The invention refers, in addition, to a plant for implementing such a process.

Background Art Geothermal fluids of endogenous origin, used for generating electric power, are made by a mixture of steam and non-condensable gas or by pressurised saturated or sub-cooled water with dissolved solids and non-condensable gas.

In the first case the steam is saturated or slightly super heated and may be directly used as motive fluid of a turbo generator unit, of various typology, which generates electric power.

In the second case the endogenous fluid is initially flashed, in one or more stages, in order to produce saturated steam, which may directly feed a turbo generator unit.

Sometimes the heat content of the endogenous fluid is transferred to a secondary fluid, which feeds the electric power plant.

The process scheme of a geothermal plant, normally adopted, is represented in Figure 1. Geothermal fluid FE, produced by one or more geothermal wells, feeds normally a unit SE that produces the motive fluid FM of the power plant CE.

The unit SE belongs to various typologies: if the endogenous fluid is steam with entrained solids then SE is used to remove the solids or SE may be a washing apparatus for eventually neutralising the chlorides contained in the fluid.

This unit operates as a separator of the liquid phase if the fluid is a mixture of steam and liquid water.

If FE is mainly liquid then SE carries out the separating process in one or two flashing steps in order to generate a vapour phase at one or two pressure levels.

The geothermal plant CE adopts a consolidated technology: its simpler scheme includes a steam turbine, with a single inlet and without bleeding, that drives an electric generator.

Usually the outlet of the steam turbine is connected to a steam condenser whose pressure is kept below the atmospheric value and whose temperature is that corresponding to the saturation value for the fluid. Since the fluid features a certain amount of non-condensable gases, these gases must be exhausted from the condenser with the help of special compressors. If the gas content in the fluid is above 10-15 % wt. , it could be convenient to adopt a conversion process without steam condenser and the fluid is discharged from the turbine directly to the atmosphere.

The rate of the motive fluid of a geothermal plant undergoes a slow or rapid decline, which entails a temporal decline of the generated power. It comes out that during the life of the plant the steam turbine will operate at a rate lower than the nominal (see: N. Gennai, G. Sestini: Studio sulle cause del declino della portata di vapore endogeno nel tempo. Ricerche di Termotecnica, Vol. 14.1964 ; G. Neri: Flow rate Decline and Pressure Transients in the Larderello Geothermal Field. Proc. 13° Workshop on Geothermal Energy. Stanford University, Stanford 1988 ; M. A. Grant, I. A. Donaldson, P. F. Bixley : Geothermal Reservoir Engineering, Academic Press 1982).

The geothermal power plants in operation exploit the heat content of the endogenous geothermal fluid and do not use, for the generation of the electric power, other forms of chemical energy.

Disclosure of Invention The process, which is the object of this invention, consists in superheating of the motive fluid of the geothermal power plant, by means of a renewable source of energy, before the fluid enters the plant itself. For renewable source of energy is here intended any solid, liquid or gaseous material having a non-fossil origin and featuring a chemical energy that can be converted into thermal energy.

The invention may apply to new geothermal power plants but also to existing power plants when this is allowed by their operative conditions and their characteristics.

The main advantage of this invention is that the same plant can generate electric power by means of two renewable energy sources (i. e. the geothermal fluid and one of the renewable above mentioned), having an overall efficiency equivalent to that of two distinguished plants each fed by only one renewable source, with an investment much lower than that required by two separated plants.

The idea of using geothermal energy, together with a fossil fuel in the same plant for generating electric power is not new. Such idea has been investigated by R. Di Pippo and others in the years between 1978 and 1981 and is the basis of a few of process schemes having in common, with the process here proposed, only the concept of super heating (see J. Kestin, R. DiPippo, H. E.

Khalifa : Hybrid Geothermal-Fossil Power Plant, Mechanical Engineering Vol.

100, Pag. 28-35, Dec 1978).

The known process named Fossil Superheat Hybrid System (FSHS) is similar to the invention here claimed and represented in Figure 2, but it is different for the following substantial differences : a) the process of the invention here claimed does not require modification of the thermal cycle of the power plant, but it is applicable to existing plants or to new plants without modifications of the thermal cycle. The process FSHS entails a substantial modification of the cycle of the geothermal plant by adoption of special materials that can operate up to 540 °C in an environment containing hydrogen sulphide. b) the ratio between the power generated by the plant with superheating and the power generated without superheating is greater than or equal to 1.4 in the process FSHS, while in the process here proposed the maximum value is 1.3.

