Energy Conversion Concepts Research Articles

Spectral modeling of an oscillating surge wave energy converter with control surfaces

The aim of this research is to use spectral techniques in evaluating the irregular wave performance of a novel wave energy converter concept that combines an oscillating surge wave energy converter with control surfaces. The control surfaces allow the wave energy converter to have a time-varying geometry that enables the hydrodynamic exciting and radiation coefficients to be altered. In the current state of development the device geometry is controlled on a sea state-to-sea state time scale and combined with control of the power take-off (PTO) on a wave-to-wave time scale to maximize power capture, increase capacity factor, and reduce design loads. Analysis begins with the application of linear hydrodynamic theory to evaluate the device performance in terms of absorbed power, foundation loads, and accumulation of fatigue damage on the PTO. To determine the linear PTO damping coefficient that maximizes the time-averaged absorbed power for a given sea state, an optimization problem was constructed while incorporating a motion constraint on the maximum pitch amplitude of motion. The inclusion of the motion constraint prevents linear scaling of the performance results with the significant wave height. Previous studies on the modeling of oscillating surge wave energy converter designs have included nonlinear hydrodynamics. Therefore, a quadratic viscous drag moment was added to the system dynamics through use of a quasi-linear viscous damping coefficient. The same performance quantities were calculated for both the linear and nonlinear models while assuming an irregular wave surface elevation described by a Bretscheider spectrum. One major effect of including the viscous drag moment was flattening of the capture width and structural load curves with respect to the wave spectrum peak frequency while reducing the sensitivity with respect to the significant wave height compared to the linear analysis.

Experimental and numerical comparisons of hydrodynamic responses for a combined wind and wave energy converter concept under operational conditions

The spar torus combination (STC) concept is a combined wind and wave energy converter concept that is composed of a spar floating wind turbine and a torus-shaped, heaving-body wave energy converter (WEC). The WEC is installed on the spar floater. Wave power can be absorbed by a power-take off (PTO) system through the relative heave motions between spar and torus. Numerical model was established to predict dynamic responses of the STC concept ​under different sea states. To validate the numerical model, a model test of the STC concept under operational conditions was performed. A two-body physical model at a 1:50 scaling ratio was built. A series of tests were performed to assess the performance of the concept. During the tests, different PTO damping levels were applied. When large power output was achieved, air compressibility of the PTO damper in the model matters, making relevant a suitable nonlinear PTO modeling in the numerical simulations. Wind conditions were considered to model the effect of the thrust force on the rotor using a wind drag disc. Numerical and experimental results are presented and compared. Good agreements are achieved.

Examination of spent fuel radiation energy conversion for electricity generation

Supply of electricity inside nuclear power plant is one of the most important considerations for nuclear safety and security. In this study, generation of electric energy by converting radiation energy of spent nuclear fuel was investigated. Computational modeling work by using MCNPX 2.7.0 code along with experiment was performed to estimate the amount of electric energy generation. The calculation using the developed modeling work was validated through comparison with an integrated experiment. The amount of electric energy generation based on a conceptual design of an energy conversion module was estimated to be low. But the amount may be useful for nuclear security applications. An alternative way of utilizing the produced electric energy could be considered for nuclear safety application through energy storage. Further studies are needed to improve the efficiency of the proposed energy conversion concept and to examine the issue of radiation damage and economic feasibility.

Interfacial effects on the catalysis of the hydrogen evolution, oxygen evolution and CO2-reduction reactions for (co-)electrolyzer development

Our progressive transition from a society energetically dependent on fossil fuels towards one relying on renewable sources requires novel, environmentally friendly energy conversion and storage concepts. Hydrogen is widely regarded as an energy carrier that could circumvent this need, particularly in sight of the foreseeable spread of fuel cell cars that would use this “renewable H2”. The latter would be produced using electrolyzers, which in their better established form cannot fulfill the targeted H2-price due to the low current densities (<0.5Acmgeom−2) associated to their liquid electrolyte. Alternatively, devices based on proton- and anion-exchange membranes are currently under development, and a new kind of co-electrolysis cell in which CO2 is reduced into hydrocarbons is also envisaged. Electrocatalysts play a crucial role in all of these systems, but the interplay between their surface and the reaction medium (in the so-called interface) is often overlooked in the quest towards better performance. With this motivation, this review discusses the current knowledge of the interfacial catalysis of the three (co-)electrolysis relevant reactions (i.e., the evolution of H2 and O2, and the reduction of CO2). From this, we identify pH and surface oxidation state as the key electrolyte- and surface-related parameters for which further understanding could lead to improved kinetics, and propose strategies for the tentative design of better electrocatalysts.

