FUTURE RESEARCH ACTIONS IN PASSIVE COOLING

The PASCOOL program was the most important European project on passive cooling of buildings. The project addressed topics included solar control, the combined effect of ventilation and thermal mass, thermal comfort during summer and the potential of natural cooling techniques. PASCOOL put in evidence also the axes towards which future research on passive cooling should be oriented. This research, giving the continuously increasing trend of energy consumption for cooling purposes, is absolutely necessary in order to take advantage of the complete potential that passive cooling can offer to buildings while maintaining the living standards, health and comfort of the occupants. This paper presents these future issues that comprise (a) research on the microclimatic scale in order to address the impact of outdoor environment on the cooling load of buildings, (b) investigation of comfort requirements under transient conditions during summer, (c) research on natural ventilation in urban environments and the impact of outdoor pollution on indoor air quality, (d) development of alternative cooling systems and techniques, (e) development of integrating design concepts optimising the use of solar heating, passive cooling and natural light in buildings.     

Nonlinear coupling between thermal mass and natural ventilation in buildings

An ideal naturally ventilated building model that allows a theoretical study of the effect of thermal mass associating with the non-linear coupling between the airflow rate and the indoor air temperature is proposed. When the ventilation rate is constant, both the phase shift and fluctuation of the indoor temperature are determined by the time constant of the system and the dimensionless convective heat transfer number. When the ventilation rate is a function of indoor and outdoor air temperature difference, the thermal mass number and the convective heat transfer air change parameter are suggested. The new thermal mass number measures the capacity of heat storage, rather than the amount of thermal mass. The analyses and numerical results show that the non-linearity of the system does neither change the periodic behaviour of the system, nor the behaviour of phase shift of the indoor air temperature when a periodic outdoor air temperature profile is considered. The maximum indoor air temperature phase shift induced by the direct outdoor air supply without control is 6 h.     

An experimental tool for research and educational applications on HVAC systems

This paper describes an experimental tool elaborated at the building sciences laboratory (LASH/DGCB). The objective of this tool is to develop control strategies for heating, cooling and ventilation, and test them on heating, ventilating and air conditioning equipment. The tests are conducted in a large room of the laboratory equipped with a double-flow ventilation system and a global control/data acquisition system. The tool is used for research and some experiments are led by engineering students. It enables the identification of optimal parameters for each developed control strategy, in order to reach good thermal comfort, acceptable indoor air quality and good stability of actuators at the least energy cost. In this paper we present an example of global control strategy for heating and cooling periods based on interior temperature control (fuzzy regulator). This strategy includes an indoor air quality controller (proportional integral or fuzzy regulator) based on indoor CO₂ level control. A measurements campaign during an occupied period is shown with an overview of the actuators’ behaviour. This tool also highlights the key role of advanced control techniques to manage heating, cooling and ventilation systems.     

Performance evaluation of a radiant floor cooling system integrated with dehumidified ventilation

The radiant floor cooling system can be used as an alternative to all-air cooling systems, using the existing Ondol system (a radiant floor heating system) in Korea to save energy and maintain indoor thermal comfort. Unfortunately, a radiant floor cooling system may cause condensation on the floor surface under hot and humid conditions during the cooling season. In addition, the radiant floor system does not respond quickly to internal load changes due to the thermal storage effect of the concrete mass, which is usually present in radiant floor cooling systems.This study proposes a radiant floor cooling system integrated with dehumidified ventilation, which cools and dehumidifies the outdoor air entering through the cooling coil in the ventilator by lowering the dew-point temperature to prevent condensation on the floor surface. Furthermore, outdoor reset control was used to modulate the temperature of chilled water supplied to the radiant floor, and indoor temperature feedback control was then used to respond to the internal load changes.To evaluate the performance of the radiant floor cooling system integrated with dehumidified ventilation, both a physical experiment in a laboratory setting and TRNSYS simulation for an apartment in Korea have been conducted. As a result, it was found that the proposed system was not only able to solve the problem of condensation on a floor surface but also to control the indoor thermal environment within the acceptable range of comfort. Furthermore, the proposed system improved the responsiveness to internal load changes.     

