Global demand for energy has risen inexorably in the last 150 years in step with industrial development and population growth. Hunger for energy is predicted to continue to rise, by at least 50% by 2030, as developing countries like China and India seek to fuel their rapid economic growth.
The lion's share of global energy (about 80% at present) is supplied by coal, oil and gas - the 'fossil fuels' that formed long ago from the carbon-rich remains of dead plants and animals. However these are non-renewable sources that will one day be exhausted.
Concern has also grown in recent decades about environmental impact. Mainstream climate scientists warn greenhouse gas emissions, caused by burning fossil fuels and other human activities, must be substantially reduced to avoid dangerous climate change.
Pressure to replace fossil fuels has focused more attention on renewable sources - eg solar and wind. Non-polluting and enduring, they offer an attractive alternative for many. But they face challenges: Viable technologies are still developing and start-up costs tend to be high. Renewables (barring hydroelectric) are not expected to achieve a significant market share within the next 25 years.