Scenariosforexaminationofhighlydistributed powersystems · sions Trading Scheme (EU ETS), CCGT is assumed to be the preferred generation technology. This vision is in accordance with

SPECIAL ISSUE PAPER 643

Scenarios for examination of highly distributedpower systemsC N Jardine1∗ and G W Ault2

1Environmental Change Institute, University of Oxford, Oxford, UK2Department of Electronic and Electrical Engineering, Institute for Energy and Environment, The University ofStrathclyde, Glasgow, UK

The manuscript was received on 23 August 2007 and was accepted after revision for publication on 17 March 2008.

DOI: 10.1243/09576509JPE517

Abstract: A set of three scenarios has been created in order to examine the incorporation ofextensive penetrations of micro-generators into electricity networks (termed ‘highly distributedpower systems’). The scenarios have been created as a synthesis of the Future Network Technolo-gies scenarios and the UK domestic carbon model, and yields energy use and carbon dioxideemissions of the UK housing stock from inputs of household numbers, house type, thermalefficiency, appliance efficiency, as well as the number and efficiency of micro-generators used.The centralized supply mix also varies between scenarios and features extensive penetrations oflarge-scale renewables.

The scenarios illustrate the scale of change required to reduce CO2 emissions by 60 per cent by2050, which has substantial impacts for electricity network operation. Moving from a centralizedsystem to the one where one-third of electricity comes from distributed sources poses significantchallenges including: reverse power flow on networks, load balancing, storage requirements,phase unbalance, harmonics, and ancillary services.

Keywords: micro-generation, electricity networks, scenarios, low carbon

1 INTRODUCTION

Micro-generation is seen as a key means of reducingcarbon dioxide emissions and utilizing energy moreefficiently [1]. As such, the use of many distributedlow or zero carbon generators offers an attractivemeans of achieving national energy policy goals.However, the integration of many small-scale gener-ators into electricity networks, and coordinating theiroperation, remains a key technical challenge beforemicro-generation can form a major share of the UKelectricity generation mix. The SUPERGEN consor-tium on highly distributed power systems (HDPS) iscurrently researching this issue.

It is useful to examine the scenarios describing arange of contrasting futures to allow sensitivity to keyparameters to be determined. By examining a range of

∗Corresponding author: Environmental Change Institute, Univer-

sity of Oxford, Dyson, Perrrins, Building, South Parks Road, Oxford

OX1 3QY, UK. email: [email protected]

possible futures, the need to make a single long-termprediction of the future is removed. However, althoughmany existing sets of scenarios, such as the Royal Com-mission on Environmental Pollution’s scenarios [2] orthe Tyndall centre scenarios [3], feature extensive useof renewable and low carbon generation, they do notexplicitly allocate this capacity to micro-generation.It is therefore a need to develop a set of scenariosspecifically for looking at the deployment of micro-generation capacity, its environmental attributes, andnetwork impacts.

Two existing sets of scenarios were identified, whichexhibited high levels of micro-generation. The FutureNetwork Technologies (FNT) scenarios [4] describea range of six high-level scenarios of electricity net-works in 2050. Each of the scenarios considers arange of technical, economic, environmental, andregulatory possibilities and predicts generation andcapacity mixes and network infrastructures. The UKdomestic carbon model (UKDCM) was developed tomodel energy and carbon use within housing between1996 and 2050 and looked at the influence of energy

JPE517 © IMechE 2008 Proc. IMechE Vol. 222 Part A: J. Power and Energy

644 C N Jardine and G W Ault

efficiency measures and micro-generation as a meansof progressing to a low carbon future in the 40 per centhouse project [5]. The model has since been developed(UKDCM2) [6] to produce three scenarios [7], allowinga more detailed sensitivity analysis of key parameters,which include household numbers; insulation levelsof walls, windows, roofs, and floors; internal temper-ature; appliance ownership; and use of low and zerocarbon technologies (LZCs) within the home.

The FNT scenarios are supply-side focused, whereasthe UKDCM takes a demand-side approach. The FNTscenarios and the UKDCM were therefore combined toproduce a set of three ‘SUPERGEN HDPS scenarios’ foruse within the research consortium. This integrationof the two set of scenarios has kept the level of detailpresent in each approach and allows a representationof the housing stock, appliance usage, electricity andheat demand, and the centralized generation mix – allkey parameters for the study of micro-generation andits influence on electricity networks.

2 CREATING THE SCENARIOS

The combination of the two parent scenarios wascomplicated by the differences in approach in theirrespective development. Although they are wellmatched in overlying ethos, the numbers themselvesdo not tally exactly, so the hybrid SUPERGEN HDPSscenarios required choices to be made as to which par-ent scenario to use for any given parameter. In general,the FNT scenarios were used to provide relative pen-etrations of centralized generators, while the UKDCMcontributed domestic demand and penetrations ofthe different micro-generation types (Table 1). TheFNT scenarios ‘Business as Usual’ (BAU), ‘Strong Opti-mism’, and ‘Green Plus’ were combined with UKDCMscenarios A, B, and a variant on B, respectively, to

produce the three SUPERGEN HDPS scenarios. Theexisting estimates of non-domestic electricity demand[8] and the forthcoming retirement of nuclear [9] andcoal plant [10, 11] were also incorporated into the finalscenarios. It is worth remembering that the scenar-ios were chosen solely for their high micro-generationcomponents and that other visions of centralized gen-eration options (notably a high nuclear, future) areexplicitly not examined here.

