US20090095012A1

US20090095012A1 - Double-effect adsorption refrigeration device

Info

Publication number: US20090095012A1
Application number: US11/974,307
Authority: US
Inventors: Atsushi Akisawa; Takao Kashiwagi
Original assignee: Individual
Current assignee: Tokyo University of Agriculture and Technology NUC
Priority date: 2007-10-12
Filing date: 2007-10-12
Publication date: 2009-04-16

Abstract

The present invention relates to an adsorption chiller using double-effect cycle utilizing middle driving heat source of 100-150 degree centigrade. The cycle consists of two cycles such as high temperature cycle (HTC) and low temperature cycle (LTC). Zeolite-water system and silica gel-water system are used as adsorbent-adsorbate pairs of HTC and LTC respectively. Waste heat of zeolite generated at during adsorption period of the zeolite is re-used for pre-heating and desorbing the silica gel.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a device for refrigeration by evaporation and adsorption, whose principle consists in evaporating a liquid, that is, water in the present invention, under the effect of a depression sustained by adsorption of the vapors of said liquid. The present invention also relates to a refrigeration device utilizing a double-effect adsorption cycle in which a high temperature cycle (HTC) and low temperature cycle (LTC) is operated in particular interval range.
2. Description of the Prior Art
The principle of these types of refrigeration by evaporation of a refrigeration liquid and adsorption of vapor of this liquid has been well known and various types of devices have also been developed. A big obstacle to the development of adsorption cycle technology is its low coefficient of performance. Many advanced cycles have been proposed, focusing on improving the performance of the adsorption refrigeration system. For example, in Japanese laid open patent No. 1990-230068 discloses a refrigeration device having a pair of beds, each of which consists of solid adsorbent material such as Silica gel, Zeolite or activated carbon. Several sobers used in developing adsorption refrigeration cycle are silica gel or zeolite with water acts as refrigerant. Silica gel-water pair is widely used and able to produce refrigerating effect at heat source temperature below 100 degree centigrade. Several prior art or literature such as Y, Liu, and K. C. Leong, Applied Thermal Engineering 25(2004), and G. Magio, A. Freni, and G. Restuccia, International Journal of Refrigeration 29 (4) (2006) show that Zeolite-water pair is also able to produce refrigerating effect. However, heat source temperature needed in the refrigeration device described in the prior art is above 150 degree centigrade. On the other hand, according to the research of Ishibashi et al. (2004) (K. Ishibashi, K. Sato, Y. Ito, M. Harada, and M. Nakano., JSRAE, Technical, Shikaku Division, 2004), HPA zeolite is potential to be used as an adsorbent material for adsorption refrigeration device using heat source temperature lower than 150 degree centigrade.
In those prior arts, however, a double-effect adsorption refrigerating device operating in a temperature between 100 degree centigrade to 150 degree centigrade with a high performance is not shown. Particularly, in a conventional device, waste heat of around 120 degree centigrade of steam can not be used, thus the waste heat of the steam is lowered to hot water of 90 degree centigrade and then used.

SUMMARY OF THE INVENTION

The aim of the present invention is to overcome the disadvantage of the prior art. For that purpose, present invention proposes a double-effect adsorption cycle. More particularly, the double-effect adsorption cycle of the present invention consists of two cycles, one of which is High Temperature Cycle (HTC) and other is Low Temperature Cycle (LTC). Further more, in the present invention, Zeolite-water system is used for the high temperature cycle (HTC) operation and Silica gel-Water system is used for the low temperature cycle (LTC) operation, and adsorption heat of the zeolite is transferred and reused as heat source of the Silica gel-Water system.
According to the present invention, waste heat of 100 to 150 degree centigrade which is supplied from an external heat source is directly used without lowering the temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The particular features and advantages of the present invention will clearly be understood from the following description and drawings appended herewith, in which:

FIG. 1 is a schematic view of the double-effect adsorption refrigerating device of the present invention.

