WO2013004901A1

WO2013004901A1 - Heat exchanger, ventilation device and method for recovering heat from fluid flow and releasing heat to fluid flow

Info

Publication number: WO2013004901A1
Application number: PCT/FI2012/050673
Authority: WO
Inventors: Jani Moilanen
Original assignee: Indifine Development Oy
Priority date: 2011-06-29
Filing date: 2012-06-28
Publication date: 2013-01-10
Also published as: FI124740B; FI20115681A; FI20115681A0

Abstract

The invention relates to a heat exchanger, particularly the invention relates to a heat exchanger which exploits a phase changing material in heat recovery and release. Furthermore, the invention relates to a ventilation device in which heat exchangers according to the present invention is arranged as a pair, and a method for recovering heat from the fluid flow and releasing heat to the fluid flow. By means of the present invention, the energy recovery of the heat exchanger which exploits a phase change can be improved. The heat exchanger according to the present invention is placed in conjunction with the first and the second spaces so that the fluid flow between the first space and the second space can be directed through the heat exchanger. Furthermore, the heat exchanger comprises a heat transfer space, in which heat transfer space is arranged a phase changing material for providing heat exchange between the fluid flow and the heat exchanger. In conjunction with the heat transfer space, are provided means for adjusting the volume of the heat transfer space, with which adjustment of the volume the phase change of the phase changing material can be controlled.

Description

HEAT EXCHANGER, VENTILATION DEVICE AND METHOD FOR RECOVERING HEAT FROM FLUID FLOW AND RELEASING HEAT TO FLUID FLOW

TECHNICAL FIELD

The invention relates to a heat exchanger, particularly the invention relates to a heat exchanger which exploits a phase changing material in heat recovery and release. Furthermore, the invention relates to a ventilation device and a method for recovering heat from the fluid flow and releasing heat to the fluid flow.

BACKGROUND OF THE INVENTION

According to the effective statutes and regulations in Finland, an adequate ventilation must be provided for residential buildings so that healthy, safe and comfortable indoor air can be guaranteed in all weather conditions and operating situations. Ad- ditionally in countries which have cold winter months, it is preferable to use heat recovery in conjunction with ventilation so that energy is not needlessly lost via the ventilation. Similarly, it is not desirable to let the indoor air which has already been cooled, straight out during the hot seasons when energy must be used for cooling the replacement air. The publication FI119705B discloses a regenerative, phase change exploiting heat recovery in which two heat exchangers have been provided in conjunction with a ventilation device, and the direction of incoming and exhaust air travelling via these heat exchangers is changed periodically. Furthermore, a phase changing material has been provided for the heat exchangers in order to intensify the heat recovery. Unfortunately, in spite of the solution disclosed in the above publication and other similar solutions, significant amounts of heat energy are, however, lost in conjunction with ventilation. As a result, the incoming air must be heated during cold periods or cooled during warm periods, and this additional heating/cooling is usually implemented, for example, through direct electric heating or with air-conditioners, which naturally increases the energy consumption of the ventilation and thereby also costs. SUMMARY OF THE INVENTION

The objective of the present invention is to provide a solution in which the above disadvantages have been eliminated or there has been an aim to reduce them. Particularly, the invention aims to solve how the energy recovery of a heat exchanger, which exploits phase change, can be intensified.

The heat exchanger according to the invention is characterized by features disclosed in the characterizing part of claim 1.

According to one embodiment of the invention, the heat exchanger has been placed in conjunction with a first and a second space so that the fluid flow between the first space and the second space can be directed through the heat exchanger. Additionally, the heat exchanger comprises a heat transfer space, in which heat transfer space a phase changing material has been provided for intensifying the heat exchange between the fluid flow and the heat exchanger. Means for adjusting the volume of the heat transfer space have been provided in conjunction with the heat transfer space, with which adjustment of the volume can be controlled the phase change of the phase changing material.

According to another embodiment of the invention, the means for changing the volume of the heat transfer space are, for example, a piston.

