Low-temperature Generator Research Articles

Thermodynamic analysis of an absorption refrigeration machine with new working fluid for solar applications

A theoretical analysis was undertaken to examine the efficiency characteristics of acetone-zinc bromide solutions for an absorption refrigeration machine, using low generator temperatures (47–60°C), which allows the use of flat plate solar collectors. The results of the simulation were confirmed with an experimental investigation. The main results showed that the solution is well suited to operate the machine at low temperatures (higher than 50°C).

Simulation of an air conditioning absorption refrigeration system in a co-generation process combining a proton exchange membrane fuel cell

In this work, a computer simulation program was developed to determine the optimum operating conditions of an air conditioning system during the co-generation process. A 1 kW PEMFC was considered in this study with a chemical/electrical theoretical efficiency of 40% and a thermal efficiency of 30% applying an electrical load of 100%. A refrigeration-absorption cycle (RAC) operating with monomethylamine–water solutions (MMA–WS), with low vapor generation temperatures (up to 80 °C) is proposed in this work. The computer simulation was based on the refrigeration production capacity at the maximum power capacity of the PEMFC. Heat losses between the fuel cell and the absorption air conditioning system at standard operating conditions were considered to be negligible. The results showed the feasibility of using PEMFC for cooling, increasing the total efficiency of the fuel cell system.

Experimental study of mass recovery adsorption cycles for ice making at low generation temperature

This paper presents the experimental results of an adsorption icemaker operating without refrigerant mass recovery and under two kinds of mass recovery cycles. The generation temperatures used in the experiments were 85 and 115 °C. At the lower generation temperature, the cycle with mass recovery in double stage produced the highest cycled mass, rate of ice production, cooling capacity and its COP was similar to that obtained in the cycle with conventional mass recovery. At generation temperature of 115 °C, the best performance was obtained with the cycle that employed conventional mass recovery. The cycle with mass recovery in double stage generated 42% more refrigerant mass than the cycle without mass recovery when the generation temperature was 85 °C, while at generation temperature of 115 °C, the cycle with conventional mass recovery generated 37% more mass than the latter. These results show that mass recovery can be an efficient process to enhance the performance of adsorption machines for ice making.

Thermodynamic Investigation of Two-Stage Absorption Refrigeration System Connected by a Compressor

The present work is to analyze a two-stage cycle based on the ammonia-water absorption system, with intermediate compression. The two generators of the system are heated by geothermal energy at low temperature. The study shows that this system makes it possible at lower generator temperature, under the limits permitted by the systems suggested up to now. For Tg = 335 K, Tc = Ta = 308 K and Te = 263 K, based on the electric consumption, the system efficiency is 8.2. The comparative study of the hybrid system and vapor compression systems shows the superiority of the proposed system. Supplied by the geothermal sources of the Tunisian south, the system makes it possible to obtain for a pilot geothermal station, a production of 75 tons of ice per day. The greenhouse gas emissions should thus be reduced by about 2.38 tons of CO2 per day. Therefore, based on the typical geothermal energy sources in Tunisia which present a global refrigeration potential of 4.4 MW, the daily quantity of ice that could be produced is about 865 tons. The greenhouse gas emissions should thus be reduced by about 10,000 tons of CO2 per year.

Modelling petroleum formation from heterogeneous source rocks: the influence of frequency factors on activation energy distribution and geological prediction

