Naturally ventilated and mixed-mode buildings—Part II: Optimal control

Natural ventilation and a combination of natural ventilation and fan-assisted cooling, in lieu of or as a supplement to air conditioning, offer significant reductions in building energy use in appropriate climates. In current practice, such buildings are operated with heuristic control strategies, involving the opening of windows under suitable indoor and outdoor conditions. Such methods are sub-optimal because they do not account for building thermal dynamics and predicted weather and therefore do not make decisions on the basis of estimated future conditions. This paper uses building thermal predictions from a data-driven thermal model to assess the impact of window and internal openings and fan operation. It then develops a means of ranking and choosing among a set of cooling strategies, with the objective of maintaining occupied-period temperatures within a specified range and minimizing fan energy use. The control algorithms were assessed with data from a test building.     

Predicting thermal and energy performance of mixed-mode ventilation using an integrated simulation approach

Mixed-mode ventilation can effectively reduce energy consumption in buildings, as well as improve thermal comfort and productivity of occupants. This study predicts thermal and energy performance of mixed-mode ventilation by integrating computational fluid dynamics (CFD) with energy simulation. In the simulation of change-over mixed-mode ventilation, it is critical to determine whether outdoor conditions are suitable for natural ventilation at each time step. This study uses CFD simulations to search for the outdoor temperature thresholds when natural ventilation alone is adequate for thermal comfort. The temperature thresholds for wind-driven natural ventilation are identified by a heat balance model, in which air change rate (ACH) is explicitly computed by CFD considering the influence of the surrounding buildings. In buoyancy-driven natural ventilation, the outdoor temperature thresholds are obtained directly from CFD-based parametric analysis. The integrated approach takes advantage of both the CFD algorithm and energy simulation while maintaining low levels of complexity, enabling building designers to utilize this method for early-stage decisionmaking. This paper first describes the workflow of the proposed integrated approach, followed by two case studies, which are presented using a three-floor office building in an urban context. The results are compared with those using an energy simulation program with built-in multizone modules for natural ventilation. Additionally, adaptive thermal comfort models are applied in these case studies, which shows the possibility of further reducing the electricity used for cooling.     

Identification of key factors for uncertainty in the prediction of the thermal performance of an office building under climate change

There is growing concern about the potential impact of climate change on the thermal performance of buildings. Building simulation is well-suited to predict the behaviour of buildings in the future, and to quantify the risks for prime building functions like occupant productivity, occupant health, or energy use. However, on the time scales that are involved with climate change, different factors introduce uncertainties into the predictions: apart from uncertainties in the climate conditions forecast, factors like change of use, trends in electronic equipment and lighting, as well as building refurbishment / renovation and HVAC (heating, ventilation, and air conditioning) system upgrades need to be taken into account. This article presents the application of two-dimensional Monte Carlo analysis to an EnergyPlus model of an office building to identify the key factors for uncertainty in the prediction of overheating and energy use for the time horizons of 2020, 2050 and 2080. The office has mixed-mode ventilation and indirect evaporative cooling, and is studied using the UKCIP02 climate change scenarios. The results show that regarding the uncertainty in predicted heating energy, the dominant input factors are infiltration, lighting gain and equipment gain. For cooling energy and overheating the dominant factors for 2020 and 2050 are lighting gain and equipment gain, but with climate prediction becoming the one dominant factor for 2080. These factors will be the subject of further research by means of expert panel sessions, which will be used to gain a higher resolution of critical building simulation input.     

