HYDRAULIC ENERGY The present invention relates to an energy cell, a device, and a method for converting heat into hydraulic energy.

More specifically, the invention relates to an energy cell, device, and method that use the properties of a phase change material of which the volume by definition changes with each phase change from a solidified to a molten phase and vice versa, and wherein volume changes of the phase change material are used as a motor for generating hydraulic energy.

For example, such an energy cell is known from US 2011/0024075, which describes an energy cell in the form of a cylinder and a piston movable in the cylinder that seals a chamber in the cylinder that is filled with such a phase change material that, on changing from a solidified to a molten form, expands through heating and thus moves the piston in the cylinder, which movement can be used to exert a mechanical force.

By alternately heating and cooling the phase change material in the cylinder, one can alternately make the phase change material melt and solidify and thus alternately expand and shrink in volume, resulting in an inward and outward movement of the piston, which movement can be converted into a movement for driving a motor or other device.

Another example of an energy cell is known from WO 2015/184516. The energy cell in WO 2015/184516 is provided with a pressure vessel with two chambers separated from each other by an elastic membrane. A first chamber is filled with a phase change material. The energy cell has a shell and tube heat exchanger for alternately heating and cooling of the phase change material, wherein the first chamber alternately increases and decreases in volume so that the membrane tensions and relaxes. A second chamber is filled with a hydraulic fluid. A wall of the pressure vessel is provided with a passage that is configured to allow, at a volume increase and/or volume decrease in the first chamber, a flow of hydraulic fluid respectively out of and/or into the second chamber.

In the energy cell described in WO 2015/184516 A2, the first chamber with the phase change material is located around the tubes of the shell and tube heat exchanger. The tubes of the shell and tube heat exchanger are configured to be connected to a supply of hot medium with a temperature higher than a melting temperature of the phase change material and to a supply of cold medium with a temperature lower than the melting temperature of the phase change material.

Such an energy cell is more efficient than the piston-cylinder embodiment since no friction losses occur when the membrane is tensioned and relaxed.

In order to realize a sufficiently large and uniform heat transfer between a heatexchanging medium in the tubes of the shell and tube heat exchanger and the phase change material in the first chamber of the energy cell, the tubes are preferentially provided on their outer wall with fins radially extending outward from this outer wall.

The presence of these fins makes it possible to keep the structure of the shell and tube heat exchanger compact.

However, a disadvantage of said fins is that they divide the first chamber into compartments, and thus also may cause axial pressure gradients between these compartments if a change in temperature and, consequently, a change in volume of the phase change material in several of the compartments is still not completely uniform. A second possibility to obtain a sufficiently large heat transfer between the medium in the tubes of the shell and tube heat exchanger and the phase change material in the first chamber, is the application of a so-called ‘multipass’ shell and tube heat exchanger in which a flow of heat exchanging medium in the tubes of the shell and tube heat exchanger passes through the first chamber with the phase change material several times in succession, wherein the flow of heat exchanging medium is reversed half a turn after each passage along the first chamber.

As a result, the heat exchanging medium will travel over a longer distance in the tubes of the shell and tube heat exchanger than in a ‘single-pass’ shell and tube heat exchanger where the heat exchanging medium is only routed once through a tube of the shell and tube heat exchanger, so the total heat exchange between this heat exchanging medium and the phase change material in the first chamber in a multi-pass shell and tube heat exchanger is greater.

However, by covering this longer distance in the tubes of a multi-pass shell and tube heat exchanger, the pressure drop in this flow is greater after the flow of heat exchanging medium has passed through the entire shell and tube heat exchanger. On the one hand this is due to friction with inner tube walls, and on the other hand it is also mainly due to friction when the flow of heat exchanging medium reverses after each passage of the heat exchanging medium along the first chamber.

In addition, in a multi-pass shell and tube heat exchanger, the heat transfer between the heat exchanging medium in the tubes and the phase change material in the first chamber is greater, but therefore not necessarily more uniform than in a single-pass shell and tube heat exchanger.

