ASHRAE Journal - Absorption Refrigeration

14 AS HRAE Jou rna l ash rae .o rg S e p t e m b e r 2 0 1 1

Adsorption (also called “solid sorption”) refrigeration systems use

solid sorption material such as silica gel and zeolite to produce

cooling effect. These systems are attracting increasing attention because

they can be activated by low-grade thermal energy and use refriger-

ants having zero ozone depletion potential and low global warming

potential. The adsorption refrigeration system has several advantages

compared to the absorption refrigeration system.

Wide range of operating tempera-tures.1 Adsorption systems can be acti-vated by a heat source with a temperature as low as 50°C (122°F), while the heat source temperature for an absorption system should be at least 90°C (194°F). Also, adsorption systems have less cor-rosion issues for the adsorbent−refriger-ant working pairs when they incorporate high temperature heat sources compared to an absorption system, while severe

corrosion might occur in absorption sys-tems when the regeneration temperature is greater than 200°C (392°F).

No crystallization issue. In the lith-ium bromide (LiBr) /water absorption system, there is a specific minimum solu-tion temperature for any given LiBr solu-tion concentration below which the salt begins to crystallize out of the solution.2 Crystallization results in interruption of machine operation and possible damage

to the unit. By contrast, in adsorption systems the adsorbent remains in a solid state, which means no crystallization is-sues.

Suitability for application where se-rious vibration occurs.3,4 Absorption systems cannot operate normally under conditions where serious vibration oc-curs, such as in fishing boats and loco-motives, because the absorbent in these systems, which is in a liquid state, may flow from the generator to the condenser or from the absorber to the evaporator. Adsorption systems are suitable for such applications, because their adsorbents stay in a solid state.

Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be clas-sified as physical adsorption or chemi-

About the AuthorsKai Wang, Ph.D., is a postdoctoral research associ-ate, and Edward A. Vineyard, P.E., is group man-ager of the Building Equipment Research Group at Oak Ridge National Laboratory, Oak Ridge, Tenn.

By Kai Wang, Ph.D., Member ASHRAE; Edward A. Vineyard, P.E., Fellow ASHRAE

AdsorptionRefrigeration

New Opportunities for Solar

This article was published in ASHRAE Journal, September 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

Sep tember 2011 ASHRAE Jou rna l 15

cal adsorption. In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the Van der Waals’ type. Since the forces of attraction are weak, the process of physical adsorption can be easily re-versed by heating. In chemical adsorption, the forces of attrac-tion and chemical bonds between the adsorbate and adsorbent molecules are strong. The adsorbate and adsorbent molecules change their original state after the adsorption process, e.g., complexation occurs between chlorides and ammonia. More-over, chemical adsorption also exhibits the phenomena of salt swelling and agglomeration, which are critical to heat and mass transfer performance.1 The major drawbacks of adsorp-tion systems are their low energy efficiency, the COP (coeffi-cient of performance: the ratio of cooling capacity to thermal energy supplied to the system) is usually less than 0.4, due to the thermal coupling irreversibility.5

Adsorbents and RefrigerantsThe adsorbents used in adsorption systems are categorized

as physical, chemical, or composite adsorbents, according to the nature of the forces involved in the adsorption process. The types, characteristics, advantages, and disadvantages of differ-ent adsorbents are summarized in this section. Two parameters are widely used to evaluate the performance of an adsorption system and adsorbents, namely, COP and SCP (specific cool-ing power: the ratio of cooling capacity to mass of adsorbent in the adsorbers).

Physical AdsorbentsThe commonly used physical adsorbents for adsorption re-

frigeration systems are activated carbon, silica gel and zeolite.Activated carbon is a form of carbon that has been pro-

cessed to make it extremely porous, and it has a large surface area available for adsorption. Methanol and am-monia are the most common refrigerants paired with ac-tivated carbon. Activated carbon−methanol is one of the most promising working pairs in practical systems because of its large adsorption quantity and low adsorption heat (about 1800 to 2000 kJ·kg–1 (773.9 to 859.8 Btu/lb).1 Low adsorption heat is beneficial to the system’s COP because the majority of heat consumption in the desorption phase is the adsorption heat. Another advantage of activated car-bon−methanol is low desorption temperature (about 100°C [212°F]), which is within a suitable temperature range for using solar energy as a heat source. However, activated carbon will catalyze methanol to decompose into dimethyl ether when the temperature is higher than 120°C (248°F).6 Since typical pressures in an activated carbon−methanol system are subatmospheric, a hermetically sealed outer vessel is required.

