Patent Application
20110203267
The invention relates to a method for operating a Stirling cycle process in which an operating medium is respectively compressed in an isothermal manner, subsequently heated in an isochoric manner subsequently, expanded in an isothermal manner and subsequently cooled in an isochoric manner which completes the cycle process.
The invention furthermore relates to a device for operating a Stirling process including a compressor for essentially isothermal compression of an operating medium under heat dissipation, a heat transfer device through which heat can be essentially transferred to the compressed operating medium essentially in a isochoric manner, an expansion device for essentially isothermal expansion of the operating medium under heat absorption, wherein heat is transferable in a heat exchanger from the expanded operating medium to the compressed operating medium and wherein the cooled operating medium is subsequently supplyable to the compressor again.
The Stirling process and devices to perform the Stirling process have been known in the art for a long time. The Stirling process is one of the cycle processes in which the efficiency of a clockwise Carnot process can be achieved for a clockwise power machine process, or the figure of merit of a counter clockwise Carnot process can be reached for a counterclockwise Stirling process (heat pump, refrigeration machine). Based on multiple restrictions in practical applications of the method and based on engineering and material limitations the actually achieved efficiency or figure of merit is always not as good as theoretically possible.
The language “essentially” isothermal compression or expansion and “essentially” isochoric heating or cooling recited supra therefore shall also include changes of state which deviate from the thermodynamic ideal process due to practical restrictions which, however, are at least approximated to the isothermal or isochoric changes of state.
A disadvantage of the Stirling process typically performed through piston compressors or piston expanders is the comparatively bad heat transfer from the operating medium to an ambient medium that surrounds the operating medium or is in contact with the operating medium. In practical applications therefore the compression process and also the expansion process occur comparatively remote from the idealized isothermal state change. This affects the efficiency of the power machine process or the figure of merit for a refrigeration machine- or heat pump process.
A liquid piston engine is known from U.S. 2008/0072597 A1 in which an electrically or electronically conducted liquid is being used. The known motor includes a first “hot” cylinder, in whose upper section a gas is supplied with heat through an external heat source. The gas is disposed above the level of a liquid piston whose liquid is electrically or electronically conductive. Another cylinder is designated as a “cold cylinder” and gas is disposed in this cylinder also above the level of a liquid piston which is formed by the same liquid as in the hot cylinder. A gas exchange can be performed between the hot cylinder and the cold cylinder respectively through a connection conduit opening at a top side of both cylinders. Through another connection conduit opening at a respective bottom side of the two cylinders liquid can be pumped from a hot cylinder into a cold cylinder or vice versa. A second distributor conduit branches off from the upper gas connection conduit, wherein the distributor conduit is run to a generator which is placed in a type of siphon and in which an electrically or electronically conductive liquid is disposed. When the hot cylinder is mostly filled with gas and the gas is heated by a heat source an expansion occurs and the gas loads the liquid surface through the divider conduit on one side of the magneto-hydrodynamic generator, which causes the magneto-hydrodynamic generator to generate electrical energy from work. After the end of the expansion the hot gas is transferred into the cold cylinder through filling the hot cylinder with the fluid using the magneto-hydrodynamic pump, wherein a volume reduction occurs as a consequence of the cooling and conductive liquid can also flow back into the magneto-hydrodynamic generator. After a subsequent filling of the hot cylinder with cold gas and activating the heat source the process can start again.
The known motor has the advantage that no moving mechanical components like valves, flaps, or similar are required which yields low maintenance requirements and high service life. The gaseous operating medium, however, is not run in a cycle in the known process, but it oscillates back and forth between the two cylinders and includes an open conduit for the generator which is open at its free end towards ambient for utilizing the expansion work.
Thus it is an object of the invention to improve a method for operating a Stirling cycle process and a device for performing a method of this type, so that the efficiency of the power machine process or the figure of merit of the refrigeration machine or heat pump process are increased.
Based on the method described supra the object is achieved in that isothermal compression is performed through a liquid piston compressor and/or isothermal expansion is performed through a liquid piston expander.
Liquid pistons have an advantage over pistons with solid rigid components with exactly defined geometry in that the cylinders in which the compression or expansion process occurs can have any geometry, since the liquid piston always adapts self acting and thus provides absolute tightness for the operating cavity. Therefore cylinders with a very good surface/volume ratio can be implemented, which is not possible for classic pistons with fixed geometry, since the sealing problem would not be solvable in this case. Thus, for example, the cylinder can be permeated by a heat exchanger bundle, so that very large surfaces are obtained for a heat transition between the operating medium and a second medium. The better the heat transition from the operating medium to another medium, the better an isothermal state change can be reached for the compression and also for the expansion. The closer this comes to implementing an ideal isothermal state change, the more the efficiency or the figure of merit of the process approaches the values possible in the respective Carnot process. As a result the method according to the invention can provide significantly improved energy efficiency for the clockwise Stirling cycle process and also for the counterclockwise Stirling cycle process.
