Double-effect Absorption Chiller Research Articles

A study of the coupled heat and mass transfer during absorption process in a spiral tubular absorber

In this study, a fine local modeling of the coupled heat and mass transfer during absorption process of water vapor by a LiBr solution in a spiral tubular absorber of an absorption chiller was made. We have taken into account of the real hydrodynamics of the liquid solution that reveals the presence of three different flow regimes: a falling film region existing on the tube in which circulates the cooling water, a droplet formation region at the tube's bottoms and a droplet free fall region between the spaced tubes. The equations governing the coupled heat and mass transfer are established for the three flow regimes. The numerical simulation has given the distributions of different absorption parameters along the absorber tubes. This numerical simulation has shown up the relationship between the heat and the mass transfers and also highlighted the importance of the droplet formation and fall regimes in improving the absorption process, when the droplets are considered fully mixed. The validity of the model was improved using experimental data performed on a spiral tubular absorber of a double effect absorption chiller.

Design and modeling of 1–10 MWe liquefied natural gas-fueled combined cooling, heating and power plants for building applications

Decentralized, liquefied natural gas-fueled, trigeneration plants are considered as alternatives to centralized, electricity-only generating power plants to improve efficiency and minimize running costs. The proposed system is analyzed in terms of efficiency and cost. Electrical power is generated with a gas turbine, while waste heat is recovered and utilized effectively to cover heating and cooling needs for buildings located in the vicinity of the plant. The high quality of cooling energy carried in the LNG fluid is used to cool the air supply to the air compressor. Waste heat is recovered with heat exchangers to generate useful heating in the winter period, while in the summer period an integrated double-effect absorption chiller converts waste heat to useful cooling. For the base system (10MWe), net electrical efficiency is up to 36.5%, while the primary energy ratio reaches 90%. The payback period for the base system is 4 years, for a lifecycle cost of 221.6 million euros and an investment cost of 13 million euros. The base system can satisfy the needs of more than 21,000 average households, while an equivalent conventional system can only satisfy the needs of 12,000 average households.

Optimization of a lithium bromide–water solar absorption cooling system with evacuated tube collectors using the genetic algorithm

Nowadays absorption chillers are of more interest due to considerable saving in energy consumption and using thermal energy sources. In this paper, a sensitivity analysis is accomplished on a double-effect absorption chiller with 100t of cooling capacity to study the effect of different parameters on auxiliary energy. The input parameters taken into account in the sensitivity analysis are the volume of storage tank, area of evacuated tube collector, and mass flow rates of water passing through the collector and generator. It is also supposed that the evacuated tube collector is mounted at the monthly optimum angle to get as much solar radiation as possible. Two objective functions are considered for the genetic algorithm namely the auxiliary energy and the net profit obtained from the solar system. An economical analysis is required to calculate the optimum area of collector. The computer code is developed to minimize the auxiliary energy, and maximize the profit. Since the auxiliary energy costs money, it is advantageous to use the same system with minimum auxiliary energy. However, the results show that the optimum mass flow rates of hot water passing through the generator and collector have an important role on reducing the auxiliary energy.

Experimental performance investigation of small solar air-conditioning systems with different kinds of collectors and chillers

Different types of solar collectors should be integrated with different sorption chillers. Traditional evacuated tube U pipe solar collector, high efficient CPC (Compound Parabolic Concentrating) solar collector and PTC (Parabolic Trough Collector) solar collector can provide 60–85°C, 85–125°C and 125–150°C of hot water. So, they can drive silica gel–water adsorption chillers, single-effect LiBr absorption chillers and double-effect LiBr absorption chillers, respectively. This paper will present the experimental performance investigation and economic analysis of three small solar cooling systems with these different kinds of collectors and chillers. The test results show that the solar COP (Coefficient of Performance) of the solar cooling system with adsorption chiller and U pipe solar collectors is about 0.15 and the silica gel–water adsorption chiller can be driven by 55°C of hot water. The solar COP of the system with CPC solar collector and single effect absorption chiller can reach 0.24 in the sunny weather condition. The solar COP of the solar cooling system with PTC solar collectors is about 0.5 and the double-effect LiBr absorption chiller is more attractive because of its higher solar COP (Coefficient of Performance) and better economy.