Activated carbon−ammonia has almost the same adsorp-tion heat as the activated carbon−methanol working pair. The main difference is the much higher operating pressure (about 1600 kPa [232 psia] when the condensing temperature is 40°C [104°F]) of activated carbon−ammonia. The high operating

pressure leads to rather small pipe diameters and relatively compact heat exchangers, as compared to activated carbon−methanol. Another advantage of activated carbon−ammonia is the possibility of using heat sources at 200°C (392°F) or above.7 The drawbacks of this working pair are the toxicity and pungent smell of ammonia.

Silica gel is a granular, highly porous form of silica made synthetically from sodium silicate. For the silica gel−wa-ter working pair, the adsorption heat is about 2500 kJ/kg (1074.8 Btu/lb) and the desorption temperature could be as low as 50°C (122°F).1 Such a low desorption tempera-ture makes it suitable for solar energy use. There is about 4% to 6% (by weight) of water connected with a single hy-droxyl group on the surface of a silica atom, which cannot be removed; otherwise the silica gel would lose its adsorp-tion capability. Thus, the desorption temperature cannot be higher than 120°C (248°F), and it is generally lower than 90°C (194°F).1 One of the drawbacks of the silica gel−water working pair is its low adsorption quantity (about 0.2 kg water/kg [0.2 lb water/lb] silica gel). Another drawback is the limitation of evaporating temperature due to the freezing point of water.

Zeolite is a type of alumina silicate crystal composed of alkali or alkali soil. The adsorption heat of zeolite−water is higher than that of silica gel−water, at about 3300 to 4200 kJ·kg–1 (1418.7 to 1805.7 Btu/lb).1 The desorption tempera-ture of zeolite−water is higher than 200°C (392°F) due to its stable performance at high temperatures. The drawbacks of zeolite−water are the same as for silica gel−water, low adsorp-tion quantity and inability to produce evaporating tempera-tures below 0°C (32°F).

Chemical AdsorbentsChemical adsorption is characterized by the strong chemical

bond between the adsorbent and the refrigerant. The chemical bond mainly includes the functions of complexation, coordi-nation, hydrogenation and oxidization.1 The chemical adsorp-tion reaction is represented in Equation 1:8

(1)

The equilibrium of this reaction is monovariant. Since the liquid-vapor equilibrium is also monovariant, the solid−gas and liquid−vapor equilibrium lines can be calculated using the Clausius-Clapeyron equation,8

Ln PH

RT

S

Req( ) = − ∆ +∆

(2)

∆H is the reaction enthalpy, ∆S is the reaction entropy, R is the gas constant. The most commonly used chemical adsor-bent−refrigerant pair is metal chlorides and ammonia, which exhibits the complexation force. The metal chlorides include calcium chloride (CaCl2), strontium chloride (SrCl2), magne-sium chloride (MgCl2), barium chloride (BaCl2), manganese chloride (MnCl2), and cobalt chloride (CoCl2), among others.

< > < >S v G S v H+ ( )→ ′ + ∆

16 AS HRAE Jou rna l S e p t e m b e r 2 0 1 1

As an example, the complexation reaction of CaCl2 and am-monia (NH3) can be written as

CaCl NH NH CaCl NH2 1 2 3 2 3 2 1 3 2× −( ) + ↔ × + ∆n n n n n H (3)

where the numbers of n1 and n2 could be 2, 4 and 8. The advantage of metal chloride−ammonia working pairs

is the higher adsorption quantity than that of physical adsor-bent−refrigerant pairs. The drawbacks of metal chloride−am-monia pairs are: 1) they require more energy to remove the adsorbed molecules than in physical adsorption, and 2) ad-sorption performance is degraded because of salt swelling and agglomeration in repeated adsorption/desorption processes.

Composite AdsorbentsThe composite adsorbents (or complex compounds)9,10 are

made from porous media, and chemical sorbents are commonly a combination of metal chlorides and expanded graphite, acti-vated carbon, active carbon fiber, zeolite or silica gel. The ob-jectives of using composite adsorbents are: 1) improve heat and mass transfer of chemical adsorbents11, 2) increase the adsorp-tion quantity of physical adsorbents.12 The addition of chemi-cal sorbents to the physical adsorbents could result in higher adsorption quantity than that of physical adsorbents alone.

The main composite adsorbents−refrigerants in the recent literature can be categorized as silica gel and chloride−water, and chlorides and porous media−ammonia.

