WO2007003929A1

WO2007003929A1 - Neck cooler

Info

Publication number: WO2007003929A1
Application number: PCT/GB2006/002455
Authority: WO
Inventors: Per. D. Sollie; Lars Kjosbakken; Sven Erik Fossum; Eldar Onsoyen; Sverre Johansen
Original assignee: Disenco Limited
Priority date: 2005-07-01
Filing date: 2006-06-30
Publication date: 2007-01-11
Also published as: GB0513590D0

Abstract

A neck cooler assembly (8) within a Stirling engine, which assembly consists of a neck cooler cylinder (8.3) and a neck cooler outer shell (8.4) both parts welded together to form a shroud with voids (V) in conjunction with inlet and outlet cooling ports that allow the passage of a coolant which can control the operating temperature of the power piston crosshead and power piston. The neck cooler assembly (8) includes a cylinder bore (D2) where the Stirling engine power piston is located and another bore (Dl) that retains and guides the Stirling engine Power Crosshead.

Description

NECK COOLER Technical Field of the Invention

The invention relates to a cooler assembly to surround the neck of a piston/cylinder arrangement in a Stirling engine. Background of the Invention

Stirling engines offer advantages of multi-fuel capabilities (geothermal, solar, bio-, fossil- and nuclear fuel), very low NO_x and HC emissions when burning fossil fuels, high total efficiency (particularly when used with a combined heat and power CHP unit), and very low maintenance intervals compared to internal combustion engines.

The principle of operation of a Stirling engine can be described with reference to FIG. 1. The displacer (a) and power piston (b) reciprocate within a cylinder with a fixed charge of working gas (e.g. air, nitrogen, helium or hydrogen). The displacer and power piston are connected to a crankshaft (c) via crossheads, connecting rods (d) and wristpins. As the displacer (a) reciprocates, it displaces the working gas (usually nitrogen or helium in production engines) through the heater head tubes (e), regenerator (f) and cooler (g) that are placed in the hot and cold portions of the engine. The displacer (a) and power piston (b) have different phase angles so that more work is put into the power piston during the expansion stroke, when most of the gas is in the hot space, than the work the piston returns to the gas a cycle later to compress cold gas back to the hot part of the engine. The net surplus of expansion work over compression work is extracted as useful work by the power piston, which in turn is transferred to the crankshaft (c) with its outgoing shaft. All external heat is supplied at the heater head (e) and rejected in the cooler (g). The regenerator (f) absorbs heat from the working gas as the gas moves from the hot end to the cold end. It returns the stored heat to the working gas when the gas is pushed from the cold end to the hot end. One can say that the regenerator acts as a "thermal dynamic sponge".

There exist several types of Stirling engines; α-, β- and γ-type. In addition there are engines with oil lubrication and non-lubricated (or lubricated for life). Next, there are engines that are hermetically sealed and ones that have a so-called "atmospheric" crankcase whereas there is a need for a dynamic seal between the oil lubricated crankshaft assembly, displacer rod, crosshead and power piston rings.

In a traditional β-type (or commonly called displacer type) engine, there is a power piston and displacer piston coaxially located within the same working cylinder. In order to move the displacer piston back and forth a displacer rod is coaxially positioned through the centre bore of the power piston. On the top side the displacer rod is fastened to the displacer base, which in turn can be threaded to the displacer piston (or dome). On the lower side the displacer rod is fastened to the displacer crosshead.

Since conventional β-type Stirling engines use a crankshaft and connecting rod mechanism to convert oscillating motion to rotary motion, side forces that originate during the Stirling cycle need to be accommodated. This is particularly necessary if the Stirling engine is of the dry running type.

JP63038669 discloses a β-type (displacer-type) Stirling engine that is lubricated by use of an oil film. In order to avoid leakage of oil into the hot Stirling circuit, an oil-separating device is located between the power piston and crankcase. This oil separating device is placed within a cooling jacket that is located around the circumference of the power piston's cylinder. The cooling jacket forms part of the Stirling engines crankcase structure. The cooling jacket serves two purposes; 1) oil cooler and 2) power piston cooler.

This solution can be an expensive and difficult part to make due to casting and tolerance issues that may arise during fabrication.

For non-lubricated high performance Stirling engines with crossheads there is a need to reject the heat generated from both crossheads (i.e. power and displacer) and power piston during operation. In addition, in order to reduce the total height of the Stirling engine, the power crosshead and displacer crosshead oscillate concentrically partly within each other and are placed within the same component as the power piston. This in turn creates a need to have provision to cool the power piston cylinder wall as well as both crossheads. In addition, for fabrication purposes, it is beneficial to be able to produce the cooler as a simplified separate unit.

