System for a rankine thermodynamic cycle

Abstract

The present invention relates to a system for performing a Rankine thermodynamic cycle, comprising a working fluid suitable to undergo thermodynamic transformations. This working fluid comprises silicon tetrachloride.

Description

[0001] The present invention is in the technical field of thermodynamic cycles for producing mechanical, electrical and / or thermal energy. In particular, it forms an object of the present invention to provide a system and a method of operating said system to produce energy. It is also an object of the present invention to use a working fluid in said system.

[0002] Nowadays, as is well known, the energy transition requires actions to substantially reduce the emission of greenhouse gases from burning fossil fuels. However, improved living standards are driving a gradual increase in global energy consumption, which makes the road to full decarbonization more complex. In order to cope with sustainable development, two paths need to be taken in parallel: conversion to a renewable energy system on the one hand, and on the other, increasing the efficiency of systems / installations that require fossil fuel consumption. In fact, there are highly energy-intensive industrial sectors in which the zeroing of greenhouse gas emissions is hardly feasible. Such sectors are termed "hard to abate" because their production processes require high-temperature heat, which can be obtained - to date - by combustion of fossil sources, and involve, for example, the chemical, steel, land, etc. industries involved in the manufacture of steel, ceramics, glass, cement, and paper.

[0003] In this context, thermal recovery from waste heat from industrial processes turns out to be an attractive solution to increase the conversion efficiency of primary energy (i.e., combustion of fossil fuels). Starting with waste heat energy, simply called "waste heat," thermal recovery is generically carried out with the purpose of producing further useful energy (mechanical and / or electrical and / or thermal) for the same amount of fuel consumed. Such waste heat energy is present in production processes in various forms such as, for example, heat emitted from hot surfaces or exhaust gases from combustion. Such combustion flue gases are exploited to fuel high-temperature industrial processes and are typically released to the stack with high energy content.

[0004] According to the Eurostat 2020 report, the industrial sector in Europe is responsible for 26.1% of primary energy consumed (or 37,086 petajoules), where 70.6% of this consumption is allocated to heating processes, resulting in a significant amount of waste heat rejected to the environment (up to 50%). From this, it is evident how the utilization of waste heat is a key factor in moving closer to zero emissions and climate change impact reduction targets.

[0005] At the state of the art, it is known to the technician in the field that the most widely used technology for waste heat recovery is organic fluid Rankine cycles also known by the acronym ORC, or Organic Rankine Cycle.

[0006] Organic working fluid Rankine cycles consist of a closed thermodynamic cycle in which an organic working fluid remains bound within the system where it undergoes thermodynamic transformations. The strength of the technology lies in the choice of the appropriate working fluid to be coupled to a heat source, typically exploited between 120°C and 400°C. In fact, if waste heat - for example, in the form of hot flue gas - is available at a higher thermal level, i.e., at a temperature above 120-400°C, the available thermal power is typically transferred to an intermediate heat-transfer fluid, which in turn transfers it to the system / plant working fluid.

[0007] Disadvantageously, a limitation of ORC technology is related to the thermal stability of the organic working fluids used in the systems, which constrains the maximum thermodynamic cycle temperature around 300°C, not guaranteeing optimal utilization of higher temperature heat sources. This constraint on the maximum thermodynamic cycle temperature is representative of the thermal stability limit of the hydrocarbon and siloxane families, i.e., the main organic working fluids. This is one of the many reasons why indirect heat-transfer through the use of the intermediate heat-transfer fluid is favored; for example, a diathermic heat-transfer oil is used that is heated by the heat source up to a maximum temperature of about 400°C, and in turn the heat-transfer fluid transfers heat to the working fluid. The efficiency of the thermodynamic cycle is in fact related to the average temperature at which heat is introduced into the working fluid; therefore, thermodynamic efficiency is directly proportional to the maximum temperature that the working fluid reaches within the system.

[0008] Therefore, bridging the technology gap at high temperatures by using thermo-chemically stable working fluids would ensure more efficient utilization of the thermal energy made available from the high temperature thermal source. This would result in greater production of mechanical and / or electrical and / or thermal energy. The benefit of using thermo-chemically stable working fluids could be exploited not only in the field of high-temperature heat recovery, but also in multiple other applications such as, for example, solar thermodynamics, biomass and waste combustion, nuclear power, etc.

[0009] "Thermal stability," or "thermochemical stability," refers to the ability of a compound to keep intact the bonds of which it is made, without going through chemical reactions of bond splitting and subsequent recombination. Thermal degradation of the working fluid is a phenomenon to be avoided because it results in the formation of secondary compounds, typically incondensable and solid residues, which alter the operation of the system (or plant) causing it to deteriorate in performance, malfunction, and blockage.

[0010] The high thermochemical stability of demineralized water is such that it has made it the most widely used working fluid for power generation in Rankine cycles, particularly in the high-power range (i.e., above 10 electric MW) . In fact, demineralized water is typically used for power generation in thermal power systems using fossil fuels or Concentrated Solar Power (CSP) systems at a maximum cycle temperature between 550-620°C. However, water as a working fluid requires high system engineering complexity, high maximum operating pressures (in the range of 90-300 bar), and high capital costs that make the use of demineralized water as a working fluid disadvantageous in the case of medium to low temperature thermal sources and in the power range of tens of kW to a few electric MW. In addition, the high heat of vaporization of water does not make it the best choice in the case of thermal recovery from sources of finite heat capacity. For this reason, working fluids of a different nature, typically organic, that provide better thermodynamic coupling with the thermal source are preferred.

