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.     

Application of thermal battery in the ice storage air-conditioning system as a subcooler

This article experimentally investigates the thermal performance of a thermal battery used in the ice storage air-conditioning system as a subcooler. The thermal battery utilizes the superior heat transfer characteristics of two-phase closed thermosyphon and eliminates the drawbacks found in convectional energy storage systems. Experimental investigations are first conducted to study the thermal behavior of thermal battery under different charge temperatures (−5 °C to −9 °C) in which water is used as the energy storage material. This study also examines the thermal performance of the subcooled ice storage air conditioner under different cooling loads. Experimental data of temperature variation of water, ice fraction, refrigerant mass flow rate and coefficient of performance (COP) are obtained. The results show that supercooling phenomenon appears in the water and it can be ended when the charge temperature is lower than −6 °C. The system gives 28% more cooling capacity and 8% higher COP by the contribution of the thermal battery used as a subcooler.     

Carbon–ammonia pairs for adsorption refrigeration applications: ice making, air conditioning and heat pumping

A thermodynamic cycle model is used to select an optimum adsorbent-refrigerant pair in respect of a chosen figure of merit that could be the cooling production (MJ m⁻³), the heating production (MJ m⁻³) or the coefficient of performance (COP). This model is based mainly on the adsorption equilibrium equations of the adsorbent–refrigerant pair and heat flows. The simulation results of 26 various activated carbon–ammonia pairs for three cycles (single bed, two-bed and infinite number of beds) are presented at typical conditions for ice making, air conditioning and heat pumping applications. The driving temperature varies from 80 °C to 200 °C. The carbon absorbents investigated are mainly coconut shell and coal based types in multiple forms: monolithic, granular, compacted granular, fibre, compacted fibre, cloth, compacted cloth and powder. Considering a two-bed cycle, the best thermal performances based on power density are obtained with the monolithic carbon KOH-AC, with a driving temperature of 100 °C; the cooling production is about 66 MJ m⁻³ (COP = 0.45) and 151 MJ m⁻³ (COP = 0.61) for ice making and air conditioning respectively; the heating production is about 236 MJ m⁻³ (COP = 1.50).     

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.     

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.     

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.     

Evaluation of the cooling performance of a consolidated expanded graphite–calcium chloride reactive bed for chemisorption icemaker

The cooling performance of a consolidated composite reactive bed made from expanded graphite impregnated with CaCl₂ was experimentally assessed under different evaporation and heat sink temperatures. The compound presented a specific cooling power (SCP) higher than 1000 W kg_Salt⁻¹ at several studied conditions. The calculated coefficient of performance (COP) was about 0.35 when the amount of refrigerant consumed in the reaction was 0.80 kg kg_Salt⁻¹. Both SCP and COP changed with the cycle time, and thus, with the degree of the reaction. The synthesis time to maximise the SCP, under any studied condition, was about 5 min, and the absorbed quantity greatly varied among the different operation conditions. When compared to the time necessary to obtain an absorbed amount of 0.80 kg kg_Salt⁻¹, the synthesis time of 5 min could improve the SCP in about 15–68%, however, COP would be deployed in about 14–50%.     

Performance of an HCFC22-based vapour absorption refrigeration system

A single-stage vapour absorption refrigeration system (VARS) is tested with monochlorodifluoromethane (HCF22) as refrigerant and different absorbents: dimethylether of tetraethylene glycol (DMETEG) and dimethyl acetamide (DMA). The influence of generator temperatures in the range 75–95°C, which represents low-grade heat sources, is studied. Cooling water temperatures were varied between 20 and 30°C. Two cases of cooling water flow paths are considered, i.e. water entering either absorber or condenser, which are connected in series. For HCFC22-DMETEG, COP values in the range 0.2–0.36 and evaporator temperatures between 0 and 10°C are obtained. For HCF22-DMA, COP values in the range 0.3–0.45 and evaporator temperatures between −10 and 10°C are obtained. It is observed that HCFC22-DMETEG can work at lower heat source temperatures than HCFC22-DMA. However, at the same operating conditions HCFC22-DMA is better from the viewpoints of circulation ratio and COP. Experiments also show that at low heat source temperature, cooling water temperature has strong influence on circulation ratio but does not affect COP significantly. Preferably, cooling water should first flow through the condenser and then through the absorber in order to achieve improved overall performance.     

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.     

