WO2006037178A1

WO2006037178A1 - Reverse peltier defrost systems

Info

Publication number: WO2006037178A1
Application number: PCT/AU2005/001533
Authority: WO
Inventors: Montag Christian Davis; Brett Robson Manners; Benjamin Peter Banney; Peter Terence Clarke; Robert Michael Weymouth
Original assignee: Hydrocool Pty Ltd
Current assignee: Hydrocool Pty Ltd
Priority date: 2004-10-01
Filing date: 2005-09-30
Publication date: 2006-04-13
Anticipated expiration: 2007-04-01
Also published as: JP2008514895A

Abstract

A refrigerator (10) having a freezer compartment (12) and a refrigerator compartment (13) includes a freezer compartment heat pump (14) which has a first heat exchanger (15) on one side and a second heat exchanger (16) on the other side. The first heat exchanger (15) is in fluid communication with a freezer compartment heat exchanger (17). Within the refrigerator compartment (13) there is a refrigeration compartment heat exchanger (19) which is in fluid communication with the second heat exchanger (16) in the freezer compartment (12). External of the refrigerato compartment (13) there is an electronic heat pump (20) having a first heat exchanger (21) on one side and a second heat exchanger (22) on the other side. The first heat exchanger (21) is in fluid communication with the refrigerator compartment heat exchanger (19) and the second heat exchanger (16) in the freezer compartment (12). The second heat exchanger (22) is in fluid communication with the hot side heat exchanger (23). Also discussed is a dehumidifier (50) which includes a plurality of thermoelectric modules (51) with ambient air passing through the cool side heat exchanger (52) and then the hot side heat exchangers (53). When the polarity of th thermoelectric modules is reversed, the previously cold sides (52) become hot and melt any ice that has formed on the heat exchangers.

Description

REVERSE PELTIER DEFROST SYSTEMS TECHNICAL FIELD

This invention relates to the efficient removal of ice build up on heat exchangers used with thermoelectric or similar electronic heat pumping devices to provide the cooling means. These heat exchangers include those found in domestic refrigerator freezer compartments and appliances that freeze water from air to produce potable water. BACKGROUND ART

Generally domestic refrigerators have separate compartments for the refrigerated section, maintained at 5°C, and the freezer compartment, maintained at -15°C to -20⁰C. These compartments are either completely independent or linked thermally in some way. Therefore, the frosting of the freezer compartment heat exchanger can impact on the performance of the refrigerator section as well.

Ice forms on the heat exchanger in the freezer compartment as a result of fresh ambient air entering the cabinet when the door is opened. Ambient air will contain much more moisture as water vapour than it will when cooled to freezer compartment temperatures (-15⁰C to -20⁰C). The water vapour present in ambient air introduced from a door opening will condense to water on any surface cooler than the dew point of the air. Because frost free freezer compartments use a fan to actively recirculate air through a heat exchanger (which is the coldest object in the freezer compartment) most of the water vapour is condensed onto the surface of the heat exchanger, where it then freezes.

In direct cooling systems there is no fan directing air through the heat exchanger however the heat exchanger is the coldest part of the freezer compartment and is generally made of a highly thermally conductive material. Water vapour tends to migrate to the heat exchanger and is condensed and frozen.

The amount of ice build up depends on two factors. These are the humidity of the ambient air and the number and duration of door openings during the day. Typically, 25 door openings per day is used as a standard in the refrigeration industry. In frost free refrigerators defrosting cycles are usually set at every 11 to 12 hours.

These defrosting cycles are either at fixed intervals or in some refrigerators there are sensors which can measure the amount of ice build up. Defrosting cycles are controlled more intelligently in these refrigerators and assist in reducing energy consumption because defrost cycles add significant heat into the freezer compartment.

In direct cooling systems defrosting is not automatic and the freezer compartment has to be defrosted manually.

The build up of ice on a heat exchanger is detrimental to its operation. A layer of ice between the thermally conductive heat exchanger surface and the air inside the cabinet provides an additional thermal resistance to the transfer of heat, requiring a lower temperature within the heat exchanger for the same heat flux. Under this condition the heat pump requires more electrical input energy for the same amount of cooling.

Additionally, ice build up reduces the clear space for air to flow through in frost free freezer compartments and therefore increases resistance to air flow. Eventually, ice can bridge across the gaps between fins and completely block off sections of the heat exchanger.