带天井高层住宅建筑热环境试验和分析

为研究带天井的高层住宅建筑的热环境现状以及更好地利用天井的自然通风来改善建筑内的热环境,采用实测和问卷调查相结合的方法,对长沙市的一栋带天井的高层住宅建筑进行了热性能试验和分析。在不同的夏日天气状况下,该建筑物居住区房间的平均温度和走廊的平均温度都比室外平均温度低,天井的自然通风在夏季对其周围空间具有良好的降温效果。     

A probabilistic analysis of the future potential of evaporative cooling systems in a temperate climate

The probabilistic projections of climate change for the United Kingdom (UK Climate Impacts Programme) show a trend towards hotter and drier summers. This suggests an expected increase in cooling demand for buildings – a conflicting requirement to reducing building energy needs and related CO₂ emissions. Though passive design is used to reduce thermal loads of a building, a supplementary cooling system is often necessary. For such mixed-mode strategies, indirect evaporative cooling is investigated as a low energy option in the context of a warmer and drier UK climate.Analysis of the climate projections shows an increase in wet-bulb depression; providing a good indication of the cooling potential of an evaporative cooler. Modelling a mixed-mode building at two different locations, showed such a building was capable of maintaining adequate thermal comfort in future probable climates. Comparing the control climate to the scenario climate, an increase in the median of evaporative cooling load is evident. The shift is greater for London than for Glasgow with a respective 71.6% and 3.3% increase in the median annual cooling load.The study shows evaporative cooling should continue to function as an effective low-energy cooling technique in future, warming climates.     

Analysis of annual heating and cooling energy requirements for office buildings in different climates in Turkey

A great amount of world energy demand is connected to the built environment. Electricity use in the commercial buildings, accounts for about one-third of the total energy consumption in Turkey and fully air-conditioned office buildings are important commercial electricity end-users since the mid-1990s. In the presented paper, the interactions between different conditions, control strategies and heating/cooling loads in office buildings in the four major climatic zones in Turkey – hot summer and cold winter, mild, hot summer and warm winter, hot and humid summer and warm winter – through building energy simulation program has been evaluated. The simulation results are compared with the values obtained from site measurements done in an office building located in Istanbul. The site-recorded data and simulation results are compared and analyzed. This verified model was used as a means to examine some energy conservation opportunities on annual cooling, heating and total building load at four major cities which were selected as a representative of the four climatic regions in Turkey. The effect of the parameters like the climatic conditions (location), insulation and thermal mass, aspect ratio, color of external surfaces, shading, window systems including window area and glazing system, ventilation rates and different outdoor air control strategies on annual building energy requirements is examined and the results are presented for each city.     

Inter-related effects of cooling strategies and building features on energy performance of office buildings

The paper compares effects on thermal performance and energy use of various pre-cooling and ventilation strategies, which might be used for reducing peak power demands in typical office buildings located in moderately warm climatic regions. Simulations were performed for different features of the building envelope, and for two levels of internal heat load.Results indicate: significant reductions of required daytime peak power loads may be obtained by cooling strategies that contribute to lowering internal mass temperatures. For buildings with large internal heat loads, intensive night pre-cooling is the most effective strategy for smoothing required power loads. However, for non-loaded buildings, it largely increases total energy loads, and night-time peak power loads. Intensive night ventilation reduces required peak power loads as well as total cooling energy loads for both building types. For non-loaded buildings, it is an extremely efficient strategy, whereas the efficacy of other pre-cooling strategies is highly questionable. Further research should include secondary effects (on required peak power loads, total energy loads, and electricity consumption) as they may decrease the efficiency differences between the two strategies.     

The role of adaptive thermal comfort in the prediction of the thermal performance of a modern mixed-mode office building in the UK under climate change

Climate change is becoming a serious issue for the construction industry, since the time scales at which climate change takes place can be expected to show a true impact on the thermal performance of buildings and HVAC systems. In predicting this future building performance by means of building simulation, the underlying assumptions regarding thermal comfort conditions and the related heating, ventilating and air conditioning (HVAC) control set points become important. This article studies the thermal performance of a reference office building with mixed-mode ventilation in the UK, using static and adaptive thermal approaches, for a series of time horizons (2020, 2050 and 2080). Results demonstrate the importance of the implementation of adaptive thermal comfort models, and underpin the case for its use in climate change impact studies. Adaptive thermal comfort can also be used by building designers to make buildings more resilient towards change.     

