Low Inertia Research Articles

A two-material thermal barrier coating spatially tailored for high-efficiency GCI combustion

Continued research into thermal barrier coatings (TBCs) for internal combustion engines has generated insights into the design of TBCs that can increase fuel conversion efficiency. In this work, two low thermal inertia TBCs (a commercially available Gen 1 material, Gadolinium Zirconate, and a proprietary Gen 2 material) were experimentally tested in gasoline compression ignition (GCI) on a light-duty single cylinder research engine. Compared to a metal baseline, a 250-micron thick Gen 1 coated piston produced an efficiency benefit of 0.4 percentage points (pp) at 6 and 10 bar net indicated mean effective pressure (IMEPn) and a 0.2 pp penalty at 15 bar IMEPn. The Gen 1 coating also showed an uHC and smoke penalty. An 80-micron thick Gen 2 coated piston showed an efficiency benefit of 0.9 pp at 6 bar IMEPn and 0.4 pp at 10 bar IMEPn, but an efficiency penalty of 0.5 pp at 15 bar IMEPn. The Gen 2 coating, which has a lower thermal inertia than the Gen 1 coating, displayed durability problems while the Gen 1 coating did not. A third piston was coated with a 120-micron thick Gen 2 coating everywhere except the high failure outer bowl region, which was coated with a 120-micron Gen 1 coating. This hybrid coating displayed good durability, an uHC emission penalty, a smoke benefit, an efficiency benefit of 0.1 pp at 6 bar IMEPn, an efficiency benefit of 0.4 pp at 10 bar IMEPn, and no change in efficiency at 15 bar IMEPn. These results indicate that a durable Gen 1/Gen 2 coating can provide an efficiency benefit to GCI combustion, and that further improvements on piston-spray interaction optimization and surface sealing of the TBC, as well as increasing the coated combustion chamber surface area (e.g. head and valves) can add further improve efficiency.

Supplementary Frequency Control for Low Inertia Power Systems

Supplementary Frequency Control for Low Inertia Power Systems

Detecting safe operational regimes of synchronous motor‐generator pair for wind integration: A non‐linear perspective

AbstractIn this article, the stability of a synchronous motor generator pair (SMGP) used for improving the inertia of grid‐connected renewable energy systems is investigated. The useful operational regime for different sets of system parameters is identified, such as electromagnetic torque, damping co‐efficient, and inertia by employing bifurcation analysis to detect stability boundaries. For the first time, the existence of bi‐stable regimes for the SMGP with non‐linear stability analysis is revealed. The authors' analysis unravels the possibility of the system getting transited to unsafe operation even when the system is in the linearly stable region. The existence of the bistable regime indicates the possibility of the system becoming unstable even when the eigenvalues are in the left half plane. The authors also identify globally stable and globally unstable regimes in the parameter space. The safe operating range of inertia and damping co‐efficient values helps in the design of a suitable MGP set that is robust to frequency deviations, even with a low inertia source. With the recommended values of electromagnetic torque, the authors' analysis provides a safe operational regime for power generation from renewable energy sources.

On data-driven modeling and control in modern power grids stability: Survey and perspective

Modern power grids are fast evolving with the increasing volatile renewable generation, distributed energy resources (DERs) and time-varying operating conditions. The DERs include rooftop photovoltaic (PV), small wind turbines, energy storages, flexible loads, electric vehicles (EVs), etc. The grid control is confronted with low inertia, uncertainty and nonlinearity that challenge the operation security, efficacy and efficiency. The ongoing digitization of power grids provides opportunities to address the challenges with data-driven and control. This paper provides a comprehensive review of emerging data-driven dynamical modeling and control methods and their various applications in power grid. Future trends are also discussed based on advances in data-driven control.

Dynamic frequency control strategy for the CSP plant in power systems with low inertia

AbstractVariable renewable energy integration poses a challenge to the secure operation of the power system, especially for frequency security. How to provide frequency response to maintain satisfactory frequency performance is still an open question. Concentrating solar power, with renewable energy characteristics and synchronous units, is a potential solution to the frequency response problem. This paper discusses how concentrating solar power plants can be employed to ensure frequency security and proposes its dynamic frequency control strategy accordingly. The proposed strategy is contained in a novel unit commitment model with a primary frequency response and a secondary frequency response to co‐optimize the energy reserve. The validity of the unit commitment model and proposed control strategy is verified by two case studies including a modified 6‐bus and RTS‐79 system.

