EP1998013A2

EP1998013A2 - Apparatus for generating electric energy using high temperature fumes

Info

Publication number: EP1998013A2
Application number: EP07425218A
Authority: EP
Inventors: Mario Gaia; Roberto Bini; Andrea Duvia
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2007-04-16
Filing date: 2007-04-16
Publication date: 2008-12-03
Also published as: EP1998013A3

Abstract

This invention concerns an apparatus for the production of electric energy by a turbine operating according to the Rankine cycle using an organic work fluid made to evaporate through a heat exchange with a source of heat. In the work fluid circuit, upstream of the main turbine, an additional electric generator is inserted having a rotation speed aimed at optimising the extraction of power from a pre-expansion of the work fluid.

Description

Field of Invention

This invention concerns in general a system for producing electric energy starting from high temperature fumes or gas coming from any type of heat source. In particular, the invention concerns an apparatus for generating electric energy with a turbogenerator operating according to the Rankine cycle with organic work fluid (ORC).

State of the Technique

A system for the production of electric energy of the type taken into consideration basically comprises: a source of fumes or gas at a high temperature, a heat exchanger between the fumes and a thermovector fluid circulating in an intermediate circuit, a heat exchanger between the intermediate thermovector fluid and an organic work fluid for the evaporation of the latter, a turbine (hereafter named the main turbine) fed by the work fluid vapour and connected to an electric generator, a regenerator for the recovery of the thermal content of the work fluid vapour, a condenser of the working fluid before it is recycled.
Usually, diathermic oil is used as an intermediate thermovector fluid and silicone oil as an operating fluid. The diathermic oil is made to circulate in a coil around which circulate the fumes or gasses at high temperature. Then it heats the operating fluid so as to generate vapour which feeds the turbo-generator.
In the Rankine cycles with organic operating fluid (ORC) with a high temperature difference between the hot and cold source, the volume flow rate of the vapour which passes through the turbine is much higher at discharge than at input.
This difference in volume flow rate has a negative effect on the efficiency of the turbine.
For example, in the particular case, but of great practical importance, of a system for the combustion of biomass, feeding an ORC cycle through an intermediate diathermic oil circuit with oil temperatures of 310°C at input and 240°C at output, the fluid vapour temperature is usually around 270°C and that of the condensation about 100°C.
Even though the input temperature of the diathermic oil would allow a higher evaporation temperature to be used, the output temperature specified is not normally exceeded.
The aim of this is to limit the expansion ratio of the turbine, which is often equipped with a small number of stages (for example two) and to avoid too great a difference in length of the blades in the passage from the first to the last stage.
In fact a rapid increase in the length of the blades in passing from the first stator to the last rotor involves a number of problems in the mechanical and fluid dynamic design of the turbine, such as the sharp deviation of the flow downstream of the rotors, the non-optimal length/diameter ratio of the blades, etc.
On the other hand, the use of higher input temperatures into the turbine would allow higher efficiency of the thermo dynamic cycle and consequently higher production of electric power in relation to the thermal power introduced.
The same considerations are valid for the lowering of the condensation temperature, which also involves an increase in the expansion ratio, besides an improvement in the cycle efficiency.

Objectives of the Invention

Starting from this preamble, one objective of this invention is to create the conditions for lowering the feed pressure of the main turbine compared to the evaporation pressure or otherwise to increase the evaporation pressure to noticeably improve the cycle efficiency.
The objective is reached according to the present invention by the adoption on the work fluid path, upstream of the main turbine, of at least one auxiliary turbine in which a pre-expansion of the work fluid is realised and an additional coaxial electric generator, assembled on the auxiliary turbine drive with a high speed rotation shaft aimed at optimising the power extraction from the pre-expansion of the work fluid.

Brief Description of the Drawings

The invention will however be described in greater detail made in reference to the enclosed indicative and not limiting drawings, in which;

Fig. 1 shows a circuit diagram of the system including an auxiliary turbine;
Fig. 2 shows the same diagram as in Fig. 1, but implemented with a secondary evaporator;
Fig. 3 shows a cross section of a possible configuration of the auxiliary turbine; and
Figs. 4 and 5 shows the views of a main turbine and an auxiliary turbine integrated in different ways.

