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Experimental study of cascading adsorption cycles
Chemcal Engineerrng Science, Vol. 44, No. 2, pp. 225-235, Printed in Great Britain.
EXPERIMENTAL
000%2509/89 $3.00 + 0.00 0 1989 Pergamon Press pls
1989.
STUDY
OF CASCADING CYCLES
N. DOUSS LIMSI
CNRS,
ADSORPTION
and F. MEUNIER
Bat 508, BP 30,91406
(Received 22 February 1988; accepted
Orsay, France
for publication 30 June 1988)
lot of speculation exists on the possibilities of cascading cycles while very few experimental data are available. Herein, experiments on a cascading adsorptive heat pump are reported. The cascading cycle consists of a two adsorber zeolite-water high temperature stage and an intermittent active carbon-methanol low temperature stage. Driving heat is supplied by a boiler to zeolite adsorbers while active carbon adsorber is heated by heat recovered from zeolite adsorber under adsorption. Evaporators from both basic cycles operate at the same temperature and contribute to the evaporating load. Experimental cooling COP is found to be 1.06, much more than the COP of an intermittent cycle (x0.5) and more than the COP of a two adsorber zeolite water cycle( ~0.75). Despite of the discontinuous operation of the cycle, evaporating rate is nearly constant (x2.35 kW or 37 W per kg of adsorbent). An analysis of the results shows that the components which limit the power of the unit are the evaporators and basically water evaporator. The COP of this cascading cycle is very sensitive to the evaporating temperature lift. If the temperature lift is higher than 45”C, a two adsorber zeolite water cycle has to be preferred. This cycle seems to be well adapted to air conditioning as long as the evaporation temperature lift is less than 45°C. Abstract-A
(1) their need of circulating pumps reduces the reliability; (2) LiBr-H,O can operate only at evaporating temperatures higher than 0°C and crystallization may occur if absorber temperature is out of a certain range: this makes delicate the control of those units; (3) H,O-NH, units need to use high pressures so that safety rules limit their temperature range of utilization. Thus, it appears that other solutions may be forecast and adsorptive heat pump is one of the most attractive alternative solutions. Two directions seem very interesting for adsorptive heat management processes:
BACKGROUND
studied for gas separation, catalysis, etc. It is only recently that adsorptive processes have been proposed for heat management (refrigeration, heat pump etc.). Let us cycles present some recall briefly why adsorptive interest in that field. Refrigerating or heat pump units use traditionally electrically driven compressors. Nevertheless, in some cases, thermally driven units are required [either because the electrical network does not exist or because one wants to use thermal heat as input energy (solar energy, waste heat or heat coming from burning refuses for example)]. Moreover, recent studies on ecological impact of traces of halocarbons in the atmosphere have shown that the emission of chlorofluorocarbons CFCl, and CF,Cl, affects the ozone layer around the globe and the greenhouse effect. It has been shown that refrigerating units are responsible up to 50% for the CF,Cl, emission in the atmosphere and that chlorofluorocarbons are responsible for about one third and carbon dioxide for about half of the greenhouse effect (Edmonds et al., 1987). Thus, there exists, on one hand, a technical interest to develop thermally driven refrigerators or heat pumps and, on the other hand, there exists an ecological interest to find other solutions than compressors using chlorofluorocarbons. For these two Adsorptive
reasons,
processes
have been extensively
it is important
to consider
the possibilities
~ soft technology [e.g. solar technology, Guilleminot (1980), Pons and Guilleminot (1986), or use of waste heat]. In this case, mechanical simplicity and high reliability will prevail on efficiency. -advanced technology. In this case, mechanical simplicity is no more a requirement while high efficiency is required in priority. Recent developments have proved that adsorptive units are well adapted for soft technology applications since they can operate without moving parts (which favours high reliability) and with low heat grade (which favours solar applications as well as the use for waste heat). But, adsorptive processes may also be adapted for advanced cycles since they can operate at high temperatures. This paper will present a study of advanced adsorptive cycles and in particular a cascading cycle in which an active carbon-methanol intermittent cycle is topped by a two adsorber zeolite-water cycle.
of
sorption units. The most commonly thermally driven refrigerators or heat pumps are absorption units and two pairs are dominating: LiBr-H,O and H,O-NH,. These pairs present a lot of advantages for specific applications. However, they also have some inconvenience: 225
226
N. Douss and F. CASCADING
CYCLES
Several ways to build up a cascading cycle exist. Usual two stage cycles Alefeld (1983) has demonstrated that there existed 26 topologically different two stage cycles. All of those cycles do not present the same interest. Two double stage cycles leading to high COP are shown in Fig. 1. Figure L(a) represents a double effect cycle commonly used with LiBr-H,O pair. This cycle is the superposition of two single stage cycles, a single evaporator and a single absorber serve both cycles; the heat of condensation of the first cycle is used as generator heat for the second cycle. With that cycle, two condensing temperatures are used so that pressure in the high temperature generator is higher than in the low temperature generator: this is not convenient in some instances (H,O-NH, for example). With the cycle presented in Fig. l(b) a single condenser is used so that the pressure in the double stage unit is not higher than in the single stage unit. In this case, two absorbers are necessary and absorption heat of the high temperature cycle is used as generator heat for the low temperature cycle. Nevertheless, as discussed by Sharfe et al. (1986) in the case of absorption heat pumps utilizing advanced cycles operating with LiBr-H,O pair, such cycles
MEUNIER
present their own following facts:
-
-
-
-
-
-
-
-
-
D
n-Adsorber
-----
F
E
1 P ev1.2
A
ilH I I i
IT1
I
Ln P
I
I I I
I I
, I
T2
i
I
!
IB
cycles
Adsorptive units differ from absorption systems since they operate with fixed bed adsorbers so that the process is discontinuous and the adsorbers are submitted to thermal swings. Consequently, a certain amount of heat is used as sensible heat during the thermal swings. To minimize the contribution of sensible heat, special care has been attached to the heat management of the adsorbers; n-adsorber cycles operating with a single evaporator and a single condenser have been proposed in Orsay (Fig. 2) with sequences of heat recovery between adsorbers. Such cycles offer the same advantages as the cascade cycle presented in Fig. l(b): a single condenser is used and pressure in the n-adsorber unit is not higher than that
C
I P condl
due to the
-the heat of desorption is a function of concentration and temperature, -the differential amount of refrigerant to be desorbed (or absorbed) depends on concentration, -temperature drops (whatever small), necessary for heat transfer between absorber and generator, may reduce drastically the fraction of heat of absorption which can be transferred for desorption.
Ln P Pcond2
inherent limitations
T3
T4
)
I
I
I
Tl
I
T2
T3
I
l
T4
(b)
(a)
Fig. 1. Two typical cascading absorption cycles in a Clapeyron diagram. (a) Double stage cycle: basic cycles: ABCD + ABEF; generatorheat in E provided by condensingheat in D. (b) Double stage cycle: basic cycles: ABCD + AEFD; generator heat in C provided by absorber heat in E.
I
I
T ev
Tcond
’ 1solid
I
Tgen
Tads
Fig. 2. n-Adsorber cascading cycle.
*
227
Experimental study of cascading adsorption cycles in the unit operating an intermittent cycle; moreover, adsorption heat at high temperature is used as desorption heat at low temperature. Counteracting heat transfer fluid circuits between adsorbers reduces entropy generation in comparison with what happens in intermittent cycles. The driving heat supplied to the cycle at the high temperature source is used only at a high temperature level (Fig. 2) so that the entropy generationAue to the inadaptation between the temperature levels of the source and of the adsorber-is much less in an n-adsorber cycle than it is in an intermittent cycle [for a detailed analysis of the entropy generation due to the coupling between an adsorptive heat pump and external heat sources see Meunier ‘(1985)]. The same thing happens for the rejected heat: the rejection temperature is much closer from the utility temperature with an n-adsorber cycle than with an intermittent cycle. Very similar conclusions to that drawn by Scharfe et al. (1986) have been presented by Meunier (1985) in the case of heat recovery between adsorbers. In a particular case, Meunier (1985) has shown that using an infinite number of adsorbers with ideal heat recovery between adsorbers, the maximum achievable-with given conditions of operating temperatures-would be a cooling COP equal to 1.85 corresponding to 68% of ideal Carnot COP. Cascading n-adsorber cycles At the moment, two working fluids and two adsorbents are mostly used in adsorptive heat pumps: water and methanol for the fluids (although some authors propose ammonia (Critoph), . . ) and zeolite and active carbon for the adsorbents. Two pairs have been studied in details: zeolite-water (Guilleminot et al., 1980; Guilleminot and Meunier, 1981) and active carbon-methanol (Delgado et al., 1982; Pons and Guilleminot, 1986). Zeolite-methanol pair is not stable since catalysis occurs and results in the production of dimethylether. The physical-chemical properties of the two pairs are quite different: active carbon desorbs easily methanol, while zeolite retains much more water. Consequently, active carbon-methanol pair is well adapted to operate cycles with small evaporating temperature lifts (up to 40°C) (the evaporating temperature lift is the difference between adsorbing temperature and evaporating temperature) while zeolite-water pair is able to operate cycles with large evaporating temperature lifts (70°C or even more). Two other characteristics of those pairs are important: (1) due to water freeze, zeolite-water pair can operate only at evaporating temperatures higher than 0°C; (2) active carbon-methanol is limited to regenerating temperatures of the order of 150°C due to methanol instability. For these reasons, cascading cycles have been designed in order to involve low temperature cycles for active carbon-methanol pair and high temperature cycles for zeolite-water pair.
