The invention relates to a plunger pressure accumulator, comprising: an elongated shell; a plunger which is adapted to move in a longitudinal direction of the shell into an interior space of the shell; said space being divided into at least two subspaces, the first subspace of which is suppliable with the hydraulic fluid of an external system and the second subspace is provided with a pressurized gas.

Pressure accumulators are typically used as a part of the energy recovery system in some hydraulic system to improve the hydraulic system's overall efficiency. Hydraulic energy is stored in a pressure accumulator by using a hydraulic fluid of the hydraulic system to compress the pressure accumulator's gas present in one subspace. In a compression phase, the gas temperature rises. Available today are prior known pressure accumulators of various designs of which can be mentioned diaphragm accumulators, bladder accumulators and piston accumulators. These are all identical in terms of basic design and operation, i.e. the shell houses two subspaces, the first subspace of which is supplied with a hydraulic fluid for compressing a gas present in the second subspace. Operation of the diaphragm and bladder accumulator is based on the deformation of a diaphragm or bladder between the spaces, allowing in the compression phase a reduction of the second subspace volume (compression of the gas). These are employed typically for the equalization of pressure fluctuations in hydraulic systems.

The piston accumulator, on the other hand, includes a piston, which is capable of sliding in contact with an internal surface of the shell in response to a force generated by a hydraulic fluid, and which also divides an interior space of the shell into two aforesaid subspaces.

In currently available pressure accumulators, the heat generated in a compression phase strives and begins to flow from pressure accumulator to environment. This constitutes a factor impairing the efficiency of a pressure accumulator. Especially in bladder and diaphragm accumulators, the elimination of this drawback is difficult because of deformations of the diaphragm or bladder. It is prior known to employ a foam type material in the gas space of bladder accumulators as presented on the internet site: httD://www.hvdac.com.au/www.hvdac.com.au/news technews bladderwithfoam.as px. In this solution, after the heat has transferred both into the gas and into the foam type material, the heat generated in the compression phase begins to migrate freely to its surroundings, i.e. into a hydraulic fluid enclosing the bladder. As for piston pressure accumulators, the publication US 8201582 B2 discloses interconnected leaf elements, which are provided between the end of one of the subspaces and the piston and which open in the direction of a piston movement and which function as a compressible regenerator. In one embodiment (fig. 3) of the publication there is presented an insulating layer provided on the internal surface of one subspace and having each leaf element fastened thereto. The piston is also provided with bellows, which operate as a vibration damper and which, due to the design and location thereof (in a direct contact with hydraulic fluid), adversely transfer heat from the gas directly to the hydraulic fluid of the first subspace and further into the environment. Such a structure, in which a complicated compressible regenerator is in contact with and in motion relative to many elements, such the shell, the piston, and the deforming insulator, is susceptible to damage. In addition, this imposes high quality requirements on the entire internal surface of a shell and on the external surface of a piston, which in turn increases manufacturing costs. It is an objective of the present invention to provide a plunger pressure accumulator, in which the aforesaid drawbacks can be eliminated or at least substantially alleviated. An objective of the invention is to provide a plunger pressure accumulator, wherein, with a structure more cost efficient than before, it is possible to maintain the heat generated in one of the subspaces and to release it at a correct mo- ment, for example in a discharge phase.

The aforesaid objective of the invention is attained according to the invention in such a way that between the plunger and the shell is arranged a slide element upon which the plunger is supported to move to a distance apart from an internal surface of the first subspace and from an internal surface of the second subspace, and that the plunger pressure accumulator is provided with at least one regenerator which is stationary relative to the shell or the plunger.

What is achieved with this type of plunger pressure accumulator is that a construc- tion simpler than before enables one of the subspaces (the shell) as well as the plunger to tie provided with a regenerator, for example between the movable plunger and an internal surface of the shell, as the plunger is discrete (not in contact with) from the shell. What is avoided with the plunger pressure accumulator construction according to the invention is an expensive operation of machining the internal shell surfaces into a sliding surface for the plunger, whereby finishing work is not required.

Preferred embodiments of the invention are presented in the dependent claims. These disclose additional features capable of improving the functionality and effi- ciency of a plunger pressure accumulator of the invention.

