WO2014012586A1

WO2014012586A1 - Heat to mechanical energy converter

Info

Publication number: WO2014012586A1
Application number: PCT/EP2012/064094
Authority: WO
Inventors: Maxim GLUSHENKOV; Alexander KRONBERG
Original assignee: Glushenkov Maxim; Kronberg Alexander
Priority date: 2012-07-18
Filing date: 2012-07-18
Publication date: 2014-01-23

Abstract

Converter of heat into mechanical energy, comprising: at least one working space comprising a hot part with at least one heater, a cold part with at least one cooler, and at least one regenerator positioned in between the hot part and the cold part;at least one auxiliary space; at least one power piston positioned in a power cylinder which separates hot part of the working space and the auxiliary space;at least one displacing differential piston positioned in a displacing differential cylinder which separates the cold part of the working space and the auxiliary space;valve means arranged in between the working space and auxiliary space for connecting and disconnecting the spaces cyclically in dependence of the movement of the power piston;and power conversion means for converting the power piston movement into power.

Description

Heat to mechanical energy converter The present invention relates to a convertor of heat into mechanical energy.

Stirling engines, drawing a remarkable attention due to their highest theoretically possible efficiency, are still under development starting from Robert Stirling invention in 1816. For almost two hundred years of the development they have not found a commercial application. This is caused by the very high price, and unsolved problems with sealing of working fluid and wear at high temperature and without oil lubrication. In addition Stirling engines cover only a rather narrow power range. This is because a working volume of the engine is proportional to third power of size whereas a heat transfer area is proportional to second power of size .

An internal space of heat exchangers (heater, cooler and regenerator) is a dead, parasitic volume decreasing thermal efficiency of the engine. An increase of heat exchange surface leads to an increase of the dead volume. As a result power range of most of Stirling engines developed does not exceed several tens kW.

To have an acceptable specific power a working fluid in

Stirling engine must have a high pressure (up to 20-30 MPa) . The best working fluids are helium and hydrogen; that makes sealing extremely difficult. Engines with such a pressure can be also dangerous. To make them less dangerous,

especially in domestic applications, the pressure of working fluid is sometimes decreased to 1-2 MPa. But that results in a decrease in specific power. A two piston (or piston and displacer) design of

Stirling engines with a phase lag involves a complicated and expensive kinematics, problems with oil sealing, pyrolysis of oil inside heaters and regenerators etc. A free piston Stirling engines require an outstanding manufacturing accuracy and involve problems with stable operation, sealing, start-up etc. The phase lag makes a balancing of Stirling engine difficult and expensive.

There also exist engines, usually called thermo

compressors, using some principles of Stirling engines such as cyclic regime of operation, forcing working gas through a cooler, regenerator and heater by a displacer etc. These engines have only one moving part; usually it is a

displacer, sometimes integral with a regenerator. In some cases the displacer can be driven by a separate driver such as a solenoid, a pneumatic or hydraulic drive etc. [Pat. USA 2157229, 3248870, 3513659, 3604821, 3678686, 3990246,

4215548, 5878571] . Then the engine operates as a compressor delivering a pressurized gas with energy that can be

converted to shaft power in a pneumatic motor or turbine, or pressure variations generated by engine can drive a liquid pump. One of the main disadvantages of such systems is a difficulty to match a frequency of the displacer

reciprocation and heat supply to the heater. If the

frequency is too high or too low, efficiency of the engine drops down tremendously; too low frequency can lead to overheating of the heater.

Displacer, sometimes integral with regenerator, can also be self-oscillating, driven directly by a working gas itself [Pat. USA 3788772, Soviet Union Author's certificates 438838, 879194, 922451, 974062, 1030627, 1019187, 1060890, 775544] . In these engines hot and cold chambers can be communicated near limiting positions of the displacer corresponding to its top and bottom dead centers. The communication leads to a momentary working gas pressure equalization in both chambers. The limiting positions are unstable and any disturbance results in a disconnection of the chambers and a forced movement of the displacer to another limiting position (dead center) . Such a disturbance can be caused or created for instance by a spring connected to the displacer. In these designs, working as compressors, a part of energy of gas-phase working fluid is used to maintain a reciprocating motion of the displacer directly in working cycle; the other, useful part is used in form of a pneumatic power. However the actuation of the displacer by working fluid is arranged not efficiently. Actuating

chambers also play a role of gas springs and have very high dead volumes. The dead volumes decrease efficiency of both the actuation of the displacer and main thermo compressor cycle resulting in a too low total efficiency of the engine. Since harmful influence of large dead volume increases with size of engines it prevents from scaling them up. Moreover, they can operate only using gas-phase working fluids.

In US patent 3484616 a self-oscillating regenerator driven more efficiently is used to decrease the parasitic dead volumes of the connection/disconnection valve and working fluid passages. However the internal regenerator is applicable only for a micro-power range.

Therefore, although the abovementioned engines are simpler and cheaper compared with conventional Stirling engines, low efficiency and restricted application area and power range prevent them from a widespread application.

The present invention relates to new type of

regenerative thermal machine. These machines are referred to in the text as heat to mechanical energy converters, heat converter or heat engines or simply engines or converters. In the heat converter according to the invention the flow of working fluid is controlled by the internal volume changes and by valve means and the working fluid transmits its energy to the power conversion means directly via the working fluid, and/or directly or indirectly via a power piston .

Heat converter according to the present invention uses two-phase working fluids which are liquids in cold state and gas-phase/supercritical fluids at a high working cycle temperature. The converter can function with any working fluid including gases suitable from the point of view thermal stability and suits perfect for subcritical and transcritical cycles.

Liquid working fluid has the advantage of easier sealing and can serve as lubricant for all moving or rubbing parts. In addition, the liquid working fluid being

incompressible decreases remarkably the adverse influence of dead volumes. It also makes the engine much safer since in operation a volume of gas phase is lower, whereas out of operation a high-pressure gas phase can be absent at all. Some of designs, being out of operation, can be not

pressurized at all.

