To Mustafa Kamal Chowdhury, Professor, Department of Mathematics, Bangladesh University of Engineering and Technology, Dhaka, and Dr. Elias, Professor, Department of Mathematics, Bangladesh University of Engineering and Technology, Dhaka for their valuable comments and advice.
SUBSCRIPTS
INTRODUCTION
- Introduction
- History of Air Conditioner and Refrigeration
- Natural refrigeration
- Nocturnal cooling
- Evaporative cooling
- Cooling by salt solutions
- Vapour compressor refrigeration systems
- Domestic refrigeration systems
- Air conditioning systems
- Vapour absorption refrigeration systems
- Ejector refrigeration systems
- Gas cycle refrigeration
- Solar energy based refrigeration systems
- Steam jet refrigeration systems
- Thermoelectric refrigeration systems
- Vortex tube systems
- Literature Review
- Objectives and Scopes of the Present Thesis Work
- Outline of the Thesis
Most current air conditioners use either a vapor compression refrigeration system or a vapor absorption refrigeration system. Although discontinuous absorption cooling systems were among the first absorption systems to be developed (eg the water cooling system invented by Edmond Carré2 in 1866), they met with very little success.
CHRONOLOGICAL DEVELOPMENT OF ADSORPTION SYSTEM
The Vapour Compression Refrigeration System
- Vapour compression refrigeration cycle
In this phase, the refrigerant enters the compressor as a gas under low pressure and low temperature. The refrigerant is then compressed adiabatically so that the liquid leaves the compressor under high pressure and high temperature.
The Vapour Absorption Refrigeration System
- Absorption process
- Vapour absorption refrigeration cycle
As the process water loses heat to the coolant, it can be cooled to significantly low temperatures. At the absorber, the coolant is "absorbed" by an absorbing lithium bromide (LiBr) solution. This heat, along with the heat of dilution produced when the refrigerant condensate mixes with the absorbent, is transferred to the cooling water and released in the cooling tower.
This is achieved by constantly pumping (4) diluted solution from the absorber to the low temperature generator (5), where addition of residual heat (hot water, steam or natural gas) boils the coolant from the absorber. Once the refrigerant is removed, the re-concentrated lithium bromide solution returns to the absorber, ready to resume the absorption process, and the free refrigerant is sent to a condenser (6). The refrigerant vapor boiled off in the generator (5) returns to the condenser (6) where it returns to its liquid state as the cooling water absorbs the heat of vaporization.
The Vapour Adsorption Refrigeration System
- Adsorption and desorption process
- Types of adsorption
- Factors of adsorption
- Basic types of industrial adsorbents
- Silica gel as adsorbent
- Applications of adsorption
- Single stage adsorption system
- Single bed single stage adsorption refrigeration cycle
- Two bed single stage adsorption refrigeration cycle
The bed is connected to the condenser by opening valve V2 and the heat input from an external source continues in this process. The desorbed refrigerant is continuously fed to the condenser, where condensation takes place at Tc. The temperature of the adsorbent rises, causing a pressure increase from the evaporation pressure to the condensation pressure.
During this time, the adsorber continues to receive heat while connected to the condenser, which now builds up its pressure. The temperature of the adsorbent decreases, causing a drop in pressure from the condensing pressure to the evaporating pressure. During this time, the adsorber continues to emit heat while connected to the vaporizer, which now covers its pressure.
Advanced Adsorption Refrigeration Cycle
- Mass recovery adsorption cycle
- Cascading adsorption cycle
- Two stage adsorption cycle
- Two stage adsorption cycle using reheat
In mode B, the absorber (HX4) is heated by hot water and the absorber (HX3) is cooled by cooling water. In mode E, HX2 is heated by hot water and cooling water cools HX1. In mode F, HX3 (at the end position of the adsorption-evaporation process) and HX4 is at the end position of the desorption-condensation process) are connected to each other by continuing cooling water and hot water respectively.
In G mode, the HX3 is heated by hot water and the cooling water cools the HX4. In mode I, HX1 (at the end position of the adsorption-evaporation process) and HX2 at the end position of the desorption-condensation process) are connected in such a way that they are mutually equivalent continuations of cooling water and hot water. In J mode, HX1 is heated by hot water and cooling water cools HX2.
Solar Adsorption Cooling System
- Solar thermal collector
- Flat plate solar collectors (FPC)
- Compound parabolic concentrating solar collectors (CPC)
The CPC Solar Collector Reflector is constructed from a cylindrical, high-gloss rolled electro-anodized pure aluminum reflector that concentrates penetrating solar insulation onto a vertically mounted absorber. The collector absorber is made of highly selective coated copper with an ultrasonically welded heat transfer tube. The CPC consists of two parabolic reflectors at both ends (left and right) of the absorber plate.
These have the ability to reflect to the absorber all the incident radiation within wide limits. The main contribution of the compound parabolic concentrating solar collector is the direct (not the diffuse) solar radiation, which differs somewhat from the flat collector. Here, after reflection from the reflector, the incident rays are not focused on a point, but simply collected on the absorber surface.
ADSORPTION SYSTEM MODELLING
Introduction
System Description
The heat transfer fluid from the solar thermal collector goes to the desorber and returns to the collector to obtain heat from the collector. The valve between desorber and condenser and the valve between adsorber and evaporator are closed during preheating and precooling periods. Where the valve between desorber and condenser and the valve between adsorber and evaporator are open during adsorption and desorption process.
