EP4083393A1

EP4083393A1 - Cogenerative organic rankine cycle with vapor extraction from the turbine

Info

Publication number: EP4083393A1
Application number: EP21212837.5A
Authority: EP
Inventors: Mario Gaia; Roberto Bini; Andrea Duvia; Claudio Pietra
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2020-12-11
Filing date: 2021-12-07
Publication date: 2022-11-02

Abstract

Partially cogenerative organic Rankine cycle plant (100) comprising, along a main path (5) in which a main flow of organic working fluid flows:- an evaporator (10),- at least one turbine (20, 21, 22),- a first condenser (40),- a first supply pump (50),- at least one preheater (60),at least two heat exchangers enslaved to a heat user, said two heat exchangers being placed in series along a path (85) of the heat user and in which:- a first heat exchanger is a further condenser (45) 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 (80) is fed by a partial flow of the primary heat source.

Description

Technical field of the invention

The present invention relates to an innovative cogenerative organic Rankine cycle plant with steam extraction from the turbine.

Known art

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 5-10 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 20 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 50-60 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 15°C, a cooling fluid could be cooled down up to 18-25°C) .
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.
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.

Summary of the invention

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 1.
Further preferred and/or particularly advantageous embodiments of the invention are described according to the characteristics set out in the attached dependent claims.

Brief description of the drawings

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

Figure 1 represents a diagram of a cogenerative organic Rankine cycle with steam extraction from the turbine, according to a first embodiment of the present invention,
Figure 2 represents a diagram of a cogenerative organic Rankine cycle with steam extraction from the turbine, in a second embodiment of the present invention,
Figure 3 represents a diagram of a cogenerative organic Rankine cycle with steam extraction from the turbine, according to a third embodiment of the present invention, and
Figure 4 represents a diagram of a cogenerative organic Rankine cycle with steam extraction from the turbine, in a fourth embodiment of the present invention.

Detailed description

A cogenerative organic Rankine cycle plant 100, according to the present invention is shown in Figure 1 and comprises, in fluid-dynamic connection between them, along a main path 5:

an evaporator 10 where the organic working fluid under pressure is heated, vaporized and eventually superheated or brought to supercritical conditions using the heat of an external primary heat source, for example a geothermal source which flows along a path 70 of the primary heat source, from a GEO IN input end to a GEO OUT output end;
a first turbine 20 where the organic working fluid is subjected to a first expansion;
a second turbine 21, mechanically connected to the first turbine 20, in which the organic working fluid is subjected to a second expansion. The second turbine 21 is also mechanically connected to an electric generator 30 used for the production of electrical energy or to another operating machine which uses the mechanical power produced by the turbine, such as a compressor, a pump, etc;
a first condenser 40, for example an air condenser, which receives a main vapor flow of the organic working fluid coming from the outlet of the second turbine 21, therefore after having fully completed the expansion phase, and returns it to the liquid phase;
a first supply pump 50 which pressurizes the organic working fluid;
at least one preheater 60, which using for example the same geothermal source supplies heat to the organic working fluid bringing it to a temperature close to the evaporation one.

In order to fulfill the high temperature cogeneration function, the organic cogenerative Rankine cycle plant 100 also includes:

a first branch line 25 equipped with a regulating valve V2, in which a partial flow of organic working fluid vapor extracted from an intermediate stage of the expansion phase flows, for example as shown in Figure 1, at the outlet of the first turbine 20;
a further condenser 45 in which the partial flow of organic working fluid vapor extracted downstream of the first turbine 20 flows and condenses; it is to be understood that the further condenser 45 can also perform the function of a de-superheater, should the partial flow of organic working fluid vapor not arrive in saturated conditions;

a second supply pump 55 which pressurizes the partial flow of organic working fluid and sends it to the preheater 60. Depending on the application and the temperature conditions, such partial flow rejoins the main flow, coming from the first supply pump 50, either:
- * upstream of the preheater 60,
- * downstream of the preheater 60, or
- * preferably, in an intermediate position, so as to make the temperature of the condensed partial flow and the temperature of the preheated main flow as similar as possible at the mixing point. In this way the overall efficiency of the cycle is maximized since, as is known, mixing at different temperatures involves a loss of exergy in the system.