The invention will now be described more precisely with reference to the accompanying drawings, in which: Fig. 1 shows a cross-section for a plunger pressure accumulator according to one preferred embodiment of the invention, which is provided with a ram, and

Fig. 2 shows a cross-section for a plunger pressure accumulator according to a second preferred embodiment of the invention,

Fig. 3 shows a cross-section for a plunger pressure accumulator according to a third preferred embodiment of the invention, which is provided with an expanded shell and plunger, and

Fig. 4 shows a cross-section for a plunger pressure accumulator according to a fourth preferred embodiment of the invention, which is provided with a heat exchanger and cooling means. Hence, in fig. 1 there is shown a plunger pressure accumulator according to one preferred embodiment of the invention, which is denoted with reference numeral 1. In this embodiment, the plunger pressure accumulator 1 comprises an elongated shell 2 with an interior space defined by its walls. The space is provided with a plunger having its body denoted with reference numeral 3. In this embodiment, the plunger 3 is an elongated hollow sleeve type element with a closed first end. Thus, in this case, the plunger is annular in its cross-section, but the shape is not necessarily limited to that. The cross-section can also be for example a square or rectangle. Likewise, the shell 2 can have a correspondingly deviant cross-sectional shape. Between the plunger 3 and the shell 2 is provided a slide element 2b of the invention, upon which the plunger 3 is supported to move in a space. Therefore, the structure of a plunger pressure accumulator 1 shown in fig. 1 indeed comprises a plunger as far as its piston is concerned. The slide element may consist of one or more annular slide bearing elements 2b disposed successively in a longitudinal di- rection of the shell. In this case, the slide element 2b is made stationary relative to the shell 2, but it can also be made stationary relative to the plunger 3. Fig. 1 presents two slide bearing elements.

Between the plunger 3 and the shell 2 is further provided a sealing element 2c or some other element, which establishes a sealing effect and, together with the plunger 3, divides the space in a lengthwise direction of the shell 2 into two sub- spaces 4 and 5. In this case, the sealing element 2c is made stationary relative to the shell 2, thereby leaving the sealing surface in engagement with an external surface 3' of the plunger 3.

Of these, the first subspace 4 is suppliable with a hydraulic fluid by way of a port 2a provided in connection with the shell 2. The source of hydraulic fluid is typically some external system, including a hydraulic circuit that the plunger pressure accumulator 3 is in communication with. The second subspace 5 is provided with a pres- surized gas. Further in this embodiment, the space inside the walls of the hollow plunger 3 establishes a third subspace 6 which is in communication with the second subspace 5. Hence, the third subspace 6 also contains pressurized gas at a pressure equal to that of the second subspace 5. The pressurized gas consists of a compressible gas. This compression takes place as the first subspace 4 is supplied with an incompressible or substantially incompressible hydraulic fluid from an external system. As a result of this, the plunger 3 or some other corresponding element moves (to the right in fig. 1), thereby reducing a combined volume of the second subspace 5 and the third subspace 6. Another result of this is the warming of pressurized gas in the second and third subspaces 5 and 6. In a preferred embodiment of the invention, the plunger 3 comprises a first insulating layer, which is denoted with reference numeral 10a. The first insulating layer 10a is preferably disposed in engagement with an inner surface of the walls of the plunger 3 so as to cover the entire internal surface of the plunger 3.

