HYBRID AIR-CONDITIONING SYSTEM FOR DECOUPLED SENSIBLE AND LATENT HEAT REMOVAL AND METHOD

BACKGROUND
Technical Field

Embodiments of the subject matter disclosed herein generally relate to a system and method for cooling or heating an input air stream, and more particularly, to a hybrid air-conditioning system that combines cooling coils and indirect evaporative technology for decoupled sensible and latent heat removal.

Discussion of the Background

Energy consumption for air conditioning has been increasing exponentially due to population growth and increase of living standard. From 2000 to 2015, the energy consumption for air conditioning has increased from 3.6 EJ to 7 EJ, representing about 15% of peak electricity demand worldwide. In the Gulf Cooperation Council (GCC) countries, the cooling demand is more than 36 million Rton, and it consumes 50-70% of the peak electricity generation. Consequently, a significant portion of natural energy resources (oil and gas) produced in these countries are consumed for cooling. To tackle the challenges of global energy shortage, it is critical to reduce energy consumption of air conditioning systems

The demand for cooling is usually addressed by mechanical-vapor-compression (MVC) chillers. The energy consumption of these chillers has been measured to be around 0.85+0.03 kW/Rton. In the GCC countries, the energy consumption for chillers is over 3 times as compared to that of other regions. This is attributed to the severe weather conditions, i.e., hot and humid conditions. To remove the moisture from the air to be cooled, the evaporator's temperature (5-12° C.) has to be lower than the dew point temperature of the supplied air. Meanwhile, the condenser temperature is very high (larger than 45° C.) due to the high heat rejection temperature. As a result of the high thermal lift between the evaporator and the condenser, the energy consumption of the MVC chillers is significant for humid and hot environments.

To improve the energy efficiency of the cooling units, decoupling of sensible and latent loads has been proposed, where the latent load is associated with the humidity removal and the sensible load is associated with the heating of the dry air. Traditionally, the moisture is first removed from the incoming air stream using a dehumidifier (latent load), after which the hot and dry air is cooled (sensible load) to the desired temperature. The sensible cooling of the dry air is usually achieved using an indirect evaporative cooler (IEC), which uses the evaporative potential of the dry air as the driving force for cooling and consumes little energy. A well-designed IEC can achieve a COP (coefficient-of-performance) of more than 20 when the air is dry. Therefore, the overall COP of the air conditioning system is dependent on the performance of the dehumidifier.

Existing dehumidification technologies include liquid and solid desiccant systems. Most of these systems have a COP of less than 1 due to a lack of heat recovery. Recently, it was proposed a desiccant coated heat pump, which adds desiccant coating on the heat exchangers of conventional chillers. This system achieved a COP larger than 6. However, operation of the system is complex. It requires cyclic switching between dehumidification and regeneration modes for components, which needs many valves and air dampers, which are prone to failing.

As the IEC performance degrades when the incoming air has a high humidity, several groups have proposed to combine the IEC with MVC. For example, [1] evaluated the energy-saving potential of the hybrid IEC-MVC system under the climatic condition of Beijing, China. Compared with a standalone MVC, the hybrid system demonstrated a seasonal energy saving of 38.2%. [2] studied the performance of the hybrid IEC-MVC system in four cities of Iran. According to this study, the IEC could reduce the cooling load and electricity consumption by 75% and 55%, respectively. [3] experimentally evaluated the energy-saving potential of the hybrid IEC-MVC system for an office building in North Italy. The total electricity consumption according to this study was 38% lower. [4, 5] presented an experimental and analytical study of a hybrid IEC-MVC system under the climatic conditions of Singapore. According to these studies, the IEC can reduce the cooling load of MVC by 32%. The authors in [6, 7] conducted a numerical study on IEC that works as a pre-cooler for an MVC system. Their results revealed that the channel gap and the cooler height had the most influence on the cooling performance.

However, all these studies indicate that the hybrid process is still at an undeveloped stage with several technical gaps. Firstly, most of the existing studies are based on the climatic data of a specific area, and there is no systematic evaluation of the system's performance that covers different weather conditions. Secondly, as the IEC operates with room exhaust air in the wet channels, there is simultaneous cooling and dehumidification in the dry channels, making the system behavior different from that in a regular IEC. Further, most of the existing studies on such a process are based on numerical simulation, and there is a lack of actual experimental investigation and actual devices being built.

Thus, there is a need for an improved system that is configured for decoupled sensible and latent heat removal that is efficient and capable of cooling the incoming air stream under extreme temperature conditions.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment, there is a hybrid air conditioning system for cooling a chamber and the system includes a mechanical vapor compression, MVC, unit configured to cool, through evaporation and condensation of a medium, a first air stream (MA1, MA, MA2), and an indirect evaporative cooling, IEC, unit configured to cool a second air stream (CA, MA2, OA), which is related to the first air stream (MA1, MA, MA2), through direct heat exchange between wet channels that are placed adjacent to dry channels. The MVC unit is fluidly connected to the IEC unit so that a generated supply air stream SA is cooled by decoupling sensible and latent heat removal.

According to another embodiment, there is a hybrid air conditioning system for cooling a chamber, and the hybrid system includes an indirect evaporative cooling, IEC, unit configured to cool a first air stream (CA, MA2, OA), through direct heat exchange between wet channels that are placed adjacent to dry channels, and a mechanical vapor compression, MVC, unit configured to cool, through evaporation and condensation of a medium, a second air stream (MA1, MA, MA2), which is related to the first air stream (CA, MA2, OA). The IEC unit is fluidly connected to the MVC unit so that a generated supply air stream SA is cooled by decoupling sensible and latent heat removal.

