Investigation of a hybrid system of nocturnal radiative cooling and direct evaporative cooling

In this paper, the results of a study on a hybrid system of nocturnal radiative cooling, cooling coil, and direct evaporative cooling in Tehran have been discussed. During a night, the nocturnal radiative cooling provides required chilled water for a cooling coil unit. The cold water is stored in a storage tank. During eight working hours of the next day, hot outdoor air is pre-cooled by means of the cooling coil unit and then it enters a direct evaporative cooling unit. In this period, temperature variation of the conditioned air is investigated. This hybrid system complements direct evaporative cooling as if it consumes low energy to provide cold water and is able to fulfill the comfort condition whereas direct evaporative alone is not able to provide summer comfort condition. The results obtained demonstrate that overall effectiveness of hybrid system is more than 100%. Thus, this environmentally clean and energy efficient system can be considered as an alternative to the mechanical vapor compression systems.     

Application of the control methods for radiant floor cooling system in residential buildings

In applying radiant floor cooling, its control system must prevent the floor surface condensation in hot and humid weather conditions. With no additional dehumidification system, only the radiant floor cooling system prevents floor condensation. In this case, the effects of the control of the cooling system on the indoor conditions can be changed because of the thermal inertia of the systems. Also different types of control system can be composed according to the control methods, which can affect the construction cost in the design stage. Therefore, the control methods for the radiant cooling system with respect to floor surface condensation must be studied. Furthermore, because Korean people's lifestyle involves sitting on the floor, it is necessary to evaluate if a floor cooling system will influence the thermal comfort of the occupants. This study intends to clarify the control methods of the radiant floor cooling system and to analyze the control performance and applicability of each control method with regard to the floor surface condensation and comfort by computer simulations and experiments on the control methods of the radiant floor cooling system. The results of computer simulations and experiments show that water temperature control is better than water flow control with respect to temperature fluctuations in controlling room air temperature. To prevent floor surface condensation, the supply water temperature could be manipulated according to the dew point temperature in the most humid room, and in individual rooms, the water flow rate (on/off control) can be controlled. Also, the results of radiant cooling experiments show that the floor surface temperature remained above 21 °C, the temperature difference among surfaces remained below 6 °C, and the vertical air temperature difference remained below 1.9 °C, conforming well to comfort standards.     

Experimental investigation of two-stage indirect/direct evaporative cooling system in various climatic conditions

Cooling performance of two-stage indirect/direct evaporative cooling system is experimentally investigated in the various simulated climatic conditions. For this purpose, a two-stage evaporative cooling experimental setup consisting of an indirect evaporative cooling stage (IEC) followed by a direct evaporative cooling stage (DEC) was designed, constructed and tested. Due to the wide variety of climatic conditions in Iran, two air simulators were provided to simulate outdoor design condition of different cities in primary and secondary air streams. Results show that under various outdoor conditions, the effectiveness of IEC stage varies over a range of 55–61% and the effectiveness of IEC/DEC unit varies over a range of 108–111%. Aspects of achieving comfort conditions and power saving have been investigated with related excess water consumption. Considering the evaporative comfort zone, this system can provide comfort condition in a vast region in Iran where direct evaporative alone is not able to provide summer comfort condition. More than 60% power saving could be obtained by this system in comparison with mechanical vapor compression systems with just 55% increase in water consumption with respect to direct evaporative cooling systems. This system can fill the gap between direct evaporative cooling systems and mechanical vapor compression systems as an energy efficient and environmentally clean alternate.     

蒸发式冷气机性能参数的试验研究

在改造后的风管式焓差法试验台上对样机进行了热工性能试验。试验分两个阶段:将喷淋水温度设定为进口空气湿球温度,试验研究不同空气流速和水汽比对阻力和效率的影响,得出最佳空气流速和水汽比;在此基础上测试喷淋水温度对水蒸发冷却器效率的影响。试验数据的整理与分析结果表明,最佳迎面空气流速为1.0m/s,最佳水汽比为0.1,蒸发冷却效率和显热制冷量随着喷淋水温度的升高而降低。根据冷气机制冷量实测值不低于明示值95%的规定,经计算得出,喷淋水温度的波动范围为20.13～25.86℃。     

转轮除湿与单元式空调机相结合的空气处理系统

本文介绍了转轮除湿与直接膨胀式单元机组相结合的复合空调系统的2种紧凑型组成形式:前置预冷与后置冷却,并在几种不同室外工况下,针对应用于独立新风系统,对比分析了这2种典型组成方案的性能特点。结果表明,这2种除湿方式在4种典型工况下的制冷能耗相差不明显,在10%以内;但后置冷却除湿在高温高湿工况下难以实现较大的单位除湿量,且再生空气温度高;而前置预冷除湿系统采用较小的除湿转轮尺寸,结构更紧凑,可明显降低初投资,且再生空气温度较低。因此,前置预冷除湿系统在较小流量空气处理应用方面更具优势,更具商品化发展潜力。     

