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Hydroelectric power
Heat capacity
The ability of a substance to absorb or give out heat without changing temperature. It is measured as the ratio of heat absorbed by a material to its temperature change.
The heat capacity in calories
per gram is called specific heat. The calorie is based on the specific heat of water, defined as the heat required to raise the temperature of one gram of water by one degree Celsius.
Water has the highest heat capacity of all common materials. The heat capacity of water is, for example, five times that of sand. This is an extremely important property. It explains why the oceans heat up slowly each summer and cool down slowly each winter.
The ability of water to absorb heat in all of its forms is important in the atmosphere. Water vapor in the air soaks up heat radiated from the ground and releases this heat slowly into the air. This slows the loss of heat into space. In this way the temperature of the atmosphere is kept 15°C
higher than it
would be if
there was no
water vapor
present.
The high heat capacity of water also explains why the dry soil close to a pond heats up more quickly than the surface water of the pond and
why people use water to store heat in
environmentally friendly central heating systems. Indeed, anywhere heat needs to be stored, water is usually chosen.
Salty water has a slightly lower heat capacity than fresh water because the salt itself has a low heat capacity. (See also: Water cycle.)
Hot springs
Sources of warm water that flow naturally from the ground.
The warming effect is usually produced by the water flowing near areas of hot volcanic rock. Some hot springs have been harnessed for geothermal power (the production of electric power by using the
steam from hot springs to turn the shaft of a generator).
(See also: Seawater.)
Humidity
The amount of water vapor in
the air. The more water vapor, the higher the humidity. It is most usually expressed as relative humidity—the amount of water in the air compared to the maximum amount it could hold at that temperature. Air that has all of
the water vapor it can absorb is said to be saturated and has a relative humidity of 100%. Air in a centrally heated home may only have a relative humidity of 20
to 30%.
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 Hydroelectric power—Hydroelectric power is suited to naturally fast flowing rivers on steep slopes. A dam is built to create a pond. The outlet of the dam runs past the turbines. In this picture surplus water is being released from the top of the dam. It gives an idea of the immense power in falling water.
Hydroelectric power
Electricity produced by forcing water to turn turbine blades that are connected to a shaft that turns an electric generator.
Hydroelectricity is a clean form of power because it generates
no waste gases and uses natural supplies of water. The only effect it has on the landscape is that water has to be stored behind dams so there can be a reliable supply of electricity (see: Reservoir).
Hydroelectric turbines are placed at the foot of a dam, not high up in it. This allows water
to drop from a maximum height and therefore build up the greatest speed (see: Head).
It is also possible to generate hydroelectric power from rivers that flow slowly, as is done on rivers where there are barrages. However, to compensate for the lack of speed, large volumes of water have to flow past the turbine blades, so this system is only suited to large rivers. (See also: Water power.)