What is it?
Solar energy comes from the sun in the form of electromagnetic rays. Only a tiny fraction of the sun's rays reach the Earth, but those that do provide enough energy to warm the planet and make it hospitable for life. On a sunny day at noon, each square metre of the Earth's surface receives around 1kW of solar power.
How do we use it?
Solar energy can be used to generate heat or electricity. It is common for houses to have solar panels fitted to their roofs to provide warm water. The panels heat up in the sun and are used to warm water that it is pumped through them.
Solar, or photovoltaic, cells are used to generate electricity from sunlight. When particles of light are absorbed by a solar cell, they dislodge electrons, which flow as an electrical current. Another way of making electricity from sunlight is to use a bank of mirrors and lenses to focus the sun's rays on to a pipe filled with water. When it is sunny enough, the water becomes hot enough to evaporate and can be used to drive an electric turbine.
Solar energy has enormous advantages over many other sources of energy. It is free, inexhaustible and clean.
Sunlight is a very diffuse source of energy, meaning it is not very concentrated like petrol or gas. To make good use of it, large areas of land need to be covered with solar panels or photovoltaic cells. Generating electricity from sunlight is not very efficient.
Where does wind come from?
Wind is caused by sunlight unevenly heating the surface of the Earth. During the day, air over the land heats up more quickly than air over the water, making it expand and rise. As it does so, cooler, more dense air rushes in beneath it, creating an air current. Some giant wind currents are driven by hot air at the equator and cool air at the poles. In Britain, we have enough wind to power the country several times over.
How do we use it?
The most common type of wind turbine is a bit like an old-fashioned windmill, with rotor blades that face into the wind. When the blades are spinning, they drive a shaft that is connected to an electrical generator by a gearbox. Most wind turbines produce electricity when the wind is blowing at 10-30mph. One 1.8MW wind turbine produces enough electricity for 1,000 households every year.
Wind is really a form of solar power, so it has similar benefits of being clean, abundant and free. Small wind turbines can be used in remote places to power homes that are too far away from the national grid. Some estimates suggest there is enough wind to generate one third of the world's electricity.
The major problem with wind power is that it is intermittent, so it can only be used to generate electricity when the wind is blowing strongly enough. Wind turbines have little impact on the environment, but some people think they look ugly and complain that they make a low-pitched humming noise. Good sites for wind turbines are often quite remote, either offshore or up on mountainsides, far from the cities where the energy is most needed.
Wave and tidal power
What is it?
There is an enormous amount of energy locked up in the oceans. First, there are tides, which are caused by the gravitational pull of the moon and the sun, and the Earth's rotation. Then there are waves, which are caused by wind blowing over the ocean surface. Waves breaking around the world's coastlines are estimated to release around 3m MW of power.
How does it work?
Tide energy is harnessed by building barrages that allow large basins to fill up with water as the tides comes in, and then empty through turbines that generate electricity. There are also tide fences, which have built-in turbines that spin as water flows past them. Wave energy can be turned into electricity by using channels to steer waves directly on to turbines. Instead, the crashing waves can fill up a tank and, as with tidal power, drive turbines as the water runs back out.
As with many other renewables, tidal power is clean, free and abundant. Tidal power is also very predictable, because the time and height of tides are easy to work out. Some tide barriers can protect coastlines from storm damage by absorbing energy from surges of water.
Tidal power stations can only generate electricity when the tide is coming in or going out. To make electricity economically, the tide needs to rise and fall at least 3 metres. Only around 20 inlets in the world are thought to have good enough conditions for tide power.
Hydroelectric powerWhat is its source?
Hydroelectric power uses the water cycle as a source of power. The sun's rays heat water on the Earth's surface, which evaporates and rises into the air. High up, the warm water vapour cools and condenses into clouds. The water then falls back down as rain or snow, and flows through rivers back into the oceans.
How does it work?
Hydroelectric plants often use dams that force river water to flow through large pipes containing turbines. As the water rushes past the turbines, they spin and generate electricity.
Hydropower is clean and abundant and, unlike wind, the water needed to drive turbines can be saved up for when it is most needed.
Hydroelectic power plants disrupt the environment by blocking up rivers. In some countries, dams used for hydropower have flooded large areas of land, forcing people to move elsewhere.
Source: The Guardian| by Ian Sample