Field of the invention

The invention relates to the supercooling of water.

Background This background section is provided to describe the utility of the invention and may thus be considered as a part of the invention and its possible characteris tics. Any of the solutions described below can be uti lized, together or separately, in connection with a so- lution suitable for the supercooling of water according to the invention.

Flowing water may be, so to speak, supercooled in the liquid phase to temperatures below zero degrees by a heat exchanger, and the crystallization of such a su percooled water can be triggered after the fact, for example, by directing the current to strike against a perpendicular wall, or by using tiny ice crystals as crystallization nuclei. The result is a so-called ice slurry which is a slush-like water-ice mixture in which the ice is in the form of exceptionally tiny crystals. Ice slurry has interesting characteristics, due to which its efficient production has long been researched. Be cause the crystals are tiny, it can be pumped like a liquid but, on the other hand, due to the heat of fusion of the ice crystals, it is capable of absorbing large amounts of heat and nonetheless retaining its tempera ture. Hence, it functions as an ideal cooling liquid and, according to estimates, in some applications, its use decreases pumping expenses to a fraction of current expenses. On the other hand, by producing ice slurry, cold can be efficiently stored in the form of ice slurry during times of cheap electricity, and this has, for example in Japan, already been done for a long time.

In addition to the production of ice slurry, a contin uous, efficient supercooling of water would enable the use of heat pumps for collecting heat by heat pumps from natural waters or another water source also at such times when the temperature of the water is close to zero degrees, for example, in the winter when layers of ice cover the water bodies leaving almost zero-degree water below the ice layer. This hides a great deal of potential because, in theory, the amounts of heat to be obtained from the water bodies are exceptionally great. Heat may also be collected from ordinary water, e.g. from a water reserve below a building and, from it, ice slurry can be produced during the winter. The slurry produced can then be melted in the summer either by using it for cooling or, for example, by means of solar heat.

Ice slurry may also be produced in many other ways than just by supercooling, for example, by freezing water to the surface of a heat exchanger and mechanically scrap ing the ice off, or by spraying water as droplets into a cold gas, but these other ways are due to the high energy consumption and/or wearing of materials for the time being inefficient and too expensive for the use of ice slurry to have become more common. A disadvantage with the supercooling of water is again the tendency of the water to freeze to the surfaces of the supercooling heat interchanger. Because the super cooled state is metastable, supercooled water has a given freezing frequency which means that, at a random moment, supercooled water eventually freezes to the sur face of the heat exchanger. Freezing frequency is expo nentially dependent on the degree of supercooling and also strongly on impurities in contact with the super cooled water which function as crystallization nuclei. In ordinary heat exchangers, supercooling of even just over 2 degrees at the coldest point on the surface often causes a fairly high freezing frequency (more than 0.5 times per hour) because a heat exchanger made from a solid material, usually metal, itself functions along its surface area as one exceptionally large crystalli zation nucleus. This influence is worsened by the fact that the water in the heat exchanger is at its coldest exactly at the contact surface with the heat exchanger. For these reasons, also the degree of supercooling to be achieved for the outgoing water is low; even at best, supercooling of clearly less than 2 degrees in contin uous use (=no freezings in two hours of use) is achieved in experiments.

Description of prior art

The function of a supercooling heat exchanger depends substantially on the characteristics of the surface of the heat exchanger. Ernst & Kauffeld (2016) ¹ studied the influence of surface roughness on the degree of super cooling to be achieved in water flowing inside a copper pipe. They observed that a slightly higher momentary degree of supercooling can be achieved with a smoother surface (a surface, at the maximum, 3 degrees smoother vs. a surface 2.3 degrees rougher). Wang et al. (2012) ¹¹ studied the influence of a superhydrofobic fluoro hy drocarbon coating and observed that it improved the de gree of supercooling to be achieved. However, in this case as well, the improvement was only slight, even though the superhydrofobic surfaces are generally also considered to be ice-repellent, i.e. they effectively prevent water droplets from freezing. The low rate of improvement is due to the fact that, although hydropho bic surfaces quite effectively prevent the freezing of droplets, this influence is based primarily on that they dramatically decrease the contact surface area of the droplet with the solid surface (Figs. 1 and 2). The probability of freezing to any surface depends directly on the size of the contact surface area, thus by mini mizing the surface area, the probability of freezing of the droplets can also be minimized. However, as far as a surface that is entirely underwater, such as a super cooling heat exchanger, is concerned the situation changes. In this case, the hydrophobicity of the surface no longer makes the surface very ice-repellent because the angle of contact is no longer of any significance, rather the surface is, in any case, entirely in contact with the water. A slight effect weakening ice formation in comparison to an ordinary metal surface can of course be achieved, for example, by a smoother surface (lower surface roughness decreases the contact surface area with water which causes a lower freezing frequency), as was pointed out by Ernst & Kauffeld, or by that the composition of the surface, due to its chemistry, is somewhat less compatible with ice. It is most likely due to this latter effect that, for example, in the studies of Wang et al. such a slight improvement was achieved.