Accordingly, the present invention relates to a

convertor of heat into mechanical energy, such as a heat engine, comprising:

at least one working space comprising a hot part with at least one heater, a cold part with at least one cooler, and at least one regenerator positioned in between the hot part and the cold part;

at least one auxiliary space;

at least one power piston positioned in a power cylinder which separates hot part of the working space and the auxiliary space; at least one displacing differential piston positioned in a displacing differential cylinder which separates the cold part of the working space and the

auxiliary space;

valve means arranged in between the working space and auxiliary space for connecting and disconnecting the spaces cyclically in dependence of the movement of the power piston;

power conversion means for converting the power piston movement into power.

The cyclic pressure-volume change of the working fluid in the working space occurs due to:

- heating of the working fluid in the hot part and cooling in the cold part;

- change of the volume of the working space;

- change of the amount of the working fluid in the working space .

Cyclic heating and cooling of the working fluid occurs by movement via the regenerator of the working fluid between the hot part and cold part of the working space. The working fluid is transferred between the hot part and cold part by the displacing differential piston during movement of the power piston and also due to forced working fluid flow during the equalizing of the pressure in the working space and the auxiliary space when the power piston stands still in its limiting positions (ends of the strokes of the power piston) .

The volume of the working chamber changes due to the movement of the power piston and displacing differential piston. Both the power piston and displacing differential piston move due to the pressure difference between the working space and auxiliary space, while the differential displacing piston moves towards a space with higher pressure due to the different piston areas exposed to the working fluid in the working space and in the auxiliary space.

The amount of the working fluid in the working space changes during the equalizing of the pressure in the working space and the auxiliary space when the working fluid flows between the two spaces and the power piston stands still in its limiting positions (ends of the strokes of the power piston) .

The working fluid transmits its energy to the power piston when the piston moves between the limiting positions and directly to the fluid in the auxiliary space during the equalizing of the pressure in the working space and the auxiliary space.

Useful work is performed by the liquid contained in the auxiliary space and/or directly by the power piston and/or the cold part of the working space.

The engine cycle includes expansion stroke and

compression stroke of the power piston positioned in the power cylinder.

During the expansion stroke the working fluid is heated in the hot part, the pressure in the working space

increases, the power piston moves as to increase the volume of the working space, the volume of the working space changes and the working fluid is displaced by the displacing piston from the cold part through the regenerator into the hot part.

The expansion stroke can be performed only if the pressure in the working space is higher than the pressure in the auxiliary space.

At the end of the expansion stroke the valve means open and an open connection is formed between the cold part of the working space and the auxiliary space. Since at this stage the pressure in the working space is higher than the pressure in the auxiliary space the cold working fluid flows from the cold part into the auxiliary space and the hot working fluid flows towards the cold part through the regenerator. The hot working fluid cools down and condenses. The condensation heat generated dispenses heat to the regenerator. This stage can be accompanied by formation of additional vapour in the hot part, due to boiling of the remaining working fluid. This boiling of working fluid contributes to the displacement of the cold working fluid into the auxiliary space and the reduction of the amount of the working fluid in the working space. The liquid working fluid from the auxiliary space can be displaced to the power conversion means adding to the useful work.

The valve means close as soon as the pressure in the spaces equalizes. Under influence of an external force on the power piston, such as a spring or inertia of a crank gear, the power piston starts moving in opposite direction performing the compression stroke. The associated movement of the displacing differential piston will result in a suction of working fluid from the hot part of the working space to the regenerator and the cold part. In the

regenerator the working fluid condenses and the heat of condensation is dispensed by the regenerator. The

condensation reduces the pressure in the working space and contribute to the energy regeneration by heat transfer to the regenerator.

At the end of the compression stroke the valve means open again. Since the pressure in the working space at this moment is lower than in the auxiliary space the cold working from the auxiliary space will flow from the auxiliary space into the working space through the cold part and regenerator to the hot part with taking up heat in the regenerator and evaporation of the liquid working fluid in the hot part and result in a pressure increase in the working space. The amount of the working fluid in the working space will also increase. The valve means close as soon as the pressure in the spaces equalizes and the expansion stroke begins.

According to another embodiment, the displacing piston is connected to the power piston. Accordingly, both pistons move together as an common piston assembly. Preferably the common piston assembly forms a unitary or monolithic piston assembly. These alternatives avoid the use of a separate displacing cylinder, separate differential piston and connection to each cylinder.

Any of two step surfaces of a differential piston can be used as a displacing surface giving a flexibility in the converter design.

It may be advantages to control or regulate the flow of working fluid through the displacing line between the displacing differential cylinder and the cold part. So that the heat transfer processes in the working space could take place in a desired and efficient manner. Thereto, valve means are arranged in the displacing line.

According to another embodiment the power conversion means is connected directly to the power piston.

When the power piston is not directly (mechanically) connected to the power conversion means it might be that the working fluid is not optimal for powering the power

conversion means. For instance, a hydraulic motor, is better powered by a hydraulic oil than by a working fluid such as water, propane, ammonia, carbon dioxide and the like. Thus, it may be advantages when interface means are arranged between the auxiliary space and the power conversion means. Such interface means may be a flexible diaphragm impermeable to the working fluid and the powering fluid, and preferably inert to both the fluids, a sealed piston, and the like. A mechanically optimal and compact heat converter is provided with less separate moving parts when preferably the valve means are integrated into the power piston and/or displacing differential piston. Such piston integrated with valve means may comprise a duct formed in the piston and may form an open communication between the working space and auxiliary space depended on the position of the piston in the cylinder.

