EP1416233B1 - Adsorption refrigerator with heat accumulator - Google Patents

Abstract

A condenser (13), which leads a condensed working medium to an evaporator (4), takes up heat from the working medium to be cooled during an adsorption phase. The condenser is coupled to a buffer reservoir (14) which buffers a portion of the condensation heat of working medium vapor. The buffer reservoir can dissipate stored heat into the surroundings even during the adsorption phase. An Independent claim is also included for an adsorption cooling apparatus operating method.

Description

The invention relates to a periodically operating adsorption cooling apparatus with buffer memory and a method for its operation according to the preamble of claim 1.

Adsorption chillers are devices in which a solid sorbent sorbs a vaporous working medium with heat release at medium temperature level (adsorption phase). The working fluid evaporates in an evaporator while absorbing heat at a lower temperature level. After the adsorption phase, the working medium can be desorbed again by supplying heat at a high temperature level (desorption phase). In this case, working fluid evaporates from the sorbent and flows into a condenser. There, the working fluid is reliquefied and then evaporated again in the evaporator.

Adsorption refrigerators with solid sorbents are known from EP 0 368 111 and DE-OS 34 25 419. Sorbent container, filled with sorbent, while sucking agent vapor, which is produced in an evaporator, and sorb it in the Sorptionsmittelfüllung under heat release. The heat of sorption must be removed from the sorbent filling. The chillers can be used to cool and keep food warm in thermally insulated containers. Between the evaporator and the sorbent these refrigerators contain a shut-off device. This allows evaporation and sorption of the working fluid at a later date.

The adsorption cooling apparatus known from EP 0 368 111 consists of a transportable cooling unit and a stationary charging station which can be separated therefrom. The cooling unit contains a sorption tank filled with a solid sorbent and an evaporator with liquid working fluid. Again, evaporator and sorption are connected to each other via a shut-off steam line. Through a heat exchanger embedded in the evaporator flow liquid media, which are cooled by temperature-controlled opening and closing of the shut-off device to the desired temperature level. After the sorbent is saturated with working fluid, it can be heated in the charging station. The working fluid vapor flowing out is reliquefied in the evaporator. The condensation heat is dissipated by cooling water flowing through the embedded heat exchanger.

The shut-off serve on the one hand during the desorption phase to decouple the evaporator from the rest of the refrigerator to prevent hot working medium vapor flowing into the cold evaporator and on the other to regulate the refrigeration in the evaporator during the adsorption or move it to a later date. Without shut-off device, the evaporator becomes hot during the desorption phase and thus warm the medium to be cooled in contact therewith.

From US-A-5 272 891 and US-A-2 055 669 it is known to arrange the evaporators so that they are also flooded with working fluid during the desorption phases. Fluid vapor can then not flow into the evaporator and heat it. However, this solution is not feasible for the working fluid water with its very low vapor pressure, since the hydrostatic water pressure counteracts evaporation. From both documents is also known to temporarily store the heat of condensation in a heat buffer until the next sorption phase.

Object of the present invention is to protect in an adsorption refrigerator without shut-off device according to the preamble of claim 1, the medium to be cooled in the desorption against excessive heating.

This object is achieved in an adsorption cooling apparatus of the type according to the invention by the characterizing features of claims 1 and 11. The subclaims show advantageous embodiments of the invention.

The coupling of the condenser to a buffer storage allows a much faster desorption and, consequently, a higher desorption performance, since the liquefaction heat, e.g. can be derived more effectively due to incipient convection. The desorption phase can thus be significantly shortened compared to the adsorption phase. The medium to be cooled is less exposed to the high liquefaction temperatures. With a suitably sized buffer storage, the desorption phase can be reduced to a few minutes, while the adsorption phase can take several hours to several days. During this long adsorption phase, the buffer store can dissipate the heat load absorbed with high power slowly and over small heat exchanger surfaces.

In principle, all storage media known from heat storage technology, such as liquids, phase change materials (PCM) and solids, are suitable as buffer storages. Inexpensive is water, which also allows a high heat transfer performance. The condenser can be integrated directly into a water reservoir. Over the outer surface of the tank, the buffered heat is then removed during the long Adsorptionsphase without additional heat exchanger to the ambient air.

Since the evaporator system is raised to the temperature level of liquefaction at each desorption and must be cooled to the low temperature level of evaporation at the beginning of the adsorption by evaporation of part of the working fluid, it makes sense to keep the thermal mass of the evaporator low and the amount set liquid working fluid so that at the end of the adsorption as possible, the entire working fluid is evaporated. Towards the end of the adsorption, the amount of working fluid in the evaporator becomes ever smaller, and consequently the wetting of the heat exchanger surface for absorbing heat from the medium to be cooled becomes increasingly difficult. According to the invention, the evaporator contains wetting agents for this operating state, which distribute the remaining working medium homogeneously over the inner evaporator surface. For this purpose, glass fiber webs, which are applied as a thin layer on the corresponding evaporator surfaces, have proved suitable.