2.1 Assumptions in the scenarios

The scenarios were created in three stages: electricitydemand was predicted out to 2050, electricity gener-ation from decentralized sources was predicted, andthe remaining electricity generation requirement wasmet from centralized sources.

2.1.1 Electricity demand

For each scenario, electricity demand for householdswas generated from the relevant UKDCM scenariobased on the assumed stock of lights and appliancesas well as building type and occupancy. Non-domesticelectricity demand was split into two broad categories:industrial and other uses. Scenarios for future energyuse were taken from Department ofTrade and Industry(DTI) [8]. The baseline case was used for the BAU sce-nario and data were scaled to the ‘behavioural change’case for use in scenarios where demand reduction wasconsidered critical. Total electricity demand was cal-culated to be the sum of domestic and non-domesticdemands.

2.1.2 Decentralized electricity generation

Each scenario assumes different penetrations andaverage capacities of LZCs (Stirling combined heatand power (CHP), fuel cell CHP, district heating, heat

Table 1 How FNTs and UKDCM scenarios combine to form SUPERGEN HDPS scenarios

FNT scenarios UK domestic carbon model Other sources SUPERGEN HDPS scenarios

Total electricity demand Domestic heat demand Domestic heat demandDomestic electricity

demandNon-domestic electricity

demand [8]Total electricity demand

(domestic and non-domestic)

Micro-generationtechnologiespenetrations



Domestic micro-generation heatprovision

Domestic micro-generation heatprovision

Decentralized electricitygeneration

Decentralized domesticelectricity generation

Assumptions madeabout non-domesticelectricity generation

Decentralized electricityprovision

Centralized electricitytechnologies

Centralized generationtreated as singleemissions factor

Nuclear [9] and coal[10, 11] plant retirementrenewables obligation

Centralized electricitytechnologies

Socio-technological vision Socio-technological vision Socio-technological vision

Proc. IMechE Vol. 222 Part A: J. Power and Energy JPE517 © IMechE 2008

Scenarios for examination of HDPS 645

pumps, solar thermal, solar photovoltaics (PV), andμ-wind) over time to 2050. The UKDCM calculatesthe electricity generated by decentralized sources:rooftop technologies are capacity multiplied by annualyield, whereas thermal technologies operate as heat-led devices and meet total space and water heatingrequirements in the home. Average thermal efficien-cies and annual electricity generation for all technolo-gies increase over time as technologies develop.

Decentralized electricity sources will also provideheat and electricity to non-domestic properties. TheUKDCM does not explicitly model non-domesticbuildings at present, so assumptions were madeabout electricity generation in such properties. Non-domestic properties were assumed to be able to gen-erate 50 per cent of electricity demand on-site as theyare typically larger buildings with proportionally lowerheat requirements (i.e. less CHP or solar thermal),smaller roof areas (i.e. less PV or wind), and higherelectricity demand (i.e. high occupancy). Comparingthe non-domestic and domestic sectors, it is assumedthat the former produces 32 per cent of the electric-ity of the latter in 2050. However, current economicconditions (economies of scale, smaller proportionaloverheads per kilo watt installed, access to up-frontcapital, dedicated energy managers, and promotion ofgreen-image for corporate social responsibility aims)favour the installation of a larger equipment in non-domestic properties rather than many small instal-lations at the domestic level. It is therefore assumedthat non-domestic micro-generation will be relativelymore significant in the shorter term, so electricitygeneration from non-domestic micro-generation isassumed to be 100 per cent of domestic generationacross all scenarios in 1996. This declines to the 32 percent outlined above by 2050.

2.1.3 Centralized electricity generation

Centralized sources provide the balance of electricityprovision (total demand - distributed generation). TheFNT scenarios provide the percentage mix of electric-ity generation from centralized sources in 2050. Micro-generation, PV, μ-wind, and biomass were modelledwithin decentralized electricity generation, as above.The relative proportions of offshore wind, onshorewind, marine, Combined Cycle Gas Turbine (CCGT),CCGT with carbon capture and storage (CCS), nuclear,and coal in 2050 are taken from the appropriate FNTscenarios and scaled to fill the total residual electricitydemand. Trajectories are estimated for offshore wind,onshore wind, marine, and CCGT with CCS out to2050. Each technology was assigned a take-off date,final generation level, and maturity date, and an s-curve was fitted to model the trajectory. For example,marine generation was assumed to be 0 until 2020 andincrease in capacity until maturity in 2050.

Nuclear generation was assumed to decrease until2025 in line with the foreseen programme of nuclearplant closures [9]. Coal-fired generation was alsopredicted to decrease to 2016 in line with the expectedimpacts of the large combustion plant directive [10]and is assumed to be an average of the DTI’s high andlow case [11] resulting in a decline from 118 to 75 TWhof electricity supplied per annum. Large hydro [12] wasassumed to be constant across all scenarios to 2050.Other sources, mainly landfill gas, are also significantin the current generation mix, but this was assumed todecline to 2050, under the influence of the EU Land-fill Directive [13]. Other biofuels are assumed to beincreasingly important to 2050, but these are envis-aged operating as community scale CHP to minimizefuel transport. Such biomass use is therefore includedin the decentralized generation sector.