The power conversion means to be used in relation to the present invention may have any suitable form primary depended on the direct or indirect connection of the power piston to the power conversion means, the type of energy conversion and the desired power of the converter.

Accordingly, the power conversion means may comprise an hydraulic motor, a compressor, accumulators, and/or crank gear .

The heat converter according to the invention, may also comprise an energy accumulator, such as spring of any type or linear to rotational motion converter with a rotating part, such as a crank gear or scotch joke with a rotating mass, connected to the power or displacing piston. The accumulator stores energy when the pistons approaches a limiting position and returns the stored energy back to the piston when the piston passes over the limiting position. The accumulator makes the limiting positions of the piston mechanically unstable.

The flow of the working fluid towards the power

conversion means may be not constant. Various types of damping means may be used to damp the flow pulses.

In addition or in the alternative two or more power pistons and displacing pistons may be used which are

operated in the counter phase or in phase (synchronously) . In relation to this alternative embodiment it is advantageous when the two of the power pistons and the displacing differential pistons, are arranged in a common cylinder. This results in a compact and robust heat

convertor according to the invention. Obviously, the two power cylinders could be constructed such that they have a common cylinder part. More preferably, the pairs of the power piston and the displacing differential piston have a common working space and auxiliary space and move in phase. Positioning the pistons coaxially eliminates vibrations of the converter.

Any type of power piston and displacing differential piston may be used moving under the influence of a pressure difference over the piston. Such movement a piston may be a reciprocating movement, oscillating movement and/or rotary movement.

Not mentioned in relation to features of the heat convertor of the invention is that the spring means of any suitable type could be used for starting, supporting or damping the stroke, providing a push to the pistons in the limiting positions. Such spring means comprise mechanical springs, hydraulic springs and the like. The diameter of the power piston and of the displacing piston may be selected such that the conversion of heat into mechanical energy is efficient. Similarly, accumulators can be used to

temporarily accumulate pressurized working fluid for delayed supply to the power conversion means whereby the conversion proceeds more continuously.

Mentioned and other features and characteristics of the invention will be apparent from a description of various embodiments of the heat to mechanical energy convertor, which description is given for illustrative purposes, without limiting the invention to any extent. Thereto reference is made to the drawings wherein: Figure 1 shows schematically a heat to mechanical energy convertor of the invention;

Figure 2 shows another convertor of the invention comprising a piston unit comprising a power piston connected to a displacing piston and mechanically connected to the power conversion means;

Figures 3-5 show alternatives of the convertor as shown in the figures 1 and 2;

Figures 6-8 show alternative heat to mechanical energy convertors of the invention comprising at least two piston units ;

Figures 9 and 10 show alternative arrangements of heat regeneration of the invention;

Figure 11 shows heat convertor of the invention with piston integrated valve means, in different stages of the cycle .

Figures 12 shows heat convertor of the invention with piston integrated valve means and heat regenerator

integrated with the displacing piston and wall of the engine body, in different stages of the cycle.

The heat converter 1000 shown in figure 1 is a heat engine which converts heat to shaft power of a hydraulic motor .

The heat engine 1000 comprises a power cylinder 1 within which is arranged a power piston 2. The power piston 2 separates an internal volume of the cylinder into a cold part 3 and a hot part 4. The cold and hot parts are

communicated by a heat circuit 5 which includes a shutoff valve 6, the cooler 7, the regenerator 8 and the heater 9. The heater 9 can be combined with the hot part 4 to form an integral unit. The heat engine 1000 of the invention also

includes a displacing differential cylinder 14 with a displacing differential piston 15 inside. The displacing differential piston 15 separates the cold part of the power cylinder 3 and the cooler 7, and divides the cylinder 14 into three parts: a driving part 16, an intermediate part 17 and a displacing part 18. The piston 15 is spring-loaded by means of a spring 19. The spring can be located not only in the displacing part of the differential cylinder as it shown in figure 1 but also in the intermediate or the driving parts. Not only mechanical spring but also magnetic, pneumatic, hydraulic one can be used.

The intermediate part 17 can be communicated to

atmosphere to vent a possible leakage from the driving and displacing parts.

Optionally instead of the displacing differential piston 15 a combination of a piston and a diaphragm or two kinematically linked diaphragms with different diameters can be used. The driving part 16 of the cylinder 14 is

communicated with the cold part 3 of the power cylinder 1 by a line (as shown) or can be a direct extension of the cylinder 1. The displacing part 18 is communicated by a line 20 with the heat circuit 5 in between the valve 6 and the cooler 7. The line 20 has a regulating valve 21.

Both the power cylinder 1 and displacing cylinder 14 are equipped with piston restrictors/dampeners (not shown in the figure) for shock-free stopping the pistons in an upper and a lower limiting positions (top and bottom dead centers, TDC and BDC) .

The cold part of the power cylinder 3 is connected to a power circuit 10 comprising two check valves 11, two

hydraulic accumulators 12 and a hydraulic motor 13. The power piston 2 is maintained in an intermediate position in between two limiting positions by a spring 22. Different kinds of springs can be used such as mechanical, magnetic etc. The spring 22 is an optional element of the design. In principle one of the hydraulic accumulators 12 and the hot part 4 can perform the function of the spring (pneumatic) whereas an auxiliary intelligent start-up system can start the engine at any position of the piston.

The hot part 4 of the power cylinder, heater 9, regenerator 8, cooler 7, displacing part 18 of the

displacing cylinder, and communicating lines form a

compartment where the working fluid undergoes cyclic

pressure-volume-temperature-amount change. This compartment is defined as a working space of the engine.

The cold part 3 of the power cylinder and the driving part 16 of the displacing cylinder form an engine

compartment in which the working fluid is not involved directly in generation of useful work. This compartment can serve for transmission of power between the working space and the power circuit and driving of the displacing piston 15 (as shown in figure 1) and for temporal storage of the working fluid during the cycle. This compartment is defined as an auxiliary space of the engine.

