WO2018062627A1

WO2018062627A1 - Stirling engine using supercritical fluid

Info

Publication number: WO2018062627A1
Application number: PCT/KR2016/014222
Authority: WO
Inventors: 이정익; 손성민
Original assignee: 한국과학기술원
Priority date: 2016-09-29
Filing date: 2016-12-06
Publication date: 2018-04-05
Also published as: KR20180035571A; KR101899466B1

Abstract

A stirling engine using supercritical fluid is designed such that working fluid operates in an environment near a critical point during a compression stroke by applying supercritical carbon dioxide to the working fluid, and thus a high expansion ratio near the critical point, a high compression efficiency, and a low compression load can be used for the engine.

Description

Stirling engine with supercritical fluid

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Stirling engine. In particular, the superfluous carbon is applied to the working fluid so that the working fluid is designed to operate in an environment near the critical point during the compression stroke. It relates to a Stirling engine using a supercritical fluid that can be utilized in the engine.

The Stirling engine is a heat engine that converts heat energy into kinetic energy by compressing and expanding a gas in a closed space at each of the high and low temperatures.

The Stirling engine was not attracted by the rapid development of steam engines and internal combustion engines at the time of development, but recently, the development of heat-resistant materials and sealing technologies, energy and environmental issues have emerged as important issues around the world.

Since the Stirling engine is an external combustion engine, it is free to select a heat source, and there is no explosion stroke, which makes it less noise than internal combustion engines such as diesel engines and gasoline engines. It has excellent properties to produce work and electricity from its heat source.

However, Stirling engines are not widely used in industry because they use low power-to-size ratios and gas expansion, making them difficult to increase power.

In order to solve this problem, the present invention is designed to operate the working fluid in the environment near the critical point during the compression stroke by applying supercritical carbon dioxide to the working fluid, so that the large expansion ratio, high compression efficiency, low compression work near the critical point It is an object of the present invention to provide a Stirling engine using a supercritical fluid that can be utilized in an engine.

Stirling engine using a supercritical fluid according to a feature of the present invention for achieving the above object,

Reciprocating a piston to compress and expand the working fluid, and including at least one cylinder having a high temperature portion for heating the working fluid and a low temperature portion for cooling the working fluid, and filling the working fluid with a supercritical fluid. In addition, the supercritical fluid is characterized in that it operates in the vicinity of the critical point that is the point where the discontinuous change in the physical properties of the supercritical fluid is observed during the compression stroke.

The Stirling engine of the present invention performs a Stirling cycle in a pressure range of at least 7 to 8 MPa, at a maximum of 20 to 25 MPa, and at a temperature range of at least 25 to 35 degrees and at a maximum of 600 to 700 degrees when the supercritical fluid is operated near a critical point. Characterized in that.

The Stirling engine of the present invention is characterized in that when the supercritical fluid is operated near the critical point, the low temperature section performs a Stirling cycle operating in a temperature range of 25 to 35 degrees and a pressure range of 7 to 8 MPa.

By the above-described configuration, the present invention has the effect of having higher efficiency and output in the same volume by applying supercritical carbon dioxide designed to operate near the critical point to the Stirling engine as a working fluid.

According to the present invention, the supercritical carbon dioxide, which is a working fluid, operates in an environment near a critical point in a compression stroke, thereby making it possible to utilize a large expansion ratio, a high compression efficiency, and a low compression work near a critical point in a Stirling engine.

1 is a view showing the configuration of a Stirling engine according to an embodiment of the present invention.

2 is a view showing a Stirling cycle process of the Stirling engine according to an embodiment of the present invention.

3 is a graph showing the density of supercritical carbon dioxide according to temperature and pressure according to an embodiment of the present invention.

4 is a graph showing the temperature and compression coefficient of the supercritical carbon dioxide according to the embodiment of the present invention.

5 is a graph showing the temperature and compression coefficient of helium according to the conventional embodiment.

6 is a view showing the pressure and volume of a Stirling engine using a supercritical carbon dioxide as a working fluid and a conventional Stirling engine using helium as a working fluid according to an embodiment of the present invention.

7 is a view showing the work of compression and expansion of the Stirling engine using supercritical carbon dioxide according to an embodiment of the present invention.

8 is a view showing the work to the compression and expansion of the Stirling engine using helium according to the prior art.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless specifically stated otherwise.

Stirling engine is an external combustion engine that seals the working fluid in a space consisting of a cylinder and a piston, and mechanically moves the piston up and down to obtain mechanical energy by heating and cooling the working fluid from the outside through heat exchange.

The Stirling engine 100 according to the exemplary embodiment of the present invention reciprocates in the first cylinder 111 and the second inner space 122 having the first piston 113 reciprocating in the first inner space 112. The second cylinder 121 having the second piston 123 and the first cylinder 111 or the second cylinder 121 are filled with the working fluid 101 to be sealed.

