Steady-state model of centrifugal liquid chillers: Modèle pour des refroidisseurs de liquide centrifuges en régime permanent

A new steady-state model of vapour-compression type centrifugal liquid chillers is presented. The model has a number of advanced features and is capable of simulating both hermetic and open-drive centrifugal compressors. The model accounts for the real process phenomena such as superheating and subcooling in the heat exchangers as well as a capacity control formulation of the inlet guide vanes. The model algorithm is developed with the aim of requiring only those inputs that are readily known to the design engineer, e.g. the general parameters of the chiller, the chilled water flow temperature out of the evaporator and the return water temperature to the condenser inlet. The outputs include the condenser capacity, the refrigeration capacity (at the evaporator), the coefficient of performance, and also the mass flow rates and thermodynamic states of the refrigerant throughout the cycle. The model is validated with the experimental data on part load to full load performance of three different chillers operating at the University of Auckland and the agreement is found to be within ±10%. The model also demonstrates that the COP of the chillers increases with increasing cooling capacity.     

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.     

Refrigeration below zero °C: Adsorption chillers using water with ethylene glycol as antifreeze

Adsorption chilling with water as working fluid is environmentally friendly, but limited to cooling temperatures above 0°C. To expand the operational range of water based adsorption to and below the freezing point of water, we propose to add ethylene glycol as anti-freezing agent to the evaporator. According to vapour liquid equilibrium data, ethylene glycol remains in the evaporator and thus should not be adsorbed. Therefore, only water is still serving as refrigerant. Experiments generating cooling power at 0°C demonstrate that the adsorption process works with ethylene glycol as anti-freezing agent. Furthermore, measured ethylene glycol concentrations in condenser and evaporator confirm that water serves as refrigerant. The experiments demonstrate the potential of adding anti-freezing agents to the evaporator to expand the application range of water based adsorption chillers.     

Performance evaluation of a two-stage silica gel + water adsorption based cooling-cum-desalination system

The first objective of this paper is to show that a single-stage adsorption based cooling-cum-desalination system cannot be used if air cooled heat rejection is used under tropical conditions. This objective is achieved by operating a silica gel + water adsorption chiller first in a single-stage mode and then in a 2-stage mode with 2 beds/stage in each case. The second objective is to improve upon the simulation results obtained earlier by way of empirically describing the thermal wave phenomena during switching of operation of beds between adsorption and desorption and vice versa. Performance indicators, namely, cooling capacity, coefficient of performance and desalinated water output are extracted for various evaporator pressures and half cycle times. The improved simulation model is found to interpret experimental results more closely than the earlier one. Reasons for decline in performance indicators between theoretical and actual scenarios are appraised.     

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.     

A new cycle time allocation for enhancing the performance of two-bed adsorption chillers

The present study proposed a new cycle time allocation in silica gel-water based adsorption chillers to enhance their performances. The new cycle time allocation not only improves the cooling capacity and coefficient of performance (COP), but also contributes in the reduction of delivered chilled water fluctuations. In a conventional adsorption chiller operation, the appropriate pre-heating/pre-cooling process enhances the cooling capacity, whereas the excess pre-heating/pre-cooling time abates the average cooling capacity. The proposed new cycle time allocation allows the continuous cooling effect over the cycle without sacrificing the effect of pre-heating/pre-cooling. Simulation results showed that the new cycle time was effective for both RD type silica gel-water and CaCl₂-in-silica gel-water pairs, and the cooling capacity was increased as much as by 6%.     

