The cellular cooling system based on the phase change material, PMC, can be used for passive cooling of the interior of buildings without refrigerating machines. The system is based on so-called phase change cells that are installed in the building or room and that store the extra heat in the room. The system can be installed both in new and renovated buildings.

Sunshine, heavy lighting, people, devices, and machines often overheat buildings.

The increase in indoor temperature in offices and other public utilities, such as schools, in particular, is too high. For the indoor temperature conditions, it is important in the initial stage to provide protection against excessive solar heating and to minimize the heat loads produced. However, even in the climatic conditions in Finland, buildings must be cooled mechanically.

It has been estimated that the share of refrigerated building stock in Finland is about 20 million m3, i. e., 6% of the business, industrial, and public buildings in our country. The amount of electric energy used for mechanical cooling is about 57 GWh per year and the maximum power about 130 MW. It has further been estimated that by 2010, the refrigerated building stock will grow to fourfold from what it is at present. The majority of the old office and public utility buildings are not provided with mechanical refrigeration. Older buildings also require cooling but the high investment costs of mechanical refrigeration often become an obstacle to implementation. To add refrigeration to the entire ventilation of a building requires a large renovation. On the other hand, space-specific refrigerating plants are expensive and energy consuming. The continuous growth of the internal loads of devices and the recent restrictions in the use of refrigerants make it important to develop alternative solutions to mechanical refrigeration.

It would be possible to decrease the temperature of office, business, industrial, and other public buildings by installing passive phase change cells (PCM cells) in the building, in its rooms, for example, for storing in the daytime any excess heat in the

room and decreasing the inner temperature of the room. The cells can be installed in the room even afterwards. The cells are easy to install, and there is no need to make any changes in the old ventilation system of the building. It is a more advantageous and environmental-friendly alternative for indoor cooling than room-specific refrigerating machines or the ventilation of the building.

The phase change materials (PCM) can store heat on a short-term basis. The phase change material stores heat, when changing its state, for example, from solid to liquid and from liquid to steam. Correspondingly, the latent heat is released, when the matter changes, for example, from steam to liquid and from liquid to solid. The operation of the system is based on the heat storage capacity of the phase change material.

Heat is charged in the phase change cells, when the room temperature is higher than the melting heat of the material. Correspondingly, heat is released from the phase change cell, when the room temperature is lower than the crystallization temperature of the cell phase change material.

One disadvantage of the phase change cells is the poor thermal conductivity of the phase change material inside the cell and the low convection heat-transfer coefficient on the surface of the cell. Heat does not transfer effectively enough from the air into the cell and from the cell into the air. The phase change material is easy to charge but difficult to discharge, as phase change materials generally exhibit a super-cooling phenomenon. Super cooling means that the crystallization temperature of the material is lower than its melting temperature.

If there is normal supply and exhaust air ventilation in the room, it is difficult to make the cell discharge in a daily cycle. An ideal situation for the functioning of the cell is when, in the morning, the phase change material is in a solid state inside the cell, ready to be charged. Consequently, the cell should be discharged fully during the night.

Publication WO 88/06216 discloses an emergency cooling system based on a phase change material, which operates, if the cooling of a room containing electric appliances fails and the room temperature becomes too high. The system cannot be operated continuously. Publication DE 4 209 251 discloses Venetian blinds containing a phase change material, storing radiating solar heat into melting heat in the day time and releasing the corresponding heat for the night. A disadvantage of this system is that it only works in sunshine.

U. S. Pat. No 5,501,268 suggests adjusting the temperatures by heat transfer between a phase-changing PCM and warm/cold air, in which the PCM is mixed with a wall material. As disclosed by column 4, lines 31-37, of the publication, compared with natural convection, triple convection is needed to effectively utilize the heat storage capacity of the PCM. According to the publication, this requires an air flow so strong that it has an adverse effect on the people in the room.

The purpose of the present invention is to provide an indoor air conditioning method, wherein heat is continuously and reliably transferred from warm air into a phase change material, in which the phase change stores the said heat. In this way, the objective of the invention has mainly been reached so that continuous cyclic charging and discharging of heat is effected so that, during the charge, heat is transferred from warm air into the phase change material in order to change the phase and, during the discharge, heat is transferred from the phase change material into cold air to change the phase back.

