2000_Zhang_Design and Testing of an Automobile Waste Heat Adsorption Cooling System

Design and testing of an automobile waste heat adsorptioncooling system

L.Z. Zhang*

HVAC Division, Department of Thermal Engineering, Tsinghua University, Beijing, 100084, People's Republic ofChina

Abstract

This paper describes an experimental adsorption cooling system driven by the waste heat of a dieselengine. Zeolite 13 � ±water is used as the working pair and a ®nned double-tube heat exchanger is usedas the adsorber. To evaluate the performance of the system, some control and instrumentation facilitiesare also designed. The time evolutions of the ¯uids and bed temperature are measured. The coe�cientof performance and the speci®c cooling power of the prototype are obtained. Some challenges facing thecommercialization of the automobile waste heat driven adsorption cooling system are discussed. # 1999Elsevier Science Ltd. All rights reserved.

1. Introduction

In general, diesel engines have an e�ciency of about 35% and thus the rest of the inputenergy is wasted. In a water-cooled engine about 35 and 30% of the input energy is wasted inthe coolant and exhaust gases, respectively. Because the wasted energy represents about two-thirds of the input energy, it is relevant to investigate various possibilities for the recovery ofsuch energy. Among those technologies, adsorption cooling is an excellent alternative, becausethe supply of waste heat and the need for air conditioning both reach maximum levels at thesame time (just like solar refrigeration). With waste heat-driven adsorption cooling, reductionof gas consumption and easy maintenance could also result for an automobile, compared to

Applied Thermal Engineering 20 (2000) 103±114

1359-4311/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.PII: S1359-4311(99)00009-5

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* Tel.: +86-10-6277-2072; fax: +86-10-6277-0544.E-mail address: [email protected] (L.Z. Zhang)

Nomenclature

COP coe�cient of performancedio outer diameter of the inner tube (m)doo outer diameter of the outer tube (m)dp diameter of the adsorbent particles (m)ge fuel consumption coe�cient (g/kWh)H®n height of ®ns (m)L length of the adsorbent bed (m)me fresh air required by fuel for complete combustion (kg/kg)mz mass of zeolite in the adsorber (kg)mÇwst mass ¯ow rate of waste gas (kg/s)N shaft power of engine (kW)Qe evaporating heat (kJ)Qin heat transmitted from the heating ¯uid to the adsorber (kJ)Qwst waste heat that can be recovered without dew point corrosion (kJ)r0 radius of the inner tube (m)r1 radius of the net (m)SCP speci®c cooling power (W/kg adsorbent)tcyc cycle time (s)T temperature (K)w water uptake (kg/kg adsorbent)WCOP coe�cient of waste heat coolingz axial coordinate (m)

Greek lettersa excess air coe�cientrz density of zeolite (kg/m3)l thermal conductivity (W/m/K)d thickness (m)f angle (rad)f1 half the angle between two ®ns (rad)

Subscriptsad adsorptionc condensere evaporatorg regenerate®n ®nfhi heating ¯uid inletfci cooling ¯uid inleti inner tube, inletinert insulationo outer tube, outlet

L.Z. Zhang / Applied Thermal Engineering 20 (2000) 103±114104

one with a vapor compression air conditioner, which has the additional problem of CFCemissions.The concept of automobile waste heat-driven adsorption cooling looks very attractive and a

couple of studies have already been conducted [1±3]. However, until now, very fewexperimental results have been reported, most of them dealing with simulations. A successfulpractical application of the system requires a sound understanding of the operational behaviorof the system, from experiments as well as from modeling.To evaluate the system performance and investigate the challenges facing the practical

application of the system, an experimental adsorption cooling unit driven by the waste heat ofa diesel engine has been built and tested in our laboratory. It is an intermittent cycle whereexhaust gas and cooling air ¯ow through the adsorber alternately. A detailed description of theunit and the experimental results are presented and interpreted in this paper.

2. System description

2.1. Characteristics of the engine

The engine is a four-stroke, non turbo-charged, water-cooled, direct inject diesel engine. Thestandard speed of the engine is 1500 rpm. The shaft power output can be adjusted in the range0±30 kW by adding/decreasing the load through a power gage. The rotating speed is controlledby a computer. Performance experiments of the engine have been done to investigate the fuelconsumption coe�cient ge and the excess air coe�cient a of the engine. Then the waste gas¯ow rate is estimated as

_mwst � Ngeme�1� a�3600� 103

�1�

where N is the shaft power measured; me is the fresh air required by unit mass of fuel forcomplete combustion. The value of ge is measured as 253.6 g/kWh. The excess air coe�cient isdeduced as 1.24 by analyzing the gas composition. For diesel engine, it has been found that thevalue of me is 14.36 kg(air)/kg(oil).