A Technique for Localized Rapid Soot Oxidation Using Metal Aided Microwave Radiation

The regeneration of soot filter using microwave heating has several drawbacks, which include non-uniform regeneration, ineffective on very small soot thickness, and could lead to excessive heating, which damages the filter that is loaded with high layer of soot. This paper proposes a novel technique for localized rapid heating and oxidation of any thickness of solid soot, and saving of the consumed microwave energy. This technique was applied by inserting a metal in the accumulated soot, located inside a closed multi-mode microwave cavity, attached to a mono-mode waveguide, connected with a magnetron with a capacity of 2.45 GHz and 800 W. The electric field, generated heat, and temperature variation with time till the attaining of the oxidation temperature of the soot during microwave heating were simulated. The simulation methodology is based on the concept of electromagnetic-thermal energy conversion by coupling Maxwell with heat transfer equations. Finite element method was used with frequency domain for electromagnetic analysis and time domain for thermal analysis. In agreement with experimental results, the predicted microwave heating time to attaining the soot ignition/oxidation temperature was reduced from 8.5 to 0.3 seconds. Based on the presented technique, metallic electrodes can be inserted on the soot filter surfaces as local heating electrodes to enhance the microwave heating of the soot filter regeneration.

Comparative experimental study of the survivability of a combined wind and wave energy converter in two testing facilities

Combining the offshore wind and wave energy on an integrated structure or in a farm configuration is beneficial for cost reduction since they could share the infrastructure and the ocean space. The spar torus combination (STC) is a combined wind and wave energy converter concept, which is composed of a spar floating wind turbine and a torus-shape heaving-body wave energy converter (WEC). Numerical simulations have shown a positive synergy between the WEC and the spar floating wind turbine under operational conditions. However, in extreme sea states, it is challenging to maintain structural integrity due to severe wave loads. Three survival modes have been proposed to study the survivability of the STC, and two of them are selected for further study by model testing. Two model tests were performed to investigate the performance of the STC under the two survival modes in extreme conditions: one test is carried out in the CNR-INSEAN towing tank, and the other one is performed in the MARINTEK towing tank. The two survival modes considered are when the torus is fixed to the spar at the mean water level (the MWL mode) and when the torus is fixed to the spar at a submerged position (the SUB mode). In this paper, the two model tests are described and then the measured responses in the two model tests are compared for each survival mode. In addition, the performance of the STC in the two survival modes for each model test is also compared. The differences in the two model tests are explained in detail. The focus of the model tests was wave-induced loads and responses, and wind was also included to model the mean wind thrust on the wind turbine rotor. In the model tests, the rigid body motions in six degree of freedom, the mooring line tensions, and the forces between the spar and torus in three directions (X, Y and Z) were measured. Finally, validation of a numerical model against the model test measurements is also briefly discussed.

