太阳能驱动风冷喷射式绝热吸收制冷循环研究

为改善太阳能吸收制冷循环性能并解决太阳能吸收制冷机风冷化问题,提出一种太阳能驱动的风冷喷射式绝热吸收制冷循环,通过喷射式绝热吸收器实现吸收过程的传热传质分别强化,同时回收高压溶液节流损失和再循环溶液的余压,以进一步增强吸收效果。构建组成循环各部件热力学数学模型,探讨了环境温度、发生温度、蒸发温度以及喷射器对新循环的影响,并与2种传统吸收制冷循环进行了比较分析。结果表明,存在最佳发生温度使得新循环最大热性能系数（COP_T）;引入喷射式绝热吸收器显著改善了极端条件下新循环性能,设计工况下新循环的热性能系数（COP_T）和电性能系数（COP_E）较风冷绝热吸收制冷循环分别提高了7.69%和43.59%,较水冷非绝热吸收-喷射制冷循环分别提高了2.44%和273.86%。     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

风冷式太阳能吸收式制冷机性能模拟研究

在环境温度和太阳辐射动态变化的情况下,对制冷量为5 kW的风冷式太阳能吸收式制冷机的性能进行了模拟,得出了集热器出口水温和热水储槽温度随时间变化的规律曲线以及在此规律的影响下吸收式制冷机的性能曲线.模拟结果表明风冷式太阳能吸收式制冷机在理论上是切实可行的,但是环境温度的变化以及风冷系统的散热能力对系统性能有较大的影响,环境温度的升高会使需要风冷降温的冷凝器、吸收器温度升高,从而提高了发生温度的要求,这不利于太阳能的利用和系统的制冷.为此需要强化冷凝器和吸收器的散热效果,来降低境温度对系统的不利影响.     

Experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator

This paper is concerned with experimental research on a new solar pump-free lithium bromide absorption refrigeration system with a second generator. By using the second generator together with a lunate thermosiphon elevation tube, the required minimum driving temperature of the heat source is only 68 °C compared to above 100 °C in traditional absorption refrigeration systems. Based on the horizontal-tube falling-film method, the performance of the absorber can be enhanced by the second generator due to an increase in the differential concentration of the solution between the inlet and the outlet of the absorber and an increase in the temperature difference between the inlet and the outlet of the cooling water in the absorber. The yield of condensate with the second generator open is increased by 68% compared to that with the second generator closed. The performance of the evaporator is significantly improved due to the increase in temperature drop of the chilled water and the decrease in the outlet temperature of the chilled water. This leads to an improvement of the performance of the overall refrigeration system. The maximum coefficient of performance (COP) approaches 0.787.     

溴化锂溶液空气预冷却绝热吸收过程研究

溴化锂吸收式制冷机的空冷化是吸收式制冷机发展的主要方向之一，该文基于传热与传质的相异性，对溴化锂吸收式制冷机空冷化的关键部件－吸收器提出了空冷预冷却绝热吸收的方法，建立了吸收器的空冷预冷却绝热吸收模型，按照所建立的模型进行热力计算并与一般空冷吸收器进行比较，最后分析了预冷却温差对溴化锂溶液空冷预冷却绝热吸收过程的影响，研究结果表明空冷预冷却绝热吸收是一种可行的空冷化方法，与传统空冷相比具有较强的优势。     

Theoretical analysis of ammonia-water absorption cycles for refrigeration and space conditioning systems

This paper presents an investigation of an ammonia-water absorption cycle for solar refrigeration, airconditioning and heat pump operations at higher heat supply temperatures. The system consists of a solar driven generator, rectifier, condenser, evaporator, absorber and heat exchangers for preheating and subcooling within the system. A steady state thermodynamic cycle analysis based on mass and heat balances along with the state equations for the thermodynamic properties of the ammonia-water mixture has been carried out. A numerical computer simulation of the system with input component temperatures, refrigerant concentration/mass flow rate and effectiveness of the heat exchangers has been made to evaluate the relative heat transfer rates (i.e. coefficients of performance) and the mass flow rates for the cooling/heating modes. It is found that unlike the low generator temperature behaviour the coefficients of performance for both cooling and heating modes are reduced at higher generator temperatures. However, an increase of condenser temperature for each mode of operation improves the performance of the systems at higher generator temperatures. A choice for keeping the absorber temperature equal to/lower than that of the condenser is also predicted at lower/higher generator temperatures, respectively. In general the results are more pronounced for the refrigeration mode than for the heat pump mode and are least effective for the airconditioning mode.     

