EP3213013B1

EP3213013B1 - Refrigerator with a phase change material as a thermal store

Info

Publication number: EP3213013B1
Application number: EP14799842.1A
Authority: EP
Inventors: Marcel Van Beek; Hans De Jong
Original assignee: Enviro-Cool Commercial Ltd
Current assignee: Enviro-Cool Commercial Ltd
Priority date: 2014-10-29
Filing date: 2014-10-29
Publication date: 2020-06-10
Anticipated expiration: 2034-10-29
Also published as: DK3213013T3; HRP20201393T1; HUE051889T2; MX2017005634A; WO2016066980A1; US20170314839A1; AU2014410347A1; JP2017537300A; CN107003050A; SI3213013T1; CA2965749A1; EA201790918A1; ES2813651T3; EP3213013A1; KR20170078705A; BR112017008803A2; PT3213013T

Description

The present invention relates to a refrigerator which uses a phase change material as a thermal store. The invention is thought to be of particular relevance to commercial bottle coolers as found in bars and pubs because they experience relatively high peak cooling loads.
Refrigerated display cabinets, of which bottle coolers are an example, are used extensively in entertainment venues to store and cool beverages for sale to customers. They are usually provided with a transparent or see-through door so that the beverages inside can be displayed to customers.
Bottle coolers experience periods of relatively high cooling loads e.g. when the door is opened frequently to remove drinks for customer and/or when the fridge is restocked with a large number of yet-to-be-cooled containers of beverage.
To cope with the high cooling loads, bottle coolers intended for commercial premises are equipped with larger vapour compression refrigeration systems than usually found in domestic fridges of similar volume. This makes them less economical to run than domestic fridges of comparable size.
Other than because of their size, the larger compressors typically used in commercial fridges are less efficient than the smaller compressors used for domestic fridges because the larger market for domestic fridges has driven development for greater efficiency in the compressors used.
In addition, the overall efficiency of bottle coolers is compromised by the need to use materials of relatively low thermal insulating properties to make the door see-through/transparent. This issue is particularly acute with display cabinets intended to operate with an open front (i.e. no door) during trading hours as are common in shops for the sale of refrigerated/frozen goods.
CA2103978 relates to a system having two refrigeration chambers, one for a fridge and the other for a freezer, each having an evaporator. A control device is used to direct refrigerant flow between the evaporators to control the temperature of the chambers. A phase change material may be used in conjunction with either evaporator.
DE202006010757 relates to a refrigerated display cabinet having a phase change material on an inside wall to act as a thermal accumulator.
US2014/0130536 (Joppolo et al. ) discloses a refrigerator with two evaporators in different compartments, one of which is in a heat exchange relationship with a phase change material.
The present invention was conceived with the aim to increase the efficiency of refrigeration display cabinets, though the invention is thought to be of benefit to any refrigeration device that experiences periodic large load variations.
In accordance with the invention, a refrigerator is provided having a cooling chamber for containing an object to be cooled, the refrigerator comprising: a thermal store comprising a phase change material for cooling the cooling chamber; a vapour compression refrigeration system including a first evaporator for cooling the cooling chamber and a second evaporator for cooling the phase change material; and means to control the flow of refrigerant to the first and second evaporators depending on the cooling load on the refrigerator, wherein, when the refrigerator is subject to a relatively low cooling load, refrigerant flows to the second evaporator to cool the phase change material and, when the refrigerator is subject to a relatively high cooling load, refrigerant flows to the first evaporator such that increased cooling is provided to the cooling chamber by the first evaporator and the phase change material.
When the refrigerator is subject to a relatively low cooling load, it is envisaged that refrigerant may flow substantially only to the second evaporator to cool the phase change material, since the phase change material and the second evaporator will provide some cooling effect. However, in a preferred embodiment refrigerant flows to both the first and second evaporators when the refrigerator is subject to a relatively low cooling load.
When the refrigerator is subject to a relatively high cooling load, it is preferred to maximise the cooling effect provided by the first evaporator by directing the refrigerant substantially only to the first evaporator.
By directing refrigerant through the second evaporator during periods when the cooling load is low, the spare capacity in the system can be used to cool the phase change material (PCM) into the lower energy state, e.g. from a gas to liquid or a liquid to a solid.
When the refrigerator experiences a high cooling load, both the first evaporator and the thermal store can be used simultaneously (sequentially) to cool the air. As the first evaporator and thermal store are separate, the cooling surface area is increased enabling faster cooling. Since the PCM is at least partially if not completely in the lower energy state from the previous low cooling load period, the refrigerant flow can be directed (primarily or completely) to the first evaporator in favour of the second evaporator, so that the cooling of the air provided by the first evaporator is increased. Even though the refrigerant flow to the second evaporator is restricted or turned off, the PCM in its lower energy state will continue to cool the air as it gradually transitions to the higher energy state. Therefore, the refrigerant can be used primarily to cool the air rather than the PCM during periods of peak loading.