The hydraulic fluid forming the liquid piston of the liquid piston compressor, wherein the hydraulic fluid must not be mixable with the operating medium under any circumstances, is pumped by a hydraulic pump with work being added. Accordingly, a hydraulic fluid forming the liquid piston of the liquid piston expander is expanded by a hydraulic motor while performing work. Typically, the liquid piston compressor and also the liquid piston expander operate in the same hydraulic fluid cycle.
According to an advantageous embodiment of the method according to the invention hydraulic fluid exiting from the liquid piston expander alternatively impacts the liquid piston compressor and/or a hydraulic motor and/or it can be stored in a pressure container, from which either the liquid piston compressor and/or the hydraulic motor is loadable with hydraulic fluid.
In order the be able to compensate shifts on a time basis between the expansion process and the compression process a regenerative heat transfer device can be used, through which heat from the operating medium is transferred after isothermal compression in an isochoric manner to the operating medium in particular of the same operating medium cycle, before the operating medium is expanded in an isothermal manner. When no phase shifts have to be compensated, a recuperative heat transfer device can also be used and a heat transfer can be performed to an operating medium of another cycle.
Alternatively thereto it is also possible to run the operating medium in two cycles that are separated from one another from a material point of view and respectively include a liquid piston compressor and a liquid piston expander and wherein heat is transferred in a first heat exchanger in an isochoric manner from the operating medium leaving the liquid piston expander of the first cycle to the operating medium leaving the liquid piston compressor of the second cycle and heat is transferred in a second heat exchanger in an isochoric manner from the operating medium leaving the liquid piston expander of the second cycle to the operating medium leaving the liquid piston compressor of the first cycle, wherein the cycle processes in both cycles are performed phase shifted by half a phase relative to one another. The hydraulic cycles can be implemented separately, but also coupled to one another.
In order to achieve high efficiency or a high figure of merit in a refrigeration machine/heat pump process it is helpful to select a temperature level of the upper (isothermal compression) or expansion as high as possible. In order to avoid problems with thermal stability of the hydraulic fluid in this case, it is useful that two Stirling cycle processes are performed that are separated from one another from a material point of view with respect to their operating media and also with respect to their hydraulic fluids, wherein the lower temperature level of a high temperature process coincides with the upper temperature level of a low temperature process and the heat dissipated during isothermal compression of the operating medium of the high temperature process is absorbed by the operating medium of the low temperature process during its isothermal expansion. In case of a counterclockwise refrigeration machine/heat pump process the heat absorbed by isothermal expansion of the operating medium of the high temperature process is dissipated by the operating medium of the low temperature process during its isothermal compression. In particular a liquid metal can be used as a hydraulic medium for the high temperature process, whereas typically mineral oils are being used for the low temperature process.
From a device point of view the object is achieved through a device according to the invention as described supra in that the compressor is a liquid piston compressor and/or the expander is a liquid piston expander. This facilitates optimizing the energy efficiency of the process by optimizing the heat transfer in combination with the cylinders of the compressor or expander that are configured with the respective sizes.
According to an embodiment of the device according to the invention a hydraulic cycle is provided which is operable by the liquid piston of the liquid piston compressor and/or the liquid piston expander, wherein the hydraulic cycle includes a hydraulic motor and/or a hydraulic pump and/or a container, in particular a pressure vessel. Furthermore a regenerative or recuperative heat transfer device can be used through which heat is transferable from the operating medium after its isothermal expansion to the operating medium after its isothermal compression. In the refrigeration machine/heat pump process the conditions are reversed accordingly.
An improvement from a device point of view is using two liquid piston compressors and tow liquid piston expanders, wherein one liquid piston compressor and one liquid piston expander are respectively tied into an independent operating medium cycle and a heat exchange between the two operating media cycles is performed through at least one heat exchanger tied into both cycles.
In the switching variant recited supra it is also possible that the heat transfer device is jointly formed by the liquid piston compressor of the first operating medium cycle with the liquid piston expander of the second operating medium cycle, wherein the liquid piston compressor and liquid piston expander include common heat exchanger surfaces, so that when the operating medium is expanded in the first operating medium cycle, the operating medium is compressed in the second operating medium cycle and thus with a respective heat exchange between the two operating medium cycles.