Experimental based energy performance analysis and life cycle assessment for solar absorption cooling system at University of Californian, Merced

Traditional air conditioning and heating systems in buildings are fossil fuel based energy systems, which take the main responsible for the carbon emissions. In contrast, using solar energy for air conditioning becomes one of the promising approaches to reduce energy consumptions and negative environmental impacts from buildings. University of California, Merced built a test facility to investigate the technology: solar cooling. The solar system has 54m2 external compound parabolic concentrator (XCPC) solar collectors to drive a 23kW double-effect absorption chiller. This paper first provides the detailed energy performance analysis of the experiments conducted in August, 2012. The data collected from the experiments shows that the system could provide adequate cooling for a test facility between 11AM to 5PM in both sunny and cloudy days. The daily average collector efficiency is at the range of 36% to 39%. The average coefficient of performance (COP) of the LiBr absorption chiller is between 0.91 and 1.02 with an average of 1.0, and the daily solar COP is approximately at 0.374.In addition to the experimental investigation, a detailed life cycle economic and environmental assessment was also performed by comparing the solar systems to the conventional systems in two types of office buildings at three locations at California. Two different solar cooling system configurations were considered: (i) configuration 1 sizes the area of solar collectors and absorption chiller to meet the peak cooling demand, and uses natural gas as the only backup energy source; (ii) configuration 2 sizes the area of solar collectors and absorption chiller to meet half of the peak cooling demand, and uses natural gas as the backup energy source for the absorption chiller, while incorporates an electrical vapor compression chiller to meet the rest half of peak cooling demand. The annual performance predicts that the systems can achieve the annual solar fraction around 55–68%. And the configuration 2 achieves better life cycle economic and environmental performance than the configuration 1. Specifically, the configuration 2 can achieve lower present worth cost during the entire life span than the conventional systems. And both configuration 1 and 2 can reduce the life time carbon footprint by 35–70%.

Solar cooling between thermal and photovoltaic: An energy and economic comparative study in the Mediterranean conditions

This paper considers different cooling systems and investigates the most promising alternatives when solar energy is to be used to supply the cooling demand. All the systems are evaluated during a summer cooling season by the energetic and economic point of view by dynamic simulation for two different climates. For Milan (Cfb climate) the highest OSE (overall system efficiency) is reached by LiBr (lithium-bromide) double effect absorption chiller driven by parabolic through collector (0.53). In terms of the collecting surface area, the best systems for Milan feature 0.08 m2 MJ−1 per day both for electric system (mono-crystalline photovoltaic coupled to water cooled chiller) and thermal system (PTC (parabolic trough collectors) coupled to double effect water-LiBr absorption chiller). Southern latitudes like Trapani (Csa climate) allow a quite better performance for thermal solar cooling solutions. The NPV (net present worths) of electric solar cooling solutions are favorable with respect to the traditional solution and the DPV (discounted payback periods) are all lower than the period of economic analysis above all for water cooled chillers. Finally a sensitivity analysis of the specific investment cost (€ MJ−1 per day) is carried out regarding the investment cost of collectors, the solar ratio and the interest rate.

Performance of the Merced Demonstration XCPC Collector and Double Effect Chiller

A solar thermal cooling system using novel nontracking external compound parabolic concentrators (XCPC) has been built at the University of California, Merced and operated for two cooling seasons. Its performance in providing power for space cooling has been analyzed. This solar cooling system is comprised of 53.3 m2 of XCPC trough collectors which are used to power a 23 kW double effect (LiBr) absorption chiller. This is the first system that combines both XCPC and absorption chilling technologies. Performance of the system was measured in both sunny and cloudy conditions, with both clean and dirty collectors. It took, on average, about 2 h for the collector system to reach operating temperatures between 160 and 180 °C. When operated in this temperature range, the XCPC collector array collected solar energy with an average daily efficiency of 36.7% and reached instantaneous efficiencies up to 40%. The thermal coefficient of performance (COP) of the system (including thermal losses and COP of absorption chiller) averaged at 0.99 and the daily solar COP of the entire system averaged at 0.363. It was found that these collectors are well suited at providing thermal power to drive absorption cooling systems and that both the coinciding of available thermal power with cooling demand and the simplicity of the XCPC collectors compared to other solar thermal collectors makes them a highly attractive candidate for cooling projects. Consequently, the XCPC technology is currently being commercialized in the U.S. and India. The XCPC's numerous potential applications include solar heating, cooling, desalination, oil extraction, electricity generation, and food processing.

