Evaluation of a combined ejector–vapour-compression refrigeration system

A new refrigeration cycle based on the combination of an ejector cycle with a vapour compression cycle is described. This integration maximizes the performance of the conventional ejector cycles and provides high COP for refrigeration. The analyses show that the new cycle has a significant increase in system performance over the conventional systems, its COP values are competitive to the absorption machines. If the system is powered by waste heat and the cost of its supply can be neglected, the COP values will be much higher. The system performance can be further improved if dual refrigerants are used and the dual refrigerants giving high performance are identified. © 1998 John Wiley & Sons, Ltd.     

Temperature–enthalpy/entropy charts of combined ejector–absorption cycle using NH3–H2O

This paper describes the performance of an ammonia–water combine ejector–absorption cycle as refrigerator and heat pump. This combination brings together the advantages of absorption and ejector systems. Also, thermodynamic cycles on the temperature–enthalpy and temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycles are simulated by a suitable method and a corresponding computer code, based on analytic functions describing the behaviour of the binary mixture NH₃–H₂O. It is found that in the case of the refrigerator and heat pump, the theoretical coefficient of performance (COP) or the theoretical heat gain factor (HGF) vary from 1.6 to 90.4 per cent and 0.7 to 37.6 per cent, greater than those of the conventional absorption system, respectively. The operation conditions were: generator temperature (205.5 to 237.1°C), condenser temperature (25.9 to 37.4°C) and evaporator temperature (−8.4 to 5°C). Copyright © 2000 John Wiley & Sons, Ltd.     

Thermodynamic analysis of novel heat pump cycles for drying process with large temperature lift

Heat pump–assisted drying has been recognized a prospective technology to meet the requirement of energy saving. However, large temperature lift will be resulted by the single‐stage heat pump cycle during the high‐temperature drying, especially operating with low ambient temperature for open‐loop drying, which leads to insufficient heat output, high compression ratio, and low coefficient of performance (COP). Two heat pump cycles, namely, multitemperature cascade cycle and combined single‐stage cycle, are proposed to address the above problems in the drying process with large temperature lift in this paper. The effects of varying operation parameters on the heat pump cycles are analyzed to optimize the cycle performance. Afterwards, the above two cycles as well as a conventional cascade cycle, a two‐stage compression cycle, and a single‐stage compression cycle are compared with each other in terms of cycle performance and drying performance under specified drying conditions. It is indicated by the results that the COPs of the multitemperature cascade cycle and combined single‐stage cycle are about 95% and 88% higher than that of the single‐stage compression cycle, respectively. As for the two cascade cycles (ie, conventional cascade cycle and multitemperature cascade cycle), 49% more water evaporation with the same power input can be resulted by the added condenser and evaporator. Among the five analyzed cycles, the multitemperature cascade cycle is the most promising to be used in the retrofitting of the drying equipment with large temperature lift.     

Performance analysis of a waste‐heat‐powered thermodynamic cycle for multieffect refrigeration

A multieffect refrigeration system that is based on a waste‐heat‐driven organic Rankine cycle that could produce refrigeration output of different magnitudes at different levels of temperature is presented. The proposed system is integration of combined ejector–absorption refrigeration cycle and ejector expansion Joule–Thomson (EJT) cooling cycle that can meet the requirements of air‐conditioning, refrigeration, and cryogenic cooling simultaneously at the expense of industrial waste heat. The variation of the parameters that affect the system performance such as industrial waste heat temperature, refrigerant turbine inlet pressure, and the evaporator temperature of ejector refrigeration cycle (ERC) and EJT cycles was examined, respectively. It was found that refrigeration output and thermal efficiency of the multieffect cycle decrease considerably with the increase in industrial waste heat temperature, while its exergy efficiency varies marginally. A thermal efficiency value of 22.5% and exergy efficiency value of 8.6% were obtained at an industrial waste heat temperature of 210°C, a turbine inlet pressure of 1.3 MPa, and ejector evaporator temperature of 268 K. Both refrigeration output and thermal efficiency increase with the increase in turbine inlet pressure and ERC evaporator temperature. Change in EJT cycle evaporator temperature shows a little impact on both thermal and exergy efficiency values of the multieffect cycle. Analysis of the results clearly shows that the proposed cycle has an effective potential for cooling production through exploitation of lost energy from the industry. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermodynamic analysis of the performance of a solar absorption heat transformer at maximum coefficient of performance

