A NOVEL SOLAR-POWERED ADSORPTION REFRIGERATION

C.SURESHM140539ME

INTRODUCTIONThe use of solar energy for environmental control is

receiving much attention as a result of the projected world energy shortage

Solar refrigeration has the potential to improve the quality of life of people who live in areas with electricity insufficient.

Solar cooling to produce ice accumulates latent heat, thus leading to smaller volume of ice-makers.

PRINCIPLE OF ADSORPTIONAdsorption occurs at the surface interface of two phases,

in which cohesive forces including electrostatic forces and hydrogen bonding, act between the molecules of all substances irrespective of their state of aggregation

The process of adsorption involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another.

The adsorbing phase is the adsorbent, and the material concentrated or adsorbed at the surface of that phase is the adsorbate.

An adsorbate is any substance that has undergone adsorption on the surface. During the process of adsorption, charge transfer occurs between the adsorbate and the metal.

REFRIGERANTS AND ADSORBENTS

There are several working pairs for solid adsorption. For the successful operation of a solid adsorption system, careful selection of the working medium is essential.

It is because, the performance of the system varies over a wide range using different working pairs at different temperatures.

the choice of the adsorbent will depend mainly on the following factors:

high adsorption and desorption capacity, to attain high

cooling effect;good thermal conductivity, in order to shorten the cycle

time;low specific heat capacity;chemically compatible with the chosen refrigerant;low cost and widely available.

The selected adsorbate (working fluid) must have most of the following desirable thermodynamics and heat transfer properties:

high latent heat per unit volume;molecular dimensions should be small enough to

allow easy adsorption;high thermal conductivity;good thermal stability;low viscosity;low specific heat;non-toxic, non-inflammable, non-corrosive; andchemically stable in the working temperature

rang.

Based on the above criteria, some of the appropriate working pairs are:

zeolite–water, zeolite–organic refrigerants, silica gel–water, zeolite–water and activated carbon–methanol in solid

adsorption systems

BASIC ADSORPTION CYCLE

A basic adsorption cycle consists of four steps :

1.Heating and pressurization, 2.Desorption and condensation, 3.Cooling and depressurization, and 4.Adsorption and evaporation

1.HEATING AND PRESSURIZATION

In the first step ,the adsorber is heated by a heat source at a temperature ofTH.The pressure of the adsorber increases from the evaporating pressure up to the condensing pressure while the adsorber temperature increases.

This step is equivalent to the “compression” in the vapor-compression cycle.

2.DESORPTION AND CONDENSATION

In the second step, the adsorber continues receiving heat and its temperature keeps increasing, which results in the desorption (generation) of refrigerant vapor from adsorbent in the adsorber.

This desorbed vapor is liquefied in the condenser and the condensing heat is released to the first heat sink at a temperature of TC

3.COOLING AND DEPRESSURIZATION

At the beginning of the third step, the adsorber is disconnected from the condenser. Then, it is cooled by heat transfer fluid at the second heat sink temperature of TM.

The pressure of the adsorber decreases from the condensing pressure down to the evaporating pressure due to the decrease in the adsorber temperature.

4.ADSORPTION AND EVAPORATION

In the last step, the adsorber keeps releasing heat while being connected to the evaporator. The adsorber temperature continues decreasing, which results in the adsorption of refrigerant vapor from the evaporator by adsorbent, producing the desired refrigeration effect .

This step is equivalent to the “evaporation” in the vapor-compression cycle.

Advanced Adsorption Cycle Since the efficiency of the basic adsorption refrigeration

cycle is low, and the cooling output is not continuous.

Due to this many advanced adsorption refrigeration cycles such as the

heat recovery cycle, mass recovery cycle, thermal wave cycle, forced convective thermal wave cycle, etc. have been

developed to improve efficiency and practicability.

HEAT RECOVERY CYCLE

The heat recovery cycle is an advanced adsorption cycle used in a system with two or more adsorbers.

After the adsorption phase and desorption phase are finished in the adsorbers, the heat from the hot adsorber is transferred to the cold adsorber by circulating heat transfer fluid between them in a closed loop.

The experimental results show that the COP of the system will increase by up to 25% with the heat recovery cycle

OPERATION AND ANALYSIS OF THE ADSORPTION CYCLE

Operation principle of solid adsorption refrigeration system utilizing solar heat. A: Sorption bed (solar collector);B: condenser; C: evaporator

PROPOSED MODULE

The modified glass tube is shown in Fig. 3a and b. It consists of the sorption bed, the condenser and the evaporator.

The sorption bed has been modified from a tube to a circular container with small thickness to increase the exposed area to the sun and insure uniform temperature distribution inside the bed.

The sorption bed contains small granules of domestic charcoal (granules diameters are between 5 and 7 mm). The specific surface area of the used charcoal is 55.7 m2/g and its porosity is 46.45%).

To increase bed conductivity, four types of bed techniques are proposed. They are;

Type 1, black metallic meshes are placed on both faces of the circular bed,

Type 2, black metallic plates are placed on both faces of the bed,

Type 3, charcoal grains are mixed with small pieces of blackened steel, and

Type 4 charcoal grains are bonded with small pieces of blackened steel.

Four types of plane reflector arrangements are used to heat the sorption bed. The details of each type are shown in Fig. 4a–d.

Inclination angles and concentration ratio cr for different reflectors arrangements

RESULTS AND DISCUSSION

Measured average temperature of the four types of bed techniques (February).

It is also clear from this figure that, by virtue of mixing charcoal with metallic pieces, bed techniques Types 3 & 4 attained higher temperatures than Types 1 & 2.

Maximum temperature recorded is for the bed Type 4, it reaches 95oC for about 2 h.

This temperature is higher by 60% than the temperature of the plain bed heated directly by solar energy.

This figure also shows that the temperatures of all beds during cooling at night are very close to the ambient temperature.

Effect of reflector arrangements on the temperature of bed Type 4 (February)

The bed temperatures heated by reflector Type C stay 1000C for time duration 4 h and the maximum temperature recorded is 1150C.

This temperature is higher by about 13.5% than the maximum temperature recorded when this bed is heated by reflector arrangement Type D.

Although reflector Type D does not realize the maximum bed temperature during heating, it has the advantage of fixed orientation over the whole year

The orientation of Types A–C must be changed monthly to attain maximum energy input at noon).

CONCLUSIONSIt is found that using the four types of bed techniques

heated by reflector Type D in cold climate can increase maximum bed temperature by a ratio between 13% and 60% than the plain bed temperature heated directly by solar energy.

The bed Type 4 heated by reflector arrangement Type D achieve the maximum increase. This bed achieved an increase of 13.5% when heated by reflector arrangement Type C.

The maximum bed temperature of the module composed of bed Type 4 and reflector arrangement Type C was found to be 120 and 1330C in cold and hot climate respectively.

The minimum bed temperature at night was found to be is very close to ambient temperature in both winter and summer