1. 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!

2. Variation in UK electricity demand Electricity 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

3. 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

4. Hydraulic pumped storage In its generating mode, the plant operates as a conventional hydro power plant. But the generator can function as a motor, and the turbine as a pump. When demand for electricity is low, power may be taken from the grid to pump water into the upper reservoir, to act as an energy store for future use when demand for electricity is high. Generator Turbine Motor Pump

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

6. Aerial view of Cruachan site showing upper reservoir and dam. Administration and visitor centre is on the shore of Loch Awe below; power station is underground, some 400 m from the loch-side

7. High-grade and low-grade energy An indication of how easily one form of energy may be converted into another. e.g. electric kettle Electricity → 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.

8. Energy storage using batteries Chemical processes involved, import / export efficiency is limited as a result ~ Rectifier Battery Inverter Rectifier and inverter convert AC to DC for storage, and DC to AC for final use

9. Energy storage using flywheels Energy Motor / Flywheel source 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.

10. 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.

11. 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.

12. The ‘hydrogen economy’ H2H2 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 H 2 is volatile and has a low energy density, so its storage and transportation are both problematic

13. 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

14. Illustration of comparative performance 100% 0 Pumped hydro Compressed air Flywheel Battery Scale Efficiency 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.