7.2 Energy sources — uses and managementIB Environmental Systems and Societies SL: Revision notes

Renewable energy sources are replaced naturally at least as fast as they are used. Examples are solar, wind, hydroelectric, tidal, wave, geothermal and biomass (when replanted).

Non-renewable sources are finite stocks used far faster than they form. The main examples are coal, oil and natural gas, which formed from ancient organisms over millions of years.

Renewable does not mean impact-free: every source has some environmental and social cost.

Global energy consumption is rising for two linked reasons:

• Population growth: more people need energy.
• Rising per capita demand: as incomes rise, people use more transport, electricity and manufactured goods.

About four-fifths of the world's energy still comes from fossil fuels, so rising consumption has usually meant rising emissions unless the mix of sources changes.

The sustainability of energy sources varies significantly. Judge each one against several criteria:

• Is it renewable or finite?
• Greenhouse gas and pollutant emissions during operation and over its whole life.
• Land use and habitat impacts, e.g. dams flood valleys, wind farms affect birds.
• Reliability, e.g. wind and solar are intermittent.

Wind and solar have very low operating emissions but use materials and land. Coal has high emissions and air pollutants and is finite. Biomass is only near carbon neutral if replanted or from waste.

A country's choice of energy mix depends on many factors:

• Availability of resources: coal, gas, wind, sun, rivers, geothermal heat.
• Cost and wealth: capital cost of building power stations and grids.
• Government policy: subsidies, taxes, targets and planning rules.
• Technology available and affordable.
• Public attitudes: acceptance of wind farms, dams or nuclear sites.
• Environmental concerns, such as climate change targets.
• Existing infrastructure such as grids and pipelines.

Example: Morocco has little fossil fuel but strong sunshine, so it built the Noor solar complex at Ouarzazate. Denmark's Samsø used its windy coast and biomass to reach 100% renewable electricity by 2007.

Wind, solar and tidal power are intermittent: output varies with weather and time, so supply does not always match demand. This creates a need for energy storage systems:

• Pumped hydroelectric storage: water pumped uphill when electricity is surplus and released through turbines when needed (e.g. Dinorwig in Wales).
• Batteries: store electrical energy chemically.
• Molten salt: stores heat from concentrated solar plants for use after sunset.
• Hydrogen: made using surplus electricity and used later as a fuel.

Interconnectors and backup supply also help, as on Samsø.

Energy conservation means using less energy, e.g. turning down heating or switching off appliances. Energy efficiency means getting the same service from less energy, e.g. LED lighting, building insulation and efficient appliances.

Both reduce demand, so a country needs to generate or import less energy. This makes it less dependent on imported fuel, less exposed to price changes, and lowers emissions. Limits include upfront costs and the need for people to change behaviour.