WO2021259401A1

WO2021259401A1 - Stirling engine

Info

Publication number: WO2021259401A1
Application number: PCT/CZ2021/000027
Authority: WO
Inventors: Oto Musálek
Original assignee: MUCKA, Jiri; SIRY, Rene
Priority date: 2020-06-23
Filing date: 2021-06-18
Publication date: 2021-12-30
Also published as: SK1812022U1; SK9783Y1; CZ2020360A3; AT17981U1; PL131203U1; DE212021000406U1; CZ308724B6; UA154724U

Abstract

Cylinders (3, 4) of the engine comprise a piston (1, 2) with a recess (12) where a heat exchanger (10, 11) is situated. The heat exchanger (10) of the first cylinder (3) is connected to a heater (5), the heat exchanger (11) of the second cylinder (4) to a cooler (6). From both cylinders (3, 4), a two-way passage (9) and a one-way outlet (13) are conducted to a regenerator (8) situated in a connecting channel (7). Heat exchangers (10, 11) are attached to the forefronts of the cylinders (3, 4), and are provided with a through-opening (17) through which a pin (15) formed on the piston (1, 2) passes. When the piston (1, 2) is in the top dead centre, the pin (15) closes the two-way passage (9), and the body of the piston (1, 2) closes the oneway outlet (13). The heat exchangers (10, 11) are preferably lamellar provided with a fluid circuit. Pistons (1, 2) are parallel, preferably connected to a cam mechanism of the stroke-drop-delay type.

Description

Stirling engine

Field of the Invention

The invention relates to a new design solution of an a-type Stirling engine enabling to reach higher efficiency, especially for use in power engineering.

Background of the Invention

A typical a-type Stirling engine is a two-cylinder hot air engine. Its design is for example described in Wikipedia. Here, the Stirling engine is described as an external combustion engine working with cyclic compression and expansion in the work cycle. This engine has two working pistons in separated cylinders, each having its piston. One cylinder is hot, also known as an expansion cylinder, and this cylinder is connected to a heater located on the outside of the cylinder. The second cylinder, also known as a compression cylinder, is cold and this cylinder is connected to a cooler located on the outside of the cylinder. Using pistons, the working gas is compressed at a low temperature and expanded at a high temperature. The working gas is transferred from the hot cylinder to the cold one, and back via a regenerator that is connected between the cylinders. The connection between the cylinders is ensured via a connecting channel with a oneway passage into the working area of cylinders. The regenerator is a heat- exchanger that keeps the heat energy in the time between the expansion and the compression of the working gas. The regenerator may be formed by a filling located in the connecting channel between the cylinders, or optionally a suitable individual regenerator is engaged in a two-way connecting channel. The gas in the hot cylinder expands at high temperatures of the heater. In the cold cylinder, the gas is compressed at low temperatures of the cooler. The pistons are provided with a movement mechanism which is usually a crankshaft. The pistons are at an angle against each other, commonly 90°. It is a closed-cycle engine with conversion of heat energy to mechanical work.

An example of another design solution of an a-type Stirling engine is the engine according to CZ PV 2002-2455. Here, two pistons which are at an acute angle to each other are described, and these pistons are connected using a Hook joint set at an obtuse angle, more than 120 and less than 180°. Here, the piston cylinders are connected by a simple connecting piping. The regenerator and the cooler are located outside the connecting piping on a channel formed by a so-called expansion piping having the inlet and the outlet into the cold cylinder. The inlets and outlets to/from the connecting piping are effectuated by two-way passages. The heat exchanger consists of a heating chamber arranged as an end part of the cylinder above a cooled cold piston, and a connecting piping.

In all known Stirling engine types, the pistons have the form of a simple solid body in the shape corresponding to the shape of a piston rod in which the piston is situated, wherein the piston has a flat front-end and a flat bottom. In the terminology of the field, the Stirling engine piston rod is usually called a cylinder, although the piston rod sometimes has a different shape, e.g. a vessel having an oval cross-section or a regular polygon. The shape of the piston then corresponds to this, and its body may be in the shape of a cylinder, a prism, an octagon, etc.