All the installation is filled with a liquid working fluid shown in gray. The fluid is pressurized by a

compressed gas in the hydraulic accumulators 12.

At the beginning of a start-up procedure the valve 6 is closed. The piston 2 is maintained at the intermediate position by the spring 22. As soon as heating and cooling are arranged, the liquid working fluid in the heater 9 is heated and boiled. Pressure in the working space rises; the piston moves, compresses the spring 22 and displaces the cold liquid working fluid to the power circuit 10. The fluid flows through the first check valve 11a to the first

hydraulic accumulator 12a then through the hydraulic motor 13 to the second hydraulic accumulator 12b. The motor converts mechanical energy of the liquid to a shaft power. The pressure in the driving part 16 of the displacing cylinder 14 also increases and the differential piston 15 overcomes a force of the spring 19 and moves, displacing the liquid working fluid through the line 20 to the heat circuit 5. The liquid flows through the cooler 7, the regenerator 8 and the heater 9 to the hot part 4. As a result the liquid is heated and evaporated maintaining a high pressure of the liquid during the movement of the piston 2. The regulating valve 21 sets a desired flow rate of the liquid working fluid that is displaced by the piston 15.

Near the upper limiting position (TDC) of the piston 2 the valve 6 opens, hot vapor of the working fluid flows through the regenerator 8 and the cooler 7 displacing the liquid working fluid to the cold part 3 and then delivering it to the first hydraulic accumulator 12a and the hydraulic motor 13.

After equalization of pressure inside the engine the spring 22 pushes the piston 2 down, to the lower limiting position (BDC) and the valve 6 closes. At the same time the spring 19 pushes the piston 15 down, to the initial

position. The piston 15 sucks the working fluid from the hot part 4 through the regenerator 8 and the cooler 7. The vapor cools down and condenses in the regenerator 8 and cooler 7. Pressure in the working space drops down resulting in downward movement of the piston 2 and a suction of the working fluid from the power circuit 10 through the second check valve lib. The liquid flow in the power circuit 10 through the hydraulic motor 13 and two hydraulic accumulators 12 caused by this suction generates a shaft power of the hydraulic motor.

Near BDC the valve 6 opens again, equalizing pressure of the working fluid in the engine. During this step the cold working fluid from the auxiliary space flows through the cooler 7, the regenerator 8 and the heater 9 to the hot part of the power cylinder and the pressure in the working space increases. After the equalization of pressure inside the engine the piston 2 starts moving towards its TDC due to the spring 22. Simultaneously the valve 6 closes. The liquid working fluid in the working space heats up and evaporates. The pressure in the working space increases, resulting in pushing the power piston up and actuation of the piston 15 which displaces the working fluid from the cold part to the hot part of the working space. Then the cycle repeats itself .

The heat converter shown in figure 1 can be used for pumping liquids in case the liquid to be pumped is a

suitable working fluid in a desired range of temperatures. The power circuit 10 as in figure 1 is not needed in this case .

The displacing piston-cylinder unit can be arranged differently, for instance the intermediate part 17 of the cylinder 14 can be interchanged with the displacing part 18. The displacing piston 15 can also be combined with the power piston 2. Then they move as a single unit. An example 2000 is shown in figure 2.

The cylinder 23 is combined with a crankcase 24 having a crank gear 25. A differential piston 26, attached to the crank gear 25, consists of two parts: a lower, displacing piston 26a and an upper, power piston 26b.

The piston divides the internal volume of the cylinder into three parts: a hot part 23a, a cold part 23b and a cold displacement part 23c. The hot part 23a and the cold power part 23b are communicated by a heat circuit 27 which

includes a shut-off valve 28, a cooler 29, a regenerator 30 and a heater 31. The cold displacement part 23c is

communicated with the heat circuit 27 in between the valve 28 and the cooler 29. The crankcase 24 is filled with a gas The working space of the engine 2000 in figure 2 includes the hot part 23a, heater 31, regenerator 30, cooler 29, displacement part 23c and connecting tubes. The cold part 23b forms the auxiliary space.

At the beginning of a start-up procedure the valve 28 is closed. As soon as cooling and heating are arranged the liquid working fluid is heated and boiled. Pressure in the working space rises, the piston 26 moves up transmitting power via the crank gear 25 to a shaft. At the same time the piston 26a displaces the liquid working fluid from the cold displacement part 23c of the cylinder 23 to the heat circuit 27 and then to the hot part 23a. The heated and evaporated working fluid maintains high pressure in the working space during the upward stroke.

Near the upper limiting position of the piston the valve 28 opens equalizing pressure inside the engine. During this step the hot working fluid flows through the regenerator 30 and the cooler 29 to the cold part of the cylinder.

Due to the inertia of the crank gear the piston is pushed down moving towards its bottom dead center. Valve 28 closes and the moving piston 26a displaces the working fluid from hot part 23a to the regenerator 30 and cooler 29 and then to the displacement part 23c. Cooling and the following

condensation result in an additional pressure decrease in the working space. A pressure drop appearing between hot 23a and cold power part 23b of the cylinder generates a force moving the piston down and transmitting its energy to the shaft power.

Near the lower limiting position of the piston valve 28 opens again equalizing pressure in the engine. Inertia of the crank gear pushes the piston up, valve 28 closes and the cycle repeats itself.

The start-up procedure in case of non-ideal, non-positive sealing of the pistons (such as a clearance seal) might also require cranking of the crank gear.

The positions of the power piston and the displacing piston, when they are made as a single unit, can be interchanged as shown in figure 3. The power produced can be also used differently .

The heat converter 3000 shown in figure 3 consists of a differential cylinder 32 with a differential piston 33 inside. The piston includes two parts - a lower, power piston 33a and an upper, displacing piston 33b moving as a single unit.

