Impianti chimici II Monographic Thesis

ADSORPTION HEAT PUMPS USING THE ZEOLITE-WATER PAIR: Operating principle and applications

Author: Federica FURNARI

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SUMMARY

Introduction ___________________________________________________________________ 3

Chapter 1 Why choosing water-zeolite? ____________________________________________________ 5

Chapter 2 Operating principle of water-zeolite heat pump ____________________________________ 8

Chapter 3 Technological applications _____________________________________________________ 11
Solar-powered adsorption icemakers _____________________________________________________ 12 Self chilling beer kegs _________________________________________________________________ 15 Solar-powered adsorption air conditioners ________________________________________________ 16

Chapter 4 Key issues for future development _______________________________________________ 18
The heat/mass transfer issue ___________________________________________________________ 19 Heat recovery _______________________________________________________________________ 21

Bibliography _________________________________________________________________ 24

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Introduction

Although adsorptive processes have been extensively studied for gas separation, catalysis, etc., it is only recently that they have been proposed for heat management. The interest in adsorption systems started to increase, firstly due to the oil crisis in the 1970s that lead to a concern about the energy shortage, and then later, in the 1990s, because of ecological problems related to the use of CFCs and HCFCs as refrigerants. Such refrigerants when released into the atmosphere, deplete the ozone layer and contribute to the greenhouse effect. Furthermore, with the increase of the energy consumption worldwide, it is becoming even more urgent to find ways of using the energy resources as efficiently as possible. Thus, machines that can recover waste heat at low temperatures level such as adsorption machines, can be an interesting alternative for a wiser energy management. In comparison with mechanical vapour compression systems, adsorption systems have the benefit of saving energy, if powered by waste heat or solar energy, simpler control, no vibration and lower operation costs. In comparison with liquid absorption systems, adsorption systems can be powered by a large range of heat source temperatures, starting at 50°C and going up to 600°C or even higher. Moreover, the latter system does not need a liquid pump or rectifier for the refrigerant, does not present corrosion problems due to the working pairs normally used, and it is less sensitive to shocks and to the installation position. Although adsorption systems offer all the benefits listed above, they usually also have the drawbacks of low coefficient of performance (COP) and low specific cooling power (SCP). 3

However, these inconveniences can be overcome by enhancing of the heat transfer properties in the adsorber, by increasing the adsorption properties of the working pairs and by better heat management during the adsorption cycle. In the past few decades a great deal of work has been carried out in developing countries to implement such cycles in cold storage and ice making application. More recently, interest has grown in developed areas as well, especially in the context of air conditioning. In this work a small overview on adsorbent-adsorbate pairs is presented (chapter 1), then the operating principle for water-zeolite adsorption heat pumps is illustrated (chapter 2). In chapter 3 some applications of this technology are shown and finally, in chapter 4, a few issues and future technical development are presented.

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Chapter 1

Why choosing water-zeolite?

Choosing the most appropriate adsorbent–adsorbate pair is one of the important factors determining the efficiency of the adsorption heat pumps. An adsorbent suitable to be employed in adsorption heat pumps should have a large adsorption capacity for the selected adsorbate and be easily regenerated regarding the pressure and temperature ranges of operation; it needs to release a good amount of energy during adsorption, to have good thermal conductivity, mass diffusivity, thermal stability; whereas a proper adsorbate should have a high latent heat of vaporization and a suitable boiling point. The lower the temperature at which adsorption occurs relative to the boiling point of the adsorbate, the larger will be the amount adsorbed. Since the desirable lowest adsorption temperature for the adsorption heat pumps is room temperature, the boiling point should be preferentially higher than 20°C. In addition to that, cost, availability and toxicity should be taken into account. CaCl2–ammonia, CaCl2–methanol, silica gel–SO2, zeolite–water, zeolite–methanol, zeolite– freons, activated charcoal–freons and activated charcoal–methanol might be mentioned among the adsorbent–adsorbate combinations tested. In this work particular attention will be posed to the couple zeolite-water. 5

Zeolites (figg.1a-b) are micro-porous crystalline alumina silicates, having internal voids consisting of channels or cavities; various condensable vapours and gases (water, ammonia, carbon dioxide, methanol) are strongly adsorbed on their large internal surface area (≈ 1000 m2/g) and they have special affinity for water.