According to yet another embodiment, there is a method for cooling air with a hybrid air conditioning system, and the method includes cooling a first air stream (MA1, MA, MA2), with a mechanical vapor compression, MVC, unit, through evaporation and condensation of a medium, to generate a second air stream (CA, MA2, OA), and cooling the second air stream (CA, MA2, OA) with an indirect evaporative cooling, IEC, unit through direct heat exchange between wet channels that are placed adjacent to dry channels. The MVC unit is fluidly connected to the IEC unit so that a generated supply air stream SA is cooled by decoupling sensible and latent heat removal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a novel hybrid air conditioning system that combines a mechanical-vapor-compression system with an indirect evaporative cooler system;

FIG. 2 illustrates various parameters of the hybrid air conditioning system of FIG. 1 and their operating values;

FIG. 3 illustrates the various air streams that flow through the hybrid air conditioning and their characteristics;

FIG. 4 illustrates the state of the refrigerant at various points along the hybrid system of FIG. 1;

FIG. 5 shows comparative data between the novel hybrid air conditioning system of FIG. 1 and traditional mechanical-vapor-compression systems;

FIG. 6 is a schematic diagram of another novel hybrid air conditioning system that combines a mechanical-vapor-compression system with an indirect evaporative cooler system;

FIG. 7 is a schematic diagram of yet another novel hybrid air conditioning system that combines an indirect evaporative cooler system with a mechanical-vapor-compression system;

FIG. 8 illustrates various parameters of the hybrid air conditioning system of FIG. 7 and their operating values;

FIG. 9 is a schematic diagram of another novel hybrid air conditioning system that combines an indirect evaporative cooler system with a mechanical-vapor-compression system;

FIG. 10 illustrates various parameters of the hybrid air conditioning system of FIG. 9 and their operating values;

FIG. 11 is a schematic diagram of yet another novel hybrid air conditioning system that combines an indirect evaporative cooler system with a mechanical-vapor-compression system;

FIG. 12 is a psycho-metric chart for the hybrid air conditioning system illustrated in FIG. 11;

FIG. 13 shows the temperature profiles of the air entering and leaving the indirect evaporative cooler system;

FIGS. 14A and 14B show the change of outdoor air temperature and humidity ratio in the dry channels for the indirect evaporative cooler system under different outdoor air temperatures;

FIGS. 15A and 15B show the drop of the outdoor air temperature and humidity ratio under varying outdoor humidity ratios for the indirect evaporative cooler system;

FIGS. 16A and 16B show the effect of dry channel air flowrate on the changes in temperature and humidity ratio for the indirect evaporative cooler system;

FIGS. 17A and 17B show the enthalpy recovery effectiveness and COP of the indirect evaporative cooler system under different outdoor air temperatures;

FIGS. 18A and 18B show the effect of the outdoor humidity ratio on the indirect evaporative cooler system's effectiveness and COP;

FIG. 19A shows that a higher air flowrate promotes heat and mass transfer coefficients and leads to higher indirect evaporative cooler system's effectiveness, and FIG. 19B shows the change of the COP with the flowrate;

FIG. 20A shows the percentage of cooling load that is handled by the indirect evaporative cooling system;

FIG. 20B shows that the higher the outdoor temperature, the higher the overall COP of the system; and

FIG. 21 is a flow chart of a method for cooling air with the hybrid air conditioning system.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a hybrid air conditioning system that includes a mechanical chiller and an IEC system. However, the embodiments to be discussed next are not limited to cooling the air, but may also be applied to heating the air.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

According to an embodiment, a novel hybrid air conditioning system includes a mechanical chiller and an indirective evaporative cooler system. The mechanical chiller may include an evaporator, an expansion valve, a compressor and one or two condensers. One of the condensers may be placed in series with the evaporator, and it is cooled by the off-coil air of the evaporator to lower the saturation pressure of condenser. A portion of the off-coil air leaving the evaporator is extracted as a purged air stream of the IEC system and the outlet of purged air is directed to the de-superheat condenser for heat rejection. In this de-coupled manner, the sensible load of the dry channels is removed from the IEC (at more than twice COP of the MVC), and the MVC removes the latent load at a much-improved COP. The mixed air (condenser outlet and returned air) is supplied to the IEC system for the sensible heat removal to achieve the desired supply air temperature but at a lower absolute humidity, to achieve human comfort in the room space. The details of this embodiment and other embodiments are now discussed with regard to the figures.

FIG. 1 illustrates a hybrid air conditioner 100 that includes an MVC system 110 fluidly connected to an IEC system 160. The term “fluid” is used herein to indicate that an air stream from the MVC system is shared with the IEC system and vice versa, but not a refrigerant stream. The refrigerant stream of the MVC system is fully contained within the MVC system. However, the term “fluidly” is also used to indicate that various elements of the MVC system are connected to each other through dedicated piping to handle the refrigerant, not the air streams. The MVC system 110 may be provided as an independent unit and the IEC system 160 may be provided as another independent unit, i.e., although the two units are autonomous, they are fluidly connected to each other for achieving the hybrid air conditioning system. Thus, these two units may be manufactured at different locations and then shipped to the final destination where they need to be assembled together. Piping connects the two units to each other and also to the chamber 170, which is desired to be cooled. The piping is design to handle air streams, not the refrigerant or other liquids. Each of these units/systems are now discussed with regard to the figures.

