Nano Grid Research Articles

Adaptive Active Power Sharing Techniques for DC and AC Voltage Control in a Hybrid DC/AC Microgrid

This paper deals with the ac and dc dynamic voltage control in a hybrid dc/ac microgrid (MG) with central and distributed battery energy storage systems (BESSs), applying a power sharing mechanism between the different devices in the MG. The MG is composed by a multiport transformation center and two fixed frequency three phase ac nanogrids (NGs) coupled to a dc bus through three-phase power electronic converters. The system pursues to minimize the dependence on the utility grid and the stress in the MGs central BESS, while increasing the power handling capability and the overall system stability during islanding condition. In order to approach the proposed aim, two main concerns are studied in this paper: An adaptive power sharing mechanism between the dc bus and the ac NGs for dc voltage control, and the design and implementation of an ac dynamic local voltage compensator based on distributed energy storage system. The proposed techniques are validated through simulations and experimental results.

Integrated battery model in cost‐effective operation and load management of grid‐connected smart nano‐grid

This paper presents a comprehensive model of dynamic optimal operation management of the smart nano-grids (NGs) including the micro wind turbines (WTs) and micro photovoltaics (PVs) as renewable energy sources (RESs) while micro turbines (MTs) and fuel cell (FC) are considered as non-RESs. Furthermore, two types of lead-acid and lithium-ion batteries are considered besides the three types of controllable, curtailment-able and must run loads to increase the flexibility of the proposed formulation. The different objective functions of NG operation problem to be minimised include operating cost, environmental damage cost of pollution gases and exchanged power cost with the main grid. Also the power losses of batteries are modelled using the quadratic functions based on types and output powers of considered batteries, while these losses impose additional cost to operation cost functions of batteries. A modified teaching-learning-based optimisation (MTLBO) algorithm is used to cope with the multiobjective problem considering the constraints.

A Self-Sustained and Flexible Control Strategy for Islanded DC Nanogrids Without Communication Links

This paper proposes a self-sustained and flexible control strategy for autonomous dc nanogrids (NGs) in remote and rural areas without the need for a communication system. The proposed control strategy of NGs is based upon a hierarchical control, in which the primary control manages the power balance inside the NG and the secondary control is responsible for removing deviation of the dc bus voltage caused by the droop operation. The state of charge of the battery and the external dc bus signal are taken into account in the proposed control strategy in order to avoid the overcharge/deep discharge of the battery as well as the collapse of the external dc bus. Bidirectional power flow among multiple NGs is implemented through a dedicated interconnected bidirectional dual-active-bridge dc/dc converter installed inside the NG to ensure a galvanic isolation among multiple interconnected NGs. Finally, the small-signal model is developed, in which the small-signal transfer function of an entire NG is derived from the small-signal transfer functions of every single converters of the system. From the attained transfer function, the appropriate secondary controller is designed, and the system stability is analyzed. The proposed control strategy is validated through simulations and experiments.

Principle and control strategy of pulse width modulation rectifier for hydraulic power generation system

According to the characteristics of the hydraulic power generation system (HPGS), HPGS can be directly fed into a DC nano grid through using an uncontrolled rectifier. However, determined only by the DC voltage, the motor speed of the HPGS is a constant rather than an optimal speed, whose curve is unknown yet during the whole power generation process. In order to realize the control of the HPGS's motor speed and reduce the armature current harmonics, a pulse width modulation (PWM) rectifier topology is introduced to replace the uncontrolled rectifier. First, the principle of the PWM rectifier is studied. Second, a control structure of this PWM rectifier is established, including a current decoupling control in the inner loop and a speed feedback control in the outer loop. After this, a maximum efficiency converting control strategy is adopted. Based on the above, the simulation model of the PWM rectifier which was fed into DC nano grid was established and a series of experiments were carried out. The experimental results show that the armature current waveform is sine wave and its rectification is flexible. Furthermore, the maximum efficiency converting control strategy is realized with HPGS's always working at the optimal speed curve.

Experimental investigation on small capacity compressed air energy storage towards efficient utilization of renewable sources

Abstract Energy storage has become a matter of great concern in the increasing Renewable Energy (RE) scenario and it plays a vital role in the renewable based micro and nano grids. Among the various energy storage systems, Compressed Air Energy Storage (CAES) system has received the attention of scientists during the recent years due to its long life cycle and the scope and potential of increasing roundtrip efficiency by introducing the concept of polygeneration along with other recent inventions in this technology. In the present work, an experimental investigation has been carried out on small capacity CAES system by constructing a 400 L capacity storage tank and the round trip efficiency is quantified by conducting several charging and discharging experiments. The results reveal that the expander efficiency is 41.79%, 54.25% and 68.3% for the mass flow rates of 0.005 kg/s, 0.0075 kg/s and 0.01 kg/s respectively and the roundtrip efficiency was 13.4%, 17.4% and 22.02% for the mass flow rates of 0.005 kg/s, 0.0075 kg/s and 0.01 kg/s respectively.

