Study on adsorption refrigeration cycle utilizing activated carbon fibers. Part 1. Adsorption characteristics

Thermal heat driven adsorption systems using natural refrigerants have been focused on the recent energy utilization trend. However, the drawbacks of these adsorption systems are their poor performance in terms of system cooling capacity and coefficient of performance (COP). The objective of this paper is to improve the performance of thermally powered adsorption cooling system by selecting new adsorbent–refrigerant pair. Adsorption capacity of adsorbent–refrigerant pair depends on the thermophysical properties (pore size, pore volume and pore diameter) of adsorbent and isothermal characteristics of the pair. In this paper, the thermophysical properties of two PAN types of activated carbon fibers (FX-400 and KF-1000) are determined from the nitrogen adsorption isotherms. The standard nitrogen gas adsorption/desorption measurements on various adsorbents at liquid nitrogen of temperature 77.3 K were performed. Surface area of each adsorbent was determined by the Brunauer, Emmett and Teller (BET) plot of nitrogen adsorption data. Pore size distribution was measured by the Horvath and Kawazoe (HK) method. As of the adsorption/desorption isotherms, FX-400 shows very small hysteresis when the value of P/P_o exceeds 0.4, while KF-1000 has no hysteresis in the whole range of P/P_o. The adsorption capacity of FX-400 is about 30% higher than that of KF-1000. The adsorption equilibrium data of activated carbon fiber (ACF)-methanol are presented and correlated with simple equations. The adsorption equilibrium data of ACF (KF-1000)-water also presented in order to facilitate comparison with those of ACFs-methanol pair. The results will contribute significantly in designing the adsorber/desorber heat exchanger for thermally driven adsorption cooling system.     

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.     

Study of a novel silica gel–water adsorption chiller. Part II. Experimental study

The prototype of a novel silica gel–water adsorption chiller is built and its performance is tested in detail. The experimental results show that the refrigerating capacity (RC) and COP of the chiller are 7.15 and 0.38 kW, respectively, when the hot water temperature is 84.8 °C, the cooling water temperature is 30.6 °C, and the chilled water outlet temperature is 11.7 °C. The RC will reach 6 kW under the condition of 65 °C hot water temperature, 30.5 °C cooling water temperature and 17.6 °C chilled water temperature. The results confirm that this kind of adsorption chiller is an effective refrigerating machine though its performance is not as fine as the prediction results. Also it is well effectively driven by a low-grade heat source. Therefore, its applications to the low-grade heat source are much attractive.     

Study of a novel silica gel–water adsorption chiller. Part I. Design and performance prediction

A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working condition with hot water temperature of 85 °C, the cooling water temperature of 31 °C and the chilled water inlet temperature of 15 °C. The COP exceeds 0.5 even under a heat source temperature of 65 °C.     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

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.     

Design and experimental study of a silica gel-water adsorption chiller with modular adsorbers

A silica gel-water adsorption chiller driven by low-grade heat is developed. System configuration without any vacuum valves includes two sorption chambers, a 4-valve hot/cooling water coupled circuit and a 4-valve chilled water circuit. Each sorption chamber is composed of one adsorber, one condenser and one evaporator. The design of this chiller, especially the design of modular adsorber, is suitable for low-cost industrial production. Efficient and reliable heat and mass recovery processes are also adopted. This chiller is tested under different conditions and it features the periodic variations of temperatures and cooling power. Through the experimental study, the optimal cooling time, mass recovery time and heat recovery time are 720 s, 40 s and 24 s, respectively. Besides, the obtained cooling power, COP and SCP are 42.8 kW, 0.51 and 125.0 W kg⁻¹, respectively, under typical conditions of 86/30/11 °C hot water inlet/cooling water inlet/chilled water outlet temperatures, respectively.     

Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.     

