Process for operating a working fluid according to a steam thermodynamic cycle to product work

Abstract

Process for operating a working fluid according to a steam thermodynamic cycle to produce energy comprising the cyclic steps of: a) compressing a working fluid by means of a pumping unit (10) increasing temperature and pressure of the working fluid, the working fluid presenting initial temperature and pressure conditions; b) heating the compressed working fluid by means of a first heating unit (20); c) evaporating the working fluid at the heated and compressed liquid state in an evaporation unit (30) generating a steam flow; d) expanding the steam flow from step c) in an expansion unit (40); e) condensing at least a portion of the expanded steam fluid from step d) by means of a condensation unit (50) to obtain the working fluid substantially at the initial temperature and pressure conditions; f) sending the working fluid from step e) to step a); Step c) provides for superheating the steam flow by at least partially dissolving at least one non-volatile solute in the working fluid and heating the solution obtained until the boiling temperature of the solution is reached and consequent production of superheated steam essentially consisting of the working fluid.

Description

[0001] “Process for operating a working fluid according to a steam thermodynamic cycle to produce work”

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates to a process for operating a working fluid by means of a steam thermodynamic cycle to produce work. The present invention also relates to a system for operating a working fluid according to a steam thermodynamic cycle to produce work. Preferably, the steam thermodynamic cycle of the present invention used for power generation is a new superheated steam Rankine-type thermodynamic cycle.

[0005] State of the art

[0006] As is well known, thermodynamic cycles for power generation are constantly being developed in order to find more efficient cycles / units, as well as more intensified or smaller cycles / units for the same power output.

[0007] Thermodynamic cycles are divided into gas cycles (e.g. the Brayton-Joule cycle) and steam cycles (e.g. the Rankine cycle). Steam cycles, in particular, exploit the phase change of the selected fluid in order to pump a liquid back to its starting point. The cost of the pumping operation is significantly lower than the cost of compressing a gas and the energy used by the pump is usually two orders of magnitude less than the energy generated by the turbine.

[0008] The Rankine cycle is the reference thermodynamic cycle for steam cycles (Figures 1 and 2). A carrier fluid is pumped by a pumping unit 1 at a higher pressure. It is then heated in a heat exchanger 2 until it reaches the boiling temperature and is sent to a boiler 3 for steam generation. The saturated steam is then sent to the turbine 4 and expanded to generate power. Afterwards, the expanded steam is condensed in a condenser 5 and sent back to the pumping unit for the subsequent cycle.

[0009] The Rankine cycle is used in thermal power plants for electricity production and is still required today for nuclear power systems such as generating stations, ships and submarines, as well as in limited power generation systems, up to 500 kW.

[0010] Its main use is the Superheated Steam Rankine Cycle, where the turbine can operate entirely with the fluid in the vapour phase, thus avoiding the formation of liquid during expansion and preserving its structure (Figures 3 and 4). In this regard, a superheater 6 is required to raise the temperature of the steam away from saturation conditions just enough to prevent the turbine 5 from expanding within the two-phase zone.

[0011] The three heating units 2, 3 and 6 have different conditions and efficiencies. Assuming three classic heat exchangers, shell and tube and / or coil, the first unit, which brings the temperature of the liquid leaving the pump up to boiling conditions, involves a heat exchange with a liquid phase and the heat exchange coefficient can usually be in the order of 400-800 W / m2 / K. The boiler unit, on the other hand, can achieve even higher coefficients by having a boiling liquid on one side, up to 1100-1200 W / m2 / K. The superheater unit, on the other hand, must heat a vapour. In this case, the overall exchange coefficient can be of the order of 200-400 W / m2 / K, but can sometimes be even lower. In addition to the lower density of the vapour compared to the liquid phase, the superheating temperature is also higher and the temperature difference may be lower, further reducing the efficiency of the unit.

[0012] In essence, for the heat exchange train dedicated to steam generation, the superheater is the least performing element, reducing the efficiency of the entire process.

[0013] Summary of the invention

[0014] In order to overcome the aforementioned problems, a process and related system was devised to operate a working fluid producing superheated vapour without the use of additional critical units or intermediate phases to reduce the apparatus footprint and improve process efficiency.

