Performance improvement of absorption heat transformer

In this study, a mathematical model of absorption heat transformer (AHT) operating with the aqua/ammonia was 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 and used a special ejector located at the absorber inlet. By the use of the ejector, the obtained absorber pressure becomes higher than the evaporator pressure and thus the system works with triple-pressure-level. The ejector has two functions: (i) aids pressure recovery from the evaporator and (ii) upgrades the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. The other advantage of the system with ejector is increased absorber temperature. Therefore, pressure recovery and pre-absorption in the ejector improves the efficiency of the AHT. Under the same circumstances, when compared to an AHT with and without an ejector, the system's COP and exergetic coefficient of performance (ECOP) were improved by 14% and 30%, respectively and the circulation ratio (f) was reduced by 57% at the maximum efficiency condition. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The maximum upgrading of the solar pond's temperature by the AHT was obtained at 57.5 °C and gross temperature lift at 97.5 °C with coefficients of performance of about 0.5. The maximum temperature of the useful heat produced by the AHT was 150 °C. In addition, exergy losses for each component in the system were calculated at different working temperatures and the results of both systems with and without an ejector were compared. Exergy analysis emphasised that both the losses and irreversibilities have an impact on the system performance and exergy analysis can be used to identify the less efficient components of the system. Exergy analyses also showed that the exergy loss of the absorber of AHT with ejector was higher than those of other components.     

Comparative substitution of butane by hydrogen/oxygen mixture in a domestic room heating convector

The search for new energy sources has a fundamental importance concerning better quality of life for the mankind. Looking for a substitute for the existing pollutant fueled heating equipment, the Hydrogen Group of the Federal University of Paraíba (Brazil), has studied the application of hydrogen to this purpose. This work shows performance tests realized with a convective commercial heater exchanger, adapted to be fueled by hydrogen and applied for the environmental heating. The study also compared its gain and efficiency using butane. The results showed that the commercial heat exchanger adapted for hydrogen has a high gain and efficiency taking into account fuel consumption and that can be improved if the suggested alterations are implemented.     

Performance of one and a half-effect absorption cooling cycle of H2O/LiBr system

The performances of half-effect, single-effect and double-effect H₂O/LiBr absorption cooling cycles were analyzed, and it was found that there is an obvious blank for generation temperature between the maximum generation temperature of the single-effect cycle and the minimum generation temperature of the double-effect cycle. It was proposed that the one and a half-effect (1.5-effect) cycle can fill up the blank perfectly. The state of the art in the 1.5-effect cycles was reviewed and analyzed, and two new configurations of 1.5-effect cycles were proposed. Three configurations of 1.5-effect cycles, which are suitable for H₂O/LiBr as working fluids, were selected to be analyzed in detail. The 1.5-effect cycle shows the optimum performance at the foregoing blank of generation temperature. For example, under the conditions of evaporation temperature t_E is 5 °C, and condensation temperature t_C is 42 °C, and absorption temperature t_A is 37 °C, the optimum range of generation temperature t_G for the 1.5-effect cycle is from 110 °C to 140 °C. The coefficient of performance of the 1.5-effect cycle is about 1.0, which is more than 30% higher than that of the single-effect cycle at the same condition. The effects of the efficiency of solution heat exchanger, the generation temperature, the absorption temperature (or the condensation temperature) and the evaporation temperature on the performances of the three configurations of 1.5-effect cycle were analyzed. It was shown that the configuration II, which is composed with a high-temperature single-effect subcycle and a low-temperature half-effect subcycle, has the highest coefficient of performance and the best operational flexibility. Among the four parameters analyzed, the performances of 1.5-effect cycles are most sensitive to the change of absorption temperature (or condensation temperature), and then to the change of generation temperature.     