FIGS. 2 a and 2 b are schematic diagram showing an first stage or cycle of the operation of the present invention.

FIGS. 3 a and 3 b are schematic diagram showing an second stage or cycle of the operation of the present invention.

FIGS. 4 a and 4 b are schematic diagram showing an third stage or cycle of the operation of the present invention.

FIGS. 5 a and 5 b are schematic diagram showing an fourth stage or cycle of the operation of the present invention.

FIG. 6 shows a characteristic curve of an influence of heat source temperature on performance.

FIG. 7 shows a characteristic curve of an influence of cycle time on performance.

FIG. 8 shows a characteristic curve of an influence of adsorbent mass ratio on performance.

FIG. 9 shows a characteristic curve showing an influence of time ratio on performance.

MORE DETAILED DESCRIPTION

The present invention proposes a double-effect adsorption refrigerating device having a high Coefficient of Performance (COP) close to 1 while a COP of the conventional device is around 0.7.
FIG. 1 shows an example of double-effect adsorption refrigerating device 1 of the present invention in a simple manner but a person skilled in this art could easily understand an actual refrigerating device from the description of the present invention.
Referring to FIG. 1, the refrigerating device of the present invention includes a first and second bed 11 and 12 consisting of adsorbent materials of zeolite 110 and silica gel 120 respectively. First bed 11 is designed and used in a high temperature adsorption cycle (HTC), while the second bed 12 is designed and used in a low temperature adsorption cycle (LTC).
In the double-effect cycle, the high temperature adsorption cycle (HTC) is used as the driving heat for the low temperature adsorption cycle (LTC).
Back to the FIG. 1, a heater 13 is connected to first bed 11 through an appropriate device having a flow control device (hereinafter referred to as faucet) 18, so that an external waste heat is used for heating the first bed 11. In this particular case, the waste heat is obtained from steam a temperature of which is 100-150 degree centigrade, preferably around 120 degree centigrade. A second bed 12 is connected to cooling device 14 through an appropriate device having a faucet 22 for cooling the second bed 12. A condenser 15 is connected to first bed 11 by an appropriate device 25 having a faucet 19 so as to be supplied with an evaporated refrigerant, for example water in this case, from the first bed 11, and also connected to second bed 12 by an appropriate device 26 having a faucet 20 so as to be supplied with the evaporated refrigerant from the second bed 12. An evaporator 16 is connected to first bed 11 through a faucet 23 so that the evaporated refrigerant is supplied to the first bed 11 from the evaporator 16, and is also connected to second bed 12 through a faucet 24 so as to supply the evaporated refrigerant from the evaporator 16 to second bed 12. An actual refrigerating device 17 is for example, an air conditioning device in connection with the evaporator 16 as a load of this device 1 shown in FIG. 1, but not limited therein, however this device 17 is not an essential in the present invention.
A fluid supply device 27 having a faucet 21 and pump 30 is provided so that a waste heat generated by the first bed 11 during its pre-cooling and adsorption cycle is transferred to the second bed 12 for pre-heating and desorbing the refrigerant.
FIG. 2 a shows an schematic diagram and indicates a initial cycle stage of the double-effect adsorption refrigerating device shown in FIG. 1. Each number shown in FIG. 2 a for indicating a function block is corresponding to a same number shown in FIG. 1. FIG. 2 b is a schematic diagram provided for indicating an operation of the double-effect adsorption refrigerating device of the present invention together with a pressure and temperature of one of beds 11 and 12. Referring to FIG. 2 a and 2 b, two adsorbents proceed through steady state process 1-2-3-4 in the high temperature cycle (HTC) and 1′-2′-3′-4′ in the low temperature cycle (LTC). The cycle begins with pre-heating process (1-2) for zeolite adsorbent 110 and pre-cooling process (3′-4′) for silica gel adsorbent 120 and this stage is defined as cycle A. In this cycle A, steam having a temperature around 120 degree centigrade heated by external heat source (not shown) is provided through faucet 18 to the first bed 11, thus the external heat is transferred to zeolite adsorbent 110 for pre-heating, while cool water is supplied through the faucet 22 to second bed 12 for pre-cooling.
