Plants for the Distributed Generation: geothermal systems

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geothermal plant
by Maria Rosaria Barbato

Our planet produces a continuous flow of heat to the surface layer. From a depth of about 20 meters, the temperature increases by about three degrees Celsius for every 100 m depth; in certain areas, called geothermal areas, this change is much more rapid and, just 6-7 kilometers deep, temperatures of 200 ° C can also be reached. 

In 1905, with the construction of the first geothermal power plant in Larderello in Tuscany, Italy was the first country to exploit the great potential of this renewable source; in fact, on the Italian territory, the areas with the greatest potential for geothermal energy are four: 

- The first one is Tuscany, with its geothermal fields of Larderello,  the Bolsena caldera and the sea, for several miles; 

- The second one is Campi Flegrei, in Campania; 

- The third, very large (and still little-known) geothermal area is  the southern Tyrrhenian Sea; 

- The fourth is the channel of Sicily, in the submerged area of volcano Empedocles and Lampedusa. 

 

CLASSIFICATION OF GEOTHERMAL SYSTEMS 

A geothermal system converts the heat of geothermal fluid, which is produced by the contact of the water with layers of hot rocks, into electrical energy. This may possibly be combined with the production of house heating (cogeneration). 

A geothermal power plant consists of a system of collection, treatment and conveyance of geothermal fluid up to the power plant: here the thermal energy is converted into electrical energy, while the steam is sent to a treatment system . 

Geothermal can be used at "high temperature", in hot water and temperatures above 150 ° C, or, as is widely spreading in recent years, at "average enthalpy", using the terrestrial heat present in the most superficial layers of the earth crust, with lower temperatures. 

In the high-temperature geothermal systems there may be either an ascent of dry and superheated steam (vapor-dominated system) ar an ascent of liquid water mixed with steam (water-dominated systems). The vapor-dominated systems are the rarest but, at the same time, they are also the most efficient, since, in the dominant water system, the liquid phase must be separated from the steam thus resulting in a greater energy consumption. 

The vapor is drawn through wells and piping systems and then it is used to activate the turbines that produce electricity. 

Besides the production of electricity, in the medium-enthalpy plants, the heat extracted can be directly used for the production of useful thermal energy, to fuel heating systems for buildings, greenhouses, industrial processes and etc.. 

Because of the lower temperature, the energy production of a plant that uses the average enthalpy can not be compared to that of a high enthalpy plant, but it can still meet the request of a few thousand households and it shows, due the several average temperature anomalies detectable on the territory, further development perspectives, both from an energy point of view and for the environmental and economic impact. 

At temperatures below 90 ° C, a plant is said to work at "low enthalpy" and, in this case, the production of energy is used for heating and cooling of buildings and in the production of sanitary hot water for residential or industrial use.

 

OPERATION OF A GEOTHERMAL POWER PLANT 

The warmth of the subsoil is exploited through the installation of geothermal probes coupled to geothermal heat pumps, the latter allowing the exchange of heat between the ground and the area to be heated. 

The low-enthalpy systems can be divided into closed-loop systems, which draw heat from the earth through a heat transfer fluid circulation, and the open-circuit systems, which are based on the sampling and subsequent return of water (groundwater, river or lake); the two systems exploit the same principle of operation.

The system uses a temperature of about 10-15 ° C, detectable at a depth of about 100 m from the earth's surface. Through a heat pump, the heat withdrawn from the soil is brought to a temperature suitable to power the heating system of the building and supply warm air in winter and cool air in the summer period. 
Despite the use of the heat pump, which requires a power supply, the use of a low-enthalpy geothermal system can be convenient: with a consumption of 1kWh, needed to operate the heat pump, 4 kWh of thermal energy can be produced, with 3kWh taken from the soil.

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