Performance evaluation of passive cooling in office buildings based on uncertainty and sensitivity analysis

Natural night ventilation is an interesting passive cooling method in moderate climates. Driven by wind and stack generated pressures, it cools down the exposed building structure at night, in which the heat of the previous day is accumulated. The performance of natural night ventilation highly depends on the external weather conditions and especially on the outdoor temperature. An increase of this outdoor temperature is noticed over the last century and the IPCC predicts an additional rise to the end of this century. A methodology is needed to evaluate the reliable operation of the indoor climate of buildings in case of warmer and uncertain summer conditions. The uncertainty on the climate and on other design data can be very important in the decision process of a building project.The aim of this research is to develop a methodology to predict the performance of natural night ventilation using building energy simulation taking into account the uncertainties in the input. The performance evaluation of natural night ventilation is based on uncertainty and sensitivity analysis.The results of the uncertainty analysis showed that thermal comfort in a single office cooled with single-sided night ventilation had the largest uncertainty. The uncertainties on thermal comfort in case of passive stack and cross ventilation were substantially smaller. However, since wind, as the main driving force for cross ventilation, is highly variable, the cross ventilation strategy required larger louvre areas than the stack ventilation strategy to achieve a similar performance. The differences in uncertainty between the orientations were small.Sensitivity analysis was used to determine the most dominant set of input parameters causing the uncertainty on thermal comfort. The internal heat gains, solar heat gain coefficient of the sunblinds, internal convective heat transfer coefficient, thermophysical properties related to thermal mass, set-point temperatures controlling the natural night ventilation, the discharge coefficient C_d of the night ventilation opening and the wind pressure coefficients C_p were identified to have the largest impact on the uncertainty of thermal comfort.The impact of the warming climate on the uncertainty of thermal comfort was determined. The uncertainty on thermal comfort appeared to increase significantly when a weather data set with recurrence time of 10 years (warm weather) was applied in the transient simulations in stead of a standard weather data set. Natural night ventilation, designed for normal weather conditions, was clearly not able to ensure a high probability of good thermal comfort in warm weather. To ensure a high probability of good thermal comfort and to reduce the performance uncertainty in a warming climate, natural night ventilation has to be combined with additional measures. Different measures were analysed, based on the results of the sensitivity analysis. All the measures were shown to significantly decrease the uncertainty of thermal comfort in warm weather. The study showed the importance to carry out simulations with a warm weather data set together with the analysis under typical conditions. This approach allows to gain a better understanding of the performance of a natural night ventilation design, and to optimize the design to a robust solution.     

Prediction of hybrid ventilation performance using two simulation tools

This paper describes the results of a collaboration between ENTPE-LASH and LEPTAB within the framework of the IEA Annex 35 “Hybrid Ventilation in New and Retrofitted Buildings”. The aim of the work is to carry out a cross-simulation study and to identify optimal control strategies for ventilation systems in order to provide a comfortable thermal indoor environment and a good indoor air quality with energy efficiency. Two models were developed by ENTPE-LASH and LEPTAB in order to carry out hybrid system simulations taking into account air flows, heat transfers and CO₂ concentrations, and numerical results are compared in this study. The models were first adjusted to an experimental cell, HYBCELL, created in the same project. The simulations were carried out using a fictive classroom. The test room was assumed to be in Copenhagen and to be equipped with a natural ventilation system (two inlet grilles and an exhaust chimney) or with mechanical ventilation systems (fans with or without heat recovery). This work also reveals what are the differences in results between the two tools and outlines some conclusions on relative performance of the specific control strategies chosen in this study.     

Cooling load reduction in office buildings of hot-arid climate,combining phase change materials and night purge ventilation

Night purge ventilation is a well-known passive technique for conserving cooling energy by storing night coolth in the thermal mass of the building fabric. We study the effect of phase change materials (PCM) as a light thermal mass, on the cooling load of a typical office building with HVAC system and night purge ventilation in hot-arid climate. In this paper the proper conditions to start night ventilation and the ventilation rate by fans is determined. Additionally, the effect of melting point temperature of the PCMs on the cooling load of the building is investigated. PCMs with proper melting temperature were applied to various building elements, and to the whole model. It was revealed that, application of PCMs will significantly contribute in reducing the cooling load, except for the floor on the ground, which resulted in an increase of the load.     