The application of basin modelling programs permits the prediction of organic matter transformation to oil and gas under variable geological conditions. Simplified kinetic models are commonly applied with a distribution of activation energies and a single average frequency factor. Here it will be shown that such simplified kinetic models are not sufficient to correctly predict organic matter transformation in heterogeneous kerogens under geological conditions. In the present paper, individual pairs of frequency factors and activation energies were calculated from the temperature shift of nine different levels of organic matter transformation ranging from 0.1 to 0.9 TR at different heating rates (0.1, 0.7 and 5.0 K/min). This type of extended kinetic evaluation has been applied on samples from the Green River Shale, the Toarcian Shale, the Duvernay Formation, the Draupne Formation, a type II-S source rock, petroleum asphaltenes, and a Tertiary coal from the Mackenzie Delta, respectively. For terrestrial and heterogeneous marine source rocks the frequency factors systematically increase with increasing level of transformation, while such an increase was insignificant or absent when homogeneous source rocks were studied. By applying geological heating rates it was possible to illustrate the significance of the extended calculation of individual frequency factors. The predicted onset temperature of petroleum formation was displaced to 60–70 °C in case of lignites/coals and type II-S source rocks, which was up to 30–40 °C lower than predicted by the use of a conventional single frequency factor model. To a lesser extend such a shift was also documented for some marine shales (e.g. Draupne Formation and the Duvernay Formation). The reason for this phenomenon is that an average single frequency factor suppresses the initial formation of petroleum at geological heating rates because high activation energies are needed to maintain the kinetic equation at low generation temperatures. In contrast, correct activation energies derive from individual frequency factors which keep the activation energies low at low levels of organic matter transformation and result in a more reliable description of the low energy bonds in the heterogeneous organic matter types.

Monomethylamine–water vapour absorption refrigeration system

Thermodynamic equations for the performance evaluation of the monomethylamine–water vapour absorption refrigeration system have been obtained, analysed and reported in this paper. These equations have been derived from experimental data with a good agreement with the expected values and have been expressed in polynomial form. These equations were utilized on the energy and mass balances for obtaining the coefficient of performance (COP) with this pair and compared with the ammonia–water absorption system. It has been observed that this refrigerant–absorbent pair has a greater coefficient of performance at low generation temperatures and moderate condenser, absorber and evaporator temperatures.

Performance evaluation of a monomethylamine–water solar absorption refrigeration system for milk cooling purposes

This study reports the applicability and the theoretical thermodynamic simulation of a solar driven monomethylamine–water single-stage absorption refrigeration cycle for milk cooling purposes in the rural regions of Mexico. A solar heating system using evacuated tube collectors coupled with a conventional auxiliary heating system is proposed. The simulation of the yearly dynamic behaviour of the solar system is carried out with the solar radiation and climate data of typical Mexican dairy farms in the semi-tropical regions. The theoretical coefficients of performance present moderate values at relative low generator and high evaporator temperatures and low absorption and condenser temperature values. The use of the solution heat exchanger and the milk precooler improve the thermodynamic efficiency and reduces the solar heating load fraction FSOL.

Performance improvement of absorption refrigeration system using triple-pressure-level

In the absorption refrigeration system (ARS) working with aqua–ammonia, the ejector is commonly located at the condenser inlet. In this study, the ejector was located at the absorber inlet. Therefore, the absorber pressure becomes higher than the evaporator pressure and the system works with triple-pressure-level. The ejector has two main functions: (i) aiding pressure recovery from the evaporator, (ii) upgrading the mixing process and the pre-absorption by the weak solution of the ammonia coming from the evaporator. In addition to these functions, it can also act to lower the refrigeration and heat-source temperatures. Energy analyses show that the system’s coefficient of performance (COP) and exergetic coefficient of performance (ECOP) were improved by 49% and 56%, respectively and the circulation ratio ( f) was reduced by 57% when ARS is initiated at lower generator temperatures. Due to the reduced circulation ratio, the system dimensions can be reduced; consequently, this decreases overall cost. The heat source and refrigeration temperatures decreased in the range of 5–15 °C and 1–3 °C, respectively. Exergy analyses show that the exergy loss of the absorber of ARS with ejector had a higher exergy loss than those of the other components. Therefore, a multiple compartment absorber can be proposed to reduce the exergy loss of the absorber of ARS with ejector.

Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COP h for the basis cycle and COP SHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio F R has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.