Possible solutions will be covered in the following description of the invention.

Summary of the Invention

It is an object of the present invention to provide a neck cooler assembly that is able to control and regulate the power crosshead and power piston wall temperature. It is another object of the present invention to provide a neck cooler assembly that comprises only one part. It is a further object of the present invention to provide a neck cooler assembly, which in conjunction with a Stirling engine neck, can constitute a void or jacket that allows coolant to circulate in order to absorb heat from the power crosshead and piston cylinder wall. The invention provides a neck cooler assembly for a Stirling engine, which assembly includes a cylinder bore for the power piston of the Stirling engine and another bore capable of guiding and retaining the power piston crosshead of the Stirling engine, in which the assembly has a peripheral shroud surrounding a void and there are inlet and outlet coolant ports arranged on or within an outer shell of the neck cooler assembly to allow coolant to pass through the void in order to control the operating temperature of the power piston crosshead and power piston, and the assembly includes a neck cooler cylinder and a neck cooler outer shell that is placed concentrically around and joined to the neck cooler cylinder It is preferred that said ports are in connection with at least two channels in the inner or outer boundaries of the void.

It is further preferred that the channels run in directions generally parallel to the axes of the bores.

In this form it is preferred that the cylinder and outer shell are joined together by welding or brazing.

In another form of the invention, the neck cooler comprises a single casting, forging or similar which includes voids.

Preferably the inlet and outlet coolant ports are formed in the material of the assembly from diametrically opposed points on the periphery of the assembly. The ports may be formed in the neck cooler outer shell or drilled into the crankcase.

In one preferred from, the neck cooler comprises a single casting, forging or similar which includes a void, and in which the void id tapered and widens downwardly, and the outer shell of the void comprises the crankcase bore.

The invention includes a Stirling engine incorporating a neck cooler assembly according to any one of the preceding paragraphs.

Brief Description of the Drawings

A specific embodiment of the invention, and two alternative variants thereof, will now be described by way of example with reference to the accompanying drawings, in which: FIG. 1 shows a simplified Stirling engine gas process.

FIG. 2 is a cross section of a Stirling engine.

FIG. 3 is a cross section of a neck cooler assembly with engine components.

FIG. 4 is a cross section of the neck cooler assembly, showing part sections taken at right angles to each other radially away from the main axis of the assembly. FIG. 5 is a cross section of the neck cooler cylinder. FIG. 6 is a cross section of the neck cooler outer shell. FIG. 7 is a perspective view of the neck cooler outer shell. FIG. 8 is a cross section of an alternative neck cooler solution. FIG. 9 is a cross section of a second alternative neck cooler solution.

Detailed Description

FIG. 2 is a cross section of a hermetically sealed β-type Stirling engine 1. A non- lubricated hermetically sealed β-type (or commonly called displacer type) Stirling engine has a power piston and displacer piston coaxially disposed within the same cylinder. In addition, all rotating components including generator rotor and the generator stator are placed within a pressurised container; in this case covered by a crankcase 2, generator housing 3, bottom lid 5, flywheel cover 6 and hot end (comprising cooler, regenerator, heater head and burner). For clarity reasons the gas cooler, regenerator, heater head, burner and its accessories are not shown. This version of engine is a very compact type of design, especially compared to e.g. the V-type version or other α- or γ-type Stirling engine.

A hermetically sealed β-type Stirling engine has a helium gas reservoir that is contained within the crankcase 2. This results in the crankcase volume being pressurised to the mean cycle pressure.

During its operation the working gas (in this instance helium; however hydrogen and nitrogen are also commonly known working gases) will become quite hot. It is not unusual to see helium gas temperatures within the Stirling engine crankcase in the region of 100⁰C. Therefore there is a need to find means to cool the working gas. For this reason it is common to install cooling shrouds on the outer surfaces of the crankcase. In this case a cooling shroud 3.1 is placed around the generator housing 3 and another cooling shroud 6.1 (not shown in detail), is placed on the surface of the flywheel cover 6. By doing so, it has been observed that the thermodynamic stability of the Stirling engine is improved. Cooling of the generator housing can be critical if the temperature of the stator windings gets too high. This can be detrimental to the generator performance and also, in the long run, can cause breakdown (embrittlement) within the insulation of the windings.

