Reducing the Energy Carbon Content - Energy and Climate: How to Achieve a Successful Energy Transition - page 76

has been studied for many years. There are various ways of harnessing the
energy from the sea:
- Mechanical energy of the waves, tides or sea currents exploited
by water turbines similar to wind turbines, but located under the
sea.
- Thermal energy of the seas in the inter-tropical regions, based on the
temperature difference between surface water and deep water.
- Energy of the salinity gradients based on the difference in salinity
between fresh water and sea water.
Due to the difficulties in producing and transporting the energy produced,
developments in this field have not gone beyond the stage of design,
isolated facilities (theRance tidal power plant) or prototypes (wave energy
and water turbines). For the time being, offshore production of wind
energy seems to be the only option feasible on a large-scale basis.
The promise of solar energy
Solar energy is the main renewable energy resource throughout the world.
Other renewable energy sources, e.g. biomass energy andwind energy, are
derived directly from it. It is an abundant energy source. Our planet
receives from the sun the equivalent of 15 000 times the energy consumed
in the world, but this energy is diffuse and intermittent.
The power received at noon with no cloud cover is about 1 kW/m 2 .
Given the day-night intermittence and weather fluctuations, the energy
received in 24 h is between 2 kWh/m 2 and 3 kWh/m 2 in northern Europe,
and between 4 kWh/m 2 and 6 kWh/m 2 in southern Europe or between the
tropics.
Solar energy can be captured as either heat or electricity using the
photovoltaic effect.
There is considerable scope for the development of low-temperature
thermal solar energy in the short term. Heat is supplied by solar sensors
consisting of a black absorbent surface which transfers the heat to a heat
exchange fluid, generally a mixture of water and glycol to prevent the
possibility of freezing. A glazed surface is fitted over the absorbent surface
to block the infrared radiation re-emitted.
Selective coatings such as chromiumoxide deposited on copper are used
to reach temperatures of 70-90 C, in order to limit the re-emission of
infrared radiation. To reach temperatures of more than 100 C, sensors