Fundamental thermodynamics of ideal absorption cycles

Starting from the representation of a real absorption refrigeration cycle on a temperature-entropy diagram, step-by-step idealisations of the binary mixture, together with the thermodynamic transformations are considered, in order to derive the ideal thermodynamic absorption cycle performance and temperature formulae. It is demonstrated that the ideal absorption cycle is the combination of a Carnot driving cycle with a reverse Carnot cooling cycle. The resorption cycle is analysed in the same manner. Information is included on absorption cooling with heat recovery cycles, heat pumps and temperature amplifiers. From the analysis of single-stage cycles, and by superimposing absorption cycles operating at different temperatures and utilising specific residual heat of the higher temperature sub-cycles, the performance and temperature relations of double, triple and multistage cycles are derived. Special attention is given to three types of triple-stage cycle and their ideal equivalence is demonstrated and represted on the pressure-temperature-concentration (PTX) diagram. A simple hybrid absorption-compression cycle is analysed and the results are compared with those of ideal cold generation cycles (combinations of driving and cooling cycles). Consideration is also given to cold generation systems. Finally, the validation of the fundamental thermodynamics of absorption cycles is presented by applying an exergy analysis. This paper presents the thermodynamic principles involved to obtain simple formulae, in a similar way to the Carnot cycle in order to convey the ideal theoretical limitations.     

General method for the evaluation of complex heat pump cycles

Whenever the fractional temperature lift ΔT/T_c of a heat pump is 0.15, simple cycles with one-stage throttling exhibit unsatisfactory energy performance. The adoption of multi-stage throttling, both in non-regenerative and regenerative cycles, is the most direct way of improving the cycle coefficient of performance (COP). The performance of these complex cycles is found to be a function of the molecular complexity of the working fluid, the reduced evaporation temperature, the fractional temperature lift and the number of stages of throttling. Furthermore, complex cycles are shown to be equivalent to a combination of simple cycles and their performance may be directly inferred by this route. Such calculations show that for a given fractional temperature lift an optimum molecular complexity (between that of R12 and n-butane) exists. Fluids with simpler molecules exhibit excessive vapour superheating during compression, and those with more complex molecules have excessive throttling losses. Also, with complex cycles, regeneration should be applied only to the cycle at the lowest temperature in order to improve the cycle COP and to prevent condensation during compression. As a general trend, however, complex cycles suffer a significant loss in performance compared to optimized simple cycles due to the adverse area of the two-phase diagram in which they work.     

Performance of compression/absorption hybrid refrigeration cycle with propane/mineral oil combination

This paper discusses the feasibility of a vapor compression/absorption hybrid refrigeration cycle for energy saving and utilization of waste heat. The cycle employs propane as a natural refrigerant and a refrigeration oil as an absorbent. A prototype of the cycle is constructed, in which a compressor and an absorption unit are combined in series. The performance of the cycle is examined both theoretically and experimentally. Although the solubility of the propane with the oil is not enough as a working pair in the absorption unit, the theoretical calculation shows that the hybrid cycle has a potential to achieve a higher performance in comparison with a simple vapor compression cycle by using the waste heat. In the experiment, the prototype cycle is operated successfully and it is found that an improvement of an absorber is necessary to achieve the good performance close to the theoretical one. The application of an AHE (absorber heat exchanger) can reduce the heat input to a generator. Further examinations on some other combinations of refrigerant/refrigeration oil and additives are desirable.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

Twin screw oil-free wet compressor for compression–absorption cycle

The performance of a twin screw compressor operating under wet (two-phase) compression conditions in an ammonia–water compression absorption heat pump cycle is investigated both theoretically and experimentally. The paper reports on the influence of the location of liquid intake or, depending what applies, injection angle and mass flow rate of the injected liquid on compressor performance. Labyrinth seals separate the oil-free process side from oil lubricated bearing housing. Labyrinth seals leakage is modelled and its impact on performance is theoretically and experimentally investigated. The need for liquid injection from the discharge side to obtain acceptable performance is discussed based on experimental results.     

