Building and Environment 107 (2016) 263e273 Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv 10 Questions Ten questions about natural ventilation of non-domestic buildings * Guilherme Carrilho da Graça a, , Paul Linden b a Instituto Dom Luiz, Faculdade de Ci^encias, Universidade de Lisboa, Lisboa, Portugal b Dept. of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, Univ. of Cambridge, United Kingdom article info abstract Article history: Throughout history, natural ventilation has remained the preferred choice for the majority of residential Received 30 May 2016 buildings, while, in commercial buildings, natural ventilation went from being the single option to Received in revised form somewhat of a lost art as mechanical ventilation systems and air conditioning became the standard 4 August 2016 during the second half of the twentieth century. Recently, as a result of environmental concerns, in Accepted 5 August 2016 particular the greenhouse gas emissions from buildings, interest in natural ventilation in commercial Available online 9 August 2016 buildings has seen a resurgence. Unfortunately, the hiatus in natural ventilation use in these buildings has resulted in the loss of existing design know-how and consequently limited new developments in a Keywords: Natural ventilation period during which comfort and indoor air quality performance standards have continuously risen. fi fl Single sided ventilation Nevertheless, the past 25 years has seen signi cant advances in our understanding of the uid mechanics Stack of natural ventilation and Architectural Fluid Mechanics has developed as a new subject. In response to Daylighting these new scientific advances and in an attempt to restore confidence in the applicability of natural Vortex shedding ventilation in practice, this paper presents ten questions about building natural ventilation that span the different scales of the problem, from an urban context down to the neighbourhood and the building itself. These questions are commonly asked when a designer is considering natural ventilation as the preferred means of cooling a non-domestic building, and the answers are intended to provide succinct links to the latest knowledge, identify areas that require additional research and assist designers in making appropriate decisions. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction other non-domestic buildings use mechanical cooling even when an optimized NV system could meet cooling and fresh air re- Natural ventilation (NV) occurs when pressure differences quirements. In the best contemporary design examples NV is able generated by wind (wind-driven NV) or buoyancy forces (stack- to replace mechanical cooling systems in the milder months of the driven NV) act on one or more openings in the building envelope. In year [2,3], reducing ventilation and cooling related energy demand contrast with the controllable energy source used in mechanical as well as sick building syndrome [4,5]. The motivation for ventilation, the variable pressure differences that drive NV make increased NV use is clear: typical mechanical ventilation fan energy designing these systems a difficult task [1]. With the steady in- consumption is comparable to indoor lighting, with power den- crease in use of mechanical ventilation and air conditioning during sities in the range 5e15 W/m2 [6]. Further, the average energy the second half of the 20th century, existing knowledge in the consumption of a mechanical cooling system has similar magni- design and architectural integration of NV systems became obso- tude (or up to twice as much in hot and humid climates), com- lete. During the same period, we have seen an increase in user pounding a heating ventilation and air conditioning (HVAC) related expectations and thermal comfort and indoor air quality standards, energy consumption of 50%e60% of total building energy con- creating a scenario where natural ventilation became rare in sumption [7,8]. In light of these numbers a successful NV cooling modern non-domestic buildings as designers and building owners system could halve office building energy consumption. This choose the apparently more reliable mechanical ventilation option. capability could be a driver for increase NV use if energy costs were As a result, in most developed countries the majority of office and not one to two orders of magnitude less than rent costs. In this context, NV needs to impose itself by its capability to improve the work environment and worker productivity as well as * Corresponding author. its contribution to climate change mitigation. Most electrical grids E-mail address: [email protected] (G. Carrilho da Graça). http://dx.doi.org/10.1016/j.buildenv.2016.08.007 0360-1323/© 2016 Elsevier Ltd. All rights reserved. 