Speed control of oil-hydraulic power take-off system for oscillating body type wave energy converters

The variable displacement oil-hydraulic pumps for the Power Take-Off (PTO) of wave energy converters must work above 80% of maximum displacement in order to have an overall efficiency of approximately 94.5%. This is achieved by controlling their rotational speed when the oil-hydraulic power fluctuates in time. Three speed control strategies have been presented, the first fixing the maximum possible speed in each sea state, the second by slowly varying the pump speed between speed peak values and average ones, and the third by working with highly variable speed reference values. The worst pump efficiency is achieved with the first strategy while the best one with the third strategy. However, the first has less impact than the third one in the pump lifecycle. On the other hand, the second strategy is used to make a trade-off between pump efficiency and lifecycle. However, this paper presents a fourth speed control strategy, which is a hybrid of the second and third strategies. So, the objectives of this paper were to know if these strategies are implementable in a test rig and also on a new PTO concept and determining what modifications should be introduced in these PTO strategies and hardware. This paper also contributes with the application of new methodologies in this field of research for the modelling of pump efficiency and pressure control, such as Neuro-Fuzzy modelling and Fuzzy Logic control systems.     

Power output variations of co-located offshore wind turbines and wave energy converters in California

The electric power generation of co-located offshore wind turbines and wave energy converters along the California coast is investigated. Meteorological wind and wave data from the National Buoy Data Center were used to estimate the hourly power output from offshore wind turbines and wave energy converters at the sites of the buoys. The data set from 12 buoys consists of over 1,000,000 h of simultaneous hourly mean wind and wave measurements. At the buoys, offshore wind farms would have capacity factors ranging from 30% to 50%, and wave farms would have capacity factors ranging from 22% to 29%. An analysis of the power output indicates that co-located offshore wind and wave energy farms generate less variable power output than a wind or wave farm operating alone. The reduction in variability results from the low temporal correlation of the resources and occurs on all time scales. Aggregate power from a co-located wind and wave farm achieves reductions in variability equivalent to aggregating power from two offshore wind farms approximately 500 km apart or two wave farms approximately 800 km apart. Combined wind and wave farms in California would have less than 100 h of no power output per year, compared to over 1000 h for offshore wind or over 200 h for wave farms alone. Ten offshore farms of wind, wave, or both modeled in the California power system would have capacity factors during the summer ranging from 21% (all wave) to 36% (all wind) with combined wind and wave farms between 21% and 36%. The capacity credits for these farms range from 16% to 24% with some combined wind and wave farms achieving capacity credits equal to or greater than a 100% wind farm because of their reduction in power output variability.     

Model predictive control of sea wave energy converters – Part II: The case of an array of devices

This paper addresses model predictive control (MPC) of highly-coupled clusters of sea wave energy converters (WECs). Since each WEC is not only a wave absorber but also a wave generator, the motion of each WEC can be affected by the waves generated by its adjacent WECs when they are close to each other. A distributed MPC strategy is developed to maximize the energy output of the whole array and guarantee the safe operation of all the WECs with a reasonable computational load. The system for an array is partitioned into subsystems and each subsystem is controlled by a local MPC controller. The local MPC controllers run cooperatively by transmitting information to each other. Within one sampling period, each MPC controller performs optimizations iteratively so that a global optimization for the whole array can be approximated. The computational burden for the whole array is also distributed to the local controllers. A numerical simulation demonstrates the efficacy of the proposed control strategy. For the WECs operating under constraints explored, it is found that the optimized power output is an increasing function of degree of WEC–WEC coupling. Increases in power of up to 20% were achieved using realistic ranges of parameters with respect to the uncoupled case.     

Multilevel converters control for renewable energy integration to the power grid

The paper deals with the multilevel converters control strategy for renewable energy resources integration in distribution grids. The proposed control scheme ensures the injection of the generated power in the distribution grid with fast dynamic response, while providing an additional active power filtering capability providing the required harmonic and reactive currents to the considered non-linear loads. The proposed control scheme is applicable to a general multilevel converter and to any types of the renewable energy resources. The control scheme is validated by means of simulations with a three-level diode-clamped converter which interfaces a wind power generation system to a distribution grid supplying non-linear loads. From extensive simulation results, high performance of this control strategy in renewable energy application is demonstrated with reduced total harmonic distortion, increased power factor and compensated load’s reactive powers.     