Online Energy Sharing for Nanogrid Clusters: A Lyapunov Optimization Approach

Nanogrid (NG) cluster (NGC) has the potential to act as one type of basic structure for the future low voltage distribution networks. In this paper, an online energy sharing method is proposed for improving the self-sufficiency and photovoltaic (PV) consumption of NGC. First, a hybrid cyber-physical peer-to-peer (P2P) energy sharing framework is proposed, which is a combination of P2P physical system (i.e., NG-to-NG) and client–server cyber system (i.e., NG controllers-central controller). Moreover, an energy sharing strategy with the classification of energy exporting and importing NGs is designed. Considering the stochastic features of PV energy and end user load, an online optimization model and algorithm is formulated based on Lyapunov optimization, as to maximize the self-sufficiency and guarantee the stability of energy storage queues. Finally, in a case study using the realistic data from the residential community, numerical experiments show the effectiveness of the proposed method in improving the self-sufficiency of NGC.

Calculation and Analysis of Graphene Nanoribbons Field Effect Transistor Structure and Characteristics

Because there is no gap between conduction band and valence band of graphene, it is difficult to achieve switching characteristics when transistors are fabricated. In order to open the graphene band gap, researchers have explored many methods, such as tailoring Shi Mocheng quantum dots, nanobelts, nano grids or laying graphene on a special substrate, one of the feasible ways is to regulate the electrical properties of graphene by doping. Through the analysis on the structure and properties of graphene band calculation, with the help of computer aided simulation, the ID and VG characteristics analysis of the Schottky barrier graphene field effect transistor (SBFET) and MOS structure of graphene field effect transistor (MOSFET) are investigated in this study. The results are of high reference value for the study of the structure and properties of graphene nanoribbons.

Decentralised and interlink-less power interchange among residences in microgrids using virtual synchronous generator control

When numerous inertia-less distributed power supplies (DPSs) are connected to a microgrid (MG), the inertial force of the entire system may be insufficient. The lack of inertial force will cause the system frequency and voltage to be transiently unstable; thus, parallel operation of multiple inverters may be difficult. As a means of solving these problems, the use of virtual synchronous generator (VSG) control, in which the inverter has a virtual inertial force and simulates the inertial behaviour of a synchronous generator (SG), is attracting interest. In residential applications, photovoltaics (PVs) and fuel cells (FCs) are examples of home DPSs that are connected to grids via inertia-less inverters. The virtual inertial force in a VSG is produced by a storage battery (BAT). Therefore, when using a VSG-controlled BAT as the main power supply in the islanded MG of a residential building, FCs can be employed as stable charging power sources for BATs instead of PVs, which are weather- and time-dependent. In addition, FCs and BATs are complementary in the sense that BATs transiently compensate for the slow responsiveness of FCs. In this paper, an autonomous power management (APM) approach, in which electric power is interchanged within an islanded MG, is proposed, where an MG consists of a set of nanogrids (NGs), meaning residential units. The power sources in the NG are FCs and BATs connected via conventional current- and VSG-controlled inverters, respectively. Both the VSG and conventional current controls are primary controls, while APM acts as a secondary control. As a VSG provides an autonomous governor-free function, interlinks between the NGs and centralised control in a higher layer are not required, and all of the abovementioned controls are installed in each NG. The advantages of communication-less, decentralised autonomous power interchange among NGs are easy operation and improved flexibility and scalability of stable MGs.

Solar electric cooking in Africa: Where will the transition happen first?

Whilst the rapid spread of solar photovoltaics (PV) across Africa has already transformed millions of lives, it has yet to have an impact on the main energy need of poor households: cooking. In the context of falling global PV prices, recent advancements in battery technology and rising charcoal/fuelwood prices in severely deforested regions, the door is opening for a potentially transformative alternative – solar electric cooking (PV-eCook). While initial investigations focused on solar home systems sized for cooking (cooking device, battery storage, charge controller and PV array), it has since been shown that battery-supported electric cooking (eCook) can also strengthen national, mini, micro and nano grids. This paper presents a multi-criteria decision analysis (MCDA) based methodology, accounting for a wide variety of socio-cultural, political, technical and economic factors which are expected to affect the uptake and potential impact of eCook across a variety of African contexts. It shows the concept has considerable viability in many African countries, that there are significant sizeable markets (millions of potential users), and that within the next five years the anticipated costs of eCook are highly competitive against existing ‘commercialised polluting fuels’.