2.2. The working pair

Adsorbent±adsorbate pairs usually used in adsorption cooling include zeolite±water, zeolite±ammonia, activated carbon±methanol and silica gel±water. The zeolite±water combination issuperior when the temperature lift (adsorption-evaporating temperature) exceeds 458C [4] andit is also the most suitable pair for air conditioning [5]. Another bene®t of using zeolite±wateris that it can withstand high regenerating temperatures. This is important since the temperatureof exhaust gas of an engine is relatively high (>2508C). In this study, synthetic zeolite 13�pallets with diameters of 2±3 mm are selected as adsorbent and distilled water is used asadsorbate.The adsorption equilibrium for water in zeolite 13� are measured at various temperatures

and pressures. The equilibrium data were correlated with the linear driving force model [6] as

L.Z. Zhang / Applied Thermal Engineering 20 (2000) 103±114 105

follows:

ln�Pz� � a�weq� � b�weq�=Tz �2�

a�weq� � a0 � a1weq � a2w2eq � a3w

3eq �3�

b�weq� � b0 � b1weq � b2w2eq � b3w

3eq �4�

where Tz is the adsorbent temperature (K); Pz is the pressure of the adsorber (mbar); and weq

is the equilibrium adsorption mass of water for unit mass of adsorbent. Numerical values of aiand bi (i = 0±3) are: a0=13.4167; a1=1.1197; a2=ÿ73.205 � 10ÿ3; a3=1.7211 � 10ÿ3;b0=ÿ7373.04; b1=ÿ67.3361; b2=0.56291; b3=ÿ3.5003 � 10ÿ3. These values are similar tothose presented by [7].Using this model, the isosteres and isopars of the zeolite-13� pair are drawn in Figs. 1 and

2, respectively. They are more convenient to use than isotherms since an ideal adsorption cycleconsists of two isosteres and two isopars.

2.3. Adsorber and heat exchangers

The resistance of the adsorber to exhaust gas must be controlled to within a limit (4000±8000 Pa for non turbo-charged engines and 2400±3000 Pa for turbo-charged engines), toalleviate the in¯uence of waste heat recovery on an engine's performance. For this reason, onepassage of exhaust gas in the adsorber is designed. The con®gurations of the adsorber areshown in Fig. 3. The adsorber is a 0.8 m long cylindrical steel double-tube. The heating or

Fig. 1. Isosteres of zeolite 13 � ±water.


cooling ¯uid ¯ows through the inner tube to supply or to extract heat to/from the adsorberthat is insulated outside the outer tube. Twelve radial ®ns that are symmetrically distributed inthe adsorbent and are welded to the inner tube are used to intensify heat conduction in thebed. And the height of the ®ns is equal to the thickness of the adsorbent. The ®ns are made incopper. The adsorbent particles are packed outside the inner tube and between the ®ns andsurrounded by a stainless steel net to give a space for water vapor passage between the net andthe outer tube. The adsorber is alternatively heated and cooled by engine's exhaust gas andambient air pumped by a blower, respectively. Note that the water vapor in the adsorber canbe transferred to or from the bed radially or axially through the net which could minimize themass transfer resistance in adsorbent. The main characteristics of the adsorber are: dio=57 mm,di=4 mm, doo=159 mm, do=5 mm, H®n=32 mm, d®n=0.5 mm and rz=700 kg/m3. The mass

Fig. 2. Isopars of zeolite 13 � ±water.

Fig. 3. Schematic of the adsorber showing the two tubes, the ®ns, the net, the insulation and the adsorbent.


of zeolite in the adsorber is 6.2 kg and the total mass of the adsorber is 31 kg. Due to thickinsulation (linert=0.017 W/mK, dinert=50 mm) outside the adsorber, the heat ¯ux to ambientis negligible.The evaporator and the condenser are air-®nned-tube heat exchangers. Table 1 gives the

main characteristics of the evaporator and the condenser. Other components used in theexperimental set-up include diesel engine, power gage (0±55 kW), blower (centrifugal, 2.4 m3/min, 30 mmH2O), graduated bottle (0±1800 ml), and fans.

