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The focus of this paper is on the use of natural ventilation in the Senedd, home of the National Assembly for Wales. The aim of this paper is to debate on the impact of natural ventilation on the building's energy use and internal comfort as well as on the potential for further reducing the building's carbon footprint through improvements or alternatives to natural ventilation.
Keywords: Senedd, mixed mode ventilation, natural ventilation, funnel, comfort, energy use, carbon footprint, control systems, cowl, Cardiff.
The Senedd is located at Cardiff bay's newly developed waterfront and houses the national Assembly for Wales (51Â°27â€²50.05â€³N, 3Â°9â€²43.49â€³W). It was designed by Richard Rogers Partnership, Arup and BDSP partnership. After many phases and delays the projected was completed in 2006. (Jeremy Colman, 2008)
It is developed in three levels (Fig.1). The first level houses the committee rooms as well as the debating chamber (Siambr). The second level hosts the main entrance and the reception area (Neuadd) while the third level contains a café and seating areas open to the public (Oriel).The Senedd is a steel construction with large scale glass facades. Its most striking building element is its wavy wooden roof. From the Neuadd level visitors can view the committee rooms, an architectural expression of the idea that the assembly's procedures are open and transparent. (Richard Rogers Partnership, 2005)
The Senedd is a very important public building for Wales and for it to reflect the Assembly's commitment to address and implement sustainable development was a priority. In effect, the building was designed to be highly sustainable and consume little energy living up to a minimum life of 100 years. (Richard Rogers Partnership, 2005) In order to achieve this, architects and engineers worked together from early design stages in order to produce an integrated design. (Dejan Mumovic, Matheos Santamouris, 2009) As a result, the building demonstrates a low energy environmental architectural approach. The main techniques and technologies employed include a) space zoning for better regulation of the indoor environment b) natural ventilation in all spaces c) optimized daylighting to all areas d) cooling and heating through an earth exchange system e) a 360kW gas fired back up biomass boiler e) a rainwater harvesting system supplying water for W.C.'s, window washing and landscaping f) use of local materials and labor. (Richard Rogers Partnership, 2005), (Fig.2). Upon completion the Senedd was awarded the BREEAM rating of "Excellent". (Department for Environment, Food and Rural Affairs. 2007)
Description of ventilation
The key principle that characterizes the Senedd's design is the achievement of an important reduction in the overall energy consumption through the use of natural ventilation and daylighting. (Richard Rogers Partnership, 2005) In detail, public spaces rely completely on natural ventilation through the use of operable windows on the building's glazed facades. All windows are made from reinforced and insulated glass and have an automatic operation in order to maintain a constant temperature in the building. This minimizes the use of air conditioning during the summer and helps the building retain heat in the winter. (M.Winter, 2006)
The Siambr, the committee Rooms and the public galleries utilize a mixed-mode ventilation system providing both natural ventilation and air-conditioning. Above the Siambr, the roof shapes into a funnel allowing this central space to be naturally ventilated. According to Mumovic and Santamouris "at the top of the funnel, a six meter wind cowl rotates according to the prevailing wind creating a negative pressure to the leeward side where outlets have been placed, drawing warm air upwards and speeding up the warm air exhaustion from the building". (Dejan Mumovic, Matheos Santamouris, 2009) Cfd modeling was used extensively to test the wind cowl operation and ensure the required air movement in the chamber, under different scenarios. (M.Winter, 2006) In summer, the exhausted air is replaced by cooler air provided at floor level by two underground ventilation plenums that are part of a natural displacement ventilation strategy while dampers located in a high level inside the funnel controlling the airflow. (M.Winter, 2006) These three spaces are tightly controlled and when required they use a VAV air condition system with heat recovery. The VAV system uses the existing diffusers located under the floor to provide cool or warm air. (M.Winter, 2006) Similarly, offices use underground ventilation trenches with floor outlets to prove cool air into the rooms during summer as well as roof vents and operable windows to exhaust air. (Dejan Mumovic, Matheos Santamouris, 2009)
It's worth noting that the minimization of air conditioning was possible through the parallel development of environmental and structural design which produced an intergraded building. Examples of this relationship are the funnel itself as well as the use of the exposed concrete's and slate's thermal mass along with natural ventilation in order to temper internal conditions. (Dejan Mumovic, Matheos Santamouris, 2009)
In sum, the Senedd employs natural ventilation to provide comfort while using reduced energy.
Energy Use and Comfort
In the absence of specific data addressing the energy use attributed to ventilation we can speculate on the relation between energy use and natural ventilation using existing knowledge.