Eventually, it is also provided according to the invention to implement a device with eight cylinders, this means a device with four liquid piston compressors and four liquid piston expanders, wherein four groups respectively including a liquid piston compressor and a liquid piston expander respectively include an independent operating medium cycle, wherein hydraulic fluid of all four liquid piston compressors and of all four liquid piston expanders is run in a common cycle with a single hydraulic motor or a single hydraulic pump and the Stirling processes in the four operating medium cycles are preformed with a phase shift of a quarter phase relative to one another.
The method according to the invention and the associated device are subsequently described in more detail with reference to embodiments illustrated in the drawing figured wherein:
An idealized Stirling process illustrated in
For a heat pump/power machine process the same process is performed in an opposite direction (counterclockwise Stirling process). As a result mechanical work is added, whereas mechanical work is generated in a power machine process.
Thus,
During the compression stroke in the liquid piston compressor 2 the hydraulic fluid 5 is pumped into the interior 7 of the cylinder 4 under the require pressure. Thus, the hydraulic fluid is removed from a pressure vessel 12 in the required quantity and run through a motorically actuated valve 13 and a conduit 14 into the inner cavity 7 of the cylinder 4.
After a compression of the operating medium in the liquid piston compressor 2a valve 15 in a conduit 16 and a valve 18 in a conduit 19 are simultaneously opened. Thereafter the operating medium flows through a heat exchanger 17. Therein the operating medium is heated in an isochoric manner and flows onward into the liquid piston expander 3, where an isothermal expansion occurs while lowering the hydraulic fluid level 6 therein. Thus, heat is transferred through a heat transfer medium to the operating medium through a tube bundle 20 and a cavity 21 configured as a double jacket about the cylinder 22.
The hydraulic fluid displaced from the cylinder 22 of the liquid piston expander 3 under high pressure flows through a conduit 23 and the valve 13 into a hydraulic motor 24 which drives a generator 25 for generating electrical energy. The hydraulic fluid then flows through another valve 26 and a conduit 27 into the pressure vessel 12 or through a conduit 28 into the liquid piston compressor 2.
After the isothermal expansion of the operating medium a valve 30 disposed in a conduit 29 opens and the valve 31 simultaneously opens. Thereafter the operating medium flows through the heat exchanger 17 where it transfers heat in an isochoric manner to the operating medium flowing from the liquid piston compressor 2 to the liquid piston expander 3.
The cycle process is completed in that the cooled operating medium flows back into the liquid piston compressor 2 until the level 6 of the hydraulic fluid is at its bottom dead center, so that a new compression stroke can begin after the valve 31 is closed.
Due to the phase shift of the flow through of the heat exchanger 17 it has to be provided in a regenerative configuration. In order to compensate for the cyclic fluctuations in the loading of the hydraulic motor 24 and the generator 25 connected therewith, a flywheel 32 is arranged on the common shaft of the two recited units wherein the large mass of the flywheel sufficiently smoothes the rotation of the generator 25. Sufficient energy is always provided in this manner in order to pump hydraulic fluid into the liquid piston compressor during a compression stroke.
By using the liquid piston compressor 2 and the liquid piston expander 3, the state changes occurring therein are approximated very well to the isotherms of the Stirling process. This is illustrated in
Another embodiment of the device 41 according to the invention according to
In the first operating cycle the operating medium after its compression in the liquid piston compressor 2.1 flows through a conduit 44 to a heat exchanger 43 where it absorbs heat and subsequently moves through a conduit 45 into the liquid piston expander 3.1. From there it flows after expansion through a conduit 46 to a heat exchanger 42 where it dissipates heat. Subsequently the fluid returns again through a conduit 47 into the liquid piston compressor 2.1.
In the second cycle the operating medium after its compression in the liquid piston compressor 2.2 flows through a conduit 48 to the heat exchanger 42 where it absorbs heat and subsequently moves through a conduit 49 to the liquid piston expander 3.2. The operating medium leaves the expander 3.2 after its expansion through a conduit 50 in a direction towards the heat exchanger 43, from which it moves after heat dissipation through a conduit 51 back into the liquid piston compressor 2.2.
Separating the two cycles facilitates simultaneously loading the two heat exchangers which are respectively flowed through by the operating medium, so that simple recuperative heat exchangers can be used.
Based on the different temperature levels in the two operating media cycles also the hydraulic cycles should be materially separated from one another. Thus, selecting a liquid metal as a hydraulic fluid is useful for the high temperature cycle HT, whereas mineral oils can typically be used in the low temperature cycle NT.
This way it is prevented that the hydraulic fluid causes a temperature shift between the high temperature cylinders and the low temperature cylinders. This would negatively influence the temperature diagrams during compression and expansion which would yield very low efficiency.
