Maria Kolokotroni

The comfort, energy and health implications of London's urban heat island

The urban heat island (UHI) is a well-known effect of urbanisation and is particularly important in world megacities. Overheating in such cities is expected to be exacerbated in the future as a result of further urban growth and climate change. Demonstrating and quantifying the impact of individual design interventions on the UHI is currently difficult using available software tools. The tools developed in the LUCID (‘The Development of a Local Urban Climate Model and its Application to the Intelligent Design of Cities’) research project will enable the related impacts to be better understood, quantified and addressed. This article summarises the relevant literature and reports on the ongoing work of the project. Practical applications: There is a complex relationship between built form, urban processes, local temperature, comfort, energy use and health. The UHI effect is significant and there is a growing recognition of this issue. Developers and planners are seeking advice on design decisions at a variety of scales based on scientifically robust, quantitative methods. The LUCID project has thus developed a series of tools that (1) quantify the effect of urbanisation processes on local environmental conditions, and (2) quantify the impact of such conditions on comfort, energy use and health. The use of such tools is vital, both to inform policy but also to be able to demonstrate compliance with it.

The London Heat Island: results from summertime monitoring

This paper reports results from a study investigating the variation of air temperature across London. The paper focuses on the description of temperature measurements in summer 1999 and the analysis of the results. The intensity of the London Heat Island has been assessed using a radial grid of 68 stations recording simultaneous hourly air temperatures. The urban heat island has been found to be predominantly a nocturnal phenomenon and its intensity reaches 7 K on occasion. It was found that the thermal centre is in the City of London which is characterized by tall buildings and high anthropogenic heat emissions. The nighttime temperature pro” le of London tends to follow a simple pattern with contours arranged roughly circularly around the centre; it is found that 77% of the variance of the mean night-time temperature across London is related to distance from the thermal centre. The more complicated pattern in the daytime precludes such a simple relationship.

ETFE foil cushions in roofs and atria

Abstract Building fenestration can be responsible for a significant impact on the environment created in a building, affecting, either adversely or beneficially, both the health and perceptions of the occupants. Alternative to traditional fenestration solutions have been available for a great many years, one of which is ethylenetetrafluoroethylene (ETFE) a co-polymer of PE and tetrafluoroethylene which has been used for the past 20 years for atria and other overhead glazing. This study examines both the effects of ETFE manufacture and its use in buildings. This study has considered both its performance in terms of fitness for purpose and in comparison to glass, the common alternative. Some built examples of ETFE foil roofs are presented. It is concluded that ETFE foil is an appropriate technology for certain building applications, in particular those where the volume of space is large and high light levels are important. ETFE foils can improve the environmental performance of a building and may reduce the overall environmental burden incurred from the construction process itself and the burden of the building during its lifetime.

Summer cooling with night ventilation for office buildings in moderate climates

Abstract The suitability of night ventilation for cooling offices in moderate climates such as that of the UK is first assessed by presenting plots of summer weather data on the bioclimatic chart for three locations within the country. These indicate that most of the external weather conditions lie within the thermal mass and ventilation effectiveness areas of the charts. To confirm this, thermal simulations of a typical office module under a variety of internal conditions and summer weather data were performed. Predictions have shown that internal temperatures can be maintained below the external values for solar and internal gains of up to about 50 W/m 2 of conditioned floor area. Field measurements in a refurbished office have confirmed that temperatures in night-ventilated spaces are generally lower during the following day, especially during the early hours of the working day. Finally, the development of a pre-design tool in the context of IEA Annex 28 on ‘Low-Energy Cooling Systems’ is discussed. The main aim of the tool is to increase the awareness of designers for the energy benefits and the range of parameters for the application of night ventilation as the first means of cooling, so that the need for artificial cooling is minimised or avoided altogether.

The balance of the annual heating and cooling demand within the London urban heat island

In London, the urban heat island increases the average and peak air temperatures which in turn affect the demand for heating and cooling. To assess this, the simultaneous hourly air temperature in London has been measured continuously for a year at 80 locations, on a radial grid covering an area of 500 square miles. These have shown that central areas of London are significantly warmer than the surrounding areas (2 K warmer over the year). The measured data have been used as input to a thermal simulation model to assess the heating and cooling load of a standard air-conditioned office building positioned at different locations within the heat island. It is found that the urban cooling load is up to 25% higher over the year, and the annual heating load is reduced by 22%. Minimum CO2 is emitted at a rural location. The net rate of increase of CO2 with temperature is found to be 2.8%/K.