Integration of variable refrigerant flow and heat pump desiccant systems for the cooling season

Energy saving and indoor air condition enhancing potentials by integrating the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems were investigated in a field performance test during a cooling season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–dehumidification assisted VRF systems were investigated. The HPD systems operated in the ventilation–dehumidification mode dehumidify the outdoor air and supply it to the indoor air during the ventilation. It was found that the VRF systems provided an average of 97.6% of the total cooling energy for the HPD ventilation assisted mode. The remainder was the recovered cool by the HPD systems during ventilation. The VRF systems provided an average of 78.9% of the total cooling energy for the HPD ventilation–dehumidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent cooling. Overall, among the three operating modes, it is concluded that the HPD ventilation–dehumidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones, but more than the non-ventilated ones, while providing the best indoor thermal comfort and indoor air quality conditions. For the total system, the HPD ventilation–dehumidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.     

Effectiveness of the earth tube heat exchanger system coupled to a space model in achieving thermal comfort in rural areas

The aim of this work is to investigate by modelling the possibility of reducing the operational energy of a typical house without negatively affecting its embodied energy. This is done through consideration of different building materials coupled with the use of an earth to air heat exchanger (EAHE) for fresh air supply and cooling. For known indoor and outdoor conditions and for given building materials (thermal capacity and conductance), a ventilation controller determines the amount of flow rate needed to temperate the indoor air temperature to achieve thermal comfort. Different wall configurations are assumed for each of the living zone and the bedroom zone of the apartment. It is found that the use of an optimal wall configuration in each zone coupled with the EAHE results in 76.7% energy savings compared with the reference case with conventional cooling.     

Residential energy savings through modified control of space-conditioning equipment

Energy requirements for heating and cooling homes depend strongly on how the space conditioning equipment is operated. Using detailed computer simulations for a typical house, the energy savings for decreasing winter thermostat settings and for further reductions during the night are given as a function of climate. Similarly, the energy savings for increased thermostat settings during the air-conditioning season are given for the coterminous states. Air-conditioning requirements can also be reduced by allowing natural ventilation to cool homes under certain outdoor conditions. A comparison between air-conditioning only and a combination of air-conditioning and opening windows is given for the coterminous states. Energy savings for all of these options are highly dependent upon local climates. Total savings from adoption of these behavioral changes are equivalent to one-fourth of U.S. imported petroleum.     

Performance of a low-energy house in a mild cooling season. Part 1: Thermal performance of the house

A low-energy house located in Halifax, Canada, was monitored for a year using a computerized data acquisition system. Data on indoor and outdoor temperatures, relative humidities, and power consumption were collected for a whole year. The results of the analysis of cooling season data are presented. It was found that indoor temperature variations in the house were generally small, indicating a high level of comfort. The cooling load, and the cooling energy requirement of the house were low owing to the high level of insulation, and could further be reduced by increasing the thermostat setting. This however would reduce the comfort level in the house. Temperature set-up during unoccupied periods did not reduce daily cooling energy requirement, and addition of an economizer control would not be feasible owing to the small magnitude of potential savings.     

空调新风问题的探讨

本文对新风量的大小与空气品质的关系,新风送风方式与空气品质的关系进行了探讨,指出一味增大新风量并不能达到显著改善室内空气品质的效果,应结合新风的送风方式,并指出置换通风是一种较好的新风送风方式。     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

Energy saving by integrated control of natural ventilation and HVAC systems using model guide for comparison

Integrated control by controlling both natural ventilation and HVAC systems based on human thermal comfort requirement can result in significant energy savings. The concept of this paper differs from conventional methods of energy saving in HVAC systems by integrating the control of both these HVAC systems and the available natural ventilation that is based on the temperature difference between the indoor and the outdoor air. This difference affects the rate of change of indoor air enthalpy or indoor air potential energy storage. However, this is not efficient enough as there are other factors affecting the rate of change of indoor air enthalpy that should be considered to achieve maximum energy saving. One way of improvement can be through the use of model guide for comparison (MGFC) that uses physical-empirical hybrid modelling to predict the rate of change of indoor air potential energy storage considering building fabric and its fixture. Three methods (normal, conventional and proposed) are tested on an identical residential building model using predicted mean vote (PMV) sensor as a criterion test for thermal comfort standard. The results indicate that the proposed method achieved significant energy savings compared with the other methods while still achieving thermal comfort.     