Mountain gazelle optimisation‐based 3DOF‐FOPID‐virtual inertia controller for frequency control of low inertia microgrid

AbstractThe primary objective of the authors is to design a new robust and improved virtual inertia controller (VIC) for renewable energy dominated inverter interfaced low inertia microgrid (LIMG). Increasing penetration of inertia‐less renewable generation in microgrid leads to increased frequency deviation during and after a disturbance. To improve the frequency response of the LIMG, conventional VIC added with different second stage and third stage controllers are proposed in existing works. Higher degree‐of‐freedom (DOF) PID controller synchronised with fractional‐order (FO) operators are used with conventional VIC controllers. These controllers work in addition with conventional VIC and the multi‐stage controllers make the system more complex. To reduce the number of controller stages and, subsequently, reduce cost and complexity of the system, a single stage 3DOF‐FOPID controller is proposed to mitigate the frequency deviation after a disturbance in a LIMG. Performance of the proposed single stage controller is compared with that of the existing controllers to establish the advantages of the proposed controller. The parameters of the proposed 3DOF‐FOPID controller are optimised by Mountain Gazelle Optimsation. The robustness of this controller is also tested for random load fluctuation and renewable power variations in presence of system non‐linearities.

Review of Low Inertia in Power Systems Caused by High Proportion of Renewable Energy Grid Integration

With the power industry moving toward a green and low-carbon direction, renewable energy is occupying an increasingly larger share in the power system. However, compared with traditional thermal power generation, the instability of new energy generation is very prominent, which also leads to a decrease in the inertia of the power system after the grid integration of a high proportion of renewable energy. If no measures are taken, this may lead to frequency collapse accidents. Therefore, this paper first introduces two international major power outage accidents that have occurred in recent years, analyzing the causes, and then summarizes the inspiration obtained from the accidents. Subsequently, some research results on low inertia-related issues in the power system caused by the high proportion of new energy grid integration in recent years were summarized and analyzed from three aspects: inertia evaluation methods, optimal operation measures for the power system, and under frequency load-shedding (the abbreviation “ULFS” in the following text stands for it) schemes. Finally, suggestions were made for future research directions.

Thermodynamic analysis of heat transfer reduction in spark ignition using thermal barrier coatings

This work uses a 0D thermodynamic engine model coupled to a 1D surface temperature solver to study the potential of low thermal inertia thermal barrier coatings (TBCs) on combustion chamber surfaces to increase the efficiency of spark ignition engines. Under ideal conditions, coating the piston crown, head, and valve faces with a TBC with a thermal inertia of 640 J/m2 K s1/2 resulted in less than a 1% relative improvement in efficiency. Despite using a low thermal inertia coating to avoid open cycle charge heating, the reduction in closed cycle heat transfer, which is the pathway to increasing efficiency, increased knock propensity. Therefore, any efficiency gain through closed cycle heat transfer reduction in spark ignition is offset by the need to retard spark timing to counter knock. An exergy analysis of completely blocking heat transfer for a 10 crank-angle degree window showed that on a low compression engine, like those used for stoichiometric gasoline spark ignition, the maximum efficiency gain achievable was limited compared to a higher compression ratio engine. Furthermore, the high gas temperatures of stoichiometric operation mean that even state-of-the-art TBCs cannot elevate surface temperatures enough purely through temperature swing to achieve a significant reduction in heat transfer near top dead center, where work availability is highest. Overall, these results indicate that low thermal inertia TBCs are ill-suited for achieving an efficiency benefit in spark ignition through a heat transfer reduction pathway. Instead spark ignition TBC research should explore ways to use low thermal inertia TBCs to achieve open cycle charge cooling to reduce knock propensity.

A consensus based adaptive virtual capacitor control strategy for reactive power sharing and voltage restoration in microgrids

In the islanded microgrid, virtual synchronous generator (VSG) control can solve the problems of insufficient damping and low inertia of the system. However, reactive power among distributed generation (DG) units controlled by VSG scannt be shared according to capacity due to the mismatch of line impedance, and the bus voltage can be significantly below the rated value due to the influence of droop control and line impedance. In this paper, an adaptive virtual capacitor control strategy (AVSSC) based on consensus is proposed to solve this problem. The strategy ensures the accurate sharing of reactive power among VSGs and adjusts the estimated bus voltage to the rated value by paralleling virtual capacitors. The voltage estimates used in AVSSC are obtained by the proposed discrete dynamic consensus algorithm (DDCA). The algorithm can obtain the average value of the local voltage estimates of all VSG units to eliminate the limitation of local information and the influence of accidental factors on voltage estimation. The multi-agent consensus theory proves that AVSSC can achieve the expected goal. The simulation results of a microgrid system with four parallel VSGs show that the AVSSC can accurately share reactive power, and the bus voltage error decreases from 0.81% to 0.39%.

