Designing A Solar Driven Two Bed Silica

Published Date: 02 Nov 2017

Faculty of Engineering

Department of Mechanical Engineering

B. Sc. Graduation Project

DESIGNING A SOLAR DRIVEN TWO-BED SILICA GEL/WATER AIR-CONDITIONING UNIT

Presented By:

Elias Nazmi Mohamad

Ibrahim Ahmad Abdo

Mahmoud Ahmad El-Hajj Daoud

Mariam Mounir Itani

RayanKamel Hussein

Said Abdel MajidAsaad

Supervised By:

Dr. Ossama Al-Samni

Table of Contents

Table of Figures

Table of Tables

2.4.1 Previous Experimental Studies

The adsorption systems are classified in the literature based on the application and the source of heat needed for desorption. Ice-making, refrigeration, air conditioning are among the applications tested by adsorption systems. Most of the applications use solar energy to drive the units and few researches rely on the energy waste recovery from boilers or vehicle exhausts. Details are given in the following subsections including the various pairs of adsorbent/absorber used in each experiment.

Solar Powered Adsorption Ice Maker

Ice makers place very large demand to preserve food, vaccine, and drugs. Research has been conducted to develop machines that could employ solar energy efficiently for this purpose Error: Reference source not found. In the late of 1970s, the development of sorption refrigeration systems powered by solar protruded after the pioneering work of Tchernev[15] who studied a basic solid sorption cycle with the working pair zeolite-water. Since then, a number of studies have been carried out, but the cost of these systems still makes them non-competitive for commercialization. Therefore, the focus of some research is concerned about cost reduction and increasing the efficiency of the machine.

Pons and Guilleminot[16] concluded that the solid sorption systems could be the basis for efficient solar powered refrigerators, and they developed a prototype with the pair Activated carbon-Methanol. The machine produced 6 kg of ice per m2 of solar panel with C.O.P of 0.12, the rate of ice production remains one of the highest obtained by solar icemaker.

Li et al. [17] developed a simpler solar powered icemaker without valves; the adsorber was placed inside an insulated case, which was covered by two transparent plastic fiber sheets. The experiments with this prototype were performed both under indoor and outdoor conditions. Under indoor conditions, with insulation from 17 to 20 MJm-2 the ice production was between 6 and 7kg per m2and the C.O.P between 0.13 and 0.15.Under outdoor conditions the system could produce 4kg of ice per m2 with C.O.P about 0.12. Based on these experiments two prototypes were developed that could produce 4 to 5 kg of ice per m2 with C.O.P 0.12 to 0.14, the cost of such a machine was estimated to be no more than 250$ per m2 of solar panel.

Hildebrand et al. [18] developed an adsorption icemaker in which water was used as refrigerant and the ice was produced within the evaporator. The adsorbent was silica gel and the total solar collector was 2 m2, for insulation higher than 20 MJm-2 the C.O.P was between 0.12 and 0.23 when the mean outdoor temperature was between 12 and 25°C, C.O.Ps higher than 0.15 were obtained with outdoor

An innovative modular icemaker was tested by Khattab[19], it placed the adsorbent in a glass container, which was positioned between reflector panels, in order to improve the thermal properties of the adsorbent bed, and four types of bed techniques were proposed. The configuration with granular carbon bonded with blackened steel had C.O.P. of 0.16 and a daily ice production of 9.4 kg per m2 of adsorber when the insulation was about 20 MJm-2 and the average outdoor temperature was 29°C. Under winter conditions with insulation of 17 MJm-2 and an average outdoor temperature of 20°C, the C.O.P. obtained was 0.14 with a daily ice production of 6.9kg per m2. Oliveira [20] tested an adsorption icemaker with refrigerant mass recovery process that had daily ice production of 1.2 and 1.6 kg per kg adsorbent when the generating temperatures were75°C and 85°C, respectively. The C.O.P in both cases was about 0.08.

Solar Powered Adsorption Air Conditioners

Air-conditioning is an excellent application of solar energy, because the supply of sunshine and the need of refrigeration reach their maximum levels in the same season. Adsorption air conditioning has been of increasing interest particularly during summer since the demand of electricity is greatly increased due to the intense use of air conditioners. Thus the use of solar powered air conditioners seems to be an attractive solution.

At the end of 1980s, Grenier et al. [21] presented a solar adsorption air conditioning system that had 20m2 of solar panel and used the working pair zeolite and water, was designed to refrigerate a 12 m3 room for food preservation. When the insulation received by the solar collectors was about 22 MJm-2, the cold room the C.O.P was 0.10.