变风量空调系统中的实时优化节能控制

在对变风量空调系统局部控制的分析基础上,利用其变风量末端风门的开度作为各区域相对负荷的指示信号,提出送风静压的实时优化控制方案;同时,针对新的ＡＳＨＲＡＥ通风标准,还提出了基于室内人数检测和焓控制的新风实时优化控制方案。试验结果证明,同常规的控制方案相比,在保证室内热舒适性和空气质量的前提下,这两个方案分别有较好的节能作用。综合采用两种优化方案,系统不仅能够达到节能的目的,而且在较小负荷情况下能够提高室内空气的品质。     

汽车发动机冷却系统节温器的智能控制

分析汽车发动机冷却系统节温器的开度采用智能控制的必要性,并详细阐述发动机冷却系统节温器智能控制系统的设计,包括控制方式设计、模糊控制器设计、控制系统结构设计等.     

Variable air volume ventilation control strategies analysed in six climate zones

This paper presents eight ventilation control strategies and their annual energy and indoor air quality simulation results for an academic building as if it were situated in each of six geographic locations. The results show that without tempering at the terminal boxes, no ventilation strategy could satisfy the outdoor air requirements when the thermal loads are low. The fixed outdoor air percentage method is the worst one. From an economic perspective, strategies using optimization techniques minimize the operating energy demand and consumption. Supply air temperature (SAT) and primary airflow rate are the two proper optimization parameters on the air side of heating, ventilating, and air‐conditioning systems. In addition to control strategies, geographic locations or weather patterns influence the benefits of optimization. Generally, a mild‐dry climate intensifies the advantages of the SAT reset and encourages the primary airflow optimization. Inversely, hot‐humid weather minimizes the benefits. Copyright © 1999 John Wiley & Sons, Ltd.     

Night ventilation control strategies in office buildings

In moderate climates night ventilation is an effective and energy-efficient approach to improve the indoor thermal environment for office buildings during the summer months, especially for heavyweight construction. However, is night ventilation a suitable strategy for office buildings with lightweight construction located in cold climates? In order to answer this question, the whole energy-consumption analysis software EnergyPlus was used to simulate the indoor thermal environment and energy consumption in typical office buildings with night mechanical ventilation in three cities in northern China. The summer outdoor climate data was analyzed, and three typical design days were chosen. The most important factors influencing night ventilation performance such as ventilation rates, ventilation duration, building mass and climatic conditions were evaluated. When night ventilation operation time is closer to active cooling time, the efficiency of night ventilation is higher. With night ventilation rate of 10 ach, the mean radiant temperature of the indoor surface decreased by up to 3.9 °C. The longer the duration of operation, the more efficient the night ventilation strategy becomes. The control strategies for three locations are given in the paper. Based on the optimized strategies, the operation consumption and fees are calculated. The results show that more energy is saved in office buildings cooled by a night ventilation system in northern China than ones that do not employ this strategy.     

Natural cooling of stand-alone houses using solar chimney and evaporative cooling cavity

In this study a low-energy-consumption technique to enhance passive cooling and natural ventilation in a solar house, using a system consisting of a Solar Chimney (SC) and an Evaporative Cooling Cavity (ECC) has been proposed. The capability of the system to meet the required thermal needs of individuals and the effects of main geometric parameters on the system performance has been studied. The dependence of the system performance on outdoor air temperature has been studied to determine the operative conditions for appropriate effectiveness, regarding thermal comfort criteria. To determine the heat and mass transfer characteristics of the system, a mathematical model based on conservation equations of mass and energy has been developed and solved by an iterative method. The findings show that the system is capable of providing good indoor air condition at daytime in a living room, even with poor solar intensity of 200 W/m². The results show that when the relative humidity is lower than 50%, the system can make good indoor air condition even at 40 °C, and a higher performance is achieved using ECC with cocurrent configuration. It is found that the proposed system may be applied successfully in hot arid climates to fulfill the indoor thermal comfort expectations.     

