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from the averaging nature of a number ofhouseholds, and from the smoothing effectsof aggregation. Mixed neighbourhoodscontaining different types of loads wouldmake matters even easier. The dailymismatch between generation and load canbe bridged over with the use of energystorage: about 2–3 kWh (equivalent to 3–4car batteries) per household will be sufficientfor a typical microgrid.For an appropriate ‘mix’ of generators –about 1.5 kW photovoltaic array perhousehold and 50% of householdownership of micro-CHP generators – themicrogrid will become self-sufficient in bothelectricity and heat. It could also operate as ‘stand-alone’ from the main utility – 24INGENIA MICROGRIDS: POWER SYSTEMS FOR THE 21ST CENTURYAlmost all the electricity currently produced in the UK is generated from a centralisedpower system designed around large fossil fuel or nuclear power stations. This powersystem is robust and reliable but the efficiency of power generation is low, resulting inlarge quantities (around 60%) of primary energy being wasted as heat. Tom Markvart andRay Arnold argue that smaller scale power supply networks could deliver substantialenvironmental benefits via higher energy efficiency and by facilitating the integration ofrenewable sources.MICROGRIDSPOWER SYSTEMS FOR THE 21ST CENTURYInstalling networks of small generators powered by solar or other renewable sources would significantly reduce carbon dioxide emissions from buildings © University of Southampton balance over longer time scales. A diversityof generation matched to the load will needto be employed if the microgrid is to becapable of stand-alone operation.Microgrids can exist as a remote powersystem 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. Anarrangement which would permit themicrogrid operator the choice to operate in the ‘grid connected’ or ‘stand-alone’mode is an uncharted territory forconventional power utility engineers, andissues remain to be resolved at both thetechnical and regulatory level.Conventional utility supply operates onthe principle that power is generated when itis required. Energy storage introduces a novelcomponent in utility supply and broadens thedesign criteria. On a quantitative level, the sizeof the energy store is intimately linked to the energy balance and to the requiredsecurity of supply provided by the microgrid.This temporal mismatch betweengeneration and load can be alleviated bymanaging the demand. The shifting of loadfacilitates achieving the energy balance andhelps reduce the size of energy storage.Whilst experience exists of demand-sidemanagement at the industrial level, furtherexperience is needed in the domestic setting.A MICROGRID MODELBy way of example, we may consider amicrogrid of domestic users powered bymicro-CHP generators and photovoltaics.Solar cells which generate mainly duringsummer combine well with micro-CHPdriven by the production of heat. Severalcompanies are about to introduce micro-CHP products onto the market. Powergen’sWhispergen has been pilot tested and is nowavailable commercially. BG group’s Microgenis expected to go on sale in spring 2007.These units, powered by natural gas, aredesigned to supply heat for a typical house(8–10 kWh) and produce 1–3 kW of electricity.Because of its variability, domestic load isnot easily supplied by a single generator (seeFigure 1). The microgrid, however, benefitsEMERGING TECHNOLOGIESISSUE 24 SEPTEMBER 200525The UK electricity system has evolvedaround the abundance of fossil fuel,principally coal. Large power stations havetraditionally fed the National Grid – a highvoltage interconnected transmissionnetwork – with low-voltage distributionnetworks supplying power from sub-stations to the end users.The evolution of energy technologiesand the consequent awareness of theenvironmental impacts that this can bringhas painted a markedly different picture ofhow energy could be generated andsupplied. There is now a trend towardsdeveloping smaller generators, withemphasis on high efficiency and lowemissions. These distributed generatorsconnected to the distribution network donot fit easily into the traditional powergeneration hierarchy. There are problemscaused by the intermittent nature of thegeneration: the output from a wind farm ora photovoltaic array depends on theclimatic conditions, and Combined Heatand Power (CHP) plants are usuallycontrolled by the production of heat.Microgrids offer a solution which canprovide a stable and reliable power supplydespite a high penetration of intermittentenergy sources.MICROGRIDS EXPLAINEDA microgrid is a small-scale power supplynetwork that is designed to provideenergy for a small community. The ‘smallcommunity’ may be a typical housingestate, an isolated rural community, amixed suburban environment,an academicor public community such as a universityor school, a commercial area, an industrialsite, a trading estate, or a municipal region.The key concept that differentiates thisapproach from a conventional powerutility is that the power generators aresmall (often referred to as microgenerators,of a similar size to the loads within themicrogrid). They are also located in closeproximity to the energy users. Thegenerators, and possibly also loads, arethen managed to achieve a local energyand power balance.This development in distributedgeneration has been compared by theEconomist with the changes currently takingplace in the telephone industry and it hasbeen estimated that, within a decade, themarket for such equipment will exceed$60 billion a year. The US Electric PowerResearch Institute and ABB (formerly AseaBrown Boveri) can foresee the emergenceof ‘virtual utilities’ which, by analogy withthe internet, will allow intelligent meteringand switching. The anticipated resultsinclude reduced environmental impact,greater system reliability and loweroperating cost.Microgrids promise substantialenvironmental benefits through higherenergy efficiency and by facilitating theintegration of renewable sources such asphotovoltaic arrays or wind turbines. Byvirtue of a good match between generationand load, microgrids have a low impact onthe electricity network, despite theirpotentially significant level of generation.However, to achieve this, a number oftechnical, regulatory and economic issueshave to be


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Smith EGR 325 - Micro Grids

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