By Steven Morris
Two visions for a future electric grid include the ‘super grid’ and the ‘dispersed grid’, which both strive for near perfect electricity delivery service. The super grid concept attempts to improve each individual element of the centralised power delivery system – the end result being a homogeneous level of power quality, reliability, and availability, higher than that which exists today. The dispersed grid focuses on improving the local distribution system in order to serve sensitive loads. This is achieved partly by placing supply sources closer to the loads to protect them from service disruptions. This allows for power quality, reliability, and availability to be tailored to meet end-use requirements.
A microgrid is a small-scale, local, integrated energy system that can work with the electric grid or operate by itself in island mode. Microgrids are one of the key tools for achieving the dispersed grid’s objectives, providing a way for generation resources to work parallel to the grid or in island mode during disruptions, thereby increasing power quality, reliability, and availability. A microgrid is comprised of multiple distributed energy resources (DERs) and multiple loads. It can control itself independently or operate in conjunction with the main grid, connecting or disconnecting itself seamlessly as needed, without disrupting service.
BENEFITS OF MICROGRIDS
The potential benefits of microgrids are wide reaching, ranging from reduced costs to increased security. One of the major benefits that can be achieved with microgrids is reduced emissions. Because they are located close to load, microgrids easily support combined heat and power (CHP) which has higher efficiency than central station generation, thereby reducing greenhouse gas emissions. Efficiency is also increased through reduced line losses.
Microgrids can also increase the reliability of the grid by making it more able to resist outages during critical situations and providing electricity free of interruption through their islanding capabilities. Having small, more manageable sources of generation offers more reliability than having one large, centralised energy generator. Finally, microgrids will provide different levels of power quality, reliability, and availability to match customers’ unique energy requirements. This will enable customers who value reliability to choose the level they wish to pay for.
BARRIERS TO MICROGRIDS
However, there are barriers blocking the widespread implementation of microgrids and discouraging industry participants from fully exploring this concept. Lack of knowledge about microgrid performance and design must be rectified through demonstration projects. Monitoring and control systems need to be developed that enable microgrids to handle a large variety of circumstances created by changes in generation and load. Because they are integrated into the grid, microgrids need to be open to communications from system operators and have the ability to react quickly. Finally, microgrids must make economic sense; they must provide a lower cost, higher level of service, or both in order for adoption to take place.
ENERGY MANAGEMENT IN THE MICROGRID
A microgrid consists of numerous DERs which can produce CHP with differing levels of efficiency. This means that in order to ensure maximum efficiency, an energy management system (EMS) is needed. The EMS controls which DER is operating and when it is operating. It makes the complex decisions needed to maximise the efficiency and minimise the cost of the microgrid. This means taking into account production costs, emission levels, and heat requirements, among other variables.
The EMS needs to identify all the various options available to the DER – serving native load, exporting power to the grid, providing ancillary services to the grid, and meeting process heat or steam demands. The cost and value of providing these services must be calculated and compared. If not all generation is needed, decisions need to be made on which DER to run.
These decisions are complex. Should load be reduced to sell power to the grid at high prices during system emergencies? Should off-peak power be purchased from the grid if it is less expensive than self-generation? Should gas be burned directly to generate heat or used to generate electricity and waste heat used? To determine the appropriate response, both the economics of the microgrid and the load requirements for heat and power must be known. The priorities and costs of decisions on business operations must also be understood. The EMS must be able to evaluate all this information in order to make the optimal decision.
MICROGRID RESEARCH, DEVELOPMENT AND DEMONSTRATIONS
A number of industry participants are exploring and developing microgrid technologies with major microgrid research efforts and implementation projects occurring primarily in Europe, the US and Japan. These efforts are intended to evaluate and assess the performance of microgrids, including how they improve reliability and decrease costs. Some of these projects are experimental in nature, or are attempting to demonstrate how microgrids can be used.
One key microgrid project in the US is being performed by General Electric, which is working on a research programme to integrate energy efficiency, cogeneration, and renewable energy in order to bring about cost effective, reliable, and higher quality electricity delivery services. The project is working on the development of business models, design specifications, and controls for microgrids. One expected outcome of the project is an EMS which will serve as a building block for microgrids in the future. In another American effort, the Consortium for Electric Reliability Technology Solutions (CERTS) is pursuing development of the CERTS Microgrid concept which seeks to enable the integration of DER into the grid. The concept is being tested at an American electric power facility.