In addition to above said two studies, from scientific literature there were found no other studies in which various coatings or the modifying of a surface to im prove the function of a water supercooling heat ex changer was clarified, tested or even suggested. From the patents there were also not found any solutions in which the function of a supercoooling heat exchanger would be improved by modifying the surface material of the heat exchanger.

Patent publications US 4671077 A, JP H09126593 and JP 3039462 B2 describe apparatuses which represent the cur rent state of the art.

US 4671077 A: the original patent for a device which supercools water and thus creates ice slurry. Herein, freezing is prevented, in practice, by just taking care that the surface temperature does not fall too low. A majority of water supercooling heat exchangers are of this type.

JP H09126593: In this solution, an attempt was made to assure that the temperature of the surface of the heat exchanger is not too low in the vicinity of the water outlet, where the degree of supercooling is at its max imum. This is based on that, in the vicinity of the water outlet, the probability of freezing is at its maximum, presumably because the water is supercooled the most and, in the vicinity of the outlet, the flow may contain various whirls of turbulence which facilitate freezing. In practice, control of the temperature of the surface of the heat exchanger is implemented by insu lating the water side surface of the heat exchanger in the vicinity of the water outlet, whereupon the surface in this critical area is not so cold. The solution as such is not an anti-icing coating, instead it is a so lution seeking to control the surface temperature in order that it would, even at its best, be able to only slightly reduce the freezing freguency. The solution thus does not in any way interfere with the high freezing degree caused by the solid-water interface, but instead attempts to just control the surface temperature in a most critical point. Hence, its anti-icing effect, even at its best, is slight.

JP 3039462 B2: in this solution, an attempt is made to achieve the improved function of a supercooling heat exchanger by controlling currents by means of the struc ture of a plate heat exchanger such that the water tem peratures would remain as steady as possible. The object is to avoid a situation in which, at some location, the temperature of the flowing water would drop locally to be lower than the average. Such a situation is not de sirable because freezing frequency is exponentially de pendent on the degree of supercooling, thus even small, locally increased degrees of supercooling may signifi cantly increase the freezing frequency of the entire heat exchanger. Summary of the invention

The description of the invention uses the terms "liquid coating" and "coating liquid" as well as their deriva tives, such as compound words. This can mean not only material in a liquid form, but also some other liquid like fluid, such as a Newtonian and/or non-Newtonian fluid/liquid, for example gel. This applies also to the use of the term "liquid" and its derivatives, wherein the term in question refers to a coating according to the invention as such.

In the invention, onto a solid heat exchanger surface on the side in contact with water is applied a thin water-insoluble liquid coating which remains in place between the water and the solid surface by some mecha nism, such as by forces between the interfaces, or by magnetic force if the liquid is a magnetic liquid, or possibly by a combination of these forces. In this con nection, the liquid may be a pure Newtonian flowing liquid, or some kind of non-Newtonian fluid, the vis cosity of which depends on shear stress and which may behave like an elastic material, at least at low shear stresses. In other words, the liquid may also be, for example, a gelatinous, elastic gel which is composed of the liquid phase and a support structure, but in which the liquid forms the majority of the material.

The core of the invention is then the use of so-called slippery liquid-infused porous surfaces (SLIPS) and/or magnetic slippery surfaces (MAGSS) as the coating of a supercooling heat exchanger. These coating types were originally disclosed in scientific articles in the years 2011 ⁱⁱⁱ and 2016 ^iv , and both of these are known to effi ciently prevent the freezing of droplets, but in a heat exchanger they are not known to have previously been suggested or tried, nor has their anti-icing function previously been tested on moving water or in a situation in which the supercooling water is not a droplet, but the water covers a large surface area.

