WO2017059520A1

WO2017059520A1 - Thermoelectric generator using in situ passive cooling

Info

Publication number: WO2017059520A1
Application number: PCT/CA2016/000246
Authority: WO
Inventors: Gerard CAMPEAU
Original assignee: Campeau Gerard
Priority date: 2015-10-07
Filing date: 2016-10-05
Publication date: 2017-04-13
Also published as: CA2996470A1

Abstract

A passively cooled portable electrical power generation system for on-demand power or for charging batteries, using thermoelectric modules, which require a heat source for hot side heat injection and absorption, and a cold side heat exchanger interface for heat flux maintenance.

Description

THERMOELECTRIC GENERATOR USING IN SITU PASSIVE COOLING

FIELD OF THE INVENTION

[0001] The invention relates to a portable generator of electrical energy using solid state Seebeck Effect thermoelectric modules with a passive liquid cooled cold side, facilitating on- demand energy for immediate consumption or for charging of batteries.

BACKGROUND OF THE INVENTION

[0002] Modern Seebeck Effect thermoelectric modules exploit the thermodynamic property of heat transfer between properly arranged n-type and p-type semiconductors, to create the thermoelectric effect. A thermoelectric module will cause a potential energy EMF to be generated in the presence of a sustained heat differential across the module, between the hot and cold surfaces of the device. In situations where the heat resource exists, it can be directed to a thermoelectric module or an array of modules to generate electrical energy.

[0003] With a sufficient heat differential across the thermoelectric module, the conditions exist for the generation electrical energy without the need for any external elements such as storage batteries or the like. A thermoelectric generator can be used to charge external batteries on demand if desired, however a battery is not required for operation once the hot side has absorbed as much heat energy as possible from the heat source that it is exposed to, in order to maintain the heat flux. With a sustained cold side interface wherein the heat is removed from the cold surface of the thermoelectric module, we can refer to this type of electrical energy generator as an on-demand electrical energy generation system.

[0004] The operation of thermoelectric generation dictates that both the hot and cold surfaces of the thermoelectric module should be maintained at sustained temperatures for continuous energy generation with acceptable power output deviation over time. The heat introduced to the hot side must be removed at the cold side surface. [0005] Previous attempts to generate adequate levels of sustained electrical energy from thermoelectric modules using a cooking pot heat exchanger format using passive liquid cooling have resulted in commercial implementations that are a compromise in terms of the maintenance of sufficient heat flux due to their inability to maintain a stable cold side surface that is necessary to qualify as on-demand power generation. The coolant is subject to accelerated evaporation due to the water boiling, thereby causing the thermoelectric module to fail due to excessive heat concentration on the hot side of the module. Further, the hot side of the module is stressed beyond the recommended temperature rating when the pot is used with electric stoves as a heat source. The thermoelectric module is in contact with the heat source through a relatively thin aluminum interface. Since aluminum excels with regard to heat conduction, the high temperature can easily overwhelm the hot side of the thermoelectric module resulting in premature failure.

[0006] Still other attempts to use thermoelectric modules for power generation that burn biomass in a combustion site to provide a heat source don't qualify as on-demand power sources either. These attempts frequently use an internal battery which must be initially charged before it can be used to provide power for an internal fan for the cold side interface to facilitate the establishment of heat flux through the module. In this category of thermoelectric generators, if the internal battery is discharged there exists a very high probability of thermoelectric module failure due to excessive heat buildup on the hot side surface that cannot be removed at the cold side surface for dissipation to ambient.

[0007] To be commercially successful, the generator should allow a number of fuel types to be accommodated. This versatility in terms of fuel options allows long chain hydrocarbon sources or shorter chain biomass sources to be used as heat sources. Further, at least one fuel source should burn cleanly enough that it can be used indoors including alcohol based fuels. Waste heat resources associated with a combustion site also can be used which requires the addition of a heat pipe or similar conductive heat transfer means, to the heat injection port to allow the heat resource to be safely and conveniently directed to the thermoelectric module hot side. In this regard, there are many possible configurations for the heat injection port physical implementation. SUMMARY

[0008] Some aspects of this disclosure may provide a method and apparatus to overcome some of the drawbacks of known techniques, and-or provide the public with a useful alternative.