2.4. Instrumentation

The heating source is the exhaust gas from the engine. Its mass ¯ow rate is estimated by theoutput power of the engine with the help of Eq. (1). In the adsorbent, the temperature ismeasured at ®ve di�erent positions (shown in Table 2) by a number of thermocouples (3 mmin diameter, type K). The accuracy of temperature measurement is 0.58C. The temperature isalso measured at the inlet and outlet of the heating/cooling ¯uids, as well as near theevaporator and the condenser. Vapor pressure is measured in the space between the adsorbentand the outer tube, and in the condenser/evaporator, by vacuum meters. The mass ofdesorbed/adsorbed adsorbate is estimated by the variations of water level in the graduatedbottle. All data acquisition is realized by a micro-computer. A real-time measurement isrealized with a data-sampling interval of 0.001 s. The data is stored in hard disk for every2 min. The diesel engine is operated and controlled automatically through the power gage. Theplacement of the components in the test rig is shown in Fig. 4.

3. The experimental procedure

3.1. Preparations

All valves are closed. Before the adsorption cooling, vacuum test and the addition ofadsorbate to the system must be done. First, the adsorber is evacuated while valves 16 and 17

Table 1Characteristics of the evaporator and the condenser

Evaporator Condenser

Number of tubes 1 4Number of tube passage 8 6Length of a passage (mm) 300 160

Number of ®ns 186 53Fin thickness (mm) 0.2 0.2Fin spacing (mm) 2.5 2.5Area of a ®n (mm2) 102 � 26 102 � 152

Dimensions (mm3) 300 � 114 � 52 160 � 102 � 152


are closed, and valves 14 and 15 are opened. A vacuum pump is used to bring the pressure ofthe system to 10 Pa. Then other parts of the system are evacuated to 10 Pa while valve 15 isclosed, and valves 16 and 17 are opened. After evacuations, valves 14, 16 and 17 are all closed.The variations of pressure in the adsorber and other parts of the system are monitored byvacuum meters 22 and 23, respectively. After 24 h, if those variations of pressure are smallerthan 50 Pa, then the vacuum test is passed.Then the adsorber is heated by exhaust gas to 2008C to purge water vapor and other gases

adsorbed on zeolite in the adsorber, prior to the experiments. The adsorber is evacuated againwhile valves 14 and 15 are opened. The purge process continues for about 3 h. After that,valves are closed. Then 1.5 kg of distilled water is introduced into the graduated bottle throughvalve 18. After this step, valve 18 is closed, and valves 15, 19 and 21 are opened. The blower isstarted at the same time. For 12 h after that, water is adsorbed in the adsorber. Then the

Table 2The positions of thermocouples in the bed

Index f/f1 (rÿr0)/(r1ÿr0) z/L

Dot 1 0.5 0.2 0Dot 2 1 0.5 0.25Dot 3 1 0.8 0.5

Dot 4 1 0.5 0.75Dot 5 0.5 0.2 1.0

Fig. 4. Schematic diagram of the test rig: (1) power gage; (2) engine; (3) blower; (4) adsorber; (5) computer; (6)vacuum pump; (7) condenser; (8) graduated bottle; (9) evaporator; (10±13) valves; (14±21) vacuum valves; (22±23)pressure gages; (24±32), thermocouples; (33±34) fans.


blower is stopped, and all the valves are closed. Write down the initial water level in the bottle.It is ready now for experiments.

3.2. Desorption

The engine is started at ®rst, while valve 10 is opened, and valves 11±13 are closed. Theexhaust gas temperature is adjusted slowly to a prescribed value by increasing/decreasing theload of the engine. Then the valves 11 and 13 are opened, and valve 10 is closed, the exhaustgas is introduced through the inner tube of the adsorber. The pressure in the adsorberincreases with increasing bed temperature. When the pressure in adsorber exceeds the designedcondensing pressure, valves 15 and 17 are opened and the fan 33 is started. The water vaporbegins to ¯ow from the adsorber and condense in the condenser. The condensed water liquid isthen stored in the graduated bottle. When the average temperature of adsorbent is higher thanthe designed regenerating temperature, the engine and the fan 33 are stopped. Then the valves15 and 17 are closed, and the desorption process is complete. At this time, write down thewater level in the bottle again.

3.3. Adsorption

This is the second half of the whole cycle. Instantly after the desorption process, valve 10 isopened and then valve 11 is closed, and the valves 12 and 13 are opened. The blower is startedto pump the ambient air to cool down the adsorber. The pressure in the adsorber goes downwith decreasing bed temperature. When the bed pressure goes below the evaporating pressure,the fan 34 is started and valves 15, 16 and 19 are all opened. Water in the bottle ¯ows throughthe throttle valve 21, evaporates in the evaporator, and then gets adsorbed by the adsorbent.When the average temperature in adsorbent is smaller than the designed adsorptiontemperature, the blower and the fan are stopped. All the valves are also closed. The adsorptionprocess is complete. The water level at this moment minus the water level instantly after thedesorption process is the variation of water level during a whole adsorption cooling cycle.

