LiBr Water Solution Research Articles

Determination of operating temperature in main generator of a two-stage absorption refrigeration machine working on an aqueous solution of lithium bromide

This paper examines the thermal characteristics of a two-stage absorption refrigeration machine. When carrying out a numerical analysis, it turned out that the secondary generator’s temperature was not constant, and it was also found that the coefficient of thermal performance of this cycle increased significantly at the beginning, then stabilized at a certain value for the temperature degree of the primary generator. But this result is not the right value for calculating the absorption refrigeration cycle; it might cause crystallization inside the cycle and This was verified based on the thermodynamic relationship chart (h-X-T) for LiBr-water solution. In addition, the decisive point for determining the appropriate temperature is to study the change in refrigeration capacity in the process when the temperature of the generator changes. In this case, the design temperature for the primary generator should be in the range between the temperature corresponding to the highest refrigeration capacity and the temperature far from the crystallization phenomenon.

Extended exergy analysis of a novel integrated absorptional cooling system design without utilization of generator for economical and robust provision of higher cooling demands

The focus of this study is on designing a novel system for the provision of high-capacity cooling and heating loads (4000 kW) with the utilization of absorption technology to increase economic viability and COP value of existing cooling plants via lower-grade waste heat sources (70 °C-90 °C). To achieve this aim, in the novel system, an integration including the LiBr-water solution based absorptional heat transformer (AHT), and absorptional cooling cycle (ACC), and flat plate solar collector (FPSC) systems was proposed. In the integration, the utilization of the generator in the cooling cycle was avoided with the interaction of the high-temperature LiBr-water solution (120 °C-150 °C) from the AHT system and ACC system evaporator. In this way, both the additional cost of the boiler and heat source and the enhancement of economic viability and COP value were achieved. Energy, economic, traditional, and extended exergy, sustainability, and environmental analyses were implemented in this novel system. The COP value for the cooling system was determined to be 3.10 from energy analysis. This result forms a significant indicator for achieving of the main focus of the current study with the proposed novel system. The annual heating and cooling duty generations with this novel system were computed as 52.37 GWh and 52.40 GWh, respectively. In the context of economically comparing the proposed system to other plants with similar scale that already exist, the initial overall expenditure, yearly operational expenses, and the time it takes to recover the investment for the proposed system were set at $4.56 million, $3.12 million, and 1.75 years, respectively. It is worth noting, though, that these figures fall within the range of $6–8 million, $5–7 million, and 5–10 years, respectively, for the currently operational plants. This result indicated that the proposed system provides a robust alternative to the existing cooling-heating cogeneration systems in terms of main output generation and is more economically viable. Also, the novel system gained annually US$3.89 million in energy costs. The conventional exergy analysis results were summarized by forming an exergy flow and loss diagram, namely, the Grassmann diagram. In addition, in this current study, the novel extended exergy flow diagram indicating extended exergy content components, energy carriers of the proposed system, and exergy product rate streams was also proposed and drawn for the proposed system.

The effect of small perturbation on dynamics of absorptive LiBr–water solution

In a binary solution of lithium bromide–water, even a small disturbance in the initial homogeneous mass fraction at the absorbing interface has profound effects on the entire system dynamics. This perturbation of absorption disrupts the equilibrium, leading to the formation of surface tension gradients and subsequently, Marangoni flows. While these flows are relatively weak, they result in a non-uniform distribution of density within the bulk, initiating buoyant convection. We investigate complexities of the Marangoni, solutal, and buoyant convection caused by localized disruptions in uniform absorption, all in the absence of any surfactants. We have conducted numerical simulations to explore fluid dynamics and heat and mass transfer, revealing three different regimes. Initially, shortly after disturbance, variations in mass fraction and flow within the cell are primarily governed by the Marangoni force. After a finite period, the emergence of buoyant convection leads to the strong growth of velocity and significant changes in temperature and mass fraction. Finally, the destabilization of the boundary layer becomes so significant that the emission of plumes is observed. At later times, the parallel existence of two types of patterns takes on a spatially fixed form. The central part, occupied by bands (visible on space-time maps), exhibits minimal changes in time, while a periodic structure is established near the wall. This behavior can be characterized as a relaxation–oscillation mode of instability.

