Partial Load Range Research Articles

Increased power gains from wake steering control using preview wind direction information

Abstract. Yaw controllers typically rely on measurements taken at the wind turbine, resulting in a slow reaction to wind direction changes and subsequent power losses due to misalignments. Delayed yaw action is especially problematic in wake steering operation because it can result in power losses when the yaw misalignment angle deviates from the intended one due to a changing wind direction. This study explores the use of preview wind direction information for wake steering control in a two-turbine setup with a wind speed in the partial load range. For these conditions and a simple yaw controller, results from an engineering model identify an optimum preview time of 90 s. These results are validated by forcing wind direction changes in a large-eddy simulation model. For a set of six simulations with large wind direction changes, the average power gain from wake steering increases from only 0.44 % to 1.32 %. For a second set of six simulations with smaller wind direction changes, the average power gain from wake steering increases from 1.24 % to 1.85 %. Low-frequency fluctuations are shown to have a larger impact on the performance of wake steering and the effectiveness of preview control, in particular, than high-frequency fluctuations. From these results, it is concluded that the benefit of preview wind direction control for wake steering is substantial, making it a topic worth pursuing in future work.

Suitability of a Profile with Tubercles for Axial Pumps—Investigation Using Flow Simulation

Even if wind tunnel tests and simulations have confirmed that tubercles can influence the behaviour of a profile, research in the field of axial pumps has so far been lacking. However, previous studies cannot be transferred to the application of axial pumps, since the requirements for the profile geometry as well as the Reynolds number range differ. The present study aims to address this research gap by performing a CFD simulation with a profile common for axial pumps, the Goe11K, testing four different tubercle configurations. At the same time, this simulation is a preliminary study for experimental tests. The results show that certain tubercle configurations improve the behaviour of the profile in the post-stall area, i.e., increase the lift of the profile at large angles of attack (α). In general, the curve of the profiles with tubercles runs more evenly, without the drastic drop in lift. This improved property comes at the expense of lower maximum lift and increased drag at lower α. With regard to the use of axial pumps, it can be concluded that there are advantages, particularly in the partial load range. These could ultimately enlarge the operation range of an axial pump.

Atomic Pt‐N4 Sites in Porous N‐Doped Nanocarbons for Enhanced On‐Site Chlorination Coupled with H2 Evolution in Acidic Water

AbstractAcidic water electrolysis (AWE) has the potential to revolutionize green H2 generation with flexible partial load range, high gas purity, and rapid system response. However, the extensive usage of noble Ru/Ir metals and sluggish oxygen evolution reaction (OER) with inexpensive O2 products pose significant challenges in anodes. Herein, it is demonstrated that ultralow Pt single atoms in highly porous N‐doped carbons (Pt1/p‐NC@CNTs) can effectively catalyze chlorine evolution reaction (CER) for on‐site chlorination to replace OER in AWE, with 200 mV potential saving at 10 mA cm−2. As a result, various organic halide motifs of pharmaceutical molecules by chlorinating anisole, ketones, and olefins can be realized, along with H2 coproduction. Combined physicochemical characterizations including synchrotron X‐ray absorption spectroscopy, finite element methodsimulations, and theory calculations indicate that atomic Pt‐N4 active sites balance the adsorption/desorption of Cl intermediates (Volmer step) and the plentiful porosity of Pt1/p‐NC@CNTs with high specific surface area of 313 m2 g−1 enriches Cl− around active sites (Heyrovský step), collectively promoting the rate‐limiting Volmer–Heyrovský pathway for improved CER.

An investigation of spatial wind direction variability and its consideration in engineering models

