Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.     

Field test investigation of a double-stage coupled heat pumps heating system for cold regions

This paper reports a double-stage coupled heat pumps (DSCHP) heating system, which couples air source heat pump (ASHP) and water source heat pump (WSHP) together. The system is presented for the first time in open literature with the objective to improve the working condition and heating performance of the ASHP under cold environment. A practical project in Beijing firstly installed this system and field test has been performed for one month. The test results indicate that the DSCHP system can be smoothly and efficiently used for heating in cold regions. Compared with the traditional ASHP heating system, the operating characteristics of the DSCHP heating system are greatly improved, demonstrating that the system can offer considerable application potential in cold regions.     

Measuring procedure and heat transfer modelling of a four-compartment isothermal liquid foodstuff tank tested according to the international ATP Agreement

The preservation of the cold chain is one of the most important requirements in order to maintain quality in the transport of perishable foodstuffs. A significant part of the aggregate of the universally transported perishable foodstuffs and more specific liquid foodstuffs like milk, wine, juice, etc. is carried out by multi-compartment isothermal tanks. According to the international ATP Agreement these special equipment used for international transport of perishable foodstuffs have to be tested periodically in officially recognized national ATP Test Stations to ensure that they meet the specifications. Because of the number of the compartments that have to be measured the testing of a multi-compartment isothermal tank presents special difficulties in comparison to the testing of a refrigerated truck. The instrumentation needed for testing a specimen in these cases is complex and that is why only few ATP Test Stations conduct such tests.In this study a heat transfer numerical simulation is described by a four compartment isothermal tank for the transport of perishable liquid foodstuffs tested inside an ATP chamber. The simulated tests were conducted according to the regulations specified in the United Nations ATP Agreement. A transient finite difference model is developed for a typical four compartment isothermal tank tested inside an ATP Testing Chamber with dimensions and specifications like that of the ATP chamber of the Zografou Campus of the National Technical University in Athens. For the control of the cooling/heating system a proportional–integral controller is employed. The study describes a method for measuring the k-coefficient of a four compartment isothermal tank according to ATP specifications using ordinary laboratory equipments. With the suggested method, the testing of a four compartment tank is possible using only one P–I controller and one thyristor instead of the normally needed four independent P–I controllers and four thyristors in order to adjust the heat power needed inside each compartment of the four compartment isothermal tank.     

An absorption based miniature heat pump system for electronics cooling

The development of an absorption based miniature heat pump system is motivated by the need for removal of increasing rates of heat from high performance electronic chips such as microprocessors. The goal of the present study is to keep the chip temperature near ambient temperature, while removing 100 W of heat load. Water/LiBr pair is used as the working fluid. A novel dual micro-channel array evaporator is adopted, which reduces both the mass flux through each micro-channel, as well as the channel length, thus reducing the pressure drop. Micro-channel arrays for the desorber and condenser are placed in intimate communication with each other using a hydrophobic membrane. This acts as a common interface between the desorber and the condenser to separate the water vapor from LiBr solution. The escaped water vapor is immediately cooled and condensed at the condenser side. For direct air cooling of condenser and absorber, offset strip fin arrays are used. The performance of the components and the entire system is numerically evaluated and discussed.     

Absorption chiller crystallization control strategies for integrated cooling heating and power systems

The concept of an air-cooled absorption chiller system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr–H₂O solution then becomes the main challenge in the operation of the chiller, since the air-cooled absorber tends to operate at a higher temperature and concentration level than the water-cooled absorber due to the relative heat transfer characteristics of the coolant. This leads to crystallization of the working fluid. The paper focuses on the crystallization issues and control strategies in LiBr–H₂O air-cooled absorption chillers. As a result a novel application opportunity is proposed for the integration of absorption chillers into cooling, heating and power (CHP) systems. This new methodology allows for air cooler operation while avoiding crystallization.     

大型LNG储罐罐底保冷层结构优化研究

针对大型LNG储罐罐底漏热量大及基础保冷性能差等问题,对储罐底部保冷结构进行了优化,并对不同储罐底部基础的温度场分布和漏热量进行了数值模拟和现场对比分析。结果表明:储罐冷损失与泡沫玻璃砖厚度呈双曲线函数关系,在内外壁温差为185 K时泡沫玻璃砖保冷层厚度设置为600 mm保冷效果显著,再增加厚度时保冷损失减小缓慢;储罐基础采用夯土基础时保冷性能优于柱桩基础,而夯土基础需在罐底设置加热系统,因此工程上大型LNG储罐需根据实际情况进行选择。     

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.     

Methodology for thermal design of novel combined refrigeration/power binary fluid systems

Refrigeration cogeneration systems which generate power alongside with cooling improve energy utilization significantly, because such systems offer a more reasonable arrangement of energy and exergy “flows” within the system, which results in lower fuel consumption as compared to the separate generation of power and cooling or heating. This paper proposes several novel systems of that type, based on ammonia–water working fluid. Importantly, general principles for integration of refrigeration and power systems to produce better energy and exergy efficiencies are summarized, based primarily on the reduction of exergy destruction. The proposed plants analyzed here operate in a fully-integrated combined cycle mode with ammonia–water Rankine cycle(s) and an ammonia refrigeration cycle, interconnected by absorption, separation and heat transfer processes. It was found that the cogeneration systems have good performance, with energy and exergy efficiencies of 28% and 55–60%, respectively, for the base-case studied (at maximum heat input temperature of 450 °C). That efficiency is, by itself, excellent for cogeneration cycles using heat sources at these temperatures, with the exergy efficiency comparable to that of nuclear power plants. When using exhaust heat from topping gas turbine power plants, the total plant energy efficiency can rise to the remarkable value of about 57%. The hardware proposed for use is conventional and commercially available; no hardware additional to that needed in conventional power and absorption cycles is needed.     