A numerical simulation tool for predicting the impact of outdoor thermal environment on building energy performance

A building affects its surrounding environment, and conversely its indoor environment is influenced by its surroundings. In order to obtain a more accurate prediction of the indoor thermal environment, it is necessary to consider the interactions between the indoor and outdoor thermal environments. However, there is still a lack of numerical simulation tools available for predicting the interactions between indoor and outdoor microclimate that take into account the influences of outdoor spatial conditions (such as building forms and tree shapes) and various urban surface materials. This present paper presents a simulation tool for predicting the effect of outdoor thermal environment on building thermal performance (heating/cooling loads, indoor temperature) in an urban block consisting of several buildings, trees, and other structures. The simulation tool is a 3D CAD-based design tool, which makes it possible to reproduce the spatial forms of buildings and constructed surface materials in detail. The outdoor thermal environment is evaluated in terms of external surface temperature and mean radiant temperature (MRT). Simulated results of these temperatures can be visualized on a color 3D display. Building heating/cooling loads and indoor air temperature (internal surface temperature) can also be simulated. In this study, a simulation methodology is described, and a sensitivity analysis is conducted for a wooden detached house under different outdoor conditions (building coverage, adjacent building height, surrounding with trees or no-trees). Simulation results show that the simulation tool developed in this study is capable of quantifying the influences of outdoor configurations and surface materials on both indoor and outdoor environments.     

Frosting of heat pump with heat recovery facility

This paper aims to prolong the heat pump frost time and reduce its growth with heat recovery facility, which should mix the exhausted indoor and outdoor air before entering the evaporator. An ideal mathematic model is developed for heat transfer, frost generation and airside pressure drop. The properties of the mixture would be obtained by solving the mass and energy conservation equations. A parametric analysis is performed to investigate the effects of air inlet temperature, relative humidity and air mass flow rate on total heat transfer coefficient, frost thickness and airside pressure drop, respectively. The results show that rationalizing the ratio of indoor and outdoor air could prolong frosting time and reduce the frost thickness greatly. The total heat transfer coefficient, frost thickness and airside pressure drop increase monotonically with time going, but are not proportional. Decreasing the mixture inlet air temperature and relative humidity could essentially reduce frost growth on the tube surfaces. This can also be observed when increasing the air mass flow rate.     

发动机电控冷却系统设计及试验

提出取消节温器,水泵、风扇分别电控化的发动机新型电控冷却系统型式,运用集总参数法建立系统的数学模型,并搭建电控冷却系统试验装置;运用反馈线性化方法设计非线性控制器,进行仿真和试验研究;分别设计基本模糊控制器和变论域模糊控制器,进行发动机冷却液温度控制试验。最后综合比较非线性控制、基本模糊控制和变论域模糊控制3种控制策略的控制效果,结果表明:发动机电控冷却系统采用变论域模糊控制效果较优。     

Review on free cooling of buildings using phase change materials

The concept of Green building is gaining importance in the present energy scenario and related environmental issues. Free cooling or ventilation cooling is truly a green concept since even 1 g of carbon is not burnt for the purpose of cooling. Also it ensures that a good indoor air quality is maintained in the building. In this paper a detailed review of work carried out by various researchers on free cooling or ventilation cooling is presented. In addition the major challenges and facts posed in the use of phase change material for free cooling system design such as thermal resistance of air and phase change materials, geometry of encapsulation are discussed in detail. Also the method of energy efficient charging and discharging, effect of phase change temperature, insulation and geographical location are also discussed in this paper. This paper also provides lists the PCM candidates used for free cooling.