Patent Application
20130192221
This invention concerns a heat machine arranged to operate with an external heat source and an external heat sink, and also to a method of operating such a heat machine. In particular, though not exclusively, this invention relates to Stirling engines (as defined herein), though various aspects of this invention may find application to other reciprocating displacer machines.
In the following, this invention will be described primarily with reference to heat engines operating on cycles approximating to the Stirling cycle, and such engines will be referred to as “Stirling engines”, though it is to be understood that the invention is not limited to such engines. Moreover, when describing a Stirling engine it is to be understood that no practical Stirling engine can operate strictly on the Stirling cycle.
A Stirling engine is a heat engine deriving that heat from an external source. Often, a Stirling engine takes the form of an external combustion engine, but may use other heat sources such as waste heat from another process, or heat from isotopes, the sun or the like. A Stirling engine operates by cyclic compression and expansion of a working fluid in such a way that there is a net conversion of heat energy to mechanical work. The transfer of heat from the heat source (usually a combustion process) to the working fluid in a hot displacer-and-cylinder combination is through the walls of that combination, as is the transfer of heat from the working fluid to a heat sink in a cold displacer-and-cylinder combination.
Mostly, the displacer takes the form of a conventional circular piston working within a bore formed in a cylinder though the displacer could take other forms such as a diaphragm. For convenience throughout this specification reference will be made to engines having a displacer in the form of a piston sliding in a cylinder, but it is to be understood that the term “piston” should be interpreted broadly and so encompass other kinds of displacer. The engine includes a mechanism to cause reciprocating movement of the displacer, and mostly that mechanism has a rotary output shaft by means of which mechanical energy may be extracted from the engine.
In a Stirling engine, the working fluid is normally a gas which is compressed in the colder piston-and-cylinder combination and is expanded in the hotter piston-and-cylinder combination, the compression and expansion taking place by the movement of the pistons within the cylinders. The pistons are coupled to associated crankshafts to achieve the required reciprocating movement thereof and though frequently conventional crankshafts are employed these can have disadvantages. A crankshaft needs to be contained within a relatively large volume for a given working fluid displacement, hence leading to structural problems when the crankcase is pressurised. Also, the piston motion with a crankshaft is less than ideal and performance improvements can be achieved with other kinds of mechanism for connecting rotating and reciprocating components, such as an eccentric mechanism which is capable of producing true sinusoidal motion of the piston. Such mechanisms are employed in specific embodiments of this invention.
It is well known to improve the efficiency of a Stirling engine by employing a regenerator for the working fluid transferring between the hot and cold piston-and-cylinder combinations. A regenerator is a temporary heat store disposed internally of the engine in the path of the working fluid between the hot and cold chambers of the respective piston-and-cylinder combinations. The regenerator retains within the overall system heat which otherwise would be lost to the environment at a temperature between the maximum and minimum cycle temperatures. In this way, the thermal efficiency may approach the limiting value defined by these maximum and minimum temperatures of the cycle.
As a regenerator increases the thermal efficiency, this allows a higher mechanical power output from the engine for a given set of heat exchangers associated with the hot and cold piston-and-cylinder combinations. Conversely, there are losses associated with the incorporation of a regenerator in a Stirling engine and these can be significant. The regenerator increases the unswept volume of the piston-and-cylinder combinations and there are also pumping losses for the working fluid passing through the regenerator. If designed with care, a regenerator can still increase the overall efficiency of the system, as a whole.
Though Stirling engines have been known for nearly 200 years, there has been only very limited commercial take-up of such engines, and they are often regarded as novelty items. Problems associated with Stirling engines include: lack of power density unless a highly pressurised working fluid is employed; sealing problems which become more significant the higher the pressure of the working fluid and also when sliding seals are employed, especially if the crank space is not pressurised; structural weight; volume changes within the crank space if pressurised; heat shunt by the structure and by the crankcase fluid, from the hot piston-and-cylinder combination to the cold; a lack of controllability and especially the lack of rapid response to a change in power demand; the inability of the engine to operate over a wide range of rotational speeds; and a lack of a self-starting capability and reversibility. Further, a Stirling engine needs to reject more heat to the environment than a comparable internal combustion engine, and this leads to higher capital costs, weight and engineering complexity.