Abstract. We propose that considering mesoscale wind direction changes in the computation of wind farm cluster wakes could reduce the uncertainty of engineering wake modeling tools. The relevance of mesoscale wind direction changes is investigated using a wind climatology of the German Bight area covering 30 years, derived from the New European Wind Atlas (NEWA). Furthermore, we present a new solution for engineering modeling tools that accounts for the effect of such changes on the propagation of cluster wakes. The mesoscale wind direction changes relevant to the operation of wind farm clusters in the German Bight are found to exceed 11∘ in 50 % of all cases. Particularly in the lower partial load range, which is associated with strong wake formation, the wind direction changes are the most pronounced, with quartiles reaching up to 20∘. Especially on a horizontal scale of several tens of kilometers to 100 km, wind direction changes are relevant. Both the temporal and spatial scales at which large wind direction changes occur depend on the presence of synoptic pressure systems. Furthermore, atmospheric conditions which promote far-reaching wakes were found to align with a strong turning in 14.6 % of the cases. In order to capture these mesoscale wind direction changes in engineering model tools, a wake propagation model was implemented in the Fraunhofer IWES wind farm and wake modeling software flappy (Farm Layout Program in Python). The propagation model derives streamlines from the horizontal velocity field and forces the single turbine wakes along these streamlines. This model has been qualitatively evaluated by simulating the flow around wind farm clusters in the German Bight with data from the mesoscale atlas of the NEWA and comparing the results to synthetic aperture radar (SAR) measurements for selected situations. The comparison reveals that the flow patterns are in good agreement if the underlying mesoscale data capture the velocity field well. For such cases, the new model provides an improvement compared to the baseline approach of engineering models, which assumes a straight-line propagation of wakes. The streamline and the baseline models have been further compared in terms of their quantitative effect on the energy yield. Simulating two neighboring wind farm clusters over a time period of 10 years, it is found that there are no significant differences across the models when computing the total energy yield of both clusters. However, extracting the wake effect of one cluster on the other, the two models show a difference of about 1 %. Even greater differences are commonly observed when comparing single situations. Therefore, we claim that the model has the potential to reduce uncertainty in applications such as site assessment and short-term power forecasting.

Influence of drivetrain efficiency determination on the torque control of wind turbines

Decreasing the levelized cost of energy is a major design objective for wind turbines. Accordingly, the control is generally optimized to achieve a high energy production and a high-power coefficient. In partial load range, speed and torque are controlled via the generator torque but the rotor torque determines the power coefficient of the turbine. High uncertainties for the uncalibrated low-speed shaft torque measurement and varying drivetrain efficiencies which depend on the speed, load and temperature lead to a torque control error that reduces the power coefficient of the wind turbine. In this paper the rotor torque control error and the impact on the power coefficient of wind turbines is quantified. For this purpose, the variation of drivetrain efficiency is analyzed. An efficiency model for the wind turbine drivetrain is build and validated on the test bench. Then, the influence of the drivetrain speed, torque loads, non-torque loads, and temperature on the efficiency is quantified. Finally, the influence of the rotor torque control error on the power coefficient was simulated with an aerodynamic model. The results show that of all examined influences only torque and temperature significantly impacting the efficiency leading to rotor torque control errors that reduce the power coefficient and consequently increase the levelized cost of energy. Improved efficiency measurement on WT test benches or drivetrain efficiency modelling can reduce the rotor torque control error and therefore decrease the LCOE.

Inductive Electrically Excited Synchronous Machine for Electrical Vehicles—Design, Optimization and Measurement

The demand for electric machines has been rising steadily for several years—mainly due to the move away from the combustion engine. Synchronous motors with rare earth permanent magnets are widely used due to their high power densities. These magnets are cost-intensive, cost-sensitive and often environmentally harmful. In addition to dispensing with permanent magnets, electrically excited synchronous machines offer the advantage of an adjustable excitation and, thus, a higher efficiency in the partial load range in field weakening operation. Field weakening operation is relevant for the application of vehicle traction drive. The challenge of this machine type is the need for an electrical power transfer system, usually achieved with slip rings. Slip rings wear out, generate dust and are limited in power density and maximum speed due to vibrations. This article addresses an electrically excited synchronous machine with a wireless power transfer onto the rotor. From the outset, the machine is designed with a wireless power transfer system for use in a medium-sized electric vehicle. As an example, the requirements are derived from the BMW’s i3. The wireless power transfer system is integrated into the hollow shaft of the rotor. Unused space is thus utilized. The overall system is optimized for high efficiency, especially for partial load at medium speed, with an operation point-depending optimization method. The results are compared with the reference permanent magnet excited machine. A prototype of the machine is built and measured on the test bench. The measured efficiency of the inductive electrically excited synchronous machine is up to 4% higher than that of the reference machine of the BMW i3.