Thermally Regenerable Redox Flow Battery.

The efficient production of energy from low-temperature heat sources (below 100 °C) would open the doors to the exploitation of a huge amount of heat sources such as solar, geothermal, and industrial waste heat. Thermal regenerable redox-flow batteries (TRBs) are flow batteries that store energy in concentration cells that can be recharged by distillation at temperature <100 °C, exploiting low-temperature heat sources. Using a single membrane cell setup and a suitable redox couple (LiBr/Br2 ), a TRB has been developed that is able to store a maximum volumetric energy of 25.5 Wh dm-3 , which can be delivered at a power density of 8 W m-2 . After discharging 30 % of the volumetric energy, a total heat-to-electrical energy conversion efficiency of 4 % is calculated, the highest value reported so far in harvesting of low-temperature heat.

Comparative Energetic and Exergetic Assessment of Different Cooling Systems in Vegetable Cold Storage Applications

In this study, four different cooling systems have been thermodynamically analysed for vegetable cold storage application, considering a fixed cooling demand of 10 kW. Both vapour absorption refrigeration (VAR) and compression refrigeration (VCR) cycles are considered, each with two different working fluids. For the VCR cycles, R410A and R134a are considered as refrigerants, whereas for the VAR cycles, NH3–water and LiBr–water solutions are considered as refrigerant–absorbent pairs. Both energetic and exergetic performances are assessed and compared between the cycles. The system performances are assessed by varying the conditions of the evaporator and condenser. The effects of ambient temperature on the system performances are also studied. The study reveals that VAR systems can provide feasible alternatives to VCR systems for the cold space temperature range applicable for vegetable storage with the exception that LiBr–H2O cycle cannot be used for cold space temperature below 2 °C. Exergetic performance (ECOP) of the LiBr–H2O-based cooling system matches well with that of VCR systems for evaporator temperature 2–5 °C, and it exceeds the ECOP of R410A cycle at 2 °C evaporator temperature. When the ambient temperature is reached above 35 °C, the exergetic performance of LiBr–H2O-based system is better than that of both the VCR systems. For higher condenser temperature also (40 °C and above), the VAR and VCR cycles show closer ECOP values.

Modeling of conjugated heat and mass transfer during film desorption

The heat and mass transfer in the process of desorption on a free falling liquid film of the LiBr-water solution is analyzed analytically. The dependences of the Nusselt number on the surface on the longitudinal coordinate downstream of the film were found, and the thickness of the film, which changed due to desorption, was determined.The problem was investigated for different values of Froude and Peclet numbers, for different values of initial film thickness.

Absorption refrigeration system using engine exhaust gas as an energy source

A single-effect absorption refrigeration system that uses LiBr-water solution and engine exhaust gas is investigated. The generator is a spiral fin-and-tube heat exchanger, while the condenser, evaporator, and absorber are shell and coil heat exchangers. Experiments are conducted at engine speeds of 1000, 1200, 1400, and 1600 rpm; expansion valve opening percentages of 54.5%, 72.7%, and 90.9% at the separator outlet and 3.41%, 4.55%, and 5.68% at the condenser outlet; refrigerant temperatures at the condenser outlet of 25, 30, and 35 °C; and LiBr-water solution flow rates of 0.35 and 0.7 LPM. The results show that the system could work with an engine speed of 1200–1400 rpm. The cooling load and coefficient of performance (COP) increase with increasing engine speed. The highest COP of 0.275 is reached at an engine speed of 1400 rpm, opening percentage of 72.7% at the separator outlet and 4.55% at the condenser outlet, water temperature of 25 °C at the condenser outlet, and LiBr-water flow rate of 0.7 LPM. The decreased refrigerant temperature at the condenser outlet helps to increase both cooling load and COP. The increase of the LiBr-water solution flow rate helps to increase cooling load but decrease COP.

