Patent Description:
As is known, a thermodynamic cycle is defined as a finite succession of thermodynamic (for example isothermal, isochoric, isobaric or adiabatic) transformations at the end of which the system returns to its initial state. In particular, an ideal Rankine cycle is a thermodynamic cycle made of two adiabatic and two isobaric transformations, with two phase changes, from liquid to vapor and from vapor to liquid. Its purpose is to turn heat into work. This cycle is generally adopted mainly in thermoelectric power plants for the production of electricity and uses water as the driving fluid, both in liquid and steam form, and the corresponding expansion takes place in the so-called steam turbine.

Together with the Rankine cycles with water as the working fluid, organic Rankine cycles (ORC) have been conceived and implemented which use high molecular mass organic fluids for the most diverse applications, in particular also for the exploitation of low-medium temperature thermal sources. As in other steam cycles, the plant for an ORC cycle includes, by way of example, one or more pumps for feeding the organic working fluid, one or more heat exchangers for carrying out the preheating, vaporization and eventually superheating or heating phases in supercritical conditions of the same working fluid, a steam turbine for the expansion of the fluid, mechanically connected to an electric generator or an operating machine. ORC cycles are also used for the production of electrical energy and for exploitation of the heat recovered from the organic working fluid in the condenser.

This is the well-known cogeneration process which provides for the simultaneous production of mechanical energy (usually transformed into electrical energy) and heat. The heat produced can be used, for example, for heating or district heating of buildings and/or for production-industrial processes. Cogeneration uses traditional generation systems, internal combustion engines, water steam turbines, gas turbines, combined cycles and ORC cycles.

Reciprocating internal combustion engines, plants with gas turbines or steam turbines are mostly powered by fossil sources or suitable for large powers (above <NUM>-<NUM> MWel), whereas ORCs are used either in the field of renewable energy (biomass or geothermal energy) or of industrial heat recovery, with powers ranging from a few hundred kW up to about <NUM> MWel per unit. In the following, a medium-high temperature cogeneration means the production of steam, water, air or any other liquid or gaseous substance, at a temperature higher than <NUM>-<NUM> C, i.e. higher than the temperatures at which a cooling fluid of the condenser could be maintained, should said fluid be cooled by giving heat directly to the environment (for example with an ambient temperature of <NUM>, a cooling fluid could be cooled down up to <NUM>-<NUM>).

If the thermal power required in cogeneration is the total one discharged by the thermodynamic cycle (ORC or water vapor), the solution to be adopted is to adapt the condensing temperature to the temperature requested by the heat user. Obviously, as the condensation temperature increases (i.e. in order to satisfy heat users at temperatures equal to or higher than the minimum ones that could be obtained by transferring heat directly to the environment) the thermodynamic conversion efficiency decreases (based on the second principle of thermodynamics).

Should instead the thermal power required by the heat user at a medium-high temperature be only a fraction of the total one available to the condenser, the need arises to adopt a solution that penalizes the cycle conversion efficiency as little as possible, i.e. that allows to discharge to the non-cogenerative condenser the fraction of the total power not requested by the user, at the lowest possible temperature (in relation to the ambient temperature), at the same time extracting from the plant the fraction of thermal power needed at a higher temperature (defined by the user), thus optimizing the electrical conversion efficiency and respecting the heat user's request.

Document <CIT> describes a method of supplying a part of an organic working medium to a condenser of a power-heat coupling system without supplying the part of the working medium to an expansion machine (<NUM>) when a condition is fulfilled.

Document <CIT> discloses an organic Rankine cycle system, the condensation heat of which is used for cogeneration purposes for temperatures higher than <NUM>.

Document <CIT> describes an ORC system provided with an auxiliary opening, which is interposed between an inlet and an outlet of a turbine and is in fluid connection with an auxiliary circuit, such as to extract from the turbine the organic working fluid at an intermediate pressure between an injection pressure and a discharge pressure.

Finally, document <CIT> relates to an incinerator and a district heating generator in which heat is transferred to a district heating medium.

There is therefore the need to define an organic Rankine cycle plant suitable for partial cogeneration of medium-high temperature energy and free of the aforementioned drawbacks, i.e., with optimized efficiency of the entire cycle.

The aim of the present invention is therefore to define an organic Rankine cycle plant of the partially cogenerative type for delivering heat to a heat user.

In particular, the heat supplied to the heat user, for example a district heating plant, is obtained using both a partial flow of organic working fluid vapor extracted from an intermediate stage of the expansion phase in at least one turbine, and a partial flow of a primary heat source, for example a geothermal source, used as a primary heat source for the organic working fluid of the ORC plant.

