GB2483912A

GB2483912A - Recuperated micro gas turbine

Info

Publication number: GB2483912A
Application number: GB1016110.7A
Authority: GB
Inventors: Alister Clay
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-09-24
Filing date: 2010-09-24
Publication date: 2012-03-28
Anticipated expiration: 2030-09-24
Also published as: GB2483912B; GB201016110D0

Abstract

The present invention relates to a coiled pipe in pipe recuperator (1, fig. 1) incorporating a micro gas turbine engine within its central void. The gas turbine engine may further comprise a compressor stage 16, a turbine stage, a bearing platform 19, a generator 25, control electronics 26 and an inverter. The gas turbine engine may further comprise a centrifugal compressor impeller 17, a centrifugal turbine impeller 21, and a generator shaft connected by a common shaft (56, fig. 8) supported by an automotive oil cooled journal bearing platform. The gas turbine engine may further comprise a combustion chamber 24 with ignition source, a compressor shroud 16, a turbine shroud 23 and electronic circuitry. There may be two or more compressor stages. There may be two or more turbine stages. A heat shield 22 may be incorporated behind the turbine and against the bearing platform.

Description

SIMPLE, LOW COST, RECUPERATED MICRO GAS TURBINE

FIELD OF THE INVENTION

This invention relates to a simple, low cost, recuperated micro gas turbine. The micro gas turbine components are aligned axially and surrounded by a coiled pipe-in-pipe recuperator.

BACKGROUND OF THE INVENTION

Micro gas turbines (MGT5) (<5 kW) have lower thermal efficiencies than other heat engines of similar size. To address this, MGT5 make use of recuperators to raise thermal efficiency. Recuperators are heat exchangers which transfer thermal energy from the post turbine exhaust gas fluid stream to the post compressor compressed air fluid stream to pre-heat before combustion and reduce the amount of fuel reguired to reach Turbine Inlet Temperature (TIT) . At smaller scales recuperators increase unit size, cost and cycle pressure-drops to the extent that alternative prime mover configurations become more appealing. The use of an effective, simple, low cost recuperator is necessary for a commercial micro gas turbine (NOT) system to be realised. Until now, recuperator proposals have introduced a cost and volume burden whilst failing to integrate successfully within the NOT system to compliment performance.

The largest proportion of heat exchanger cost is associated with manufacturing. Heat exchangers have a physical reguirement for a specified heat transfer surface area that is calculated to provide the necessary energy transfer between each fluid stream. The heat exchanger design then seeks to provide a maximum surface area/volume ratio in order to reduce the overall size.

Also, the design must be considerate of available materials and manufacturing techniques to provide a low cost system. When priority is given to function rather than cost, intricate designs are developed which require the use of complex and time-consuming manufacturing processes such as forming and welding. Forming processes such as bending and cutting require new factory tooling.

Welding processes are common because they represent the best method of producing a pressure tight seal but are very labour intensive. Hence, a low risk recuperator which can use stock material without fabrication of new tooling and minimal welding processes is financially attractive if a viable commercial recuperator is to be realised.

SUMMARY OF THE INVENTION

This invention describes a system which achieved by successfully integrating a simple, low cost, coiled pipe-in-pipe recuperator with a micro gas turbine. The gas turbine components are arranqed in an axial alignment to fit within the void volume of the coiled pipe-in-pipe recuperator for a compact system. Available space is used efficiently and separate connections between components are sparse, reducing potential sources of pressure loss.

Arranging the components this way provides maximum distance between the hottest and coldest components to reduce the temperature gradient and heat transfer across the NOT which can be detrimental to performance.

ADVANTAGES OF THE INVENTION

1. Cost: the use of a pipe-in-pipe heat exchanger as a NOT recuperator may use readily available stock material to provide a barrier between different fluid streams and the surrounding atmosphere. Coiling the pipes may use existing manufacturing practises, such as a CNC pipe bending machine, tc manipulate the heat exchange length tc a practical and usable volume. A minimal number of fixing processes are required for the pipe extensions to produce separate gas-tight fluid streams.