Research into Stirling engines has shown that the known designs suffer particularly from three problems. The most important of these is leakage of the working fluid past the seals of the engine, and particularly from the working space (that is, the space within the cylinder on the side of the piston remote from the crank mechanism) into the crank space. It is known to pressurise the crank space in an attempt to reduce that leakage but then there will be pumping losses within the crank space caused by the reciprocating motion of the pistons. The problems of leakage with a Stirling engine can be differentiated from those of an internal combustion engine because on each working cycle of an internal combustion engine, by virtue of the induction of a fresh charge of the working fluid for each power cycle, the conditions within the engine are effectively reset, and the consequence of any leakage within a cycle are restricted to that cycle. As the consequences of leakage are not carried forward to the next cycle, there is no adverse effect on the starting conditions of the next working cycle. By contrast, with an external heat engine having a fixed charge of working fluid, any deleterious effect of normal operation, such as working fluid leakage, is not compensated for within the normal operation and so the effect is cumulative. This leads to a rapid decline in the performance of the engine. Progressive wear over the working life of the engine significantly compounds the problem associated with leakage.
This invention has resulted from research into and development of known forms of Stirling engine, bearing in mind the above known issues of existing external combustion heat engines. Taking as a starting point, the mechanism described in my earlier WO96/23991 (granted as EP-0807219-B) has been developed so as to be capable of operating as a Stirling engine, and in the course of that, further aspects and improvements of the mechanism have been realised, as will be apparent from the following description of the inventive concepts and specific embodiments.
According to one aspect of this invention, there is provided a heat machine operating with an external heat source and an external heat sink and having:
According to a second but closely related aspect of this invention, there is provided a method of operating a heat machine with an external heat source and an external heat sink, the machine having:
As has been described above, leakage of the working fluid past a displacer in a heat engine will lead to greatly reduced efficiency. In part, there is a loss of heat consequent upon the loss of heated fluid by leakage and it is necessary to replenish the lost fluid. That must be done at the operating pressure of the engine and so requires the expenditure of work to drive the fluid into the chambers of the machine.
By having a substantially closed and pressurised casing for the interconnecting mechanism, and controlling the pressure in that casing to be related to (and preferably slightly less than) the sensed pressure in the working fluid chamber on the side of the displacer remote from the casing, the leakage past the displacer can greatly be reduced. Optimally, the pressure in the casing is maintained at a value of not more than, and preferably slightly less than, the pressure in the working fluid chamber though in practice the pressure in the working fluid chamber will be varying cyclically and it may not be possible for the casing pressure accurately to track the pressure in the working fluid chamber. As such, the casing pressure should be maintained at slightly less than the minimum pressure in the working fluid chamber.
By maintaining the casing pressure at below the minimum working fluid pressure, leakage occurs only from the working space into the casing, thus mitigating any movement of oil from the casing spaces to the working spaces. By maintaining the casing pressure to slightly below the minimum pressure in the working fluid, leakage of the working fluid can be minimised. It can be seen that in this invention, if the casing pressure rises due to leakage past the displacer seals and also consequent upon any temperature rise within the casing, fluid is moved out of the casing to maintain the casing pressure at the required value. Also, if the working fluid pressure drops due to leakage past the displacers, fluid can be moved into the working fluid space. The trigger for these fluid movements may be the lowest working space transient pressure when the machine is in operation.
The relative pressures to each side of the displacer may be small compared to the absolute pressure in the working space so that there is only a small pumping requirement to move working fluid from the casing space to the working space. Relatively small reservoirs for low-pressure and high-pressure working fluid may be provided. Preferably a filter is arranged to remove oil from fluid withdrawn from the casing, before the fluid is returned to the working space.
The monitoring and pressure adjustment may be achieved purely mechanically by having automatically operating valve arrangements. In this way, it is possible to achieve stable control of the working fluid pressure and casing pressure. In the alternative, this may be obtained electronically or electro-mechanically, perhaps using a computerised system.
