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Title:
COMBINED COOLING AND HEATING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2022/013655
Kind Code:
A1
Abstract:
A combined cool storage and heating (CCHD) device comprising: at least one cooled compartment; at least one heated compartment; at least one sealed thermal reservoir compartment; and at least two thermoelectric modules, the first thermally connecting the at least one cooled compartment to the thermal reservoir transferring heat from the at least one cooled compartment to the reservoir, the second thermally connecting the reservoir to the at least one heated compartment transferring heat from the reservoir to the at least one heated compartment.

Inventors:
WYLLIE NICHOLAS (GB)
Application Number:
PCT/IB2021/055668
Publication Date:
January 20, 2022
Filing Date:
June 25, 2021
Export Citation:
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Assignee:
WYLLIE NICHOLAS (GB)
International Classes:
F25B21/04; F25D1/00
Foreign References:
US20130174580A12013-07-11
JP2008025920A2008-02-07
KR20110059477A2011-06-02
Attorney, Agent or Firm:
ROBERTSON, Christopher (GB)
Download PDF:
Claims:
Claims

1. A combined cooling and heating (CCHD) device comprising: at least one cooling compartment; at least one heating compartment; at least one sealed thermal reservoir compartment; and at least two thermoelectric modules; wherein a first one of the thermoelectric modules thermally connects one of the cooling compartments to one of the thermal reservoir compartments, and a second one of the thermoelectric modules thermally connects one of the thermal reservoir compartments to one of the heating compartments.

2. A CCHD according to claim 1 comprising an outer casing and a plurality of internal partitions defining the compartments.

3. A CCHD according to any preceding claim further comprising at least one additional thermoelectric module thermally connecting the sealed reservoir compartment to the ambient temperature outside the CCHD device.

4. A CCHD according to any preceding claim wherein each of the plurality of thermoelectric modules each comprise at least one Peltier device.

5. A CCHD according to any preceding claim wherein each thermoelectric device has at least one heat sink on one or both sides of the thermoelectric device.

6. A CCHD according to any preceding claim wherein each thermoelectric device has at least one associated fan to circulate air in a connected compartment. 7. A CCHD according to any one of claims 2 to 6 wherein the casing and/ or partitions comprise insulation in the form of an insulating coating and/ or filling, or wherein the casing and/or partitions comprise internal vacuum sealed chambers.

8. A CCHD according to any preceding claim comprising a power supply for providing DC current to the at least two thermoelectric modules.

9. A CCHD according to any preceding claim further comprising a control system configured to control the temperatures of the compartments respectively by selectively activating the thermoelectric elements between heating and cooling modes. 10. A CCHD according to claim 9 wherein the control system is further configured to selectively switch at least one of the at least two thermoelectric module between a heating and cooling mode and an electricity generating mode in which a temperature gradient between said hot and cold sides causes the at least one Peltier module to generate electricity. 11. A CCHD according to either one of claims 8 to 10 further comprising a chargeable battery and/ or a supercapacitor.

12. A CCHD according to any one of claims 8 to 11 further comprising solar panels electrically connected to the power supply to supplement power needs.

13. A CCHD according to any preceding claim wherein the at least one reservoir compartment has a larger volume than the at least one heated compartment and/ or the at least one cooled compartment.

14. A CCHD according to any preceding claim comprising a plurality of cooled compartments.

15. A CCHD according to any preceding claim comprising a plurality of heated compartments.

16. A CCHD according to any preceding claim comprising a plurality of thermal reservoir compartments.

17. A domestic or industrial kitchen comprising a CCHD as claimed in any one of claims 1 to 12.

Description:
Combined Cooling and Heating System

[001] The present invention relates to a combined cooling and heating device (CCHD). More specifically it relates to an energy efficient CCHD, having a stepped heat exchange between heating and cooling chambers.

Background of the Invention

[002] It is common or even standard practice in both domestic and industrial settings to store food and drink etc in a refrigerator. In domestic and some industrial settings it is common for the refrigerator to be located in a kitchen area, which typically also has an oven/ cooker situated in the kitchen area too. It is the norm that these two devices are entirely separate from one another.