[0011] With reference to the attached figures 1a and 1b, an organic-fluid Rankine cycle (ORC) is depicted in the temperature-entropy diagram (or T-s diagram). Specifically, figure 1a shows a subcritical ORC cycle, while figure 1b presents a supercritical ORC cycle. In both cases (figures 1a and 1b), the ORC cycle is regenerative, that is, steam leaving the turbine is used for preheating the working fluid before it is exposed to the heat source.

[0012] In accordance with the attached figure 1a, the transformations that the working fluid undergoes in the different components of the system are as follows: pumping carried out straddling a first point 1 and a second point 2 by means of pressure increasing means, such as a pump or compressor, such pumping allows a change from condensing pressure to maximum cycle pressure; preheating carried out between the second point 2 and a third point 3 within a regenerator; heating carried out between the third point 3 and a fourth point 4 to slightly subcooled liquid conditions. This heating is due to the interaction of the working fluid with a heat source, such as fumes produced by the combustion of a fossil fuel; evaporation realized at the turn of the fourth point 4 and a fifth point 5 due to the interaction of the working fluid with the heat source; expansion carried out inside a turbine straddling the fifth point 5 and a sixth point 6. Expansion begins from saturated steam conditions (as in fifth point 5 of figure 1a) or superheated steam; cooling of the working fluid between the sixth point 6 and a seventh point 7. Such cooling is achieved by circulating the exhausted working fluid flow out of the turbine into the regenerator; cooling and condensation of the working fluid between the seventh point 7 and the first point 1.

[0013] The working fluid receives heat from the heat source between the third point 3 and the fifth point 5.

[0014] In contrast, in the case of supercritical ORC cycle as shown in figure 1b, the working fluid undergoes the following steps: pumping carried out between the first point 1 and the second point 2 by the means of pressure boosting to go from condensing pressure to maximum cycle pressure; Preheating carried out between the second point 2 and the third point 3 within a regenerator; heating realized between the third point 3 and the fourth point 4. In the supercritical ORC cycle, there is no phase transition (evaporation), because the heating of the working fluid takes place at a pressure higher than the critical pressure of the working fluid, and thus there is a transition from subcooled liquid conditions in the third point 3 to supercritical vapor conditions in the fourth point 4, without the formation of a liquid-vapor meniscus. Therefore, the fourth point 4 coincides with the fifth point 5, because the supercritical ORC cycle lacks the phase transition. Also, in the supercritical ORC cycle (as in the subcritical one), heating is due to the interaction of the working fluid with the heat source; expansion made inside the turbine straddling the fourth point 4 and the sixth point 6; cooling of the working fluid between the sixth point 6 and the seventh point 7. Such cooling is achieved by circulating the exhausted working fluid flow out of the turbine into the regenerator; cooling and condensation of the working fluid between the seventh point 7 and the first point 1.

[0015] The advantages of a supercritical setup can be improved thermodynamic performance and better coupling with the cooling curve of the thermal source, at the expense of high operating pressures and thermodynamic properties that are more difficult to predict at the critical point of the working fluid.

[0016] There are many advantages due to the use of an organic working fluid over water. Among them, there is the possibility of dry expansion in the turbine (between the fifth point 5 and sixth point 6 of figure 1a), unlike water-vapor cycles where the expansion ends under liquid-vapor mixture conditions (i.e., within the Andrews bell), which has a negative effect on the life and efficiency of the vane. More importantly, the possibility of employing low-boiling organic working fluids has a positive effect on the coupling with the thermal source, since the molecular complexity of the organic working fluid is higher than that of water and warrants the use of compact turbines with low numbers of stages (typically no more than three) because of the limited enthalpy jumps in the turbine.

[0017] In the state of the art, a further working fluid that has recently been the subject of much research, as it is considered as a potential working fluid in new generation thermoelectric plants, is carbon dioxide or carbon dioxide, in the formula CO2. Although from the system point of view there are only a few small demonstration systems to date, there is considerable attention from the scientific community and commercial use is expected in the coming years. The advantages of using carbon dioxide as a working fluid for power plants are basically as follows: non-toxicity, low global warming potential (GWP=1), zero ozone depletion potential (ODP=0), low cost, good thermochemical stability, high plant compactness (due to high CO2 density and good heat-transfer properties), and finally high thermodynamic efficiency / performance. The high thermodynamic efficiency is related to the low critical temperature of the fluid (31°C), which makes it possible to operate compression (pumping) of the working fluid near the critical point (31°C, 73.8 bar) thus exploiting the so-called real gas effects.

[0018] Disadvantageously, the operating conditions of a CO2 cycle are typically supercritical, that is, both the lower isobar and the upper isobar of the thermodynamic cycle are above the critical fluid pressure: this implies that the maximum operating pressures are on the order of 250 bar, conditions that result in significant thermal-mechanical stresses on the materials and components of the system. In addition, the cycle configuration for CO2 - considered in accordance with the state of the art - is supercritical and involves recompression, i.e., the aid of a second compressor that compresses a fraction of the working fluid, and two regenerators (a primary regenerator 51 and a secondary regenerator 52); thus, a not inconsiderable degree of system engineering and control complexity.

[0019] Figure 2a shows the temperature-entropy diagram of a supercritical CO2 Rankine cycle in accordance with the state of the art. Between a primary point 1' and a secondary point 2', CO2 compression occurs with a first compressor. Between secondary point 2' and a tertiary point 3', first regeneration (first preheat) occurs with primary regenerator 51. Between the tertiary point 3' and a quaternary point 4' the second regeneration (second preheating) occurs with the secondary regenerator 52. Between quaternary point 4' and a quinary point 5' CO2 interaction with the thermal source occurs. Between the quinary point 5' and a senary point 6' the expansion of CO2 into the turbine occurs. Between the senary point 6' and a septenary point 7' the passage of CO2 leaving the turbine through the secondary regenerator 52 occurs. Between the septenary point 7' and an octonary point 8' the passage of CO2 leaving the secondary regenerator 52 through the primary regenerator 51 occurs. At this point, a major fraction of the CO2 passes through the condenser / radiator to cool (between octonary point 8' and primary point 1'); instead, a minority fraction of the CO2 undergoes recompression with a second compressor between octonary point 8' and tertiary point 3'.