The MVC system 110 includes an evaporator 112 that is fluidly connected to a compressor 114, which is fluidly connected to a first condenser 116. The piping that achieves these fluid connections is configured to handle the refrigerant or medium of the MVC system, not the various air streams discussed later. The first condenser 116 is further fluidly connected to a second condenser 118, which in turn is fluidly connected to a valve 120. The valve 120 is fluidly connected to the evaporator 112, which completes the fluid path of the MVC system 110. Note that the path described above is a closed fluid path through which an appropriate medium 122 (e.g., the refrigerant) flows. No air stream enters into this path. The compressor 114 forces the medium 122 to continuously move through the evaporator and the two condensers.

When the medium 122 enters the evaporator 112, the medium 122 experiences a latent heat exchange as it transforms from liquid to vapor, due to the heat provided by the outside air stream OA that enters the evaporator. As a consequence, condensed water 112C is formed inside the chamber 112A from the entering air stream OA, which has a high humidity. The condensed water forms on the outside of the coil 113 and accumulates at the bottom of the chamber 112A. In one application, no fabric material is used inside or outside the coils to prevent or limit biofouling. Thus, the condensed water 112C in the chamber 112A may be used either as water supply to the IEC system 160 (to be discussed later) or for other purposes. Due to this process, the incoming air stream OA loses moisture, i.e., experiences dehumification. This process absorbs energy (the latent heat) from a mixture air stream MA1, which is obtained by mixing the incoming air stream OA (hot outdoor air) with a first partial return air RA1 from the chamber 170. Note that the partial return air RA1 is just a fraction of the total return air RA from the chamber 170. This means that an initial temperature T1 of the incoming air stream OA is decreasing to a temperature T1 of the mixture MA1 due to the mixing with the cool air RA1, which has a final temperature Tf, i.e., Tf<T1<Ti. After the air mixture stream MA1 passes through the chamber 112A of the evaporator 112, and contacts the outside wall of a coil 113 present inside the chamber 112A, which holds the medium 122, heat is transferred from the mixture MA1 to the medium 122, which results in an air stream (EA) leaving the evaporator that has a temperature T3 lower than the temperature T1, i.e., T3<T1. In one application, the temperature T3 is at or below the dew point temperature to remove the moisture from the air. Thus, the evaporator 112 cools the outside air OA, before being supplied to the IEC system 160 and before being supplied to the chamber 170, while evaporating the medium 122 of the MVC system 110.

Next, the air stream EA leaving the evaporator is split into two streams at a pipe junction 126, a purge air stream PA0 and a first mixture air stream MA2. The air stream PA0 is directly supplied to the wet channels 162 of the IEC system 160 as purge air, while the remaining part of the air stream EA is mixed with the remaining fraction RA2 of the return air RA, to generate the second air stream mixture MA2, which is supplied to the second condenser 118. Note that a temperature T4 of the second air stream mixture MA2 is lower than the temperature T3 of the EA stream as the temperature Tf of the RA stream is low (the coldest temperature of the system is achieved inside the chamber 170 if the system runs long enough and this temperature is the same with the supply air stream SA's temperature, which is supplied by the IEC system 160).

In one application, the percentages of the air volume that are mixed for obtaining the air streams OA, MA2, PA0, RA1, and RA2 are illustrated in FIG. 1, and they are 15%, 85%, 15%15%, and 85%, respectively. These percentages have been found by the inventors as being conducive to a best COP for the overall system. It is noted that due to the large number of air streams, just varying the corresponding percentages would effectively produce an infinite number of possibilities, which suggests that the found numbers are not simply part of an optimization process, but rather the result of extensive research and innovation. In another application, these percentages may vary by up to +/−20% relative to the above reference values and still achieve comparable advantages.

The air stream MA2 thermally interacts with the medium 122 that flows through the coil 119, inside a housing 118A of the second condenser 118. The second condenser 118 condenses the vapor medium into liquid medium 122, which results in heat release into the air stream MA2. This means that the air stream CA exiting the second condenser 118 has a temperature T5 higher than the temperature T4 of the incoming air stream MA2. This air stream CA enters next the dry channel 164 of the IEC system 160 and loses heat to the wet channel 162, which makes the final air stream SA to arrive at the desired temperature Tf. This air stream SA is then supplied at an input 172 of the chamber 170 as the cooling air.

The return air streams RA1 and RA2, previously discussed, are extracted from the room 170, at an output 174. Thus, the room 170, whose air temperature is desired to be controlled, not only receives the air stream SA, at the input 172, which cools the room, but also supplies from output 174 part of the ambient air to be mixed with the hot outdoor air stream OA to be cooled again.

The purge air stream PA0 receives the heat from the air stream CA, inside the IEC system 162 and then is supplied as heated air stream PA1 to the first condenser 116. The air stream PA1 thermally interacts with the vapor medium 122, which flows through a coil 117 located inside the chamber 116A of the second condenser 116. The air stream PA1 takes the heat from the vapor that condenses inside the first condenser 116 and is discharged outside as heated air stream PA2.

Within the MVC system 110, the medium 122 flows through the closed piping circuit 124 as a vapor (1) from the evaporator 112 to the compressor 114, then still as a vapor (2) from the compressor 114 to the first condenser 116, then to the second condenser 118 as a mixture (3) of liquid and vapor, then to the valve 120 as liquid (4), and then to the evaporator 112 as liquid (5), where it is transformed from liquid to vapor (1). The valve 120 may be a throttling valve that controls the amount of liquid passing from the second condenser to the evaporator. A controller 140, for example, a processor or a smart device, may be used to control the compressor 114 and the valve 120. Various temperature and/or humidity sensors S may be distributed at one or more of the elements discussed above to monitor their temperature and/or humidity. Based on these measurements, the controller 140 may slow down or increase the speed of the compressor 114 and/or may close or open the valve 120 and/or may increase or decrease the air flowrates through the various ducts (for example, using one or more fans 180) and/or may increase or reduce an amount of water that is applied to the wet channels (for example, with a pump 182). In one application, as illustrated in the Table 1 in FIG. 2, temperature ranges and air stream fractions may be selected and stored in the controller 140 for being maintained during the operation of the system 100.