[003] Refrigerators typically rely on a phase change process in which a suitable refrigerant is pumped around a circuit. As the refrigerant is moved around the circuit its pressure is changed to cause it to switch between its vapour and liquid states. The circuit includes a heat exchanger located on the outside of the refrigerator so that as the heated vapour passes through it, heat can be rejected to the surrounding atmosphere. At best, this heat is wasted. In some confined locations, the heat build-up around the heat exchanger may even adversely affect the performance of the refrigerator.

[004] Improved energy efficiency is always beneficial to the cost of running a system, and with the growing climate emergency, energy efficiency is becoming a more and more important focus of day to day life.

[005] It is an object of the present invention to provide a CCHD device which can operate as both a cooling device and a heating device as required, with increased energy efficiency.

Statement of Invention

[006] According to a first aspect of the invention there is provided a combined cooling and heating (CCHD) device comprising: at least one cooling compartment; at least one heating compartment; at least one sealed thermal reservoir compartment; and at least two thermoelectric modules; wherein a first one of the thermoelectric modules thermally connects one of the cooling compartments to one of the thermal reservoir compartments, and a second one of the thermoelectric modules thermally connects one of the thermal reservoir compartments to one of the heating compartments.

[007] An embodiment of the first aspect comprising an outer casing and a plurality of internal partitions defining the compartments.

[008] An embodiment of the first aspect further comprising at least one additional thermoelectric module thermally connecting the sealed reservoir compartment to the ambient temperature outside the CCHD device.

[009] An embodiment of the first aspect wherein each of the plurality of thermoelectric modules each comprise at least one Peltier device.

[010] An embodiment of the first aspect wherein each thermoelectric device has at least one heat sink on one or both sides of the thermoelectric device.

[011] An embodiment of the first aspect wherein each thermoelectric device has at least one associated fan to circulate air in a connected compartment.

[012] An embodiment of the first aspect wherein the casing and/ or partitions comprise insulation in the form of an insulating coating and/ or filling, or wherein the casing and/ or partitions comprise internal vacuum sealed chambers.

[013] An embodiment of the first aspect comprising a power supply for providing DC current to the at least two thermoelectric modules.

[014] An embodiment of the first aspect further comprising a controller configured to switch at least one of the at least two thermoelectric module between a heating and cooling mode in which the at least one thermoelectric module transfers heat from one compartment to another compartment, and an electricity generating mode in which a temperature gradient between said one compartment and said another compartment causes the at least one Peltier module to generate electricity. The device may further comprise a chargeable battery and/ or a supercapacitor.

[015] An embodiment of the first aspect further comprising solar panels electrically connected to the power supply to supplement power needs.

[016] An embodiment of the first aspect wherein the at least one reservoir compartment has a larger volume than the at least one heated compartment and / or the at least one cooled compartment.

[017] A domestic or industrial kitchen comprising an embodiment of the first aspect.

Brief Description of the Drawings

[018] The invention will be described in more detail, by way of example, with reference to the following drawings:

[019] Figure 1 depicts an embodiment of the invention; [020] Figure 2 depicts an embodiment of the invention having a plurality of thermal reservoirs;

[021] Figure 3 depicts an embodiment of the inventio having a plurality of heating and cooling compartments.

Detailed Description [022] Figure 1 depicts the simplest embodiment of the invention, comprising a

CCF1D device 1 comprising a cooled compartment 2, a heated compartment 3, a thermal reservoir compartment 4, at least one thermoelectric module 5 between the cooled compartment 2 and the thermal reservoir 4 for transferring heat between the cooled compartment 2 and the reservoir 4, and at least one thermoelectric module 6 for transferring heat between the reservoir 4 and the heated compartment 3. [023] Compartments 2, 3 and 4 may be defined by a casing 7. The casing 7 is provided with a partition 8 separating the cool compartment 2 from the heated compartment 3. There is also provided another partition 9 separating both compartments 2 and 3 from the thermal reservoir compartment 4. The partitions hold the at least two thermoelectric modules 5, 6.

[024] The casing 7 comprises insulation therein to inhibit, or prevent, the transfer of heat energy by the casing 7. The partitions 8, 9 are also insulated to inhibit, or prevent heat transfer between compartments. The casing 7 and the partitions 8, 9 can be insulated by any suitable means known to the skilled person; this can include, but is not limited to at least one of: an insulating filling inside the casing and partitions; an insulating material coating, or vacuum sealed evacuated chambers therein.