[0020] Figure 2b shows a schematic of a system for the above-mentioned supercritical CO2 Rankine cycle with recompression.

[0021] A further disadvantage may be due to the high density of supercritical CO2, which tends to favor a high compactness of the cycle components, but this goes against its use in the field of small power (i.e., less than 3-5 electric MW), since the size of the turbomachines would be such that it would be comparable with the present clearances. The resulting fluid dynamic losses would have a negative impact on machine efficiencies and thus on the thermodynamic efficiency of the cycle.

[0022] In addition, CO2 cycles are found to be highly recuperative because the expansion of the fluid in the turbine results in a small temperature jump and thus the need to employ large regenerators to internally recover the heat available at the turbine exhaust. The high degree of internal recovery through the two regenerators is not conducive to the optimal utilization of an available external heat source (thermal source); thus, supercritical CO2 cycles are not particularly well suited for heat recovery (exception made for particular cycle configurations, e.g., the so-called "cascade" cycle).

[0023] Technical progress has meant that in recent years carbon dioxide mixtures have also been considered as a potential working fluid in power plants, as an alternative to pure CO2. Particular attention has been paid to binary mixtures that are rich in CO2 and comprise - in addition to CO2 - a dopant fluid characterized by high critical temperature. The composition of CO2 and dopant fluid results in a binary mixture having higher critical temperature than pure CO2 and the possibility of condensing the working fluid (around 50-60°C) even with ambient air in locations characterized by high temperature (typical conditions of solar concentrating power plants). The possibility of condensing the binary mixture even at high ambient temperature ensures compression of the working fluid under liquid conditions, using a pump, with an advantage over the energy expended for the compression phase compared to compression of pure CO2 around 50°C, and thus also ensures higher thermodynamic efficiency of the cycle.

[0024] CO2-based cycles (either pure or blended) have the advantage of a significant reduction in power plant layout, when compared to the Rankine water vapor cycle, and a lower plant complexity that benefits capital investment costs and the levelized cost of electricity (LCoE) produced. For this reason, CO2 blends have been introduced to bring down the costs of concentrating solar power (e.g., in the H2020 Scarabeus project). In fact, most blend cycles do not require the use of a second compressor (or secondary compressor) or a second regenerator (or secondary regenerator); therefore, the cycle architecture using a CO2 blend as the working fluid is similar to that of a supercritical ORC cycle.

[0025] However, it is possible to say that CO2 mixtures actually occupy the same market sector, in terms of sizes and temperatures, as supercritical CO2 cycles.

[0026] For these reasons, there is a need to identify further working fluids that are potentially applicable in the temperature and power range not currently exploited by either organic fluids in ORC cycles / engines, water, or supercritical CO2.

[0027] According to the study M. Papapetrou, G. Kosmadakis, A. Cipollina, U. La Commare, G. Micale, Industrial waste heat: Estimation of the technically available resource in the EU per industrial sector, temperature level and country, Appl. Therm. Eng. 138 (2018) 207-216, on the potential for heat recovery in Europe by sector and temperature level, estimates that there are 304.13 TWh / year technically exploitable, of which one-third is available below 200°C (low temperature), 25% in the 200-500°C range (medium-low temperature), and the rest above 500°C (high temperature heat recovery).

[0028] The purpose of the present invention is to propose a system, method, and use capable of at least partially obviating the drawbacks complained of above in relation to the state of the art. For the purpose of the present discussion, it is intended to identify a working fluid applicable in a typical ORC system scheme, capable of efficiently exploiting heat sources of limited thermal capacity, even for net electro-mechanical outputs of the order of kW to a few MW, i.e., in sizes where both sCO2 (supercritical CO2 cycles) and CO2 mixtures and Rankine water cycles are not economically and / or technically viable.

[0029] Said purpose is achieved by a system according to claim 1, by use according to claim 6, and by a method according to claim 7. The dependent claims describe preferred forms of embodiment of the invention.

[0030] However, the features and advantages of the system, method and use according to the invention will be evident from the description below of its preferred examples of implementation, given by way of illustration and not limitation, with reference to the attached figures, wherein: figure 1a is the temperature-entropy diagram of a subcritical organic fluid Rankine cycle (subcritical ORC cycle) according to the state of the art; figure 1b is the temperature-entropy diagram of a supercritical organic fluid Rankine cycle (supercritical ORC cycle) according to the state of the art; figure 2a illustrates the temperature-entropy diagram of a supercritical CO2 Rankine cycle in accordance with the state of the art; figure 2b shows a schematic of a system for the supercritical CO2 Rankine cycle in figure 2a with recompression. figures 3a-3b show an experimental apparatus for performing thermal stability tests; figure 4 is the pressure-temperature diagram obtained from the first thermal stability test; figure 5 is the pressure-temperature diagram obtained from the second thermal stability test; figures 6a-6b illustrate the metallographic analyses conducted on a test cylinder at the conclusion of the second thermal stability test; figure 7 is the pressure-temperature diagram obtained from the third thermal stability test; figure 8 is a schematic of a system for a Rankine thermodynamic cycle using pure SiCl4 as the working fluid; figure 9 is a diagram wherein thermodynamic efficiency is represented as a function of maximum pressure considering different turbine inlet temperatures (TIT), minimum temperature of 58°C and minimum pressure of 1.05 bar; figure 10 illustrates the temperature-entropy diagram of a supercritical Rankine ORC thermodynamic cycle using SiCl4; figure 11 is a diagram showing thermodynamic efficiency (left y-axis) and minimum pressure (right y-axis) as a function of condensing temperature at the conditions of maximum pressure 60 bar and turbine inlet temperature (TIT) of 550°C; figure 12 shows the system schematic of an ORC cycle applied to a biomass boiler.

