CO2-BASED MIXTURES AS WORKING FLUID IN THERMODYNAMIC CYCLES

Abstract

Described herein is a working fluid for a thermodynamic cycle that includes CO.sub.2 as main component and one or more of the compounds selected from the group including: TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, metal carbonyls, by way of example Ni(CO).sub.4.

Claims

1. A working fluid for a thermodynamic cycle that comprises CO.sub.2 as a main component and one or more of the compounds selected from the group consisting of TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, and metal carbonyls.

2. The working fluid for a thermodynamic cycle according to claim 1, which comprises CO.sub.2 and one of the compounds selected from the group consisting of TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, GeCl.sub.4, and metal carbonyls.

3. The working fluid for a thermodynamic cycle according to claim 1, wherein the one or more compounds is added with a concentration between 1 and 30 mol %.

4. The working fluid for a thermodynamic cycle according to claim 1, wherein the one or more compounds is added as a pure compound in 98% by weight.

5. The working fluid for a thermodynamic cycle according to claim 1, wherein the one or more compounds is added with a purity of 99.9% by weight.

6. The working fluid for a thermodynamic cycle according to claim 1, wherein the one or more compounds is TiCl.sub.4.

7. The working fluid for a thermodynamic cycle according to claim 3, wherein the one or more compounds is added with a concentration between 2 and 20 mol %.

8. The working fluid for a thermodynamic cycle according to claim 7, wherein the one or more compounds is added with a concentration between 2.5 and 10 mol %.

9. The working fluid for a thermodynamic cycle according to claim 4, wherein the one or more compounds is added at purities higher than 99%.

10. The working fluid for a thermodynamic cycle according to claim 9, wherein the one or more compounds is added at purities higher than 99.5%.

11. The working fluid for a thermodynamic cycle according to claim 5, wherein the one or more compounds is added with a purity of 99.99% by weight.

12. The working fluid for a thermodynamic cycle according to claim 11, wherein the one or more compounds is added with a purity of 99.9999% by weight.

13. The working fluid for a thermodynamic cycle according to claim 1, wherein the metal carbonyl is Ni(CO).sub.4.

Description

BRIEF DESCRIPTIONS OF THE FIGURES

[0012] FIG. 1: Efficiency of a thermodynamic cycle at temperatures between 400 and 700° C. using as working fluid either CO.sub.2 (dashed line) or a mixture which comprises CO.sub.2 and TiCl.sub.4 2.5 mol % (solid line).

[0013] FIG. 2: Efficiency of a thermodynamic cycle at temperatures between 300 and 700° C. using as working fluid either CO.sub.2 (dashed line) or a mixture which comprises CO.sub.2 and TiCl.sub.4 2.5 mol % (solid line), or CO.sub.2 and TiCl.sub.4 10 mol % (solid bold line).

[0014] FIG. 3: Diagram of a regenerative system which uses a mixture according to the present invention as a working fluid.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The mixture which is the object of the present invention comprises CO.sub.2 as main component and one or more of the compounds selected from the group which comprises: TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, metal carbonyls, by way of example Ni(CO).sub.4.

[0016] In a preferred embodiment, said mixture comprises CO.sub.2 and one of the compounds selected from the group which comprises: TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, metal carbonyls, by way of example Ni(CO).sub.4.

[0017] In a further embodiment, said mixture comprises CO.sub.2 and one of the compounds selected from the group which comprises: TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, GeCl.sub.4, GeCl.sub.4.

[0018] Said additional compound is present in the mixture of CO.sub.2 with a concentration comprised between 1 and 30 mol %, or between 2 and 20 mol %, or between 2.5 and 10 mol %.

[0019] In a preferred embodiment, the mixture according to the present invention consists in CO.sub.2 and one of the compounds selected from the group which comprises: TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4, wherein said additional compound is present at a concentration comprised between 1 and 30 mol %, or between 2 and 20 mol %, or between 2.5 and 10 mol %.

[0020] Said additional compound is added to the mixture according to the present invention as a pure compound in 98%, or 99%, or 99.5% by weight. In a preferred embodiment, the compound is added with a purity of 99.9% by weight, preferably 99.99%, even more preferably 99.9999%. The high degree of purity guarantees the absence of impurities which may have a low thermal stability and cause, at high processing temperatures, undesired and even violent reactions, such as for example explosions.

[0021] In a particularly preferred embodiment, said additional compound is TiCl.sub.4.

[0022] TiCl.sub.4, TiBr.sub.4, SnCl.sub.4, SnBr.sub.4, VCl.sub.4, VBr.sub.4 can be mixed with the liquid CO.sub.2 and surprisingly elevate its critical temperature, leading to a surprising advantage in terms of efficiency when the mixtures according to the present invention are employed in thermodynamic cycles which operate at temperatures equal to higher than 400° C.

[0023] The mixture according to the present invention has the surprising advantage of significantly modifying the critical temperature of the CO.sub.2, whereby obtaining a fluid which is liquid up to 40° C., or up to 50° C., i.e. higher than 31° C. Furthermore, the mixture obtained is thermodynamically stable and is able to withstand operating temperatures of 400° C. and higher without significant alterations and without originating undesired degradation products, increasing operational efficiency of thermodynamic cycles in which it is used as working fluid in replacement of CO.sub.2 as such.

