Simplified steady-state modeling for variable speed compressor

This paper presents a detailed analysis of semi-empirical methods to calculate mass flow rate, shaft power and discharge temperature for three types of variable speed compressors: reciprocating, scroll and piston rotary. The proposed methods are an integration of physical-based models for constant speed compressor and the physical characteristics of volumetric efficiency and isentropic efficiency between different speeds. The physical-based models were first validated with good agreement with experimental data from publication for the three types of constant speed compressors. The comparison between modeling results and experimental data from publication for the three types of variable speed compressors shows the RMS errors are less than 3%, 3% and 3 °C for refrigerant mass flow rate, compressor power input and discharge temperature, respectively. The model of variable speed compressor will allow the reduction of the number of experimental data required to characterize variable speed compressor behavior in the modeling of refrigeration systems because of its physical mechanisms.     

Study on performance of a heat pump water heater using suction stream liquid injection

Liquid refrigerant injection into a suction line is an effective and practical method to reduce the discharge temperature when a scroll compressor operates at high compression ratios. In the present study, correlations among the compressor suction temperature, discharge temperature, heat pump heating capacity, power consumption, coefficient of performance (COP) and the quantity of suction liquid injection are established. The paper presents experimental analysis and a comparison with calculated results of the heat pump water heater (HPWH) performance with suction liquid injection in different conditions. It is found that the suction liquid injection explicitly lowers the discharge temperature of the compressor and the heating capacity of the unit, but the power consumption increases with COP decreasing. In addition, the highest injection ratio must be controlled fewer than 5%. The suction liquid injection has a better effect on the HPWH at the temperature ranging from ?15 °C to 20 °C. Within this temperature range, the 5% ratio suction liquid injection decreases the discharge temperature of the compressor by 10 °C, while the heating capacity of the HPWH decreases by less than 5%, power consumption increases by less than 1.5%, and COP decreases by less than 7%.     

Investigation of the effects of vapour quality and oil concentration on performance of a swash plate compressor

The main objective of the present study is to experimentally investigate the effects of vapour quality and oil concentration on the performance of a swash plate compressor for automotive air conditioning systems. R‐134a is used as refrigerant. The compressor used is a typical automotive swash‐plate‐type compressor driven by a 10 hp variable‐speed electric motor and lubricated by polyalkylene glycol (PAG) oil. The variables measured during the experiment are pressure, temperature, oil concentration, total mass flow rate and vapour mass flow at the inlet and outlet of the compressor. The experiment was performed at varying compressor speeds, compression ratios and vapour quality. The results revealed some unknown aspects of the compression process in an automotive air conditioning system. The vapour quality does not affect volumetric efficiency, but influences isentropic efficiency of the compressor. In the vapour quality range of 80–90%, isentropic efficiency decreases with increasing vapour quality. During the compression process, only a portion of the liquid refrigerant evaporates. However, at the outlet of the compressor, refrigerant/oil mixture never reaches steady state. The evaporation ratio decreases with increasing compressor speed, and with increasing vapour quality as well. Copyright © 2005 John Wiley & Sons, Ltd.     

Research on oil-free air scroll compressor with high speed in 30 kW fuel cell

To supply the clean compressed air to a fuel cell system, an oil-free compressor is needed. The working process of the oil-free air scroll compressor and its cooling feature are discussed in this paper. A prototype, delivering 100 m³/h and having a speed of 5000 rpm, is researched. The water from the fuel cell system is used as the lubricant of the bearings and the moving parts of the compressor. The prototype of the oil-free air scroll compressor has been designed, fabricated and tested, and the performances are satisfactory. Whatever material the scroll parts are constructed of, for example, aluminum alloy or titanium alloy, the trend of the compressor delivery with rotary speed is similar. However, the power consumption varies with the materials and the surface coating of the scroll parts because different materials and surface coating have different friction coefficients.     

Improving dynamic performance of proton-exchange membrane fuel cell system using time delay control

Transient behaviour is a key parameter for the vehicular application of proton-exchange membrane (PEM) fuel cell. The goal of this presentation is to construct better control technology to increase the dynamic performance of a PEM fuel cell. The PEM fuel cell model comprises a compressor, an injection pump, a humidifier, a cooler, inlet and outlet manifolds, and a membrane-electrode assembly. The model includes the dynamic states of current, voltage, relative humidity, stoichiometry of air and hydrogen, cathode and anode pressures, cathode and anode mass flow rates, and power. Anode recirculation is also included with the injection pump, as well as anode purging, for preventing anode flooding. A steady-state, isothermal analytical fuel cell model is constructed to analyze the mass transfer and water transportation in the membrane. In order to prevent the starvation of air and flooding in a PEM fuel cell, time delay control is suggested to regulate the optimum stoichiometry of oxygen and hydrogen, even when there are dynamical fluctuations of the required PEM fuel cell power. To prove the dynamical performance improvement of the present method, feed-forward control and Linear Quadratic Gaussian (LQG) control with a state estimator are compared. Matlab/Simulink simulation is performed to validate the proposed methodology to increase the dynamic performance of a PEM fuel cell system.     