The piston 33 divides the internal volume of the cylinder into three parts: a hot part 32a, a cold power part 32b and a cold displacement part 32c. The hot part 32a and the cold power part 32b are communicated by a heat circuit 34 which includes a shut-off valve 35, a cooler 36, a regenerator 37 and a heater 38. The cold displacement part 32c is

communicated to the heat circuit 34 in between the valve 35 and the cooler 36.

A two-stage free piston compressor 39 shown schematically is connected by a line to the cold power part 32b of the cylinder. The piston 33 is maintained in the middle position by a spring 40.

The hot part 32a, heat circuit 34, cold displacement part 32c and connecting tubes form the working space. The cold part 32b forms the auxiliary space serving for the transmission of pressure in the working space to the power conversion means, e.g. the free piston compressor 39.

The working and auxiliary space are filled with a working fluid shown in gray.

At the beginning of a start-up procedure the valve 35 is closed. As soon as a heating and cooling are arranged the liquid working fluid is heated and boiled. Pressure in the working space rises, the piston 33 moves up displacing the working fluid from the cold power part 32b and performing a compression stroke of the compressor 39. At the same time the piston displaces the working fluid from the cold

displacement part 32c of the cylinder to the heat circuit 34 and then to the hot part 32a. The working fluid heated and evaporated in the regenerator and the heater maintains a high pressure in the working space providing upward motion of the piston.

Near the upper limiting position of the piston the valve 35 opens equalizing pressure in the cylinder. During this step the hot working fluid flows back through the regenerator 37 and the cooler 36 to the cold power part of the cylinder 32b where it cools down and condenses.

Then the spring 40 pushes piston down moving it to the bottom dead center and the valve 35 closes. The moving piston displaces the working fluid from the hot part 32a to the regenerator 37 and the cooler 36 and then to the

displacement part 32c. Cooling and the following

condensation result in an additional pressure decrease in the working space and movement of the piston down. The power piston 33a sucks the working fluid from the compressor 39 providing a suction stroke of the compressor.

Near the lower limiting position of the piston the valve 35 opens equalizing pressure in all parts of the converter. After that the spring 40 pushes piston up, the valve 35 closes and the cycle repeats itself.

The start-up in case of a non-ideal, non-positive sealing of the pistons might also require an auxiliary pneumatic, hydraulic or electromagnetic push-up starting system.

Instead of the two-stage compressor one- or multi-stage compressor, pump or any other reciprocating or oscillating hydraulically driven device can be applied. In addition the pressurized working fluid can be pumped into a hydraulic accumulator and then be used to drive compressors, pumps, mining machines, vibrators and the like.

Working fluid under pressure can be used to drive any liquid working fluid consuming machines such as reciprocating, oscillating or rotary hydraulic motors and the like.

The heat converter 4000 shown in figure 4 drives a piston hydraulic motor shown schematically.

Basic principles and operation of the engine are the same as those shown in figure 3. However, instead of the piston compressor, hydraulic motor 41 is used to transmit energy of the liquid to shaft power. In addition the piston 42a plays a role of the displacer piston whereas the piston 42

functions as power one.

Conventional hydraulic oils suit perfect as working fluid for hydraulic motors or other consumers of the hydraulic power, generated by heat converters of the invention.

However they cannot be used as working fluids for the heat converters due to their improper characteristics such as thermal stability, vapor pressures etc.

Basic principles and operation of the heat converter 5000 shown in figure 5 are the same as those of the heat engine shown in figure 3. However, instead of the compressor, a hydraulic motor 43 in figure 5 is used to convert energy of the working fluid to shaft power. A power consuming loop similar to that in figure 1 includes two hydraulic

accumulators 44a and 44b and check valves 47a and 47b to provide a pulseless flow of a hydraulic liquid and a

constant pressure drop across the hydraulic motor. The power loop uses hydraulic oil (shown in dark gray) whereas a suitable working fluid (shown in light gray) is applied in the heat converter (water, carbon dioxide, propane, pentane, ammonia etc) . To separate the liquids and transmit power (force) from the working fluid to the hydraulic oil, a diaphragm unit 45 with a flexible diaphragm 46 impermeable for both the liquids is used.

In the general case, instead of a diaphragm, a sealed piston can be used. In case of immiscible liquids an interface between the liquids can play the role of the diaphragm or the piston. In that case special packings preventing a formation of an emulsion of both liquids can be used.

To provide a more stable pulseless flow two converters shown in figure 5 can be placed in series whereas the pistons may be rigidly connected to each other. Such an embodiment 6000 is shown in figure 6. Pistons 48a and 48b are connected by a rod 49; the rod is sealed with a sealing unit 50. The design permits to have only one hydraulic motor producing double power; size of hydraulic accumulators can be reduced due to decreased flow pulsations.

If diameter of the connecting rod is equal to the diameter of the displacing piston a simpler design 7000 shown in figure 7 is possible. In the embodiment shown only one shut- off valve 51 is sufficient to serve both pistons simplifying the design. In contrast to the engines shown in figures 1 - 6 the power conversion means is connected to the cold displacement parts of the cylinder and the pressure

equalizing occurs between the two working spaces i.e. one working space serves as an auxiliary space for the second working space.

Instead of the hydraulic circuit containing the check valves and hydraulic accumulators, a very simple hydraulic motor 52 with double-acting piston 53 connected to a crank gear 54 is used. The strokes of the hydraulic motor 52 are equal working strokes.

In contrary to heat engines with a gas-phase working fluid the heat converters with a liquid working fluid can be balanced easily. The simplest way is to use pairs of

identical converters with a synchronized, opposite movement of the pistons. All well-known methods of synchronization of hydraulic cylinders can be applied.

