A thermal nonlinear dynamic model for water tube drum boilers

A thermal model for the prediction of possible tube overheating was developed. The model incorporates a nonlinear state space dynamic model that captures the important physical interactions of the main variables of steam generation in naturally circulated water tube drum boilers. This paper provides an investigation of the dynamic effects of rapid rise in fuel flow rate (heat input) on the thermal and flow characteristics of the riser tubes in natural circulation water tube boilers. The system under consideration includes the drum, riser and downcomer as its major components. The dynamic response of the system's state variables due to rapid rises in fuel flow rates was investigated. The results show that the sudden rise in the firing rate is followed by an increase in the steam quality, which is accompanied by a decrease in the circulation rate as a result of increase in the pressure. The riser temperature increases slightly above the saturation temperature due to the increase in the steam temperature and due to the dynamic influence resulting from sudden increase in the heat flux. The present calculations of the water level in the drum provide good comparison with those in the literature. Copyright © 2009 John Wiley & Sons, Ltd.     

The natural circulation solar water heater model with linear temperature distribution

This paper presents an analysis of natural circulation of a compact thermosyphon solar domestic hot water (SDHW) system produced and commercialised locally in Algeria. Calculations and measurements were performed on the mass flow rate, temperature rise fluid and absorber temperatures inside the thermosyphon of parallel tube design. Comparison between experimental and theoretical results is presented.     

A solid polymer water electrolysis system utilizing natural circulation

Solid Polymer Water Electrolysis (SPWE) is a method to efficiently produce high-purity hydrogen gas using a polymer electrolyte membrane-based system. SPWE systems that utilize natural water circulation (resulting from a difference in buoyancy) are a promising technology, which need no additional circulation pump for water supply to the electrolysis cells, and generate no pressure difference between the hydrogen generation and oxygen generation chambers. However, despite not needing an accurate pressure control, gas bubbles formed and trapped within the cell stacks can inhibit heat convection, leading to hot-spot formation and consequent destructive degradation. Improving the reliability is therefore one of the most important technological issues in natural circulation SPWEs. In this study, hot-spot formation is studied both by numerical heat and flow analysis, and by experimental in-situ visualization. This leads to insights into the degradation mechanisms of SPWE stacks, and their possible solutions. An improved design for an SPWE cell stack is successfully fabricated, and reliable operation up to 5000 h is demonstrated.     

Analysis of natural circulation solar water heating systems

This paper presents an analysis of the performance of a solar water heating system with natural thermosyphon circulation between the collector and the storage tank. The analysis is based on the formulation by Ong except that provision for withdrawal of hot water from the tank (for domestic/ industrial use) has been made in the energy balance equation; further in contrast to the use of the finite difference method by Ong, explicit expressions have been obtained. The results of the present analysis (in the absence of withdrawal of hot water from the tank) are seen to be in better agreement with experiments than the corresponding results of Ong, obtained by use of the finite difference method.

Numerical results, corresponding to hot water retrieved from the storage tank, have been presented for two modes of hot water withdrawal viz. the constant flow rate and constant mean storage tank water temperature.     

Forced versus natural circulation solar water heaters: A comparative performance study

An experimental study has been carried out to compare the performance of natural and forced circulation domestic solar water heaters. Several measurments have been made for the two cases which included; the collector water inlet and outlet temperatures, the mass flow rate, the tank temperature, the ambient temperature and the solar insolation. The main parameters for the solar collector are calculated for the natural and forced circulation systems. These included; the top, back, and overall loss coefficients, the heat removal factor, the efficiency factor, the useful energy gain and the instantaneous efficiency. The comparison showed that the efficiency of the forced circulation system could be 35 to 80% higher.     

Similarity theory of solar water heater with natural circulation

The similarity theory of solar thermosyphon collector is developed in the present paper. Ten dimension-less groups or system characteristic parameters which uniquely determine the performance of the collector are derived. The solution shows that the mean efficiency generally increases with increasing incident solar radiation and relative height of the tank. For the frictional parameters N_e and N_f higher than 10⁵, the efficiency appears to be independent of the incident radiation and the relative height of the tank. Therefore, for parallel plate absorber, the tank may be designed to sit on the floor without sacrificing the efficiency since the values of N_e and N_f are usually larger than 10⁵ in most designs.     

连接方式对自然循环太阳能热水系统的影响

以国标GB／T18708—2002《家用太阳热水系统热性能试验方法》为依据,针对自然循环平板式太阳能热水系统在连接方式不同的情况下,其热性能的差异进行了对比试验,并从自然循环系统循环动力——热虹吸压力的角度分析了两种连接方式下热性能差异的原因。     

自然循环锅炉水冷壁温度计算方法及影响因素分析

研究了自然循环锅炉膜式水冷壁管的传热，采用编制程序对水冷壁进行温度场分布的计算，在研究方法上考虑了欠热沸腾起始点的问题，探讨了入口工质状态、炉内热流密度、质量流量、质量舍汽率等对水冷壁温度的影响。计算结果表明，入口水温的变化会影响欠热沸腾起始点的高度，但对温度的影响较小；炉内热流密度、质量流量和质量舍汽率的波动都会对温度产生影响，这主要是由于它们影响了换热系数的大小。     

自然循环热水锅炉进水装置的优化设计

在热水锅炉的优化设计中，要保证上升管、下降管水流速，增加循环动力压头，以保证锅炉安全运行，提高锅炉寿命，通过对引射和插入两种装置的分析计算，总结出在不改变锅炉结构的情况下，增加循环动力压头的方法。     

Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water

The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Our observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.     