[025] The at least two thermoelectric modules 5, 6 comprise Peltier/Seebeck elements, heat sinks/cold plates, and optional fans. Peltier elements are known heat pumps which transfer heat from one side of the element to the other, the direction of heat flow dependent on the polarity of the voltage applied across the element.

[026] The at least two thermoelectric modules 5, 6 may comprise at least one respective heat sink/heat exchanger on one or both sides of the thermoelectric module in direct contact with the Peltier elements. The at least two thermoelectric modules may have one or more fans associated with one or both sides of the thermoelectric modules, the fans aiding in air circulation, orientated as such to maximise heat transfer between the thermoelectric modules and the respective compartment. The heatsink/heat exchangers may be made of, for example, metals, such as aluminium or an aluminium alloy. In other examples, the heat exchangers may be made of a non-metallic thermally conducting material, such as ceramic.

[027] The cooled compartment 2 and the heated compartment 3 are not thermally coupled via thermoelectric devices for direct movement of heat from one to the other. Instead the thermoelectric devices transfer heat to/ remove heat from the sealed thermal reservoir compartment 4.

[028] The purpose of the thermal reservoir compartment 4 is to provide a sealed reservoir that is not openable/ easily accessible for the storage of thermal energy for transfer within a device. It may be advantageous to provide an access hatch to the thermal reservoir 4 for the purposes of maintenance and repair, but suitable insulation provisions must be made. Heat can be moved into, stored and transferred from the thermal reservoir on demand, for immediate use or for later use. This means that the cooled compartment 2 and the hot compartment 3 can operate independently from one another.

[029] It will be clear that the configuration of compartments in this embodiment is not essential. The arrangement and number of compartments depends on the intended use of the device, and is variable within the scope of the invention. For example, there may be more than one heating chamber, and/ or more than one cooling chamber. There may also be more one thermal reservoir. The chambers need not be adjacent, as long as they are in direct or indirect thermal communication with one another by means of thermoelectric modules.

[030] A larger thermal reserve/heat reservoir(s) may reduce the electrical energy needed to transfer the heat from the cold compartment and when transferring heat to a smaller heater compartment may allow for a greater heat pressure. This is because it costs more energy to move heat energy from a cooler to a warmer compartment, the greater the difference in ambient temperature in each compartment. A larger thermal reservoir will heat up more slowly, and so the temperature difference will increase more slowly.

[031] Providing a smaller heating chamber 3 reduces the energy required to heat the heating chamber 3 to a given temperature. In some embodiments a larger heating chamber and a smaller heating chamber can be provided. In such embodiments it may be preferable to use the smaller heating chamber when this is large enough, in order to reduce the energy cost.

[032] An electrical current applied to the thermoelectric modules creates a heat flux drawing heat from one compartment (e.g. the cooler 2), leaving it cooler. The resultant heat is transferred to another compartment (e.g. the heat reservoir 4), increasing the heat therein. In this example, wherein heat energy has been transferred from the cooling chamber 2 to the thermal reservoir 4, the heat inside the thermal reservoir can then be transferred to another compartment (e.g. the heating compartment 3) increasing the heat within that compartment. The presence of the heat reservoir 4 allows a stepped transfer of heat between the heating chamber 3 and the cooling chamber 2, meaning the temperature difference across any given thermoelectric module is generally less than it would be if the heating chamber 3 and the cooling chamber 2 were directly coupled by means of a thermoelectric module. This leads to a less costly transfer of heat.

[033] A control and DC power supply unit (not shown) may be disposed in the casing/ partitions, to control operation of the CCHD system and distribute electrical power to the thermoelectric modules and therefore control the conditions in each heated/ cooled compartments and thermal reservoir. The control and power supply unit may comprise a suitable form of electric charge storage device, and/ or may be powered from a mains electrical supply or external battery. Thus, for example, the control and power supply unit may be connected with a power socket to receive power from a suitable transformer plug (not shown). Alternatively, the control and power supply unit may comprise suitable conditioning circuitry for converting power from an AC power supply to a suitable DC voltage to power the thermoelectric modules or charge an electric charge storage device. The electric charge storage device may comprise a battery or supercapacitor. [034] The control and power supply unit may comprise a processor or dedicated control circuitry and may be connected to an on/ off switch and respective thermostats 10 provided in the cooled, heated, and reservoir compartments. The control and power supply unit may incorporate a timer. It is to be understood that although the control and power supply unit is described and shown herein as one unit, this is not essential and it may be made up of separate circuits, components or sub-assemblies located at different points within the casing that are suitably connected as required.