Multi-bed regenerative adsorption chiller — improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation

A multi-bed regenerative adsorption chiller design is proposed. The concept aims to extract the most enthalpy from the low-grade waste heat before it is purged into the drain. It is also able to minimise the chilled water temperature fluctuation so that downstream temperature smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. The design also avoids a master-and-slave configuration so that materials invested are not under-utilised. Because of the nature of low-grade waste heat utilization, the performance of adsorption chillers is measured in terms of the recovery efficiency, η instead of the conventional COP. For the same waste heat source flowrate and inlet temperature, a four-bed chiller generates 70% more cooling capacity than a typical two-bed chiller. A six-bed chiller in turn generates 40% more than that of a four-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator during start-up is greatly reduced compared with that of a two-bed chiller.     

Use of compound desiccant to develop high performance desiccant cooling system

The paper is aimed to develop a high performance rotary solid desiccant cooling system using a novel compound desiccant wheel (DW). The unique feature of the desiccant wheel is that it can work well under a lower regeneration temperature and have a higher dehumidification capacity due to the contribution of the new compound desiccant materials. Experimental results indicate that the novel desiccant wheel under practical operation can remove more moisture from the process air by about 20–40% over the desiccant wheel employing regular silica gel. A mathematical model that is used to predict the system performance has been validated with the test results. By integrating the desiccant wheel with evaporative cooling, heat recovery and heating for regeneration sections, a solid desiccant cooling system can be formed. Simulation results show that because of the use of the new compound desiccant, the desiccant cooling system can work under much lower regeneration temperature and have a relative high COP, thus low grade thermal energy resources, such as solar energy, waste heat, etc., can be efficiently utilized to drive such a cooling cycle.     

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.     

Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system

Over the past few decades there have been considerable efforts to use adsorption (solid/vapor) for cooling and heat pump applications, but intensified efforts were initiated only since the imposition of international restrictions on the production and utilization of CFCs and HCFCs. In this paper, a dual-mode silica gel–water adsorption chiller design is outlined along with the performance evaluation of the innovative chiller. This adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. The drawback of this operational mode is its poor efficiency in terms of cooling capacity and COP. Simulation results show that the optimum COP values are obtained at driving source temperatures between 50 and 55 °C in three-stage mode, and between 80 and 85 °C in single-stage, multi-bed mode.     

Experimental study on a continuous adsorption water chiller with novel design

A newly developed adsorption water chiller is introduced and tested. In the new adsorption refrigeration system, there are no refrigerant valves, the problem of mass transfer resistance resulting in pressure drop along refrigerant passage in conventional systems when methanol or water is used as refrigerant can be absolutely solved. Silica-gel–water is used as working pair and mass recovery-like process is adopted in order to use low temperature heat source ranging from 70 to 85 °C effectively. The experiment results demonstrate that the chiller (26.4 kg silica-gel in each adsorber) has a cooling capacity of 2–7.3 kW and COP ranging 0.2–0.42 according to different evaporating temperatures. Based on the experimental tests of the first prototype, the second prototype is designed and tested; the experimental data demonstrate that the chiller performance has been greatly improved, with a heat source temperature of 80 °C, a COP over 0.5 and cooling capacity of 9 kW has been achieved at evaporating temperature of 13 °C.     

Thermodynamic limits to thermal regeneration in adsorption cooling cycles

Energy and exergy models for ideal adsorption cycles with isothermal beds and no mass recovery are developed to predict the limits to COP enhancement using thermal regeneration. The models are applied to compare the performance of zeolite–water and silica gel–water adsorbent–refrigerant pairs over a range of maximum bed temperatures. The thermodynamic consistencies of several alternate adsorption property assumptions are quantified. Differences in adsorption characteristics between zeolite–water and silica gel–water result in a significantly larger potential to enhance COP by implementing thermal regeneration for zeolite–water. Based on COP, the zeolite–water pair is preferred when both thermal regeneration and a high temperature thermal energy source (>150 °C) are used, while the silica gel–water pair is preferred when thermal regeneration is not used and/or a low temperature thermal energy source (<100 °C) is used.     