2. Packaging: the void created within the coiled pipe-in-pipe recuperator provides the necessary volume to house and contain all the other MGI components to form a single, compact system.

3. Temperature gradient: heat transfer (a system loss) between components is minimised by creating hot' and cold' ends which positions the hottest and coldest components as far away from each other as possible.

4. Pressure loss (a system loss) from numerous and difficult connections is reduced by an axial component alignment by direct coupling between components.

BRIEF DESCRIPTION OF THE DRAWINGS

shows a sectioned view of one embodiment of the coiled pipe-in-pipe recuperator 1 with nomenclature: outer pipe 2, inner pipe 3, annulus cold fluid zone 4, inner hot fluid zone 5, outer pipe diameter 6, inner pipe diameter 7, coil pitch 8, neutral axis bend diameter (NABD) 9, central void diameter 10.

shows an exploded view of an embodiment of the coiled pipe-in-pipe MGI to describe the fundamental components: outer pipe 2, inner pipe 3, cold end outer pipe extension 11, cold end inner pipe extension 12, hot end outer pipe extension 13, hot end inner pipe extension 14, additional cold end outer pipe extension 15, compressor shroud 16, compressor impeller 17, compressor thrust bearing 18, bearing unit 19, common shaft 20, turbine impeller 21, turbine heat shield 22, turbine shroud 23, combustor 24, generator 25, control unit 26.

shows a sectioned view of an embodiment of the coiled pipe-in-pipe MGI system to illustrate how the MGI components fit together within the coiled pipe-in-pipe recuperator central void.

Error! Reference source not found. shows an embodiment of an recuperator 1 end pipe extension assembly: cold end circular hole 27 in the cold end outer pipe extension 11 to allow the cold end inner pipe extension 12 to protrude, a bend 28 in the cold end outer pipe extension 11 to provide separate connectivity to the annulus cold fluid zone 4 and inner hot fluid zone 5. A gas tight fixing method perhaps but not restricted to welding 29, to join the cold end inner pipe extension 12, to the inner pipe 3, A gas tight fixing method perhaps but not restricted to welding 30, to join the outer pipe extension 11, to the outer pipe 2, A gas tight fixing method perhaps but not restricted to welding 31, to join the cold end outer pipe extension 11, with the cold end inner pipe extension 12.

Error! Reference source not found. shows the compressor shroud 16, with a compressor inlet 32, volute 33, diffuser 34, and axial discharge 35. Ihis embodiment shows a vaneless diffuser, but further embodiments may use a vaned or other diffuser type.

Error! Reference source not found. shows the turbine shroud 23, with turbine inlet 36, turbine discharge 37.

Ihe design includes a 90° bend 38, in the turbine shroud 23 for an axial inlet from the combustor 24. In the preferred embodiment, the turbine shroud 23, is designed for a direot axial coupling to the oombustor 24. This embodiment shows a vaneless spaoe 57, but further embodiments may use vanes, nozzles or other method of flow control.

Error! Reference source not found. shows the oombustor 24, with oombustor discharge 39, hollow cylinder entry 40, hollow cylinder 41, and hollow cylinder exit 42. In the preferred embodiment the combustor 24 is designed for a direct axial coupling to the turbine shroud 23.

Error! Reference source not found. shows the preferred embodiment of a coupling technigue to join the generator shaft 56 of the generator 25 to the common shaft 20 which holds the compressor impeller 17 and turbine impeller 21 (not shown) . The coupling uses the common shaft thread 43 to attach the compressor impeller 17 to the common shaft 20 to connect with the generator shaft 56 with a die or internal thread 44 within the generator shaft. Other embodiments may use this or other coupling technigues.

Error! Reference source not found. Shows an embodiment that combines the turbine shroud and combustor to form a single part 45, to move fluid directly through the hollow cylinder 41 and out the combustor discharge 42.

Error! Reference source not found. Shows an embodiment for a turbulence promoter 46 which disrupts the flow in the annulus cold fluid zone 4.