Investigating of Helmholtz wave energy converter

This study examines a new concept of wave energy conversion inspired by natural wave pools which are able to trap ocean wave energy into a basin. A novel wave energy converter that uses the Helmholtz resonance frequency is introduced. This device which is called Helmholtz wave energy converter (HWEC) amplifies the oscillation amplitude of a fluid in a basin that is connected to the sea through a narrow channel. The main focus of the current study is to understand the mechanism of wave amplification at Helmholtz resonance frequency in a basin and experimentally investigate the geometry of an ideal wave pool in a wave tank utilizing the theoretical method previously has been done on tidal Helmholtz resonance. The geometry causes amplification in oscillating fluid velocity within the channel at the Helmholtz frequency. Geometric characteristics of HWEC were experimentally studied to determine the dimensions of the basin for inducing Helmholtz resonance. A 1/25th and a 1/7th scale models were used to correlate the Helmholtz frequency with the device geometry. In addition, the effects of the device's basin length and angle of winglets at the inlet of the channel were explored. Furthermore, Particle image velocimetry (PIV) technique was used to determine the velocity fields through the channel of the 1/25th scale model. The results suggest that reducing the angle of the winglets leads to fewer higher order harmonics resulting in better performance of the device in extracting the energy of the incoming waves. It is also shown that the alignment of the axis of the device relative to the incoming waves plays an important role in the overall efficiency of the device. The results of the current study can be used to optimize the design of HWEC devices for extracting wave energy. Moreover, the results reveal that resonance in the channel results in amplification of wave height relative to incoming waves and the corresponding wave power available for absorption. Appropriate location of the power take-off (PTO) in the channel can be empirically determined using PIV data. However this work does not examine the effect of the PTO in the device's energy absorption performance.

On the concept of sloped motion for free-floating wave energy converters.

A free-floating wave energy converter (WEC) concept whose power take-off (PTO) system reacts against water inertia is investigated herein. The main focus is the impact of inclining the PTO direction on the system performance. The study is based on a numerical model whose formulation is first derived in detail. Hydrodynamics coefficients are obtained using the linear boundary element method package WAMIT. Verification of the model is provided prior to its use for a PTO parametric study and a multi-objective optimization based on a multi-linear regression method. It is found that inclining the direction of the PTO at around 50° to the vertical is highly beneficial for the WEC performance in that it provides a high capture width ratio over a broad region of the wave period range.

Experimental and numerical study of hydrodynamic responses of a combined wind and wave energy converter concept in survival modes

The Spar Torus Combination (STC) concept consists of a spar floating wind turbine and a torus-shaped heaving-body wave energy converter (WEC). Numerical simulations have shown a positive synergy between the WEC and the spar floating wind turbine under operational conditions. However, it is challenging to maintain structural integrity under extreme wind and wave conditions, especially for the WEC. To ensure the survivability of the STC under extreme conditions, three survival modes have been proposed. To investigate the performance of the STC under extreme conditions, model tests with a scaling factor of 1:50 were carried out in the towing tank of MARINTEK, Norway. Two survival modes were tested. In both modes, the torus WEC was fixed to the spar. In the first mode, the torus WEC is at the mean water surface, while in the second mode, it is fully submerged to a specified position. The model tests recorded the 6 degrees of freedom (D.O.F.s) rigid body motions, mooring line tensions, and the forces between the spar and torus in 3 directions (X, Y and Z). The wind speed was also measured by a sensor in front of the model and the wind force on the wind turbine disk was measured by a load cell installed on top of the tower. This paper describes the model test setup for the two survival modes, and presents and compares the results from the tests and the numerical simulations. Several nonlinear phenomena were observed during the tests, such as wave slamming, Mathieu instability and vortex induced motion. The current numerical model based on linear potential theory cannot capture these nonlinear phenomena and requires a further development.

Inkjet Printing Meets Electrochemical Energy Conversion.

Inkjet printing is a very powerful digital and mask-less microfabrication technique that has attracted the attention of several research groups working on electrochemical energy conversion concepts. In this short review, an overview is given about recent efforts to employ inkjet printing for the search of new electrocatalyst materials and for the preparation of catalyst layers for polymer electrolyte membrane fuel cell applications. Recent approaches of the Laboratory of Physical and Analytical Electrochemistry (LEPA) at the École Polytechnique Fédérale de Lausanne for the inkjet printing of catalyst layers and membrane electrode assemblies are presented and future energy research directions of LEPA based on inkjet printing in the new Energypolis campus in the Canton of Valais are summarized.