The present invention aims at solving at least one of the said and/or other disadvantages. In particular, the present invention aims at providing an energy cell, device, and method of for the conversion of heat into hydraulic energy, involving the conversion of heat into hydraulic energy in the most energy-efficient way possible, that is with the greatest possible heat transfer and as little pressure and friction losses as possible.

To this end, the invention relates to an energy cell for converting heat into hydraulic energy, which energy cell is provided with a pressure vessel, wherein the pressure vessel comprises two chambers separated from each other by an impermeable and elastic membrane, respectively a first chamber filled with a phase change material of which the density changes at each phase change from a solid to a molten phase and vice versa, and a second chamber that is filled with a hydraulic fluid when the energy cell is in use, wherein the energy cell is provided with means for alternately heating and cooling the phase change material to make the phase change material alternately change from the solidified to the molten phase and vice versa, in such a way that a volume of the first chamber alternately increases and decreases, wherein a wall of the pressure vessel is provided with at least one passage configured to allow a flow of hydraulic fluid to flow respectively out of and/or into the second chamber at a volume increase and/or volume decrease in the first chamber, wherein the means for alternately heating and cooling of the phase change material includes a shell and tube heat exchanger, wherein the shell and tube heat exchanger comprise a number of straight or almost straight tubes, which pass through the first chamber filled with the phase change material, characterized in that the tubes of the shell and tube heat exchanger have an external diameter of not more than 3.0 millimeters. The advantages of such an energy cell according to the invention are numerous.

Firstly, the small outer diameter of the straight or almost straight tubes of the shell and tube heat exchanger allows these tubes to be arranged in large numbers within a limited space at a small mutual distance. In other words, the tubes in the energy cell according to the invention can be arranged in a higher density than in known energy cells.

That way, the tubes within the limited space create a large external heat exchanging surface on their outer walls and a large internal heat exchanging surface on their inner walls.

This allows the use of fin structures on the tubes of the shell and tube heat exchanger in the first chamber to be reduced or even completely eliminated in an energy cell with dimensions that do not need to be excessively larger than those of known energy cells. This way, any axial pressure differences occurring in an energy cell that in many cases does comprise these fin structures on the tubes of the shell and tube heat exchanger can be reduced or even avoided altogether.

In addition, the absence of bends in the straight or almost straight tubes results in only a limited pressure drop in the energy cell.

In addition, for generating hydraulic energy with the energy cell according to the invention, only elastic deformations of the membrane occur, wherein less friction losses occur than with an energy cell that would include a piston cylinder instead of a membrane for generating hydraulic energy.

To further enhance the above advantages, it is of course also possible to opt for a shell and tube heat exchanger with tubes of an even smaller external diameter: preferably a maximum of 2.5 millimeters, more preferably a maximum of 2.0 millimeters, even more preferably a maximum of 1.5 millimeters, still even more preferably a maximum of 1 .0 millimeter.

In a preferred embodiment of the energy cell according to the invention, the tubes of the shell and tube heat exchanger are free of external fins.

As stated above, any resulting axial pressure forces in the energy cell can thus be reduced or even avoided altogether, nor can they act on these external fins.

In a subsequent preferred embodiment of the energy cell according to the invention, the wall of the pressure vessel is implemented as a first tube around a longitudinal axis, wherein openings on the longitudinal axis on both sides of the pressure vessel are hermetically sealed by means of two covers held in the pressure vessel at a distance from each other.

That way, the energy cell is formed as a non-complex structure, which simplifies assembly of the energy cell and maintenance, repair, and/or replacement of components in the energy cell.

In this embodiment, preferably at least one of the two covers is removable, while preferentially both covers are removable, and one or several seals are provided between each removable cover and the pressure vessel.

This makes assembly and maintenance, repair, and/or replacement of the components of the energy cell even easier.

In addition, in this embodiment preferably each of the tubes has a first end mounted sealed in the first cover of the two covers, and a second end opposite this first end mounted sealed in a second cover of the two covers opposite the first cover. Furthermore, in this embodiment, the membrane is preferably also implemented as a second tube which is mounted coaxial within the pressure vessel wall, such that the first chamber is surrounded by the membrane and the second chamber extends around the membrane between the pressure vessel wall and the membrane.

More preferably, the membrane is mounted with its free edges sealed in the two covers or between the covers and the pressure vessel wall.