Error! Reference source not found. Shows an embodiment of a MGT setup incorporating a generator turbine impeller 52 within the turbine shroud 23 connected by a common shaft to the compressor impeller 17 within the compressor shroud 16 supported by a bearing platform 19. In this embodiment the fluid part expands in the generator turbine and enters the power turbine shroud 50 and fully expands via the power turbine impeller 51 which is connected to the generator 25 by the power shaft 55 supported by the power bearing block.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

OVERALL DESIGN

This invention details the use of a system consisting of necessary gas turbine components to extract electrical power from a rotating shaft via the Brayton cycle (Saravanamutto, H.I.H., G.F.C. Rogers, and H. Cohen, Gas Turbine Theory. 5th ed. 2001, Harlow: Pearson Education Limited. 491) and in a first preferred embodiment comprising: compressor, turbine, bearing platform, combustor, electrical generator, inverter and control electronics packaged inside a coiled pipe-in-pipe recuperator to produce a single compact micro gas turbine (MCT) system. Pipe-in-pipe heat exchangers are the simplest and usually cheapest method of heat exchange.

However, the large heat exchange surface area produces unpractical overall pipe length. Coiled pipe-in-pipe heat exchangers manipulate the overall heat exchanger length to a more practical and useable volume along with providing additional fluid mixing from a toroidal flow pattern.

A coiled pipe-in-pipe heat exchanger involves coiling 2 pipes of different diameter on the same pitch 8 and neutral axis bend diameter (NABD) 9 and screwing' the smaller inner pipe 3 inside the larger outer pipe 2 see. Pipe extensions 11, 12, and 13, 14 are fixed at their respective cold and hot ends of recuperator 1 to separate in a gas tight fashion the annulus cold fluid zone 4 and inner hot fluid zone 5 whilst holding the inner pipe 3 inside the outer pipe 2. An embodiment system using pipe extensions is shown but not limited to and. The pipe extensions 11, 13, and 12, 14, have similar diameters to the outer pipe 2 and inner pipe 3 respectively of the recuperator 1 to which they're affixed. It is preferred though not limited to for each outer pipe extension 11 and 13 to have a similar side wall hole 27 and a similar bend 28 to allow the inner pipe extensions 12 and 14 to protrude and separate the annulus cold fluid zone 4 and inner hot fluid zone 5.

An embodiment recuperator 1 pipe extension fixing method is shown in Error! Reference source not found.

where the cold end inner pipe extension 12 is fixed 29 on to the inner pipe 3. The cold end outer pipe extension 11 is positioned and fixed 30 on to the outer pipe 2. A final fixing process 3 around the side wall hole 27 of the cold end outer pipe extension 11. The fixing methods provide a gas tight seal to separate the annulus cold fluid zone 4 and inner hot fluid zone 5. Affixation and jointing may be by welding or any other method suited to the material or otherwise from which the pipes are fabricated.

An alternative embodiment may have a straight outer pipe extension with a hole in the side wall which allows the inner pipe extension with a bend to protrude through.

RECUPERATOR

The coil NABD 9 must be a minimum size to provide a void diameter 10 to house the other MOT components 16 - 26 within. The embodiment illustrated in Figures 1 -9 will produce 1 kW electrical power with a pressure ratio of 2.15 and electrical efficiency cf 11.5%. The embodiment has an optimum outer pipe diameter 6 of 63.5 mm and inner pipe diameter 7 of 50.8 mm. At 5 turns the recuperated MOT is approximately 350 mm long and 230 mm in height and depth which approximately occupies a volume of 22 Litres. Embodiments with smaller pressure ratio will be larger, as larger hydraulic diameters or pipe diameters will be reguired for smaller pressure drops.

Smaller embodiments will require a higher pressure ratio since smaller pipe diameters will produce larger pressure drops.

Devices, objects or material can be placed within the annulus cold fluid zone 4 or inner hot fluid zone 5 to disrupt the cold working fluid or hot exhaust gas fluid to promote turbulence. An embodiment turbulence promoter 46 is shown in but is not restricted to Error! Reference source not found.. Turbulence promoters 46 are any device, object or material that assists purposively or otherwise in disturbing the fluid flow. A disturbed flow will amongst other things, improve the energy distribution within the fluid stream and reduce or disrupt the boundary layer to improve the heat transfer between the fluid streams. If the heat transfer rate can be improved the recuperator size can be reduced by pipe length and or pipe diameter.