The alpha-type Stirling engine is significant for its effectiveness. Currently, its importance is increased with the use of alternative and renewable energy sources. The problem with these devices is represented by the dead space of the pistons that decreases the reached efficiency.

The disadvantage of currently known alpha-type Stirling engines is that they do not enable to reach an ideal cycle with respect to efficiency. Above the pistons, a temporary dead space causing the decreased compression ratio of pistons is formed. Some types of regenerators with a relatively low heat-exchanging surface are used. The working gas path is relatively long. The heat-exchanging surfaces where working gas heating or cooling is effectuated are relatively small. Another disadvantage is represented by the fact that the heat source cannot be removed from the engine. The engine may be connected to a single heat source only. No regulation using cyclic closing of the expansion and compression area, or a oneway flow of the working gas is possible. Summary of the Invention

The above stated disadvantages are substantially eliminated by the invention. The invention substantially improves and structurally modifies the alpha-type Stirling engine comprising two movable pistons, each in one cylinder, a regenerator, a heater as a source of thermal energy outside the cylinder, and a cooler as an offtake point of thermal energy outside the cylinder. The heater and the cooler are located outside the cylinders, and the regenerator is located in a connecting channel which connects the working area of the first cylinder and the working area of the second cylinder, wherein the connecting channel is connected to the working area of cylinders by two-way passages. The summary of the new solution is that both pistons are provided with at least one recess, wherein the first heat exchanger is connected to the heater and the second heat exchanger is connected to the cooler, where these heat exchangers, or their heat-exchanging surfaces, are located in the working area of cylinders, and they correspond with their shape, dimensions and position to the recess so that they at least partially fit into the piston recess. The first heat exchanger fits into the piston recess in the first cylinder, and the second heat exchanger fits into the piston recess in the second cylinder.

A one-way outlet is preferably led out from the working area of each cylinder into the regenerator. The one-way outlets are led out from the working area of cylinders always at a place other than the place where the two-way passage runs into the working area.

The one-way outlet is preferably provided with a one-way shutoff in the point of exit from the cylinder.

Preferably, the piston body comprises a pin which closes the two-way passage from the cylinder of that piston to the connecting channel, in the piston position where the piston is situated in the top dead centre. The two-way passage is then situated above the pin. Pin dimensions, shape and position are selected to enable the pin to tightly fit to the two-way passage when the piston is in its top dead centre, and to close the two-way passage.

The pin preferably protrudes from the recess bottom. The first and the second heat exchangers are preferably provided with an opening enabling the movement of the pin and the medium. Via this opening, the pin runs through the heat exchanger during its movement, wherein it goes over the whole heat exchanger at least in the top dead centre of the piston.

The heat exchangers preferably tightly fit with their upper part to the cylinder front end, namely at a place other than the place where the two-way passage runs into the working area. Shapes, dimensions and location of elements correspond to each other so that the whole heat exchangers are situated in the piston recess when in their top dead centre, and they fit into it only with a gap necessary for the medium flow. The gap size may be calculated and determined as the size necessary for the working gas flow.

The one-way outlets from cylinders are then preferably situated above the piston, in a place where the one-way outlet is closed by the pin in the top dead centre.

The lamellar heat exchangers are preferably selected for the cylinders. In that case, a lamellar heat exchanger with a fluid circuit is located in both cylinders.

The pistons are preferably in the position where they have parallel axes, and they are provided with a cam mechanism of a stroke-drop-delay type in this position.

The cam-mechanism cams of the stroke-drop-delay type are preferably mechanically connected for a revolving motion in the same direction and with the same number of revolutions. This is for examples enabled by selection of a mechanism with disc cams on the shaft, or the mechanism of drum cams mechanically connected via a gearing.

The proposed new solution of the alpha-type Stirling engine has increased output in comparison with the prior art, and is applicable especially for electricity generators. It enables to efficiently use the waste heat from combustion exhaust gases. It is also suitable for solar heat sources. When using the solar heat energy, up to 40% of heat can be converted to electric energy. The proposed Stirling engine can be used for production of electricity in co-generation units, in buildings, for example in municipal buildings, households or business premises. It can also enable use of waste heat from industry processes, e.g. bakeries, roasting facilities, etc., which can be utilized for production of electricity using this engine. The main advantage of this engine is its high engine efficiency, higher than with the prior art. An almost perfect Stirling cycle is achieved. The presented solution achieves a reduction of the dead space in cylinders and an increase of their compression ratio. By selecting the regenerator forming the filling of the connecting piping, a large heat-exchanging surface of the engine regeneration element is formed. The minimum path of the working gas with a large heatexchanging surface of heat exchangers is achieved. The heat source may be distant from the engine. Another advantage is that the presented engine may be connected to more heat sources at one time. The presented engine enables cyclic closing of the expansion and compression area, and a one-way flow of the working gas.

