• Energy Research

Abstract The absorption cooling/heating system is an old technology that has been relegated by the more efficient mechanical vapour compression systems. However, if they were driven by residual heat or solar thermal energy, advanced absorption technologies for cooling or heating could supply current demand and have a much lesser impact on the environment. With the cost of electricity rising and the climate change more and more in evidence, it would be a positive move towards energy saving. Since the absorber is recognized the key component of the absorption system due to the complex heat and mass transfer process that take place there, the improvement of the absorption process would mean reducing the absorber and desorber sizes to make them more compact, or reducing the system driving temperature for low grade temperature applications. The objective of this paper is to identify, summarize, and review the experimental studies dealing with the enhancement of vapour absorption processes in absorbers by means of passive techniques i.e. advanced surface designs and the use of additives and nanofluids. This review also includes an exhaustive and detailed scrutiny on absorption processes in falling film, spray and bubble mode absorbers for different working fluids, evidencing the experimenting techniques, operating conditions, and latest advances in terms of heat and mass transfer enhancement in absorbers. Finally, the paper contains suggestions for future work to be carried out to obtain mass transfer enhancement in absorbers and absorption cooling/heating systems.

At present, much interest is being shown in absorption refrigeration cycles driven by low temperature heat sources, such as solar energy or low-grade waste-heat. Double-lift absorption cycles working with ammonia-water have been recommended for refrigeration applications which require cold at 0°C and which are activated by waste heat between 70 and 100°C. This paper discusses the potential of the organic fluid mixtures trifluoroethanol (TFE)-tetraethylenglycol dimethylether (TEGDME or E181) and methanol-TEGDME as working pairs in series flow and vapour exchange double-lift absorption cycles. The ammonia-water mixture was used for comparison purposes. The results show that the performances of these cycles improve significantly when they have the above mentioned organic fluid mixtures as working pairs. For example, the coefficient of performance of the vapour exchange cycle working with TFE-TEGDME is 15% higher than with ammonia-water. In this study, we used a modular software package, which we developed for the thermodynamic properties and cycles simulation of absorption systems.

A theoretical model was developed to investigate a falling-film absorber on horizontal tubes with an aqueous alkaline nitrate solution as working fluid. The absorbent, composed of an aqueous solution of nitrates (Li, K, Na) in salt mass percentages of 53%, 28%, and 19% respectively, offers favourable thermal stability, corrosiveness, and heat and mass transfer conditions which can be appropriate for absorption cooling cycles driven by high-temperature heat sources. The mathematical model developed characterises the heat and mass transfer processes and the flow regime effect (droplet-formation, droplet-fall, and falling-film) on the falling-film absorber. The results show the importance of the falling-film and droplet-formation flow regimes in the absorption process. The solution temperature and concentration profiles inside the absorber were established together with their values at the exit. The results obtained by the theoretical model were well in agreement with the experimental data obtained by the authors in a previous study. Deviations in predicting the solution and cooling water temperatures at the absorber exit were around 1 °C and for the concentration of the solution leaving the absorber, around 0.49%. The mathematical model also predicts the absorption rate at 4.7 g·m−2·s−1 for the absorber design and operating conditions used in the present work. This value is 22% higher than the experimental value obtained by the authors in their previous experimental work. The deviation is attributed to approximations incorporated into the model, especially as regards surface wettability and calculation of the mass transfer coefficients for each flow regime.

Abstract A steady-state simulation model of a commercial 3-ton ammonia/water absorption chiller is developed and validated using the flow-sheeting software Aspen-Plus. First an appropriate thermodynamic property model for the ammonia/water fluid mixture is selected. To this purpose nine methods from the software library are pre-selected and tested, but none of the methods predicts the VLE (vapour–liquid equilibrium) with sufficient accuracy. The interaction parameters of these models are then determined by fitting the equations of state (EOS) to VLE data. It is finally found that the Boston–Mathias modified Peng–Robinson EOS with fitted parameters predicts most accurately the VLE for the temperature and pressure ranges encountered in commercial chillers. A simulation model of the machine is then developed. The simulation results are found to be in good agreement with data from literature at a cooling air temperature of 35 oC. The heat transfer characteristics ( UA ) of the various heat exchangers of the machine are then determined and the model modified to make it accept these ( UA ) as input parameters. The comparison of the simulation predictions at cooling air temperatures of 26.7 and 38 oC with the bibliographical data showed good concordance. The proposed model could be very useful for the analysis and performance prediction of the commercial absorption chiller.

