Sorptive heat exchanger and related cooled sorption process

A sorptive heat exchanger (E), which presents a plurality of heat exchange channels (10) in thermal contact with respective sorption channels (11), where sorption material (12) is fixed on the internal surfaces of channels (11).

The present invention relates to a sorptive heat exchanger and related cooled sorption process.

Particularly the invention relates to an equipment where a cooled sorption process takes place on a solid sorption material and to the related cooled sorption process on a solid sorption material.

In various industrial applications a sorption process is used in order to eliminate or reduce the presence of at least one component from a gas mixture for example wet gas used in an industrial process from which a liquid must be extracted.

In the case of air, i.e. gas mixture including water vapour, during air conditioning, cooling and dehumidification processes take place. The air dehumidification implies the partial extraction of the gas component water vapour from the air. Therefore the cooled sorption process of water vapour from air on a solid sorption material, could be used for air conditioning purposes, extracting the water vapour (i.e. dehumidifying) from the air stream.

Half of the energy consumption of office buildings is due to air conditioning. In the last years, air conditioning plants using solar energy and employing sorption components have been developed, built, and monitored. For example, sorption processes were implemented in thermodynamic open cycles (Desiccant and Evaporative cooling, DEC plants), where the sorption material is regenerated, by means of desorption process, using the thermal energy produced for instance with solar collectors. Many refrigerant compounds are hazardous for the environment, on the contrary water used as refrigerant does not cause any risks for the atmosphere. The sorption material regeneration is carried out by means of a warm air stream, which can come, for example, from solar air collectors. In a successive phase the regenerated sorption material dehumidifies the external air that is then further cooled and humidified and then blown into the building. In order to realise the open cycle, up to now the sorption material is regenerated with hot air and then brought into contact with external air causing its dehumidification.FIG. 1presents the layout of a conventional DEC plant according to prior art. In the simplified scheme ambient air1flows through the sorption wheel SR. The ambient air is dehumidified and heated in the SR. The air is then blown towards position2. Afterwards the air reaches the heat recovery wheel WR, in which the air is cooled down. The air, which leaves the wheel WR by means of the channel3, is further cooled down by means of humidification in the humidifier4using the effect of evaporative cooling and afterwards the air is transferred into the interior of the building. In the interior of the building the air takes up humidity M and heat Q. The air leaves the interior building5and is again humidified and cooled down in the humidifier6. In the heat recovery wheel WR the air takes up heat and then reaches the channel7. In a heating unit which is preferably a solar heating unit8(e.g. solar air heating collector) the air is further heated and is afterwards transferred to the sorption wheel SR. In the SR the hot air dries the sorption material. The air leaves the sorption wheel SR warm and humid, by means of a channel9.

This kind of plant, where the rotary dehumidifiers technology is used, results economically feasible only if their size is bigger than about 10.000 m3/h. In sorption air conditioning systems, where the air treatment takes place in a heat exchanger, the process is optimised, costs are reduced and it is advantageous to realise sorptive air conditioning systems even of small size (air volume flow considerably lower than 10.000 m3/h).

The process implementation of conventional sorption air conditioning plants, as the one described inFIG. 1, faces problems, which are not solved in a satisfying way yet. This becomes obvious at two states in the physical process.

The sorption rotor (desiccant wheel) is heated up remarkably after thermal desorption. This heat is an obstacle in the subsequent adsorption step, i.e. the step of water uptake, because the sorption material can take up less amount of water from the incoming air stream at higher temperatures. The sorption potential (and thereby the cooling capacity) would be higher, if the sorption material would be cooled during the sorption process.

When ambient air gets in the sorption rotor humidity from the ambient air is taken up. Thereby chemical heat is set free leading to a temperature increase of the sorption material. This heat is taken up from the streaming air and is transported in direction of the stream. The sorption material following in the direction of the stream takes up part of this heat. This again leads to a reduction of the potential for uptake (sorption) of the sorption material. Besides this the air is heated up in an unfavourable way since this contradicts to the main purpose of the entire process, namely cooling of the air.

Again, it is more favourable, if the sorption material is cooled during the sorption process and remains on a lower temperature level. Thereby also the temperature of the air leaving the process can be reduced remarkably.

Because of the described disadvantages in the process implementation lots of operation states occur, during which the sorptive air conditioning plant delivers only an insufficient or even not any cooling capacity.

A further disadvantage of usual sorptive air conditioning systems (desiccant systems employing rotors) is the requirement of two rotating components (wheels SR and WR). This construction causes high cost and furthermore unavoidably a mixing of the air streams occurs. For the above mentioned reasons such type of systems are not economically competitive, at least at low capacity (i.e. size).