The novel hybrid system 100 is able to provide a lower supply air temperature (20° C.) due to the use of cold and dry air in the wet channels of the IEC system 160. In contrast, traditional IEC systems usually supply air at a temperature higher than 22° C. Further, the hybrid system 100 has a much higher energy efficiency than the existing systems. This is because the MVC system 110 is mainly used to remove the latent load, while the sensible load is mainly handled by the IEC system 160. In the MVC system 110, the cooling medium is the cold air stream EA leaving the evaporator 112 and the air stream PA1 leaving the wet channels 162, which lowers the condensation pressure and reduces the compressor power. Meanwhile, the sensible load is handled by the IEC system, which has a high COP. The hybrid system 100 may also have a high compactness form factor. The evaporator 112 and the condensers 116 and 118 may be integrated in line with the air ducting system 130, while the compressor 114 and the expansion valve 120 can be installed in the spare space of the IEC system 160. The operation of the hybrid system 100 does not require any switch of air or refrigerant flow.

The ducting system 130 includes plural ducts which are fluidly connected to each other for handling the various air streams as now discussed. A first duct 130-1 communicates with the ambient and provides the outside air stream OA. The first duct is fluidly connected to a second duct 130-2, which is fluidly connected to the chamber 170, and provides the air from this chamber. The first duct 130-1, after being merged with the second duct 130-2, is fluidly connected at an input 1121 of the evaporator 112. A third duct 130-3 is fluidly connected to an output 1120 of the evaporator 112 and extends up to an input 1181 of the second condenser 118. The third duct 130-3 is fluidly connected to a fourth duct 130-4 and to a fifth duct 130-5, in this order. The fourth duct 130-4 is connected with the other end to the wet input 1621 of the IEC system 160. The fifth duct 130-5 is connected with the other end to the second duct 130-2. The output 1180 of the second condenser 118 is fluidly connected to one end of a sixth duct 130-6, while the other end of the sixth duct is connected to the dry input 1641 of the IEC system 160. The dry output 1640 of the IEC system is fluidly connected to a seventh duct 130-7, which is also connected to the input 172 of the chamber 170. An eight duct 130-8 fluidly connects the wet output 1620 of the IEC system 160 to an input 1161 of the first condenser 116. A final ninth duct 130-9 fluidly connects the output 1160 of the first condenser 116 to the ambient. It is noted that the inputs and outputs of the evaporator and condensers are fluidly connected to a corresponding chamber, and they do not fluidly communicate with the medium 122 that flows through the corresponding coils 113, 117, and 119.

Table 2 in FIG. 3 shows the flowrate of the various air streams, their temperatures, humidity and enthalpy while Table 3 in FIG. 4 show the states of the refrigerant 122 at different locations along the piping 124 in the MVC system 110. It is noted that these values may depart by up to 20%, up or down, and the hybrid system 100 still can achieve one or more of the advantages discussed above. The hybrid system 100 can effectively treat the outdoor air (35° C., 15 g/kg moisture) to the desired condition (22° C., 10 g/kg) before being supplied to the room 170. In one application, the mechanical chiller 110 works at a low thermal lift (compression ratio 2.9) due to the use of cold air in the condensers, which effectively reduces the compressor work. The COP (coefficient-of-performance) of the mechanical chiller is 4.2 considering isentropic compression with a compressor efficiency of 0.65, while overall COP of the MVC+IEC process in the system 100 being 6.0.

FIG. 5 illustrates a comparison of the electricity and water consumption between the hybrid system 100 and a conventional MVC system. The COP for water-and air-cooled MVC are 3.2 and 2.88, respectively. The air-cooled MVC 110 does not consume any water. However, for comparison purposes, it is assumed that its extra electricity consumption with respect to the water-cooled MVC is used for desalination with an efficiency of 4.5 kWh/ton. As clearly shown in the figure, the electricity consumption of the hybrid system 100 is only about 50% of a conventional MVC, while the water consumption is 24% or less than the conventional system.

The configuration shown in FIG. 1 may be changed as illustrated in FIG. 6 to have the return air stream RA2 being mixed with the air stream CA existing the second condenser 118. This means that the piping 130-5 that brings the return air stream RA2 is connected to the piping 130-6 just before entering the IEC system 160, and not just before the second condenser 118 as in the embodiment of FIG. 1. All other elements of the system 100 and the air stream remain the same as in FIG. 1.

In yet another variation of the embodiment of FIG. 1, the IEC system 160 may be placed upstream of the MVC system 110, as illustrated in FIG. 7, and one condenser of the MVC system 110 may be removed. According to this hybrid system 700, the hot outdoor air stream OA passes first through the dry channels 164 of the IEC system 110 to get a pre-cooled air stream DA at a temperature T6 and then this air stream is mixed with a portion of the return air stream RA2 having the chamber 170's final temperature Tf. The resulting mixture air stream MA1, having a lower temperature T7 than T6, is fed through the evaporator 112 to be cooled to a temperature T8, and dehumidified, which results in the air stream EA. The air stream EA that exits the evaporator 112 is mixed with another portion of a return air stream RA3 and the temperature of the mixture is T9. The temperature T9 can be just a little bit smaller than the final temperature Tf. Note that the percentage of the return air streams RA2 and RA3 along corresponding pipes 730-5 and 710, respectively, can be controlled by the controller 140, through corresponding dampers D2 and D3. In one embodiment, the dampers are instructed to make the RA2 to be 40% of the total air stream RA and RA3 to be the balance, i.e., 100-30−40=30%. Other values may be used for increasing the COP of the system.

