Load-sharing Strategy Research Articles

Load sharing strategy for parallel-operated variable speed centrifugal compressors in chillers

The building space cooling is accounting for a significant portion of global energy use, and it is essential to enhance chiller efficiency for energy reduction and climate change mitigation. Chiller systems with multiple compressors provide the flexibility to meet varying cooling demands. The load-sharing strategy controls how the cooling load is distributed among the compressors. The load-sharing strategy optimization can significantly affect the chiller overall power consumption. This paper presents a mathematical analysis of the power consumption of centrifugal compressors under specific ambient temperatures. The influence of the inlet guide vane and the variable frequency drive efficiency on the compressor power consumption are considered and discussed briefly. An optimal load-sharing strategy for commonly used systems with identical compressors is proposed. Additionally, the scenarios involving a mix of different compressors are investigated. To validate the proposed strategies, a case study using the established performance data of a commercial centrifugal compressor is conducted. The case study results align with the mathematical analysis and the proposed strategy. This study provides valuable insights for designing and managing efficient chiller systems.

Thermodynamic performance analysis of direct methanol solid oxide fuel cell hybrid power system for ship application

Methanol is a fuel that can be directly supplied to solid oxide fuel cell (SOFC) without carbon deposition. This paper introduces the direct methanol SOFC-Internal combustion engine (ICE) hybrid power and SOFC-Gas turbine (GT) hybrid power systems. Thermodynamic performance analysis of the hybrid power systems and an investigation into the impact of various parameters were conducted. The results indicate that the electric efficiency of the SOFC-ICE system is 56.07 %, which is 3 percentage points higher than the SOFC-GT system but 3–4 percentage points lower than the SOFC-ICE system with pre-reforming. Increasing fuel utilization rate, operation temperature, and excess air ratio is beneficial to the performance of the SOFC, and raising current density helps improve the system's power density. Load-sharing strategy research demonstrates that in a scenario where the power ratio between SOFC and the ICE is 35:65, compared to a methanol engine, the efficiency of the SOFC-ICE hybrid power system increases by 4.3 %. Despite a 1.4-fold increase in weight and a 1.66-fold increase in cost, the fuel consumption rate and carbon emissions decrease by 8.8 %. Therefore, the strategy of combining a high-power engine with a low-power SOFC is currently a suitable load-sharing strategy for ship applications.

Load Sharing Design of a Multi-legged Adaptable Gripper With Gecko-Inspired Controllable Adhesion

Gecko-inspired controllable dry adhesion has promising applications in various fields such as transfer printing, advanced robotics, and space technology. In this study, we proposed a multi-legged self-adaptive gripper based on gecko-inspired controllable adhesion to manipulate both flat and curved objects. It consists of four symmetric adhesive units, each modulated by two adaptive-locking mechanisms for compression and rotation, respectively, and one peeling mechanism. The two adaptive-locking mechanisms adapt to surfaces with height and curvature differences to ensure intimate contact and then lock the adaption configuration to enable equal load sharing for strong attachment. The peeling mechanism rapidly peels the adhesive surfaces from the substrate for easy detachment. Therefore, this gripper can achieve controllable load sharing to reliably grasp and easily release smooth objects with various surface shapes. Furthermore, the effects of object posture and structural parameters on its grasping performance were analyzed, which can guide its optimal design by improving equal load sharing among adhesive units. This work provides an equal load-sharing strategy for multi-unit adhesive system design and demonstrates its potential for emerging industrial applications.

Cost and Availability Improvements for Fault-Tolerant Systems Through Optimal Load-Sharing Policies

The demand for highly-available systems is continuously increasing. Fault-tolerant systems which are composed of several redundant sub-units allow for continuous operation of such critical systems by distributing the system load across the functional units. However, these systems can fail if several redundant sub-units break down simultaneously. To address this, we propose a modeling and simulation approach to study the probability of simultaneous breakdown of redundant sub-units under different load distribution strategies. This allows for choosing the load-sharing strategy which achieves the highest availability for a certain system load profile and operational lifetime. We show that different load-sharing strategies can change the system availability and the operational and maintenance costs by almost an order of magnitude for a redundant power-supply unit of industrial controllers.

An Improved Proportional Load-Sharing Strategy for Meshed Parallel Inverters System With Complex Impedances

Inverters and loads can freely connect to any node in a meshed parallel inverters system, leading to variations of line impedance between two nodes and the structure of the electric network. More challenges exist for proportional load-sharing control in such a system. An improved proportional load-sharing strategy for a meshed parallel inverters system with complex impedances is proposed in this paper. The RL -type droop method is modified through injecting the power droop information of all other inverters, which can improve the load-sharing accuracy to some extent and completely compensate the voltage amplitude and frequency deviations caused by droop equations under accurate load-sharing conditions. Besides, an adaptive virtual resistance tuned by the integral of the linear combination of output active and reactive powers, is added to eliminate load-sharing errors. In addition, the central controller broadcasts the control variables to all inverters. This improves the simultaneity of control data and minimizes the mismatch of control delay. Three modified droop methods are compared with the small-signal analysis and simulation. Experimental results are presented to investigate the performance of the proposed control strategy.

A Load-Sharing Strategy for the State of Charge Balancing Between the Battery Modules of Integrated Reconfigurable Converter

In the integrated reconfigurable converter (IRC) topology for high-voltage series-connected battery module systems, the converter can be reconfigured into a range of operating modes: “feeding a load” mode, “battery modules balancing” mode, “regenerative” mode, “charging” mode, and “backup” mode. In most battery applications, a battery-balancing system operates while another converter is used as a controllable power interface between the battery system and a load. However, during the conventional IRC operation, the “battery modules balancing” and “feeding a load” modes cannot be operated simultaneously. In this paper, a new balancing scheme is proposed that allows balancing the battery modules while they are powering the load. The proposed balancing strategy is able to distribute different amount of load between the battery modules based on their state of charge (SOC). When the load is disconnected, the battery modules can be still balanced using the conventional “battery modules balancing” mode. A scaled down prototype of the IRC is implemented with three series-connected battery modules. Experimental results are provided to verify the viability of the proposed balancing method.

Modeling a shared-load k-out-of-n:G system

A Markov model for analyzing the reliability and availability of an n-unit shared-load repairable k-out-of-n:G system with imperfect switching is presented. The equations for both time-dependent and steady-state system availability are given. An inverse Laplace transform is used to solve the simultaneous differential equations for the nonrepairable case. A generalized analytic function for system reliability is obtained. Examples are provided to demonstrate the model and the impact of a load-sharing strategy on the reliability. The load-sharing strategy can improve system reliability and availability, if the controller and switching parameters are adequate. The proposed approach and solution are helpful to system engineers and reliability analysts.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>