Comparative study of various optimization criteria for SDHWS and a suggestion for a new global evaluation

This study compares various optimization criteria for a solar domestic hot water system (SDHWS). First of all, we present the various parameters used to evaluate a SDHWS. We consider the energetic, exergetic, environmental (CO₂ emissions) and financial (life cycle cost) analysis. Various optimization criteria of a standard solar hot water system are then proposed. The optimized solutions are compared with a standard hot water system. The most suitable criteria take into account both energetic (therefore environmental) and financial evaluations. The most powerful solutions tend to increase the collector area - increasing the solar fraction during the mid-season - and reduce the tank volume, thereby decreasing the thermal losses and financial cost.Some of the usual evaluation criteria for SDHWs cannot be used as optimization criteria because they do not consider the auxiliary heater, resulting in inaccurate indications of the system’s performance. Therefore, it seemed important to propose a new evaluation method which integrates the life cycle savings, primary energy savings and CO₂ emission savings with regard to a referenced solution based on a radar diagram of these three fractions. This mode of representation is particularly useful when various auxiliary heaters are compared.     

A study on a ventilation stack integrated with a light pipe

A theoretical and experimental study has been conducted on the performance of a vertical light pipe that also functions as an air flow stack for night ventilation. The rectangular light pipe of height 3 m and cross-section area 0.0625 m² surrounded by an air duct of total cross-section area 0.23 m² is situated above a room of height 3.8 m and floor area 9 m². Heat transfer from the hot water in the wraparound hot water jacket to the air in the duct is assisted by stainless steel fins. The ventilation of the room, due partly to the buoyancy of the air in the duct and partly to the wind effect, amounted to nearly 10 air changes per hour which is sufficient for passive cooling during cooler night periods. The light pipe has specular reflecting walls. It was found that the transmission of daylight through the light pipe in the middle of a partly cloudy day was sufficient for illuminating the room to general illumination level.     

Thermo-economic optimization of hot water piping systems: A comparison study

Four different thermo-economic techniques for optimum design of hot water piping systems are presented. They are as follows: the first one is a sequential optimization of pipe diameter based on minimization of total cost without considering heat losses and then of insulation thickness based on minimization of cost of insulation and heat losses. The second is simultaneous optimization of pipe diameter and insulation thickness based on the first law of thermodynamics and cost. The third is simultaneous determination of pipe diameter and insulation thickness based on maximization of exergy efficiency without considering cost. Finally, the fourth is simultaneous determination of pipe diameter and insulation thickness based on maximization of exergy efficiency and cost minimization. A case study is carried out for a hot water pipe segment, and the differences and merits of each method are discussed.     

两种太阳能集热器的瞬时效率对比分析

对强制循环全玻璃真空管太阳能热水系统和强制循环热管式真空管太阳能热水系统的瞬时效率进行了对比分析,发现全玻璃真空管热水系统的效率曲线的斜率大于热管式真空管热水系统,这说明两种热水系统在运行温度相同时,热管式真空管的热损失较小,瞬时效率较高,有较好的高温特性和保温特性,并且热管式真空管太阳集热器的集热效率波动较小,能稳定在较高的水平。     

A study on the new separate heat pipe refrigerator and heat pump

A new separate heat pipe refrigerator and heat pump is suggested based on the general three temperature thermal jet refrigerator and heat pump cycle. Sub-cooled hot water or other appropriate liquid heated by low grade heat sources forms the hot end and another heat pipe containing evaporator and condenser ends, adiabatic section of two-phase ejector and throttling tube is as the cold end of the separate heat pipe system. Performance relations for the thermal jet refrigerator and heat pump of such system is analyzed and a method of thermodynamic performance analysis is recommended. Primary prediction shows the feasibility of such heat pipe system for cold and warm water supply.     

水泥回转窑余热热水系统的设计

针对水泥回转窑的大量余热资源,提出了余热热水系统的回收方式。着重对系统的设计思想和方法进行研究,提出了设计过程必须遵循的几个原则。在此基础上编制计算程序,通过迭代计算,获得了不同条件下集热管几何参数的选择范围。通过对比分析,确定最优结构下的管径和单根管长,为合理设计水泥回转窑余热热水系统提供了理论依据,具有工程应用价值。     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.     

