1. Technical field of the invention
The invention relates to a system for the recovery of energy from exhaust gases.
5 In particular, the invention relates to a system for the recovery of energy from exhaust
gases from a turbos haft engine fitted to an aircraft, for example a helicopter.
2. Technological background
Aircraft, including helicopters, are generally fitted with one or a plurality of
10 turboshaft engines, the principle of which is to drive a turbine in rotation through the
combustion of a gas into which fuel is injected.
At the outlet from the turbine, the burnt gas that has driven the turbine in
rotation, known as exhaust gas, is discharged to the exterior through an exhaust pipe.
15 The cycles ofthe turbos haft engine result in exhaust gas temperatures of approximately
GOO"C. The theoretical thermal ~energy contained in tl 1is flow of exhaust gases is
estimated to be 60% ofthe potential energy contained in the fuel injected at the turbine
inlet.
20 It is therefore advantageous to attempt to recover part of this thermal energy in
order to increase the efficiency of the turboshaft engine. To do this, solutions have been
proposed, in particular the use of heat exchangers situated in the exhaust pipe of the
turboshaft engine, allowing part of the thermal energy to be recovered. This recovered
thermal energy is used, for example, to preheat the gas supplied to the turboshaft
25 engine before it is burned, or to reheat a gas from a secondary machine present in the
aircraft, of a turbine engine or piston machine type.
However, these solutions lead to a multitude of disadvantages. This is because
the presence of a heat exchanger in the exhaust pipe results in pressure losses that
30 affect the operation of the turbine. This heat exchanger can lead to fouling that affects
the performance of the turboshaft engine, and requires appropriate washing
5
2
procedures, and also to degradation in the event of blade loss and blade-shedding.
Blade-shedding is a mechanical protection against overs peed of the free turbine of the
turbos haft engine.
In addition, the use of such a heat exchanger requires the turboshaft engine to
be calibrated for operation with this heat exchanger. In the context of use of the heat
exchanger to preheat the gas fuelling the gas turbine, the presence of the heat
exchanger requires an engine operating point that is different from the operating point
without a heat exchanger, which means that the engine performance is heavily affected
10 if the heat exchanger is not used. This non-use of the heat exchanger, if it is accidental
(due to a failure of the heat exchanger), can furthermore cause degradation of the heat
exchanger and non-operation of the engine.
Finally, the constraints on the operation of the heat exchanger in an exhaust
15 pipe (high temperatures greater than, or equal to, 600"C, pressure between 4 and 8 bar,
etc.), require an appropriate sizing of the heat exchanger resulting, in particular, in an
increase in its size and weight, ana the use of materials that can withstand these
constraints. However, the thermal conduction performance of these materials that are
adapted to withstand the constraints is generally poor, which reduces the efficiency and
20 usefulness of the heat exchanger.
25
3. Objectives of the invention
The aim of the invention is to overcome at least some of the disadvantages of
known systems for recovering energy from exhaust gases.
In particular, the invention also aims to provide, in at least one embodiment of
the invention, an energy recovery system that does not result in pressure losses that
affect the operation of a turboshaft engine.
30 The invention also aims to provide, in at least one embodiment, an energy
recovery system of which a failure does not affect the operation of the turboshaft
5
10
15
20
3
engine.
The invention also aims to provide, in at least one embodiment of the invention,
an energy recovery system that can be put in place on existing turboshaft engines.
The invention also aims to provide, in at least one embodiment, an energy
recovery system that allows the use of heat exchangers made of materials with a better
heat exchange efficiency.
4. Summary of the invention
To that end, the invention relates to a system for the recovery of energy from
exhaust gases from at least one turboshaft engine, comprising:
a turbine fitted rotatably around a recovery shaft, adapted to bleed off
at least a part of the exhaust gases, known as bleed gases, and to
expand said bleed gases to become expanded gases at a pressure below
atmospheric pressu_re_.;_ __
a first heat exchanger, adapted to use a cold source to cool said
expanded gases to become cooled gases;
a compressor fitted rotatably around said recovery shaft, adapted to
compress said cooled gases to atmospheric pressure;
a fan configured to bring the cold source to the first heat exchanger, the
fan being driven in rotation by the recovery shaft.