It is proven that a solar absorption heat transformer affected by the irreversibility of finite-rate heat transfer may be modelled as an equivalent combined system consisting of a solar collector and an endoreversible absorption heat transformer, the latter being further treated as a combined cycle having an endoreversible heat pump driven by an endoreversible heat engine. The maximum coefficient of performance of the system is determined, based on the linear heat loss model for solar collectors and the general optimum relation for endoreversible absorption heat transformers. The optimality problems concerning the primary performance parameters of the system are discussed. The results obtained here may serve as a good guide for the evaluation of existing real solar absorption heat transformers or provide some theoretical bases for the optimal design of future solar absorption heat transformers. © 1997 by John Wiley & Sons, Ltd.     

Cooling performance and energy saving of a compression–absorption refrigeration system driven by a gas engine

The prototype of combined vapour compression–absorption refrigeration system was set up, where a gas engine drove directly an open screw compressor in a vapour compression refrigeration chiller and waste heat from the gas engine was used to operate absorption refrigeration cycle. The experimental procedure and results showed that the combined refrigeration system was feasible. The cooling capacity of the prototype reached about 589 kW at the Chinese rated conditions of air conditioning (the inlet and outlet temperatures of chilled water are 12 and 7°C, the inlet and outlet temperatures of cooling water are 30 and 35°C, respectively). Primary energy rate (PER) and comparative primary energy saving were used to evaluate energy utilization efficiency of the combined refrigeration system. The calculated results showed that the PER of the prototype was about 1.81 and the prototype saved more than 25% of primary energy compared to a conventional electrically driven vapour compression refrigeration unit. Error analysis showed that the total error of the combined cooling system measurement was about 4.2% in this work. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of an absorption machine driven by the heat recovery on an I.C. reciprocating engine

The present work studies an absorption machine driven by the heat recovery on an internal combustion (i.c.) reciprocating engine. The thermal energy recovered from the i.c. engine exhaust is used to drive a double‐effect water–lithium bromide cycle, while the heat recovered from the cooling jacket of the engine drives a single‐effect water–lithium bromide cycle. The two absorption cycles are integrated into a single unit with a common evaporator and absorber. The absorption unit was first evaluated by a cycle analysis determining the sensitivity to the main boundary conditions and to the internal parameters. Then a specific simulation code of all the different devices of the absorption machine was developed to evaluate the real performance and size of the unit together with its operating condition limits. The absorption machine shows a coefficient of performance around 1, very close to the performance of a traditional double‐effect absorption chiller driven by steam or by a gas burner. The absorption unit could operate with cooling water inlet temperature lower than 35–36°C and refrigerated outlet temperature higher than 3°C. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance trends and heat transfer considerations in an ammonia–water resorption cycle

An experimental test facility was constructed to examine the potential of ammonia–water mixtures as the working fluid in high‐temperature heat pumps. The nature of the working fluid necessitates an alternative design to the conventional vapour compression cycle. The addition of a solution circuit in parallel with the compressor leads to the resorption cycle. The composition of the working fluid can be altered by varying the flow ratio between the compression and solution pump circuits. Changes in the composition of the circulating fluid are accompanied by changes in the dryness fraction at the end of the heat transfer process in the desorber. Higher rates of heat transfer from the source to the working fluid were measured at higher concentrations of ammonia in the circulating fluid, though this was accompanied by lower overall flow rates of the circulating fluid. A 70/30 ammonia/water mass concentration is thought to be the optimum composition of the working fluid due to a combination of temperature glide and circulation ratio. Significant differences were observed in the overall heat transfer coefficient achieved in the two heat exchangers, which may restrict the range of likely applications. Copyright © 2001 John Wiley & Sons, Ltd.     