Integration of a molten carbonate fuel cell with a direct exhaust absorption chiller

A high market value exists for an integrated high-temperature fuel cell-absorption chiller product throughout the world. While high-temperature, molten carbonate fuel cells are being commercially deployed with combined heat and power (CHP) and absorption chillers are being commercially deployed with heat engines, the energy efficiency and environmental attributes of an integrated high-temperature fuel cell-absorption chiller product are singularly attractive for the emerging distributed generation (DG) combined cooling, heating, and power (CCHP) market. This study addresses the potential of cooling production by recovering and porting the thermal energy from the exhaust gas of a high-temperature fuel cell (HTFC) to a thermally activated absorption chiller. To assess the practical opportunity of serving an early DG-CCHP market, a commercially available direct fired double-effect absorption chiller is selected that closely matches the exhaust flow and temperature of a commercially available HTFC. Both components are individually modeled, and the models are then coupled to evaluate the potential of a DG-CCHP system. Simulation results show that a commercial molten carbonate fuel cell generating 300 kW of electricity can be effectively coupled with a commercial 40 refrigeration ton (RT) absorption chiller. While the match between the two “off the shelf” units is close and the simulation results are encouraging, the match is not ideal. In particular, the fuel cell exhaust gas temperature is higher than the inlet temperature specified for the chiller and the exhaust flow rate is not sufficient to achieve the potential heat recovery within the chiller heat exchanger. To address these challenges, the study evaluates two strategies: (1) blending the fuel cell exhaust gas with ambient air, and (2) mixing the fuel cell exhaust gases with a fraction of the chiller exhaust gas. Both cases are shown to be viable and result in a temperature drop and flow rate increase of the gases before the chiller inlet. The results show that no risk of cold end corrosion within the chiller heat exchanger exists. In addition, crystallization is not an issue during system operation. Accounting for the electricity and the cooling produced and disregarding the remaining thermal energy, the second strategy is preferred and yields an overall estimated efficiency of 71.7%.     

Heat transfer of a row of three butane/air flame jets impinging on a flat plate

Experiments were performed to investigate the heat transfer characteristics of a row of three premixed, laminar, butane/air flame jets impinging on a water-cooled flat plate. The between-jet interference was found to reduce the heat transfer rate in the jet-to-jet interacting zone due to the depressed combustion. The interference became stronger when the jet-to-jet spacing and/or the nozzle-to-plate distance were/was small. The positive pressure existed in the between-jet interacting zone caused the asymmetric flame and heat transfer distribution of the side jet. The meeting point of the spreading wall jets of the central and the side jets did not occur at the midpoint of the neighboring jets, but at a location shifted slightly outwards. The maximum local heat flux and the maximum area-averaged heat flux occurred at a moderate nozzle-to-plate distance of 5d with a moderate jet-to-jet spacing of 5d. The lowest area-averaged heat flux was produced when both the jet-to-jet spacing and the nozzle-to-plate distance were small. Comparing with a single jet under the same experimental conditions, the heat transfer rates in both the stagnation point and the maximum heat transfer point were shown to be enhanced in a row of three-jet-impingement system. The present study provided detailed information on the heat transfer characteristics of a row of three in-line impinging flame jets, which had rarely been reported in previous study.     

Experimental investigation of integrated refrigeration system (IRS) with gas engine,compression chiller and absorption chiller

An integrated refrigeration system (IRS) with a gas engine, a vapor-compression chiller and an absorption chiller is set up and tested. The vapor-compression refrigeration cycle is operated directly by the gas engine. The waste heat from the gas engine operates the absorption refrigeration cycle, which provides additional cooling. The performance of the IRS is described. The cooling capacity of the IRS is about 596 kW, and primary energy ratio (PER) reaches 1.84 at air-conditioning rated conditions. The refrigerating capacity of the prototype increased and PER of prototype decreased with the increase of the gas engine speed. The gas engine speed was preferably regulated at part load condition in order to operate the prototype at high-energy efficiency. The refrigerating capacity and PER of the prototype increased with the increase of the outlet temperature of chilled water or the decrease of the inlet temperature of cooling water. The integrated refrigeration chiller in this work saves running costs as compared to the conventional refrigeration system by using the waste heat.     