New concepts and approach for developing energy efficient buildings: Ideal specific heat for building internal thermal mass

The shortcomings or limitations of the traditional approach to developing energy efficient buildings are that they can not determine: (1) the ideal thermophysical properties of building envelope material, where “ideal” means that such material can use ambient air temperature variation and/or solar radiation efficiently to keep the indoor air temperature in the thermal comfort range with no additional space heating or cooling; (2) the best natural ventilation strategy; (3) the minimal additional energy consumption for space heating in winter or air-conditioning in summer. To overcome these problems, some new concepts for developing energy efficient buildings are put forward in this paper. They are the ideal thermophysical properties of the building envelope material, the ideal natural ventilation rate, and a minimal additional space heating or cooling energy consumption. A new approach for determining these properties is also developed. In contrast to the traditional approach (the thermophysical properties of building envelope material are known and constant so that the relating equations describing the indoor air temperature tend to be linear differential equations), the new approach solves the inverse problem (thermophysical properties, etc. of a buildings are unknown), whose solution can be a function instead of a value. As a first step, the ideal specific heat of the building envelope material for internal thermal mass is analyzed for buildings located in various cities in different climatic regions of China, such as Beijing, Shanghai, Harbin, Urumchi, Lhasa, Kunming and Guangzhou. We found that the ideal specific heat is composed of a basic value and an excessive one which is of δ function for the cases studied. Some limitations that would need further study are introduced in the end of the paper.     

Combined thermal and natural ventilation modeling for long-term energy assessment: validation with experimental measurements

The role of ventilation in the energy balance of buildings is very important, as well as the need to model the use of large openings by occupants in energy simulation. Examples of detailed assessment of natural ventilation in overall building thermal simulation procedures are rather rare, especially for long-term evaluation and different types of ventilation. This study addresses this need focusing on natural ventilation modeling in building energy simulation and introducing coupled thermal and ventilation nodal modeling in 1 h simulation steps. All the related parameters are taken into consideration, from wind pressure data to building envelopes and user behavior. The results are validated by conducting experiments on existing constructions and the thermal behavior is monitored and compared in detail in specific building elements. Coupled thermal and ventilation modeling is then implemented to model and assess ventilation strategies for optimum energy-efficient ventilation in a common apartment. These strategies are compared and analyzed for mild Mediterranean and oceanic climates and optimum design procedures are proposed.     

Control of natural ventilation for aerodynamic high-rise buildings

The aim of the paper is to present a framework for the optimal control of natural ventilation. To account for the significant crossflow between zones, the notion of ASHRAE-equivalent airflow is proposed which is used to evaluate the quality of ventilation provided to different zones within a building. A model-based predictive control approach is developed which is used to simulate different ventilation strategies and select the most appropriate strategy. To this end, the rationale behind choosing an objective function that captures the goals of a ventilation system is described, and candidate objective functions are discussed. Finally, an illustrative example with an elliptical cross-section building is discussed in detail. The framework for model-predictive control developed in this paper is applicable beyond natural ventilation and high-rise buildings.     

The effects of roof covering on the thermal performance of highly insulated roofs in Mediterranean climates

While the EU Directive 2002/91/CE on the Energy Performance of Buildings (EPBD) clearly establishes regulations for the thermal insulation of buildings for saving energy in winter, the summer strategy is described by a little more than qualitative provisions. As a consequence, in the national requirements, the high insulation of the building envelope is considered as the principal strategy to control energy consumption even in summer, regardless of the different climates. This approach leads to a homologation of the building trade, and imposes construction technology and materials which do not adhere to the traditional way of making buildings, like in Southern Europe. Here, the “over insulation” of buildings runs the risk of reducing the effectiveness of traditional passive cooling strategies (thermal mass, air permeability of the roof covering, roof ventilation) and could have adverse effects on internal comfort. In this paper, we focus on the effects of over insulation on the thermal performance of roofs in summer, by analyzing experimental data from monitoring a full-scale mock-up in Italy. Results show how an increase in insulation thickness reduces the effectiveness of traditional passive cooling strategies, as an effect of the thermal decoupling between the interior and the upper layers of the roofs.     