According to still another embodiment of the invention, the phase change of said phase changing material occurs between the liquid state and the gaseous state of the phase changing material.

The ventilation device according to the invention is characterized by features disclosed in the characterizing part of claim 5.

According to one embodiment of the invention, the ventilation device comprises at least two heat exchangers according to the present invention arranged as a pair so that the fluid flows travelling through the heat exchangers are opposite to each other.

According to still another embodiment of the invention, the directions of the fluid flows to said heat exchanger pair are changed periodically, and the length of this pe- riod, according to one embodiment, is preferably, for example, approximately 10 s - 5 min, more preferably, for example, approximately 20 s - 3 min, most preferably, for example, approximately 30 s - 1 min.

According to still another embodiment of the invention, the means provided in conjunction with the heat exchangers of the ventilation device for adjusting the volume of the space comprising the phase changing material of the heat transfer space is arranged to connect to each other and to separate from each other the heat transfer spaces of the heat exchangers for adjusting the volume of the space which comprises the phase changing material.

The method according to the present invention is characterized by features disclosed in the characterizing part of claim 10.

Preferable embodiments of the invention is also disclosed in dependent claims.

The utility of the heat exchanger according to the present invention is based on numerous facts. By means of the adjustment of the volume of the phase changing material and the heat transfer space it is possible to further intensify the heat exchange between the fluid flow and the heat exchanger. By means of the present heat exchanger, it can be possible to implement also a latent heat recovery in addition to the heat exchange. In this way it can be possible to achieve significant energy savings, particularly in areas which have extremely cold winter months. The present invention is suitable also for cooling the indoor air, so that the heat exchanger and/or the ventilation device according to the present invention can be exploited also during the summer months and/or in areas which are warm. In this way, significant energy savings can be achieved also in cooling, because by means of the present invention it may be possible to improve energy transfer from humid air to dry air. The heat exchanger according to the present invention can be applied to be used in many different ways in all heat recovering systems in which the change of direction of the flow is used.

By means of a ventilation device according to one embodiment of the invention, in which the phase changing material is transferred between a heat exchanger pair, it may be possible to achieve energy savings also at lower powers of a compressor or similar means for transferring the phase changing material, since with the method according to the present invention it may be possible to exploit the pressure provid- ed in the phase changing material, as a result of which the phase changing material will be able to transfer more efficiently between the two ventilation devices.

In this application, the term "fluid flow" means, for example, air, gas or liquid from which heat energy is desired to be recovered and/or to which heat energy is desired to be released.

Furthermore, the term "phase changing material" in this application means such materials whose change of state, particularly melting, solidification, evaporation, condensation, sublimation and/or desublimation, at a certain temperature and pressure binds or releases energy, particularly heat energy.

Still further, the term "heat transfer space" in this application refers to the space which comprises a phase changing material and possibly air or some other gas with which the heat transfer space can be filled.

BRIEF DESCRIPTION OF THE FIGURES In the following, preferable embodiments of the invention are described in more detail with reference to the attached figures in which

Fig. 1 shows an exemplary view of the principle of a heat exchanger according to one embodiment of the present invention,

Fig. 2 shows an exemplary view of the principle of a ventilation device which has a heat exchanger pair according to one embodiment of the present invention,

Fig. 3 shows an exemplary view of the principle of the heat exchanger pair of the ventilation device according to one embodiment in which the heat transfer spaces of the heat exchangers can be combined,

Fig. 4 shows a flowchart of a method according to one embodiment of the present invention for recovering heat from fluid flow and releasing heat to fluid flow,