[0015] Thus, the subject matter of the present invention is a process for operating a working fluid according to a steam thermodynamic cycle to produce work comprising the cyclic steps of: a) compressing a working fluid by means of a pumping unit (10) increasing temperature and pressure of the working fluid, the working fluid having initial temperature and pressure conditions; b) heating the compressed working fluid by means of a first heating unit (20); c) evaporating the working fluid at the heated and compressed liquid state in an evaporation unit (30) generating a steam flow; d) expanding the steam flow from step c) in an expansion unit (40); e) condensing at least a portion of the expanded steam fluid from step d) by means of a condensation unit (50) to obtain the working fluid substantially at the initial temperature and pressure conditions; f) sending the working fluid from step e) to step a); characterised in that step c) provides for superheating the steam flow by at least partially dissolving at least one non-volatile solute in the working fluid and heating the solution obtained until the boiling temperature of the solution is reached and consequent production of superheated steam essentially consisting of the working fluid.

[0016] Advantageously, the process and system of the present invention make it possible to improve the efficiency of heat exchange in the cycle and to reduce the total volume of the system for the same power compared to the prior art.

[0017] Advantageously, the process and system of the present invention allow for improved efficiencies compared to Rankine cycles with superheated steam.

[0018] Advantageously, the process and system of the present invention for the respective system on which the process is implemented also allow a:

[0019] • Reduction in investment costs

[0020] • Reduction in operating costs

[0021] • Reduction in maintenance costs

[0022] • Reduction in monitoring and control costs

[0023] LIST OF FIGURES

[0024] Figure 1: Schematic block representation of an apparatus adopting a Rankine cycle according to the prior art;

[0025] Figure 2: Representation of the T-s diagram of the Rankine cycle in accordance with the heat pump in Figure 1 ;

[0026] Figure 3: Schematic block representation of an apparatus adopting a superheated steam Rankine cycle according to the prior art;

[0027] Figure 4: Representation of the T-s diagram of the Rankine superheated steam cycle according to the heat pump in Figure 3;

[0028] Figure 5: Schematic block representation of a system according to an embodiment of the present invention;

[0029] Figure 6: Representation of the T-s diagram of the steam thermodynamic cycle in accordance with the system in Figure 5, wherein the dotted line illustrates the course of the thermodynamic cycle in accordance with the present invention with respect to the prior art in Figure 2; Figure 7: Graphical representation of Duhring’s rule for an NaOH / water solution taken from McCabe et al., 2005.

[0030] Figure 8: Graphical representation of Duhring's rule for a water / NaCl solution taken from Elsayed, Wu, Chow Desalination Volume 504, 15 May 2021, 114955 High salinity seawater boiling point elevation: Experimental verification

[0031] DETAILED DESCRIPTION

[0032] The process of the present invention allows steam, preferably saturated, to be superheated for energy production. Specifically, the process of the present invention makes it possible to remove critical units present in the prior art and to avoid additional steps for superheating the steam, such as the superheater.

[0033] It should be noted that the process of the present invention allows a new Rankine- type steam thermodynamic cycle to be defined.

[0034] The process according to the present invention can be implemented in a system collectively designated with 100 in the figures to operate a thermodynamic cycle of the refrigeration type, described below.

[0035] Specifically, the process of the present invention involves operating the working fluid in a cyclic manner in a series of phase transitions of the same working fluid by receiving heat and releasing it Qin and Qout shown in the figures, in particular in Figure 6, and by expending work for compressing the fluid Win, in the figures for the relative production of energy Wout in the figures.

[0036] It should be noted that the process of the present invention involves exploiting the phase change of the working fluid in order to pump a liquid back to its starting point following the generation of energy.

[0037] Preferably, the working fluid is water in the liquid and vapour state. The process for operating a working fluid according to a steam thermodynamic cycle to produce energy includes a step a) of compressing a working fluid by increasing temperature and pressure of the working fluid.

[0038] Step a) is carried out in a pumping unit 10 of the system 100.

[0039] Preferably, the working fluid in step a) is in the liquid state.

[0040] It should be noted that the working fluid has initial temperature and pressure conditions.