波浪能发电装置能量转换系统多级负载试验

根据波浪的特点,设计了一种应用在波浪能转换装置上的多级负载系统。对安装该系统的鹰式波浪能转换装置,按照功能和能量转换两种分类方法介绍机构组成和工作原理,并对液压系统的多级负载系统的设计进行了描述,包括系统的元件、试验原理及方法,最后通过试验对系统进行验证。试验结果表明,多级负载系统设计合理,能高效合理地利用及转换液压能。     

Shape optimisation of floating wave energy converters for a specified wave energy spectrum

Ocean wave energy is one of the world's most powerful forms of energy and the energy density in ocean waves is the highest among renewable energy sources. Wave energy converters are employed to harness this energy and convert it into usable electrical energy. However, in order to efficiently extract the energy, the wave energy converter must be optimised in the design stage. Therefore, in this paper, a methodology is presented which aims to optimise the structural geometric configuration of the device to maximise the average power extraction from its intended deployment site. Furthermore, a case study of the Atlantic marine energy test site, off the west coast of Ireland, is undertaken in order to demonstrate the methodology. Using the average annual wave energy spectrum at this site as the input, the optimum structural geometric configuration was established, along with an analysis of the optimum configuration for different radius devices. In addition, the optimum damping coefficient of the PTO mechanism is determined and the total mean absorbed power for the structure at the site over the entire scatter diagram of data is calculated.     

Master-slave wave farm systems based on energy filter with smoothed power output

Wave energy is an important renewable energy source. Previous studies of wave energy conversion (WEC) have focused on the maximum power take-off (PTO) techniques of a single machine. However, there is a lack of research on the energy and power quality of wave farm systems. Owing to the pulsating nature of ocean waves and popular PTO devices, the generated electrical power suffers from severe fluctuations. Existing solutions require extra energy storage and overrated power converters for wave power integration. In this study, we developed a master-slave wave farm system with rotor inertia energy storage; this system delivers self-smoothed power output to the grid and reduces the number of converters. Two control methods based on the moving average filter (MAF) and energy filter (EF) are proposed to smooth the output power of wave farms. RTDS simulations show that the proposed systems and control methods facilitate simple and smooth grid integration of wave energy.     

On the park effect in arrays of oscillating wave energy converters

This paper aims to provide guidelines for designing the layout of arrays of oscillating Wave Energy Converters (WECs) based on a review of the literature of wave interactions and park effect in WEC arrays that has been published over the past 30 years.First, the fundamentals of wave energy absorption by oscillating bodies are summarised, and the principal differences between the park effect in arrays of wave energy converters and wind turbines are highlighted. Then, the numerical approaches commonly used to deal with WEC arrays are outlined briefly and their limitations are discussed. It is argued that, at present, only Boundary Element Methods (BEM) are capable of the appropriate analysis. Finally, previous work on wave interactions and park effect in WEC arrays is reviewed. Similar trends are found in these studies, which allow conclusions to be drawn regarding the significance of the park effect as a function of the number of WECs in the array and their spacing. Based on these conclusions, the following tentative guidelines are proposed:For small arrays of conventional devices (fewer than 10 devices of typical dimension 10–20 m) with usual layouts (regular or shifted grids with separating distance of order 100–200 m), the park effect appears to be negligible. For larger arrays (more than 10 devices), a negative park effect seems to be increasingly important with increasing number of rows (the lines of WECs perpendicular to the incident wave direction). Therefore, the number of rows should remain as small as possible, with a separating distance as large as possible. For arrays of non-conventional WECs (WECs of typical dimensions much larger than 10–20 m), no information has been found. However, trends similar to the previous cases could be expected, provided that aspect ratios are maintained.     

数字化牵引变流器控制系统建模方法研究

针对常规建模方法的仿真系统通常只采用一个时间周期完成所有模块的采样和计算并没有考虑数字控制系统的多信号处理周期时序控制问题,提出一种基于时序控制逻辑的建模方法。该方法模拟数字控制系统的运行时序,对SVM-DTC牵引逆变器控制系统进行建模仿真。结果表明,该方法可以更好地模拟实际数字控制系统的运行状态和计算过程,使仿真结果更接近于实际系统输出,更好地指导控制算法改进和目标系统开发。     

Generalized predictive control for fractional order dynamic model of solid oxide fuel cell output power

An adaptive generalized predictive control (GPC) system is presented for the management of output power of solid oxide fuel cells (SOFCs). The dynamics of SOFC output power are characterized by a fractional order model, which is more accurate than an integer order model to depict the dynamics; the fractional order dynamic model is taken as the controlled plant of the GPC system. The GPC algorithm adopts a linear approximation method that uses a linear predictive model to approximate locally and dynamically the nonlinear dynamics of SOFC output power at each sampling period. Moreover, the parameters of the predictive model are identified online to overcome the time-varying dynamics of SOFC output power via introducing a forgetting factor recursive least squares (FFRLS) algorithm. Finally, according to the future power outputs predicted by the predictive model, an optimal current control sequence is obtained by solving a multistage cost function. The results demonstrate that the dynamic responses of the GPC system are quick and smooth, and the change of the current control sequence is slow and smooth. The quick and smooth dynamics are important for satisfying the rapid load following of SOFC generating systems and for prolonging the lifetime of SOFC stack.     

