THERMAL STORAGE INTEGRATED WITH STIRLING MOTOR

The present invention relates to a thermo-mechanical system for transforming thermal energy into mechanical energy .

The Stirling motor is a thermal machine in which a constant mass of fluid is contained inside cylinders, which expands when subjected to heating and which contracts when subjected to cooling, thereby alternately moving force pistons.

According to certain variables such as for example, the displacement, the rapidity with which the heat exchange occurs and the differential temperature between maximum hot and maximum cold, the power produced by such a machine may be varied.

The Stirling motor has a simple construction, which does not need complicated servicing, and the noise emissions during the operation thereof are highly contained. At least such features would make the use of this motor desirable in the field of renewable energy, but the experiments attempted in this background have been unsuccessful to date.

By way of example, the following literature references are mentioned:

Renewable Energy 33 (2008) 77-87, Simulation, construction and testing of a two-cylinder solar Stirling engine powered by a flat-plate solar collector without regenerator ;

- Renewable and Sustainable Energy Reviews 7 (2003) 131-154, A review of solar-powered Stirling engines and low temperature differential Stirling engines.

The conclusions of this second article are particularly fitting for the background of the present invention because they note how the dependency of solar energy alone is not satisfactory mainly due to . the discontinuity of such a source, making the recourse to hybrid solutions inevitable.

The Stirling motor is applied in the field of renewable energy to the Dish Stirling technology, wherein however the thermal storage is inexistent, making the availability of the electrical energy produced uncertain.

The present invention therefore is included in the preceding background, aiming at providing a thermo mechanical system in which at least one hot cylinder of the driving unit is integrated in a thermal storage body which, although being charged by a source in intermittent manner, for example a renewable source, ensures a heating continuity of such at least one cylinder also when the source is not available.

Such an object is achieved by means of a system according to claim 1. The dependent claims thereof show preferred alternative embodiment.

The object of the present invention is now described in detail with the aid of accompanying figures, in which:

-figures 1A and IB show a solar array with a plurality of reflectors and the system according to two possible embodiments comprising a plurality of hot cylinders;

-figure 2 shows a variant of the cooling unit according to a different embodiment, which can be used in the system in figure 1A, figure IB, figure 5A, figure 5B or in figure 6A and figure 6B;

-figures 3 and 4 show alternatives of cooling units according to different embodiments, comprising a single hot cylinder, which cooling units can be used in the system in figure 1A, figure IB, figure 5A, figure 5B or in figure 6A and figure 6B;

- figures 5A, 5B, 6A, 6B show alternatives of heating units of the system of the present invention, according to a second and a third possible embodiment;

figures 7, 10 show further alternatives of the heating units according to different embodiments, which can be used in the system in figure 1A, figure IB, figure 5A, figure 5B or in figure 6A and figure 6B;

- figures 8 and 11 represent an alternative of zone VIII noted in figure 7 and of zone XI noted in figure 10, respectively;

- figure 9 corresponds to the detail in figure 8 according to a different embodiment;

-figure 12 represents the management of the energy through "Power Management System" (PMS) techniques, according to one embodiment .

With reference to the aforesaid figures, a thermo mechanical system comprising a driving unit 10, a heating unit 20 and optionally, at least one cooling unit 30, is indicated as a whole with 1. Said thermo-mechanical system 1 has a so-called upper portion 15 and a so-called lower portion 16. Said driving unit 10 and said heating unit 20 are positioned on said upper portion 15 and on said lower portion 16, respectively, or vice versa.

The driving unit 10 is configured to transform thermal energy into mechanical energy by means of a working fluid expanding and contracting inside a closed circuit.

The driving unit 10 comprises at least one hot cylinder 2 (or "first cylinder") and at least one cold cylinder 4 (or "second cylinder") accommodating the working fluid at mutually different temperatures and fluidly connected by means of the closed circuit.

Thereby, pistons of such driving unit 10 are alternately movable (in particular: translatable) in the respective cylinders 2, 4 due to the different volume of such a fluid at the different temperatures.

It is worth noting that the terms "hot" and "cold" in the present description are to be intended as an at least relative connotation; thus, a so-called "hot" cylinder has a higher temperature with respect to the so-called "cold" cylinder.

By way of example, the hot cylinder could have a temperature higher than about 300°C, optionally higher than about 500°C, optionally again higher than 600°C, for example of about 650-850°C.