Exergy-Based Efficiency Assessment of Fans vs. Isentropic Efficiency

The efficiency definition allows us to compare two machines with each other. In general, the efficiency is defined as the ratio of usable power to the required power. This raises the question: what is the usable power? Most engineers discuss efficiency on the basis of the energy balance, i.e., the first law of thermodynamics. In this paper, we derive the exegetic efficiency taking the second law of thermodynamics into account. Exergy analysis takes into account work and heat and is able to model reality very accurately. On this basis, a comparison between the isentropic and exergetic efficiencies is given. A high-pressure radial fan is used as an example, and the differences are discussed. Therefore, measurements of a non-adiabatic fan are evaluated, and the role of the heat flux in the environment is discussed. The investigations show that a relevant difference between the isentropic and exergetic efficiencies becomes apparent in the partial-load range with high-pressure build-up. The thermal energy contained in the flow belongs proportionally to the exergy, i.e., the working capacity of the gas relative to its environment. For a standard such as ISO 5801 “Fans—Performance testing using standardized airways”, the efficiency must not only be physically correct, it must also be simple and practical. Against this background, the outlook of this paper discusses when and which efficiency definition is appropriate and best suited for a standard.

Development and performance evaluation of a chiller plant predictive operational control strategy by artificial intelligence

Traditionally, chiller plants are controlled and monitored by a predetermined control strategy to ensure appropriate operation based on the designed system configuration. With the use of new technology of variable speed drive (VSD) for compressors, smart control strategies could be leveraged to enhance the system efficiency in lieu of traditional control strategies. For example, using orderly and straightforward switching procedures without considering various factors in switching the units, including the high-efficiency partial load range benefitted from the VSD, the actual performance of the units as a whole and the variable chilled water flow rate, result in the chiller plant not operating at maximum performance and efficiency. To address these issues, a hybrid predictive operational chiller plant control strategy is proposed to optimize the performance of the chiller plant. Artificial intelligence is employed as the data mining algorithm, with big data analysis based on the actual acquired voluminous operation data by fully considering the characteristics of chiller plants without additional installation of large-sized and high-priced equipment. Artificial neural network (ANN) was employed in the control strategy to predict the future outdoor temperature, building cooling load demand and the corresponding power consumption of the chiller plants. At the same time, particle swarm optimization (PSO) was applied to search for the optimized setpoints, e.g., chilled water supply temperature, operating sequence, chilled water flow rate, for the chiller plants. The developed control strategy has been launched in a chiller plant with a cooling capacity of 7,700 kW installed in a hospital in Hong Kong. The system coefficient of performance (COP) and overall energy consumption of the chiller plants were enhanced by about 8.6% and reduced by about 7.9%, respectively, compared with the traditional control strategy. This real-time, continuous, automatic optimization control strategy can determine the most efficient combination of operating parameters of a chiller plant with different control settings. This ensures that the chiller plant operates in its most efficient mode year-round under various operational conditions.

Improved design of experiments method for machine-learning-based modelling of gearbox efficiency in a test rig environment

Knowledge of the efficiency of the subsystems in the drive train is essential for development in drive technology. Experiments have confirmed that the efficiency gradient of gearboxes is particularly high in the partial load range. Conventional test planning does not adequately reflect the influence of the efficiency gradient in the partial load range. To adequately represent the partial load range, it must be recorded with a particularly high number of measuring points. Map ranges with a low gradient are recorded with a smaller number of measuring points. This paper describes a method for the design of experiments, not with but for neural networks, with regard to modelling the efficiency of a two-stage spur transmission. In order to determine the test data, an optimized design of experiments is carried out. The test planning is divided into preliminary tests and main tests. The measuring points are optimally distributed in the characteristic diagram for use in neural networks. The measurement data is generated with the planned test data on a road to rig vehicle test rig. Suitable methods are used to prepare the data for further processing. The modelling is done experimentally with the help of neural networks. The result is a function for the continuous description of the efficiency at each operating point. The procedure is compared with the conventional method and validated with statistical methods. It was proven that the optimised experimental design method for neural networks gives better results than the conventional approach.