This allows seals to be located between the tubes and/or the membrane in the covers rather than in the pressure vessel wall. As a result, the pressure vessel wall does not have to be modified to accommodate seals of this type. As a result, a standard tube structure can be used as pressure vessel wall, while maintaining the full mechanical strength of pressure vessel wall.

This even further simplifies assembly and maintenance of the energy cell.

Furthermore, in view of the fact that the hydraulic fluid in the second chamber of the energy cell is on the outside of the tubular membrane, a smaller expansion of the membrane diameter is required for the same volume displacement of the hydraulic fluid than if the second chamber with the hydraulic fluid were to be on the inside of the membrane. This results in less stresses in the membrane and, consequently, it reduces the risk of membrane failure.

In a next preferred embodiment of the energy cell according to the invention, the phase change material has a melting temperature between 25°C and 90°C, preferably between 25°C and 60°C.

The advantage of this is that heat can be recovered from a low temperature residual heat flow, for example a compressed gas flow at 60°C heated by compression heat in a compressor installation. In this embodiment, the phase change material is preferably selected from a group comprising - a wax,

- a fatty acid or a mixture of fatty acids, preferably palmitic acid or lauric acid,

- a glyceride or a mixture of glycerides, or

- a mixture of these. In this context, ‘wax’ refers to a mixture of organic compounds consisting mainly of alkyl chains with 12 or more carbon atoms, which is malleable around ambient temperature, but typically harder and more brittle than fats, and melts around a melting temperature or interval of melting temperatures typically above 35°C to a low-viscous fluid with a dynamic viscosity typically lower than 1000 mPa.s.

Alternatively, the phase change material is preferentially a paraffin, preferentially an alkane with an even number of carbon atoms or a mixture of alkanes with an even number of carbon atoms, and more preferentially octadecane.

In this context, ‘paraffin’ refers to a mixture of alkane chains with 15 or more carbon chains. Wax, a fatty acid, a glyceride, paraffin or a mixture thereof is an appropriate phase change material for an intended application which, depending on the type of wax, fatty acid, glyceride or paraffin used, has a low melting temperature of, for instance around 45°C, and the volume of which increases significantly on changing from the solidified phase to the molten phase and which returns to its original volume on solidification. Moreover, a suitable selection of materials of this type, based on a number of carbon atoms in the molecules in the material, the melting temperature or the melting temperature interval, can be adapted to the intended application.

In this connection, it is not excluded that a different type of material or mixture of materials is used as phase change material, which also has such a low melting temperature, the volume of which increases considerably on changing from the solidified to the molten phase, and that resumes its original volume on solidification.

An alkane with an even number of carbon atoms or a mixture of alkanes with an even number of carbon atoms provides a maximum absolute ratio of the volume change of the phase change material on a phase change relative to latent heat stored or released by the phase change material on a phase change, which absolute ratio will be referred to below as the expansion capacity of the phase change material.

Octadecane offers the advantages of having a relatively great expansion capacity, is compatible with all kinds of metals and has a relatively low melting temperature of 27°C compared to most waxes. Because of the relatively low melting temperature, octadecane is ideally suited for heat recovery from a residual heat flow at a relatively low temperature, because of the relatively large temperature difference between the residual heat flow and the phase change material on a phase change of the phase change material, which temperature difference is the driving force for the heat transfer between the residual heat flow and the phase change material.

Because of the relatively large expansion capacity, octadecane is extremely suitable for a relatively large conversion of heat from the residual heat flow into hydraulic energy. In a next preferred embodiment of the energy cell according to the invention, the tubes of the shell and tube heat exchanger are made of stainless steel, preferably an AISI 304 stainless steel, or copper.

An advantage of these types of materials is their excellent thermal conductivity, which is beneficial to the heat transfer between a heat exchanging medium and the phase change material. Moreover, materials of this type have a good mechanical strength and rigidity, which means that the tubes of the shell and tube heat exchanger can withstand high pressures. Furthermore, these types of materials are characterized by their good workability, which allows smooth production of large numbers of tubes using standard production techniques.

In another preferred embodiment of the energy cell according to the invention, the membrane is made of an elastic material.