Composite adsorbents of silica gel and chloride are usually produced using the impregnation method. The silica gel is im-mersed in a chloride salt solution and is then dried to remove the water. The adsorption characteristics of silica gel and chlo-ride composite adsorbents could be modified by 1) changing the silica gel pore structure, 2) changing the type of salt, and 3) changing the proportions of chloride and silica gel.13 Daou, et al.,14 impregnated silica gel with calcium chloride, which improved the COP by 25% and increased the SCP by 283% compared to pure microporous silica gel.

Four types of porous media reported in the recent literature were used to produce composite adsorbents with chlorides: ex-panded graphite,11,12,15 activated carbon,16 and activated car-bon fiber as well as vermiculite.17,18 Han, et al.,19 measured the effective thermal conductivity and gas permeability of a composite adsorbent made from expanded graphite impreg-nated with MnCl2 using the consolidation method. The mea-sured effective thermal conductivities ranged from 14.0 to 25.6 W·m–1·K–1 (8.1 to 14.8 Btu/h·ft·°F) and permeability ranged from 8.1×10–15 to 2.5×10–13 m2 (8.7×10 – 14 to 2.7×10 – 12 ft2). Wang, et al.,11 used the same method to produce the composite

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Sep tember 2011 ASHRAE Jou rna l 17

adsorbent of expanded graphite and CaCl2. Effective thermal conductivities of the expanded graphite–CaCl2 consolidated composite adsorbent are in the range of 7.05 to 9.2 W·m–1·K–1 (4.07 to 5.3 Btu/h·ft·°F). The obtained results indicated that the thermal conductivity of the composite adsorbent has a strong dependence on the bulk density, the mass fraction of expanded graphite and the ammoniated state of CaCl2. Wang, et al.,20 investigated the effective thermal conductivity of a compos-

ducing the desired refrigeration effect. This step is equivalent to the “evaporation” in the vapor-compression cycle. The ba-sic adsorption refrigeration cycle is an intermittent system and the cooling output is not continuous. A minimum of two adsorbers are required to obtain a continuous cooling effect (when the first adsorber is in the adsorption phase, the second adsorber is in desorption phase). These adsorbers will sequen-tially execute the adsorption-desorption process.

ite consolidated adsorbent of expanded graphite and activated carbon, and test results showed that its thermal conductiv-ity could reach as high as 30 W·m–1·K–1 (17.3 Btu/h·ft·°F).

Adsorption Cycle DescriptionBasic Adsorption Cycle

A basic adsorption cycle consists of four steps (Figure 1): heating and pres-surization, desorption and condensa-tion, cooling and depressurization, and adsorption and evaporation. In the first step, the adsorber is heated by a heat source at a temperature of TH. The pres-sure of the adsorber increases from the evaporating pressure up to the condens-ing pressure while the adsorber temper-ature increases. This step is equivalent to the “compression” in the vapor-com-pression cycle. In the second step, the adsorber continues receiving heat and its temperature keeps increasing, which results in the desorption (or generation) of refrigerant vapor from adsorbent in the adsorber. This desorbed vapor is liquefied in the condenser and the con-densing heat is released to the first heat sink at a temperature of TC. This step is equivalent to “condensation” in the vapor-compression cycle.

At the beginning of the third step, the adsorber is disconnected from the con-denser. Then, it is cooled by heat trans-fer fluid at the second heat sink tempera-ture of TM. The pressure of the adsorber decreases from the condensing pressure down to the evaporating pressure due to the decrease in the adsorber tem-perature. This step is equivalent to the “expansion” in the vapor-compression cycle. In the last step, the adsorber keeps releasing heat while being connected to the evaporator. The adsorber tempera-ture continues decreasing, which results in the adsorption of refrigerant vapor from the evaporator by adsorbent, pro-



Advanced Adsorption CycleSince the efficiency of the basic adsorption refrigeration

cycle is low, and the cooling output is not continuous, many advanced adsorption refrigeration cycles (such as the heat re-covery cycle, mass recovery cycle, thermal wave cycle, forced convective thermal wave cycle, etc.) have been developed to improve efficiency and practicability.