[0041] Preferably, the working fluid in the liquid state in step a) has initial pressure and temperature conditions wherein the pressure is at or near atmospheric pressure, and more preferably in a range of 80 and 120 kPa, the temperature in a range of 20 and 45°C. After pumping, the working fluid in the liquid state, leaving step a) and thus leaving the pumping unit preferably has a pressure in a range of 1000 to 15000 kPa and temperature comparable or slightly higher than the initial temperature.

[0042] As illustrated in Figure 5, the solid line on the T-s diagram shows in the a-b section a temperature increase from the bell curve.

[0043] The process comprises the step b) of heating the compressed working fluid preferably to a temperature comparable or close to the boiling temperature of the working fluid.

[0044] Step b) is carried out in a first heating unit 20 of the system 100.

[0045] Preferably, the compressed and heated working fluid has pressure and temperature conditions where the pressure is in a range of 1000 to 15000 kPa, however with a slight decrease from the pumping pressure due to pressure drops, and the temperature in the range of 170 to 340°C, however not exceeding the boiling temperature of the working fluid. It should be noted that the pressure of the compressed and heated working fluid essentially corresponds to the pressure at which the subsequent boiling takes place, and thus essentially corresponds to the saturation pressure.

[0046] As illustrated in Figure 5, the dashed and continuous line on the T-s diagram shows in the b-c section a temperature increase from the working fluid curve until the relative boiling temperature is reached. It should be noted that the dashed section at essentially constant entropy in Figure 6 differs from the state-of-the-art thermodynamic cycles section b-c’ and b-c”, in that the temperature of the working fluid is expected to be raised to the related boiling temperature.

[0047] The process of the present invention comprises a step c) of evaporating the working fluid to a heated and compressed liquid state by generating a vapour flow. Preferably, the working fluid in step c) is initially in the liquid state until it evaporates to the gaseous state. Specifically, during step a) the working fluid heats up by receiving heat until it evaporates.

[0048] Preferably, the vapour flow generated in step c) has pressure and temperature conditions where the pressure is substantially in the range between the saturation condition of the pure solvent and the same saturation pressure of the solution, i.e. between 1000 kPa and 15000 kPa.

[0049] It should be noted that step c) is carried out in an evaporation unit 30.

[0050] In accordance with the preferred embodiment, step c) involves superheating the vapour flow by dissolving at least one non-volatile solute in the working fluid and heating the resulting solution until the solution’s boiling temperature is reached, resulting in superheated vapour essentially consisting of the working fluid.

[0051] Specifically, the solution obtained by combining the working fluid and the nonvolatile solute has a boiling temperature higher than the boiling temperature of the working fluid.

[0052] Preferably, step c) involves adding at least one non-volatile solute to the working fluid in the liquid state to form a solution with a boiling temperature higher than the boiling temperature of the working fluid alone in the liquid state. It should be noted that this addition of solute can be accomplished in a start-up cycle, with possible additions in subsequent cycles, since due to its chemical and physical properties the solute is nonvolatile and is therefore kept confined for the most part, preferably all of it, within the evaporation unit 30 and / or circulated in units in which the working fluid is at least partly in the liquid state.

[0053] Thus, the boiling of the solution and subsequent evaporation of the working fluid results in the generation of a superheated working fluid vapour.

[0054] It is noted that the enthalpy gap to go from the boiling temperature of the working fluid from step b) to the boiling temperature of the solution is bridged by the evaporation unit 30. In this way, the outgoing steam is in an overheated condition ready for the subsequent steps.

[0055] As illustrated in Figure 6, the continuous dotted line at a substantially constant temperature on the T-s diagram shows in section c-d the transition of state of the working fluid passing into the superheated steam zone, as known from the bell-shaped curve of the T-s diagram. This transition into the superheated zone is absent in the prior art of Figure 2 section c’-d’ e carried by means of a superheater combined with the heating unit sections c”-d” and d”-d’”.

[0056] Due to the solution of working fluid and at least one non-volatile solute, the average boiling temperature of the solution increases and thus when the solution reaches boiling point, the vapour produced, consisting essentially of working fluid in a gaseous state since by definition the solute is non-volatile, has a higher temperature than it would have without the solute.

[0057] It should be noted that the non-volatile solute is of the solid type and can be added to step c) in the liquid state by pre-heating depending on the type of solute and / or in the solid state configured to dissolve during the heating that takes place in step c).