Detailed Description of the Invention

The system here proposed basically comprises, in association with a source of high temperature fumes - not shown, a turbogenerator group 10 using organic work fluid running in a relative circuit 11 and, between the fumes source and the turbogenerator, an intermediate circuit for a thermal carrier fluid 12.
The turbogenerator group 10 comprises a main turbine 13 with a respective electric generator 13'. The turbine is downstream of the heat exchanger group that comprises a pre-heater 17, an evaporator 14 the work fluid and a possible superheater 15 of the feed vapour of said turbine.
The fluid on exiting the main turbine 13 is directed into a work fluid condenser 16 and at least one regenerator 16', using the heat of the vapour to preheat the work fluid.
The high temperature fumes, by means of a primary heat exchanger - not shown, heat the thermal carrier fluid circulating in the intermediate circuit 12 and are directed immediately to a chimney or through a pre-heater used to pre-heat the comburent air to be fed to the combustor and a possible economizer used to heat a liquid for various purposes - not shown.
On exiting the intermediate heat exchanger, the heated thermal carrier fluid is made to circulate, in the direction of the arrows F in Fig. 1, in the superheater 15, if provided, and in the evaporator 14 to produce the feed vapour of the main turbine 13 before returning in cycle in the intermediate circuit through the pre-heater 17 for the work fluid.
In the intermediate circuit of the thermal carrier fluid 12 and in the work fluid circuit 11 pumps will be provided in the usual way and control valves 18, 18' to control the circulation of the respective thermal carrier and work fluids, plus other useful components.
The heat exchangers indicated may be set up as separate bodies or may be integrated in a single unit fulfilling the indicated functions.
According to an innovative aspect of the system, in the organic work fluid path, upstream of the main turbine 13, between the latter and the evaporator 14 or superheater 15, where provided, is inserted at least one auxiliary turbine 19, which creates a first expansion, or pre-expansion, of the fluid and the output of which is connected to the input of the main turbine.
Said auxiliary turbine 19 is equipped with a respective coaxial electric generator 19', assembled on the shaft 20 of the turbine, with a high rotation speed sufficient to optimise the power extraction from the pre-expansion.
The electric generator 19' connected to the auxiliary turbine 19 is preferably the permanent magnets type and with a rated capacity sufficiently high to enable operating without the intervention of adjustment devices in all the function field of the main turbine.
The auxiliary turbine 19 is preferably equipped with variable section nozzles to optimise its operating to the different loads that is in the presence of varying vapour flow-rate.
Furthermore it will have inlet and discharge volutes 21, 21' -Fig. 3- designed to keep their volume as small as possible so that the fluid content of these volutes does not increase the overspeed of the main turbine in the case of sudden lack of load, with consequent rapid closure of the input valves to the turbines.
In this way it is possible to avoid having to insert a cutoff valve, which would be very large and costly, between the discharge of the auxiliary turbine 19 and input of the main turbine 13.
The auxiliary turbine 19, besides, must be equipped with a sealing system so as to avoid access of high temperature work fluid into the area the coaxial electric generator turns in. For this purpose and in conjunction with another innovative aspect, around the shaft 20 of the auxiliary turbine 19, an annular chamber 22 is provided which is isolated with regards to the chamber in which the rotor turns by a labyrinth seal 23 made according to the known techniques and maintained at a pressure close to the one of the condenser 16 thanks to a duct 24 connected to the condenser itself. The cross section of the duct 24 must be such so as to enable the fluid leaking through the labyrinth seal to be transferred to the condenser at an acceptable loss of pressure.
