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New Opportunities for Solar

By Kai Wang, Ph.D., Member ASHRAE; Edward A. Vineyard, P.E., Fellow ASHRAE to the unit. By contrast, in adsorption systems the adsorbent remains in a solid dsorption (also called “solid sorption”) refrigeration systems use state, which means no crystallization is- sues. solid sorption material such as silica gel and zeolite to produce Suitability for application where se- A 3,4 rious vibration occurs. Absorption cooling effect. These systems are attracting increasing attention because systems cannot operate normally under conditions where serious vibration oc- they can be activated by low-grade thermal energy and use refriger- curs, such as in fishing boats and loco- motives, because the absorbent in these ants having zero ozone depletion potential and low global warming systems, which is in a liquid state, may flow from the generator to the condenser potential. The adsorption refrigeration system has several advantages or from the absorber to the evaporator. compared to the absorption refrigeration system. Adsorption systems are suitable for such applications, because their adsorbents Wide range of operating tempera- corrosion might occur in absorption sys- stay in a solid state. tures.1 Adsorption systems can be acti- tems when the regeneration temperature Depending on the nature of attractive vated by a heat source with a temperature is greater than 200°C (392°F). forces existing between the adsorbate as low as 50°C (122°F), while the heat No crystallization issue. In the lith- and adsorbent, adsorption can be clas- source temperature for an absorption ium bromide (LiBr) /water absorption sified as physical adsorption or chemi- system should be at least 90°C (194°F). system, there is a specific minimum solu- Also, adsorption systems have less cor- tion temperature for any given LiBr solu- About the Authors rosion issues for the adsorbent−refriger- tion concentration below which the salt Kai Wang, Ph.D., ant working pairs when they incorporate begins to crystallize out of the solution.2 is a postdoctoral research associ- ate, and Edward A. Vineyard, P.E., is group man- high temperature heat sources compared Crystallization results in interruption of ager of the Building Equipment Research Group at to an absorption system, while severe machine operation and possible damage Oak Ridge National Laboratory, Oak Ridge, Tenn.

14 ASHRAE Journal ashrae.org September 2011 cal adsorption. In physical adsorption, the forces of attraction pressure leads to rather small pipe diameters and relatively between the molecules of the adsorbate and the adsorbent compact heat exchangers, as compared to activated carbon− are of the Van der Waals’ type. Since the forces of attraction methanol. Another advantage of activated carbon− are weak, the process of physical adsorption can be easily re- is the possibility of using heat sources at 200°C (392°F) or versed by heating. In chemical adsorption, the forces of attrac- above.7 The drawbacks of this working pair are the toxicity tion and chemical bonds between the adsorbate and adsorbent and pungent smell of ammonia. molecules are strong. The adsorbate and adsorbent molecules Silica gel is a granular, highly porous form of silica made change their original state after the adsorption process, e.g., synthetically from sodium silicate. For the silica gel−wa- complexation occurs between chlorides and ammonia. More- ter working pair, the adsorption heat is about 2500 kJ/kg over, chemical adsorption also exhibits the phenomena of salt (1074.8 Btu/lb) and the desorption temperature could be swelling and agglomeration, which are critical to heat and as low as 50°C (122°F).1 Such a low desorption tempera- mass transfer performance.1 The major drawbacks of adsorp- ture makes it suitable for solar energy use. There is about tion systems are their low energy efficiency, the COP (coeffi- 4% to 6% (by weight) of water connected with a single hy- cient of performance: the ratio of cooling capacity to thermal droxyl group on the surface of a silica atom, which cannot energy supplied to the system) is usually less than 0.4, due to be removed; otherwise the silica gel would lose its adsorp- the thermal coupling irreversibility.5 tion capability. Thus, the desorption temperature cannot be higher than 120°C (248°F), and it is generally lower than Adsorbents and 90°C (194°F).1 One of the drawbacks of the silica gel−water The adsorbents used in adsorption systems are categorized working pair is its low adsorption quantity (about 0.2 kg as physical, chemical, or composite adsorbents, according to water/kg [0.2 lb water/lb] silica gel). Another drawback is the nature of the forces involved in the adsorption process. The the limitation of evaporating temperature due to the freezing types, characteristics, advantages, and disadvantages of differ- point of water. ent adsorbents are summarized in this section. Two parameters Zeolite is a type of alumina silicate crystal composed of are widely used to evaluate the performance of an adsorption alkali or alkali soil. The adsorption heat of zeolite−water is system and adsorbents, namely, COP and SCP (specific cool- higher than that of silica gel−water, at about 3300 to 4200 ing power: the ratio of cooling capacity to mass of adsorbent kJ·kg–1 (1418.7 to 1805.7 Btu/lb).1 The desorption tempera- in the adsorbers). ture of zeolite−water is higher than 200°C (392°F) due to its stable performance at high temperatures. The drawbacks of Physical Adsorbents zeolite−water are the same as for silica gel−water, low adsorp- The commonly used physical adsorbents for adsorption re- tion quantity and inability to produce evaporating tempera- frigeration systems are activated carbon, silica gel and zeolite. tures below 0°C (32°F). Activated carbon is a form of carbon that has been pro- cessed to make it extremely porous, and it has a large Chemical Adsorbents surface area available for adsorption. Methanol and am- Chemical adsorption is characterized by the strong chemical monia are the most common refrigerants paired with ac- bond between the adsorbent and the . The chemical tivated carbon. Activated carbon−methanol is one of the bond mainly includes the functions of complexation, coordi- most promising working pairs in practical systems because nation, hydrogenation and oxidization.1 The chemical adsorp- of its large adsorption quantity and low adsorption heat tion reaction is represented in Equation 1:8 (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 <>Sv+(GS) → <>′ +∆vH (1) the majority of heat consumption in the desorption phase is the adsorption heat. Another advantage of activated car- The equilibrium of this reaction is monovariant. Since the bon−methanol is low desorption temperature (about 100°C liquid-vapor equilibrium is also monovariant, the solid−gas [212°F]), which is within a suitable temperature range for and liquid−vapor equilibrium lines can be calculated using the using solar energy as a heat source. However, activated Clausius-Clapeyron equation,8 carbon will catalyze methanol to decompose into dimethyl 6 ∆H ∆S ether when the temperature is higher than 120°C (248°F). Ln()Peq =− + (2) Since typical pressures in an activated carbon−methanol RT R system are subatmospheric, a hermetically sealed outer ∆H is the reaction enthalpy, ∆S is the reaction entropy, R is vessel is required. the gas constant. The most commonly used chemical adsor- Activated carbon−ammonia has almost the same adsorp- bent−refrigerant pair is metal chlorides and ammonia, which tion heat as the activated carbon−methanol working pair. The exhibits the complexation force. The metal chlorides include main difference is the much higher operating pressure (about calcium chloride (CaCl2), strontium chloride (SrCl2), magne- 1600 kPa [232 psia] when the condensing temperature is 40°C sium chloride (MgCl2), barium chloride (BaCl2), manganese [104°F]) of activated carbon−ammonia. The high operating chloride (MnCl2), and cobalt chloride (CoCl2), among others.