Preferably, this elastic material is an elastomer or a composite material or a rubber, preferably a nitrile rubber.

The advantage of a membrane made of an elastic material is that the membrane easily and uniformly follows the volume change of the phase change material in the first chamber, which makes the membrane less prone to failure.

In another preferred embodiment, the tubes of the shell and tube heat exchanger are grouped into one or several modular units, wherein each modular unit has the tubes arranged around a reference axis of this modular unit.

This facilitates the production of parts and assembly on constructing the energy cell, and also facilitates dismantling for maintenance, repair, and/or replacement of parts of the energy cell. Preferably, each modular unit has the tubes arranged in parallel around the reference axis.

This facilitates a dense stacking of the tubes, which is necessary for uniform heat transfer between a heat exchanging medium and the phase change material.

Alternatively, in each modular unit, the tubes are arranged diagonally toward each other around the reference axis.

This makes it possible to choose the distance between neighboring tubes in a modular unit smaller at an outlet of the energy cell than at an inlet of the energy cell. Since on feeding hot heat exchanging medium the temperature of this heat exchanging medium and, consequently, the temperature difference between the heat exchanging medium and the phase change material in the first chamber, is higher at the inlet than at the outlet of the energy cell, heating of the phase change material can thus occur more uniformly over an entire length of the energy cell.

More preferably, in each modular unit, in a plane perpendicular to the reference axis, the centers of the tubes are arranged according to a regular pattern.

In this context, ‘regular pattern’ refers to a pattern in a two-dimensional plane that, according to two intersecting dimensions in this two-dimensional plane, consists of a self-replicating simple figure, such as for instance a triangle or rectangle.

This provides further improvements in the uniform heat transfer between a heat exchanging medium and the phase change material.

Preferentially, the regular pattern is a hexagonal pattern. Even more preferably, in each modular unit, in a plane perpendicular to the reference axis, the centers of neighboring tubes are located at a fixed first distance from each other.

This provides further improvements in the uniform heat transfer between a heat exchanging medium and the phase change material.

Even more preferably, the tubes of the pipe heat exchanger are grouped as several modular units with parallel-oriented reference axes, and in a plane perpendicular to these reference axes, a second distance between the tubes of one of the several modular units and the tubes of a neighboring one of the several modular units is greater than said first distance.

This offers the possibility to create reservoirs between the modular units that continuously contain molten phase change material that is less subject to heat transfer between the heat exchanging medium and the phase change material. These reservoirs that at all times contain molten phase change material can easily collect melting and, consequently, expanding phase change material in the modular units. This way, radial force action of this expanding phase change material on the tubes in the modular units is relieved, thus reducing or even avoiding dislocation of these tubes during expansion of the phase change material.

In a next preferred embodiment of the energy cell according to the invention, the tubes of the shell and tube heat exchanger have a wail thickness of at least 0.075 millimeters, preferably at least 0.080 millimeters, more preferably at least 0.090 millimeters, even more preferably at least 0.100 millimeters.

The advantage of such a minimum wall thickness is that the tubes are resistant to high pressure, typically a pressure of 250 bar or more. In a next preferred embodiment of the energy according to the invention, the tubes of the shell and tube heat exchanger are configured to be connected to

- a supply and discharge of a hot medium, which medium is capable of melting the phase change material, and/or

- a supply and discharge of a cold medium, which cold medium is capable of solidifying the phase change material.

This has the advantage that heat can be used that is recovered from waste flows that are in many cases produced in the form of hot water or the like as by-product of an industrial process, and that is usually lost as unusable heat since the temperature of these waste flows is in many cases insufficient to economically recover energy using existing energy recovery systems.

In this preferred embodiment, the hot medium is preferably a gas flow compressed by a compressor installation.

Here the compression heat that has heated the compressed gas flow in the compressor installation, can be recovered by means of the energy cell according to the invention.

Furthermore, in this preferred embodiment the energy cell is preferably provided with two collectors between which the tubes of the shell and tube heat exchanger extend, wherein each of these two collectors is provided with two connections, respectively a first connection for a hot circuit with the hot medium and a second connection to a cold circuit with the cold medium.