Fig. 5 shows a flowchart of a method according to another embodiment of the present invention for recovering heat from fluid flow and releasing heat to fluid flow. DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows an exemplary view of the principle of a heat exchanger 100 according to one embodiment of the present invention. The heat exchanger according to the present invention is placed in conjunction with the first space 102 and the second space 104 so that the fluid flow 106 between the first space 102 and the second space 104 can be directed through the heat exchanger. In Fig. 1, the fluid flow 106 has been drawn to travel from the first space 102 to the second space 104, but it will be clear to a person skilled in the art that the fluid flow can also be arranged to travel from the second space 104 to the first space 102, or the direction of the fluid flow 106 can be reversed whenever desired. Said first space 102 and second space 104 can be, for example, the interior of a residence and the outside air or some other space outside the interior, for example, an outer hall. It will be obvious to a person skilled in the art that said first space 102 and second space 104 do not need to be exactly the spaces mentioned above between which fluid flow has been provided. The fluid flow 106 between the first space 102 and the second space 104 can be provided, for example, by means of mechanical ventilation, or the fluid flow can be some other flow of fluid, for example, flow of liquid between two spaces.

Additionally, according to one embodiment, e.g. pipes, such as Vortex pipe/pipes can be used, for directing the fluid flow through the heat exchanger. The heat exchanger 100 comprises at least one heat transfer space 108, in which heat transfer space 108 a phase changing material 110 is arranged for providing heat recovery and/or release between the fluid flow 106 and the heat exchanger 100. The heat transfer space 108 can be formed, for example, as a pipework which can be provided to circulate with the fluid flow in the heat exchanger so that the heat ex- change with the fluid flow will become as efficient as possible. It will also be obvious to a person skilled in the art that the shape of the heat transfer space does not need to be exactly this kind of pipework. There can also be more heat transfer spaces in the heat exchanger so that at least not all heat transfer spaces are connected with each other, and the phase changing material arranged in the heat transfer spac- es can also be different material in the different heat transfer spaces.

According to one embodiment, the phase changing material 110 arranged in the heat transfer space/spaces is a material whose phase change occurs between the liquid state and the gaseous state, for example, but not limiting, various fluorinated hydro- carbons, isobutane, propane, ammonia and carbon dioxide. According to another embodiment, a phase changing material is used whose phase change occurs between the solid state and the liquid state, for example, but not limiting, linear alkyl hydrocarbons, fatty acids and esters, polyethylene glycols, quarternary ammonium com- pounds, inorganic salt hydrates, eutectic alloys, such as bismuth, cadmium, indium and lead, and polyhydric alcohol-water solutions, such as DMP and HMP. It will be clear to a person skilled in the art that also other materials can be used which will be suitable for the purpose, and the phase changes can also be other than those used in the above embodiments. In addition, means is also provided in conjunction with at least one heat transfer space 108 in the heat exchanger 100 according to the present invention for adjusting the volume of the heat transfer space 108. According to one embodiment, said means can comprise of at least one piston 112, which piston 112, in one embodiment, is connected to control means 114, such as to a motor, and by means of this motor, the piston 112 can be moved for adjusting the volume of the heat transfer space 108. According to another embodiment, some other matter is used for adjusting the volume, which matter is provided in the heat transfer space or in conjunction with it. In this case, the adjustment of the volume of the heat transfer space will be focused on the space where there is phase changing material and gas, e.g. air. In the present embodiment, the matter is chosen so that it is incompressible and expands appropriately when becoming warm. Depending on the embodiment, the chosen matter can be solid matter or liquid.

Depending on the embodiment, also more means can be connected to one or more heat transfer spaces for adjusting the volume of the heat transfer space. The differ- ent means can be, depending on the embodiment, connected to be controlled either by one motor or by different motors, or then part of the means can be manually operated.

According to another embodiment, the control means can comprise additionally a sensor/sensors (not in the figure) to measure, for example, the temperature, pressure and/or flow from the fluid flow and/or from the phase changing material. Depending on the embodiment, the control means can additionally comprise conventional data transfer means and storing means for transferring and storing data collected by the sensor/sensors, and conventional processing means for controlling the possible motors, pistons and/or sensor/sensors etc. The sensor/sensors of the control means can be provided to collect data from the fluid flow and/or from the phase changing material in real time and/or continuously, for example, preferably at 1-4 second intervals, or the collection of data can be implemented in such a way that the variables measured by the sensor/sensors are measured/read only when it is desired that the phase changing material will change its state or keep its prevailing state, and the volume of the heat transfer space can be adjusted to attain the desired effect.