Numerical and experimental development of integrated electrostatic precipitator concepts for small-scaled biomass furnaces

Downstream electrostatic precipitators (ESP) represent the state of the art separation technology in medium and large biomass combustion plants. However, these technologies are often difficult to implement in smaller furnaces due to economic aspects and space constraints. This study deals with the integration of ESP systems into the heat exchanger of a small-scale biomass furnace. In order to accelerate the costly and time-consuming development process, preliminary numerical investigations of different ESP configurations were conducted using computational fluid dynamics (CFD). A prototype of the most promising variant were manufactured based on the results of the CFD simulations and experimentally tested with regard to the collection efficiency. In addition to the full load behaviour of the firing system, further test arrangements with several partial-load situations were tested to analyse the particle precipitation under realistic plant conditions with regard to flue gas properties and flow conditions. Furthermore, the collection efficiency for the combustion of different types of biomass was evaluated. Both discontinuous and time-resolved aerosol measuring methods are used to determine particulate matter (PM) emissions. Discontinuous measurements show that at least 55% of the PM2.5 particles are separated with the integrated ESP, both at full and partial load operation of the boiler, irrespective of the fuel used. In the partial load range, collection efficiencies for PM2.5 can be increased by up to 75%, depending on the fuel being fired. In order to enable a more precise observation of the separation behaviour with regard to particle size, additional continuous high-resolution aerosol measurements were carried out for a selected fuel (wood chips w=20%). The results show that over 50% (full load) and over 80% (part load) of small particles (PM1) can be separated.

Improving the effective efficiency of a spark ignition engine through the use of a fully independent valve control system

The article presents theoretical research of the proposed system of fully independent valve control (FIVC) of the SI engine. The analysis included controlling the movement of the intake valves, which results in adjusting the mass of the fresh charge to the current engine load, as well as the movement of the exhaust valves, where the main aim is to keep the rest of the exhaust gas in the cylinder, i.e. implementation of internal EGR. The open theoretical Seiliger-Sabathe cycle with the classic throttle regulation of load is the reference cycle for assessment of benefits and study of the effectiveness of obtaining work as a result of application of the FIVC system. A comparative analysis of the effectiveness of application of the proposed system was carried out based on the selected quantities: fuel dose, cycle work, relative work of charge exchange and cycle efficiency. The use of the FIVC to regulate the SI engine load makes it possible to eliminate the throttle and thus reduce the charge exchange work, especially in the partial load range. And this then leads to an increase in internal and effective work, which in turn results in an increase in the effective energy efficiency of an engine operation.

Increasing the Theoretical Efficiency of The Spark-Ignition Engine Cycle by Adding Inert Gas at Part-Load

The article presents calculations of the theoretical efficiency of the spark ignition engine cycle. Calculations of the efficiency of the Otto cycle were carried out with the use of throttling to a certain value of manifold vacuum in relation to atmospheric pressure, which is to simulate the operation of a real engine in the partial load range. The determined efficiency is the benchmark for determining the efficiency of the same cycle with a decreasing of throttling by adding an inert gas in the form of argon. This approach, apart from limiting the pumping work in the comparative cycle, increases the isentropic exponent in the compression and expansion processes, and thus translates into an increase in the efficiency of the considered cycle. The calculations were performed for various ranges of throttling pressure with the use of air and air with argon addition. The results are compiled in the form of a 3D diagram, taking into account the limiting ranges of throttling pressures and maintaining the ratio of argon mass to air mass at a certain level not exceeding 20% of the air mass involved in the process. Up to about 5.4% increase in the cycle efficiency was observed with an argon addition of 20% by weight. The obtained results are the basis for conducting research on a real object.

Simulation of Optimizing the Partial Load Performance of a Gas Turbine Combined Cycle Using Exhaust Heat Recuperation and Inlet Bleed Heating