The simplest method of synchronization is shown in figure 8 for the converter 8000. Two identical converters based on the same principle as the converter shown in figure 4 are combined in one unit. They share one cylinder, valve, regenerator, cooler and heater. A hydraulic motor 55

converting energy of liquid working fluid to rotary shaft power has a differential cylinder 56 with a differential piston 57 inside so that two working spaces 58a and 58b with equal volumes are formed. Since liquid is incompressible, the power pistons 59a and 59b of the converter 8000 can move only synchronously in opposite directions. A variant of such a principle of synchronizing is an application of two equal piston hydraulic motors with rigidly connected crankshafts. Such a variant can also be realized by combining two and more heat converters shown in figure 2.

Balancing of the converters shown in figures 1 and 5

requires more sophisticated methods such as application of flow dividers/combiners, electro hydraulic servo drives and the like or mechanical synchronizers. Depending on origin and grade of heat source and sink, the heater and cooler of the converters according to the

invention can be conventional heat exchangers/tubular steam generator or to have advanced designs such as micro- and mini-channel heat exchanger/steam generators which provide the lowest internal volume (i.e. dead volume) and footprint and the highest heat exchange surface.

The simplest regenerative heat exchanger typical of Stirling engines can be used as a regenerator. The typical

regenerator is a volume filled with a packing such as wire mesh, random wires, convoluted foils, metal or ceramic balls and the like.

An alternative design of regenerator 9000 together with the heat circuit is shown in figure 9a. It consists of a

conventional countercurrent flow heat exchanger 60 and two check valves 61; positions of a heater 62 and a cooler 63 are also shown. Since liquid is incompressible, a liquid working fluid can flow in each passage of the heat exchanger only in one direction. The combination of the heat exchanger with the check valves can diminish a negative influence of the regenerator dead volume on the engine performance and characteristics .

The check valves 61 can be installed in the heat circuit 10000 also in between the cooler 63 and a cold part of the converters (figure 9b) . It prevents back flow (and shuttle motion) of working fluid from the regenerator 60 and the heater 62. The most proper places to install the check valves are parts of the heat circuit filled with liquid rather than vapor. For instance one of the check valves 61 can be installed in between the cooler 63 and the

regenerator 60, figure 9c, heat circuit 11000.

The heat circuit designs shown in figure 9 when applied for a liquid working fluid allow ease scaling of the engine in the power range from several watts to several megawatts per power cylinder.

Generally for all the aforementioned embodiments heat can be supplied and removed at several different temperatures in contrary to the Stirling engines, where usually only two temperature levels are used. Typically temperature of exhaust gases leaving a heater of the Stirling engine can be rather high. Heat of the exhaust can be used to preheat combustion air but it results in an excessive NOx emission. If there is no any exhaust heat consuming equipment, the exhaust heat is wasted.

The engines of the invention can have several heaters, coolers and regenerators i.e. depending on application heat can be supplied and rejected at several temperature levels remarkably increasing efficiency of the engines. For

example, the heat circuit shown in figure 10 has two heaters 64a and 64b, two regenerators 65a and 65b and one cooler 66. In this case a heating agent such as combustion products can be used twice, resulting in a better efficiency of the engines. This type of scheme can be used for instance in case of a waste heat utilization in industrial plants such as chemical plants and refineries, where different heat sources with different temperature levels can be used for each heater. In the similar manner several coolers in combination with several regenerators can be used.

The engine of the invention can be based not only on the reciprocating pistons but also apply an oscillating or rotary piston designs. Hydraulic motors converting energy of a fluid to shaft power can also be based on oscillating or rotary piston designs.

The shut-off valve communicating the working space and auxiliary space of the engine can be a conventional 2-way (poppet) valve as well as a reciprocating or rotary spool valve. A stem or spool of the valve can be driven by an electrical, hydraulic (electro-hydraulic) , pneumatic, etc actuators. Opening and closing of the valve are defined by position of the power piston or some members kinematically connected to the piston. When the power piston reaches predetermined upper and lower limiting position (near TDC and BDC correspondingly) the valve opens; when the piston leaves the limiting position the valve closes. The position of the piston can be detected by different kinds of

displacement, position, proximity or limit sensors/switches and the valve can be opened and closed by a control system processing the sensors/switches signals. In case of the rotary piston design or different reciprocating-to-rotary motion converting mechanisms, corresponding angular position sensors can be used.

The valve can also be driven mechanically, for instance being kinematically linked to the piston or being an

integral part with the piston. In the latter case the power piston itself can play the role of the valve. An example of such an embodiment 13000 is shown in figure 11 for an intermediate, figure lib, and two limiting positions of the piston, figure 11a and figure 11c.

Figure 11 shows a sketch of a successful working model of a heat converter made in accordance with this invention explained in figure 5. It consists of a power cylinder 67, a displacing cylinder 68, a heater 69, regenerator 70 and a cooler 71. The power cylinder, heater and regenerator have a common cylindrical body. It is joined to the displacing cylinder 68 and the cooler 71 by connectors 72 and 73. The converter also includes a piston 74. The piston divides the cylinder 67 into two parts - a cold part 67a and a hot part 67b. The piston consist of two parts - a power piston 74a and displacing piston 74b made as one unit. Passages 75 are made inside the piston to provide the piston with a function of a valve spool. The heater comprises an insert 76 with a plurality of heating channels 77 in form of axial slots. The insert 76 and the surrounding cylinder are brazed together to provide a high rigidity of the heater. To enhance heat flux to the channels 77, fins 78 are machined on the

external surface of the heater wall.