Fig.1a-Structure of zeolite crystal

Fig.1b-Zeolite crystal

In fact the zeolite–water pair is very suitable to be used in adsorption heat pumps because of the extremely non-linear pressure dependence of its adsorption isotherms (fig.2). The isotherms saturate at low partial pressure, after which the amount adsorbed becomes almost independent of pressure. That is, at ambient temperature zeolites can adsorb large quantities of water vapor even at low partial pressures, and when they are heated, they desorb most of the vapor even at high partial pressures, corresponding to high condenser temperatures. This unique property of the zeolites is especially important in the case where a high condenser temperature and only a moderate regeneration temperature might be employed.

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Fig. 2 - Adsorption Isotherms for water vapour on a zeolite

Since water has a high latent heat of vaporization and a convenient boiling point, the zeolite–water pair is one of the most preferred adsorbent–adsorbate pairs. Although activated charcoal and silica gel have higher adsorption capacities for water, zeolites are preferred in many cases due to the shape of their isotherms and their extremely hydrophilic nature. NaX, NaY, 4A, 5A, 3A, chabazite, clinoptilolite, mordenite and erionite are the types of zeolites tested for their suitability as adsorbents in the adsorption heat pumps up to date. The best results using the zeolite–water pair are said to be achieved with zeolite 13X which has the highest adsorption capacity for water among the types of zeolites that are commonly employed and readily available. Zeolite 4A follows closely owing to its hydrophilic nature.

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Chapter 2

Operating principle of water-zeolite heat pump

A heat pump may provide either a heating or a cooling effect. Refrigeration, ice making, air-conditioning and provision of hot water are the prominent applications of heat pumps. An adsorption heat pump mainly consists of an adsorber, an evaporator and a condenser. The zeolite bed is placed in the adsorber and the water circulates throughout the system. The flow diagram of an adsorption heat pump is illustrated in fig. 3.

Fig. 3-The flow diagram of an adsorption heat pump.

As the first step, the zeolite placed in the adsorber is saturated and the whole system is outgassed and sealed off. In this first phase, the valves between the adsorbent bed and the condenser and between the adsorbent bed and the evaporator are closed. Then the adsorber temperature is allowed to increase with the aid of an available energy source 8

(isosteric heating). When the vapor pressure within the adsorber reaches the pressure determined by the condenser temperature, the valve between the adsorber and the condenser is opened, allowing the desorption of water. Water vapor is liquified in the condenser, releasing its latent heat of condensation (isobaric desorption and condensation). In the meantime, the heating of the zeolite bed goes on until the maximum temperature is attained. The liquified water is introduced into the evaporator after the cooling of the adsorber has commenced. When the vapor pressure inside the adsorber falls down to the pressure determined by the evaporator temperature (isosteric cooling), the valve between the adsorber and the evaporator is opened, allowing the vaporization of water which is cooling down to the adsorption temperature (isobaric cooling and adsorption). The function of the zeolite is to adsorb the vapor produced by the evaporator, maintaining the partial pressure below a certain value so that the circulation of water may continue without any disturbance and complete the cycle. An idealized adsorption heat pump cycle is illustrated in fig. 4. According to the operating principle of an adsorption heat pump, the system might be analyzed by dividing it into four different operational stages that can be outlined as follows: T 1→T 2 isosteric heating T 2→T 3 isobaric desorption T 3→T 4 isosteric cooling T 4→T 1 isobaric adsorption

Fig. 4-The adsorption heat pump cycle.