Non-isothermal desorption and nucleate boiling in a water-salt droplet LiBr

Experimental data on desorption and nucleate boiling in a droplet of LiBr water solution were obtained. An increase in salt concentration in a liquid layer leads to a considerable decrease in the rate of desorption. The significant decrease in desorption intensity with a rise of initial mass concentration of salt has been observed. Evaporation rate of distillate droplet is constant for a long time period. At nucleate boiling of a water-salt solution of droplet several characteristic regimes occur: heating; nucleate boiling; desorption without bubble formation; formation of the solid, thin crystalline-hydrate film on the upper droplet surface, and formation of the ordered crystalline-hydrate structures during the longer time periods. For the final stage of desorption there is a big difference in desorption rate for initial salt concentration (C0) 11 % and 51 %. This great difference in the rate of desorption is associated with significantly more thin solution film for C0 = 11 % and higher heat flux.

Bubble boiling in droplets of water and lithium bromide water solution

The dynamics of growth and interaction of vapor bubbles in droplets of pure water and LiBr water solution on a horizontal wall were investigated in a wide superheating range. The growth rates of bubbles were determined both in a distillate droplet and in a salt solution droplet. The bubble growth rate in a pure water droplet at the final stage is somewhat lower than in pool boiling. The bubble growth rate in a salt solution is substantially lower than for pure water. Due to the bubble density maldistribution, the vapor flow density is appreciably higher at the droplet edges than on the droplet axis. Collective behavior of the bubbles possesses both stochastic character and elements of self-organization. The thermal measurements were carried out by means of high-speed video and blowup thermal imager.

Modeling, Design and Construction of a Micro-scale Absorption Chiller

In last decades, much effort has been made to drive cooling cycles exploiting renewable energy sources. The use of solar energy is one of the most attractive solutions especially for air conditioning, as availability of solar radiation and cooling loads are approximately in phase.Solar cooling based on water-lithium bromide absorption chillers is the most promising technology for low-medium temperature heat sources (80-100°C). Small (15-40kW cooling capacity), medium (50-250kW) and large scale (up to tens of MW) units are currently at commercial stage. In the present work, the development of a novel micro-scale LiBr absorption chiller (around 5kW) is presented. The objective is to demonstrate the technical feasibility and to investigate the performance under different operating conditions.A computer code has been developed to simulate a LiBr absorption chiller. The model computes mass flow rates, temperatures, pressures and mass concentration of LiBr-water solution in all the chiller components, both in design and off-design conditions. Giving as inputs inlet temperature and mass flow rate of the external circuits (hot water source, cooling water and chilled water), the computer code is able to evaluate the efficiency (COP) and the actual cooling capacity.The simulation code has been used to size the heat exchangers and to design a prototype of a micro-scale chiller. The chiller prototype with a 5kW nominal cooling capacity has been manufactured and fully instrumented in order to monitor all physical quantities in the internal and external circuits. The prototype is installed on a test rig at the Energy System and Turbomachinery Laboratory of Bergamo University. Measurement devices, data acquisition system and in-house monitoring software are described in the paper. Preliminary results of the experimental investigation are presented.

High temperature nonisothermal desorption in a water–salt droplet

Experimental data on dynamics of nucleate boiling in the droplets of LiBr water solution were obtained. Time dependences for a change in bubble geometry were determined and characteristic desorption regimes were distinguished. The rate of bubble growth decreases with a rise in salt concentration and boiling stops completely at high concentration of LiBr. An increase in salt concentration in a liquid layer of the bubble bottom leads to a considerable decrease in the rate of desorption, and the rate of bubble diameter increase decreases significantly, and a dry spot is not formed. The growth features of the crystalline-hydrate film depend on the rate of crystallization. Thermal measurements were carried out by means of thermal imager lenses with ten-fold magnification. The video and thermal recording allowed us to distinguish the self-organized crystalline-hydrate structures at high crystallization rates: rod-like, dendrite, and dome-shaped ones. Complex morphology of these structures promotes an increase in the rate of water desorption from the surface of crystalline-hydrate film.