[0058] At least one non-volatile solute has a boiling temperature above the boiling temperature of the working fluid, thus increasing the boiling temperature of the solution.

[0059] Preferably, the steam flow generated in step c) has pressure and temperature conditions where the pressure is in the range of 1000 to 15000 kPa and the temperature in the range of 170 to 340 °C.

[0060] In accordance with a preferred embodiment, step c) involves increasing the boiling temperature of the solution by increasing the concentration of the non-volatile solute in the solution in accordance with Duhring’s rule. Specifically, steam superheating is a function of the non-volatile solute and its concentration in the solution. It is noted that for the same non-volatile solute, the steam is more superheated the more the solution is concentrated in the solute up to the solubility limit. Preferably, the maximum solute concentration is governed by the supersaturation and Miers, Ostwald and Bereich conditions.

[0061] In accordance with a preferred embodiment, the non-volatile solute is NaOH. The boiling point variation of the solution comprising water and NaOH varies as a function of solute concentration as derived from the diagram shown in Figure 7.

[0062] In accordance with an alternative embodiment, the non-volatile solute can for example be selected from CaC12, NaCl, KC1, KC1O3, nitrates, phosphates or sulphates in combination when the working fluid is water, preferably pure. It should be noted that the list of non-volatile solutes is not limiting. By way of example, the boiling temperature of water as a function of NaCl concentration is shown in Figure 8.

[0063] The process of the present invention comprises a step d) of expanding the steam flow from step c) producing energy. Step d) therefore involves expanding the superheated steam flow, thereby reducing the temperature and pressure of the fluid itself.

[0064] Step d) is carried out in the expansion unit 40 of the system 100. Preferably, the expanded steam flow generated in step d) has pressure and temperature conditions where the pressure is close to or a few tens of kPa higher than the fluid pressure in step a), and where the temperature is in the range of 20 to 100°C.

[0065] It should be noted that in the T-s diagram in Figure 6, there is an expansion of the steam flow in the superheated steam field for energy production Wout in Figure 6.

[0066] The process of the present invention comprises step e) of condensing at least a portion of the expanded vapour fluid from step d) to obtain the working fluid substantially at the initial temperature and pressure conditions.

[0067] Step e) is carried out in a condensation unit 50 of the system 100.

[0068] During step e), the working fluid in the gaseous state releases sensible and latent heat until it condenses.

[0069] It should be noted that in the T-s diagram in Figure 6, step e) provides for section e-a involving a drop in temperature until the bell-shaped change-of-state curve is reached and the subsequent change of state of the working fluid from vapour to liquid.

[0070] The process of the present invention comprises step f) of sending the working fluid from step e) to step a).

[0071] In this way, the process is repeated cyclically for energy production.

[0072] It should be noted that the thermodynamic cycle defined by the process of the present invention thus makes it possible to exploit the boiling temperature variation of a solution with one or more non-volatile solutes to produce a overheated vapour of the working fluid without introducing critical heat exchange units.

[0073] In accordance with a preferred embodiment, step c) comprises the sub-step cl .1) of mixing in the evaporation unit 30 at least one non-volatile solute in the working fluid in the liquid state until a solution is obtained.

[0074] Specifically, sub-step cl. l) involves adding at least one non-volatile solute to the working fluid, at least in the start-up cycle, to regulate its concentration. Subsequently, step c) includes a sub-step cl.2) of heating the solution obtained in step cl.l) until the boiling temperature of this solution is reached. In the case of solute added in the solid state, substep cl .2) involves dissolving the relevant non-volatile solute and then reaching the boiling temperature of the solution, e.g. in a start-up step and / or in subsequent cycles to adjust the concentration. It is noted that the non-volatile solute can be added in liquid form from a recycle and / or added in a solid / liquid state in the initial start-up step and / or in the control step during subsequent cycles. Preferably, the working fluid entering step c) has a temperature substantially equal to its boiling temperature. Once the boiling temperature of the solution is reached, step c) comprises sub-step cl.3) of forming vapour consisting mainly of the working fluid to be sent to step d). Preferably, the superheated vapour essentially consisting of the working fluid is in thermal equilibrium with the boiling solution. Specifically, superheated vapour is generated as the boiling of the solution occurs at a higher temperature than the boiling temperature of the working fluid, so that in the liquid-gas state transition, vapour is produced from the already overheated working fluid, as can be seen from the straight line c-d state transition in Figure 6. Finally, step c) comprises the sub-step cl .4) of separating by evaporation the non-volatile solute flow from the superheated vapour flow before sending the superheated vapour to step b). Preferably, the solute flow is kept inside the evaporation unit and / or in a tank so that it cyclically mixes with the working fluid from step b) of the subsequent cycle.