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A typical idealized cycle starts at a temperature T 1 and at a pressure P e. T 1 is equal to the minimum temperature of the system while P e represents the pressure of the evaporator. The cycle moves along the isosteres to reach the temperature T 2 and the pressure P c which represents the pressure of the condenser. The first stage of the cycle, isosteric heating, operates at a constant amount of gas adsorbed. At the next stage the pressure is kept constant at P c while the temperature is raised to T 3 which is equal to the maximum temperature attainable in the system. This is the isobaric desorption stage which results with the condensation of the adsorbate. The following step involves decreasing the temperature of the system fromT 3 to T 4. Meanwhile, the pressure drops to P e once again. There is no mass transfer during this stage of isosteric cooling. The final stage occurs at a constant pressure P e while the temperature decreases from T 4 to T 1. This is the isobaric adsorption stage and the adsorbate vaporized in the evaporator is once again adsorbed by the adsorbent.

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Chapter 3:

Technological applications

This chapter presents the results obtained with some prototypes and adsorption technologies applications already on the market. The prototypes in question were designed to use waste heat or solar energy as the main heat source. The applications under examination are ice making and air conditioning. Air conditioning systems are those with closed cycles as well as those systems with open cycles, such as in desiccant systems. The working principle they have in common is the mere application of what was illustrated in chapter 2. It can be used for efficient generation and storage of heat and/or cooling power. As said before, in the isobaric adsorption phase, zeolite attracts water vapor and incorporates it in its internal crystal lattice while releasing heat at the same time.

Fig. 5 – Adsorption phase

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If this process proceeds in an evacuated (airless) environment the attraction of water by the zeolite is so forceful that the internal pressure drops dramatically. The remaining water in an attached vessel evaporates, cools down and freezes immediately due to the heat of evaporation. The resulting ice can be used for cooling and air conditioning while the simultaneously produced heat of adsorption within the zeolite tank can be utilized for heating. If a valve is included between the two vessels, the heat or cold production can be interrupted for any periods without loss of energy. The first phase of this process proceeds up to the point when the zeolite is saturated with water. The reverse process is initiated by heating the zeolite at high temperatures in the second phase (isosteric heating).

Fig. 6 – Desorption phase

The adsorbed water molecules are forced to evaporate (isobaric desorption). Condensation takes place in the water tank (condensor). The sequence of adsorption/desorption processes is completely reversible and can be repeated indefinitely.

Solar-powered adsorption icemakers
Sites with high insolation usually have a large demand for cooling to preserve food, drugs and vaccines, and considerable research has been devoted to develop machines that could employ solar energy efficiently for such purposes. A commercial example of this process is the adsorption-desorption solar refrigerator developed by NPI.

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Solar collector box

Black painted solar collector Black PVC pipe Zeolite containing pipes

Two –layers of glass

SECTION

Copper heat exchanger

Insulated cooler box

Ground Airtight cap Sealed container

Fig. 7 – Adsorption-desorption solar refrigerator developed by NPI: scheme

This refrigerator produces about 5 lbs of ice daily per cubic ft of storage space. NPI’s seeks to help meet food storage needs for Third World families, with locally constructed solar refrigerators having the following parts: 1. An insulated, open-top solar collector box, about 7 ft. x 7 ft. (painted black), with two-layers of glass (or other glazing) attached to the open-top. This box is used to 13

collect solar heat via exposure to direct sunlight. A single 2 1/16-inch hole is drilled, on one side, to allow the exit of a 2-inch PVC pipe (see drawing). 2. A container, for zeolites, made of 6-inch PVC pipe generally forming a square that fits inside the above collector box ---with four added cross-sections as shown on the drawing to follow. (Pipe should be black PVC, or paint the outside black.) . 3. A vacuum hand-pump is used to help maintain a partial vacuum in the above described pipe container system sealed with PVC glue to prevent air leaks. 4. An attached heat exchanger, made from copper or brass tubing, also acts as a condenser to convert hot water vapor into water, or water droplets. 5. An insulated cooler (refrigerator) box of at least 4 cu. ft. may be located in the ground or partly below ground to improve insulation (also use an insulated top door). This size is suggested to meet the food cooling needs of a family of four. 6. A one (1) cu. ft. sealed container to be located inside the above cooler box. At the various stages of operation, this container holds a combination of water, water vapor, and ice. There is an airtight cap, over a 2-inch hole, that may be opened to add water as needed to keep this container at least 2/3rds full of water and/or ice. A sealed, attached float-gauge indicates the level of the water/ice without opening this container. (Loss of water vapor, and loss of the partial vacuum, may occur if any of the above components develop a leak.) 7. An adapter and appropriate glue or sealing materials are used to connect the 2-inch PVC pipe to the copper or brass tubing used for the condenser (item 4 above). Air leaks will greatly reduce the cooling efficiency of this refrigerator. Concept of operation is based on the fact that when cool (at night) the zeolite acts somewhat like a sponge soaking up or adsorbing the water vapor ---then when heated during the solar day, this water vapor is desorbed or released. Operating under a partial vacuum, the water vapor moves with high efficiency under low pressures. At about 37°C, water vapor begins to desorb from the zeolite. This water vapor is condensed into water droplets as heat is given off by the heat exchanger. The resulting water runs down, using gravity, into a sealed storage tank inside the cooler. 14