Still, in one embodiment, the heat exchanger further comprises a fixed heat exchanger unit, which fixed heat exchanger unit can be shaped, for example, in a cellular manner. The fixed heat exchanger unit can be made of known, good endo- thermal or exothermal materials, for example, such as, but not limiting, aluminium. Usually, the fixed heat exchanger unit is provided in conjunction with the heat transfer space so that heat exchange with the fluid flow and/or with the phase changing material occurs also in said heat exchanger unit. According to one embodiment, the fixed heat exchanger unit is provided to function as a medium between the fluid flow and the phase changing material, for example so that the heat transfer space with its phase changing material is placed inside the fixed heat exchanger unit, at least partly.

Fig. 2 shows a view of the principle of a ventilation device 200 into which is provided at least two heat exchangers 100 according to one embodiment of the present invention. The heat exchangers 100 is arranged in the ventilation device as a pair so that incoming air 202 is directed through one heat exchanger and exhaust air 204 through the other heat exchanger. The incoming air 202 and the exhaust air 204 is marked with arrows in Fig. 2 so that the incoming air 202 goes through the upper heat exchanger and the exhaust air 204 through the lower one, but it will be obvious to a person skilled in the art that the directions of the fluid flows can also be re- versed.

According to another embodiment, the directions of the fluid flows to the heat exchangers is arranged to be reversible, however in such a way that they are always opposite to each other. In this way, the heat collected from the warmer fluid flow by the heat exchanger can be released into the colder fluid flow. The directions of the fluid flows can be changed at regular periods, such as preferably, for example, at intervals of approximately 10 s - 5 min, more preferably, for example, approximately 20 s - 3 min, most preferably, for example, approximately 30 s - 1 min.

According to another embodiment, the period of the change of direction of the fluid flow is provided to be dependent on the phase change of the phase changing materi- al, so that when the phase changing material in either of the heat exchangers has changed its state, then the directions of the fluid flows are changed.

According to yet another embodiment, the adjustment of the volume of the heat transfer space of one heat exchanger is arranged to be dependent on the phase change of the phase changing material of the other heat exchanger, for example, in such a way that when the phase changing material of the other heat exchanger has changed its state, the volume of the heat transfer space of the above mentioned heat exchanger is adjusted so that also in this heat exchanger, the phase changing material changes its state, after which the directions of the fluid flows can be reversed. Figure 3 shows an exemplary view of the principle of the heat exchanger pair of the ventilation device according to one embodiment in which the heat transfer spaces of the heat exchangers can be combined. In the present embodiment, both heat transfer spaces 108 of the heat exchangers 100 of the ventilation device which is provided as a pair is thus arranged in such a way that with the means 402 provided in conjunc- tion with the heat transfer spaces 108, the heat transfer spaces 108 can be connected to each other and separated from each other in the different stages of the heat recovery and release. Then, the phase change of the phase changing material 1 10 can be controlled by increasing the volume of the space which comprises the phase changing material 110 to comprise the heat transfer spaces 108 which is provided in both heat exchangers. By separating the spaces 108 from each other by using the means 402, the volume of the space which comprises the phase changing material 110 can be correspondingly reduced to comprise the heat transfer space 108 of only one of the heat exchangers.

It will be clear to a person skilled in the art that the means 402 can be implemented in many different ways. According to one embodiment, the means 402 comprise a controllable four-way valve by means of which the heat transfer spaces can be connected. It is preferable to place the heat transfer spaces of the heat exchanger pair near each other so that the length of the pipes/channels connecting the heat transfer spaces remains short, because in that case, the energy absorbing phase change of the phase changing material creates a pressure rise in the phase changing material, and the adjustment of the volume of the space comprising the phase changing material further intensifies the phase change of the phase changing material, as a result of which the phase changing material spreads efficiently also to the heat transfer space of the other heat exchanger, and the transfer of the phase changing material by using the means provided in conjunction with the heat transfer spaces, from the heat trans- fer space of the first heat exchanger to the heat transfer space of the second heat exchanger can be implemented by using lower power.