Design of Nano Grid using Solar and Hydro System for Smart Homes

Design of Nano Grid using Solar and Hydro System for Smart Homes

Cooperative Distributed Control Implementation of the Power Flow Coloring Over a Nano-Grid With Fluctuating Power Loads

To realize efficient and versatile energy management systems for future homes, buildings, and local communities, nano-grid (NG) finds possibility for integrating distributed energy resources. This paper implements the power flow coloring, which gives a unique ID to each power flow between a specific power source and a specific power load. It enables us to design versatile power flow patterns between distributed power sources and loads, taking into account energy availability, cost, and carbon dioxide emission. To implement the power flow coloring, this paper proposes a cooperative distributed control method, where a master–slave role assignment scheme of power sources and a time-slot based feedback control are introduced to cope with power fluctuations while keeping the voltage stability of the NG. Experimental results show the practical feasibility of our proposed method in managing distributed power sources and fluctuating loads.

A Multi-Tenant Cloud-Based DC Nano Grid for Self-Sustained Smart Buildings in Smart Cities

Energy is one of the most valuable resources of the modern era and needs to be consumed in an optimized manner by an intelligent usage of various smart devices, which are major sources of energy consumption nowadays. With the popularity of low-voltage DC appliances such as-LEDs, computers, and laptops, there arises a need to design new solutions for self-sustainable smart energy buildings containing these appliances. These smart buildings constitute the next generation smart cities. Keeping focus on these points, this article proposes a cloud-assisted DC nanogrid for self-sustainable smart buildings in next generation smart cities. As there may be a large number of such smart buildings in different smart cities in the near future, a huge amount of data with respect to demand and generation of electricity is expected to be generated from all such buildings. This data would be of heterogeneous types as it would be generated from different types of appliances in these smart buildings. To handle this situation, we have used a cloudbased infrastructure to make intelligent decisions with respect to the energy usage of various appliances. This results in an uninterrupted DC power supply to all low-voltage DC appliances with minimal dependence on the grid. Hence, the extra burden on the main grid in peak hours is reduced as buildings in smart cities would be self-sustainable with respect to their energy demands. In the proposed solution, a collection of smart buildings in a smart city is taken for experimental study controlled by different data centers managed by different utilities. These data centers are used to generate regular alerts on the excessive usage of energy from the end users' appliances. All such data centers across different smart cities are connected to the cloud-based infrastructure, which is the overall manager for making all the decisions about energy automation in smart cities. The efficacy of the proposed scheme is evaluated with respect to various performance evaluation metrics such as satisfaction ratio, delay incurred, overhead generated, and demand-supply gap. With respect to these metrics, the performance of the proposed scheme is found to be good for implementation in a realworld scenario.

A Kind of DC-DC Converter Based on LLC Resonant in Rural Nano Grid

The load of the rural grid is changeable and irregular, it is easily affected by the large power grid. This paper proposed the concept of “rural nano grid”, which can combines the power generation system, energy storage system and control system into unified whole. The rural nano grid can work in the grid-connected mode and off-grid mode when there are some faults in large power grid, so that power supply in rural areas can be sustainable. Then this paper proposed a new kind of LLC resonant DC-DC converter, which can ensure the power generation system of the rural nano grid supply power for the load and energy storage system under wide load range with high efficiency that can be appropriate for characteristics of the rural nano grid. At last a prototype with a power rating of 5kW is designed and built up to verity validity and correctness of the converter proposed in this paper.

Connection technology of HPTO type WECs and DC nano grid in island

Wave energy fluctuating a great deal endangers the security of power grid especially micro grid in island. A DC nano grid supported by batteries is proposed to smooth the output power of wave energy converters (WECs). Thus, renewable energy converters connected to DC grid is a new subject. The characteristics of WECs are very important to the connection technology of HPTO type WECs and DC nano grid. Hydraulic power take-off system (HPTO) is the core unit of the largest category of WECs, with the functions of supplying suitable damping for a WEC to absorb wave energy, and converting captured wave energy to electricity. The HPTO is divided into a hydraulic energy storage system (HESS) and a hydraulic power generation system (HPGS). A primary numerical model for the HPGS is established in this paper. Three important basic characteristics of the HPGS are deduced, which reveal how the generator load determines the HPGS rotation rate. Therefore, the connector of HPTO type WEC and DC nano grid would be an uncontrollable rectifier with high reliability, also would be a controllable power converter with high efficiency, such as interleaved boost converter-IBC. The research shows that it is very flexible to connect to DC nano grid for WECs, but bypass resistance loads are indispensable for the security of WECs.

Dynamic operation and control of a hybrid nanogrid system for future community houses

A hybrid NanoGrid (NG) can be considered as the power supply system, for future community houses. In this study, the operation and control of an NG are presented. The NG consists of an AC bus and a DC bus, interconnected through a tie-converter (TC). All AC loads and distributed energy resources (DERs) of the community houses are connected to the AC bus while all their DC loads and DERs are connected to the DC bus. The NG has an adequate generation capacity to supply its loads in off-grid status as well as the capability to exchange power with the grid. The TC exchanges the power between two buses and regulates the voltage in both buses, in the case of off-grid operation. The NG dynamics, with the proposed structure and developed primary and secondary control algorithms are validated through extensive computer simulations in PSCAD/EMTDC.

