تجزیه و تحلیل عملکرد ترمودینامیکی فشرده سازی بخار ؛ جذب سیستم تبرید آبشاری

In the present study, a thermodynamic model for cascaded vapor compression–absorption system (CVCAS) has been developed which consists of a vapor compression refrigeration system (VCRS) coupled with single effect vapor absorption refrigeration system (VARS). Based on first and second laws, a comparative performance analysis of CVCAS and an independent VCRS has been carried out for a design capacity of 66.67 kW. The results show that the electric power consumption in CVCAS is reduced by 61% and COP of compression section is improved by 155% with respect to the corresponding values pertaining to a conventional VCRS. However there is a trade-off between these parameters and the rational efficiency which is found to decrease to half of that for a VCRS. The effect of various operating parameters, i.e., superheating, subcooling, cooling capacity, inlet temperature and the product of effectiveness and heat capacitance of external fluids are extensively studied on the COP, total irreversibility and rational efficiency of the CVCAS. Besides, the performance of environment friendly refrigerants such as R410A, R407C and R134A is found to be almost at par with that of R22. Hence, all the alternative refrigerants selected herein can serve as potential substitutes for R22. Furthermore, it has been found that reducing the irreversibility rate of the condenser by one unit due to decrease in condenser temperature depicted approximately 3.8 times greater reduction in the total irreversibility rate of the CVCAS, whereas unit reduction in the evaporator’s irreversibility rate due to increase in evaporator temperature reduced total irreversibility rate by 3.4 times for the same system. Since the changes in the inlet temperatures of external fluid in the condenser and the evaporator contribute significant changes in system’s overall irreversibility, due consideration is required in condenser and evaporator temperatures to improve the system performance.