Phenolic antioxidants in alder smoke during industrial meat curing

Alder wood smoke in a large sausage-curing chamber was analysed with regard to its content of methoxyphenol antioxidants. The use of Tenax adsorbent cartridges permitted simultaneous quantitative sampling of phenols condensed on smoke particles as well as gaseous compounds. The analytical determinations were performed by thermal desorption combined with gas chromatography and mass spectrometry. In addition to methoxyphenols, several other key components were assessed, including 1,6-anhydroglucose, 2-furaldehyde, furans and hydrocarbons. Benzene was the most prominent hydrocarbon. The concentrations of polycyclic aromatic compounds were low, due to a low smoke generation temperature. Predominant smoke components were the 2,6-dimethoxyphenols, which are characteristic thermal degradation products from hardwood. The 2,6-dimethoxyphenols are stronger antioxidants than the 2-methoxyphenols present in lower amounts. The particularly active antioxidants with a 4-alkenyl side-chain constituted 20–30% of total methoxyphenols, which is much more than normally reported for liquid smoke. The phenolic antioxidants may be important not only for the preservation of foods, but also for health as dietary components.

Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems

The equilibrium temperatures at the low pressure (LP) generator for double-effect series flow lithium bromide–water vapour absorption chiller are evaluated and the system performance is estimated at these temperatures. Influence of temperatures at high pressure (HP) generator, evaporator, condenser and absorber, and the effectiveness of heat exchangers on equilibrium temperature and internal heat transfer at LP generator, circulation ratio, and coefficient of performance are studied. Dual-heat mode of operation of the system is also investigated utilising low grade waste heat at the LP generator. Correlations are presented for equilibrium LP generator temperature, internal heat transfer at the low pressure generator, circulation ratio, coefficient of performance, optimum HP generator temperature and optimum circulation ratio for maximum coefficient of performance in terms of operating temperatures, which are useful in the design and control of absorption system even at the off-design conditions.

Effect of component pressure drops in two-fluid pumpless continuous vapour absorption refrigerator

Miniaturisation of the vapour absorption refrigerator requires replacement of the solution pump by a heat operated bubble pump and air cooled condenser and absorber. The replacement necessitates the selection of working media restricted to vacuum operation, and the air cooling poses the problem of high pressure drops in the refrigerator. Thermodynamic analysis of the absorption refrigerator with such a suitable working medium is performed considering the pressure drops in the system as parameters. The analysis shows that the effect of the evaporator to absorber pressure drop on the system performance is more significant than that of the generator to condenser pressure drop, and it becomes more predominant at the low generator temperature normally encountered in solar operated systems.

Experimental studies of a three-pressure absorption refrigeration cycle

A novel experimental technique has been developed to study a three-pressure absorption refrigeration cycle using R600-DMF as the working pair. The third (or the intermediate) pressure is obtained by employing a liquid-gas ejector between the absorber and the evaporator. Pilot plant studies showed that it is possible to obtain high COP (0.4) when compared with a conventional two-pressure cycle. Parametric studies on the COP and the recirculation ratio have been made. A mathematical model has been developed and the results obtained have been compared with those obtained experimentally. The important result is that higher COP values could be obtained at lower generator temperatures.

Comparison of various working pairs for absorption refrigeration systems: application of R21 and R22 as refrigerants

This paper reports a study of the applicability of R21 and R22 as refrigerants in an absorption refrigeration system. A simulation has been performed for the working pairs R21-DMF, R21-DMETEG and R22-DMF and these were compared with R22-DMETEG. The results show that the coefficient of performance of the refrigerant-absorbent working pairs R21-DMF and R21-DMETEG are superior to those with R22 and these working pairs are thus particularly suitable for the refrigeration of foodstuffs, vegetables and fruits in tropical regions using low generator temperatures.

Feasibility studies on an alcohol-salt mixture for absorption refrigeration systems

This paper presents an investigation on the feasibility of a methanol-LiI.ZnBr 2 mixture as a potential working fluid in vapour absorption refrigeration systems. At the first stage, the thermodynamic properties' state equations, viz. P- T- X and H- T- X, for the proposed mixture have been developed, and then, a numerical computer simulation, based on mass, material and heat balance equations for various components of the single/double effect absorption cycles have been carried out for the methanol-LiI.ZnBr 2 mixture. A comparative study with methanol-LiBr.ZnBr 2 and water-LiBr mixtures has also been undertaken. It has been found that the COP of the methanol-LiI.ZnBr 2 and methanol-LiBr.ZnBr 2 mixtures are almost the same, while for the water-LiBr mixture, the COP is slightly higher than for the two above mentioned mixtures. The alcohol-salt mixtures can work at lower generator temperatures and can be used for air-conditioning as well as for low temperature applications. The advantage of the methanol-LiI.ZnBr 2 mixture over the methanol-LiBr.ZnBr 2 mixture is that the former is having a wide working range and lower viscosity, which may be feasible for vapour absorption solar refrigeration and air-conditioning applications.