[0009] In an aspect, there is provided a heat exchanger apparatus, allowing a plurality of thermoelectric modules, each comprising a hot side and a cold side to be coupled such that the cold side interface is in intimate contact with an external planar surface of the heat exchanger with adequate compression such that heat is removed from the cold side of the thermoelectric module.

[0010] In another aspect, there is provided a utility system comprising a plurality of thermoelectric modules, a first heat exchanger and a first heat injection means for the hot side interface of the thermoelectric module, consisting of an extruded aluminum heat sink with integral fins whose dimensions and spacing are conducive to the efficient absorption and transfer of heat to the hot side interface of the thermoelectric modules from direct contact with the heat source.

[0011] In another aspect, there is provided a system comprising a plurality of

thermoelectric modules, a plurality of second heat exchangers of similar configuration to a first heat exchanger, whose dimensions may be increased lengthwise to allow the vessel to retain at least 10 litres of the liquid coolant, and a second heat injection means to direct the heat resource from a combustion site such as the exhaust flue stream of a hearth device such as a wood fired heating stove or the like, to the hot side interface of the thermoelectric modules.

[0012] In another aspect, there is provided a system comprising a plurality of first heat exchangers and a third heat injection means, and a plurality of thermoelectric modules to direct the heat resource from a combustion site including, but not limited to a hearth device such as a natural gas fireplace appliance, to the hot side interface of the thermoelectric modules.

[0013] In another aspect, there is provided an apparatus comprising an external Direct

Current (D.C.) fan utilizing air entrainment to multiply the observed air flow compared to that from only a fan, increasing the convective air flow directed to the exterior fins of a first heat exchanger tower to which is conducive to the efficient flow of air thus removing the heat from the exterior fins particularly in conditions of elevated temperature and humidity.

BRIEF DESCRIPTION OF THE FIGURES

[0014] An illustrative embodiment of the present invention is described by way of example only, with reference to the appended drawing figures, wherein:

[0015] FIG.1 is an isometric view of a preferred embodiment of a heat exchanger;

[0016] FIG. 2 is an isometric view of a preferred embodiment of a portable, on-demand

thermoelectric power generator;

[0017] FIG. 3 is a side elevation view of a preferred embodiment of a portable, on-demand

thermoelectric power generator;

[0018] FIG. 4 is an isometric exploded view of a preferred embodiment of a portable, on-demand thermoelectric power generator;

[0019] FIG. 5 is an isometric view of a preferred embodiment of a permanently installed, on- demand thermoelectric power generator from a flue stream of a combustion site;

[0020] FIG. 6 is an isometric exploded view of a preferred embodiment of a permanently installed, on-demand thermoelectric power generator from the flue stream of a combustion site;

[0021] FIG. 7 is an isometric view of a preferred embodiment of a permanently installed, on- demand thermoelectric power generator from the combustion site of a natural gas fireplace appliance;

[0022] FIG. 8 is an isometric exploded view of a preferred embodiment of a permanently installed, on-demand thermoelectric power generator from the combustion site of a natural gas fireplace appliance;

[0022] FIG. 9 is a section view of a preferred embodiment of a convective air flow multiplier utilizing air entrainment for a thermoelectric power generator;

[0023] FIG. 10 is side elevation view of a preferred embodiment of a convective air flow

multiplier utilizing air entrainment for a thermoelectric power generator;

[0024] FIG. 11 is an isometric view of a preferred embodiment of a convective air flow multiplier utilizing air entrainment for a thermoelectric power generator;

DETAILED DESCRIPTION

[0025] It should be understood that the present invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "includes", "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms "connected," "coupled," "configured" and "mounted" and variations thereof herein are used broadly and encompass direct and indirect connections, couplings and mountings. In addition, the terms "connected" and "coupled" and variations thereof are not restricted to physical, mechanical or electrical connections or couplings. Furthermore, and as described in subsequent paragraphs, the specific mechanical and/or other configurations illustrated in the drawings are intended to exemplify embodiments of the invention. However, other alternative mechanical and/or electrical and other configurations are possible which are considered to be within the teachings of the disclosure.