The return air stream RA1 is provided to the wet channels 162 of the IEC system 160, where water collected from the evaporator 112 and also from an external water source (provided through pipe 140) is sprayed on the channels to obtain the evaporative cooling effect. The wet air stream WA leaving the IEC system 160 is mixed with an additional outdoor air stream OA2 to obtain the mixed air stream MA2, which is then circulated through the condenser 116 for heat rejection, i.e., to condense the liquid medium 122 circulating through the condenser 116. The resulting air flow PA is then discharged into the ambient, outside the chamber 170. The flow of the medium 122 through the MVC system 110 is similar to that discussed above with regard to FIG. 1 and thus, it is not repeated herein. In one application, the values of the various parameters used for running the hybrid system 700 of FIG. 7 are illustrated in the table of FIG. 8.

The ducting system for the system of FIG. 7 is different from the ducting system 130 of the embodiments shown in FIGS. 1 and 6. More specifically, the ducting system 130 in FIG. 7 includes a first duct 730-1 that fluidly connects the ambient to the dry input 1641 of the IEC system 160. A second duct 730-2 fluidly connects the output 174 of the chamber 170 to the wet input 1621 of the IEC system 160. A third duct 730-3 fluidly connects the dry output 1640 of the IEC system 160 to the input 1121 of the evaporator 112. The duct 708 connects the second duct 730-2 to the third duct 730-3. A fourth duct 730-4 fluidly connects the output 1120 of the evaporator 112 to the input 172 of the chamber 170. The duct 710 fluidly connects the second duct 730-2 to the fourth duct 730-4. A fifth duct 730-5 fluidly connects the wet output 1620 of the IEC system 160 to the input 1161 of the condenser 116. A sixth duct 730-6 is connected to the fifth duct 730-5 and brings air from outside. A seventh duct 730-7 fluidly connects the output 1160 of the condenser 116 to the ambient.

The hybrid system 700 shown in FIG. 7 may be configured to also heat the chamber 170, as illustrated in FIG. 9. For this embodiment, the damper D3 is closed to not allow any air to enter the duct 710, and thus, this mode works as the duct 710 is not present. Further, for the heating mode, the condenser and evaporator of FIG. 7 are swapped as shown in the figure, i.e., the air stream MA1 is used to condense the medium 122 while the air stream MA2 is used to evaporate the medium 122. FIG. 9 shows the temperature of each air stream and its corresponding humidity. For this configuration, the cold outdoor air OA is preheated inside the dry channels 164 of the IEC system 160, where the heat is provided by the return air stream RA1. The preheated air stream DA is mixed with the return air stream RA2 to further increase its temperature and the mixture air stream MA1 then passes through the condenser 116 to again increase its temperature, which results in the supply air stream SA, which is provided to the chamber 170 for heating. The air stream WA leaving the wet channels 162 of the IEC 110 is mixed with another stream of outdoor air OA2 and passes through the evaporator 112 to evaporate the medium 122. The cycle of the medium 122 through the MVC system 110 is described by steps (4) to (1), which is in the reverse direction to where discussed above with regard to FIG. 1. A possible set of parameters for the configuration shown in FIG. 9 is illustrated in Table 4 of FIG. 10. Note that the numerical values shown in the table may be modified by up to 20%, up or down and still achieve advantages of the hybrid system 100.

A variation of the embodiment illustrated in FIG. 7 is now discussed with regard to FIG. 11. Different from the hybrid system 700 of FIG. 7, the current hybrid system 1100 does not have any duct connecting the third duct 730-3 or the fourth duct 730-4 to the return air stream RA (i.e., duct 730-2). This embodiment shows the IEC system 160 having plural walls 166 that define the wet and dry channels 162, 164. Note that the wet channels 162 are effectively provided with a water spray or film 168 from an exterior water source 196. In one embodiment, the wet channels are run as wet as possible (i.e., the wettability in the wet surfaces is 100%) so that there are no wet and dry regions where scaling can occur. In one embodiment, the water source 196 is the condensed water 112C that forms inside the evaporator 112, as discussed with regard to the embodiment of FIG. 1. In this embodiment, as the condensed water is distilled water, there is less microbial contamination and thus, less chances of biofouling on the coils of the heat exchangers. In one implementation, the condensed water from the evaporator 112 was about 80% of the water needed for the wet channels. During a one year run of an experimental unit, no scaling and/or biofouling has been observed on the coils of the heat exchangers.

According to this embodiment, the room exhaust air (A4), which is cold and dry, flows through the wet channels 162 to cool down the outdoor air stream (A1) in the dry channels 164. The wet channels 162 are supplied with water, which evaporates and absorbs heat from the air to further cool down the outdoor air (A4). When the temperature in the dry channels 164 is lower than the dew-point temperature of the outdoor air (A1), condensation occurs, and the humidity ratio of the outdoor air drops. The pre-cooled and dehumidified outdoor air (A2) is then passed through the evaporator coils 113 of the mechanical chiller 110 to further bring down its temperature and humidity to the desired values and it is supplied as supply air (A3) to the room 170. Meanwhile, the exhaust air (A5) leaving the wet channels 162 is mixed with outdoor air (A1′) and channeled as mixed air (A6) to the condenser 116 for heat rejection, after each, the resulting air (A7) is purged to the ambient.

