Refrigerant-charged integrated solar water heater

The performance of two R-11-charged integrated solar water heater collectors was investigated experimentally for forced and natural circulation water flows. The heat transfer from the refrigerant loop to the hot water storage tank took place through a condenser of novel design integrated within the collector frame. The effect of the condenser inclination on the system efficiency was remarkable for natural circulation water flow but had no significant effect for forced circulation flow. The difference in thermal response between the refrigerant and water loops caused buildup of stored energy in the condenser. This energy affected the buoyancy forces and generated flow pulsation that caused a harmonic-like variation of the system efficiency. This effect vanished with forced water circulation flow. The system efficiency varied between 20 and 50% depending on the solar insolation.     

Improving the performance of a Seawater Greenhouse desalination system by assessment of simulation models for different condensers

The main aim of this paper was the development of a mathematical model for a new proposed passive condenser in order to enhance the performance of a humidification–dehumidification Seawater Greenhouse desalination system. Seawater Greenhouse desalination is used to create a cool environment and at the same time to produce fresh water for irrigation of crops grown inside the unit. The condenser in particular is currently one of the main bottlenecks in the commercialization of the technology. In addition to the current pump driven condenser, two new designs were considered: a passive cooling system with a condenser immersed in a water basin, and an external passive condenser connected to a basin of water placed on top of the cooling unit. The simulated condensate values for the proposed passive cooling condenser were compared with that of the actual measured values of the installed condenser. Preliminary results suggest that the passive condenser has a much greater water production capacity than the existing pump driven system. While the model for the proposed system still needs to be validated experimentally the initial study indicates that the passive containment cooling system is a promising improvement in the further development of greenhouse desalination.     

Simulation of packaged solar heat-pump water heaters

The performance of heat-pump water heaters with solar boosted evaporators is investigated experimentally, and a simulation model in the TRNSYS package is developed for assessing annual performance. A packaged heat-pump system with a passive evaporator and condenser in the one unit, which eliminates parasitic energy consumption of the usual circulating pump and fan-coil unit, is assessed for a range of climatic conditions.     

考虑凝汽器压力的火电厂循泵出口阀启闭规律优化

当火电厂循环冷却水系统中发生水力瞬变时,凝汽器可能出现冷却水失水现象,这将导致凝汽器压力增高,从而影响汽轮机组运行的稳定性及安全性。循泵出口阀启闭规律的优化设定对凝汽器最大失水量能起到一定的控制作用。本文提出水力过渡过程计算结合凝汽器变工况计算的方法来优化设定阀门规律。经应用于300MW汽轮机组实例表明,采用该法确定的最优规律,凝汽器最小过流量从1.991m3/s增加到2.271m3/s,凝汽器最大压力从22.111kPa降低为16.911kPa。这样既能保证循环水系统的水力安全,又考虑了瞬变过程中凝汽器的动态特性,有利于汽轮机组的安全稳定运行。     

Computational study of water transport in proton exchange membrane fuel cells

A computational fuel cell dynamics framework is used to develop a unified water transport equation for a proton exchange membrane fuel cell (PEMFC). Various modes of water transport, i.e., diffusion, convection and electro-osmotic drag, are incorporated in the unified water transport equation. The water transport model is then applied to elucidate water management in three-dimensional fuel cells with dry-to-low humidified inlet gases after its validation against available experimental data for dry oxidant and fuel streams. An internal circulation of water with the aid of counter-flow design is found to be of vital importance for low-humidity operation, for example, in the portable application of a PEMFC without an external humidifier. The general features of water transport in PEMFCs are discussed to show various water transport regimes of practical interest, such as anode water loss, cathode flooding, and the equilibrium condition of water at the channel outlets, particularly for limiting situations where anode and cathode water profiles acquire an equilibrium state. From the practical point of view, the effects of the flow arrangement, membrane thickness, and inlet gas humidity as important determinants of fuel cell performance are also analyzed to elucidate fuel cell water transport characteristics.     

EFFECT OF ADDING A PASSIVE CONDENSER ON SOLAR STILL PERFORMANCE

A theoretical and experimental study was carried out to investigate the effect of adding a passive condenser on the performance of the single slope, basin type solar still. A theoretical model based on Dunkle [1] mass transfer (evaporation) rate was developed. The model assumes that the transfer of water vapour from the still to the condenser is due to one or more of the following mass transfer modes; (i) diffusion, (ii) purging and (iii) natural circulation. The theoretical results indicate that diffusion contribution is relatively small. The contribution through purging represents the fraction [V(cond)/(V(cond) + V(s))] of the still yield, while it represents 75% of the still yield through natural circulation. An experimental study that simulates the purging mass transfer mode was investigated. The experimental results show good agreement with the theoretical predictions and an increase of 45% in solar still efficiency was obtained.     

