Renewable energy techniques (intero corso per Environmental engineering)

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This is the only for of “renewable” energy that is independent of the Sun. Its ultimate source

is within the Earth. The amount of energy flowing through the Earth surface is around 10^21

J/y, that is a tiny in comparison with the massive 5.4*10^24 J/y coming from the sun.

There are many places where the Earth heat flow is sufficiently concentrated to have

generated natural resources in the form of steam and hot water (180÷250 °C), available

rocks within feasible drilling distance of the ground surface and suitable for electricity

generation.

Geothermal wells are drilled to depths from 700 m to 3 km. Power generation is the

preferred way. Non-electrical direct use is also implemented, around 50 GW (thermal).

In many cases the heat can be use directly, in other, “low-enthalpy” resources, the T may be

increased by the use of heat pumps. In 2010, 122000GWh, more or less 10 Mtoe (million

tonnes of oil equivalent). heat

In almost all these situation the heat is being removed faster than it is replaced:

mining. In general, geothermal resources are non-renewable on the scale of human

lifetimes, however they share many features with true renewable resources

(geothermalplant lifespan ± 100y).

However, the new technology (e.g. EGS) potential of geothermal energy is growing

remarkably.

The heat flows out of the Earth because of the massive temperature difference between the

surface and the interior. Temperature at the centre is around 7000°C. Why the Earth is hot?

Two reasons:

when the Earth formed 4.6Gy ago, the interior was heated up rapidly as the kinetic

energy and gravitational energy of accumulated material was converted into heat. But

this is not sufficient!

the Earth contains tiny quantities of long-lived radioactive isotopes: Thorium 232,

Uranium 238 and Potassium 40 all of them liberate heat as they decay

These radiogenic elements are concentrated in the upper crustal rocks. Cumulative heat

production from these radioactive isotopes accounts for ½ the surface heat flow.

Heat is transferred through the main body of the Earth principally by convection, involving

motion of material mainly by creep processes in hot deformable body. This is a very

efficient heat transport process resulting in rather small variations of T across the depth of

the convective layer.

Closer to the surface, across the outer ~ 100 km of the Earth, the material is too rigid for

convection because it is colder, so the heat is transferred by conduction and there are much

larger increases of T with depth. This rigid outer boundary layer, or shell, is broke into a

number of fragments, lithospheric plates, which move around the surface at the speed of a

few cm/y. Only this, is of direct relevance to geothermal exploitation.

It is mainly at the boundaries of the plates that the heat flow reaches a maximum. The

energy is around 300 mW/m2 compared to the average 60 mW/m2 of the Earth. Storage of

molten rock at about 1000°C just a few km beneath the surface strongly augments the heat

flow around even dormant volcanoes.

The industrial geothermal plants in Larderello, Tuscany, have turned 200 years old. It was 8

May 1818 when, near the village of Montecerboli, the French-born merchant Francesco

Giacomo Larderel started work on the first plant able to use geothermal waters for the

production of boric acid.

Enel Green Power’s plants in the Larderello area have an installed capacity of 0.8 GW and

provide energy to more than 10,000 users, including private homes, public services and

industrial activities.

The use of geothermal energy to produce electricity started at the beginning of the 20th

century, when the Prince Piero Ginori Conti succeeded in channelling the heat from the earth

into a dynamo able to light five light bulbs. In 1916, the power plant was already able to

produce 2750 kW, enough electricity to power the village of Larderello and nearby Volterra.

Geothermal resources of most types must have 3 important characteristics:

an aquifer containing water that can be accessed by drilling;

a cap of rock to retain the geothermal fluid;

a heat source

Natural aquifer are porous rocks that can

store water through which water will flow.

Porosity refers to he cavities present in the

rock. Permeability is the ability to transmit

water and is measured by its hydraulic

conductivity (Darcy Law):

The heat supply for a high entalphy field is usually derived from a cooling and solidifying

body of magma, which not necessarily be centered directly beneath the geothermal field.

There are geothermali fields which successfully extract heat directly from molten lava lake.

Several of the world’s most advanced geothermal sites are located on extinct volcanic areas.

Rocks are good insulators, magm