Heat transfer and wall pressure characteristics of a twin premixed butane/air flame jets

Experimental studies were carried out to investigate the flame shape and the heat transfer and wall pressure characteristics of a pair of laminar premixed butane/air flame jets impinging vertically upon a horizontal water-cooled flat plate at jet Reynolds numbers of 800, 1000 and 1200, respectively. Equivalence ratio of the butane/air mixture was maintained constantly at unity. The flame shape, the pressure distribution on the impingement plate and the heat transfer from the flame to the plate were greatly influenced by the interference occurred between the two flame jets. This interference caused a sharp pressure peak at the between-jet midpoint and the positive pressures at the between-jet area, which led to the separation of the wall jet from the impingement plate after collision. Such interference became more significant when the non-dimensional jet-to-jet spacing (S/d) and the nozzle-to-plate distance (H/d) were reduced. Heat transfer in the interaction zone between the jets was at the lowest rate due to this interference at the smallest S/d ratio of 2.6, resulting from the separation of the high-temperature inner reaction zone of the flame from the impingement plate. On the other hand, the interference enhanced the heat transfer in the interaction zone between the jets when the S/d ratio was greater than 5, by enhancing the heat transfer coefficient. The average heat flux of the impingement plate was found to increase significantly with the increasing H/d ratio until H/d=6. The present study provided detailed information on flame shape and the heat transfer and wall pressure characteristics of a twin laminar pre-mixed impinging circular flame jets, which has rarely been reported in previous studies.     

Modeling and simulation of a 100 kWe HT-PEMFC subsystem integrated with an absorption chiller subsystem

A 100 kW_e liquid-cooled HT-PEMFC subsystem is integrated with an absorption chiller subsystem to provide electricity and cooling. The system is designed, modeled and simulated to investigate the potential of this technology for future novel energy system applications. Liquid-cooling can provide better temperature control and is preferable for middle-scale transport applications, such as commercial vessels, because stack cooling can be achieved within smaller volumes. A commercial ship requiring cooling and electricity is taken as the case study for the application of the proposed system. All system components are described and analyzed in detail, in terms of modeling assumptions and configuration topology. The results show the conceptual feasibility of the proposed system configuration, since high net electrical efficiencies are accomplished. The calculated net electrical efficiency is 43.8% for a net electrical power output of 100 kW_e. The heat exhausted to the absorption chiller subsystem is 107 kW and can satisfy a cooling duty of up to 128 or 64.5 kW for a LiBr–water double-effect system or a water–NH₃ single-effect system, respectively. Finally, the projected total cost is comparable to conventional systems, i.e., diesel engines integrated with vapor-compression chillers, and therefore justifies further development of the proposed system.     

Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons

We propose in this work a thermodynamic feasibility study of absorptiondiffusion refrigerating machine working with hydrocarbon mixtures. We used a machine of low power (300 W) that operates with generator temperatures lower than 150 °C (fossil energy or solar energy can be used) and where the condenser and absorber temperatures are taken equal to 42 °C. The inert gas used is helium and the total functioning pressure is about 17.5 bars. A modeling on suitable software was made to simulate the machine functioning for four binary mixtures which are: propylene/hexane, propylene/heptane, propylene/octane and propylene/nonane. The validation of our model was made by comparison with the results taken from literature and the optimal operating conditions are determined.     

Thermodynamic and economic performance of the LiBr–H2O single stage absorption water chiller

The LiBr–H₂O single stage absorption water chiller, installed at the Municipal CHP plant of the city of Poznan, Poland was examined in order to find its energy performance. The 495 kW water chiller is used for CHP plant technological rooms air conditioning (e.g. control rooms, power supply rooms). The superheated steam (p = 1.0 MPa, t = 250 °C) is the heat source for the generator and cooling water for the absorber and condenser is supplied from the existing cooling water installation. The chilled water temperature is set at 6 °C. On the basis of online temperature and media flow measurements, the system’s energy balance was created. Employing the first law of thermodynamics the energy balance equation was solved and used for the derivation of the chiller’s COP factor. The work’s main goal was to establish the influence of the chiller’s actual load on the energy efficiency of the system.The economic evaluation of cooling energy unit price was carried out on the basis of the measured chiller’s COP factor.     

Performance improvement of the combined cycle power plant by intake air cooling using an absorption chiller

This paper studies how to improve the capacity of the combined cycle (CC) power plant which has been operated for 8 years. The most popular way is to lower intake air temperature to around 15 °C (ISO) and 100% RH before entering the air compressor of a gas turbine (GT). Thailand has 3 seasons: winter, summer and rainy season. According to 2003 Bangkok monthly weather data, all year ambient temperature is higher than 15 °C. This research proposes a steam absorption chiller (AC) to cool intake air to the desired temperature level. It could increase the power output of a GT by about 10.6% and the CC power plant by around 6.24% annually. In economic analysis, the payback period will be about 3.81 years, internal rate of return 40%, and net present value 19.44 MUS$.     