These collectors provide a uniform supply and discharge of hot and/or cold medium over the various tubes, which will improve the uniform heating and/or cooling of the phase change material in the energy cell. Preferably, the two connections of each of the two collectors are provided with a check valve which is configured to alternately provide the shell and tube heat exchanger with hot and cold medium.

The check valve ensures that either hot medium or cold medium flows into the tubes of the shell and tube heat exchanger.

By avoiding that the tubes of the shell and tube heat exchanger would simultaneously receive a supply of hot and cold medium, a uniform temperature and therefore a uniform density of the phase change material in the first chamber is guaranteed, thus avoiding major pressure gradients in the first chamber.

The invention also refers to a device for the conversion of heat into hydraulic energy, characterized in that the device comprises one or several energy cells according to any preceding claim, wherein each shell and tube heat exchanger of one or several energy cells is connected via a valves system to a supply of a cold medium of which the supply temperature is lower than a melting temperature of the phase change material and to a supply of a hot medium of which the supply temperature is higher than the melting temperature of the phase change material, wherein the valves system is configured so that alternately the cold medium and hot medium are each routed through the shell and tube heat exchanger for a specific adjustable duration.

If this device comprises several energy cells, the advantage of such a device is that the supply of cold medium and hot medium does not have to be alternately switched on and off, but instead can alternately be routed to various energy cells, so the supply of cold and hot medium can remain flowing continuously, while the alternating phase changes that are necessary for the operation of the separate energy cells will not come to a halt. In a preferred embodiment of the device according to the invention, the second chamber of the one or several energy cells is connected to a hydraulic circuit for driving a hydraulic consumer.

Preferably the hydraulic consumer is a hydraulic motor used to drive an electric generator.

That way, the device can generate useful electrical energy, for instance for driving parts of the device itself of for other nearby devices in an industrial installation.

In a next preferred embodiment of the invention, the device comprises an even number of energy cells, and the valves system is configured so that during operation of the device every time a first half of the number of energy cells have a supply of hot medium and another second half of the number of energy cells have a supply of cold medium.

That makes it possible that supply of cold and hot medium keeps flowing at the same flow rate.

Moreover, that way, there are always cells that send hydraulic fluid to the consumer so the latter can be driven at greater regularity and always in the same direction.

In the following preferred embodiments of the device according to the invention, the valves system is controllably connected to a controller, which is provided with a means for setting the specific adjustable duration and which is further provided with an algorithm for alternately routing the cold medium and the hot medium through the shell and tube heat exchanger, each during said specific adjustable duration. Such a controller will automatically arrange the control and operation of the valves system with only minimal supervision required by a human operator.

Furthermore, the invention also relates to a method for converting heat into hydraulic energy, characterized in that an energy cell according to any of said embodiments is used.

It goes without saying that such a method offers the same advantages as the advantages of said described embodiments of energy cell according to the invention.

In a preferred embodiment of the method according to the invention, use is made of a device in accordance with any of said embodiments, comprising one or several energy cells in accordance with any of said embodiments, wherein the valves system alternately routes cold medium and hot medium suited cube heat exchanger of the one or several energy cells, each during the specific adjustable duration.

Preferably, the device comprises an even number of energy cells, and during the operation of the device each time the first half of the number of energy cells each has a supply of hot medium and another second half of the number of energy cells has a supply of cold medium.

More preferably, at the same time and with simultaneous periods, the first half of the energy cells has a supply of hot medium and another second half of the energy cells has a supply of cold medium, wherein the supply of the first half of the energy cells and the supply of the other second half of the energy cells is simultaneously switched respectively from hot to cold medium and vice versa. That way, the valves system control is very simple and only has to be performed at a minimum number of times.

Alternatively, more preferably, during a period of twice the specific adjustable duration, the energy cells are switched successively with equal interval periods from a supply of hot medium to a supply of cold medium, wherein the interval period has a duration equal to the period divided by the number of energy cells.

As a result, any transient phenomena that occur when the energy cells are switched from a supply of hot medium to supply of cold medium and vice versa are evenly distributed and, consequently, spread over the above period. As a result, every time a uniform amount of hydraulic fluid is sent to the consumer and this consumer is every time uniformly driven in the same direction.