Thermodynamic study of multistage absorption cycles using low‐temperature heat

The use of low‐temperature heat (between 50 and 90°C) is studied to drive absorption systems in two different applications: refrigeration and heat pump cycles. Double‐ and triple‐stage absorption systems are modelled and simulated, allowing a comparison between the absorbent–refrigerant solutions H₂O–NH₃, LiNO₃–NH₃ and NaSCN–NH₃. The results obtained for the double‐stage cycle show that in the refrigeration cycle the LiNO₃–NH₃ solution operates with a COP of 0.32, the H₂O–NH₃ pair with a COP of 0.29 and the NaSCN–NH₃ solution with a COP of 0.27, when it evaporates at ?15°C, condenses and absorbs refrigerant at 40°C and generates vapour at 90°C. The results are presented for double‐ and triple‐stage absorption systems with evaporation temperatures ranging between ?40 and 0°C and condensation temperatures ranging from 15°C to 45°C. The results obtained for the double‐stage heat pump cycle show that the LiNO₃–NH₃ solution reaches a COP of 1.32, the NaSCN–NH₃ pair a COP of 1.30 and the H₂O–NH₃ mixture a COP of 1.24, when it condenses and absorbs refrigerant at 50°C, evaporates at 0°C and generates vapour at 90°C. For the double‐ and triple‐stage cycles, the results are presented for evaporation temperatures ranging between 0 and 15°C. The minimum temperature required in the generators to operate the refrigeration and heat pump cycles are also presented. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance analysis of an irreversible cogeneration heat pump cycle

An irreversible heat engine-driven vapour compression and absorption heat pump system is considered as a cogeneration cycle. The effects of thermal resistances and internal irreversibilities on the coefficient of performance (COP) of this cogeneration cycle were investigated using finite-time thermodynamic approach. An improved equation for the COP of the system under consideration was obtained. The results obtained here may serve as a good guide for the evaluation of existing real cogeneration heat pumps or provide some theoretical bases for the optimal design of future cogeneration heat pumps. © 1998 John Wiley & Sons, Ltd.     

Parametric analysis for a new combined power and ejector–absorption refrigeration cycle

A new combined power and ejector–absorption refrigeration cycle is proposed, which combines the Rankine cycle and the ejector–absorption refrigeration cycle, and could produce both power output and refrigeration output simultaneously. This combined cycle, which originates from the cycle proposed by authors previously, introduces an ejector between the rectifier and the condenser, and provides a performance improvement without greatly increasing the complexity of the system. A parametric analysis is conducted to evaluate the effects of the key thermodynamic parameters on the cycle performance. It is shown that heat source temperature, condenser temperature, evaporator temperature, turbine inlet pressure, turbine inlet temperature, and basic solution ammonia concentration have significant effects on the net power output, refrigeration output and exergy efficiency of the combined cycle. It is evident that the ejector can improve the performance of the combined cycle proposed by authors previously.     

Preliminary investigation of a transcritical CO2 heat pump driven by a solar‐powered CO2 Rankine cycle

In this paper, a transcritical carbon dioxide heat pump system driven by solar‐owered CO₂ Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar‐powered heat pump cycle using CO₂ as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO₂ heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO₂ heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO₂ flow rate of two sub‐cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental study of operating characteristics of compression/absorption high-temperature hybrid heat pump using waste heat

This research describes the development of a compression/absorption hybrid heat pump system that utilizes a mixture of NH₃ and H₂O as a working fluid. The heat pump cycle is based on a hybrid combination of vapor compression cycle and absorption cycle. The system consists of major components of two-stage compressors, absorbers, and a desorber. There are also auxiliary parts like a desuperheater, solution heat exchangers, a solution pump, a rectifier, and a liquid/vapor separator to support stable operation of the heat pump. This compression/absorption hybrid heat pump provides many advantages of performance over conventional vapor compression heat pumps including a large temperature glide, an improved temperature lift, a flexible operating range, and greater capacity control. These benefits are optimized by changing the composition of the mixture. In this study, the effect of the composition on the operating characteristics of the compression/absorption hybrid heat pump was experimentally observed.     

Thermodynamic and thermoeconomic analyses of an irreversible combined Carnot heat engine system

A combined cycle model which includes the irreversibilities of finite‐rate heat transfer in heat‐exchange processes and heat leak loss of the heat source is used to analyse the performance of a multi‐stage Carnot heat engine system. The efficiency, power output, ecological function and profit of operating the combined system are optimized. The optimally operating region of the combined system is determined. The optimal combined conditions between two adjacent cycles in the combined system are obtained. Moreover, the cycle model is generalized to include the internal irreversibilities of the working fluids so that the results obtained here become more general. Copyright © 2001 John Wiley & Sons, Ltd.     

