Exergy analysis of multi-effect water–LiBr absorption systems: From half to triple effect
Introduction
The thermal refrigeration or thermally activated refrigeration, is based on the use of a heat driven absorption unit [1]. In this cooling system the energy input to the generator can be heat from renewable energies such as solar thermal energy and biomass (Fig. 1). Depending on the available temperature level different cycle configurations can be used. These cycles are generally evaluated in terms of their Coefficient of Performance (COP). Compression cycles have significant higher COP’s than absorption cycles. Among the absorption cycles, multi-stage cycles have higher COP’s than the basic configuration, but need higher driving heat temperatures. Best energetic efficiency is obtained by triple effect configurations, followed by double effect and single effect. For very low heat input temperatures the half effect (also called double lift) configuration can be applied, but it presents the lowest COP.
This change in driving temperature and thus quality of the input energy is not taken into account by the COP, but it is considered in the exergy analysis [2], [3], [4]. Using exergy efficiencies we can compare on a rational basis cycles with different types of energy input, in form of heat at different temperature levels or work. The exergy analysis of absorption cycles started in the eighties with publications describing the methodology and the evaluation of the exergy destruction rates and exergy efficiencies [5], [6], [7]. Studies focussing on the effect of operation temperatures and heat exchangers effectiveness has been done for single effect cycles with both the water–LiBr [8], [9], [10] or the ammonia-water working pair [11], [12]. The working pairs have also been directly compared [5], [13]. Anand et al. [5] considered the influence of the effectiveness of the heat exchangers on the cycle, and found the highest increase of the exergetic efficiency by improving solution and refrigerant heat exchangers. Koehler et al. [6] reported for a water–LiBr cycle a large effect of the solution heat exchanger, while the refrigerant heat exchanger was less important. They also stated a high interdependence between various components. Meunier et al. [14] compared for different sorption systems, namely, adsorption, chemical reaction and liquid absorption heat pumps, the main contributions to entropy generation.
Besides the single effect configuration different authors determined also for double effect [5], [15], [16], [17], [18], [19], [20] and triple effect cycles [16] exergy destruction rates and exergetic efficiencies. Jeong et al. [21] used the exergy analysis to obtain an optimum design for an absorption cycle. Sencan et al. [22] evaluated for a water–LiBr single effect cycle the effect of the operating conditions on COP and exergetic efficiency and discussed the main factors which cause the exergy destruction in absorption systems.
Results of the different studies are often difficult to compare and differ in the conclusions obtained, especially if different cycle configurations are analyzed. Basically this is due to the different methodologies and assumptions considered in each analysis. An approach comparing three different cycles has been proposed by [16]. Their study evaluates for a single, a double and a triple effect cycle COP's and exergetic efficiencies for given UA value of the components. The results obtained are valid for these designs, but may change for other design specifications.
Indeed, in the former analysis the exergy destruction rates were always evaluated without distinguishing the avoidable and unavoidable part. Morosuk and Tsatsaronis [23] proposed splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts, in order to facilitate the understanding and the improvement of the considered systems. The exergy destruction rate, which cannot be further reduced by design improvements, represents the unavoidable part. The designer has to focus on the avoidable part, which represents the potential for improving. In thermoeconomic analysis its cost can be compared with the avoidable investment costs, and improvements can center on the most relevant components [24]. Parameters as the modified exergoeconomic factor based on the avoidable costs can be introduced [25].
The purpose of this paper is to compare different configurations of absorption cycles taking into account only the unavoidable exergy destruction. Thus, the results represent the maximum obtainable performance and are not affected by design specifications. The exergy analysis of seven different multi-stage absorption cycles (Table 1) has been achieved applying the same methodology and assumptions for all cycles. The exergetic efficiencies for the different cycles and exergy destruction rates in the main components depending on the heat source temperature have been obtained. Thus, the origin of the exergy destruction in the different cycles can be quantified and compared. The analysis will consider typical cooling conditions with fixed temperatures of the chilled water and cooling water, while the heat source temperature will be varied.
Section snippets
Description of the absorption cycle configurations
The operation and the configuration of absorption cycles already have been described in detail elsewhere [1]. Therefore, only the schematics of the different configurations will be presented (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8), starting with the basic single effect configuration. The cycles are presented in pressure–temperature diagrams. Based on the single effect cycle, more complicated cycles can be obtained in order to improve the energy efficiency or the achievable
Methodology of the simulation
A computer code for simulating the cycles has been established using the program Engineering Equation Solver [27]. Properties for water–LiBr have been evaluated by the correlations from [28]. These correlations are valid for temperatures up to 210 °C, as required in the triple effect cycles. Properties for all state point have been evaluated.
The input data, output data and main assumptions are presented below. For this study, typical cooling operating conditions have been chosen [29], [30]. In
First law analysis
The further calculation have been achieved with the assumptions presented above, especially fixing for the heat exchangers the minimum temperature difference. Thermodynamics properties at each state points of some of the cycles are given in (Table 5, Table 6, Table 7, Table 8). The tabulated values correspond approximately to the generator temperature which yields to the maximum COP. COP and exergetic efficiency are plotted versus the generator temperature in order to compare the different
Conclusions
The performance of seven different water–LiBr absorption cooling cycles has been evaluated applying first and second law analysis. Only unavoidable exergy destruction is considered in order to compare the cycles on a rational basis. Effects of the avoidable exergy destructions are eliminated at this stage of the theoretical analysis, but should be considered at the design stage. The present study enables us to distinguish and quantify these parts. The avoidable part shows where the main
Acknowledgements
Berhane H. Gebreslassie expresses his gratitude for the financial support received from University Rovira i Virgili. The authors also wish to acknowledge support of this research work from the Spanish Ministry of Education and Science (projects DPI2002-00706, DPI2008-04099, PHB2008-0090-PC and BFU2008-863 00196) and the Spanish Ministry of External Affairs (projects A/8502/07, HS2007-864 0006 and A/020104/08).
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