Energy conversion and storage

Some energy sources have storage ‘built in’• Fossil fuels• Biofuels • Hydro power (to some extent)

Others are available on demand• Geothermal energy• Ocean thermal energy

But most renewable sources are intermittent, and the times at which they are plentiful will not necessarily coincide with consumer demand. So we have a problem!

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Variation in UK electricity demandElectricity demand in the UK varies continually (actual data for two recent week-days shown here).

There is a strong seasonal influence on demand, as might be expected

Matching of supply to demand

The electricity network has no capacity for storing energy. If demand changes, the amount supplied must change – almost immediately.

Power stations must be switched on (or off) to follow the changes in demand.

Some types of power station will tolerate this better than others

Hydraulic pumped storage

In its generatingmode, the plantoperates as a conventionalhydro power plant.But the generator can function as a motor, and the turbineas a pump. When demand for electricity is low, power maybe taken from the grid to pump water into the upper reservoir, to act as an energy store for futureuse when demand for electricity is high.

Generator

Turbine

Motor

Pump

Cruachan pumped storage scheme, Argyll: 365 m head,400 MW rated output; 4 Francis pump/turbine units

Aerial view ofCruachan siteshowing upper

reservoir and dam.Administration and

visitor centre ison the shore of

Loch Awe below;power station isunderground,

some 400 m fromthe loch-side

High-grade and low-grade energy

An indication of how easily one form of energy may be converted into another.e.g. electric kettleElectricity → heat, efficiency > 90%

But the reverse process, hot water into electricity, cannot effectively be accomplished.We say that electricity is a high-grade, and heat is a low-grade form of energy.Converting heat into electricity, or just into mechanical energy, is always an inefficient process.

Energy storage using batteriesChemical processes involved, import / export efficiency is limited as a result

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Rectifier Battery Inverter

Rectifier and inverter convert AC to DC for storage, and DC to AC for final use

Energy storage using flywheelsEnergy Motor / Flywheelsource generator

Clutch

A massive wheel turning at high speed can store a large amount of energy in kinetic form.

CHARGING: -

Electrical energy from an intermittent source (such as a wind turbine) drives an electric motor to spin up the flywheel.

STORING: -Clutch is disengaged, electric motor stopped, flywheel spins freely. Longevity of storage is affected by friction in bearings; can be avoided by magnetic levitation!

RECOVERY: -Clutch is engaged, electrical machine now functions as a generator and supplies energy to meet the demand. Speed of generator will not be constant, so it may be necessary to process its output using an inverter.

Compressed air energy storage

heat

Motor Compressor Compressed air storage

Air turbine

Intermittent energy source supplies an electric motor, which drives an air compressor. Stored air may be used to drive a turbine / generator set to produce electricity. Waste heat from compressor may be put to use.

The ‘hydrogen economy’

H2

Fuel cell

Direct heating

Primary energy source

Electrolysis

IC engines

Electricity

It has been suggested that hydrogen can take the place of all fossil fuels, acting as an energy store to serve all possible demands. But H2 is volatile and has a low energy density, so its storage and transportation are both problematic

Characterisation of energy conversion and storage systems

• Energy capacity per unit weight volume cost

• Import / export efficiency• Limits on rate of charge / discharge• Energy ‘leakage’ rate• Working life of hardware• Environmental impact

Illustration of comparative performance

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Pumped hydro

Compressed air

Flywheel

Battery

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Efficiency figures here are for the processing of electrical energy via the store, from import to export.

Some types of energy storage system only make sense over a certain range of scales, while for others the efficiency varies with scale. So for domestic applications batteries, flywheels or compressed air might be appropriate – and also of course thermal storage in water or building fabric.