The main aim of this invention is to realise an equipment where a cooled sorption process of a component from a gas mixture on a solid sorption material takes place. The equipment should make possible to reach high efficiencies and to achieve low costs even for small size devices.

Another aim of the present invention is to realise an air conditioning or climatization apparatus presenting high efficiency, which is employing the equipment where takes place a cooled sorption process of a component from a gas mixture on a solid sorption material. The apparatus will then present low costs and result economically convenient for small air volume flow (i.e. low capacity of the apparatus).

Another aim of the present invention is to realise an air conditioning or climatization apparatus, which can be employed, for example as unitary system (i.e. not centralised) in particular as alternative to unitary air conditioning systems based on vapour compression chillers.

It is among the aims of this invention to provide a sorptive process of a component from a gas mixture on a solid sorption material and in particular the cooled sorption process of water vapour from an air stream on a solid sorption material.

The above mentioned and other aims of the present invention are reached by the sorptive heat exchanger and related cooled sorption process according to the independent claims.

The sorptive heat exchanger according to the invention, includes a heat exchanger, which consists of a plurality of separated channels which are in thermal contact and in part of them a sorption material is fixed. According to the invention the sorption material is fixed on the internal surface of part of the channels.

As schematically shown inFIGS. 2 to 8, a sorptive heat exchanger E includes at least two separated systems of channels in thermal contact.

The heat exchanger, preferably a cross-counter-flow heat exchanger or a counter-flow heat exchanger presents a plurality of heat exchange channels10in thermal contact with respective sorption channels11. The sorption material12is fixed on the internal surface of each of the sorption channels11.

FIG. 2shows two channels in thermal contact, and the path of the two fluids through a cross-counter-flow heat exchanger E. If for example the heat exchanger would be used for air conditioning purposes the fluids going through the heat exchanger would be air, but the exchanger is also suitable for treating a generic wet gas used in an industrial process from which a liquid or at least a component has to be extracted.

In each heat exchange channel10the cooling fluid F2, which for example in case of an air conditioning or climatization apparatus, can be air, flows according to the direction of the arrow, in the sorption channel11the gas mixture F1from where at least a component has to be extracted, which for example in case of an air conditioning or climatization apparatus can be humid hot air, flows from left to right according to the direction of the arrow.

The sorption material12, is located on the internal walls of the sorption channel11. The sorption material has to be chosen among the materials which can better serve the realisation, for example in the case of air conditioning proper materials for air dehumidification are Silica-gel, Zeolite and some hygroscope salts like for instance lithium chloride.

If the fluid F2, which flows in channel10is a gas, the equipment will include humidifier components19for the possible humidification of the fluid F2before entering the heat exchanger E, for example ultrasonic humidifiers. In a favourable way, it is possible, to install humidifiers19in order to humidify substantially continuously the fluid F2during its passage in the channels10.

In this way the fluid is over-saturated or this air is continuously humidified during its way through the heat exchanger channel such that evaporation takes place as soon as the air takes up heat and thereby cooling capacity is provided continuously. This is done, for example, by means of injectors installed at entrance section or inside the channel10.

FIG. 3shows a sorption air conditioning apparatus, realised using the sorptive exchanger according to the present invention.

In the operation during sorption phase (i.e. cooling), ambient air flows, according to arrow of fluid F1, in the sorption channel11along regenerated sorption material12and is thereby dehumidified. The heat which is thereby created is to a large extent taken up from the cool air in the heat exchanger channel10. In a favourable way the air in the heat exchanger channel10is over-saturated or this air is continuously humidified during its way through the heat exchanger channel such that evaporation takes place as soon as the air absorbs heat and thereby cooling capacity is provided continuously during the passage in channel10. After the air leaves the sorption channel by means of a channel15the air is relatively cold and dry. Optionally the air is further cooled by means of humidification in the humidifier16and afterwards it is conducted to the air conditioned interior building17, by means of the fan13. Room air is taken from the interior building, by means of the fan14, and further humidified in the humidifier18, this time preferably up to over-saturation. Then the air is conducted to the heat exchanger channel10. In the heat exchanger channel the air can—by means of a respectively suitable device (humidification device)—be continuously humidified during its way through the heat exchanger channel.

FIGS. 4 to 6show different methods for the sorption material12regenerating phase. In general a wide variety of heat sources can be employed for the regeneration of the sorption material, e.g. waste heat, heat from a district heating system, heat from cogeneration plants or heat from solar thermal collectors. When using heat from a heat source20, for example solar thermal collectors for desorption the one or other method for desorption is applied depending on the characteristic of the solar collector20, the type of sorption material12and the climatic and meteorological boundary conditions. Another possibility for the desorption of the sorption material12(desorption phase) could be to circulate in channel10a fluid, preferably close to evaporation condition, for example steam at 100° C. In case of desorption of the sorbens the steam would condense in channel10and deliver the energy of condensation for desorption. The condensate preferably could stay in channel10and later in the phase of the dehumidification of the gas in channel11the occurring sorptive energy would preferably be absorbed by the energy of evaporation of the condensate (the system is similar to heat-pipe systems). In this case the humidifier components19would not be necessary.