The duration of the preheating and desorption steps is called the half cycle time and is equal to the duration of the precooling and adsorption steps.
Formulation
- Energy balance for the adsorber and desorber
- Energy balance for the condenser
- Energy balance for the evaporator
- Mass balance
- Adsorption rate
- Solar system
- Measurement of system performance
The left side of equation (3.4) represents sensible heat of materials used in condenser heat exchanger and the condensed refrigerant inside condenser. The first term on the right side of equation (3.4) is energy released by the vapor during condensation, the second term is for the energy transport due to vapor transfer from bed to condenser and third term is for energy release from condenser by the heat transfer fluid . The left side of equation (3.6) corresponds to the sensible heat of materials used in the evaporator's heat exchanger and the amount of refrigerant inside the evaporator.
Where Tf is the heat transfer fluid and is taken as: Tf Tf,in Tf,out/2 Solar radiation is taken as a sine function:. 3.17). The ambient temperature is calculated based on the following equation: 3.18) Where, Dj Sunsettime Sunrisetime , t DaytimeSun, time difference between maximum radiation and maximum temperature, here the value is taken as 1. Wcon, Weight of refrigerant condensed inside the condenser 0.0kg Heat transfer coefficient Ub bed 1724.14W/(m2K) Ueva Evaporator heat transfer coefficient 2557.54 W/(m2K) Ucon Condenser heat transfer coefficient 4115.23W/(m2K).
PERFORMANCE COMPARISON WITH DIFFERENT CYCLE TIME AND COLLECTORS
Introduction
Results and Discussion
- Adsorption unit performance for different number of collectors
- Adsorption unit performance for different cycle time
The average outlet temperature for different number of collectors with different cycle times is presented in figure 4.3. In this study, the half-cycle time is kept varied while 20, 22 and 24 numbers of collectors are used. It is observed that with fewer collectors, a longer cycle time is needed to increase the bed temperature and cooling capacity.
It is found that the maximum cycle COP, COPsc and COPsolar.net are respectively 0.21, when the optimal number of collectors and cycle time are in use. It is seen that the maximum cold water flow rate is 0.35 kg/ with a cycle time of 800 seconds when 22 collectors are in use. The average cold water outlet temperature for different numbers of collectors with different cycle times is illustrated in Figure 4.7.
Conclusion
EXPLORATION OF OPTIMUM CYCLE TIME AND COLLECTORS
Introduction
Results and Discussion
- Investigation of optimum cycle time
- Investigation of optimum number of collectors
The collector outlet temperature profile for different cycle times with 13 collectors is depicted in Figure 5.1. It is observed that as the cycle time increases, the collector outlet temperature also increases. It is also observed that better performance is possible with increasing cycle time.
It is observed that fewer collectors are required with longer cycle time to increase bed temperature and cooling capacity. The collector outlet temperature for different number of collectors with 1100s cycle time is illustrated in Figure 5.5. The performance of the cooler for different number of collectors with their optimum cycle time is reported in Figure 5.6(a), (b), (c) and (d).
Conclusion
PERFORMANCE COMPARISON WITH OPTIMAL CYCLE TIME AND COLLECTORS IN DIFFERENT MONTHS
Introduction
Results and Discussion
For December the collector outlet temperature is too low and the chiller performance is too low with this choice. This study was carried out taking into account the optimal design area and cycle time considered in the previous chapter for a typical hot day in April. In the month of March, it is average and maximum 7 kW in peak hours.
It is also understood that during peak times there is very little variation in the values of COPcycle for different months. The variation of chilled water flow rate for different months is also illustrated in Figure 6.3. This study was conducted to investigate the feasibility of installing the optimum number of collectors and cycle time, found in chapter 5, for different months of the year.
OVERALL CONCLUSION
In general, it is observed that the COP behavior of the adsorption cycle increases with increasing cycle time, but exhibits irregularities at too much longer cycle time. However, with a smaller number of collectors the cycle time turned out to be longer, because at a lower temperature of the driving heat source a longer time is required for effective desorption. It was determined that the optimal number of collectors is 16, each with an area of 2,415 m2 and an optimal cycle time of 1100 seconds.
The highest cooling capacity is 8.1 kW and the highest COP cycle is 0.5 with a cycle time of 1100 s and 16 collectors in the month of April. In this regard, the comparative performance for the months of March, April, June, August, October and December was studied. It is found that better performance is achieved in the month of April and then in the month of March because this time period is the beginning of hot summer.
FUTURE WORK
C., "Experimental Study of Solid Adsorption Refrigeration System Using Flat Tube Heat Exchangers as Adsorption Beds", Applied Thermal Engineering, Vol. 34; Modeling the performance of two-bed, silica gel-water adsorption chillers", International Journal of Refrigeration, Vol. 12] History of Air Conditioning: www.airconditioning-systems.com/history-of-air conditioner.html (accessed 6 May 2015) .
16] Meunier, F., "Solid Sorption Heat Cycles for Refrigeration and Heat Pump Applications", Applied Thermal Engineering, Vol. Mizanur, Development of Advanced Multistage Adsorption Refrigeration Cycles, PhD Dissertation, Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2013.