A combined effect absorption chiller for enhanced performance of combined cooling heating and power systems

Most industrial waste heat (e.g. waste heat from engines) is available as two or more heat sources or in a wider temperature range. Additionally, solar thermal energy has a higher harnessing efficiency at low temperatures while its work potential increases with temperature. However, well-established absorption cooling technologies, such as single and double effect absorption chillers, operate in relatively narrow firing temperature ranges. The use of the maximum temperature range of the sources or of multiple sources together increases the energy harnessing efficiency as well as the productivity of the absorption technology.This paper introduces a combined effect absorption cycle that can utilize two energy sources at different temperature ranges or a single source at a wide temperature range. This new method reduces the capital investment and the operation costs of two conventional cycles that would otherwise have to be deployed in a series for an efficient energy conversion from heat to the cooling energy. The simulation results show that the proposed cycle improved the coefficient of performance (COP) of the system by 11% compared to the conventional separate production of the same capacity at the same temperature intervals.

Multi-Effect Plants and Ionic Liquids for Improved Absorption Chillers

State-of-the-art absorption chillers using conventional working pairs still suffer from problems like crystallization, corrosiveness, and a relatively low efficiency. To improve this technology, different working pairs as well as plant designs are investigated using the simulation tool AspenPlus. The simulation is validated by comparing the results of single-effect absorption chillers using the current commercially applied working pairs water/lithium bromide and ammonia/water with literature data. To increase the efficiency, double-effect absorption chillers are implemented and analyzed. The performance of two kinds of double-effect cycles, series and parallel, is compared using the working pair water/lithium bromide. In addition, ionic liquids (ILs) are investigated as a sorbent in order to improve the technology. So far, ILs have not been implemented in AspenPlus yet. Therefore, a guideline for the implementation of ILs in AspenPlus is outlined and the accordant phase equilibria results are validated with literature data. Simulations of single-effect cycles using the ILs 1,3-dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and 1-ethyl-3-methylimidazolium dimethylphosphate ([EMIM][DMP]) in combination with water as a refrigerant are performed and the results are compared to conventional working pairs. It is shown that by using ILs, similar or even higher coefficients of performance (COPs) can be achieved in comparison to conventional working pairs. Moreover, the findings reveal that the main benefit of using ILs as a sorbent consists in providing a broader operating range with respect to heat source temperature.

The exergy and energy level analysis of a combined cooling, heating and power system driven by a small scale gas turbine at off design condition

This paper presents the off design performance analysis of a combined cooling, heating, and power (CCHP) system consisting of a small-scale gas turbine, an exhaust-fired double-effect absorption chiller, and a heat exchanger. The energy and exergy analyses of the CCHP system are investigated under the rated and part-load conditions. Energy level analysis is implemented on the energy conversion processes to reveal the mechanisms of the deterioration of the CCHP performance under part-load conditions. The results show that the CCHP system is energy saving when the power output of the gas turbine exceeds 30% of the full load. It is also found that the CO2 emission of the CCHP system reduced by 66.7%–70.5%, compared with conventional separation system, when the power output of gas turbine increased from about 30% to 100%. Energy level results reveal that the combustor of the small-scale gas turbine mainly contributed to the deteriorated performance of the CCHP system. In addition, a case study is carried out to illustrate the advantage of using dynamic data in the performance assessment. The case results indicate that using off-design data leads to a more realistic evaluation of the CCHP system.

Thermoeconomic multi-objective optimization of a novel biomass-based integrated energy system

Both thermoeconomic modeling and multi-objective optimization studies are undertaken for a novel integrated multigeneration system, containing a biomass combustor, an organic Rankine cycle to produce electricity, a double-effect absorption chiller for cooling, a heat exchanger, a proton exchange membrane electrolyzer to produce hydrogen, a domestic water heater to produce hot water and a reverse osmosis desalination unit to produce fresh water. Energy and exergy analyses and an environmental impact assessment are included. A multi-objective optimization method based on a fast and elitist NSGA-II (non-dominated sorting genetic algorithm) is developed and employed to determine the best design parameters for the system. The two objective functions utilized in the optimization study are the total cost rate of the system, which is the cost associated with fuel, component purchasing and environmental impact, and the system exergy efficiency. The total cost rate of the system is minimized while the cycle exergy efficiency is maximized using an evolutionary algorithm. To provide insight, the Pareto frontier is shown for a multi-objective optimization. In addition, a closed form equation for the relationship between exergy efficiency and total cost rate is derived. A sensitivity analysis is performed to assess the effects of several design parameters on the system total exergy destruction rate, CO2 emission and exergy efficiency.