FIG. 4shows the most simple way of desorption. Thereby in the heat exchanger E according to a first regenerating method R′ in channel10there is no fluid blown. Instead the fluid after being heated from the heat source20is blown in the sorption channel11.

InFIG. 5according to a second regenerating method R″ both channel systems,10and11, in the heat exchanger E are flown through in the same direction. The two fluid streams are respectively G1and G2and they are previously heated by the heat source20, for example a solar thermal collector. This variant has the advantage of an improved heat transfer from the fluid to the sorption material12, since the sorption material is heated from both, the sorption channel11and the heat exchanger channel10of the heat exchanger E. The heated fluid from the heat exchanger channel10is mixed, for example with ambient air24and conducted to the heat source20. Thereby the fluid by means of the heat source20reaches higher temperatures, before being used for the desorption process.

A different third regenerating method R′″ of the sorption material is described inFIG. 6. When conducting the process according toFIG. 6approximately a linear temperature profile will occur during desorption in the heat exchanger E: at the left entrance I1of the heat exchanger the fluid has a lower temperature and at the right entrance I2a higher temperature. This distribution means, for example for air conditioning, that the sorption material during operation in cooling mode on the side where the fluid leaves the sorption channel11is higher dehumidified. Therefore the air is during the sorption phase during its flow through the sorption channel11continuously in contact with a drier sorption material12, which results in a higher dehumidification potential for the further cooling phase. The absolute value of dehumidification of ambient air can be optimised by the implementation of this process. The desorption methods described inFIGS. 4 and 5are called “Concurrent Flow Desorption” and the desorption method according toFIG. 6is called “Counter Flow Desorption”.FIG. 7shows in a qualitative manner the temperature profiles in the sorption channel11after desorption phase, according toFIGS. 4,5,6and where the three profiles of the regenerating methods are respectively indicated with R′, R″ and R′″. In a first approximation high temperatures mean a high drying of the sorption material12.

FIG. 8shows the pre-cooling phase of the heat exchanger E after desorption. The fluid24, for example for air conditioning applications ambient air, which as desired has been humidified or not humidified or for example room return air F2which as desired has been humidified or not humidified, is conducted in the heat exchanger channel10and takes up the heat from the sorption channel11, whereby the sorption channel is pre-cooled for the subsequent sorption phase.

A complete cycle of desorption, pre-cooling and sorptive cooling, for example of external ambient air, can be realised by means of subsequent combination of the different operation modes of the devices as inFIGS. 3 to 6andFIG. 8. If for instance one minute would be available for desorption, in a part of this time desorption can be arranged following the process ofFIG. 6and another part following the process ofFIG. 4and afterwards the heat exchanger could be cooled according toFIG. 8. After this sequence of processes the sorption material12in the sorption channel11of the heat exchanger shown in the above mentioned figures would be particularly highly dried and well pre-cooled for the subsequent phase of sorption (air cooling). These conditions are favourable for the process.

In order to realise a sorption process after the desorption or regenerating phase follows the sorption phase.

For example for the purpose of air conditioning the cooled sorption process will result in the dehumidification and possibly cooling of the airflow F1inFIG. 3. The cold and humid air flow F2inFIG. 3is responsible for the cooling of the sorption material12and consequently of the fluid F1.

Sorption phase and regeneration phase realised by means of desorption are carried out alternately in the equipment, namely the heat exchanger built according to the present invention. For example in air conditioning applications, in order to realise a continuous provision of cold, dehumidified air to the building and for a continuous use of the heat source, e.g. the solar air heating collector and of the humidifiers at least two exchangers, i.e. sorptive heat exchangers, are necessary.

Thereby the two heat exchangers are each time alternately in the operation states “sorption phase” and “regenerating phase”. The air streams are diverted depending on the actual operation phase by means of control of respective fluid diverters.

The equipment, according to present invention, if applied for air conditioning would give the chance to achieve higher dehumidification rates and air temperature reductions in comparison with other sorption air conditioning apparatus employing solid sorption material, avoiding any possibility of mixing of the exhaust—i.e. coming from the building—stream and process air.

In comparison to a conventional sorption air conditioning apparatus the construction incorporating the heat exchanger according to the invention is able to achieve a higher air dehumidification and a higher temperature decrease of ambient air without any mixing between fresh air and room return air.