Summary of the Drawings

The invention will be more clear with the aid of the drawings depicting in Fig. 1, the scheme of the exemplary Stirling engine according to the invention, looking at a vertical section through the pistons and the connecting channel. in Fig. 2, a top cross-section view through one cylinder with a piston and a heat exchanger, in the place designated as A-A in Fig. 1 in Figs. 3, A, B, C and Figs. 4 D, E, different work stages of the presented engine according to the position and movement direction of pistons, with indication of medium and cam movement direction in Fig. 5, a heat exchanger detail indicated in previous figures only in block for the sake of simplicity, in a front perspective view of a vertical section taken through the centre of the heat exchanger. in Fig. 6, a front perspective oblique-bottom view of the whole heat exchanger from Fig. 5 Example of Invention Embodiments

The example of the best embodiment is represented by the Stirling engine depicted in Figs. 1 to 6.

It is a Stirling engine of the basic alpha type that is structurally modified according to the invention. It comprises two pistons 1, 2. each movable in one cylinder 3, 4. a heater 5 constituting the heat energy source, and a cooler 6 as an offtake point of the thermal energy. The cylinders 3, 4 have the working area above the pistons 1_^2, wherein the working area of the first expansion cylinder 3 is connected with the working area of the second compression cylinder 4 by a connecting channel 7. In the connecting channel 7, a regenerator 8 formed by the accumulation material that is able to absorb and accumulate heat from the flowing medium is fitted. The accumulation materials are known, e.g. glass balls or crushed material based on porous ceramic particles or a system of sheet metal slats, may be used. Gas is used as the working medium. The heater 5 and the cooler 6 are situated outside the cylinders 3, 4. The connecting channel 7 is connected to the working area of the cylinders 3. 4 via two-way passages 9. One heat exchanger 10 is connected to the heater 5, and another heat exchanger H is connected to the cooler 6. Both pistons 1, 2 are provided with a recess 12. The first heat exchanger 10 is located in the working area of the first expansion cylinder 3; the second heat exchanger 11 is located in the working area of the second compression cylinder 4. In both cylinders, the heat exchanger 10, 11 is located in the recess 12. In this exemplary embodiment of an ideal engine, such a specific shape of the elements is selected to correspond to the established nomenclature in the given field for the sake of clarity and ease of explanation. Therefore, the cylinders 3, 4 have the cylindrical shape, and thus, have the form of a hollow body with a circular cross-section. Adequately to that, the exemplary pistons 1, 2 have the form of solid bodies with the circumferential wall also in the circular shape. In practice, all shapes used and referred to as engine cylinders in the art, i.e. bodies having elliptical cross-section or polygons, etc. may be used, and thus understood as cylinders 3_^.4 within the meaning of the invention.

From the working area of each cylinder 3, 4. a one-way outlet 13 to the regenerator 8 is located in a place other than the place where the two-way passage 9 is located. These one-way outlets 13 are in the place of exit from cylinders 3,_4 provided with a one-way shutoff 14, e.g. a simple closing valve.

The piston 1, 2 body comprises a pin 15 enabling to temporarily close the two- way passage 9 to the connecting channel 7. This option is achieved by the shape and the mutually convenient location of the pin 15 and the two-way passage 9. The two-way passage 9 is situated above the pin 15, and dimensions of these elements are selected so that the pin 15 in the top dead centre of the piston 1, 2 tightly fits to the two-way passage 9 of the respective piston 1, 2. and the stated two-way passage 9 is closed by the pin 15.