Conceptual Study of Central Receiver Systems with Liquid Metals as Efficient Heat Transfer Fluids

Abstract Concentrated Solar Power plants (CSP) provide several advantages to other energy conversion concepts. However, they are still not competitive in price, compared to conventional power plants. But there is still potential to reduce the cost. The higher heat conductivity of liquid metals leads to very high heat transfer coefficients – ten times higher as with molten salts. This big advantage could increase the efficiency and lower the cost of CSP plants. In order to optimize such a power plant regarding efficiency and cost, detailed information of each component of the system is necessary. This paper shows several system concepts in which liquid metals can be used as heat transfer fluid. In addition, several liquid metal pump types are investigated as well as thermal energy storage concepts. With the assumptions and boundary conditions taken in this model, direct thermal energy storage with liquid metal is prohibitive. But some innovative indirect storage systemswith liquid metals show costs in the same range as with a 2-Tank molten salt storage.

Performance analysis and optimization of a box-hull wave energy converter concept

In this paper we consider a wave energy converter concept which is created by linking a box barge to the mechanical reference by linear dampers. The response to incident wave action in terms of power take-off is expressed explicitly as the solution of a linear frequency-domain model. The simplicity of the model combined with the possibility of the application of theory allows for a nested, and so manageable, procedure of optimization. We find that for any geometry, i.e., a combination of e.g. the breadth-to-length and breadth-to-draught aspect ratios of the box, the optimum is characterized by resonance at least in one of the two degrees of freedom, heave or pitch. Furthermore, optimal geometries turn out to be extremal: either long attenuator-type or wide terminator-type devices perform the best. We find also that optimal wavelengths, which are comparable to the device length in case of attenuators, emerge either due to the progressively increasing buoyancy restoring force characteristic, or due to the finite bandwidth of irregular waves. In particular, diffraction forces are more significant under optimal conditions for performance in irregular seas in comparison with conditions necessary for the most intensive displacement response of the free-floating box barge exposed to regular waves.

Defect states at organic–inorganic interfaces: Insight from first principles calculations for pentaerythritol tetranitrate on MgO surface

Light-responsive organic–inorganic interfaces offer experimental opportunities that are otherwise difficult to achieve. Since laser light can be manipulated very precisely, it becomes possible to engineer selective, predictive, and highly controlled interface properties. Photochemistry of organic–inorganic energetic interfaces is a rapidly emerging research field in which energy absorption and interface stability mechanisms have yet to be established. To explore the interaction of the laser irradiation with molecular materials, we performed first principle calculations of a prototype organic–inorganic interface between a nitroester (pentaerythritol tetranitrate, PETN, C5H8N4O12) and a magnesium oxide (MgO) surface. We found that the light absorption is defined by the band alignment between interface components and interfacial charge transfer coupled with electronic states in the band gap, generated by oxide surface defects. Hence the choice of an oxide substrate and its morphology makes the optical absorption tunable and governs both the energy accumulation and energy release at the interface. The obtained results offer a possible consistent interpretation of experiments on selective laser initiation of energetic materials, which reported that the presence of metal oxide additives triggered the photoinitiation by excitation energy much lower than the band gap. We suggest that PETN photodecomposition is catalyzed by oxygen vacancies (F0 centers) at the MgO surface. Our conclusions predict ways for a complete separation of thermo- and photo-stimulated interface chemistry of molecular materials, which is imperative for highly controllable fast decomposition and was not attainable before. The methodology described here can be applied to any type of molecular material/wide band gap dielectric interfaces. It provides a solid basis for novel design and targeted improvements of organic–inorganic interfaces with desired properties that promise to enable vastly new concepts of energy storage and conversion, photocatalysis, and molecular electronics.

Reliability-Based Structural Optimization of Wave Energy Converters

More and more wave energy converter (WEC) concepts are reaching prototypelevel. Once the prototype level is reached, the next step in order to further decrease thelevelized cost of energy (LCOE) is optimizing the overall system with a focus on structuraland maintenance (inspection) costs, as well as on the harvested power from the waves.The target of a fully-developed WEC technology is not maximizing its power output,but minimizing the resulting LCOE. This paper presents a methodology to optimize thestructural design of WECs based on a reliability-based optimization problem and the intentto maximize the investor’s benefits by maximizing the difference between income (e.g., fromselling electricity) and the expected expenses (e.g., structural building costs or failure costs).Furthermore, different development levels, like prototype or commercial devices, may havedifferent main objectives and will be located at different locations, as well as receive varioussubsidies. These points should be accounted for when performing structural optimizationsof WECs. An illustrative example on the gravity-based foundation of the Wavestar deviceis performed showing how structural design can be optimized taking target reliability levelsand different structural failure modes due to extreme loads into account.