FIG. 12 demonstrates the above-mentioned processes in the psycho-metric chart. The room exhaust air (A4) is simultaneously heated and humidified in the wet channels 162 of the IEC 160, which recovers both the sensible and latent energy for pre-cooling and dehumidifying the outdoor air (A1). The exhaust air (A5) leaving the wet channels 162 is still colder than the outdoor air (A1) and is used as part of the cooling media of the MVC condenser 116 to recover its cooling potential further. In this manner, the sensible and latent potential of the room exhaust air (A4) is effectively reused, which will significantly reduce the overall energy consumption of the system 1100.

In one implementation, the inventors designed and manufactured a 1-Rton IEC unit to experimentally test the efficiency of the new system 1100. The unit has a cross-flow configuration with a dimension of 1 m×1 m×0.7 m. 200 mm× 300 mm chamfers are cut at the four corners to form the entrances and exits of dry and wet channels. The air flow channels are connected to acrylic ducts. At the inlet of the wet channels 162, two rows of spray nozzles are installed to supply fine water droplets to form the water films 168. The heat and mass exchanger in the IEC system 110 includes in this implementation 50 dry channels and 50 wet channels arranged in an alternating manner. The channel walls are made of aluminum plates with a thickness of 300 μm. These plates are separated with spacers (5 mm thickness) to form the flow channels.

Employing this experimental setup, the energy recovery performance of the IEC was evaluated. Hot and humid air, provided by an environmental chamber (a chamber that can control the temperature and humidity of a supplied air), is supplied to the dry channels to simulate the outdoor air. Its temperature is ranged between 30 and 42° C., and the humidity ratio is 10-20 g/kg. The wet channel is supplied with room air, which has a temperature of 23±1° C. and a humidity ratio of 11±1 g/kg. The air flowrate in the dry channels is varied between 280 and 420 CMH by controlling the fan speed, while that in the wet channels is fixed at 230 CMH.

FIG. 13 shows the temperature profiles of the air entering and leaving the IEC system 160 during one set of experiments. The humidity ratio of the outdoor air is fixed at 15 g/kg, while the room air condition is 23.5° C. and 10.5 g/kg. With the step-wise increment of the outdoor air temperature, the air temperatures leaving the dry and wet channels increase accordingly. After the outdoor air temperature is stabilized, it takes another 1000 s for the exiting temperatures to reach a steady state (i.e., temperature change over 10 min is <0.1° C.). The outdoor air temperature drops by 11-14° C. in the dry channels, and the wet channel air temperature is always ˜ 2.5° C. higher than that in the dry channels. It should be noted that the wet-bulb air temperature in the wet channels is almost the same as its dry-bulb temperature. In other words, the air in the wet channels is fully saturated when leaving the wet channels, indicating effective recovery of the latent potential from the room exhaust air.

FIGS. 14A and 14B show the change of outdoor air temperature and humidity ratio in the dry channels under different outdoor air temperatures. Each data point is the average value of more than 3 measurements. For each measurement, the value is recorded after the temperatures are stabilized for more than 600 s, as previously shown in FIG. 13. Under a constant humidity ratio, the changes in temperature and humidity ratio are linearly proportional to the outdoor air temperature. This is consistent with the observations reported in the literature. This is so because when the outdoor air temperature is higher, the exhaust air in the wet channels can be heated to a higher temperature, which provides more sensible cooling effects. More importantly, a higher air temperature in the wet channels increases the saturation humidity ratio and allows more evaporation, leading to more latent cooling effects. The enhanced sensible and latent cooling effects result in a larger drop of temperature and humidity ratio in the dry channels. The temperature drop is in the range of 6-15° C. When the outdoor air temperature is low at 31° C., it can be pre-cooled to ˜ 23° C., which is very close to the desired supply air temperature (18-20° C.). In this case, the IEC system is able to undertake the majority of the sensible cooling load, and the MVC system only needs to handle the latent load.

FIGS. 15A and 15B show the drop of the outdoor air temperature and humidity ratio under varying outdoor humidity ratios. When the outdoor humidity is high, there is more potential for condensation, and the drop of air humidity in the dry channels is larger, as can be seen from FIG. 15B. With more moisture condensation in the dry channels, more condensation heat is released, which heats up the air in both dry and wet channels. Consequently, the air temperature drop in the dry channels is smaller, as shown in FIG. 15A. When the outdoor air humidity ratio is low at 13.46 g/kg, the IEC system 160 is able to bring down the humidity to about 11 g/kg, a value that is close to the room air humidity. In other words, the IEC system handles most of the latent load by recovering energy from the room exhaust air.

FIGS. 16A and 16B show the effect of dry channel air flowrate on the changes in temperature and humidity ratio. The air flowrate in the wet channels is fixed at 230 CMH. When the air flowrate in the dry channels increases from 280 to 350 CMH, the drops of temperature and humidity ratio become less significant. The reason is that the total amount of cooling capacity is fixed under a given wet channel air flowrate. Therefore, the higher the dry channel air flowrate, the smaller the enthalpy change for the unit mass of air. When further increasing the air flowrate from 350 to 420 CMH, the descending trend of temperature change remains, while the humidity change is slightly higher. This is attributed to the improvement of heat and mass transfer coefficients under a higher air flowrate in the dry channels.

As there are simultaneous heat and mass transfer in the dry channels, the enthalpy exchange effectiveness and COP allow better measurement of the IEC system's performance, as they account for both temperature and humidity change. In this regard, FIGS. 17A and 17B show the enthalpy recovery effectiveness and COP of the IEC system under different outdoor air temperatures. As defined, the enthalpy recovery effectiveness is the ratio of actual enthalpy transfer to maximal possible enthalpy transfer. When the outdoor temperature is higher, the available cooling potential is also higher, while the IEC unit is unable to completely recover such potential due to a limited surface area for heat and mass transfer. As a result, the effectiveness is lower at a higher outdoor temperature, as revealed by FIG. 17A. The COP, in contrast, is higher when the outdoor temperature is higher, as illustrated in FIG. 17B. With a higher outdoor air temperature, there is a larger drop of temperature and humidity in the dry channels, providing more cooling effects. On the other hand, the energy consumption for the fans and the water pump is fixed with the air flowrate. Thus, the higher the outdoor temperature, the higher the COP is. The COP is in the range of 8-14, despite a high outdoor humidity ratio of 20 g/kg. Such a high COP value is the result of effective sensible and latent heat recovery from the room exhaust air.