Through the control of the refrigerant in this way the efficiency of the system is improved. The thermal store provides an effect analogous to spreading the cooling load over a longer period. This allows the system to use a smaller more efficient compressor with minimal if any reduction in the effectiveness of the refrigerator. The thermal store may also enable the refrigerator to continue cooling in the event of temporary power failure.
The state of the cooling load is typically identified by sensing a temperature difference between the actual temperature and the desired temperature.
To improve the cooling rate of air within the cooling chamber, it is preferred that there comprises means to circulate air in the cooling chamber past a cooling surface of the thermal store, and preferably also past the first evaporator. Forced air cooling is considered to be particularly favourable for display refrigerators where cooling through convection or conduction may not be practical to provide the cooling rate needed to cope with the higher thermal load. Preferably, the means to circulate air comprises a fan. It is preferred that the air is circulated out of the cooling chamber, past the cooling surface of the thermal store, past the first evaporator and back into the cooling chamber.
The refrigerator may comprise a duct which is interposed between a wall of the cooling chamber and an external insulating wall of the refrigerator. The thermal store may be mounted within the duct. To provide the greatest surface area, it is preferred that the thermal store is mounted within the duct so that two opposing external sides of the thermal store are exposed to air flowing through the duct. In other words, air passes on both sides of the thermal store. Preferably, the thermal store is elongate with its elongate axis parallel to the axis of the duct, so that the thermal store presents a large proportion of its surface area to the passing air.
Preferably the first evaporator is positioned downstream of the thermal store so that the full cooling effect of the PCM can be used during periods of high cooling load, to cool the air before it passes over the first evaporator.
The vapour compression refrigeration system preferably further comprises a compressor, a condenser, at least one expansion device, a first pathway through which refrigerant flows to the first evaporator and back to the compressor, and a second pathway through which refrigerant flows to the second evaporator and back to the compressor. In a preferred embodiment, the second pathway downstream of the second evaporator merges with the first pathway upstream of the first evaporator.
The means to control the flow of refrigerant between the first and second evaporators may act to route refrigerant to both evaporators (in equal or unequal rates) or completely or substantially to one or the other
Preferably, the means to control the flow of refrigerant to the first and second evaporators depending on the cooling load comprises a valve and a controller. Preferably, the controller controls the position of the valve depending upon the cooling load. As mentioned above, the cooling load is preferably determined by determining the difference between the temperature of the air within the cooling chamber and/or duct and the desired temperature. The temperature of the air within the cooling chamber and/or duct is preferably determined by a temperature sensor.
Preferably, the controller also controls the position of the valve depending on the relative proportions of the two phases of the PCM.
In a preferred embodiment, the valve has only two positions, a first position in which the refrigerant is directed to both evaporators and a second position in which the refrigerant is directed only to the first evaporator. The valve is preferably a bistable valve. This allows a simplified control system.
The controller preferably also controls the operation of the compressor, and/or the operation and/or speed of the means to circulate air mentioned above.
It is preferred that the PCM comprises water as a primary constituent, which may also include one or more solutes to adjust the freezing point.
The second evaporator may be arranged to be partially or completely located within the PCM, so that the PCM is cooled from the inside outwards. It is preferred that the thermal store comprises means to identify the extent to which the PCM has changed state. This information can be used by the controller to control the flow of refrigerant to the second evaporator and/or to control the compressor.
In an alternative aspect, the refrigerator has a cooling chamber for holding an object to be cooled, comprising: a thermal store including a phase change material to cool the cooling chamber during a period when the refrigerator is subject to relatively high cooling load; a vapour compression refrigeration system comprising: a first evaporator through which a refrigerant flows to cool the cooling chamber; and a second evaporator through which refrigerant flows to cool the phase change material when the refrigerator is subject to a relatively low cooling load; and means to control the flow of refrigerant to the first and second evaporators depending on the cooling load on the refrigerator.
The invention will now be described by example with reference to the following figures in which:

Fig. 1 is a part schematic side sectional view of a refrigeration display cabinet in accordance with the invention;
Fig. 2 is a schematic drawing of a refrigeration circuit in accordance with the invention; and
Fig. 3 is a part schematic side view illustrating the thermal store.