Performance of ejector heat pumps

An ejector-compression heat pump can use low-grade thermal energy in the neighbourhood of 93.3°C (200°F) to provide space cooling and heating. This paper applies the existing ejector theory to estimate the performance of an ejector heat pump system at various operating conditions. The study includes parametric, sensitivity and off-design analyses of the heat pump performance. The performance enhancement options and desired ejector geometry are also examined. Refrigerants 11, 113 and 114 are three of the halocarbons most suitable for the ejector heat pump system. The estimated coefficients of performance for a simple ejector heat pump are 0.3 for the cooling mode and 1.3 for the heating mode at a sample operating condition in which the refrigerant (R-11) boiling temperature is 93.3°C (200°F), condensing temperature 43.3°C (110°F) and evaporating temperature 10°C (50°F). A 24 per cent performance improvement is predicted for a heat pump with two-stage ejectors and regenerative heat exchangers. The off-design performance is relatively insensitive to the evaporator temperature variations.     

燃气机热泵变负荷特性的试验研究

燃气机热泵是一项高效节能技术，在试验条件下其一次能源利用率PER为1．13～1．79。为了解交负荷时燃气机热泵的性能，通过试验得到了燃气机热泵的发动机负荷特性、发动机余热回收和燃气机热泵的总体特性曲线。结果表明：随着发动机转速的增加，燃气机热泵的COP和PER是下降的，但下降的幅度较为平缓，且保持较高的数值。通过对IPL Vcop值的分析，发现燃气机热泵的IPL Vcop比热泵系统的大，这说明燃气机热泵的部分负荷性能好，可以很好地实现交负荷运行。     

A combined power and ejector refrigeration cycle for low temperature heat sources

A combined power and ejector refrigeration cycle for low temperature heat sources is under investigation in this paper. The proposed cycle combines the organic Rankine cycle and the ejector refrigeration cycle. The ejector is driven by the exhausts from the turbine to produce power and refrigeration simultaneously. A simulation was carried out to analyze the cycle performance using R245fa as the working fluid. A thermal efficiency of 34.1%, an effective efficiency of 18.7% and an exergy efficiency of 56.8% can be obtained at a generating temperature of 395 K, a condensing temperature of 298 K and an evaporating temperature of 280 K. Simulation results show that the proposed cycle has a big potential to produce refrigeration and most exergy losses take place in the ejector.     

Performance analysis of a closed regenerated Brayton heat pump with internal irreversibilities

This paper analyses the performance of a real heat pump plant via methods of entropy generation minimization or finite‐time thermodynamics. The analytical relations between heating load and pressure ratio, and between coefficient of performance (COP) and pressure ratio of real closed regenerated Brayton heat pump cycles coupled to constant‐ and variable‐temperature heat reservoirs are derived. In the analysis, the irreversibilities include heat transfer‐irreversible losses in the hot‐ and cold‐side heat exchangers and the regenerator, the non‐isentropic expansion and compression losses in the compressor and expander, and the pressure drop loss in the pipe and system. The optimal performance characteristics of the cycle may be obtained by optimizing the distribution of heat conductances or heat transfer surface areas among the two heat exchangers and the regenerator, and the matching between working fluid and the heat reservoirs. The influence of the effectiveness of regenerator, the effectiveness of hot‐ and cold‐side heat exchangers, the efficiencies of the expander and compressor, the pressure recovery coefficient and the temperature of the heat reservoirs on the heating load and COP of the cycle are illustrated by numerical examples. Published in 1999 by John Wiley & Sons, Ltd.     

Performance study of a heat pump dryer system for specialty crops—Part 1: Development of a simulation model

This research is concerned with the technology of heat pump assisted drying of specialty crops. A simplified procedure for modelling the performance of a low temperature heat pump dryer was developed. The system modelled consists of a vapour compression heat pump coupled to a continuous cross flow bed dryer. The model takes into account the detailed heat and mass transfer phenomena taking place in the heat pump and dryer circuits. Copyright © 2002 John Wiley & Sons, Ltd.     

Optimization study of combined refrigeration cycles driven by an engine

In order to utilize the waste heat efficiently for a gas engine-driven heat pump running in a cooling mode, this paper studies two combined absorption/compression refrigeration cycles using ammonia and water as the working fluid. By analyzing the operating characteristics of the combined cycles that make efficient use of both the work and the heat output of an engine, this paper puts forward an optimal mathematical model with an objective function of the primary-energy ratio (PER). The model has been calculated for typical cooling applications. Analysis of the results indicates that optimization can make the combined cycle fully achieve the sought-after energy saving advantage. It was also found that the PERs of the combined cycles increase considerably compared with a conventional engine-driven compression cycle working with pure ammonia. The combined cycle, with two solution circuits, is the best.