Resilience of naturally ventilated buildings to climate change: Advanced natural ventilation and hospital wards
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Mitigating climate change
It is now recognised that climate change, caused by carbon (CO2) emissions that result from the burning of fossil fuels, poses the single greatest threat to humanity in the 21st century. The UK, in keeping with other developed countries has set ambitious CO2 reduction targets. The 2003 energy white paper [1] set a long term target of a 60% carbon emission reduction by 2050 and more recently, the Secretary of State for Energy and Climate Change, announced that the target would be increased to
Advanced natural ventilation and refurbishing hospital wards
Simple natural ventilation involves opening windows or other apertures between a space and the outdoors and many buildings, including hospital wards, are ventilated in this way. The strategy use apertures on one side of a space, single-sided natural ventilation (SNV), or on two (or more), usually opposing, sides of a space, cross-flow ventilation (Table 1). These strategies are well understood by building occupants, they are low cost, and many types of window with different opening formats are
Thermal comfort and ventilation guidelines and criteria
Determining the criteria to be used for quantifying the risk of day and nighttime overheating in new or refurbished hospital wards proved more complicated than expected. It is useful therefore to spend a little time on the topic so that others might benefit from the study.
In the UK, guidelines are provided in the DoH's own documents, and in particular HTM03-01 [29], in the recommendations contained in the CIBSE design guides, in particular CIBSE Guide A [39], and, most importantly in British
Modelling the ward rooms
Thermal simulations were undertaken using the IES simulation system, virtual environment version 5.8 [49] in five phases: firstly, using the current London DSY, studies were conducted to refine the control of the heating and ventilation systems in the ward with ANV (Fig. 2) and to understand the hour-by-hour internal conditions (internal temperatures, CO2 levels, ventilation rates, heating energy demands; and relative humidity13
Phase 1: predicted environmental conditions in the ward
Simulations to explore the hourly performance of the ANV design during the summer period were conducted using the London DSY produced by the CIBSE in 2005 (LonDSY05). Aspects investigated included the inclusion and control of a fan in the exhaust stack and alternative nighttime temperature set-backs.
Initial simulations showed that the basic ANV proposition worked well, however when the internal temperature was close to, or below, the external temperature, the ventilation flow reversed, that is
Phase 2: comparison of temperatures in wards with advanced natural ventilation with those in wards with simple natural ventilation
The results for the basic ANV design suggest that it has the potential to keep the ward thermally comfortable in the SE of England15 as judged by the HTM03-01 criterion. However, to gauge the significance of the predicted internal temperatures, it is helpful to compare them with those predicted for conventional ward designs that use simple single-sided natural ventilation. Many possible ‘conventional’ designs could be
Phase 3: sensitivity to location and internal heat gains
It is evident from the previous results that ANV has the potential to yield acceptable temperatures in hospital wards even in the SE of England, whereas only SNV spaces that have a large area of opening window and solar heat control glazing can yield acceptable conditions. To understand the wider applicability of the two ventilation strategies the risk of overheating in other UK locations and with additional internal heat gains was studied.
Three other UK locations were examined using the 2005
Phase 4: resilience to future elevated temperatures
To assess the risk of overheating in the future, two important steps are required; firstly the establishment of an appropriate criterion by which to decide if a space is comfortable, and secondly the development of credible hourly future weather data.
As discussed above, the BS EN 15251 adaptive standard provides the basis for assessing future comfort but it is necessary to establish an overheating risk criterion based on either the number of hours over the daily temperature limit and/or for the
Phase 5: comparison of energy demands and CO2 emissions for the ANV and alternative ventilation strategies
The ANV strategy is more resilient to climate change and to increases in internal heat gain, than other simpler natural ventilation strategies. It is important, however, to ascertain whether the strategy will contribute to mitigating climate change and to helping the NHS meet its energy demand targets for new build and refurbishment: 35–55 GJ/100 m3 and 55–65 GJ/100 m3 respectively. Therefore, predictions were made of the likely energy demands for heating and ventilation and for lights and
Discussion
In reflecting on the work undertaken, it is most useful to focus on aspects that have generic value, i.e. beyond the confines of healthcare buildings, and which are especially pertinent when investigating the resilience of existing and new buildings to climate change. In the UK, most existing buildings are naturally ventilated, and there are merits in retaining these as they invariably produce lower CO2 emissions than mechanically conditioned spaces; as the research reported here demonstrates.
Assessing overheating risk
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Naturally ventilated buildings have a key role to play in mitigating climate change because they tend to precipitate lower CO2 emissions than mechanically ventilated alternatives; as has been demonstrated in this paper. However, climate change has the potential to tip naturally ventilated buildings that currently have a low risk of overheating into a position where the risk will be deemed unacceptable at some point within the coming century. Therefore, the resilience of naturally ventilated
Acknowledgements
The authors are grateful to the Department of Health who funded this work, to Short and Associates who led the project and from whom much stimulus was gained, and to Prof V I Hanby, of the Institute of Energy and Sustainability at De Montfort University, for generating the morphed weather files. The paper was written whilst a Visiting Fellow at Clare Hall Cambridge kindly supported by a Research Fellowship from the Leverhulme Trust.
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