With reference to Fig. 1, a refrigeration display cabinet comprises a thermally insulating casing 1 with a glass door 2. The casing is preferably made from vacuum-formed insulating panel combined with high-density polyurethane for structural rigidity. The glass door may be double-glazed or preferably triple-glazed. Krypton gas may be provided between the glass plates to increase insulation. The cabinet rests on a base which holds components of a vapour compression refrigeration including a compressor 3, a condenser 4, and a fan 5 associated with the condenser 4.
The cabinet 1 has a compartment 6 in which products to be held are cooled. Lighting may be provided for the compartment 6, which is preferably energy-efficient LED lighting. The lighting power supply is preferably located outside the compartment 6. In order to decrease the appliance heat load still further, the LED light source may also be located outside the compartment 6 and the light guided in to the compartment 6 by appropriate means, such as light guides, fibre optics, aerogels, etc.
Air is drawn from compartment 6 into ducting 7 by a fan 7A in order to be cooled. The ducting 7 is defined in part at least by a gap between internal walls 6A which form the compartment 6 and the inner wall of the insulating casing 1. The base of the compartment 6 may be defined by a first evaporator 10 (referred to below) or casing thereof.
Figs. 1 and 2 show the refrigerant circuit. Condensed refrigerant from the condenser 4 can flow optionally along one of two path ways back to the compressor 3. A first pathway 8 carries refrigerant through a first expansion device 9A and first evaporator 10, typically a fin and tube evaporator. The second pathway 11 carries refrigerant through a second expansion device 9B and a second evaporator 12 which is embedded within a thermal heat storage unit 13 holding a phase change material (PCM) 14, in this case water.
Flow of refrigerant is controlled by a valve 15 downstream of the condenser 4. The position of the valve 15 is controlled by a controller 16, which is also used to control compressor 3, condenser fan 5 and ducting fan 7A.
As shown in Figs. 1 and 2, the system is arranged such that refrigerant that has flowed from the second evaporator 12 then flows back to the compressor 3 via the first evaporator 10. Other arrangements are possible, including two separate pathways which merge up-stream of the compressor 3.
Returning to Fig. 1, both the first evaporator 10 and the thermal storage unit 13 are mounted so that the air circulating through the ducting 7 passes across cooling surfaces of the thermal storage unit 13 and the first evaporator 10 to cool the air.
A temperature sensor 17 senses the temperature of the air coming out of the compartment 6 and provides a corresponding signal to controller 16.
The first evaporator 10 is positioned downstream of the thermal store 13, so that the warmest air passes over the thermal store 13 increasing thermal transfer from the PCM 14 during periods of high cooling loading.
As seen in Fig. 3, the second evaporator 12 is embedded within the thermal store 13 so that freezing of the PCM 14 occurs first in a central region 14A, which is primarily ice, outside of which are outer regions 14C formed primarily of water. The extent to which the PCM 14 has frozen/melted is detected through registering the position of the ice/water interface 14B. This is achieved by a sensor(s) 18 which measures the electrical conductivity of the PCM 14 at points between the outer wall of the thermal store 13 and the evaporator 12. The signals from the sensor(s) 18 are received by the controller 16. Such arrangements are known in the art of thermal stores incorporating PCMs.
To maximise the cooling surface presented by the thermal store 13 to the air within duct 7, the thermal store 13 is spaced from both wall 6A and casing 1 so that air can pass across either side of it.
Returning to Figs. 1 and 2, the operation of the refrigerator will now be described. When the refrigerator is operating in a steady state mode, i.e. the temperature of the air flowing out of the compartment is at or near the desired temperature, the controller 16 operates valve 15 so that refrigerant is pumped along the second pathway 11 through the second evaporator 12 so as to cool and freeze the PCM 14 within the thermal store 13. Once the PCM 14 has frozen as determined by sensor 18, the controller 16 can cause the compressor 3 and condenser fan 5 to turn off/slow down to save energy. Typically, such compressors are either on or off, but a reduction in speed may be possible in some cases.
By applying such control, the state of freezing of the PCM 14 during steady state operation can be controlled for example between completely frozen and 20% melted to ensure sufficient PCM 14 is frozen to provide additional cooling during the next period of high cooling load. If during steady state operation it is determined that the PCM 14 is sufficiently frozen, the controller 16 can cause the compressor 3 to turn off or reduce in speed to reduce energy consumption.
In addition, the controller may control the operation or speed of fan 7A during this steady state, as a further way of adjusting and controlling the compartment air temperature.
In a specific embodiment of steady-state operation, refrigerant flows through both evaporators and the product temperature is controlled by regulating the air temperature leaving the compartment. The air temperature is controlled by adjusting the speed of the fan 7A and switching the compressor 3 and condenser fan 5 on and off. On top of this, if the quantity of frozen PCM as measured by sensor 18 drops below a threshold value, the compressor 3 and condenser fan 5 are activated. If the air temperature becomes too low, the speed of fan 7A is reduced. Once the PCM reaches or nears 100% frozen, the compressor/fan are deactivated and the thermal store/PCM cools the air. As air temperature increases, the fan speed is increased until another threshold temperature is reached and the compressor/fan are then activated.
During periods of relatively high thermal loading, as determined by sensor 17 detecting that the temperate of air from the compartment 6 is above the desired temperature (perhaps by more than an accepted range from the desired temperature), the controller 16 adjusts valve 15 so that refrigerant is preferentially directed to the first evaporator 10. This provides the first evaporator 10 with greater cooling power to cool the circulating air. The cooling load on the first evaporator 10 is also lessened by the cooling effect of the thermal storage unit 13 on the air which first passes across it.
Once it is sensed that the temperature has fallen to or around the desired temperature, the controller 16 will operate valve 15 to cause the or a portion of the flow of refrigerant to be directed through the second evaporator 12 to refreeze the PCM 14, and ultimately steady-state conditions will be reached.
It will be appreciated that there are numerous possible variations to the embodiments described above without departing from the scope of the invention, which is defined by the claims. For example, the refrigerator may comprise more than two evaporators. The temperature sensor may instead be mounted in the compartment 6.
As mentioned above, the thermal store may enable the refrigerator to continue cooling in the event of a temporary power failure. However, a battery may also be provided to run the vapour compression system in the event of power failure.

