35465_Integration of Renewable Energy Suppli

8/6/2019 35465_Integration of Renewable Energy Suppli

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Integration of Renewable Energy Supplies


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Map of UK PowerSupply System


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Daily load duration curve


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Frequency control and Reserve (SpareCapacity)

At any given time the amount of plant inoperation and connected to the system should

be exactly equal to the system demand.However spare capacity must be available toallow for:

loss of the largest system generator due to afault

to allow for load changes to allow for inaccuracies in load forecasting


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Estimates of renewable-energy resource and cost in2025 for the UK


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Renewable electricity generation: resource-cost curve for the UK in 2025, based on an8% discount rate (source: ETSU, 1999)


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Are renewable-energy supplies available wherewe want them?

It would be helpful if renewable energy supplies were located at thepoints of maximum energy demand, the major cities.

Some can be. Solar thermal or photovoltaic panels can be fitted to

the roofs of buildings

Energy crops are likely to be available in rural areas but not wherefood production is considered more important. Fuels such asforestry wastes and SRC need to be gathered and transported fromwhere they are grown (possibly remote country areas) to where they

will be used (probably in towns). Remember, wood only has half theenergy density of coal so transportation can be a problem.


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The roof of this petrol station in London has a large PV array, providing enough electricity topower the lights and petrol pumps. BP has installed several hundred such arrays on petrolstations around the world


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Wave, wind and tidal power

the locations of the best wind, wave and tidal energysources in the British Isles can be seen on the next slide

these electricity-generating sources are not uniformlydistributed.

Both the UK and the Irish Republic have enormouspotential for renewable-energy supplies.

Comparison of this map with that for the UK electricitydistribution system raises the question of how theexisting National Grids will need to evolve in order tomatch the new energy sources to the loads.


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Wave Power

The best wave power resources are in Scotland, the

Atlantic coast of Ireland and the south-west of England.

The best onshore wind resources are in Scotland,Wales, Cornwall, the north and west of England and thewest of Ireland.

All of these are areas currently under rapid development.


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The prime areas for offshore wind development are the shallow waters of theNorth and Irish Seas, possible areas are necessarily restricted by shippinglanes. Yet it would only require an area of sea 30 km by 40 km to supply10% of the UKs electricity needs.

The DTI have identified three strategic areas for development: in MorecambeBay on the west coast, and around the Wash and the Thames Estuary onthe east coast.

These are all conveniently close to existing major grid links.

In Ireland, a major offshore wind farm has been approved for constructionat Arklow Bank, south of Dublin. This project alone could provide morethan 10% of the Republics electricity.


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The potential for tidal barrages is concentrated ona few large estuaries, particularly the Severn,though lagoon type structures could be built in

more open sea areas.

The potential for tidal current devices is in similarestuary locations, but there are many

possibilities around prominent headlands,

notably the Race of Alderney between Alderney

and Cap la Hague on the French coast.


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Are renewable-energy supplies available whenwe want them?

Our demand for energy is not constant. It varies widely over theday, the week and the year.

We need more energy for heating buildings in winter than insummer.

As a result, the UK consumes three times as much natural gas ina typical December as it does in a summer month.

Connecting the renewables

Where do the renewables fit into all this? The answer depends onthe particular source and the extent to which the timing andquantity of its output matches demand.


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Biofuel plant

Generation plants using MSW, waste wood or landfill gasare relatively small, typically in the output range 100 kWto 50 MW.

As such, they are likely to be connected to the system at 11kV or 33 kV and run fairly continuously.

Apart from breakdowns, their output is highly predictableand, as such, their electricity is every bit as valuable asthat from larger power stations.


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Solar powerIn the UK, photovoltaic systems are most likely to

be at the kilowatt scale and connected locally at

the 230 V or 400 V level.

Naturally, they only produce electricity during theday and their output will be higher in summer

than in winter.

Given their relative expense, they are only likely to

make up a small proportion of any renewable

electricity mix for the UK in the near future.