During the night, zeolite is cooled close to ambient temperature and starts adsorbing water vapor. Two sides of the solar collector box are opened to allow night air to help cool the zeolite inside the PVC pipes. Liquid water, in the storage tank (a tank also operating as an evaporator) adsorbs heat from the space to be cooled and is converted into water vapor. Since the system is sealed under very low pressure, any water remaining in the storage tank is then frozen into ice. This ice then melts slowly during the next day providing sustained cooling at reasonably constant temperatures. NPI’s cooler (as above described) was developed to provide food storage for the poorest of the poor, and is not intended to be a commercial system. The NPI cooler has been designed to allow village users to easily obtain their own inexpensive means of local food preservation, with no power other than solar.

Self chilling beer kegs
An example of a Zeolite ice maker has been incorporated into the Self Cooling Beer Keg. The technology is licensed to Cool-System Bev. GmbH and is being used by Germany’s Tucher Brau brewery for 20-liter barrels of beer. The self-cooling keg contains three chambers. A reservoir of water in an evacuated chamber surrounds the inner chamber containing the beer. This water reservoir is connected by a tube to the outer chamber containing Zeolite. Since this tube is also evacuated it contains water vapor. By opening a valve the water vapor flows to the Zeolite where it will be absorbed. As this happens, the Zeolite warms up, absorbing heat from the water reservoir as it does so. The reaction is sufficiently intense to cool the water in the reservoir enough so that it freezes. After 30 minutes, a cold glass of beer can be tapped and the keg will keep a perfect drinking temperature for at least 12 hours.
Fig. 8 - Self chilling beer kegs

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Solar-powered adsorption air conditioners
In many countries the demand for electricity greatly increases in summer due to the intense use of air conditioners. Blackouts can occur if power plant capacities are not sufficient to meet demand, especially during peak hours. As this period usually coincides with higher insolation hours, the use of solar-powered air conditioners seems to be an attractive solution. At the end of the 1980s, Grenier et al. presented a solar adsorption air conditioning system with 20 m2of solar panel, which used the working pair zeolite–water. This system, shown in Fig. 9, was designed to refrigerate a 12-m3 room for food preservation. When the insolation received by the solar collectors was about 22 MJ m2, the cold room could
Fig.9 – Cold storage room

store 1000 kg of vegetables with a rotation of 130 kg day-1 for a temperature difference of 20 °C between the ambient outside and the cold room. The COP, in this case, was 0.10.

Another interesting usage of the zeolite-water pair for air conditioning was made by Lu et al. who developed an air conditioner that could be powered by the exhaust gases of a locomotive. This system, which scheme is shown in Fig. 10, was based on a laboratory prototype developed by Jiangzhou et al. It was designed to refrigerate the driver’s cabin of a locomotive that ran in the Zhejiang province, East China. The cooling power of such a system under typical running conditions ranged from 3 to 5 kW, with a COP of 0.21.

