Design Strategy 64: Solar Air Collectors

THERMAL COLLECTOR WALLS and ROOFS capture solar heat at the edge of a room in a layer of air, which carries the heat to storage in the building's structure. (heating)

This excerpt is presented with permission. You can purchase Sun, Wind and Light from the Iris Catalog. Follow the link above to see a detailed description of the book and a complete table of contents.

Air collectors may be manufactured products or may be site-built and integrated with the architectural design. If the building's section is designed in an open scheme between levels, distribution can be by natural convection, similar to the function of vented thermal storage walls. However, because the performance of thermosiphoning systems are so hard to predict, a fully-sized back-up mechanical ventilation system should be designed. When rooms must be closed to each other, when manufactured collectors are used, or when storage is within voids of the structure, distribution must be assisted by fans (Strategy 105) and may use ducts or plenums (Strategy 107). Warm air collectors are especially well suited to buildings with high daytime heat loads or those that require high levels of fresh ventilation air (Strategy 87, Technique 17).

In the Housing Development in Passau, architects Schroder and Widmann used as warm air collectors glazed, unconditioned airlock entries connected to thin glazed shafts above, covering portions of the exterior wall and windows. Doors and windows are used as the primary means of regulating air flow from the collector to rooms behind. A central internal air promotes a convective loop within the section of each dwelling. Thermal storage is in the massive floors and ceilings. In summer, fabric shading is used to reduce heat gain, lower glass panels are removed to make an outdoor room and ventilation flaps at the top of the collector can be opened to release excess heat by stack effect.

Heat from air collectors can be transferred to mass using the room air, or, as in the Lutzstrassed Apartments in Berlin, warmed air can be blown, using fans, through hollow cores in a massive floor, called a hypocaust (Hastings, 1999, pp. 92-95). Architects from the Institute for Building, Environment, and Solar Research designed the building with a closed loop from collectors in the south facade (see images in Strategy 61) through tubes embedded in the concrete floor, and back to the collector. Discharge is by radiant transfer through the slab. This has the advantage of keeping the indoor air temperature from rising rapidly when the collector is heated by the sun.

In a dense urban context, full solar access to walls may not be possible, but the roof often retains solar access all winter. Warm air collectors allow the heat to be captured high on the roof, as it is in the retrofit of the Apartment Block in Gothenburg, Sweden, by Christer Nordstrom (Hastings, 1999, pp. 113-117). Air warmed from rooftop collectors is ducted by mechanical ventilation to a murocaust cavity in the external walls, formed by adding an insulated layer outside the existing uninsulated masonry wall. The detail sketch of the site-built air collector shows typical features: a clear layer of double glazing over a thin air space, backed up by a black-painted corrugated sheet metal absorber plate that captures heat and transfers it to air circulated behind the plate and over an insulating layer.