In the preferred embodiment the recuperator acts as a diffuser by expanding area principle to convert the kinetic energy pressure component of a fluid stream from a previous stage to a static pressure component. This improves heat exchange effectiveness by reducing fluid speed and raising static density. In the embodiment a significant kinetic pressure component may exist after using a low diffusion or accelerating compressor stage to improve the compressor and so micro gas turbine operating performance. In other embodiments the recuperator may accelerate or otherwise the fluids streams and use high diffusion compressor stages.

BEARING PLATFORM

The function of the bearing platform 19 is to provide the shaft 20 with rotational freedom about its axis whilst restricting movement elsewhere. The preferred embodiment uses an oil-cooled journal bearing platform taken from the Garrett GT1241 turbo-charger series with a published maximum speed of 220,000 rev/mm. As an established off-the-shelf technology it's representative of a low risk, low cost commercial solution. Other NOT proposals of 1 kM or less have shaft speeds in excess of 500,000 rev/mm to increase compressor performance by using non-contact bearings such as air or electromagnetic. Different embodiments with other bearing platforms like air or electromagnetic will have similar functionality, even at lower speeds, and not effect the interpretation of the layout described herein.

Air bearings consist of aerostatic which uses externally supplied pressurised air, aerodynamic which use viscous forces created in the air between the rotor and casing generated by the shearing motion during rotor rotation, air-foil, foil-air or foil have the same working principle as aerodynamic but with a compliant foil casing, Hydroinertia are a hybrid between aerostatic and aerodynamic where the lubricating air goes supersonic which creates a vacuum to pull rather than push the rotor from the case.

Electromagnetic or active magnet bearings use electric current to induce a magnetic flux to lift the rotor from the casing.

-10 -The preferred embodiment will combine all rotating components; compressor impeller 17, turbine impeller 21 and generator 25 on a single shaft and bearing platform.

Other embodiments may include, but are not restricted to, separate and/or additional bearing platforms for the generator and/or additional turbomachinery with or without shaft coupling technigues. All additional MCT components resulting from other embodiments which are not explicitly explained come under the term MGI components' and shall be housed within the coiled pipe-in-pipe recuperator 1 void volume much the same way as the preferred embodiment illustrated in and

COMPRESSOR

The compressor consists of a rotating impeller 17 and compressor shroud 16. The compressor shroud 16 contains a compressor inlet 32, volute 33, diffuser 34 and axial discharge 35. The preferred embodiment shows a vaneless diffuser. Other embodiments may use a vaned or other diffuser type. The preferred embodiment uses but isn't restricted to drawing atmospheric air into the compressions stage to use as the working fluid.

Atmospheric air is drawn into the compressor inlet 32 is compressed and exits via the axial discharge 35. From here the compressed air enters the annulus cold fluid zone 4 of the recuperator 1.

Conventional compressors discharge in a tangential direction. The preferred embodiment has the flow turned through 90° in the shroud to provide an axial discharge 35 which supports the compact axial component alignment using a direct cold end outer pipe extension 15 to couple via the cold end outer pipe extension 11 with the coiled pipe-in-pipe recuperator 1.

-11 -COP4BUS TOR In the preferred embodiment after warming in the annulus cold fluid zone 4 of the recuperator 1, the warm compressed air enters the combustor 24. In the combustor 24 the warm compressed air is mixed with fuel and ignited. The fluid now an exhaust gas reaches its maximum or Turbine Inlet Temperature (TIT) during this stage of the cycle. The exhaust gas fluid exits the combustor through the combustor discharge 39 and enters the turbine shroud 23 through the turbine inlet 36. A higher TIT will raise the system efficiency but maximum temperature is limited by metallurgical limits of the combustor and turbine. For the preferred embodiment a TIT of 1200K may or may not incorporate standard engineering materials and turbine components such as those found on automotive turbochargers to support a simple, low cost system. Other embodiments using a broader range of material and manufacturing technology may raise or lower TIT.