Abstract This paper proposes a novel method to extend the operating range and improve the partial load efficiency of the gas turbine combined cycle (GTCC). The combination of exhaust heat recuperation and inlet bleed heating (IBH) was evaluated through a cycle simulation. The degree of heat recuperation was modulated during partial load operation to enhance the cycle efficiency. The recuperation ratio was modulated before control of the variable inlet guide vane (VIGV) began. This means that the recuperation control covers the high partial load regime. The gas turbine power remained almost constant in this regime because the inlet flow rate and turbine inlet temperature (TIT) were kept constant. In contrast, the power of the bottoming cycle decreased with increasing recuperation ratio due to the decrease in exhaust gas energy. After the recuperation ratio reached a limit, the load control was the same, as in conventional plants: VIGV control followed by fuel only control. The purpose of using IBH was to reduce CO emissions in the low load regime. Some of the compressor discharge air was recirculated to the compressor inlet, and the combustion temperature was maintained at a high level. Both IBH and recuperation were effective in extending the operating range. The turndown ratio was predicted to decrease by approximately 10%p. The efficiency remained higher than the full load efficiency over a wide partial load range. The efficiency of the recuperated GTCC was 4.1%p higher at 50% power than that of the conventional GTCC.

Sound reduction in heat exchanger modules by integrating plate absorbers with sub-millimeter openings

In air conditioning systems heat exchangers are frequently installed upstream of ventilators. These modules are part of the daily life of humans and should therefore create an environment that is as quiet as possible. The heat exchanger generates increased turbulence, which leads to increased sound emissions of the ventilator. Due to installation space limitations, it is not possible to connect silencers to the heat exchanger module. A novel concept is presented, which allows to reduce the sound emissions of heat exchanger due to plate absorbers without the need of additional installation room. The plates contain sub-millimeter openings. They are integrated into the heat exchanger housing together with back volumes. This arrangement guarantees compactness and generates no additional flow resistance. The emitted sound could be reduced by up to 2.9 dB in the partial load range. The main advantages of the concept presented are individual adjustment, cost-effective design and simple installation.

LSTM-NN Yaw Control of Wind Turbines Based on Upstream Wind Information

Based on wind lidar, a novel yaw control scheme was designed that utilizes forecast wind information. The new scheme can reduce the power loss caused by the lag of accurate measurement data in the traditional yaw control strategy. A theoretical analysis of the power loss caused by the traditional wind measurement inherent error and the wind direction based traditional yaw control strategy was conducted. The yaw angle error and yaw stop/start frequency in an actual wind field were statistically analyzed, and a novel Long Short Term-Neural Network (LSTM-NN) yaw control strategy based on wind lidar information was proposed. An accurate forecast of the wind direction could reduce the power loss caused by the inherent yaw misalignment, while an accurate forecast of wind speed could increase the stop/start frequency in the medium speed section within the partial load range and reduce the frequency in the low speed section within the partial load range. Thus, the power captured could be increased by 3.5% under certain wind conditions without increasing the yaw duty. Based on a simple wind evolution model and a novel yaw control strategy, the validity of the yaw control strategy was verified in a FAST/Simulink simulation model.

Tumble Flow Enhancement Applied for Low-Load Condition of Engines by Utilizing Reverse Flow Phenomenon in Intake Port

<div class="section abstract"><div class="htmlview paragraph">We established a technology that can enhance the tumble flow in the cylinder only in a partial load range of the engine without the need to use any intake path switching mechanisms.</div><div class="htmlview paragraph">Firstly, we attempted to understand the basic phenomena of intake flow by using a CFD model, while using a butterfly throttle valve in a straight pipe. By doing this, we were able to observe the reverse flow of intake air that appears after the intake air has passed the throttle valve when the throttle valve opening is 30% or less. This reverse flow is generated mainly in the flow that has passed the trailing edge of the throttle valve. At both sides of the trailing edge opening, the flow is slowed down by diffusing. The flow is then pulled into the low-pressure zone created behind the throttle valve. In addition, a part of the reverse flow merges with the air flowing on the leading-edge side.</div><div class="htmlview paragraph">Next, we confirmed that installing a flow separator behind the throttle valve that vertically divides the flow can successfully capture the reverse flow into one of the two flow paths. Furthermore, we confirmed that optimizing the separator position can capture most of the flow into one path, thereby gaining the required amount of flow that can generate tumble in the combustion chamber.</div><div class="htmlview paragraph">By applying the above results to an actual engine, we validated the effect through a CFD flow analysis and also steady flow tests. As a result, we confirmed that this system can enhance tumble within a partial load range of the engine to a level that is equivalent to that obtained by a tumble port that has a flow path switching mechanism.</div></div>

Reducing SoC-Management and losses of battery energy storage systems during provision of frequency containment reserve