[0024] The advantage in terms of operational efficiency recovery in thermodynamic cycles has been found to be particularly significant with mixtures which comprise concentrations higher than 2 mol % of compound. The advantage tends to be imperceptible to concentrations close to 40-50 mol % of compound. For each concentration, as the temperature increases, in particular at temperatures higher than 700° C., we have hereby demonstrated that the difference in efficiency between a thermodynamic cycle operating with pure CO.sub.2 and a thermodynamic cycle operating with a mixture according to the present invention tends to be lower, while maintaining a non-insignificant advantage for the mixture.

[0025] By way of example, FIG. 1 shows the performance of a thermodynamic cycle operating with a mixture of CO.sub.2 and TiCl.sub.4 2.5 mol % (solid line) and the performance of the same operating cycle with pure CO.sub.2 (dashed line). The mixture shows higher performance at all temperatures, at 400° C. the performance increase is greater than the increase observed at higher temperatures.

[0026] An increase in the concentration of the compound leads to better performance, but increments are greater at low molar concentrations (2-5%). An excessive increase of the concentration of the compound causes slightly better performance, with the disadvantage of operating with a greater amount of substance which is less easy to manage than CO.sub.2.

[0027] By way of example, FIG. 2 shows an example of comparison between the performance of a thermodynamic cycle operating with a mixture of CO.sub.2 and TiCl.sub.4 10 mol % (solid bold line), with a mixture of CO.sub.2 and TiCl.sub.4 2.5 mol % (solid line) and with pure CO.sub.2 (dashed line). It may be noted from the comparison between the solid bold line and the solid line, that the increase in performance is less evident as the concentration of TiCl.sub.4 in the mixture increases.

[0028] It is particularly interesting to observe that the advantages in terms of efficiency were observed at relatively low compound concentrations, between 2 and 5 mol %. This is particularly significant because it allows the use of CO.sub.2 mixtures with minimal additions of one of the indicated compounds. The toxicity, although limited, of the added compounds in the CO.sub.2 mixture and their cost compared to the cost of CO.sub.2 have very limited impact because of the small volumes used.

[0029] The performance of the thermodynamic cycles to which reference is made in the examples were evaluated by calculating the thermodynamic properties of the mixture. Cubic Peng-Robinson was used as thermodynamic equation; the equation parameters were inferred from experimental data.

[0030] The mixture according to the present invention is prepared according to methods known to a person skilled in the art. By way of example, the amount of one or more of the compounds to be added to the CO.sub.2 is introduced in an anhydrous tank and flushed with an inert gas, such as N.sub.2 or He. Said tank is connected to a CO.sub.2 bottle, at a pressure higher than the final pressure of the tank, whereby obtaining the desired mixture.

[0031] The mixture according to the present invention is validly applied as working fluid in systems for the generating electric and possibly even thermal energy. It is also an alternative to fluids commonly used in ORC systems and in refrigeration systems.

[0032] By way of example, it may be used in electric and thermal energy generation and distribution, in organic fluid Organic Rankine Cycle engines (ORC), in co-generative engines and in refrigeration systems.

[0033] The mixture according to the present invention is advantageously used in geothermal heat systems, solar heat systems, in particular solar towers, in waste incinerators or biomass systems. Advantageously, the mixture according to the present invention is used in aerospace applications.

[0034] FIG. 3 diagrammatically shows, by way of example, a regenerative system operated with the mixture according to the present invention. The system (1) comprises a circuit (2) in which the mixture according to the present invention circulates. Said circuit (2) comprises two exchange points with the outside environment, a heat exchanger (3) which acquires heat from the outside and a condenser (4) which exchanges heat with the environment to obtain saturated liquid. Said mixture is circulated in said circuit (2) by a pump (5). Since said mixture is in liquid state, the circuit is operated by a pump and not by a compressor, which requires higher energy costs for operation. Having acquired heat from the heat exchanger, the mixture actuates a turbine (6) which generates the energy after its crosses a recuperator (7). The passage in the recuperator (7) exchanges heat between the mixture exiting from the turbine and the mixture entering into the primary heat exchanger, whereby allowing a recovery of the remaining heat, as well as a lowering of the temperature of the mixture before it enters into the condenser. From the recuperator, the mixture flows into the condenser (4), in which it condenses and resumes the cycle.

[0035] Advantageously, the solution suggested in the present invention, by replacing the CO.sub.2 with a mixture having a critical temperature greater than the CO.sub.2, makes it possible to have the same mixture in liquid phase also at temperatures higher than the critical value of CO.sub.2, wherein said mixture is in a liquid phase even at temperatures of 50° C., or 40° C. The mixture according to the present invention, makes it possible to have a working fluid in liquid phase, whereby permitting the adoption of the Rankine cycle, in which the compression phase occurs advantageously by using a pump and not a compressor.

[0036] TiCl.sub.4 is particularly advantageous since it is an intermediate synthesis product. The product is therefore available at an affordable price.