The plunger pressure accumulator 3 according to the invention is provided with at least one regenerator, which is stationary relative to the shell 2 or the plunger 3. Fig. 1 shows two regenerators 7a and 7b, which can be alternatives to each other or which can be applied concurrently in the plunger pressure accumulator 3 of the in- vention. It should be noted that these are merely examples of regenerator configurations, nor is the invention limited to these configurations and positions. Thus, the regenerator 7a presents one solution for a type of regenerator which is stationary relative to the shell 2. The elongated regenerator 7a is attached to an end face of the shell 2 and adapted to extend a distance into the third subspace 6. Hence, the first insulating layer 10a provided on the plunger 3 surrounds the regenerator 7a at least partially. As the plunger 3 is moving to the right in fig. 1, the regenerator will be respectively surrounded by the first insulating layer 10a over a longer distance. The regenerator 7a and/or the plunger 3 can also be designed in terms of its length so as to be surrounded by the insulating layer 10a only in a compression phase, i.e. as the piston 3 is being displaced by hydraulic pressure (to the right in fig. 1). The structure and/or material of the regenerator 7a is such that the pressurized gas present in the third subspace 6 is able to flow into the regenerator 7a and, if necessary, through the regenerator 7a into the second subspace 5. As an alternative or in addition to the regenerator 7a, the plunger pressure accumulator 1 may include a regenerator 7b which is stationary relative to the plunger 3. Fig. 1 shows a regenerator 7b, which has a cavity type structure and which is disposed in engagement with the insulating layer 10a so as to have the first insulating layer 10a completely covered by the regenerator 7b. Thus, when the plunger pres- sure accumulator 1 is in a compressed condition, the regenerator 7a (if present), which is stationary relative the shell 2, has moved into a cavity of the regenerator 7b which is stationary relative to the plunger 3. In this embodiment, it is the cavity of the regenerator 7b which establishes the third subspace 6. The insulator layer 10a of the plunger 3 can be alternatively omitted and optionally replaced for exam- pie with a thicker layer of regenerator material, whereby the regenerator 7b will have a greater mass and heat capacity.

In a preferred embodiment of the invention, the internal surface of the shell 2 de- fined by the second subspace 5 is provided with a second insulating layer 10b. This can be implemented in such a way that the second insulating layer 10b is attached to the shell 2 in a manner making it stationary relative to the shell 2. This is made possible in a particularly advantageous way by having the plunger 3 supported to move with the assistance of the slide elements 2b to a distance apart from an inter- nal surface 4a of the first subspace 4 and from an internal surface 5a of the second subspace 5. Accordingly, as opposed to prior known solutions, for the second insulating layer there is left, between the plunger 3 and the internal surface 4a, a space (which therefore in this case is a part of the second subspace 5) in which the second insulating layer 10b can be easily accommodated inside the shell 2

What can be further seen in fig. 1 is one further embodiment for a second regenerator, which is stationary relative to the shell 2. The second regenerator is denoted with reference numeral 8, and it is in this case constructed as a layer of desired thickness around the insulating layer 10b. It should be noted that, from the opera- tional standpoint of the plunger pressure accumulator 1 of the invention, the second insulating layer 10b is not an absolute necessity, which is why the second regenerator 8 can be optionally disposed in a direct engagement with the internal surface 5a of the second subspace 5 of the shell 2. This is made possible by having the plunger 3 supported to move, according to the invention, with the assistance of the slide elements 2b to a distance apart from the internal surface 4a of the first subspace 4 and from the internal surface 5a of the second subspace 5.

The plunger type structure of a plunger pressure accumulator according to the invention enables the use of diverse materials in regenerators. The employed material can be for example a metal, ceramic, composite and/or polymer. It is also possible to use a material, such as paraffin, based on phase transition. In addition, the structure of regenerators can be preferably sintered, mesh-like, fibrous, granular and/or foamy. The implementation of structurally other types of regenerators is possible. The purpose of such structures is to provide an interior space of the shell 2, espe- daily the second subspace 5, at desired locations, with a regenerator sufficient in terms of its thermal capacity, but also in terms of its heat transfer capacity. Particularly the regenerator, which is in communication with a gas of the second subspace 5, as well as with a gas of the third subspace 6, must have an area which is large in comparison with that of the second subspace's internal surface 5a. An objective is to collect from the gas as thoroughly as possible the heat generated during the compression phase and to deliver it back into the gas during a discharge phase or expansion phase. At the same time, there is provided an effective blockage of heat flows towards the shell by binding the heat as well as by using its appropriate structure and materials for impeding and stopping the flow of heat into the shell struc- ture. This objective is attained particularly well with a plunger pressure accumulator construction of the invention, since the plunger 3 does not hinder the positioning of regenerators particularly in the second subspace 5. Depending on the material and structure of a regenerator or regenerators, there will be achieved for the regenerator a surface area which is approximately 10 to 1000-fold compared to the internal surface 5a while the thermal capacity of the regenerator or regenerators is nevertheless sufficient for the recovery of heat generated in the gas. However, the regenerator's surface area with respect to the internal surface 5a can be other than this. Further in a plunger pressure accumulator 1 of the invention, the regenerator 7a, 7b, 8, in terms of its structure and with material selections, can be constructed as a heat transfer device or something like a heat transfer device. Hence, the regenerator 7a; 7b; 8 also works as an element which delivers the heat stored therein as desired. Thus, the regenerator or heat transfer device allows the thermal energy, stored in the regenerator 7a; 7b; 8 in the compression phase of pressurized gas, to be released at the latest when the plunger pressure accumulator 1 terminates its discharge phase. In a typical case, the duration of a discharge phase is 1-60 seconds but, depending on the application and the plunger pressure accumulator's capacity, it may deviate from the aforesaid time frame, being for example 0,5-600 seconds. Naturally, the regenerator or heat transfer device can be constructed so as to deliver thermal energy even after the discharge phase has terminated.

