Pressurizθd-Cycle Based Electro-Adsorption Chiller

Background and Field of the Invention

This invention relates to a pressurized-cycle based electro-adsorption chiller (PEAC) for dissipating heat from a location.

Owing to its niche in converting waste-heat at low temperatures to useful cooling, adsorption chillers have been commercially employed for almost two decades. In such a chiller, an adsorption cycle employs environmental-benign working pairs such as silica gel-water pair [2,3] and there is practically no moving part and hence, maintenance costs are usually low. Extensive research has been channelled into ways of improving the performance of adsorption chillers, as well as lowering of the heat source temperatures by using multistage cycles [4].

On the macro-scale, studies have been made on adsorption chillers to improve the conversion efficiency and system performance (i.e. Coefficient of Performance or COP). Saha et al. [5] proposed a multi-bed and multi-stage scheme for thermal compression of the refrigerant and such an operational scheme enables the desorption temperature to be lowered to 55 ⁰ C. Chua et al. [6], on the other hand, proposed a single-stage multi-bed system to enhance the heat extraction capability of adsorption cycle and the heat source temperature can be reduced to 65 ⁰ C.

To further improve the COP of an adsorption heat pump in an adsorption chiller, Pons et al. [7] employs longer cycle-time processes where heat propagates slower, and hence experiences slower thermal gradients across adsorbent beds of the adsorption chiller, which minimizes dissipative losses. To render the scheme effective, Pons et. al. proposed using a higher temperature heat source and the COP of the cycle is reported to be about unity. Alternatively, the adsorption cycle is proposed to operate at a low temperature heat source, typically less than 85 ⁰ C, and the cycle employs the silica gel-water pair as the working adsorbent-adsorbate pair. Such designs tend to have two characteristics [8]: firstly, the optimum design region hovers about the point of maximum cooling, and hence, minimizes the initial capital payback period of chillers. Secondly, the associated chiller cycles have shorter cycle times. Although the COP of heat-driven chillers is obstinately low by comparison with the work-driven machines, the performance of adsorption chillers can be improved with suitable energy recovery schemes, in particular during the switching time intervals [9].

On the micro or mini scale, Ng et al [1] proposed an electro-adsorption chiller (EAC), which is predicated on bi-directional flows of current to a thermoelectric that is sandwiched between desorption and adsorption beds, providing concomitantly, the necessary heating and cooling processes. However, the EAC as proposed has a number of limitations: Firstly, low efficiency of the thermoelectric generates unbalance flows of energy or heat between the thermoelectric's hot and cold junctions and consequently, cooling rates at the adsorber bed is much lower as compared to the heating rates of the desorber.

Secondly, the EACs working fluid operates in semi-vacuum or low pressure, making the cycle susceptible to air leakages from ambient.

It is an object of the invention to provide a pressurized-cycle based electro- adsorption chiller (PEAC) which addresses at least one of the disadvantages of the prior art.

Summary of the Invention

In a first aspect of the invention, there is provided a pressurized-cycle based electro-adsorption chiller (PEAC) comprising: a condenser for cooling refrigerant; an evaporator for dissipating heat from a location; two or more reactors in fluid communication with the condenser and the evaporator, each of the reactor comprising an amount of absorbent and capable of operating in adsorption and desorption modes; and two or more thermoelectric devices, each thermoelectric device being received within a corresponding reactor so that each device is in thermal communication with the adsorbent of the reactor.

Since the thermoelectric device is placed within the reactor device, there is practically little thermal-mass effect from reactor's cover plate(s) and thus improves the performance of the chiller.

Preferably, each of the two or more thermoelectric device is connected to each heat exchanger assembly containing the adsorbent within the reactor.

Advantageously, the chiller further comprises an energy storage for storing heat energy from the reactors. . It is preferred that the energy storage device is disposed between two of the reactors. Also, the energy storage may be arranged immediately adjacent to an external surface of a wail of a said reactor, and if so, then one of the thermoelectric device is arranged immediately adjacent to an inner surface of said wall. With the above arrangement, the thermoelectric devices and the storage medium can be operated in tandem to enhance energy regeneration across the reactor plates and a pair of thermoelectrics. Also, the heat energy stored in the medium can be employed to raise the temperature of the cold junctions of thermoelectric devices and maintain a lower temperature difference across the hot and cold junctions thus, reduces power consumption.

