Naturally ventilated buildings typically use less than half the energy of air conditioned buildings, and reduce capital costs by some 15 percent, and associated maintenance costs. Natural ventilation is also proven to improve occupant performance and reduce incidence of sick building syndrome. Building Regulations are increasingly focussing on thermal efficiency and air tightness leading to the creation of sealed envelopes. Designers must now balance these requirements with the need for ventilation in order to control CO2 levels, remove pollutants and prevent overheating.

Many Government departments and agencies now include energy efficiency and sustainability within the specification for public buildings in the continuing drive to help reduce global warming as part of our commitment to the Kyoto Agreement. Natural ventilation is also the preferred method of ventilation in new school buildings, and will gain points under the BREEAM assessment method for offices and other commercial buildings.

Further, there is the new Energy Performance of Building Directive to create a 22 percent saving in energy by 2010 by focussing designers’ attention on making buildings more energy efficient. To meet these requirements cost effectively, it is better to work with the outdoor environment, rather than shutting it out: “build tight, ventilate right”.

Low energy natural ventilation can provide both occupant ventilation and free cooling, thus reducing the energy consumption of buildings. In principle natural ventilation uses controllable openings and planned air routes through the building to harness natural air movement and driving forces. Stack effect arises from the decrease in density of air as its temperature increases.

Pressure differences will exist between separate volumes of air at differing temperatures. Thus, cooler air will tend to flow into the lower part of a building, and warm, ‘used’ air will rise out of the higher part of a building. This natural buoyancy effect is further enhanced by wind pressure acting on the building façade and by the venturi effect as air is drawn out through openings at high level. In this way, fresh air is drawn into the building continuously- the passive stack or chimney principle- working 24/7, 365 days a year. Control of natural ventilation is easily achieved by incorporating controllable elements in the façade or exhaust outlets. To work effectively, the principle needs to be planned into a building at the design stage, although it can be retro-fitted with some reconstruction work- typified at City and Islington College, where a 1960s building has been redesigned internally to create a central chimney.

There are several types of natural ventilation strategy that can be used, depending on building size, depth, and use: passive stack ventilation, cross ventilation, single sided ventilation, displacement ventilation and mixed mode ventilation. Other factors also need to be taken into account to ensure the success of the natural ventilation strategy, such as night cooling and solar protection.

Passive Stack Ventilation

PSV is the most effective natural ventilation strategy as it uses a combination of cross ventilation, buoyancy (warm air rising) and the venturi effect (wind around a high level opening creating suction). Passive stacks can comprise stairwells, atria or ductwork to take the warm air from within the building to exhaust outlets at high level. Because a stack can be located in the middle of the building, it can ventilate twice the depth of traditional cross ventilation, and can also provide effective night cooling as the difference between internal and external temperatures at night tends to increase the buoyancy effect. One of the most famous examples of PSV is the Houses of Parliament, with the towers forming the chimneys, with a more recent example at the Building Research Establishment’s Office of the Future building at Garston.

Caerphilly County Borough Council has achieved an Excellent Rating under BREEAM 2002 Design and Procurement Assessment for Offices using a passive stack ventilation strategy at its Tredomen Innovation & Technology Centre. Designed by Passivent, the system uses passive stack ducts discreetly sited within the service cupboard to the rear of each office encouraging a cross flow ventilation path from windows in the façade. The ducts then terminate into circular Passivent High Capacity Terminals that feature double bank louvres combining excellent weather rejection with high volume air flow rates.

Cross ventilation achieves good air change rates driven by pressure differences across the building. It uses controllable high capacity inlets/outlets on the building facades, typically created by opening windows and can achieve penetration depths up to five times the floor to ceiling height. However, this strategy will require two preferably opposing facades with openable windows that may not allow the fine control of ventilation in winter to provide adequate draught-free ventilation. Passivent Aircool façade ventilators offer both controllable ventilation and weather protection in one integrated unit, and can be sited on both facades either at the time of build, or retro-fitted.

Single sided ventilation

Single sided ventilation, again typically created by opening windows, is driven by air turbulence, and creates relatively low levels of ventilation and penetration up to 2.5 times the floor to ceiling height. However, openable windows may not allow the fine control of ventilation in winter to provide adequate draught-free ventilation.

