EP0772757B1 - Energy transfer system between a hot source and a cold source - Google Patents

Description

The present invention relates to a system for energy transfer between a hot source and a source cold, using a pumped two-phase loop capillary.

Two-phase capillary pump loops provide profit the following physical phenomenon: if we send, to a end of a heated capillary tube, a liquid having suitable properties, this liquid enters the tube capillary to a point where it completely vaporizes. The liquid and vapor phase separation surface has a curved shape, and is called "meniscus". We observe, at meniscus level, in the vapor phase, an increase significant pressure, which can be used to the fluid circulating in a closed circuit comprising, in addition to the evaporator capillaries, a suitable condenser.

The phenomena are the same if, instead of a tube capillary, we use a "capillary mass", that is to say a material having an open porosity with passages of substantially homogeneous dimensions, typically 2 to 20 micrometers.

This increase in pressure results from surface tension. It depends on the temperature and the nature of the fluid and the solid walls with which it is in contact, and it is inversely proportional to the radius of the meniscus, or the equivalent radius in case the meniscus is not spherical. The radius of the meniscus or the equivalent radius are themselves very closely related to radius of the capillary, and more generally the radius of curvature of the solid surface in contact with which is made change of state. The pressure increase is therefore negligible if the liquid-vapor interface is in contact with solid surfaces with radii of curvature of a few hundreds of micrometers.

In this text, we talk about evaporators capillaries and condensers. These terms can each times apply to groups of capillary evaporators or of condensers arranged in parallel in the closed circuit.

To fix ideas, we built on this principle systems using ammonia between -10 and + 60 ° C, with equivalent meniscus radii on the order of 10 micrometers, the pressure generated at the menisci was around 5 kPa, which is sufficient to compensate for the circuit pressure losses. The condensers could be constituted either by radiators which radiate energy towards space, either by exchangers coupled with other analogous systems, either by change-over devices phase such as boilers or evaporators.

Such systems are used today in the space domain.

These systems have the disadvantage of not being able operate in a closed circuit only in one direction, the still in the hot spring. AT on board spacecraft, sometimes transfers thermal must be carried out one way and sometimes in the opposite direction, for example in the case of daily or seasonal variations in sunshine. It is necessary in this case to implant two loops independent operating alternately and in direction reverse, which complicates the apparatus and increases its size.

The object of the present invention is to provide a apparatus which allows energy transfers in two opposite directions, in a simple way and in a limited volume.

To obtain this result, the invention provides a energy transfer system between a hot source and a cold source, the system comprising an evaporator capillary located in the hot spring and in which a fluid is introduced in the liquid state and passes completely in the vapor state, a vapor pipe, a condenser located in the cold source where the fluid returns to the state liquid, and a liquid conduit that brings the fluid back to the capillary evaporator, the fluid circulating in the circuit closed under the effect of the pressure generated at meniscus constituting the liquid / vapor interfaces in the evaporator capillaries, this system having for particularity that the closed fluid circuit includes two assemblies each formed by a capillary evaporator connected to the liquid conduit and condenser interposed between the capillary evaporator and the vapor duct, one of the sets located in the hot spring and the other in the cold source, and that the amount of fluid is calculated in such a way that all of the evaporation takes place in capillary passages of capillary evaporation located in the hot spring and that all of the condensation is done in the condenser located in the cold source.

It will be understood that, in the hot spring, the evaporation in the capillary evaporator creates the increase in pressure required to set the fluid in motion. In the cold source, if the condensation took place in the capillary evaporator, it would appear therein reverse pressure difference, which could be of the same order of magnitude, the difference depending mainly temperature differences between hot springs and cold. In fact, as the condensation takes place in the cold source condenser, the capillary evaporator which follows it in the direction of circulation of the fluid behaves as a simple passive resistance, because its passages are completely filled with condensate. The condensation on surfaces with a large radius of curvature condenser generates practically only reverse pressures negligible.

The circuit filling must be done with precision, so that changes in fluid state occur do in the places provided. Some latitude is provided by the length of the passages in the evaporator capillary and the dimensions of the condenser. This latitude may be exceeded in the case, for example, of a lowering of the temperature of the liquid, causing contraction of it. We found, in a way surprising, that even in this case, which corresponds to a "underfill" the system continues to operate from correctly when a vapor bubble has formed from the side of the capillary evaporator which is normally in contact with the liquid, and this as long as this bubble is completely separated from the vapor duct by retained liquid by capillarity in the capillary evaporator.

We can therefore predict that the quantity of fluid is calculated so that under all conditions of operation, at least one liquid-vapor interface is found in the capillary evaporator, a vapor bubble without communication with the steam pipe which can nevertheless possibly be on the liquid side of the capillary evaporator.