FIGS. 18A and 18B present the effect of the outdoor humidity ratio on the IEC system's effectiveness and COP. Different from the effect of temperature, a higher humidity ratio leads to higher energy recovery effectiveness. This is mainly because higher outdoor humidity induces more condensation, which significantly enhances heat and mass transfer in the dry channels. The COP of the system is also higher when the outdoor humidity ratio is higher, as there is not only higher cooling potential, but also more effective energy recovery.

A higher air flowrate also promotes heat and mass transfer coefficients and leads to higher IEC effectiveness, as shown in FIG. 19A. With the increase of the dry channel air flowrate from 280 to 420 CMH, the effectiveness can be increased by 4%, regardless of the outdoor humidity ratio. FIG. 19B shows the change of the COP with flowrate, which demonstrates a different trend from the effectiveness. It firstly decreases when increasing the air flowrate from 280 to 350 CMH, while further increasing the flowrate to 420 CMH leads to a higher COP. This is attributed to several competing effects when increasing the flowrate. On one hand, a higher flowrate reduces the total cooling potential for the dry channels and causes the decrease of COP, as previously discussed. On the other hand, heat and mass transfer are promoted under a higher air flowrate, which allows for better recovery of the cooling potential. The trend of COP change with flowrate is the result of the trade-off between these two effects.

Based on the experimental results on the IEC system discussed above, the inventors compared the COP of the hybrid IEC-MVC process with that of a standalone MVC. The effectiveness of energy recovery from the IEC system is firstly calculated. Then, the energy consumption to further cool down the outdoor air to the supply condition is calculated. For comparison purposes, the energy consumption of a standalone air-cooled MVC chiller, which directly cools down the outdoor air to the supply condition is also calculated. For both systems, the isentropic efficiency (ncomp) is assumed to be fixed at 0.65 under all the operating conditions.

FIG. 20A shows the percentage of cooling load that is handled by the IEC system. The desired supply air condition is assumed to be 20° C. and 9 g/kg. As the IEC system is suitable for sensible cooling, the percentage is higher when the sensible load is higher, i.e., the outdoor air has higher temperature and lower humidity. When the outdoor humidity ratio is low at 10 g/kg, the IEC system is able to deal with 70-77% of the overall cooling load. With the increase of the outdoor humidity ratio to 15 g/kg, the percentage of cooling load that is undertaken by the IEC system decreases to 45-58%. This value further drops to 34-48% when the outdoor humidity ratio is 20 g/kg.

With different percentages of cooling load undertaken by the IEC system, the overall system COP is also different. When the outdoor air is dry, the overall energy efficiency is dominated by the COP of the IEC system. Therefore, the higher the outdoor temperature, the higher the overall COP, as illustrated in FIG. 20B. In contrast, the COP of the standalone MVC system, as plotted by the dashed line 2010, decreases with higher outdoor temperatures, as the refrigerant 122 has to be compressed to a higher pressure for heat rejection. The COP of the hybrid IEC-MVC system (100, 700, or 1100) is 39-135% higher than that of a standalone MVC under a low outdoor humidity ratio of 10 g/kg.

When the outdoor humidity ratio is higher then 15 g/kg, more cooling load has to be handled by the MVC system. In this case, the overall COP changes marginally with the temperature, which is a result of the completing effects between the IEC and MVC systems. The improvement over standalone MVC system reduces to 24-79%, which is still significant. The overall COP starts to drop with outdoor temperature when the outdoor humidity ratio is higher at 20 g/kg, while the values are still 19-59% higher than that of standalone MVC system.

A method for operating one of the hybrid systems discussed above is illustrated in FIG. 21. The method includes a step 2100 of cooling a first air stream (MA1, MA, MA2), with the MVC system 110, through evaporation and condensation of the medium 122, to generate a second air stream (CA, MA2, OA), and a step 2102 of cooling the second air stream (CA, MA2, OA) with the IEC system 160 through direct heat exchange between the wet channels 162 that are placed adjacent to dry channels 164. The MVC system is fluidly connected to the IEC system so that a generated supply air stream SA is cooled by decoupling sensible and latent heat removal.

In one application, the hybrid air conditioning system includes a MVC unit configured to cool the first air stream (MA1, MA, MA2) and the IEC unit is configured to cool the second air stream (CA, MA2, OA), which is related to the first air stream (MA1, MA, MA2). As illustrated in FIG. 1, the first air stream OA, which is ambient air, is mixed with a first return air stream RA1 from the chamber, prior to entering the MVC unit, to generate a first mixed air stream MA1. The MVC unit in this embodiment includes one evaporator and first and second condensers. The first mixed air stream MA1 enters the evaporator to evaporate a refrigerant of the MVC unit. An air stream EA leaving the evaporator is split into first and second EA streams, the first EA stream is supplied to the wet channels of the IEC unit, and the second EA stream is mixed with a second return air stream RA2, from the chamber, to form a second mixed air stream MA2. The second mixed air stream MA2 is supplied to the second condenser of the MVC unit to condense the medium of the MVC unit. The air stream CA is supplied to the dry channels of the IEC unit. The air stream CA becomes the supply air stream SA when exiting the dry channels of the IEC system and the supply air stream SA is supplied to the chamber while the wet channels of the IEC unit generate a first heated air stream PA1, which is supplied to the first condenser of the MVC unit to condense the medium, and a second heated air stream PA2, which is generated by the first condenser, is discharged into the ambient.