Monitoring martian slope streaks in the northeast of Lycus sulci

Slope streaks are dark features found in low-latitude regions of Mars, formed from rapid-duration events in low thermal inertia terrain. The sudden appearance of low reflectance factor (I/F) patches followed by gradual whitening is a planet-relevant environmental indicator. The present work aims to quantify the variation in the fading of slope streaks using HiRISE/MRO images. We carried that out by analyzing I/F contrast of 31 streaks about their surroundings north of Olympus Mount, arranged in 16 images dated from 2007 to 2021, used as a dataset for measuring the formation and brightening of the streaks in the region. For the contrast calculations, we used I/F data points extracted from within the streaks and their brighter surroundings. We conducted a statistical analysis on the contrast distribution of each slope streak to test for normality and homogeneity of variances. Using Welch's t-Test, we found that the average albedo contrast values of the slope streaks were statistically different across all pairwise comparisons between dates. Our study provides some insight on the contrast variation (due to sediment deposition) of the streaks along time and seasonal correlation. The results point out to a possible dry formation process, such as dust avalanches, but are not sufficient to rule out wet mechanisms.

Unit Combination Scheduling Method Considering System Frequency Dynamic Constraints under High Wind Power Share

Power systems with a high wind power share are characterized by low rotational inertia and weak frequency regulation, which can easily lead to frequency safety problems. Providing virtual inertia for large-scale wind turbines to participate in frequency regulation is a solution, but virtual inertia is related to wind power output prediction. Due to wind power prediction errors, the system inertia is reduced and there is even a risk of instability. In this regard, this article proposes a unit commitment model that takes into account the constraints of sharp changes in frequency caused by wind power prediction errors. First, the expressions of the equivalent inertia, adjustment coefficient, and other frequency influence parameters of the frequency aggregation model for a high proportion wind power system are derived, revealing the mechanism of the influence of wind power prediction power and synchronous machine start stop status on the frequency modulation characteristics of the system. Second, the time domain expression of the system frequency after the disturbance is calculated by the segment linearization method, and the linear expressions of “frequency drop speed and frequency nadir” constraints are derived to meet the demand of frequency regulation in each stage of the system. Finally, a two-stage robust optimization model based on a wind power fuzzy set is constructed by combining the effects of wind power errors on power fluctuation and frequency regulation capability. The proposed model is solved through affine decision rules to reduce its complexity. The simulation results show that the proposed model and method can effectively improve the frequency response characteristics and increase the operational reliability of high-share wind power systems.

Design, Control, and Validation of a Symmetrical Hip and Straight-Legged Vertically-Compliant Bipedal Robot.

This paper presents the development, modeling, and control of L03, an underactuated 3D bipedal robot with symmetrical hips and straight legs. This innovative design requires only five actuators, two for the legs and three for the hips. This paper is divided into three parts: (1) mechanism design and kinematic analysis; (2) trajectory planning for the center of mass and foot landing points based on the Divergent Component of Motion (DCM), enabling lateral and forward walking capabilities for the robot; and (3) gait stability analysis through prototype experiments. The primary focus of this study is to explore the application of underactuated symmetrical designs and determine the number of motors required to achieve omnidirectional movement of a bipedal robot. Our simulation and experimental results demonstrate that L03 achieves simple walking with a stable and consistent gait. Due to its lightweight construction, low leg inertia, and straight-legged design, L03 can achieve ground perception and gentle ground contact without the need for force sensors. Compared to existing bipedal robots, L03 closely adheres to the characteristics of the linear inverted pendulum model, making it an invaluable platform for future algorithm research.

Transient energy transfer control of frequency-coupled energy storage devices in low inertia prosumer energy systems

If the energy source of rotational inertia is expanded to include the stored static energy, the transient stability of prosumer energy systems is enhanced by the energy transfer between frequency-coupled hybrid energy storage device (HESD) and synchronous generator (SG). In this paper, first, the conversion relationships between the stored energy in the battery and capacitor, and the mechanical kinetic energy of SG are established. Subsequently, the virtual inertia hidden in HESD is obtained for the frequency-coupled capability. Second, the small disturbance model of a prosumer energy system with virtual inertia is derived, and the impact of frequency-coupled HESD on frequency stability and damping characteristics is analyzed. Third, based on the mechanism analysis of system transient stability, a novel energy transfer control strategy adapted to the HESD is proposed. By sharing transient energy between the SG and HESD, both the frequency variation and rotor angle oscillation can be prevented using a unified controller. Last, a typical test system with high penetration of photovoltaic (PV) arrays is implemented on a hardware-in-the-loop platform. The results demonstrate that under the proposed control strategy, the transient stability of prosumer energy systems with frequency-coupled HESD can be significantly improved.