Recently, Meunier (1986) has proposed several cascading cycles utilizing those two pairs for: -cogeneration -cogeneration -refrigeration -cogeneration
of chilled water and hot water of chilled water and steam ’ of refrigeration
and hot water.
These cascading cycles consist of the superposition of a twin adsorber zeolite-water cycle and of either an intermittent or a twin adsorber active carbonmethanol cycle. Other cascading cycles Hybride solutions have been proposed: A triple effect machine operating a cascade between a water zeolite heat pump and a single stage LiBr-H,O refrigerator has been tested in Munich (Ziegler et al., 1985); Moss and Shahidullah (1985) have studied a staged system employing an adsorptive zeolite-water heat pump to “bootstrap” desorption of a univariant chemical heat pump (CaCl,-methanol unit). THE CASE
UNDER
STUDY
The experimental cascading cycle presented herein consists of a twin adsorber zeolite water cycle topping a single adsorber active carbon methanol cycle. The objective of this cycle is the production of chilled water for air conditioning. Main characteristics of the unit (Fig. 3) Numbers here after refer to components Fig. 3.
shown in
Adsorbers (I, 2, 3). Copper “hair pen” type heat exchangers are used. They consist of rectangular fins crossed by eight passages of tubes. Cylindrical pellets of about 1.5 mm in diameter of active carbon or zeolite are distributed between the fins. The temperature of adsorbent is taken as the averaged temperature of different temperature probes (0.5 mm dia) located in different places of the adsorber between pellets. Table 1 gives the main characteristics of adsorbers. The mass of the metallic part of each adsorber (including casing) is 118 kg which corresponds to a ratio mass of adsorber/mass of adsorbent ranging from 4.3 to 9.5 for the three adsorbers under study. If one uses larger units, this ratio decreases: in a demonstration unit built in France using 750 kg of zeolite the mass of the adsorber is 1622 kg corresponding to a ratio 2.2. Condensers (4). The heat transfer fluid (water) flows inside the tubes of a shell and tube exchanger (Young F303 HY, two passages) while the working fluid (methanol or water) condenses outside the tubes; the transfer area is 1.05 mz. Evaporators (5). Two different types of evaporators are used for methanol and water. Methanol evaporates inside the tubes of a shell and tube exchanger
N. Douss and F. MEUNIER
228
: zeolite adsorbers : active carbon adsorber 4 : condenser 5 : evaporator 6 : graduated bottle 7 : water-oil heat exchanger : thermostated baths with 6.9 12
3
immersed Ai Vi-j
:
working
:
heat
pump fluid
recovery
adorber
valves valves
i to adsorbsr
-heat
recovery
-heat
recovery
line line
from j
between between
zeolite zeolite
adsorbers adsorbers
and
active
carbon
adsorber
Fig. 3. Scheme of the experimental unit.
Table 1. Characteristics of adsorbers Adsorber Zeolite Number of fins Fin thickness (mm) Fin spacing (mm) Exchange area on the fin side (m’)
112 0.15 5.5
Exchange area on the oil side (m’) Mass of adsorbent (kg)
(Young F303 HY, one passage) standing in a vertical position so as to get a vapour lift evaporator; the transfer area is 0.85 m*. Water evaporates inside the tubes of a coaxial evaporator whose transfer area is 2.5 m’. Heat sources. To the adsorbers. Heating phase (zeolite adsorbers), a thermostated bath (Parmilleux, 9) is used as the heat source (electrical heat rate: 10 kW). Cooling phase (active carbon adsorber), a water oil heat exchanger, WOHE: 7, is used to cool down the adsorber through water from the network. The heat exchanger area of that WOHE is 1.7 m2 and the flow rates of oil and water are respectively 18 and 60lmin-‘. Heat recovery phase: The circulating pump used during cooling and heat recovery phases is the pump immersed in a reservoir of thermostated bath. The contribution of the sensible heat of the reservoir (due to the temperature variation of the reservoir during the recovery phase) to the heat balances is taken into account (see below).
20 1.31 23.5
1
Adsorber 2 Zeolite 112 0.15 5.5
20 1.31 27.5
Adsorber 3 Active carbon 231 0.15 2.5
32.5 I::?
To the condensers. Recirculated water from a thermostated bath (Facis) with a flowrate of 60 1min- ‘. To the ezjaporator. A thermostated electrical heater (Secasi, heat rate: 2 kW) is used with a flowrate of 40lmin’ (heat transfer fluid: water +ethylene glycol 24% by weight). Sensors. Pressure. Membrane type (Schlumberger), &300 mb and &600 mb precision: 0.5%.
scale:
Flow rate. A variable area flow-meter (Brooks: scale C&60 1 min- I, precision 2%) is used in the water heat transfer circuit. A turbine flow-meter (Schlumberger; scale: &84 i min- I, precision: 0.5%) is used in the oil heat transfer circuit. Volume of cycled adsorbate. Direct measurement of the volume in a graduated bottle (6), precision: 0.0251. Temperature. A four conductors technique with platinum probes is used so as to get the highest
229
Experimental study of cascading adsorption cycles precision. The probes are systematically calibrated and the precision of the measurements is 0.03”C. Heat balances. They are determined from the measurements of the flow rates and temperature drops between inlet and outlet temperatures on the exchangers. The temperature drops are of the order of 5°C for the adsorbers but only 1°C and 0.5”C for condensers and evaporators, it is the reason why heat balances, for evaporators and condensers, have been determined preferably from the measurement of the volume of cycled adsorbate (which is measured with a good precision). Determination of heat balances on components whose temperature varies a lot with time is rather delicate. For example, when a heat recovery phase between the two zeolite adsorbers is started, oil at low temperature (1oo”C) is introduced in an exchanger where oil is at a high temperature (220°C); if one measures inlet and outlet temperatures at that initial time, a 120°C temperature drop will be measured which does not correspond at all to the instantaneous heat balance of the adsorber. To avoid such errors, data acquisition program measures temperatures in the exchangers taking into account the fluid velocity in the exchangers so that temperature is assumed to be measured at the same time than the passage of the mass unit of oil in the heat exchangers. With that technique, after calibration, a good precision for the balances was obtained.
Operation of the cycle The main characteristics
T
4ot
film
of this cycle are: WorkingJIuid connections. Zeolite adsorber 1 to water evaporator and zeolite 2 to water condenser. Active carbon adsorber is connected to methanol evaporator. Second sequence. During the second sequence, active carbon adsorber is heated up (lines: 345; Fig. 4) by adsorber heat drawn from zeolite adsorber 1 (line: 12-7; Fig. 4) and at the same time zeolite adsorber 2 is heated up by high temperature heat supplied by the boiler (line: 9-10; Fig. 4).
T cond Zeolite -Water Tev
10
First sequence. Starting with the three adsorbers (zeolite 1, zeolite 2 and active carbon) respectively at 220, 105 and 100°C. During this first sequence, zeolite adsorber 2 is heated up by adsorber heat drawn from zeolite adsorber 1 (lines: 7-8-9 for zeolite adsorber 2 and l&1.1-12 for zeolite adsorber 1; Fig. 4). Active carbon adsorber is cooled down by external heat sink (lines: 5-63; Fig. 4).
(“”
30
20
Description of the four sequences of the three adsorber cascading cycle In this presentation, numbers will be referred to Fig. 4.
Thermal connections. The two zeolite adsorbers are under heat recovery. Active carbon- adsorber is connected to the WOHE (water oil heat exchanger).
-The same evaporating temperature (25°C) and the same condensing temperature (35°C) are used in both cycles. -Low temperature adsorptive heat of the zeolite cycle is used as generator heat for the active carbon cycle -Heat rejected at the heat sink comes from condensers of both cycles and from active carbon adsorber but not from zeolite adsorbers.
T
The cycle may be decomposed in four sequences. Figure 4 represents the adsorbers evolution during a cycle in a Clapeyron type diagram. As pressure of water and methanol are distinct, instead of presenting In P vs adsorbent temperature, we present liquid saturating temperature corresponding to pressure in adsorbers versus adsorbent temperature. In such a diagram, vapour-liquid equilibrium is represented by a straight line (line l-2). In this figure, lines 345-6-3 represent the active carbon-methanol cycle and lines 7-8-9-10-l 1-12-7 represent the two-adsorber zeolite-water cycle. One has to notice that the two zeolite adsorbers describe the same cycle out of phase (7-8-9-l&1 1-12-7).
Active 1
carbon methanol 6
7
12
11
t --
II
0
50
Fig.
4. The proposed
I 1
100
three adsorber
T
I
I
150
200
cascading cycle. Arrows represent heat flows.
ads (Oc)
e
N. Douss and
230
F.
MEUNIER
Thermai connections. Zeolite adsorber 1 and active carbon adsorber are under heat recovery. Zeolite adsorber 2 is connected to the boiler.