Consequently, the invention deviates from the methods of the above-mentioned publications in that it is continuously operating and, at the same time, based on the heat transfer between air and the phase change material. As mentioned earlier, cooling is effected when air that is warmer than the phase change material is brought into heat-transferring contact (direct or indirect contact) with the phase change material to change its phase, and heating is effected, when the phase change material is brought into heat-transferring contact with air that is colder than it and its phase is changed in another direction.

Thus, the phase change temperature of the phase change material is between warm and cold air temperatures. Henceforth, when talking about warm and cold air, we refer to the temperatures on both sides of the phase change temperatures.

The method according to the invention thus applies to any situations, in which the said warm and said cold air are available at least periodically. For practical purposes, however, it is preferable to perform the said heat charge in the daytime and the said heat discharge by night. In the daytime, there are a lot of sources of warm air, while by night the temperature; the outdoor temperature in particular, decreases.

The said heat charge is preferably performed by transferring heat from the warm indoor air of an indoor space and/or, possibly, from warm incoming air, preferably from warm indoor air, into the phase change material to change its phase. This is

effected, for example, by bringing the warm indoor air into contact with the phase change cell. Warm incoming air is mainly used in warm countries, where the outdoor air in the daytime is warmer than the indoor air. Otherwise, preferably warm indoor air is used.

The said heat discharge is preferably performed by transferring heat from the phase change material into cold incoming air and/or cold indoor air of the indoor space, preferably into cold incoming air, to change the phase back. This is preferably effected by bringing the flow of cold supply air into contact with the phase change cell. By selectively choosing cold air for the regeneration of the phase change material enables the regeneration of the cell during the night, among others. If we want to avoid mixing the supply of air with the indoor air, the said supply air flow should immediately be directed out from connection with the phase change cell, and not into the indoor air.

The method according to the invention can be applied to all types of air condition- ing, which require heating and/or cooling and which have cold or warm air available.

According to one embodiment of the invention, the indoor space comprises supply and exhaust air ventilation, wherein the supply of air is fed into the indoor air and the exhaust air is removed from the indoor air. In this case, the said heat charge is carried out by transferring heat from the warm indoor air and/or, possibly, from warm incoming air, preferably warm indoor air, into the phase change material to change its phase.

Generally, heat is transferred from warm indoor air into the phase change material by bringing the warm indoor air into contact with the phase change cell. In this case, the cell must be located and/or the indoor space shaped so that the contact surface becomes as large as possible. Direction of air is preferably used for assistance.

When supply and exhaust air ventilation is used, cold air from the outside is generally fed into the indoor space in the daytime. In that case, it is preferable to direct, by using a guide plate, the flow of cold supply air directly into the indoor air, and not via the phase change cell. Thus, the phase change cell receives as warm air as possible in the daytime, while as much cold air as possible is mixed with the indoor air.

The said heat discharge of the phase change material by changing the phase takes place, when cold air is available, for example, at night. The heat discharge is

performed by transferring heat from the phase change material into the supply of cold air and/or into cold indoor air, preferably into the supply of cold air, to change the phase back. With respect to the supply and exhaust air ventilation, this is effected by bringing the flow of cold supply air into contact with the phase change cell with the aid of the guide plate.

Unless the intention is to lead the regeneration air of the cell into the indoor space, the guide plate can be used to direct the said supply air flow immediately out from connection with the phase change cell or the like, and not into the open indoor space.

According to another embodiment of the invention, natural (gravity) ventilation is provided in the indoor space, wherein the exhaust air flows along the ducts built for it because of the differences in density or pressure differences caused by the wind, so that the supply of air is obtained from the outdoor air flowing through walls, windows, and other slots in the form of leakage air. In that case, the said heat charge is performed by transferring heat from the warm indoor air to the phase change material to change the phase, preferably by allowing the warm indoor air to come into contact with the phase change cell.

The said heat discharge, i. e., the regeneration of the cell is performed by transferring heat from the phase change material into the supply of cold air and/or the cold indoor air, preferably into the supply of cold air to change the phase back.

This is preferably effected by bringing the flow of cold supply air into contact with the phase change cell by using a fan. Unless the intention is to lead the regeneration air into the indoor space, the flow of cold supply air can immediately be directed into the exhaust air from connection with the phase change cell by using the fan and/or the guide plate.

