A combined double-way chemisorption refrigeration cycle based on adsorption and resorption processes

An innovative combined double-way chemisorption refrigeration cycle based on adsorption and resorption processes is presented. Two different reactive salts were used as sorbents and ammonia was utilized as the refrigerant in the proposed cycle. The useful cold was obtained from the evaporation heat of the refrigerant during the adsorption process and from the reaction heat of the low-temperature salt during the resorption process. The proposed combined double-way cycle has a distinct advantage of higher coefficient of performance (COP) in comparison with conventional adsorption cycle or resorption cycle. Experimental verification indicated that the advanced combined double-way cycle is feasible for refrigeration application, and the ideal COP of the basic cycle was about 1.24. Theoretical results showed that the proposed combined double-way cycle could improve COP by 167% and 60% when compared with conventional adsorption cycle and resorption cycle, respectively.     

Thermodynamic study of a combined double-way solid–gas thermochemical sorption refrigeration cycle

A combined double-way thermochemical sorption refrigeration thermodynamic cycle was proposed and tested. Both adsorption refrigeration and resorption refrigeration processes were combined in order to improve the system performance. Two different consolidated composite materials were used as the reactive sorbents and ammonia was used as the refrigerant. Experimental results showed that a system operating with such proposed cycle can have two useful cold productions during one cycle at the expense of only one heat input at high temperature. The average specific cooling power (SCP) during the adsorption refrigeration phase was 301 W kg^?1. Analysis of the experimental data showed that the driving equilibrium drop during the resorption process was much lower than that during the adsorption process, when the cold production temperature was similar. The proposed combined double-way sorption cycle has a larger cooling capacity per unit of heat input and the maximum theoretical coefficient of performance (COP) is 1.24 when MnCl₂ and BaCl₂ are used as the reactive sorbents.     

双效双重热化学吸附制冷性能实验研究

建立了基于吸附-再吸附原理和内部回热技术的双效双重热化学吸附制冷实验系统,对其可行性及工作性能进行了实验研究。测试结果表明:双效双重热化学吸附制冷热力循环技术用于制冷空调领域是完全可行的,在每次循环过程中由外界热源输入一次高温解吸热可实现四次冷量输出;当采用NiCl2为高温盐吸附剂、MnCl2为中温盐吸附剂、BaCl2为低温盐吸附剂、NH3为制冷剂时,在加热温度为265℃、制冷温度为15℃、冷却温度为30℃的工况下,双效双重热化学吸附制冷循环的COP达到1.1。在此基础上分析了吸附制冷阶段和再吸附制冷阶段冷量输出过程的制冷功率变化特性,发现再吸附过程吸附反应强于吸附反应。     

Numerical study of a novel cascading adsorption cycle

A novel cascading adsorption cooling cycle for refrigeration purposes is proposed in this paper. This cycle consists of two zeolite adsorbent beds and a silica gel adsorbent bed. The working refrigerant for the three adsorbers is water. The zeolite adsorbent bed is configured as the high temperature stage while the silica gel adsorbent bed acts as the low temperature stage. Both heat and mass recovery are carried out between the two zeolite adsorbent beds. In addition, heat is also exchanged between the zeolite adsorbent and the silica gel adsorbent beds. A lumped model is assumed for this cascading cycle. The COP for the base case is found to be 1.35, which is much higher than the COP of an intermittent cycle (about 0.5) and a two-bed combined heat and mass recovery cycle (about 0.8). However, its specific cooling power (SCP) of 42.7 W/kg is much lower than that of the intermittent cycle. The numerical results indicate that an optimal middle temperature exists for a prescribed driven temperature. The optimal COP increases with an increase in the driven temperature. However, when the driven temperature increases beyond 503 K, there is negligible change in the COP.     

Performance evaluation of combined adsorption refrigeration cycles

This paper presents the results of an investigation on the performance of combined adsorption refrigeration cycles. The novel combined cycle amalgamates the activated carbon (AC)-R507A as the bottoming cycle and AC-R134a cycle as the topping cycle and deliver refrigeration load at as low as −10 °C at the bottoming cycle. The cycle simulation is based on the experimentally confirmed adsorption isotherms, kinetics and isosteric heat of adsorption data for R134a and R507A on highly porous based activated carbon of type Maxsorb III. The optimum cooling capacity, coefficient of performance (COP) and chiller efficiency are calculated in terms of cycle time, switching time, regeneration and brine inlet temperatures. Results show that the combined adsorption cycles are feasible even when low-temperature heat source is available.     