[0031] In the following description, elements common to the various forms of implementation represented in the drawings are indicated by the same reference numbers.

[0032] In a general form of implementation, a system is proposed for performing a Rankine thermodynamic cycle that comprises a working fluid suitable to undergo thermodynamic transformations. Such a working fluid comprises silicon tetrachloride, in formula SiCl4.

[0033] The proposed working fluid, comprising silicon tetrachloride (SiCl4), will be better presented later.

[0034] In the past, other working fluids belonging to the same family of metal halides have been considered as working fluids for high-temperature heat pipes. Among them, moreover, titanium tetrachloride (TiCl4) has also been proposed as a working fluid for high-temperature Rankine cycles up to 500°C (see C.M. Invernizzi, P. Iora, D. Bonalumi, E. Macchi, R. Roberto, M. Caldera, Titanium tetrachloride as novel working fluid for high temperature Rankine Cycles: Thermodynamic analysis and experimental assessment of the thermal stability, Appl. Therm. Eng. 107 (2016) 21-27), and also in mixture with CO2 for CSP applications (see G. Manzolini, M. Binotti, D. Bonalumi, C. Invernizzi, P. Iora, CO2 mixtures as innovative working fluid in power cycles applied to solar systems. Techno-economic assessment, Sol. Energy. 181 (2019) 530-544). The major limitation to the exploitation of TiCl4, as well as most metal halides, is the high reactivity with water and moisture in the air (hydrolysis), leading to the formation of TiO2 and corrosive HCl.

[0035] However, the boiling point of silicon tetrachloride is 57.6°C at atmospheric pressure and thus allows it to operate above ambient pressure and at a reasonable condensation temperature so as to avoid problems of air infiltration and subsequent hydrolysis.

[0036] In contrast, avoiding sub-atmospheric expansion and condensation is actually problematic in the case of TiCl4 since at ambient pressure its boiling point is 136.4°C. In addition, SiCl4 is less reactive with water than TiCl4, as reported in the literature, and as qualitatively confirmed by laboratory results. So, silicon tetrachloride appears to be less problematic than other metal halides in this respect: once the moisture has been removed and a vacuum has been obtained in the system / plant, loading the system with SiCl4 follows, but reactivity with water becomes a marginal problem especially when operating above ambient pressure.

[0037] Silicon tetrachloride (SiCl4), pure and in mixture, is proposed as a working fluid for closed, subcritical or supercritical Rankine thermodynamic cycles operating at high temperature. SiCl4 is an inorganic compound of the metal halide family, which appears as a colorless liquid under ambient conditions, as its boiling point is 57.6°C. Such a working fluid has high thermal stability as, in the past, it was formed by reacting SiO2 and chlorine at a temperature of 1300°C. In addition, SiCl4 is widely found industrially mostly as a waste product of the high-purity silicon production process: the pyrolysis of SiHCl3 to obtain silicon results in the production of SiCl4 as a by-product, which needs to be reduced above 900°C with hydrogen to increase silicon production. It is known that the production of one ton of polycrystalline silicon involves the formation of about four tons of silicon tetrachloride, which is why it is commercially widely available at a low cost.

[0038] In one form of realization, the working fluid consists only of pure silicon tetrachlorid.

[0039] According to one form of implementation, the working fluid is a mixture comprising silicon tetrachloride and at least one further substance.

[0040] Preferably, the at least one further substance is carbon dioxide.

[0041] In an embodiment form, the working fluid is a binary mixture of carbon dioxide (CO2) and silicon tetrachloride (SiCl4), in which the silicon tetrachloride has molar percentage less than 30% and the carbon dioxide has a molar percentage greater than 70%.

[0042] In the literature, namely in the publication United States. Bureau of Mines., Mineral Facts and Problems: 1985 Edition, Washington, D.C., 1985, SiCl4 is reported to be stable even at 1200°C. However, it was necessary to investigate the thermo-chemical stability of the working fluid when it comes into contact with materials typically used in power plants. In fact, the thermal stability of a working fluid does not depend only on the chemical bonds of which the molecule is made, as the materials with which it comes into contact can act as catalysts to overcome the activation energy required to cleave the chemical bonds.

[0043] An experimental campaign aimed at establishing the applicability of SiCl4 with commercially used materials for making system components was carried out at the Fluid Test Lab of the University of Brescia. "Applicability" means the simultaneous testing of the thermal stability of the fluid and a low-level interaction with a material to make the components, such that no significant damage is induced over the life of the system. It is necessary to analyze both the fluid and the material, since the thermal stability of the working fluid alone sometimes does not reveal the interaction (even vigorous interaction) that might exist between the working fluid and the material of the system components.

[0044] For the purpose of this discussion, the thermal stability of the working fluid was analyzed in the laboratory by the static and isochoric (i.e., closed-volume) method described in C.M. Invernizzi, THERMAL STABILITY EVALUATION FOR RANKINE CYCLES WORKING FLUIDS: EXPERIMENTAL APPARATUS AND CALIBRATION RESULTS, The Thermotec. No. 4 (1990) 69-76, which is based on the change in vapor pressure between the virgin fluid and the thermally stressed fluid.