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.     

Online performance evaluation of alternative control strategies for building cooling water systems prior to in situ implementation

This paper presents the online test and evaluation of the performance of five practical control strategies (fixed set-point control method, fixed approach control method, two near optimal strategies and one optimal strategy) for building cooling water systems to identify the best strategy for future field validation. All of these strategies were tested and evaluated in a simulated virtual environment similar to the situation when they are actually implemented in practice. A virtual building system representing the real building and its central chilling system was developed and used to test the operational performance of the system controlled by different strategies. The packages of each control strategy are separately computed by the application program of Matlab, as the control optimizers to identify the necessary control settings for the given condition based on the collected operation data. The data exchanger between the virtual building system and the control optimizer was managed by a software platform through a communication interface. The results showed that the optimal control strategy is more energy efficient and cost effective than the other strategies, and its computational cost is manageable and can satisfy the requirements of practical applications. This strategy is being implemented in a super high-rise building for field validation.     

Experimental study of vertical ground-source heat pump performance evaluation for cold climate in Turkey

Heat pump systems are recognized to be outstanding heating, cooling and water heating systems. They provide high levels of comfort as well as offering significant reductions in electrical energy use. In addition, they have very low levels of maintenance requirements and are environmentally attractive. The purpose of this study is to evaluate the experimentally performance and energy analysis of vertical ground-source heat pump (GSHP) for winter climatic condition of Erzurum, Turkey. For this aim, an experimental analysis was performed on GSHP system made up in the Energy Laboratory in the campus of Ataturk University. The experimental apparatus consisted of a ground heat exchanger, the depth of which was 53 m, a liquid-to-liquid vapor compression heat pump, water circulating pumps and other measurement and control equipments. Tests were performed under laboratory conditions for space heating, in which experimental results were obtained during January–May within the heating season of 2007. The experimentally obtained results were used to calculate the heat pump coefficient of performance (COP) and the system performance (COPs). The COP and COPs were found to be in the range of 2.43–3.55 and 2.07–3.04, respectively. This study also shows that the system proposed could be used for residential heating in the province of Erzurum which is one of the coldest climate region of Turkey.     

Passive cooling in a low-energy office building

In office buildings, the use of passive cooling techniques combined with a reduced cooling load may result in a good thermal summer comfort and therefore save cooling energy consumption. This is shown in the low-energy office building ‘SD Worx’ in Kortrijk (Belgium), in which natural night ventilation and an earth-to-air heat exchanger are applied. In winter, the supply air is successively heated by the earth-to-air heat exchanger and the regenerative heat exchanger, which recovers the heat from the exhaust air. In summer, the earth-to-air heat exchanger cools the ventilation air by day. In addition, natural night ventilation cools down the exposed structure which has accumulated the heat of the previous day. In this article the overall thermal comfort in the office building is evaluated by means of measuring and simulation results. Measurements of summer 2002 are discussed and compared to simulations with a coupled thermal and ventilation simulation model TRNSYS-COMIS. The simulations are used to estimate the relative importance of the different techniques. The evaluation shows that passive cooling has an important impact on the thermal summer comfort in the building. Furthermore, natural night ventilation appears to be much more effective than an earth-to-air heat exchanger to improve comfort.     