Split heat pipe type compound adsorption ice making test unit for fishing boats

In order to settle the problem of the corrosion between sea water and the steel adsorber for ammonia system, a split heat pipe type adsorption ice making test unit, which use compound adsorbent of CaCl₂ and activated carbon to improve the adsorption performance, is designed and constructed. For this test unit there is mass recovery function between two beds and the CaCl₂ in compound adsorbent per bed is 1.88 kg, and there is only one pump for the whole heating and cooling phase for adsorbers. Performances of this system are tested; the lowest evaporating temperature is as low as −42 °C. At the evaporating temperature of −35 and −25 °C, the cooling powers are 0.89 and 1.18 kW, respectively. At the evaporating temperature of −15 °C, its average cooling power is 1.37 kW, which corresponds coefficient of performance of refrigeration COP=0.41 and specific cooling power per kilogram CaCl₂ of each adsorber SCP=731 W kg⁻¹. The mass recovery process has improved SCP and COP for the system by 15.5 and 24.1%, respectively. Heat transfer performance is also improved by the split heat pipe construction; the average heat transfer coefficient for a whole cycle is 155.8 W m⁻² °C⁻¹.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

An ammonia-water absorption refrigerator with a large temperature lift for combined heating and cooling

The COP (Coefficient of Performance) of an ammonia-water absorption refrigerator can be significantly improved by incorporating a mixing column and a second absorber which is combined with a two-stage refrigerant expansion. At an internal temperature lift of 85 K (−20°C to + 65°C), these modifications result in a 50% higher COP for cooling—assuming ideal components. With real components, a 40% improvement may be achieved. The proposed cycle was realised in a laboratory test plant with a refrigeration capacity of 10 kW at −20°C evaporator temperature. This machine was operated at a solution temperature up to 225°C and a condenser temperature of 65°C corresponding to a pressure up to 3.0 MPa. COP versus temperature lift and load behaviour was tested. A COP of 0.38 was achieved at a temperature lift of 85 K. An efficient high-temperature lift cycle like that described in this paper may find applications for deep freezing, as a topping cycle to achieve triple-effect performance, or as a device to produce simultaneous heating and cooling.     

Attempt of integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator: Experience and results

The integration of a small commercial ammonia-water absorption refrigerator with a solar concentrator as heat source was analyzed theoretically and experimentally. Operation parameters of the fridge were determined by parametric fitting experimental data using a thermodynamic model of the cycle leading to a working pressure of 25 bar, weak and strong solution concentrations of 15% and 30% respectively and a COP of ∼0.18. As sold, the fridge can operate with a reduction in the electric power supply of up to 40%. Results showed that the Parabolic Concentrator (PC) used can deliver temperatures above 200 °C for almost 6 h with an average useful power delivery and efficiency of 530 W and 26% respectively. During laboratory test runs it was possible to hold the refrigeration cycle running in a very unstable regime despite the inappropriate heat distribution in the generator unit due to the lower temperature heat source used (220 °C). However, outdoor testing with the sun as energy source proved unsuccessful due to the higher heat losses that occur at the pipes connecting the PC with the fridge.     

Two-stage heat pump system with vapor-injected scroll compressor using R410A as a refrigerant

Refrigerant vapor-injection technique has been well justified to improve the performance of systems in refrigeration applications. However, it has not received much attention for air conditioning applications, particularly for air conditioning in hot climates and for heat pumping in cold climates. In this study, the performance of an 11 kW R410A heat pump system with a two-stage vapor-injected scroll compressor was experimentally investigated. The vapor-injected scroll compressor was tested with the cycle options of both flash tank and internal heat exchanger configurations. A cooling capacity gain of around 14% with 4% COP improvement at the ambient temperature of 46.1 °C and about 30% heating capacity improvement with 20% COP gain at the ambient temperature of −17.8 °C were found for the vapor-injected R410A heat pump system as compared to the conventional system which has the same compressor displacement volume.     