New absorption chiller and control strategy for the solar assisted cooling system at the German federal environment agency

Typically the cooling capacity of absorption chillers is controlled by adjusting the driving hot water temperature according to the load. Meanwhile the cooling water temperature is controlled to a constant set value. In order to increase the solar cooling fraction and/or to decrease the operating costs of solar assisted cooling systems (SAC-systems) a new control strategy has been developed which controls hot and cooling water temperature simultaneously. Hereby the specific cost of cold – generated from solar or conventional heat – can be reduced. The basic concept of the strategy is explained and results are shown for the SAC-system at the Federal Environment Agency in Dessau, Germany. Here a recently developed absorption chiller is now used instead of a former adsorption chiller. With the new absorption chiller and the control strategy the seasonal energy efficiency ratio SEER is above 0.75, electric efficiency is 35% higher and water consumption is reduced by 70%.     

System options for cooling of buildings making use of district heating heat

Issues stemming from district heating utilization during summer periods and the conversion of low-temperature heat into cold in adsorption chillers have been investigated in this paper. Due to the high vulnerability of adsorption chillers to ambient conditions, in the case of relatively low ambient temperatures, adsorption-based air-conditioning systems would be characterized by excessive cooling power. Moreover, adsorption chillers are also characterized by high investment costs and big time constants, and the vulnerabilities found in their regulatory processes have yet to be sufficiently investigated. The authors recommend the application of hybrid air-conditioning systems, consisting of adsorption and compressor chillers. The adsorption chiller works as a base while the compressor chiller contributes missing cooling power, working as a regulation unit. Sixteen configurations of the hybrid air-conditioning system have been analysed. It has been shown that 100 kW cooling power hybrid air-conditioning system, with respect to its configuration, enables the utilization of 0.5 to 0.9 TJ of low-temperature heat per year, while simultaneously providing comfortable air-conditioning. The authors have concluded that the adsorption share in the analysed hybrid system should not exceed 50%.     

膜接触器用于氨吸收式制冷精馏过程

氨水吸收式制冷装置是一种可以有效利用余热制取0℃以下低温的制冷系统,且氨作为一种天然工质,有很好的环保特性,但氨-水的标准沸点相差133.4℃,在较低蒸发温度的要求下需加精馏装置.介绍了目前传统分离单元传质性能的研究进展,强调了加强精馏器传质性能的必要性,提出采用能提供更大传质比表面积的膜接触器提高传质性能与效率,为改进氨水精馏塔的设计提供参考.     

Solar refrigeration options – a state-of-the-art review

A state-of-the-art review is presented of the different technologies that are available to deliver refrigeration from solar energy. The review covers solar electric, solar thermal and some new emerging technologies. The solar thermal systems include thermo-mechanical, absorption, adsorption and desiccant solutions. A comparison is made between the different solutions both from the point of view of energy efficiency and economic feasibility. Solar electric and thermo-mechanical systems appear to be more expensive than thermal sorption systems. Absorption and adsorption are comparable in terms of performance but adsorption chillers are more expensive and bulkier than absorption chillers. The total cost of a single-effect LiBr–water absorption system is estimated to be the lowest.     

Performance simulation of multi-bed silica gel-water adsorption chillers

Adsorption chiller technology is one of effective means to convert waste thermal energy into cooling, which substantially improves energy efficiency and lowers environmental pollution. This article develops an improved lumped-parameter model for multi-bed silica gel-water adsorption chillers. It is validated by experimental results stemming from a four-bed silica gel-water adsorption chiller at various operating conditions. It is found that the performance predictions from this model compare favourably with experimental results. At all tested conditions and over a wide range of cycle times, the cooling capacity and COP can be predicted to within 10% and 12%, respectively.     

Part-load characteristics of a new ammonia/lithium nitrate absorption chiller

A pre-industrial prototype of a new water-cooled ammonia/lithium nitrate absorption chiller was characterised at part-load operation mode. The chiller was built using brazed plate heat exchangers in all its components, including the absorber and the generator.A test campaign was carried out varying the thermal load in the chilled water circuit and keeping the hot and cooling water temperatures constant.Part-load curves of the thermal and electrical coefficients of performance were obtained, plotted and compared with data from the literature on small capacity absorption chillers with conventional working pairs, namely ammonia/water and water/lithium bromide. The experimental results showed that to achieve a higher electrical coefficient of performance at part-load operation, it was much more convenient to use an ON-OFF control than to modify the hot water temperature. Furthermore, using a simple ON-OFF control strategy, the behaviour of the new absorption chiller was more agile and responded more quickly.The part-load curve of the electrical coefficient of performance was obtained by adjusting the experimental data to the shape of the curve proposed in the standard prEN-14825:2011 for air-to-water chillers. The Cc coefficient was 0.7985 matching the value obtained dividing the remaining electrical consumption measured during the OFF half cycles by the total energy consumption generated.     