A micro tri-generation system based on direct flame fuel cells for residential applications

A novel micro tri-generation system which combines a direct flame fuel cell, a boiler and a double-effect absorption chiller is proposed and analyzed for residential applications. Parametric analyses are conducted to investigate the effects of operating parameters (i.e. the equivalence ratio and the fuel utilization factor of the fuel cell) on the system efficiency and the thermal-to-electric ratio. Then optimum operating parameters are determined based on the typical energy demand of Hong Kong in the summer and the typical energy demand of Beijing in the winter, respectively. It is found that very high efficiency (over 90%) can be achieved by this novel tri-generation system for both Hong Kong and Beijing. Besides, the system is modified for combined heat and power cogeneration, combined cooling and power cogeneration and their efficiencies are compared with the tri-generation system to evaluate the effect of each single unit on the whole system.

Performance of a 23KW Solar Thermal Cooling System Employing a Double Effect Absorption Chiller and Thermodynamically Efficient Non-tracking Concentrators

A solar thermal cooling system using novel non-tracking External Compound Parabolic Concentrators (XCPC) has been built and operated for two cooling seasons (summers of 2011 and 2012). Its performance in providing power for space cooling has been analyzed. This solar cooling system is comprised of 53.3 m2of XCPC trough collectors which are used to power a 23kW double effect (LiBr) absorption chiller. This is the first system that combines both XCPC and absorption chilling technologies. Performance of the system was measured in both sunny and cloudy conditions. The collector system maintained operating temperatures between 160-200°C. When operated in this temperature range, the XCPC collector array collected solar energy with an average daily efficiency of 36.7% and reached instantaneous efficiencies up to 40%. The thermal COP of the system (including thermal losses and COP of absorption chiller) averaged at 0.99 and the daily solar COP of the entire system averaged0.363. It was found that these collectors are well suited at providing thermal power to drive absorption cooling systems and that both the coinciding of available thermal power with cooling demand and the simplicity of the XCPC collectors compared to other solar thermal collectors makes them a highly attractive candidate for cooling projects. XCPC technology has numerous potential applications and is currently being commercialized in the U.S. and India

The Pumping Up Phenomenon of Double-Stage Bubble Pump with Water and Aqueous LiBr Solution

The double-stage bubble pump, using thermal energy as driving force to transport the solution, can replace the mechanical solution pump in the double-effect lithium bromide absorption chiller. By building a bench, a lot of experimental research and analysis were conducted with water and different concentrations of lithium bromide solution as the working fluid of the bubble pump. The first-stage bubble pump in the experiment pumps up by the external heat source. The heat for driving the second-stage bubble pump is provided by refrigerant steam produced from the first-stage bubble pump. The experiment data shows that the heating of refrigerant vapor is only one of the elements of pump-up phenomenon. Another is that the intermediate solution flashes to vapor to become bubbles. The pump-up phenomenon of double-stage bubble pump has much to do with the pressure difference of intermediate solution and first-stage refrigerant vapor. With water as the working fluid, when the pressure difference between refrigerant vapor and the intermediate liquefied refrigerant is 3.5-3.9 kPa, the bubble pump can pump up and run for some time and the start-up time decreases with the driving head. When the working fluid is lithium bromide solution, the pressure difference of the double-stage bubble pump increases with the solution concentration and is bigger than that of water. The start-up time increases with the concentrations of lithium bromide solution within the range of 45.5 to 54% and decreases within the range of 54-59.5%. The start-up time is largest at 54% under this experimental condition. The experimental result is also compared with the single-stage bubble pump. The start-up time of double-stage bubble pump decreases with the driving height, which is contrary to the single-stage bubble pump.

Design of Heat Storage for a Solar Concentrator Driving an Absorption Chiller

The feasibility of employing stand-alone solar energy systems to meet demand-side loads depends strongly on providing appropriate solar energy storage. The present paper presents an efficient and economical, underground, thermal storage design to store hot water at a temperature of around 180?C required for running a double effect absorption chiller to cool a zero-energy-house in a desert environment. The performance of the design is evaluated employing a specially developed efficient mathematical model, for simulating the steady state radiation, convection and conduction processes occurring within the storage unit. The model is presented and analyzed, and employed to investigate the effects of various design parameters on storage efficiency. It is demonstrated that high storage efficiency may be reached, providing that appropriate insulation materials are used. It is also revealed that the soil conductivity has little effect on storage efficiency.