Preferably, the chiller's adsorbent-adsorbate pair comprises activated carbon-n- butane (AC-C ₄ H ₁₀ ) and this allows the chiller pressure to be operated above that of ambient pressure throughout the batch-operated cycle. Since the evaporator and condenser pressures are preferably about 1.2 bars and 6 bars respectively, which corresponds to effective evaporator temperatures and condensing temperatures obtained by ambient air cooling, a pressurized cycle of the chiller eliminates a need for vacuum equipment which makes the chiller almost maintenance free.

The thermoelectric devices 106,107 may be "fired up" independently by controlling the power supply. Thus, the two or more thermoelectric devices may be operable simultaneously or one at a time (i.e. timed and alternating manner). If fast cooling is required, then all the thermoelectric devices may be operated simultaneously but this would result in a low COP. If on the other hand, the rate of cooling is not of utmost concern, then selectively operating one but not the other would result in a higher COP of operation of the pressurized-cycle based EAC.

The chiller may comprise means for removing heat form the adsorbent of the reactors, particularly from the reactor configured in adsorption mode. In the preferred embodiment, the heat removal means comprises cooling channels for circulating a coolant through the adsorbents.

If the chiller includes the energy storage device, then the chiller may also comprise cooling channels for circulating a coolant through the energy storage device.

The present invention also relates to a method of removing heat from a location using the chiller herein described and which forms further aspects of the present invention. In one further aspect, heat is dissipated from the reactor(s) via the cooling channels and in another further aspect, heat is dissipated from the reactor(s) via the corresponding thermoelectric device to the energy storage device.

In another further aspect of the invention, there is provided a pressurised-cycle based electro-adsorption chiller comprising: a condenser for cooling refrigerant; an evaporator for dissipating heat from a location; two or more reactors in fluid communication with the condenser and the evaporator, each reactor comprising an amount of absorbent and capable of operating in adsorption and desorption modes; one or more thermoelectric device, a said thermoelectric device being arranged between two said reactors so that the device is in thermal communication with the absorbent of the reactors; and cooling channels for circulating a coolant through the adsorbents to remove heat therefrom.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which,

Figure 1 is a schematic representation of a pressurized-cycle based electro- adsorption chiller (PEAC) according to a preferred embodiment of the invention which illustrates the chiller operating in a first half-cycle; and

Figure 2 is the pressurized-cycle based electro-adsorption chiller of Figure 1 operating in a second half-cycle.

Detailed Description of the Preferred Embodiment

Figure 1 is a schematic representation of a pressurized-cycle based electro- adsorption chiller 100 according to a preferred embodiment of the invention which comprises an evaporator 102, two reactors 104,105, thermoelectric devices 106,107 and a condenser 108. The evaporator 102 is arranged to be coupled to a heat source Q| _0aC ι to dissipate heat therefrom, for example, a surface of an electronic device. In this embodiment, each of the reactor device 104,105 comprises a reaction chamber 112,114 which receives a corresponding thermoelectric device 106,107, and a first pipe 116 and a first on- off valve 118 connect the evaporator 102 to the first chamber 112 and a second pipe 120 and a second on-off valve 122 connect the evaporator 102 to the second chamber 114.

Each of the chambers 112,114 comprises a pre-determined amount of adsorbent 124,126 housed in a series of fins 128, 130 with one ends of the fins connected to a perforated stainless steel mesh 132,134. Each reactor device 104,105 also includes cooling channels 181 ,182 running through the adsorbent 124,126.