The CNPM thermal heat pump. Part 1 — general description and thermodynamic analysis

A research programme, funded by CNR (National Research Council), has been undertaken by CNPM since 1973. The aim of the programme is the construction and testing of a prototype thermal heat pump. The most significant component is an organic Rankine cycle engine, driving the compressor of a heat pump. Since the heat rejected by the engine is supplied to the user — water for domestic heating — the whole system performs as a ‘heat multiplier’, converting the high temperature heat given to the engine into a larger amount of low temperature heat, to be used for domestic heating.In this paper, the selection criteria for the working fluid — a completely fluoro-substituted hydrocarbon — and the main thermodynamic data of both power and heat pump cycles, are discussed; the finally adopted plant configuration is described, with particular emphasis on the influence exterted by the working fluid nature on the heat exchangers and turbo-machinery dimensions and performance. A discussion on the merits of the single fluid solution (ie the same working fluid in the power and the heat pump cycle) and dual fluid solution is also carried out. The feasibility of a low-temperature heat distribution, based on compact-surface, natural-draft convectors, with the relevant advantages on the Rankine and heat-pump cycles, is also investigated.Finally, the expected overal; system performance is given, both at design and part-load conditions. As a premium for the rather complex but efficient thermodynamicscv of the system, significant energy savings are obtained in all situations.     

Fuzzy control of the compressor speed in a refrigeration plant

In this paper, referring to a vapor compression refrigeration plant subjected to a commercially available cold store, a control algorithm, based on the fuzzy logic and able to select the most suitable compressor speed in function of the cold store air temperature, is presented. The main aim is to evaluate the energy saving obtainable when the fuzzy algorithm, which continuously regulates the compressor speed by an inverter, is employed to control the compressor refrigeration capacity instead of the classical thermostatic control, which imposes on/off cycles on the compressor that works at the nominal frequency of 50 Hz. The variation of the reciprocating compressor speed is obtained by controlling the compressor electric motor supply current frequency in the range 30–50 Hz, as it is not possible to consider values smaller than 30 Hz because of the lubrication troubles due to the splash system. In this range, two among the most suitable working fluids proposed for the R22 substitution, such as the R407C (R32/R125/R134a 23/25/52% in mass) and the R507 (R125/R143A 50/50% in mass) are tested. Comparing the compressor speed fuzzy control with the classical thermostatic control, frequently used in the cold stores and in other refrigeration systems, the experimental results show a meaningful energy saving equal even to about 13% when the R407C is used as a working fluid. In particular, to explain from the energy saving point of view the best performances of the refrigeration plant when the compressor speed varies, an exergetic analysis is realized. Besides, with regard to the inverter cost, the pay-back period determined is more than acceptable for the plant size examined.     

Mapping of optimum operating condition for LiBr–water refrigeration cycles

In this work, optimum operating condition maps are generated covering wide ranges of refrigeration and sink temperatures for single- and double-effect LiBr–water vapour absorption refrigeration cycle. These optimum condition maps will be useful to choose optimum operating conditions while designing LiBr–water cycle for desired applications. Methodology for generating such maps is discussed in detail, which can also be used for other absorption refrigeration cycles with various working fluids. Three configurations of LiBr–water absorption refrigeration cycles, single effect, double-effect series flow and double-effect parallel flow, are analysed with the most accurate thermodynamic property correlation available in the literature. Sensitivity of cycle performance to various operating variables such as generator, absorber and condenser temperatures is determined. Second law analysis shows that when a higher temperature heat source is available, double-effect cycles are more effective over single effect as they have higher coefficient of performance.     

Coefficient of performance of multistage absorption cycles

A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H₂O/LiBr and NH₃/H₂O.

Résumé

A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H₂O/LiBr and NH₃/H₂O.     

吸收式制冷(热泵)循环流程研究进展

吸收式制冷作为最早的人工制冷方法,诞生至今已有200多年。在民用和工业中的实际应用有60多年。近20余年来,吸收式制冷在理论与应用等方面都取得了迅速发展,并在制冷机市场上占有相当的份额,得到国内外厂商和学者的广泛关注与研究。随着人类能源消耗量的不断增加,需要进一步深入研究新能源、分布式能源及能源的高效利用。余热、废热、可再生的太阳能、地热能等的利用使得热能驱动的吸收式制冷(热泵)技术得到越来越多的关注。与采用电驱动蒸气机械压缩式制冷(热泵)系统不同,吸收式制冷(热泵)技术可利用采用低品位热源的热能直接驱动,运行成本远低于电驱动系统。吸收式系统多采用H2O-LiB r溶液、NH3-H2O溶液等自然工质作为制冷剂,具有环境友好特性,同时具有安全、可无噪音运行、可靠性高等显著优点。但也具有占地面积大、初投资高,冷却负荷高,一次能源效率低(直燃形式)等不足。针对这些特性,现阶段的主要研究方向包括:循环设计优化、工质对选择、系统部件热质传递强化、系统控制策略优化等。狭义的吸收式循环是指闭式、溶液吸收制冷剂蒸气的吸收式制冷(热泵)循环。该类循环按照循环形式分类包括单吸收循环、多吸收循环和复合循环。单吸收循环主要包括基本单效吸收循环、扩散吸收循环、膜吸收循环、热变换器循环、重力驱动的阀切换循环以及自复叠循环;多吸收循环主要包括再吸收循环、多效循环、中间效循环、多级循环、中间级循环以及GAX循环;复合循环主要包括喷射-吸收复合、压缩-吸收复合和膨胀-吸收复合等复合形式。现有吸收式制冷技术研究热点主要包括且不局限于太阳能、中低温余热利用、冷热电联产、储能(蓄冷、蓄热),膜交换材料、高温下耐腐蚀材料,塑料热交换器等方面。吸收式循环现有循环结构的提出针对的是一定温度和浓度下循环,面对新的应用场景、新材料以及新吸收工质对,吸收式循环可以提出多种更高效、更宽热源驱动温度范围和溶液浓度范围的新循环。     