As mentioned above, the invention also relates to an air conditioning system for indoor use, comprising a phase change cell for storing the heat coming from warm air by using the phase change. The air conditioning system is characterized in equipment for the continuous and cyclic transfer of heat from warm air to the phase change material cell for storing it in the cell by using the phase change, and from the phase change material cell into the cold air to discharge it in the air by using the phase change.

Thus, the air conditioning system according to the invention comprises a phase change cell and equipment for using and regenerating the same.

In accordance with the previous model, the phase change cell is a structure containing a phase change material, to which heat can transfer from the air and from which heat can transfer into the air. The phase change temperature of its phase change material is between the temperatures of warm and cold air. As the normal operating temperature is close to the room temperature, the phase change temperature of the phase change material is preferably between about 18°C and about 35°C, preferably between about 20°C and about 30°C. Furthermore, it is preferably selected from paraffins, salt hydrates, and fatty acids, the solid/liquid phase change temperature of which is within the said range and the latent heat, i. e., the heat storage capacity is high. Typical paraffins include hexadecane, hepta- decane, octadecane, and nonadecane. See details in WO-98/42929. Typical salt hydrates include calcium chloride hexahydrate, lithium nitrate trihydrate, and sodium sulfate decahydrate.

When talking about heat transfer between air and the phase change cell, the phase change cell preferably consists of a structure made of at least metal, graphite or another material with high heat conductivity, comprising cavities, the cavities containing the said phase change material. The structure is preferably one with as large a surface area as possible, such as a honeycomb or a similar structure. The exterior can be provided with fins or the like, which are intended to maximize the contact surface and heat transfer with the ambient air.

In addition to the phase change material, the cavities of the said structure can also comprise metal, graphite or other material with high heat conductivity, which is intended to enhance the heat transfer between the phase change material and the said structure. The said material is preferably in the form of chips, sawdust, fibr-s, thread or the like. By adding an additive to the phase change material of the phase change cell, enhancing the heat transfer, the charging and discharging phases of the cell can be shortened so that they can be carried out within one day.

It is advantageous for the air conditioning system according to the invention, if the phase change cell is installed in the ceiling or on the wall of the indoor space, preferably the ceiling, and in contact with the indoor air. As we know, spontaneous convention lifts hot air upwards in the indoor space, so that the warmest air rises to the ceiling. On the other hand, in spontaneous convention, the air moves fastest along the walls, whereupon the heat transfer is at its maximum. Consequently, a professional can conclude on the basis of temperatures and streams of material, whether to install the cell in the ceiling, on the wall, or perhaps both.

The equipment of the air conditioning system according to the invention can comprise, for example, an electronic control circuit, which is preferably excited by a clock and/or the air temperature, and mechanical guiding devices, preferably guide plates, dampers and/or fans or the like, to direct the flows of air so that heat is continuously and discontinuously transferred in the way described above, i. e., so that the heat is transferred from the warm air to the phase change material cell for charging it by the phase change, and from the phase change material cell into the cold air to discharge the same by the phase change.

It is especially preferable, if the control circuit and the mechanical guiding devices are adjusted to operate so that heat is charged in the daytime and discharged at night.

The indoor space has preferably been shaped and, during the heat charge, the said equipment is adjusted to bring the warm indoor air into contact with the phase change cell to charge the heat in the cell. For the time of discharging the heat, the indoor space and the equipment has been shaped and adjusted to bring the flow of cold supply air into contact with the phase change cell to discharge the heat from the cell. If the regeneration air of the cell is not intended for the indoor air, a sensor can be installed to direct the said supply air flow immediately out from connection with the phase change cell, and not into the indoor air.

According to one embodiment, the air conditioning system according to the invention operates on supply and exhaust air ventilation, comprising a supply and exhaust air ventilating installation for leading the incoming air into the indoor air and leading the exhaust air from the indoor air.

In that case, the indoor space is preferably shaped and, during the heat charge, the said supply and exhaust air ventilation installation is adjusted to bring the warm indoor air into contact with the phase change cell to charge the heat in the cell. At the same time, it is preferable, if the said installation comprises a guide plate or the like to lead cold incoming air directly into the indoor air, and not to the phase change cell.

Correspondingly, during the heat discharge, the supply and exhaust air ventilation installation has preferably been adjusted to bring the supply of cold air into contact with the phase change cell by using the guide plate or the like, to discharge heat from the cell. If the regeneration air of the cell is not intended for the indoor air, another guide plate or the like can be installed to direct the said supply air flow

immediately our from connection with the phase change cell, and not into the indoor air.