The two heat sources feed two separate heat exchangers (for example, an additional condenser for the vapor of the organic working fluid extracted during expansion and a high temperature heat exchanger for the partial flow of the primary heat source) placed in series and in counter-flow with respect to the heat carrier of the user.

In this way the overall efficiency of the organic Rankine cycle, as well as the performance of the cogeneration and the exploitation of the heat of the thermal source are optimized as the fraction of thermal power not required in cogeneration is discharged to the condenser at the lowest possible temperature, with respect to room temperature.

Advantageously, the organic working fluid vapor extracted during expansion will have a lower temperature than the temperature of the partial flow of the primary heat source.

In particular, the present invention defines a partially cogenerative organic Rankine cycle with steam extraction from the turbine, according to independent claim <NUM>.

Further preferred and/or particularly advantageous embodiments of the invention are described according to the characteristics set out in the attached dependent claims.

The invention will now be described with reference to the annexed drawings, which illustrate some nonlimiting exemplary embodiments, in which:.

A cogenerative organic Rankine cycle plant <NUM>, according to the present invention is shown in <FIG> and comprises, in fluid-dynamic connection between them, along a main path <NUM>:.

In order to fulfill the high temperature cogeneration function, the organic cogenerative Rankine cycle plant <NUM> also includes:.

The mixing point can be decided either at the design level or it can be adjusted by arranging several inlet points in the preheater <NUM> and by deciding where to convey the partial flow of the organic working fluid coming from the further condenser <NUM> by means of suitable valves. The second option is preferable if the working conditions and temperatures of the cogeneration heat user change over the course of the year.

Finally, still in order to optimize the high temperature cogeneration, a second branch line <NUM> is provided, equipped with a regulation valve V1, in which a partial flow of the geothermal source flows which feeds a heat exchanger <NUM> at high temperature. The partial flow withdrawal from the geothermal source that feeds the heat exchanger <NUM> may be either:.

Suitably, also the return point of the geothermal source withdrawn after having been cooled in the heat exchanger <NUM> may also be either:.

The criterion for deciding at which point to withdraw and return to the path <NUM> the flow rate of the geothermal source that releases heat to the heat exchanger <NUM> is conveniently defined in order to subtract the heat required at the minimum possible temperature (compatibly with the needs of the heat exchanger <NUM> and therefore of the heat user), it being evident that a withdrawal at a point of the circuit <NUM> where the temperature is lower and a return at a point of the circuit <NUM> where the temperature is higher favor the thermodynamic cycle that feeds the turbine and therefore improve the system efficiency.

Therefore, the heat supplied to the heat user, for example a district heating plant, is obtained using both a partial flow of organic working fluid vapor extracted from an intermediate stage of the expansion phase, for example at the outlet of the turbine <NUM>, and a partial flow of the primary heat source, for example a geothermal source, used as a primary heat source for the organic working fluid of the ORC plant.

The two heat sources feed two separate heat exchangers which transmit the heat to a heat user, for example to the working fluid of a district heating system which flows along a path <NUM> from a TELE IN inlet end to a TELE OUT outlet end.

In the examples described, such heat exchangers are the further condenser <NUM> in which the working fluid of the district heating system and the partial flow of the organic working fluid vapor extracted downstream of a first expansion and the heat exchanger <NUM> at a high temperature, in which the working fluid of the district heating system and the partial flow of the geothermal source are flowing in countercurrent. The further condenser <NUM> and the heat exchanger <NUM> are placed in series and in countercurrent with respect to the heat carrier of the user.

Advantageously, the partial flow of the organic working fluid vapor extracted during the expansion will be at a lower temperature with respect to temperature of the partial flow of the primary heat source.

The partial flow control of the geothermal source which feeds the high temperature heat exchanger <NUM> is carried out by the first valve V1, whereas the control of the partial flow of the organic working fluid vapor extracted during the expansion and which feeds the further condenser <NUM> is realized by the second valve V2.

In general, as a consequence of variable heat requests of the heat user, several combinations of the opening degree of the two valves V1, V2 are possible. The two valves and the corresponding opening degree can be managed by a suitable PLC control unit which will also be electrically connected to the TC control unit of the district heating network. The PLC control unit can be equipped with suitable optimization algorithms in order to make the ORC system work, as the user requests vary, always at maximum efficiency, so maximizing the supply of electrical energy and at the same time satisfying the heat user in cogeneration.

