"Semi-closed cycle internal combustion prime mover and semi-closed thermodynamic process for the production of power"

Description

Field of the Invention

The present invention relates to a semi-closed cycle internal combustion prime mover and a semi-closed thermodynamic process for the production of power. The present invention falls within the field of power production, for example, from fossil fuels, hydrogen or other fuels derived from fossil and/or organic fuels. The present invention can be included within the field of the production of mechanical and/or electrical power on board vessels, typically with powers ranging from hundreds of kW to tens of MW for large naval engines and/or the production of mechanical and/or electrical power in fixed applications, for example, for driving operating machines (pumps, compressors, etc.) and/or for the production of electricity for extended and/or isolated networks, also with powers from hundreds of kW to tens and hundreds of MW per single unit. Background of the invention

Today, in the naval sector, the use of two- or four-stroke Otto and/or diesel cycle engines are known, more in general Otto and diesel cycle engines, gas turbines and Rankine steam cycles fuelled with fossil and/or vegetable fuels and/or hydrogen. Such known prime movers can have application with direct drive of propellers or via an electric generator and electric propulsion motors.

Known instead in fixed applications (fixed power plants) are:

- Rankine steam power plants, also ultra-supercritical, fuelled with coal, oil, gas or biomass;

- combined-cycle plants fuelled with gas or a refined liquid fuel (GTCC), and/or fuelled with coal and/or fuel oil through integration of the gasification process

(IGCC: Integrated Gasification CC);

- simple-cycle gas turbine; - internal combustion engines according to the Otto and/or diesel cycle fuelled with, oil, gasoil, gas and other petroleum derivatives, or vegetable-derived fuels such as rapeseed and sunflower oils, etc.

Recently, due to the ever growing demand for plants that provide for the containment of emissions of CO2 and other pollutants and energy storage needs, systems providing for the use of hydrogen as fuel have been introduced (gas turbines and engines).

Sub-critical and/or supercritical CO2 cycle oxy-combustion systems, which are currently being developed, have also been proposed and are known, though not yet commercially applied (Maximizing the performance of semi-closed O2/CO2 gas turbine combined cycles for power generation, Thesis by Lome G. Allaby - Master of Applied Science in Mechanical Engineering - Carleton University - April 2006; CO2 capture in power plant using oxy-combustion principle - Thesis by Ricardo Llorente Manso - Norwegian University of Science and Technology - July 2013; Semi-closed cycle gas turbine with carbon dioxide-argon as working fluid - Uniza, Pilidis - The American Society of Mechanical Engineers - 96-GT-345). Among them, one may also mention S-Graz cycles (Thermodynamic and Economic Investigation of an improved Graz Cycle Power Plant for CO2 Capture - Sanz et al.

- Institute for thermal turbomachinery and machine dynamics, Graz University of Technology - 2004), the Matiant cycle (Zero emission Matiant cycle - The

American Society of Mechanical Engineers - 98-GT-383 - 1998), the Allam cycle (US7596075), and also internal combustion engines that have been converted or newly conceived to work with a semi-closed cycle (WO 2014/036256 - Enhanced Energy Group LLC).

In this regard the following three systems for the capture and sequestration (and storage) of carbon produced by combustion (Carbon Capture and Sequestration - CCS) (www. ccsassociation.org) are known: pre-combustion capture, post- combustion capture and oxy-fuel combustion.

Also known, albeit for different applications, are closed-cycle noble gas internal combustion engines, functioning according to Otto cycles, and possibly diesel cycles and variants of the same: US 41 12875; WO2016/019357; Killingsworth, Rapp et al.: Increase efficiency in SI engine with air replaced by oxygen in argon mixture, 33rd International Symposium on Combustion, Beijing, China, 2010. These known documents relate to a new type of semi-closed-cycle internal combustion engine that uses a working fluid distinct from air, preferably monoatomic (such as argon, neon, helium or xenon), again with an Otto or diesel- type process or variations thereof. Said processes are always carried out, according to the above-mentioned prior art, with intake, compression, combustion, expansion and exhaust phases in the same volumetric machine, typically piston driven, and therefore, in practice, in the same cylinder.

Constant volume combustion gas turbine cycles are likewise known, illustrated, for example, in the following documents: Constant Volume Combustion: the ultimate gas turbine cycle - Gulen - Gas Turbine World - November December 2013; US2095984; US3525214; US4815294. These known documents refer to gas turbine cycles/processes with combustion at a constant volume or in any case with a pressure increase. In fact, all of the above-mentioned prior art documents refer to open-cycle gas turbine cycles.

With the aim of overcoming the limits of use of constant volume combustors in gas turbine cycles, so-called "free-piston combustors" have been proposed and are known; not being connected to a rotating shaft, by means of crank-connecting rod mechanisms, they can reach very high pumping frequencies such as to enable the attainment of volumetric flow rates that may also be considerable. However, the "free-piston combustor" also needs to have the operating machine, i.e. the compression phase, incorporated within it.

Public document WO2008/065036 discloses a semi-closed CO2 cycle with an expansion turbine preceded by a combustor and mentions the fact that a piston engine can be used as a compression and expansion machine.

Also known in this field is public document WO2014/005229, which, in one embodiment, illustrates an open-cycle system comprising an isothermal compressor configured to receive and compress air, an internal combustion engine, of the two- or four-stroke type, which receives the compressed air and in which a compression, combustion and exhaust cycle takes place, and a heat exchanger configured to recover heat from the exhaust and heat the compressed air coming from the compressor before it enters the engine. A different embodiment, again illustrated in WO2014/005229, shows a closed-cycle system in which air or another gas, e.g. nitrogen, argon, helium or hydrogen, is compressed in an isothermal compressor, passes through a heat exchanger, where it is heated, enters a volumetric adiabatic expander and is then recirculated in the isothermal compressor. Summary

The Applicant has observed that current naval engines have efficiencies of up to as much as 50%, combined cycles up to 58-60% and oil and coal plants up to 45%; however, they do not enable CO2 emissions to be avoided in a manner that has a low economic impact compared to current mechanical or electrical energy production costs.