T (oC1
Wurking.fluiJ connections. Zeolite adsorber 1 to water evaporator and zeolite adsorber 2 to water condenser. Active carbon adsorber is connected to methanol condenser. The sequences 3 and 4 are analogous to respectively first and second sequences exept that zeolite adsorbers are interchanged (adsorber 1-adsorber 2 and vice versa) Third sequence. Zeolite adsorber 1 is preheated by adsorber heat drawn from zeolite adsorber 2 (lines: 7-8-9 for zeolite adsorber 1 and l&11-12 for zeolite adsorber 2; Fig. 4). Active carbon adsorber is cooled down by external heat sink for a second time (lines: 5-6-3; Fig. 4). Thermal connections. The two zeolite adsorbers are under heat recovery. Active carbon adsorber is connected to the WOHE.
Fig. 5. Experimental adsorbent temperature variations -zeolite adsorber 1, in adsorbers: the three ---zeolite adsorber 2,. . . . . . . . . active carbon adsorber.
1
film(T)
50
Workingfluid connections. Zeolite adsorber 1 to water condenser and zeolite adsorber 2 to water evaporator. Active carbon adsorber is connected to methanol evaporator. Fourth sequence. Zeolite adsorber 1 is heated up by the boiler (line: 9-10; Fig. 4). Active carbon adsorber is heated up (lines: 345; Fig. 4) by heat drawn from zeolite adsorber 2 (line: 12-7; Fig. 4). Thermal connections. Zeolite adsorber 2 and active carbon adsorber are under heat recovery. Zeolite adsorber 1 is connected to the boiler.
Tads
6. Representation of the cycle in a liquid saturation temperature vs adsorbent temperature diagram.
Fig.
d 30
(kW) F
2E2 1
1
SE@2
Workingjluid connections. Zeolite adsorber 1 to water condenser and zeolite adsorber 2 to water evaporator.’ Active carbon adsorber is connected to methanol condenser. At the end of these four sequences, final state corresponds zeolite
to
initial
adsorber
has
state
of
operated
sequence the
same
1.
( 9.3
1
j
(El3
sEEl4
. ‘--.“-I,,_----ii
Each
.“.
cycle
Fig. 4), but the active carbon adsorber has operated two cycles (3-4-S-&3; Fig. 4). Heat is produced either when the adsorbers are connected to their condensers, or when the active carbon adsorber is connected to the WOHE. Cooling is produced when the adsorbers are connected to their evaporators.
Fig. 7. Experimental adsorbers: -zeolite 5 .. . ..
Results Experimental results of the three adsorber cycle are presented in Fig. 5-8: adsorbent temperature variations (Fig. 5), operating cycle (Fig. 6), adsorbers heating/cooling rates (Fig. 7) and evaporators and condensers heating/cooling rates (Fig. 8) (numbers used in Fig. 6 will be referred to Fig. 4). Figure 5 shows that a temperature drop of the order of 5”C, at the end of the heat recovery phases, is
necessary to limit the length of the recovery sequences. It has to be noted that during the heat recovery phases, heat rates are very large when heat recovery starts (Fig. 7) (heating and cooling rates reach respectively 28 and 17 kW at the beginning of the sequences; while with the external heat sources they are less than 8 kW). Consequences of those high heat rates are
(7-8-9-l&11-12-7;
I
x .
I
TIME
(min)
5:
heat rate variations out of and in adsorber 1, - - - - zeolite adsorber . active carbon adsorber.
231
Experimental study of cascading adsorption cycles b
Figure 8 presents the heating (cooling) rates variations of water and methanol condensers (evaporators). The evaporating rate of the three-adsorber cycle is the sum of the water and methanol evaporation rates (Fig. 8); while the heating rate is the sum of the water and methanol condensation and the active carbon adsorption rates (Figs 7 and 8). One must note that both cooling and heating are produced continuously during the three adsorber cycle, with the following characteristics:
CkW)
6
2EQl
j
IEI
I
I
-90t
.E,l
2
I
1
SEl I
I
I
I
TIME
tmin)
I
5;
8
%
Fig. 8. Experimental heat rate variations out of condensers water, - - - - methanol. and in evaporators: ~ irreversibilities on condensers and evaporators. This effect is particularly important during sequences 2 and 4 when zeolite adsorber is cooled down through heat exchange with active carbon adsorber: during this phase, the water evaporator is unable to provide water vapour LO the Leolile adsorber so that pressure in the adsorber (or film temperature in the evaporator) decreases a lot (line: 12-7; Fig. 6). A temperature drop as high as 20°C between heat transfer fluid in water evaporator and film temperature of refrigerant fluid is observed in that case. No doubt that if water evaporator was more efficient, heat rates on water evaporator and zeolite adsorber under adsorption would be higher [a detailed discussion of the influence of the heat exchange coefficient of evaporator on the dynamics of a cycle has been presented by Karagiogas and Meunier (1987)] and consequently heat rate on active carbon adsorber would also be higher. The irreversibility is much more important on the zeolite-water cycle than on the active carbonmethanol cycle (line: 6-3; Fig. 6). This very strong irreversibility on the water evaporator could probably be reduced if one used a vapour lift evaporator as used for methanol.
-cooling ratez2.35 kW which corresponds to 37 W per kg of adsorbent and to an evaporating load z 364 kJ per kg of adsorbent. -heating rate x 3.95 kW which _ corresponds to 62 W per kg of adsorbent and to a heating load z 612 kJ per kg of adsorbent. Heat
balance
Experimentai heat balance for the cascading cycle is presented in Fig. 9 and Table 2. Input heat (Fig. 9) consists of: (1) high temperature driving heat supplied to zeolite adsorbers (15,536 kJ), (2) heat provided by the reservoirs in which are immersed the pumps during heat recovery phases (4835 kJ to zeolite adsorbers and 1500 kJ to active carbon adsorber) and (3) low temperature heat provided to evaporators (15,120 kJ to vaporize 6.3 kg of water, and 8000 kJ to vaporize 7 kg of methanol). Output heat is supplied by: (1) active carbon adsorber (15,360 kJ), (2) condensation of water (15,385 kJ) and methanol (8110 kJ) and (3) losses on adsorbers estimated to 6136 kJ so as to equalize output and input heat. Model
The model used in Table 2 is an equilibrium model (Guilleminot et al., 1980) based on heat and mass balances drops
when neither kinetics
are taken
effects nor temperature
into account.
heat
reservoirs to
ADSORBER RNAL RCE
METHANOL
HEAT TO
CARBON ORPTION
Z
METHANOL CONDENSATION
EVAPORATIO
WATER
INPUT
EVAPORATION
WATER
CONDENSATION
OUTPUT
HEAT
HEAT
Fig. 9. Heat flowing out of and into the three adsorber cascading heat pump during a cycle (experimental heat balance).
232
N. bouss
and F. MEUNIER
Table 2. Global heat balance for the cascade
Zeolite adsorhers input (kJ)
Experiment
Model
external heat 15,536& 1090 source reservoir 4835 + 340
17,713 4818
(I) (2)
16,514+ 1150
18,127
(3)
16,514+ 1150 1500+ 150
13,149 2ooo
(4) (5)
Zeolite adsorhers output (kJ) Active carbon adsorbers input (kJ)
zeolite output reservoir
Active carbon adsorbers output (kJ)
15,360& 1100
14,175
(6)
Condensation (kJ)
23,495 + 700
23,595
(7)
Evaporation (kJ)
23,120+700
23,343
(8)
1.06 iO.07
0.95
(9)
1.78f0.07
1.54
(IO)
Cooling COP COA (heating COP)
(8) (l)+(2)+(5) (6)+ (7) (I)+(2)+(5)
Cycle-period (min)
The index by which performances of refrigerating cycles can be evaluated is the coefficient of performance (COP). The cooling coefficient of performance of an adsorptive cycle is equal ti the heat load in the evaporator per unit heat load (supplied by external heat source and reservoirs) in the adsorbers, while the heating coefficient of performance (or coefficient of amplification: COA) is the heat load out of condensers plus heat load out of adsorbers per unit heat load in the adsorbers: COP =
Q.v/(Qg.n + &A
COA = (Qcond+ QadAQgen + &A. The COP and COA are readily calculated if thermodynamic data for the adsorbent-refrigerant pair are available and if heat capacities of all components are known. Detailed analysis of a two adsorber zeolite-water cycle (Douss et al., 1988) and of an intermittent active carbon-methanol cycle (Douss and Meunier, 1988) operating on the unit studied here give all the information needed by the model. Experimental as well as computed COA are less than 1 +COP, owing to heat losses on the adsorbers. Heat balances of both elementary theoretical cycles (zeolite-water and active carbon-methanol) are presented in Figs 10 and 11. In both figures, estimated losses-from previous studies on both cycles (Douss et al., 1988; Douss and Meunier, 1988)-are indicated. Input heat to zeolite adsorbers (Fig. 10) comes from boiler (Qi =9551 kJ, Q,=8162 kJ) but also from heat reservoir (K,, =2598 kJ and K,, =2220 kJ) in which is immersed the circulating pump. On Fig. 10, evaporating load on water evaporator is divided into contributions from adsorber l(8169 kJ) and from adsorber 2 (6980 kJ); all data between zeolite adsorbers are different since zeolite masses differ.