As shown in Figure 1 , each thermoelectric device 106,107 is placed within a corresponding reactor device 112,114 so that they are in direct thermal communication with the adsorbent 124,126 of the reactor device. The reactor devices 112,114 are spaced from each other and sandwiched therebetween is an energy storage device 140 with each thermoelectric device 106,107 arranged immediately adjacent to the energy storage device 140 (separated by a chamber wall 136,138 of the reactors 112,114) for efficient heat transfer.

In the present embodiment, the energy storage device 140 includes a housing 142 made of metal, and flow channels (not shown but similar in structure to the flow channels 181 ,182 for the adsorbents 124,126) embedded within to provide uniform flow of coolant. Since the thermoelectric device 107 (and 106) and the energy storage device 140 are arranged immediately on either side of the corresponding chamber wall 136 (and 138), heat can propagate between them with greater efficiency. Further, the energy storage device 140 is coupled to reaction chambers 112,114 via pipes 150,152 and on-off valves 154,156,158,160,162,164.

The thermoelectric devices 106,107 are connected to a DC power supply 144 via leads 146,148 which supply DC current to the device 106 during a designated cycle. Depending on the operation mode of the reactor device 104 in a batch-operated cycle, the direction of the DC current can be reversed depending on which of the adsorption chamber 112,114 is functioning as an adsorber or desorber.

As the thermoelectric devices 106,107 alone may only provide partial cooling during the adsorption process, in this embodiment, further external cooling is supplied through a re-circulating stream of coolant, and in this embodiment, water is used, that provide the regenerative as well as the rejection of heat from the adsorption process to the ambient environment. In this respect, the chiller 100 further includes an air-water exchanger 166 connected to the cooling channels 181 ,182 and the cooling channel embedded within the energy storage device 140 via a pump 168 and valves 156, 167,169 with the pump operable to supply a coolant from the exchanger 166 to cool the adsorbents 124,126 in the adsorption chambers 112,114. The coolant that is returned to the exchanger 166 carries with it heat from the reaction chambers 112,114 which is given off as heat Q _∞0 |. The operation of valves is synchronised with the designation of reactors, eg. valve 167 is opened (and valve 154 is closed) when 112 is designated as adsorber. Concomitantly, valves 158 and 169 are closed when reactor 114 is designated as desorber.

The adsorption chambers 112,114 are connected to the condenser 108 via pipes 170,172 and on-off valves 174,176 so as to channel refrigerant (vapour) to the condenser 108 for dissipation (as Q _re j _ect ) away from the heat source Q| _Oad . The condenser 108 is connected to the evaporator 102 via a feedback path 178 and pressure-modulated needle valve 180 to re-circulate the refrigerant (liquid). The condenser 108 may further include an air-cooled fan or by circulation of a coolant fluid into a heat exchanger within the condenser 108.

In this embodiment, the PEAC chiller 100 uses an adsorbent-adsorbate pair of activated carbon-n-butane (AC-C ₄ Hi ₀ ). This combination is considered to be most suitable as the evaporator and condenser pressures are preferably about 1.2 bars and about 5-6 bars respectively, which corresponds to the effective freezing and condensing temperatures obtained by amb'ient air cooling. With this, a pressurised cycle of the chiller eliminates a need for vacuum equipment which makes the chiller almost maintenance free.

An operation of the chiller 100 will now be described by referring to figure 1 with the first reaction chamber 112 configured in adsorption mode (and thus suitably called an adsorption chamber) and the second reaction chamber 114 configured in desorption mode (and thus suitably called a desorption chamber).

The evaporator 102 draws heat from the heat source Qi _oad and after sufficient heat transfer has taken place, boiling of the adsorbate (or refrigerant and in this embodiment, this is in the form of liquid butane) takes place within the evaporator 102 which is pressurised at about 1.2 bars. With the on-off valve 118 opened (shown in Figure 1 as unshaded) and the on-off valves 122,174 closed (shown in Figure 1 as shaded), the generated vapour flows into the first adsorption chamber 112 via the pipe 116. The absorbent 124 adsorbs the vaporised refrigerant and heat generated by the exothermic process is rejected by fins 128, cooling water channels 181 and the thermoelectric 136. The hot junction of the thermoelectric 136 thus rejects heat into the energy storage device 140.