Investigation of the effect of the structure of large-area carbon nanotube/fuel composites on energy generation from thermopower waves.

Thermopower waves are a recently developed energy conversion concept utilizing dynamic temperature and chemical potential gradients to harvest electrical energy while the combustion wave propagates along the hybrid layers of nanomaterials and chemical fuels. The intrinsic properties of the core nanomaterials and chemical fuels in the hybrid composites can broadly affect the energy generation, as well as the combustion process, of thermopower waves. So far, most research has focused on the application of new core nanomaterials to enhance energy generation. In this study, we demonstrate that the alignment of core nanomaterials can significantly influence a number of aspects of the thermopower waves, while the nanomaterials involved are identical carbon nanotubes (CNTs). Diversely structured, large-area CNT/fuel composites of one-dimensional aligned CNT arrays (1D CNT arrays), randomly oriented CNT films (2D CNT films), and randomly aggregated bulk CNT clusters (3D CNT clusters) were fabricated to evaluate the energy generation, as well as the propagation of the thermal wave, from thermopower waves. The more the core nanostructures were aligned, the less inversion of temperature gradients and the less cross-propagation of multiple thermopower waves occurred. These characteristics of the aligned structures prevented the cancellation of charge carrier movements among the core nanomaterials and produced the relative enhancement of the energy generation and the specific power with a single-polarity voltage signal. Understanding this effect of structure on energy generation from thermopower waves can help in the design of optimized hybrid composites of nanomaterials and fuels, especially designs based on the internal alignment of the materials. More generally, we believe that this work provides clues to the process of chemical to thermal to electrical energy conversion inside/outside hybrid nanostructured materials.

Advanced thermopower wave in novel ZnO nanostructures/fuel composite.

Thermopower wave is a new concept of energy conversion from chemical to thermal to electrical energy, produced from the chemical reaction in well-designed hybrid structures between nanomaterials and combustible fuels. The enhancement and optimization of energy generation is essential to make it useful for future applications. In this study, we demonstrate that simple solution-based synthesized zinc oxide (ZnO) nanostructures, such as nanorods and nanoparticles are capable of generating high output voltage from thermopower waves. In particular, an astonishing improvement in the output voltage (up to 3 V; average 2.3 V) was achieved in a ZnO nanorods-based composite film with a solid fuel (collodion, 5% nitrocellulose), which generated an exothermic chemical reaction. Detailed analyses of thermopower waves in ZnO nanorods- and cube-like nanoparticles-based hybrid composites have been reported in which nanostructures, output voltage profile, wave propagation velocities, and surface temperature have been characterized. The average combustion velocities for a ZnO nanorods/fuel and a ZnO cube-like nanoparticles/fuel composites were 40.3 and 30.0 mm/s, while the average output voltages for these composites were 2.3 and 1.73 V. The high output voltage was attributed to the amplified temperature in intermixed composite of ZnO nanostructures and fuel due to the confined diffusive heat transfer in nanostructures. Moreover, the extended interfacial areas between ZnO nanorods and fuel induced large amplification in the dynamic change of the chemical potential, and it resulted in the enhanced output voltage. The differences of reaction velocity and the output voltage between ZnO nanorods- and ZnO cube-like nanoparticles-based composites were attributed to variations in electron mobility and grain boundary, as well as thermal conductivities of ZnO nanorods and particles. Understanding this astonishing increase and the variation of the output voltage and reaction velocity, precise ZnO nanostructures, will help in formulating specific strategies for obtaining enhanced energy generation from thermopower waves.

