Energy and exergy analysis of a double absorption heat transformer operating with water/lithium bromide

In the present study, the first and second law of thermodynamics have been used to analyze in detail the performance of a double absorption (lift) heat transformer operating with the water–lithium bromide mixture. A mathematical model was developed to estimate the coefficient of performance (COP), the exergy coefficient of performance (ECOP), the total exergy destruction in the system (Ψ_TD) and the exergy destruction (Ψ_D) in each one of the main components, as a function of the system temperatures, the efficiency of the economizer (EF_EC), the gross temperature lift and flow ratio (FR). The results showed that the generator is the component with the highest irreversibilities or exergy destruction contributing to about 40% of the total exergy destruction in the whole system, reason why this component should be carefully designed and optimized. The results also showed that the COP and ECOP increase with increase in the generator, the evaporator and the absorber–evaporator temperatures and decrease with the absorber and condenser temperatures. Finally, it was observed that the COP and ECOP are very dependent of the FR and the economizer efficiency (EF_EC) values. Also the optimum operating region of the analyzed system is shown in the present study. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical comparison of single stage and advanced absorption heat transformers operating with water/lithium bromide and water/Carrol mixtures

A thermodynamic analysis was carried out to compare the theoretical performance of single stage, two stage and double-absorption heat transformers operating with the water/lithium bromide and the water/Carrol mixtures, where Carrol is a mixture of lithium bromide and ethylene glycol [(CH₂OH)₂] in the ratio 1:4·5 by weight. A mathematical model to predict the theoretical performance of single stage and the advanced heat transformers is also described. Coefficients of performance and gross temperature lifts are compared for the different heat transformers and plotted against the main temperatures of the system for both mixtures. The water/Carrol mixture showed in general to have a better performance than the water/lithium bromide mixture. © 1998 John Wiley & Sons, Ltd.     

Experimental study of the use of additives in the performance of a single‐stage heat transformer operating with water–lithium bromide

In the present paper, the additives 1‐octanol and 2‐ethyl‐1‐hexanol, have been used for the first time in a 2 kW single‐stage heat transformer installed in the Energy Research Centre of the National University of Mexico utilizing H₂O/LiBr, operating at absorber temperatures in a range between 70 and 110°C. The results showed that at the same conditions, absorber temperatures increased about 5°C by adding 400 ppm of 2‐ethyl‐1‐hexanol to the lithium bromide mixture. Also it was shown that the coefficient of performance increases up to 40% with the same additive. Copyright © 2005 John Wiley & Sons, Ltd.     

温度参数对升温型溴化锂吸收式热泵性能系数影响程度分析

通过对升温型溴化锂吸收式热泵进行理论分析,得到在相同工况条件下,发生温度变化对系统性能系数的影响程度最大,但小于吸收温度和冷凝温度影响程度之和.并利用焓浓度图,结合前人数据对理论分析进行了验证.     

Boiling heat transfer coefficients inside a vertical smooth tube for the water/lithium bromide mixture

This paper describes the experimental results obtained on the heat transfer in forced convective boiling for the water/lithium bromide mixture flowing upward in a vertical tube uniformly heated. The concentration range for the mixture was from 48.1 to 57.7 wt%. Correlations were proposed to correlate the experimental local heat transfer coefficients. The results showed that the local heat transfer coefficients are strongly dependent on Bo, 1/X_tt and 1/x at the analysed conditions. It was observed that the average heat transfer coefficients increased for the mixture with a decrease of the solution concentration or an increase of the mass flux. Copyright © 2002 John Wiley & Sons, Ltd.     

Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature

Mathematical models of single-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/Carrol™ mixtures were developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. Plots of coefficients of performance and gross temperature lifts are shown against the temperatures of the heat supplied by the solar pond. The results showed that the single-stage and the double absorption heat transformer are the most promising configuration to be coupled to solar ponds. With single-stage heat transformers it is possible to increase solar pond's temperature until 50°C with coefficients of performance of about 0.48 and with double absorption heat transformers until 100°C with coefficients of performance of 0.33.     

Theoretical and experimental comparison of the performance of a single‐stage heat transformer operating with water/lithium bromide and water/Carrol™

This paper compares under the same operating conditions, the theoretical and experimental performance of a single‐stage heat transformer operating with the water/lithium bromide and the water/Carrol? mixtures, where Carrol? is a mixture of lithium bromide and ethylene glycol [(CH₂OH)₂] in the ratio 1:4.5 by weight patented by Carrier Corp. Flow ratios, gross temperature lifts, useful heat, and coefficients of performance are plotted for the heat transformer against temperatures and solution concentrations. Because the water/Carrol? mixture has higher solubility than water/lithium bromide and high experimental values were obtained for the gross temperature lift, it seems to be a better alternative mixture to be used in absorption heat transformers. Copyright © 2002 John Wiley & Sons, Ltd.     