The mixing point can be decided either at the design level or it can be adjusted by arranging several inlet points in the preheater 60 and by deciding where to convey the partial flow of the organic working fluid coming from the further condenser 45 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 75 is provided, equipped with a regulation valve VI, in which a partial flow of the geothermal source flows which feeds a heat exchanger 80 at high temperature. The partial flow withdrawal from the geothermal source that feeds the heat exchanger 80 may be either:

* at the GEO IN inlet of the geothermal source, or
* in an intermediate point of the path 70, for example between the preheater 60 and the evaporator 10 (as shown in Figure 1) or between two adjacent preheaters if more than one of them are installed.

Suitably, also the return point of the geothermal source withdrawn after having been cooled in the heat exchanger 80 may also be either:

* at the GEO OUT outlet of the geothermal source (as indicated in Figure 1), or
* in an intermediate point of the path 70 but certainly downstream of the withdrawal point, according to the path 70.

The criterion for deciding at which point to withdraw and return to the path 70 the flow rate of the geothermal source that releases heat to the heat exchanger 80 is conveniently defined in order to subtract the heat required at the minimum possible temperature (compatibly with the needs of the heat exchanger 80 and therefore of the heat user), it being evident that a withdrawal at a point of the circuit 70 where the temperature is lower and a return at a point of the circuit 70 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 20, 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 85 from a TELE IN inlet end to a TELE OUT outlet end.
In the examples described, such heat exchangers are the further condenser 45 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 80 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 45 and the heat exchanger 80 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 80 is carried out by the first valve VI, whereas the control of the partial flow of the organic working fluid vapor extracted during the expansion and which feeds the further condenser 45 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 VI, 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 40 of the ORC plant (or main condenser) can supply heat to a first heat user (for example at 80-90 C), whereas the further condenser 45 and the high temperature heat exchanger 80 feed a heat user at a higher temperature (for example 100-120 C) requiring only a fraction of the thermal energy available to the main condenser. In this double cogeneration scheme, the first condenser 40 instead of being an air condenser is preferably a water-cooled condenser.
With reference to Figure 2, the cogeneration ORC plant is the same as that of Figure 1 with a further variant. The partial flow of the organic working fluid coming from the further condenser 45 is cooled in a further heat exchanger 90 - said exchanger being able to be installed both upstream and downstream of the second supply pump 55 - and then be mixed with the flow outgoing from the first supply pump 50, which pumps the main flow of organic working fluid out of the first condenser 40. Downstream of this mixed flow there is a branch line 95 equipped with a regulating valve V3, in which a partial liquid flow of organic working fluid which is preheated in the additional heat exchanger 90 before being returned to the path 5 and being mixed with the main flow of the organic working fluid at the preheater 60, as well as in the solution illustrated in Figure 1.
This scheme can be convenient to have the possibility of decoupling the flow coming from the further condenser 45 from the flow sent through the further exchanger 90 and from there back to the cycle or to sub-cool the fluid before being pumped by the second supply pump 55, 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 Figure 3 differs from the one illustrated in Figure 2 due to the further presence of a regenerator 105. The regenerator 105 receives the main flow of the organic working fluid vapor, that is the one coming from the turbine 21 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 50. 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 Figure 4, a fourth variant of a cogenerative ORC cycle plant is now illustrated. The cycle is similar to that of Figure 1 and differs from it in that there is only one turbine 22, 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 22, as illustrated in Figure 4, flows in the branch line 25 provided with a regulating valve V2.
Advantageously, the turbine 22 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 European patent EP3455465B1 , and the branch line 25 is located near the angular stator stage.
In addition to the ways of implementing the invention, as described above, it should be understood that there are numerous further variants. For example, the plant illustrated in Figure 1 may also be equipped with a regenerator, as can be seen in the plant of Figure 3. Furthermore, the single turbine plant in Figure 4 may be equipped with a regenerator or a further heat exchanger (as in the system of Figure 2) or of both devices (as in the plant of Figure 3). It should also be understood that such implementing modes are only exemplary and do not limit neither the object of the invention, nor its applications, nor its possible configurations. On the contrary, although the above description makes it possible for the skilled person to implement the present invention at least according to an exemplary configuration thereof, it must be understood that numerous variations of the components described are conceivable, without thereby departing from the object of the invention, as defined in the appended claims, literally interpreted and/or according to their legal equivalents.