[035] In some examples, the control and power supply unit may be configured to switch the thermoelectric modules between a heating mode and a cooling mode. This is done by changing the direction of the current supplied to the Peltier modules. The control unit may also be able to switch one or more of the Peltier modules to a third, power generating mode in which the supply of electricity to the thermoelectric modules is interrupted and the Peltier modules, via the Seebeck effect, generate an electric current as they move heat from a warmer compartment to a cooler compartment. This functionality may be used to provide a control strategy in which the Peltier modules are switched between the two/three modes to regulate the temperature in the cooled and heated compartments. Switching may be based on signals from the thermostats 10. Alternatively, the combined cool storage and heating device 1 may be provided with other forms of temperature sensor 10 disposed in the compartments and the control and power supply unit may comprise a processor or processing circuity configured to cause switching between the two operating modes based on temperature indicating signals received from the temperature sensors 10. The CCHD device 1 may be provided with a user interface to allow a user to enter heating and cooling parameters used to control operation of the combined cool storage and heating device. Such parameters may include other options, such as desired temperature of each compartment. [036] The combined cool storage and heating device 1 may be provided with a user interface 11 that is configured to allow a user to input control commands. The user interface 11 may, for example, comprise one or more of a keypad, a touchpad, dials and switches. The control and power supply unit may be network enabled to enable remote control by a remote control device such as a smartphone or a dedicated handheld controller using, for example, infra-red control signals. Thus, for example, the control and power supply unit may be provided with a wireless communications module that enables wireless communication with a remote device using a WIFI or Bluetooth protocol. The device control system may additionally or alternatively be incorporated into an external management system.

[037] A number of factors affect the transfer of heat within the CCHD device and between the device and locations exterior to the device. As the thermal gradient increases between adjacent compartments, the resistance to the generation of thermal flux increases. For example, if the temperature difference between adjacent compartments is 100°C there is more resistance to the generation of a thermal flux than if the differential temperature is 10°C. The provision of a reservoir allows a staged or incremental temperature increase or decrease between the cooled and heated compartments that in turn allows compartment temperatures to be attained more readily and efficiently.

[038] By way of example of use of the combined cool storage and heating device 10, the control and power supply unit may be set, or commanded, to activate the thermoelectric modules to achieve a Tridge temperature’ of around 4°C in the cooled compartment 2 and an ‘oven temperature’ between 80°C and 240°C in the heated compartment 3.

[039] Although it may be possible in this Tridge-oven’ example to heat the oven to a temperature at the lower end of the quoted range using only the heat energy extracted from the fridge 2 and the ambient heat in the thermal reservoir 4 and the oven 3, depending on the respective volumes of the compartments, the ambient temperature and the specific heat capacities of the contents of the compartments, it is likely that more energy will be required to heat the oven 3 to the higher temperatures in its operating range. An additional, conventional heating element may be provided. Alternatively or additionally, the thermal reservoir 4 can be in thermal communication with the external environment by means of another thermoelectric module 12 and can extract additional heat energy from the external environment for provision to the oven 3.

[040] Until such time as a user resets the temperature, the fridge temperature will ideally be kept at least substantially constant, whereas an elevated oven temperature may only be required for a relatively brief periods. The external, ambient, temperature may be 20°C, which is the temperature found in a typical home. There will typically be relatively regular transfers of thermal energy from the cooled compartment 2 to the thermal reservoir 4. In cases in which there is no demand for additional heat to be supplied to the heated compartment 3, if the temperature in the thermal reservoir 4 rises above a desired level, the thermoelectric module 12 may be activated to transfer heat out to the thermal reservoir 4 into the air surrounding the combined cool storage and heating device 1.

[041] When, for example, cooking is required at 180°C the respective temperatures in the cooled compartment 2, thermal reservoir 4 and heated compartment 3 may be 4°C, 100°C and 180°C. Thermal flux is generated from the cooled compartment 2 to the thermal reservoir 4 by the thermoelectric module 5. Thermal energy is then transferred from the thermal reservoir 4 to the heated compartment 3 by the thermoelectric module 6. As described above, additional heat energy may be obtained by activating the thermoelectric module 12 to transfer heat from the air surrounding the combined cool storage and heating device 1.