The remaining centralized generation to beaccounted for could have been filled by either CCGTor coal with flue gas desulphurization. In accordancewith the low carbon visions of most scenarios, and thecontinued influence of policies such as the EU Emis-sions Trading Scheme (EU ETS), CCGT is assumed tobe the preferred generation technology. This vision isin accordance with the DTI’s own energy scenarios to2020 [14].

3 THE SCENARIOS

3.1 The HDPS BAU scenario

This scenario represents a future where change isincremental and reflects the continuation of near termtrends, technological development, environmentalattitudes, and policy. The scenario has been createdas a combination of FNTs BAU scenario with ‘Sce-nario A’ from the UKDCM. There is some increasein small-scale generation technologies, which is con-sumer demand led, rather than the result of significantpolicy intervention. Overall, the electricity networkremains dominated by centralized fossil fuel gener-ation, and network control continues to operate inthe same manner as present. However, large cen-tralized renewables continue to be a growth area, inparticular offshore wind, resulting in a decarboniza-tion of electricity supply. As a result, consumptioncontinues to rise, but carbon dioxide emissions arereduced.

Society remains unrestricted in its consumption ofenergy and improvements in energy efficiency con-tinue to be outweighed by an increase in demand forenergy services. In particular, the consumer electron-ics sector is assumed to continue to be a major growtharea, with plasma televisions, PCs, and digital ser-vices commonplace. An increasingly wealthy societywill demand more luxury goods such as cooling and



Fig. 1 Electricity supplied by different sources under theBAU scenario

outdoor goods such as patio heaters and jacuzzis. Elec-tricity demand in the domestic sector rises from 116 to177 TWh per annum, with similar increases observedin industrial and commercial sectors resulting in atotal electricity demand of 480 TWh in 2050.

Demand for heating is also expected to increasemainly as a result of the increase in numbers ofhouseholds. Furthermore, average internal tempera-tures are expected to rise and then saturate at 23 ◦C,so the incremental improvements in efficiency of thehousing stock and heating technologies are largelytaken back in the form of additional comfort. Similarly,demand for hot water continues its strong growth andis predicted to rise by 28 per cent by 2050.

Centralized electricity supply remains dominatedby fossil fuel plant, with CCGT (nearly half with

Fig. 2 Proportion of heat supplied to homes by differentsources under the BAU scenario

CCS), coal, and nuclear important sources of gener-ation. However, there is also an increase in renewablegeneration, with offshore wind showing especiallystrong growth (Fig. 1).

Under the BAU scenario, the growth in utilizationof distributed generation technologies follows currenttrends. This sees a marginal penetration of all tech-nologies, with solar thermal being installed on 10 percent of properties, with 5 per cent of houses havingPV and μ-wind. There is also a slight trend away fromgas central heating towards Stirling and fuel cell CHP,heat pumps, and district heating, but no technologymakes a major impact by 2050 (Fig. 2). Nonetheless,the cumulative effect of these small changes is signif-icant and coupled with demand reduction measures,decentralized sources supply 29 per cent of heat and23 per cent of electricity to homes.

Fig. 3 Energy use by fuel and CO2 emissions from the domestic sector under the BAU scenario



Within the domestic sector, energy use from all fuelsrises 12 per cent to 512 TWh by 2050, but greater useof renewables and CHP sees CO2 emissions decreaseto 30.5 MtC per annum (71 per cent of 1996 levels), asshown in Fig. 3.

3.2 The HDPS low carbon scenario

This scenario presents a vision of a low carbon future,featuring extensive penetrations of distributed gener-ation, by combination of the FNT ‘Strong Optimism’scenario and the UKDCM ‘Scenario B’. Environmen-tal objectives are a priority and electricity and heatdemand are reduced compared with the BAU scenarioas a result of strong energy efficiency measures. Dis-tributed generation contributes ≈44 per cent of overallelectricity supply, and residential dwellings are netexporters of electricity in 2050. Centralized genera-tion remains diverse but is completely decarbonizedby 2050. Renewables (offshore wind, onshore wind,and marine) contribute 71 per cent of centralized gen-eration, with nuclear and CCGT making up the balance(Fig. 4).

Under this scenario, the society is assumed to valuecarbon and adopt energy-saving measures accord-ingly. Strong product standards mean that the tech-nical potential of appliances is reached, with vacuuminsulated panel fridges and light emitting diode (LED)lighting the norm. Restrictions are also placed on newproducts (e.g. patio heaters and air conditioners) andfuel switching to gas is encouraged where appropri-ate. The net result is that domestic electricity demandpeaks in 2015, and reduces to 111 TWh in 2050. Mildgrowth in electricity demand is seen in the industrialand commercial sectors. Overall electricity demandpeaks in 2020 at 376 TWh per annum, reducing to345 TWh in 2050.