Seawater lubricated polymer journal bearings for use in wave energy converters

This study investigates the wear characteristics of polymeric journal bearings while immersed in seawater, and their applicability towards wave energy conversion. A block on ring wear machine was used to test four commercially available bearing materials under unidirectional and oscillatory sliding conditions at low pressure. It was observed that wear generally increases with counterface roughness; however, major deviations to this trend exist, depending on the bearing’s composition. Stable wear rates were shown to vary widely depending on velocity profile, though a general trend could not be established for the sample group as a whole. It was shown that polymer wear rates cannot be attributed to any one parameter, and that detailed testing at several characteristic pressures and velocities is needed to determine a material’s applicability towards wave energy converters operating in real seas.     

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.     

Net power and energy output of the German LWR nuclear power system

Net energy analysis is used to calculate the (annual) net energy output and the power and energy efficiencies of the German nuclear power system from the beginning of the construction program until 1982. For each individual power plant, actual construction time, start of operation and annual energy yield data are taken into account. The power expenditure data for construction, first core and reloading are taken from the literature on energy analysis, and the sensitivity of the results to the pertinent data uncertainties is shown. The problem of evaluation of thermal and electric forms of energy is also addressed. The results clearly show that an appreciable fraction of the power produced in the German nuclear power system is consumed within the nuclear industry, especially in the first period of expansion when small power plants are in operation and large ones under construction. This finding is contrary to what may be expected from the small payback time associated with individual power plants. However, nuclear power is not an energy sink. If the issue were to compare a nuclear power system with an alternative option such as an oil or coal-based technology, the same efficiency assessment would have to be carried out for the alternative system before a proper comparison can be made.     

A novel radial self-rectifying air turbine for use in wave energy converters

The flow through the air turbine of an oscillating water column (OWC) wave energy converter is reciprocating and is random and highly variable. It is not surprising that the time-averaged efficiency of the air turbine is substantially lower than that of a conventional turbine working in nearly steady conditions. A new type of radial-flow self-rectifying turbine (named here biradial turbine) is described in the paper. The two inlet/outlet openings of the rotor are axially offset from each other and face radially the surrounding space. The turbine is symmetrical with respect to a plane perpendicular to its axis of rotation. The rotor blades are surrounded by a pair of radial-flow guide-vane rows. Each guide vane row is connected to the rotor by an axisymmetric duct whose walls are flat discs. A two-dimensional flow method is used first as a preliminary design tool for the turbine geometry. More detailed numerical results are then obtained with the aid of a commercial three-dimensional real-fluid CFD code, which allows a more refined geometry optimization to be carried out, and yields results for flow details through the turbine and for the turbine overall performance under several operating conditions.     

基于FMPC的储能系统跟踪光伏发电计划控制策略

为了最大限度地提高跟踪光伏发电计划的能力,文章提出一种适用于储能系统的模糊模型预测控制(FMPC)充放电控制策略。基于模型预测控制(MPC)方法,建立了以储能系统剩余容量偏离理想值、并网功率与发电计划偏差最小为目标的控制模型,通过引入模糊控制(Fuzzy Control),即时调节目标函数中的权重系数以获得最佳跟踪效果。以光伏电站实际运行数据进行仿真分析,结果表明,与传统MPC和普通控制策略相比,所提出的基于FMPC的储能系统控制方法具有独特的灵活性和适应性。     

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.     

提高风出力预测精度的储能系统模糊控制策略

风气象信息的精细化程度不够高会造成风电场风出力预测精度的降低,给电网调度增加了难度。在风电场中配置锌溴电池储能系统是提高风电场日前预报精度的有效措施,对储能装置控制是其关键问题。文章采用模糊控制方法,搭建了储能系统模糊控制规则库,根据电池储能系统荷电状态SOC的变化来控制储能充放电功率;并将所提出的控制策略在新疆达坂城风电场-储能联合发电系统中进行了仿真验证,与传统控制策略进行了对比分析。研究结果表明,采用模糊控制策略的储能系统能够更有效地提高风出力短期预测精度,85%的预测值达到了国家电网要求。