Abstract Battery energy storage systems (BESS) are increasingly being used to provide Frequency Containment Reserve (FCR) due to the gradual decline in prices of Li-ion cells. Compared to conventional generators providing FCR, the limited energy reservoir of BESS requires suitable management of the state of charge (SoC-management). The energy to restore the SoC of BESS can either be traded on the energy market or balanced by other generation units inside the same balance group. In both cases costs arise, which can be minimized by reducing the energy for SoC-management. The possibility to use “degrees of freedom” (DoF) during provision of FCR allows such a reduction of SoC-management. This paper analyses the influence of these DoF on the SoC-management via simulation in Matlab/Simulink and presents measurements of an implementation of these DoF at a BESS test site. The second topic in this paper investigates the losses that arise during provision of FCR and SoC-management. Since the frequency deviations in the European power grid are usually very low, the power demand of FCR is carried out in the partial load range of the BESS and its converters. As the efficiency of converters in the partial load range is rather small, the losses increase. In case of BESS consisting of several subunits it is possible to reduce the overall losses by only activating the number of subunits that is needed to cover the actual power demand. This paper presents an algorithm to implement this idea and investigates its effectiveness by simulation in Matlab/Simulink . The results proof that the use of DoF reduces the energy needed for SoC-management. But the consideration of losses reduces the effect of the DoF. The presented algorithm to minimize losses can be applied to reduce losses and also makes it possible to fully exploit the potential of the DoF.

Application of IMEP and MBF50 indexes for controlling combustion in dual-fuel reciprocating engine

Engine control must take into account both the torque values and the combustion process. The first of these values can be controlled using the knowledge of IMEP, while the combustion process can be evaluated using the angle at which 50% of heat is released (CoC − Center of Combustion). The inclusion of these two values in the control strategy was the main objective of the study presented in this article. The tests were performed on a single-cylinder test engine. The share of gaseous fuel was set from 0 to 80% (for IMEP = 5 bar) and up to 99% (for IMEP = 9 bar). Tests of the combustion process were performed for three different cases: control with fixed IMEP settings, constant combustion angle CoC, and using both control strategies simultaneously. Utilizing a full engine control strategy allows for increased combustion rates: the maximum cylinder pressure is increased by 23% at low loads and 19% at high loads. The influence of such control on the heat release rate in the range of partial load was shown.As a result of using a two-parameter engine control strategy, it is possible to obtain optimum settings for the diesel oil and natural gas doses at which the Pareto optimum is obtained. It has been found, that at partial engine load maintaining constant IMEP or alternatively a constant CoC is difficult. Operation at heavy load indicates bigger compatibility with strategy to keep both IMEP and CoC constant. Operation in such conditions, with a large share of gas fuel energy, requires a larger dose of diesel oil to keep the values of thermodynamic parameters within the acceptable range, as well as to limit the increase in THC emission.

Real-Life Load Profiles of PV Battery Systems from Field Measurements

The definition of universal load profiles for Photovoltaic (PV) battery systems is an important factor in understanding and optimizing system efficiency, sizing and operating strategies. This publication gives detailed information on the load distribution of 16 households featuring PV battery systems in Germany that were measured in the course of a scientific monitoring program since mid-2015. Statements on the frequencies of high- or partial-load range of the different electronic components (for both, AC- and DC-coupled systems) as well as the batteries are given. Using the given data, a method to develop a generally accepted definition of an averaged operating efficiency for those systems is proposed.

Throttleless control of SI engine load by fully flexible inlet valve actuation system

A system with independent, early inlet valve closure (EIVC) has been analysed. The open, theoretical cycle has been assumed as a model of processes proceeding in the engine with variable inlet valve actuation. The system has been analysed individually and comparatively with open Seiliger-Sabathe cycle which is theoretical cycle for the classic throttle governing of engine load. The influence of EIVC on fuel economy, cycle work, relative charge exchange work and cycle efficiency has been theoretically investigated. The use of the analysed system to governing of an engine load will enable to eliminate a throttling valve from inlet system and reduce the charge exchange work, especially within the range of partial load. The decrease of the charge exchange work leads to an increase of the internal and effective works, which results in an increase of the effective efficiency of the spark ignition engine.