Fig. 4 Electricity supplied by different sources under thelow carbon scenario

Fig. 5 Heat supplied to homes by different sourcesunder the low carbon scenario

In a more carbon aware society, householders areassumed to be more aware of their energy use forheating in the home. As a consequence, indoor tem-peratures continue to rise in line with current trendsbut saturate at 22 ◦C. Heat demand is increasingly pro-vided by LZCs: in 2050, Stirling engine CHP accountsfor 15 per cent of heat provision; fuel cell CHP 20 percent; heat pumps 5 per cent; and district heating 15 percent (Fig. 5). The heat supplied by gas boiler systemsdeclines to just 26 per cent, but electric heating risesto 13 per cent, mainly to provide highly efficient newbuild properties with their minimal heating require-ments (<2000 kWh per annum) – for such properties,it is not considered cost-effective to install wet centralheating systems. The rooftop technologies also showstrong growth with solar thermal on 12 per cent ofproperties, PV on 10 per cent, and μ-wind on 7 percent, and heat pumps accounting for a further 5 percent of heat demand.

Micro-generation makes a substantial contributionto the electricity supply mix in 2050 (Fig. 4). House-holds are net exporters, and overall micro-generationprovides 117 TWh out of an economy-wide demand of345 TWh. Onshore and offshore wind grows stronglyin the short term under the influence of the renew-ables obligation, as does marine in the longer term. Allcoal plants are closed by 2050, but there remains someresidual nuclear generation. Gas use makes up theshortfall of electricity between 2015 and 2030, peak-ing at a maximum of 206 TWh in 2021, but is usedless as renewables come on line. By 2050, the remain-ing gas-fired plant is used in conjunction with carbonsequestration technologies.

Within the domestic sector, energy use from all fuelsdecreases 1 per cent to 379 TWh by 2050, despite the 33per cent increase in households (Fig. 6). The increaseduse of centralized renewables and decentralized gen-eration yields domestic CO2 emissions of 18.7 MtC



Fig. 6 Energy use by fuel and CO2 emissions from the domestic sector under the low carbonscenario

per annum in 2050 (43 per cent of 1996 levels). Thisis in line with the 60 per cent emissions reductionestimated to be necessary by current government pol-icy. The remaining emissions are all for meeting heatdemand within homes – it is important to note thatdecarbonizing the electricity supply is not enough onits own to reduce CO2 emissions by 60 per cent.

3.3 The HDPS deep green scenario

This scenario presents an alternative vision of a lowemissions future and one in which renewable genera-tors play a more important role in electricity provision.It has been created from the FNT ‘Green Plus’ sce-nario and a variant on UKDCM scenario B. Onceagain, demand for heat and electricity is minimizedby improvements to the building fabric and demandreduction measures. Electricity and heat demandsare identical to the low carbon scenario; the onlydifferences arise from how the energy is provided.Centralized generation is completely renewable, withoffshore wind providing 80 per cent of centralized gen-eration and the remainder from onshore wind andmarine sources. Fossil fuel generation does not existat all as a consequence of high fossil fuel prices –either because supply is short or financially penalizedon environmental grounds (Fig. 7).

As with the low carbon scenario, internal tempera-tures are assumed to saturate at 22 ◦C. LZCs providethe majority of heating, but because of the high gasprices under this scenario, CHP plays a smaller role.In 2050, Stirling engine CHP accounts for only 10 percent of heat requirements, fuel cell CHP 15 per cent,heat pumps 10 per cent, and district heating 20 percent (Fig. 8). Importantly, biomass also plays a moreimportant role in heat provision, supplying 10 per cent

Fig. 7 Electricity supplied by different sources under thedeep green scenario

Fig. 8 Heat supplied to homes by different sourcesunder the deep green scenario



of homes directly, and 30 per cent of Stirling enginesand 50 per cent of district heating schemes are alsoassumed to be fuelled by biomass or waste. The heatsupplied by gas boiler systems declines to just 21 percent, and electric heating rises to 13 per cent. Domesticelectricity generation becomes more important withPV and μ-wind on 20 per cent and 10 per cent ofhouseholds by 2050.

Electricity demand under the deep green scenariois assumed to be identical to that under the low car-bon scenario. These two scenarios present the samesocietal vision, and demand is therefore identical.These scenarios only differ in the means in which theelectricity demand is met.

Under the deep green scenario, micro-generationagain plays a major role in 2050, providing 147 of345 TWh (Fig. 7). The centralized electricity genera-tion is completely decarbonized by 2050, and sourcedentirely from renewables. The majority comes fromoffshore wind, with onshore wind and marine tech-nologies providing the balance. Once again, extragas-fired generation capacity is needed between 2015and 2030 to cover the declining output of coal andnuclear plants, but this is lower than under other sce-narios due to the extensive offshore wind capacityinstalled in the near to medium term.

Overall, LZCs provide 66 per cent of heat to house-holds. Electricity generation is slightly lower thanunder the low carbon scenario at 111 TWh perannum, mainly due to the lower penetration of fuelcell CHP.

Within the domestic sector, energy use from all fuelsdecreases 1 per cent to 379 TWh by 2050 carbon diox-ide emissions reduce to 13.7 MtC per annum (32 percent of 1996 levels), less than the low carbon scenariodue to the greater use of biomass for heat provision(Fig. 9). This clearly emphasizes the importance of

renewable sources of heat for meeting GovernmentCO2 emissions reduction targets.

3.4 Summary of scenarios

The key features of the scenarios are summarized inTable 2.