The heat recovery cycle is an advanced adsorption cycle used in a system with two or more adsorbers. Figure 2 shows the heat recovery system on the P-T diagram. After the adsorption

phase and desorption phase are finished in the adsorbers, the heat from the hot adsorber is transferred to the cold adsorber by circulating heat transfer fluid between them in a closed loop. The experimental results show that the COP of the system will increase by up to 25% with the heat recovery cycle.21,22

The mass recovery cycle uses refrigerant mass recovery between two adsorbers to effectively increase cooling output and COP of the system. Figure 3 presents a diagram of the mass recovery cycle of an adsorption system. In the end of the desorption−adsorption phase, the high-pressure adsorber

Figure 1: Basic adsorption refrigeration system. A. Heating and pressurization. B. Desorption and condensation. C. Cooling and depressurization. D. Adsorption and evaporation.29

–1/T

Pc

PE

Pc

PE

–1/T

Adsorption and Evaporation

TH

TM

TC

TE

TH

TM

TC

TE

Throttling Valve

Throttling Valve

Adsorber Condenser

Absorbed Vapor

Evaporator

Absorbed Vapor

Evaporator

Adsorber Condenser

QH

QM

QE

QC

QH

QM

QE

QC

–1/T

Ln(P)

Pc

PE

Ln(P)

Pc

PE

Heating and Pressurization

–1/T

TH

Desorption and Condensation

TM

TC

TE

TH

TM

TC

TE

Desorber Condenser

Throttling Valve

Throttling Valve

Desorber Condenser

Desorbed Vapor

Desorbed Vapor

QH

QM

QE

QC

QH

QM

QE

QC

B

CD

A

Ln(P) Ln(P)Cooling and

Depressurization



is connected to the low-pressure adsorber in a closed loop. The refrigerant in the high-pressure adsorber will be re-adsorbed by the adsorbent in the low-pressure adsorber due to the pres-sure difference between the two adsorbers. In a mass recov-ery process, the adsorption quantity of adsorbent is increased, which causes the cooling capacity and COP to increase. The experimental results showed the mass recovery cycle may help obtain a COP increase of more than 10%.21

The concept of thermal wave cycle, proposed by Shelton, et al.,23,24 is shown in Figure 4.25 The heat transfer fluid circu-lates through four components: (1) Adsorber 1 in adsorption phase, (2) the heat source; (3) Adsorber 2 in desorption phase, and (4) heat sink. The adsorption heat released from Adsorber 1 is recovered by the heat transfer fluid and transferred to Ad-sorber 2, and only limited thermal energy is required from the heat source since about 65% of the total energy received by each adsorber can be internally recovered.26 Experimental re-sults showed the COP of a two-bed adsorption air conditioner (zeolite−water) with thermal wave cycle was approximately 1.0 in cooling season.27 Critoph28 invented and theoretically investigated the convection thermal wave cycle, which uses re-frigerant as a heat transfer medium for internal heat recovery. The simulation results predicted a COP of 0.95 for this system when the evaporating temperature and condensing tempera-ture are 0°C and 42°C (32°F and 107.6°F), respectively.

Performance of Adsorption SystemsTable 125 summarizes the performance of some typical ad-

sorption refrigeration systems that were manufactured and tested in the last 20 years for use of waste heat and solar en-ergy. These results were obtained under various operating con-ditions; hence they should not be compared to one another.

However, they could be used as a reference to what can be expected from adsorption refrigeration systems.

SummaryCompared to the vapor compression refrigeration systems,

adsorption systems have the following advantages: 1) they can be driven by waste heat and low-grade heat such as solar en-ergy; 2) they use environmentally friendly fluids such as water or ammonia as refrigerants. The major drawbacks of adsorption systems are their low energy efficiency (low COP and SCP).

Silica gel−water and activated carbon−methanol are suitable working pairs for low temperature waste heat and solar energy due to their relatively low desorption temperatures. Zeolite−water, activated carbon−ammonia, and metal chlorides−am-monia, as well as composite adsorbents−ammonia can be used in adsorption systems driven by high temperature waste heat. Since the typical pressures in silica gel−water, zeolite−water, and activated carbon−methanol systems are subatmospheric, a hermetically sealed outer vessel is essential to maintain good machine performance.

The basic adsorption refrigeration cycle is an intermittent system and the cooling output is not continuous. A minimum of two adsorbers are required to obtain a continuous cooling effect (when the first adsorber is in the adsorption phase, the second adsorber is in the desorption phase). Several advanced adsorption cycles (such as heat recovery cycle, mass recov-

Heating and Pressurization

Cold Adsorber –1/T

Hot Adsorber –1/T

Cooling and Depressurization

Ln(P)

Pc

PE

Ln(P)

Pc

PE

Figure 2: Pressure-temperature diagram of heat recovery cycle.