The challenge of reducing vehicle energy consumption and greenhouse gas emissions has become a major orientation of automotive industry research throughout the world. Improving and optimizing power consumption by electric vehicles is of special concern. A novel use of thermoelectric generators in vehicle braking is presented. Thermal analysis of brake pads and discs using finite elements was applied to evaluate the energy potentially available in the form of heat produced by the friction involved in braking. We present stimulations of disc heating during and after braking at three ambient temperatures and reflect on the possibilities of energy recovery in warm as well as cold climates. The results show that although the yield of electrical energy from typical thermoelectric generators is about 0.3% of the total thermal energy associated with braking, at least 4 W can be made available, enough to power on-board instrumentation and vehicle devices and thereby improve the energy efficiency of motor vehicles.

This study discusses the technical, environmental, and economic feasibility of using absorption chillers driven by solar energy and/or natural gas, in selected shopping malls in Barranquilla, Caribbean region of Colombia. The high solar irradiation and the low prices of natural gas in the cities of the Caribbean region of Colombia are attractive conditions for the use of absorption chillers. To prove the feasibility of absorption chillers in the Caribbean region of Colombia, the use of water/LiBr absorption chillers of 352 kW cooling capacity was investigated considering the cooling loads in selected malls. A thermodynamic model was developed to study the performance of the absorption chiller and evaluate different scenarios proposed. The results evidenced that the absorption chiller could reach a maximum COP and SCOP of 0.77 and 0.52, respectively. The different alternatives could reduce gas emissions between 17% and 76% depending on the cooling load covered by the absorption chillers and driving energy input as compared to the current use of mechanical compression chillers. The economic results indicated that the best scenario, considering a lifetime of 20 years, is the gas-driven absorption chiller with IRR varying from 40% to 54.6% depending on the mall cooling load covered.

Abstract A pre-industrial prototype of a new water-cooled ammonia/lithium nitrate absorption chiller was characterised at part-load operation mode. The chiller was built using brazed plate heat exchangers in all its components, including the absorber and the generator. A test campaign was carried out varying the thermal load in the chilled water circuit and keeping the hot and cooling water temperatures constant. Part-load curves of the thermal and electrical coefficients of performance were obtained, plotted and compared with data from the literature on small capacity absorption chillers with conventional working pairs, namely ammonia/water and water/lithium bromide. The experimental results showed that to achieve a higher electrical coefficient of performance at part-load operation, it was much more convenient to use an ON-OFF control than to modify the hot water temperature. Furthermore, using a simple ON-OFF control strategy, the behaviour of the new absorption chiller was more agile and responded more quickly. The part-load curve of the electrical coefficient of performance was obtained by adjusting the experimental data to the shape of the curve proposed in the standard prEN-14825:2011 for air-to-water chillers. The Cc coefficient was 0.7985 matching the value obtained dividing the remaining electrical consumption measured during the OFF half cycles by the total energy consumption generated.

This study describes and analyses the performance of a commercial 3-ton ammonia/water absorption chiller under steady-state regime. A steady-state simulation model is developed and validated using the flow-sheeting software Aspen-Plus. First an appropriate thermodynamic property model from the software library for the ammonia/water system is tested. The interaction parameters of this model are determined by fitting the equation of state (eos) to VLE data. It is found that the Boston-Mathias modified Peng-Robinson eos with fitted parameters predicts accurately the VLE for the temperature and pressure ranges encountered in commercial chillers. A simulation model of the machine is then developed. The simulation results are found to be in good agreement with data from literature for 35°C cooling air temperature.

The potential of the fluid mixture water-(LiBr + Lil+ LiNO3 +LiCl) (5:1:1:2 molar) is studied for air-cooled absorption air-conditioning systems. This multicomponent system shows a considerably higher solubility than that of water-LiBr and is also less corrosive. It is, therefore, one of the potential alternatives to replace water-LiBr in future air-cooled absorption chillers. A comparative study based on thermodynamic simulation of the single-and double-effect cycles with water-LiBr and the new fluid mixture is first reported. Once the operating conditions were established in terms of temperature and concentration in the generator and the absorber, the absorption process of a falling film flowing on the inner surface of a vertical tube at typical air cooling thermal operating conditions of the absorber was modelled in order to compare the absorption rates of the multicomponent salt mixture with those of water-LiBr. This study was completed by an experimental characterization of the absorption process in a vertical falling film tube. The results show that the multicomponent salt solution is more suitable than water-LiBr if the temperatures in the generator and absorber/condenser are low and high, respectively. Therefore this new working fluid can be recommended for air-cooled absorption air-conditioning systems driven by low temperature heat sources.