According to another embodiment, the air conditioning system according to the invention comprises an installation for natural ventilation. It is used to provide ventilation, wherein exhaust air flows along the ducts built for it because of the differences in density or the pressure differences caused by the wind, and the supply of air is obtained from outdoor air flowing through the walls, windows or other slots.

In that case, the indoor space is preferably shaped and, during the heat charge, the natural ventilation installation is adjusted to bring the warm indoor air into contact with the phase change cell to charge the heat in the cell.

Furthermore, the installation is shaped and, during the heat discharge, the natural ventilation installation is adjusted to bring the flow of cold supply air into contact with the phase change cell to charge the heat into the cell. During discharge, an air fan or the like is preferably needed to lead the flow of cold supply air into contact with the phase change cell to discharge the heat from the cell. If the intention is not to lead the discharge air of the cell into the indoor air, another air fan or the like is needed to direct the said supply air flow immediately out from connection with the phase change material cell, and not to the indoor space.

The air conditioning system according to the invention is well suited to both larger interior complexes and to separate indoor spaces, such as halls, auditoriums, entrance halls, vestibules, and ordinary rooms. The said indoor space is preferably a room.

Thus, the PCM cell cooling system of the present invention is based on the different directing of air flows during the charge and discharge stages, the heat storage capacity of the phase change material, and the structure that improves the heat transfer inside the cell. In the discharging stage of the cell, the supply air jet is directed at the surface of the cell, whereupon the convection heat-transfer coefficient of the cell surface increases, while the air flow increases on the cell surface. In that case, the cell can be more effectively discharged overnight. A matrix structure made of aluminium, steel or another metal or graphite, which improves the heat transfer, is installed inside the cell. The metal can also be in the form of sawdust or chips among the phase change material. The cell is now effectively charged or discharged.

In the following, the invention is described in detail with reference to the drawings, wherein Fig. 1 shows the travel of the air flows in the supply and exhaust air ventilation in the daytime, when the phase change cells installed in a lower ceiling are charged, Fig. 2 shows the travel of the air flows in the supply and exhaust air ventilation at night, when the phase change cells installed in the lower ceiling are discharged, Fig. 3 shows the travel of the air flows in the supply and exhaust air ventilation in the daytime, when the phase change cells installed on the walls are charged, Fig. 4 shows the travel of the air flows in the supply and exhaust air ventilation at night, when the phase change cells installed on the walls are discharged, Fig. 5 shows the travel of the air flows in natural ventilation in the daytime, when the phase change cells installed in the lower ceiling are charged, Fig. 6 shows the travel of the air flows in natural ventilation at night, when the phase change cells installed in the lower ceiling are discharged, Fig. 7 shows the travel of the supply and exhaust air flow rates in the supply and exhaust air ventilation separately in the daytime, when the balk-shaped phase change cells installed in the ceiling are charged, Fig. 8 shows the travel of the supply and exhaust air flow rates in the supply and exhaust air ventilation separately at night, when the balk-shaped phase change cells installed in the ceiling are discharged, and Fig. 9 shows various phase change cell structures that improve the heat transfer.

As already mentioned earlier, the operation of the PCM cell system is based on guiding the air flows in different ways during the charging and discharging stages, the heat storage capacity of the phase change material, and the structure inside the cell, which improves the heat transfer. The cells can be installed in a room having supply and exhaust air ventilation. The cells are installed in the lower ceiling in place of the lower ceiling boards (Figs. 1, 2) or on the wall on special support structures (Figs. 3,4). The cells are made of aluminium or steel and they contain a

phase change material, the melting temperature of which is 20-25°C. The outer surfaces of the cells are provided with heat transfer fins that are intended for enhancing the heat transfer between air and the cell. Inside the cell, there is a matrix structure made of aluminium, steel or other metal or graphite, improving the heat transfer. The metal can also be in the form of sawdust or chips among the phase change material (Fig. 9).

A damper is connected to the supply air duct of the room. Depending on the position of the damper, the air jet is either directed at the room or the space between the structure and the PCM cell. Correspondingly, a damper is also connected to the exhaust air duct. In that case, the damper can be used to control whether the air is taken from between the lower ceiling and the ceiling or from the room.

On the basis of the ventilation needed, the amounts of supply and exhaust air are kept unchanged.