[0075] In accordance with a preferred embodiment, step d) involves tapping a portion of the superheated vapour flow to heat the working fluid from step a). In this way, energy integration can be carried out.

[0076] Further subject matter of the present invention to provide a system 100 for operating a previously described steam thermodynamic cycle. Specifically, the process according to the present invention is preferably conducted in the system 100, which can be attributed to an energy production plant in which steam is used.

[0077] For the purposes of the present invention, a system is defined as an assembly of one or more operating units associated with each other and configured to perform one or more steps of a process. It should be noted that the operating units can be in fluid communication so that the outflows are the inflows of one or more subsequent operating units.

[0078] As will be clear from the description, each operating unit carries out one or more steps of the process of the invention.

[0079] The system comprises operating units connected in series to define a cycle.

[0080] Specifically, the system 100 comprises a pumping unit 10 configured to compress the working fluid. Preferably, the pumping unit 10 comprises at least one pump. The system 100 comprises a first heating unit 20 configured to heat the compressed working fluid from the pumping unit 10. Preferably, the first heating unit 20 comprises at least a first heat exchanger. The system comprises an evaporation unit 30 configured to evaporate the working fluid from the first heating unit 20 and generate a steam flow. Preferably, the evaporation unit 30 comprises at least one evaporator. The system 100 comprises an expansion unit 40 configured to expand the steam flow from the evaporation unit 30 and generate power. Preferably, the evaporation unit 30 comprises at least one turbine. The system 100 comprises a condensation unit 50 configured to condense the expanded steam flow generated by the expansion unit 40 and obtain the working fluid to be sent to the pumping unit 10. Preferably, the condensation unit 50 comprises a second heat exchanger.

[0081] It should be noted that the pumping unit 10 is in fluid communication downstream with the first heating unit 20 and upstream with the condensation unit 50, the evaporation unit 30 is in fluid communication downstream with the expansion unit 40 and upstream with the first heating unit 20 and the expansion unit 40 is in fluid communication upstream with the condensation unit 50 thus defining a cycle. Preferably, it is noted that the connections between the units are hydraulic.

[0082] In accordance with the preferred embodiment, the evaporation unit 30 is configured to generate a superheated steam flow by heating to evaporation a solution obtained preferably by dissolution of a non-volatile solute in the working fluid. This solution, as described above, has a higher boiling temperature than the temperature of the working fluid, thus allowing the production of superheated steam.

[0083] In accordance with a preferred embodiment, the system comprises a separation unit configured to separate the superheated steam flow and the non-volatile solute flow to keep the non-volatile flow within the evaporation unit 30 or to be sent to a tank of solute. Specifically, the separation unit comprises anti-aerosol and anti-drag devices configured to separate the non-volatile solute flow from the steam flow to prevent any residual solute from being sent to subsequent units.

[0084] In accordance with a preferred embodiment that can be combined with the previous one, the system 100 comprises a tapping unit configured to draw a portion of superheated steam flow. This tapping is used to preheat the compressed working fluid. Preferably, the system 100 comprises a second heating unit interposed between the pumping unit 10 and the first heating unit 20 and configured to receive the portion of superheated steam flow tapped to heat the compressed working fluid.

Claims

CLAIMS1. Process for operating a working fluid according to a steam thermodynamic cycle to produce energy comprising the cyclic steps of: a) compressing a working fluid by means of a pumping unit (10) increasing temperature and pressure of the working fluid, the working fluid presenting initial temperature and pressure conditions; b) heating the compressed working fluid by means of a first heating unit (20); c) evaporating the working fluid at the heated and compressed liquid state in an evaporation unit (30) generating a steam flow; d) expanding the steam flow from step c) in an expansion unit (40); e) condensing at least a portion of the expanded steam fluid from step d) by means of a condensation unit (50) to obtain the working fluid substantially at the initial temperature and pressure conditions; f) sending the working fluid from step e) to step a); characterized in that step c) provides for superheating the steam flow by at least partially dissolving at least one non-volatile solute in the working fluid and heating the solution obtained until the boiling temperature of the solution is reached and consequent production of superheated steam essentially consisting of the working fluid, said solution having a boiling temperature higher than the boiling temperature of the working fluid.