The two combined hydraulic motors or hydraulic pumps 52.1, 52.2 thus impact separate shafts 53.1, 53.2 respectively with one generator 53.1, 54.2 and one flywheel 56.1, 56.2.
Each hydraulic loop has its own container 55.1, 55.2. When the device 61 illustrated as a power machine in
On the other hand the operating medium is compressed in an isothermal manner in the low temperature cycle NT starting at point IN towards IIN subsequently heated in an isochoric manner towards point IIIN (=IIH). An isothermal expansion occurs from point IIIN to point IVN along the same line IH-IIH which represented the isothermal compression of the high temperature cycle HT. The heat dissipated during the compression in the high temperature cycle HT is thus absorbed during the isothermal expansion occurring in the low temperature cycle NT.
Eventually
Thus a heat exchange occurs in the heat exchanger 84.1 between the operating media of the cycle of the liquid piston compressors/expander 82.1, 83.1 and the liquid piston compressors/expanders 82.3, 83.3 in the heat exchanger 84.2 between the cycles of the liquid piston-compressors/expanders 82.2, 83.2 and the liquid piston compressors-expanders 82.4, 83.4 in the heat exchanger 84.3 between the cycles of the liquid piston compressors/expanders 82.1, 83.1 and the liquid piston—compressors/expanders 82.3, 83.3 and the heat exchanger 84.4 between the cycles of the liquid piston compressors/expanders 82.2, 83.2 and the liquid piston compressors/expanders 82.4, 83.4.
From a hydraulic point of view the hydraulic cycles of the four liquid piston compressors 82.1, 82.282.3, 82.4 on one side and the four liquid piston expanders 83.1, 83.2, 83.3, 83.4 on the other side are separated from one another from a material point of view, so that different hydraulic media can be selected as required. In any case this hydraulic separation prevents a temperature drag between the liquid piston expanders 83.1, 83.2, 83.3, 83.4 operating at a higher temperature level and the liquid piston compressors 82.1, 82.2, 82.3, 82.4 operating at the lower temperature level.
Controlling the four liquid piston compressors 82.1, 82.2, 82.3, 82.4 and the four liquid piston expanders 83.1, 83.2, 83.3, 83.4 is respectively performed through a hydraulic control block 57 on the low temperature side and through a hydraulic control block 58 on the high temperature side. The hydraulic medium in the high temperature cycle impacts a shaft through two hydraulic motors 59, 60, wherein two hydraulic pumps 62, 63 are also arranged on the shaft, wherein the hydraulic pumps supply the liquid piston compressors 82.1, 82.2, 82.3, 82.4 through the hydraulic control block 57 with the hydraulic fluid of the low temperature cycle. A generator 64 is also disposed on the common shaft of the two hydraulic pumps 62, 63 and of the two hydraulic motors 59, 60, wherein the generator has to be replaced with an electric motor when the device 81 is used as a heat pump/refrigeration machine. In the present case in which the device 81 is operated as a power machine heat is absorbed at a high temperature level in the liquid piston expanders 83.1, 83.2, 83.383.4 and dissipated again by the liquid piston compressors 82.1, 82.2, 82.3, 82.4 at a low temperature level. The generator 64 delivers electrical energy. When operated as a heat pump/refrigeration machine the conditions are reversed accordingly. For the purposes of clarity the hydraulic motors 59, 60 disposed on a single shaft and the hydraulic pumps 62, 63 on the two opposite sides of the system diagram are illustrated twice, wherein the units on one respective side of the diagram are drawn in dashed lines and drawn in full lines on the other side.
While the hydraulic motor 59 is used for expanding high pressures at low volume flows it is an object of the hydraulic motor 60 to use the energy which is released during isochoric displacement of the operating medium by the associated heat exchanger into the respective liquid piston expander. Thus, the hydraulic motor 60 is configured for high pressures and large volume flows. The same applies for the pump side. Thus, the pump 62 is configured for feeding small volume flows under high differential pressures and the pump 63 on the other hand side is configured for feeding high volume flows at below pressure differences, as they occur during “push over” of the operating medium from the compressor side to the expander side. The hydraulic blocks 57, 58 and the system control controlling the hydraulic blocks provide that the required hydraulic path is switched at the correct point in time.
It is appreciated that the principle of separating the hydraulic cycles can already be implemented for a “simple” device with two cylinders according to
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 102008042828.0 | Oct 2008 | DE | national |
This patent application is a continuation of International patent application PCT/EP2009/062112, filed on Sep. 18, 2009 claiming priority from and incorporating by reference German patent application DE 10 2008 042 828.0, filed on Oct. 14, 2008, both of which are incorporated herein by this reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2009/062112 | Sep 2009 | US |
| Child | 13065993 | US |