空气源热泵承压热水供应与全新风空调联供系统研究

针对华南地区某连锁酒店客房热水供应及该酒店商场空调需求,提出了空气源热泵承压热水供应与全新风空调联供系统方案,介绍了系统具体构成及关键部件的匹配设计计算方法,利用蓄热水罐、蓄冷风柜等蓄能装置和以PLC为核心的数据采集及控制手段,探讨了冷热需求不同步、冷热负荷不均衡的冷热联供固有局限问题的一种解决方案,并对实施的工程系统进行了全年性现场运行性能测试和分析调试。结果表明:该系统在可靠地满足该酒店热水供应的同时,又满足了该酒店商场夏季全新风降温空调的需求,该系统年平均综合能效比大于4.7,为空气源热泵冷热联供提供了一种工程实例。     

双分层水箱太阳能喷射制冷循环特性

本文提出一种采用双分层水箱的太阳能喷射制冷循环,分层水箱热分层显著,颇具可用能储存优势,结合大小水箱各自的优势弥补因太阳日辐射量波动而导致太阳能利用率不高、太阳能驱动的喷射制冷效率较低等问题。采用逐时冷负荷分析法分析了双分层水箱太阳能喷射制冷系统特性,结果表明:该制冷循环高品位能耗约为普通机械压缩制冷循环的1/5,较传统水箱太阳能喷射制冷循环全天工作时间约多4 h,日产冷量提高36.8%,且分层水箱喷射制冷系统的逐时制冷量与办公室逐时冷负荷更吻合。     

以H2O/LiBr为工质的潜能储存系统分析与研究

节约能源和缓解高峰用电的紧张状况是制冷空调领域的专家非常关注的问题,采用蓄能系统是一种行之有效的方法.制冷潜能储存系统是以H2O/LiBr为工质的蓄能系统,通过分析其工作原理、过程、热力性并与现有其它蓄能系统进行经济性比较得知:该蓄能系统不仅能起到平衡电网峰谷负荷即移峰填谷的作用,且具有其它蓄能系统所无法比拟的优点:如蓄能密度大,当性能系数COP为4.3时,蓄能密度为400kJ/kg;可以利用各种能源如太阳能、地热能及工业废热等作为系统的驱动热源;系统简单;工作介质对环境友好,无任何污染;技术成熟;在经济性方面具有优势,投资少,在10年使用时间内的总收益是冰蓄冷空调系统的7倍.     

Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is difficult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H₂O–LiBr and HN₃–H₂O. The new organic working pair, 2,2,2-trifluoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H₂O–LiBr and NH₃–H₂O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH₃–H₂O. For example, there is a lower boiling temperature difference between TFE and NMP, so a rectifier is needed in refrigeration and heat pump systems. Because TFE–NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator–absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its flow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water flows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle.     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.     

“Heat pumps — status and trends” in Asia and the Pacific

In Asian and Pacific regions, economic growth in the last decade has propelled the use of air-conditioners for space cooling along with the use of reversible heat pumps for year round space conditioning. This has led to the rapid increase of electricity demand for air conditioning in summer. To cope with the increasing power demand and the requirement for efficient energy use for space conditioning, governments and energy supply utilities have encouraged effective use and leveling of power load using a heat pump with thermal storage systems and gas cooling systems, by enacting financial and promotional supports. Status and trends of heat pumps in Asian and Pacific regions, related to the use of heat pumps for space heating and cooling were surveyed from the view points of climate, energy consumption, technologies, markets and promotion measures.     

Regional report Europe: “heat pumps — status and trends”

By 1997 about 90 million heat pumps have been installed worldwide, only less than 5% are located in Europe, historically the cradle of this “thermodynamic heating and cooling process”. The majority of the approximately 4 million installed heat pumps are imported reversible air-to-air systems in southern Europe and only 30% represent the typical European-made heating only electric driven compression systems for space and water heating in buildings in central and northern Europe. The first and second oil crises has been the main cause for a first European heat pump “boom” at the end of the seventies. Consequently the following drop in energy prices negatively influenced the market in some countries. The new renaissance in Europe in the middle of the nineties was initiated by the understanding of sustainable development for a more efficient energy use and the related protection of the environment.     

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.     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.     

Estimation of differential heat of dilution for aqueous lithium (bromide,iodide, nitrate,chloride) solution and aqueous (lithium,potassium, sodium) nitrate solution used in absorption cooling systems

The differential heat of dilution data are estimated theoretically using Duhring's diagrams for water/LiBr, water/(LiBr + LiI + LiNO₃ + LiCl) with mass compositions in salts of 60.16%, 9.55%, 18.54% and 11.75%, respectively, and water/(LiNO₃ + KNO₃ + NaNO₃) with mass compositions in salts of 53%, 28% and 19%, respectively, as these can be potentially utilized as working fluids in absorption cooling systems. The differential heat of dilution data obtained were correlated with simple polynomial equations for the three working fluids as a function of the solution concentration and temperature. The results showed that the differential heat of dilution of the non-conventional working fluid mixtures is lower than that of water/LiBr at typical operating temperature and concentration of interest in absorption cooling cycles employing these working fluid mixtures. The correlations developed could be useful in predicting the differential heat of dilution value while performing heat and mass transfer analyses of these potential non-conventional working fluid mixtures in absorption cooling systems.