Conventionally, pistons are sealed to cylinder walls by means of rings mounted in grooves formed circumferentially around the head of a piston. With the displacers of this invention taking the form of conventional pistons and with the casing pressure being maintained at just less than the minimum working fluid pressure, such ring seals may be sufficient to minimise working fluid leakage into the casing. It has been proposed to use a known form of annular rolling seal between the displacer and the cylinder, in which a highly flexible annular diaphragm has its inner periphery sealed to the displacer and its outer periphery sealed to the cylinder wall. On account of the working conditions, and in particular the temperatures and pressures prevailing in a Stirling engine, such rolling seals have been found wholly impractical and fail very quickly.
In order to facilitate the maintenance of the required pressure in the casing, it is advantageous to reduce the volume of the casing as much as possible. If the mechanism interconnecting each displacer (piston) with an output shaft is in the form of a conventional crankshaft and connecting rod, a relatively large volume is required within the casing to accommodate that mechanism. Moreover, there will be variations in the volume within the casing as the displacers are moved by the mechanism.
The above problems may be addressed by providing a mechanism in the form described in WO 96/23991, employing eccentrics connected to the mounts for the displacers. This mechanism has the advantage of producing true sinusoidal motion in the displacers and moreover the mechanism may be contained within a casing having a very small internal volume. These measures together facilitate the maintenance of the pressure within the casing to the required value.
Advantageously, means are provided to maintain the pressures in the first and second casings substantially the same and this may be achieved by having a pipe or duct extending between those casings. Alternatively, the first and second casing could be integrated into a single casing.
A preferred form of heat machine of this invention has: a third pair of displacers provided on a common third mount and working in opposed third bores formed in third cylinders; a third casing enclosing a volume between the third pair of displacers; a fourth pair of displacers provided on a common fourth mount and working in opposed fourth bores formed in fourth cylinders; a fourth casing enclosing a volume between the fourth displacers; and a mechanism interconnecting the third and fourth mounts and arranged to maintain a phase angle between the third and fourth pairs of displacers, the mechanism associated with the third and fourth pairs of displacers being arranged to maintain a phase angle with respect to the first and second pairs of displacers.
With this preferred form of machine, the first and third pairs of displacers may have directly linked interconnecting mechanisms disposed in a common casing (that is, the first and third casings are common) and the second and fourth pairs of displacers may have directly linked interconnecting mechanisms disposed in a common casing (that is, the second and fourth casings are common). When this preferred arrangement is arranged as a Stirling engine, the first and third displacer and cylinder combinations may serve as the hot combinations and the second and fourth displacer and cylinder combinations may serve as the cold combinations.
If the first and third casings are integrated into a single common casing and the second and fourth casings are integrated into a further single common casing, means may be provided to maintain the pressures in the two common casings to be substantially the same and this may be achieved by having a pipe or duct extending between those casings. Alternatively, the first, second, third and fourth casings could all be integrated into a single common casing.
By employing a mechanism which produces true sinusoidal motion of the displacers for each of the combinations, the volume within each casing will not change upon operation of the machine and the mean pressure within each casing may be maintained at the required value, just below the minimum pressure within the working space of the machine.
The preferred form of heat machine of this invention as described above has directly linked interconnecting first and second mechanisms respectively for the first and third pairs of displacers and for the second and fourth pairs of displacers. The first and second mechanisms advantageously are coupled together for synchronous operation in order to allow the machine to operate as a Stirling engine. The performance of such an engine may be enhanced by providing means to adjust the phase angle between the first and third pairs of displacers with respect to the second and fourth pairs of displacers, by adjustment of the relative phase of the first and second mechanisms. In addition, a Stirling engine provided with phase adjustment allows the starting characteristics of the engine to be improved by appropriate adjustment of the relative phase of the first and second mechanisms.
From the foregoing, it will be appreciated that in its most preferred form, this invention relates to a Stirling engine having hot and cold pairs of displacers, wherein there are:
It is known to control the power output of a Stirling engine by varying the working fluid pressure. In general, the greater the working fluid pressure, the higher the achievable power output but of course the maximum pressure is limited by other mechanical factors such as the strength of the engine housing and crank casings, seals and so on. The power output of a Stirling engine of this invention as just described above may also be controlled by varying the absolute pressure in the working fluid but in this case the pressure in the casings should also be varied in a corresponding manner, as defined by this invention.