掘进工作面降温参数拟合分析

为了掌握矿井掘进工作面降温参数变化规律,更好的指导现场实施与应用,以存在高温热害现象的长距离掘进巷道(掘进巷道长度1 000 m)为研究对象,现场实际测定矿井降温系统运行后部分测点风流温度,拟合分析得到了部分参数之间较准确的定量关系。拟合结果表明:赵楼煤矿掘进工作面迎头温度与风筒出口风流温度(干、湿球温度)近似呈线性关系;与空冷器后处理风流温度(干球温度,相对湿度近似为100%)在一定范围内近似呈二次抛物线关系。此外,掘进工作面附近干湿球温度差值较大,最大可达5~7 ℃。风流在回头过程中热湿交换严重,很容易形成放大的回头增热、增湿现象。     

Evaluation of alternative arrangements of a heat pump system for plume abatement in a large-scale chiller plant in a subtropical region

Heat pumps could be used to produce hot water for hybrid cooling towers for preventing the occurrence of plume in subtropical regions. The evaporative side of the heat pump system could be arranged either at the inlet side or at the outlet side of these cooling towers for cooling down the cooling water temperature. Alternatively, the evaporative side of the heat pump system could also be arranged at the evaporative side of chillers to reduce return chilled water temperature and therefore to reduce the cooling load of chillers. This study presents the evaluation of the impacts of these three arrangements of the heat pump system on the plume control performance and the energy performance in a large-scale chiller plant in Hong Kong. The performance prediction and evaluation of the chiller plant and the plume abatement system were conducted on a dynamic simulation platform. The results show that these three arrangements have almost the same plume control performance with sufficient plume control capability. The results also show that the arrangement of the evaporative side of the heat pump system for cooling down return chilled water temperature has much better performance that the other two arrangements for improving the overall energy efficiency.     

Feasibility of using various kinds of cooling systems in a multi-climates country

There are few countries in the world where they have special characteristics including weather conditions, wind direction, and topography. In other words, some countries have multi-climate regions, and needed to be investigated more precisely. Based on these conditions, several cooling systems can be used for each region. In this study, the potential of the direct evaporative cooling (DEC) systems, indirect evaporative cooling (IEC) systems, desiccant wheel (DW) with DEC systems, and combination of theses methods for cooling in the cities of Iran are investigated. In the remaining regions, mechanical cooling systems and absorption systems can be used. According to weather condition and natural resources for water in Iran, DEC can be considered as a serious alternative for cooling. In this paper, based on numerical simulation and long-term meteorological measurements, general set of criteria is obtained. Results show that according to population dispersal DEC systems can provide comfort conditions for 53% of people of Iran. IEC, DW, combination of these systems, and mechanical cooling systems can supply comfort condition in remaining major cities of Iran consequently. Moreover, for showing the uniqueness of weather condition which leads to these varieties of climates, other important cities in the neighboring countries are compared with this numerical simulation code.     

环境工况对空调器运行性能的影响

通过试验研究了房间室内侧和室外侧温湿度的变化对空调器运行性能的影响。结果表明,在制冷系统不变的情况下,室内侧湿球温度和室外侧干球温度的变化对空调器运行性能影响较大,而室内侧干球温度和室外侧湿球温度的影响则稍小。     

地板辐射与置换通风空调系统运行参数

建立了基于EnergyPlus的地板辐射供冷加置换通风空调系统模型,模拟得到的室内温度和辐射地板所承担冷量与实验结果的误差小于±7%。在此模型基础上,改变送风参数和供水参数,得到置换通风供冷量、辐射地板供冷量、地板表面温度、室内空气平均温度、AUST温度等参数的变化规律。结合热舒适性模型,得到满足室内热舒适性(-0.5≤PMV≤0.5)条件下,置换通风的送风参数和辐射地板的供水参数范围,为复合系统设计和应用提供依据。     

混合式地源热泵系统的运行控制策略研究

针对目前比较流行的混合式地源热泵系统,提出了三种运行控制策略并建立了基于圆柱热源理论的评价模型。模拟运行结果表明,运行控制策略面临地下热平衡与冷却塔运行时间之间的协调优化问题。从冷却塔运行节能及地下冷热平衡的角度考虑,建议采用设定地埋管换热器流体平均温度或设定地埋管换热器流体温度与环境湿球温度的差值的控制策略。混合式系统应综合考虑各种影响因素以确定合适的运行控制策略,必要时也可采用三种控制策略相互组合的运行方式。     