In Europe, the Technical University of Denmark, the Centre for Electric Technology (CET) is spearheading efforts to develop a smart grid in Denmark. As part of that effort, CET has undertaken two microgrid-specific projects. The first project, ‘Microgrids: Large scale integration of microgeneration on low voltage grids’, was focused on methods of controlling microgrids and demonstrating their feasibility. A follow up project, ‘EU more microgrids: Advanced architectures and control concepts for microgrids’, is focused on taking microgrids from the laboratory to the field in a series of pilot projects.
In Japan, the New Energy and Industrial Technology Development Organisation (NEDO) has initiated three demonstration projects of microgrids under the ‘Regional power grid with renewable energy resources project’. These three demonstration projects are the Aomori project in Hachinohe, the Aichi project near Central Japan Airport, and the Kyoto project at Kyotango. In addition, numerous projects are being conducted in order to move microgrid research beyond the laboratory and into a real world setting. These projects are attempting to demonstrate the feasibility of a microgrid, as well as its benefits, so that companies will become more interested in investing in these technologies.
DEMONSTRATION PROJECTS IN JAPAN
The Aichi microgrid, NEDO’s first demonstration project, was implemented at the 2005 World Exposition in Aichi, Japan. Its purpose was to generate electricity from renewable energy sources at the Nagakute Japan Pavilion, as well as to demonstrate how multiple energy sources could be combined and controlled in a microgrid. A second project, the Aomori microgrid in Hachinohe city, started operations in October 2005 and continued though early 2008 with 100 kW of installed photovoltaic (PV) and wind capacity, 510 kW of digester gas-fired reciprocating engines, and 100 kW of battery storage. These generating units are creating CHP for seven buildings in the city of Hachinohe, including the City Hall and several schools. Since the microgrid is interconnected with the main electric grid, the project will include testing islanding functions.
In Europe, Continuon Netbeheer has built a microgrid consisting of 315 kW on installed PV capacity to support load consisting of over 200 holiday cottages in a camp. When load is light, excess power is exported to the grid and during peak hours needed electricity is imported to the microgrid. Another European project is taking place on Kythnos Island in Greece. The DER being used consists of 10 kW of installed PV capacity, a 5 kW diesel generator, and 53 kWh of battery storage. The microgrid supplies power to 12 houses in the village of Gaidouromantra, which is 4 km from the nearest medium voltage power line. Additional capacity of 5 kW from a wind turbine is planned for the future.
In the US, Northern Power Systems has been developing a microgrid in Waitsfield, Vermont since 2003. The project will consist of a 267 kW propane-fired reciprocating engine, a 100 kW biodiesel engine generator, a 30 kW propane-fired microturbine, and 1.5 kW of PV. The load consists of five commercial and industrial buildings and 12 residential homes, all customers of Washington Electric Cooperative (WEC).
Also in the US, Stamford, Connecticut has passed an ordinance establishing an Energy Improvement District to provide local businesses an opportunity to join with the city in a microgrid. A key driver of this project is the city’s emergency operating centre which requires a higher level of power quality, reliability, and availability. The city intends to build a microgrid to support critical operations and sell excess power to local businesses. A feasibility study of the project is currently underway.
The electric grid served end-users well throughout the latter half of the 20th century, providing sufficient and affordable electricity to customers across the country. However, society has entered a new digital age. As the economy revolves more and more around digital operations, greater amounts of electricity are required. Along with a requirement for more electricity comes a requirement for better electricity service. Companies whose primary activities are based on the creation, dissemination, or storage of information have a need for continuous, uninterrupted, high-quality electricity. These end-users are seeking five 9s (i.e. 99.999%) reliability, which implies that disruptions and outages would be only a few minutes per year.
The electric grid that was built in the 1950s is outdated and unable to accommodate these growing needs. The grid is centred on the use of large, centralised generation units that are often located far away from end-users, making it vulnerable to disruption. The Northeastern Blackout of 2003 demonstrated this vulnerability, which can negatively impact communities and businesses. The growing threat of terrorism, cyber attacks, natural disasters, and other unforeseen events pose great challenges to preventing outages and disruption and the current grid is not structured to provide the necessary level of power quality, reliability, and availability for the digital age. The potential benefits of microgrids are wide-reaching, ranging from reduced costs, reduced environmental pollution, and greater reliability to enhanced security. However, there are barriers currently blocking the widespread use of microgrids and discouraging industry participants from fully exploring these technologies.
To achieve the benefits and overcome the barriers a significant amount of research, development, and demonstration is still needed. Commercial implementation must be achieved which proves that the operating capabilities of a microgrid do in fact deliver higher levels of power quality, reliability, and availability. In addition, real world economics must be proven if companies are to invest in microgrids.