A system according to the invention therefore allows the recovery of at least a
25 part of the energy from the exhaust gases from at least one turboshaft engine, this
being performed in an offset manner, unlike the prior art. This is because here, a part of
the exhaust gases is bled off in order to perform the heat exchange via the heat
exchanger in conditions more favourable than in the prior art, where the heat exchanger
is situated in an exhaust pipe of the turboshaft engine. The turbine, allowing the exhaust
30 gases to be bled off, also allows the pressure of the exhaust gases to be reduced, and
therefore the temperature of the exhaust gases to fall. The heat exchanger is therefore
4
subject to a lower pressure and a lower temperature, allowing the use of materials with
a better heat exchange efficiency. Likewise, the fact that the pressure of the gases
circulating between the turbine and the compressor is low, below atmospheric pressure,
generally limits the internal stresses by these gases on the components of the energy
5 recovery system.
Preferably, the energy recovery system does not bleed off all the exhaust gases,
in order to maintain a good level of efficiency by taking off mainly the high-temperature
gases, the portion of the exhaust gases bled off being dependent on the aerodynamics
10 of an exhaust pipe of the turboshaft engine allowing the exhaust gases to be discharged.
15
The bleeding-off can advantageously be performed in an elbowed portion of the exhaust
pipe. An elbowed portion of this kind is, for example, generally present in the exhaust
pipes of the turbos haft engines fitted to helicopters.
The energy recovered from the exhaust gases originates from the difference
between the mechanical energy produced when the exhaust gases pass through the
turbine and transmitted to the recovery shaft, and the energy consumed ·by the
recovery shaft to drive the compressor in rotation in order to compress the exhaust
gases cooled by the first heat exchanger. Any energy needed to bring the cold source to
20 the exchanger may be taken from this recovered energy if necessary. The recovered
energy can therefore be used in the form of mechanical energy transmitted by the
recovery shaft. The recovery shaft is then, for example, connected to other shafts,
through a relay box, to provide these other shafts with additional mechanical energy.
The shafts capable of using the recovered energy at the recovery shaft are, for example,
25 a shaft of the free turbine of the turboshaft engine, a shaft of the gas generator of the
turboshaft engine, a shaft of the main transmission box of a helicopter, a rear shaft
connected to the tail rotor of a helicopter, etc. This recovered energy at the recovery
shaft is in the form of mechanical energy, but can subsequently be converted into
anotherform (electrical, pneumatic, etc.).
30
Furthermore, bleeding off part of the exhaust gases does not cause pressure
losses in the turbos haft engine. Unlike the prior art, in which the exchanger was situated
5
in the exhaust pipe of the turboshaft engine, the bleeding of exhaust gases through the
turbine does not disrupt the normal operation of the turboshaft engine and therefore
limits pressure losses. In addition, any failure of the energy recovery system will not
affect the operation of the turboshaft engine, all the exhaust gases of which will be
5 discharged via the exhaust pipe. Furthermore, the energy recovery system can in this
way be adapted to turboshaft engines that already exist, and does not require any
change in the operating point of the turboshaft engine on which it is installed and
therefore does not affect its performance.
10 Advantageously, a system according to the invention comprises a second heat
exchanger, adapted to perform a preliminary cooling of the expanded gases, before they
pass into the first heat exchanger.
According to this aspect of the invention, the second exchanger allows the
15 temperature of the exhaust gases to be reduced before they pass into the first heat
exchanger, to allow an additional reduction in the temperature constraints of the first
heat exchanger, which can thus be designed so as to allow a high heat exchange
performance via more efficient materials and a reduced sizing. Advantageously and
according to the invention, the material used by the first exchanger is aluminium,
20 allowing a good compromise between good heat exchange performance (thermal
conductivity of approximately 150 W/mrC) for a reduced weight (density of
approximately 2700kg/m3
).
Advantageously, a system according to the invention comprises an external air
25 intake, adapted to perform a preliminary cooling of the expanded gases, before they
pass into the first heat exchanger.