In the preferred embodiment the combustor 24 is positioned axially and directly coupled to the turbine shroud 23 to form a hot end at maximum distance from the cold components or cold end. The coiled pipe-in-pipe recuperator described in this embodiment allows the NOT components to be strategically positioned in this axial configuration without substantially increasing the system volume. This permits maximum or near maximum distance between hot and cold components to lowers the temperature gradient across the NOT and reduce heat transfer which is detrimental to performance.

In the preferred embodiment hot expanded turbine fluid passes through the turbine discharge 37 on the turbine shroud 23 into the hollow cylinder entry 40 and through the hollow cylinder 41 of the combustor 24. The hot end inner pipe extension 14 fixed to the hollow -12 -cylinder exit 42 collects the expanded fluid. Heat loss from the comhustor 24 through the hollow cylinder 41, which can reduce efficiency and performance, can be minimised with the warm expanded exhaust gas fluid passing through. The use of a hollow cylinder 41 supports a direct coupling between the combustor 24 and turbine shroud 23 for the NOT axial alignment configuration.

A further embodiment may combine the turbine shroud and combustor to form a single part 45 to remove the combustor discharge/turbine inlet connection 39/36 and the turbine discharge/hollow cylinder inlet 37/40 connection to move exhaust gas fluid directly through the hollow cylinder 41 and out the combustor discharge 42.

In other embodiments atmospheric air and fuel maybe be mixed before entering compressor stage.

TURBINE

The turbine consists of a rotating turbine impeller 21, a stationary heat shield 22 and stationary shroud 23. In the preferred embodiment after combustion the exhaust gas fluid is delivered to the turbine stage through the axial turbine inlet 36. Similar to the compressor outlet the turbine inlet 36 is axial with a 90° bend within the shroud 38 to support the axial component alignment within the coiled pipe-in-pipe recuperator. The exhaust gas fluid energy (enthalpy) produces power by expanding through the turbine stage causing the common shaft 20 to rotate. In the preferred embodiment some of the rotating power will drive the compressor stage along the common shaft 20 whilst the remainder or net power will turn the generator shaft 56 on the electrical generator 25. In the embodiment the generator shaft 56 is attached to the common shaft 20 by coupling with an internal thread 44 on the generator shaft as previously described.

-13 -In a further embodiment as shown Error! Reference source not found., the exhaust gas fiuid maybe part expanded in a generator turbine 52 to drive the compressor impeller 17 along a common shaft 20 and fully expanded in the power turbine 51 to generate external power. The turbine shaft 55 supported by a turbine bearing platform 53 connects the power turbine 51 to the generator 25 by a coupling 54.

The heat shield 22 reduces heat transfer from the turbine into the bearing case. It is therefore fabricated out of a material with lower thermal conductivity compared to the surrounding components.

After the turbine, the exhaust gas fluid enters the hot end inner pipe extension 14 and passes through the inner hot fluid zone 5 of the coiled pipe-in-pipe recuperator 1. Heat exchange from the hot exhaust gas fluid to the cold compressed air will occur as the turbine exit temperature (beginning of inner hot fluid stream 5) is higher than the combustion inlet temperature (end of annulus cold fluid stream 4) . In this embodiment the streams are arranged; hot inner, cold outer, to minimise heat loss to the atmosphere and surrounding MGT components. Other embodiments may arrange the streams alternatively: hot annulus, cold inner which would not affect the MGT layout nor the claims made herein.

GENERATOR

The preferred embodiment is to combine all rotating components on a single shaft. Other embodiments a separate generator shaft 56 will require a coupling technique to the common shaft 20 similar but not restricted to that shown in Error! Reference source not found.. The electrical generator 25 converts the mechanical power (rotating shaft) into electrical power -14 -by electromagnetic induction. The preferred embodiment is for 1 kW electrical power. Other embodiments are not restricted by power outputs.