164
Comparison
between
model and experiment
The agreement is good for condensation and evaporation, this proves that experimental and theoretical mass balances coincide. Agreement is not so good on output heat from zeolite adsorbers and is poor on input heat on active carbon adsorber: during heat recovery phases, heat rates show very large variations so that uncertainties larger than expected may exist on heat balances. It would be very difficult to get a better agreement for two reasons: (1) contribution of heat losses in the heat balance is all the more important as driving heat supplied by boiler decreases, which is the case with cascading cycles, (2) Dubinin’s law used to represent active carbon methanol isotherm underestimates methanol cycling. This is probably due to the fact that evaporating and adsorbing temperatures are close each other. The main discrepancy between experiments and model comes from the coupling between the two cycles: according to the model, 18,127 kJ are delivered by zeolite adsorbers and only 13,149 kJ are supplied to active carbon adsorber, heat losses would be 4978 kJ which corresponds to 38% of input heat in active carbon adsorber. In fact, we are unable to say exactly where heat losses appear. Equalizing input and output heat for the unit leads to estimate losses on the adsorbers to 5126 kJ (21% of the total input heat supplied by external heat sources and reservoirs to adsorbers). In Table 2, experimental COP is found to be equal to 1.06 f 0.07 while theoretical COP is 0.95.
Predicted
performances
For technical reasons (inefficiency of the water evaporator at low temperature), it was not possible to
233
Experimental study of cascading adsorption cycles K02
Ads
2
(23.5
kg)
ds
1
1
(27.5
kg)
01
KOl 2220
’
CONDENSER
EVAPORATOR
INPUT
OUTPUT
HEAT
HEAT
Fig. 10. Heat flowing out of and into the two adsorber zeolite-water heat pump during a cycle (theoretical analysis of experimental results). K,i represents the heat reservoir contribution to input heat in zeolite adsorbers, QGVi(QCi) represents contribution of ith evaporator (condenser). Adsorbers heat losses are included in input heat and excluded from output heat.
heat
reservoir
ACTIVE
CARBON CTIVE
ADSORBER QC
CARBON
ADSOABER
Qev z
INPUT
OUTPUT
HEAT
( to heat
HEAT sink
)
Fig. 1 I. Heat flowing out of and into the intermittent active carbon-methanol heat pump during a cycle (theoretical analysis of experimental results). Adsorbers heat losses are included in input heat and excluded
from output heat.
perform experiments at an evaporating temperature equal to 2°C. We can use the model to predict what would be the efficiency of a heat pump operating in the cooling mode for air conditioning. In the model, we took the same losses as those used in the preceeding study for the zeolite water and the active carbon methanol units; we introduced coupling losses between the two units; the value of those losses is quite arbitrary, but the order of magnitude is reasonable. We eliminated the sensible heat term supplied by the reservoir associated to the circulating pump: we assume that the circulating pump is no more immersed. Figure 12 shows the heat flowing out of and into the cascading unit during a complete cycle for following operating temperatures (T,, = 2°C; heat sink temperature: 30°C; T,,, = 250°C). Heat losses on the adsorbers have been taken into account and may be divided into
two parts: 1050 kJ during heating of zeolite adsorbers with external heat source and 4407 k.J in other phases (heat recovery and cooling of active carbon). If heat losses on adsorbers were cancelled, the COP would be 0.91. Large units operating with larger adsorbers would then probably lead to higher COPS than what has been computed. In fact heat losses depend on the ratio between external area of the adsorber and mass of adsorbent. This ratio decreases when mass increases. Figure 13 shows variation of COP as a function of heat sink temperature (T,, = 2°C; r,,,; T,,, = 250°C) for the cascading cycle and for the two adsorber cycle operating the same temperatures (T,, = 2°C; Ths; T,,, =250°C). It is seen that for small temperature lifts, cascading cycle is better than two adsorber zeolite water cycle but for large temperature lifts, it is the reverse: once more, we observe that zeolite-water cycles are still efficient while active carbon-methanol cycles are no more efficient for large temperature lifts.
N. Douss
234 Cleat
and F. MEUNIER
lasses
LOSSES
\
ONADSORBERS ACTIVE
METHANOL
CARBON
ADSORBER
EVAPORATIO
METHANOL CONDENSATION
WATER EVAPORATIO
WATER
INPUT HEAT Fig. 12.
12-
OUTPUT
= 25O”C, T,,,, aink = 3O”C, i-,, =2”C.
COP
WOK 0*4T
02 20
HEAT
Predicted heat flows out of and into a three adsorber heat pump. Operating temperatures:T,,,
1,0-
o,o
CONDENSATION
30
40
50
h.s 60
Fig. 13. Predicted cooling COP versus heat sink temperature. COPl: cascading cycle without losses on adsorbers. COP2: cascading cycle with losses on adsorbers. COP3: two adsorber zeolite-water cycle.
CONCLUSION
Speculations exist on the possibilities of cascading cycles while very few experimental data are available. In that paper, a cascading adsorptive cycle consisting of a twin adsorber zeolite-water cycle topping a single adsorber active carbon cycle has been operated with given temperature conditions (T,, = 25”C, Thr= 35”C, T,,, = 220”C).The cycle under study operates with the same evaporating temperature and the same condensing temperature in both basic cycles. The cascade is carried out on the adsorbers: heat added to active carbon is supplied by adsorber heat of zeolite cycle. This cycle is characterized by the heat recovery phases between adsorbers. Experimental as well as numerical results allow to draw some conclusions: (1) The heat recovery phases between adsorbers (either between the two zeolite adsorbers or between a zeolite adsorber and the active carbon adsorber) were very efficient: high heat rates were obtained. However, to reduce the irreversibilities caused by the high heat rates obtained at the beginning of the heat recovery phases, we should increase the heat exchange coefficient of the adsorbers, condensers and particularly the water evaporator.
(2) Water evaporator is the limiting component in the unit which compelled us to use a high evaporating temperature (25°C): a vapour lift evaporator would probably be much more efficient than coaxial evaporator. (3) The experimental cooling COP was found to be 1.06. This COP is higher than what would be obtained with a two adsorber cycle (x 0.75) and much higher than the COP of intermittent cycles (~0.5) in the same operating conditions. (4) The model shows that the advantage of the cascading cycle (compared to a two adsorber zeolitewater cycle) depends on the evaporating temperature lift. If this temperature lift is higher than 45”C, then this cascading cycle is not advantageous; on the opposite, for low temperature lifts (2&3O”C) the COP of the cascading cycle is higher by 25 % than that of the two adsorber zeolite-water cycle for air conditioning. (5) Scaling up may be considered since specific heat losses on adsorbers should decrease when the mass of adsorbent would increase. (6) This cycle enables to build air conditioning units with a cooling COP equal to or higher than 1, depending on heat losses on adsorbers. NOTATION
AC COA COP Gi M Q’ T
W WOHE Z
active carbon adsorber coefficient of amplification coefficient of performance energy coming from reservoir i, kJ methanol pressure, mbar energy, kJ temperature, “C water water oil heat exchanger zeolite adsorber
Subscripts
ads cond ev
adsorption condenser evaporator
Experimental
gen hs
study of cascading adsorption cycles
generator heat sink
REFERENCES
Alefeld, G., 1983, Double effect, triple effect and quadruple effect absorption machines. Proc. idth Inc. Congress on Refrigeration Paris, Ed IIR. Critoph, R. E., 1988, Performance limitations of adsorption cycles for solar cooling. Solar Energy, in press. Delgado, R., Choisier, A., Grenier, Ph., Ismail, I., Meunier, F. and Pons, M., 1982, Proc. IIR, Jerusalem, Ed. IIR, pp. 181-187. Douss, N. and Meunier, F., 1988, Effect of operating temperatures on the coefficient of performance of active carbon methanol systems. J. Heat Recovery Syst., in press. Douss, N., Meunier, F. and Sun, L. M., 1988, Predictive model and experimental results for a two adsorber solid adsorption heat pump. Ind. engng Chem. Res. 27, 310-316. Edmonds, J. A., Wuebbles, D. L. and Scott, M. J., 1987, Energy and radiative precursor emissions. 8th Miami Int.
235
Conf Alternative Energy Sources, 1416 December 1987. Guilleminot, J. J., Meunier, F. and Mischler, B., 1980, Rev. Phys. Appl. 15, 441452. Guilleminot, J. J. and Meunier, F., 1981, Rev. Gen. Therm. Fr. 239, 825-834. Karagiorgas, M. and Meunier, F., 1987, J. Heat Recovery Syst. 7, 285-299. Meunier, F., 1985, J. Heat Recoverv Svst. 5. 133-141. Meunier, F., 1986, J. Heat Recover; S;st. 6; 491498. Moss, G. and Shahidullah, M., 1985. in Proc. Znt. Workshoo on Heat Transformation and Storage, CEC, Ispra, 9-11 October, pp. 295-309. Pons, M. and Guilleminot, J. J., 1986, ASME JJSE 108, 332-337. Scharfe, J., Ziegler, F. and Radermacher, 1986, Analysis of advantages and limitations of absorber-generator heat exchange. Int. J. Refrig. 9, November. Ziegler, F., Brandl. F., Viilkel, J. and Alefeld, G., 1985, A cascading two-stage sorption chiller system consisting of water-zeolite high temperature stage and a water-LiBr low-temperature stage. .Absorption Heat Pumps Cong., Paris, 2&22 March 1985, pp. 231-238.