Shower water heat recovery in high-rise residential buildings of Hong Kong

This paper investigates the potential for shower water heat recovery from bathrooms equipped with instantaneous water heaters in high-rise residential buildings of Hong Kong. A simple single-pass counter-flow heat exchanger installed horizontally beneath the shower drain is employed as a localized heat recovery measure for preheating cold water going to a water heater. The thermal energy exchange is evaluated using the effectiveness-number of transfer units (ε-NTU) approach. Shower usage patterns including shower operating time and water flow rate sampled from an interview survey via the Monte-Carlo sampling technique, together with the water temperatures at the shower heads, shower drains and cold water supply mains recorded in sample shower operations, are the input parameters. The results indicate that 4–15% shower water heat can be recovered through a 1.5 m long single-pass counter-flow heat exchanger for a drainage pipe of diameter 50 mm.     

Experimental study on the immersion cooling of an upward-facing multichip module with an opposing condensing surface

The heat transfer characteristics of a simulated immersion cooling module were studied experimentally. The test module consisted of three doped silicon chips mounted on a horizontal substrate and a downward-facing finned condenser with rectangular fins that were assembled in a rectangular box. FC-72 was used as a test fluid. Experimental data showing the effects of the space height between the chip surface and the fin tip of the condenser, the fin spacing of the condenser, and the cooling water temperature on the boiling curve, are presented. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 497–508, 1998     

Study on performance of solar assisted air source heat pump systems for hot water production in Hong Kong

This paper reports the investigation results on application of the solar assisted air source heat pump systems for hot water production in Hong Kong. A mathematical model of the system is developed to predict its operating performance under specified weather conditions. The optimum flow rate from the load water tank to the condenser is proposed considering both the appropriate outlet water temperature and system performance. The effect of various parameters, including circulation flow rate, solar collector area, tilt angle of solar collector array and initial water temperature in the preheating solar tank is investigated, and the results show that the system performance is governed strongly by the change of circulation flow rate, solar collector area and initial water temperature in the preheating solar tank.     

基于凝汽器强化传热技术的循环水系统节水研究

从提高凝汽器换热系数出发,在保证真空不变的前提下,引出临界污垢热阻的概念,通过分析临界污垢热阻和循环浓缩倍率的关系,将强化传热和循环水节水问题结合起来研究,并通过实例计算出相应的节水量。研究表明,在凝汽器真空和其它换热条件不变时,增大凝汽器换热管汽侧换热系数可以增大临界污垢热阻,从而提高循环水的浓缩倍率,达到循环水系统节水的目的,并且带来可观的节水效益和环保效益。文中分析了某300 MW机组凝汽器采用强化传热管后,汽侧换热系数显著增大,在达到原设计真空下,该机组每年因此可以节水约86.9×104t。     

TP316L不锈钢管在以中水作为循环水补水的凝汽器中的应用

中水对凝汽器铜管腐蚀性强,导致凝汽器频繁发生泄漏。通过采用TP316L不锈钢管替代铜管,并做好不锈钢管的安装及运行维护工作,解决了中水对凝汽器管的腐蚀问题,保证了凝汽器安全运行。     

Theoretical model for evaluation of variable frequency drive for cooling water pumps in sea water based once through condenser cooling water systems

In sea water based once through cooling water system for power plants, sea water is pumped through the condenser and the return hot water is let back to sea. The cooling water pumps (CWP) in power plants generally operate at constant speed, pumping variable quantities of water depending on the tide level in the sea. The variable flow causes variation in condenser back pressure resulting in changes in the turbine cycle heat rate. If the pump speed is controlled using a variable frequency drive (VFD) to maintain design flow irrespective of the tide level, the CWP power consumption can be reduced compared to the case with constant speed CWP. However, the turbine cycle heat rate benefit that could have accrued at tide levels above the pre defined level (for fixing the CWP head) with constant speed CWP would have to be sacrificed. This paper provides a theoretical model with a typical case study to establish viability of providing VFD for CWPs in power plants with sea water based once through condenser cooling water system.     

