Enrique Rodriguez-Diaz

Intelligent DC Homes in Future Sustainable Energy Systems: When efficiency and intelligence work together.

The evidence that climate change is real and most likely caused by human-related activities has made the international community consider a new energy model. Europe has led the initiative, moving away from fossil fuels toward renewable energies, while powerful countries such as the United States and China are lagging behind and still rely heavily on coal, gas, and oil as energy sources. Europe has set ambitious goals for 2020 regarding the increase of renewable energy production, energy efficiency, and greenhouse gas emission reduction. The concept of a microgrid is perfectly aligned with the new energy strategy. A microgrid eases the integration of renewable energy sources (RESs) and energy-storage systems (ESSs) at the consumption level, aiming to increase power quality, reliability, and efficiency. The increasing importance of dc-based loads has reopened the discussion of dc versus ac distribution systems. As a consequence, much research has been done on dc distribution systems and their potential for residential applications. Furthermore, the growing presence and use of smart devices in homes reveal a promising future for intelligent homes integrated in the Internet of Things (IoT), where residential electrical power systems work in cooperation with smart devices to achieve smarter, more sustainable, and cleaner energy systems.

Voltage-Level Selection of Future Two-Level LVdc Distribution Grids: A Compromise Between Grid Compatibiliy, Safety, and Efficiency

Environmental concerns and new energy policies are causing energy systems to shift toward decentralization and sustainability. Electricity generation has been historically based on large-scale fossil and nuclear sources, even though in the last decade, the share of renewables has grown significantly. Microgrids (MGs) come as a suitable solution for the installation of distributed sources in the low-voltage (LV) grid, where most consumers are sparsely located. MGs ease the integration of distributed generators (DGs) with energy storage systems (ESSs) at a consumption level, especially renewable energy sources (RESs), such as solar panels and small wind turbines (WTs). By decentralizing electricity generation, it can now be produced in closer proximity to the consumer, thereby avoiding transmission and distribution losses and increasing the efficiency of the electricity grid, as well as higher power reliability.

An overview of low voltage DC distribution systems for residential applications

The concept of a microgrid has drawn the interest of research community in recent years. The most interesting aspects are the integration of renewable energy sources and energy storage systems at the consumption level, aiming to increase power quality, reliability and efficiency. On top of this, the increasing of DC-based loads has re-open the discussion of DC vs AC distribution systems. As a consequence a lot of research has been done on DC distribution systems and its potential for residential applications. This paper presents an overview of the LVDC distribution systems used in residential applications. Several publications that study the potential energy savings and overall advantages of the LVDC distribution systems are analysed. Different power architectures and topologies are discussed. The existing demonstration facilities where LVDC distribution systems have been implemented are also shown.

A Root-Locus Design Methodology Derived From the Impedance/Admittance Stability Formulation and Its Application for LCL Grid-Connected Converters in Wind Turbines

This paper presents a systematic methodology for the design and the tuning of the current controller in LCL grid-connected converters for wind turbine applications. The design target is formulated as a minimization of the current loop dominant time constant, which is in accordance with standard design guidelines for wind turbine controllers (fast time response and high stability margins). The proposed approach is derived from the impedance/admittance stability formulation, which, on one hand, has been proved to be suitable for the controller design when the active damping is implemented and, on the other hand, has also been proved to be very suitable for system-level studies in applications with a high penetration of renewable energy resources. The tuning methodology is as follows: first, the physical system is modeled in terms of the converter admittance and its equivalent grid impedance; then, a sensitivity transfer function is derived, from which the closed-loop eigenvalues can be calculated; finally, the set of control gains that minimize the dominant time constant are obtained by direct search optimization. A case study that models the target system in a low-power scale is provided, and experimental verification validates the theoretical analysis. More specifically, it has been found that the solution that solves the minimization of the current controller time constant (wind turbine controller target) also corresponds to a highly damped electrical response (robustness provided by the active damping).

Multi-level energy management and optimal control of a residential DC microgrid

Extensive exploitation of renewable energies together with the increased role of low-voltage DC (LVDC) micro-sources in the generation mix of the future electricity networks, have become the driving force behind the DC microgrid applications. In this paper, an optimal dispatch model of a residential DC microgrid (R-DCMG) with different distributed generations (DGs) and loads is proposed and implemented as an optimal hierarchical control strategy. A system-level optimizer is designed to calculate the optimal operating points of the controllable energy sources (CESs) when needed, while lower-level controllers are utilized to enforce the CESs to follow optimal set-points.