The Applicant has further observed that several known systems, including those with supercritical CO2 cycles, in particular, are potentially capable of ensuring conversion efficiencies in line with current technologies, but, due to their very high temperatures combined with pressures of hundreds of bars (typically up to 1 ,200°C with 250-300 bar), they are technologically challenging. Furthermore, using turbomachines they introduce very complex problems in order to be able to accept direct combustion also of heavy fuels like fuel oils, etc.

The Applicant has observed, moreover, that the known constant volume combustion gas turbine cycles without an integrated expander are limited in their maximum temperature because of the limits in the materials of the expander, which will in any case see the maximum temperature of the cycle. Due to limits in the materials and cooling techniques, this in fact limits the maximum temperature of the cycle and hence the maximum efficiency.

Furthermore, in normal gas turbines, the volumetric flow rates are very considerable and normally do not allow the use of volumetric machines, as also pointed out in the public document cited above, "Constant Volume Combustion: the ultimate gas turbine cycle - Gulen - Gas Turbine World - November December 2013", on page 3, left-side column, first paragraph, after figure 3.

In this context, the Applicant has perceived the necessity of introducing new prime movers capable of replacing the prime movers presently in use and capable of delivering conversion efficiencies equal to or greater than those in use. The Applicant has perceived the need to propose new prime movers that enable carbon capture and sequestration (CCS) to be achieved without deteriorating, but rather even increasing, efficiency compared to current systems.

More in general, the Applicant has set itself the following objectives:

" to conceive a cycle/process and a prime mover/plant for the production of mechanical or electrical energy, from fossil and non-fossil fuels, with conversion efficiencies equal to or greater than those of the ones in use;

^■ to conceive a cycle/process/ prime mover/plant with the possibility/capability of sequestering the products of combustion (for example CO2, H2O and pollutants) and which thus enable the containment of polluting emissions to be improved;

^■ to conceive a cycle/process/prime mover/plant characterized by fewer irreversibilities and higher efficiency compared to closed or open Brayton cycles, typical of gas turbines;

■ to conceive a cycle/process/prime mover/plant capable of also converting fuels with a high level of impurities and pollutants;

^■ to conceive a cycle/process/prime mover/plant capable of offering what has been specified above with modest manufacturing costs and capable of simultaneously ensuring a competitive cost of electricity production and carbon capture and sequestration;

^■ to conceive a cycle/process/prime mover/plant capable of offering the above using oxygen as the oxidant (oxy-combustion) in some cases and in other cases also air;

^■ to conceive a cycle/process/prime mover/plant that achieve the above with known parts and components already in use or similar to components already in use;

^■ to conceive a cycle/process/prime mover/plant that achieve the above without the need to develop new materials and without having to develop expanders and/or turbines suitable for very high temperature and pressure combinations.

The Applicant has found that the above-mentioned objectives and still others can be achieved by means of a semi-closed internal combustion cycle (preferably with oxy-combustion, ma in some cases also with air, in particular when the working fluid used is nitrogen, with small percentages of argon and other impurities), in which a working fluid is subjected to an intercooled or nearly isothermal compression in a compressor, a recuperative temperature increase in a recuperator, a combustion with a pressure increase in a volumetric expander distinct from the compressor, and a recovery of heat in said recuperator, which is operatively interposed between the compressor and the volumetric expander.

In particular, the above-mentioned objectives and still others are substantially achieved by a semi-closed cycle internal combustion prime mover, a semi-closed thermodynamic process for the production of power and an internal combustion thermodynamic cycle of the type claimed in the appended claims and/or described in the following aspects.

In an independent aspect, the present invention relates to a semi-closed cycle internal combustion prime mover, comprising: a compression device; a volumetric expander (in order to enable combustion with a pressure increase) distinct from the compression device and preferably connected to a generator; first feeding devices connected to the volumetric expander (directly or placed upstream thereof) and configured to feed a fuel into the volumetric expander and/or second feeding devices connected to the volumetric expander (directly or placed upstream thereof) and configured to feed an oxidant into the volumetric expander, wherein the volumetric expander is configured to perform a combustion with a pressure increase and an expansion; a closed circuit in which at least a working fluid circulates; wherein the compression device and the volumetric expander are in fluid communication with each other through said closed circuit; a recuperator disposed in the closed circuit and operatively interposed between the compression device and the volumetric expander; wherein the recuperator is configured to transfer heat from the working fluid coming from the volumetric expander, and directed towards the compression device to the working fluid coming from the compression device and directed into the volumetric expander.

The terminology used above is intended to mean that the oxidant and/or fuel feeding devices may also not be directly connected to the volumetric expander, but instead be connected to the closed circuit so as to feed the oxidant and/or fuel into the closed circuit or into an auxiliary circuit connected to the closed circuit. In an independent aspect, the present invention relates to a semi-closed thermodynamic process for the production of power, preferably implemented with the engine according to at least one of the aspects listed and/or at least one of the appended claims.

The thermodynamic process comprises: compressing a working fluid in a compression device; feeding the already compressed working fluid coming from the compression device into a volumetric expander distinct from the compression device; performing in the volumetric expander a combustion with a pressure increase by feeding an oxidant and a fuel into the volumetric expander and without performing a further compression of the working fluid in said volumetric expander after feeding and before combustion; expanding the working fluid in the volumetric expander; discharging the working fluid from the volumetric expander and feeding it again into the compression device; wherein, in a recuperator, heat is transferred from the working fluid coming from the volumetric expander and directed towards the compression device to the working fluid coming from the compression device and directed into the volumetric expander, so as to heat the working fluid before combustion.