According to one embodiment, the means comprise a mechanical device, for example, a compressor by means of which the transfer of the phase changing material to- tally to the other heat transfer space can be confirmed before separating the spaces from each other. According to another embodiment, in the heat transfer space/spaces, a heat expanding material has been provided which, when absorbing heat, pushes the phase changing material away from the heat transfer space to the other heat transfer space, and when it cools down, it contracts and permits the phase changing material to return back to the heat transfer space. It will be clear to a person skilled in the art that the heat exchanger according to the present invention can also comprise the combination of these, or the transfer of the phase changing material to the heat transfer space of the other heat exchanger is arranged in some other way. Furthermore, the means can comprise, for example, other valves, such as three-way valves, for example, for measurements and/or sensoring. It will be clear to a person skilled in the art that also safety means, such as for example a double pressure controller, can be incorporated into the circuit to prevent possible situations of danger due to excess pressure, just to mention one example. According to one preferable embodiment, the above described ventilation device comprising a heat exchanger pair in which the provided heat transfer spaces can be connected and separated, is a direction changing device, i.e. the direction of the fluid flows led to the heat exchangers is changed periodically according to the above embodiments. According to still one preferable embodiment, the above described ventilation device comprising the heat exchanger pair in which the provided heat transfer spaces can be connected and separated, comprises additionally fixed heat exchanger units arranged in conjunction with the heat exchangers. The method connected to the present embodiment for heat recovery and release will be described in more detail below. The above described ventilation device can be used, for example, for heat recovery from indoor air during those times when indoor spaces need to be heated and for cooling down outdoor air at those times when it is desired that the apartment or space should be cooled down. According to one embodiment, there is also the pos- sibility for the user to control whether he/she wants to use the heat recovery of the indoor air or the cooling of the incoming air.

The heat exchanger according to the present invention can be applied for use also in other ventilation devices than the one described above. The heat exchanger can be used in all heat recovery and release systems, but the heat exchanger is particularly applicable for use in ventilation devices in which a flow reverser is being used.

Figure 4 shows a flowchart of a method according to one embodiment of the present invention for recovering heat from the fluid flow and releasing heat to the fluid flow. It will be clear to a person skilled in the art that the present method is intended only as an example and not for limiting the present invention. At step 302, the fluid flow is directed to a heat exchanger to be in connection with the phase changing material and, depending on the embodiment, to be linked with a fixed heat exchanger unit, by using, for example, Vortex pipe/pipes.

At step 304, procedures relating to the fluid flow and/or to the phase changing mate- rial are performed, such as data are obtained from the sensor/sensors of the control devices and/or procedures are performed, such as arithmetic operations, for example, on the basis of the data. In one embodiment, the performed procedure is the measuring of time, after which predetermined time is moved to step 306. According to another embodiment, the procedures of step 304 comprise the measuring of pres- sure at least from one heat exchanger, or when it concerns the present ventilation device, from one or both heat exchangers, and when a specific, predetermined pressure value is exceeded, move to step 306.

On the basis of the above procedures, the volume of the space comprising the phase changing material of the heat transfer space is adjusted in order to achieve the de- sired effect (step 306) by means of control means, such as a piston. The desired effect can be, for example, the reduction of the volume of the space comprising the phase changing material of the heat transfer space, in which case, the pressure directing to the phase changing material increases and the phase changing point moves to a higher temperature. Then, the phase changing material changes its state, for example, from gas into liquid, in a higher temperature releasing heat energy into the fluid flow. It will be clear to a person skilled in the art that the above gas-liquid phase change does not exclude the gas-solid phase change in the embodiments in which the desired phase change of the phase changing material takes place between gas and solid matter. Another desired effect can be, for example, increasing the volume of the space comprising the phase changing material of the heat transfer space, at which the pressure directed to the phase changing material decreases and the phase changing point moves to a lower temperature. Then the change of state of the phase changing material, for example, from liquid into gas, can be arranged to take place at a lower temperature, and thus it can be possible to improve the heat recovery from the fluid flow. Steps 304 and 306 can be repeated as long as the fluid flow is directed to the heat exchanger (marked with a dashed line in Fig. 4).

