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Smart walls set to shape sustainable structures

19/8/2012

Façade technologies are creating numerous  ways to maintain comfortable interiors while reducing energy use. Both windows and walls can incorporate systems that manage light ingress and control temperatures sustainably. Andy Pearson looks at the latest ideas on the market

Façades are the skin of buildings and can be important moderators of the environment. Any improvement in a façade’s energy performance will contribute towards reducing the 40% of the world’s carbon emissions that are used to maintain a comfortable interior environment. So what kind of façade technologies will tomorrow’s building be hidden behind, and how will they help lessen the need for energy?

Glazed sections

Developments in window glass will enable the wavelength of ultraviolet solar radiation to be transformed into visible rays. The technology was developed to increase the output of photovoltaic cells and is being adapted for use in buildings. The new glass will have higher light transmission to provide more natural light all year round, which will reduce the need for artificial light.

Vacuum insulated double glazing is another technology starting to become affordable. Vacuum glazing units, such as Pilkington’s Spacia, consist of an outer pane of low-emissivity glass and an inner pane of clear float glass, with just a vacuum in between. The units are very thin which means the single-glazed windows of historic buildings can be replaced with more energy efficient units without altering their appearance.

Several dynamic glazing technologies can help control the amount of light and heat transmitted into a building, effectively by changing colour. Technologies include:

• Electrochromic glazing, which can be changed from translucent to transparent by applying a voltage to thin film within the unit;
• Gasochromic units which change colour when a gas (usually diluted hydrogen) is injected into the sealed unit; exposure to oxygen returns the window to transparent;
• Suspended particle devices and liquid crystals devices, which rely on a voltage to align molecules within a substrate contained within the unit to change from transparent to opaque.

Opaque sections

Passive solutionsinclude the use of a lightweight, translucent panelised wall system manufactured using aerogel. Aerogel is a synthetic material derived from a gel, in which the liquid component is removed through supercritical drying to be replaced by a gas. The resulting walls give a similar U-value to opaque walls and transmit diffuse sunlight internally without the need for blinds or external solar control, helping reduce the need for artificial light.

Aerogels are also being used to form the rigid core of vacuum insulated cladding panels. Here the core is surrounded by a gas-tight, airless to further improve its insulation performance.

In the future, transparent silica aerogel could also be applied to windows as a thermal insulation material. U-values as low as 0.1 W/m²K have been suggested for such windows, although the products are still in the laboratory.

Last month scientists at Germany’s Kiel University and the Hamburg University of Technology announced the development of aerographite as possible successor to aerogel. This is claimed to be the world’s lightest material with a weight of just 0.2 milligrams per cubic centimetre. It is early days, but with such a material architects could create otherwise impossible structures.

Another passive solution gaining ground is breathable walls. The principle behind the system is that the building envelope acts as a heat exchanger. Ventilation air is drawn through the walls of the building, which warms the incoming air by removing heat from the walls that would otherwise be lost.

More dynamic are walls built with a variable U-value so that energy can flow out of the building when the interior gets too warm. According to consultant Arup, one way this can be done is with blown fibre insulation, modified with spray-applied, microencapsulated phase change materials (PCMs). The PCMs store large amounts of heat energy when the ambient temperature rises and release it when the temperature drops, improving comfort and reducing the energy demand of HVAC systems. See below for how PCMs can be used inside a building.

Arup is also developing a bio-reactive façade, which relies on algae to reduce solar gain. More impressive still, when less solar protection is needed, the algae can be harvested and used as an energy source. The system works by cladding the building’s façade in series of linked flat transparent containers. These are filled with water and fed with carbon from combustion processes in the building’s plant room. Algae are circulated through the panels along with water and nutrients, absorbing light and carbon to produce biomass.

Harvesting the algae controls the amount of light that passes through the façade and into the building. On a sunny day the algae is left to reproduce and reduce solar gain; it can then be harvested in the evening as the power of the sun diminishes, which will allow more light into the building. The harvested algae are transformed into methane; generating heat which is stored goethermally or fed back into the building. In addition, the part of the solar spectrum not absorbed by the algae will heat the circulating water in the facade; this heat too is removed for use in the building. The first application of the system is planned for a four-storey residential building in Hamburg Wilhelmsburg at the end of 2013.

Incorporating building-integrated photovoltaics (BIPVs) into the façade generates energy more directly. The PV’s can also provide some solar shading to reduce the cooling load. can often have PVs can often be applied to high cost facades at no additional cost, while electricity generation is at point of use. In the future thin-film solar cells could allow PV generation to be added to typical façade elements at a marginal additional cost, making the technology far more ubiquitous.