In an independent aspect, the present invention relates to an internal combustion thermodynamic cycle, preferably performed by the prime mover and/or in the process according to at least one of the aspects listed and/or at least one of the appended claims.

The thermodynamic cycle comprises, in succession:

a compression of a working fluid

an isobaric recuperative heating;

a nearly isochoric combustion with a pressure increase;

an adiabatic expansion;

an isobaric recovery corresponding to the isobaric recuperative heating;

an isobaric cooling.

In one aspect, the volumetric expander is configured to receive the already compressed working fluid from the compression device and does not perform any further compression before combustion, but the combustion and subsequent expansion take place therein. The phrase "does not perform any compression of the working fluid" means that compression is absent or is less than 15%, preferably lower than 10%, more preferably less than 5%, of the actual volumetric compression ratio that characterizes the volumetric expander.

In one aspect, the volumetric expander has at least one working chamber and is movable between a first configuration, wherein the working chamber has a maximum volume, and a second configuration, wherein said working chamber has a minimum volume.

In one aspect, the already compressed working fluid is fed when the volumetric expander is in the second configuration or near the second configuration.

In one aspect, an inlet opening for the working fluid is open when the working chamber is in the second configuration or near the second configuration, to allow entry of the fluid under pressure coming from the compression device just before combustion without performing a further compression of the compressed working fluid from the compressor before said combustion.

In one aspect, the inlet opening of the working fluid is opened and then closed when the working chamber is near the second configuration.

In one aspect, the volumetric expander is configured to allow the discharge of the working fluid, after combustion and expansion, without performing any compression of the working fluid.

In one aspect, during expansion, the working chamber moves from the second configuration towards the first configuration.

In one aspect, during discharge, the working chamber moves from the first configuration towards the second configuration.

In one aspect, during discharge, at least one discharge opening of the volumetric expander is open to allow the outflow of the working fluid, after combustion and expansion, and without performing any compression before the following combustion.

In one aspect, the volumetric expander is of the piston type, but could also be of other types, e.g. a Wankel or in any case a volumetric expander in general.

In one aspect, the volumetric expander comprises several pistons/cylinders.

In one aspect, the first configuration corresponds to the bottom dead center (bdc). In one aspect, the second configuration corresponds to the top dead center (tdc). In one aspect, the inlet and discharge openings are provided with valves. In one aspect, the inlet and discharge openings are defined by a single opening, preferably provided with a single valve.

In one aspect, at least part of the working fluid passing through the compression device is cooled.

In one aspect, the prime mover comprises a cooling device operatively associated with the compression device and configured to cool at least part of the working fluid passing through the compression device.

In one aspect, the compression is intercooled.

In one aspect, the compression device comprises a plurality of compressors arranged in series.

In one aspect, the cooling device is operative between the compressors, so as to perform an intercooled compression.

In one aspect, the compression is nearly isothermal, preferably achieved by water spray.

In one aspect, the cooling device comprises a recirculation circuit in fluid communication with the closed circuit and provided with at least one tank and a heat exchanger.

In one aspect, the cooling device comprises nozzles arranged in said at least one tank so as to spray into the tank a liquid phase (preferably water) resulting from the combustion and cooled in the heat exchanger.

In one aspect, the recirculation circuit comprises a plurality of tanks, each in fluid communication with an inlet of a respective compressor.

In one aspect, the recirculation circuit comprises nozzles arranged in the compression device so as to spray into said compression device a liquid phase (preferably water) resulting from the combustion and cooled in the heat exchanger (spray cooling).

In a different aspect, the cooling device comprises at least one heat exchanger configured to exchange heat between the working fluid and a refrigerant fluid (surface cooling).

In one aspect, combustion products, such as, for example CO2, H2O and pollutants, are extracted. In one aspect, the engine comprises at least one extraction apparatus for extracting combustion products, e.g. CO2, H2O and pollutants resulting from combustion and/or contained in the oxidant and in the fuel, and which is operatively associated with the closed circuit and located downstream and/or upstream of the compression device.

In one aspect, systems are provided for the extraction of H2O and any other minor pollutants formed in liquid form during the combustion phase before or after the compression phase.

The proposed system preferably comprises the possibility of extracting CO2 (carbon capture and sequestration - CCS) and falls within the oxy-fuel combustion category. The required oxygen is separated from air prior to combustion and the fuel is burnt in oxygen diluted with recycled exhaust gas rather than with air.

In another embodiment, the cycle envisages that the oxidant is air and that CO2 is extracted together with nitrogen and argon. In such a case, the oxidant is ambient air that is compressed and introduced into the cycle; the oxygen is consumed upon combustion, whereas the nitrogen and argon mix with the working fluid and are then discharged together with the CO2, which is likewise in a percentage that will depend on the number of concentrations.

In one aspect, the second oxidant feeding devices comprise an oxidant air compressor operatively connected to the closed circuit and configured to feed compressed ambient air into said closed circuit.

In one aspect, the prime mover comprises an extraction expander having an inlet operatively connected to the closed circuit in at least one point downstream of the volumetric expander and, preferably, of the auxiliary expander, so as to receive the expanding nitrogen and argon coming out of the closed circuit. In one aspect, the extraction expander is mechanically coupled to the combustion air compressor so as to actuate said compressor through the expansion of the expanding nitrogen and argon.

In one aspect, systems are provided for the separation/extraction of CO2 and possibly other pollutants using different possible methods (cryogenic, molecular sieves, pressure swing adsorption, temperature swing adsorption, simple extraction, etc.) depending on the working fluid and design conditions. In one aspect, an outlet of the extraction expander is connected to a CO2 separation/extraction system, wherein the CO2 is separated so that nitrogen and argon are discharged into the atmosphere.