[0026] FIG.1 shows an exemplary embodiment of a heat exchanger 30 accommodating a plurality of thermoelectric modules 7, comprising a vessel 1 having a first end and a second end, capable of retaining a liquid, water being a typical liquid although a glycol based coolant can be substituted if the potential exists for the water to freeze in ambient environments at or below zero degrees Celsius. The heat exchanger is formed by extrusion of aluminum into a vertically oriented rectangular vessel of such dimensions and height to retain at least 1.5 litres of coolant in one embodiment, which corresponds to a height of approximately 381 mm. There is provided a cover plate 3 for the first end which is used to reduce the rate of evaporation of the coolant in conjunction with an upper silicon rubber based sealing gasket 4 to be used in a permanent installation. Condenser tube 5 is coupled to cover plate 3. Should a heat imbalance begin to form in favour of heat absorption due to higher than ideal ambient conditions; the condenser tube 5 will restore the imbalance condition. The condenser tube 5 has a plurality of densely spaced fins which cause the heat to radiate over the increased surface area of the fins, thereby condensing before any significant amount of vapour can develop. There is provided on the top of each condenser tube a vent plug, which prevents an undesired pressure rise by an increased volume of expanding liquid. The vent plug ensures an atmospheric pressure equalization condition outside and inside the vessel. The condenser tube 5, cover plate 3 and upper sealing gasket 4 are optional when used in portable or non-permanent applications. The second end is joined to an aluminum plate 6 or the like, which serves to retain the liquid coolant, and includes a lower silicon rubber based sealing gasket 2 capable of operating up to 500 degrees Celsius. The heat exchanger vessel 1 is extruded in such a way that the opposing internal surfaces of the vessel that are in contact with the liquid are not solid aluminum, but are formed by extrusion into a series of protrusions from the flat metallic surface and having a first group and a second group of closely spaced, thin ribs called fins that conduct heat from each liquid to fin interface surface, through the heat sink fin. Each group of fins increases the overall surface area of the liquid to metal interface. The first group of fins is extruded such that they protrude only from the interior wall of the vessel which is in intimate contact with the coolant. The thermoelectric modules 7 are coupled to the outer planar surface of the vessel 1, to which the cold sides of the thermoelectric modules are attached. Further, the fins directly opposite the thermoelectric modules and on the remaining two interior surfaces are contiguously joined from the inside of the vessel to the outside surface of the heat exchanger which is the interface to the external ambient air. Through a first means consisting of conduction of heat through the fins, and a second means consisting of air convection, the heat is dissipated to the external ambient air. To accelerate the rate at which the heat is dissipated, an external fan blower utilizing air entrainment may optionally be installed to enhance the efficiency during higher than ideal ambient temperature or humidity conditions.

[0027] The heat exchanger efficiency is additionally enhanced due to the vertical orientation of the extrusion to take advantage of thermal gradients from the top to the bottom of the heat exchanger resulting from the thermosiphon effect. A cyclical temperature gradient effect is observed from the top to the bottom of the heat exchanger due to the fact that heated liquid rises and as it does, the heat is absorbed by the liquid to fin interface as it rises, wherein the cooled liquid falls to the bottom of the heat exchanger. An internal liquid flow loop is created by this action thereby operating as a passive cooling system without the need for an external energy consuming liquid pump, preventing the liquid from reaching the boiling point.

[0028] FIG.2 shows one embodiment of a portable thermoelectric power generator 40 in accordance with the teachings of the present invention. The system includes a vessel 1 that functions as a heat exchanger, and retains various liquid coolants, including water that absorbs heat for dissipation to ambient. A heat injection means includes heat injector 2, and is comprised of an aluminum heat sink extrusion with large fins relative to the diameter of the heat source physical container, that absorbs heat from a heat source mounted directly beneath the heat sink. The heat source introduces the open flame from a fuel source 12 from a hydrocarbon based fuel derivative which varies from sources including gel based alcohol, ethanol, methanol or the like. There is provided, an aluminum heat shield 6 that encloses the heat sink fins in such a way that the thermal energy of the fuel source is contained within the heat sink structure to prevent ambient heat loss, and enforces direct convection through the heat sink fins vertically as the heat passes the fin surface and is absorbed by each fin.