For the invention to function, the solid material of the heat exchanger should be selected such that the liquid coating actually remains in place. In this case, when magnetic forces are used, the heat exchanger surface on the water side may either be a permanent magnet, or alternatively, a permanent magnet should be placed below the surface and, utilizing forces between the inter faces, the heat exchanger surface on the water side should in turn be suchlike that the forces between the interfaces are adequate to keep the water away from the solid surface and the liquid layer in place.

When a permanent liquid coating is achieved, the only contact formed with the water is found between another water-insoluble liquid, such as oil, and the water. In this case, the freezing frequency, i.e. in everyday lan guage, the tendency of the water to freeze, decreases a great deal because the freezing frequency on the liquid- water interface is much lower than on the solid-water interface. Somewhat simplified thermodynamically, this results from that as water freezes on a solid surface, the solid-water interface is replaced by a solid-ice interface, i.e. by a solid-solid interface, whereas when water freezes on a liquid coating, the liquid-water in terface is replaced by a liquid-ice interface, i.e. by a liquid-solid interface. Energetically, the interfaces between solid substances are generally more favourable than liquid-solid interfaces, thus a new solid-ice in terface is formed much more easily than a new liquid- ice interface, due to which the use of a liquid coating decreases the freezing frequency.

The anti-icing function of such coatings is thus based directly on the low compatibility of liquid and ice (=solid phase) and not on the large contact angle, so the influence is the same on both the droplets and the large surface, such as a heat exchanger. Due to this, such solutions creating a liquid layer function in su percooling heat exchangers distinctly better than pre viously tried solutions and, due to innovation, flowing water can be supercooled significantly more than in the past. In preliminary experiments, by the method of this invention, there was achieved already approximately 40% higher degrees of supercooling than with an uncoated control, and the potential is probably significantly greater. Naturally, it is worthwhile to cover with a coating liquid layer all the locations of the heat ex changer in which the freezing frequency would otherwise rise impractically high.

According to that disclosed in this application, the heat exchanger may be a supercooling heat exchanger coated with a slippery liquid coating. A heat exchanger according to the invention may be used for supercooling water, particularly flowing water. According to that disclosed in this application, a su percooling heat exchanger may comprise a solid heat ex changer surface with a water-insoluble coating, wherein said coating is a liquid-containing Newtonian or non- Newtonian fluid which remains in place between the water and said solid surface.

According to that disclosed in this application, there is also provided a method for supercooling water. In the method, onto a solid heat exchanger surface on the side in contact with water is applied a water-insoluble coat ing, wherein said coating is a liquid-containing Newto nian or non-Newtonian fluid which remains in place be- tween the water and said solid surface. As said in the above, the coating may be entirely or partially in a liquid state, for example, as a gel. The method enables the supercooling of flowing water. For supercooling wa ter, it can utilize a supercooling heat exchanger ac- cording to this application, disclosed above, or any one of its embodiment alternatives.

The following embodiments may relate to a supercooling heat exchanger according to the invention as well as to a method.

In one embodiment, said coating is kept in place by magnetic force and/or forces between the interfaces. In one embodiment, in order to keep the coating in place, magnetic force is applied such that the heat exchanger surface is magnetized in one or more of the following manners: the heat exchanger surface is of permanent mag netic material, there is permanent magnetic material below the heat exchanger surface, or the heat exchanger surface can be magnetized by means of an external mag netic field.

In one embodiment, in order to keep the coating in place, magnetic force is applied by using a Halbach-type mag netization. In this case, the solid surface may be mul tipolar, but the magnetic field is directed primarily to only one side of the solid surface.

In one embodiment, in order to keep the coating in place, magnetic force is applied such that the strength of the magnetic field varies regionally. In critical locations in which the freezing frequency would otherwise have been at its highest, a stronger magnetic field may be used.

In one embodiment, said coating is or comprises fer- rofluid and/or magnetic gel.