A typical application for such cogenerative ORC cycle plant can comprise a geothermal application (as in the example described where the primary heat source is represented by a flow of liquid water) and the heat user is a district heating network.

However, the same plant can be conveniently applied in biomass cogeneration applications. For these applications, in which the primary source has normally a temperature higher than in an ORC geothermal application, a double cogeneration system can advantageously be obtained. In fact, the first condenser <NUM> of the ORC plant (or main condenser) can supply heat to a first heat user (for example at <NUM>-<NUM> C), whereas the further condenser <NUM> and the high temperature heat exchanger <NUM> feed a heat user at a higher temperature (for example <NUM>-<NUM> C) requiring only a fraction of the thermal energy available to the main condenser. In this double cogeneration scheme, the first condenser <NUM> instead of being an air condenser is preferably a water-cooled condenser.

With reference to <FIG>, the cogeneration ORC plant is the same as that of <FIG> with a further variant. The partial flow of the organic working fluid coming from the further condenser <NUM> is cooled in a further heat exchanger <NUM> - said exchanger being able to be installed both upstream and downstream of the second supply pump <NUM> - and then be mixed with the flow outgoing from the first supply pump <NUM>, which pumps the main flow of organic working fluid out of the first condenser <NUM>. Downstream of this mixed flow there is a branch line <NUM> equipped with a regulating valve V3, in which a partial liquid flow of organic working fluid which is preheated in the additional heat exchanger <NUM> before being returned to the path <NUM> and being mixed with the main flow of the organic working fluid at the preheater <NUM>, as well as in the solution illustrated in <FIG>.

This scheme can be convenient to have the possibility of decoupling the flow coming from the further condenser <NUM> from the flow sent through the further exchanger <NUM> and from there back to the cycle or to sub-cool the fluid before being pumped by the second supply pump <NUM>, in order to increase the NPSH (English acronym for "net pressure suction head") upstream of the pump itself.

The cogenerative ORC cycle plant illustrated in <FIG> differs from the one illustrated in <FIG> due to the further presence of a regenerator <NUM>. The regenerator <NUM> receives the main flow of the organic working fluid vapor, that is the one coming from the turbine <NUM> and which has processed the entire pressure drop, and in countercurrent the main flow of organic working fluid in the liquid phase, coming from the first supply pump <NUM>. The addition of a regenerator further increases the overall efficiency of the ORC cycle, in relation the characteristics of the working fluid used and the temperature of the source.

With reference to <FIG>, a fourth variant of a cogenerative ORC cycle plant is now illustrated. The cycle is similar to that of <FIG> and differs from it in that there is only one turbine <NUM>, instead of two turbines placed in series. Therefore the expansion of the organic working fluid vapor takes place entirely in the single turbine. A partial flow of organic working fluid vapor extracted from an intermediate stage of the expansion phase which takes place in the single turbine <NUM>, as illustrated in <FIG>, flows in the branch line <NUM> provided with a regulating valve V2.

Advantageously, the turbine <NUM> can be a mixed flow (radial and axial) turbine with injection and/or extraction of organic working fluid in an angular stator stage, such as that described in the <CIT>, and the branch line <NUM> is located near the angular stator stage.

Claim 1:
Partially cogenerative organic Rankine cycle plant (<NUM>) suitable for a heat user and comprising a primary heat source and, along a main path (<NUM>) in which a main flow of organic working fluid flows:
- an evaporator (<NUM>) where the organic working fluid under pressure is heated, vaporized and eventually superheated or brought to supercritical conditions using the heat of a main flow of the primary heat source which flows along a path (<NUM>) of the primary heat source,
- at least one turbine (<NUM>, <NUM>, <NUM>) where the organic working fluid is expanded,
- a first condenser (<NUM>) which returns to the liquid phase a main vapor flow of the organic working fluid which has fully completed the expansion phase
- a first supply pump (<NUM>) which pressurizes said main flow of organic working fluid;
- at least one preheater (<NUM>), which supplies heat to the organic working fluid bringing it to a temperature close to the evaporation one,
said partially cogenerative organic Rankine cycle plant (<NUM>) being characterized by at least two heat exchangers enslaved to the heat user, said two heat exchangers being placed in series along a path (<NUM>) of the heat user and in which:
- a first heat exchanger is a further condenser (<NUM>) fed by a partial flow of organic working fluid, in the vapor phase extracted from an intermediate stage of the expansion phase, and
- a second heat exchanger (<NUM>) is fed by a partial flow of the primary heat source.