[0045] It should be noted that the phrase "virgin fluid" is intended to refer to a fluid that is essentially new, i.e., not subjected to thermal stress of any kind.

[0046] The experimental apparatus is presented in the attached figures 3a 3b. The choice of analyzing the vapor pressure is particularly strategic for investigating the presence of partial or total decomposition products of the working fluid because it is a property of the fluid, independent of the amount of working fluid fed into the closed volume, and is extremely sensitive to the presence of any decomposition products, especially if these decomposition products are incondensable.

[0047] The thermal stability test procedure involves the following steps: a) after vacuuming a system (to remove air and unwanted products), an arbitrary sample of virgin working fluid is placed in a test cylinder 100, closed; b) the test cylinder 100 is placed vertically in a thermostatic bath 200 to measure its reference vapor pressure at different temperatures to obtain the REF reference curve in figure 4; c) the test cylinder 100 is placed vertically in a 300 furnace, such as a muffle furnace, to perform thermal stress at a constant temperature for a predetermined residence time (such as 100 hours); d) following thermal stress, the test cylinder 100 is placed back into the thermostatic bath 200 to measure the vapor pressure at the same temperatures at which the vapor pressure of the virgin working fluid was measured; e) if no appreciable deviation from the reference vapor pressure (i.e., the vapor pressure of the virgin working fluid) is observed, a thermal stress is performed at a higher temperature than the preceding one; otherwise, it can be said that the preceding thermal stress was performed at a temperature beyond the thermal stability limit of the fluid. In other words, steps c)-d) are repeated until a deviation is observed between the vapor pressure of the stressed working fluid and the vapor pressure of the virgin working fluid.

[0048] In addition, before step a), i.e., before loading the working fluid into Test Cylinder 100, a leak test with helium at 30 bar for 30 hours is performed to verify that there is neither pressure change in the system nor weight change.

[0049] An initial thermal stability test was performed, in which test cylinder 100 of the system is made of AISI 316L, i.e., a material commonly used in power systems. A 22-gram sample of working fluid consisting exclusively of SiCl4 was loaded into the 150 cm3 volume test cylinder, after vacuuming the system using a special vacuum pump. The temperature of the thermostatic bath 200 is controlled by a PID 201 and can be varied between -40°C and 150°C with a resolution of 0.1°C and a stability of ±0.2°C.

[0050] During vapor pressure measurement, first with the virgin working fluid and then with the stressed working fluid, both pressure and temperature are measured every 5 seconds. The acquisition of the measurements takes place5 in the LabVIEW environment, which is also responsible for monitoring the system.

[0051] The results of the thermal stability test are presented in figure 4, following which it can be stated that SiCl4 is thermally stable in AISI 316L at 650°C. Subsequent thermal stress, carried out at 700°C, does not provide any further useful information on the stability of the working fluid at this temperature and in contact with AISI 316L, since test cylinder 100 was damaged by overpressure. However, from an application point of view, the result obtained at 650°C with a relatively inexpensive material such as AISI 316L, when compared with superalloys such as Inconel, is promising and highlights the potential of the working fluid consisting of SiCl4.

[0052] It should be noted that the test performed at 700°C does not necessarily signify a degradation of silicon tetrachloride, but indicates a situation worthy of further investigation, as the unexpected outcome of the 700°C test could be attributable to the test cylinder (material, welds, geometry, surface finish, etc.).

[0053] As mentioned earlier, for application in power systems, it is necessary not only that the working fluid does not degrade at the high temperature sections of the system, but also that the material with which it is in contact does not experience severe corrosion or deposition.

[0054] For this reason, a second medium-term thermal stability test was conducted, i.e., with maximum residence time in the furnace of 300 hours, maximum temperature of 550°C, and AISI 316L as the material of test cylinder 100. In addition, a metallographic analysis was conducted following the test.

[0055] The thermal level chosen for that second test is compatible with many high-temperature applications (concentrating solar power, biomass combustion, high-temperature heat recovery, etc.).

[0056] After the thermal stress was completed, the stability of the working fluid - which consisted exclusively of SiCl4 - was compared with the reference vapor pressure of the virgin working fluid, after which the test cylinder 100 was sectioned and analyzed optically to visualize any interaction between the working fluid and the material.

[0057] The attached figure 5 shows the experimental results of the second thermal stability test, in which the vapor pressure of the working fluid stressed at 550°C for 300 hours is compared with the vapor pressure of the virgin working fluid, i.e., not thermally stressed and denoted by REF.

[0058] Metallographic analysis following prolonged exposure of the material to silicon tetrachloride was carried out with a MeF3 optical microscope equipped with LAS4.12 image analysis software.

[0059] As shown in the attached figures 6a-6b, the optical microscope analysis offers good results since the interaction between the AISI 316L walls and silicon tetrachloride appears mostly negligible both in the base metal (figure 6a) and in the thermally altered zone of the welds. Only in one of the analyzed welds (figure 6b) was a crack detected that could potentially be pre-existing. The wall surface of test cylinder 100 appears jagged due to a rather coarse finish. Thus, the analysis confirms the potential applicability of pure silicon tetrachloride (not in a mixture) at 550°C and under contract with AISI 316L.

[0060] A third thermal stability test was conducted on a mixture comprising silicon tetrachloride (SiCl4) with a molar rate of 20% and carbon dioxide (CO2) with a molar rate of 80%.

[0061] The interest in binary mixtures comprising CO2, as introduced earlier, is due to the possibility of condensing the working fluid at 50-60°C, with ambient air also available at temperatures typical of arid and desert areas.