By way of further example, the cold cylinder could have an ambient temperature or could have a lower temperature than the ambient temperature or again, higher than the ambient temperature.

According to one embodiment, the driving unit 10 comprises a hot cylinder 2.

According to one embodiment, the driving unit 10 comprises a plurality of hot cylinders 2, for example mutually arranged in parallel (in particular, with respect to the extension of a drive shaft, described later) .

According to one embodiment, the driving unit 10 comprises a single cold cylinder 4.

According to one embodiment, the driving unit 10 comprises a plurality of cold cylinders 4, for example mutually arranged in parallel.

According to one embodiment, the driving unit 10 comprises an equal number of hot cylinders 2 and cold cylinders 4.

According to one embodiment, the pistons accommodated in the at least one hot cylinder 2 and in the at least one cold cylinder 4 are mechanically connected to the above-mentioned drive shaft of the driving unit 10.

According to one embodiment, such pistons are connected to the drive shaft by means of a rod and crank kinematism.

The heating unit 20 comprises an energy receiver 6 and a thermal storage body 8 configured to store the energy of receiver 6 and convey it (for example, by conduction) to the hot cylinder 2.

In other words, the thermal storage body 8 stores the thermal energy input through the energy receiver 6 and makes it available to the hot cylinder 2 or to the plurality thereof. The further result is that the "receiver" is the functional component of the heating unit 20 through which the energy is put into the thermal storage body 8, for example through convection, conduction and/or radiation mechanisms.

In an alternative embodiment (not depicted in the figure) , the thermal storage body 8 stores thermal energy provided by an electrical resistance inserted in the thermal storage body, where the electrical energy available is converted into thermal energy due to the Joule effect.

According to one embodiment, the thermal storage body 8 is monolithic or is obtained by monolithically joining separate body portions, for example modular portions.

In other words, such a variant provides for the thermal storage body 8 to comprise or consist of a mass or a block of a material which, in the light of the physical features thereof, lends itself to thermal storage. Advantageously, such a mass or block substantially does not have moving functional components.

According to one embodiment, the thermal storage body 8 at least partially consists of at least a high thermal capacity material.

It is worth noting that in the present description, "high thermal capacity" means a material characterized in the range of temperatures above 200 °C, for a thermal capacity equal to or greater than 1.40-kJ/kgK, for example for a thermal capacity equal to 1.84 kJ/kg°C.

According to one embodiment, the thermal storage body 8 is at least partially made of materials having high conductivity and thermal capacity such as for example, copper, aluminum, graphite or combinations thereof, and has tetragonal, cylindrical shape with circular or elliptic section. The side surface is thermally insulated with insulating materials such as rock wool, glass materials, mica, pumice and/or combinations thereof. A base is in contact with the receiver body and takes by conduction the heat collected from the receiver body, and the other base is in contact with the hot cylinder 2 and transmits by conduction the heat thereto. The material which forms the thermal storage body is arranged, oriented, positioned, made or poured so that a heat flow according to preferential directions is created therein, from the receiving module towards the hot cylinders and not towards the side walls, which instead promote heat losses .

According to the invention, the hot cylinder 2 is at least partially integrated in the thermal storage body 8.

In other words, at least one cylinder seat 12 delimited by the thermal storage body 8 or associated with/connected to such a body 8 creates a housing for the hot cylinder 2.

That innovatively results in the energy stored in the thermal storage body 8 being made available to the hot cylinder 2 without needing to transport heat at such a cylinder, but such a heating is carried out continuously and in a thermo-dynamically spontaneous manner. According to one embodiment, the hot cylinder 2 is at least partially heated by conduction by the thermal storage body 8.

Indeed, a thermal gradient is established inside such a thermal storage body 8, which decreases from the energy receiver 6 (which has a higher temperature) to the hot cylinder 2, which works at a lower temperature because the heat is then transferred to the working fluid.

According to one embodiment, the heating unit 20 is designed so that the thermal gradient between the energy receiver 6 and the hot cylinder 2 is lower than about 200°C, optionally lower than 180°C, for example about 50- 150 °C .

According to one embodiment, the hot cylinder 2 and the thermal storage body 8 are in direct thermal contact.