Energy and exergy comparison of a cascade air conditioning system using different cooling strategies

A cascade air conditioning system consisting of a compression and an absorption chiller working in a parallel arrangement has been proposed. This system is powered up by a micro-gas turbine. Different cooling strategies are studied in order to recognize the best configuration. The system components have been modeled and analyzed through the energy and exergy approaches. The performance parameters of the systems and the second law efficiency have been calculated in different operating conditions. Water consumption of the systems has also been investigated, considering water as a source of exergy. The results revealed that a system with water-cooled chillers has the highest second law efficiency and water consumption. On the other hand, the water consumption of a system with an air-cooled absorption and a water-cooled compression chiller is about 50% less than that of the system with two water-cooled chillers while its second law efficiency is only about 10% less.     

冷却水压差对离心式冷水机的影响分析

通过对离心式冷水机组冷凝器构造的分析,按管壳式热交换器Kern方程,由管程压降计算出冷却水流速,而后得出冷凝器的传热系数,进而算出冷凝器的热负荷和冷水机组的制冷量;建立冷却水压差和离心式冷水机组制冷量的关系,对冷水机组的设计和运行提供了一些参考。     

自复叠制冷循环试验中的特殊现象讨论

在自复叠制冷循环中,经过大量的实验测试,发现在实验中存在气液分离器液体温度偏低、蒸发器出现换热系数过小以及蒸发器制冷量过小,但仍能循环等特殊的现象,这些现象在理论基础上无法解释。针对这些现象,进行了详细的描述,分析并结合实验数据给出一些见解与推测,这有助于对自复叠系统更加深入的研究。     

Feasibility study and performance evaluation of low capacity water–LiBr absorption cooling systems functioning in different Algerian climate zones

A performance analysis was carried out on water–LiBr absorption chillers performing in the five different climate zones in Algeria. A 17.6 kW single-effect and a 16 kW double-effect commercial absorption chillers were simulated. In climate zones E1 and E2, the single-effect and double-effect chillers supplied 37% and 91%, respectively, of their nominal capacity to produce chilled water at 7 °C. In the hot climate zones E3, E4 and E5, it was not feasible for either of the chillers to produce chilled water at 7 °C. By increasing the chilled water temperature to 12 °C both absorption chillers were able to operate in climate zones E3 and E4. The single-effect chiller reached 45% of its nominal capacity in zone E3 and 33% in zone E4. The double-effect chiller delivered 80% of its nominal capacity in both climate zones. Neither of the chillers was able to operate under the thermal conditions of climate zone E5.     

Novel absorption refrigeration system with a hollow fiber membrane-based generator

Vapor absorption refrigeration systems using the LiBr–water pair can be an alternative to existing automobile air-conditioning systems. To this purpose, the new system must be compact enough to fit into automobiles, and robust against sloping roads and unexpected vibrations while driving. The system uses a micro-porous hydrophobic hollow fiber membrane-based generator (HFM-G) instead of the conventional generator in the absorption cycle. HFM-G extracts water vapor from the LiBr solution that is mechanically constrained by a hydrophobic membrane. This paper presents the results of a parametric steady-state simulation investigating how the HFM-G affects the proposed absorption cooling cycle. We first describe the mechanism of the HFM-G, and the system performance of the proposed cycle under various operating conditions is examined. Simulation results show that the proposed system can achieve a cooling capacity and COP of approximately 2.88 kW and 0.63, respectively, by incorporating the solution recirculation process.     