Nonisothermal Desorption of the Libr Aqueous Salt Solution in Minichannels

This work is devoted the research for two (three)-phase flows of LiBr water solution in minichannels with different heat flux and wall thicknesses. Six flow patterns have been observed: a bubble flow, plug flow, laminar and oscillating laminar flow, mist flow, and flow locking. The physical solution properties and the equilibrium conditions change in time. The desorption rate depends not only on the heat flow and speed ratio of vapor to liquid, but also on the total area of the interface (liquid-vapor). The third phase (solid crystal hydrates) are formed under high heat fluxes and in the presence of boiling crisis. A variation in the wall thickness leads to a change in the desorption mode. With increasing wall thickness the boiling crisis is realized at higher heat fluxes.

Two-phase nonisothermal flows of LiBr water solution in minichannels

Two-phase flows of LiBr water solution in minichannels with heat exchange is experimentally investigated. Six flow patterns have been revealed: a bubble flow, plug flow, separate and wavy separate flow, dispersed flow, and flow blocking. Contrary to one-component liquids, an additional third phase may appear in desorption of salt solutions, that is, a crystal-hydrate shell on channel walls. The maximum desorption efficiency corresponded to minichannels of 3 mm in diameter and in the presence of a dispersed flow. The desorption rate depends not only on the ratio of liquid, gas, and heat flux velocities, but also on the overall interface area. The maximal interfacial evaporation surface corresponds to the dispersed flow due to the presence of a high concentration of fine-dispersed droplets. Minichannels with a high desorption rate can be effectively applied for high-temperature desorbers of thermal pumps.

Direct molecular diffusion and micro-mixing for rapid dewatering of LiBr solution

A slow molecular diffusion rate often limits the desorption process of an absorbate molecule from a liquid absorbent. To enhance the desorption rate, the absorbent is often boiled to increase the liquid–vapor interfacial area. However, the growth of bubbles generated during the nucleate boiling process still remains mass-diffusion limited. Here, it is shown that a desorption rate higher than that of boiling can be achieved, if the vapor–absorbent interface is continuously replenished with the absorbate-rich solution to limit the concentration boundary layer growth. The study is conducted in a LiBr–water solution, in which the water molecules' diffusion rate is quite slow. The manipulation of the vapor–solution interface concentration distribution is enabled by the mechanical confinement of the solution flow within microchannels, using a hydrophobic vapor-venting membrane and the implementation of microstructures on the flow channel's bottom wall. The microstructures stretch and fold the laminar streamlines within the solution film and produce vortices. The vortices continuously replace the concentrated solution at the vapor–solution interface with the water-rich solution brought from the bottom and middle of the flow channel. The physics of the process is described using a combination of experimental and numerical studies.

Investigation of a hybrid solar and geothermal driven absorption cooling system for district applications

In this paper, a new hybrid geothermal and solar driven absorption cooling system (ACS) is proposed for low–temperature district energy applications. Two approaches are proposed in this study, namely: a) a new model for determining the specific entropy values of LiBr–water solution and (b) two new energetic and exergetic parameters, namely geothermal contribution index and solar contribution index. A feasibility study of incorporating a solar driven ACS into Bigadic Geothermal District Heating System is considered and investigated in this regard. Some parametric studies are conducted to highlight the importance of the present study and make a practical illustration.

Identification of LiBr-Water Solution Thermodynamic Properties using the ANN Technique

In this study, Artificial Neural Networks (ANNs) technique is applied for the determination of thermodynamic properties of the Lithium Bromide-Water solution, which is widely used in the thermodynamic simulations. For training of the ANNs the simulation results of a thermodynamic analysis are used. The presented ANN model provides simpler and faster results comparing to complex differential equations and exsiting limited experimental data. Using the ANN technique, the thermodynamic properties of LiBr-Water solution are derived as the mathematical relations. Simulation results show the effectiveness of ANN in identification of LiBr-Water solution thermodynamic properties.