In addition, the temperature of the power crosshead and power piston cylinder wall within a hermetically sealed β-type Stirling engine can get quite hot. The thermal loading on these critical components can be detrimental to the performance of the Stirling engine process. In order to reduce the thermal loading, a neck cooler assembly 8 is mounted around and oscillating assembly 7 comprising a power crosshead 7.2 and a power piston 7.1. This neck cooler assembly 8 cools the oscillating components, and in addition it retains them in position. FIG. 3 is a cross section of the neck cooler assembly 8 with oscillating Stirling engine components. The power piston 7.1 that oscillates within the power cylinder is treaded to the power crosshead 7.2. The power crosshead 7.2 oscillates and is guided within the neck cooler assembly. The engines operating speed can be in the magnitude of 1200 - 3600 rpm. During operation, friction will be present and heat is generated. The best method of transferring and rejecting heat is by means of a cooling jacket that surrounds the components in question.

As seen in FIG. 3 a cooling jacket or void V can be seen as part of a neck cooler assembly 8. The coolant entrance 11 and coolant exit 12 are depicted in this view. Most of the coolant will flow circumferentially around within the void (V) but some of the coolant will also be able to circulate in a downwardly axial direction and accept heat from the lower part of the power crosshead. The main functions of the neck cooler assembly 8 are threefold:

1. To control the temperature of the Power Crosshead and also (partly) the temperature of the power piston.

2. To contain the power piston and constitute the power piston cylinder.

3. To contain the Power Crosshead Displacer and constitute the power crosshead bore.

FIG. 4 is a cross section of the neck cooler assembly 8. As partly shown in FIG. 2, the neck cooler assembly 8 is fixed between the top of the Stirling engine crankcase flange (or neck) and the bottom of the Stirling engine cooler flange (not shown). A flange F constitutes part of the neck cooler assembly. Flange F is fastened to the

Stirling engine crankcase 2 is by means of bolts (not shown). Fig 4 shows two cross sections at right angles to each other. As shown below the sections, the Left Hand Side is taken in a plane out of the page, and the Right Hand Side is in the plane of the page.

The neck cooler assembly 8 consists of two components, neck cooler cylinder 8.3 and neck cooler outer shell 8.4. In this case these two components are joined together by welding W. In other cases the joining process could be done by brazing.

The power crosshead bore is depicted as Dl₅ and power piston cylinder bore as D2.

FIG. 5 is a cross section of the neck cooler cylinder 8.3. This figure shows the neck cooler cylinder 8.3 after rough machining. The inner bore of the neck cooler cylinder is slightly smaller than the target diameter of the power piston bore D2. This bore will be machined to its correct diameter after the neck cooler cylinder 8.3 is fastened to the neck cooler outer shell 8.4. FIG. 6 is a cross section of the neck cooler outer shell 8.4. This figure shows the neck cooler outer shell 8.4 after rough machining and tapping of the inlet and outlet ports 8.5, 8.6. The outer shell 8.4 is manufactured with two internal channels P that are formed (cast, milled, grinded or spark eroded) adjacent to ports 8.5,8.6 on opposite sides of the neck cooler outer shells internal diameter D4' spaced at an angle of approximately 180°. These pockets help direct the cooling water in an axially downward direction within the void V instead of just flowing circumferentially within the void V. This geometry can be observed from FIG. 4.

The neck cooler outer shell 8.4 has two inner diameters that are important for the assembly and fabrication of the neck cooler assembly. Both inner diameters d4 and D4 correspond to the outer diameters d3 and D3. Inner diameters d4, D4 are slightly larger than outer diameter d3, D3 in order for the parts to slip concentrically into each other during assembly for subsequent welding.

FIG. 7 is a perspective view of the neck cooler outer shell 8.4. This view depicts a better picture showing the placement and geometry of one of the channels P. In addition, the inlet and outlet ports 8.5, 8.6 can be clearly seen. The inlet and outlet ports 8.5, 8.6 are in direct communication with the machined channels P.

The inner diameter D4' is at its largest measured between the channels (P). If the inner diameter D4' is measured 90 degrees with respect to the channels (P), the inner diameter D4' is smaller. This means that the channels will let coolant flow axially downwards as well as circumferentially within the void.

Both neck cooler cylinder 8.3 and outer shell 8.4 can be made from a solid bar, by machining or forging, or by casting. Another embodiment of the neck cooler assembly 8 could comprise just one casting including the void V.

FIG. 8 is a cross section of an alternative neck cooler solution. This concept simplifies the design of the neck cooler assembly 8. This solution only requires one component instead of two as shown in the previous figures. The inlet and outlet ports 8.5, 8.6 are drilled in the flange F in the same manner as previously. However, the void V is located between the neck cooler's outer wall and the Stirling engine's neck bore (which corresponds to diameter D5). In order to seal off any possible leakages, an O-Ring 8.9 is placed in a groove between the bottom flange F of neck cooler assembly 8 and the top flange of the Stirling engine crankcase neck. In addition, another O-Ring 8.9 is placed in a groove in the bottom part of the neck cooler F' and the crankcase 2. This is to hinder the Stirling engine's working gas (helium or nitrogen) from entering into the cooling system, which would result in catastrophic consequences, such as loss of working gas and pressurisation of cooling system. The design in FIG. 8 compared to the solution in FIG. 3 will increase coolant flow and allow better control of the power crosshead and piston wall temperature. This is due to the fact that there is no need for an extra outer wall on the neck cooler.