A novel experimental investigation of a solar cooling system in Madrid

This paper reports novel experimental results derived through field testing of a part load solar energized cooling system for typical Spanish houses in Madrid during the summer period of 2003. Solar hot water was delivered by means of a 49.9 m² array of flat-plate collectors to drive a single-effect (LiBr/H₂O) absorption chiller of 35 kW nominal cooling capacity. Thermal energy was stored in a 2 m³ stratified hot water storage tank during hours of bright sunshine. Chilled water produced at the evaporator was supplied to a row of fan coil units and the heat of condensation and absorption was rejected by means of a forced draft cooling tower. Instantaneous, daily and period energy flows and energy balance in the installation is presented. System and absorption machine temperature profiles are given for a clear, hot and dry day's operation. Daily and period system efficiencies are given. Peak insolation of 969 W m⁻² (at 12:30 solar time on 08/08/03) produced 5.13 kW of cooling at a solar to cooling conversion efficiency of 11%. Maximum cooling capacity was 7.5 kW. Cooling was provided for 8.67 h and the chiller required a threshold insolation of 711 W m⁻² for start-up and 373 W m⁻² for shut-down. A minimum hot water inlet temperature to the generator of 65 °C was required to commence cold generation, whereas at 81 °C, 6.4 kW of cooling (18.3% of nominal capacity) was produced. The absorption refrigeration machine operated within the generation and absorption temperature ranges of 57–67 and 32–36 °C, respectively. The measured maximum instantaneous, daily average and period average COP were 0.60 (at maximum capacity), 0.42 and 0.34, respectively. Energy flows in the system are represented on a novel area diagram. The results clearly demonstrate that the technology works best in dry and hot climatic conditions where large daily variations in relative humidity and dry bulb temperature prevail. This case study provides benchmark data for the assessment of other similar prototypes and for the validation of mathematical models.     

Thermodynamic based comparison of sorption systems for cooling and heat pumping: Comparaison des performances thermodynamique des systèmes de pompes à chaleur à sorption dans des applications de refroidissement et de chauffage

A comparison of thermodynamic performances of sorption systems (liquid absorption, adsorption, ammonia salts and metal hydrides) is carried out for typical applications (deep-freezing, ice making, air-conditioning and heat pumping) with either air-cooled or water-cooled heat sink. The results are given in terms of cooling coefficient of performance (COP) (heating COP or coefficient of amplification (COA) for the heat pump), cooling (heating) power versus reactor volume or weight and thermodynamic efficiency. LiBr–water systems show the best results for air-conditioning except when small units are required (metal hydride systems lead to more compact units). Other systems, however, show better results for other applications (chemical reaction with ammonia salts for deep-freezing, adsorption for heat pumping).     

Determining the optimum cyclic operation of adsorption chillers by a direct method for periodic optimal control

Adsorption refrigeration systems provide a sustainable possibility to reduce the environmental impact of refrigeration and air-conditioning as they allow for sources of otherwise unused excess waste heat to be reused for cooling purposes. Adsorptive cooling is a discontinuously operated cycling process, and it is well known that the determination of an optimal cycling time yielding maximum cooling power is a key to the design of an efficient mode of operation. The optimal cycle time however strongly depends on operating conditions such as ambient air temperature, available heat source temperature, desired target cooling temperature, and achievable volume flow rates of the secondary heat transfer circuits. In this contribution, we apply a direct method for periodic optimal control to optimize two-bed adsorption chillers. We present a first principles dynamic model of the underlying thermal process. We show that direct methods for periodic optimal control allow for quick and reliable computation of optimal cycle times for a given set of parameters. Contrary to pre-existing methods, fast computation times and guaranteed optimality of the solutions computed by our approach makes it viable to extensively study the simulated optimal cyclic operation of two-bed adsorption chillers under a wide range of varying conditions.