In this cycle A, referring to FIG. 1, the faucets 13 and 14 are opened and another faucets 19, 20, 21, 23 and 24 are closed. After a pressure of evaporator (Pe) of the zeolite reaches to a pressure of condenser (Pc), and a pressure of condenser (Pc) of the silica gel reaches to a pressure of evaporator (Pe), then the process goes into a next stage, that is, cycle B as shown in FIGS. 3 a and 3 b. Referring to FIGS. 3 a and 3 b, when the pressure of zeolite adsorbent (110) reaches the pressure of condenser (Pc), the first (zeolite) bed 11 is connected to condenser 15 for desorption-condensation process (2-3). At the same time, when the pressure of the second (silica gel) bed 12 reaches the pressure of evaporator (Pe), the second (silica gel) bed 12 is connected to evaporator 16 for adsorption-evaporation Process (4′-1′). The refrigerant (water) will be released from the first bed 11 (zeolite) and liquefied in the condenser 15 and then flow to the evaporator 16 through an appropriate supply device (tube) 28. The refrigerant in the evaporator 16 is evaporated and supplied to the second bed 12 (silica gel) through faucet 24 so as to be adsorbed by the silica gel adsorbent (cycle B). In this cycle B, the faucet 19 and 24 shown in FIG. 1 are opened, thus in this cycle B, the load (refrigeration device) 17 works as refrigerator. After the cycle B completed, then next process (cycle C) starts. Referring to FIGS. 4 a and 4 b, this process is pre-cooling (3-4) process at the first (zeolite) bed 11 and pre-heating (1′-2′) process at the second (silica gel) bed 12. In this process, the pressure of zeolite will be reduced from condenser pressure (Pc) to evaporator pressure (Pe) while the pressure of the silica gel adsorbent will be expanded from evaporator pressure (Pe) to condenser pressure (Pc). For this cycle operation, temperature of the zeolite adsorbent should be decreased and temperature of the silica gel adsorbent should be increased. In this cycle stage, referring to FIG. 1, the faucet 13 is closed to stop a supply of the external heat source such as steam to the first (zeolite) bed 11. The faucet 22 is also closed to stop a supply of the cool water to the second (silica gel) bed 12. Then the faucet 21 is opened and the pump 30 starts to operate so that the heat from the zeolite adsorbent is transferred to silica gel adsorbent through the circulation of heat exchange fluid. In this cycle, heat generated by the zeolite is re-used for pre-heating the silica gel.
In this cycle C, the faucet 18, 19, 20, 22, 23, and 24 are closed and only faucet 21 is opened. A detail control system or device itself of the faucets are not shown because it can be adequately designed by a person skilled in this art if he or she understands the feature of the present invention.
After the cycle C completed, next cycle (cycle D) starts. Referring to FIG. 1, FIG. 5 a and 5 b, the faucets 20 and 23 are re-opened in addition to the faucet 21 thus first bed 11 is connected to evaporator 16 and the second bed 12 is connected to the condenser 15. Another faucets 18, 19, 22 and 24 are closed.
As shown in FIGS. 5 a and 5 b, zeolite 110 adsorbs the evaporated refrigerant thus zeolite works as the adsorbent material while the silica gel 120 desorbs the refrigerant and provides the same to the condenser 15 as evaporated manner. In this cycle (cycle D), the evaporator 16, thus the load 17, works for a refrigerating device.
After the cycle D, the double-effect adsorption device 1 returns to the initial stage of cycle A.
From the above description, it is understood that the zeolite operates as adsorber and desorber alternatively and the silica gel also operates as desorber and adsorber alternatively.
Numerical analysis of the above described operation may be useful for understanding the advantage and good performance of the present invention.
The energy balance for adsorber/desorber can be written as;
$\begin{matrix} \frac{\partial}{\partial t} {(W_{s, z} C_{s, z} + W_{s, z} C_{w} q_{s, z} + W_{hex} C_{hex}) T_{s, z}} = W_{s, z} Q_{{st}_{s, z}} \frac{\partial q_{z, s}}{\partial t} - δ W_{z, s} C_{v} {(T_{z, s} - T_{e})} \frac{\partial q_{z, s}}{\partial t} + Q_{s, zin} & (1) \end{matrix}$
where δ is either 0 or 1, depends on whether the bed 11 or bed 12 is working as desorber or adsorber. In the equation (1),