The efficiency of thermoelectric devices is very sensitive to temperature difference between the hot side of the thermoelectric module and the cold side. Therefore any additional thermal resistance, which directly increases this temperature differential, will reduce efficiency. Reduced efficiency makes this technology less competitive when compared with conventional vapour compression technology.

The most common method of defrosting heat exchangers in domestic refrigerators is by using an electric resistance heater wire located underneath the heat exchanger. Both radiant heat and convective heat transfer are used to convey heat onto the heat exchanger and melt the ice. A temperature sensor is usually located on the heat exchanger and when this sensor reaches a preset temperature limit (substantially above 0⁰C to ensure complete defrosting), the electric heater is switched off. In other more complex refrigeration systems, which use vapour compression, the compressor can be switched to a reverse cycle mode which directs hot gas through the evaporator.

The problem with these methods is that they are either energy intensive, reducing overall efficiency, or they require complex controls and valves.

The fact that the electric wire heater adds to the heat load that the refrigeration system has to handle is particularly detrimental for efficient operation of a thermoelectric refrigerator/freezer.

Thermoelectric modules are easily reversible, i.e. by reversing the polarity of the electric voltage across the module, a reversal in the direction of heat flow is achieved. Therefore it is relatively easy to provide heating onto the normally cold side of the module.

A thermoelectric module which uses reversed polarity for defrosting is described in US Patent Application US 2002/0116933. However this unit is applied to relatively small heat exchangers which are used to dehumidify air for cooling electronic devices. The arrangements shown in that patent application are not suitable for domestic refrigerators because of their small size. The process of reversing the polarity of the voltage across the heat pump can be used with a two compartment refrigerator to provide heat into the freezer compartment heat exchanger, raising the temperature until ice melts.

In cases where the heat exchanger is directly connected to the thermoelectric module, the heat passes directly into the heat exchanger.

In cases where a liquid is used as a heat exchange medium between the thermoelectric module and freezer compartment heat exchanger, this liquid must be heated to above 0⁰C and pumped around the circuit.

Heat exchangers directly attached to the thermoelectric module are necessarily small in size and this reduces their effectiveness and makes them less suitable for applications such as in the freezer compartment of a domestic refrigerator. A liquid heat exchange medium allows a much larger and therefore much more efficient freezer compartment heat exchanger to be used. DISCLOSURE OF INVENTION

In a first embodiment of the invention, the refrigerator has:-

(i) a freezer compartment having an electronic heat pump, a heat exchanger and a fluid circuit connecting the heat pump and the heat exchanger, and

(ii) a refrigerator compartment having an electronic heat pump, a heat exchanger and a fluid circuit connecting the heat pump and the heat exchanger.

With the fluid circuit of the freezer compartment being coupled to the fluid circuit of the refrigerator compartment through the freezer compartment heat pump and the fluid circuit of the refrigerator compartment being coupled through the refrigerator compartment heat pump to a hot side heat exchanger. In the defrost mode for this embodiment, the refrigerator compartment heat pump is operated at a minimum pre-set voltage and the polarity of the freezer compartment heat pump is reversed so that heat is pumped from the refrigerator compartment to the freezer compartment through the freezer compartment heat pump to heat the freezer compartment heat exchanger. In a second embodiment of the invention, the refrigerator has:- (i) a freezer compartment having an electronic heat pump, a heat exchanger within the compartment and a fluid circuit connecting the heat pump and the heat exchanger,

(ii) a refrigerator compartment having an electronic heat pump, a heat exchanger within the compartment and a fluid circuit connecting the heat pump and the heat exchanger, and

(iii) a hot side heat exchanger and a hot side fluid circuit connecting the hot side heat exchanger to the freezer compartment heat pump and the refrigerator compartment heat pump, with the two heat pumps being in series.

In the defrost mode for this embodiment of the invention, the polarity of the freezer compartment heat pump is reversed and heat is transferred from the hot side fluid circuit to the freezer compartment heat exchanger.

A thermoelectric module is very efficient when used as a heater. It is a heat pump and when used as a heater it will have a COP of greater than 1.0. In other words, the heat energy delivered to the hot side will be greater than the electrical energy consumed. This means that it will use less electrical energy than the heater wire to achieve the same level of heating.

An advantage of this reversed polarity defrost compared to an electric wire heater is that while the heater wire is remote from the heat exchanger, the system proposed hereunder is in direct contact with the heat exchanger. When using the heater wire heat must be transferred by a combination of radiant and convective heat transfer. In convective heat transfer the heat is carried up to the heat exchanger by air which recirculates within the cabinet, warming up all of the contents.