In the presented ideal embodiment, the pistons 1, 2 have a circumferential wall in the shape of a cylindrical shell, and the pin 15 has the shape of a cylinder with a bevelled upper edge. The recess 12 also has the circumferential wall in the shape of a cylindrical shell, and the pin 15 protrudes from the bottom 16 of the recess 12; thus, the recess 12 forms a cavity in the shape of an annulus between the pin 15 and the cylindrical wall of the recess 12 in piston 1, 2 bodies. The shape and the type of both heat exchangers 10, 11 are adapted to the presence of the pin 15 in the recess 12. Lamellar heat exchangers 10, 11 with heat-exchanging surfaces of a plurality of lamellas are used, and these lamellar heat exchangers 10, 11 are provided with an opening 17 forming the pass-through tunnel for movement of the piston 15. The opening 17 in each of the heat exchangers 10, 11, the central together with the recess 12 in this particular example, enables more efficient pushing-through of the working medium around the heat-exchanging surfaces of heat exchangers 10, 11 during the movement of pistons 1, 2. The pin 15 slides in and out into/from this opening 17 during the movement of pistons 1, 2.

During the movement of the piston 1, 2 upwards, the pin 15 moves gradually from the bottom to the top and pushes the medium around the respective heat exchanger 10, 11, up to the two-way passage 9 and the one-way outlet 13. During the movement of the piston 1, 2 downwards, the medium flows into the cylinder ^ 4 only through the two-way passage 9, and flows down around the heatexchanging surfaces of the respective heat exchanger 10, 11 in the cylinder 3, 4. Depending on which cylinder 3, 4 is concerned, the working medium is either heated and expanded, or cooled with reduction of the working medium volume. As the working medium, hydrogen, helium, nitrogen or air are particularly suitable. In the position, where the piston 1, 2 is in the top dead centre, its pin 15 goes through the respective heat exchanger 10, 11 in the total height dimension of the heat exchanger 10, 11. and concurrently, the pin 15 closes the two-way passage 9. It is not a prerequisite for the pin 1J5 to be in the centre of the recess 12 or the cylinder 3, 4. or to be as high as the edge of the piston 1, 2; it depends on the selection of shapes and location of the engine elements, e.g. it may also be higher as seen in the figures in this exemplary embodiment.

In the optimal patent execution, as presented in the exemplary embodiment in the figures, the heat exchangers 10, 11 in the working area of the cylinders 3, 4 are located so that they tightly fit with their upper part to the forefront of the cylinders 3, 4. and at the top dead centre position of the pistons 1, 2, the heat exchangers 10, 11 fully fit into the recess 12 in the piston 1, 2 with the circumferential gap 18 necessary for the working medium flow during the movement of pistons 1, 2. In the presented optimal patent execution, the one-way outlets 1_3 from the cylinders 3, 4 to the regenerator 8 are situated above the piston 1, 2. and the pistons 1, 2 have such dimensions and shape that the oneway outlet 13 at this piston 1, 2 is closed in the top dead centre position of the respective piston 1, 2. Thus, in the upper position of the first piston 1 in the expansion cylinder 3, the one-way outlet 13 from the expansion cylinder 3 is closed, and in the upper position of the second piston 2 in the compression cylinder 4, the one-way outlet 13 from the compression cylinder 4 is closed. In the presented example, the front-ends of the cylinders 3, 4 and upper sides of pistons 1, 2 are flat, which is however not necessary, they can be for example convex, concave or having different shapes protuberant, but corresponding in shape to each other.

For the engine according to the presented invention, it is optimal to select lamellar heat exchangers 10, 11 having a fluid circuit structurally adjusted to the shape and the dimensions of the recess 12 in pistons 1, 2 and in the shape and position of pins 15 for the cylinders ^4, as shown in Fig. 5 and Fig. 6.

The invention further solves the provision of optimal movement of pistons 1, 2. The pistons 1, 2 according to the invention have parallel axes contrary to the standard alpha-type Stirling engine. In order to achieve the required movement, they are preferably provided with a cam mechanism of the stroke-drop-delay type. The cams 19, 20 of the cam mechanism are mechanically connected for a rotational movement in the same direction and with the same number of revolutions, e.g. by coupling on a shaft or as in the previous example, using two shafts 21, 22 and a belt 23. The pistons 1, 2 are connected to the cam mechanism e.g. using pulleys 24, 25.