Refrigeration is the process of maintaining the system temperature to a value lower than that of surrounding [1] by means of a refrigeration cycle. Vapor compression refrigeration system is widely used in domestic as well as industrial refrigerating and air-conditioning equipments such as in restaurants, hotels, hospitals and theatres. It is also used for manufacturing of ice, dehydration of gases, lubricating oil purification, low temperature reactions, and separation of volatile hydrocarbons, etc. [1]. Many developing countries like India currently suffer from a major shortage of electricity. The demand for electricity in India is very high both in terms of base load energy and peak availability. During the year 2010–2011, base load requirement was 861,591MU against the availability of 788,355 MU – an 8.5% deficit. During peak loads, the demand was for 122 GW against the availability of 110 GW, a 9.8% shortfall. Out of the 1.4 billion people in the whole world with no access to electricity, India accounts for over 300 million [2]. The refrigerating and air conditioning equipment based on VCRS consume a considerable amount of electric power. In India, 56% of the total electrical capacity is generated using coal [2]. Not only does it exacerbate the depletion of fossil fuel, but also results in the production of harmful gases such as CO2, nitrogen oxides and sulfur oxides etc. It is well known that these gases cause green house effect and deteriorate the environment. In various industrial sectors like liquid milk processing, chilled ready meals, frozen food, cold storage etc. the electricity employed for refrigeration is 25%, 50%, 60% and 85% respectively of their total energy consumption [3]. However, due to poor availability of electric power, the total installed refrigeration capacity is inadequate to meet the refrigeration requirements. Nearly 30% of fresh fruits and vegetables produced in India are wasted due to lack of refrigeration technology [4]. One of the alternatives to reduce the dependence on electrically powered VCRS is the use of VARS in which the compressor is replaced by absorber, pump and generator [1]. VARS demands a substantial amount of heat energy for the generator; however, heat is low grade energy and its demand can be fulfilled using non-conventional sources such as solar, geothermal and waste heat. The electrical energy consumed by this system is merely 10% of the total energy requirements [5]. Several researchers [6], [7], [8], [9], [10] and [11] have studied the performance of VARS considering H2O–LiBr as a working pair. This system is generally used for air conditioning purposes as it can maintain evaporator temperature up to 5 °C [5]. For low temperature applications i.e. below 5 °C, working pair of NH3–H2O can be used in vapor absorption system [12]. But NH3–H2O does not form an ideal pair for absorption system because the difference in their Normal Boiling Points (N.B.Ps) is not large enough (138 °C). There should be a sufficiently large difference in the N.B.Ps of the two substances (at least 200 °C) [1] so that the absorbent exerts negligible vapor pressure at the generator temperature. This system produces ammonia mixed with water vapor at the exit of the generator. Water in the refrigerant stream can cause operational problems in the evaporator of the system. Thus H2O–LiBr pair is suitable from the view point of solubility and boiling point requirements but it cannot be used for low temperature refrigeration [1]. The application of cascaded refrigeration system maintains the advantages of both vapor compression and vapor absorption systems while minimizing the limitations of both simultaneously. The main advantage of CVCAS over VCRS is that it saves considerable amount of high grade energy (electrical energy). While the structure of the cascaded system is more complex and bulky, the overall operating cost is relatively lower because of simultaneous usage of electricity and heat energy for refrigeration. Furthermore, the non-conventional sources of energy such as solar, geothermal etc. can be used to supply heat energy for this system. The literature on CVCAS is summarized in Table 1 with key findings. Table 1. Summary of literature survey of CVCAS. Author System selection Refrigerants Salient features Cimsit and Ozturk [13] Cascade cycle H2O–LiBr and NH3–H2O pairs in absorption section and R134A, R410A and NH3 in vapor compression section • Electrical energy consumption in the cascade refrigeration cycle is 48–51% lower than classical vapor compression refrigeration cycle • Heat energy requirement at generator is reduced by 35% and overall COP of system is improved by 33% with H2O–LiBr fluid pair instead of NH3–H2O pair • Categorized hybrid vapor compression and absorption system into two groups called as combined cycle and cascaded cycle. Many researchers [5], [14], [15], [16], [17], [18], [19] and [20] had worked on combined cycle. Kairouani and Nehdi [21] Cascade cycle powered by geothermal energy in absorption section and combined cycle NH3–H2O pair in absorption section and R717, R22 and R134A in vapor compression section • COP of proposed cascade system is reported to be 5.5 which is superior to that presented by Ayala et al. [5] for combined system. Ayala et al. [5] indicated a COP of 3.1 with NH3–H2O as pair of refrigerant • COP of cascaded cycle was 37–54% higher than the vapor compression cycle. Fernandez-Seara et al. [22] Cascade cycle powered by cogeneration system CO2 and NH3 in compression section and NH3–H2O working pair in the absorption section • COP of compression section is reported to be 2.602 and 2.463 with CO2 and NH3 respectively for low evaporator temperature of −45 °C • Intermediate temperature defined as the condensation temperature of the compression system is an important design parameter that determines the overall COP of system Seyfouri and Ameri [23] Cascade system in which the compressor was powered by a microturbine R22 in compression section and H2O-LiBr working pair in the absorption section • Four configurations of the integrated refrigeration system were considered and highest energy consumption was reported to reduce up to 133% • Use of absorption system improves the COP of compression chiller by 440% Wang et al. [24] Solar assisted cascaded refrigeration system R134A in compression section and H2O–LiBr working pair in the absorption section • COP of the system can be attained up to 6.1 with the solar intensity of 700 W/m2 • Power consumption was reported to be lowered by 50% in comparison with conventional mechanical vapor compression systems Garimella et al. [25] Cascade refrigeration system for naval ship application CO2 in compression section and H2O–LiBr working pair in the absorption section • 31% electricity demand was found to be reduced when compared to an equivalent vapor compression system Chinnappa et al. [26] Solar operated cascade system for air conditioning R22 in compression section and NH3–H2O working pair in the absorption section • Considerable savings in electrical energy consumption by the compression system Table options Many researchers [13], [21], [22] and [26] have considered NH3–H2O pair of working fluid in absorption system which does not form an ideal pair [1]. Unlike water, ammonia is both toxic and flammable. The previous work [13], [21], [22], [23], [24], [25] and [26] reported on CVCAS is based on first law analysis (energy conservation). In the present study, a CVCAS consisting of VCRS coupled with VARS (H2O–LiBr as working pair) has been proposed as an alternative to reciprocating vapor compression chiller [27]. Besides comparative energy as well as exergy analysis supported with preliminary economic analysis has also been performed. The concepts of coefficient of structural bond (CSB), heat utilization factor (HUF), electrical energy utilization factor (EEUF), fuel energy saving ratio (FESR) and heat to work ratio have been applied for better understanding of performance of the system with wide range of cooling capacities. The effect of alternative refrigerants (R134A, R410A and R407C), superheating, subcooling, and size of heat exchanger are also investigated in detail.

In this communication, an extensive thermodynamic study of vapor compression–absorption cascaded refrigeration system has been presented. From the comparative performance study of VCRS and CVCAS, the following conclusions are drawn: 1. Electric power requirement in VCRS is reduced substantially by cascading it with absorption system. However, the total size of the VCRS will increase when it is cascaded, but the running cost is expected to decrease due to the utilization of waste heat available at lower cost. In addition, the COP of vapor compression section will also increase because of low electric power requirement. 2. Subcooling improves system performance, whereas, superheating is found to degrade the system performance. 3. The larger the temperature difference in cascade heat exchanger, the lower the COP of the system; however, a lower temperature difference will lead to increased heat exchanger size and cost. 4. Increasing the size of heat exchanger increases the overall performance of system, but it also increases the system cost. 5. Using the technique of CSB, the thermodynamic performance of condenser is found to be more sensitive to external fluid temperature as compared to evaporator. 6. From the comparative performance of various refrigerants, it can be concluded that the performance of all the selected alternative refrigerants is nearly same as R22. Hence, it may be concluded that the refrigerants considered here have the potential to substitute R22. 7. The initial installation cost of the proposed CVCAS is high but the shorter payback period makes it a commercially viable option.