EXPERIMENTAL
000%2509/89 $3.00 + 0.00 0 1989 Pergamon Press pls
1989.
STUDY
OF CASCADING CYCLES
N. DOUSS LIMSI
CNRS,
ADSORPTION
and F. MEUNIER
Bat 508, BP 30,91406
(Received 22 February 1988; accepted
Orsay, France
for publication 30 June 1988)
lot of speculation exists on the possibilities of cascading cycles while very few experimental data are available. Herein, experiments on a cascading adsorptive heat pump are reported. The cascading cycle consists of a two adsorber zeolite-water high temperature stage and an intermittent active carbon-methanol low temperature stage. Driving heat is supplied by a boiler to zeolite adsorbers while active carbon adsorber is heated by heat recovered from zeolite adsorber under adsorption. Evaporators from both basic cycles operate at the same temperature and contribute to the evaporating load. Experimental cooling COP is found to be 1.06, much more than the COP of an intermittent cycle (x0.5) and more than the COP of a two adsorber zeolite water cycle( ~0.75). Despite of the discontinuous operation of the cycle, evaporating rate is nearly constant (x2.35 kW or 37 W per kg of adsorbent). An analysis of the results shows that the components which limit the power of the unit are the evaporators and basically water evaporator. The COP of this cascading cycle is very sensitive to the evaporating temperature lift. If the temperature lift is higher than 45”C, a two adsorber zeolite water cycle has to be preferred. This cycle seems to be well adapted to air conditioning as long as the evaporation temperature lift is less than 45°C. Abstract-A
(1) their need of circulating pumps reduces the reliability; (2) LiBr-H,O can operate only at evaporating temperatures higher than 0°C and crystallization may occur if absorber temperature is out of a certain range: this makes delicate the control of those units; (3) H,O-NH, units need to use high pressures so that safety rules limit their temperature range of utilization. Thus, it appears that other solutions may be forecast and adsorptive heat pump is one of the most attractive alternative solutions. Two directions seem very interesting for adsorptive heat management processes:
BACKGROUND
studied for gas separation, catalysis, etc. It is only recently that adsorptive processes have been proposed for heat management (refrigeration, heat pump etc.). Let us cycles present some recall briefly why adsorptive interest in that field. Refrigerating or heat pump units use traditionally electrically driven compressors. Nevertheless, in some cases, thermally driven units are required [either because the electrical network does not exist or because one wants to use thermal heat as input energy (solar energy, waste heat or heat coming from burning refuses for example)]. Moreover, recent studies on ecological impact of traces of halocarbons in the atmosphere have shown that the emission of chlorofluorocarbons CFCl, and CF,Cl, affects the ozone layer around the globe and the greenhouse effect. It has been shown that refrigerating units are responsible up to 50% for the CF,Cl, emission in the atmosphere and that chlorofluorocarbons are responsible for about one third and carbon dioxide for about half of the greenhouse effect (Edmonds et al., 1987). Thus, there exists, on one hand, a technical interest to develop thermally driven refrigerators or heat pumps and, on the other hand, there exists an ecological interest to find other solutions than compressors using chlorofluorocarbons. For these two Adsorptive
reasons,
processes
have been extensively
it is important
to consider
the possibilities
~ soft technology [e.g. solar technology, Guilleminot (1980), Pons and Guilleminot (1986), or use of waste heat]. In this case, mechanical simplicity and high reliability will prevail on efficiency. -advanced technology. In this case, mechanical simplicity is no more a requirement while high efficiency is required in priority. Recent developments have proved that adsorptive units are well adapted for soft technology applications since they can operate without moving parts (which favours high reliability) and with low heat grade (which favours solar applications as well as the use for waste heat). But, adsorptive processes may also be adapted for advanced cycles since they can operate at high temperatures. This paper will present a study of advanced adsorptive cycles and in particular a cascading cycle in which an active carbon-methanol intermittent cycle is topped by a two adsorber zeolite-water cycle.
of
sorption units. The most commonly thermally driven refrigerators or heat pumps are absorption units and two pairs are dominating: LiBr-H,O and H,O-NH,. These pairs present a lot of advantages for specific applications. However, they also have some inconvenience: 225
226
N. Douss and F. CASCADING
CYCLES
Several ways to build up a cascading cycle exist. Usual two stage cycles Alefeld (1983) has demonstrated that there existed 26 topologically different two stage cycles. All of those cycles do not present the same interest. Two double stage cycles leading to high COP are shown in Fig. 1. Figure L(a) represents a double effect cycle commonly used with LiBr-H,O pair. This cycle is the superposition of two single stage cycles, a single evaporator and a single absorber serve both cycles; the heat of condensation of the first cycle is used as generator heat for the second cycle. With that cycle, two condensing temperatures are used so that pressure in the high temperature generator is higher than in the low temperature generator: this is not convenient in some instances (H,O-NH, for example). With the cycle presented in Fig. l(b) a single condenser is used so that the pressure in the double stage unit is not higher than in the single stage unit. In this case, two absorbers are necessary and absorption heat of the high temperature cycle is used as generator heat for the low temperature cycle. Nevertheless, as discussed by Sharfe et al. (1986) in the case of absorption heat pumps utilizing advanced cycles operating with LiBr-H,O pair, such cycles
MEUNIER
present their own following facts:
-
-
-
-
-
-
-
-
-
D
n-Adsorber
-----
F
E
1 P ev1.2
A
ilH I I i
IT1
I
Ln P
I
I I I
I I
, I
T2
i
I
!
IB
cycles
Adsorptive units differ from absorption systems since they operate with fixed bed adsorbers so that the process is discontinuous and the adsorbers are submitted to thermal swings. Consequently, a certain amount of heat is used as sensible heat during the thermal swings. To minimize the contribution of sensible heat, special care has been attached to the heat management of the adsorbers; n-adsorber cycles operating with a single evaporator and a single condenser have been proposed in Orsay (Fig. 2) with sequences of heat recovery between adsorbers. Such cycles offer the same advantages as the cascade cycle presented in Fig. l(b): a single condenser is used and pressure in the n-adsorber unit is not higher than that
C
I P condl
due to the
-the heat of desorption is a function of concentration and temperature, -the differential amount of refrigerant to be desorbed (or absorbed) depends on concentration, -temperature drops (whatever small), necessary for heat transfer between absorber and generator, may reduce drastically the fraction of heat of absorption which can be transferred for desorption.
Ln P Pcond2
inherent limitations
T3
T4
)
I
I
I
Tl
I
T2
T3
I
l
T4
(b)
(a)
Fig. 1. Two typical cascading absorption cycles in a Clapeyron diagram. (a) Double stage cycle: basic cycles: ABCD + ABEF; generatorheat in E provided by condensingheat in D. (b) Double stage cycle: basic cycles: ABCD + AEFD; generator heat in C provided by absorber heat in E.
I
I
T ev
Tcond
’ 1solid
I
Tgen
Tads
Fig. 2. n-Adsorber cascading cycle.
*
227
Experimental study of cascading adsorption cycles in the unit operating an intermittent cycle; moreover, adsorption heat at high temperature is used as desorption heat at low temperature. Counteracting heat transfer fluid circuits between adsorbers reduces entropy generation in comparison with what happens in intermittent cycles. The driving heat supplied to the cycle at the high temperature source is used only at a high temperature level (Fig. 2) so that the entropy generationAue to the inadaptation between the temperature levels of the source and of the adsorber-is much less in an n-adsorber cycle than it is in an intermittent cycle [for a detailed analysis of the entropy generation due to the coupling between an adsorptive heat pump and external heat sources see Meunier ‘(1985)]. The same thing happens for the rejected heat: the rejection temperature is much closer from the utility temperature with an n-adsorber cycle than with an intermittent cycle. Very similar conclusions to that drawn by Scharfe et al. (1986) have been presented by Meunier (1985) in the case of heat recovery between adsorbers. In a particular case, Meunier (1985) has shown that using an infinite number of adsorbers with ideal heat recovery between adsorbers, the maximum achievable-with given conditions of operating temperatures-would be a cooling COP equal to 1.85 corresponding to 68% of ideal Carnot COP. Cascading n-adsorber cycles At the moment, two working fluids and two adsorbents are mostly used in adsorptive heat pumps: water and methanol for the fluids (although some authors propose ammonia (Critoph), . . ) and zeolite and active carbon for the adsorbents. Two pairs have been studied in details: zeolite-water (Guilleminot et al., 1980; Guilleminot and Meunier, 1981) and active carbon-methanol (Delgado et al., 1982; Pons and Guilleminot, 1986). Zeolite-methanol pair is not stable since catalysis occurs and results in the production of dimethylether. The physical-chemical properties of the two pairs are quite different: active carbon desorbs easily methanol, while zeolite retains much more water. Consequently, active carbon-methanol pair is well adapted to operate cycles with small evaporating temperature lifts (up to 40°C) (the evaporating temperature lift is the difference between adsorbing temperature and evaporating temperature) while zeolite-water pair is able to operate cycles with large evaporating temperature lifts (70°C or even more). Two other characteristics of those pairs are important: (1) due to water freeze, zeolite-water pair can operate only at evaporating temperatures higher than 0°C; (2) active carbon-methanol is limited to regenerating temperatures of the order of 150°C due to methanol instability. For these reasons, cascading cycles have been designed in order to involve low temperature cycles for active carbon-methanol pair and high temperature cycles for zeolite-water pair.