According to one embodiment, at step 308, the direction of the fluid flow is re- versed and the implementation of the method is started again from step 302.

Figure 5 shows a flowchart of a method according to another embodiment of the present invention for recovering heat from fluid flow and releasing heat to fluid flow. The method according to another embodiment of the invention comprises a ventilation device according to one embodiment of the present invention which comprises at least two heat exchangers which is arranged as a pair and the heat transfer spaces of which is arranged to be connected to each other and to be separated from each other for adjusting the volume of the space comprising the phase changing material. In the exemplary starting situation of the present embodiment, the phase changing material which is in a heat absorbing state, such as for example, in a liquid state, is arranged totally or at least to a significant extent in the heat transfer space of the first heat exchanger of the heat exchanger pair of the ventilation device.

At step 502, the fluid flows which are opposite to each other are directed to each of the heat exchangers of the heat exchanger pair. Usually, one fluid flow is the incom- ing air of the ventilator, for example, outdoor air, and the other fluid flow is exhaust air. It will be clear to a person skilled in the art that depending on the season, the warmer fluid flow can be incoming air (in summer) or exhaust air (in winter). In the present example, the warmer fluid flow, from which it is desired to recover heat, is directed to the first heat exchanger unit, and the colder fluid flow, into which it is desired to release heat, is directed to the second heat exchanger. In the heat exchangers of the ventilation device according to the present invention, also the above fixed heat exchanger units is preferably arranged for the recovery of heat from the fluid flow and for the release of heat to the fluid flow. This feature makes the heat exchange possible also in that heat exchanger where there is none or only very little of the phase changing material at this stage of the method. Whereas, in the heat ex- changer which comprises the phase changing material, in this example in the first heat exchanger, the energy absorbing phase change of the phase changing material is starting due to the warmer fluid flow which has been directed to the first heat exchanger. At step 504, procedures related to the fluid flow and/or to the phase chang- ing material are implemented, and they is described above in connection with the second embodiment.

Next, at step 506, as a result of the procedures implemented at step 504, by using the means provided in conjunction with the heat transfer spaces, the volume of the space comprising the phase changing material is adjusted by connecting the heat transfer spaces of the heat exchanger pair, at which the energy absorbing phase change of the phase changing material is intensified, because the volume of the space comprising the phase changing material increases, at the same time decreasing the pressure at which the phase changing material is directing to the space comprising the phase changing material, thus simultaneously lowering the temperature of the phase changing point of the phase changing material. In the present example, the phase changing material evaporating from the liquid state is able to spread freely also to the part of the heat transfer space of the second heat exchanger.

At step 508, by using the means provided in conjunction with the heat transfer spaces, the transfer of the phase changing material from the heat transfer space of the first heat exchanger to the heat transfer space of the second heat exchanger is implemented. The phase changing material is transferred, depending on the embodiment, either totally to the second heat transfer space or at least for the significant parts. The phase change of the energy absorbed phase changing material to the energy emitting material begins in the heat transfer space of the second heat exchanger as a result of the colder fluid flow as the phase changing material reaches the heat transfer space of the second heat exchanger.

Thereafter, at step 510, by using the means provided in conjunction with the heat transfer spaces, the volume of the space comprising the phase changing material of the heat transfer space is adjusted by separating the heat transfer spaces of the heat exchanger pair from each other, at which the energy emitting phase change of the phase changing material provided in the heat transfer space of the second heat exchanger intensifies, resulting from the decrease of the volume i.e. the increase of the pressure. In the present example, the phase changing material thus experiences the phase change from gas into liquid. Depending on the embodiment, also the above described step concerning the procedures related to the fluid flow or to the phase changing material can be incorporated into this step.