Economic analysis of the daylight-linked lighting control system in office buildings

The objective of this study is to perform an economic analysis of the daylight-linked automatic on/off lighting control system installed for the purpose of energy savings in office buildings. For this, a building was chosen as a typical example, and the energy cost was calculated by using the daylight and building energy analysis simulation. When the lighting control was utilized, an economic analysis was performed using a payback period that was calculated by comparing the initial cost of installing the lighting control system with the annual energy cost which was reduced thanks to the application of the lighting control.The results showed that the lighting energy consumption, when the lighting control was applied, was reduced by an average of 30.5% compared with the case that there was not lighting control applied. Also, the result for total energy consumption showed that, when lighting control was applied, this was reduced by 8.5% when the glazing ratio was 100%, 8.2% for 80%, and 7.6% for 60% when compared to non-application. The payback period was analyzed in terms of the number of floors in a building; 10 floors, 20 floors, 30 floors, and 40 floors. Hence, the building with 40 floors and glazing ratio 100% resulted in the shortest payback period of 8.8 years, the building with 10 floors and glazing ratio 60% resulted in the longest period of 12.7 years. In other words, the larger the glazing ratio and the number of building floors are, the shorter the payback period is.     

Effect of ventilation strategies on air contaminant concentrations and energy consumption in buildings

Considering the diversity of indoor contaminant characteristics and generation patterns, finding an appropriate ventilation strategy that can secure acceptable indoor air quality with minimum energy consumption is a challenging task for HVAC system designers and operators. This study theoretically models and investigates the impact of various ventilation strategies on contaminant concentration behaviour and corresponding ventilation cooling energy requirements for a single‐zone enclosure. Two types of contaminants are considered; carbon dioxide as an occupancy dependent and formaldehyde, which is independent of occupancy. An airflow model is used to predict space pressure and air leakage rates across the enclosure envelope, and an air quality model is used to predict time‐varying contaminant concentrations. In addition, a building energy simulation model is utilized to predict the corresponding ventilation cooling energy requirements under hot climatic conditions. Results from this study show that acceptable contaminant concentrations during occupied periods can be achieved by different ventilation strategies but at substantially different ventilation energy requirements. More than 50 per cent reduction in ventilation energy requirements can be obtained while maintaining acceptable IAQ if proper ventilation strategy is employed. Copyright © 2001 John Wiley & Sons, Ltd.     

The role of corporate reputation and employees' values in the uptake of energy efficiency in office buildings

Although office market actors in the United Kingdom show a growing interest in energy efficiency, the pace of takeup of energy efficient office features is slow. Previous studies have highlighted the roles of limited direct financial costs and benefits (‘efficiency gaps’) and market barriers in limiting the rate of technology adoption. This study provides further evidence on the importance of these factors, but the primary contribution is focused on the role of corporate reputation and on the importance of individuals' values in shaping corporate behaviour. The paper presents a theoretical framework to explain environmental decision making in firms and we present qualitative evidence drawing from sixteen semi-structured individual and group interviews with office market stakeholders in London, Glasgow and Edinburgh. The research finds that companies, despite gradually becoming more energy conscious, still regard energy costs as a negligible part of their business costs. Nevertheless, an increasingly important driver is the reputational gain obtained by corporate businesses implementing sustainable practices. All the interviewees agreed that the pace of change in the office market is slow and that only further policy interventions will accelerate it.     

Assessing the natural ventilation cooling potential of office buildings in different climate zones in China

This paper presents an investigation of the natural ventilation cooling potential (NVCP) of office buildings in the five generally recognised climate zones in China using the Thermal Resistance Ventilation (TRV) model, which is a simplified, coupled, thermal and airflow model. The acceptable operative temperature for naturally conditioned space supplied by the ASHARE Standard 55-2004 has been used for the comfort temperature setting. Dynamic simulations for a typical office room in the five representative cities, which are Harbin, Beijing, Shanghai, Kunming and Guangzhou, have been carried out. The study demonstrates that the NVCP depends on the multiple impacts of climate, the building's thermal characteristics, internal gains, ventilation profiles and regimes. The work shows how the simplified method can be used to generate detailed, indoor, operative temperature data based on the various building conditions and control profiles which are used to investigate the NVCP at the strategic design stage. The simulation results presented in this paper can be used as a reference guideline for natural ventilation design in China.     