2. The process according to claim 1, wherein step c) provides for increasing the boiling temperature of the solution by increasing the concentration of the non-volatile solute in the solution in accordance with Duhring's rule.

3. The process according to claim 1 or 2, wherein step c) comprises the sub-steps of:cl .1) mixing in the evaporation unit (30) a solute at the liquid or non-volatile solid state in the working fluid until a solution is obtained; cl.2) heating the solution obtained in step cl. l) until the boiling temperature is reached; cl.3) forming steam consisting mainly of the working fluid to be sent to step d); cl.4) separating by evaporation the non-volatile solute in a solute flow from the superheated steam flow before sending the superheated steam to step b).

4. The process according to any one of claims 1 to 3, wherein step d) provides for tapping a portion of the superheated steam flow to heat the working fluid from step a).

5. The process according to any one of claims 1 to 4, wherein- the working fluid has initial pressure and temperature conditions where the pressure is in a range between 80 and 120 kPa and the temperature in a range between 20 and 45°C;- the compressed working fluid has pressure and temperature conditions where the pressure is in the range between 1000 and 15000 kPa and the temperature substantially corresponds to or is slightly higher than the initial temperature;- the compressed and heated working fluid has pressure and temperature conditions where the pressure substantially corresponds to or is slightly lower than the pumping pressure and the temperature in a range of 170 and 340°C;- the steam flow generated in step c) has pressure and temperature conditions where the pressure substantially corresponds to or is slightly lower than the pumping pressure and the temperature in the range of 170 and 340°C;- the expanded steam flow generated in step d) has pressure and temperatureconditions where the pressure substantially corresponds to or is slightly higher than the initial pressure and the temperature in a range of 20 and 100°C6. The process according to any one of claims 1 to 6, wherein the working fluid is water, preferably pure, and the non-volatile solid solute is NaOH.

7. The process according to any one of claims 1 to 5, wherein the working fluid is water, preferably pure, and the non-volatile solute is selected from CaC12, NaCl, KC1, KC1O3, nitrates, phosphates, sulfates.

8. System (100) for operating a steam thermodynamic cycle to generate work comprising connected in series in a cycle:- a pumping unit (10) configured to compress the working fluid;- a first heating unit (20) configured to heat the compressed working fluid from the pumping unit (10)- an evaporation unit (30) configured to evaporate the working fluid from the first heating unit (20) and generate a steam flow;- an expansion unit (40) configured to expand the steam flow from the evaporation unit (30) and generate energy;- a condensation unit (50) configured to condense the expanded steam flow generated by the expansion unit (40) and obtain the working fluid to be sent to the pumping unit (10); characterized in that the evaporation unit (30) is configured to generate a superheated steam flow by heating until evaporation a solution obtained by dissolving a non-volatile solid solute in the working fluid, the solution obtained having a boiling temperature greaterthan the temperature of the working fluid.

9. System (100) according to claim 8 wherein:- the evaporation unit (30) comprises an evaporator;-the pumping unit (10) comprises a pump;- the first heating unit (20) comprises a first heat exchanger;-the condensation unit (50) comprises a second heat exchanger;- the expansion unit (40) comprises a turbine.

10. System (100) according to claim 8 or 9, further comprising- a separation unit configured to separate the superheated steam flow and the flow of non-volatile solute to maintain the flow of non-volatile solute within the evaporation unit (30); said separation unit comprising anti-aerosol and anti-entrainment devices configured to separate the flow of non-volatile solute from the steam flow.

11. System (100) according to any one of claims 8 to 10, further comprising- a tapping unit configured to withdraw a portion of the superheated steam flow;- a second heating unit interposed between the pumping unit (10) and the first heating unit (20) and configured to receive the portion of superheated steam flow tapped to heat the compressed working fluid.

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