Modeling and control of air stream and hydrogen flow with recirculation in a PEM fuel cell system—I. Control-oriented modeling

Transient behavior is one of the key requirements for the vehicular application of proton exchange membrane (PEM) fuel cell. The goal of this study is to develop a dynamic model of PEM fuel cell system (FCS) that is capable of characterizing the mixed effects of gas flow, pressure and humidity. In addition to the model of air supply system, the anode recirculation is also presented in this paper by an analytical model of injection pump. A steady-state, isothermal analytical fuel cell model is adopted to analyze the mass transfer in the diffusion layer and water transportation in the membrane. The liquid water accumulation in the cathode flow channel is described by a finite-rate phase-change model and the cathode flooding in the diffusion layer is also discussed. The transient phenomena in FCS are captured by the mechanical inertia of compressor and flow filling in lumped-parameter volumes of manifolds, anode and cathode.     

Numerical analysis on the effects of refrigerant injection on the scroll compressor

Refrigerant injection is an effective method to improve the performance of the scroll compressor and its system under high compression ratio working conditions. This paper intends to find the exhaustive relationship between the injection parameters and the compressor’s performance. Based on a thermodynamic model, the effects of various parameters of refrigerant injection on general performance and inner compression process of scroll compressor have been investigated. As a result, it is found that the injected scroll compressor will get the maximum indicated efficiency when the ratio of inner compression ratio and outer compression ratio is a right value. The right value is 1 for the isentropic compression process, and smaller than 1 for a real compression process. Finally, the effects of all the injection factors on the compression work, refrigerant mass flow rate, p–h diagram, volumetric efficiency, and indicated efficiency are investigated detailedly.     

Design and partial load exergy analysis of hybrid SOFC–GT power plant

This paper presents a full and partial load exergy analysis of a hybrid SOFC–GT power plant. The plant basically consists of: an air compressor, a fuel compressor, several heat exchangers, a radial gas turbine, mixers, a catalytic burner, an internal reforming tubular solid oxide fuel cell stack, bypass valves, an electrical generator and an inverter. The model is accurately described. Special attention is paid at the calculation of SOFC overpotentials. Maps are introduced, and properly scaled, in order to evaluate the partial load performance of turbomachineries. The plant is simulated at full-load and part-load operation, showing energy and exergy flows trough all its components and thermodynamic properties at each key-point. At full-load operation a maximum value of 65.4% of electrical efficiency is achieved. Three different part-load strategies are introduced. The off-design operation is achieved handling the following parameters: air mass flow rate, fuel mass flow rate, combustor bypass, gas turbine bypass, avoiding the use of a variable speed control system. Results showed that the most efficient part-load strategy corresponded to a constant value of the fuel to air ratio. On the other hand, a lower value of net electrical power (34% of nominal load) could be achieved reducing fuel flow rate, at constant air flow rate. This strategy produces an electrical efficiency drop that becomes 45%.     

System analysis of a protonic ceramic fuel cell and gas turbine hybrid system with methanol reformer

The performance of a methanol-fed protonic ceramic fuel cell (PCFC)/gas turbine (GT) hybrid system is investigated in this work. To build the system, Thermolib software is employed with input parameters obtained from references. Effects of air stoichiometry on system performance are analyzed. Results show that, as air stoichiometry is increased, the reformer temperature and CO concentration decrease, while H₂ concentration increases. High air stoichiometry decreases PCFC temperature and performance. GT output power increases with increasing air flow. But, the power consumption by compressor also increases. Overall, to achieve higher system efficiency for this hybrid system, the optimum values of air stoichiometry are from 2.7 to 2.9. An additional heat recovery steam generator can also improve the overall system efficiency from 66.5% to 71.7%. This work helps in understanding the modeling and optimum functioning parameters of high power generation systems.     

Heat and power management of a direct-methanol-fuel-cell (DMFC) system

In this paper, we describe the heat and the power management of a direct methanol fuel cell system. The system consists mainly of a direct methanol fuel cell stack, an anode feed loop with a heat exchanger and on the cathode side, a compressor/expander unit. The model calculations are carried out by analytical solutions for both mass and energy flows. The study is based on measurements on laboratory scale single cells to obtain data concerning mass and voltage efficiencies and temperature dependence of the cell power. In particular, we investigated the influence of water vaporization in the cathode on the heat management of a direct-methanol-fuel-cell (DMFC) system. Input parameters were the stack temperature, the cathode pressure and the air flow rate. It is shown that especially at operating temperatures above 90 °C, the combinations of pressure and air flow rate are limited because of heat losses due to vaporization of water in the cathode.     