Large Area Silica Nano Grids by Homogeneous High Resolution Laser Patterning of SiOx-Films

Structured excimer laser back side irradiation of thin SiOx-films on fused silica substrates is used for the formation of complex micro- and nano-patterns. A polymeric superstrate serving as a confinement layer on top of the SiOx-film allows the formation of very smooth and regular patterns. Depending on laser processing parameters, either arrays of blisters or cups, or grids with free standing wires below 100 nm in diameter can be fabricated. To obtain these nano-features with high structure resolution on a comparatively large area with a single KrF-laser pulse (wavelength 248 nm), an optical system combining a beam homogenizer with a high resolution mask imaging optical system has been established. At fluences in the range of 300 to 400 mJ/cm² very regular grid patterns with wires detached from the substrate and nodes attached to the substrate are obtained. The free standing wires have diameters down to less than 100 nm. The patterned SiOx-material can be oxidized to SiO2 by thermal or laser treatment.

Nano scale depth reconstruction based on optical diffraction

Traditional depth from defocus (DFD) is based on the imaging principle in geometric optics to calculate blurring degree, influence of optical diffraction has never been considered. However, diffraction is a basic property of all kinds of waves, it must happen in an optical imaging system based on pinhole imaging principle. In this paper, a high resolution DFD method with a single vision and optical diffraction is given. Firstly, the basic principle of Fresnel diffraction in an optical system is analyzed, and the relationship curve between Fresnel diffraction and depth information is constructed. Secondly, a defocus imaging model with optical diffraction is supposed through curve fitting, and with respect to relative blurring and heat diffusion, a new DFD method with optical diffraction and two defocused images is proposed. Finally, a standard nano grid is used to validate the proposed DFD method on nana scale. The results show that the new algorithm, compared with traditional DFD algorithms, is a more effective method to reconstruct depth information on nano scale.

Nanopattern fabrication via AP-UVO exposure of a self-organized block copolymer

Block copolymers were used to achieve nanoscale patterning. Block copolymers, particularly polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), produce patterns from which one block polymer may be selectively etched. However, the selective wet etching of PMMA with acetic acid can induce the remaining PS structures to collapse due to capillary forces between adjacent PS domains during drying. This study presents an alternative approach to the formation of nanoscale patterns by using an atmospheric UV-ozone (AP-UVO, λ = 365 nm) exposure system. Compared with the general etching system, AP-UVO exposure is relatively low cost, can potentially be scaled up, and proceeds via a simple process. In addition, we demonstrate that the line and space (L/S) sizes of the block copolymer pattern can be controlled by using the AP-UVO etch time. Also, we were able to obtain well-ordered patterns from the combining the AP-UVO exposure process with a control process of block copolymers conditions. These results suggest that the AP-UVO exposure method with block copolymers may be used to form etch masks, particularly for the fabrication of graphene nanoribbons, quantum dots, nano grid wires, and nanopores.

Global shape reconstruction of nano grid with singly fixed camera

Depth from defocus (DFD), as a typical shape reconstruction method, has been widely researched in most recent years. However, all the existing DFD algorithms require at least two defocused images with different camera parameters. Unfortunately, in micro/nano manipulation, any change on visual sensor’s parameters is absolutely forbidden. Therefore, a novel DFD method to reconstruct the shape of a nano grid on micro/nano scale is researched in this paper. First, the blurring imaging model is constructed with the relative blurring and the diffusion equation. Second, the relationship between depth and blurring is discussed from four aspects. Subsequently, depth measurement problem is transformed into an optimization issue which is solved using the gradient flow algorithm. Finally, experiment results and error analysis are conducted to show the feasibility and effectiveness of the proposed method.

FIB micro/nano mould machining process for productivity improvement

FIB has been commonly used as a very powerful tool in the semiconductor industry. It is mainly used for mask repair, device correction, failure analysis, and IC error correction, etc. Currently, FIB is not being applied to the fabrication of the micro and nano-structured mould because of low productivity and sputtering rate has also been required to fabricate the 3D shape. In this paper, we studied the FIB-sputtering rate according to mould materials. As well, surface roughness characteristics were analysed for micro and nano mould fabrication. The Si wafer, GC (glassy carbon), STAVAX, and DLC that have been normally considered as good micro or nano mould materials were used in this study. And also we carried out experiments for productivity of improvement of FIB process. The blu-ray DVD pattern and the nano grid pattern were made using the GC material. In addition, the wave guide mould was fabricated using the DLC material as a test sample to improve the productivity.