Saha et al. [22] experimentally investigated a double-stage four bed, non-regenerative adsorption chiller that could be powered by solar/waste heat sources at between 50ËšC and 70ËšC. The studied prototype produced cold water at 10ËšC and had a cooling power of 3.2kW with a C.O.P of 0.36 when the heating source and sink temperatures of 55 and30ËšC respectively. Flat plate collectors could easily produce hot water to regenerate the adsorbent of the chiller at these levels of temperature difference of 20ËšC between the ambient outside and the cold room, the C.O.P was 0.10.

Liu et al. [23] developed an adsorption chiller with the pair silica gel-water, which had no refrigerant valves. This feature reduces the cost of the chiller, and makes it more reliable as there are fewer moving parts that could lead to air infiltration. The whole chiller contains 52.8 kg of silica gel divided between two adsorbent beds, which operate out of phase and thus produce constant cooling. Experiments with this prototype showed that a cooling power of 3.56 kW and C.O.P of 0.26 could be obtained when the mass and heat recovery processes are employed under the following operation conditions: an evaporation temperature of 7˚C, a heat sink temperature of 28˚C, and a heat source temperature of 85°C. To enhance the performance of the chiller, the research team developed a new prototype with some improvements. The new prototype had less non-continuous and movable pieces to reduce the number of possible places for inward leakage. The condenser was changed to avoid undesirable refrigerant evaporation that occurred inside this device during the operation of the first machine. The configuration of the adsorber was changed to improve the heat and mass transfer. The second prototype had a cooling and C.O.P about 34% and 28% higher than the first one. Experiments performed at a generation temperature of 80˚C and an evaporation temperature of 13˚Cshowed that C.O.P and the cooling power of this new system could reach 0.5 and 9 kW, respectively.

Xia et al. [24] developed a silica gel-water adsorption chiller driven by low temperature heat source. This chiller has two identical chambers and a second stage evaporator with methanol as working fluid. Each chamber contains one adsorber, one condenser and one evaporator. There is also a mass recovery tube between the two chambers. The solar powered water-heating unit includes almost 50m2 of evacuated tube collectors, a water pump and a hot water storage tank. Experiments performed when hot water at 85ËšC was used to drive the chiller, resulted in a cooling power close to 4.96 kW, and with the corresponding cycle C.O.P around 0.32. When the hot water temperature was 65ËšC, the cooling power and cycle C.O.P were 2.97 kW and 0.23, respectively. Two chillers similar to the one described previously, but with a higher nominal capacity are used in the air conditioning system of a "green" building located in the Shanghai Research Institute of Building Science.

Nunez et al. [25] developed and tested a silica gel-water adsorption chiller with nominal cooling power of 3.5 kW. It had two adsorbers filled, each one, with 35 kg of adsorbent. The chiller operated at generation temperatures between 75 and 95°C, heat sink temperatures between 25 and 35°C, and evaporation temperature ranging from 10 to 20°C, the C.O.P varied from 0.4 to 0.6 according to the experimental condition.

Restuccia et al. [26] developed an adsorption chiller that employed silica gel impregnated with CaCl2 as sorption material. This kind of adsorbent was used due to its high sorption ability (up to 0.7 kg of water per kg of dry sorbent) and due to the fact that most of the water content can be desorbed at generation temperatures between 90 and 100°C.When the condensation temperature was 35°C, the C.O.P of the chiller was close to 0.6 in the range of generation temperatures from 85 to 95°C, but it varied between 0.3 and 0.4 when the condensation temperature was 40°C. The evaporation temperature during these experiments was 10°C, the SCP was20W kg-1 when the generation temperature was 95°Cand the condensing temperature was 40°C. Another kind of solar powered air conditioner employs open adsorption system instead of closed ones. These system are called open because the refrigerant, which is water from the air is released to the atmosphere after the desorption process.

Ismail et al. [27] studied a system with silica gel designed to reduce the absolute humidity and enthalpy of the air used to cool grains. In such a system, the air passed through two silica gel beds and two heat exchangers to provide dry air during the night to grain storage room. The utilization of this system could keep the temperature of the grain at about 16°C, while this temperature would be closer to 21°C without the system. The C.O.P based only on the electricity supplied to the system was greatly influenced by the airflow rate, and it ranged from 3.9 to 30.3 for an airflow rate between 0.065 kg/s and 0.021 kg/s.

