Olov Ågren

Multiphysics simulation of wave energy to electric energy conversion by permanent magnet linear generator

The possibility to use three-phase permanent magnet linear generators to convert sea wave energy into electric energy is investigated by multiphysics simulations. The results show a possibility, which needs to be further verified by experimental tests, for a future step toward a sustainable electric power production from ocean waves by using direct conversion. The results suggest that wave energy can have an impact on tomorrows new sustainable electricity production, not only for large units, but also for units ranging down to 10 kW. This gives wave power a larger economical potential than previously estimated. The study demonstrates the feasibility of computer simulations to give a broad, and in several aspects a detailed, understanding of the energy conversion. The simulation results also give a useful starting point for future experimental work.

Economical considerations of renewable electric energy production—especially development of wave energy

Investments in renewable energy plants normally only take standard economic key figures into account, such as installed rated power, the market price of energy and the interest rate. The authors propose that the degree of utilisation, i.e. the ratio of yearly produced energy in the installation to the installed power, must be included due to its significant impact on the present value of the investment. A site with a limited average wave height could be of economic interest if the utility factor for the installation is high, since the investment cost (associated with the power installed) can be better adjusted to conditions at the particular site. In the case of wave power from the Baltic Sea with its limited variation in wave height (and limited average wave height), this indicates that the economic potential is best for smaller units.

Catch the wave to electricity

The ocean are largely an untapped source of energy. However, compared to other energies, power fluctuations for ocean waves are small over longer periods of time. This paper present a grid-oriented approach to electricity production from ocean waves, utilizing a minimal amount of mechanical components.

Simulation of wave-energy converter with octagonal linear generator

To extract electrical energy from sea waves in a commercially and technologically acceptable manner, a number of issues have to be solved. Electricity generation by means of direct conversion of the oscillating gravitational potential energy of a floating buoy can be anticipated, provided a proper design of a generator could be made. This paper deals with the simulation of a novel design for a linear generator aimed for the extraction of energy from ocean waves. The ocean waves are modeled by 4-m-height sinusoidal waves with a characteristic period of 7 s. A wide range of the geometrical sizes, permanent magnets, stator winding, and spring forces acting on the buoy are possible. This paper presents simulations of octagonal three-phase linear generators in the 100-kW power range. The beneficial effects of a stator of octagonal shape are briefly investigated, but not studied in depth. The main emphases in the present study have been to decrease power fluctuations and suppress voltage harmonics. In conventional rotating machines, well-known measures are to use a fractional number of slots per pole and phase, and an additional method is to make the pole edges smoother. These methods are here simulated for the first time on a linear machine aimed for ocean wave-energy conversion and a substantial reduction in power fluctuations and voltage harmonics are predicted.

Magnetic mirror minimum B field with optimal ellipticity

Closed form expressions for the ellipticity and the magnetic field of a conventional mirror machine, constrained to fulfill a minimum B stability criterion, are derived. The best result for zero-β plasmas is obtained with a marginally stable magnetic field, and the ellipticity is 13.9 with the representative value 4 for the mirror ratio. The gyro centers move on a single filed line, and thereby the perpendicular drift is zero and the radial transport is minimized. The flux tube has quadrupolar symmetry with straight (nonparallel) field lines in the confinement region.

Simulation of cogging in a 100 kW permanent magnet octagonal linear generator for ocean wave conversion

Fluctuations in the torque acting on the rotor in a rotating electric generator i.e., cogging, gives unstable rotor motion and can venture reliable operation of the machine. Cogging also gives a similar fluctuation in the output power. This paper presents a study of the cogging force in a linear permanent magnet generator. A constant output power is possible in a rotating machine with three phases winding. In a linear generator, due to the reciprocate piston motion, power fluctuations on the slow time scale of the piston bounce period cannot be avoided. Associated with power variations on a time scale shorter than the piston bounce motion is fluctuations on the force (torque) acting on the piston, i.e., cogging. Cogging is not less dangerous in linear generators compared to rotation machines and the problem requires investigation. The paper investigates means to reduce cogging by improving the geometrical design of the stator. In particular, rational number of the slots per pole and phase reduce the cogging by one order of magnitude

STUDIES OF A STRAIGHT FIELD LINE MIRROR WITH EMPHASIS ON FUSION-FISSION HYBRIDS

Abstract The straight field line mirror (SFLM) field with magnetic expanders beyond the confinement region is proposed as a compact device for transmutation of nuclear waste and power production. A design with reactor safety and a large fission-to-fusion energy multiplication is analyzed. Power production is predicted with a fusion Q = 0.15 and an electron temperature of ~500 eV. A fusion power of 10 MW may be amplified to 1.5 GW of fission power in a compact hybrid mirror machine. In the SFLM proposal, quadrupolar coils provide stabilization of the interchange mode, radio-frequency heating is aimed to produce a hot sloshing ion plasma, and magnetic coils are computed with an emphasis on minimizing holes in the fission blanket through which fusion neutrons could escape. Neutron calculations for the fission mantle show that nearly all fusion neutrons penetrate into the fission mantle. A scenario to increase the electron temperature with a strong ambipolar potential suggests that an electron temperature exceeding 1 keV could be reached with a modest density depletion by two orders in the expander. Such a density depletion is consistent with stabilization of the drift cyclotron loss cone mode.

A time-dependent potential flow theory for the aerodynamics of vertical axis wind turbines

The Betz factor, i.e., the value 16∕27 for the power coefficient, is widely expected to give an upper limit for the performance of any wind turbine. In the present study, an analytical model of a vertical-axis wind turbine with straight vertical wings is developed. A goal of the work is to study if the one-dimensional Betz theory gives an upper limit of the performance of wind turbines when two-dimensional effects are included. The two-dimensional and time-dependent potential flow is solved by a conformal map of the wing sections to circles. The stagnation points are determined by the Kutta condition. The calculated power coefficient exceeds the Betz limit by a large factor. This is due to a completely different flow pattern compared to the one-dimensional Betz theory. In aerodynamic potential flow, the expanding flux tube of Betz is replaced by an asymptotic flow consisting of a superposition of homogeneous flow and a circulation around the wings. Moreover, the total torque on a turbine with three or mor...

Theoretical study of increased electron temperature in mirror machines by tuned ion cyclotron resonance heating cycles

The end loss is a serious problem for a mirror fusion device. The purpose of this paper is to study an idea for improvement of end confinement of electrons and ions for a conventional singe mirror machine with a marginal minimum B stable confining magnetic field. By creating an electrostatic electron barrier that confines electrons sufficiently well, an electron temperature of the order of the ion temperature may be reached. This electrostatic electron barrier may be obtained by decreasing the fuel ion density at the mirror plug sufficiently much compared to the central cell density. Microinstabilities may be suppressed or even stabilized by tuned ICRH (ion cyclotron resonance heating) cycles with resonant absorption of the wave just outside one of the mirrors, since a distribution of sloshing ions would be created. Qualitative arguments indicate the possibility with tuned ICRH to reach an energy gain factor well above unity.

Sloshing ion distribution function in a minimum B mirror field

Combinations of the energy and magnetic moment invariants, such as the longitudinal bounce time, the longitudinal action integral, and the longitudinal turning point, are investigated to find a pair of invariants that are convenient to use for analytical determinations of the distribution function in a mirror magnetic field. The energy and the arclength along a flux line for the longitudinal turning point are found to be the most convenient pair of invariants. In terms of these invariants, the distribution function and the density are related to each other by a pair of Abel transforms. The calculation is done for a three-dimensional minimum B field. The simplicity of the model is illustrated with a construction of a sloshing ion distribution with a realistic density profile.