In one embodiment in particular, the hot cylinder 2 is at least partially accommodated in a cylinder seat 12 delimited in the thickness of the thermal storage body 8. By way of example, see the alternatives in figure 1A, figure IB, figure 3, 4, or figure 5A and figure 5B.

According to a further embodiment, the hot cylinder 2 is at least partially accommodated in a cylinder seat 12 associated with the thermal storage body 8. By way of example, see the alternative in figure 7 and figure 10.

In other words, according to this embodiment, the integrated nature between the thermal storage body 8 and the hot cylinder originates from the presence of a body 14, which may be defined intermediate or supplementary, in thermal contact with such a body 8 and with such a cylinder 2.

More precisely, the cylinder seat 12 could be obtained in the body 14, advantageously a sintered or a three- dimensionally molded body (or other processing which facilitates accommodating the hot cylinders) made of a material which is equal to or different from the material with which the thermal storage body 8 is at least partially (for example: completely) made.

The heat flow in the alternatives in figure 8 and 9 is mainly directed to the hot cylinder 2, or to the plurality thereof, due to the advantageous shaping of the thermal storage body 8 in the alternative in figure 8, or of the supplementary body 14 alone in the alternative in figure 9. The alternative in figure 9 provides a construction advantage, requiring a shaping of the supplementary body 14 alone and not of the entire thermal storage body 8.

According to one embodiment, such as that represented in figure 9, body 14 could have a tapered geometry in direction away from the thermal storage body 8.

According to one embodiment, such as that represented in figure 10, the hot cylinder 2 and the thermal storage body 8 are in direct thermal contact through a first heat transfer fluid FI.

Thus, according to one variant, the thermal storage body 8 could transfer part of the heat thereof to the first heat transfer fluid FI so that the latter comes into thermal contact with the hot cylinder 2.

According to one embodiment, such as that represented in figure 10, the heating unit 20 could comprise a head portion 56 in which at least one hot cylinder 2 is at least partially obtained, for example a plurality of such cylinders .

According to one embodiment, a fluid chamber 58 configured to accommodate, or be crossed by, the first heat transfer fluid FI could be delimited between the head portion 56 and the thermal storage body 8.

According to one embodiment, the energy receiver 6 is at least partially arranged externally to the thermal storage body 8.

According to one embodiment, the energy receiver 6 could be fastened to a surface of the thermal storage body 8.

More precisely, according to one embodiment, at least one contact surface 18 of the energy receiver 6 lies in abutment with an outer surface 22 of the thermal storage body 8.

According to one embodiment, the energy receiver 6 delimits at least one inlet surface 24 through which the energy is introduced into receiver 6.

According to one embodiment, the inlet surface 24 is at least partially surrounded by a thermal insulating material 26 shaped so as to guide a heat flow from the inlet surface 24 mainly to the thermal storage body 8.

According to one embodiment, the inlet surface 24 is opposite to, or arranged incident to, the contact surface 18.

According to one embodiment, the energy receiver 6 could be at least partially made of one or more materials with a conductivity greater than 25 W/mK, optionally greater than 35 W/mK, for example between 28-460 W/mK or between 28-200 W/mK.

According to one embodiment, the energy receiver 6 could be at least partially made of copper, aluminum, graphite, cast iron, steel, silver, aluminum, ceramic materials, metal carbides, or combinations thereof.

Said energy receiver 6 receives the thermal energy from the solar array or from another natural and/or artificial source and conveys it by conduction towards the thermal storage module 8; said receiver preferably is pyramidal or conical, or truncated-pyramidal or truncated-conical in shape, having a rectangular or circular or elliptic section, with the larger base facing said thermal storage module 8. A cylindrical- or conical shaped opening is obtained on the smaller base of said energy receiver, having a circular or elliptic section, which acts as a black receiving body 7. The shape of said receiver maximizes the transfer by conduction to the thermal storage module 8. The side surfaces of said receiver are thermally insulated with insulating materials such as by way of example, rock wool, glass materials, mica, pumice and/or combinations thereof. The internal opening (black receiving body 7) faces the external environment at the smaller base of the truncated-conical or truncated-pyramidal body and in the absence of an incoming heat flow, is also thermally insulated through a door or plug type device made of insulating materials such as rock wool, glass materials, mica, pumice and/or combinations thereof.

The internal surface of the black body is shaped, processed and/or covered so as to maximize the incoming heat flow to the receiving body and minimize that emitted externally .