Here, the first internal space 112 of the first cylinder 111 is a space where the working fluid 101 is heated, and is called a high temperature unit 110, and the second internal space 122 of the second cylinder 121 is operated. A space in which the fluid 101 is cooled is divided into a low temperature part 120.

The high temperature unit 110 is the working fluid 101 is heated by the heating in the first internal space 112 of the first cylinder 111 is filled with the high temperature working fluid 101, the low temperature unit 120 is the second cylinder In the second internal space 122 of 121, the working fluid 101 is cooled by cooling to fill the low temperature working fluid 101.

The first piston 113 is connected to the flywheel 102 by the first crankshaft 114, and the second piston 123 is connected to the flywheel 102 by the second crankshaft 124.

The outer circumferential surface of the first cylinder 111 forming the first inner space 112 is provided with an external heat source 115 for heating the working fluid 101, and the second cylinder forming the second inner space 122 ( On the outer circumferential surface of 121, an external cooling source 125 for cooling the working fluid 101 is installed.

The working fluid 101 filled in the second cylinder 121 is delivered to the cooling medium in a liquid state stored in the external cooling source 125 and cooled.

The Stirling engine 100 according to the embodiment of the present invention constitutes a Stirling cycle with two power pistons, the first piston 113 and the second piston 123.

The Stirling engine 100 is referred to as the phase angle of the difference between the operation between the first piston 113 and the second piston 123, the Stirling cycle using the phase angle of the first piston 113 and the second piston 123 Configure the compression, expansion, and movement of the working fluid 101 required.

The first cylinder 111 and the second cylinder 121 communicate with each other by the flow path 103 to operate both spaces according to the volume change (compression and expansion) of the first cylinder 111 and the second cylinder 121. As the fluid 101 moves, it performs a Stirling cycle that repeats the state change.

The working fluid 101 repeatedly performs expansion and compression to enable high efficiency and high power in the expansion and compression process, and uses a supercritical fluid.

Supercritical fluid refers to a fluid at a point in time at which the liquid and gas cannot be distinguished due to reaching a state exceeding a certain limit of high temperature and high pressure. The molecular density is close to liquid, but the viscosity is low and close to gas.

At normal temperatures and pressures, gaseous and liquid materials also exceed the limits of certain high temperatures and pressures, called critical points, so that no evaporation occurs, resulting in a state in which gas and liquid are indistinguishable. A substance in a state is called a supercritical fluid.

Supercritical is a state in which the boundary between a gas and a liquid disappears at temperatures and pressures above the critical point, with a characteristic in between.

Supercritical fluids represent a state of matter at temperatures and pressures above the critical point, with gas diffusivity and liquid solubility.

Supercritical fluids include water, carbon dioxide, methane, ethane, propane, ethylene, propylene, methanol and the like.

Among the supercritical fluids having this property, the critical temperature is relatively close to room temperature, and carbon dioxide is used in the present invention as one embodiment of the supercritical fluid. The critical point of carbon dioxide is known as 7.37 MPa, 31.1 ° C.

Carbon dioxide becomes supercritical carbon dioxide at conditions above the critical temperature and the critical pressure. Supercritical carbon dioxide has high density and low viscosity. That is, supercritical carbon dioxide has a dense gas characteristic.

Supercritical carbon dioxide has high density and thermal conductivity like liquids, but expands like gas, occupies space, has low viscosity, and has properties such as density and heat capacity that change rapidly near the critical point.

In particular, supercritical carbon dioxide has the feature of compressing fluids at lower work and with higher efficiency than when compressing near conventional critical gases.

Hereinafter, the Stirling cycle of the Stirling engine of the present invention will be described in detail with reference to FIG. 2.

The present invention proposes a Stirling engine 100 in which supercritical carbon dioxide near a critical point is set as a working fluid 101 in a cooling process and a compression process in order to solve a low power-to-output ratio of the Stirling engine 100. do.

The Stirling engine 100 of the present invention has a pressure range of at least 7 to 8 MPa, a maximum of 20 to 25 MPa, and a minimum of 25 degrees Celsius for operating the Stirling engine 100 when the working fluid 101 is set to supercritical carbon dioxide near a critical point. Higher volume-to-volume power ratios and thermal efficiencies can be achieved in the temperature range of from 35 degrees to 35 degrees Celsius, up to 600 degrees Celsius.

The Stirling engine 100 of the present invention performs the Stirling cycle while repeating the sequence of the expansion zone, the heating zone, the cooling zone, and the compression zone.

As shown in (a) and (b) of FIG. 2, the first cylinder 111 is heated by an external heat source 115 so that the high temperature unit 110 expands and pushes up the first piston 113 ( Expansion zone).

