results were the use of a globally optimised power curve smoothing parameter and the correction of seasonal and diurnal bias.

    Because of bottlenecks in the Swedish transmission system it is relevant to model certain areas separately. For the two southern areas the MAE were 3.7 and 4.2%. The northern area was harder to model and had a MAE of 6.5%. This might be explained by a low installed capacity, more complex terrain and icing losses not captured in the model.

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  • 41.
    Olauson, Jon
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bergström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Bergkvist, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Restoring the missing high-frequency fluctuations in a wind power model based on reanalysis data2016In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 96, p. 784-791Article in journal (Refereed)
    Abstract [en]

    A previously developed model based on MERRA reanalysis data underestimates the high-frequency variability and step changes of hourly, aggregated wind power generation. The goal of this work is to restore these fluctuations. Since the volatility of the high-frequency signal varies in time, machine learning techniques were employed to predict the volatility. As predictors, derivatives of the output from the original “MERRA model” as well as empirical orthogonal functions of several meteorological variables were used. A FFT-IFFT approach, including a search algorithm for finding appropriate phase angles, was taken to generate a signal that was subsequently transformed to simulated high-frequency fluctuations using the predicted volatility. When comparing to the original MERRA model, the improved model output has a power spectral density and step change distribution in much better agreement with measurements. Moreover, the non-stationarity of the high-frequency fluctuations was captured to a large degree. The filtering and noise addition however resulted in a small increase in the RMS error.

  • 42.
    Polatidis, Heracles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Haralambopoulos, Dias
    Renewable Energy Systems; A Societal & Technological Platform2007In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682Article in journal (Refereed)
  • 43.
    Rossander, Morgan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fjellstedt, Christoffer
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Bernhoff, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Multiple Vertical Axis Wind Turbines with Passive Rectification to a Common DC-link2018In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 127, p. 1101-1110Article in journal (Refereed)
    Abstract [en]

    Wind turbines are commonly placed in wind farms, usually operating as separate units. Possible benefits could be found by allowing turbines to share a common DC-link. Diode rectifiers are a robust and cost effective way to rectify variable speed wind turbines, with loss of direct control of the generator. This paper studies the electromechanical interactions between four passively rectified vertical axis wind turbines connected to a common DC-link. Two different load approaches for the DC-link are compared using simulations in terms of performance and stability: a power source and a voltage source. The optimal torque (or optimal power) control is implemented for the two loads approaches. In addition, three-phase and dual stator winding (six-phase) generators are compared. The results show that all suggested solutions work with similar performance. However, the power load requires a large DC-link capacitance to achieve stability. More generatorphases improve the system with passive rectification in most cases. The simulations suggest that the common DC-link systems are expected to have a few percent lower energy capture due to the lack of individual turbine control. On the other hand, there is a significant reduction in peak power and a potential for smoother output power.

  • 44.
    Saarinen, Linn
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Vattenfall AB, Älvkarlebylaboratoriet, Älvkarleby, Sweden.
    Dahlbäck, Niklas
    Vattenfall AB, Älvkarlebylaboratoriet, Älvkarleby, Sweden.
    Lundin, Urban
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Power system flexibility need induced by wind and solar power intermittency on time scales of 1-14 days2015In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 83, p. 339-344Article in journal (Refereed)
    Abstract [en]

    This article describes a method to assess the needed production flexibility to adapt the power system to the production from variable renewable energy sources such as wind power and photovoltaics over time horizons of 1-14 days. Load and production data from the German power system is used to quantify the flexibility need in terms of power and energy storage requirement due to higher shares of renewable energy (20-80%). It is found that with an 80% variable renewable energy share in the German system, the average power need from flexible sources decreases by 31 GW (59%) while the peak power need only decreases by 3 GW (4%). In terms of energy, the storage need over a two week horizon increases by 2.6 TWh, which is 14% of the average load per day. If the European plans for 100 GW wind power in the North Sea region are realised, this would mean an increase of the energy storage need in the region with 2.2 TWh over a two week horizon.

  • 45.
    Sjökvist, Linnea
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Center for Natural Disaster Science (CNDS), Uppsala, Sweden.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Peak forces on a point absorbing wave energy converter impacted by tsunami waves2019In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 133, p. 1024-1033Article in journal (Refereed)
    Abstract [en]

    Although a tsunami wave in deep sea can be simulated using linear shallow water theory, the wave dynamics of a tsunami running up a continental shelf is very complex, and different phenomena may occur, depending on the width and profile of the shelf, the topography of the coast, incident angle of the tsunami and other factors. How to simulate tsunami waves at an intermediate depth is studied in this paper by using three different simulation approaches for tsunamis, a soliton, a simulated high incident current and a dam-break approach. The surface wave profiles as well as the velocity- and pressure profiles for the undisturbed waves are compared. A regular Stokes 5th wave of the same amplitude is simulated for comparison. A wave energy converter model, previously validated with wave tank experiment, is then used to study the survivability of the Uppsala University wave energy device for the different waves. The force in the mooring line is studied together with the resulting force on a bottom mounted column, corresponding to the linear generator on the seabed.