In fig. 1 is shown an elongated plunger, whose length parallel to the direction of motion is about 2,7-fold with respect to its width transverse to the lengthwise di- mension, in this case with respect to the outer diameter of a plunger with circular cross-section. In fig. 3 is shown a plunger pressure accumulator structurally similar to that of fig. 1, but the length to transverse width ratio of its plunger 3 is less, being about 1,4. It can be seen from fig. 3 that, when the shell 2 is dimensioned in keeping with the width of a plunger, the second subspace 5, and the optional third subspace 6 shown in fig. 3, increase in volume considerably with respect to the internal surface 5a (which is the heat transfer area) of the subspace 5. As a result, the shell 2 has even more favorable facilities in its interior space of providing the plunger pressure accumulator 1 with a necessary number of regenerators 7a, 7b, 8. In other words, the mass of regenerators will be sufficient for achieving a desired recovery of heat. For example, the third subspace 6 internal of the hollow plunger 3 can be set up with several layers of regenerators 7b. It is preferred that, in terms of its length parallel to the direction of motion, the plunger 3 be 0,01 to 1000-fold in proportion to its transverse width. Consequently, the plunger 3 of the invention, equipped with the regenerator 7b, can have a width which even exceeds its length parallel to the direction of motion. Generally speaking, such a plunger or floating piston solution provides a cost effective way of constructing truly heavy-duty and structurally durable plunger pressure accumulators because, with the exception of an outer surface of the plunger 3, its structure does not require perfectly finished surfaces, nor are there moving parts between the shell 2 and the plunger 3.

In fig. 2 is shown another preferred embodiment of the invention, wherein the slide element, which is denoted with reference numeral 2b', is structurally different from the annular slide bearing elements 2b presented in figs. 1 and 3. Fig. 2 shows primarily the features essential for understanding this particular embodiment. It can be seen from fig. 2 that the slide element 2b' is preferably a thin-walled tube. The thin- walled tube 2b' extends preferably co-directionally with the motion of a plunger 30 substantially across or all the way across the interior space of the shell 2. In this case, the internal surface of the tube 2b' constitutes a sliding surface upon which the plunger 30 is resting while moving in the interior space of the shell 2. The plunger 30 is depicted here as a piston included in a traditional piston accumulator, but it can also be structurally similar to the plunger 3 shown in fig. 1. To ensure operation as favorable as possible, the slide element 2b' is made of material as small as possible in terms of its specific heat capacity and wall thickness. Depending on the material, it is in any case preferred for the wall thickness not to exceed 0,5 millimeters. A lower limit for the wall thickness is only defined by the characteristics of existing materials as well as those to be developed in the future. The sleeve 2b' is preferably dimensioned in such a way that its outer surface remains a distance apart from the internal surface 4a of the first subspace 4 and from the internal surface 5a of the second subspace 5. This enables a regenerator 8 to be disposed be- tween the outer surface of the sleeve 2b' and at least the internal surface 4a and optionally the internal surface 5a. The sleeve 2b' is dimensioned in the second sub- space 5 so as to leave a gap 2b" between en end face of the shell 2 and an edge of the sleeve. Therefore, in the second subspace 5, and between the sleeve 2b' and the internal surfaces 5a and 5a of the second subspace 5 and the first subspace 4, exists an equal pressure. It is possible to provide the slide element 2b' with separate support elements (not shown) for supporting it on the internal surfaces 5a and 4a. What should also be noted is an embodiment, which has not been shown but in which the sleeve 2b' can have its outer surface dimensioned in such a way that the sleeve's outer surface is in contact with the internal surfaces 4a and 5a. Hence, the distance from sliding surfaces of the piston 30 to the internal surfaces 4a and 5a is at least equal to the wall thickness of the slide element 2b'.

