Adsorption Refrigeration

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. New Opportunities for Solar Adsorption Refrigeration 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 issues. 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−ammonia 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 Refrigerants 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 refrigerant. 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

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