Heating and cooling degree day prediction within the London urban heat island area

This paper describes the London Site Specific Air Temperature prediction model, which comprises of a suite of artificial neural network (ANN) models to predict site-specific hourly air temperature within the Greater London Area (GLA). The model was developed using a back-propagation ANN model based on hourly air temperature measurements at 77 fixed temperature stations (FTS) and hourly meteorological data (off-site variables) from Heathrow; it also includes six on-site variables calculated for each FTS. The temporal and spatial validity of the model was tested using data measured 7 years later from the original dataset, which include new FTS locations. It was found that site-specific hourly air temperature prediction is within accepted range and improves considerably for average daily and monthly values. Therefore, the model can be used with confidence to predict daily and seasonal variations of air temperature within the GLA and in particular for the calculation of monthly and annual heating degree days (HDD) and cooling degree hours (CDH). It was found that as expected HDD increase and CDH decrease with distance from the urban heat island centre point; however, all variations cannot be explained with distance and six key on-site variables namely aspect ratio, surface albedo, plan density ratio, green density ratio, fabric density ratio and thermal mass have been identified to explain the remaining variation. Practical applications: Research studies have confirmed the extent of Urban Heat Island (UHI) within many cities in Europe. Studies have also confirmed the impact of the UHI on energy demand by buildings. There is therefore need to consider this in the design of building by using site-specific external temperatures in the energy calculations for urban buildings. This paper describes the development of a model, which can generate site-specific air temperature in a large number of locations in London. The models predictions can be used for the calculation of HDD and CDH for any base temperature across London using any Heathrow weather file for a specific year, design years or future climate years; such values can be used for the calculation of site specific building heating and cooling loads.

An investigation of passive ventilation cooling and control strategies for an educational building

Abstract Many non-domestic buildings, built recently in the UK, use natural means to provide ventilation for indoor air quality and thermal comfort. This paper presents monitoring results obtained from such a purpose built naturally ventilated educational building. Its performance during the summer is discussed based on monitored results. Using thermal and ventilation modelling, the paper also discusses the optimisation of the building’s summer performance. Recommendations on the selection of appropriate ventilation strategies in relation to the prevailing external conditions are derived and the appropriateness of the control methods is discussed.

Monitoring and Modelling Indoor Air Quality and Ventilation in Classrooms within a Purpose-Designed Naturally Ventilated School

This paper investigates the performance in use of ther mal passive ventilation stacks in the classrooms of a school during the summer period. Measurements of air temperature, carbon dioxide (CO2) and air velocity were carried out from which ventilation rates in the class rooms and stacks were calculated. It was found that the airflow through the passive stacks was a significant pro portion of the classroom air exchange rate and metabolic CO2 was lower in classrooms with passive stacks. The predictions of a multi-zone airflow computer model were compared with measured values. Multi-zone modelling indicated that passive stacks could significantly increase ventilation in classrooms, which would otherwise be ventilated via single side ventilation through openable windows on one external wall. It was found that if the zones are accurately represented, multi-zone modelling is able to provide a good indication of the general trends of airflow rates both for the whole classroom and the flow through passive stacks.

Using localised weather files to assess overheating in naturally ventilated offices within London's urban heat island:

Urban environments typically experience increased average air temperatures compared to surrounding rural areas – a phenomenon referred to as the Urban Heat Island (UHI). The impact of the UHI on comfort in naturally ventilated buildings is the main focus of this article. The overheating risk in urban buildings is likely to be exacerbated in the future as a result of the combined effect of the UHI and climate change. In the design of such buildings in London, the usual current practice is to view the use of one generic weather file as being adequate to represent external temperatures. However, the work reported here demonstrates that there is a considerable difference between the overheating performance of a standard building at different sites within London. This implies, for example, that a building may wrongly pass or fail criteria used to demonstrate compliance with building regulations as a result of an inappropriate generic weather file being used. The work thus has important policy implications. Practical application: The Greater London Authority has recently developed, with the Chartered Institute of Building Services Engineers, guidance for developers to address the risk of overheating in buildings via the provision of weather files for London relating to three zones. While such an initiative is welcomed, it may be that a weather file tailored to the building location would be preferable. Of course, this would add further complexity to the process and a view would have to be taken as the viability of such an approach. The work presented in this article, however, suggests that serious consideration should be given to the use of tailored weather data for regulatory purposes.

Environmental impact analysis for typical office facades

The design of a building facade influences internal thermal and lighting conditions and energy use associated with the provision of these conditions. Key decisions about the building facade are usually taken during the concept design stage of a building, while decisions about the method of providing the environmental conditions are often made later in the design process. This dilemma is addressed by the development of a concept design tool that allows the design team to investigate the effect of facade design on the resulting internal environmental conditions, energy use and environmental impact. The concept design tool was developed by performing detailed thermal, lighting and environmental modelling for a number of generic office building facade designs and a range of parameters that affect directly the environmental performance of an office building. The results are presented in a user-friendly interface requiring a minimum number of inputs. Key parameter outputs (such as temperature, lighting levels, heating/cooling energy demand, embodied energy and eco-points) can then be viewed, while a more detailed analysis can also be created for specified facade designs. A parametric analysis of the summary result outputs for selected facade parameters indicates that natural ventilation and cooling can reduce the environmental impact of offices by up to 16%, although heating energy demand could increase significantly. Improving the construction standard of the facade and reducing the internal heat loads can reduce the environmental impact by up to 22%. Use of this tool at early design stages will benefit the design team through an improved understanding of the dynamics between facade design and building services and assist with a more integrated approach.