September 2011 ASHRAE Journal 15 As an example, the complexation reaction of CaCl2 and am- The main composite adsorbents−refrigerants in the recent monia (NH3) can be written as literature can be categorized as silica gel and chloride−water, and chlorides and porous media−ammonia.

CaCl21×−()nn23NH+↔nn23 NH CaCl21×+NH32nH∆ (3) Composite adsorbents of silica gel and chloride are usually produced using the impregnation method. The silica gel is im- where the numbers of n1 and n2 could be 2, 4 and 8. mersed in a chloride salt solution and is then dried to remove The advantage of metal chloride−ammonia working pairs the water. The adsorption characteristics of silica gel and chlo- is the higher adsorption quantity than that of physical adsor- ride composite adsorbents could be modified by 1) changing bent−refrigerant pairs. The drawbacks of metal chloride−am- the silica gel pore structure, 2) changing the type of salt, and monia pairs are: 1) they require more energy to remove the 3) changing the proportions of chloride and silica gel.13 Daou, adsorbed molecules than in physical adsorption, and 2) ad- et al.,14 impregnated silica gel with calcium chloride, which sorption performance is degraded because of salt swelling and improved the COP by 25% and increased the SCP by 283% agglomeration in repeated adsorption/desorption processes. compared to pure microporous silica gel. Four types of porous media reported in the recent literature Composite Adsorbents were used to produce composite adsorbents with chlorides: ex- The composite adsorbents (or complex compounds)9,10 are panded graphite,11,12,15 activated carbon,16 and activated car- made from porous media, and chemical sorbents are commonly bon fiber as well as vermiculite.17,18 Han, et al.,19 measured a combination of metal chlorides and expanded graphite, acti- the effective thermal conductivity and gas permeability of a vated carbon, active carbon fiber, zeolite or silica gel. The ob- composite adsorbent made from expanded graphite impreg- jectives of using composite adsorbents are: 1) improve heat and nated with MnCl2 using the consolidation method. The mea- mass transfer of chemical adsorbents11, 2) increase the adsorp- sured effective thermal conductivities ranged from 14.0 to 25.6 tion quantity of physical adsorbents.12 The addition of chemi- W·m–1·K–1 (8.1 to 14.8 Btu/h·ft·°F) and permeability ranged cal sorbents to the physical adsorbents could result in higher from 8.1×10–15 to 2.5×10–13 m2 (8.7×10 – 14 to 2.7×10 – 12 ft2). adsorption quantity than that of physical adsorbents alone. Wang, et al.,11 used the same method to produce the composite