一种间接式自然冷却风冷冷水循环及节能研究

简要介绍不用压缩机的而采用风机及冷冻水泵变频控制的一种间接式自然冷却风冷冷水机组的配置和系统原理,将其与采用压缩机的低环境温度风冷冷水机组的适用性进行分析对比。依据实际运行效果,论述其节能性,并阐述其与采用压缩机的低环境温度风冷冷水机组联合使用时,可满足环境温度从常温到极低气温下的四季运行需求并提供稳定的制冷量输出。     

Energetic and exergetic performance evaluation of natural circulation solar water heating systems

This study deals with the energy and exergy analyses of natural circulation solar water heating (SWH) systems. The system comprises of a single glazed flat plate solar collector (FPSC) with absorber plate of 2 m², and a separate insulated well-mixed vertical water storage tank (WST) of 125 liters. The variable heat transfer coefficients, water inlet and outlet temperatures of the FPSC; and temperature of heated water stored in the WST are predicted theoretically for each interval. The daily energy and exergy efficiency of the FPSC, WST and SWH system are estimated to be about 39 and 4.36%, 67 and 38.55%, 27 and 1.01%, respectively. It is found that the water inlet temperature, optical efficiency and the solar radiation strongly influence the performance of the FPSC both energetically and exergetically. It is observed that change in the mass flow rate of water improves the exergy efficiency of the FPSC significantly. FPSC has been identified as a critical component of the system where exergy destruction of 308 W/m² takes place daily as compared to 24 W/m² in the WST against available solar exergy of about 663 W/m².     

Energy conservation and payback periods of natural circulation type solar water heaters

Energy savings in relation to different fuels, namely firewood, coal, kerosene, LPG and electricity have been calculated for a pressurized natural circulation type solar water heater with blackboard paint and a selective surface on the absorber. The payback periods have been computed by considering 10% compound annual interest, 5% annual maintenance cost and 5% inflation per year in maintenance cost and fuel prices. The cash flow has also been worked out for both solar water heaters. The cash flow is more for a solar water heater with a selective surface. The payback periods are 2.08–8.67 years for a solar water heater with a selective surface and 2.13–8.96 years for a solar water heater with blackboard paint. The estimated life of a heater is about 15 years. This shows that use of a solar water heater for heating water is very economical.     

Performance and testing of improved natural circulation type solar water heater in Arid areas

A conventional natural circulation type solar water heater has been improved upon by eliminating the float valve and using a sealed pressure tank instead of an open tank. Performance of the improved natural circulation type solar water heater has been compared with and without solar-selective surfaces. The detailed design, fabrication and testing have been reported in this paper. The efficiency of the improved natural circulation type solar water heater with and without solar-selective surfaces is 57.5 and 51.0%, respectively.     

Flow reversal in natural circulation systems

Boiling flow in natural circulation systems can be susceptible to thermohydrodynamic instabilities which can be the cause of mechanical damage. This paper presents an analysis of a circulation system where two unequal heated riser circuits are connected to a common downcomer. It can be shown that for such systems a critical heat absorption ratio exists which is dependent on the geometry and the overall heating conditions. Based on computer simulations, the static and dynamic behaviour of the riser systems with emphasis on the lower heat absorption circuit is discussed. Criteria to avoid flow reversal are presented.     

Energy conservation and field performance of a natural circulation type solar water heater

A natural circulation type solar water heater has been fabricated and installed at the CAZRI Guest House. Its field performance has been evaluated. The heater provides 2001 of hot water at 50–60 °C at all times. This solar water heater is economical.     

Thermal characteristics of a building-integrated dual-function solar collector in water heating mode with natural circulation

As a modified building-integrated solar thermal system, building-integrated dual-function solar collector here proposed is able to provide passive space heating in cold winter, and water heating in warm seasons. In this study, evaluations were made on this modified collector system for the warm period operation under the water heating mode with natural circulation of flow. A dynamic numerical model has been developed and validated by experimental data. Based on practical air-conditioned room design conditions, numerical analysis was performed to study the water heating performance, as well as to compare the solar transmission through building facade in different seasons with or without its presence. The results show that when working in the water heating mode, the system performs well in providing services hot water in the warm seasons without bringing in summer overheating problem.     

Analytical studies on drop of H-W-B constants due to scaling in natural circulation flat plate solar water heater

It is observed that the performance of the natural circulation flat plate solar water heater (FSH) deteriorates faster due to scaling. This paper quantifies the effect of scaling on instantaneous efficiency as variation in Hottel-Whillier-Bliss (H-W-B) constants. The drop in a0 and a1 values with scale growth is evaluated and brought out as the H-W-B plot. The comprehensive analysis using developed software FLATSCALE reflects the deviation of circulating ratio with scale thickness. The individual and combined effect of mass flow rate and thermal resistance on the performance of FSH is also discussed.