Experimental study on hydrodynamic performance of a wave energy converter within multi‐heaving‐buoys

A compact buoy‐array‐type wave energy converter called multi‐heaving‐buoys (MHB) is introduced in this study. The hydrodynamic performance of MHB under regular wave conditions was first investigated experimentally in a wave tank located in Ocean University of China. It was found that a limited number of heaving buoys had little effect on the wave fields around the device. The small period of the incident waves caused an intense interaction between the waves and the buoys. The phase difference between the buoys in different rows was determined by the distance between the buoys. It was found that the response amplitude operator of the buoys varied from 0.6 to 1.2. Correspondingly, the range of the averaged relative velocity of the heaving buoys was 0.6–1.3. The upper limit of the acceleration of the buoys' motion was 0.2 times that of gravity. All of the experimental results provide valuable information for the future design of the hydraulic pressure power take‐off systems. Copyright © 2017 John Wiley & Sons, Ltd.     

A review on front end conversion in ocean wave energy converters

Harvesting the energy from ocean waves is one of the greatest attractions for energy engineers and scientists. Till date, plenty of methods have been adopted to harvest the energy from the ocean waves. However, due to technological and economical complexity, it is intricate to involve the majority of these energy harvesters in the real ocean environment. Effective utilization and sustainability of any wave energy harvester depend upon its adaptability in the irregular seasonal waves, situation capability in maximum energy extraction and finally fulfilling the economic barriers. In this paper, the front end energy conversions are reviewed in detail which is positioned in the first stage of the wave energy converter among other stages such as power take off (PTO) and electrical energy conversion. If the recent development of these front end energy conversion is well known then developing wave energy converter with economic and commercial viability is possible. The aim of this review is to provide information on front end energy conversion of a point absorber and emphasize the strategies and calamity to be considered in designing such kinds of devices to improve the energy harvesting competence. This will be useful to the engineers for speeding up the development of a matured point absorbing type wave energy converter.     

Novel ocean energy permanent magnet linear generator buoy

This paper describes the research, design, construction and prototype testing process of a novel ocean energy direct drive permanent magnet linear generator buoy. The buoy employs the vertical component of the motion of ocean waves to power a linear generator. The generator consists of a permanent magnet field system (mounted on the central translator shaft) and an armature, in which the power is generated (mounted on the buoy). The translator shaft is anchored to the sea floor, and the buoy/floater moves armature coils relative to the permanent magnet translator to induce voltages. The electrical and mechanical structures of the buoy generator are provided, along with performance characteristics, including voltage, current and developed power.     

Testing and control of a power take-off system for an oscillating-water-column wave energy converter

The paper concerns the development of the PTO (power take-off) control of an OWC (oscillating-water-column) spar-buoy wave energy converter. The OWC spar-buoy is an axisymmetric device consisting of a submerged vertical tail tube open at both ends, rigidly fixed to a floater that moves essentially in heave. The oscillating motion of the internal free surface relative to the floater-tube set, produced by the incident waves, makes the air flow through a novel self-rectifying air turbine: the biradial turbine. To reduce the losses of the PTO system at partial load, an electrical generator with a rated power twice the maximum expected average power conversion of the buoy was adopted. The control of the turbine-generator set under highly energetic sea-state conditions was experimentally investigated by means of tests performed in a PTO test rig. In the reported tests, the hydrodynamics of the OWC spar-buoy and the aerodynamics of the air turbine were numerically simulated in real-time and coupled with the experimental model of the turbine/electrical generator set in a hardware-in-the-loop configuration. The experimental results allowed the dynamic behaviour of the PTO to be characterized and provided validation of the proposed control algorithms that ensure operation within safe limits.     

Steady-state analysis of a conceptual offshore wind turbine driven electricity and thermocline energy extraction plant

A system for using offshore wind energy to generate electricity and simultaneously extract thermal energy is proposed. This concept is based on an offshore wind turbine driven hydraulic pump supplying deep seawater under high pressure to a land based plant consisting of a hydroelectric power generation unit and heat exchanger. A steady-state system model is developed using empirical formulae. The mathematical model comprises the fundamental system sub-models that are categorised as the rotor, hydraulic pump, pipeline, hydroelectric turbine and heat exchanger. A means for modelling the seawater temperature field across a two-dimensional bathymetry is also discussed. These mathematical models are integrated into a computational tool and a brief parametric static analysis is undertaken. The results illustrate the effect of pipeline diameter, rotational speed of the grid connected hydroelectric turbine, and the turbine distance from shore on the overall performance of the system. Through adequate parameter selection, the total rate of energy output for such a system, consisting of both electricity and thermal energy, is shown to increase by as much as 84%, when compared to a conventional wind turbine having an identical rotor diameter but which supplies only electrical energy.     

