Thermomineral Water and Geothermal Energy - Hydrogeology: Objectives, Methods, Applications - page 83

2.2.4 Problems in the exploitation of thermomineral water

Water originating at depth can pass through other aquifers as it rises
towards the surface and can mix with water of varying quality. It can
therefore lose its original characteristics and see more or less alteration of
its quality. Establishments exploiting this type of resource must be careful
to maintain a constant and irreproachable standard of quality. Catchment
systems must therefore extract the thermomineral water before it mixes
with surface water.

3 GEOTHERMAL ENERGY

Thermal energy being present below ground, it is tempting to use it as is or
to transform it into electrical energy. The fi rst attempts to produce electricity
took place in Italy at the beginning of the 20th century. Geothermal energy
is greatly used in Iceland, where, thanks to heated greenhouses, bananas
can be produced for the European market. 40% of the population depends
on geothermal energy for heating, and this energy also serves to produce
30% of the country's electricity. Iceland benefi ts from a fortuitous location,
on the Mid-Atlantic Ridge, the site of active volcanism, but in other areas
of Europe, the temperature gradient can reach 1°C for 20 m.
The heat held in 1 km 3 of granite at a depth of 5,000 m can, through
the injection and heating of water to 200°C from an initial temperature of
20°C, produce the same amount of energy as 1.3 million tons of petroleum,
or the consumption of a 10 MWe electric plant for 20 years. The potential
resources in Europe have been estimated by Shell international to be
125,000 km 3 . Geothermal energy could therefore become an important
European resource. It currently provides 1.5% of Italy's energy.
Depending on the temperature, resources are generally classifi ed as
one of 5 different types (Table 9).

Table 9 Different Types of Geothermal Energy.

Type Temperature

in °C

Geologic setting Depth in m Use

Very low energy < 30 Shallow aquifers heat pumps

Low energy 30-100 aquifers 500-2,500 thermal heating

Medium energy 90-150 deep aquifers,

thermal anomalies

2,000-3,000 heating, electricity

High energy > 150 aquifers in volcanic

structures

electricity

Deep geothermal
energy

> 150 deep dry rock 3,000-5,000 electricity