Lu and Yan [28] studied another kind of open sorption system for air dehumidification and that could be regenerated by solar energy. The system identified as solar desiccant enhanced radiated cooling (SDERC), used silica gel as sorbent. The European project Clismol[29] presented examples of buildings that already use solar powered sorption air conditioners. The examples include applications of solid sorption chillers and solid desiccant systems. Some of the chillers were installed in a university hospital located in Freiburg, Germany, and in cosmetic company Sarantis S.A., in Greece, while some of the solid desiccant systems were installed in the Chamber of Commerce in Freiburg, and in the Renewable Energies Department, in Lisbon.

2.4.2 Working Pairs and Their Properties

This study aims to draw a map for the application of adsorption pairs in cooling. The study introduces a classification and a comparison for the working pairs in order of its use. The selection of any pair of adsorbent– adsorbate for refrigeration applications depends on certain desirable characteristics of their constituents. These characteristics range from their thermodynamic and chemical properties to their physical properties and even to their costs or availability.

The adsorbate or refrigerant should have the following properties:

Evaporation temperature below 0 °C.

Small molecular size to enable it to be adsorbed into the adsorbent.

High latent heat of vaporization and low specific volume.

Thermally stable with the adsorbent at the cycle operating temperature ranges.

Non-toxic, non-corrosive and non-flammable.

Low saturation pressures (above atmospheric) at normal operating temperature.

The important considerations influencing the choice of a suitable adsorbent are:

Adsorption of large amount of the adsorbate under low temperature conditions.

Desorption of most of the adsorbate when exposed to thermal energy.

Possession of high latent heat of adsorption compared to sensible heat.

No deterioration with age or use.

Non-toxic and non-corrosive.

Low cost and widely available.

The adsorption process is divided into physical adsorption and chemical adsorption.

2.4.2.1 Working Pairs

Activated carbon / Ammonia:

Physical properties of activated carbon:

Activated carbons are made by carbonizing source materials

Activated carbons are available in many forms including powders, micro-porous, granulated, molecular sieves and carbon fibers.

The specific area of activated carbon is between 500 and 1500 m2/g.

Physical properties of ammonia:

Ammonia has a relatively high latent heat of about 1365 kJ/kg at -30 °C

maximum adsorption quantity in activated carbon is 0.29 g/g

The heat of adsorption for carbon-ammonia pair is in range of 2000 to 2700 kJ/kg

Desorption pressure of the activated carbon–ammonia reaches 1.6 MPa so that high pressure.

Pair behavior:

Maximum Specific cooling power (SCP) 600→2000 w/kg

Coefficient of performance (COP) 0.12→0.6

Driving temperature varied from 80 °C to 200 °C

Cycle time about 12 s.

Activated carbon / Ethanol:

Physical properties of ethanol:

Maximum adsorption quantity in activated carbon is 1.2 g/g

Pair behavior:

The specific cooling effect was about 420kJ/kg

The driving temperature is between 60 and 95 °C.

Coefficient of performance (COP) 0.6→0.8

Activated carbon / Methanol:

Physical properties of methanol:

The maximum adsorption quantity in activated carbon is 0.45 g/g and the latent heat at -30 °C is about 1229.1 kJ/kg °C

Methanol decomposes at 120 °C through either a dehydrogenation or a dehydration mechanism to form formaldehyde (HCHO) or dimethyl ether (CH3OCH3).

Pair behavior:

Lower adsorption heat, which is about1800–2000 kJ/kg.

Has the disadvantage of operating under sub-atmospheric.

For the evaporator temperature of 15 °C, the Maxsorb III (A type of activated carbon) can adsorb methanol of 1.2 g/g within about 160 min. The maximum COP was 0.78 with Maxsorb III/methanol at regeneration temperature of 90 °C.

Zeolite /Water:

Physical properties of zeolite:

Zeolite is a type of alumina-silicate crystal composed of alkali or alkali soil. The porosity of the alumina-silicate skeletal is between 0.2 and 0.5.

There are about 40 types of natural zeolites, and the main types for adsorption refrigeration are chabazite, sodium chabazite, cowlesite and faujasite. About 150 types of zeolites can be artificially synthesized, and they are named by one letter or a group of letters, such as type A, type X, type Y, type ZSM, etc.

Artificially synthesized zeolite molecular sieves have micro pores with uniform size, and different sizes can be obtained by different manufacturing methods. 4A, 5A, 10X and 13X zeolite molecular sieves are the main types used for adsorption refrigeration.

Pair behavior:

The adsorption temperature of zeolite pairs is about 250–300 °C.

The heat of adsorption for the Zeolite/water is in range of 330-4200 where Natural zeolites have lower values than synthetic zeolites.

The maximum amount of water could be adsorbed by zeolite were estimated to be 0.12 g/g using zeolite 13X.