Entropy generated and exergy destroyed in lithium bromide thermal compressors driven by the exhaust gases of an engine

This work is devoted to the study of the entropy generated and the exergy destroyed in the lithium bromide absorption thermal compressors of single and multiple effects, driven by the heat of the exhaust gases of an engine, when the absorption heat is directly transferred either to water or to air. Air‐cooled systems work with temperature and pressure gradients higher than those cooled by water. The absorption temperature in air‐cooled systems can reach and even exceed 50°C. Under these conditions, boiling temperature within the high desorber of the double and triple effect systems can exceed 200 and 300°C, respectively. Maximum pressures reach values of 1.7 and 15 bar, respectively. The thermal compressor cooled by air generates more entropy and destroys more exergy than the one cooled by water. The triple‐effect thermal compressor destroys less exergy than the one of double effect and the latter destroys less exergy than the one of single effect. The lithium bromide thermal compressor of single effect cooled by air is not feasible when working with absorption temperatures around 50°C. The one of double effect is feasible since the high‐pressure desorber can work at higher temperatures. Under these conditions, the solution cycle described within the high‐pressure desorber remains out of the zone of crystals formation, and offers the possibility of producing more refrigerant than the one of single effect. Also, in the double‐effect compressor less entropy is generated, and therefore less exergy is destroyed than in the single effect. The triple‐effect compressor cooled by air offers the possibility of producing more refrigerant than the one of double effect, but at higher expenses of temperatures and boiling pressures of the solution. This creates corrosion and control problems, which do not have an easy solution yet. Less exergy destruction does not compensate for the increase of these problems. In any case, the compression process of the cooling steam occurs with entropy reduction. Copyright © 2000 John Wiley & Sons, Ltd.     

Thermodynamic analysis of a lithium bromide/water absorption system for cooling and heating applications

Absorption systems have the potential of employing thermal energy such as waste heat to produce both chilled water and hot water for building cooling and heating applications. In the present study, a lithium bromide/water (LiBr/H₂O) absorption system for cooling and heating applications was analysed on the basis of the first and second laws of thermodynamics. Simulation was employed to determine the coefficient of performance (COP) and the exergetic efficiency of the absorption system under different operating conditions such as the heat source, cooling water, chilled water, and supply hot water temperatures. Copyright © 2001 John Wiley & Sons, Ltd.     

Comparison of the theoretical performance of a solar air conditioning system operating with water/lithium bromide and an aqueous ternary hydroxide

This paper compares the theoretical performance of the modelling of a solar absorption air conditioning system operating with water/lithium bromide and an aqueous ternary hydroxide mixture consisting of sodium, potassium and cesium hydroxides in the proportions 40 : 36 : 24 (NaOH : KOH : CsOH). In this paper, plots of the coefficients of performance of a solar air conditioning system operating with these two mixtures are presented. The results showed that similar coefficients of performance are obtained for both mixtures, however, it was found that the system operating with the hydroxides may operate with a higher range of condenser and absorber temperatures and the heat delivered by these components can be removed by air.     

Optimization of energy plants including water/lithium bromide absorption chillers

In this paper a methodology for the optimal integration of water/lithium bromide absorption chillers in combined heat and power plants is proposed. This method is based on the economic optimization of an energy plant that interacts with a refrigeration cycle, by using a successive linear programming technique (SLP). The aim of this paper is to study the viability of the integration of already technologically available absorption chillers in CHP plants. The results of this alternative are compared with the results obtained using the conventional way of producing chilled water, that is, using mechanical vapour compression chillers in order to select the best refrigeration cycle alternative for a given refrigeration demand. This approach is implemented in the computer program XV, and tested using the data obtained in the water/LiBr absorption chiller of Bayer in Tarragona (Catalonia, Spain). The results clearly show that absorption chillers are not only a good option when low‐cost process heat is available, but also when a cogeneration system is present. In this latter case, the absorption chiller acts as a bottoming cycle by using steam generated in the heat recovery boiler. In this way, the cogeneration size can be increased producing higher benefits than those obtained with the use of compression chillers. Copyright © 2000 John Wiley & Sons, Ltd.     

Performance study of a double-absorption water/calcium chloride heat transformer

In order to increase the temperature lift and efficiency of single-absorption heat transformers there are other possible arrangements. Double-absorption heat transformers have a relatively simple design and smaller size compared to two-stage heat transformers. In this work, the thermodynamic performance of the water/calcium chloride system was modelled for a double-absorption heat transformer. Results indicate that temperature lifts of up to 40°C are possible with coefficients of performance close to 0·3. © 1998 John Wiley & Sons, Ltd.     