Investigation of heat transfer and pressure drop in a concentric heat exchanger with snail entrance

In this study, in order to increase heat transfer in concentric double-pipe heat exchangers by passive method, snail which is mounted at inlet of the inner pipe and assumed as a swirl generator was used. In the experimental set-up, cold air in ambient conditions was passed through the inner pipe while hot water was flowing through the annulus. The effects of a snail on the heat transfer and pressure drop were investigated for parallel and counter-flow, and obtained Nusselt numbers (Nu) were compared with those found, using a standard correlation such as Dittus–Boelter equation given for axial flows in smooth pipes. The results were correlated in the form of Nusselt number as a function of Reynolds number, Prandtl number and the swirling angle. An augmentation of up to 120% in Nusselt number was obtained in the swirl flow for counter-flow and 45° swirling angle. Though the swirl flow effect of the snail caused some increase in pressure drop, this effect was unimportant compared with the improvement in heat transfer capacity.     

通水除热管板应用于屋面结构中的模型实验研究

针对夏热冬暖地区的屋顶在夏季受太阳辐射强度大的问题,开发一种新型通水除热管板,将该通水除热管板应用于屋面结构中,构成新型通水除热屋面,可以有效改善夏热冬暖地区顶层屋面的热工性能,在间歇空调状况下,同时达到节能和舒适的目的。通过搭建模型实验台进行模型实验来研究通水除热屋面的隔热性能,得出主要结论如下:将铝制通水除热管板埋入屋顶找坡层,在17:00左右通水,此工况下对屋顶内表面有一定的降温效果,阻止晚间屋顶内表面温度的升高,降低了晚间空调的冷负荷,对夏热冬暖地区居住建筑有着重要的应用价值。     

Dishwasher and washing machine heated by a hot water circulation loop

Electric energy (70–90%) used by electrically heated dishwashers and washing machines is used for heating the water, the crockery, the laundry and the machine and could as well be replaced by heat from other sources than electricity. This article evaluates prototypes of a dishwasher and a washing machine, where the machines are heated by a hot water circulation loop and the heat is transferred to the machines via a heat exchanger. The machine therefore uses water from the cold water pipe. Measurements and simulations have been performed showing that all energy for heating can be replaced if the supply water temperature is 65–70 °C. An alternative and common way to save electricity is to connect the machines to the domestic hot water pipe, but the electrical savings with this measure are much smaller, especially for the dishwasher. Computer modelling has been performed and the model has proved to have a high agreement with measured data. However comparison with manufacturers’ data indicates that the computer models overestimate the energy demand by about 10%.     

Performance prediction of refrigerant-DMF solutions in a single-stage solar-powered absorption refrigeration system at low generating temperatures

A theoretical analysis of the coefficient of performance was undertaken to examine the efficiency characteristics of R22 + DMF, R134a + DMF, R32 + DMF as working fluids, respectively, for a single-stage and intermittent absorption refrigerator which allows the use of heat pipe evacuated tubular collectors. The modeling and simulation of the performance considers both solar collector system and the absorption cooling system. The typical meteorological year file containing the weather parameters for Hangzhou is used to simulate the system. The results show that the system is in phase with the weather. In order to increase the reliability of the system, a hot water storage tank is essential. The optimum ratio of storage tank per solar collector area for Hangzhou’s climate for a 1.0 kW system is 0.035-0.043L. Considering the relative low pressure and the high coefficient of performance, R134a + DMF mixture presents interesting properties for its application in solar absorption cycles at moderate condensing and absorbing temperatures when the evaporating temperatures in the range from 278 K to 288 K which are highly useful for food preservation and for air-conditioning in rural areas.     

Fuel cells and energy networks of electricity,heat, and hydrogen: A demonstration in hydrogen-fueled apartments

A demonstration was performed to evaluate our proposal of a residential energy system based on fuel cells and energy networks of electricity, hot water, and hydrogen. The demonstration was conducted from April 2007 to March 2009 in a small apartment building constructed for experimental purposes in Osaka City. Three small proton exchange membrane fuel cells were installed, and the electricity and hot water from the fuel cells were shared among 6 units via an internal electricity grid and hot water pipe. A hydrogen production facility, a small storage device, and a hydrogen pipe were installed to supply hydrogen to the fuel cells. Six families went about their normal daily lives using this system. The energy flow from hydrogen production to consumption was demonstrated. The results of fuel cell operation, energy supply, and energy demand, as well as an analysis of primary energy saving and CO₂ emission mitigation are presented.     

Aqueous propylene-glycol concentrations for the freeze protection of thermosyphon solar energy water heaters

Using a validated dynamic simulation model, the thermal performance of an indirect thermosyphon solar energy water heater was examined. The heat transfer fluids employed were aqueous solutions of propylene glycol. The effect of varying the glycol concentration on the hot water output and efficacy of freeze protection was determined for a specific pattern of hot water withdrawal and weather for the temperate maritime climate of London, England. The heat output is compared with that of a drain-down direct system.     