Natural ventilation strategies for indoor thermal comfort in Mediterranean apartments

Natural ventilation strategies as effective low energy refurbishment solutions are identified within this research study, for an existing urban multi-storey apartment building in Athens, representative of over four-million Greek urban residential buildings. Retrofit strategies were evaluated using occupant comfort criteria and the existing ventilation strategy, for a single apartment using dynamic thermal simulations. These strategies included individual day and night ventilation, a wind-catcher and a dynamic façade. Suitable openings operation in response to environmental parameters provided sufficient day and night ventilation and occupant comfort. The inclusion of a wind-catcher yielded very little improvement to the ventilation performance. However, the combined operation of the wind-catcher and the dynamic façade delivered operative temperature reductions of up to 7 °C below the base-case strategy, and acceptable ventilation rates for up to 65% of the cooling period. The successful performance of the proposed strategies highlights their potential for reducing energy consumption and improving thermal comfort in a large number of buildings in hot climates.     

Simulation of wind-driven ventilative cooling systems for an apartment building in Beijing and Shanghai

This paper presents a performance evaluation of two passive cooling strategies, daytime ventilation and night cooling, for a generic, six-story suburban apartment building in Beijing and Shanghai. The investigation uses a coupled, transient simulation approach to model heat transfer and airflow in the apartments. Wind-driven ventilation is simulated using computational fluid dynamics (CFD). Occupant thermal comfort is accessed using Fanger’s comfort model. The results show that night cooling is superior to daytime ventilation. Night cooling may replace air-conditioning systems for a significant part of the cooling season in Beijing, but with a high condensation risk. For Shanghai, neither of the two passive cooling strategies can be considered successful.     

Thermal mass activation by hollow core slab coupled with night ventilation to reduce summer cooling loads

This study deal with the analysis of the effectiveness of free cooling ventilation strategies coupled with thermal mass activation to reduce peak cooling loads.A numerical simulation of the temperatures distribution of an office placed in Milan, Italy, during the month of July, is conducted on a Simulink^® dynamical model.No air-conditioned system is present but two different free cooling systems are analysed and compared. Both systems act a primary ventilation during the day and a night ventilation during the non-occupancy period but the first is a traditional mixing ventilation system, the other is a thermal mass activation system, i.e. the outdoor ventilation air, before entering the room, flows through the ducts of the hollow core concrete ceiling slab.The performances of the two systems are investigated by means of time profile analyses of indoor operative temperatures and by means of frequency temperature distributions during the occupancy period.The cooling performances are measured by two different discomfort indexes: one represents the discomfort time percentage during occupation period, the other the discomfort weighted on the distance of calculated operative temperature from the acceptable temperature interval.This paper, in last analysis, tries to highlight the possibilities on cooling loads reduction and on thermal comfort increase in Mediterranean climate, connected to new strategies for thermal mass activation and night ventilation.     

Climate change,thermal comfort and energy: Meeting the design challenges of the 21st century

This paper addresses the dual challenge of designing sustainable low-energy buildings while still providing thermal comfort under warmer summer conditions produced by anthropogenic climate change—a key challenge for building designers in the 21st century. The main focus is towards buildings that are ‘free running’ for some part of the summer, either being entirely naturally ventilated or mixed-mode (where mechanical cooling is only used when thought to be essential). Because the conditions in these buildings will vary from day to day it is important to understand how people react and adapt to their environment. A summary is made of recent developments in this area and of the climate data required to assess building performance. Temperatures in free running buildings are necessarily closely linked to those outside. Because the climate is changing and outside summer temperatures are expected to increase, the future will offer greater challenges to the designers of sustainable buildings aiming to provide either entirely passive or low-energy comfort cooling. These issues are demonstrated by predictions of the performance of some case study buildings under a climate change scenario. The examples also demonstrate some of the important principles associated with climate-sensitive low-energy design.     

