Smith EGR 325 - Micro Grids (4 pages)

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Micro Grids



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Micro Grids

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4
School:
Smith College
Course:
Egr 325 - Electric Power Systems
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MICROGRIDS POWER SYSTEMS FOR THE 21ST CENTURY EMERGING TECHNOLOGIES MICROGRIDS POWER SYSTEMS FOR THE 21ST CENTURY Almost all the electricity currently produced in the UK is generated from a centralised power system designed around large fossil fuel or nuclear power stations This power system is robust and reliable but the efficiency of power generation is low resulting in large quantities around 60 of primary energy being wasted as heat Tom Markvart and Ray Arnold argue that smaller scale power supply networks could deliver substantial environmental benefits via higher energy efficiency and by facilitating the integration of renewable sources The UK electricity system has evolved around the abundance of fossil fuel principally coal Large power stations have traditionally fed the National Grid a high voltage interconnected transmission network with low voltage distribution networks supplying power from substations to the end users The evolution of energy technologies and the consequent awareness of the environmental impacts that this can bring has painted a markedly different picture of how energy could be generated and supplied There is now a trend towards developing smaller generators with emphasis on high efficiency and low emissions These distributed generators connected to the distribution network do not fit easily into the traditional power generation hierarchy There are problems caused by the intermittent nature of the generation the output from a wind farm or a photovoltaic array depends on the climatic conditions and Combined Heat and Power CHP plants are usually controlled by the production of heat Microgrids offer a solution which can provide a stable and reliable power supply despite a high penetration of intermittent energy sources 24 INGENIA MICROGRIDS EXPLAINED A microgrid is a small scale power supply network that is designed to provide energy for a small community The small community may be a typical housing estate an isolated rural community a mixed suburban environment an academic or public community such as a university or school a commercial area an industrial site a trading estate or a municipal region The key concept that differentiates this approach from a conventional power utility is that the power generators are small often referred to as microgenerators of a similar size to the loads within the microgrid They are also located in close proximity to the energy users The generators and possibly also loads are then managed to achieve a local energy and power balance This development in distributed generation has been compared by the Economist with the changes currently taking place in the telephone industry and it has been estimated that within a decade the market for such equipment will exceed 60 billion a year The US Electric Power Research Institute and ABB formerly Asea Brown Boveri can foresee the emergence of virtual utilities which by analogy with the internet will allow intelligent metering and switching The anticipated results include reduced environmental impact greater system reliability and lower operating cost Microgrids promise substantial environmental benefits through higher energy efficiency and by facilitating the integration of renewable sources such as photovoltaic arrays or wind turbines By virtue of a good match between generation and load microgrids have a low impact on the electricity network despite their potentially significant level of generation However to achieve this a number of technical regulatory and economic issues have to be resolved before microgrids can become commonplace ENABLING MICROGRIDS There needs to be precise energy and power balance within microgrids on a timescale ranging from milliseconds to years Over a short timescale the power balance is linked to the question of control and frequency over longer time scales one needs to consider the relationship between energy supply demand and storage Sufficient energy must be available from the generators to ensure energy balance over longer time scales A diversity of generation matched to the load will need to be employed if the microgrid is to be capable of stand alone operation Microgrids can exist as a remote power system in regions where utility supply is not available They may on the other hand be embedded in a larger electrical utility this would be the typical scenario in the UK with its mature utility power system An arrangement which would permit the microgrid operator the choice to operate in the grid connected or stand alone mode is an uncharted territory for conventional power utility engineers and issues remain to be resolved at both the technical and regulatory level Conventional utility supply operates on the principle that power is generated when it is required Energy storage introduces a novel component in utility supply and broadens the design criteria On a quantitative level the size of the energy store is intimately linked to the energy balance and to the required security of supply provided by the microgrid This temporal mismatch between generation and load can be alleviated by managing the demand The shifting of load facilitates achieving the energy balance and helps reduce the size of energy storage Whilst experience exists of demand side management at the industrial level further experience is needed in the domestic setting A MICROGRID MODEL By way of example we may consider a microgrid of domestic users powered by micro CHP generators and photovoltaics Solar cells which generate mainly during summer combine well with micro CHP driven by the production of heat Several companies are about to introduce microCHP products onto the market Powergen s Whispergen has been pilot tested and is now available commercially BG group s Microgen is expected to go on sale in spring 2007 These units powered by natural gas are designed to supply heat for a typical house 8 10 kWh and produce 1 3 kW of electricity Because of its variability domestic load is not easily supplied by a single generator see Figure 1 The microgrid however benefits Installing networks of small generators powered by solar or other renewable sources would significantly reduce carbon dioxide emissions from buildings University of Southampton from the averaging nature of a number of households and from the smoothing effects of aggregation Mixed neighbourhoods containing different types of loads would make matters even easier The daily mismatch between generation and load can be bridged over


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