It is particularly advantageous if the evaporator is arranged with respect to the medium to be cooled in such a way that it releases relatively little heat during the desorption phase. This is achieved, e.g. in that relatively little medium to be cooled is in contact with the evaporator or is not circulated during the desorption phase. If the medium to be cooled is gaseous, e.g. in refrigerators, it is advantageous to place the evaporator under the ceiling of the cabinet. Since warm air is lighter than cold, the cold air mass remains in the lower part of the cabinet while only the air surrounding the evaporator gets warm. The goods stored in the cabinet then remain cold during the relatively short desorption phase. This effect can be further enhanced by cold storage media and / or radiation screens, which are arranged below the evaporator.

For high desorption a high heat conduction in the sorbent and a good heat transfer from the heat source are necessary. It may be particularly advantageous if the heating power in the sorbent during the desorption phase is much higher than the heat losses to the environment. In this case, thermal isolation from the sorbent container sheaths facing the environment may be eliminated. Through this, the heat of adsorption is released during the adsorption phase, without the need for additional measures.

The use of the zeolite / water adsorption substance pair is particularly advantageous. Zeolite is a crystalline mineral that consists of a regular skeletal structure of silicon and aluminum oxides. This framework structure contains small cavities in which water molecules can be adsorbed by releasing heat. Within the framework structure, the water molecules are exposed to strong field forces, which bind the molecules in the lattice in a liquid-like phase. The strength of the binding forces acting on the water molecules depends on the already pre-adsorbed amount of water and the temperature of the zeolite. For practical use, up to 25 grams of water per 100 grams of zeolite can be sorbed. Zeolites are solids without disturbing thermal expansion in the adsorption or desorption reaction. The framework structure is freely accessible from all sides for the water vapor molecules. The devices are therefore operational in every situation.

The use of water as a working fluid makes it possible to reduce the required regulatory effort to a minimum. Upon evaporation of water under vacuum, the water surface cools to 0 ° C and freezes on continued evaporation to ice. The ice layer can be advantageously used to control the temperature of the medium to be cooled. With low heat input the ice layer grows, with very high heat it melts off. The formation of ice reduces the heat transfer from the medium to be cooled into the evaporator, so that the medium can not cool below 0 ° C. With continued evaporation, the entire water supply can freeze in the evaporator. The sublimation temperature of the ice layer then drops below 0 ° C.

The aqueous evaporator content may also be added to the freezing point lowering substances, if the temperature of the medium to be cooled should be lowered below 0 ° C.

Also usable are other sorbent pairings in which the sorbent is solid and remains solid during the sorption reaction. Solid sorbents have low heat conduction and limited heat transfer. Since the heat transfer from the sorbent container to the heat-absorbing ambient air is of the same order of magnitude, heat exchangers without ribs, such as plates, tubes and corrugated metal hoses, are generally recommended. Some solid sorbents, such as zeolites, are stable enough to compensate for external pressures on thin-walled heat exchanger surfaces. Additional stiffeners or thick-walled heat exchanger surfaces are therefore not necessary.

Solid sorbents can also be processed into moldings. A single or a few moldings can form a complete, low-cost sorbent filling.

For economical operation, desorption end temperatures of 200 to 300 ° C. and adsorption end temperatures of 40 to 80 ° C. are recommended for zeolite / water systems. Since in particular zeolite granules have a low heat conduction, the sorbent containers are to be designed so that the heat conduction path for the amounts of heat converted does not exceed 3 cm.

As a heat source for the desorption all known means are suitable, provided that the required temperature level for the desorption reaction is achieved. Advantageously, electrically heated plates or cartridges, which are adapted to the geometry of the sorbent container. Heating devices which heat the sorbent charge by means of radiation or induction (eddy currents) are also very suitable. When heating the sorbent by means of a flame, the heating surface can also be used as a heat exchanger surface for heat dissipation during the adsorption phase. Thus, one of the usually double-installed heat exchanger surfaces can be saved.

It can also be advantageous to tailor the geometry of the sorbent container specifically to the heat output during the sorption phase. In the case of heat emission to the ambient air, large, flow-favorable heat exchanger surfaces are to be preferred.