In one embodiment, in order to keep the coating in place, forces between the interfaces are applied such that the solid surface possesses at the micro level a three- dimensional support structure, to which the coating ad heres and locks. Alternatively, instead of the support structure, the surface may just be rough, whereupon the roughness of the surface functions as a support struc ture. In one embodiment, in order to keep the coating in place, forces between the interfaces are applied such that the coating is or comprises a thermoreversible gel.

In one embodiment, the heat exchanger is coated with a layer which enables the attachment of the gel to the heat exchanger.

In one embodiment, the heat exchanger comprises a water outlet with an uncoated extension.

In one embodiment, the heat exchanger comprises a heater to prevent freezing from beginning from said uncoated extension.

In one embodiment, the heat exchanger is a coaxial heat interchanger .

In one embodiment, the heat exchanger is a vaporizer.

The embodiments disclosed above may be used in any com binations with each other and they are suitable for use with both the disclosed heat exchanger as well as with the method. The embodiments may also be combined in number without limit to form further developed embodi ments.

According to an example, the invention may relate to efficient continuous supercooling of water by a coated heat exchanger, for example, for producing ice slurry and/or heat. In order to improve the function of the heat exchanger, between the water and the solid heat exchanger surface is a constantly permanent, thin, wa ter-insoluble liquid layer at least in those locations in which the water is strongly supercooled in order that the freezing frequency would remain low.

In all the locations of the solid surface of the heat exchanger in which freezing otherwise would occur at a significantly high freezing frequency, there may be an adequately strong magnetic field and a vertical gradient of a magnetic field to the surface such that the magnetic field weakens when moved outwards from the surface.

In one embodiment, the invention comprises an apparatus for continuous supercooling of water using slippery liq uid surfaces. In the apparatus, the heat transfer sur faces of the heat exchanger may be manufactured from a permanent magnet material. In the apparatus, below the heat exchanger surface may be installed a permanent mag net material. In the apparatus, the heat exchanger sur face may also be magnetized by means of an external magnetic field.

List of figures

Fig. 1 shows the definition of the contact angle Q.

Fig. 2 shows the influence of the contact angle on the contact surface area.

Fig. 3 shows a cross-section of the surface of an exem plary heat exchanger, the coating liquid being a fer- rofluid and the heat exchanger material being permanent magnetic. F _m is the magnetic force exerted on the fer- rofluid.

Fig. 4 shows a cross-section of the surface of an exem plary heat exchanger, the coating liquid being a fer- rofluid and magnetization occurring by a permanent mag net installed below the solid heat exchanger. F _m is the magnetic force exerted on the ferrofluid.

Fig. 5 shows a cross-section of the surface of an exem plary heat exchanger, the coating liquid being a ferro- gel and the heat exchanger material being permanent mag netic. F _m is the magnetic force exerted on the ferrogel.

Fig. 6 shows a cross-section of the surface of an exem plary heat exchanger, the coating liquid being a ferro gel and magnetization occurring by a permanent magnet installed under the solid heat exchanger. F _m is the mag netic force exerted on the ferrogel.

Fig. 7 shows a cross-section of the surface of an exem plary heat exchanger when the coating liquid is kept in place by attractive forces between the coating liquid and the solid heat exchanger and by means of the three- dimensional support structure or roughness of the sur face.

Fig. 8 shows a cross-section of the surface of an exem plary heat exchanger when the coating liquid is kept in place by attractive forces between the coating liquid and the support structure/adhesive phase as well as the support structure/adhesive phase and the intermediate coating. The intermediate coating is not necessarily required, instead it may be directly replaced by a solid heat exchanger surface.

Fig. 9 shows an example of a supercooling heat exchanger which is in type of a coaxial heat interchanger in which the water flows in an inner, coated tube.

Fig. 10 shows a cross-section of the inner pipe of the exemplary heat interchanger of Fig. 9, coated with a slippery liquid surface.

Fig. 11 shows an exemplary system in which could be used, for example, the supercooling heat exchanger of Figs. 9 and 10.

Fig. 12 shows a problem in the outlet of a heat exchanger that manifests in some cases, wherein the coating liquid leaks out due to surface tensions, and the exemplary extension to fix the problem, not coated with a coating liquid.

Detailed description

The detailed description set forth below in connection with the drawing is provided to present possible exem plary embodiment solutions. It is not intended to rep resent the only forms in which the examples may be im plemented or utilized. The same or comparable functional and structural features may also be implemented by other examples.