In the year 1994 Louis J. Blaize obtained in the United States of America the patent n. 5,311, 741 with reference to a process named"Hybrid Electric Power Generation". The Blaize's process features a few of similarities with the process here proposed but is substantially different for the following reasons: a) The Blaize's process is conceived for exploiting the heat contained in the hot and dry rocks of the terrestrial crust by means of thermal exchange between the rocks and pressurised water, pumped from surface to depth trough suitable injection wells and withdrawn at temperature of 350 °C by means of appropriate production wells. The process, according the invention here claimed, is applicable to geothermal fluids of endogenous origin and not to exogenous fluids injected into deep rocks from the hearth surface with the purpose of extracting the heat stored in them. Moreover this concept never found application at yet. b) In the Blaize's process the temperature of the fluid is assumed equal to 350 °C at the head of the production wells. The heat stored in the fluid is exploited by means of a battery of heat exchangers that generates saturated steam. This steam is first superheated by the use of fossil fuel, and then it feeds a closed loop electric power plant based on the Rankine cycle. In the process of this invention the geothermal fluid is endogenous, it does not exchange heat with other fluids and it feeds directly, after one or two stage of flashing, the geothermal power plant that adopts an open cycle. The temperature of the saturated steam, before superheating, is about 350 °C in the Baize's process, while in the process of this invention the temperature is about 350 °C downstream the superheating process. c) According to the process of this invention the power plant can be operated even if the equipment performing the superheating, i. e. the superheater is not operating, in the Blaize's process any outage to the superheater causes the outage of the power plant. d) The process of this invention has been conceived for using a renewable fuel and the invention provides advantages only with such a typology of fuel and not, for instance, if natural gas or coal would be used for superheating the fluid.

The process and the plant of this invention are represented in the schemes of Figure 2 and Figure 3 and include following equipment and/or systems and/or components: FE-Geothermal fluid of endogenous origin SE-Eventual apparatus for processing the endogenous fluid. According to the nature and to the composition of fluid the apparatus may be: i) A separator of solid particles or an apparatus for washing the fluid if it is in vapour phase. ii) A single or double flash reactor with phase separator if the fluid is mainly in liquid phase. iii) A simple centrifugal separator of the liquid phase, if the fluid is mainly in vapour phase SL-Liquid phase from unit SE BM-Renewable fuel, as defined above in this document, i. e. biomass or RDF (Refused Derived Fuel) or others.

SH-Superheater: apparatus, which increases the temperature of the motive fluid.

FM-Motive fluid of the geothermal plant.

CE-Geothermal power plant.

Mode for Carrying out the Invention In the process represented in Figure 2 the geothermal fluid FE, of endogenous origin, feeds normally a unit SE. This unit, described above, generates a flow of gaseous material, which is the motive fluid of the plant and generates also a flow SL of liquid or solid material. The motive fluid FM, produced by the unit SE enters the superheater SH, which is a device that increases the temperature of the motive fluid FM.

The superheater increases the fluid temperature up to the maximum value allowed by the geothermal plant, which is about 400 °C, or up to a lower value considered more appropriate. The increase of temperature occurs by using the heat content of a renewable fuel, as for instance biomass or RDF (Refuse Derived Fuel), represented in the schemes of Figure 2 and Figure 3 by stream BM. The superheater may adopt various technologies as direct combustion of renewable fuels or combustion of an intermediate liquid or gaseous product, which comes from the thermal conversion of the fuel.

The motive fluid, exiting the superheater, features an enthalpy higher than that at the inlet and feeds the geothermal power plant CE.

Lets consider a geothermal fluid with following characteristics: Nature of fluid Mixture of slightly super-heated steam and inert gas Flow rate of fluid 30 kg/s Pressure of fluid 6,5 bar Temperature of fluid 170 °C Gas content 3% by weight Gas composition Carbon dioxide The fluid feeds a geothermal plant equipped by: i) a condensing turbine ii) direct contact condenser iii) battery of centrifugal inter-cooled compressors for gas extraction from condenser iv) evaporative cooling towers The condenser operates at the pressure of 0,08 bar; in such a condition the plant may generate a net power of 15 MWe.

Superheating the motive fluid up to the temperature of 370 °C, prior of its expansion through the steam turbine, allows an increase of the electric power generated up to about 19 MWe. The superheating may be carried out according to the scheme of Figure 3. In such a scheme appear the following devices: V0, V1-On-Off valves SP-Device diverting the direction of motive fluid FM MX-Gathering device of motive fluid FM.

The plant configuration shown in Figure 3 allows the geothermal plant to operate with (V1 open and VO closed) or without (VO open and V1 closed) superheating. As a consequence the plant can be operated at base load during any shut down of superheater.

The superheater, based on direct fuel combustion, is fed by biomass with a heat content of 15 thermal MWh. In such a condition the superheating allows to generate electric power with a marginal efficiency of about 25 %. The advantage of the process here proposed is that the investment required for converting the biomass to electric power is much lower than that necessary for the construction of a power plant fuelled only by biomass. The superheater may be built adopting also technologies different from the direct combustion and in particular could be based on combustion of a synthesis gas derived from thermal conversion of the primary renewable fuel.

The process of superheating may be applied to existing power plants when their effective electric load becomes, as a consequence of the natural flow rate decline during operation, much lower than the nominal maximum load. The process can find also application in geothermal plants of novel construction.

From a functional point of view the superheating process is situated upstream the heat conversion process of the motive fluid to power, while from the point of plant lay out view, the superheating unit is separated from the power plant and may be located also at a great distance.