• Energy Research

Abstract The present study aims to quantify experimentally the individual and simultaneous effects of CNTs (carbon nanotubes) and advanced surfaces on the performance of an NH 3 /LiNO 3 tubular bubble absorber. Operating conditions are those of interest for use in air-cooled absorption chillers driven by low temperature heat sources. Firstly, experimental tests were performed with the tubular absorber fitted with an inner smooth surface to analyze the effect of adding carbon nanotubes (0.01 wt%) to the base mixture NH 3 /LiNO 3 . Then, the tubular absorber was tested using an inner advanced surface tube both with and without adding carbon nanotubes to the base mixture NH 3 /LiNO 3 . The advanced surface tube is made of aluminum and has internal helical micro-fins measuring 0.3 mm in length. Results show that the maximum absorption mass flux achieved with the CNT binary nanofluid and the smooth tube is up to 1.64 and 1.48 times higher than reference values at cooling-water temperatures of 40 and 35 °C, respectively. It is also found that simultaneous use of CNT nanoparticles and advanced surfaces resulted in a more pronounced increase in the absorption mass flux and solution heat transfer coefficient with respect to the smooth tube absorber with NH 3 /LiNO 3 as a working pair.

In this study, ammonia vapor absorption with NH3/LiNO3 was assessed using correlations derived from a semi-empirical model, and artificial neural networks (ANNs). The absorption process was studied in an H-type corrugated plate absorber working in bubble mode under the conditions of an absorption chiller machine driven by low-temperature heat sources. The semi-empirical model is based on discretized heat and mass balances, and heat and mass transfer correlations, proposed and developed from experimental data. The ANN model consists of five trained artificial neurons, six inputs (inlet flows and temperatures, solution pressure, and concentration), and three outputs (absorption mass flux, and solution heat and mass transfer coefficients). The semi-empirical model allows estimation of temperatures and concentration along the absorber, in addition to overall heat and mass transfer. Furthermore, the ANN design estimates overall heat and mass transfer without the need for internal details of the absorption phenomenon and thermophysical properties. Results show that the semi-empirical model predicts the absorption mass flux and heat flow with maximum errors of 15.8% and 12.5%, respectively. Maximum errors of the ANN model are 10.8% and 11.3% for the mass flux and thermal load, respectively.

10.1016/j.ijrefrig.2014.06.005 Two pre-industrial prototypes of a new ammonia/lithium nitrate absorption chiller, a water-cooled one and an air-cooled one, have been built and experimentally characterized. The single-effect configuration of the absorption refrigeration cycle was selected for both prototypes in which brazed plate heat exchangers were used in all thermal components. These prototypes, designed for air conditioning applications, were tested under various operating conditions to assess their performance. The water-cooled prototype yields 12.9 kW of cooling capacity and an electrical COPelec of 19.3, when operating at a 15 °C chilled water temperature, 90°C hot water temperature and a 35 °C cooling water temperature. In the case of the air-cooled prototype, at a 15 °C chilled water temperature, 90 °C hot water temperature and a 35 °C ambient air temperature, the cooling capacity is 9.3 kW and the electrical COPelec is 6.5.

Abstract In air-cooled water–LiBr absorption chillers the working conditions in the absorber and condenser are shifted to higher temperatures and concentrations, thereby increasing the risk of crystallisation. To develop this technology, two main problems are to be addressed: the availability of new salt mixtures with wider range of solubility than water–LiBr, and advanced absorber configurations that enable to carry out simultaneously an appropriate absorption process and an effective air-cooling. One way of improving the solubility of LiBr aqueous solutions is to add other salts to create multicomponent salt solutions. The aqueous solution of the quaternary salt system (LiBr + LiI + LiNO3 + LiCl) presents favourable properties required for air-cooled absorption systems: less corrosive and crystallisation temperature about 35 K lower than that of water–LiBr. This paper presents an experimental study on the absorption of water vapour over a wavy laminar falling film of an aqueous solution of (LiBr + LiI + LiNO3 + LiCl) on the inner wall of a water-cooled smooth vertical tube. Cooling water temperatures in the range 30–45 °C were selected to simulate air-cooling thermal conditions. The results are compared with those obtained in the same experimental set-up with water–LiBr solutions. The control variables for the experimental study were: absorber pressure, solution Reynolds number, solution concentration and cooling water temperature. The parameters considered to assess the absorber performance were: absorber thermal load, mass absorption flux, degree of subcooling of the solution leaving the absorber, and the falling film heat transfer coefficient. The higher solubility of the multicomponent salt solution makes possible the operation of the absorber at higher salt concentration than with the conventional working fluid water–LiBr. The absorption fluxes achieved with water–(LiBr + LiI + LiNO3 + LiCl) at a concentration of 64.2 wt% are around 60 % higher than those of water–LiBr at a concentration of 57.9 wt%.

The vapor pressure of ammonia + lithium nitrate + water and ammonia + lithium nitrate mixtures was measured by a static method from (293.15 to 353.15) K in ammonia mass fractions ranging from 0.2 to 0.6. The experimental vapor pressure data were correlated with the temperature and the liquid-phase composition using an analytical polynomial equation. The capability of the electrolyte nonrandom two liquid (E-NRTL) model to predict the vapor−liquid equilibrium (VLE) of the ternary mixture was evaluated by comparing predicted and experimental data of the ammonia + lithium nitrate + water solutions. The binary interaction parameters of ammonia + lithium nitrate needed for the prediction of ternary VLE were determined from binary experimental data.