Permanent magnet magnetic refrigerator design and experimental characterization

Magnetic refrigeration (MR) using permanent magnets is being investigated for near-room temperature applications. An MR prototype is described and results using gadolinium as refrigerant are presented. Important design features are simple sealing, compactness, high operating frequencies, and ease of use. Using a total of 110 g of refrigerant, the device produces a maximum temperature span under no thermal load of 29 °C, and 10 °C under 50 W. The overall COP determined by using the power to the drive motor is between 0.3 and 0.8 under most operating conditions and temperature spans. The maximum COP measured is 1.6 with a span of 2.5 °C when operating at 1.4 Hz and utilization of 1.28. If the inefficiency of the motor is removed the maximum COP is 2.2, and, when the magnet drive losses are excluded, the maximum COP becomes 10. The peak specific exergetic cooling power is 0.085 W T⁻¹ cm⁻³.     

Forced convection adsorption cycle with packed bed heat regeneration: Cycle à adsorption à convection forcée avec régénération thermique du lit fixe

The convective thermal wave is part of a patented cycle which uses heat transfer intensification to achieve both high efficiency and small size from a solid adsorption cycle. Such cycles normally suffer from low power density because of poor heat transfer through the adsorbent bed. Rather than attempting to heat the bed directly, it is possible to heat the refrigerant gas outside the bed and to circulate it through the bed in order to heat the sorbent. The high surface area of the grains leads to very effective heat transfer with only low levels of parasitic power needed for pumping. The new cycle presented here also utilises a packed bed of inert material to store heat between the adsorption and desorption phases of the cycle. The high degree of regeneration possible leads to good coefficients of performance (COPs). Thermodynamic modelling, based on measured heat transfer data, predicts a COP (for a specific carbon) of 0.90 when evaporating at 5°C and condensing at 40°C, with a generating temperature of 200°C and a modest system regenerator effectiveness of 0.8. Further improvement is possible. Experimental heat transfer measurements and cycle simulations are presented which show the potential of the concept to provide the basis of a gas-fired air conditioner in the range 10–100 kW cooling. A research project to build a 10-kW water chiller is underway. The laboratory system, which should be operational by June 1997, is described.     

Performance improvement and comparison of mass recovery in CaCl2/activated carbon adsorption refrigerator and silica gel/LiCl adsorption chiller driven by low grade waste heat

One CaCl₂/activated carbon-ammonia adsorption refrigerator and one silica gel/LiCl-methanol chiller was designed and tested. The comparison of performance improvement of mass recovery process on the two adsorption systems was studied. The results show that the COP (coefficient of performance) and SCP (specific of cooling power) can be improved by 15.4% and 10.5% by mass recovery process in silica gel/LiCl-methanol chiller, while they can be improved by 53.8% and 51.5% in CaCl₂/acitvated carbon-ammonia refrigerator, because the latter has larger pressure difference between the hot and cold bed. Both the CaCl₂/acitvated carbon-ammonia refrigerator and the silica gel/LiCl-methanol chiller can provide continuous and stable cooling capacity. Under nominal working condition, the evaporator temperature, COP and SCP can reach −21 °C, 0.26 and 474 W kg⁻¹ in CaCl₂/activated carbon-ammonia refrigerator, and they are 15 °C, 0.41 and 244 W kg⁻¹ in the silica gel/LiCl-methanol chiller.     

Mass recovery adsorption refrigeration cycle—improving cooling capacity

The study investigates the performance of two-bed, silica gel-water adsorption refrigeration cycle with mass recovery process. The cycle with mass recovery can be driven by the relatively low temperature heat source. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The chiller with mass recovery process utilizes the pressure difference to enhance the refrigerant mass circulation. Cooling capacity and coefficient of performance (COP) were calculated by cycle simulation computer program to analyze the influences of operating conditions. The mass recovery cycle was compared with conventional cycle such as the single stage adsorption cycle in terms of cooling capacity and COP. The results show that the cooling capacity of mass recovery cycle is superior to that of conventional cycle and the mass recovery process is more effective for low regenerating temperature.     