The efficient and economic design of PEM fuel cell systems by multi-objective optimization

Since the efficiency of fuel cells is the ratio of the electrical power output and the fuel input, it is a function of power density, system pressure, and stoichiometric ratios of hydrogen and oxygen. Typically, the fuel cell efficiency decreases as its power output increases. In order for the fuel cell system to obtain highly efficient operation with the same power generation, more cells and other auxiliaries such as a high-capacity compressor system, etc. are required. In other words, fuel cell efficiency is closely related to fuel cell economics. Therefore, an optimum efficiency should exist and should result in the definition of a cost-effective fuel cell system. Using a multi-objective optimization technique, the sequential quadratic programming (SQP) method, the efficiency and cost of a fuel cell system have been optimized under various operating conditions. This paper has obtained some analytical results that provide a useful suggestion for the design of a cost-effective fuel cell system with high operation efficiency.     

Anti-disturbance control of oxygen feeding for vehicular fuel cell driven by feedback linearization model predictive control-based cascade scheme

The accurate control of automotive fuel cell oxygen excess ratio (OER) is necessary to improve system efficiency and service life. To this end, an anti-disturbance control driven by a feedback linearization model predictive control (MPC)-based cascade scheme is proposed. It considers strong nonlinear coupling and disturbance injection of fuel cell oxygen supply. A six-order nonlinear fuel cell oxygen feeding model is presented. It is further formulated using an extended state observer to rapidly reconstruct the OER, to overcome the slow response and interference errors of sensor measurements. In the proposed cascade control, the outer loop is the anti-disturbance control which is used to realize the optimized OER tracking and the inner loop via the feedback linearization to linearize the oxygen feeding behaviors conducts MPC to regulate the air compressor output mass flow. The feedback linearization demonstrates a robust tracking performance of nonlinear outputs, and the integral absolute error of anti-disturbance control is 0.3021 lower than that of PI control under a custom test condition. Finally, the numerical validation on a hybrid driving cycle indicates that the proposed cascade control can regulate the fuel cell OER with an average absolute error of 0.02313 in the high air compressor operation efficiency zone.     

Modelling and validation of a gas engine heat pump working with R410A for cooling applications

Gas engine heat pumps play an important role in energy saving and environment protection in both cooling and heating applications. In the present work, a thermal modelling of the gas engine driven heat pump in cooling mode is performed and system main parameters such as cooling capacity, gas engine energy consumption and primary energy ratio (PER) are computed. The modelling of the gas engine heat pump includes modelling of the scroll compressor, the plate evaporator and the gas engine. Discharged refrigerant mass flow rate and compressor power represent the main output parameters of the compressor semi-empirical model. Using the discharged refrigerant mass flow rates along with the available evaporation heat transfer correlations, the system cooling capacity is deduced. Based on the present experimental data, a correlation of gas engine energy consumption as function of compressor power, engine speed and ambient air temperature is obtained. Furthermore, the gas engine heat pump model is validated by comparing experimental and simulation data. The model error percentages to predict the cooling capacity, the gas engine energy consumption and the PER are 7%, 5%, 6% respectively.     

Investigation of two-phase transport phenomena in microchannels using a microfabricated experimental structure

Microchannels (0.05–1 mm) improve gas routing in proton exchange membrane fuel cells, but add to the complexities of water management. This work microfabricates experimental structures with distributed water injection as well as with heating and temperature sensing capabilities to study water formation and transport. The samples feature optical access to allow visualization and distributed thermometry for investigation of two-phase flow transport phenomena in the microchannels. The temperature evolution along the channel is observed that the temperature downstream of the distributed water injection decreases as the pressure drop increases. As the water injection rate is lower than 200 μl/min, there exists a turning point where temperature increases as the pressure drop increases further. These micromachined structures with integrated temperature sensors and heaters are key to the experimental investigation as well as visualization of two-phase flow and water transport phenomena in microchannels for fuel cell applications.     

