Performance improvement of absorption refrigeration system using triple-pressure-level

doi:10.1016/S1359-4311(03)00106-6

Applied Thermal Engineering

Volume 23, Issue 13, September 2003, Pages 1577-1593

https://doi.org/10.1016/S1359-4311(03)00106-6 Get rights and content

Abstract

In the absorption refrigeration system (ARS) working with aqua–ammonia, the ejector is commonly located at the condenser inlet. In this study, the ejector was located at the absorber inlet. Therefore, the absorber pressure becomes higher than the evaporator pressure and the system works with triple-pressure-level. The ejector has two main functions: (i) aiding pressure recovery from the evaporator, (ii) upgrading the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. In addition to these functions, it can also act to lower the refrigeration and heat-source temperatures. Energy analyses show that the system’s coefficient of performance (COP) and exergetic coefficient of performance (ECOP) were improved by 49% and 56%, respectively and the circulation ratio (f) was reduced by 57% when ARS is initiated at lower generator temperatures. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The heat source and refrigeration temperatures decreased in the range of 5–15 °C and 1–3 °C, respectively. Exergy analyses show that the exergy loss of the absorber of ARS with ejector had a higher exergy loss than those of the other components. Therefore, a multiple compartment absorber can be proposed to reduce the exergy loss of the absorber of ARS with ejector.

Introduction

Despite a lower coefficient of performance (COP) as compared to the vapour compression cycle, absorption refrigeration systems (ARS) are attractive for using waste energy from industrial processes, geothermal energy, solar energy, etc. In addition, the ARS use natural substances, which do not cause ozone depletion and has no fossil fuel consumption as working fluids [1], [2].

There are many parameters affecting the overall performance of the absorption thermal systems. The performance of the ARS is dependent upon the choice of appropriate refrigerant/absorbent pair. Because of this reason, the choice of the most appropriate refrigerant/absorbent pair is as important as system design and optimisation of parameters. Utilization of low-potential heat-sources (50–130 °C) for cooling and refrigeration is limited by the properties of the working fluids. The aqua–ammonia combination has acceptable thermo physical properties for the ARSs. In order to improve the system performance, researches on the design of system components and applications of various configurations on the system have been constantly going on.

In the ARS, an ejector is located at the absorber inlet [3] for the purpose of increasing the absorber pressure with respect to the evaporator pressure and of improving the mixing process and the pre-absorption by the weak solution of the ammonia coming from evaporator. It should be noted that the ejectors can increase the pressure without consuming mechanical energy directly, which are the main characteristics of ejectors [4], [5], [6]. Due to these characteristics, applying an ejector may be simpler and safer technologically than applying mechanical devices, which can increase pressure, such as a compressor, pump, etc. Besides the very simple configuration ejector’s, the systems combining ejectors and other devices are also very simple [7].

In this work, a combination of computerized simulation programs, one for the ARS without ejector (WEARS) and the other for the ejector, was developed to examine the influence of the ejector on performance of the ARS with ejector (EARS). In addition, exergy losses for each component in the EARS and WEARS were calculated at different working temperatures. The results of the exergy analyses can be used to identify the less efficient components of the system.

Section snippets

System description

The EARS operated with aqua–ammonia consists of a condenser, an evaporator, an absorber, a generator, an ejector, a pump, expansion valves, a refrigerant heat exchanger (precooler, RHE) and a mixture heat exchanger (solution heat exchanger, MHE). When these two heat exchangers are used in the system, the overall performance is increased [8], [9]. A schematic representation of a triple-pressure-level absorption refrigeration cycle and the ejector are given Fig. 1, Fig. 2. An ejector cycle is

Analysis of the ejector

The ejector is characterized by the facts that there are no moving parts and there is no requirement for an additional energy sources. The heated solution leaving the heat exchanger (stage 6) as a primary fluid enters the nozzle, of the ejector in which it expands rapidly. In the exit of the nozzle the solution gets very rapid with very low pressure and draws the working fluid vapor (stage 12) from the evaporator as the secondary fluid. The two fluids mix fully in the mixing tube of ejector.

Effect of the ejector on the COP of the ARS cycle

The calculated COP for the different generator, absorber, evaporator and condenser temperatures in EARS and WEARS are shown in Fig. 3, Fig. 4. For a given evaporator, absorber and condenser temperature there is a minimum generator temperature, which corresponds to equalization of ammonia concentrations of the solution flowing into and out of the generator. This temperature is called cut off temperature. For the generator temperature above this value, the COP increases until its maximum value.

Conclusions

The available energy analysis of the ARS is carried out in this work and exergy loss of each component as well as the f, COP, and ECOP of the EARS and WEARS are calculated. The ejector facilitated pressure recovery and upgrading the mixing between the weak solution and the refrigerant vapour coming from the evaporator. The effect of the ejector on the overall performance of the ARS cycle was evaluated and the performance of the EARS was compared with that of the WEARS.

The maximum upgrading

Acknowledgements

This study was supported as the Research Project of Gazi University (Project no: 07/2001-16).

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Citation Excerpt :
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The fact that diffusion absorption refrigeration (DAR) systems can also work with low power plants has increased the interest in these cooling systems again. The DAR systems are mostly analyzed using energy analysis. Although, energy analysis is not adequate itself, so exergy analysis is required. In this study, the effects of nanofluid use on the DAR system performance are investigated by performing exergy and thermoeconomic analyses. Some nanoparticles like MgOAl₂O₃, ZnOAl₂O₃ and TiO₂ in various weight ratios are used in the DAR system. In experiments with different nanoparticles, the effect of utilizing hybrid nanoparticles on enhancing system performance is also evaluated. The highest exergy destruction rate occurs in the solution heat exchanger, although the lowest exergy destruction rate happens in the condenser. The highest exergy efficiency with 0.798 is found in the base fluid (25% NH₃–H₂O). Besides, it is clearly observed that exergy efficiency decreased in the DAR systems using nanofluids. Moreover, the economic analysis of the DAR system is added to the study in order to evaluate the relationship between the exergy destruction rate and initial investment costs. It is found that the DAR system with ZnOAl₂O₃ (2 wt%) is preferable compared to other DAR systems.

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Applied Thermal Engineering

Abstract

Introduction

Section snippets

System description

Analysis of the ejector

Effect of the ejector on the COP of the ARS cycle

Conclusions

Acknowledgements

References (20)

Study and control of the optimal generation temperature in NH₃–H₂O absorption refrigeration systems

Applied Thermal Engineering

A new adsorption–ejector refrigeration and heating hybrid system powered by solar energy

Applied Thermal Engineering

Performance of a triple-pressure-level absorption cycle with R125-N,N-dimethylethylurea

Applied Energy

Evaluation of a novel combined ejector–absorption refrigeration cycle––I: computer simulation

International Journal of Refrigeration

Thermo-economical analysis between new absorption–ejector hybrid refrigeration system and small double-effect absorption system

Applied Thermal Engineering

Collector selection for solar ejector cooling system

Solar Energy

Variable geometry ejectors and their applications in ejector refrigeration systems

Energy

Effect of heat exchangers of performance on absorption refrigeration system

Energy Conversion and Management

Development and testing a prototype absorption heat pump operated by solar energy

Applied Thermal Engineering

Study of design parameters of a jet ejector for absorption systems

Applied Energy

Cited by (58)

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Exergy and thermoeconomic analyses of the diffusion absorption refrigeration system with various nanoparticles and their different ratios as work fluid