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.     

Theoretical and experimental study on characteristics of a novel silica gel–water chiller under the conditions of variable heat source temperature

In this paper, a transient model of a silica gel–water adsorption chiller, which is developed in Shanghai Jiao Tong University (SJTU), is developed in order to simulate the evaporating, condensing, and adsorption temperature. Furthermore, this model is verified by a series of experiments. The theoretical studies and experimental data show that the coefficient of performance (COP) is influenced significantly by the variation rates of the heat source temperatures. The results also show that when this chiller is driven by solar energy, a buffer tank should be adopted in the system in order to get better performance when solar insolation is low, and should not be utilized when solar insolation is high, otherwise low COP will be gotten for the reason of the consumption of high electric energy.     

Experimental comparison of the sorption and refrigerating performances of a CaCl2 impregnated composite adsorbent and those of the host silica gel

In this paper, the adsorption and refrigerating performances of a composite adsorbent (S40) and its host microporous silica gel matrix (S0) are investigated comparatively in which water is used as refrigerant. The composite adsorbent is developed by impregnating the silica gel (S0) with calcium chloride. A lab-scale single-bed adsorption chiller system, functioning without any valve on its refrigerant circuit, is designed and used as test rig. The mass ratio (MR), defined as the ratio of the specific cooling power (SCP) of S40 to that of S0, is found to be higher than 2, while the COP has been improved by 25%, in average. The S40 has been tested to have, not only the capacity of adsorbing water vapour more than twice as much as the S0 does, but also, kinetically, to adsorb and desorb faster. The cycled amounts of refrigerant (CAR), calculated from measured isobaric adsorption levels, further show that the S40 can be regenerated at lower temperatures, with respect to S0.     

A dynamic simulation model for transient absorption chiller performance. Part II: Numerical results and experimental verification

This paper is the second paper out of two which present the development of a dynamic model for single-effect LiBr/water absorption chillers. The first part describes the model in detail with respect to the heat and mass balances as well as the dynamic terms. This second part presents a more detailed investigation of the model performance, including performance analysis, sensitivity checks and a comparison to experimental data. General model functionality is demonstrated.A sensitivity analysis gives results which agree very well to fundamental expectations: it shows that an increase in both external and internal thermal mass results in a slower response to the step change but also in smaller heat flow oscillations during the transient period. Also, the thermal mass has been found to influence the heat flow transients more significantly if allocated internally. The time shift in the solution cycle has been found to influence both the time to reach steady-state and the transients and oscillations of the heat flow. A smaller time shift leads to significantly faster response.A comparison with experimental data shows that the dynamic agreement between experiment and simulation is very good with dynamic temperature deviations between 10 and 25 s. The total time to achieve a new steady-state in hot water temperature after a 10 K input temperature step amounts to approximately 15 min. Compared to this, the present dynamic deviations are in the magnitude of approximately 1–3%.     

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.     

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 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.     

Study on an activated carbon fiber–ethanol adsorption chiller: Part I – system description and modelling

This article presents the transient modelling for a two-bed, activated carbon fiber (ACF)–ethanol adsorption chiller. This innovative adsorption chiller employs pitch based ACF of type A-20 as adsorbent which is a fibrous adsorbent having the advantages of fast adsorption rate, high porosity and ease of handling when compared with granular adsorbents and powdered adsorbents. Ethanol is used as refrigerant as it has no harm to environment, it is a non-toxic substance, moreover, ethanol has comparatively higher vapor pressure even at low temperature. This innovative system utilizes effectively low-temperature waste heat sources of temperature between 60 and 95 °C along with a coolant at 30 °C. We have found that, regardless of the initial mass distribution, the ACF–ethanol adsorption chiller is able to achieve the same cyclic-steady-state within three cycles or 1890 s.     

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.     

Simulation results of triple-effect absorption cycles

A simulation analysis was carried out for three kinds of triple-effect absorption cycles of parallel-flow, series-flow and reverse-flow using a newly developed simulation program. The cycles investigated in this paper are similar to the alternate double-condenser coupled cycles of Grossman. The coefficient of performance, the maximum pressure and the maximum temperature of each cycle were calculated. The sensitivity analysis of UA of each component was also carried out. The results show that the parallel-flow cycle yields the highest coefficient of performance among the cycles, while the maximum pressure and temperature in the reverse-flow cycle are lower than those of other cycles.     

A dynamic simulation model for transient absorption chiller performance. Part I: The model

This paper is the first of two which presents the development of a dynamic model for single-effect LiBr/water absorption chillers. The model is based on external and internal steady-state enthalpy balances for each main component. Dynamic behaviour is implemented via mass storage terms in the absorber and generator, thermal heat storage terms in all vessels and a delay time in the solution cycle. A special feature is that the thermal capacity is partly connected to external and partly to internal process temperatures.In this paper, the model is presented in detail. For verification, the model has been compared to experimental data. The dynamic agreement between experiment and simulation is very good with dynamic deviations around 10 s. General functionality of the model and a more detailed comparison with experimental data are presented in Part II of this paper.     