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Fig.10 – Scheme of the adsorption air conditioner installed in the locomotive

The temperature inside the cabin was between 4 and 6°C lower than the ambient temperature, while this same cabin without refrigeration usually had a temperature of 2– 5°C higher than the ambient temperature. The authors remarked that the velocity of the locomotive and the rotating speed of the engine have significant influence on the cooling and heating power, respectively, of the air conditioner. This system is feasible and practical to be applied in a locomotive except when it pulls passenger cars, because in this condition, the train runs slowly and stops at every railway station. The cost of such a system is expected to be about US$5000.

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Chapter 4:

Key issues for future development

The adsorption systems must have their size and cost reduced to become more commercially attractive. The most promising alternatives to achieve these goals include finding an economic compromise between mass and heat transfer (both internal and external) to increase the SCP, and the improvement of the heat management to increase the COP.

and the power of the machine which is generally expressed as specific cooling power (SCP) per volume or mass of the apparatus or volume or mass of the adsorbent

usually determine the efficiency of the process (high COP and SCP ensure high process efficiency). To minimize thermal loads or size of the apparatuses, high sorption capacities of the adsorbent during operating conditions and fast sorption kinetics are required. Therefore, the adsorbent and the adsorber configuration have a key impact on the performance, as well as a good management of heat exchange. So optimization of the adsorber units among these aspects is an elementary approach to improve adsorption refrigerators.

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The heat/mass transfer issue
There are mainly two types of resistance to heat transfer in a system comprising a solid adsorbent bed and heat exchanger tubes: external heat transfer and internal heat transfer resistance. The first one occurs at the metal–adsorbent interface and depends on the physical contact between the materials. The lack of good contact between the metal surface and the adsorbent creates a steep thermal gradient at the interface. The inefficient heat exchange is mainly due to the shape of the adsorbent particles, generally spheres or cylinders, which do not allow a good contact between the adsorbent solid surface and the metal of the heat exchanger. In order to reduce this thermal resistance, a suitable shape of the solid bed with a smooth surface should be sought. One of the solutions proposed is the utilization of coated adsorbers, which consists in the coating of the heat exchanger surface with the adsorbent material, such as zeolites, silica gel etc.). Therefore, there is an increase of the wall heat transfer coefficient by the effective decrease of the contact thermal resistance between the heat exchange surface and the adsorbent. This technology is particularly suited for applications where high COP is not as important as high SCP. In fact, the main disadvantage of using coated adsorber is the very high ratio between the inert mass and the adsorbent mass, which spoils the COP. In order to overcome this drawback, a very effective heat management is required. Restuccia and Cacciola compared by simulation the performances of two heat pumps in which one of them had an adsorber with coated surface and in the other one, the adsorber was filled with pelleted zeolite. The coated surface adsorber had a thin layer (1 mm) of zeolite, which was synthesized outside the tubes of a tube and shell heat exchanger. The system with pelleted zeolite was from a previous study performed by Benthem et al.. The system with coated tube had a cooling power about 3.6 times higher than that obtained with the system which employed pelleted zeolite. Due to the utilization of heat recovery process in both systems, the COP of the former was only 2% lower than the COP of the latter. The second type of resistance is associated with heat transfer inside the adsorbent bed and is inversely proportional to the thermal conductivity of the bed. The low thermal 19

conductivity of the adsorbent bed limits the efficiency of the adsorption heat pumps. Although many efforts were made to improve the thermal conductivity of the system, none of them proved to be on the right track. As a result of some studies, it may be concluded that putting metallic spheres or strips into the bed is useless; the connection between grain and grain must be as large as possible; binders with high thermal conductivity must be well bound with the adsorbent and consolidated samples must be used.