参数变化对LiBr吸收式热泵性能的影响

针对溴化锂吸收式热泵的特点,在一定参数变化范围内研究驱动热源、冷媒水以及冷却水进出口变化对其性能的影响。结果表明,不同参数对COP及具体设备的热负荷、传热温差造成不同程度的影响,在热泵的设计过程中应综合考虑各参数的取值以获得最佳的效果。研究结果为热泵系统的优化设计提供理论支持和参考。     

Comparison of energy,exergy, and entropy balance methods for analysing double‐stage absorption heat transformer cycles

The energy, exergy and entropy balance methods are used to analyse a double‐stage LiBr‐water absorption heat transformer cycle. An energy balance comparing component energy transfer is used to determine energy calculations. An exergy balance is employed to evaluate exergy destruction, and an entropy balance to verify entropy generation. A comparison of the results by the second law exergy and entropy balances indicates that they are consistent in identifying the location and relative significance of key non‐idealities within the system. The results obtained clearly show the influence of irreversibilities of individual components on deterioration of the effectiveness and the coefficient of performance of the system. The second law analysis offers an alternative view of cycle performance and provides an insight that the first law analysis cannot. The differences between the first law analysis by energy balance method and second law analysis by exergy and entropy balance methods are illustrated quantitatively for the double‐stage absorption heat transformer cycle, and the limitations and advantages of these methods are presented and discussed. Copyright © 2004 John Wiley & Sons, Ltd.     

Exergy analysis of an experimental heat transformer for water purification

First and second law of thermodynamics have been used to analyze the performance of an experimental heat transformer used for water purification. The pure water is produced in the auxiliary condenser delivering an amount of heat, which is recycled into the heat transformer increasing the heat source temperatures and also the internal, external and exergy coefficients of performance. The theoretical and experimental study was divided into two parts. In the first part, a second law analysis was carried out to the experimental system showing that the absorber and the condenser are the components with the highest irreversibilities. In the second part, with the results obtained from the second law analysis, new test runs were carried out at similar conditions than the former but varying only one selected temperature at the time. Comparing the COP (coefficient of performance) between the old and new test runs, it was shown that higher internal, external and exergy coefficients of performance were obtained in all the new test runs. Also it was shown that the ECOP (exergy coefficient of performance) increases with an increment of the amount of the purified water produced and with the decrease of the flow ratio.     

Heat transfer coefficients in two phase flow for the water/lithium bromide mixture used in solar absorption refrigeration systems

This paper describes the experimental results obtained from the heat transfer in saturated nucleate boiling for the water/lithium bromide mixture flowing upward in a uniformly heated vertical tube, which is the generator of a solar absorption refrigeration system. The concentration range for the mixture was from 48 to 56 wt.% Plots of local and average heat transfer coefficients are shown against solution concentration, heat flux and the temperature difference between the wall tube and the fluid. It was observed that the average heat transfer coefficients increased for the mixture with an increase of the heat flux and with the decrease of the solution concentration and the temperature difference. The average heat transfer coefficients varied from 1.0 to 4.0 kW/m² °C.     

Single stage and double absorption heat transformers in an industrial application

An industrial application of the single stage and double absorption heat transformer (AHT) systems using water–lithium bromide solutions with water as the refrigerant was analyzed. First, a basic single stage AHT system was described, the operating sequence was explained and thermodynamic system analysis was presented. Next, an application of the single stage AHT system to an industrial company was analyzed. A computer code was prepared to determine the effect of different parameters on the AHT system performance and the results were presented in graphical form. Additionally, the series and parallel double absorption AHT systems were introduced, the operating sequences were explained and thermodynamic system analysis was included. All results were presented in tabular form for comparison. It is concluded that about 50% of the waste heat can be utilized and the hot process water and vapor could be produced by applying single stage and double AHT systems, respectively. The parallel double AHT system could generate more vapor than the series double AHT system. Copyright © 2009 John Wiley & Sons, Ltd.     

Proposal of humid air turbine cycle incorporated with absorption heat transformer

A detailed graphical exergy study based on the energy‐utilization diagram (EUD) is applied to humid air turbine (HAT) cycle incorporated with a modified two‐stage absorption heat transformer (TAHT). Employing the sensible and latent heat exchange modes in this TAHT and then introducing to the HAT cycle have clarified that this method can intensively recover the waste heat of HAT cycle and improve the system performance. Compared to a conventional HAT cycle, the overall cycle efficiency can be increased by 2 per cent points and the specific work can be increased by 7.3 per cent. Copyright © 2000 John Wiley & Sons, Ltd.     

Experimental performance of the water-lithium chloride system in a heat transformer

Heat tranformers are devices with the unique capability of raising the temperature of part of a low-grade heat source whilst simultaneously delivering the rest of the heat at a lower temperature. The gross temperature lift that could be attained in the process depends on the characteristics of the working pair. Many combinations of working fluid/absorbent have been proposed although until now the water/lithium bromide system is the most widely used. In order to study the performance of combinations of environmentally friendly working pairs, an absorption heat transformer was constructed and tested. The experimental equipment is described in this work. The performance of the water/lithium chloride system is discussed. The results showed that gross temperature lifts of more than 30°C can be obtained for absorber temperatures higher than 110°C. The enthalpic coefficient of performance indicated that more than 45% of the waste heat can be upgraded for flow ratios less than 10.