The proposed system uses a liquid medium which is heated by the thermoelectric module and then passes through the heat exchanger, passing its heat directly by conduction through to the ice. No extra heating of the freezer compartment air space or contents occurs.

Because the liquid is pumped throughout the whole heat exchanger, heating is very even, and all parts of the heat exchanger are raised above 0⁰C at substantially the same time. This means that there is no need to overheat one part of the heat exchanger to ensure that all parts are above the melting point of ice, reducing the amount of heat needed for defrost.

A very significant advantage of the defrosting system proposed is that the heat introduced into the freezer compartment for defrosting can be drawn from an associated cooled enclosure and then substantially returned there afterwards, leading to a near zero nett energy expenditure. Essentially the process follows the laws of Conservation of Energy and the only losses will be irreversibilities associated with the heat pump device.

This feature is made possible because of the close conjunction of the refrigerator and the freezer compartments. No other method of defrosting can accomplish this with such simplicity.

In a third embodiment of this invention the purpose is to extract moisture from the air and freeze it to enable recovery later as potable water. This embodiment has:-

(i) heat exchangers attached directly to each side of an electronic heat pump such as a thermoelectric module with these heat exchangers in direct communication with inlet and outlet air or, (H) heat exchangers on either or both sides of the heat pump communicating with inlet and outlet air through a liquid circuit and a second radiator heat exchanger.

After the dehumidifier has been operating for a period of time ice will build up on the final cold side heat exchanger. In order to recover this moisture as water the polarity of current flowing through the thermoelectric module is reversed. The heat exchanger covered with ice is now heated and the ice will melt. In addition the air flow can be reversed so that the cooled air off the iced heat exchanger is used to reduce the hot side temperatures of subsequent modules, improving efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a schematic diagram of a two compartment refrigerator incorporating a defrost system according to one embodiment of the invention,

Fig 2 is a schematic diagram of a two compartment refrigerator incorporating a defrost system according to a second embodiment of the invention, Fig 3 is a schematic diagram of a dehumidifier operating in the dehumidifying mode, Fig 4 is a schematic diagram of a dehumidifier operating in its defrost mode, and Fig. 5 is a schematic diagram of a dehumidifier circuit with both air and current reversed when in defrost mode. MODES FOR CARRYING OUT THE INVENTION

The refrigerator 10 shown in Fig. 1 includes a cabinet 11 having a freezer compartment 12 and a refrigerator compartment 13. Within the freezer compartment 12 there is a freezer compartment electronic heat pump which in this instance is a thermoelectric module or convector 14, which has a first heat exchanger 15 on one side and a second heat exchanger 16 on the other side. The first heat exchanger 15 is in fluid communication with the freezer compartment heat exchanger 17.

Within the refrigerator compartment 13 there is a refrigeration compartment heat exchanger 19 which is in fluid communication with the second heat exchanger 16 in the freezer compartment 12. External of the refrigerator compartment 13 there is a refrigerator compartment electronic heat pump which in this instance is a thermoelectric module or convector 20 having a first heat exchanger 21 on one side and a second heat exchanger 22 on the other side. The first heat exchanger 21 is in fluid communication with the refrigerator compartment heat exchanger 19 and the second heat exchanger 16 in the freezer compartment 12. The second heat exchanger 22 is in fluid communication with the hot side heat exchanger 23.

Thus, the embodiment of Rg.1 includes a circuit where during normal operation the heat load of the freezer compartment 11 is transferred, via the freezer compartment thermoelectric module 14, into the refrigerator compartment liquid circuit and then out to the hot side heat exchanger 23 which disposes of the heat to ambient. Freezer compartment temperature is typically -18°C, the refrigerator compartment temperature is typically 5°C and the ambient temperature is typically 25°C.

During defrost, the refrigerator compartment thermoelectric module 20 operates at a minimum pre-set voltage (to prevent heat leakage through the modules) and the freezer compartment module 14 is operated with a reverse polarity. The effect of this is to heat the freezer compartment liquid (and hence the freezer compartment heat exchanger) by drawing heat from the refrigerator compartment 13 through the thermoelectric heat pump 14. The refrigerator compartment circuit is cooled below 5°C by this action. This means that the only energy entering the system is that required to move the heat (which is less than the amount that would be required to generate that amount of heat).

Because the freezer compartment module 14 is moving heat from a region of higher temperature (5°C) to lower temperature (-18⁰C) this is accomplished at very high Coefficient of Performance (COP) and therefore very high efficiency.