Feasibility of hydraulic power recovery from waste energy in bio-gas scrubbing processes

This paper investigates the feasibility of recovering waste energy from typical bio-gas upgrading facilities by means of a hydraulic turbine, and presents analysis of different types of hydraulic power recovery turbines. A selection method and analysis is developed which can be applied to an existing process to determine the effectiveness and energy savings of such a system for its economic viability. A practical testing rig was designed and constructed to verify the reliability and consistency of data for the both selection and optimization techniques. It was found that a centrifugal pump operating in reverse flow, essentially as a turbine, could be a possible option for waste energy recovery.     

A novel direct-drive ocean wave energy extraction concept with contact-less force transmission system

This paper describes the design and testing of a new rotary direct-drive ocean wave energy extraction system. The device employs a contact-less force transmission system (CFTS) to couple a float to the power take-off (PTO) mechanism made up of a ball screw, unidirectional clutch and a permanent magnet generator. Comprehensive simulation, design and testing of both the force transmission system and the ocean energy extraction system are presented along with preliminary wave flume test results of the buoy in irregular waves.     

Design and performance evaluation of a pump-as-turbine micro-hydro test facility with incorporated inlet flow control

In the context of micro-hydro power schemes the initial cost of conventional Francis turbine units is often prohibitive. As such there is growing interest in pump-as-turbine (PAT) technology offering a more cost effective, yet still highly efficient, power generating alternative, finding uses in remote area power supply and energy recovery systems. However, the implementation of a PAT is highly problematic in terms of predicting the installed best operating point coupled with poor off-design performance due to the fixed geometry and absence of inlet flow control. In the current work a micro-hydro test facility and turbine unit is developed utilising a commercially available pump impeller together with a customised housing for incorporation of inlet flow control. Working initially from established PAT theory, this paper presents the design and performance testing of a hydraulic turbine unit suitable for use in rural micro-hydro, and energy recovery installations. Maximum efficiency of the unit was found to be 79%, marginally higher than that of the parent pump, while the off-design efficiency offered considerable improvement over previously published data of traditional PAT systems. The design provides a cost effective power generator in comparison to small scale Francis turbines, while providing a greater operational range than traditional PAT units.     

Mass and energy storage analysis of an absorption heat pump with simulated time dependent generator heat input

This communication presents an assessment of the feasibility of energy storage via refrigerant mass storage within an absorption cycle heat pump with simulated time dependent generator heat input. The system consists of storage volumes with the condenser and absorber of the conventional absorption cycle heat pump to store liquid refrigerant, weak and strong solutions during the generation period, which are required for the heat pump operation during the generation off period. A time dependent mass and energy storage analysis based on mass and energy balance equations for various components of the heat pump system has been carried out to evaluate energy storage concentration and storage efficiency for combined and separate storage schemes for the weak and strong solutions. Two possible performance modes, viz constant pumping ratio or the constant flow of the strong solution from the absorber to the generator have been analysed: the latter is preferable over the former from a practical point of view. Numerical computer simulation has been made for a typical winter day in Melbourne (Australia) with the desired heating load specified. It is found that the concept of refrigerant storage within the absorption cycle heat pump is technically feasible for efficient space heating. The energy storage concentration in the condenser store is slighly higher while that in absorber store is slightly lower for the separate storage mode as compared to the combined storage. However, the combined storage has an advantage of less storage volume and hence is more cost effective than separate storage and the disadvantage of limited system operation due to the decrease of solution concentrations.     

Wells turbine blade profile optimization for better wave energy capture

Despite the fact that wave energy is available at no cost, it is always desired to harvest the maximum possible amount of this energy. The axial flow air turbines are commonly used with oscillating water column devices as a power take‐off system. The present work introduces a blade profile optimization technique that improves the air turbine performance while considering the complex 3D flow phenomena. This technique produces non‐standard blade profiles from the coordinates of the standard ones. It implements a multi‐objective optimization algorithm in order to define the optimum blade profile. The proposed optimization technique was successfully applied to a biplane Wells turbine in the present work. It produced an optimum blade profile that improves the turbine torque by up to 9.3%, reduces the turbine damping coefficient by 10%, and increases the turbine operating range by 5%. The optimized profile increases the annual average turbine power by up to 3.6% under typical sea conditions. Moreover, new blade profiles were produced from the wind turbine airfoil data and investigated for use with the biplane Wells turbine. The present work showed that two of these profiles could be used with low wave energy seas. Copyright © 2017 John Wiley & Sons, Ltd.     