Fig 11 – metallic foam coated with zeolite crystals

One of the proposals to increase the thermal conductivity of the adsorbent bed involves the formation of a composite material made of metallic foam and zeolite. In order to achieve such a result the zeolite powder is suspended in a silico-aluminate gel and a metallic foam mesh is filled with the paste obtained. The resulting compound is compressed at some tens of MPa and the composite is withdrawn from the mould after consolidation at 1000°C for 3 h. The thermal conductivity of the composite material thus obtained is reported to increase 92-fold with respect to that of a granular bed. However, a crucial point is overlooked, namely the resistance to mass transfer. The composite material compressed at high temperatures and pressures will be deprived of the high porosity necessary for a good mass transfer since the zeolite will no longer retain its microporous crystalline structure under those conditions and the operation of the system will again be limited to a great extent. It has been reported that an optimum compromise should be achieved between the high porosity necessary for fast vapor diffusion and the high density required for good thermal conductivity. The resistances to mass transfer which vary in accordance with the extent of the porosity and the thickness of the adsorbent bed might 20

play an equally important role in limiting the performance of the system. Various other designs of adsorption heat pumps have been proposed, all aiming to improve the heat and mass transfer inside the system. Most of these systems introduce new drawbacks and cannot fully achieve the desired results.

Heat recovery
The intermittent nature of the energy supplied to the user and the low thermodynamic efficiencies obtained, compelled researchers to develop advanced cycles with heat management with the aim to increase COP, since in the conventional adsorption cycle is usually smaller than 0.4. Higher COPs can be expected with cycles that employ heat regeneration process, which is also called thermal wave. The inlet fluid temperatures (points A and C in Fig. 12) are the same as the heat sink and heat source temperatures, respectively. The outlet fluid temperatures (points B and D) change with the time. In the heat regeneration process, the heat transfer fluid flows successively through: (i) one adsorber, which is being cooled; (ii) the heat source; (iii) another adsorber, which is being heated; and (iv) the heat sink. When the gradient of temperature between the inlet and outlet heat transfer fluid from the first adsorber is large, only a limited heat power is required from the heat source in order to provide a large heat power to the second adsorber.

Fig. 12. Scheme of the heating/cooling thermal fluid loop with eight adsorbers

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According to Pons and Poyelle, this is possible because the large amount of energy recovered from the first adsorber is regenerated (according to the thermodynamics second law) by the energy supplied at high temperature. The heat regeneration can be a very efficient heat storage process because about 65% of the total energy received by each adsorber can be internally recovered. Furthermore, the heat transfer fluid circuit can be very simple. It needs only a reversible pump, and control valves are not necessary.

Wade et al. simulated a regenerative adsorption cycle with eight beds that could recover 76% of the waste energy from the adsorption process. During this cycle, the hot thermal fluid that left the hot adsorber (Adsorber 1 in Fig. 13) pre-heated the adsorbers 2–4, before be completely cooled by the heat sink. The cold heat transfer fluid flowed from the heat sink towards the adsorber at the adsorption phase (Adsorber 5). After removing the heat from the adsorber 5, the heat transfer fluid continued its path to pre-cool the adsorbers 6– 8, and be pre-heated. Then, the thermal fluid was heated until the generation temperature by an external heat source, before it entered the adsorber 1.

Fig 13 - Scheme of the heating/cooling thermal fluid loop with eight adsorbers

This fluid route was maintained during a specific period, before the adsorbers switch position. The previous work was the base for the design of a prototype with four beds that was used to produce cooling at 136 °C . The energy input necessary to produce 1 W of

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cooling in this prototype was 76.6 W, which was much lower than the 165 W required in the system without heat regeneration The Table 1 shows some of the best performances obtained by different prototypes manufactured until 2006, for the applications of some adsorption heat pumps. The results should not be compared to one another, as they were obtained under different working conditions, but they should be used as a reference of what can be expected from these systems.

Table 1 - Performance of adsorption systems for different applications

In conclusion, adsorption heat pumps which make internal heat recovery possible, cause an improvement in the efficiency of the system and have the advantage of being able to operate in the continuous mode. Moreover, the use of diverse kinds of energy sources, especially of waste heat originating from various processes, has created additional opportunities of progress.

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“Evaluation of thermal parameters and simulation of a solar-powered, solidsorption chiller with a CPC collector” Manuel I. Gonzalez, Luis R. Rodrıguez, Jesus H. Lucio, Renewable Energy

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“Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy” R.Z. Wang, R.G. Oliveira, Progress in Energy and Combustion Science 32 (2006) 424–458

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