4 CONSEQUENCES OF THE SCENARIOS

The scenarios presented allow some conclusions to bedrawn about the environmental benefits of distributedgeneration and the electricity network consequencesof such changes. The extent of change, especiallyunder the low carbon and deep green scenarios is vast– moving from a centralized generation model in thepresent day to one in which households are self suffi-cient in electricity. However, the changes illustratedhere are indicative of the scale of change requiredto reduce CO2 emissions by 60 per cent by 2050. Itis clear that these changes will pose significant chal-lenges for network operation, especially in the form ofuncontrolled generation. There is a potential conflictbetween reducing CO2 emissions and network oper-ation (and costs), and forthcoming work will addressthe issues raised in the rest of this paper. The followingsection highlights potential issues, but does not seekat this stage to provide any definitive answers.

4.1 Emission factors

An emissions factor for electricity from the grid canbe calculated from the emissions factor of the indi-vidual plant in the centralized generation mix. The‘grid emissions factor’ varies with what technologiesare taken up, at what rate, and the emissions factor for

Fig. 9 Energy use by fuel and CO2 emissions from the domestic sector under the deep greenscenario



Table 2 Summary of key features of the scenarios

1996 BAU Low carbon Deep green

OverviewParent scenario (FNT) – BAU Strong optimism Green plusParent scenario

(UKDCM)– A B B

Approach – Current policyand incrementaltechnologicalchange

Medium gas price,some central-ized generation,substantialmicro-generation

High gas price, cen-tralized generation100% renew-able, substantialmicro-generation

Gas price Low Low Low High

Demographics and demandPopulation (2050) 59 million 66.8 million 66.8 million 66.8 millionHouseholds 23.8 million 31.8 million 31.8 million 31.8 millionDemolition rate Current levels Current levels Increased four-fold Increased four-foldInternal temperatures 18.5 ◦C existing

propertiesSaturates at 23 ◦C Saturates at 22 ◦C Saturates at 22 ◦C

19.5 ◦C new buildSpace heating – Improvements in

efficiency takenas comfort untilinternal temperaturesaturates

Greater improvementsto stock anddecreases morerapidly as saturationreached earlier

Greater improvementsto stock anddecreases morerapidly as saturationreached earlier

Hot water – Increases by 28% to2050 in line withcurrent trends

Increases 14% to 2050 Increases by 14% by2050

Energy of lights andappliances

– Increases due to higherownership and newenergy intensiveproducts

Decreases due toimprovements inefficiency and fuelswitching

Decreases due toimprovements inefficiency and fuelswitching

Micro-generation Micro-generationmarkets justbeginning to take off

Gas boilers still dom-inant technology.LZC ownership at38% in 2050

Strong uptake ofmicro-generationespecially CHP

Strong uptake of micro-generation, but CHPcontribution smallerdue to high gasprices

Total electricitydemand (TWh)

382.2 512.9 379.7 379.7

Centralized generation in 2050 (FNT)Offshore wind (TWh) 0 71.5 76.7 132.4Onshore wind (TWh) 0 26.0 19.2 33.1Marine (TWh) 0 19.5 38.4 29.4Nuclear (TWh) 83.3 32.5 26.8 0CCGT (TWh) 72.2 130.1 0 0CCGT + CCS (TWh) 0 97.6 26.8 0Coal (TWh) 128.4 45.5 0 0Emissions factor of

centralizedgeneration,(tCO2/MWh)

0.155 0.24 0 0

Decentralized generation in 2050 (UKDCM)% heat from Stirling

engine CHP0 5 15 10

% heat from fuel cellengine CHP

0 4 20 15

% heat from heat pump 0 2.5 5 10% heat from district

heating0.2 5 15 20

% properties with Solarthermal

0.03 10 12 25

% properties with PV 0 5 10 15% properties with

μ-wind0 5 7 10

Domestic model outputsDomestic electricity

demand (TWh)116.8 178.0 111.3 112.1

(Continued)



Table 2 Continued

1996 BAU Low carbon Deep green

Domestic spaceheating demand(TWh)

214.0 245.8 191.8 191.8

Total domestic demand(TWh)

305.8 480.2 345.6 364.4

Domestic micro-generation (TWh)

0.18 40.4 116.5 111.8

% domestic demandfrom LZC

0 22 104 99

Mt C (2050) 43.0 30.5 18.7 14.1CO2 emissions

compared with 1996(%)

100 71 43 32

Network issuesReverse power flow Not an issue Some power flow up

to medium-voltagenetwork

A reversal of traditionalpower flows – fromlow to high

A reversal of traditionalpower flows – fromlow to high

Variability Minimal Some local effects fromweather fronts

Balancing micro-generation andvariable central-ized renewableschallenging

Balancing micro-generation andvariable central-ized renewablesextremely challeng-ing. All controllablegenerators are on thelow voltage network

Ancillary services Provided by control-lable centralizedplant

Provided by control-lable centralizedplant

Provided by bothcentralized andcoordinateddistributed plant

Provided solely by con-trollable distributedgenerators

Supply gap – An expansion inCCGT capacity thatremains utilised outto 2050

Some more CCGTcapacity, but mostlyretired by 2050

Some more CCGTcapacity, butcompletely retiredby 2050

each technology. The grid emissions factor (expressedas kgC per kilowatt-hour electricity supplied) underthe three scenarios are shown in Fig. 10 and is a mea-sure of the carbon offset by micro-generation or energysaved through efficiency measures within the home.All scenarios follow a comparatively similar trajectoryuntil 2025 as this period is dominated by the closure ofcoal and nuclear plants, and the incorporation of some

Fig. 10 Emission factors for electricity and gas under thedifferent scenarios

renewables into the electricity mix, which reducesemissions to ca. 0.12 kgC/kWh. Beyond 2025, the BAUscenario retains coal and gas plants but both low car-bon and deep green scenarios completely decarbonizetheir central electricity mix, the former through CCGTwith CCS and extensive marine, which mature later.