According to this aspect of the invention, the air intake allows the expanded
gases to mix with air from the exterior in order to reduce their temperature. The
30 injection of external air through the air intake is facilitated by the fact that the expanded
gases are at a iower pressure than the atmospheric pressure of the external air. The
gases cooled in this way are sent to the first exchanger. The air intake can replace or
6
supplement the second exchanger described previously.
Advantageously, a system according to the invention comprises a plurality of
ducts connecting the turbine to a plurality of exhaust pipes for bleeding off exhaust
5 gases originating from a plurality ofturboshaft engines.
10
According to this aspect of the invention, a single energy recovery system allows
the recovery of part of the energy of the exhaust gases originating from a plurality of
turboshaft engines.
The invention also relates to a turboshaft engine, fitted with an energy recovery
system according to the invention.
A turboshaft engine according to the invention allows a better overall efficiency
15 of operation through the recovery of a part of the potential energy in the form of heat
contained in the exhaust gases, through the energy recovery system.
Advantageously, a turboshaft engine according to the invention further
comprises a gas generator driven in rotation by a gas generator shaft, and the recovery
20 shaft is connected to the gas generator shaft.
According to this aspect of the invention, the mechanical energy recovered by
the energy recovery system is used at the gas generator shaft, thus increasing the
performance of the turbos haft engine. In the event of failure of the energy recovery
25 system, the gas generator can operate normally, the only consequence being a
reduction in the performance of the turboshaft engine.
Advantageously, a turboshaft engine according to the invention further
comprises a free turbine driving in rotation a free turbine shaft, and the recovery shaft is
30 connected to the free turbine shaft.
According to this aspect of the invention, the mechanical energy recovered by
7
the energy recovery system is used at the free turbine shaft, intended, for example, to
drive the rotation of a propeller, thus increasing the performance of the turboshaft
engine. In the event of a failure of the energy recovery system, the gas generator can
operate normally, the only consequence being a reduction in the performance of the
5 turboshaft engine.
10
The invention also relates to a helicopter comprising a turboshaft engine
according to the invention.
Advantageously and according to the invention, the helicopter further
comprising a tail rotor driven by a rear shaft, and the recovery shaft is connected to said
rear shaft.
The invention also relates to a method for the recovery of energy from
15 turbos haft engine exhaust gases, comprising:
20
a step of bleeding off at least a part of the exhaust gas;
a step of expandingthe exhaust gas bledoff at the bleeding-off step;
a step of cooling the exhaust gas expanded at the expansion step;
a step of compressing the exhaust gas cooled at the cooling step.
Advantageously, the energy recovery method according to the invention is
implemented by the energy recovery system according to the invention.
Advantageously, the energy recovery system according to the invention
25 implements the energy recovery method according to the invention.
30
The invention also relates to an energy recovery system, an energy recovery
method, a turboshaft engine and a helicopter characterised in combination by all or part
of the features mentioned above or below.
5. List of figures
5
10
15
8
Other aims, features and advantages of the invention will become apparent on
reading the description that follows, given in a purely non-restrictive manner and which
refers to the accompanying figures, wherein:
Fig. 1 is a diagrammatic view of a turboshaft engine fitted with an energy recovery
system according to an embodiment of the invention;
Fig. 2 is a diagrammatic view of an energy recovery system according to a first
embodiment of the invention;
Fig. 3 is a diagrammatic view of an energy recovery system according to a second
embodiment of the invention;
Fig. 4 is a diagrammatic representation of an energy recovery method according
to an embodiment of the invention;
Fig. 5 is a diagrammatic view of a helicopter comprising a turboshaft engine
according to an embodiment of the invention.
6. Detailed description of an embodiment of the invention
The following embodiments are examples. Although the description refers to
one or a plurality of embodiments, this does not necessarily mean that each reference
relates to the same embodiment, or that the features apply only to a single
embodiment. Single features of different embodiments can also be combined in order to
20 provide other embodiments.
25
Figure 1 is a diagrammatic representation of a turboshaft engine 10 fitted with
an energy recovery system 12 according to an embodiment of the invention.