By adopting all of the above measures, it becomes possible to produce a practical embodiment of a Stirling engine, able to self-start and then run efficiently to produce useful output work, with minimal leakage of working fluid past the seals of the displacers. This invention therefore extends to a method of operating such a Stirling engine, in which the pressure in the working fluid to both sides of the displacers is monitored and controlled, and also the relative phase of the hot and cold pairs of displacers is adjusted to optimise engine performance and also the direction of rotation.
By way of example only, certain specific embodiments of Stirling engine, reciprocating piston mechanisms and other engines and pumps constructed and arranged in accordance with the various aspects of this invention will now be described in detail, reference being made to the accompanying drawings in which:
The heat machines to be described hereinafter include mechanisms which are developments of the mechanism described in WO96/23991 aforesaid. Some of these machines are intended to operate as Stirling engines whereas others may operate as pumps requiring mechanical energy input in order to move fluids, and yet others may operate as electrical generators. Reference should be made to WO96/23991 for the basic operating principles of the eccentric mechanism which is incorporated in the machines described hereinbelow, either exactly as has been described in WO96/23991 or in modified forms.
Referring initially to
As shown in
The eccentrics 22 of each mechanism 20 are furnished with an internally-toothed bore 23 eccentric to the outer surface of the eccentrics 22 and which are received in the circular openings 19 of the connecting elements. An output shaft 24 is journalled in the casing 21 and has an externally-toothed gear 25 meshed with the internally-toothed bore 23 of the eccentrics. Rotation of the eccentrics around the output shaft 24 will cause rotation of that output shaft, allowing power to be extracted from the machine.
As mentioned above, the mechanism of
The general operation of a Stirling engine with the transfer of fluid between hot and cold piston-and-cylinder combinations, though not the mechanical arrangement described above, is well known within the field of Stirling engines and will not be described in more detail here.
In a so-called alpha Stirling engine, the pistons of the interconnected hot and cold cylinders making up one pair normally reciprocate at 90° out of phase though it is known to provide a mechanism to allow adjustment of the out-of-phase angle. In the arrangement of
The two output shafts 35 and 36 (equivalent to the output shafts 24 of the
Annular grooves 43A,43B are formed around the central region of the slug 40, groove 43A communicating with an axial passageway extending to the left (in
There are several well known problems associated with Stirling engines as has been discussed hereinbefore. One of those is leakage of the working fluid past the piston seals and also past the output shaft-to-casing seals. Though this merely leads to a loss of efficiency if that working fluid is air, far better efficiency can be achieved with hydrogen or helium as the working fluid, but it is much more difficult to prevent leakage with gases of such low molecular weight. It is moreover more expensive to replenish lost gas of this kind.
The machine described above addresses this problem by providing sealed casings around the eccentric mechanisms and then seeking to maintain the pressure drop across any one piston to a minimum value. It will be appreciated that as each set of four piston-and-cylinder combinations has the combinations arranged in opposed pairs, the pressure variation in the sealed casing will be minimised: as one piston moves from bottom dead centre to top dead centre, the 180° opposed piston moves from top dead centre to bottom dead centre and thus the total volume within the casing should not change but when operating dynamically, due to the movement of the gas within the casing, there will in fact be pressure changes therein. This effect is minimised by use of the eccentric mechanism as described above, since this allows the contained volume within the casing to be minimised. Moreover, the eccentric mechanism controls the movement of the pistons to be truly sinusoidal, unlike the case with a conventional crank and connecting rod assembly.
Conversely to the pressure within the casing, the pressure of the working fluid on the sides of the pistons remote from the eccentric mechanisms will vary, as the working fluid moves from the hot piston-and-cylinder combinations to the cold piston-and-cylinder combinations, and vice versa. This will lead to some leakage past the pistons but in an attempt to minimise that, in accordance with this invention the pressure in each casing is monitored as well as the pressure in each transfer duct, and then the casing pressure is adjusted as required in an attempt to maintain that pressure difference within a narrow band, to minimise leakage. The regime is that the pressure in the casing should always be slightly less than the pressure in the transfer ducts so that any leakage of working fluid will be from the sides of the piston remote from the eccentric mechanisms, into the casing.