The regenerator 70 is a packing of a wire mesh, steel balls, etc. The cooler 71 encloses a cooling coil 79. The cooler, power cylinder and displacing cylinder have ports 80, 81 and 82 correspondingly. The port 81 in the power cylinder plays a role of a valve orifice; together with the power piston 74a and the passages 75 they form a reciprocating spool valve. The ports 80, 81 and 82 are interconnected by a line 83. The connector 72 joining the cylinders 67 and 68 has a power port 84. This port is connected to a power conversion means such as hydraulic motor, pump, compressor etc (not shown in figure 11) . The displacing piston 74b is sealed by a sealing unit 85 located in between the connector 72 and the displacing cylinder 68. A cushioning member 86 connected to the displacing piston 74b provides shock-free stops of the piston in the limiting positions. It also plays a role of a holder of a spring 87; the spring maintains the piston in a middle position before start up and disturbs the piston from equilibriums at the both limiting positions.

Heat can be supplied to the external surface of the heater 69 for instance by combustion (the burner is not shown) . Water as a cooling agent circulates through the cooling coil 79.

All installation is filled with a liquid working fluid.

Depending on the temperature levels available it can be water, water-ammonia mixtures, carbon dioxide, typical organic Rankine cycle fluids such as propane, pentane and the like.

At the beginning of start-up the piston 74 is maintained by the spring 87 in the middle position (figure lib) ; in this position the piston totally closes the port 81, i.e. the shut-off valve is closed. When heat is supplied, the liquid working fluid is heated and boiled. Pressure in the working space rises, the piston 74 moves up and the power piston 74a displaces the working fluid from the cold part 67a through the power port 84 to a power conversion means (not shown) . At the same time the displacing piston 74b displaces the working fluid from the displacing cylinder 68 through the port 82, the line 83 and the port 80, the cooler 71, the regenerator 70 and the heater 69 to the hot part 67b. The working fluid is heated and evaporated in the regenerator and heater; the heater maintains a high temperature and pressure during the upward stroke.

When the piston reaches the upper limiting position (figure 11a) the internal passages 75 coincide with the port 81 and communicate the hot 67b and the cold 67a parts of the power cylinder 67 and the displacing cylinder 68. In this

position, corresponding to the top dead centre (figure 11a) , pressure in the hot and cold parts of the power cylinder and the displacing cylinder equalizes. During this step the hot working fluid flows back through the heater 69, the

regenerator 70 and the cooler 71 to the cold part 67a of the power cylinder and through the power port 84 to the power conversion means. Doing this the working fluid cools down and condenses.

Then the spring 87 pushes the piston back, the piston disconnects the cold and the hot parts of the power cylinder and moves down to its bottom dead center expanding the centering spring 87. The moving piston displaces the working fluid vapor from the hot part 67b through the heater, the regenerator and the cooler and then to the displacement cylinder 68. The cooling and condensation result in an additional pressure decrease in the hot part of the power cylinder. A pressure drop appearing between the hot 67b and the cold power part 67a of the cylinder results in moving the piston down. The piston sucks the working fluid from the power conversion means.

Near the lower limiting position (figure 11c) the piston opens the port 81 and communicates again the hot 67b and the cold 67a parts of the power cylinder and equalizes the pressure. After that the spring pushes the piston to the top dead centre again, disconnects the cold and the hot parts of the power cylinder and the cycle repeats itself.

For mini- and micro-power ranges hot parts of an external heat circuit such as heaters, regenerators and connecting lines, can involve excessively high heat losses. In that case a converter with an internal regenerator can be

applied. A simplified example 14000 of the successful heat converter is shown in figure 12. It is based on the same principles and operates the same way as the converter shown in figure 4.

The converter includes a power cylinder 88 secured to a displacing cylinder 89. The power cylinder 88 is closed by a cover 90. A cooling jacket 91 with inlet and outlet ports for a cooling agent is placed around a finned external surface 92 of the cylinder 89 so that a cooler 93 is formed. A lower part of cylinder 89 is provided with heat exchange enhancing fins 94 and plays a role of a heater 95.

The power and displacing cylinders have flow passages 96, 97, 98 and 99 for a working fluid. The passages 96 and 97 are communicated by a line 100; the passages 98 and 99 are communicated by a line 101. Line 100 is communicated with power line 102 to transmit the power of liquid working fluid to a hydraulic power conversion means such as a hydraulic motor, piston pump etc (not shown in figure 12) .

The power piston 103 is placed inside the power cylinder 88 with a very small clearance. The power piston is rigidly connected to the displacing piston 104 to operate as one unit. A part of external surface of the displacing piston 104 functions as a regenerator 105. It can be made as a well-developed surface, for instance as an array of radial pins machined on the side surface of the displacing piston. An alternative and supplement is an unmovable regenerator placed in the cylinder wall in between the heater and cooler. The regenerator can also be placed inside the displacing piston 104.

The power piston 103 has a flow passage 106 machined as an open slot. A centering spring 107 is placed into an axial bore machined inside the piston 103; ends of the spring are secured to the piston 103 and the cover 90. The centering spring maintains the piston in a middle position before start up and tends to returns it to the middle position from both the limiting positions.

The power and displacing pistons divide the internal volume of the converter into a hot part 108a, a cold displacement part 108b and a cold power part 108c (figure 12b) .

A fillet 109 and parts 110 and 111 in figure 12c are

cushioning elements providing shock-free stops of the piston in the limiting positions. The part 110 also plays a role of a holder of the centering spring 107.

Before start-up the power piston 103 is in the middle position (figure 12b) disconnecting the hot and cold parts of the cylinder 108a and 108b from the power part 108c.