At step 512, the directions of the fluid flows directed to each heat exchanger are reversed after the phase change of the phase changing material which occurred in the second heat transfer space. Thereafter, the method can be repeated from the beginning as many times as desired.

It will be clear to a person skilled in the art that the heat transfer space or the connected heat transfer spaces is separated from the surroundings so that the change in the volume of the heat transfer space causes a change in pressure in the heat transfer space, and the fluid and/or the phase changing material arranged in the heat transfer spaces are not able to flow away from the spaces, and fluid or some other material from outside the space or the connected spaces is not able to enter the space.

Only some of the embodiments of the solution according to the invention have been disclosed above. The principle according to the invention can naturally be modified within the framework of the scope as defined by the claims, for example, considering the details of the implementation and methods of use.

Claims

1. A heat exchanger for recovering heat from fluid flow and/or releasing heat to fluid flow, which heat exchanger is placed in conjunction with the first and the se- cond spaces so that the fluid flow between the first space and the second space can be directed through the heat exchanger, which heat exchanger comprises one or more heat transfer spaces, in which said heat transfer space is arranged a phase changing material for providing heat exchange between the fluid flow and the heat exchanger, characterized in that in conjunction with the heat transfer space are provided means for adjusting the volume of the space comprising the phase changing material of the heat transfer space.

2. A heat exchanger according to claim 1, wherein the means for adjusting the volume of the heat transfer space additionally comprise a temperature sensor and/or a pressure sensor/sensors.

3. A heat exchanger according to any of the preceding claims, wherein the phase change of said phase changing material occurs between the liquid state and the gaseous state of the phase changing material.

4. A heat exchanger according to any of the preceding claims, which heat exchanger further comprises a fixed heat exchanger unit provided in conjunction with the heat transfer space for providing heat exchange with the fluid flow and/or the phase changing material.

5. A ventilation device, which ventilation device comprises at least two heat exchangers according to any of claims 1-4 arranged as a pair so that the fluid flows travelling through the heat exchangers are reversed in relation to each other.

6. A ventilation device according to claim 5, wherein the directions of said fluid flows are changed periodically.

7. A ventilation device according to claim 6, wherein said period of the change of direction of the fluid flows is preferably, for example, approximately 10 s - 5 min, more preferably, for example, approximately 20 s - 3 min, most preferably, for example, approximately 30 s - 1 min.

8. A ventilation device according to claim 6, wherein said change of direction of the fluid flows depends on the phase change of the phase changing material occurring in at least one heat exchanger.

9. A ventilation device according to any of claims 5-8, wherein the means for ad- justing the volume of the space comprising a phase changing material of the heat transfer space of the heat exchanger is arranged to connect to each other and to separate from each other the heat transfer spaces of the heat transfer pair of the ventilation device for adjusting the volume of the space comprising the phase changing material.

10. A method for recovering heat from the fluid flow and/or releasing heat to the fluid flow, characterized in that the method comprises at least the following steps:

- directing the fluid flow to the heat exchanger according to any of claims 1-4 or to the ventilation device according to any of claims 5-9,

- implementing procedures related to the fluid flow and/or to the phase changing material, such as obtaining of data from a sensor/sensors and/or arithmetic operations,

- adjusting the volume of the space comprising a phase changing material of the heat transfer space for achieving the desired effect.

11. A method according to claim 10 comprising the ventilation device according to claim 8, wherein the adjustment of the volume of the space comprising a phase changing material of the heat transfer space is performed by connecting/separating the volumes of the heat transfer spaces of the heat exchangers comprising additionally the following steps:

- transferring the phase changing material from the heat transfer space of one heat exchanger to the heat transfer space of the other heat exchanger after connecting the heat transfer spaces,

- separating the heat transfer spaces of the heat exchangers from each other.

12. A method according to any of claims 10-11 comprising additionally the step of reversing the direction/directions of the fluid flow/flows after the phase change of the phase changing material which occurred in the heat transfer space.