The thermal reservoir 4 functions as thermal stage or increment between the cooled and heated compartments 2, 3. It will be clear that these figures are purely exemplary and depend on a number of factors, such as compartment volume, specific heat capacity and ambient temperature. [042] The change in temperature in the cooled and heated compartments 2, 3 and the reservoir 4 obtained by activating the thermoelectric modules is in part dependent on the specific heat of the fluid contained in the respective compartments. The cooled and heated compartments typically contain air (specific heat capacity around 1 J/g.K), particularly in cases in which the combined cool storage and heating device 1 is used as a domestic appliance. In examples used in manufacturing or scientific research, the compartments may contain other fluids, for example, an inert gas such as nitrogen or argon or gases containing an etching component for etching a surface in photoetching. In medical applications, the compartments may contain a therapeutic fluid. The molar content of the contained gas affects the rate of temperature change and molar content is dependent on volume. For example, the transfer of heat to the thermal reservoir causes a smaller temperature change if the volume or pressure in the reservoir is higher. In some examples, it may be desirable to increase the volume of the thermal reservoir relative to the volume of the cooled or heated compartments. For example, a volumetric ratio greater 2:1 may be preferred. As the thermal reservoir is a sealed compartment, it may be desirable to increase the pressure of gas contained therein, or to use a gas other having a lower specific heat capacity than air.

[043] The thermal reserve(s) may contain air. Alternatively, depending on the intended application, the thermal reserve may contain another gas, or a liquid, or a suitable solid material. In other embodiments, the thermal reserve(s) may contain a substance which changes phase between solid, liquid and gas across the intended range of operating temperatures and pressures. The material should be selected according to the required specific heat capacity. The optimal specific heat capacity will vary according to the application. Each thermal reserve may contain a plurality of different materials. In embodiments with more than one thermal reserve, some or each of them may contain different materials.

[044] Above was a description of a simple embodiment of the CCF1D device 10 and is not considered limiting. There can be a plurality of hot compartments and / or a plurality of cooled compartments. There may also be a plurality of sealed thermal reservoirs. There may be thermoelectric modules which can transfer heat between the thermal reservoir and the ambient air outside the casing. Thermoelectric modules may be incorporated into the fabric of the system or some parts of the thermoelectric module(s) and/ or fans may protrude into the compartments, thermal reserves or outside the device. Thermoelectric modules may be with or without fans or with fans either on both sides or on a single side.

[045] The system, individual compartments, doors/drawers and thermal reserve will be insulated. Insulation could be materials that are good insulators and or vacuum insulation. Insulation may vary within a system, for example between the hot and cold faces insulation might be thicker, other material or vacuum insulated.

[046] Access to compartments may be by door(s) (top, side, bottom hung or other hinging), drawers or other method.

[047] The system may have a controller/ controllers and a temperature sensor/ sensors within some/ all compartments, thermal reserves and externally.

Some devices may utilize some or all the thermoelectric modules to generate power and some devices may also feature a battery or supercapacitor (or other storage device) to store energy.

[048] In some devices the temperature differential existing on either side of the thermoelectric elements can be used to generate electrical energy, the greater the gradient between the hot and cold sides the greater the power generation.

[049] For some applications (portable, etc.) a photo voltaic device (solar panel) can be incorporated in or connected to the system for power generation to use within the device, power storage and other uses. [050] This dynamic heat engine conversion device uses the movement of electrons (rather than fluids/gases) to move heat energy within the system and to create energy to maintain the system and for energy storage in supercapacitors. [051] The CCHD device in figure 1 is shown having three thermoelectric modules 5, 6, 12. It is to be understood that this is not to be taken as limiting. A combined cool storage and heating device may comprise only one thermoelectric module or two or more thermoelectric modules as required. Furthermore, a thermoelectric module may comprise a single Peltier module or a plurality of connected Peltier modules. The stacking of the Peltier modules to provide enhanced performance is known to those skilled in the art and so will not be described in detail herein.

[052] A combined cool storage and heating device may be a standalone unit configured for portability or configured for installation as a fitted unit in a kitchen. A combined cool storage and heating device designed for installation as a fitted unit in a kitchen may be configured to fit an industry standard footprint. Although not limited to such uses, it is envisaged that the cooled compartment(s) may be used for food storage for preserving and cooling food while the heated compartment(s) may be used for at least one of defrosting, warming, heating or cooking food.