The sealing system on the shaft will then have, according to the known technique, a mechanical seal, a further labyrinth seal.
A further innovative aspect consists in the injection in the zone of said ring shaped chamber 23, by means of another duct 25, of a small amount of liquid work fluid, correctly filtered, coming from circuit 11 and which evaporating at a pressure close to that of the condenser, guarantees to cool the shaft 20 and adjacent devices.
Furthermore, an interesting aspect deriving from the use of the auxiliary turbine, is the possibility of using an auxiliary turbine with a few robust blades, for example made by milling from a solid piece in the rotor disk, or by casting, maintaining between the length of the blade and axial chord of the blade a low ratio, for example less than a unit.
In this way the auxiliary turbine becomes a robust device capable to smooth the flow of vapour fed to the main turbine, above all in relation to the risk of dragging "plugs" of liquid during transient periods.
To reduce the volume of its load and discharge volutes 21, 21', the construction and dimension costs, the auxiliary turbine 19 can be made with the discharge volute integrated with the input volute of the main turbine according to the illustrative diagrams in Figs. 4 and 5.
It is also possible to foresee - Fig. 1- an additional duct 26 with feed valve 27 of the auxiliary turbine 19, which is opened when the main input valves 18, 18' are rapidly closed. Through this duct 26 the auxiliary turbine can be fed with a fluid delivery sufficient to maintain the respective electric generator 19' at a generating level equal to the power absorbed by the auxiliaries and to keep the electric generator at no-load at the rated speed , which will be maintained in rotation at a controlled speed.
This avoids having to stop the plant should there be a short time interruption in the mains.
Furthermore, the work fluid circuit, downstream of the evaporator and possible superheater, can be equipped with a shunted line 28 with a control valve 28' to bypass the auxiliary turbine 19, both in the case of a breakdown of the latter and for its maintenance, and to feed the main turbine 13 with an increase in delivery.
Furthermore, it will be possible to feed the main turbine 13 with a shunted delivery of vapour product in an exchanger/secondary evaporator 29 - Fig. 2- that receives heat from the intermediate circuit of the thermal carrier fluid 12, downstream compared with the main evaporator 14.
This delivery has a lower pressure compared to the evaporation temperature of the remaining delivery crossing through the main evaporator. Therefore it will be possible to produce a shunted delivery with an exchanger/secondary evaporator with a smaller surface compared to the surface which would be needed for a counter requirement to evaporate the same delivery of work fluid at the pressure of the main evaporator.
Said exchanger/secondary evaporator 29 can be fed by a separate pump and pre-heater, positioned in parallel compared with the main evaporator 14, but as an alternative and preferably, as shown in Fig.2, the secondary evaporator 29 will be fed with the liquid work fluid collected downstream of the pre-heater 17 and appropriately reduced in pressure in a throttle valve 30 upstream of the secondary evaporator.
Downstream of the secondary evaporator will b e inserted an interceptor valve 31 to avoid over-speed of the main turbine in case of lack of load.