A supercooling heat exchanger coated with a slippery liquid coating is composed of a refrigerant side, a solid heat exchanger and its liquid coating located on the water side and the water side itself, from which heat flows to the refrigerant side as water is cooled. The liquid coating may also include some third sub stance, for example, a gelator or other solid phase and, further, the solid heat exchanger itself may be coated with some solid, thin layer which enables the attachment of the gel to the heat exchanger. On the refrigerant side may flow any liquid, the temperature of which is below zero degrees. The liquid flowing on the refriger ant side may also vaporize as heat flows from the water to be supercooled into the refrigerant, i.e. the super cooling heat exchanger may also function as a vaporizer. The shape of the heat exchanger itself can be anything, as long as its water side is coated in all those loca tions in which the freezing frequency of the water would otherwise be impractically high, i.e. in which the su percooling degree of the water is high. Because the "slippery liquid surface" that is formed prevents freez ing by a mechanism which scales also to large surfaces entirely covered by water as efficiently as to droplets, by using these coatings, it is possible to achieve with a heat exchanger significantly higher degrees of super cooling than has been possible in the past.

The function of a supercooling heat exchanger may be improved by a constantly permanent, thin, water-insol uble liquid layer between the water and the solid heat exchanger surface, for example, at least in those loca tions in which the water is strongly supercooled in order that the freezing frequency would remain low. In order that the liquid coating would remain in place between the solid heat exchanger surface and the water, it has to be kept in place by some force. This force may be, for example, magnetic, the coating liquid being a magnetic liquid, i.e. a ferrofluid, in which the liquid contains evenly mixed magnetic, coated nanoparticles, such as iron oxide particles. In this case, the coating is a so-called magnetic slippery surface, i.e. a MAGSS. Instead of pure ferrofluid, there may also be used a magnetic gel, of which the majority is still liquid, but in which, for example, using a gelator, a three-dimen sional support structure is created inside the liquid, to which the liquid adheres. The magnetic particles are inside the gel, either in the liquid phase, or in the support structure, and the force exerted on them keeps the gel in place between the water and the solid surface. An example of a magnetic gel is a ferrogel which can be manufactured by using ferrofluid and, for example, a polymer gelator. The advantage of the gel is improved durability .

In a heat exchanger utilizing magnetic force, the heat exchanger surface is magnetized either by constructing the heat exchanger from a permanent magnetic substance, for example, from a ferrite- or neodymium-based material which is permanently magnetized, i.e. it has been ren dered to a permanent magnet, or alternatively, by in stalling permanent magnetic material below the heat ex changer surface, or yet alternatively, by magnetizing the heat exchanger surface by means of an external mag netic field. A permanent magnet may be a bipolar magnet with only one north and south pole, but, in practice, for broad surfaces is best suited a multipolar permanent magnet in which there are south and north poles side- by-side on the surface, similarly as, for example, in ordinary refrigerator magnets. It is especially effi cient to use a so-called Halbach-type magnetization in which the surface is multipolar, but the magnetic field is directed primarily to only one side of the surface. In practice, the heat exchanger may thus be manufactured from the same type of material as, for example, refrig erator magnets, and may also be magnetized in the same manner.

The heat exchanger itself may thus be of any shape, provided that, in a solution using magnetic force, all locations of the solid surface in which freezing would otherwise occur at a significantly higher freezing fre quency, there is an adequately strong magnetic field and a vertical gradient of the magnetic field to the surface such that the magnetic field weakens when moved outwards from the surface. A gradient of a magnetic field is required because it causes an attractive force towards the surface that is exerted on the nanoparticles and, by this means, on the liquid. This magnetic field gra dient will prevent the movement of the magnetic liquid away from the solid heat exchanger surface, and the water from pressing through the liquid layer into con tact with the solid surface. The stronger the magnetic field on the solid surface, the greater is the gradient of the magnetic field away from the surface, and thus the more strongly the magnetic force prevents contact from forming between the water and solid substance. Hence, the strength of the magnetic field may also be varied regionally such that in critical locations in which freezing frequency would otherwise be at its high est, a stronger magnetic field may be used. This can be achieved, for example, by using materials that are lo cally more strongly magnetizable, or by using a thicker permanent magnet. Locations in which freezing frequency would otherwise be higher may be created in such areas in which, for example, due to strong flow turbulence or a change of direction, the water strikes against the heat exchanger surface with greater than average kinetic energy. By regional strengthening of the magnetic field, it is possible to avoid a situation in which the water would in these locations push through the liquid coating into contact with the solid surface, at least occasion ally.