High Pressure Desorber

Low Pressure Adsorber

Open

Refrigerant Vapor

TH TM

Figure 3: Diagram of mass recovery cycle.

Reversible Pump

Tem

pera

ture

A

Adsorber 1

Adsorber 2

Condenser

Evaporator

Energy Supplied by The Heat SourceHeat

Source

Energy Released To the Adsorber

Heat Sink

B C

D

Figure 4: Thermal wave adsorption cycle.

A



ery cycle, and thermal wave adsorption cycle) have been de-veloped to improve efficiency and practicality. Although the advanced cycles can improve the adsorption system perfor-mance, the complexity and the initial costs of the system also increase. In these advanced cycles, the mass recovery cycle has the potential to be a cost-effective way to boost the COP and SCP of the adsorption systems.25

Although the adsorption refrigeration systems have several advantages over vapor compression refrigeration systems, there are several challenges (such as improvement in systems’ energy efficiency and/or reduction of manufacturing costs, ad-vanced cycles with less thermal coupling irreversibilities, and formulation of new composite adsorbents with enhanced ad-sorption capacity and improved heat and mass transfer prop-erties) to overcome before they can be considered as possible alternatives to replace the present vapor compression systems, especially in regions with abundant waste heat or solar en-ergy resources available. These challenges also point to new research and development opportunities and leave opportunity for considerable creativity.

Application Heat Source Temperature or Insolation Working Pair COP SCP or Ice Production Year

Ice Making

20 MJ m–2 day–1 AC – Methanol 0.12 6 kg day–1m–2 1986

105°C AC – NH3 0.10 35 W kg–1 1997

18.1 to 19.2 MJ m–2 day–1 AC – Methanol 0.12 to 0.14 5.0 to 6.0 kg day–1m–2 2002

17 to 20 MJ m–2day–1 AC – Methanol 0.13 to 0.15 6.0 to 7.0 kg day–1m–2 2004

15.4 MJ m–2 day–1 Silica Gel – Water 0.16 a 2.05 MJ m–2day–1 2004

20 MJ m–2 day–1 AC – Blackened Steel – Methanol 0.16 9.4 kg day–1m–2 b 2004

<120°C AC – Methanol 0.18 27 W kg–1 2005

115°C AC+CaCl2 – NH3 0.39 770 W kg–1 c 2006

Chilled Water

55°C Silica Gel – Water 0.36 3.2 kW Unit–1 2001

100°C AC – Methanol 0.40 73.1 W kg–1 2001

65°C Silica Gel – Water 0.28 12.0 kW Unit–1 2004

75 to 95°C Silica Gel – Water 0.35 to 0.60 15.0 kW m–3 2004

80 to 95°C Silica Gel – Water 0.30 to 0.60 20 W kg–1 d 2004

80°C Silica Gel – Water 0.33 to 0.50 91.7 to 171.8 W kg–1 2005

Air Conditioning

232°C AC – NH3 0.42 to 1.19 NI e 1996

204°C Zeolite – Water 0.60 to 1.60 36 to 144 W kg–1 1988

230°C Zeolite – Water 0.41 97 W kg–1 1999

310°C Zeolite – Water 0.38 25.7 W kg–1 2000

100°C AC – NH3 0.20 600 W kg–1 2003

230 to 300°C Zeolite – Water 0.20 to 0.21 21.4 to 30 W kg–1 2004

a Average value obtained during 30 days of continuous operation; b Based on the area of the adsorber, which was different from the area of the reflector panels;c The SCP is based on the mass of CaCl2 inside one adsorbent bed and only for the duration of the adsorption phase; d At generation temperature of 95°C; e Not informed.

Table 1: Performance of adsorption refrigeration systems for different applications.25

AcknowledgmentsThe authors would like to acknowledge Dr. Liwei Wang

and Dr. Ruzhu Wang of Shanghai Jiao Tong University, Shanghai, and Dr. Abdolreza Zaltash, Dr. Moonis R. Ally and Erica Atkin of Oak Ridge National Laboratory, Oak Ridge, Tenn., for their support, enlightening discussions and insights.

NoteFigure 4 and Table 1 are reprinted from Progress in Energy

and Combustion Science, 32(4), R.Z. Wang, R.G. Oliveira, “Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy,” pp. 424 – 458 with per-mission from Elsevier.

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ASHRAE Journal - Absorption Refrigeration