[0062] In the third thermal stability test, the presence of CO2 necessarily implies the use of high-grade materials for the test cylinder (such as Inconel 625 or 617) from about 500°C, because above that temperature (500°C) CO2 reacts with stainless steel (e.g., AISI 316L). For this reason, the third thermal stability test was conducted with a test cylinder 100 made of Inconel 625. The methodology by which the test was conducted on the mixture comprising SiCl4 and CO2 is the same as that already described for the pure working fluid, the only difference being that the curve obtained in the pressure-temperature plane represents a unique isochoric curve, thus dependent on the mass of the mixture loaded in the closed volume of the test cylinder; therefore, the repeatability of this curve following thermal stress at constant temperature remains the criterion for establishing the thermal stability of the fluid. Following the third thermal stability test on the mixture, it can be stated that it is stable in contact with Inconel 625 below 650°C, as evident in figure 7.

[0063] As already mentioned, the presence of CO2 necessarily implies the use of valuable materials such as Inconel 625, which, however, may not be the optimal solution for the dopant fluid (SiCl4) that, in preliminary analysis, showed better behavior with AISI 316L. However, an extensive experimental material compatibility campaign might uncover a material that is more compatible with both mixing fluids.

[0064] Preferably, the system comprises a plurality of elements (i.e., system components) suitable to come into contact with the working fluid. This plurality of elements is made of a metal material, e.g., AISI 316, AISI 316L, carbon steel, Inconel 617, or Inconel 625.

[0065] It also forms an object of the present invention, a use of a working fluid comprising silicon tetrachloride in a system for performing a Rankine thermodynamic cycle, preferably an organic-fluid Rankine (ORC) system.

[0066] Following the thermal stability tests, a preliminary analysis was conducted to evaluate the performance of a Rankine thermodynamic cycle operating with pure SiCl4 as the working fluid.

[0067] As experimentally proven, pure SiCl4 is thermally stable up to 650°C in contact with stainless steel, thus applicable well beyond the current thermal stability limit of ORC systems. Furthermore, analysis on the material following prolonged thermal stress (second thermal stability test) demonstrated the applicability of a relatively low-grade metal such as AISI 316L steel in power plants.

[0068] The mixture of SiCl4 with CO2 was also found to be stable up to 650°C in Inconel 625.

[0069] Such working fluids, i.e., pure SiCl4 or mixtures comprising SiCl4, can be coupled with high-temperature sources such as biomass boilers, turbogas, concentrating solar power, waste heat from glassworks and industrial processes, in order to produce electricity and useful heat with higher efficiency than today's technologies. Several turbine inlet temperatures (TIT) were considered, representative of coupling with high-temperature thermal sources.

[0070] A representative schematic of a system for a Rankine thermodynamic cycle using pure SiCl4 as the working fluid is shown in figure 8, wherein heat recovery from hot gases of a high-temperature process has been represented as the heat source.

[0071] As illustrated in figure 8, there is no heat-transfer fluid interposed between the hot industrial waste gases (i.e., heat recovery from high-temperature process gas) and the steam generator (also known to the industry technician as a heat exchanger), because the maximum thermal level attainable by SiCl4 (650°C) in the cycle is higher than conventional commercially used heat-transfer fluids (e.g., diathermic oil used below 400°C, molten salts used up to 565°C).

[0072] In one form of implementation, if a heat-transfer fluid is used, sodium or ternary salts, which are already being considered in next-generation CSP systems, could be used.

[0073] According to one form of realization, the system for the realization of the Rankine thermodynamic cycle comprises a plurality of elements, such as: pressure increasing means 10, such as a pump or compressor, to compress / pump the working fluid up to a maximum pressure value; a heat exchanger 20 so as to heat the working fluid by means of a heat source 15; a turbine 30 to expand the working fluid; a condenser 40 to cool the working fluid.

[0074] Preferably, the plurality of elements comprises at least one regenerator 50 that harnesses the heat of the working fluid exiting the turbine 30 to heat the working fluid compressed by the pressure increasing media 10 before entering the heat exchanger 20.

[0075] In an embodiment form, the at least one regenerator 50 comprises a primary regenerator 51 and a secondary regenerator 52 arranged downstream of the primary regenerator 51. Preferably, when the secondary regenerator 52 is present, the pressure increasing means 10 comprise a primary compressor 11 (or a primary pump) and a secondary compressor 12 (or a secondary pump) arranged in parallel with the primary compressor 11, such that the primary compressor 11 is fluidically connected to the primary regenerator 51 and then to the secondary regenerator 52, while the secondary compressor 12 is fluidically connected only to the secondary regenerator 52.

[0076] Preferably, following the heating of the working fluid in the heat exchanger 20, where hot industrial waste gases are thermally recovered, the gases are expelled through a chimney 60.

[0077] A method for producing mechanical and / or electrical and / or thermal energy comprising the following steps is also an object of the present invention: providing a system for performing a Rankine thermodynamic cycle; compressing the working fluid by the means of pressure increasing means 10 such as a pump or compressor up to a maximum pressure value; heating the working fluid up to a maximum temperature value by means of heat source 15 or by means of at least one regenerator 50 and heat source 15; expanding the working fluid by the turbine 30; cooling the working fluid by condenser 40 or by the at least one regenerator 50 and condenser 40.

[0078] In a form of realization, the maximum pressure value obtained by the means of pressure increase 10 is between 10 and 300 bar, preferably it is between 40 and 300 bar.

[0079] According to one form of realization, heat source 15 exchanges heat directly with the working fluid; alternatively, heat source 15 exchanges heat with an intermediate heat-transfer fluid and in turn that intermediate heat-transfer fluid exchanges heat with the working fluid.