This strategy relies on specifically designed roof mounted devices to capture prevailing wind and channel it down separate chambers into the building. The cooler, heavier incoming air displaces warmer, lighter air upwards, which is drawn out through the leeward chambers of the terminal. Passivent Airscoop displacement ventilation terminals are specifically designed to maximise wind energy to efficiently ventilate deep plan or large spaces. They can provide ventilation where air cannot be drawn in from the building façade and are ideal for sports halls, drama studios, call centres, libraries etc. The new £2m Caveill Primary School in County Antrim uses displacement ventilation, relying on wind power alone. Wind passing across the roof is channelled through the Passivent Airscoop chambers into the building below, providing an air flow rate equating to 8l/s per person in line with Government guidelines.

The terminals are designed to ensure that even on days when the external wind speeds are low, air still circulates through the school as the Aiscoop will operate as a passive stack system under buoyancy forces. The ventilation is proving so effective that independent testing showed the school has one of the lowest measured internal carbon dioxide levels in the region.

Outdoor temperatures are predicted to rise as a consequence of global warming which will impact on summer time thermal comfort. Building heat gains are also increasing as a result of the growth in IT equipment and associated electrical energy use: each single computer screen, for example, is equivalent to burning a 60W light bulb continuously through the day! In order to balance the requirement for thermal comfort with the need to minimise energy consumption and subsequent carbon emissions, designers will increasingly need to utilise innovative passive designs and technologies in order to meet these challenges. Providing the best of both worlds, mixed mode combines natural ventilation with mechanical comfort cooling.

Natural ventilation is used all year round to provide the fresh air and maintain good indoor air quality. It can also provide a large proportion of the cooling requirement especially when combined with a night cooling strategy where daytime heat build up inside is dissipated using cool external air naturally drawn into the building, and the cooled thermal mass of the building helps reduce internal temperature build up the following day. This reduces demand on comfort cooling, which only needs to run for short periods to deal with peak demand.

Modern energy efficient air conditioning equipment such as Mitsubishi Electric’s City Multi VRF uses the latest technology with R410A refrigerant to offer what is claimed to be one of the most energy efficient air conditioning units. Its flexibility in offering the highest levels of efficiency at full and part load make it ideal for mixed mode applications. It can also utilise excess heat from high gain spaces and direct it to areas where it is required, reducing overall heating demand.

Properly designed and controlled mixed mode ventilation maximises the passive cooling available from natural ventilation and uses the air conditioning to heat or cool only when required, even operating it at partial load to reduce energy and associated emissions. Mixed mode relies on natural ventilation as much as possible to create a flow of fresh air keeping the building at comfortable internal temperatures. Testing at Brunel University showed the system produced an annual energy saving of 40+ percent against a fully mechanical system.

Night cooling

Night cooling refers to the operation of natural ventilation at night to purge excess heat from occupied spaces. It also uses the lower external temperature to reduce the temperature of the building fabric. A building with sufficient thermal mass that can be exposed to night time ventilation can reduce peak internal temperatures by 2-3°C. Night cooling offers the potential to minimise reliance on air conditioning and improve the internal conditions in naturally ventilated buildings. Good control of night cooling is required in order to achieve maximum free cooling whilst avoiding overcooling and subsequent re-heating or thermal discomfort the following day.

Solar control

Solar radiation through windows is the highest external contributor to heat gain. Solar shading deflects the radiation from the windows, reducing heat gain whilst allowing optimum natural daylight to enter the building. Barton Peveril College in Eastleigh, one of the UK’s top 6th Form Colleges, has applied this principle on its IT and classroom suite. The new blocks face south, so control of solar heat gain was a major issue, to help create a comfortable atmosphere for the students particularly in the IT suites, where there was the additional consideration of preventing the computers from overheating. Solar shading is reducing solar heat gain within the building by up to 85 percent.

The options are there, to create environmentally-friendly, efficient natural ventilation solutions. All that is needed is for specifiers and installers to understand the different types, so the most effective strategy for each building is incorporated, to create an environment that is pleasant to work and play in, regardless of the weather outside.

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