To better demonstrate the characteristics of the invention, the following describes, as an example without any restrictive character, some preferred versions of an energy cell according to the invention, with reference to the accompanying drawings, wherein: figure 1 is a schematic representation, in perspective, of the energy cell according to the invention; figure 2 shows a cross-section of the energy cell of figure 1 , but with partial omission; figure 3 very schematically represents a device according to the invention that is provided with an energy cell according to the invention with a phase change material in a solidified state; figure 4 shows the layout of figure 3 but with the phase change material in molten state; figure 5 shows a cross-section of an alternative embodiment of an energy cell according to the invention. In this case, the energy cell 1 shown in figure 1 is composed of a tubular pressure vessel 2 with at both ends 21 and 22 a removable cover 3, which in this case is held in the pressure vessel 2 by a retaining ring 4 in the form of a nut.

The pressure vessel 2 is configured to withstand very high pressures, for instance pressures up to 25,000 kPa (250 bar), as a function of the desired pressures in a particular application.

The space demarcated by pressure vessel 2 and by the covers 3 is divided into two chambers by means of a tubular membrane 5, respectively a first chamber 6 surrounded by the membrane 5 itself and a second chamber 7 which extends around the membrane 5 between the pressure vessel 2 and the membrane 5, as best seen in figure 2.

The membrane 5 is made of an impervious elastic material such as rubber, for instance nitrile rubber, or an elastomer or a composite material or similar, and is mounted sealed at each end with a free edge 8 in a relevant cover 3.

For that purpose, said edges 8 of the membrane 5 may be provided with a boss 9 which can serve as an integrated seal and the covers 3, in the embodiment of figure 2, are implemented in two parts with a first part 3A held by said retaining ring 4 in the pressure vessel 2 and a second part 3B mounted in or against the first part 3A with clamping of a said edge 8 with boss 9 of the membrane 5 in a chamber 10 enclosed between both parts 3A and 3B.

The second part 3B of each cover 3 is clamped against the first part 3A by means of bolts 11 or similar.

Seals 12 and 13 are provided between cover 3 and pressure vessel 2 and retaining ring 4. On the outside facing the second chamber 7, the membrane 5 may be provided with one or several ribs 14, in this case circumference ribs 14, with a certain thickness that can act locally as spacers between membrane 5 and pressure vessel 2 and also act as reinforcement ribs 14 of membrane 5. In the example shown, ribs 14 and membrane 5 are made of one piece in the same material, although this is not a strict necessity.

Similarly, on the inside facing the membrane 5, the pressure vessel 2 may be provided with ribs 15 of a certain thickness, wherein these ribs 15 are preferably also designed as circumference ribs 15 and provided opposite the corresponding ribs 14 of the membrane 5.

In addition, the energy cell 1 is provided with a shell and tube heat exchanger 16 in the form of a bundle of tubes 17 which extend axially through the first chamber 6. For a clear representation, the radial dimensions such as the outside and inside diameters of the tubes 17 are shown larger than in reality.

The tubes 17 with their ends 18 may be mounted by means of a sealing O-ring in passages 19 in the respective covers 3. However, it is also possible that the ends 18 of the tubes 17 are welded into the passages 19 in the respective covers 3.

The space around the tubes 17 in the first chamber 6 is filled with a phase change material 23 which, when the energy cell 1 is not used, is in a solidified state and in this state occupies a volume just sufficient to fill the first chamber 6 when the energy cell 1 is empty and not in use or slightly larger than this empty volume of the first chamber 6, so that the membrane 5 in this state is not tensioned in radial direction or only slightly tensioned.

Tubes 17 form a connection between both ends 21 and 22 of the pressure vessel 2, which ends 21 and 22 can act as inlet and/or outlet for a cold or hot medium that can be passed through it for heating up or cooling of phase change material 23 in order to melt or allow this phase change material 23 to solidify.