Recently, Meunier (1986) has proposed several cascading cycles utilizing those two pairs for: -cogeneration -cogeneration -refrigeration -cogeneration
of chilled water and hot water of chilled water and steam ’ of refrigeration
and hot water.
These cascading cycles consist of the superposition of a twin adsorber zeolite-water cycle and of either an intermittent or a twin adsorber active carbonmethanol cycle. Other cascading cycles Hybride solutions have been proposed: A triple effect machine operating a cascade between a water zeolite heat pump and a single stage LiBr-H,O refrigerator has been tested in Munich (Ziegler et al., 1985); Moss and Shahidullah (1985) have studied a staged system employing an adsorptive zeolite-water heat pump to “bootstrap” desorption of a univariant chemical heat pump (CaCl,-methanol unit). THE CASE
UNDER
STUDY
The experimental cascading cycle presented herein consists of a twin adsorber zeolite water cycle topping a single adsorber active carbon methanol cycle. The objective of this cycle is the production of chilled water for air conditioning. Main characteristics of the unit (Fig. 3) Numbers here after refer to components Fig. 3.
shown in
Adsorbers (I, 2, 3). Copper “hair pen” type heat exchangers are used. They consist of rectangular fins crossed by eight passages of tubes. Cylindrical pellets of about 1.5 mm in diameter of active carbon or zeolite are distributed between the fins. The temperature of adsorbent is taken as the averaged temperature of different temperature probes (0.5 mm dia) located in different places of the adsorber between pellets. Table 1 gives the main characteristics of adsorbers. The mass of the metallic part of each adsorber (including casing) is 118 kg which corresponds to a ratio mass of adsorber/mass of adsorbent ranging from 4.3 to 9.5 for the three adsorbers under study. If one uses larger units, this ratio decreases: in a demonstration unit built in France using 750 kg of zeolite the mass of the adsorber is 1622 kg corresponding to a ratio 2.2. Condensers (4). The heat transfer fluid (water) flows inside the tubes of a shell and tube exchanger (Young F303 HY, two passages) while the working fluid (methanol or water) condenses outside the tubes; the transfer area is 1.05 mz. Evaporators (5). Two different types of evaporators are used for methanol and water. Methanol evaporates inside the tubes of a shell and tube exchanger
N. Douss and F. MEUNIER
228
: zeolite adsorbers : active carbon adsorber 4 : condenser 5 : evaporator 6 : graduated bottle 7 : water-oil heat exchanger : thermostated baths with 6.9 12
3
immersed Ai Vi-j
:
working
:
heat
pump fluid
recovery
adorber
valves valves
i to adsorbsr
-heat
recovery
-heat
recovery
line line
from j
between between
zeolite zeolite
adsorbers adsorbers
and
active
carbon
adsorber
Fig. 3. Scheme of the experimental unit.
Table 1. Characteristics of adsorbers Adsorber Zeolite Number of fins Fin thickness (mm) Fin spacing (mm) Exchange area on the fin side (m’)
112 0.15 5.5
Exchange area on the oil side (m’) Mass of adsorbent (kg)
(Young F303 HY, one passage) standing in a vertical position so as to get a vapour lift evaporator; the transfer area is 0.85 m*. Water evaporates inside the tubes of a coaxial evaporator whose transfer area is 2.5 m’. Heat sources. To the adsorbers. Heating phase (zeolite adsorbers), a thermostated bath (Parmilleux, 9) is used as the heat source (electrical heat rate: 10 kW). Cooling phase (active carbon adsorber), a water oil heat exchanger, WOHE: 7, is used to cool down the adsorber through water from the network. The heat exchanger area of that WOHE is 1.7 m2 and the flow rates of oil and water are respectively 18 and 60lmin-‘. Heat recovery phase: The circulating pump used during cooling and heat recovery phases is the pump immersed in a reservoir of thermostated bath. The contribution of the sensible heat of the reservoir (due to the temperature variation of the reservoir during the recovery phase) to the heat balances is taken into account (see below).
20 1.31 23.5
1
Adsorber 2 Zeolite 112 0.15 5.5
20 1.31 27.5
Adsorber 3 Active carbon 231 0.15 2.5
32.5 I::?
To the condensers. Recirculated water from a thermostated bath (Facis) with a flowrate of 60 1min- ‘. To the ezjaporator. A thermostated electrical heater (Secasi, heat rate: 2 kW) is used with a flowrate of 40lmin’ (heat transfer fluid: water +ethylene glycol 24% by weight). Sensors. Pressure. Membrane type (Schlumberger), &300 mb and &600 mb precision: 0.5%.
scale:
Flow rate. A variable area flow-meter (Brooks: scale C&60 1 min- I, precision 2%) is used in the water heat transfer circuit. A turbine flow-meter (Schlumberger; scale: &84 i min- I, precision: 0.5%) is used in the oil heat transfer circuit. Volume of cycled adsorbate. Direct measurement of the volume in a graduated bottle (6), precision: 0.0251. Temperature. A four conductors technique with platinum probes is used so as to get the highest
229
Experimental study of cascading adsorption cycles precision. The probes are systematically calibrated and the precision of the measurements is 0.03”C. Heat balances. They are determined from the measurements of the flow rates and temperature drops between inlet and outlet temperatures on the exchangers. The temperature drops are of the order of 5°C for the adsorbers but only 1°C and 0.5”C for condensers and evaporators, it is the reason why heat balances, for evaporators and condensers, have been determined preferably from the measurement of the volume of cycled adsorbate (which is measured with a good precision). Determination of heat balances on components whose temperature varies a lot with time is rather delicate. For example, when a heat recovery phase between the two zeolite adsorbers is started, oil at low temperature (1oo”C) is introduced in an exchanger where oil is at a high temperature (220°C); if one measures inlet and outlet temperatures at that initial time, a 120°C temperature drop will be measured which does not correspond at all to the instantaneous heat balance of the adsorber. To avoid such errors, data acquisition program measures temperatures in the exchangers taking into account the fluid velocity in the exchangers so that temperature is assumed to be measured at the same time than the passage of the mass unit of oil in the heat exchangers. With that technique, after calibration, a good precision for the balances was obtained.
Operation of the cycle The main characteristics
T
4ot
film
of this cycle are: WorkingJIuid connections. Zeolite adsorber 1 to water evaporator and zeolite 2 to water condenser. Active carbon adsorber is connected to methanol evaporator. Second sequence. During the second sequence, active carbon adsorber is heated up (lines: 345; Fig. 4) by adsorber heat drawn from zeolite adsorber 1 (line: 12-7; Fig. 4) and at the same time zeolite adsorber 2 is heated up by high temperature heat supplied by the boiler (line: 9-10; Fig. 4).
T cond Zeolite -Water Tev
10
First sequence. Starting with the three adsorbers (zeolite 1, zeolite 2 and active carbon) respectively at 220, 105 and 100°C. During this first sequence, zeolite adsorber 2 is heated up by adsorber heat drawn from zeolite adsorber 1 (lines: 7-8-9 for zeolite adsorber 2 and l&1.1-12 for zeolite adsorber 1; Fig. 4). Active carbon adsorber is cooled down by external heat sink (lines: 5-63; Fig. 4).
(“”
30
20
Description of the four sequences of the three adsorber cascading cycle In this presentation, numbers will be referred to Fig. 4.
Thermal connections. The two zeolite adsorbers are under heat recovery. Active carbon- adsorber is connected to the WOHE (water oil heat exchanger).
-The same evaporating temperature (25°C) and the same condensing temperature (35°C) are used in both cycles. -Low temperature adsorptive heat of the zeolite cycle is used as generator heat for the active carbon cycle -Heat rejected at the heat sink comes from condensers of both cycles and from active carbon adsorber but not from zeolite adsorbers.
T
The cycle may be decomposed in four sequences. Figure 4 represents the adsorbers evolution during a cycle in a Clapeyron type diagram. As pressure of water and methanol are distinct, instead of presenting In P vs adsorbent temperature, we present liquid saturating temperature corresponding to pressure in adsorbers versus adsorbent temperature. In such a diagram, vapour-liquid equilibrium is represented by a straight line (line l-2). In this figure, lines 345-6-3 represent the active carbon-methanol cycle and lines 7-8-9-10-l 1-12-7 represent the two-adsorber zeolite-water cycle. One has to notice that the two zeolite adsorbers describe the same cycle out of phase (7-8-9-l&1 1-12-7).
Active 1
carbon methanol 6
7
12
11
t --
II
0
50
Fig.
4. The proposed
I 1
100
three adsorber
T
I
I
150
200
cascading cycle. Arrows represent heat flows.
ads (Oc)
e
N. Douss and
230
F.
MEUNIER
Thermai connections. Zeolite adsorber 1 and active carbon adsorber are under heat recovery. Zeolite adsorber 2 is connected to the boiler.