[0029] In addition to providing a containment function for the heat source, heat shield 6 can be raised or lowered vertically. The heat shield is raised to allow the fuel source to be placed directly below the heat sink 2 and to ignite the fuel source initially. The heat shield 6 is lowered when the fuel is ignited to constrain the heat from the fuel within the confines of the area of the heat sink 2, and to prevent external ambient air current from a windy environment from affecting the efficient heat flow from the fuel to the heat sink which is the heat injection port means. In addition, the heat shield constrains or provides shelter for the fuel source vessel itself, in the event that the entire assembly is accidentally tipped over while in operation, and prevents it from being dislodged from the bottom plate. There is provided on the side of the vessel 1, a tap 4 that allows liquid that has become heated to be conveniently removed from the vessel. In order to prevent damage to the thermoelectric modules 7 resulting from excessive temperature rise on the hot side, the tap 4 is strategically located on the side of the heat exchanger just above the physical location of the thermoelectric modules 7. This allows approximately 50% of the liquid to be removed, and replaced with cooler liquid to once again increase the heat flux across the thermoelectric modules without risk of damage to the thermoelectric modules, since the other 50% of the liquid in the heat exchanger is always present to allow the cold side to function. There is provided a Direct Current (henceforth referred to D.C.) voltage regulator, and a D.C. to D.C. converter, to condition the resulting output voltage and current generated by the thermoelectric modules to charge a plurality of external batteries in the absence of conventional power sources during power failure situations, or in remote environments that do not provide access to electrical power. There is provided a means to charge cellular handset batteries, which typically provide an attachment mechanism for communications and battery charging consisting of a Universal Serial Bus (USB) connector referred to as a Micro B connector by the industry.

Other cellular telephone manufacturers will use a proprietary connection scheme for charging their battery, the Apple Lightning connector being another such format. By using a suitable USB to Lightning adapter device, it is possible to charge Apple iPhone devices and iPad devices. Typical USB connectors will accept charging voltages of 5 Volts D.C., to allow the internal charge controller of the cellular handset to charge the internal battery. There is provided an electronic interface circuit board mounted on the exterior surface of the heat exchanger that facilitates the connection of the conditioned D.C. output voltage to the external cellular handset. The USB connector is a preferred cellular battery charging format due to the availability of 5 Volts D.C. from the USB A connector, thus there is provided on the interface circuit board for this device, a USB A connector providing 5 Volts D.C. for the purpose of charging the cellular telephone battery. There is also provided, an additional current-limited output voltage that allows the direct connection of a rechargeable battery or a Light Emitting Diode (LED) light source. The external battery that can be recharged with this connection can vary from 1.2 Volts to 12 Volts, including 3.6 Volts, 5 Volts, 6 Volts, 7.2 Volts and 9 Volts. The charging current for any battery is fixed at a maximum of 125 milliamps. LED light sources are commonly available that are used at 12 Volts thereby supporting the charging of a cellular handset and the use of an LED lamp simultaneously.

[0030] FIG.3 is a side elevation view revealing the electrical interconnection of the electronics control assembly 3 and the thermoelectric modules 7. The thermoelectric modules 7 are mounted directly between the heat injector 2 and the heat exchanger 1, with the hot side of the thermoelectric modules 7 connected in direct physical contact with the heat injector 2 and the cold side of the thermoelectric modules 7 connected in direct contact with the heat exchanger 1 thereby establishing a condition of heat flow from the hot side to the cold side of the

thermoelectric modules 7, also thereby creating the conditions for electrical energy to be available that supports electric current flow in an external load. [0031] FIG.4 is an exploded view revealing the schematic detail and assembly of the heat injection means 2, thermoelectric modules 7, and the heat exchanger 1 in detail.

[0032] FIG.5 is an isometric view of an exemplary embodiment of the invention, revealing the detail and assembly of a configuration suitable for insertion of a heat absorbing sink that is part of a first heat exchanger of FIG.1 into the waste heat exhaust flue stream chimney 4 of a biomass stove such as wood or pellets. The heat injection means 2, thermoelectric modules 7, and the heat exchanger 1 are arranged with at least two heat injector heat sinks 2 and at least two heat exchanger assemblies 1, sampling the exhaust flue heat stream and directing the heat into the heat sinks 2 and into the hot side of the thermoelectric modules 7.