Thermodynamic analysis of an integrated power generation system driven by solid oxide fuel cell

A new integrated power generation system driven by the solid oxide fuel cell (SOFC) is proposed to improve the conversion efficiency of conventional energy by using a Kalina cycle to recover the waste heat of exhaust from the SOFC-GT. The system using methane as main fuel consists an internal reforming SOFC, an after-burner, a gas turbine, preheaters, compressors and a Kalina cycle. The proposed system is simulated based on the developed mathematical models, and the overall system performance has been evaluated by the first and second law of thermodynamics. Exergy analysis is conducted to indicate the thermodynamic losses in each components. A parametric analysis is also carried out to examine the effects of some key thermodynamic parameters on the system performance. Results indicate that as compressor pressure ratio increases, SOFC electrical efficiency increases and there is an optimal compressor pressure ratio to reach the maximum overall electrical efficiency and exergy efficiency. It is also found that SOFC electrical efficiency, overall electrical efficiency and exergy efficiency can be improved by increasing air flow rate. Also, the largest exergy destruction occurs in the SOFC followed by the after-burner, the waste heat boiler, the gas turbine. The compressor pressure ratio and air flow rate have significant effects on the exergy destruction in some main components of system.     

低品位热能驱动有机朗肯循环的变工况特性

构建有机朗肯循环变工况分析模型,研究热源条件对系统变工况性能的影响规律。结果表明:随着热源温度升高,系统的最佳蒸发压力线性增大,而涡旋膨胀机的等熵效率逐渐减小。相比额定工况,热源温度变化-30.0K与30.0K时,净输出功率变化了-32.4%与18.4%,热效率降低了4.0%与11.4%,热回收效率变化幅度分别为-9.8%及8.9%;当热源温度从423增大至483K时,系统不可逆损失的变化率为-37.1%与45.5%,火用效率的变化率为6.7%与-17.5%。相比热源流量,热源温度对系统变工况性能的影响更大。     

Research on the performance of water-injection twin screw compressor

Due to the development of the automotive fuel cell systems, the study on water-injection twin screw compressor has been aroused again. Twin screw compressors with water injection can be used to supply the clean compressed air for the Proton Exchange Membrane (PEM) fuel cell systems. In this research, a thermodynamic model of the working process of water-injection twin screw compressor was established based on the equations of conservation of mass and energy. The effects of internal leakage and air–water heat transfer were taken into account simultaneously in the present mathematical model. The experiments of the performance of a prototype compressor operating under various conditions were conducted to verify the model. The results show that the predictions of the model are in reasonable agreement with the experimental data.     

丙酮-丁醇-乙醇(ABE)/汽油混合燃料对高速发动机性能影响的试验研究

以发动机4000r/min、节气门开度35%为试验工况,对纯汽油及不同掺混体积分数丙酮-丁醇-乙醇(acetone-butanol-ethanol,ABE)与汽油混合物开展了不同点火提前角和喷油量的试验研究。分析了不同ABE混合比、点火提前角和过量空气系数对发动机性能的影响,并对每种燃料发动机最大功率工况的性能参数进行了比较。结果表明：点火提前角和过量空气系数相同时,混合燃料中ABE含量越高,燃油流量越大,发动机功率越大,有效热效率越高;燃油流量的总热量增大和热-功转换效率提高是促使发动机功率增大的主要原因;随ABE掺混比增加,NO比排放明显降低,CO比排放略有增加,碳氢化合物比排放先增后减。浓混合气工况增加ABE含量比在当量空燃比状态下增加ABE含量,发动机的有效热效率增大更明显,发动机的NO比排放降低更加明显。研究表明高速汽油机掺混ABE燃料具有较好的应用前景。     

Performance analysis for the part-load operation of a solid oxide fuel cell–micro gas turbine hybrid system

In this paper, the performance evaluation of a solid oxide fuel cell (SOFC)–micro gas turbine (MGT) hybrid power generation system under the part-load operation was studied numerically. The present analysis code includes distributed parameters model of the cell stack module. The conversions of chemical species for electrochemical process and fuel reformation process are considered. Besides the temperature distributions of the working fluids and each solid part of cell module by accounting heat generation and heat transfers, are taken into calculation. Including all of them, comprehensive energy balance in the cell stack module is calculated. The variable MGT rotational speed operation scheme is adopted for the part-load operation. It will be made evident that the power generation efficiency of the hybrid system decreases together with the power output. The major reason for the performance degradation is the operating temperature reduction in the SOFC module, which is caused by decreasing the fuel supply and the heat generation in the cells. This reduction is also connected to the air flow rate supplement. The variable MGT rotational speed control requires flexible air flow regulations to maintain the SOFC operating temperature. It will lead to high efficient operation of the hybrid system.     

开式燃气轮机中冷回热再热循环功率和效率优化

建立了开式燃气轮机中冷回热再热（ICRR）循环有限时间热力学模型,导出了循环功率和效率解析式,优化了气流沿通流部分的压降（或低压压气机进口空气质量流率）和中间压比,得到最大功率;并在给定燃油流率的情况下,优化了气流沿通流部分的压降和中间压比,得到最大热效率,进一步在给定低压压气机进口和动力涡轮出口总面积的情况下,优化两者面积分配比,得到双重最大热效率.