Claims

CLAIMS: 1. A coiled pipe-in-pipe recuperator incorporating a micro gas turbine (MOT) within its void.
2. The recuperated MOT of claim 1 wherein the coiled pipe-in-pipe recuperator which has a central void to contain the gas turbine components required to form a working system to deliver power via the Brayton cycle: a compressor stage, a turbine stage, bearing platform, generator, control electronics and inverter.a centrifugal compressor impeller, centrifugal turbine impeller and generator shaft connected by a common shaft supported by an automotive oil cooled journal bearing platform; and a combustion chamber with ignition source, compressor shroud, turbine shroud; and electronic circuitry, cables, wires, hose to supply fuel, oil and move electrical current.
3. The recuperated MCT of claim 1 wherein working fluid is compressed through the action of 2 or more compressor stages.
4. The recuperated NOT of any of claIms 1 to 3 whereIn the combusted gases are expanded via 2 or more turbine stages.
5. The recuperated NOT of any preceding claim in which the combusted gases are at first part expanded via a generator turbine' as revealed in the description of this invention and whose purpose in the cycle is to generate power to drive the compressor or compressors for the entire cycle. The combusted gases are then fully expanded via what is revealed as a power turbine' whose purpose in the cycle is to generate power for external use.
6. The recuperated MGT of any proceeding claim wherein the bearing platform is of oil cooled journal bearing type.
7. The recuperated NOT of any proceeding claim wherein the bearing platform is of electromagnetic bearing type
8. The recuperated NOT of any proceeding claim wherein the bearing platform is of electro-active bearing type.
9. The recuperated NOT of any proceeding claim wherein the bearing platform is of aerostatic bearing type wherein the lubricating fluid could be bled from a compression stage.
10. The recuperated NOT of any proceeding claim wherein the bearing platform is of aerostatic bearing type wherein the lubricating fluid supply could be supplied from additional or external devices.
11. The recuperated NOT of any proceeding claim wherein the bearing platform is of aerodynamic bearing type.
12. The recuperated NOT of any proceeding claim wherein the bearing platform is of air-foil bearing type.
13. The recuperated NOT of any proceeding claim wherein the bearing platform is of hydroinertia bearing type.
14. The recuperated NOT of any previous claim wherein all working fluid from the compression stages is used for lubrication in the bearing platforms before recuperation.
15. The recuperated NOT of any previous claim wherein the combustion fuel whether gas or liguid is used to provide lubrication in the bearing platforms before the fuel enters the combustion stage.
16. The recuperated NOT of any preceding claim wherein the working fluid begins as atmospheric air.
17. The recuperated NOT of any preceding claim wherein the working fluid is formed from a mixture of combustion fuel and atmospheric air.
18. The recuperated NOT of any preceding claim wherein the working fluid is formed of Hydrogen.
19. A coiled pipe-in-pipe reouperator by incorporating pipe extensions to locate in a gas tight fashion an inner coil within an outer coil.
20. A recuperated NOT wherein 90° bends in the compressor and turbine shrouds aiiow a substantially axial arrangement of the NOT components within the coiled pipe-in-pipe recuperator.
21. The recuperated NOT of any preceding claim wherein the hottest components are situated a far distance away from the coldest components as the outer dimensions of the system will allow.
22. A recuperated MGT of any previous claim wherein a separate heat shield is incorporated behind the turbine and against bearing platform.
23. A recuperated MGI of any previous claim wherein the combustor is directly attached to the turbine shroud.
24. A recuperated MGI of any previous claim wherein a single part forms the turbine shroud and combustor.
25. A recuperated MGI of any previous claim wherein threads are used to join a separate generator shaft to the common shaft holding the compressor and turbine impellers.
26. A recuperated MGI of any previous claim wherein turbulence promoters are incorporated within a coiled pipe-in-pipe recuperator.
27. A recuperated MGI of any previous claim wherein the coiled pipe-in-pipe recuperator converts kinetic energy of the working fluid to static pressure by diffusion.
28. A recuperated MGI of any previous claim wherein compressor stages incorporate a low diffusion or accelerating flow regime.