Since the second piston 123 of the low temperature part 120 is coupled to the first piston 113, when the first piston 113 is pushed up, the second piston 123 is pushed up together to expand the low temperature part 120.

As shown in FIG. 2 (c), when the first piston 113 is compressed again, the high temperature part 110 has the heated working fluid 101 along the flow path 103 in the second of the second cylinder 121. It is moved to the interior space 122 (heating area).

Subsequently, as shown in FIG. 2D, the low temperature part 120 cools the working fluid 101 introduced into the second inner space 122 of the second cylinder 121 by the external cooling source 125. (Cooling zone).

As shown in FIG. 2A, as the cooling unit 120 cools, the second piston 123 compresses the working fluid 101, and the cooled working fluid 101 passes through the flow path 103. Is moved to the first internal space 112 of the first cylinder 111 along the (compression area).

When the second cylinder 121 is cooled by the external cooling source 125 and the filled working fluid 101 of the second internal space 122 is compressed, one cycle is completed.

The compressed working fluid 101 of the second cylinder 121 moves to the first inner space 112 of the first cylinder 111 through the flow path 103, and then heat is applied by the external heat source 115. The sterling cycle is in progress.

The Stirling engine 100 is connected to the flywheel 102 in which the high temperature part 110 and the low temperature part 120 rotate in a constant phase difference, and use the rotational force transmitted from the flywheel 102 to the first piston 113. The expansion and compression process is performed while repeatedly raising and lowering the second piston 123.

The Stirling engine 100 is designed to operate near the critical point in the compression stroke because it can take the greatest benefit near the critical point in the compression stroke. More details will be described with reference to FIGS. 6 and 7.

3 is a graph showing the density of supercritical carbon dioxide according to temperature and pressure according to an embodiment of the present invention, Figure 4 is a graph showing the temperature and compression coefficient of supercritical carbon dioxide according to an embodiment of the present invention, Figure 5 Is a graph showing the temperature and compression coefficient of helium according to the conventional embodiment.

In the vicinity of the critical point, the supercritical fluid shows nonlinear properties, and FIG. 3 shows the density with temperature and pressure near the critical point of the supercritical carbon dioxide (31.04 degrees Celsius, 7380 kPa).

Nonlinearly rapidly changing physical properties of the supercritical fluid include a compression factor.

Compression coefficient is an index showing how the actual gas has a different compression properties than the ideal gas is defined by the following equation (1).

Where p is pressure, V is volume, n is molar number, R is gas constant, and T is temperature.

The compression coefficient of the ideal gas is 1, and a large compression coefficient indicates that the intermolecular repulsive force of the actual gas is superior to the ideal gas, and a small compression coefficient indicates that the intermolecular attraction is superior.

Thus, the preponderance of repulsive force means greater work for compression, and the predominance of gravitational force means less work for compression.

In the present invention, the supercritical carbon dioxide is used as the working fluid 101 to increase the efficiency by applying a reduction in compression work that can be taken in a sudden change in physical properties near the critical point to the Stirling engine 100.

As shown in FIG. 4, the supercritical carbon dioxide has a small compression coefficient near the critical point, and thus, a small amount of work may be achieved in compression, thereby obtaining a large gain.

As shown in Figure 5, helium has a compression coefficient of 1 or more, it can be seen that the intermolecular repulsive force is superior to the ideal gas.

Since the working fluid 101 of the present invention intends to use the compression effect near the critical point, the critical point does not deviate significantly from room temperature and selects a fluid whose pressure is not high.

As shown in Table 1 below, supercritical carbon dioxide is selected among the selected working fluids 101, which is the cheapest fluid without toxicity or explosiveness and shows a sudden change in physical properties near the critical point.

6 is a view showing the pressure and volume of a Stirling engine using another supercritical carbon dioxide as a working fluid and a conventional Stirling engine using helium as a working fluid, according to an embodiment of the present invention, and FIG. FIG. 8 is a diagram showing the work of compression and expansion of the Stirling engine using supercritical carbon dioxide, and FIG. 8 is a view of the work of compression and expansion of the Stirling engine using helium according to the prior art.

As shown in FIG. 6, the Stirling engine 100 of the present invention includes a low temperature unit 120 operating at 32 ° C. near a critical point of supercritical carbon dioxide, and a high temperature unit 110 having a higher temperature than the low temperature unit 120. The minimum pressure of the engine within the piston stroke is designed to be at or above 7.37 MPa, the critical point of supercritical carbon dioxide.

Stirling engine 100 of the present invention is designed so that the cooling zone and the compression zone to operate in the environment near the critical point of the supercritical carbon dioxide can realize a large expansion ratio, high compression efficiency, low compression work near the critical point.

