Adsorption Refrigeration Technology

Theory and Application

Ruzhu Wang, Liwei Wang, Jingyi Wu

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9781118197431
• eBook ISBN: 9781118197479
• Edition: 1
• Topic: Technologie et ingénierie
• Subtopic: Ingénierie de la construction et de l'architecture

Gives readers a detailed understanding of adsorption refrigeration technology, with a focus on practical applications and environmental concerns

Systematically covering the technology of adsorption refrigeration, this book provides readers with a technical understanding of the topic as well as detailed information on the state-of-the-art from leading researchers in the field. Introducing readers to background on the development of adsorption refrigeration, the authors also cover the development of adsorbents, various thermodynamic theories, the design of adsorption systems and adsorption refrigeration cycles. The book guides readers through the research process, covering key aspects such as: the principle of adsorption refrigeration; choosing adsorbents according to different characteristics; thermodynamic equations; methods for the design of heat exchangers for adsorbers; and the advanced adsorption cycles needed. It is also valuable as a reference for professionals working in these areas.

• Covers state-of-the art of adsorption research and technologies for relevant applications, working from adsorption working pairs through to the application of adsorption refrigeration technology for low grade heat recovery
• Assesses sustainable alternatives to traditional refrigeration methods, such as the application of adsorption refrigeration systems for solar energy and waste heat
• Includes a key chapter on the design of adsorption refrigeration systems as a tutorial for readers new to the topic; the calculation models for different components and working processes are also included
• Takes real-world examples giving an insight into existing products and installations and enabling readers to apply the knowledge to their own work

Academics researching low grade energy utilization and refrigeration; Graduate students of refrigeration and low grade energy utilization; Experienced engineers wanting to renew knowledge of adsorption technology, Engineers working at companies developing adsorption chillers; Graduate students working on thermally driven systems; Advanced undergraduates for the Refrigeration Principle as a part of thermal driven refrigeration technology.

Chapter 1
Introduction

Sustainable development is a common pursuit for people worldwide and energy utilization is a key element. Generally, energy will be consumed in large amounts as the economy of society develops rapidly, and a careful eye needs to be kept on environmental pollution. How to coordinate the balance between energy utilization, economy development, and environmental protection is one of the most important strategies for sustainable development.

With regards to environmental protection, the ozonosphere depletion by chlorofluorocarbons (CFCs), which causes the ultraviolet rays of the sun to be insufficiently blocked and thus threatens life on the earth, has been commonly recognized worldwide. CFCs are very important substances in compression refrigeration. As a type of substitute substance, HCFCs can only be temporally utilized because they also have a negative influence on the ozonosphere. Meanwhile, with regards to central heating systems, the combustion of gases and coal releases CO₂ into the environment. Similarly, CFCs produce the greenhouse effect that is becoming more and more serious as the desire for comfortable living conditions all over the world becomes greater and greater. Finding a type of green technology that can be used in air conditioning and heat pumps is very important with regards to solving the problems caused by traditional compression refrigeration technology.

Another critical problem for refrigeration and heat pumps is energy utilization. Traditional compression refrigerators and heat pumps are commonly driven by electricity. Demands for electricity increase as societies develop. According to data provided by the energy department of the US between 2003 and 2004, the electricity consumed by air conditioners in the summer is 15.4% of the total electricity consumption. In China too, for example, in Shanghai City, in summer electricity consumption by air conditioning reached 45–56% according to data collected from 2010. If we analyze the energy utilized through the electricity generation process we find that energy efficiency for electrical generation is only about 40–50%, and there is a large amount of energy being released into the environment as waste heat at temperatures of around 70–200 °C. Meanwhile solar energy and geothermal heat also exist in large amounts in the environment as a low grade energy. Developing refrigeration and heat pump technologies driven by such low grade heat is a solution for energy conservation.

Sorption refrigeration and heat pump technology which is driven by low grade heat and utilizes the green refrigerants, is coordinated with the sustainable requirements of current energy and environmental developments. Firstly, the sorption technology requires little electricity, secondly, the refrigerants for the sorption refrigeration generally are the substances of water, ammonia, and methanol, and so on, which are green refrigerants with zero ODP (Ozonosphere depletion potential) and zero GWP (Greenhouse warming potential).