C_s,zis a specific heat capacity of silica gel or zeolite,
C_wis a specific heat capacity of water,
q_s,zis an amount of water adsorbed by the silica gel or zeolite,
W_hexis a weight of a heat exchanger,
C_hexis a specific heat capacity of the heat exchanger,
T_s,zis temperature of silica gel or zeolite,
Q_{st s,z}is heat generated by the silica gel or zeolite when silica gel or zeolite is in an adsorbent cycle,
C_vis specific heat capacity of steam, and
Q_{s,z in}is an amount of input heat for silica gel or zeolite.
W_s,zrefers to mass of adsorbent of silica gel or zeolite respectively.

Equation 1 is expressed together with a case of silica gel and zeolite in order to reduce a number of equation.
The mass of silica gel (W_s) can be expressed as;
W_s=k_mW_z (2)
where k_mis the ratio between mass of silica gel and zeolite.
During desorption process of cycle B, the zeolite is heated by the external heat source. So the heat input is given as;
$\begin{matrix} \begin{matrix} Q_{s - in} = {\dot{m}}_{oil} {Cp}_{oil} (T_{in, s} - T_{out, s}) \\ = {\dot{m}}_{oil} {Cp}_{oil} (T_{in, s} - T_{s}) [1 - \exp (- {NTU}_{s})] \end{matrix} & (3) \\ \begin{matrix} Q_{z - in} = {\dot{m}}_{oil} {Cp}_{oil} (T_{in, z} - T_{out, z}) \\ = {\dot{m}}_{oil} {Cp}_{oil} (T_{in, z} - T_{z}) [1 - \exp (- {NTU}_{z})] \end{matrix} & (4) \end{matrix}$
During heat recovery process as cycle C shown in FIG. 4 a, the heat exchange fluid, for example oil, circulates between first and second bed 11 and 12. If heat loss and sensible heat of the fluid are neglected, hence the input temperature of the first bed 11 (T_{in s}) should be equal to outlet temperature of the second bed 12 (T_{out z}). Thus T_{in s}and T_{out z}can be written as;
T _in,s =T _out,z =T _s+(T _in,s −T _s)exp(−NTU _s) (5)
T _in,z =T _out,s =T _z+(T _in,z −T _z)exp(−NTU _z) (6)
The sum of heat input of first bed 11 and second bed 12 can be expressed as;
Q _z-in +Q _s-in=0 (7)
Substitute equation 5, 6 to equation 3, 4 so that the temperature input of the first bed 11 can be written as;
$\begin{matrix} T_{in, z} = \frac{T_{z} \exp (- {NTU}_{s}) [1 - \exp (- {NTU}_{z})] + T_{s} [1 - \exp (- {NTU}_{s})]}{1 - \exp (- {NTU}_{z}) \exp (- {NTU}_{s})} & (8) \end{matrix}$
The energy balance for condenser can be written as
$\begin{matrix} \frac{\partial}{\partial t} {(W_{cw} C_{w} + W_{c, hex} C_{c, hex}) T_{c}} = (1 - δ_{z}) (- L - C_{v} (T_{des, z} - T_{c})) W_{z} \frac{\partial q_{des, z}}{\partial t} + (1 - δ_{s}) (- L - C_{v} (T_{des, s} - T_{c})) W_{s} \frac{\partial q_{des, s}}{\partial t} + {\dot{m}}_{cw} C_{w} (T_{cw, in} - T_{cw, out}) & (9) \end{matrix}$
In this equation (9), the value δ_zor δ_sare either 0 or 1, and it depends on whether or not the adsorbent, that is zeolite or silica gel, is connected to condenser. For example, when the first bed 11 or second bed 12 is connected to the condenser 15, δ_z=0, and δ_s=0. The energy balance for evaporator can be as
$\begin{matrix} \frac{\partial}{\partial t} {(W_{ew} C_{w} + W_{e, hex} C_{e, hex}) T_{e}} = (- L - C_{w} (T_{c} - T_{e})) (γ_{z} W_{z} \frac{\partial q_{ads, z}}{\partial t} + γ_{s} W_{s} \frac{\partial q_{des, s}}{\partial t}) + {\dot{m}}_{chill} C_{w} (T_{chill, in} - T_{chill, out}) & (10) \end{matrix}$
wherein, γ_zor γ_sis either 0 or 1, and it also depends on whether or not the adsorbent, that is zeolite or silica gel, is connected to evaporator. If the first bed 11 or second bed 12 is connected to the evaporator 17, γ_zor γ_sis 1.