According to an interesting realization, in the case where the capillary evaporator consists of a mass controlled porosity in which the liquid can spray with formation of menisci with a small radius or equivalent radius, this mass being placed in an enclosure between two chambers, one connected to the liquid pipe and the other to the vapor conduit, the source condenser cold is constituted at least in part by that of said chambers which is connected to the steam duct. In case all the condensation can be done in this room, that is to say inside the enclosure of the device capillary evaporation in the current sense of the term, we results in a remarkably simple and compact package.

According to a more sophisticated embodiment, it there are several hot springs and / or several sources cold, and there is at least one of said assemblies formed of a capillary evaporator and condenser in each source hot and each cold source.

It was unexpectedly found that the system self-stabilized even with appreciable differences in temperature between hot springs or between sources cold.

The invention will be explained in more detail in using practical examples illustrated by the drawings, among which :

Figure 1 is a block diagram of a system of prior art.

Figure 2 is a block diagram of a system according to the invention.

Figures 3 and 4 are sections respectively longitudinal and transverse of an evaporation device capillary of the usual technique.

Figure 5 is a perspective diagram of the arrangement several capillary evaporation devices.

Figure 6 is a diagram showing a meniscus.

Figure 1 shows a block diagram of a system intended to transfer thermal energy from zone A, called "hot spring", to zone B, at temperature lower, called "cold source".

This system includes a closed circuit in which circulates a fluid which can be, depending on temperatures of use, water, ammonia, a "Freon" etc. This circuit includes capillary evaporation devices 1 connected in parallel, condensers 2, also connected in parallel (or parallel series), a steam circulation 3 and a circulation duct of liquid 4. The direction of circulation of the fluid is indicated by arrows 5.

Figures 3 and 4 show the structure of a capillary evaporation device in common use.

This device comprises a metal tube 6 having a inlet 7 at one end and outlet 8 at the end opposite. Inside the tube, a cylinder of material porous 9 is held by spacers 10 coaxially to tube 6. This porous material consists of fibers parallels arranged so as to constitute between them passages of maximum controlled dimension, for example of on the order of 20 micrometers, and forming what is called a "capillary wick".

The porous material can consist of any material having pores of suitable dimensions and substantially homogeneous, for example sintered materials metallic or plastic, or ceramic.

Figure 5 shows a hot spring consisting of a plate 11 on one side of which are mounted equipment 12 which gives off heat and / or which is desired cool. On the opposite side of the plate are fixed capillary evaporation devices 1 whose input 7 is connected to a liquid line 5 and communicates with the vacuum inside 13 (see FIG. 4) of the capillary wick 9, and whose outlet 8 is connected to a steam pipe 3 and communicates with the annular space 14 located between the tube 6 and the hair wick 9.

In normal operation, the internal vacuum 13 is filled with liquid, and the annular space 14 is filled with steam. The liquid-vapor interface consists of a set of menisci 15 (see Figure 6), rays substantially equal equivalents, all of which are found in the thickness of the porous mass 9.

In the usual technique, the devices capillary evaporation which we have just described are known under the name of "capillary evaporators". It emerges from this which precedes that, within the meaning of this text, only the mass porous 9 therefore constitutes the capillary evaporator properly said, the vacuum 13 and the space 14 being, functionally, liquid pipe or vapor pipe extensions.

The circulation of the fluid is due to the increase in steam pressure, in capillary evaporators, which is generated at the menisci where the total vaporization of the liquid takes place. during the passage of the capillary wick, the liquid gets heats up very quickly (the flow rates are very low) and vaporizes completely at the menisci at temperature almost constant. The increase in pressure is proportional to the surface tension of the fluid and inversely proportional to the equivalent radius of menisci (we work with radii less than 10µm). The fluid flow in each evaporator is thus constantly self-adjusting to have only steam pure at the outlet of each evaporator.

To have correct operation of the evaporators capillaries, it is imperative to have only liquid to the inlet of each capillary evaporation device. These devices can therefore only be arranged in parallel. In addition, an insulator 16 (Figure 1) must be positioned at the inlet of each evaporator. The role of this isolator is to prevent a return of vapor (in the main tube of liquid from the loop) which could occur in a evaporator during accidental defusing (during too strong power injection for example).

Pure steam is transported to condensers 2 where the energy acquired by the fluid is extracted, either by radiators (which radiate energy to space), either by exchangers coupled to other loops, either by phase change systems such than boilers or evaporators.

Returning to the device of FIG. 1, a sub-cooler 17 is positioned on the outlet tube liquid. The role of this sub-cooler is to condense vapor which, accidentally, for situations not nominal, would not have been fully condensed at the exit of one of the last condensers.

The operating temperature of the loop is controlled by a two-phase pressurizer tank 18. This tank is thermally controlled (heating system and cooling) so as to ensure control of its vaporization temperature which is also the temperature vaporization at the "cold plates" 11 and exchangers (except for pressure losses, which are minimal).

With this type of loop, you can control with a good precision (better than the degree in the majority of case) a set temperature, whatever the power variations undergone by the loop at the level evaporators or condensers.