Pyroelectric Crystals: Structure, Properties and Applications

As defined by the IUCr, a material is a crystal if it has essentially a sharp diffraction pattern. Crystalline materials are wide spread in our today's life. More than 98 % of the solid fraction of the earth comprises crystalline matter, most of which are oxides. Single-crystals in particular are the basis for many applications – lasers, LEDs, sensors, etc. – and play an important role in fundamental research – for instance in materials science. The discipline that elucidates the impact of the crystal structure on the physical properties of particularly crystalline materials – crystallography – is of specific importance for the design of new materials. Moreover, crystallography can be utilized to establish new concepts and thus may contribute solving today's challenges in science and technology. Several technologies exist for the conversion of electric energy into, e. g. heat, light and motion or vice versa. In this context, this work highlights an approach based on the crystal coupling phenomenon pyroelectricity that can be adopted for energy conversion concepts. By means of pyroelectric crystals, waste heat can be converted into surface charges, which provide a manifold of applications. First, a comprehensive overview of more than 3000 known pyroelectric materials is given, including a categorization in terms of properties and crystallographic characteristics. In order to provide a high pyroelectric coefficient at certain temperatures, taking economic, ecologic and further material properties into account, promising materials are suggested and verified by a computer controlled thermal/electrical stimulation set-up. Several possible applications as, for instance, disinfection [1], and anti-icing are presented. Finally, an approach to convert waste heat into chemical energy, i. e. the generation of hydrogen, is introduced.

Nanowires enabling strained photovoltaics

Photovoltaic nano-devices have largely been relying on charge separation in conventional p-n junctions. Junction formation via doping, however, imposes major challenges in process control. Here, we report on a concept for photovoltaic energy conversion at the nano scale without the need for intentional doping. Our approach relies on charge carrier separation in inhomogeneously strained germanium nanowires (Ge NWs). This concept utilizes the strain-induced gradient in bandgap along tapered NWs. Experimental data confirms the feasibility of strain-induced charge separation in individual vapor-liquid-solid grown Ge NW devices with an internal quantum efficiency of ∼5%. The charge separation mechanism, though, is not inherently limited to a distinct material. Our work establishes a class of photovoltaic nano-devices with its opto-electronic properties engineered by size, shape, and applied strain.

Photo-Ionic Cells: Two Solutions to Store Solar Energy and Generate Electricity on Demand

A solar energy conversion concept based on the photoinduced separation of a pair of redox species in a biphasic liquid cell is presented. The redox pair is subsequently discharged in an electrochemical flow cell to generate electricity. To illustrate this generic concept, we have revisited the thionine/cobalt EDTA system where, upon light excitation, the excited thionine dye is quenched in the aqueous solution by the (Co(II)EDTA) 2� complex to form both (Co(III)EDTA) � and reduced thionine, namely leucothionine, that partitions to the organic phase. As a result, solar energy is converted to a redox pair, leucothionine/(Co(III)EDTA) � . The two immiscible liquid phases are separated, and the redox energy is stored in the respective electrolyte solutions. These two solutions can then be electrochemically discharged in a flow cell to generate electricity on demand. The electrode reactions involved are the reoxidation of leucothionine to thionine in the organic solvent and the reduction of the Co(III) complex in water.

Variable Speed Wind Turbines Based on Electromechanical Differential Systems

This paper introduces a new electromechanical energy conversion concept for use in wind-based power generation systems. Modern wind turbines use frequency converters to meet the speed control requirement. But voltage source-based frequency converters typically have limited overload capability. In addition, they fully or partially decouple the rotating masses of wind energy converter systems, thereby removing altogether or reducing the inertia as a medium of energy storage and modulation from the perspective of the grid during transient conditions. In the proposed concept, a drive system based on electromechanical differential gear including a servo motor is used for generator torque control, while using a synchronous machine as the main generator. The system thus provides the speed variability required for optimal utilization of wind energy combined with the advantages of a directly grid-connected synchronous generator. It as a result permits the supply of high currents during faults to the grid to provide more effective voltage support as there are no electronic circuits in the main power path, which would limit the overload capability. Furthermore, the inertia of the system comes fully to bear, thus contributing to the overall grid inertia. The performance of the system has been studied experimentally on a prototype system and the results of the conceptual analysis have been verified. Using the mathematical model of the system, a grid fault simulation has been performed, and the response during fault and the damping behavior has been studied. The results show the excellent damping behavior due to the active damping control by the differential drive.