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16 ASHRAE Journal September 2011 adsorbent of expanded graphite and CaCl2. Effective thermal ducing the desired refrigeration effect. This step is equivalent conductivities of the expanded graphite–CaCl2 consolidated to the “evaporation” in the vapor-compression cycle. The ba- composite adsorbent are in the range of 7.05 to 9.2 W·m–1·K–1 sic adsorption refrigeration cycle is an intermittent system (4.07 to 5.3 Btu/h·ft·°F). The obtained results indicated that the and the cooling output is not continuous. A minimum of two thermal conductivity of the composite adsorbent has a strong adsorbers are required to obtain a continuous cooling effect dependence on the bulk density, the mass fraction of expanded (when the first adsorber is in the adsorption phase, the second 20 graphite and the ammoniated state of CaCl2. Wang, et al., adsorber is in desorption phase). These adsorbers will sequen- investigated the effective thermal conductivity of a compos- 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 Description Basic 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- Advertisement formerly in this space. 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-

September 2011 ASHRAE Journal 17 Desorption and Ln(P) Ln(P) Condensation Heating and Pressurization A B P c Pc

P E PE

–1/T –1/T

Desorber Condenser Desorber Condenser Desorbed Q Desorbed QH T T H T TH C H Vapor C Vapor Q QC C

Q QM T M TM Throttling M Throttling Valve Valve

T TE Q E QE E

Cooling and Ln(P) D Ln(P) Depressurization C P Pc c

P PE E Adsorption and Evaporation

–1/T –1/T

Adsorber Condenser Adsorber Condenser

QH QH T TC TH TC H QC QC

Q QM T M TM Throttling M Throttling Valve Valve

Absorbed Vapor Absorbed Vapor TE TE Q QE E Evaporator Evaporator

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

Advanced Adsorption Cycle phase and desorption phase are finished in the adsorbers, the Since the efficiency of the basic adsorption refrigeration heat from the hot adsorber is transferred to the cold adsorber cycle is low, and the cooling output is not continuous, many by circulating heat transfer fluid between them in a closed loop. advanced adsorption refrigeration cycles (such as the heat re- The experimental results show that the COP of the system will covery cycle, mass recovery cycle, thermal wave cycle, forced increase by up to 25% with the heat recovery cycle.21,22 convective thermal wave cycle, etc.) have been developed to The mass recovery cycle uses refrigerant mass recovery improve efficiency and practicability. between two adsorbers to effectively increase cooling output The heat recovery cycle is an advanced adsorption cycle used and COP of the system. Figure 3 presents a diagram of the in a system with two or more adsorbers. Figure 2 shows the mass recovery cycle of an adsorption system. In the end of heat recovery system on the P-T diagram. After the adsorption the desorption−adsorption phase, the high-pressure adsorber

18 ASHRAE Journal ashrae.org September 2011 Ln(P) High Pressure Low Pressure Heating and Pressurization Desorber Adsorber

Pc Open

TH TM Refrigerant Vapor

P E Figure 3: Diagram of mass recovery cycle.

Energy Supplied by Energy Released Cold Adsorber –1/T The Heat Source To the Adsorber Heat Source Ln(P) B C Adsorber 1

Pc Heat Sink Adsorber 2 A

Temperature D Cooling and Condenser Depressurization PE

Hot Adsorber –1/T

Figure 2: Pressure-temperature diagram of heat recovery cycle. Evaporator 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- Reversible Pump ery process, the adsorption quantity of adsorbent is increased, Figure 4: Thermal wave adsorption cycle. which causes the cooling capacity and COP to increase. The experimental results showed the mass recovery cycle may help However, they could be used as a reference to what can be obtain a COP increase of more than 10%.21 expected from adsorption refrigeration systems. The concept of thermal wave cycle, proposed by Shelton, et al.,23,24 is shown in Figure 4.25 The heat transfer fluid circu- Summary lates through four components: (1) Adsorber 1 in adsorption Compared to the vapor compression refrigeration systems, phase, (2) the heat source; (3) Adsorber 2 in desorption phase, adsorption systems have the following advantages: 1) they can and (4) heat sink. The adsorption heat released from Adsorber be driven by waste heat and low-grade heat such as solar en- 1 is recovered by the heat transfer fluid and transferred to Ad- ergy; 2) they use environmentally friendly fluids such as water sorber 2, and only limited thermal energy is required from the or ammonia as refrigerants. The major drawbacks of adsorption heat source since about 65% of the total energy received by systems are their low energy efficiency (low COP and SCP). each adsorber can be internally recovered.26 Experimental re- Silica gel−water and activated carbon−methanol are suitable sults showed the COP of a two-bed adsorption air conditioner working pairs for low temperature waste heat and solar energy (zeolite−water) with thermal wave cycle was approximately due to their relatively low desorption temperatures. Zeolite− 1.0 in cooling season.27 Critoph28 invented and theoretically water, activated carbon−ammonia, and metal chlorides−am- investigated the convection thermal wave cycle, which uses re- monia, as well as composite adsorbents−ammonia can be used frigerant as a heat transfer medium for internal heat recovery. in adsorption systems driven by high temperature waste heat. The simulation results predicted a COP of 0.95 for this system Since the typical pressures in silica gel−water, zeolite−water, when the evaporating temperature and condensing tempera- and activated carbon−methanol systems are subatmospheric, a ture are 0°C and 42°C (32°F and 107.6°F), respectively. hermetically sealed outer vessel is essential to maintain good machine performance. Performance of Adsorption Systems The basic adsorption refrigeration cycle is an intermittent Table 125 summarizes the performance of some typical ad- system and the cooling output is not continuous. A minimum sorption refrigeration systems that were manufactured and of two adsorbers are required to obtain a continuous cooling tested in the last 20 years for use of waste heat and solar en- effect (when the first adsorber is in the adsorption phase, the ergy. These results were obtained under various operating con- second adsorber is in the desorption phase). Several advanced ditions; hence they should not be compared to one another. adsorption cycles (such as heat recovery cycle, mass recov-