An alternate porting solution for the coolant inlet and outlet is to drill and tap ports AI, AO in the top part of the Stirling engine crankcase 2. This would remove the ports 8.5, 8.6 and reduce the flange thickness of the neck cooler assembly 8. In addition, this would reduce fabrication costs on the neck cooler assembly since two operations (drilling and tapping) are removed. The extra drilling and tapping that would be imposed on the neck portion of the Stirling engine crankcase 2 would not cost very much since the crankcase has several features that in any case require drilling and tapping. The neck cooler assembly in FIG. 8 can be fabricated from solid bar by machining or forging, or formed by casting.

FIG. 9 is a cross section of a second alternative neck cooler solution 8. Again, this concept simplifies the design of the neck cooler assembly 8. This solution only requires one component instead of two as shown in the previous figures (3 through 7). The inlet and outlet ports 8.5, 8.6 are drilled in the flange F in the same manner as previously. However, the void V is located between the neck cooler's outer wall and the Stirling engine's neck bore (which corresponds to diameter D5 or per definition is the neck coolers "outer shell"). In this case the void V is tapered. It is tapered in a way that it widens downwardly in order to promote coolant fluid circulation. The taper T is formed either in a lathe or in a milling machine. In order to seal off any possible leakages, an O-Ring 8.9 is placed in a groove between the bottom flange F of neck cooler assembly 8 and the top flange of the Stirling engine crankcase neck. In addition, another O-Ring 8.9 is placed in a groove in the bottom part of the neck cooler F and the crankcase 2. This is to hinder the Stirling engine's working gas (helium or nitrogen) from entering into the cooling system, which would result in catastrophic consequences, such as loss of working gas and pressurisation of cooling system.

The design in FIG. 9 compared to the embodiments in FIG. 3 and FIG. 8 will increase coolant flow and allow better control of the power crosshead and piston wall temperature. First of all this is due to the fact that there is no need for an extra outer wall on the neck cooler. By doing so, a larger void V (more volume) is possible. Secondly, the taper T is introduced in the coolant part of the neck cooler's outer surface that promotes coolant circulation.

The neck cooler assembly in FIG. 9 can be fabricated from solid bar by machining or forging, or formed by casting.

Claims

1. A neck cooler assembly (8) for a Stirling engine (1), which assembly includes a cylinder bore (D2) for the power piston (7.1) of the Stirling engine and another bore (Dl) capable of guiding and retaining the power piston crosshead (7.2) of the

Stirling engine, in which the assembly has a peripheral shroud surrounding a void (V) and there are inlet and outlet coolant ports (8.5, 8.6) arranged on or within an outer shell (8.4) of the neck cooler assembly to allow coolant to pass through the void in order to control the operating temperature of the power piston crosshead (7.2) and power piston (7.1), and the assembly includes a neck cooler cylinder (8.3), and the neck cooler outer shell (8.4) is placed concentrically around and joined to the neck cooler cylinder (8.3)

2. An assembly as claimed in claim 1, in which said ports are in connection with at least two channels (P) in the inner or outer boundaries of the void.

3. An assembly as claimed in claim 2, in which the channels run in directions generally parallel to the axes of the bores Dl and D2.

4. An assembly as claimed in and one of the preceding claims in which the cylinder and outer shell are joined together by welding or brazing.

5. An assembly according to any one of claims 1 to 3, in which the neck cooler (8) comprises a single casting, forging or similar, which includes voids (V).

6. An assembly as claimed in any one of the preceding claims in which the inlet and outlet coolant ports are formed in the material of the assembly from diametrically opposed points on the periphery of the assembly.

7. An assembly as claimed in claim 6, in which the ports (8.5, 8.6) are formed in the neck cooler outer shell (8.4).

8. An assembly as claimed in claim 6, in which the ports (A.I. and A.O) are drilled into the crankcase (2).

9. An assembly according to any one of the preceding claims, in which the neck cooler (8) comprises a single casting, forging or similar which includes a void (V), and in which the void is tapered (T) and widens downwardly, and the outer shell of the void comprises the crankcase bore (D5).

10. A Stirling engine incorporating a neck cooler assembly according to any one of the preceding claims.