The fluid circulation in the fluid circuit of heat exchangers 10, 11 is provided in a standard manner, using a pump 26.

A specific example of engine operation is illustrated in individual stages of the engine in Fig. 3 and Fig. 4 and is as follows.

Fig. 3 A illustrates the engine condition in extreme positions of the pistons 1, 2. The first hot piston is above in the top dead centre position; the second cold piston 2 is at the bottom. The cam mechanism, in particular the cam 20 in Fig. 3 A on the right, provides a short remaining of the pistons 1, 2 in this position. In the position, where the piston 1_ is in the top dead centre, its pin 15 goes through the respective heat exchanger 10 in the total height dimension of the first heat exchanger 10, and closes the two-way passage 9 from the expansion cylinder 3. The two-way passage 9 from the compression cylinder 4 situated above the second piston 2 is open. The one-way outlet 13 from the expansion cylinder 3 to the regenerator 8 has closed the upper edge of the first piston 1 The one-way shutoff 14 closes the one-way outlet 13 from the compression cylinder 4. The working medium flow stops.

In Fig. 3 B, the first hot piston 1 remains in the top dead centre and closes the connecting channel 7 on the side of the expansion cylinder 3, while the second piston 2 moves upwards. During this movement, the second piston 2 compresses the cooled working medium contained in the compression cylinder 4 and pushes it through the two-way passage 9 and the one-way outlet 13 to the connecting channel 7 and here to the regenerator 8. When the second piston 2 reaches the top dead centre, its pin 5 closes the two-way passage 9 from the compression cylinder 4 and the edge of the second piston 2 closes the one-way outlet 13 from the compression cylinder 3. The engine changes over to the next stage according to Fig. 3 C. In the next stage according to Fig. 3 C, the piston 1 moves downwards from the top dead centre to the bottom dead centre. The second piston 2 remains up in the top dead centre position and concurrently closes the connecting channel 7 on the side of the compression cylinder 4. Due to the compression pressure and suction resulting from the movement of the first piston 1, the compressed working medium is pushed from the connecting channel 7, and it flows to the expansion cylinder 3 only through the two-way passage 9. The one-way outlet 13 from the expansion cylinder 3 is closed using the one-way shutoff 14. e.g. in the form of a return valve. The working medium in the expansion cylinder 3 flows to the opening 17 in the first heat exchanger 10 and around its heat-exchanging surfaces, thus, among its lamellas and through the gap 18. The working medium is heated by the heatexchanging surfaces of the first exchanger 10; it expands, gradually fills the working area of the expansion cylinder 3, and compresses the first piston down up to its lower limit position. The engine changes over to the stage illustrated in Fig. 4 D.

Fig. 4 D shows the engine condition at limit positions of pistons 1, 2. where the first hot piston 1 is down, and the second cold piston 2 is in the top dead centre. The cam mechanism, in particular the cam 9 in Fig. 4 D on the left, provides a short remaining of the pistons 1, 2 in this position. In the position, where the piston 2 is in the top dead centre, its pin 15 goes through the second heat exchanger H in the total height dimension of the second heat exchanger H, and closes the two- way passage 9 from the compression cylinder 4. The two-way passage 9 from the expansion cylinder 3 situated over the first piston 1 is open. The one-way outlet T3 from the compression cylinder 4 to the regenerator 8 has closed the upper edge of the second piston 2. The one-way shutoff 14 closes the one-way outlet 13 from the expansion cylinder 4. The working medium flow stops.

Then, the first piston 1 moves upwards, and concurrently, the second piston 2 moves downwards, as illustrated in Fig. 4 E. In the expansion cylinder 3, the pin 15 moves gradually through the opening 17 from the bottom upwards during the movement of the first pin 1, and pushes the working medium up. The heated medium flows through the first heat exchanger 10 to the expansion cylinder 3 through the one-way outlet 13 and the two-way passage 9 to the connecting channel 7, and in it, to the regenerator 8. This is followed by stage according to the Fig. 3 A. The above stated stages 3A to 4E repeat in the given sequence.