In the present invention, two different adsorbent, one is zeolite and other is silica gel, are used alternatively for adsorption and desorption respectively in the operation cycle, thus the optimum adsorption-desorption rate of each adsorbent can be arranged by adjusting desorption time of zeolite and adsorption time of silica gel (cycle B) and adsorption time of zeolite and desorption time of silica gel (cycle D).
The ratio time allows gaining optimum setting time in cycle B and D if pre-heating and pre-cooling time is fixed. The equation can be written as
$\begin{matrix} k_{time} = \frac{{ads}_{time} (CycleD)}{{des}_{time} (CycleB)} & (11) \end{matrix}$
A value of coefficient of performance (COP) is the most important and interesting index in this invention. The value shows an efficiency of the refrigerating device of the present invention. During heating mode (cycle A), the heat input from the heat source, that is, heat supplied through steam providing devise 13 in FIG. 1 to zeolite 110 can be estimated as
$\begin{matrix} Q_{in} = m_{hot} C_{w} \int_{0}^{tcycle} (T_{hot, in} - T_{hot, out}) \partial t & (12) \end{matrix}$
and heat released by the evaporator 116 that works as a cooling device can be written as
$\begin{matrix} Q_{eva} = m_{chil} {lC}_{w} \int_{0}^{tcycle} (T_{chill, in} - T_{chill, out}) \partial t & (13) \end{matrix}$
Therefore, coefficient of performance (COP) can be written as ratio between the heat released by evaporator and the heat input from the heat source. An equation of the COP can be written as
$\begin{matrix} COP = \frac{Q_{eva}}{Q_{in}} & (14) \end{matrix}$
Another major index for expressing the cooling device of the present invention is a specific cooling power (SCP). A value of specific cooling power measures chilling capacity to produce cooling effect in its relation with amount of adsorbent used.
The equation of SCP can be expressed as
$\begin{matrix} SCP = \frac{Q_{eva}}{t_{cycle} M_{s}} & (15) \end{matrix}$
FIG. 6 shows a characteristic curve of COP and SCP in relation to heat source temperature under a condition of fixed cycle time of 1200 second. As shown in FIG. 6, the SCP increases as heat source temperature increases thus it shows that the SCP improves with the increase of heat source temperature.
The same observation may be done on the COP. As shown in FIG. 6, the COP increases until certain point of the heat source temperature that is almost 120 and then decreases as the heat source temperature decreases. It can be obtained from FIG. 6 that maximum value of COP reaches at the heat source temperature around 120 degree centigrade.
FIG. 7 shows a characteristic curve of COP and SCP in relation to the cycle time. It can be seen that longer cycle time produces better COP value but the SCP value decreases slightly
If the cooling device of the present invention is operated at shorter cycle time, heat input to bed 11 is relatively higher than cooling output thus it causes low COP.
FIG. 3 also shows that there is an optimum cycle time to produce a maximum SCP value. As shown in FIG. 7, the maximum SCP value obtained at heat source temperature 130 degree centigrade is in interval range of 900 to 1200 cycle time and the SCP value decreases drastically if the cycle time is longer than 1200 second. From the Equation 15, it can also be explained that the cycle time is strongly engaged in the SCP value, that is, longer cycle time causes low SCP value. As mentioned above, the cycle uses two different adsorbents, with different properties and adsorption capacity. For standard condition, effect of distribution adsorbent mass and time ratio are presented in FIG. 8 and FIG. 9. FIG. 8 shows a characteristic curve of COP and SCP value in relation to a mass adsorbent ratio, and FIG. 9 shows also COP and SCP value in relation to the time ratio described in equation 11. As shown in FIG. 8 and FIG. 9, a change of COP value of the cycle is relatively stable in the range of 0.8-1. However, it can be seen that mass adsorbent ratio and time ratio are dominant factor on SCP value.