Experimental investigation and ANN modeling on improved performance of an innovative method of using heave response of a non-floating object for ocean wave energy conversion

To convert wave energy into usable forms of energy by utilizing heaving body, heaving bodies (buoys) which are buoyant in nature and float on the water surface are usually used. The wave exerts excess buoyancy force on the buoy, lifting it during the approach of wave crest while the gravity pulls it down during the wave trough. A hydraulic, direct or mechanical power takeoff is used to convert this up and down motion of the buoy to produce usable forms of energy. Though using a floating buoy for harnessing wave energy is conventional, this device faces many challenges in improving the overall conversion efficiency and survivability in extreme conditions. Up to the present, no studies have been done to harness ocean waves using a non-floating object and to find out the merits and demerits of the system. In the present paper, an innovative heaving body type of wave energy converter with a non-floating object was proposed to harness waves. It was also shown that the conversion efficiency and safety of the proposed device were significantly higher than any other device proposed with floating buoy. To demonstrate the improvements, experiments were conducted with non-floating body for different dimensions and the heave response was noted. Power generation was not considered in the experiment to observe the worst case response of the heaving body. The device was modeled in artificial neural network (ANN), the heave response for various parameters were predicted, and compared with the experimental results. It was found that the ANN model could predict the heave response with an accuracy of 99%.     

Measurements on a PV solar pump equipped with a piston pump with a matching valve

The work on a simple high efficient solar pump equipped with a piston pump with a matching valve, reported at the Solar World Congress in Budapest, has been continued. Quasi-static and dynamic models of the solar pump have been derived with which the operation of the system is simulated. A test rig has been built at ECN in Petten (The Netherlands) to perform some preliminary measurements. These tests show that the piston with a matching valve indeed ensures a good matching of the system components over a wide range of solar insolations. Daily average solar panel maximum power point tracking efficiencies of over 80% were measured. Daily average subsystem efficiencies (solar panel power output to hydraulic power output) measured were about 40%, which is lower than expected. These low efficiencies were caused by substantial power losses in the motor, transmission and pump. The losses can easily be reduced by appropriate design of each component; subsystem efficiencies of 50% should be attainable.     

Performance investigation of a novel near-isothermal compressed air energy storage system with stable power output

As one of the grid-scale energy storage technologies, compressed air energy storage (CAES) is promising to facilitate the permeability of renewable energies. By integrating CAES into renewable sources, the fluctuation and intermittence of renewable energies could be effectively restrained. Among various CAES system configurations, isothermal CAES (I-CAES) is considered as a most competitive technology with expected high efficiency. However, most of the existing I-CAES systems have trouble in keeping a stable power output. To address this issue, a novel near-isothermal CAES system is proposed in this article to acquire a near stable power output. Imitating the concept of hydraulic accumulator, a two pressure vessels structure is employed to maintain the gas pressure stable during discharging. Besides, the turbine power output can be controlled by adjusting the liquid flow rate of the Pelton turbine under this near constant pressure condition. Based on the system transient model and economic model, the system components transient behavior, parametric analysis, off-design performance analysis and economic evaluation issues are also conducted. Results show that system round trip efficiency (RTE) with 61.42% and energy density (ED) with 0.2015 kWh/m³ can be achieved under design condition. In the discharge process, the gas pressure in vessel varies in a small range, from 68 to 72 bar, which is relatively stable. The power output from Pelton turbine can be maintained around 1 kW. Meanwhile, the initial pressure, the pipe diameter, and the spraying flow rates of circulating pumps have significant effects on system RTE and ED. Furthermore, the Pelton turbine power output level can be adjusted by adding jets number, and the higher storage pressure can make the power output unsteady.     

A linearized model for estimating the performance of submerged resonant wave energy converters

A realistic performance analysis of oscillating water column wave energy converters (WECs) addresses to a set of non-linear differential equations that need to be integrated in time, by using a stochastic approach, under the hypothesis of random wind-generated sea waves, for all the sea states which characterize the location of the system. Non-linearities of the differential equations have several origins:

• minor and major losses of the unsteady flow of water and air;

• compressibility of air and heat exchange with the walls of the air chamber;

• non-linear characteristics of the turbine.

Under the hypothesis of random sea waves with Gaussian distribution, the authors propose an original methodology for linearizing the differential equations that describe the flow motion inside a wholly submerged WEC. Under such hypothesis, the linearized model can be used for predicting the power output by means of the calculations in the frequency domain and for control design. The developed methodology has been applied to the estimation of the performance of the new “resonant sea wave energy converters”, called REWEC, patented by Boccotti in 1998, and consisting of several caissons, characterized by a structure similar to the caissons of the traditional breakwaters and placed on the seabed, close one to each other, to form a submerged breakwater. Each caisson is connected to a vertical duct wholly beneath the sea level, where a hydraulic Wells turbine is placed.The matching between turbine and resonance characteristic of the system is carefully analysed in order to maximize the energy conversion efficiency.Some results, given for a small installation in the Mediterranean sea, confirm that the REWEC system is able to absorb a large share of the incident wave energy due to a very simple regulation system which permits the tuning on sea states with different significant heights.     