264 G. Carrilho da Graça, P. Linden / Building and Environment 107 (2016) 263e273 have a large incorporation of fossil fuels in their energy mix, models is bringing increased precision and reliability to design- creating a direct link between building energy efficiency and phase predictions of NV system performance [23e25]. Improved climate change mitigation. Limiting the global temperature rise to software models integrated in 3D CAD-based tools should allow for less than 2 C requires, among other things, drastically reducing a better collaborative environment and more feedback in the fossil fuel use [9]. In the design and research community there is a crucial early design phase. growing consensus that NV is a key component in the mix of so- lutions that will deliver, by 2020, nearly zero energy buildings [10]. 2. Ten questions (and answers) concerning building natural In cooler and mild climates, equipping non-domestic buildings ventilation with natural ventilation offers the most potential for reducing en- ergy costs and CO2 emissions associated with cooling. This can also There many interesting open questions in the area of building be the case in hot climates if NV is used appropriately. natural ventilation, ranging from design application to detailed In spite of its many advantages NV is still a rare feature in simulation and fluid mechanics question. The ten questions pre- modern buildings. This lack of successful application is surprising in sented in this section span several scales and topics within the area. a world where, when given the choice, most people prefer natural The authors hope that these questions prove not only relevant to ventilation and operable windows and tend to show increased design practitioners, but also stimulating to researchers in this area. thermal tolerance when in NV buildings [11,12]. There is also increasing evidence that worker productivity is enhanced in 2.1. “Why is the challenge of indoor climate control of non-domestic buildings with low CO2 levels [13], a characteristic of NV buildings buildings cooling rather than heating?” where the occupants have control over their environment by the use of operable windows. Clearly, most design practitioners and Continuous improvements in building envelope thermal insu- sustainable architecture consultants struggle to integrate NV sys- lation, combined with an increase in internal thermal gains insure tems in modern designs [14]. According to [15] this is due to several sufficient heating in winter and create a need for cooling in the problems, that occur in the design phase, such as: late involvement remainder of the year. fi of energy ef ciency consultants, low fees to support a more com- Natural ventilation is due to two free driving forces: wind and plex design analysis, lack of NV design experience, higher risk and stack (buoyancy) generated by solar and internal gains. These forces in some cases a design approach that is not accommodating of the have different characteristics and combine to drive nearly all NV requirements of NV and low energy buildings. In these latter cases systems. An 8 C temperature difference in a 3 m stack generates a the requirements that low energy architecture places on building flow driving pressure of 1 Pa, a value that is exceeded by a wind form and facade are not well integrated by the leading architectural speed of 2 m/s. Thus stack is weaker than wind but is more reliable practices that are sought by building developers seeking their due to its wonderful self-adjusting nature: sensible heat internal spectacular design approach. Adequate external shading and NV loads drive the ventilation flow in a proportional way, increasing it [16] must be integrated in the early design stages and can have with every increase in load. Wind is stronger but suffers from fi fi signi cant impact in the nal design. In some cases, the numerous fluctuations in intensity and direction on yearly, daily and minute operable windows that a typical NV solution requires are thought time scales. Further, the typical urban atmospheric boundary layer not to be attractive. Further, NV systems often require advanced turbulence intensity is 10e20%, leading to wind pressure fluctua- control strategies with the ability to vary the openings area tions of up to 40% or even more when there are fluctuating changes throughout the year as well as adequate commissioning and post in wind direction. Since every location will have periods of nearly occupancy interaction with the building users [17]. As a result, NV zero wind pressure, every wind-driven NV system needs to be solutions have a reduced penetration and the energy use intensity designed for a ‘worst case’ scenario of operating only with stack of new non-domestic buildings has remained nearly constant effect. during the last decade [18,19], even in places with a suitable climate The challenge of NV in non-domestic buildings is best illustrated for NV [20]. In addition to these design issues there is also the by considering a typical office space, shown in Fig. 1. An occupant in problem that opening windows in current city environments can such an office produces about 100 W of sensible heat and uses office lead to excessive air and noise pollution exposure.