The energy recovery system 12 is adapted to recover at least a part 14 of the
exhaust gases of the turboshaft engine 10 in order to recover a part of the thermal
energy originating from the exhaust gases 16. These exhaust gases 16 are formed by
gases 18 that enter the turboshaft engine 10 via an inlet duct, are then mixed with fuel
and burned in the turboshaft engine 10 in order to drive the rotation of a free turbine 20
30 at the outlet from the turboshaft engine 10. Most of the kinetic energy of the burned
gases is recovered by the free turbine 20 being set in rotation, the residual kinetic
5
9
energy allowing the exhaust gases 16 to be discharged at the outlet from this free
turbine 20. The exhaust gases 16 are discharged via an exhaust pipe 22, allowing, in the
context of the use of the turboshaft engine 10 in an aircraft, the exhaust gases 16 to be
discharged to the open air, outside the aircraft.
The energy recovery system 12 allows a part of these exhaust gases,
represented by the arrow 14, to be bled off, for example by means of a bleed duct 24
connected to the exhaust pipe 22 of the turboshaft engine 10. The bleeding-off of the
part 14 of the exhaust gases is performed by virtue of a turbine that allows the bled-off
10 exhaust gases to expand, allowing the part 14 of the exhaust gases in the exhaust pipe
to be aspirated via the bleed duct 24. According to another embodiment of the
invention, the energy recovery system 12 comprises a plurality of bleed ducts connected
to a plurality of exhaust pipes, in order to bleed off a part of the exhaust gases
originating from a plurality of turboshaft engines.
15
The energy recovery system 12 also comprises a duct 26 for the admission of
external air 28, allowing the gas -bled offtnroughthe turbine to be cooled by means of a
heat exchanger present in the energy recovery system 12. Once the energy from the
part 14 of the exhaust gases has been recovered, the outlet gases 30 are discharged
20 through a discharge duct 32. This discharge duct 32 can also be used to discharge the
external air 28 after it has passed into the heat exchanger.
Fig. 2 is a diagrammatic representation of the energy recovery system 12
according to a first embodiment. In this first embodiment, the turbine 34, a compressor
25 36 and a fan 38 are connected to a recovery shaft 40. The turbine 34 bleeds off a part 14
of the exhaust gases from the exhaust pipe 10, and allows these exhaust gases to
expand and therefore their temperature to reduce, thus forming expanded gases,
represented by the arrow 42. The pressure of the expanded g3ses 42 resulting from the
expansion through the turbine 34 is less than the atmospheric pressure. The expanded
30 gases 42 pass through a first heat exchanger 44 to allow them to cool, thus forming
cooled gases, represented by the arrow 46. The cooling of the expanded gases 42 is
performed at the first heat exchanger 44 by virtue of a cold source 45, here brought to
10
the first heat exchanger 44 by the fan 38 driven in rotation by the recovery shaft 40 and
allowing the addition of external air 28.
The gases 46 cooled by the first heat exchanger 44 are sent to the compressor
5 36, connected to the recovery shaft 40. The compressor compresses the cooled gases 46
in order to obtain gases at a pressure substantially equal to atmospheric pressure,
known as outlet gases 30, which are, for example, discharged through a discharge duct
32 as shown previously with reference to Fig. 1. The discharge duct also allows the
discharge of the cold source 45 after it has passed into the first heat exchanger 44. The
10 discharge of the cold source 45 and the outlet gases 30 is represented by the arrow 48.
The energy recovered by the energy recovery system 12 can be used in the form
of mechanical energy transmitted by the recovery shaft 40. This recovered mechanical
energy originates from the difference between the mechanical energy brought to the
15 recovery shaft 40 by the rotation of the turbine 34 because of the passage of the part 14
of the exhaust gases, and the mechanical energy originating from the recovery shaft 40
and consumed by the compressor 36 in order to compress the cooled gases 46 to a
pressure substantially equal to atmospheric pressure. The cooling of the expanded gases
by the first heat exchanger 44 makes it possible to reduce the temperature of the
20 expanded gases and thus to reduce the energy needed by the compressor 36 to
compress the cooled gases to atmospheric pressure. The amount of energy recovered is
therefore dependent on the efficiency of the cooling by the first heat exchanger 44. The
energy consumed by the fan 38 also has to be taken from the recovered energy. The
recovered energy at the recovery shaft 40 can then, for example, be transmitted to
25 other shafts of an aircraft, by means of a relay box, or converted into another form of
energy.