Integration of Heat Pipe into Fuel Cell Technology

This paper describes recent applications of heat pipe technology in fuel cell systems, which include new stack designs with heat pipes to improve heat transfer as well as work on fuel cell system level design and engineering with adopting the heat pipe concept. In one design, micro-heat pipes are inserted and bonded in bipolar plates for thermal control in the fuel cell stack. In another design, flat heat pipes are integrated with a carbon bipolar plate for improving thermal control in the fuel cell stack. Finally, based on the heat pipe concept, we specifically developed a series of direct methanol fuel cell (DMFC) systems characterized as passive technology for methanol fuel delivery, water recirculation, and air and thermal management. Long-term durability and stability of the passive DMFC systems have been proved experimentally.     

Numerical and Experimental Investigation of Heat Transfer of α-Al2O3/Water Nanofluid in Double Pipe and Shell and Tube Heat Exchangers

This research presents an experimental and numerical study on the heat transfer of α-Al₂O₃/water nanofluid flowing through the double pipe and shell and tube heat exchangers, under laminar flow conditions. Effects of important parameters such as hot and cold volume flow rates, nanofluid temperature, and nanoparticles concentration on the heat transfer characteristics are investigated. The results indicated that the heat transfer performance of both double pipe and shell and tube heat exchangers increases with increasing the hot and cold volume flow rates, as well as the particle concentrations and nanofluid inlet temperature. Compared with pure water, the results indicated that the heat transfer coefficients of nanofluid in the double pipe and shell and tube heat exchangers are higher than those of water by 13.2% and 21.3%, respectively. Also, the heat transfer performance of nanofluid in a shell and tube heat exchanger is 26.2% higher than the double pipe heat exchanger. A computational fluid dynamics (CFD) technique was used for heat transfer simulation in the previously mentioned heat exchangers. Computed overall heat transfer coefficients of the nanofluids are in good agreement with the experimental data.     

Use of controlled water circulation in tap water heat pumps

Condensers used in tap water heat pumps are either of the integrated passive immersion type or separate units with active water circulation. The use of the active circulation configuration together with a counter-flow condenser and water flow control has a number of advantages compared to passive immersion condensers. In particular, hot water temperatures above the saturation temperature of the condensing refrigerant are possible. The counter-flow arrangement also permits sub-cooling gains in cycle efficiency. These two features offset the pumping power penalty which would not be necessary in a system with a passive immersion condenser.     

采用200H减压阀由一级管网回水向二级管网补水定压方案及经济分析

本文简要叙述200H先导式减压阀的工作原理，重点介绍用其作为由一级管网回水向二级管网补水定压主要阀件时与热网的连接方式、选型计算，与用补给水泵向二级管网补水定压方案的经济比较等问题。     

被动式生态建筑中庭的自然通风设计策略

分析中庭设计存在的问题,将被动式生态设计与中庭结合,归纳总结了寒冷气候区、温和气候区、干热气候区、湿热气候区等4种典型气候类型对中庭被动式生态自然通风设计的影响,提出有效的通风策略和中庭自然通风的季节性调控技术。另外,还提出自然通风控制技术和构造策略,提供了一些新的视角,赋予中庭空间新的意义,为生态建筑中庭的自然通风设计提供参考。     

EVALUATION OF THE DYNAMIC PERFORMANCE OF A HOT WATER TANK WITH BUILT-IN HEATING COIL

Hot water tanks with a built-in water-heating coil are commonly used in district heating house stations in Denmark for domestic hot water (DHW) production and storage. In this study, an evaluation of the dynamic performance of a hot water tank with built-in heating coil is carried out by applying a dynamic simulation programme which has been made previously, based on a simple dynamic model developed by the authors. System evaluation of the way in which system parameters, such as control valve size, heat loss coefficient of the DHW circulation pipe, position of the temperature sensor (for DHW temperature control) and fouling of the heating coil, affect the domestic hot water capacity and the average district heating water cooling for a given hot water tank is presented and discussed in this paper. The evaluation results show the importance of the correct design of the control valve size, the reduction of heat loss from DHW circulation pipes, the careful adjustment of temperature sensor position and temperature sensor set-point, and the reduction of the heat coil fouling growth rate in order to operate the hot water tank in an efficient way and to achieve significant cooling of the district heating water. © 1997 by John Wiley & Sons, Ltd.     

平板式微型热管的温度特性实验研究

平板热管是一种较好的电子散热元件 ,本文对平板热管不同工质、不同的输入功率及不同的充液率的换热特性进行试验。结果表明平板热管有较好的散热效应 ,可以有效地降低物体温度。