4. Results and discussion

The various heat duties are calculated by the following. The heat condensed

Qc ��

des

mz

�dw

dt

�L�Tc� dt �5�

The cooling production

Qe ��

ads

mz

�dw

dt

�L�Te� dt �6�


The heat transferred to the adsorber during the isosteric heating and the desorption periods is

Qin ��

iso�des

_mwstcpwst�Twst,i ÿ Twst,o� dt �7�

The performance parameters are de®ned as: the coe�cient of performance,

COP � Qe=Qin �8�the speci®c cooling power during a cycle on the basis of the unit weight of adsorbent,

SCP � Qe=�tcycmz� �9�the coe�cient of waste heat cooling,

WCOP � Qe=Qwst �10�where Qwst is the potential waste heat energy that can be recovered without dew pointcorrosion, and it can be estimated by

Qwst � cpwstmwst�Twst,i ÿ Tdew� �11�where mwst is the total mass of exhaust gas ¯owing through the adsorber during the isostericheating and the desorption periods, and Tdew is the dew point temperature of the exhaust gas,which is considered to be 1808C for a diesel engine. Among the coe�cients, WCOP determinesthe cooling capacity an engine can produce with its exhaust gas, while SCP yields the requiredsize of a cooling unit.Many tests have been conducted. One prototype test will be discussed here. Table 3 shows

the operating parameters of the engine and the cycle. Table 4 shows the results of thecorresponding experiment. The evolutions of ¯uids and bed temperature are shown in Figs. 5and 6, respectively. To compare the experimental data with the simulated results, a three-

Table 3Operating parameters used in the test

Symbol or name Value Unit

Engine speed 1500 rpmEngine power 15 kWEngine coolant inlet 63 8CEngine coolant outlet 68 8CTemperature of lubrication 48 8CTad 80 8CTg 200 8CTfhi 310 8CTfci 25 8Ctc 45 8Cte 10 8C


dimensional nonequilibrium model has been set up [8]. The simulated ¯uids outlet temperatureand bed temperature are also plotted in Figs. 5 and 6.It can be shown that the agreement of the results calculated and experimentally obtained is

generally good. From Fig. 6, it is obvious that the slopes of time evolutions of bed temperaturefor isosteric heating or cooling processes are much steeper than those for isobaric heating orcooling processes. The total cycle time is 131.5 min. The adsorption process (76.5 min) is muchlonger than the adsorption process (55 min), so a bigger mass ¯ow rate of cooling air from theblower is bene®cial for an increased SCP and a shortened cycle time. From Fig. 5, we knowthat the heating ¯uid (waste gas) outlet temperature increases from 255 to 2968C with a rise inbed temperature, while the cooling ¯uid (air) outlet temperature decreases from 56 to 308Cwith a decrease in bed temperature, but the temperature variation slope of the waste gas issteeper than that of the cooling ¯uid.The COP of the system is 0.38. Surely, the COPs can be increased with heat recovery, but it

must do so at the expense of a rise in complexity of system. For an intermittent cycle with nointernal heat recovery, the value of 0.38 can be accepted. The WCOP of the system is 0.31; andthe SCP is 25.7 W/kg.For a standard bus (12.2 m long, 2.6 m wide, 3 m high, 49 seats) with a 207 kW diesel

engine, the cooling load is about 17.6 kW, and a conventional air conditioner weighs about300 kg [9]. The waste heat that can be recovered through the exhaust gas from such a bus is atleast 70 kW. So a WCOP of 0.25 is required to meet the demand for cooling load and a SCPof the order of 200 W/kg is desired to keep the bulk and cost of the equipment within theeconomic limits demanded by commercial applications. From the experimental results, it isclear that the demand of WCOP can be easily satis®ed. However, the SCP currently availableis very low. This is due to the low thermal conductivity of the bed (0.2 W/mK) and the lowwall heat transfer coe�cient between the bed and the exchanger (25 W/m2K). To increase thesystem performance, in terms of SCP, an adsorbent with high heat transfer parameters must beused. Fortunately, many e�orts [10±12] have been made to obtain an improved conductivity (avalue of 40 W/m/K for thermal conductivity was reported by Mauran et al. [10]), and it seemspossible that an overall heat transfer coe�cient of more than 100 kW/m3/K can be achieved[13]. If so, the SCP of the system could be greatly increased.However, the intensi®cation of heat transfer in the adsorbent bed leads in general to lower