Study on indoor thermal environment of office space controlled by cooling panel system using field measurement and the numerical simulation

The purpose of this study was to analyze the performance of a cooling panel system installed in the vertical plane with condensation (hereinafter a cooling panel system) using field measurement and coupled simulation of convection and radiation (CFD). Unlike an all-air cooling system, the cooling panel system could remove the cooling load by convection and radiation. Since the surface temperature of the cooling panel analyzed in this study can be controlled under the dew point, this system is expected to be energy-saving and dehumidifying. In the first step of this study, the indoor thermal environment of the office space, which is air-conditioned by a cooling panel system, was analyzed by field measurement and CFD. By comparing both results, the expected precision of CFD for an indoor space with a cooling panel was examined and the CFD method verified from a practical point of view. In the next step, the thermal environment of the model office space, which is cooled by three types of HVAC system (cooling panel system, all-air cooling system, and hybrid air cooling system composed of cooling panel and natural ventilation), was analyzed using CFD. Installing a human model in the office, the characteristics of heat transportation from the human model were also analyzed. The analysis deals with only sensible heat in this study. The operative temperature in the cooling panel system was lower than that in the all-air cooling system when each of the sensible cooling loads of all types of HVAC system was the same. In conclusion, the cooling panel system was found to be very energy-efficient.     

地板辐射供冷-置换通风的实验研究

为了研究地板辐射供冷的热工性能,测试了北京地区不同室外气温下地板辐射供冷系统的运行工况,得到了该系统的制冷量,地板表面温度,室内温度场分布等参数,并且把单独地板辐射供冷系统的运行参数与地板辐射供冷-置换通风复合式系统进行了对比,提出了将地板辐射供冷与置换通风配合用于夏季空调室内供冷除湿的新型空调方式,置换通风系统在近地面处形成干燥空气层,可有效防止夏季热湿天气在地板表面出现结露现象,并且使这种新型空调系统条件下地面与室内的换热得到强化.通过理论分析和实验研究指出这是一种舒适、节能的空调方式.     

The effects of climate change on energy consumption of cooling systems in Tehran

Energy consumption in buildings is heavily affected by climate change. It causes significant variations of the outdoor design conditions, cooling and heating loads. In this paper, a total of 40 years (1967-2006) of hourly temperature data were considered in four decades and outdoor design conditions for each decade were calculated. It was found that wet-bulb temperature (WBT) has increased by 3.28 °C during the mentioned period, which impacts on design and selection of the cooling equipment. The results show that cooling hours is projected to increase around 15% over the whole 1967-2006 period. The most important result found in this study is the inability of direct evaporative cooling (DEC) systems in preparing comfort condition after 1997, whereas these systems provided thermal comfort in Tehran's buildings from 1967 to 1996. The prediction of climate change impacts on the outdoor design conditions and cooling systems performance until 2020 demonstrates dry bulb temperature (DBT) and WBT increase 1.05 °C and 2.42 °C respectively. Hence, indirect-direct evaporative cooling systems (IDEC) may not be responsible for the years after 2016. Besides, the additional energy for absorption and vapor compression (VC) cooling systems will grow quickly in the future.     

Energy performance optimization of radiant slab cooling using building simulation and field measurements

Few field studies of energy performance of radiant cooling systems have been undertaken. A recently constructed 17,500 m² building with a multi-floor radiant slab cooling system in the tower was investigated through simulation calibrated with measured building energy use and meteorological data. For the very cold, dry region where the building was located, it was found that a typical floor of the tower would have had 30% lower annual energy use with a conventional variable air volume system than with the as-built radiant cooling-variable air volume combination. This was due to (1) simultaneous heating and cooling by the existing radiant cooling and air systems, (2) the large amount of free cooling possible in this climate, and (3) suboptimal control settings. If these issues were remedied and combined with improved envelope and a dedicated outdoor air system with exhaust air heat recovery, a typical floor could achieve annual energy use 80% lower than a typical floor of the existing building HVAC system. This shows that radiant thermal control can make a significant contribution to energy-efficiency, but only if the building design and operating practices complement the strengths of the radiant system.     