A combined-cycle refrigeration system using ejector-cooling cycle as the bottom cycle

A combined-cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector-cooling cycle (EJC) is proposed and studied. The EJC is driven by the waste heat from the RAC/MC and acts as the bottom cycle of the RAC/MC. A system analysis shows that the COP of a CCRS is significantly higher than a single-stage refrigeration system. Improvement in COP can be as high as 18.4% for evaporating temperature of the RAC/MC T_e at −5°C. A prototype of the CCRS was built and tested in the present study. Experimental results show that at T_e=−4.5°C, COP is improved by 14% for a CCRS. For T_e at 5°C, COP can be improved by 24% for a CCRS with higher condensing temperature of the RAC/MC. The present study shows that the CCRS using the ejector-cooling cycle as the bottom cycle of the RAC/MC is viable. Further improvement in COP is possible since the prototype is not designed and operated at an optimal condition.     

Application of an ejector in autocascade refrigeration cycle for the performance improvement

This paper presented a novel autocascade refrigeration cycle (NARC) with an ejector. In the NARC, the ejector is used to recover some available work to increase the compressor suction pressure. The NARC enables the compressor to operate at lower pressure ratio, which in turn improves the cycle performance. Theoretical computation model based on the constant pressure-mixing model for the ejector is used to perform a thermodynamic cycle analysis for the NARC with the refrigerant mixture of R23/R134a. The effects of some main parameters on cycle performance were investigated. The results show the NARC has an outstanding merit in decreasing the pressure ratio of compressor as well as increasing the COP. For NARC operated at the condenser outlet temperature of 40 °C, the evaporator inlet temperature of −40.3 °C, and the mass fraction of R23 is 0.15, the pressure ratio of the ejector reaches to 1.35, the pressure ratio of compressor is reduced by 25.8% and the COP is improved by 19.1% over the conventional autocascade refrigeration cycle.     

Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling

CO₂ transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO₂ has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO₂ transcritical refrigeration plant, experimentally. It tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO₂ transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO₂ cycle (0 and −10 °C) and three heat rejection temperatures (24, 30 and 40 °C). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7% and 30.3% on COP.     

Experimental studies on air-cooled NH3-H2O based modified gax absorption cooling system

An experimental investigation on the performance of an air-cooled modified generator absorber heat exchange (GAX) absorption cooling system has been carried out and presented in this paper. The conventional system is modified by incorporating high pressure GAX, low pressure GAX, a solution cooler and an additional solution heat exchanger to reduce the heat input to the system. The system is designed for a cooling capacity of 10.5 kW using ammonia-water (NH₃-H₂O) as the working fluid. The performance of the system in terms of the circulation ratio, internal heat recovery and coefficient of performance (COP) has been obtained. The system is capable of producing a low evaporator temperature of −5 °C, at a sink temperature of 35 °C, under no load conditions. The results indicate that at a generator and evaporator temperature of 120 °C and 2 °C respectively, the system delivers a maximum cooling capacity of about 9.5 kW with a fuel and total COP of 0.61 and 0.57 respectively.     

氯化钙-氨吸附式制冷的实验研究

在以氯化钙-氨为工质对的两床吸附式制冷系统上进行了实验研究,得出了不同热源温度下,以氯化钙-氨为工质对的连续循环制冷系统的制冷量、性能参数COP随时间的变化关系。所得结果可为进一步的实验研究和工程设计提供指导。     

Research on performance of regenerative room temperature magnetic refrigeration cycle

On the basis of classical Langevin theory along with statistical mechanics, thermodynamics and magnetism, a new expression of magnetocaloric parameters used for room temperature magnetic refrigeration is proposed, which is briefer and more accurate than the existing one, providing a new way for studying performance of regenerative room temperature magnetic Ericsson refrigeration cycle. Influences of temperature of heat reservoirs and magnetic intensity on cycle refrigeration capacity and coefficient of performance are analyzed. The results show that the maximal temperature span of the cycle increases but its increasing rate decreases with the increase of magnetic field strength. In addition, there exists only one maximum value of effective refrigerating capacity. Two cycles with the same COP can reach a same temperature span under a certain magnetic field strength. A large magnetic field strength can improve COP but the increase rate of COP decreases.