In one aspect, the compression device comprises at least one turbo-compressor. In one aspect, the compression device comprises at least one volumetric compressor. The volumetric compressor is preferably of the piston type.

In one aspect, there is an auxiliary expansion of the working fluid in an auxiliary expander, after discharge from the volumetric expander, before reintroduction into the compression device and preferably before the recuperator.

In one aspect, there is an auxiliary adiabatic expansion after the adiabatic expansion and before the isobaric recovery.

In one aspect, the expansion work of the adiabatic expansion is greater than the expansion work of the auxiliary adiabatic expansion, the latter corresponding to the compression work.

In one aspect, the prime mover comprises an auxiliary expander, preferably located downstream of the volumetric expander and upstream of the recuperator.

In one aspect, the auxiliary expander is an auxiliary turbo-expander.

In one aspect, the auxiliary turbo-expander is mechanically connected to the compression device in order to receive motion from it.

In one aspect, said turbo-expander is endowed with a variable geometry system

(preferably of the IGV type), for adjusting the pressure upstream thereof, and thus downstream of the volumetric expander, in order to adjust the pressure to the natural discharge value of the volumetric expander (imposed by the geometric expansion ratio) under each load condition.

In one aspect, the working fluid is a noble gas.

In one aspect, the working fluid is monoatomic, preferably selected from the group comprising: Ar, He, Xe, Ne.

In one aspect, the working fluid is biatomic, preferably selected from the group comprising H2, O2 and N2. In the event that oxygen and hydrogen are used, respectively, the oxidant (oxygen) will be the working fluid, and combustion will be facilitated thanks to the large excess of oxidant, whereas in the latter case the fuel will be the working fluid, and in this case as well combustion may be particularly facilitated. In a further aspect, the working fluid is triatomic, preferably CO2.

In a further aspect the working fluid can be a mixture of the above-mentioned fluids, in particular, but not only, an argon/C02, nitrogen/argon/C02 mixture, etc., with percentages optimized in order not only to optimize the cycle perfromance, but also to make the CO2 and pollutant extraction and capture systems more cost- effective and efficient.

Mono-, bi- or tri-atomic fluids enable high efficiencies even with low compression ratios. Mono-atomic fluids above all enable high cooling/heating of the fluid even with modest compression ratios. This makes it possible to have, in the volumetric expander, large temperature increases in isochoric combustion or a pressure increase and high subsequent cooling, even with limited compression ratios. In practical terms, once a maximum discharge temperature value of the volumetric expander has been fixed, for example about 700 - 1 , 100°C, with a monoatomic gas it is possible to have theoretical combustion temperatures even greater than 2,200-2,500°C with consequent high efficiencies.

In one aspect, the working fluid is argon and an air separation system is provided for the production of oxygen.

It shall be noted, moreover, that the cycle with argon has the advantage of not requiring the separation of argon, normally present at a percentage of about 0.93%, from the air in order to produce oxygen for oxy-combustion (or of requiring only a small separation), and this means a lower cost (and/or self-consumption) for the production of oxygen.

It is further noted that in order to facilitate the removal of any combustion products and pollutants (i.e. CO2, CO, NO _x , etc.) and any other gases present in the oxygen or fuel, it is accepted that a percentage of the same may circulate in the plant and in the cycle, so that in reality the cycle works according to a mixture of primary working fluid (noble gas (Ar, Ne, Xe, Kr or CO2, N2, H2) and combustion products and pollutants, typically up to as much as 10-20%. This is determined in order to balance the loss in cycle performance due to the presence of pollutants with the advantage of containing the Capex and Opex costs for oxygen production and combustion product and pollutant separation systems. In one aspect, the working fluid is used under subcritical or transcritical or slightly supercritical conditions at the end of compression to avoid excessively high pressures.

In one aspect, the fuel is selected from the group comprising: natural gas, fuel oil and derivatives, fuels and/or slurry derived from coal, gas from the gasification of coal and/or biomass, H2 and CO.

In one aspect, the oxidant is preferably selected from the group comprising: O2, air and oxygen-enriched air. Oxygen-enriched air is simpler to obtain and more cost- effective.

In one aspect, at the end of compression the working fluid has a temperature ranging from about 50°C to about 150°C.

In one aspect, at the end of recuperative isobaric heating, the working fluid has a temperature ranging from about 400°C to about 800°C and a pressure ranging from about 20 bar to about 150-180 bar.

In one aspect, the combustion temperature is a few thousand degrees.

In one aspect, at the end of expansion, the working fluid has a temperature ranging from about 400°C to about 800°C.

In one aspect, it is envisaged to adjust the minimum pressure of the cycle and thus the volumetric flow rates in play.

The prime mover/process/cycle of the present invention, unlike Brayton cycles or gas turbine cycles or supercritical CO2 cycles, comprises a combustion/heating with a pressure increase or which is nearly isochoric; unlike Otto and diesel cycles and derivatives thereof, it comprises a recuperator which heats the cold fluid entering the volumetric expander with the fluid discharged from the volumetric expander and/or auxiliary expander; unlike Otto and diesel cycles and the like, it can have an intercooled compression.

The Applicant has verified that the present invention is capable of offering performances that match or are superior to those of the known cycles/processes and plants and of ensuring an easy recovery of a large part of the combustion products, i.e. typically H2O and CO2, in addition to any other minor pollutants. The prime mover, process and cycle according to the present invention thus effectively meet modern needs to reduce greenhouse gas emissions by enabling carbon capture and sequestration (CCS) and the containment of pollutant emissions, as well as the recovery of water vapor from combustion and the consequent possible recycling of the water.

Additional features and advantages will become more apparent from the detailed description of preferred, but non-exclusive, embodiments of a semi-closed cycle internal combustion prime mover, a semi-closed thermodynamic process for the production of power and an internal combustion thermodynamic cycle in accordance with the present invention.