The maximum power that can be transported is conditioned by the maximum pressure rise that can supply capillary evaporators and by the sum circuit pressure losses for maximum power considered. With ammonia and equivalent radii of menisci of 10µm, we can reach lifts of pressure of the order of 5000Pa.

Figure 2 shows the diagram of a transfer system of energy according to the invention.

In each of sources A and B, the circuit includes sets each consisting of a capillary evaporator 1A, 1B in series with a condenser 2A, 2B, a steam 3 being connected to each of the condensers 2A, 2B, and a liquid conduit 4 being connected to each of the evaporators capillaries 1A, 1B. A means of reheating the duct low power steam 20 is expected. There is no pressurizer tank 18 or insulators 16.

When the temperature of source A is higher than that of source B, the direction of circulation of the fluid is the one indicated by the arrows 21. The evaporators lA are active. The liquid at the inlet of the evaporators crosses the capillary strands 9 and vaporizes there. Steam leaves each evaporator device (with an increase of capillary pressure) and passes through the "hot" condensers 2A which are therefore inactive. Steam is collected at the output of these condensers and is transported in a tube 3 up to the inlet of the "cold" condensers 2B. The steam is partially or fully condenses in these condensers. A two-phase or single-phase liquid mixture therefore enters in evaporator devices 1B with "counter-direction" by compared to normal operation for an evaporator. The remaining vapor condenses completely in space ring 14 of the evaporator devices 1B. Some cash only comes out of these evaporators. The liquid is collected and is transported in tube 4 to the entry of 1A evaporators, which closes the loop. In the tube liquid; you can temporarily allow a spray partial liquid.

When source B becomes hotter than source A, the direction of circulation of the fluid is that of the arrows 22. The 1B evaporators play their role evaporators, the 2B condensers are inactive, the 2A condensers are active and the evaporator devices 1A act as additional condensers at the level of their annular space 14.

These annular spaces, which are included in the capillary evaporation devices, then, from the point from a functional point of view, part of the condensers 2A.

When you want to perform a heat transfer between the different sources and that the transfer does not take place no, the steam tube 3 must be slightly heated (typically with 1W / m) using the heating means 20 typically for an hour to expel the liquid which could be there.

In cases where the condensing capacities of annular spaces 14 of the inactive evaporators are sufficient, we can remove all condensers. The loop is then only constituted by devices conventional evaporators operating in evaporators, the others in condensers.

The concept proposed for two heat sources can be extended to a multi-source concept (we can have a different source by "evaporator-condenser", the system will self-adapt). Nor is it necessary for capillary evaporators 1A, 1B or condensers 2A, 2B sources A and B are identical in number or in performance, nor that the number of evaporator-condenser assemblies be the same in all sources.

In the space domain, the system according to the invention can be used to transfer heat between the different parts of a spacecraft subject to different heat fluxes as a function of time (daily or seasonal sunshine, dissipation thermal, ..). The advantages of this type of loop by compared to the current concept basically consist of the possibility of heat transfers bidirectional with a single loop, which contributes to simplification and reduction of the mass balance.

Claims (4)

1. A system for transfer of energy between a hot source (A) and a cold source (B), the system including a capillary evaporator situated in the hot source, and in which a fluid is introduced in the liquid state and changes integrally into the vapor state inside capillary passages, a vapor conduit (3), a condenser (2) situated in the cold source where the fluid changes back into the liquid state while condensing on surfaces of large radius of curvature, and a liquid conduit (5) which returns the fluid to the capillary evaporator, the fluid circulating in closed circuit under the effect of the pressure generated at the meniscus constituting the liquid/vapor interfaces in the capillary passages of the evaporator,
   in which:

   the closed fluid circuit includes two units each formed by a capillary evaporator connected to the liquid conduit and by a condenser inserted between the capillary evaporator and the vapor conduit, one of the units being placed in the hot source and the other in the cold source;

   and the quantity of fluid is calculated in such a way that the evaporation takes place integrally in the capillary passages of the capillary evaporator situated in the hot source and that the condensation takes place in the condenser situated in the cold source.
2. The system as claimed in claim 1, wherein the quantity of fluid is calculated in order that, in all the conditions of operation, at least one liquid-vapor interface is present, it being nevertheless possible for a bubble of vapor without communication with the vapor conduit to be present, possibly on the liquid side of the capillary evaporator.
3. The system as claimed in claim 1, wherein the capillary evaporator consists of a mass with controlled porosity in which the liquid can be vaporized with formation of menisci (15) of small radius or equivalent radius, this mass being placed in a vessel between two chambers (13, 14), one being connected to the liquid conduit and the other to the vapor conduit (3), and the condenser of the cold source consists at least partially of that one (14) of said chambers which is connected to the vapor conduit (3).
4. The system as claimed in claim 1, wherein there are a number of hot sources and/or a number of cold sources, and at least one of said units formed by a capillary evaporator and by a condenser is provided in each hot source and each cold source.

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