According to one embodiment, the thermo-mechanical system 1 could comprise a plurality of solar reflectors 28 arranged and oriented so as to concentrate solar energy on the energy receiver 6. See the schematization in figure 1A for example, which shows a solar array with a plurality of such reflectors 28. Again, see the schematization in figure IB for example, which shows a solar array with a plurality of such reflectors 28, where the inlet surface 24 of said energy receiver 6 is tilted by an angle a with respect to the horizontal plane, where said horizontal plane refers to the contact surface 18 of said thermal storage body 8, allows the optical efficiency to be optimized, optical efficiency meaning the ratio between the radiant energy intercepted by surface 24 and the total energy radiated by the solar array. In a solar farm, where solar farm means an assembly of solar arrays, each solar array being associated with a thermo-mechanical system, the receivers preferably have tilted inlet surfaces 24 with different angles a from each other so as to optimize the optical efficiency of each solar array also in the presence of a non-uniform orography of the site, such as a hilly territory, for example.

According to alternative embodiments of the present invention, such a heat may be directly or indirectly introduced .

According to one embodiment, the thermo-mechanical system 1 could comprise one or more burners 32 configured to directly generate thermal energy by combustion (figures 5A and 5B) and to transfer combustion energy to the energy receiver 6, in the embodiment in figure 5A, or directly to the thermal storage body 8, in the embodiment in figure 5B.

According to one embodiment, at least one burner could be gas-fed, e.g. methane, hydrogen, biofuel, flare gas, tail gas, syngas or combinations thereof.

According to one embodiment, such a burner could be fed with at least one liquid fuel, or with solid, liquid and gaseous waste.

According to one embodiment, the heating unit 20 comprises a heating circuit 34 of the thermal storage body 8 inside of which a second heat transfer fluid F2 circulates .

More precisely, a first zone 36 of the heating circuit 34 is at least partially in thermal contact with an inlet surface 24 of the energy receiver 6, while a second zone 38 of such a circuit 34 crosses at least one heat exchange stage 40 in which at least part of the thermal combustion energy of burner 32 is transferred to the second heat transfer fluid F2.

Whether it originates directly or indirectly, the heat may also be that of the exhaust fumes of a gas turbine, from the combustion gases of an existing furnace; this condition exploits the residual thermal capacity and may provide the addition of a further post combustion .

According to a further embodiment again, the thermal power (heat) may be provided by the products of a high temperature chemical reaction such as for example, the steam methane reforming (SMR) reaction and gasification.

When the heat is indirectly transferred through a heat transfer fluid (figure 6A) or from combustion fumes (figure 6B) , said receiver 6 serves the function of heat exchange module between heat transfer fluid and/or combustion fumes (and/or reaction products) and said thermal storage body 8.

In particular, the heat exchange module comprises means for transferring heat from the working fluid or carrier fluid and is made of a material having high thermal conductivity, such as for example copper, aluminum, graphite for the body, and tubes made of steel, copper, aluminum accommodated inside cavities made in the body; a part of the heat may also be transmitted by radiation between thermal storage module (as seen above) and the body and between the body itself and the tubes. In one embodiment, one or more fluids are in the heat exchange module, which fluids enter and exit and which transfer heat, also at different heat levels, to the thermal storage body 8.

The heat exchange module has one or more sections in contact with the thermal storage module 8 and the remaining outer surfaces are thermally insulated. The heat flow may be interrupted at the sections in contact with the thermal storage module 8, creating a gap in which a gas with a low heat exchange coefficient, e.g. argon, may be caused to flow.

In an alternative configuration, the heat exchange module may consist of a plate-type exchanger and be conveniently machined to come into contact with the thermal storage module 8.

In an alternative configuration again, the tubes may be absent and may be replaced by the same cavities in the heat exchange module when for example, the heat exchange module is obtained through 3D molding techniques (additive manufacturing) ; with the same 3D molding technique again, in addition to having circular section, the cavities may also have rectangular, hexagonal, elliptic section, and other geometrical shapes which optimize the heat exchange; in addition to being rectilinear, the axis of the cavities may also be helical, curvilinear in general. In an alternative configuration again, the heat exchange module may be an exchanger obtained with techniques which are typical of PCHEs (printed circuit heat exchanger) , where instead of having two fluids which exchange heat with each other, there are several fluids entering and exiting, which transfer heat at various heat levels .