Thermodynamic modeling of an ammonia–water absorption chiller

This article develops a general thermodynamic framework for the modeling of an irreversible absorption chiller at the design point, with application to a single-stage ammonia–water absorption chiller. Component models of the chiller have been assembled so as to quantify the internal entropy production and thermal conductance (UA) in a thermodynamically rigorous formalism, which is in agreement with the simultaneous heat-and-mass transfer processes occurring within the exchangers. Local thermodynamic balance (viz. energy, entropy, and mass balance) and consistency within the components is respected, in addition to the overall thermodynamic balance as determined by the inlet and outlet states of the components. For the absorbers, Colburn-and-Drew mass transfer equations are incorporated to describe the absorption process. Furthermore, the impact of various irreversibilities on the performance of chiller is also evaluated through the use of a general macroscopic equation.     

Experimental investigation of silica gel–water adsorption chillers with and without a passive heat recovery scheme

We experimentally show that for the same heat exchanger inventory allocation, a four-bed adsorption chiller delivers a 12% higher ultimate cooling capacity than its two-bed counterpart. In addition it delivers a significantly improved quality of instantaneous cooling than a two-bed chiller at the same cooling capacity. The COP-enhancing feature of a passive heat recovery scheme that does not involve additional pumping action or valves is experimentally proven. It improves the COPs of a two-bed chiller and a four-bed chiller by as much as 38 and 25%, respectively, without any effect on their cooling capacities. The highest COPs achieved with a two-bed and four-bed chillers are 0.46±0.02 and 0.45±0.02, respectively. These are measured at a hot-water inlet temperature of 85 °C, cooling-water inlet temperature of 29.4 °C and chilled-water inlet temperature of 12.2 °C.     

A general thermodynamic framework for understanding the behaviour of absorption chillersCadre thermodynamique facilitant la compréhension du comportement des refroidisseurs à absorption

In this article, a general definition of the process average temperature has been developed, and the impact of the various dissipative mechanisms on 1/COP of the chiller evaluated. The present component-by-component black box analysis removes the assumptions regarding the generator outlet temperature(s) and the component effective thermal conductances. Mass transfer resistance is also incorporated into the absorber analysis to arrive at a more realistic upper limit to the cooling capacity. Finally, the theoretical foundation for the absorption chiller T–s diagram is derived. This diagrammatic approach only requires the inlet and outlet conditions of the chiller components and can be employed as a practical tool for system analysis and comparison.     

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.     

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%.     

An experimental investigation and modelling of the thermodynamic properties of isobutane–compressor oil solutions: Some aspects of experimental methodology

This paper presents experimental data for the solubility, density and capillary constant for solutions of natural refrigerant isobutane with commercial mineral compressor oil Azmol over a wide range of temperatures and concentrations. Based on information for the capillary constant, the surface tension of the solutions isobutane/Azmol is determined. The experimental data were obtained in the temperature range from 303 K to 363 K and at pressures up to 1.7 MPa using static methods. The experimental data obtained for the solutions of the natural refrigerant isobutane with the commercial mineral compressor oil Azmol are sufficiently described with the help of correlations based on the theory of thermodynamic similarity. The paper reports variation of the vapor pressure, density, capillary constant and surface tension as a function of concentration for the isobutane/Azmol solutions. The enthalpy of liquid phase of the isobutane/Azmol solutions is calculated. The analysis of the behaviour of the excess thermodynamic functions is carried out. The paper examines experimental and methodical uncertainties in the investigation of thermodynamic properties of the refrigerant/oil solutions (ROS). The influence of the time taken to establish thermodynamic equilibrium in the experimental cell on the uncertainty of the experimental data for gas-saturated mixtures such as ROS is discussed. Information about the changing concentration of refrigerant in the liquid phase of the ROS and in the surface layer of the liquid phase of the ROS at increasing temperature is presented. In addition, the experimental data for the density, surface tension and refractive index of the mineral compressor oil Azmol are reported.     

Experimental results of a solar powered cooling system at low temperature

A solar thermochemical prototype producing low-temperature cold has been built and tested during the summer and autumn 2005 in Perpignan, France. It cools a 560 L cold box down to about −25 °C using only low-grade heat produced by two simple flat plate solar collectors. The process involves two cascaded thermochemical systems using BaCl₂ salt reacting with ammonia. Its working mode is discontinuous, as it alternates between one decomposition mode at high pressure (daytime) and one cold production mode at low pressure (nighttime). Experimental results prove the feasibility of this new concept of solar cold production, with temperatures as low as −30 °C, demonstrate its potential use in housing, by the acceptable size and weight of the system and show the system performances during the sunniest months of the year, with a rough solar coefficient of performance (COP) of about 0.031 over the test period. The major meteorological parameters influencing the process efficiency are the solar irradiation and the outside temperature.     

Two bed silica gel–water adsorption chillers: An effectual lumped parameter model

This article develops an improved lump-parameter design model to investigate the water-circulation heat recovery scheme as applied to the two-bed silica gel–water adsorption chillers. We demonstrate that performance predictions stemming from this improved lump-parameter formalism compare favorably with experimental results at various conditions, particularly at the industrial rated conditions. We find that the present lump-parameter formalism adequately elucidates the water-circulation heat recovery scheme as does the distributed-parameter formalism. In the studied working condition of a two-bed silica gel–water adsorption chiller, the differences in cooling capacities and coefficients of performance (or COP) by using the two different formalisms are typically less than 10%. This gives rise to a useful and rapid design tool for the industry.     

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.