Claims

Partially cogenerative organic Rankine cycle plant (100) comprising, along a main path (5) in which a main flow of organic working fluid flows:
- an evaporator (10) 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 a primary heat source which flows along a path (70) of the primary heat source,

- at least one turbine (20, 21, 22) where the organic working fluid is expanded,

- a first condenser (40) 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 (50) which pressurizes said main flow of organic working fluid;

- at least one preheater (60), 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 (100) being characterized by at least two heat exchangers enslaved to a heat user, said two heat exchangers being placed in series along a path (85) of the heat user and in which:
- a first heat exchanger is a further condenser (45) 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 (80) is fed by a partial flow of the primary heat source.
Organic Rankine cycle plant (100) according to claim 1 wherein the partial flow of organic working fluid flows along a first branch line (25) of the main path (5) provided with a first regulating valve (V2).
Organic Rankine cycle plant (100) according to claim 1 or 2, wherein the partial flow of the primary heat source flows along a second branch line (75) of the path (70) of the primary heat source provided with a second regulating valve (V1).
Organic Rankine cycle plant (100) according to any of the preceding claims, wherein the working temperature range of the further condenser (45) is lower than the working temperature range of the second heat exchanger (80).
Organic Rankine cycle plant (100) according to any of the preceding claims, wherein the partial flow of organic working fluid rejoins the main flow of organic working fluid upstream of the at least one preheater (60) .
Organic Rankine cycle plant (100) according to any of the claims 1 to 4, wherein the partial flow of organic working fluid rejoins the main flow of organic working fluid downstream of the at least one preheater (60) .
Organic Rankine cycle plant (100) according to any of the claims 1 to 4, wherein the partial flow of organic working fluid rejoins the main flow of organic working fluid in an intermediate position of the at least one preheater (60).
Organic Rankine cycle plant (100) according to any of the preceding claims, wherein a taking point of the partial flow of the primary heat source is located at an inlet end (GEO IN) of the path (70) of the primary heat source.
Organic Rankine cycle plant (100) according to any of the claims 1 to 7, wherein a taking point of the partial flow of the primary heat source is located at an intermediate point of the path (70) of the primary heat source, downstream of the evaporator (10).
Organic Rankine cycle plant (100) according to any of the claims 8 or 9, wherein a return point of the partial flow of the primary heat source is located at an outlet end (GEO OUT) of the path (70) of the primary heat source.
Organic Rankine cycle plant (100) according to any of the claims 8 or 9, wherein a return point of the partial flow of the primary heat source is located at an intermediate point of the path (70) of the primary heat source, downstream of the taking point of the partial flow of the primary heat source, along the path (70).
Organic Rankine cycle plant (100) according to any of the preceding claims, comprising a control unit (PLC) configured to control the flow rate of the partial flow of organic working fluid by means of the first regulating valve (V2) and the flow rate of the partial flow of the primary heat source by means of the second regulating valve (V1).
Organic Rankine cycle plant (100) according to any of the preceding claims comprising a further heat exchanger (90) fed by the partial flow of organic working fluid leaving the further condenser (45) and, in countercurrent, by a partial flow of liquid of organic working fluid drawn along a branch line (95) of the main path (5) provided with a regulating valve (V3) upstream of the at least one preheater (60).
Organic Rankine cycle plant (100) according to any of the claims 2 to 13, wherein said at least one turbine is exactly two in number and in a first turbine (20) the organic working fluid vapor is subjected to a first expansion and in a second turbine (21), mechanically connected to the first turbine (20), the organic working fluid vapor is subjected to a second expansion and in which the first branch line (25) of the main path (5) it is positioned at the outlet of the first turbine (20).
Organic Rankine cycle plant (100) according to any of the claims 2 to 13, wherein said at least one turbine is a single turbine (22) and the first branch line (25) of the main path (5) is positioned between two consecutive stages of the single turbine (22).
Organic Rankine cycle plant (100) according to claim 15, wherein the single turbine (22) is a mixed radial and axial flow turbine and the first branch line (25) is located in proximity to an angular stator stage of the single turbine (22).
Organic Rankine cycle plant (100) according to any of the preceding claims comprising a recuperator (105) fed by the main vapor flow of the organic working fluid and, in countercurrent, by the main flow of the organic working fluid in liquid phase.
Organic Rankine cycle plant (100) according to any of the preceding claims in which the heat user is a district heating network.
Organic Rankine cycle plant (100) according to any of the preceding claims in which the primary heat source is a geothermal source.
Organic Rankine cycle plant (100) according to any of the claims 1 to 17 wherein the primary heat source is a biomass.
Organic Rankine cycle plant (100) according to claim 20, wherein the first condenser (40) is a water-cooled condenser and supplies heat to a first heat user, while the further condenser (45) and the heat exchanger (80) provide higher temperature heat to a second heat user.