Optimal decision making in ventilation control

In this paper, a two-mode ventilation control of a single facility is formulated as a scheduling model over multiple time horizons. Using the CO₂ concentration as the major indoor air quality index and expected room occupancy schedule, optimal solutions leading to reduced CO₂ concentration and energy costs are obtained by solving the multi-objective optimization model formulated in the paper. A modified evolutionary strategy algorithm is used to solve the model at different time horizons. The optimized ventilation schedules result in energy savings and maintain an acceptable level of indoor CO₂ concentration.     

Promoting conservation in buildings: A promising route

More attention would be paid to energy conservation if the property market in the UK reflected the true benefits of energy conservation improvements on the value of buildings. The benefits extend beyond the saving in fuel bills during the investor's period of occupation to a real appreciation in property value as the saving generally continues during the lifetime of the building. When the full extent of this is recognized a free market approach to investing in these measures can be more properly effective and the potential for saving up to £6 billion and for creating up to 150 000 jobs can be realized.     

Energy consumption,energy savings,and emission analysis in Malaysian office buildings

This paper is concerned with the estimation of energy use in office buildings in Malaysia and with the energy use of major equipment. Energy intensity (EI) – a measure of a building's energy performance – is estimated for Malaysia and compared with a number of selected countries. Air conditioners are shown to be the major energy users (57%) in office buildings, followed by lighting (19%), lifts and pumps (18%) and other equipment (6%). It is estimated that 77,569 MWh of energy can be saved and a huge reduction of emissions achieved through the application of advance glazing, compact fluorescent lamps (CFL), insulation, housekeeping, and by raising thermostat set point temperature of air conditioners, and reducing EI.     

济南办公建筑自然通风工况下自然室温的研究

针对济南地区一办公房间,讨论不同的换气次数、围护结构和建筑朝向对自然室温影响的变化规律。指出夜间通风和节能50%的围护结构能获得适宜的自然室温,建筑朝向对自然室温的影响较小。     

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.     

Energy and cost analysis of semi-transparent photovoltaic in office buildings

Solar energy conversion systems and daylighting schemes are important building energy strategies to produce clean energy, reduce the peak electrical and cooling demands and save the building electricity expenditures. A semi-transparent photovoltaic (PV) is a building component generating electricity via PV modules and allowing daylight entering into the interior spaces to facilitate daylighting designs. This paper studies the thermal and visual properties, energy performance and financial issue of such solar facades. Data measurements including solar irradiance, daylight illuminance and output power for a semi-transparent PV panel were undertaken. Using the recorded results, essential parameters pertaining to the power generation, thermal and optical characteristics of the PV system were determined. Case studies based on a generic reference office building were conducted to elaborate the energy and cooling requirements, and the cost implications when the PV facades together with the daylight-linked lighting controls were being used. The findings showed that such an integrated system could produce electricity and cut down electric lighting and cooling energy requirements to benefit the environmental, energy and economic aspects.     

Policy issues in improving energy efficiency

Today many urgent policy issues need addressing within the overall topic of improving energy efficiency. Also, they need to be addressed at many levels in society, each level possibly requiring a different approach: the overall national economy; the national energy sector; the energy utility; and the energy consumer. This article looks at some of the more important common issues, which apply at all levels, and gives some options on how these issues might be addressed.     

A numerical comparison of control strategies applied to an existing ice storage system

While ice storage systems are designed according to a defined strategy for warm day loads, it is interesting to consider other conventional control strategies for mid-season day loads. Three different charging–discharging control strategies are applied to an existing cooling plant and compared in terms of operating costs and energy consumption. A cooling plant model is built. A time stage equal to 15 min is considered to simulate numerically a whole charging–discharging process and compare the different control strategies. These simulations take into account existing technical constraints and set points. EES software is used. The operating costs of the cooling plant are evaluated by taking into account both the energy and the demand cost rate. It is shown that an ice storage system can allow savings of operating costs. However, they can increase energy consumption.