Comparison of the solar-operated two-stage and single-stage LiBrH2O absorption cycles

Analysis of a solar operated two-stage LiBrH 2 O cycle is presented. The cycle has the advantages that it can operate at a higher ambient temperature and at a lower generator temperature than those required by the single stage cycle. The performance of the cycle is predicted and compared to the performance of the single stage cycle at various design conditions. Four design parameters are identified to control the performance of the cycle. These parameters are the heat sink temperature, the evaporator temperature, the generator temperature, and the intermediate pressure. Numerical correlations are developed to specify the minimum allowable sink temperature, the minimum and maximum allowable generator temperature and the coefficient of performance of both the single-stage and the two-stage cycles. By defining a system thermal ratio for the combined absorption machine and a flat plate collector, the performances of the integrated systems are evaluated and compared when using both cycles. Conditions at which the performance of the solar operated two-stage cycle is superior to that of the solar operated single stage cycle are defined.

Comparison of performances of single stage and two stage intermittent ammonia-water solar refrigeration systems

Abstract The paper explains briefly the operations of a single stage and a two stage ammonia-water intermittent solar refrigeration system with the help of relevant schematic and cycle diagrams. The performances of these two systems are compared based on the results of the thermodynamic analyses of the corresponding cycles. It is inferred that a stage system is capable of producing the required refrigeration with relatively high overall coefficient of performance at even low generation temperatures. Hence, it is a viable refrigeration system, especially when flat collectors are used for the collection of solar energy.

Analysis of solar absorption cooling systems with low generator temperatures

Closed cycle absorption cooling systems have been analysed using water-lithium bromide and ammonia-water as working fluid pairs. An open cycle system using water-lithium bromide has also been analysed and compared with the closed cycle system. A flat plate collector has been considered for heat supply to the generator of the closed cycle systems. The effect of high flow ratios on the overall solar collector area required has been studied. It has been shown that the daily operating time of a cooling system can be increased by employing relatively high flow ratios.

Thermal modelling and parametric study of two stage absorption refrigeration and air-conditioning systems

This paper presents a thoermodynamic assessment of two stage absorption refrigeration and air-conditioning systems. Two working fluids, namely water-LiBr and NH3-H2O as refrigerant-absorbent combinations have been considered for the production of different cooling temperatures. Both the systems are assumed to be operated with hot water available from solar collectors. Thermal modelling and a parametric study of a two stage absorption system, based on thermodynamic analysis aided by computer, have been carried out in detail. It is found that the cascading of two stage absorption systems (in which the first stage evaporator produces cooling water to be circulated in the absorber of the second stage) can be easily operated to produce much lower temperatures suitable for refrigeration and air-conditioning applications. The COP of a two stage absorption system is lower than that of a single stage; however, the second stage can be operated with lower generator temperatures than the first stage. However, there is an advantage for a two stage system because of the slow fall-off in COP at higher generator temperatures.

Thermodynamic study of a two-stage vapour absorption refrigeration system using NH 3 refrigerant with liquid/solid absorbents

A systematic investigation is made of the two-stage vapour absorption refrigeration system employing the refrigerant absorbent combinations of NH 3H 2O and NH 3LiNO 3. The system consists of coupling two conventional absorption cycles so that the first-stage evaporator produces cooling water to circulate in the absorber of the second stage. The effect of operating variables such as generator temperature, evaporator temperature, absorber temperature and condenser temperature on the coefficient of performance (COP), heat transfer rates and relative circulation have been studied for both single-stage and two-stage absorption refrigeration systems. It is found that the COP is higher for NH 3LiNO 3 than for NH 3H 2O, in both single-stage and two-stage absorption systems, especially at higher generator temperatures. Furthermore, the minimum evaporator temperature achieved is lower for NH 3LiNO 3, and the system can be operated at lower generator temperatures.