20 ASHRAE Journal ashrae.org September 2011 Application Heat Source Temperature or Insolation Working Pair COP SCP or Ice Production Year

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

–1 105°C AC – NH3 0.10 35 W kg 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 Ice Making 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

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

e 232°C AC – NH3 0.42 to 1.19 NI 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 Air Conditioning 310°C Zeolite – Water 0.38 25.7 W kg–1 2000

–1 100°C AC – NH3 0.20 600 W kg 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 d e The SCP is based on the mass of CaCl2 inside one adsorbent bed and only for the duration of the adsorption phase; At generation temperature of 95°C; Not informed.

Table 1: Performance of adsorption refrigeration systems for different applications.25 ery cycle, and thermal wave adsorption cycle) have been de- Acknowledgments veloped to improve efficiency and practicality. Although the The authors would like to acknowledge Dr. Liwei Wang advanced cycles can improve the adsorption system perfor- and Dr. Ruzhu Wang of Shanghai Jiao Tong University, mance, the complexity and the initial costs of the system also Shanghai, and Dr. Abdolreza Zaltash, Dr. Moonis R. Ally increase. In these advanced cycles, the mass recovery cycle and Erica Atkin of Oak Ridge National Laboratory, Oak has the potential to be a cost-effective way to boost the COP Ridge, Tenn., for their support, enlightening discussions and and SCP of the adsorption systems.25 insights. Although the adsorption refrigeration systems have several advantages over vapor compression refrigeration systems, Note there are several challenges (such as improvement in systems’ Figure 4 and Table 1 are reprinted from Progress in Energy energy efficiency and/or reduction of manufacturing costs, ad- and Combustion Science, 32(4), R.Z. Wang, R.G. Oliveira, vanced cycles with less thermal coupling irreversibilities, and “Adsorption refrigeration—An efficient way to make good formulation of new composite adsorbents with enhanced ad- use of waste heat and solar energy,” pp. 424 – 458 with per- sorption capacity and improved heat and mass transfer prop- mission from Elsevier. erties) to overcome before they can be considered as possible alternatives to replace the present vapor compression systems, References especially in regions with abundant waste heat or solar en- 1. Wang, L.W., R.Z. Wang, R.G. Oliveira. 2009. “A review on ad- ergy resources available. These challenges also point to new sorption working pairs for refrigeration.” Renewable and Sustainable Energy Reviews, 13(3):518 – 534. research and development opportunities and leave opportunity 2. Wang, K., O. Abdelaziz, P. Kisari, E.A. Vineyard. 2011. “State- for considerable creativity. of-the-Art Review on Crystallization Control Technologies for Water/

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Oliveira. 2006. “Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy.”Prog - ress in Energy and Combustion Science 32(4):424 – 458. 26. Pons, M., F. Poyelle. 1999. “Adsorptive machines with advanced cycles for heat pumping or cooling applications.” International Journal of Refrigeration 22(1):27 – 37. 27. Tchernev, D.I., and D.T. Emerson. 1988. “High-efficiency regen- erative zeolite heat pump.” ASHRAE Transactions 94(2):2024 – 2032. 28. Critoph, R.E. 1998. “Forced convection adsorption cycles.” Ap- plied Thermal Engineering 18(9 – 10):799 – 807. 29. Pons, M. “Principle of adsorption cycles for refrigeration or heat pumping.” www.limsi.fr/Individu/mpons/pricyc.htm. Latest access 08/15/2011.

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