Exergy Analysis of a Double-effect Parallel-flow Commercial Steam Absorption Chiller

Exergy Analysis of a Double-effect Parallel-flow Commercial Steam Absorption Chiller

Thermodynamic modelling of a double-effect LiBr-H2O absorption refrigeration cycle

The goal of this paper is to estimate the conductance of components required to achieve the approach temperatures, and gain insights into a double-effect absorption chiller using LiBr-H2O solution as the working fluid. An in-house computer program is developed to simulate the cycle. Conductance of all components is evaluated based on the approach temperatures assumed as input parameters. The effect of input data on the cycle performance and the exergetic efficiency are investigated.

Investigation of a solar cooling installation in Tunisia

In this paper, we present a solar cooling installation located at the Center of Researches and Energy Technologies (CRTEn), in Bordj-Cédria, Tunisia. It is composed mainly of parabolic trough solar collectors, a 16kW LiBr double effect absorption chiller associated with a cooling tower, a backup heater, two tanks for storage and drain-back storage and a set of fan-coils installed in the building to be conditioned. The system was run in the summers 2008 and 2009 using a water solution as heat transfer medium (HTF) without storage. In summer 2010, thermal oil was used as a HTF instead of water and two tanks were added, one for temperatures stabilities and the other one as a backup tank for night storage. Two running strategies of the installation were adopted; starting with or without backup heater. The site specifications, the building loads, the chillier temperatures and heat flows, the parabolic trough solar collector efficiencies, the installation powers and performances were analyzed. It was found that the coefficient of performance COP was ranged between 0.8 and 0.91 during the installation running while the solar coefficient of performance SCOP was between 0.1 and 0.43. The maximum output of the absorption chiller was around 12kW. The drain backup night storage has improved the average solar fraction SF of the system which increased from 0.54 to 0.77. The avoided CO2 emission during a cooling season was estimated to be about 3000kg corresponding to an equivalent energy saving of 1154l of gasoil.

Building integration of concentrating systems for solar cooling applications

The high energy consumption in buildings in Mediterranean countries, especially in the spring and summer months due to the extensive use of air conditioning, requires immediate actions to minimise energy costs and environmental impact given the current energy crisis. Solar cooling systems offer an attractive solution, but the main drawbacks of this type of systems are the low efficiency of the currently used single-effect absorption chillers and the large areas of thermal collectors needed to produce the thermal energy. These large solar installations make difficult their building integration. A way to overcome these difficulties is the use of high efficient integrated solar concentrator systems able to achieve temperatures around 150 °C that could be used to activate the more energy efficient double-effect absorption chillers. In the frame of this concept, in the present work a comparison between two cooling systems for a specific three-floor building, with and without solar concentration, is performed. The first is a conventional system which consists of evacuated tube collectors feeding a single-effect absorption chiller. On the other hand, a Fresnel reflective solar concentrating system, integrated on the building façade, is coupled to a double-effect absorption chiller. The results show an important reduction of the solar collectors absorber area in the concentrating system compared with the standard solar thermal installation. However, the solar concentrating system requires a large aperture area. In addition, the rejected heat in the double-effect chiller is lower, implying that the investment and operation costs of the solar concentrating cooling system can be reduced significantly.

Experimental validation of an optical and thermal model of a linear Fresnel collector system

This paper describes the design and validation of a mathematical model for a solar Fresnel collector. The function of the model is to simulate the optical and thermal dynamics of a Fresnel system for heating water. The model is validated using real data gathered from a cooling plant with double effect absorption chiller located in the School of Engineering University of Seville, Spain (Experimental cooling plant is also described in the paper). Comparison of calculated and plant measured data shows that the error is lower than 3% in the optical model and within 7% in the thermal model.The model uses a new approach to include a solar tracking mirror mechanism in one axis. This tracking has been designed to maximise the reception of available solar radiation by the absorption pipe. The thermal model used is based around classical models for solar receivers and it is validated with real operating data gathered from a supervisor system.The Fresnel model has been designed with sufficient flexibility to consider different geometries and thermal parameters, and may be used to simulate the performance of a proposed Fresnel collector system at any location.