As a type of sorption technology, adsorption refrigeration and heat pumps have been paid more and more attention since the 1970s. If compared with other types of sorption technology driven by low grade heat, firstly, adsorption refrigeration has a wide variety of adsorbents, including different physical and chemical adsorbents; which can be used with low grade heat across a large range of temperatures, and generally we find these adsorbents are driven by low grade heat in the range of 50–400 °C. Secondly, adsorption refrigeration doesn't need the solution pump and rectification equipment, and it also doesn't have the problems of refrigerant pollution and solution crystallization that often happens in absorption refrigeration technology. But, generally, adsorption refrigeration is not as efficient as absorption, and it also has the disadvantages of being a large volume system. Because of these advantages and disadvantages, adsorption refrigeration is recognized by academics as an essential complementary technology for absorption refrigeration.

1.1 Adsorption Phenomena

According to the different types of adsorption processes, adsorption is divided into physical adsorption and chemical adsorption [1]. Physical adsorption is driven by the van der Waals force among the molecules, and generally happens on the surface of adsorbents. Physical adsorption is not selective, which means multi-layer adsorption can be formed. The phenomena of physical adsorption can be treated as the condensation process of the refrigerant inside the adsorbents, and for most adsorbents the adsorption heat is similar to the condensation heat of the refrigerant. The molecules for the physical adsorption won't be decomposed in the desorption process.

Chemical adsorption is different to physical adsorption. A chemical reaction will happen between the adsorbent and the adsorbate, and new types of molecules will be formed in the adsorption process. Commonly, the monolayer of the adsorbate will react with the chemical adsorbent, and after this reaction the chemical adsorbents cannot adsorb more layers of molecules. The newly formed molecules will be decomposed in the desorption process. The adsorption/desorption heat produced will be much larger than the physical adsorption heat. The chemical adsorption is selective. For example, H₂ can be adsorbed by W, Pt, and Ni, but cannot be adsorbed by Cu, Ag, and Zn. It is recognized by academics that physical adsorption will happen before chemical adsorption because the effective distance of the van der Waals force is inversely proportional to the power of 7 of distance, and it is much longer than the effective distance for the chemical reaction. Thus, when the adsorbate molecules approach the solid adsorbent the physical adsorption will proceed first, and will transfer into the chemical adsorption when the distance decreases.

The physical adsorption/desorption mainly depends on the heat and mass transfer performances of the adsorbents. For the desorption process, because the pressure is high, correspondingly the mass transfer process will be accelerated by the high pressure, and the heat transfer performance will be the main criterion for the performance. If the heat transfer performance is intensified the main problem for the adsorption systems will be the permeability of the gas inside the adsorbents. Generally, the permeability is higher when the adsorbent granules are smaller. The kinetic reaction rate will also influence the adsorption/desorption rate.

Because the chemical reaction happens in the chemical adsorption process, the chemical adsorption will be influenced by the heat and mass transfer process of the adsorbents, as well as the chemical reaction process and the reaction kinetics of the molecules. Meanwhile, the adsorption hysteresis also exists for the chemical adsorption because the adsorption activated energy is different from the desorption activated energy. The desorption activated energy is always much larger than the adsorption activated energy because it is the sum of the adsorption activated energy and the adsorption heat, and such a phenomenon will lead to a serious hysteresis phenomenon between adsorption and desorption [2].

For adsorption refrigeration most refrigerant molecules are polar molecular gases that can be absorbed under the van der Waals force, such as ammonia, methanol, and hydrocarbons that can be adsorbed by activated carbon, zeolite, and silica gel. For physical adsorption the cycle adsorption quantity is generally from 10 to 20%. The chemical adsorption has greater cycle concentrations than that of physical adsorbents, for example, for CaCl₂ the cycle adsorption quantity is always larger than 0.4.

The advantage of chemical adsorption refrigeration is the larger adsorption/desorption quantity, which is essential for the improvement of the specific cooling capacity per kilogram adsorbent (SCP, specific cooling power). But the expansion and agglomeration will happen in the chemical adsorption process, and the expansion space always needs to be kept at two times of the adsorbent volume to ensure high mass transfe...

Table of contents

• Cover
• Title Page
• Copyright
• About the Authors
• Preface
• Acknowledgments
• Nomenclature
• Chapter 1: Introduction
• Chapter 2: Adsorption Working Pairs
• Chapter 3: Mechanism and Thermodynamic Properties of Physical Adsorption
• Chapter 4: Mechanism and Thermodynamic Properties of Chemical Adsorption
• Chapter 5: Adsorption Mechanism and Thermodynamic Characteristics of Composite Adsorbents
• Chapter 6: Adsorption Refrigeration Cycles
• Chapter 7: Technology of Adsorption Bed and Adsorption Refrigeration System
• Chapter 8: Design and Performance of the Adsorption Refrigeration System
• Chapter 9: Adsorption Refrigeration Driven by Solar Energy and Waste Heat
• Index
• End User License Agreement