Novel partial admission radial compressor for CO2 applications

A novel partial admission turbo compressor concept is proposed as an alternative to a conventional radial oil-free CO₂ compressor. The concept aims at the improvement of the overall performance through the reduction of the non-stage windage and cooling losses enabled by compression at significantly reduced shaft speeds. Transient CFD analysis gives fairly optimistic prediction of more than 80% of base stage efficiency at around 1.4 total pressure ratio. The study shows potential for efficiency improvement by optimization of the shape and number of blades. The conceptual compressor may be an interesting alternative for commercial CO₂ applications operating at close to critical pressures, provided that the deceleration of the gas in the diffuser is efficient.     

Supercritical heat pump cycles

Supercritical heat-pump cycles suited for high-temperature heat generation and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evaluated. The main variables governing the energy efficiency of the process and the temperatures of the heat produced are recognized to be the fluid critical temperature, the molecular complexity, the top cycle pressure and the amount of internal regeneration of heat. Two cycle configurations are examined: one featuring fluid compression after a regenerative preheating and one that also includes turbine expansion of a fraction of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics of a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF₆, C₃F₈, C₂HF₅, c-C₄F₈), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is given. An example application in which a conventional high-temperature cycle is compared with two supercritical solutions is presented. The following conclusions summarize the findings of the thermodynamic analysis. (1) In supercritical cycles high heat-output temperatures are achievable with moderate compressor pressure ratios and with a comparatively simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pressures, which may be required in conventional cycles, can be avoided. (2) As heat is available in supercritical cycles within a certain temperature range, applications implying the use of heat at variable temperature could benefit from the natural matching between temperature availability and process requirements. (3) The comparatively high pressures at which heat is produced in supercritical cycles could represent a drawback for small-capacity plants but are probably acceptable or even beneficial for large systems. (4) The internal regeneration of a sizeable amount of heat, which is requested in supercritical cycles, represents a definite cost item for this type of heat pump.     

The commercial feasibility of the use of water vapor as a refrigerant

This paper investigates the economic feasibility of a water-based vapor compression chiller with a nominal capacity of 3520 kW (1000 ton). Simplified models of potential cycle configurations are developed and used as a screening tool to identify a baseline cycle, the most attractive configuration for a water-based refrigeration machine. More detailed component-level models are developed to accurately size equipment and predict both the performance and cost of the baseline chiller. These component models address issues that are particularly crucial when water is used in refrigeration cycles, such as compression ratio, compressor discharge superheat and refrigerant-side pressure drop. Where possible, these component models are verified through comparison against the current state-of-the-art technology for large chillers that use R-134a as the refrigerant. The capital cost and expected operating costs are determined in order to quantify the payback and life-cycle costs associated with using water as a refrigerant, relative to traditional halocarbon refrigerants currently in use. Other issues that may have an economic impact on the feasibility of water as a viable alternative to traditional synthetic refrigerants are discussed, including purging and condensation within the compressor.The results show that water-based vapor compression refrigeration systems will not be economically attractive without substantial and successful efforts to develop low-cost, high capacity compressors. The paper provides an indication of the cost targets that must be met in order to make water vapor refrigeration systems practical.     

