Each of these terms describes a variety of glazing in which the transmission properties are variable. Extensive opportunities exist for the development of some of these technologies to allow dynamic control of light and heat gain to match building and occupant requirements. Photochromic devices change transmission in response to prevailing radiation levels. Small examples have been in everyday use for some years in the form of sunglasses and spectacles. These react automatically to light levels. There are considerable technical problems to scaling up photochromic glass to normal window size.
Thermochromic glass has changing optical properties in response to temperature variations. It has a laminated structure incorporating a chemical which turns opaque at around 30oC, reducing insolation by about 70 per cent. For this reason an ideal application is as external solar shading. As it reacts to heat it may not be so suitable for windows since it could react to the internal temperature and again cannot be independently controlled.
The most refined and controllable of the three options is elec-trochromic glass, the properties of which can be changed by the application of a small electrical current. Their construction consists of complex multi-layered transparent coatings. The electrical signal reduces the transmission capacity of the electrochromic layer between two sheets of glass affecting not only daylight but also solar heat.
The latest version from Pilkington is EControl glass which can, at the flick of a switch, cut out over 80 per cent of solar radiation. It can also achieve an airborne sound insulation level of 42 dB using thicker internal glass and a special sound insulating gas between the panes. Pilkington is also developing a solid state electrochromic glass, in other words, without any applied coatings.
Capital cost savings in terms of reduced cooling requirements and the exclusion of blinds plus revenue savings in respect of lower energy costs make electrochromic glass an attractive option, especially since it can be controlled by the occupants - a major factor in workplace satisfaction.
Pilkington has recently marketed a self-cleaning or 'hydrophilic' glass known commercially as 'Pilkington Activ'. Rainwater forms an overall film on the glass rather than collecting in drops that deposit dirt which remains after drying. This should offer significant revenue cost savings in maintenance, especially for commercial buildings.
Romag, a company specialising in laminated glass, has joined with BP Solar to produce a composite glass which incorporates PV cells. It will be marketed as PowerGlaz and should be available towards the end of 2004. It will be available in a range of sizes up to 3.3 X 2.2 metres.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.