In another application, as illustrated in FIG. 6, an air stream EA leaving the evaporator is split into first and second EA streams, the first EA stream is supplied to the wet channels of the IEC unit, and the second EA stream is supplied to the second condenser of the MVC unit to condense the medium of the MVC unit and to generate a cold air stream CA. The air stream CA is mixed with a second return air stream RA2 from the chamber to form a second mixed air stream MA2, which is supplied to the dry channels of the IEC unit. The cold air stream MA2 becomes the supply air stream SA when exiting the dry channels of the IEC system and the supply air stream SA is supplied to the chamber to be cooled while the wet channels of the IEC unit generate a first heated air stream PA1, which is supplied to the first condenser of the MVC unit to condense the medium, and a second heated air stream PA2, which is generated by the first condenser, is discharged into the ambient.

The method may also be implemented with the hybrid air conditioning system 700 or 1100 for cooling the chamber. For this configuration, the method uses first the IEC unit to cool a first air stream (CA, MA2, OA), through direct heat exchange between the wet channels 162 that are placed adjacent to dry channels 164, and then uses the MVC unit 110 to cool, through evaporation and condensation of the medium 122, a second air stream (MA1, MA, MA2), which is related to the first air stream (CA, MA2, OA).

The first air stream enters the dry channels of the IEC unit to be pre-cooled and a first return air stream RA1, from the chamber, is supplied to the wet channels of the IEC unit. In one application, as shown in FIG. 7, a pre-cooled air stream DA exiting the dry channels of the IEC unit is mixed with a second return air stream RA2 from the chamber to generate a first mixed air stream MA1. The first mixed air stream MA1 is provided to an evaporator of the MVC unit for evaporating the medium of the MVC unit, an air stream EA leaving the evaporator is mixed with a third return air stream RA3 from the chamber to generate the supply air stream SA. A wet air stream WA exiting the wet channels of the IEC unit is mixed with another outside air stream OA2 to generate a second mixed air stream MA2. The second mixed air stream MA2 is provided to an input of a condenser of the MVC unit to condense the medium, and an output stream PA from the condenser is discharged into the ambient.

In another application, as illustrated in FIG. 9, the first mixed air stream MA1 is provided to a condenser of the MVC unit for condensing the medium of the MVC unit, and an air stream leaving the condenser is the supply air stream SA. A wet air stream WA exiting the wet channels of the IEC unit is mixed with another outside air stream OA2 to generate a second mixed air stream MA2. The second mixed air stream MA2 is provided to an input of an evaporator of the MVC unit to evaporate the medium, and an output stream PA from the evaporator is discharged into the ambient.

In yet another application, as illustrated in FIG. 11, a pre-cooled air stream DA exiting the dry channels of the IEC unit is provided directly to an evaporator of the MVC unit for evaporating the medium of the MVC unit, and an air stream leaving the evaporator is the supply air stream SA. A wet air stream WA exiting the wet channels of the IEC unit is mixed with another outside air stream OA2 to generate a second mixed air stream MA2. The second mixed air stream MA2 is provided to an input of a condenser of the MVC unit to condense the medium, and an output stream PA from the condenser is discharged into the ambient.

The disclosed embodiments provide a hybrid air conditioning system that uses an indirect evaporative cooler system and a mechanical vapor compression system, in this or a reversed order, for cooling and/or heating the air in a chamber. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications, and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

REFERENCES

The entire content of all the publications listed herein is incorporated by reference in this patent application.

[1] Duan Zhiyin, Zhao Xudong, Liu Jingjing, Zhang Qunli. Dynamic simulation of a hybrid dew point evaporative cooler and vapour compression refrigerated system for a building using EnergyPlus. J Build Eng 2019; 21:287-301.
[2] Delfani Shahram, Esmaeelian Jafar, Pasdarshahri Hadi, Karami Maryam. Energy saving potential of an indirect evaporative cooler as a pre-cooling unit for mechanical cooling systems in Iran. Energy Build 2010; 42(11):2169-76.
[3] Zanchini Enzo, Naldi Claudia. Energy saving obtainable by applying a commercially available M-cycle evaporative cooling system to the air conditioning of an office building in North Italy. Energy 2019; 179:975-88.
[4] Cui X, Islam MR, Chua KJ. An experimental and analytical study of a hybrid air-conditioning system in buildings residing in tropics. Energy Build 2019;201: 216-26.
[5] Cui X, Chua KJ, Islam MR, Ng KC. Performance evaluation of an indirect pre-cooling evaporative heat exchanger operating in hot and humid climate. Energy Convers Manage 2015; 102:140-50.
[6] Chen Yi, Yang Hongxing, Luo Yimo. Parameter sensitivity analysis and configuration optimization of indirect evaporative cooler (IEC) considering condensation. Appl Energy 2017; 194:440-53.
[7] Chen Yi, Luo Yimo, Yang Hongxing. A simplified analytical model for indirect evaporative cooling considering condensation from fresh air: Development and application. Energy Build 2015; 108:387-400.

HYBRID AIR-CONDITIONING SYSTEM FOR DECOUPLED SENSIBLE AND LATENT HEAT REMOVAL AND METHOD

Information

Publication Number

Date Filed

Date Published

Inventors

Original Assignees

CPC

International Classifications

Abstract

Description

Claims

CROSS-REFERENCE TO RELATED APPLICATIONS

PCT Information

Provisional Applications (1)