Air-cooled LiBr–water absorption chillers for solar air conditioning in extremely hot weathers

A low temperature-driven absorption cycle is theoretically investigated for the development of an air-cooled LiBr–water absorption chiller to be combined with low-cost flat solar collectors for solar air conditioning in hot and dry regions. The cycle works with dilute LiBr–water solutions so that risk of LiBr crystallization is less than for commercially available water-cooled LiBr–water absorption chillers even in extremely hot ambient conditions. Two-phase heat exchangers in the system were modelled taking account of the heat and mass transfer resistances in falling film flows by applying the film theory in thermal and concentration boundary layers. Both directly and indirectly air-cooled chillers were modelled by properly combining component models and boundary conditions in a matrix system and solved with an algebraic equation solver. Simulation results predict that the chillers would deliver chilled water around 7.0 °C with a COP of 0.37 from 90 °C hot water under 35 °C ambient condition. At 50 °C ambient temperature, the chillers retained about 36% of their cooling power at 35 °C ambient. Compared with the directly air-cooled chiller, the indirectly air-cooled chiller presented a cooling power performance reduction of about 30%.

Flow patterns and heat and mass transfer coefficients of low Reynolds number falling film flows on vertical plates: Effects of a wire screen and an additive

The effects of the surface geometry and of a surfactant on the characteristics of LiBr–water solution falling films are experimentally investigated. Two different surfaces (flat copper plate and the same copper plate covered with a copper wire screen) are tested with four fluids (pure water and 50% lithium bromide–water solution with or without 100 ppm of 2-ethyl-1-hexanol) for Reynolds numbers from 40 to 110. Flow patterns and heat and mass transfer coefficients are discussed. Marangoni convection was observed in water-cooled LiBr–water flows resulting in significant heat and mass transfer enhancement. 2-ethyl-1-hexanol enhances heat and mass transfer in LiBr–water flows by more than a factor of two. 2-ethyl-1-hexanol in water makes copper surface strongly hydrophobic. The wire screen promotes absorption process in adiabatic conditions but hinders the process in non-adiabatic conditions by reducing the Marangoni convection induced by 2-ethyl-1-hexanol. The experimental heat and mass transfer coefficients are presented in dimensionless forms.

Water sorption on composites “LiBr in a porous carbon”

Water sorption equilibrium of LiBr confined to pores of a mesoporous synthetic carbon Sibunit and a macroporous expanded graphite (samples SWS-2C and SWS-2EG, respectively) was studied. Isobars of water sorption on these composites are measured at vapor pressure 6–81 mbar and temperature 30–145 °C. The type of sorption equilibrium for the two composites appears to be quite different. The isobars for SWS-2EG have a plateau corresponding to one molecule of H 2O adsorbed by one molecule of LiBr, which indicates the formation of crystalline hydrate LiBr·H 2O inside pores with a monovariant type of equilibrium. At lower temperatures, the equilibrium becomes divariant that is typical for LiBr–water solutions. On the contrary, the water sorption equilibrium for SWS-2C is divariant over the whole temperature and pressure range which means that no crystalline hydrates are formed inside Sibunit pores. In our opinion, this distinction results from differences in a pore structure of the host carbons. The composite sorption capacity can reach 0.6–1.1 g H 2O per 1 g of the dry sorbent at relative humidity 70%. The advanced sorption capacity makes the sorbents promising for gas drying, thermal storage of energy and other applications.

Thermodynamic properties of lithium bromide–water solutions at high temperatures

Emerging triple-effect LiBr–water absorption chillers operate at higher temperatures and pressures than traditional double-effect chillers. However, there is not enough data about thermodynamic properties of LiBr–water solutions at such high temperatures. Using recently measured data of vapor pressure and heat capacity, we have developed the equations which can calculate the vapor pressure, enthalpy and entropy of LiBr solutions at such high temperatures. The developed equations are valid from concentrations of 40–65 wt.% and also from temperatures of 40–210°C. These equations will be very helpful for the modeling and design of triple-effect LiBr–water chillers.