The working fluid condenses predominantly in the condenser. The condensate must be directed from there into the evaporator. If the adsorption chiller is simple in design, the condensate must be able to tile back into the evaporator without additional assistance. This is always easy to realize when the condenser and thus also the heat buffer are higher than the evaporator. The condensate can then already during the desorption phase due to gravity tile back. In refrigerators where this is not possible, it may be advantageous if the condensate is stored in the condenser or a collection tank, to then be sucked up into the evaporator at the beginning of the adsorption phase when the vapor pressure in the evaporator drops.

In low-cost refrigerators must be dispensed with a complex, electronic control. However, since adsorption devices inevitably give off a strongly fluctuating cooling capacity, it is advantageous if the cooling capacity can be limited in a simple manner. According to the invention, the cross section of the working medium vapor line to the sorbent is reduced for this purpose. This can be done, for example, by means of expansion bodies, which reduce the line cross-section to the sorbent as the temperature falls. Particularly inexpensive are bi-metal elements that are installed in the evaporator, narrow the output of the evaporator at decreasing evaporator temperatures.

• Fig. 1 zeigt ein Schnittbild durch einen Adsorptions-Kühlapparat mit tiefer liegendem Verflüssiger, während
• Fig. 2 den oberen Teil eines Adsorptions-Kühlapparat mit einem in Bezug auf den Verdampfer höher liegenden Verflüssiger aufzeigt.

In the drawing, the invention is illustrated by the example of two electrically heated refrigerators.

• Fig. 1 shows a sectional view through an adsorption chiller with lower condenser, while
• Fig. 2 shows the upper part of an adsorption refrigerator with a higher condenser with respect to the evaporator.

A refrigerator 1 shown in FIG. 1 consists of a thermally insulated hollow body 2, which closes a door 3 on its front side and which cools food and beverage bottles 11 in the interior and stores them cooled. The medium to be cooled by the evaporator in this application is the air in the interior of the refrigerator.

Under the ceiling of the refrigerator 1, an evaporator 4 is arranged, from which the working fluid evaporates water 5. The evaporator 4 is connected via a working medium vapor line 9 with a sorbent container 12 and via a further connecting line 10 with a condenser 13. The evaporator 4 is coated on its lower inner surface with an absorbent non-woven fabric 6, which distributes the working fluid water homogeneously over the heat-absorbing surface. Outside it contains several cooling fins 7, which absorb heat from the medium to be cooled air. Below the cooling fins 7 is a layer cold storing elements 8 are inserted, which contain water and can also ice. Before the mouth of the working medium vapor line 9, a bi-metal element 23 is arranged, which narrows the outlet opening to the sorbent container at decreasing evaporator temperatures. The condenser 13 provided with heat exchanger fins 15 is located in the lower region of a buffer reservoir 14 which is filled with water 16. The sorbent container 12 consists of two metallic Sorberhüllen 17, which embed an electric heater 18 in the middle. The Sorberhüllen 17 each contain a sorbent filling 19, which is constructed of zeolite molded bodies.

A controller 20 controls the operation of the heater 18, depending on the temperature of the refrigerator air and the temperature of the sorbent charge 19. Input variables to the controller 20 are the air temperatures in the refrigerator, which are detected by a temperature sensor 21 and the zeolite temperature, which is a zeolite Temperature sensor 22 is reported.

The function of the refrigerator according to the invention can be subdivided into a relatively short desorption phase and a significantly longer adsorption phase.

The desorption phase begins with the heating of the sorbent filling 19. The temperature sensor 21 reports to the controller 20, the exceeding of the preselected temperature of the refrigerator air. Thereafter, the electric heater 18 is put into operation until the zeolite temperature sensor 22 detects the reaching of the desorption end temperature. During the heating phase, water vapor is expelled from the ever warmer sorbent charge 19, which flows through the working medium vapor line 9, the evaporator 4 and the connecting line 10 into the liquefier 13. In this, the steam is liquefied by heat through the heat exchanger fins 15 to the buffer water 16. The condensate collects in the lower part of the condenser 13. A small part of the water vapor liquefies in the evaporator 4 until it has risen to the temperature level of the condenser 13. The air masses around the evaporator 4 heat up. Since this amount of air is lighter than the cold air in the lower refrigerator area, there is no mixing. In addition, the cold storage elements 8 prevent the beverage bottles 11 from being markedly heated in the refrigerator (e.g., by heat radiation).