  • 46.
    Skoglund, Annika
    et al.
    Dep. of Industrial Economics, Royal Institute of Technology, Stockholm.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rehn, Alf
    Lindahl, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Industrial Engineering & Management.
    Waters, Rafael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    On the physics of power, energy and economics of renewable electric energy sources - Part II2010In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 35, no 8, p. 1735-1740Article in journal (Refereed)
    Abstract [en]

    Renewable Energy Technologies (RETs) are often recognized as less competitive than traditional electric energy conversion systems. Obstacles with renewable electric energy conversion systems are often referred to the intermittency of the energy sources [1] and the relatively high maintenance cost. However, due to an intensified discourse on climate change and its effects, it has from a societal point of view, become more desirable to adopt and install CO2 neutral power plants. Even if this has increased the competitiveness of RETs in a political sense, the new goals for RET installations must also be met with economical viability. We propose that the direction of technical development, as well as the chosen technology in new installations, should not primarily be determined by policies, but by the basic physical properties of the energy source and the associated potential for inexpensive energy production. This potential is the basic entity that drives the payback of the investment of a specific RET power plant. With regard to this, we argue that the total electric energy conversion system must be considered if effective power production is to be achieved, with focus on the possible number of full loading hours and the Degree of Utilization [2]. This will increase the cost efficiency and economical competitiveness of RET investments, and could enhance faster diffusion of new innovations and installations without over-optimistic subsidies. This paper elaborates on the overall problem of the economy of renewable electric energy conversion systems by studying the interface between physics, engineering and economy reported for RET power plants in different scientific publications. The core objective is to show the practical use of the Degree of Utilization and how the concept is crucial for the design and economical optimization disregarding subsidies. The results clearly indicate that the future political regulative frameworks should consider the choice of renewable energy source since this strongly affects the economical output from the RET power plants.

  • 47.
    Sörensen, Jens Nörkaer
    et al.
    Tech Univ Denmark, DTU Wind Energy, Lyngby, Denmark..
    Nilsson, Karl
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Ivanell, Stefan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Asmuth, Henrik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Mikkelsen, Robert Flemming
    Tech Univ Denmark, DTU Wind Energy, Lyngby, Denmark..
    Analytical body forces in numerical actuator disc model of wind turbines2020In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 147, no Part 1, p. 2259-2271Article in journal (Refereed)
    Abstract [en]

    An analytical model for representing body forces in numerical actuator disc models of wind turbines is developed and validated. The model is based on the assumption that the rotor disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in actuator disc simulations without any prior knowledge of the wind turbine being analysed.

  • 48.
    Tang, Renbo
    et al.
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Hubei, Peoples R China;Zhongnan Engn Corp Ltd, Power Construct Corp China, Changsha 410014, Hunan, Peoples R China.
    Yang, Jiandong
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Hubei, Peoples R China.
    Yang, Weijia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Hubei, Peoples R China.
    Zou, Jin
    China Southern Power Grid, EPRI, Guangzhou 510663, Guangdong, Peoples R China.
    Lai, Xu
    Wuhan Univ, State Key Lab Water Resources & Hydropower Engn S, Wuhan 430072, Hubei, Peoples R China.
    Dynamic regulation characteristics of pumped-storage plants with two generating units sharing common conduits and busbar for balancing variable renewable energy2019In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 135, p. 1064-1077Article in journal (Refereed)
    Abstract [en]

    Pumped-storage plants (PSPs) are becoming increasingly important for balancing variable renewable energy (VRE) in power systems. A large portion of PSPs consist of multiple generating units (GUs) with shared water conduits and busbar. In this paper, an integrated transfer function (ITF) model for such PSPs in generation mode under primary frequency control (PFC) is proposed. Based on a real PSP in China, this ITF model is validated with both simulation results using the method of characteristics (MOC) as well as data from on-site measurements. For a series of governor parameter (K-p and K-i) settings under different governor control modes, stability limits expressed by gain and phase margins are evaluated and time domain simulations for balancing wind power variations are carried out to assess the quality of regulation. The results reveal that the dynamic regulation characteristics of the studied PSP system is clearly effected by the governor control mode. Besides, due to its complexity, a general range of the gain and phase margins is not viable for the studied PSP system under different governor control modes. Therefore, a thorough time domain sensitivity analysis using both step and realistic load disturbance as inputs is required for the studied PSPs for balancing VRE.

  • 49.
    Temiz, Irina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Leijon, Jennifer
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ekergård, Boel
    Boström, Cecilia
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Economic aspects of latching control for a wave energy converter with a direct drive linear generator power take-off2018In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 128, no Part A, p. 57-67Article in journal (Refereed)
    Abstract [en]

    A wave energy converter (WEC) should be controlled in order to increase the average output power. In this paper, economic aspects of latching applied to a point absorbing WEC with a linear generator power take-off (PTO) are discussed. The capacity utilisation factor (CUF) is suggested to be used along with average absorbed power for control optimisation. Optimum and suboptimum latching controls are assessed for the WEC and compared with a constant damping PTO force control. The WEC performance is simulated using monochromatic waves for the wave conditions of the Wave Hub test site, UK. The linear wave theory is used in a hydro-mechanical two-body simulation model. It is shown that the latching controls possess considerable practical challenges significantly increasing the return of investment time periods.

  • 50.
    Thorburn, Karin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Leijon, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Case study of upgrading potential for a small hydro power station2005In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 30, no 7, p. 1091-1099Article in journal (Refereed)
    Abstract [en]

    Schemes for upgrading hydro power are formed in many parts of the world. Small hydro power (<15 MW) constitutes a fraction of all hydro power, but upgrading can still be worthwhile. In this article, a small generating station in Sweden, with two generators, is simulated with new generators. The voltage is increased by introducing a cable wound stator, thereby the transformers can be excluded, and more efficient generators are introduced with a higher power factor. These improvements lead to an active power increase from 8.9 to 9.4 MW per generator, which means an increased total annual production by 4.2 GW h.

12 1 - 50 of 57
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