The DC power unit 144 supplies a DC voltage to the thermoelectric devices 106,107 which supply a fraction of cooling requirement Q _ad but the full heating requirement of Q _de of the adsorbent beds respectively. For high COP, no power is supplied to the thermoelectric device 107, whilst chamber cooling is carried out by the coolant from the air-water exchanger 166.

During switching, regeneration of the residual water that remains in the cooling channels are alternated to conserve electricity during desorption cycle and this is achieved by opening valves 169, 162, 154 and closing valves 158, 156, 160, 164 167,169 in the re-circulating coolant circuit. Although active cooling (from thermoelectrics 106,107) can be supplied to the designated adsorption chamber but this procedure may not be energy efficient. The energy storage device 140 which is sandwiched between the reactors 112, 114 acts as a heat store and it is in thermal communication to the circulating coolant and the active unit of thermoelectrics.

With valve 174 open (and valve 118 closed), vapour flows from the adsorption chamber 112 and into the condenser 108, pressurised at about 5-6 bars, under the effect of a positive pressure gradient. Next, the valves 118, 122, 174, 176 are closed and the reactors 112, 114 are now isolated from the condenser 108 and evaporator 102. The condensate inside the condenser 108 flows back to the evaporator 102 via the needle valve 180 which maintains different pressures inside the evaporator 102 and condenser 108.

In another arrangement of the embodiment, the roles of the chambers 112,114 are reversed, that is, the adsorption chamber now becomes a desorber, and the desorption chamber is configured as a adsorber, as shown in Fig. 1 (b). Concomitantly, the valve 176 connecting the adsorption chamber 114 to the air or coolant cooled condenser 108 is closed, whilst the valve 122 is opened to the pipe 120 linking the adsorption chamber 114 to the evaporator 102. Similarly, valve 118 isolates the link between the desorption chamber 112 and the evaporator 102. The valve 122 that controls the pipe between the adsorption chamber 114 and the evaporator 102 is timed to open to enable vapour to be adsorb by the adsorbent 126.

It should be noted that the powered thermoelectric 107 is set in tandem with its designated desorption process of reactor 112, with the other thermoelectric 106 on the opposite side of the energy storage device 140 either set to idle (for high overall efficiency) or be powered to cool the designated adsorption process of the reactor 114. In other words, the chiller 100 may be configured such that only one of the two thermoelectric pair is fired in tandem with the designated desorption cycle whilst concomitantly, the other thermoelectric unit remains either in an idle (consume no power) condition or alternatively powered with reverse polarity to boost cooling of the designated adsorber reactor. When the processes in the beds are alternated, then power to the thermoelectric pair 106,107 is switched correspondingly. Cooling of the adsorption process is further boosted by the pump 168 where a stream of coolant removes the excess heat from adsorption process could be rejected to the ambient via the air-water heat exchanger 166.

It is noted that regeneration of energy from the storage device 140 is most effective in the switching period as it is assisted by the favourable thermal gradients. When the thermal lift across the hot and cold junctions of a thermoelectric device 106,107 is large, regeneration becomes less effective. As both pairs of thermo-electrics 136, 138 are decoupled from the reactor covers 112, 114 by the energy store 140, current demand patterns of thermoelectric would be self-adjusting with respect to the temperatures of the beds 112, 114 and the sandwiched energy store 140.

From the described embodiment, it can be understood that numerous advantages can be achieved. Since the thermoelectric devices 106,107 are placed within the reactors 112,114 and placed in direct thermal contact with the adsorbents 124,126, there is practically little thermal-mass effect from the chamber walls of the reactors 112, 114 and thus improves the performance of the chiller. Further, the thermoelectric pairs 106,107 and the storage device 140 can be operated in tandem to enhance energy regeneration . Also, the heat energy stored in the device 140 can be employed to reduce the thermal lift (the hot junction and cold junction temperatures) of the thermoelectric devices and thus reduces power consumption. The use of activated carbon-n-butane (AC- C ₄ Hio) as the adsorbent-adsorbate pair allows the chiller pressure to be operated above that of ambient pressure. Since the evaporator and condenser pressures are preferably about 1.2 bars and 5-6 bars respectively, which

corresponds to effective evaporating (about 3-5 ⁰ C) and condensing temperatures obtained by ambient air cooling, a pressurized cycle of the chiller eliminates a need for vacuum equipment for the regeneration process which makes the chiller almost maintenance free.