Performance Evaluation of R134a/DMF-Based Vapor Absorption Refrigeration System

This article describes an experimental investigation to measure performances of a vapor absorption refrigeration system of 1 ton of refrigeration capacity employing tetrafluoro ethane (R134a)/dimethyl formamide (DMF). Plate heat exchangers are used as system components for evaporator, condenser, absorber, generator, and solution heat exchanger. The bubble absorption principle is employed in the absorber. Hot water is used as a heat source to supply heat to the generator. Effects of operating parameters such as generator, condenser, and evaporator temperatures on system performance are investigated. System performance was compared with theoretically simulated performance. It was found that circulation ratio is lower at high generator and evaporator temperatures, whereas it is higher at higher condenser temperatures. The coefficient of performance is higher at high generator and evaporator temperatures, whereas it is lower at higher condenser temperatures. Experimental results indicate that with addition of a rectifier as well as improvement of vapor separation in the generator storage tank, the R134a/DMF-based vapor absorption refrigeration system with plate heat exchangers could be very competitive for applications ranging from –10°C to 10°C, with heat source temperature in the range of 80°C to 90°C and with cooling water as coolant for the absorber and condenser in a temperature range of 20°C to 35°C.     

Research on performance of mixed absorption refrigeration for solar air-conditioning

A novel lithium bromide/water mixed absorption refrigeration cycle that is suitable for the utilization of solar air-conditioning and can overcome the drawbacks of low system overall efficiency of traditional solar absorption refrigeration air-condition systems is presented. The accessorial high pressure generator was added in the cycle. The lithium bromide solution flowing out from the high pressure generator was mixed with the solution from the low pressure absorber to increase lithium bromide solution concentration and decrease pressure in the high pressure absorber. The performance of a mixed absorption refrigeration cycle was analyzed. The theoretical analysis shows that the highest COP is 0.61, while the highest available temperature difference of heat resource is 33.2°C. The whole coefficient of performance of the solar air-conditioning using mixed absorption cycle is 94.5% higher than that of two-stage absorption. The advantages of solar air-conditioning can be markedly made use of by the cycle.     

Analysis of an air cooled ammonia–water vertical tubular absorber

This paper presents a detailed analysis of an ammonia–water vertical tubular absorber cooled by air. The absorption process takes place co-currently upward inside the tubes. The tubes are externally finned with continuous plate fins and the tube rows are arranged staggered in the direction of the air flow. The air is forced over the tube bank and circulates between the plain fins in cross flow with the ammonia–water mixture. The analysis has been carried out by means of a mathematical model developed on the basis of mass and energy balances and heat and mass transfer equations. The model takes into account separately the churn, slug and bubbly flow patterns experimentally forecasted in this type of absorption processes inside vertical tubes and considers the simultaneous heat and mass transfer processes in both liquid and vapour phases, as well as heat transfer to the cooling air. The model has been implemented in a computer program. Results based on a representative design and nominal operating conditions of an absorber for a small capacity ammonia–water absorption refrigeration system are shown. A parametric analysis was realised to investigate the influence of the design parameters and operating conditions on the absorber performance. The noteworthy results that have effect on practical design of the absorber are presented and commented.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents

A systematic investigation is made of the two-stage vapour absorption refrigeration system employing the refrigerant absorbent combinations of NH₃---H₂O and NH₃---LiNO₃. 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 NH₃---LiNO₃ than for NH₃---H₂O, in both single-stage and two-stage absorption systems, especially at higher generator temperatures. Furthermore, the minimum evaporator temperature achieved is lower for NH₃---LiNO₃, and the system can be operated at lower generator temperatures.     

Conservation of energy estimated by second law analysis of a power-consuming process

A refrigeration system removing heat from a cold storage is analyzed to determine exergetic losses of the thermodynamic cycle and the power needed for the flows of the refrigerant and the media transferring heat at a specified temperature. How much the overall power consumption of the system can be decreased by lowering the condensing temperature, by either increasing the external heat transfer at the condenser or by lowering the inlet temperature of the heat exchanging media, is investigated. The latter depends on the relative humidity of the air. The performance of air, water and evaporative-cooled condensers are evaluated as a function of relative humidity of the ambient air. It is shown that the evaporative condenser operates at the lowest condensing temperature and, therefore, the least power consumption of the total system is achieved. Wetting the condenser with water requires only 1% of the overall power consumption but reduces the consumption by 30% as compared with the air-cooled condenser. Precooling the air by a water spray before it enters an air-cooled condenser is of benefit only at relative humidities of 65% or less. At other state conditions of the air, a higher power consumption will result. Lowering the temperature of the surroundings lowers P_rev and the second law efficiency must be properly defined so that, for cases of lowest power consumption, highest values of the efficiency will be obtained. Means for the design of least power-consuming air-conditioners are briefly stated, as well as the advantages of evaporative condensers for fog-free operation.     