In certain situations, the temperature of the expanded gases 42 remains high at
the outlet from the turbine and on their entry into the first heat exchanger 44.
30 Consequently, the dimensions and the material of first heat exchanger 44 must be
compatible with these high temperatures, although they are lower than the
temperatures encountered in the exhaust pipe 22 of the turboshaft engine 10.
11
In order to allow the use of a more efficient first heat exchanger 44, one solution
is to reduce the temperature of the expanded gases 42 beforehand. To do this, the
energy recovery system 12 comprises a second heat exchanger 50 allowing the
5 expanded gases 42 to be cooled before passing into the first heat exchanger 40, as
shown with reference to Fig. 3. In Fig. 3, the elements that are unchanged relative to the
embodiment shown in Fig. 2 bear the same reference numbers. The preliminary cooling
in the second heat exchanger 50 is performed using a second cold source 52, consisting,
for example, of outlet gases 30 leaving the compressor and of the cold source 45 after it
10 has passed into the first heat exchanger 44. Thus, by virtue of this preliminary cooling by
the second heat exchanger 50, the temperature of the gases 53 entering the first heat
exchanger is lower. The first heat exchanger 44 can therefore be made of a material
with more appropriate temperature limits and allowing a more efficient cooling and/or
smaller dimensions and lower weight.
15
For example, a material resistant to high temperatures, such as steel, has a
thermal conduCtivity ofapproximately 15 W/mtc ano a density of approximately
7800 kg/m3
. The second heat exchanger 50 can therefore be made of steel, for example.
Aluminium has a lower resistance to high temperatures, but a higher thermal
20 conductivity, approximately 150 W/mtc, and a density of approximately 2700 kg/m3
•
The first heat exchanger 44 can therefore be made of aluminium, for example, allowing
a more efficient cooling of the gases 53 passing through it, for a reduced weight.
Other types of metals or metal alloys can be used for the manufacture of the
25 first heat exchanger 44 and the second heat exchanger 50, depending on the constraints
on temperatures and sizing, and the desired performance, which may vary according to
the turboshaft engines from which at least a part of the exhaust gases are bled off, and
according to the embodiments, with one or two exchangers. Preferably, the heat
exchangers and/or the materials used in these heat exchangers have already been
30 tested for an application in an aircraft. For example, the energy recovery system can use
heat exchangers of the type that are used in aircraft cabin air conditioning systems,
which have already been tested for aviation use.
12
A preliminary cooling before entry into the first heat exchanger 44 is also
performed, in this embodiment, through the use of an air intake 51, allowing external air
to be injected into the gases going to the first heat exchanger 44. The mixing of the
5 gases with external air thus allows a reduction in temperature. The injection of external
air is furthermore facilitated by the fact that the gases present in the energy recovery
system 12 are at a pressure that is lower than atmospheric pressure.
Depending on the embodiments, the energy recovery system 12 may comprise
10 the first heat exchanger 44 only, or the first heat exchanger 44 accompanied by the
second heat exchanger SO, or accompanied by the air intake 51, or accompanied by a
combination of the second heat exchanger 50 and the air intake 51.
Fig. 4 is a diagrammatic representation of a method 54 for energy recovery
15 according to an embodiment of the invention. The energy recovery method 54
comprises a step 55 of bleeding off at least a part of the exhaust gases, known as bleed
gases, originating from a turboshaft engine, as described with reference to Fig. 1. The
bleed gases are then expanded in a step 56 of expanding the bleed gases, for example
through the turbine 34 in the energy recovery system 12 as described previously. This
20 expansion step 56 allows the formation of expanded gases, the temperature of which is
lower than that of the bleed gases because of the reduction in pressure. The expanded
gases are then cooled during a cooling step 58, to form cooled gases. These cooled gases
are then compressed during a compression step 60.