When defrosting has completed the freezer compartment liquid is now above 0⁰C (typically between 5°C and 10⁰C) and it has to be cooled down again to -18°C or slightly below. In addition, the body of the heat exchanger 17 has to be cooled back down to its operational temperature. The original polarity on the freezer compartment thermoelectric module 14 is then restored and heat is pumped out of the freezer compartment liquid back into the refrigerator compartment liquid.

The refrigerator compartment liquid is now colder than the freezer compartment liquid and heat is being pumped along a positive thermal gradient again, making it a very efficient process. In essence heat has been pumped into the freezer compartment and it is now being reversed. The heat which has gone into melting the ice is contained in the condensate which automatically drains out of the freezer compartment, thus no longer imposing a heat load.

In a preferred embodiment this condensate (at less than 5°C) is delivered to the hot side heat exchanger coils where it assists is removing heat from the refrigerator compartment modules.

Thus this process is one which is very efficient and achieves the defrosting process more rapidly than would otherwise occur. This helps to limit the effect of defrosting on food products in the freezer compartment and improves keeping quality. Staging the freezer compartment heat pump 14 through the refrigerator compartment 13 is particularly appropriate for this system. The temperature difference through which the heat is moved (from the refrigerator compartment 13 to the freezer compartment 12 and then back again) in order to achieve defrost is less with a staged system than with a non-staged system. Therefore, the associated irreversibilities will be less.

The refrigerator 30 shown in Fig. 2 includes a cabinet 31 having a freezer compartment 32 and a refrigerator compartment 33. Within the freezer compartment 32 there is the freezer compartment heat exchanger 34 but in contrast to the first embodiment, the freezer compartment thermoelectric module or convector 35 is mounted externally of the freezer compartment 32. On one side of the thermoelectric module 35 there is a first heat exchanger 36 in fluid communication with the freezer compartment heat exchanger 34 and on the other side of the thermoelectric module 35 there is a second heat exchanger 37.

Within the refrigerator compartment 33 there is a refrigerator compartment heat exchanger 38. External of the refrigerator compartment 33 there is a refrigerator compartment thermoelectric module or convector 39. On one side of the thermoelectric module 39 there is a first heat exchanger 40 in fluid communication with the heat exchanger 38 and on the other side of the thermoelectric module 39 there is a second heat exchanger 41 which is in fluid communication with the second heat exchanger 37 of the freezer compartment thermoelectric module 35 and the hot side heat exchanger 42.

The refrigerator shown in Fig. 2 is a variation on the thermoelectric module combinations where the freezer compartment heat is pumped directly out of the freezer compartment 32 and into the hot side circuit. The freezer compartment thermoelectric modules have a much higher temperature difference to pump the heat across and this affects module efficiency. It is often useful to use multistage modules for this purpose.

In this case the freezer compartment 32 and refrigerator compartment 33 are not directly linked on the cold side. However they are linked through the hot side circuit (37, 41 and 42) and heat can be transferred beneficially between them during the defrost.

In defrosting, the polarity of the freezer compartment module 35 is reversed and heat is transferred from the hot side liquid. The hot side liquid is cooled in this process and it then passes on to the refrigerator compartment module 35. Because it has been cooled on the hot side to lower than ambient (by the freezer compartment module 35) the refrigerator compartment module operates at a lower temperature difference and for the same input electrical power will cool the refrigerator compartment (33) to below 5°C.

If the temperature of the hot side liquid out of the refrigerator compartment module 39 is lower than ambient then this liquid should not be directed through the hot side heat exchanger 42 but instead re-routed back to the freezer compartment module 35. In this way the temperature of the liquid entering the hot side of the refrigerator compartment module 39 will be kept at the lowest level, allowing the refrigerator 31 to operate at improved efficiencies.

When defrosting has been completed the freezer compartment module 35 is restored to its original polarity and the refrigerator compartment module 39 can be switched off. Conduction through the module will cool the hot side liquid. The direction of flow of the hot side liquid is reversed so that it passes through the refrigerator compartment module 39 first before passing through the freezer compartment module 35. In this way the heat transferred in overcooling the refrigerator compartment 33 is returned to assist the operation of the freezer compartment module 35 in pulling the freezer compartment 32 back down to the desired temperature.

As before, the condensate is drained from the freezer compartment 32 and onto the hot side heat exchanger coils.