[053] It will be understood that the combined cool storage and heating device allows use to be made of the heat withdrawn from the cooled compartment and moved to the heated compartment by the thermoelectric modules, together with any heat generated within thermoelectric module of, that would otherwise be wasted to atmosphere. Thus, the thermoelectric module functions as both a cooler cooling the cooled compartment and a heater heating the heated compartment. Furthermore, by suitable switching, a thermoelectric module may be switched from a heating and cooling mode into an electricity generating mode. In the electricity generating mode, the electric supply to the thermoelectric module is switched off and due to the temperature differential between the hot and cold sides of the thermoelectric module, electricity is generated. In examples of combined cool storage and heating devices that comprise a plurality of thermoelectric modules, there may be operating conditions in which some of the modules are used to move heat from the cooled compartment to the heated compartment and one or more is switched to electricity generating mode. Some examples may even include thermoelectric modules configures solely for operation as electricity generators.

[054] For some examples, the combined cool storage and heating device may be powered by electricity generated by one or more solar panels. Thus, for example, a portable unit may be connected with one or more solar panels that generate electricity to power the thermoelectric module or modules.

[055] Advantageously, aside from the or each fan, the heating and cooling system of the combined cool storage and heating device may have no moving parts and as a result the maintenance requirements should be minimal.

[056] In principle the combined cool storage and heating device may be used for any application that calls for the provision of cooling and heating. For example: in hospitals and laboratories to store chemicals or medicines, or as incubators; to provide a temperature-controlled transport device; as food or drink displays.

[057] Figure 2 discloses another exemplary embodiment, in which like features are given like reference numbers to those in Figure 1. Rather than a single thermal reservoir, three thermal reservoirs 13-15 are provided. The first thermal reservoir 13 is in thermal communication with the cooling chamber 2, and the second thermal reservoir 14. The second thermal reservoir 14 is also in thermal communication with the heating chamber 3 and the third thermal reservoir 15.

The third thermal reservoir 15 is also in thermal communication with the heating chamber 3 and the external environment. The thermal communication between the chambers is effected by thermoelectric modules 5, 6, 12, 16, 17 and 19. It may be particularly advantageous to have a plurality of thermal reservoirs in order to increase the benefits discussed above of having stepped heat transfer. It costs less energy to transfer heat against a smaller temperature gradient. It may be advantageous to have a plurality of thermal reservoirs in thermal communication with the heating chamber(s) 3 in order to increase the rate of heat transfer into the heating chamber(s). [058] Figure 3 depicts an embodiment with a plurality of heating chambers 3, 20 and a plurality of cooling chambers 2, 21. Two thermal reservoirs 4, 15 are provided, in thermal communication with each other. The first thermal reservoir is in thermal communication with each of the heating and cooling chambers 2, 3, 20, 21. The second thermal reservoir is in thermal communication with one of the heating chambers 20 and the external environment. The thermal communication is effected by means of thermoelectric modules 5, 6, 12, 16, 17, 22 and 23. An additional chamber 24 is optionally provided for storing control and / or power supply apparatus. [059] The plurality of heating chambers 3, 20 could have different operating ranges for temperature. For example, chamber 20 could have a higher range of operating temperatures. This can be facilitated by thermal communication with both thermal reservoirs 4, 15.

[060] Alternatively or additionally, the plurality of heating chambers 3, 20 could have different sizes. It might be particularly preferable for chamber 20 to be smaller. This would facilitate faster heating or a higher range of operating temperatures. It would also or alternatively enable cheaper operation, since less energy would be required to heat it to a given temperature than the larger chamber 3. [061] It will be clear that the chambers as described above are interchangeable.

For example, in some oven-related embodiments, it may be preferable to cook something larger in chamber 20.

[062] The plurality of cooling chambers could have different operating ranges for temperature. For example, one chamber 2 could be a refrigerator (having an operating temperature about 4 degrees, for example) and another chamber 21 could be a freezer (having an operating temperature about -18 degrees, for example). [063] The plurality of cooling chambers 2, 21 could also have different sizes. It may be preferable, for example, to have a larger fridge unit and a smaller freezer unit.

[064] The invention has been described with reference to preferred embodiments. The description is intended to enable a skilled person to make the invention, not to limit the scope of the invention. The scope of the invention is determined by the claims.