Both these scenarios are capable of supplying elec-tricity to end-users with a lower grid emissions factorthan burning natural gas in the home post-2040. Thishas behavioural implications, not modelled here, inwhich householders could revert to electric heatingtechnologies, which would have overall lower carbonemissions than conventional boilers and gas fuelledCHP. Such fuel switching would then cause an increasein demand on the electricity network, and greatergeneration and network capacity requirements.

4.2 Import/export issues

The BAU scenario has the least amount of domes-tic scale electrical generation. However, even with 22per cent of domestic electricity supplies coming frommicro-generation there will be a significant number oftimes throughout the year when there is export fromindividual houses and from groups of houses, low-voltage power circuits and even reverse power through



high-voltage and medium-voltage electrical substa-tions. In this scenario, the domestic electrical con-sumption rises very substantially; major investmentwould be required in circuit capacity if this rise inconsumption also led to a rise in the peak perioddemand level. If the majority of consumer load wasdirected to comfort (e.g. heating and lighting) andentertainment (plasma screen televisions), then thewinter evening peak demand would undoubtedly risecausing the major network capacity investment. Ifhowever, the majority of the rise in demand arosefrom other luxury items such as outdoor goods (e.g.hot tubs, lighting) and ‘trade-up’ domestic appliances(e.g. larger sized refrigerators), then the contributionto system peak demand would not be as significantand the situation would be more manageable from apower network perspective.

The ‘low carbon’ and ‘deep green’ scenarios have avast amount of micro-generation in comparison withthe situation in 2007. The operation of this domesticgeneration portfolio fuelled by diverse sources (solar,natural gas, wind, and biomass) will have major impli-cations for matching electricity generation to demand.For periods of low electricity demand period (e.g.summer days), there would at some times also be a sig-nificant production of electricity from solar and windsources that would cause excess power to flow fromlow-voltage networks into higher voltage networks.This reverses the general pattern of power flow fromhigher to lower voltage networks. The implications forplanning and operating power systems are substantialwith obvious changes to the level of required circuitcapability to individual consumers and groups of con-sumers and to power network control systems (e.g. forvoltage control and circuit protection). In the case of ahigh-demand period (e.g. winter evening) there wouldbe no solar power output and if this coincided withlittle or no wind output then the burden of meetingdemand would be placed on CHP units (which wouldbe required to meet the heating load in any case)and imports into distribution networks from centralgenerating units. The key issue in this case wouldbe the lack of contribution to high system demandfrom a large section of the micro-generation portfo-lio. Some reserve capability would still be requiredfrom central generating units unless effective demandmanagement or energy storage was available.

4.3 Generation production and demand variability

The power output variability of the primary sourcesof energy at a domestic level will also be important.Many of the identified issues could be resolved dueto diversity in location and customer behaviour. Forexample, it might be supposed that clouds passingover solar panels, changes in wind speed, timing of

individual customers starting CHP units would eachcause major changes in power production. However, inreality, the diversity in the timing of these occurrencesand their location would likely lessen (or smooth) theirimpact. The implication of the variability would bringmore emphasis onto flexibility in operation of con-trolled generating unit output to track the variations.The difference in electrical generation with the pass-ing of weather fronts can be imagined: reduced outputfrom solar devices with cloud cover, reduced outputfrom wind generation with lowering wind speed. Atthe same time, there might be a reduction in electri-cal output from CHP units due to time of the day. Thechange in power output over a short period could bevery dramatic. Flexible generating units (most likelycentral units connected to the main transmission sys-tem) would have more balancing duty placed uponthem in these situations.The domination of the centralgenerating pool by wind power in the ‘low carbon’ and‘deep green’ scenarios presents an issue here. Thus, inthe scenarios that need most flexibility in central gen-eration, the central generation portfolio is dominatedby less flexible generation.

Alternatives for balancing generation and demandat a regional or local level might lean on energystorage technologies or smarter demand-side man-agement than is employed at present. In particular,it is envisaged that coordinated control of the installedmicro-generation capacity utilizing more active net-work management could make a major contributiontowards the balancing process. Given the likely inflex-ibility in a central generation pool based on windgeneration, the obligation to balance demand andsupply would have to be pushed down the system tolocal or regional areas. The location of energy stor-age or demand-side measures to manage the rapidlychanging energy imbalance is a key issue. Some of thelarge load demand-side potential is already expendedto cover power system emergencies. Greater exploita-tion of medium to large loads might be possible, but ata cost. Demand management at the small commercialand domestic customer level has much potential butrequires a serious effort in adoption and subsequentcoordination. However, given futures where such largenumbers of customers have installed generation capa-bility on their property, there is good opportunity alsoto implement effective demand management rangingfrom optimally timed operation of wet goods to waterand the use of space heating (electric, heat pumps, andmicro-CHP) with a heat storage component.

