ASHRAE Journal - September 2011 - 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 reversed by heating. In chemical adsorption, the forces of attraction 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. Moreover, 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 adsorption systems are their low energy efficiency, the COP (coefficient 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 Refrigerants
The 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 different 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 cooling power: the ratio of cooling capacity to mass of adsorbent in the adsorbers).
Physical Adsorbents
The commonly used physical adsorbents for adsorption refrigeration systems are activated carbon, silica gel and zeolite. Activated carbon is a form of carbon that has been processed to make it extremely porous, and it has a large surface area available for adsorption. Methanol and ammonia are the most common refrigerants paired with activated 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 carbon−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 adsorption 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
September 2011
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−water 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 temperature makes it suitable for solar energy use. There is about 4% to 6% (by weight) of water connected with a single hydroxyl group on the surface of a silica atom, which cannot be removed; otherwise the silica gel would lose its adsorption 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 temperature 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 adsorption quantity and inability to produce evaporating temperatures below 0°C (32°F).
Chemical Adsorbents
Chemical adsorption is characterized by the strong chemical bond between the adsorbent and the refrigerant. The chemical bond mainly includes the functions of complexation, coordination, hydrogenation and oxidization.1 The chemical adsorption reaction is represented in Equation 1:8
+ v(G ) → + v∆H
(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 ( Peq ) = − ∆H ∆S + RT R
(2)
∆H is the reaction enthalpy, ∆S is the reaction entropy, R is the gas constant. The most commonly used chemical adsorbent−refrigerant pair is metal chlorides and ammonia, which exhibits the complexation force. The metal chlorides include calcium chloride (CaCl2), strontium chloride (SrCl2), magnesium chloride (MgCl2), barium chloride (BaCl2), manganese chloride (MnCl2), and cobalt chloride (CoCl2), among others.
ASHRAE Journal 15
ASHRAE Journal - September 2011
Table of Contents for the Digital Edition of ASHRAE Journal - September 2011
Contents
Commentary
Industry News
Meetings and Shows
Feature Articles
Adsorption Refrigeration: New Opportunities for Solar
Optimizing Design & Control of Chilled Water Plants—Part 2: Condenser Water System Design
Designing for Comfort: Selecting Air-Distribution Outlets
Technology Award Case Studies
Library Sets Example
Sustainable Campus
Standing Columns
Building Sciences
IAQ Applications
Emerging Technologies
Products
Classified Advertising
Advertisers Index
Special Supplement: AMCA International inmotion Magazine
ASHRAE Journal - September 2011 - Intro
ASHRAE Journal - September 2011 - Cover1
ASHRAE Journal - September 2011 - Cover2
ASHRAE Journal - September 2011 - 1
ASHRAE Journal - September 2011 - 2
ASHRAE Journal - September 2011 - Contents
ASHRAE Journal - September 2011 - Commentary
ASHRAE Journal - September 2011 - 5
ASHRAE Journal - September 2011 - Industry News
ASHRAE Journal - September 2011 - 7
ASHRAE Journal - September 2011 - 8
ASHRAE Journal - September 2011 - 9
ASHRAE Journal - September 2011 - Meetings and Shows
ASHRAE Journal - September 2011 - 11
ASHRAE Journal - September 2011 - 12
ASHRAE Journal - September 2011 - 13
ASHRAE Journal - September 2011 - Adsorption Refrigeration: New Opportunities for Solar
ASHRAE Journal - September 2011 - 15
ASHRAE Journal - September 2011 - 16
ASHRAE Journal - September 2011 - 17
ASHRAE Journal - September 2011 - 18
ASHRAE Journal - September 2011 - 19
ASHRAE Journal - September 2011 - 20
ASHRAE Journal - September 2011 - 21
ASHRAE Journal - September 2011 - 22
ASHRAE Journal - September 2011 - 23
ASHRAE Journal - September 2011 - 24