夜间通风与建筑蓄热的非线性耦合

夜间通风是一种冷却建筑物的低能耗手段,其冷却效果和建筑蓄热材料之间的关系非常密切。研究夜间通风和建筑蓄热能力之间的关系对指导建筑物的设计意义重大。在以往的研究中,一般将夜间通风和建筑蓄热之间的关系看成是线性变化的,但是在实际情况中,它们的关系往往是呈非线性变化的。本文独立提出了夜间通风和建筑蓄热的非线性耦合模型并编制了计算程序,最后结合实验进行对比分析,发现数值解和实验数据能够较好吻合。说明该模型可以有效的预测夜间通风的效果。     

Evaluation of environmental design strategies for university buildings

ABSTRACT

This paper examines the performance of environmental strategies in seven recently constructed or refurbished university buildings in the UK. These buildings contain a range of administrative spaces, classrooms, libraries and studios, reflecting their often complex, multi-use, heterogeneous nature. The key features of each environmental strategy are described (including passive, mixed-mode or active systems), in the context of the occupants and spaces they serve and the level of interaction that they afford. Energy performance and occupant thermal comfort (assessed by user surveys) are analysed and compared with studies of other non-domestic buildings, which have typically focused on more predictable single administrative uses (e.g. government offices), and unusually effective operation scenarios (e.g. continuous monitoring by expert building managers). The paper concludes by examining two of the case studies that reflect an increasingly common model of ‘flexible’ environmental design in more detail, identifying key features of the strategies for each building that have had a significant impact on their performance. The design assumptions leading to these features will be explored, and key lessons identified, contributing towards the development of a more robust evidential basis for choosing appropriate environmental strategies for university and other non-domestic buildings in the UK.     

A combined system of chilled ceiling,displacement ventilation and desiccant dehumidification

Different types of heating, ventilation, and air-conditioning (HVAC) systems consume different amounts of energy yet they deliver similar levels of acceptable indoor air quality (IAQ) and thermal comfort. It is desirable to provide buildings with an optimal HVAC system to create the best IAQ and thermal comfort with minimum energy consumption. In this paper, a combined system of chilled ceiling, displacement ventilation and desiccant dehumidification is designed and applied for space conditioning in a hot and humid climate. IAQ, thermal comfort, and energy saving potential of the combined system are estimated using a mathematical model of the system described in this paper. To confirm the feasibility of the combined system in a hot and humid climate, like China, and to evaluate the system performance, the mathematical model simulates an office building in Beijing and estimates IAQ, thermal comfort and energy consumption. We conclude that in comparison with a conventional all-air system the combined system saves 8.2% of total primary energy consumption in addition to achieving better IAQ and thermal comfort. Chilled ceiling, displacement ventilation and desiccant dehumidification respond consistently to cooling source demand and complement each other on indoor comfort and air quality. It is feasible to combine the three technologies for space conditioning of office building in a hot and humid climate.     

An investigation into the thermal performance of office buildings in Ghana

This paper comprises the outcome of a long-term monitoring of the thermal conditions in a selected number of office buildings in Kumasi, Ghana. The observed data was not only used to assess indoor environmental conditions in these offices, but also to calibrate a number of thermal simulation models of the buildings. Thus, a simulation-based exploration of thermal retrofit options towards a general reduction of cooling requirements could be conducted. Moreover, the impact of thermal retrofit measures towards reducing carbon dioxide (CO₂) emissions was assessed and the amortization times for investments in such retrofit measures were estimated. The results suggest that improvements in building fabric and controls (with payback times of 3–12 years) can reduce buildings’ cooling loads by around 20–35% and CO₂ emissions around 27%. Additionally, the outcome of interviews conducted showed that 45% and 70% of occupants in mixed-mode and naturally ventilated buildings were uncomfortable with the air quality during the dry season. The highest dissatisfaction with indoor environment was reported by 85% of the occupants in the naturally ventilated building. The importance attached to the operation of windows and shades was relatively high, 55–80%, depending on building type.