[0080] In accordance a form of realization, between the heat source 15 and the heat exchanger 20 there is a fluidic circuit 70 containing the intermediate heat-transfer fluid that receives heat from the heat source and then releases that heat to the working fluid.

[0081] Preferably, the maximum temperature value of the working fluid - obtained before entering turbine 30 - is between 300 and 700°C.

[0082] It should be noted that the optimum condition at the level of turbine inlet temperature (TIT) and maximum pressure should be selected following an economic analysis taking into account the application, the materials required to achieve these pressures and temperatures, and the thermodynamic cycle efficiency. Therefore, should the working fluid be used below 550°C, the use of commercially mature molten salts as an intermediate heat-transfer fluid is also possible.

[0083] The thermodynamic model used to evaluate the optimal condition is the Peng Robinson cubic equation of state. The assumptions made about the non-ideality of the components of the ORC cycle, namely turbomachinery (pump, turbine) and exchangers (recuperator, exchanger), are shown in the following table. PARAMETERVALUEMinimum temperature58°CMinimum pressure1.05 barTurbine inlet temperature TIT450-650°CΔTmin in the recuperator15°CHot / cold branch recuperator pressure drops2 / 1%PHE / Condenser Pressure Drop3 / 2%Turbine isentropic efficiency≈88%Pump isentropic efficiency70%

[0084] In particular, an isentropic efficiency of 70% was assumed for the pump, a reasonable value given that the optimal conditions of the Rankine cycle involve pumping at supercritical pressures. In addition, the weight of the pump efficiency on the thermodynamic efficiency of the cycle is low, since the pumping work is significantly less than the work extracted at the turbine blades.

[0085] In accordance with the publication of E. Macchi, M. Astolfi, Organic Rankine Cycle (ORC) Power Systems: Technologies and Applications, 2016, the turbine isentropic efficiency is derived, under each operating condition, with Astolfi and Macchi's correlation obtained following the design of N axial turbines for ORC cycles. Since single-stage axial turbines are highly penalized by high volumetric expansion ratios - which in the case of SiCl4 is high (around 38) - the correlation relative to two-stage axial turbines was considered. The low enthalpy jump (around 137 kJ / kg under isentropic conditions) astride the turbine favors a good isentropic efficiency of the aforementioned two-stage turbine, close to 88% under the optimum thermodynamic cycle conditions.

[0086] The cycle proposed in figure 8, which uses pure SiCl4, involves the use of a regenerator 50, which allows the thermal energy still available at the turbine exhaust 30 to be recovered internally in the cycle: this heat allows the working fluid to be preheated after the pump, before entering the heat exchanger 20 (also known to the technician in the field as the evaporator or primary exchanger). For the cycle in question, given the high TIT, the use of a regenerator 50 is inevitable to achieve good thermodynamic efficiency.

[0087] A minimum temperature of 58°C was considered since the "normal" boiling point, at atmospheric pressure, of the fluid is 57.7°C, so condensing at 58°C means avoiding sub-atmospheric conditions in the system. This choice was made for the following reasons: (i) to avoid the aid of an expensive incondensable extraction and working fluid recirculation system; (ii) avoid infiltration of atmospheric air, known the reactivity of SiCl4 with water in moist air.

[0088] Thermodynamic cycle efficiency at different TIT, according to the assumptions in the table above, is presented in figure 9, wherein thermodynamic efficiency is diagrammed as a function of maximum pressure considering different turbine inlet temperatures (TIT).

[0089] Although the thermodynamic performance optimum may result above 90 bar maximum pressure for certain TIT, above 60 bar there is a kind of plateau such that the increase in maximum pressure does not result in a significant improvement in thermodynamic performance. Therefore, it is permissible to consider 60 bar as the maximum design pressure of the cycle in the first analysis, since a further increase in maximum pressure would result in a corresponding increase in metal thicknesses to cope with mechanical stresses, in addition to the difficulty of commercially sourcing heat exchangers capable of sustaining such stresses especially in the high-temperature sections.

[0090] Since the critical pressure of SiCl4 is 35.9 bar, this design point corresponds to thermodynamic conditions of a supercritical Rankine cycle (or rather transcritical, since the lower isobar is below the critical pressure). Supercritical Rankine cycles are, at the state of the art, little adopted commercially, although they may be advantageous for coupling with thermal sources of finite heat capacity. The relatively low critical pressure of SiCl4 allows the fluid to be applied under transcritical conditions.

[0091] In accordance with the state of the art, the critical pressure of working fluids adopted in the few existing supercritical systems is slightly higher than that of SiCl4. In 2012, a manufacturing company made a 500 kWel prototype with supercritical R134a (critical pressure 40.06 bar) to improve conversion efficiency in geothermal applications. In the same year, the company TAS Energy brought to market a 22 MWel system with the same working fluid, for the Neal Hot Springs geothermal system. In 2014, Granite Power built a 30 kWel parabolic trough CSP system to demonstrate the use of supercritical GRANEX technology, but the fluid used was not revealed. In 2012, a 7.5 MWel ORC for thermal recovery was installed at a natural gas compressor station used supercritical butane (critical pressure 38 bar).

[0092] Figure 10 shows the T-s diagram of the supercritical Rankine thermodynamic cycle using pure SiCl4 as the working fluid. This is a simple regenerative cycle (i.e., with only one regenerator), maximum pressure of 60 bar and turbine inlet temperature (TIT) of 550°C.