Another unique feature of the present invention is the independent firing or power supply to the thermoelectric devices 106,107, namely either to both thermoelectric devices 106,107 simultaneously or only to a selected thermoelectric device 106 or 107 attached with the designated desorber, etc., and this permits the fast cooling or_high COP operation of the pressurized-cycle based EAC.

The described embodiment should not be construed as limitative. For example, in the preferred embodiment, for high cooling efficiency, the reactors operate in pairs and one thermoelectric device per reactor. Other arrangements are also possible. Further, the option to fire both thermoelectric devices 106,107 or only firing one depends if high efficiency is needed. Firing both thermoelectric devices would expedite the cycle operation with a slightly shorter cycle time.

In the preferred embodiment, the chiller 100 includes the energy storage device 140 but the storage device is not absolutely necessary. Without the energy storage device, this would result in a lower efficiency for the chiller.

It is preferred for the chiller 100 to include the external cooling system comprising the heat exchanger 166 and the cooling channels embedded in the

adsorbents 124,126 and the heat storage device 140 (if present), but not absolutely necessary. However, the omission of the external cooling system may render the PEAC not working properly in a long run because the fins 128,130 and the thermoelectric devices might not be able to dissipate the heat sufficiently and the temperature in the reactor devices 112,114 might rise with time to unacceptable levels. Also, it is envisaged that the cooling channels 181 ,182 and the air-water heat exchanger 166 may be employed for in a chiller having just one thermoelectric device, similar to the preferred configuration discussed in [1], the disclosure of which is incorporated herein by reference.

Having now fully described the invention, it should be apparent to one of ordinary skill in the art that many modifications can be made hereto without departing from the scope as claimed.

References

[1] Ng KC, JM Gordon, HT Chua and Chakraborty A, An electro- adsorption chiller: a miniaturized cooling cycle with applications from microelectronics to conventional air conditioning, USPO 6434955 (2002).

[2] Yonezawa Y, Matsushita M, Oku K, Nakano H, Okumura S, Yoshihara M, Sakai A and Morikawa A, Adsorption refrigeration system. US patent no. 4881376, 1989

[3] Yonezawa Y, Ohnishi T, Okumura S, Sakai A and Nakano H, Matsushita M, Morikawa A and Yoshihara M, Method of operating adsorption refrigerator. US patent no. 5024064, 1991 [4] KC Ng, Experimental and prototype development of multi-bed regenerative adsorption chiller using industrial waste heat and renewable energy source, Final Report, WBS no: 265-000-083- 112/303/592, ME Dept, National University of Singapore, 2000. [5] Saha, B. B, EC Boelman and T Kashiwagi, Computational analysis of an advanced adsorption-refrigeration cycle, Energy, Vol. 20(10): 983- 994, 1995

[6] HT Chua, KC Ng, A. Malek, T. Kashiwagi, A. Akisawa and B. B. Saha, Multi-reactor regenerative adsorption chiller, Int. J. Refrigeration., Vol. 24:124-136, 2001

[7] Pons M, Laurent D and Meunier F. Experimental temperature fronts for adsorptive heat pump applications. Appl Thermal Engng, Vol. 16(5):395-404, 1996

[8] Ng KC, Recent developments in heat-driven silica gel-water adsorption chiller, Editorial, Heat Transfer Engineering, 24(3): 1-3, 2003.

[9] A. Akahira, K.C.A Alam, Y. Hamamoto, A. Akisawa and T. Kashiwagi, Mass recovery adsorption refrigeration cycle - improving cooling capacity, Int. J. Refrigeration., Vol. 27, pp: 225-234, (2004).