Claims

Apparatus for generating electric energy with Rankine cycle using an organic work fluid, comprising a high temperature fumes or gas source, a heat exchanger between the high temperature fumes and a thermal carrier fluid circulating in an intermediate circuit, a heat exchanger between the thermal carrier fluid of the intermediate circuit and the organic work fluid in a relative circuit including at least an evaporator for the evaporation of the work fluid, a main turbine fed by the vapour of the work fluid and connected to a respective electric generator, a recovery regenerator of the thermal content of the work fluid vapour on exiting the main turbine and a condenser group of the work fluid before it returns into circulation, characterised in that in the work fluid circuit, upstream of the main turbine, at least one auxiliary turbine is inserted in which a pre-expansion of the work fluid takes place, and in that an additional electric generator is connected to the output shaft of the auxiliary turbine, having a rotation speed aimed at optimising the extraction of power from the pre-expansion of the work fluid.
Apparatus for the production of electric energy according to claim 1, in which the auxiliary and main turbines are connected without the use of interposed means of interception.
Apparatus for the production of electric energy according to claims 1 and 2, in which the auxiliary turbine has a discharge volute which is integrated and permanently connected to an input volute of the main turbine.
Apparatus for the production of electric energy according to claims 1 and 2 or 3, in which the auxiliary turbine shaft is associated with a labyrinth sealing system to avoid the high temperature work fluid entering the electric generator connected to the turbine.
Apparatus for the production of electric energy according to claim 4, in which said sealing system comprises a chamber provided around the auxiliary turbine shaft, isolated on the electric generator side and connected by an inserted duct to the condenser group in the fluid circuit to evacuate any leaked fluid and to maintain a pressure in said chamber close to the one in said condenser.
Apparatus for the production of electric energy according to claim 5, in which said sealing system comprises a chamber provided around the auxiliary turbine shaft, isolated on the electric generator side and connected by a supply duct of a small amount of work fluid designed to evaporate to maintain a pressure in said chamber close to the pressure in the condenser group circuit in the work fluid circuit.
Apparatus for the production of electric energy according to any of the previous claims in which the auxiliary turbine has a rotation speed higher than the main turbine speed.
Apparatus for the production of electric energy according to any of the claims from 1-6, in which the auxiliary turbine has a variable rotation speed to manage the efficiency at different loads.
Apparatus for the production of electric energy according to one of the claims 7 and 8, in which the auxiliary turbine is equipped with variable cross-section nozzles to operate with varying fluid flow-rate.
Apparatus for the production of electric energy according to any of the previous claims, in which the work fluid circuit, downstream of the evaporator and a possible superheater, has a derived line with a control valve to bypass the auxiliary turbine both in case of a breakdown of the latter and for an increased delivery of vapour to the main turbine.
Apparatus for the production of electric energy according to any of the previous claims, in which the work fluid circuit, downstream of the evaporator and a possible superheater, has a derived line with a control valve to feed the auxiliary turbine with a varying delivery of work fluid vapour, said valve being open when the main valves are closed.
Apparatus for the production of electric energy according to any of the previous claims, in which the work fluid circuit, includes a secondary evaporator in a heat exchanger condition with the thermal carrier fluid in the intermediate circuit and designed to feed the main turbine with an additional delivery of vapour.
Apparatus for the production of electric energy according to claim 11, in which said secondary evaporator has an input connected downstream of the pre-heater in the work fluid circuit using a throttle valve to adjust the vapour pressure going to said main turbine and an outlet connected to the main turbine by means of a cut-off valve.
A method for generating electric energy in a plant comprising a high temperature fumes or gas source, a heat exchanger between high temperature fumes or gas and a thermal carrier fluid circulating in an intermediate circuit, a heat exchanger between the thermal carrier fluid of the intermediate circuit and an organic work fluid for the evaporation of the latter, a turbine fed by the work fluid vapour coming from said heat exchanger and connected to a respective electric generator, a recovery regenerator of the thermal work fluid content at output from the main turbine and a work fluid condenser group before it returns into circulation, wherein the expansion of the fluid in the turbine of the system is preceded by pre-expansion in an auxiliary turbine in which a modest fraction of the available enthalpy drop is elaborated, and the auxiliary turbine is connected directly to a coaxial electric generator.
Method according to claim 14, wherein the auxiliary turbine has a higher rotation speed than the main turbine.
Method according to claim 14, wherein the auxiliary turbine has a variable rotation speed.
Method according to claim 14, wherein the auxiliary turbine is fed with a variable delivery of work fluid vapour through an additional line derived from the work fluid circuit, downstream of the evaporator and equipped with an adjustment valve.
Method according to any of claims from 14-17, wherein the main turbine can be fed directly with work fluid bypassing the auxiliary turbine using a derived line with a control valve.
Method according to any of claims 14 - 17, wherein the main turbine is fed by an additional delivery of work fluid vapour through an additional line that leads to an added evaporator that receives heat from the thermal carrier fluid downstream of the main evaporator.
Method according to any of the claims from 14 to 19, wherein the coaxial generator to the auxiliary turbine is connected to an electronic power device to generate a network frequency starting from a high and/or variable frequency at the output of the auxiliary turbine generator, dissipating the excess energy on a resistance.
Method according to any of claims from 14 to 19, wherein the generator associated with the auxiliary turbine is connected to a conversion electronic device capable of generating a reactive/capacitive power with a compensation at least partial of the main generator power factor.