As shown in FIG. 6, the Stirling engine 100 of the present invention may have efficiency and output more than 1.5 times higher in the same volume than a Stirling engine using a high pressure helium as a working fluid.

For example, a phase diagram of 90 degrees, a volume of 160 ml of the low temperature part 120, a volume of 80 ml of the high temperature part 110, a low temperature part 120 at 32 ° C., and the high temperature part 110 at 650 ° C. at a maximum pressure of 23 MPa can be designed. When a high-pressure helium sterling engine achieves an efficiency of 25.0% at an average of 150.9 atmospheres, it does 797.2J per piston stroke. Under the same conditions, the Stirling engine 100 with supercritical carbon dioxide set as the working fluid 101 operates at 32 ° C. and 7.49 MPa near the critical point in the low temperature section 120, and has an efficiency of 39.6% and a piston stroke at an average pressure of 109.1 atm. Doing 1299.3J per day.

The Stirling engine 100 operates at a lower average pressure than conventional high pressure helium when the working fluid 101 is replaced with supercritical carbon dioxide under the above operating conditions, and is 1.58 times more efficient and 1.63 times more efficient when operated in the same volume. Can work.

In the Stirling engine 100 of the present invention, when the working fluid 101 is used as supercritical carbon dioxide, the lower end 104 is a cooling zone and a compression zone, and the upper end 105 is heated with an expansion zone in the pressure volume graph of FIG. 6. Area.

As shown in FIG. 6, when comparing a pressure volume graph of a Stirling engine to which high pressure helium is applied as the working fluid and a Stirling engine 100 to which supercritical carbon dioxide is applied to the working fluid 101, the expansion zone and the heating area of the upper part 105 are heated. There is less performance in the area, but much less energy in the cooling and compression areas at the lower end 104 increases the overall work.

The pressure-volume graphs of FIGS. 7 and 8 show the work taken to expand by combining work (A), work per cycle (B), and A + B for compression.

As shown in FIG. 7 and FIG. 8, when comparing the pressure volume graph of the Stirling engine to which the high pressure helium is applied as the working fluid and the Stirling engine 100 to which the supercritical carbon dioxide is applied to the working fluid 101 (A) + B) is similar, and there is no difference in gain, but the work of compression (A) increases the overall work because it gains an overwhelming gain with much less energy near the critical point. This is due to the nature of supercritical carbon dioxide, which requires less compression near the critical point.

The vicinity of the critical point represents the point where discontinuous changes in the physical properties of supercritical carbon dioxide are observed. Therefore, the Stirling engine 100 of the present invention is designed to operate near the critical point where the supercritical carbon dioxide is observed at the discontinuous change of physical properties of the supercritical fluid during the compression stroke and the cooling stroke.

The embodiments of the present invention described above are not implemented only by the apparatus and / or method, but may be implemented through a program for realizing a function corresponding to the configuration of the embodiments of the present invention, a recording medium on which the program is recorded, and the like. Such implementations may be readily implemented by those skilled in the art from the description of the above-described embodiments.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

The present invention aims to increase efficiency by applying supercritical carbon dioxide as a working fluid to the Stirling engine by applying a reduction in the compression work that can be taken in the rapid change in physical properties near the critical point.

Claims

In a Stirling engine that generates power through heating and cooling of working fluid,

Reciprocating a piston to compress and expand the working fluid, and including at least one cylinder having a high temperature portion for heating the working fluid and a low temperature portion for cooling the working fluid, and filling the working fluid with a supercritical fluid. And the supercritical fluid operates near a critical point at which a discontinuous change in physical properties of said supercritical fluid is observed during the compression stroke.
The method of claim 1,

When the supercritical fluid is operated near the critical point, a sterling cycle is performed in a pressure range of at least 7 to 8 MPa, at a maximum of 20 to 25 MPa, and at a temperature of at least 25 to 35 degrees and at a maximum of 600 to 700 degrees. Stirling Engine.
The method of claim 1,

Stirling engine, characterized in that when operating the supercritical fluid near a critical point, the low temperature section performs a Stirling cycle operating in a temperature range of 25 to 35 degrees and a pressure range of 7 to 8 MPa.
The method of claim 1,

Stirling engine, characterized in that the working fluid is supercritical carbon dioxide.
The method of claim 1,

Each cylinder has an external heat source for heating the working fluid on an outer circumferential surface thereof, a first cylinder having a first piston reciprocating in the first inner space, and an external cooling source for cooling the working fluid on an outer circumferential surface thereof. And a second cylinder having a second piston reciprocating in a second internal space, wherein the first cylinder is a space for heating the working fluid, and the second cylinder cools the working fluid. Stirling engine, characterized in that the setting to the low temperature portion.
The method of claim 1,

And said supercritical fluid operates near said critical point when performing a compression stroke for compressing said working fluid and a cooling stroke for cooling said working fluid.