On-site real-time evaluation of an air-conditioning direct-fired double-effect absorption chiller

This work presents a procedure for calculating the COP and heat transfer rates, based on on-site experimental temperature measurements, of a lithium–bromide/water direct-fired double-effect absorption chiller in reverse parallel flow configuration, running on natural gas. The chiller was equipped with a set of thermocouples which allowed measuring its working temperature levels through all its operating stages. The chiller analysed in this work is the central cooling system of the air-conditioning installation of the Principe Felipe Science Museum, located at the Valencia’s City of Arts and Sciences (Spain). This installation is capable of providing a cooling capacity of 4.5 millions of kcal/h (5.2 MW), by means of three direct-fired double-effect absorption chillers. From the experimental measurements a calculation procedure, based on energy and mass balances, has been developed, which allows estimating the specific powers by unit of mass flow rate through the evaporator. From these power values the instantaneous COP of the chiller could be obtained. Additionally, the paper analyzes different aspects that were not possible to be considered and details the actions taken in order to take them into account.     

Proposal of a hybrid CHP system: SOFC/microturbine/absorption chiller

Distributed generation is becoming an attractive option for industrial and commercial scale customers. The main advantage of this on‐site power generation is that it offers a more efficient, reliable and cost‐effective power supply. In addition, waste heat can be used for local heating or cooling. This is known as cogeneration or combined heat and power (CHP). In the present work, a hybrid‐CHP system for a 230 kWe demand building is proposed and analyzed. The system considers the coupling of:

• A Solid Oxide Fuel Cell stack with an output of 200 kWe
• A Microturbine with an output of 30 kWe
• A single effect Absorption cooling system providing 55 kWt for air conditioning using water chillers

This plant would use natural gas as the primary fuel. The SOFC module is fed with the gas fuel and the whole stack generates the main power while acting as a combustor. The product gases exit the anode at a temperature of 900°C and are directly injected to the Micro Gas Turbine unit to produce additional power. Finally, the waste heat available at the turbine's exhaust fires a single effect Absorption Water‐Chiller to provide cooling for air conditioning in the building. This proposed system would generate up to 230 kWe and 55 kWt with high thermal efficiencies of around 70–75%. Currently, Hybrid SOFC/GT and Microturbine/CHP systems are being considered or tested at several facilities. However, a combination of both, which would yield to trigeneration, has not been considered yet. Here we present a conceptual model based on specific proposals and investigations done by other researchers. A theoretical analysis on the proposed model is conducted to evaluate the potential and possibilities of such Hybrid CHP system and further discussions based on the economical considerations is also presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Remarkable improvement of NOx–PM trade-off in a diesel engine by means of bioethanol and EGR

In order to realize a premixed compression ignition (PCI) engine, the effects of bioethanol–gas oil blends and exhaust gas recirculation (EGR) on PM–NO_x trade-off have been investigated focusing on ignition delay, premixed combustion, diffusion combustion, smoke, NO_x and thermal efficiency. The present experiment was done by increasing the ethanol blend ratio and ethanol and by increasing the EGR ratio in a single cylinder direct injection diesel engine. It is found that a remarkable improvement in PM–NO_x trade-off can be achieved by promoting the premixing based on the ethanol blend fuel having low evaporation temperature, large latent heat and low cetane number as well, in addition, based on a marked elongation of ignition delay due to the low cetane number fuel and the low oxygen intake charge. As a result, very low levels of NO_x and PM, which satisfies the 2009 emission standards imposed on heavy duty diesel engines in Japan, were achieved without deterioration of brake thermal efficiency in the PCI engine fuelled with the 50% ethanol blend diesel fuel and the high EGR ratio. It is noticed that smoke can be reduced even by increasing the EGR ratio under the highly premixed condition.     