Description of the drawings

This description is provided herein below with reference to the attached drawings, which are provided solely for purpose of providing approximate and thus non- limiting examples, and of which:

^■ figure 1A illustrates a T (temperature) - S (entropy) diagram of a thermodynamic cycle according to the present invention;

■ figure 1 B illustrates a P (pressure) - V (volume) diagram of the thermodynamic cycle of figure 1A;

^■ figure 1 C schematically illustrates a semi-closed cycle internal combustion prime mover according to the present invention, configured to perform the thermodynamic cycle as per figures 1A and 1 C;

■ figure 2A illustrates a T (temperature) - S (entropy) diagram of a variant of the thermodynamic cycle according to the present invention;

^■ figure 2B illustrates a P (pressure) - V (volume) diagram of the thermodynamic cycle of figure 2A;

^■ figure 2C schematically illustrates a variant of the semi-closed cycle internal combustion prime mover according to the present invention, configured to perform the thermodynamic cycle as per figures 2A and 2C;

^■ figures 3, 4 and 5 schematically illustrate some elements of further variants of the semi-closed cycle internal combustion prime mover according to the present invention;

■ figures 6A, 6B, 6C illustrate respective operating phases of a volumetric expander belonging to the prime movers of the preceding figures;

^■ figure 7 illustrates a further variant of the prime mover of figure 1 C. Detailed description

With reference to figure 1 C, the reference number 1 denotes in its entirety a semi- closed cycle "oxy-combustion" internal combustion prime mover according to a first embodiment of the invention.

The prime mover 1 comprises a first, second and third turbo-compressor 2, 3, 4 coaxial to one another and operatively connected to a common shaft. The turbo- compressors 2, 3, 4 are preferably single-stage centrifugal compressors but may also be multi-stage in various configurations. In some cases, one or more compression stages, typically the high pressure one (the third 4), can work under supercritical conditions with a saturated intake fluid. Said turbo-compressors 2, 3, 4 together define a compression device 5.

The prime mover 1 comprises a volumetric expander 6 which, as illustrated, is distinct from the compression device 5 and is of the piston type. The volumetric expander 6 comprises a cylinder 7 inside which a piston 8 slides; the piston is connected, by means of a crank-connecting rod mechanism, to a power shaft connected in turn to a driven machine and/or an electricity generator 9. The piston 8 and cylinder delimit a working chamber 10. As illustrated in figures 6A, 6B and 6C, the piston 6 is movable between a first configuration (figure 6C), wherein the working chamber 10 has a maximum volume and the piston is in the bottom dead center (bdc), and a second configuration (figure 6B), wherein said working chamber has a minimum volume and the piston is in the top dead center (tdc). In other embodiments, the volumetric expander can also have several pistons/cylinders, in various configurations, also to minimize the downstream and upstream pressure pulses and enable a more stable operation and better coupling with any turbomachines that may be present upstream and/or downstream.

An auxiliary expander 1 1 defined by a turbo-expander is mechanically connected to the shaft of the compression device 5 so as to define a compressor-expander or "compander". Preferably, all of the rotors of the "compander" are mounted on a single shaft.

The compression device 5, the volumetric expander 6 and the auxiliary turbo- expander 1 1 are in fluid communication with one another through a closed circuit 12, which comprises respective pipes and in which a working fluid flows. In particular, a discharge opening 13 of the volumetric expander 6, provided with a respective valve, communicates via a respective pipe with an inlet 14 of the auxiliary turbo-expander 1 1 ; an outlet 15 of the auxiliary turbo-expander 1 1 communicates, via a respective pipe, with an inlet 16 of the first compressor 2; an outlet 17 of the first compressor 2 communicates, via a respective pipe, with an inlet 18 of the second compressor 3; an outlet 19 of the second compressor 3 communicates, via a respective pipe, with an inlet 20 of the third compressor 4; an outlet 21 of the second compressor 3 communicates, via a respective pipe, with an inlet opening 22, fitted with a respective valve, of the volumetric expander 6.

The auxiliary turbo-expander 1 1 can be provided with a variable geometry system 1 1 a with IGV (inlet guide vanes) in order to optimize the pressures between the volumetric expander 6 and auxiliary turbo-expander 1 1 and avoid losses at the time of opening the discharge valve (figure 6C).

First feeding devices 23 are connected to the volumetric expander 6 and are configured to feed a fuel (for example gasoil or other petroleum derivatives) into said volumetric expander 6. In the embodiment illustrated in figure 1 C, the first feeding devices 23 are connected both to the pipe of the closed circuit 12 located immediately upstream of the inlet opening 22 and directly to the cylinder 7.

Second feeding devices 24 are connected to the volumetric expander 6 and are configured to feed an oxidant (for example oxygen) into said volumetric expander 6. In the embodiment illustrated in figure 1 C, the second feeding devices 24 are connected both to the pipe of the closed circuit 12 located upstream of the inlet opening 22 and directly to the cylinder 7.

The volumetric expander 6 is configured to perform a combustion with a pressure increase and an expansion, as will be better detailed below.

A recuperator 25 is disposed in the closed circuit 12 and operatively interposed between the compression device 5 and the volumetric expander 6. The recuperator 25 is for example of the surface or heat pipe type. A number of recuperators in series can also be provided.

Figure 1 C illustrates the use of an additional recuperator 25' typically located above the head of the volumetric expander 5, which makes it possible to have outlet temperatures of the volumetric expander 5 (typically completely cooled) even exceeding 750°C, without the need for conduits and tubing and exchangers with particularly difficult and costly materials. In the recuperator 25, the pipe that extends between the outlet 15 of the auxiliary turbo-expander 1 1 and the inlet 16 in the first compressor 2 is operatively coupled to the pipe that extends between the outlet 21 of the third compressor 4 and the inlet opening of the volumetric expander 6. The recuperator 25 is configured to transfer heat from the working fluid coming from the volumetric expander 6 and directed towards the compression device 5 to the working fluid coming from the compression device 5 and directed into the volumetric expander 6.