In
Referring now to
Pressure transducers 51 are connected to the casing pressure tappings 46,47 and provide electrical inputs to the control unit 49, which typically is in the form of a microcomputer or PLC. Similarly, further pressure transducers 52 are connected to the duct pressure tappings 48 and also provide electrical inputs to the control unit 49. There are further pressure tappings 53 and 54 respectively to the casings and ducts, respective three-position valves 55 being provided on each such further pressure tapping.
The system includes a low-pressure fluid reservoir 56 and a high-pressure fluid reservoir 57 with a pump 58 arranged to transfer fluid from the low-pressure reservoir to the high-pressure reservoir, the pump being optionally driven from the output shaft 24 of the machine. A pressure by-pass valve 59 is arranged across the pump to ensure that the fluid pressure difference between the two reservoirs does not exceed a pre-set value.
The high-pressure fluid reservoir is connected through pipes 60 to one side of the three-position valves 55 and the low-pressure fluid reservoir to the other side of those valves through pipes 61, with the control unit 49 providing a control signal to each of those valves as required. That control signal may maintain the associated valve in a closed setting, or may either allow the introduction of fluid from the high-pressure reservoir into the associated space through the pressure tapping or allow fluid to flow from that space to the low-pressure reservoir.
The control unit 49 is programmed to monitor the inputs from the casing and duct transducers and provide outputs to the valves 55 in an attempt to maintain a pressure regime within the working fluid and casings to ensure that there is a minimum leakage of working fluid from the working spaces of the pistons, into the casings. By maintaining the pressure difference at a predetermined minimal value, that leakage can be minimised. As the pressure in the casings rises due to leakage past the pistons and also on account of a temperature rise when in operation, fluid is moved out of those casings. As the pressure in the working fluid drops due to leakage past the pistons, fluid is moved into the working spaces. The use of the eccentric mechanisms allows the volume in the casings to be minimised and moreover the movement of the opposed pairs of pistons is strictly sinusoidal. As such, pressure variations in the casings are minimised and though the pressure in the working fluid will vary with operation of the machine, the casing pressure may easily be maintained below the minimum working pressure of the fluid. The control unit 49 may operate with an appropriate algorithm to achieve this result.
Also shown in
A control 65 is provided for the valve chest, to switch the operation from the normal configuration as shown in
The arrangement of
The machine is started by setting the control 65 of the valve chest to the start-up configuration and then heat is applied to the machine so that the temperature and pressure of the working fluid rises. Simultaneously there will be some leakage of that working fluid to the casing, so increasing the pressure in that casing. Then, rotation commences on moving the valve chest control 65 to the normal operating position together with the shifting of the phase angle between the hot piston-and-cylinder combinations and cold piston-and-cylinder combinations from 180° to the working angle, which typically will be at or about 90° or at or about 270°, depending upon the required direction of rotation. Operation of the machine should then commence and will continue with automatic pressure adjustment within a stable loop.
With the mechanism of
The mechanism of
The output shaft for this mechanism is shown in
The assembly of one side of this mechanism is shown in
The eccentrics 93,94 are carried on ball races 99 arranged between those eccentrics and the counterweights, with one ball race to each side of the central gear 91. Each counterweight has an off-centre balance weight which will rotate around the axis of the output shaft in synchronism with the rotation of the eccentrics 93,94 and thus serve to balance the reciprocating mass of the sliding elements and the eccentrics, with the counterweights linking together the eccentrics and the output shafts.
The arrangement of
In operation, the eccentrics are driven by the reciprocating movement of the sliding elements, so that the eccentrics rotate around the axis of the output shafts. This rotates the associated eccentric gears 110,111 to drive the output shaft gears 106,107, so effecting rotation of the output shafts. Moreover, the movement of the eccentrics around the output shafts also causes the counterweights to rotate in synchronism with the rotation of the eccentrics, thus balancing the reciprocating masses, with the counterweights linking together the eccentrics and the output shafts.
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 1016522.3 | Oct 2010 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2011/051876 | 10/3/2011 | WO | 00 | 3/28/2013 |