Table 4

Experimental results of performance

Symbol Simulated Experimental Unit Error (%)

Qin 2640.1 2956.8 kJ 10.71Qc 1340.2 1367.8 kJ 8.5

Qe 1140.1 1113.6 kJ 2.3tcyc 130.1 131.5 min 1.1wcyc 0.089 0.082 kg/kg 7.9COP 0.43 0.38 13.2

SCP 26.6 25.7 W/kg 3.5


permeability. Studies have shown that the system performance could be seriously deterioratedby external mass transfer resistance if the bed's permeability is smaller than a critical value[14]. This problem can be solved by such methods as reducing the bed dimensions and usingarteries to increase vapor passage space. It is certain that much work needs to be done.

Fig. 5. Time evolutions of heating/cooling ¯uid outlet temperature.

Fig. 6. Time evolutions of bed temperature at two di�erent bed positions: Dot 1, f/f1=0.5, (rÿr0)/(r1ÿr0)=0.2, z/L = 0; Dot 3, f/f1=1.0, (rÿr0)/(r1ÿr0)=0.8, z/L = 0.5.


5. Conclusions

In this paper, an experimental intermittent adsorption cooling system driven by the exhaustgas of a diesel engine is designed and tested. The adsorber is a double-tube pipe packed withzeolite 13� pallets inside. To intensify heat transfer, 12 radial ®ns are welded to the inner tube.To evaluate the performance, the control and instrumentation facilities of the test rig are alsodeveloped. Experimental results show that this prototype can be successfully used for wasteheat driven air conditioning. The COP of the system is 0.38, and the SCP is 25.7 W/kg. For astationary system (like heat-cooling-electricity co-generation), these parameters may be veryencouraging. However, for a practical automobile waste heat adsorption cooling system(mobile), the demand for WCOP is satis®ed, but for SCP (concerning bulk and cost), furtherresearch is needed. Modi®cations to the system are being made in our laboratory.

References

[1] R.Q. Zhu, B.Q. Han, M.Z. Lin, Y.Z. Yu, Experimental investigation on an adsorption system for producing

chilled water, Int. J. Refrig. 15 (1992) 31±34.[2] M. Suzuki, Application of adsorption cooling systems to automobiles, Heat Recovery Systems CHP 13 (1993)

335±340.

[3] L.Z. Zhang, L. Wang, Performance estimation of an adsorption cooling system for automobile waste heatrecovery, Appl. Therm. Engng 17 (1997) 1127±1139.

[4] R.E. Critoph, Performance limitations of adsorption cycles for solar cooling, Solar Energy 41 (1988) 21±31.

[5] G. Cacciola, G. Restuccia, Progress on adsorption heat pumps, Heat Recovery Systems CHP 14 (1994) 409±420.

[6] M. Pons, Ph Grenier, A phenomenological adsorption equilibrium law extracted from experimental and theor-etical considerations applied to the activedcarbon+methanol pair, Carbon 24 (1986) 615±625.

[7] G. Cacciola, G. Restuccia, Reversible adsorption heat pump: a thermodynamic model, Int. J. Refrig. 18 (1995)100±106.

[8] L.Z. Zhang, Z. Wang, Momentum and heat transfer in the adsorbent of a waste-heat adsorption cooling sys-

tem, Energy (in press).[9] A.A. Pesaran, Y.O. Parent, D. Bharathan, Non-CFC air conditioning for transit buses, SAE Trans. 101 (1992)

750±759.

[10] S. Mauran, P. Prades, F. L'haridon, Heat and mass transfer in consolidated reacting beds for thermochemicalsystems, Heat Recovery Systems CHP 13 (1993) 315±319.

[11] J.J. Guilleminot, A. Choisier, J.B. Chalfen, S. Nicolas, J.L. Reymoney, Heat transfer intensi®cation in ®xedbed adsorbers, Heat Recovery Systems CHP 13 (1993) 297±300.

[12] R.E. Critoph, R. Thorpe, Momentum and heat transfer by forced convection in ®xed beds for granular activecarbon, Appl. Therm. Engng 16 (1996) 419±427.

[13] F. Meunier, Solid sorption: an alternative to CFCs, Heat Recovery Systems CHP 13 (1993) 289±295.

[14] L.Z. Zhang, L. Wang, E�ects of coupled heat and mass transfers in adsorbent on the performance of a wasteheat adsorption cooling unit, Appl. Therm. Engng 19 (1999) 195±215.


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2000_Zhang_Design and Testing of an Automobile Waste Heat Adsorption Cooling System