[0033] FIG.6 is an isometric exploded view of an exemplary embodiment of the invention of FIG.5, revealing the internal configuration and assembly of this embodiment.

[0034] FIG.7 is an isometric view of an exemplary embodiment of a first heat exchanger 30 of FIG.1, revealing its use and configuration as part of an apparatus to generate electrical energy from the heat from a combustion site of a hearth device such as a natural gas fireplace appliance. A plurality of heat exchangers 1 are configured to dissipate heat that has been absorbed from a heatsink injection port 2 through the thermoelectric modules 7 (not visible in this view) flowing through to the cold side of each thermoelectric module 7. There is provided a plurality of 12 Volt Direct Current (D.C.) blower fan motors 6 that are primarily intended to direct the heat from the inside of the fireplace to warm the external air in the area where the fireplace is installed, by means of forced air through a front panel vent. There is also provided an electronic control means to allow the air flow to be adjusted using an external user control according to the desired comfort level of the user. Internally the fan motors are oriented in such a way that the air flow is directed over the heat exchanger vessel 1 as the air is forced through the front panel vent opening to the room. When the EMF voltage developed by the thermoelectric modules 7 is sufficient to cause fan motors 6 to operate, the additional secondary cooling provided by the forced air will cause the temperature differential across the thermoelectric modules 7 to increase and result in a higher EMF voltage which allows greater load currents to be accommodated. This electrical energy can be used to provide power for external lighting, battery charging and cell phone charging as may be desired by the user of the appliance during power outages or interruptions. Each heat exchanger 1 shares a common condenser tube 9 through a common condenser coupling tube 4 arranged in a Y configuration.

[0035] FIG.8 is an isometric exploded view of FIG.7 an exemplary embodiment of a first heat exchanger 1 revealing its use and configuration as part of an apparatus to generate electrical energy from the heat from a combustion site of a hearth device such as a natural gas fireplace appliance. A cold air intake port for the fireplace appliance allows additional cooling air flowing past the condenser tube 9 thus supporting the operation of the condenser with cooled air.

[0036] FIG.9 is a cross section view of a device that uses the principle of entrainment in accordance with the teachings of the present invention to enhance the efficiency of a first heat exchanger as well as providing a means to aid in the entrapment of airborne insects. This enhancement improves the convective air flow on the exterior fins of the tower to remove the heat from the exterior fins to ambient. In a preferred embodiment there is provided a Direct Current (D.C.) fan motor 3, a rectangular enclosure 1 to contain and direct the air flow from the fan, and a metallic fine pitch mesh assembly that will entrap insects such as mosquitos to provide insect control and elimination. The enclosure is fabricated from a suitable material for the operating environment and may be a plastic or metallic material that is open on both ends to allow air to flow through it. The D.C. fan motor 3 is mounted on one entry port end, and is oriented in such a way that airflow from the fan will be directed through the blades of the fan from outside the enclosure, to the inside of the enclosure. The dimensions of the enclosure 1 containing the fan should be greater than the dimensions of the fan. Through a process referred to as inducement, the air directly behind the blades of the fan will be drawn into the fan and will be directed with the air being forced forward from the front of the fan, to the exit of the enclosure. Additionally through a process referred to as entrainment, the air around the edges of the enclosure will also be drawn into the enclosure in direct proportion to the circumference of the enclosure relative to the circumference of the fan motor. The advantage of this arrangement is that a small fan that consumes a relatively small amount of energy behaves as a larger fan, thereby multiplying its effectiveness proportionally. Additionally, the location of the entry port of the enclosure 1 is physically oriented towards the direction of the hydrocarbon fuel source towards the front of the tower. The C0₂ that is emitted from the burning of a hydrocarbon fuel attracts insects sensitive to such emissions such as mosquitos. As the insect is drawn towards the emission, there is a greater probability that the insect will fly towards the open fan port end and be drawn into the enclosure by the increased air flow from the edges of the enclosure created by the entrainment process, and into the enclosure. Once inside the enclosure insects are trapped by an integral mesh filter.

[0037] FIG. 10 shows a side or elevation perspective view of a preferred embodiment of a first heat exchanger of FIG.9, including the entrainment air flow enhancement device attached to a heat exchanger.