The sorbent container shells 17, which are in contact with the ambient air, also give off heat during the desorption phase. However, since this phase can be kept short according to the invention and the heat losses are low relative to the high heat output, can be dispensed with a thermal insulation of the outer sorbent casing 17. In addition, forms within the sorbent filling 19, a relatively high temperature gradient. Thus, 18 temperatures up to 400 ° C can be measured near the electric heater, while the zeolite temperatures in contact with the outer sorption container shells 17 are only 140 ° C hot. The heat losses to the environment are much lower from this low temperature level. In addition, these temperatures occur only at the end of the desorption phase. Upon reaching the desorption end temperature, the heating is turned off. The buffer has its highest temperature at this time. This now decreases continuously during the following adsorption phase, as heat slowly flows out of the container walls to the environment.

Heat also flows through the non-thermally insulated sorption container shells 17 to the ambient air flowing past. The temperature of the sorbent charge 19 thereby drops and working medium vapor flows back into the sorbent container 12. The vapor pressure in the evaporator 4 then decreases until the condensate is sucked up from the condenser. Immediately, the entire amount of liquid working fluid is in the evaporator 4. With continued cooling of the sorbent 19, this water mass in the evaporator evaporate during the adsorption phase, absorbing the heat of vaporization. At evaporation temperatures below the freezing point, the remaining amount of water will gradually freeze. Possible cooling far below the freezing point prevents the bi-metal element 23, which narrows the inflow opening in the working medium vapor line 9. The end of the adsorption phase is reached when the regulator 20 registers an excessively high air temperature in the refrigerator. By heating the sorbent filling 19, the desorption phase then starts from the beginning.

In the adsorption cooling apparatus shown in Fig. 2, the buffer memory 30 is above the evaporator 32. From the sorbent tank 33, the working medium vapor line 34 passes through the buffer memory 30 to the water content 35 to be able to effectively derive the liquefaction heat. The part of the working medium vapor line 34, which can deliver heat to the water content 35, therefore, at the same time has the function of the condenser. The working medium vapor line 34 is arranged inclined, so that the condensate 39 can already flow directly into the evaporator 32 during the desorption phase without additional precautions, following the gravitational force. The sorption container 33 consists in this embodiment of an inner heating cartridge 38 and a sorbent filling 37, which is enclosed by a cylindrical Sorber-shell 36. Again, this does not require thermal insulation, since the heat losses due to the short desorption phase and the large temperature gradient within the Sorptionsmittelfüllung 37 are low.

The operation of the cooling apparatus of FIG. 2 is identical to the above-described operation of the apparatus of FIG. 1. The only difference is that the condensate 39 does not remain in the condenser, but can flow into the evaporator 32 during the desorption phase.

Claims (11)

1. Adsorption cooling apparatus including an intermittently heated sorption agent container (12), which contains a sorption agent (19), which exothermically adsorbs a working agent during an adsorption phase and desorbs during a subsequent desorption phase with the addition of heat at relatively high temperatures, a condenser (13), which draws off condensed working agent via a connecting conduit (10) into the vaporiser (4), which is in turn connected to the sorption agent (19) via a working agent vapour conduit (9) and, during the adsorption phase, absorbs heat from a medium to be cooled, and the condenser (13) is coupled to an accumulator (14), which stores at least a proportion of the heat of condensation of the working medium vapour and can conduct the stored heat to the environment even during the adsorption phase, characterised in that the vaporiser (4) contains wetting agents (6),which produce a homogeneous distribution of the liquid working agent within the vaporiser.
2. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the vaporiser (4) is so arranged that it transfers relatively little heat to the medium to be cooled during the desorption phase.
3. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the vaporiser (4) is arranged in the upper region of the medium to be cooled and that the medium to be heated during the desorption phase does not mix with the cooler medium located beneath it due to the lower density.
4. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that arranged beneath the vaporiser (4) there is an element (8) storing cold or a radiation shield.
5. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the medium to be cooled is prevented by means of barrier devices from exchanging heat with the medium that has already been cooled during the desorption phase.
6. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the desorption heat supplied during the desorption phase is supplied from a burner.
7. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the sorption agent contains zeolite and the working agent contains water.
8. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the condensate is collected at a lower level in a condensate buffer and at the beginning of the adsorption phase is drawn into the higher level of the vaporiser (4) by suction.
9. Adsorption cooling apparatus as claimed in one of the preceding claims, characterised in that the working medium vapour conduit (9) includes a control element, which restricts the flow cross-section at vaporiser temperatures which are too low.
10. Adsorption cooling apparatus as claimed in Claim 10, characterised in that the control element includes a bimetallic element (23).
11. A method of operating an adsorption cooling apparatus as claimed in Claim 1, wherein the desorption phase constitutes less than one-third of the time of the adsorption phase, characterised in that during the adsorption phase, a temperature gradient of above 100 K is produced between the heat-receiving surface and the heat-emitting surface, caused by high heat output.

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