Environmental Engineering Reference
In-Depth Information
heating systems. In the Earth's interior, temperatures are somewhere between
3000°C and 10,000°C. Radioactive decay releases enough energy to produce
such temperatures. The heat flow in the Earth's crust is relatively low and
increases with depth. Volcanic eruptions demonstrate the enormous activity in
the Earth's interior. The option of geothermics is demonstrated persuasively in
Iceland and the Philippines, where it provides more than 20 per cent of the
electricity supply.
The temperature differences between the Earth's interior and crust cause a
continuous heat flow. The mean global value of this heat flow at the Earth's
surface is 0.063 W/m
2
. The total energy flow is of the same magnitude as the
world's primary energy consumption. Applying the constraint that the upper
strata of the Earth's crust should not be cooled down significantly and that the
technical effort required must not be prohibitively high, only a portion of the
available geothermal energy is usable. Today geothermal energy is only
exploited in regions with geothermal anomalies. These regions record high
temperatures at low depths.
Only very few regions exist with such high temperatures directly under the
Earth's surface. Geysers can indicate such places. Geothermal heat pumps do
not require high temperature differentials, although such differentials do help
to make them more economical. Electrically driven compression heat pumps
can be used to boost temperature differentials and these have reached technical
maturity. However, the ecological benefits of heat pumps driven with electrical
energy produced by power plants burning fossil fuels are low. On the other
hand, if renewable sources provide the power required to drive the heat pump,
the system becomes one avenue for providing a zero-carbon heating system.
These systems are rarely used today.
Another technique that uses geothermal energy from hot, dry rocks at great
depths is the so-called hot dry rock method (HDR). First a cavity is drilled
into hot rocks (300°C) at a depth between 1000 and 10,000 m. Pressurized
cold water is pumped into the cavity, heated up, and transported to the surface
where a steam power plant generates electricity. This technology is still
experimental.
Planetary energy
The different celestial bodies, in particular our moon, exchange mutual forces
with Earth. The motion of the celestial bodies results in continuously varying
forces at any specific point on the Earth's surface. The tides are the most
obvious indicator of these forces. The movement of enormous water masses in
the oceans creating the tides involves enormous amounts of energy.
Tidal energy can be used by power plants on coasts with high tidal ranges.
At high tide, water is let into reservoirs and is prevented from flowing back as
the tide ebbs, creating a potential difference between the collected water and
water outside the reservoir. The collected water is then released though
turbines into the sea at low tide. The turbines drive electric generators to
produce electricity. Today there are only a few tidal power plants in operation.