25
30
The energy recovery method 54 implemented in this way follows a
thermodynamic cycle that is a reverse Brayton cycle with pressure reduction. The
mechanical energy recovered by this cycle originates from the thermal energy contained
in the exhaust gases that are bled off.
Fig. 5 is a diagrammatic representation of a helicopter 61 comprising a
turboshaft engine 10 according to an embodiment of the invention. The turboshaft
engine is fitted with an energy recovery system 12 according to an embodiment of the
13
invention. In this embodiment, the energy recovery system 12 is connected to a rear
shaft 62 of the helicopter 61. This rear shaft 62 allows a tail rotor 64 of the helicopter 61
to be set in rotation, allowing the helicopter to be stabilised, in particular by
compensating for the torque exerted by the main rotor 66 driven by the turboshaft
5 engine 10 via a main transmission box. The mechanical energy recovered by the energy
recovery system 12 is thus transmitted by the recovery shaft 40 to the rear shaft 62 of
the helicopter 61.
The starting of the energy recovery system 12 requires the recovery shaft 40
10 connected to the turbine 34 and the compressor 36 to be set in rotation beforehand by
the addition of an external energy source, for example through the rear shaft 62 of a
helicopter. The energy recovery system 12 is therefore an energy receiver during startup.
Once the operating point has been reached, the energy recovery system 12 reaches
an equilibrium in which it becomes motive by virtue of the bleeding-off of at least a part
15 of the exhaust gases allowing the recovery of mechanical energy.
The invention is not restricted only to the embodiments described. In particular,
the cold source used at the exchanger or exchangers can take different forms, for
example a supply of air via an electric fan or via a pump, etc. In addition, the mechanical
20 energy generated by the system can be reused in a different form, for example at the
main transmission box of the helicopter, or by conversion into pneumatic energy,
electrical energy, etc. Furthermore, the energy recovery system can comprise more than
two exchangers.

CLAIMS
1. System for the recovery of energy from exhaust gases (16) from at least one
turboshaft engine (10), comprising:
a turbine (34) fitted rotatably around a recovery shaft (40), adapted to
bleed off at least a part (14) of the exhaust gases, known as bleed gases,
and to expand said bleed gases to become expanded gases (42) at a
pressure below atmospheric pressure;
a first heat exchanger (44), adapted to use a cold source (45) to cool said
expanded gases (42) to become cooled gases (46);
a compressor (36} fitted rotatably around said recovery shaft (40),
adapted to compress said cooled gases (46) to atmospheric pressure;
a fan (38} configured to bring the cold source (45) to the first heat
exchanger (44), the fan (38} being driven in rotation by the recovery
shaft (40).
2. Energy recovery system according to claim 1, characterised in that it comprises a
second heat exchanger (50}, adapted to perform a preliminary cooling of the expanded
gases (42), before they pass into the first heat exchanger (44).
3. Energy recovery system according to claim 1 or claim 2, characterised in that it
comprises an air intake (51), adapted to perform a preliminary cooling of the expanded
gases (42). before they pass into the first heat exchanger (44).
25 4. Energy recovery system according to any of claims 1 to 3, characterised in that it
comprises a plurality of ducts (24) connecting the turbine (34) to a plurality of exhaust
pipes (22) for bleeding off exhaust gases (16) originating from a plurality of turboshaft
engines (10).
30 5. Turboshaft engine, fitted with an energy recovery system (12) according to any of
claims 1 to 4.
15
6. Turboshaft engine according to claim 5, further comprising a gas generator driven
in rotation by a gas generator shaft, characterised in that the recovery shaft (40) is
connected to the gas generator shaft.
5 7. Turboshaft engine according to claim 5 or claim 6, further comprising a free
10
turbine (20) driving in rotation a free turbine shaft, characterised in that the recovery
shaft (40) is connected to the free turbine shaft.
8. Helicopter comprising a turboshaft engine (10) according to any of claims 5 to 7.
9. Helicopter according to claim 8, further comprising a tail rotor (64) driven by a
rear shaft (62), characterised in that the recovery shaft (40) is connected to said rear
shaft (62).