The prime mover 1 comprises a cooling device 26 operatively associated with the compression device 5 and configured to cool at least part of the working fluid passing through the compression device 5.

In the embodiment of figure 1 C, the cooling device 26 comprises a first, a second and a third heat exchanger 27, 28, 29 placed on the pipes of the closed circuit 12 which extend between the recuperator 25 and the third compressor 4. The first, second and third heat exchangers 27, 28, 29 are respectively located: between the recuperator 25 and the first compressor 2, between the first and second compressors 2, 3 and between the second and third compressors 3, 4. The cooling device 26 is operative between the compressors 2, 3, 4 in order to perform an intercooled compression of the working fluid. In each of the heat exchangers 27, 28, 29, the working fluid exchanges heat with a cooling fluid (surface cooling). The prime mover 1 further comprises a system 30 for the extraction and treatment of water (H2O) and, if necessary, of other minor pollutants that form in the combustion phase and or have precipitated and/or dissolved in liquid form. In figure 1 C, the extraction and treatment system 30 is operatively connected to the pipes located immediately downstream of each exchanger 27, 28, 29.

Also provided for is a system 31 for the separation/extraction of carbon dioxide (C02) and any other pollutants. In figure 1 C, the separation/extraction system 31 is operatively connected to the pipes located downstream of the third compressor 29 and upstream of the recuperator 25, but it could also be placed in connection with other points of the cycle.

A storage tank 32 for the working fluid is connected, by means of respective pipes and valves, to the closed circuit 12 upstream and downstream of the compression device 5 in order to allow adjustment of the pressure in said closed circuit 12. Figures 1 A and 1 B illustrate the thermodynamic cycle of the prime mover 1 in T-S and P-V diagrams.

In accordance with the cycle and process according to the present invention, the working fluid (for example argon) enters the first compressor 2 (point A) and follows the following cycle/process steps.

- Intercooled compression from A to B (in the compression device 5 and in the second and third heat exchangers 28, 29), with cooling/discharge of the resulting heat. As shown in figures 1A and 1 B, the compression process is divided into several phases (three in the case represented), with three compressions, ideally adiabatic, and a number of intercooling phases (two in this specific case) in between. The three vertical dashed lines from A to B', A' to B", and A" to B graphically represent the three adiabatic compressions, whilst the lines from B' to A' and B" to A" represent the two intercooling phases. This makes it possible to minimize the increase in the temperature of the gas during compression and to contain, accordingly, the compression work/power, which, as is known, depends on the specific volume of the gas, which increases with temperature. At the end of compression the working fluid has a temperature ranging from about 50°C to about 150°C. This is particularly useful where the working fluid is CO2 with a compression point close to the critical point and a hypercritical pressure at the end of compression.

- Recuperative isobaric heating from B to C (in the recuperator 25) by cooling of the hot flow discharged from the expander from E to F. At the end of recuperative isobaric heating the working fluid has a temperature ranging from about 400°C to about 800°C and a pressure ranging from about 20bar to about 150-180bar.

- Combustion from C to D (in the volumetric expander 6). Depending on the combustion rate (flame speed) of the fuel used, the geometry of the working chamber, the number of revolutions, the ignition point and other secondary parameters, the combustion will be of the nearly isochoric type, with a possible increase in volume during combustion but in any case always with a pressure increase from C to D. The D point in the diagram in figure 1 C is inside the working chamber 10 and has not been represented. In this case, in the T-S diagram, the process line is represented by a line comprised between the isobar corresponding to the pressure C below and the ideal isochore corresponding to the volume in C. The combustion temperature is also above 1 ,500°C in the case of C0 ₂ , but even as high as 2,000-2,500°C in the case of a noble gas, e.g. argon.

- Adiabatic expansion from D to E, divided into a primary expansion between D and E' (in the volumetric expander 6) and a secondary expansion between E' and E (in the auxiliary expander 1 1 ) represented in the ideal case by a vertical line and in the real case by a nearly straight and nearly vertical line. The cycle is characterized in that the expansion work between D and E' is greater than the expansion work between E' and E, the latter corresponding to the compression work between A and B. At the end of expansion the working fluid has a temperature comprised between about 500°C and about 900°C.

- Isobaric recovery from E to F (in the recuperator 25), with heating of the cold pressurized flow from B to C.

- Cooling (ideally isobaric) from F to A (in the first heat exchanger 27).

With reference to the part of the process carried out by the volumetric expander 6 (figures 6A - 6D), the working fluid, already compressed by the compression device 5 and coming from the recuperator 25, is fed into the working chamber 10 of the volumetric expander 6 (intake) when the volumetric expander 6 is in the second configuration or near the second configuration (figure 6A). For example, the inlet opening 22 is opened (by means of the opening of an intake valve) when the piston 8 is about to reach the top dead center (tdc) and is subsequently closed again (figure 6B) when the piston 8 is at the top dead center (tdc). Based on the technical specifications, the working fluid intake phase can in any case take place through the opening of the intake valve just before or just after or at the top dead center and with the closure of the same at or just before or just after the top dead center.