Fig. 3 shows a dehumidifier 50 which includes an arrangement of several thermoelectric modules 51 with ambient air passing through the cold side heat exchangers 52 in series. Air is cooled as it passes from location 1 to location 2 through successive heat exchangers 52. After passing through the cold side of all heat exchangers 52 the air is then recirculated from location 3 to location 4 past the hot side 53 of the thermoelectric modules 51. As the air has been cooled the temperature is below ambient at location 3 and the working temperature difference across each module 51 is reduced. This feature enables better efficiency to be obtained since thermoelectric module efficiency in pumping heat is highly dependent on the temperature difference.

By the time air reaches the last module 52 at location 2 it has been cooled down from ambient and because the heat exchanger temperature is maintained below 0⁰C the moisture that condenses on to its surface is immediately frozen. Any bacteria present have their membranes ruptured when internal moisture forms ice crystals. Freezing the water therefore improves sterility.

After a period of time ice builds up and will block the heat exchanger unless it is removed. Thermoelectric modules are well suited to a defrosting action because to convert the cooling side to heating simply requires a reversal of the direction of electrical current.

Fig. 4 shows the direction of heat flow (arrows Qi ) when current is reversed. The previously cold sides 52 become hot, melting any ice that has formed. After the ice has been defrosted the current is returned to its original polarity and the dehumidifying/freezing process commences again. With this configuration the defrosting period is as short as possible because while the current polarity is reversed there is no dehumidification occurring on the water collection side of the modules.

Fig. 5 shows the situation when both the current and airflow are reversed in defrosting mode. In this case useful dehumidification can occur during defrosting because the incoming ambient air is now directed across the cold side of all the thermoelectric modules. Provision now needs to be made to collect water from both sides of the final thermoelectric module in the series. Once this is done then the dehumidifying/freezing operation can happen simultaneously with defrosting, improving overall efficiencies.

Not all of the recirculated air needs to pass through the hot side heat exchangers. Because moisture has been removed from the air its specific heat is lower and its temperature will rise at a faster rate than on the cold side. When the air temperature exceeds ambient it is obviously preferable to use ambient air for cooling the hot side heat exchangers. This can be accommodated with a bypass arrangement which is temperature activated. Alternative cooling fluid paths on the hot side are shown in Fig 3.

Various modifications may be made in details of the design and circuit configuration without departing from the scope and ambit of the invention.

Claims

1. A refrigerator comprising :-

(i) a freezer compartment having a heat pump, a heat exchanger and a fluid circuit connecting the heat pump and the heat exchanger, and (ii) a refrigerator compartment having a heat pump, a heat exchanger and a fluid circuit connecting the heat pump and the heat exchanger.

2. A refrigerator according to claim 1 wherein the fluid circuit of the freezer compartment is coupled to the fluid circuit of the refrigerator compartment through the freezer compartment heat pump and the fluid circuit of the refrigerator compartment is coupled through the refrigerator compartment heat pump to a hot side heat exchanger.

3. A refrigerator according to claim 1 wherein in the defrost mode the refrigerator compartment heat pump is operated at a minimum pre-set voltage and the polarity of the freezer compartment heat pump is reversed so that heat is pumped from the refrigerator compartment to the freezer compartment through the freezer compartment heat pump to heat the freezer compartment heat exchanger.

4. A refrigerator comprising:-

(i) a freezer compartment having a heat pump, a heat exchanger within the compartment and a fluid circuit connecting the heat pump and the heat exchanger, (N) a refrigerator compartment having a heat pump, a heat exchanger within the compartment and a fluid circuit connecting the heat pump and the heat exchanger, and

5. A refrigerator according to claim 4 wherein in the defrost mode, the polarity of the freezer compartment heat pump is reversed and heat is transferred from the hot side fluid circuit to the freezer compartment heat exchanger.

6. A humidifier for extracting moisture from the air and for freezing it to enable later recovery as potable water, the dehumidifier comprising,

(i) a chamber having an air inlet thereto and an air outlet therefrom, (ii) a thermoelectric module within the chamber, and a heat exchanger attached to each side of the thermoelectric module with the heat exchangers in direct communication with the flow of air from the inlet to the outlet.

7. A dehumidifier according to claim 6 wherein the heat exchanger on either or both sides of the module communicate with the air inlet and air outlet through a liquid circuit having a second radiator heat exchanger.

8. A dehumidifier according to claim 6 wherein ice which builds up on the heat exchangers in direct communication with the inflow of air is melted by reversing the polarity of the electronic heat pump.