Claims

A refrigerator having a cooling chamber (6) for containing an object to be cooled, the refrigerator comprising:
a thermal store (13) comprising a phase change material (14) for cooling the cooling chamber (6);

a vapour compression refrigeration system including a first evaporator (10) for cooling the cooling chamber (6) and a second evaporator (12) for cooling the phase change material (14); and

means (15, 16) to control the flow of refrigerant to the first and second evaporators (10, 12) depending on the cooling load on the refrigerator;

wherein, when the refrigerator is subject to a relatively low cooling load, refrigerant flows to the second evaporator (12) to cool the phase change material (14) and, when the refrigerator is subject to a relatively high cooling load, refrigerant flows to the first evaporator (10) such that increased cooling is provided to the cooling chamber (6) by the first evaporator (10) and the phase change material (14).
The refrigerator of claim 1, wherein refrigerant flows to both the first and second evaporators (10, 12) when the refrigerator is subject to a relatively low cooling load.
The refrigerator of claim 1 or 2, wherein refrigerant flows substantially only to the first evaporator (10) when the refrigerator is subject to a relatively high cooling load.
The refrigerator of any preceding claim, wherein the means to control the flow of refrigerant comprises a valve (15) and a controller (16) for controlling the valve position depending on the cooling load on the refrigerator.
The refrigerator of claim 4, wherein the valve (15) is a bistable valve.
The refrigerator of claim 4 or 5, wherein the cooling load on the refrigerator is determined by means of a temperature sensor (17) to determine the temperature of the cooling chamber (6).
The refrigerator of any of claims 4 to 6, wherein a sensor (18) is provided for determining the relative proportions of the phases of the phase change material (14), and wherein the controller controls the valve (15) position depending on the relative proportions of the phases of the phase change material (14).
The refrigerator of any of claims 4 to 7, wherein the vapour compression refrigeration system includes a compressor (3), and wherein the controller (16) controls the amount of cooling provided to the cooling chamber (6) by controlling the compressor (3).
The refrigerator of any of claims 4 to 8, wherein the refrigerator further comprises a fan (7A) for circulating air from the cooling chamber (6) to the thermal store (13) and the first evaporator (10) for cooling, and wherein the controller controls the amount of cooling provided to the cooling chamber by controlling the operation or speed of the fan (7A).
A refrigerator according to any preceding claim, wherein the thermal store (13) comprises two opposing cooling surfaces such that air can flow over either side of the thermal store (13) to pass across both cooling surfaces.
A refrigerator according to claim 10, wherein the first evaporator (10) is positioned downstream of the thermal store (13).