Performances of adsorption systems for ambient heating and air conditioning: Performances de systèmes à adsorption pour le chauffage et le conditionnement d'air d'ambiance

Two adsorption systems are considered: zeolite–water and activated carbon–methanol, both consisting of two ‘uniform temperature' adsorbent beds operating with internal heat recovery by a heat carrier circuit. Regarding the zeolite–water system, the performance obtained with a new adsorbent bed, with good heat transfer properties, is compared with a traditional tube and fin exchanger embedded with zeolite pellets. The performances were calculated by using a dynamic model developed and validated previously. Results show that the system based on the new adsorber has a higher specific power and the same Coefficient of Performance. Results of simulation for adsorbers consisting of finned tube heat exchangers and utilising the activated carbon–methanol pair are also presented.     

Experimental investigation of a micro-combined cooling, heating and power system driven by a gas engine

An experimental investigation of the performance of a micro-combined cooling, heating and power (CCHP) system is described. The natural gas and LPG-fired micro-CCHP system uses a small-scale generator set driven by a gas engine and a new small-scale adsorption chiller, which has a rated electricity power of 12 kW, a rated cooling of 9 kW and a rated heating capacity of 28 kW. Silica gel–water is used as working pair in the adsorption cooling system. The refrigeration COP of the adsorption chiller is over 0.3 for 13 °C evaporation temperature. The test facility designed and built is described, which supplies better test-rig platform for cooling, heating and power cogeneration. Experimental methodology of this system is presented and the results are discussed. An energetic analysis of micro-CCHP system is performed as well. The overall thermal and electrical efficiency is over 70%.     

Experimental investigation of mass recovery adsorption refrigeration cycle

The study investigates the performance of silica gel–water adsorption refrigeration cycle with mass recovery process by experimental prototype machine. In an adsorption refrigeration cycle, the pressures in adsorber and desorber are different. The mass recovery cycle utilizes the pressure difference to enhance the refrigerant mass circulation. Moreover, novel cycle was proposed for improvement of cooling output. In our previous study, simulation analysis shows that mass recovery cycle has the advantage over conventional single-stage. Experiments with prototype machine were conducted to investigate the performance improvement of mass recovery cycle in the present paper. Specific cooling power (SCP) and coefficient of performance (COP) were calculated with experimental data to analyze the influences of operating conditions. The proposed cycle was compared with the single-stage cycle in terms of SCP and COP. The results show that SCP of mass recovery cycle is superior to that of conventional cycle and mass recovery cycle is effective with low temperature heat source.     

Experimental investigation of a novel multifunction heat pipe solid sorption icemaker for fishing boats using CaCl2/activated carbon compound–ammonia

The performance of a solid sorption icemaker is investigated. CaCl₂/activated carbon was used as compound adsorbent and ammonia was employed as adsorbate. The influence of operating conditions (cooling water temperature, mass recovery and heat pipe heat recovery, etc.) on the mass of ice, SCP (specific cooling power) and COP (coefficient of performance) was experimentally assessed. At the desorption temperature of 126 °C, cooling water temperature of 22 °C, ice produced temperature of −7.5 °C, 40 s of mass recovery and 2 min of heat pipe heat recovery, the mass of ice, SCP and COP values are 17.6 kg/h, 369.1 W/kg and 0.2, respectively.     

Performance improvement of adsorption cooling by heat and mass recovery operation

An adsorption cooling system was developed and tested and various operation procedures have been tried. The experimental results show that the heat recovery operation between two adsorption beds will increase the COP by about 25% if compared with one adsorber basic cycle system. It was also proved that mass recovery is very effective for heat recovery adsorption cooling operation, which may help to obtain a COP increase of more than 10%. Theoretical analyses on the COP have been completed for various heat and mass recovery cycles, such as basic intermittent adsorption cycle, continuous two-adsorber heat recovery cycle, mass recovery cycle, mass recovery with sensible heat recovery, and mass recovery with both sensible heat and heat of adsorption recovery. The theoretical results are in good agreement with experimental values. Based upon the developed theoretical model, it is possible to predict the COP for various operation procedures of a real adsorption cooling system.