This concept of active system management has beendeveloped as the virtual power plant concept [15].Here, instead of a traditional ‘fit and forget’ approachto micro-generation, the virtual power plant enhancesthe visibility and control of small-scale generators tosystem operators and other market actors. Under a ‘fit



and forget’ approach, the micro-generation displacesthe energy from centralized generation, but cannotreplace capacity, as the central generation is stillrequired to provide system support and security. Byfully integrating micro-generation into system oper-ation, and allowing it to participate in energy mar-kets and system management, retirement of centralplant is possible. This approach makes the best useof all assets on the system, resulting in lower costgeneration.

4.4 Phase unbalance issues

With so much domestic generation in the scenarios,there are important power system engineering issuesto be addressed. One of these is the nature of three-phase electrical systems. One underlying requirementin exploiting the economic and technical benefits ofthree-phase power systems is to maintain balance inthe loading across the three phases branching out fromsubstations. Without this balance, currents will flowin neutral conductors and voltage levels are likely todrift – neither of these conditions is desirable andcan damage power system plant as well as customerequipment. The greater variability in the magnitude ofnet loads from properties on the three phases (not tomention the direction of flow as noted previously) isvery likely to increase the incidence of phase unbal-ance problems. Some of the solutions noted earlier(e.g. demand management, energy storage) are likelyto have a role to play in solving this problem.

4.5 Reactive power issues

A problem with small-scale rotating generating units(e.g. squirrel cage induction generators) is theirrequirement for reactive power (i.e. currents out ofphase with active power flows). Arrangements forcompensating the additional reactive power load canhave unusual effects on local voltage level depend-ing on whether the generation operates at periods ofhigh- or low-demand and on what basis any reac-tive compensation is deployed. Careful coordinationof the introduction of high levels of small-scale rotat-ing generation into low voltage power systems will berequired. Other technical electrical issues include theincidence of harmonic currents and voltages in lowervoltage distribution networks.These currents and volt-ages not only emanate from ‘switched mode’ powersupplies (already significantly penetrating power net-works as power supplies to electronic appliances)but also from the power electronic interfaces of fuelcell, photovoltaic, and energy storage technologies.The negative effects of harmonics include abnormalheating and mal-operation of electrical equipments.High-quality power electronic converters and effective

filtering can potentially solve this problem but atadditional cost.

4.6 Ancillary services

The existing power system operates efficiently andsecurely on the foundation of large central generationplant, which provides various ancillary services. Theseservices include voltage control, frequency response,reserve, and black start capabilities. In the ‘low car-bon’ and ‘deep green’ scenarios, there is a massive shiftof generating plant away from the larger transmissionconnected units. What generation is left connected tothe transmission system is dominated by the variablesources of wind and marine energy. Providing effec-tive ancillary services to support the operation of thepower system in these scenarios is a major challenge.It could be argued that voltage control is more effec-tively provided closer to loads but there is still a need tosupport the voltage in an interconnected transmissionsystem.

Frequency response, reserve, and black start capa-bility are more challenging. Frequency response is theservice of providing additional generating output tomeet the changing supply–demand balance and torespond to unplanned outages of operating generatingplant. This is essential to maintain supply frequencywithin tight limits. There are possibilities to arrest fre-quency drops through demand curtailment but thisdepends on a well-coordinated system with signifi-cant magnitude of demand willing to be chopped andchanged at short notice to respond to system require-ments. Large-scale energy storage can play a part herein much the same manner as pumped storage hydrogeneration does at present. Reserve capacity is thegeneration capacity that operates at a reduced out-put, is rotating at minimum output or is ready tostart at short notice. It is not clear which generatingunits would operate in this mode in the ‘low car-bon’ and‘deep green’ scenarios. Perhaps biomass unitscould operate in this mode or coordinated dispatch-ing of small and very small-scale generation in timesof system emergencies combined with demand-sidemeasures and energy storage could fill this gap.

4.7 Meeting the medium-term supply gap

Under all the three scenarios, the combined effectof increased demand and the retirement of coal andnuclear plant in the medium-term lead to a supply gapand the need for new capacity. In each of the scenar-ios, it has been assumed that this gap will be met by anincreased capacity of CCGT plant. However, under thelow carbon and deep green scenarios, which featureextensive renewable sources and micro-generationpost-2030, the majority of this gas plant is then retired.



The shape of these curves is the result of the back-casting approach taken to scenario development, butnonetheless illustrates the problem of stranded assetsif we are to both reach the 2050 futures illustrated andkeep the lights on in the medium term. Such plantwould still have value in terms of providing backup tothe electricity system, so would not be truly ‘stranded’.Once again, it is worth noting that other electricity sys-tem features are possible, where this would not be anissue.

There is therefore a need for both technologiesand policies to avoid the requirement for a new gasplant in the medium term. Energy efficiency mea-sures that could be implemented rapidly to reducethe demand such as phasing out incandescent light-bulbs or implementing either centralized renewableor decentralized generation technologies more rapidlythan illustrated would avoid the need for new gascapacity that could become obsolete. It would alsohelp prevent the UK becoming over-reliant on a sin-gle fuel for the majority of its electricity and heatingrequirements, which is detrimental in fuel securityterms and leaves the economy vulnerable to fluctu-ations in the wholesale gas price.