The combustion phase (C - D) takes place when the volumetric expander 6 is in the second configuration or near the second configuration (figure 6B), with the inlet and discharge openings closed (intake and discharge valves closed) and, as already expressed, it takes place with a pressure increase. Depending on the fuels used, the combustion may be with controlled ignition (spark plug or pre-chamber or another means) or spontaneous. The fuel can be injected by the first feeding devices 23 into the closed circuit 12 before entering the working chamber 10 or directly into the working chamber 10 of the expander 6 (for example by means of the injector 100 illustrated in figure 6B), as can the oxidant, by means of the second feeding devices 24 (figure 1 C). In this regard, the combustion and ignition systems may be among the wide variety also in use in known engines. In some cases, it could be useful to mix the fuel with water to favor atomization and, in such a case, the water can be recycled by means of the extraction and treatment system 30. In some cases different ignition systems can be used, controlled or not controlled, also with pre-chambers, etc.

As may be noted, inside the working chamber 10 and just before combustion, the volumetric expander 6 substantially does not perform any compression of the working fluid. The phrase "substantially does not perform any compression of the working fluid" means that compression is absent or is less than 15%, preferably less than 10%, more preferably less than 5%, of the actual volumetric compression ratio that characterizes the volumetric expander 6.

The primary expansion between D and E' takes place in the volumetric expander 6 while the inlet opening 22 and discharge opening 13 are closed and the working chamber 10 moves from the second configuration (tdc) towards the first configuration (bdc). The opening of the discharge valve and movement of the piston 8 from the bottom dead center to the top dead center subsequently determine the discharge of the working fluid, which is thus fed into the auxiliary expander 1 1 . During discharge, the working fluid, after combustion and expansion, is discharged without any compression being performed before the following combustion.

In the variant of the internal combustion prime mover 1 of figure 2C, the compression device 5 comprises, in place of the three turbo-compressors 2, 3, 4 of figure 1 C, a volumetric compressor of the piston type and has no auxiliary expander 1 1 . The elements corresponding to those in figure 1 C have been numbered with the same numbers and will not be described again in detail below. A tank 34 of a cooling device 26 is located on the pipe of the closed circuit 12 extending between the discharge opening 13 of the volumetric expander 6 and an inlet opening 33 of the volumetric compressor 5 and downstream of the recuperator 25. In this embodiment, the cooling device 26 comprises a recirculation circuit 35 in fluid communication with the closed circuit 12 and provided with said tank 34 and a heat exchanger 36.

In detail, the tank 34 is placed in line on the pipe of the closed circuit 12 and has an inlet 37 for the working fluid coming from the recuperator 25 and an outlet 38 in fluid communication with the inlet opening 33 of the volumetric compressor 5.

The recirculation circuit 35 is connected to a lower outlet 39 of the tank 34 and a plurality of nozzles 40 arranged inside the tank 34 near an upper portion thereof. The heat exchanger 36 is placed on the recirculation circuit 35 and a first pump 41 is operatively located between the lower outlet 39 and said heat exchanger 36. Moreover, the water extraction and treatment system 30 is operatively connected to the recirculation circuit 35, for example downstream and upstream of the first pump 41 .

In the tank 34, the liquid phase and gaseous phase of the working fluid with the combustion products coming from the volumetric expander 6 are separated. The liquid phase is recirculated in the recirculation circuit 35 and cooled in the heat exchanger 36 and then again sprayed into the tank 34 by means of the nozzles 40. The water resulting from the combustion and contained in the liquid phase can be extracted by means of the extraction and treatment system 30, treated to remove pollutants and at least in part reintroduced into the recirculation circuit 35. The recirculation circuit 35 further comprises an auxiliary branch 42 provided with a respective second pump 43. The auxiliary branch 42 branches off from a main portion of the recirculation circuit 35 downstream of the heat exchanger 36 and terminates with nozzles 44 placed in the compression device 5 in order to spray into said compression device 5 the liquid phase (preferably water) resulting from the combustion and cooled in the heat exchanger 36 (spray cooling). In particular, the nozzles 44 of the compression device 5 open in a compression chamber 45 delimited by a cylinder 46 and a piston 47 of said volumetric compression device 5.

The internal combustion prime mover 1 of figure 2C also comprises a separator/demister 48 placed in line on an auxiliary circuit 49 that connects a pipe of the closed circuit 12, which extends between an outlet opening 50 of the volumetric compressor 5, with the tank 34 of the cooling device 26.

The storage tank 32 for the working fluid of the prime mover of figure 2C is connected, by means of respective pipes and valves, to the closed circuit 12 upstream of the compression device 5 and the auxiliary circuit 49.

The separation/extraction system 31 of the prime mover of figure 2C is likewise operatively connected to the auxiliary circuit 49.

Figures 2A and 2B illustrate the thermodynamic cycle of the prime mover 1 of figure 2C in T-S and P-V diagrams.

In accordance with the cycle and process according to the present invention, the working fluid (for example argon) enters the volumetric compressor 5 (point A) and follows the following cycle/process steps, similar to the ones already illustrated above.

Nearly isothermal compression from A to B (in the volumetric compressor 5), with cooling/discharge of the resulting heat.

Recuperative isobaric heating from B to C (in the recuperator 25) by cooling of the hot discharge flow from the expander from E to F.

Combustion from C to D (in the volumetric expander 6).

Adiabatic expansion from D to E, all taking place in the volumetric expander

6.

Cooling (ideally isobaric) from F to A (in the cooling device 26).

Figure 3 illustrates a variant of the compressor-expander or "compander" 5, 1 1 of figure 1 C, wherein in place of the first, second and third heat exchangers 27, 28, 29 there are a first, second and third tank 51 , 52, 53 of a spray cooling device 26. The structure of said tanks 51 , 52, 53 is similar or identical to that of the tank 34 of figure 2C.

The cooling device 26 further comprises a recirculation circuit 35 provided with a heat exchanger 36 and a pump 41 and in fluid communication with each of the tanks 51 , 52, 53. In particular, the recirculation circuit 35 is connected to a lower outlet 39 of each tank 51 , 52, 53 and a plurality of nozzles 40 arranged inside every tank 34 near an upper portion thereof. The extraction and treatment system 30 is operatively connected to the recirculation circuit 35, for example downstream and upstream of the heat exchanger 36.