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Wind and wave power

With the current large-scale development of onshore andoffshore wind farms, there is much interest in exactlyhow much wind-generated electricity can be absorbed bythe existing infrastructure.

Although wave power is much less developed, it shares thesame basic problems.

Modern individual wind and wave generators are likely tohave power ratings of between 50 kW and 5 MW.

Current wind farms and future large wave power devicescould have total outputs in excess of 100 MW.

Thus they are likely to be connected to the grid within thelocal distribution network at voltages of 11 kV, 33 kV oreven higher.


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Wind and wave power

The output of wind and wave power generators is not perfectlypredictable (although detailed weather forecasting can help) but itpays to use the output when it is there, since there are no fuel costs.

Typically, a 1 MW wind turbine will produce 300400 kW on average.

It will produce full output on a windy day but nothing on a calm one.

At modest wind speeds, its output may vary considerably from minuteto minute.

Modern large turbine designs incorporate sophisticated power

electronics that can reduce this variability.


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Wind and wave power

The output of a wave power device is dependent on the variableintensity of the waves.

On a stormy day it will run at full power, on a flat calm one it willproduce nothing, and on an intermediate day, its output will bevariable.

If such sources are widely spaced, then just as diversity of demandadds up to a smoothly varying total demand on the National Grid, sodiversity of supply can also smooth out the local variations ofoutput of various renewable-energy sources.

When the wind stops blowing in Scotland, it may still be blowing inWales.


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Tidal Power

Tidal power is intermittent, buthighly predictable. A schemesuch as the proposed Severn

Barrage, generating only onthe ebb tide, could produce apulse of power of up to 8 GWabout six hours long every

12.4 hours


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Tidal Power

The output of a single turbine might consist of arather unpromising sequence of pulses ofpower three to four hours long. However, theoutput of twoidentical devices in differentlocations where the times of high tidesdiffered by about 3 hours (such as PortlandBill in Dorset and Dover in Kent) could addup to an almost constant supply .in practice,it is likely that such turbines would bedeployed at a large number of different

locations.

In total, they might produce a supply that variedlittle over the day, but would have longmonthly cyclic variations with the spring-tideto neap-tide changes in tide amplitude.

The price that might have to be paid for anylarge-scale deployment would be astrengthening of the National Grid to make

use of the differing times of the tides atdifferent locations.


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Tidal Current Turbines

The position with tidal current turbines seemsmore promising but, at present, it is onlypossible to estimate their performance.

Theoretically, such a turbine is likely to producepower in proportion to the cube of the speed ofthe water flowing through it (this is analogous tothe performance of wind turbines).

The output will peak every 6.2 hours, on theincoming tide and again on the ebb tide.


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Some system solutions

Grid strengthening

The existing grid has grown up around the power stations of the past.Connecting large amounts of power from new renewable sources willundoubtedly require strengthening the National Grid.

This will not come cheaply. Typically, an overhead 400 kV line costs about150 per MW per kilometre.

On this basis one capable of carrying 2 GW would cost about 300 000 perkilometre.

Moreover, a new line from, for instance, Scotland to England would almostcertainly face environmental objections.

Placing sections of the cable underground would be extremely expensive, withprices of up to 5 million per km.


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Some system solutions

Demand managementIt would be convenient for fossil-fuelled, nuclear and most

renewable electricity supplies if electricity demandvaried as little as possible over the day and night. Thiscan be encouraged through the use of off-peak

electricity tariffs.There are many electrical loads that are not immediately

needed. One example is large-scale water pumpingwhich could be placed under remote control. At timesof any impending shortage of renewable electricitysupply, pumps could be turned off.

Large scale energy storage would also be anotherpossibility


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Some system solutionsMicro-power

It is often argued that large numbers (possibly millions) of

small embedded generators (i.e. at the low voltage end

of the grid) could cut the need for large scale electricitygrids and provide backup for wind generators.

Small-scale generators are designed to operate completely

automatically. They will connect and disconnect from thegrid in response to their local energy needs or

circumstances.

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35465_Integration of Renewable Energy Suppli

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