Frequency-based demand side response considering the discontinuity of the ToU tariff

Demand side response (DSR) according to the time-of-use (ToU) tariff can bring a number of benefits to consumers and networks. However, for microgrids with high penetrations of renewable energy and flexible load, the sudden and drastic variations of loads triggered by the discontinuity of the ToU tariff could be fatal to the frequency stability of this low inertia system. This paper proposes a new frequency-based DSR (FB-DSR) strategy through a two-stage model with the frequency dynamics expressed analytically as the exchange power. The first stage optimizes the day-ahead DSR strategy based on the ToU tariff and equipment degradation cost. At the second stage, frequency response is constrained through the whole process of inertia support, frequency nadir and quasi-steady-state by integrating time-domain optimization and frequency-domain control. Numerical results demonstrate that the proposed strategy can effectively reduce the volatility of flexible load when responding to the ToU tariff, and thus significantly lowering the cost of maintaining the system frequency stability. At the same time, the negative impact of this strategy on the economic benefits of DSR is negligible. The overall operating cost of the test system is reduced by 7.34% even considering the increased degradation cost.

Development of Virtual Inertia Control with State-of-Charge Recovery Strategy Using Coordinated Secondary Frequency Control for Optimized Battery Capacity in Isolated Low Inertia Grid

Integrating renewable energy through inverter-based generators has decreased the power system’s inertia. Reduced inertia may lead to frequency instability during power imbalance disturbances, particularly in an isolated power system with limited inertia. The Battery Energy Storage System (BESS) and a virtual inertia (VI) emulation control system have become popular to mitigate this issue. Nonetheless, the BESS utilization for VI emulation is highly dependent on the availability of BESS capacity, which may affect the energy cost. Therefore, developing a VI emulation control strategy that requires less energy and can recover the state of charge (SoC) to a desired level to optimize BESS utilization is required. This paper proposes a VI control with an SoC recovery strategy through coordination with the generators’ secondary frequency control. Instead of relying on the frequency, such as in the conventional approach, the controlled signal of the generators’ secondary frequency control also includes the VI power and BESS SoC. Hence, the generators can contribute to lowering the VI required energy and recovering the BESS SoC. The results show that the proposed method outperforms the conventional method by requiring around 36% lower energy and the ability to maintain the BESS SoC.

Simulated methane emissions from Arctic ponds are highly sensitive to warming

Abstract. The Arctic is warming at an above-average rate, and small, shallow waterbodies such as ponds are vulnerable to this warming due to their low thermal inertia compared to larger lakes. While ponds are a relevant landscape-scale source of methane under the current climate, the response of pond methane emissions to warming is uncertain. We employ a new, process-based model for methane emissions from ponds (MeEP) to investigate the methane emission response of polygonal-tundra ponds in northeastern Siberia to warming. MeEP is the first dedicated model of pond methane emissions which differentiates between the three main pond types of the polygonal-tundra, ice-wedge, polygonal-center, and merged polygonal ponds and resolves the three main pathways of methane emissions – diffusion, ebullition, and plant-mediated transport. We perform idealized warming experiments, with increases in the mean annual temperature of 2.5, 5, and 7.5 ∘C on top of a historical simulation. The simulations reveal an approximately linear increase in emissions from ponds of 1.33 g CH4 yr−1 ∘C−1 m−2 in this temperature range. Under annual temperatures 5 ∘C above present temperatures, pond methane emissions are more than 3 times higher than now. Most of this emission increase is due to the additional substrate provided by the increased net productivity of the vascular plants. Furthermore, plant-mediated transport is the dominating pathway of methane emissions in all simulations. We conclude that vascular plants as a substrate source and efficient methane pathway should be included in future pan-Arctic assessments of pond methane emissions.