Investigation on performance of an integrated solid oxide fuel cell and absorption chiller tri-generation system

An integrated tri-generation system incorporating a solid oxide fuel cell (SOFC) and a double-effect water/Lithium Bromide absorption chiller is presented in this paper. The proposed tri-generation system can provide power, cooling or heating simultaneously with a typical gas produced from a gasication process. The system conguration and design are discussed, and the energy and mass balances are obtained through the matrix representation method and integrated into a simulation program by MATLAB soft package. The developed model comprises of three modules: SOFC module, exhaust combusting and HRSG module, and the absorption chiller module. Validation of the SOFC model is performed by comparison with a single tubular cell of Siemens-Westinghouse, and a specific case study of the system is presented. For parametric analysis, the fuel utilization ratio, fuel flow ratio and air inlet temperature are investigated and the results are discussed in detail.     

Exergy analysis of lithium bromide/water absorption systems

Exergy analysis of a single-effect lithium bromide/water absorption system for cooling and heating applications is presented in this paper. Exergy loss, enthalpy, entropy, temperature, mass flow rate and heat rate in each component of the system are evaluated. From the results obtained it can be concluded that the condenser and evaporator heat loads and exergy losses are less than those of the generator and absorber. This is due to the heat of mixing in the solution, which is not present in pure fluids. Furthermore, a simulation program is written and used for the determination of the coefficient of performance (COP) and exergetic efficiency of the absorption system under different operating conditions. The results show that the cooling and heating COP of the system increase slightly when increasing the heat source temperature. However, the exergetic efficiency of the system decreases when increasing the heat source temperature for both cooling and heating applications.     

Study of the sub-bandgap absorption and the optical transitions in CuInSe2 polycrystalline thin films

Optical transitions near the fundamental band edge are studied for CuInSe₂ films having various Cu/In ratios by analysing the variations of the absorption coefficient with incident photon energy. The results indicate different transitions depending upon the Cu/In ratio. There are sub-bandgap absorption for near stoichiometric and Cu-rich films. The results are compared to some literature data.     

Theoretical analysis and optimization of a double-effect series-flow-type absorption chiller

A thermodynamic analysis was carried out to study the effect of design parameters, including heat recovery ratio and solution circulation ratio, on the performance of a double-effect absorption chiller of the series-flow-type using water-lithium bromide as the working fluid. Increases in the heat recovery ratios of the high-temperature heat exchanger and the low-temperature heat exchanger and/or decreases in the solution circulation ratio improved the coefficient of performance. An increase in the heat recovery ratio of the high-temperature heat exchanger increased the total heat transfer area of the absorption chiller. The optimum design and operating conditions of a double-effect absorption chiller are suggested based on this cycle simulation analysis.     

Theoretical study of a solar powered absorption/MED combined system

In this article, a theoretical study is presented for a solar powered combined system comprising a LiBr---H₂O absorption cooling machine and a multiple-effect distillator (MED). The MED has 8 VTE of the falling film type, and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions, and the condensate follows its usual route towards the evaporator of an A/C unit. Thus, the MED is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly.

Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power the system, and a transient simulation for the combined arrangement is presented.

Results are given for a typical design summer day in Jeddah, Saudi Arabia, for a range of firing temperatures 150–190°C with a storage temperature amplitude of 10–20°C over a daily working period of 12 h. For a given cooling load of 100 ton refrigeration, the system can produce up to 40m³ of fresh water at a specific collector area of 12.41. H₂O plus 0.03 TR/m². The overall COP_o reaches 1.44, which is more than twice that of a conventional absorption machine at the same temperature levels.     

Performance prediction of refrigerant-DMF solutions in a single-stage solar-powered absorption refrigeration system at low generating temperatures

A theoretical analysis of the coefficient of performance was undertaken to examine the efficiency characteristics of R22 + DMF, R134a + DMF, R32 + DMF as working fluids, respectively, for a single-stage and intermittent absorption refrigerator which allows the use of heat pipe evacuated tubular collectors. The modeling and simulation of the performance considers both solar collector system and the absorption cooling system. The typical meteorological year file containing the weather parameters for Hangzhou is used to simulate the system. The results show that the system is in phase with the weather. In order to increase the reliability of the system, a hot water storage tank is essential. The optimum ratio of storage tank per solar collector area for Hangzhou’s climate for a 1.0 kW system is 0.035-0.043L. Considering the relative low pressure and the high coefficient of performance, R134a + DMF mixture presents interesting properties for its application in solar absorption cycles at moderate condensing and absorbing temperatures when the evaporating temperatures in the range from 278 K to 288 K which are highly useful for food preservation and for air-conditioning in rural areas.