As soon as heat is supplied to the external surface of the heater 95, for instance by combustion of a fuel, and a flow of a cooling agent is arranged through the jacket 91 the working fluid is heated and boiled in the hot part 108a of the cylinder 89. The pressure generated in the hot and cold parts 108a and 108b pushes both of the pistons 103 and 104 up. It results in displacing the liquid working fluid from the power part 108c through the passage 96 and the line 100 to the power line 102 and then to the power conversion means. When moving up the piston 104 displaces the liquid working fluid from the cold part 108b to the hot part 108a. Passing through the regenerator 105 and the heater 95 the liquid working fluid is heated up and boiled. This permits to maintain a high pressure of the liquid fluid in the power line 102 and perform useful work. Near the top limiting position (TDC) the piston 103 opens the flow passage 97 and communicates the cold part 108b and the power part 108c of the power cylinder, equalizing pressure in both parts

(figure 12c) . The working fluid boils, vaporizes and heats up in the hot part and flows from the hot part 108a to the cold part 108b and then through the passage 97 and the line 100 to the power line 102 providing an additional power in the power conversion means. The amount of the working fluid and pressure in the power and displacing cylinders 88 and 89 decrease. An additional pressure decrease occurs due to cooling of the working fluid in the regenerator.

When the pressure in all parts of the converter are

equalized the spring 107 pushes the piston 103 down. The piston closes the opening 97, disconnecting the power part 108c, and the line 100 and 102 from the parts 108b and 108a. Moving down, the piston 104 displaces the working fluid from the hot part 108a to the cold part 108b. Passing through the regenerator and the cooler the working fluid cools down and condenses. Pressure in the parts 108a and 108b drops down creating a pressure difference between the power part 108c and the cold part 108b. This difference pushes the power and displacing pistons further and provides a suction of the liquid working fluid from the power conversion means. At the lower limiting point (figure 12a) the power piston

communicates the cold part 108b and the power part 108c by means of the passage 106, 98 and 99 and the line 101. The pressure in different parts of the converter equalizes again, the spring 107 pushes the pistons upwards and the cycle repeats itself.

Spring holding the power piston in the middle position is not mandatory part for the all converters of the invention. The main reason to use it is to provide a more stable start^¬ up and operation of the engine and avoid a sophisticated start-up system.

To prevent a parasitic heat flux from the hot to the cold area the piston can be made of low thermal conductivity materials (such as glass, glass ceramic etc) or to be similar to typical Stirling engine displacers.

To seal the piston all types of seals such as lip seals, piston rings, stuffing box and the like can be used. The pistons can be also sealed by means of clearance sealing (small gap) with tight clearances between the piston and cylinder. In case of a clearance sealing a fluid hydrostatic bearing technology can be used to provide a wearless

reciprocation of the piston.

The application of liquid working fluids solves main

problems of the Stirling engines such as unlubricated friction and wear as well as sealing of working fluid.

Liquid acts as a lubricant extending dramatically lifetime of rubbing materials. Sealing of liquids is simpler then sealing of gases, especially such as helium or hydrogen. Liquid, being incompressible, eliminates a remarkable part of dead volume of the engine. If a countercurrent heat exchanger in the combination with check valves is used as the regenerator and/or heater and/or cooler the influence of dead volume decreases additionally. It results in increasing of thermodynamic efficiency of the engine, permits to scale it up and gives a more freedom in design and arrangement of the engine parts and units. For instance it permits a less tight integration and more room for different parts and units especially such as heat exchangers and regenerators. That makes it possible to use more economical, conventional heat exchangers.

In addition, an amount of the hot gas-phase working fluid is restricted by an internal volume of the hot chamber, heater and sometimes a part of a regenerator. Apparently that also makes the engine much safer.

If the pressure in the engine exceeds the critical pressure of the working fluid the engine cycle remains the same_but the evaporation and condensation stages are eliminated and the cycle is not accompanied with phase changes.

By selecting of appropriate working fluids the heat sources with temperature from about 50 to 1000 °C can be used. The lowest temperature of the heat source is determined by the boiling point of the working fluid, whereas the highest temperature - by materials of construction.

A liquid working fluid, particularly in supercritical state, may enhance remarkably heat exchange in the heater, cooler and regenerator which can result in great improvements in design, size and composition of the engine.

Claims

1. Converter of heat into mechanical energy, comprising:

at least one auxiliary space;

at least one power piston positioned in a power cylinder which separates hot part of the working space and the auxiliary space;

at least one displacing differential piston positioned in a displacing differential cylinder which separates the cold part of the working space and the

auxiliary space;

power conversion means for converting the power piston movement into power.

2. Converter as claimed in claim 1, wherein the power conversion means are connected to the auxiliary space and/or directly to the power piston and/or to the cold part of working space.

3. Converter as claimed in claim 1 or 2, wherein the displacing differential piston is connected to the power piston, and preferably forming a unitary piston.

4. Converter as claimed in any of the claims 1-3, wherein valve means are arranged in a displacing line connecting the displacing differential cylinder and the cold part of the working space.

5. Converter as claimed in any of the claims 1-4, wherein interface means are arranged between the auxiliary space and the power conversion means.

6. Convertor as claimed in any of the claims 1-5, comprising at least two working spaces, at least two

auxiliary spaces, at least two power pistons and at least two displacing pistons.

7. Convertor as claimed in claim 6, wherein the power pistons and the displacing pistons are arranged in a common cylinder.

8. Convertor as claimed in claim 6 or 7 wherein a cold part of one working space serves as an auxiliary space for the second working space.

9. Convertor as claimed in any of the claims 6-8, wherein the pairs of power pistons and the displacing pistons have a common working space and auxiliary space.

10. Convertor as claimed in any of the claims 1-9, wherein the regenerator comprises a combination of a

recuperative heat exchanger and at least two non-return valves .

11. Converter as claimed in any of the claims 1-

10, wherein the valve means are integrated into the power piston.

12. Convertor as claimed in any of the claims 1-

11, wherein the regenerator is integrated into the power piston and/or into the power piston cylinder.

13. Convertor as claimed in any of the claims 1-12, wherein instead of a differential piston a combination of a piston and a diaphragm or two kinematically linked

diaphragms with different areas are used.

14. Convertor as claimed in any of the claims 1-

13, wherein the movement of a piston is a reciprocating movement, oscillating movement and/or rotary movement .