Performance evaluation of a two-stage adsorption refrigeration cycle with different mass ratio

The performance of a two-stage adsorption chiller with different mass allocation between upper and bottom beds has been investigated numerically. It is found that the chiller can be driven effectively by the waste heat of temperature 55 °C with the heat sink at environment temperature. Results show that cooling capacity can be improved with the optimum allocation of adsorbent mass to the bottom beds than that to the upper beds. The improvement in Coefficient of Performance (COP) values, however, is less significant. It is also seen that the improvement in cooling capacity is more significant for the relatively higher heat source temperature. It is shown that the cooling capacity can be improved up to 20% if the heat source temperature is 80 °C and the average outlet temperature is fixed at 7 °C.     

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.     

Modelling the performance of a transcritical CO2 heat pump for high temperature heating

A prototype transcritical CO₂ heat pump was constructed for heating water to temperatures greater than 65°C while providing refrigeration at less than 2°C. The heating capacity was 115 kW at an evaporation temperature of +0.3°C and a hot water temperature of 77.5°C, with a heating coefficient of performance (COP) of 3.4. Performance data is presented for each of the compressor, the gas cooler, and the recuperator as well as for the overall heat pump system. Equipment performance data was incorporated into a computer model to enable parametric investigations of heat pump performance. Model predictions showed that the hot water temperature could be increased from 65 to 120°C with a relatively small reduction in heating capacity and heating COP of 33 and 21%, respectively. Model predictions also highlight the potential for significant capacity improvements by eliminating the recuperator in favour of a larger gas cooler.     

Study on adsorption refrigeration cycle utilizing activated carbon fibers. Part 2. Cycle performance evaluation

Thermal heat driven adsorption systems have been gained considerable attention on the recent energy utilization trend. However, the drawbacks of these adsorption systems are their poor performance. It is urgently necessary to improve the system performance of the adsorption cycles. There are two major ways for the system performance improvement. One is to develop new adsorbent material well suited to low temperature heat regeneration. The other is to enhance heat and mass transfer in the adsorber/desorber heat exchanger. The objective of the paper is to investigate the system performance of an adsorption cycle. The cycle utilizes activated carbon fiber (ACF)/methanol as adsorbent/refrigerant pair. In this paper, specific cooling effect SCE and COP of the system are numerically evaluated from the adsorption equilibrium theory with different hot, cooling and chilled fluid inlet temperatures. It is confirmed that the influences of hot, cooling and chilled fluid inlet temperatures on the system performance are qualitatively similar to those of silica gel/water pair. Even though, the driving temperature levels of ACF/methanol and silica gel/water are different. There is an optimum condition for COP to reach at maximum for ACF/methanol pair. Particularly, the ACF/methanol system shows better performance with lower chilled fluid inlet temperature between −20 and 20 °C.     

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.     

Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats

Adsorption performances and thermal conductivity were tested for three types of adsorbent: Pure CaCl₂ powder, simple composite adsorbent and consolidated composite adsorbent. The simple composite adsorbents show better adsorption performance because the additive of expanded graphite in CaCl₂ powder has restrained the agglomeration phenomenon in adsorption process and improved the adsorption performance of CaCl₂. The consolidated composite adsorbent are suitable to be used as adsorbent for ice maker on fishing boats because they have higher thermal conductivity, larger volumetric cooling capacity, higher SCP values and better anti-sway performance than simple composite adsorbents. Thermal conductivity of the consolidated composite adsorbent is 6.5–9.8 W m⁻¹ K⁻¹ depending on the molding pressure, ranging from 5 to 15 MPa, which is about 32 times higher than the thermal conductivity of CaCl₂ powder. The volumetric cooling capacity of consolidated composite adsorbent is about 52% higher than the best result obtained for CaCl₂ at the evaporating temperature of −10 °C. The SCP of the consolidated adsorbent increases of about 353% than CaCl₂ powder from simulation results at T_ad=30 °C and T_ev=−10 °C. The consolidated composite adsorbents have good anti-sway performance and they are not easy to be scattered out when the fishing boats sway on the sea.     

Hybrid absorption heat pump with ammonia/water mixture – Some design guidelines and district heating application

An ammonia/water mixture can be used as an efficient working fluid in industrial-type heat recovery heat pumps and heat transformers. Several configurations of such systems are possible depending on the availability of the waste (thermal) and primary (thermal or electrical) energy sources. This article presents the configurations, the main thermodynamic and hydraulic parameters, and some design guidelines and operating experiences of a medium-temperature, ammonia/water-based compression/re-sorption heat recovery system for district domestic hot water production. In-field experiments have proven the advantages of the concept and its applicability limits in a particular economical environment, while hot water was produced at 55 °C with industrial cooling water at 36 °C as a waste heat source.     

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.     

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.