Abstract Ammonia/water absorption–resorption refrigeration systems are an interesting technological alternative to conventional absorption systems thanks to their additional degree of freedom, which makes operation more flexible and can significantly reduce the typically high working pressure, so cheaper assembly materials can be used. This paper aims to review the theoretical and experimental works carried out so far on these systems, paying special attention to the experimental plants currently operating and recent progress in improving system control. This paper reports how to determine the operational pressure range when the temperatures of the main components of the ammonia/water single-stage cycle are fixed and how the pressure range changes when these working temperatures are varied. As a result of the theoretical study of the base case selected, the cycle performance was evaluated without rectification after the generator in order to reduce the size and cost of the system without reducing the performance. Finally, it should be pointed out that the effectiveness of the resorption solution heat exchanger plays an important role in the performance and operating conditions of the cycle because the cycle cannot work below certain values of effectiveness.

It is well known that the absorber is the key component in energy conversion systems that are based on absorption cycles. This paper describes an experimental investigation into the absorption process of organic fluid mixtures in an absorption system which has a spray and a plate heat exchanger. The absorber consists of an adiabatic mixing chamber with a spray, where the solution that is weak in refrigerant is sprayed into the refrigerant vapour. A two-phase mixture is formed and enters a plate heat exchanger, where the solution is cooled to complete the absorption process. We carried out experiments with different types of spray nozzles using the organic fluid mixtures methanol–tetraethyleneglycol dimethylether (TEGDME) and trifluoroethanol (TFE)–TEGDME. We analyse how the solution mass flow rate, absorber pressure and cooling water temperature affected the absorber performance and we discuss the results in terms of the absorber load, absorbed mass flux, degree of subcooling of the solution at the absorber outlet, solution film heat and mass transfer coefficients. The results indicate that the absorption system proposed is suitable for relatively low pressures. For water temperatures of 30 °C and absorber pressures between 2 and 6 kPa, the absorption rates for TFE–TEGDME were 1 to 2.5 g·s−1·m−2. The corresponding values for methanol–TEGDME with absorber pressures between 10 and 15 kPa were 0.4 to 1.2 g·s−1·m−2.

Abstract In this work, the use of waste heat energy of jacket water in diesel engines of fishing ships was analysed for use as a heat source for absorption refrigeration systems. The thermodynamic simulation of an absorption refrigeration cycle with three different working fluid mixtures that use ammonia as a refrigerant was carried out. This analysis was assessed in terms of the cooling demand and cycle performance as a function of the evaporator, condenser and generator temperatures. Moreover, the need for rectifying the vapour stream leaving the generator was analysed together with the drag of the fraction of non-evaporated liquid to the absorber. The results show that the NH3/(LiNO3 + H2O) and NH3/LiNO3 fluid mixtures have higher values of COP as compared to NH3/H2O fluid mixture, the differences being more pronounced at low generation temperatures. If the activation temperature is set to 85 °C, the minimum evaporation temperatures that can be achieved are −18.8 °C for the cycle with NH3/LiNO3, −17.5 °C for the cycle with NH3/(LiNO3 + H2O) cycle and −13.7 °C for the NH3/H2O cycle at a condensing temperature of 25 °C. Also, for the NH3/(LiNO3 + H2O) fluid mixture, it has been demonstrated that the absorption refrigeration cycle can be operated without a distillation column and in this case the water content in the refrigerant stream entering the evaporator is less than 1.5% in weight at the operating conditions selected.

The movement toward the 4th generation district heating (4GDH) embraces a great opportunity to support the future smart energy development concept. However, its development calls for addressing technological and economic obstacles aligning with the need for a reformation of the energy market to ensure the quality of service. In this context, our paper presents a comprehensive analysis based on a multi-objective optimization framework incorporating an artificial neural network-based model for the possibility of integrating heat pump (HP) into solar assisted district heating system (SDHS) with seasonal thermal energy storage to support the sustainable transition toward 4GDH. The study evaluates the performance of the proposed system with the help of key performance indicators (KPI) related to the 4GDH characteristics and key stakeholders for possible market growth with consideration for the environmental benefits. The proposed analysis is applied to a small neighbourhood of 10 residential buildings located in Madrid (Spain) to investigate the optimal integration of HP under different control strategies into a SDHS. Inherent the SDHS operator perspective, the results reveal a significant improvement in the stabilization of the SDHS performance due to the HP integration where the solar field temperature never exceeds 80 ◦C, and the seasonal storage tank (SST) temperature stands at 85.4 ◦C. In addition, the share of solar energy stands above 86.1% with an efficiency of 73.9% for the SST, while the seasonal HP performance factor stands above 5.5 for all optimal scenarios. From the investor viewpoint, an energy price of 59.1 Euro/MWh can be achieved for the proposed system with a payback period of 26 years. Finally, from the policymaker perspective, along with the significant economic and sustainable improvement in the SDHS performance, a substantial environmental improvement of 82.5% is achieved when compared to the conventional boiler heating system. The proposed analysis reflects a great motivation for different stakeholders to propose this system as a path toward the 4GDH in the future district energy systems. The work is funded by the Spanish government RTI2018-093849-B-C31 and RTI2018-093849-B-C33. The authors would like to thank the Catalan Government for the quality accreditation given to their research group (GREiA – 2017 SGR 1537, AGACAPE – 2017 SGR 1409). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme. This work is partially funded by the Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación (AEI) (RED2018-102431-T). This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 713679 and from the Universitat Rovira i Virgili (URV).