EXERGETIC ANALYSIS OF A SOLAR ABSORPTION HEAT TRANSFORMER

1KHERRIS Sahraoui, 2ZEBBAR Djallel, AZIZI Aicha, BENADI Noura et AYAD Laila

Institut des Sciences et Technologies

Centre Universitaire de Tissemsilt

Abdelhak Benhamouda, BP. 182, 38000, Tissemsilt, Algérie

E-mails:,

Abstract:

In the present study, the first and second laws of thermodynamic have been used to analyzea solar absorption heat transformer (SAHT). A mathematical model of a single stage solar absorption heat transformer using ammonia-water as binary mixtures has been discussed. Furthermore, in this study new thermodynamics properties of ammonia-water mixtures have been used. An analyze of the performances of the SAHT highly affected by the energy efficiency,solar coefficient of performance, and circulation ratio has been carried out in this study.

Keywords- exergy; heat transformer; solar; modelling ; absorption.

2.Introduction

Very large quantities of waste heat at low temperatures are discarded into the atmosphere from power plants and industrial processes. Many attempts have been made in most industrial sectors to recover this useful energy by heat pumps, vapor absorption refrigeration systems and heat transformers.

Upgrading this low-level heat can make possible its use in different forms. Among the different possibilities, heat transformers present an attractive solution for upgrading low temperature waste heat to higher temperature useful heat with minimum consumption of external energy [1-3]. The general principles of heat transformation were studied by Altenkirch [1, 4] in 1913, and developed later by Nesselmann [1, 5]. In “1982” Wilkinson [1, 3] proposed different types of single and multi-stage absorption heat transformers. A comparative study of different working fluid combinations with R22 as refrigerant and six absorbents in a single AHT was performed by Fatouh and Srinivasain “1992”, [6]. A new type of AHT operating with reverse rectification with water-glycol and ammonia-water mixtures was presented by Le Goff in 1992 [7-8].

In the same year, Rivero and Le Goff described and compared the different performance criteria available for analyzing heat pump and heat transformers experimental work using an ammonia-water heat transformer has been reported by Mostofizadeh [3].

Stephan and Seher [9-10] have discussed the heat transformer cycles for single and double-stage processes. Kripalani et al. [11] have studied the performance analysis of a vapor absorption heat transformer with different working fluid combinations.

Exergy analysis permits to know energy quality into heattransformer for the optimization of the operating parameters to yield a better performance in the AHT [12-20].

Zebbar et al. [12] have elaborated a mathematical modeling of an AHT to find out the optimal operating parameters using the so-called structural analysis, for the thermodynamic optimization.

Lee and Sherif [13] utilized thesecond law analysis to know the performance of multi stage

water-lithium bromide absorption heat transformers. The resultsprovided theoretical basis for the optimal operation conditions anddesign of absorption heat transformers.

Sozen [14] studied theirreversibilities in a single-stage heat transformer used to increasesolar pond’s temperature. The results showed that the absorber andthe generator need to be thermally improved in order to increasethe efficiency of the system.

Fartaj [15] compared the energy,exergy and entropy balance methods for analysing double stageabsorption heat transformer cycle which is in fact a modification ofa two stage heat transformer cycle. The results obtained show theinfluence of irreversibilities of individual components on deteriorationof the effectiveness and the coefficient of performance of thesystem.

In this study a new thermodynamics properties of ammonia-water mixtures have used [21]. The proposed correlations cover equilibrium conditions of phases at high pressures and temperatures.

The performances of absorption heat transformer are defined by the energy efficiency, thesolar coefficient of performance and circulation ratio have been analyzed and compared.

3.Mathematical model

A schematic representation of SAHT is shown in Fig. 1. It consists from the following elements: generator, evaporator, condenser, absorber, solution heat exchanger (SHE), refrigerant heat exchanger (RHE), two pumps and throttling valve. The generator and evaporator receive waste heat at the same medium temperature. The absorber delivers useful heat at a higher delivery temperature, where as part of the heat flowing into the process is rejected at ambient temperature from the condenser.

The discussion of the mathematical modelling is based on the laws of the mass and energy balances for each SAHT element and the properties of ammonia-water mixtures at various points of the system.

Fig. 1 Schematic representation of solar absorption heat transformer

3. Results and discussions

For the simulation of the solar ammonia-water absorption heat transformer, the software "SARM" (Simulation of Absorption Refrigeration Machine) is used [22-25] in this study. The performances of the system have been evaluated by varying the operating parameters.

3.1.Variation of the COP with different heat source temperatures

The comparison of the results obtained for the coefficient of performance according to the absorber and generator temperatures, with those of Ismail I.M., [20] for three temperature values ​​of generator and evaporator on (60, 70 and 80 ° C) is carried out (Figs. 2: a, b and c). The operating conditions chosen are:

  • The condenser temperature Tc = 25 °C;
  • The generator an evaporator temperatures Tg =Te= (60, 70, 80) °C;
  • The efficiency of the two exchangers  = 0.8.

The comparison shows a good agreement of the coefficient of performance obtained in this study with those of Ismail I.M., [20], for three values ​​of temperatures. The average error is less than 2.82%.


«a» /
«b»

«c» /
Fig. 2 Comparison of COP with different heat source temperatures / Fig. 3 Variation of fc=f(Ta, Tc) with Tg=Te=70 °C

3.2.Effect of the absorber and condenser temperatures to the circulation ratio

This analysis is performed when the variation of the absorber and condenser temperatures for the evaporator and generator temperatures set at 70 ° C. The efficiency of the two exchangers is assumed equal to 75 %.

From Fig. 3 it can be seen that the increase of the absorber temperature leads to an increase in the circulation ration. The value of this last is higher in the lower range of the condenser temperatures. This increase reflects the growth of low pressure.

3.3.Effect of the absorber, generator and condenser temperatures to the exergy efficiency

The exergy efficiency is plotted in Fig. 4 as a function of generator, condenser and heat delivery temperatures.

It is clear that the exergy efficiency decrease with an increase in generator, condenser and heat delivery temperatures.

Fig. 4Variation of exergy efficiency with different heat source temperatures

4.Conclusion

The conclusions of this study are summarized as follows:

  • The performance of the heat transformer strongly depends on the properties of the refrigerant-absorbent solutions;
  • A mathematical modeling of solar ammonia-water AHT was carried out;
  • The COP of the SAHT system was analyzed and compared with other results;
  • The variation of COP of the SAHT system with different waste heat source temperatures against the temperature boost (absorber) for the given condenser, generator and evaporator temperatures show that the increase in temperature boost causes a decrease in COP and COPs;
  • The variation of delivery temperature has very little influence over the COP of the system but the exergy efficiency varies significantly.

Finally, it is necessary to note the good agreement of the calculated of the COP of the AHT with Ismail I.M.

Nomenclature

/

Subscripts

T / temperature, [°C, K] / a / Absorber
COP / coefficient of performance / g / Generator
 / heat rate [kJ/ s, kW] / e / Evaporator
x / solution concentration / c / condenser
NH3 / Ammonia / s / Solution
H2O / Water / r / Refrigerant
fc / circulation ratio / RHE / refrigerant heat exchanger
AHT / absorption heat transformer / SHE / solution heat exchanger
SAHT / Solar absorption heat transformer / TST / thermal storage tank
η / exergy efficiency
 / exchanger efficiency

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