Influence of PEMFC gas flow configuration on performance and water distribution studied by SANS: Evidence of the effect of gravity

A non intrusive method based on small angle neutron scattering (SANS) has been developed to determine the water concentration profile through the thickness of Nafion^® 117 membrane during fuel cell operation. This technique was used to study the effect of gas flow configuration, co- or counter-flow, on water repartition within the fuel cell both within and outside the membrane. As it has been reported previously in the literature the counter-flow configuration gives better performance than co-flow but more surprisingly we evidence a significant difference in performance between symmetric configurations either in co- or counter-flow. Indeed, for a given current density, cell voltage is higher when the cathode inlet is at the bottom of the cell. We demonstrate that the gravity retains liquid water within the cell which leads to a better membrane hydration. Moreover, we have been able to correlate the average water content within the membrane with the performance and especially with the voltage drop resulting from the membrane resistance.     

Experimental study of a refrigerant charged solar collector

The performance of a heat-pipe solar collector was investigated experimentally using refrigerants R11 as the working fluid. The unit is fabricated locally and its performance is evaluated under Beirut Solar conditions. The heat transfer from the heat pipes to the hot-water storage tank took place through a circular end condenser section of the heat-pipe integrated within the collector frame. Tests of single heat pipes showed that the thermal performance of the heat pipe were dependent on its tilt angle, condenser section length and configuration, and type of internal wick used. A circular condenser end of the heat-pipe performed better than a straight condenser due to increased surface area for heat transfer. The R11-charged solar collector with integrated condenser for secondary cooling of water had an efficiency in early operation hours that reached values higher than 60% for the forced circulation mode. The instantaneous system efficiencies varied from 60 to 20%, which are in the range of conventional water solar collectors. System response was fast and sensitive to the incident solar radiation. The thermosyphonic mode of the system operation generated build up of stored energy in the condenser, resulting in oscillating-type flow thus reducing system efficiency below values obtained with forced circulation. © 1998 John Wiley & Sons, Ltd.     

Condensation in a vertical tube bundle passive condenser – Part 2: Complete condensation

An experimental study of the tube bundle effect on heat removal capabilities in complete condensation mode of a passive condenser was performed. A full scale test section, with four condenser tubes, was designed and constructed to simulate operating conditions of a passive containment cooling system. For complete condensation analysis, pure steam was supplied to the test section and heat transfer properties were measured for pressure from 100 to 280 kPa. The condensation heat transfer results were similar to the findings from single tubes, except for a slightly higher condensate mass flux. This was determined to be a result of turbulent mixing in the secondary boiling water caused by the tube bundle.     

多背压凝汽器的平均冷凝温度

通过理论分析,得出考虑传热系数随凝汽器结构和冷却水入口温度变化条件下多背压凝汽器的平均冷凝温度的关系式,分析了各级传热面积和循环倍率对平均冷凝温度的影响,并以多背压凝汽器的平均冷凝温度为目标函数,得出多背压凝汽器的最优结构。     

Correspondence between solar load ratio method for passive water wall systems and f-Chart performance estimates

A simple correspondence is demonstrated between passive solar system performance and active system f-Chart estimates. The equations describing a thermal network model of a passive water wall solar system are compared term by term with similar equations governing the heat balance in an active solar heating system, making possible an identification of appropriate passive system parameters with active system parameters comprising inputs to the f-Chart procedure. Comparisons of f-Chart predictions and results using the Solar Load Ratio method are made for sample cities in sixteen climatic zones. Results indicate a discrepancy of at most about 9 per cent solar fraction between the two methods in the cases studied.     

Numerical modeling of a zeolite/water adsorption cooling system with non-constant condensing pressure

A new transient two-dimensional model with non-constant condensing pressure for a zeolite/water adsorption cooling cycle is proposed in this paper. This numerical model focuses on the heat and mass transfer behaviors in the adsorber and is solved by the control volume method. Due to the heat transfer limitation in the condenser, the simulated pressure during the isobaric generation phase of the cycle is not constant and will decrease with time. Compared with the model for constant condensing pressure, the cycle duration and cycled adsorbate for the base case are increased. Furthermore, the effect of mass flow rate of condenser cooling water on system performance is also investigated. It is found that both COP and SCP increase with an increase in the mass flow rate of cooling water in the condenser.     

循环水流量变化后端差的变化方向判别

借助凝汽器热平衡关系,对凝汽器理论端差进行了再研究。结果表明,理论端差公式变换后,可进行同一负荷、同一进水温度下循环水流量变化后的端差变化方向判别和端差变化程度计算。与之对应的公式[式(5)和式(4)],可用于凝汽器运行工况的诊断分析。