4.8 Energy storage issues

The ownership, management, and location of energystorage devices are more problematic – notwithstand-ing the unattractive economics at present. If own-ership were left with the power network operator,their goals would include managing equipment load-ing away from peak demand periods and maintain-ing/enhancing supply security. Individual customerswould also be interested in maximizing energy cap-ture from their generation devices (through an energystorage unit) to reduce power purchase costs. Energyservice companies would likely focus on reducing theoverall fuel purchase costs for a population of cus-tomers. The different objectives of these stakeholdersallow partial but not fully complementary objectivesfor use of energy storage capacity. The location of thedevices depends to some degree on ownership andmanagement but the individual customer approach toenergy storage does not allow for larger and more eco-nomic energy storage solutions. Larger devices wouldbe located in medium- and high-voltage power net-works and thus would suit operation for an entiregroup of customers. The commercial arrangementsfor sharing a larger energy storage resource would becomplex.

5 CONCLUSIONS

This paper has introduced a set of three scenariosthat take previous scenario work further through,

focusing specifically on the micro-scale. The scenariospresent a challenging picture of changes that wouldbe required in the use of energy in buildings andin the power system at large. The issues of massivedeployment of wind and marine renewables (as in the‘low carbon’ and ‘deep green’ scenarios) are discussedin other literatures [16–21]. One major factor is theexpense and difficulty of developing the power sys-tem to cater for their needs. The massive change ingeographical spread and capability of the transmis-sion system to reach all regions with high levels ofrenewables (likely to be offshore and in the north ofthe country) would require very substantial planningsystem amendments, incentive schemes, use of sys-tem pricing mechanisms, and investment funds. Theradical shifts required in these areas cannot be over-emphasized. Considering transitions towards the ‘lowcarbon’ and ‘deep green’ scenarios really does requirea complete rethink of the nature of power systems. Thescenarios presented here provide a stimulus to furtheranalysis of this and other questions as to the nature offuture energy supplies.

REFERENCES

1 Costyn, J. Ofgem and microgeneration: next steps, 2006(Ofgem, London).

2 Energy – the changing climate, 2000 (The Royal Commis-sion on Environmental Pollution, London).

3 Anderson, K., Shackley, S., Mander, S., and Bows, A.Decarbonising the UK: energy for a climate consciousfuture, 2005 (Tyndall Centre, Norwich).

4 Ault, G., Elders, I., Galloway, S., McDonald, J., Leach,M., Lampaditou, E., and Koehler, J. Electricity networkscenarios for 2050, 2005 (SUPERGEN Future NetworkTechnologies Consortium, Glasgow).

5 Boardman, B., Darby, S., Killip, G., Hinnells, M., Jar-dine, C., Palmer, S., and Sinden, G. 40% house, 2005 (ECI,University of Oxford, Oxford).

6 Layberry, R. UK domestic carbon model, 2007 (ECI,University of Oxford, Oxford).

7 Palmer, J., Boardman, B., Bottrill, C., Darby, S., Hinnells,M., Killip, G., Layberrg, R., and Lovell, H. Reducing theenvironmental impact of housing, 2006 (ECI, Universityof Oxford, Oxford).

8 UK energy and CO2 emissions projections, 2006 (DTI,London).

9 Laughton, M. and Watkiss, P. Narrow margins: address-ing the issue of overcapacity in the UK electricity sector,2003 (Royal Academy of Engineering, London).

10 EU. Directive 2001/80/EC of the European Parliament andof the council of 23 October 2001 on the limitation ofemissions of certain pollutants into the air from largecombustion plants, 2001.

11 UK coal production outlook: 2004-16, 2004 (DTI,London).

12 Digest of United Kingdom energy statistics 2006, 2006(DTI, London).



13 EU. Council Directive 1993/31/EC of 26 April 1999 on thelandfill of waste, 1999.

14 The energy challenge: the energy review report, 2006 (DTI,London).

15 Pudjianto, D., Ramsay, C., and Strbac, G. Virtual powerplant and system integration of distributed energyresources. IET Renew. Power Gener., 2007, 1, 10.

16 Anderson, D. Power system reserves and costs with inter-mittent generation (Ed. UK Energy Research Centre),2006, p. 47 (UKERC, London).

17 Dale, L., Milborrow, D., Slark, R., and Strbac, G. Totalcost estimates for large-scale wind scenarios in the UK.Energy Policy, 2004, 32, 8.

18 Milborrow, D. Penalties for intermittent sources of energy,2001 (Performance and Innovation Unit, London).

19 MacDonald, M. Renewable network impact study. Annex4: intermittency literature survey and roadmap, 2004(The Carbon Trust and the Department of Trade andIndustry, London).

20 Sinden, G. Diversified renewable energy resources (Ed.The Carbon Trust), 2006, p. 42 (Environmental ChangeInstitute and The Carbon Trust, London).

21 UKERC. The costs and impacts of intermittency: anassessment of the evidence on the costs and impacts ofintermittent generation on the British electricity network,2006, p. 112 (UK Energy Research Council, London).


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Scenariosforexaminationofhighlydistributed powersystems · sions Trading Scheme (EU ETS), CCGT is assumed to be the preferred generation technology. This vision is in accordance with