Comparison of the performance of falling film and bubble absorbers for air-cooled absorption systems

Small capacity, air-cooled NH₃–H₂O absorption systems are becoming more attractive in applications where the input energy can be obtained for free (e.g., solar energy, exhaust gases of engines), due to the increasing price of the primary energy. One of the main difficulties for a wider use of absorption machines is the necessary high initial investment. For this reason, the development of air-cooled systems could be an important achievement for low capacity applications. In this work, two types of air-cooled absorber have been modelled: (i) falling film flow; (ii) bubble flow. The two models have been validated with experimental data obtained from a developed testing device and published numerical results of other authors from another model. The agreement is acceptable for both cases. Finally, a parametric study has been done for air-conditioning and refrigeration in a mobile application taking advantage of the exhaust gases of the engine. In both cases, the performance of the bubble absorber has been higher.     

Absorption of organic fluid mixtures in plate heat exchangers

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

Computer-aided conceptual thermodynamic design of a two-stage dual fluid absorption cycle for solar refrigeration

This communication presents thermodynamic assessment of a two-stage dual fluid absorption refrigeration system using H₂O---LiBr and NH₃---H₂O as working fluids at the first and second stage, respectively. Both stages are assumed to be operated with hot water available from separate solar collectors. In the cascading of two-stage absorption systems, the evaporator of the first stage produces cooling water which is circulated in the absorber of the second stage. It is found that two-stage systems can be used for the production of very low temperatures using moderate generator temperatures at the first stage. The effects of generator temperature, absorber temperature and condenser temperature on the coefficient of performance, minimum evaporator temperature and effective refrigeration produced are also presented.     

Exergy analysis of double effect vapor absorption refrigeration system

Energy and exergy analyses previously performed by the authors for a single effect absorption refrigeration system have been extended to double effect vapor absorption refrigeration system with the expectation of reducing energy supply as well as an interest in the diversification of the motive power employed by HVAC technologies. The total exergy destruction in the system as a percentage of the exergy input from a generator heating water over a range of operating temperatures is examined for a system operating on LiBr–H₂O solution. The exergy destruction in each component, the coefficient of performance (COP) and the exergetic COP of the system are determined. It is shown that exergy destructions occur significantly in generators, absorbers, evaporator2 and heat exchangers while the exergy destructions in condenser1, evaporator1, throttling valves, and expansion valves are relatively smaller within the range of 1–5%. The results further indicate that with an increase in the generator1 temperature the COP and ECOP increase, but there is a significant reduction in total exergy destruction of the system for the same. On the other hand, the COP and ECOP decrease with an increase in the absorber1 temperature while the total exergy destruction of the system increases significantly with a small increase in the absorber1 temperature. The results show that the exergy method can be used as an effective criterion in designing an irreversible double effect absorption refrigeration system and may be a good tool for the determination of the optimum working conditions of such systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance of film absorbers under the effect of a contaminant non-absorbable gas

This paper presents a method of calculating the effects of the presence of non-absorbable gases on the performance of the absorber of vapour absorption refrigeration systems. The paper, which uses data for lithium bromide/water system with air as a contaminant, shows the detrimental effects of such gases on the overall effectiveness of the absorber. It is shown that the reduction in performance, due to the presence of a non-absorbable gas, can be offset by either introducing turbulence into the vapour/gas mixture in the absorber or choosing a larger absorber. Calculations of the reduction in COP, due to hydrogen build-up in the absorber of a LiBr/ H₂O system, are also presented.     

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.     

Choice of absorbent-refrigerant mixtures

Twenty-six absorbent—refrigerant combinations, holding good promise as fluid systems, have been considered for single stage absorption air conditioning system. These fluids have been compared on the basis of solution characteristics, life expectancy characteristics and refrigeration cycle characteristics. The mass flow rates of rich and poor solutions per ton of refrigeration capacity and the coefficient of performance (CP) were compared for an evaporator temperature of 5°C, absorber and condenser temperatures of 35°C and a generator temperature of 120°C (low grade energy sources). More than half of the waste energy available in industry happens to be at a temperatures below 200°C. Other types of low grade thermal energy such as solar energy and geothermal energy can be used in operating vapour absorption refrigeration and air-conditioning systems.     

An air-cooled solar collector using all-cylindrical elements in a low-loss body

A novel IX air-cooled solar collector has a phenolic polymer structural foam body, a black painted fiberglass mat absorber and a coplanar, parallel, close-packed array of evacuated cylindrical glass tubes for a cover. Modeling suggests, and measurements confirm, high efficiency performance as well as a substantial dispersion of performance characteristic arranged so that the specific temperature difference increases as the radiant intensity decreases. This dispersion suggests greatly improved cloudy weather performance. In a steady state simulation at full sun under northern midwinter conditions this collector can deliver a minimum of 1.9 times more heat per day to Glauber's Salt storage than can a IX double glazed water-cooled collector with a non-selective black flat absorber plate. Under cloudy conditions this advantage ratio rises to 12.1 at sun. Effective heat collection continues at intensities as low as sun. Under summertime conditions the collector can operate an absorption air conditioner at sun.