T (oC1
Wurking.fluiJ connections. Zeolite adsorber 1 to water evaporator and zeolite adsorber 2 to water condenser. Active carbon adsorber is connected to methanol condenser. The sequences 3 and 4 are analogous to respectively first and second sequences exept that zeolite adsorbers are interchanged (adsorber 1-adsorber 2 and vice versa) Third sequence. Zeolite adsorber 1 is preheated by adsorber heat drawn from zeolite adsorber 2 (lines: 7-8-9 for zeolite adsorber 1 and l&11-12 for zeolite adsorber 2; Fig. 4). Active carbon adsorber is cooled down by external heat sink for a second time (lines: 5-6-3; Fig. 4). Thermal connections. The two zeolite adsorbers are under heat recovery. Active carbon adsorber is connected to the WOHE.
Fig. 5. Experimental adsorbent temperature variations -zeolite adsorber 1, in adsorbers: the three ---zeolite adsorber 2,. . . . . . . . . active carbon adsorber.
1
film(T)
50
Workingfluid connections. Zeolite adsorber 1 to water condenser and zeolite adsorber 2 to water evaporator. Active carbon adsorber is connected to methanol evaporator. Fourth sequence. Zeolite adsorber 1 is heated up by the boiler (line: 9-10; Fig. 4). Active carbon adsorber is heated up (lines: 345; Fig. 4) by heat drawn from zeolite adsorber 2 (line: 12-7; Fig. 4). Thermal connections. Zeolite adsorber 2 and active carbon adsorber are under heat recovery. Zeolite adsorber 1 is connected to the boiler.
Tads
6. Representation of the cycle in a liquid saturation temperature vs adsorbent temperature diagram.
Fig.
d 30
(kW) F
2E2 1
1
SE@2
Workingjluid connections. Zeolite adsorber 1 to water condenser and zeolite adsorber 2 to water evaporator.’ Active carbon adsorber is connected to methanol condenser. At the end of these four sequences, final state corresponds zeolite
to
initial
adsorber
has
state
of
operated
sequence the
same
1.
( 9.3
1
j
(El3
sEEl4
. ‘--.“-I,,_----ii
Each
.“.
cycle
Fig. 4), but the active carbon adsorber has operated two cycles (3-4-S-&3; Fig. 4). Heat is produced either when the adsorbers are connected to their condensers, or when the active carbon adsorber is connected to the WOHE. Cooling is produced when the adsorbers are connected to their evaporators.
Fig. 7. Experimental adsorbers: -zeolite 5 .. . ..
Results Experimental results of the three adsorber cycle are presented in Fig. 5-8: adsorbent temperature variations (Fig. 5), operating cycle (Fig. 6), adsorbers heating/cooling rates (Fig. 7) and evaporators and condensers heating/cooling rates (Fig. 8) (numbers used in Fig. 6 will be referred to Fig. 4). Figure 5 shows that a temperature drop of the order of 5”C, at the end of the heat recovery phases, is
necessary to limit the length of the recovery sequences. It has to be noted that during the heat recovery phases, heat rates are very large when heat recovery starts (Fig. 7) (heating and cooling rates reach respectively 28 and 17 kW at the beginning of the sequences; while with the external heat sources they are less than 8 kW). Consequences of those high heat rates are
(7-8-9-l&11-12-7;
I
x .
I
TIME
(min)
5:
heat rate variations out of and in adsorber 1, - - - - zeolite adsorber . active carbon adsorber.
231
Experimental study of cascading adsorption cycles b
Figure 8 presents the heating (cooling) rates variations of water and methanol condensers (evaporators). The evaporating rate of the three-adsorber cycle is the sum of the water and methanol evaporation rates (Fig. 8); while the heating rate is the sum of the water and methanol condensation and the active carbon adsorption rates (Figs 7 and 8). One must note that both cooling and heating are produced continuously during the three adsorber cycle, with the following characteristics:
CkW)
6
2EQl
j
IEI
I
I
-90t
.E,l
2
I
1
SEl I
I
I
I
TIME
tmin)
I
5;
8
%
Fig. 8. Experimental heat rate variations out of condensers water, - - - - methanol. and in evaporators: ~ irreversibilities on condensers and evaporators. This effect is particularly important during sequences 2 and 4 when zeolite adsorber is cooled down through heat exchange with active carbon adsorber: during this phase, the water evaporator is unable to provide water vapour LO the Leolile adsorber so that pressure in the adsorber (or film temperature in the evaporator) decreases a lot (line: 12-7; Fig. 6). A temperature drop as high as 20°C between heat transfer fluid in water evaporator and film temperature of refrigerant fluid is observed in that case. No doubt that if water evaporator was more efficient, heat rates on water evaporator and zeolite adsorber under adsorption would be higher [a detailed discussion of the influence of the heat exchange coefficient of evaporator on the dynamics of a cycle has been presented by Karagiogas and Meunier (1987)] and consequently heat rate on active carbon adsorber would also be higher. The irreversibility is much more important on the zeolite-water cycle than on the active carbonmethanol cycle (line: 6-3; Fig. 6). This very strong irreversibility on the water evaporator could probably be reduced if one used a vapour lift evaporator as used for methanol.
-cooling ratez2.35 kW which corresponds to 37 W per kg of adsorbent and to an evaporating load z 364 kJ per kg of adsorbent. -heating rate x 3.95 kW which _ corresponds to 62 W per kg of adsorbent and to a heating load z 612 kJ per kg of adsorbent. Heat
balance
Experimentai heat balance for the cascading cycle is presented in Fig. 9 and Table 2. Input heat (Fig. 9) consists of: (1) high temperature driving heat supplied to zeolite adsorbers (15,536 kJ), (2) heat provided by the reservoirs in which are immersed the pumps during heat recovery phases (4835 kJ to zeolite adsorbers and 1500 kJ to active carbon adsorber) and (3) low temperature heat provided to evaporators (15,120 kJ to vaporize 6.3 kg of water, and 8000 kJ to vaporize 7 kg of methanol). Output heat is supplied by: (1) active carbon adsorber (15,360 kJ), (2) condensation of water (15,385 kJ) and methanol (8110 kJ) and (3) losses on adsorbers estimated to 6136 kJ so as to equalize output and input heat. Model
The model used in Table 2 is an equilibrium model (Guilleminot et al., 1980) based on heat and mass balances drops
when neither kinetics
are taken
effects nor temperature
into account.
heat
reservoirs to
ADSORBER RNAL RCE
METHANOL
HEAT TO
CARBON ORPTION
Z
METHANOL CONDENSATION
EVAPORATIO
WATER
INPUT
EVAPORATION
WATER
CONDENSATION
OUTPUT
HEAT
HEAT
Fig. 9. Heat flowing out of and into the three adsorber cascading heat pump during a cycle (experimental heat balance).
232
N. bouss
and F. MEUNIER
Table 2. Global heat balance for the cascade
Zeolite adsorhers input (kJ)
Experiment
Model
external heat 15,536& 1090 source reservoir 4835 + 340
17,713 4818
(I) (2)
16,514+ 1150
18,127
(3)
16,514+ 1150 1500+ 150
13,149 2ooo
(4) (5)
Zeolite adsorhers output (kJ) Active carbon adsorbers input (kJ)
zeolite output reservoir
Active carbon adsorbers output (kJ)
15,360& 1100
14,175
(6)
Condensation (kJ)
23,495 + 700
23,595
(7)
Evaporation (kJ)
23,120+700
23,343
(8)
1.06 iO.07
0.95
(9)
1.78f0.07
1.54
(IO)
Cooling COP COA (heating COP)
(8) (l)+(2)+(5) (6)+ (7) (I)+(2)+(5)
Cycle-period (min)
The index by which performances of refrigerating cycles can be evaluated is the coefficient of performance (COP). The cooling coefficient of performance of an adsorptive cycle is equal ti the heat load in the evaporator per unit heat load (supplied by external heat source and reservoirs) in the adsorbers, while the heating coefficient of performance (or coefficient of amplification: COA) is the heat load out of condensers plus heat load out of adsorbers per unit heat load in the adsorbers: COP =
Q.v/(Qg.n + &A
COA = (Qcond+ QadAQgen + &A. The COP and COA are readily calculated if thermodynamic data for the adsorbent-refrigerant pair are available and if heat capacities of all components are known. Detailed analysis of a two adsorber zeolite-water cycle (Douss et al., 1988) and of an intermittent active carbon-methanol cycle (Douss and Meunier, 1988) operating on the unit studied here give all the information needed by the model. Experimental as well as computed COA are less than 1 +COP, owing to heat losses on the adsorbers. Heat balances of both elementary theoretical cycles (zeolite-water and active carbon-methanol) are presented in Figs 10 and 11. In both figures, estimated losses-from previous studies on both cycles (Douss et al., 1988; Douss and Meunier, 1988)-are indicated. Input heat to zeolite adsorbers (Fig. 10) comes from boiler (Qi =9551 kJ, Q,=8162 kJ) but also from heat reservoir (K,, =2598 kJ and K,, =2220 kJ) in which is immersed the circulating pump. On Fig. 10, evaporating load on water evaporator is divided into contributions from adsorber l(8169 kJ) and from adsorber 2 (6980 kJ); all data between zeolite adsorbers are different since zeolite masses differ.