Reduced CO2 footprint of buildings in agro-industrial communities in Qatar with improved insulation standards and solar cooling

Food production in arid countries like Qatar is strongly linked to water and energy resources to grow plants or rear cattle in a protected and controlled environment. The high energy demand for air conditioning in agro-industrial communities leads to significant CO2 emissions. In a case-study on buildings that are typical in the agro-industrial sector, the reduction potential for these emissions is investigated. Therefore, greenhouses and community buildings with an insulation according to the Passivhaus standard and solar cooling with photovoltaic driven compression chillers are compared to conventional buildings.An innovative solar cooling system is developed: Power is generated in hybrid photovoltaic / thermal collectors to enhance the electric efficiency of the photovoltaic cells and to deliver heat for food processing or other purposes at a high temperature level. A compression chiller model is designed, that is based on the experimentally determined performance of the refrigeration cycle for the harsh operating conditions in Qatar. Different refrigerants are compared regarding their cycle performance and Global Warming Potential.A result of the study at hand is that greenhouses are very well suited for solar cooling since their cooling load is nearly congruent to photovoltaic power generation due to their low thermal inertia and radiative cooling during the night.The case study reveals that an insulation according to the Passivhaus standard combined with the proposed solar cooling system reduces the CO2 emissions caused by the air conditioning of greenhouses by 98% and of residential buildings by 94.7% compared to conventional buildings and air conditioning systems.

A State-of-the-Art Review on Recent Load Frequency Control Architectures of Various Power System Configurations

Load frequency management (LFM) work has become more difficult due to hybrid power systems. These power systems are responsible for increased power quality issues, exacerbating control problems. Frequency changes and power quality issues are the central concerns caused by these modern power systems. Frequency deviations occur mainly due to variations in generation, consumption, or both, and one of the critical reasons is the low inertia of the modern renewable penetrated hybrid power systems. So, to handle all these issues related to frequency and many more, the need for the best controller arises, which can easily handle any nonlinearities. This article comprehensively analyzes various load frequency control (LFC) architectures in various power system configurations. It focuses on the frequency management features of various power systems, including microgrids that use renewable energy and electric vehicles, with a complete understanding of current development states. Different controllers and tactics underwent comparative investigations. The detailed tabular analysis of frequency control architectures provides a deep insight into the LFC assessment of various power system configurations. The importance of better controllers for LFC planning in complicated power system domains is also emphasized in this work.

Fluidic self-assembly for MicroLED displays by controlled viscosity.

Displays in which arrays of microscopic 'particles', or chiplets, of inorganic light-emitting diodes (LEDs) constitute the pixels, termed MicroLED displays, have received considerable attention1,2 because they can potentially outperform commercially available displays based on organic LEDs3,4 in terms of power consumption, colour saturation, brightness and stability and without image burn-in issues1,2,5-7. To manufacture these displays, LED chiplets must be epitaxially grown on separate wafers for maximum device performance and then transferred onto the display substrate. Given that the number of LEDs needed for transfer is tremendous-for example, more than 24 million chiplets smaller than 100 μm are required for a 50-inch, ultra-high-definition display-a technique capable of assembling tens of millions of individual LEDs at low cost and high throughput is needed to commercialize MicroLED displays. Here we demonstrate a MicroLED lighting panel consisting of more than 19,000 disk-shaped GaN chiplets, 45 μm in diameter and 5 μm in thickness, assembled in 60 s by a simple agitation-based, surface-tension-driven fluidic self-assembly (FSA) technique with a yield of 99.88%. The creation of this level of large-scale, high-yield FSA of sub-100-μm chiplets was considered a significant challenge because of the low inertia of the chiplets. Our key finding in overcoming this difficulty is that the addition of a small amount of poloxamer to the assembly solution increases its viscosity which, in turn, increases liquid-to-chiplet momentum transfer. Our results represent significant progress towards the ultimate goal of low-cost, high-throughput manufacture of full-colour MicroLED displays by FSA.

Intelligent frequency stabilization of low-inertia islanded power grids-based redox battery

Renewable energy sources (RES)-generated electricity is increasing at a fast pace and is becoming more prevalent in microgrids (MGs). System inertia is much decreased due to the fact that MGs are mostly powered by RES rather than synchronous generators. As a result, system stability suffers, which is especially problematic with MGs. A suitable solution for this problem is an energy storage system and an appropriate inertial control technique. Redox flow batteries (RFBs) have an unusually long charge-discharge life cycle and a fast reaction time, which allows them to reduce oscillations in the power system when there is a sudden disturbance in the power system. This paper suggests a unique concept for VIC based on RFB in an isolated MG, which emulates the primary frequency control, virtual inertia, and damping all at once. In addition to the VIC concept, we suggest a novel nonlinear model predictive control (NMPC) method for MG secondary control. The NMPC utilizes non-integer order calculus and interval type-3 (IT3) fuzzy logic systems (FLSs). Finally, OPAL-RT-based real-time hardware-in-the-loop (HIL) simulation results are used to examine the validity and applicability of the suggested techniques in low inertia islanded MG.