During the period of non-heating, the operation of the system is based on two steps : the charging and discharging stages of the cells. In the daytime, there is a lot of heat load in the room, whereupon the temperature of the indoor air increases. Then, the supply air jet is mainly directed at the room. The air is at the temperature of the outdoor air or, if the outdoor temperature is too low, it is heated. The excess heat in the room is absorbed into the phase change cells in the room, the material in which melts and stores the heat. Upon the heat absorbing into the cells, the temperature of the indoor air decreases.

The heat stored in the cells is to be discharged at night. Then, the supply air jet is directed to travel in the space between the structure and the PCM cell. Although the amount of air does not change, the velocity of the air in the space between the structure and the cell increases, and the convection heat-transfer coefficient of the surface increases. In that case, the heat transfer between the cell surface and air is enhanced and the phase change material in the cells is crystallized, delivering the heat into the air flow. The excess heat is removed along with the exhaust air. In the morning, the cells are again in a solid state, ready for a recharge.

During the heating period, there is no need for ventilation at night. The excess heat is released into the room and it can be used to heat the room.

The PCM cell is also suitable for implementation in a building that has natural ventilation. The phase change cells are installed in the lower ceiling of the room. A supply air fan and an exhaust air fan are installed on the outer walls of the room. In

the daytime, excess heat is stored in the phase change cells without extra directing of the air. At night, a supply air jet is directed at the space between the cells and the ceiling by using the supply air fan installed on the wall. The exhaust air fans are used to exhaust, by suction, the air heated by the phase change cells. The supply and exhaust air fans are used to increase the convection heat-transfer coefficient of the cell surface, and the discharge of the phase change cells is enhanced.

As the heat releasing from the cells can be used to heat the room, there is no need to use the supply and exhaust air fans during the heating period. The building now works on natural ventilation around the clock.

During the night, the supply air is taken from the outdoor air directly through the wall, and the exhaust air is directed out by using the exhaust air fan.

The phase change cells can also be suspended from the ceiling of the room. See Figs. 7 and 8, wherein the supply air flow is directed at the room in the daytime by using the supply air terminal devices. At night, the air jet is directed straight at the cell surfaces in the discharging stage by using dampers, whereby the heat transfer is enhanced and the discharge of the cells is effective.

At night, the supply and exhaust air can also be taken from the outdoor air by using the supply and exhaust air fans in a building, which has natural ventilation, in the same way as shown in Figs. 5 and 6.

A structure of aluminium, steel or another metal or graphite is located inside the phase change cell. The purpose of the structure is to increase the heat conductivity of the phase change material and, consequently, to improve the charging and discharging of the cells. Fig. 9 shows a metal structure that improves the heat transfer: a matrix of metal or graphite, and a structure, in which the structure that improves the heat transfer is in the form of sawdust or chips or in some other form, which is homogeneously spread among the phase change material.

Advantages of the system include, among others, that it can be installed in rooms that need cooling, either new buildings or old buildings in connection with renovation. In Finland, there are a lot of office, service, and business buildings that suffer from too high room temperatures. At present, mechanical cooling is used in the buildings. The spaces are cooled by refrigerating machines, which use refrigerants that are harmful to the environment. The share of mechanical cooling in the energy consumption of a building is fairly high. By using the PCM cells, we can

eliminate the use of harmful substances and the cells do not need any electricity. In this way, energy costs are reduced.

The cells are easy to install in various spaces and the number of cells can be dimensioned in accordance with the cooling demand for each space separately.

Rooms that face north do not need as many cells as the ones facing south.

In the winter, the cells store the surplus heat that is generated in the daytime.

However, the heat should not be removed but used to heat the room in the winter.

However, the prerequisite for installing the system is that the building is provided with supply and exhaust air ventilation, or the air must be taken to the room from the outdoor air by using supply and exhaust air fans. The PCM cells also operate without ventilation but the heat is discharged back to the room, increasing the room temperature at night. However, even in this case, the cells generally decrease the room temperature by 1-2°C.

PCM cells are best suited to renovation projects, which do not need achieve the S t class of the present indoor classifications, which determine the room temperature in Finland to be less than 24°C in the summer and less than 21°C in the winter. The PCM system is a partly passive system that cannot be adjusted in the same way as the mechanical cooling system. The PCM cells are well suited to projects, which aim at reducing the summer temperature levels because of comfort and to improve working capacity.