164
Comparison
between
model and experiment
The agreement is good for condensation and evaporation, this proves that experimental and theoretical mass balances coincide. Agreement is not so good on output heat from zeolite adsorbers and is poor on input heat on active carbon adsorber: during heat recovery phases, heat rates show very large variations so that uncertainties larger than expected may exist on heat balances. It would be very difficult to get a better agreement for two reasons: (1) contribution of heat losses in the heat balance is all the more important as driving heat supplied by boiler decreases, which is the case with cascading cycles, (2) Dubinin’s law used to represent active carbon methanol isotherm underestimates methanol cycling. This is probably due to the fact that evaporating and adsorbing temperatures are close each other. The main discrepancy between experiments and model comes from the coupling between the two cycles: according to the model, 18,127 kJ are delivered by zeolite adsorbers and only 13,149 kJ are supplied to active carbon adsorber, heat losses would be 4978 kJ which corresponds to 38% of input heat in active carbon adsorber. In fact, we are unable to say exactly where heat losses appear. Equalizing input and output heat for the unit leads to estimate losses on the adsorbers to 5126 kJ (21% of the total input heat supplied by external heat sources and reservoirs to adsorbers). In Table 2, experimental COP is found to be equal to 1.06 f 0.07 while theoretical COP is 0.95.
Predicted
performances
For technical reasons (inefficiency of the water evaporator at low temperature), it was not possible to
233
Experimental study of cascading adsorption cycles K02
Ads
2
(23.5
kg)
ds
1
1
(27.5
kg)
01
KOl 2220
’
CONDENSER
EVAPORATOR
INPUT
OUTPUT
HEAT
HEAT
Fig. 10. Heat flowing out of and into the two adsorber zeolite-water heat pump during a cycle (theoretical analysis of experimental results). K,i represents the heat reservoir contribution to input heat in zeolite adsorbers, QGVi(QCi) represents contribution of ith evaporator (condenser). Adsorbers heat losses are included in input heat and excluded from output heat.
heat
reservoir
ACTIVE
CARBON CTIVE
ADSORBER QC
CARBON
ADSOABER
Qev z
INPUT
OUTPUT
HEAT
( to heat
HEAT sink
)
Fig. 1 I. Heat flowing out of and into the intermittent active carbon-methanol heat pump during a cycle (theoretical analysis of experimental results). Adsorbers heat losses are included in input heat and excluded
from output heat.
perform experiments at an evaporating temperature equal to 2°C. We can use the model to predict what would be the efficiency of a heat pump operating in the cooling mode for air conditioning. In the model, we took the same losses as those used in the preceeding study for the zeolite water and the active carbon methanol units; we introduced coupling losses between the two units; the value of those losses is quite arbitrary, but the order of magnitude is reasonable. We eliminated the sensible heat term supplied by the reservoir associated to the circulating pump: we assume that the circulating pump is no more immersed. Figure 12 shows the heat flowing out of and into the cascading unit during a complete cycle for following operating temperatures (T,, = 2°C; heat sink temperature: 30°C; T,,, = 250°C). Heat losses on the adsorbers have been taken into account and may be divided into
two parts: 1050 kJ during heating of zeolite adsorbers with external heat source and 4407 k.J in other phases (heat recovery and cooling of active carbon). If heat losses on adsorbers were cancelled, the COP would be 0.91. Large units operating with larger adsorbers would then probably lead to higher COPS than what has been computed. In fact heat losses depend on the ratio between external area of the adsorber and mass of adsorbent. This ratio decreases when mass increases. Figure 13 shows variation of COP as a function of heat sink temperature (T,, = 2°C; r,,,; T,,, = 250°C) for the cascading cycle and for the two adsorber cycle operating the same temperatures (T,, = 2°C; Ths; T,,, =250°C). It is seen that for small temperature lifts, cascading cycle is better than two adsorber zeolite water cycle but for large temperature lifts, it is the reverse: once more, we observe that zeolite-water cycles are still efficient while active carbon-methanol cycles are no more efficient for large temperature lifts.
N. Douss
234 Cleat
and F. MEUNIER
lasses
LOSSES
\
ONADSORBERS ACTIVE
METHANOL
CARBON
ADSORBER
EVAPORATIO
METHANOL CONDENSATION
WATER EVAPORATIO
WATER
INPUT HEAT Fig. 12.
12-
OUTPUT
= 25O”C, T,,,, aink = 3O”C, i-,, =2”C.
COP
WOK 0*4T
02 20
HEAT
Predicted heat flows out of and into a three adsorber heat pump. Operating temperatures:T,,,
1,0-
o,o
CONDENSATION
30
40
50
h.s 60
Fig. 13. Predicted cooling COP versus heat sink temperature. COPl: cascading cycle without losses on adsorbers. COP2: cascading cycle with losses on adsorbers. COP3: two adsorber zeolite-water cycle.
CONCLUSION
Speculations exist on the possibilities of cascading cycles while very few experimental data are available. In that paper, a cascading adsorptive cycle consisting of a twin adsorber zeolite-water cycle topping a single adsorber active carbon cycle has been operated with given temperature conditions (T,, = 25”C, Thr= 35”C, T,,, = 220”C).The cycle under study operates with the same evaporating temperature and the same condensing temperature in both basic cycles. The cascade is carried out on the adsorbers: heat added to active carbon is supplied by adsorber heat of zeolite cycle. This cycle is characterized by the heat recovery phases between adsorbers. Experimental as well as numerical results allow to draw some conclusions: (1) The heat recovery phases between adsorbers (either between the two zeolite adsorbers or between a zeolite adsorber and the active carbon adsorber) were very efficient: high heat rates were obtained. However, to reduce the irreversibilities caused by the high heat rates obtained at the beginning of the heat recovery phases, we should increase the heat exchange coefficient of the adsorbers, condensers and particularly the water evaporator.
(2) Water evaporator is the limiting component in the unit which compelled us to use a high evaporating temperature (25°C): a vapour lift evaporator would probably be much more efficient than coaxial evaporator. (3) The experimental cooling COP was found to be 1.06. This COP is higher than what would be obtained with a two adsorber cycle (x 0.75) and much higher than the COP of intermittent cycles (~0.5) in the same operating conditions. (4) The model shows that the advantage of the cascading cycle (compared to a two adsorber zeolitewater cycle) depends on the evaporating temperature lift. If this temperature lift is higher than 45”C, then this cascading cycle is not advantageous; on the opposite, for low temperature lifts (2&3O”C) the COP of the cascading cycle is higher by 25 % than that of the two adsorber zeolite-water cycle for air conditioning. (5) Scaling up may be considered since specific heat losses on adsorbers should decrease when the mass of adsorbent would increase. (6) This cycle enables to build air conditioning units with a cooling COP equal to or higher than 1, depending on heat losses on adsorbers. NOTATION
AC COA COP Gi M Q’ T
W WOHE Z
active carbon adsorber coefficient of amplification coefficient of performance energy coming from reservoir i, kJ methanol pressure, mbar energy, kJ temperature, “C water water oil heat exchanger zeolite adsorber
Subscripts
ads cond ev
adsorption condenser evaporator
Experimental
gen hs
study of cascading adsorption cycles
generator heat sink
REFERENCES
Alefeld, G., 1983, Double effect, triple effect and quadruple effect absorption machines. Proc. idth Inc. Congress on Refrigeration Paris, Ed IIR. Critoph, R. E., 1988, Performance limitations of adsorption cycles for solar cooling. Solar Energy, in press. Delgado, R., Choisier, A., Grenier, Ph., Ismail, I., Meunier, F. and Pons, M., 1982, Proc. IIR, Jerusalem, Ed. IIR, pp. 181-187. Douss, N. and Meunier, F., 1988, Effect of operating temperatures on the coefficient of performance of active carbon methanol systems. J. Heat Recovery Syst., in press. Douss, N., Meunier, F. and Sun, L. M., 1988, Predictive model and experimental results for a two adsorber solid adsorption heat pump. Ind. engng Chem. Res. 27, 310-316. Edmonds, J. A., Wuebbles, D. L. and Scott, M. J., 1987, Energy and radiative precursor emissions. 8th Miami Int.
235
Conf Alternative Energy Sources, 1416 December 1987. Guilleminot, J. J., Meunier, F. and Mischler, B., 1980, Rev. Phys. Appl. 15, 441452. Guilleminot, J. J. and Meunier, F., 1981, Rev. Gen. Therm. Fr. 239, 825-834. Karagiorgas, M. and Meunier, F., 1987, J. Heat Recovery Syst. 7, 285-299. Meunier, F., 1985, J. Heat Recoverv Svst. 5. 133-141. Meunier, F., 1986, J. Heat Recover; S;st. 6; 491498. Moss, G. and Shahidullah, M., 1985. in Proc. Znt. Workshoo on Heat Transformation and Storage, CEC, Ispra, 9-11 October, pp. 295-309. Pons, M. and Guilleminot, J. J., 1986, ASME JJSE 108, 332-337. Scharfe, J., Ziegler, F. and Radermacher, 1986, Analysis of advantages and limitations of absorber-generator heat exchange. Int. J. Refrig. 9, November. Ziegler, F., Brandl. F., Viilkel, J. and Alefeld, G., 1985, A cascading two-stage sorption chiller system consisting of water-zeolite high temperature stage and a water-LiBr low-temperature stage. .Absorption Heat Pumps Cong., Paris, 2&22 March 1985, pp. 231-238.