According to one embodiment, the thermo-mechanical system 1 comprises at least one cooling unit 30 of the cold cylinder 4 comprising an impeller 42 configured to generate a forced air flow A for directly or indirectly cooling the cold cylinder 4.

According to one embodiment, impeller 42 is directed so that the forced air flow A hits the heat exchange finning 44 of the cold cylinder 4.

According to one embodiment, the cooling unit 30 comprises a coolant circuit 46 which extends from a first zone 48 close to the cold cylinder 4, to a second zone 50 in which the forced air flow A of such an impeller 42 partially hits such a circuit for extracting heat from the liquid.

According to one embodiment, the coolant circuit 46 could contain a water-ethylene glycol mixture.

According to one embodiment, the thermal storage body 8 is at least partially accommodated in a thermal insulating shell 52 comprising one or more gaps 54 which can be crossed by a flow of an insulating fluid, for example in the gaseous state.

By way of example, the insulating fluid could be or comprise argon, helium or nitrogen.

According to the embodiment depicted in figure 12, several thermo-mechanical systems according to the present invention are connected to a system for managing energy through "Power Management System" (PMS) techniques with the possibility of adjusting the power output from the solar array (or system of thermal storage modules in an industrial plant) to the network requirements. Advantageously, this embodiment allows creating a solar farm which supplies power in a manner dependent on the network requirements without the need for pipes.

The system of the present invention may be installed on a dedicated structure or single structure accommodating more than one; the structure (s) may also be a pre-existing structure (s) in a plant, a disused platform or a floor of an industrial plant or of a renewable park.

The present invention further relates to a process for producing mechanical energy carried out in a thermal storage body 8, integrated with a driving unit 10, which uses a working fluid in a closed circuit, comprising a step in which said working fluid is heated by the heat stored by said thermal storage body 8.

Said process comprises the following steps:

- storing the energy of an energy receiver 6 in a thermal storage body 8 and transferring the energy to at least one hot cylinder 2;

- achieving a temperature difference between said at least one hot cylinder 2 and said at least one cold cylinder 4 which are in fluid connection in a closed circuit ;

- triggering a cyclic pulse, turned into alternate movement by pistons contained in the same cylinders, by means of a working fluid expanding and contracting inside said closed circuit;

- optionally, cooling said at least one cold cylinder

4.

Innovatively, the system object of the present invention is capable of achieving the preset purposes, in particular it allows an operating continuity within the daily timeframe.

Advantageously, the system object of the present invention serves as thermal flywheel because the thermal storage body may be sized and/or insulated so as not to cool during the night or in the periods in which there is no energy source. Advantageously, the system object of the present invention has a hot cylinder with a bulky base, having large thermal capacity, which serves as reliable source.

Advantageously, the functions are separate from one another in the system of the present invention, so as not to mutually affect one another in a negative manner.

Advantageously, a receiver arranged externally to the thermal storage body allows an increased amount of thermal energy to be stored, being the overall volume of the system equal.

Advantageously, the thermal storage body could be a spherical body, or a body in the shape of a parallelepiped .

Those skilled in the art could make variations to the embodiments of the aforesaid system and replace elements with others which are functionally equivalent in order to meet specific needs.

Such alternatives are also included within the scope of protection as defined by the following claims.

Moreover, each alternative described as belonging to a possible embodiment may be obtained irrespective of the other variants described. LIST OF REFERENCE NUMERALS

1 Thermo-mechanical system

2 Hot cylinder

4 Cold cylinder

6 Energy receiver

7 Black receiving body

Thermal storage body

10 Driving unit

12 Cylinder seat

14 Sintered or three-dimensionally molded body

15 Upper portion of the thermo-mechanical system

16 Lower portion of the thermo-mechanical system 18 Contact surface

20 Heating unit

22 Outer surface

24 Inlet surface

26 Thermal insulating material

28 Solar reflector

30 Cooling unit

32 Burner

34 Heating circuit

36 First zone

38 Second zone

40 Heat exchange stage

42 Impeller 44 Heat exchange finning

46 Coolant circuit

48 First portion

50 Second portion

52 Thermal insulating shell

54 Gap

56 Head portion

58 Fluid chamber

FI First heat transfer fluid

F2 Second heat transfer fluid

A Forced air flow for cooling