Analysis and distributed control of power flow in DC microgrids to improve system efficiency

DC Microgrid attains popularity in integrating renewable energy sources and batteries. It also has the potential to achieve higher efficiency than ac power grid with optimized power flow. In this paper, a general dc microgrid is modeled based on a cluster of general dc nodes, which includes constant power renewables generation, droop-controlled voltage source and different kinds of load. Then the dc power flow is solved for optimization. A voltage restoration method based on consensus algorithm is used to restore the voltage deviation from droop characteristic. An enhanced current regulator is adopted to guarantee the accurate load sharing even under the influence of sensor drift and line resistance. A tie line power flow control method is proposed to regulate the tie line power directly and increase the system efficiency at light load condition. All the considered methods only need the local information and the information from its nearest neighbor thus the system expendability is guaranteed. Simulation and experiment results are provided to validate the proposed methods.

Using smart meters data for energy management operations and power quality monitoring in a microgrid

Smart metering devices have become an essential part in the development of the current electrical network toward the paradigm of Smart Grid. These meters present in most of the cases, functionalities whose analysis capabilities go further beyond the basic automated meter readings for billing purposes, integrating home or building area networks (HAN/BAN), alarms and power quality indicators in some cases. All those characteristics make this widely spread equipment a free, accurate and flexible source of information that can replace expensive and dedicated devices. Therefore, this paper presents the integration of a commercial advanced metering infrastructure (AMI) in the context of a smart building with an energy management system (EMS). Furthermore, power quality monitoring based on this AMI is explained. All the details regarding the implementation in a laboratory scale application, as well as the obtained results, are provided.

Input-Admittance Passivity Compliance for Grid-Connected Converters With an LCL Filter

This paper presents a design methodology and its experimental validation for the input-admittance passivity compliance of LCL grid-connected converters. The designs of the LCL filter parameters and the discrete controller are addressed systematically, and suitable design guidelines are provided. The controller design is developed in the Z-domain, with capacitor-voltage-based active damping used as a degree of freedom to compensate for system delay effects. The role of resistive components in the circuit, which have inherent dissipative properties, is also discussed. As an outcome of the design, a passive input-admittance shaping is obtained. The theoretical development is further verified in a low-scale prototype supplied from a controllable grid simulator. For the sake of generality, different combinations of resonant to sampling frequency are tested. Experimental results fully prove the input-admittance passivity compliance.

Real-time Energy Management System for a hybrid AC/DC residential microgrid

This paper proposes real-time Energy Management System (EMS) for a residential hybrid ac/dc microgrid. The residential microgrid is organized in two different distribution systems. A dc distribution bus which interconnect the renewable energy sources (RES), energy storage systems (ESS) and the buildings common facilities; while the apartments are supplied by an ac distribution system connected to the grid. This architecture avoids any modifications in the electrical installation that supplies energy to the apartments. A pure dc voltage supply is not yet a feasible approach for residential buildings. This architecture increases the overall efficiency of the distribution by interconnecting the RES and ESS thorough a dc distribution bus, and therefore avoiding unnecessary dc/ac conversion stages. The real-time EMS performs an 24 hours ahead optimization in order to schedule the charge/discharge of the ESS, and the energy injection/consumption from the grid. The EMS estimates the RES generation based on the weather forecasting, together with stochastic consumption modelling of the building. The EMS architecture and the residential microgrid have been implemented and tested in a laboratory scale setup. The results shown how the operational costs of the system are effectively decreased by 28%, even with non-accurate estimation of the RES generation or building parameters.

Potential energy savings by using direct current for residential applications: A Danish household study case

This paper presents a study of the potential energy savings by implementing dc distribution systems for residential applications. In general, it is commonly accepted that the use of dc voltage improves the efficiency of the distribution, due to a decrease in the conduction losses and an efficiency improvement in the power converter units. However, for residential applications, the efficiency is not always improved. A grid connected residential microgrid, with renewable energy sources (RES), energy storage systems (ESS) and local loads, is presented in this work. The microgrid has been modelled for an ac-based and dc-based distribution system, in order to simulate and assess the overall efficiency of the system, for different distribution technologies. Commercially available power supplies for home electronics have been modelled and modified to work with both ac and dc distribution voltages. A Danish household has been used as study case. This study has shown that, depending on the application, architecture, consumption and generation profiles, a dc distribution system might not bring any efficiency improvement, especially when the dc voltage is supplied from a grid rectifier that works at low loads. However, for isolated microgrids, the use of dc voltage has the potential to bring a significant efficiency improvement. Nevertheless the potential for cost reduction in all scenarios is very promising.