When the energy cell 1 is in use, the second chamber 7 is filled with a hydraulic fluid 24 from a hydraulic circuit 31 which is hydraulically connected with the second chamber 7 via two connecting nipples 25 each screwed into a passage 26 of the pressure vessel 2 and which are provided with a hood 27 which prevents the membrane 5 from being pushed out of the pressure vessel 2 in a radial direction in connection nipple 25 or passage 26.

The membrane 5 serves as an impermeable separation between the phase change material 23 in the first chamber 6 and the hydraulic fluid 24 in the second chamber 7.

The use of an energy cell 1 according to the invention is very simple and is explained below by figure 3 in which the energy cell 1 is shown as part of a device 28 according to the invention for recovering heat from a feed A of a hot medium having a temperature higher than the melting temperature of the phase change material 23.

This hot medium supply A is connected via a valves system 29 with end 21 of the energy cell 1 , while the other end 22 of the energy cell 1 is connected with a hot medium outlet B after it has been routed through the shell and tube heat exchanger 16 of the energy cell 1.

Similarly, the energy cell 1 is connected through said valves system 29 with a cold medium feed C and a discharge D for the same medium after passage through the shell and tube heat exchanger 16.

The cold medium being fed has a temperature lower than the melting temperature of the phase change material 23. The valves system 29 is such that alternately the cold medium and hot medium can be routed through the shell and tube heat exchanger 16 for a specific adjustable duration.

The valves system 29 is controllably connected to a controller 20. This controller 20 is provided with a setting tool for setting the specific adjustable duration. Furthermore, this controller 20 is provided with an algorithm for alternately routing the cold medium and the hot medium, each during the specific adjustable duration, through the shell and tube heat exchanger 16.

The energy cell 1 is also connected through the connection nipples 25 and another valves system 30 with a hydraulic circuit 31 for driving a hydraulic consumer 32 which, as an example, is represented here as a hydraulic motor 33 for driving an electric generator 34.

The valves system 30 is designed to ensure that the fluid in the hydraulic circuit 31 at all times circulates in the same direction.

The device 28 functions as follows.

Starting from a state as shown in figure 3 wherein the phase change material 23 is in a solidified state, for instance for an initial period of about fifteen seconds, the valves system 29 is controlled in such a way that during this period hot medium from the supply A is sent through the shell and tube heat exchanger 16 to the discharge B while the inlet and outlet C and D of the cold medium are shut off.

The heat of the hot medium heats the phase change material 23 so it starts to melt, thus increasing the volume of phase change material 23 and causing the membrane 5 to be pushed away radially outward, reducing the volume of the second chamber 7 and forcing the hydraulic fluid 24 out of this second chamber 7 into the hydraulic circuit 31 at a pressure that depends on a hydraulic resistance of this hydraulic circuit 31 and in particular on a load demanded by the consumer 32.

During a subsequent period, as shown in figure 4, now the cold medium instead of the hot medium is routed through the shell and tube heat exchanger 16 through a suitable control of the valves system 29.

This causes the molten phase change material 23 to solidify and decrease in volume, allowing the hydraulic fluid to flow back from the hydraulic circuit 31 to the energy cell 1 .

That way, the energy cell 1 functions as it were as a beating heart that alternately sends hydraulic oil to the consumer 32.

In practice, every time a device 28 with an even number of energy cells 1 in the hydraulic circuit 31 will be activated, while the valves system 29 will ensure that during operation of the device 28 every time a first half of the number of energy cells 1 has a supply of warm medium and another half of the number of energy cells 1 has a supply of cold medium.

Figure 5 shows a variant of an energy cell 1 wherein part of the valves system 29 is integrated into a collector 35 with a double connector at each end 21 and

22 of the energy cell 1 in the form of a check valve 36 in each of two connections 37 and 38 provided in each collector 35 for connection to a hot circuit and a cold circuit.

Although in the example shown, the pressure vessel 2 and the membrane 5 are implemented as coaxial cylinders, other forms are not excluded that allow the membrane 5 to be elastically stretched when the phase change material

23 expands. Instead of using a phase change material 23 that expands when melting, it is not excluded to use a phase change material that shrinks when melting.

The present invention is by no means limited to the embodiments described as examples and shown in the figures, but an energy cell according to the invention can be implemented in all shapes and sizes without going beyond the scope of the invention as defined in the claims.