The Senedd has great potential of demonstrating low energy consumption through the ventilation techniques which include the implementation of zoning, natural and mixed mode ventilation. Zoning divides the building into zones serviced by different ventilation systems according to ventilation needs that rise from occupancy and use. (CIBSE Guide F, 2004) Along with the implementation of natural ventilation in public spaces and the installation of mixed mode ventilation in the remaining spaces, they can lead to significant reduction of the energy used to cool or heat the building in comparison to the use of a general mechanical system (CIBSE Sustainability Tool, 2011), a more popular solution in a building of that scale. This is also supported by the fact that according to CBE's research natural ventilation can reduce the building's energy use by 15% to 80% depending on climate, load size and building type. (Center for the Built Environment, University of California, Berkeley, 2005) The introduction of an intelligent façade in conjunction with a BEMS system are important components of both natural and mixed mode ventilation's efficiency, as effective manual control would be possibly hard to achieve. (Peter Frederick Smith, 2005) Effectively, a well designed and properly operated building using natural or mixed mode ventilation can contribute greatly to the reduction of operating cost and pollution as well as significantly minimize its carbon footprint. (Dejan Mumovic, Matheos Santamouris, 2009)
Observing the Senedd's Energy certificate for the period 12/2008-12/2009 we can draw some interesting conclusions on the building's energy use. First, it's evident that most of its emitted CO2 emissions are a result of the electricity used in the building, exceeding by far the emissions from heating and renewable energy. Second, the energy performance operation rating falls under category C making the Senedd a relatively more efficient building than the set standard. This presents a decrease in comparison to the period 12/2008 indicating a possible progressive improvement of its efficiency. This is related to the fact that in 2009 the electrical annual use dropped at 158 kWh/m2/year, presenting a reduction of 11% in comparison to 2008 data. We could assume that the progressive optimization of the BEMS operating the building is one of the reasons of such a reduction as this system is based on self learning control algorithms using feedback from employees to improve through time the operation of natural ventilation, night-time cooling, shading as well as daylighting and artificial lighting that are expected to consume a large amount of electricity. (M.Winter, 2006) In sum, we could conclude that the use of an intelligent façade, coupling passive design with BEMS, plays an important role in the Senedd's decreased energy use.
However, the complexity of such control system might discourage employees from using them efficiently. (CIBSE Guide F, 2004, p.3-6) As a result, on one hand, users might feel that their environment is automated creating the need for tighter comfort bands that are harder to satisfy with passive means. (CIBSE Guide F, 2004, p.3.5-3.6) On the other hand, in spaces equipped with a mechanical backup system, users will be tempted to employ VAV ventilation, increasing energy use. Moreover we should take in consideration that intelligent facades present inherent problems like increased room temperatures during heat waves and draughts during winter. (Ulrich Knaack, 2007, p.85)
Indeed, after visiting the Senedd and questioning the users, complaints of poor comfort conditions were recorded in public spaces while the remaining spaces were said to have satisfying comfort conditions. Specifically the employees described that public spaces are rather cold during the winter, very hot during the summer and the internal air tends to be significantly dry. The reported conditions could be a sign that the ventilation techniques used are inefficient for the reasons developed in the previous paragraph. This is also supported by the fact that the employees have different opinions on the comfort of naturally and mixed mode ventilated spaces but further research should be made to support this theory. However, we should not rule out the possibility of the BEMS itself being inefficient. (location of sensors, conflict of mechanical and passive systems) (CIBSE Guide F, 2004, p.6-2) To complete the assessment of the relation between natural ventilation and energy use, we should also examine the electricity required to operate the BEMS.
In brief, although the building has succeeded in having a small carbon footprint and limited energy use, we have to carefully assess the efficiency of the ventilation components employed in respect of the comfort they produce and the final electrical energy they consume.
Improvements and Alternatives
According to the aforementioned inefficiencies, the natural ventilation system could receive improvements. First, the control systems could be optimized either by allowing users more control on ventilation or by changing the location of sensors if assessed that they are not efficiently placed in the building. (J.Shimmin, I.Khoo, 2002, p.1) Effectively, those improvements would create the opportunity to keep the energy use at the present levels providing simultaneously more comfortable spaces. (CIBSE Guide F, 2004, p.3-6) Otherwise, prolonged discomfort might lead to the future installation of additional systems, increasing the energy use.
A passive design solution would be the installation of a double glass façade to temper the air before it enters the spaces. The Senedd could install an active wall which according to Kragh M. (2000) is constructed by adding "an extra skin is applied to the inside of the building envelope; inside return air is passing through the cavity of the façade and returning to the ventilation system. In periods with solar radiation the energy, which is absorbed by the blinds, is removed by ventilation. In periods with heating loads, solar energy can be recovered by means of heat exchangers. Both during cold periods with no or little solar irradiation and during periods with solar gains or cooling loads, the surface temperature of the inner glass is kept close to room temperature, leading to increased occupant comfort in the perimeter zone, near the façade." This solution has the advantage of offering increased comfort during the cold season as well as the recovery of solar energy which is suitable for Britain's cold climate. (Kragh M., 2000)
In sum, the optimization of the natural ventilation's control system as well as the installation of a double skin facade will require additional capital costs but it's more probable that in the long run those costs will be recouped by the building's efficient energy performance. Any addition of mechanical systems could improve the spaces' comfort but with an energy penalty. Any "after-thought" systems will rarely be as efficient as the intended design and certainly lead to additional cost, a typical example of the problem created by inefficient design process. (D.Alexander, 2011) Effectively, if we are targeting a reduction in the energy use the later is not an option.
Natural ventilation techniques employed in the Senedd have a major role on its efficient energy performance but there is certainly room for improvement. Operating, monitoring and reassessing a building's performance is as equally important as its integrated design face in order to ensure the building's efficiency. Since reduced energy use is the first step to a low carbon footprint (Department of Energy and Climate Change, 2011), the Senedd's natural ventilation scheme has the potential to greatly contribute to its low carbon footprint, given its operating efficiently.