Optimization study of the compression/absorption cycle

For each external situation optimum working conditions for the compression/absorption cycle can be found. The improvement in cycle performance which is gained by optimizing the temperature gradient in the absorber is considerable, particularly for situations with small external temperature gradients. Theoretically, the external and internal temperature gradients should be equal to maximize the cycle performance. The introduction of a solution loop, however, changes this and the optimum internal temperature gradient is always larger than the external gradients. The optimum point of operation is found by studying the changes in the compressor and pump and the heat loss obtained in the solution heat exchanger with the working conditions. A comparison of a compression/absorption cycle, using NH₃-H₂O, and a compression cycle working with pure R12, always results in a higher coefficient of performance for the former. The capacity of the NH₃-H₂O system is also considerably higher.     

Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagés

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

Evaluation of absorption cycles with respect to COP and economics

The calculation of the performance of absorption heat pump cycles or the comparison of different types of machine cannot be done in a reasonable way without considering the first cost of the machine and especially the cost of the heat exchangers, as their area and particularly the distribution of the area between the respective components of the heat pump determine the COP. Therefore it makes sense only to compare machines that are optimized in this respect. A good way to evaluate cycles is to calculate the maximum COP in terms of the total cost of the heat exchangers. For that purpose a computer program was developed for different absorption heat pump cycles with water as refrigerant. The calculation method is simple and thus the result reliable. The program is suitable for evaluating double-lift, single-, double- and some triple-effect cycles, each one with different absorption fluids and with different options, such as different solution flows (parallel, serial), different types of absorber (spray- or falling-film absorber), and different types of generator (pool- or falling-film generator). With this instrument different cycles or similar cycles with different features can be compared. An economically significant estimation of the performance of a cycle working under defined conditions is possible.     

无油涡旋压缩机的关键技术综述及其发展展望

首先对国内外无油涡旋压缩机的发展历史及现状进行了全面的概述,尔后介绍无油涡旋压缩机的工作原理及组成,对无油涡旋压缩机的密封与润滑、冷却系统及其防自转等关键技术作出了概述。为解决无油涡旋压缩机的关键技术问题做出了系统的归纳并提出相关关键技术的延伸拓展。通过对无油涡旋压缩机的综合分析,详细的展望了无油涡旋压缩机的发展前景,为拓宽无油涡旋压缩机的应用领域提出了见解,指出了质子交换膜燃料电池技术与无油涡旋压缩机相辅相成的发展关系。     

Development of one-dimensional model for initial design and evaluation of oil-free Co2 turbo-compressor

A 1-dimenional tool for preliminary design and performance prediction of oil-free CO₂ compressor is presented. The model describes high speed centrifugal compressor in a hermetic configuration supported on foil gas bearings. To give possibly comprehensive overview of the technology, a wide range of loss mechanisms is considered. The model predicts aerodynamic performance of the compressor as well as losses related to the windage of rotor and bearings and due to the internal cooling. Numerical investigation of different compressor stages was used to validate aerodynamic predictions of the 1D model. Maximal prediction discrepancy amounted 2% for efficiency and 5% for pressure ratio. The prediction of the total compressor efficiency was compared with test data from a 50 kW compressor published Sandia Laboratories. The predicted peak compressor efficiencies are between 66 and 67.5% while experimentally measured values are within 65–70% region.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 1: System design and modeling

The advanced energy storage technology proposed and patented by authors can be applied for cooling, heating, dehumidifying, combined cooling and heating, and so on. It is also called the variable mass energy transformation and storage (VMETS) technology in which the masses in one or two storage tanks change continuously during the energy charging and discharging processes. This paper presents an advanced energy storage system using aqueous lithium bromide (H₂O–LiBr) as working fluid. As one of VMETS systems, this system is a closed system using two storage tanks. It is used to shift electrical load and store energy for cooling, heating or combined cooling and heating. It is environmental friendly because the water is used as refrigerant in the system. Its working principle and process of energy transformation and storage are totally different from those of the traditional thermal energy storage (TES) systems. The electric energy in off-peak time is mostly transformed into the chemical potential of the working fluid and stored in the system firstly. And then the potential is transformed into cold or heat energy by absorption refrigeration or heat pump mode when the consumers need the cold or heat energy. The key to the system is to regulate the chemical potential by controlling the absorbent (LiBr) mass fraction or concentration in the working fluid with respect to time. As a result, by using a solution storage tank and a water storage tank, the energy transformation and storage can be carried out at the desirable time to shift electric load efficiently. Since the concentration of the working solution in the VMETS cycle varies continuously, the working process of the VMETS system is dynamic. As the first part of our study, the working principle and flow of the VMETS system were introduced first, and then the system dynamic models were developed. To investigate the system characteristics and performances under full-storage and partial-storage strategies, the numerical simulation will be performed in the subsequent paper. The simulation results will be very helpful for guiding the actual system and device design.