SOLAR AND GREEN BUILDINGS |
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Objective |
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Objective |
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The use of solar gains is one possibility to increase energy efficiency in buildings and thus to reduce their impact on environment. It consists of collecting solar radiation, preferably storing the heat produced and distributing it where and when it is needed. Furthermore, attention should be paid to comfort. Many components of a building envelope have a thermal function (transmission of solar gains and light through windows, heat storage by masonry, insulation, ventilation, solar protection etc.). Employing them appropriately allows cost reduction during construction, e.g. by storing heat in a slab instead of adding a water tank, and during utilisation. Aesthetic concerns also require a good integration of components. Thus, collaboration between architects and engineers is essential. The concept of transparent insulation is an examplary contribution of science in architecture. The idea is to maximize net gains by increasing transparency and reducing heat losses. TIMs (transparent insulation materials) can replace classical opaque external insulation. They can also be used like glazings in various configurations (windows, sunspaces, air collectors and Trombe walls) for various purposes : daylighting, passive or active solar space heating, domestic water heating, active cooling,etc. Their special light diffusion with diurnal and seasonal variations of light intensity, their mysterious and cloudy aspect on opaque facades give new creative possibilities to architects. After a short technical presentation, application possibilities and practical advice will be given. |
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New technologies |
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The translucent fur of polar bears allows them to collect the scanty radiation of a low sun, while protecting them from cold. It took a long time to men to achieve this artificially. Glass was invented in Egypt or Mesopotamia around 1500 B.C., and used for jewelry long before the first leaded glass windows appeared in buildings. It was only in the early 60's that Giovanni Francia formed thin glass honeycombs, aiming at increasing the efficiency of solar collectors. At the same time Felix Trombe developed with the architect Jacques Michel the well known "Trombe wall" concept. In Israel, honeycomb glass was replaced by extruded plastics for applications in solar ponds. Adolf Goetzberger (Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany) applied these materials as transparent insulation for buildings in the early 80's. The search for higher transparency and lower heat transfer is a continuous challenge for physicists, and various approaches exist in parallel (e.g. evacuated glazings, aerogels, etc.). We will focus here on manufactured products proposed on the market, i.e. extruded plastic structures.
Appropriate materials should be chosen according to the expected temperature level : PMMA can be used for glazings, but polycarbonate, having a higher melting point, is needed in case of higher temperatures (e.g. when applied on opaque walls, air or water collectors). In collectors without thermal mass, problems may occur in stagnation conditions. More sophisticated materials exist for still higher temperatures (solar cookers, etc.), but this does not concern applications in the building sector. A thickness L of 100 mm, offering insulative capabilities similar to 50 mm of glasswool, is prefered if heat is stored near the transparent cover (e.g., solar walls and integrated collector storage systems for domestic hot water in cold climates). A lower thickness (around 50 mm) is enough for windows, air collectors and solar water heaters in mild climates. The section of a cell can be either rectangular or circular, according to the extrusion process (addresses of manufacturers are given in a table below). The energy transmission factor can reach more than 90% at normal incidence and up to 75% for the diffuse transmission. The geometrical properties of cellular structures can be further optimized according to a specific project using a building simulation tool, in order to find the best compromise between transparency and insulation. |
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At the service of the architect |
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Used like glazings, TIMs provide interesting daylighting conditions as they diffuse light, avoiding glare effects and distributing light deeper into rooms. Attention should be paid to the high density of transmitted light. Because the landscape cannot be seen through them, TIMs do not replace classical glazings but rather complement them. Good examples of such utilizations are some office buildings or libraries. Various possibilities exist concerning integration in space heating systems. Glazed areas constitute the simplest system but gains must be stored, otherwise effciciency and thermal comfort might be poor. TIMs can form the roof of a sunspace in order to decrease radiation losses towards the sky, while vertical glazings offer a good view. It is essential that openings allow sufficient ventilation in order to avoid overheating in summer. TIMs can also be integrated in the wall between the sunspace and the building, allowing light and solar radiation to go through. They can be used as external insulation layer. Beside solar gains, the reduction of damp problems in humid climates is another advantage. Retrofitting applications are very effective provided that the old wall has a high conductivity (i.e. no insulation, hollow bricks, or such materials). The large need for renovation is an opportunity for such applications. Two other possible solar systems follow as examples. ACTIVE SOLAR HEATING BY AIR COLLECTORS WITH TIMS
TROMBE WALLS WITH A TRANSPARENT INSULATION COVER
Part of south facades can incorporate collectors for preheating of ventilation air. This application gives one of the highest productivities, and is particularly adapted to educational buildings where the ventilation demand is very high. Other south facing areas can include a domestic water heater. An integrated collector storage system can also be built on site : a cylindrical water tank is covered by transparent insulation on the front and surrounded by a mirror on the rear side, allowing solar beams to converge onto the black storage. Such a design is simpler than conventional devices, which in cold regions need an additionnal circuit with antifreeze. In warm climates, collectors covered by TIMs can also be used to constitute the boiler of an absorption heat pump for active cooling applications. |
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Practical advice |
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TIMs can be ordered from the manufacturers listed in the table below. They may be encapsulated in the factory between two glazings and an aluminium frame. This avoids problems with dust and water on the building site, but of course the product is more expensive (around 210 Euros per m2) than the simple plastic structure (around 50 Euros per m2 for 96 mm thickness).
Small holes should be left in the frame so that a light ventilation evacuates
condensation water on the outer glazing. Trombe walls should not be higher than
3m in order to avoid chimney effects in the air gap or alternatively, this gap
should be divided into separate cells. CONTACT OF MANUFACTURERS
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Demonstration project |
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Industrialized construction process (wooden frame, cf fig. 4) have allowed the construction of six houses in the French Ardennes for social housing (architect: Jacques Michel). One house costs around 70,000 Euros and the solar overcost is between 4,500 and 14,500 Euros according to the system (passive systems are cheaper in this project) and the transparent cover.
The monitoring of the houses gave a heating load of 46 to 83 kWh/m2/yr according to the system (active systems are more efficient in this project) and the users behaviour.
Many other TI projects were built in various countries, e.g. in Germany, England, Switzerland, Denmark and China. The measured productivity of solar walls/roofs varies between 40 and 200 kWh/m2/yr according to many factors (construction, users behaviour, climate,...). |
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Conclusions |
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The progress achieved concerns both the material itself and its integration in buildings. Manufactured components are already available on the market and industrialization of new products (e.g. air collectors) is possible. This will allow a still larger diversification of application possibilities. The large variety of demonstration projects shows how architects can use the technology and how a physicist's concept can become a building material used for creative purposes. Monitored results demonstrate the on site efficiency of transparent insulation projects. Greenhouse effect on the scale of a building prevents from greenhouse effect and global warming on a planetary level. |
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Contact |
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Bruno PEUPORTIER |