Silica Gel /Water:

Physical properties of Silica gel:

Silica gel is a granular, vitreous, porous form of silicon dioxide made synthetically from sodium silicate.

Silica gel is tough and hard; it is more solid than common household gels like gelatin or agar.

Can absorb up to 40% of its weight in water.

Extremely efficient at temperatures below 77°F (25°C), but will lose some of its adsorbing capacity as temperatures begin to raise, much like clay.

Polar and can form hydrogen bonds with polar oxides, such as water and alcohol. The adsorption ability of silica gel increases when the polarity increases. One hydroxyl can adsorb one molecule of water.

Each kind of silica gel has only one type of pore, which usually is confined in narrow channels. The pore diameters of common silica gel are 2, 3 nm (A type) and 0.7 nm (B type), and the specific surface area is about 100–1000 m2/g.

Silica gel is widely used for desiccation because of its large adsorption ability. Type A silica gel could be used for all desiccation conditions, but type B silica gel can only be used when the relative humidity is higher than 50%.

Pair behavior:

Water could be considered a very good refrigerant but it has the disadvantage of impossibility to get an evaporation temperature less than 0 °C.

The heat of adsorption for silica gel/water pair is about 2800 kJ/kg [30].

One disadvantage of silica gel/water working pair is the low adsorption quantity, which is about 0.2 g/g [31].

2.4.2.2 Comparison between Working Pairs

To choose the appropriate pair in adsorption cooling applications the running conditions and the aim of the application should be taken into account. For example when using the adsorption pair in solar cooling the important parameter governing the choice is the driving temperature as it should be the minimum allowable temperature. The most important running conditions are the driving temperature, evaporation temperature and COP.

Table 2 Physical Properties for Some Adsorption Pairsintroduces a comparison between the working adsorption pairs on the basis of the best reached operating conditions. The comparison covers the already constructed systems and conducted the best results of them. For every pair the running conditions were brought from existing systems and compared together to introduce the best. For every Row in the table, the shown data is not for one system but it is brought from many systems to get the limits of using the pair in cooling application.

Table 2 Physical Properties for Some Adsorption Pairs

Working Pair

COP

Te ( ÌŠC)

Td ( ÌŠC)

SCP (W/kg)

Physical adsorbent

Activated carbon/ammoina

0.61

-5

100

2000

Activated carbon/ethanol

0.78

15

90

16

Activated carbon/water

0.8

3

80

N.A

Silica gel/water

0.61

12

82

208

Zeolite/water

0.25

6.5

350

200

Chemical adsorbent

Metal Chloride/ammonia

0.6

-10

52

N.A

Metal hydrides/hydrogen

0.83

-50

85

300

Metal oxides/water

N.A

10

200

78

Composite adsorbents

Silica gel and chlorides/water

1.65

7

70

N.A

Silica gel and chlorides/methanol

0.33

-10

47

N.A

Chlorides and pourous media/ammonia

0.35

-15

117.5

493.5

Zeolite and foam aluminum/water

0.55

10

250

500

2.4.3 Adsorption Measurement Facilities

The aim of adsorption measurement is to determine the properties of the adsorbent-adsorbate working pair, such as the isotherms, adsorption kinetics and adsorption heat data. All these properties are basic information that is helpful for industrial applications. There are several techniques of measuring adsorption data, and many researchers have proposed their machines to measure adsorption. The two techniques often used are the volumetric and gravimetric techniques.

In this section, only adsorption isotherm and kinetics measurement techniques are discussed.

2.4.3.1 Volumetric Technique

The volumetric technique is based on the pressure change of adsorbate in the constant volume container. Once the vapor is isolated from the system, the total amount of vapor introduced into the chamber is fixed. Due to the adsorption of adsorbent, the pressure of vapor in the chamber or container would decrease. By tracking the pressure change of reaction gas, the adsorption percentage of adsorbate can be calculated under the measured pressure and temperature during the equilibrium state.

2.4.3.2 Gas Adsorption Gravimetry

In gas adsorption gravimetry, the weight of adsorbent ismeasured directly. Gas adsorption gravimetry is quite suitable for adsorption ofcondensable vapor because the condensation of vapor on the wall of container willhave no influence on the results. However the condensation on the movingbalance parts should be prevented, because this will affect the results due to the weightincrease by condensation, not by adsorption. The gas adsorption gravimetry canmeasure the adsorption directly and quickly, but there are also disadvantages,including the buoyancy effect, the need of maintaining the temperature of adsorbentand the electrostatic effect might cause systematic errors.