Figure 4 illustrates a further variant of the compressor-expander or "compander" 5, 1 1 of figure 1 C, wherein the first and second compressors 2, 3 have coaxial, aligned shafts, and the third compressor 4 and the auxiliary turbo-expander 1 1 have coaxial, aligned shafts. The shafts of the first and second compressors 2, 3 and of the third compressor 4 and auxiliary turbo-expander 1 1 are parallel and connected by a group of gears 54.

Figure 5 illustrates a further variant of the compressor-expander or "compander" 5, 1 1 of figure 1 C which comprises two shafts (multi-shaft). The second and third compressors 3, 4 and the auxiliary turbo-expander 1 1 have coaxial, aligned shafts. A further auxiliary turbo-expander 1 1 ' is also present, located downstream of the auxiliary turbo-expander 1 1 , so as to receive from it the working fluid and expand it further, and having a shaft in common with the first compressor 2.

Figure 7 illustrates a further variant of the prime mover 1 of figure 1 C, which differs from the prime mover of figure 1 C insofar as the oxidant feeding devices 24 are concerned. In this variant embodiment, the oxidant is ambient air (i.e. 20% oxygen, 79% nitrogen and about 1 % argon) and the working fluid is a mixture with a prevalence of nitrogen and with argon, CO2 and oxygen and water vapor in some areas of the cycle. In a further embodiment, the oxidant could consist of air enriched with oxygen, up to 30-60%, and the composition could therefore have an increase in CO2 content, as a result of the enrichment of the air, even arriving at percentages of CO2 prevailing over the other components.

In place of the oxidant feeding devices 24 schematically illustrated in figure 1 C, the prime mover 1 of figure 7 comprises an extraction expander 55 and an oxidant air compressor 56 possibly connected to an electric motor 57. The extraction expander 55 and the oxidant air compressor 56 are mechanically coupled to a same shaft so as to form a compressor-expander. The compressor 56 receives incoming ambient air, compresses it and introduces it, for example, through a feed pipe 58, into a point of the closed circuit 12 located between the recuperator 25 and the third heat exchanger 27. The compressor 56 and feed pipe 58 thus define a particular embodiment of the oxidant feeding devices 24. The compressor 56 is driven in rotation by means of the expander 55 and the electric motor 57, where present. The extraction expander 55 is moved using the expanding nitrogen and argon coming out of the closed circuit 12 before or after the extraction of CO2. In the embodiment of figure 7, an extraction pipe 59 extends from one point of the closed circuit 12, located immediately downstream of the auxiliary expander 1 1 and before the recuperator 25, to an inlet of the extraction expander 55. Once it has passed through the extraction expander 55, the mixture is fed, through a respective pipe 60, to the separation/extraction system 31 , in which the CO2 is separated, for example by cryogenic condensation, and nitrogen and argon are thus released into the atmosphere through an outlet pipe 61 .

In further unillustrated variants, the points of introduction of ambient air into the closed circuit 12 may also differ from the one indicated and/or the CO2 extraction system may be placed before the extraction expander 55.

Furthermore, the oxidant air compressor 56 could be driven electrically or with another driving motor and the extraction expander 55 could be uncoupled from compressor 56 and actuate a generator or a separate driven machine.

The present invention, in particular in the embodiments illustrated above, enables the following advantages to be obtained:

fewer irreversibilities compared to a normal Otto and/or diesel cycle thanks to the presence of the recuperator and the possible intercooled compression and thus better conversion efficiencies/yields;

acceptability of dirty and heavy fuels (including natural gas, fuel oil and derivatives, fuels and/or slurry derived from coal, gas from the gasification of coal and/or biomass, hydrogen, CO, etc.) similar to that of internal combustion engines, which are much more tolerant than gas turbines;

very hot parts at high pressures confined within the cooled cylinder, unlike in the case of supercritical CO2 cycles and plants where the expander must withstand inlet pressures of hundreds of bar with temperatures simultaneously exceeding 1000°C;

- fewer irreversibilities compared to Brayton cycles; in particular thanks to the combustion with a pressure increase and the higher maximum cycle temperatures obtainable; greater efficiency compared to "free piston combustors"; in fact, in the present invention the compression phase absorbs much less power than the expansion phase and, given a maximum combustion temperature equal to that of a "free-piston combustor", we would have very high discharge temperatures at point E' of figure 1A; in other words, for the purposes of the operating principle according to the present invention, a very high maximum cycle temperature can be reached (at point D of figure 1A) simultaneously with an acceptably low temperature at point E' (i.e. between 700-1 , 100°C) only if enough work is extracted through the piston combustor, by expansion, and this work is greater than the compression work.

good size scalability thanks to: the possibility of adjusting the minimum pressure of the cycle and thus the volumetric flow rates in play (being typically an ideal or nearly ideal gas cycle, it enables the cycle pressures to be chosen as desired, while the choice of the working fluid and its molecular weight enable the design of turbomachines to be optimized); the possibility of combining turbomachines, typically better suited to high volumetric flow rates in the low/medium pressure part, and a volumetric expander in the high pressure part, where the flow rates are limited and thus the volumetric machine does not constitute an element limiting the increase in power, the cycle being semi-closed and pressurized;

possibility of constructing the prime mover with components largely already in use and already known, without the need to develop new materials or new expanders with very high pressure and temperature parameters;

possibility of easily capturing carbon dioxide and combustion water;

- containment of polluting emissions;

possibility of constructing very compact engines which are also suitable for ships where space is limited and specific power is an important parameter, thanks to the compactness of the hot high-pressure parts and compactness of the cooling systems;

- possibility of choosing non-flammable working fluids, with a consequent reduction in the risks of fire or explosions and a smaller investment necessary for fire safety systems, etc.