冷却塔回流影响数值分析及优化研究

从建筑外观、空间限制、运行噪音等角度考虑,冷却塔一般被布置在隐蔽的区域。冷却塔的布置决定了热湿空气回流的情况,并直接影响冷却塔的运行性能。横流冷却塔侧面进风,逆流是塔身下部四周进风。冷却塔回流导致进风温湿度增高,同时,恶化周边微环境。采用数值模拟方法研究逆流与横流冷却塔布置方式的回流及影响。通过对比2种塔进风口空气的温升,来判断两种塔的回流量情况,并提出可行的措施来减少回流量。模拟表明逆流的进风温升和回流率都低于横流的,是因为逆流布置使进风气流更均匀,且排风风速大。模拟了横流冷却塔出风圆管口加15°向上百叶、加2.2 m进风百叶的优化措施,该措施可有效减少回流量。研究结果可为工程设计提供参考,以降低由于冷却塔设计不当引起不利的回流影响。     

Energy savings potential of a desiccant assisted hybrid air source heat pump system for residential building in hot summer and cold winter zone in China

In hot summer and cold winter zone in China, air conditioning system has four running modes yearly including cooling with dehumidification, cooling, dehumidification and heating in residential buildings. The conventional air source heat pump (ASHP) system is not designed to independently control temperature and humidity, and is not very suitable for the dehumidification mode in the view of building energy consumption. A novel ASHP system combining radiant cooling/heating for residential buildings was presented. The main feature of this hybrid ASHP system is that desiccant wheel and cooling coil accomplish dehumidification process together, and the regenerative heat needed by the desiccant wheel is supplied by the condenser dissipated heat. Based on simulation studies and performance analysis, this paper predicts the primary energy consumption of the hybrid ASHP system in comparison with the conventional ASHP system during the cooling and heating seasons. It was found that primary energy requirement can be reduced by more than 8% in cooling with dehumidification mode, by 50% in dehumidification mode, and by more than 14% in heating mode. The study results prove that the hybrid ASHP system can keep great energy saving and running cost saving yearly, especially in the dehumidification process.     

Effects of utilizing seawater as a cooling source system in a commercial complex

This paper treats energy and cost performance of a cooling source system with indirect seawater utilization for air conditioning in a commercial complex. Seawater utilization has merits as a cooling source, because seawater temperature is lower than outdoor air in summer and it is cost effective because there is no water monetary cost. Actual operating data has been measured for about 2 years and the chiller and system co-efficient of performance (COP) have indicated about 4.77 and 2.93, respectively, even in summer season and the mean efficiency of the thermal storage system was about 89.9% taking into account heat loss of pumps. In addition, we have constructed simulation models for cooling tower systems, air cooling chiller systems and direct seawater utilization systems then compared them to this system. The electric power consumption of the indirect seawater utilization system was almost the same as the other systems except the air cooling chiller system, because using lower seawater temperature made the chiller efficiency higher. In conclusion, our results showed the indirect seawater utilization system was able to improve the system COP compared to air cooling chiller system, and cancel water consumption compared with the cooling tower system, and cut down an initial and maintenance costs compared with the direct seawater utilization system.     

Experimental study of cooling effects of a passive evaporative cooling wall constructed of porous ceramics with high water soaking-up ability

As a passive cooling strategy aimed at controlling increased surface temperatures and creating cooler urban environments in summer, the authors developed a passive evaporative cooling wall (PECW) constructed of porous ceramics. These ceramics possess a capillary force to soak water, which means that their vertical surface is wet up to a level higher than 100 cm when their lower end is placed in water. The present paper describes an experiment that clarifies the cooling effects of a prototype PECW constructed of pipe-shaped ceramics. The PECW is capable of absorbing water and allows wind penetration, thus reducing its surface temperature by means of water evaporation. Passive cooling effects such as solar shading, radiation cooling, and ventilation cooling can be enhanced by incorporating PECWs into the design of outdoor or semi-outdoor spaces in parks, pedestrian areas and residential courtyards. The following findings were understood from an experimental data collected over a summer period. Wet vertical surfaces of the ceramic pipe reached a height of over 1 m at an outdoor location exposed to solar radiation. Wet surface conditions can be maintained throughout successive sunny days during summer. A slight difference in the vertical surface temperatures of the ceramic pipe was found. The air passing through the PECW was cooled, and its temperature can be reduced to a minimum value by several degrees during summer daytime. It was also found that the surface temperature of the shaded ceramic pipe can be maintained at a temperature nearly equal to the wet-bulb temperature of outdoor air.     

Overall cooling efficiency of a solar desiccant plant powered by direct-flow vacuum-tube collectors: simulation and experimental results

Using solar thermal energy is an interesting option for heat-driven air conditioning, e.g. desiccant cooling. In this article, the autonomous operations of a solar desiccant cooling plant powered by direct-flow vacuum-tube collectors are investigated. A model of the solar installation and the desiccant air handling unit is presented and implemented in the SPARK simulation environment and then it is validated experimentally. The overall cooling efficiency of the system is evaluated using simulation for humid and moderately humid climates and the effect of increasing the regeneration temperature on the cooling capacity, the overall cooling efficiency is studied and finally the overall efficiency of the collectors is calculated for the studied cases.