The liquid coating may be applied onto the desired area evenly, by which means it can also be made to function better. The application onto the heat exchanger surface depends on what substance is used. Clean ferrofluids can be poured directly onto the heat exchanger surface, where they will thereafter by themselves spread evenly onto the entire magnetized surface area, provided that the magnetic field is constant on the surface and that there is enough ferrofluid. If the magnetic field is not constant, the ferrofluid can, as needed, be poured sep arately onto each area of the constant magnetic field.

When various magnetic gels are used, such as ferrogels, the production method depends on which type of gel is used. It may be necessary, for example, to heat the heat exchanger surface when producing the gel coating in or der that the gel would spread evenly. For example, a thermoreversible ferrogel may be used that is produced by using a polymer gelator and an oil-based ferrofluid. In this case, the gel is first produced by mixing the polymer gelator and ferrofluid, and heating them in a furnace, until the gelator has dissolved into the fer rofluid. After dissolution, the liquid mixture is poured onto the magnetic heat exchanger surface, where it spreads like ferrofluid. However, because the mixture forms an elastic, i.e. non-flowing, gelatinous gel as it cools, the heat exchanger surface can be kept hot in order that the mixture would have time to spread evenly before formation of the gelatinous phase. Further, the coating may possibly be applied mechanically onto the surface, or when the heat exchanger is a pipe, the hot pipe may be rotated around its axis, whereupon the gel spreads evenly.

A second alternative in addition to utilizing magnetic forces is to utilize the interface forces between the coating liquid, the solid heat exchanger surface and a possible third substance which may be, for example, a polymer forming a gel structure in the liquid, the forces being caused by atomic-level attractive forces. In this case, an ordinary slippery liquid-infused porous surface (SLIPS) is concerned. For example, the heat ex changer solid surface may possess at the micro level a three-dimensional support structure, to which the coat ing liquid adheres and locks when there is a high degree of attraction between the liquid and the solid surface. Alternatively, instead of the support structure, the surface may just be rough, whereupon the roughness of the surface functions as a support structure. When the interface between the solid substance and the coating liquid has a high mutual attractive force, water is not able to come into contact with the solid substance and, on the other hand, when the liquid adheres to the support structure of the surface, surface tension prevents it from moving away from the solid surface, and a permanent liquid coating is formed.

In practice, as a condition for that the water does not displace the coating liquid in such a case where the preventing force is only the forces between the inter faces, is the equation

R Ukn— ( YNV + R YKN ) > 0 where g _kn is the interface energy between the solid heat exchanger surface and the water, g _Nn is the interface energy between the coating liquid and the water, g _KN is the interface energy between the solid heat exchanger surface and the coating liquid, and R is the local sur face roughness of the solid heat exchanger or its sup port structure which is defined as a ratio of the actual surface area of the rough surface and the surface area of an ideal, perfectly smooth surface. The liquid-solid substance pair should thus be selected such that the condition of the above equation is satisfied.

In a second alternative, the support structure is formed by a third phase, for example some gelator, such as a polymer which forms inside the coating liquid a three- dimensional, permanent support structure to which the coating liquid adheres. If the support structure is a gelator, one can speak of a gel coating. When the support structure-solid heat exchanger coating combination is selected such that between them is a strong molecular level attractive force, the support structure may also be attached to the solid surface below, whereupon the liquid coating remains in place, the support structure functioning as the adhesive between the coating liquid and the solid heat exchanger. In such a case, the solid heat exchanger surface is replaced by the support struc- ture/adhesive phase in the equation disclosed above, i.e. it is achieved