液压自由活塞发动机的能量平衡分析

液压自由活塞发动机是将一次动力机－内燃机与二次动力机－液压泵的集成为一体，以液体为工作介质，利用油液压力能来实现动力非刚性传输的复合发动机，针对稳态运转的液压自由活塞发动机，分析了其能量输入，耗散，分配及输出情况，给出了各能量组分所占的比例，并估算了活塞的最大运动速度和负载压力，为HFPE的结构设计，控制和效率提高指明方向。     

A twin unidirectional impulse turbine for wave energy conversion

A twin unidirectional impulse turbine has been proposed in order to enhance the performance of wave energy plant.This turbine system uses two unidirectional impulse turbines and their flow direction is different from each other.However,the turbine characteristics have not been clarified to date.The performances of a unidirectional impulse turbine under steady flow conditions were investigated experimentally by using a wind tunnel with large piston/cylinder in this study.Then,efficiency of the twin impulse turbine have been estimated by a quasi-steady analysis using experimental results.     

Development and thermodynamic assessment of a novel solar and biomass energy based integrated plant for liquid hydrogen production

In this paper, a combined power plant based on the dish collector and biomass gasifier has been designed to produce liquefied hydrogen and beneficial outputs. The proposed solar and biomass energy based combined power system consists of seven different subplants, such as solar power process, biomass gasification plant, gas turbine cycle, hydrogen generation and liquefaction system, Kalina cycle, organic Rankine cycle, and single-effect absorption plant with ejector. The main useful outputs from the combined plant include power, liquid hydrogen, heating-cooling, and hot water. To evaluate the efficiency of integrated solar energy plant, energetic and exergetic effectiveness of both the whole plant and the sub-plants are performed. For this solar and biomass gasification based combined plant, the generation rates for useful outputs covering the total electricity, cooling, heating and hydrogen, and hot water are obtained as nearly 3.9 MW, 6584 kW, 4206 kW, and 0.087 kg/s in the base design situations. The energy and exergy performances of the whole system are calculated as 51.93% and 47.14%. Also, the functional exergy of the whole system is calculated as 9.18% for the base working parameters. In addition to calculating thermodynamic efficiencies, a parametric plant is conducted to examine the impacts of reference temperature, solar radiation intensity, gasifier temperature, combustion temperature, compression ratio of Brayton cycle, inlet temperature of separator 2, organic Rankine cycle turbine and pump input temperature, and gas turbine input temperature on the combined plant performance.     

A heaving point absorber‐based triboelectric‐electromagnetic wave energy harvester: An efficient approach toward blue energy

This paper presents a hybridized triboelectric‐electromagnetic generator based on heaving point absorbers to harvest the energy of water waves. The device consists of a cylindrical freestanding grating triboelectric generator (TENG) and a 3‐phase tubular electromagnetic generator (EMG). The proposed system incorporates a slider which is capable of moving through a stator under the motion of a floating buoy. The floating component can heave up and down while facing water waves without being affected by the wave direction. The performance of the TENG and EMG units and corresponding electrical outputs are evaluated under various structural, dynamical, and electrical conditions. It is shown that the number of segments in the TENG unit, phase number in the EMG unit, and motion frequency in both harvesters are the key elements in the outputs of the hybridized system. For the first time, the effect of irregular wave motion on the TENG harvester performance is systematically explored using a well‐known wave spectrum. Also, the performance of the hybridized system for charging a storage unit is evaluated in details. The presented energy harvester shows a great potential toward harvesting the energy of water waves as well as hydrodynamic sensing applications. In addition, this research provides a framework for the exploration of irregular wave motion in TENG‐based energy harvesters.     

单浮筒振荡浮子式波浪能装置的动力特性分析

针对振荡浮子式波浪能利用技术,提出一种单浮筒式波浪转换设计方案,对单浮筒随波浪的运动特性展开研究,将波浪能转换为振荡浮筒的摆动机械能,传递给PTO能量转换系统。通过采用Star-CCM流体仿真软件分析浮筒装置在不同PTO能量转换系统参数下的运动特性及受力情况,得到不同弹簧阻尼工况下浮筒装置的运动特性,以期为波浪能技术的装置结构优化及真实海况运行提供理论基础。     

漂浮式波浪发电系统波浪能模拟及参数计算

基于线性叠加法模拟了不同风速下的波浪波形,提取了波浪力能,分析了浮筒的受力情况,进而在保证漂浮式波浪发电系统稳定运行的情况下计算了不同波浪浪高下的浮筒尺寸、质量与发电机功率之间的关系,并应用仿真软件进行了仿真研究,结果对漂浮式波浪发电系统的各参数设计具有重要的指导意义。