[0093] In addition, on the cycle shown in figure 10, a sensitivity analysis was conducted on the minimum cycle temperature to analyze its impact on the thermodynamic efficiency of the cycle by setting the maximum pressure at 60 bar and the TIT at 550°C. Figure 11 is a diagram showing thermodynamic efficiency (left y-axis) and minimum pressure (right y-axis) as a function of condensing temperature under the conditions of maximum pressure 60 bar and turbine inlet temperature (TIT) of 550°C.

[0094] As expected, minimum cycle temperature has a significant impact on efficiency. An efficiency gain of 2% occurs at a minimum cycle temperature reduction of about 20°C. From figure 11, it can be seen that condensation at 35°C, which is typically achievable if chilled water or ambient air at a suitable temperature is available, results in a minimum cycle pressure of around 0.5 bar, which is not excessively sub-atmospheric: with appropriate sealing precautions, it is possible to operate under such conditions with an efficiency gain.

[0095] In contrast, considering titanium tetrachloride (TiCl4) in accordance with the state of the art, condensation at 53°C would result in a minimum cycle pressure of 0.05 bar, making it more difficult to handle dangerous air infiltration as well as higher compression and volume ratios.

[0096] According to the NFPA 704 Hazard Identification System standard, SiCl4 is toxic; like any chlorine gas, it is classified as a material that following brief exposure can cause temporal or residual harm (Health Hazard 3). Its toxicity is mainly associated with the formation of HCl in contact with moisture in the ambient air. Therefore, the fluid needs to be applied in the closed cycle with proper precautions, such as an auxiliary suction and neutralization system.

[0097] From laboratory experience, it was possible to completely neutralize the SiCl4 evacuated from the system by a solution of NaOH and water. So, any emptying of a portion of the system (e.g., for overpressure or maintenance) can be totally neutralized in this manner. Through proper procedures, it is possible to operate safely with such a working fluid despite its toxicity. Since it is in fact widely produced industrially, it stands to reason that there are numerous safety protocols already implemented, especially in the silicon industry where about 4 tons of SiCl4 is produced for every ton of polycrystalline silicon.

[0098] Advantageously, silicon tetrachloride is nonflammable, so it does not require the installation of an expensive safety system in this regard. In addition, it has zero "global warming potential" (GWP).

[0099] Typically, biomass boilers employ siloxane fluids (such as MM or MDM) as they are capable of operating at high temperatures, about 300-350°C. From the heat output made available from biomass combustion, the ORC engine can operate either in electric-only mode or, more commonly, in cogeneration mode. In electric-only mode, the thermal power entering the cycle is converted to electric power, through the thermo-electric efficiency of the ORC engine. In cogeneration mode, on the other hand, there is both electric power and thermal power as dual outputs, where the latter is made available by the thermal power rejected from the cycle through capacitor 40. In order for the ORC engine to provide this thermal power at a temperature appreciable by the thermal user, usually consisting of a district heating network, isothermal condensation of the fluid is carried out around 120°C, thus penalizing the thermodynamic efficiency of the cycle.

[0100] Figure 12 shows the system schematic of an ORC cycle - using pure or blended SiCl4 - applied to a biomass boiler, where at condenser 40 the transfer of useful thermal power to the utility can occur.

[0101] In this regard, the use of a cogeneration configuration, with condensation around 120°C, would in any case provide a thermodynamic advantage over the cogeneration engines used in the state of the art.

[0102] Innovatively, the present discussion illustrates the exploitation of pure or blended silicon tetrachloride for the production of useful mechanical, electrical, and thermal energy.

[0103] In particular, the use in CO2 mixture is of interest, as its high critical temperature (234°C), allows a condensing mixture to be obtained even at high ambient temperature.

[0104] To the forms of realization of the system, use and method according to the invention a branch technician, in order to meet contingent needs, may make modifications, adaptations and replacements of elements with functionally equivalent ones, without leaving the scope of protection of the following claims. Each of the features described as belonging to a possible form of embodiment can be realized independently of the other described forms of embodiment.

Claims

1. A system for performing a Rankine thermodynamic cycle, comprising a working fluid suitable to undergo thermodynamic transformations, wherein said working fluid comprises silicon tetrachloride.

2. A system according to the preceding claim, wherein the working fluid consists only of pure silicon tetrachloride.

3. A system according to claim 1, wherein the working fluid is a mixture comprising silicon tetrachloride and at least one further substance.

4. A working system according to claim 3, wherein the at least one further substance is carbon dioxide.

5. A system according to any one of the preceding claims, comprising a plurality of elements suitable to come into contact with the working fluid, said plurality of elements being made of a metal material, such as AISI 316, AISI 316L, carbon steel, Inconel 617, or Inconel 625.

6. Use of a working fluid comprising silicon tetrachloride in a system for performing a Rankine thermodynamic cycle according to any one of the preceding claims.

7. A method for producing mechanical and / or electrical and / or thermal energy comprising the following steps: - providing a system according to any one of claims 1 to 5; - compressing the working fluid by means of pressure increasing means (10), such as a pump or a compressor, up to a maximum pressure value; - heating the working fluid up to a maximum temperature value by means of a heat source (15) or by means of at least one regenerator (50) and the heat source (15); - expanding the working fluid by means of a turbine (30); - cooling the working fluid by means of a condenser (40) or by means of the at least one regenerator (50) and the condenser (40).

8. A method according to the preceding claim, wherein the maximum pressure value is between 10 and 300 bar, preferably between 40 and 300 bar.

9. A method according to any one of claims 7 to 8, wherein the heat source (15) exchanges heat directly with the working fluid, or the heat source (15) exchanges heat with an intermediate heat-transfer fluid, and in turn said intermediate heat-transfer fluid exchanges heat with the working fluid.

10. A method according to any one of claims 7 to 9, wherein the maximum temperature value of the working fluid is between 300 and 700°C.

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