These sources are grouped together because all are ultimately forms of solar power. Solar energy drives the atmosphere, causing wind and is largely responsible for powering the large scale oceanic currents. The wind creates ocean waves and helps drive surface ocean currents. Solar heating maintains the vertical temperature gradient, which can be used as an energy source, in deep oceans and lakes.
Solar and wind energy are possible solutions for stationary use provided the ecowarriors allow sufficient acreage to be installed to handle the total electricity and heating requirements of domestic, public and commercial buildings. The problem is that using these energy sources requires a rather dilute form of energy to be concentrated into a more useful form: hence the large size of most installations.
Solar power's main disadvantage is that it is only available for a third of the day, so some form of energy storage is needed to allow its use at other times or during times of heavy cloud. The power available depends on the sun's orbital position and hence its distance from the sun. The earth receives an average 1366 W/m^2, though it fluctuates by about 6.9% during a year - from 1412 W/m^2 in early January to 1321 W/m2 in early July. In addition the Earth's axial tilt and the lattitude affect the energy delivered to a particular location on the surface because these affect the distance the the rays must travel through the atmosphere before they reach the collector. Despite this, if a collector is perpendicular to the incident rays the power level it receives remains fairly independent of the season and place on the Earth's surface. At midday in mid lattitudes this is about 1 kW/m^2.
Solar and wind energy cannot be directly used to power land or air transport. Solar powered cars are not a practical proposition and the thought of sailing traffic tacking into the wind on a motorway is alarming. Shipping is a different case. Sailing vessels date back to the Greeks and were used for cargo up to the start of World War 2. After the war they were replaced, for the first time in history, first by coal and then oil powered ships, but this may start to change back within the 10 year oil deadline. At present there is significant effort going into reviving large cargo sailing ships in Germany and Scandinavia.
Wind power is becoming a reality in New Zealand and solar
power certainly makes sense locally, e.g., on the buildings that
use the power, but, the only long-term answer will be to learn to
live within renewable resources.
- source: David Gregorie, New Zealand.
Germany, whose native fossil energy source consists entirely
of low-grade coal, already derives 8% of its energy requirements
from renewable sources. It is the world's biggest consumer of
PV cells, which
generate electricity directly from sunlight. Germany is on target
to have half of its energy needs provided by renewables by 2050.
This will reduce its carbon dioxide emissions to 20% of their
- source: New Scientist editorial, 14 May 2005.
Infrared capture is a new and quite speculative energy source and most remain in this category until a pilot installation has breen shown to produce usable amounts of electricity at a competitive price.