R Ύkn— ( YNV + R ΎKN ) > 0 where g _kn is the interface energy between the support structure/adhesive phase and the water, g _Nn is the in terface energy between the coating liquid and the water, g _KN is the interface energy between the support struc ture/adhesive phase and the coating liquid and R is as in the earlier equation. This functions thus as a con dition for the function of the coating, and the coating liquid-support structure/adhesive phase combinations should be selected such that the equation is satisfied and, further, the adhesive phase should be attached to the solid heat exchanger surface. In some cases, the solid heat exchanger surface may even be coated with a very thin layer of another solid substance in order that the adhesive phase would attach to the surface. In one such coating alternative, the coating is a ther- moreversible gel which is formed by mixing into water the insoluble coating liquid and a gelator which dis solves into the coating liquid when heated. The liquid mixture thus formed is poured onto the heat exchanger surface and the mixture or the surface is mixed or oth erwise agitated such that the mixture spreads onto the surface as evenly as possible. Upon cooling, the gelator crystallizes at least partially and forms a continuous three-dimensional support structure inside the liquid. Because the gelator also attaches to the solid surface or to the solid coating on top of it, the liquid also remains in place on the surface. Attachment to the solid surface can be achieved, for example, by using a SEBS- type gelator in which a polystyrene-poly(ethylene bu tylene)-polystyrene molecule chain functions as the gelator, and as the solid surface or, alternatively, as the solid thin coating, is used polystyrene, whereupon a portion of the polystyrene ends of the polymer chain of the gelator attaches to the solid polystyrene as it crystallizes. However, because the majority of the gel is still liquid, and the support structure is located inside the liquid, water running over the coating in the heat exchanger is not able to come into contact with any solid surface.

The slippery liquid coatings in a heat exchanger de scribed above are shown in Figs. 3-8. An example of a water supercooling heat exchanger coated with a slippery liquid surface is shown in Figs. 10-11, and Fig. 12 shows one possible application for the supercooling heat exchanger coated with a slippery liquid surface of Figs. 11-12. If desired, the two previous alternatives may also be combined, i.e. there may be used such a coating liquid- solid-heat exchanger surface combination in which the coating liquid both attaches to the heat exchanger sur face either directly or via a gelator/other support structure and a possible intermediate coating and also contains magnetic particles, and the solid surface is also magnetized. Such a solution is obtained, if there is used a thermoreversible ferrogel and such a solid surface to which the gelator polymer of the ferrogel attaches.

If the heat exchanger coating liquid is a substance which satisfies the so-called spreading condition

Yvi— (YNI + YNV) > 0 where g _ni is the interface energy between the water and the air, g _NI the interface energy between the coating liquid and the air, and g _Nn the interface energy between the coating liquid and the water, the coating liquid tends to spread between the water and the air always when an interface of three phases is formed between the coating liquid, the water and the air. If the coating extends to the water outlet from the heat exchanger, this will lead to the coating liquid "leaking" contin uously along with the outgoing water spray in the outlet of the heat exchanger. In order to fix the problem, to the water outlet may, in this case, be placed a very short uncoated extension, the surface of which can be heated slightly, for example, by an electrical resistor in order that freezing would not begin from this un coated area. Because the uncoated area may be, in prac tice, for example, less than a millimetre in length, its surface area is exceptionally small and it does not have any practical influence on the efficiency of the heat exchanger. The solution is shown in Fig. 12. The invention is not limited solely to the embodiment examples described above, but instead many variations are possible within the scope of the inventive concept defined by the claims.

References

(i) Ernst, G. & Kauffeld, M. (2016) Influence of the wall surface roughness on the supercooling degree of water flowing inside a heat exchanger. In 11th H R Con ference on Phase-Change Materials and Slurries for Re frigeration and Air Conditioning. Karlsruhe, Germany. May 15-20, 2016. International Institute of Refrigera- tion. ISSN/ISBN 01511637.

(ii) Wang, H., He, G. & Tian, Q. (2012) Experimental study of the supercooling heat exchanger coated with fluorocarbon coating. Energy and Buildings. Vol. 55p. 526-532. DOI:10.1016/j.enbuild.2012 .09.012. ISSN/ISBN 0378-7788.

(iii) Wong, T., Kang, S.H., Tang, S., Smythe, E., Hat ton, B., Grinthal, A., Aizenber, J. (2011) Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity . Nature 477, 443-447 DOI:10.1038/na- turel0447

(iv) Irajizad, P., Hasnain, M., Farokhnia, N., Sajadi, S.M. & Ghasemi, H. (2016) Magnetic slippery extreme icephobic surfaces. Nature Communications. Vol. 7:1. DOI:10.1038/ncommsl3395