Monodraught delivers energy-free ventilation

Monodraught discusses how natural ventilation can deliver energy-free cooling and zero carbon in buildings.

The UK Climate Projections (UKCP09) predict an average UK temperature rise of 1.5C to 6C annually, and 1C to 8C in summer months by 2100.

Over time we will all adapt to these changes by amending elements of our lifestyles such as the clothes we wear.

Personal changes can only have a finite effect because most of us spend 80-90 per cent of our time indoors, where the behaviour of buildings determines our ability to adapt to a changing climate.

The temperature projections show that summer warming is a greater problem than winter cooling.

Rather than concentrating on the energy lost from buildings in winter, Monodraught suggests we should concentrate on the amount of energy spent keeping buildings cool in summer.

Zero carbon buildings need to be constructed that meet the demands imposed by climate change.

The term zero carbon building is not yet fixed, and in this article, only the energy consumed by a functioning building is considered, rather than its embodied energy.

Therefore a zero carbon building is one that consumes as much energy as it produces.

In order to achieve low energy consumption in a warming climate, it may be helpful to look backwards to vernacular buildings to help us move forwards.

These buildings have developed over time through trial and error to produce generic designs that are efficient and a function of their environment.

In the city of Yazd in the Iranian Plateau, the buildings are orientated to avoid significant solar heat gains, exploit wind, and use the cooling effects of a local micro-climate provided by vegetation and pools of standing water.

They are built of heavyweight materials such as brick and stone that have an ability to store latent thermal energy, known as thermal mass, which reduces internal diurnal temperature differences.

It is well known that the thermal comfort of occupants is related to the external temperature, but less well known that it is also related to the effect of radiation from surfaces in a room and the air speed in a room.

Consequently, the thermal mass of a building and the ventilation rate are very important for thermal comfort.

Yazdi buildings are often naturally ventilated using a wind-catcher, a roof mounted device that channels fresh air into a room under the action of wind pressure, while simultaneously drawing air out of the room by virtue of a low pressure region created downstream from the element.

It does this whatever the direction of the wind and without mechanical assistance.

Monodraught asks how can these ancient principals be reapplied to a zero carbon building to keep us cool in hot weather? Heat gains must be reduced below levels of 30-40 W/m2.

A natural ventilation strategy should then be derived based upon the local environment and the type and use of the building to provide ventilation for typical conditions.

The thermal comfort of occupants is maintained by a steady ventilation rate encouraged by an obstruction-free flow path between the natural ventilation inlets and outlets.

Perspiration efficiency increases with the speed of the internal air flow, which on very hot days can be augmented by electric fans.

At night natural ventilation can reduce the air temperature at the beginning of the working day and assist available thermal mass to dissipate heat stored during the day.

Many modern non-domestic buildings are built from lightweight materials because of costs and material scarcity, and so contain little thermal mass.

The road to zero carbon buildings will involve the development of new modern materials, but ventilation and cooling strategies should invoke proven practices present in vernacular architecture, which need little or no daily energy input.

Natural ventilation strategies are widely recognised as reliable and effective alternatives to energy-hungry air conditioning systems.

Systems such as Monodraught's Windcatcher require no mechanical input to function, but on abnormally hot days, solar-powered fans can supply additional fresh air at up to 250 l/s.

At other times of the year, volume control dampers can be set to allow trickle ventilation, helping to maintain a healthy environment without cold draughts.

The Carbon Trust also finds that naturally ventilated buildings have half the energy costs and associated CO2 emissions of comparable mechanically ventilated buildings.

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