[0038] FIG. 11 shows an isometric perspective view of a preferred embodiment of a heat exchanger of FIG.10, including the entrainment air flow enhancement device attached to the heat exchanger.

Claims

An apparatus for generating electrical energy, comprising a vessel configured to retain a liquid coolant which is configured as a first heat exchanger with high thermal efficiency, a plurality of thermoelectric modules, a heat injection means to absorb thermal energy from a heat source, coupled to the thermoelectric modules located between boundary of the heat injection means and the planar surface of the vessel.

An apparatus as defined in claim 1, wherein the heat injection means comprises an anodized aluminum heatsink extrusion of sufficient dimensions and configuration to absorb heat from the direct contact of the fins to the open flame of a heat source, to efficiently conduct the heat from each fin which terminates at a storage means on the hot side boundary of the thermoelectric modules.

An apparatus as defined in claim 2, wherein the heat exchanger comprises an anodized aluminum vessel having a first and second end, and oriented in an upright vertical configuration capable of retaining a liquid coolant, which is bonded to a bottom plate constructed of anodized aluminum, a silicon sealing gasket attached to the bottom plate of the vessel such that the vessel is capable of retaining a liquid coolant without leaking.

An apparatus as defined in claim 3, wherein the heat exchanger heat sink fins conduct heat stored in the liquid coolant in the interior of the vessel to the exterior of the heat exchanger, the efficiency of which is aided by the thermosiphon effect causing a thermal gradient to be established, whereby heated water at the bottom rises and is cooled at the top of the vessel due to heat radiation to ambient external, the cooled water thus sinking towards the bottom of the vessel thus preventing the coolant from reaching its boiling point.

An apparatus as defined in claim 4, wherein a thermally non-conductive separation plate is attached to the interior of the vessel thus providing a first thermosiphon cellular region, and a second thermosiphon cellular region to enhance the thermosiphon effect between the hot and cold side of the vessel.

A device comprising a first heat exchanger as defined in claim 4, for the purpose of generating electrical energy using thermoelectric devices, wherein the vessel includes an integral water tap or valve, located on a side of the vessel above the thermoelectric modules allowing the liquid coolant to be replaced as required, draining the coolant from the vessel until it reaches a level just above the thermoelectric modules, thus ensuring that the coolant level remains sufficiently high enough to prevent damage to the thermoelectric modules due to insufficient coolant, including a heat shield surrounding the heat injection means to both contain the heat resource within the boundaries of the heat injection heat sink.

7. A system for the purpose of generating electrical energy, comprising a plurality of first heat exchange means as defined in claim 1 , a plurality of thermoelectric devices, and a plurality of heat injection means wherein the heat injection interface is a semi-circular heat sink of sufficient mass with integral fins thus increasing the surface area of the interface, to absorb heat from the exhaust flue stream of a combustion site resulting from the use of a stove using a biomass fuel source such as wood or the like, wherein each thermoelectric module is in intimate contact through mechanical compression force with the planar surface of the vessel of the heat exchanger and the planar surface of the heat injection interface.

8. A system for the purpose of generating electrical energy, comprising a plurality of first heat exchange means as defined in claim 1 , a plurality of thermoelectric devices, and a plurality of heat injection means wherein the heat injection interface is a heat sink of sufficient mass with integral fins thus increasing the surface area of the interface, to absorb heat from the combustion site of a natural gas fireplace or the like, wherein each thermoelectric module is in intimate contact through mechanical compression force with the planar surface of the vessel of the heat exchanger and the planar surface of the heat injection interface, and including a finned condenser tube.

9. A device used with a first heat exchanger as defined in claim 4, comprising a rectangular enclosure, a Direct Current D.C. fan motor attached at one end of the containing rectangular enclosure, open at both ends, through which air is free to flow through the enclosure, wherein the fan is mounted physically in such a way that the air flow from the fan blades is directed towards the interior of the enclosure, and through the property of inducement, the fan will draw air from outside the enclosure directly behind the fan, which is free to exit the enclosure, wherein through the property of entrainment, the fan will also cause air at the edges of the enclosure to enter the containing enclosure, thereby increasing the overall air flow by a multiplying factor greater than that of only the fan, thus increased air flow is available to provide a convective air flow increase to the external fins of the cooling tower which assists the rate at which the water can release the absorbed heat to external ambient.