From FIG. 8, the optimum mass ratio of two types of adsorbents is observed at 0.8. If total mass of adsorbent (zeolite+silica gel) is 28 kg, the optimum mass of zeolite and silica gel is 15.6 kg and 12.4 kg respectively. If the zeolite adsorbent mass is larger than optimum amount, more water vapor will be released. However, the ability of silica gel adsorbent for adsorption is low because of low amount of silica gel adsorbent. Besides that, more heat input is needed thus will cause lower COP. Therefore, the ratio of adsorbent mass should be optimum to produce greater cooling effect with less heat input. The same trend also observed in the effect of time ratio. The optimum value on performance is observed in the range of 0.6-0.8 as shown in FIG. 9. Since two abadsorbents work simultaneously in opposite process for each cycle, desorption or adsorption rate should be adjusted to obtain optimum performance. If the time ratio is higher than optimum value, the rate of refrigerant release by zeolite is not balanced compared to the rate of refrigerant adsorption by the silica gel. As a result, the SCP value decreases drastically if time ratio is higher than optimum value.
The performance of cycle has been analyzed in terms of COP and SCP. It can be concluded that cycle time, time ratio and mass adsorbent ratio are influential factors on cycle performance. That is, longer cycle time produces higher COP but produces lower SCP. In the observation of effect mass adsorbent and ratio time, it can be concluded that optimum mass adsorbent and time ratio in the range of 0.6-0.8 produces optimum SCP and COP value.
As described above, it is concluded that the two bed double-effect adsorption refrigerating device includes zeolite-HPA and silica gel works at higher performance if the cycle time, time ratio and mass adsorbent ratio are preferably adjusted.

Claims

1. A double effect adsorption refrigeration device comprising a first and a second bed provided for alternatively adsorbing and desorbing a refrigerant liquid in a predetermined one cycle time, in which the first bed comprises a first adsorbent material and the second bed comprises a second adsorbent material which is different from the first adsorbent material, said device further comprising a liquid circulation device provided between the first and second beds so that a heat generated by the first bed is transferred to the second bed for heating the same.

2. A double effect adsorption refrigeration device according to claim 1, in which said first adsorbent material is zeolite and said second adsorbent material is silica gel.

3. A double effect adsorption refrigeration device according to claim 2, further comprising an external heat source for heating the zeolite, a temperature of the external heat is 100 to 150 degree centigrade.

4. A double effect adsorption refrigeration device according to claim 3, in which said predetermined one cycle time is around 1200 seconds.

5. A double effect adsorption refrigeration device according to claim 4, in which an amount ratio of the zeolite and the silica gel (silica gel/zeolite) is from 0.8 to 1.0.

6. A double effect adsorption refrigeration device according to claim 5, in which an time ratio of a time of desorption time and adsorption time of the zeolite, or adsorption time and desorption time of the silica gel is 0.6 to 0.8.