In the embodiment shown in fig. 2, it is possible that at least the second subspace 5, and optionally the first subspace 4, be provided with a regenerator 8 which can be fitted between the outer surface of the sleeve 2b' and the internal surface 5a

(and optionally the internal surface 4a of the first subspace 4). Hence, the regenerator 8 participates in supporting the slide element 2b' in place in a direction transverse to the motion direction of the piston 30. Further in this case, a regenerator 7b, which is thus only presented by way of example, can be arranged to be station- ary relative to the piston 30. In addition to this, to the arrangement of fig. 2 can be applied solutions, which are shown in figs. 1 and 3 and involve regenerators 7a and 7b as well as insulating layers 10a and 10b, and which need not be described in this context. Moreover, the plunger pressure accumulator 1 according to the invention can be provided with other equipment for improving a plunger pressure accumulator of the invention in terms of its functionality, as well as for improving the overall efficiency of a hydraulic external system or other external system communicating with the plunger pressure accumulator. As an example, fig. 4 shows a heat exchanger de- noted with reference numeral 11. The heat exchanger 11 is disposed in the second subspace 5 within a zone between an outer surface of the plunger 3 and an internal surface of the second subspace 5, for example inside a mesh-like regenerator 8 (a heat transfer device). This is made possible by a plunger pressure accumulator of the invention, wherein the motion of the plunger 3 allows the positioning of regen- erators and other stationary additional features. The heat exchanger 11 has a function of bringing thermal energy from an external system into the second subspace 5 so as to replace, whenever necessary, the thermal energy displaced from gas and regenerators or heat transfer devices into the environment (this occurring to a certain extent especially in long compression phases and in the static compression phase of a plunger pressure accumulator). The thermal energy is brought to the heat exchanger for example from the cooling fluid or exhaust gases of internal combustion engines, from power plants based on renewable energy, such as solar energy, and/or from industrial processes in general. Operation of the heat exchanger 11 can be made automatic (active) for example by means of control elements 22a between the heat exchanger 11 and an external system 22, which are used for controlling the supply of thermal energy contained for example in a fluid substance (gas, liquid) to the heat exchanger 11 and thereby into the second subspace 5. The supply of thermal energy can be controlled, for example with the control elements 22a, on the basis of parameters obtained for example from the second subspace 5, such as gas temperature, pressure and/or a working phase of the plunger pressure accumulator 1. Whenever necessary, the heat exchanger 11 enables a removal of heat from the second subspace 5.

Fig. 4 shows also cooling means 12a and 12b. The cooling means 12a consist of cooling ribs or the like disposed on an outer surface of the shell 2 at the first sub- space 4. Alternatively, or in addition to the ribs 12a, in engagement with the first subspace 4, for example on its internal surface 4a, can be disposed conduits 12b for liquid cooling. The purpose of these is to prevent the temperature of a hydraulic fluid present in the first subspace 5 from increasing too much. In other words, the purpose thereof is to cool the hydraulic fluid as may be needed. The thermal energy that has transferred from hydraulic fluid to the cooling means 12a and/or 12b can be recovered and exploited for example in the heat exchanger 11. On the other hand, it is also possible to provide means from the heat exchanger 11 to the cooling means 12a and/or 12b, which can be used, as necessary, for cooling the hydraulic fluid present in the subspace 5. These actions can be controlled for example by means of the control elements 22a the same way as in relation to the heat exchanger 11. It is also conceivable that, in extreme conditions, such as at sub-zero temperatures, it is possible to introduce, particularly by way of the cooling means 12b, a medium capable of warming up the hydraulic fluid.

The present invention is not limited merely to the foregoing embodiments but can be applied within the scope of protection defined by the appended claims.