In the expansion cylinder 3, the working medium is heated by means of the first heat exchanger 10 using the heat brought from an external source, here designated as the heater 5, e.g. in the form of a combustion engine, furnace or a solar heat source. The heat is accumulated in the regenerator 8 through which the working medium flows alternately from one and the other side. In the compression cylinder 4, the heat is transferred from the heat-transfer medium to the second heat exchanger 1 ., and from here, the heat is conducted to the external takeoff via the cooler 6. The maximum heat inlet for the external consumption occurs when the second piston 2 moves down.

15

List of Reference Marks

1 first piston

2 second piston

3 expansion cylinder

4 compression cylinder

5 heater

6 cooler

7 connecting channel

8 regenerator

9 two-way passage

10 first heat exchanger

11 second heat exchanger

12 recess

13 one-way outlet

14 one-way shutoff

15 pin

16 bottom

17 opening

18 gap

19, 20 cam 21, 22 shaft 23 belt 24, 25 pulley 26 pump

Claims

12

1. A Stirling engine comprising two pistons (1, 2) each movable in one cylinder (3, 4), a regenerator (8), a heater (5) as the source of thermal energy, and a cooler (6) as the takeoff point for the thermal energy, where the heater (5) and the cooler (6) are situated outside the cylinders (3, 4), and the regenerator (8) is situated in a connecting channel (7), which connects the working area of the first cylinder (3) with the working area of the second cylinder (4), wherein the connecting channel

(7) is connected to the working area of the cylinders (3, 4) by means of two-way passages (9), characterized in that both pistons (1, 2) are each provided with at least one recess (12), and, in the working area of each cylinder (3, 4), at least one heat exchanger (10, 11) located at least partially in this recess (12) at least in some of the working positions of the piston (1, 2), wherein the heat exchanger (10) situated in the recess (12) of the first piston (1) is connected to the heater (5), and the heat exchanger (11) situated in the recess (12) of the second piston (2) is connected to the cooler (6).

2. The Stirling engine according to claim 1 characterized in that a one-way outlet (13) is conducted from the working area of each cylinder (3, 4) to the regenerator

(8), in a place other than the place where the two-way passage (9) is located.

3. The Stirling engine according to claim 2 characterized in that each one-way outlet (13) from the cylinder (3, 4) is provided with a one-way shutoff (14) in the point of exit from the cylinder (3, 4).

4. The Stirling engine according to claims 2 and 3 characterized in that the body of pistons (1, 2) comprises a pin (15), wherein the two-way passage (9) from the respective cylinder (3, 4) to the connecting passage (7) is located above this pin (15), and wherein, the pin (15) tightly fits to the two-way passage (9) in the position of the top dead centre of the piston (1 , 2) so that the two-way passage (9) is closed by the pin (15) in the indicated position of the piston (1, 2). 13

5. The Stirling engine according to claim 4 characterized in that the pin (15) protrudes from the bottom (16) of the recess (12).

6. The Stirling engine according to claim 5 characterized in that the heat exchangers (10, 11) of each cylinder (3, 4) are provided with an opening (17) for the movement of the pin (15), and the pin (15) is located in this opening (17) in the position of the piston (1 , 2) in the top dead centre, and goes through the heat exchanger (10, 11) in the height direction.

7. The Stirling engine according to claim 6 characterized in that the heat exchangers (10, 11) with their upper part tightly fit to the forefront of cylinders (3, 4), and the heat exchangers (10 ,11) completely fit into the recess (12) of the piston (1, 2) in the top dead centre of pistons (1, 2), and a circumferential gap (18) is situated all around to enable the working medium flow, wherein the one-way outlets (13) to the regenerator (8) are located above the pistons (1, 2), and the one-way outlet (13) of the piston (1, 2) in the top dead centre of the piston (1, 2) is closed by the piston (1, 2) in this position.

8. The Stirling engine according to claims 1 to 7 characterized in that the heat exchanger (10, 11) of a lamellar type with a fluid circuit is located in both cylinders (3, 4).

9. The Stirling engine according to claims 1 to 8 characterized in that pistons (1, 2) have parallel axes, and are provided with a cam mechanism of the stroke-drop- delay type.

10. The Stirling engine according to claim 9 characterized in that the cams (19, 20) of the cam mechanism are mechanically connected for a rotational movement in the same direction and with the same number of revolutions.