ASHRAE Journal - September 2011 - 25
ASHRAE Journal - September 2011 - Optimizing Design & Control of Chilled Water Plants—Part 2: Condenser Water System Design
ASHRAE Journal - September 2011 - 27
ASHRAE Journal - September 2011 - 28
ASHRAE Journal - September 2011 - 29
ASHRAE Journal - September 2011 - 30
ASHRAE Journal - September 2011 - 31
ASHRAE Journal - September 2011 - 32
ASHRAE Journal - September 2011 - 32A
ASHRAE Journal - September 2011 - 32B
ASHRAE Journal - September 2011 - 33
ASHRAE Journal - September 2011 - 34
ASHRAE Journal - September 2011 - 35
ASHRAE Journal - September 2011 - 36
ASHRAE Journal - September 2011 - 37
ASHRAE Journal - September 2011 - Designing for Comfort: Selecting Air-Distribution Outlets
ASHRAE Journal - September 2011 - 39
ASHRAE Journal - September 2011 - 40
ASHRAE Journal - September 2011 - 41
ASHRAE Journal - September 2011 - 42
ASHRAE Journal - September 2011 - 43
ASHRAE Journal - September 2011 - 44
ASHRAE Journal - September 2011 - 45
ASHRAE Journal - September 2011 - 46
ASHRAE Journal - September 2011 - 47
ASHRAE Journal - September 2011 - Library Sets Example
ASHRAE Journal - September 2011 - 49
ASHRAE Journal - September 2011 - 50
ASHRAE Journal - September 2011 - 51
ASHRAE Journal - September 2011 - 52
ASHRAE Journal - September 2011 - 53
ASHRAE Journal - September 2011 - Sustainable Campus
ASHRAE Journal - September 2011 - 55
ASHRAE Journal - September 2011 - 56
ASHRAE Journal - September 2011 - 57
ASHRAE Journal - September 2011 - 58
ASHRAE Journal - September 2011 - 59
ASHRAE Journal - September 2011 - 60
ASHRAE Journal - September 2011 - 61
ASHRAE Journal - September 2011 - 62
ASHRAE Journal - September 2011 - 63
ASHRAE Journal - September 2011 - Building Sciences
ASHRAE Journal - September 2011 - 65
ASHRAE Journal - September 2011 - 66
ASHRAE Journal - September 2011 - 67
ASHRAE Journal - September 2011 - 68
ASHRAE Journal - September 2011 - 69
ASHRAE Journal - September 2011 - 70
ASHRAE Journal - September 2011 - 71
ASHRAE Journal - September 2011 - 72
ASHRAE Journal - September 2011 - 73
ASHRAE Journal - September 2011 - 74
ASHRAE Journal - September 2011 - 75
ASHRAE Journal - September 2011 - IAQ Applications
ASHRAE Journal - September 2011 - 77
ASHRAE Journal - September 2011 - 78
ASHRAE Journal - September 2011 - 79
ASHRAE Journal - September 2011 - 80
ASHRAE Journal - September 2011 - Special Supplement: AMCA International inmotion Magazine
ASHRAE Journal - September 2011 - AMCACover2
ASHRAE Journal - September 2011 - AMCA3
ASHRAE Journal - September 2011 - AMCA4
ASHRAE Journal - September 2011 - AMCA5
ASHRAE Journal - September 2011 - AMCA6
ASHRAE Journal - September 2011 - AMCA7
ASHRAE Journal - September 2011 - AMCA8
ASHRAE Journal - September 2011 - AMCA9
ASHRAE Journal - September 2011 - AMCA10
ASHRAE Journal - September 2011 - AMCA11
ASHRAE Journal - September 2011 - AMCA12
ASHRAE Journal - September 2011 - AMCA13
ASHRAE Journal - September 2011 - AMCA14
ASHRAE Journal - September 2011 - AMCA15
ASHRAE Journal - September 2011 - AMCA16
ASHRAE Journal - September 2011 - AMCA17
ASHRAE Journal - September 2011 - AMCA18
ASHRAE Journal - September 2011 - AMCA19
ASHRAE Journal - September 2011 - AMCA20
ASHRAE Journal - September 2011 - AMCA21
ASHRAE Journal - September 2011 - AMCA22
ASHRAE Journal - September 2011 - AMCA23
ASHRAE Journal - September 2011 - AMCA24
ASHRAE Journal - September 2011 - AMCA25
ASHRAE Journal - September 2011 - AMCA26
ASHRAE Journal - September 2011 - AMCACover3
ASHRAE Journal - September 2011 - AMCACover4
ASHRAE Journal - September 2011 - Emerging Technologies
ASHRAE Journal - September 2011 - 82
ASHRAE Journal - September 2011 - 83
ASHRAE Journal - September 2011 - 84
ASHRAE Journal - September 2011 - 85
ASHRAE Journal - September 2011 - 86
ASHRAE Journal - September 2011 - 87
ASHRAE Journal - September 2011 - 88
ASHRAE Journal - September 2011 - Products
ASHRAE Journal - September 2011 - 90
ASHRAE Journal - September 2011 - 91
ASHRAE Journal - September 2011 - 92
ASHRAE Journal - September 2011 - 93
ASHRAE Journal - September 2011 - Classified Advertising
ASHRAE Journal - September 2011 - 95
ASHRAE Journal - September 2011 - Advertisers Index
ASHRAE Journal - September 2011 - Cover3
ASHRAE Journal - September 2011 - Cover4
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